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UBC Theses and Dissertations

In-situ testing of soil with emphasis on its application to liquefaction assessment Robertson, Peter Kay 1982

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IN-SITU TESTING OF SOIL WITH EMPHASIS ON ITS APPLICATION TO LIQUEFACTION ASSESSMENT by PETER KAY ROBERTSON B.Sc., The U n i v e r s i t y of Nottingham, 1972 M.A.Sc, The U n i v e r s i t y of B r i t i s h Columbia, 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF APPLIED SCIENCE DEPARTMENT OF CIVIL ENGINEERING We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA December 1982 © Peter Kay Robertson, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Ct <~ /rt€£Ylirt The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) i i ABSTRACT The major objective of t h i s research was to advance the state of the art i n i n t e r p r e t a t i o n and a p p l i c a t i o n of r e s u l t s from i n - s i t u t e sting of s o i l , i n p a r t i c u l a r the Cone Penetration Test (CPT), the Self-boring Pressuremeter Test (SBPMT) and the Flat Plate Dilatometer Test (DMT). This study c r i t i c a l l y examines the equipment, f i e l d procedures and methods of test i n t e r p r e t a t i o n so that improvements can be made i n t h e i r a p p l i c a t i o n to f i e l d l i q u e f a c t i o n assessment. Improvements to i n - s i t u t e s t equipment and procedures are proposed. Improvements for the i n t e r p r e t a t i o n of CPT data i n sands for evaluating r e l a t i v e density, f r i c t i o n angle and modulus are made. A method for prediction of deformation c h a r a c t e r i s t i c s of clay from CPT data i s proposed by incorporating the influence of s o i l s t i f f n e s s . A c o r r e l a t i o n between c y c l i c stress r a t i o to cause l i q u e f a c t i o n (10 percent double amplitude shear strain) and cone penetration resistance i s proposed f o r sands and s i l t y sands. The proposed CPT l i q u e f a c t i o n c o r r e l a t i o n i s substantiated using data from B.C., Japan, China and U.S.A. and appears to represent a good lower bound. The addition of continuous pore pressure measurements during cone penetration i s shown to s i g n i f i c a n t l y improve the i n t e r p r e t a t i o n of the CPT. Data i s also presented that c l a r i f i e s the c o r r e l a t i o n between the Standard Penetration Test (SPT) and the CPT. Improvements are suggested f o r the i n t e r p r e t a t i o n of SBPMT data i n sands for f r i c t i o n angle and modulus. These improvements are applied to the assessment of l i q u e f a c t i o n resistance using the SBPMT. i i i New c o r r e l a t i o n s are proposed f o r estimating the r e l a t i v e d e n s i t y and l i q u e f a c t i o n r e s i s t a n c e of sand using the r e s u l t s from DMT. A f i e l d and l a b o r a t o r y study i s c a r r i e d out to evaluate e x i s t i n g and proposed methods of i n t e r p r e t a t i o n of i n - s i t u t e s t s and t h e i r a p p l i c a t i o n to the assessment of l i q u e f a c t i o n r e s i s t a n c e . In general the proposed new c o r r e l a t i o n s produce good r e s u l t s , although f u r t h e r f i e l d v e r i f i c a t i o n i s r e q u i r e d . i v TABLE OF CONTENTS Page 1 INTRODUCTION 1.1 I n - s i t u T e s t i n g i n Geotechnical Engineering 1 1.2 Mechanism of S o i l L i q u e f a c t i o n . . 2 1.3 E x i s t i n g Methods f o r E v a l u a t i o n of L i q u e f a c t i o n P o t e n t i a l . 5 1.4 Report Objectives . . . . . . . . . . . . . . . . 9 2 REVIEW OF EXISTING IN-SITU TEST METHODS 2.1 I n t r o d u c t i o n 11 2.2 Standard P e n e t r a t i o n Test 13 2.3 Cone Penetration Test 17 2.4 F l a t P l a t e Dilatometer 21 2.5 E l e c t i c a l R e s i s t i v i t y 23 2.6 Pressuremeter 25 2.7 Acoustic Emission 31 2.8 Geophysical Tests 31 2.9 P l a t e Load Tests and Screw Pl a t e Tests 33 3 REVIEW OF EXISTING IN-SITU TESTING APPLICATIONS FOR ASSESSMENT OF LIQUEFACTION POTENTIAL 3.1 I n t r o d u c t i o n 35 3.2 Standard Penetration Test 35 3.3 Cone P e n e t r a t i o n Test 42 3.4 E l e c t r i c a l R e s i s t i v i t y 44 3.5 Pressuremeter . . . 4 6 3.6 Geophysical Method 49 3.7 General Review 50 V Page 4 CONE PENETRATION TESTING 4.1 Equipment . . . . . . . . . . . 52 4.2 Factors Affecting Measured Parameters from E l e c t r i c Cone 4.2.1 Bearing and F r i c t i o n 54 4.2.2 Pore Pressure E f f e c t s on Measured Parameters . . . 55 4.2.3 Friction-Bearing Offset 58 4.2.4 Piezometer Tip Saturation 59 4.2.5 Rate of Penetration 59 4.2.6 Slope Sensor 63 4.2.7 F r i c t i o n Sleeve Measurement 64 4.3 Interpretation: E l e c t r i c F r i c t i o n Cone 67 4.3.1 S o i l C l a s s i f i c a t i o n 69 4.3.2 Stratigraphy 73 4.3.3 Density 74 4.3.4 Drained Shear Strength of Sand 80 4.3.5 Undrained Shear Strength of Clay 86 4.3.6 Deformation C h a r a c t e r i s t i c s of Sand 91 4.3.7 Deformation C h a r a c t e r i s t i c s of Clay 101 4.4 Interpretation: E l e c t r i c Piezometer F r i c t i o n Cone . . . 102 4.4.1 S o i l Type and Stress History 104 4.4.2 C o e f f i c i e n t of Consolidation and Permeability . . 107 4.5 L i q u e f a c t i o n Resistance 4.5.1 Introduction 112 4.5.2 Normalized Cone Resistance 113 4.5.3 Relative Density C o r r e l a t i o n 113 4.5.4 Shear Strength Correlation 115 v i Page 4.5.5 SPT-CPT Correlation 115 4.5.6 Proposed CPT Liquefaction Relationship 121 5 PRESSUREMETER TESTING 5.1 Equipment 133 5.2 I n s t a l l a t i o n 133 5.3 Factors A f f e c t i n g Results from Pressuremeter Testing . . . 136 5.3.1 I n s t a l l a t i o n 136 5.3.1.1 Water Pressure Control 138 5.3.1.2 Cutter Geometry 138 5.3.1.3 Insertion Rate 141 5.3.1.4 V e r t i c a l i t y 142 5.3.2 Testing Procedures 5.3.2.1 Temperature E f f e c t s 142 5.3.2.2 Rate of Expansion 143 5.3.3 Piezometer Saturation 145 5.3.4 Comments on Equipment 146 5.4 Interpretation 147 5.4.1 In-Situ Stress 149 5.4.2 Drained Shear Strength of Sand 152 5.4.3 Shear Modulus 169 5.4.4 C y c l i c Testing 177 5.5 Cone-Pressuremeter 183 5.6 Liquefaction 5.6.1 Proposed D i l a t i o n Angle C o r r e l a t i o n . . . . . . . . 185 5.6.2 Proposed Cycl i c Testing Correlation 192 v i i Page 6 FLAT PLATE DILATOMETER TESTING (DMT) 6.1 Equipment 198 6.2 Factors Affecting Results from DMT 201 6.2.1 Slope 201 6.2.2 Pore Pressure Effects 202 6.3 Interp r e t a t i o n 6.3.1 Introduction 204 6.3.2 Theoretical Considerations 6.3.2.1 General 205 6.3.2.2 DMT i n Sand 209 6.3.2.3 DMT i n Clay 212 6.4 Liquefaction Resistance 6.4.1 E x i s t i n g DMT Liquefaction C o r r e l a t i o n 213 6.4.2 Proposed DMT Liquefaction Correlation 214 7 FIELD AND LABORATORY STUDY 7.1 Introduction 217 7.1.1 F i e l d Study 217 7.1.2 Laboratory Study 217 7.2 McDonalds Farm, Richmond 221 7.2.1 S i t e Description and Geology 221 7.2.2 F i e l d Study 223 7.2.3 Laboratory Study 230 7.2.3.1 C l a s s i f i c a t i o n and Grain Size 230 7.2.3.2 Density 235 7.2.3.3 Shear Strength 236 v i i i Page 7.2.3.4 Modulus 237 7.2.3.5 C y c l i c Resistance 237 7.2.4 Results 7.2.4.1 S o i l C l a s s i f i c a t i o n 240 7.2.4.2 I n - S i t u Stress 241 7.2.4.3 R e l a t i v e Density 245 7.2.4.4 Shear Strength 7.2.4.4.1 Sand 250 7.2.4.4.2 Clayey S i l t 254 7.2.4.5 Modulus 7.2.4.5.1 S t a t i c Shear Modulus 258 7.2.4.5.2 Dynamic Shear Modulus 268 7.2.4.6 L i q u e f a c t i o n 273 7.2.4.6.1 Standard P e n e t r a t i o n Test . . . 276 7.2.4.6.2 Cone P e n e t r a t i o n Test 276 7.2.4.6.3 S e l f - b o r i n g Pressuremeter Test 282 7.2.4.6.4 Dilatometer Test 282 7.2.4.6.5 Seismic Test 282 7.2.5 Summary 284 7.3 Fraser Landing, New Westminster 7.3.1 S i t e D e s c r i p t i o n and Geology 284 7.3.2 F i e l d Study 289 7.3.3 Laboratory Study 289 7.3.4 Results 291 ix Page 7.4 Other S i t e s 7.4.1 McDonald's Farm 306 7.4.2 N i i g a t a , Japan 309 7.4.3 Tangshan, China 312 7.4.4 Imperial V a l l e y , USA 314 7.4.5 SPT/CPT C o r r e l a t i o n 316 7.4.6 Summary 323 8 SUMMARY AND CONCLUSIONS 8.1 Cone Pe n e t r a t i o n Test 327 8.1.1 Equipment and procedures 327 8.1.2 I n t e r p r e t a t i o n 329 8.1.3 L i q u e f a c t i o n Assessment 330 8.2 S e l f - B o r i n g Pressuremeter Test 333 8.2.1 Equipment and procedures 333 8.2.2 I n t e r p r e t a t i o n . 333 8.2.3 L i q u e f a c t i o n Assessment 335 8.3 F l a t P l a t e Dilatometer Test 336 8.3.1 Equipment and procedures 336 8.3.2 I n t e r p r e t a t i o n 337 9 RECOMMENDATIONS FOR FURTHER RESEARCH 9.1 Cone Pe n e t r a t i o n Testing 338 9.2 Se l f - B o r i n g Pressuremeter Testing 339 9.3 F l a t P l a t e Dilatometer Testing 340 X Page REFERENCES 342 APPENDIX 1. C h a r a c t e r i s t i c Behaviour o f Cohesionless S o i l s . . 356 APPENDIX 2. Kinematics of Cavity Expansion 376 x i LIST OF TABLES Table Number T i t l e Page 2.1. Perceived A p p l i c a b i l i t y of In - s i t u Test 14 Methods - Update 1982 (After Campanella and Robertson, 1982) 2.2. General Types of Cone Penetration Tests 17 (Adapted from Schmertmann, 1975) 4.1. Properties of Sand Tested i n C a l i b r a t i o n 77 Chamber Studies ( A f t e r Robertson and Campanella, 1982) 4.2. Summary of Ca l i b r a t i o n Chamber Results for 92 Constrained Modulus Factor, a (After Robertson and Campanella, 1982) 4.3. Estimation of Compression Index, C c, from 101 C u / a ' v o ^ t i o ( A f t e r Schmertmann, 1978) 4.4. Anisotropic Permeability of Clays 112 (After Baligh and Levadoux, 1980) 5.1. Comparison of French and English Self-Boring 137 Pressuremeters 5.2. Summary of Cy c l i c Pressuremeter Tests, 193 Beaufort Sea 7.1. Summary of Self-Boring Pressuremeter Testing 229 McDonald's Farm, Richmond 7.2. Summary of Push-in Cone-Pressuremeter Testing 231 McDonald's Farm, Richmond 7.3. Summary of Static Drained T r i a x i a l Compression 232 Test Results, McDonald's Farm, Richmond 7.4. Summary of Cy c l i c Undrained T r i a x i a l Test 233 Results, McDonald's Farm, Richmond 7.5. Summary of Self-Boring Pressuremeter Results, 243 McDonald's Farm, Richmond 7.6. Self-Boring Pressuremeter Results for Shear 252 Strength, McDonald's Farm 7.7. Self-Boring Pressuremeter Results - 257 Clayey S i l t , McDonald's Farm x i i LIST OF TABLES (cont'd) Table Number T i t l e Page 7.8. Summary of the Laboratory Moduli and Moduli 260 Numbers, McDonald's Farm 7.9. Summary of the Pressuremeter Test Results f o r 261 Shear Modulus, McDonald's Farm 7.10. Cone-Pressuremeter R e s u l t s , McDonald's Farm 267 7.11. Summary of Standard Penetration Test 274 L i q u e f a c t i o n Assessment, McDonald's Farm 7.12. Summary of Se l f - B o r i n g Pressuremeter Assessment 279 of L i q u e f a c t i o n Resistance, McDonald's Farm 7.13. Summary of C y c l i c S e l f - B o r i n g Pressuremeter 281 Res u l t s f o r L i q u e f a c t i o n Resistance, McDonald's Farm 7.14. Comparison of S i l t Parameters Before and A f t e r 292 Dynamic Compaction, New Westmister S i t e 7.15. Summary of F i e l d and C y c l i c Laboratory R e s u l t s , 307 McDonald's Farm 7.16. Summary of F i e l d and Laboratory R e s u l t s , N i i g a t a , Japan 310 x i i i LIST OF FIGURES T i t l e Estimated range of l i m i t i n g Shear S t r a i n Developed under Undrained C y c l i c Shear f o r Saturated Sands (Adapted from Seed, 1 9 7 6 ) Examples of Mechanical and E l e c t r i c Cones F l a t P l a t e Dilatometer and Readout Unit Schematic of Menard Pressuremeter Comparison of Menard Pressuremeter Test Result and S e l f - b o r i n g Pressuremeter Test R e s u l t Schematic of S e l f - b o r i n g Pressuremeter C o r r e l a t i o n Between F i e l d L i q u e f a c t i o n Behaviour of Sands (D^Q > 0 . 2 5 mm) Under L e v e l Ground Conditions and Standard P e n e t r a t i o n Resistance ( A f t e r Seed and I d r i s s , 1 9 8 1 ) R e l a t i o n s h i p Between C N and E f f e c t i v e Overburden Pressure ( A f t e r Seed and I d r i s s , 1 9 8 1 ) Comparison Between Seed and I d r i s s ( 1 9 8 1 ) and Iwasaki et a l . ( 1 9 7 5 ) SPT Based Methods C o r r e l a t i o n Between F i e l d L i q u e f a c t i o n Behaviour of Sands f o r Level Ground Conditions and E l e c t r i c a l Parameter ( A f t e r A r u l m o l i et a l . , 1 9 8 1 ) C o r r e l a t i o n Between Resistance to L i q u e f a c t i o n o Sand as a Function of R e l a t i v e Density and Corrected D i l a t i o n Angle ( A f t e r Vaid et a l . , 1 9 8 1 ) V a r i a t i o n of Threshold A c c e l e r a t i o n with Shear Wave V e l o c i t y ( A f t e r Dobry et a l . , 1 9 8 1 ) 5-Channel Cone Penetrometer ( A f t e r Campanella & Robertson, 1 9 8 1 ) Influence of Unequal End Areas ( A f t e r Campanella Robertson and G i l l e s p i e , 1 9 8 3 ) Tip Design to Relocate Porous F i l t e r and Allow Easy S a t u r a t i o n w i t h G l y c e r i n ( A f t e r Campanella, Robertson and G i l l e s p i e , 1 9 8 3 ) x i v LIST OF FIGURES (cont'd) Figure No. T i t l e Page 4.4 Penetration Rate A f f e c t s i n Clayey S i l t Deposit at 61 McDonald's Farm ( A f t e r Campanella, Robertson and G i l l e s p i e , 1983) 4.5 F r i c t i o n Along Shaft During Cone Penetration i n 65 Sand at McDonald's Farm ( A f t e r Campanella and Robertson, 1981) 4.6 Change i n H o r i z o n t a l Stress C o e f f i c i e n t due to Cone 68 P e n e t r a t i o n i n Sand 4.7 S o i l C l a s s i f i c a t i o n Chart f o r Standard E l e c t r i c 70 Cone ( A f t e r Douglas and Olsen, 1981) 4.8 S o i l C l a s s i f i c a t i o n Chart f o r Mechanical F r i c t i o n 72 Cone ( A f t e r S e a r l e , 1979) 4.9 Comparison of D i f f e r e n t R e l a t i v e Density 76 R e l a t i o n s h i p s ( A f t e r Robertson and Campanella, 1982) 4.10 R e l a t i v e Density R e l a t i o n s h i p f o r Uncemented and 79 Unaged Quartz Sands (Adapted from B a l d i e t a l . , 1982) 4.11 R e l a t i o n s h i p Between Bearing Capacity Number and 84 F r i c t i o n Angle from Large Chamber Tests ( A f t e r Robertson and Campanella, 1982) 4.12 Proposed C o r r e l a t i o n Between Cone Bearing and 85 F r i c t i o n Angle f o r Uncemented, Quartz Sands ( A f t e r Robertson and Campanella, 1982) 4.13 E f f e c t of R i g i d i t y Index and Cone Angle on the 88 P e n e t r a t i o n Resistance of Clay ( A f t e r B a l i g h , 1975) 4.14 R e l a t i o n s h i p Between Cone Bearing and Constrained 94 Modulus f o r Normally Consolidated, Uncemented, Quartz Sands ( A f t e r Robertson and Campanella, 1982) 4.15 R e l a t i o n s h i p Between Cone Bearing and Drained 97 Young's Modulus f o r Normally Consolidated, Uncemented, Quartz Sands ( A f t e r Robertson and Campanella, 1982) 4.16 C o r r e l a t i o n Between Dynamic Shear Modulus Number 99 and R e l a t i v e Density ( A f t e r Robertson and Campanella, 1982) X V LIST OF FIGURES (cont'd) Figure No. T i t l e Page 4 . 1 7 Proposed R e l a t i o n s h i p Between Cone Bearing and 1 0 0 Dynamic Shear Modulus f o r Normally Consolidated, Uncemented, Quartz Sands ( A f t e r Robertson and Campanella, 1 9 8 2 ) 4 . 1 8 S e l e c t i o n of S o i l S t i f f n e s s Ratio f o r Clays 1 0 3 (Adapted from Ladd et a l . , 1 9 7 7 ) 4 . 1 9 Summary of E x i s t i n g Solutions f o r Pore Pressure 1 0 8 D i s s i p a t i o n (Adapted from G i l l e s p i e , 1 9 8 2 ) 4 . 2 0 Proposed V a r i a t i o n of C o r r e c t i o n Factor, C Q, with 1 1 4 E f f e c t i v e Overburden Pressure 4 . 2 1 C o r r e l a t i o n Between L i q u e f a c t i o n Resistance and 1 1 6 Cone P e n e t r a t i o n Resistance f o r Sands Based on R e l a t i v e Density C o r r e l a t i o n 4 . 2 2 A n a l y s i s of F i e l d Records of S i t e s Where 1 1 7 L i q u e f a c t i o n d i d and d i d not Occur ( A f t e r Vaid et a l . , 1 9 8 1 ) 4 . 2 3 C o r r e l a t i o n Between L i q u e f a c t i o n Resistance and 1 1 8 Cone Pe n e t r a t i o n Resistance i n Sands Based on F r i c t i o n Angle C o r r e l a t i o n 4 . 2 4 V a r i a t i o n of q c/N Ratio with Mean Grain Size 1 2 0 ( A f t e r Robertson et a l . , 1 9 8 2 ) 4 . 2 5 C o r r e l a t i o n Between L i q u e f a c t i o n Resistance and 1 2 2 Cone Pe n e t r a t i o n Resistance i n Sands Based on SPT C o r r e l a t i o n 4 . 2 6 C o r r e l a t i o n Between L i q u e f a c t i o n Resistance and SPT 1 2 3 Showing Proposed Lower Bound (Data from Seed and I d r i s s , 1 9 8 1 ) ~ ' 4 . 2 7 Summary of C o r r e l a t i o n s Between L i q u e f a c t i o n 1 2 4 Resistance and Cone P e n e t r a t i o n Resistance i n Sands 4 . 2 8 S o i l C l a s s i f i c a t i o n Chart f o r E l e c t r i c Cone Showing 1 2 7 Proposed Zone of L i q u e f i a b l e S o i l s 4 . 2 9 Proposed C o r r e l a t i o n s Between L i q u e f a c t i o n Re s i s t a n c e Under Le v e l Ground Conditions and Cone Penetration Resistance f o r Sands and S i l t y Sands 1 2 9 x v i LIST OF FIGURES (cont'd) Figure No. T i t l e Page 5.1 I l l u s t r a t i o n of Self-boring Pressuremeter 134 I n s t a l l a t i o n at McDonald's Farm 5.2 Schematic to Show Eff e c t s of Cutter P o s i t i o n on 140 Degree of Disturbance (Adapted from Wroth, 1982) 5.3 Examples of "Good" and "Poor" Self-boring 150 Pressuremeter Test Results 5.4 Example of Potential Error i n Assessment of I n - s i t u 153 Stress Due to I n i t i a l Disturbances i n Sand 5.5 Stress-Strain and Volumetric Strain-Shear Strain 155 Curves f o r (a) Simple Shear Test Results (Stroud, 1971), (b) Idealized by Hughes et a l . , (1977). 5.6 Approximate Var i a t i o n of Shear Strain with Radial 158 Distance from Pressuremeter Probe. 5.7 Stress-Strain Behaviour of Ottawa Sand i n Drained 159 Simple Shear (After Vaid et a l . , 1981) 5.8 Volumetric Strain-Shear Strain Curves for Loose and 160 Dense Ottawa Sand i n Drained Simple Shear (Data from Vaid et a l . , 1981) 5.9 Va r i a t i o n of D i l a t i o n Angle with Relative Density 162 for Ottawa Sand i n Drained Simple Shear 5.10 Proposed Correlation Between Self-boring 163 Pressuremeter Data and Peak F r i c t i o n Angle, Corrected for Strain Level 5.11 Proposed Correlation Between Self-boring 164 Presuremeter Data and Maximum D i l a t i o n Angle, Corrected for Strain Level 5.12 Ef f e c t s of Large I n i t i a l Disturbance on 167 Pressuremeter Data i n Sand at McDonald's Farm 5.13 Comparison Between Proposed Correlation and Hughes 168 et a l . (1977) Cor r e l a t i o n f o r Peak F r i c t i o n Angle from Self-boring Pressuremeter Test Data i n Sands 5.14 Idealised Behaviour of Free Draining Sand During 171 Self-boring Pressuremeter Test 5.15 Self-boring Pressuremeter Test Data i n Sand at a Depth of 10.9 m at McDonald's Farm 172 x v i i LIST OF FIGURES (cont'd) Figure No. T i t l e Page 5.16 Hyperbolic Stress Strain Curve 176 5.17 Examples of C y c l i c Pressuremeter Tests i n Loose and 178 Dense Sand (Adapted from Hughes et a l . , 1980) 5.18 Cumulative Strain Under Several Cycles of Loading 180 i n Pressuremeter Test (Adapted from Hughes et a l . , 1980) 5.19 Cumulative Strain with Number of Cycles with 182 D i f f e r e n t C y c l i c S t r a i n Amplitudes 5.20 Schematic of Cone-Pressuremeter 184 5.21 Cone-Pressuremeter Test Result at a Depth of 7.6 m 186 at McDonald's Farm 5.22 V a r i a t i o n of D i l a t i o n Angle with Relative Density 188 f o r Ottawa Sand i n Drained Simple Shear 5.23 Proposed Correlation Between Self-boring 189 Pressuremeter Data and Tangent D i l a t i o n Angle at Y = 10% 5.24 V a r i a t i o n of Maximum D i l a t i o n Angle, v, with Mean 191 Normal Stress f o r Various Sands 5.25 Correlation Between Cumulative Strain at 10 cycles 195 and Corrected D i l a t i o n Angle from Self-boring Pressuremeter Test Data, Beaufort Sea 5.26 Proposed Correlation Between C y c l i c Stress Ratio 196 and Cumulative St r a i n at 10 c y c l e s . 6.1 Schematic of Dilatometer 199 6.2 Intermediate Parameters from Dilatometer, DMT-2, 206 McDonald's Farm 6.3 Tabular Output from Dilatometer, DMT-2, 207 McDonald's Farm 6.4 Proposed Correlation Between Horizontal Stress 211 Index from DMT and Relative Density f o r Normally Consolidated, Uncemented Sand 6.5 Proposed Correlation Between Liquefaction 215 Resistance Under Level Ground Conditions and Dilatometer Horizontal Stress Index for Sands x v i i i LIST OF FIGURES (cont'd) Figure No. T i t l e Page 7.1 UBC F i e l d Research Vehicle, Supported and Leveled 218 on Large Pads, Raised Mast Houses Penetration Device (After Campanella and Robertson, 1981) 7.2 Schematic of T r i a x i a l Testing Apparatus 220 7.3 S o i l P r o f i l e for Research Site at McDonald's Farm, 222 Sea Island ( A f t e r Campanella, Robertson and G i l l e s p i e , 1983) 7.4 Site Plan of McDonald's Farm 224 7.5 Summary of Borehole Record, McDonald's Farm 225 7.6 Comparison of 4 CPT P r o f i l e s at McDonald's Farm 227 7.7 Intermediate Parameters from DMT-1, McDonald's 228 Farm 7.8 Range of Grain Size D i s t r i b u t i o n of Sand and Clayey 234 S i l t Deposits, McDonald's Farm 7.9 Self-boring Pressuremeter Horizontal E f f e c t i v e 244 Stresses Versus Depth, McDonald's Farm 7.10 Comparison of Laboratory and CPT Relative 246 Densities, McDonald's Farm 7.11 Comparison of Laboratory and DMT Relative 247 Dens i t i e s , McDonald's Farm 7.12 Change i n Horizontal Stress Index with Increases i n 249 I n - s i t u Horizontal Stress, Tincino Sand (Data from B e l l o t t i et a l . , 1979) 7.13 Comparison of Laboratory T r i a x i a l Peak F r i c t i o n 251 Angle with CPT and Self-Boring Pressuremeter Values, McDonald's Farm 7.14 Interpreted Geotechnical Parameters, DMT-1, 255 McDonald's Farm 7.15 Comparison of Laboratory and Self-boring 262 Pressuremeter S t a t i c Shear Moduli, McDonald's Farm 7.16 Comparison of Laboratory and Self-boring 264 Pressuremeter S t a t i c Shear Moduli Number, McDonald's Farm xix LIST OF FIGURES (cont'd) Figure No. T i t l e Page 7.17 Comparison of S e l f - b o r i n g and Cone-Pressuremeter 266 S t a t i c Shear Moduli Numbers, McDonald's Farm 7.18 Comparison of CPT, S e l f - b o r i n g and Cone- 269 Pressuremeter and Laboratory S t a t i c Shear Moduli Numbers, McDonald's Farm 7.19 Comparison Between CPT Predicted and Seismic 270 Measured Dynamic Shear Moduli, McDonald's Farm 7.20 Comparison Between CPT and Pressuremeter P r e d i c t e d 272 and Seismic Measured Dynamic Shear Moduli, McDonald's Farm 7.21 Comparison Between SPT Predicted and Laboratory 275 L i q u e f a c t i o n Resistance, McDonald's Farm 7.22 Comparison Between SPT and CPT P r e d i c t e d and 277 Laboratory L i q u e f a c t i o n Resistance, McDonald's Farm 7.23 Comparison Between S e l f - b o r i n g Pressuremeter and 280 CPT P r e d i c t e d and Laboratory Measured L i q u e f a c t i o n Resistance, McDonald's Farm 7.24 Comparison Between Dilatometer P r e d i c t e d and 283 Laboratory Measured L i q u e f a c t i o n Resistance, McDonald's Farm 7.25 C y c l i c Stress Ratio to Cause L i q u e f a c t i o n from a l l 285 I n - s i t u Test Methods versus Depth, McDonald's Farm 7.26 T y p i c a l S o i l P r o f i l e Before Treatment at New 287 Westminster S i t e 7.27 S i t e Plan, New Westminster S i t e 288 7.28 Standard Penetration Resistance Before and A f t e r 290 Dynamic Compaction, New Westminster 7.29 T y p i c a l Range of Grain Size D i s t r i b u t i o n of S i l t 293 Layer, New Westminster S i t e 7.30 Piezometer Cone Logging Before and A f t e r Dynamic 294 Compaction, New Westminster ( A f t e r Campanella, Robertson and G i l l e s p i e , 1983) 7.31 Laboratory Tests on S i l t Sample A f t e r Dynamic 296 Compaction, New Westminster S i t e X X LIST OF FIGURES (cont'd) Figure No. T i t l e Page 7.32 C o r r e l a t i o n Between C y c l i c Stress Ratio and Cone 298 Resistance i n S i l t a t New Westminster S i t e 7.33 A f t e r Dynamic Compaction at Sample L o c a t i o n , 299 Comparison of Porous Element L o c a t i o n , New Westminster ( A f t e r Campanella, Robertson and G i l l e s p i e , 1983) 7.34 Control Area (No Treatment) Comparison of Porous 301 Element L o c a t i o n , New Westminster ( A f t e r Campanella, Robertson and G i l l e s p i e , 1983) 7.35 Intermediate Parameters from Dilatometer i n Co n t r o l 302 Area (No Treatment), New Westminster S i t e 7.36 Intermediate Parameters from Dilatometer A f t e r 303 Dynamic Compaction, New Westminster 7.37 Intermediate Parameters from Dilatometer A f t e r 304 Vibrocompation, New Westminster S i t e 7.38 C o r r e l a t i o n Between C y c l i c Stress Ratio and Cone 308 Resistance at McDonald's Farm 7.39 C o r r e l a t i o n Between C y c l i c Stress Ratio and Cone 311 Resistance at N i i g a t a , Japan (Data from I s h i h a r a and Koga, 1981) 7.40 C o r r e l a t i o n Between C y c l i c Stress Ratio and Cone 313 Resistance at Tangshan, China (Data from Zhou 1980, 1981) 7.41 C o r r e l a t i o n Between C y c l i c Stress Ratio and Cone 315 Resistance i n 1979, Imperial V a l l e y Earthquake, C a l i f o r n i a , U.S.A. (Data from Youd and Bennett, 1981) 7.42 V a r i a t i o n of q c/N Ratio with Mean Grain S i z e , 318 I n c l u d i n g SPT Energy Measurements ( A f t e r Robertson et a l . , 1982) 7.43 Measured Average Energy Ratio f o r SPT N Values Using 319 Donut Hammer, T i l b u r y I s l a n d ( A f t e r Robertson et a l . , 1982) 7.44 Comparison of SPT N Values Using A l t e r n a t e Donut and 321 Safety Hammer w i t h Energy Corrected N c Values ( A f t e r Robertson, et a l . , 1982) xx i LIST OF FIGURES (cont'd) Figure No. T i t l e Page 7.45 C l a s s i f i c a t i o n Chart f o r E l e c t r i c Cones (Adapted 324 from Douglas and Olsen, 1981) ( A f t e r Robertson et a l . , 1982) 7.46 Summary of CPT L i q u e f a c t i o n Data 326 xx i i APPENDIX 1 A.1.1 I d e a l i z e d Behaviour of Sand i n Drained T r i a x i a l Compression ( A f t e r Vaid et a l . , 1981) A.1.2 S t r e s s - S t r a i n Behaviour of Ottawa Sand i n Drained Simple Shear ( A f t e r Vaid et a l . , 1981) A.1.3 Angle of I n t e r n a l F r i c t i o n f o r Chattahoochee Sand Tested at D i f f e r e n t Stress Levels i n T r i a x i a l Compression ( A f t e r Vesic and Clough, 1968) A.1.4 V a r i a t i o n of I n t e r n a l F r i c t i o n Angle with Stress f o r Monterey Sand #0 ( A f t e r V i l l e t and M i t c h e l l , 1981) A.1.5 Components of Shear i n Granular S o i l s A.1.6 V a r i a t i o n of D i l a t i o n Angle, v, with Mean Normal Stress f o r Various Sands A.1.7 I s o t r o p i c Compression of Chattahoochee Sand ( A f t e r Vesic and Clough, 1968) A.1.8 I d e a l i z e d Planes that Represent Changes i n Behaviour f o r Cohesionless S o i l s A.1.9 I d e a l i z e d S o i l Model Showing Surfaces that Represent Changes i n Behaviour f o r Cohesionless S o i l s xx i i i ACKNOWLEDGEMENT S I would l i k e to thank my advisor, Dr. R.G. Campanella for his constant i n t e r e s t and guidance during t h i s study. P a r t i c u l a r appreciation must also be extended to Dr. J.M.O. Hughes for his enthusiastic encouragement and assistance throughout t h i s study. I would also l i k e to extend my appreciation to my colleagues Don G i l l e s p i e , Steve Brown, Ian McPherson and Tony Rice f o r t h e i r assistance during the data c o l l e c t i o n . The talents of the C i v i l Engineering Technical Staff, Dick Postgate and Art Brookes, are grea t l y appreciated. The assistance and advice of Professors Y.P. Vaid and P.M. Byrne i s also g r a t e f u l l y acknowledged. Appreciation i s extended to Carol Lore f o r her typographical s k i l l s and patience during the preparation of t h i s d i s s e r t a t i o n . Mention must also be made of the support of the Izaak Walton Killam Memorial Scholarships. The assistance of NSERC and EMR, Canada; Ertec Western; Fugro B.V.; B.C. Minis t r y of Highways and Transportation; M i n i s t r y of Transport, Canada; MacLeod Geotechnical; Klohn Leonoff and the C i v i l Engineering Technical Staff at the Univ e r s i t y of B.C. i s most appreciated. A s p e c i a l thanks i s extended to my wife, Terry, for her constant support and encouragement. This d i s s e r t a t i o n i s dedicated to her. 1. 1 INTRODUCTION 1.1. I n - s i t u Testing i n Geotechnical Engineering The continued growth of many c i t i e s has l e d to increased c o n s t r u c t i o n o f l a r g e r more complex s t r u c t u r e s on s i t e s w i t h d i f f i c u l t ground c o n d i t i o n s . In s i t u a t i o n s where complex s t r u c t u r e s are founded on s o f t , s t r a t i f i e d s o i l s there i s r e l a t i v e l y l i t t l e evaluated experience and the u n c e r t a i n t i e s i m p l i c i t i n s i m p l i f i e d and h i g h l y e m p i r i c a l design methods have become extremely s i g n i f i c a n t . The use of more s o p h i s t i c a t e d and r e l i a b l e design procedures, and t h e i r continued development, has t h e r e f o r e become i n c r e a s i n g l y important. T h i s , i n t u r n , n e c e s s i t a t e s improved c a p a b i l i t i e s f o r logging, measurement and s e l e c t i o n of s o i l parameters, i n c r e a s i n g l y by i n - s i t u techniques. The requirement f o r b e t t e r logging methods and i n - s i t u t e s t s has created increased i n t e r e s t i n i n - s i t u t e s t i n g techniques. I n - s i t u t e s t i n g has a long h i s t o r y i n g e o t e c h n i c a l engineering. Load bearing t e s t s have been a part of foundation design even p r i o r to modern s o i l mechanics. The standard p e n e t r a t i o n t e s t and e a r l i e r forms of the cone p e n e t r a t i o n t e s t where both i n use before 1930 and represented the main methods f o r e a r l y subsurface e x p l o r a t i o n , and e v e n t u a l l y l e d to widely used design procedures based on e m p i r i c a l c o r r e l a t i o n s . E v a l u a t i o n of these t e s t s and development of new, more s o p h i s t i c a t e d i n - s i t u t e s t i n g techniques have r e c e n t l y become the subject of renewed i n t e r e s t and research. Since the Alaska and N i i g a t a earthquakes of 1964 g e o t e c h n i c a l engineers have taken a serious i n t e r e s t i n the general phenomenon of earthquake induced l i q u e f a c t i o n or c y c l i c m o b i l i t y and the c o n d i t i o n s r e s p o n s i b l e f o r causing them i n the f i e l d . S i g n i f i c a n t i n t e r e s t has been 2 d i r e c t e d toward the use of i n - s i t u t e s t i n g a p p l i e d to the assessment of f i e l d l i q u e f a c t i o n c h a r a c t e r i s t i c s . Japanese engineers were the f i r s t to use i n - s i t u t e s t i n g i n the form of the Standard P e n e t r a t i o n Test, to d i f f e r e n t i a t e between l i q u e f i a b l e and n o n l i q u e f i a b l e areas i n N i i g a t a . Considerable progress has been made i n recent years to develop new more s o p h i s t i c a t e d i n - s i t u t e s t i n g techniques and to apply them to the assessment of f i e l d l i q u e f a c t i o n c h a r a c t e r i s t i c s . This research i s d i r e c t e d towards improving our knowledge and understanding of i n - s i t u t e s t i n g w i t h p a r t i c u l a r a t t e n t i o n to i t s a p p l i c a t i o n i n the assessment of l i q u e f a c t i o n p o t e n t i a l i n the f i e l d . 1.2 Mechanism of S o i l L i q u e f a c t i o n It i s now g e n e r a l l y recognised t h a t the b a s i c cause of l i q u e f a c t i o n or c y c l i c m o b i l i t y i n cohesionless s o i l s during earthquake lo a d i n g i s the tendency f o r volume decrease due to the a p p l i c a t i o n of c y c l i c shear s t r e s s e s induced by ground shaking. I f the s o i l i s s a t u r a t e d , volume decrease cannot occur because water i s e s s e n t i a l l y incompressible and cannot escape from the s o i l pores during the short period of shaking. There i s , t h e r e f o r e , a r e s u l t i n g t r a n s f e r of s t r e s s to the pore water w i t h a corresponding r e d u c t i o n i n s t r e s s on the s o i l g r a i n s and thus a r e d u c t i o n i n shear s t r e n g t h . The accumulation of pore water pressure i s probably r e l a t e d to i r r e v e r s i b l e microscopic s l i p s a t i n t e r g r a n u l a r c o n t a c t s . There i s a balance between the volume r e d u c t i o n caused by c y c l i c shear, and rebound from the r e d u c t i o n i n the e f f e c t i v e s t r e s s . If the s o i l i s l o o s e , the pore pressure w i l l i n c r e a s e to a value equal to or c l o s e to the c o n f i n i n g pressure and l a r g e deformations w i l l occur a t a constant low r e s i d u a l s t r e s s . I f the s o i l develops e s s e n t i a l l y u n l i m i t e d 3 deformation i t i s s a i d to have l i q u e f i e d . The a c t u a l magnitude of the deformations r e s u l t i n g from l i q u e f a c t i o n depend on the s t a t i c d r i v i n g f o r c e s . I f on the other hand the s o i l i s dense, the pore water pressure does accummulate, s i n c e even dense sand i s c o n t r a c t i v e at small shear s t r a i n s . The dense sand may develop pore pressures equal to the c o n f i n i n g pressure and a c o n d i t i o n of I n i t i a l l i q u e f a c t i o n can occur. However, when the shear s t r a i n exceeds a c e r t a i n l e v e l , the sand w i l l tend to d i l a t e and pore pressures w i l l decrease and the s o i l w i l l develop increased s t r e n g t h . If the subsequent shearing m o b i l i z e s a s t r e n g t h equal to the a p p l i e d shear s t r e s s e s , then deformation w i l l be l i m i t e d , and a complete c o l l a p s e of the s t r u c t u r e w i t h u n l i m i t e d flow cannot take place i n dense sands. The term " c y c l i c m o b i l i t y " i s used to d e s c r i b e the behaviour of dense sands during c y c l i c shear (Castro, 1975). In d i l a t i v e s o i l s , the p r a c t i c a l problems are w i t h drainage and w i t h deformation and not w i t h s t r e n g t h r e d u c t i o n due to pore pressure accumulation. F i g u r e 1.1 shows the upper l i m i t of shear s t r a i n that i s expected to develop during undrained c y c l i c shear of saturated sands (Seed, 1976). Following the c y c l i c s t r e s s a p p l i c a t i o n s there w i l l be a r e s i d u a l pore water pressure i n the s o i l and t h i s may lea d to m i g r a t i o n of water i n the s o i l that may cause problems i n the o v e r l y i n g s o i l l a y e r s . This can lea d to the e x p u l s i o n of water and s o i l a t the ground surface i n the form of sand b o i l s . Upward seepage can cause delayed l i q u e f a c t i o n of the surface s o i l o v e r l y i n g a l i q u e f i e d l a y e r a f t e r earthquake ground motions have ceased. Such a time l a g f o r the surface s o i l to l i q u e f y was observed during the N i i g a t a earthquake of 1964. The mechanism of s o i l l i q u e f a c t i o n or c y c l i c m o b i l i t y i n the f i e l d 4 0 20 40 60 80 100 RELATIVE DENSITY, D r , % FIG. 1.1. ESTIMATED RANGE OF LIMITING SHEAR STRAIN DEVELOPED UNDER UNDRAINED CYCLIC SHEAR FOR SATURATED SANDS. (Adapted from Seed, 1976) 5 i s complicated by s o i l v a r i a b i l i t y , mass s o i l p e r m e a b i l i t y and v a r i a t i o n i n f i n e s content. Fine grained clayey s o i l s tend not to develop l a r g e i n c r e a s e s i n pore pressure but do s o f t e n and develop increased c y c l i c shear s t r a i n s under c y c l i c l o a d s . Laboratory r e s u l t s i n d i c a t e that clayey s o i l s do not s u f f e r a s i g n i f i c a n t s t r e n g t h l o s s unless the c y c l i c s t r a i n s induced are very l a r g e and the s o i l i s very s e n s i t i v e , (Castro and C h r i s t i a n , 1976, K o u t s o f t a s , 1978). However, a s i g n i f i c a n t r e d u c t i o n i n s t i f f n e s s can occur. The amount of s t i f f n e s s r e d u c t i o n depends on the l e v e l of c y c l i c s t r a i n . Thus, the use of the word " l i q u e f a c t i o n " should only r e a l l y be a p p l i e d to loose cohesionless s o i l s . For most other s o i l s c y c l i c m o b i l i t y or the development of c y c l i c shear s t r a i n s i s a more meaningful d e s c r i p t i o n of what occurs under c y c l i c l o a d i n g . A more d e t a i l e d d i s c u s s i o n of the c h a r a c t e r i s t i c behaviour of cohesionless s o i l s under s t a t i c and c y c l i c l o a d i n g c o n d i t i o n s i s given i n Appendix 1. 1.3. E x i s t i n g Methods f o r E v a l u a t i o n of L i q u e f a c t i o n P o t e n t i a l Some of the most comprehensive work on l i q u e f a c t i o n and c y c l i c m o b i l i t y assessment during earthquake loading has been reported by Seed (1976, 1979) and r e c e n t l y expanded upon by F i n n (1981). As a r e s u l t of extensive l a b o r a t o r y s t u d i e s over the past 15 years, the l i q u e f a c t i o n r e s i s t a n c e of saturated sands i s known to depend on the f o l l o w i n g : 1) the i n i t i a l s t r e s s e s , 2) the number of shear s t r e s s c y c l e s , 3) the magnitude of the c y c l i c shear s t r e s s , 4) the s o i l c h a r a c t e r i s t i c s , such as, d e n s i t y , s t r u c t u r e , aging, e t c In general, most methods a v a i l a b l e f o r e v a l u a t i n g l i q u e f a c t i o n or .6 c y c l i c m o b i l i t y p o t e n t i a l of a s o i l d e p o s i t subjected to earthquake l o a d i n g i n v o l v e two p a r t s : i ) E s t i m a t i o n of the c y c l i c s t r e s s or s t r a i n c o n d i t i o n developed i n the f i e l d due to the design earthquake, i i ) E s t imation of the f i e l d l i q u e f a c t i o n c h a r a c t e r i s t i c s . For s o i l under l e v e l ground c o n d i t i o n s , the c y c l i c c h a r a c t e r i s t i c s a r e best represented by the average c y c l i c s t r e s s r a t i o , T/O^ ; that i s , the r a t i o of the average c y c l i c shear s t r e s s , T, developed on h o r i z o n t a l planes i n the s o i l as a r e s u l t of the earthquake motions to the i n i t i a l v e r t i c a l e f f e c t i v e s t r e s s , °y 0* T h i s parameter has the advantage of t a k i n g i n t o account the depth of the s o i l l a y e r i n v o l v e d , the depth of the water t a b l e and the i n t e n s i t y of earthquake or other c y c l i c l o a d i n g phenomena (Seed and I d r i s s , 1981). Most methods compare the average c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n o r a c e r t a i n l e v e l o f c y c l i c m o b i l i t y , T /a' , w i t h the X. vo average c y c l i c s t r e s s r a t i o generated by the earthquake ground motions, T/O^ . By assuming undrained c o n d i t i o n s most methods ignore the p o s s i b l e e f f e c t s o f d i s s i p a t i o n of excess pore pressures. The c y c l i c s t r e s s r a t i o s developed i n the f i e l d due to earthquake l o a d i n g are, i n g e n e r a l , computed from e i t h e r a complex dynamic a n a l y s i s or a s i m p l i f i e d procedure based on a knowledge of the maximum ground surface a c c e l e r a t i o n . The dynamic analyses can be e i t h e r t o t a l s t r e s s analyses, where the i n f l u e n c e of changes i n pore pressures are neglected as the earthquake progresses (Seed and I d r i s s , 1971) or e f f e c t i v e s t r e s s a nalyses, where changes i n pore pressures are accounted f o r as the earthquake progresses ( F i n n et a l . , 1977). In ge n e r a l , the t o t a l s t r e s s approach, which does not take i n t o account changes i n pore pressures, w i l l p r e d i c t 7, s l i g h t l y l a r g e r c y c l i c s t r e s s r a t i o s (Seed, 1979). The e f f e c t i v e s t r e s s approaches have the major advantage that they lea d to d i r e c t e v a l u a t i o n o f the c y c l i c m o b i l i t y or l i q u e f a c t i o n p o t e n t i a l without f u r t h e r s t u d i e s . However, they r e q u i r e the determination of con s i d e r a b l y more s o i l parameters i n order to make the analyses. Many of these s o i l parameters are strange to many ge o t e c h n i c a l engineers and tend to be s e n s i t i v e to t e s t i n g e r r o r s . However, i n recent years the complex e f f e c t i v e s t r e s s dynamic analyses are g r a d u a l l y being introduced i n t o engineering p r a c t i c e . For many r o u t i n e engineering p r o j e c t s many engineers use s i m p l i f i e d procedures f o r the e s t i m a t i o n of c y c l i c s t r e s s r a t i o s developed due to earthquake l o a d i n g . The s i m p l i f i e d approach developed by Seed and I d r i s s , 1971, computes the average c y c l i c s t r e s s r a t i o i n the f i e l d due to earthquake l o a d i n g from an equation of the form: a a T _ ,_ max o — r — = 0.65 • —=— • r j a' g a' d vo vo maximum a c c e l e r a t i o n a t ground su r f a c e , t o t a l o v e r b u r d e n p r e s s u r e on s o i l l a y e r u n d e r c o n s i d e r a t i o n , i n i t i a l e f f e c t i v e overburden pressure on s o i l l a y e r under c o n s i d e r a t i o n , a s t r e s s r e d u c t i o n f a c t o r v a r y i n g from a v a l u e o f 1 at ground surface to a value of about 0.9 at a depth of about 10 m. where a max a o vo r d 8 A convenient r e l a t i o n s h i p proposed by Iwasaki et a l . , 1981, f o r the reduction f a c t o r , r ^ , i s given by r = 1 - 0.015 z d where z i s the depth i n meters. The estimation of the f i e l d l i q u e f a c t i o n c h a r a c t e r i s t i c i s determined either; i ) by use of f i e l d c o r r e l a t i o n s using i n - s i t u t e s t s , or i i ) by means of laboratory t e s t s on representative samples of the s o i l deposit. Many problems e x i s t i n laboratory t e s t i n g , such as, adequate simulation of f i e l d c o n d i t i o n s , system compliance and membrane p e n e t r a t i o n and development of uniform shear stresses. Because of the d i f f i c u l t y i n obtaining and te s t i n g undisturbed samples of cohesionless s o i l s , many engineers prefer to adopt the f i e l d performance c o r r e l a t i o n approach (Peck, 1979). Several f i e l d performance c o r r e l a t i o n s have been developed using i n -s i t u t e s t i n g techniques as an index of f i e l d l i q u e f a c t i o n c h a r a c t e r i s t i c s . Examples of these are: the Standard Penetration Test (Seed and I d r i s s , 1981, Iwasaki et a l . , 1978), the e l e c t r i c a l r e s i s t i v i t y probe (Arulmoli et a l . , 1981) and the Cone Penetration Test (Zhou, 1980). Other methods for assessment of l i q u e f a c t i o n p o t e n t i a l have been developed using i n - s i t u t e s t s such as, the se l f - b o r i n g pressuremeter (Vaid et a l . , 1981) and the i n - s i t u measurement of shear wave v e l o c i t y (Dobry et a l . , 1980), but these have not yet been f u l l y correlated to f i e l d performance. In general, there i s very l i t t l e f i e l d data a v a i l a b l e to e s t a b l i s h good c o r r e l a t i o n s of f i e l d performance with i n - s i t u test methods other than the Standard Penetration Test (SPT). This s i t u a t i o n w i l l probably change 9 w i t h time as other i n - s i t u t e s t methods are c o r r e l a t e d to l i q u e f a c t i o n r e s i s t a n c e and these c o r r e l a t i o n s improved and developed by comparison w i t h a c t u a l f i e l d performance. D e t a i l s of e x i s t i n g i n - s i t u t e s t methods f o r assessment of l i q u e f a c t i o n r e s i s t a n c e w i l l be given i n Chapter 3. 1.4. Research Objectives The research contained i n t h i s r e p o r t has been d i r e c t e d towards improving our knowledge and understanding of i n - s i t u t e s t i n g and i t s a p p l i c a t i o n to f i e l d l i q u e f a c t i o n assessment. At present the SPT i s the most widely used i n - s i t u t e s t method f o r assessment of l i q u e f a c t i o n p o t e n t i a l . The SPT method has the advantage th a t i t i s based on a l a r g e amount of f i e l d experience and i s widely accepted and used. However the SPT provides discontinuous data w i t h a low l e v e l of r e l i a b i l i t y and r e p e a t a b i l i t y . The Cone Pe n e t r a t i o n Test (CPT) o f f e r s considerable improvement over the SPT i n the form of f a s t e r , l e s s expensive, continuous records of p e n e t r a t i o n r e s i s t a n c e w i t h a greater degree of r e l i a b i l i t y and r e p e a t a b i l i t y . A major o b j e c t i v e of t h i s r esearch has been to g a i n a b e t t e r understanding of the CPT, i t s equipment, procedures, and methods of i n t e r p r e t a t i o n and how these r e l a t e to the a p p l i c a t i o n of l i q u e f a c t i o n assessment. A r e c e n t l y developed i n - s i t u t e s t , the f l a t p l a t e d i l a t o m e t e r which i s s i m i l a r to the CPT i n that i t i s a logging t o o l , has a l s o been s t u d i e d . The d i l a t o m e t e r t e s t , (DMT) i s a f a s t , low cost t e s t that provides near continuous data w i t h a great d e a l of r e p e a t a b i l i t y . At present the DMT has not been used to assess l i q u e f a c t i o n p o t e n t i a l and so part of t h i s research has been d i r e c t e d toward t h i s p o s s i b i l i t y . 10 Both the CPT and DMT are f a s t , low cost logging tools that are e s s e n t i a l l y interpreted using empirical c o r r e l a t i o n s . The s e l f - b o r i n g pressuremeter t e s t , (SBPMT), however, i s one of the only i n - s i t u test methods that can be analysed using fundamental theories and can provide d i r e c t measurement of basic s o i l parameters. The SBPMT has therefore been included i n th i s research since i t o f f e r s the most promising p o t e n t i a l f o r a s p e c i f i c i n - s i t u t e s t method. The objective of the research i n t o the SBPMT has been to gain an increased understanding of the equipment, procedures and methods of i n t e r p r e t a t i o n and to suggest improvements where necessary. It i s f e l t that some of the theories developed f o r the SBPMT may provide further i n s i g h t into the analyses and i n t e r p r e t a t i o n of the CPT and DMT. In summary, the objective of t h i s research has been; to review i n d e t a i l some of the a l t e r n a t i v e i n - s i t u test methods, t h e i r equipment, procedures, and methods of i n t e r p r e t a t i o n and t h e i r a p p l i c a t i o n to f i e l d l i q u e f a c t i o n assessment. Also to suggest new co r r e l a t i o n s that may, i n time, be used as a basis f o r comparison with actual f i e l d performance and to perform l i m i t e d f i e l d and laboratory studies to evaluate these i n - s i t u test methods. The phenomena of l i q u e f a c t i o n i n the f i e l d i s very complex. This research i s l i m i t e d to the study of i n - s i t u t e s t i n g and i t s possible a p p l i c a t i o n to l i q u e f a c t i o n assessment. This research does not include other aspects of l i q u e f a c t i o n assessment, such as, the estimation of c y c l i c stresses developed i n the f i e l d due to an earthquake. The research i s p r i n c i p l y d i r e c t e d toward l i q u e f a c t i o n due to earthquake loading, although some of the i n - s i t u t e s t methods could be applied to other forms of l i q u e f a c t i o n . 11 2 REVIEW OF EXISTING IN-SITU TEST METHODS 2.1. I n t r o d u c t i o n The measurement of s o i l p r o p e r t i e s by i n - s i t u t e s t methods has developed r a p i d l y during the l a s t decade. Improvements i n apparatus, i n s t r u m e n t a t i o n , measurement techniques and a n a l y s i s procedures have been s i g n i f i c a n t . Several conferences have provided vast amounts of i n f o r m a t i o n about i n - s i t u t e s t methods and t h e i r i n t e r p r e t a t i o n ; namely the European Symposium on P e n e t r a t i o n Testing (ESOPT I , 19 74 and I I , 1982) and the ASCE G e o t e c h n i c a l E n g i n e e r i n g D i v i s i o n S p e c i a l t y Conference on I n - s i t u Measurement o f S o i l P r o p e r t i e s , R a l e i g h , 1975. Incr e a s i n g i n t e r e s t has been generated by i n - s i t u t e s t methods because of t h e i r s i g n i f i c a n t c a p a b i l i t i e s , that i n c l u d e : i ) The a b i l i t y to determine p r o p e r t i e s o f s o i l s , such as sands and o f f -shore d e p o s i t s , that cannot be e a s i l y sampled i n the undisturbed s t a t e . i i ) The a b i l i t y to t e s t a l a r g e r volume o f s o i l than can conveniently be te s t e d i n the l a b o r a t o r y . i i i ) The a b i l i t y to avoid some of the d i f f i c u l t i e s o f l a b o r a t o r y t e s t i n g , such as sample disturbance and the proper s i m u l a t i o n of i n - s i t u s t r e s s e s , temperature, and chemical and b i o l o g i c a l environments. i v ) The increased cost e f f e c t i v e n e s s of an e x p l o r a t i o n and t e s t i n g program using i n - s i t u methods. However, an understanding of i n - s i t u t e s t i n g l i m i t a t i o n s i s e s s e n t i a l s i n c e sampling and l a b o r a t o r y t e s t i n g w i l l s t i l l p l a y an important r o l e f o r some g e o t e c h n i c a l problems. The s i g n i f i c a n t l i m i t a t i o n s o f i n - s i t u t e s t i n g are: 12 1) Stress d i r e c t i o n and stress path cannot be independently varied i n most cases. P r i n c i p a l stress d i r e c t i o n s and stress path i n the tests may d i f f e r from those i n the r e a l problem. Rotation of p r i n c i p a l stresses may occur i n the test but not i n the r e a l problem. i i ) Drainage conditions cannot be co n t r o l l e d independently. i i i ) The possible e f f e c t s of future changes i n environmental conditions cannot be r e a d i l y determined. These l i m i t a t i o n s , however, are often outweighed by the s i g n i f i c a n t c a p a b i l i t i e s of many i n - s i t u t e s t s . I n - s i t u t e s t methods currently a v a i l a b l e can be divided into two basic groups: i) logging methods, i i ) s p e c i f i c test methods. The logging methods are usually penetration type t e s t s and are usually f a s t and economical and provide q u a l i t a t i v e estimates, based on empirical c o r r e l a t i o n s , of various geotechnical parameters. S p e c i f i c test methods are usually more s p e c i a l i z e d and, therefore, often slower and more expensive to perform than the logging methods. The s p e c i f i c test methods are usually c a r r i e d out to obtain s p e c i f i c s o i l parameters, such as shear strength or modulus. The two basic groups are often complementary i n t h e i r use. The logging method i s best suited f o r s t r a t i g r a p h i c logging and preliminary evaluation of s o i l parameters. The s p e c i f i c test methods are best suited f o r use i n c r i t i c a l areas, as defined by the logging methods, where more d e t a i l e d assessments are required of s p e c i f i c s o i l parameters, which of course may include undisturbed sampling and laboratory t e s t i n g . The logging method should therefore be f a s t , economic, continuous and most important, repeatable. Whereas, the s p e c i f i c method should be better 13 suited to fundamental analyses to provide the required parameter. One of the best examples of a combination of logging and s p e c i f i c test methods i s the s t a t i c cone and the pressuremeter. Table 2.1 presents an updated version of the table presented by M i t c h e l l et a l . (1978) of i n - s i t u test methods and t h e i r a p p l i c a b i l i t y (Campanella and Robertson, 1982). Each method i s l i s t e d i n approximate order of i t s cost or complexity and with i t s s u i t a b i l i t y f o r determining various d i f f e r e n t geotechnical parameters. The s u i t a b i l i t y of each method f o r determining various d i f f e r e n t parameters i s indicated by a grade of A, B or C, with A i n d i c a t i n g high a p p l i c a b i i t y , B i n d i c a t i n g moderate a p p l i c a b i l i t y , C i n d i c a t i n g l i m i t e d a p p l i c a b i l i t y and a blank i n d i c a t i n g l i t t l e or no a p p l i c a b i l i t y . The grade i s based on a q u a l i t a t i v e evaluation of the confidence l e v e l assessed f o r each method i n determining the various geotechnical parameters. The t e s t methods l i s t e d i n the upper half of the table tend to be logging methods, whereas the methods i n the lower half tend to be s p e c i f i c methods. It i s not possible, or necessary, to comment here on a l l the test methods shown i n Table 2.1. Comments w i l l be r e s t r i c t e d to test methods that are a p p l i c a b l e to assessment of l i q u e f a c t i o n r e s i stance. In any disc u s s i o n of i n - s i t u t e s t i n g , i t i s important to discuss the equipment and procedures used since these have a s i g n i f i c a n t bearing on the r e s u l t s obtained. 2.2. Standard Penetration Test (SPT) The Standard Penetration Test (SPT) was developed i n 1927 and i s practised worldwide to a greater extent than any other s o i l t e s t . The test i s made by dropping a free f a l l i n g hammer weighing 63.5 kg (140 lb) onto o te U H 8 H fe 8 i 1 s 6< O < U K Vi Q Gu 3 S I I to CO CO M a CO g I < g to co co p Q o •a H H n M CO CO g fi! 8 | & M < t J O M o 5 co £ 2 S o w CO CO co CO CO M CO u a z o M B < a. Dynamic cone Static cone: Mechanical Elec. Friction Elec. Piezo Elec. Piezo/Friction Acoustic probe Dilatometer Vane Shear Standard Penetration Test Seismic CPT downhole K Q Blade Resistivity Probe Borehole Permeability Hydraulic fracture Screw Plate Seismic downhole Impact cone Borehole shear Menard Pressuremeter Selfboring Pressuremeter Selfboring devices: KQ meter Lateral penetrometer Shear vane Seismic crosshole Nuclear tests Plate load tests c A B C C - C C B A B c B — c B C B B A B c B - c B C - - — B A A B B B A A B B A B B A A A A B B A A B B A B B A C B B C C - C C - - _ _ C B A B C B - B B C — — C B B C - - A - B - — — _ _ _ B B B c C - - - c — — A C C C — — - - A - - - B B B B A B c - C C c _ _ C A C — - - - A - - - B A - -— - - - B B - C C - _ c C B C B - B A B c C B B c C C - - - - A - - - B B c B c c C - C C c — — C c C - B B - c C - - - C _ B B c B B - c B B - - C C B B A A A A A A A A B A A C C B B B B B C B c - - A - B - — — _ _ — C c B - - - - A - — - B B — - A B - - - C - - - — C C c B B C — B A B . C C B B A - High applicability B - Moderate applicability C - Limited applicability TABLE 2.1. Perceived ApplicaBili.ty of In-situ Test Methods - Update 1982. (After Campanella and Robertson, 1982) 15 the d r i l l rods from a height of 0.76 m (30 i n ) . The number of blows, N, necessary to achieve a penetration of 0.30 m (below the seating drive of 0.15 m) of a standard sample tube, i s regarded as the penetration resistance. Because of the dynamic nature of the SPT there are considerable problems regarding i t s r e p e a t a b i l i t y and r e l i a b i l i t y . Numerous studies have shown considerable v a r i a b i l i t y i n the procedures and equipment used i n t h i s supposedly standardized t e s t . However, the SPT, with a l l i t s problems of r e p e a t a b i l i t y and r e l i a b i l i t y , i s s t i l l the most commonly used i n - s i t u t e s t today. Considerable improvements i n our understanding of the dynamics of the SPT have occurred i n recent years (Schmertmann and Palacios, 1979, Kovacs et a l . , 1981, Kovacs and Salomone, 1982). Schmertmann, (1978a), concludes that SPT r e s u l t s may be s i g n i f i c a n t l y influenced by such factors as: 1) the s i z e of the d r i l l hole, 2) the number of turns of the rope around the cathead, 3) the length of the d r i l l rods, 4) the use of d r i l l i n g mud versus casing to support the walls of the d r i l l hole, 5) the use of non-standard sampling tubes, 6) the depth range over which the penetration resistance i s measured (0 to 12 i n . or 6 i n . to 18 in.) Kovacs et a l . (1981), have shown that the energy delivered to the rods during a SPT can vary from about 30% to 80% of the t h e o r e t i c a l maximum po t e n t i a l energy 475J (4200 i n . l b . ) . The energy delivered to the d r i l l stem varies with the number of turns of rope around the cathead and v a r i e s with the f a l l height, d r i l l r i g type, hammer type, and operator 16 c h a r a c t e r i s t i c s . The type of hammer appears to have an influence on the energy transfer mechanism between the a n v i l and the d r i l l stem. Kovacs and Salomone (1982), suggest that two turns of a rope around the cathead would minimize the e f f e c t of operator performance c h a r a c t e r i s t i c s on the delive r e d energy. When using the rope and cathead procedure with two turns of the rope the t y p i c a l energy i n a standard donut hammer at impact i s about 50 to 60% of the t h e o r e t i c a l maximum (Kovacs and Salomone, 1982). Schmertmann has suggested that based on l i m i t e d data, an e f f i c i e n c y of about 55% appears to be the norm f o r which i t i s believed that many current c o r r e l a t i o n s were developed. It i s c l e a r there i s a need f o r increased standardization when using the SPT. Schmertmann, 1978, and Kovacs and Salomone, 1982, i d e n t i f y the two most s i g n i f i c a n t f a c t ors a f f e c t i n g the measured N-value as: 1) the amount of energy delivered to the d r i l l rods, and 2) the method used to d r i l l and support the borehole. Several studies have concluded that an energy standard should be adopted as a c r i t e r i o n f o r SPT. With the existence of a f a i r l y inexpensive and easy to use energy c a l i b r a t i o n unit ( H a l l , 1982) many researchers f e e l that measured energy c o r r e c t i o n factors w i l l lead to more r e p e a t a b i l i t y and r e l i a b i l i t y of SPT N-values i n the future (Campanella and Robertson, 1982, Kovacs and Salomone, 1982). One of the other major areas of concern with the SPT i s i t s poor r e s o l u t i o n i n soft s o i l s , such as s i l t and cl a y , where N-values of l e s s than one can be encountered. The main advantages the SPT has i s that i t i s commonly used and that i t provides a s o i l sample for i d e n t i f i c a t i o n . 17 2.3. Cone Penetration Tests (CPT) Many of the cone penetration devices were developed and used i n Europe but are now gaining increasing acceptance i n North America. The main reasons f o r the increasing i n t e r e s t i n cone penetration tests are the s i m p l i c i t y of tes t i n g , r e p r o d u c i b i l i t y of r e s u l t s and the greater amenability of the test data to r a t i o n a l a n a l y s i s . The general types of cone penetration devices are summarized i n Table 2.2. TABLE 2.2 General Types of Cone Penetration Tests (Adapted from Schmertmann 19 75) Type Tip Advance Method Where Used Rate Notes S t a t i c (Quasi-s t a t i c ) Dynamic Screw I n e r t i a l (Impact) Hydraulic or mechanical jacking Impact of drive weight Rotation of a weighted h e l i c a l cone Dropped or propelled into s o i l surface 2 cm/ s ec variable v a r i a b l e variable-measured deceler-a t i o n world-wide world-wide Sweden No rway Offshore, M i l i t a r y Usually 10 cm2, 60° cone Great v a r i e t y of s i z e s and weights Useful f o r s o i l s i n ina c c e s s i b l e areas 18 The s t a t i c method i s the most commonly used technique i n engineering p r a c t i c e . Dynamic cones are subject to the same disadvantages as the SPT. The s t a t i c cone penetration test w i l l hereafter be referred to as the cone penetration test (CPT). The proceedings of ESOPT I (19 74) and II (1982) provide excellent sources of information on t e c h n i c a l d e t a i l s of the CPT and v a r i a t i o n s i n equipment, and a v a i l a b l e methods f o r i n t e r p r e t a t i o n of CPT r e s u l t s . A cone with a 10 cm2 base area cone t i p with an apex angle of 60° i s accepted as standard and has been s p e c i f i e d i n the European and American Standards (ISSMFE, 1977; ASTM, 1971). The f r i c t i o n sleeve, located above the c o n i c a l t i p , has a standard area of 150 cm 2. The mechanical cones (Begemann, 1965) require a double rod system for t h e i r t elescopic a c t i o n . The e l e c t r i c a l cones (De Ruiter, 1971) have the f r i c t i o n sleeve and t i p advanced continuously with a s i n g l e rod system. F i g . 2.1 i l l u s t r a t e examples of mechanical and e l e c t r i c a l cones. The mechanical cones o f f e r the advantage of an i n i t i a l low cost f o r equipment and s i m p l i c i t y of the operation. However, i t does have the disadvantage of a rather slow incremental procedure, in e f f e c t i v e n e s s i n s o f t s o i l s , requirement of moving parts, labour intensive data handling and presentation, and generally poor accuracy and shallow depth c a p a b i l i t y . The e l e c t r i c cones have b u i l t - i n load c e l l s that record continuously the end resistance (q ) and side f r i c t i o n (f ). The load c e l l s can be made c s i n a v a r i e t y of c a p a c i t i e s from 50 to 150 kN for end bearing and 7.5 to 15 kN f o r side f r i c t i o n , depending on the strength of the s o i l s to be penetrated. An e l e c t r i c cable usually connects the cone with the recording equipment at ground surface. (b) ELECTRIC CONES FIG. 2.1. EXAMPLES OF MECHANICAL AND ELECTRIC CONES. 20 The e l e c t r i c cone o f f e r s obvious advantages, such as, a more rapid procedure, continuous recording, higher accuracy and r e p e a t a b i l i t y , p o t e n t i a l for automatic data logging, reduction and p l o t t i n g , and the p o s s i b i l i t y of incorporating a d d i t i o n a l sensors i n the cone. However, the e l e c t r i c cones do have an i n i t i a l high cost f or equipment and require well s k i l l e d operators with a knowledge of e l e c t r o n i c s . They also require adequate back up i n technical f a c i i t i e s f o r c a l i b r a t i o n and maintenance. The most s i g n i f i c a n t advantage that e l e c t r i c cones o f f e r i s t h e i r r e p e a t a b i l i t y and accuracy (Schmertmann, 1975, Schaap and Zuidberg, 1982). The most s i g n i f i c a n t recent development i n CPT i s the addition of a pore pressure element to the cone. The add i t i o n of pore presure measurements during CPT has added a new dimension to the i n t e r p r e t a t i o n of geotechnical parameters p a r t i c u l a r l y i n loose or s o f t , saturated, d e l t a i c deposits. The continuous measurement of pore pressures along with bearing and f r i c t i o n has enhanced the e l e c t r i c cone penetrometer as the premier t o o l f o r s t r a t i f i c a t i o n logging of s o i l deposits (Campanella and Robertson, 1982). The excess pore pressure (Au) measured during penetration i s a useful i n d i c a t i o n of the s o i l type and provides an excellent means for detecting d e t a i l s i n stratigraphy. The d i f f e r e n t i a l pore pressure r a t i o (Au/q^) also appears to be a good index of s o i l type and r e l a t i v e consistency and a rough i n d i c a t o r of s t r e s s - h i s t o r y . In addition, when the steady penetration i s stopped, the excess pore pressure decay with time can be used as an i n d i c a t o r of the c o e f f i c i e n t of consolidation. F i n a l l y the e q u i l i b r i u m p ore pressure v a l u e ( U q ) , a f t e r complete d i s s i p a t i o n i s reached, provides important data on the ground water conditions. Cone resistances and pore pressures as measured i n the CPT are determined by a large number of variables such as s o i l type, density, 21 s t r e s s l e v e l , s o i l f a b r i c and mineralogy, e t c . Many t h e o r i e s e x i s t to a s s i s t i n b e t t e r understanding the process of a penetrating cone, but the c o r r e l a t i o n s with s o i l c h a r a c t e r i s t i c s remain l a r g e l y e m p i r i c a l . One of the most important a p p l i c a t i o n s of CPT remains i t s use f o r an accurate determination of the s o i l p r o f i l e . Extensive use i s made of the f r i c t i o n r a t i o (F.R. = f /q x 100%) as a means of s o i l c l a s s i f i c a t i o n s c (Begemann, 1965, Schmertmann, 1975, Douglas and Olsen, 1981). A more d e t a i l e d s t r a t i f i c a t i o n can be obtained w i t h the a d d i t i o n of a pore pressure element (Campanella and Robertson, 1981). Many t h e o r i e s and e m p i r i c a l c o r r e l a t i o n s have been developed to r e l a t e CPT r e s u l t s to s o i l parameters such as r e l a t i v e d e n s i t y , shear s t r e n g t h , c o m p r e s s i b i l i t y and modulus. A d e t a i l e d review and d i s c u s s i o n of these c o r r e l a t i o n s w i l l be presented i n Chapter 4. 2.4. F l a t P l a t e Dilatometer Test The F l a t P l a t e Dilatometer was developed i n I t a l y by S. M a r c h e t t i and introduced i n t o North America through h i s 1980 p u b l i c a t i o n i n the Geo t e c h n i c a l D i v i s i o n , J o u r n a l of the ASCE. The dilatometer t e s t (DMT) i s extremely easy to perform and provides an impressive range of e m p i r i c a l l y p r e d i c t e d s o i l parameters from only two quick measurements at each depth. The f l a t p l a t e i s 14 mm t h i c k by 95 mm wide by 200 mm long w i t h a f l e x i b l e s t e e l membrane 60 mm i n diameter on one face. F i g . 2.2 i l l u s t r a t e s the F l a t P l a t e Dilatometer and readout u n i t . The pressure f o r l i f t - o f f of the diaphragm and the pressure required to d e f l e c t the centre of the diaphragm 1 mm i n t o the s o i l are recorded at each depth. Readings can be made every 20 cm i n depth and the d i l a t o m e t e r , which has a sharpened edge, i s advanced by a cone p e n e t r a t i o n r i g or 22 FIG. 2.2. FLAT PLATE DILATOMETER AND READOUT UNIT. 23 s i m i l a r pushing apparatus. C o r r e l a t i o n s have been developed by Mar c h e t t i between t h e s e r e a d i n g s and s o i l t y p e , K q , OCR, u n d r a i n e d s t r e n g t h , constrained modulus and f r i c t i o n angle, The DMT has been found to be a h i g h l y repeatable t e s t t h a t i s almost operator independent (Lacasse and Lunne, 1982). The DMT o f f e r s the advantage o f an i n i t i a l low cost f o r equipment and i n the s i m p l i c i t y o f the op e r a t i o n and maintenance. However, i t does have the disadvantage o f a r a t h e r slow incremental procedure and a short h i s t o r y of experience f o r i n t e r p r e t a t i o n . The other disadvantage r e l a t e d to i n t e r p r e t a t i o n i s that the purchase of the equipment i n c l u d e s a computer program t h a t contains the data i n t e r p r e t a t i o n and p r e s e n t a t i o n software. This tends to r e s t r i c t the user and discourage improvements to the e x i s t i n g c o r r e l a t i o n s as more experience i s gained with the t e s t . A d e t a i l e d review and d i s c u s s i o n of the DMT and of some of the c o r r e l a t i o n s developed w i l l be given i n Chapter 6. 2.5. E l e c t r i c a l R e s i s t i v i t y The p r i n c i p l e of e l e c t r i c a l r e s i s t i v i t y methods i s based on the f a c t t h a t sand g r a i n s c o n s i s t of e l e c t r i c a l l y non-conducting m i n e r a l s , whereas the pore water i s e l e c t r i c a l l y conducting, e s p e c i a l l y i f i t contains d i s s o l v e d s a l t s . The more pores that occur i n a mass of sand; i . e . the more pore water i s present, the lower i s the e l e c t r i c a l r e s i s t a n c e of the t o t a l mass of sand g r a i n s plus water. A r e s i s t i v i t y probe used as a p e n e t r a t i o n cone has been developed and i s i n use by the Delf S o i l Mechanics Laboratory (Kroezen, 1981) . I t has apparently been s u c c e s s f u l l y used i n o f f - s h o r e i n v e s t i g a t i o n s i n Holland and r e c e n t l y i n the Beaufort Sea, Canada. The s p e c i f i c e l e c t r i c a l 24 r e s i s t a n c e of the s o i l i s measured by means of a s o i l probe, c o n s i s t i n g of four e l e c t r o d e s f i t t e d around a standard CPT sounding tube, from which they are i n s u l a t e d . The probe i s pushed i n t o the s o i l by means of a sounding apparatus or a modified d r i l l r i g and a reading i s taken every 20 cm i n depth. A voltage d i f f e r e n c e i s a p p l i e d to the two outer e l e c t r o d e s and the e l e c t r i c a l r e s i s t a n c e of the s o i l i s measured at the two inner e l e c t r o d e s under a non-load c o n d i t i o n . The s o i l probe i s c a l i b r a t e d i n water of a known e l e c t r i c a l r e s i s t a n c e . Since r e s i s t i v i t y methods measure the e l e c t r i c a l r e s i s t i v i t y of the pore water, a reference measurement i s r e q u i r e d of the ground water. The D e l f method measures the e l e c t r i c a l r e s i s t a n c e of the ground water using a separate water probe which i s a l s o pushed i n t o the s o i l . At the d e s i r e d depth a small amount of the pore water i s drawn v i a a f i l t e r i n t o a measuring c e l l i n the probe. A t h e o r e t i c a l determination of the r e l a t i o n between measured e l e c t r i c a l r e s i s t a n c e and p o r o s i t y i s not p o s s i b l e , s i n c e a mass of sand c o n s i s t s of g r a i n s of varying s i z e s and shapes. The above r e l a t i o n , however, can be e s t a b l i s h e d i n the l a b o r a t o r y by t e s t i n g samples of the sand under i n v e s t i g a t i o n . This complicates the procedure as a bulk sample i s required at s p e c i f i c l o c a t i o n s . Another form of the e l e c t r i c a l r e s i s t i v i t y probe has been developed at the U n i v e r s i t y of C a l i f o r n i a a t Davis by P r o f . K. Arulanandan (Arulanandan, 1977). D e t a i l s of the probe and i t s o p e r a t i o n are given by A r u l m o l i et a l . , 1981. The probe c o n s i s t s of a 120 cm (4 f t ) long, 7.62 cm (3 in) o u t s i d e diameter, s t e e l tube which houses the e l e c t r o n i c s and a 12 v o l t pump. The probe a l s o has a 30 to 45 cm long r e p l a c e a b l e s t e e l , t h i n walled (1.6 mm t h i c k ) tube which i s screwed to the main probe. The t h i n walled tube c a r r i e s symmetrically placed e l e c t r o d e s . A microprocessor c o n t r o l u n i t f o r t r a n s m i t t i n g the e l e c t r i c a l s i g n a l and r e c e i v i n g the measured e l e c t r i c a l p r o p e r t i e s i s connected to the e l e c t r o n i c s i n the probe v i a a s t i f f c a b l e . The probe i s placed i n t o a p r e v i o u s l y bored hole to the d e s i r e d depth and the t i p i s pushed about 15 to 25 cm i n t o the s o i l . The Davis probe has the advantage, over the Delf probe, of t e s t i n g a l e s s d i s t u r b e d sample of s o i l and measuring the e l e c t r i c a l r e s i s t a n c e i n a more uniform e l e c t r i c a l f i e l d . However, i t does have the disadvantage of a very slow incremental procedure that r e q u i r e s a pre-bored hole f o r each t e s t . R e s i s t i v i t y methods, i n gen e r a l , a r e complicated by the need to o b t a i n measurements on a pore water sample as w e l l as a bulk, sample at a s p e c i f i c l o c a t i o n . The measurements on a pore water sample are complicated by p o s s i b l e contamination during repeated t e s t s . The Davis probe appears to have reduced the bulk, sample problem by u s i n g g e n e r a l e m p i r i c a l c o r r e l a t i o n s based on l a b o r a t o r y t e s t s . 2.6. Pressuremeter Tests In p r i n c i p l e , the pressuremeter i s an expandable tube which i s placed i n the s o i l and then expanded under c o n t r o l l e d c o n d i t i o n s against the s o i l . From t h i s t e s t a pressure expansion curve of the s o i l i s obtained. The advantages of the t e s t are: i ) The t e s t models the axisymmetric expansion of an i n f i n i t e c y l i n d r i c a l c a v i t y ; t h i s problem has w e l l developed e l a s t i c and e l a s t o - p l a s t i c s o l u t i o n s . 26 i i ) The t e s t data provides d i r e c t measures of i n - s i t u s t r e s s e s and s t r e s s -s t r a i n response i n the d i r e c t i o n perpendicular to the a x i s of the borehole w a l l s . The pressuremeter t e s t (PMT), of the type developed by L. Menard, i s widely used i n Europe and i s r e c e i v i n g i n c r e a s i n g acceptance i n North America. The instrument i s expanded by applying a i r pressure to the l i q u i d which f i l l s the instrument and the lead l i n e s . The volume expansion I s measured by measuring the amount of l i q u i d f o r ced i n t o the expanding s e c t i o n under a given pressure. To ensure that the expansion recorded i s a measure of the behaviour of the c e n t r a l s e c t i o n of the instrument, two guard c e l l s are provided. A schematic r e p r e s e n t a t i o n of the Menard type pressuremeter i s shown i n F i g . 2.3. A t y p i c a l pressure expansion curve i s shown i n F i g . 2.4. The Menard pressuremeter i s placed i n t o a prebored hole. During the d r i l l i n g process to form the hole there i s some unloading of the sides o f the hole. At the s t a r t of the t e s t the probe i s not n e c e s s a r i l y i n f u l l c ontact w i t h the s i d e s of the prebored hole and no pressure i s a p p l i e d to the s o i l . From po i n t 0 to A on the curve ( F i g . 2.3) i t i s assumed that the w a l l s of the borehole are pushed back to t h e i r o r i g i n a l p o s i t i o n p r i o r to forming the h o l e . P o i n t A i s t h e o r e t i c a l l y the s t a r t of the expansion process. From A to B the t e s t curve i s u s u a l l y a s t r a i g h t l i n e , and the s o i l i s assumed to be behaving e l a s t i c a l l y . A modulus of deformation, E^, f o r the s o i l i s derived from the slope of AB and the volume of the c a v i t y a t the mid-point of the s t r a i g h t l i n e p o r t i o n of the curve. I t has been found that the slope of AB i s dependent upon the method used to form the prebored h o l e . To overcome the problems of s o i l d isturbance, Menard 27. i FIG. 2.3. SCHEMATIC OF MENARD PRESSUREMETER. V.VOLUME INCREASE, c m 3 FIG. 2.4. COMPARISON OF MENARD PRESSUREMETER TEST RESULT AND SELF-BORING PRESSUREMETER TEST RESULT. 28 developed a s e r i e s of e m p i r i c a l c o r r e l a t i o n s to enable data from a Menard pressuremeter t e s t (PMT) to be a p p l i e d to design. The use of the FMT i n foundation design i s based on a number of e m p i r i c a l c o r r e l a t i o n s which were e s t a b l i s h e d from a l a r g e number of t e s t s and observations on a c t u a l s t r u c t u r e s . Consequently, the q u a l i t y of foundation design based on the PMT i s o f t e n very good, provided the t e s t s are c a r r i e d out according to the standard method and i n s o i l s s i m i l a r to those which have been stud i e d i n the development of the e m p i r i c a l methods. An e x c e l l e n t reference that d e s c r i b e s the t e s t method and e m p i r i c a l c o r r e l a t i o n s i s the book by Bag u e l i n , Jezequel and S h i e l d s , 1978. The standard Menard type PMT i s , however, h i g h l y s e n s i t i v e to v a r i a t i o n s i n t e s t procedure. The instrument i s lowered or pushed i n t o a pre-bored hole and i n f l a t e d to provide a pressure-volumetric expansion curve. S o i l d i s t u r b a n c e , e s p e c i a l l y i n l o o s e , s a t u r a t e d , cohesionless s o i l s , i s u s u a l l y s i g n i f i c a n t . In an e f f o r t to minimize s o i l d isturbance s e l f - b o r i n g pressuremeters (SBPMT) were developed independently i n both France and England (Baguelin et a l . , 1972, Wroth and Hughes, 1973). The s e l f - b o r i n g pressuremeter c o n s i s t s e s s e n t i a l l y of a t h i c k walled tube w i t h a f l e x i b l e membrane attached to the o u t s i d e . The instrument i s pushed i n t o the ground and the s o i l d i s p l a c e d by a sharp c u t t i n g shoe i s removed up the centre of the instrument by the a c t i o n of a r o t a t i n g c u t t e r j u s t i n s i d e the shoe of the instrument. The c u t t i n g s are f l u s h e d to the surface by d r i l l mud which i s pumped down to the c u t t i n g head. A schematic o u t l i n e o f the s e l f - b o r i n g pressuremeter developed by Dr. J.M.O. Hughes i s shown In F i g . 2.5. Once the instrument i s at the d e s i r e d depth, the membrane surrounding the instrument i s expanded against the s o i l . The expansion at the centre 29 HSB SELF-BORING PRESSUREMETER Drilling Fluid Drill Rod Signal Wires to Surface -Pressure Hose-Casing Support -Air (Pressurized)^ Electrical Signal Conditioners -Pressure Transducer-Pore Pressure Cell -Displacement -Transducers Flexible Membrane-Return Drilling Fluid-. FIG. 2.5. SCHEMATIC OF SELF-BORING PRESSUREMETER. 30 of the instrument i s measured by displacement transducers. Pore pressure c e l l s can be incorporated i n t o the membrane to monitor changes i n pore water pressures. An example of a s e l f - b o r i n g pressuremeter expansion curve i s shown i n F i g . 2.4. By d e f i n i t i o n s e l f - b o r i n g i m p l i e s t h a t the pressuremeter i s capable o f i n s e r t i o n i n t o the ground w i t h minimal d i s t u r b a n c e . I d e a l l y the o r i g i n a l i n - s i t u s t r e s s e s are maintained around the probe, so that the expansion curve s t a r t s o f f from the a t r e s t , t o t a l h o r i z o n t a l s t r e s s shown at D i n F i g . 2.4. A d i r e c t comparison between the r e s u l t s from the two kinds of pressuremeter t e s t s i s shown i n F i g . 2.4. The borehole r e l o a d i n g (OA) i s completely missing from the s e l f - b o r i n g curve. The i n i t i a l p o r t i o n DE of the s e l f - b o r i n g curve i s much steeper than the s t r a i g h t l i n e p o r t i o n of the Menard curve (AB) and the s e l f - b o r i n g t e s t reaches a pressure e q u i v a l e n t to the Menard l i m i t pressure (C) at a much smaller s t r a i n . The Menard pressuremeter t e s t and the s e l f - b o r i n g pressuremeter t e s t should be thought of as two d i s t i n c t and separate t e s t s . The Menard pressuremeter t e s t r e s u l t s i n s o f t s o i l s are u s u a l l y analysed using e m p i r i c a l c o r r e l a t i o n s r e l a t e d to s p e c i f i c design r u l e s . In very s t i f f s o i l s or r o c k s , where a preformed hole can be made w i t h only e l a s t i c unloading of the s o i l , the Menard type pressuremeter data can be analysed from a more fundamental b a s i s . The s e l f - b o r i n g pressuremeter can, of course, only be i n s t a l l e d i n t o r e l a t i v e l y s o f t s o i l s . The kinematics of c a v i t y expansion theory are given i n Appendix 2. The development of s e l f - b o r i n g instruments w i t h t h e i r greater accuracy and the e l i m i n a t i o n of the i n i t i a l s o i l disturbance has brought a new dimension to the i n t e r p r e t a t i o n o f pressuremeter r e s u l t s . The g r e a t e s t problem w i t h the SBPMT appears to be the high cost a s s o c i a t e d w i t h the 31 equipment and i n s t a l l a t i o n and the need f o r h i g h l y t r a i n e d personnel. The equipment a l s o r e q u i r e s adequate back up i n t e c h n i c a l f a c i l i t i e s f o r c a l i b r a t i o n and maintenance. The instrument, however, i s c u r r e n t l y being s u c c e s s f u l l y used commercially. A d e t a i l e d d i s c u s s i o n of the s e l f - b o r i n g pressuremeter and i t s associated t h e o r i e s developed w i l l be given i n Chapter 5. 2.7. Acoustic Emission Tests The a c o u s t i c probe and a c o u s t i c emission method i s s t i l l very much i n the development stage and has a t t r a c t e d i n t e r e s t i n recent years i n both North America and Japan. The method e s s e n t i a l l y i n v o l v e s p l a c i n g m i n i a t u r e microphones i n t o probes, commonly a CPT probe, and recording the changes i n sound during steady p e n e t r a t i o n . The major t h r u s t a t present i n research i n t o a c o u s t i c probing i s the i n t e r p r e t a t i o n of the a c o u s t i c "language 1. Recent research i n d i c a t e s t h a t the a c o u s t i c response i s very r a t e s e n s i t i v e i n sands, s i g n f i c a n t l y more so than CPT. This would r e q u i r e very uniform and standard r a t e s of p e n e t r a t i o n . The a c o u s t i c emission methods show i n t e r e s t i n g p o t e n t i a l e s p e c i a l l y i n the area of monitoring f o r changes i n i n - s i t u c o n d i t i o n s and f o r d e t a i l e d s o i l p r o f i l i n g . 2.8. Geophysical Tests Seismic methods to determine the i n - s i t u shear wave v e l o c i t i e s have gone through r a p i d developments i n recent years. The b a s i s f o r a l l seismic techniques i s an i n t e r p r e t a t i o n of the time r e q u i r e d f o r p a r t i c u l a r types of e l a s t i c waves to t r a v e l known or i n f e r r e d d i s t a n c e s through the subsurface. Several comprehensive summary papers have been presented at conferences that d e t a i l the various seismic methods f o r determination of s o i l moduli (Woods, 1978, B a l l a r d and McLean, 1975). The two most common methods used f o r assessment of i n - s i t u shear wave v e l o c i t i e s are the crosshole and uphole/downhole methods. Crosshole surveys are c a r r i e d out using two or more bo r i n g s , cased or uncased, i n t o which a seismic source and transducers are placed a t known e l e v a t i o n s . The spacing of borings and shot e l e v a t i o n s are v a r i e d according to s i t e c o n d i t i o n s . Uphole surveys are made by l o c a t i n g a geophone at the top of the shothole, o f t e n during a crosshole survey. Downhole surveys are made by p l a c i n g a geophone or geophone s t r i n g i n a boring and i n i t i a t i n g a seism i c wave near the top of the borehole. The uphole/downhole survey methods are p a r t i c u l a r l y u s e f u l i n i d e n t i f y i n g low v e l o c i t y zones und e r l y i n g zones of higher v e l o c i t y . Sources and geophones are o f t e n se l e c t e d i n order to enhance shear wave a r r i v a l time determination. The shear wave v e l o c i t y , V , and shear modulus, G, are r e l a t e d by s the b a s i c wave propagation r e l a t i o n s h i p . G = p V 2 s where p i s the mass d e n s i t y of the s o i l . The m a j o r i t y of seismic methods measure the shear wave v e l o c i t y a t very small s t r a i n l e v e l s (shear s t r a i n , y < 10 - 1 + % ) . Thus, the very low s t r a i n l e v e l dynamic shear modulus, G , i s u s u a l l y measured. Since max l a r g e earthquakes o f t e n cause shear s t r a i n l e v e l s i n the range 1 0 - 3 to 1 0 - 1 percent a borehole h i g h energy impulse t e s t was developed by M i l l e r e t a l . , 19 75. This t e s t uses the crosshole technique a t c l o s e spacing w i t h a high energy impulse device jacked against the si d e s o f an uncased borehole. The t e s t a l l o w s the measurement o f shear modulus as a f u n c t i o n of s t r a i n l e v e l ( 1 0 - 5 to 1CT1 percent). 33 The seismic methods f o r measuring shear modulus avoid many of the problems of s o i l disturbance r e l a t e d to l a b o r a t o r y s t u d i e s , and measure parameters which are r e p r e s e n t a t i v e of a l a r g e volume of s o i l . However, the h i g h cost o f these techniques o f t e n l i m i t i t s wide use. A new type of device that i s under development (Campanella and Robertson, 1982), that can s i g n i f i c a n t l y reduce the cost a s s o c i a t e d w i t h most seismic methods, i s a seismic CPT. The device combines a bearing-pore pressure cone w i t h a set of miniature 28 Hz. seismometers b u i l t i n t o the cone. The bearing and pore pressure measurements can be used to l o g the s t r a t i g r a p h y of a s i t e during p e n e t r a t i o n and downhole (or crosshole) s e i s m i c methods performed at appropriate depths i n the s o i l p r o f i l e when the cone i s being removed. This t e s t i s c u r r e n t l y i n the e a r l y stages of development. 2.9. P l a t e Load Tests and Screw-Plate Tests P l a t e load t e s t s have been a t r a d i t i o n a l i n - s i t u method f o r e s t i m a t i n g the s o i l modulus f o r purposes of e s t i m a t i n g the settlement of spread f o o t i n g s . The v e r t i c a l modulus of deformation i s obtained from the load settlement behaviour of a s u i t a b l e bearing p l a t e placed at a d e s i r e d depth. Experience has shown th a t the p l a t e l o a d t e s t can provide r e l i a b l e estimates of v e r t i c a l modulus f o r settlement c a l c u l a t i o n s . However, the l a r g e cost i n v o l v e d has stimulated i n c r e a s i n g i n t e r e s t i n a l t e r n a t e methods. One a l t e r n a t e method developed i n Europe i s the Screw-plate t e s t . The screw-plate i s a f l a t p i t c h auger device that can be screwed to the d e s i r e d depth i n the s o i l and loaded i n a s i m i l a r manner to a p l a t e load t e s t . The h o r i z o n t a l l y p r o j e c t e d area over the s i n g l e 360° auger f l i g h t i s taken as the loading p l a t e area. A v a r i e t y of l o a d i n g procedures can be a p p l i e d depending on the s o i l type and data r e q u i r e d . Constant r a t e of load or deformation can be a p p l i e d and load versus deformation p l o t t e d to o b t a i n modulus and s t r e n g t h of the s o i l . The l o a d can be a p p l i e d i n increments and maintained constant t o o b t a i n c o n s o l i d a t i o n d a t a (Janbu and Senneset, 1973). The p r e c o n s o l i d a t i o n pressure can a l s o be obtained from screw-plate t e s t s (Dahlberg, 1975). Screw-plate t e s t s are w e l l s u i t e d f o r use i n sandy s o i l s where undisturbed sampling i s d i f f i c u l t . However, the t e s t i s o f t e n performed by hand and thus i t s e f f e c t i v e depth i s q u i t e l i m i t e d . Recent research a t the U n i v e r s i t y of B r i t i s h Columbia (UBC) has i n v o l v e d the development of an automatic i n s t a l l a t i o n and t e s t i n g procedure f o r the screw-plate t e s t ( B e r z i n s and Campanella, 1981). This t e s t and procedure i n v o l v e s a 500 cm 2 area, double h e l i x screw-plate i n s t a l l e d w i t h a high torque, h y d r a u l i c torque motor. The double h e l i x p l a t e allows symmetrical l o a d i n g on the p l a t e and e a s i e r advancement than a s i n g l e h e l i x . The double h e l i x p l a t e has been i n s t a l l e d to depths i n excess of 20 m through medium dense sands (Campanella and Robertson, 1982). 35 3. REVIEW OF EXISTING IN-SITU TEST APPLICATIONS FOR ASSESSMENT OF  LIQUEFACTION RESISTANCE 3.1. I n t r o d u c t i o n Laboratory c y c l i c t e s t s have been used as a b a s i s f o r procedures f o r the assessment of l i q u e f a c t i o n p o t e n t i a l . However, the d i f f i c u l t i e s i nherent i n o b t a i n i n g and t e s t i n g undisturbed samples of cohesionless s o i l s has l e d to the development of s e v e r a l methods f o r assessment of l i q u e f a c t i o n p o t e n t i a l using i n - s i t u t e s t methods. F i e l d l i q u e f a c t i o n and c y c l i c m o b i l i t y due to earthquake l o a d i n g i s a complex phenomena. E x i s t i n g i n - s i t u t e s t i n g techniques a r e used to provide an Index of the l i q u e f a c t i o n c h a r a c t e r i s t i c s o f s o i l . Because of drainage c o n t r o l problems i t i s extremely d i f f i c u l t to perform an i n - s i t u t e s t that a c t u a l l y l i q u e f i e s a s o i l . The f o l l o w i n g s e c t i o n s w i l l d e s c r i b e and review the e x i s t i n g i n - s i t u t e s t a p p l i c a t i o n s f o r assessment of l i q u e f a c t i o n r e s i s t a n c e . 3.2. Standard Penetration Tests The most widely used and accepted i n - s i t u t e s t f o r assessment of l i q u e f a c t i o n p o t e n t i a l i s the Standard P e n e t r a t i o n Test (SPT). In North America the most commonly used procedure f o r the assessment o f l i q u e f a c t i o n p o t e n t i a l i s the SPT based method suggested by Seed, 1979. The method i s based upon a r e l a t i o n s h i p between the c y c l i c s t r e s s r a t i o , T./O' , to i, vo cause l i q u e f a c t i o n and standard p e n e t r a t i o n r e s i s t a n c e , N, obtained from the SPT. The r e l a t i o n s h i p i s shown on F i g . 3.1. The r e l a t i o n s h i p was developed by examining a l a r g e number of s i t e s a t which l i q u e f a c t i o n had and had not occurred and where both the shaking l e v e l s and SPT N values 36 0 10 20 30 40 Modified Penetration Resistance, N, - blows/ft FIG. 3.1. CORRELATION BETWEEN FIELD LIQUEFACTION BEHAVIOUR OF SANDS ( D 5 0 > 0.25 mm) UNDER LEVEL GROUND CONDITIONS AND STANDARD PENETRATION RESISTANCE. (After Seed and I d r i s s , 1981) 37 were known. To overcome the e f f e c t of i n c r e a s i n g N value w i t h i n c r e a s i n g c o n f i n i n g s t r e s s the SPT N-values were c o r r e c t e d or normalized to a c o n f i n i n g s t r e s s of 1 atmosphere and termed N^. The c o r r e c t i o n can be determined from the r e l a t i o n s h i p : N x = C N • N where C>T i s a f u n c t i o n of the e f f e c t i v e overburden pressure, as shown on N F i g . 3.2. The c h a r t shown i n F i g . 3.1 was developed f o r earthquakes of magnitude, M = 7.5 and f o r sands. The method has r e c e n t l y been extended (Seed and I d r i s s , 1981) to i n c l u d e the e v a l u a t i o n of l i q u e f a c t i o n r e s i s t a n c e of s i l t y sands and to provide g u i d e l i n e s f o r other s o i l s , such as c l a y s . The procedure can be summarized i n the f o l l o w i n g steps: i ) For s o i l s a t depths shallower than 3 m (10 f t . ) m u l t i p l y measured N values by 0.75 to a l l o w f o r energy l o s s i n the d r i v e rods, i i ) Convert N values to N^ values using the C^ c o r r e c t i o n curves shown i n F i g . 3.2. i i i ) F o r sands w i t h > 0.25 mm use the standard c o r r e l a t i o n curves f o r sand shown i n F i g . 3.1. i v ) For s i l t y sands and s i l t s p l o t t i n g below the A - l i n e ( U n i f i e d S o i l C l a s s i f i c a t i o n P l a s t i c i t y Chart) and w i t h Dc_ < 0.15 mm use N. = (N.,) , + 7.5 and then use the standard 50 1 1 measured c o r r e l a t i o n curves f o r sands ( F i g . 3.1). v) I f the c o n f i n i n g pressure exceeds 1.5 kg/cm 2 (1.5 tons/sq. f t . ) , reduce the s t r e s s r a t i o causing l i q u e f a c t i o n to a l l o w f o r the r e d u c t i o n due to increased c o n f i n i n g pressure. Such r e d u c t i o n may be determined by l a b o r a t o r y t e s t s or on the b a s i s of experience, v i ) Consider some c l a y s o i l s as p o t e n t i a l l y l i q u e f i a b l e . Based on Chinese data these s o i l s would appear to have the f o l l o w i n g c h a r a c t e r i s t i c s : 38 C N ,0 0.2 04 0.6 08 1.0 1.2 14 16 31 r - 1 1 1 1 1 1 1 FIG. 3.2. RELATIONSHIP BETWEEN C N and EFFECTIVE OVERBURDEN PRESSURE. (After Seed and I d r i s s , 1981) 39 Percent f i n e r than 0.005 mm < 15% L i q u i d L i m i t (LL) < 35% Water content > 0.9 LL. v i i ) I f the c l a y content (determined by 0.005 mm) > 20%, consider the s o i l n o n - l i q u e f i a b l e . v i i i ) I f the water content of any clayey s o i l ( c l a y , sandy c l a y , s i l t y c l a y , c l a y e y sand, etc.) < 0.9 LL, consider the s o i l n o n - l i q u e f i a b l e . The main advantages o f t h i s method are that i t i s r a p i d , i t i s based upon a l a r g e amount of f i e l d experience, and i t i s widely used and a c c e p t e d . The main d i s a d v a n t a g e s a r e t h a t SPT measurements a r e discontinuous and are not always r e l i a b l e and repeatable. The SPT a l s o has very poor r e s o l u t i o n i n s o f t f i n e grained s o i l s , such as s i l t or sandy s i l t . The r e p e a t a b i l i t y of the SPT can be somewhat improved i f the t e s t i s performed according to the c o n d i t i o n s suggested by Seed and I d r i s s , 1981. The SPT should the r e f o r e be performed under the f o l l o w i n g c o n d i t i o n s : 1) the use of a rope and drum system, w i t h two turns of the rope around the drum (cathead), to l i f t the f a l l i n g weight, 2) d r i l l i n g mud to support the borehole, 3) a r e l a t i v e l y small diameter hole, approximately 4 inches i n diameter, and 4) p e n e t r a t i o n r e s i s t a n c e measured over the range 6 inches to 18 inches p e n e t r a t i o n i n t o the ground. P r a c t i c e i n North America would i n d i c a t e that the l i n e r i n the sampler should be removed (Schmertmann, 1979). The work by Kovacs et a l . , 1981, would i n d i c a t e that the e f f i c i e n c y of the energy d e l i v e r e d to the rods using the above procedure i s i n the range 50 to 60 percent. 40 A method developed by Iwasaki et a l . , 1975, i s widely used i n Japan and i s based on numerous undrained c y c l i c t r i a x i a l t e s t r e s u l t s on undisturbed samples of saturated sands. The method c a l c u l a t e s the i n - s i t u r e s i s t a n c e of a s o i l element to dynamic loads u s i n g the f o l l o w i n g : T * = 0.0573 [ ^ f(D )] °vo /a' + 0.7 5 0 vo i n which . 2.55 l o g f o r 0.04 mm < D.n < 0.6 mm £ < D 5 0 > = 0.35 5 0 0.567 f o r 0.6 mm < D < 1.5 mm D = the average or 50 percent g r a i n diameter i n mm. a' = v e r t i c a l e f f e c t i v e s t r e s s i n kg/cm 2, vo Iwasaki's method takes i n t o acount the e f f e c t of f i n e s content on the SPT N value, i . e . , f o r the same stre n g t h N tends to decrease as the sand cont a i n s more f i n e s . Because the Iwasaki method i s based on the t e s t data on samples of normally c o n s o l i d a t e d sands of a l l u v i a l d e posits or f i l l s , i t s a p p l i c a b i l i t y i s l i m i t e d to the f o l l o w i n g range, (Tatsuoka et a l , 1978): 0.2 < a* < 1.7 kg/cm 2 vo 0.1 < x / o ' < 0.26 SL vo 15% < D < 80%. r A comparison between the Seed method and the Iwasaki method i s shown on F i g . 3.3. s i n c e the Iwaski method i s based on undrained c y c l i c t r i a x i a l 41 0 10 20 30 40 MODIFIED SPT N-VALUE, N, FIG. 3.3. COMPARISON BETWEEN SEED AND IDRISS (1981) AND IWASAKI ET AL. (1975) SPT BASED METHODS. t e s t s on "undisturbed" samples i t i s of sampling and t e s t i n g has produced r e s i s t a n c e f o r dense samples and samples. 42 reasonable to suspect that the process c o n s e r v a t i v e estimates of l i q u e f a c t i o n unconservative estimates f o r loose 3.3. Cone Penetration Test L i q u e f a c t i o n s t u d i e s i n China have l e d to a c o r r e l a t i o n between earthquake shaking c o n d i t i o n s causing l i q u e f a c t i o n or c y c l i c m o b i l i t y and the cone p e n e t r a t i o n r e s i s t a n c e o f sands (Zhou, 1980). In t h i s c o r r e l a t i o n the c r i t i c a l v a l u e o f cone p e n e t r a t i o n r e s i s t a n c e , q . , separating c r i t l i q u e f i a b l e from n o n - l i q u e f i a b l e c o n d i t i o n s to a depth of 15 m i s determined by: q M = q [l-0.065(H -2)1[l-O.05(H -2)1 c r i t co w J L o J where = depth o f water l e v e l below ground surface i n meters, H = depth to top of sand l a y e r under c o n s i d e r a t i o n , o q„ r t = f u n c t i o n of the shaking i n t e n s i t y as f o l l o w s : M o d i f i e d M e r a l l i I n t e n s i t y V I I V I I I IX C r i t i c a l p e n e t r a t i o n 47 117 180 r e s i s t a n c e q (kg/cm 2) co Reduced e p i c e n t r a l d i s t a n c e (km) 80.5 38.0 18.6 Maximum surface a c c e l e r a t i o n , a 0.1 g 0.2 g 0.4 g max (Chinese Code) Thi s e m p i r i c a l equation was the r e s u l t o f f i e l d t e s t data from the Tangshan earthquake area where the sand was p r i m a r i l y a c l e a n sand w i t h l i t t l e f i n e s c o n t e n t . The mean g r a i n s i z e was D^^ = 0.25 mm. The method was l a t e r (Zhou, 1981) expanded to incor p o r a t e data from a s i l t y sand s i t e ( L u t a i ) , 43 where D, .Q = 0 . 0 7 mm. The p r a c t i c a l a p p l i c a t i o n of t h i s method may present some d i f f i c u l t y s i n c e the e p i c e n t r a l d i s t a n c e and i n t e n s i t y o f shaking i s i n v o l v e d . The e l e c t r i c cone used i n the Chinese study was a 1 6 cm 2, 6 0 degree cone w i t h a 1 0 0 cm2 f r i c t i o n sleeve immediately behind the t i p . Immediately behind the f r i c t i o n sleeve the cone was reduced i n diameter to couple w i t h the smaller diameter push rods. The r e d u c t i o n i n diameter of the s h a f t c l o s e behind the cone t i p may produce p e n e t r a t i o n r e s i s t a n c e s s l i g h t l y d i f f e r e n t from those using a cone w i t h a uniform shaft diameter meeting the European and ASTM Standards. A f u r t h e r d i s c u s s i o n of the data from the Chinese study w i l l be given i n Chapter 5 . Another method that uses cone p e n e t r a t i o n r e s i s t a n c e f o r assessment of l i q u e f a c t i o n p o t e n t i a l was developed by Douglas e t a l . , 1 9 8 1 . This method i n v o l v e s the conversion of CPT data to equ i v a l e n t SPT blowcounts. Douglas e t a l . showed that the SPT and CPT p e n e t r a t i o n values were s i m i l a r l y i n f l u e n c e d by the same s o i l c h a r a c t e r i s t i c s and that CPT data could t h e r e f o r e be d i r e c t l y used i n SPT based l i q u e f a c t i o n p o t e n t i a l assessments. The main disadvantage w i t h t h i s method i s that i t re q u i r e s on s i t e c a l i b r a t i o n w i t h SPT data to a l l o w CPT conversion. However, the blowcounts p r e d i c t e d from CPT measurements can be adjusted, once the CPT i s c a l i b r a t e d a g a i n s t a c t u a l blowcount hammer energy measurements, to account f o r the dependence of any p a r t i c u l a r SPT-based design procedure upon a s p e c i f i c SPT-sampler energy. As discussed e a r l i e r , i t appears that the energy l e v e l a s s o c i a t e d w i t h the Seed SPT-based method f o r l i q u e f a c t i o n assessment i s about 5 0 to 6 0 percent o f the maximum p o t e n t i a l energy. Further d e t a i l s concerning conversion of CPT data to equ i v a l e n t SPT data, and v i s a v e r s a , 44 w i l l be discussed i n Chapters 4 and 7. The main advantages the CPT methods have over the SPT methods are: the CPT i s f a s t e r and l e s s expensive, i t provides an increased d e s c r i p t i o n of s i t e v a r i a b i l i t y and a greater degree of measurement r e p e a t a b i l i t y . However, the cone r e s i s t a n c e , s i m i l a r to the SPT, i s an i n s e n s i t i v e parameter i n s o f t f i n e grained s o i l s , such as s i l t . 3.4. E l e c t r i c a l R e s i s t i v i t y A new method f o r the i n - s i t u measurement of l i q u e f a c t i o n p o t e n t i a l was developed by Arulanandan et a l . , (1981) based on the e l e c t r i c a l p r o p e r t i e s o f s o i l . A summary d e s c r i p t i o n o f the equipment was given i n Chapter 2. F u l l d e t a i l s o f the equipment and procedures are given i n A r u l m o l i e t a l . , 1981. The c y c l i c s t r e s s r a t i o r e q uired to cause l i q u e f a c t i o n was c o r r e l a t e d to an e l e c t r i c a l parameter (^ 3/^*^ m e a n) using c y c l i c l a b o r a t o r y t e s t s . The e l e c t r i c a l parameter combines three e l e c t i c a l parameters, defined as f o l l o w s : A = Anisotropy Index F = Average Formation Factor f = Average Shape Factor mean The v a l i d i t y o f the c o r r e l a t i o n was checked using i n - s i t u measurements from a l i m i t e d number o f s i t e s where l i q u e f a c t i o n had and had not occurred. The c o r r e l a t i o n appears t o p r o v i d e r e a s o n a b l e p r e d i c t i o n s o f whether l i q u e f a c t i o n would occur or not a t three major earthquake s i t e s , although the data p o i n t s were a s i g n i f i c a n t d i s t a n c e from the boundary separating l i q u e f i a b l e from n o n - l i q u e f i a b l e s i t e s , as shown on F i g . 3.4. 45 z o 3 o 0.6 0.5 0.4 I/) ,u O * 10 o 6 < 8 0.3 0.2 0.1 1 1 (Lawson's Landing Site San Francisco E.Q. • 1906, M>8td ) 1 I • Liquefaction O No Llquofoction (Agn«« Hospital Site • San Francisco E.Q. 1906, M 'SU. ) (Touho Reservoir Site, Tongshan E.Q. 1976, M • 7.5 (Embarcadero Site / ' SonFrancisco E.Q. ' 1906, M - 8 ' 4 ) / (River Sits, t Niigata E.Q. 1964, M'7tfa) (Road Site, • Niigata E.Q. 1964, M>~' (30 CYCLES from Arulmoli (7)) 0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0 12 0.10 ELECTRICAL PARAMETER ( y • J" ) FIG. 3.4. CORRELATION BETWEEN FIELD LIQUEFACTION BEHAVIOUR OF SANDS FOR LEVEL GROUND CONDITIONS AND ELECTRICAL PARAMETER. (After Arulmoli et a l . , 1981) 46 The procedure r e q u i r e s a p r e v i o u s l y bored hole i n t o which the probe i s placed. The t i p of the probe i s then pushed about 15 to 25 cm i n t o the s o i l to be t e s t e d . The method thus has a slow incremental procedure that r e q u i r e s a pre-bored hole f o r each t e s t . The t e s t i s f u r t h e r complicated by the need to o b t a i n measurements on a pore water sample and a bulk sample a t each s p e c i f i c l o c a t i o n . The measurements on a pore water sample are complicated by p o s s i b l e contamination during repeated t e s t s . The e l e c t r i c a l p a r a m e t e r s , average f o r m a t i o n f a c t o r , F, and a v e r a g e shape f a c t o r , f , a r e not uniqu e f o r a l l sands and thus r mean r e q u i r e c a l i b r a t i o n i n the l a b o r a t o r y using bulk samples. The need f o r bulk samples can be reduced i f general e m p i r i c a l c o r r e l a t i o n s are used. The e l e c t r i c a l method shows s i g n i f i c a n t p o t e n t i a l as a s p e c i f i c i n -s i t u t e s t method f o r assessment o f l i q u e f a c t i o n p o t e n t i a l s i n c e i t provides a measure of g r a i n and aggregate p r o p e r t i e s from a nondestructive t e s t . However, i t s use would almost always f o l l o w the a p p l i c a t i o n of a logging t o o l , which would be re q u i r e d to evaluate the s i t e v a r i a b i l i t y . Thus, f o r many p r o j e c t s the high cost of the e l e c t r i c a l r e s i s t i v i t y technique may l i m i t i t s wide use. This i s f u r t h e r complicated by the f a c t that the equipment and methodology required f o r the technique are patented. The technique, at present, i s only a p p l i c a b l e to uniform sands. 3.5. Pressuremeter Test A recent method proposed f o r the assessment of l i q u e f a c t i o n p o t e n t i a l i s based upon a c o r r e l a t i o n between c y c l i c s t r e s s r a t i o and d i l a t i o n angle, v, (Vaid et a l . , 1981). The r e l a t i o n s h i p i s shown i n F i g . 3.5. The d i l a t i o n angle i s derived from s e l f - b o r i n g pressuremeter data using the theory by Hughes e t a l . (1977). The c o r r e l a t i o n was e s t a b l i s h e d from a 47 Liquefaction or 10% double amplitude shear strain in 10 cycles 20 40 60 80 100 Rela t ive density, D r , % 0 6 12 18 24 Corrected dilation angle,v,,degrees (After Vaid et al. 1981 ) FIG. 3.5. CORRELATION BETWEEN RESISTANCE TO LIQUEFACTION OF SAND AS A FUNCTION OF RELATIVE DENSITY AND CORRECTED DILATION ANGLE. (After Vaid et a l . , 1981) 48 r e l a t i o n s h i p between r e l a t i v e d e n s i t y and d i l a t i o n angle f o r Ottawa sand. The d i l a t i o n angle was considered to be a u s e f u l parameter to represent the i n - s i t u s t a t e of a sand and was computed using a tangent a t a shear s t r a i n Y = 10%. The d i l a t i o n angle was obtained from simple shear t e s t s c o r r e c t e d to a normal pressure of 1 T / f t 2 (100 kPa). The d i l a t i o n r a t e of a s o i l i s a d i r e c t measure of the volume change c h a r a c t e r i s t i c s , which have long been considered a primary f a c t o r f o r l i q u e f a c t i o n p o t e n t i a l . However, there i s some disagreement as to whether the d i l a t i o n angle at 10% shear s t r a i n t r u l y represents the behaviour of the s o i l under earthquake c y c l i c l o a d i n g c o n d i t i o n s where shear s t r a i n s are g e n e r a l l y very s m a l l . Considerable research i s s t i l l r e q u i red to e s t a b l i s h i f the d i l a t i o n angle at Y = 10% i s a r e l i a b l e measure of the i n - s i t u s t a t e and can e f f e c t i v e l y account f o r f a c t o r s such as s t r e s s h i s t o r y , aging and cementation. The main advantage o f the pressuremeter method i s that i t uses a parameter (v) that can be measured i n the f i e l d and i n the l a b o r a t o r y . This enables d i r e c t comparison of f i e l d and l a b o r a t o r y data. The disadvantages to i t s use are: the high cost a s s o c i a t e d w i t h the equipment and i n s t a l l a t i o n , s e n s i t i v i t y to disturbance during i n s t a l l a t i o n and i t s present l i m i t e d a v a i l a b i l i t y . There i s a l s o a l a c k o f f i e l d data to support the technique. S i m i l a r to the e l e c t r i c a l r e s i s t i v i t y method, the s e l f - b o r i n g pressuremeter i s a s p e c i f i c i n - s i t u t e s t method. However, the t e s t i s w e l l s u i t e d to fundamental analyses to provide b a s i c s o i l parameters such as f r i c t i o n angle, <t>', and d i l a t i o n angle, v. Further d e t a i l s regarding the i n t e r p r e t a t i o n of s e l f - b o r i n g pressuremeter data w i l l be given i n Chapter 5. 49 3.6. Geophysical Method A recent method (Dobry e t a l . , 1980) has been proposed f o r e v a l u a t i o n o f l i q u e f a c t i o n p o t e n t i a l based upon the c y c l i c shear s t r a i n developed du r i n g earthquake l o a d i n g . I t has long been recognized that shear s t r a i n s p l a y a key r o l e i n the generation of pore pressure. S t o l l and Kald (19 76) advanced the concept that there might be a l i m i t i n g shear s t r a i n below which no porewater pressures would develop r e g a r d l e s s of the number of lo a d i n g c y c l e s . Dobry e t a l . , 1981, show r e s u l t s to i n d i c a t e that t h r e s h o l d shear s t r a i n , y , ranges between l x l O - 2 % to 3*10 - 2% f o r many sands. Dobry et a l . , 1980, proposed the use of a thr e s h o l d shear s t r a i n to estimate l i q u e f a c t i o n p o t e n t i a l of saturate d sands. In t h i s approach, the s i m p l i f i e d method of Seed and I d r i s s , 19 71, I s used i n terms of shear s t r a i n s r a t h e r than shear s t r e s s e s so th a t a th r e s h o l d o f peak a c c e l e r a t i o n ( a t ) i s def i n e d i n terms of the thr e s h o l d s t r a i n . I f the a n t i c i p a t e d peak a c c e l e r a t i o n exceeds the threshold a c c e l e r a t i o n , pore pressures w i l l be generated and l i q u e f a c t i o n may be p o s s i b l e . The method, t h e r e f o r e , appears to e s t a b l i s h i f pore pressures w i l l develop during a given earthquake l o a d i n g , but i t does not d e f i n e i f these pore pressures w i l l e v e n t u a l l y cause l i q u e f a c t i o n . The method r e q u i r e s the measurement of i n - s i t u shear wave v e l o c i t y , V , to determine the maximum shear modulus, G , from the expression, s indx G = p V 2 max s where p = mass d e n s i t y of the s o i l under c o n s i d e r a t i o n . The maximum shear modulus i s then r e l a t e d to the maximum surface a c c e l e r a t i o n using the Seed and I d r i s s s i m p l i f i e d method^ r e s u l t i n g i n the expression 50 a t V G / G m a x > t v 2 g g z r d s w here (G/G ) = modulus r e d u c t i o n f a c t o r a t the t h r e s h o l d shear max't s t r a i n , Y g = a c c e l e r a t i o n of g r a v i t y r = r e d u c t i o n c o e f f i c i e n t to account f o r s o i l f l e x i b i l i t y d z = depth of s o i l element considered. The shear wave v e l o c i t y , V , i s u s u a l l y measured by the c r o s s - h o l e o r uphole/downhole techniques. The main disadvantages w i t h these methods a r e : the h i g h c o s t a s s o c i a t e d w i t h the f i e l d measurement of V and the s d i f f i c u l t y a s s o c i a t e d w i t h determination of V g i n the upper 5 m o f s o i l . The cost of the i n - s i t u t e s t may be reduced i n the f u t u r e w i t h the development of techniques such as the seismic CPT. The method enables e v a l u a t i o n of l i q u e f a c t i o n s u s c e p t i b i l i t y , based on the e x i s t e n c e of a threshold s t r a i n f o r sands, but does not determine i f l i q u e f a c t i o n w i l l or w i l l not occur. The method, however, does demonstrate the increase i n l i q u e f a c t i o n r e s i s t a n c e observed f o r o l d e r sands ( o l d e r than 10,000 y e a r s ) , as shown i n F i g . 3.6. F i g . 3.6 i l l u s t r a t e s the very l o w t h r e s h o l d a c c e l e r a t i o n , a^ _ , f o r young, n a t u r a l or f i l l sand d e p o s i t s . 3.7. General Review I t i s c l e a r from the previous s e c t i o n s t h a t there i s considerable confusion as to what i n - s i t u method should be used f o r assessment of l i q u e f a c t i o n p o t e n t i a l . The research contained i n t h i s r e p o r t has been d i r e c t e d toward reducing some of t h i s confusion and reviewing, i n d e t a i l , some of the more promising i n - s i t u t e s t methods and t h e i r i n t e r p r e t a t i o n and a p p l i c a t i o n to l i q u e f a c t i o n assessment. 51 S H E A R WAVE VELOCITY IN FT/SEC FIG. 3.6. VARIATION OF THRESHOLD ACCELERATION WITH SHEAR WAVE VELOCITY. (After Dobry et a l . , 1981) 52 4. CONE PENETRATION TESTING 4.1. Equipment The cone used f o r cone p e n e t r a t i o n t e s t i n g i n t h i s study was a 5-channel cone developed a t UBC. A d e t a i l e d d e s c r i p t i o n of the cone i s given by Campanella and Robertson (1981). The cone i s i l l u s t r a t e d i n F i g . 4.1. The 5-channel cone enables continuous monitoring of bearing, f r i c t i o n , pore pressure, slope and temperature. The cone has a 60 degree apex angle, w i t h a 10 cm2 p r o j e c t e d base area. The dimensions conform to the European Standard and ASTM Standard f o r e l e c t r i c cones. The porous f i l t e r element i s l o c a t e d immediately behind the cone t i p and i s connected h y d r a u l i c a l l y to the pressure transducer f o r measuring the pore pressure. An interchangeable t i p can be used to r e l o c a t e the porous element onto the face of the cone. The pressure transducer i s a h e r m e t i c a l l y sealed, a l l welded, f l u s h diaphragm, absolute pressure type. A very small c a v i t y i s used and surrounded by a porous p l a s t i c f i l t e r element. The c a v i t y i s sa t u r a t e d w i t h g l y c e r i n . D e t a i l s o f the s a t u r a t i o n procedure w i l l be given l a t e r . S i g n a l s from the load c e l l s and pressure transducer are sent to the surf a c e through a 10 conductor c a b l e . The use of common e x i t a t i o n v o l t a g e permits the simultaneous measurement of the 5 channels. A 15 v o l t r e g u lated power supply i s used to provide a f i x e d 10 v o l t e x c i t a t i o n . Balance r e s i s t o r s are used to o b t a i n zero output a t the reference s e t t i n g f o r each transducer i n order to change range on the chart recorder without an o f f s e t v o l t a g e . S c a l i n g r e s i s t o r s are used to d i r e c t l y p l o t i n t o engineering u n i t s on the chart r e c o r d e r s . A three channel recorder d r i v e n by a pulse d r i v e d i g i t a l s h a f t encoder, i s a u t o m a t i c a l l y a c t i v a t e d during p e n e t r a t i o n to record bearing (QC), f r i c t i o n (FC) and dynamic pore 53 -swage fitting to lock 10 conductor cable strain gages for-friction load cell thermister-pressure transducer-porous plastic small cavity -wires spliced to cable inside tube — slope sensor /-equal end area friction sleeve (I50cm'areo ) -strain gages for cone bearing load cell 60° cone 35 68mm0.D. 5-CHANNEL CONE PENETROMETER. FIG. 4.1. 5-CHANNEL CONE PENETROMETER. (After Campanella and Robertson, 1981) 54 pressures (U) w i t h depth. A second chart recorder i s used to monitor pore pressure and slope w i t h time. The slope sensor i s a simple device that monitors the i n c l i n a t i o n of the cone t i p during p e n e t r a t i o n . This i n f o r m a t i o n can be used to c o r r e c t the depth p r o f i l e during deep soundings. The continuous records of bearing, f r i c t i o n , pore pressure and slope r e q u i r e c o n s i d e r a b l e handling and r e d u c t i o n , i n c l u d i n g c a l c u l a t i o n s of f r i c t i o n r a t i o and d i f f e r e n t i a l pore pressure r a t i o . To f a c i l i t a t e the c a l c u l a t i o n s and p l o t t i n g r e q u i r e d , the s t r i p c h a r t records are d i g i t i z e d on a graphics t a b l e t . During the d i g i t i z i n g process, s u f f i c i e n t data p o i n t s are used to f u l l y d e s cribe the f i e l d r e c o r d s . Computer graphics f a c i l i t i e s are used to generate f i n a l p l o t s . 4.2. Factors A f f e c t i n g Measured Parameters from E l e c t r i c Cone Before a n a l y z i n g any e l e c t r i c f r i c t i o n cone data i t i s important to r e a l i z e and account f o r the p o t e n t i a l e r r o r s that each element of data may c o n t a i n . During development and use of the UBC cone i n t h i s study s e v e r a l s i g n i f i c a n t aspects concerning the data c o l l e c t i o n and i n t e r p r e t a t i o n have been observed. Some of these points are summarized here. 4.2.1. Bearing and F r i c t i o n The t o l e r a n c e i n machining the standard (Fugro) e l e c t r i c f r i c t i o n cone I s such t h a t the d i f f e r e n c e i n diameter between the t i p and the sleeve can be up to 0.25mm. This combined w i t h wear during usage o f t e n r e s u l t s i n s i g n i f i c a n t d i f f e r e n c e s i n diameter between the t i p and sleeve. I t has been found that d i f f e r e n c e s i n diameters, e s p e c i a l l y when the t i p i s l a r g e r than the s l e e v e , can r e s u l t i n s i g n i f i c a n t v a r i a t i o n s i n measured f r i c t i o n 55 v a l u e s . This v a r i a t i o n can be reduced by c a r e f u l machining during con-s t r u c t i o n and r e g u l a r tolerance checks during the l i f e of the cone. The O.D. of the cone should be i d e n t i c a l or l e s s than O.D. of f r i c t i o n sleeves (+ 0.00 mm to + 0.25 mm). The cones used i n t h i s study meet these s p e c i f i c a t i o n s . The l o a d c e l l s w i t h i n the cone are o f t e n temperature dependent and are almost always c a l i b r a t e d at room or a i r temperature. However, s o i l and groundwater are o f t e n c o n s i d e r a b l y c o o l e r than the c a l i b r a t i o n temperature and a s h i f t i n the zero can occur f o r both lo a d c e l l s during p e n e t r a t i o n . T h i s u s u a l l y i s of l i t t l e consequence i n sands i n which measured values of f r i c t i o n and bearing are l a r g e compared w i t h the zero s h i f t but can be s i g n i f i c a n t i n s o f t s o i l s . Temperature a f f e c t s should the r e f o r e be accounted f o r when penetrating i n s o f t s o i l s . A temperature sensing element i n the cone such as a thermister or m i n i a t u r e i n t e g r a t e d c i r c u i t can provide the b a s i s f o r c a l i b r a t e d zero s h i f t c o r r e c t i o n s due to temperature and i s p a r t i c u l a r l y u s e f u l i n northern environments. 4.2.2. Pore Pressure E f f e c t s on Measured Parameters I t has been observed that when the cone i s subjected to an a l l around water pressure there i s u s u a l l y a s h i f t i n the zeros f o r both the f r i c t i o n and t i p measurements. The f r i c t i o n s h i f t i s o f t e n due to unequal end area of the f r i c t i o n s leeve (see F i g . 4.2) and i s u s u a l l y negative or opposite to the s o i l f r i c -t i o n but can be p o s i t i v e . Most f r i c t i o n cones i n use today have unequal end areas, but c o r r e c t i o n s are not u s u a l l y made, perhaps because dynamic pore pressure measurements are l a c k i n g . F r i c t i o n c o r r e c t i o n s are espe-c i a l l y s i g n i f i c a n t i n deep p r o f i l e s beneath the water t a b l e and i n low 56 FIG. 4.2. INFLUENCE OF UNEQUAL END AREAS. (After Campanella, Robertson and G i l l e s p i e , 1983) 57 p e r m e a b i l i t y saturated s o i l s where very l a r g e dynamic pore pressures are generated during p e n e t r a t i o n . High pore pressure zero s h i f t s i n f r i c t i o n e x p l a i n why some normally c o n s o l i d a t e d and s e n s i t i v e c l a y s may give low or negative f r i c t i o n r a t i o s . This w r i t e r b e l i e v e s that f r i c t i o n cones should be c a l i b r a t e d against a l l around pressure and readings c o r r e c t e d f o r dynamic pore pressure e f f e c t s , i f one i s to develop confidence i n i t s use. Of course, the best s o l u t i o n i s the design and use of a f r i c t i o n sleeve which has equal end area r e q u i r i n g no pore pressure c o r r e c t i o n s such as the cone used I n t h i s study, and shown i n F i g . 4.1. Since the bearing t i p i s a t o t a l s t r e s s element, i t should record a bearing s t r e s s equal to an a l l around a p p l i e d pressure. This i s never the case and the t i p always records a s t r e s s l e s s than the a p p l i e d a l l around pressure. Water can enter the j o i n t s between the f r i c t i o n sleeve and the cone t i p (see F i g . 4.2), which r e s u l t s i n water pressure a c t i n g on the back of the t i p . The net area over which the water pressure a c t s i s l e s s than the cone t i p area. Thus, every cone has a g i v e n net area r a t i o a s s o c i a t e d w i t h i t s design and dimensions. Most cones have net area r a t i o s of from about 0.6 to 0.8, but a 20 cm2 bulbous cone t i p l i k e the one shown i n F i g . 4.2 would have a net area r a t i o l e s s than 0.5 and probably c l o s e to 0.4. This w r i t e r s t r o n g l y recommends that bearing cones be c a l i b r a t e d f o r a l l around pressure e f f e c t s and when p o s s i b l e a l l bearing values be reported as t o t a l s t r e s s where qT = q c + u T ( 1 " a ) ( 4 , 1 ) and q_ i s t o t a l s t r e s s , q i s measured bearing, u„ i s t o t a l dynamic I c i pore pressure and "a" i s net area r a t i o . This c o r r e c t i o n can not be e l i -minated except w i t h a u n i t i z e d , j o i n t l e s s d e s i g n where the sleeve i s s t r a i n gauged to measure the t i p l o a d . Such a design i s not yet a v a i l a b l e . The 58 b e a r i n g r e s i s t a n c e v a l u e s , q^, shown i n t h i s t h e s i s have been correct e d according to the formula i n equation 4.1. A l s o , the use of t o t a l bearing, q^, may reduce some of the reported wide v a r i a t i o n s of the c a l c u l a t e d bearing c a p a c i t y f a c t o r , N^, required to determine undrained shear strength from cone bearing (Lunne and Kleven, 1981). 4.2.3. F r i c t i o n - B e a r i n g O f f s e t The center of the f r i c t i o n sleeve i s approximately 10 cm behind the cone t i p . To c a l c u l a t e the f r i c t i o n r a t i o (F.R.), the average f r i c t i o n r e s i s t a n c e (FC) and bearing r e s i s t a n c e (QT) are compared at the same depth. This u s u a l l y i n v o l v e s an o f f s e t of the f r i c t i o n r e s i s t a n c e by the p h y s i c a l d i s t a n c e of 10 cm. However, the bearing r e s i s t a n c e i s e f f e c t e d by the s o i l ahead of the t i p , whereas, the f r i c t i o n i s only e f f e c t e d by the s o i l i n d i r e c t contact w i t h the f r i c t i o n s l e e ve. Thus, the standard o f f s e t d i s t a n c e of 10 cm may not always produce r e a l i s t i c f r i c t i o n r a t i o p l o t s , e s p e c i a l l y i n h e a v i l y interbedded s o i l s and i n r e l a t i v e l y s t i f f s o i l s where the o f f s e t can be c o n s i d e r a b l y more than 10 cm. The computer program developed at UBC f o r data p r e s e n t a t i o n has the f a c i l i t y to use any f r i c t i o n - b e a r i n g o f f s e t . I t a l s o allows the user to c a l c u l a t e the o f f s e t d i s t a n c e by using computer graphics to match peaks o r troughs i n the f r i c t i o n - b e a r i n g p r o f i l e . In g eneral, however, the standard 10 cm f r i c t i o n - b e a r i n g o f f s e t u s u a l l y provides adequate f r i c t i o n r a t i o p l o t s . A l l f r i c t i o n r a t i o p l o t s shown i n t h i s t h e s i s have used a 10 cm f r i c t i o n - b e a r i n g o f f s e t . 59 4.2.4. Piezometer T i p S a t u r a t i o n I t has p r e v i o u s l y been shown by Campanella and Robertson, 1981, that complete s a t u r a t i o n of the piezometer t i p i s e s s e n t i a l . Pore pressure response was compared f o r saturated and entrapped a i r piezometer systems. Both the maximum pore pressure and d i s s i p a t i o n times are s e r i o u s l y e f f e c t e d by a i r entrapment. Furthermore, i t was a l s o shown that g l y c e r i n worked e f f e c t i v e l y as a s a t u r a t i n g f l u i d which i s m i s c i b l e w i t h water yet develops a high a i r en t r y t e n s i o n to prevent l o s s o f s a t u r a t i o n during use and penetration through s o i l s above the water t a b l e . U n f o r t u n a t e l y , i t i s not p o s s i b l e to check s a t u r a t i o n before penetra-t i n g the s o i l . F i g . 4.3 shows a system used during t h i s study and which a l l o w s easy s a t u r a t i o n . The f i g u r e a l s o shows the design of the UBC equal end area f r i c t i o n sleeve cone and the interchangeable t i p to r e l o c a t e the porous f i l t e r . A simple cup was made to s l i p over the f r i c t i o n s l eeve and i s sealed w i t h an C—ring. With the t i p and f i l t e r removed, g l y c e r i n i s introduced i n t o the cup and a hypodermic i s used to f l u s h a i r from i n t e r i o r c a v i t i e s . The presaturated f i l t e r and t i p are assembled, the excess g l y c e r i n poured o f f and the cup removed. This technique has worked w e l l . 4.2.5. Rate of Pe n e t r a t i o n The standard r a t e of pe n e t r a t i o n f o r a s t a t i c cone t e s t i s 2 cm/sec. T r a d i t i o n a l l y cone p e n e t r a t i o n i n sands has been considered to be drained and p e n e t r a t i o n i n c l a y s undrained. However, f o r mixed s o i l s such as s i l t s and clayey s i l t s , the drainage c o n d i t i o n during p e n e t r a t i o n i s not w e l l d e f i n e d . F i g . 4.4 summarizes cone data c o l l e c t e d by the w r i t e r , as a f u n c t i o n o f p e n e t r a t i o n r a t e i n a c l a y e y s i l t a t the UBC research s i t e near Richmond. F u l l d e t a i l s of the s i t e w i l l be gi v e n i n Chapter 7. The clayey 60 Porous Filter uad Ring Sl ip-on_ Saturation Cup Pressure Transducer Strain Gauges Bearing Load Cell Equal End Area Friction Sleeve Strain Gauges _ Friction Load Cell Glycerin Slope Sensor FIG. 4.3. TIP DESIGN TO RELOCATE POROUS FILTER AND ALLOW EASY SATURATION WITH GLYCERIN. !(After Campanella, Robertson and G i l l e s p i e , 1983) j e 0.41 l l o s | -o • 0.2 | c . h o n I " 0.0\ J I L I II III 0 1 I I 111 III 1 1 1 1 1 10 10 e» £ 8 22 6 1 Bearing corrected for temperature and water pressure effects I I I I 11 III '"I • ' ' • • LLL > 0.01 I 0 | 8 6 0.1 10 - V Z °* A Effect ive bearing «i * " "TOTAL J I I I Mill I i i i i i J I I I I I 0.01 0.1 10 2 o 10 8 6 [ ^ E q u i l i b r i u m pert i i i i n i l J i i 1 1 1ul i 1 1 1 1 1 1 1 o . o i 0.1 I Penetration rote,cm/sec. 10 Ai l measurements at 20m depth FIG. 4.4. PENETRATION RATE EFFECTS IN CLAYEY SILT DEPOSIT AT MCDONALD'S FARM. (After Campanella, Robertson and G i l l e s p i e , 1983) 62 s i l t has a p e r m e a b i l i t y i n the order of 8xl0~ 7cm/sec. F i g . 4.4 shows that the p e n e t r a t i o n i s undrained down to a p e n e t r a t i o n speed of about 0.2 cm/sec. As the p e n e t r a t i o n speed i s p r o g r e s s i v e l y decreased below t h i s speed the t o t a l pore pressure (measured behind the t i p ) during p e n e t r a t i o n decreases and a corresponding increase i s observed f o r the t o t a l cone bearing and f r i c t i o n , both c o r r e c t e d f o r zero s h i f t . The increase i s p a r t i c u l a r l y n o t i c e a b l e i n the f r i c t i o n . The increase i s l e s s n o t i c e a b l e f o r the bearing, i n p a r t , because the bearing a l s o records the water pressure. Thus, as the water pressure decreases the bearing tends to decrease, but t h i s i s o f f s e t by the i n c r ease i n e f f e c t i v e s t r e s s i n the s o i l which increases the bearing. To i l l u s t r a t e t h i s behaviour the effective bearing i s a l s o shown on F i g . 4.4. The e f f e c t i v e bearing i s d e f i n e d as the t o t a l bearing ( c o r r e c t e d f o r temperature and a l l around pressure e f f e c t s ) minus the t o t a l water pressure. It i s i n t e r e s t i n g to note that t h i s e f f e c t i v e bearing data shows how s m a l l an e f f e c t i v e pressure (bearing) was r e q u i r e d to penetrate the s o f t c l a y e y s i l t under undrained c o n d i t i o n s ; the e f f e c t i v e bearing was about 1/4 of the t o t a l bearing. These values are s t i l l extremely small when compared to the almost two orders of magnitude l a r g e r bearing values f o r the o v e r l y i n g sand which i s being sheared under drained c o n d i t i o n s ( F i g . 7.3). The data a l s o i l l u s t r a t e s the marked increase i n the e f f e c t i v e s t r e s s e s around the t i p as the r a t e of p e n e t r a t i o n decreases and the pore pressures drop by over 15 m of water pressure or about 1.5 bar (150 kPa) . The r e s u l t i n g change i n e f f e c t i v e s t r e s s e s due to p a r t i a l l y drained condi-t i o n s around the t i p produce an almost twofold increase i n the e f f e c t i v e bearing required to penetrate the s i l t . This behaviour i s analogous to observed t r i a x i a l t e s t behaviour of normally c o n s o l i d a t e d c l a y s when com-paring undrained w i t h drained strength r e s u l t s . 6 3 The proposed concept o f e f f e c t i v e bearing defined as the t o t a l bearing s t r e s s minus the t o t a l water pressure represents a f i r s t order attempt a t i n t e r p r e t i n g cone data i n e f f e c t i v e s t r e s s terms. This may a l l o w comparison of measured bearing s t r e s s e s i n undrained and p a r t i a l l y drained s o i l s w i t h those i n drained s o i l s . Of course, the i n - s i t u e f f e c t i v e normal s t r e s s a t the cone t i p i s s t i l l a missing e s s e n t i a l parameter and must be estimated i f one i s to attempt a complete e f f e c t i v e s t r e s s a n a l y s i s . I t i s b e l i e v e d that the f r i c t i o n sleeve measurement may c o r r e l a t e w e l l w i t h the l a t e r a l e f f e c t i v e normal s t r e s s and may provide the missing parameter. The concepts of e f f e c t i v e bearing and e f f e c t i v e s t r e s s i n t e r p r e t a t i o n of cone soundings are c u r r e n t l y t o p i c s of intense research. However, s i n c e the pore pressure i s dependent on the l o c a t i o n of the porous element, e f f e c t i v e bearing must be defined i n r e l a t i o n to the porous element l o c a t i o n . I t has become apparent, however, t h a t i t i s e s s e n t i a l to co n t i n u o u s l y monitor both pore pressure and bearing during p e n e t r a t i o n and to c o n s i s t e n t l y work w i t h t o t a l bearing ( c o r r e c t e d f o r net area) i n undrained s o i l s . 4.2.6. Slope Sensor Several cones today have simple slope sensors incorporated i n the desi g n to enable a measure of the n o n - v e r t i c a l i t y of the sounding (De R u i t e r , 19 82). This i s p a r t i c u l a r l y u s e f u l f o r deep soundings when t i p i n c l i n a t i o n s i n excess of 45° are not uncommon e s p e c i a l l y i n s t r a t i f i e d s o i l . The maximum depth f o r which a slope sensor can be omitted depends on the acceptable e r r o r i n recorded depth provided o b s t r u c t i o n s do not e x i s t . However, f o r most CPT work the maximum depth without a slope sensor, f o r which n e g l i g i b l e e r r o r i n recorded depth can be assumed, i s about 15 m (Van de Graaf and J e k e l , 1982). 64 Experience during t h i s study suggests that once a cone t i p i s d e f l e c t e d , i t continues along a path w i t h a r e l a t i v e l y c o n s i s t e n t r a d i u s of curv a t u r e . The standard equipment tends to accept about 1° of d e f l e c t i o n per meter length without n o t i c e a b l e damage. 4.2.7. F r i c t i o n sleeve measurement The f r i c t i o n measurement made w i t h the standard f r i c t i o n cone i s u s u a l l y used f o r s o i l c l a s s i f i c a t i o n and p i l e design. However, a b e t t e r understanding of the f r i c t i o n measurement i s r e q u i r e d . A study to t h i s aim was c a r r i e d out by the w r i t e r by p r o g r e s s i v e l y moving the f r i c t i o n sleeve f u r t h e r away from the t i p . The measured f r i c t i o n versus d i s t a n c e from t i p i s shown on F i g . 4.5 f o r pene t r a t i o n through a lo o s e and a dense n a t u r a l sand d e p o s i t . The r e s u l t s show that there i s a marked increase i n the f r i c t i o n measurement between 10 and 25 cm ( i . e . 3 to 7 cone diameters) behind the t i p . This increase appears more pronounced i n the dense sand. Beyond 40 cm (11 diameters) behind the t i p , the f r i c t i o n appears reasonably constant. It i s i n t e r e s t i n g that the average f r i c t i o n s t r e s s measured by the standard f r i c t i o n sleeve immediately behind the t i p appears to gi v e a good estimate of the a c t u a l f r i c t i o n away from the t i p even though the f r i c t i o n s leeve i s l o c a t e d i n a complex non-uniform s t r a i n f i e l d . This i s important Information f o r those f r i c t i o n sleeve r e s u l t s used f o r p i l e design. The reason f o r the increase i n f r i c t i o n a t about 20 cm behind the t i p i s unclear. I t may have something to do w i t h the f a i l u r e mode during continuous p e n e t r a t i o n . Further research i s r e q u i r e d to f u l l y evaluate the reason and s i g n i f i c a n c e of t h i s phenomenon. 65 20 Penetration Rate 2cm/sec. < • \ • 1 SAND (at 8m ) \ (Very Dense D r « 80%) \ SAND (at 5m ) ^ \ / 1 1 1 / 1 1 _ l 0 10 20 30 40 50 60 Sleeve Friction, f c , ( bars x I0~ 2 ) 4.5. FRICTION ALONG SHAFT DURING CONE PENETRATION IN SAND AT MCDONALD'S FARM. (After Campanellaand Robertson, 1981) 66 Considerable i n s i g h t i n t o the e f f e c t on i n - s i t u h o r i z o n t a l s t r e s s e s can be achieved by a c a r e f u l review of the c a l i b r a t i o n chamber t e s t s p e rformed u s i n g the CPT. The f r i c t i o n s l e e v e measurement, f g , made du r i n g drained cone p e n e t r a t i o n i n sands i s a d i r e c t measure o f the average i n - s i t u h o r i z o n t a l s t r e s s around the f r i c t i o n sleeve, since f = a! tan 6 s h where o^ = average i n - s i t u h o r i z o n t a l s t r e s s around the f r i c t i o n s l e e v e , 6 = f r i c t i o n angle between cone s t e e l and sand. I t would be expected t h a t , because of the d i l a t a n t behaviour of most sands, the h o r i z o n t a l s t r e s s e s around the f r i c t i o n sleeve would g e n e r a l l y i n c r e a s e due to the pen e t r a t i o n process. Thus, the more d i l a t a n t the sand the l a r g e r the increase i n h o r i z o n t a l s t r e s s e s due to pen e t r a t i o n . A c a r e f u l review was performed by the w r i t e r of the c a l i b r a t i o n chamber t e s t r e s u l t s by B a l d i e t a l . (1981). This review enables an estimate of the changes i n h o r i z o n t a l s t r e s s e s due to cone pe n e t r a t i o n to be made. The i n i t i a l h o r i z o n t a l s t r e s s e s i n the chamber were c o n t r o l l e d and measured b e f o r e p e n e t r a t i o n . D u r i n g p e n e t r a t i o n t h e s l e e v e f r i c t i o n , f g , was recorded. By assuming a value of the f r i c t i o n angle between the metal and the sand (<5) i t i s p o s s i b l e to estimate the average changes i n h o r i z o n t a l s t r e s s e s during p e n e t r a t i o n . Since the changes i n h o r i z o n t a l s t r e s s e s can be expected to be r e l a t e d to the d i l a t a n t behaviour of the sand the r e s u l t s of the c a l i b r a t i o n t e s t s are c o r r e l a t e d to the maximum d i l a t i o n angle (v ) . The maximum d i l a t i o n angle was i n f e r r e d from the cone r e s i s t a n c e max ° values (<l c) using the f r i c t i o n angle c o r r e l a t i o n (shown l a t e r i n F i g . 4.10) and Rowe's s t r e s s d i l a t a n c y . The r e s u l t i n g c o r r e l a t i o n between changes In h o r i z o n t a l s t r e s s e s due to cone p e n e t r a t i o n and maximum d i l a t i o n angle i s 67 shown i n F i g . 4.6. A remarkable c o r r e l a t i o n appears to e x i s t between d i l a t i o n r a t e and changes i n h o r i z o n t a l s t r e s s e s due to pe n e t r a t i o n . The d i l a t a n t behaviour of sand increases w i t h i n c r e a s i n g d e n s i t y and w i t h decreasing c o n f i n i n g pressure (Appendix 1 ) . I t i s c l e a r from F i g . 4.6 that a c o r r e l a t i o n would not have e x i s t e d between changes i n h o r i z o n t a l s t r e s s e s and d e n s i t y s i n c e the d i l a t a n t behaviour of a sand a t constant d e n s i t y v a r i e s with c o n f i n i n g s t r e s s . 4.3. I n t e r p r e t a t i o n : E l e c t r i c F r i c t i o n Cone During cone p e n e t r a t i o n i n s o i l s the v a r i a t i o n of s t r e s s e s and s t r a i n s are extremely complex. Stresses and s t r a i n s are very l a r g e immediately around the cone t i p but r a p i d l y decrease to t h e i r i n - s i t u values some d i s t a n c e away. The st r e s s e s generated near the t i p i n sands are u s u a l l y c o n s i d e r a b l y l a r g e r than normally encountered i n engineering p r a c t i c e and g r a i n crushing can, and o f t e n does, occur. A phenomenon t h a t occurs during p e n e t r a t i o n i n sands i s volume change, s i n c e f o r most saturated sands the standard p e n e t r a t i o n r a t e of 2 cm/sec i s u s u a l l y s u f f i c i e n t f o r f u l l drainage to occur. In f i n e sands o r sands w i t h a s i g n i f i c a n t f i n e s content p e n e t r a t i o n may be p a r t i a l l y drained and pore pressures can be generated. The volume changes that take place during drained p e n e t r a t i o n are caused by changes i n mean normal s t r e s s and shear s t r e s s e s . Because of these volume changes, the p e n e t r a t i o n r e s i s t a n c e i s p r i m a r i l y dependent on two s o i l parameters, namely, shear s t r e n g t h and c o m p r e s s i b i l i t y . The shear s t r e n g t h i n c o r p o r a t e s the e f f e c t s of d i l a t i o n due to changes i n shear s t r e s s e s and the c o m p r e s s i b i l i t y i n c o r p o r a t e s the volume changes due to changes i n mean normal s t r e s s . These two s o i l parameters, however, are not constant f o r any one sand, but vary w i t h both s t r e s s and s t r a i n l e v e l . 68 6" 4A 3A baldi of al (l93l) Normally coneolidafcd Ticino Band O Medium Dime, Dr = 4Q>% D ' X Dzntxt , Dr = 70% • Very D&ntc / Dr ~ OO % I A Assume $= 30° &HO ~ l^-o &so f5= kcrve?'fan& <?„'{> = in if ial horizonfal s]ret>6 f$s sleeve friofian 0~v'o~ inifial vtrfic#l sfretrb (assumed contfan-j) 0- ~i 1 1 r 20 0 ~i r 10 MAXIMUM P1LAT/0N AN6LBy j ) ^ (<f>cv- 34-° A*?wme.ds) FIG. 4.6. CHANGE IN HORIZONTAL STRESS COEFFICIENT DUE TO CONE PENETRATION IN SAND. 69 Thus, i t i s extremely d i f f i c u l t to develop a complete t h e o r e t i c a l s o l u t i o n f o r p e n e t r a t i o n i n t o sands. A comprehensive t h e o r e t i c a l s o l u t i o n to t h i s problem has not yet been developed, although the t h e o r i e s based on c a v i t y e x p a n s i o n have come c l o s e to a complete s o l u t i o n . In g e n e r a l , i n t e r p r e t a t i o n of cone p e n e t r a t i o n data i s made by e m p i r i c a l c o r r e l a t i o n s to r e q uired g e o t e c h n i c a l parameters. A review by the w r i t e r of e x i s t i n g c o r r e l a t i o n s and t h e o r i e s w i l l be presented along with suggested improvements. 4.3.1. S o i l C l a s s i f i c a t i o n The most comprehensive recent work on s o i l c l a s s i f i c a t i o n using e l e c t r i c cone penetrometer data i s that by Douglas and Olsen (1981). A copy o f t h e i r proposed s o i l - b e h a v i o u r type c l a s s i f i c a t i o n c h a r t i s shown i n F i g . 4.7. The chart shows how cone p e n e t r a t i o n t e s t data has been c o r r e l a t e d w i t h other s o i l type i n d i c e s , such as those provided by the U n i f i e d S o i l C l a s s i f i c a t i o n System. The c o r r e l a t i o n was based on extensive data c o l l e c t e d from areas i n C a l i f o r n i a , Oklahoma, Utah, Ar i z o n a and Nevada. The usual progression of s i t e i n v e s t i g a t i o n using cone p e n e t r a t i o n t e s t (CPT) i s to perform the CPT soundings, develop d e t a i l e d s i t e p r o f i l e s w i t h the s o i l c l a s s i f i c a t i o n chart ( F i g . 4.7), and then s e l e c t i v e l y sample and t e s t to provide any a d d i t i o n a l i n f o r m a t i o n regarding ambiguous c l a s s i f i c a t i o n s . With l o c a l experience t h i s l a t t e r step i s o f t e n not necessary. As discussed i n the previous s e c t i o n s , cone design and water pressures have a s i g n i f i c a n t e f f e c t on the measured bearing and f r i c t i o n due to unequal end areas ( F i g . 4.2). Thus cones of s l i g h t l y d i f f e r e n t designs 70 FIG. 4.7. SOIL CLASSIFICATION CHART FOR STANDARD ELECTRIC CONE. (After Douglas and Olsen, 1981) 71 w i l l g i v e d i f f e r e n t bearing, f r i c t i o n and f r i c t i o n r a t i o s . The data used to compile the c l a s s i f i c a t i o n chart ( F i g . 4.7) used bearing and f r i c t i o n v alues that had not been correct e d f o r pore pressure e f f e c t s , s i n c e , i n g e n e r a l , pore pressure measurements were not made. It appears that there i s l i t t l e d i f f e r e n c e between c o r r e c t e d and uncorrected f r i c t i o n r a t i o s f o r most s o i l types except f o r those s o i l s t h a t c l a s s i f y i n the lower l e f t p o r t i o n of the c h a r t ( F i g . 4.7). These s o i l s u s u a l l y generate l a r g e p o s i t i v e pore pressures during p e n e t r a t i o n and have very low measured b e a r i n g (q c<10 kg/cm 2) and f r i c t i o n values (FR<2%). These s o i l s a l s o tend to have hig h l i q u i d i t y index v a l u e s , as noted by Douglas and Olsen (1981). However, i n o f f - s h o r e i n v e s t i g a t i o n s where s i g n i f i c a n t h y d r o s t a t i c water pressures e x i s t , i t may be important to account f o r these e f f e c t s f o r most s o i l types. Most standard e l e c t r i c cone data do not i n c l u d e pore pressure measurements and the measured bearing and f r i c t i o n values are t h e r e f o r e not c o r r e c t e d f o r pore pressure e f f e c t s . For t h i s type of data the chart i n F i g . 4.7 can be used d i r e c t l y to provide a reasonable estimate of s o i l type. I f pore pressure measurements are included and the necessary c o r r e c t i o n s a p p l i e d to the data the usefulness of F i g . 4.7 f o r s o i l c l a s s i f i c a t i o n i s reduced somewhat to that of a guide o n l y , e s p e c i a l l y f o r s o f t saturated s o i l s . S e v e r a l recent p u b l i c a t i o n s have suggested that s o i l c l a s s i f i c a t i o n be based on pore pressure and bearing data (Jones e t a l . , 1981, B a l i g h et a l . , 1980). S i g n i f i c a n t improvements i n c l a s s i f i c a t i o n are made i f pore pres-sure, bearing and f r i c t i o n are used (Campanella & Robertson 1981). A comprehensive c l a s s i f i c a t i o n c h a r t f o r use w i t h a mechanical cone was proposed by Searle (1979) and i s reproduced i n F i g . 4.8. The c h a r t i s s i m i l a r to that proposed by Schmertmann (1978) although considerably more 72 1000 100 FRICTION RATIO, F R , % FIG. 4.8. SOIL CLASSIFICATION CHART FOR MECHANICAL FRICTION CONE. (After Searle, 197 9) 73 i n f o r m a t i o n i s contained on Searle's c h a r t . I t i s important to note that the c l a s s i f i c a t i o n of s o i l s t i f f n e s s or d e n s i t y i s r a t h e r q u a l i t a t i v e as these parameters depend on i n - s i t u s t r e s s l e v e l . This t o p i c w i l l be expanded f u r t h e r i n the next s e c t i o n s . Recently the CPT has a l s o been used f o r c l a s s i f i c a t i o n and i n t e r p r e -t a t i o n of unconventional s o i l s such as carbonate sediments ( S e a r l e 1979; and Power, 1982). Experience during t h i s study suggests that the f r i c t i o n r a t i o f o r some f i n e grained s o i l s may decrease w i t h i n c r e a s i n g e f f e c t i v e overburden pressure. 4.3.2. St r a t i g r a p h y The cone p e n e t r a t i o n r e s i s t a n c e i s i n f l u e n c e d by the s o i l p r o p e r t i e s ahead and behind the t i p . Chamber s t u d i e s by Schmertmann (19 78b) showed tha t the t i p senses an i n t e r f a c e between 5 to 10 cone diameters ahead and behind. The d i s t a n c e over which the cone t i p senses an i n t e r f a c e i n c r e a s e s w i t h i n c r e a s i n g s o i l s t i f f n e s s . For interbedded d e p o s i t s , the t h i n n e s t l a y e r the cone b e a r i n g can respond f u l l y ( i . e . q to reach f u l l value c w i t h i n the l a y e r ) i s about 10 to 20 diameters. For the standard 10 cm2 e l e c t r i c cone, the minimum l a y e r t h i c k n e s s to ensure f u l l t i p r e s i s t a n c e i s between 36 cm to 72 cm. Since the cone t i p i s advanced co n t i n u o u s l y , the t i p r e s i s t a n c e w i l l sense much thi n n e r l a y e r s , but not f u l l y . This has s i g n i f i c a n t i m p l i c a t i o n s when i n t e r p r e t i n g cone bearing, f o r example, r e l a t i v e d e n s i t y determination i n sand. I f a sand l a y e r i s l e s s than about 70 cm t h i c k and l o c a t e d between, say, two s o f t c l a y d e p o s i t s , the cone p e n e t r a t i o n r e s i s t a n c e may not reach i t s f u l l value w i t h i n the sand because of the c l o s e proximity of the adjacent i n t e r f a c e s . Thus, the r e l a t i v e 74 density i n the sand may be underestimated. The continuous monitoring of pore pressures during cone penetration can s i g n i f i c a n t l y improve the i d e n t i f i c a t i o n of s o i l stratigraphy (Campanella, Robertson and G i l l e s p i e , 1983). The pore pressure responds to the s o i l type i n the immediate area of the cone t i p . To aid i n the i d e n t i f i c a t i o n of very t h i n s i l t or sand layers within clay deposits, some researchers (Torstensson, 1982) have proposed and successfully used t h i n (2.5 mm) pore pressure elements located immediately behind the cone t i p . 4.3.3. Density For cohesionless s o i l s the density, or more commonly the r e l a t i v e density, i s often used as an intermediate s o i l parameter. Recent research has shown that the s t r e s s - s t r a i n and strength behaviour of cohesionless s o i l s i s too complex to be represented s o l e l y by the r e l a t i v e density of the s o i l . Several papers i n ASTM (1973) have discussed d i f f i c u l t i e s associated with determination of maximum, minimum and i n - s i t u d e n s i t i e s as well as problems i n c o r r e l a t i n g r e l a t i v e density with measured s o i l pro-p e r t i e s . However, because many engineers continue to use r e l a t i v e density as a guide i n design and because r e l a t i v e density i s an important parameter f o r l i q u e f a c t i o n studies some discussion i s given here on recent work r e l a t i n g cone penetration resistance to s o i l r e l a t i v e density. Recent work i n large c a l i b r a t i o n chambers (Veismanis, 1974, Chapman and Donald, 1981, B a l d i et a l . 1981, Parkin et a l . , 1980 and V i l l e t and M i t c h e l l , 1981) has provided numerous c o r r e l a t i o n s of cone resistance (q ) c w i t h s o i l r e l a t i v e density (D^). Most of these works have also shown that no si n g l e unique r e l a t i o n s h i p e x i s t s between r e l a t i v e density, i n - s i t u e f f e c t i v e stress and cone resistance for a l l sands. Recent work by Parkin 75 and Lunne (1982) has a l s o shown that the measured r e l a t i o n s h i p s between r e l a t i v e d e n s i t y and cone r e s i s t a n c e i s i n f l u e n c e d by the small c a l i b r a t i o n chamber s i z e , p a r t i c u l a r l y at higher d e n s i t i e s . It i s not s u r p r i s i n g that no unique r e l a t i o n s h i p e x i s t s between cone r e s i s t a n c e , i n - s i t u e f f e c t i v e s t r e s s and r e l a t i v e d e n s i t y s i n c e other f a c t o r s such as s o i l c o m p r e s s i b i l i t y a l s o i n f l u e n c e cone r e s i s t a n c e . A review c a r r i e d out by the w r i t e r of the numerous c a l i b r a t i o n chamber t e s t s performed on a v a r i e t y of d i f f e r e n t sands shows a s i g n i f i c a n t range o f versus q c r e l a t i o n s h i p s . However, on d e t a i l e d i n s p e c t i o n part of the v a r i a t i o n can be accounted f o r due to d i f f e r e n c e s i n chamber s i z e and boundary c o n d i t i o n s . A l l the chamber t e s t r e s u l t s , however, show that the curves are a l l s i m i l a r i n shape and most show that the cone r e s i s t a n c e can be more uniquely r e l a t e d to r e l a t i v e d e n s i t y , f o r any given sand, i f c o r r e l a t e d w i t h the i n - s i t u i n i t i a l h o r i z o n t a l e f f e c t i v e s t r e s s (a,' ). If ho the h o r i z o n t a l e f f e c t i v e s t r e s s i s used the r e l a t i o n s h i p can be expected to apply to both normally and overconsolidated sand. F i g . 4.9 shows a comparison between the curves proposed by Schmertmann (1978b), V i l l e t and M i t c h e l l , (1981) and B a l d i et a l . (1981) f o r two l e v e l s of r e l a t i v e d e n s i t y . A l l the curves have been co r r e c t e d f o r chamber s i z e . The curves proposed by B a l d i et a l (1981) appear to represent a reasonable average o f the three s t u d i e s and a reasonable average f o r most of the other s t u d i e s . D e t a i l s of the sands used i n the numerous c a l i b r a t i o n chamber s t u d i e s are given i n Table 4.1. The c a l i b r a t i o n t e s t data ( F i g . 4.9) shows the importance of sand c o m p r e s s i b i l i t y . The curves by Schmertmann (1978b) represent the r e s u l t s of t e s t s on H i l t o n Mines sand, which I s a r e l a t i v e l y compressible quartz, f e l d s p a r , mica mixture w i t h angular g r a i n s . The curves by V i l l e t and 76 .2 (SI CONE R E S I S T A N C E , q c , kg/cm' 0 100 200 300 400 500 E o V i cn tn LJ tr & 2 < or txl > Ixl > O IxJ U. L_ UJ (1 4 0 % © 1 D r=80% © SCHMERTMANN (1970) Hilton Mint* Sond - High ComprtMibility © B A L 0 I «1 01.(1982) Ticin© Sond - Modtrott Comprtitib.lity (D VILLET ft MITCHELL(1981) Monttrty Sond-Low Comoro ttibility FIG. 4.9. COMPARISON OF DIFFERENT RELATIVE DENSITY RELATIONSHIPS. (After Robertson and Campanella, 1982). M i t c h e l l (1981) represent r e s u l t s on Monterey Sand which i s a r e l a t i v e l y i n c o m p r e s s i b l e quartz sand w i t h subrounded p a r t i c l e s . Schmertmann (1978b) Reference Sand Name Mineralogy Shape Gradation (mm) Porosit y D60 D10 nmax nmin B a l d i et a l . , (1981,1982) Ticino Mainly quartz 5%* mica Subangular to angular 0.65 0.40 0.50 0.41 V i l l e t & M i t c h e l l (1981) Monterey Mainly quartz some feldspar Subrounded to subangular 0.40 0.25 0.45 0.36 Schmertmann (1978b) Ottawa #90 quartz Bounded 0.24 0.13 0.44 0.33 H i l t o n mines quartz + mica + feldspar Angular 0.30 0.15 0.44 0.30 Parkin et a l (1980) Hokksund 35% quartz 45% feldspar 10%* mica Bounded to subangular 0.5 0.27 0.48 0.36 Veismanis (1974) Edgar Mainly quartz Subangular 0.5 0.29 0.48 0.35 " Ottawa Quartz Subangular 0.54 0.45 0.42 0.32 Uolden (1971) South Oakleigh Quartz Subangular 0.19 0.12 0.47 0.35 - -. " Quartz Subangular 0.37 0.17 0.43 0.29 Chapman & Donald (1981) Franks ton Mainly Quart Bounded to Subangular 0.37 0.18 * Percent mica by volume TABLE 4.1: Properties of Sand Tested i n C a l i b r a t i o n Chamber Studies (After Robertson and Campanella, 1982) a l s o performed t e s t s on Ottawa sand, which i s a l s o an incompressible quartz sand w i t h rounded p a r t i c l e s , and obtained curves almost i d e n t i c a l to those of V i l l e t and M i t c h e l l (1981). Thus, i t appears t h a t sands w i t h a low c o m p r e s s i b i l i t y have a ^ r - c l c r e l a t i o n s h i p s i m i l a r to that shown by V i l l e t and M i t c h e l l (1981) and sands w i t h a high c o m p r e s s i b i l i t y have a r e l a t i o n s h i p s i m i l a r to that shown by Schmertmann (1978b). The sand used by B a l d i e t a l . (1981) ( T i c i n o Sand) was a q u a r t z , f e l d s p a r , mica mixture 78 w i t h subangular p a r t i c l e s . The T i c i n o Sand appears to have a moderate c o m p r e s s i b i l i t y somewhere between the two extremes of H i l t o n Mines and Monterey Sand. A l a r g e p o r t i o n of CPT work i s o f t e n c a r r i e d out i n sands where the g r a i n m i n erals are predominately quartz and f e l d s p a r . These are sands s i m i l a r to those t e s t e d i n most of the c a l i b r a t i o n chamber work. Research has shown th a t there i s r e l a t i v e l y l i t t l e v a r i a t i o n i n the c o m p r e s s i b i l i t y f o r most quartz sands, although t h i s depends on the a n g u l a r i t y of the g r a i n s ( J o u s t r a & de G i j t , 1982). Angular quartz sands tend to be more compressible than rounded quartz sands. I f an estimate of r e l a t i v e d e n s i t y i s r e q u i r e d f o r a predominantly quartz sand of moderate compressibity, the w r i t e r recommends that the r e l a t i o n given by B a l d i et a l . (1981 & 1982) be used. F i g . 4.10 shows B a l d i 1 s r e l a t i o n s h i p between r e l a t i v e d e n s i t y (D ) r v e r t i c a l e f f e c t i v e s t r e s s ( a ' ) and cone r e s i s t a n c e (q ), c o r r e c t e d f o r vo c chamber t e s t s i z e ( B a l d i et a l . , 1982). The r e l a t i o n s h i p i s f o r normally c o n s o l i d a t e d , where K q = 0.45, uncemented and unaged sands. I f overcon-s o l i d a t e d or aged sands are encountered, the h o r i z o n t a l e f f e c t i v e s t r e s s (o,' ) s h o u l d be used i n s t e a d of a' . However, the a p p l i c a t i o n of t h i s v ho' vo r e l a t i o n s h i p to overconsolidated sands appears, a t present, very d i f f i c u l t because of the inherent d i f f i c u l t i e s i n measuring or choosing an appro-p r i a t e a' i n - s i t u and a s s e s s i n g the s t r e s s h i s t o r y o f n a t u r a l sand r ho d e p o s i t s . The w r i t e r suggests that F i g . 4.10 should be used only as a guide to i n - s i t u r e l a t i v e d e n s i t y , but can be expected to provide reasonable estimates f o r c l e a n normally c o n s o l i d a t e d moderately compressible quartz FIG. 4.10:. RELATIVE DENSITY RELATIONSHIP FOR UNCEMENTED AND UNAGED QUARTZ SANDS'. CAdapted from Baldi et a l . , 1982) 80 sands. In an e f f o r t to overcome some of these problems, V i l l e t and M i t c h e l l (1981) extended the bearing capacity theory developed by Durgunoglu and M i t c h e l l (1975) so that cone resistance, r e l a t i v e density, v e r t i c a l stress curves f o r any sand could be constructed based on a knowledge of the s o i l f r i c t i o n angle (<(>) and i t s v a r i a t i o n with stress l e v e l and a current l a t e r a l e a r t h p r e s s u r e c o e f f i c i e n t (K ). However, t h i s theory takes no o account of the s o i l c o m pressibility. It would also seem l i k e l y that d e t a i l e d information concerning the f r i c t i o n a l strength of the s o i l and the l a t e r a l e a r t h p r e s s u r e (& Q) i s not o f t e n a v a i l a b l e , and i f s u f f i c i e n t information were a v a i l a b l e , the requirement for a knowledge of r e l a t i v e density would probably not e x i s t . 4.3.4. Drained Shear Strength of Sand Many theories and empirical or semi-empirical c o r r e l a t i o n s f o r the i n t e r p r e t a t i o n of drained shear strength of sand from cone resistance have been published. The theories can be divided into two categories; namely those based on bearing capacity theory (Janbu and Senneset, 1974, Durgunoglu and M i t c h e l l , 1975) and those based on c a v i t y expansion theory (Vesic, 1972). Work by Vesic (1963) has shown that no unique r e l a t i o n s h i p e x i s t s between f r i c t i o n angle for sands and cone resistance, since s o i l compres-s i b i l i t y influences the cone resistance. The curvature of the Mohr-Coulomb f a i l u r e envelope for granular s o i l s has been observed repeatedly by numerous i n v e s t i g a t o r s and i s presently recognized as a t y p i c a l material behaviour. Most of the a v a i l a b l e bearing capacity theories on deep penetration neglect both the curvature of the shear strength envelope 81 and the c o m p r e s s i b i i t y of the s o i l . These two f a c t o r s tend to reduce the t i p r e s i s t a n c e . Based on c a v i t y expansion concepts, V e s i c (19 72) developed a theory f o r t i p r e s i s t a n c e t a k i n g account of s o i l c o m p r e s s i b i l i t y and volume change c h a r a c t e r i s t i c s . B a l i g h (1976) developed t h i s f u r t h e r to i n c o r p o r a t e the curvature of the s t r e n g t h envelope. The comprehensive c a l i b r a t i o n chamber t e s t work by B a l d i et a l . (19 81) showed th a t the c a v i t y expansion theory appeared to model the measured response extremely w e l l . The c a v i t y expansion theory by V e s i c , however, cannot i n c o r p o r a t e volume expansion. At f i r s t t h i s appears to be a major disadvantage s i n c e almost a l l sands d i l a t e (expand) during shear. Work by V e s i c and Clough (1968) showed that a t h i g h s t r e s s e s ( g r e a t e r than 50 kg/cm 2) dense sand (D^ = 80%) w i l l compress during shear. The s t r e s s e s around the cone t i p during p e n e t r a t i o n i n t o sand o f t e n exceeds 200 kg/cm 2. Thus i t seems reasonable that the high s t r e s s e s developed during cone p e n e t r a t i o n i n sands cause a compressive punching f a i l u r e around the t i p . This agrees w e l l w i t h the observed behaviour from model t e s t s of deep p e n e t r a t i o n (Robinsky and Morrison, 1964; Mikasa and Takada, 1974). The c a v i t y expansion a n a l y s i s , however i s complex and r e q u i r e s considerable input data regarding c o m p r e s s i b i l i t y and shear s t r e n g t h . C a l i b r a t i o n chamber r e s u l t s i l l u s t r a t e the complex nature of cone p e n e t r a t i o n i n sands and show th a t simple c l o s e d form s o l u t i o n s to d e r i v e the shear s t r e n g t h are not p o s s i b l e . In a d d i t i o n , chamber t e s t s provide v a l u a b l e i n s i g h t i n t o the r e l a t i v e importance of the various f a c t o r s t h a t i n f l u e n c e cone p e n e t r a t i o n i n sands. In g e n e r a l , i t would be expected that the bearing c a p a c i t y t h e o r i e s , which cannot take account of s o i l c o m p r e s s i b i l i t y , c o u l d not provide r e l i -a b l e p r e d i c t i o n s of f r i c t i o n angle. However, the work by V i l l e t and 82 M i t c h e l l (1981) showed that the bearing c a p a c i t y theory developed by Durgunoglu and M i t c h e l l (1975) provided reasonable p r e d i c t i o n s f o r a v a r i e t y of d i f f e r e n t sands. The chamber t e s t study by B a l d i e t a l . , (1981) showed that Durgunoglu and M i t c h e l l ' s theory gave e x c e l l e n t agreement w i t h measured f r i c t i o n angle at a f a i l u r e s t r e s s l e v e l approximately equal to the average s t r e s s around the cone. The average s t r e s s around the cone was assumed to be about 9 times the i n - s i t u h o r i z o n t a l s t r e s s ( B a l d i e t a l . , 1982). Thus, due to the curvature of the s t r e n g t h envelope, the Durgunoglu and M i t c h e l l theory appears to underestimate the f r i c t i o n angle a t the i n -s i t u s t r e s s l e v e l by about 2 d e g r e e s . — As discussed e a r l i e r , the two main parameters that c o n t r o l p e n e t r a t i o n r e s i s t a n c e i n sands are shear str e n g t h and c o m p r e s s i b i l i t y . Work by A l -Awkati (19 75) showed t h a t , f o r the predominantly quartz sands he t e s t e d , shear s t r e n g t h had s i g n i f i c a n t l y more i n f l u e n c e on cone r e s i s t a n c e than c o m p r e s s i b i l i t y . This i s probably due to the f a c t that f o r most n a t u r a l quartz sands the v a r i a t i o n i n c o m p r e s s i b i l i t y i s not th a t l a r g e , e s p e c i a l l y when compared to the p o s s i b l e v a r i a t i o n of shear s t r e n g t h . This o b s e r v a t i o n i s p a r t i c u l a r l y important when one considers t h a t a l a r g e p o r t i o n of n a t u r a l sands encountered i n the northern hemisphere c o n s i s t predominantly of quartz and f e l d s p a r s w i t h small amounts of mica. These sands are s i m i l a r to those tes t e d i n the c a l i b r a t i o n chamber s t u d i e s (see Table 4.1). Thus i t has been p o s s i b l e to use bearing c a p a c i t y t h e o r i e s , i n which the i n f l u e n c e of c o m p r e s s i b i l i t y i s neglected, and produce reasonable estimates of f r i c t i o n angle. I t i s i n t e r e s t i n g to note that such t h e o r i e s w i l l g i v e c o n s e r v a t i v e l y low estimates of f r i c t i o n angle f o r more compres-s i b l e sands ( i . e . carbonate sands). A review of the c a l i b r a t i o n chamber t e s t r e s u l t s was c a r r i e d out to 83 compare the measured cone p e n e t r a t i o n r e s i s t a n c e to measured f r i c t i o n angle from drained t r i a x i a l t e s t s . The f r i c t i o n angle values were obtained from t r i a x i a l t e s t s performed at c o n f i n i n g s t r e s s e s approximately equal to the h o r i z o n t a l e f f e c t i v e s t r e s s i n the c a l i b r a t i o n chamber before cone p e n e t r a t i o n ( i . e . , i n - s i t u a ' ^ Q ) * Th e r e s u l t s o f the comparison are shown on F i g . 4.11. D e t a i l s of the sands used i n the s t u d i e s are given i n Table 4.1. The s c a t t e r i n the r e s u l t s i l l u s t r a t e the l i m i t e d i n f l u e n c e o f s o i l c o m p r e s s i b i l i t y . A l s o shown i n F i g . 4.11 are the t h e o r e t i c a l r e l a t i o n s h i p s proposed by Janbu and Senneset (1974) and Durgunoglu and M i t c h e l l (1975). The Durgunoglu and M i t c h e l l method i n c l u d e s the e f f e c t of i n - s i t u h o r i z o n t a l s t r e s s e s . The d i f f e r e n c e between the normally c o n s o l i d a t e d s t a t e , where KQ = l-sin<|>, and the overconsolidated s t a t e (OCR K 6 ) , where Kg = 1.0, i s l e s s than 2 degrees, as shown on F i g . 4.11. Since the s o l u t i o n by Janbu and Senneset (1974), f o r B = 0, (see F i g . 4.9) tends to s l i g h t l y over-estimate <j> and Durgunoglu and M i t c h e l l tends to under-estimate <(>, an average e m p i r i c a l r e l a t i o n s h i p i s proposed by the w r i t e r , as shown on F i g . 4.11. I f the average r e l a t i o n s h i p i s taken, a u s e f u l design chart f o r e s t i m a t i o n of f r i c t i o n angle from cone p e n e t r a t i o n r e s i s t a n c e can be developed, as shown i n F i g . 4.12. The proposed c h a r t i n F i g . 4.12 can be expected to provide reasonable estimates of f r i c t i o n angle f o r normally c o n s o l i d a t e d , moderately incompressible, predominantly quartz sands, s i m i l a r to those used i n the chamber s t u d i e s . For h i g h l y compressible sands, the c h a r t would tend to p r e d i c t c o n s e r v a t i v e l y low f r i c t i o n angles. Durgunoglu and M i t c h e l l ' s theory shows that there i s l i t t l e change i n p r e d i c t e d f r i c t i o n angle f o r r e l a t i v e l y l a r g e changes i n s t r e s s h i s t o r y . I t i s important to note that the f r i c t i o n angle p r e d i c t e d from F i g . 4.12 i s r e l a t e d to the i n - s i t u i n i t i a l h o r i z o n t a l s t r e s s l e v e l 84 II or LU CD. 15 O < CL < o o or < LU CD 1000 8 0 0 6 0 0 4 0 0 2 0 0 h LEGEND • CHAPMAN 8 DONALD (1981) + BALDI et 01 . (1981) H0LDEN ( 1976) VEISMANIS (1974) O PARKIN et al . (1980) <f> s 3 0 ° 3 2 o 3 4 0 3 6 o 3 8 0 4 0 ° 4 2 ° 4 4 ° 4 6 ° 4 8 ° 0.0 0.2 0.4 0.6 0.8 T A N G E N T <£' 1.0 1.2 FIG. 4.11. RELATIONSHIP BETWEEN BEARING CAPACITY NUMBER AND FRICTION ANGLE FROM LARGE CHAMBER TESTS. (After Robertson and Campanella, 1982) FIG. 4.12. PROPOSED CORRELATION BETWEEN CONE BEARING AND FRICTION ANGLE FOR UNCEMENTED, QUARTZ SANDS. (After Robertson and Campanella, 1982) 8 6 before cone p e n e t r a t i o n . I t i s i n t e r e s t i n g to note that the f r i c t i o n r a t i o increases w i t h i n c r e a s i n g c o m p r e s s i b i l i t y . Many compressible carbonate sands have f r i c t i o n r a t i o s of about 3 percent ( J o n s t r a and de G i j t , 1982) whereas, t y p i c a l incompressible quartz sands have f r i c t i o n r a t i o s of about 0 . 5 percent. Thus, the presence of compressible sands may be i d e n t i f i e d using the f r i c t i o n r a t i o . 4 . 3 . 5 . Undrained Shear Strength of Clay One of the e a r l i e s t a p p l i c a t i o n s o f the cone p e n e t r a t i o n t e s t was i n the e v a l u a t i o n o f u n d r a i n e d shear s t r e n g t h (C^) of c l a y s . Comprehensive reviews o f C^ e v a l u a t i o n from CPT data have been presented by B a l i g h e t a l . ( 1 9 8 0 ) , Jamiolkowski e t a l . ( 1 9 8 2 ) , and Lunne and Kleven ( 1 9 8 1 ) . Note that the undrained shear s t r e n g t h of c l a y depends s i g n i f i c a n t l y on the type of t e s t used, the r a t e of s t r a i n and the o r i e n t a t i o n of the f a i l u r e planes. E s t i m a t e s o f C^ from CPT r e s u l t s u s u a l l y employ an equation of the f o l l o w i n g form: q = C N. + o c u k o where a i s the i n - s i t u t o t a l overburden pressure o i s the cone f a c t o r . The c o n t r i b u t i o n o f the t o t a l o v e r b u r d e n p r e s s u r e (a ) has been o i n t e r p r e t e d as e i t h e r the i n - s i t u v e r t i c a l s t r e s s ( a V Q ) > o r t n e i n - s i t u h o r i z o n t a l s t r e s s ( o^ ), or the i n - s i t u o c t a h e d r a l s t r e s s ( a ^ = i ( a + v ho oct 3 vo 2a ) ) . T h e o r e t i c a l s o l u t i o n s f o r N, have been based on bearing c a p a c i t y ho k t h e o r i e s (e.g., Meyerhof, 1961) and more r e c e n t l y by use of c a v i t y expansion t h e o r i e s (e.g., Landanyi, 1967 , and V e s i c , 1 9 7 2 ) . B a l i g h (1975) combined these two approaches i n approximate form to o b t a i n the r e s u l t s 87 shown i n F i g . 4 . 1 3 . The r i g i d i t y index ( I r ) i s the r a t i o o f the undrained s h e a r modulus ( u s u a l l y a t the 5 0 % s t r e s s l e v e l , G^Q) to undrained str e n g t h and the v e r t i c a l a x i s g i v e s N = (q - a, )/C . Note use of the i n - s i t u k c ho u t o t a l h o r i z o n t a l s t r e s s C0"^) r a t h e r than and that the theory a p p l i e s to the standard (Fugro) type e l e c t r i c cone. For the case of the standard c o n e , where 2 9 = 6 0 ° , N^ = 1 6 ± 2 over the f u l l range o f l i k e l y 1 ^ v a l u e s . The s o l u t i o n by B a l i g h ( 1 9 7 5 ) i n v o l v e s s e v e r a l s i m p l i f y i n g assump-t i o n s , such as neglect of undrained s t r e n g t h a n i s o t r o p y and s t r a i n s o f t e n -i n g behavior. The former can be be adequately approximated by using the average of the v e r t i c a l and h o r i z o n t a l s t r e n g t h s . Neglecting s t r a i n -s o f t e n i n g , on the other hand, can lead to a se r i o u s e r r o r f o r s e n s i t i v e c l a y s , Landanyi ( 1 9 7 2 ) . Other f a c t o r s such as cone type and r a t e of pene-t r a t i o n may s i g n f i c a n t l y a f f e c t the p e n e t r a t i o n r e s i s t a n c e . However, N i s g e n e r a l l y o b t a i n e d from e m p i r i c a l c o r r e l a t i o n s . The r e f e r e n c e i s u s u a l l y measured from unconfined t r i a x i a l compression or f i e l d vane t e s t s . The overburden pressure (° Q) i s u s u a l l y taken as the i n -s i t u t o t a l v e r t i c a l s t r e s s (a ) s i n c e the i n - s i t u h o r i z o n t a l s t r e s s i s vo u s u a l l y not known. Data presented by Lunne and Kleven ( 1 9 8 1 ) shows that f o r normally c o n s o l i d a t e d marine c l a y s using c o r r e c t e d f i e l d vane s t r e n g t h ( i . e . B j e r r u m ' s , 1 9 7 2 , c o r r e c t i o n ) , the cone f a c t o r N^ f a l l s between 1 1 and 1 9 w i t h an average of 1 5 . These r e s u l t s were obtained using a standard (Fugro) type e l e c t r i c cone a t a standard r a t e of p e n e t r a t i o n of 2 cm/sec. I t i s more d i f f i c u l t to e s t a b l i s h s i m i l a r c o r r e l a t i o n s i n s t i f f over-c o n s o l i d a t e d c l a y s because of the e f f e c t s of f a b r i c and f i s s u r e s on the response of the c l a y . 88 FIG. 4.13. EFFECT OF RIGIDITY INDEX AND CONE ANGLE ON THE PENETRATION RESISTANCE OF CLAY. (After Baligh, 1975) 89 I n v e s t i g a t i o n s by Kjekstad et a l . (19 78) i n non-fissured overconso-l i d a t e d c l a y s i n d i c a t e an average cone f a c t o r = 17. In t h i s case the ref e r e n c e C u was obtained by t r i a x i a l compression t e s t s . The value of N appears to be independent of overconsolidated r a t i o . K. The C v a l u e d e t e r m i n e d as a f u n c t i o n o f cone r e s i s t a n c e (q ) i n u c h i g h l y overconsolidated c l a y d e p o s i t s must be considered w i t h great c a u t i o n s i n c e i t i s d i f f i c u l t to e s t a b l i s h the extent f i s s u r e s e f f e c t drainage and t h e i r e f f e c t on progressive f a i l u r e . Some people have used the r e l a t i o n s h i p c -u N c where N v a r i e s from 9 to 20. c Senneset et a l . (19 82) have r e c e n t l y suggested the use of e f f e c t i v e bearing (q^,) to determine from q' C = u N» c where q' i s defined as the cone r e s i s t a n c e (q ) minus the t o t a l measured n c c dynamic water pressure (u ) . They propose = 9 w i t h a l i k e l y v a r i a t i o n of ±3. One of the major drawbacks of t h i s method i s the r e l i a b i l i t y to wh i c h q^ can be d e t e r m i n e d . In s o f t normally c o n s o l i d a t e d c l a y s , the t o t a l dynamic water pressure generated on the t i p during cone p e n e t r a t i o n i s o f t e n approximately 90 percent of the measured cone r e s i s t a n c e . Thus q^ i s an e x t r e m e l y small q u a n t i t y . Because o f cone design ( i . e . unequal end a r e a s ) the measured q i s sometimes o b s e r v e d to be l e s s than the c measured u^, which i s p h y s i c a l l y impossible and would make the method by 90 Senneset e t a l . , (1982) unusable unless q c i s c o r r e c t e d . This w r i t e r has sug g e s t e d t h a t a l l measured cone r e s i s t a n c e , q^, should be c o r r e c t e d f o r measured dynamic pore pressures using the net area r a t i o ( F i g . 4.2), to g i v e a true t o t a l s t r e s s measure, q T« I t should a l s o be p o s s i b l e to estimate C u from the excess pore pressure generated during p e n e t r a t i o n using the c a v i t y expansion t h e o r i e s . However, the l o c a t i o n of the pore pressure element becomes extremely important. I f the pore pressure i s measured on the cone t i p the maximum excess pore pressure could be estimated using the s p h e r i c a l c a v i t y expan-s i o n theory ( V e s i c , 1972) and would be i n the range, 4 < |a < 7 u where the value 4 a p p l i e s to h i g h l y p l a s t i c s o i l s ( P I > 80) and 7 a p p l i e s to s o i l s of low p l a s t i c i t y ( PI = 15). These values are only a p p l i c a b l e to normally c o n s o l i d a t e d s o i l s and tend to s l i g h t l y overestimate C u. The semi-em p i r i c a l s o l u t i o n proposed by Torstenssen (1977) and Massarch (1978) would enable t h i s approach to be a p p l i e d to overconsolidated c l a y s , p r o v i d e d an e s t i m a t e o f Skempton's pore pressure parameter (A^) could be made. Schmertmann (1975) w i s e l y comments th a t the best procedure i s to make i n d i v i d u a l c o r r e l a t i o n s f o r based on the s p e c i f i c c l a y s and CPT procedures. T h i s , o f course, r e q u i r e s a r e l i a b l e estimate of the i n - s i t u C u appropriate to the p a r t i c u l a r design problem. U n c e r t a i n t i e s i n v o l v e d i n t h i s assessment may be reduced by making reference to values of C u b a c k - f i g u r e d from w e l l documented case h i s t o r i e s (e.g. Bjerrum 1972) i n s t e a d of using other i n - s i t u t e s t s or l a b o r a t o r y measured v a l u e s . Senneset e t a l . (1982) have a l s o suggested a method to determine the 91-drained e f f e c t i v e s t r e s s shear s t r e n g t h parameter, <j>', from the cone p e n e t r a t i o n r e s i s t a n c e and the measured t o t a l pore pressures. However, t h e i r method, as w i t h any method f o r determining e f f e c t i v e s t r e s s parameters from undrained cone p e n e t r a t i o n d a t a , i s subject to serious e r r o r due t o the problems of cone design and the i n a b i l i t y o f most cones to a c c u r a t e l y measure the req u i r e d parameters. An important problem, which i s not i d e n t i f i e d by Senneset et a l . , 1982, i s the l o c a t i o n of the porous element, s i n c e d i f f e r e n t l o c a t i o n s g i v e d i f f e r e n t measured t o t a l pore pre s s u r e s . As discussed before, a l l the measured cone r e s i s t a n c e values should be c o r r e c t e d using a net area r a t i o to g i v e a true t o t a l s t r e s s measure, q^ ,. 4.3.6. Deformation C h a r a c t e r i s t i c s of Sand As already discussed, the cone p e n e t r a t i o n r e s i s t a n c e i n sand i s a complex f u n c t i o n of both str e n g t h and deformation p r o p e r t i e s . Hence, no g e n e r a l l y a p p l i c a b l e a n a l y t i c a l s o l u t i o n f o r cone r e s i s t a n c e as a f u n c t i o n of deformation modulus i s a v a i l a b l e . Instead, many e m p i r i c a l c o r r e l a t i o n s between cone r e s i s t a n c e and deformation modulus have been e s t a b l i s h e d . M i t c h e l l and Gardner (1975) made a comprehensive review of the e x i s t i n g c o r r e l a t i o n s f o r sand. The c o r r e l a t i o n s g e n e r a l l y take the form M = a q c where M i s the drained constrained modulus (equal to 1/m from oedometer v t e s t s ) . The f a c t o r a i s g e n e r a l l y recommended i n the range of 1.5 to 4.0. Considerable confusion appears to e x i s t as to whether or not a s h o u l d remain c o n s t a n t w i t h depth. V e s i c (1970) proposed a = 2(1+0^^), where D = r e l a t i v e d e n s i t y . Dahlberg (1974) found a to increase w i t h q based on M values obtained from screw p l a t e t e s t s f o r precompressed sand. Other r e f e r e n c e s by M i t c h e l l and Gardner use decreased a values when q c exceeds a c e r t a i n l i m i t . Review of c a l i b r a t i o n chamber t e s t s (Lunne and Kleven, 1981) are shown i n Table 4.2. Results i n d i c a t e that a = 3 should provide conservative estimates of one-dimensional settlement. Be ference N.C. Sand O.C. Sand No • sands a No. sands a Veismanis (1974) 2 3 - 1 1 3 5 - 3 0 Parkin et a l . , (1980) 1 3 - 1 1 1 5 - 3 0 Chapman & Donald (1981) 1 3 - 4 3 absolute lower l i m i t 1 8 - 1 5 (12 -average) Baldi et a l . , (1979) 1 >3 1 3 - 9 TABLE 4.2: Summary of Calib r a t i o n Chamber Results f o r Constrained Modulus Factor a. (After Robertson and Campanella, 1982) The w r i t e r f e e l s that c onsiderable i n s i g h t i n t o the r e l a t i o n s h i p between one-dimensional deformation modulus and cone r e s i s t a n c e can be obtained from a c a r e f u l review of c a l i b r a t i o n chamber t e s t s . B a l d i e t a l . (19 81) r e p o r t tangent moduli corresponding to the l a s t load increment f o r normally c o n s o l i d a t e d samples, and apply them to the e m p i r i c a l formula proposed by Janbu (1963): where M = tangent constrained modulus 93 k = modulus number, which v a r i e s w i t h r e l a t i v e d e n s i t y m n = modulus exponent, which may be approximately 0.4 a' = v e r t i c a l e f f e c t i v e s t r e s s vo Pa = reference s t r e s s ( i . e . Pa = 1 kg/cm 2) The t e s t r e s u l t s by B a l d i et a l (1981) show a r e l a t i o n s h i p between the modulus number, k and r e l a t i v e d e n s i t y , D as f o l l o w s : ' m J r Medium dense, D = 46% k = 575 r m Dense , D = 70% k = 753 r m Very dense , D = 90% k = 815 J ' r m S i m i l a r values were reported by P a r k i n (19 77) and Byrne and E l d r i d g e (1982). If the correspondence between r e l a t i v e d e n s i t y and modulus number i s used i n cooperation w i t h the c o r r e l a t i o n developed by B a l d i e t a l . (1981), shown i n F i g . 4.10, a s e r i e s of curves r e l a t i n g tangent constrained modulus, M , t o cone r e s i s t a n c e , q , f o r d i f f e r e n t l e v e l s of v e r t i c a l ' t n c e f f e c t i v e s t r e s s , can be developed. This has been performed by the w r i t e r and i s shown i n F i g . 4.14. Review of F i g . 4.14 i l l u s t r a t e s the apparent reason f o r the wide range i n a values reported i n Table 4.2. Some of the confusion concerning use of CPT f o r i n t e r p r e t a t i o n of d e f o r m a t i o n modulus can be overcome i f the f o l l o w i n g p o i n t s a r e considered. a) S o i l i s not l i n e a r e l a s t i c and modulus v a r i e s w i t h both s t r e s s and s t r a i n l e v e l . b) Modulus i s o f t e n derived from or a p p l i e d to non one-dimensional load-ing c o n d i t i o n s . 94 2000i 500 1000 500 BALDI at 01.(1961) NORMALLY CONSOLIDATED TICINO SAND O MEDIUM DENSE , Dr « 46% + DENSE , Df = 70% A VERY DENSE , Df * 9 0 % O"' 0 • 8 kg /cm2 4 Kg/cm 100 200 300 400 CONE B E A R I N G , q ,kg / c m 2 500 FIG. 4.14. RELATIONSHIP BETWEEN CONE BEARING AND CONSTRAINED MODULUS FOR NORMALLY CONSOLIDATED, UNCEMENTED, QUARTZ SANDS. (After Robertson and Campanella, 1982) 95 c) D i f f e r e n t t h e o r e t i c a l methods were a p p l i e d when o b t a i n i n g c o r r e l a -t i o n s . The simple f a c t that s o i l i s not a l i n e a r e l a s t i c m a t e r i a l makes the assumption of a constant modulus u n r e a l i s t i c . This i s f u r t h e r complicated by the f a c t that many of the c o r r e l a t i o n s where d e r i v e d from non one-dimensional l o a d i n g c o n d i t i o n s f o r which " e l a s t i c " s o l u t i o n s were a p p l i e d to b a c k - f i g u r e a modulus. Thus, reasonable agreement can be expected only i f the r e q u i r e d problem i n v o l v e s s i m i l a r boundary c o n d i t i o n s and the same t h e o r e t i c a l method i s r e a p p l i e d . Schmertmann's (1970) CPT method f o r p r e d i c t i n g settlements i n sand under s t r i p or spread foundations i s a t y p i c a l example. Schmertmann a p p l i e d h i s s t r a i n i n f l u e n c e e l a s t i c theory to analyse the r e s u l t s of screw p l a t e t e s t s . An equ i v a l e n t Youngs modulus ( E g ) was c a l c u l a t e d u s i n g a secant slope over the 1 t s f - 3 t s f (1 kg/cm 2 -3 kg/cm 2) increment of p l a t e l o a d i n g . This i n t e r v a l was chosen p r i n c i p a l l y because r e a l f o o t i n g pressures commonly f a l l w i t h i n t h i s i n t e r v a l . Thus, Schmertmann* s d e s i g n method, where E = 2 q can be expected to produce s o good r e s u l t s i f the proposed design problem has s i m i l a r l o a d i n g c o n d i t i o n s to the screw p l a t e ( i . e . c i r c u l a r spread f o o t i n g loaded from 1-3 t s f ) and the same s t r a i n i n f l u e n c e theory i s r e a p p l i e d . A common problem, however, appears to be the use of the one-dimen-s i o n a l c o n s t r a i n e d modulus (M^) a p p l i e d to non one-dimensional lo a d i n g c o n d i t i o n s . For non one-dimensional cases an equivalent Youngs modulus, as suggested by Schmertmann (1970), would appear to be a more l o g i c a l parameter. A review, performed by the w r i t e r , of the c a l i b r a t i o n chamber r e s u l t s ( B a l d i e t a l . 1981) provides a r e l a t i o n s h i p between the drained secant Youngs modulus a t the 50 and 25 percent f a i l u r e s t r e s s e s , E^^ and E , r e s p e c t i v e l y , and cone r e s i s t a n c e , q , f o r d i f f e r e n t l e v e l s of 96 v e r t i c a l e f f e c t i v e s t r e s s ( F i g . 4.15). Since the o v e r a l l s a f e t y f a c t o r a g ainst bearing c a p a c i t y f a i l u r e i s u s u a l l y around 4 f o r foundations on sand, the designer i s u s u a l l y i n t e r e s t e d i n a Youngs modulus f o r an average m o b i l i z e d s t r e s s l e v e l around 25 percent of the f a i l u r e s t r e s s . Thus, the c a l i b r a t i o n chamber r e s u l t s on normally c o n s o l i d a t e d sand g i v e values of ^2S^c v a r y i n 8 between 1.5 and 3.0 w h ich are i n good agreement w i t h the recommended value of 2 by Schmertmann (1970) f o r computation of settlements of shallow foundations on sand. A c a r e f u l review of F i g . 4.15 shows that i n Schmertmann's study the l o a d increment of 2 to 3 kg/cm 2 (2 to 3 t s f ) was probably c l o s e r to the 50 percent f a i l u r e s t r e s s l e v e l f o r loose to medium dense sands and c l o s e r to the 25 percent s t r e s s l e v e l f o r medium dense to dense sands. For very dense sands the l o a d increment (2 to 3 kg/cm 2) was only a s m a l l percentage of the f a i l u r e s t r e s s . R e s u l t s from chamber t e s t s suggest the r a t i o o f E^^/q o v e r c o n ~ s o l i d a t e d sands i s i n the range of 3 to 6 times l a r g e r than those f o r normally c o n s o l i d a t e d sands ( i . e . 6 < a < 18). However, the a p p l i c a t i o n of these l a r g e r f a c t o r s to overconsolidated sands should be used w i t h c a u t i o n , s i n c e the i n c r e a s e i s dependent on degree of o v e r c o n s o l i d a t i o n and d e n s i t y ( B a l d i et a l . , 1982). The use of F i g . 4.15 may underestimate the i n - s i t u Young's Modulus because i t i s based on l a b o r a t o r y measured moduli using r e - c o n s t i t u t e d samples. Many i n - s i t u sand deposits have had some past s t r e s s h i s t o r y t h a t can cause a s i g n i f i c a n t increase i n s o i l s t i f f n e s s . A s i m i l a r approach can be a p p l i e d to develop a c o r r e l a t i o n between cone r e s i s t a n c e and shear modulus, G, f o r sands. Extensive l a b o r a t o r y work has been conducted by s e v e r a l researchers (Seed and I d r i s s , 1970, Handin and Drnevich, 1972) to r e l a t e dynamic shear modulus, G to s o i l max 97 is 600 5 0 0 400 £2 300 3 g LU < Li-o if) 200 I 00 BALDI ct 01(1981) NORMALLY CONSOLIDATED TICINO SAND O + A MEDIUM DENSE , Dr = 46% DENSE , D, = 7 0 % VERY DENSE C v ^ k g / c m 2 . A 900 co z> _ i _ i LU 750 = > Q LU O _J CO CO CO 600 'o LU z or N E \— o CO 4 5 0 ^ RE — CP AN ILU *> CM a < LU 3 0 0 ^ o in LU CM Z 1 5 0 5 i -< or o 100 200 300 400 500 C O N E B E A R I N G q c , k g / c m ' FIG. 4.15. RELATIONSHIP BETVJEEN CONE BEARING AND DRAINED YOUNG'S MODULUS FOR NORMALLY CONSOLIDATED, UNCEMENTED, QUARTZ SANDS. ( A f t e r R o b e r t s o n and Campanella, 1982) 98 index p r o p e r t i e s . When expressed i n the form, 6 = k Pa ( ^ ) 0 ' 5 max G *• P a J where k = modulus number G o' = mean e f f e c t i v e s t r e s s m Pa = reference s t r e s s ( i . e . Pa = 1 kg/cm 2) the e m p i r i c a l equations can be compared, as shown on F i g . 4.16. I f the proposed r e l a t i o n s h i p f o r k shown i n F i g . 4.16 i s combined w i t h the G r e l a t i v e d e n s i t y , cone r e s i s t a n c e r e l a t i o n s h i p developed by B a l d i e t a l . (1981) a s e r i e s o f curves r e l a t i n g G to q can be developed. This HlclX C has been done by the w r i t e r and i s shown on F i g . 4.17. Once a c o r r e l a t i o n has been developed f o r the dynamic shear modulus i t should be p o s s i b l e to estimate the shear modulus a t any s t r a i n l e v e l by using the r e d u c t i o n curves suggested by Seed and I d r i s s (1970). Byrne and E l d r i d g e (1982) suggest that the i n i t i a l tangent modulus under s t a t i c l o a d i n g c o n d i t i o n s i s about 1/5 the dynamic modulus. This i s because of the combined e f f e c t of s t r a i n l e v e l and repeated l o a d i n g a s s o c i a t e d w i t h the resonant column t e s t s to o b t a i n G max A l s o shown on F i g . 4.17 i s the r e l a t i o n s h i p developed i n Japan (Imai and Tononchi, 1982) between dynamic shear modulus and SPT N value f o r sands. The SPT N value has been converted to cone bearing, q c , using the r e l a t i o n s h i p f o r sands ¥- = 4 ' 5 Further d e t a i l s regarding SPT/CPT c o r r e l a t i o n s w i l l be given l a t e r . 99 A HARDIN 8 DRNEVICH(l972),em o x =0 .9 ,e m j n = 0.4 O SEED a IDRISS0970) 0 ' 1 1 1 1 1 i i I i I 0 50 100 R E L A T I V E D E N S I T Y , D r % FIG. 4.16. CORRELATION BETWEEN DYNAMIC SHEAR MODULUS NUMBER AND RELATIVE DENSITY'. (After Robertson and Campanella, 1982) 100 FIG. 4.17. PROPOSED RELATIONSHIP BETWEEN CONE BEARING AND DYNAMIC-: SHEAR MODULUS FOR NORMALLY CONSOLIDATED, UNCEMENTED, QUARTZ SANDS. ( A f t e r R o b e r t s o n and Campanella, 1982) 101 4.3.7. Deformation C h a r a c t e r i s t i c s of Clay The e s t i m a t i o n of drained parameters such as the one dimensional compression index, C^, or c o m p r e s s i b i l i t y , m^ , from an undrained t e s t i s l i a b l e to s e r i o u s e r r o r , e s p e c i a l l y when based on general e m p i r i c a l c o r r e l a t i o n s . Conceptually, t o t a l s t r e s s undrained measurements cannot y i e l d parameters f o r drained c o n d i t i o n s . M i t c h e l l and Garder (1974) made a comprehensive review of the numerous c o r r e l a t i o n s between cone r e s i s t a n c e and drained modulus. Most of these take the general form 1 M = = a q m c v Sanglerat e t a l . (1972) developed a comprehensive a r r a y of a values f o r d i f f e r e n t cohesive s o i l types w i t h d i f f e r e n t cone r e s i s t a n c e v a l u e s . Schmertmann developed a s l i g h t l y more l o g i c a l method that r e l a t e d the C / a ' r a t i o t o the o v e r c o n s o l i d a t i o n r a t i o (OCR) and then to the one u vo dimensional compression index of the s o i l , C , as shown on Table 4.3. C / o ' u ' v o a p p r o x . O C R C c / ( 1 + e x ) 0 - 0 . 1 l e s s t h a n 1 g r e a t e r t h a n 0 . 4 ( s t i l l c o n s o l i d a t i n g ) 0 . 1 - 0 . 2 5 1 • 0 . 4 0 . 2 6 - 0 . 5 0 1 t o 1 . 5 ( a s s u m e 1 ) 0 . 3 0 . 5 1 - 1 . 0 0 3 0 . 1 5 1 - 4 6 0 . 1 0 o v e r 4 g r e a t e r t h a n 6 0 . 0 5 T A B L E 4 . 3 : E s t i m a t i o n o f C o m p r e s s i o n I n d e x , C c > f r o m c u / o ' r a t i o ( A f t e r S c h m e r t m a n n , 1 9 7 8 ) . These methods provide only a rough estimate of s o i l c o m p r e s s i b i l i t y . 102 A d d i t i o n a l data from Atterberg l i m i t t e s t s (PI) or undisturbed sampling and oedometer t e s t s are required f o r more r e l i a b l e estimates. The e s t i m a t i o n of undrained Young's modulus, E^, i s u s u a l l y made using e m p i r i c a l c o r r e l a t i o n s w i t h the undrained shear s t r e n g t h , C^, i n the form E = n C u u where n i s a constant that depends on s t r e s s l e v e l , o v e r c o n s o l i d a t i o n r a t i o , c l a y s e n s i t i v i t y and other f a c t o r s (Ladd et a l . 1977). As discussed e a r l i e r , because s o i l behaviour i s n o n - l i n e a r , the choice of r e l e v a n t s t r e s s l e v e l i s v ery important. F i g . 4.18(a) presents data f o r normally c o n s o l i d a t e d s o i l s from Ladd et a l . (1977) that shows the v a r i a t i o n of the r a t i o E /C w i t h s t r e s s l e v e l f o r seven d i f f e r e n t cohesive s o i l s , (15 < u u PI < 75). F i g . 4.18(b), shows the v a r i a t i o n of ^ / C ^ w i t h o v e r c o n s o l i d a t i o n r a t i o (OCR) at two s t r e s s l e v e l s f o r the same s o i l types shown i n F i g . 4.18(a). T h e w r i t e r recommends a procedure f o r the e s t i m a t i o n of the undrained Young's modulus (E ) by f i r s t e s t i m a t i n g the undrained shear s t r e n g t h (C^) from CPT p r o f i l e s , as discussed before, then estimate the s t r e s s h i s t o r y (OCR) using the r a t i o , c u / a ^ 0 (Table 4.3), then, u s i n g F i g . 4.18, estimate E f o r the r e l e v e n t s t r e s s l e v e l appropriate f o r the p a r t i c u l a r problem. A knowledge of the p l a s t i c i t y index (PI) would s i g n i f i c a n t l y improve the estimate. 4.4. I n t e r p r e t a t i o n : E l e c t r i c Piezometer F r i c t i o n Cone The a d d i t i o n of pore pressure measurements during s t a t i c cone pene-t r a t i o n t e s t i n g has added a new dimension to the i n t e r p r e t a t i o n of geo-t e c h n i c a l parameters. The continuous measurement of pore pressures along w i t h bearing and f r i c t i o n has enhanced the e l e c t r i c penetrometer as the 103 2000 No. DESCRIPTION cu/p' ^ 2 i 4 •A 6 Portsmouth CL Cloy PI =15 st-IO LL= 35 Boston CL Clay LL = 41 PI = 22 Bangkok CH Clay LL=65 PI = 41 Maine CH OH Clay LL=65 PI = 38 AGS CH Clay L L =7| P I = 4 0 Atcho faloyo CH Clay LL=95 PI = 75 Tailor River Peat w = 5 0 0 % .20 .20 .27 .29 .26 .24 — CK U simple shear tests — All soils normally consolidated 0.2 0.4 0.6 0.8 APPLIED SHEAR STRESS RATIO T h / c „ (a) © 1 I 2 4 6 8 10 0 C R = <„/O-v'c I 2 4 6 8 10 OCRsoVn/Ov'c (b) FIG. 4.18. SELECTION OF SOIL STIFFNESS RATIO FOR CLAYS. (Adapted from Ladd et a l . , 1977) 104 premier t o o l f o r s t r a t i g r a p h i c l o g g i n g o f s o i l d e p o s i t s . The excess pore pressure (Au) measured during p e n e t r a t i o n i s a u s e f u l i n d i c a t i o n o f the s o i l type and provides an e x c e l l e n t means f o r d e t e c t i n g d e t a i l s i n s t r a t i g r a p h y . The d i f f e r e n t i a l pore pressure r a t i o (Au/q c) a l s o appears to be a good index of s o i l type and r e l a t i v e consistency and a rough i n d i c a t o r of s t r e s s - h i s t o r y . In a d d i t i o n , when the steady penetra-t i o n i s stopped, the excess pore pressure decay w i t h time can be used as an i n d i c a t o r of the c o e f f i c i e n t of c o n s o l i d a t i o n . F i n a l l y the e q u i l i b r i u m pore p r e s s u r e v a l u e ( U Q)» a f t e r complete d i s s i p a t i o n i s reached, provides important data on the ground water c o n d i t i o n s . These p o i n t s w i l l be discussed i n more d e t a i l i n the f o l l o w i n g sec-t i o n s . 4.4.1. S o i l Type and Stress H i s t o r y During cone p e n e t r a t i o n , s o i l s tend to generate pore pressures. For sandy s o i l s these pore pressures d i s s i p a t e almost as f a s t as they are g e n e r a t e d and the h i g h cone r e s i s t a n c e C^,,) i Q sands g e n e r a l l y g i v e s d i f f e r e n t i a l pore pressure r a t i o s (Au/q^) of e s s e n t i a l l y zero. S i l t y and cl a y e y s o i l s , because of t h e i r r e l a t i v e l y low p e r m e a b i l i t y , can generate s i g n i f i c a n t excess pore pressures during cone p e n e t r a t i o n . The volume change c h a r a c t e r i s t i c s are a d i r e c t measure o f the s t r e s s h i s t o r y of a s o i l . Normally c o n s o l i d a t e d s i l t s and c l a y s tend to develop l a r g e p o s i t i v e pore pressures during shear, whereas, overconsolidated s i l t s and c l a y s tend to develop smaller p o s i t i v e or even negative pore pressures during shear. Therefore, i f the p e r m e a b i l i t y of a s o i l deposit i s r e l a t i v e l y low such that drainage during cone p e n e t r a t i o n i s s m a l l , the d i f f e r e n t i a l pore pressure r a t i o i s a d i r e c t measure of the s o i l deposit 105 s t r e s s h i s t o r y . Thus, the d i f f e r e n t i a l pore pressure r a t i o (—) g i v e s an q c e x c e l l e n t i n d i c a t i o n of both the volume change c h a r a c t e r i s t i c s and r e l a t i v e p e r m e a b i l i t y . This l o g i c a p p l i e s e q u a l l y w e l l to sandy s o i l s i n that loose sands tend to generate p o s i t i v e pore pressures and dense sands negative pore pressures during shear. However, because of t h e i r r e l a t i v e l y h i g h perme-a b i l i t y these pore pressures o f t e n d i s s i p a t e as f a s t as they are generated and very l i t t l e excess pore pressures are recorded. Use of the pore pressure or d i f f e r e n t i a l pore pressure r a t i o i s very dependent on the d e t a i l s of the cone design. The two s i g n i f i c a n t aspects of cone design i n r e l a t i o n to pore pressure measurements are: i ) Pore pressure element l o c a t i o n , i i ) Unequal end area e f f e c t s . Because of the complex v a r i a t i o n of s t r e s s e s and s t r a i n s around a cone t i p , the l o c a t i o n of the pore pressure element can s i g n i f i c a n t l y a f f e c t the measured pore pressure during cone p e n e t r a t i o n . In normally c o n s o l i d a t e d c l a y s and s i l t s , where l a r g e p o s i t i v e pore pressures are generated during shear, pore pressures measured on the face of the t i p are g e n e r a l l y only 10-20 percent l a r g e r than pore pressures measured immediately behind the t i p (Roy e t a l . , 1982, B a l i g h et a l . , 1978, Campanella, Robertson and G i l l e s p i e , 1983). In overconsolidated c l a y s and s i l t s , and f i n e sands, where s m a l l p o s i t i v e or negative pore pressures are generated during shear, pore pressures on the face of the t i p tend to be p o s i t i v e whereas pore pressures measured immediately behind the t i p may be negative (Campanella, Robertson and G i l l e s p i e , 1983) . This i s because the area along the face of the cone t i p i s i n a zone of maximum shear and compression; u n l i k e the 106 area immediately behind the t i p which i s i n a zone o f t o t a l s t r e s s r e l i e f . I t appears t h a t , because of the s t r e s s r e l i e f experienced by a s o i l element as i t passes behind the t i p , the pore pressure element behind the t i p encourages the measurement of low or negative dynamic pore pressures. Thus, w i t h the element l o c a t e d immediately behind the t i p the d i f f e r e n t i a l pore pressure r a t i o may be a more s e n s i t i v e measure o f s t r e s s h i s t o r y . I f the t o t a l t i p r e s i s t a n c e , q^, i s used, the d i f f e r e n t i a l pore p r e s s u r e r a t i o , Au/q^, can be ex p e c t e d to r e l a t e more uniquely to the s t r e s s h i s t o r y of s o i l d e p o s i t s . Some researchers p r e f e r to use the r a t i o , u,j,/q,j, and t h i s r a t i o can not p h y s i c a l l y exceed one. For a uniform normally c o n s o l i d a t e d d e p o s i t of s i l t or c l a y the d i f f e r e n t i a l pore p r e s s u r e r a t i o , Au/q^,, measured b e h i n d the t i p i s constant w i t h depth and has a value i n the range of 0.4 - 0.6 depending on r i g i d i t y index ( 1 ^ = G/C^) , where G i s the shear modulus a t the 50% f a i l u r e s t r e s s ( i . e . G , . Q ) . A n a l y s i s , by the w r i t e r , of a v a i l a b l e experimental r e s u l t s (Ladd et a l . 1977) i n d i c a t e s : i ) I r f o r n a t u r a l c l a y d e p o s i t s vary between 50 and 300. i i ) I tends to decrease as OCR i n c r e a s e s , r i i i ) G e n e r a l l y , f o r the same OCR, I incre a s e s w i t h decreasing P I . The h i g h e r (Au/q^,) r a t i o value i s generated by s o i l s w i t h a high r i g i d i t y index ( i . e . a low p l a s t i c i t y i n d e x ) . The t o t a l pore pressure r a t i o , u^/q^, i s o n l y c o n s t a n t w i t h depth f o r normally c o n s o l i d a t e d s o i l s i f the groundwater c o n d i t i o n s are h y d r o s t a t i c ( i . e . l i n e a r w i t h depth). I t s h o u l d be noted t h a t the t i p r e s i s t a n c e (q^) can a l s o g i v e an approximate i n d i c a t i o n of s t r e s s h i s t o r y (see Table 4.3). For normally c o n s o l i d a t e d c l a y d e p o s i t s w i t h h y d r o s t a t i c groundwater c o n d i t i o n s the t i p 107 r e s i s t a n c e i s l i n e a r w i t h depth. For most young c l a y s where overconso-l i d a t i o n has been caused by e r o s i o n or d e s i c c a t i o n , the OCR w i l l decrease w i t h depth u n t i l the deposit i s approximately normally c o n s o l i d a t e d . In these cases, the t i p r e s i s t a n c e w i l l be approximately constant or even decrease w i t h depth u n t i l the depth where the d e p o s i t i s normally c o n s o l i d a t e d i s reached and w i l l then increase l i n e a r l y w i t h depth. For aged c l a y s where the OCR i s constant w i t h depth the t i p r e s i s t a n c e may continue to s t a y constant w i t h depth. 4.4. C o e f f i c i e n t of C o n s o l i d a t i o n and P e r m e a b i l i t y Upon the a r r e s t of steady p e n e t r a t i o n , excess pore pressures generated during cone p e n e t r a t i o n immediately s t a r t to d i s s i p a t e . The r a t e of d i s s i p a t i o n depends upon the c o e f f i c i e n t of c o n s o l i d a t i o n of the s o i l . By monitoring the r a t e of d i s s i p a t i o n of the excess pore pressure, an estimate of the c o e f f i c i e n t of c o n s o l i d a t i o n of the s o i l may be obtained. Several t h e o r e t i c a l s o l u t i o n s are a v a i l a b l e to o b t a i n the c o e f f i c i e n t from d i s s i p a t i o n of excess pore pressures generated by c a v i t y expansion. A summary of these s o l u t i o n s are shown on F i g . 4.19(a), which h i g h l i g h t s the major d i f f e r e n c e s between them. In order to compare r e s u l t s of the d i f f e r e n t s o l u t i o n s , they have been non-dimensionalized and shown i n F i g . 4.19(b). F i g . 4.19(b) shows the decay of excess pore pressure, Au, p l o t t e d a g a i n s t a n o n - d i m e n s i o n a l time f a c t o r , T = c ^ t / r 2 . Use of the time f a c t o r , T, a l l o w s a quick c a l c u l a t i o n f o r the c o e f f i c i e n t of c o n s o l i d a t i o n , c ^ t / r 2 . The s o l u t i o n by B a l i g h ,and Levadoux (1980) and the c y l i n d r i c a l s o l u t i o n by Torstensson (1977), y i e l d e s s e n t i a l l y the same r e s u l t . The s o l u t i o n by Randolph and Wroth (1979) i s not shown because of i t s s i m i l a r i t y to that of Torstensson (1977). Author Cavity Type Material Model I n i t i a l Fore Pressure Dis t r i b u t i o n Proposed Applications ReoarkB Baligh t Levadouz 1980 coablned r a d i a l and spherical non-linear Boston Blue clay froa F.E. studies using s t r a i n path aethod consolidation ch a r a c t e r i s t i c s shows very small influence of spherical coaponent of dis s i p a t i o n Randolph i Wroth 1979 c y l i n d r i c a l e l a s t i c - p l a s t i c tuL - 2 cu l n ( | ) | - (G/cu)1'2 o consolidation around p i l e s pressureaeter analysis a n a l y t i c a l solution Soderberg 1962 c y l i n d r i c a l • l a s t l c - p l a s t l c r i u i consolidation around p i l e s Torstensson 1977 c y l i n d r i c a l e l a s t i c - p l a s t i c i i j • 2 t u l n ( * j — - ( G / c u ) 1 / 2 0 consolidation ch a r a c t e r i s t i c s proposes average of two results Torstensson 1977 spherical e l a s t i c - p l a s t i c \ - 4 cu ln(§) f - - ( G / c u ) 1 ' 3 o consolidation c h a r a c t e r i s t i c s v e r t i c a l drains (a) FIG. 4.19.. SUMMARY' OF EXISTING SOLUTIONS FOR PORE PRESSURE DISSIPATION. (Adapted from G i l l e s p i e , 1982) 109 The s o l u t i o n s by Randolph and Wroth and by Torstensson r e q u i r e an e s t i m a t e of the s o i l s t i f f n e s s r a t i o or r i g i d i t y index (G/C^). The reason f o r t h i s i s that a s t i f f s o i l w i l l have a much l a r g e r zone of i n f l u e n c e than a s o f t s o i l . The r e s u l t of a l a r g e r zone of i n f l u e n c e i s to decrease the r a t e of decay of excess pore pressures at the cone. The s o i l s t i f f n e s s r a t i o can be expressed as e i t h e r the r a t i o o f undrained Young's modulus, E , o r the s h ear modulus, G, to the u n d r a i n e d shear s t r e n g t h , C . The u ° u undrained Young's modulus and shear modulus are r e l a t e d by E r - u 2(1+M) since u = 0.5 f o r undrained c o n d i t i o n s . E G -S e l e c t i o n of an exact s t i f f n e s s r a t i o i s complicated by the v a r i a t i o n i n moduli w i t h s t r a i n l e v e l , as shown i n F i g . 4.18. With the complex v a r i a t i o n i n s t r a i n s around the cone i t seems reasonable to s e l e c t a s t i f f n e s s r a t i o at an intermediate s t r e s s l e v e l (say, Gc_ or E , . _ ) . 50 u50 Although some doubt surrounds the s e l e c t i o n of an appropriate s t i f f n e s s r a t i o the s o l u t i o n s are not very s e n s i t i v e to s o i l s t i f f n e s s . For a four-f o l d i n c r e a s e i n s t i f f n e s s r a t i o , the p r e d i c t e d c o e f f i c i e n t of c o n s o l i d a t i o n changes by a f a c t o r of about 2. Provided e q u i l i b r i u m pore pressures are not r e q u i r e d , i t i s not necessary, f o r the purpose of o b t a i n i n g c o n s o l i d a t i o n c h a r a c t e r i s t i c s , to wait past the 50 percent l e v e l of d i s s i p a t i o n . The a p p l i c a b i l i t y and meaning of the s o l u t i o n s are complicated by s e v e r a l phenomena. These phenomena i n c l u d e : - the importance of v e r t i c a l as w e l l as h o r i z o n t a l d i f f u s i o n , - the e f f e c t of s o i l disturbance, 110 - u n c e r t a i n t y over the d i s t r i b u t i o n , l e v e l and change of t o t a l r a d i a l s t r e s s e s , - s o i l a n i s o t r o p y and n o n l i n e a r i t y of s o i l c o m p r e s s i b i l i t y , and - non-homogenity due to s o i l l a y e r i n g or nearness t o a l a y e r boundary. In s p i t e of these l i m i t a t i o n s , the use of the t h e o r e t i c a l s o l u t i o n s i s encouraged by the r e p e a t a b i l i t y of the t e s t and the vast range i n d i s s i p a t i o n r a t e s measured f o r v a r i o u s s o i l s encountered. The i n f l u e n c e of v e r t i c a l d i s s i p a t i o n was shown by G i l l e s p i e and Campanella (1981) to be i n s i g n i f i c a n t and that h o r i z o n t a l d i s s i p a t i o n appears to dominate the c o n s o l i d a t i o n process, a t l e a s t , f o r the pore pres-sure element l o c a t e d immediately behind the t i p . Hence, c y l i n d r i c a l d i s s i p a t i o n s o l u t i o n s , such as that by Torstensson (1977) can be expected to g i v e reasonable r e s u l t s . Results from a study by G i l l e s p i e and Campanella (1981) showed that the t h e o r e t i c a l s o l u t i o n s appear to give a c o e f f i c i e n t o f c o n s o l i d a t i o n , c, , i n the h o r i z o n t a l d i r e c t i o n f o r a s o i l i n h the s l i g h t l y overconsolidated s t a t e (OCR « 2 ) . This r e s u l t seems reasonable s i n c e the s o i l around the t i p , e s p e c i a l l y behind the t i p , has been preloaded due to the process of p e n e t r a t i o n . In s p i t e of these l i m i t a t i o n s , the d i s s i p a t i o n t e s t provides a u s e f u l means of e v a l u a t i n g approximate c o n s o l i d a t i o n p r o p e r t i e s and macrofabric and r e l a t e d drainage paths o f n a t u r a l c l a y d e p o s i t s . The t e s t a l s o appears to provide very important i n f o r m a t i o n f o r the design of v e r t i c a l d r a i n s ( B a t t a g l i o e t a l . , 1981). I t i s u s e f u l here to comment on the procedure f o l l o w e d when recording the pore pressure d i s s i p a t i o n s . Some researchers have reported that they found i t necessary to clamp the p e n e t r a t i o n rods a t the ground surface w h i l e r e c o r d i n g pore pressure d i s s i p a t i o n . I t appears that i f the rods I l l were not clamped a drop i n the measured pore pressure would r e s u l t when lo a d was r e l e a s e d from the t i p . I t appears the l o c a t i o n of the sensing element e x p l a i n s the s e n s i t i v i t y of decay response to procedure. When load i s r e l e a s e d , pore pressures at the t i p immediately drop i n response to the decrease i n t o t a l s t r e s s . Whereas, behind the t i p , i n the zone of f a i l e d s o i l the s t r e s s l e v e l does not change s i g n i f i c a n t l y when load i s r e l e a s e d . I t t h e r e f o r e appears t h a t , f o r standard 60° cones, the l o c a t i o n of the piezometer element behind the t i p i s l e s s s e n s i t i v e to t e s t procedure used. This i s an important point because the amount of load a p p l i e d to the t i p , even w i t h the rods clamped, w i l l change w i t h time due to s t r e s s r e l a x a t i o n . A crude estimate of p e r m e a b i l i t y can be made from the s o i l type c l a s s i f i c a t i o n . A more r e l i a b l e estimate of p e r m e a b i l i t y , e s p e c i a l l y f o r f i n e grained s o i l s , can be made from the c o n s o l i d a t i o n and c o m p r e s s i b i l i t y c h a r a c t e r i s t i c s . Since: k = c m Y v v v w k = c,m Y h h h w where k^ and k^ are the c o e f f i c i e n t of p e r m e a b i l i t y i n the v e r t i c a l and h o r i z o n t a l d i r e c t i o n s , r e s p e c t i v e l y . R e s u l t s of l i m i t e d past experience suggests that s o i l c o m p r e s s i b i l i t y can be regarded as approximately i s o t r o p i c , mv = m h ( M i t c h e l l et a l . , 1978; Ladd e t a l . , 1977). I f i t i s assumed that s o i l c o m p r e s s i b i l i t y i s i s o t r o p i c , then: k v c = c x — v h k, h An e s t i m a t e o f t h e r a t i o k /k can be o b t a i n e d from Table 4.4, a f t e r v h B a l i g h and Levadoux, 1980. Evidence of s o i l heterogeneity can be obtained from examination of the bearing, f r i c t i o n and dynamic pore pressure 112 records. Nature of Clay k l / k v 1. No evidence of la y e r i n g 1.2 ± 0.2 2. Slight l a y e r i n g , eg., sedimentary 2 to 5 clays with occasional s i l t dust-ings to random lenses 3. Varved clays i n north-eastern U.S. 10 ± 5 TABLE 4.4: Anisotropic Permeability of Clays ( A f t e r : B a l i g h and Levadoux, 1980) S i n c e an estimate of m can be made, then estimates of h o r i z o n t a l and v v e r t i c a l p e r m e a b i l i t i e s can be obtained. Estimates of m^  can be made using Table 4.3 or using an a f a c t o r based on l o c a l experience. 4.5. L i q u e f a c t i o n Resistance 4.5.1. I n t r o d u c t i o n The p e n e t r a t i o n r e s i s t a n c e of both the SPT and CPT and the r e s i s t a n c e of s o i l to l i q u e f a c t i o n are s i m i l a r l y i n f l u e n c e d by most s o i l compositional and environmental v a r i a b l e s (Schmertmann, 1978). These v a r i a b l e s i n c l u d e ; s o i l d e n s i t y , s o i l s t r u c t u r e , cementation, aging, s t r e s s s t a t e and s t r e s s h i s t o r y . Most of these v a r i a b l e s a l s o i n f l u e n c e the more fundamental s o i l p r o p e r t i e s of shear s t r e n g t h and c o m p r e s s i b i l i t y . Thus a knowledge of how these v a r i a b l e s i n f l u e n c e the r e s i s t a n c e to cone p e n e t r a t i o n can be ap p l i e d to provide a c o r r e l a t i o n between CPT r e s i s t a n c e and l i q u e f a c t i o n r e s i s t a n c e . As discussed e a r l i e r , the r e s i s t a n c e to cone p e n e t r a t i o n i s in f l u e n c e d 113 by the shear s t r e n g t h and c o m p r e s s i b i l i t y of the sand. However, as pointed out e a r l i e r , f o r most n a t u r a l q u a r t z sands the shear s t r e n g t h has s i g n i f i c a n t l y more i n f l u e n c e on cone r e s i s t a n c e than the c o m p r e s s i b i l i t y . Since a l a r g e p o r t i o n of l i q u e f a c t i o n problems occur i n a l l u v i a l quartz sands i t should be p o s s i b l e to use CPT c o r r e l a t i o n s developed f o r these sands to provide a CPT method f o r assessment of l i q u e f a c t i o n r e s i s t a n c e . 4.5.2. Normalised Cone Resistance Since the SPT method proposed by Seed has proven extremely s u c c e s s f u l , i t would appear l o g i c a l to produce a CPT method along s i m i l a r l i n e s . Thus one of the f i r s t requirements would be to modify the cone bearing, q^, to an overburden s t r e s s l e v e l of 1 kg/cm 2 (1 t s f ) using the r e l a t i o n : Q = C »q . c Q n c S i n c e most c a l i b r a t i o n chamber t e s t s t u d i e s show that the q versus D c r r e l a t i o n s h i p s a l l have s i m i l a r shapes i t should be p o s s i b l e to modify q c using one of these c o r r e l a t i o n s . The w r i t e r has c a r r i e d t h i s out using the very comprehensive data produced by B a l d i e t a l . (1981) and i s shown i n F i g . 4.20. The c o r r e c t i o n f a c t i o n , C^, shown i n F i g . 4.20 i s very s i m i l a r t o t h a t proposed by Schmertman (1976) and to the f a c t o r C^ proposed by Seed and I d r i s s (1981). This i l l u s t r a t e s that CPT and SPT r e s i s t a n c e vary i n a s i m i l a r manner w i t h depth. 4.5.3. R e l a t i v e Density C o r r e l a t i o n As a f i r s t step i n a CPT method f o r l i q u e f a c t i o n assessment use can be made o f the r e l a t i v e d e n s i t y c o r r e l a t i o n shown i n F i g . 4.10. This has been done by the w r i t e r by combining the l i q u e f a c t i o n r e s i s t a n c e data produced by Vaid et a l (1981) (see F i g . 3.5) to produce the CPT r e s i s t a n c e curve 114 115 shown i n F i g . 4.21. The l i q u e f a c t i o n r e s i s t a n c e curve by Vaid et a l . (1981) has been chosen because i t appears to represent q u i t e c l o s e l y the observed f i e l d l i q u e f a c t i o n behaviour, as shown i n F i g . 4.22. The r e s u l t s by Vaid et a l . (1981) al s o enable a l i q u e f a c t i o n and c y c l i c m o b i l i t y r e s i s t a n c e curve to be developed f o r a more s p e c i f i c l e v e l of c y c l i c s t r e s s r a t i o . For t h i s purpose a c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n or 10 percent double amplitude shear s t r a i n i n 15 c y c l e s has been chosen. The 10 percent double amplitude shear s t r a i n l e v e l was chosen because dense sands may develop i n i t i a l l i q u e f a c t i o n ( i . e . = 0) but w i t h l i m i t e d shear s t r a i n . 15 c y c l e s was chosen because t h i s can be considered to represent approximately a magnitude 7.5 earthquake (Seed and I d r i s s , 1981). 4.5.4. Shear Strength C o r r e l a t i o n A f u r t h e r r e l a t i o n s h i p between cone r e s i s t a n c e and l i q u e f a c t i o n r e s i s t a n c e can be developed using the proposed f r i c t i o n angle c o r r e l a t i o n shown i n F i g . 4.12. The l i q u e f a c t i o n r e s i s t a n c e data by Vaid et a l . (1981) (See F i g . 3.5) usi n g the d i l a t i o n angle has been combined, by the w r i t e r , w i t h the proposed f r i c t i o n angle c o r r e l a t i o n using Rowes s t r e s s d i l a t a n c y to convert f r i c t i o n angle to d i l a t i o n angle. The r e s u l t i n g r e l a t i o n s h i p i s shown i n F i g . 4.23 f o r two l e v e l s of f r i c t i o n angle a t constant volume, <b . The <j) v a l u e s of 30° and 32° are considered to represent uniform cv cv f i n e to medium sands. I t i s c l e a r from F i g . 4.23 t h a t t h i s approach cannot d e f i n e c l e a r l y a r e l a t i o n s h i p f o r dense sands where q^ > 150 kg/cm2 at a ' = 1 kg/cm 2. vo 4.5.5. SPT-CPT C o r r e l a t i o n A f u r t h e r , and p o s s i b l y more l o g i c a l , method to d e r i v e a CPT LU 0 100 200 M O D I F I E D C O N E B E A R I N G , Q c , k g / c m 2 FIG. 4.21. CORRELATION BETWEEN LIQUEFACTION RESISTANCE AND CONE PENETRATION RESISTANCE FOR SANDS BASED ON RELATIVE DENSITY CORRELATION. 117 f—i—i—i—i—i—i—i—r 0 10 20 SO 40 50 60 70 80 90 100 Apparent Relative Density,D r (%) Bosed on Dato by Cnrittion ond Swiger, 1975 FIG. 4.22. ANALYSIS OF FIELD RECORDS OF SITES WHERE LIQUEFACTION DID AND DID NOT OCCUR. (A f t e r Vaid et a l . , 1981) 118 in UJ FIG. 4.23. CORRELATION BETWEEN LIQUEFACTION RESISTANCE AND CONE PENETRATION RESISTANCE IN SANDS BASED ON FRICTION ANGLE CORRELATION. 119 l i q u e f a c t i o n r e l a t i o n i s by conversion of SPT to CPT. This has s e v e r a l advantages: the SPT l i q u e f a c t i o n method i s based on a l a r g e amount of experience from observed cases of l i q u e f a c t i o n (Seed and I d r i s s , 1981); the SPT and CPT are both s i m i l a r l y i n f l u e n c e d by most s o i l v a r i a b l e s . Thus, c o r r e c t conversion of SPT data to CPT can b e t t e r account f o r f a c t o r s such as a g i n g and s t r e s s h i s t o r y t h a t the d e n s i t y o r f r i c t i o n a n g l e r e l a t i o n s h i p s cannot. Considerable research has taken place over the years to q u a n t i f y the r e l a t i o n s h i p between SPT N values and CPT t i p r e s i s t a n c e , q . A summary, produced by the w r i t e r , of many of the de r i v e d q /N c c r e l a t i o n s h i p s i s shown i n F i g . 4.24, as a f u n c t i o n of mean g r a i n s i z e < D5C> I t i s c l e a r t h a t the 1 C/N r a t i o d e c r e a s e s w i t h decreasing g r a i n s i z e . The s c a t t e r i n r e s u l t s appears to increase w i t h i n c r e a s i n g g r a i n s i z e . T h i s i s not s u r p r i s i n g s i n c e p e n e t r a t i o n i n g r a v e l l y sand ( u ^ Q B 1.0 mm) i s s i g n i f i c a n t l y i n f l u e n c e d by the l a r g e r i n d i v i d u a l g r a v e l s i z e d p a r t i c l e s not to mention the v a r i a b i l i t y of d e l i v e r e d energy i n the SPT data. A l s o sand dep o s i t s i n general are u s u a l l y s t r a t i f i e d or non-homogeneous causing r a p i d v a r i a t i o n s i n CPT t i p r e s i s t a n c e . There was a l s o some d i f f i c u l t y i n d e f i n i n g the D^ ^ from some of the ref e r e n c e s . Work by M a r t i n s and Furtado (1963) and Douglas (1982) has shown that the q c / N r e l a t i o n s h i p a l s o v a r i e s w i t h SPT hammer type and s o i l d e n s i t y . The 9 C/N r e l a t i o n i s s i g n i f i c a n t l y a f f e c t e d by SPT hammer type s i n c e t h i s e f f e c t s the energy transmitted to the rods. As discussed e a r l i e r the SPT l i q u e f a c t i o n method by Seed was based on a standard c y l i n d e r hammer operated by 2 turns of a rope around a cathead drum. I t appears that much of the data shown i n F i g . 4.24 was obtained using the standard hammer. Thus, to convert standard 2 rope-turn-around-the-cathead hammer SPT N-120 q e . b o r * j N , blows/foot ( I b o r » lOOkPo) C L A Y CLAYEY SILTS ft SILTY CLAY SANDY SILT ft SILT SILTY SAND SAND CT o < tr. O.OOI 0.01 0.1 MEAN GRAIN S I Z E , D™,mm D O 1.0 1. Meyerho f (1956) 2 . M e i g h and Nixon (1961) 3. Rodin (1961) 4. De Alencar Velloeo (1959) 5. Schmertmann (1970) 6 . Suther land (1974) 7. Thornburn (|j*70) FIG. 4.24. VARIATION OF q /N c (After Robertson et 8. Componello et o l . (1979) 9. N i i o n (1962) 10. Kruizingo(l982) 11. Douglas (1982) 12. Muromochi B Koboyoshi (1982) 13. Goel (1982) I4.lthihara and K o g o ( l 9 8 l ) RATIO WITH MEAN GRAIN SIZE, a l . , 1982) v a l u e s t o e q u i v a l e n t CPT t i p r e s i s t a n c e values f o r medium sands = 0.25 mm) a q /N r a t i o of 4.5 to 5.0 can be considered r e p r e s e n t a t i v e , c For s i l t y s o i l s ( u ^ Q = u , l m S R) a r a t i o of 4.0 can be considered more r e p r e s e n t a t i v e . As discussed e a r l i e r the SPT and CPT are s i m i l a r l y i n f l u e n c e d by most s o i l v a r i a b l e s and thus N and q c appear to vary i n a s i m i l a r manner w i t h depth ( i . e . a' ). Thus the q /N r a t i o of 4.5 can be vo c used to c onvert normalized N, to Q values d i r e c t l y w i t h l i t t l e e r r o r . 1 c The r e s u l t i n g CPT l i q u e f a c t i o n r e l a t i o n i s shown i n F i g . 4.25 based on Seeds l i q u e f a c t i o n c h a r t f o r sands w i t h D^ ^ > 0.2 5 mm. The recent SPT and l i q u e f a c t i o n data s t u d i e d by Seed and I d r i s s (1981) show s e v e r a l data p o i n t s where l i q u e f a c t i o n d i d occur that p l o t below the o r i g i n a l d i v i d i n g l i n e proposed by Seed. A p o s s i b l e a l t e r n a t i v e lower bound to the SPT data p o i n t s i s shown i n F i g . 4.2 6. The r e s u l t i n g lower bound appears to agree b e t t e r w i t h the l a b o r a t o r y and f i e l d data shown i n F i g s . 3.5 and 4.22. A d d i t i o n a l data, i n c l u d i n g SPT energy measurements, obtained during t h i s s t u d y on p o s s i b l e <1C/N r a t i o s w i l l be presented i n Chapter 7. 4.5.6. Proposed CPT L i q u e f a c t i o n R e l a t i o n s h i p A comparison between the d i f f e r e n t CPT l i q u e f a c t i o n r e l a t i o n s suggested i s shown i n F i g . 4.27. A proposed c o r r e l a t i o n i s suggested by the w r i t e r based on the f o l l o w i n g : i ) f a c t o r s such as aging, cementation and s t r e s s h i s t o r y tend to increase the r e s i s t a n c e to l i q u e f a c t i o n . Hence, the r e l a t i o n s based on l a b o r a t o r y d a t a (Dr,<|)) may tend to u n d e r e s t i m a t e the l i q u e f a c t i o n r e s i s t a n c e , i i ) SPT c o r r e l a t i o n i s based on a l a r g e amount of f i e l d observations which 122 < or co co UJ or i-</> o o o a. UJ o UJ CO 0.5 0.4 0 . 3 J -0.2 o 0.1 h-B a s e d o n : / Seed S Id ri s (1981) SPT, N, vs. T/OJ S t a t i c / d y n a m i c correlation N vs. q 0 100 200 M O D I F I E D C O N E B E A R I N G , Q r , k g / c m 2 FIG. 4.25. CORRELATION BETWEEN LIQUEFACTION RESISTANCE AND CONE PENETRATION RESISTANCE IN SANDS BASED ON SPT CORRELATION. 123 FIG. 4.2 6. CORRELATION BETWEEN LXQUEFACTON RESISTANCE AND SPT SHOWING PROPOSED LOWER BOUND. (Data from Seed and I d r i s s , 1981) co UJ o or UJ Q_ o >-o If) or o o 2 or I-or < UJ i CO UJ o V) UJ </) o -U J _ _ l or _ j a . CO or < UJ X to UJ CO < o < UJ _ l m O o 0 . 5 i -0.4 0.3 0.2 0.1 SPT lower bound ^=4.5* N Proposed cor re la t ion J I L 100 200 MODIFIED CONE BEARING, Q r ,kg /cm' FIG. 4.27. SUMMARY OF CORRELATIONS BETWEEN LIQUEFACTION RESISTANCE AND CONE PENETRATION RESISTANCE IN SANDS. 125 i n c l u d e f a c t o r s such as aging, cementation and s t r e s s h i s t o r y . Hence the r e l a t i o n based on SPT data w i l l tend to be more r e p r e s e n t a t i v e of f i e l d behaviour. However, the r a t i o of 4.5 i s an average low v a l u e f o r sands ( u^Q * 0 . 2 5 mm) based on standard 2 rope-turn-around-the-cathead hammer energy l e v e l . Some of the d a t a , i n p a r t i c u l a r , the Japanese da t a , reviewed by Seed and I d r i s s ( 1 9 8 1 ) was almost c e r t a i n l y obtained using higher energy hammers where r a t i o s may be c l o s e r to 5.5 ( I s h i h a r a and Koga, 1 9 8 1 ) . A l s o , f o r medium to coarse sand ( D 5 Q > 0 . 2 5 mm) the r a t i o w i l l tend towards 5 . 5 . The proposed CPT r e l a t i o n shown i n F i g . 4 . 2 7 can be used i n the same manner as the Seed SPT method. For any given s i t e and a given value of maximum ground a c c e l e r a t i o n , the p o s s i b i l i t y of l i q u e f a c t i o n or c y c l i c m o b i l i t y can be evaluated on an e m p i r i c a l b a s i s w i t h the a i d of F i g s . 4 . 2 0 and 4 . 2 7 . By determining the appropriate values of f o r the sand d e p o s i t l o w e r bound v a l u e s o f c y c l i c s t r e s s r a t i o , x /a' , to cause t vo l i q u e f a c t i o n or c y c l i c m o b i l i t y can be obtained and compared w i t h the c y c l i c s t r e s s r a t i o induced by the design earthquake ( x / o 1 ). vo The proposed c o r r e l a t i o n was based on c y c l i c s t r e s s r a t i o s to cause l i q u e f a c t i o n or 1 0 percent double amplitude shear s t r a i n i n 1 5 c y c l e s . The c o r r e l a t i o n can be extended to earthquakes where the number of c y c l e s are more or l e s s than 1 5 c y c l e s . Seed and I d r i s s ( 1 9 8 1 ) have presented a r e p r e s e n t a t i v e shape f o r the r e l a t i o n s h i p between c y c l i c s t r e s s r a t i o and numbers of c y c l e s to cause l i q u e f a c t i o n and i s summarized below: 126 E a r t h q u a k e No.' o f R e p r e s e n t a t i v e ^x av^avo^i~CjT = c^ M a g n i t u d e C y c l e s a t 0.65 x m a x ( T a v / o ^ Q ) £ - 1 5 = 15 8-1/2 26 0.89 7-1/2 15 1.0 6-3/4 10 1.13 6 5 1.32 5-1/4 2-3 1.5 Thus by m u l t i p l y i n g the proposed curve i n F i g . 4.2 7 by the s c a l i n g f a c t o r s shown above i n column 3, boundary curves s e p a r a t i n g s i t e s where l i q u e f a c t i o n i s l i k e l y to occur or u n l i k e l y to occur may be determined f o r earthquakes with d i f f e r e n t numbers of equivalent uniform c y c l e s . The proposed c o r r e l a t i o n shown i n F i g . 4.27, however, i s only a p p l i c a b l e to c l e a n sands w i t h D^ ^ > 0.25 mm. To i d e n t i f y sands of t h i s nature w i t h CPT data alone use can be made of the s o i l c l a s s i f i c a t i o n chart developed by Douglas et a l . (1981) but adapted by the w r i t e r as shown i n F i g . 4.28. Work by Douglas (1982) and experience gained at UBC during t h i s study would suggest that s o i l s susceptable to l i q u e f a c t i o n f a l l w i t h i n an area on the s o i l c l a s s i f i c a t i o n c h a r t designated Zone A. Clean sands w i t h a D c„ > 0.25 m tend to f a l l w i t h i n the upper area of Zone A w i t h q > 10 50 c kg/cm2 and f r i c t i o n r a t i o FR < 1.0. S o i l s t h a t f a l l w i t h i n the lower area of Zone A are the s i l t y sands and s i l t s . These s o i l s tend to have higher r e s i s t a n c e to l i q u e f a c t i o n f o r the same p e n e t r a t i o n r e s i s t a n c e values and tend to develop more pore pressures during p e n e t r a t i o n because of t h e i r lower p e r m e a b i l i t y . F i e l d and l a b o r a t o r y observations have shown that the l i q u e f a c t i o n r e s i s t a n c e tends to increase w i t h decreasing g r a i n s i z e below a mean g r a i n I bar > lOOkPa = 1.02 kg/cm 2 FRICTION RATIO , F R , % FIG. 4.28. SOIL CLASSIFICATION CHART FOR ELECTRIC CONE SHOWING PROPOSED ZONE OF LIQUEFACTION SOILS. 128 s i z e (D ^ Q) of a p p r o x i m a t e l y 0 . 2 5 mm. Thus, f o r the same p e n e t r a t i o n r e s i s t a n c e the c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n i n c r e a s e s w i t h decreasing g r a i n s i z e . I t appears, however, that t h i s i n c r e a s e i s a l s o a s s o c i a t e d w i t h an increase i n the cohesive nature o f the s o i l . This e f f e c t was in c o r p o r a t e d i n t o the SPT based method proposed by Iwasaki et a l . ( 1 9 7 5 ) by i n c r e a s i n g the c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n by an amount dependent on the mean g r a i n s i z e , using the r e l a t i o n D A T / O ' = - 0 . 1 4 6 l o g • - T T ^ V (mm) vo ° 0 . 3 5 T h i s was based on l a b o r a t o r y r e s u l t s and shows there i s a steady gradual increase i n l i q u e f a c t i o n r e s i s t a n c e w i t h decreasing g r a i n s i z e below a mean g r a i n s i z e of D,_Q = 0 . 6 mm. The f i e l d data reviewed by Seed and I d r i s s ( 1 9 8 1 ) showed an increase i n c y c l i c s t r e s s r a t i o of about 0 . 0 7 5 w i t h a d e c r e a s e i n mean g r a i n s i z e ( D ^ Q ) from 0 . 2 5 mm to 0 . 1 5 mm. This corresponds to a decrease i n SPT N value of about 7 . 5 f o r a constant c y c l i c s t r e s s r a t i o . For comparison, the Iwasaki et a l . ( 1 9 7 5 ) method p r e d i c t s a 0 . 0 4 i n c r e a s e w i t h a decrease i n mean g r a i n s i z e from 0 . 2 5 mm to 0 . 1 5 mm. Data from Chinese CPT work (Zhou, 1 9 8 2 ) i n s i l t y sands i n d i c a t e a decrease i n cone r e s i s t a n c e o f about 4 0 kg/cm 2 f o r a decrease i n g r a i n s i z e from a medium sand (D^Q K 0 . 2 5 mm) to a s i l t y sand (O^Q " 0 . 1 5 mm). The w r i t e r has combined these observed responses to generate a second c o r r e l a t i o n f o r CPT based l i q u e f a c t i o n by decreasing the proposed sand c o r r e l a t i o n shown i n F i g . 4 . 2 7 by a cone r e s i s t a n c e of 4 0 kg/cm 2. This i s equivalent to an increase i n c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n of about 0 . 0 5 f o r constant cone r e s i s t a n c e . The combined proposed CPT based l i q u e f a c t i o n c o r r e l a t i o n s are shown i n F i g . 4 . 2 9 . The c o r r e l a t i o n s proposed i n F i g . 4 . 2 9 f o r CPT data are based on 129 FIG. 4.29. PROPOSED CORRELATIONS BETWEEN LIQUEFACTION RESISTANCE UNDER LEVEL GROUND CONDITIONS AND CONE PENETRATION RESISTANCE FOR SANDS AND SILTY SANDS. 130 e m p i r i c a l r e l a t i o n s h i p s and r e q u i r e c o n s i d e r a b l e f i e l d v e r i f i c a t i o n . The CPT based method should be used i n the same manner proposed by Seed and I d r i s s (1981) f o r the SPT based method. The CPT data can be used to provide a p r e l i m i n a r y i d e n t i f i c a t i o n of l i q u e f a c t i o n s u s c e p t i b l e s o i l s using the c h a r t shown i n F i g . 4.28. S o i l s t h a t f a l l w i t h i n the upper shaded a r e a o f Zone A can be c o n s i d e r e d as sands w i t h a > 0.25 mm. S o i l s that f a l l w i t h i n the lower hatched area of Zone A can be considered as s i l t y sands or s i l t s w i t h a D^ ^ < 0.15 mm. The CPT data would provide a continuous and repeatable measure of the p e n e t r a t i o n r e s i s t a n c e and the c o r r e l a t i o n s i n F i g . 4.28 and 4.29 can be expected to provide a p r e l i m i n a r y estimate of l i q u e f a c t i o n p o t e n t i a l . The CPT data would provide data to i d e n t i f y p o t e n t i a l c r i t i c a l areas where d e t a i l e d assessment may be r e q u i r e d , which may i n c l u d e sampling and/or f u r t h e r i n - s i t u t e s t i n g . The e x i s t i n g SPT and proposed CPT methods use p e n e t r a t i o n r e s i s t a n c e to assess l i q u e f a c t i o n r e s i s t a n c e i n sand and s i l t y sands. However, i n s i l t s , and to some extent s i l t y sands, p e n e t r a t i o n o f t e n takes place under undrained or p a r t i a l l y drained c o n d i t i o n s and l a r g e pore pressures can be generated. The p e n e t r a t i o n r e s i s t a n c e i n these s o i l s i s o f t e n extremely small and becomes s e n s i t i v e to Instrument or t e s t e r r o r s . The recent a d d i t i o n of continuous pore pressure measurements during cone p e n e t r a t i o n has the p o t e n t i a l to s i g n i f i c a n t l y improve our i n t e r p r e t a t i o n and understanding of CPT data f o r l i q u e f a c t i o n assessment i n f i n e grained s o i l s such as s i l t s . Parameters such as the d i f f e r e n t i a l pore pressure r a t i o can provide an i n d i c a t i o n of both the volume change c h a r a c t e r i s t i c s and r e l a t i v e p e r m e a b i l i t y . The d i f f e r e n t i a l pore pressure r a t i o or e f f e c t i v e cone b e a r i n g , q^, may be extremely u s e f u l i n f i n e g rained s o i l s such as s i l t y sands and s i l t s where the r e l a t i v e p e r m e a b i l i t y 131 i s low enough to enable s i g n i f i c a n t pore pressure response to be measured. From the s t a n d p o i n t of l i q u e f a c t i o n r e s i s t a n c e , volume change c h a r a c t e r i s t i c s are very important. To the w r i t e r s knowledge, nobody has yet completely q u a n t i f i e d the measured pore pressure response during cone p e n e t r a t i o n to l i q u e f a c t i o n r e s i s t a n c e , mainly because of the f o l l o w i n g : i ) Pore pressure response i s dependent on s o i l p e r m e a b i l i t y and r a t e of cone p e n e t r a t i o n . i i ) Measured pore pressure depends on cone design and l o c a t i o n of pore pressure element. The standard r a t e f o r cone pe n e t r a t i o n t e s t i n g of 2 cm/sec u s u a l l y generates s i g n i f i c a n t pore pressures i n s i l t s or sandy s i l t s w i t h approximate D^ ^ < 0.1 mm. Because of the complex v a r i a t i o n of s t r e s s e s and s t r a i n s around a cone t i p , the l o c a t i o n of the pore pressure element can s i g n i f i c a n t l y a f f e c t the measured pore pressures during cone p e n e t r a t i o n . As discussed e a r l i e r , the pore pressures around a cone penetrating normally c o n s o l i d a t e d c l a y s and s i l t s i s r e l a t i v e l y evenly d i s t r i b u t e d , w i t h h i g h p o s i t i v e pore pressures measured on the face of the t i p approximately 10-20 percent l a r g e r than pore pressures measured immediately behind the t i p . Ln overconsolidated c l a y s and s i l t s and i n s i l t y f i n e sands the pore pressure d i s t r i b u t i o n around a cone during p e n e t r a t i o n i s complex, w i t h pore pressures tending to be p o s i t i v e on the face of the cone t i p and negative immediately behind the t i p . This i s because of the complex v a r i a t i o n o f pore pressure response due to changes i n s t r e s s and s t r a i n l e v e l s f o r overconsolidated s o i l s and sands, and because of the t o t a l s t r e s s r e l i e f behind the t i p . Thus, i f the measured pore pressures are to be used f o r l i q u e f a c t i o n 132 assessment i n f i n e grained s o i l s a c o n s i s t a n t l o c a t i o n f o r the pore pressure element must be e s t a b l i s h e d . Because the face of a cone t i p i s i n a zone of maximum compression, pore pressures w i l l always tend to p o s i t i v e . Thus, a pore pressure element l o c a t e d on the face of a cone t i p may not p r o v i d e a v e r y s e n s i t i v e measure o f the s o i l s volume change c h a r a c t e r i s t i c s under shear. I t i s considered by the w r i t e r that the pore pressure element l o c a t e d immediately behind the t i p w i l l provide a b e t t e r l o c a t i o n to measure the pore pressure during p e n e t r a t i o n . Chapter 7 w i l l present f i e l d and l a b o r a t o r y data to evaluate the proposed c o r r e l a t i o n s given i n the above s e c t i o n s . 133 5. PRESSUREMETER TESTING 5.1. Equipment The pressuremeter t e s t i n g i n t h i s study was performed w i t h the a i d of S i t u Technology Inc., Vancouver, B.C., using a s e l f - b o r i n g pressuremeter developed by Dr. J.M.O. Hughes. E a r l y development and research on s e l f -b oring pressuremeters was c a r r i e d out by Dr. Hughes at Cambridge U n i v e r s i t y , England, i n the e a r l y 1970's. Since that time, Dr. Hughes has continued the development of the s e l f - b o r i n g pressuremeter. The s e l f -b o r ing pressuremeter used i n t h i s study i s used e x c l u s i v e l y by S i t u Technology Inc. i n North America. A schematic of the instrument was shown i n F i g . 2.3. The Hughes s e l f - b o r i n g pressuremeter has a l e n g t h to diameter r a t i o of 6. The o u t s i d e diameter of the probe i s 75 mm. A Chinese l a n t e r n type metal sheath i s placed over the rubber membrane f o r p r o t e c t i o n during t e s t s i n cohesionless s o i l s . Two pore pressure c e l l s are l o c a t e d 90 degrees from the two displacement transducers. The gas l i n e a l s o contains the m u l t i p l e cable from the instrument to the output u n i t a t the surface. The se p a r a t i o n o f the cable and gas l i n e i s achieved by a g a s - t i g h t union. 5.2. I n s t a l l a t i o n The s e l f - b o r i n g instrument was attached to the f i r s t l e n g t h of c u t t e r d r i v e rod and casing and the gas l i n e taped to the casing a t about 20 cm i n t e r v a l s . The c a s i n g and c u t t e r d r i v e rods were then attached to the top d r i v e u n i t of the d r i l l r i g v i a a small t h r u s t bearing and spacer. The t h r u s t bearing enables the inner c u t t e r rods to r o t a t e w h i l e the outer c a s i n g and instrument are pushed i n t o the s o i l without r o t a t i o n . The 134 FIG. 5.1. ILLUSTRATION OF SELF-BORING PRESSUREMETER INSTALLATION AT McDONALD*S FARM. 135 spacer c o n t r o l s the d i s t a n c e between the l e a d i n g edge of the c u t t i n g shoe and the r o t a t i n g c u t t e r l o c a t e d i n s i d e the c u t t i n g shoe. The choice of r o t a t i n g c u t t e r type and s i z e of spacer depend on ground c o n d i t i o n s , i . e . s o i l s t i f f n e s s and g r a i n s i z e . A s w i v e l T-connector i s l o c a t e d below the t h r u s t bearing to discharge the d r i l l mud r e t u r n . A photograph of the s e l f - b o r i n g instrument being i n s t a l l e d i s shown i n F i g . 5.1. The s e l f -b o r i n g technique was g e n e r a l l y s t a r t e d at ground surface w i t h no s p e c i a l guide holes provided. The pushing f o r c e and mud pressure were c o n t r o l l e d by the d r i l l e r . The speed of the r o t a t i n g c u t t e r was g e n e r a l l y kept constant. A s m a l l flow o f d r i l l mud i s allowed to e x i t behind the instrument to support the open hole formed by the instrument. The d r i l l e r observes the mud r e t u r n from both the r e t u r n pipe ( i . e . up the centre of the casing) and the borehole. A steady r e t u r n should take place from both sources, w i t h most flow o c c u r r i n g a t the r e t u r n pipe. I f the r a t e of r e t u r n of d r i l l m u d from the r e t u r n pipe suddenly decreases and the r a t e of r e t u r n from the borehole i n c r e a s e s , the mud has escaped around the l e a d i n g edge of the c u t t i n g shoe and broken the s e a l around the instrument. I f t h i s should occur the p e n e t r a t i o n i s stopped and a s e a l r e - e s t a b l i s h e d . P e n e t r a t i o n should be at a constant r a t e , which req u i r e s v a r y i n g the pushing f o r c e to s u i t the v a r y i n g ground c o n d i t i o n s . The r a t e of p e n e t r a t i o n depends on the ground c o n d i t i o n s and the pushing c a p a c i t y of the d r i l l r i g . Ghionna et a l . (1981) suggested a maximum rate of p e n e t r a t i o n not g r e a t e r than 2 cm/min i n c l a y s to minimise s o i l d i sturbance. The t e s t i n g performed during t h i s study showed th a t a r a t e of p e n e t r a t i o n of about 2 to 4 cm/min was adequate i n sands. However, t h i s r a t e tended to be c o n t r o l l e d by the pushing c a p a c i t y o f the d r i l l r i g and the g r a i n s i z e and d e n s i t y of the sand. 136 5.3. Factors A f f e c t i n g Results from Pressuremeter T e s t i n g The recent development of s e l f - b o r i n g pressuremeters has generated c o n s i d e r a b l e i n t e r e s t around the world. Numerous research s t u d i e s have been c a r r i e d out i n t o the i n s t r u m e n t a t i o n , i n s t a l l a t i o n and t e s t i n g procedures of v a r i o u s s e l f - b o r i n g pressuremeter models (Ghionna et a l . , 1981, C l a r k e , 1981, FHWA TS-80-209, 1980, Stenssy, 1980, Denby, 1978). The s e l f - b o r i n g pressuremeters were developed simultaneously i n France (Je z e q u e l , Le Mehaute and Le Mec, 1970) and i n England (Wroth and Hughes, 1972), and most s e l f - b o r i n g pressuremeters f o l l o w the b a s i c French or E n g l i s h designs. The main d i f f e r e n c e s between the French and E n g l i s h d evices are summarized i n Table 5.1. The Hughes s e l f - b o r i n g pressuremeter i s a f u r t h e r development of the E n g l i s h design. Before a n a l y z i n g any pressuremeter data, i t i s important to r e a l i z e and account f o r the p o t e n t i a l e r r o r s that each element of data may c o n t a i n . Because of the wide v a r i e t y of designs i n i n s t r u m e n t a t i o n i t i s not p o s s i b l e w i t h i n the scope of t h i s r esearch to d i s c u s s i n d e t a i l a l l the f a c t o r s that a f f e c t the measured r e s u l t s from s e l f - b o r i n g pressuremeter t e s t i n g . However, s e v e r a l important observations have been made during the course of t h i s r esearch that should be discussed. 5.3.1. I n s t a l l a t i o n The s e l f - b o r i n g refinement of the pressuremeter represents a s i g n i f i c a n t step forward i n measuring the undisturbed p r o p e r t i e s of s o i l . However, unless the d r i l l i n g technique i s used j u d i c i o u s l y and c o n s i s t e n t l y the r e a l advantages may be l o s t through d i s t u r b e d and i n c o n s i s t e n t t e s t 137 Table 5.1. Comparison of French and E n g l i s h S e l f - B o r i n g Pressuremeters C h a r a c t e r i s t i c French E n g l i s h I n f l a t i o n f l u i d Water Nitrogen Membrane measurement Increase i n volume R a d i a l displacement Membrane displacement monitor Flowmeter Feeler arms and e l e c -t r o n i c recorder C o n s t r u c t i o n Modular S i n g l e u n i t Cutter d r i v e Hydraulic mounted on probe H y d r a u l i c from surface Type of t e s t Mainly s t r a i n con-t r o l l e d Stress c o n t r o l l e d easy Mainly s t r e s s c o n t r o l l e d Recently developed more complicated s t r a i n c o n t r o l l e d E f f e c t of leak Termination of t e s t Test may be continued Temperature e f f e c t s Requires s p e c i a l precautions E l e c t r o n i c equipment may be s e n s i t i v e Pore pressure measurement Hydraulic E l e c t r o n i c Cutter geometry Not adjustable during i n s e r t i o n Adjustable during i n s e r t i o n 138 r e s u l t s . The w r i t e r was f o r t u n a t e during t h i s study to have the s u p e r v i s i o n and advise of the experienced s t a f f of S i t u Technology Inc. (STI) during the i n s t a l l a t i o n and t e s t i n g . However, the i n s t a l l a t i o n was performed predominantly by the d r i l l e r , under the c l o s e s u p e r v i s i o n of STI s t a f f and the w r i t e r . There are many aspects that c o n t r o l the q u a l i t y of the i n s t a l l a t i o n procedure, some of which w i l l be discussed i n the f o l l o w i n g s e c t i o n s . 5.3.1.1. Water pressure c o n t r o l The p o s s i b i l i t y of h y d r a u l i c a l l y f r a c t u r i n g the s o i l w i t h excessive water pressures was pointed out by Windle (1977). This c o n d i t i o n may be brought on by clogging of the area behind the c u t t e r . With the r e t u r n passage thereby c o n s t r i c t e d , the water pressure i s g e n e r a l l y increased at the c u t t i n g head to maintain f l o w . This may l e a d to h y d r a u l i c f r a c t u r i n g i n c l a y or a blowout i n sand. While the pressure a t which t h i s phenomena w i l l occur may be t h e o r e t i c a l l y c a l c u l a t e d , i t i s d i f f i c u l t to estimate i n the f i e l d e x a c t l y what pressure l o s s i s o c c u r r i n g i n the c u t t e r rods and e x a c t l y when f r a c t u r e or blowout i s induced. The s i t u a t i o n may be avoided by c a r e f u l l y monitoring the water pressure r e q u i r e d to produce a c e r t a i n f l o w . Any decrease i n flow w i t h i n c r e a s e i n pressure s i g n i f i e s a blockage. This procedure r e q u i r e s operator experience and v i g i l a n c e . 5.3.1.2. Cutter geometry The term " c u t t e r geometry" i s used to d e f i n e the c o n f i g u r a t i o n of the c u t t e r r e l a t i v e to the l e a d i n g edge of the c u t t i n g shoe and has an e f f e c t on the amount of disturbance produced during d r i l l i n g . Recent work a t Cambridge ( C l a r k e , 1981) has i n v e s t i g a t e d the i n f l u e n c e 139 of the c u t t e r p o s i t i o n on the amount of disturbance caused during i n s t a l l a t i o n . The problem i s best described i n q u a l i t a t i v e terms by means of F i g . 5.2. F i g . 5.2(a) i s a schematic s e c t i o n through a t h i c k - w a l l e d c y l i n d r i c a l instrument, w i t h sharp c u t t i n g edge, at a stage during i n s e r t i o n i n t o the ground at a steady pushing f o r c e . S o i l w i l l enter the instrument u n t i l a plug i s formed, so that i n e f f e c t i t becomes a closed-ended c y l i n d e r . The i n f l u e n c e on s o i l elements ahead of the c u t t i n g shoe i s an increase i n s t r e s s l e v e l , w i t h movements r a d i a l l y outwards and downwards as shown i n F i g . 5.2(a). If an i n t e r n a l c u t t i n g device i s introduced i n t o the instrument r i g h t a t the bottom, F i g . 5.2(b), so that i t d r i l l s a hole as the instrument advances, the o v e r a l l e f f e c t on s o i l elements i s a r e d u c t i o n i n s t r e s s l e v e l , w i t h movements r a d i a l l y inwards and upwards. By v a r y i n g the p o s i t i o n of the c u t t e r r e l a t i v e to the c u t t i n g edge, F i g . 5.2(c), i t i s p o s s i b l e that a balance can be made between the two opposing e f f e c t s r e s u l t i n g i n minimal disturbance. The optimum c u t t e r geometry w i l l vary w i t h s o i l s t i f f n e s s and i n - s i t u s t r e s s c o n d i t i o n s . These f a c t o r s may vary c o n t i n u o u s l y i n a s o i l p r o f i l e , e s p e c i a l l y i n interbedded d e l t a i c d e p o s i t s . Thus, i t i s d i f f i c u l t to e s t a b l i s h a standard procedure and c u t t e r geometry t h a t w i l l ensure minimal disturbance i n a l l ground c o n d i t i o n s . Considerable s k i l l and experience i s thus r e q u i r e d by the operator to minimise disturbance during i n s t a l l a t i o n . I t i s important when t e s t i n g i n a v a r y i n g s o i l p r o f i l e to have the f a c i l i t y to vary the c u t t e r geometry without complete removal of the instrument. The s e l f - b o r i n g equipment used i n t h i s study had t h i s f a c i l i t y by varying the s i z e of the spacer above the t h r u s t bearing. 140 i FIG. 5.2. SCHEMATIC TO SHOW EFFECTS OF CUTTER POSITION ON DEGREE OF DISTURBANCE. (Adapted from Wroth, 1982) 141 5.3.1.3. I n s e r t i o n Rate If the i n s e r t i o n r a t e i s too f a s t the c u t t e r and mud f l u s h cannot remove a l l the s o i l and a plug i s formed. The a b i l i t y of the c u t t e r and mud f l u s h to remove s o i l i s dependent on: the s o i l s t i f f n e s s and g r a i n s i z e , the rate and consistency of mud fl o w , the s i z e of the instrument and c u t t e r rods, the r a t e of i n s e r t i o n and the r o t a t i o n r a t e of the c u t t e r . When i n s e r t i n g the s e l f - b o r i n g instrument i n t o coarse sand there i s o f t e n a problem removing the s o i l from the c u t t e r area. The gap between the instrument and r o t a t i n g c u t t e r rods i s very s m a l l and l a r g e g r a i n s are e a s i l y jammed. An undesirable s i t u a t i o n can also a r i s e i f the i n s e r t i o n r a t e i s too slow r e l a t i v e to the c u t t e r speed when i n s e r t i n g i n t o c l a y . In t h i s event, a s l u r r y can be formed a t the c u t t e r head, and blockages can occur. Again, considerable s k i l l and experience i s re q u i r e d by the operator to c o n t r o l the i n s e r t i o n r a t e , c u t t e r speed and mud flow to minimise s o i l d isturbance. When i n s t a l l i n g the instrument i n t o dense sands, the f r i c t i o n a l forces developed along the sides of the instrument can become very l a r g e , e s p e c i a l l y a t depth. These l a r g e f o r c e s r e q u i r e c o n s i d e r a b l e pushing r e a c t i o n from the d r i l l r i g to ensure a steady r a t e of p e n e t r a t i o n . Many sm a l l e r t o p - d r i v e d r i l l r i g s are unable to provide s u f f i c i e n t r e a c t i o n from t h e i r dead weight alone. This may lead to a c y c l i c f o r c e a p p l i c a t i o n as the d r i l l e r attempts to push and l i f t s h i s r i g i n t o the a i r . This c y c l i c l o a d i n g a p p l i e d to help advance the instrument can cause s i g n i f i c a n t disturbance i n the form of v i b r a t i o n s i n the surrounding sand adjacent to the probe. The r e a c t i o n o f smaller d r i l l r i g s can be improved by pr o v i d i n g 142 ground anchors, such as augers, and hol d i n g the r i g down to avoid the c y c l i c movement of the pushing head. The requirement f o r adequate r e a c t i o n f o r c e to provide a steady p e n e t r a t i o n r a t e i s extremely important when t e s t i n g i n sands. 5.3.1.4. V e r t i c a l i t y As f a r as the w r i t e r i s aware no s e l f - b o r i n g instrument incorporates a slope sensor. This i s unfortunate, s i n c e i t i s almost impossible to i n s t a l l any instrument i n t o the ground without some n o n - v e r t i c a l i t y , e s p e c i a l l y f o r deep holes. The a f f e c t of n o n - v e r t i c a l i t y can be important w i t h regard to i n t e r p r e t a t i o n of i n - s i t u s t r e s s e s . A simple slope sensor s i m i l a r to those i n c o r p o r a t e d i n t o many cone pen e t r a t i o n devices could a l s o be included i n t o s e l f - b o r i n g instruments. Many d r i l l e r s do not pay enough a t t e n t i o n t o the v e r t i c a l i t y o f t h e i r d r i l l mast before i n s e r t i o n of s e l f - b o r i n g d e v i c e s . C a r e f u l a t t e n t i o n to the i n i t i a l v e r t i c a l i t y would s i g n i f i c a n t l y improve the o v e r a l l v e r t i c a l i t y of the instrument during i n s t a l l a t i o n , e s p e c i a l l y f o r r e l a t i v e l y deep holes. 5.3.2. Testing Procedures W i t h the i n c r e a s e d use of p r e s s u r e m e t e r s t h e r e i s need f o r s t a n d a r d i z a t i o n o f t e s t p r o c e d u r e s . In a t t e m p t i n g to d e v e l o p a standardized t e s t procedure, i t i s important to d e f i n e the p o s s i b l e e r r o r s involved with pressuremeter expansion t e s t i n g . 5.3.2.1. Temperature e f f e c t s Two e f f e c t s are p o s s i b l e from temperature v a r i a t i o n on the e l e c t r o n i c 143 output. F i r s t , s m a l l zero s h i f t s may occur i n the t o t a l s t r e s s , s t r a i n and pore pressure outputs. The zero s h i f t i s not important f o r the t o t a l s t r e s s and s t r a i n output as a r e l a t i v e value i s recorded during the t e s t , i . e . , the zero i s r e s e t before the beginning of each t e s t . However, a zero s h i f t i n the pore pressure output i s p o t e n t i a l l y more of a problem. The i n i t i a l value of t h i s parameter i s needed as w e l l as i t s r e l a t i v e excess value d u r i n g shear. F o r t u n a t e l y , the i n i t i a l pore pressure value i s o f t e n known before pressuremeter t e s t i n g because of previous s i t e i n v e s t i g a t i o n s . However, i t would be of great value i f the pore pressure could be continuously monitored, w i t h confidence, to assess the amount of disturbance generated around the probe, e s p e c i a l l y f o r t e s t i n g i n s o i l s with a low p e r m e a b i l i t y . A temperature sensing element such as a t h e r m i s t e r or miniature i n t e g r a t e d c i r c u i t could be incorporated i n t o the instrument and could provide the b a s i s f o r c a l i b r a t e d zero s h i f t c o r r e c t i o n s due to temperature. This would be p a r t i c u l a r l y u s e f u l i n northern environments. A second p o s s i b l e problem w i t h temperature e f f e c t s i s that the c a l i b r a t i o n constants may be a f f e c t e d . This problem can be reduced by using transducers and s t r a i n guages that are i n s e n s i t i v e to temperature v a r i a t i o n s . Temperature changes can a l s o be minimized by maintaining the gas l i n e at a constant temperature. 5.3.2.2. Kate of Expansion In developing a standardized t e s t procedure the r a t e of expansion and method of data c o l l e c t i o n i s p a r t i c u l a r l y important. The t e s t i n g performed during t h i s study was c a r r i e d out i n a s t r e s s c o n t r o l l e d manner. The pressure was a p p l i e d i n increments at r e g u l a r i n t e r v a l s . The r e g u l a t o r , 144 however, r e q u i r e s small adjustments to maintain the re q u i r e d pressure. Readings were made approximately every 30 seconds depending on s o i l c o n d i t i o n s and t e s t type. Results were g e n e r a l l y recorded by hand. This u s u a l l y r e q u i r e d two operators, one to adjust the pressure r e g u l a t o r and p l o t the data i n approximate form and the other to record the data onto data sheets. Whenever p o s s i b l e t e s t s were c a r r i e d out using X-Y-Y reco r d e r s . The t e s t procedure was consider a b l y e a s i e r and gave b e t t e r on s i t e e v a l u a t i o n of the t e s t when using X-Y-Y reco r d e r s . The recorders were g e n e r a l l y set up to record r a d i a l s t r a i n , from both arms, against t o t a l s t r e s s and pore pressure, from both pore pressure c e l l s , a g ainst t o t a l s t r e s s . The use of X-Y-Y recorders s i g n i f i c a n t l y improved the e v a l u a t i o n of the l i f t - o f f pressures and the r e t u r n pressure r e q u i r e d to completely d e f l a t e the membrane. The r a t e of expansion can be very important f o r some s o i l s . In f i n e g r a i n e d s o i l s where undrained s o i l parameters are re q u i r e d , the choice of the r a t e of t e s t i n g has to be a compromise between ac h i e v i n g undrained c o n d i t i o n s and i n t r o d u c i n g r a t e e f f e c t s . When performing s t r e s s c o n t r o l l e d t e s t s i n these s o i l s , i t i s not always c l e a r when to take readings. This problem can be overcome by performing s t r a i n c o n t r o l l e d t e s t s . A leakage In the membrane can a l s o c r e a t e problems f o r s t r e s s c o n t r o l l e d t e s t s i f the leakage i s f a s t e r than the steady supply from the r e g u l a t o r s . With the advent of microprocessors and computer c o n t r o l l e d equipment, i t would be p r e f e r a b l e to perform s t r a i n c o n t r o l l e d t e s t s w i t h automatic data l o g g i n g . Some form of v i s u a l d i s p l a y , such as X-Y-Y r e c o r d e r s , would be a u s e f u l part of t e s t e v a l u a t i o n i n the f i e l d , e s p e c i a l l y f o r unloading-145 reloading c y c l e s . However, s o p h i s t i c a t e d e l e c t r o n i c equipment r e q u i r e s c a r e f u l design f o r f i e l d use and a high l e v e l of e x p e r t i s e from the operator. There i s an urgent need f o r more s t a n d a r d i z a t i o n i n t e s t procedures and data c o l l e c t i o n . This w i l l become more apparent when d i s c u s s i n g i n t e r p r e t a t i o n i n subsequent s e c t i o n s . 5.3.3. Piezometer S a t u r a t i o n Because the pore pressure elements are f l u s h w i t h the membrane surface they are not e a s i l y saturated. The procedure f o r d e - a i r i n g the pore pressure c e l l s has changed l i t t l e from the standard recommended by Wroth and Hughes (19 73). The procedure i n v o l v e s l a y i n g the intrument on i t s s i d e w i t h one pore pressure element f a c i n g upwards. The c a v i t y behind the porous d i s c i s f i l l e d w i t h water, using a s y r i n g e , u n t i l water flows out through the d i s c . The syringe i s placed i n t o the c a v i t y v i a a hole l o c a t e d i n the centre of the porous d i s c . The instrument i s turned around and the process repeated f o r the other pore pressure element. The instrument i s then lowered or s e l f - b o r e d i n t o the ground as soon as p o s s i b l e . The procedure i s p r a c t i c a l i n the f i e l d but by no means f o o l p r o o f . D e - a i r i n g the second c e l l i n v o l v e s t u r n i n g the f i r s t c e l l upside down, p o s s i b l y a l l o w i n g water to escape. In a d d i t i o n water w i l l t r y to flow out during the time period before submerging below groundwater l e v e l . Denby (1978) used a pat of mud smeared over the c e l l s i n an e f f o r t to reduce t h i s problem. Another p o s s i b l e a l t e r n a t i v e i s the use of g l y c e r i n instead of water. G l y c e r i n has been shown to work e f f e c t i v e l y as a s a t u r a t i n g agent f o r cone p e n e t r a t i o n t e s t i n g (Campanella and Robertson, 1981). G l y c e r i n has the advantage i n that i t i s m i s c i b l e w i t h water yet develops a high a i r 146 entry t e n s i o n to prevent l o s s of s a t u r a t i o n during i n s t a l l a t i o n . The push-in cone-pressuremeter holes performed during t h i s study used g l y c e r i n , i n s t e a d of water, as the s a t u r a t i n g agent. Bet t e r r e s u l t s were obtained using t h i s technique. When using water i t was found that the pore pressure c e l l s d i d not behave w e l l u n t i l the instrument was about 4 m below the ground water l e v e l . I t appears that some small a i r bubbles that were entrapped i n the element went i n t o s o l u t i o n under a water pressure equivalent to about 4 m of water. 5.3.4. Comments on Equipment S i g n i f i c a n t developments i n pressuremeter t e s t i n g have taken place over the l a s t 1 0 y e a r s , p a r t i c u l a r l y i n the area of measurement techniques. With the development of accurate and r e l i a b l e e l e c t r i c a l transducers and s t r a i n guages placed w i t h i n the pressuremeter i t has been p o s s i b l e to improve the accuracy of these measurements. With the advancement of micro e l e c t r o n i c s i t has a l s o become p o s s i b l e to measure many parameters at one time without a l a r g e number of e l e c t r i c a l w i r e s . However, s i g n i f i c a n t improvements are s t i l l required i n pressuremeter i n s t r u m e n t a t i o n and data c o l l e c t i o n . One area of improvement required f o r most s e l f - b o r i n g pressuremeters, and i n p a r t i c u l a r the pressuremeter used i n t h i s study, i s the number of f e e l e r arms (displacement transducers). The instrument used i n t h i s study had two f e e l e r arms. In almost a l l t e s t s performed, the two arms d i d not l i f t - o f f at the same pressure. The pressure expansion curve used f o r i n t e r p r e t a t i o n was based on an average of the two arms. The average was c a l c u l a t e d based on the s t r a i n measurement, s i n c e the t e s t s were s t r e s s 147 c o n t r o l l e d . The problem w i t h using only two arms i s that i t i s not always p o s s i b l e to e s t a b l i s h the cause f o r the d i f f e r e n t amounts of movement. The membrane may appear to expand i n a n o n - c y l i n d r i c a l manner f o r s e v e r a l reasons, i ) n a t u r a l a n i s o t r o p i c s t r e s s c o n d i t i o n s of the s o i l , i i ) o b s t r u c t i o n of one arm, i n s i d e or o utside the instrument, i i i ) f a i l u r e of one arm, i v ) n o n - v e r t i c a l i t y of instrument. S i g n i f i c a n t improvement i n the equipment can be made by i n c o r p o r a t i n g three f e e l e r arms arranged at 120 degrees. This would provide b e t t e r system r e l i a b i l i t y . Recent work i n B r i t a i n (Dalton and Hawkins, 1982) suggests that s i x s t r a i n sensing arms e q u a l l y spaced at 60 degrees would provide adequate redundancy and two simultaneous measurements of the i n -s i t u s t r e s s f i e l d . Several researchers (Denby, 1978, Ghionna et a l . , 1981) have observed an i n i t i a l movement of the f e e l e r arms i n E n g l i s h (Cambridge) type s e l f -b o r i n g pressuremeters, i . e . when the pressure i s i n c r e a s i n g but below the i n - s i t u s t r e s s the f e e l e r arms i n d i c a t e a steady outward movement. This phenomenon may be a t t r i b u t e d to equipment performance (Dalton and Hawkins, 1982). The design of the f e e l e r arms was such t h a t when a pressure was a p p l i e d they tended to r o t a t e and i n d i c a t e an apparent displacement. This problem was c o r r e c t e d i n the Hughes s e l f - b o r i n g pressuremeter used i n t h i s study, by designing equal area displacement arms. 5.4. I n t e r p r e t a t i o n The advantages of the pressuremeter t e s t are a l l too apparent. The t e s t simulates a plane s t r a i n c y l i n d r i c a l c a v i t y expansion which has w e l l 148 defined e l a s t i c and e l a s t i c - p l a s t i c s o l u t i o n s . This unique c h a r a c t e r i s t i c b r i n g s the p o t e n t i a l f o r d e r i v a t i o n of the t r u e s t r e s s s t r a i n c h a r a c t e r i s t i c s of s o i l one step c l o s e r . An a d d i t i o n a l important aspect of the t e s t i s the opportunity to measure the i n - s i t u h o r i z o n t a l s t r e s s i n the ground. The i n - s i t u measurements of the s t r e s s s t r a i n c h a r a c t e r i s t i c s together wi t h the value of the i n - s i t u s t r e s s e s are e s s e n t i a l i f deformation analyses are to be c a r r i e d out using r e a l i s t i c n o n - l i n e a r s o i l models. The b a s i c disadvantage of the t e s t i s that i t measures the deformation c h a r a c t e r i s t i c of the s o i l i n the h o r i z o n t a l , r a t h e r than the more rel e v a n t v e r t i c a l d i r e c t i o n . S o i l i s i n h e r e n t l y a n i s o t r o p i c w i t h the degree of a n i s o t r o p y tending to decrease w i t h i n c r e a s i n g p l a s t i c i t y (Bjerrum, 1973). There are a l s o problems when making c o r r e l a t i o n s between pressuremeter data and standard l a b o r a t o r y t e s t d a t a , such as t r i a x i a l t e s t data due to the f o l l o w i n g f a c t o r s : 1) f a i l u r e mode, 2) r o t a t i o n of p r i n c i p l e s t r e s s e s , 3) s t r a i n r a t e . The pressuremeter more c l o s e l y resembles the simple shear t e s t , w i t h an i n f i n i t e number of shear planes surrounding the probe. The r o t a t i o n of p r i n c i p l e s t r e s s e s i n the pressuremeter t e s t may i n f l u e n c e the measured s t r e s s s t r a i n c h a r a c t e r i s t i c s when compared to a s i t u a t i o n where no r o t a t i o n of p r i n c i p l e s t r e s s e s takes place. T y p i c a l pressuremeter t e s t s i n f i n e grained s o i l s may be c a r r i e d out up to 100 times f a s t e r than those used i n c o n v e n t i o n a l t r i a x i a l t e s t s . At any stage of a pressuremeter t e s t , each annulus of s o i l i s experiencing a d i f f e r e n t s t r a i n r a t e . The i n t e r p r e t a t i o n of the t e s t n e c e s s a r i l y assumes s t r e s s s t r a i n 149 c h a r a c t e r i s t i c s independent of s t r a i n r a t e e f f e c t s . A problem a s s o c i a t e d w i t h the i n t e r p r e t a t i o n o f s e l f - b o r i n g pressuremeter data i s the l a c k of a c o n s i s t e n t and g e n e r a l l y recognized c r i t e r i a f o r assessment of the q u a l i t y of the pressure expansion curves. Based on the w r i t e r s experience obtained during t h i s study a pressure expansion curve may be judged as "good" i f i t meets the f o l l o w i n g requirements: i ) there i s no i n f l e c t i o n point near the beginning of the curve, i i ) a l l f e e l e r arms move i n a c o n s i s t e n t manner, i . e . one arm does not appear to l i f t - o f f or move s i g n i f i c a n t l y d i f f e r e n t from the other arm(s). i i i ) the pore pressure measured before the pressure expansion t e s t i s c l o s e to the i n - s i t u water pressure. However, s i g n i f i c a n t l y more documented experience i s required to ensure c o n s i s t e n t c r i t e r i a f o r assessment of good q u a l i t y pressure expansion data. Examples of "good" and "poor" pressure expansion curves, obtained during t h i s study, are shown i n F i g . 5.3. The i n t e r p r e t a t i o n methods reviewed i n t h i s study are p r i n c i p a l l y those r e l a t e d to drained pressuremeter t e s t s i n granular m a t e r i a l s . The kinematics of c a v i t y expansion theory i s given i n Appendix 2. 5.4.1. I n - s i t u Stress The p o s s i b i l i t y of e v a l u a t i n g the i n - s i t u h o r i z o n t a l s t r e s s represents one of the most i n t e r e s t i n g features of the s e l f - b o r i n g pressuremeter t e s t . A d e t a i l e d d i s c u s s i o n concerning i n - s i t u h o r i z o n t a l s t r e s s i s beyond the scope of t h i s study. However, s e v e r a l s i g n i f i c a n t p o ints w i l l be IG. 5.3. EXAMPLE: OF "GOOD" AND "POOR" SELF-EORING PRESSUREMETER TEST RESULTS. 151 discussed. In order to assess the r e l i a b i l i t y of the i n - s i t u h o r i z o n t a l s t r e s s , a, , e s t i m a t e d from s e l f - b o r i n g pressuremeter t e s t s , i t i s necessary to ho d e t e r m i n e t h e m a g n i t u d e o f a, by means of o t h e r i n - s i t u and/or ho l a b o r a t o r y t e s t s ; these r e s u l t s are a l s o subject to considerable u n c e r t a i n t i e s . Hence the c a p a b i l i t y of the s e l f - b o r i n g pressuremeter t e s t to p r o v i d e a c o r r e c t measurement of o L i s assessed i n comparison to the ho best estimate of o, obtained from other t e s t s . ho Se v e r a l s t u d i e s (Denby, 1978, Chionna et a l . 1981) have shown that r e a s o n a b l y good e s t i m a t e s o f a can be obtained from good s e l f - b o r i n g pressuremeter t e s t s i n s o f t , normally to l i g h t l y overconsolidated c l a y d e p o s i t s . However, f o r t e s t s i n c l a y d e p o s i t s a r e l a x a t i o n time i s r e q u i r e d to allow a l l excess pore pressures generated during i n s t a l l a t i o n t o d i s s i p a t e . This r e l a x a t i o n time may take s e v e r a l days f o r very low p e r m e a b i l i t y d e p o s i t s . There appears to be very l i t t l e published data concerning the success of the s e l f - b o r i n g pressuremeter i n e s t i m a t i n g a i n sands. This may, i n p a r t , be due to the extreme d i f f i c u l t y of o b t a i n i n g other estimates of a, f o r sands, ho Hughes (1973) showed t h a t , under c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s , the r a d i a l displacement of the s o i l immediately adjacent t o the s u r f a c e of the probe i n sand was l e s s than 0.5% of the r a d i u s of the probe i n an outward d i r e c t i o n . Thus, under c o n t r o l l e d c o n d i t i o n s , the e r r o r i n the c i r c u m f e r e n t i a l s t r a i n , e Q , a x i s i s about 0.5%. This i s approximately equivalent to 1% shear s t r a i n ( y ) • Therefore, f o r s t i f f s o i l s , i . e . s o i l s w i t h a s t e e p i n i t i a l s t r e s s s t r a i n c u r v e , the e r r o r i n estimating a from s e l f - b o r i n g pressuremeter t e s t s i s l i k e l y to be s i g n i f i c a n t . Most 152 sands ar e co n s i d e r a b l y s t i f f e r than normally c o n s o l i d a t e d c l a y s . Thus, the w r i t e r c o n s i d e r s the e s t i m a t i o n o f a, i n sands u s i n g a s e l f - b o r i n g ho pressuremeter i s l i k e l y to be i n e r r o r even w i t h "minimal" d i s t u r b a n c e . The s i z e of the e r r o r w i l l depend on the d e n s i t y of the deposit and the degree of disturbance generated during i n s t a l l a t i o n . I f the disturbance i s always i n an outward d i r e c t i o n the e s t i m a t e of a w i l l always be too high, as shown i n F i g . 5.4. The measurement of i n - s i t u h o r i z o n t a l s t r e s s e s i n sands using a s e l f -b oring pressuremeter would appear to be h i g h l y s e n s i t i v e to i n i t i a l d i sturbance. 5.4.2. Drained Shear Strength of Sand T h e o r e t i c a l methods f o r determination of the f r i c t i o n angle of sands from pressuremeter t e s t data have been proposed by s e v e r a l authors, i . e . Gibson et a l . (1961), Ladanyi (1963), Vesic (1972) and Hughes et a l . (1977). Each method r e l i e s on a model f o r the sand behavior. A l l the above methods consider that sand has a constant f r i c t i o n angle at f a i l u r e . However, not a l l methods a l l o w f o r the f a c t that sand changes i n volume during shearing. In Ladanyi's method the volume change i s considered to be constant a t the point the f a i l u r e s t r e s s r a t i o i s reached. This volume change i s introduced i n t o the assessment of the f r i c t i o n angle by a t r i a l and e r r o r method. Vesic's s o l u t i o n uses the r e s u l t s of l a b o r a t o r y t e s t s d i r e c t l y . However, the problem of determining the appropriate l a b o r a t o r y d e n s i t y to perform t e s t s a t , i s not easy to r e s o l v e . A l s o , the l a b o r a t o r y t e s t s may not produce r e l i a b l e volume change behavior because the i n - s i t u s t r u c t u r e and f a b r i c cannot be reproduced i n the l a b o r a t o r y . 153 31 8 Prelum m&far Curve P0 lift'off prt-wrc O I RADIAL DI5PLACEMEMT % 0.5% Initial disturbance FIG. 5.4. EXAMPLE OF POTENTIAL ERROR IN ASSESSMENT OF IN-SITU STRESS DUE TO INITIAL DISTURBANCES IN SAND. 154 The s o l u t i o n developed by Hughes et a l . (1977) r e l i e s on the f a c t that the volume changes are oc c u r r i n g during the expansion of the c a v i t y and the amount of volume change ( d i l a t i o n ) i s c l o s e l y r e l a t e d to the current d i l a t a n c y concept of Rowe (1962) and the observed behavior of sand i n simple shear as observed by Vaid et a l . (1981) and Stroud (1971). Consider the r e s u l t s of simple shear t e s t s conducted by Shroud (1971) shown i n F i g . 5.5.(a) and (b) . Several important f e a t u r e s can be observed from such r e s u l t s . When sand reaches f a i l u r e , a t peak p r i n c i p a l s t r e s s r a t i o , the volume change i s approximately l i n e a r w i t h shear s t r a i n and the r a t e of volume change i s a t a maximum. The volu m e t r i c s t r a i n s are i n c r e a s i n g even though the sand i s f a i l i n g a t a constant s t r e s s r a t i o . For loose sand the peak s t r e n g t h i s not reached u n t i l shear s t r a i n l e v e l s of between 20 to 30 percent. Before f a i l u r e the volume change behaviour i s no n - l i n e a r , p a r t i c u l a r l y f o r loose sands. Most s e l f - b o r i n g pressuremeter probes expand to a maximum of about 10 percent c i r c u m f e r e n t i a l s t r a i n (£„) 6 ( i . e . approximately 20 percent shear s t r a i n , y). Therefore, i n loose sands, only the elements of s o i l immediately adjacent to the probe reach f a i l u r e . The model proposed by Hughes et a l . (1977) assumes th a t the i d e a l s o i l behaves as shown i n F i g . 5.5(c) and ( d ) . The r a t e of volume change i s p r o p o r t i o n a l to the f r i c t i o n angle, as suggested by Rowe (1962). In the method proposed by Hughes et a l . (1977) i t was shown t h a t : f r i c t i o n angle developed. This approach brings together the s t r e s s l o 8 ^ + f ) -n+1 1-N • log(P-u ) + constant o 155 Results of Simple Shear Tests (After Stroud 1971) FIG. 5.5. STRESS-STRAIN AND VOLUMENTRIC STRAIN-SHEAR STRAIN CURVES FOR (a) SIMPLE SHEAR TEST RESULTS (Stroud, 1971), (b) IDEALIZED BY HUGHES ET AL. (1977). 156 i n i t i a l r a dius of pressuremeter change i n radius of pressuremeter c i r c u m f e r e n t i a l s t r a i n , e_. o i n t e r c e p t shown on F i g . 5.5(c) and (d) t o t a l pressuremeter pressure pore water pressure (1+sin v) , ,, . ,, . .—TT\ * S I N 4> = slope s (1+sin <|>' ) r maximum d i l a t i o n r a t e . In the above method a p l o t of the pressuremeter data i n terms of l o g ( P - u ) ( e f f e c t i v e pressure) against l o g (;r-+-£) w i l l tend towards a O a. t O s t r a i g h t l i n e w i t h a slope s. This slope i s r e l a t e d to the i n - s i t u f r i c t i o n angle (<t>') and the maximum d i l a t i o n r a t e ( s i n e v°). For very dense sands the c o r r e c t i o n term "c" i s n e g l i g i b l e and f o r a l l p r a c t i c a l purposes can be ignored. The r e s u l t s of the l a b o r a t o r y s t u d i e s c o n d u c t e d by J e w e l et a l . (1980) i n very dense sands (D^ = 90%) using the s e l f - b o r i n g pressuremeter probe show that the above technique works very s u c c e s s f u l l y . In loose m a t e r i a l s the method i s not so convenient as the pressuremeter does not expand s u f f i c i e n t l y f o r the sand around the probe to reach the l i n e a r p o r t i o n of i t s volumetric s t r a i n / s h e a r s t r a i n curve, (see F i g . 5.5(b)). I f pressuremeter r e s u l t s , where the maximum c i r c u m f e r e n t i a l s t r a i n i s 10%, are analysed w i t h the assumption that c = 0 the r e s u l t s obtained w i l l be accurate f o r very dense sands, as discussed above, but f o r medium dense and loose sand a r t i f i c i a l l y low d i l a t i o n and f r i c t i o n angles w i l l be where R = o AR R o c = P u = o n+1 1-N s i n v = 157 obtained. To i l l u s t r a t e t h i s p o i n t , consider the behavior of the s o i l surrounding the probe when expanded to maximum c a p a c i t y , i . e . i n the r e g i o n A R of c i r c u m f e r e n t i a l s t r a i n , e = —— = 10 percent (approximately 20 percent o shear s t r a i n , y ) . The shear s t r a i n s i n the s o i l surrounding the probe w i l l f o l l o w approximately the form shown i n F i g . 5.6, i . e . they w i l l be l a r g e near the face of the probe and decrease r a p i d l y w i t h d i s t a n c e away from the probe. The shear s t r a i n one r a d i u s away from the probe w i l l have dropped to about one quarter of the shear s t r a i n at the f a c e . Beyond one r a d i u s the s t r a i n s are probably not s u f f i c i e n t to f a i l medium dense to loo s e s o i l , however i n s i d e t h i s zone the s o i l can be assumed to be f a i l i n g a t constant s t r e s s r a t i o , which i s r e l a t e d to the f r i c t i o n angle (<j>')« The average shear s t r a i n around the probe i n the f a i l e d r e g i o n i s approximately 10 percent, as shown on F i g . 5.6. The volume changes i n the shear s t r a i n range of 5 to 20 percent are n o n - l i n e a r , as shown by the simple shear t e s t data on Ottawa sand conducted by Vaid et a l . (1981), shown on F i g . 5.7. Consider the simple shear r e s u l t s obtained by Vaid e t a l . (1981) on a loose sand a t a r e l a t i v e d e n s i t y of 26.9 percent, as shown on F i g . 5.8. The volume changes around the probe f o l l o w s a path ABCDE. I f the pressuremeter data i s analysed assuming the "c" i n t e r c e p t i s zero then the c a l c u l a t e d d i l a t i o n r a t e w i l l be based on a s t r a i g h t l i n e passing through the o r i g i n A and going through the average shear s t r a i n p o i n t a t C. The d i l a t i o n angle derived by t h i s method w i l l be conside r a b l y smaller than the l a r g e s t r a i n l e v e l d i l a t i o n angle that i s normally a n t i c i p a t e d f o r the sand at that d e n s i t y and s t r e s s l e v e l . For t h i s p a r t i u c l a r sand the a n t i c i p a t e d d i l a t i o n r a t e would probably be based on the s t r a i g h t l i n e s e c t i o n DEF, s i n c e i t i s t h i s 158 O 4 — r — —T ° IB. 2Z Face of Protx, RADIAL DISTANCE. FROM PflOBB (H\ = inifial radius of Probe,) FIG. 5.6. APPROXIMATE VARIATION OF SHEAR STRAIN WITH RADIAL DISTANCE FROM PRESSUREMETER PROBE. 159 12 16 20 24 Sheor S t ra in , 28 32 FIG. 5.7. STRESS-STRAIN BEHAVIOUR OF OTTAWA SAND IN DRAINED SIMPLE SHEAR.. (After Vaid et a l . , 1981). 160 FIG. 5.8. VOLUMETRIC STRAIN-SHEAR STRAIN CURVES FOR LOOSE AND DENSE OTTAWA SAND IN DRAINED SIMPLE SHEAR. (Data from Vaid et a l . , 1981) 161 maximum d i l a t i o n r a t e that r e l a t e s to the peak, f r i c t i o n angle (see F i g . 5.7) . If the data shown i n F i g , 5.7 were a v a i l a b l e f o r any p a r t i c u l a r sand then i t would be p o s s i b l e to c o r r e c t the c a l c u l a t e d d i l a t i o n angle. F i g . 5.9 (a) i s a p l o t of r e l a t i v e d e n s i t y a gainst d i l a t i o n angle f o r Ottawa Sand obtained from the data by Vaid et a l . (1981) presented i n F i g . 5.7. This p l o t shows two l i n e s r epresenting d i f f e r e n t d e f i n i t i o n s of d i l a t i o n . L i n e AB i s the maximum r a t e of d i l a t i o n , i . e . r e l a t e d to the slope of the s t r a i g h t p o r t i o n of the volume shear curve, GH i n F i g . 5.9(b). The maximum d i l a t i o n r a t e i s a s s o c i a t e d w i t h the peak f r i c t i o n angle. Line CD i s the average d i l a t i o n r a t e around the probe ( i . e . the secant I J , i n F i g . 5.9(b)) when the probe i s expanded to a maximum of about 10 percent c i r c u m f e r e n t i a l s t r a i n (e ). o The c a l c u l a t e d d i l a t i o n angles u s i n g the method of Hughes et a l . (1977) are based on a l i n e a r r a t i o between volume change and shear s t r a i n , i . e . L i n e CD. Combining pressuremeter r e s u l t s a t a c i r c u m f e r e n t i a l s t r a i n l e v e l of 10% w i t h the data on Ottawa Sand ( F i g . 5.9) the maximum rate of d i l a t i o n and peak f r i c t i o n angle can be determined. For example, i f the s l o p e o f t h e l o g l o g c u r v e at K 10% i s measured to be 0.34 the c a l c u l a t e d d i l a t i o n angle w i l l be 0° (based on <b = 32°). The correcte d cv maximum d i l a t i o n w i l l be + 6°. Design c h a r t s developed by the w r i t e r f o r c a l c u l a t i n g the peak f r i c t i o n angle and maximum d i l a t i o n r a t e that i n c o r p o r a t e the proposed s t r a i n l e v e l c o r r e c t i o n discussed above are shown i n F i g s . 5.10 and 5.11. To c a r r y out the above analyses a knowledge of the f r i c t i o n angle at constant volume, <b' , i s re q u i r e d . The range of values of d>' f o r most cv cv sands u s u a l l y l i e s i n the range of 30 to 40 degrees. 162 FIG. 5.9., VARIATION OF DILATION ANGLE WITH RELATIVE DENSITY FOR OTTAWA SAND IN DRAINED SIMPLE SHEAR. 163 FIG.. 5.10. PROPOSED CORRELATION BETWEEN SELF-BORING PRESSUREMETER DATA AND PEAK" FRICTION ANGLE, CORRECTED FOR STRAIN LEVEL.. 164 FIG. 5.11. PROPOSED CORRELATION BETWEEN SELF-BORING PRESSUREMETER DATA AND MAXIMUM DILATION ANGLE, CORRECTED FOR STRAIN LEVEL. 165 An estimate of <J>' can be made from the cv W e l l graded g r a v e l - s a n d - s i l t Uniform coarse sand W e l l graded medium sand Uniform medium sand Well graded f i n e sand Uniform f i n e sand Assign lower values f o r w e l l rounded p a r t i c l e s . A s s i g n higher values f o r angular p a r t i c l e s . I f a b e t t e r e s t i m a t e of <)>' i s r e q u i r e d i t should be p o s s i b l e to cv e s t i m a t e <b ( f o r p l a n e s t r a i n c o n d i t i o n s ) from d r a i n e d t r i a x i a l c v compression t e s t s c a r r i e d out on loose samples r e c o n s t i t u t e d from d i s t u r b e d samples. The c a l c u l a t i o n of shear s t r e n g t h and d i l a t i o n angle from s e l f - b o r i n g pressuremeter t e s t r e s u l t s appears to be l e s s s e n s i t i v e to i n i t i a l d i s t u r b a n c e than the measurement of i n - s i t u s t r e s s . This i s mainly because the c a l c u l a t i o n f o r shear strength uses the l a r g e s t r a i n p o r t i o n of the p r e s s u r e m e t e r c u r v e (£„ = 10%), where i n i t i a l minor disturbance i s l i k e l y to have l e s s i n f l u e n c e . S i g n i f i c a n t i n i t i a l d i s t u r b a n c e , on the other hand, can be expected to have an i n f l u e n c e on the c a l c u l a t e d shear s t r e n g t h . The i n f l u e n c e of a l a r g e i n i t i a l disturbance can be shown by performing two pressure expansion t e s t s a t the same l o c a t i o n . F i g . 5.12 f o l l o w i n g t a b l e . <f>' cv 40° 37° 37° 34° 34° 30° 166 summarizes two pressure expansion t e s t s performed by the w r i t e r i n sand a t a depth of 9.3 m at the UBC research s i t e . F u l l d e t a i l s of the s i t e w i l l be g iven i n Chapter 7. The f i r s t expansion t e s t ( F i g . 5.12(a)) was performed a f t e r the instrument was s e l f - b o r e d to the req u i r e d depth. The expansion curve i s t y p i c a l of a s e l f - b o r i n g pressuremeter t e s t i n sand. A f t e r completion o f the t e s t the pressuremeter membrane was allowed to come back to i t s i n i t i a l p o s i t i o n w i t h no i n t e r n a l pressure. Another pressure expansion t e s t was performed without moving the instrument, F i g . 5.12(b). The r e - e x p a n s i o n t e s t shows the e f f e c t s of c o n s i d e r a b l e i n i t i a l d i s t u r b a n c e . The c a l c u l a t e d i n - s i t u s t r e s s and shear strength are con s i d e r a b l y d i f f e r e n t f o r the re-expansion t e s t as compared to the f i r s t s e l f - b o r i n g expansion t e s t . I t i s i n t e r e s t i n g to note the s i m i l a r i t y between the d i s t u r b e d re-expansion curve and a t y p i c a l Menard type pressure expansion curve ( F i g . 2.4). This comparison provides considerable i n s i g h t i n t o the problems a s s o c i a t e d w i t h i n t e r p r e t a t i o n of Menard type pressuremeter t e s t r e s u l t s . I t i s a l s o i n t e r e s t i n g to note that the slope of the unload-reload and unload s e c t i o n of the two curves i n F i g . 5.12(a) and (b) are remarkably s i m i l a r . Further d i s c u s s i o n of t h i s observation w i l l be made i n a l a t e r s e c t i o n . The s t r a i n l e v e l c o r r e c t i o n proposed here i s a p p l i c a b l e to s e l f - b o r i n g pressuremeter data where the maximum c i r c u m f e r e n t i a l s t r a i n i s about 10%. The c o r r e c t i o n would be s i g n i f i c a n t l y l e s s i n loose sands i f the pressuremeter were able to expand to a c i r c u m f e r e n t i a l s t r a i n of about 40%. A comparison between the o r i g i n a l Hughes et a l . (1977) method and the proposed s t r a i n l e v e l c o r r e c t i o n method i s shown i n F i g . 5.13. This confirms the work by Jewel et a l . (1980) that shows the Hughes et a l . (1977) method to work s u c c e s s f u l l y f o r very dense sands ( i . e . S > 0.55). 167 FIGX12. EFFECTS OF LARGE INITIAL DISTURBANCE ON PRESSUREMETER DATA IN SAND AT McDONALD'S FARM. 168 FIG. 5.13. COMPARISON BETWEEN PROPOSED CORRELATION AND HUGHES et a l . (.1977) CORRELATION FOR PEAK FRICTION ANGLE FROM SELF-BORING PRESSUREMETER TEST DATA IN SANDS. 169 The d i l a t i o n angle (v) measured by the pressuremeter i s r e l a t e d to the mean e f f e c t i v e s t r e s s l e v e l during the t e s t . Values of v co r r e c t e d f o r s t r a i n l e v e l are r e l a t e d to the average e f f e c t i v e s t r e s s l e v e l around the probe during the t e s t . When the s o i l reaches f a i l u r e the mean e f f e c t i v e s t r e s s i n c r e a s e s i n pr o p o r t i o n to the e f f e c t i v e r a d i a l s t r e s s , a^, sin c e f a i l u r e i s t a k i n g p l a c e a t a p p r o x i m a t e l y constant s t r e s s r a t i o (o'/o'). r o The average e f f e c t i v e s t r e s s around the probe during the t e s t i s th e r e f o r e somewhat l a r g e r than the i n - s i t u mean e f f e c t i v e s t r e s s before the t e s t . Since the angle of f r i c t i o n decreases w i t h i n c r e a s i n g s t r e s s l e v e l (see Appendix 1), the f r i c t i o n angle c a l c u l a t e d from the pressuremeter data w i l l be somewhat l e s s than the maximum p o t e n t i a l f r i c t i o n angle a t the i n - s i t u s t r e s s l e v e l . However, the f r i c t i o n angle values from the pressuremeter are obtained under approximately p l a i n s t r a i n c o n d i t i o n s which would r e s u l t i n v alues g e n e r a l l y higher than under t r i a x i a l c o n d i t i o n s . These e f f e c t s may p a r t i a l l y c a n c e l one another w i t h the r e s u l t that the c a l c u l a t e d f r i c t i o n angle, 4>, and d i l a t i o n angle, v, obtained during a pressuremeter t e s t i n sand and correcte d f o r s t r a i n l e v e l may be approximately equal to that obtained from a t r i a x i a l t e s t . 5.4.3. Shear Modulus One of the most common uses of a s e l f - b o r i n g pressuremeter i s f o r the d e r i v a t i o n of the s o i l moduli (Wroth, 1982). However, s o i l moduli v a r i e s with both s t r e s s l e v e l and s t r a i n . The " e l a s t i c " shear modulus of a s o i l can be measured by performing an unloading-reloading c y c l e during a pressuremeter expansion t e s t . I f the s o i l i s p e r f e c t l y e l a s t i c i n unloading then the unloading-reloading c y c l e w i l l have a g r a d i e n t of 2G. D e t a i l s of the theory are given i n Appendix 2. 170 In carrying out such a cycle care must be taken not to exceed the e l a s t i c l i m i t of the s o i l during the unloading phase; t h i s r e s t r i c t s the amplitude of the stress cycle that can be c a r r i e d out. The i d e a l i s e d behaviour of a free draining sand during a pressuremeter te s t Is shown i n F i g . 5.14. The stress path of the sand i n terms of the shear s t r e s s x = 4-(o' - a') and a mean e f f e c t i v e stress -kco' + a') i s 2 r 9 ' 2 r 8 represented i n F i g . 5.14(b) . AX i s the e f f e c t i v e stress path during e l a s t i c unloading at constant mean e f f e c t i v e s t r e s s . F u l l d e t a i l s of the theory behind determination of e l a s t i c shear moduli from unloading-reloading cycles are given i n Hughes (1982) and Wroth (1982). A summary of the theory i s given i n Appendix 2. A t y p i c a l s e l f - b o r i n g pressuremeter t e s t r e s u l t i s shown i n F i g . 5.15. The test was performed by the writer at a depth of 10.9 m i n sand at the UBC research s i t e (McDonald's Farm). The measured value of the e l a s t i c shear modulus f o r c y c l e AB i s 340 kg/cm2. For the example shown i n F i g . 5.15, another unloading-reloading cycle (EF) was performed on the decreasing pressure side of the pressure expansion curve. The stress l e v e l s f or the two cycles are approximately equal and the remarkable s i m i l a r i t y of the slopes of the cycles i s c l e a r l y evident. It i s also i n t e r e s t i n g to note that the slope of the unloading (CD) at the end of the increasing pressure side of the test i s i n i t i a l l y l i n e a r with a slope s l i g h t l y greater than the other two c y c l e s , which were c a r r i e d out at a lower stress l e v e l . Since sand i s free draining and no excess pore pressures are recorded d u r i n g the t e s t , i t follows that the mean e f f e c t i v e stress (o') increases m as the test progresses. Therefore, i t would be expected that the measured 171 FIG. 5.14. IDEALISED BEHAVIOUR OF FREE DRAINING SAND DURING SELF-BORING PRESSUREMETER TEST. 172 lOOOi ' - i — ,— Zac/ial Displacz-menf % ^ FIG. 5.15. SELF-BORING PRESSUREMETER TEST DATA IN SAND AT A DEPTH OF 10.9 m AT McDONALD'S FARM. 173 shear modulus would increase as Che p o s i t i o n of the unloading-reloading c y c l e moves further from the s t a r t of the t e s t . It i s well established that for granular materials the value of e l a s t i c moduli are dependent on the value of the mean e f f e c t i v e stress (o^) , and i s u s u a l l y p r o p o r t i o n a l to ( o ^ ) n , where n i s t y p i c a l l y 0.5. The moduli may b e expressed In a dimensionless form that i s independent of stress l e v e l b y the equation: G = kG * ( 5 a ) where k = modulus number Cx n = modulus exponent, t y p i c a l l y 0.5 Pa = reference s t r e s s ( i . e . Pa = 1 kg/cm2) a' = mean e f f e c t i v e stress m Hence, r e s u l t s are more c l e a r l y expressed i n dimensionless form using the modulus number, k , rather than the actual shear modulus, G. G During the very early stages of a pressure expansion test when the s o i l i s approximately e l a s t i c the mean e f f e c t i v e stress i s unchanged (see F i g . 5.14). However, when the s o i l around the pressuremeter reaches f a i l u r e the mean e f f e c t i v e stress increases i n proportion to the e f f e c t i v e r a d i a l s t r e s s ( a ^ ) , since f a i l u r e i s taking place at constant stress r a t i o (a^/Og). For most pressuremeter t e s t s , the mean e f f e c t i v e stress when the s o i l i s at f a i l u r e i s approximately equal to one hal f the e f f e c t i v e r a d i a l s t r e s s . Thus, the writer believes that i t would be more l o g i c a l to express the moduli values from pressuremeter t e s t s i n sands i n terms of the modulus number, k , t a k i n g the mean e f f e c t i v e s t r e s s e q u a l to one h a l f t he e f f e c t i v e r a d i a l s t r e s s ( o ^ ) a t the s t a r t of the unloading c y c l e and the modulus exponent, n, equal to 0.5. As the behaviour of s o i l i s n o n - l i n e a r the appropriate modulus w i l l a l s o depend on the s t r a i n range. For computer analyses i t may be more appropriate to f i t a mathematical curve to the pressure expansion curve so the modulus can a u t o m a t i c a l l y be c a l c u l a t e d at any s t r a i n l e v e l . Denby (1978) used t h i s approach f o r c a l c u l a t i n g c l a y moduli. However, f o r pressure expansion t e s t s i n c l a y the mean e f f e c t i v e s t r e s s i s e s s e n t i a l l y unchanged. Many s t a t i c n o n - l i n e a r computer analyses employ a n o n - l i n e a r s t r e s s s t r a i n curve represented by the h y p e r b o l i c f u n c t i o n (Duncan and Chan, 1970) introduced by Kondner (1963), and of the form, x = ! (5.2) G T. i max where G^  = i n i t i a l tangent modulus x = shear s t r e n g t h max Y = shear s t r a i n . I t appears reasonable to assume that the e l a s t i c shear modulus obtained from the unload-reload c y c l e i s equivalent to the I n t i a l tangent modulus, G^, s i n c e the s t r a i n l e v e l s are comparable at about Y = 1 0 - 1 % , and since most n a t u r a l sand deposits have experience some past l o a d i n g . 175 Thus, the w r i t e r b e l i e v e s the f u l l s t r e s s s t r a i n curve f o r a sand d e p o s i t can be q u i c k l y evaluated by ta k i n g the unload-reload modulus and app l y i n g i t to the h y p e r b o l i c f u n c t i o n (eq. 5.2). The v a r i a t i o n of s t r e s s s t r a i n c h a r a c t e r i s t i c s w i t h s t r e s s l e v e l could be incorp o r a t e d by using the modulus number, k , to c a l c u l a t e the i n i t i a l shear modulus. The re l e v a n t CJ secant modulus a t the appropriate working s t r e s s or s t r a i n l e v e l can then be derived from the s t r e s s s t r a i n curve, as shown on F i g . 5.16. A s i m i l a r approach can be a p p l i e d to o b t a i n the shear modulus from pressuremeter t e s t s i n c l a y . However, s i n c e the c l a y i s sheared i n an undrained manner, the e f f e c t i v e s t r e s s i s constant a t f a i l u r e , so that the deduced shear modulus should remain constant and be independent of the stage of the t e s t a t which i t i s measured. In r e a l i t y some drainage w i l l take place during the t e s t . The r a d i a l d i s t r i b u t i o n of excess pore pressure i s approximately l i n e a r w i t h the logarithm of r a d i a l d i s t a n c e . T h i s g i v e s r i s e to s u b s t a n t i a l h y d r a u l i c g r a d i e n t s i n the r a d i a l d i r e c t i o n . Some c o n s o l i d a t i o n w i l l occur, l e a d i n g to an increase i n e f f e c t i v e s t r e s s near the pressuremeter, w i t h the consequence that the observed values of G may in c r e a s e m a r g i n a l l y w i t h the value of s t r a i n a t which the unloading c y c l e i s commenced. An e s t i m a t e of the maximum dynamic shear modulus, G , can be made max w i t h a knowledge of the i n i t i a l s t a t i c tangent modulus, G^. As suggested e a r l i e r , the i n i t i a l s t a t i c tangent modulus f o r sands i s about 1/5 of the maximum shear modulus values obtained from resonant column or i n - s i t u shear wave v e l o c i t y t e s t s . This occurs f o r two reasons; the s t r a i n s f o r the i n i t i a l tangent modulus are around 0.1% whereas the maximum modulus occurs at s t r a i n s of 10 - l t%, and repeated l o a d i n g such as occurs i n the resonant column t e s t i n c r e a s e s the modulus by a f a c t o r of about 2. These two 176 FIG. 5.16. HYPERBOLIC STRESS STRAIN CURVE. 177 factors therefore lead to an i n i t i a l modulus of about 1/5 of the maximum modulus. A s i m i l a r approach has been suggested by Massarsch and Drnevich (1979) f o r estimation of the maximum shear modulus f or c l a y s . The v a r i a t i o n of shear modulus with s t r a i n appears to be dependent on the p l a s t i c i t y of the c l a y . The shear modulus decreases more r a p i d l y with continued s t r a i n i n g f o r s o i l s with low p l a s t i c i t y . Thus, a simple f a c t o r of 5 to convert the i n i t i a l tangent modulus to the maximum shear modulus i s not possible f o r c l a y s . The p l a s t i c i t y of the c l a y must also be taken i n t o account. 5.4.4. Cy c l i c Testing The p o s s i b i l i t y of using slow c y c l i c pressuremeter tests to assess l i q u e f a c t i o n p o t e n t i a l i n sands was put forward by Hughes et a l . (1980). They observed that the accumulated s t r a i n s during pressuremeter c y c l i c t e s t s were s i g n i f i c a n t l y greater i n loose material than dense. Although the tests are run slowly, under drained conditions, the test gives a measure of the degradation of the sand from cycle to c y c l e . If the c y c l i c loading occurred q u i c k l y , such as during earthquake motions, the excess pore pressures generated would l i k e l y be rela t e d to the amount of st r u c t u r a l degradation i n the s o i l . Examples of c y c l i c t e s t s i n dense and loose sand are shown on F i g . 5.17. The test i s usually c a r r i e d out maintaining the applied pressure above the i n - s i t u s t r e s s . The f i r s t c ycle i s applied a f t e r a r a d i a l displacement of about 2 percent of the i n t i a i l radius has been achieved. The cycles are c a r r i e d out i n a stress c o n t r o l l e d manner by reducing the applied pressure to a c e r t a i n l e v e l then reapplying the pressure back to the i n i t i a l l e v e l . During the test the magnitude of the applied pressure FIG. 5.17. EXAMPLES OF CYCLIC PRESSUREMETER TESTS IN LOOSE AND DENSE SAND. (Adapted from Hughes et a l . , 1980) 179 i s normally kept high enough to prevent shear stress r e v e r s a l . The test i s performed slowly and no excess pore pressures are recorded. However, i t i s c l e a r there i s appreciable successive movement from c y c l e to cy c l e although the moduli for each cycle are s i m i l a r ( F i g . 5.17). Because of the nature of the t e s t , the expansion process i s always stable with the s t r a i n per cycle decreasing as the number of cycles increases. The cumulative increase i n s t r a i n with number of cycles f o r the tests shown i n F i g . 5.17 are shown i n F i g . 5.18. It can be seen that successive s t r a i n per cycle f o r the dense sand i s much l e s s than for the loose sand. This response of cumulative s t r a i n with cycles from slow c y c l i c pressuremeter tests i s s i m i l a r to the observed cumulative volume changes during drained c y c l i c loading i n simple shear ( S i l v e r and Seed, 1971). It i s generally recognised that the r i s e i n pore pressure during undrained c y c l i c loading i s rel a t e d to the cumulative volume changes during drained c y c l i c loading (Martin et a l . , 1975). Thus, i t i s reasonable to assume that the observed cumulative s t r a i n with cycles from slow c y c l i c pressuremeter t e s t i n g can be related to the increase i n pore pressure during c y c l i c undrained loading and hence, to l i q u e f a c t i o n p o t e n t i a l . It should, therefore, be possible to r e l a t e the cumulative strain-from c y c l i c pressuremeter tests to l i q u e f a c t i o n p o t e n t i a l . However, there are several problems with t h i s approach. Research has shown that volume changes during c y c l i c loading are dependent on c y c l i c shear s t r a i n amplitudes rather than c y c l i c shear stress amplitudes ( S i l v e r and Seed, 19 71). The pressuremeter cycles are usually c a r r i e d out i n a st r e s s c o n t r o l l e d manner with varying amounts of induced shear s t r a i n . However, t h i s problem can be reduced by performing e i t h e r s t r a i n c o n t r o l l e d cycles or stress c o n t r o l l e d cycles at approximately equal induced s t r a i n 180 \ I t a n M t •» C y c l * * FIG. 5.18. CUMULATIVE STRAIN UNDER SEVERAL CYCLES OF LOADING IN PRESSUREMETER TEST. (Adapted from Hughes et a l . , 1980) 181 l e v e l s . The st r e s s c o n t r o l l e d cycles shown i n F i g . 5.17(b) induced an average c y c l i c c i r c u m f e r e n t i a l s t r a i n amplitude of around ± 0.2%. The l e v e l of induced c y c l i c s t r a i n i s very important i f cumulative s t r a i n values are to be compared. F i g . 5.19 shows cumulative s t r a i n with number of cycles f o r two slow c y c l i c pressuremeter tests performed i n a sand of s i m i l a r density and i n - s i t u stress l e v e l s . Both te s t s were stress c o n t r o l l e d , but test A was performed with large stress cycles and an average induced c y c l i c s t r a i n amplitude of ± 0.35%. Test B, on the other hand, was performed with the more t y p i c a l smaller stress cycles and had an average induced c y c l i c s t r a i n amplitude of ± 0.1%. It i s clear that the la r g e r c y c l i c s t r a i n amplitude test produced a s i g n i f i c a n t l y larger cumulative s t r a i n i n 10 cycle s . Thus, slow c y c l i c pressuremeter tests should i d e a l l y be performed i n a s t r a i n c o n t r o l l e d manner. However, i f s t r e s s c o n t r o l l e d tests are performed an e f f o r t should be made to induce sim i l a r s t r a i n l e v e l s . To aid i n comparison of data the number of cycles performed should also be consistent, such as 10 c y c l e s , as shown i n F i g . 5.19. Laboratory studies have shown that c y c l i c shear stress r e v e r s a l has a s i g n i f i c a n t influence on the c y c l i c behaviour of sands. The slow c y c l i c pressuremeter tests does not induce c y c l i c shear stress r e v e r s a l . This would be a serious problem i f the r e s u l t s from such te s t s were applied i n some t h e o r e t i c a l model to calculate the p o t e n t i a l induced pore pressures. However, i f the r e s u l t s from such c y c l i c pressuremeter tests are used i n some empirical c o r r e l a t i o n to l i q u e f a c t i o n p o t e n t i a l t h i s would not present a problem. A method proposed by the writer to r e l a t e the r e s u l t s from c y c l i c pressuremeter tests to l i q u e f a c t i o n resistance w i l l be presented i n Section 182 NUtJibBtL OF CYCLES, N FIG. 5.19. CUMULATIVE STRAIN WITH NUMBER OF CYCLES WITH DIFFERENT CYCLIC STRAIN AMPLITUDES. 183 5.6.2. 5.5. Cone-Pressuremeter Hughes (1982) has shown that the shear moduli values obtained from unloading-reloading cycles during pressuremeter tests appear to be i n s e n s i t i v e to the manner i n which the instrument i s i n s t a l l e d . This i s confirmed by the data presented i n F i g . 5.12, which shows two pressure expansion tests performed i n sand at the same l o c a t i o n . The re-expansion te s t ( F i g . 5.12(b)) was performed without moving the instrument following the f i r s t expansion test ( F i g . 5.12(a)). The re-expansion test shows considerable i n i t i a l disturbance with a r e s u l t i n g s i g n i f i c a n t d i f f e r e n c e i n c a l c u l a t e d i n - s i t u stress and shear strength. However, the unloading section of the disturbed re-expansion test i s remarkably s i m i l a r to the unload-reload and unload section of the f i r s t undisturbed t e s t . Thus, i t would appear that the requirement f o r minimal s o i l disturbance during i n s t a l l a t i o n i s not necessary i f the only parameter required i s the shear modulus, G. To investigate t h i s further a ser i e s of tests were c a r r i e d out by the writer using a push-in cone-pressuremeter. The tests were performed at the UBC research s i t e at McDonald's Farm, near Richmond. The push-in cone-pressuremeter i s e s s e n t i a l l y a standard self-boring pressuremeter with the cutting shoe and i n t e r n a l cutter removed and a s o l i d 60° cone t i p attached i n place of the cutt i n g shoe. A schematic of the push-in cone-pressuremeter instrument i s shown i n F i g . 5.20. The cone-pressuremeter was pushed into the ground at a constant rate of 2 cm/sec using the UBC i n - s i t u t e s t i n g v e h i c l e . Standard pressuremeter t e s t s incorporating unload-reload c y c l e s were performed at selected depths. The unload-reload cycles of the pressure expansion te s t s were analysed to 185 give the e l a s t i c shear moduli, G, as discussed i n Section 5.4.3. A t y p i c a l push-in cone-pressuremeter test r e s u l t at a depth of 7.6 m i s shown i n F i g . 5.21. Note again the s i m i l a r i t y i n slopes between the unloading-reloading cycles and the unloading at the end of the t e s t . F u l l d e t a i l s of the testing and r e s u l t s are given i n Chapter 7. 5.6. Liquefaction 5.6.1. Proposed D i l a t i o n Angle C o r r e l a t i o n As discussed i n Chapter 3, Vaid et a l . (1980) proposed a method for assessment of l i q u e f a c t i o n p o t e n t i a l based upon a c o r r e l a t i o n between c y c l i c stress r a t i o and d i l a t i o n angle, v. The d i l a t i o n angle was to be measured i n the f i e l d using a s e l f - b o r i n g pressuremeter and applying Hughes et a l . (19 77) method to obtain v. The c o r r e l a t i o n was established from a r e l a t i o n s h i p between r e l a t i v e density and d i l a t i o n angle f o r Ottawa sand. The c o r r e l a t i o n was based on testing performed i n a simple shear device. The l i q u e f a c t i o n r e s i s t a n c e was obtained from constant volume c y c l i c simple shear t e s t s . The laboratory d i l a t i o n angle values were obtained under drained simple shear conditions at a constant v e r t i c a l confining s t r e s s , o' = 200 kPa . The d i l a t i o n angle values were then corrected to a normal vo pressure of 100 kPa. The d i l a t i o n angle values were computed as a tangent at a shear s t r a i n y = 10%. As discussed e a r l i e r (Section 5.4.2) the method by Hughes et a l . (19 77) appears to measure an average d i l a t i o n angle around the probe. If the probe i s expanded to a maximum of about 10 percent ci r c u m f e r e n t i a l s t r a i n ( i . e . approximately 20 percent shear strain) the average d i l a t i o n angle c a l c u l a t e d i s approximately equivalent to the secant d i l a t i o n angle at a shear s t r a i n y = 10%. Thus, a c o r r e c t i o n i s required to convert the F I G . 5.21. C O N E - P R E S ' S U M E T E R T E S T R E S U L T A T A D E P T H O F 7.6m A T M C D O N A L D * s F A R M . 187 calculated d i l a t i o n angle from the pressuremeter ( i . e . secant at y = 10%) to the d i l a t i o n angle proposed by Vaid et a l . (1981) ( i . e . tangent at y = 10%). F i g . 5.22(a) i s a plot of r e l a t i v e density against d i l a t i o n angle f o r Ottawa sand obtained from the data by Vaid et a l . (1981) presented i n F i g . 5.7. This pl o t i s s i m i l a r to that shown i n F i g . 5.9 but shows three l i n e s representing d i f f e r e n t d e f i n i t i o n s of d i l a t i o n . Line AB i s the maximum rate of d i l a t i o n , i . e . related to the slope of the st r a i g h t portion of the volume shear curve, GH i n F i g . 5.22(b). The maximum rate of d i l a t i o n i s associated with the peak f r i c t i o n angle. Line CD i s the average d i l a t i o n rate around the probe ( i . e . the secant IJ) when the probe i s expanded to a maximum of about 10 percent c i r c u m f e r e n t i a l s t r a i n (e ). Line EB i s the tangent d i l a t i o n rate at 10% shear s t r a i n used by Vaid et a l . (1981) ( i . e . KL i n F i g . 5.22(b)). Combining pressuremeter data at a circumferential s t r a i n l e v e l of 10% with the data on Ottawa Sand ( F i g . 5.22) the tangent d i l a t i o n angle at 10% shear s t r a i n used by Vaid et a l . (1981) can be determined. For sands with a r e l a t i v e density greater than 50% the co r r e c t i o n i s the same as that suggested e a r l i e r f o r the determination of the peak f r i c t i o n angle and maximum rate of d i l a t i o n . A chart developed by the writer, f o r c a l c u l a t i n g the tangent d i l a t i o n angle at y = 10% from s e l f - b o r i n g pressuremeter data that incorporates the proposed s t r a i n l e v e l correction i s shown i n F i g . 5.23. One problem s t i l l remains before the d i l a t i o n angle obtained from the pressuremeter, using F i g . 5.23, can be applied to determine the c y c l i c s tress r a t i o using F i g . 3.5. The d i l a t i o n angle determined from the pressuremeter r e l a t e s to the average mean e f f e c t i v e stress l e v e l around the probe, whereas, the d i l a t i o n angle used by Vaid et a l . (1981) was corrected 188 20 AO c?o &o loo ££LAT/V£ OBN5JTY, Dr, % FIG. 5.22. VARIATION OF DILATION ANGLE WITH RELATIVE DENSITY FOR OTTAWA SAND IN DRAINED SIMPLE SHEAR. FIG. 5.23. PROPOSED CORRELATION BETWEEN SELF-BORING PRESSUREMETER DATA AND TANGENT DILATION ANGLE AT Y = 10%. 190 to a normal pressure of 100 kPa (1 T / f t 2 ) . As discussed i n Appendix 1 the d i l a t i o n angle v a r i e s w i t h c o n f i n i n g s t r e s s . F i g . 5.24 i s a review performed by the w r i t e r of a v a i l a b l e data showing the v a r i a t i o n of the maximum d i l a t i o n r a t e , v , w i t h mean normal s t r e s s f o r a v a r i e t y of max d i f f e r e n t sands a t an i n i t i a l r e l a t i v e d e n s i t y of 80 percent. The data was obtained from published t r i a x i a l t e s t r e s u l t s and converted to maximum d i l a t i o n r a t e using Rowes s t r e s s d i l a t a n c y theory. I t i s i n t e r e s t i n g to note t h a t the v a r i a t i o n i n v f o r a l l the sands l i e w i t h i n a r e l a t i v e l y max narrow band and that the v a r i a t i o n i s approximately l i n e a r w i t h the logar i t h m of mean normal s t r e s s . The r a t e of decrease i n d i l a t i o n angle i s about 6 degrees per l o g sale increase i n mean e f f e c t i v e s t r e s s . As discussed e a r l i e r ( S e c t i o n 5.4.3) the mean e f f e c t i v e s t r e s s around a pressuremeter increases i n p r o p o r t i o n to the e f f e c t i v e r a d i a l s t r e s s when the s o i l reaches f a i l u r e during a pressuremeter t e s t . For most pressuremeter t e s t s , the mean e f f e c t i v e s t r e s s when the s o i l i s at f a i l u r e i s approximately equal to one h a l f the e f f e c t i v e r a d i a l s t r e s s . Since the e f f e c t i v e r a d i a l s t r e s s decreases w i t h the square of the r a d i a l d i s t a n c e ( s i m i l a r to the decrease i n shear s t r a i n shown i n F i g . 5.6) the average e f f e c t i v e r a d i a l s t r e s s around the probe i s approximately one h a l f the maximum e f f e c t i v e r a d i a l s t r e s s . Thus, the average mean e f f e c t i v e s t r e s s around the probe i s approximately 1/4 the maximum e f f e c t i v e r a d i a l s t r e s s , a ' . r To use s e l f - b o r i n g pressuremeter data i n sands to assess l i q u e f a c t i o n or c y c l i c m o b i l i t y p o t e n t i a l , the w r i t e r b e l i e v e s the f o l l o w i n g steps are req u i r e d : i ) p l o t l o g e f f e c t i v e pressure against l o g c i r c u m f e r e n t i a l s t r a i n to o b t a i n the slope s at or near the 10% c i r c u m f e r e n t i a l s t r a i n <f>' " (degrees ) T>1.--&oZ 3 2 4 3 2 j 3 5 3 7 3 3 3 5 • o + X V Chattahoochee Sand Mol S a n d Monterey S a n d G l a c i a l Sand S A T A F Le igh ton Buzzard Sand Vesie and Clough I 9 6 8 D e B e e r I 9 6 5 Villet and Mitchell I98I Hirshfield and Poulos 1963 B a l d i et a l . I 9 8 I Cole I 9 6 7 -V. Hr -9+Q : l 0.5 10 50 I00 500 I000 M E A N NORMAL S T R E S S , 0 - m kg /cm V A R I A T I O N OF D I L A T I O N A N G L E , V W I T H M E A N N O R M A L S T R E S S F O R V A R I O U S S A N D S . 192 l e v e l (see F i g . 5.23). i i ) Using F i g . 5.23 o b t a i n the tangent d i l a t i o n angle ( v 1 (-)) at y = 10%. i i i ) Calculate the average mean e f f e c t i v e stress l e v e l around the probe by t a k i n g the maximum e f f e c t i v e r a d i a l s t r e s s , and d i v i d i n g by 4. i v ) Correct the calculated d i l a t i o n angle (obtained from F i g . 5.23) to a mean e f f e c t i v e stress l e v e l of 100 kPa (1 km/cm2) using F i g . 5.24. v) Apply the c o r r e c t e d d i l a t i o n angle, v , to the r e l a t i o n s h i p proposed by Vaid et a l . (1981) shown i n F i g . 3.5. It i s c l e a r from the above discussion that using the se l f - b o r i n g pressuremeter to measure the i n - s i t u d i l a t i o n angle and then to apply that measurement to the assessment of l i q u e f a c t i o n or c y c l i c m o b i l i t y p o t e n t i a l i s a complex procedure with several semi-empirical c o r r e l a t i o n s . 5.6.2. Proposed C y c l i c Testing Correlation An a l t e r n a t i v e method of using the se l f - b o r i n g pressuremeter to assess l i q u e f a c t i o n or c y c l i c m o b i l i t y p o t e n t i a l i s to perform slow c y c l i c pressuremeter t e s t s . As discussed i n Section 5.4.4 i t i s important that the slow c y c l i c pressuremeter tests be performed i n a consistent manner. It i s suggested that 10 cycles be performed maintaining an approximately constant c y c l i c s t r a i n amplitude. Situ Technology Inc. (STI) performed 7 such t e s t s , at the request of the writer, during an i n v e s t i g a t i o n at a s i t e i n the Beaufort Sea area i n Northern Canada. The r e s u l t s from these tests are summarized i n Table 5.2 The s i t e where the te s t i n g was performed contained a sand deposit of TABLE 5.2 SUMMARY OP CYCLIC PRESSUREMETER TESTS, BEAUFORT SEA Test Depth Slope D i l a t i o n angle Max. e f f e c t i v e Corrected Cumulative s t r a i n Average c y c l i c No. (m) log-log at Y - 10% r a d i a l s t r e s s , d i l a t i o n at 10 cycles s t r a i n amplitude e 6 - 10Z angle (100 kPa) • °IW> ^ eN-10 * e c y * 1 8.53 0.28 - 1 750 + 2 0.43 0.02 2 4.26 0.27 - 1-1/2 600 + 1/2 0.50 0.26 3 11.17 0.24 - 4 450 - 3 0.68 0.18 TEST B 4 12.69 0.39 + 8 650 +10 0.25 0.29 5 7.51 0.32 + 2-1/2 450 + 3-1/2 0.46 0.20 6 8.25 0.41 +10 750 +13 0.19 0.08 7 5.07 0.22 - 5-1/2 850 - 2 2.04* 0.72 TEST A * Projected to 10 cycles. See F i g . 5.19 - 32° (Assumed). 194 v a r i a b l e density. The v a r i a t i o n i n cumulative s t r a i n a f t e r 10 cycles was s i g n i f i c a n t . In one test ( t e s t 7) the magnitude of the c y c l i c stresses and thus, the induced c y c l i c s t r a i n amplitudes, were s i g n i f i c a n t l y larger than i n the other t e s t s . The cumulative s t r a i n s at 10 cycles from the low s t r a i n amplitude c y c l i c tests ( t e s t s 1 to 6) were plotted against the corrected d i l a t i o n angle, v , and are shown i n F i g . 5.25. The c o r r e c t e d d i l a t i o n angle values were determined using F i g s . 5.23 and 5.24, and corrected to a normal pressure of 100 kPa. There i s c l e a r l y a strong c o r r e l a t i o n between the cumulative s t r a i n developed i n c y c l i c pressuremeter tests and the corrected d i l a t i o n angle. Combining the c o r r e l a t i o n shown i n F i g . 5.25 with the l i q u e f a c t i o n resistance curve by Vaid et a l . (1981) ( F i g . 3.5) an a l t e r n a t i v e pressuremeter l i q u e f a c t i o n resistance c o r r e l a t i o n can be obtained. This has been performed by the writer and i s shown i n F i g . 5.26. The c o r r e l a t i o n shown i n F i g . 5.2 6 could be used as an independent check as to the l i q u e f a c t i o n resistance of a sand deposit using a s e l f - b o r i n g pressuremeter. The l i q u e f a c t i o n resistance could be determined from both the corrected d i l a t i o n angle and the cumulative s t r a i n . It i s c l e a r from F i g . 5.26 that sands that develop cumulative s t r a i n s i n 10 cycles of greater than about 0.3% may develop large s t r a i n s during undrained c y c l i c loading. For correct i n t e r p r e t a t i o n of c y c l i c pressuremeter tests i t i s important that the c y c l i c s t r a i n amplitude values during each cycle be approximately constant at about e _ » 0.2%. 195 1.0-\ S K ^ as-8 1 0 -5 - i — i 1 1—i 1 — i — i 1 — i — i — i — P — | — i — i — i — i 1 — i — i — i 1—| O 5 10 15 20 CORZECTSa DILATION AN6L5r i>c , degree* FIG. 5.25. CORRELATION BETWEEN CUMULATIVE STRAIN AT 10 CYCLES AND CORRECTED DILATION ANGLE FROM SELF-BORING PRESSUREMETER TEST DATA, BEAUFORT SEA. 196 0 O.S i.o CUMULATIVE STRAIN AT IO CYCLES ZN^CO / Percent FIG. 5.26. PROPOSED CORRELATION BETWEEN CYCLIC STRESS RATIO AND CUMULATIVE STRAIN AT 10 CYCLES. 197 The correlations and method of data analyses presented in the previous sections require considerable field verification. The application of the self-boring pressuremeter to the assessment of liquefaction or cyclic mobility is complex and requires several semi-empirical correlations. Chapter 7 will present field and laboratory data to evaluate the proposed analyses and correlations. 198 6. FLAT PLATE DILATOMETER TESTING 6.1. Equipment The f l a t plate dilatometer used f o r the dilatometer t e s t i n g (DMT) i n t h i s study was developed i n I t a l y by S. Marchetti. The dilatometer i s a f l a t p l a t e 14 mm thi c k , 95 mm wide by 220 mm i n length. A f l e x i b l e s t a i n l e s s s t e e l membrane 60 mm i n diameter i s located on one face of the blade. Beneath the membrane i s a measuring device which turns a buzzer o f f i n the c o n t r o l box at the surface when the membrane s t a r t s to l i f t o f f the sensing d i s c and on again a f t e r a d e f l e c t i o n of 1 mm at the centre of the membrane. A schematic of the dilatometer i s shown i n F i g . 6.1. Readings are made every 20 cm i n depth. The membrane i s i n f l a t e d using high pressure nitrogen gas supplied by a tube pre-threaded through the rods. As the membrane i s i n f l a t e d , the pressures required to ju s t l i f t the membrane o f f the sensing d i s c (reading A), and to cause 1 mm d e f l e c t i o n at the centre of the membrane (reading B), are recorded. Readings are made from a pressure gauge i n the control box and entered on a standard data form. F u l l d e t a i l s of the test procedure are given i n the Dilatometer Users Manual (Marchetti and Crapps, 1981). The dilatometer was pushed into the ground during t h i s study using the UBC i n - s i t u t e s t i n g v e h i c l e at a rate of penetration of 2 cm/sec. Before and a f t e r each sounding the dilatometer was c a l i b r a t e d for membrane s t i f f n e s s . The dilatometer data (readings A and B) were corrected f o r o f f s e t i n the measuring gauge and for membrane s t i f f n e s s . Another small correction i s required because of the configuration of the measuring system. A f u l l d i s c u s s i o n on corrections i s given by Marchetti and Crapps, 1981. Si m p l i f i e d expressions f o r the corrected data are: FIG. 6.1. SCHEMATIC OF DILATOMETER. 200 P = A + AA o P = B - AB . Where AA i s the vacuum required to keep the membrane i n contact with i t s seating, since a f t e r several readings the membrane acquires a permanent outward curvature. AB i s the a i r pressure required to cause a 1 mm de f l e c t i o n i n free a i r . Using the P q and P^ the f o l l o w i n g three index parameters were proposed by Marchetti: ( prV I, = — = Material Index d P -u o o P -u K, = — ; = Horizontal Stress Index d a' vo E = 34.6(P -P ) = Dilatometer Modulus, d 1 ° where u i s the h y d r o s t a t i c water p r e s s u r e and a' i s the i n - s i t u o vo v e r t i c a l e f f e c t i v e s t r e s s . The data was reduced using a computer program supplied with the instrument (Crapps and Schmertmann, 1981) and adapted at UBC by Ian McPherson. Computer graphics f a c i l i t i e s were used to generate the completed p l o t s . The dilatometer equipment used during t h i s study was extremely simple to operate and maintain. The s i m p l i c i t y and low i n i t i a l cost of the 201 equipment i s one of the main advantages of the f l a t plate dilatometer as an i n - s i t u t e s t method. However, the s i m p l i c i t y of the equipment does generate some d i f f i c u l t y with i n t e r p r e t a t i o n of the r e s u l t s . D e t a i l s of these problems w i l l be discussed i n the following sections. 6.2. Factors Affecting Results from DMT Before using any data from f l a t plate dilatometer t e s t i n g i t i s important to r e a l i z e and account f o r p o t e n t i a l errors that the data may contain. During the use of the f l a t plate dilatometer i n th i s study se v e r a l s i g n i f i c a n t aspects concerning data c o l l e c t i o n and i n t e r p r e t a t i o n have been observed. Some of these points are summarized i n the next sections. 6.2.1. Slope As discussed i n Chapters 4 and 5, i t i s almost impossible to push an instrument i n t o the ground without some n o n - v e r t i c a l i t y , e s p e c i a l l y f o r deep holes. This problem i s p a r t i c u l a r l y important i f the instrument measures a l a t e r a l s t r e s s , such as the pressuremeter and dilatometer. The i n i t i a l l i f t - o f f p r e s s u r e f o r the di l a t o m e t e r (P Q) can be s i g n i f i c a n t l y influenced by n o n - v e r t i c a l i t y . A simple slope sensor s i m i l a r to those incorporated into many cone penetration devices could also be included into the f l a t plate dilatometer. However, i t i s not c l e a r how the data could be adjusted to allow f o r non-v e r t i c a l i t y . The problem can be reduced, somewhat, by paying c a r e f u l a ttention to the i n i t i a l v e r t i c a l i t y at ground surface and by r e s t r i c t i n g the maximum depth of penetration. Work by Van de Graaf and Jeke l (1982) using CPT has shown that n e g l i g i b l e error i n recorded depth can be assumed for a maximum 202 penetration depth of 15 m, provided no obstructions e x i s t . Experience at UBC would suggest that good v e r t i c a l i t y can be maintained i n soft uniform deposits f o r penetration depths i n excess of 15 m. However, for le s s uniform deposits the suggested maximum depth of 15 m by Van de Graaf and Je k e l (1982) would appear reasonable. The incorporation of a d d i t i o n a l sensors to the e x i s t i n g dilatometer would s i g n i f i c a n t l y complicate the equipment and thus detract from i t s main advantage, s i m p l i c i t y . 6.2.2. Pore Pressure E f f e c t s The d i l a t o m e t e r records t o t a l stress measurements (P and P n ) . This o 1 i s an important a s p e c t r e g a r d i n g the t e s t procedure and data i n t e r p r e t a t i o n . If the dilatometer was submerged i n water, the l i f t - o f f p r e s s u r e P q should equal the hydrostatic water pressure ( U q ) . Marchetti has attempted to take t h i s i n t o consideration by normalizing the horizontal s t r e s s index, K, . However, there are s t i l l s e v e r a l problems with the d e x i s t i n g approach. The e x i s t i n g procedure assumes the membrane i n f l a t i o n i s performed "without delay" when pushing i s stopped. The rate of pressure increase i s set so that expansion occurs i n 15 to 30 seconds. The data a n a l y s i s assumes the e x i s t i n g s t a t i c water pressure to be hydrostatic. However, the i n - s i t u s t a t i c water pressure i s r a r e l y hydrostatic and the rate of t e s t i n g , i . e . time between stopping penetration and f u l l i n f l a t i o n , i s not always constant. The assumption of hydrostatic water pressure (u ) o can have a s i g n i f i c a n t influence on the index parameters e s p e c i a l l y i n deep s o f t d e p o s i t s where P and P, are small i n r e l a t i o n to the assumed u . o 1 o It i s not always possible to maintain a constant rate of t e s t i n g since the r a t e of expansion i s g e n e r a l l y c a r r i e d out at a c o n s t a n t r a t e but P o 203 and P, may v a r y considerably, thus the time to reach P and P, w i l l 1 o 1 vary. Results from cone penetration t e s t i n g with piezometer measurements have shown that penetration into s o f t , saturated, cohesive deposits can generate very large pore pressures. The r a d i a l d i s t r i b u t i o n of these excess pore pressures gives r i s e to substantial hydraulic gradients i n the r a d i a l d i r e c t i o n . D i s s i p a t i o n of the excess pore pressures commences immediately a f t e r stopping penetration. The value of the high pore pressures around the dilatometer when t e s t i n g i n s o f t , saturated cohesive deposits w i l l have a s i g n i f i c a n t influence on the measured t o t a l stress values of P and P, . o 1 Research by I. McPherson at UBC has shown that i f the rate of t e s t i n g i n a s a t u r a t e d c o h e s i v e d e p o s i t i s v a r i e d , the index parameters I,, K, d d and E, w i l l a l s o v a r y . McPherson performed dilatometer t e s t s a t the UBC d research s i t e (McDonald's Farm) i n the uniform clayey s i l t deposit from a depth of 15 m to 33 m at a v a r i e t y of r a t e s . The rate of t e s t i n g was progressively decreased to allow pore pressure d i s s i p a t i o n . As the pore p r e s s u r e decreased the measured values P and P, also decreased. The o 1 r e s u l t i n g d e c r e a s e i n P and P n caused an i n c r e a s e i n the index o 1 parameters I, and E, but a decrease i n K.. The decrease i n K, i s a d d d d d i r e c t r e s u l t of the decreasing pore pressures around the dilatometer membrane. The increase i n I, and E, i n d i c a t e s that the drop i n P i s d d o greater than the drop i n P, , since r and E, both depend on ( P - P ). 1 d d 1 o In many low permeability cohesive (clay) deposits the generally accepted rate of t e s t i n g w i l l have l i t t l e influence on the measured values. This has been confirmed by the remarkably consistent dilatometer test 204 r e s u l t s obtained i n the Norwegian c l a y s (Lacasse and Lunne, 1982). However, when t e s t i n g i n r e l a t i v e l y high p e r m e a b i l i t y d e p o s i t s such as s i l t o r s i l t y f i n e sand where s i g n i f i c a n t pore pressures can be generated during p e n e t r a t i o n , the e x i s t i n g t e s t i n g procedure may not produce such c o n s i s t e n t r e s u l t s due to v a r i a t i o n s i n pore pressure d i s s i p a t i o n . Further d i s c u s s i o n regarding the i n f l u e n c e of pore pressures w i l l be made i n the subsequent s e c t i o n s on i n t e r p r e t a t i o n . 6.3. I n t e r p r e t a t i o n 6.3.1. I n t r o d u c t i o n M a r c h e t t i performed DMT a t about 10 w e l l documented s i t e s i n I t a l y and developed e m p i r i c a l c o r r e l a t i o n s based on these r e s u l t s . C o r r e l a t i o n s were d e v e l o p e d between the t h r e e i n d e x p a r a m e t e r s , I , , K, and E, and s o i l d a d t y p e , s o i l u n i t w e i g h t , , OCR, undrained shear s t r e n g t h , constrained modulus and f r i c t i o n angle. A l l of the s o i l parameters were obtained from l a b o r a t o r y t e s t r e s u l t s . The m a j o r i t y of the s i t e s c o n s i s t e d of c l a y d e p o s i t s w i t h only two s i t e s i n v o l v i n g sand. At both sand s i t e s the sand was very loose w i t h r e l a t i v e d e n s i t i e s around 30 to 40%. D e t a i l s o f the s i t e s and the e m p i r i c a l c o r r e l a t i o n s are given by M a r c h e t t i (1980). The i n t e r p r e t a t i o n of the DMT r e s u l t s centers around the three index p a r a m e t e r s , I , K, and E . The p a r a m e t e r s , I and K, r e q u i r e a a d d d d knowledge o f the i n - s i t u water p r e s s u r e (u ) and the i n - s i t u v e r t i c a l o e f f e c t i v e s t r e s s ( a ' ) . The i n - s i t u water p r e s s u r e i s assumed to be vo h y d r o s t a t i c and the only data r e q u i r e d i s the depth of the ground water l e v e l . The s i g n i f i c a n c e of t h i s assumption was discussed i n the previous s e c t i o n . The i n - s i t u v e r t i c a l e f f e c t i v e s t r e s s ( a ' ) i s c a l c u l a t e d using vo s o i l u n i t weights obtained from an e m p i r i c a l c o r r e l a t i o n u s i n g 1A and E^ 205 and u s i n g the assumed h y d r o s t a t i c water pressure. The index parameter K, d can be s i g n i f i c a n t l y i n f l u e n c e d by the assumed v a l u e s of u and o' o vo s i n c e , P -u K = - V 2 -d a' vo e s p e c i a l l y i n s o f t saturated cohesive s o i l d e p o s i t s where P i s s m a l l . o The purchase of the dilatometer equipment i n North America i n c l u d e s a computer program t h a t c o n t a i n s the e m p i r i c a l c o r r e l a t i o n s f o r i n t e r p r e t a t i o n and data p r e s e n t a t i o n . An example of DMT r e s u l t s analysed and d i s p l a y e d by the computer i s shown i n F i g . 6.2 and 6.3. The c o r r e l a t i o n s proposed by Ma r c h e t t i (1980) were based on a l i m i t e d amount of data. In h i s c l o s u r e to h i s 1980 ASGE paper M a r c h e t t i suggested th a t "the data base f o r a l l the c o r r e l a t i o n s discussed i n the paper w i l l expand w i t h the expanding use of the dil a t o m e t e r t e s t " . U n f o r t u n a t e l y , the w r i t e r b e l i e v e s t h a t the development o f the computer program to analyse and d i s p l a y the DMT r e s u l t s tends to r e s t r i c t the user and discourage improvements or m o d i f i c a t i o n s to the e x i s t i n g c o r r e l a t i o n s as more experience i s gained w i t h the t e s t . 6.3.2. T h e o r e t i c a l Considerations 6.3.2.1. General The f l a t p l a t e d i l a t o m e t e r i s a p e n e t r a t i o n t e s t that i n c l u d e s a l a t e r a l expansion a f t e r p e n e t r a t i o n . The t e s t (DMT) th e r e f o r e combines many of the fe a t u r e s contained i n the cone p e n e t r a t i o n t e s t (CPT) and the pressuremeter t e s t (PMT). I t seems reasonable that many of the observations and t h e o r i e s developed f o r the CPT and PMT r e l a t e to the i n t e r p r e t a t i o n and understanding of the DMT r e s u l t s . 2 0 6 £ 7 J 3 . C \ INSITU TESTING. LDcatlon; HRCDQNRLDS- FRRII. McDonald's Farm DHT 2. I N T E R M E D I A T E G E O T E C H N I C A L P A R A M E T E R S Test No. DOT 2 Test Date; MAY 18,19821 LO '—I % a u ^° oi a, OJ •—' e a +-1 01 <—i •-\ Q ro - o C H a •H in CO u -a OJ c ro j i i a to • n • m • (U) qidaQ O'L c e O ' U O'EI I I I I I 1 L O'E O'S O'B (U) mteQ 0' Ct o ' e i 0' St L r 1 1 1 1 1 : Mil SILT - >• - 3 o O'St S3 3 a. F I G . 6 . 2 . I N T E R M E D I A T E P A R A M E T E R S F R O M D I L A T O M E T E R , D M T - 2 , M C D O N A L D ' S F A R M . M o S o o o 6 o a .H hr| ,hrj g H • TI 8' It-1 > iH IS H : M O H I Z <m) F i l e Name:dmt2 LocatIon:MACOONALDS FARM 1J B C.INSITU TESTING RESEARCH GROUP. — Record of Dilatometer test No:DMT 2 Date:MAY 18.1982 C a l i b r a t i o n Informat1on:DA= 0.15 Bars Gamma=Bulk unit weight Sv Uo Id Ed Kd DB= 0.50 Bars ZM= 0.10 Bars ZW= 1.00 metres ' E f f e c t i v e over.stress =Pore pressure =Matertal index 'Dilatometer modulus 'Horizontal stress Index INTERPRETED GEOTECHNICAL PARAMETERS Ko =Ins1tu earth press.coeff. OCR'Overconsol idatlon Ratio M 'Constrained modulus Cu =Undrained cohesion(cohesive) PHI'Friction Angle(cohesionless) PO P1 (Bar) (Bar) Ed (Bar) Uo (Bar) Id Gamma (t/CM) Sv (Bar) Kd OCR Pc (Bar) Cu PHI M (Bar) (Deg) (Bar) So i l Type 0.60 0.§0 2 2 2 2 1 3 3 3 3 4 4 4 00 00 20 40 GO 40 60 80 40 4.ab oo 20 f»P 6 0 o§ 2b 4b 60 z , (m) 0. 61 1 . 50 31 . 0. 0 1 . 47 1 . 60 0. 78 2 . 30 53. 0. 0 1 • 96 1 . 70 1. 03 3. 50 85. 0. 0 2. 39 1 . 70 0. 85 3. 00 74 . 0. 02 2. 60 1 . 70 0. 92 3. 70 96. 0.04 3. 17 1 . 70 0. 80 1 . 90 38. 0. 06 1 . 49 1 .60 0. 78 2. 30 53. 0. 08 2. 18 1 .70 1. 03 3. 60 89. 0. 10 2. 77 1 .70 1. 48 7. 10 194. 0. 12 4. 12 1 .80 1. 20 4 . 30 107. 6 . 14 2. 92 1 .80 0. 95 5. 10 144 . 0. 16 5. 23 1 .70 1. 27 5. 00 129. ft- 18 3. 41 1 .80 1. .50 6. 70 180. 0. 20 3. 99 1 .80 1. .35 5. .60 147. 0. 22 3. 77 1 .80 2 .03 8 .80 234. 0. 24 3. .79 1 .80 1 .43 10 .20 303 . 0. 26 7 .48 1 .80 2 .24 10 .80 296. 0. .28 4 .36 1 .90 2 .74 1 1 .30 296. 0 .30 3 .50 1 .90 1 .91 9 .00 245. 0 .32 4 .45 1 .80 2 . 15 8 .40 216. 0 .34 3 .45 1 .80 1 .86 7 .90 209. 0 .36 4 .02 1 .80 2 .28 8 .00 198 . 0 .38 3 .02 1 .80 2 .55 8 .80 216. 0 .40 2 .90 1 .90 2 .63 9 .40 234. 0 .42 3 .07 1 .90 0 .75 2 .90 74. 0 .44 7 .00 1 .70 2 .74 11 .30 296. 0 .46 3 .75 1 .90 2 .40 9 .80 256. 0 .48 3 .86 1 .90 1 .95 10 .30 289 . 0 .50 5 .75 1 .80 2 . 16 14 .50 427. 0 .52 7 .51 1 .90 3 . 15 13 .70 365. 0 .54 4 .05 1 .90 3 .00 14 .60 401 . 0 .56 4 .76 1 .90 32 1 .90 PO P1 Ed Uo Id Gamma ._ — -0. 114 5. 3 6. 21 0. 71 1 . 21 28 . 3 58. SANDY SILT 0. 148 5. 3 10. 54 1 . 56 1 . 20 29 . 6 100. SILTY SAND 0. 182 5. 7 12. 81~ 2. 33 1 . 27 31 . 1 169. SILTY SAND 0. 196 4 . 2 7 . 29 1 . 43 1 . 03 30. 7 129 . SILTY SAND 0. 210 4 . 2 7. 14 1 . 50 1 . 02 32. 0 168. SILTY SAND 0. 222 3. 3 2. 88 0. 64 0. 85 27. 4 55. SANDY SILT 0. 236 3. 0 3. 69 0. 87 0. 78 28. 7 73. SILTY SAND 0. 250 3. 7 5. 69 1 . 42 0. 93 30. 7 145. SILTY SAND 0. 266 5. 1. 10. 54 2. 80 1. 18 35. 0 373. SAND 0. 282 3. 8 5. 86 1 . 65 0. 94 31 . 0 176. SILTY SAND 0. 296 2. 7 3. 05 0. 90 0. 71 34. 4 195. SAND 0. 312 3. 5 5. 10 1 . 59 0. 89 31 . 9 205. SAND 0. 328 4 . 0 6. 48 2 . 13 0. 98 33. 7 305. SAND 0. 344 3. .3 4. 49 1 55 0. 84 32. 4 225. SAND 0. 360 5. .0 9 93 3 58 1. 16 34 . 1 443. SAND 0: 376 3 . 1 4. .09 1 .54 0. .81 39. 1 451 . SAND 0. 394 5 .0 9 .99 3 .94 1. . 16 35. .4 561 . SAND 0. 412 5 .9 13 .94 5 .74 1. .31 34 . 2 606. SAND 0. 428 3 . 7 5 .72 2 .45 0 .93 34 .4 402 . SAND 0. 444 4 . 1 6 .83 3 .03 1 .00 32 .5 372. SAND 0. .460 3 . 3 4 .46 2 .05 0 .84 32 .9 319. SAND 0 .476 4 .0 6 .53 3 . 1 1 0 .98 31 .5 337 . SILTY SAND 0 .494 4 .4 7 .74 3 .82 1 .05 31 .5 383. SILTY SAND 0 .512 4 . 3 7 .58 3 .88 1 .04 31 .9 415. SILTY SAND 0 .526 0 . 6 0 . 17 0 .09 0 .04 30 .7 63. SAND 0 .544 4 .2 7 .20 3 .92 1 .02 33 .4 517. SAND 0 .562 3 .4 4 .85 2 .73 0 .87 32 .7 401 . SAND 0 . 578 2 .5 2 .70 1 .56 0 .67 35 .0 376. SAND 0 .596 2 .8 3 .23 1 .92 0 .73 38 .8 589. SAND 0 .614 4 .2 7 .37 4 .52 1 .03 . 34 . 1 641 . SAND 0 .632 3 .9 6 . 13 3 .87 0 .96 35 .3 671 . SAND -0 .650 4 .5 8 .08 5 .25 1 .07 39 .8 1 140. SAND Description COMPRESSIBLE LOOSE LOOSE LOOSE LOOSE COMPRESSIBLE LOOSE LOOSE LOW RIGIDITY LOW RIGIDITY LOOSE LOW RIGIDITY LOW RIGIDITY LOW RIGIDITY LOW RIGIDITY LOW RIGIDITY MEDIUM RIGIDITY MEDIUM RIGIDITY LOW RIGIDITY LOW RIGIDITY LOW RIGIDITY LOW RIGIDITY MEDIUM RIGIDITY MEDIUM RIGIDITY LOOSE MEDIUM RIGIDITY MEDIUM RIGIDITY LOW RIGIDITY MEDIUM RIGIDITY MEDIUM RIGIDITY MEDIUM RIGIDITY MEDIUM RIGIDITY (Bar) (Bar) (Bar) (Bar) Sv (t/CM) (Bar) OCR Pc (Bar) Cu PHI M So i l Type (Bar) (Deg) (Bar) DescriptIon z (m) 60 80 1 .00 1 . 20 1 .40 1 .60 1 .80 00 20 40 60 80 00 20 40 60 80 00 20 40 4 .60 4 .80 5 5 5 5 5 6 6 6 6 6 00 20 40 60 80 00 20 40 60 .80 Z (m) o 208 The p e n e t r a t i o n phase of the DMT i s very s i m i l a r to the pe n e t r a t i o n of a cone. Chapter 4 provided d e t a i l s on the curr e n t understanding of the penet r a t i o n process f o r CPT. Experience w i t h CPT r e s u l t s has shown that very l a r g e s t r e s s e s are generated during cone p e n e t r a t i o n . Although the dil a t o m e t e r i s wedge shaped and only 14 mm t h i c k i t can be expected that l a r g e changes i n s t r e s s e s w i l l a l s o occur around the blade during p e n e t r a t i o n . I t can th e r e f o r e be assumed that the p e n e t r a t i o n process w i l l have some i n f l u e n c e on t h e me a s u r e d v a l u e s P and P, d u r i n g d i l a t o m e t e r membrane o 1 expansion. The expansion of the dilatometer membrane i s s i m i l a r to the expansion of a pressuremeter. Thus, many o f the observations made from pressuremeter t e s t i n g may apply to the expansion phase of dil a t o m e t e r t e s t i n g . F u l l d e t a i l s of the pressure expansion phase of pressuremeter t e s t i n g was given i n Chapter 5 and Appendix 2. The membrane on the dilatometer i s l o c a t e d i n the center of one si d e of the f l a t p l a t e , a short d i s t a n c e behind the sharpened t i p . Observations and c a v i t y expansion t h e o r i e s have shown that there i s some t o t a l s t r e s s r e l i e f behind the t i p of a penetrating e l e c t r i c cone. This i s because the t o t a l s t r e s s e s r e q u i r e d to open the c a v i t y a t the t i p are l a r g e r than those r e q u i r e d to maint a i n the c a v i t y . In the case o f the e l e c t r i c cone, the t h e o r i e s of s p h e r i c a l c a v i t y expansion r e l a t e approximately to the t i p and c y l i n d r i c a l c a v i t y expansion to the cone s h a f t . I t seems reasonable that a s i m i l a r process e x i s t s f o r pe n e t r a t i o n of the f l a t p l a t e d i l a t o m e t e r . However, the l e v e l of s t r e s s e s and s t r a i n s developed around the dilatometer are probably smaller than those around a cone, when penetrating the same m a t e r i a l , because of the thinner (14 mm) wedge shape. The element of s o i l 209 that i s i n contact w i t h the dilatometer membrane, however, has undergone some s t r e s s r e l i e f ( i . e . unloading). Observations from pressuremeter t e s t i n g have shown that the e l a s t i c modulus can be measured by performing an unload-reload c y c l e . The membrane expansion of a di l a t o m e t e r appears to be a r e l o a d i n g and t h e r e f o r e may be as s o c i a t e d w i t h an e l a s t i c modulus. However, the expansion of 1 mm a t the center of the membrane may exceed the previous unloading and f u r t h e r shearing may take p l a c e , r e s u l t i n g i n a modulus s o f t e r than the e l a s t i c modulus. The w r i t e r b e l i e v e s that the p e n e t r a t i o n process and the phenomena of s t r e s s r e l i e f can be expected to have a s i g n i f i c a n t i n f l u e n c e on the measured values P and P. and the d i f f e r e n c e ( P - P ). o 1 1 o 6.3.2.2. DMT i n Sand Observations made during CPT and SBPMT i n t h i s study would i n d i c a t e t h a t DMT p e n e t r a t i o n and membrane i n f l a t i o n i n c l e a n sands u s u a l l y takes place under drained c o n d i t i o n s . Experience has shown that the values of P q , P^ and ( P i ~ p 0 ) a r e u s u a l l y r e l a t i v e l y l a r g e i n sands, e s p e c i a l l y dense sands. Thus, e r r o r s i n assumed values of u and a' have a l e s s o vo s i g n i f i c a n t i n f l u e n c e on the index parameters than i n s o f t c l a y s where the values of P , P and (P,-P ) are u s u a l l y s m a l l . The membrane of the dilatometer i s located i n a s i m i l a r p o s i t i o n r e l a t i v e to the t i p as the f r i c t i o n sleeve on the cone t i p . Thus, the changes i n i n - s i t u stresses adjacent to the dilatometer membrane due to penetration can be expected to vary i n a s i m i l a r manner to those around the f r i c t i o n sleeve of a cone as shown In F i g . 4.6. Thus, the measurement, P - U q , can be expected to increase with increasing d i l a t a n c y of the sand deposit. Data presented i n Appendix 1 indicates that the maximum d i l a t i o n angle for a sand decreases l i n e a r l y with the logarithm of increasing confining s t r e s s . However, for a l i m i t e d stress range i t can be assumed tha t the maximum d i l a t i o n angle (v ) decreases l i n e a r l y with max increasing confining pressure. Thus, i t can be expected that the h o r i z o n t a l s t r e s s index parameter, K,, should be re l a t e d to r e l a t i v e d density f o r normally consolidated, uncemented sand. Recent chamber test r e s u l t s i n sand using the DMT suggest t h i s to be true (Marchetti, 1982). Results presented by Marchetti (1982) are shown on F i g . 6.4. Results from the two sand s i t e s presented by Marchetti i n h i s ASCE 1980 paper are also i n c l u d e d i n F i g . 6.4. The i n - s i t u v e r t i c a l e f f e c t i v e stress (a' ) for the vo data presented i n F i g . 6.4 was i n the range 0.5 to 2.2 kg/cm 2. The i n - s i t u r e l a t i v e density values of the sand deposits presented by Marchetti (1980) were estimated by the writer from CPT data using the c o r r e l a t i o n shown i n F i g . 4.10. However, Marchetti (1980 & 1982) suggests that the sand at the two s i t e s have r e l a t i v e d e n s i t i e s around 60 to 70%. It seems u n l i k e l y , however, based on the c a l i b r a t i o n test r e s u l t s , that a sand with a r e l a t i v e d e n s i t y of 60 to 70%, and at a low c o n f i n i n g p r e s s u r e , would have K, d values of as low as 1.5. Experience gained during t h i s study appears to confirm t h i s view. 211 10 I s 5 H • Chomb&r f&,fo QAarchcffi) 1632.) A Dam^n Site - \ ( M a r a ) i f t l - I W ) • Torre Qglio 5ifd J / / O O 50 too RELATIVE DENSITY / P r , FIG. 6.4. PROPOSED CORRELATION BETWEEN HORIZONTAL STRESS INDEX FROM DMT AND RELATIVE DENSITY FOR NORMALLY CONSOLIDATED, UNCEMENTED, SAND. (Data from Marchetti, 1982) 212 6.3.2.3. DMT i n Clay Observations made during CPT and PMT would i n d i c a t e that DMT p e n e t r a t i o n and membrane expansion i n c l a y s takes place under undrained c o n d i t i o n s . Observations and c a v i t y expansion t h e o r i e s would a l s o suggest t h a t the pe n e t r a t i o n and membrane expansion generate very l a r g e pore pressures during DMT i n s o f t , normally c o n s o l i d a t e d cohesive s o i l s . The c l a y s i t e s used by Mar c h e t t i (1980) to develop the e m p i r i c a l c o r r e l a t i o n s were mostly composed of s o f t saturated d e p o s i t s where l a r g e p o s i t i v e pore pressures could be expected during DMT. Experience a t UBC has shown th a t the values of P.P., and (P,-P ) are o 1 1 o u s u a l l y s m all i n s o f t c l a y d e p o s i t s . Thus, e r r o r s i n assumed values of u o and can have a s i g n i f i c a n t i n f l u e n c e on the de r i v e d index parameters and subsequent i n f e r r e d g e otechnical parameters. C a v i t y expansion t h e o r i e s have shown that a l i m i t pressure e x i s t s f o r undrained c a v i t y expansion i n s o f t c l a y s . I t seems reasonable to assume tha t the p e n e t r a t i o n process i n a DMT i s s u f f i c i e n t to induce pressures e q u i v a l e n t to the l i m i t pressure. Because of the s t r e s s r e l i e f phenomena, c r e e p and pore p r e s s u r e d i s s i p a t i o n , the l i f t - o f f pressure P q i s l e s s than the l i m i t pressure. However, the expansion of 1 mm i s probably s u f f i c i e n t to r e - e s t a b l i s h a l i m i t p r e s s u r e . Thus the value (P -P ) i s 1 o probably r e l a t e d to the l i m i t pressure f o r some form of c a v i t y expansion. The c a v i t y expansion t h e o r i e s have shown that the l i m i t pressures are r e l a t e d to the E / r a t i o (Appendix 2 ) . Ladd et a l . (1977) have shown t h a t the E / C u r a t i o v a r i e s a p p r o x i m a t e l y w i t h p l a s t i c i t y index ( P I ) . Thus, the index parameters I , and E, from DMT r e s u l t s i n cohesive s o i l s d d may r e l a t e to the PI of the s o i l . Since experience has shown that g e o t e c h n i c a l parameters such as u n d r a i n e d shear s t r e n g t h and c o m p r e s s i b i l i t y can be r e l a t e d i n some manner to P I , i t seems reasonable 213 t h a t the i n d e x parameters I , and E, can s i m i l a r l y be r e l a t e d to these d d g e o t e c h n i c a l parameters. M a r c h e t t i (1980), Schmertmann (1980) and Lacasse and Lunne (1982) have reported good c o r r e l a t i o n s i n s o f t c l a y deposits using DMT r e s u l t s . For DMT r e s u l t s i n overconsolidated cohesive s o i l s , the c o r r e l a t i o n s may not be so s u c c e s s f u l . This may be due to the pore pressure e f f e c t s . During p e n e t r a t i o n i n overconsolidated cohesive s o i l s , s m a ll p o s i t i v e or p o s s i b l y negative (of h y d r o s t a t i c ) water pressures may be generated. These s m a l l e r pore p r e s s u r e s may i n f l u e n c e the measured P q and v a l u e s , s i n c e both values are t o t a l s t r e s s measurements. An example of t h i s phenomenon w i l l be presented i n Chapter 7 and i t s i m p l i c a t i o n s on the i n t e r p r e t a t i o n d i s c u s s e d . 6.4. L i q u e f a c t i o n Resistance 6.4.1. E x i s t i n g DMT L i q u e f a c t i o n C o r r e l a t i o n M a r c h e t t i (1982) suggested t h a t the h o r i z o n t a l s t r e s s index, K,, d could be used as a parameter to assess the l i q u e f a c t i o n r e s i s t a n c e of sands. appears to r e f l e c t the f o l l o w i n g s o i l v a r i a b l e s ; i ) R e l a t i v e d e n s i t y , D^ ; i i ) i n - s i t u s t r e s s e s , K ; o i i i ) s t r e s s h i s t o r y and p r e - s t r e s s i n g ; i v ) aging; v) cementation. However, i t i s not p o s s i b l e to i d e n t i f y the i n d i v i d u a l r e s p o n s i b i l i t y o f each v a r i a b l e . On the o t h e r hand, when K, i s low none of t h e s e d v a r i a b l e s i s h i g h , i . e . the sand i s l o o s e , uncemented, i n a low h o r i z o n t a l s t r e s s environment and has l i t t l e s t r e s s h i s t o r y . A sand under these 214 c o n d i t i o n s may be a l i q u e f a c t i o n problem. B a s e d on the a s s u m p t i o n t h a t K = 1.5 f o r n a t u r a l , s a t u r a t e d , d n o r m a l l y c o n s o l i d a t e d sands w i t h a = 60 to 70%, M a r c h e t t i (1982) suggested a c o r r e l a t i o n between the c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n (T./O* ) and h o r i z o n t a l s t r e s s index, K,, as f o l l o w s , a' 10 6.4.2. Proposed DMT L i q u e f a c t i o n C o r r e l a t i o n I t i s c o n s i d e r e d by the w r i t e r t h a t t h e a s s u m p t i o n Kj = 1.5 f o r n a t u r a l s a n d s w i t h a D = 60-70% i s i n c o r r e c t . F i g . 6.4 shows a r r e l a t i o n s h i p between K, and D f o r normally c o n s o l i d a t e d , uncemented d r sands. I f t h i s r e l a t i o n s h i p i s combined w i t h the c y c l i c r e s i s t a n c e curve developed by Vaid et a l . (1981) and shown i n F i g . 3.5 a l i q u e f a c t i o n c o r r e l a t i o n can be developed. This has been performed by the w r i t e r and i s shown i n F i g . 6.5. M a r c h e t t i ( 1 9 8 2 ) has shown t h a t K, appears to i n c r e a s e w i t h d i n c r e a s e s i n , a g i n g , cementation, and s t r e s s h i s t o r y . Experience has shown that the l i q u e f a c t i o n r e s i s t a n c e a l s o increases w i t h these f a c t o r s . Although the c o r r e l a t i o n shown i n F i g . 6.5 i s based on a - Dr r e l a t i o n s h i p f o r normally c o n s o l i d a t e d , uncemented sands any i n c r e a s e i n the above f a c t o r s w i l l produce an increase i n apparent d e n s i t y and thus be r e f l e c t e d by an increase i n l i q u e f a c t i o n r e s i s t a n c e . The c o r r e l a t i o n proposed i n F i g . 6.5 f o r DMT data i s based on l i m i t e d e m p i r i c a l data and r e q u i r e s c o n s i d e r a b l e f i e l d v e r i f i c a t i o n . The method can be used i n the same manner proposed f o r the CPT and SPT based methods. 215 0.5-1 0.4 H 0.5 H 0.2 H o.i H 0 / X / / / X / 10 HORIZONTAL STZBSS /NPEX, Kd £<j - Po - Up FIG. 6.5. PROPOSED CORRELATION BETWEEN LIQUEFACTION RESISTANCE UNDER LEVEL GROUND CONDITIONS AND DILATOMETER HORIZONTAL STRESS INDEX FOR SANDS. 216 The c o r r e l a t i o n i s only a p p l i c a b l e f o r t e s t i n g i n sands where p e n e t r a t i o n and expansion occur under drained c o n d i t i o n s . T e s t i n g i n s i l t y sands or s i l t s may generate s i g n i f i c a n t pore pressures which would i n f l u e n c e the measured K, v a l u e s . d Chapter 7 w i l l present f i e l d and l a b o r a t o r y data to evaluate the proposed DMT c o r r e l a t i o n . 217 7. FIELD AND LABORATORY STUDY 7.1. I n t r o d u c t i o n A f i e l d and l a b o r a t o r y study was undertaken by the w r i t e r to evaluate the I n - s i t u t e s t methods st u d i e d i n t h i s r e search. The study was conducted mainly a t the U n i v e r s i t y of B r i t i s h Columbia (UBC) research s i t e (McDonald's Farm) near Richmond, but s e v e r a l other s i t e s i n B r i t i s h Columbia were a l s o s t u d i e d . A d d i t i o n a l t e s t data was obtained from a v a i l a b l e l i t e r a t u r e and from l o c a l g e o t e c h n i c a l c o n s u l t a n t s . 7.1.1. F i e l d Study Most of the f i e l d work was performed using the UBC i n - s i t u t e s t i n g v e h i c l e . A complete d e s c r i p t i o n o f the v e h i c l e , i t s c a p a c i t y and h y d r a u l i c c o n t r o l s i s g i v e n by Campanella and Robertson, 1981. The v e h i c l e , shown i n F i g . 7.1, was designed as a low c o s t , v e r s a t i l e v e h i c l e f o r both research and teaching i n the f i e l d . The v e h i c l e was used during t h i s study to perform cone p e n e t r a t i o n t e s t s , f l a t p l a t e dilatometer t e s t s and s e v e r a l push-in cone-pressuremeter t e s t s . 7.1.2. Laboratory Study S t a t i c and c y c l i c t r i a x i a l t e s t s were performed to determine the s t a t i c and dynamic c h a r a c t e r i s t i c s of s o i l s from v a r i o u s s i t e s . The t e s t s were g e n e r a l l y c a r r i e d out on f u l l s i z e , 86 mm I.D. (3-3/8 inch) Shelby tube p i s t o n samples. A schematic layout of the t r i a x i a l t e s t i n g apparatus i s shown i n F i g . 7.2. For c y c l i c t e s t i n g , the a x i a l c y c l i c l o a d was a p p l i e d to the sample by means of a double a c t i n g a i r p i s t o n . C y c l i c l o a d was c o n t r o l l e d by 218 U B C Field Research V e h i c l e , supported and leveled on large pads , ra ised mast h o u s e s penetrat ion d e v i c e . FIG. 7.1. UBC FIELD RESEARCH VEHICLE, SUPPORTED AND LEVELED ON LARGE PADS, RAISED MOST HOUSES PENETRATION DEVICE. (After Campanella and Robertson, 1981) 219 pneumatic s o l e n o i d v a l v e s a c t i v a t e d by an e l e c t r i c timer s w i t c h . The c y c l i c l o a d i n g t e s t s were g e n e r a l l y c a r r i e d out a t a frequency o f 1 Hz. During each c y c l i c t e s t , c y c l i c l o a d , porewater pressure and a x i a l s t r a i n were continuously monitored by e l e c t r i c transducers and records obtained on a l i g h t beam recorder. In order to maintain a constant c y c l i c load amplitude when l a r g e a x i a l s t r a i n s develop near onset of l i q u e f a c t i o n , a constant backpressure r e g u l a t o r was i n s t a l l e d on the bottom chamber of the a i r p i s t o n . R e s t r i c t e d drainage from or to the c e l l can cause f l u c t u a t i o n i n the c e l l pressure due to the t h r u s t i n g or e x t r a c t i n g l o a d i n g ram when l a r g e s t r a i n s develop. Therefore, a s u f f i c i e n t l y l a r g e drainage l i n e was used to connect r e s e r v o i r A to the t r i a x i a l c e l l (see F i g . 7.2) through which the c e l l pressure was a p p l i e d during c y c l i c l o a d i n g . Hence porewater pressures measured were due only t o the shearing of the sample. S t a t i c t e s t s were c a r r i e d out under s t r a i n c o n t r o l l e d c o n d i t i o n s . The constant r a t e of s t r a i n was c a l c u l a t e d from the c o n s o l i d a t i o n stage of the t e s t . Records of l o a d , pore pressure, a x i a l s t r a i n and volume change were recorded using a s t r a i n i n d i c a t o r box, d i a l gauge and p i p e t t e . M u l t i - s t a g e t e s t i n g was c a r r i e d out whenever p o s s i b l e to o b t a i n data on changes i n s t a t i c c h a r a c t e r i s t i c s w i t h i n c r e a s i n g s t r e s s l e v e l . Each stage was completed a t or s l i g h t l y before f a i l u r e , where f a i l u r e was defined as the point of maximum p r i n c i p l e s t r e s s r a t i o . P r i o r to c o n s o l i d a t i o n and shearing a l l samples were saturated i n stages under backpressure, keeping a small p o s i t i v e e f f e c t i v e c o n f i n i n g pressure at a l l times. S a t u r a t i o n was considered to have been achieved when the pore pressure parameter, B, reached a value of 0.98 o r , g r e a t e r . F o l l o w i n g s a t u r a t i o n , samples were co n s o l i d a t e d and volume changes 220 A/5 SUPPLY T/MEZ t SOLENOID VALVES li DOUbLE ACTING AIH Pie-TDN LVD1-727 ZECOUDEIZ& TRIAYJAL CELL £04 0 C£LL LEGZND (?) REGULATORS 5o VALVES t 4/2 SUPPLY J J TRANSDUCER TO RECORDER TO RECORI>ER POIZE PR£56U££ TRANSDUCER. STRAIN CONTROLLED FIG. 7.2. SCHEMATIC OF TRIAXIAL TESTING APPARATUS. 221 recorded. For drained, s t a t i c t e s t s , s a t i s f a c t o r y d i s s i p a t i o n of excess pore pressures during shearing was checked by monitoring pore pressures a t the top of the sample. 7.2. McDonald's Farm, Richmond 7.2.1. S i t e D e s c r i p t i o n and Geology A research s i t e f o r i n - s i t u t e s t i n g i s l o c a t e d on an abandoned farm (McDonald's Farm) near the Vancouver I n t e r n a t i o n a l A i r p o r t . The s i t e i s l o c a t e d on the n o r t h s i d e of Sea I s l a n d on M i n i s t r y o f Transport, Canada land near the M u n i c i p a l i t y of Richmond. Sea Is l a n d i s l o c a t e d between the North Arm and Middle Arm of the Fraser River on the n o r t h s i d e o f the main Fraser River D e l t a . The s i t e i s approximately l e v e l w i t h the n a t u r a l ground a t e l e v a t i o n +1.6 m. Sea I s l a n d i s contained by a system of dykes to protect against f l o o d i n g from the Fraser R i v e r . A summary of the s o i l p r o f i l e based on sampling, l a b o r a t o r y and cone pe n e t r a t i o n t e s t i n g (CPT) i s shown i n F i g . 7.3. The upper 2 m of s o i l c o n s i s t s of s o f t , compressible c l a y s and s i l t s . The sand from 2 m to 13 m was deposited i n a tu r b u l e n t environment and i s t h e r e f o r e r e l a t i v e l y non-uniform i n d e n s i t y . In general however, the sand inc r e a s e s i n d e n s i t y w i t h depth as i n d i c a t e d by the constant r e l a t i v e d e n s i t y r e l a t i o n s h i p by B a l d i et a l . , 1982. The sand has medium to coarse g r a i n s i z e s w i t h t h i n l a y e r s of medium to f i n e sand and some lenses of s i l t y sand. A t h i n t r a n s i t i o n l a y e r of f i n e sand with some s i l t e x i s t s from 13 m to 15 m. The sand i s u n d e r l a i n by a t h i c k d e p o s i t o f s o f t , n o r m a l l y c o n s o l i d a t e d clayey s i l t . The clayey s i l t i s estimated to extend to a depth of more than 300 m (Blunden, 1973). Groundwater i s approximately 1 m below e x i s t i n g ground surface and PORE PRESSURE FRICTION RESISTANCE BEARING RESISTANCE FRICTION RATIO DIFFERENTIAL P.P. SOIL U ( B A R ) . . . FC (BAR). QT (BAR) , M R f :FC /QT(%) RATIO Al l /QT PROFILE v ZOO 0 2 0 .80 h 10 D r » 6 0 % (Boldi ot ol,1982) i i i i i i i i i Solt CLAY & SILT Coort* SAND Loose to Dense with layers ot lino Sand Fin* SAND, M n t »Ht Soft,normolly consolidated cloyty SILT Sand * 10% Silt « 7 0 % Clay » 2 0 % L.L. * 3 8 % P.I. « I 5 % W„ « 3 5 % k38X|0"7cm/WcJ c^o.s 'Equilibrium pore pressure I B A R « I O O k P o 2 8 I k g f / c m 2 * I ton / f t . 2 223 groundwater pressures are approximately h y d r o s t a t i c to the depth shown i n F i g . 7.3. 7.2.2. F i e l d Study In A p r i l , 1981 a standard s i t e i n v e s t i g a t i o n borehole was made using a BBS-37A r o t a r y d r i l l r i g . The d r i l l r i g and operators were provided by the B.C. M i n i s t r y of Highways and Tr a n s p o r t a t i o n , whose a s s i s t a n c e i s g r a t e f u l l y acknowledged. The hole was d r i l l e d to a t o t a l depth of 24.5 m. Standard P e n e t r a t i o n Tests (SPT) and undisturbed sampling were c a r r i e d out at approximately 1.5 m i n t e r v a l s . A s i t e p l a n showing the t e s t l o c a t i o n s and sample borehole i s shown i n F i g . 7.4. The SPT was c a r r i e d out according to ASTM 1586:19 67. A standard sample tube was d r i v e n 600 mm (24 i n s . ) i n t o undisturbed s o i l at the bottom of the d r i l l h o l e , by blows from a 63.5 kg (140 lb ) hammer, dropped 760 mm (30 i n s . ) . The number o f blows per 150 mm (6 i n s . ) p e n e t r a t i o n were recorded. The SPT N-value was recorded over the range 150 mm (6 in s . ) to 450 mm (18 i n s . ) p e n e t r a t i o n . The hammer was a standard c y c l i n d e r s a f e t y hammer operated using a manila rope and cathead system. One t u r n of the rope around the cathead was used to l i f t the hammer. The borehole was approximately 100 mm (4 i n s . ) i n diameter and supported by d r i l l i n g mud. Undisturbed samples were taken using a 86 mm I.D. (3-3/8 i n c h ) , t h i n w a l l e d , f i x e d p i s t o n , Shelby tube sampler. The borehole l o g , SPT N-values and sample l o c a t i o n s are shown i n F i g . 7.5. Six CPT's were conducted using the UBC 5 channel cone (see Chapter 4 ) , a t l o c a t i o n s shown i n F i g . 7.4. To i n v e s t i g a t e s o i l v a r i a b i l i t y , the w r i t e r performed two CPT's conducted w i t h i n 1 m of each other (PC3 and N STEEL 3 REFERENCE etx>r PCZ A ebpMT-3 O 5&PMT-2 o PCS A * 0 PPMT-I LEGEND SBPMT-I 3CHI PPMT-Z PMT-I ±PC4 APCZ • A 8 A PIEZOMETER. COhJE 7E£>7 • /=Z47 PLKTE D/LA70METEZ. TE&T O bOZJUA PP.E66UIZEME7EZ. TEST fj PU$H-IH CONE- W££*?UZPh1E7EJ^TEt>7 E BOZeHOLE, STANDARD PZNE7ZATI0N TES7, SAMPLED FIG. 7.4. SITE PLAN OF McDONALD'S FARM. 225 S STANDARD PEUE18STI0N TEST N blows J ft-10 20 30 — . 1 1 1-10 -20 SAMPLE No l • 9LZ1 SOIL PROFILE Soft CL&f* SILT Loose f° mzdlum demA, grey t medium fo coarse SAND vifh some layers of fine sand. .Loose, grey fine SAND, some si If Scff, grey clayey S/LT V ID End of- hole. 2 4 . 5*7. 104 FIG. 7.5. SUMMARY OF BOREHOLE RECORD, MCDONALD'S FARM. P C 4 ) . The CPT d a t a showed no s i g n i f i c a n t d i f f e r e n c e i n the b e a r i n g p r o f i l e s , q^,, e s p e c i a l l y i n the underlying clayey s i l t d e p o s i t . The sand d e p o s i t from 2 m to 15 m showed s l i g h t v a r i a b i l i t y i n a c t u a l peaks and troughs of the two bearing p r o f i l e s . A comparison between four cone bearing p r o f i l e s to a depth o f 15 m i s shown i n F i g . 7.6. I t i s c l e a r that the sand deposit i s r e l a t i v e l y c o n s i s t e n t over the t e s t i n g area but that s l i g h t v a r i a t i o n s e x i s t due to the complex d e p o s i t i o n a l environment i n which the sand was l a i d down. Two d i l a t o m e t e r t e s t s (DMT) were performed at l o c a t i o n s shown i n F i g . 7.4. A t y p i c a l p r o f i l e o f the intermediate parameters ( I . , K, and E.) from d d d DMT-1 i s shown i n F i g . 7.7. Three s e l f - b o r i n g pressuremeter holes were made under the s u p e r v i s i o n o f the w r i t e r a t l o c a t i o n s shown i n F i g . 7.4. A t o t a l of 19 s e l f - b o r i n g pressuremeter t e s t s (SBPMT's) were performed to a maximum depth of 19 m. A summary of the SBPMT's performed a t McDonald's Farm i s given i n Table 7.1. SBPMT-1 was made using a l a r g e t r a c k mounted Chapman 1988 H e l i d r i l l operated by Jay-Dee D r i l l e r s . SBPMT-2 and 3 were made using a smaller t r u c k mounted Gardener Denver TD4 d r i l l r i g operated by Associated D r i l l e r s . Considerable d i f f i c u l t y was experienced during the i n s t a l l a t i o n of the pressuremeter In the sand using the l i g h t e r t r u c k mounted d r i l l r i g (SBPMT-2 ) . This i s evident from the poor t e s t r e s u l t s from SBPMT-2. To i n v e s t i g a t e a p o s s i b l e new i n - s i t u t e s t method s e v e r a l push-in cone-pressuremeter holes were made (PPMT-1 and 2 ) , a t l o c a t i o n s shown i n F i g . 7.4. The cone-pressuremeter i s e s s e n t i a l l y the s e l f - b o r i n g 227 FIG. 7.6. COMPARISON OF 4 CPT PROFILES AT MCDONALD'S FARM. 228 U.B.C. IN SITU TESTING. location: MACDQNALD'S FARM McDonald's Farm DMT INTERMEDIATE GEOTECHNICAL PARAMETERS Test No. DH-1 Test Date; 1 4 / 5 / 8 1 O OJ CL. QJ ' — ' s a a ro x QJ -o C C M o N in •H m i_ QJ a L. x CO ro x •H QJ l_ - o OJ C m M ro CZ J. LO • co-in • cn • m Mid3Q 0 9 DDI O'M O'BI O'ZZ 0 9 2 J I I I I 0, LU * V Y i i i i i i i 1 1 1 1 1 1 1 1 1 1 1 1 1 b •a* T 0 9 1 1 1 r-O'OI C M (U) qidaa i r -C8I 1 r-C22 1 I C9Z SI FIG. 7.7. INTERMEDIATE PARAMETERS FROM DMT-1, McDONALD'S FARM. 229 TABLE 7.1 SUMMARY OF SELF-BORIKG PRESSUREMETER TESTING MCDONALD'S FARM, RICHMOND HOLE DATE TEST TEST TEST DETAILS REMARKS NO. NO. DEPTH (m) SBPMT--1 03-12-81 1 3.0 1 c y c l e good t e s t 2 3.8 10 cy c l e s •• 3 4.6 2 c y c l e s 4 5.3 Expansion 5 6.3 10 c y c l e s SBPMT--2 11-02-81 1 4.9 Expansion l e a k 2 6.2 1 c y c l e poor t e s t - one arm 3 7.0 Expansion good t e s t 4 8.5 10 c y c l e s poor t e s t 5 9.3 1 c y c l e poor t e s t 5a 9.3 Re-expansion Re-exansion SBPMT--2 12-02-81 6 11.0 1 c y c l e good t e s t 7 11.7 10 c y c l e s poor t e s t - one arm 8 12.8 No expansion p o s s i b l e o b s t r u c t i o n SBPMT--3 13-02-81 9 16.2 1 c y c l e good t e s t 10 16.9 Expansion tt 11 17.7 Expansion t< 12 18.4 Holding Test «• 13 18.9 Expansion i * 230 pressuremeter w i t h the c u t t i n g shoe and r o t a t i n g c u t t e r removed and a s o l i d 60 degree cone f i t t e d i n place o f the shoe. The instrument was pushed i n t o the ground a t a constant r a t e o f 2 cm/sec. and standard pressuremeter t e s t s performed a t s e l e c t e d depths. A l t o g e t h e r 9 push-in cone-pressuremeter t e s t s were c a r r i e d out. A summary of the push-in cone-pressuremeter t e s t i n g i s gi v e n i n Table 7.2. The push-in t e s t h o l e , PPMT-1, was made using the same d r i l l r i g t hat c a r r i e d out SBPMT-1. The push-in t e s t h o l e , PPMT-2, was made using the UBC i n - s i t u t e s t i n g v e h i c l e ( F i g . 7.1). 7.2.3. Laboratory Study. Laboratory t e s t i n g was c a r r i e d out on undisturbed samples obtained from the borehole (BCH1) at McDonald's Farm. The l a b o r a t o r y program was d i r e c t e d towards determination of the s t a t i c and dynamic c h a r a c t e r i s t i c s of the sand deposit from a depth o f 2 m to 15 m. A summary of the s i g n i f i c a n t l a b o r a t o r y r e s u l t s i s given i n Table 7.3 and 7.4. D e t a i l s o f the t e s t procedures and i n t e r p r e t a t i o n o f r e s u l t s are given i n the f o l l o w i n g s e c t i o n s . 7.2.3.1. S o i l C l a s s i f i c a t i o n and Grain S i z e During sample e x t r a c t i o n samples were v i s u a l l y c l a s s i f i e d and de s c r i b e d . F o l l o w i n g t e s t i n g , standard s i e v e analyses were performed to o b t a i n g r a i n s i z e d i s t r i b u t i o n . The s i g n i f i c a n t g r a i n s i z e data f o r each sample are shown i n Table 7.3 and 7.4. An envelope of the g r a i n s i z e curves i s shown on F i g . 7.8. The sand was subangular to angular and predominantly of uniform medium TABLE 7.2 SUMMARY OF PUSH-IN CONE-PRESSUREMETER TESTING  MCDONALD'S FARM, RICHMOND HOLE DATE TEST TEST NO. NO. DEPTH TEST DETAILS REMARKS PPMT-1 04-12-81 PPMT-2 20-01-82 (m) 6 2.7 1 c y c l e 7 3.8 10 c y c l e s 8 4.6 Expansion 1 2.75 1 c y c l e 2 3.8 1 c y c l e 3 4.6 1 c y c l e 4 5.5 1 c y c l e 5 6.7 Expansion 6 7.6 2 cy c l e s good t e s t TABLE 7.3 SUMMARY OF STATIC DRAINED TRIAXIAL COMPRESSION TEST RESULTS, McDONALD'S FARM, RICHMOND Sample No. Depth (m) Gradation Void Ratio e Relative Density Dr (%) E f f e c t i v e Confining Stress °3 C (kg/cm) Maximum F r i c t i o n Angle <t>m (deg) Young's Modulus A x i a l S t r a i n at f a i l u r e e f (%) D50 (mm) D10 (mm) ^50 (kg/cm 2) E l (kg/cm 2) 1 3.5 0.45 0.15 0.66 35 0.40 41 100 250 3.0 2 4.8 0.15 0.09 0.90 29 0.50 42 100 400 5.1 3 6.3 0.50 0.18 0.64 45 0.62 43.5 310 385 3.0 5 10.9 0.30 0.12 0.9* 37* 0.70 42 250 435 5.5 7 14.0 0.12 0.06 0.7 67 0.75 41 200 455 6.2 * Some wood l n sample - e & Dr not considered r e l i a b l e . Samples 4 and 6 were damaged during preparation and not tested. TABLE 7.4 SUMMARY OF CYCLIC UNDRAINED TRIAXIAL TEST RESULTS, MCDONALD'S FARM, RICHMOND Sample No. Depth (m) Grade i t l o n Void Ratio e Relative Density Dr (%) I n - s i t u V e r t i c a l E f f e c t i v e Stress o; Q( kg/cm2) C y c l i c Stress Ratio to 10% P-P D50 (mm) D10 (mm) 10 cycles 20 cycles °dy/ 2 o3c ' vo <V2o3c T'°vo 1 3.5 0.45 0.15 0.64 45 0.40 0.22 0.14 0.19 0.12 2 4.8 0.15 0.09 0.87 41 0.53 0.27 0.18 0.24 0.16 3 6.3 0.50 0.18 0.61 59 0.68 0.36 0.23 0.32 0.21 4 9.4 0.60 0.45 0.59 68 1.00 0.37 0.24 0.33 0.21 5 10.9 0.30 0.12 0.90* 13* 1.14 0.35 0.23 0.30 0.20 6 12.4 0.50 0.20 0.63 51 1.29 0.38 0.25 0.33 0.21 7 14.0 0.12 0.06 0.60 50 1.43 0.32 0.21 0.28 0.18 * Some wood i n sample - e & Dr not considered r e l i a b l e . FIG. 7.8. RANGE OF GRAIN SIZE DISTRIBUTION OF SAND AND CLAYEY SILT DEPOSITS, McDONALD'S FARM. 235 g r a i n s i z e with some lenses of uniform f i n e to s i l t y sand. The samples were generally homogeneous i n grain si z e although several samples did contain wood fragments. The wood fragments made dry density estimates rather u n r e l i a b l e . Visual petrographic analyses were performed on several samples to i d e n t i f y the predominant grain minerals. T y p i c a l data from the analyses are given below. A t y p i c a l g r ain s i z e d i s t r i b u t i o n i n the underlying clayey s i l t i s also shown on F i g . 7.8. Atterberg l i m i t values f o r the clayey s i l t are given on Fig . 7.3. 7.2.3.2. Density During sample preparation s u f f i c i e n t measurements were made to enable moisture content and un i t dry weights to be determined. Maximum and minimum void r a t i o s were determined according to ASTM: D2049. Several attempts were required to obtain r e l a t i v e l y consistent maximum and minimum void r a t i o s . Calculated r e l a t i v e density values f o r each sample are given i n Tables 7.3 and 7.4. Some d i f f i c u l t y was encountered with several samples because of the presence of wood fragments. The n o n - c y l i n d r i c a l shape of the samples a f t e r preparation also had some influence on the calculated void r a t i o s . In general, the d i f f i c u l t i e s associated with determination of maximum, minimum and sample d e n s i t i e s cast some uncertainty over the calculated r e l a t i v e density values. Percentage by volume Quartz Feldspar and rock fragments Mica 10 20 70 236 7 . 2 . 3 . 3 . S t a t i c Shear Strength Standard drained t r i a x i a l compression t e s t s were c a r r i e d out on s e v e r a l samples. A l l t e s t s were c a r r i e d out under i s o t r o p i c s t r e s s c o n d i t i o n s . M u l t i - s t a g e t e s t i n g was c a r r i e d out whenever p o s s i b l e to o b t a i n a d d i t i o n a l data on the v a r i a t i o n of shear s t r e n g t h and modulus w i t h i n c r e a s i n g s t r e s s l e v e l . Each stage of a mu l t i - s t a g e t e s t was stopped at or s l i g h t l y before f a i l u r e , where f a i l u r e was defined as the point of maximum p r i n c i p l e s t r e s s r a t i o . Maximum f r i c t i o n angles a t c o n f i n i n g s t r e s s e s approximately equal to i n - s i t u h o r i z o n t a l s t r e s s e s are given i n Table 7 . 3 . The i n - s i t u K q was assumed to be 0.5. The maximum f r i c t i o n angle was found to decrease approximately l i n e a r l y w i t h the logarithm of the e f f e c t i v e c o n f i n i n g s t r e s s . The decrease i n f r i c t i o n angle f o r 1 l o g c y c l e was g e n e r a l l y about 4 degrees. R e s u l t s were a l s o p l o t t e d i n terms of Rowe's s t r e s s - d i l a t a n c y equation from which an e s t i m a t e o f the c o n s t a n t volume f r i c t i o n angle <1> was cv obtained. R e s u l t s , however, were not very c o n s i s t e n t , p o s s i b l y due to the f o l l o w i n g reasons: i ) Non-uniform s t r a i n p a t t e r n w i t h i n a t r i a x i a l sample, i i ) Membrane p e n e t r a t i o n , i i i ) Small sample s i z e . R e s u l t s , however, i n d i c a t e an average constant volume f r i c t i o n angle of about 3 6 degrees. This value appears reasonable c o n s i d e r i n g the r e l a t i v e a n g u l a r i t y and the g e n e r a l l y uniform g r a i n s i z e d i s t r i b u t i o n of the sand. 2 3 7 7.2.3.4. Modulus To determine s t r e s s s t r a i n parameters, the t r i a x i a l compression t e s t s were i n t e r p r e t e d using the procedures described by Duncan and Chang (1970). A transformed h y p e r b o l i c p l o t was drawn to enable the i n i t i a l tangent Young's M o d u l u s , , t o be d e t e r m i n e d f o r e a c h t e s t . The E ^ , c a l c u l a t e d i n t h i s manner, a t an e f f e c t i v e c o n f i n i n g pressure approximately equal to the assumed i n - s i t u h o r i z o n t a l s t r e s s i s shown i n Table 7.3. The Secant Young's Modulus a t 50 percent of the f a i l u r e s t r e s s , ^$Q> w a s a l s o obtained and shown i n Table 7.3. The v a r i a t i o n o f and E,_Q w i t h the mean c o n s o l i d a t i o n s t r e s s (o^) appears to f i t w e l l the f o l l o w i n g e m p i r i c a l r e l a t i o n s h i p (Janbu, 1963) E = ^ Pa A)n Pa where K^, = modulus number n = modulus exponent, where n « 0.5 Pa = reference s t r e s s (Pa = 1 kg/cm 2) o' = mean c o n s o l i d a t i o n s t r e s s , c 7.2.3.5. C y c l i c Resistance The c y c l i c r e s i s t a n c e of the samples, i n terms o f the c y c l i c s t r e s s r a t i o , was determined from c y c l i c t r i a x i a l t e s t s performed under i s o t r o p i c s t r e s s c o n d i t i o n s ( K q = 1 ) . A c o r r e c t i o n f a c t o r i s re q u i r e d to convert the c y c l i c s t r e s s r a t i o from the c y c l i c t r i a x i a l t e s t , o, /2a' , to the dy 3 c y c l i c s t r e s s r a t i o , T/O^ , a p p l i c a b l e to the f i e l d , f o r l e v e l ground c o n d i t i o n s . The r e s u l t s of extensive l a b o r a t o r y s t u d i e s over the past 15 238 years have r e s u l t e d i n an improved understanding of the f a c t o r s that i n f l u e n c e the r e s u l t s from c y c l i c t r i a x i a l t e s t s on undisturbed samples. The s i g n i f i c a n t f a c t o r s are as f o l l o w s : i ) I n c o r r e c t i n i t i a l c o n s o l i d a t i o n s t r e s s e s ; i i ) E f f e c t s of m u l t i d i r e c t i o n a l shaking i n the f i e l d ; i i i ) Membrane pe n e t r a t i o n ; i v ) E f f e c t s of sample d i s t u r b a n c e . For c y c l i c l o a d i n g t e s t s to provide a r e l i a b l e index of s t r e s s c o n d i t i o n s causing l i q u e f a c t i o n or c y c l i c m o b i l i t y i n the f i e l d , i t i s necessary that they reproduce the f i e l d c o n d i t i o n s . The simple shear t e s t appears to provide the c l o s e s t r e p r e s e n t a t i o n of f i e l d c o n d i t i o n s . However, c y c l i c simple shear devices are complex and d i f f i c u l t f o r t e s t i n g undisturbed samples. The c y c l i c t r i a x i a l t e s t s i s co n s i d e r a b l y more p r a c t i c a l and makes a convenient a l t e r n a t i v e . However, the c y c l i c t r i a x i a l t e s t does not reproduce the c o r r e c t i n i t i a l s t r e s s c o n d i t i o n s f o r normally c o n s o l i d a t e d s o i l i n theground or i n a simple shear t e s t s i n c e i t i s performed w i t h i n i t i a l l y i s o t r o p i c s t r e s s c o n d i t i o n s (K^ = 1) to produce the symmetrically r e v e r s i n g shear s t r e s s c o n d i t i o n s r e p r e s e n t a t i v e of l e v e l ground c o n d i t i o n s . The s t r e s s r a t i o s to cause l i q u e f a c t i o n or a given c y c l i c s t r a i n l e v e l are u s u a l l y r e l a t e d by ^T^°vo^simple shear ^r^°dy^°3c^triaxial V a l u e s o f have been proposed by F i n n e t a l . (1971), Seed and Peacock (1971), Castro (1975) and I s i h a r a e t a l . (1977). The values o f C f d e p e n d on the i n i t i a l c o n s o l i d a t i o n s t r e s s e s ( K Q)« For n o r m a l l y c o n s o l i d a t e d sands (K^ = 0.5) a reasonable value appears to be C_ = 0.7. 239 In the f i e l d , s o i l elements are subjected to m u l t i d i r e c t i o n a l shaking whereas i n the l a b o r a t o r y only u n i d i r e c t i o n a l shaking i s c a r r i e d out. Experimental and a n a l y t i c a l s t u d i e s have shown that peak c y c l i c pore pressure r a t i o s of 100% occur under m u l t i d i r e c t i o n a l shaking c o n d i t i o n s a t a s t r e s s r a t i o 10% lower than those causing the same pore pressure r a t i o under u n i d i r e c t i o n a l shaking. Thus; ( T/ a' )*< s 0.9(T/O' 1 vo f i e l d vo'lab-simple shear The e f f e c t s of membrane p e n e t r a t i o n on measured pore pressures during undrained t r i a x i a l t e s t s on saturated sand has been recognised f o r a number of years. I n v e s t i g a t i o n s by El-Sohby and Andrawes (1972), Roscoe et a l . (1963) and Rajn et a l . (1974) have shown th a t f o r uniform sands and a given sample s i z e , membrane p e n e t r a t i o n c h a r a c t e r i s t i c s are p r i m a r i l y a f u n c t i o n o f p a r t i c l e s i z e ( a s c h a r a c t e r i z e d by the mean g r a i n s i z e , ^ Q ) , A N < * A R E reasonably independent of sample d e n s i t y . Membrane p e n e t r a t i o n i s s i g n i f i c a n t l y reduced w i t h i n c r e a s i n g sample s i z e and w i t h more w e l l graded sands. Data presented by Finn (1981) would i n d i c a t e t h a t f o r a uniform sand w i t h a D^ of about 0.3 mm and sample s i z e of 86 mm i n diameter the e r r o r i n c y c l i c s t r e s s r a t i o causing l i q u e f a c t i o n would be i n the order of 20% too high. Recent s t u d i e s (Mori et a l . , 1978) have shown that s t r e n g t h increases due to p r i o r s t r a i n h i s t o r y are u s u a l l y l o s t i n the process of sampling and handling of c l e a n sands p r i o r to t e s t i n g . Studies conducted by the Corps of Engineers Waterways Experiment S t a t i o n have a l s o shown that sampling i s l i k e l y to lead to some d e n s i f I c a t i o n i n loose to medium dense sands and some loosening i n dense to very dense sands. Sampling i s a l s o l i k e l y to 240 r e s u l t i n the l o s s of cementation between g r a i n s . Seed (19 79) has suggested t h a t , because of a l l these e f f e c t s , the c y c l i c r e s i s t a n c e of medium dense to dense undisturbed samples i n l a b o r a t o r y t e s t s w i l l o f t e n be very much lower than those i n - s i t u . The e f f e c t s o f membrane p e n e t r a t i o n and sample d i s t u r b a n c e tend to counterbalance each other, e s p e c i a l l y f o r the type of medium dense, uniform, medium g r a i n s i z e sands tested from McDonald's Farm. Combining a l l the above f a c t o r s , a c o r r e c t i o n f a c t o r of 0.65 has been a p p l i e d to the c y c l i c s t r e s s r a t i o from t r i a x i a l t e s t s to o b t a i n the f i e l d l i q u e f a c t i o n s t r e s s r a t i o , T/O' . vo The c y c l i c s t r e s s r a t i o s to cause 10 percent double amplitude shear s t r a i n i n 10 and 20 c y c l e s are given i n Table 7.4. During the c y c l i c t e s t i n g program no s i g n i f i c a n t necking of the samples was observed a t s t r a i n s l e s s than about 5% double amplitude shear s t r a i n . The process of extruding and preparing the samples was found to be extremely d i f f i c u l t , s i n c e the samples were very f r a g i l e . The o v e r a l l success r a t e during the e x t r u s i o n and preparation process was about 70 percent. Improvements i n technique were developed w i t h experience but some samples were s t i l l l o s t . 7.2.4. Results A comparison between p r e d i c t e d s o i l parameters from i n - s i t u t e s t i n g and measured values from l a b o r a t o r y t e s t i n g w i l l be presented i n the f o l l o w i n g s e c t i o n s . 7.2.4.1. S o i l C l a s s i f i c a t i o n Using the s o i l c l a s s i f i c a t i o n chart shown i n F i g . 4.7, the CPT data 241 provided a good estimate of the s o i l type, as shown i n F i g . 7.3. The sand de p o s i t from a depth o f 2 m to 13 m had a f r i c t i o n r a t i o of about 0.4% w i t h bearing values v a r y i n g from 20 to 200 kg/cm 2. The measured dynamic pore pressures were c o n s i s t e n t l y c l o s e to h y d r o s t a t i c , i n d i c a t i n g a f r e e d r a i n i n g , coarsed grained m a t e r i a l . The f i n e sand from a depth of 13 m to 15 m developed s l i g h t l y negative pore pressures, i n d i c a t i n g a s l i g h t l y f i n e r grained m a t e r i a l . The underlying clayey s i l t had a f r i c t i o n r a t i o of about 1.5% and bearing values of 8 to 12 kg/cm 2. The c l a s s i f i c a t i o n c h a r t showed the deposit to be a s i l t c l a y mixture (ML to CL). The l a b o r a t o r y r e s u l t s show the deposit to p l o t c l o s e to the A - l i n e on the p l a s t i c i t y c h a r t i n the CL/ML re g i o n . The measured dynamic pore pressures were very high w i t h t o t a l pore pressure values approximately 3 times l a r g e r than the e q u i l i b r i u m h y d r o s t a t i c v a l u e s . The d i f f e r e n t i a l pore pressure r a t i o (Au/q^,) was c o n s t a n t w i t h depth at about 0.6. The constant Au/q^, value i n d i c a t e s a normally c o n s o l i d a t e d d e p o s i t . The l i n e a r i n c r e a s e i n cone bearing (q^,) w i t h depth a l s o i n d i c a t e s a normally c o n s o l i d a t e d d e p o s i t . The dilatometer data provided a r a t h e r poor estimate of s o i l type, as shown on F i g . 7.7. The m a t e r i a l from 2 to 15 m was d e f i n e d as a sand w i t h some s i l t l a y e r s . The m a t e r i a l from a depth of 15 m was defined as a c l a y although the samples show the deposit to be a c l a y e y s i l t . The DMT r e s u l t s d i d not i d e n t i f y c l e a r l y the s u r f i c i a l (0-2 m) deposit of c l a y and s i l t . The s e l f - b o r i n g pressuremeter provided a q u a l i t a t i v e assessment of the s o i l type based on the d r i l l m u d and c u t t i n g s returned during i n s t a l l a t i o n . 7.2.4.2. I n - s i t u s t r e s s The s e l f - b o r i n g pressuremeter was the only i n - s i t u t e s t capable of 242 a s s e s s i n g the i n - s i t u h o r i z o n t a l s t r e s s e s . However, both the CPT and DMT data i n d i c a t e the underlying clayey s i l t to be normally c o n s o l i d a t e d . A complete summary of the s e l f - b o r i n g pressuremeter r e s u l t s i s shown i n Table 7.5. The v a r i a t i o n of measured h o r i z o n t a l e f f e c t i v e s t r e s s w i t h depth from the s e l f - b o r i n g pressuremeter i s shown on F i g . 7.9. Considerable v a r i a t i o n i n measured e f f e c t i v e h o r i z o n t a l s t r e s s was obtained. The geology of the d e l t a would suggest that the sand i s normally c o n s o l i d a t e d , which would i n d i c a t e a K = 0.4 to 0.5. Because of the t u r b u l e n t environment i n which o the sand was l a i d down and past seismic a c t i v i t y , some a d d i t i o n a l h o r i z o n t a l s t r e s s e s may have been locked i n t o the sand. This would suggest a p o s s i b l e K q of about 0.6 to 0.7. However, the very l a r g e v a r i a t i o n of measured i n - s i t u s t r e s s e s i n d i c a t e s a v a r i a t i o n of K from 0.35 to 3.5. o I t i s the w r i t e r s o p i n i o n t h a t the very l a r g e values of measured i n - s i t u s t r e s s are due to s o i l disturbance during i n s t a l l a t i o n . This appears to confirm the d i s c u s s i o n g iven i n s e c t i o n 5 . 4 . 1 . M a r c h e t t i ( 1 9 8 0 ) has suggeted t h a t the from the DMT can be d c o r r e l a t e d t o i n - s i t u K q f o r s a n d s . U s i n g the M a r c h e t t i ( 1 9 8 0 ) c o r r e l a t i o n , K Q - ( K D / 1 . 5 ) ° * 4 7 - 0.6 the DMT d a t a would i n d i c a t e a v a r i a t i o n of K from 0.6 to 2.0. Based on o the d i s c u s s i o n i n Chapter 6 and these r e s u l t s , the w r i t e r c onsiders i t u n l i k e l y t h a t the DMT d a t a a l o n e can be used to i d e n t i f y the i n - s i t u K o i n sands. Table 7.5 Summary of S e l f - b o r i n g Pressuremeter R e s u l t s - McDonald's Farm Hole Test Depth V e r t i c a l Measured H o r i z o n t a l Comeback C y c l i c Shear Slope No. No. (m) E f f e c t i v e Stress E f f e c t i v e Stress Pressure Modulus l o g - l o g a' , (kg/cm 2) a' , (kg/cm 2) (kg/cm 2) G, (kg/cm 2) S vo Ho SBPMT-1 1 3.0 0.40 0.45 0.40 130 0.33 2 3.8 0.48 0.87 0.45 260 0.35 3 4.6 0.56 0.40 0.40 170 0.32 4 5.3 0.63 2.20 0.45 275 0.34 5 6.3 0.73 2.13 0.45 300 0.33 SMPMT-2 1 4.9 0.49 0.17 _ _ _ 2 6.2 0.62 0.28 0.60 287 0.50 3 7.0 0.70 0.70 0.65 425 0.40 4 8.5 0.85 1.55 1.10 520 0.29 5 9.3 0.93 2.73 0.95 750 0.26 6 10.9 1.10 0.76 1.10 340 0.36 7 11.7 1.17 0.48 1.25 360 0.28 MEASURED HORIZONTAL EFFECTIVE STZEbS, &H , % /cm2 15 -J FIG. 7 . 9 . SELF-BORING PRESSUREMETER HORIZONTAL EFFECTIVE STRESSES VERSUS DEPTff, McDONALD'S FARM. 245 7.2.4.3. R e l a t i v e Density The r e l a t i v e d e n s i t y of the sand deposit was estimated u s i n g F i g . 4.10 and the CPT data. R e l a t i v e d e n s i t i e s p r e d i c t e d by the CPT and those measured i n the l a b o r a t o r y are shown on F i g . 7.10. The CPT data appears to provide an e x c e l l e n t p r e d i c t i o n of r e l a t i v e d e n s i t y . However, i t i s important to remember that the l a b o r a t o r y measured d e n s i t i e s are not very r e l i a b l e due to sample disturbance and the d i f f i c u l t y i n o b t a i n i n g the maximum and minimum val u e s . The r e l a t i v e d e n s i t y r e l a t i o n s h i p shown i n F i g . 4.10 was developed by B a l d i et a l . (1981) using a sand ( T i c i n o ) that i s very s i m i l a r i n g r a i n c h a r a c t e r i s t i c s ( i . e . g r a i n shape, s i z e and mineralogy) to the McDonald's Farm sand. Thus, a reasonable agreement could be expected. Results from the chamber t e s t s performed by B a l d i et a l . (1981) showed that a unique r e l a t i o n s h i p e x i s t s between cone r e s i s t a n c e and r e l a t i v e d e n s i t y f o r one sand i f t h e i n - s i t u h o r i z o n t a l e f f e c t i v e s t r e s s (a,' ) i s used. The ho c a l c u l a t e d r e l a t i v e d e n s i t i e s shown i n F i g . 7.10 co n t a i n the assumption t h a t K = 0.45 at McDonald's Farm. I f the a c t u a l i n - s i t u c o e f f i c i e n t of o e a r t h p r e s s u r e , , was c l o s e r t o , s a y , 0.6, the p r e d i c t e d r e l a t i v e d e n s i t y values would decrease by up to 10% (see F i g . 4.10). The r e l a t i v e d e n s i t y of the sand can a l s o be estimated from the DMT data using F i g . 6.4. D e n s i t i e s p r e d i c t e d by DMT and those measured i n the l a b o r a t o r y are compared i n F i g . 7.11. The DMT data appear to p r e d i c t an average r e l a t i v e d e n s i t y around 70%, w i t h a maximum of c l o s e to 100%. The CPT and l a b o r t o r y data i n d i c a t e a r e l a t i v e d e n s i t y of about 40% at 4 m r i s i n g to about 60% at 12 m. The r e l a t i o n s h i p between r e l a t i v e d e n s i t y and h o r i z o n t a l s t r e s s index, 2 4 6 F I G . 7.10. COMPARISON OF LABORATORY AND CPT R E L A T I V E D E N S I T I E S , MCDONALD'S FARM. 247 RELATIVE DEN5ITY, Dr, % FIG. 7.11. COMPARISON OF LABORATORY AND DMT RELATIVE DENSITIES^ MCDONALD'S FARM. 248 , shown i n F i g . 6.4 was based on d a t a from chamber t e s t s t u d i e s ( B e l l o t t i et a l . , 1979) using the same T i c i n o sand used by B a l d i et a l . (1981). As discussed before, the g r a i n c h a r a c t e r i s t i c s of the T i c i n o sand are very s i m i l a r to those of the sand at McDonald's Farm. However, the d i l a t o m e t e r h o r i z o n t a l s t r e s s index, K,, i s more s e n s i t i v e to changes i n i n - s i t u h o r i z o n t a l s t r e s s c o n d i t i o n s than i s cone r e s i s t a n c e . I f i t i s assumed that the l a b o r a t o r y values and CPT r e s u l t s p r e d i c t approximately the c o r r e c t i n - s i t u r e l a t i v e d e n s i t i e s , the higher DMT K, r e s u l t s may be due to a h o r i z o n t a l i n - s i t u s t r e s s l a r g e r than t h a t used i n the chamber t e s t s . The chamber t e s t r e s u l t s ( B e l l o t t i et a l . , 1979) used to c o s t r u c t F i g . 6.4 were obtained w i t h K Q = 0.40. The chamber t e s t work, by B e l l o t t i e t a l . (1979) a l s o i n c l u d e d some t e s t s where the sand was ov e r c o n s o l i d a t e d ( i . e . K > 0 . 4 ) . These r e s u l t s were reviewed by the w r i t e r and i n d i c a t e a o p r o p o r t i o n a t e i n c r e a s e i n f o r any increase i n K q , as shown i n F i g . 7.12. Thus, i f the r e l a t i v e d e n s i t i e s from the CPT and l a b o r a t o r y measurements are assumed approximately c o r r e c t the DMT data would i n d i c a t e an i n - s i t u K = 0 . 5 5 at McDonald's Farm, o M a r c h e t t i (1982) has shown t h a t K, i s s e n s i t i v e to the f o l l o w i n g d f a c t o r s ; r e l a t i v e d e n s i t y , i n - s i t u K^, cementation, aging, p r e s t r e s s i n g and s t r e s s h i s t o r y . The cone r e s i s t a n c e i s a l s o i n f l u e n c e d by these f a c t o r s but appears to be l e s s s e n s i t i v e to changes i n the i n - s i t u K q and s t r e s s h i s t o r y . Thus, i t appears that the combined use of the CPT and DMT i n sand dep o s i t s may g i v e an estimate of the i n d i v i d u a l r e s p o n s i b i l i t i y of e a c h o f t h e m a i n f a c t o r s ( i . e . , r e l a t i v e d e n s i t y and i n - s i t u K ) . o However, the i n d i v i d u a l t e s t s on t h e i r own cannot i d e n t i f y the r e s p o n s i b i l i t y of each f a c t o r . 249 10 I 3 I -i r~ O 0.5 IN-SITU STZESS COEfFICItNT, Kc 1.0 FIG. 7.12. CHANGE IN HORIZONTAL STRESS INDEX WITH INCREASES IN IN-SITU HORIZONTAL STRESS, TINCINO SAND. (Data from B e l l o t t l et a l . , 1979) 250 7.2.4.4. Shear Strength 7.2.4.4.1. Sand The peak, shear s t r e n g t h of the sand deposit can be determined from the CPT data using F i g . 4.12. Shear strengths p r e d i c t e d by the CPT and those measured i n the l a b o r a t o r y are shown on F i g . 7.13. Since the peak f r i c t i o n angle v a r i e s w i t h c o n f i n i n g pressure, the l a b o r a t o r y measured values shown i n F i g . 7.13 are the peak f r i c t i o n angles measured i n t r i a x i a l t e s t s where t h e e f f e c t i v e c o n f i n i n g p r e s s u r e (o^) was a p p r o x i m a t e l y the i n - s i t u h o r i z o n t a l s t r e s s . The i n - s i t u K was assumed to equal 0.5. Because of o the d i f f i c u l t y i n performing t r i a x i a l t e s t i n g a t very low e f f e c t i v e c o n f i n i n g p r e s s u r e s , the t e s t i n g c a r r i e d out on the shallower samples had c o n f i n i n g pressures somewhat l a r g e r than the assumed i n - s i t u h o r i z o n t a l s t r e s s (see Table 7.3). The average l a b o r a t o r y measured peak f r i c t i o n angle was about 42°. The CPT p r e d i c t e d an average peak f r i c t i o n angle of about 38° at a depth of 4 m i n c r e a s i n g to about 41° at 6 m and remaining approximately constant at 41° u n t i l 13 m. A l l o w i n g f o r the inherent v a r i a b i l i t y of the sand deposit (see F i g . 7.6), the CPT appears to provide an e x c e l l e n t continuous p r e d i c t i o n of the peak shear s t r e n g t h . The most s i g n i f i c a n t aspect about the p r e d i c t i o n i s that i t i s continuous and repeatable. The w r i t e r b e l i e v e s that the continuous nature of the p r e d i c t i o n provides an e x c e l l e n t assessment of the s o i l v a r i a b i l i t y . The shear s t r e n g t h of the sand deposit was a l s o p r e d i c t e d using s e l f -b o r ing pressuremeter data. A summary of the s e l f - b o r i n g pressuremeter shear s t r e n g t h r e s u l t s i s shown i n Table 7.6. Tests 1,2,4,5 and 7 i n SBPMT h o l e No. 2 were of a poor q u a l i t y and shear strengths were not c a l c u l a t e d . The reason f o r the poor q u a l i t y data i s b e l i e v e d to be the disturbance 251 MAXIMUM FZICTION AN6LE 4^ MAX. j (degree^ 20 5 -10-5ILT 40 50 _ i LEGEND SbPMT O SE>PMT-I LAb A TZ/AMAL CPT — PCI 15 -i FIG. 7.13. COMPARISON OF LABORATORY TRIAXIAL PEAK FRICTION ANGLE WITH CPT AND SELF-BORING PRESSUREMETER VALUES, MCDONALD'S FARM. 252 Table 7.6. S e l f - b o r i n g Pressuremeter Results f o r Shear Strength, McDonald's Farm Hole No. Test No. Depth (m) Slope l o g - l o g S Hughes et a l . , 1977 Corrected Maximum D i l a t i o n v° F r i c t i o n D i l a t i o n v° max F r i c t i o n max SBPMT-1 1 3.0 0.33 - 4 33 +3.5 39 2 3.8 0.35 - 2 35 +4.5 40 3 4.6 0.32 -4.5 32.5 + 3 38.5 4 5.3 0.34 - 3 34 + 4 39.5 5 6.3 0.33 - 4 33 +3.5 39 SBPMT-2 1 4.9 0.45* _ 2 6.2 0.50* - - - -3 7.0 0.40 +2.5 38.5 + 8 42 4 8.5 0.29* - - - — 5 9.3 0.26* - - - -6 10.9 0.36 - 1 35.5 + 5 40 7 11.7 0.28* Test not accepted f o r f r i c t i o n angle c a l c u l a t i o n . F r i c t i o n Angle at Constant Volume, A = 36° . 253 caused by the i n s t a l l a t i o n of the pressuremeter i n the sand using the l i g h t e r t ruck mounted d r i l l r i g (see Sectio n 7.2.2). The d r i l l r i g was f r e q u e n t l y l i f t e d o f f the ground a t the rear due to the l a r g e pushing f o r c e r e q u i r e d to i n s t a l l the pressuremeter. The v i b r a t i o n caused by the i n t e r m i t t e n t p e n e t r a t i o n r a t e appears to have caused some disturbance and t h i s was i d e n t i f i e d i n the pressure expansion curves. The c o r r e c t e d values of maximum f r i c t i o n angle c a l c u l a t e d using the s t r a i n l e v e l c o r r e c t i o n proposed by the w r i t e r and discussed i n Se c t i o n 5.4.2 are a l s o shown i n F i g . 7.13. The average value p r e d i c t e d from the pressuremeter data using the proposed c o r r e c t i o n i s about 40°. This agrees w e l l w i t h the CPT values and i s about 2° l e s s than the l a b o r a t o r y t r i a x i a l v a l u e s . The d i f f e r e n c e between the SBPMT r e s u l t s and the l a b o r a t o r y values may be, i n p a r t , due to the higher average s t r e s s l e v e l that e x i s t s around the pressuremeter during the t e s t . However, as discussed i n S e c t i o n 5.4.2, t h i s was expected to be o f f s e t by the f a c t that the pressuremeter f r i c t i o n angle i s obtained under approximately p l a i n s t r a i n c o n d i t i o n s which may r e s u l t i n values g e n e r a l l y higher than under t r i a x i a l c o n d i t i o n s . The f r i c t i o n angle values p r e d i c t e d using the Hughes et a l . (1977) method are co n s i d e r a b l y smaller than those obtained i n the l a b o r a t o r y o r p r e d i c t e d from the CPT. The average value p r e d i c t e d using the Hughes et a l . (19