THE PIEZOMETER CONE PENETRATION TEST by DONALD G. GILLESPIE BASc, The U n i v e r s i t y of B r i t i s h Columbia, 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES 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, 1981 © Donald G. G i l l e s p i e , 1981 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 a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or pu b l i c a t i o n 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 c i v ^ gn^ ingg^ iA^ The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 i i A b s t r a c t Deep q u a s i - s t a t i c cone p e n e t r a t i o n t e s t s i n s a t u r a t e d s o i l s can develop l a r g e excess pore p r e s s u r e s . The l e v e l and sig n of t;he excess pore p r e s s u r e s depend on the volume change c h a r a c t e r i s t i c s , t h e s t r e n g t h and the p e r m e a b i l i t y of the s o i l . The recent a d d i t i o n of pore pressure measurements during cone p e n e t r a t i o n t e s t i n g adds a new dimension to the i n t e r p r e t a t i o n f o r g e o t e c h n i c a l parameters. T h i s r e p o r t shows how the pore pressure measurements enhance the s t r a t i g r a p h i c l o g g i n g a b i l i t i e s of the cone p e n e t r a t i o n t e s t . C a l i b r a t i o n s performed on the cone penetrometer are used to show the importance of c o r r e c t i n g bearing and f r i c t i o n measurements f o r pore pressure e f f e c t s . As w e l l as pore pressure e f f e c t s the importance of procedure and equipment i s d i s c u s s e d . The use of pore pressure d i s s i p a t i o n s recorded while p e n e t r a t i o n i s h a l t e d i s i l l u s t r a t e d . T h e o r e t i c a l s o l u t i o n s f o r pore pressure decay r a t e s are i l l u s t r a t e d and r e s u l t s p r e d i c t i n g 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 are compared to l a b o r a t o r y measured v a l u e s . 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 p r e d i c t e d from decay r a t e s i s shown to compare w e l l with l a b o r a t o r y measured valu e s f o r the s o i l i n the o v e r c o n s o l i d a t e d s t a t e . III Contents A b s t r a c t ii L i s t of Tables v L i s t of F i g u r e s vi Acknowledgement Vii Chapter 1. I n t r o d u c t i o n 1 1.1 H i s t o r i c a l Development of the Piezometer Cone Test 1 1.2 Report O r g a n i z a t i o n 2 Chapter 2. Equipment and Procedure 4 2.1 I n t r o d u c t i o n 4 2.2 The Probe and Recording U n i t s 4 2.3 D r i v i n g U n i t 7 2.4 Test Procedure 7 2.4.1 C a l i b r a t i o n 9 2.4.2 S a t u r a t i o n of the Porous Element 11 2.4.3 Cone P e n e t r a t i o n Test Procedure 12 2.4.4 Data Reduction 13 Chapter 3. Dynamic Pore Pressures and S t r a t i g r a p h i c Logging 16 3.1 I n t r o d u c t i o n .16 3.2 Pore Pressures Generated d u r i n g Cone P e n e t r a t i o n T e s t i n g 16 3.3 Dynamic Pore Pressure R a t i o 17 3.4 S t r a t i g r a p h i c Logging 21 3.4.1 CPT Example 1 F i g u r e 5 21 3.4.2 CPT Example 2 F i g u r e 6 24 3.4.3 CPT Example 3 F i g u r e 7 26 3.4.4 CPT Example 4 F i g u r e 8 28 3.5 S t a t i c Pore Pressures 30 3.6 C o n c l u s i o n s 31 Chapter 4. A n a l y t i c a l E x p r e s s i o n s f o r Dynamic Pore Pressure 33 4.1 I n t r o d u c t i o n 33 4.2 A n a l y t i c a l E x p r e s s i o n s 34 4.2.1 Comparison Between Models 36 Chapter 5. F i e l d O bservations of Dynamic Pore Pressure ....45 5.1 I n t r o d u c t i o n 45 5.2 F i e l d T e s t s Conducted 45 5.3 E f f e c t of P e n e t r a t i o n Speed 46 5.3.1 F i e l d Evidence 49 5.4 L o c a t i o n of the Porous Element 52 5.5 E f f e c t of S a t u r a t i o n 56 5.6 E f f e c t of Pore Pressure D i s s i p a t i o n 59 5.7 H y d r o f r a c t u r e : F i e l d Evidence 60 IV 5.7.1 R a d i a l E f f e c t i v e S t r e s s e s d u r i n g Cone P e n e t r a t i o n T e s t i n g 62 5.7.2 Pore Pressure Response and H y d r o f r a c t u r e ...67 Chapter 6. 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 from Pore Pressure D i s s i p a t i o n a f t e r Piezometer Cone P e n e t r a t i o n 69 6.1 I n t r o d u c t i o n 69 6.2 C o n s o l i d a t i o n Process a f t e r P e n e t r a t i o n 70 6.3 S o l u t i o n s P o s s i b l e f o r 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 73 6.4 Data Reduction 79 6.5 Comparison of E x i s i t i n g S o l u t i o n 79 6.6 Laboratory T e s t s Performed 81 6.6.1 T e s t i n g Procedure 82 6.6.2 R e s u l t s : Burnaby S i t e 82 6.6.3 R e s u l t s : Mcdonald S i t e 86 6.7 F i t of E x i s t i n g S o l u t i o n s 92 6.8 Comparison between P r e d i c t e d and Laboratory Measured Values 100 6.9 S e l e c t i o n of S o i l S t i f f n e s s R a t i o 102 6.10 Procedure Used During D i s s i p a t i o n T e s t s 103 6.11 C o n c l u s i o n 106 Chapter 7. E s t i m a t i n g S o i l P e r m e a b i l i t y from Cone P e n e t r a t i o n Test Data 108 7.1. I n t r o d u c t i o n 108 7.2 Laboratory T e s t s to Obtain C o m p r e s s i b i l i t y 110 7.3 F i e l d Loading 111 7.4 Estimates of C o m p r e s s i b i l i t y from Bearing Values ...111 7.5 Laboratory Index Te s t s to Obtain C o m p r e s s i b i l i t y ...113 7.6 I n s i t u T e s t s to Obtain S o i l C o m p r e s s i b i l i t y 113 Chapter 8. Con c l u s i o n s 114 8.1 C o n c l u s i o n s 114 8.2 Recommendations f o r F u r t h e r Research 117 References 119 Appendix A Complete CPT Logs Macdonald S i t e 122 Appendix B Table of Time F a c t o r s f o r D i s s i p a t i o n A n a l y s e s . . . 136 V L i s t of Tables Table I Input Parameters Used to Compare Dynamic Pore Pressure Models 37 Table II P u b l i s h e d R e s u l t s Showing the E f f e c t of P e n e t r a t i o n Rate on Bearing and Dynamic Pore Pressure 50 Table III Summary of E x i s t i n g S o l u t i o n s f o r Pore Pressure D i s s i p a t i o n 76 Table IV Comparison between Laboratory Data and C a l c u l a 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 : McDonald S i t e 20.5 m 94 Table V Comparison between Laboratory Data and C a l c u l a 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 : Mcdonald S i t e 25.5 m 95 Table VI Comparison between Laboratory Data and C a l c u l a 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 : Burnaby S i t e 15.5 m 96 Table VII A n i s o t r o p i c P e r m e a b i l i t y of c l a y s 109 Table VIII E s t i m a t i n g the C o n s o l i d a t i o n of Clay from cu/p 112 VI L i s t of F i g u r e s F i g u r e T i t l e 1 5-Channel E l e c t r i c B e a r i n g - F r i c t i o n Piezometer Cone Penetrometer 5 2 T i p s Extensions Used 8 3 R e s u l t s of Piezometer F r i c t i o n Cone C a l i b r a t i o n ...10 4 S o i l P r o f i l e f o r Research S i t e at Mcdonalds Farm . . 1 8 5 CPT Example 1 22 6 CPT Example 2 25 7 Complete CPT Log and D e t a i l of Dynamic Pore Pressure 27 8 Before and A f t e r Compaction CPT Example 4 29 9 Example of Piezometer, F r i c t i o n Cone Log through T a i l i n g s Deposit 32 10 Comparison between A n a l y t i c a l S o l u t i o n s and Measured Dynamic Pore Pressures 41 11 Excess Pore Pressure D i s t r i b u t i o n around I n s t a l l e d P i l e s 44 12 Rate E f f e c t s , Clayey S i l t 48 13 P e n e t r a t i o n Rate E f f e c t s i n Clayey S i l t Deposit ...51 14 Pore Pressure D i s t r i b u t i o n along Shaft d u r i n g Cone P e n e t r a t i o n T e s t i n g 54 15 I n f l u e n c e of S a t u r a t i o n on Pore Pressure Response .58 16 E f f e c t of Pore Pressure D i s s i p a t i o n s d u r i n g CPT i n Clayey S i l t 61 17 Pore Pressure and F r i c t i o n D i s t r i b u t i o n along Shaft d u r i n g CPT 64 18 Estimated R a d i a l S t r e s s e s and Dynamic Pore Pressure 66 19 Excess Pore Pressure D i s t r i b u t i o n around a C y l i n d r i c a l C a v i t y with Time 72 20 I n f l u e n c e of Porous Stone L o c a t i o n 74 21 Time F a c t o r s f o r P r e d i c t i n g 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 77 22 Pore Pressure D i s s i p a t i o n s at Mcdonald and Burnaby S i t e s 80 23 C o m p r e s s i b i l i t y of Burnaby Clay 83 24 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 : Burnaby Clay 84 25 P e r m e a b i l i t y of Burnaby Clay 85 26 C o m p r e s s i b i l i t y of Richmond Clayey S i l t 87 27 C o e f f i c i e n t of Consolidation:Richmond Clayey S i l t .88 28 P e r m e a b i l i t y of Richmond Clayey S i l t 89 29 E f f e c t of Remolding on the C o m p r e s s i b i l i t y of Richmond Clayey S i l t 90 30 E f f e c t of Remolding on 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 : Richmond Clayey S i l t 91 31 F i t of T h e o r e t i c a l Curves to F i e l d Data: Mcdonald S i t e 20.5 m 97 32 F i t of T h e o r e t i c a l Curves to F i e l d Data: Mcdonald S i t e 25.5 m 98 33 F i t of T h e o r e t i c a l Curves to F i e l d Data: Burnaby S i t e 15.5 m 99 34 S e l e c t i o n of S o i l S t i f f n e s s R a t i o 104 VII Acknowledgements I would l i k e to express my s i n c e r e a p p r e c i a t i o n and g r a t i t u d e to Dr.Campanella fo r suggesting the study and p r o v i d i n g both l o g i s t i c a l support and guidance throughout. Much of the work was undertaken with Mr.P.K.Robertson who p r o v i d e d many suggestions and hours of f i e l d work. Thanks a l s o go to M r . B i l l B e r z i n s e s p e c i a l l y f o r w r i t i n g the computer g r a p h i c s r o u t i n e s . L a s t l y , s p e c i a l thanks go to my wife Brenda f o r her support and h e l p with p r e s e n t a t i o n . F i n a n c i a l support was p r o v i d e d by the Department of Energy Mines and Resources,Canada. Chapter 1 I n t r o d u c t i o n 1.1 H i s t o r i c a l Development of the Piezometer Cone Test The s t a t i c cone p e n e t r a t i o n t e s t has become the premier l o g g i n g t o o l f o r i n s i t u i n v e s t i g a t i o n s of s o i l d e p o s i t s . I t s main advantages over the standard p e n e t r a t i o n t e s t are the r a p i d i t y of the t e s t , i t s high r e p e a t a b i l i t y , and the continuous nature of the p r o f i l e s o b t a i n e d . Recent r e s e a r c h on the s t a t i c cone p e n e t r a t i o n t e s t has focussed on the measurement and i n t e r p r e t a t i o n of pore water p r e s s u r e s at and around the probe. Knowledge of pore water pr e s s u r e i s fundamental to any 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 the measured parameters. In a d d i t i o n , i t has been r e a l i z e d that the measurement of pore water pressure p r o v i d e s an a d d i t i o n a l parameter to d e s c r i b e the s o i l b ehavior. The proceedings of the European Symposium on P e n e t r a t i o n T e s t i n g , ESOPT I, i n c l u d e d works by Janbu and Senneset 1974 and by Schmertmann 1974. Janbu and Senneset proposed an 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 the s t a t i c cone p e n e t r a t i o n t e s t , but concluded that t h e i r knowledge of the d i s t r i b u t i o n of pore water p r e s s u r e s around the cone was inadequate. Schmertmann in t r o d u c e d the e f f e c t s of p e n e t r a t i o n pore pressure on cone r e s i s t a n c e . He d i s c u s s e d some of the parameters that e f f e c t the measured pore pressure and showed how the l e v e l of pore pressure would change the p e n e t r a t i o n r e s i s t a n c e measured. 2 At another conference, the ASCE S p e c i a l t y Conference on I n s i t u Measurement of S o i l P r o p e r t i e s , two very s i g n i f i c a n t papers i l l u s t r a t e d the use of the piezometer cone. Torstensson 1975 d e s c r i b e d how continuous i n f o r m a t i o n on the s o i l p r o p e r t i e s c o u l d be obtained from only the dynamic pore pressure r e c o r d . Wissa et a l . 1975 showed how the piezometer cone c o u l d be used to o b t a i n e q u i l i b r i u m pore p r e s s u r e s , as w e l l as 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 from the pore pressure decay r a t e . U n t i l r e c e n t l y , a l l piezometer cones were e i t h e r made f o r the s o l e purpose of measuring pore p r e s s u r e s or were mo d i f i e d v e r s i o n s of the Fugro e l e c t r i c f r i c t i o n cone, i n which the pore p r e s s u r e measuring system was s u b s t i t u t e d f o r the f r i c t i o n s l e e v e . More recent cones such as the one used i n t h i s i n v e s t i g a t i o n i n c l u d e both the pore pressure measurement c a p a b i l i t y , as w e l l as bearing and f r i c t i o n measurements. T h i s paper shows how the a d d i t i o n of the pore p r e s s u r e measuring system can enhance the cone p e n e t r a t i o n t e s t . 1.2 Report O r g a n i z a t i o n A great v a r i e t y of i n f o r m a t i o n can be obtained from d i f f e r e n t a spects of the cone p e n e t r a t i o n t e s t ; the data can be used i n e i t h e r a q u a n t i t a t i v e or a q u a l i t a t i v e manner, depending on the r e s u l t s r e q u i r e d . T h i s r e p o r t i s d i v i d e d i n t o seven f o l l o w i n g c h a p t e r s , each of which o u t l i n e s some aspect of the t e s t . Chapter 2 d e s c r i b e s the equipment and procedure used dur i n g the i n v e s t i g a t i o n . A d d i t i o n a l d e s c r i p t i o n of s p e c i f i c d e t a i l s of the t e s t i s i n c l u d e d i n other chapters, where a p p r o p r i a t e . Chapter 3 i l l u s t r a t e s some examples of dynamic pore pressure records and shows how they can be used i n a q u a l i t a t i v e manner to enhance the s t r a t i g r a p h i c l o g g i n g c a p a b i l i t y of the cone p e n e t r a t i o n t e s t . Chapter 4 i n v e s t i g a t e s t h e o r e t i c a l s o l u t i o n s f o r the development of pore p r e s s u r e d u r i n g steady p e n e t r a t i o n and compares them to measured v a l u e s . Chapter 5 d e s c r i b e s the e f f e c t s of equipment and procedure v a r i a t i o n on the r e s u l t s obtained and c o n s i d e r s the p o s s i b i l i t y of h y d r o f r a c t u r e during cone p e n e t r a t i o n . Chapter 6 compares r e s u l t s obtained from c o n s o l i d a t i o n t e s t s to 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 obtained from the rate of excess pore pressure d i s s i p a t i o n . Chapter 7 o f f e r s some recommendations on the s e l e c t i o n of 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 t h a t , when combined with 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 , can be used to c a l c u l a t e a p e r m e a b i l i t y v a l u e . Chapter 8 i n c l u d e s some a d d i t i o n a l c o n c l u s i o n s and recommendations f o r f u r t h e r r e s e a r c h . 4 Chapter 2 Equipment and Procedure 2.1 I n t r o d u c t i o n The purpose of t h i s paper i s not to d e s c r i b e the d e t a i l s of the r e s e a r c h and development that have gone i n t o the equipment necessary f o r the research undertaken, but rather to d e s c r i b e and i n t e r p r e t the r e s u l t s o b t a i n e d . Some d e s c r i p t i o n of the equipment and procedure i s , however, n e c e s s i t a t e d by the tremendous range in equipment and procedure used, a l l of which are termed piezometer cone l o g g i n g . T h i s chapter g i v e s a short d e s c r i p t i o n of the probe, the d r i v i n g u n i t , and the r e c o r d i n g d e v i c e s . I t then b r i e f l y d e s c r i b e s the procedures used i n c a l i b r a t i o n , set up, f i e l d t e s t i n g , and data r e d u c t i o n . A more complete d i s c u s s i o n of d e t a i l s , e s p e c i a l l y concerning the equipment, i s p r o v i d e d by Campanella and Robertson 1981. The importance of some of the procedures o u t l i n e d i n t h i s Chapter are f u r t h e r d i s c u s s e d in Chapters 5 and 6. 2.2 The Probe and Recording U n i t s The piezometer f r i c t i o n probe used i s i l l u s t r a t e d i n F i g u r e 1. The cone has a 60 apex angle, with a 10 cm p r o j e c t e d base area. The porous f i l t e r element s i t s immediately behind the cone 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 p r e s s u r e . The pressure transducer used i s a h e r m e t i c a l l y s e a l e d , a l l welded, f l u s h diaphragm, a b s o l u t e pressure type. F r i c t i o n i s measured on a f r i c t i o n s l e e v e having FIG. I 5- CHANNEL E L E C T R I C B E A R I N G - FRICTION -P I E Z O M E T E R C O N E P E N E T R O M E T E R wires -t her mister-stroin gages for cone^ bearing load cell porous polythylene-\i.< swoge fitting to lock lOconductor electric coble wires spliced to cable inside tube •wires -slope sensor -QUAD ring •stroin gouges for friction load cell - f r ic t ion sleeve (150cm areo) - 0 - rings pressure transducer -QUAD r ing small cavity 6 0 ° c o n e , 35.6mm 0.D. 6 an area of 150 cm 2. The c a p a c i t y of the t i p load c e l l used was 6 tons (about 540 b a r ) , and the c a p a c i t y of the pressure transducer was 150 p s i (10 b a r ) . Although i t was not necessary to do so, the pressure transducer was designed to be e a s i l y r e p l a c e d i n the f i e l d f o r one of a d i f f e r e n t c a p a c i t y . S i g n a l s from the load c e l l s and pressure transducer were sent to the s u r f a c e through a 10 conductor c a b l e . The use of common e x c i t a t i o n p ermitted the simultaneous measurement of the channels recorded (while only using 8 conductor c a b l e s ) . A 15 v o l t r e g u l a t e d power supply was 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 were used to o b t a i n zero output at the r e f e r e n c e s e t t i n g f o r each transducer i n order to change range on the c h a r t 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 were used to d i r e c t l y p l o t i n t o e n g i n e e r i n g u n i t s on the c h a r t r e c o r d e r s . Two c h a r t r e c o r d e r s were used i n the i n v e s t i g a t i o n . One, a three channel r e c o r d e r 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, was 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 re c o r d b e a r i n g , f r i c t i o n , and dynamic pore p r e s s u r e s with depth. The other recorder was run on a time b a s i s to r e c o r d pore pressure response d u r i n g rod breaks and pore pressure d i s s i p a t i o n t e s t s . To d i f f e r e n t i a t e between the p a r t of the r e c o r d that was recorded d u r i n g p e n e t r a t i o n from the p a r t that was recorded d u r i n g rod breaks, each metre of p e n e t r a t i o n was manually i n d i c a t e d on the r e c o r d . To i n v e s t i g a t e the d i s t r i b u t i o n of pore p r e s s u r e s (and 7 f r i c t i o n ) up the cone s h a f t , s u c c e s s i v e l y longer t i p extensions were used to r e p l a c e the standard cone t i p . A d d i t i o n of the longer cone t i p s e f f e c t i v e l y moved both the porous element and the f r i c t i o n s leeve up the cone. The length and arrangement of the cone extensions used i s shown i n F i g u r e 2. R e s u l t s of t e s t i n g conducted with the longer t i p s i s presented i n Chapters 5 and 6. 2.3 D r i v i n g u n i t A complete d e s c r i p t i o n of the i n s i t u t e s t i n g 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 given by Campanella and Robertson 1981. 2.4 Test Procedure During the i n v e s t i g a t i o n , i t was found necessary to pay c o n s i d e r a b l e a t t e n t i o n to c e r t a i n d e t a i l s i n the t e s t i n g procedure. In p a r t , t h i s was n e c e s s i t a t e d by the extreme range i n the parameters measured. For example, the t i p loa d c e l l had to measure bearing v a l u e s from 5 to 500 bars. Although not necessary f o r each f i e l d t e s t , the complete procedure r e q u i r e d can be broken i n t o four s t e p s : c a l i b r a t i o n ; s a t u r a t i o n of the pore pressure measuring system; f i e l d t e s t i n g ; and data r e d u c t i o n . Each of these steps i s d e s c r i b e d here because of the s e n s i t i v i t y of the r e s u l t s obtained to t h e i r proper completion. 8 S T A N D A R D D 10' CM 15" E X T E N S I O N S to P O R E P R E S S U R E •* E L E M E N T P R O B E F O R C E S T I P S L E E V E S T A N D A R D 10' 15' T A + S A , T A + S A + S B TA + S A + S B + S , * B sD FIGURE 2 TIP E X T E N S I O N S U S E D 9 2.4.1 C a l i b r a t i o n C a l i b r a t i o n of each of the load c e l l s and the pressure transducer was p e r i o d i c a l l y undertaken to ensure the accuracy of r e s u l t s . To t r y to reproduce as many f i e l d c o n d i t i o n s as p o s s i b l e , a l l c a l i b r a t i o n s were done i n the i n s i t u t e s t i n g v e h i c l e . During c a l i b r a t i o n , a l l v o l t a g e s were measured on both the c h a r t r e c o r d e r s and a 6 d i g i t multimeter using a 1 m i c r o v o l t r e s o l u t i o n . One of the most important f i e l d c o n d i t i o n s necessary to d u p l i c a t e was the use of the 10 channel conductor c a b l e intended f o r use i n the f i e l d . The b e a r i n g and f r i c t i o n l o a d c e l l s were c a l i b r a t e d a g a i n s t a r e f e r e n c e l o a d c e l l u s i n g a 7 ton c a l i b r a t i o n jack i n s t a l l e d i n the t e s t i n g v e h i c l e . L i n e a r i t y and s t a b i l i t y were found to be e x c e l l e n t . The pressure transducer was c a l i b r a t e d using a dead weight pressure t e s t e r connected h y d r a u l i c a l l y to the pressure t r a n s d u c e r . During c a l i b r a t i o n of each load c e l l , c r o s s - t a l k onto the other channels was recorded and found to be i n s i g n i f i c a n t . During e a r l y stages of the i n v e s t i g a t i o n , i t was found that the t i p recorded only about 71% of any water pressure around i t . As w i l l be d i s c u s s e d i n Chapters 3, 4, and 5, cone p e n e t r a t i o n t e s t i n g i n cohesive s o i l s generates high pore p r e s s u r e s . Since the t i p i s a t o t a l s t r e s s c e l l , i t should r e c o r d both the s o i l and water pressure a c t i n g upon i t . In f a c t , only 71% of the water pressure was recorded. The e x p l a n a t i o n of t h i s problem, from which a l l piezometer cones are thought to s u f f e r , can be 10 8 0 0 r CHAMBER PRESSURE (kPa) CHAMBER PRESSURE (kPa) FIGURE 3 RESULTS OF PIEZOMETER FRICTION CONE CALIBRATION. II seen i n F i g u r e 3. Some of the pore pressure i n the porous element r e a c t s a g a i n s t the f r i c t i o n s leeve and not through the bearing l o a d c e l l . F i g u r e 3 shows t h i s e f f e c t , recorded d u r i n g c a l i b r a t i o n of the load c e l l i n a pressure chamber. S i m i l a r h y d r o s t a t i c pressure c o r r e c t i o n s were found to be necessary to c o r r e c t f o r h y d r o s t a t i c p r e s s u r e s a c t i n g on unequal end areas of the f r i c t i o n s l e e v e . T h i s c a l i b r a t i o n , again performed with the cone i n a pressure chamber, i s shown i n F i g u r e 3. S l i g h t n o n l i n e a r i t y i n t h i s c a l i b r a t i o n i s due to c o m p r e s s i b i l i t y of the 0 and quad r i n g s . These c a l i b r a t i o n f a c t o r s are f u r t h e r d i s c u s s e d i n S e c t i o n 2.4.4. 2.4.2 S a t u r a t i o n of the Porous Element Proper s a t u r a t i o n of the pore water pressure measuring system was found to be extremely important f o r proper i n t e r p r e t a t i o n of t e s t r e s u l t s and w i l l be f u r t h e r d i s c u s s e d i n Chapter 5. G l y c e r i n e was found to be most e f f e c t i v e f o r s a t u r a t i o n . The c a v i t y between the pressure transducer and the porous element was s a t u r a t e d with g l y c e r i n e using a hypodermic tube when the cone was i n v e r t e d with the t i p arid f i l t e r removed. When no f u r t h e r a i r bubbles were seen emerging from the connecting c a v i t i e s , a s a t u r a t e d porous f i l t e r was put i n t o p l a c e and the hypodermic entrance was c l o s e d with a screw, as shown i n F i g u r e 1. Porous p o l y e t h y l e n e and polypropylene f i l t e r s were used 12 throughout the i n v e s t i g a t i o n . S a t u r a t i o n and c l e a n i n g of the elements was achieved by s o n i c a l l y v i b r a t i n g the porous elements while submerged i n g l y c e r i n e . 2.4.3 Cone P e n e t r a t i o n Test Procedure F o l l o w i n g a short warm up p e r i o d and s a t u r a t i o n of the pore p r e s s u r e measuring system, the cone was atta c h e d to the f i r s t rod, and the e x c i t a t i o n v o l t a g e s and zeros were a d j u s t e d with the cone hanging i n a v e r t i c a l d i r e c t i o n . When v e r t i c a l alignment was ensured, p e n e t r a t i o n was s t a r t e d . Unless otherwise d i s c u s s e d , the p e n e t r a t i o n r a t e used was 2 cm/sec. At one metre i n t e r v a l s , the pushing u n i t was r a i s e d and an a d d i t i o n a l rod at t a c h e d . T h i s delay u s u a l l y took approximately one minute, d u r i n g which time, pore p r e s s u r e s were recorded. At p e r i o d i c i n t e r v a l s , p e n e t r a t i o n was h a l t e d , and pore pressure d i s s i p a t i o n s were recorded with time. During t h i s p e r i o d , no load was p l a c e d on the rods by the pushing head. At the completion of the t e s t h o l e , the rods were withdrawn, and, with the cone hanging i n the v e r t i c a l p o s i t i o n , the zero l o a d readings were recorded by each channel. Zero d r i f t s were found to be completely the r e s u l t of temperature v a r i a t i o n between the l a b o r a t o r y , where c a l i b r a t i o n was performed, and the f i e l d . The zero used f o r i n t e r p r e t a t i o n of the r e s u l t s obtained was the zero recorded at the t e s t completion, while the cone was s t i l l at the ground temperature. Measurement of ground temperatures and p o s s i b l e temperature c o r r e c t i o n s i s given i n Campanella and Robertson 1981, who d e s c r i b e a 5 channel cone*that i n c l u d e s a t h e r m i s t e r . 13 2.4.4 Data Reduction The continuous records of bearing f r i c t i o n and pore pressure r e q u i r e c o n s i d e r a b l e h a n d l i n g 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 the f r i c t i o n r a t i o , the pore pressure r a t i o , and the 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 r e q u i r e d , the s t r i p c h a r t records were d i g i t i z e d on a g r a p h i c s 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 were used to f u l l y d e s c r i b e the f i e l d r e c o r d s ; i n a d d i t i o n , a l l unwanted data were e l i m i n a t e d . These unwanted data were caused by l o a d i n g and unloading at rod breaks. Each of the c a l c u l a t i o n s and c o r r e c t i o n s performed i s d e s c r i b e d here. F r i c t i o n c o r r e c t i o n Before c a l c u l a i n g the f r i c t i o n r a t i o , the f r i c t i o n values were c o r r e c t e d f o r the pore pressure a c t i n g on the exposed d i f f e r e n t i a l end areas of the f r i c t i o n s l e e v e . The equation used was: F C C o r r e c t e d = F C Measured + 0.01 1U C o n s i s t e n t u n i t s The pore pressure c o r r e c t i o n f a c t o r , 0.011, was a r r i v e d at by a p p l y i n g a h y d r o s t a t i c pressure to the cone and measuring i t s e f f e c t on the f r i c t i o n s l e e v e , as d e s c r i b e d i n S e c t i o n 2.4.1. One assumption inherent i n t h i s equation i s that the pore pr e s s u r e , measured j u s t behind the t i p , i s r e p r e s e n t a t i v e of the pore pressure at the top of the f r i c t i o n s l e e v e . T h i s important 14 assumption i s f u r t h e r d i s c u s s e d i n Chapter 5, i n which the d i s t r i b u t i o n of pore p r e s s u r e s up the cone i s i n v e s t i g a t e d . Bearing C o r r e c t i o n The cone p e n e t r a t i o n t i p i s a t o t a l s t r e s s c e l l ; as such, i t should r e c o r d both pore pressure and e a r t h pressure r e a c t i n g a g a i n s t i t . As mentioned i n S e c t i o n 2.4.1, the cone used was found to r e c o r d approximately 71% of the water pressure a c t i n g around i t . The proper way, t h e r e f o r e , to account f o r t h i s i s to add to the bearing reading that p o r t i o n of the water pressure that i s not recorded, i . e . : Q cCorrected = Q c Measured + 0.29U C o n s i s t e n t u n i t s T h i s t i p c o r r e c t i o n i s s u b s t a n t i a l when t e s t i n g i n s o f t c l a y e y s o i l s , which have high dynamic pore p r e s s u r e s and low bearing v a l u e s . The importance of t h i s t i p c o r r e c t i o n i s f u r t h e r d i s c u s s e d i n Chapter 5. I t should, however, be noted that the c o r r e c t i o n f a c t o r used, 0.29, was a r r i v e d at by c a l i b r a t i n g the f r i c t i o n piezometer cone C6FPS-2UBC; other cones, a l l of which s u f f e r from t h i s problem, may have d i f f e r e n t c o r r e c t i o n f a c t o r s . I t i s , i n f a c t , the v a r i a t i o n i n c o r r e c t i o n f a c t o r s that n e c e s s i t a t e s proper b e aring and f r i c t i o n c o r r e c t i o n s . D i f f e r e n t i a l Pore Pressure D i f f e r e n t i a l pore pressure i s i n t r o d u c e d by Campanella et a l . 1981 as being a more fundamental parameter than the dynamic pore p r e s s u r e . T h i s d i f f e r e n t i a l pore pressure i s c a l c u l a t e d by: U Differential = U Measured ~ U S t a r i c S t a t i c pore pr e s s u r e s were i n t e r p o l a t e d between measured e q u i l i b r i u m v a l u e s . F r i c t i o n R a t i o F r i c t i o n r a t i o s were c a l c u l a t e d using an o f f s e t depth of 10 cm between simultaneous ( c o r r e c t e d ) bearing and f r i c t i o n v a l u e s . F r i c t i o n r a t i o s were p l o t t e d at the depth of the o f f s e t f r i c t i o n r e a d i n g and c a l c u l a t e d using a l i n e a r l y i n t e r p o l a t e d value f o r the b e a r i n g . Pore Pressure R a t i o Pore pressure and d i f f e r e n t i a l pore pressure r a t i o s were c a l c u l a t e d and p l o t t e d at the depth of the pore pressure reading using i n t e r p o l a t e d v a l u e s between the nearest d i g i t i z e d bearing v a l u e s . Chapter 3 Dynamic Pore Pressures and S t r a t i g r a p h i c Logging 16 3.1 I n t r o d u c t i o n The n e c e s s i t y f o r monitoring dynamic pore p r e s s u r e s to c o r r e c t the measured bearing and f r i c t i o n values was shown in Chapter 2. In a d d i t i o n to c o r r e c t i n g the bearing and f r i c t i o n v a l u e s , the dynamic pore pressure l o g and d i s s i p a t i o n records can enhance the s t r a t i g r a p h i c l o g g i n g a b i l i t i e s of the cone p e n e t r a t i o n t e s t . The use of b e a r i n g and f r i c t i o n measurements fo r s t r a t i g r a p h i c l o g g i n g i s thoroughly e x p l a i n e d by Schmertmann 1978 and o t h e r s ; t h i s chapter d i s c u s s e s only the ways i n which the a d d i t i o n of pore pressure measurements can be u s e f u l . The g e n e r a t i o n of pore p r e s s u r e s i s d i s c u s s e d , and the dynamic pore pr e s s u r e r a t i o i n t r o d u c e d . For i l l u s t r a t i o n purposes, s p e c i f i c examples are drawn upon and e x p l a i n e d . 3.2 Pore Pressures Generated During Cone P e n e t r a t i o n T e s t i n g Pore p r e s s u r e s generated d u r i n g cone p e n e t r a t i o n t e s t i n g depen.d upon the m a t e r i a l t e s t e d and the procedure used. Experience has shown that cone p e n e t r a t i o n t e s t i n g i n c l e a n sands i s e s s e n t i a l l y a d r a i n e d t e s t . In c o n t r a s t , cone p e n e t r a t i o n t e s t i n g i n c l a y s o i l s i s a f u l l y undrained t e s t . The degree of pore p r e s s u r e d i s s i p a t i o n d u r i n g t e s t i n g i n i ntermediate s o i l s i s f u r t h e r d i s c u s s e d i n Chapter 5. The magnitude and s i g n (greater or l e s s than the e q u i l i b r i u m pore pressure) i s a f u n c t i o n of the t e s t procedure, p r e e x i s t i n g 1 7 s t r e s s l e v e l , and the m a t e r i a l t e s t e d . Cone p e n e t r a t i o n t e s t i n g i n dense sands with s u f f i c i e n t f i n e s content ( p a r t i a l l y d r a i n e d ) o f t e n r e s u l t s i n the g e n e r a t i o n of negative pore p r e s s u r e s , whereas cone p e n e t r a t i o n t e s t i n g i n loose sands o f t e n r e s u l t s i n g e n e r a t i o n of p o s i t i v e pore p r e s s u r e s . As s o i l s become more p l a s t i c , p o s i t i v e pore p r e s s u r e s are generated; the magnitude of the pore pressure i s d i s c u s s e d i n Chapter 4. Pore pressures can a l s o be expressed as a r a t i o of the b e a r ing value (which should a l s o i n c l u d e the water p r e s s u r e a c t i n g on the t i p ) ; t h i s t o p i c i s o u t l i n e d i n the next s e c t i o n . The CPT logs from the Mcdonald s i t e are shown i n F i g u r e 4. The s i t e i s c h a r a c t e r i z e d by s i l t o v e r l y i n g c l e a n sands to 13 metres. Below 13 metres, the sands grade i n t o s i l t y sands with low bearing v a l u e s . Below 15 metres, a s o f t c l a y e y s i l t d e p o s i t can e a s i l y be i d e n t i f i e d by the bearing and pore pressure response, as w e l l as by the f r i c t i o n r a t i o . Very s l i g h t l y n egative pore pr e s s u r e s ,recored behind the t i p , c a n be seen i n the f i n e s i l t y sand from 13 to 15 metres. Although the sand above has much hig h b e a r i n g v a l u e s , i t i s f r e e d r a i n i n g and no negative pore pr e s s u r e s are observed. Even though the two sand d e p o s i t s , t h e r e f o r e , have s i m i l a r f r i c t i o n r a t i o s , an i n d i c a t i o n of t h e i r g r a d a t i o n a l c h a r a c t e r i s t i c s can be o b t a i n e d . The s o f t c l a y e y s i l t d e p o s i t s below 15 metres generate l a r g e p o s i t i v e pore p r e s s u r e s and can e a s i l y be d i s t i n g u i s h e d . 3.3 Dynamic Pore Pressure R a t i o An example showing l a r g e pore p r e s s u r e s generated d u r i n g FIG. 4S0IL PROFILE FOR RESEARCH SITE AT McDONALDS FARM, SEA ISLAND. 19 cone p e n e t r a t i o n t e s t s have been shown. For most c l a y e y s o i l s , the undrained shearing r e s u l t s i n l a r g e pore pr e s s u r e s and accompanying decrease i n the e f f e c t i v e s t r e s s l e v e l . The pore p r e s s u r e s generated r e s u l t from a combination of two f a c t o r s : The l a r g e shear induced pore p r e s s u r e ; and the compressive s t r e s s induced pore p r e s s u r e . B a l i g h et a l . 1980 introduced the pore pressure r a t i o U/QC as a means of no n d i m e n s i o n a l i z i n g the pore pressure generated. I t was proposed t h a t , when p e n e t r a t i n g s o f t c l a y e y s o i l s , the shear induced pore pressures would be p o s i t i v e , whereas the shear induced pore pressures generated d u r i n g cone p e n e t r a t i o n t e s t i n g i n o v e r c o n s o l i d a t e d s o i l s would be small or even n e g a t i v e . T h i s does not imply that the t o t a l pore pressure generated need n e c e s s a r i l y be negative because of the pore p r e s s u r e s generated by the compressive s t r e s s e s . As w i l l be shown i n Chapter 4, the magnitude of the t o t a l pore pr e s s u r e s generated i n c l a y e y s o i l s i s , to a degree, a f u n c t i o n of the undrained shear s t r e n g t h . By e x p r e s s i n g the' pore p r e s s u r e as a r a t i o of the be a r i n g , which i s a l s o a f u n c t i o n of the undrained shear s t r e n g t h , i t was hoped that the e f f e c t s of shear induced pore pressure and, t h e r e f o r e , s t r e s s h i s t o r y c o u l d be emphasized. Thus, i t was hypothesized that o v e r c o n s o l i d a t e d s o i l s t h a t generate small p o s i t i v e or negative pore p r e s s u r e s when sheared and have high bearing v a l u e s should have a low pore pressure r a t i o U/QC, whereas s o f t s o i l s should have a pore pressure r a t i o approaching 1.0. Since the bearing value i s a combination of s o i l r e s i s t a n c e and water p r e s s u r e a c t i n g on the t i p , the upper l i m i t of the 2 0 pore pressure r a t i o i s 1.0. In s p i t e of t h i s , pore pressure r a t i o s g r e a t e r than 1.0 are commonly r e p o r t e d . A l i s t of pore pressure r a t i o s i s shown with measured o v e r c o n s o l i d a t i o n r a t i o s : OCR U/QC B a l i g h et a l . 1980 6 0.6 60° T i p 4 1.0 2 1.2 B a l i g h et a l . 1980 6 0.15 18° T i p 4 0.4 2 0.75 Lacasse et a l . 1981 2.5 0.5 1 . 5 - 1 . 1 5 0.6 1.2 0.7 Roy et a l . 1980 2 . 2 - 2 . 5 0.56 T h i s I n v e s t i g a t i o n 1.0 0.8 - 1.0 The d i f f e r e n t i a l pore pressure r a t i o — t h e r a t i o of that pore pressure i n excess of the h y d r o s t a t i c value to the bearing v a l u e — w a s introduced by Campanella et a l . 1981 as being a more fundamental parameter. Cone p e n e t r a t i o n t e s t i n g i n granula r m a t e r i a l s may a l s o generate e i t h e r p o s i t i v e or negative pore p r e s s u r e s . In sands, 21 the b e a r i n g value i s u s u a l l y so l a r g e and the induced pore pressure so low that the r a t i o ( e i t h e r p o s i t i v e or negative) i s u s u a l l y very s m a l l . Mixed s o i l s may, however, r e s u l t i n l a r g e r pore p r e s s u r e r a t i o s , which c o u l d have a value s i m i l a r to that of an o v e r c o n s o l i d a t e d c l a y . D i f f e r e n t i a t i o n between an o v e r c o n s o l i d a t e d c l a y s o i l and a s i l t y s o i l may not be p o s s i b l e i f o nly the dynamic pore p r e s s u r e and b e a r i n g values are a v a i l a b l e . D i f f e r e n t i a t i o n between such s o i l s i s , however, p o s s i b l e with the a i d of f r i c t i o n measurements and the f r i c t i o n r a t i o . P o s s i b l e a d d i t i o n a l means of d i f f e r e n t i a t i n g such s o i l s u s i ng pore pressure decay r a t e s i s d e s c r i b e d i n Chapter 6. 3.4 S t r a t i g r a p h i c Logging S p e c i f i c examples of the u s e f u l n e s s of simultaneous measurements of dynamic pore p r e s s u r e s , r e c o r d e d behind the t i p , as w e l l as f r i c t i o n and b e a r i n g v a l u e s , are presented i n t h i s s e c t i o n . 3.4.1 CPT Example 1 F i g u r e 5 The complete cone l o g o b t a i n e d from a s i t e of the F r a s e r R i v e r d e l t a i s shown i n F i g u r e 5. The interbedded sands and s i l t s were d e p o s i t e d s i n c e the l a s t g l a c i a l p e r i o d . The s t r a t i g r a p h i c v a r i a t i o n shown r e s u l t s , i n p a r t , from v a r i a t i o n i n the l o c a t i o n of the r i v e r channel w i t h i n the prograding d e l t a . The m a t e r i a l types i n t e r p r e t e d from the cone logs are shown i n F i g u r e 5. Within each u n i t shown, the g r e a t e s t d e t a i l can be seen from the pore pressure l o g . For example, w i t h i n the P O R E F R I C T I O N B C A P . I N O F R I C T I O N P R E S S U C RE9STANCE R E S I S T A N C E R A T I O U(bor) FC(bar) QCtbor) RFl%) Q 2 5 0 2 0 250 0 5 O F F R r. S O I L R A T I O P R O F I L E MJ/QC 0 030 TTTT PIEZOMETER FRICTION CONE LOGGING IN STRATIFIED SOILS / / / (ANNACIS ISLAND SITE) KJ-LJ SAND SILT SILTY SAND F I G U R E 5 CPT E X A M P L E I 23 s i l t l e n s e s , the bearing and f r i c t i o n v a l u e s , which r e f l e c t the p r o p e r t i e s of a l a r g e r zone of s o i l , are n e a r l y constant, whereas the pore pressure l o g r e v e a l s small sandier p a r t i n g s w i t h i n each s i l t l e n s . The m a t e r i a l c l a s s i f i c a t i o n shown i n F i g u r e 5 i n c l u d e s the d i s t i n c t i o n between sand, s i l t , and s i l t y sand d e p o s i t s . Although the sand and s i l t d e p o s i t s are e a s i l y d i s t i n g u i s h e d using only the bearing and f r i c t i o n measurements, the s i l t y sand l a y e r s can o n l y be d i s t i n g u i s h e d from the s i l t l a y e r s by the pore p r e s s u r e measurements. The bearing and f r i c t i o n r a t i o f o r the two m a t e r i a l s are s i m i l a r ; each has a value of approximately 20 bar and 3.0 r e s p e c t i v e l y . I f only the f r i c t i o n and b e a r i n g measurements are a v a i l a b l e , each would be c l a s s i f i e d as a s i l t . I n s p e c t i o n of the dynamic pore pressure l o g s , however, r e v e a l s a d i s t i n c t l y d i f f e r e n t behavior of the two m a t e r i a l s . Compared to the s i l t , the s i l t y sand has a much lower dynamic pore p r e s s u r e . The v a r i a t i o n i n dynamic pore p r e s s u r e can occur because e i t h e r the s i l t y sand i s l e s s compressible or i s a more f r e e l y d r a i n i n g m a t e r i a l . Since the m a t e r i a l s have e q u i v a l e n t b e a r i n g v a l u e s , i t appears t h a t p e n e t r a t i o n of the s i l t y sand r e s u l t s i n a lower dynamic pore pressure because of p a r t i a l drainage of the dynamic pore p r e s s u r e s . A more complete i n d i c a t i o n of the g r a d a t i o n a l c h a r a c t e r i s t i c s can thus be o b t a i n e d from the pore pressure response. An a l t e r n a t e means of i d e n t i f y i n g the three s o i l types can be o b t a i n e d from the time to reach d i s s i p a t i o n of 50% of the excess pore p r e s s u r e , t 5 0 , such as those marked at t h e i r 24 a p p r o p r i a t e depths i n F i g u r e 5. For a given probe, the time f o r d i s s i p a t i o n of 50% of the excess pore p r e s s u r e i s a f u n c t i o n of the 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 of the s o i l , and c l e a r l y d i s t i n g u i s h e s the three s o i l types. The l o c a t i o n of the pore p r e s s u r e d i s s i p a t i o n s shown i n F i g u r e 5 a l s o h i g h l i g h t s the n e c e s s i t y of i n s p e c t i n g the complete cone lo g s when i n t e r p r e t i n g d i s s i p a t i o n t e s t r e s u l t s . D i s s i p a t i o n s conducted w i t h i n the s i l t l e n s e s , but near sand boundaries, are a f f e c t e d by the g r e a t l y reduced drainage path l e n g t h . The time to reach t ^ 0 i s a u s e f u l parameter f o r d i s t i n g u i s h i n g s t r a t i g r a p h i c types. For many s o i l types such as those shown i n F i g u r e 5, the time to reach 50% d i s s i p a t i o n of excess pore pressure i s o f t e n achieved d u r i n g rod breaks, with very l i t t l e standby time. S i m i l a r r e s u l t s can be obtained u s i n g a s m a l l e r degree of d i s s i p a t i o n i n l e s s permeable s o i l s . 3.4.2 CPT Example 2 F i g u r e 6 A g e n e r a l i z e d s o i l p r o f i l e f o r t h i s s i t e would i n d i c a t e that the s i t e c o u l d be broken i n t o two b a s i c u n i t s : a sand d e p o s i t o v e r l y i n g s i l t , with interbedded sand l e n s e s . The sand l e n s e s are e a s i l y d i s t i n g u i s h e d by t h e i r h i g h b e a r i n g , low f r i c t i o n r a t i o , and low 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 . The s i l t has b e a r i n g v a l u e s ranging from 20 to 30 bar, i n c r e a s i n g with depth, and i s e a s i l y d i s t i n g u i s h e d by i t s h i g h dynamic pore pr e s s u r e r a t i o . In c o n t r a s t to the s i l t d e p o s i t s , a very low bea r i n g value m a t e r i a l can be seen between 28.5 and 30.0 metres. The b e a r i n g and f r i c t i o n r a t i o s alone i n d i c a t e t h at the m a t e r i a l PORE PRESSURE f l l C T I N RES1STRMCE BCRRING RESISTANCE FR1CTIOM RATIO BIFFEfiENTIflL P .P . B IBRRI Ft IBflRI DC IBRR) RF=FC/QC ('/) RRIIO &U/QC I 25 II 2 I 250 0 5 0 .80 F IGURE 6 C P T E X A M P L E 2 26 i s a s o f t c l a y s o i l ; examination of the pore pressure record, however, shows that no pore p r e s s u r e was generated d u r i n g p e n e t r a t i o n . Two very d i s t i n c t types of m a t e r i a l do not generate excess pore p r e s s u r e s when sheared: those that are so f r e e d r a i n i n g that the pore p r e s s u r e s immediately d i s s i p a t e ; and those t h a t are of a p a r t i c u l a r s t i f f n e s s such that n e i t h e r p o s i t i v e nor negative pore pre s s u r e s r e s u l t . Since m a t e r i a l with a much higher bearing v a l u e — t h e s i l t l e n s e s between 45 and 70 m e t r e s — r e s u l t s i n very l a r g e pore p r e s s u r e s , i t appears that the m a t e r i a l between 28.5 and 30.0 metres i s a f r e e d r a i n i n g , but very s o f t , m a t e r i a l . The b e a r i n g value i s too low and the f r i c t i o n r a t i o too high f o r the m a t e r i a l to be a sand. I t would appear, t h e r e f o r e , that the m a t e r i a l i s a f r e e d r a i n i n g organic d e p o s i t such as f i b r o u s peat. 3.4.3 CPT Example 3 F i g u r e 7 Pore pressure r e c o r d s , e s p e c i a l l y d i s s i p a t i o n s , are s e n s i t i v e to both the m a t e r i a l that the piezometer element l i e s w i t h i n and the m a t e r i a l surrounding the cone. To i l l u s t r a t e some of the e f f e c t s of s o i l s t r a t i g r a p h y on the pore pressure response, F i g u r e 7 shows a p o r t i o n of the dynamic pore pressure r e c o r d . As i n d i c a t e d i n F i g u r e 7, the r e c o r d i n c l u d e s both the dynamic pore pressure r e c o r d with depth, as w e l l as the d i s s i p a t i o n r e c ords o b t a i n e d d u r i n g rod breaks. D i s s i p a t i o n s conducted at 73.6 and 75.6 metres are of the type g e n e r a l l y recorded i n a c l a y e y d e p o s i t . D i s s i p a t i o n s of the type recorded at 74.6 metres, however, have a l s o been shown i n the l i t e r a t u r e . PDBE PRESSURE fRIMN RES1STRNCE "ERRING RESISTHCE FlltUII RATIO IIFFEfiEMIRL P.P. RF=F£/OC ID RRIIO U/IC D 52 18 U (bars) 72.6 73.6 73.6 E 74.6 x k UJ 74.6 Q 75.6 75.6 766 LEGEND DATA RECORDED DURING PENETRATION 0.5 m DATA RECORDED DURING DISSIPATION T DETAIL OF DISSIPATION LOCATION FIGURE 7- COMPLETE CPT LOG AND DETAIL OF DYNAMIC PORE PRESSURE RECORD 28 Many e x p l a n a t i o n s e x i s t f o r the apparent i n c r e a s e i n pore p r e s s u r e b e f o r e the u l t i m a t e decay of excess pore p r e s s u r e . In t h i s case, however, examination of the complete depth r e c o r d i n g s r e v e a l s that the piezometer element at 74.6 metres was w i t h i n a sand l e n s . As shown i n F i g u r e 7, pore p r e s s u r e s generated i n the s i l t l e n s , both above and immediately below the cone, r e s u l t e d i n a delayed pore pressure r i s e i n the sand lens a f t e r p e n e t r a t i o n was h a l t e d . Subsequent r a p i d d i s s i p a t i o n of pore p r e s s u r e was c o n t r o l l e d , i n p a r t , by the higher p e r m e a b i l i t y of the sand l e n s . Other d i s s i p a t i o n s of t h i s type are o f t e n seen i n l a y e r e d s o i l s ; c a r e f u l examination of the complete r e c o r d s , both with depth and with time, can f u r t h e r e x p l a i n such phenomena. 3.4.4 CPT Example 4 F i g u r e 8 F i g u r e 8 shows the b e a r i n g and pore pressure l o g s obtained before and a f t e r compaction i n adjacent h o l e s . The m a t e r i a l was h y d r a u l i c a l l y p l a c e d . I t has been s t a t e d that when she a r i n g s o i l s of s u f f i c i e n t l y low p e r m e a b i l i t y , dynamic pore p r e s s u r e s r e s u l t , the l e v e l of which depends on the volume change c h a r a c t e r i s t i c s of the s o i l . F i g u r e 8 i l l u s t r a t e s the change i n both magnitude and s i g n of the dynamic pore pressure that accompanies an i n c r e a s e i n compactiveness of the s o i l . Before compaction, l a r g e p o s i t i v e pore p r e s s u r e s are generated i n the s o f t s i l t between 2.5 and 7.5 metres; a f t e r compaction, the dynamic pore p r e s s u r e s become n e g a t i v e . Although the magnitude and s i g n of the dynamic pore pressure has only been used to date 62 30 i n a q u a l i t a t i v e manner, i t may prove to be a u s e f u l parameter to d e s c r i b e the p r o p e r t i e s of p a r t i a l l y d r a i n e d or undrained m a t e r i a l s . The s i g n of the dynamic pore p r e s s u r e probably depends on the l o c a t i o n of the measuring element. As s t a t e d i n Campanella et a l . 1981, the l o c a t i o n of the porous element used i n t h i s i n v e s t i g a t i o n probably encourages the measurement of negative pore p r e s s u r e s i n a l l but very loose s o i l s . F u r t h e r i n v e s t i g a t i o n of the e f f e c t of l o c a t i o n of the porous element i s ongoing. 3.5 S t a t i c Pore Pressures Piezometer cone l o g g i n g can be stopped at any depth and the e q u i l i b r i u m ground water c o n d i t i o n s measured. Although the cone generates dynamic pore p r e s s u r e s d u r i n g p e n e t r a t i o n , c a r e f u l m o n i t o r i n g of the decay of excess pore pressure with time can ensure that f u l l e q u i l i b r i u m has been achieved. In t h i s manner, cone p e n e t r a t i o n t e s t i n g can be a c o s t e f f e c t i v e means of o b t a i n i n g e q u i l i b r i u m ground water c o n d i t i o n s . As w i l l be shown in Chapter 6, the time to achieve any degree of d i s s i p a t i o n i s a f u n c t i o n of the 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 of the s o i l and the r a d i u s of the probe squared. The f a c t that the time to reach e q u i l i b r i u m c o n d i t i o n s i n c r e a s e with the r a d i u s of the probe squared i s one of the reasons why e a r l y piezometer probes, which only measured pore p r e s s u r e s , were of very small r a d i u s . An example of the use of the piezometer cone f o r o b t a i n i n g 31 e q u i l i b r i u m ground water c o n d i t i o n s can be shown from Campanella and Robertson 1981. E q u i l i b r i u m ground water pre s s u r e s are shown with depth i n F i g u r e 9. Knowledge of the e q u i l i b r i u m pore p r e s s u r e s i s u s e f u l f o r flow net, seepage, and s t a b i l i t y a n a l y s i s , as w e l l as p r o p e r l y e v a l u a t i n g the cone bearing data. For example, present methods f o r e v a l u a t i n g bearing values f o r r e l a t i v e d e n s i t y or f r i c t i o n angle of sands r e q u i r e s an estimate of the i n s i t u e f f e c t i v e s t r e s s l e v e l . I f , i n t h i s example, h y d r o s t a t i c c o n d i t i o n s had been estimated, the l e v e l of e f f e c t i v e s t r e s s would be g r e a t l y underestimated and r e l a t i v e d e n s i t y d e t e r m i n a t i o n s from b e a r i n g v a l u e s overestimated. 3.6 C o n c l u s i o n s Examples showing how the dynamic pore p r e s s u r e and dynamic pore p r e s s u r e r a t i o change with both volume change c h a r a c t e r i s t i c s as w e l l as p e r m e a b i l i t y have been shown Although p e n e t r a t i o n of very d i f f e r e n t m a t e r i a l s may r e s u l t i n s i m i l a r dynamic pore p r e s s u r e by c a r e f u l l y i n s p e c t i n g the b e a r i n g and f r i c t i o n measurements as w e l l as the dynamic pore p r e s s u r e the m a t e r i a l type can be d i s t i n g u i s h e d . The e f f e c t of p e r m e a b i l i t y on the dynamic pore pressure w i l l be f u r t h e r d i s c u s s e d i n Chapter6. FIG. 9 E X A M P L E OF P I E Z O M E T E R , FRICTION C O N E L O G T H R O U G H T A I L I N G S D E P O S I T . 01 33 Chapter 4 A n a l y t i c a l E x p r e s s i o n s f o r Dynamic Pore Pressure 4.1 I n t r o d u c t i o n During cone p e n e t r a t i o n t e s t i n g , pore pr e s s u r e s have only been measured at one l o c a t i o n . The l o c a t i o n chosen during t h i s i n v e s t i g a t i o n was d i r e c t l y behind the t i p . In a d d i t i o n , pore p r e s s u r e s were a l s o measured up the s h a f t . Other r e s e a r c h e r s , Roy et a l . 1980 and B a l i g h et a l . 1980, have measured pore p r e s s u r e s at the t i p of the cone penetrometer. Alignment problems precl u d e the p o s s i b i l i t y of a c t u a l l y measuring pore p r e s s u r e s around the cone d u r i n g cone p e n e t r a t i o n t e s t i n g . P r e v i o u s experience i n measurement of pore p r e s s u r e s around d r i v e n p i l e s can, however,, be drawn upon to compare pore p r e s s u r e s with r a d i a l d i s t a n c e from the cone to those measured at the cone s u r f a c e . T h i s chapter compares a n a l y t i c a l e x p r e s s i o n s f o r the magnitude and d i s t r i b u t i o n of pore pr e s s u r e s with f i e l d o b s e r v a t i o n s of dynamic pore p r e s s u r e s around d r i v e n p i l e s . In a d d i t i o n , a n a l y t i c a l e x p r e s s i o n s f o r the magnitude of the pore pressure generated are compared with values measured d u r i n g cone p e n e t r a t i o n t e s t i n g at the Mcdonald s i t e . A knowledge of the magnitude and d i s t r i b u t i o n of pore p r e s s u r e s i s important f o r s e v e r a l reasons, i n c l u d i n g : 1) The d i s t r i b u t i o n of dynamic pore pr e s s u r e s r e p r e s e n t s the i n i t i a l c o n d i t i o n s f o r pore p r e s s u r e d i s s i p a t i o n a n a l y s i s used to c a l c u l a t e c o n s o l i d a t i o n 34 c h a r a c t e r i s t i c s . 2) P e n e t r a t i o n t e s t i n g i n f l u e n c e s a l a r g e zone of s o i l ; the d i s t r i b u t i o n of pore pr e s s u r e s i s fundamental to any 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 bearing and f r i c t i o n measurements. 3) If a n a l y t i c a l e x p r e s s i o n s f o r the generated pore pressure that r e l y on fundamental s o i l p r o p e r t i e s compare w e l l with measured pore p r e s s u r e s generated -at the s u r f a c e of the cone, then dynamic pore pressure measurement may prove u s e f u l i n e v a l u a t i n g s o i l parameters. 4.2 A n a l y t i c a l E x p r e s s i o n s The t h e o r e t i c a l p r e d i c t i o n of the pore pressure d i s t r i b u t i o n induced d u r i n g steady cone p e n e t r a t i o n t e s t i n g i n s a t u r a t e d c l a y e y s o i l s i s the s u b j e c t of c o n s i d e r a b l e a t t e n t i o n because of i t s relevance to many problems, i n c l u d i n g p i l e i n s t a l l a t i o n and the 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 , which i n c l u d e the piezometer cone and the pressuremeter t e s t s . Pore pr e s s u r e d i s t r i b u t i o n around the cone penetrometer i s complicated by: 1) complex boundary c o n d i t i o n s around the cone; 2) complex s t r e s s paths f o l l o w e d by the s o i l , e s p e c i a l l y near the cone; 3) very l a r g e s t r a i n s ; and 4) complexity of s o i l b ehavior. 3 5 In order to s i m p l i f y the problem, s e v e r a l assumptions must be made re g a r d i n g both the s o i l behavior and geometry. Experience gained by mon i t o r i n g of pore pr e s s u r e s induced by d r i v e n p i l e s has l e d to the r e l a t i v e l y widely accepted idea t h a t the complex geometry of cone i n s t a l l a t i o n can be modelled by e i t h e r expansion of a c y l i n d r i c a l or s p h e r i c a l c a v i t y . Past experience seems to i n d i c a t e t h at pore p r e s s u r e s generated a t the t i p can best be estimated by expansions of a s p h e r i c a l c a v i t y , whereas pore p r e s s u r e s up the sleeve or s h a f t can best be modelled by expansion of a c y l i n d r i c a l c a v i t y (Roy et a l . 1980 and V e s i c 1972). In a d d i t i o n to s i m p l i f i c a t i o n of the geometry, d i f f e r e n t s o i l models can be used t o d e s c r i b e the s o i l b e h avior. T h i s s e c t i o n summarizes a n a l y t i c a l e x p r e s s i o n s f o r pore pressure magnitude and d i s t r i b u t i o n . Chapter3 showed examples of very low dynamic pore p r e s s u r e r e c o r e d when p e n e t r a t i n g c e r t i a n c l a y e y s o i l s . Negative dynamic pore pr e s s u r e s were observed when p e n e t r a t i n g a compacted s i l t y s o i l . Other r e s e a r c h e r s have shown how the dynamic pore p r e s s u r e , e s p e c i a l l y when expressed as the dynamic pore pressure r a t i o , d e c r e a s e s with i n c r e a s i n g o v e r c o n s o l i d a t i o n r a t i o f o r a c l a y s o i l . Some of the a n a l y t i c a l e x p r e s s i o n s i l l u s t r a t e d i n t h i s Chapter apply only to normally c o n s o l i d a t e d s o i l s . Others attempt to account f o r the volume change c h a r a c t e r i s t i c s of the s o i l , t h e d i f f e r e n c e between the ex p r e s s i o n s should be noted. 4.2 .1 Comparison between Models A r i g o r o u s comparison between models i s dependent upon the s e l e c t i o n of input parameters. Recognizing that no s i n g l e t e s t procedure can r e s u l t i n a r e f e r e n c e parameter, Table I summarizes the s e l e c t i o n of input parameters and the p r e d i c t e d l e v e l of excess pore pressure f o r each of the models o u t l i n e d . The r e s u l t s are shown f o r the s i l t at 20 metres depth. Comparison between models and measured pore p r e s s u r e s at the Mcdonald s i t e i s d i s c u s s e d f u r t h e r . Parameters Used cu undrained shear s t r e n g t h I r r i g i d i t y index = E / 3 c u E e l a s t i c modulus - undrained U pore pressure r r a d i a l s t r e s s p average s t r e s s A^ Skempton's pore pressure parameter k at r e s t e a r t h pressure r a t i o Geometric Terms Plastic Zone Elastic Zone 3 7 MODEL Parameters Used (bar) cu I r Af 0' p' c s U c y l Usph P'nc 1. E l a s t i c p l a s t i c 0.55 167 2.81 3.75 2. E l a s t i c p l a s t i c 0.55 167 0.4 3.26 4.20 with volume hardening 3. Semi e m p i r i c a l 0.55 167 1.0 3.45 4.49 Torstensson 1977 4. Semi e m p i r i c a l 0.55 32° 1.94 Hagerty and Garlanger 1972 5. Semi e m p i r i c a l U/p 2.33 Roy et a l ^ 1979 =0.75 M a t e r i a l Tested Clayey S i l t QC = 8.2 bar rj v ' =2.0 bar PI = 1.5% .% sand 10 Uo = 2.0 bar LL = 38% % s i l t 70 Wn = 35% % c l a y 20 -cu = QC/NC = 8.2/15 =0.55 bar la - I r = Estimated from C o r r e l a t i o n s et a l . 1977 -Af = Estimated from c o r r e l a t i o n s Whitman 1969 -0' = Estimated from c o r r e l a t i o n s Whitman 1969 p n c = Estimated from c o r r e l a t i o n s E s r i g and K i r b y 1978 > vane - 5 - - 6 bar between cu and I r from Ladd between OCR and Af, Lambe and between PI and 0 ', Lambe and between p^.5 /p' n c a n a " C c ' T a b l e I Input P a r a m e t e r s Used to Compare D y n a m i c Pore P r e s s u r e M o d e l s 38 Method 1 E l a s t i c p e r f e c t l y p l a s t i c s o i l V e s i c 1972; Randolph and Wroth 1979; Torstensson 1977 C y l i n d r i c a l c a v i t y A err = cut 1 + In I r ) (1 ) a U = 2cu • In R/r (2) R/r = ( I r ) 1 ' 2 (3) S p h e r i c a l c a v i t y A^rr = 4/3CU(1 + In I r ) (4) A U = 4cu l n ( R / r ) (5) R/r = I r 1 / 3 (6) Method 2 E l a s t i c p l a s t i c s o i l with volume hardening s o i l s with OCR < 2.0 S c h o f i e l d and Wroth 1968; Roscoe and Burland 1968 A U s (p'j - p ' f ) + A p (7) M = q/p = 6 s i n 0 ' / ( 3 - sin0) (8) q = / 3 cu (9) A p = from egns 2 + 4 (10) C y l i n d r i c a l c a v i t y : a f t e r E s r i g and K i r b y 1978 A U = (p'j - p'f ) + Ap (11) A U = ( p ' n c - p; s) + 2cu In I r ' / 2 (12) JfcLnc ~ J2.cs A U = p'nc P'nc + 2cu In I r l / a (13) P'nc 3 9 S p h e r i c a l c a v i t y : a f t e r E s r i g and K i r b y 1978 A U = (p'j - p'f ) + AP (14) A U = (p'nc - Pes > + 4cu In I r 1 / 3 (15) JP_hc ~ p 'eg A U = phe £'nc + 4 c u In I r •/ 3 (16) P'nc Method 3 Semi e m p i r i c a l Torstensson 1977;Massarch 1978 A u = 1 /3 ( A cr, + A cr2+ A cjj) + a f [ ( A a, - A a 3 ) 2 + ( A cr 2- A fj 3)2 + ( a C T | - A f j 2 ) 2 ] " f c (17) a f = 1/ 2(3A f - 1) (18) C y l i n d r i c a l c a v i t y A U = c u [ l n I r + 0.577(3A f - 1)3 (19) S p h e r i c a l c a v i t y A U = cu[4/31n I r + 0.667(3A f - 1)] (20) Method 4 Semi e m p i r i c a l Hagerty and Garlanger 1972 C y l i n d r i c a l c a v i t y A U = pj> [ 1 - ( s i n 0 ' / s i n 0 ' ) • Cu/p",) ] (21 ) Method 5 Semi e m p i r i c a l Roy et a l . 1979 4 0 C y l i n d r i c a l c a v i t y A U = (1 - k ) crv'o + Uu/p)ma'p (22) C a l c u l a t e d values f o r the d i f f e r e n t i a l pore p r e s s u r e , ^ M e a s u r e d ~ u S t a f i c > a r e shown f o r the c l a y e y s i l t at the Mcdonald s i t e i n Table I. As can be seen i n the a n a l y t i c a l e x p r e s s i o n s f o r the dynamic pore p r e s s u r e , the models are not s e n s i t i v e to the s e l e c t i o n of a r i g i d i t y index ( s t i f f n e s s r a t i o ) ; they are, however s e n s i t i v e to the s e l e c t i o n of an Undrained shear s t r e n g t h . The undrained shear s t r e n g t h used was c a l c u l a t e d from the measured bearing v a l u e . The e f f e c t of i n c r e a s e d r i g i d i t y index i s mainly to extend the zone of s o i l i n f l u e n c e d by the c a v i t y expansion. F u r t h e r d i s c u s s i o n of the importance of the r i g i d i t y index w i l l be i n c l u d e d i n Chapter 6. A n a l y t i c a l estimates of the dynamic pore pressure can be compared to the f i e l d measured values shown i n F i g u r e 10 . For the purposes of i l l u s t r a t i o n , only the s o l u t i o n s obtained using the e l a s t i c p l a s t i c model are shown i n F i g u r e 10 . The s p h e r i c a l c a v i t y expansion s o l u t i o n compares fa v o u r a b l y with the pore p r e s s u r e s measured immediately behind the cone t i p . The s o l u t i o n f o r the c y l i n d r i c a l expansion compares w e l l with pore p r e s s u r e s measured 25 cm ( 7 . 1 d ) behind the cone t i p . An a b s o l u t e comparison between the r e s u l t s obtained with each model i s complicated by the s e l e c t i o n of input parameters, e s p e c i a l l y the undrained shear s t r e n g t h . The e l a s t i c p e r f e c t l y p l a s t i c model p r e d i c t s v a l u e s very c l o s e to the f i e l d measured 4 1 U (bar) 0 5 10 • • • » 1 • • « » in i_ E a. a LEGEND Hydrostatic pressure conditions Dynamic pore pressure measured with piezometer element immediately behind tip Spherical solution* for dynamic pore pressure Dynamic pore pressure measured with piezometer element 25 cm behind tip Cylindrical solution* for dynamic pore pressure Analytical solutions shown after Torstensson 1977. Assumption for input data derailed in Table I . FIGURE 1 0 COMPARISON BETWEEN ANALYTICAL SOLUTIONS AND MEASURED DYNAMIC PORE PRESSURES. 42 v a l u e s . The e l a s t i c p l a s t i c model with volume hardening p r e d i c t s s l i g h t l y high l i m i t and pore p r e s s u r e s . The semi e m p i r i c a l method o u t l i n e d by Massarch 1978, and Torstensson 1977 i n c l u d e s Skempton's A f pore pressure parameter. The i n c o r p o r a t i o n of an Af v a l u e , which v a r i e s with past l o a d i n g h i s t o r y , i n t o the e x p r e s s i o n f o r dynamic pore pressures i s c o n s i s t e n t with the b e l i e f t h a t the dynamic pore pressure r a t i o U/QC i s a good i n d i c a t o r of the l o a d i n g h i s t o r y of the s o i l . The semi e m p i r i c a l method proposed by Roy et a l . 1979 r e q u i r e s an estimate f o r U/p, best obtained from 'an undrained t r i a x i a l t e s t . As i n the case of models by V e s i c 1972, Massarch 1978, and Torstensson 1977, which i n c o r p o r a t e an Af value, the model by Roy et a l . 1979 attempts to i n c o r p o r a t e the e f f e c t of p r e v i o u s l o a d i n g h i s t o r y . The semi e m p i r i c a l method proposed by Hagerty and Garlanger 1972 tends to u n d e r p r e d i c t the dynamic pore p r e s s u r e s . Use of e i t h e r of the f i r s t three models o u t l i n e d appears to c o n f i r m that the pore p r e s s u r e s at the t i p can best be modelled by s p h e r i c a l c a v i t y expansion, whereas those measured some d i s t a n c e behind the t i p can best be modelled by c y l i n d r i c a l c a v i t y expansion. The d i s t r i b u t i o n of pore pr e s s u r e s p r e d i c t e d by the models can be compared to pore pr e s s u r e s measured during the i n s t a l l a t i o n of p i l e s . F i g u r e 11 shows the d i s t r i b u t i o n of pore p r e s s u r e around d r i v e n p i l e s . Each of the case h i s t o r i e s s e l e c t e d confirms the d i s t r i b u t i o n of pore p r e s s u r e p r e d i c t e d by 43 the a n a l y t i c a l e xpression (egns 2, 5). The excess pore pre s s u r e s shown in F i g u r e 11 a l l appear to decrease with the l o g a r i t h m of the r a d i u s . As p r e v i o u s l y s t a t e d , a knowledge of the i n i t i a l d i s t r i b u t i o n of excess pore p r e s s u r e s i s fundamental to the proper i n t e r p r e t a t i o n of pore pressure decay r a t e s . UJ >-cr -r O * a- UJ cr UJ 10 u ui UJ 0-Ul >-O 4 o- UJ cr Ul i/l O UJ X cr ui &• 2-5n 20 15H to 5H REFERENCE Lo and Stermac 1965 DEPTH (m)1 0-7 cu(bar) 2-25 o c r 1 DIAMETER (m) .09 0 1 20-1 1-5' \0 < a- UJ cr ^ Ul io U UJ x cr Ul 0. Ul >• a u u> § w Ul i/i U Ul x a Ul 0. \ \ • \ REFERENCE Koizumi and Ito 1967 DEPTH(m) 5 cu(bar) 3-35 o c r 25 DIAMETER (m) .3 0 25-, 20 15H \0 •5H REFERENCE Ismael and Klym 1979 DEPTH(m) 5 8 cu (bar) 13 OCT — DIAMETER H pile 12x12' 0 25-20-t 15-1 TO 0 REFERENCE ROY et ol 1979 DEPTH(m)6 cu (bar) 2 OCT 23 DIAMETER (m) .22 5 T 10 20 NORMALIZED RADIUS 50 r/R 100 Figure H Excess Pore Pressure Distribution Around Installed Piles: Case Histories 45 Chapter 5 F i e l d Observations of Dynamic Pore Pressure 5.1 I n t r o d u c t i o n T h e o r e t i c a l estimates of the magnitude of the pore pressure generated both at the s u r f a c e of the cone penetrometer and with r a d i a l d i s t a n c e from the cone have been presented i n Chapter 4. In a d d i t i o n , measured values of pore pressure generated d u r i n g f i e l d t e s t s have been compared to the t h e o r e t i c a l l y o b t a i n a b l e v a l u e s . T h i s chapter summarizes r e s u l t s of f i e l d t e s t i n g conducted by both the UBC i n s i t u r e s e a r c h team and o t h e r s , showing the importance of both t e s t i n g procedure and equipment. By changing both the speed of p e n e t r a t i o n and the l o c a t i o n of the porous element, measurements of the dynamic pore pressure at the cone t i p and up the s h a f t were obt a i n e d . Estimates of the r a d i a l e f f e c t i v e s t r e s s were made by u s i n g r e s u l t s of the f r i c t i o n s l e e v e and pore p r e s s u r e measurements. By using the c a l c u l a t e d l e v e l s of r a d i a l e f f e c t i v e s t r e s s and other f i e l d evidence, the p o s s i b i l i t y of h y d r o f r a c t u r e i s d i s c u s s e d . 5.2 F i e l d T e s t s Conducted F i e l d t e s t s were conducted at the Mcdonald s i t e to e v a l u a t e the e f f e c t s of t e s t i n g procedure and equipment. The t e s t s were conducted i n h o l e s spaced 1-2 metres a p a r t . The complete cone log s showing p e n e t r a t i o n r a t e and cone geometry used are i n c l u d e d i n Appendix A. 4 6 During cone p e n e t r a t i o n sounding pore pre s s u r e s are measured at one l o c a t i o n on the cone. C o n s i d e r a b l e debate s t i l l e x i s t s over the optimum l o c a t i o n of the porous f i l t e r (Roy et a l . 1980). A study was conducted i n which s u c c e s s i v e l y longer t i p e x tensions r e p l a c e d the standard cone t i p . T h i s e f f e c t i v e l y moved both the porous element and the f r i c t i o n s l e e v e f u r t h e r up the s h a f t . In a d d i t i o n to v a r y i n g the l o c a t i o n of the porous element, cone p e n e t r a t i o n r a t e s were v a r i e d . In order to optimize the f i e l d t e s t i n g time, most of the comparative data was c o l l e c t e d at a depth of 20.0 metres. T h i s procedure was n e c e s s i t a t e d by the extremely slow p e n e t r a t i o n r a t e used (as low as 0.027 cm/sec). Examples from other s i t e s have a l s o been drawn upon when the.y i l l u s t r a t e s p e c i a l f e a t u r e s . 5.3 E f f e c t of P e n e t r a t i o n Speed P e n e t r a t i o n r a t e a f f e c t s the cone p e n e t r a t i o n process by i n f l u e n c i n g the r a t e of s t r a i n i n g i n the s o i l around the cone. During t h i s study, the r a t e of p e n e t r a t i o n was v a r i e d between 5 cm/sec and 0.027 cm/sec and the r e s u l t a n t t i p , f r i c t i o n , and pore pressure measured si m u l t a n e o u s l y . The pore pressure measurements were found necessary to understand the r e s u l t a n t changes i n both the be a r i n g and f r i c t i o n measurements. The r a t e of p e n e t r a t i o n has been widely accepted at 2 cm/sec; i n s p i t e of t h i s , the e f f e c t of p e n e t r a t i o n r a t e i s important f o r s e v e r a l reasons: 4 7 1) At a p e n e t r a t i o n r a t e of 2 cm/sec, cone p e n e t r a t i o n t e s t i n g can be e i t h e r t o t a l l y undrained, d r a i n e d , or p a r t i a l l y d r a i n e d depending on the s o i l t e s t e d . 2) Use of l a r g e r or smal l e r cone t i p s at the standard r a t e of 2 cm/sec r e s u l t s i n a v a r i a t i o n i n s t r a i n r a t e . 3) Use of cone data f o r p r e d i c t i o n of p i l e performance i n v o l v e s r a t e changes over many ord e r s of magnitude. During cone p e n e t r a t i o n t e s t i n g at 2 cm/sec, experience has shown that some s o i l s ( c l e a n , medium-coarse sands) behave i n a completely d r a i n e d manner. Others such as c l a y e y s o i l s behave i n an e s s e n t i a l l y undrained manner. Mixed s o i l s having g r a i n s i z e d i s t r i b u t i o n midway between these two extremes have p e r m e a b i l i t i e s such that the measured pore p r e s s u r e s are i n f l u e n c e d by both the t i p p e n e t r a t i o n and simultaneous c o n s o l i d a t i o n . An example of each of these s o i l s i s provided at the Mcdonald s i t e . F i g u r e 12 shows r e s u l t s of one t e s t performed at the standard r a t e of 2 cm/sec and others performed at f a s t e r and slower r a t e s . S i l t d e p o s i t s between 0 and 3.0 metres behave e s s e n t i a l l y undrained. Sand d e p o s i t s between 3 and 13 metres behave i n a completely d r a i n e d manner, even at p e n e t r a t i o n r a t e s up to 5 cm/sec. Between 13 and 15 metres, a le n s of f i n e s i l t y sand generates pore p r e s s u r e s , the magnitude of which depends on the r a t e of p e n e t r a t i o n . The c l a y e y s i l t below 15 metres behaves i n an undrained manner when t e s t e d at r a t e s f a s t e r then 0.2 cm/sec (Campanella et a l . 1981). Only at p e n e t r a t i o n r a t e s l e s s than 0.2 cm/sec does p a r t i a l 4 8 UD BAR QC BAR U D / Q C 10 10 20 30 100 I 200 Penetration speed 5 cm/sec Penetration speed 2 cm/sec Penetration speed .04 cm/sec Penetration speed .027 Penetration speed 2 cm/sec Penetration speeds shown .0 _ i _ .5 _L_ 1.0 STANDARD CONE C6FPS-2UBC FIGURE 12 RATE EFFECTS, CLAYEY SILT. RICHMOND, BC 49 c o n s o l i d a t i o n i n f l u e n c e the measures v a l u e s . E x t e n s i v e t e s t i n g of t h i s d e p o s i t i s d i s c u s s e d next. 5.3.1 F i e l d Evidence The e f f e c t of p e n e t r a t i o n r a t e on t i p p e n e t r a t i o n r e s i s t a n c e and pore pressure measurements has been d i s c u s s e d by p r e v i o u s r e s e a r c h e r s . For p e n e t r a t i o n t e s t i n g i n c l a y s o i l s , p u b l i s h e d r e s u l t s showing the e f f e c t of v a r y i n g r a t e are summarized i n Table I I . In each of the cases shown in Table I I , the pore p r e s s u r e s generated w e r e - p o s i t i v e and the m a t e r i a l of low p e r m e a b i l i t y . In s p i t e of t h i s , l i t t l e agreement can be seen. The e x p l a n a t i o n f o r the apparent d i s c r e p a n c y can only be seen i f pore pressure measurements are a v a i l a b l e . I f , when the p e n e t r a t i o n r a t e i s reduced, the t e s t i s s t i l l e s s e n t i a l l y an undrained t e s t , then i t can be expected that a decrease in s t r a i n r a t e c o u l d r e s u l t i n a decrease i n apparent end bearing v a l u e . T h i s c o n c l u s i o n can only be e s t a b l i s h e d i f o b s e r v a t i o n s of the dynamic pore pressure show no decrease i n the pore pressure with decreased r a t e of p e n e t r a t i o n . An example of t h i s i s shown i n F i g u r e 13, which shows l i t t l e change i n bearing or dynamic pore p r e s s u r e between r a t e s of 0.2 cm/sec and 2 cm/sec. I f , however, a decreased r a t e of p e n e t r a t i o n r e s u l t s i n p a r t i a l c o n s o l i d a t i o n and, t h e r e f o r e , lower dynamic pore p r e s s u r e s , then an i n c r e a s e i n b e a r i n g v a l u e s can be expected. 5 0 Reference M a t e r i a l Type Re s u l t of Decreased Rate of P e n e t r a t i o n Roy et a l . 1980 s o f t s i l t y c l a y St-Alban s i t e - b e a ring decreases to a minimum, then i n c r e a s e s -dynamic pore pressure decreases Schmertman 1978 c l a y e y sand k = 10" 5 cm/sec OCR = 2 - b e a r i n g i n c r e a s e s B a l i g h et a l 1978. Janbu and Senneset 1 974 Boston blue c l a y moraine c l a y low p e r m e a b i l i t y - b e a r i n g decreases - b e a r i n g decreases -dynamic pore pressure i n c r e a s e s Casson 1978 s o f t c l a y - b e aring decreases T a b l e n : P u b l i s h e d R e s u l t s Showing the E f f e c t of P e n e t r a t i o n Rate on Bearing and Dynamic Pore P r e s s u r e . 0.4 0.3 0.2 0.1 0 v ' i i i n 11 I u i | | | I I I 11 0.01 0.1 I 10 10 8 6 4 2 Bearing corrected for temperature and water pressure effects I i i i i i 111 i i i i i i II I i l i l l 0.01 0.1 I 10 1 o i — Effective bearing °-c " O - c e " " T O T A L I I I I 1 1 1 1 1 . 1 1 I—I I I 11111 0.01 0.1 I 10 10 8 - 6 - i < S 4 t- [ y_ Equilibrium pore pressure i i i i i i 11 I i 1 1 1 i i i i i i 11 0.01 0.1 I 10 Penetration rate, cm/sec. All measurements at 20m depth FIG.I 3 PENETRATION RATE EFFECTS IN CLAYEY SILT DEPOSIT (McDonalds farm,Sea Island). 52 T h i s i s probably the case when t e s t i n g s o i l s of p e r m e a b i l i t y midway between that of a t o t a l l y undrained c l a y d e p o s i t and a t o t a l l y d r a i n e d sand d e p o s i t . R e s u l t s of measurements taken at 20 metres depth at the Mcdonald s i t e , shown i n F i g u r e 13, i l l u s t r a t e t h i s phenomenon. As p e n e t r a t i o n speed i s decreased below about 0.2 cm/sec, the pore pressure d u r i n g p e n e t r a t i o n decreases. Corresponding i n c r e a s e s i n bearing and f r i c t i o n are shown i n F i g u r e 13. The i n c r e a s e i s e s p e c i a l l y n o t i c e a b l e f o r the f r i c t i o n ; with i t s l o c a t i o n up from the t i p , a d d i t i o n a l time permits g r e a t e r c o n s o l i d a t i o n . The i n c r e a s e i s l e s s n o t i c e a b l e f o r the b e a r i n g , i n p a r t because the bearing i s a measure of both s o i l r e s i s t a n c e and pore water p r e s s u r e . Thus, as the water p r e s s u r e 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 e a s e 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 i n c r e a s e s the b e a r i n g . To i l l u s t r a t e t h i s behavior, the e f f e c t i v e b e a r i n g i s a l s o shown i n F i g u r e 13; the e f f e c t i v e b e a r i n g i s d e f i n e d as the c o r r e c t e d measured bearing l e s s the t o t a l water p r e s s u r e . A much l a r g e r i n c r e a s e i n s o i l r e s i s t a n c e i s e v i d e n t . In s o i l s with pore pressure r a t i o s c l o s e to u n i t y , v a r i a t i o n s i n p e n e t r a t i o n r a t e may r e s u l t i n such sm a l l changes i n the measured b e a r i n g value that they cannot be d e t e c t e d by the b e a r i n g measurement alone . Only simultaneous measurement of pore water pressure and b e a r i n g permits t h i s s e p a r a r t i o n of s o i l and water p r e s s u r e s . 5.4 L o c a t i o n of the Porous Element O p t i m i z i n g the dynamic response of the pore pressure probe 5 3 has been the sub j e c t of s e v e r a l recent papers, i n c l u d i n g Roy et a l . 1980 and B a l i g h et a l . 1978. P u b l i s h e d r e s u l t s i n d i c a t e that maximum response should be ob t a i n e d when the porous element i s l o c a t e d the the t i p . The l o c a t i o n used f o r t h i s study, however, immediately behind the time has been found (Campanella et a l . 1981; Roy et a l . 1980) to r e c o r d dynamic pore p r e s s u r e s very c l o s e to maximum i n s o f t s o i l s . The area behind the t i p i s a l s o b e t t e r p r o t e c t e d f o r p e n e t r a t i o n through dense a b r a s i v e sands or impact onto bou l d e r s , c o n c r e t i o n s , wood, dense t i l l s , or bedrock. Recognizing that peak performance may be ob t a i n e d when the porous element i s l o c a t e d at the t i p , the porous element l o c a t e d behind the t i p appears to o f f e r a reasonable compromise. I t does appear, however, that l o c a t i o n of the porous element behind the t i p may r e s u l t i n a more d i l a t e n t dynamic pore pressure response (Campanella et a l . 1981). The f i e l d t e s t i n g conducted i l l u s t r a t e s the v a r i a t i o n i n pore p r e s s u r e s up the cone. F i g u r e 14 i n c l u d e s the pore pressure d i s t r i b u t i o n up the cone f o r t e s t s conducted at the standard r a t e of 2 cm/sec at the Mcdonald s i t e . As d i s c u s s e d i n Chapter 4, pore pressure development at the t i p and up the sl e e v e can probably best be modelled by s p h e r i c a l and c y l i n d r i c a l c a v i t y expansions. The r e d u c t i o n i n pore p r e s s u r e s up the cone can be due to two separate reasons: 54 150 40 E u Q_ I— O or u_ ui o 1 30 20 10 PENETRATION RATES 2.00 cm/sec 0.20 cm/sec 0.04 cm/sec RICHMOND CLAYEY SILT DEPTH 20 M 2 3 4 5 PORE PRESSURE (BAR) FIGURE 14 PORE PRESSURE DISTRIBUTION ALONG SHAFT DURING CONE PENETRATION TESTING. VARIABLE PENETRATION RATE. 55 1) changes i n t o t a l s t r e s s , s p h e r i c a l vs c y l i n d r i c a l c a v i t y expansions; or 2) pore pressure d i s s i p a t i o n . For the c l a y e y s i l t t e s t e d , at the standard r a t e of p e n e t r a t i o n , v i r t u a l l y no pore pressure d i s s i p a t i o n can take p l a c e and the decrease i n pore pressure up the s h a f t represents the t r a n s i t i o n from s p h e r i c a l to c y l i n d r i c a l c a v i t y expansion. At slower r a t e s of p e n e t r a t i o n , however, p a r t i a l pore pressure d i s s i p a t i o n was recorded. V a r i a t i o n i n the measured pore pressure f o r t e s t s conducted at d i f f e r e n t p e n e t r a t i o n speeds and with d i f f e r e n t cone t i p s i s shown i n F i g u r e 14. Since the cone geometry i s repeated at d i f f e r e n t p e n e t r a t i o n speeds, the d i f f e r e n c e i n measured pore pr e s s u r e between the t e s t s conducted at d i f f e r e n t r a t e s r e p r e s e n t s a measure of the pore pressure d i s s i p a t i o n . T h i s s u b j e c t i s d i s c u s s e d f u r t h e r i n Chapter 6. R e s u l t s showing the pore pressure d i s t r i b u t i o n up the cone from the t i p at the standard r a t e of p e n e t r a t i o n (Figure 14) r e v e a l an important c o n c l u s i o n r e g a r d i n g the pore water pressure c o r r e c t i o n a p p l i e d to the f r i c t i o n s l e e v e . As e x p l a i n e d i n S e c t i o n 2.3.4, the c o r r e c t i o n a p p l i e d to the f r i c t i o n s leeve reading was n e c e s s i t a t e d by unequal end area of the f r i c t i o n s l e e v e . As e x p l a i n e d i n S e c t i o n 2.3.4, an assumption inherent i n the c o r r e c t i o n i s that the pore pressure used i n the c o r r e c t i o n , measured at the f r o n t of the f r i c t i o n s l e e v e , i s the 5 6 same as the pore pressure at the back of the f r i c t i o n s l e e v e . I n s p e c t i o n of F i g u r e 14 shows that t h i s assumption i s approximate and may need r e v i s i o n . T e s t i n g s o i l s that have g r e a t e r p e r m e a b i l i t y and r e s u l t i n g l a r g e r pore pressure decay between the f r o n t and back of the f r i c t i o n s l e e v e may be g r e a t l y hampered by t h i s problem. An experimental procedure s i m i l a r to the one d e s c r i b e d above but conducted i n such a s o i l may prove u s e f u l . I t must s t i l l be remembered, however, than at unequal d i s t r i b u t i o n of pore p r e s s u r e s up the cone would r e s u l t i n a net l o a d on the f r i c t i o n s leeve having equal end a r e a s . T h i s load may prove f a i r l y l a r g e c o n s i d e r i n g F i g u r e 14, which shows pr e s s u r e s of 6.0 bar on the bottom and only 5.3 bar on the top of the f r i c t i o n s l e e v e . Only the design of cones having minimal and equal end areas prevent t h i s problem. 5.5 E f f e c t of S a t u r a t i o n D e t a i l s of the s a t u r a t i o n procedure used i n t h i s study are i n c l u d e d i n Chapter 1. The design d e t a i l s are d i s c u s s e d i n Campanella and Robertson 1981 and i n Campanella et a l . 1981. Experience has shown that i n order to measure proper dynamic and s t a t i c v a l u e s , complete s a t u r a t i o n must be maintained. As d i s c u s s e d i n Campanella et a l . 1981, g l y c e r i n has been found to work admirably; i t s v i s c o s i t y permits easy s a t u r a t i o n , a l l o w i n g i n v e r s i o n of the cone without the n e c e s s i t y of e n c a p s u l a t i n g the cone i n d e a i r e d water. In a d d i t i o n , g l y c e r i n maintains s a t u r a t i o n when t e s t i n g above the water t a b l e . As d i s c u s s e d i n Campanella et a l . 1981, the design used i n t h i s study was found 5 7 e f f e c t i v e even a f t e r being pushed f o r over 40 metres through unsaturated f i n e sands before encountering the water t a b l e . C o n f i r m a t i o n of the readings below the water t a b l e were obtained from a nearby standpipe piezometer. Response of the dynamic pore p r e s s u r e s appears to be s i g n i f i c a n t l y a f f e c t e d by entrapped a i r w i t h i n the sensing element. T h i s i s p a r t i c u l a r l y t r u e f o r s o f t low p e r m e a b i l i t y s o i l s . An i l l u s t r a t i o n of t h i s e f f e c t i s shown from Campanella et a l . 1981 i n F i g u r e 15. An e a r l i e r model piezometer cone that has a l a r g e sensing element c a v i t y t hat c o u l d not s a t i s f a c t o r i l y be s a t u r a t e d showed that when pushing was stopped d u r i n g p e n e t r a t i o n through a s o f t c l a y , the measured pore pressure continued to r i s e f o r a s h o r t p e r i o d , f o l l o w e d by the expected decrease due to pore p r e s s u r e d i s s i p a t i o n . When us i n g the more r e c e n t l y designed piezometer cone i n the same d e p o s i t , however, the continued i n c r e a s e i n measured pore pressure was not observed f o l l o w i n g a stop i n p e n e t r a t i o n . I t was concluded that the c o n t i n u e d i n c r e a s e i n measured pore p r e s s u r e observed i n the e a r l i e r model was due to time e f f e c t s due to water flow caused by some entrapped a i r w i t h i n the element. I t should a l s o be noted t h a t the dynamic pore pressure measured with the unsaturated piezometer i s s i g n i f i c a n t l y l e s s than the pore pressure measured by the s a t u r a t e d piezometer. S i m i l a r r e s u l t s are shown by B a t t a g l i o et a l . 1981, who found that with c a r e f u l s a t u r a t i o n procedure, they c o u l d minimize response d e l a y s . BURNABY SITE -VERY SOFT SILTY CLAY q c « 4kPa , k =IO"7cm/sec a) PIEZOMETER E L E M E N T Time (minutes) 90 r 80 h Penetrotion stopped ot 17.7m b) S A T U R A T E D PIEZOMETER E L E M E N T 70 60 Penetrotion from 16.7m to 17.7m Equilibrium pore pressure 20m 50 1 1 I 2 3 Time (minutes) FIG. 1 5 INFLUENCE OF SATURATION ON PORE PRESSURE RESPONSE OF PIEZOMETER CONE. 5 9 E a r l y piezometer cone r e s u l t s p u b l i s h e d by Janbu and Senneset 1974 and by Schmertmann 1974 i n d i c a t e l a r g e i n c r e a s e s in pore pressure before the s t a r t of pore pressure decay s i m i l a r to r e s u l t s shown i n F i g u r e 15. E x p l a n a t i o n s f o r t h i s response i n c l u d e : 1) Mandel Cryer e f f e c t , proposed by Tumay et a l . 1981; and 2) a zone of pore p r e s s u r e s adjacent to the cone that have sm a l l e r magnitude than pore p r e s s u r e s at some d i s t a n c e from the t i p . Only with c a r e f u l s a t u r a t i o n can i t be ensured that t h i s kind of response i s not the r e s u l t of poor s a t u r a t i o n procedure. 5.6 E f f e c t of Pore Pressure D i s s i p a t i o n During normal cone p e n e t r a t i o n l o g g i n g p a r t i a l or complete pore p r e s s u r e , d i s s i p a t i o n may take p l a c e at rod changes to e v a l u a t e 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 , or to e v a l u a t e 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 s . Upon the resumption of p e n e t r a t i o n , some displacement i s necessary i n order to generate e q u i l i b r i u m dynamic pore p r e s s u r e s . During t h i s i n t e r v a l , the measured b e a r i n g and f r i c t i o n parameters are a f u n c t i o n of the surrounding pore p r e s s u r e s , which may be e i t h e r higher or lower than the steady s t a t e p e n e t r a t i o n pore p r e s s u r e s . Care must be taken i n order to account f o r t h i s e f f e c t , e s p e c i a l l y i n s o f t s o i l s , which generate l a r g e pore p r e s s u r e s . An example of t h i s 6 0 e f f e c t i s shown in F i g u r e 16. Rod changes conducted on metre i n t e r v a l s and a d i s s i p a t i o n conducted at 20.0 metres r e s u l t i n p a r t i a l pore pressure d i s s i p a t i o n . On the resumption of p e n e t r a t i o n , some displacement i s recorded before pore p r e s s u r e s b u i l d to t h e i r steady p e n e t r a t i o n l e v e l s . T h i s does not appear to be the r e s u l t of a response l a g of the measuring system, as evidenced by the i n c r e a s e d f r i c t i o n and b e a r i n g v a l u e s . The i n c r e a s e i n f r i c t i o n i s due to the i n c r e a s e d s t r e s s e s caused by pore pressure d i s s i p a t i o n , as d i s c u s s e d i n S e c t i o n 5.3.1. In a d d i t i o n , g r e a t e r displacement i s necessary i n order to a t t a i n p r e v i o u s f r i c t i o n l e v e l s , because of the l o c a t i o n of the f r i c t i o n s l e e v e , which must f i r s t pass through s o i l having a l r e a d y undergone p a r t i a l c o n s o l i d a t i o n . The e f f e c t of p a r t i a l pore p r e s s u r e d i s s i p a t i o n s , as shown in F i g u r e 16, was recorded while p e n e t r a t i n g through r e l a t i v e l y impermeable (k = 10 7 cm/sec) c l a y e y s i l t . The e f f e c t of d i s s i p a t i o n s at rod breaks may be more pronounced when t e s t i n g s o i l s of higher p e r m e a b i l i t y . In some s o i l s , i t may even be the case that i t takes the e n t i r e next metre to f u l l y recover the dynamic pore p r e s s u r e s . 5.7 H y d r o f r a c t u r e : F i e l d Evidence The p o s s i b i l i t y of h y d r o f r a c t u r e must be addressed when a s s e s s i n g e i t h e r the magnitude of dynamic pore pre s s u r e s generated dur i n g cone p e n e t r a t i o n t e s t i n g , or the use of pore pressure d i s s i p a t i o n s f o r the p r e d i c t i o n of 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 . Two types of f i e l d evidence suggest that 61 PORE PRESSURE FRICTION RESISTANCE BEARING RESISTANCE . U (BAR) FC (BAR) QC (BAR) 0 10 0 0.5 0 5 1 0 —»DEPTH OF DISSIPATIONS FIGURE 1 6 EFFECT OF PORE PRESSURE DISSIPATIONS DURING CONE PENETRATION TESTING IN CLAYEY SILT. RICHMOND, B.C. 6 2 h y d r o f r a c t u r e d i d not occur i n the s o i l s t e s t e d d u r i n g t h i s study. An estimate of the r a d i a l e f f e c t i v e s t r e s s c a l c u l a t e d from f r i c t i o n s leeve v a l u e s and pore pressure measurements suggests t h a t the r a d i a l e f f e c t i v e s t r e s s i s p o s i t i v e . A d d i t i o n a l evidence i s p r o v i d e d by the measurement of pore pressure decay and comparing measured valu e s to the expected response that would r e s u l t from h y d r o f r a c t u r e . 5.7.1 R a d i a l E f f e c t i v e S t r e s s e s During Cone P e n e t r a t i o n T e s t i n g An e f f e c t i v e s t r e s s approach to cone p e n e t r a t i o n t e s t i n g remains a h i g h l y d e s i r e d g o a l . Present t h e o r i e s of cone p e n e t r a t i o n t e s t i n g have r e s t r i c t e d themselves to t o t a l s t r e s s approaches f o r s e v e r a l reasons, i n c l u d i n g : 1) inadequate t h e o r e t i c a l a n a l y s i s f o r the h i g h l y complex d i s t r i b u t i o n of s t r e s s e s and pore p r e s s u r e s around the cone; and 2) p r e v i o u s i n a b i l i t y to measure pore p r e s s u r e . Although an 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 bearing and f r i c t i o n measurements i s beyond the scope of t h i s paper, the a d d i t i o n of the pore pressure element does permit an estimate of the r a d i a l e f f e c t i v e s t r e s s e s . The cone p e n e t r a t i o n t e s t i n g conducted at the Mcdonald s i t e , Richmond B.C., i n c l u d e d v a r i a t i o n i n both p e n e t r a t i o n speed and i n the l o c a t i o n of the porous element and f r i c t i o n 63 s l e e v e . V a r i a t i o n i n pore pressures due to p a r t i a l c o n s o l i d a t i o n has al r e a d y been d i s c u s s e d . For the m a t e r i a l t e s t e d , c l a y e y s i l t , p e n e t r a t i o n r a t e s slower than about 0.2 cm/sec r e s u l t i n p a r t i a l c o n s o l i d a t i o n . An e f f e c t i v e s t r e s s approach remains h i g h l y d e s i r a b l e i n m a t e r i a l t h a t , when t e s t e d at standard p e n e t r a t i o n r a t e s , behave i n a p a r t i a l l y d r a i n e d manner. The v a r i a t i o n i n pore p r e s s u r e s and f r i c t i o n with d i f f e r e n t t i p e x t e n s i o n s and p e n e t r a t i o n speeds i s show p l o t t e d in Fi g u r e 17. Since the l o c a t i o n of the f r i c t i o n s l e e v e does not correspond to the exact l o c a t i o n of the porous element, some judgement must be made to estimate the pore pre s s u r e s c o r r e s p o n d i n g to the c e n t r e of f r i c t i o n s l e e v e . Some judgement i s a l s o necessary to e x t r a p o l a t e measured pore pressures i n order to o b t a i n an estimate of the pore pressure at the ce n t r e of the f r i c t i o n s leeve behind the 38.0 cm t i p e x t e n s i o n . If an estimate i s made of the e f f e c t i v e angle of f r i c t i o n between the s o i l and the s t e e l f r i c t i o n s l e e v e , then the e f f e c t i v e r a d i a l s t r e s s can be c a l c u l a t e d from: o r ' = FC/tan h ' where S' = M0' M = c o e f f i c i e n t to account f o r the s u r f a c e roughness of the s t e e l f r i c t i o n s l e e v e 0' = angle of f r i c t i o n of the s o i l T h i s assumes that t h e . m a t e r i a l can be modelled with a cohesion 64 PENETRATION RATES 2.00 cm/sec 0.20 cm/sec 0.04 cm/sec p 0.1 0,2 0-3 0 4 0.5 0-6 0-7 j I 1 1 1 1 1 1 FRICTION (BAR) 7 PORE PRESSURE (BAR) FIGURE 17 PORE PRESSURE AND FRICTION DISTRIBUTION ALONG SHAFT DURING CONE PENETRATION TESTING. VARIABLE PENETRATION RATE. i n t e r c e p t of zero; i n a d d i t i o n , the estimate of the value f o r the angle of f r i c t i o n between the s o i l and the f r i c t i o n sleeve i s c o m p l i cated by not knowing e i t h e r the e f f e c t i v e angle of f r i c t i o n of the s o i l or the i n t e r a c t i o n between the f r i c t i o n s l e e v e and the s o i l . An estimate of the f r i c t i o n angle should probably account f o r the l a r g e s t r a i n s a s s o c i a t e d with cone p e n e t r a t i o n t e s t i n g . Schmertmann 1978 estimates the undrained shear s t r e n g t h from the f r i c t i o n s l e e v e midway between CU peak and CU r e s i d u a l . An 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 should account f o r a s i m i l a r r e d u c t i o n i n 0 ' . For the purposes of i l l u s t r a t i o n , an angle of f r i c t i o n 8' between the s o i l and the s t e e l f r i c t i o n sleeve has been assumed to be 10 degrees. Using t h i s v a l u e , the e f f e c t i v e r a d i a l s t r e s s has been c a l c u l a t e d and i s shown i n F i g u r e 18. S t r e s s e s may vary with l o c a t i o n up the s h a f t due to the complex t r a n s i t i o n from s p h e r i c a l to c y l i n d r i c a l c a v i t y expansion. For each l o c a t i o n of the f r i c t i o n s l e e v e , however, the l e v e l to t o t a l s t r e s s should be independent of the p e n e t r a t i o n speed, s i n c e the p a r t i a l d i s s i p a t i o n of pore p r e s s u r e s should r e s u l t i n i n c r e a s e d f r i c t i o n s l e e v e readings. An i n d i c a t i o n of the v a l i d i t y of the estimate of the angle of f r i c t i o n between the p o l i s h e d s t e e l and the s o i l can be made by comparing the c a l c u l a t e d t o t a l s t r e s s at each l o c a t i o n up the s h a f t . The c a l c u l a t e d t o t a l s t r e s s e s shown i n F i g u r e 18 correspond reasonably w e l l , c o n s i d e r i n g the p o s s i b l e v a r i a t i o n r e s u l t i n g from n a t u r a l s o i l v a r i a b i l i t y . Although the c a l c u l a t e d v a l u e s of e f f e c t i v e r a d i a l s t r e s s 66 PENETRATION RATES 2.00 cm/sec 0.20 cm/sec 0.04 cm/sec Or PORE PRESSURE 0> 7 PORE PRESSURE AND STRESS (BAR) FIGURE 18 ESTIMATED RADIAL STRESSES AND DYNAMIC PORE PRESSURE. VARIABLE PENETRATION RATE. 0.8 0.7 10 REMOLDED SAMPLE —I 1 1—| I I I F SAMPLE DEPTH 25.5 m -I 1 I I I I I a v o= 200 kPa UNDISTURBED SAMPLE VERTICAL COMPRESSION J I I I I I 100 EFFECTIVE STRESS (kPa) 1000 FIG. 2 9 EFFECT OF REMOLDING ON T H E COMPRESSIBILITY OF RICHMOND CLAYEY SILT. CRSC T E S T S . (0 O 1 1 r | ? f M | 1 - i r 7 i i r T - SAMPLE DEPTH 25.5 m -UNDISTURBED SAMPLE -- VERTICAL COMPRESSION -- -_ -REMOLDED — 1 L_ 1 1 1 1 1 1 1 - l I 1 1 1 1 1 JL. 10 100 1000 EFFECTIVE STRESS (kPa) FIG. 3 0 E F F E C T OF REMOLDING ON THE COEFFICIENT OF CONSOLIDATION 5 RICHMOND CLAYEY SILT. CRSC TESTS 92 i d e n t i c a l . Both the h o r i z o n t a l and t h e , v e r t i c a l c o e f f i c i e n t s of c o n s o l i d a t i o n can be c h a r a c t e r i z e d by two reasonably d i s t i n c t r e g i o n s ; i n t h e i r o v e r c o n s o l i d a t e d s t a t e , d u r i n g p r e l i m i n a r y stages of l o a d i n g , they vary between 2.0 and 5.0 cm 2/min, then decrease at a s t r e s s l e v e l near t h e i r p r e c o n s o l i d a t i o n pressure to a s i n g l e e q u i l i b r i u m value l e s s than 1.0 cm 2/min. The change i n 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 i s due to changes in both the c o e f f i c i e n t of c o m p r e s s i b i l i t y and 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 . 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 i s shown i n F i g u r e 26; the p r e c o n s o l i d a t i o n p ressure i s l e s s evident in the case of h o r i z o n t a l l o a d i n g , perhaps due to gr e a t e r sample d i s t u r b a n c e while trimming the sample. The e f f e c t of complete remolding was i n v e s t i g a t e d because of i t s p o s s i b l e e f f e c t d u r i n g cone p e n e t r a t i o n t e s t i n g . R e s u l t s are shown i n F i g u r e s 29 and .30. In comparison to the und i s t u r b e d sample, the remolded sample i s much more compressible i n e a r l y stages of l o a d i n g , causing a decrease i n the measured 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 . R e s u l t s of the t e s t i n g conducted f o r the Mcdonald c l a y e y s i l t i n d i c a t e two d i s t i n c t v a l u e s 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 : one f o r i t s behavior at s t r e s s e s l e s s than i t s p r e c o n s o l i d a t i o n p r e s s u r e ; the other f o r i t s behavior at s t r e s s e s g r e a t e r than the p r e c o n s o l i d a t i o n p r e s s u r e . 6.7 F i t of E x i s t i n g S o l u t i o n s For the d i s s i p a t i o n curves shown i n F i g u r e 22, p r e d i c t e d 93 values 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 have been compared i n Tables IV, V, and VI. To compare the f i t of the d i f f e r e n t s o l u t i o n s to the f i e l d data, F i g u r e s 31, 32, and 33 compare Torstensson's 1977 s p h e r i c a l and c y l i n d r i c a l s o l u t i o n s to the f i e l d d i s s i p a t i o n curves. Torstensson's c y l i n d r i c a l s o l u t i o n i s shown because of i t s s i m i l a r i t y to the s o l u t i o n s by B a l i g h and Levadoux 1980 and by Randolph and Wroth 1979. To p l o t the d i s s i p a t i o n curves on the same s c a l e as the t h e o r e t i c a l s o l u t i o n s , time f a c t o r s , T, have been c a l c u l a t e d using the l a b o r a t o r y measured values f o r c. The l a b o r a t o r y values have been s e l e c t e d only as a means of comparing the shapes of the d i s s i p a t i o n c u r v e s . F i g u r e 31 compares the shape of the a n a l y t i c a l s o l u t i o n s with the data from the Mcdonald s i t e at 20.5 metres. The f i e l d data compares reasonably w e l l with e i t h e r the s p h e r i c a l or the c y l i n d r i c a l s o l u t i o n . 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 i s , t h e r e f o r e , n e a r l y independent of the degree of d i s s i p a t i o n . F i g u r e 32 compares the a n a l y t i c a l s o l u t i o n s with the data c o l l e c t e d from the Mcdonald s i t e at a depth of 25.5 metres. The f i e l d curves show a more r a p i d r a t e of d i s s i p a t i o n at e a r l y stages than e i t h e r the s p h e r i c a l or c y l i n d r i c a l s o l u t i o n s . 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 , t h e r e f o r e , complicated by the n e c e s s i t y of d e c i d i n g which l e v e l of d i s s i p a t i o n i s best used to p r e d i c t the a c t u a l 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 . Probable causes f o r departure of the f i e l d data from the 94 Table IV : Comparison between Laboratory Data and C a l c u l a 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 : Mcdonald s i t e 20.5 m Laboratory Test R e s u l t s -CRSC Test c v - o v e r c o n s o l i d a t e d r e g i o n 3.50 cm 2/min c v -normally c o n s o l i d a t e d r e g i o n 1.10 cm 2/min c n - o v e r c o n s o l i d a t e d r e g i o n 4.50 cm 2/min c n -normally c o n s o l i d a t e d r e g i o n 0.75 cm 2/min P r e d i c t e d Values f o r 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 (cm z/min) Method B a l i g h and Levadoux 1980 Torstensson 1977 S p h e r i c a l (E/cu = 500) S p h e r i c a l (E/cu = 200) C y l i n d r i c a l (E/cu = 500) C y l i n d r i c a l (E/cu = 200) Percent D i s s i p a t i o n 20 40 50 60 80 2.32 2.00 2.29 2.60 3.55 0.57 0.49 0.52 0.51 0.43 0.35 0.31 0.30 0.31 0.40 1 .78 2.26 2.72 3.34 3.12 0.95 1.12 1 .47 1 .53 1 .34 95 Table V : Comparison between Laboratory data and C a l c u l a 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 : Mcdonald S i t e 25.5 m Laboratory R e s u l t s -CRSC Tes t s Undisturbed Sample: c v - o v e r c o n s o l i d a t e d region c v -normally c o n s o l i d a t e d region Remolded Sample: c -1.0 cm 2/min 6.00 cm 2/min 0.75 cm 2/min P r e d i c t e d Values f o r 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 (cm 2/min) Method B a l i g h and Levadoux 1980 Torstensson 1977 S p h e r i c a l (E/cu = 500) S p h e r i c a l (E/cu = 200) C y l i n d r i c a l (E/cu = 500) C y l i n d r i c a l (E/cu = 200) Percent D i s s i p a t i o n 20 40 50 60 80 1.17 2.14 2.73 3.31 5.67 0.29 0.52 0.61 0.64 0.69 0.18 0.31 0.36 0.39 0.40 0.90 2.42 3.42 4.26 4.98 0.48 1 .20 1 .75 1 .95 2.14 TableVI : Comparison between Laboratory Data and C a l c u l a 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 : Burnaby S i t e 15.5 m Laboratory R e s u l t s -CRSC T e s t s c v - o v e r c o n s o l i d a t e d region 0.60 cm 2/min c v -normally c o n s o l i d a t e d region 0.08 cm 2/min P r e d i c t e d Values f o r 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 (cm z/min) Percent D i s s i p a t i o n 20 40 50 Method B a l i g h and Levadoux 1980 Torstensson 1977 S p h e r i c a l (E/cu = 500) S p h e r i c a l (E/cu = 200) C y l i n d r i c a l (E/cu = 500) C y l i n d r i c a l (E/cu = 200) 0.23 0.27 0.27 0.056 0.065 0.060 0.035 0.039 0.035 0.18 0.30 0.32 0.095 0.15 0.17 T = c t / r 2 FIGURE 3 1 FIT OF THEORETICAL C U R V E S TO FIELD D A T A : McDONALD SITE 2 0 - 5 m to - 4 1 T = c t / r 2 F IGURE 3 2 FIT OF T H E O R E T I C A L CURVES TO F IELD DATA- McDONALD S ITE 25.5 m CD 1 .9 .8 .7 !- 6 I -5 - A .3 .2 .1 01 MEASURED CURVE c = 6.0 MEASURED CURVE c = 0.08 TORSTENSSON 1977 SPHERICAL SOLUTION TORSTENSSON 1977 CYLINDRICAL SOLUTION M M ' 1 i i i i i i i i I _ L i t i i i i i .1 10 1 100 T = c t / r 2 F I G U R E 3 3 FIT OF T H E O R E T I C A L C U R V E S TO F I E L D D A T A - B U R N A B Y S I T E 15 .5m CD CD 1 0 0 t h e o r e t i c a l s o l u t i o n s i n c l u d e : 1) i n a p p r o p r i a t e c o n s o l i d a t i o n a n a l y s i s , p o s s i b l y due to an i n c o r r e c t assumption of the i n i t i a l excess pore p r e s s u r e ; and 2) s i g n i f i c a n t m a t e r i a l nonhomogeneities near the cone. F i g u r e 33 compares the t h e o r e t i c a l s o l u t i o n s to the f i e l d data from the Burnaby s i t e . '.The f i e l d data f i t s e i t h e r of the t h e o r e t i c a l s o l u t i o n s w e l l . 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 i s , t h e r e f o r e , n e a r l y independent of the degree of d i s s i p a t i o n . 6.8 Comparison between P r e d i c t e d and Laboratory Measured Values Constant r a t e of s t r a i n c o n s o l i d a t i o n t e s t s performed on samples obtained at the two s i t e s can be used to o b t a i n r e f e r e n c e c o n s o l i d a t i o n parameters. For the Burnaby s i t e , u n d i s t u r b e d samples were used . to measure only the v e r t i c a l 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 . An absence of v i s i b l e l a y e r i n g i n the sample t e s t e d , however, i n d i c a t e d that the r a t i o of the v e r t i c a l to h o r i z o n t a l 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 should be c l o s e to 1.0. For the Mcdonald s i t e , c o n s o l i d a t i o n t e s t s were performed i n both the h o r i z o n t a l and v e r t i c a l d i r e c t i o n s f o r the sample c o l l e c t e d at 20.5 metres; r e s u l t s were s i m i l a r . In a d d i t i o n , v e r t i c a l and remolded c o n s o l i d a t i o n t e s t s were conducted i n samples c o l l e c t e d from 25.5 metres. The 101 c o n s o l i d a t i o n t e s t r e s u l t s are considered here as reference v a l u e s . A comparison between the l a b o r a t o r y v a l u e s and 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 i s best done at one degree of d i s s i p a t i o n . At the e a r l y stages of d i s s i p a t i o n , c o n s o l i d a t i o n d e f i n i t e l y takes p l a c e along the r e c o n s o l i d a t i o n curve in e f f e c t i v e s t r e s s space. A f t e r more d i s s i p a t i o n has occurred, some u n c e r t a i n t y about the t o t a l s t r e s s space confuses the e f f e c t i v e s t r e s s l e v e l and, t h e r e f o r e , which parameter c , normally c o n s o l i d a t e d , or c , o v e r c o n s o l i d a t e d , i s more a p p r o p r i a t e . Torstensson 1977 recommends c a l c u l a t i n g 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 at 50% l e v e l of d i s s i p a t i o n . Using only the 50% l e v e l of d i s s i p a t i o n , T ables IV, V, and VI can be used to compare the d i f f e r e n t methods. For the Mcdonald s i t e at 20.5 metres, the s o l u t i o n s 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 each p r e d i c t s a 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 measured in the o v e r c o n s o l i d a t e d s t a t e . For t e s t i n g conducted .at the Mcdonald s i t e at 25.5 metres, 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 at the 50% l e v e l of d i s s i p a t i o n p r e d i c t e d by the s o l u t i o n s of B a l i g h and Levadoux and the c y l i n d r i c a l s o l u t i o n of Torstensson each compare reasonably w e l l with the l a b o r a t o r y value measured i n the o v e r c o n s o l i d a t e d s t a t e . S i m i l a r r e s u l t s are obtained from the Burnaby s i t e , where the s o l u t i o n s by B a l i g h and Levadoux and by Torstensson again 102 compare reasonably w e l l with the l a b o r a t o r y value measured i n the o v e r c o n s o l i d a t e d s t a t e . C o n s i d e r i n g the e f f e c t of p a r t i a l remolding, which i s to reduce 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 , the f i e l d r e s u l t s obtained compare reasonably w e l l with l a b o r a t o r y measured values of the h o r i z o n t a l 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 i n the o v e r c o n s o l i d a t e d state.. 6.9 S e l e c t i o n of S o i l S t i f f n e s s R a t i o The pore pressure d i s s i p a t i o n s o l u t i o n s proposed by Torstensson 1977 and Randolph and Wroth 1979 r e l y on an assumption of the i n i t i a l d i s t r i b u t i o n of excess pore pressure . As was shown i n Chapter 4 (eqns.1-6) the i n i t i a l excess pore pressure was assumed to decrease l i n e a r l y with the l o g a r i t h m of the r a d i u s . In a d d i t i o n , t h e s i z e of the zone of i n c r e a s e d pore pressure was shown to i n c r e a s e with the square root of the s o i l s t i f f n e s s r a t i o from c y l i n d r i c a l c a v i t y expansion theory and with the cube root of the s o i l s t i f f n e s s r a t i o f r o n s p h e r i c a l c a v i t y expansion theory . The s o i l s t i f f n e s s can be expressed as e i t h e r , the undrained e l a s t i c modulas E or the undrained shear modulas G, to the undrained shear s t r e n g t h . The e l a s t i c modulas and the shear modulas can be r e l a t e d by: G=E/2(1+u) where u =0.5 f o r undrained l o a d i n g of s a t u r a t e d c l a y I n c r e a s i n g the s o i l s t i f f n e s s has the e f f e c t of extending the zone of excess pore pressure and s u b s i q u e n t l y reducing the pore p r e s s u r e g r a d i e n t f o r any given excess pore pressure at the c a v i t y f a c e . Use of e i t h e r the s o l u t i o n by Torstensson 1977 or 103 Randolph and Wroth 1979 thus r e q u i r e s an estimate of the s o i l , s t i f f n e s s r a t i o . 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 modulas with s t r a i n l e v e l . F i g u r e 34 shows the v a r i a t i o n i n s t i f f n e s s r a t i o f o r s e v e r a l d i f f e r e n t s o i l s . I t can a l s o be seen i n Fi g u r e 34 how s t i f f n e s s v a r i e s with s t r e s s l e v e l . 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 intermediate s t r e s s l e v e l s . Although some doubt surrounds the s e l e c t i o n of an a p p r o p r i a t e 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 s e n s i t i v e to the s o i l s t i f f n e s s used. Appendix B i n c l u d e s the time f a c t o r s , T , used to c a l c u l a t e 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 . For example the time f a c t o r at the 50% l e v e l of d i s s i p a t i o n i n Torstensson's c y l i n d r i c a l s o l u t i o n v a r i e s from T=1.37 to T=4.29 as E/cu v a r i e s from 100 to 500. 6.10 Procedure Used During D i s s i p a t i o n T e s t s There i s very l i t t l e a v a i l a b l e l i t e r a t u r e d e s c r i b i n g the procedures used duri n g d i s s i p a t i o n t e s t s . B a t t a g l i o et a l . 1981 and others report that they found i t necessary to clamp the rods at the ground s u r f a c e d u r i n g d i s s i p a t i o n t e s t s . They found that i f the rods were not clamped that a drop i n the measured pore pressure would r e s u l t when loa d was r e l e a s e d from the t i p . V a r i a t i o n s i n the amount of loa d a p p l i e d to the top end of the rods were made durin g the i n v e s t i g a t i o n at the Mcdonald s i t e . No d i f f e r e n c e i n decay response was noted when e i t h e r removing the l o a d e n t i r e l y or m a i n t a i n i n g a loa d only s l i g h t l y l e s s than 104 2000 c ^ 2 0 0 No, DESCRIPTION cu/p' Portsmouth CL C l o y PI = 15 -20 st-IO L L = 35 I _ Boston C L Cloy 2 L L = 4I Pl=22 3 Bongkok CH Cloy LL=65PI=4I 4 Maine CH OH Clay LL=65PI=38 AGS CH Cloy LL=7I Pl = 4 0 J 5 •20 •27 29 •26 •24 Atchafalaya 6 CH Clay LL=95 Pl = 75 j Taylor River P e a t w =50 0 % CKo U simple shear tests all soils normally consol i da ted 0-2 0-4 0-6 0-8 APPLIED SHEAR STRESS RATIO T h / c u Figure 3 5 Selection of Soi l Stiffness Rat io Adapted from Ladd et al 1977 105 that r e q u i r e d to resume p e n e t r a t i o n . Part of the ex p l a n a t i o n f o r the i n s e n s i t i v i t y of procedure v a r i a t i o n probably l a y s i n the d i s t r i b u t i o n of s t r e s s e s and s t r a i n s along the rods, e s p e c i a l l y those between the f r i c t i o n reducer and the cone t i p . Even when the pushing head i s f u l l y withdrawn from the rods, some loa d i s maintained on the t i p by the f r i c t i o n m o b i l i z e d along the s h a f t between the f r i c t i o n reducer and the t i p . T h i s t i p l o a d was measured d u r i n g s e v e r a l d i s s i p a t i o n s . In f a c t , the decay of the t i p load d i r e c t l y p a r a l l e l e d the decay of excess pore pressure recorded by the piezometer u n i t . T h i s i s not s u r p r i s i n g , c o n s i d e r i n g that the t i p i s a t o t a l s t r e s s c e l l . The amount of load maintained on the t i p when p e n e t r a t i o n i s h a l t e d probably depends on many f a c t o r s i n c l u d i n g v e r t i c a l i t y of the h o l e , squeeze of the s o i l above the f r i c t i o n reducer onto the rods, and the l o c a t i o n and s i z e of the f r i c t i o n reducer used. The apparent c o n t r a d i c t i o n i n the importance i n mai n t a i n i n g l o a d on the rod s t r i n g found i n t h i s i n v e s t i g a t i o n and that r e p o r t e d by B a t t a g l i o et a l . 1981 may be the r e s u l t of v a r i a t i o n i n the l o c a t i o n of the piezometer element. B a t t a g l i o et a l . 1981 employed a cone with the piezometer element l o c a t e d on the 60° face of the cone; when loa d was r e l e a s e d , the pore p r e s s u r e dropped immediately because of the v a r i a t i o n i n t o t a l s t r e s s at the cone. In other words, the pore pressure had to be in d i r e c t e q u i l i b r i u m with the bearing m o b i l i z e d . The piezometer element used i n t h i s i n v e s t i g a t i o n was l o c a t e d immediately behind the cone t i p i n the zone of f a i l e d s o i l . 106 V a r i a t i o n s i n the t o t a l s t r e s s at the t i p do not seem to have any e f f e c t on the response recorded up the s h a f t . 6.11 C o n c l u s i o n The two c y l i n d r i c a l s o l u t i o n s f o r pore pressure d i s s i p a t i o n around the cone by Randolph and Wroth and by Torstensson, and the two-dimensional s o l u t i o n by B a l i g h and Levadoux, provide a reasonable ^ p r e d i c t i o n of the h o r i z o n t a l 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 i n the o v e r c o n s o l i d a t e d s t a t e . Torstensson's s p h e r i c a l s o l u t i o n u n d e r p r e d i c t s 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 ; t h i s i s probably due to the assumption of the i n i t i a l pore pressure d i s t r i b u t i o n a r r i v e d at d u r i n g s p h e r i c a l c a v i t y expansion. The s o l u t i o n s by Randolph and Wroth and by Torstensson that r e q u i r e a s t i f f n e s s r a t i o are not s e n s i t i v e to the s t i f f n e s s r a t i o chosen. For a f o u r - 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. The s o l u t i o n by Soderberg should be d i s c o u n t e d , as i t showed the poorest f i t to the f i e l d d a t a. Provided that e q u i l i b r i u m pore p r e s s u r e s 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% l e v e l of d i s s i p a t i o n . The i n t e r p r e t a t i o n of r e s u l t s r e q u i r e s some judgement i n the event that d i f f e r e n t c o e f f i c i e n t s of c o n s o l i d a t i o n are 107 p r e d i c t e d at d i f f e r e n t stages of d i s s i p a t i o n . C o n s i d e r i n g the v a r i a b i l i t y of l a b o r a t o r y r e s u l t s and s o i l nonhomogeneity, t h i s d i f f i c u l t y may be of small consequence. A t a b l e of time f a c t o r , T, from e x i s t i n g s o l u t i o n s i s i n c l u d e d i n Appendix B. Some recommendations of p o s s i b l e methods fo r o b t a i n i n g s o i l p e r m e a b i l i t y are d i s c u s s e d in Chapter 7. I 08 Chapter 7 E s t i m a t i n g S o i l P e r m e a b i l i t y from Cone P e n e t r a t i o n Test Data 7.1 I n t r o d u c t i o n T h i s paper has addressed the p o s s i b i l i t y of o b t a i n i n g 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 from the pore pressure decay a f t e r p e n e t r a t i o n i s h a l t e d . As d i s c u s s e d i n Chapter 1, there i s o f t e n a need to o b t a i n a r e l i a b l e estimate of s o i l p e r m e a b i l i t y . Provided that an estimate of s o i l c o m p r e s s i b i l i t y can be obtained, an estimate of p e r m e a b i l i t y can be obtained from: k v = c vm v tfw (1 ) k h = C h m h t f w (2) 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 u s u a l l y be regarded as i s o t r o p i c : mv = mn ( M i t c h e l l et a l 1978; Ladd et al.1977). If 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 , i . e . mv=mn, then: C V = C h X jCy (3) kh An estimate of the r a t i o k v / k h can be obtained from Table V I I , a f t e r B a l i g h and Levadoux 1980. In a d d i t i o n , evidence of the 109 Anisotropic Permeability of Clays Nature of Clay ^ h ^ v 1. No evidence of layering 1.2 ± 0.2 2 . S l i g h t layering, e . g . , sedimentary clays with 2 to 5 occasional s i l t dustings to random lenses 3. Varved clays i n north- 10 ± 5 eastern U.S. T a b l e V I I A n i s o t r o p i c P e r m e a b i l i t y of C l a y s a f t e r : B a l i g h and Levadoux 1980 no h e t e r o g e n e i t y can be obtained from examination of the bearing f r i c t i o n and dynamic pore pressure l o g s . Methods that can be used to estimate s o i l c o m p r e s s i b i l i t y i n c l u d e : 1) Laboratory T e s t i n g ; 2) F i e l d Loading Test R e s u l t s ; 3) C o r r e l a t i o n s between QC and mv; 4) C o r r e l a t i o n s between Index T e s t s and mv,Cc 5) Other I n s i t u T e s t s . P r o v i d e d that an estimate of mv i s obtained, then estimates of h o r i z o n t a l and v e r t i c a l p e r m e a b i l i t y can be obtained from equations 1, 2, and 3. 7.2 Laboratory T e s t s to Obtain C o m p r e s s i b i l i t y S ince the purpose of t h i s paper i s to t r y to extend the use of the cone p e n e t r a t i o n t e s t , a d i s c u s s i o n of l a b o r a t o r y t e s t i n g i s unwarranted. Employment of a l i m i t e d number of l a b o r a t o r y t e s t s , however, combined with a l a r g e number of pore pressure d i s s i p a t i o n s , may be a very u s e f u l technique. C o n s o l i d a t i o n t e s t s performed on samples s e l e c t e d near d i s s i p a t i o n t e s t l o c a t i o n s would provide c o n f i r m a t i o n of 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 from d i s s i p a t i o n t e s t s r e s u l t s , as w e l l as an estimate of s o i l c o m p r e s s i b i l i t y f o r the s i t e . D i s s i p a t i o n t e s t r e s u l t s c o u l d then be used to extend r e s u l t s of ILL the c o n s o l i d a t i o n t e s t s . 7.3 F i e l d Loading B a c k c a l c u l a t e d v a l u e s from the performance of f i l l s and s t r u c t u r e s may permit the best estimate of an average value of s o i l c o m p r e s s i b i l i t y . 7.4 Estimates of C o m p r e s s i b i l i t y from Bearing Values Although no a n a l y t i c a l s o l u t i o n can be used to r e l a t e b e a r i n g to c o m p r e s s i b i l i t y , an estimate of deformation c h a r a c t e r i s t i c s of a s o i l can be obtained from QC; i t must, however, be recognized that QC i s an u l t i m a t e " s t r e n g t h " parameter that depends upon s o i l deformation, s t r e n g t h 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 . In a d d i t i o n , as has a l r e a d y been d i s c u s s e d , the p r e d i c t i o n of d r a i n e d parameters from an undrained t e s t i s not a p p e a l i n g . Any r e l a t i o n , t h e r e f o r e , that g i v e s a deformation c h a r a c t e r i s t i c from QC w i l l be p u r e l y e m p i r i c a l and c a r r y a c e r t a i n degree of i m p r e c i s i o n . The most commonly used r e l a t i o n between bearing and c o m p r e s s i b i l i t y i s : mv = J_ aQC where a = 7 - 10 f o r c l a y e y s o i l s a = 2 - 3.5 f o r sand Values f o r a as a f u n c t i o n of s o i l type have been compiled by 112 M i t c h e l l et a l 1978. An a l t e r n a t e procedure to o b t a i n c o m p r e s s i b i l i t y from cone p e n e t r a t i o n t e s t s given by Bri a u d 1980 i s : 1) estimate cu from QC; 2) estimate p 0 ' ; 3) r e f e r to r e l a t i o n s between cu/po' and c o m p r e s s i b i l i t y . One such r e l a t i o n i s shown below. c /p* u' p ov approx. OCR V * 1 + e o ) 0 - 0.1 0.1 - 0.25 0.26 - 0.50 0.51 - 1.00 1 - 4 over 4 less than 1 1 1 to 1.5 (assume 1) 3 6 greater than 6 greater than 0.4 (still consolidating) 0.4 0.3 0.15 0.10 0.05 TABLE VIII - Estimating the consolidation of clay from T - T * -pov F R O M B R I A U D 1 9 8 0 1 1 3 The r e l a t i o n between cu/po' and OCR was suggested as a method of c a l c u l a t i n g OCR from QC by Schmertmann 1974. Since t h i s method i n v o l v e s three separate c o r r e l a t i o n s , o b t a i n i n g cu from QC, OCR from cu/po', and Cc from OCR, i t l a c k s the appeal of simply c o r r e l a t i n g mv and QC. 7.5 Laboratory Index T e s t s to Obtain C o m p r e s s i b i l i t y Laboratory Index t e s t s have been used r o u t i n e l y i n the past to o b t a i n s o i l c o m p r e s s i b i l i t y . Although s o i l sampling i s n e c e s s i t a t e d , commonly used c o r r e l a t i o n s are i n c l u d e d here from B a l i g h and Levadoux 1980 f o r completeness: Cc = 0.009 (Wl% - 10%) T e r z a g h i and Peck 1967 Cc = 0.54 ( e 0 - 0.35) N i s h i d a 1958 Cc = 0.01 to 0.15 (Wn%) MPMR 1958 Cc = 0.6 ( e 0 - 1) ( K e 0 < 6 ) Kapp 1966 Cc = 0.013 PI Atkinson and Bransby 1978 7.6 I n s i t u T e s t s to Obtain S o i l C o m p r e s s i b i l i t y An a c c u r a t e e v a l u a t i o n of s o i l c o m p r e s s i b i l i t y can best be obtained by i n s i t u t e s t s , i n c l u d i n g screw p l a t e , p l a t e load, pressuremeter, and d i l a t o m e t e r t e s t s . An e v a l u a t i o n of these t e s t s i s i n c l u d e d i n M i t c h e l l et a l 1978 and B r i a u d 1980 and w i l l not be d i s c u s s e d here. As i n the case of l a b o r a t o r y t e s t i n g , the use of pore pressure d i s s i p a t i o n t e s t s may be enhanced, i f necessary, with other t e s t s that are more e x a c t i n g f o r e v a l u a t i n g s o i l c o m p r e s s i b i l i t y . 114 Chapter 8 C o n c l u s i o n s Some c o n c l u s i o n s have been presented w i t h i n the previous c h a p t e r s ; the c o n c l u s i o n s presented i n t h i s chapter e i t h e r r e i t e r a t e the most important c o n c l u s i o n s or t r y to t i e the d i f f e r e n t chapters t o g ether. 8.1 C o n c l u s i o n s 1) Much of t h i s r e p ort has c e n t r e d around the n e c e s s i t y f o r c a r e f u l c a l i b r a t i o n , s a t u r a t i o n , and proper data r e d u c t i o n . Although t h i s may seem to c o n t r a d i c t the b e l i e f that the cone p e n e t r a t i o n t e s t i s a simple, h i g h l y repeatable t e s t , i t has. been found that with c a r e , anomalous data c o u l d be c o r r e c t e d and meaningful r e s u l t s obtained. C o r r e c t i o n s and procedures found to be most s i g n i f i c a n t i n c l u d e : A) pore pressure e f f e c t s of unequal end areas of the f r i c t i o n s l e e v e ; B) pore pressure e f f e c t s on bearing measurements; C) e f f e c t s of s a t u r a t i o n ; and D) e f f e c t s > of r a t e . C o r r e c t i o n s found to be of secondary s i g n i f i c a n c e i n c l u d e : A) e f f e c t s of d i s s i p a t i o n s ; and B) unequal pore pressure d i s t r i b u t i o n up the s l e e v e , and subsequent net pore pressure c o r r e c t i o n to the 115 f r i c t i o n r e a dings. The a d d i t i o n of the dynamic pore pressure has been found to be a u s e f u l parameter to a s s i s t s t r a t a g r a p h i c l o g g i n g f o r two reasons: A) i t shows the g r e a t e s t d e t a i l ; and B) a knowledge of the p e r m e a b i l i t y can be obtained. The dynamic pore pressure and pore p r e s s u r e r a t i o has been shown to depend on both the p e r m e a b i l i t y and the volume change c h a r a c t e r i s t i c s of the s o i l . The infuuence of p e r m e a b i l i t y was shown by v a r y i n g p e n e t r a t i o n r a t e . The i n f l u e n c e of volume change c h a r a c t e r i s t i c s was shown i n an example that showed the change i n dynamic pore pressure that accompanied compaction. Because of the dependancy of the dynamic pore pressure on both of these s o i l parameters c o n s i d e r a b l e care must be used i n i n t e r p r e t i n g dynamic pore pressure logs.Use of simultaneous bearing , f r i c t i o n and pore pressure measurements reduces p o s s i b l e ambiguity i n i n t e r p r e t a t i o n s . The piezometer cone can be used to o b t a i n s t a t i c pore pressure v a l u e s . T h i s i n f o r m a t i o n , although e a s i l y o b tained, appears to be underemployed. A n a l y t i c a l e x p r e s s i o n s f o r the pore pressure generated d u r i n g s p h e r i c a l c a v i t y expansion compare w e l l with v a l u e s measured at the t i p , whereas val u e s measured up the sleeve compare w e l l with c y l i n d r i c a l c a v i t y expansion theory. The e f f e c t of r a t e on the bearing measurements has been 116 shown. To i l l u s t r a t e t h i s e f f e c t , the e f f e c t i v e b e a r i n g concept was i n t r o d u c e d . The e f f e c t of rate on f r i c t i o n s l e e v e readings was found to be more s i g n i f i c a n t because of the g r e a t e r time f o r c o n s o l i d a t i o n . 7) Estimates of the r a d i a l e f f e c t i v e and t o t a l s t r e s s were made from f r i c t i o n and pore pressure measurements obtained from v a r i a b l e p e n e t r a t i o n r a t e s and t i p e x t e n s i o n s . The s t r e s s d i s t r i b u t i o n up the s h a f t has important i m p l i c a t i o n s regarding the use of f r i c t i o n s l eeve readings f o r p r e d i c t i o n of p i l e s k i n f r i c t i o n c a p a c i t y . 8) The pore pressure d i s s i p a t i o n s o l u t i o n s used p r e d i c t e d c o e f f i c i e n t s of c o n s o l i d a t i o n that compared w e l l with l a b o r a t o r y measured values of the h o r i z o n t a l 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 in the o v e r c o n s o l i d a t e d s t a t e . 9) C o n s i d e r a b l e r e s e a r c h undertaken by others has shown that pore pressure measurements on the t i p face r e s u l t i n the hi g h e s t measured v a l u e s . Previous work has been done only i n c l a y e y s o i l s . For s e v e r a l reasons, the l o c a t i o n of the porous stone used i n t h i s study has been found to be an a c c e p t a b l e compromise that a l s o has the f o l l o w i n g advantages: A) The stone i s p r o t e c t e d both from wear and impact damage. B) The l o c a t i o n used r e s u l t s i n d i s s i p a t i o n s that compare to those measured f u r t h e r up the s h a f t and 117 to c y l i n d r i c a l d i s s i p a t i o n t h e o r i e s . C) The d i s s i p a t i o n s conducted do not appear to be s e n s i t i v e to procedure used. D) The l o c a t i o n used corresponds e x a c t l y to the pore pressure r e q u i r e d f o r c o r r e c t i n g the b e a r i n g value fo r pore pressure e f f e c t s . 8.2 Recommendations f o r Further Research C e r t a i n t o p i c s f o r f u r t h e r r e s e a r c h became apparent during t h i s i n v e s t i g a t i o n . These t o p i c s i n c l u d e : 1) One of the primary uses of the cone p e n e t r a t i o n t e s t i s to o b t a i n undrained shear s t r e n g t h . Undrained shear s t r e n g t h i s c a l c u l a t e d u sing c o r r e l a t i o n f a c t o r s Nc or Nk, which were obtained both t h e o r e t i c a l l y and e x p e r i m e n t a l l y . C o n s i d e r a b l e s c a t t e r i n a the a p p r o p r i a t e Nc f a c t o r has been obtained from comparing f i e l d vane to cone bearing r e s u l t s . I t has been suggested i n t h i s r e p o r t that p r e v i o u s p u b l i s h e d bearing values have s u f f e r e d (to v a r y i n g degrees) from pore pressure e f f e c t s . I t may be that much of the s c a t t e r i s Nc v a l u e s i s caused by i n c o r r e c t and i n c o n s i s t e n t b e a r i n g v a l u e s . In f u t u r e , bearing values should be c o r r e c t e d ; t h i s may r e s u l t i n b e t t e r agreement between t h e o r e t i c a l l y and e x p e r i m e n t a l l y obtained Nc v a l u e s . 2) Good c o r r e l a t i o n s were found between 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 obtained from f i e l d d i s s i p a t i o n records 1 1 8 and r e f e r e n c e v a l u e s . F u r t h e r comparisons should be made when p o s s i b l e . In a d d i t i o n , simple c o r r e l a t i o n s between p e r m e a b i l i t y , 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 , and the time f o r d i s s i p a t i o n of 50% of the excess pore •pressure should be e s t a b l i s h e d . 3) The pore pressure r a t i o and the excess pore pressure r a t i o show promise i n p r e d i c t i n g p r evious l o a d i n g h i s t o r y of c l a y s o i l s ; f u r t h e r work i n t h i s f i e l d i s warranted. 4) I t has been suggested t h a t the l o c a t i o n of the porous stone used i n t h i s i n v e s t i g a t i o n o f f e r s c o n s i d e r a b l e advantages. Other r e s e a r c h e r s p r e f e r to l o c a t e the porous element on the cone f a c e . Comparison between r e s u l t s obtained with each cone has only been done f o r c l a y s o i l s . L i t t l e d i f f e r e n c e i s seen i n the r e s u l t s o b t a i n e d . 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Wissa,A.E.Z.,Christian,J.T.,Davis,E.H.,and Heiberg,S.,1971. " C o n s o l i d a t i o n at Constant Rate of S t r a i n " , J o u r n a l of the S o i l Mechanics and Foundation D i v i s i o n , A S C E , 97(SM8) pp 1393-1412. Wissa, A.E.Z., M a r t i n , R.T., and Garlanger, J.E., 1975. "The Piezometer Probe", Proceedings, ASCE S p e c i a l t y Conference on In S i t u Measurement of S o i l P r o p e r t i e s , R a l e i g h , N.C., Vo l I, pp 536 - 545. 122 Appendix A Complete CPT Logs Mcdonald S i t e 1 2 3 0 UD BAR 10 5 0 J i i_ QC BAR 10 20 30 UD/QC 200 . 0 . 5 1.0 _j i i PENETRATION SPEED 2 cm/sec PENETRATION SPEED .04 cm/sec PENETRATION SPEED .027 cm/sec DENOTES E Q U I L I B R I U M PORE 1 PRESSURE PH3 STANDARD CONE C6FPS-2UBC 124 « •» DENOTES C Q U I L I O n i U M P O R E 1 P R E S S U R E PH4 STANDARD CONE C6FPS-2UBC WITH 5 IN. TIP EXTENSION 1 2 5 PH 5 STANDARD CONE C6FPS-2UBC 1 2 6 0 UD B A R 10 5 0 J L L QC BAR 0 1 0 2 0 3 0 1 0 0 i ± U D / Q C 2 0 0 . 0 . 5 1.0 PENETRATION S P E E D 2 c m / s e c i " D E N O T E S E Q U I L I B R I U M POR E P R E 5 5 PH6 STANDARD CONE C6FPS-2UBC WITH 10 IN. TIP EXTENSION 1 2 8 UD B A R 10 5 0 _! I !_ QC B A R I 0 20 30 P H 8 U D / Q C 2 0 0 . 0 . 5 ! ' I— l o O PENETRATION S P E E D 2 c m / s e c .2 c m / s e c .0 4 c m / s e c i D E N O T E S E Q U I L I B R I U M P O R E P R E S S U R E STANDARD CONE C6FPS-2UBC WITH 15 IN. TIP EXTENSION 129 UD BAR QC BAR UD/QC PH10 STANDARD CONE C6FPS-2UBC PH3 STANDARD CONE C6FPS-2UBC 131 FS BAR QC BAR RF 7. STANDARD CONE C6FPS-2UBC WITH 5 IN. TIP EXTENSION 1 3 2 FS BflR i n 1 • 0 0 i i QC BflR 0. 0 50 100 150 200 RF '/. PENETRATION S P E E D 2 c m / s e c i d . 2CL i . 3 d PH6 STANDARD CONE C6FPS-2UBC WITH 10 IN. TIP EXTENSION 1 3 3 PH 7 STANDARD CONE C6FPS-2UBC WITH 10 IN. TIP EXTENSION 134 FS BAR QC BAR RF "/. P H 8 STANDARD CONE C6FPS-2UBC WITH 15 IN. TIP EXTENSION 135 FS BRR QC BAR RF 7. P H 1 0 STANDARD CONE C6FPS-2UBC Appendix B Time F a c t o r s f o r D i s s i p a t i o n Analy I 3 7 Values of Time F a c t o r T f o r V a r i o u s Percent D i s s i p a t i o n s Method 20 Percent 40 D i s s i p a t i o n 50 60 80 B a l i g h and Levadoux 1980 0 .44 1 .89 3.62 6.47 26 .85 Torstensson 1977 S p h e r i c a l (E/cu = 500) 0 . 1 1 0.46 0.81 1 .26 3 .28 S p h e r i c a l (E/cu = 400) 0 .10 0.40 0.68 1.12 2 .85 S p h e r i c a l (E/cu = 300) 0 .085 0.35 0.61 0.98 2 .36 S p h e r i c a l (E/cu = 200) 0 .66 0.28 0.47 0.77 1 .91 s p h e r i c a l (E/cu = 1 00) 0 .057 0.20 0.32 0.50 1 .16 Torstensson 1977 C y l i n d r i c a l (E/cu = 500) 0 .34 2.14 4.29 8.33 23 .60 C y l i n d r i c a l (E/cu = 400) 0 .30 1 .75 3.57 6.79 21 .00 C y l i n d r i c a l (E/cu = 300) 0 .24 1 .38 2.81 5.37 16 .29 C y l i n d r i c a l (E/cu = 200) 0 .18 1 .06 2.32 3.82 10 . 1 3 C y l i n d r i c a l (E/cu = 1 00) 0 . 1 4 0.83 1 .37 2.49 5 .03 Note: T i s the Time F a c t o r d e f i n e d as T = c h t / r 2