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Pore pressure characteristics of an extrasensitive clay Glynn, Thomas Edward 1960

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PORE PRESSURE CHARACTERISTICS OP AN EXTRASENSITIVE CLAY by THOMAS EDWARD GLYNN B. E., UNIVERSITY COLLEGE OP GALWAY, IRELAND, 1948 A THESIS SUBMITTED IN PARTIAL FULFILMENT OP THE REQUIREMENTS FOR THE DEGREE OP M. A. Sc. IN THE DEPARTMENT of CIVIL ENGINEERING WE ACCEPT THIS THESIS AS CONFORMING TO THE REQUIRED STANDARDS. THE UNIVERSITY OP BRITISH COLUMBIA APRIL I960 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada. i i ABSTRACT The r e s u l t s of a laboratory Investigation into the pore pressure c h a r a c t e r i s t i c s of an extrasensltive marine cl a y are presented. The s o i l samples were obtained from the Port Mann area of B r i t i s h Columbia. Experimental work consisted of the performance of long-duration t r i a x i a l shear tests with pore-pressure measurements. A st r e s s - c o n t r o l l e d t r i a x i a l machine equipped with a n u l l - i n d i c a t i n g type pore-pressure device was employed f o r a l l shear t e s t s . The observed data show that f o r t h i s s o i l a slow build-up of pore pressure occurs f o r both increases i n e e l l pressure and a x i a l stresses i n the t r i a x i a l t e s t . Even i n saturated speci-mens the slow build-up e f f e c t prevailed. The rates of build-up observed for changes i n a x i a l stress were slower than those recorded f o r changes i n c e l l pressure* Measurements at the upper end of some specimens, and at the centre of others, i n d i -cated that the pore pressure required more time to reach e q u i l -ibrium, at the ends of c y l i n d r i c a l specimens. The hypothesis i s put forward that the observations can be explained by p l a s t i c deformations o f the adsorbed layers surrounding the p a r t i c l e s . Strength and pore pressure parameters have been obtained for the s o i l . An automatic control has been developed to a s s i s t i n the performance of long-duration tests. The apparatus employs the photo-electric e f f e c t to control movements of pore water. A detailed description of this device i s presented* i i i CONTENTS Page CHAPTER I PHYSICAL PROPERTIES OP SOILS 1 A, INTRODUCTION 1 B. SOILS - GENERAL 2 1. Geological Aspects 2 2. Size and Shape of S o i l P a r t i c l e s 4 3« Mineralogical Composition of Pine-Grained S o i l s 5 4. Surface A c t i v i t y and Adsorbed Layers 6 5» Plo c c u l a t i o n 10 6 # Structure of Marine Clay 10 CHAPTER II STRENGTH THEORY 14 A. INTRODUCTION 14 B. EFFECTS OF PORE PRESSURE 15 1. Relationship Between Pore Pressure and Shear Strength 15 2* Relationship Between Compressive Strength and Pore Pressure 18 3. Total Stress Parameters 20 4» T o t a l F r i c t i o n Angle and True Cohesion 20 C. FACTORS AFFECTING PORE PRESSURE: THE PORE PRESSURE PARAMETERS A AND B . 23 CHAPTER I I I APPARATUS: DEVELOPMENT AND OPERATION 30 A. TRIAXIAL SHEAR TESTS WITH PORE PRESSURE MEASUREMENTS 30 1, General 30 2. Stress-Controlled T r i a x i a l Apparatus 31 3. The T r i a x i a l C e l l 32 4« L a t e r a l Pressure Control 33 5. Load and Deformation Measuring Devices 35 6, Apparatus f o r Measuring Pore Pressure 35 7« Automatic Control 38 8 # Fabrication of Membranes 38 9« Preliminary Testing of Apparatus 40 10. Preparation of S o i l Specimens 41 11* S e t t i n g up Specimen i n T r i a x i a l C e l l 42 12» Temperature Control 43 CHAPTER IV SHEAR TESTS WITH PORE PRESSURE MEASUREMENTS 44 A. INTRODUCTION 44 B. PREVIOUS RESEARCH 45 C. SCOPE OP PRESENT INVESTIGATION 46 D. DESCRIPTION OP SAMPLES 47 E. DESCRIPTION OP SHEAR TESTS AND RESULTS 47 1. (a) Test 1 51 (b) Test 2 52 (c) Test 3 55 (d) Test 4 58 (e) Test 5 60 (f) Test 6 62 (g) Test 7 67 2» Apparent Strength Parameters 69 P. OBSERVATION 70 0. MISCELLANEOUS TESTS 70 1. Mineralogical Composition of P a r t i c l e s 70 2. S e n s i t i v i t y 71 3. Atterberg Limits 72 CHAPTER V DISCUSSION OF TEST RESULTS AND CONCLUSIONS 73 A* EFFECTS OP OVERBURDEN PRESSURE 73 V B . SENSITIVITY 74 C. STRESS-STRAIN CURVES 75 D. PORE PRESSURE CHARACTERISTICS 76 E. STRENGTH PARAMETERS 82 P. DRAINAGE CHARACTERISTICS 83 CHAPTER VI AUTOMATIC CONTROL 84 A. INTRODUCTION 84 B . DETAILS OP APPARATUS FOR AUTOMATIC CONTROL OP PORE PRESSURE 88 CHAPTER VII SUMMARY OP CONCLUSIONS 97 APPENDIX I Test' r e s u l t s I APPENDIX II Correction of Compressive Strength on Basis of Experimental Factors v i APPENDIX III Sample Calculations v i i i BIBLIOGRAPHY x i PHOTOGRAPHIC SUPPLEMENT x i i i v i GRAPHS To Follow Page Graph 4 mm 1 Build-up Stage: Test 1. 51 Graph 4 - 2 Build-up Stage: Test 2. 54 Graph 4 3 Drainage Stage: Test 2. 54 Graph 4 - 4 Stress vs S t r a i n : Test 2. 54 Graph 4 - 5 Loading Stage: Test 2. 54 Graph 4 - 6 Build-Up Stage: Test 3. 57 Graph 4 - 7 Drainage Stage: Test 3w 57 Graph 4 - 8 Stress vs S t r a i n : Test 3. 57 Graph 4 m 9 Loading Stage: Test 3* 57 Graph 4 - 10 Build-Up Stage: Test 4. 59 Graph 4 - 11 Stress vs S t r a i n : Test 4. 59 Graph 4 - 12 Loading Stage: Test 4* 59 Graph 4 - 13 Stress vs S t r a i n : Test 5. 61 Graph 4 mm 14 Build-Up Stages: Test 6. 66 Graph 4 mm 15 Drainage Stage: Test 6. 66 Graph 4 - 16 Stress vs S t r a i n : Test 6. 66 Graph 4 mm 17 Loading Stages Test 6. 66 Graph 4 - 18 Build-Up Stages: Test 7. 68 Graph 4 - 19 Stress vs S t r a i n : Test 7. 68 Graph 4 - 20 Loading Stage: Test 7. 68 Graph 4 - 21 Mohr Diagram of E f f e c t i v e Stresses 72 Graph 4 - 22 Mohr Diagram of E f f e c t i v e Stresses 72 Graph 4 «. 23 Mohr C i r c l e s of E f f e c t i v e Stresses vs Time 72 Graph 4 m 24 Mohr Diagram: T o t a l Stresses 72 APPENDIX : I Consolidation Test Results i i i Borehole Logs v v i i ILLUSTRATIONS To Follow Page F i g . 1 Clay Minerals 5 F i g . 2 Helmholtz and Diffuse Layers 7 F i g . 3 Clay-Water System 8 F i g . 4 Structure of Marine Clay 12 F i g . 5 Mohr Diagram: Total and E f f e c t i v e Stresses 19 F i g . 6 Mohr Diagram: Total Stresses 19 F i g . 7 Consolidation H i s t o r i e s 21 F i g . 8 Mohr Diagram: True Parameters 21 F i g . 9 Stress-Controlled T r i a x i a l Machine 31 Fig.10 T r i a x i a l C e l l 31 Fig.11 Proving-ring Correction f o r C e l l Pressure 43 Fig.12 Rubber Membrane: Stress vs S t r a i n Curve 43 Fig.13 Pore-Pressure Apparatus 37 Fig.14 Clay-Water System: E f f e c t s of P l a s t i c Deformation of Adsorbed Layers 77 Fig,15 Pore-Pressure Device 86 Fig.16 C i r c u i t of Automatic Control 86 Fig.17 Layout of Automatic Control 89 Fig.18 Automatic Control 91 v i i i LIST OP TABLES Page TABLE I L i q u i d Limit of Putnam Clay 9 TABLE II Properties of Shellhaven Clay 24 TABLE I I I Description of Test Samples 49 TABLE IV Schedule of Shear Tests 50 TABLE V Determination o f C o e f f i c i e n t of Consolidation 64 TABLE VI Apparent Strength Parameters • Port Mann Clay 69 TABLE VII Mineralogical Composition of P a r t i c l e s - Port Mann Clay 71 TABLE VIII S e n s i t i v i t y Indices - Port Mann Clay 71 TABLE IX Atterberg Limits - Port Mann Clay 72 ACKNOWLEDGMENT The w r i t e r wishes to express h i s a p p r e c i a t i o n and indebtedness to h i s s u p e r v i s o r , Mr. R. A. Spence, f o r a s s i s t a n c e i n a l l stages o f t h i s undertaking. He a l s o wishes to thank P r o f e s s o r J . P. Muir, of the Department of C i v i l E n g ineering, f o r making arrangements to procure equipment r e -q u i r e d f o r the experimental work of t h i s t h e s i s . X NOTATIONS A pore pressure parameter:axial. A* Angstrom u n i t a area 8^, contact area of p a r t i c l e s B pore pressure parameter:all round Y surface tension density of water C w compressibility of water Crt compressibility of s o i l skeleton C v c o e f f i c i e n t o f consolidation c cohesion c' apparent conesIon:effeetive stresses c^j cohesion undrained:total stresses c r true cohesion A incremental change; p r e f i x E Young's modulus 6 u n i t s t r a i n f at/to f a i l u r e : s u f f i x 7j porosity 6 angle of f a i l u r e plane to plane of major p r i n c i p a l stress k c o e f f i c i e n t of permeability L Length u Poisson's r a t i o CT stress NOTATIONS (Cont'd.) CT, major p r i n c i p a l s t r e s s : t o t a l 0~3 minor p r i n c i p a l s t r e s s : t o t a l matfor p r i n c i p a l s t r e s s : e f f e c t i v e (fj minor p r i n c i p a l s t r e s s : e f f e c t i v e T shearing stress t time u pore pressure V volume 1 CHAPTER I .  PHYSICAL PROPERTIES OF SOILS A. I n t r o d u c t i o n In the course of a l a b o r a t o r y i n v e s t i g a t i o n c a r r i e d out by R. A. Spence, C o n s u l t i n g Engineers, on the mechanical p r o p e r t i e s o f Po r t Mann marine c l a y , anomalies were observed i n the pore pressure vs. a p p l i e d s t r e s s r e l a t i o n s h i p s , obtained from t r i a x i a l t e s t s . The r e s u l t s of these t e s t s i n d i c a t e d that the behavior of the s o i l departed from accepted theory concerning pore pressures of c l a y s from submerged s t r a t a , w i t h consequent e f f e c t on shear s t r e n g t h . I n p a r t i c u l a r , i t was noted that a time l a g e x i s t e d i n attainment of e q u i l i b r i u m between pore pressure and the a p p l i e d t o t a l s t r e s s e s i n undrained t r i a x i a l shear t e s t s . The research r e p o r t e d i n t h i s t h e s i s was under-taken to i n v e s t i g a t e f u r t h e r the pore pressure vs. a p p l i e d s t r e s s r e l a t i o n s h i p , by c o n c e n t r a t i n g on long d u r a t i o n t e s t s . S p e c i a l apparatus and t e s t i n g procedures were developed f o r the long d u r a t i o n t r i a x i a l t e s t s r e p o r t e d i n t h i s t h e s i s . 2 By way of abstracts from the l i t e r a t u r e , background information is presented i n t h i s , and the following Chapter. Although part of the discussion i n Chapters I and II applies to s o i l s i n general, p a r t i c u l a r emphasis i s placed on topics pertinent to marine cla y . Chapter I I I and subsequent Chapters deal with the present i n v e s t i g a t i o n . B. S o i l s - General 1. Geological Aspects. Inorganic s o i l s may be broadly separated Into two major groups, r e s i d u a l and transported* Residual s o i l s are found i n close proximity to the parent rock. Physical and chemical weathering are the main factors governing t h e i r formation and c h a r a c t e r i s t i c s . D e t r i t a l accumulations remote from the source of o r i g i n constitute the transported s o i l s , i r r e s p e c t i v e o f the transporting agency. Thus g l a c i a l , a l l u v i a l , c o l l u v i a l , lacustrine, eolian and marine deposits are examples of transported s o i l s . Usually, the history of a transported s o i l has a marked ef f e c t on Its c h a r a c t e r i s t i c s . Parent material, erosion and transportation agencies, depositional conditions, time and geographic f a c t o r s may a l l contribute to the observed properties* Organic s o i l s are not included i n the above s i m p l i f i e d c l a s s i f i -c a tion. The organic component i n most s o i l s develops i n - s i t u as a r e s u l t of the growth and decay of plants and organisms* In contrast to r e s i d u a l and transported deposits, organic accumulations are commonly confined to comparatively thin s t r a t a * 3. Organic matter, however, may be a c o n s t i t u e n t of e i t h e r r e s i d u a l or t r a n s p o r t e d s o i l s . The foundation engineer i s u s u a l l y most i n t e r e s t e d In the s t r e n g t h and/or l o a d - c a r r y i n g a b i l i t y of the s o i l . Except i n r a r e i n s t a n c e s , r e s i d u a l s o i l s have h i g h s t r e n g t h and s t a b i l i t y , e s p e c i a l l y i n temperate c l i m a t i c zones. On the other hand, tr a n s p o r t e d s o i l s show considerable v a r i a t i o n s from place t o p l a c e , even w i t h i n the same stratum. Loose or s o f t deposits commonly occur f o r considerable depths. Organic s o i l s must be c a r e f u l l y considered f o r foundations, because of the high c o m p r e s s i b i l i t y a s s o c i a t e d w i t h organic d e p o s i t s . Research on the engineering p r o p e r t i e s has been focussed mainly on s o i l s of the transported type. Deposits which have been subjected to stresses greater than the present overburden pressure are termed p r e c o n s o l i d a t e d or precompressed. U n l i k e normally c o n s o l i d a t e d deposits which have not been subjected to excess pressures i n the past, the p r o p e r t i e s o f precompressed s o i l s are g r e a t l y Influenced by the extent of the p r e l o a d i n g . The e f f e c t i s most pronounced on the s t r e n g t h and settlement c h a r a c t e r i s t i c s , Terzaghi and Peck (1948). The temporary excess pressure may have been caused by the weight of s o i l s t r a t a which was l a t e r removed by e r o s i o n a l agencies. G l a c i a l r e c e s s i o n has a s i m i l a r a c t i o n and i s o f t e n r e s p o n s i b l e f o r precompression. G e o l o g i c a l evidence i s o f much a s s i s t a n c e In a s c e r t a i n i n g whether a s o i l i s normal or p r e c o n s o l i d a t e d . 4* 2. S i z e and Shape o f S o i l P a r t i c l e s . Grain s i z e and shape l a r g e l y determine the behavior of s o i l s as an engineering m a t e r i a l . Coarse grained s o i l s , such as g r a v e l s , sands and s i l t s , are u s u a l l y c o h e s i o n l e s s ; there i s l i t t l e or no tendency f o r the grains to adhere to one another. P r i m a r i l y , t h e i r s t r e n g t h depends on f r i c t i o n a l p r o p e r t i e s . Packing d e n s i t y , p a r t i c l e s i z e and shape l a r g e l y determine t h e i r f r i c t i o n a l r e s i s t a n c e to a p p l i e d l o a d s . As the g r a i n s i z e diminishe s , the a c t i o n of i n t e r - p a r t i c l e f o r c e s becomes more pronounced; these f o r c e s are manifested i n the property known as cohesion. T e n t a t i v e l y , cohesion w i l l be considered as that property which enables a s o i l mass to r e t a i n i t s shape i n the absence o f e x t e r n a l c o n f i n i n g s t r e s s e s . I f cementation of i n d i v i d u a l p a r t i c l e s i s excluded, cohesive s o i l s are predominant-l y f i n e g r a i n e d . By v i r t u e of cohesion, such s o i l s are o f t e n capable of c a r r y i n g considerable e x t e r n a l l o a d s , i n the absence of l a t e r a l support. I t has been observed that cohesive s o i l s are composed mainly o f f l a k e - l i k e p a r t i c l e s , whereas granular s o i l s tend to have mostly c u b i c a l or bulky fragments. The f l a k y p a r t i c l e s r e s u l t from the weathering of the l e a s t s t a b l e minerals o f the parent rock. E v i d e n t l y , cohesion i s r e l a t e d to the m i n e r a l -o g i c a l composition o f the p a r t i c l e . Thus, quartz f o r example, independent o f the fineness o f the g r a i n s , behaves as a cohesion-l e s s m a t e r i a l whether dry or f u l l y s a t u r a t e d . Other things being equal, the A t t e r b e r g l i m i t s increase 5. w i t h decrease In g r a i n s i z e . Skempton r e p o r t s that a l i n e a r p r o p o r t i o n a l i t y p r a c t i c a l l y e x i s t s between the p l a s t i c i t y index (1) and the c l a y f r a c t i o n of c o l l o i d a l s i z e . The r a t i o p l a s t i c i t y i n d e x / c l a y f r a c t i o n i s termed the " a c t i v i t y " o f the s o i l , Skempton (1954). 5« M l n e r a l o g l c a l Composition of F i n e - g r a i n e d S o i l s . The p r i n c i p a l c l a y - f o r m i n g minerals are m o n t m o r i l l o n i t e , i l l i t e and k a o l i n i t e . Chemically, they are a l l c r y s t a l l i n e arrangements of s i l i c o n , aluminum, potassium, oxygen and water molecules, Terzaghi and Peck (1948)* Clay minerals have a laminated s t r u c t u r e . Recent work r e p o r t e d by R. E. Grim i n d i c a t e s that the laminae are composed of two fundamental b u i l d i n g b l o c k s ; a t e t r a h e d r a l u n i t and an octohedral l a t t i c e , F i g . 1 ( a ) , ( b ) . S i m i l a r elemental blocks combine to form a s h e e t - l i k e s t r u c t u r e as shown i n F i g . 1 ( c ) , ( d ) . The p a r t i c u l a r atoms present and the arrangement of the sheets determine the mineral type. The sheets adhere to one another, thus forming the i n d i v i d u a l p a r t i c l e s . The f l a k i n e s s c h a r a c t e r i s t i c s o f c l a y p a r t i c l e s can be t r a c e d to the mineral s t r u c t u r e . M o n t m o r i l l o n i t e i s composed of two s i l i c a t e t r a h e d r a l sheets separated by one o c t o h e d r a l u n i t . The t h i c k n e s s of the l a y e r i s about 9*5 w h i l e the dimensions i n the other two o d i r e c t i o n s are i n d e f i n i t e , Grim (1959)• The 9.5 A l a y e r s are stacked one above the other to form the m o n t m o r i l l o n i t e p a r t i c l e . (1) E q u i v a l e n t diameter l e s s than 0.002 m i l l i m e t e r s . To foLLow pa,ye i ~ Crys taltine Components of Clay Minerals. (b) (d) O ccnoL. '^i HydroxyLs (^Aluminums f magnesiums etc. F I G 1 . C L A Y M I N E R A L S . (After R.E. G rim. / 959 ) 6. There i s l i t t l e bonding f o r c e between l a y e r s of m o n t m o r i l l o n i t e . The h i g h s w e l l i n g c a p a c i t y of s o i l s formed of t h i s m i n e r a l , i s b e l i e v e d t o be evidence o f the weak bonding. Apparently water can penetrate between the l a y e r s , enter the c r y s t a l l a t t i c e and promote s w e l l i n g , Terzaghi and Peck (1948). I l l i t e has a s i m i l a r s t r u c t u r e t o m o n t m o r i l l o n i t e , but there i s a s u b s t a n t i a l r e p l a c e -ment of the s i l i c o n by aluminum i n the t e t r a h e d r a l l a y e r s . Potassium i s present between l a y e r s where I t serves as a bonding l i n k . Clays w i t h a predominance of i l l i t e are not n e a r l y so subject to s w e l l i n g as those formed of m o n t m o r i l l o n i t e . K a o l i n i t e i s the l e a s t a c t i v e o f the three m i n e r a l s . I t s s t r u c t u r e c o n s i s t s of an alumina octohedral sheet i n t e r l o c k e d w i t h a p a r a l l e l s i l i c a t e t r a h e d r a l sheet to form a l a y e r about 7 A* t h i c k ^ . The l a y e r s are?stacked l i k e the leaves o f a book to form the k a o l i n i t e c r y s t a l , Grim (1959)• Consequently, a l l c l a y p a r t i c l e s tend to have cleavage planes i n the d i r e c t i o n of the l a r g e r dimensions. Forces of the type that b i n d the m i n e r a l l a y e r s , a l s o act at the boundaries of the p a r t i c l e s . T. W. Lambe (1958) a t t r i b -utes the boundary fo r c e s i n s o i l s to "bhe nonsymmetrical d i s t r i -b u t i o n o f e l e c t r o n s i n the s i l i c a t e c r y s t a l s ( a r i s i n g from h e t e r p o l a r bonds), the c r y s t a l s a c t as a l a r g e number of d i p o l e s . " This gives the p a r t i c l e magnet-like p r o p e r t i e s which are r e f l e c t e d i n surface a c t i v i t y . 4. Surface A c t i v i t y and Absorbed Layers. The chemical and p h y s i c a l m a n i f e s t a t i o n s of the surface charge c o n s t i t u t e the surface a c t i v i t y of the m i n e r a l . Surface (2) Angstrom Unit 1 S . - 10~ 8cms. a c t i v i t y i s dependent on both the m i n e r a l o g i c a l composition and fineness of the p a r t i c l e s . Bulky p a r t i c l e s such as q u a r t z , e x h i b i t l i t t l e surface a c t i v i t y . M o n t m o r i l l o n i t e , on the other hand, i s most a c t i v e among the c l a y m i n e r a l s . I t can be shown (3) e x p e r i m e n t a l l y that c l a y p a r t i c l e s c a r r y a surface charge. The e l e c t r i c a l charge r e s u l t s from the u n s a t i s f i e d bonds of the m i n e r a l m a t r i x , Baver (1956), Lambe (1958). G e n e r a l l y , c l a y p a r t i c l e s are n e g a t i v e l y charged. To n e u t r a l i z e t h i s charge, substances possessing p o s i t i v e p o t e n t i a l s are a t t r a c t e d to the p a r t i c l e . This r e s u l t s i n an envelope o f net p o s i t i v e charge e n c l o s i n g the n e g a t i v e l y charged p a r t i c l e . I n c o l l o i d a l chemistry t h i s e l e c t r i c a l arrangement i s known as the Helmholtz double l a y e r . P i g . 2 ( a ) . In n a t u r a l s o i l s the p o s i t i v e charges are s u p p l i e d by ions of e l e c t r o l y t e s i n aqueous s o l u t i o n , and by the water molecules themselves. Water i s a t t r a c t e d because of i t s permanent p o l a r p r o p e r t i e s , and a l s o the f a c t t h a t i t i s a weak e l e c t r o l y t e , Baver (1956), Terzaghi and Peck (1948). The a t t r a c t i o n between the p a r t i c l e s and the surrounding medium r e s u l t s i n an i o n -water complex bonded to the s o i l p a r t i c l e s . P i g . 2 ( b ) . That p a r t of the complex c l o s e r t o the surface of the p a r t i c l e than 10 & i s termed the adsorbed l a y e r . F u r t h e r a f i e l d , but s t i l l under the i n f l u e n c e o f the surface charge, i s the double l a y e r water. Outside the double l a y e r i s the f r e e f l u i d , which i s (3) Experiments on the e l e c t r o p h o r e s i s e f f e c t show that c l a y s are a t t r a c t e d to the anode I f a p o t e n t i a l g r adient i s a p p l i e d to a disperse suspension. To /oilo*/ p<xye 7 0 <0 • 0 Al " u . < + + + + + + + + + Liquid. (a) He.L/nholfz Double. Layer v. v. \ d <o " o . » u 0 »< k. Co.riQn ^ 0 ^ e \ 0 & 0 % Wcufer molecuLe (b) lon-wafer complex. F I G 2. H E L M H O L T Z A N D D I F F U S E L A Y E R S 8. not a f f e c t e d by the presence of the s o i l p a r t i c l e . There being no d e f i n i t e p h y s i c a l boundary between the three, the water i n a s o i l mass i s more or l e s s a r b i t r a r i l y d i v i d e d i n t o adsorbed  water and f r e e water. The st r e n g t h of the bond between the adsorbed l a y e r and the p a r t i c l e i s b e l i e v e d to be so great that i t produces a s o l i d or a h i g h l y viscous substance i n the v i c i n -i t y of the i n t e r f a c e . As the distance from the p a r t i c l e surface i n c r e a s e s , the hel d water r e v e r t s t o normal water. According t o Lambe (1958) p r a c t i c a l l y a l l the pore water i n a c l a y under normal f i e l d c o n d i t i o n s i s w i t h i n the double l a y e r . An idea o f the dimensions i n v o l v e d may be obtained from F i g . 3» The adsorbed l a y e r s ( i n c l u d i n g the double l a y e r ) have a marked i n f l u e n c e on the behavior of f i n e - g r a i n e d s o i l s . P r o p e r t i e s such as cohesion, p l a s t i c i t y , s e n s i t i v i t y * ' and (5) t r i x o t r o p y w / are b e l i e v e d to depend on the nature of the adsor p t i o n complex. For i n s t a n c e , the cohesion of a c l a y may be removed by r e p l a c i n g the water by a non-polar l i q u i d such as carbon t e t r a c h l o r i d e . The thic k n e s s of the adsorbed l a y e r s has thus a considerable e f f e c t on the p r o p e r t i e s * The dimensions of the l a y e r are i n f l u e n c e d by the nature of the adsorbed i o n s . According t o Baver (1956), both the valence Unconfined compressive s t r e n g t h undisturbed Unconfined compressive s t r e n g t h remoulded R e v e r s i b l e s o l - g e l t r a n s f o r m a t i o n . (4) S e n s i t i v i t y -(5) T r i x o t r o p y To foLLow pa.ge 8 —V Double l a y e r Water Adsorbed. Wafer M on tm oriClonife Crystal. Double Layer Water \ M o n f m o r i L L o n f - f e S h e e i " I O A — £ ^Double L a y e r Water^ Adsorbed. Wa.ter^ kaoLinife Cr ys t u t K a o C i n i t e Sheet F I G 3 . C L A Y - W A T E R S Y S T E M . ( A f t e r T.W. Lambe. . I9SB) 9. and size of the ion i s important. The e l e c t r i c - f i e l d inten-s i t y of an ion i s known to increase d i r e c t l y with the charge and inversely with the radius squared. In other words, some ions a t t r a c t more water molecules towards the adsorbed layer than others, thereby increasing i t s thickness. Sodium, calcium, hydrogen and potassium are the p r i n c i p a l adsorbed ions i n natural c l a y s . Sodium tends to produce thick layers, while on the other hand, hydrogen ions are adsorbed i n comparatively t h i n layers, Tschebotarioff (1951). I f a p a r t i c u l a r ion predominates, the cl a y i s sometimes given the name of this element, for example, Na-clay or Ca-clay. Ions of one element may be removed and replaced by those of another; the process i s known as base exchange. Generally, exchange of ions leads to change In proper-t i e s . To quote one example, Winterkorn (1941) found the l i q u i d l i m i t o f a sample of Putnam clay to vary with the adsorbed c a t i o n as follows: , , Natural » Na t Ca t A l i H i t K J L i q u i d l i m i t 64.5 t 88 i 61.9 ,60.2 156.4 .56.3 r52.8 t TABLE I. LIQUID LIMIT OF PUTNAM CLAY. The percolation of pure water through a s o i l reduces the salt content. Adsorbed ions can be l a r g e l y exchanged by this means; complete exchange producing an H-clay. Certain marine clays have been subjected to leaching by fresh water. Attempts 10. are being made to account f o r the unusual p r o p e r t i e s of such c l a y s i n terms of the degree of l e a c h i n g , Bjerrura (1954) and Rosenquist (1959). 5. F l o c c u l a t i o n . F l o c c u l a t i o n i s another t o p i c connected w i t h the adsorbed l a y e r s . Marine c l a y l a r g e l y owes i t s s t r u c t u r e to t h i s phenomenon. When a n e u t r a l e l e c t r o l y t e I s int r o d u c e d i n t o a dispersed suspension of s o i l p a r t i c l e s i n water, the negative charges which h i t h e r t o tended to separate the p a r t i c l e s , are n e u t r a l i z e d . The mass a t t r a c t i o n , or Van der Waal's f o r c e s are then capable o f c o l l e c t i n g the p a r t i c l e s i n t o a f l o e l a r g e enough to s e t t l e under the a c t i o n of g r a v i t y . The f l o c c u l e n t a c t i o n i s not very s e l e c t i v e r e g a r d i n g s i z e ; s i l t s as w e l l as c l a y being taken up by the f l o e s . Hence, the o r i g i n of the uniform texture observed i n some marine d e p o s i t s . 6. S t r u c t u r e o f Marine Clay. In S o i l Mechanics l i t e r a t u r e the term " s t r u c t u r e " i s used to denote the arrangement of the p a r t i c l e s i n a s o i l mass. Thus the s o i l s k e l e t o n may be r e f e r r e d to as having a s i n g l e - g r a i n e d , honeycomb, or a f l o c c u l e n t s t r u c t u r e , depending on the p r e v a i l -i n g g r a i n assemblage, T a y l o r (1948). With f i n e - g r a i n e d s o i l s i n mind, T. W. Lambe (1958) has extended the d e f i n i t i o n to read: " S t r u c t u r e means the arrangement of the s o i l p a r t i c l e s and the e l e c t r i c a l f o r c e s a c t i n g between adjacent p a r t i c l e s . , r The importance of the e l e c t r i c a l f orces i n promoting and m a i n t a i n i n g s t r u c t u r e i s thus- emphasized. 11 As noted e a r l i e r , marine c l a y s are sedimentary deposits r e s u l t i n g from the settlement of f l o e s on to the sea f l o o r . T h e i r formation i n a s a l i n e environment gave ready access to the d i s s o c i a t e d ions contained i n sea water. Consequently, the f i n e r p a r t i c l e s were enveloped i n r e l a t i v e l y t h i c k adsorbed l a y e r s . B u r i a l under f u r t h e r depths of sediment " t i g h t e n e d up*1 the f l o c c u l e n t s t r u c t u r e . With s u f f i c i e n t overburden pressure and time, the p a r t i c l e s may be v i r t u a l l y brought i n t o c ontact w i t h one another, over a t l e a s t p a r t o f t h e i r s u r f a c e s , Skempton and Northerley (1952). The str e n g t h c h a r a c t e r i s t i c s of such a deposit are determined by the degree of c o n s o l i d a t i o n i n the normal manner, Taylor (1948) . The s e n s i t i v i t y i s not abnormal; i t may be anywhere i n the range one to e i g h t . A d i f f e r e n t s i t u a t i o n a r i s e s however, i f f o r any reason the o r i g i n a l s a l t content of a marine deposit i s lowered. Leaching reduces the thicknesses of the adsorbed l a y e r s , w h i l e the water content remains almost unchanged; the volume o f the fr e e pore f l u i d i n creases at the expense of the a d s o r p t i o n complex, Skempton and Northey (1952) . This has a two-fold e f f e c t on the engineering p r o p e r t i e s : I t lowers the undisturbed s t r e n g t h , but more important, i t g r e a t l y increases the s e n s i t i v -i t y . At the same time, I t has been observed t h a t the A t t e r b e r g l i m i t s are reduced, Bjerrum (1954) . Leaching of n a t u r a l deposits i s accomplished by p e r c o l a t i o n of f r e s h water through the pores. In most cases, l e a c h i n g i s the r e s u l t o f the u p l i f t of marine deposits to above sea l e v e l . In Europe and North America, g e o l o g i c a l evidence i n d i c a t e s that 12. u p l i f t took p l a c e f o l l o w i n g the r e t r e a t o f P l e i s t o c e n e g l a c i a t i o n . The c l a s s i c a l example o f t h i s type o f development i s along the coast o f Norway. I n v e s t i g a t i o n of the Norwegian c l a y s by Bjerrum, i n d i c a t e s t h a t the l o s s of shear s t r e n g t h mentioned above, does not take place i n l i n e a r p r o p o r t i o n to the r e d u c t i o n i n s a l t c o n c e n t r a t i o n . Bjerrum r e p o r t s that even a decrease i n s a l t c o n c e n t r a t i o n from the o r i g i n a l 35 grams down to 10 - 15 grams per l i t r e , r e s u l t s only i n a n e g l i g i b l e r e d u c t i o n i n shear st r e n g t h . Further l e a c h i n g , however, r e s u l t i n g In s a l t concen-t r a t i o n s below 10 grams, produces a considerable decrease i n str e n g t h . The r e d u c t i o n i n undisturbed strength has taken place over a l o n g p e r i o d of time; t h e r e f o r e , except i n cases where f u r t h e r l e a c h i n g i s p o s s i b l e , t h i s r e d u c t i o n i n shear s t r e n g t h i s mainly o f academic i n t e r e s t . Of greater s i g n i f i c a n c e , i s the increase i n s e n s i t i v i t y f o l l o w i n g l e a c h i n g . "Quick" c l a y s , as they are c a l l e d , are among the most d i f f i c u l t s o i l s encountered by engineering p r o j e c t s . The e x p l a n a t i o n f o r the s e n s i t i v i t y can be found, i n p a r t a± l e a s t , from the s t r u c t u r e . Rosenquist has i n v e s t i g a t e d the s t r u c t u r e of Norwegian "quick" c l a y s . With the a i d of e l e c t r o n microscopy, he has shown that the s t r u c t u r e corresponds remark* ably cbse to a c o n f i g u r a t i o n proposed e a r l i e r by W. T. Tan. The essence o f the s t r u c t u r e i s that the edges of some p a r t i c l e s are v i r t u a l l y i n contact w i t h the f l a t surfaces of others to give a s o r t of card-house framework. F i g . 4. The meta-stable s t r u c t u r e , i l l u s t r a t e d i n F i g . 4, breaks down i f subjected to repeated s t r e s s e s or shock. Undisturbed samples of some marine deposits are so s e n s i t i v e t h a t they can To /oUotv f>aye /£ F I G 4. S T R U C T U R E O F M A R I N E C L A Y (A-f rer T.K. Tan. /9S7.) 13. be transformed by remoulding alone, from a f i r m c l a y , to one w i t h the consistency of a viscous f l u i d . The c u r r e n t i n v e s t i g a t i o n i s concerned w i t h the p r o p e r t i e s of a "quick" c l a y . As the I n v e s t i g a t i o n i s mainly concerned w i t h shear s t r e n g t h c h a r a c t e r i s t i c s , a b r i e f d i s c u s s i o n on s t r e n g t h theory f o l l o w s . 14 CHAPTER I I .  STRENGTH THEORY A. I n t r o d u c t i o n Foundation and earthwork engineering o f t e n r e q u i r e a knowledge of the behavior of s o i l s l o c a t e d below the water t a b l e . When e x t e r n a l loads are a p p l i e d to a submerged stratum, i n t e r n a l s t r e s s e s r e s u l t . P a r t o f the i n t e r n a l s t r e s s e s i s taken by the s o l i d c o n s t i t u e n t s , or s o i l s t r u c t u r e , while the i n t e r s t i t i a l medium ( f o r the most p a r t water) absorbs the remainder. The str e s s e s taken by the s o i l s k e l e t o n are known as e f f e c t i v e s t r e s s e s . Pore pressure, or n e u t r a l s t r e s s , i s a p p l i e d to the p o r t i o n taken by the " f r e e f l u i d s " ^ ^ occupying the pores. Thus pore pressures are p e c u l i a r to the i n t e r s t i t i a l water only, w h i l e e f f e c t i v e s t r e s s p e r t a i n s to the p a r t i c l e and i t s adsorbed l a y e r . (1) "Free f l u i d s " r e f e r s to the water,gases, e t c . , which are not i n t i m a t e l y h e l d to the surface of the p a r t i c l e s . I t i s considered here to i n c l u d e the double l a y e r water. 15 B. E f f e c t s of Pore Pressure 1, R e l a t i o n s h i p Between Pore Pressure and Shear Strength. The question a r i s e s as t o how the pore pressure e f f e c t s the s t r e n g t h p r o p e r t i e s . For the purpose of t h i s d i s c u s s i o n , no d i s t i n c t i o n w i l l be made between the various types of s t r u c t u r e and textures which may be found i n n a t u r a l s o i l s . G e n e r a l l y , p o s i t i v e pore pressures reduce the contact s t r e s s e s , whereas negative pressure leads to an increase i n the contact or e f f e c t i v e s t r e s s between p a r t i c l e s . The i n t e r s t i t i a l water i s capable of t a k i n g e i t h e r compression or t e n s i l e s t r e s s e s , but a l l shearing s t r e s s e s must be taken by the s o i l s k e l e t o n , Bishop and Henkel (1957). I f an e x t e r n a l l o a d i s to produce f a i l u r e , i t must overcome the shearing r e s i s t a n c e of the s o i l mass. Moreover, the l o a d must be capable o f m a i n t a i n i n g the deformation. Hence, there i s a d i s t i n c t i o n between the apprec-i a b l e amount of deformation t h a t may occur without causing f a i l u r e , and the prolonged deformation known as creep which may u l t i m a t e l y l e a d to f a i l u r e over a p e r i o d o f time. Elementary mechanics shows that the r e l a t i o n s h i p between the f o r c e necessary -to produce r e l a t i v e motion of two surfaces i n contact, i s a f u n c t i o n of the normal l o a d and the f r i c t i o n angle. The r e l a t i o n -ship i s expressed by the formula F 2 u t a n o i where F denotes the force r e q u i r e d to produce s l i d i n g , N corresponds to the normal f o r c e and cL denotes the f r i c t i o n angle p e c u l i a r to the m a t e r i a l s i n c o n t a c t . In S o i l Mechanics these concepts are 16. embodied In the well-known Coulomb-Terzaghi equation: s a c' + (p - u) tan 0» Eqn. ( 2 - 1 ) where s denotes the maximum r e s i s t a n c e to sheer on any place c' denotes the apparent cohesion ) I n terras ) of 0' denotes the angle of shearing ) e f f e c t i v e r e s i s t a n c e ) s t r e s s p denotes the t o t a l pressure normal to the plane considered, and u denotes the pore pressure. The above equation i s used i n most problems i n v o l v i n g the shear s t r e n g t h of s o i l s . However, an a n a l y s i s of the terms appearing i n the formula, i s of i n t e r e s t i n order to gain an understanding of i t s v a l i d i t y . (2) The s t r e s s e s ' (equation 2-1) are defined i n respect to a plane ( i n the geometrical sense) passing through the pore space and the p o i n t s o f contact of the s o i l p a r t i c l e s . S t r e s s e s , and areas, are then considered as p r o j e c t e d on to t h i s p lane. The e f f e c t i v e s t r e s s on the plane i s assumed to be represented by the term (p «* u) . A more exact expression f o r the e f f e c t i v e s t r e s s would be p - u ( l - a r ) , where a r denotes the contact area o f the p a r t i c l e s per u n i t area of the plane. A p r e c i s e e v a l u a t i o n of the e f f e c t i v e s t r e s s t h e r e f o r e , r e q u i r e s a knowledge of contact areas. In p r a c t i c e , d i r e c t measurement of the contact areas of a l l p a r t i c l e s on a s l i p plane, i s an extremely d i f f i c u l t problem. I n d i r e c t methods however, i n d i c a t e t h a t (1 - a p) i s i n f a c t (2) Compression s t r e s s e s considered p o s i t i v e , and t e n s i l e n e g a t i v e . 17. close to u n i t y f o r both sands and c l a y s , Bishop and E l d i n (1950). The v a l i d i t y o f the assumption t h a t the f r i c t i o n com-ponent of shear s t r e n g t h i s (p - u) tan jzS' i s based on t h i s t h i s o b s e r v a t i o n . The f r i c t i o n angle (#') i s known to depend p r i m a r i l y on the s i z e , shape, packing d e n s i t y ^ i n t e r l o c k i n g and m i n e r a l o g i c a l composition o f the p a r t i c l e s . Bishop and Henkel (1957) r e p o r t that fl1 a l s o depends somewhat on the r a t e o f s t r a i n ; high r a t e s tend to increase 0*. I n c l a y s , the decrease i n the value of tan <py Is about 5% f o r each t e n f o l d increase i n the d u r a t i o n of a shear t e s t * The cohesion appears In equation 2-1 as the component o f the shear s t r e n g t h independent of the normal s t r e s s . For any one cohesive s o i l the apparent cohesion depends on the water content, s t r e s s h i s t o r y and r a t e of deformation under l o a d . G e n e r a l l y , the cohesion increases as the moisture content i s lowered. P r e c o n s o l i d a t i o n leads to an increase i n cohesion be-cause the s o i l i s i n e q u i l i b r i u m w i t h a lower s t r e s s than the o r i g i n a l c o n s o l i d a t i o n pressures. High r a t e s of deformation tend to increase the observed cohesion, T a y l o r (1943). This i s due to the m o b i l i z a t i o n o f the viscous component of the i n t e r s t i t r i a l water. Consequently, the p r e f i x 'apparent 1 i s used, i n connection w i t h f r i c t i o n angles and cohesion, i n order to s i g n i f y the dependence o f these parameters on f a c t o r s which are not n e c e s s a r i l y b a s i c s o i l p r o p e r t i e s . L a t e r , the more fundamental p r o p e r t i e s of true f r i c t i o n angle and true cohesion w i l l be con-s i d e r e d . 18 2. Relationship Between Compressive Strength and Pore Pressure. The shearing resistance on any plane may be obtained from the Coulomb-Terzaghi equation provided the pore pressure, strength parameters and normal stress on the plane are known. However, the t o t a l stresses on the p r i n c i p a l planes are more re a d i l y accessed i n prac t i c e , therefore equation 2-1 w i l l be derived i n terms of the t o t a l p r i n c i p a l stresses. Gne purpose of t h i s i s to emphasize the r o l e played by pore water pressure i n the engineering performance of the s o i l . The re l a t i o n s h i p can be conveniently derived from the Mohr diagram, Pig* 5» In F i g . 5 the t o t a l p r i n c i p a l stresses at f a i l u r e are represented by o~, and cr3 w / The compressive strength i s then (C~, — 6 j ) ; PGH represents the corresponding Mohr c i r c l e . For this system of t o t a l stresses, l e t the e f f e c t i v e stresses be represented by (T, and (T3. The Mohr c i r c l e P'G'H' associated with the l a t t e r w i l l be located to the l e f t by a distance corres-ponding to the pore pressure (u). Assuming the f a i l u r e envelope for e f f e c t i v e stress i s represented by the l i n e AB, then the c i r c l e P'G'H' w i l l touch AB at f a i l u r e . The shear stress at f a i l u r e ( 7 ? ) i s represented by OL. It Is required to express the compressive stress ((T, - <3}) i n terms of the strength parameters ( c f , 0') and the pore pressure u. (3) In t r i a x i a l test <?, i s t o t a l a x i a l stress and (T3 repre-sents the c e l l pressure or the confining s t r e s s . In a natural s o i l deposit <?, represents v e r t i c a l stress on an element, and O j » K.0~, where K = c o e f f i c i e n t of l a t e r a l earth pressure. 19. Prom the geometry of the diagram i t follows that ^ _ (c< - 0"? ) cos cf>' _ (<r, - Q-3 ) Also 7/- = [( 0~3 - u ) + c~i - CM Q~> - Q a s l n 0' J tan 0' + e«. (<J7 - 01) cos^>' z ( CT^ - u) tan <f>' + 2 (6T - O 3 ) tan - (CJ7 - 6 3 ) s i n ^' tan + C . 2 2 (CH - 0~3) (cos $ - tan 0' •+• s i n <p' tan ^ ) s c' + (0^ • u) tan0' . 2" . .. or (CT - o p - e* + (0"3 • tan <p' ^ cos </>' 2 cos <p' - tan <p' + sin<£' tan cos <p' r c' cos + (0~3 - u) s i n ^ ' cos 20' - s i n 0' (1 - sin^') Since s i n a^'+ co&z<f>' r 1 (CT, - (r 3) g ? f c « cos <f>' -f ( C T 3 - u) s i n ^  ] E q n # 2 . 2 I 1 - s i n <p' J Furthermore (CT, - 0"3) r 2 c 1 °°g 0' + 2(£sin0' - 2 s i n ft' u 1 - sinq6' 1 - sin0' 1 - s i n ^ ' sgfc' cps <[>' -+ o~3 s i n <f>' I - 2,( s i n 0' )„ I - 1 - s i n <P' J 1 - s i n ft' ( C T , - 6~3) : Y - Z u where Y and Z are constants f o r any one value of o~3 Equation 2-2 et sequo shows that the compressive strength comprises a constant term minus a function o f the pore water pressure. The measured compressive strength i s therefore l a r g e l y determined by the pore pressure. To follow page IS £ff ec rive Stresses Total Stresses Nor m a.L St resses Fig 5. Mohr Diagram . Tot ai <xncL E fre ctive Stresses • V. a Normal. Stresses Fig 6 . Mohr Diagram : Tofai Stresses F I G S . 5 a n d 6 20. The 'apparent* s t r e n g t h parameters c' and 0' are obtained i n the l a b o r a t o r y , from a s e r i e s o f t r i a x i a l t e s t s w i t h pore pressure measurements. I n d i v i d u a l specimens are f i r s t c o n s o l i d a -ted a t d i f f e r e n t c e l l pressures (corresponding t o 6~3)» Shear t e s t s are then performed on the specimens, which i n general w i l l have d i f f e r e n t moisture contents due to the d i f f e r e n c e s i n c o n s o l i d a t i o n pressures. The higher shear strengths w i l l normally be obtained from the specimens w i t h low moisture contents and high c o n f i n i n g p r essure. By p l o t t i n g the e f f e c t i v e s t r e s s c i r c l e s on a Mohr diagram s i m i l a r to P i g . %. the f a i l u r e envelope can be e s t a b l i s h e d . The slope of the envelope y i e l d s the 'apparent' f r i c t i o n angle ( 0 ' ) , while the i n t e r c e p t o f the envelope on the Y a x i s gives the 'apparent' cohesion ( c ' ) . g». T o t a l S t r e s s Parameters. Two other 'apparent' parameters, c u and </u are sometimes quoted. These are obtained from the envelope o f the Mohr c i r c l e s f o r t o t a l s t r e s s e s a t f a i l u r e . An example o f t h i s type o f p l o t i s shown i n P i g . 6. As remarked e a r l i e r , the magnitude of the 'apparent' parameters depends on the s t r e s s h i s t o r y and the r a t e of deform-a t i o n . Consequently, i n an e f f o r t to obviate t h i s dependence, the i d e a of true f r i c t i o n angle and true cohesion made i t s appearance i n S o i l Mechanics l i t e r a t u r e * 4. True F r i c t i o n Angle and True Cohesion. So f a r , the s t r e n g t h p r o p e r t i e s have been expressed i n terms of the 'apparent' cohesion and f r i c t i o n angle. For most 21 purposes these are s u f f i c i e n t ? b u t a more fundamental approach i s d e s i r a b l e i f the b a s i c s o i l p r o p e r t i e s are to be e l u c i d a t e d . I t has been suggested, Shempton and Bishop ( 1 9 5 4 ) that the true f r i c t i o n angle and true cohesion can be obtained under c e r t a i n c o n d i t i o n s which w i l l be discussed p r e s e n t l y . The d i s c u s s i o n a p p l i e s to cohesive s o i l s only. The concept of true f r i c t i o n and true cohesion i s - b a s e d on Hvorslev's c o n t e n t i o n t h a t the cohesion should be a f u n c t i o n of the water content only, and t h a t the f r i c t i o n angle i s a f u n c t i o n of any increase In s t r e n g t h w i t h increase i n e f f e c t i v e s t r e s s at constant water content. I t i s p o s s i b l e to have two samples of a s o i l at i d e n t i c a l water contents ( v o i d r a t i o s the same i f saturated) but In e q u i l i b r i u m w i t h d i f f e r e n t e f f e c t i v e s t r e s s e s . This can be seen by r e f e r r i n g to the r e s u l t s o f a conventional c o n s o l i d a t i o n t e s t on a normally c o n s o l i d a t e d c l a y . P i g . 7. I t i s evident from F i g . 7 that the s t a t e corresponding to p o i n t X on the l o a d i n g curve i s i n e q u i l i b r i u m w i t h c o n s o l i -d a t i o n pressure p-j, and that the v o i d r a t i o i s e^ . At p o i n t Y on the unloading curve the sample has been p r e c o n s o l i d a t e d to the extent of pressure pg but i s i n e q u i l i b r i u m w i t h p^. (p-^ Pg p^ are e f f e c t i v e s t r e s s e s ) . The v o i d r a t i o corresponding to p r e s s -ure p^ i s e^ a l s o , t h e r e f o r e , the v o i d r a t i o s are I d e n t i c a l but the e f f e c t i v e s t r e s s e s d i s s i m i l a r . Assuming s a t u r a t i o n , the cohesion w i l l then have the same magnitude f o r s t a t e s represented by X and Y. I f shear t e s t s w i t h pore pressure measurements are c a r r i e d out on two samples, whose c o n s o l i d a t i o n h i s t o r i e s correspond to To foLLow page 2/ Consolidaf/qn Pressure F I G 7- C O N S O L I D A T I O N H I S T O R I E S Effective Stress <xt F<xi L u r e F I G 8 . M O H R D I A G R A M . T R U E P A R A M E T E R S F I G S 7 a n d 8 22. conditions represented by points X and Y. P i g . 7, the Mohr diagram of e f f e c t i v e stresses would resemble F i g . 8. Provided that a rate o f s t r a i n i s chosen which Is slow enough to minimize viscous e f f e c t s , the true f r i c t i o n angle <f>r and true cohesion c r are obtained by the method indicated i n Pig. 8. The f a i l u r e envelope of the two Mohr c i r c l e s can be represented by the equation: 7> r e r + (CT- u) tan ^ where 7/: ~ shear stress on the plane of f a i l u r e 0~ - t o t a l normal stress on the same plane u • pore pressure cp - true cohesion <p - angle of true Internal j At the water con-f r i c t i o n j tent at f a i l u r e By analogy with equation 2-1 the compressive strength i s given by: (4) ( 0 7 - 0 3 ) = 2 f e r cos + (0"]j - u) s i n j [ 1 - s i n j where 0", and 0~3 are the major and minor t o t a l p r i n c i p a l stresses at f a i l u r e r e s p e c t i v e l y . The performance o f such tests presents experimental d i f f i c u l t i e s , due to the requirement that the specimens have Identical moisture contents but d i f f e r e n t stress h i s t o r i e s . An al t e r n a t i v e procedure f o r obtaining i s to measure the i n -c l i n a t i o n of the shear plane at f a i l u r e . The angle of i n c l i n a t i o n (4) (<T, - 0~ ) i s equivalent to (07 - (T3) — devlator s t r e s s . 23. of the shear plane to the plane of major p r i n c i p a l stress i s given b y 0 r 4 5 + 2 Not a l l samples, however, f a i l on a single shear plane. There-fore, this approach i s not fea s i b l e i n a l l cases. End r e s t r a i n t produced by loading caps also a f f e c t s the angle The true parameters have a s i g n i f i c a n t c o r r e l a t i o n with the p l a s t i c i t y index and mineralogical composition of clays. Test r e s u l t s reported by Skempton ( 1 9 5 4 ) indicate the order of magnitude o f the true and apparent f r i c t i o n angles. Table I I . C . Factors Affee ting Pore Pressure: The Pore- Pressure Parameters, A and B. The magnitude of the pore pressure developed i n a stressed s o i l mass depends primarily on two factors: (a) The compressibility of the s o i l skeleton. (b) The consituents o f the f l u i d occupying the pore space. In order to i l l u s t r a t e the dependence on the above factors, the s o i l i s assumed to behave as an e l a s t i c isotropic m a terial. The v a l i d i t y of such an assumption f o r the case of r e a l s o i l s i s discussed l a t e r . Consider a small cube AB o f e l a s t i c material stressed i n the manner indicated i n the following sketch: where <T~X, o~y and o~ are compressive stresses o f equal magnitude. j SOIL TYPE ' LIQUID LIMIT ' PLAS TIC 1 INDEX ' ACTIVITY 1 MOISTURE CONTENT RANGE 1 TRUE ' FRICTION APPARENT j 1 FRICTION j ' ANGLE j j Shellhaven Clay 1 « undisturbed. \ 123 ; 87 \ 1*42 ] 52~60# \ 18° ! 23° j TABLE I I . PROPERTIES OF SHELLHAVEN CLAY. 2 5 . cr (Tyy y z. From e l a s t i c theory and the p r i n c i p a l s t r a i n s € xx>-£"yy a n d € 2 Z are r e l a t e d by: £ zz » € y y s ^ z z = £" (1 - 2//) E where yt/ - Poisson's r a t i o E - Young's Modulus. I f L i s the o r i g i n a l l e n g t h of a s i d e of the cube and (L - AL) the s t r a i n e d l e n g t h , then the s t r a i n p a r a l l e l to any one a x i s of the co-ordinates i s : - A L = £ L xx = - £ (1 - 2/>) E Hence: A L = L 6~ (1 - 2 />) i " The volume of the compressed cube i s then: (L - A L ) 3 a f l - £" (1 - 2 / ^ ) | 3 E = 1? j l - 3 _ f (1 - 2 f) j f o r small value o f AL. 26. - 3 Since the o r i g i n a l volume of the cube i s V s L , the change In the volume (-AV) i s given by: -AV = L 3 - (L - A L ) 3 - A V - L 3 r 3 j r ( i - 2 / 0 - I s or - AV ^ V [30~(1 -ap) 1 IE S i m i l a r l y i f ( T n ^=0~jj ± (Tzz Volume change: - A V r Vj" 1 - 2f(ff„ + 0~yy + <7~z^ (1) Turning now to a s o i l mass subjected to incremental changes i n the t o t a l stresses on p r i n c i p a l planes - equivalent to ACT, AGgand A 0 ~ 3 the rel a t i o n s h i p s between the changes i n t o t a l and eff e c t i v e stresses are given by: ACJ1 = A O T - A U ; (a) A ? 2 =A0"2-4u j (b) &<T5 = A C T J - A U ; (c) Where ACT, 7 Ad^, and A0" 3 represent the change l n e f f e c t i v e stresses on the p r i n c i p a l planes — A U denotes the pore pressure change. Then from expression (1) the decrease i n volume ( A V ) of the s o i l skeleton i s approximated by: -AV = v u _ - _ 2 £ ) , + £ara t A ^ ) (2) E where and E are Poisson's r a t i o and Young* s modulus respectively f o r the s o i l skeleton. The decrease i n the volume o f the s o i l skeleton i s almost (5) e n t i r e l y due to a decrease i n the volume o f the voids. I f the I n i t i a l p orosity i s denoted b y i ? , ^ and C w the compress-i b i l i t y o f the pore f l u i d , the volume change (assuming no drainage occurs} i s given by: -AV - V 7 C w A U (3) Combining equations (2) and (3) rj C „ * u . 1 - I A F F . + A ( R I + A ( R I ) ( 4 ) E ( 7 ) For the case where A ( T 2 r A<X 3 expression (a) (b) (c) can be written: A C T ; +- A U = A c r 3 ( Acr, - A C T 3 ) A O " 2 + A U = A C T 3 (5) Compressibility of the s o i l grains i s n e g l i g i b l e . (6) Porosity 77 s Volume of Voids ' T o t a l Volume (7) T r i a x i a l test and most p r a c t i c a l problems A6~Z = AG~3 2 8 . By addition: (ACT, + A0~£ + 4 0~3) + 3 A U = 3AtX 3 +-(A(71 - A<r3) P r o m ( 4 ) Y] C w 1 - 2 ^ E 3 ACT, + ( ACT, - AG\) - 3 A U | I f the compressibility o f the s o i l skeleton f o r u n i t stress change i s denoted by C c, then: G C r 3 ( 1 - 2 / 0 E Introducing C c, rearranging the terms and dividing by 3 , the change i n pore pressure f o r an all-round change In stress Is given by the expression: A u * 1 + T £ ACT, + =-3 ( A C T , - ACT3) ( 5 ) The term outside the bracket i s known as the pore pressure parameter B, Bishop and Henkel ( 1 9 5 7 ) . The c o e f f i c i e n t j- i n Exp. ( 5 ) only applies, of course, i n the case/an i d e a l i z e d e l a s t i c s o i l . Real s o i l s are not even approximately e l a s t i c , therefore, i t i s necessary to replace the c o e f f i c i e n t of the deviator stress by a parameter A. Equation ( 5 ) then becomes: z i u = B [ A(T3 + A( ACT, - A 0 ~ 3 ) J (Eqn. 2 - 3 ) 29. The value of the parameter A at f a i l u r e ranges from about -0.1 f o r normally consolidated s o i l s to about 1.3 f o r pre-consolidated c l a y s , Bishop and Henkel (1957). IFer f u l l y saturated s o i l s the value of C w - that ©f water alone - i s so small that B r 1. When the pore water contains a i r and other gases, the value of B i s l e s s than unity, but depends on the stress range; B approaches u n i t y as the t o t a l stresses increase. Where ACT, = ACT^ as i s the case i n b u i l d -up of pore pressure at the i n i t i a l stages o f a t r i a x i a l shear tes t , B i s given by the expression B s A U wherezm denotes A 0 " 3 the change i n pore pressure, and A<T3 denotes the change i n c e l l pressure. 30. CHAPTER I I I . APPARATUS: DEVELOPMENT AND OPERATION A * T r i a x i a l Shear Tests with Pore Pressure Measurements. 1. General. The t r i a x i a l apparatus used i n the i n v e s t i g a t i o n was designed to accommodate specimens o f 2-g-" diameter w i t h a l e n g t h up to 6 n. Previous r e s e a r c h ^ ^ i n d i c a t e s that best r e s u l t s are obtained w i t h a l e n g t h to diameter r a t i o o f 2:1* A l l r e s u l t s r e p o r t e d here are f o r specimens 2jr n diameter and 5 n l o n g . Excessive b u c k l i n g of the specimen during t e s t i s thus avoided. D i r e c t l o a d i n g was used because the s t r e s s remains v i r t u a l l y constant f o r any l o a d increment. This enables the development of pore pressure to be compared w i t h the incremental change i n a x i a l s t r e s s . A l s o t h i s t e s t set-up c l o s e l y corresponds to the l o a d i n g of s o i l s i n p r a c t i c e , where the s o i l i s normally allowed to deform at w i l l under an almost uniform s t r e s s . Changes i n cross s e c t i o n a l area, i n the course of a t e s t , w i l l tend t o reduce the a x i a l s t r e s s e s , but the magnitude o f such changes w i l l not (1) Bishop and Henkel (1957). 31. be very great, i f the specimen f a i l s at small s t r a i n . I t i s more convenient to measure the pore pressure at the top, or at the base o f a specimen, than at intermediate p o i n t s i n i t s l e n g t h . I t was f e l t , however, t h a t end r e s t r a i n t may have a bear i n g on the observed r a t e of pore pressure development. Therefore, some t e s t s were run w i t h pore pressure measurements at or about the centre o f the specimen. 2. S t r e s s - C o n t r o l l e d T r i a x i a l Apparatus The l o a d i n g apparatus i s shown i n P i g . 9. The frame i s made o f 2" x 2" aluminum box s e c t i o n , b o l t e d down to a 30 n x 1" x •§" t h i c k aluminum base. A wooden bench supports the frame. P r o v i s i o n i s made f o r l e v e l l i n g the frame, by the I n c o r p o r a t i o n of a d j u s t a b l e screws i n the bench l e g s . The l o a d i n g yoke i s c o n s t r a i n e d to move i n a v e r t i c a l plane by guide t r a c k s f i x e d t o the frame u p r i g h t s . A counter-balance system keeps the yoke and p r o v i n g r i n g i n a " f l o a t i n g " p o s i t i o n , thereby reducing to a minimum the i n i t i a l l o a d on the specimen. The t r i a x i a l c e l l i s mounted on a c e n t e r i n g column. With the c e l l i n p o s i t i o n , the u n i t was a l i g n e d by means of a surveyor's t h e o d o l i t e . A x i a l l o a d i n g or as n e a r l y so as p o s s i b l e , i s thus obtained. Although the l o a d i n g yoke i s f i t t e d w i t h r o l l e r bearings and the counter* balance weights are suspended from low f r i c t i o n p u l l e y s , the proving r i n g i s Incorporated f o r the purpose of e l i m i n a t i n g f r i c t i o n e r r o r s from the estimated l o a d . The loads are a p p l i e d to the yoke v i a the shackle and pan, l o c a t e d beneath the bench. Jo follow p&je Jl F I G 1 0 . T R I A X I A L C E L L 32. 3. The T r i a x i a l C e l l The pressure chamber of the t r i a x i a l c e l l c o n s i s t s of a " l u c i t e " c y l i n d e r (^tt w a l l thickness) capped top and bottom by 3/4" t h i c k p l a t e s , P i g . 10. The c y l i n d e r i s seated on grooves In the p l a t e s and i s sealed by s y n t h e t i c rubber washers. Clamping down b o l t s are f i t t e d w i t h wing nuts f o r easy assembly. The l o a d i n g plunger i s i n s e r t e d through a brass bushing i n the upper p l a t e . I n order to minimize f r i c t i o n , and at the same time prevent excessive leakage, the s t a i n l e s s s t e e l plunger i s f i n i s h e d to give a clearance of 0.0003 inches. E x t e r n a l loads f o r the plunger are t r a n s m i t t e d to the specimen by the l o a d i n g cap. The lower end of the plunger i s machined to a hemispherical shape which r e g i s t e r s i n a c e n t r a l coned s e a t i n g i n the cap. The cap i s f r e e to t i l t through angles up to 10 degrees to the h o r i -z o n t a l . Greater tendency to t i l t i s prevented by a guide which forms an i n t e g r a l p a r t of the cap. A p e d e s t a l , which can be s l i p p e d over the c e n t e r i n g column of the l o a d i n g frame, protrudes through the lower p l a t e o f the chamber. The base f o r the specimen i s s c r e w - f i t t e d to the top o f the p e d e s t a l . Both cap and base have shallow r a d i a l grooves, on the specimen s i d e , which l e a d to drainage o u t l e t s . Two o u t l e t s are provided i n the top cap and there i s one c e n t r a l o u t l e t i n the base. One o u t l e t i n the cap may be connected to e i t h e r the pore pressure apparatus or to a vacuum pump, depending on the requirements o f the t e s t . The other l e a d i n the cap and the l e a d from the base are intended f o r drainage purposes. "Saran" 33. tubing i s used f o r a l l l e a d s . Plow i n the tubing i s c o n t r o l l e d by p i s t o n valves l o c a t e d outside the c e l l . The o p e r a t i o n of t h i s type of valve does not introduce undesirable volume changes any-where i n the drainage system. Drainage from the specimen i s measured by means of two b u r e t t e s . A i r e x p e l l e d from unsaturated samples i s measured i n an i n v e r t e d U-tube f i t t e d i n the l i n e to the base b u r e t t e . 4. L a t e r a l Pressure C o n t r o l The c o n t r o l of chamber pressure to the degree of p r e c i s i o n demanded i n t r i a x i a l t e s t i n g , i s not an easy matter. This i s p a r t i c u l a r l y true i n the case of long d u r a t i o n t e s t s . Commercial pressure r e g u l a t o r s u s u a l l y employ a spring-loaded diaphragm. This mechanism i s prone t o i n s t a b i l i t y , en d In the absence of a u x i l i a r y equipment cannot be used f o r p r e c i s e pressure c o n t r o l over l o n g periods of time. A number of systems have been devised to meet the problem. Most, however, r e q u i r e elaborate i n s t r u m e n t a t i o n . Two systems commonly used, one developed at I m p e r i a l C o l l e g e , London, and bhe other a t the Norwegian G-eotechnical I n s t i t u t e , Oslo, have proved s a t i s f a c t o r y under c e r t a i n c o n d i t i o n s . The method used at I m p e r i a l College employs a self-compensating mercury manometer, one limb of which can be r a i s e d , to provide the r e q u i r e d pressure head. A drawback to t h i s equipment i s t h a t i t r e q u i r e s c o n s i d e r -able head-room i n the l a b o r a t o r y , i f s u f f i c i e n t l y h i g h pressures are t o be obtained. I f head-room i s l i m i t e d , the apparatus must 3 4 . be d u p l i c a t e d , which adds to the co s t of the i n s t a l l a t i o n . The Norwegian apparatus i s e s s e n t i a l l y a h y d r a u l i c system, A ram loaded by means of dead weights maintains the pressure i n an o i l f i l l e d c y l i n d e r . The c y l i n d e r must be c a r e f u l l y a l i g n e d other-wise f r i c t i o n e r r o r s a r i s e . Leakage o f o i l past the ram and the l i m i t e d weight c a p a c i t y are the main disadvantages. Both the above systems are, t h e r e f o r e , most convenient f o r low c e l l p r e s s u r e s t For i n v e s t i g a t i o n s , l i k e the one r e p o r t e d here, pressures up to 100 pounds per square i n c h are d e s i r a b l e . To o b t a i n pressures of t h i s magnitude, a diaphragm-type r e g u l a t o r was used f o r the main c o n t r o l , A method of c o u n t e r a c t i n g pressure f l u c t u a -t i o n s was devised. The m o d i f i c a t i o n c o n s i s t s of a l l o w i n g a con-tinuous b l e e d i n g o f a i r to the atmosphere from the main r e g u l a t o r . This b l e e d i n g i s c o n t r o l l e d by a f i n e adjustment a u x i l i a r y r e g u l a t o r . The l a t t e r i s f i t t e d downstream of the main c o n t r o l . Using t h i s method, r e g u l a t i o n b e t t e r than 0,15 pounds per square i n c h was obtained f o r the medium and high c e l l p ressures. At pressures below 15 pounds per square i n c h , the system i s l e s s e f f i c i e n t ; presumably t h i s i s due to the lowered momentum of the a i r , r e n d e r i n g the a u x i l i a r y r e g u l a t o r i n e f f e c t i v e , A r e s e r v o i r , f e d from an a i r compressor, maintains the d e s i r e d p r essure. The c a p a c i t y of the r e s e r v o i r i s l a r g e , compared w i t h p o s s i b l e volume changes i n the specimen, or leakage from the c e l l . Deaired water was used as the chamber f l u i d f o r a l l t e s t s r e p o r t e d i n t h i s t h e s i s , I n order to minimize flow through 35. p r o t e c t i v e membranes. A Bourdon gauge ( t o t a l range 0-100 l b . / sq.in.) i n d i c a t e s the pressure. A deduction of 2»5 l b . / s q . i n . from the gauge r e a d i n g i s necessary to a l l o w f o r the l o s s o f head between the r e s e r v o i r and c e l l . The Bourdon gauge was checked against a standard gauge t e s t e r . A random discrepancy of not greater than 0.3 l b . / s q . i n . was observed In t h i s guage. The c e l l pressure may be r e l i e v e d by opening the needle valve l o c a t e d In the upper p l a t e of the chamber. The c e l l p ressure, a c t i n g on the plunger, decreases the l o a d on the specimen to values lower than those r e g i s t e r e d by the p r o ving r i n g . The c o r r e c t i o n to be a p p l i e d to the r i n g d e f l e c t i o n , i n order to o b t a i n the a c t u a l l o a d , i s shown i n P i g . 15. 5, Load and Deformation Measuring Devices. A d i a l gauge, reading t o 0,0001" i s used f o r i n d i c a t i n g the p r o v i n g r i n g d e f l e c t i o n . For loads i n the range 5-220 l b s , the r i n g constant K « 0.444 lbs./0,0001" d e f l e c t i o n . The deformation of the specimen i s measured by another d i a l gauge (0.001"/div) set between the lower clamp on the proving r i n g and the upper p l a t e of the chamber, 6. Apparatus f o r Measuring Pore Pressure. In undrained t r i a x i a l t e s t s , I t i s e s s e n t i a l that moisture changes i n the specimen be prevented during the l o a d i n g stage. Consequently, any apparatus used f o r measuring pore pressures must be capable of o p e r a t i n g on a minimum of pore water movement. The u s u a l l a b o r a t o r y methods o f measuring pressure - the mercury 36 manometer and the Bourdon gauge - cannot be a p p l i e d d i r e c t l y t o the measurement of pore pressure, owing to the volume of pore water which would have to flow from the specimen to cause the instrument to r e g i s t e r . For l a r g e specimens, a transducer-type apparatus may be used. This device measures the d e f l e c t i o n of a metal diaphragm by means o f e l e c t r i c a l s t r a i n gauges. Changes i n h y d r o s t a t i c pressure produce d e f l e c t i o n s o f the diaphragm which lends i t s e l f to pore pressure a p p l i c a t i o n s , Plantima (1953)• This method departs somewhat from the no-flow c o n d i t i o n . Permanent moisture changes can be e n t i r e l y avoided, however, by the use of the n u l l method o f pressure measurement o r i g i n a l l y devised by Rendulic (1937)• The apparatus used i n the present I n v e s t i g a t i o n employs the l a t t e r system. The method adopted f o r the measurement of pore pressure i s e s s e n t i a l l y that developed at Imperi a l C o l l e g e , London. I t i s eq u a l l y e f f i c i e n t f o r a l l specimen s i z e s . The apparatus and procedure are des c r i b e d i n d e t a i l by Bishop and Henkel i n t h e i r book "The T r i a x i a l Test™. Therefore, only a b r i e f d e s c r i p t i o n w i l l be given here. Main f e a t u r e s o f the apparatus are a n u l l i n d i c a t o r , a c o n t r o l c y l i n d e r , and a Bourdon gauge coupled to a manometer, F i g . 13. The n u l l I n d i c a t o r employs a mercury column i n a glass c a p i l l a r y tube. This column i s maintained throughout the dura-t i o n o f the t e s t a t a predetermined l e v e l (the n u l l p o s i t i o n ) i n the c a p i l l a r y tube. Any pore pressure developed i n the specimen 37. i s brought to act on the upper surface of the mercury. Changes i n pore pressure tend to d i s l o c a t e the mercury maniscus from the n u l l p o s i t i o n . The c o n t r o l c y l i n d e r i s used to r e s t o r e the mercury column to the i n i t i a l p o s i t i o n , and a t the same time provide f l u i d to actuate the Bourdon gauge ** manometer u n i t . I n t h i s manner, drainage from the specimen Is prevented, and the pore pressure i s r e g i s t e r e d on the Bourdon gauge or the manometer. The Bourdon gauge i s used f o r i n d i c a t i n g pressures higher than atmospheric pressure (considered p o s i t i v e ) . Before i t was put i n t o s e r v i c e , the gauge was checked a g a i n s t a standard gauge t e s t e r w i t h which i t agreed, w i t h i n the l i m i t s of accuracy of r e a d i n g the d i a l s . Pore pressures below atmospheric pressure (negative) are i n d i c a t e d by the manometer. The manometer may be used a l s o f o r c e l l pressures up t o 20 l b . / s q . i n . Pore pressures w i t h i n the range -15 to + 100 l b . / s q . i n . can be measured w i t h t h i s i n s t a l l a t i o n . Changes i n pressure o f 0,1 l b . / s q . i n . can be detected. I t i s o f the utmost importance that the system be completely f i l l e d w i t h water and f r e e from l e a k s . To f a c i l i t a t e the r e -moval of a i r from the various tubes and f i t t i n g s , a vacuum i s a p p l i e d . F r e s h l y b o i l e d water i s then f l u s h e d through the apparatus, u n t i l a l l trapped a i r i s taken i n t o s o l u t i o n by the water as i t c o o l s . F i n a l l y , deaired water i s pumped i n t o the system. The mercury trough (a p a r t o f the n u l l i n d i c a t o r u n i t ) can be lowered, thereby a l l o w i n g water to be pumped from the c o n t r o l c y l i n d e r to the l o c a t i o n i n the specimen, where the measurement of pore pressure i s d e s i r e d . The l a t t e r f e a t u r e i s , To foLLow p<xje 3 7. F I G 15. P O R E - P R E S S U R E A P P A R A T U S 38. perhaps, the greatest s i n g l e advantage o f t h i s apparatus; i t ensures l i q u i d to l i q u i d c o n t i n u i t y between the pore water and the measuring gauges. The apparatus, as used at Imperi a l C o l l e g e , measures the pore pressure at the upper, or lower, ends of the specimen. I n the l a t t e r stages o f the present i n v e s t i g a t i o n , t h i s method was mod i f i e d i n order t h a t measurements may be obtained anywhere on the l o n g i t u d i n a l a x i s of the specimen. The only a l t e r a t i o n to the apparatus, c o n s i s t s o f the i n c o r p o r a t i o n of a porous probe (sometimes c a l l e d a p i l o t ) of the type developed at the Massachu-s e t t s ' I n s t i t u t e o f Technology, Lambe (1951). The probe i s i n s e r t e d i n the specimen, pore pressure being measured i n the regi o n of the t i p . 7. Automatic C o n t r o l . The pore pressure apparatus, discussed i n the preceding paragraphs, r e q u i r e s the f u l l time a t t e n t i o n of an operator. The main duty of the operator i s to m a i n t a i n the mercury column at the n u l l p o s i t i o n - by manually a d j u s t i n g the c o n t r o l c y l i n d e r . On l o n g d u r a t i o n t e s t s t h i s can be tedious and time-consuming. Consequently, the development of an automatic c o n t r o l was under-taken. The system of automatic c o n t r o l f i n a l l y adopted operates i n c o n j u n c t i o n w i t h the e x i s t i n g pore pressure apparatus. D e t a i l s of the new device are presented i n Chapter VT of t h i s t h e s i s . 8. F a b r i c a t i o n of Membranes. The specimen i n a t r i a x i a l t e s t must be p r o t e c t e d a g a i n s t the ingress o f chamber f l u i d by some form of f l e x i b l e membrane* 39. The most appropriate method of p r o t e c t i o n i n l o n g d u r a t i o n t e s t s was the subject of an extensive i n v e s t i g a t i o n conducted by Casagrande and Wilson (1949) a t Harvard U n i v e r s i t y . The outcome of t h i s r e s e a r c h p o i n t s to the d e s i r a b i l i t y o f s e a l i n g the specimen I n a j a c k e t comprising an inner and outer membrane w i t h a l a y e r of hydrophobic compound between the sheathes. In accordance w i t h these recommendations s p e c i a l membranes were f a b r i c a t e d f o r the present t e s t i n g program. The membranes were formed by d i p p i n g a wooden mandril i n rubber l a t e x emulsion. The surfaces of the mandril were p r e t r e a t e d w i t h s i l i c o n e grease and c a s t o r o i l ; i n order to s e a l the wood and f a c i l i t a t e removal of the f i n i s h e d membrane. Each c o a t i n g o f l a t e x a p p l i e d , was allowed to a i r dry f o r a t l e a s t eight hours. The membranes were given about ten dips to o b t a i n a w a l l thickness o f 0.03 inches. Two s i z e s were r e q u i r e d , the outer membrane was formed on a 2.55 i n c h diameter m a n d r i l , whereas the m a n d r i l f o r the Inner membrane was 2.40 inches i n diameter. The membranes are normally soaked In water before use, which has a tendency to produce s t r e t c h i n g , hence the $ a n d r i l diameters f o r the 2.5 i n c h specimen s i z e . Advantage can be taken, w i t h t h i s method o f f a b r i c a t i o n , to i n c l u d e sleeves which are used f o r the a i r - t i g h t s e a l a t the p o i n t o f entrance o f the pore-pressure probe. One sleeve was cast as an i n t e g r a l p a r t of the inner membrane. Another sleeve used w i t h the outer membrane was c a s t s e p a r a t e l y . The u t i l i z a t i o n of membranes possessing the above f e a t u r e s , i s shown i n the photographic supplement to t h i s t h e s i s . 40. The measured compressive s t r e n g t h of the specimem must be c o r r e c t e d , to a l l o w f o r the e f f e c t s of the p r o t e c t i v e j a c k e t . The c o r r e c t i o n to be a p p l i e d can be estimated from the deforma-t i o n c h a r a c t e r i s t i c s of the membranes. A s t r e s s / s t r a i n curve f o r membranes formed o f "Aerotex" rubber l a t e x (used throughout the I n v e s t i g a t i o n ) i s shown i n P i g . 12. Assessment of the c o r r e c t i o n i s d i s c u ssed i n Appendix I I . 9. P r e l i m i n a r y T e s t i n g o f Apparatus Before the performance of any s o i l t e s t s , the apparatus was put through the f o l l o w i n g p r o v i n g t r i a l s : A dummy specimen, made of s t e e l , was set up i n the t r i -a x i a l c e l l . Two membranes, w i t h a f i l m of c a s t o r o i l between, p r o t e c t e d the s t e e l block from the chamber f l u i d ( i n t h i s case, deaired water). Rubber bands were used to s e a l the membranes to the end f i t t i n g s . A chamber pressure o f 50 l b . / s q . i n . was a p p l i e d f o r a p e r i o d o f 72 hours. At the end o f t h i s time, the block was c a r e f u l l y removed from the c e l l and examined f o r any evidence of leakage through the membranes or end f i t t i n g s . No trace o f water was observed, so i t was concluded that the p r o t e c t i v e measures were adequate. The d e a i r i n g o f the pore pressure apparatus passed the t e s t p r e s c r i b e d by Bishop and Henkel (1957). The f u n c t i o n i n g o f the c e l l pressure c o n t r o l was a l s o observed; i t was found to be f r e e from undesirable f l u c t u a t i o n s . 41 10. P r e p a r a t i o n of S o i l Specimens. A l l specimens were prepared i n a humid room, i n order to prevent moisture l o s s e s . The s o i l was trimmed from the o r i g i n a l sample s i z e of 2.8 inches diameter down to the r e q u i r e d diameter of 2.5 i n c h e s . A s o i l l a t h e and wire saw were used f o r trimming (see photographic supplement). The surfaces were shaped to produce a c y l i n d r i c a l block, or as n e a r l y so as p o s s i b l e , care being taken not to unduly d i s t u r b the s o i l s t r u c t u r e . Before removal from the l a t h e , a t h i n p l a s t i c wrap (somewhat l e s s than 5 inches long) was p l a c e d around the specimen. The specimen was then gripped i n a s p l i t - m o u l d and removed from the l a t h e . The s p l i t - m o u l d permits trimming the ends to o b t a i n a specimen 5 inches l o n g . Due to the p l a s t i c wrap pr e v e n t i n g adhesion be-tween s o i l and mould, the specimen can be e x t r a c t e d from the mould w i t h a minimum of disturbance. The specimens were then weighed and a v i s u a l c l a s s i f i c a t i o n of the s o i l type recorded. (2) F i l t e r pads v were placed a t both upper and lower ends of the specimen. V e r t i c a l s ide drains made of ^ n wide f i l t e r paper s t r i p s were p l a c e d around the perimeter w i t h a spacing of about % i n c h between d r a i n s . T h i s arrangement of f i l t e r s i s intended to f a c i l i t a t e drainage and d i s t r i b u t e the pore pressure uniformly throughout the specimen. The measured compressive strength must be c o r r e c t e d f o r the e f f e c t of the s i . i e d r a i n s as discussed i n Appendix I I . (2) Reeve Angel No. 202 F i l t e r Paper. 42 11. S e t t i n g Up Speclmen-ln T r i a x i a l C e l l . P r i o r to p o s i t i o n i n g the specimen i n the t e s t i n g machine, a l l drainage connections to the c e l l were f r e e d of a i r by f l u s h i n g w i t h deaired water. The pore-pressure apparatus was connected to the c e l l at t h i s stage. Porous d i s c s were place d at each end of the specimen. The drains were made to overlap the d i s c s , thus p r o v i d i n g un-i n t e r r u p t e d drainage from the sides to both ends. The specimen was seated on the base and the inner membrane placed i n p o s i t i o n by means of a membrane s t r e t c h e r . A i r trapped between the mem-brane and the specimen was removed by a l l o w i n g a l i t t l e water to f low back from the base b u r e t t e . The appropriate number of rubber bands (or rubber f t 0 f t r i n g s ) was i n p o s i t i o n while d e s i r i n g * A f i l m o f c a s t o r o i l was a p p l i e d to the in n e r membrane. The second membrane was then p l a c e d over the specimen and sealed In a s i m i l a r manner to the f i r s t . Any excess water which may have accumulated around the specimen during the d e a i r i n g operation was withdrawn by lowering the base burette to o b t a i n a s l i g h t negative pressure i n the pore water. The chamber was f i l l e d w i t h deaired water and a low p o s i t i v e c e l l pressure ( 0 . 5 - 1 « 0 l b . / s q . i n . ) a p p l i e d . Any negative pore pressure remaining, was then r e l i e v e d by a l l o w i n g the pore water access to atmospheric pressure* In t e s t s where pore pressure measurements were obtained by means of the probe, a c a v i t y was formed i n the specimen w i t h the a i d of a d r i l l b i t . The d r i l l was r o t a t e d i n t o the s o i l by hand, producing a duct of the same diameter as the probe. 43 D e a i r i n g the c a v i t y was accomplished by having the probe connected to the pressure l e a d from the c o n t r o l c y l i n d e r of the pore-press-ure apparatus. By lo w e r i n g the mercury trough of the n u l l i n d i c a t o r , and o p e r a t i n g the c o n t r o l c y l i n d e r o f the pore-pressure apparatus, water was made to flo w through the probe. I n s e r t i o n of the probe wh i l e m a i n t a i n i n g a steady flow of water, deaired the c a v i t y . F i n a l l y , the stem of the probe was sealed from chamber f l u i d by means of rubber bands t i g h t l y s t r e t c h e d around the membrane s l e e v e s . This method o f i n s e r t i o n prevents the formation o f a h i g h l y compressed zone of s o i l i n the neighbour-hood o f the probe. Moreover, the small amount o f water r e q u i r e d f o r d e a i r i n g i s not l i k e l y to have d e l e t e r i o u s e f f e c t s on the specimen. 12. Temperature C o n t r o l . The temperatures i n the l a b o r a t o r y were maintained i n the range 18 - 22 degrees centigrade throughout the duration of the t e s t s . 44. CHAPTER IV. SHEAR TESTS WITH PORE PRESSURE MEASUREMENTS A. I n t r o d u c t i o n As s t a t e d at the outset of the t e x t , the primary purpose of the i n v e s t i g a t i o n was to e s t a b l i s h the p a t t e r n of pore pressure, changes w i t h a p p l i e d s t r e s s and time, i n long d u r a t i o n t r i a x i a l t e s t s on Po r t Mann c l a y . In the course of the i n -v e s t i g a t i o n , a d d i t i o n a l i n f o r m a t i o n has a l s o been obtained on stre n g t h parameters and drainage c h a r a c t e r i s t i c s o f the s o i l . The r e p o r t summarizes the r e s u l t s of a l l l a b o r a t o r y t e s t s con-nected w i t h the above assignment. The t e s t i n g program extended from May, 1959 through September, 1959. A l l t e s t s were per-formed i n the S o i l Mechanics Laboratory at the U n i v e r s i t y of B r i t i s h Columbia. The s o i l samples r e q u i r e d f o r the i n v e s t i g a -t i o n were s u p p l i e d by R. A. Spence, C o n s u l t i n g Engineers, Vancouver. More s p e c i f i c a l l y , the problem concerns the l o s s i n shear s t r e n g t h which would r e s u l t from a slow build-up of pore pressure; a phenomenon n o t i c e d i n e a r l i e r r e s t s performed 45. by R. A. Spence, C o n s u l t i n g Engineer. I t was a n t i c i p a t e d that l o n g d u r a t i o n shear t e s t s would accentuate the slow build-up e f f e c t i f i t were a r e a l i t y . The present i n v e s t i g a t i o n was planned w i t h t h i s i n mind. B. Previous Research E a r l i e r works, reported by Casagrande and Wilson (1949),' T a y l o r (1943) and other s , i n d i c a t e that the r a t e of l o a d i n g In la b o r a t o r y t e s t s has a marked i n f l u e n c e on the measured shear stren g t h of f i n e - g r a i n e d s o i l s . For any one c l a y higher strengths are g e n e r a l l y obtained at the f a s t e r l o a d i n g r a t e s . To c i t e one example, Taylor found t h a t the s t r e n g t h Increased by about 50$ as a consequence of i n c r e a s i n g the r a t e of de-formation from 1$ per minute to 1000$ per minute. A s i m i l a r change from 1$ to 0.001$ per minute l e d to a r e d u c t i o n of 20$ i n observed shear s t r e n g t h . Decrease i n the deformation r a t e below 0.001$ per minute had only n e g l i g i b l e e f f e c t s . R e s u l t s s i m i l a r to Taylor's have been obtained i n t e s t s on c l a y samples from widespread l o c a l i t i e s . Although th© present i n v e s t i g a t i o n i s concerned only w i t h a p a r t i c u l a r marine c l a y , i t i s p o s s i b l e t h a t the observations have a more general a p p l i c a t i o n to c l a y s , i n view of the f i n d i n g s of the above i n v e s t i g a t o r s . The dependence of the measured shear s t r e n g t h on the r a t e o f deformation has g e n e r a l l y been a t t r i b u t e d to viscous l a g i n the pore f l u i d a t high deformation r a t e s , and, to the p l a s t i c flow o f the s o i l mass at the slower r a t e s . Viscous e f f e c t s are most s i g n i f i c a n t at high deformation r a t e s , which are outside 46 the scope of t h i s i n v e s t i g a t i o n . P l a s t i c flow, on the other hand, i s of considerable i n t e r e s t . The r e s u l t s of the shear t e s t s w i l l demonstrate t h a t p l a s t i c flow (or "creep") and pore pressure are, probably, inter-dependent, i n the case o f Port Mann c l a y , at any r a t e . C. Scope of the Present I n v e s t i g a t i o n Samples obtained from two borings at the s i t e of the New Port Mann Bridge were s e l e c t e d ; keeping i n mind that samples w i t h as uniform a texture as p o s s i b l e were r e q u i r e d . The samples were from depths ranging from 135 t o 150 f e e t below the e x i s t i n g ground l e v e l . A t t h i s l o c a t i o n , the c l a y stratum was found t o be q u i t e homogeneous. T r i a x i a l t e s t s of t h i s t h e s i s extended over periods o f one to twenty days. The t r i a x i a l apparatus used throughout the i n v e s t i g a t i o n was a c o n t r o l l e d - s t r e s s type machine (loads a p p l i e d i n increments). C e l l pressures up to 80 l b . / s q . i n . were em-ployed. Rates of development of pore pressure were observed throughout the d u r a t i o n of t e s t s on seven specimens. Pore pressure was measured e i t h e r a t the top or at the centre o f the specimen. Pour t e s t s were c a r r i e d out w i t h the pore pressure measurements taken at the top; i n the remaining t h r e e , i t was measured i n the v i c i n i t y of the cen t r e . Shear st r e n g t h s , at d i f f e r i n g degrees of c o n s o l i d a t i o n , were determined f o r s i x o f these specimens. S t a i n t e s t s , to o b t a i n an i n d i c a t i o n o f m i n e r a l o g i c a l 47 composition o f the s o i l p a r t i c l e s , were performed. The s e n s i t i v i t y has been estimated from vane t e s t r e s u l t s provided by R. A. Spence, C o n s u l t i n g Engineers. A t t e r b e r g l i m i t s were determined f o r : (a) the c l a y i n the n a t u r a l s t a t e and (b) the c l a y t r e a t e d w i t h sea water. The A t t e r b e r g l i m i t s obtained i n these t e s t s are assumed to be i n d i c a t i v e of the l e a c h i n g , D. D e s c r i p t i o n o f Samples A l l shear t e s t s r e p o r t e d i n t h i s Chapter, apply to un-d i s t u r b e d samples of the Port Mann c l a y . Swedish F o i l Samplers were used to recover the 2,8 inch diameter samples from the b o r i n g s . S h o r t l y a f t e r sampling, the s o i l was extruded from the sampler, wrapped i n polyethlene f i l m and thoroughly waxed. This moisture s e a l was not removed u n t i l the samples were r e -q u i r e d f o r t e s t i n g . This method o f p r o t e c t i o n appeared to have been very e f f e c t i v e ; no d i s c e r n i b l e change i n p r o p e r t i e s being observed during the p e r i o d the samples were s t o r e d before t e s t i n g . The sampling ope r a t i o n was c a r r i e d out during the summer of 1958, The l a b o r a t o r y i n v e s t i g a t i o n f o r t h i s t h e s i s commenced i n May, 1959. Table I I I shows the order of t e s t i n g , l o c a t i o n o f samples, v i s u a l d e s c r i p t i o n of m a t e r i a l , e t c . E. D e s c r i p t i o n of Shear Tests and R e s u l t s . In the d i s c u s s i o n which f o l l o w s , each t e s t i s t r e a t e d s e p a r a t e l y . R e s u l t s dependent on the c o r r e l a t i o n of a number of 48. t e s t s are presented a t the end of the s e c t i o n d e a l i n g w i t h the i n d i v i d u a l t e s t s . The sequence of the stages, e t c . , i s summarized i n Table IV. SHEAR TEST} NUMBER J BORING NUMBER ! SAMPLE ! ! NUMBER J DEPTH BELOW GROUND LEVEL 1 NATURAL 1 ! MOISTURE ! CONTENT VISUAL DESCRIPTION . OP SAMPLES 1 | BN23P i 2 2 i 137'-6" 1 3 8 ' - 4 " to | Dark Grey Clay w i t h darker markings. 2 ! BN23P j 25 ; 1 3 9 1 - 6 " 140'-00" to ! 66.8$ tt 3 ! BN23P I 23 ! 138'-4" 1 3 9 t to ! 67.6$ tt 4 | BN23P j 27 j 140'-6" 141 ' ~ 4 M to ; 61.8$ tt 5 ; BS2P ! 40 I 147' -9" 148 ' - 5 M to ! 6 8 . 1 $ tr 6 J BS2P ! 42 J 148' - 7 n : 149' -5" to ! 5 9 . 4 $ Dark Grey Clay with L i g h t Grey Dis» i c o l o r a t i o n on top. 7 ; BS2F j 4 6 | 1 5 1 t ^ 3 " 152 ' - 1 " to ! 58.1$ ! Dark Grey Clay w i t h | .darker markings. TABLE I I I ~ DESCRIPTION OP TEST SAMPLES. TABLE IV — SCHEDULE OF SHEAR TESTS SHEAR TEST NO. DURA-TION OF TEST Days TIME ELAPSED BETWEEN PREP-ARATION OF SPECIMEN AND APPLICATION OF CELL PRESSURE Hours Build-UD Staaes Drainaae Staaes Loadina Staaes TOTAL NO. CELL PRESS-URES lb./sq. in. SEQUENCE TOTAL NO. CELL PRESS-URES lb./sq. in. SEQUENCE TOTAL NO. CELL PRESS-URES lb./sq, in. TYPE OF LOADING 1 1 None 2 20 50 Consecu-tive None None 2 6 5 4 12 30 50 60 ditto 1 60 Follow-ing build-up stages 1 60 Incre-mental 3 9 13 1 20 1 20 Follow-ing build-up stages 1 20 ditto 4 10 15 4 20 40 60 80 ditto 1 80 Follow-ing build-up stages 1 80 ditto 5 10 3 20 40 60 ditto 1 60 ditto 1 60 ditto 6 17 144 2 20 40 Separa-ted by a Drainage Stage 1 40 Inter-mediate between build-up stages 1 40 ditto 7 LOCATIC 20 )N OF POF 18 :E PRESSURE MFAST 2 RRMRNTS? 40 60 Separa-ted by a Drainage Stage act ffl _ a* 2 40 80 One inter-mediate and one after build-up stages T r n -1 z 1 80 ditto 51. l . ( a ) Test 1. Test Procedure: Test 1 was performed as a p i l o t t e s t and was consequently of short d u r a t i o n . The pore pressure was measured at the top of the specimen. The c e l l pressure' was r a i s e d immediately the specimen was set up f o r t r i a x i a l apparatus. Pore pressure changes r e s u l t i n g from the increase i n c e l l pressure were recorded. The r e s u l t s are shown i n Graph 4-1. Drainage was prevented throughout the build-up stage. The specimen was not loaded. R e s u l t s : The r a t e of build-up of pore pressure, and the magni-tudes of the pore pressure parameter B are as f o l l o w s : INCREASE IN CELL PRESSURE 6~3 l b . / s q . i n Prom To TIME REQUIRED TO REACH EQUILIBRIUM PORE PRESSURE PARAMETER B (1) MINUTES AT EQUILIBRIUM 5 20 25 0.87 20 50 20 0.92 (1) Here the values o f B apply to t o t a l changes i n c e l l and pore pressures. 52. (b) Test 2. Test Procedure: The specimen was allowed to stand i n the c e l l f o r a p e r i o d o f f i v e hours before a p p l i c a t i o n of c o n f i n i n g pressure. Pore pressure was measured at the top of the specimen throughout the dur a t i o n o f the t e s t . F i l t e r paper side drains were employed i n the manner discussed e a r l i e r . A x i a l l o a d i n g was a p p l i e d i n increments u n t i l the specimen f a i l e d . The mois-ture content was determined at three l o c a t i o n s l n the specimen a f t e r the shear t e s t . Duration o f the t e s t was s i x days; the l o a d i n g stage accounting f o r one and one-half days o f t h i s time. Pore Pressure Build-Up Stage: A negative pore pressure of 4«8 l b . / s q . i n . developed during the f i v e hour p e r i o d before the c e l l pressure was a p p l i e d . The c e l l pressure was r a i s e d In f o u r increments to a maximum of 60 l b . / s q . i n . , no drainage from specimen being permitted. Graph 4-2 shows the r a t e of build-up of pore pressure f o r each incremental change I n c e l l pressure. The time r e q u i r e d f o r the pore pressure to reach e q u i l i b r i u m w i t h the c e l l pressures and the corresponding magnitudes of B are l i s t e d below: INCREASE IN CELL j PRESSURE J o~3 l b . / s q . i n . J From To i TIME REQUIRED TO REACH EQUILIBRIUM MINUTES i PORE PRESSURE ! PARAMETER B J AT i EQUILIBRIUM 0 12 | 110 1 0.57 12 30 * 45 ! 0.78 50 50 j 40 ; 0.87 50 60 | 35 J 0.88 53. The value of B however, equals u n i t y when based on subsequent changes i n c e l l pressures exceeding 30 l b . / s q . i n . ; i n d i c a t i n g that the specimen was f u l l y s a t u r a t e d before the drainage stage commenced. This was the only t e s t i n which the specimen was f u l l y s aturated p r i o r to l o a d i n g . Drainage Stage: F o l l o w i n g the build-up stage, drainage from the specimen was allowed, while the c e l l pressure' was maintained at 60 l b . / s q . i n . The f l o w of water was d i r e c t e d towards the base of the specimen. Volume changes due to expulsion of pore water were measured i n the b u r e t t e . A sudden drop i n pore pressure was expected at the onset of drainage, but t h i s d i d not m a t e r i a l -i z e as i s evident from the pore pressure/time curve shown i n Graph 4-3. During the drainage stage the pore pressure dropped to 11.8 l b . / s q . i n . from the i n i t i a l 51.7. Primary c o n s o l i d a t i o n was 16% complete a t end of drainage stage (based on d i s s i p a t i o n of pore p r e s s u r e ) . Loading Stage: The c e l l pressure (o"3) was maintained at 60 l b . / sq. i n . throughout the l o a d i n g stage. A x i a l l o a d i n g produced f a i l u r e when the d e v i a t o r s t r e s s (o"~ 1 - 6~ 3) a t t a i n e d a value of 31.0 l b . / s q . i n . During the l o a d i n g stage the pore pressure g r a d u a l l y i n c r e a s e d from 11.9 to 35»1 l b . / s q . i n . "Creep" accounted f o r the greater p a r t o f the deformation before f a i l u r e ; the instantaneous deformation being very small i n comparison. The s t r e s s vs. s t r a i n curve f o r the specimen i s shown i n Graph 4-4. The manner i n which the pore pressure changed w i t h de v i a t o r s t r e s s , time and deformation, i s shown i n Graph 4**5. Time to f a i l u r e t f ; 34 hours. 5 4 . Type o f F a i l u r e : F a i l u r e occurred on a s i n g l e shear plane i n c l i n e d at 60° to the plane of major p r i n c i p a l s t r e s s , ( h o r i z o n t a l ) . Moisture Content Determinations: The f o l l o w i n g are the r e s u l t s of the moisture content t e s t s performed a f t e r the shear t e s t . LOCATION MOISTURE CONTENT % OF DRY WEIGHT Top 61.0 Shear Zone 60.0 Base 5 5 . 5 55 (c) Test 5. Test Procedure: The specimen was allowed to stand i n the c e l l f o r a p e r i o d of 13 hours before a p p l y i n g the c e l l pressure. Pore pressure was measured at the top of the specimen throughout the d u r a t i o n of the t e s t . Side drains were employed to a s s i s t drainage. The specimen was t e s t e d to f a i l u r e ; the loads being a p p l i e d i n increments. Duration of t e s t was nine days, s i x o f which were devoted to the l o a d i n g stage. Pore Pressure Build-Up Stage: No water was allowed to escape . from the specimen during the build-up stage. A negative pore pressure o f 5.5 l b . / s q . i n . had developed before the c e l l pressure was a p p l i e d . The c e l l pressure was r a i s e d i n one o p e r a t i o n to a value of 20 l b . / s q . i n . A p e r i o d of 210 minutes elapsed before the pore pressure came to e q u i l i b r i u m w i t h the i n c r e a s e d c e l l p ressure. The value of B a t e q u i l i b r i u m was found to be 0.76. Pore pressure gauge readings vs. time are p l o t t e d f o r the b u i l d -up stage of t h i s t e s t i n Graph 4-6. Drainage Stage: Drainage was permitted by opening the valve to the base bur e t t e . The manner i n which the pore pressure, and volume, changed during the drainage stage, i s shown In Graph 4-7. There i s a marked s i m i l a r i t y between the curves obtained f o r the drainage stages of t h i s t e s t and those of Test 2. In t h i s t e s t , however, the volume change vs. time i n d i c a t e that the average primary c o n s o l i d a t i o n was complete at about 1,100 m i n u t e s ^ ' from the commencement of the drainage stage. (2) Curve f i t t i n g to o b t a i n tioo f o l l o w s the method proposed by Bishop and Henkel (1957). 5 6 . The curve r e l a t i n g decrease of pore pressure t o time (Graph 4-7) shows that 70$ d i s s i p a t i o n had occurred i n 1,100 minutes. A r e s i d u a l pore pressure o f 3.2 l b . / s q . i n . was recorded at the end of the drainage stage (top of specimen). Average o f pore pressure would be somewhat l e s s . Loading Stage: The pore pressure showed a gradual increase f o l l o w i n g the a p p l i c a t i o n of each l o a d increment. F a i l u r e o f the specimen occurred when the deviator s t r e s s a t t a i n e d a value of 18.1 l b . / s q . i n . During the l o a d i n g stage the pore pressure increased by 7.6 l b . / s q . i n . to 10.8 l b . / s q . i n . C e l l pressure was maintained at 20 l b . / s q . i n . throughout the l o a d i n g stage. Graphs 4-8 and 4-9 p e r t a i n to the l o a d i n g stage of t h i s t e s t . Time to f a i l u r e t f : 114 hr s . Type of F a i l u r e : F a i l u r e occurred on a s i n g l e shear plane which was i n c l i n e d a t an angle of 60° to the plane of m a j o r ' p r i n c i p a l s t r e s s . Moisture Content Determinations: Moisture contents a f t e r t e s t s were as f o l l o w s : t MOISTURE CONTENT LOCATION J % OF DRY WEIGHT Top of Specimen | 6'1.6 Shear Zone 66.1 Base o f Specimen i 68.0 57. Water drawn i n t o the specimen when the c e l l pressure was lowered may account f o r the high moisture content recorded at the base of the sample. 58. (d) Test 4. Procedure: The specimen was allowed to stand i n the t r i a x i a l c e l l f o r a p e r i o d of 15 hours before c e l l pressure was a p p l i e d . Pore pressure was measured at the top of the specimen throughout the d u r a t i o n o f the t e s t . F o l l o w i n g the i n i t i a l standing p e r i o d , c e l l pressure was a p p l i e d i n increments; no change i n the water content of the specimen being permitted. On e q u i l i b r i u m being beached between the pore pressure and c e l l pressure, drainage was permitted i n order to increase the degree of c o n s o l i d a t i o n . F i n a l l y the specimen was t e s t e d to f a i l u r e ; no drainage being allowed during the l o a d i n g stage. Duration o f t e s t - 10 days. Pore Pressure Build-Up Stage: A negative pore pressure of 5.8 l b . / s q . i n . developed before the c e l l pressure was a p p l i e d . The c e l l pressure was r a i s e d i n four increments, each 20 l b . / sq. i n . The manner i n which the pore pressure i n c r e a s e d , f o l l o w i n g the a p p l i c a t i o n of c e l l pressures, i s shown i n Graph 4-10. The p e r t i n e n t data from the build-up stage are l i s t e d INCREASE IN CELL J PRESSURE } 6~ 3 l b . / s q . i n i From To \ TIME REQUIRED . TO REACH EQUILIBRIUM MINUTES j PORE PRESSURE i PARAMETER B ! AT ! EQUILIBRIUM 0 20 j 90 ! 0.60 20 40 | 45 0.79 40 60 j 25 j 0.86 60 80 ! 17 ! 0.88 5 9 . Drainage Stage: The specimen drained to the base burette f o r a p e r i o d o f 77 hours. Daring t h i s time the pore pressure dropped from the i n i t i a l value of 70 down to 10,8 l b . / s q . i n . The behavior of the pore pressure during the drainage stage f o l l o w e d a s i m i l a r p a t t e r n to tha t observed i n previous t e s t s . The volume decrease f o r the drainage stage represents 11$ of the o r i g i n a l volume of specimen. Loading Stage: The c e l l pressure was maintained at 80 l b . / s q . i n . throughout the l o a d i n g stage. Incremental l o a d i n g produced f a i l u r e at 4,2% s t r a i n . The de v i a t o r s t r e s s at f a i l u r e was 39.3 l b . / s q . i n . and the pore pressure 46>»0 l b . / s q . i n . Pore pressure increases were gradual f o r periods up to 20 hours a f t e r the a p p l i c a t i o n of an increment; the r a t e of increase f a l l i n g o f f sharply w i t h time. The s t r e s s vs s t r a i n curve i s shown i n Graph 4-11. Graph 4-12 shows the manner i n which the pore pressure changed with d e v i a t o r s t r e s s , time and deformation. Time of f a i l u r e t ^ ; 75 hours. Type o f F a i l u r e : The specimen f a i l e d on a s i n g l e shear plane i n c l i n e d at 52° to the h o r i z o n t a l . Moisture contents a f t e r t e s t s were as f o l l o w s : MOISTURE CONTENT LOCATION 1 % OF DRY WEIGHT Top of Specimen 55.6 Shearing Zone 53.1 Base o f Specimen 54.3 60. (e) Test 5. Procedure: T h i s t e s t i s the f i r s t o f a s e r i e s o f three where pore pressure measurements were made i n the v i c i n i t y o f the centre o f the specimen. For t h i s purpose a porous probe was employed i n the manner described i n Chapter I I I . Otherwise, the procedure was s i m i l a r to that of previous t e s t s . U n f o r t u n a t e l y , a f a u l t developed i n a valve a s s o c i a t e d w i t h the probe equipment. Some drainage occurred during the stage intended f o r the observation of pore b u i l d - u p . The defect i n t h i s valve was not remedied u n t i l the l o a d i n g stage was i n progress. Consequently, pore pressures recorded i n the build-up stage, and the i n i t i a l stages o f l o a d i n g , were e r r a t i c . However, the trends are i n d i c a t i v e that a s i m i l a r r e l a t i o n s h i p p r e v a i l e d to that recorded f o r previous t e s t s . In t h i s t e s t the only r e l i a b l e i n f o r m a t i o n on the pore pressure i s that obtained from the f o u r t h l o a d increment onwards. The shape o f the s t r e s s - s t r a i n curve i s v i r t u a l l y u n a f f e c t e d by t h i s i n c i d e n t . Furthermore, the stre s s e s at f a i l u r e are appropriate when i t comes to p l o t t i n g Mohr diagrams. C e l l pressure was maintained at 60 l b . / s q . i n . through-out the l o a d i n g stage. F a i l u r e occurred when the dev i a t o r s t r e s s a t t a i n e d a value of 34.7 l b . / s q . i n . , a t which s t r e s s the pore pressure was 35«2 l b . / s q . i n . The s t r e s s - s t r a i n r e l a t i o n s h i p i s shown i n Graph 4-13• Time to f a i l u r e : t f - 126 hours. LOCATION ! MOISTURE CONTENT % OF DRY WEIGHT Top of Specimen { 53.2 Shearing Zone i 56.2 Base o f Specimen j 57.8 62 . ( f ) Test 6, Procedure: The specimen was allowed to stand i n the t r i a x i a l c e l l f o r a p e r i o d o f s i x days before c e l l pressure was a p p l i e d . Deaired water surrounded the specimen and i t s p r o t e c t i v e membranes during t h i s time. Pore pressure measurements were made through-out the d u r a t i o n o f the t e s t ; the probe being l o c a t e d a t mid height o f the specimen. The procedure adopted i n previous t e s t s , that o f a b u i l d -up stage f o l l o w e d by drainage, was s l i g h t l y m o d ified i n t h i s case. Instead, the c e l l pressure was r a i s e d i n two increments w i t h a drainage stage intermediate between the two b u i l d - u p stages. As a r e s u l t of t h i s approach, f u r t h e r i n f o r m a t i o n was obtained on the pore pressure parameters. The specimen was t e s t e d to f a i l u r e i n the u s u a l manner. Duration o f t e s t , 17 days, i n c l u d e d the 6 days standing time. Pore Pressure Build-Up Stage: Although not i n c h r o n o l o g i c a l sequence, the build-up stages w i l l be d i s cussed together i n t h i s paragraph; the d i s c u s s i o n on drainage stage appearing under a separate heading. The r e s u l t s of the f i r s t b u i l d - u p stage are s i m i l a r t o those r e p o r t e d f o r previous t e s t s . No negative pore pressure, however, developed i n the i n i t i a l p e r i o d before the specimen was subjected to c e l l p r essures. The second stage y i e l d e d the c h a r a c t e r i s t i c pore pressure vs time r e l a t i o n s h i p , but the value of B was c o n s i d e r a b l y lower, as might be expected i n view of the i n t e r v e n i n g drainage stage. The r e s u l t s o f the b u i l d - u p stages are p l o t t e d i n Graph 4-14. The f o l l o w i n g t a b u l a t i o n shows some r e s u l t s f o r the build-up stage: 63. INCREASE IN CELL PRESSURE 6~ 3 l b . / s q . i n . Prom To TIME REQUIRED TO REACH EQUILIBRIUM MINUTES PORE PRESSURE PARAMETER B AT EQUILIBRIUM 0 20 15 0.86 (Drainage Stage Between 20 40 12 0.51 Drainage Stage: I n t e s t s where the pore pressures are determined at the centre o f the specimen, drainage may be pe r m i t t e d from the top and base of the specimen simultaneously. This procedure r e -duces the time r e q u i r e d to o b t a i n f u l l d i s s i p a t i o n of the pore pressure. Such a drainage arrangement simulates the c o n d i t i o n s e x i s t i n g i n the l a b o r a t o r y c o n s o l i d a t i o n t e s t ; i n f a c t , the r e s u l t s of the drainage stage of a t r i a x i a l t e s t can be t r e a t e d as i f they were obtained from a c o n s o l i d a t i o n t e s t on a l a r g e specimen. The r e s u l t s of drainage stage may be used i n determining the c o e f f i c i e n t o f c o n s o l i d a t i o n ( c 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 ( k ) . In the present t e s t , drainage was permitted from the top and base of the specimen. The r e s u l t s f o r the drainage stage are p l o t t e d i n Graph 4-15. At the end o f the drainage stage primary c o n s o l i d a t i o n was v i r t u a l l y complete a t the centre of the specimen, as can be observed from the pore pressure d i s s i p a t i o n vs time curve, Graph 4-15. The volume change vs time curves on the same graph i n d i c a t e t h a t on the average primary c o n s o l i d a t i o n was complete at about 760 minutes from the commencement of drainage. 64. The c o e f f i c i e n t of c o n s o l i d a t i o n ( c v ) , based on the pore pressure vs time r e l a t i o n s h i p , i s estimated to be 0.0054 i n s . 2 per minute. (Sample c a l c u l a t i o n s i n c l u d e d i n Appendix I I I ) . Previous t e s t s have i n d i c a t e d that the side drains are not e f f e c t i v e i n promoting drainage. This t e s t a f f o r d e d an opportunity of checking the e f f i c i e n c y of the d r a i n s . The value o f c y i s computed from the volume change vs time curve on the basis o f : (a) r a d i a l drainage, which takes f o r granted e f f e c t i v e side drainage «* and (b) drainage towards the upper and lower ends o f the specimen only. By comparison w i t h the c v value obtained independently from pore pressure/time r e l a t i o n s h i p f o r the same s t r e s s c o n d i t i o n s , i t ^ a s s e r t e d t h a t the c v values ob-t a i n e d i n (a) and (b) w i l l i n d i c a t e the drainage c o n d i t i o n a c t u a l l y p r e v a i l i n g during the drainage stage. The value of c y d e r i v e d from the three c o n s i d e r a t i o n s are l i s t e d below: 1 COEFFICIENT OF CONSOLIDATION/ CU.INS.2/MIN. • Based on d i s s i p a t i o n of { pore pressure i 0.00540 Based on r a d i a l and end i drainage { 0.00026 End drainage only j 0.00650 TABLE V. DETERMINATIONS OF COEFFICIENT OF CONSOLIDATION. 65 Further d i s c u s s i o n on the e f f e c t s of side d r a i n s i s i n c l u d e d i n the next Chapter. 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 (k) i s estimated to be 4.7 x IO"? ins./min. This value a p p l i e s , of course, only to the s t r e s s e s corresponding to a c e l l press of 40 l b . / s q . i n . ; the s t r e s s a c t i n g during the drainage stage. The pore pressure dropped from 17.2 down to 0.75 l b . / s q . i n . during the drainage stage. Loading Stage: The c e l l pressure was maintained at 40 l b . / s q . i n . throughout the l o a d i n g stage. F a i l u r e occurred when the d e v i a t o r s t r e s s a t t a i n e d a value of 19.1 l b . / s q . i n . The pore pressure at f a i l u r e was 31.0 l b . / s q . i n . The value of the pore pressure parameter A i s estimated at 1.01 a t f a i l u r e . The c a l c u l a t i o n i s based on the assumption that B remained constant throughout the l o a d i n g stage. In f a c t , the magnitude o f B increases during l o a d i n g , but the increase i n t h i s case would be s m a l l , because the specimen had a t t a i n e d a high degree of s a t u r a t i o n p r i o r to the lo a d i n g stage. The r e s u l t s o f the l o a d i n g stage are shown i n Graph 4-16 and 4-17. I t i s evident from Graph 4-17 that the pore pressure showed a gradual increase f o r some time a f t e r the a p p l i c a t i o n of each l o a d increment. Time to f a i l u r e , t f 5 167 hours. Type o f F a i l u r e : F a i l u r e occurred on a t l e a s t two planes as i n d i c a t e d on the sketch accompanying Graph 4-16. Moisture contents a f t e r t e s t s were as f o l l o w s : LOCATION J MOISTURE CONTENT % OP DRY WEIGHT Top o f Specimen J 62.2 Middle Zone | 62.6 Base o f Specimen » 66.0 67. (g) Test 7-Procedure: The procedure adopted was s i m i l a r to that em-ployed i n Test 6. Table IV i n d i c a t e s the sequence o f the stages. Duration o f t e s t - 20 days. Pore Pressure Build-Up Stage: The c e l l pressure was r a i s e d i n two increments, namely, 0 to 40 l b . / s q . i n . and 40 to 80 Ib./sq. i n . F o l l o w i n g the build-up p e r i o d f o r the f i r s t i ncrease i n c e l l p r e s s u r e , drainage was permitted from the base. The second increment was a p p l i e d when drainage had been i n progress f o r a p e r i o d o f 4 days. The r e s u l t s o f the build-up stages are shown i n G-raph 4-18. P e r t i n e n t data are l i s t e d below: 1 INCREASE IN CELL j PRESSURE j 6 3 l b . / s q . i n . { From To } TIME REQUIRED J TO REACH J EQUILIBRIUM { MINUTES • PORE PRESSURE PARAMETER B AT EQUILIBRIUM 0 40 J 15 j 0.96 (Drainage Stage Between)} J 40 80 j 25 j 0.65 j D i s s i p a t i o n o f pore pressure was o n l y 63$ complete at the end of the drainage stage; t h i s may account f o r the r a t h e r h i g h value o f 0.65 obtained f o r B i n the 40-80 l b . / s q . i n . range. In a d d i t i o n to the drainage stage mentioned above, a second drainage p e r i o d was permitted p r i o r to l o a d i n g . This l a t t e r stage which was conducted at a c e l l pressure of 80 l b . / sq. i n . produced a high e f f e c t i v e s t r e s s In the specimen before 68. the commencement of l o a d i n g . T h i s i s an advantage when Mohr diagrams are r e q u i r e d . At the end o f the second drainage stage the pore pressure was 15.7 l b . / s q . i n . Loading Stage: C e l l pressure was maintained at 80 l b . / s q . i n . throughout the l o a d i n g p e r i o d . The s t r e s s / s t r a i n r e l a t i o n s h i p f o r the l o a d i n g stage i s shown i n Graph 4-19. F a i l u r e occurred at a de v i a t o r s t r e s s of 39.4 l b . / s q . i n . Pore pressure at f a i l u r e was 52.7 l b . / s q . i n . Graph 4-20 shows the manner i n which the pore pressure changed w i t h l o a d , deformation and time. The build-up of the pore pressure during the l o a d i n g stage was somewhat e r r a t i c . T h i s was probably due to the hi g h c e l l pressure and to the high degree of c o n s o l i d a t i o n at which t h i s specimen was t e s t e d . The s o i l was very compact i n the neighbourhood of the probe, conse-quently, the time r e q u i r e d f o r the pore pressure to reach e q u i l -i b r i u m showed a random v a r i a t i o n from increment to increment. The o v e r a l l trends, however, f o l l o w the p a t t e r n of previous t e s t s . Type of "F a i l u r e : F a i l u r e occurred on a s i n g l e shear plane, i n c l i n e d a t 60° to the h o r i z o n t a l . Time to f a i l u r e , t f - 120 hours. Moisture contents a f t e r t e s t s were as f o l l o w s : ' i MOISTURE CONTENT LOCATION 2 % OF DRY WEIGHT Top o f Specimen j 43.9 Shearing Zone 43.5 Base of Specimen j 46.8 69. 2. Apparent Strength Parameters For the purpose of d e r i v i n g the 'apparent 1 s t r e n g t h parameters c and o' the t e s t s are a r b i t r a r i l y d i v i d e d Into two groups on the ba s i s of the methods employed f o r pore pressure measurement, e.g. t e s t s where pore pressure measurements were made at the top o f the specimen, c o n s t i t u t e one group; l i k e w i s e , the t e s t s where the pore pressure was measured at the ce n t r e , w i l l be grouped together. Graph 21 and Graph 22 show the Mohr c i r c l e s f o r the e f f e c t -i v e stresses at f a i l u r e . The apparent parameters, d e r i v e d from the f a i l u r e envelopes f o r the two groups, d i f f e r s l i g h t l y as shown below: POSITION OF PORE } PRESSURE J MEASUREMENTS i FRICTION ANGLE (APPARENT) 0' DEGREES J COHESION { (APPARENT) i c 1 l b . / s q . i n . At top of Specimen. i Tests 2, 3 and 4. ! 17 ! 4.0 At centre o f Specimen.i Tests 5, 6 and 7. \ i 1 21 ! 3.0 TABLE V I . APPARENT STRENGTH PARAMETERS - PORT MANN CLAY. Considering the r e s u l t s o f Test 4 as t y p i c a l of the trends observed i n a l l t e s t s , the e f f e c t s o f the slow build-up of pore 70. pressure during the l o a d i n g stage o f t h i s t e s t w i l l be e l u c i d a t e d w i t h the a i d o f a Mohr diagram. The manner i n which the e f f e c t -i v e s t r e s s on the f a i l u r e plane changes w i t h time i s shown f o r two increments of a x i a l l o a d i n g Graph 4-23« I t i s evident from t h i s p l o t that the c i r c l e s r epresenting a s i n g u l a r value of the d e v i a t o r s t r e s s w i l l advance towards the f a i l u r e envelope w i t h an Increase i n the elapsed time from the a p p l i c a t i o n of a l o a d increment. I n other words, the compressive s t r e n g t h decreases w i t h time, even i f no other e f f e c t but the slow build-up of pore pressure I s taken i n t o c o n s i d e r a t i o n . The t o t a l s t r e s s e s at f a i l u r e are shown i n the Mohr diagrams, Graph 4-24. The value of c u and 0U are estimated to be 4.5 l b . / s q . i n . , and 9 degrees r e s p e c t i v e l y . F. Observation. No b u c k l i n g , or t i l t i n g of top cap, occurred i n any of these t e s t s . G. Miscellaneous T e s t s . 1. M i n e r a l o g i c a l Composition of P a r t i c l e s . The r e s u l t s o f s t a i n t e s t s shown i n Table VII i n d i c a t e that the s o i l i s predominately an i l l i t e c l a y . S t a i n t e s t s , however, provide only an i n d i c a t i o n o f m i n e r a l o g i c a l composition. A more exact a n a l y s i s demands the elaborate techniques of x-ray d i f f r a c t i o n , or d i f f e r e n t i a l thermal a n a l y s i s - which have not been attempted. 71 STAIN TEST (#) ! COLOUR REACTION ! MINERAL INDICATED Crys t a l - V i o l e t J Dark-Green ] I l l l t e predominates Safranine y i Purple i I l l i t e predominates Malachite Green j L i g h t Brown | I n d e f i n i t e (#) "Subsurface Methods" LeRoy 1949 pp. 164-166 TABLE V I I . MINERALOGICAL COMPOSITION OP PARTICLES - PORT MANN CLAY. 2, S e n s i t i v i t y . The s e n s i t i v i t i e s shown i n Table V I I I , have been c a l c u -l a t e d on the r e s u l t s of vane t e s t s performed i n boreholes BS2 and BS1B. These borings are l o c a t e d not more than 100 f e e t away from boring BS2P. I t seems reasonable to assume that r e s u l t s of the vane t e s t s r e f l e c t the s e n s i t i v i t y o f the m a t e r i a l i n BS2P and BN23P. BORING I DEPTH BELOW j SENSITIVITY NUMBER j GROUND LEVEL i INDEX BS2 « 135' - 6" J 64 BS2 136' - 3« ! 31 BS2 • 137' - 0" I 7 7 BS1B { 143' - 0"' j 76 BS1B j 143» *. 9" ! 39 BS1B j 147? - 9" | 34 TABLE V I I I . SENSITIVITY INDICES - PORT MANN CLAY. 72. 3j A t t e r b e r g L i m i t s . A t t e r b e r g l i m i t s were determined f o r sample No. 23 -borehole BN23F. The purpose o f these t e s t s was to determine i f l e a c h i n g o f salf' from the pore water had taken p l a c e i n the nat-u r a l c l a y stratum. As s t a t e d i n Chapter I , l e a c h i n g i s b e l i e v e d to reduce the values o f the A t t e r b e r g l i m i t s . The index t e s t s were therefore performed on the n a t u r a l c l a y , and on c l a y which had been p r e t r e a t e d w i t h sea water. The treatment c o n s i s t e d o f mixing the c l a y with excess s a l t water and a l l o w i n g i t t o stand f o r 24 hours. The f l o c c u l a t e d s o i l was then separated by de-canting o f f the superfluous l i q u i d . Both the n a t u r a l and t r e a t e d s o i l s were allowed to a i r dry f o r the purpose of o b t a i n i n g the consistency of the A t t e r b e r g l i m i t t e s t s . The r e s u l t s o f the t e s t s are l i s t e d below: BORING BN23P SAMPLE NO. 23 LIQUID LIMIT ! PLASTIC ! LIMIT Na t u r a l S o i l j 74.5 } 32.6 S a l t - T r e a t e d S o i l 1 89.0 J 31.2 TABLE IX. ATTERBERG LIMITS - PORT MANN CLAY. These r e s u l t s show that a s i g n i f i c a n t change i n l i q u i d l i m i t occurs when the s o i l i s allowed access to the s a l t ions d i s s o l v e d i n sea water. 73 CHAPTER V. DISCUSSION OF TEST RESULTS AND CONCLUSIONS A, E f f e c t s of Overburden Pressure At the outset o f the t e x t , a t t e n t i o n was drawn to the e f f e c t s of p r e c o n s o l i d a t i o n on the s o i l p r o p e r t i e s - notably the shear.strength. For p r e c o n s o l i d a t e d c l a y s the f a i l u r e envelope of e f f e c t i v e s t r e s s e s shows a cohesion i n t e r c e p t on the shear s t r e s s a x i s , whereas f o r normally c o n s o l i d a t e d c l a y s t e s t e d at c e l l pressures greater than the overburden pressure, the e f f e c t i v e s t r e s s envelope passes through the o r i g i n . G e o l o g i c a l evidence i n d i c a t e s that the P o r t Mann c l a y i s normally c o n s o l i d a t e d , but the e f f e c t i v e s t r e s s e s a t the depth o f the samples t e s t e d are such that i n l a b o r a t o r y t e s t s , the s o i l would be expected to behave l i k e a p r e c o n s o l i d a t e d m a t e r i a l u n t i l the equivalent over** burden stresses of 50 - 57 l b . / s q . i n . have been e x c e e d e d . ^ Due to l i m i t a t i o n imposed by the shear t e s t apparatus, (1) E f f e c t i v e s t r e s s e s are given i n Appendix I . 74. only i n Testis 4, 5 and 7 d i d the a x i a l s t r e s s exceed the over-burden pressure. Therefore, the values o f 'apparent' cohesion l i s t e d i n Chapter IV are to be expected. B. S e n s i t i v i t y The r e s u l t s of the c o n s o l i d a t i o n t e s t s (e vs l o g p graphs) given i n Appendix I show that the r e l a t i o n s h i p between v o i d r a t i o and pressure i s t y p i c a l of e x t r a s e n s i t i v e c l a y s , Terzaghi and Peck (1949). The r e s u l t s o f the vane t e s t s f u r t h e r confirm the high s e n s i t i v i t y o f t h i s s o i l d e p o s i t . (Table VTII and Appendix I ) . S e n s i t i v i t i e s g r e a t e r than 8 are considered high. This s o i l w i t h s e n s i t i v i t i e s i n the range 30 to 80, undoubtedly, belongs to the e x t r a s e n s i t i v e group of c l a y s . The vane t e s t r e s u l t s emphasize the importance of disturbance o f s t r u c t u r e on shear str e n g t h ; t h i s s o i l would be c l a s s i f i e d as a f i r m c l a y , i n the undisturbed s t a t e , y e t breaking down the s t r u c t u r e by remoulding transforms the s o i l i n t o a s l u r r y of n e g l i g i b l e shear s t r e n g t h . In comparison w i t h the Norwegian c l a y s , the A t t e r b e r g l i m i t s of the samples used f o r these t e s t s are high; 74 and 31 as against 26 and 18 f o r some Norwegian c l a y s , Bjerrum (1954). The s e n s i t i v i t y of the Norwegian c l a y i s a l s o higher; s e n s i t i v i t y i n d i c e s of 300 to 500 have been reported by the same author. The n a t u r a l moisture content o f the Port Mann c l a y i s clo s e to the l i q u i d l i m i t . 75. I f A t t e r b e r g l i m i t s are accepted as an i n d i c a t i o n of the degree of l e a c h i n g , the process i s not i n an advanced stage i n t h i s deposit. The f a c t that the l i q u i d l i m i t was r a i s e d by (2) 14.5 when the s o i l was immersed i n sea water, i n d i c a t e d that some l e a c h i n g has taken p l a c e . As the deposit Is below sea l e v e l and appears to have never been subjected to s u b a e r i a l l e a c h i n g , the most l i k e l y source of l e a c h i n g water i s an a r t e s i a n (3) head i n the previous l a y e r beneath the c l a y d e p o s i t . I t i s i n f e r r e d from the s e n s i t i v i t y t h a t the Port Mann c l a y possesses a "cardhouse" s t r u c t u r e o f a s i m i l a r c o n f i g u r a -t i o n to that confirmed i n the case of the Norwegian c l a y s , Rosenquist (1959). C. S t r e s s - S t r a i n Curves A l l the shear t e s t s e x h i b i t n o n - l i n e a r r e l a t i o n s h i p between s t r e s s and s t r a i n d u r i n g the i n i t i a l stages o f l o a d i n g e.g. Graph 4-8. Rarely have s t r e s s s t r a i n curves of t h i s shape been re p o r t e d . The cause(s) of the h i g h i n i t i a l deformation i s not known. Precautions were taken during the t e s t s to o b t a i n a p o s i t i v e s e a t i n g of the c e l l plunger on the specimen cap, before a p p l y i n g the a x i a l l o a d s . I t i s a l l the more d i f f i c u l t to under-stand i n vie'.w of the drainage stages having seated the end f i t t i n g s (porous d i s c s , etc.) p r i o r to the l o a d i n g stages. Note-worthy i s the f a c t that a considerable p a r t o f the i n i t i a l de-(2) See Table IX. (3) See borehold l o g s , Appendix I . 7 6 formation occurred i n the form of *creepnKq\ p o s s i b l y some r e -arrangement o f s t r u c t u r e occurs i n the i n i t i a l stages of l o a d i n g . No peak d e v i a t o r s t r e s s appears on the s t r e s s - s t r a i n curves. Membrane r e s t r a i n t a t the higher s t r a i n s may have overcome the tendency to develop the peak s t r e s s u s u a l l y a s s o c i a t e d with e x t r a s e n s i t i v e c l a y s . D, Pore Pressure C h a r a c t e r i s t i c s The r e s u l t s o f the shear t e s t s l e a d to the c o n s l u c i o n that i n t h i s s o i l , pore pressure r e q u i r e s considerable time to reach e q u i l i b r i u m w i t h the a p p l i e d s t r e s s e s . R e s u l t s presented i n Chapter IV show that the e f f e c t Is common to both the i n -creases i n a l l r o u n d and a x i a l s t r e s s e s produced i n the t r i a x i a l t e s t . The time l a g i s evident i n the build-up stages, but i s seen to best advantage i n the l o a d i n g stages, Graph 4 - 1 7 , f o r inst a n c e . The deformation vs time curves resemble an i n v e r t e d image o f the pore pressure vs time r e l a t i o n s h i p i n p l o t s such as Graph 4 - 1 7 . ^he s i m i l a r i t y suggests that the r a t e of b u i l d -up of pore pressure i s r e l a t e d to the creep of the s o i l s k eleton (up to i n s i p i e n t f a i l u r e ) . Considering the remarks made e a r l i e r (Chapter I) re g a r d i n g the a d s o r p t i o n and s t r u c t u r a l c h a r a c t e r i s t i c s of marine c l a y , i t i s reasonabke to assume that the cardhouse type of s t r u c t u r e i s capable o f withstanding considerable e f f e c t -ive s t r e s s e s at the "edge to f l a t " contacts o f the p a r t i c l e s ; ( 4 ) Creep and p l a s t i c flow are regarded as synonymous terms 77. the e f f e c t i v e s t r e s s e s r e s u l t i n g from a combination of the e f f e c t s of i n t e r p a r t i c l e f orces and the e x t e r n a l l o a d s . Apparently, w i t h the passing of time, the e f f e c t i v e s t r e s s e s are r e l i e v e d by p l a s t i c deformations of the adsorbed l a y e r s -the p a r t i c l e s are r e a l i g n e d to become more n e a r l y p a r a l l e l . I t i s b e l i e v e d that an increase i n the spacing o f the p a r t i c l e s occurs i n the process o f realignment; regions which were i n i t i a l l y i n contact tending t o separate. This hypothesis i s I l l u s t r a t e d d i a g r a m a t i c a l l y i n Figure 14. Realignment would then permit a red u c t i o n i n the e f f e c t i v e s t r e s s e s , while at the same time the s t r e s s changes would be t r a n s m i t t e d to the pore f l u i d . The w r i t e r suggests that the time l a g i n pore pressure build-up can be a t t r i b u t e d to realignment of the p a r t i c l e s , r e s u l t i n g from p l a s t i c deformations of the adsorbed l a y e r i n the r e g i o n of contact areas. Supporting t h i s view are i n v e s t i g a t i o n s r e p o r t e d by Michel and Lambe. Mi c h e l (1956) observed that repeated s t r e s s i n g of a s o i l a l i g n s the p a r t i c l e s i n almost p a r a l l e l formation, assuming, of course, that the p a r t i c l e s possess the c h a r a c t e r i s -t i c f l a k y shapes o f true c l a y s . Moreover, M i c h e l proved w i t h measurements th a t not only remoulding, but even shear s t r a i n s , arranges p a r t i c l e s i n p a r a l l e l a r r a y , Lambe (1959) drew s i m i l a r conclusions regarding the o r i e n t a t i o n of p a r t i c l e s i n mechanically compacted s o i l s . In t h i s s e r i e s the r e s u l t s o f Test 2 s u b s t a n t i a t e the p o s t u l a t i o n concerning realignment. Although f u l l y s a t u r a t e d at c e l l pressures higher than 30 l b . / s q . i n . , the t e s t y i e l d e d To follow p a-a e 77 D/recdon of External. Pressure Time t0 : £ xternal Pressure taken partly by In fe r g r a. n u I a. r Stresses. Time t, : External Pressure, taken ent/f-ely by Pore Water F I G 14 C L A Y - W A T E R S Y S T E M . ' E F F E C T S OF P L A S T I C D E F O R M A T I O N O F T H E A D S O R B E D L A Y E R S . 78. the c h a r a c t e r i s t i c pore pressure vs time r e l a t i o n s h i p f o r f u r t h e r increases i n c e l l pressure, and a l s o f o r the l o a d i n g stage, Graphs 4-2 and 4-5. The other t e s t specimens were very c l o s e to s a t u r a t i o n p r i o r to l o a d i n g - p o s s i b l y reaching s a t u r a t i o n during the long periods devoted to the l o a d i n g stages. The r a t e of pore pressure build-up a l s o depends to a l a r g e extent on degree of s a t u r a t i o n . I t has h i t h e r t o been assumed that i n completely s a t u r a t e d s o i l s the e l a s t i c deforma-t i o n of the s k e l e t o n i s s u f f i c i e n t to b u i l d up the pore pressure instantaneously to i t s f i n a l v a l u e , (due to the low compressi-b i l i t y o f water i n comparison to that o f the s o i l s k e l e t o n ) . This assumption e n t a i l s t h a t the pore pressure change has the same magnitude as the a p p l i e d s t r e s s provided no drainage occurs. I f t h i s assumption i s v a l i d , i t does not take i n t o c o n s i d e r a t i o n : (a) the i n t e r p a r t i c l e s t r e s s e s i n c o l l o i d a l m a t e r i a l s ^ ) , and (b) the p o s s i b i l i t y t h a t gas bubbles may be trapped i n the pore water of submerged s o i l s t r a t a . The r e l a t i o n between i n t e r p a r t i c l e s t resses and s t r u c t u r e has been discussed a l r e a d y , so a t t e n t i o n w i l l now be d i r e c t e d to the e f f e c t s o f trapped gases. Complete s a t u r a t i o n does not seem to be a j u s t i f i a b l e assumption i n a l l cases of submerged s t r a t a . Transported s o i l s may be expected to c o n t a i n some a i r trapped a t the time of d e p o s i t i o n . A l s o , b a c t e r i a may be r e s p o n s i b l e f o r i n c r e a s i n g the gas content of the pore f l u i d . Values o f B s l i g h t l y l e s s (5) S a t u r a t i o n as i n d i c a t e d by B values. (6) I n t e r p a r t i c l e s t r e s s e s are deemed to i n c l u d e both s t r e s s e s a r i s i n g from Van der Waal's forces and e f f e c t i v e s t r e s s e s due to e x t e r n a l f o r c e s . 7 9 . than u n i t y have been recorded f o r l a b o r a t o r y t e s t s on s o i l s from submerged s t r a t a , Skempton (1954). The values o f B re p o r t e d i n Chapter IV, i n d i c a t e that the samples o f Port Mann c l a y were not f u l l y s a t u r a t e d on r e c e i p t at the l a b o r a t o r y . The pore pressure developed i n the shear t e s t s are then governed by f a c t o r s l i s t e d below. An e v a l u a t i o n of the r o l e played by each o f these f a c t o r s i s a complicated study; r e q u i r i n g a knowledge of the amount and composition of the gases, r a t e of deformation of adsorbed l a y e r s , e t c . Only a b r i e f d i s c u s s i o n of the t o p i c s i s presented here: (a) Deformation of the s o i l s k e l e t o n . (b) C o m p r e s s i b i l i t y of the pore f l u i d . (c) S o l u b i l i t y o f trapped gases. (d) Surface t e n s i o n at the g a s - l i q u i d i n t e r f a c e . (e) Vapour pressure of water. (f) Temperature. In view of e a r l i e r d i s c u s s i o n s v , i t i s evident that the pressure developed i n the pore f l u i d i s a f u n c t i o n of the deformation of the s o i l s k e l e t o n . The pressure developed i n the gas phase w i l l depend p r i m a r i l y on decrease i n volume of the eeil s k e l e t o n . The gases w i l l r e a c t instantaneously to the reduced volume, b u i l d i n g up the pore pressure i n accordance w i t h Boyle's law. Prom t h i s vjewpoint then, the r a t e o f development of pore pressure i s d i c t a t e d by the r a t e of deformation of the s k e l e t o n . ( 7 ) Refer to Chapter I I I , and e a r l i e r statements i n t h i s Chapter. 80. The other f a c t o r s l i s t e d are of secondary importance as s h a l l be seen p r e s e n t l y . An increase i n the pore pressure d r i v e s an i n c r e a s e d p r o p o r t i o n o f the trapped gases i n t o s o l u t i o n i n accordance w i t h Henry's law of s o l u b i l i t y . A time e f f e c t i s introduced here, as i t r e q u i r e s an i n t e r v a l f o r an e q u i l i b r i u m s t a t e to be e s t a b l i s h e d between the pore pressure and the c o n c e n t r a t i o n of gas i n s o l u t i o n . Gases going i n t o s o l u t i o n tend to lower the pore pressure, but the drop i n pressure w i t h time, due to t h i s phenomenon, i s not r e a d i l y observed because the s o i l skeleton i s deforming simultaneously. A f t e r a c e r t a i n increase i n pore pressure (corresponding to a l l gases i n s o l u t i o n ) f u l l s a t u r a t i o n i s reached (then B w i l l t h e r e a f t e r be equal to u n i t y w i t h respect to f u r t h e r s t r e s s changes). Therefore, s o l u b i l i t y o f the gases does not c o n t r i b u t e to the g r a d u a l l y i n c r e a s i n g pore pressure observed i n t h i s s e r i e s of t e s t s . Surface t e n s i o n produces a d i f f e r e n c e i n pressure between the l i q u i d and gas phases of the pore f l u i d . The pressure d i f f e r e n c e p i s given by the expression p = 2JL where Y denotes r the surface t e n s i o n of water and r r e f e r s to the r a d i u s of the entrapped bubble. This e f f e c t i s u s u a l l y ignored - the assumption being made that the pore water and the gases are a t the same pressure. P a r t of the pressure i n the gas phase i s derived from the vapour pressure o f the water. As the vapour pressure of water i s about 0 .35 l b . / s q . i n . at 20° C , pore pressures higher than t h i s f i g u r e are not a f f e c t e d . 81. Temperature e f f e c t s are inherent i n a l l f i v e f a c t o r s discussed. In t h i s s e r i e s o f t e s t s the temperatures were r e s t r i c t e d to the range 18-22° C , which i s not l i k e l y to m a t e r i a l l y a f f e c t the r e s u l t s . The slower rates of build-up recorded f o r those t e s t s w i t h pore pressure measurements a t the top cap, are most l i k e l y caused by r e s t r a i n t imposed by the end f i t t i n g (porous d i s c s , e t c . ) . Shear s t r e s s e s are introduced a t the ends, which r e s t r i c t s the deformation of the s o i l , p a r t i c u l a r l y under the a c t i o n of c e l l pressure. The observed data provide f u r t h e r evidence to support the view that the r a t e of deformation o f the s o i l skeleton i s the prime f a c t o r determining the r a t e of pore pressure b u i l d -up i n t h i s s o i l . The p o s s i b i l i t y of a s i g n i f i c a n t time l a g being inherent i n the response of the measuring apparatus, i s discounted by the o bservation that the pore pressure measurements obtained by us i n g the probe, l e a d to sh o r t e r times to e q u i l i b r i u m than do measurements taken at the surface of the specimen. The movement of water at the t i p of the probe i s r e s t r i c t e d by the small area of the p e r f o r a t i o n s , which would have the e f f e c t of emphasizing-any l a g i n the response o f the pore pressure apparatus. The tendency :;of the s o i l to expand on the r e l i e f of overburden pressure i n sampling may be re s p o n s i b l e f o r b r i n g i n g some gas out o f s o l u t i o n p r i o r to t e s t i n g . Negative pore pressures, however, were recorded at the surface o f the specimen, despite the f a c t that water was used i n preparing the specimens -to f r e e the surface a i r . I t appears t h a t the surface tension i n 82. the outermost pore water exceeds co n s i d e r a b l y the observed negative pressures of 5 l b . / s q . i n . (Tests* 2, 3, 4 ) . Large negative pore pressures at the surface i n d i c a t e t h a t expansion has been r e s t r a i n e d i n the inner parts of the sample. E. Strength Parameters Although the primary o b j e c t i v e o f the i n v e s t i g a t i o n was to determine the p a t t e r n of pore pressure changes w i t h a p p l i e d s t r e s s , a d d i t i o n a l data has been obtained concerning the s t r e n g t h parameters and drainage c h a r a c t e r i s t i c s o f the s o i l . The 'apparent' s t r e n g t h parameters (c' and o') d e r i v e d from Tests 2, 3 and 4, d i f f e r from those obtained fromt Tests 5, 6 and 7 (Table V I ) . The disagreement i s b e l i e v e d t o be due to the higher r a t e of l o a d i n g adopted i n Tests 2 and 4. I t i s apparent from Graphs 4-5 and 4-12 that the pore pressure had not reached e q u i l i b r i u m w i t h every l o a d increment i n the case of those t e s t s . The deformation/time r e l a t i o n s h i p was the only matter considered i n d e c i d i n g on l o a d i n g r a t e s , but i t now appears that the pore pressure/time r e l a t i o n s h i p i s a more r a t i o n a l c r i t e r i o n ; e q u i l i b r i u m between the pore pressure and the previous increment should be e s t a b l i s h e d before a d d i t i o n a l increments are a p p l i e d . In the absence of i n f o r m a t i o n to the c o n t r a r y , the values c' = 3.0 l b s . / s q . i n . and ^' s 21 degrees, obtained from Tests 5, 6 and 7, are accepted as the most approp-r i a t e 'apparent' s t r e n g t h parameters. On the assumption that the e f f e c t i v e s t r e s s c i r c l e s o f Tests 2 and 4 should come somewhat c l o s e r to the o r i g i n i n 83. Graph 4-21, i t appears that the two systems employed f o r measur-in g pore pressure w i l l y i e l d r e s u l t s i n s u b s t a n t i a l agreement w i t h respect to s t r e n g t h parameters. The true f r i c t i o n angle <f>v could not be determined from the i n c l i n a t i o n f o r the f a i l u r e plane(s) i n any t e s t o f t h i s s e r i e s . The i n c l i n a t i o n s observed appear to be i n f l u e n c e d by the r e s t r a i n t developed at the end f i t t i n g , which leads to an overestimate of the true f r i c t i o n angle. P. Drainage C h a r a c t e r i s t i c s The c o e f f i c i e n t o f p e r m e a b i l i t y k = 4.7 x 10"? ins./min. determined from the data on the drainage stage o f Test 6, places the c l a y i n the category of p r a c t i c a l l y impervious s o i l s , Terzaghi and Peck, (1948). The r e s u l t s shown i n Table V l e a d to the c o n c l u s i o n that f i l t e r paper s t r i p s do not form e f f e c t i v e side drains when sub-j e c t e d to hig h c e l l p r essures. This c o n c l u s i o n i s supported by work r e p o r t e d by Rowe, p u b l i s h e d about the time the t e s t i n g program f o r t h i s t h e s i s terminated, Rowe (1959). The Mohr diagram, Graph 4-23, i l l u s t r a t e s the r e d u c t i o n i n shear strength with time, ^ o r f u r t h e r i n v e s t i g a t i o n of t h i s aspect, a s t r a i n - c o n t r o l l e d t r i a x i a l machine has many advantages over the s t r e s s - c o n t r o l l e d type used i n the present test s e r i e s . A s t r a i n - c o n t r o l l e d machine, g i v i n g a wide range of t e s t r a t e s , has been designed, and i s a t present being f a b r i c a t e d . I t i s hoped to c a r r y out f u r t h e r experiments on the r e d u c t i o n of shear s t r e n g t h w i t h time to f a i l u r e when the s t r a i n - c o n t r o l l e d machine becomes a v a i l a b l e . 84 CHAPTER VI.  AUTOMATIC CONTROL A. Introdue t i o n Automatic c o n t r o l of pore water movements i s a decided advantage where the n u l l method of pore pressure measurement i s employed. I n p a r t i c u l a r , t h i s i s the case i f t e s t s are to be run over long periods o f time. Adjustment of the n u l l I n d i c a t o r must be made so f r e q u e n t l y during a t e s t that i n the absence of automatic c o n t r o l an operator i s r e q u i r e d t o devote almost h i s f u l l time to the a t t e n t i o n o f t h i s d e t a i l . In commercial l a b o r a t o r i e s t h i s i s o f f s e t somewhat by a s s i g n i n g one person to take charge of a number of t e s t s . Overnight, how-ever, the pore pressure apparatus has to be i s o l a t e d from the specimen, which means that the i n t e r v e n i n g pore pressure changes must be estimated by e x t r a p o l a t i o n before the apparatus i s r e -connected. A disadvantage o f t h i s approach i s that i f a poor estimate I s made, undesirable movement of the pore water r e s u l t s . Consequently, the development of automatic c o n t r o l systems has a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n . 85. I d e a l l y , an automatic c o n t r o l system should be capable of r e s t r i c t i n g the pore water movements to the very l i m i t e d range that can be t o l e r a t e d i n t r i a x i a l t e s t i n g . Furthermore, i t should not add complications to the already d i f f i c u l t prob-lem of d e a i r i n g the pore-pressure apparatus. Before d i s c u s s i n g the d e t a i l s of the new device, a review of e x i s t i n g automatic c o n t r o l s and remarks on t h e i r performance w i l l be presented. I n s o f a r as the author i s aware, only two servomechanisms have been developed f o r use w i t h the n u l l method of pore-pressure measurement. They are based on somewhat d i f f e r e n t p r i n c i p l e s , but both operate towards the same purpose; that of m a i n t a i n i n g the mercury column of the pore pressure apparatus a t a p r e s e l e c t e d l e v e l thoughout the d u r a t i o n o f the t e s t . The apparatus o r i g i n a l l y designed at D e l f t S o i l Mechanics Laboratory and l a t e r m o d i f i e d by Penman, employs a thermostatic-a l l y c o n t r o l l e d o i l bath to maintain a back pressure on the pore water v i a the mercury column. A schematic of the apparatus i s shown i n F i g . 15. The mercury column of the n u l l i n d i c a t o r makes contact w i t h an e l e c t r o d e placed i n s i d e the c a p i l l a r y tube. An increase i n pore pressure w i l l break the contact between mercury and e l e c t r o d e . Breaking the contact operates a r e l a y which switches i n the heating u n i t i n s t a l l e d i n the o i l bath. The thermal expansion of the o i l r e s t o r e s the mercury to the n u l l p o s i t i o n (contact w i t h e l e c t r o d e ) . At t h i s p o i n t the r e l a y cuts out the heater. In p r a c t i c e , a continuous make and break a c t i o n (10 times per second) occurs i n the v i c i n i t y 86. of the e l e c t r o d e . By t h i s means the movement of pore water i s r e s t r i c t e d to as l i t t l e as 0,1 m i l l i m e t e r s of mercury i n the 1 m i l l i m e t e r diameter c a p i l l a r y tube. The apparatus i s there-fore capable of very e f f i c i e n t c o n t r o l , p a r t i c u l a r l y i f the pore pressure i s i n c r e a s i n g . On a f a l l i n g pore pressure, how-ever, the o i l bath must be cooled, which leads to mechanical complications - a pump to c i r c u l a t e c o l d water through the o i l bath and some time delay switches must be i n s t a l l e d . I t i s claimed t h a t even f o r the case of f a l l i n g pore pressure the movement of pore water can be r e s t r i c t e d to about 0.5 m i l l i m e t e r s , Penman (1953)• When d e a i r i n g the system, the presence o f three l i q u i d i n t e r f a c e s , namely, o i l , water and mercury, seems to be a disadvantage to t h i s method. In 1956, Burton r e p o r t e d a design f o r an automatic c o n t r o l based on the a p p l i c a t i o n of p h o t o - e l e c t r i c c e l l s . The c e l l s are used to monoiter the l e v e l s of a mercury column i n a U-tube. The pore pressure ac t s on the mercury i n one limb of the U-tube. To the other side o f the U-tube i s f i t t e d a d i s -placement system c o n s i s t i n g o f an a c t u a t o r operating h y d r a u l i c bellows, and a pressure gauge. Any movement of the mercury causes f l u c t u a t i o n s i n the q u a n t i t y of r a d i a n t energy reaching the c e l l s from a l i g h t source. E l e c t r i c a l impulses r e c e i v e d from the photo c e l l s , c o n t r o l the operation o f the servo-mechanism. The e l e c t r i c a l c i r c u i t o f t h i s apparatus i s shown i n P i g . 16. E s s e n t i a l l y , the c i r c u i t i s that of an e l e c t r o n i c bridge c o n s i s t i n g o f twin a m p l i f i e r s . When the mercury i s at to Specimen Confro L r~o Re Lay Merc ur y To follow pa-o& 86 From Rett F I G 15. P O R E - P R E S S U R E D E V I C E . (Afrer A.D. P enmcxn 1953) F I G IB . C I R C U I T OF A U T O M A T I C C O N T R O L (After L.J. Burton 1356) F I G S 15 a n d I 6 . 87. same l e v e l i n both limbs o f the U-tube, the two a m p l i f i e r s can be adjusted so th a t t h e i r outputs are equal and opposite. The pressure feedback u n i t i s then i d l e . Any change i n the mercury l e v e l unbalances the a m p l i f i e r s , which c l o s e s one o f the two r e l a y s i n the anode c i r c u i t of the output stage. The r e l a y s c o n t r o l the movement o f the actuator and h y d r a u l i c bellows. D i r e c t i o n of movement depends on which r e l a y i s c l o s e d ; the h y d r a u l i c bellows works i n the sense to oppose the change i n mercury l e v e l . I t i s claimed that the device i s capable of r e s t r i c t i n g the pore water movement to about 1 p a r t i n 400,000. This f i g u r e i s quoted f o r a sat u r a t e d sample 4 inches i n diameter, 8 inches long and having a p o r o s i t y o f 20%, Burton (1956). Prom the s o i l - t e s t i n g viewpoint, a drawback to the use of t h i s apparatus i s the d i f f i c u l t y of d e a i r i n g the U-tube arrangement. Furthermore, the h y d r a u l i c bellows operates almost immediately a r e l a y c l o s e s . T h i s introduces the problem of "hunting" when working w i t h s o i l s of low p e r m e a b i l i t y . Perhaps minor o b j e c t i o n s t o the design i s the need f o r two power packs and a w e l l r e g u l a t e d power l i n e v o l t a g e . Nevertheless, the apparatus provides a remarkable degree o f c o n t r o l over pore water movements. In order to overcome the problem of converting e x i s t i n g s e r v o - c o n t r o l s , to work i n c o n j u n c t i o n with Bishop's pore-pressure apparatus, f u r t h e r p o s s i b i l i t i e s r e r e i n v e s t i g a t e d , as pa r t of the present program. The use o f p i e z o - e l e c t r i c c r y s t a l s was considered. At f i r s t s i g h t , p i e z o - c r y s t a l s , of quartz or of Rochelle s a l t , appear to o f f e r an approach t o the problem. 88. Such c r y s t a l s have been e f f e c t i v e l y used f o r the measurement of impulse pressures, shock wave i n t e n s i t i e s , e t c . Measurable p o t e n t i a l s are developed on the c r y s t a l faces when subjected to stre s s e s o f the magnitudes o c c u r r i n g i n pore pressure work. The deformation o f the c r y s t a l would be n e g l i g i b l e and they are a v a i l a b l e i n convenient s i z e s . This suggested the p o s s i b i l i t y of developing a pressure c e l l to a s s i s t i n the oper a t i o n o f the e x i s t i n g pore pressure apparatus. A study of the p r o p e r t i e s of these c r y s t a l s , however, r e v e a l e d that they are u n s u i t a b l e f o r use i n systems where pressure changes are gradual, as i s the case i n t r i a x i a l t e s t i n g . In. f a c t , the o r i g i n a l mechanical problem becomes an e l e c t r i c a l one of an analgous nature. I t was then decided to r e v e r t to the a p p l i c a t i o n o f photo-e l e c t r i c c e l l s , but t o design a c o n t r o l u n i t that preserved, as f a r as p o s s i b l e , the advantages d e r i v e d from the use o f B i s h i p ' s pore pressure apparatus. The new device has proved s a t i s f a c t o r y i n a c h i e v i n g t h i s o b j e c t i v e , B. D e t a i l s of Apparatus f o r Automatic C o n t r o l of Pore Pressure Turning again to the n u l l i n d i c a t o r of Bishop's pore-pressure apparatus, i t w i l l be r e c a l l e d that i t embodies a glass c a p i l l a r y tube which terminates i n a mercury r e s e r v o i r . The height to which the column of mercury w i l l r i s e i n the tube depends upon a st a t e of e q u i l i b r i u m being a t t a i n e d , between the pore pressure which acts on the meniscus of the column, and the back pressure a c t i n g on the surface o f the mercury i n the 89. r e s e r v o i r . At the commencement of a t e s t , the mercury i s r a i s e d to a convenient height i n the c a p i l l a r y tube, w i t h the pore pressure connection to the specimen i s o l a t e d from the u n i t . Before a l l o w i n g the pore water access to the mercury column, the t r i a x i a l s t r e s s e s on the specimen are so arranged as to produce zero pore pressure - or as nea r l y so as p o s s i b l e . On admission o f the pore water i n t o contact w i t h the mercury column, the i n i t i a l s t a t e o f e q u i l i b r i u m i s obtained. The l e v e l of the mercury i n the c a p i l l a r y i s then taken as the n u l l p o s i -t i o n . The n u l l p o s i t i o n i s a l s o made to c o i n c i d e w i t h atmospher-i c pressure, by opening the appropriate valves momentarily, throughout the system. An increase i n pore pressure tends to dr i v e the mercury column down - towards the r e s e r v o i r , and s i m i l a r l y a decrease has the tendency to r a i s e the mercury to a higher l e v e l than the n u l l p o s i t i o n . To maintain the mercury at the n u l l p o s i t i o n (no flow of pore water from specimen) the back pressure on the mercury r e s e r v o i r must be adjusted. The new device a u t o m a t i c a l l y makes the adjustments. The design o f the automatic c o n t r o l centres around the c h a r a c t e r i s t i c s of a p h o t o - v o l t a i c type photo c e l l , which i s used to detect changes i n the q u a n t i t y of l i g h t a r r i v i n g from a l i g h t source focused on the n u l l p o s i t i o n of the mercury column. The s i g n a l s from the photo c e l l are i n t e r p r e t e d by a s e n s i t i v e a m p l i f i e r f o r the purpose of r e g u l a t i n g the back pressure on the mercury r e s e r v o i r . A block diagram o f the apparatus i s shown i n P i g . 17. Any change i n the mercury l e v e l from the n u l l p o s i t i o n , r e f l e c t s i n the oper a t i o n of the To -folLow page 89. To Spec i 'rn e n AmpCf-f/er. Volfaye-goin^ Stage Null Indicator c a Power S rage Povse r Pa CM Wafer Ser vomoror £3 \ Auxiliary- Con f r o L Cy L i not er F I G 17. L A Y O U T O F A U T O M A T I C C O N T R O L 90. a m p l i f i e r . Two r e l a y s i n the output stage of the a m p l i f i e r , c o n t r o l the course o f a c t i o n of the pressure feedback, so that i t always r e a c t s towards r e s t o r i n g the mercury column to the n u l l p o s i t i o n . E x c i t e r Lamp: The concentrated l i g h t beam r e q u i r e d f o r e x c i t a -t i o n of the photo c e l l , i s obtained from a f i l m s l i d e p r o j e c t o r housing a 75 watt lamp. The condenser lens and concave m i r r o r of the p r o j e c t o r c o n s t i t u t e the o p t i c a l system; a l l other lenses are removed i n order to o b t a i n near p a r a l l e l rays from the p r o j e c t o r . A t h i n metal sleeve w i t h v e r t i c a l s l i t s cut on opposite s i d e s , f i t s over the c a p i l l a r y tube of the n u l l i n d i c a t o r . I t s purpose i s to minimize l i g h t s c a t t e r i n g which would otherwise occur i n the w a l l s of the c a p i l l a r y tube. Due to the presence of the sleeve, l i g h t r e a c h i n g the photo c e l l must f i r s t pass through the bore of the tube, i n a d i r e c t i o n at r i g h t angles to the l o n g i t u d i n a l a x i s of the bore. Photo E l e c t r i c C e l l : The l i g h t - s e n s i t i v e c e l l i s made of two i d e n t i c a l p h o t o - v o l t a i c elements enclosed i n a metal housing. The v o l t a i c c e l l s (commercially a v a i l a b l e ) are formed by d e p o s i t i n g a t h i n l a y e r of selenium on an i r o n p l a t e : the p l a t e c o n s t i t u t e s the p o s i t i v e e l e c t r o d e ; the negative e l e c t r o d e being a transparent f i l m of metal evaporated onto the selenium s u r f a c e . L i g h t f a l l i n g on the selenium a c t i v a t e s the c e l l to produce d i r e c t l y an E.M.P. between the e l e c t r o d e s . Although the a m p l i f i e r i s capable of ope r a t i n g on a s i n g l e element, two p h o t o - v o l t a i c c e l l s are p r e f e r a b l e i n t h i s case. The t w o - c e l l system a r i s e s 9 1 . from the requirement of an observation aperture, to enable a v i s u a l check to be made on the mercury l e v e l . I t i s not p r a c t -i c a b l e to cut the aperture i n the f r a g i l e surface coatings o f the i n d i v i d u a l c e l l . Instead, a s l i t i s formed by l e a v i n g a space between the upper and lower c e l l s . Due to the s m a l l dimensions of the i n d i v i d u a l c e l l s (0.72'* x 0.44": a c t i v e area 0.26 square inches) no d i f f i c u l t y was experienced i n accommodat-ing them i n c l o s e p r o x i m i t y to the c a p i l l a r y tube. In passing, i t may be o f i n t e r e s t to note that the p h o t o - v o l t a i c c e l l i s p r e f e r a b l e to e i t h e r g a s - f i l l e d , or vacuum-type photo c e l l s f o r t h i s a p p l i c a t i o n . The c e l l s r e q u i r e no e x t e r n a l source of energy (except l i g h t ) , are not s e n s i t i v e to i n f r a r e d (heat) r a y s , and because of t h e i r m i n i a t u r e s i z e , are easy to i n s t a l l . C o n t r o l U n i t : For d e s c r i p t i v e purposes the remaining p a r t s of the c o n t r o l u n i t w i l l be t r e a t e d under two headings: (a) the e l e c t r o n i c system and (b) the electro-mechanical system, (a) The E l e c t r o n i c System*, embodies the a m p l i f i e r and r e l a y switching components. An a m p l i f i e r of the d i r e c t current type i s employed, because a D.C. output i s more r e a d i l y obtainable from the photo c e l l . The use of a D.C. a m p l i f i e r e l i m i n a t e s the need f o r a l i g h t "chopper™ or other means of producing p u l s a t i n g inputs from the c e l l The a m p l i f i e r c o n s i s t s o f a h i g h v o l t a g e gain stage, f o l l o w e d by a power a m p l i f y i n g stage. The r e l a y s are connected i n the output c i r c u i t of the power stage. The c i r c u i t of the a m p l i f i e r i s shown i n F i g . 18(a). Conventional symbols are used to denote the various components i n the c i r c u i t diagrams (1) Symbols as o u t l i n e d i n handbook of American Radio Relay League. T o P o w e r PacM 3 0 0 V ( + ) R 9 • V v V W V • T A W ^ W V — I Rio Ria R u R13 7"o /"ower PacMC—) Relay Poles VI C 3 1 Motor lb ] C I R C U I T O F S E R V O M O T O R FiLame n fs 6 6 < p # eating rs (CO C I R C U I T O F A M P L I F I E R FIG 1 8 . A U T O M A T I C C O N T R O L 92. The c e l l s (2 No. B2M.'s) I n t e r n a t i o n a l R e c t i f i e r Corp. form part of a c l o s e d c i r c u i t which i n c l u d e s the r e s i s t o r s R^ and Rg. The negative terminals of the c e l l s , and the j u n c t i o n of R^, R2 , are brought to a common ground, by a connection to the a m p l i f i e r c h a s s i s . T h i s , i n e f f e c t , y i e l d s two independent loops, each comprising a c e l l w i t h a r e s i s t a n c e across i t s t e r m i n a l s . This arrangement provides a p a r a l l e l i n p u t . A change i n flow of cu r r e n t from a c e l l to ground, pro-duces a corresponding change i n the g r i d p o t e n t i a l o f the therm-i o n i c tube a s s o c i a t e d w i t h that loop. Thus, changes i n the p o t e n t i a l of X on R^, e f f e c t the g r i d p o t e n t i a l of V^ only. S i m i l a r l y , a voltage change at p o i n t Y r e f l e c t s on the g r i d p o t e n t i a l o f Vg. Consider, f o r the present, the case where an equal increase has occurred i n the i l l u m i n a t i o n of both c e l l s . The g r i d s o f V^ and Vg w i l l then acquire a p o s i t i v e charge, which i s p r o p o r t i o n a l to the change i n i l l u m i n a t i o n a t the c e l l . The p o s i t i v e charge w i l l i n c rease the c o n d u c t i v i t y of the tubes (V-^  and Vg) so that more c u r r e n t w i l l flow through the lo a d r e s i s t a n c e s (RiQ-' - I*i2» R ^ l " ^13^* Increase i n p l a t e current w i l l produce a voltage drop across the l o a d r e s i s t a n c e s i n accordance w i t h Ohm's Law. Voltage changes are tapped o f f R^Q and R-^ and a p p l i e d to the g r i d s o f the power tubes (V3 and V ^ ) . Here a voltage drop has the e f f e c t of reducing the current flow through the r e l a y s A and B which are incorporated i n the anode c i r c u i t of V 3 , V^. The r e l a y contacts w i l l open i f the current f a l l s below a preset v a l u e . Reducing the i l l u m i n a t i o n a t the 93. c e l l has the opposite e f f e c t - tends to cl o s e the c o n t a c t s . When the mercury l e v e l i s at the n u l l p o s i t i o n r e l a y "Art i s open and "B^ i s c l o s e d (closed i n t h i s sense means that the contacts are p u l l e d i n towards the r e l a y c o i l ) . The power supply to the feedback motor i s connected to the poles o f r e l a y switches i n such a way that the motor i s i d l e f o r t h i s c o n d i t i o n . The motor i s switched on only when both relaysr.are e i t h e r i n the open or c l o s e d p o s i t i o n s . As may be surmised, t h i s occurs when the mercury l e v e l departs from the n u l l p o s i t i o n . The c i r c u i t r e l a t i n g to the motor and r e l a y switches i s shown i n P i g . 18(b). The f u n c t i o n s of the other components i n the a m p l i f i e r w i l l now be discussed b r i e f l y . The condensers C-^  and Cg are intended to smooth out r i p p l e from the p h o t o - c e l l output, caused by the a l t e r n a t i n g c u r r e n t supply to the e x c i t e r lamp. The g a i n of the vo l t a g e a m p l i f y i n g stage may be r e g u l a t e d by s e t t i n g the screen p o t e n t i a l s o f and Vg, by means of potentiometers R^, Rg. The r e s i s t a n c e s R^, R^ and R Q provide g r i d b i a s f o r V-^  and Vg. Considerable d i f f i c u l t y was experienced i n d e v i s i n g a means of b i a s i n g the power tubes V^, V^. This problem i s en-countered i n m u l t i s t a g e d i r e c t c u r r e n t a m p l i f i e r s , because the g r i d of a f o l l o w i n g stage i s at a p o t e n t i a l c l o s e to the anode voltage of the preceding stage. The d i f f i c u l t y was overcome i n t h i s case, by employing neon voltage droppers (V7, V Q) i n the g r i d c i r c u i t s ; and, by i n t r o d u c i n g a voltage r e g u l a t o r tube (V/r) i n t o the cathode c i r c u i t , i n s t e a d o f the u s u a l cathode 9 4 . r e s i s t o r . In f a c t , i t i s the l a t t e r i n n o v a t i o n that makes the operation of the a m p l i f i e r p o s s i b l e . "Bleeder" r e s i s t o r s (%4> R i 5 * R i 6 ^ a r e P r o v i < i e d i * 1 order t h a t V 7, V 0 and V remain " f i r e d " while the a m p l i f i e r i s I o y i n o p eration. The p l a t e p o t e n t i a l s o f V3, may be e q u a l i z e d by a d j u s t i n g the potentiometer R17. (b) The Electr o m e c h a n i c a l System. The electromechanical system c o n s i s t s of an e l e c t r i c motor, geared to d r i v e a p i s t o n i n an a u x i l i a r y c o n t r o l c y l i n d e r - the a u x i l i a r y c y l i n d e r being connected d i r e c t l y to the main c o n t r o l c y l i n d e r of the pore pressure apparatus. The motor which i s r e v e r s i b l e , gives an output speed of 1^ - r.p.m. IPurther r e d u c t i o n i n speed i s pro-v i d e d by a worm gear r e d u c t i o n u n i t . The design of the a u x i l i a r y c y l i n d e r i s patterned on that of the main c o n t r o l c y l i n d e r . For s o i l s o f low p e r m e a b i l i t y , such as the Port Mann c l a y , a feed-back r a t e o f 0.077 cubic inches per minute has been found s a t i s -f a c t o r y ; "hunting" being e l i m i n a t e d e n t i r e l y at t h i s r a t e . A f a s t e r r a t e of feedback would perhaps be an advantage i n more permeable s o i l s . The i n s t a l l a t i o n of the automatic c o n t r o l has not i n t e r f e r e d w i t h the manually-operated system. The only modi-f i c a t i o n s to the e x i s t i n g pore-pressure apparatus c o n s i s t e d o f inter c h a n g i n g the 1 mm bore c a p i l l a r y tube f o r a 1.5 mm tube. The l a r g e r boro c a p i l l a r y tube i s r e q u i r e d to provide a p o s i t i v e gate to the p h o t o - c e l l u n i t . L i g h t s c a t t e r i n g i n the w a l l s o f 1 mm. ID.(8 mm 0D) tube introduced d i f f i c u l t i e s i n t o the 95. adjustments of the a m p l i f i e r . Provided t h a t the mercury i s maintained at the n u l l p o s i t i o n , i n c r e a s i n g the bore of the tube i s not a serious disadvantage. A tube w i t h square e x t e r n a l cross s e c t i o n would overcome t h i s s c a t t e r problem and thus per-mit use of smaller i n t e r n a l bore. L i m i t switches are provided f o r breaking the power supply to the motor i n the event of e x c i t e r lamp f a i l u r e , or adverse operation of the c o n t r o l . The t e s t specimen i s thus p r o t e c t e d a g a i n s t i r r e g u l a r i t i e s a r i s i n g i n e l e c t r i c a l system. Performance; The c o n t r o l has been found to r e s t r i c t the pore water movements to somewhat l e s s than 2 ram from the n u l l p o s i t i o n . This represents about 1 p a r t i n 200,000 of the pore water f o r Port Mann c l a y . On a r e l a y c l o s i n g , the mercury i s r e s t o r e d to the n u l l p o s i t i o n i n about 45 seconds, which allows s u f f i c i e n t time to e q u a l i z e the pressure a t the t i p of the probe. Pressure e q u a l i z a t i o n i s l e s s c r i t i c a l when measuring pore pressures at the ends of the specimen. Operation: The p h o t o - c e l l u n i t must be p o s i t i o n e d on the c a p i l l a r y tube i n such a manner fchat the mercury a t the n u l l p o s i t i o n appears i n view at the observation s l i t . The c o n t r o l s Rc» R » R,,* & r e then adjusted to give the optimum s e n s i -5' 6' 10 11 t i v i t y to movements of the mercury column. A warming-up p e r i o d of 15 - 30 minutes i s r e q u i r e d to o b t a i n s t a b l e o p e r a t i o n of the a m p l i f i e r . Power Pack: The power pack f o l l o w s conventional design, and su p p l i e s 300 v o l t s r e g u l a t e d output to the a m p l i f i e r . Two 96. voltage r e g u l a t o r tubes i n c o r p o r a t e d i n the power pack provide a s t a b l e output voltage - independent of the l o a d and moderate f l u c t u a t i o n s i n main's p o t e n t i a l . P o s s i b l e Improvements: Although the s e n s i t i v i t y of the c o n t r o l u n i t to changes i n mercury l e v e l i s adequate f o r the present purpose, there appears to be l i t t l e d i f f i c u l t y i n modifying the apparatus to o b t a i n even greater s e n s i t i v i t y . For i n s t a n c e , r e p l a c i n g the selenium c e l l s by the new s i l i c o n type photo c e l l s , would increase the s e n s i t i v i t y c o n s i d e r a b l y . However, the l a t t e r c e l l s are more expensive, and u n t i l r e c e n t l y were not a v a i l a b l e commercially. An o p t i c a l system r e p l a c i n g the present o b s e r v a t i o n s l i t would perhaps be more e f f e c t i v e i n checking the operation of the u n i t . In order that the c o n t r o l apparatus may be of s e r v i c e i n t e s t s on a l l s o i l types, a v a r i a b l e speed gearbox i s p r e f e r a b l e to the present system o f f i x e d g e a r i n g between motor and a u x i l -i a r y c o n t r o l c y l i n d e r . A smaller bore c a p i l l a r y would be o f a s s i s t a n c e i n the d e a i r i n g o p e r a t i o n . The complete i n s t a l l a t i o n i s shown i n the photographic supplement. 97 CHAPTER VII.  SUMMARY OF CONCLUSIONS (a) The samples ;were not saturated on rec e i p t at the laboratory. B values indicate that the samples were probably saturated i n the loading stage of the shear t e s t s . (b) The s o i l i s an extrasensitive clay with s e n s i t i v i t y indices i n the range 30 - 80. Some leaching of the natural deposit appears to have occurred, (c) A slow build-up of pore pressure was observed i n a l l t r i a x i a l t e s ts. The rate of build-up was l e a s t for a x i a l loading. End r e s t r a i n t retarded the build-up of pore pressure at top and base of specimen; a slower rate being recorded at the ends than at the centre. The pore-pressure l a g i n saturated specimens has been a t t r i b u t e d to p l a s t i c deformations of the adsorbed layers surrounding the p a r t i c l e s . P l a s t i c deformation i s assumed to lead to a realignment of the p a r t i c l e s with time - an e f f e c t most noticeable for applications of u n i d i r e c t i o n a l stresses, (d) In t r i a x i a l tests, the specimens f a i l e d on eithe r one or 98 two shear planes. The strains at f a i l u r e ranged from 3 to 7$. Values of the strength parameters are as follows: C = 3 lb./sq. i n . f = 21° c u r 4«5 lb./sq. i n . The true parameters could not be derived from the results of the shear t e s t s . (e) Th. c o e f f i c i e n t of permeability wa s estimated at 4.7 s 1 0 ^ inches per minute - a value to be expected i n clays of this type. (f) Side drains are not e f f e c t i v e when subjected to high c e l l pressures. (g) The automatic c o n t r o l provides s a t i s f a c t o r y regulation of the pore water movements i n t r i a x i a l tests on clayey s o i l s . i APPENDIX I. The following data was supplied by R. A. Spence, Consulting Engineers. Results of vane te s t s : BORING NO. i DEPTH i BELOW J GROUND 1 LEVEL J SHEARING • ' RESISTANCE ! J UNDISTURBED J J l b . / s q . f t , • SHEARING RESISTANCE REMOULDED l b . / s q . f t . BS2 j 135,-6»» i 1930 ; 30 tt i i36« - 3 n j 1860 j 60 ti ! 1371-Ott j 2300 j 30 BS1B j 143 ,-0" j 2275 i 30 « • i43t - 9 « j 2330 J 60 n • 147*-9 t t J 2045 { • 1 60 BORING l SAMPLE* NO. J NO. I WATER ! CONTENT ! % i LIQUID LIMIT { PLASTIC ! LIMIT J GRAIN SIZE: MI.T SCALE ! SAND% SILT % CLAY % BN23F ! 21 ! 68.3 | 66.6 j 38.6 j 0 J 10 J 90 j * 22 ! 66.4 j 67.4 ! 35.3 1 1 1 j * 23 j 67.6 j 62.8 i 34.1 ! 1 I ! 2 4 ! 65.1 j | I i j * 25 j 66.6 j j * i j ! 26 j 57.2 j 68.5 ! 35.6 i 0 J 7 i 93 ! *27 j 61.8 | ! ! ! ! BS2F ! * 40 ! 68.1 j | ! i i 1 * 42 | 61.3 j j i S i ! 43 J 59.4 j 63.7 j 29.4 ! 11 j 9 j 80 ! 44 { 61.4 ! | i J ! j 45 j 57,8 j i i i i j * 46 j 58.1 j | j { I * Samples available for the tests undertaken for this thesis. DRY DENSITY lb./cn.ft. 61.3 63.8 67.9 61.7 63.6 67.2 ESTIMATED ? OVERBURDEN PRESSURE: EFFECTIVE 56.5 51.5 DEPTH BELOW GROUND LEVEL 137'6W to I S S M " 138*4n to 139'2M 139,6n to 140*6" to 141'4" 147'9M to 148,5M 148*7M to 149,5M 151*3" 152*1" to APPENDIX I (Cont'd.) i i i P O R T M A N N . C O N S O L I D A T I O N T E S T J O B N O : R . A . S P E N C E , P. E N G . H O L E ^ N ^ i E S A M P L E 3 7 DATE-. iv o . i ui at -j a> to -P R E S S U R E - K G / C M 2 I.O IO.O M u & ui o> sj co <D - w w A ui o> si co ID -O > 0 - 9 0 8 UI 01 s| CO (0 . M U Ul 01 vl CO O.I N U Ul 31 sj CO <£> -I.O IO.O P R E S S U R E - KG / C M 2 10 (il tt UI c P O R T M A N N . C O N S O L I D A T I O N T E S T J O B N O : R . A . S P E N C E , P. E N G . H O L E B5 a F. S A M P L E 43. D A T E : V B N £ 3 F so IOO /so /30 /90 /93' III Peat and Organic Si it S <xn d. Cla.yey SiL f 5ancly Gra-veL MARINE CLAY Pervious GnnveL (artesian) . GLCLCICLL t i LL o' 3' 16 lOl lib' I6S 16$' IBZ' "t *•>•/] * x 1 Pear Organic Silt O . • I D ' , *o ° i o S a. n ci with 5i Lt L ayers. 5 (xncl CLncl CroveL MARINE CLAY band and CrcureL ia.r te s i an). GLCLCICLL f i L L TILL and isater-S or ted. L ayers . Borehole Logs C /'de a. L ix e d) v i APPENDIX I I CORRECTION OF COMPRESSIVE STRENGTHS ON  BASIS OF EXPERIMENTAL FACTORS The corrections described below were made to the apparent compressive strengths (deviator stresses) as determined by a x i a l loading. 1. Area Correction. The cross sectional area was correct-ed, f o r both..the e f f e c t s of drainage and deformation of the specimen, i n accordance with the expression: a a a G (1 - AV) T 1 - £ Where(a)denotes the area on which true deviator stress i s calcul a t e d [ins^] a Q denotes the i n i t i a l area [ins 2] AV denotes volume reduction by drainage V denotes i n i t i a l volume of specimen [ins*] £ denotes a x i a l s t r a i n [ins/in] The area correction was ignored once a shear plane i n the specimen became v i s i b l e . 2» Rubber Membrane Correction. According to Bishop and Henkel (1957) only the a x i a l stresses i n the t r i a x i a l test need be corrected for membrane r e s t r a i n t . The correction to be applied v i i to c y l i n d r i c a l specimens i s then given by the expression: D Where (0% - ( T l ) denotes the corrected deviation stresses J - o m (O-^ - Oj) denotes the apparent devlator stress w denotes the thickness of membrane (s) [ins] ^ denotes Young*s modulus of rubber l b s . / i n s . ^ € denotes the s t r a i n [ins./in.] D denotes diameter of specimen at s t r a i n € Calculations based on F i g . 18 showed this correction to be in the order of 0.25 l b . / s q . i n . at 5% s t r a i n , for the twin membranes employed i n the tests. 3. Drain Corrections. The correction f o r r e s t r a i n t a r i s -ing from f i l t e r drains i s also made to the a x i a l stresses. I t i s not dependent on the s t r a i n however and i s normally considered to be le s s than 1 lb . / s q . i n . , Bishop and Henkel (1957). A nominal co r r e c t i o n of 1 l b . / s q . i n . was applied to the a x i a l stresses to include the e f f e c t s of both rubber membranes and f i l t e r drains. No correction was deemed necessary for plunger f r i c t i o n , but the weight of the end f i t t i n g s were i n -cluded i n the a x i a l loads. APPENDIX I I I . Calculations based on data obtained i n Test 6. 1. Coe f f i c i e n t o f Consolidation (C v) Coe f f i c i e n t of consolidation based on pore pressure vs. time r e l a t i o n s h i p : G v = T v H 2 * t 5 0 = 0*38 x 2.5) 2 - 0.0054 ins2/min time fa c t o r of 0.38 length of drainage path : neglecting I n i t i a l deformations time for 50% consolidation: minutes from Graph 4*15. Where T y -H -2.5tt * 5 0 -(21) 2 # "Fundamentals of S o i l Mechanics'*, Taylor (1948), pp.235. ix C o e f f i c i e n t of consolidation based on volume change vs time r e l a t i o n s h i p , assuming r a d i a l and end drainage. C v s TT H 2 ** i o o t 1 0 O = 3.14 x (2.5) 2 10G x (27.5) 2 = .00026 ins 2/min Where H = 2.5 n as before t, « 0 - time for 100$ primary consolidation, x Bishop and Henkel (1957). t 1 0 o - (27.5) 2 minutes from Graph 4-15. **• "The T r i a x i a l Test", Bishop and Henkel (1957), pp 126 X C o e f f i c i e n t of consolidation based on volume change vs. time r e l a t i o n s h i p , assuming end drainage only. C = 77" H 2 ** v •• 4 t100 - 3.14 x( 2 . 5 ) 2 4 x (27.5) 2 0.0065 ins 2/min. Where symbols have same meaning as those on preceding page. Calcula t i o n of pore pressure parameter A at f a i l u r e . A f s A u - B(£>0~j) B(A0~^ * A 0 ~ 5 ) f - 30.25 - 40(0.51) 0.51 x 19.1 - 1.01 x i BIBLIOGRAPHY A. Books Baver, L. D . , 1 9 5 6 " , S o i l Physics, 3rd. ed., New York, Wiley. Bishop, A.W. and Henkel, D . J . , 1 9 5 7 , The T r l a x i a l Test. London: Edward Arnold. Lambe, T.W., 1 9 5 1 * S o i l Testing f o r Engineers. New York: John Wiley. Taylor, D.W., 1 9 4 8 , Fundamentals of S Q U Mechanics. New York: John Wiley. Terzaghi, K. and Peck, R.B., 1 9 4 8 , S o i l Mechanics i n Engineering Practice . New York: John Wiley. Tschebotarioff, G.P., 1 9 5 1 , S o i l Mechanics, Foundations and Earth Structures. New York: MoGraw H i l l . B. Journals and Reports Bishop, A.W. and E l d i n , G., 1950, "Undrained T r i a x i a l Tests on Saturated Sands and their Significance i n the General Theory of Shear Strength" Geotechnlque, 2, 13-32. Bjerrum, L., 1954* "Geotechnieal properties of Norwegian -Marine Clays" Geotechnlque, 4 f 49-69• Burton, L. J . , 1956, "A New Device f o r Automatic Pore-Water Pressure Determination™ C i v i l Engineering  and Public Works Review. Casagrande, A. and Wilson, S.D., 1949, Report to U.S. Waterways Experimental Station. Grim, R.E., 1959, "Physico-chemical properties of S o i l s : Clay Minerals". Jour A.S.C.E. Vol.85 No. SM2. 1998; 1-TFl A l l Lambe, T.W., 1958, "The Structure o f Compacted Clay", Jour A.S.C.E. Vol. 84, No. SM2, 1654; 2-32. Miehell, J.K., 1956 "The Importance o f Structure to the Engineer-ing Behavior o f Clay. Sc.D. Thesis M.I.T. Penman, A.D.M., 1953, "Shear Characteristics of a Saturated S i l t " Geoteehnique I I I : 8, 312-328. Plantema, G., 1953, " E l e c t r i c a l Pore-Water Pressure C e l l s : Some Designs and Experiences" Proc. 3rd. Int. Conf. S o u Mech., 1:279^ 28"2*7 Rendulic, L., 1937, " E i n Grundgesetz der Tonmechanik und sein experlmentaler Beweis" Bauingenieur, 18: 459-467. Rosenquist, I.T., 1959, Physico-chemical Properties of S o i l s : Soil-Water Systems* Jour A.S.C.E. Vol.85, No. SM2. 2G00; 31-52. Rowe, P.W., 1959, "Measurement of the C o e f f i c i e n t of Con-s o l i d a t i o n of Lacustrine Clay". Geoteehnique IX: 3, 107-118. . Skempton, A.W., 1954, Geoteehnique 4 pp 143-147. Skempton, A.W., and Bishop, A.W., 1950. "The Measurement of the Shear Strength of S o i l s " . Geoteehnique I I : 2,90-108. Skempton, A.W., and Bishop, A.W., 1954, " S o i l s " Building Materials '* * .... Amsterdam ,their E l a s t i c i t y and i n e l a s t i c i t y . : North-Holland Publishing Company. Skempton, A.W., and Northey, R.D., 1952, "The S e n s i t i v i t y of Clays"• Geoteehnique 3, 30-54. Taylor, D.W., 1943, Ninth Progress Report on Shear Research to U.S. Engineers, M.I.T. Publication. Winterkorn, H.P., 1941, "Study of Changes i n Physical Properties of Putnam S o i l Induced by Ionic S u b s t i t u t i o n " . Proc. Highway Res. Bd. 21, 415-434. SPECIMEN PREPARATION Specimen w i t h Side Drains i n P o s i t i o n . SPECIMEN PREPARATION (Cont'd.) A p p l i c a t i o n o f Twin Membranes and S e a l s . SPECIMEN PREPARATION (Cont'd.) Prepared Specimen. Complete I n s t a l l a t i o n Showing T r i a x i a l Machine, P o r e -P r e s s u r e Apparatus and Automatic C o n t r o l . To rollow pa-ge S4-CeLL pressure o~j = 60^6 per sq.fn-V~, = SO 6\* 3 0 <r3 -. 12. ffauge. 10 so 30 40 so &O 7 0 E L a p se ci . -Time -minu-tes. so so IOO 110 D R A I N A G E S T A G E - T E S T £ . J~t in m / n s 3 2 £ 8 2 4 2 0 t> tn mins 48 O 70 CeLL pr&.ssure 0~3~ 8 0 Lb per sq.in. © TofoLLow pcuge S3 (A — /r> <0 Q Cell p r e s s u r e 0~s - 2 0 lb per sq. in--&— ; : , : ; o Dra-i nag e : Por e -pressure oCrOj z.er o o-f po r e - p ressure gauge-To foLl.oliv page 66 0~~3 .= A-O lb p e r sq. in-to a o 3 0 4 0 . SO £ L exp s e aL time. - minutes. 10 zo 3 0 4-0 SO 61 E Lapsed, time - minutes D R A I N A G E S T A G E - T E S T j6. 3a 36 40 44 46 o 3 o . M o h r D i c x q r a m - Eff e c l r i v e Sfesses. To foLLowG "ra.ph 4-G R A P H 4-gg T o -follow Graph 4-G R A P H ^Pa^" lO X lO TO THE INCH 3 5 9 - 5 D G KEUFFEL & ESSER CO. OAOE t(f U s.A. 1 Vi ?: r ?\ r T e £ s CIO 1 1 1 1 L A 0 1 T - fa • " i " u 7 7 W fa ^, — j 9" 5 c r 1 %• c 3T/7C 1 J 7 -\ u 2. Q A, | « 0 «a ,— — / N t -[/ f s c A ( > r 0 r 1 1 0 ?. 0 4 0 & .0 8 0 LOC -4. c\f r r / t -b- > S > 7 O T r e 5 S S 5 ' in u X /V, 1 > * ro > S ' .V V »l 1 -f ,1 To foliow pcLje 5 Crystalline Comp (a) QancCCj Oxygens. (b) Q ccnoL C^i Hydroxyls FIG 1 . orients ot Clay Minerals. (C) Oa.ncL O Si'Cicons-(cL) Q AL amin u m s f moLonesfums etc. C L A Y M I N E R A L S . (After RE. G rim. I95S) To folio*/ pa.ye 7. 'N V I — 0 <0 -s. _ 0 QI -<J _ V _ <o -+ + + + + + + + 4-+ + Liquid (a) HeL/nholtz Double. Layer v. v. \ _ >s d Vj • s. o . 0 u a ^ 0 ^ C a. rio n 0 \ 0 ^ Wafer tnolecuLe (b) lon-wctfer compie F I G 2. H E L M H O L T I A N D D I F F U S E L A Y E R S To foUou/ page 7. — 4 - + -v - + •Si — + O <o - + X - + I J _ + <3 S, - 4-v _ + + (cx) HeLmholfz Double Layer. u CJ .^ 0 . <!) v <0 C o . rion S 0 \ <^^> Liauid X 0 A. (b) Ion-wafer complex. F I G £ . H E L M H O L T Z A N D D 1 F F U 5 E L A Y E R S . (Sc he rncxt/c) To follow pa.ye 8 K Double L a y e r Wafer Adsorbed. Wafer ont rn or i Li on/ fe Crystal Double Layer Wafer Monfm or i lions'-re S h e e f ioA ^Double L a y e r V / a f e r ^ Adsorbed Water^ v- ~ k a o l i n ite Crys t a l K a o C i n i f e Sheet V 6 F I G 3 . C L A Y - W A T E R S Y S T E M . ( A f t e r T.W. Lambe.. I9SB) To -follow pctje /2. F I G 4 . S T R U C T U R E OF M A R I N E C L A Y . (After T.K.Ta.n. /9S~7.) To follow page IS B \ r 0-3 ^ ; H cr, H NormcuL Stresses- «-FigS. Mohr Diagram : Tofai and Effective Stresses Normal. Stresses Fig 6 . Mohr Diagram : Tofai Stresses FIGS. 5 and 6 T o foLLow page 2.1 ConsoL idat ion Pressure F I G 7. C O N S O L I D A T I O N H I S T O R I E S Effective Stress at failure F I G 8 . M O H R D I A G R A M . T R U E P A R A M E T E R S F 1 GS 7 and. 8 To foiiow paye 31 F I G 9 . S T R E S S - C O N T R O L L E D T R I A X I A L M A C H I N E FIG 10. T R I A X I A L C E L L To foLL ow po.j e 3 7. Layout of Appa ret t us for Meas u ring Pore Pr ess ur & . F I G 13. P O R E - P R E S S U R E A P P A R A T U S :" • To f-oLLow page 4-3 F?:i'Cf II. Proving-rIn g Co rr e c tio,n •IO zo so 40x10"* inches Proving- r/nq Oeliect/on Fig IE . Rubber Membrane Stress vs. srrain Curve. o 5 /o /s~ BO 2 5 Straih-per cent o L o F I G S - II a n d . 12 To follow pa-oe 77 Direction of E x t e r n a. I Pressure Time tQ : External. Pre ssut-e taken partly by Tn fe r g r a.n u I a. r Stresses . Time t, : External Pr ess u re. take n e.ntir-e.ly by Pore Water. F I G 14. C L A Y - W A T E R S Y S T E M : E F F E C T S OF P L A S T I C D E F O R M A T I O N OF T H E A D S O R B E D L A Y E R S .  To Specimen Con tr o I f"o Re Lay Ko, r&r Merc ur y. To follow pa ye. 86 From Relai IH_3 F I G 1 5 . PORE-PRESSURE DEVICE {After A.D. Pe n m a.n 1953) P. E.G. T L - W V V - T J 5 T L A W - r j Acfua-tor proy Ac f oaf or P. E.G. F I G 16. C I R C U I T OF A U T O M A T I C C O N T R O L (After L.J. Button IBS6) F I G S 15 a n d I 6 . To Joltow page 89. To Spec/men AmpC i J /'er. Null Indicator] F I G 1 7 . L A Y O U T O F A U T O M A T I C C O N T R O L To Power Pack 3QOV(+) R 9 • A / W V W R i o R I& W W W — 1 v 5 To Power Po.ckC—') Relay Poles <0 o f Motor lb ] C I R C U I T O F S E R V O M O T O R FiLam e « t s j> |> |> |> <*> ^  (CU C I R C U I T O F A M P L I F I E R FIG 1 8 . A U T O M A T I C C O N T R O L IS Cr-L to A . *J IO Q) «o co <0 i» S To /otto uv'Graph A--16 Pore pressure. - u. r Deviafor sfr ess-(07 - c3) 10 ao zo 4-0 so 60 10 71 me ep hours so 100 12.0 13 O 14-0 ISO IbO St rev in - £ L O A D I N G S T A G E T E S T &• G R A P H 4 - 1 7 To -foLLow Graph 4--19 L O A D I N G S T A G E - T E S T 71 1 1 G R A P H 4-20 / C M 2 . 2-4-6 cms 1-52 6 Grey Clay with darker streaks. /•6 IS /•4 0-9 0-& P O R T M A N N . BS a F. 4 3 ^ 3 6 . 1 5+ P6 2-4B cms ..'ifr'itt.'*.- .J-'iMS--iq-o -a Grey Clay with darker streaks. 1-8-1-2.' I-l • 0 - 8 t ^ £ pj . P. E L s i C ; £ JB.N £ 3 F- SAMPLE. . MX - - ff^o ^ 7 • ffl^ • T^S5& J "~ r - — . - — :- i *^|^ B s e B N £ 3 F O' so' IOO /BO /SO f90 / 9 3 ' a to o Peat and Organic Silt S and Clayey SiLf Sanely G ra ve L MARINE CLAY Pervious GRavel (artesian). Glacial t i l l 3 ' 16' l i f t 1 Vf'/y X X l " ' X K vitr X lor I6S 7 6 9 ' /92* • 0 t> 'a o • © Peat Organic Silt Sand With Silt layers. Sand and Grovel MARINE CLAY Sand and Gravel .la-rtes /an). Glacial f i l l Till and vater-s or ted layers. Bore ho L e Logs C fete a. Li 7. e ct). 

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