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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 I N PARTIAL FULFILMENT OP THE REQUIREMENTS FOR THE DEGREE OP M. A. S c . I N THE DEPARTMENT of C I V I L 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 the  this thesis  r e q u i r e m e n t s f o r an  of  B r i t i s h Columbia,  it  freely available  agree that for  Department  copying  gain  shall  or  not  his  University  Library  s h a l l make  for reference  and  study.  I  for extensive be  copying  granted  representatives.  publication be  the  of  the  p u r p o s e s may  o r by  that  advanced degree a t  fulfilment  I agree that  permission  scholarly  in partial  the  It i s  of t h i s t h e s i s  allowed without  Department The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a .  by  Columbia  my  of  further this  Head o f  thesis my  understood  for financial  written  permission.  ii  ABSTRACT The r e s u l t s 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 i n t o the pore p r e s s u r e c h a r a c t e r i s t i c s o f an e x t r a s e n s l t i v e marine c l a y are presented.  The  s o i l samples were o b t a i n e d from the P o r t Mann  area o f B r i t i s h Columbia.  E x p e r i m e n t a l work c o n s i s t e d o f the  performance o f l o n g - d u r a t i o n t r i a x i a l p r e s s u r e measurements.  shear t e s t s w i t h p o r e -  A s t 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 employed f o r a l l shear  device  was  tests.  The observed data show t h a t f o r t h i s s o i l a slow b u i l d - u p of pore p r e s s u r e occurs f o r both i n c r e a s e s i n e e l l p r e s s u r e a x i a l s t r e s s e s i n the t r i a x i a l t e s t .  and  Even i n s a t u r a t e d s p e c i -  mens the slow b u i l d - u p e f f e c t p r e v a i l e d .  The r a t e s of b u i l d - u p  observed f o r changes i n a x i a l s t r e s s were slower than r e c o r d e d f o r changes i n c e l l p r e s s u r e *  those  Measurements a t the  upper end o f some specimens, and a t the c e n t r e o f o t h e r s ,  indi-  c a t e d that the pore p r e s s u r e r e q u i r e d more time t o r e a c h e q u i l ibrium, a t the ends o f c y l i n d r i c a l specimens.  The hypothesis i s  put forward t h a t the o b s e r v a t i o n s can be e x p l a i n e d by deformations  o f the adsorbed  plastic  l a y e r s s u r r o u n d i n g the p a r t i c l e s .  S t r e n g t h and pore p r e s s u r e parameters have been o b t a i n e d for  the s o i l . An automatic  c o n t r o l has been developed t o a s s i s t i n the  performance o f l o n g - d u r a t i o n t e s t s .  The  apparatus  employs the  p h o t o - e l e c t r i c e f f e c t to c o n t r o l movements o f pore water. d e t a i l e d d e s c r i p t i o n of t h i s device i s presented*  A  iii  CONTENTS Page CHAPTER I PHYSICAL PROPERTIES OP SOILS  1  A,  INTRODUCTION  1  B.  SOILS - GENERAL  2  1. 2. 3« 4. 5» 6  #  Geological Aspects S i z e and Shape o f S o i l Particles M i n e r a l o g i c a l Composition o f Pine-Grained S o i l s S u r f a c e A c t i v i t y and Adsorbed Layers Plocculation S t r u c t u r e o f Marine Clay  2 4 5 6 10 10  CHAPTER I I STRENGTH THEORY  14  A.  INTRODUCTION  14  B.  EFFECTS OF PORE PRESSURE  15  1. 2* 3. 4» C.  R e l a t i o n s h i p Between Pore P r e s s u r e and Shear S t r e n g t h R e l a t i o n s h i p Between Compressive S t r e n g t h and Pore Pressure T o t a l S t r e s s Parameters T o t a l F r i c t i o n Angle and True Cohesion  FACTORS AFFECTING PORE PRESSURE: THE PORE PRESSURE PARAMETERS A AND B .  15 18 20 20 23  CHAPTER I I I APPARATUS: DEVELOPMENT AND OPERATION A.  TRIAXIAL SHEAR TESTS WITH PORE PRESSURE MEASUREMENTS 1, 2.  General 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  30 30 30 31  3. 4« 5. 6, 7« 8 9« 10. 11* 12» #  The T r i a x i a l C e l l L a t e r a l Pressure C o n t r o l Load and Deformation Measuring Devices Apparatus f o r Measuring Pore Pressure Automatic C o n t r o l F a b r i c a t i o n o f Membranes P r e l i m i n a r y T e s t i n g o f Apparatus P r e p a r a t i o n o f S o i l Specimens S e t t i n g up Specimen i n T r i a x i a l C e l l Temperature C o n t r o l  32 33 35 35 38 38 40 41 42 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.  2»  (a) T e s t 1  51  (b) (c) (d) (e) (f)  Test Test Test Test Test  2 3 4 5 6  52 55 58 60 62  (g)  Test 7  67  Apparent S t r e n g t h Parameters  69  P.  OBSERVATION  70  0.  MISCELLANEOUS TESTS  70  1.  M i n e r a l o g i c a l Composition o f P a r t i c l e s  2. Sensitivity 3. Atterberg Limits CHAPTER V DISCUSSION OF TEST RESULTS AND CONCLUSIONS A*  EFFECTS OP OVERBURDEN PRESSURE  70 71 72 73  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 A.  INTRODUCTION  B.  DETAILS OP APPARATUS FOR AUTOMATIC CONTROL OP PORE PRESSURE  84 84  88  CHAPTER V I I SUMMARY OP CONCLUSIONS APPENDIX I  Test' r e s u l t s  97 I  APPENDIX I I C o r r e c t i o n o f Compressive Strength on B a s i s o f Experimental F a c t o r s APPENDIX I I I Sample C a l c u l a t i o n s BIBLIOGRAPHY PHOTOGRAPHIC SUPPLEMENT  vi viii xi xiii  vi  GRAPHS To  Follow Page  1  Build-up Stage: T e s t 1.  51  2  Build-up Stage: T e s t 2.  54  Graph 4  3  Drainage Stage: T e s t  54  Graph 4  4  S t r e s s vs S t r a i n : T e s t  Graph  5  L o a d i n g Stage: T e s t  Graph 4  6  Build-Up Stage: T e s t  3.  57  Graph  7  Drainage Stage: T e s t 3w  57  8  S t r e s s vs S t r a i n : T e s t 3.  57  Loading Stage: T e s t 3*  57  10  Build-Up Stage: T e s t  59  11  S t r e s s vs S t r a i n : T e s t  12  L o a d i n g Stage: T e s t 4*  13  S t r e s s vs S t r a i n : T e s t  Graph 4 Graph 4  mm  -  4 -  4 -  Graph 4  -  Graph 4 m 9 Graph 4 Graph 4 Graph 4 Graph 4  -  -  2.  4.  mm  14  Build-Up Stages: T e s t  Graph 4  mm  15  Drainage Stage: Test  16  S t r e s s vs S t r a i n : T e s t  17  L o a d i n g Stages T e s t  Graph 4  mm  54  2.  Graph 4  Graph 4  54  2.  59  4.  59 61  5.  66  6.  66  6.  66  6.  66  6.  Graph 4  - 18  Graph 4  19  S t r e s s vs S t r a i n : T e s t  Graph  20  Loading Stage: T e s t  21  Mohr Diagram of E f f e c t i v e  Stresses  72  22  Mohr Diagram o f E f f e c t i v e  Stresses  72  Graph Graph  4 4 4 -  Graph 4 «. 23 Graph 4 m 24 APPENDIX :I  Build-Up Stages: T e s t  68  7.  68  7.  68  7.  Mohr C i r c l e s o f E f f e c t i v e S t r e s s e s vs Time Mohr Diagram: T o t a l  Stresses  Consolidation Test Results Borehole Logs  72 72 iii v  vii  ILLUSTRATIONS To F o l l o w Page Fig. 1  Clay Minerals  5  Fig. 2  Helmholtz and D i f f u s e Layers  7  Fig. 3  Clay-Water System  8  Fig. 4  S t r u c t u r e o f Marine C l a y  Fig. 5  Mohr Diagram: T o t a l and E f f e c t i v e Stresses  19  Fig. 6  Mohr Diagram:  Fig. 7  Consolidation Histories  21  Fig. 8  Mohr Diagram: True Parameters  21  Fig. 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 Machine  31  Fig.10  Triaxial Cell  31  Fig.11  Proving-ring Correction f o r C e l l Pressure Rubber Membrane: S t r e s s vs  43  Fig.12  Total Stresses  12  S t r a i n Curve  19  43  Fig.13  Pore-Pressure Apparatus  37  Fig.14 Fig,15  Clay-Water System: E f f e c t s o f P l a s t i c Deformation o f Adsorbed Layers Pore-Pressure Device  77 86  Fig.16  C i r c u i t o f Automatic C o n t r o l  86  Fig.17  Layout o f Automatic C o n t r o l  89  Fig.18  Automatic C o n t r o l  91  viii  L I S T OP TABLES Page  TABLE  I  TABLE  II  TABLE  III  TABLE  IV  TABLE  V  TABLE  TABLE  TABLE  TABLE  VI  VII  VIII  IX  L i q u i d L i m i t o f Putnam C l a y  9  Properties o f Shellhaven Clay  24  D e s c r i p t i o n o f Test Samples  49  Schedule o f Shear T e s t s  50  Determination o f 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  64  Apparent S t r e n g t h Parameters • P o r t Mann C l a y  69  M i n e r a l o g i c a l Composition o f  P a r t i c l e s - P o r t Mann C l a y  71  S e n s i t i v i t y I n d i c e s - P o r t Mann C l a y  71  A t t e r b e r g L i m i t s - P o r t Mann C l a y  72  ACKNOWLEDGMENT  The w r i t e r w i s h e s t o e x p r e s s h i s a p p r e c i a t i o n and i n d e b t e d n e s s t o 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 u n d e r t a k i n g .  He a l s o wishes  t o thank P r o f e s s o r J . P. M u i r , o f t h e Department o f C i v i l E n g i n e e r i n g , f o r making arrangements  t o p r o c u r e equipment r e -  q u i r e d f o r t h e e x p e r i m e n t a l work o f t h i s  thesis.  X  NOTATIONS  A  pore p r e s s u r e  parameter:axial.  A*  Angstrom u n i t  a  area  8^,  contact area o f p a r t i c l e s  B  pore p r e s s u r e p a r a m e t e r : a l l round  Y  surface  tension  d e n s i t y o f water C  w  C  rt  C  v  c  c o m p r e s s i b i l i t y o f water compressibility of s o i l  skeleton  coefficient of consolidation cohesion  c'  apparent c o n e s I o n : e f f e e t i v e  c^j  cohesion u n d r a i n e d : t o t a l s t r e s s e s  c  true  r  stresses  cohesion  A  incremental  change; p r e f i x  E  Young's modulus  6  unit strain  f  at/to f a i l u r e :  7j  porosity  6  angle  k  c o e f f i c i e n t of p e r m e a b i l i t y  L  Length  u  Poisson's  CT  stress  suffix  o f f a i l u r e plane  ratio  t o plane of major p r i n c i p a l s t r e s s  NOTATIONS (Cont'd.)  CT,  major p r i n c i p a l  stress:total  0~  minor p r i n c i p a l  stress:total  matfor p r i n c i p a l  stress:effective  (fj  minor p r i n c i p a l  stress:effective  T  shearing stress  t  time  u  pore p r e s s u r e  V  volume  3  1  CHAPTER I . PHYSICAL PROPERTIES OF SOILS  A.  Introduction  I n the c o u r s e o f 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 o u t by R. A. Spence, C o n s u l t i n g E n g i n e e r s , on the m e c h a n i c a l p r o p e r t i e s o f P o r t Mann marine c l a y , anomalies were o b s e r v e d i n t h e pore p r e s s u r e from t r i a x i a l  vs. applied stress r e l a t i o n s h i p s , obtained  tests.  The r e s u l t s o f these t e s t s i n d i c a t e d t h a t  the b e h a v i o r o f the s o i l d e p a r t e d from a c c e p t e d theory pore p r e s s u r e s  concerning  o f c l a y s f r o m 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 n o t e d t h a t a  time l a g e x i s t e d i n a t t a i n m e n t of e q u i l i b r i u m between pore pressure  and t h e a p p l i e d t o t a l s t r e s s e s i n u n d r a i n e d  shear t e s t s .  triaxial  The r e s e a r c h r e p o r t e d i n t h i s t h e s i s was u n d e r -  taken t o i n v e s t i g a t e f u r t h e r t h e pore p r e s s u r e 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  vs. applied  on l o n g d u r a t i o n t e s t s .  S p e c i a l a p p a r a t u s and t e s t i n g p r o c e d u r e s were d e v e l o p e d f o r the long duration t r i a x i a l  tests reported i n this thesis.  2  By way information  of a b s t r a c t s from t h e  l i t e r a t u r e , background  i s p r e s e n t e d i n t h i s , and  the f o l l o w i n g Chapter.  A l t h o u g h p a r t of the d i s c u s s i o n i n Chapters I and I I a p p l i e s to s o i l s i n general,  p a r t i c u l a r emphasis i s p l a c e d on  p e r t i n e n t t o marine c l a y .  Chapter I I I and  topics  subsequent Chapters  d e a l w i t h the p r e s e n t i n v e s t i g a t i o n . B. 1.  S o i l s - General  Geological Aspects. Inorganic  s o i l s may  groups, r e s i d u a l and close proximity weathering are  be  transported*  separated Into  Residual  to the parent r o c k .  the  t r a n s p o r t i n g agency.  s o i l s are found i n  P h y s i c a l and  transported  the h i s t o r y o f a t r a n s p o r t e d  s o i l has  geographic f a c t o r s may  organic  source the  soils.  a marked  Parent m a t e r i a l , e r o s i o n  transportation agencies, depositional conditions,  Organic s o i l s are not  and  soils, irrespective of  are examples o f t r a n s p o r t e d  e f f e c t on I t s c h a r a c t e r i s t i c s .  a l l contribute  time  and  and  t o the observed p r o p e r t i e s *  i n c l u d e d i n the above s i m p l i f i e d c l a s s i f i component i n most s o i l s develops  as a r e s u l t o f the growth and In c o n t r a s t  chemical  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 , l a c u s t r i n e ,  e o l i a n and marine d e p o s i t s  The  major  D e t r i t a l accumulations remote from the  of o r i g i n c o n s t i t u t e  cation.  two  the main f a c t o r s governing t h e i r f o r m a t i o n  characteristics.  Usually,  broadly  to r e s i d u a l and  decay o f p l a n t s and  transported  accumulations are commonly c o n f i n e d  deposits,  in-situ  organisms* organic  to comparatively thin s t r a t a *  3.  Organic m a t t e r , or t r a n s p o r t e d  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  soils.  The f o u n d a t i o n e n g i n e e r  i s u s u a l l y most i n t e r e s t e d I n 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 o f 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 e s p e c i a l l y i n temperate c l i m a t i c zones.  stability,  On the o t h e r hand,  t r a n s p o r t e d s o i l s show c o n s i d e r a b l e v a r i a t i o n s from p l a c e t o p l a c e , even w i t h i n the same s t r a t u m .  Loose or s o f t  commonly o c c u r f o r c o n s i d e r a b l e d e p t h s .  deposits  Organic s o i l s must be  c a r e f u l l y c o n s i d e r e d f o r f o u n d a t i o n s , because of the h i g h 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 . the e n g i n e e r i n g p r o p e r t i e s has been f o c u s s e d m a i n l y the t r a n s p o r t e d  R e s e a r c h on on s o i l s  of  type.  D e p o s i t s w h i c h have been s u b j e c t e d to s t r e s s e s g r e a t e r than the p r e s e n t or precompressed.  o v e r b u r d e n p r e s s u r e are termed p r e c o n s o l i d a t e d U n l i k e normally consolidated deposits which  have not been s u b j e c t e d to excess p r e s s u r e s i n the p a s t ,  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 I n f l u e n c e d by  the  e x t e n t o f the p r e l o a d i n g .  the  The  e f f e c t i s most pronounced on  s t r e n g t h and s e t t l e m e n t c h a r a c t e r i s t i c s , T e r z a g h i and Peck The  temporary excess p r e s s u r e may  of s o i l s t r a t a which was  (1948).  have been c a u s e d by the weight  l a t e r removed by e r o s i o n a l a g e n c i e s .  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 e v i d e n c e i s o f much a s s i s t a n c e  I n a s c e r t a i n i n g whether a s o i l i s normal o r 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  Particles.  G r a i n s i z e and shape l a r g e l y determine the b e h a v i o r o f s o i l s as an e n g i n e e r i n g m a t e r i a l .  Coarse g r a i n e d s o i l s ,  as g r a v e l s , sands and s i l t s , a r e u s u a l l y c o h e s i o n l e s s ;  such  there i s  l i t t l e o r no tendency f o r the g r a i n s t o adhere t o one a n o t h e r . 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 . P a c k i n g 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 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 .  their  As the g r a i n s i z e  d i m i n i s h e s , t h e a c t i o n o f 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 a r e m a n i f e s t e d as c o h e s i o n .  i n the p r o p e r t y known  T e n t a t i v e l y , c o h e s i o n w i l l be c o n s i d e r e d as t h a t  p r o p e r t y w h i c h e n a b l e s a s o i l mass t o 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 n d i v i d u a l p a r t i c l e s i s excluded, l y fine grained.  I f cementation of  cohesive  By v i r t u e o f c o h e s i o n ,  s o i l s a r e predominant-  such s o i l s a r e o f t e n  c a p a b l e o f c a r r y i n g c o n s i d e r a b l e 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 o b s e r v e d t h a t c o h e s i v e mainly  s o i l s a r e composed  o f f l a k e - l i k e p a r t i c l e s , whereas g r a n u l a r s o i l s tend to  have m o s t l y c u b i c a l o r b u l k y f r a g m e n t s .  The f l a k y p a r t i c l e s  r e s u l t from t h e w e a t h e r i n g o f the l e a s t s t a b l e m i n e r a l s o f the parent rock.  E v i d e n t l y , cohesion  o g i c a l composition  i s r e l a t e d t o the m i n e r a l -  o f the p a r t i c l e .  Thus, q u a r t z  f o r example,  independent o f t h e f i n e n e s s o f the g r a i n s , behaves as a l e s s m a t e r i a l whether d r y o r f u l l y  cohesion-  saturated.  Other t h i n g s b e i n g e q u a l , the A t t e r b e r g l i m i t s  increase  5.  w i t h decrease I n g r a i n s i z e .  Skempton r e p o r t s t h a t 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 t h e s o i l , Skempton (1954).  5«  M l n e r a l o g l c a l Composition  of Fine-grained 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 m i n e r a l s a r e 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 .  C h e m i c a l l y , they are a l l c r y s t a l l i n e  arrangements o f s i l i c o n , aluminum, p o t a s s i u m , m o l e c u l e s , T e r z a g h i and Peck (1948)* laminated s t r u c t u r e .  oxygen and water  C l a y m i n e r a l s have a  Recent work r e p o r t e d by R. E. Grim  i n d i c a t e s t h a t the laminae a r e composed o f 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 o c t o h e d r a l l a t t i c e , Fig.  1 (a), (b).  S i m i l a r e l e m e n t a l b l o c k s combine t o 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  particular  atoms p r e s e n t and the arrangement o f the s h e e t s determine mineral type.  The  s h e e t s adhere to one a n o t h e r , thus  the i n d i v i d u a l p a r t i c l e s .  The  the  forming  flakiness characteristics  of  c l a y p a r t i c l e s can be t r a c e d to the m i n e r a l 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 o f two s h e e t s s e p a r a t e d by one o c t o h e d r a l u n i t . l a y e r i s about 9*5  s i l i c a tetrahedral The t h i c k n e s s of t h e  w h i l e the dimensions i n the o t h e r o  d i r e c t i o n s are i n d e f i n i t e , G r i m (1959)•  The 9.5  A l a y e r s are  s t a c k e d one above the o t h e r t o form the m o n t m o r i l l o n i t e (1)  E q u i v a l e n t diameter  two  l e s s than 0.002 m i l l i m e t e r s .  particle.  To foLLow  Crys  taltine Components  of  Clay  pa,ye i ~  Minerals.  (d)  (b)  O ccnoL. '^i HydroxyLs  FIG  (^Aluminums  1.  CLAY  f  magnesiums  etc.  M I N E R A L S .  (After R.E. G rim. / 9 5 9 )  6.  There i s l i t t l e The  bonding f o r c e between l a y e r s o f m o n t m o r i l l o n i t e .  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 o f t h i s m i n e r a l , i s  b e l i e v e d t o be e v i d e n c e o f t h e weak b o n d i n g . can penetrate  Apparently  water  between the l a y e r s , e n t e r the c r y s t a l l a t t i c e  promote s w e l l i n g , T e r z a g h i and Peck (1948).  I l l i t e has a  and  similar  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 t h e r e i s a s u b s t a n t i a l r e p l a c e ment o f 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 link.  between l a y e r s where I t serves as a bonding  C l a y s w i t h a predominance o f i l l i t e  are not n e a r l y  s u b j e c t to s w e l l i n g as those formed o f m o n t m o r i l l o n i t e . i s the l e a s t a c t i v e o f the t h r e e m i n e r a l s .  so Kaolinite  I t s structure consists  o f an a l u m i n a o c t o h e d r a l 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  silica  t e t r a h e d r a l s h e e t t o f o r m a l a y e r about 7 A* t h i c k ^ .  layers  The  a r e ? s t a c k e d l i k e the l e a v e s o f a book t o f o r m the k a o l i n i t e c r y s t a l , Grim (1959)• have c l e a v a g e Forces  planes  C o n s e q u e n t l y , a l l c l a y p a r t i c l e s tend  to  i n the d i r e c t i o n o f the l a r g e r d i m e n s i o n s .  o f the type t h a t b i n d the m i n e r a l l a y e r s , a l s o a c t  a t the b o u n d a r i e s o f the p a r t i c l e s .  T. W.  u t e s the boundary f o r c e s i n s o i l s t o "bhe  Lambe (1958) a t t r i b -  nonsymmetrical  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  distri-  ( a r i s i n g from  h e t e r p o l a r b o n d s ) , the c r y s t a l s a c t as a l a r g e number o f d i p o l e s . " This g i v e s the p a r t i c l e m a g n e t - l i k e i n surface 4.  p r o p e r t i e s w h i c h are r e f l e c t e d  activity.  S u r f a c e A c t i v i t y and A b s o r b e d L a y e r s . 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 o f the  charge c o n s t i t u t e the s u r f a c e a c t i v i t y o f the m i n e r a l . (2)  Angstrom U n i t 1 S . -  10~ cms. 8  surface Surface  a c t i v i t y i s dependent on b o t h the m i n e r a l o g i c a l f i n e n e s s o f the p a r t i c l e s . exhibit l i t t l e  composition  B u l k y p a r t i c l e s such as  surface a c t i v i t y .  quartz,  M o n t m o r i l l o n i t e , on the  hand, i s most a c t i v e among the c l a y m i n e r a l s .  and  I t can be  other shown  (3)  experimentally The  that c l a y p a r t i c l e s carry a surface charge.  e l e c t r i c a l charge r e s u l t s from t h e u n s a t i s f i e d bonds of  m i n e r a l m a t r i x , B a v e r (1956), Lambe (1958). p a r t i c l e s are n e g a t i v e l y  charged.  enclosing  clay  To n e u t r a l i z e t h i s c h a r g e ,  substances p o s s e s s i n g p o s i t i v e p o t e n t i a l s particle.  Generally,  the  are a t t r a c t e d  to  the  T h i s r e s u l t s i n an envelope o f n e t p o s i t i v e charge the n e g a t i v e l y  charged p a r t i c l e .  In 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 H e l m h o l t z double layer.  Pig. 2  (a).  In natural  s o i l s the p o s i t i v e c h a r g e s are  o f e l e c t r o l y t e s i n aqueous s o l u t i o n , and themselves. properties,  by  supplied  by  ions  the w a t e r m o l e c u l e s  Water i s a t t r a c t e d because o f i t s permanent p o l a r and  a l s o the f a c t t h a t  Baver (1956), T e r z a g h i and the p a r t i c l e s and  i t i s a weak e l e c t r o l y t e ,  Peck (1948).  The  a t t r a c t i o n between  the s u r r o u n d i n g medium r e s u l t s i n an  water complex bonded t o the s o i l p a r t i c l e s . p a r t o f the complex c l o s e r t o the  ion-  Pig. 2 (b).  That  s u r f a c e of the p a r t i c l e than  10 & i s termed the a d s o r b e d l a y e r . F u r t h e r a f i e l d , but  still  under the i n f l u e n c e  o f the s u r f a c e c h a r g e , i s the double l a y e r  water.  double l a y e r i s the f r e e f l u i d , w h i c h i s  (3)  O u t s i d e the  E x p e r i m e n t s on the e l e c t r o p h o r e s i s e f f e c t show t h a t 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 a d i e n t i s a p p l i e d to a d i s p e r s e s u s p e n s i o n .  To /oilo*/ p<xye 7  +  0 <0 • 0 Al " u.  <  (a)  + + + + Liquid. + +  + +  He.L/nholfz  Double.  Layer  Co.riQn  v. v. \  ^0^  d  e  <o " o. » u  0 »< k.  \ 0 &  Wcufer  molecuLe  0% (b) lon-wafer  FIG  2.  HELM HOLT Z  complex.  AND DIFFUSE  LAYERS  8.  not a f f e c t e d by t h e p r e s e n c e o f t h e s o i l p a r t i c l e .  There b e i n g  no d e f i n i t e p h y s i c a l boundary between the t h r e e , t h e w a t e r i n a s o i l mass i s more o r 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 a d s o r b e d water and f r e e w a t e r .  The s t r e n g t h o f t h e bond between t h e  a d s o r b e d 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 t o be so g r e a t  that  i t produces a s o l i d o r a h i g h l y v i s c o u s s u b s t a n c e i n the v i c i n i t y of the i n t e r f a c e . As t h e d i s t a n c e from the p a r t i c l e s u r f a c e i n c r e a s e s , the h e l d w a t e r r e v e r t s t o normal w a t e r .  A c c o r d i n g 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 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 . i n v o l v e d may be o b t a i n e d  field  An i d e a o f t h e dimensions  from F i g . 3»  The a d s o r b e d l a y e r s ( i n c l u d i n g t h e double l a y e r ) have a marked i n f l u e n c e on the b e h a v i o r o f 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 s u c h as c o h e s i o n , p l a s t i c i t y , s e n s i t i v i t y *  ' and  (5)  trixotropy  w /  a r e b e l i e v e d t o depend on the n a t u r e o f the  a d s o r p t i o n complex.  F o r i n s t a n c e , the c o h e s i o n o f a c l a y may  be removed by r e p l a c i n g the water by a n o n - p o l a r l i q u i d s u c h as carbon t e t r a c h l o r i d e . thus a c o n s i d e r a b l e  The t h i c k n e s s o f the a d s o r b e d l a y e r s has  e f f e c t on the p r o p e r t i e s *  The dimensions o f the l a y e r a r e i n f l u e n c e d by the n a t u r e o f the a d s o r b e d i o n s . (4)  Sensitivity  (5)  Trixotropy  -  A c c o r d i n g t o B a v e r (1956), b o t h the v a l e n c e Unconfined compressive s t r e n g t h U n c o n f i n e d compressive s t r e n g t h Reversible  undisturbed remoulded  s o l - g e l transformation.  To foLLow  pa.ge 8  —V  Double  layer  Adsorbed.  Wafer  M on tm oriClonife  Double  Water  Crystal.  Layer  Water  MonfmoriLLonf-fe  ^Double  Sheei"  Layer  Adsorbed. IOA  \  Water^  Wa.ter^  — £  kaoLinife Cr ys t u t  KaoCinite FIG  3.  Sheet  C L A Y - W A T E R (After  S Y S T E M . T.W. Lambe. . I9SB)  9.  and s i z e o f the i o n i s important.  The e l e c t r i c - f i e l d  inten-  s i t y o f an i o n i s known to i n c r e a s e d i r e c t l y w i t h the charge i n v e r s e l y w i t h the r a d i u s squared. a t t r a c t more water molecules towards  I n o t h e r words, some ions the adsorbed l a y e r than  o t h e r s , thereby i n c r e a s i n g i t s t h i c k n e s s . hydrogen clays.  and  Sodium, c a l c i u m ,  and potassium are the p r i n c i p a l adsorbed ions i n n a t u r a l Sodium tends to produce  t h i c k l a y e r s , w h i l e on the o t h e r  hand, hydrogen  ions are adsorbed i n comparatively t h i n l a y e r s ,  Tschebotarioff  (1951).  I f a p a r t i c u l a r i o n predominates,  c l a y i s sometimes given the name o f t h i s element, Na-clay o r C a - c l a y .  Ions of one element may  the  f o r example,  be removed and  r e p l a c e d by those o f another; the p r o c e s s i s known as base exchange. ties.  G e n e r a l l y , exchange of ions l e a d s to change In p r o p e r -  To quote one example, Winterkorn (1941) found the l i q u i d  l i m i t o f a sample o f Putnam c l a y to vary w i t h the adsorbed c a t i o n as  follows:  ,  , N a t u r a l » Na  J Liquid limit  64.5  t 88  t Ca  t Al  i 61.9 ,60.2  TABLE  i  i t K  H  156.4  .56.3  r52.8 t  I.  LIQUID LIMIT OF PUTNAM CLAY.  The p e r c o l a t i o n of pure water salt content.  through a s o i l reduces the  Adsorbed i o n s can be l a r g e l y exchanged by  means; complete exchange p r o d u c i n g an H-clay.  this  C e r t a i n marine  c l a y s have been s u b j e c t e d to l e a c h i n g by f r e s h water.  Attempts  10.  are  b e i n g made to account f o r the u n u s u a l p r o p e r t i e s of s u c h  c l a y s i n terms o f the degree of l e a c h i n g , Bjerrura (1954) and Rosenquist 5.  (1959).  Flocculation. F l o c c u l a t i o n i s a n o t h e r t o p i c c o n n e c t e d w i t h the adsorbed  layers.  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 t o 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 i n t r o d u c e d i n t o a  d i s p e r s e d s u s p e n s i o n o f s o i l p a r t i c l e s i n w a t e r , the n e g a t i v e charges w h i c h h i t h e r t o tended t o s e p a r a t e the p a r t i c l e s , a r e neutralized.  The mass a t t r a c t i o n , or Van der Waal's f o r c e s a r e  then c a p a b l e 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 enough to s e t t l e under the a c t i o n o f g r a v i t y .  The  large  flocculent  a c t i o n i s n o t v e r y 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 b e i n g taken up by the f l o e s .  Hence, the o r i g i n o f the  u n i f o r m t e x t u r e o b s e r v e d i n some marine 6.  deposits.  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 t h e term " s t r u c t u r e " i s used  to denote the arrangement the  o f the p a r t i c l e s i n a s o i l mass.  Thus  s o i l s k e l e t o n may be r e f e r r e d to as h a v i n g 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 ing  g r a i n assemblage, T a y l o r (1948).  With fine-grained soils  i n mind, T. W. Lambe (1958) has extended the d e f i n i t i o n t o r e a d : " S t r u c t u r e means the arrangement o f 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 a d j a c e n t p a r t i c l e s .  , r  The  importance o f t h e e l e c t r i c a l f o r c e s i n p r o m o t i n g 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 n o t e d e a r l i e r , marine c l a y s a r e s e d i m e n t a r y d e p o s i t s r e s u l t i n g from t h e s e t t l e m e n t o f f l o e s on t o t h e sea f l o o r . T h e i r f o r m a t i o n i n a s a l i n e environment gave ready a c c e s s t o t h e d i s s o c i a t e d i o n s c o n t a i n e d i n sea w a t e r .  C o n s e q u e n t l y , the  f i n e r p a r t i c l e s were e n v e l o p e d i n r e l a t i v e l y t h i c k adsorbed layers.  B u r i a l under f u r t h e r depths o f sediment " t i g h t e n e d up*  the f l o c c u l e n t s t r u c t u r e .  1  W i t h s u f f i c i e n t overburden p r e s s u r e  and t i m e , 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 o n t a c t w i t h one a n o t h e r , 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 N o r t h e r l e y ( 1 9 5 2 ) .  The s t r e n g t h c h a r a c t e r i s t i c s  o f s u c h a d e p o s i t a r e d e t e r m i n e d by t h e degree o f c o n s o l i d a t i o n i n the normal manner, T a y l o r ( 1 9 4 8 ) .  The s e n s i t i v i t y i s n o t  abnormal; i t may be anywhere i n the range one t o 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 r e a s o n the o r i g i n a l s a l t c o n t e n t o f a marine d e p o s i t i s l o w e r e d . L e a c h i n g r e d u c e s t h e t h i c k n e s s e s o f the adsorbed l a y e r s , w h i l e the water c o n t e n t remains almost unchanged; the volume o f t h e f r e e pore f l u i d i n c r e a s e s a t the expense o f the a d s o r p t i o n complex, Skempton and N o r t h e y ( 1 9 5 2 ) .  T h i s has a t w o - f o l d  e f f e c t on t h e e n g i n e e r i n g p r o p e r t i e s :  I t l o w e r s the u n d i s t u r b e d  s t r e n g t h , but more i m p o r t a n t , i t g r e a t l y i n c r e a s e s t h e s e n s i t i v ity.  A t t h e same t i m e , I t has been o b s e r v e d t h a t the A t t e r b e r g  l i m i t s are reduced, Bjerrum  (1954).  L e a c h i n g o f n a t u r a l d e p o s i t s i s a c c o m p l i s h e d by p e r c o l a t i o n of f r e s h water t h r o u g h the p o r e s .  I n most c a s e s , 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 o f marine d e p o s i t s t o above s e a l e v e l . I n Europe and N o r t h A m e r i c a , g e o l o g i c a l e v i d e n c e i n d i c a t e s t h a t  12.  u p l i f t took p l a c e f o l l o w i n g t h e r e t r e a t o f P l e i s t o c e n e The  glaciation.  c l a s s i c a l example o f t h i s type o f development i s a l o n g t h e  c o a s t o f Norway.  I n v e s t i g a t i o n o f t h e Norwegian c l a y s by B j e r r u m ,  i n d i c a t e s t h a t t h e l o s s o f shear s t r e n g t h mentioned above, does not take p l a c e i n l i n e a r p r o p o r t i o n t o the r e d u c t i o n i n s a l t concentration.  Bjerrum r e p o r t s t h a t even a decrease i n s a l t  c o n c e n t r a t i o n from t h e o r i g i n a l 35 grams down t o 10 - 15 grams per l i t r e , r e s u l t s o n l y 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 strength.  F u r t h e r l e a c h i n g , however, r e s u l t i n g I n s a l t concen-  t r a t i o n s below 10 grams, produces a c o n s i d e r a b l e d e c r e a s e i n strength.  The r e d u c t i o n i n u n d i s t u r b e d  s t r e n g t h has t a k e n p l a c e  over a l o n g p e r i o d o f time; t h e r e f o r e , e x c e p t 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 g r e a t e r s i g n i f i c a n c e , i s t h e i n c r e a s e i n s e n s i t i v i t y following leaching.  " Q u i c k " c l a y s , as they a r e c a l l e d , a r e among  the most d i f f i c u l t s o i l s e n c o u n t e r e d by e n g i n e e r i n g 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 f o u n d , i n p a r t a±  l e a s t , from t h e s t r u c t u r e .  R o s e n q u i s t has i n v e s t i g a t e d t h e  s t r u c t u r e o f Norwegian " q u i c k " c l a y s .  With the a i d o f e l e c t r o n  m i c r o s c o p y , he has shown t h a t t h e s t r u c t u r e c o r r e s p o n d s remark* a b l y cbse t o a c o n f i g u r a t i o n p r o p o s e d e a r l i e r by W. T. Tan. The  essence o f t h e s t r u c t u r e i s t h a t t h e edges o f some p a r t i c l e s  are v i r t u a l l y i n c o n t a c t w i t h t h e f l a t s u r f a c e s o f o t h e r s to g i v e a s o r t o f card-house framework.  F i g . 4.  The m e t a - s t a b l e 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 s u b j e c t e d t o r e p e a t e d  s t r e s s e s o r shock.  Undisturbed  samples o f some m a r i n e d e p o s i t s a r e so s e n s i t i v e t h a t they c a n  To /oUotv  FIG  4.  S T R U C T U R E  OF  f>aye  MARINE (A-f rer  /£  C L A Y  T.K. Tan.  /9S7.)  13.  be t r a n s f o r m e d  by r e m o u l d i n g a l o n e , from a f i r m c l a y , t o  w i t h the c o n s i s t e n c y of a v i s c o u s  one  fluid.  The c u r r e n t i n v e s t i g a t i o n i s c o n c e r n e d 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 m a i n l y  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.  Introduction  F o u n d a t i o n and earthwork e n g i n e e r i n g  often require a  knowledge o f t h e b e h a v i o r o f s o i l s l o c a t e d below t h e water t a b l e . When e x t e r n a l l o a d s a r e a p p l i e d t o a submerged s t r a t u m , i n t e r n a l stresses 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 t a k e n by the  s o l i d constituents, or s o i l structure, while medium  the i n t e r s t i t i a l  ( f o r the most p a r t w a t e r ) a b s o r b s the r e m a i n d e r .  The  s t r e s s e s t a k e n by the s o i l s k e l e t o n a r e known as e f f e c t i v e stresses.  Pore p r e s s u r e ,  o r neutral s t r e s s , i s applied to the  p o r t i o n t a k e n by t h e " f r e e f l u i d s " ^ pore p r e s s u r e s  ^ occupying the pores.  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,  Thus while  e f f e c t i v e s t r e s s p e r t a i n s t o the p a r t i c l e and i t s a d s o r b e d l a y e r .  (1)  "Free f l u i d s " r e f e r s t o t h e w a t e r , g a s e s , e t c . , w h i c h a r e not i n t i m a t e l y held to the surface of the p a r t i c l e s . I t i s c o n s i d e r e d here t o i n c l u d e t h e double l a y e r w a t e r .  15  B. 1,  E f f e c t s of Pore  Pressure  R e l a t i o n s h i p Between Pore P r e s s u r e The  q u e s t i o n a r i s e s as t o how  the s t r e n g t h p r o p e r t i e s .  and Shear  Strength.  the pore p r e s s u r e  effects  F o r the purpose o f 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 v a r i o u s t y p e s of be f o u n d i n n a t u r a l  s t r u c t u r e and t e x t u r e s w h i c h may  G e n e r a l l y , p o s i t i v e pore p r e s s u r e s whereas n e g a t i v e  pressure  leads  soils.  reduce t h e c o n t a c t  stresses,  to an i n c r e a s e i n the  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  contact  i n t e r s t i t i a l water  i s c a p a b l e o f t a k i n g e i t h e r c o m p r e s s i o n or t e n s i l e s t r e s s e s , but a l l s h e a r i n g s t r e s s e s must be t a k e n by the s o i l Bishop and Henkel (1957).  skeleton,  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 s h e a r i n g r e s i s t a n c e of the  soil  mass.  the  Moreover, the l o a d must be c a p a b l e o f m a i n t a i n i n g  deformation.  Hence, t h e r e i s a d i s t i n c t i o n between the a p p r e c -  i a b l e amount of d e f o r m a t i o n t h a t may f a i l u r e , and  occur without  causing  the p r o l o n g e d d e f o r m a t i o n known as c r e e p w h i c h  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 t i m e .  may  Elementary  mechanics shows t h a t the r e l a t i o n s h i p between the f o r c e n e c e s s a r y to produce r e l a t i v e m o t i o n o f two f u n c t i o n o f the normal l o a d and s h i p i s e x p r e s s e d by the f o r m u l a  surfaces  i n contact, i s a  the f r i c t i o n a n g l e . F  The  2 u t a n o i where F denotes  the f o r c e r e q u i r e d t o produce s l i d i n g ,  N c o r r e s p o n d s 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 materials i n contact.  relation-  the  I n S o i l Mechanics these c o n c e p t s are  16.  embodied I n the well-known Coulomb-Terzaghi e q u a t i o n : s where  s  a  c' + (p - u) t a n 0»  (2-1)  denotes the maximum r e s i s t a n c e to sheer on any p l a c e  c' denotes the apparent  and  Eqn.  cohesion  0'  denotes the angle o f s h e a r i n g resistance  p  denotes the t o t a l p r e s s u r e normal to the p l a n e c o n s i d e r e d ,  u  denotes the pore p r e s s u r e .  ) ) ) )  I n terras of effective stress  The above e q u a t i o n i s u s e d i n most problems i n v o l v i n g the shear s t r e n g t h o f s o i l s .  However, an a n a l y s i s o f the terms  a p p e a r i n g i n the f o r m u l a , i s o f i n t e r e s t i n o r d e r to g a i n an understanding of i t s v a l i d i t y . (2)  The a plane  stresses  ' ( e q u a t i o n 2-1)  are d e f i n e d i n r e s p e c t t o  ( i n the g e o m e t r i c a l sense) p a s s i n g t h r o u g h the pore space  and the p o i n t s o f c o n t a c t o f the s o i l p a r t i c l e s .  S t r e s s e s , and  a r e a s , are t h e n c o n s i d e r e d as p r o j e c t e d on t o t h i s p l a n e .  The  e f f e c t i v e s t r e s s on the p l a n e i s assumed t o be r e p r e s e n t e d by the term  (p «* u) .  would be  A more e x a c t e x p r e s s i o n f o r the e f f e c t i v e s t r e s s  p - u ( l - a ) , where r  a  r  denotes the c o n t a c t a r e a o f  the p a r t i c l e s p e r u n i t a r e a o f the p l a n e .  A precise evaluation  o f 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 o f c o n t a c t areas.  I n p r a c t i c e , d i r e c t measurement o f the c o n t a c t a r e a s  of  a l l p a r t i c l e s on a s l i p p l a n e , i s an e x t r e m e l y 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 ) i s i n f a c t (2) Compression s t r e s s e s c o n s i d e r e d p o s i t i v e , and t e n s i l e negative. p  17.  c l o s e t o u n i t y f o r both sands and c l a y s , B i s h o p and E l d i n (1950).  The  v a l i d i t y o f t h e assumption  ponent o f shear s t r e n g t h i s  t h a t the f r i c t i o n com-  (p - u) t a n jzS'  i s based on  this  this observation. 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, p a c k i n g 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 c o m p o s i t i o n o f the p a r t i c l e s . fl  that  <p  (1957) r e p o r t  a l s o depends somewhat on the r a t e o f s t r a i n ; h i g h r a t e s  1  t e n d t o i n c r e a s e 0*. tan  B i s h o p and Henkel  I n c l a y s , the decrease i n the v a l u e of  I s about 5% f o r each t e n f o l d i n c r e a s e i n the d u r a t i o n o f  y  a shear  test*  The c o h e s i o n appears I n e q u a t i o n 2-1 as the component o f the shear s t r e n g t h independent  o f the normal s t r e s s .  For  any  one c o h e s i v e s o i l the a p p a r e n t c o h e s i o n depends on the w a t e r c o n t e n t , s t r e s s h i s t o r y and r a t e o f d e f o r m a t i o n under l o a d . G e n e r a l l y , the c o h e s i o n i n c r e a s e s as the m o i s t u r e c o n t e n t i s lowered.  P r e c o n s o l i d a t i o n l e a d s t o an i n c r e a s e i n c o h e s i o n 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 l o w e r s t r e s s than the o r i g i n a l consolidation pressures.  H i g h r a t e s of  deformation  tend t o i n c r e a s e the observed c o h e s i o n , T a y l o r (1943). due  t o the m o b i l i z a t i o n o f the v i s c o u s component o f the  i a l water.  Consequently,  the p r e f i x ' a p p a r e n t  1  This i s interstitr  i s used, i n  c o n n e c t i o n w i t h f r i c t i o n a n g l e s and c o h e s i o n , i n o r d e r t o s i g n i f y the dependence o f these parameters  on f a c t o r s w h i c h a r e n o t  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 o f t r u e f r i c t i o n a n g l e and t r u e c o h e s i o n w i l l be sidered.  con-  18 2.  R e l a t i o n s h i p Between Compressive S t r e n g t h and Pore P r e s s u r e . The s h e a r i n g r e s i s t a n c e on any plane may be o b t a i n e d from  the Coulomb-Terzaghi equation p r o v i d e d the pore p r e s s u r e , s t r e n g t h parameters and normal s t r e s s on the plane a r e known. However, the t o t a l s t r e s s e s on the p r i n c i p a l planes a r e more r e a d i l y a c c e s s e d i n p r a c t i c e , t h e r e f o r e e q u a t i o n 2-1 w i l l be d e r i v e d i n terms o f the t o t a l p r i n c i p a l s t r e s s e s .  Gne purpose  o f t h i s i s t o emphasize the r o l e p l a y e d by pore water p r e s s u r e i n the e n g i n e e r i n g performance o f the s o i l .  The r e l a t i o n s h i p  can be c o n v e n i e n t l y d e r i v e d from the Mohr diagram, P i g * 5» In F i g . 5 the t o t a l p r i n c i p a l s t r e s s e s a t f a i l u r e are r e p r e s e n t e d by o~, and cr  w  The compressive  /  3  then (C~, — 6 j ) ; For  strength i s  PGH r e p r e s e n t s the c o r r e s p o n d i n g Mohr c i r c l e .  t h i s system o f t o t a l s t r e s s e s , l e t the e f f e c t i v e s t r e s s e s be  r e p r e s e n t e d by (T, and (T . 3  The Mohr c i r c l e P'G'H' a s s o c i a t e d  w i t h the l a t t e r w i l l be l o c a t e d to the l e f t by a d i s t a n c e c o r r e s ponding to the pore p r e s s u r e for e f f e c t i v e  envelope  s t r e s s i s r e p r e s e n t e d by the l i n e AB, then the  c i r c l e P'G'H' w i l l failure  ( u ) . Assuming the f a i l u r e  touch AB a t f a i l u r e .  The shear s t r e s s a t  ( 7 ? ) i s r e p r e s e n t e d by OL.  I t I s r e q u i r e d t o express  the compressive  stress  ((T, - <3}) i n terms o f the s t r e n g t h parameters ( c , 0') and f  the pore p r e s s u r e u .  (3)  I n t r i a x i a l t e s t <?, i s t o t a l a x i a l s t r e s s and (T r e p r e sents the c e l l p r e s s u r e o r the c o n f i n i n g s t r e s s . I n a n a t u r a l s o i l d e p o s i t <?, r e p r e s e n t s v e r t i c a l s t r e s s on an element, and O j » K.0~, where K c o e f f i c i e n t o f l a t e r a l earth pressure. 3  =  19. Prom the geometry o f the diagram i t f o l l o w s t h a t __ (<r, Q(c< -- 0"? ))  ^ Also  7/- = [( 0~ - u ) +  c~i -  3  (<J7 - 01) cos^>' 2 (6T 2  tan  O3)  CM  Q~> - Q a s l n 0' J tan 0' +  e«.  - (CJ7  -63)  s i n ^' tan  +  C.  2 - t a n 0' •+• s i n <p' t a n ^ )  3  (CT - o p 2  cos cf>'  ( CT^ - u) t a n <f>' +  z  (CH - 0~ ) (cos $ . 2" . ... or  3  s c ' + ( 0 ^ • u)  - e* + (0" • t a n <p' cos <p' - tan <p' + sin<£' t a n 3  tan0'  ^ cos </>' cos <p'  r c' cos  + (0~3 - u) s i n ^ ' c o s 0 ' - s i n 0' (1 - s i n ^ ' ) 2  S i n c e s i n ^ ' + co& <f>' r 1 a  (CT, - (r ) 3  z  g ? f c « cos <f>' -f ( C T - u) s i n ^ ] I 1 - s i n <p' J 3  Furthermore (CT, - 0" ) r 2 c 3  1  °°g 0'  1 - sinq6'  +  E  q  n  #  2(£sin0' - 2 s i n ft' 1 - sin0' 1 - s i n ^ '  2  .  2  u  sgfc' cps <[>' -+ o~ s i n <f>' I - 2,( s i n 0' )„ I 1 - s i n <P' J 1 - s i n ft' 3  (CT,  -  6~ ) 3  :  Y  -  Zu  where Y and Z are c o n s t a n t s f o r any one v a l u e o f o~ 3  E q u a t i o n 2-2 e t sequo shows that the compressive  strength  comprises a c o n s t a n t term minus a f u n c t i o n o f the pore water pressure.  The measured compressive  determined by the pore p r e s s u r e .  strength i s therefore l a r g e l y  To follow  £ff ec rive Stresses  Nor  Fig 5. Mohr  m a.L  Diagram  St  page  Total  . Tot ai  <xncL E fre ctive  Stresses  a  Fig  6 . Mohr  Stresses  resses  • V.  Normal.  IS  Stresses  Diagram  FIGS. 5and 6  : Tofai  Stresses  20.  'apparent* s t r e n g t h parameters c' and 0'  The  are o b t a i n e d  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 p r e s s u r e measurements.  I n d i v i d u a l specimens a r e f i r s t  ted a t d i f f e r e n t c e l l p r e s s u r e s ( c o r r e s p o n d i n g t o 6~3)»  consolidaShear  t e s t s a r e then p e r f o r m e d on the specimens, w h i c h i n g e n e r a l w i l l have d i f f e r e n t m o i s t u r e c o n t e n t s due to the d i f f e r e n c e s i n consolidation pressures.  The h i g h e r s h e a r s t r e n g t h s w i l l n o r m a l l y  be o b t a i n e d from the specimens w i t h l o w m o i s t u r e c o n t e n t s and h i g h confining pressure.  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 t o P i g . %. the f a i l u r e envelope can be established.  The s l o p e o f the envelope y i e l d s the 'apparent'  f r i c t i o n angle ( 0 ' ) ,  w h i l e the i n t e r c e p t o f the envelope on the  Y a x i s g i v e s the 'apparent' c o h e s i o n ( c ' ) . g».  T o t a l S t r e s s Parameters. Two  quoted.  o t h e r 'apparent' p a r a m e t e r s , c  u  and </ a r e sometimes u  These are o b t a i n e d from the envelope o f the Mohr c i r c l e s  f o r t o t a l stresses at 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 parameters ation.  o f the 'apparent'  depends on the s t r e s s h i s t o r y and the r a t e o f deform-  C o n s e q u e n t l y , i n an e f f o r t t o o b v i a t e t h i s dependence,  the i d e a o f t r u e f r i c t i o n a n g l e and t r u e c o h e s i o n made i t s appearance 4.  i n S o i l Mechanics  literature*  True F r i c t i o n Angle and True C o h e s i o n . 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 e x p r e s s e d i n  terms o f the 'apparent' c o h e s i o n and f r i c t i o n a n g l e .  F o r 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 t o be I t has been s u g g e s t e d , Shempton and Bishop friction  elucidated.  (1954)  t h a t the  true  a n g l e and t r u e c o h e s i o n can be o b t a i n e d under c e r t a i n  c o n d i t i o n s w h i c h w i l l be d i s c u s s e d p r e s e n t l y . a p p l i e s to cohesive s o i l s The  discussion  only.  concept of true f r i c t i o n  Hvorslev's  The  and  true cohesion is-based  on  c o n t e n t i o n t h a t the c o h e s i o n s h o u l d be a f u n c t i o n o f  the w a t e r c o n t e n t o n l y , 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 i n c r e a s e I n s t r e n g t h w i t h i n c r e a s e i n e f f e c t i v e s t r e s s a t c o n s t a n t water c o n t e n t .  I t i s p o s s i b l e t o have two  samples o f  a s o i l a t i d e n t i c a l water c o n t e n t s ( v o i d r a t i o s the same i f saturated)  but I n 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 .  T h i s 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 c o n s o l i d a t i o n t e s t on a n o r m a l l y  conventional  consolidated clay.  P i g . 7.  I t i s e v i d e n t f r o m F i g . 7 t h a t the s t a t e c o r r e s p o n d i n g t o p o i n t X on the l o a d i n g c u r v e 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 dation pressure the u n l o a d i n g  p-j, and  t h a t the v o i d r a t i o i s e^.  At p o i n t Y  c u r v e the sample has been p r e c o n s o l i d a t e d  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^.  are e f f e c t i v e s t r e s s e s ) .  The  on  to the (p-^ Pg  void r a t i o corresponding to  p^  press-  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 ,  c o h e s i o n w i l l then have the same magnitude f o r s t a t e s by X and  the  represented  Y.  I f shear t e s t s w i t h pore p r e s s u r e 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 c o r r e s p o n d t o  To foLLow  Consolidaf/qn  FIG  7-  F I G  Pressure  C O N S O L I D A T I O N  Effective 8.  M O H R  Stress D I A G R A M  F I GS  page 2/  H I S T O R I E S  <xt F<xi L u r e . T R U E  7 and  8  P A R A M E T E R S  22. c o n d i t i o n s r e p r e s e n t e d by p o i n t s X and Y. diagram  P i g . 7, the Mohr  o f e f f e c t i v e s t r e s s e s would resemble  F i g . 8.  Provided  that a r a t e o f s t r a i n i s chosen which I s slow enough t o minimize v i s c o u s e f f e c t s , the true f r i c t i o n angle <f> and true c o h e s i o n r  c  r  a r e o b t a i n e d by the method i n d i c a t e d i n P i g . 8. The f a i l u r e envelope  of the two Mohr c i r c l e s can be  r e p r e s e n t e d by the e q u a t i o n :  7> r e where  r  +  (CT- u) t a n ^  7/: ~ shear s t r e s s on the plane o f f a i l u r e 0~  - t o t a l normal s t r e s s on the same plane  u  • pore p r e s s u r e  c  - true cohesion  p  <p  - angle o f t r u e I n t e r n a l j friction j  A t t h e water c o n tent a t f a i l u r e  By analogy w i t h e q u a t i o n 2-1 the compressive  strength i s  given by: (4) (07-03)  = 2 fe  r  cos  [  +  (0"]j - u)  1 - sin  sin  j  j  where 0", and 0~ a r e the major and minor t o t a l p r i n c i p a l 3  stresses at f a i l u r e  respectively.  The performance difficulties,  o f such t e s t s p r e s e n t s experimental  due t o the requirement  that the specimens have  I d e n t i c a l moisture contents but d i f f e r e n t s t r e s s h i s t o r i e s . a l t e r n a t i v e procedure f o r o b t a i n i n g  i s to measure the i n -  c l i n a t i o n o f the shear plane at f a i l u r e .  (4)  An  The angle o f i n c l i n a t i o n  (<T, - 0~ ) i s e q u i v a l e n t to ( 0 7 - (T ) 3  —  devlator stress.  23. of the shear plane given b y 0  r 45  to the plane of major p r i n c i p a l s t r e s s i s  + 2  Not a l l samples, however, f a i l on a s i n g l e shear p l a n e . fore,  t h i s approach i s n o t f e a s i b l e i n a l l c a s e s .  produced  End  Thererestraint  by l o a d i n g caps a l s o 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 w i t h  the p l a s t i c i t y index and m i n e r a l o g i c a l composition o f c l a y s . Test r e s u l t s r e p o r t e d by Skempton  (1954)  i n d i c a t e the order o f  magnitude o f the true and apparent f r i c t i o n a n g l e s . C.  F a c t o r s A f f e e t i n g Pore P r e s s u r e : The P r e s s u r e Parameters, A and B.  Table I I .  Pore-  The magnitude of the pore p r e s s u r e developed i n a s t r e s s e d s o i l mass depends p r i m a r i l y on two  factors:  (a)  The c o m p r e s s i b i l i t y o f the s o i l s k e l e t o n .  (b)  The  c o n s i t u e n t s o f the f l u i d  the pore  occupying  space.  I n order to i l l u s t r a t e the dependence on the above f a c t o r s , the s o i l i s assumed to behave as an e l a s t i c isotropic m a t e r i a l . The v a l i d i t y of such an assumption  f o r the case o f r e a l s o i l s i s  discussed l a t e r . C o n s i d e r a s m a l l cube AB o f e l a s t i c m a t e r i a l s t r e s s e d i n the manner i n d i c a t e d i n the f o l l o w i n g s k e t c h : and  o~  are compressive  where <T~,  s t r e s s e s o f e q u a l magnitude.  X  o~y  j  SOIL TYPE  j Shellhaven Clay « undisturbed.  '  LIQUID LIMIT  ' PLAS TIC INDEX  ' ACTIVITY  1  1  MOISTURE CONTENT RANGE  TRUE ' FRICTION  1  \  !  1  '  APPARENT j FRICTION j ANGLE j  1  \  123  ;  87  \  1*42  ] 52~60#  TABLE I I . PROPERTIES OF SHELLHAVEN CLAY.  18°  23°  j  25.  cr  y  (Tyy  z.  From e l a s t i c t h e o r y and the p r i n c i p a l and€  2 Z  s t r a i n s € xx>-£"yy  a r e r e l a t e d by: £ zz » €  s ^zz  y y  =  £" (1 -  2//)  E where yt/ -  Poisson's  E -  ratio  Young's Modulus.  I f L i s the o r i g i n a l l e n g t h o f a s i d e o f the cube and (L - A L ) 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 t o any one a x i s o f the c o - o r d i n a t e s i s : - AL £ xx = L =  Hence:  AL =  - £ (1 E  2/>)  L 6~ (1 - 2 />)  i" The volume o f t h e compressed cube i s then: (L - A L )  =  1?  3  a  f l - £" (1 - 2 / ^ ) | E  j l - 3 _ f (1 - 2 f)  j  3  for small v a l u e o f AL.  26. S i n c e t h e o r i g i n a l volume of the cube i s V  s  -  3 L , the  change In t h e volume (-AV) i s given by: -AV = L  -AV-L  - (L - A L )  3  3  r 3jr ( i - 2/0  3  - I s or  Similarly  i f ( T  Volume change:  n  - AV ^ V [30~(1 IE  ^=0~jj ±  -A V  -ap)  1  (Tzz  r Vj" 1 - 2f(ff„ + 0~yy +  <7~ ^  (1)  z  T u r n i n g now to a s o i l mass s u b j e c t e d to i n c r e m e n t a l changes i n the t o t a l s t r e s s e s on p r i n c i p a l planes - e q u i v a l e n t t o ACT, AGgand A 0 ~ 3 the r e l a t i o n s h i p s between the changes i n t o t a l and effective  s t r e s s e s are g i v e n by: ACJ1  A?  2  = A O T - A U  ;  (a)  =A0" -4u  j  (b)  &<T Where ACT,  7  2  5  =ACTJ-AU  ;  (c)  Ad^, and A0" r e p r e s e n t the change l n e f f e c t i v e 3  s t r e s s e s on the p r i n c i p a l planes — A U denotes the pore p r e s s u r e change. Then from e x p r e s s i o n (1) the decrease  i n volume  ( A V ) of  the  s o i l s k e l e t o n i s approximated by:  -AV  =  vu_-_2£)  ,  +  £ar  t A ^ )  a  (2)  E where  and E a r e P o i s s o n ' s r a t i o and Young* s modulus  r e s p e c t i v e l y f o r the s o i l s k e l e t o n . The decrease i n the volume o f t h e s o i l  s k e l e t o n i s almost  (5) e n t i r e l y due to a decrease i n the volume o f the v o i d s . the  I n i t i a l p o r o s i t y i s denoted b y i ? , ^  i b i l i t y o f t h e pore f l u i d ,  and C  If  t h e compress-  w  the volume change (assuming no  drainage occurs} i s g i v e n by: -AV  - V7 C  w  (3)  A U  Combining equations (2) rj C „ * u  .  1  (3)  and  -  I  A  F  F  .  +  A  (  R  I  +  A  (  R  I  )  (  4  )  E (7)  For  the case where A ( T  2  r  A<X  e x p r e s s i o n (a) (b) (c) can be  3  written: +- A U =  ACT; A O "  2  + A U  =  Acr ACT  3  (  Acr,  -  ACT ) 3  3  (5)  Compressibility of the s o i l grains i s n e g l i g i b l e .  (6)  P o r o s i t y 77 s '  (7)  Triaxial  Volume o f V o i d s T o t a l Volume  t e s t and most p r a c t i c a l problems  A6~  Z  = AG~3  28.  By a d d i t i o n : (ACT, + A0~£ + 4 0~ ) + 3 A U = 3  Prom  (4)  Y] C  1 - 2 ^  w  3 ACT, +  3AtX  3  +-(A(71 - A<r ) 3  ( ACT, - AG\)  - 3AU|  E If  the c o m p r e s s i b i l i t y o f t h e s o i l  change i s denoted by C ,  then:  c  G  skeleton f o r u n i t stress  C  r  - 2 / 0  3(1  E I n t r o d u c i n g C , r e a r r a n g i n g the terms and d i v i d i n g by 3 , c  the change i n pore p r e s s u r e f o r an a l l - r o u n d change I n s t r e s s Is g i v e n by the e x p r e s s i o n : A u *  1  The  +  T  £  ACT,  =3  +  (ACT,-  ACT )  term o u t s i d e the bracket  i s known as  the pore p r e s s u r e parameter B, Bishop and Henkel The c o e f f i c i e n t in  (1957).  j - i n Exp. ( 5 ) only a p p l i e s , o f course,  the case/an i d e a l i z e d e l a s t i c  approximately  (5)  3  soil.  Real s o i l s a r e not even  e l a s t i c , t h e r e f o r e , i t i s necessary  to r e p l a c e  the c o e f f i c i e n t o f the d e v i a t o r s t r e s s by a parameter A. Equation  ( 5 ) then becomes:  ziu=  B [ A(T + A( ACT, 3  -A0~ )J 3  (Eqn. 2 - 3 )  29. The v a l u e o f the parameter A a t f a i l u r e ranges from about -0.1 f o r n o r m a l l y c o n s o l i d a t e d s o i l s t o about 1.3 f o r p r e c o n s o l i d a t e d c l a y s , Bishop and Henkel (1957). IFer f u l l y s a t u r a t e d s o i l s the v a l u e o f C alone - i s so s m a l l that B r 1.  w  - t h a t © f water  When the pore water c o n t a i n s  a i r and o t h e r gases, the value o f B i s l e s s than u n i t y , b u t depends on the s t r e s s range; stresses increase.  Where  B approaches u n i t y as the t o t a l  ACT, = ACT^ as i s the case i n b u i l d -  up o f pore p r e s s u r e a t the i n i t i a l stages o f a t r i a x i a l shear t e s t , B i s g i v e n by the e x p r e s s i o n  B s  A  A 0 "  the change i n pore p r e s s u r e , and A<T  3  pressure.  wherezm denotes  U  3  denotes the change i n c e l l  30.  CHAPTER I I I . APPARATUS:  A  1.  *  DEVELOPMENT AND  OPERATION  T r i a x i a l Shear T e s t s w i t h Pore P r e s s u r e Measurements.  General. The  t r i a x i a l a p p a r a t u s u s e d i n the i n v e s t i g a t i o n  d e s i g n e d t o accommodate specimens up t o 6 . n  o f 2-g-" diameter w i t h a l e n g t h  Previous r e s e a r c h ^ ^ indicates  that best r e s u l t s are  o b t a i n e d w i t h a l e n g t h t o diameter r a t i o o f 2:1* r e p o r t e d here are f o r specimens  was  2jr d i a m e t e r and 5 n  A l l results n  long.  E x c e s s i v e b u c k l i n g o f the specimen d u r i n g t e s t i s thus a v o i d e d . D i r e c t l o a d i n g was u s e d because the s t r e s s remains c o n s t a n t f o r any l o a d i n c r e m e n t .  virtually  T h i s enables the development o f  pore p r e s s u r e t o be compared w i t h the i n c r e m e n t a l change i n axial stress.  A l s o t h i s t e s t set-up c l o s e l y c o r r e s p o n d s t o the  l o a d i n g o f s o i l s i n p r a c t i c e , where the s o i l i s n o r m a l l y a l l o w e d to deform a t w i l l under an almost u n i f o r m s t r e s s .  Changes i n  c r o s s s e c t i o n a l a r e a , i n the c o u r s e o f a t e s t , w i l l t e n d t o reduce the a x i a l s t r e s s e s ,  (1)  b u t the magnitude o f such changes w i l l not  Bishop and Henkel  (1957).  31.  be v e r y g r e a t , i f the specimen f a i l s a t s m a l l  strain.  I t i s more c o n v e n i e n t t o measure the pore p r e s s u r e top, or at the base o f a specimen, than at i n t e r m e d i a t e i n i t s length.  I t was  points have  development.  some t e s t s were r u n w i t h pore p r e s s u r e  at or about the c e n t r e o f the 2.  the  f e l t , however, t h a t end r e s t r a i n t may  a b e a r i n g on the o b s e r v e d r a t e o f pore p r e s s u r e Therefore,  at  measurements  specimen.  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 a p p a r a t u s i s shown i n P i g . 9.  made o f 2" x 2" aluminum box x •§" t h i c k aluminum base.  The  s e c t i o n , b o l t e d down to a 3 0  A wooden bench s u p p o r t s the  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 of a d j u s t a b l e constrained  frame i s  screws i n the bench l e g s .  The  x  1"  frame.  Incorporation  l o a d i n g yoke i s  to move i n a v e r t i c a l p l a n e 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 c o u n t e r - b a l a n c e system keeps the yoke  proving 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 minimum the i n i t i a l l o a d on the specimen. mounted on a c e n t e r i n g column. u n i t was  n  The  and  to a  triaxial cell is  W i t h the c e l l i n p o s i t i o n , the  a l i g n e d by means o f a s u r v e y o r ' s t h e o d o l i t e .  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 o b t a i n e d . 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 b e a r i n g s  and the  Axial Although counter*  b a l a n c e w e i g h t s a r e suspended from low f r i c t i o n p u l l e y s , t h e proving r i n g i s Incorporated  f o r the purpose o f 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 e s t i m a t e d  load.  t o the yoke v i a the s h a c k l e and pan,  The  l o a d s are  applied  l o c a t e d beneath the bench.  Jo follow  FIG  10.  T R I A X I A L  C E L L  p&je  Jl  32.  3.  The T r i a x i a l  Cell  The p r e s s u r e 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 "lucite" 3/4"  c y l i n d e r (^  tt  w a l l t h i c k n e s s ) capped top and bottom  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 s e a t e d on  In t h e p l a t e s and i s s e a l e d by s y n t h e t i c r u b b e r  by  grooves  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 p l u n g e r i s i n s e r t e d t h r o u g h a b r a s s b u s h i n g i n the upper p l a t e .  I n o r d e r to m i n i m i z e f r i c t i o n ,  and a t the same  time p r e v e n t e x c e s s i v e l e a k a g e , t h e s t a i n l e s s s t e e l p l u n g e r i s f i n i s h e d t o g i v e a c l e a r a n c e of 0.0003 i n c h e s . for cap.  External loads  t h e p l u n g e r a r e t r a n s m i t t e d t o the specimen by the l o a d i n g The l o w e r end of the p l u n g e r i s machined t o a h e m i s p h e r i c a l  shape w h i c h 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 t o t i l t t h r o u g h a n g l e s up t o 10 degrees t o the h o r i zontal.  G r e a t e r tendency  t o t i l t i s p r e v e n t e d by a g u i d e 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 o f the l o a d i n g frame, p r o t r u d e s through the l o w e r 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 t o the top o f the p e d e s t a l . B o t h cap and base have s h a l l o w r a d i a l g r o o v e s , on the specimen s i d e , w h i c h l e a d to d r a i n a g e o u t l e t s . are  Two  outlets  p r o v i d e d i n the top cap and t h e r e 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 c o n n e c t e d to e i t h e r the  pore p r e s s u r e a p p a r a t u s or t o a vacuum pump, depending on the r e q u i r e m e n t s o f the t e s t .  The  o t h e r l e a d i n the cap and the  l e a d from the base are i n t e n d e d f o r d r a i n a g e p u r p o s e s .  "Saran"  33.  tubing i s used f o r a l l l e a d s .  Plow i n the t u b i n g i s c o n t r o l l e d  by p i s t o n v a l v e s l o c a t e d o u t s i d e t h e c e l l .  The o p e r a t i o n o f t h i s  type o f v a l v e does n o t i n t r o d u c e u n d e s i r a b l e volume changes anywhere i n the d r a i n a g e  system.  Drainage from the specimen i s measured by means o f two burettes.  A i r e x p e l l e d from u n s a t u r a t e d 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 Control The c o n t r o l o f chamber p r e s s u r e  to the degree o f 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 m a t t e r . p a r t i c u l a r l y t r u e i n the case o f l o n g d u r a t i o n t e s t s .  This i s Commercial  p r e s s u r e r e g u l a t o r s u s u a l l y employ a s p r i n g - l o a d e d diaphragm. T h i s mechanism  i s prone t o i n s t a b i l i t y , en d I n the absence of  a u x i l i a r y equipment c a n n o t be u s e d f o r p r e c i s e p r e s s u r e c o n t r o l over l o n g p e r i o d s o f t i m e . A number o f systems have been d e v i s e d t o meet t h e problem. Most, however, r e q u i r e e l a b o r a t e i n s t r u m e n t a t i o n .  Two  systems  commonly u s e d , one d e v e l o p e d at I m p e r i a l C o l l e g e , London, and bhe o t h e r a t the Norwegian G-eotechnical I n s t i t u t e , O s l o , have p r o v e d 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 u s e d  a t I m p e r i a l C o l l e g e employs a s e l f - c o m p e n s a t i n g mercury manometer, one l i m b o f w h i c h can be r a i s e d , t o p r o v i d e t h e r e q u i r e d p r e s s u r e 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 -  a b l e 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 p r e s s u r e s are t o be o b t a i n e d .  I f head-room i s l i m i t e d , the a p p a r a t u s must  34.  be d u p l i c a t e d , w h i c h adds to the c o s t o f the i n s t a l l a t i o n . Norwegian a p p a r a t u s 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  The ram  l o a d e d by means o f dead w e i g h t s m a i n t a i n s the p r e s s u r e i n an o i l f i l l e d cylinder.  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 o t h e r -  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 p a s t the ram and the  l i m i t e d weight c a p a c i t y a r e the main d i s a d v a n t a g e s .  B o t h the  above systems a r e , t h e r e f o r e , most c o n v e n i e n t f o r l o w c e l l pressurest F o r 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 h e r e , p r e s s u r e s up to 100 pounds p e r square i n c h a r e d e s i r a b l e .  To o b t a i n  p r e s s u r e s o f t h i s magnitude, a diaphragm-type r e g u l a t o r was f o r the m a i n c o n t r o l ,  used  A method o f c o u n t e r a c t i n g p r e s s u r e f l u c t u a -  t i o n s was d e v i s e d . The m o d i f i c a t i o n c o n s i s t s o f a l l o w i n g a c o n t i n u o u s 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 .  T h i s 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 regulator.  auxiliary  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 .  U s i n g t h i s method, r e g u l a t i o n b e t t e r t h a n 0,15 pounds p e r square i n c h was o b t a i n e d f o r the medium and h i g h c e l l p r e s s u r e s .  At  p r e s s u r e s below 15 pounds p e r 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 l o w e r e d 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, m a i n t a i n s the desired pressure.  The c a p a c i t y o f 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 cell.  i n the specimen, or l e a k a g e from the  D e a i r e d w a t e r was u s e d as the chamber f l u i d f o r a l l  r e p o r t e d i n t h i s t h e s i s , I n o r d e r to m i n i m i z e f l o w through  tests  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 . /  s q . i n . ) i n d i c a t e s the p r e s s u r e .  A d e d u c t i o n o f 2»5  lb./sq.in.  from the gauge r e a d i n g i s n e c e s s a r y to a l l o w f o r t h e 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 a g a i n s t a s t a n d a r d gauge t e s t e r .  A random d i s c r e p a n c y  of n o t g r e a t e r t h a n 0.3 l b . / s q . i n . was observed I n t h i s guage. The c e l l p r e s s u r e may  be r e l i e v e d by o p e n i n g the needle v a l v e  l o c a t e d I n the upper p l a t e of t h e chamber. The c e l l p r e s s u r e , a c t i n g on the p l u n g e r , decreases the l o a d on the specimen the  proving ring.  t o v a l u e s l o w e r t h a n those r e g i s t e r e d by  The c o r r e c t i o n t o be a p p l i e d t o the r i n g  d e f l e c t i o n , i n o r d e r to o b t a i n the a c t u a l l o a d , i s shown i n Pig. 5,  15. Load and D e f o r m a t i o n M e a s u r i n g D e v i c e s . A d i a l gauge, r e a d i n g t o 0,0001" i s u s e d f o r i n d i c a t i n g  the  proving ring deflection.  the  r i n g c o n s t a n t K « 0.444 l b s . / 0 , 0 0 0 1 " d e f l e c t i o n . The  F o r l o a d s i n the range 5-220 l b s ,  d e f o r m a t i o n o f the specimen i s measured by a n o t h e r  d i a l gauge (0.001"/div) s e t between the l o w e r clamp on the p r o v i n g r i n g and the upper p l a t e of the chamber, 6.  A p p a r a t u s f o r M e a s u r i n g Pore P r e s s u r e . 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 p r e v e n t e d d u r i n g the l o a d i n g s t a g e . C o n s e q u e n t l y , any apparatus u s e d f o r m e a s u r i n g pore p r e s s u r e s must be c a p a b l e o f o p e r a t i n g on a minimum o f pore water movement. The u s u a l l a b o r a t o r y methods o f m e a s u r i n g p r e s s u r e - 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 o f pore p r e s s u r e , owing t o t h e volume o f pore  water w h i c h would have t o f l o w from the specimen t o cause the instrument t o r e g i s t e r . For used.  l a r g e specimens, a t r a n s d u c e r - t y p e a p p a r a t u s may be  T h i s d e v i c e measures the d e f l e c t i o n o f a m e t a l  by means o f e l e c t r i c a l s t r a i n gauges.  diaphragm  Changes i n h y d r o s t a t i c  p r e s s u r e produce d e f l e c t i o n s o f t h e diaphragm w h i c h l e n d s i t s e l f to pore p r e s s u r e a p p l i c a t i o n s , P l a n t i m a (1953)• d e p a r t s somewhat from t h e n o - f l o w c o n d i t i o n .  T h i s method  Permanent m o i s t u r e  changes c a n be e n t i r e l y a v o i d e d , however, by t h e u s e o f the n u l l method o f p r e s s u r e measurement o r i g i n a l l y d e v i s e d by R e n d u l i c (1937)• the  The a p p a r a t u s used i n the p r e s e n t I n v e s t i g a t i o n  latter  employs  system.  The method adopted f o r the measurement o f pore p r e s s u r e i s e s s e n t i a l l y t h a t d e v e l o p e d a t I m p e r i a l C o l l e g e , London. e q 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 .  It is  The a p p a r a t u s and  procedure a r e d e s c r i b e d i n d e t a i l by B i s h o p 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  description  w i l l be g i v e n h e r e . Main f e a t u r e s o f t h e a p p a r a t u s a r e 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 c o u p l e d t o a manometer, Fig.  13.  The n u l l I n d i c a t o r employs a mercury column i n a g l a s s  c a p i l l a r y tube.  T h i s column i s m a i n t a i n e d throughout t h e d u r a -  t i o n o f the t e s t a t a predetermined l e v e l i n the c a p i l l a r y t u b e .  (the n u l l p o s i t i o n )  Any pore p r e s s u r e developed i n the specimen  37.  i s brought to a c t on the upper s u r f a c e o f the mercury.  Changes  i n pore p r e s s u r e t e n d to d i s l o c a t e the mercury maniscus from the null position.  The c o n t r o l c y l i n d e r i s u s e d t o r e s t o r e the  mercury column t o the i n i t i a l p o s i t i o n , and a t the same time p r o v i d e f l u i d t o a c t u a t e the Bourdon gauge ** manometer u n i t .  In  t h i s manner, d r a i n a g e from the specimen I s p r e v e n t e d , and the pore p r e s s u r e i s r e g i s t e r e d on the Bourdon gauge o r the manometer. The Bourdon gauge i s u s e d f o r i n d i c a t i n g p r e s s u r e s h i g h e r than a t m o s p h e r i c p r e s s u r e ( c o n s i d e r e d p o s i t i v e ) .  B e f o r e i t was  p u t i n t o s e r v i c e , the gauge was checked a g a i n s t a s t a n d a r d gauge t e s t e r w i t h w h i c h i t a g r e e d , w i t h i n the l i m i t s o f a c c u r a c y o f r e a d i n g the d i a l s .  Pore p r e s s u r e s below a t m o s p h e r i c p r e s s u r e  ( n e g a t i v e ) a r e i n d i c a t e d by the manometer.  The manometer may  u s e d a l s o f o r c e l l p r e s s u r e s up t o 20 l b . / s q . i n . w i t h i n the range this installation.  -15 to  be  Pore p r e s s u r e s  + 100 l b . / s q . i n . can be measured w i t h  Changes i n p r e s s u r e o f 0,1 l b . / s q . i n . can  be d e t e c t e d . I t i s o f t h e utmost importance t h a t the system be c o m p l e t e l y f i l l e d w i t h w a t e r and f r e e from l e a k s .  To f a c i l i t a t e t h e r e -  moval o f a i r from the v a r i o u s tubes and f i t t i n g s , a vacuum i s applied.  F r e s h l y b o i l e d w a t e r i s then f l u s h e d through the  a p p a r a t u s , u n t i l a l l t r a p p e d 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 . system.  Finally,  d e a i r e d w a t e r i s pumped i n t o the  The mercury t r o u g h (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 l o w e r e d , thereby a l l o w i n g water t o 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 p r e s s u r e 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  FIG  15.  PORE - P R E S S U R E  p<xje  3 7.  A P P A R A T U S  38.  p e r h a p s , t h e g r e a t e s t s i n g l e advantage o f t h i s a p p a r a t u s ; 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 t h e pore w a t e r and the m e a s u r i n g gauges. The a p p a r a t u s , as u s e d a t I m p e r i a l C o l l e g e , measures the pore p r e s s u r e a t t h e upper, or l o w e r , ends o f the specimen.  In  the l a t t e r s t a g e s o f t h e p r e s e n t i n v e s t i g a t i o n , t h i s method was m o d i f i e d i n o r d e r t h a t measurements may be o b t a i n e d anywhere on the l o n g i t u d i n a l a x i s o f t h e specimen.  The o n l y a l t e r a t i o n  t o the a p p a r a t u s , c o n s i s t s o f t h e i n c o r p o r a t i o n o f a porous probe (sometimes c a l l e d a p i l o t ) o f t h e type developed a t t h e Massachus e t t s ' I n s t i t u t e o f T e c h n o l o g y , Lambe ( 1 9 5 1 ) .  The probe i s  i n s e r t e d i n the specimen, pore p r e s s u r e b e i n g measured  i n the  r e g i o n o f the t i p . 7.  Automatic C o n t r o l . The pore p r e s s u r e a p p a r a t u s , d i s c u s s e d i n t h e p r e c e d i n g  p a r a g r a p h s , r e q u i r e s the f u l l  time a t t e n t i o n o f an o p e r a t o r .  The  main duty o f t h e o p e r a t o r i s to m a i n t a i n the mercury column a t the n u l l p o s i t i o n - by m a n u a l l y a d j u s t i n g t h e 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 c a n be t e d i o u s and time-consuming. C o n s e q u e n t l y , t h e development o f an a u t o m a t i c c o n t r o l was u n d e r taken.  The system o f a u t o m a t i c c o n t r o l f i n a l l y adopted o p e r a t e s  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 p r e s s u r e a p p a r a t u s . of the new d e v i c e a r e p r e s e n t e d i n Chapter VT o f t h i s 8.  Details  thesis.  F a b r i c a t i o n o f 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 i n g r e s s o f chamber f l u i d by some form o f f l e x i b l e membrane*  39.  The most a p p r o p r i a t e method o f 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 s u b j e c t o f an e x t e n s i v e i n v e s t i g a t i o n conducted by  Casagrande and W i l s o n  (1949) a t H a r v a r d U n i v e r s i t y .  The  outcome  o f 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 c o m p r i s i n g an i n n e r and o u t e r membrane w i t h a l a y e r o f 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 p r e s e n t  t e s t i n g program.  The membranes were  formed by d i p p i n g a wooden m a n d r i l i n r u b b e r l a t e x The  emulsion.  s u r f a c e s o f the m a n d r i l 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 o r d e r t o s e a l the wood and f a c i l i t a t e r e m o v a l  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 ,  a l l o w e d t o a i r dry f o r a t l e a s t e i g h t h o u r s .  The membranes were  g i v e n about t e n d i p s to o b t a i n a w a l l t h i c k n e s s o f 0.03 Two  was  s i z e s were r e q u i r e d , the o u t e r membrane was  inches.  formed on a  2.55  i n c h d i a m e t e r 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 n o r m a l l y  soaked  In water b e f o r e u s e , w h i c h has a tendency to produce s t r e t c h i n g , hence the $ a n d r i l d i a m e t e r s f o r the 2.5  i n c h specimen s i z e .  Advantage c a n be t a k e n , w i t h t h i s method o f f a b r i c a t i o n , t o i n c l u d e s l e e v e s which are u s e d 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  c a s t as an i n t e g r a l p a r t o f the i n n e r membrane. u s e d w i t h the o u t e r membrane was  sleeve  was  Another sleeve  cast separately.  The  utilization  of membranes p o s s e s s i n g the above f e a t u r e s , i s shown i n the photographic  supplement t o t h i s  thesis.  40.  The measured compressive s t r e n g t h o f the specimem must be c o r r e c t e d , t o a l l o w f o r the e f f e c t s o f the p r o t e c t i v e The c o r r e c t i o n t o be a p p l i e d can be e s t i m a t e d from the t i o n c h a r a c t e r i s t i c s o f the membranes.  jacket. deforma-  A s t r e s s / s t r a i n curve f o r  membranes formed o f " A e r o t e x " r u b b e r 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 o f the c o r r e c t i o n  i s d i s c u s s e d 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 B e f o r e the performance  o f any s o i l  t e s t s , the apparatus  was p u t t h r o u g h 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 o f s t e e l , was s e t up i n the axial c e l l .  tri-  Two membranes, w i t h a f i l m o f 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 b l o c k from the chamber f l u i d deaired water). the end f i t t i n g s .  ( i n t h i s case,  Rubber bands were u s e d to s e a l the membranes t o A chamber p r e s s u r e o f 50 l b . / s q . i n .  a p p l i e d f o r a p e r i o d o f 72 h o u r s .  was  A t the end o f t h i s time, the  b l o c k was c a r e f u l l y removed from the c e l l and examined f o r any e v i d e n c e o f l e a k a g e t h r o u g h the membranes o r end f i t t i n g s .  No  t r a c e o f water was o b s e r v e d , so i t was c o n c l u d e d t h a t 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 p r e s s u r e a p p a r a t u s p a s s e d 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 p r e s s u r e c o n t r o l was o b s e r v e d ; i t was  also  found to be f r e e from u n d e s i r a b l e 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 o f S o i l Specimens. A l l specimens were p r e p a r e d i n a humid room, i n o r d e r t o  prevent moisture l o s s e s . sample s i z e o f 2.8 of  2.5  (see  inches.  The  s o i l was trimmed from the o r i g i n a l  i n c h e s diameter down to the r e q u i r e d diameter  A s o i l l a t h e and w i r e saw were u s e d f o r t r i m m i n g  photographic supplement).  The s u r f a c e s were shaped  to  produce a c y l i n d r i c a l b l o c k , or as n e a r l y so as p o s s i b l e , c a r e b e i n g t a k e n n o t t o u n d u l y 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 5 i n c h e s l o n g ) was p l a c e d around the specimen.  The  than  specimen  was  then g r i p p e d 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 p e r m i t s t r i m m i n g 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 p r e v e n t i n g a d h e s i o n 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 o f d i s t u r b a n c e . The specimens were then weighed and a v i s u a l of  the s o i l type r e c o r d e d . (2) Filter  of  classification  pads  the specimen.  were p l a c e d a t both upper and l o w e r ends  v  V e r t i c a l s i d e d r a i n s made o f ^  n  wide f i l t e r  paper s t r i p s were p l a c e d around the p e r i m e t e r w i t h a s p a c i n g of about % i n c h between d r a i n s . intended to f a c i l i t a t e  T h i s arrangement o f f i l t e r s i s  d r a i n a g e and d i s t r i b u t e the pore p r e s s u r e  u n i f o r m l y throughout the  specimen.  The measured compressive s t r e n g t h must be c o r r e c t e d f o r the e f f e c t o f the s i . i e (2)  Reeve A n g e l No.  d r a i n s as d i s c u s s e d i n Appendix I I .  202 F i l t e r  Paper.  42  11.  S e t t i n g Up S p e c l m e n - l n T r i a x i a l  Cell.  P r i o r t o p o s i t i o n i n g t h e specimen i n the t e s t i n g machine, all  d r a i n a g e c o n n e c t i o n s to the c e l l were f r e e d o f a i r by  f l u s h i n g w i t h deaired water. The p o r e - p r e s s u r e a p p a r a t u s was c o n n e c t e d t o the c e l l a t this stage.  Porous d i s c s were p l a c e d a t each end o f the specimen.  The d r a i n s were made t o o v e r l a p t h e d i s c s , thus p r o v i d i n g uni n t e r r u p t e d d r a i n a g e from the s i d e s t o b o t h ends.  The specimen  was s e a t e d on t h e base and t h e i n n e r membrane p l a c e d i n p o s i t i o n by means o f a membrane s t r e t c h e r .  A i r t r a p p e d between the mem-  brane and t h e specimen was removed by a l l o w i n g a l i t t l e to f l o w back from t h e base b u r e t t e . r u b b e r bands ( o r r u b b e r  f  t  0  f  t  water  The a p p r o p r i a t e number o f  r i n g s ) was i n p o s i t i o n w h i l e  A f i l m o f c a s t o r o i l was a p p l i e d t o the i n n e r membrane.  desiring* The  second membrane was then p l a c e d over the specimen and s e a l e d I n a s i m i l a r manner t o the f i r s t .  Any excess water w h i c h may have  accumulated around the specimen d u r i n g t h e d e a i r i n g o p e r a t i o n was withdrawn by l o w e r i n g t h e base b u r e t t e to o b t a i n a s l i g h t n e g a t i v e p r e s s u r e i n the pore w a t e r . The chamber was f i l l e d w i t h d e a i r e d w a t e r and a low p o s i t i v e c e l l pressure ( 0 . 5 - 1 « 0 lb./sq.in.) applied.  Any  n e g a t i v e pore p r e s s u r e r e m a i n i n g , was then r e l i e v e d by a l l o w i n g the  pore water a c c e s s to a t m o s p h e r i c p r e s s u r e * In  t e s t s where pore p r e s s u r e measurements were o b t a i n e d  by means o f the p r o b e , 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 t h e s o i l  by hand, p r o d u c i n g a duct o f the same diameter as t h e probe.  43  D e a i r i n g the c a v i t y was a c c o m p l i s h e d by h a v i n g t h e probe c o n n e c t e d to ure  the p r e s s u r e l e a d from the c o n t r o l c y l i n d e r o f t h e p o r e - p r e s s apparatus.  By l o w e r i n g t h e mercury  trough o f t h e n u l l  i n d i c a t o r , and o p e r a t i n g t h e c o n t r o l c y l i n d e r o f the p o r e - p r e s s u r e a p p a r a t u s , water was made t o f l o w t h r o u g h t h e probe. of the  Insertion  the probe w h i l e m a i n t a i n i n g a s t e a d y f l o w o f w a t e r , d e a i r e d cavity.  Finally,  the stem o f the probe was s e a l e d from  chamber f l u i d by means o f r u b b e r 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 .  T h i s method o f i n s e r t i o n p r e v e n t s the  f o r m a t i o n o f a h i g h l y compressed zone o f s o i l i n the n e i g h b o u r hood o f t h e probe.  Moreover, the s m a l l amount o f water r e q u i r e d  for deairing i s not l i k e l y  t o have d e l e t e r i o u s e f f e c t s on t h e  specimen. 12.  Temperature The  Control.  temperatures i n the l a b o r a t o r y were m a i n t a i n e d i n the  range 18 - 22 degrees c e n t i g r a d e throughout t h e d u r a t i o n o f the tests.  44.  CHAPTER I V . SHEAR TESTS WITH PORE PRESSURE MEASUREMENTS  A.  Introduction  As s t a t e d a t the o u t s e t o f t h e t e x t , t h e p r i m a r y purpose of the i n v e s t i g a t i o n was t o e s t a b l i s h the p a t t e r n o f pore p r e s s u r e , changes w i t h a p p l i e d s t r e s s and time, i n l o n g t r i a x i a l t e s t s on P o r t Mann c l a y .  duration  I n 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 o b t a i n e d on s t r e n g t h parameters a n d d r a i n a g e c h a r a c t e r i s t i c s o f t h e s o i l . The r e p o r t summarizes  t h e r e s u l t s o f a l l l a b o r a t o r y t e s t s con-  n e c t e d w i t h t h e above a s s i g n m e n t . from May, 1959 t h r o u g h September,  The t e s t i n g program  1959. A l l t e s t s were p e r -  formed i n the S o i l Mechanics L a b o r a t o r y B r i t i s h Columbia.  extended  a t the U n i v e r s i t y of  The s o i l samples r e q u i r e d f o r t h e 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 w h i c h would r e s u l t from a slow b u i l d - u p o f pore p r e s s u r e ; a phenomenon n o t i c e d i n e a r l i e r r e s t s p e r f o r m e d  45.  by R. A. Spence, C o n s u l t i n g E n g i n e e r .  I t was a n t i c i p a t e d t h a t  l o n g d u r a t i o n shear t e s t s w o u l d a c c e n t u a t e e f f e c t i f i t were a r e a l i t y .  the s l o w b u i l d - u p  The p r e s e n t i n v e s t i g a t i o n was  p l a n n e d w i t h t h i s i n mind. B.  Previous Research  E a r l i e r works, r e p o r t e d by Casagrande and W i l s o n Taylor  (1949),'  (1943) and o t h e r s , i n d i c a t e t h a t the r a t e o f l o a d i n g I n  l a 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 s h e a r strength of fine-grained s o i l s .  F o r any one c l a y h i g h e r  s t r e n g t h s a r e g e n e r a l l y o b t a i n e d a t 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, T a y l o r f o u n d t h a t t h e s t r e n g t h  Increased  by about 50$ as a consequence o f i n c r e a s i n g the r a t e o f def o r m a t i o n from 1$ p e r minute to 1000$ p e r m i n u t e .  A similar  change from 1$ to 0.001$ p e r minute l e d to a r e d u c t i o n o f 20$ i n observed shear s t r e n g t h .  Decrease i n the d e f o r m a t i o n  below 0.001$ p e r minute had o n l y n e g l i g i b l e e f f e c t s .  rate  Results  s i m i l a r t o T a y l o r ' s have been o b t a i n e d i n t e s t s on c l a y samples from w i d e s p r e a d l o c a l i t i e s .  Although  th© p r e s e n t i n v e s t i g a t i o n  i s c o n c e r n e d o n l y 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 o b s e r v a t i o n s  have a more g e n e r a l a p p l i c a t i o n t o c l a y s ,  i n v i e w o f t h e f i n d i n g s o f the above i n v e s t i g a t o r s . The  dependence o f 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 t o v i s c o u s l a g i n  the pore f l u i d a t h i g h d e f o r m a t i o n  r a t e s , and, t o the p l a s t i c  f l o w o f the s o i l mass a t the slower r a t e s . most s i g n i f i c a n t a t h i g h d e f o r m a t i o n  Viscous e f f e c t s are  r a t e s , which are outside  46  the  scope o f t h i s i n v e s t i g a t i o n .  hand, i s o f c o n s i d e r a b l e i n t e r e s t .  P l a s t i c f l o w , on the o t h e r The r e s u l t s o f t h e shear  t e s t s w i l l demonstrate t h a t p l a s t i c f l o w ( o r "creep") and pore p r e s s u r e a r e , p r o b a b l y , i n t e r - d e p e n d e n t , i n the case o f P o r t Mann c l a y , a t any r a t e . C.  Scope o f the P r e s e n t I n v e s t i g a t i o n  Samples o b t a i n e d from two b o r i n g s a t the s i t e o f t h e New P o r t Mann B r i d g e were s e l e c t e d ; k e e p i n g i n m i n d t h a t samples w i t h as  u n i f o r m a t e x t u r e as p o s s i b l e were r e q u i r e d .  The samples were  from depths r a n g i n g from 135 t o 150 f e e t below the e x i s t i n g level.  ground  A t t h i s l o c a t i o n , the c l a y s t r a t u m was f o u n d t o be q u i t e  homogeneous. T r i a x i a l t e s t s o f t h i s t h e s i s extended o v e r p e r i o d s o f one t o twenty days.  The t r i a x i a l a p p a r a t u s u s e d 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 ( l o a d s a p p l i e d i n i n c r e m e n t s ) . C e l l p r e s s u r e s up t o 80 l b . / s q . i n . were employed. Rates o f development o f pore p r e s s u r e were o b s e r v e d throughout t h e d u r a t i o n o f t e s t s on seven specimens.  Pore  p r e s s u r e was measured e i t h e r a t t h e top o r a t the c e n t r e o f t h e specimen.  Pour t e s t s were c a r r i e d o u t w i t h t h e pore p r e s s u r e  measurements taken a t the t o p ; i n t h e r e m a i n i n g t h r e e , i t was measured i n t h e v i c i n i t y o f the c e n t r e .  Shear s t r e n g t h s , a t  d i f f e r i n g degrees o f c o n s o l i d a t i o n , were d e t e r m i n e d f o r s i x o f these specimens. S t a i n t e s t s , t o 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  c o m p o s i t i o n o f the s o i l p a r t i c l e s , were p e r f o r m e d . The  s e n s i t i v i t y has been e s t i m a t e d  from vane t e s t  r e s u l t s p r o v i d e d by R. A. Spence, C o n s u l t i n g E n g i n e e r s . A t t e r b e r g l i m i t s were d e t e r m i n e d 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 The  (b) the c l a y t r e a t e d w i t h sea w a t e r .  Atterberg l i m i t s obtained  i n d i c a t i v e of the D.  i n these t e s t s are assumed to  leaching,  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 C h a p t e r , a p p l y d i s t u r b e d samples o f the P o r t Mann c l a y . were u s e d t o r e c o v e r borings.  be  the 2,8  i n c h diameter samples from  the s a m p l e r , wrapped i n p o l y e t h l e n e  quired f o r t e s t i n g .  un-  Swedish F o i l Samplers  S h o r t l y a f t e r s a m p l i n g , t h e s o i l was  T h i s m o i s t u r e s e a l was  to  f i l m and  the  e x t r u d e d from thoroughly  waxed.  n o t removed u n t i l the samples were r e -  T h i s method o f p r o t e c t i o n appeared t o have  been v e r y 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 o b s e r v e d d u r i n g the p e r i o d the samples were s t o r e d  being  before  testing.  The  s a m p l i n g o p e r a t i o n was  o f 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,  c a r r i e d out d u r i n g t h e summer  1959. Table I I I shows the o r d e r 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 o f m a t e r i a l , e t c . E.  D e s c r i p t i o n o f Shear T e s t s and  Results.  I n the d i s c u s s i o n w h i c h f o l l o w s , each t e s t i s t r e a t e d separately.  R e s u l t s dependent on the c o r r e l a t i o n o f a number o f  48.  t e s t s a r e p r e s e n t e d a t t h e end o f t h e s e c t i o n d e a l i n g w i t h the individual tests.  The sequence o f the s t a g e s , e t c . , i s  summarized i n T a b l e I V .  SHEAR TEST} NUMBER J  BORING NUMBER  ! SAMPLE ! ! NUMBER J  DEPTH BELOW GROUND LEVEL  |  BN23P  i  2  !  BN23P  j  25  ;  1391-6"  3  !  BN23P  I  23  !  138'-4" to  4  |  BN23P  j  27  j  !  40  5  6  7  ;  J ;  BS2P  BS2P  BS2F  !  j  42  46  NATURAL MOISTURE CONTENT  1  VISUAL DESCRIPTION . OP SAMPLES  |  1  22  1 ! !  i  Dark Grey C l a y w i t h darker markings.  137'-6" to 138'-4"  I J |  TABLE I I I ~  to  !  66.8$  tt  !  67.6$  tt  140'-6" t o 141'~4  ;  61.8$  tt  147'-9" t o 148'-5  !  68.1$  tr  148' - 7 to 149'-5"  !  59.4$  !  58.1$  140'-00"  139t  M  M  i  Dark Grey C l a y w i t h L i g h t Grey Dis» c o l o r a t i o n on t o p .  ! |  Dark Grey C l a y w i t h .darker m a r k i n g s .  n :  151t^3"  152'-1"  to  DESCRIPTION OP TEST SAMPLES.  TABLE IV — SCHEDULE OF SHEAR TESTS  SHEAR TEST NO.  TIME ELAPSED BETWEEN PREPDURA- ARATION OF TION SPECIMEN AND OF APPLICATION OF TEST CELL PRESSURE Days Hours  Build-UD Staaes  TOTAL NO.  CELL PRESSURES lb./sq. in. SEQUENCE  1  1  None  2  20 50  2  6  5  4  12 30 50 60  3  9  13  1  20  4  10  15  4  20 40 60 80  5  10  3  6  17  144  2  20 40 60 20 40  7  20  18  2  LOCATIC)N OF POF:E PRESSURE MFAST RRMRNTS?  40 60  Consecutive  Drainaae Staaes CELL PRESSURES TOTAL lb./sq. in. SEQUENCE NO.  Loadina Staaes CELL PRESSURES TYPE TOTAL lb./sq, OF NO. in. LOADING  None  None  1  60  Following build-up stages  1  60  Incremental  1  20  Following build-up stages  1  20  ditto  ditto  1  80  Following build-up stages  1  80  ditto  ditto  1  60  ditto  1  60  ditto  Separated by a Drainage Stage  1  40  1  40  ditto  Separated by a Drainage Stage  2  40 80  1  80  ditto  ditto  act ffl  _ a*  Trn  Intermediate between build-up stages One intermediate and one after build-up stages -1 z  51.  l.(a)  T e s t 1.  Test P r o c e d u r e :  T e s t 1 was performed  consequently of short d u r a t i o n . at  the top o f t h e specimen.  as a p i l o t t e s t and was  The pore p r e s s u r e was measured  The c e l l pressure' was r a i s e d  i m m e d i a t e l y t h e specimen was s e t up f o r t r i a x i a l a p p a r a t u s . p r e s s u r e changes r e s u l t i n g from t h e i n c r e a s e i n c e l l  pressure  were r e c o r d e d .  Drainage  The r e s u l t s a r e shown i n Graph 4-1.  was p r e v e n t e d throughout  the build-up stage.  Pore  The specimen was  not loaded. Results:  The r a t e o f b u i l d - u p o f pore p r e s s u r e , and the magni-  tudes o f t h e pore p r e s s u r e parameter B a r e as f o l l o w s :  INCREASE IN CELL PRESSURE 6~3 l b . / s q . i n Prom To  (1)  TIME REQUIRED TO REACH EQUILIBRIUM MINUTES  PORE PRESSURE (1) PARAMETER B AT EQUILIBRIUM  5  20  25  0.87  20  50  20  0.92  Here the v a l u e s o f B a p p l y t o t o t a l changes i n c e l l and pore p r e s s u r e s .  52. (b)  T e s t 2.  Test Procedure: for  The specimen was a l l o w e d t o s t a n d i n the c e l l  a p e r i o d o f f i v e hours b e f o r e a p p l i c a t i o n o f c o n f i n i n g  pressure.  Pore p r e s s u r e was measured a t the t o p o f the specimen  throughout  the d u r a t i o n o f the t e s t .  F i l t e r paper s i d e d r a i n s  were employed i n the manner d i s c u s s e d e a r l i e r .  Axial loading  was a p p l i e d i n i n c r e m e n t s u n t i l t h e specimen f a i l e d .  The m o i s -  t u r e c o n t e n t was determined a t t h r e e l o c a t i o n s l n the specimen a f t e r t h e shear t e s t .  D u r a t i o n o f the t e s t was s i x days; t h e  l o a d i n g stage a c c o u n t i n g f o r one a n d o n e - h a l f days o f t h i s Pore P r e s s u r e B u i l d - U p S t a g e :  time.  A n e g a t i v e pore p r e s s u r e o f 4«8  l b . / s q . i n . developed d u r i n g the f i v e hour p e r i o d b e f o r e t h e c e l l p r e s s u r e was a p p l i e d .  The c e l l p r e s s u r e was r a i s e d I n f o u r  increments t o a maximum o f 60 l b . / s q . i n . , no d r a i n a g e specimen b e i n g p e r m i t t e d .  from  Graph 4-2 shows the r a t e o f b u i l d - u p  of pore p r e s s u r e f o r each i n c r e m e n t a l change I n c e l l p r e s s u r e . The  time r e q u i r e d f o r t h e pore p r e s s u r e t o r e a c h e q u i l i b r i u m  w i t h the c e l l p r e s s u r e s and t h e c o r r e s p o n d i n g magnitudes o f B are l i s t e d below:  INCREASE I N CELL PRESSURE o~3 l b . / s q . i n . From To  j J J i  TIME REQUIRED TO REACH EQUILIBRIUM MINUTES  i ! J  i  PORE PRESSURE PARAMETER B AT 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 v a l u e o f B however, e q u a l s u n i t y when based subsequent  changes i n c e l l p r e s s u r e s e x c e e d i n g 30  i n d i c a t i n g t h a t t h e specimen was f u l l y d r a i n a g e stage commenced. specimen was f u l l y Drainage S t a g e :  T h i s was  on  lb./sq.in.;  s a t u r a t e d before the  the o n l y t e s t i n which  the  saturated p r i o r to loading.  F o l l o w i n g t h e b u i l d - u p s t a g e , d r a i n a g e from the  specimen was a l l o w e d , w h i l e the c e l l p r e s s u r e ' was m a i n t a i n e d a t 60 l b . / s q . i n .  The f l o w o f w a t e r was  of t h e specimen.  Volume changes due  were measured i n the b u r e t t e .  d i r e c t e d towards the base to e x p u l s i o n o f pore  water  A sudden drop i n pore p r e s s u r e  was  e x p e c t e d a t the o n s e t o f d r a i n a g e , but t h i s d i d not m a t e r i a l -  ize  as i s e v i d e n t from the pore p r e s s u r e / t i m e c u r v e shown i n  Graph 4-3. t o 11.8  D u r i n g the d r a i n a g e s t a g e t h e pore p r e s s u r e  l b . / s q . i n . from the i n i t i a l 51.7.  dropped  Primary c o n s o l i d a t i o n  was 16% complete a t end o f d r a i n a g e stage (based on  dissipation  of pore p r e s s u r e ) . Loading Stage: sq.  The c e l l p r e s s u r e (o"3) was m a i n t a i n e d a t 60 l b . /  i n . throughout  the l o a d i n g s t a g e .  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 v a l u e o f 31.0  lb./sq.in.  D u r i n g t h e l o a d i n g stage the pore p r e s s u r e  g r a d u a l l y i n c r e a s e d from 11.9 for  t o 35»1 l b . / s q . i n . "Creep"  accounted  the g r e a t e r p a r t o f the d e f o r m a t i o n b e f o r e f a i l u r e ; the  i n s t a n t a n e o u s d e f o r m a t i o n b e i n g v e r y s m a l l i n comparison. The s t r e s s v s . s t r a i n c u r v e f o r the specimen i s shown i n Graph 4-4.  The manner i n w h i c h the pore p r e s s u r e changed w i t h  d e v i a t o r s t r e s s , time and d e f o r m a t i o n , i s shown i n Graph 4**5. Time t o f a i l u r e t ; f  34  hours.  54.  Type o f F a i l u r e :  F a i l u r e o c c u r r e d on a s i n g l e s h e a r p l a n e  i n c l i n e d a t 60° t o the plane o f major p r i n c i p a l  stress,  (horizontal). M o i s t u r e Content D e t e r m i n a t i o n s :  The f o l l o w i n g  a r e the r e s u l t s  of the m o i s t u r e c o n t e n t t e s t s performed a f t e r the shear  LOCATION  MOISTURE CONTENT % OF DRY WEIGHT  Top  61.0  Shear Zone  60.0  Base  55.5  test.  55  (c)  Test  5.  Test P r o c e d u r e : for  The  specimen was  a l l o w e d to s t a n d i n the  a p e r i o d o f 13 hours b e f o r e a p p l y i n g the c e l l p r e s s u r e .  Pore p r e s s u r e was measured a t the top o f the specimen the d u r a t i o n o f the t e s t . drainage.  The  w h i c h were devoted  t e s t e d to f a i l u r e ; the l o a d s b e i n g  D u r a t i o n of t e s t was  No water was  from the specimen d u r i n g the b u i l d - u p s t a g e .  applied.  n i n e days, s i x o f  t o the l o a d i n g s t a g e .  Pore P r e s s u r e B u i l d - U p S t a g e :  p r e s s u r e o f 5.5  throughout  S i d e d r a i n s were employed to a s s i s t  specimen was  a p p l i e d i n increments.  was  cell  a l l o w e d t o escape . A n e g a t i v e pore  l b . / s q . i n . had developed b e f o r e the c e l l  The  c e l l p r e s s u r e was  a v a l u e o f 20 l b . / s q . i n .  pressure  r a i s e d i n one o p e r a t i o n t o  A p e r i o d o f 210 minutes e l a p s e d b e f o r e  the pore p r e s s u r e came t o 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 pressure.  The v a l u e o f B a t e q u i l i b r i u m was  found to be  0.76.  Pore p r e s s u r e gauge r e a d i n g s v s . time are p l o t t e d f o r the b u i l d up stage o f t h i s t e s t i n Graph Drainage to  Stage:  Drainage was  the base b u r e t t e .  4-6. p e r m i t t e d by o p e n i n g  the v a l v e  The manner i n which the pore p r e s s u r e ,  and volume, changed d u r i n g the d r a i n a g e s t a g e , i s shown In Graph 4-7.  There i s a marked s i m i l a r i t y between the c u r v e s  o b t a i n e d f o r the d r a i n a g e s t a g e s o f t h i s t e s t and those o f Test In  t h i s t e s t , however, the volume change v s . time i n d i c a t e t h a t  the average p r i m a r y c o n s o l i d a t i o n was complete a t about minutes^'  (2)  from the commencement o f the d r a i n a g e  tioo  stage.  Curve f i t t i n g t o o b t a i n f o l l o w s the method p r o p o s e d by B i s h o p and H e n k e l (1957).  1,100  2.  56.  The c u r v e r e l a t i n g decrease o f pore p r e s s u r e t o time (Graph 4-7) shows t h a t 70$ d i s s i p a t i o n had o c c u r r e d i n 1,100 minutes. A r e s i d u a l pore p r e s s u r e o f 3.2 l b . / s q . i n . was r e c o r d e d a t t h e end o f the d r a i n a g e stage (top o f specimen).  Average o f  pore p r e s s u r e would be somewhat l e s s . Loading Stage: following  The pore p r e s s u r e showed a g r a d u a l i n c r e a s e  the a p p l i c a t i o n  o f each l o a d i n c r e m e n t .  Failure of  the specimen o c c u r r e d when the d e v i a t o r s t r e s s a t t a i n e d of 18.1 l b . / s q . i n .  a value  D u r i n g t h e l o a d i n g stage the pore p r e s s u r e  i n c r e a s e d by 7.6 l b . / s q . i n . t o 10.8 l b . / s q . i n .  C e l l pressure  was m a i n t a i n e d a t 20 l b . / s q . i n . throughout t h e l o a d i n g s t a g e . Graphs 4-8 a n d 4-9 p e r t a i n  t o the l o a d i n g stage o f t h i s  test. Time t o f a i l u r e t : f  Type o f F a i l u r e : was  114 h r s .  F a i l u r e o c c u r r e d on a s i n g l e  shear p l a n e  i n c l i n e d a t an angle o f 60° t o t h e p l a n e o f  which  major'principal  stress. M o i s t u r e Content D e t e r m i n a t i o n s :  Moisture contents a f t e r  were as f o l l o w s :  LOCATION Top o f Specimen  tJ |  6'1.6 66.1  Shear Zone Base o f Specimen  MOISTURE CONTENT % OF DRY WEIGHT  i  68.0  tests  57.  Water drawn i n t o the specimen when the c e l l p r e s s u r e was may account  lowered  f o r the h i g h m o i s t u r e c o n t e n t r e c o r d e d a t the base  of the sample.  58.  (d)  Test  Procedure:  4. The  specimen was  a l l o w e d t o s t a n d i n the  triaxial  c e l l f o r a p e r i o d o f 15 hours b e f o r e c e l l p r e s s u r e was a p p l i e d . Pore p r e s s u r e was measured a t t h e top o f the specimen the d u r a t i o n o f the t e s t . c e l l p r e s s u r e was  throughout  F o l l o w i n g the i n i t i a l s t a n d i n g p e r i o d ,  a p p l i e d i n i n c r e m e n t s ; no change i n the water  c o n t e n t o f the specimen b e i n g p e r m i t t e d .  On e q u i l i b r i u m b e i n g  beached between the pore p r e s s u r e and c e l l p r e s s u r e , d r a i n a g e was p e r m i t t e d i n o r d e r to i n c r e a s e the degree o f 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 d r a i n a g e b e i n g  a l l o w e d d u r i n g the l o a d i n g s t a g e . Pore P r e s s u r e B u i l d - U p S t a g e : 5.8  days.  A n e g a t i v e pore p r e s s u r e o f  l b . / s q . i n . developed b e f o r e t h e c e l l p r e s s u r e was a p p l i e d .  The c e l l p r e s s u r e was sq.  D u r a t i o n o f t e s t - 10  r a i s e d i n f o u r i n c r e m e n t s , each 20 l b . /  i n . The manner i n w h i c h the pore p r e s s u r e 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 o f c e l l p r e s s u r e s , i s shown i n Graph 4-10.  The p e r t i n e n t d a t a from the b u i l d - u p stage a r e  INCREASE IN CELL PRESSURE 6~ 3 l b . / s q . i n From To  J } i \  TIME REQUIRED . TO REACH EQUILIBRIUM MINUTES  j i ! ! !  listed  PORE PRESSURE PARAMETER B AT EQUILIBRIUM  0  20  j  90  20  40  |  45  40  60  j  25  j  0.86  60  80  !  17  !  0.88  0.60 0.79  59.  Drainage S t a g e :  The specimen d r a i n e d t o the base b u r e t t e f o r a  p e r i o d o f 77 h o u r s .  Daring t h i s time t h e pore p r e s s u r e dropped  from the i n i t i a l v a l u e o f 70 down t o 10,8 l b . / s q . i n .  The b e h a v i o r  of t h e pore p r e s s u r e d u r i n g t h e d r a i n a g e stage f o l l o w e d a s i m i l a r p a t t e r n t o t h a t observed i n previous t e s t s .  The volume  decrease  f o r the d r a i n a g e stage r e p r e s e n t s 11$ o f the o r i g i n a l volume o f specimen. Loading Stage: throughout  The c e l l p r e s s u r e was m a i n t a i n e d a t 80 l b . / s q . i n .  the l o a d i n g s t a g e .  f a i l u r e a t 4,2% s t r a i n .  I n c r e m e n t a l l o a d i n g produced  The d e v i a t o r s t r e s s a t f a i l u r e was 39.3  l b . / s q . i n . and t h e pore p r e s s u r e 46>»0 l b . / s q . i n .  Pore p r e s s u r e  i n c r e a s e s were g r a d u a l f o r p e r i o d s up to 20 hours a f t e r t h e application  o f an i n c r e m e n t ; the r a t e o f i n c r e a s e f a l l i n g o f f  sharply w i t h time. The s t r e s s v s 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 p r e s s u r e changed  w i t h d e v i a t o r s t r e s s , time a n d d e f o r m a t i o n . Time o f f a i l u r e t ^ ; Type o f F a i l u r e :  75 h o u r s .  The specimen f a i l e d on a s i n g l e shear p l a n e  i n c l i n e d a t 52° t o t h e h o r i z o n t a l . M o i s t u r e c o n t e n t s a f t e r t e s t s were as f o l l o w s :  LOCATION  1  MOISTURE CONTENT % OF DRY WEIGHT  Top o f Specimen  55.6  S h e a r i n g Zone  53.1  Base o f Specimen  54.3  60.  (e)  T e s t 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 p r e s s u r e measurements were made i n the v i c i n i t y o f t h e c e n t r e o f the specimen.  F o r t h i s purpose a porous probe was  employed i n the manner d e s c r i b e d i n Chapter I I I . O t h e r w i s e , the p r o c e d u r e was s i m i l a r t o t h a t o f p r e v i o u s  tests.  U n f o r t u n a t e l y , a f a u l t developed i n a v a l v e a s s o c i a t e d w i t h t h e probe equipment. stage  Some d r a i n a g e  occurred during the  i n t e n d e d f o r the o b s e r v a t i o n o f pore b u i l d - u p .  The d e f e c t  i n t h i s v a l v e was not remedied u n t i l t h e l o a d i n g s t a g e was i n progress.  C o n s e q u e n t l y , pore p r e s s u r e s r e c o r d e d i n the b u i l d - u p  s t a g e , 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 t r e n d s a r e i n d i c a t i v e t h a t 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  tests.  I n 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 t h e pore p r e s s u r e the f o u r t h l o a d increment onwards. curve  i s that obtained  The shape o f the  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 .  from  stress-strain  F u r t h e r m o r e , the  s t r e s s e s a t f a i l u r e a r e a p p r o p r i a t e when i t comes to p l o t t i n g Mohr diagrams. C e l l p r e s s u r e was m a i n t a i n e d out t h e l o a d i n g s t a g e .  a t 60 l b . / s q . i n .  through-  F a i l u r e o c c u r r e d 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 v a l u e o f 34.7 l b . / s q . i n . , a t w h i c h s t r e s s the pore p r e s s u r e 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 t o f a i l u r e : t  f  -  126 h o u r s .  MOISTURE CONTENT % OF DRY WEIGHT  LOCATION  !  Top o f Specimen  {  53.2  S h e a r i n g Zone  i  56.2  Base o f Specimen  j  57.8  62.  (f)  Test  Procedure:  6, The  specimen was  a l l o w e d t o s t a n d i n the  triaxial  c e l l f o r a p e r i o d o f s i x days b e f o r e c e l l p r e s s u r e was a p p l i e d . D e a i r e d water surrounded during t h i s time.  the specimen and i t s p r o t e c t i v e membranes  Pore p r e s s u r e measurements were made t h r o u g h -  out the d u r a t i o n o f t h e t e s t ; the probe b e i n g l o c a t e d a t mid h e i g h t o f the specimen. The p r o c e d u r e adopted  i n previous t e s t s , that of a b u i l d -  up s t a g e f o l l o w e d by d r a i n a g e , was case.  s l i g h t l y modified i n this  I n s t e a d , the c e l l p r e s s u r e was r a i s e d i n two  increments  w i t h a d r a i n a g e stage i n t e r m e d i a t e 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,  o b t a i n e d on t h e pore p r e s s u r e The  specimen was  f u r t h e r i n f o r m a t i o n was  parameters.  t e s t e d t o f a i l u r e i n the u s u a l manner.  D u r a t i o n o f t e s t , 17 days, i n c l u d e d the 6 days s t a n d i n g t i m e . Pore P r e s s u r e B u i l d - U p Stage:  A l t h o u g h not i n c h r o n o l o g i c a l  sequence, the b u i l d - u p s t a g e s w i l l be d i s c u s s e d t o g e t h e r i n t h i s paragraph;  the d i s c u s s i o n on d r a i n a g e stage a p p e a r i n g under a  separate heading.  The r e s u l t s o f t h e f i r s t b u i l d - u p stage a r e  s i m i l a r t o those r e p o r t e d f o r p r e v i o u s t e s t s .  No n e g a t i v e pore  p r e s s u r e , however, developed i n the i n i t i a l p e r i o d b e f o r e t h e specimen was  subjected to c e l l pressures.  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 p r e s s u r e vs time r e l a t i o n s h i p , but the v a l u e of B was  c o n s i d e r a b l y l o w e r , as might be  expected  i n v i e w o f the i n t e r v e n i n g d r a i n a g e s t a g e .  The r e s u l t s o f the  b u i l d - u p s t a g e s are p l o t t e d i n Graph 4-14.  The  following  t a b u l a t i o n shows some r e s u l t s f o r the b u i l d - u p s t a g e :  63.  INCREASE I N CELL PRESSURE 6~ 3 l b . / s q . i n . Prom To 0  TIME REQUIRED TO REACH EQUILIBRIUM MINUTES  20  PORE PRESSURE PARAMETER B AT EQUILIBRIUM  15  0.86  12  0.51  (Drainage Stage Between 20  40  Drainage S t a g e :  I n t e s t s where the pore p r e s s u r e s a r e determined  at the c e n t r e o f t h e specimen,  d r a i n a g e may be p e r m i t t e d from t h e  top and base o f the specimen s i m u l t a n e o u s l y .  T h i s procedure r e -  duces the time r e q u i r e d t o o b t a i n f u l l d i s s i p a t i o n of t h e pore pressure.  Such a d r a i n a g e arrangement s i m u l a t e s the c o n d i t i o n s  e x i s t i n g i n t h e 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 o f the d r a i n a g e stage o f a t r i a x i a l t e s t c a n be t r e a t e d as i f they were o b t a i n e d 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 o f d r a i n a g e stage may be u s e d i n d e t e r m i n i n g t h e 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 ) , and the c o e f f i c i e n t o f y  permeability (k). I n the p r e s e n t t e s t , d r a i n a g e was p e r m i t t e d from t h e t o p and base o f the specimen. are p l o t t e d i n Graph 4-15.  The r e s u l t s f o r t h e d r a i n a g e stage A t t h e end o f t h e d r a i n a g e s t a g e  p r i m a r y 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 c e n t r e o f the specimen,  as c a n be o b s e r v e d from t h e pore p r e s s u r e d i s s i p a t i o n  vs time c u r v e , Graph 4-15.  The volume change v s time c u r v e s on  the same graph i n d i c a t e t h a t on t h e average p r i m a r y c o n s o l i d a t i o n was complete a t about 760 minutes drainage.  from the commencement o f  64.  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 ) , based on the v  pore p r e s s u r e vs time r e l a t i o n s h i p , i s e s t i m a t e d t o be 0.0054 ins.  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  III). P r e v i o u s t e s t s have i n d i c a t e d t h a t the s i d e d r a i n s a r e not e f f e c t i v e i n promoting  drainage.  T h i s t e s t a f f o r d e d an  o p p o r t u n i t y o f c h e c k i n g the e f f i c i e n c y o f t h e d r a i n s . of c  y  The v a l u e  i s computed from t h e volume change v s t i m e c u r v e on the  b a s i s o f : (a) r a d i a l d r a i n a g e , which takes f o r g r a n t e d e f f e c t i v e s i d e d r a i n a g e «* and (b) d r a i n a g e towards the upper and l o w e r ends o f the specimen o n l y .  By comparison  w i t h the c  v  value  o b t a i n e d i n d e p e n d e n t l y from pore p r e s s u r e / t i m e 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 t h e c  v  v a l u e s ob-  t a i n e d i n (a) a n d (b) w i l l i n d i c a t e the d r a i n a g e 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 v a l u e o f c  d e r i v e d from the t h r e e c o n s i d e r a t i o n s a r e 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 o f pore p r e s s u r e  { i  0.00540  Based on r a d i a l and end drainage  i {  0.00026  End d r a i n a g e o n l y  j  0.00650  TABLE V. DETERMINATIONS OF COEFFICIENT OF CONSOLIDATION.  y  65  F u r t h e r d i s c u s s i o n on the e f f e c t s o f s i d e d r a i n s i s i n c l u d e d i n the n e x t  Chapter.  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) i s e s t i m a t e d t o be 4.7 x IO"? i n s . / m i n .  T h i s v a l u e a p p l i e s , o f c o u r s e , o n l y to  the s t r e s s e s c o r r e s p o n d i n g t o a c e l l p r e s s o f 40 l b . / s q . i n . ; the s t r e s s a c t i n g d u r i n g t h e d r a i n a g e s t a g e .  The pore p r e s s u r e  dropped from 17.2 down t o 0.75 l b . / s q . i n . d u r i n g the drainage stage. Loading Stage: throughout  The c e l l p r e s s u r e was m a i n t a i n e d a t 40 l b . / s q . i n .  the l o a d i n g stage.  F a i l u r e o c c u r r e d when t h e d e v i a t o r  s t r e s s a t t a i n e d a v a l u e o f 19.1 l b . / s q . i n .  The pore p r e s s u r e a t  f a i l u r e was 31.0 l b . / s q . i n . The v a l u e o f t h e pore p r e s s u r e parameter A i s e s t i m a t e d a t 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 b a s e d on t h e a s s u m p t i o n  t h a t B r e m a i n e d c o n s t a n t throughout  the l o a d i n g stage.  In f a c t ,  the magnitude o f B i n c r e a s e s d u r i n g l o a d i n g , but t h e i n c r e a s e i n t h i s case would be s m a l l , because t h e specimen had a t t a i n e d a h i g h degree o f s a t u r a t i o n p r i o r t o the l o a d i n g s t a g e . The r e s u l t s o f the l o a d i n g s t a g e a r e shown i n Graph 4-16 and 4-17.  I t i s e v i d e n t from Graph 4-17 t h a t t h e p o r e p r e s s u r e  showed a g r a d u a l i n c r e a s e f o r some time a f t e r the a p p l i c a t i o n o f each l o a d  increment.  Time t o f a i l u r e , t Type o f F a i l u r e :  f  5  167 h o u r s .  F a i l u r e o c c u r r e d on a t l e a s t two p l a n e s as  i n d i c a t e d on the s k e t c h accompanying Graph 4-16.  M o i s t u r e c o n t e n t s a f t e r t e s t s were as f o l l o w s :  LOCATION  J  Top o f Specimen  J  MOISTURE CONTENT % OP DRY WEIGHT 62.2  M i d d l e Zone  |  62.6  Base o f Specimen  »  66.0  67.  (g)  T e s t 7-  Procedure:  The p r o c e d u r e adopted was s i m i l a r t o t h a t  p l o y e d i n T e s t 6. stages.  em-  Table IV i n d i c a t e s the sequence o f the  D u r a t i o n o f t e s t - 20  Pore P r e s s u r e B u i l d - U p S t a g e :  days. The c e l l p r e s s u r e was r a i s e d i n  two i n c r e m e n t s , namely, 0 to 40 l b . / s q . i n . and 40 t o 80 I b . / s q . in.  F o l l o w i n g the b u i l d - u p p e r i o d f o r the f i r s t i n c r e a s e i n  c e l l p r e s s u r e , d r a i n a g e was p e r m i t t e d from the base. increment was  second  a p p l i e d when d r a i n a g e had been i n p r o g r e s s f o r a  p e r i o d o f 4 days. i n G-raph 4-18.  The  The r e s u l t s o f the b u i l d - u p s t a g e s are shown  P e r t i n e n t d a t a are l i s t e d below: 1  INCREASE IN CELL PRESSURE 6 3 lb./sq.in. From To 0  40  j j { }  TIME REQUIRED TO REACH EQUILIBRIUM MINUTES  J J { •  PORE PRESSURE PARAMETER B AT EQUILIBRIUM  J  15  j  0.96  (Drainage Stage Between)} 40  80  j  J  25  j  0.65  j  D i s s i p a t i o n o f pore p r e s s u r e was o n l y 63$ complete a t the of the d r a i n a g e s t a g e ; t h i s may v a l u e o f 0.65  end  account f o r the r a t h e r h i g h  o b t a i n e d f o r B i n t h e 40-80 l b . / s q . i n .  I n a d d i t i o n t o the d r a i n a g e stage mentioned  range. above, a  second d r a i n a g e p e r i o d was p e r m i t t e d p r i o r t o l o a d i n g .  This  l a t t e r stage w h i c h was c o n d u c t e d a t a c e l l p r e s s u r e o f 80 l b . / sq.  i n . produced a h i g h e f f e c t i v e s t r e s s I n the specimen b e f o r e  68.  the commencement o f l o a d i n g . diagrams a r e r e q u i r e d .  T h i s i s an advantage when Mohr  A t t h e end o f the second d r a i n a g e  stage  the pore p r e s s u r e was 15.7 l b . / s q . i n . Loading Stage: throughout for at was  C e l l p r e s s u r e was m a i n t a i n e d a t 80 l b . / s q . i n .  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  t h e l o a d i n g s t a g e i s shown i n Graph 4-19. F a i l u r e o c c u r r e d a d e v i a t o r s t r e s s o f 39.4 l b . / s q . i n .  Pore p r e s s u r e a t f a i l u r e  52.7 l b . / s q . i n . Graph 4-20 shows the manner i n which the pore p r e s s u r e  changed w i t h l o a d , d e f o r m a t i o n and t i m e .  The b u i l d - u p o f the  pore p r e s s u r e d u r i n g t h e l o a d i n g s t a g e was somewhat e r r a t i c . T h i s was p r o b a b l y due t o t h e h i g h c e l l p r e s s u r e and t o the h i g h degree o f c o n s o l i d a t i o n a t w h i c h t h i s specimen was t e s t e d . The s o i l was v e r y compact i n the neighbourhood  of the probe,  conse-  q u e n t l y , the time r e q u i r e d f o r the pore p r e s s u r e t o r e a c h 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 t o i n c r e m e n t . The o v e r a l l t r e n d s , however, f o l l o w t h e p a t t e r n o f p r e v i o u s t e s t s . Type o f " F a i l u r e :  F a i l u r e o c c u r r e d on a s i n g l e shear p l a n e ,  i n c l i n e d a t 60° to the h o r i z o n t a l . Time t o f a i l u r e , t f -  120 h o u r s .  M o i s t u r e c o n t e n t s a f t e r t e s t s were as f o l l o w s : ' i LOCATION Top  o f Specimen  2 j  MOISTURE CONTENT % OF DRY WEIGHT 43.9  S h e a r i n g Zone  43.5  Base o f Specimen  46.8  j  69. 2.  A p p a r e n t S t r e n g t h Parameters For  the purpose o f d e r i v i n g the ' a p p a r e n t  strength  1  parameters c a n d o' t h e t e s t s a r e a r b i t r a r i l y d i v i d e d I n t o two groups on the b a s i s o f t h e methods employed f o r p o r e p r e s s u r e measurement, e.g. t e s t s where pore p r e s s u r e measurements were made a t t h e t o p o f t h e 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 p o r e p r e s s u r e was measured a t the c e n t r e , w i l l be grouped  together.  Graph 21 and Graph 22 show t h e Mohr c i r c l e s ive stresses at f a i l u r e . the f a i l u r e  f o r the e f f e c t -  The a p p a r e n t p a r a m e t e r s , d e r i v e d from  envelopes f o r the  two groups, d i f f e r s l i g h t l y as  shown below:  } J i  FRICTION ANGLE (APPARENT) 0' DEGREES  J { i  COHESION (APPARENT) c lb./sq.in.  i !  17  !  4.0  A t c e n t r e o f Specimen.i T e s t s 5, 6 and 7. \  21  !  3.0  POSITION OF PORE PRESSURE MEASUREMENTS At t o p o f Specimen. T e s t s 2, 3 and 4.  i  1  1 TABLE V I . APPARENT STRENGTH PARAMETERS - PORT MANN CLAY.  C o n s i d e r i n g the r e s u l t s o f T e s t 4 as t y p i c a l  o f the t r e n d s  o b s e r v e d i n a l l t e s t s , the e f f e c t s o f the slow b u i l d - u p o f pore  70.  p r e s s u r e d u r i n g t h e 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. ive  The manner i n which the e f f e c t -  s t r e s s on the f a i l u r e p l a n e changes w i t h time i s shown f o r  two i n c r e m e n t s  o f a x i a l l o a d i n g Graph 4-23«  I t i s e v i d e n t from  t h i s p l o t t h a t the c i r c l e s r e p r e s e n t i n g a s i n g u l a r v a l u e of t h e 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  with  an I n c r e a s e i n the e l a p s e d time from the a p p l i c a t i o n of a l o a d increment.  I n o t h e r words, the compressive  s t r e n g t h decreases  w i t h t i m e , even i f no o t h e r e f f e c t b u t t h e slow b u i l d - u p o f pore p r e s s u r e 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 a t f a i l u r e a r e shown i n the Mohr diagrams, Graph 4-24.  The v a l u e o f c  u  and 0  U  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, o c c u r r e d i n any o f these  tests. G.  1.  Miscellaneous Tests.  M i n e r a l o g i c a l Composition  of Particles.  The r e s u l t s o f s t a i n t e s t s shown i n T a b l e V I I i n d i c a t e t h a t the s o i l i s p r e d o m i n a t e l y  an i l l i t e c l a y .  Stain tests,  however, p r o v i d e o n l y 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 c o m p o s i t i o n . A more e x a c t a n a l y s i s demands the e l a b o r a t e t e c h n i q u e s o f x - r a y d i f f r a c t i o n , o r d i f f e r e n t i a l t h e r m a l a n a l y s i s - which have n o t been  attempted.  71  STAIN TEST (#)  !  COLOUR REACTION !  MINERAL INDICATED  Crys t a l - V i o l e t  J  Dark-Green  ]  Illlte  predominates  Safranine y  i  Purple  i  Illite  predominates  M a l a c h i t e Green  j  L i g h t Brown  |  Indefinite  (#) " S u b s u r f a c e Methods" LeRoy 1949 pp.164-166 TABLE V I I . MINERALOGICAL COMPOSITION OP PARTICLES - PORT MANN CLAY.  2,  Sensitivity. The s e n s i t i v i t i e s shown i n T a b l e V I I I , have been c a l c u -  l a t e d on t h e r e s u l t s o f vane t e s t s p e r f o r m e d i n b o r e h o l e s BS2 and BS1B.  These b o r i n g s a r e l o c a t e d n o t more than 100 f e e t  away from b o r i n g BS2P.  I t seems r e a s o n a b l e t o assume t h a t  r e s u l t s o f 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 a n d BN23P.  BORING NUMBER BS2  I j «  BS2  DEPTH BELOW GROUND LEVEL  j i  SENSITIVITY INDEX  135' - 6"  J  64  136' - 3«  !  31  BS2  •  137' - 0"  I  7 7  BS1B  {  143' - 0"'  j  76  BS1B  j  143»  *. 9"  !  39  147?  - 9"  |  34  BS1B  j  TABLE V I I I . SENSITIVITY INDICES - PORT MANN CLAY.  72.  3j  Atterberg Limits. A t t e r b e r g l i m i t s were d e t e r m i n e d f o r sample No.  b o r e h o l e BN23F.  The  purpose o f these t e s t s was  l e a c h i n g o f salf' from the p o r e w a t e r had u r a l c l a y stratum.  23  -  t o determine i f  taken p l a c e i n the  nat-  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  t o reduce the v a l u e s o f t h e A t t e r b e r g l i m i t s .  The  index  tests  were t h e r e f o r e p e r f o r m e d on the n a t u r a l c l a y , and on c l a y w h i c h had been p r e t r e a t e d w i t h sea w a t e r . mixing for  The  treatment c o n s i s t e d o f  the c l a y w i t h e x c e s s s a l t water and a l l o w i n g i t t o  24 h o u r s .  The  f l o c c u l a t e d s o i l was  c a n t i n g o f f the s u p e r f l u o u s  liquid.  then s e p a r a t e d  stand  by  B o t h the n a t u r a l and  detreated  s o i l s were a l l o w e d t o a i r dry f o r the purpose of o b t a i n i n g the c o n s i s t e n c y o f 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  Natural  Soil  Salt-Treated S o i l  j 1  ! PLASTIC ! LIMIT  74.5  }  32.6  89.0  J  31.2  TABLE I X . 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 o c c u r s when the s o i l i s a l l o w e d a c c e s s t o t h e s a l t d i s s o l v e d i n sea w a t e r .  ions  73  CHAPTER V. DISCUSSION OF TEST RESULTS AND CONCLUSIONS  A,  E f f e c t s o f Overburden  Pressure  A t the o u t s e t o f t h e t e x t , a t t e n t i o n was drawn to t h e e f f e c t s o f 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 - n o t a b l y 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 o f  e f f e c t i v e s t r e s s e s shows a c o h e s i o n 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 n o r m a l l y pressures  consolidated clays tested at c e l l  g r e a t e r than t h e overburden p r e s s u r e ,  s t r e s s envelope passes through the o r i g i n .  the e f f e c t i v e  Geological  i n d i c a t e s that the P o r t Mann c l a y i s n o r m a l l y  evidence  consolidated, 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 t h e samples t e s t e d a r e s u c h t h a t i n l a b o r a t o r y t e s t s , t h e s o i l would be e x p e c t e d t o 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 e q u i v a l e n t  over**  burden s t r e s s e s o f 50 - 57 l b . / s q . i n . have been e x c e e d e d . ^ Due t o l i m i t a t i o n imposed by the shear t e s t a p p a r a t u s ,  (1)  E f f e c t i v e s t r e s s e s a r e g i v e n i n Appendix I .  74.  o n l y i n Testis 4, 5 and 7 d i d t h e a x i a l s t r e s s exceed t h e o v e r burden p r e s s u r e .  T h e r e f o r e , the v a l u e s o f 'apparent' c o h e s i o n  l i s t e d i n Chapter IV a r e to be e x p e c t e d .  B.  Sensitivity  The  r e s u l t s o f the c o n s o l i d a t i o n  t e s t s (e v s l o g p graphs)  g i v e n i n Appendix I show t h a t t h e r e l a t i o n s h i p between v o i d r a t i o and p r e s s u r e i s t y p i c a l o f e x t r a s e n s i t i v e c l a y s , T e r z a g h i 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 c o n f i r m the h i g h  s e n s i t i v i t y o f t h i s s o i l deposit.  (Table V T I I 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 a r e c o n s i d e r e d h i g h . with  This  soil  s e n s i t i v i t i e s i n t h e range 30 t o 80, u n d o u b t e d l y , belongs  to t h e e x t r a s e n s i t i v e group o f c l a y s .  The vane t e s t r e s u l t s  emphasize t h e importance o f d i s t u r b a n c e o f s t r u c t u r e strength;  on shear  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 t h e  u n d i s t u r b e d s t a t e , y e t b r e a k i n g down t h e s t r u c t u r e by r e m o u l d i n g t r a n s f o r m s t h e s o i l i n t o a s l u r r y o f n e g l i g i b l e shear  strength.  I n comparison w i t h t h e Norwegian c l a y s , the A t t e r b e r g l i m i t s o f t h e samples u s e d f o r t h e s e t e s t s a r e h i g h ;  74 and 31  as a g a i n s t 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 o f t h e Norwegian c l a y i s a l s o h i g h e r ;  s e n s i t i v i t y i n d i c e s o f 300 author.  t o 500 have been r e p o r t e d by t h e same  The n a t u r a l m o i s t u r e c o n t e n t o f the P o r t Mann c l a y i s  c l o s e t o the l i q u i d  limit.  75.  I f A t t e r b e r g l i m i t s a r e a c c e p t e d as an i n d i c a t i o n o f the degree o f l e a c h i n g , the p r o c e s s i s n o t i n an advanced stage i n t h i s d e p o s i t . The f a c t t h a t t h e l i q u i d l i m i t was r a i s e d by (2) 14.5 when t h e s o i l was immersed i n sea w a t e r , i n d i c a t e d t h a t some l e a c h i n g has taken p l a c e .  As the d e p o s i t I s below s e a  l e v e l and appears t o have n e v e r been s u b j e c t e d t o s u b a e r i a l l e a c h i n g , t h e most l i k e l y source o f 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 It  l a y e r beneath t h e c l a y d e p o s i t .  i s i n f e r r e d f r o m t h e s e n s i t i v i t y t h a t t h e P o r t Mann  c l a y p o s s e s s e s a "cardhouse" s t r u c t u r e o f a s i m i l a r t i o n t o that confirmed  configura-  i n the case o f the Norwegian c l a y s ,  R o s e n q u i s t (1959). C.  S t r e s s - S t r a i n Curves  All  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 t h e i n i t i a l e.g. Graph 4-8. been r e p o r t e d . not known.  R a r e l y have s t r e s s s t r a i n c u r v e s o f t h i s shape The c a u s e ( s ) o f the h i g h i n i t i a l d e f o r m a t i o n i s  P r e c a u t i o n s were taken d u r i n g t h e t e s t s t o o b t a i n a  p o s i t i v e s e a t i n g o f the c e l l p l u n g e r applying the a x i a l loads.  on the specimen cap,  before  I t i s a l l the more d i f f i c u l t to under-  s t a n d i n vie'.w o f t h e d r a i n a g e stages fittings  stages o f l o a d i n g  h a v i n g s e a t e d t h e end  (porous d i s c s , e t c . ) p r i o r t o the l o a d i n g s t a g e s .  Note-  worthy i s t h e f a c t t h a t a c o n s i d e r a b l e p a r t o f the i n i t i a l de-  (2)  See T a b l e I X .  (3)  See b o r e h o l d l o g s , Appendix I .  7 6  f o r m a t i o n o c c u r r e d i n t h e form of *creep \ nKq  p o s s i b l y some r e -  arrangement o f s t r u c t u r e o c c u r s i n t h e i n i t i a l stages o f 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 h i g h e r s t r a i n s may have overcome the tendency t o develop  t h e peak s t r e s s u s u a l l y a s s o c i a t e d w i t h  extrasensitive clays.  D,  Pore P r e s s u r e 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 t h e shear t e s t s l e a d to the c o n s l u c i o n t h a t i n t h i s s o i l , pore p r e s s u r e r e q u i r e s c o n s i d e r a b l e time t o r e a c h 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 .  Results  presented  i n Chapter IV show t h a t the e f f e c t I s common t o both the i n c r e a s e s 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 test.  The time l a g i s e v i d e n t i n the b u i l d - u p s t a g e s , b u t i s  seen t o b e s t advantage i n t h e l o a d i n g s t a g e s , Graph 4 - 1 7 , f o r instance. The  deformation  vs time c u r v e s resemble an i n v e r t e d  image o f the pore p r e s s u r e v s 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  up o f pore p r e s s u r e  t h a t the r a t e o f b u i l d -  i s r e l a t e d t o the creep o f the s o i l s k e l e t o n  (up t o i n s i p i e n t f a i l u r e ) .  C o n s i d e r i n g t h e remarks made e a r l i e r  (Chapter I ) r e 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 o f marine c l a y , i t i s reasonabke t o assume t h a t t h e cardhouse type o f s t r u c t u r e i s capable  o f withstanding considerable e f f e c t -  i v e s t r e s s e s a t t h e "edge t o f l a t " c o n t a c t s o f t h e p a r t i c l e s ; (4)  Creep and p l a s t i c f l o w a r e r e g a r d e d 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 c o m b i n a t i o n  o f the  e f f e c t s o f i n t e r p a r t i c l e f o r c e s and t h e e x t e r n a l l o a d s . A p p a r e n t l y , w i t h t h e p a s s i n g o f t i m e , 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 d e f o r m a t i o n s  o f the adsorbed l a y e r s -  the p a r t i c l e s a r e 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 t h a t an i n c r e a s e i n the s p a c i n g o f the p a r t i c l e s o c c u r s i n the p r o c e s s  o f realignment;  i n contact tending t o separate. d i a g r a m a t i c a l l y i n F i g u r e 14.  r e g i o n s w h i c h were  This hypothesis  initially  i s Illustrated  Realignment w o u l d then p e r m i t a  r e d u c t i o n i n the e f f e c t i v e s t r e s s e s , w h i l e a t the same time the s t r e s s changes would be t r a n s m i t t e d t o the pore f l u i d . The w r i t e r s u g g e s t s t h a t the time l a g i n pore p r e s s u r e b u i l d - u p c a n be a t t r i b u t e d to r e a l i g n m e n t p l a s t i c deformations contact  o f the p a r t i c l e s , r e s u l t i n g from  o f t h e a d s o r b e d l a y e r i n the r e g i o n o f  areas. Supporting  M i c h e l and Lambe.  t h i s v i e w a r e i n v e s t i g a t i o n s r e p o r t e d by M i c h e l (1956) o b s e r v e d t h a t r e p e a t e d s t r e s s i n g  of a s o i l a l i g n s t h e p a r t i c l e s i n almost p a r a l l e l assuming, o f c o u r s e ,  formation,  t h a t 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 t r u e c l a y s .  Moreover, M i c h e l p r o v e d w i t h  measurements t h a t n o t o n l y r e m o u l d i n g ,  b u t 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  c o n c l u s i o n s r e g a r d i n g the o r i e n t a t i o n o f p a r t i c l e s i n m e c h a n i c a l l y compacted  soils.  I n t h i s s e r i e s the r e s u l t s o f T e s t 2 s u b s t a n t i a t e t h e p o s t u l a t i o n concerning at c e l l pressures  realignment.  higher than  Although  fully  saturated  30 l b . / s q . i n . , the t e s t y i e l d e d  To follow  D/recdon  Time  Time  FIG  14  t  of  External.  t, : External ent/f-ely  CLAYOF THE  WATER  P L A S T I C  Pressure, by Pore  partly Stresses.  taken Water  S Y S T E M  .'EFFECTS  D E F O R M A T I O N  A D S O R B E D  77  Pressure  : £ xternal Pressure taken by In fe r g r a. n u I a. r  0  p a-a e  LAYERS.  OF  78. the c h a r a c t e r i s t i c pore p r e s s u r e increases i n c e l l pressure, Graphs 4-2  and 4-5.  saturation  The  and a l s o f o r t h e l o a d i n g  other  devoted to the l o a d i n g  r a t e o f pore p r e s s u r e  build-up  l a r g e e x t e n t on degree of s a t u r a t i o n . assumed t h a t i n c o m p l e t e l y  saturation  stages.  a l s o depends t o a  I t has  h i t h e r t o been  saturated s o i l s the e l a s t i c  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 t o b u i l d up instantaneously  stage,  t e s t specimens were v e r y c l o s e t o  p r i o r to l o a d i n g - p o s s i b l y r e a c h i n g  d u r i n g the l o n g p e r i o d s The  vs time r e l a t i o n s h i p f o r f u r t h e r  to i t s f i n a l v a l u e ,  (due  the pore  to the low  b i l i t y o f w a t e r i n comparison to t h a t o f the s o i l T h i s assumption e n t a i l s t h a t t h e pore p r e s s u r e  deformapressure  compressi-  skeleton).  change has  same magnitude as the a p p l i e d s t r e s s p r o v i d e d no d r a i n a g e  the occurs.  I f t h i s assumption i s v a l i d , i t does not t a k e 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 ^ ) , (b) t h e p o s s i b i l i t y t h a t gas bubbles may water o f submerged s o i l s t r a t a . s t r e s s e s and w i l l now  The  be t r a p p e d i n the pore  r e l a t i o n between i n t e r p a r t i c l e  s t r u c t u r e has been d i s c u s s e d a l r e a d y , so a t t e n t i o n  be d i r e c t e d t o the e f f e c t s o f t r a p p e d gases. Complete s a t u r a t i o n does n o t seem t o be a  a s s u m p t i o n i n a l l c a s e s o f submerged s t r a t a . may  and  justifiable  Transported  soils  be e x p e c t e d t o c o n t a i n some a i r t r a p p e d a t the time of  deposition.  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  the gas c o n t e n t o f the pore f l u i d .  for increasing  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 t o 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 f o r c e s 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 .  79.  than u n i t y have been r e c o r d e d 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 v a l u e s o f B r e p o r t e d i n Chapter I V , i n d i c a t e t h a t the samples o f P o r t Mann c l a y were n o t f u l l y at  the l a b o r a t o r y .  s a t u r a t e d on r e c e i p t  The pore p r e s s u r e developed i n the shear  t e s t s a r e 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 t h e r o l e p l a y e d by each o f these f a c t o r s i s a c o m p l i c a t e d s t u d y ; r e q u i r i n g a knowledge o f t h e amount and c o m p o s i t i o n o f the gases, r a t e o f d e f o r m a t i o n o f 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 o f 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 o f the pore  (c)  S o l u b i l i t y o f t r a p p e d gases.  (d)  Surface tension a t the g a s - l i q u i d i n t e r f a c e .  (e)  Vapour p r e s s u r e o f w a t e r .  (f)  Temperature.  I n view o f e a r l i e r d i s c u s s i o n s  v  fluid.  , i t i s evident that  the p r e s s u r e developed i n the pore f l u i d i s a f u n c t i o n o f the d e f o r m a t i o n o f the s o i l s k e l e t o n . The p r e s s u r e 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 o f t h e eeil skeleton.  The gases w i l l r e a c t i n s t a n t a n e o u s l y t o the r e d u c e d  volume, b u i l d i n g up t h e pore p r e s s u r e i n accordance w i t h B o y l e ' s law.  Prom t h i s v j e w p o i n t t h e n , the r a t e o f development o f pore  p r e s s u r e i s d i c t a t e d by the r a t e o f d e f o r m a t i o n o f t h e s k e l e t o n .  (7)  R e f e r t o Chapter I I I , and e a r l i e r s t a t e m e n t s i n t h i s Chapter.  80. The  o t h e r f a c t o r s l i s t e d a r e of secondary importance as  shall  be seen p r e s e n t l y . An i n c r e a s e i n the pore p r e s s u r e 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 t r a p p e d gases i n t o s o l u t i o n i n accordance w i t h Henry's law o f s o l u b i l i t y .  A time e f f e c t i s i n t r o d u c e d  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 p r e s s u r e 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 g o i n g i n t o s o l u t i o n tend t o l o w e r  the  pore p r e s s u r e , b u t the drop i n p r e s s u r e w i t h time, due t o t h i s phenomenon, i s n o t r e a d i l y observed deforming pressure  simultaneously.  because the s o i l s k e l e t o n i s  A f t e r a c e r t a i n i n c r e a s e i n pore  ( c o r r e s p o n d i n g t o 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  ( t h e n B w i l l t h e r e a f t e r be e q u a l to u n i t y w i t h r e s p e c t  to f u r t h e r s t r e s s c h a n g e s ) .  T h e r e f o r e , 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 t o the g r a d u a l l y i n c r e a s i n g pore p r e s s u r e observed  i n t h i s s e r i e s of  tests.  S u r f a c e t e n s i o n produces a d i f f e r e n c e i n p r e s s u r e between the l i q u i d and gas phases o f the pore f l u i d . d i f f e r e n c e p i s g i v e n by the e x p r e s s i o n  The  p = 2JL r  pressure where Y denotes  the s u r f a c e t e n s i o n o f w a t e r and r r e f e r s to the r a d i u s o f entrapped  bubble.  the  T h i s e f f e c t i s u s u a l l y i g n o r e d - the a s s u m p t i o n  b e i n g made t h a t the pore w a t e r and the gases are a t the same pressure. P a r t of the p r e s s u r e i n the gas phase i s d e r i v e d from the vapour p r e s s u r e o f the w a t e r . i s about 0.35  l b . / s q . i n . a t 20° C ,  t h i s f i g u r e are not a f f e c t e d .  As the vapour p r e s s u r e of water pore p r e s s u r e s h i g h e r  than  81. Temperature e f f e c t s a r e i n h e r e n t i n a l l f i v e discussed.  factors  I n t h i s s e r i e s o f t e s t s t h e temperatures were  r e s t r i c t e d t o t h e range 18-22° C , w h i c h i s n o t l i k e l y t o 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 s l o w e r r a t e s o f b u i l d - u p r e c o r d e d f o r those t e s t s w i t h pore p r e s s u r e measurements a t t h e t o p cap, a r e most caused by r e s t r a i n t imposed by the end f i t t i n g etc.). the  (porous d i s c s ,  Shear s t r e s s e s a r e i n t r o d u c e d a t t h e ends, which r e s t r i c t s  d e f o r m a t i o n o f the s o i l , p a r t i c u l a r l y under t h e a c t i o n of  c e l l pressure.  The o b s e r v e d d a t a p r o v i d e f u r t h e r e v i d e n c e t o  support t h e v i e w t h a t t h e r a t e o f d e f o r m a t i o n o f the s o i l is  likely  skeleton  the prime f a c t o r d e t e r m i n i n g the r a t e o f pore p r e s s u r e b u i l d -  up i n t h i s  soil.  The p o s s i b i l i t y o f a s i g n i f i c a n t time l a g b e i n g i n h e r e n t i n the response o f the m e a s u r i n g a p p a r a t u s , i s d i s c o u n t e d by the  o b s e r v a t i o n t h a t t h e pore p r e s s u r e measurements o b t a i n e d by  u s i n g t h e probe, l e a d t o s h o r t e r times t o e q u i l i b r i u m than do measurements t a k e n a t the s u r f a c e o f t h e specimen.  The movement  of water a t t h e t i p o f the probe i s r e s t r i c t e d by the s m a l l a r e a of  the p e r f o r a t i o n s , which would have the e f f e c t o f e m p h a s i z i n g -  any l a g i n t h e response o f the pore p r e s s u r e a p p a r a t u s . The tendency :;of  the s o i l t o expand on the r e l i e f o f  overburden p r e s s u r e i n s a m p l i n g may be r e 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 t o t e s t i n g .  N e g a t i v e pore  p r e s s u r e s , however, were r e c o r d e d a t the s u r f a c e o f the specimen, d e s p i t e t h e f a c t t h a t water was u s e d i n p r e p a r i n g t h e specimens to f r e e the s u r f a c e a i r .  I t appears t h a t the s u r f a c e t e n s i o n i n  82. the outermost pore water exceeds c o n s i d e r a b l y the o b s e r v e d negative pressures of 5 l b . / s q . i n .  (Tests* 2, 3, 4 ) .  Large  n e g a t i v e pore p r e s s u r e s a t the s u r f a c e i n d i c a t e t h a t e x p a n s i o n has been r e s t r a i n e d i n the i n n e r p a r t s o f the sample.  E.  S t r e n g t h Parameters  A l t h o u g h the p r i m a r y 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 o f pore p r e s s u r e 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 d a t a has been o b t a i n e d c o n c e r n i n g the s t r e n g t h parameters and d r a i n a g e 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')  derived  from T e s t s 2, 3 and 4, d i f f e r from those o b t a i n e d fromt T e s t s 5, 6 and 7 (Table V I ) .  The disagreement i s b e l i e v e d t o be due t o  the h i g h e r r a t e o f l o a d i n g adopted i n T e s t s 2 and 4. a p p a r e n t from Graphs 4-5 and 4-12  It is  t h a t the pore p r e s s u r e had n o t  reached e q u i l i b r i u m w i t h e v e r y l o a d i n c r e m e n t i n the case o f those t e s t s .  The d e f o r m a t i o n / t i m e r e l a t i o n s h i p was  the o n l y  m a t t e r c o n s i d e r e d 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 t h a t the pore p r e s s u r e / t i m e 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 p r e s s u r e and the p r e v i o u s increment s h o u l d be e s t a b l i s h e d b e f o r e a d d i t i o n a l increments are a p p l i e d .  I n the absence o f i n f o r m a t i o n t o the  c o n t r a r y , the v a l u e s c' = 3.0 l b s . / s q . i n . and ^'  s  21 degrees,  o b t a i n e d from T e s t s 5, 6 and 7, a r e a c c e p t e d as the most appropr i a t e 'apparent' s t r e n g t h p a r a m e t e r s . On the assumption t h a t 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 T e s t s 2 and 4 s h o u l d come somewhat c l o s e r t o the o r i g i n i n  83. Graph 4-21, i t appears  t h a t the two systems employed f o r measur-  i n g pore p r e s s u r e 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 r e s p e c t t o s t r e n g t h parameters. The t r u e f r i c t i o n a n g l e <f>  v  c o u l d n o t be determined  from  the i n c l i n a t i o n f o r t h e f a i l u r e p l a n e ( s ) i n any t e s t o f t h i s series.  The i n c l i n a t i o n s o b s e r v e d appear t o be i n f l u e n c e d by  the r e s t r a i n t developed a t t h e end f i t t i n g , which l e a d s to an o v e r e s t i m a t e o f the t r u e f r i c t i o n a n g l e .  P.  Drainage  Characteristics  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"? i n s . / m i n . determined from the d a t a on t h e d r a i n a g e s t a g e o f T e s t 6, p l a c e s the c l a y i n the c a t e g o r y o f p r a c t i c a l l y i m p e r v i o u s T e r z a g h i and Peck,  soils,  (1948).  The r e s u l t s shown i n T a b l e V l e a d t o t h e c o n c l u s i o n t h a t f i l t e r paper s t r i p s do n o t form e f f e c t i v e s i d e d r a i n s when subjected to high c e l l pressures.  T h i s c o n c l u s i o n i s s u p p o r t e d 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 t e r m i n a t e d , Rowe  (1959).  The Mohr diagram, Graph 4-23, i l l u s t r a t e s t h e r e d u c t i o n i n shear s t r e n g t h w i t h t i m e ,  ^ o r further investigation of this  a s p e c t , 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 over t h e s t r e s s - c o n t r o l l e d  advantages  type u s e d i n t h e p r e s e n t t e s t  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 o f t e s t has been d e s i g n e d , and i s a t p r e s e n t b e i n g f a b r i c a t e d .  series. rates, It i s  hoped to c a r r y o u t f u r t h e r experiments on the r e d u c t i o n o f shear s t r e n g t h w i t h time t o 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 V I . AUTOMATIC CONTROL  A.  Introdue t i o n  A u t o m a t i c c o n t r o l o f pore water movements i s a d e c i d e d advantage where the n u l l method o f pore p r e s s u r e 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 a r e to be r u n over l o n g p e r i o d s o f t i m e .  Adjustment o f 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 d u r i n g a t e s t t h a t i n the absence o f a u t o m a t i c c o n t r o l an o p e r a t o r i s r e q u i r e d t o devote almost h i s f u l l  time t o 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 p e r s o n t o take charge of a number o f t e s t s .  O v e r n i g h t , how-  e v e r , the pore p r e s s u r e apparatus has t o be i s o l a t e d from the specimen, w h i c h means t h a t the i n t e r v e n i n g pore p r e s s u r e changes must be e s t i m a t e d by e x t r a p o l a t i o n b e f o r e the a p p a r a t u s i s r e connected.  A d i s a d v a n t a g e o f t h i s approach i s t h a t i f a poor  e s t i m a t e I s made, u n d e s i r a b l e movement of t h e pore water r e s u l t s . C o n s e q u e n t l y , the development o f a u t o m a t i c c o n t r o l systems has attracted considerable attention.  85.  I d e a l l y , an a u t o m a t i c c o n t r o l system s h o u l d be c a p a b l e of r e s t r i c t i n g the pore water movements to the v e r y l i m i t e d range t h a t 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 s h o u l d n o t add c o m p l i c a t i o n s to the a l r e a d y d i f f i c u l t p r o b lem o f d e a i r i n g the p o r e - p r e s s u r e a p p a r a t u s . the d e t a i l s o f the new  Before discussing  device, a review of e x i s t i n g  c o n t r o l s and remarks on t h e i r performance  automatic  w i l l be p r e s e n t e d .  I n s o f a r as the a u t h o r i s aware, o n l y two  servomechanisms  have been developed f o r use w i t h the n u l l method o f p o r e - p r e s s u r e measurement.  They are based on somewhat d i f f e r e n t  but b o t h operate towards  principles,  the same purpose; t h a t of m a i n t a i n i n g  the mercury column o f the pore p r e s s u r e a p p a r a t u s 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 t h e  test.  The a p p a r a t u s o r i g i n a l l y d e s i g n e d a t D e l f t S o i l  Mechanics  L a b o r a t o r y and l a t e r m o d i f i e d by Penman, employs a t h e r m o s t a t i c a l l y c o n t r o l l e d o i l bath t o m a i n t a i n a back p r e s s u r e on the pore water v i a the mercury column. shown i n F i g . 15.  A schematic o f the a p p a r a t u s i s  The mercury column o f t h e n u l l  indicator  makes c o n t a c t w i t h an e l e c t r o d e p l a c e d i n s i d e the c a p i l l a r y tube.  An i n c r e a s e i n pore p r e s s u r e w i l l break the c o n t a c t  between mercury and e l e c t r o d e . B r e a k i n g the c o n t a c t o p e r a t e s a r e l a y w h i c h s w i t c h e s i n the h e a t i n g u n i t i n s t a l l e d i n the o i l bath. to  The  t h e r m a l e x p a n s i o n of the o i l r e s t o r e s the mercury  the n u l l p o s i t i o n ( c o n t a c t w i t h e l e c t r o d e ) .  the r e l a y c u t s out t h e h e a t e r .  At t h i s p o i n t  I n p r a c t i c e , a c o n t i n u o u s make  and b r e a k a c t i o n (10 times per second) o c c u r s i n the v i c i n i t y  86. of t h e e l e c t r o d e .  By t h i s means the movement o f pore water i s  r e s t r i c t e d t o as l i t t l e as 0,1 m i l l i m e t e r s o f mercury i n the 1 m i l l i m e t e r d i a m e t e r c a p i l l a r y tube.  The apparatus i s t h e r e -  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 p r e s s u r e i s i n c r e a s i n g .  On a f a l l i n g pore p r e s s u r e , how-  e v e r , the o i l b a t h must be c o o l e d , w h i c h l e a d s t o m e c h a n i c a l c o m p l i c a t i o n s - a pump t o c i r c u l a t e c o l d w a t e r through t h e o i l b a t h and some time d e l a y s w i t c h e s must be i n s t a l l e d .  It is  c l a i m e d t h a t even f o r t h e case o f f a l l i n g pore p r e s s u r e the movement o f pore water c a n be r e s t r i c t e d t o 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 t h e system, the presence o f t h r e e  l i q u i d i n t e r f a c e s , namely, o i l , water and mercury, seems t o be a d i s a d v a n t a g e t o t h i s method. I n 1956, B u r t o n r e p o r t e d a d e s i g n f o r an a u t o m a t i c c o n t r o l based on the a p p l i c a t i o n o f p h o t o - e l e c t r i c c e l l s .  The  c e l l s a r e u s e d t o m o n o i t e r t h e l e v e l s o f a mercury column i n a U-tube.  The pore p r e s s u r e a c t s on the mercury i n one l i m b of  the U-tube.  To the o t h e r s i d e o f t h e 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 o p e r a t i n g h y d r a u l i c b e l l o w s , and a p r e s s u r e gauge.  Any movement o f t h e 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 o f r a d i a n t energy r e a c h i n g the c e l l s from a l i g h t s o u r c e .  E l e c t r i c a l impulses r e c e i v e d  from t h e photo c e l l s , c o n t r o l the o p e r a t i o n o f the s e r v o mechanism. i n P i g . 16.  The e l e c t r i c a l c i r c u i t o f t h i s a p p a r a t u s i s shown E s s e n t i a l l y , t h e c i r c u i t i s t h a t o f 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 a t  To follow to  pa-o&  86  Specimen  Confro L r~o Re Lay  From  Rett  Merc ur y  F I G 15. P O R E - P R E S S U R E  DEVICE.  (Afrer  F I G IB . C I R C U I T  A.D. P enmcxn  OF A U T O M A T I C (After  FIGS  15 a n d  I6 .  1953)  CONTROL  L.J. Burton  1356)  87. same l e v e l i n b o t h l i m b s o f t h e U-tube, the two a m p l i f i e r s c a n be a d j u s t e d so t h a t t h e i r o u t p u t s a r e e q u a l and o p p o s i t e . p r e s s u r e feedback u n i t i s then i d l e . l e v e l unbalances  The  Any change i n t h e mercury  t h e a m p l i f i e r s , w h i c h c l o s e s one o f t h e two  r e l a y s i n the anode c i r c u i t o f t h e output s t a g e .  The r e l a y s  c o n t r o l the movement o f the a c t u a t o r and h y d r a u l i c b e l l o w s . D i r e c t i o n o f movement depends on w h i c h r e l a y i s c l o s e d ; the h y d r a u l i c b e l l o w s works i n the sense t o oppose t h e change i n mercury l e v e l .  I t i s claimed t h a t the device i s capable of  r e s t r i c t i n g t h e pore water movement t o about 1 p a r t i n 400,000. T h i s f i g u r e i s quoted f o r a s a t u r a t e d sample 4 i n c h e s i n d i a m e t e r , 8 i n c h e s l o n g and h a v i n g a p o r o s i t y o f 20%, (1956).  Burton  Prom the s o i l - t e s t i n g v i e w p o i n t , a drawback t o the use  of t h i s apparatus arrangement. immediately  i s the d i f f i c u l t y o f d e a i r i n g t h e U-tube  Furthermore,  the h y d r a u l i c b e l l o w s o p e r a t e s  a relay closes.  almost  T h i s introduces the problem o f  " h u n t i n g " when w o r k i n g w i t h s o i l s o f l o w 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 d e s i g n 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 . apparatus p r o v i d e s a remarkable  N e v e r t h e l e s s , the  degree o f c o n t r o l over pore  water movements. I n o r d e r t o overcome the problem o f c o n v e r t i n g e x i s t i n g s e r v o - c o n t r o l s , t o work i n c o n j u n c t i o n w i t h Bishop's  pore-  p r e s s u r e a p p a r a t u s , 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 p a r t o f t h e p r e s e n t program. was c o n s i d e r e d .  The use o f p i e z o - e l e c t r i c  crystals  A t 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 R o c h e l l e 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 u s e d f o r t h e measurement o f impulse p r e s s u r e s , 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 a r e developed on the c r y s t a l f a c e s when s u b j e c t e d t o s t r e s s e s o f the magnitudes o c c u r r i n g i n pore p r e s s u r e work. The  deformation o f  t h e 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 c o n v e n i e n t s i z e s . of  T h i s suggested  the p o s s i b i l i t y  d e v e l o p i n g a p r e s s u r e c e l l t o a s s i s t i n the o p e r a t i o n o f the  e x i s t i n g pore p r e s s u r e a p p a r a t u s .  A study o f t h e p r o p e r t i e s o f  these c r y s t a l s , however, r e v e a l e d t h a t they a r e u n s u i t a b l e f o r use i n systems where p r e s s u r e changes a r e g r a d u a l , as i s the case in t r i a x i a l testing.  In. f a c t , the o r i g i n a l m e c h a n i c a l  becomes a n e l e c t r i c a l one o f an analgous  problem  nature.  I t was then d e c i d e d t o r e v e r t t o t h e a p p l i c a t i o n o f p h o t o e l e c t r i c c e l l s , b u t t o d e s i g n a c o n t r o l u n i t t h a t p r e s e r v e d , as far  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 p r e s s u r e a p p a r a t u s . in achieving this  B.  The new d e v i c e has p r o v e d  satisfactory  objective,  D e t a i l s o f Apparatus f o r A u t o m a t i c C o n t r o l of Pore P r e s s u r e  T u r n i n g a g a i n to the n u l l i n d i c a t o r o f B i s h o p ' s  pore-  p r e s s u r e a p p a r a t u s , i t w i l l be r e c a l l e d t h a t i t embodies a g l a s s c a p i l l a r y tube which t e r m i n a t e s i n a mercury r e s e r v o i r . The h e i g h t t o w h i c h the column o f mercury w i l l r i s e i n the tube depends upon a s t a t e o f e q u i l i b r i u m b e i n g a t t a i n e d , between the pore p r e s s u r e w h i c h a c t s on the meniscus o f t h e column, and the back p r e s s u r e a c t i n g on the s u r f a c e o f t h e mercury i n the  89. reservoir.  A t the commencement o f a t e s t , the mercury i s  r a i s e d t o a c o n v e n i e n t h e i g h t i n the c a p i l l a r y tube, w i t h the pore p r e s s u r e c o n n e c t i o n t o the specimen i s o l a t e d from t h e u n i t . B e f o r e a l l o w i n g t h e pore water a c c e s s t o the mercury the  column,  t r i a x i a l s t r e s s e s on the specimen a r e so a r r a n g e d as t o  produce z e r o pore p r e s s u r e - o r as n e a r l y so as p o s s i b l e .  On  a d m i s s i o n o f t h e pore water i n t o c o n t a c t 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 o b t a i n e d . The l e v e l of  the mercury  tion.  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 -  The n u l l p o s i t i o n i s a l s o made t o c o i n c i d e w i t h  atmospher-  i c p r e s s u r e , by o p e n i n g t h e a p p r o p r i a t e v a l v e s m o m e n t a r i l y , throughout t h e system.  An i n c r e a s e i n pore p r e s s u r e tends to  d r 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 t h e tendency t o r a i s e t h e mercury to a h i g h e r l e v e l than the n u l l p o s i t i o n . at  To m a i n t a i n the mercury  the n u l l p o s i t i o n (no f l o w o f pore water from specimen) t h e  back p r e s s u r e on the mercury r e s e r v o i r must be a d j u s t e d .  The  new d e v i c e a u t o m a t i c a l l y makes t h e a d j u s t m e n t s . The d e s i g n o f t h e a u t o m a t i c c o n t r o l c e n t r e s around the c h a r a c t e r i s t i c s o f a p h o t o - v o l t a i c type photo c e l l , which i s used t o d e t e c t changes  i n t h e q u a n t i t y o f l i g h t a r r i v i n g from  a l i g h t s o u r c e f o c u s e d on the n u l l p o s i t i o n o f t h e mercury column.  The s i g n a l s from the photo c e l l a r e 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 t h e purpose o f r e g u l a t i n g t h e back p r e s s u r e on the mercury r e s e r v o i r . a p p a r a t u s i s shown i n P i g . 17.  A b l o c k diagram o f the  Any change i n the mercury  from t h e n u l l p o s i t i o n , r e f l e c t s i n the o p e r a t i o n o f t h e  level  To -folLow page 89.  To  Spec  i 'rn e n AmpCf-f/er.  Volfaye-goin^ Power Stage  S rage  Null  Indicator Wafer  Ser  £3  vomoror  \  Auxiliary- Con f r o L Cy L i not er  c a  FIG  17.  L A Y O U T  Povse r  Pa CM  OF A U T O M A T I C  C O N T R O L  90. amplifier.  Two r e l a y s i n the output stage o f the a m p l i f i e r ,  c o n t r o l t h e course o f a c t i o n o f the p r e s s u r e  feedback, so  t h a t i t always r e a c t s towards r e s t o r i n g t h e mercury column t o the n u l l p o s i t i o n . E x c i t e r Lamp:  The c o n c e n t r a t e d  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 o f the photo c e l l , i s o b t a i n e d from a f i l m s l i d e p r o j e c t o r h o u s i n g a 75 w a t t lamp.  The condenser l e n s and concave m i r r o r  of t h e 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 o t h e r  lenses  are removed i n o r d e r t o o b t a i n near p a r a l l e l r a y s from t h e projector. A t h i n m e t a l s l e e v e w i t h v e r t i c a l s l i t s c u t on o p p o s i t e s i d e s , f i t s over the c a p i l l a r y tube o f t h e n u l l i n d i c a t o r . I t s purpose i s t o m i n i m i z e l i g h t s c a t t e r i n g w h i c h would occur i n t h e w a l l s o f the c a p i l l a r y tube.  otherwise  Due t o the p r e s e n c e  of t h e s l e e v e , 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 t h r o u g h the bore o f t h e tube, i n a d i r e c t i o n a t r i g h t a n g l e s t o the l o n g i t u d i n a l a x i s o f the b o r e . 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 o f two  i d e n t i c a l p h o t o - v o l t a i c elements e n c l o s e d i n a m e t a l h o u s i n g . The voltaic cells  (commercially  a v a i l a b l e ) a r e formed by d e p o s i t i n g  a t h i n l a y e r o f s e l e n i u m on an i r o n p l a t e : t h e 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 n e g a t i v e  electrode being a  f i l m o f m e t a l e v a p o r a t e d onto the s e l e n i u m s u r f a c e . f a l l i n g on t h e s e l e n i u m a c t i v a t e s t h e c e l l an E.M.P. between the e l e c t r o d e s .  transparent  Light  to produce d i r e c t l y  A l t h o u g h the a m p l i f i e r i s  c a p a b l e o f o p e 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 preferable i n t h i s case.  The t w o - c e l l system a r i s e s  91.  from the requirement  o f an o b s e r v a t i o n a p e r t u r e , to enable  v i s u a l check to be made on the mercury l e v e l .  a  I t i s not p r a c t -  i c a b l e to c u t the a p e r t u r e i n the f r a g i l e s u r f a c e c o a t i n g s o f the i n d i v i d u a l c e l l .  I n s t e a d , a s l i t i s formed by l e a v i n g a  space between the upper and l o w e r c e l l s . dimensions o f the i n d i v i d u a l c e l l s 0.26 ing  Due  to the s m a l l  (0.72'* x 0.44": a c t i v e a r e a  square i n c h e s ) no d i f f i c u l t y was  e x p e r i e n c e d i n accommodat-  them i n c l o s e p r o x i m i t y t o the c a p i l l a r y tube.  i t may  In passing,  be o f i n t e r e s t to n o t e t h a t 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 t o 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 this application.  The  c e l l s r e q u i r e no e x t e r n a l source  energy (except l i g h t ) , a r e 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 o f t h e i r m i n i a t u r e s i z e , are easy t o Control Unit:  install.  F o r d e s c r i p t i v e purposes the r e m a i n i n g p a r t s o f  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: e l e c t r o n i c system and (a)  of  (a) the  (b) the e l e c t r o - m e c h a n i c a l system,  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  s w i t c h i n g components.  An a m p l i f i e r o f the d i r e c t c u r r e n t type  i s employed, because a D.C. f r o m the photo c e l l .  o u t p u t i s more r e a d i l y o b t a i n a b l e  The use  of a D.C.  amplifier eliminates  the need f o r a l i g h t "chopper™ o r o t h e r means o f p u l s a t i n g i n p u t s from the c e l l  producing  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 g a i n s t a g e , f o l l o w e d by a power a m p l i f y i n g s t a g e . The r e l a y s are c o n n e c t e d i n the output c i r c u i t o f 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 . 1 8 ( a ) .  C o n v e n t i o n a l symbols are u s e d t o denote the v a r i o u s 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 R e l a y League.  T o  R  P o w e r  PacM  3 0 0 V ( + )  9  •VvVWV  •TAW^WV—I Rio  Ria  Ru  R13  FiLame  Relay  Poles  7"o /"ower PacMC—)  n fs  3  6 < p # rs  6  VI  eating  C  1 lb] C I R C U I T  OF  SERVOMOTOR  Motor  (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  CONTROL  92. The c e l l s  (2 No. B2M.'s)  International Rectifier  Corp.  form p a r t o f a c l o s e d c i r c u i t w h i c h i n c l u d e s the r e s i s t o r s R^ and Rg.  The n e g a t i v e  t e r m i n a l s o f the c e l l s , and t h e j u n c t i o n  of R^, R 2 , a r e brought t o a common ground, by a c o n n e c t i o n t o 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  l o o p s , each c o m p r i s i n g a c e l l w i t h a r e s i s t a n c e a c r o s s i t s terminals.  T h i s arrangement p r o v i d e s a p a r a l l e l i n p u t .  A change i n f l o w o f c u r r e n t from a c e l l t o ground, p r o duces a c o r r e s p o n d i n g  change i n the g r i d p o t e n t i a l o f t h e therm-  i o n i c tube a s s o c i a t e d w i t h t h a t l o o p .  Thus, changes i n t h e  p o t e n t i a l o f X on R^, e f f e c t the g r i d p o t e n t i a l o f V^ o n l y . S i m i l a r l y , a v o l t a g e change a t 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.  C o n s i d e r , f o r the p r e s e n t ,  the case where an  equal i n c r e a s e has o c c u r r e d i n t h e i l l u m i n a t i o n o f b o t h c e l l s . The g r i d s o f V^ and Vg w i l l then a c q u i r e a p o s i t i v e c h a r g e , w h i c h 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 r e a s e t h e c o n d u c t i v i t y o f the tubes (V-^ and Vg) so t h a t more c u r r e n t w i l l f l o w through the l o 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 v o l t a g e drop a c r o s s 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.  V o l t a g e changes a r e tapped o f f R^Q  and R-^ and a p p l i e d t o the g r i d s o f t h e power tubes (V3 and V ^ ) . Here a v o l t a g e drop has the e f f e c t o f r e d u c i n g the c u r r e n t f l o w t h r o u g h t h e r e l a y s A a n d B w h i c h a r e i n c o r p o r a t e d i n the anode c i r c u i t o f V 3 , V^.  The r e l a y c o n t a c t s w i l l open i f the c u r r e n t  f a l l s below a p r e s e t v a l u e .  Reducing t h e i l l u m i n a t i o n a t the  93.  c e l l has t h e o p p o s i t e e f f e c t - tends to c l o s e t h e 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 "A  rt  i s open and "B^ i s c l o s e d ( c l o s e d i n t h i s sense means t h a t t h e c o n t a c t s a r e p u l l e d i n towards the r e l a y c o i l ) . s u p p l y t o the feedback motor i s c o n n e c t e d  The power  to the p o l e s o f r e l a y  s w i t c h e s i n such a way t h a t t h e 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 s w i t c h e d on o n l y when b o t h r e l a y s r . a r e e i t h e r i n t h e open o r c l o s e d p o s i t i o n s .  As may be s u r m i s e d , t h i s o c c u r s when  the mercury l e v e l d e p a r t s from t h e 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 s w i t c h e s i s shown i n P i g . 1 8 ( b ) . The f u n c t i o n s o f t h e o t h e r components i n the a m p l i f i e r w i l l now be d i s c u s s e d b r i e f l y . The condensers C-^ and Cg a r e i n t e n d e d t o smooth out r i p p l e from t h e p h o t o - c e l l o u t p u t , c a u s e d by the a l t e r n a t i n g c u r r e n t s u p p l y t o the e x c i t e r lamp. The g a i n o f t h e v o 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 t h e s c r e e n p o t e n t i a l s o f p o t e n t i o m e t e r s R^, Rg.  and Vg, by means o f  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. C o n s i d e r a b l e d i f f i c u l t y was e x p e r i e n c e d i n d e v i s i n g a means o f b i a s i n g t h e power tubes V^, V^.  T h i s problem i s e n -  c o u n t e r e d 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 o f a f o l l o w i n g stage i s a t a p o t e n t i a l c l o s e to the anode v o l t a g e o f the p r e c e d i n g s t a g e . i n t h i s c a s e , by employing  The d i f f i c u l t y was overcome  neon v o l t a g e droppers  (V7, V ) i n Q  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 v o l t a g e r e g u l a t o r tube (V/r) i n t o t h e cathode c i r c u i t , i n s t e a d o f the u s u a l cathode  94.  resistor.  I n 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 t h a t makes the  o p e r a t i o n o f the a m p l i f i e r p o s s i b l e . "Bleeder"  resistors  (%4>  R  i5*  R  i6^  a r e  P  r o v i < i e d  i*  1  o r d e r t h a t V , V and V r e m a i n " f i r e d " w h i l e the a m p l i f i e r i s I o y 7  in operation.  0  The p l a t e p o t e n t i a l s o f V3,  by a d j u s t i n g t h e p o t e n t i o m e t e r (b)  The E l e c t r o m e c h a n i c a l  may be e q u a l i z e d  R17.  System.  The e l e c t r o m e c h a n i c a l  system  c o n s i s t s o f an e l e c t r i c motor, g e a r e d t o 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 b e i n g c o n n e c t e d d i r e c t l y t o the main c o n t r o l c y l i n d e r o f the pore pressure  apparatus.  The motor w h i c h i s r e v e r s i b l e , g i v e s an  o u t p u t speed o f 1^- r.p.m.  IPurther r e d u c t i o n i n speed i s p r o -  v i d e d by a worm gear r e d u c t i o n u n i t .  The d e s i g n o f the a u x i l i a r y  c y l i n d e r i s p a t t e r n e d on t h a t o f the main c o n t r o l c y l i n d e r . F o r s o i l s o f l o w p e r m e a b i l i t y , s u c h as the P o r t Mann c l a y , a f e e d back r a t e o f 0.077 c u b i c i n c h e s p e r minute has been found s a t i s f a c t o r y ; " h u n t i n g " being e l i m i n a t e d e n t i r e l y a t t h i s r a t e .  A  f a s t e r r a t e o f f e e d b a c k would perhaps be an advantage i n more permeable The  soils. i n s t a l l a t i o n o f the a u t o m a t i c c o n t r o l has n o t  i n t e r f e r e d w i t h the m a n u a l l y - o p e r a t e d system. f i c a t i o n s to the e x i s t i n g pore-pressure  The o n l y modi-  apparatus c o n s i s t e d o f  i n t e r 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 The  tube.  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 t o p r o v i d e a p o s i t i v e  gate t o 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 i n t r o d u c e d d i f f i c u l t i e s i n t o the  95.  a d j u s t m e n t s o f the a m p l i f i e r . maintained  P r o v i d e d t h a t the mercury i s  a t 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 o f the  tube i s n o t a s e r i o u s d i s a d v a n t a g e .  A tube w i t h square e x t e r n a l  c r o s s s e c t i o n would overcome t h i s s c a t t e r problem and thus p e r mit use o f s m a l l e r i n t e r n a l b o r e . L i m i t s w i t c h e s are p r o v i d e d f o r b r e a k i n g the power supply to  the motor i n the event o f e x c i t e r lamp f a i l u r e , or adverse  o p e r a t i o n o f 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  against i r r e g u l a r i t i e s arising i n e l e c t r i c a l Performance;  system.  The c o n t r o l has been found t o 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  position.  T h i s r e p r e s e n t s about 1 p a r t i n 200,000 of the pore water f o r P o r t 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 t o  the n u l l p o s i t i o n i n about 45 seconds, w h i c h a l l o w s  sufficient  time t o e q u a l i z e the p r e s s u r e a t the t i p of t h e p r o b e .  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 p r e s s u r e s  at  the ends o f 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 v i e w a t the o b s e r v a t i o n s l i t . Rc» 5'  6'  R  » R,,* 10 11  &  The c o n t r o l s  r e then a d j u s t e d to g i v e the optimum s e n s i -  t i v i t y t o movements o f the mercury column.  A warming-up p e r i o d  o f 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 o f the  amplifier.  Power Pack:  The power pack f o l l o w s c o n v e n t i o n a l d e s i g n ,  s u 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 .  and  Two  96.  v o l t a g e 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 p r o v i d e a s t a b l e o u t p u t v o l t a g e - 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 p u r p o s e , there appears to be l i t t l e  d i f f i c u l t y i n modifying  apparatus to o b t a i n even g r e a t e r s e n s i t i v i t y . r e p l a c i n g the s e l e n i u m c e l l s by the new  not a v a i l a b l e c o m m e r c i a l l y .  the  For instance,  s i l i c o n type photo  c e l l s , would i n c r e a s e the s e n s i t i v i t y c o n s i d e r a b l y . the l a t t e r c e l l s are more e x p e n s i v e ,  present  However,  and u n t i l r e c e n t l y were  An o p t i c a l  system r e p l a c i n g the  p r e s e n t o b s e r v a t i o n s l i t w o u l d perhaps be more e f f e c t i v e i n checking  the o p e r a t i o n o f the u n i t .  I n o r d e r t h a t 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 t y p e s , 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 p r e s e n t  system o f f i x e d g e a r i n g between motor and  auxil-  iary control cylinder. A s m a l l e r bore c a p i l l a r y w o u l d be o f a s s i s t a n c e i n the deairing operation.  The in  complete i n s t a l l a t i o n i s shown the p h o t o g r a p h i c  supplement.  97  CHAPTER V I I . SUMMARY OF CONCLUSIONS  (a)  The samples ;were n o t s a t u r a t e d on r e c e i p t a t t h e l a b o r a t o r y .  B v a l u e s i n d i c a t e t h a t the samples were probably s a t u r a t e d i n the l o a d i n g stage o f the shear (b)  The s o i l i s an e x t r a s e n s i t i v e c l a y w i t h s e n s i t i v i t y  i n the range 30 - 80. appears (c)  tests. indices  Some l e a c h i n g o f the n a t u r a l d e p o s i t  t o have o c c u r r e d ,  A slow b u i l d - u p o f pore p r e s s u r e was observed i n a l l  t r i a x i a l tests.  The r a t e o f b u i l d - u p was l e a s t f o r a x i a l l o a d i n g .  End r e s t r a i n t r e t a r d e d the b u i l d - u p o f pore p r e s s u r e a t top and base o f specimen; a slower r a t e being r e c o r d e d at t h e ends than at  the c e n t r e .  The pore-pressure l a g i n s a t u r a t e d specimens has  been a t t r i b u t e d to p l a s t i c deformations surrounding t h e p a r t i c l e s .  o f the adsorbed  layers  P l a s t i c deformation i s assumed t o  l e a d t o a r e a l i g n m e n t o f the p a r t i c l e s w i t h time - an e f f e c t most n o t i c e a b l e f o r a p p l i c a t i o n s o f u n i d i r e c t i o n a l (d)  In t r i a x i a l  stresses,  t e s t s , the specimens f a i l e d on e i t h e r one or  98  two  The s t r a i n s a t f a i l u r e ranged from 3 to 7$.  shear p l a n e s .  Values o f the s t r e n g t h parameters a r e as f o l l o w s :  c  The  3 lb./sq. i n .  C  =  f  = 21°  u  r  4«5 l b . / s q . i n .  true parameters c o u l d not be d e r i v e d from the r e s u l t s  of the shear t e s t s . (e)  T h . 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 wa  inches p e r minute - a value (f)  s  estimated  a t 4.7 s 1 0 ^  t o be expected i n c l a y s o f t h i s  S i d e d r a i n s are not e f f e c t i v e when s u b j e c t e d to h i g h  type. cell  pressures. (g)  The automatic c o n t r o l p r o v i d e s  satisfactory regulation of  the pore water movements i n t r i a x i a l t e s t s on c l a y e y  soils.  i  APPENDIX  The  I.  f o l l o w i n g data was s u p p l i e d by R. A. Spence,  Consulting Engineers.  R e s u l t s o f vane t e s t s :  J SHEARING • ' RESISTANCE ! J UNDISTURBED J J lb./sq.ft, •  i i J 1  DEPTH BELOW GROUND LEVEL  BS2  j  135 -6»»  i  1930  ;  30  tt  i  i36«-3  j  1860  j  60  ti  !  137 -O  tt  j  2300  j  30  BS1B  j  143 -0"  j  2275  i  30  «  •  i43t-9«  j  2330  J  60  n  •  147*-9  J  2045  {  60  BORING NO.  ,  1  n  ,  tt  •  1  SHEARING RESISTANCE REMOULDED lb./sq.ft.  WATER ! BORING l SAMPLE* CONTENT ! LIQUID { % i LIMIT ! NO. J NO. I  PLASTICJ LIMIT !  BN23F !  21 !  68.3  |  66.6  j  38.6  j  j  * 22 !  66.4  j  67.4 !  35.3  1  j  * 23 j  67.6  j  62.8  !  65.1  j  j  * 25 j  66.6  j  j  !  26  j  57.2  j  68.5 !  !  *27 j  61.8  |  ! * 40 !  68.1  j  |  1  * 42 |  61.3  j  j  !  43 J  59.4 j  63.7 j  !  44 {  61.4  !  BS2F  j j  2 4  j  45 * 46 j  !  j  57,8 58.1 j  i  | !  | i  |  34.1  35.6  GRAIN SIZE: MI.T SCALE SAND% SILT % CLAY % 0  J  10 J  1  90  i  *  i  j  J  7 i  0  61.3  1  I  DEPTH BELOW GROUND LEVEL  137'6 to W  ! 1I i  DRY DENSITY lb./cn.ft.  ESTIMATED ? OVERBURDEN PRESSURE: EFFECTIVE  ISSM"  138*4 to n  139'2  M  63.8  56.5 139 6 to ,  93  n  67.9  140*6" to 141'4"  !  !  !  !  i  i  61.7  147'9 to 148 5  i  63.6  148*7 to 149 5  i 29.4 !  i i j  S 11  j J  9  j  M  ,  M  M  ,  M  51.5  80  !  i  i  {  I  * Samples available for thetests undertaken for this thesis. APPENDIX I (Cont'd.)  67.2  151*3" to  152*1"  iii  PORT  MANN.  R. A. S P E N C E ,  P. E N G .  C O N S O L I D A T I O N H O L E  ^N^iE  S A M P L E  T E S T 37  J O B  NO:  DATE-.  iv  PRESSURE  -  KG/CM  2  I.O  o.i ui at -j a> to -  M  u  M  U  &  ui o>  sj  Ul  vl CO  IO.O  co <D -  w  w  N  U  A  ui o>  si  co  Ul  sj CO <£> -  ID -  O >  0-9  0 8  UI  01  s| CO (0 .  PORT R.A.  MANN.  S P E N C E ,  01  I.O PRESSURE  O.I  P. E N G .  B5  a F.  10  IO.O -  KG / C M  C O N S O L I D A T I O N H O L E  31  S A M P L E  (il tt UI c  2  T E S T 43.  J O B  NO:  DATE:  V  B N £3 F  o'  Pear  3'  Peat and Organic Si  " *•>•/] t  it  * x  16  Organic  Silt  1  III  so  S a. n ci with 5i Lt L ayers.  S <xn d.  IOO  Cla.yey  SiL f  lOl  /so 5ancly  5 (xncl  lib'  Gra-veL  /30  CLncl  MARINE MARINE  CLAY  I6S 16$'  O . • ID', *o ° i o  /93'  Pervious GnnveL (artesian) . GLCLCICLL  Borehole  fiLL  IBZ' TILL  and  S or ted.  t i LL  Logs  CLAY  band and CrcureL ia.r te s i an). GLCLCICLL  /90  CroveL  C /'de a. L ix e d)  isaterL ayers  .  vi  APPENDIX  II  CORRECTION OF COMPRESSIVE STRENGTHS ON BASIS OF EXPERIMENTAL FACTORS  The  c o r r e c t i o n s d e s c r i b e d below were made to the apparent  compressive s t r e n g t h s  ( d e v i a t o r s t r e s s e s ) as determined by a x i a l  loading. 1.  Area C o r r e c t i o n .  The c r o s s s e c t i o n a l a r e a was c o r r e c t -  ed, f o r both..the e f f e c t s o f drainage and deformation o f the specimen, i n accordance with the e x p r e s s i o n : a a a  G  (1 - AV)  T 1  -  £  Where(a)denotes the area on which true stress i s calculated a  Q  deviator  [ins^]  denotes the i n i t i a l a r e a  [ins ] 2  AV  denotes volume r e d u c t i o n by drainage  V  denotes i n i t i a l volume o f specimen  £  denotes a x i a l s t r a i n [ins/in]  The  area c o r r e c t i o n was i g n o r e d once a shear plane i n the  [ins*]  specimen became v i s i b l e . 2»  Rubber Membrane C o r r e c t i o n .  According  t o Bishop and  Henkel (1957) only the a x i a l s t r e s s e s i n the t r i a x i a l be c o r r e c t e d f o r membrane r e s t r a i n t .  t e s t need  The c o r r e c t i o n to be a p p l i e d  vii  to  c y l i n d r i c a l specimens i s then g i v e n by the e x p r e s s i o n :  D Where (0% - ( T l )  o m  J-  (O ^ - O j )  denotes the c o r r e c t e d d e v i a t i o n  denotes the apparent d e v l a t o r  -  denotes the t h i c k n e s s o f membrane (s) [ins]  ^  denotes Young*s modulus o f rubber  D  denotes the s t r a i n  lbs./ins.^  [ins./in.]  denotes diameter o f specimen a t s t r a i n €  C a l c u l a t i o n s based on F i g . in  stress  w  €  stresses  18 showed t h i s c o r r e c t i o n t o be  the o r d e r o f 0.25 l b . / s q . i n . a t 5% s t r a i n , f o r the twin membranes  employed i n the t e s t s . 3. ing  Drain Corrections.  from f i l t e r  i s n o t dependent to  The c o r r e c t i o n f o r r e s t r a i n t  d r a i n s i s a l s o made t o the a x i a l s t r e s s e s .  arisIt  on t h e s t r a i n however and i s n o r m a l l y c o n s i d e r e d  be l e s s than 1 l b . / s q . i n . , Bishop and Henkel (1957). A nominal c o r r e c t i o n of 1 l b . / s q . i n . was a p p l i e d t o the  a x i a l s t r e s s e s t o i n c l u d e the e f f e c t s o f both rubber membranes and f i l t e r d r a i n s .  No c o r r e c t i o n was deemed necessary f o r  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 c l u d e d i n the a x i a l  loads.  APPENDIX I I I .  C a l c u l a t i o n s based on data o b t a i n e d i n T e s t 6. 1.  Coefficient o f Consolidation (C ) v  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 based on pore p r e s s u r e v s . time r e l a t i o n s h i p :  G  v  =  T  H  v  t  5  2  *  0  =  0*38 x 2 . 5 )  -  0.0054 ins2/min  2  Where T - time f a c t o r o f 0.38 y  - l e n g t h o f drainage p a t h :  H 2.5  #  n e g l e c t i n g I n i t i a l deformations  tt  *50  -  (21)  2  "Fundamentals  time f o r 50% c o n s o l i d a t i o n : minutes  from Graph 4*15.  o f S o i l Mechanics'*, T a y l o r (1948), pp.235.  ix  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 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 d r a i n a g e .  C  TT H  s  v  ioot  =  **  2  1 0 O  3.14  x (2.5)  2  (27.5)  2  10G x  =  Where  H  0  x  **•  2  = 2.5  t, «  t  .00026 i n s / m i n  1 0  o  n  as b e f o r e  - time f o r 100$ primary c o n s o l i d a t i o n , Bishop and Henkel (1957). -  ( 2 7 . 5 ) minutes 2  from Graph  "The T r i a x i a l T e s t " , Bishop and Henkel  4-15.  (1957), pp  126  X  Coefficient  of consolidation  based on volume change v s .  time r e l a t i o n s h i p , assuming end drainage o n l y .  C  =  v  77" H  **  2  •• 4 t  -  100  3.14 x ( 2 . 5 ) 4 x  (27.5)  2  2  0.0065 i n s / m i n . 2  Where symbols have same meaning as those on p r e c e d i n g page.  Calculation  o f pore p r e s s u r e parameter A a t f a i l u r e .  A  f  s  Au  - B(£>0~j)  B(A0~^  -  *A0~ ) 5  30.25 - 40(0.51) 0.51 x 19.1  -  f  1.01  xi  BIBLIOGRAPHY A. Baver, L.  Books D.,1956",  S o i l P h y s i c s , 3rd. ed., New York, W i l e y .  Bishop, A.W.  and Henkel, D . J . , 1 9 5 7 , The T r l a x i a l T e s t . London: Edward A r n o l d .  Lambe, T.W.,  1951*  T a y l o r , D.W.,  1948,  S o i l T e s t i n g f o r E n g i n e e r s . New John W i l e y .  York:  Fundamentals o f S Q U Mechanics. New York: John W i l e y .  T e r z a g h i , K. and Peck, R.B., 1 9 4 8 , S o i l Mechanics i n E n g i n e e r i n g P r a c t i c e . New York: John W i l e y . T s c h e b o t a r i o f f , G.P.,  B. Bishop, A.W.  1 9 5 1 , S o i l Mechanics, Foundations and E a r t h S t r u c t u r e s . New York: MoGraw H i l l .  J o u r n a l s and Reports and E l d i n , G., 1950, "Undrained T r i a x i a l T e s t s on S a t u r a t e d Sands and t h e i r S i g n i f i c a n c e i n the General Theory o f Shear S t r e n g t h " Geotechnlque, 2, 13-32.  Bjerrum, L., 1954*  " G e o t e c h n i e a l p r o p e r t i e s o f Norwegian -Marine C l a y s " Geotechnlque, 4 49-69• f  Burton, L. J . , 1956,  "A New Device f o r Automatic Pore-Water Pressure Determination™ C i v i l Engineering and P u b l i c Works Review.  Casagrande, A. and W i l s o n , S.D., 1949, Report to U.S. Experimental S t a t i o n . Grim, R.E.,  1959,  Waterways  " P h y s i c o - c h e m i c a l p r o p e r t i e s of S o i l s : C l a y M i n e r a l s " . Jour A.S.C.E. Vol.85 No. SM2. 1998; 1-TFl  A  Lambe, T.W.,  1958,  M i e h e l l , J.K., 1956  l  l  "The S t r u c t u r e o f Compacted C l a y " , Jour A.S.C.E. V o l . 84, No. SM2, 1654; 2-32. "The Importance o f S t r u c t u r e to the E n g i n e e r i n g Behavior o f C l a y . Sc.D. T h e s i s M.I.T.  Penman, A.D.M., 1953, "Shear C h a r a c t e r i s t i c s o f a S a t u r a t e d 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 E x p e r i e n c e s " Proc. 3 r d . I n t . Conf. S o u Mech., 1:279^28"2*7  R e n d u l i c , L., 1937,  " E i n Grundgesetz der Tonmechanik und s e i n e x p e r l m e n t a l e r Beweis" Bauingenieur, 18: 459-467.  Rosenquist,  I.T., 1959, P h y s i c o - c h e m i c a l P r o p e r t i e s o f S o i l s : S o i l - W a t e r Systems* Jour A.S.C.E. Vol.85, No. SM2. 2G00; 31-52.  Rowe, P.W.,  1959,  "Measurement o f the C o e f f i c i e n t of Cons o l i d a t i o n of L a c u s t r i n e C l a y " . Geoteehnique IX: 3, 107-118. .  Skempton, A.W.,  1954,  Skempton, A.W.,  and Bishop, A.W., 1950. "The Measurement o f the Shear S t r e n g t h o f 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 , t h e i r E l a s t i c i t y and inelasticity. M a t e r i a l s '* * .... : North-Holland P u b l i s h i n g Company. Amsterdam  Skempton, A.W.,  and Northey, R.D., 1952, "The S e n s i t i v i t y o f Clays"• Geoteehnique 3, 30-54.  T a y l o r , D.W.,  Geoteehnique 4  pp 143-147.  1943, N i n t h Progress Report on Shear Research to U.S. E n g i n e e r s , M.I.T. P u b l i c a t i o n .  Winterkorn, H.P., 1941, "Study of Changes i n P h y s i c a l P r o p e r t i e s of Putnam S o i l Induced by I o n i c 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 in Position.  SPECIMEN PREPARATION  (Cont'd.)  Application of Twin Membranes and S e a l s .  SPECIMEN PREPARATION  Prepared  (Cont'd.)  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 Pressure Apparatus and A u t o m a t i c C o n t r o l .  To rollow pa-ge S4-  o~j = 60^6 per sq.fn-  CeLL pressure  =  V~,  SO  6\* 3 0  <r -. 3  12.  ffauge. 10  so  30  40  so  E L a p se ci . -Time  &O  70  -minu-tes.  so  so  IOO  110  DRAINAGE in m / n s  J~t 32  £8  24  STAGE  - TEST  £.  t> tn mins 20  48  O  CeLL  pr&.ssure ©  70  (A  — /r>  0~ ~ 8 0 Lb per 3  sq.in.  TofoLLow  pcuge  S3  To foLl.oliv  0~~  3  Cell -&—  pressure ;  0~ s  2 0 lb per  :  sq.  ,  Q  z.er  o  o-f  to  po  r e - p ressure  ao £ L exp s e aL  A-O lb  per  sq.  66  in-  in-  : ;  Dra-i nag e : Por e -pressure  <0  .=  page  o  oCrOj  gauge-  30  time.  40.  -  minutes.  SO  10  zo  30  4-0  E Lapsed, time - minutes  SO  61  DRAINAGE  STAGE - TEST  j6. 3a  36  40  44  46  o  3o.  Mohr  D i c x q r a m - Eff  e clr i v e  Sfesses.  To foLLowG  "ra.ph  4-  G R A P H 4-gg  T o -follow  G R A P H  Graph 4-  ^Pa^"  lO X lO TO THE INCH KEUFFEL & ESSER CO.  359-5DG OAOE t(f U s.A.  1  Vi ?: r  L  ?\  1  1  T e£ s  r CIO  1 1  A0  1  T -fa • " i "  u  7  7 W  u 2.Q  ^,  fa  5  —  c  r  A,  0  /  «a  0  >  *  ro  SV  1  -\  j  9"  s  , ——  Nt  f  [/  -(  c A  0  u X 1  c3T/7CJ 7  |  «  r  1%•  ?. 0  40  7  >  1  rr  O T  r  r 1 &.0 e  5 SS5'  80 /t-  b-  LOC >  S  -4.  c\f  in /V, >  >  '.V  »l 1  -f ,1  To foliow  Crystalline  Comp orients  ot  Clay  (a)  QancCCj  5  Minerals.  (C)  Oa.ncL O  Oxygens.  Si'Cicons-  (b) Q ccnoL C^i  pcLje  (cL) Hydroxyls  FIG  1 .  Q AL amin  CLAY  u m s moLonesfums f  etc.  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  _  + + + + + + + +  Liquid  4+ <o - + V _  (a)  HeL/nholtz  Double.  Layer  C a. rio n  v.  v.  ^0 ^  d  0  \_ >s  Vj • s. o. 0 u a  (b)  FIG  2.  \ 0 ^ Wafer  lon-wctfer  HELMHOLTI  tnolecuLe  compie  AND DIFFUSE  LAYERS  To foUou/ page 7.  —4  -+  -v - +  •Si  —  O <o X  -  IJ  _  <3  S, v  _  + +  + +  4-  + +  (cx)  HeLmholfz  Double  u  0\  C o . rion  S CJ  <^^>  ^. 0 .  <!)  Layer.  X  Liauid  0 A.  v <0  (b) Ion-wafer  FIG  £.  HELMHOLTZ  complex.  AND D1FFU5E (Sc he  LAYERS. rncxt/c)  To follow  K Double  Layer  Adsorbed.  Monfm  Layer  Wafer  Crystal Wafer  or i lions'-re  ^Double  8  Wafer  ont rn or i Li on/ fe Double  pa.ye  S h e e f  Layer  Adsorbed  V/afer^  Water^  v- ~  k a o l i n ite Crys t a l  ioA  KaoCinife FIG V6  3.  Sheet  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  FIG 4 . S T R U C T U R E  pctje  OF M A R I N E (After  /2.  CLAY.  T.K.Ta.n.  /9S~7.)  To follow  page  IS  B  \  0-3  r  H  ^  cr,  H  NormcuL  FigS.  ;  Mohr Diagram  Normal.  Fig  6 . Mohr  Stresses-  : Tofai  «-  and  Effective  Stresses  Stresses  Diagram  : Tofai  FIGS. 5 and 6  Stresses  T o foLLow  ConsoL  F I G 7.  ion  8.  M O H R  Stress D I A G R A M  2.1  Pressure  CONSOLIDATION  Effective F I G  idat  page  at  HISTORIES  failure  . T R U E  F 1 GS 7 and. 8  P A R A M E T E R S  To foiiow  F I G 9 .  STRESS-CONTROLLED MACHINE  FIG  10. T R I A X I A L  paye  31  TRIAXIAL  CELL  To foLL ow po.j e 3 7.  Layout  of  Appa  F I G 13.  ret t us  for  Meas u ring  PORE - P R E S S U R E  Pore  Pr ess ur & .  APPARATUS  :  F? i'Cf II. :  oL o  Proving-rIn  •IO  Proving-  5  To  zo  so  Oeliect/on  page 4-3  40x10"*  inches  Membrane vs. srrain Curve.  /o  /s~  Straih-per  FIGS-  f-oLLow  g Co rr e c tio,n  r/nq  Fig IE . Rubber Stress  o  " •  II a n d . 12  BO  cent  25  To follow  Direction  Time  Time  t  of  E x t e r n a. I  pa-oe  77  Pressure  : External. Pre ssut-e taken partly by Tn fe r g r a.n u I a. r Stresses .  Q  t, : External e.ntir-e.ly  F I G 14. C L A Y -  WATER  OF P L A S T I C THE  Pr ess u re. by Pore  take n Water.  S Y S T E M : EFFECTS DEFORMATION  ADSORBED  LAYERS.  OF  To follow To  pa ye. 86  Specimen  r&r  Ko,  Con tr o I f"o Re Lay  From  Relai  IH_3  Merc ur y.  F I G 1 5 . PORE-PRESSURE D E V I C E {After  T L  -WVV-  T J  Acfua-tor  5  proy  T L  A.D. Pe n m a.n  A  W  -  1953)  r j  Ac f oaf or  P. E.G.  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 IGS  15 a n d  I6 .  To Joltow  To  Spec/men  page  89.  AmpC i J /'er.  Null Indicator]  F I G 1 7 .L A Y O U T  OF AUTOMATIC  CONTROL  To Power R  Pack  3QOV(+)  9  •A/WVW Rio  RI&  W W W —  v  1  5  To Power  Relay <0  FiLam  Poles  Po.ckC—')  e «ts  j> |> |> |> <*> ^  o Motor  (CU C I R C U I T  f lb] C I R C U I T  OF  SERVOMOTOR  OF  A M P L I F I E R  FIG 1 8 . A U T O M A T I C  CONTROL  To  Pore  IS  pressure.  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