Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Undrained time dependent behavior of a lightly overconsolidated natural clay 1982

You don't seem to have a PDF reader installed, try download the pdf

Item Metadata

Download

Media
UBC_1982_A7 Z47.pdf [ 2.95MB ]
UBC_1982_A7 Z47.pdf
Metadata
JSON: 1.0062650.json
JSON-LD: 1.0062650+ld.json
RDF/XML (Pretty): 1.0062650.xml
RDF/JSON: 1.0062650+rdf.json
Turtle: 1.0062650+rdf-turtle.txt
N-Triples: 1.0062650+rdf-ntriples.txt
Citation
1.0062650.ris

Full Text

UNDRAINED TIME DEPENDENT BEHAVIOR OF A LIGHTLY OVERCONSOLIDATED NATURAL CLAY by Mustapha Z e r g o u n B.Sc.A. C i v i l E n g i n e e r i n g , L a v a l U n i v e r s i t y , Quebec, 1976 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF M.A.Sc . i n the Department of C i v i l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g to t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A u g u s t , 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or pub l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of CIVIL ENGINEERING The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date M)6A>ST M | Wis r->T7' a I ~> /na \ ABSTRACT Time dependence of s t r e s s - s t r a i n and s t r e n g t h c h a r a c t e r i s t i c s of a s e n s i t i v e , o v e r c o n s o l i d a t e d , i n t a c t c l a y have been i n v e s t i g a t e d under u n d r a i n e d c o n d i t i o n s . V a r i o u s t i m e l o a d i n g h i s t o r i e s i n u n d r a i n e d t r i a x i a l c o m p r e s s i o n , under a g i v e n c o n s o l i d a t i o n h i s t o r y , showed t h a t i n c r e a s e i n speed of t e s t i n g r e s u l t s i n s t i f f e r s t r e s s - s t r a i n r e s p o n s e and h i g h e r s t r e n g t h , and t h a t s u s t a i n e d s t r e s s or l o a d c a u s e s a r e d u c t i o n o f s t r e n g t h w i t h t i m e . In a l l t e s t s , r u p t u r e was t r i g g e r e d at about the same c r i t i c a l l e v e l of s t r a i n . C o r r e l a t i o n among t h e v a r i o u s t e s t s s u p p o r t e d t h e v a l i d i t y of the e q u a t i o n - o f - s t a t e u n i q u e l y r e l a t i n g s t r e s s , s t r a i n and s t r a i n - r a t e . C o m p a r i s o n w i t h the same c l a y d e s t r u c t u r e d on normal c o n s o l i d a t i o n showed t h a t d e s t r u c t u r a t i o n of the c l a y does not cause any s i g n i f i c a n t change i n i t s time dependent b e h a v i o r . TABLE OF CONTENTS Page CHAPTER 1 INTRODUCTION 1 CHAPTER 2 REVIEW OF SOME CONTRIBUTIONS TO THE KNOWLEDGE OF TIME EFFECTS ON CLAYS AT CONSTANT VOLUME 5 2.1. E f f e c t of r a t e of t e s t i n g on the u n d r a i n e d s t r e n g t h of c l a y s . . . . . 5 2.2. G e n e r a l U n d r a i n e d s t r e s s - s t r a i n - t i m e b e h a v i o r of c l a y s 8 CHAPTER 3 EXPERIMENTATION 15 3.1. M a t e r i a l t e s t e d 15 3.2. A p p a r a t u s . . . . '. 18 3.3. E x p e r i m e n t a l p r o c e d u r e 23 CHAPTER 4 TESTS RESULTS 27 4.1. C o n s t a n t r a t e of s t r a i n s h e a r . . . . 27 4.2. C o n s t a n t s t r e s s c r e e p 34 4.3. C o n s t a n t r a t e of l o a d i n g s h e a r . . . 44 4.4. C o n s t a n t l o a d c r e e p 48 CHAPTER 5 CORRELATIONS OF RESULTS FROM TESTS WITH VARIOUS TIME LOADING HISTORIES 55 5 . 1 S t r e s s - S t r a i n - S t r a i n r a t e r e l a t i o n - s h i p 55 i i i Page CHAPTER 5 ( c o n t ' d ) 5.2. C o m p a r i s o n w i t h the b e h a v i o r of n o r m a l l y c o n s o l i d a t e d Haney c l a y . . 61 5.3. S t r e s s c o n d i t i o n s at maximum e f f e c t i v e s t r e s s r a t i o . . . . . . . 68 CHAPTER 6 CONCLUSION 70 LIS T OF REFERENCES 72 i v LIST OF TABLES Page 1. P r o p e r t i e s of Haney c l a y 16 2. R e s u l t s of c o n s t a n t r a t e of s t r a i n s h e a r 28 3. R e s u l t s of c o n s t a n t s t r e s s c r e e p 35 4. R e s u l t s of c o n s t a n t r a t e of l o a d i n g s h e a r . . . . 45 5. R e s u l t s of c o n s t a n t l o a d c r e e p 49 v LIST OF FIGURES Page 1. D i s t r i b u t i o n of n a t u r a l water c o n t e n t a l o n g v e r t i c a l samples of Haney c l a y 17 2. Components of t h e h y d r o s t a t i c s e a l s ystem f o r f r i c t i o n l e s s t r i a x i a l c e l l 20 3. Pore p r e s s u r e g e n e r a t e d due to the a r r e s t of s e c o n d a r y c o m p r e s s i o n a f t e r 38 h r s c o n s o l i d a t i o n at a = 0.5 kg/cm 2 24 c 4. I n f l u e n c e of r a t e of s t r a i n on u n d r a i n e d s t r e s s - s t r a i n b e h a v i o r of o v e r c o n s o l i d a t e d Haney c l a y i n c o n s t a n t r a t e of s t r a i n s h e a r 29 5. V a r i a t i o n of e x c e s s pore p r e s s u r e w i t h a x i a l s t r a i n f o r u n d r a i n e d O.C. Haney c l a y i n c o n s t a n t r a t e of s t r a i n s h e a r 31 6. V a r i a t i o n of u n d r a i n e d s t r e n g t h w i t h r a t e o f s t r a i n i n c o n s t a n t r a t e of s t r a i n s h e a r . . . . 33 7. A x i a l s t r a i n v e r s u s time r e l a t i o n s h i p i n c o n s t a n t s t r e s s c r e e p 36 8. A x i a l s t r a i n r a t e v e r s u s time r e l a t i o n s h i p i n c o n s t a n t s t r e s s c r e e p 38 9. Pore p r e s s u r e r e s p o n s e a t the base of the sample w i t h time i n c o n s t a n t s t r e s s c r e e p on O.C. Haney c l a y 40 v i LIST OF FIGURES ( c o n t ' d ) Page 10. Time dependence of u n d r a i n e d s t r e n g t h i n c o n s t a n t s t r e s s c r e e p 42 11. V a r i a t i o n of u n d r a i n e d s t r e n g t h w i t h r a t e of s t r a i n i n c o n s t a n t s t r e s s c r e e p 43 12. I n f l u e n c e of r a t e of l o a d i n g on u n d r a i n e d s t r e s s - s t r a i n b e h a v i o r of o v e r c o n s o l i d a t e d Haney c l a y i n c o n s t a n t r a t e of l o a d i n g s h e a r 46 13. V a r i a t i o n of u n d r a i n e d s t r e n g t h of O.C. Haney c l a y w i t h r a t e of l o a d i n g i n c o n s t a n t r a t e of l o a d i n g s h e a r 47 14. A x i a l s t r a i n v e r s u s time r e l a t i o n s h i p i n c o n s t a n t l o a d c r e e p 50 15. A x i a l s t r a i n r a t e v e r s u s time r e l a t i o n s h i p i n c o n s t a n t l o a d c r e e p 52 16. I n i t i a l d e v i a t o r s t r e s s v e r s u s time to f a i l u r e • r e l a t i o n s h i p i n c o n s t a n t l o a d c r e e p 53 17. I n i t i a l d e v i a t o r s t r e s s v e r s u s minimum a x i a l s t r a i n r a t e r e l a t i o n s h i p i n c o n s t a n t l o a d c r e e p 54 18. C o m p a r i s o n of s t r a i n r a t e dependence of u n d r a i n e d s t r e n g t h i n c o n s t a n t r a t e of s t r a i n s h e a r and c o n s t a n t s t r e s s c r e e p 57 v i i LIST OF FIGURES ( c o n t ' d ) 19. D e v i a t o r s t r e s s v e r s u s a x i a l s t r a i n r a t e at e q u a l s t r a i n l e v e l r e l a t i o n s h i p i n c o n s t a n t r a t e of s t r a i n s h e a r , c o n s t a n t s t r e s s c r e e p , c o n s t a n t r a t e of l o a d i n g s h e a r , and c o n s t a n t l o a d c r e e p 20. S t r e s s - s t r a i n - s t r a i n r a t e r e l a t i o n s h i p f o r u n d r a i n e d t r i a x i a l c o m p r e s s i o n on O.C. Haney c l a y u s i n g v a r i o u s time l o a d i n g h i s t o r i e s . . . 21. V a r i a t i o n of u n d r a i n e d s t r e n g t h w i t h r a t e of s t r a i n i n c o n s t a n t s t r a i n r a t e s h e a r and c o n s t a n t s t r e s s c r e e p f o r O.C., and N.C. Haney c l a y 22. Time dependence of u n d r a i n e d s t r e n g t h of O.C. and N.C. Haney c l a y i n c o n s t a n t s t r e s s c r e e p 23. S t r e s s c o n d i t i o n s f o r a l l t e s t s at ( 0 , / o J on 1 3 max O.C. Haney c l a y i n the M o d i f i e d Mohr d i a g r a m v i i i LIST OF SYMBOLS Skempton 1s pore p r e s s u r e p a r a m e t e r = E x c e s s p o r e p r e s s u r e D e v i a t o r s t r e s s i n t e r c e p t of l n e v e r s u s D, a t t = t ^ s h e a r s t r e s s l e v e l , ^a±~a2^^ai~°3^f s l o p e o f l n e v e r s u s l n t , a t a f i x e d D time e l a p s e d s i n c e the i n i t i a t i o n of c r e e p r u p t u r e l i f e s l o p e of l n e v e r s u s D, a t any f i x e d time a x i a l s t r a i n a x i a l s t r a i n r a t e minimum a x i a l s t r a i n r a t e p r i n c i p a l e f f e c t i v e s t r e s s e s d e v i a t o r s t r e s s e f f e c t i v e s t r e s s r a t i o i x ACKNOWLEDGEMENTS G r a t e f u l acknowledgement i s e x p r e s s e d to the N a t i o n a l R e s e a r c h C o u n c i l of Canada f o r f i n a n c i a l s u p p o r t of the s t u d y . The a u t h o r ' s d e e p l y f e l t g r a t i t u d e goes to h i s r e s e a r c h a d v i s e r , Dr. Y.P. V a i d , f o r h i s c o n s t a n t g u i d a n c e i n t h e d i f f i c u l t t a s k o f o b t a i n i n g r e l i a b l e and a c c u r a t e e x p e r i m e n t a l d a t a . He a l s o w i s h e s to thank the s t a f f o f the C i v i l E n g i n e e r i n g Workshop of the U n i v e r s i t y of B r i t i s h C o l u m b i a f o r t h e i r h i g h l y p r o f e s s i o n a l t e c h n i c a l a s s i s t a n c e . x 1 CHAPTER 1 INTRODUCTION Time i s one of the v a r i o u s v a r i a b l e s t h a t a f f e c t b o t h s h e a r s t r e n g t h and d e f o r m a t i o n b e h a v i o r of a l l g e o l o g i c a l m a t e r i a l s . In most s o i l s the e f f e c t of time i s e i t h e r d i f f i c u l t to i s o l a t e from o t h e r f a c t o r s or has a r e l a t i v e l y minor i m p o r t a n c e . However, f o r c l a y s , time e f f e c t s may have a c o n s i d e r a b l e i n f l u e n c e . F o l l o w i n g the t r a d i t i o n a l s e p a r a t i o n between d e f o r m a t i o n a n a l y s i s and s t a b i l i t y a n a l y s i s i n s o i l m e c h a n i c s , time e f f e c t s on c l a y s have been c o n s i d e r e d , on one hand, i n the a p p l i c a t i o n of oedometer t e s t s to s e t t l e m e n t p r o b l e m s , and on the o t h e r hand, i n the a p p l i c a t i o n of s h e a r s t r e n g t h t e s t s to s l o p e s t a b i l i t y p r o b l e m s . The f i r s t phenomenon i s c o n c e r n e d w i t h v o l u m e t r i c d e f o r m a t i o n s of c l a y s . The s e c o n d a s p e c t d e a l s w i t h the time dependence of s t r e n g t h , and w i t h the g e n e r a l r e l a t i o n s h i p between s t r e s s , s t r a i n and t i m e . D u r i n g the s h e a r i n g of a c l a y , two t y p e s of time e f f e c t s a r e e n c o u n t e r e d d e p e n d i n g on whether the volume of the m a t e r i a l i s v a r y i n g ( d r a i n e d c o n d i t i o n s ) or i s c o n s i d e r e d to be c o n s t a n t ( u n d r a i n e d c o n d i t i o n s ) . The knowledge of u n d r a i n e d time dependent b e h a v i o r of c l a y s has a c o n s i d e r a b l e p r a c t i c a l 2, i m p o r t a n c e i n the p r e d i c t i o n of end of c o n s t r u c t i o n d e f o r m a t i o n s and s t a b i l i t y of e a r t h s t r u c t u r e s i n v o l v i n g s u c h m a t e r i a l s . U n d r a i n e d time dependence of s t r e s s - s t r a i n and s t r e n g t h c h a r a c t e r i s t i c s of c l a y s have been i n v e s t i g a t e d by s e v e r a l r e s e a r c h e r s ( 1 , 2 , 3 , 8 , 1 0 , 1 4 , 2 4 , 3 2 ) . I t has been found t h a t t h e u n d r a i n e d s t r e n g t h measured i n t h e l a b o r a t o r y u s i n g the c o n v e n t i o n a l s h e a r t e s t s depends on the speed of t e s t i n g ; t h e i n c r e a s e i n s t r e n g t h w i t h speed of t e s t i n g b e i n g more pr o n o u n c e d i n c l a y s w i t h a h i g h e r p l a s t i c i t y i n d e x . S i m i l a r t i m e e f f e c t s a r e o b s e r v e d d u r i n g c r e e p l o a d i n g , where d e f o r m a t i o n i n c r e a s e s under c o n s t a n t a p p l i e d s t r e s s e s , and the r u p t u r e l i f e d e c r e a s e s as the l e v e l of c r e e p s t r e s s i n c r e a s e s (9,10,15,21,25,28,32,31). In g e n e r a l , t h e s t r e s s - s t r a i n and s t r e n g t h r e s p o n s e of a c l a y i s a f u n c t i o n of the time h i s t o r y of l o a d i n g or d e f o r m a t i o n . P r i o r to the s t u d y of time dependent b e h a v i o r of an u n d i s t u r b e d c l a y by V a i d and C a m p a n e l l a i n 1977 ( 3 2 ) , no a t t e m p t was made by the p r e v i o u s i n v e s t i g a t o r s to c o r r e l a t e s t r e s s - s t r a i n - s t r e n g t h b e h a v i o r from t e s t r e s u l t s w i t h d i f f e r e n t time d e f o r m a t i o n h i s t o r i e s . F u r t h e r m o r e , most of the i n v e s t i g a t i o n s of time d ependent c l a y b e h a v i o r have been r e s t r i c t e d to n o r m a l l y c o n s o l i d a t e d c l a y s . S i m i l a r s t u d i e s on o v e r c o n s o l i d a t e d c l a y s a r e v e r y l i m i t e d ( 1 5 , 3 1 , 3 3 ) , even though such c l a y s have a wide 3 s p r e a d o c c u r r e n c e . N a t u r a l c l a y s may be o v e r c o n s o l i d a t e d d u r i n g t h e i r g e o l o g i c a l h i s t o r y by the w e i g h t of s o i l s t r a t a t h a t were l a t e r e r o d e d , by the weight of i c e t h a t l a t e r m e l t e d , by d e s s i c a t i o n due t o t e m p o r a r y e x p o s u r e to w e a t h e r , or by a g i n g ( 6 , 7 ) . The time dependent b e h a v i o r of t h e s e s o i l s i s i m p o r t a n t i n p r e d i c t i n g the end of c o n s t r u c t i o n d e f o r m a t i o n s or a s s e s s i n g the s t a b i l i t y of c u t t i n g s , f o u n d a t i o n s , and t u n n e l o p e n i n g s . In the e x p e r i m e n t a l s t u d y of s e n s i t i v e c l a y s , i t i s i m p o r t a n t to d e f i n e c l e a r l y the o v e r c o n s o l i d a t i o n s t a t e of a c l a y . S i n c e s e n s i t i v e c l a y s undergo a r a d i c a l change i n t h e i r n a t u r a l s t r u c t u r e a t c o n s o l i d a t i o n s t r e s s e s h i g h e r t h a n the a p p a r e n t p r e c o n s o l i d a t i o n p r e s s u r e , i t i s n e c e s s a r y t h a t t h i s l i m i t i n g p r e s s u r e i s n e v e r e x c e e d e d p r i o r to s h e a r t e s t i n g i f the s t r u c t u r e of the c l a y i s to be m a i n t a i n e d i n t a c t . In t h i s i n v e s t i g a t i o n , t h e o v e r c o n s o l i d a t i o n of the c l a y i s d e f i n e d w i t h r e s p e c t to the i n - s i t u a p p a r e n t p r e c o n s o l i d a t i o n p r e s s u r e as e s t i m a t e d from the i n c r e m e n t a l o n e - d i m e n s i o n a l c o n s o l i d a t i o n t e s t . T h i s d e f i n i t i o n i s t o be d i s t i n g u i s h e d from the one used i n most s t u d i e s on o v e r c o n s o l i d a t e d c l a y s where the samples a r e f i r s t s u b j e c t e d to l a b o r a t o r y n o r m a l c o n s o l i d a t i o n , t h e n r ebounded to a l o w e r s t r e s s . The o v e r c o n s o l i d a t i o n i s t h e n d e f i n e d w i t h r e s p e c t to the maximum c o n s o l i d a t i o n s t r e s s a p p l i e d i n the l a b o r a t o r y . 4 In the p r e s e n t s t u d y , the i n f l u e n c e of r a t e of d e f o r m a t i o n and l o a d i n g h i s t o r y on the u n d r a i n e d s t r e s s - s t r a i n and s t r e n g t h c h a r a c t e r i s t i c s of an u n d i s t u r b e d , s a t u r a t e d , s e n s i t i v e , o v e r c o n s o l i d a t e d , m arine c l a y i s s t u d i e d . A v a r i e t y of time h i s t o r i e s of l o a d i n g a r e used to s h e a r i d e n t i c a l t r i a x i a l t e s t s a m p l e s . These i n c l u d e c o n v e n t i o n a l c o n s t a n t r a t e o f s t r a i n s h e a r , c o n s t a n t s t r e s s c r e e p , c o n s t a n t r a t e of l o a d i n g , and c o n s t a n t l o a d c r e e p among o t h e r s . The d e f o r m a t i o n b e h a v i o r of the c l a y i s c o r r e l a t e d among v a r i o u s t y p e s of t e s t s u s i n g the c o n c e p t of the e q u a t i o n of s t a t e used i n m e t a l c r e e p and a l s o f o u n d a p p l i c a b l e to the time d ependent d e f o r m a t i o n of c l a y s ( 3 2 , 3 3 ) . C o m p a r i s o n s w i t h the r e s u l t s of the s i m i l a r s t u d y on the same c l a y i n the n o r m a l l y c o n s o l i d a t e d s t a t e a r e a l s o a t t e m p t e d . 5 CHAPTER 2 REVIEW OF SOME CONTRIBUTIONS TO THE KNOWLEDGE OF TIME EFFECTS ON CLAYS AT CONSTANT VOLUME In the l a s t t h r e e d e c a d e s , t h e r e have been s e v e r a l c o n t r i b u t i o n s to the knowledge o f time e f f e c t s on the u n d r a i n e d b e h a v i o r o f c l a y s . These s t u d i e s may be c l a s s i f i e d i n two g r o u p s . The f i r s t group c o n c e r n s the time dependence of u n d r a i n e d s t r e n g t h o f c l a y s as e v i d e n c e d by the e f f e c t o f v a r i a t i o n i n the speed of t e s t i n g i n the l a b o r a t o r y and a l s o i n the f i e l d . The s e c o n d , d e a l s w i t h the g e n e r a l r e l a t i o n s h i p between s t r e s s , s t r a i n and time i n c l u d i n g the i n f l u e n c e o f r a t e of t e s t i n g as w e l l as c r e e p e f f e c t s on t h e d e f o r m a t i o n and s t r e n g t h b e h a v i o r of c l a y s . Some of t h e s e c o n t r i b u t i o n s w i l l be r e p o r t e d h e r e a f t e r f o l l o w i n g t h i s d i s t i n c t i o n . 2.1. E f f e c t of Rate of T e s t i n g on the U n d r a i n e d S t r e n g t h o f C l a y s The i n f l u e n c e o f r a t e o f t e s t i n g has been r e c o g n i z e d i n the measurement o f the u n d r a i n e d s t r e n g t h o f c l a y s i n the l a b o r a t o r y as w e l l as i n the f i e l d . In the l a b o r a t o r y , the^ 6 e f f e c t o f r a t e of s t r a i n on c l a y samples t e s t e d i n u n d r a i n e d c o m p r e s s i o n has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n . One o f the e a r l i e s t c o n t r i b u t i o n s to t h a t s u b j e c t was made by T a y l o r i n 1943 ( 3 0 ) . The r e s u l t s o f h i s i n v e s t i g a t i o n showed t h a t the u n d r a i n e d s t r e n g t h of remoulded B o s t o n B l u e C l a y i n c r e a s e s l i n e a r l y w i t h the l o g a r i t h m of the a x i a l s t r a i n r a t e . Numerous s t u d i e s f o l l o w e d t h e n on v a r i o u s t y p e s of c l a y s i n o r d e r to d e t e r m i n e the e f f e c t of r a t e of s t r a i n on the v a l u e o f the u n d r a i n e d s t r e n g t h , a l s o termed a p p a r e n t c o h e s i o n ( 3 , 8 , 1 4 , 2 3 , 2 4 ) . I t i s now w e l l e s t a b l i s h e d t h a t the u n d r a i n e d s t r e n g t h o f s a t u r a t e d c l a y s i n c r e a s e s by 5 to 10% f o r a 1 0 - f o l d i n c r e a s e i n r a t e o f s t r a i n . However, t h e r e i s a minimum v a l u e o f r a t e o f s t r a i n below which the u n d r a i n e d s t r e n g t h r e m a i n s e s s e n t i a l l y c o n s t a n t . In 1950, C a s a g r a n d e and W i l s o n (10) r e p o r t e d an e x p e r i m e n t a l s t u d y p a r t l y d e a l i n g w i t h the e f f e c t o f r a t e o f l o a d i n g on the s t r e n g t h of f u l l y s a t u r a t e d b r i t t l e c l a y s and c l a y s h a l e s a t c o n s t a n t water c o n t e n t . Two t y p e s o f t e s t s were u s e d : c r e e p - s t r e n g t h t e s t s i n which l o a d s were b u i l t up q u i c k l y and m a i n t a i n e d c o n s t a n t u n t i l the s p e c i m e n f a i l e d ; and l o n g - t i m e u n c o n f i n e d c o m p r e s s i o n t e s t s i n w h i c h the s p e c i m e n s were s u b j e c t e d to i n c r e m e n t a l a x i a l l o a d i n g , the e l a p s e d time between i n c r e m e n t s o f l o a d v a r y i n g f o r d i f f e r e n t t e s t s . As the pore p r e s s u r e s were not measured, the e f f e c t 7. o f the t e s t d u r a t i o n was r e l a t e d to t o t a l s t r e s s e s . A s t r e n g t h r a t i o was d e f i n e d as the r a t i o o f the c o m p r e s s i v e s t r e n g t h c o r r e s p o n d i n g to a normal r a t e o f l o a d i n g . In c r e e p - s t r e n g t h t e s t s a n ormal time to f a i l u r e o f 1 m i n u t e was u s e d , and f o r l o n g - t i m e t e s t s a normal time o f l o a d i n g o f 10 m i n u t e s . In c r e e p - s t r e n g t h t e s t s a t c o n s t a n t water c o n t e n t the s t r e n g t h was r e d u c e d to v a l u e s of between 80 and 40 p e r c e n t of i t s n o r m a l v a l u e i n about 30 d a y s . F o r l o n g - t i m e u n c o n f i n e d c o m p r e s s i o n t e s t s , the s t r e n g t h o f f u l l y s a t u r a t e d M e x i c o C i t y c l a y showed a d e c r e a s e of about 40% from i t s n o rmal v a l u e i n 30 days of t o t a l time o f l o a d i n g . These r e s u l t s c l e a r l y i n d i c a t e t h a t the u n d r a i n e d s t r e n g t h o f s a t u r a t e d c l a y s i n c r e a s e s w i t h the r a t e of l o a d i n g and d e c r e a s e s under the e f f e c t of s u s t a i n e d l o a d s . In 1972, B j e r r u m (5) r e p o r t e d s e v e r a l f i e l d s t u d i e s d e m o n s t r a t i n g t h a t the p r o c e d u r e s used t h e n f o r c o m p u t i n g the end o f c o n s t r u c t i o n s t a b i l i t y of an embankment b u i l t on s o f t c l a y were u n s a t i s f a c t o r y m a i n l y b e c a u s e of i n c o r r e c t v a l u e s of s h e a r s t r e n g t h i n t r o d u c e d i n the a n a l y s i s . The dominant f a c t o r c a u s i n g such e r r o r s i n the e s t i m a t i o n o f the s h e a r s t r e n g t h b e i n g the r a t e o f t e s t i n g . He o b s e r v e d t h a t the d i s c r e p a n c y between the u n d r a i n e d s t r e n g t h measured i n the f i e l d vane t e s t s and the s h e a r s t r e n g t h b a c k - c a l c u l a t e d from the r e p o r t e d f a i l u r e c a s e s was e s s e n t i a l l y d e p e n d e n t on the 8 p l a s t i c i t y of the c l a y . He p o i n t e d out t h a t the more p l a s t i c t h e c l a y , the l a r g e r the d i s c r e p a n c y between the vane and the f i e l d m o b i l i z e d u n d r a i n e d s t r e n g t h . He t h e n e s t a b l i s h e d a c o r r e l a t i o n between the f i e l d s h e a r s t r e n g t h and the v a l u e s measured i n a vane t e s t and p r o p o s e d a c o r r e c t i o n f a c t o r w i t h w h i c h a vane s h e a r s t r e n g t h s h o u l d be m u l t i p l i e d b e f o r e i t i s i n t r o d u c e d i n a s t a b i l i t y a n a l y s i s . T h i s c o r r e c t i o n f a c t o r v a r i e d w i t h t h e p l a s t i c i t y i n d e x of the c l a y . He f u r t h e r c a r e f u l l y s t a t e d t h a t the v a l i d i t y of the p r o p o s e d c o r r e c t i o n o f u n d r a i n e d s t r e n g t h i s l i m i t e d to s u c h c o n d i t i o n s where the f a c t o r of s a f e t y i n c r e a s e s a f t e r the c o n s t r u c t i o n due to a g a i n i n s t r e n g t h by c o n s o l i d a t i o n . The c o n t r i b u t i o n s d i s c u s s e d above were e s s e n t i a l l y c o n c e r n e d w i t h the v a r i a t i o n o f the u n d r a i n e d s t r e n g t h o f c l a y s w i t h the d u r a t i o n of s t r e s s i n g . They do not g i v e any i n d i c a t i o n on the d e f o r m a t i o n b e h a v i o r of s u c h c l a y s w i t h t i m e . C o n t r i b u t i o n s on t h a t a s p e c t o f time e f f e c t s on u n d r a i n e d c l a y s w i l l now be p r e s e n t e d . 2.2 G e n e r a l U n d r a i n e d S t r e s s - S t r a i n - T i m e B e h a v i o r of C l a y s The s t u d y o f the r e l a t i o n s h i p between s t r e s s , s t r a i n and time i n the b e h a v i o r o f c l a y s a t c o n s t a n t volume was e s s e n t i a l l y c o n d u c t e d t h r o u g h the e x p e r i m e n t a l o b s e r v a t i o n of c r e e p t e s t s on c l a y s a m p l e s . The a c c u m u l a t i o n of s t r a i n w i t h 9 time i n c o n s t a n t s t r e s s c r e e p t e s t s c o n s t i t u t e s a c l e a r m a n i f e s t a t i o n o f the time dependence of s t r e s s - s t r a i n b e h a v i o r o f c l a y s . The s t u d y of c r e e p o f c l a y s was l a r g e l y b a s e d on the e x p e r i e n c e a c c u m u l a t e d i n the a n a l y s i s o f c r e e p o f m e t a l s and o t h e r e n g i n e e r i n g m a t e r i a l s ( 1 6 , 1 8 , 3 4 ) . M a t h e m a t i c a l a n d / o r r h e o l o g i c a l models have been p r o p o s e d by v a r i o u s r e s e a r c h e r s (17,21,25,29) f o r t h e d e s c r i p t i o n of c r e e p d e f o r m a t i o n o f c l a y s . G e n e r a l l y , the c r e e p s t r a i n i s e x p r e s s e d as a f u n c t i o n of the s t r e s s , t e m p e r a t u r e and t i m e . I t i s c u s t o m a r y to assume t h a t the t h r e e e f f e c t s a r e s e p a r a b l e . Two f u n d a m e n t a l a s s u m p t i o n s l e a d i n g to two d i f f e r e n t a p p r o a c h e s have been used to d e s c r i b e time d e pendent r e s p o n s e under a r b i t r a r y s t r e s s c h a n g e s . I t may be assumed t h a t e i t h e r : a) the r e s p o n s e o f the m a t e r i a l depends on the p r e s e n t s t a t e e x p l i c i t l y , b) o r , the m a t e r i a l remembers i t s p a s t e x p l i c i t l y and r e s p o n d s to the p r e s e n t i n a manner t h a t r e f l e c t s i t s p a s t h i s t o r y . The f i r s t a s s u m p t i o n l e a d s to what i s known as the e q u a t i o n of s t a t e f o r m u l a t i o n , w h i l e the second r e s u l t s i n the s o - c a l l e d memory t h e o r y . The e q u a t i o n o f s t a t e a p p r o a c h was a d o p t e d by most r e s e a r c h e r s f o r p r a c t i c a l r e a s o n s of p a s t s u c c e s s f u l e x p e r i e n c e w i t h t h i s t h e o r y , and s i m p l i c i t y i n i t s a p p l i c a t i o n and u n d e r s t a n d i n g . V a r i o u s e x p r e s s i o n s o f the 10 e q u a t i o n of s t a t e were p r o p o s e d to r e l a t e c r e e p s t r a i n , c r e e p s t r e s s , t i m e , and t e m p e r a t u r e . In a l l t h e s e e x p r e s s i o n s the e f f e c t of t e m p e r a t u r e i s i n c o r p o r a t e d as a c o n s t a n t . One o f the most known e x p r e s s i o n s i s the s o - c a l l e d B a i l e y - N o r t o n law: . m n / i \ e = Ao t (1) where: e = c r e e p s t r a i n a = c r e e p s t r e s s t = time A,m,n = c o n s t a n t s f u n c t i o n of t e m p e r a t u r e . O t h e r more complex e x p r e s s i o n s have a l s o f o u n d u s e . G e n e r a l l y , the s t r a i n r a t e i s of i n t e r e s t and the a n a l y s i s g i v e s r i s e to two p o s s i b i l i t i e s . I f the c r e e p s t r a i n i s d i f f e r e n t i a t e d w i t h r e s p e c t to t i m e , the s o - c a l l e d t i m e h a r d e n i n g f o r m u l a t i o n i s o b t a i n e d . A n o t h e r f o r m u l a t i o n can be d e r i v e d by e l i m i n a t i n g time between the e q u a t i o n o f s t a t e and the time h a r d e n i n g e x p r e s s i o n . T h i s p r o c e d u r e l e a d s to the s o - c a l l e d s t r a i n h a r d e n i n g f o r m u l a t i o n i n w h i c h the c u r r e n t c r e e p s t r a i n r a t e depends on the c u r r e n t s t r e s s , s t r a i n and, t h r o u g h the c o n s t a n t s , t e m p e r a t u r e . Based on t h e s e f u n d a m e n t a l c o n s i d e r a t i o n s , p h e n o m e n o l o g i c a l r e l a t i o n s h i p s were d e v e l o p e d u s i n g e m p i r i c a l c u r v e - f i t t i n g t e c h n i q u e s f o r the c h a r a c t e r i z a t i o n of c r e e p of s o i l s . 11 In 1968, S i n g h and M i t c h e l l (28) used the time h a r d e n i n g a p p r o a c h f o r the c a s e of c o n s t a n t s t r e s s c r e e p , and p r o p o s e d an a p p a r e n t l y g e n e r a l f u n c t i o n r e l a t i n g c r e e p r a t e , c r e e p s t r e s s and time i n the form: t e - A1 e X p ( a D ) ( - E i ) m (2) where : • e = a x i a l r a t e o f s t r a i n t = e l a p s e d time under s h e a r s t r e s s a -a t ^ = e l a p s e d time at which the c o n s t a n t A ̂  i s d e f i n e d o 1 - o 3 D = s h e a r s t r e s s l e v e l < t f l _ 0 3 ) f ( o ^ - o ^ ) ^ i s o b t a i n e d from a c o n s t a n t r a t e o f d e f o r m a t i o n u n d r a i n e d t r i a x i a l c o m p r e s s i o n te st m = s l o p e o f l n £ vs I n t a t any f i x e d v a l u e of D a = s l o p e o f l n e vs D a t any f i x e d time t . T h i s p h e n o m e n o l o g i c a l r e l a t i o n s h i p s u f f e r s from some s e r i o u s l i m i t a t i o n s . The c r e e p s t r e s s i s assumed to r e m a i n c o n s t a n t w h i l e c r e e p d e f o r m a t i o n i s t a k i n g p l a c e , and t h e e x p r e s s i o n 12 p r o p o s e d i s not v a l i d e i t h e r at the b e g i n n i n g or end of s t r e s s i n g . I t i s t h e r e f o r e d i f f i c u l t to a p p l y t h i s f u n c t i o n to a v a r i a b l e s t r e s s s i t u a t i o n , and the form o f the r e l a t i o n s h i p r u l e s out the p o s s i b i l i t y of c r e e p r u p t u r e i n the l a b o r a t o r y t e s t s . In g e n e r a l , r h e o l o g i c a l models and e m p i r i c a l c u r v e - f i t t i n g t e c h n i q u e s do not n e c e s s a r i l y t ake i n t o a c c o u n t t h e mechanisms g o v e r n i n g the c r e e p d e f o r m a t i o n p r o c e s s . C o m p a r i s o n s of the p r e d i c t i o n from t h e s e models w i t h o b s e r v e d e x p e r i m e n t a l d a t a i n d i c a t e d t h a t the models d e v e l o p e d a r e o n l y a p p r o x i m a t e and f a i l to d u p l i c a t e the r e a l s t r e s s - s t r a i n - t i m e r e s p o n s e o f c l a y s . A g r e a t e r s u c c e s s has been a c h i e v e d by the a p p l i c a t i o n o f the e q u a t i o n o f s t a t e t h e o r y u s i n g the s t r a i n h a r d e n i n g a p p r o a c h to the c o r r e l a t i o n between c o n s t a n t s t r a i n r a t e and c o n s t a n t s t r e s s c r e e p t e s t d a t a . In 1952, Pao and M a r i n (22) o b t a i n e d a good agreement i n c o r r e l a t i n g the r e s u l t s o f the two t y p e s o f t e s t s on P l e x i g l a s s . In 1963, C o a t e s , Burn and M c R o s t i e (12) s u g g e s t e d the v a l i d i t y of s u c h a c o r r e l a t i o n f o r c l a y s but d i d not have any d a t a to s u p p o r t i t . In an e x t e n s i v e s t u d y of the time dependent b e h a v i o r of an u n d i s t u r b e d c l a y , V a i d and C a m p a n e l l a (32) p r e s e n t e d i n 1977 a c l e a r e x p e r i m e n t a l e v i d e n c e s u p p o r t i n g the e q u a t i o n of s t a t e t h e o r y . The r e s u l t s o f t r i a x i a l c o m p r e s s i o n t e s t s on 13 n o r m a l l y c o n s o l i d a t e d u n d i s t u r b e d Haney c l a y showed t h a t i n c r e a s e i n s t r a i n r a t e , r a t e o f l o a d i n g , l e n g t h of a g i n g , and t h i x o t r o p i c h a r d e n i n g a l l r e s u l t i n s t i f f e r u n d r a i n e d s t r e s s - s t r a i n r e s p o n s e and h i g h e r u n d r a i n e d s t r e n g t h . Based on the a s s u m p t i o n t h a t a t a g i v e n l e v e l of s t r a i n ( o r s t r u c t u r e ) , the s h e a r s t r e s s i s a f u n c t i o n o n l y o f the i n s t a n t a n e o u s r a t e o f s t r a i n , and i s i n d e p e n d e n t of the p a s t s t r a i n r a t e h i s t o r y , s u c c e s s f u l c o r r e l a t i o n s t i e d t o g e t h e r the t e s t r e s u l t s from v a r i o u s time l o a d i n g h i s t o r i e s . I t was a l s o shown t h a t the e f f e c t i v e s t r e s s f a i l u r e e n v e l o p e f o r the c l a y t e s t e d was u n a f f e c t e d by any k i n d of time e f f e c t s . The e q u a t i o n o f s t a t e was s i m i l a r l y f o u n d a p p l i c a b l e to the s t u d y of the s t r a i n r a t e b e h a v i o r of a h e a v i l y o v e r c o n s o l i d a t e d q u i c k Leda c l a y ( 3 3 ) . A s a t i s f a c t o r y c o r r e l a t i o n was e s t a b l i s h e d f o r t h i s c l a y between c o n s t a n t s t r a i n r a t e s h e a r and c o n s t a n t s t r e s s c r e e p l o a d i n g s . The s i m i l a r i t y i n the m a g n i t u d e o f the a x i a l s t r a i n a t f a i l u r e - d e f i n e d a t minimum s t r a i n r a t e i n c r e e p t e s t s , and at peak d e v i a t o r s t r e s s i n c o n s t a n t s t r a i n r a t e s h e a r - s u p p o r t e d the e x i s t e n c e o f a c r i t i c a l s h e a r s t r a i n l e v e l a t w h ich r u p t u r e o c c u r s as s u g g e s t e d by C o a t e s and M c R o s t i e i n 1963 ( 1 2 ) . Such a s u g g e s t i o n , s u p p o r t e d by e x p e r i m e n t a l e v i d e n c e f o r the e x i s t e n c e o f a c r i t i c a l l e v e l of s t r a i n r e s p o n s i b l e f o r t r i g g e r i n g f a i l u r e , has a l s o been made by L e o n a r d s ( 1 9 ) , C a m p a n e l l a and V a i d ( 9 ) , and V a i d e t a l . (33) . 14 In the p r e s e n t i n v e s t i g a t i o n , the time dependent b e h a v i o r o f a l i g h t l y o v e r c o n s o l i d a t e d , s e n s i t i v e c l a y w i l l be s t u d i e d i n o r d e r to o b t a i n b a s i c i n f o r m a t i o n on s u c h b e h a v i o r as the d a t a a v a i l a b l e i n the l i t e r a t u r e i s s t i l l d e f i c i e n t on t h a t s u b j e c t . The a p p l i c a b i l i t y of the e q u a t i o n o f s t a t e t h e o r y i n i t s s t r a i n h a r d e n i n g f o r m u l a t i o n w i l l be c o n s i d e r e d , and c o m p a r i s o n s of the g e n e r a l form o f b e h a v i o r w i t h the same c l a y i n the n o r m a l l y c o n s o l i d a t e d s t a t e w i l l be p r e s e n t e d . 15 CHAPTER 3 EXPERIMENTATION 3.1 M a t e r i a l T e s t e d A l o c a l u n d i s t u r b e d medium s t i f f c l a y ( c a l l e d Haney c l a y ) was used f o r the s t u d y . Haney c l a y i s b e l i e v e d to have been d e p o s i t e d i n a m a r i n e e n v i r o n m e n t and l a t e r s u b j e c t e d to p a r t i a l l e a c h i n g due to s u r f a c e i n f i l t r a t i o n . The c l a y was b l o c k sampled from an open p i t and a l l t e s t samples were trimmed to c y l i n d e r s of $> 3.5 cm x 7.5 cm from b l o c k s o b t a i n e d from the same h o r i z o n . T h i s e n s u r e d the l e a s t v a r i a t i o n among i n d i v i d u a l s a m p l e s . Some p h y s i c a l p r o p e r t i e s of the c l a y t e s t e d a r e o u t l i n e d i n T a b l e 1. I t i s a g r e y s i l t y c l a y w i t h u n i f o r m h o r i z o n t a l l a y e r s of about 0.5 cm i n t h i c k n e s s o f d a r k g r e y o r g a n i c m a t e r i a l . F i g u r e 1 shows a t y p i c a l d i s t r i b u t i o n o f i n i t i a l water c o n t e n t a l o n g a v e r t i c a l s ample. The h i g h e r v a l u e s of water c o n t e n t a r e due to the o r g a n i c l a y e r s . These l a y e r s were r e g u l a r l y e n c o u n t e r e d i n a l l samples and d i d not seem to have any n o t i c e a b l e e f f e c t on the t e s t r e s u l t s . I t may be o f i n t e r e s t to r e p o r t t h a t d u r i n g the u n c o n f i n e d c o m p r e s s i o n t e s t the samples e x h i b i t e d a b r i t t l e r u p t u r e a t v e r y low a x i a l s t r a i n . The f a i l u r e p l a n e s formed 16 TABLE 1 PROPERTIES OF HANEY CLAY N a t u r a l water c o n e n t 63 to 73% Degree o f s a t u r a t i o n 100% L i q u i d l i m i t 89% P l a s t i c l i m i t 35% P l a s t i c i t y i n d e x 54% L i q u i d i t y i n d e x 0.52 to 0.70 S p e c i f i c g r a v i t y of s o l i d s 2.80 C l a y f r a c t i o n ( d < 0.002 mm) 85% S i l t f r a c t i o n (0.002 mm < d < 0.06 mm) 13% A c t i v i t y 0.64 Maximum p a s t p r e s s u r e 3.5 kg/cm 2 U n c o n f i n e d c o m p r e s s i o n s t r e n g t h 1.4 kg/cm 2 (e = 1.1%/min, e f = 1.4%) S e n s i t i v i t y 6 to 10 17 FIG. 1 DISTRIBUTION OF NATURAL WATER CONTENT ALONG VERTICAL SAMPLES OF HANEY CLAY. 18 a wedge cone c l o s e to the top of the s p e c i m e n s and e x t e n d e d v e r t i c a l l y to the b o t t o m . T h i s t y p e of f a i l u r e o c c u r r e d r e g a r d l e s s of the d i r e c t i o n of l o a d i n g w i t h r e s p e c t to the b e d d i n g l a y e r s . 3 .2 A p p a r a t u s U n d r a i n e d s t r e n g t h of o v e r c o n s o l i d a t e d Haney c l a y at s m a l l c o n f i n i n g p r e s s u r e i s r e l a t i v e l y low. The maximum d e v i a t o r s t r e s s f o r an e f f e c t i v e c o n f i n i n g p r e s s u r e of 0.5 kg/cm 2 and o v e r c o n s o l i d a t i o n r a t i o of 7 i s about 1.0 kg/cm 2 i n c o n v e n t i o n a l c o n s t a n t r a t e of s t r a i n s h e a r . The a c c u r a t e d e t e r m i n a t i o n of the v a r i a t i o n of u n d r a i n e d s t r e n g t h w i t h t i m e a l o n e t h u s r e q u i r e s a g r e a t c o n f i d e n c e i n the measurement o f the a x i a l l o a d a p p l i e d to shear, th e s a m p l e s . In a s t a n d a r d t r i a x i a l a p p a r a t u s , f r i c t i o n i s d e v e l o p e d around the l o a d i n g ram by the 0 - r i n g s e a l . The amount of f r i c t i o n g e n e r a t e d d u r i n g l o a d i n g depends on the v a l u e o f the c e l l - p r e s s u r e and the r a t e of d e f o r m a t i o n or the r a t e of l o a d i n g . I t a l s o depends on o c c a s i o n a l b e n d i n g of the l o a d i n g ram due to any t i l t i n g o f the top cap o f the s a mple. The a c c u r a t e measurement of the amount of f r i c t i o n g e n e r a t e d i s cumbersome and i n any case q u e s t i o n a b l e . Among the s e v e r a l methods a v a i l a b l e f o r the r e d u c t i o n o f e r r o r due to f r i c t i o n e f f e c t s on the measured a x i a l l o a d o u t s i d e the t r i a x i a l c e l l , the h y d r o s t a t i c s e a l s y s t e m w i t h a c o n t i n u o u s i 19 a i r - l e a k a g e a r ound the l o a d i n g ram has p r o v e n to be q u i t e s a t i s f a c t o r y . F i g u r e 2 shows the components of the c o n t i n u o u s l y a i r - l e a k i n g s ystem o r i g i n a l l y p r o p o s e d by Chan ( 1 1 ) . A s l i g h t l y m o d i f i e d d e s i g n of t h i s h y d r o s t a t i c s e a l was i n c o r p o r a t e d i n the t r i a x i a l c e l l used i n t h i s t e s t i n g program. In u s i n g s u c h a s e a l , the c e l l water p r e s s u r e i s c o u n t e r a c t e d by an e q u a l a i r p r e s s u r e a p p l i e d t h r o u g h the top of the c e l l . T h i s s y s t e m forms a f i l m of a i r c o n t i n u o u s l y l e a k i n g upward around the l o a d i n g ram. The t o l e r a n c e between the f i x e d r i n g and the l o a d i n g ram i s s u c h t h a t f r i c t i o n may d e v e l o p o n l y at the c o n t a c t of the l o a d i n g ram and the s t a i n l e s s s t e e l b a l l b u s h i n g . An a t t e m p t was made to measure the amount of f r i c t i o n g e n e r a t e d t h e n , and i t was found to r e p r e s e n t a p p r o x i m a t e l y 10 gram f o r c e . I t i s b e l i e v e d t h a t no a d d i t i o n a l f r i c t i o n w i l l d e v e l o p s i n c e w i t h the s m a l l m a g n i t u d e o f a x i a l l o a d s i n v o l v e d b e n d i n g of the l o a d i n g ram can be d i s r e g a r d e d . The m agnitude of c e l l p r e s s u r e used (2.5 kg/cm 2) i s n o t l i k e l y to c a u s e any i n c r e a s e i n ram f r i c t i o n . The o n l y d i f f i c u l t y t h a t may be e n c o u n t e r e d d u r i n g the a p p l i c a t i o n o f the c o n f i n i n g p r e s s u r e i s a s l i g h t d e l a y i n the g e n e r a t i o n of the same p r e s s u r e i n the chamber and t h r o u g h the s e a l s y s t e m . The s e a l i n g a i r p r e s s u r e and the c e l l water p r e s s u r e must be a p p l i e d s i m u l t a n e o u s l y so t h a t the l e v e l o f the c o n t a c t between a i r and water w i l l be k e p t 20 FIG. 12'- COMPONENTS OF THE HYDROSTATIC SEAL SYSTEM FOR LOW FRICTION TRIAXIAL CELL. 21 above the top of the c e l l and below the a i r l e a k i n p u t . T h i s may be done by c o u p l i n g . t h e a i r s u p p l y f o r the c e l l w a ter p r e s s u r e and f o r the a i r l e a k i n g s e a l system to the same a i r p r e s s u r e r e g u l a t o r and by p r o v i d i n g a l a r g e bore tube c o n n e c t i o n between a t e m p o r a r y p l e x i g l a s s water r e s e r v o i r and t h e t r i a x i a l chamber f o r the i n i t i a l a p p l i c a t i o n of the c o n f i n i n g p r e s s u r e . When the l e v e l of the a i r - w a t e r c o n t a c t i s e n s u r e d to be above the top of the chamber, the c e l l w a ter p r e s s u r e c o n n e c t i o n may be s w i t c h e d to the s m a l l 0.3 cm o u t s i d e d i a m e t e r s a r a n tube d i f f u s i o n s p i r a l s y s t e m . In u n d r a i n e d t r i a x i a l t e s t s on c l a y , the measurement o f t h e pore p r e s s u r e g e n e r a t e d d u r i n g s h e a r i n g i s c r i t i c a l . For o v e r c o n s o l i d a t e d Haney c l a y t e s t e d i n c o n v e n t i o n a l c o n s t a n t r a t e of s t r a i n c o m p r e s s i o n , t h e e x c e s s pore p r e s s u r e g e n e r a t e d at peak d e v i a t o r s t r e s s i s a l m o s t e q u a l to the e f f e c t i v e c o n f i n i n g p r e s s u r e and t h i s has a tremendous e f f e c t on the r a t i o of major e f f e c t i v e s t r e s s o v e r minor e f f e c t i v e s t r e s s . D u r i n g i s o t r o p i c r e c o n s o l i d a t i o n , samples of Haney c l a y showed a volume change of about 0.5% f o r an o v e r c o n s o l i d a t i o n r a t i o of 7, a f t e r 24 h o u r s under d o u b l e d r a i n a g e c o n d i t i o n s . Volume changes d u r i n g r e c o n s o l i d a t i o n a r e t h e r e f o r e v e r y s m a l l . B o t h pore p r e s s u r e and volume change measurements must be a c c u r a t e and r e l i a b l e . An e l e c t r o n i c d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r was used f o r 22 volume change measurements. Pore water p r e s s u r e s were a l s o m o n i t o r e d w i t h an e l e c t r o n i c p r e s s u r e t r a n s d u c e r h a v i n g a c a p a c i t y of 7 kg/cm 2, a s e n s i t i v i t y of 0.005 kg/cm 2, and a r a t e d c o m p l i a n c e of 0.0044 cm 3 f o r 7 kg/cm 2 change i n p r e s s u r e . The r e l i a b i l i t y was e n s u r e d by f u l l s a t u r a t i o n o f th e m e a s u r i n g s y s t e m w i t h t h o r o u g h l y d e a i r e d water and s p i r a l d i f f u s i o n l o o p s of at l e a s t 1 metre l o n g of 0.3 cm o u t s i d e d i a m e t e r s a r a n tube between the t r a n s d u c e r s , the back- p r e s s u r e g l a s s p i p e t t e , and the c e l l - p r e s s u r e p l e x i g l a s s r e s e r v o i r . The s p i r a l d i f f u s i o n l o o p s a l l o w e d t h e v i s u a l d e t e c t i o n of any a i r b u b b l e s and p r o t e c t e d the p r e s s u r e t r a n s d u c e r s and the chamber from the p r e s e n c e of d i f f u s e d a i r from the a i r p r e s s u r e s u p p l y . The 0.3 cm o u t s i d e d i a m e t e r s a r a n t u b i n g d r a i n a g e l i n e s were k e p t as s h o r t as p o s s i b l e between the t r i a x i a l a p p a r a t u s and the e l e c t r i c a l t r a n s d u c e r s t o m i n i m i z e t h e c o m p l i a n c e of the s y s t e m . The a x i a l d e f o r m a t i o n of the t r i a x i a l sample was measured by a L i n e a r V a r i a b l e D i s p l a c e m e n t T r a n s d u c e r DCDT h a v i n g an a c c u r a c y of 3.0 x 1 0 - 1 + cm f o r a maximum d i s p l a c e m e n t of 1.5 cm. The L.V.D.T. was p l a c e d on an e x t e n s i o n of one of the t h r e e r o d s r e t a i n i n g the top of t h e t r i a x i a l chamber. The v e r t i c a l sample d e f o r m a t i o n was measured by the d i s p l a c e m e n t of a r i g i d bar clamped on t h e <(> 0.635 cm s t a i n l e s s s t e e l l o a d i n g ram. S i n c e the c e l l 23 p r e s s u r e was c o n s t a n t t h r o u g h o u t the t e s t program, no d i s p l a c e m e n t due to s t r a i n i n g of the p r e s s u r i z e d chamber w i l l o c c u r to a l t e r the r e f e r e n c e on which the L.V.D.T. was mounted. 3.3 E x p e r i m e n t a l P r o c e d u r e A l l t e s t samples were s u b j e c t e d i n one i n c r e m e n t to an a l l - r o u n d e f f e c t i v e p r e s s u r e of a p p r o x i m a t e l y 0.5 kg/cm 2 and a b a c k - p r e s s u r e of 2.0 kg/cm 2. The o v e r c o n s o l i d a t i o n r a t i o was e q u a l to 7 assu m i n g a maximum i n - s i t u p a s t p r e s s u r e of 3.5 kg/cm 2. Double d r a i n a g e was a l l o w e d f o r a p e r i o d of 24 h o u r s f o r a l l t e s t s . In a p r e l i m i n a r y t e s t , a sample was c o n f i n e d under an e f f e c t i v e p r e s s u r e of 0.5 kg/cm 2 f o r 38 h o u r s , and the po r e p r e s s u r e g e n e r a t e d due to the a r r e s t o f s e c o n d a r y c o n s o l i d a t i o n a t the end of the d r a i n a g e p e r i o d , was o b s e r v e d . F i g u r e 3 shows the amount of pore p r e s s u r e d e v e l o p e d d u r i n g an u n d r a i n e d p e r i o d of about 2000 m i n u t e s . The s m a l l f l u c t u a t i o n i n the measurement may be due p a r t l y to e l e c t r o n i c n o i s e e f f e c t s and p a r t l y to s m a l l t e m p e r a t u r e v a r i a t i o n s . N e v e r t h e l e s s , t h e changes i n t h e s e p o r e p r e s s u r e s a r e s m a l l . A s t a b l e maximum of 0.04 kg/cm 2 was a n t i c i p a t e d f o r the l o n g e s t t i m e of r e l i a b i l i t y of u n d r a i n e d c o n d i t i o n s (10,000 m i n u t e s ) , and i t was c o n s i d e r e d not s u f f i c i e n t to i n f l u e n c e the measurement of pore p r e s s u r e d u r i n g the s h e a r p r o c e s s . Hence, f o r the r e m a i n i n g t e s t s the 24 CN 6 o 6 0 400 800 1200 1600 2000 TIME (min) FIG. 3 PORE PRESSURE GENERATED DUE TO THE ARREST OF SECONDARY COMPRESSION AFTER 38 HRS CONSOLIDATION AT a =0.5 kg/cm2. 25 s h e a r l o a d i n g f o l l o w e d i m m e d i a t e l y a f t e r the c o n s o l i d a t i o n p e r i o d . T h i s p r e l i m i n a r y o b s e r v a t i o n a l s o e n s u r e d t h a t no l e a k a g e w i l l t a k e p l a c e between the c e l l and the sample t h r o u g h the two t h i n s e a l i n g r u b b e r membranes s e p a r a t e d by a c o a t o f h i g h vacuum g r e a s e . In a l l t e s t s , v e r t i c a l c o m p r e s s i o n l o a d i n g w i t h c o n s t a n t c o n f i n i n g p r e s s u r e was imposed i n u n d r a i n e d c o n d i t i o n s w i t h pore p r e s s u r e measurement at t h e base of the s p e c i m e n . The c o n v e n t i o n a l c o n s t a n t r a t e o f s t r a i n c o m p r e s s i o n t e s t s were p e r f o r m e d u s i n g a 1 tonne c a p a c i t y Wykeham F a r r a n c e l o a d i n g m a c h i n e . The r e s u l t s of t h e s e t e s t s were used to c h o o s e the r ange of s t r e s s l e v e l s f o r the c r e e p t e s t s and the r a t e s o f l o a d i n g i n the c o n s t a n t r a t e of l o a d i n g c o m p r e s s i o n t e s t s . In the c o n s t a n t s t r e s s c r e e p t e s t s , the a x i a l f o r c e was a p p l i e d i n s t a n t a n e o u s l y by a <j> 3.81 cm f r i c t i o n l e s s B e l l o f r a m a i r p i s t o n . The c o n s t a n t a i r p r e s s u r e s u p p l y f o r t h e l o a d i n g p i s t o n was c o n t r o l l e d by a p r e c i s i o n F a i r c h i l d r e g u l a t o r . The l o a d from the p i s t o n was t r a n s f e r r e d to t h e sample t h r o u g h a r i g i d m e t a l l i c frame, a l l o w i n g the a d d i t i o n o f d i s c r e t e s m a l l l e a d s h o t as dead w e i g h t s to keep the v e r t i c a l s t r e s s c o n s t a n t w h i l e the sample was c o m p r e s s i n g and the c o r r e s p o n d i n g a r e a was i n c r e a s i n g . In the c o n s t a n t r a t e of l o a d i n g t e s t s , the samples were l o a d e d i n c o m p r e s s i o n under a c o n t i n u o u s l y i n c r e a s i n g a x i a l 26 f o r c e . A <|> 5.08 cm f r i c t i o n l e s s B e l l o f r a m a i r p i s t o n was ' used to a p p l y the l o a d on the sample. The a i r s u p p l y f o r the l o a d i n g p i s t o n was c o n t r o l l e d by a v a r i a b l e speed s e r v o - m o t o r c o u p l e d to a p r e c i s i o n F a i r c h i l d a i r p r e s s u r e r e g u l a t o r . T h i s system e n s u r e d a l i n e a r i n c r e a s e of a i r p r e s s u r e s u p p l y w i t h time r e s u l t i n g i n a c o n s t a n t r a t e of v e r t i c a l l o a d i n g of the sample. The c o n s t a n t l o a d c r e e p t e s t s were p e r f o r m e d s i m i l a r l y to the c o n s t a n t s t r e s s c r e e p t e s t s . The a x i a l l o a d was i n s t a n t a n e o u s l y a p p l i e d and k e p t c o n s t a n t by a c o n t r o l l e d a i r p r e s s u r e s u p p l y a c t i n g i n a <f> 5.08 cm f r i c t i o n l e s s B e l l o f r a m p i s t o n . As c o m p r e s s i o n was t a k i n g p l a c e , the h o r i z o n t a l a r e a o f the sample was i n c r e a s i n g , r e s u l t i n g i n a c o n t i n u o u s l y d e c r e a s i n g c r e e p s t r e s s . A l l t e s t s were p e r f o r m e d i n a c o n s t a n t t e m p e r a t u r e e n v i r o n m e n t (maximum t e m p e r a t u r e v a r i a t i o n of ± 0.25°C) i n o r d e r to e l i m i n a t e the i n f l u e n c e of t e m p e r a t u r e on d e f o r m a t i o n r a t e s and pore p r e s s u r e measurement. A l l measurements were c a r r i e d out e l e c t r o n i c a l l y and t h e t e s t d a t a were a u t o m a t i c a l l y r e c o r d e d on a d i g i t a l c a s s e t t e tape u s i n g a h i g h speed V i d a r D i g i t a l Data A c q u i s i t i o n System. 27 CHAPTER 4 TEST RESULTS 4.1. C o n s t a n t Rate of S t r a i n Shear T a b l e 2 summarizes the r e s u l t s of c o n v e n t i o n a l c o n s t a n t r a t e of s t r a i n s h e a r t e s t s . S i n c e the s t r e s s e s at the end of c o n s o l i d a t i o n a r e s i m i l a r , the o n l y v a r y i n g p a r a m e t e r from t e s t to t e s t i s the d i f f e r e n c e i n the c o n s t a n t r a t e of a x i a l s t r a i n d u r i n g s h e a r l o a d i n g . The i n f l u e n c e of v a r i a t i o n i n t h e c o n s t a n t r a t e of s t r a i n on the r e s u l t i n g s t r e s s - s t r a i n r e s p o n s e of the c l a y i s shown i n F i g u r e 4. I t can be s e e n t h a t the s t r e s s - s t r a i n r e l a t i o n f o r the o v e r c o n s o l i d a t e d Haney c l a y i s dependent on t h e r a t e of s t r a i n . The s t r e s s - s t r a i n r e s p o n s e p r i o r to f a i l u r e - d e f i n e d as the peak d e v i a t o r s t r e s s - becomes s t i f f e r w i t h i n c r e a s i n g c o n s t a n t r a t e of s t r a i n . D i f f e r e n c e s i n the peak d e v i a t o r s t r e s s as h i g h as 30% may be n o t e d f o r a s t r a i n r a t e v a r i a t i o n o f a bout 3 o r d e r s of m a g n i t u d e . I t i s i n t e r e s t i n g to n o t e t h a t the a x i a l s t r a i n at peak d e v i a t o r s t r e s s was e s s e n t i a l l y i n d e p e n d e n t of the r a t e of s t r a i n and was i n the range of 1.2 to 1.5%. The s h a r p d e c r e a s e i n s t r e n g t h p a s t the peak d e v i a t o r s t r e s s i s t y p i c a l of a s e n s i t i v e and o v e r c o n s o l i d a t e d c l a y . S t r a i n i n g i n the p o s t TABLE 2 RESULTS OF UNDRAINED CONSTANT RATE OF STRAIN SHEAR FOR OVERCONSOLIDATED HANEY CLAY Test Number 6-80 2-80 29-80 4-80 8-80 Ax i a l Strain Rate (%/min) 1.03-x 10° 1.37 x 10-1 1.40 x 10" 2 2.59 x 10" 3 8.72 x 10^ Void Ratio: Before consolidation After consolidation 2.025 2.015 2.037 2.020 1.967 1.962 1.953 1.948 1.967 1.967 Consolidaton Stresses: o^(kg/cm2) 5 T 3(kg/cm 2) 0.54 0.50 1.07 0.57 0.49 1.17 0.50 0.41 1.20 0.47 0.46 1.04 0.62 0.50 1.23 Deviatoric Stress (a-j-Og) (kg/cm 2): at e = 0.8% at e = 1.0% at e = 1.2% 1.24 1.28 1.31 1.32 1.08 1.18 1.21 1.21 0.93 1.00 1.06 1.18 0.92 1.01 1.06 1.07 0.88 0.94 0.95 0.95 Maximum Stress Ratio a^/a-j 195.9 OO 7.9 15.4 9.0 Ax i a l Strain e (%): a t ( i l - ^ m a x a t ( al/°3>max 1.41 1.41 1.15 1.15 1.54 1.54 1.15 1.15 1.50 0.93 P.P. Parameter A : a t (£l-°3>max a t ( al/ a3>max 0.38 0.38 0.40 0.40 0.35 0.35 0.36 0.36 0.39 0.42 -29, AXIAL STRAIN e(%) FIG.,4.' INFLUENCE OF RATE OF STRAIN ON UNDRAINED STRESS- STRAIN BEHAVIOR OF OVERCONSOLIDATED HANEY CLAY IN CONSTANT RATE OF STRAIN SHEAR. 30 peak r e g i o n o f a s e n s i t i v e c l a y i s a s s o c i a t e d w i t h t h e c o l l a p s e o f the s t r u c t u r e o f the m a t e r i a l . As a c o n s e q u e n c e , s t r a i n s o f t e n i n g i s i n i t i a t e d , and f o r the c l a y t e s t e d t h i s a p p e a r s to t r i g g e r a t a c r i t i c a l l e v e l o f s t r a i n . S i m i l a r o b s e r v a t i o n s r e g a r d i n g a c r i t i c a l s t r a i n l e v e l have been made f o r o t h e r c l a y s by C o a t e s e t a l . ( 1 2 ) , V a i d and C a m p a n e l l a ( 3 2 ) , and V a i d e t a l ( 3 3 ) . F i g u r e 5 shows the v a r i a t i o n o f p o r e water p r e s s u r e measured a t the base o f the sample. There i s a s h a r p i n c r e a s e i n pore p r e s s u r e r e a c h i n g to a maximum v a l u e c l o s e to the i n i t i a l e f f e c t i v e c o n f i n i n g p r e s s u r e , f o l l o w e d by a r a p i d d e c r e a s e to a more s t a b l e v a l u e . The maximum p o r e water p r e s s u r e o c c u r s a t the same time as the peak d e v i a t o r s t r e s s and a t a c o n s t a n t s t r a i n l e v e l of about 1.0 to 1.5%. The v a r i a t i o n of pore water p r e s s u r e w i t h a x i a l s t r a i n i s c l e a r l y s t r a i n r a t e d e p e n d e n t p r i o r to the peak v a l u e . However, i t a p p e a r s from T a b l e 2 t h a t Skempton 1s pore p r e s s u r e p a r a m e t e r a t f a i l u r e A^ does not show any c l e a r s t r a i n r a t e d e p e n d e n c e . T h i s would i m p l y t h a t d i f f e r e n c e s i n po r e water p r e s s u r e among v a r i o u s t e s t s ( F i g u r e 5) a r e a s s o c i a t e d s o l e l y w i t h the i n f l u e n c e of s t r a i n r a t e on d e v i a t o r s t r e s s e s . The r e s u l t s on T a b l e 2 a l s o i n d i c a t e t h a t the maximum e f f e c t i v e s t r e s s r a t i o i s r e a c h e d a t about the same s t r a i n as the peak d e v i a t o r s t r e s s f o r a l l c o n s t a n t r a t e o f s t r a i n 31 i r i i i i L 2 4 . 6 8 10 AXIAL STRAIN (%) FIG. 5 VARIATION OF EXCESS PORE PRESSURE WITH AXIAL STRAIN FOR UNDRAINED O.C. HANEY CLAY IN CONSTANT RATE OF STRAIN SHEAR. 32 s h e a r t e s t s . The e x t r e m e l y h i g h v a l u e s of t h i s r a t i o a r e a c o n s e q u e n c e o f v e r y low v a l u e s of minor e f f e c t i v e s t r e s s w h i c h , i n t u r n , a r e r e l a t e d to the h i g h p ore p r e s s u r e s g e n e r a t e d a t t h a t t i m e . O t h e r i n v e s t i g a t o r s ( 27) have r e p o r t e d t h a t f o r l i g h t l y o v e r c o n s o l i d a t e d s e n s i t i v e Norwegian c l a y s the p o i n t of maximum e f f e c t i v e s t r e s s r a t i o o c c u r r e d f a i r l y e a r l y i n an u n d r a i n e d t r i a x i a l c o m p r e s s i o n t e s t , about 2% a x i a l s t r a i n , and the p o i n t of maximum d e v i a t o r s t r e s s was r e a c h e d l a t e r i n the t e s t a t an a x i a l s t r a i n d e p e n d i n g on the p r e c o n s o l i d a t i o n r a t i o . F o r t h a t s t u d y the o v e r c o n s o l i d a t e d s t a t e o f the c l a y was o b t a i n e d by c o n s o l i d a t i n g a l l the s a m p l e s , under the c o n d i t i o n s of no l a t e r a l s t r a i n , to a same s t r e s s l a r g e l y i n e x c e s s o f t h e a p p a r e n t p r e c o n s o l i d a t i o n p r e s s u r e , t h e n r e b o u n d i n g them to lo w e r c o n s o l i d a t i o n s t r e s s e s . As n o t e d p r e v i o u s l y t h i s p r o c e d u r e r e s u l t s i n a c o l l a p s e of the n a t u r a l s e n s i t i v e s t r u c t u r e of the c l a y . T h e r e f o r e , a p o s s i b l e e x p l a n a t i o n f o r the c o n t r a s t i n g b e h a v i o r o f Haney c l a y f o r which b o t h maximum d e v i a t o r s t r e s s and maximum e f f e c t i v e s t r e s s r a t i o o c c u r a t an e q u a l s m a l l l e v e l o f s t r a i n may be the u n a l t e r e d s e n s i t i v e n a t u r e of t h e c l a y s t r u c t u r e . The s t r a i n r a t e d ependence o f the u n d r a i n e d s t r e n g t h o f the c l a y t e s t e d i s b e t t e r shown i n the s e m i - l o g a r i t h m i c p l o t o f F i g u r e 6. The r e s u l t s o f t h e s e t e s t s i n d i c a t e a l i n e a r i n c r e a s e i n u n d r a i n e d s t r e n g t h w i t h the l o g a r i t h m o f a x i a l  34 s t r a i n r a t e of the o r d e r of 8% per l o g c y c l e . S i m i l a r i n c r e a s e s r a n g i n g between 5 and 10% per l o g c y c l e of s t r a i n r a t e were r e p o r t e d by s e v e r a l o t h e r r e s e a r c h e r s f o r o t h e r c l a y s , b o t h n o r m a l l y c o n s o l i d a t e d and o v e r c o n s o l i d a t e d (1,2,3,5,8,12,14,23,24,30,32,33). 4.2. C o n s t a n t S t r e s s Creep The r e s u l t s of t h i s s e r i e s of t e s t s a r e summarized i n T a b l e 3. S i n c e the end of c o n s o l i d a t i o n c o n d i t i o n s a r e e s s e n t i a l l y i d e n t i c a l , the o n l y v a r i a b l e from t e s t to t e s t i s the c o n s t a n t d e v i a t o r s t r e s s d u r i n g c r e e p l o a d i n g . The d e v elopment of a x i a l s t r a i n w i t h time a t v a r i o u s l e v e l s of c r e e p s t r e s s i s shown i n F i g u r e 7. F o r a s t r e s s l e v e l o f 0.83 kg/cm 2 a c o n t i n u o u s l y d e c r e a s i n g d e f o r m a t i o n r a t e was o b s e r v e d u n t i l the maximum time of r e l i a b i l i t y of u n d r a i n e d c o n d i t i o n s when the t e s t was t e r m i n a t e d (10,000 m i n u t e s ) . I t i s b e l i e v e d t h a t t h i s s t r e s s l e v e l a p p r o a c h e s t h e upper y i e l d s t r e n g t h v a l u e and t h a t no f a i l u r e w i l l d e v e l o p w i t h time under s t r e s s e s s m a l l e r than t h i s v a l u e . F o r the h i g h e r s t r e s s l e v e l s , the samples p r o g r e s s i v e l y s t r a i n e d w i t h time u n t i l e v e n t u a l r u p t u r e . I t i s c l e a r from F i g u r e 7 t h a t the time of f a i l u r e ( t o t a l c o l l a p s e of the c l a y sample) under a s u s t a i n e d s t r e s s d e c r e a s e s w i t h i n c r e a s i n g c r e e p s t r e s s . F o r the t e s t a t a s t r e s s l e v e l of 0.70 kg/cm 2, a s t e p change i n c r e e p s t r e s s was s u d d e n l y a p p l i e d a f t e r an e l a p s e d TABLE 3 RESULTS OF UNDRAINED CONSTANT STRESS CREEP FOR OVERCONSOLIDATED HANEY CLAY Test Number 19 r80 21-80 51-81 45-81 27-80 24-80 28-80 18-80 31-81 Deviatoric Stress Level (kg/cm 2) 1.15 1.13 1.06 1.01 0.98 0.95 0.93 0.83 0.70 1.08 Void Ratio: Before consolidation After consolidation 1.878 1.876 1.822 1.822 1.845 1.852 1.774 1.771 1.883 1.888 1.768 1.811 1.952 1.996 1.755 1.774 1.891 1.927 Consolidation stresses: a1 (kg/cm 2) 0 3 (kg/cm 2) O1/O3 0.52 0.51 1.02 0.50 0.48 1.05 . 0.57 0.51 1.11 0.51 0.50 1.02 0.51 0.49 . 1.04 0.52 0.49 1.05 0.53 0.50 1.06 0.49 0.49 1.00 0.51 0.50 1.02 Axial Strain Rate e- (%/min): at "e = 0.8% at e = 1.0% at e = 1.2% • e . min 2.2xl0 - 2 2.1 x10~ 2 1.5 X10~ 2 1.4 X10 _ 1 2.5xl0~ 2 l . l x 1 0 ~ 2 9.6 X10~ 3 6.5 X10~ 2 9.4xl0 - 3 2.7 X10~ 3 2.0 X10~ 3 1.7 X10 - 2 3.1xl0 - 3 1.0 X10 - 3 4.8 x10 - l t 6.4 X10 - 3 l . l x I O - 3 5.0 x10 _ l t 2.5 x10 _ t t 3.5X10":3 2.4xl0 _ i t. 1.4 x10 _ t f 1.2 x10 - l f 6.7 x10 - l t 1.2X10-11 8.3 X10 - 5 2.7 X10~ 5 9.0xl0- 6 8.5 X10 - 6 • Time at e m j [ n (minutes) 19 46 160 800 .1000 1300* 2100 8000 Ax i a l Strain at e m i n (%) 1.42 1.30 1.45 1.55 1.55 1.60 1.10 1.00 • P.P. Parameter A at £ m • mm 0.39 0.35 0.36 • 0.29 0.28 0.45 0.38 0.29 no rupture  37 i t ime of about 80 m i n u t e s , b r i n g i n g the s t r e s s l e v e l to 1.08 kg/cm 2. As a r e s u l t , the f a i l u r e o c c u r r e d a t a time i n t e r m e d i a t e between the time of r u p t u r e i n t e s t s at s t r e s s l e v e l s of 1.06 and 1.13 kg/cm 2, which s t r a d d l e the s t r e s s l e v e l 1.08 kg/cm 2. Such a r e s u l t would be a n t i c i p a t e d i f the h y p o t h e s i s t h a t t h e r e l a t i o n s h i p between c u r r e n t s t r e s s , s t r a i n , and s t r a i n r a t e i s assumed u n i q u e and i n d e p e n d e n t of t h e time l o a d i n g h i s t o r y used to b r i n g the c l a y to r u p t u r e . The e x i s t e n c e of s u c h a r e l a t i o n s h i p f o r the c l a y t e s t e d i s shown i n C h a p t e r 5. The t i m e h i s t o r y of d e f o r m a t i o n r a t e s i n c o n s t a n t s t r e s s c r e e p t e s t s i s i l l u s t r a t e d i n F i g u r e 8. T h i s p l o t was d e r i v e d by d i f f e r e n t i a t i n g s t r a i n - t i m e c u r v e s of F i g u r e 7. For samples which e v e n t u a l l y f a i l e d , the d e f o r m a t i o n r a t e i n i t i a l l y d e c r e a s e d u n t i l a minimum v a l u e was r e a c h e d b e f o r e i t s s u b s e q u e n t a c c e l e r a t i o n l e a d i n g to r u p t u r e . For the sample a t the upper y i e l d s t r e s s l e v e l , a c o n t i n u o u s l y d e c r e a s i n g d e f o r m a t i o n r a t e w i t h time was o b s e r v e d . As p o i n t e d out by o t h e r r e s e a r c h e r s (4,9,15,17,21,25,28,29,32) a time r e g i o n , termed s e c o n d a r y c r e e p , o v e r which the r a t e of s t r a i n i s e s s e n t i a l l y c o n s t a n t does not seem to e x i s t . I t i s i n t e r e s t i n g to n o t e on T a b l e 3 t h a t , i r r e s p e c t i v e o f the c r e e p s t r e s s l e v e l , t h e f a i l u r e of the c l a y samples - d e f i n e d at minimum s t r a i n r a t e - o c c u r r e d at an a x i a l s t r a i n l e v e l r a n g i n g between 1.0 and 1.6%. T h i s s t r a i n l e v e l i s i d e n t i c a l to the a x i a l s t r a i n a t peak d e v i a t o r s t r e s s i n t h e ,•-38 . 10= - i L 10 -2 •H 53 H H 3 10 -3 10 -4 10 -5 1.15 1.13 1.07 1.01 0.98 o±-a3 (kg/cm ) = 0.95 y 0.93 10 100 ELAPSED TIME t (min) 1000 FIG. 8. AXIAL STRAIN RATE VERSUS TIME RELATIONSHIP IN CONSTANT STRESS CREEP. 39 c o n s t a n t r a t e of s t r a i n s h e a r t e s t s d e s c r i b e d i n t h e p r e v i o u s s e c t i o n . T h i s r e s u l t i s a f u r t h e r i n d i c a t i o n of the e x i s t e n c e of a c r i t i c a l s t r a i n l e v e l beyond which t h e s t r u c t u r e of t h i s p a r t i c u l a r c l a y c o l l a p s e s and f a i l u r e e n s u e s . The v a r i a t i o n of the pore water p r e s s u r e , measured a t the base of the sample, w i t h t i m e i s shown i n F i g u r e 9. E s s e n t i a l l y , t h e r e was a sudden r i s e of pore p r e s s u r e a t the a p p l i c a t i o n of the a x i a l s t r e s s , f o l l o w e d by a slow d e c r e a s e u n t i l c o l l a p s e of the sample o c c u r r e d , when the p o r e p r e s s u r e d r o p p e d v e r y r a p i d l y to a s m a l l s t a b l e v a l u e . The v e r t i c a l a r r o w s on the p l o t of the p o r e p r e s s u r e v e r s u s time i n d i c a t e when the minimum r a t e of a x i a l s t r a i n was r e a c h e d . In c r e e p t e s t s , the pore p r e s s u r e b e h a v i o r does not g i v e any i n d i c a t i o n of imp e n d i n g f a i l u r e o f the sample. The p o r e p r e s s u r e i s s t i l l s t e a d i l y d e c r e a s i n g s l o w l y when r u p t u r e i s about to t a k e p l a c e . F o r t h e s t e p - c r e e p t e s t s t a r t i n g w i t h a c o n s t a n t s t r e s s of 0.70 kg/cm 2, the pore water p r e s s u r e was r e a c h i n g a s t a b l e v a l u e f o l l o w i n g the i n i t i a l r i s e when i t was ' i n s t a n t a n e o u s l y ' b r o u g h t up to a h i g h e r v a l u e due to t h e s t e p change i n c r e e p s t r e s s . C o n s e q u e n t l y , the p o r e p r e s s u r e s t a r t e d d e c r e a s i n g u n t i l f a i l u r e o c c u r r e d . The c u r v e f o r t h i s t e s t shows c l e a r l y the sudden i n i t i a l r i s e of pore p r e s s u r e a t the a p p l i c a t i o n of c r e e p s t r e s s as t h i s p a r t was m i s s i n g i n the p l o t of the r e m a i n i n g c o n s t a n t s t r e s s c r e e p 10 100 1000 ELAPSED TIME (min) FIG. 9 PORE PRESSURE RESPONSE AT THE BASE OF THE SAMPLE WITH TIME IN CONSTANT STRESS CREEP ON O.C. HANEY CLAY. ^ O 41 t e s t s b e c a u s e o f the low f r e q u e n c y of r e c o r d i n g a t the s t a r t o f l o a d i n g . I t i s a l s o i n t e r e s t i n g to note t h a t , i n t h e s e t e s t s , the e f f e c t i v e s t r e s s r a t i o a /a^ has a s i m i l a r v a r i a t i o n w i t h t i m e as the pore p r e s s u r e s i n c e b o t h major and minor t o t a l s t r e s s e s a r e m a i n t a i n e d c o n s t a n t d u r i n g s h e a r . Hence, t h e maximum e f f e c t i v e s t r e s s r a t i o w i l l o c c u r a t the same time as the maximum pore water p r e s s u r e , t h a t i s at the v e r y b e g i n n i n g o f the a p p l i c a t i o n of the c r e e p s t r e s s . F i g u r e 10 shows the v a r i a t i o n of r u p t u r e l i f e ( t i m e e l a p s e d from the i n i t i a t i o n o f c r e e p u n t i l f i n a l c o l l a p s e ) w i t h c r e e p s t r e s s . I t may be seen t h a t a g i v e n l e v e l o f s t r e s s c a n n o t be s u s t a i n e d by the c l a y f o r more t h a n a f i x e d time w i t h o u t u n d e r g o i n g c o l l a p s e . The p l o t of F i g u r e 10 r e p r e s e n t s , i n f a c t , the r e d u c t i o n o f u n d r a i n e d s t r e n g t h w i t h time f o r t h i s p a r t i c u l a r c l a y . In F i g u r e 11, the minimum s t r a i n r a t e s a r e p l o t t e d a g a i n s t the c o r r e s p o n d i n g s t r e s s l e v e l s . The v a r i a t i o n o f the l o g a r i t h m o f minimum s t r a i n r a t e shows e s s e n t i a l l y a l i n e a r i n c r e a s e w i t h s t r e s s l e v e l . C o m p a r i s o n of t h i s r e l a t i o n s h i p w i t h the v a r i a t i o n of u n d r a i n e d s t r e n g t h w i t h r a t e s of d e f o r m a t i o n from c o n s t a n t s t r a i n r a t e s h e a r t e s t s w i l l be d i s c u s s e d l a t e r .  I I I I I I I i o " 5 i o - 4 i o - 3 i o " 2 i o - 1 10° MINIMUM AXIAL STRAIN RATE e . (%/min) mm FIG. 11. VARIATION OF UNDRAINED STRENGTH WITH MINIMUM RATE OF STRAIN IN CONSTANT STRESS CREEP. OJ 44 4.3. C o n s t a n t Rate of L o a d i n g Shear I n t h i s s e r i e . 8 o f t e s t s , t h e samples were l o a d e d by l i n e a r l y i n c r e a s i n g the a x i a l f o r c e w i t h t i m e . The r e s u l t s o f t h e s e t e s t s a r e summarized i n T a b l e 4. As b e f o r e , the c o n d i t i o n s o f the samples a r e e s s e n t i a l l y i d e n t i c a l a t the end of the c o n s o l i d a t i o n p e r i o d , and the o n l y v a r y i n g p a r a m e t e r d u r i n g s h e a r i s the r a t e of l o a d i n g . The e f f e c t o f r a t e o f l o a d i n g on the s t r e s s - s t r a i n r e s p o n s e o f the c l a y i s shown i n F i g u r e 12. For t h i s t y p e o f t e s t s , t h e r u p t u r e of the samples o c c u r r e d a t a v e r y l a r g e r a t e o f d e f o r m a t i o n . S i n c e o n l y the maximum d e v i a t o r s t r e s s and the s t r e s s - s t r a i n r e l a t i o n p r i o r to f a i l u r e were o f p a r t i c u l a r i n t e r e s t i n t h i s s t u d y , the c o n t i n u o u s measurement of the p o s t - p e a k b e h a v i o r was not r e c o r d e d . In some o f t h e s e t e s t s , the v a r i a b l e speed s e r v o - m o t o r used to r e g u l a t e the c o n t i n u o u s i n c r e a s e of l o a d i n g a i r - p r e s s u r e was t u r n e d on a t the s t a r t of s h e a r . A s l i g h t d e l a y was e x p e r i e n c e d i n the system to r e a c h a c o n s t a n t r a t e . T h i s may a c c o u n t f o r some d i s c r e p a n c y i n the r e s u l t s a t v a r i o u s l o a d i n g r a t e s . In s p i t e o f t h e narrow r a n g e of v a r i a t i o n i n the r a t e of l o a d i n g , a t e n d e n c y to an i n c r e a s e i n s t i f f n e s s and s t r e n g t h w i t h i n c r e a s i n g speed o f t e s t i n g may be n o t e d . The e f f e c t o f r a t e o f l o a d i n g on the c l a y b e h a v i o r , as shown i n F i g u r e 13, i n d i c a t e s a d e f i n i t e i n c r e a s e i n u n d r a i n e d s t r e n g t h w i t h i n c r e a s i n g l o g a r i t h m of the l o a d i n g TABLE 4 RESULTS OF UNDRAINED CONSTANT RATE OF LOADING SHEAR FOR OVERCONSOLIDATED HANEY CLAY Test Number Loading Rate (kg/min) Void Ratio: Before consolidation After consolidation Consolidation stresses: Oj_ (kg/cm 2) a 3 (kg/cm 2) a1/a3 Deviatoric Stress o^-a.j(kg/cm2) at e = 0.8% at e = 1.0% at e = 1.2% ( 0l- a3>max Ax i a l Strain Rate e (%/min): at e = 0.8% at e = 1.0% at e = 1.2% Maximum Stress Ratio (o-^/o^) max Axial Strain (%): at (°r°$)maK at (a /a ) 1 3 max P.P. Parameter A: at at (21-^3) max max 13-80 12-80 9-80 44-81 10-80 46-81 14-80. 52-81 2.54X101 2.74x10° 2.6xl0 _ 1 1.6xl0 _ 1 1.4xl0 _ 1 4.9xl0 - 2 1.6xl0 - 2 8.3xl0 - 3 1.963 1.997 1.990 1.804 1.998 1.763 1.986 2.054 1.966 1.984 1.996 1.939 2.002 1.878 1.975 1.987 0.50 0.48 0.54 0.52 0.44 0.52 0.53 0.53 0.42 0.46 0.43 0.50 0.41 0.50 0.54 0.51 1.21 1.06 1.25 1.04 1.06 1.04 0.98 1.03 1.45 1.37 1.10 0.95 0.99 1.02 1.08 0.90 - 1.40 1.16 1.04 1.09 1.12 - 1.01 - ' - 1.19 1.10 1.16 1.14 - 1.07 .1.52 1.40 1.24 1.14 1.22 1.15 1.12 1.02 6.4X101 6.5X10"2 3.6X10"2 2.9*10 - 2 4.7X10"3 6.0 X10 - 3 1.6*10 _ 3 - l.OxlO 1 9.0xl0" 2 4.8X10" 2 3.8X10" 2 1.5X10"2 - 2.2X10 - 3 — — 1.5*10 - 1 6.5><10~2 5.0 X10 - 2 5.0 X10" 2 - 4.2 X10 - 3 21.06 10.37 13.73 50.00 11.23 33.22 10.87 23.66 0.96 1.06 1.60 1.52 1.44 1.33 0.89 1.02 0.96 1.06 1.60 1.52 1.44 1.17 0.89 1.59 0.23 0.29 0.28 0.41 0.31 0.40 0.37 0.45 0.23 0.29 0.28 0.41 0.31 0.40 0.37 0.39 46 0.4 0.8 1.2 1.6 2.0 AXIAL STRAIN (%) FIG. 12 EFFECT OF RATE OF LOADING ON THE STRESS-STRAIN RESPONSE OF O.C. HANEY CLAY IN CONSTANT RATE OF LOADING SHEAR. 10 2 10 1 10° i o 1 RATE OF AXIAL LOADING (kg/min) FIG. 13 VARIATION OF UNDRAINED STRENGTH OF O.C. HANEY CLAY' WITH.RATE OF LOADING IN CONSTANT RATE OF LOADING SHEAR. 48 r a t e . T h i s i n c r e a s e i n u n d r a i n e d s t r e n g t h i s a p p r o x i m a t e l y 8% per l o g c y c l e i n r a t e o f l o a d i n g . T h i s b e h a v i o r i s s i m i l a r to the o b s e r v a t i o n s r e p o r t e d by C a s a g r a n d e and W i l s o n on s a t u r a t e d M e x i c o C i t y c l a y (10) and by V a i d and C a m p a n e l l a on n o r m a l l y c o n s o l i d a t e d Haney c l a y ( 3 2 ) . 4.4. C o n s t a n t Load Creep Samples i n t h i s s e r i e s of t e s t s were i n s t a n t a n e o u s l y l o a d e d w i t h p r e d e t e r m i n e d l o a d s which were h e l d c o n s t a n t w i t h t i m e . As the d e f o r m a t i o n p r o g r e s s e d , the sample a r e a i n c r e a s e d , t h e r e b y r e s u l t i n g i n a c o n t i n u o u s d e c r e a s e i n c r e e p s t r e s s . T a b l e 5 summarizes the r e s u l t s o f c o n s t a n t l o a d c r e e p t e s t s . In F i g u r e 14 i t can be seen t h a t the d e f o r m a t i o n b e h a v i o r w i t h t i m e under c o n s t a n t l o a d i s s i m i l a r to t h a t under c o n s t a n t s t r e s s ( F i g u r e 7 ) . Due to the low l e v e l o f s t r a i n a t f a i l u r e , the s t r e s s d e c r e a s e d u r i n g c r e e p was not s i g n i f i c a n t . I t i s most l i k e l y t h a t the t e s t at the i n i t i a l d e v i a t o r s t r e s s l e v e l of 0.92 kg/cm 2 w i l l r e s u l t i n a c o l l a p s e of the sample had s u f f i c i e n t time f o r c r e e p been a l l o w e d . T h i s argument i s based on the f a c t t h a t the sample had a l r e a d y s t r a i n e d i n e x c e s s o f 1.0% a x i a l s t r a i n when the t e s t was t e r m i n a t e d ; and as shown e a r l i e r , a c r i t i c a l l e v e l o f a x i a l s t r a i n o f about 1.2 to 1.5% would r e s u l t i n sample u n d e r g o i n g f a i l u r e . TABLE 5 RESULTS OF UNDRAINED CONSTANT LOAD CREEP FOR OVERCONSOLIDATED HANEY CLAY Test Number 41-81 39-81 42-81 43-81 25-80 26-80 17-80 I n i t i a l Deviatoric Stress Level (kg/cm ) 1.20 1.18 1.14 1.10 1.02 0.92 0.88 1.01 Void Ratio: Before consolidation After consolidation 1.907 2.033 1.991 2.160 1.860 2.016 1.825 1.814 1.941 1.940 1.989 1.993 1.806 1.814 Consolidation stresses: 6^ (kg/cm 2) 0 3 (kg/cm 2) 0.49 . 0.48 1.02 0.51 0.49 1.02 0.49 0.48 1.03 0.54 0.49 1.11 0.52 0.50 1.04 0.49 ' 0.50 0.97 0.51 0.50 1.02 Deviatoric Stress 0-^-0^(kg/cm2 ) at e = 0.8% at e = 1.0% at e = 1.2% 1.20 1.20 1.17 1.17 1.14 1.14 1.14 1.10 1.10 1.10 1.02 1.02 1.02 0.92 0.92 A x i a l Strain Rate e (%/min): at e = 0.8% at e = 1.0% at e =1.2% • emin 8.0 X 10 - 1 2.1xl0 _ 1 l . l x l O - 1 2.8 X 10 _ 1 l . l x l O - 1 6.0x l0 - 2 3.8xl0 _ 1 5.0X10~2 l .Ox lO" 2 l . O x l O - 2 l . l x l O - 1 9.0 X 10 - 3 9.5xl0 - 3 6.0xlO - 3 3.3x l0 - 2 5.6 X 10 - 3 1.5xl0 - 3 1.3xl0 - 3 5.4X10-1* 9.0 X 10 - 5 8.4x l0 - 5 Time at e .„ (minutes) min ^ ' 3 6 20 70 215 2900 A x i a l Strain at e . (%) min v ' 1.40 • 1.50 1.30 1.50 1.40 1.00 • P.P. Parameter A at e min 0.35 0.33 0.33 0.33 0.35 0.38 no rupture 1 1 I I 10 100 1000 ELAPSED TIME t (min) | FIG. 14. AXIAL STRAIN VERSUS TIME RELATIONSHIP IN CONSTANT LOAD CREEP. ' ' ° 51 By d i f f e r e n t i a t i n g s t r a i n time c u r v e s of F i g u r e 14, t h e t i m e h i s t o r y o f c r e e p r a t e s was o b t a i n e d i n F i g u r e 15. The d e f o r m a t i o n r a t e s i n i t i a l l y d e c r e a s e d u n t i l a minimum v a l u e b e f o r e f i n a l l y i n c r e a s i n g , thus s i g n a l i n g the o n s e t of f a i l u r e . I t may be n o t e d t h a t the shape o f the a x i a l s t r a i n r a t e v e r s u s l o g a r i t h m of time r e l a t i o n s h i p under the s t r e s s l e v e l of 0.92 kg/cm 2 does p o i n t to the l i k e l i h o o d of r e a c h i n g the minimum c r e e p r a t e , and then e v e n t u a l f a i l u r e . F i g u r e 16 shows the v a r i a t i o n of r u p t u r e l i f e ( t i m e e l a p s e d from the i n i t i a t i o n o f c r e e p u n t i l f i n a l c o l l a p s e ) w i t h i n i t i a l c r e e p s t r e s s . I t can be seen t h a t t h i s r e l a t i o n s h i p i s a l s o v e r y s i m i l a r to the r e s u l t s from c o n s t a n t s t r e s s c r e e p i n F i g u r e 10. In F i g u r e 17, the minimum s t r a i n r a t e s a r e p l o t t e d a g a i n s t the s t r e s s l e v e l s at the s t a r t of s h e a r . The v a r i a t i o n o f the l o g a r i t h m of minimum s t r a i n r a t e s i s e s s e n t i a l l y a l i n e a r i n c r e a s e w i t h i n i t i a l s t r e s s l e v e l s . R e s u l t s shown i n F i g u r e 16 and F i g u r e 17 would be e x p e c t e d to be s i m i l a r to t h o s e f o r c o n s t a n t s t r e s s c r e e p , s i n c e c r e e p s t r e s s e s d i d not v a r y e x c e s s i v e l y i n c o n s t a n t l o a d c r e e p due to low l e v e l s o f a x i a l s t r a i n a c c u m u l a t i o n . I . I 1 10 100 1000 ELAPSED TIME t (min) FIG. 15. AXIAL STRAIN RATE VERSUS TIME RELATIONSHIP IN CONSTANT LOAD CREEP. FIG. 16.' INITIAL DEVIATOR STRESS VERSUS TIME TO FAILURE RELATIONSHIP IN CONSTANT LOAD CREEP. 1.2 0.8 0.4 MINIMUM AXIAL STRAIN RATE e . (%/min) min FIG. 17. INITIAL DEVIATOR STRESS VERSUS MINIMUM AXIAL STRAIN RATE RELATIONSHIP IN CONSTANT LOAD CREEP. 55 CHAPTER 5 CORRELATIONS OF RESULTS FROM TESTS WITH VARIOUS TIME LOADING HISTORIES 5.1. S t r e s s - S t r a i n - S t r a i n Rate R e l a t i o n s h i p In o r d e r to e s t a b l i s h c o r r e l a t i o n s of r e s u l t s from the t e s t s w i t h the d i f f e r e n t time l o a d i n g h i s t o r i e s c o n s i d e r e d i n t h i s i n v e s t i g a t i o n , the r a t e o f s t r a i n i s used as a u n i f y i n g v a r i a b l e among the v a r i o u s t e s t s . The u n i q u e n e s s of t h e r e l a t i o n s h i p between c u r r e n t s t r e s s and c u r r e n t s t r a i n r a t e a t any g i v e n l e v e l of s t r a i n d u r i n g s h e a r i s assumed. T h i s i s i d e n t i c a l to ass u m i n g the v a l i d i t y of the e q u a t i o n o f s t a t e i n i t s s t r a i n h a r d e n i n g f o r m u l a t i o n : e = f ( o , e ) ( 3 ) where: e = c r e e p r a t e o r a x i a l s t r a i n r a t e e = c r e e p d e f o r m a t i o n or a x i a l s t r a i n 0 = c r e e p s t r e s s or d e v i a t o r s t r e s s In o f t h i s b o t h e x p r e s s i o n , the c u r r e n t the c u r r e n t r a t e of s t r a i n i s a f u n c t i o n s t r e s s and s t r a i n and i s i n d e p e n d e n t o f 56 the p a s t s t r a i n r a t e h i s t o r y . A d i r e c t r e l a t i o n s h i p may be e s t a b l i s h e d between s t r a i n r a t e and s t r e s s p r o v i d e d t h a t the s t r a i n l e v e l i s m a i n t a i n e d f i x e d to g i v e n v a l u e s . S i n c e the s t r a i n l e v e l i s e s s e n t i a l l y the same a t maximum d e v i a t o r s t r e s s i n c o n s t a n t r a t e of s t r a i n s h e a r and a t minimum s t r a i n r a t e i n c o n s t a n t s t r e s s c r e e p , t h e f i r s t p r o o f of the v a l i d i t y of the p r o p o s e d e q u a t i o n of s t a t e i s a t t e m p t e d between the r e s u l t s o f c o n s t a n t s t r a i n r a t e s h e a r and c o n s t a n t s t r e s s c r e e p t e s t s i n terms of u n d r a i n e d s t r e n g t h v e r s u s s t r a i n r a t e . F i g u r e 18 i l l u s t r a t e s s u c h a c o r r e l a t i o n . An e x c e l l e n t agreement may be seen to e x i s t between the r e s u l t s of t h e s e two t y p e s of t e s t s . As m e n t i o n e d p r e v i o u s l y , c o m p a r a b l e r e s u l t s have been o b t a i n e d by o t h e r i n v e s t i g a t o r s f o r b o t h n o r m a l l y c o n s o l i d a t e d and o v e r c o n s o l i d a t e d c l a y s (15,32,33) as w e l l as f o r o t h e r e n g i n e e r i n g m a t e r i a l s ( 1 6 , 2 2 ) . A s i m i l a r c o r r e l a t i o n may be a t t e m p t e d among a l l the t e s t s i n t h i s i n v e s t i g a t i o n i n terms o f d e v i a t o r s t r e s s v e r s u s a x i a l s t r a i n r a t e a t f i x e d l e v e l s of s t r a i n p r i o r to f a i l u r e . These a r e shown i n F i g u r e s 19a, 19b and 19c f o r t h e c h o s e n s t r a i n l e v e l s o f r e s p e c t i v e l y 0.8, 1.0 and 1.2%. F o r each c h o s e n s t r a i n l e v e l , i t may be n o t e d t h a t t h e d a t a p o i n t f r o m t e s t s w i t h a v a r i e t y o f time l o a d i n g h i s t o r i e s f a l l e s s e n t i a l l y on the same c u r v e , thus s u p p o r t i n g the v a l i d i t y o f the e q u a t i o n o f s t a t e . A l l the c o r r e l a t i o n s a r e 10 5 10 4 10 3 10 2 10 1 10° AXIAL STRAIN RATE e (%/min) FIG. 18. COMPARISON OF STRAIN RATE DEPENDENCE OF UNDRAINED STRENGTH IN CONSTANT RATE OF STRAIN SHEAR AND CONSTANT STRESS CREEP. Ln 1 1 I J I I J i o " 5 i o " 4 i o - 3 i o - 2 i o " 1 10° AXIAL STRAIN RATE e (%/min) FIG . I 9 a - - . , DEVIATOR STRESS VERSUS AXIAL STRAIN RATE AT EQUAL STRAIN LEVEL RELATIONSHIP IN CONSTANT RATE OF STRAIN SHEAR, CONSTANT STRESS CREEP, CONSTANT RATE OF LOADING SHEAR, AND U i. CONSTANT LOAD CREEP. co i 1 r AXIAL STRAIN RATE e (%/min) FIG. 19b.-, DEVIATOR STRESS VERSUS AXIAL STRAIN RATE AT EQUAL STRAIN LEVEL RELATIONSHIP IN CONSTANT RATE OF STRAIN SHEAR, CONSTANT STRESS CREEP, CONSTANT RATE OF LOADING SHEAR, AND CONSTANT LOAD CREEP. 10 -5 10 -4 ,-3 1 .-2 10 " 10 AXIAL STRAIN RATE e (%/min) 10 -1 10 FIG. 19c. DEVIATOR STRESS VERSUS AXIAL STRAIN RATE AT EQUAL STRAIN LEVEL RELATIONSHIP IN CONSTANT RATE OF STRAIN SHEAR, CONSTANT STRESS CREEP, CONSTANT RATE OF LOADING SHEAR, AND CONSTANT LOAD CREEP. ov, o 61 summarized i n F i g u r e 20, and i t may be seen t h a t t h e e s s e n t i a l l y l i n e a r r e l a t i o n s h i p o b t a i n e d between d e v i a t o r s t r e s s and l o g a r i t h m of the a x i a l s t r a i n r a t e , a t each f i x e d l e v e l of a x i a l s t r a i n , has a p p r o x i m a t e l y the same s l o p e as the v a r i a t i o n of u n d r a i n e d s t r e n g t h w i t h l o g s t r a i n r a t e . A s i m i l a r p l o t was p r e s e n t e d by B e r r e and B j e r r u m i n a s t u d y of a p l a s t i c c l a y from Drammen (2).' However, t h a t s t u d y was r e s t r i c t e d to t r i a x i a l c o m p r e s s i o n t e s t s u s i n g c o n v e n t i o n a l c o n s t a n t r a t e of s t r a i n t e s t s o n l y . T h i s d i a g r a m e s s e n t i a l l y shows t h a t the r a t e o f s h e a r s t r a i n i n c r e a s e s e x p o n e n t i a l l y w i t h an i n c r e a s e i n the s h e a r s t r e s s p r o v i d e d the change i n s t r a i n i s i n s i g n i f i c a n t , and t h i s i n d e p e n d e n t l y of the time l o a d i n g h i s t o r y used to s h e a r the c l a y i n u n d r a i n e d t r i a x i a l c o m p r e s s i o n . 5.2. Comparison w i t h the B e h a v i o r of N o r m a l l y C o n s o l i d a t e d Haney c l a y . The s t r e s s - s t r a i n and s t r e n g t h b e h a v i o r o f Haney c l a y were p r e v i o u s l y i n v e s t i g a t e d i n the n o r m a l l y c o n s o l i d a t e d s t a t e by V a i d and C a m p a n e l l a ( 3 2 ) . I t would be i n t e r e s t i n g to compare t h e d a t a o b t a i n e d from t h a t s t u d y w i t h the p r e s e n t r e s u l t s on o v e r c o n s o l i d a t e d Haney c l a y . T h i s i s p a r t i c u l a r l y i m p o r t a n t b e c a u s e of the s e n s i t i v e n a t u r e of the c l a y u s e d . C o n s o l i d a t i o n o f a s e n s i t i v e c l a y p a s t the a p p a r e n t p r e c o n s o l i d a t i o n p r e s s u r e i n t o the n o r m a l l y c o n s o l i d a t e d T FIG." 20 STRESS-STRAIN-STRAIN RATE RELATIONSHIP FOR UNDRAINED TRIAXIAL COMPRESSION ON O.C. HANEY CLAY USING VARIOUS TIME LOADING HISTORIES. 63 r e g i o n i s a s s o c i a t e d w i t h a r a d i c a l change i n i t s s t r u c t u r e . Thus the b e h a v i o r of the n a t u r a l c l a y i n i t s o v e r c o n s o l i d a t e d s t a t e may, b e c a u s e o f the u n a l t e r e d s t r u c t u r e , r e f l e c t t i m e d e pendent c h a r a c t e r i s t i c s s i g n i f i c a n t l y d i f f e r e n t from t h o s e a f t e r i t has been d e s t r u c t u r e d upon normal c o n s o l i d a t i o n i n the l a b o r a t o r y . I t may be p o i n t e d out t h a t such a c o m p a r i s o n c a n o n l y be q u a l i t a t i v e b e c a u s e o f the s i g n i f i c a n t d i f f e r e n c e i n the b a t c h e s o f the c l a y used i n t h i s s t u d y and t h a t used i n V a i d and C a m p a n e l l a s t u d y . The n o r m a l l y c o n s o l i d a t e d c l a y had a n a t u r a l water c o n t e n t o f about 40%, a l i q u i d l i m i t o f 44%, a p l a s t i c l i m i t of 26%, and a c l a y c o n t e n t ( l e s s t h a n 0.002 mm) o f a p p r o x i m a t e l y 50%. A l l t h e s e p h y s i c a l p r o p e r t i e s p r e s e n t e d h i g h e r v a l u e s f o r the c l a y t e s t e d i n the p r e s e n t s t u d y , as shown p r e v i o u s l y on T a b l e 1. The o c c u r r e n c e o f u n i f o r m t h i n l a y e r s o f o r g a n i c m a t e r i a l s i n the p r e s e n t c l a y i s a f u r t h e r i n d i c a t i o n t h a t t h i s c l a y was o b t a i n e d from a d i f f e r e n t h o r i z o n t h a n the one t e s t e d by V a i d and C a m p a n e l l a . The s t r e s s - s t r a i n b e h a v i o r o f n o r m a l l y c o n s o l i d a t e d Haney c l a y i n c o n s t a n t r a t e of s t r a i n s h e a r d i d not e x h i b i t t h e same s h a r p d e c r e a s e i n s t r e s s p a s t the peak v a l u e as i t was o b s e r v e d on the o v e r c o n s o l i d a t e d c l a y i n F i g u r e 4. The s t r a i n s o f t e n i n g o f t h e n o r m a l l y c o n s o l i d a t e d samples o c c u r r e d more g r a d u a l l y p a s t the maximum d e v i a t o r s t r e s s . T h i s d i f f e r e n c e i n b e h a v i o r i s e s s e n t i a l l y due to the 64 d e s t r u c t u r a t i o n o f the n o r m a l l y c o n s o l i d a t e d c l a y o c c u r r i n g p r i o r to s h e a r l o a d i n g when the c l a y was s u b j e c t e d to an e f f e c t i v e c o n f i n i n g s t r e s s h i g h e r t han the a p p a r e n t p r e c o n s o l i d a t i o n p r e s s u r e . However, some s i m i l a r i t i e s were o b s e r v e d i n the b e h a v i o r o f the c l a y i n b o t h c o n s o l i d a t i o n s t a t e s . The s t r a i n a t f a i l u r e was e s s e n t i a l l y i n d e p e n d e n t o f the r a t e of s t r a i n i n b o t h c a s e s a l t h o u g h the peak d e v i a t o r s t r e s s was r e a c h e d a t an a x i a l s t r a i n o f about 2.5 to 3.0% i n the n o r m a l l y c o n s o l i d a t e d s t a t e as compared to 1.2 to 1.5% f o r the o v e r c o n s o l i d a t e d c l a y . o The v a r i a t i o n o f u n d r a i n e d s t r e n g t h w i t h l o g a r i t h m of s t r a i n r a t e from c o n s t a n t r a t e of s t r a i n and c o n s t a n t s t r e s s c r e e p i s shown i n F i g u r e 21. W h i l e the o v e r c o n s o l i d a t e d c l a y p r e s e n t e d a r e g u l a r i n c r e a s e i n u n d r a i n e d s t r e n g t h of about 8% per l o g c y c l e o f s t r a i n r a t e , the n o r m a l l y c o n s o l i d a t e d c l a y e x h i b i t e d no change i n u n d r a i n e d s t r e n g t h a t low s t r a i n r a t e s ( l e s s t h a n 5 x 1 0 ~ 3 % / m i n ) , t h e n a more p r o n o u n c e d i n c r e a s e of about 9% per l o g c y c l e of s t r a i n r a t e f o r h i g h e r speed of d e f o r m a t i o n . The c o m p a r i s o n o f the r e d u c t i o n o f u n d r a i n e d s t r e n g t h w i t h time from the c o n s t a n t s t r e s s c r e e p t e s t s i s shown i n F i g u r e 22. I t may be seen t h a t the b e h a v i o r of the c l a y has a s i m i l a r t r e n d i n b o t h c o n s o l i d a t i o n s t a t e s . However, l a r g e r r e d u c t i o n s o f s t r e n g t h were o b s e r v e d i n the n o r m a l l y c o n s o l i d a t e d c l a y f o r s h o r t time f a i l u r e s ( l e s s t h a n 100 ° Constant Strain Rate Shear at (°n-°0) • 1 3 max Constant Stress Creep at e • I mm 10 5 10 4 10 3 10 2 10 1 10° AXIAL STRAIN RATE (%/min) FIG. 21 VARIATION OF UNDRAINED STRENGTH WITH RATE OF STRAIN IN CONSTANT STRAIN RATE SHEAR AND CONSTANT STRESS CREEP FOR O.C. AND N.C. HANEY CLAY. ON Ln 3.4 2.4 1.6 NORMALLY CONSOLIDATED (Destructured) 0.8 OVERCONSOLIDATED (Intact structure) - L ± ± ± 10 0 i o J 10 10" 10 10 TOTAL RUPTURE LIFE (min) FIG. 22 TIME DEPENDENCE OF UNDRAINED STRENGTH OF O.C. AND N.C. HANEY CLAY IN CONSTANT STRESS CREEP, 67 m i n u t e s i n t o t a l r u p t u r e l i f e ) . S i n c e the v a l i d i t y o f the e q u a t i o n o f s t a t e has been p r o v e d f o r b o t h c l a y s , w hich means t h a t the u n i q u e r e l a t i o n s h i p between s t r e s s , s t r a i n , and s t r a i n r a t e i s e s s e n t i a l l y i n d e p e n d e n t of the time l o a d i n g h i s t o r y used to s h e a r the c l a y , the r e s u l t s o f c o n s t a n t r a t e o f l o a d i n g s h e a r and c o n s t a n t l o a d c r e e p t e s t s f o r the two c l a y s w i l l c e r t a i n l y be as c o m p a r a b l e as the c o n s t a n t r a t e of s t r a i n s h e a r and the c o n s t a n t s t r e s s c r e e p t e s t s . T h e r e f o r e , s u c h a c o m p a r i s o n w i l l n o t be n e c e s s a r y . From t h e c o m p a r i s o n between a c l a y t h a t has been d e s t r u c t u r e d by n o r m a l c o n s o l i d a t i o n i n the l a b o r a t o r y „and the same c l a y i n i t s i n t a c t , s t r u c t u r e d and o v e r c o n s o l i d a t e d s t a t e w i t h r e s p e c t to the i n - s i t u a p p a r e n t p r e c o n s o l i d a t i o n p r e s s u r e , i t a p p e a r s t h a t the d e s t r u c t u r a t i o n of the c l a y has n o t c a u s e d any s i g n i f i c a n t change i n the time d e p e n d e n t b e h a v i o r of the m a t e r i a l . A minor v a r i a t i o n i n t h e m a g n i t u d e of the time e f f e c t s on the u n d r a i n e d s t r e n g t h may be n o t e d , but the c h a r a c t e r i s t i c time dependent b e h a v i o r r e m a i n s b a s i c a l l y the same. Both c l a y s showed e s s e n t i a l l y t h e same i n c r e a s e i n u n d r a i n e d s t r e n g t h w i t h i n c r e a s i n g r a t e of s t r a i n o r r a t e o f l o a d i n g , and the same r e d u c t i o n i n u n d r a i n e d s t r e n g t h w i t h time under the e f f e c t of s u s t a i n e d s t r e s s o r l o a d . In b o t h c l a y s , the c u r r e n t s t r e s s was u n i q u e l y r e l a t e d to the c u r r e n t s t r a i n r a t e a t any g i v e n l e v e l of s t r a i n p r i o r 68 to f a i l u r e , r e g a r d l e s s of the time l o a d i n g h i s t o r y u s e d d u r i n g s h e a r . 5.3. S t r e s s C o n d i t i o n s a t Maximum E f f e c t i v e S t r e s s R a t i o F i g u r e 23 shows the s t r e s s c o n d i t i o n s i n the m o d i f i e d Mohr p l o t a t maximum e f f e c t i v e s t r e s s r a t i o f o r a l l t e s t s on o v e r c o n s o l i d a t e d Haney c l a y . A l l t h e t e s t d a t a p o i n t s a r e l o c a t e d i n a narrow range between the f a i l u r e e n v e l o p e of the n o r m a l l y c o n s o l i d a t e d c l a y and the 45 d e g r e e l i n e i n d i c a t i n g the l i m i t of t h e s t r e s s c o n d i t i o n s i n the t r i a x i a l t e s t s ( o ^ = 0 ) . T h i s d i a g r a m does not show any c l e a r e v i d e n c e of a time dependence of the r e s u l t s . I t i s however d i f f e r e n t f r o m the r e s u l t s of the n o r m a l l y c o n s o l i d a t e d c l a y where the d a t a p o i n t s of a l l the v a r i o u s t e s t s were l i n e d up a l o n g a u n i q u e f a i l u r e e n v e l o p e . 69 T—  1 ' 71 O . Constant s t r a i n rate shear • Constant stress creep A Constant loading rate shear A ' Constant load creep + Step change constant stress creep x Step change constant load creep FIG. 23. STRESS CONDITIONS FOR ALL TESTS AT (•o-/cO ON O.C. HANEY CLAY. 1 3 max 70 CHAPTER 6 CONCLUSION The time dependence of u n d r a i n e d s t r e n g t h and d e f o r m a t i o n b e h a v i o r o f a s a t u r a t e d , s e n s i t i v e , o v e r c o n s o l i d a t e d , n a t u r a l c l a y have been examined i n t r i a x i a l c o m p r e s s i o n , f o r a g i v e n c o n s o l i d a t i o n h i s t o r y , under a v a r i e t y of time l o a d i n g h i s t o r i e s . The t e s t s showed t h a t i n c r e a s e i n s t r a i n r a t e and r a t e o f l o a d i n g r e s u l t i n s t i f f e r u n d r a i n e d s t r e s s - s t r a i n r e s p o n s e and h i g h e r u n d r a i n e d s t r e n g t h f o r the c l a y i n v e s t i g a t e d . A c r i t i c a l low l e v e l o f s t r a i n a p p e a r s to t r i g g e r r u p t u r e i n a l l the t e s t s p e r f o r m e d i n t h i s s t u d y . A c o r r e l a t i o n among the v a r i o u s t e s t s s u p p o r t e d the v a l i d i t y o f the e q u a t i o n of s t a t e i n i t s s t r a i n h a r d e n i n g f o r m u l a t i o n , t h a t i s , the c u r r e n t s t r a i n r a t e i s u n i q u e l y r e l a t e d to the c u r r e n t s t r e s s a t any g i v e n l e v e l of s t r a i n p r i o r to f a i l u r e , r e g a r d l e s s of the time l o a d i n g h i s t o r y . A q u a l i t a t i v e c o m p a r i s o n w i t h the r e s u l t s of a s i m i l a r s t u d y o f the same s e n s i t i v e c l a y , d e s t r u c t u r e d upon l a b o r a t o r y n o r m a l c o n s o l i d a t i o n , i n d i c a t e d t h a t the d e s t r u c t u r a t i o n of the c l a y has not c a u s e d any s i g n i f i c a n t change i n i t s time d e p e n d e n t b e h a v i o r . The o n l y d i f f e r e n c e s o b s e r v e d and r e l a t e d to the n a t u r a l s t r u c t u r e of the 71 o v e r c o n s o l i d a t e d c l a y were i n the s t r e s s - s t r a i n r e s p o n s e o b t a i n e d from c o n v e n t i o n a l c o n s t a n t s t r a i n r a t e t e s t s , and i n the s t r e s s c o n d i t i o n s a t maximum e f f e c t i v e s t r e s s r a t i o o f a l l t e s t s p l o t t e d on a m o d i f i e d Mohr d i a g r a m . The o v e r c o n s o l i d a t e d c l a y e x h i b i t e d a pronounced c o l l a p s e of i t s s t r u c t u r e a t f a i l u r e , and i t d i d not show any w e l l d e f i n e d e f f e c t i v e s t r e s s f a i l u r e e n v e l o p e a t maximum e f f e c t i v e s t r e s s r a t i o , r e g a r d l e s s o f the time l o a d i n g h i s t o r y of s h e a r i n g . 72 LIST OF REFERENCES A l b e r r o , J.A. and S a n t o y o , E.Y., "Long Term B e h a v i o r o f Mexico C i t y C l a y " , P r o c e e d i n g s , 8 t h I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , Moscow 1973, V o l . 1.1, pp. 1-9. B e r r e , T. and B j e r r u m , L., "Shear S t r e n g t h o f N o r m a l l y C o n s o l i d a t e d C l a y s " , P r o c e e d i n g s , 8 t h I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , Moscow, 1973, V o l . 1.1, pp. 39-49. B i s h o p , A.W. and H e n k e l , D.J., "The Measurement of S o i l P r o p e r t i e s i n the T r i a x i a l T e s t " , 2nd e d i t i o n , Edward A r n o l d L t d . , London, E n g l a n d , 1962. B i s h o p , A.W. and L o v e n b u r y , H.T., "Creep C h a r a c t e r i s t i c s of Two U n d i s t u r b e d C l a y s " , P r o c e e d i n g s , 7th I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , M e x i c o , 1969, V o l . 1, pp. 29- 37 . B j e r r u m , L., "Embankments on S o f t Ground", P r o c e e d i n g s , ASCE S p e c i a l t y C o n f e r e n c e on E a r t h and E a r t h S u p p o r t e d S t r u c t u r e s , 1972, V o l . 2, pp. 1-54. B j e r r u m , L., " E n g i n e e r i n g G e o l o g y of Norwegian N o r m a l l y C o n s o l i d a t e d M a r i n e C l a y s as R e l a t e d to S e t t l e m e n t s o f B u i l d i n g s " , S e v e n t h R a n k i n e L e c t u r e , G e o t e c h n i q u e , V o l . 17, 1967, pp. 81-118. 73 7. B j e r r u m , L. and Lo, K.Y., " E f f e c t of A g i n g on the S h e a r S t r e n g t h P r o p e r t i e s of a N o r m a l l y C o n s o l i d a t e d C l a y " , G e o t e c h n i q u e , V o l . 13, 1963, pp. 147-157. 8. B j e r r u m , L., Simons, N. and T o r b l a a , I . , "The E f f e c t of Time on the Shear S t r e n g t h of a S o f t M a r i n e C l a y " , P r o c e e d i n g s , B r u s s e l s C o n f e r e n c e on E a r t h P r e s s u r e P r o b l e m s , V o l . 1, pp. 148-158. 9. C a m p a n e l l a , R.G. and V a i d , Y.P., " T r i a x i a l and P l a n e S t r a i n Creep R u p t u r e of an U n d i s t u r b e d C l a y " , C a n a d i a n G e o t e c h n i c a l J o u r n a l , V o l . 2, 1974, pp. 1-10. 10. C a s a g r a n d e , A. and W i l s o n , S., " E f f e c t of Rate o f L o a d i n g on S t r e n g t h o f C l a y s and S h a l e s a t C o n s t a n t Water C o n t e n t " , G e o t e c h n i q u e , V o l . 2, No. 3, 1951, pp. 251-264. 11. Chan, C.K., " L o w - F r i c t i o n S e a l System", J o u r n a l of the G e o t e c h n i c a l E n g i n e e r i n g D i v i s i o n , ASCE, V o l . 101, No. GT 9, 1975 , pp. 991-995 . 12. C o a t e s , D.F., B u r n , K.N., M c R o s t i e , G.C., " S t r a i n - T i m e - S t r e n g t h R e l a t i o n s h i p s i n a M a r i n e C l a y " , E n g i n e e r i n g I n s t i t u t e of Canada, T r a n s a c t i o n s , EJG-63, Geot. 11, O c t o b e r 1963. 13. C o u l o n , R.J., " L a n d s l i d e on the T o u l n u s t a u c R i v e r , Quebec", C a n a d i a n G e o t e c h n i c a l J o u r n a l , V o l . 3, No. 3., 1966, pp. 113-144. 74 C r a w f o r d , C.B., "The I n f l u e n c e of Rate of S t r a i n on E f f e c t i v e S t r e s s e s i n S e n s i t i v e C l a y " , P a p e r s on S o i l s , ASTM STP, No. 254, 1959, pp. 36-48. F i n n , W.D.L. and Snead, D.E., "Creep and Creep R u p t u r e of an U n d i s t u r b e d S e n s i t i v e C l a y " , P r o c e e d i n g s , 8 t h I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , Moscow, 1973, V o l . 3, pp. 135- 142 . F i n n i e , J . and H e l l e r , W.R., "Creep of E n g i n e e r i n g M a t e r i a l s " , M c G r a w - H i l l Book Company, I n c . , New Y o r k , 1959 . G o l d s t e i n , M. and T e r - S t e p a n i a n , G., "The Lon g - t e r m S t r e n g t h of C l a y s ' , P r o c e e d i n g s , 4 t h I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , London, 1957, V o l . 2, pp. 311-314. K r a u s , H., "Creep A n a l y s i s " , John W i l e y and Sons, I n c . , New Yo r k , 1980. L e o n a r d s , G.A., " S t r a i n Rate B e h a v i o u r of S a i n t - J e a n - V i a n n e y C l a y : D i s c u s s i o n " , C a n a d i a n G e o t e c h n i c a l J o u r n a l , V o l . 17, 1980, pp. 461-462. M e s r i , G., F e b r e s - C o r d e r o , E., S h i e l d s , D.R. and C a s t r o , A., "Shear S t r e s s - S t r a i n - T i m e B e h a v i o u r o f C l a y s " , G e o t e c h n i q u e , V o l . 31, No. 4, 1981, pp. 537- 552. 75 21. Murayama, S. and S h i b a t a , T., " R h e o l o g i c a l P r o p e r t i e s of C l a y s " , P r o c e e d i n g s , 5th I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , F r a n c e , 1964, V o l . 1, pp. 269-273. 22. Pao, Y.H. and M a r i n , J . , " P r e d i c t i o n of Creep C u r v e s from S t r e s s S t r a i n D a t a " , P r o c e e d i n g s ASTM, V o l . 52, 1952, pp. 51-57. 23. P e r l o f f , W.H. and O s t e r b e r g , J.O., "The E f f e c t of S t r a i n Rate on the U n d r a i n e d Shear S t r e n g t h of C o h e s i v e S o i l s " , P r o c e e d i n g s , 2nd Pan A m e r i c a n C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , V o l . 2, pp. 103-128. 24. R i c h a r d s o n , A.M. and Whitman, R.V., " E f f e c t of S t r a i n Rate Upon U n d r a i n e d Shear R e s i s t a n c e of a S a t u r a t e d Remoulded F a t C l a y " , G e o t e c h n i q u e , V o l . 13, No. 4, 1963, pp. 310-324. 25. S a i t o , M. and Uezawa, H., " F o r e c a s t i n g the Time of O c c u r r e n c e of a S l o p e F a i l u r e " , P r o c e e d i n g s , 5 t h I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , P a r i s , F r a n c e 1961, V o l . 1, pp. 315-318. 26. S c o t t , R. and Hon-Yim Ko, " S t r e s s - D e f o r m a t i o n and S t r e n g t h C h a r a c t e r i s t i c s of S o i l s " , S t a t e - o f - t h e - A r t Volume, 7th I n t e r n a t i o n a l C o n f e r e n c e on S o i l M e c h a n i c s and F o u n d a t i o n E n g i n e e r i n g , Mexico, 1969, pp. 28-38. 76 27. Simons, N.F., "The E f f e c t of O v e r c o n s o l i d a t i o n on the Shear S t r e n g t h C h a r a c t e r i s t i c s of an U n d i s t u r b e d O s l o C l a y " , ASCE R e s e a r c h C o n f e r e n c e on Shear S t r e n g t h of C o h e s i v e S o i l s , B o u l d e r , C o l o r a d o , June 1960, pp. 747- 763 . 28. S i n g h , A. and M i t c h e l l , J.K., " G e n e r a l S t r e s s - S t r a i n - Time F u n c t i o n f o r S o i l s " , ASCE, J o u r n a l of S o i l M e c h a n i c s and F o u n d a t i o n s D i v i s i o n , V o l . 94, SMI, 1968, pp. 21-46. 29. £ u k l j e , L., " R h e o l o g i c a l A s p e c t s of S o i l M e c h a n i c s " , John W i l e y and Sons L t d . , London, E n g l a n d , 1969. 30. T a y l o r , D.W., " Fundamentals of S o i l M e c h a n i c s " , John W i l e y and Sons I n c . , New York, N.Y., 1948. 31. T a v e n a s , F., L e r o u e i l , S. L a R o c h e l l e , P., Roy, M., "Creep B e h a v i o r of an U n d i s t u r b e d L i g h t l y O v e r c o n s o l i d a t e d C l a y " , C a n a d i a n G e o t e c h n i c a l J o u r n a l , V o l . 15, 1978, pp. 402-423. 32. V a i d , Y.P. and C a m p a n e l l a , R.G., "Time Dependent B e h a v i o u r of U n d i s t u r b e d C l a y " , ASCE, J o u r n a l of t h e G e o t e c h n i c a l E n g i n e e r i n g D i v i s i o n , V o l . 103, GT7, 1977, pp. 693-709. 33. V a i d , Y.P., R o b e r t s o n , P.K., C a m p a n e l l a , R.G., " S t r a i n Rate B e h a v i o u r of S a i n t - J e a n - V i a n n e y C l a y " , C a n a d i a n G e o t e c h n i c a l J o u r n a l , V o l . 16, No. 1, 1979, pp. 34-42. 34. Symposium on S t r e s s - S t r a i n - T i m e - T e m p e r a t u r e R e l a t i o n s h i p s i n M a t e r i a l s . ASTM S p e c i a l T e c h n i c a l P u b l i c a t i o n No. 325, New York, N.Y., 1962.

Cite

Citation Scheme:

    

Usage Statistics

Country Views Downloads
China 13 12
United States 5 7
Japan 5 0
City Views Downloads
Beijing 13 2
Tokyo 5 0
Burke 2 0
San Bruno 1 0
Sunnyvale 1 7
Ashburn 1 0

{[{ mDataHeader[type] }]} {[{ month[type] }]} {[{ tData[type] }]}

Share

Share to:

Comment

Related Items