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Effect of confining pressure and particle angularity on resistance to liquefaction 1983

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EFFECT OF CONFINING PRESSURE AND PARTICLE ANGULARITY ON RESISTANCE TO LIQUEFACTION by Hadi Omran Zaglut TUMI B.Sc. Eng. Al-Fatah U n i v e r s i t y , ( T r i p o l i - L i b y a ) A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE IN THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CIVIL ENGINEERING We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA JUNE, 19 83 © Hadi Omran Zaglut TUMI, 1983 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that .the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of r.it//L ^AlGiNIhJOj The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) ABSTRACT A study to investigate the e f f e c t s of p a r t i c l e angularity and high confining stresses on l i q u e f a c t i o n resistance of sands i s presented. Two quartz sands of i d e n t i c a l mineral composition and gradation but d i f f e r i n g i n p a r t i c l e angularity were used. The investigations were performed under c y c l i c simple shear condition which c l o s e l y simulates f i e l d stress conditions. Resistance to l i q u e f a c t i o n i s compared for angular and rounded sands over a range of r e l a t i v e d e n s i t i e s and confining stresses. Con- f i n i n g stress of up to 2500 kPa were applied to represent the condition of granular materials i n high dams. The change i n l i q u e f a c t i o n r e s i s t - ance with increase i n confining stress i s shown for each sand for a range of r e l a t i v e d e n s i t i e s . The data i n d i c a t e s that l i t t l e benefit i s gained i n dynamic resistance by i n i t i a l l y densifying t a i l i n g s sands which w i l l l a t e r be subjected to high confining stresses. - i i - TABLE OF CONTENTS Page ABSTRACT - i i - TABLE OF CONTENTS - i i i - LIST OF FIGURES - i v - NOTATIONS • • • - v i i - ' ACKNOWLEDGEMENT - v i i i - 1. INTRODUCTION 1 2. REVIEW OF LITERATURE 3 3. TESTING PROGRAM 7 3.1 Testing Apparatus 7 3.2 M a t e r i a l Tested 10 3.3 Testing Program 12 4. TEST RESULTS 15 4.1 C y c l i c Loading Behaviour 15 4.2 L i q u e f a c t i o n Resistance of T a i l i n g s Sand 20 4.3 L i q u e f a c t i o n Resistance of Ottawa Sand 38 5. EFFECT OF CONFINING STRESS AND PARTICLE ANGULARITY ON LIQUEFACTION POTENTIAL 53 6. CONCLUSIONS 58 REFERENCES 61 - i i i - LIST OF FIGURES Page F i g . 1 Reduction i n C y c l i c Stress Ratio Causing Liquefac- 5 tion with Increase i n Confining Pressure Suggested by Seed, H.B. F i g . 2 Constant Volume Simple Shear Apparatus 8 (Af t e r Finn and Vaid) F i g . 3 Grain Size D i s t r i b u t i o n of Ottawa and T a i l i n g s Sands 11 F i g . 4 One-Dimensional Compressibility of T a i l i n g s Sand 13 F i g . 5 One-Dimensional Compressibility of Ottawa Sand 14 F i g . 6 T y p i c a l C y c l i c Loading Behaviour of T a i l i n g s Sand 16 at Low Confining Pressure F i g . 7 T y p i c a l C y c l i c Loading Behaviour of T a i l i n g s Sand 17 at High Confining Pressure F i g . 8 T y p i c a l C y c l i c Loading Behaviour of Ottawa Sand 18 at Low Confining Pressure F i g . 9 T y p i c a l C y c l i c Loading Behaviour of Ottawa Sand 19 at High Confining Pressure F i g . 10 Number of Cycles to Liquefaction (Y ± 5%) Versus Void Ratio for T a i l i n g s Sand: (a) At Confining Pressure of 200 kPa 22 (b) At Confining Pressure of 400 kPa 22 (c) At Confining Pressure of 800 kPa 23 (d) At Confining Pressure of 1600 kPa 23 (e) At Confining Pressure of 2500 kPa 24 F i g . 11 Resistance to L i q u e f a c t i o n of T a i l i n g s Sand at Various Confining Pressures Based on Shear Strain of ±5% i n 10 Cycles F i g . 12 V a r i a t i o n of C y c l i c Stress Ratio to Cause Liquefac- ti o n of T a i l i n g s Sand With Confining Pressure at Various R e l a t i v e Density Levels F i g . 13 Percent Reduction i n Resistance to Liquefaction of T a i l i n g s Sand When Referred to the Resistance at 200 kPa 26 28 29 - i v - F i g . 14 I n i t i a l Void Ratio With Void Ratio A f t e r Consolida- 31 tio n Relationships of Ta i l i n g s Sand at Various Confining Pressures F i g . 15 E f f e c t of Confining Pressure on the Resistance to 33 Liquefaction of T a i l i n g s Sand Prepared at Various I n i t i a l Relative Densities F i g . 16 Resistance to Liquefaction of T a i l i n g s Sand at 3 5 Various Confining Pressures Based on Shear S t r a i n Level of ±5% i n 15 Cycles F i g . 17 Resistance to Liquefaction of T a i l i n g s Sand at 36 Various Confining Pressures Based on Shear S t r a i n Level of ±2.5% i n 10 Cycles Fig . 18 Resistance to Liquefaction of T a i l i n g s Sand at 37 Various Confining Pressures, Based on Shear S t r a i n Level of ±2.5% i n 15 Cycles F i g . 19 Number of Cycles to Liquefaction Versus Void Ratio fo r Ottawa Sand: (a) At Confining Pressure of 200 kPa 39 (b) At Confining Pressure of 800 kPa 39 (c) At Confining Pressure of 1600 kPa 40 (d) At Confining Pressure of 2500 kPa 40 Fig . 20 Resistance to Liquefaction of Ottawa Sand at Various 42 Confining Pressures Based on Shear S t r a i n Level of ±5% i n 10 Cycles Fig . 21 V a r i a t i o n of C y c l i c Stress Ratio to Cause Liquefac- 44 t i o n f o r Ottawa Sand With Confining Pressure at Various Relative Density Levels Fig . 22 Percent Reduction i n Resistance to Liquefaction of 4 5 Ottawa Sand When Referred to the Resistance at 200 kPa Fig . 23 I n i t i a l Void Ratio With Void Ratio After Consolida- 47 t i o n Relationships of Ottawa Sand at Various Confining Pressures Fig . 24 E f f e c t of Confining Pressure on the Resistance to 48 Liq u e f a c t i o n of Ottawa Sand Prepared at Various I n i t i a l Relative Densities F i g . 25 Resistance to Liquefation of Ottawa Sand at Various 4 9 Confining Pressures Based on Shear S t r a i n Level of ±5% i n 15 Cycles - v - F i g . 26 Resistance to Li q u e f a c t i o n of Ottawa Sand at Various 51 Confining Pressures, Based on Shear Strain Level of ±2.5% i n 10 Cycles F i g . 27 Resistance to Liqu e f a t i o n of Ottawa Sand at Various 52 Confining Pressures, Based on Shear Strain Level of ±2.5% i n 15 Cycles F i g . 28 Comparison of Resistance to Liquefaction of Angular 55 and Rounded Sands at Low and High Confining Pressures - v i - NOTATIONS Relative density a f t e r consolidation. D . I n i t i a l r e l a t i v e density before consolidation, r i e Void r a t i o a f t e r c o n s o l i d a t i o n . e^ I n i t i a l void r a t i o before consolidation. N Number of loading c y c l e s . Au Excess porewater pressure. Y Shear s t r a i n . V e r t i c a l confining pressure. T C y c l i c shear s t r e s s , cy - v i i - ACKNOWLEDGEMENTS In presenting t h i s t h e s i s , the author wishes to record h i s indebtedness to Dr. Y.P. Vaid for h i s continuous encouragement, guidance, and advice during the research course, and to Dr. P.M. Byrne for reviewing the manuscript and making valuable suggestions. The author would also l i k e to thank Mr. Fred Zurkirchen f o r h i s te c h n i c a l assistance and Mrs. K e l l y Lamb f o r the preparation of the manuscript. The Scholarship of Garyounis U n i v e r s i t y of Benghazi-Libya i s g r a t e f u l l y acknowledged. - v i i i - 1. 1. INTRODUCTION Li q u e f a c t i o n i s one of the problems associated with earthquake shaking of saturated cohesionless materials. It i s the development of l a r g e s t r a i n s i n s o i l mass during c y c l i c undrained loading when the pore water pressure i n the s o i l becomes close or equal to the e f f e c t i v e confining s t r e s s . Sands and s i l t s are the most susceptible materials to l i q u e f a c t i o n phenomena. When these materials are subjected to earthquakes, no s i g n i f i c a n t pore water pressure d i s s i p a t i o n i s expected to occur even i n r e l a t i v e l y permeable sands due to the short duration of the earthquake loading. Thus s o i l s of t h i s type can be considered undrained during earthquake shaking. Liq u e f a c t i o n resistance of cohesionless s o i l s can be determined i n the laboratory mainly by using c y c l i c t r i a x i a l or c y c l i c simple shear t e s t s . C y c l i c t o r s i o n a l shear t e s t s and shaking table t e s t s are also used but l e s s frequently. By conducting a s e r i e s of undrained c y c l i c simple shear or c y c l i c t r i a x i a l t e s t s , one can obtain a r e l a t i o n s h i p between the c y c l i c s t r e s s r a t i o (T /a1 ) to cause l i q u e f a c t i o n i n a J cy vo n s p e c i f i e d number of cycles and the sand r e l a t i v e density. This r e l a - t i o n s h i p forms the l i q u e f a c t i o n resistance curve of the s o i l . It Is f e l t that t h i s r e l a t i o n s h i p may be a f f e c t e d when the material i s loaded under high confining stresses. However, no comprehensive study has been made to seek the influence of high confining stress on l i q u e f a c - t i o n p o t e n t i a l of sands. These confining stresses i n earth dams can go as high as 2500 kPa when the dam height approaches 200 m. The past experience with the evaluation of the l i q u e f a c t i o n 2. p o t e n t i a l i s confined mainly to natural sands which consist of rounded to subrounded p a r t i c l e s . T a i l i n g s sands which are used to construct t a i l i n g s dams are d i f f e r e n t from natural sands i n that they consist of angular p a r t i c l e s . Recently there i s a growing tendency to b u i l d high t a i l i n g s dams. It i s f e l t that the performance of these t a i l i n g s sands i n high t a i l i n g s dams could be d i f f e r e n t from that of conventional sands i n t h e i r resistance to l i q u e f a c t i o n . This research work i s an attempt to investigate the e f f e c t s of confining stress and p a r t i c l e a n g u l a r i t y on l i q u e f a c t i o n resistance of sands. The constant volume c y c l i c simple shear apparatus i s used i n these laboratory investiga- t i o n s . Simple shear conditions are considered most representative of the i n - s i t u stress conditions during earthquakes. The e f f e c t of p a r t i c l e a n g u l a r i t y has been examined by t e s t i n g two d i f f e r e n t materials, rounded Ottawa sand and angular t a i l i n g s sand, having an i d e n t i c a l mineral composition and gradation. The e f f e c t of confining stress on both sands has been investigated through t e s t i n g each sand over a range of confining stresses up to a maximum of 2500 kPa. 3. 2. REVIEW OF THE LITERATURE To the author's knowledge, no comprehensive study has been made to seek the influence of large confining stresses on the l i q u e f a c t i o n r e s i s t a n c e of granular materials. The previous work c a r r i e d out by few researchers was confined to a r e l a t i v e l y low confining s t r e s s . Furthermore, no d i r e c t assessment of the e f f e c t of p a r t i c l e angularity on resistance to l i q u e f a c t i o n has been attempted. Early recommendations have been made regarding the density requirements to preclude l i q u e f a c t i o n . Sherard (13) stated that sands with r e l a t i v e density of 50% or more probably cannot l i q u e f y regardless of the gradation. A si m i l a r conclusion has been made by D'Appolonia (4) as he indicated that l i q u e f a c t i o n might occur f o r s o i l s having a r e l a t i v e density l e s s than 50% during ground motions with accelerations i n excess of approximately 0.1 g, and for r e l a t i v e d e n s i t i e s greater than 75% l i q u e f a c t i o n f o r most earthquake loading i s u n l i k e l y . Casagrande (2) had recommended a r e l a t i v e density of 60% to be s u f f i - c i e n t as f a r as the compaction of mine t a i l i n g s dams i s concerned. A minimum r e l a t i v e density of 60% was also recommended by the Department of Energy, Mines and Resources (5) as a requirement for the mine waste embankments. However, the above recommendations were based on empirical c r i t e r i a . It w i l l be shown i n a l a t e r chapter that sands can l i q u e f y even at r e l a t i v e d e n s i t i e s i n excess of 75% i f they are under high c o n f i n i n g stresses and subjected to a moderate earthquake loading. Therefore, these recommendations are not always true, as they might have been based on performance of rounded sand under low confining 4 . stresses. Data on l i q u e f a c t i o n resistance of several subangular to sub- rounded sands was presented by Castro and Poulos ( 3 ) . A general reduc- t i o n trend i n c y c l i c stress r a t i o to cause l i q u e f a c t i o n was observed as the confining stress increased from 50 to 600 kPa. The sands were tested i n t r i a x i a l apparatus with i s o t r o p i c consolidation. A s i m i l a r reduction i n l i q u e f a c t i o n resistance of compacted t a i l i n g s sand was reported by Volpe (15). The c y c l i c stress r a t i o to cause l i q u e f a c t i o n was reduced by 25% when the confining stress had increased from 100 to 3 50 kPa. Volpe had used the t r i a x i a l apparatus i n generating these data, using anisotropic c o n s o l i d a t i o n stress r a t i o k c of 2. Later, as a step i n the design procedure of dams, Seed (12) had suggested to reduce the c y c l i c stress r a t i o to cause l i q u e f a c t i o n i f the confining stress exceeds 150 kPa. The reduction increases to ~35% when the confining stress reaches 800 kPa. The reduction curve presen- ted by Seed i s shown on F i g . ( 1 ) . However, no reference was made as to ei t h e r the sand type or r e l a t i v e density l e v e l at which t h i s c o r r e c t i o n was suggested. Anyhow, t h i s reduction mode cannot be generalized and i t may be v a l i d only f o r one type of material at a c e r t a i n r e l a t i v e density l e v e l . Dorey and Byrne (6), i n dynamic s t a b i l i t y a n a l y s i s of t a i l i n g s impoundment, had shown that as the height of t a i l i n g s increases l i q u e f a c t i o n resistance decreases f o r compacted t a i l i n g s and increases f o r uncompacted t a i l i n g s . From the above l i m i t e d i n v e s t i g a t i o n s , one can r e a l i z e that the e f f e c t of confining stress has been investigated over a r e l a t i v e l y low range (maximum of 600 to 800 kPa) . For dams of ~200 m height, the confinin g stress at the lower zone of the dam can be as high as 2500 5. F i g . 1. Reduction i n C y c l i c Stress Ratio Causing Liquefaction With Increase i n Confining Pressure Suggested by Seed H.B. (10). 6. kPa. Furthermore, a c l e a r assessment of the e f f e c t of p a r t i c l e a n g u l a r i t y on l i q u e f a t i o n resistance has not been made. This thesis i s therefore an attempt to c l a r i f y the e f f e c t s of p a r t i c l e angularity and confining stress on the l i q u e f a c t i o n potential of sands. 7. 3. TESTING PROGRAM 3.1 Testing Apparatus The constant volume c y c l i c simple shear apparatus described by Finn and Vaid (9) was used i n t h i s study. The apparatus i s shown i n F i g . ( 2 ) . In the constant volume technique, the changes i n the v e r t i - c a l stresses to maintain constant volume during the c y c l i c loading are equivalent to the changes i n pore water pressures i n the corresponding undrained t e s t s . The constant volume of the simple shear specimen i s achieved by clamping the loading head to a r i g i d loading plate which i n turn i s clamped to four v e r t i c a l posts. These posts are threaded into the body of the simple shear apparatus. The v e r t i c a l stress i s applied by tightening the b o l t nut on the underside of the loading plate u n t i l the desired value of the v e r t i c a l stress i s achieved. Then the b o l t nut on the top of the loading plate i s tightened. Only a very small volume change can be introduced due to the recovery of the e l a s t i c deformation of the loading plate as the v e r t i c a l stress decreases dur- ing the constant volume t e s t s . These small changes i n the volume dur- ing the constant volume tests represents a very small percent of those associated with system compliance i n the undrained tests (9). Hence more accurate and r e l i a b l e c y c l i c loading r e s u l t s are expected from the constant volume t e s t s , since they are free from the compliance e f f e c t s which are common to a l l undrained t e s t s . The sample preparation technique described by Finn and Vaid (9) was used i n preparing dry sand specimens. In t h i s technique, the mem- brane i s f i r s t stretched i n the sample c a v i t y i n the simple shear apparatus. The sand i s then deposited within the membrane i n the 2. Constant Volume Simple She Apparatus ( a f t e r Finn & Va 9 . apparatus through a funnel t i p permitting a free f a l l u n t i l the height of the sand i n the sample c a v i t y exceeds the required sample height. The excess sand over the f i n a l grade i s then siphoned o f f using a small vacuum. The sample height i s c o n t r o l l e d by adjusting the vacuum tube length. The top ribbed plate i s then placed on the top of the sample and the membrane closed over i t . The sample i s then sealed to the loading head. The desired density i s achieved by v i b r a t i o n s using a s o f t hammer while the sample i s kept under a seating e f f e c t i v e stress of about 0.20 kg/cm2. The above sample preparation procedure r e s u l t s i n a sample of uniform density throughout (9). This i s very important e s p e c i a l l y while preparing dense samples i n which the l i q u e f a c t i o n resistance may be underestimated by the possible existence of loose surface layer during sample preparation (9). There are advantages i n using the constant volume c y c l i c simple shear apparatus. It permits the use of dry sand for c y c l i c undrained t e s t s . It i s easier and fa s t e r to handle dry sand than saturated sand. Furthermore, many problems associated with undrained t e s t i n g are avoided. This, together with the improved sample preparation technique described by Finn and Vaid (9) leads to a more accurate estimate of the l i q u e f a c t i o n potential i n the laboratory. The simple shear specimen had a square dimension with a h o r i z o n t a l cross-sectional area of =25 cm2 and a height of =2.5 cm. The c y c l i c shear stress was applied by means of an e l e c t r o - pneumatic loading system. A s i n u s o i d a l shape was used with a frequency of 0.1 Hz. This low frequency was used i n order to overcome any d e f i - ciency i n the response of the electropneumatic system or of the s t r i p 1 0 . chart recorder on which signal traces were recorded. It also enabled examination of the pore water pressures and s t r a i n development not only at the completion of the loading cycles but also during each loading c y c l e . V e r t i c a l and horizontal loads were measured by means of s t r a i n gauge type transducer whereas displacement was measured by a displace- ment transducer. The transducers were p r e c i s e l y c a l i b r a t e d . The f r i c - t i o n i n the l i n e a r bearings of the apparatus as well as the resistance due to membrane st r e t c h during simple shear deformation were measured over the f u l l working pressure and s t r a i n range using an a i r sample. A f r i c t i o n c o e f f i c i e n t of 0.006 was recorded and considered i n correcting the applied c y c l i c shear stress amplitude i n each t e s t . Continuous records of the c y c l i c shear stress, pore water pres- sure, and s t r a i n as monitored by the transducers were obtained on a s t r i p chart recorder. 3.2 Material Tested Two d i f f e r e n t granular materials, Ottawa sand and Brenda mine t a i l i n g s sand, were used i n t h i s study. Ottawa sand i s a natural s i l i c a sand consisting of rounded p a r t i c l e s . It i s a medium sand and corresponds to ASTM designation C-10 9. I t s maximum and minimum void r a t i o s are 0.82 and 0.50 r e s p e c t i v e l y , and i t s s p e c i f i c g r a v i t y i s 2.67. Brenda mine t a i l i n g s sand was the coarse f r a c t i o n of a copper mine waste which i s used i n b u i l d i n g the t a i l i n g s dam. It was treated so that i t s g r a i n s i z e when tested was e s s e n t i a l l y the same as that of Ottawa sand. This was achieved by washing through #100 sieve together with removal of some of the coarse f r a c t i o n . The grain size curves of the two sands are shown i n F i g . (3). T a i l i n g s sand had maximum and So n d Coorse Medium Fine Sieve Size Diameter i n mm Fi g . 3. Grain Size D i s t r i b u t i o n of Ottawa and T a i l i n g s Sands. 12. minimum void r a t i o s of 1.06 and 0.688 res p e c t i v e l y , and a s p e c i f i c gra- v i t y of 2.68. T a i l i n g s sand which i s composed of angular p a r t i c l e s i s more compressible than Ottawa sand. The marked d i f f e r e n c e i n t h e i r c o m p r e s s i b i l i t i e s may be noted from the r e s u l t s of one dimensional consolidation tests, Figs. (4) and (5). The mineral composition of t a i l i n g s was mainly quartz with occa- s i o n a l traces of mica and chalcopyrite. Since Ottawa sand i s composed of quartz, the mineral composition of the two sands i s i d e n t i c a l . Thus any d i f f e r e n c e i n behaviour of these sands can be a t t r i b u t e d to grain shape. 3.3 Testing Program Samples of both sands, prepared as described e a r l i e r , were subjec- ted to v e r t i c a l e f f e c t i v e confining stresses of 200, 400, 800, 1600, and 2 500 kPa. At each confining s t r e s s , samples were c y c l i c a l l y loaded u s i n g three or more d i f f e r e n t c y c l i c s t r e s s r a t i o s ( T C y / a y 0 ) ' ^ h e amplitudes of the c y c l i c stress r a t i o s were selected i n such a way that the sample l i q u e f i e d i n a reasonable number of cycles (preferably between 5 and 50 cycles) . This was achieved by making a few i n i t i a l t r i a l t ests at each confining s t r e s s . At every c y c l i c stress r a t i o l e v e l samples were prepared at d i f f e r e n t i n i t i a l r e l a t i v e d e n s i t i e s and then subjected to the c y c l i c loading u n t i l l i q u e f a c t i o n . L i q u e f a c t i o n i s considered here to be the development of a c e r t a i n shear s t r a i n amplitude. Up to t h i s l e v e l of developed s t r a i n the electropneumatic loading system provided a s a t i s f a t o r y response and d i d not r e s u l t i n attenuation of the shear stress amplitude with large s t r a i n excursions. 400 600 800 1000 2000 . 4000 6000 8000 10000 Confining Pressure, a' (kPa) F i g . 4. One Dimensional C o m p r e s s i b i l i t y of T a i l i n g s Sand. u •H O > 0.81 0.7 0.6 0.5' -A- -Q—• • i—r T — r 3—EI-13-Q-GH3.Q- 20 40 60 80 100 200 400 600 800 1000 Confining Pressure, kPa 2000 4000 6000 8000 10000 F i g . 5. One-Dimensional C o m p r e s s i b i l i t y of Ottawa Sand. 4. TEST RESULTS The basic data on c y c l i c loading behaviour was obtained i n the form of number of cycles to cause a c e r t a i n shear s t r a i n i n the sample due to the applied c y c l i c shear s t r e s s f o r various i n i t i a l r e l a t i v e d e n s i t i e s . Several such r e l a t i o n s h i p s were obtained at each confining stress l e v e l and various l e v e l s of c y c l i c stress r a t i o s (T /a' ). The cy vo rel a t i o n s h i p s between r e l a t i v e density and the c y c l i c stress r a t i o to cause l i q u e f a c t i o n i n a s p e c i f i c number of cycles were then achieved by cross p l o t t i n g the data. The test data f o r both sands, t a i l i n g s and Ottawa sand, showed extremely high degrees of r e p r o d u c i b i l i t y and l i t t l e s c a t t e r . 4.1 C y c l i c Loading Behaviour T y p i c a l r e l a t i o n s h i p s of re s i d u a l pore water pressure r a t i o (Au/a' ) as w e l l as shear s t r a i n y with the number of loading cycles v o are shown i n Figures (6, 7, 8, 9). In these f i g u r e s , the behaviour of both sands under low and high confining stresses i s shown at loose and dense r e l a t i v e d e n s i t i e s . At loose r e l a t i v e density, there was a large development of c y c l i c shear s t r a i n a f t e r a c e r t a i n number of cycles regardless of the confining stress l e v e l and p a r t i c l e angularity. The s t r a i n increased suddenly from a very low value to more than 5% i n one c y c l e . At high r e l a t i v e d e n s i t i e s , however, a gradual increase i n the c y c l i c shear s t r a i n was noted with cycles of loading. At such densi- t i e s , i t may take more than 10 a d d i t i o n a l cycles to bring up the shear s t r a i n l e v e l from 2.5% to 5% (Figure (6a)). This d i f f e r e n c e i n the shear s t r a i n development between the loose and dense states would 16. Number of Cycles, N F i g . 6. T y p i c a l C y c l i c Loading Behaviour of T a i l i n g s Sand at Low Confining Pressure, (a) Shear S t r a i n Versus Number of Cycles, (b) Pore Pressure Ratio Versus Number of Cycles. 17. 0 1.0 0.8 0.6 0.4 0.2 0.0 (b) / O D r = 55. 8% - I f x /a' cy vo = 0.120 f £> D = 88. r 1% 1 a' = 2500 kPa / vo T /a' cy vo = 0.147 • i . l . l i f . ' • 1 4 8 12 16 20 24 28 32 36 Number of Cycles, N F i g 7. T y p i c a l C y c l i c Loading Behaviour of T a i l i n g s Sand at High C o n f i n i n g Pressure. (a) Shear S t r a i n Versus Number of Cycles, (b) Pore Pressure R a t i o Versus Number of Cycles. 18. TT 1 1 ' T 0.0 a *B (a) A l.Oi 0.8U- 0.6 0.4 0.2 .A a a- (b) a' =200 kPa vo «» D = 50% T /a' = 0.127 cy vo D = 74.6% r T /a' = 0.187 cy vo J . 1 »- 6 0.0 12 16 20 24 Number of Cyc l e s , N 28 32 36 F i g . 8. T y p i c a l C y c l i c Loading Behaviour of Ottawa Sand at Low Con f i n i n g Pressure. (a) Shear S t r a i n Versus Number of Cycles, (b) Pore Pressure R a t i o Versus Number of Cycles. 19. 0.4 h 0.2 a' = 2500 kPa vo 0.0 _L _L D r = 62.2% T /a' = o 117 D r = 84.0% T /a1 = 0 3 7 7 cy vo U-J-/' 12 16 20 Number of Cycles, N 24 28 32 36 F i g . 9. T y p i c a l C y c l i c Loading Behaviour of Ottawa Sand at High Con f i n i n g Pressure, (a) Shear S t r a i n Versus Number of Cycles, (b) Pore Pressure R a t i o Versus Number of Cycles. 20. a f f e c t the c y c l i c stress r a t i o causing l i q u e f a c t i o n , depending on what l e v e l of s t r a i n development i s considered to define the occurrence of l i q u e f a c t i o n . Such differences w i l l be discussed l a t e r . For both sands, at low confining stresses, the development of pore water pressures i n loose samples was at a f a s t e r rate, which r e f l e c t s the high potential volume contraction during c y c l i c loading. Dense samples at low confining stresses, however showed a gentle increase i n pore water pressure r a t i o with the number of c y c l e s . At high confining stresses, loose samples of both sands showed a s i m i l a r increase i n pore pressure to that at low confining stresses. But, dense samples of t a i l i n g s sand showed a f a s t e r rate of pore pres- sure generation with increase i n number of cycles than s i m i l a r samples of Ottawa sand. It may be noted i n Figures (7 and 9) that although t a i l i n g s sand was at higher r e l a t i v e density and subjected to lower c y c l i c stress r a t i o than Ottawa sand, the pore pressure development i n t a i l i n g s sand was at a fa s t e r rate than that i n Ottawa sand. The breakage of sharp edges of t a i l i n g s sand during c y c l i c loading at high confining stresses might be responsible f o r larger p o t e n t i a l volumetric s t r a i n which could be the cause f o r t h i s high pore pressure development rates i n t a i l i n g s sands. A.2 L i q u e f a c t i o n Resistance of T a i l i n g s Sand In the following discussion, the c y c l i c stress required to develop ±5% shear s t r a i n i n 10 loading cycles i s defined as the resistance to l i q u e f a c t i o n . Results based on development of ±5% shear s t r a i n i n 15 cycles as well as ±2.5% s t r a i n i n 10 and 15 cycles w i l l also be presented l a t e r . The r e l a t i o n s h i p s between void r a t i o a f t e r consolidation and the number of cycles to cause l i q u e f a c t i o n of t a i l i n g s sand at various c y c l i c shear stress r a t i o s and under confining stresses of 200, 400, 800, 1600, and 2500 kPa are shown i n Figure (10). In these r e l a t i o n - ships, each contour was developed using a f i x e d l e v e l of c y c l i c shear str e s s r a t i o while the void r a t i o was varied. It may be seen that the number of cycles to l i q u e f y the t a i l i n g s sand increases very r a p i d l y with decrease i n void r a t i o at low confining stresses regardless of the l e v e l of c y c l i c shear stress r a t i o . However, at high confining stresses, a s i m i l a r decrease In void r a t i o r e s u l t s i n a much smaller increase i n number of cycles to cause l i q u e f a c t i o n . This i s apparent i n progressive steepening of the void r a t i o versus number of cycles contours i n Figure (10) as the confining stress increased. Figure (11) shows the l i q u e f a c t i o n resistance of t a i l i n g s sand as a function of r e l a t i v e density and confining s t r e s s . The l i q u e f a c t i o n r e s i s t a n c e i s defined here as the c y c l i c stress r a t i o to cause ±5% shear s t r a i n i n 10 cy c l e s . The l i q u e f a c t i o n resistance, as expected, increases with increase i n r e l a t i v e density. However, the rate of t h i s increase depends on the confining stress l e v e l and r e l a t i v e density range c o n s i d e r e d . At low c o n f i n i n g s t r e s s e s ( a' < 400 kPa) , the vo l i q u e f a c t i o n resistance b u i l d s up very r a p i d l y over a narrow range of r e l a t i v e density i n excess of about 60%. A small increase i n r e l a t i v e d e n s i t y at these confining stress l e v e l s r e s u l t s In a considerable increase i n the resistance. The resistance curves at confining stresses < 400 kPa are highly nonlinear i n contrast to the generally assumed l i n e a r increase In resistance with the increase i n r e l a t i v e density (1). 22. 2 4 6 8 10 20 40 60 80 100 200 400 600 800 1000 Number of Cycles to Liquefaction, Nl F i g . (10a). Number of Cycles to Liquefaction Versus Void Ratio f o r T a i l i n g s Sand at Confining Pressure of 200 kPa. 1.0 L 0.9 L 0.8 r 2 4 6 8 10 20 40 60 80 100 200 400 600 800 1000 Number of Cycles to Liq u e f a c t i o n , Nl F i g . (10b). Number of Cycles to Liqu e f a c t i o n Versus Void Ratio f o r T a i l i n g s Sand at Confining Pressure of 400 kPa. 23. o 0) 1.0 c o •H 4-1 CO T J •H rH O t n c 0.9 o u u OJ 4J 14-1 < o •H 4-1 0.8 es -a •H o > 0.7 1 I - I — r •' = vo Y = + 5% .T /a' • cy vo • II • II = 0.122 = 0.151 = 0.181 2 4 6 8 10 20 40 60 80 100 200 400 600 800 1000 Number of Cycles to Liquefaction, Nl F i g . (10c). Number of Cycles to Liquefaction Versus Void Ratio For T a i l i n g s Sand at Confining Pressure of 800 kPa. u 1. 0 „ c o •H 4-1 CO T J i - l i-H o ns  0. 9 o u u OJ 4-1 < O I 0 .8 4-1 CO O J T J i - l o > 0.7 a' = 1600 kPa vo V = + 5% T /a' cy vo = 0.118 - 0.132 = 0.148 = 0.178 2 4 6 8 10 20 40 60 80 100 200 400 600 800 1000 Number of Cycles to Liquefaction, Nl Fi g . (10d). Number of Cycles to Liqu e f a c t i o n Versus Void Ratio For T a i l i n g s Sand at Confining Pressure of 1600 kPa. 24. 1 1—i—r = 2500 kPa = + 5% . T /a' cy vo = 0.118 = 0.147 = 0.177 4 6 8 10 20 40 60 80 100 200 400 600 800 1000 Number of Cycles to Liquefaction, NI Number of Cycles to Liquefaction Versus Void Ratio For T a i l i n g s Sand at Confining Stress of 2500 kPa. 2 5 . At high confining stress l e v e l s , the b u i l d up i n l i q u e f a c t i o n r e s i s t a n c e with increase i n r e l a t i v e density i s much smaller than at lower confining s t r e s s e s . The resistance curves become progressively f l a t t e r as the confining pressure increases. The l i q u e f a c t i o n r e s i s t - ance, at a given r e l a t i v e density l e v e l , decreases with the increase i n c o n f i n i n g stress as i t i s apparent i n Figure (11). This decrease i n r e s i s t a n c e with increase i n confining stress seems to be confined to r e l a t i v e d e n s i t i e s i n excess of about 55%. As the r e l a t i v e density increases above t h i s value, the reduction i n resistance becomes pro- g r e s s i v e l y larger with increasing confining pressure. The most drama- t i c decrease i n resistance seems to be associated with the increase of confining stress from 200 kPa to 800 kPa. At r e l a t i v e density of 77%, the reduction i n l i q u e f a c t i o n resistance as the confining stress increases from 200 kPa to 800 kPa i s about 60%, while the t o t a l reduc- t i o n i n the resistance at the same r e l a t i v e density l e v e l as the con- f i n i n g stress increases from 200 kPa to 2500 kPa i s 68%. Samples at r e l a t i v e d e n s i t i e s greater than about 77%, at a confin- ing stress of 200 kPa could not be formed e a s i l y i n the laboratory. On the other hand, the minimum achieved r e l a t i v e density at a confining s t r e s s of 2500 kPa was about 60%. This confining stress of 2500 kPa brought the r e l a t i v e density of samples prepared at i n i t i a l r e l a t i v e density of about 20% to a r e l a t i v e density of about 60% a f t e r c o n s o l i - dation. These l i m i t a t i o n s i n the r e l a t i v e d e n s i t i e s under confining stresses of 200 kPa and 2500 kPa l i m i t s the comparison between the responses under the extreme values of confining stresses to the range of r e l a t i v e density between 60% and 77% only. Due to very high l i q u e - f a c t i o n resistance at confining stress of < 400 kPa, t a i l i n g s sand at 2 6 . Void Ratio, e 0.95 0.90 0.85 0.80 0.75 0.70 100 Relative Density, D % Fi g . 11. Resistance to Liquefaction of T a i l i n g s Sand at Various Confining Pressures Based on Shear S t r a i n Level of + 5% in 10 Cycles. r e l a t i v e d e n s i t i e s i n excess of 70% i s u n l i k e l y to l i q u e f y even under moderate to strong earthquakes, whereas at c o n f i n i n g s t r e s s of 2500 kPa, the same sand may be s u s c e p t i b l e to l i q u e f a c t i o n even at r e l a t i v e d e n s i t i e s approaching 90% under moderate earthquake shaking. The r e d u c t i o n i n l i q u e f a c t i o n r e s i s t a n c e w i t h the increase i n c o n f i n i n g s t r e s s occurs a t high r e l a t i v e d e n s i t i e s r a t h e r than low ones. At r e l a t i v e d e n s i t i e s of about 50% to 55%, a l l r e s i s t a n c e curves tend to merge together, i n d i c a t i n g l i q u e f a c t i o n r e s i s t a n c e r e l a t i v e l y independent of c o n f i n i n g s t r e s s . As the r e l a t i v e d e n s i t y drops below about 50%, the l i q u e f a c t i o n r e s i s t a n c e becomes higher at higher c o n f i n - i n g s t r e s s e s , provided t h i s r e l a t i v e d e n s i t y l e v e l i s a c c e s s i b l e a t the c o n f i n i n g pressure under c o n s i d e r a t i o n . The v a r i a t i o n of c y c l i c s t r e s s r a t i o to cause l i q u e f a c t i o n w i t h i n c r e a s e i n c o n f i n i n g s t r e s s a t v a r i o u s r e l a t i v e d e n s i t y l e v e l s i s shown i n Fi g u r e (12) while the percent r e d u c t i o n i n c y c l i c s t r e s s r a t i o to cause l i q u e f a t i o n w i t h increase i n c o n f i n i n g s t r e s s at these r e l a - t i v e d e n s i t y l e v e l s , when r e f e r r e d to a c o n f i n i n g pressure of 200 kPa, are shown i n Figure (13). I t may again be noted from Figures (12) and (13) that the l a r g e s t r e d u c t i o n i n l i q u e f a c t i o n r e s i s t a n c e occurs as a consequence of c o n f i n i n g s t r e s s increase from 200 kPa to 800 kPa. The higher the r e l a t i v e d e n s i t y , the more i s the r e d u c t i o n . Not much change i n l i q u e f a c t i o n r e s i s t a n c e occurs when the c o n f i n i n g s t r e s s i n c r e a s e s from 800 kPa to 1600 kPa, except at r e l a t i v e d e n s i t i e s i n excess of about 65%. However, a l a r g e r r e d u c t i o n occurs as the c o n f i n - ing s t r e s s exceeds 1600 kPa. It may a l s o be noted from Figure (12) that the change i n r e l a t i v e d e n s i t y a t lower c o n f i n i n g s t r e s s e s has a much l a r g e r e f f e c t on the 28. C o n f i n i n g S t r e s s , a' i n kPa ° vo F i g . 12. V a r i a t i o n of C y c l i c Stress R a t i o to Cause L i q u e f a c - t i o n of T a i l i n g s Sand With Co n f i n i n g Pressure at Various R e l a t i v e Density Levels. 29. • • • i i 1 1 • . . 800 1200 1600 2000 2400 2800 3200 Co n f i n i n g S t r e s s , a' i n kPa ° vo Percent Reduction i n Resistance to L i q u e f a c t i o n of T a i l i n g s Sand When Referred to the Resistance at 200 kPa. 3 0 . l i q u e f a c t i o n resistance of t a i l i n g s sand. At a confining stress of 200 kPa, the c y c l i c stress r a t i o ( T C y / a ^ 0 ) required to l i q u e f y a sample at r e l a t i v e density of about 50% Is only 0.13, whereas a c y c l i c stress r a t i o of about 0.45 i s needed to l i q u e f y samples at r e l a t i v e d e n s i t i e s of 77% under the same confining s t r e s s . On the other hand, a change In r e l a t i v e density at a high confining stress has only a small e f f e c t on l i q u e f a c t i o n resistance e.g., at a confining stress of 2500 kPa, the c y c l i c stress r a t i o (T /o' ) to cause l i q u e f a c t i o n increased only from J cy vo M J 0.13 to 0.155 as the r e l a t i v e density increased from 60% to 85%. The c o m p r e s s i b i l i t y c h a r a c t e r i s t i c s of t a i l i n g s , as measured i n one-dimensional consolidation tests and expressed as the r e l a t i o n s h i p between v o i d r a t i o e and l o g were shown i n Figure ( 4 ) . It may be noted that considerable volume compression occurs on a p p l i c a t i o n of large confining pressures. The e f f e c t of confining stress on r e l a t i v e density increase, however, decreases with increase i n i n i t i a l r e l a t i v e density. The substantial increase i n r e l a t i v e density i s the one asso- ci a t e d with the a p p l i c a t i o n of high confining stresses on samples of i n i t i a l l y low r e l a t i v e density. Figure (14) shows the r e l a t i o n s h i p s between the i n i t i a l void r a t i o s and the void r a t i o s a f t e r consolidation at confining stresses under consideration. It i s i n t e r e s t i n g to note that at each selected confining s t r e s s , a l i n e a r r e l a t i o n s h i p i s obtained between i n i t i a l v o i d r a t i o e. and the v o i d r a t i o a f t e r c o n s o l i d a t i o n , e . T h i s i c observed l i n e a r i t y i n the r e l a t i o n s h i p s between e, and e helped i n i c estimating the r e l a t i v e d e n s i t i e s a f t e r consolidation at each confining stress l e v e l . The v a r i a t i o n i n l i q u e f a c t i o n resistance with increase i n  32. c o n f i n i n g s t r e s s a t f i x e d values of i n i t i a l r e l a t i v e d e n s i t i e s ( D r^) i s shown i n Figure ( 1 5 ) . Each contour i n Figure (15) represents the l i q u e f a c t i o n r e s i s t a n c e of samples co n s o l i d a t e d along a t y p i c a l conso- l i d a t i o n curve i n Figure ( 4 ) . The c o n f i n i n g s t r e s s has two e f f e c t s on the l i q u e f a c t i o n r e s i s t a n c e , which are opposite to each other, 1. Increased c o n f i n i n g s t r e s s reduces l i q u e f a c t i o n r e s i s t a n c e . 2. Increased c o n f i n i n g s t r e s s increases the r e l a t i v e d e n s i t y which, i n t u r n , i n c r e a s e s the r e s i s t a n c e . The r e s i s t a n c e curves i n Figure (15) show the net i n f l u e n c e of these two f a c t o r s . I t may be seen that at low i n i t i a l r e l a t i v e d e n s i t i e s , the e f f e c t of d e n s i f i c a t i o n f a r out- weighs t h a t of i n c r e a s i n g c o n f i n i n g s t r e s s up to of about 1600 kPa. At higher c o n f i n i n g s t r e s s e s , the e f f e c t of c o n f i n i n g s t r e s s i n reduc- in g the c y c l i c r e s i s t a n c e seems to be predominant. A general continu- ous decrease i n r e s i s t a n c e i s obtained w i t h i n c r e a s e i n c o n f i n i n g s t r e s s a t higher i n i t i a l r e l a t i v e d e n s i t i e s . This i n d i c a t e s that at these hig h i n i t i a l r e l a t i v e d e n s i t i e s , the increase i n r e l a t i v e d e n s i t y due to c o n s o l i d a t i o n i s too small to o f f s e t the r e d u c t i o n due to high c o n f i n i n g s t r e s s e s . That i n i t i a l d e n s i f i c a t i o n of t a i l i n g s sand had a l a r g e e f f e c t on i t s l i q u e f a c t i o n r e s i s t a n c e a t lower c o n f i n i n g s t r e s s e s ( O ^ q < 400 kPa) may be noted from Figure ( 1 5 ) . At c o n f i n i n g s t r e s s of 200 kPa, f o r example, the r e s i s t a n c e increased by 150%, i f the sand had been i n i t i a l l y d e n s i f i e d to a r e l a t i v e d e n s i t y of 70% i n s t e a d of 35%. At h i g h e r c o n f i n i n g s t r e s s e s (°^ 0 > 800 kPa) i n i t i a l d e n s i f i c a t i o n had l i t t l e b e n e f i t to the l i q u e f a c t i o n r e s i s t a n c e . At c o n f i n i n g s t r e s s of 2500 kPa, only 19% increase i n r e s i s t a n c e was observed as the i n i t i a l r e l a t i v e d e n s i t y i n c r e a s e s from 35% to 70%. This has a considerable p r a c t i c a l s i g n i f i c a n c e i n de c i d i n g placement d e n s i t i e s of t a i l i n g s at 33. F i g . 15. E f f e c t of Confining Pressure on the Resistance to Liquefaction of T a i l i n g s Sand Prepared at Various I n i t i a l Relative Densities. 34. various l o c a t i o n s i n t a i l i n g s dams. Li q u e f a c t i o n resistance of t a i l i n g s sand based on the development of ±5% shear s t r a i n i n 15 stress cycles rather than 10 cycles i s shown i n Figure (16). The resistance curves look very s i m i l a r to those pre- v i o u s l y discussed f o r 10 cy c l e s . The r e l a t i v e p o s i t i o n s of the curves at a l l confining stresses stay about the same. The values of the c y c l i c stress r a t i o s to cause ±5% s t r a i n i n 15 cycles are as expected l e s s than those f o r 10 cycle s as shown In Figure (11). A l l the curves i n Figure (16) are e s s e n t i a l l y h o r i z o n t a l l y s h i f t e d towards higher r e l a t i v e d e n s i t i e s . This s h i f t r e s u l t s i n the resistance curves merg- ing at s l i g h t l y higher r e l a t i v e density l e v e l s than i n the case of 10 cycles. The l i q u e f a c t i o n resistance of t a i l i n g s sand based on the development of ±2.5% shear s t r a i n i n 10 cycles and 15 cycles i s shown i n Figures (17) and (18) respectively. Figure (17) shows the c y c l i c s t r e s s r a t i o required to cause ±2.5% shear s t r a i n i n 10 cycles with increase i n r e l a t i v e density at various confining stresses. The resistance curves i n Figure (17) are s i m i l a r to those f o r ±5% s t r a i n shown i n Figure (11) except that they become s l i g h t l y f l a t t e r i n the region of higher r e l a t i v e d e n s i t i e s . This i s because, a r e l a t i v e l y l a r g e number of cycles i s needed f or dense sand to bring up the shear s t r a i n from 2.5% to 5%. Such a di f f e r e n c e i s c l e a r l y shown i n Figures (6, 7, 8, and 9), where the number of cycles needed to increase the shear s t r a i n from 2.5% to 5% could be up to 10 cycles or more i n the case of dense sand. On the other hand the s t r a i n b u i l d s up from very low values to more than 5% occurs i n only one c y c l e i n loose sand. L i q u e f a c t i o n resistance curves of t a i l i n g s sand based on the 35. 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 Y = ± 5% i n 15 c y c l e s 30 F i g . 16. a ' = 200 kPa r/ ,o vo « " = 400 kPa . " = 800 kPa £ " = 1600 kPa = 2500 kPa 80 90 100 _J , I • 1 — - 40 50 60 70 R e l a t i v e Density, Dr% Resistance to L i q u e f a c t i o n of T a i l i n g s Sand at Various C o n f i n i n g Pressures Based on Shear S t r a i n L e v e l of + 5% i n 15 Cycles. 3 6 . F i g . 17. Resistance to Liquefaction of T a i l i n g s Sand at Various Confining Pressures Based on Shear S t r a i n Level of + 2.5% i n 10 Cycles. 3 7 . i—•—r i—•—r 0.22 0.20 r >• 0.18| 0 .16U 0 . 1 4 0.12H O.lOh 0.081 Y = + 2.5% In 15 cycles = 200 kPa = 400 kPa = 800 kPa = 1600 kPa" = 2500 kPa X 30 40 50 60 70 80 Relative Density, D r% 90 100 F i g . 18. Resistance to Liquefaction of T a i l i n g s Sand at Various Confining Pressures, Based on Shear S t r a i n Level of ± 2.5% i n 15 Cycles. 38. development of 2.5% s t r a i n i n 15 cycles are shown i n Figure (18). The resistance curves are s i m i l a r to those i n Figure (17) with a s l i g h t s h i f t towards the higher r e l a t i v e d e n s i t i e s . In both Figures (17 and 18), the r e l a t i v e p o s i t i o n of the curves stays the same as i n the case of 5% s t r a i n . This implies that s i m i l a r reduction i n l i q u e f a c t i o n r e s i s t a n c e occurs with increase i n confining stress, regardless of the shear s t r a i n l e v e l to define the l i q u e f a c t i o n . 4.3 L i q u e f a c t i o n Resistance of Ottawa Sand The resistance to l i q u e f a c t i o n i s f i r s t based on the development of ±5% shear s t r a i n i n 10 cycles. Data obtained at other l e v e l s of shear s t r a i n and d i f f e r e n t number of loading cycles w i l l be discussed l a t e r . The basic data on l i q u e f a c t i o n resistance of Ottawa sand i s shown i n Figure (19). The increase i n number of cycles to cause ±5% shear s t r a i n with the decrease i n void r a t i o at confining stress of 200 kPa was very rapid and s i m i l a r to that f o r t a i l i n g s at the same confining s t r e s s . At higher confining s t r e s s , even though the void r a t i o range i s d i f f e r e n t from that at low confining s t r e s s , the increase i n number of c y c l e s to cause l i q u e f a c t i o n as the void r a t i o decreases i s s t i l l at a high r a t e . This i s a d i s t i n c t i v e c h a r a c t e r i s t i c of Ottawa sand when compared to the behaviour of t a i l i n g s sand and implies a rapid buildup of resistance to l i q u e f a c t i o n with r e l a t i v e density even at high confining s t r e s s e s . The l i q u e f a c t i o n resistance of Ottawa sand as expressed by a r e l a - t i o n between the c y c l i c stress r a t i o to cause ±5% s t r a i n i n 10 cycles and r e l a t i v e density at each confining stress l e v e l i s shown i n Figure 39. .8 0.7 a' =200 kPa vo Y = ± 5% T /O' o cy vo A " a a • it = 0.085 = 0.127 = 0.157 = 0.187 4 6 8 10 20 40 60 80 100 200 Number of Cycles to Li q u e f a c t i o n , NI 400 600 800 1000 F i g . (19a) . Number of Cycles to Liquefaction Versus Void Ratio for Ottawa Sand at Confining Pressure of 200 kPa. a' = 800 kPa vo Y = + 5% x / a ' =0.105 cy vo = 0.121 = 0.151 =0.181 0.6 0.5 4 6 8 10 20 40 60 80 100 200 Number of Cycles to Liquefaction, NI 400 600 800 1000 F i g . (19b). Number of Cycles to Liqu e f a c t i o n Versus Void Ratio f o r Ottawa Sand at Confining Pressure of 800 kPa. 40. oi 0.80 c o o CO c o u u QJ 4-1 < cd o > 0.70 0.60 0.50 - i 1 1—r a' = 1600 kPa vo Y = ± 5% T /a' • cy vo = 0.118 = 0.148 = 0.178 6 8 10 20 40 60 80 100 200 400 600 800 1000 Number of Cycles to L i q u e f a c t i o n , NI F i g . (19c). Number of Cycles to L i q u e f a c t i o n Versus Void R a t i o For Ottawa Sand at Con f i n i n g Pressure of 1600 kPa. 0.80 h c o rt o c o c o u u OJ 4-1 < m pi > 0.70 h 0.60 0.50 4 6 8 10 20 40 60 80 100 200 Number of Cycles to L i q u e f a c t i o n , NI 400 600 800 1000 F i g . (19d) Number of Cycles to L i q u e f a c t i o n Versus Void R a t i o For Ottawa Sand at Co n f i n i n g Pressure of 2500 kPa. (20) . A nonlinear increase i n resistance as the r e l a t i v e density increases may be noted from Figure (20) over p r a c t i c a l l y the f u l l range of r e l a t i v e d e n s i t i e s considered. The resistance curve at confining stress of 200 kPa has also been presented i n e a r l i e r studies (see 8). No t e s t s were performed at a confining stress of 400 kPa, however, e a r l i e r studies on t h i s sand (8) have shown that i t s resistance to l i q u e f a c t i o n was not affected by the increase of confining stress from 200 kPa to 400 kPa. It may be seen i n Figure (20) that the l i q u e f a c - t i o n resistance b u i l d s up r a p i d l y as the r e l a t i v e density increases, even at high confining stresses. This occurs despite the d i f f e r e n c e i n the r e l a t i v e density range between low and high confining stresses at which samples can e x i s t . The resistance to l i q u e f a c t i o n at r e l a t i v e d e n s i t i e s i n excess of about 55% decreases with increase i n confining stress l e v e l . The reduction i n resistance i s larger at higher r e l a t i v e d e n s i t i e s . Again, as f o r t a i l i n g s , the most s i g n i f i c a n t reduction i n resistance to l i q u e f a c t i o n occurs as the confining stress increases from 200 kPa to 800 kPa. No change i n resistance seems to occur when confining stress increases from 800 kPa to 1600 kPa. Then as the con- f i n i n g stress exceeds 1600 kPa, further reduction i s noticed, regard- l e s s of the r e l a t i v e density l e v e l . At r e l a t i v e d e n s i t i e s below about 55%, the shape of the r e s i s t a n c e curve at o' of 1600 kPa seems to ' vo s u g g e s t a c r o s s o v e r with that at of 200 kPa, i m p l y i n g l a r g e r resistance at lower r e l a t i v e d e n s i t i e s associated with higher confining stresses. Ottawa sand i s u n l i k e l y to exist at r e l a t i v e d e n s i t i e s l e s s than about 50% under confining stresses i n excess of 800 kPa. At the same time, samples at r e l a t i v e d e n s i t i e s of more than 75% were extremely 42. Void Ratio, e 0.70 0.65 0.60 0.55 0.50 100 Relative Density, D r% F i g . 20 . Resistance to Liq u e f a c t i o n of Ottawa Sand at Various Confining Pressures Based on Shear S t r a i n Level of + 5% i n 10 Cycles. 4 3 . d i f f i c u l t to prepare under a consolidation pressure of 200 kPa. Thus the v a r i a t i o n of c y c l i c stress r a t i o with the increase i n confining s t r e s s at various r e l a t i v e d e n s i t i e s which i s shown i n Figure (21), spans a r e l a t i v e density range of only 55 to 75%. It may be noted from Figure (21) that the decrease i n l i q u e f a c t i o n resistance with increase i n confining stress i s larger at higher r e l a t i v e d e n s i t i e s . The increase i n resistance with increase i n r e l a t i v e density i s higher at lower confining stresses. At high confining stresses, even though the increase i n resistance with increase i n r e l a t i v e density i s l e s s than that at low confining stresses, Ottawa sand, unlike t a i l i n g s , shows a marked increase i n resistance with increase i n r e l a t i v e density. The percent reduction i n resistance due to increase of confining stress as i t i s referred to a confining stress of 200 kPa i s shown i n Figure (22). Unlike the behaviour of t a i l i n g s , the a p p l i c a t i o n of high confin- ing stress to Ottawa sand causes only a small change i n r e l a t i v e density (Figure 5). A confining stress of 2500 kPa causes the r e l a t i v e density of the loose sample to increase by only 15% and dense sample by a mere 6%. Nevertheless, even these smaller increases i n r e l a t i v e d e n s i t y are much more e f f e c t i v e , In comparison with that f o r t a i l i n g s , i n increasing i t s resistance to l i q u e f a c t i o n . This i s so because of steepness of the resistance curves shown i n Figure (20). The void r a t i o of Ottawa sand a f t e r c o n s o l i d a t i o n i s again l i n e a r l y r e l a t e d with the i n i t i a l void r a t i o at a l l confining stress l e v e l s . This r e l a t i o n s h i p , as f o r t a i l i n g s sand, was useful i n predic- t i n g the r e l a t i v e density a f t e r consolidation under a prescribed confining pressure when s t a r t i n g from a known i n i t i a l density. 44. 0.34 l 1 1 1 p o > o •H •u CO OS CO w 0) o •H a 0.18 0.16 0.14 0.12 0.10 Y = + 5% i n 10 cycles] 8 0 0 1 2 0 0 1 6 0 0 2 0 0 0 2400 2800 Conf i n i n g Pressure, a' i n kPa vo F i g . 21, V a r i a t i o n of C y c l i c Stress Ratio to Cause L i q u e f a c t i o n For Ottawa Sand With C o n f i n i n g Pressure at Various R e l a t i v e Density L e v e l s . 45. C o n f i n i n g S t r e s s , a' i n kPa F i g . 22 . Percent Reduction i n Resistance to L i q u e f a c t i o n of Ottawa Sand When Referred to the Resistance at 200 kPa. 46. Figure (24) shows the r e l a t i o n s h i p between c y c l i c stress r a t i o to cause l i q u e f a c t i o n and confining stress at various i n i t i a l r e l a t i v e d e n s i t i e s . At i n i t i a l d e n s i t i e s up to about 62%, the c y c l i c resistance i n c r e a s e s w i t h i n c r e a s i n g confining stress up to of ~1200 kPa, as the p o s i t i v e influence of d e n s i f i c a t i o n exceeds the negative e f f e c t of confinin g stress increase. At i n i t i a l r e l a t i v e density of about 62% and a confining stress range from 200 kPa to ~1600 kPa, the p o s i t i v e e f f e c t of d e n s i f i c a t i o n of the sand with confining stress seems to compensate the negative e f f e c t due to the confining stress Increase. A general reduction i n c y c l i c resistance may be noted with confining stress increase at r e l a t i v e d e n s i t i e s i n excess of 62%. A very s l i g h t change i n resistance occurs at a l l i n i t i a l r e l a t i v e d e n s i t i e s when the confining stress increases from 800 kPa to 1600 kPa. As the confining s t r e s s exceeds 1600 kPa, the l i q u e f a c t i o n resistance decreases with increase i n confining s t r e s s , regardless of the i n i t i a l r e l a t i v e density l e v e l . I n i t i a l d e n s i f i c a t i o n of Ottawa sand i s b e n e f i c i a l i n increasing i t s l i q u e f a c t i o n resistance over a broad range of confining stress as shown i n Figure (24). Regarding t h i s aspect, d e n s i f i c a t i o n i s much more e f f e c t i v e at lower confining stresses. At a confining stress of 200 kPa, the l i q u e f a c t i o n resistance i s increased by 80% as a conse- quence of increase i n i n i t i a l r e l a t i v e density l e v e l from 53% to 69%. Whereas at confining stress of 2500 kPa, the gain i n resistance f o r the same increase i n i n i t i a l r e l a t i v e density was only 32%. Results of l i q u e f a c t i o n resistance at ±5% shear s t r a i n i n 15 cy c l e s are shown i n Figure (25). The resistance curves, i n t h i s case, are s i m i l a r to that for 10 cycles and t h e i r r e l a t i v e p o s i t i o n stays 0.70 0.55 0.60 0.65 I n i t i a l Void Ratio, e± 0.70 0.75 Fig. 23. I n i t i a l Void Ratio, With Void Ratio A f t e r Consolidation, e , Relationships of Ottawa Sand at Various Confining Pressures. ° » • I , I , I , 1 , I , I—,—I 400 800 1200 1600 2000 2400 2800 3200 Co n f i n i n g S t r e s s , a' i n kPa F i g . 24. E f f e c t of Co n f i n i n g Pressure on f a c t i o n of Ottawa Sand Prepared D e n s i t i e s . the Resistance to Lique- at Various I n i t i a l R e l a t i v e Relative Density, D % F i g . 25. Resistance to Liquefaction of Ottawa Sand at Various Confining Pressures Based on Shear S t r a i n Level of + 5% i n 15 Cycles. 5 0 . almost the same. As expected, a higher r e l a t i v e density i s needed to withstand a c e r t a i n c y c l i c shear stress l e v e l for 15 loading cycles than the r e l a t i v e density required for the same shear stress l e v e l i n 10 cycles at a l l confining stress l e v e l s . The reduction i n c y c l i c stress r a t i o to cause ±5% s t r a i n i n 15 cycles at fixed r e l a t i v e density l e v e l s i s very s i m i l a r to that for 10 cy c l e s . The l i q u e f a c t i o n resistance of Ottawa sand based on shear s t r a i n l e v e l of ±2.5% i n 10 and 15 cycles i s shown i n Figures (2 6) and (2 7) r e s p e c t i v e l y . A s i m i l a r conclusion to that on l i q u e f a c t i o n resistance of t a i l i n g s sand at the same s t r a i n l e v e l can be applied here. The r e l a t i v e positions of the resistance curves at the various confining pressure l e v e l s stay very well the same as i n the case of ±5% shear s t r a i n . This implies a s i m i l a r reduction i n l i q u e f a c t i o n resistance, with increase i n confining pressure, to that at ±5% shear s t r a i n regardless of the shear s t r a i n l e v e l to define the l i q u e f a c t i o n . 5 1 . F i g . 26. Resistance to Liquefaction of Ottawa Sand at Various Confining Pressures, Based on Shear S t r a i n Level of + 2.5% i n 10 Cycles. 52. Relat i v e Density, D r% 27. Resistance to Liq u e f a c t i o n of Ottawa Sand at Vario Confining Pressures Based on Shear S t r a i n Level of ± 2.5% i n 15 Cycles. 53. 5. EFFECT OF CONFINING STRESS AND PARTICLE ANGULARITY ON LIQUEFACTION POTENTIAL The main purpose of t h i s research was to investigate the e f f e c t of c o n f i n i n g stress and p a r t i c l e angularity on l i q u e f a c t i o n p o t e n t i a l . The e f f e c t of confining stress has been investigated and discussed i n Chapter 4 by looking at the resistance of both angular and rounded sands at various confining stress l e v e l s . The e f f e c t of p a r t i c l e angu- l a r i t y w i l l be considered by comparing the response of the two sands at f i x e d l e v e l s of confining stress and r e l a t i v e density. In the follow- ing discussion, development of ±5% s t r a i n i n 10 cycles i s considered as the occurrence of l i q u e f a c t i o n . The resistance to c y c l i c loading of both t a i l i n g s and Ottawa sand, as discussed e a r l i e r , c l e a r l y i n d i c a t e a reduction with increase i n confining s t r e s s . The v a r i a t i o n of the c y c l i c stress r a t i o to l i q u e f y both sands with confining stress increase i s shown i n Figures (12 and 19). Both figures show a general reduction i n c y c l i c stress r a t i o to cause l i q u e f a c t i o n with increase i n confining s t r e s s , regardless of the sand type. The magnitude of reduction, however, depends on the sand type, confining stress l e v e l , and the r e l a t i v e density l e v e l at which t h i s evaluation i s made. The percent reduction i n l i q u e f a c t i o n resistance with confining s t r e s s increase at various l e v e l s of r e l a t i v e density i s shown i n Figures (13 and 20) for t a i l i n g s and Ottawa sand. In these f i g u r e s , the resistance at confining stress of 200 kPa was taken as the r e f e r - ence. The l i q u e f a c t i o n resistance of t a i l i n g s sand, f o r example, reduces by as much as 56% due to an increase i n confining stress from 54. 200 kPa to 2500 kPa at r e l a t i v e density of 75%. Most of the reduction occurs as the confining stress increases from 200 kPa to 800 kPa. The same trend appears to be applicable f o r Ottawa sand but with r e l a t i v e l y lower reductions. The amount of reduction decreases as the r e l a t i v e density decreases. Furthermore, there seems to be a c e r t a i n upper l i m i t of r e l a t i v e density, f o r both sands, below which the l i q u e f a c t i o n r e s i s t a n c e appears not to be negatively affected by the increase i n confining s t r e s s . The value of t h i s r e l a t i v e density i s about 50 to 55% for both sands. The e f f e c t of confining stress on the l i q u e f a c t i o n p o t e n t i a l , however, cannot be i s o l a t e d from the e f f e c t of the p a r t i c l e shape. Very large confining stresses r e s u l t i n very large d e n s i f i c a t i o n of angular sands and consequently may increase t h e i r c y c l i c resistance more by d e n s i f I c a t i o n than the reduction due to Increasing confining pressure. The two sands used i n the study had i d e n t i c a l mineral composition and gradation. Therefore the d i f f e r e n c e i n t h e i r response to c y c l i c loading can be att r i b u t e d s o l e l y to the differences i n angularity of t h e i r p a r t i c l e s . Figure (2 8) shows a d i r e c t comparison of l i q u e f a c t i o n r e s i s t a n c e of the two sands at confining stresses of 200 kPa and 2500 kPa. It may be noted that at low confining stress (200 kPa), the angu- l a r sand i s more r e s i s t a n t to l i q u e f a c t i o n than the rounded sand over the e n t i r e range of r e l a t i v e d e n s i t i e s investigated. At high confining s t r e s s (2500 kPa), the di f f e r e n c e i n t h e i r response depends on the r e l a t i v e density l e v e l under consideration. At r e l a t i v e d e n s i t i e s l e s s than about 70%, the angular sand i s more r e s i s t a n t to l i q u e f a c t i o n than the rounded sand. As the r e l a t i v e density exceeds about 70%, the 55. F i g . 28. Comparison of Resistance to L i q u e f a c t i o n of Angular and Rounded Sands at Low and High Confining Pressures. 56. rounded sand becomes more r e s i s t a n t than the angular sand. The d i f f e r e n c e between the resistance of the two sands beyond the r e l a t i v e d e n s i t y of 70% increases r a p i d l y as the r e l a t i v e density increases. T h i s may be due to the much f a s t e r rate of the resistance buildup of Ottawa sand at high confining stresses, whereas for angular sand, the r e s i s t a n c e curve at high confining stress (2 500 kPa) i s much f l a t t e r . Comparison of l i q u e f a c t i o n resistance at other confining pressures can be made i n a s i m i l a r manner, which would show a gradual transion between the behaviour at low (200 kPa) and high (2 500 kPa) confining pressures. Thus, the resistance of angular sand can be e i t h e r larger or smaller than that of i t s rounded counterpart, depending upon the l e v e l of confining stress and the magnitude of r e l a t i v e density at which the comparison i s made. At low confining stresses, both angular and rounded sands are u n l i k e l y to l i q u e f y at r e l a t i v e d e n s i t i e s i n excess o f about 70% even under strong earthquakes. This i s apparent i n Figure (2 8) which shows extremely high c y c l i c resistance for r e l a t i v e densi- t i e s i n excess of about 70%. At high confining stresses, t a i l i n g s sand may be susceptible to l i q u e f a c t i o n even at r e l a t i v e d e n s i t i e s approach- ing 100% under moderate earthquake shaking. Ottawa sand at high c o n f i n i n g stresses i s more r e s i s t a n t than t a i l i n g s sand and seems u n l i k e l y to l i q u e f y at r e l a t i v e d e n s i t i e s i n excess of about 80% even under strong earthquakes. The low resistance o f t a i l i n g s at higher confining stresses may be due to the breakage of sharp edges of t a i l i n g s sand p a r t i c l e s under c y c l i c shear s t r a i n s . The consequence of p a r t i c l e breakage during c y c l i c loading i s analogous to increased p a r t i c l e compressibility, 5 7 . which would r e s u l t i n an accelerated pore water pressure r i s e i n undrained t e s t s . The resistance to l i q u e f a c t i o n of such a sand, under co n f i n i n g stress high enough to cause p a r t i c l e breakage during c y c l i c shearing, w i l l be of course l e s s than that i n the case where no p a r t i c l e breakage would occur. Results from monotonic undrained t r i a x i a l t e s t s on both sands i n d i c a t e that gradation curve of Ottawa sand a f t e r being tested at confining stress of 2500 kPa i s v i r t u a l l y i d e n t i c a l to that f o r the untested sand. For t a i l i n g s sand, however, about 0.5% increase i n f i n e s content was observed during consolidation under the confining stress of 2500 kPa and an a d d i t i o n a l increase of about 1.5% during monotonic undrained t e s t i n g . However, l a r g e r increase i n f i n e s may be expected from c y c l i c loading of t a i l i n g s . This implies that both c o n s o l i d a t i o n and shearing of angular sands r e s u l t s i n breakage of the shape edges of the p a r t i c l e under high confining stresses. No detect- able increase i n f i n e s content was noted for tests at confining stresses l e s s than 400 kPa and consequently l i q u e f a c t i o n resistance of t a i l i n g s at these confining stress l e v e l s would not be reduced by breakage of p a r t i c l e edges during c y c l i c shearing. 58. 6. CONCLUSIONS The e f f e c t s of high confining pressure and p a r t i c l e angularity on l i q u e f a c t i o n resistance of sands have been investigated under the constant volume c y c l i c simple shear condition. Two quartz sands of i d e n t i c a l mineral composition and gradation but varying i n p a r t i c l e angularity were used i n the study. The r e s u l t s show that a decrease i n l i q u e f a c t i o n resistance of sands occurs with increase i n confining stress regardless of p a r t i c l e shapes c o n s t i t u t i n g the sand. The magnitude of reduction i n resistance to l i q u e f a c t i o n with increase i n confining s t r e s s , however, depends on the r e l a t i v e density l e v e l , p a r t i c l e shape of the sand, and the range of confining stresses of i n t e r e s t . Liquefaction resistance of both angular and rounded sands at low c o n f i n i n g stress of 200 kPa increased very r a p i d l y with increase i n r e l a t i v e density. At t h i s low confining stress l e v e l , angular sand was more r e s i s t a n t to c y c l i c loading than rounded sand. At high confining s t r e s s e s , by contrast, the buildup i n l i q u e f a c t i o n resistance of angu- l a r sand with increase i n r e l a t i v e density i s very slow compared to that of rounded sand, which maintained a rapid rate of increase i n r e s i s t a n c e with increase i n r e l a t i v e density. This makes rounded sand under high confining stresses more r e s i s t a n t to c y c l i c loading than angular sand at higher r e l a t i v e d e n s i t i e s . The decrease i n resistance to l i q u e f a c t i o n with increase i n c o n f i n i n g s t r e s s , f o r both sands, increases with increase i n r e l a t i v e density. However, such decrease seems to be more s i g n i f i c a n t f o r angular sand. Very l i t t l e increase i n resistance occurs with large 59. increase i n r e l a t i v e density at high confining stresses i n such a sand. The most dramatic decrease i n resistance to l i q u e f a c t i o n , f o r both sands, was that associated with increase i n confining stress from 200 kPa to 800 kPa. A c e r t a i n r e l a t i v e density l e v e l appeared to e x i s t , f o r both angular and rounded sands, below which the resistance to l i q u e f a c t i o n was not affected by the negative e f f e c t of confining s t r e s s , provided such r e l a t i v e density states were accessible under confining stress c o n s i d e r e d . At low confining stresses ( ° v o < 400 kPa) , both sands at r e l a t i v e density i n excess of about 70% are u n l i k e l y to l i q u e f y even under strong earthquakes because of t h e i r extremely large resistance to l i q u e f a c t i o n (D of 75%, x /a' > 0.3). n r cy vo At high confining stresses, c y c l i c shearing tends to cause break- age of sharp edges of angular sand p a r t i c l e s , which induces more poten- t i a l volumetric contraction and consequently an accelerated pore water pressure increase. This makes angular sand more susceptable to l i q u e - f a c t i o n under high confining stresses than rounded sand. Angular sand could l i q u e f y under high confining pressure during even moderate earth- quakes, despite r e l a t i v e d e n s i t i e s approaching 100%. Furthermore, i n i t i a l d e n s i f i c a t i o n of angular sand was found to be of l i t t l e b e n e f i t i n increasing i t s l i q u e f a c t i o n resistance i f i t was to perform eventu- a l l y under high confining stresses. Rounded sand at r e l a t i v e d e n s i t i e s i n excess of about 80% seemed to be non-susceptible to l i q u e f a c t i o n even at confining stresses of up to 2500 kPa under strong earthquake shaking. Because of the d i f f e r e n c e s i n the behaviour of angular and rounded 6 0 . sands discussed above, extreme caution must be exercised i n t r y i n g to p r e d i c t c y c l i c resistance of t a i l i n g sand at high confining pressures from r e s u l t s of comparable studies on natural rounded sands or from studies on t a i l i n g s sands at low confining stresses. 61. REFERENCES 1. Alba, P.D., Chan, C.K., and Seed, H.B. "Determination of S o i l L i q u e f a c t i o n C h a r a c t e r i s t i c s by Large-Scale Laboratory Tests." Report EERC 75-14, University of C a l i f o r n i a , Berkeley, 1975. 2. Casagrande L., Maclver, B.N. "Design and Construction of T a i l i n g s Dams." S t a b i l i t y i n Open P i t t Mining. Society of Mining Engineers of AIME, 1971, New York. 3. Castro, G, and Poulos, S. "Factors A f f e c t i n g L i q u e f a t i o n and C y c l i c Mobility." Journal of the Geotechnical Engineering D i v i - sion, ASCE, Vol. 103, GT6, June 1977. 4. D'Appolonia, E. "Dynamic Loading." Journal of S o i l Mechanics and Foundation D i v i s i o n , ASCE, Vol. 96, No. SMI. 5. Department of Energy, Mines and Resources. "Tentative Design Guide f o r Mine Waste Embankment i n Canada." Mines Branch, Technical B u l l . 145, Information Canada, Ottawa, 1972. 6. Dorey, R. and Byrne, P.M. "Dynamic S t a b i l i t y of Thompson Creek T a i l i n g s Impoundment." ASCE National Convention on Dynamic Stabi- l i t y of Dams, New Orleans, L.A., 1982. 7. Finn, W.D.L. and Byrne, P.M. "Liquefaction P o t e n t i a l of Mine T a i l i n g s Dams." International Conference on Large Dams, QL̂ , R9, Mexico, 1976. 8. Finn, W.D.L. and Vaid, Y.P. "Liquefaction P o t e n t i a l From Drained Consant Volume C y c l i c Simple Shear Tests." Proceedings, 6th World Conference on Earthquake Engineering, 1977, Session 6. 9. Finn, W.D.L. and Vaid, Y.P. "Constant Volume C y c l i c Simple Shear Testing." Proceedings, 2nd International Conference on Microzona- tion for Safer Construction, Vol. I I , 1978. 10. M i t t a l , Hari K. and Morgenstern, Norbert R. "Design and Perform- ance of T a i l i n g s Dams." Proceedings of the Conference on Geotech- n i c a l P r a c t i c e f o r Disposal of S o l i d Waste Materials, Ann Arbor, Michigan, 1977. 11. Pickering, D.J. "A Simple Shear Machine f o r S o i l . " Ph.D. Thesis, U.B.C., 1969. 12. Seed, H.B. "Earthquake-Resistant Design of Earth Dams." Interna- t i o n a l Conference on Recent Advances In Geotechnical Earthquake Engineering and S o i l Dynamics, 1981, St. Louis, Vol. I I I . 13. Sherard, J.L., Woodward, R.J., G i z i e n s k i , M.S., CLeverger, W.A. "Earth and Earth-Rock Dams." 1963. 62. 14. Vaid, Y.P. "Invited Discussion on Liquefaction of Cohesionless S o i l s . " International Conference on Recent Advances i n Geotech- n i c a l Earthquake Engineering and S o i l Dynamics, 1981, St. Louis, Vol. I I I . 15. Volpe, R.L. "Physical and Engineering Properties of Copper T a i l - ings." Current Geotechnical Practice i n Mine Waste Disposal, ASCE, 1979.

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