"Applied Science, Faculty of"@en . "Civil Engineering, Department of"@en . "DSpace"@en . "UBCV"@en . "Bosdet, Bruce W."@en . "2010-03-20T02:09:34Z"@en . "1980"@en . "Master of Applied Science - MASc"@en . "University of British Columbia"@en . "The purpose of this thesis has been to design and develop a practical apparatus for determining the residual strength of clay soils. To provide background for the study, current knowledge regarding residual strength is reviewed, including the following points. 1. Residual strength, defined as the lowest drained strength a soil can exhibit, is attained at large shear displacements. 2. Residual conditions result when particles located in shear bands within the failure zone become aligned in the direction of shear. 3. Residual strength, derived from interparticle bonding, is influenced by crystal structure and, in active clay minerals, by pore water chemistry. The ring shear test, performed by applying a torsional shear load to an annular shaped specimen, is particularly suited for residual strength determinations because of unlimited uni-directional strain capabilities. The UBC Ring Shear Device was designed to combine versatility with uncomplicated operation. Features of the design are as follows. 1. Variable sample height up to 0.75 inches. 2. Smoothly variable normal stress up to 200 psi delivered through an air piston. 3. Smoothly variable rate of shear from 3-2 inches per year to 9 inches per hour. 4. A non-tilting loading platen which reduces required machine\r\ntolerances and improves control of sample losses during testing.\r\n5. Automatic data acquisition.\r\n6. A simple method of sample placement. Residual strength determinations obtained with the UBC Ring Shear Device demonstrate its efficient and effective operation. Minimal\r\nsupervision is required and test results are easily interpreted. Multi-reversal direct shear tests for residual strength were undertaken for comparison with the ring shear results, but no satisfactory results were obtained due to excess pore pressures within the test specimens. Recommendations for improvements to both ring shear and direct shear devices are given."@en . "https://circle.library.ubc.ca/rest/handle/2429/22214?expand=metadata"@en . "THE UBC MNG SHEAR DEVICE BRUCE W. BOSDET B . A . S c , Un i vers i ty ,of B r i t i s h Columbji, 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of C i v i l Engineering The Univers i ty of B r i t i s h Columbia / i We accept this thes is as conforming / to the required standard / ' THE UNIVERSITY OF BRITISH COLUMBIA October, 1980 (c) Bruce W. Bosdet In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th i s thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion o f th is thes is fo r f i nanc ia l gain sha l l not be allowed without my written permission. Department of C i v i l Engineering The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date October 10, 1980 i i ABSTRACT The purpose of th is thesis has been to design and develop a p r ac t i c a l apparatus for determining the residual strength of c lay s o i l s . To provide background for the study, current knowledge regarding residual strength is reviewed, inc lud ing the fo l lowing po i nts. 1. Residual s trength, defined as the lowest drained strength a s o i l can e x h i b i t , is at ta ined at large shear displacements. 2. Residual condit ions resu l t when p a r t i c l e s located in shear bands with in the f a i l u r e zone become a l igned in the d i r e c t i o n of shear. 3. Residual s t rength, derived from i n t e r p a r t i c l e bonding, is inf luenced by c ry s ta l s t ructure and, in ac t i ve clay minera ls , by pore water chemistry. The r ing shear t e s t , performed by applying a tors iona l shear load to an annular shaped specimen, is p a r t i c u l a r l y su i ted for residual strength determinations because of unl imited un i -d i rec t iona1 s t r a i n c a p a b i l i t i e s . The UBC Ring Shear Device was designed to combine v e r s a t i l i t y with uncomplicated operat ion. Features of the design are as fol1ows. 1. Var iab le sample height up to 0.75 inches. 2. Smoothly var i ab le normal s t ress up to 200 psi de l i vered through an a i r p i s ton. 3. Smoothly var iab le rate of shear from 3-2 inches per year to 9 inches per hour. i i i *t. A n o n - t i l t i n g loading platen which reduces required machine tolerances and improves contro l o f sample losses during te s t i ng . 5. Automatic data a c q u i s i t i o n . 6. A simple method of sample placement. Residual strength determinations obtained with the UBC Ring Shear Device demonstrate i t s e f f i c i e n t and e f f e c t i v e operat ion. Min-imal superv is ion is required and test resu l ts are ea s i l y in terpreted . Mu1ti-reversa1 d i rec t shear tests for residual strength were undertaken for comparison with the r ing shear re su l t s , but no s a t i s f a c t o r y resu l t s were obtained due to excess pore pressures with in the test specimens. Recommendations fo r improvements to both r ing shear and d i r e c t shear devices are given. i v CONTENTS Page CHAPTER I INTRODUCTION 1 CHAPTER II RESIDUAL STRENGTH BEHAVIOUR OF CLAYS 2.1 INTRODUCTION ^ 2.2 SHEAR BEHAVIOUR OF CLAY 2.3 FAILURE PLANE STRUCTURE 7 2.4 CLEAVAGE 9 2.5 BONDING 11 2.6 STRESS DEPENDENT BEHAVIOUR 16 CHAPTER III RESIDUAL STRENGTH TESTING METHODS 20 3.1 INTRODUCTION 20 3.2 DIRECT SHEAR TEST 21 3.3 TRIAXIAL TEST 22 3.4 RING SHEAR TEST 23 CHAPTER IV THE UBC RING SHEAR DEVICE 26 4.1 INTRODUCTION 26 4.2 GENERAL DESCRIPTION 26 4.3 DETAILED DESCRIPTION 29 4.3-1 The Drive System 29 4.3.2 The Upper Ring Assembly 31 4.3.3 The Loading System 33 4.3.4 Measurement of Normal Loads 35 4.3.5 Measurement of Shear Forces 35 4.3.6 Displacement Measurements 37 4.4 SAMPLE PLACEMENT 37 4.5 DATA COLLECTION 40 V Page CHAPTER V TESTING PROGRAM 41 5.1 INTRODUCTION 41 5.2 RING SHEAR PERFORMANCE TESTS 42 5.3 RESIDUAL STRENGTH TESTS ON HANEY CLAY 48 5.3-1 So i l Descr ipt ion 48 5.3.2 Ring Shear Tests 49 TEST PROCEDURES AND RESULTS 49 DISCUSSION AND INTERPRETATION OF RESULTS 50 5.3.3 Direct Shear Tests 59 TEST PROCEDURES AND RESULTS 59 DISCUSSION AND INTERPRETATION OF RESULTS 59 5.3.** Review and Evaluation of Results 67 5.4 RESIDUAL STRENGTH TESTS ON HAT CREEK CLAY 69 5.^.1 So i l Descr ipt ion 69 5.4.2 Ring Shear Tests 70 TEST PROCEDURES AND RESULTS 70 DISCUSSION AND INTERPRETATION OF RESULTS 76 5.4.3 D irect Shear Tests 79 TEST PROCEDURES AND RESULTS 79 STRAIN CONTROLLED CONSOLIDATION TEST 84 DISCUSSION AND INTERPRETATION OF RESULTS 85 5.4.4 Review and Evaluation of Results 87 5.5 RESIDUAL STRENGTH TESTS ON IRANIAN CLAY 89 5.5.1 Soi l Descr ipt ion 89 5.5.2 Ring Shear Tests 90 5.6 SUMMARY OF TESTING PROGRAM 99 CHAPTER VI SUMMARY AND EVALUATION 100 6.1 RESIDUAL STRENGTH 100 6.2 UBC RING SHEAR DEVICE 101 6.3 TESTING PROGRAM 102 6.4 EVALUATION 103 REFERENCES 106 APPENDIX I: DERIVATION OF EQUATION FOR DETERMINING tan 0' 109 APPENDIX II: DIRECT SHEAR APPARATUS 110 v i TABLES Page Table 2.1 Cleavage and Residual F r i c t i o n Angle of Some Clay Minerals 10 Table 2.2 Bonding Along Cleavage Planes and Residual Strength 13 Table 5-1 Schedule of Ring Shear Tests 43 Table 5-2 Schedule of D irect Shear Tests 44 Table 5-3 Test Results : RS 2 and RS 4 on Haney Clay 54 Table 5-4 Test Results : RS 8 on Haney Clay 55 Table 5-5 Test Results : D irect Shear Tests on Haney Clay 62 Table 5.6 Test Results: Ring Shear Tests on Hat Creek Clay 74 Table 5-7 Test Results: Direct Shear Tests on Hat Creek Clay 8 3 Table 5.8 Test Results: Ring Shear Tests on Iranian Clay 97 v i i ILLUSTRATIONS Page Fi gure 2. 1 Shear Behaviour of Clays Under Fu l l y Drained Condi t ions 6 Fi gure 2. 2 Shear Zone Textures Strength State and Fabric at the Residual 8 Fi gure 2. 3 Residual Shear Strength versus Apparent E f f e c t i v e Stress for Sodium Montmori1 Ionite at Various Pore F lu id Salt (NaCl) Concentrations 15 Fi gure 2. 4 Relat ionship Between Shearing Resistance and Normal Stress for D i f fe rent Minerals 17 Fi gure 2. 5 Shearing Resistance Stress on the Shear 1/3 Power versus Normal E f f e c t i v e Plane Raised to the Minus 19 Fi gure 4. 1 The UBC Ring Shear Dev i ce 27 Fi gure 4. 2 Major Components of UBC Ring Shear Device 28 Fi gure 4. 3 Drive System 30 Fi gure 4. Upper Ring Assembly and Suspension System 32 Fi gure 4. 5 Loading System 34 Fi gure 4. 6 Sample Cutting Unit 38 Fi gure 5. ,1 Shearing Resistance vs. Displacement - RS 3 46 Fi gure 5-,2 Shearing Resistance vs. Displacement .- RS 2 51 Fi gure 5-\u00E2\u0080\u00A2 3 Shearing Resistance vs. Displacement - RS 4 52 Fi gure 5. .4 Shearing Resistance vs. Displacement - RS 8 53 Fi gure 5. ,5 Mohr Rupture Diagram: Summary of Results of Residual Strength Tests on Haney Clay 56 Fi gure 5. .6 Shearing Resistance Displacement - DS 3 vs. Cumulative 60 v i i i ILLUSTRATIONS (Cont'd) Page Figure 5-7 Figure 5-8 Figure 5-9 Figure 5-10 Figure 5-11 Figure 5.12 Fi gure 5-13 Fi gure 5 \u00E2\u0080\u00A2 1*\u00C2\u00BB Fi gure 5.15 Figure 5-16 Figure 5-17 Figure 5.18 Figure 5.19 Figure 5-20 Figure 5-21 Figure 5-22 Shearing Resistance vs. Cumulative Displacement - DS 5 Determining the Influence of an Incl ined Fai1ure Surface Shear Plane at End of Test ing - DS 3 Shearing Resistance vs. Displacement - RS 5 Shearing Resistance vs. Displacement - RS 6 Shearing Resistance vs. Displacement - RS 7 Mohr Rupture Diagram: Summary of Results of Residual Strength Tests on Hat Creek Clay Relat ion Between Clay Content and Residual Shearing Resistance for Various So i l s Shearing Resistance vs. Cumulative Displacement - DS 1 Shearing Resistance vs. Cumulative Displacement - DS 2 Shearing Resistance vs. Cumulative Displacement - DS k Shearing Resistance vs. Displacement - RS 9 Shearing Resistance vs. Displacement - RS 10 Shearing Resistance vs. Displacement - RS 11 Total Normal and Shear Stress vs. Displacement - RS 10 Mohr Rupture Diagram: Summary of Results of Residual Strength Tests on Iranian Clay 61 64 66 71 72 73 75 78 80 81 82 91 92 93 95 98 i x ACKNOWLEDGMENTS Many people have contr ibuted to the completion of th i s p ro jec t . Fred Zurkirchen, a machinist at U.B.C., pa t i en t l y advised me in the p rac t i ca l aspects of the equipment design and constructed the r ing shear device. Golder Assoc iates , Vancouver, and Sydney H i l l i s , a consu l t ing engineer in Vancouver, provided in teres t ing so i l specimens for the tes t ing program. Friends and family provided the needed encouragement to complete the wr i t ten work when the tes t ing was done. However, th i s thes i s would never have been completed without the a id of my superv i sor , R.G. Campanella, Department of C i v i l Engineering, U.B.C., who suggested the project and provided support and guidance throughout. 1. CHAPTER I INTRODUCTION A s o i l subjected to shear displacement under drained condi -t ions w i l l normally exh ib i t a r i se in shearing res i s tance with increas ing displacement unt i l a maximum res i s tance, the peak s t rength, is reached. With continued displacement, the shearing res i s tance of the s o i l w i l l decrease un t i l a minimum res i s tance, unaffected by further displacements, is a t ta ined . The minimum res i s tance, termed residual s trength, is the lowest shearing res i s tance the so i l can exh ib i t under drained cond i t ions . This general re l a t i onsh ip between shear displacement and the drained shear strength of s o i l has long been recognized. In p a r t i c u l a r , peak strength and the behaviour of s o i l s at small s t ra in s has been studied in great d e t a i l . However, the f i e l d of res idual strength and s o i l behaviour at large s t ra ins has been large ly unexplored un t i l recent years. The r e l a t i v e i n a c t i v i t y in th is f i e l d can be a t t r i bu ted to p rac t i ca l d i f f i c u l t i e s in tes t ing to large s t ra ins and to an under-assessment of the importance of residual strength to the s t a b i l i t y of earth s t ruc tures . Ear ly studies into the res idual strength behaviour of s o i l s were ca r r i ed out in the 1930's. The i n a b i l i t y o f conventional shear te s t i ng techniques to model f i e l d condit ions by producing large un i -d i r e c t i o n a l shear s t ra ins led the ear ly researchers to devise large displacement rotat iona l shear devices of varying design. Several of these devices performed tors iona l shear tests on annular s o i l specimens 2. and, as such, were the f i r s t r ing shear devices. The i n i t i a l in teres t in res idual strength apparently stemmed from the recognit ion that residual s t rength, which could be as l i t t l e as 20 percent of peak strength, governs the s t a b i l i t y of prev ious ly f a i l e d s tructures (Hvorslev, 1 939 ) . This in teres t was shor t ly lost as a t tent ion turned to the study of peak strength behaviour and i t s broader app l i c a t i on to unfa i led s lopes. However, a t tent ion returned to residual strength in the mid -1960 ' s when res idual strength was c l e a r l y l inked to the i n s t a b i l i t y o f cer ta in s o i l s which had not previous ly f a i l e d . In the Fourth Rankine Lecture, Skempton (1964) presented evidence to show that the long-term s t a b i l i t y o f slopes in overcon-so l idated f i s sured clays is governed, at least in the lower bound, by residual strength through the mechanism of progress ive f a i l u r e . A progress ive f a i l u r e may occur i f the load on a small sect ion of a large so i l mass surpasses the peak s o i l s trength. As the peak strength is exceeded, the shearing res is tance of the s o i l f a l l s toward the res idual value and the loads previous ly ca r r i ed by the small sect ion of s o i l are t rans ferred to adjacent areas, causing the stresses in these areas to exceed the peak strength a l so . In th i s manner, a f a i l u r e may progress throughout a large s o i l mass without ever causing a large sca le move-ment. This is p a r t i c u l a r l y true in heavi ly overconsol idated c lays which genera l ly pass the peak strength at r e l a t i v e l y small s t r a i n s . Bjerrum (1967) has indicated that an overconsol idated c lay need not contain f i s su res for i t s long-term s t a b i l i t y to be governed by residual s t rength. 3-The app l i c a t i on of res idual strength to the s t a b i l i t y of un fa i l ed s lopes, as ou t l i ned by Skempton, sparked new research into the changes that occur in so i l during the t r a n s i t i o n from peak to residual strength and led to the development of new rotat iona l devices for determining the res idual strength of s o i l s . The UBC r ing shear apparatus is one of these new devices. The purpose of th i s thes i s project has been to design, construct and test a p r ac t i c a l apparatus for determining the res idual strength of s o i l . During the course of the study, ava i l ab l e l i t e r a t u r e was reviewed to develop an understanding of res idual strength behaviour and the h i s t o r i c a l development of the res idual strength apparatus. Chapter II describes residual strength behaviour and reviews current knowledge and theory regarding the causes of th i s behaviour. Chapter III b r i e f l y reviews and evaluates the commonly used methods for determining residual strength and traces the h i s t o r i c a l development of r ing shear devices. The design of the UBC Ring Shear Device is described in de ta i l in Chapter IV. Chapter V presents the resu l t s of the te s t ing program undertaken to evaluate the new r ing shear device and to explore te s t ing methods and procedures. The resu l t s of repeated d i r ec t shear t e s t s , conducted for purposes of comparison, are a l so presented and discussed. A summary of the work and an evaluat ion of the equipment is provided in Chapter VI, together with recommendations for improvements to the apparatus. CHAPTER I I RESIDUAL STRENGTH BEHAVIOUR OF CLAYS 2.1 INTRODUCTION Considerable advances have been made s ince the mid-1960's in our understanding of s o i l behaviour at large s t r a i n s . It has been shown that granular and clayey s o i l s exh ib i t s im i l a r macroscopic behaviour in shear, but that only the clayey so i l s , and to a lesser extent dense sands, exh ib i t the s i g n i f i c a n t drop from peak to res idual strength which resu l t s in important engineering app l i c a t i on s . For th i s reason, c lays are the focus of a t tent ion in the study of res idual s trength. This chapter provides a review of c lay behaviour in shear and the microscopic mechanisms which inf luence the residual strength of these so i1s . 2.2 SHEAR BEHAVIOUR OF CLAY A l l s o i l s subjected to shear displacement under drained condit ions w i l l eventual ly a r r i ve at a shear strength and water content that are functions of the s o i l composition and appl ied stresses on ly , regardless o f the i n i t i a l s tate o f the s o i l . The strength of the s o i l is then the lowest that can be achieved and is termed the ult imate or residual s t rength. Further shearing w i l l not a f f e c t the strength of the so i l once the residual condi t ion is e s tab l i shed . Because res idual strength is unaffected by the i n i t i a l s t ructure or s t ress h i s to ry of the s o i l s , i t is sometimes considered to be a fundamental s o i l para-meter, an unchanging c h a r a c t e r i s t i c which i d e n t i f i e s the s o i l . 5. The s t r e s s - s t r a i n and volume change c h a r a c t e r i s t i c s exh ib i ted by a c lay before reaching the res idual condi t ion are funct ions of the s t ress h i s tory of the s o i l . Heavi ly overconsol idated c lays have high peak strengths and genera l ly exh ib i t a large decrease from peak to res idual s trength. The reduction in strength is accompanied by an increase in void ra t i o and water content. Normally consol idated clays genera l ly have lower peak strengths than overconsol idated c lays and exh ib i t a smaller decrease from peak to residual s trength. This decrease in strength is accompanied by a reduction in void r a t i o and water content. This typ ica l s o i l behaviour is i l l u s t r a t e d in Figure 2 . 1 . As with peak strength behaviour, the residual strength of a c lay is described in terms of a f r i c t i o n angle, 0^ ., and a cohesion in tercept , c^., as fo l lows: T = c 1 +G V | tan 0' L<\" r r = c 1 + ((P-u) tan 0' r r where ' 7 \ . = residual shear s tress CT\" = tota l normal s t ress on the shear plane u = pore water pressure CT1 = (T*- u = e f f e c t i v e normal s tress For most natural s o i l s , the res idual strength cohesion intercept is c lose or equal to zero, and the f r i c t i o n angle is less than the peak f r i c t i o n angle. The residual strength behaviour is commonly described by the shearing res i s tance r a t i o , TVcr1 = tan 0' ( for c ' = 0) FIGURE 2.1 Shear Behaviour of Clays Under Fu l l y Drained Conditions (from Mi tche l1, 1976) 7. 2.3 FAILURE PLANE STRUCTURE The physical process by which a f a i l u r e plane develops in c lay subjected to shear displacement has been studied by Morgenstern and Tchalenko (1967)- Their microscopic observations of kao l in c lay i n d i -cate that c lay deforms in simple shear up to the displacement at peak strength. However, beyond the peak strength, the so i l gradual ly develops a complex system of shear s t ructures and p a r t i c l e o r i en ta t i on s which lead to the establishment of a shear zone. At large displacements, as the clay approaches the residual cond i t i on , the shear zone cons is ts of two thin shear bands, in which the microscopic plate-shaped clay p a r t i c l e s are or iented in the d i r e c t i o n of shear displacement, enc los ing a region of compression textures, as shown in Figure 2.2. The compression textures resu l t from the p a r a l l e l alignment of c lay p a r t i c l e s in the regions between shear bands such that the p a r t i c l e s are or iented with the basal c rys ta l planes approximately perpendicular to the major p r inc ipa l s t re s s . The dominant mechanism of deformation at large s t ra in s is by s l i p between the basal cleavage planes of adjacent p a r a l l e l a l igned p a r t i c l e s within the two thin shear bands. A l l deformations are accounted for by basal plane s l i p or p a r t i c l e ro ta t i on . Shear f a i l u r e through p a r t i c l e s does not appear to occur. The study by Morgenstern and Tchalenko a lso showed that the p a r t i c l e s must be in v i r t u a l l y perfect p a r a l l e l alignment before the res idual strength is obta ined. Small local p a r t i c l e d i so r ien ta t i ons s i g n i f i c a n t l y increase the measured f r i c t i o n angle. Tests ca r r i ed out during the study ind icate that the i n i t i a l o r i en t a t i on of samples composed of p a r t i c l e s with a strong preferred p a r a l l e l alignment 500 ji\u00E2\u0084\u00A2 ~ 7 Shear Zone Textures and Fabric at the Residual Strength State (from M i t c h e l l , 1 9 7 6 ; a f t e r Tchalenko, 1968 ) (a) Structures formed in d i rec t shear test (b) P r inc ipa l displacement shears in d i rec t shear test S.j, i n i t i a l f a b r i c a t t i tude (fmajor p r inc ipa l stress d i rect ions at peak and ^ ^ residual strengths, respect ive ly P, thrust shears R, Riedel shears hatchings, p a r t i c l e o r ien ta t i on in compression texture white areas, p a r t i c l e s in i n i t i a l f ab r i c a t t i tude 9-does not a f f e c t the displacement required to obtain the res idual cond i t i on . However, p re - cu t t i ng the shear plane p r i o r to shearing great ly reduces the required displacement, presumably by improving c lay p a r t i c l e alignment. 2.4 CLEAVAGE The residual strength of c lay s o i l s depends, in par t , on the c ry s ta l cleavage mode. Although, as discussed in the previous s ec t i on , s l i p along the basal cleavage planes is the dominant mechanism of deformation at large s t r a i n s , some c lay minerals have no easy s l i p planes and exh ib i t higher residual strengths. For example, a t t a p u l g i t e , a f ibrous needle-shaped c lay mineral which shears p r e f e r e n t i a l l y along a stepped cleavage plane, has a very high residual angle of f r i c t i o n (see Table 2.1 ). The high shear strength of a t t apu l g i t e may a l so be re lated to the intermeshing of p a r t i c l e s which occurs even at large s t ra in s (Chattopadhyay, 1972, p. 234). K a o l i n i t e , i11 i te and mont-m o r i l l o n i t e , with a strongly preferred basal c leavage, have low residual strengths. With the poss ib le exception of montmori1lonite, s l i p does not appear to occur on cleavage planes with in the mineral c r y s t a l s , but between cleavage planes of adjacent p a r t i c l e s (Chattopadhyay, 1972, p. 61). TABLE 2.1 Cleavage and Residual F r i c t i o n Angle of Some Clay Minerals Mineral Cleavage Mode Bonding Along Cleavage Planes Residual F r i c t i o n Angle 0' r (degrees) Montmori1loni te Easy (001) basal cleavage Secondary valence plus exchangeable ion l ink 8.5 Kaoli n i te Easy (001) basal cleavage Secondary valence plus hydrogen bond i ng 11 Attapulg i te Sta i rcase 1i ke (110) cleavage Si - 0 - Si weak 1i nk 30 ( a f ter Chattopadhyay, 1972) 11. 2.5 BONDING Based on studies o f s o i l creep as a rate process, M i t c h e l l , Singh and Campanella (1969) developed a hypothesis concerning the re l a t i onsh ip between bonding, e f f e c t i v e stress and strength of s o i l . Chattopadhyay (1972) reached subs tan t i a l l y s i m i l a r conclus ions based on his study of the inf luence of pore water chemistry on the residual strength of c l ay s . M i t che l l e t al (1969) postu late that the only s i g n i f i c a n t mechanism by which e f f e c t i v e normal and shear s tresses are transmitted through a s o i l mass is by intergranular contact. The in tergranu lar contact is e s s e n t i a l l y s o l i d - t o - s o l i d , although adsorbed water layers may sometimes behave as part of the c ry s ta l s t ruc tu re . The shearing res is tance of the s o i l is derived from inter-atomic and inter -molecu lar bonding across the contact zone between adjacent p a r t i c l e s . D irect physical evidence of s o l i d - t o - s o l i d i n t e r p a r t i c l e contacts in c lays has been obtained in the form of scanning e lec t ron microscope photographs of c lay p a r t i c l e s (Matsui, Ito, M i tche l l and Abe, 1980). Microphotographs of kao l in c lay p a r t i c l e s obtained a f t e r shearing show occasional scratches on p a r t i c l e surfaces which are interpreted as tracks formed by i n t e r p a r t i c l e f r i c t i o n . No such tracks were observed on the p a r t i c l e s p r i o r to shearing. For fur ther evidence of s o l i d - t o - s o l i d contacts in clays Matsui et al (1980) point to the work of Koerner, Lord and McCabe ( 1 9 7 7 ) . who recorded the acoust i c emissions of cohesive s o i l s during shear. It is suggested that the acoust i c emissions occur as a resu l t of s o l i d - t o - s o l i d i n t e r p a r t i c l e contacts and that such emissions should not be expected i f the contacts occurred between \" s o f t \" adsorbed water layers . 12. The shear strength of c lay is governed by the strength and concentrat ion of bonds in the plane of deformation (Chattopadhyay, 1972). As shown by Morgenstern and Tchalenko (1967), beyond the peak strength displacement, deformation occurs by s l i p between mineral cleavage planes of adjacent p a r a l l e l o r iented mineral c r y s t a l s in the two th in shear bands. The s l ippage involves the continuous rupture and formation of bonds in the i n t e r p a r t i c l e contact areas. Between minerals with the same cleavage mode, the mineral with the lesser bond energy per unit o f contact area w i l l have the lower residual s t rength. The general re l a t i onsh ip between bonding and residual strength is i l l u s t r a t e d by the data tabulated in Table 2.2. For normally consol idated c l ay s , the number of bonds formed across an i n t e r p a r t i c l e contact zone appears to be proport ional to the e f f e c t i v e normal force at the contact (Mitchel l et a l , 1969)- This e f f e c t may be expla ined by the Terzaghi-Bowden and Tabor adhesion theory of f r i c t i o n which states that the i n t e r p a r t i c l e contact area is proport ional to the e f f e c t i v e normal f o rce . An increase in e f f e c t i v e normal force resu l t s in a proport ionate increase in the s i ze of the contact zone, the number of bonds per unit of contact area remaining constant. However, in the absence of shear deformation, reducing the conso l ida t ion pressure appl ied to a clay does not resu l t in a proport ionate reduction in the number of bonds, thus accounting for the higher shear strength of overconsol idated c lays . Water does not appear to a f f e c t the strength of bonds, but does a l t e r the e f f e c t i v e ( p a r t i c l e - t o - p a r t i c l e ) s t ress and, there fore , the number of bonds which form. TABLE 2.2 Bonding Along Cleavage Planes and Residual Strength Mi neral Quartz A t tapu lg i te Mi ca Kaoli n i te 111i te Montmori1 Ion i te Ta lc Graph i te MoS\u00E2\u0080\u009E Mode of Cleavage No d e f i n i t e cleavage Along (110) plane Good basal (001) Basal (001) Basal (001) Bonding Along Cleavage Planes S i-0-S i , weak Secondary valence (0.5 to 5 kcal/mole) + K- l inkages Secondary valence (0.5 to 5 kcal/mole) + H bonds Secondary valence (0.5 to 5 kcal/mole) + K l inkages Exce l lent basal (001) Secondary valence (0.5 to 5 kcal/mole) + exchangeable i on 1i nkages Basal (001) Basal (001) Basal (001) Secondary valence (0.5 to 5 ka1/mo1e) van der Waa11s Weak in ter layer Residual F r i c t i o n Angle, 0'r 3 5 degrees 30 degrees 17 to 2k degrees 12 degrees 10.2 degrees k to 10 degrees 6 degrees 3 to 6 degrees 2 degrees P a r t i c l e Shape Bui ky Fibrous and needle-shaped Sheet Platy Platy P l a t y - f i lmy Platy Sheet Sheet From M i t c h e l l , 1976; as adapted from Chattopadhyay, 1972. 14. The true e f f e c t i v e normal force by which contacts are made and bonds are formed is composed of ex te rna l l y app l ied forces ( inc lud ing the se l f -weight of s o i l ) and/or i n te rna l l y generated physico-chemical forces . In genera l , the physico-chemical forces are r e l a t i v e l y sma l l , the true e f f e c t i v e s tress is v i r t u a l l y equal to the apparent e f f e c t i v e s t re s s , and the strength of c lay s o i l s appears purely f r i c t i o n a l in character . However, in very ac t i ve c lay minerals, such as sodium montmori1lonite, large physico-chemical forces carry part of the normal s t res s . As the pore water chemistry great ly inf luences the magnitude of the physico-chemical forces in ac t i ve c l ay s , changes in pore water chemistry cause changes in the apparent residual angle of f r i c t i o n , as i l l u s t r a t e d in Figure 2.3. Studies by Chattopadhyay (1972, p. 326) ind icate that, i f the physico-chemical forces are taken into account, the behaviour of sodium montmori1 Ionite is purely f r i c t i o n a l , s im i l a r to the behaviour of less ac t i ve c l ay s . He concludes that every mineral has an unchanging true, or i n t r i n s i c , residual angle of f r i c t i o n . E l e c t r o l y t i c concentra-t ion may inf luence water content and physico-chemical s t res ses , thereby a l t e r i n g the true e f f e c t i v e s t re s s , but i t has no e f f e c t on the strength generating mechanism or the true res idual angle of f r i c t i o n . In p r ac t i c a l app l i c a t i on s , however, the apparent res idual angle of f r i c t i o n measured is the macroscopic or mass s o i l property, no allowance being made for the inf luence of physico-chemical fo rces . A de ta i l ed theoret i ca l treatment of th i s microscopic model of shear strength in c lay s o i l s is provided by Matsui et al ( 1 9 8 0 ) , inc luding cons iderat ion of the i n t r i n s i c or true angle of f r i c t i o n and 15-FIGURE 2.3 Residual Shear Strength Versus Apparent E f f e c t i v e Stress for Sodium Montmori1lonite at Various Pore F lu id Salt (NaCl) Concentrations (a f ter Chattopadhyay, 1972) 16. the e f fec t s of physico-chemical fo rces , d i l a tency , cementation, suct ion and stress h i s to ry . 2.6 STRESS DEPENDENT BEHAVIOUR Studies have shown that, for many c l ay s , the apparent res idual f r i c t i o n angle is greater at normal stresses less than 30 psi than at higher s t resses , as shown in Figure 2.k. As recorded by Mi tche l l (1976, p.315), two poss ib le explanations for th is curvature of the res idual strength envelope in the low normal s tress region are as fo l lows. 1. The work required to a t t a i n a per fect p a r t i c l e alignment in the f a i l u r e plane under low normal stresses exceeds the work required to shear the so i l without such alignment. Therefore, the p a r t i c l e s are less than p e r f e c t l y a l igned under low normal stresses and the shear strength is correspondingly increased. Under higher normal s t resses , less work is required i f the p a r t i c l e s are p e r f e c t l y a l igned and the lower res idual strength re su l t s . 2. Under low normal s tresses the p a r t i c l e contact areas behave e l a s t i c a l l y . By e l a s t i c junct ion theory, i t can be shown that the real contact area increases p ropor t i ona l l y with ((P) \u00E2\u0080\u00A2' at the e l a s t i c contact. The shearing re s i s tance , /> -1/3 tan fb'r = <-/(T~ ' , must therefore vary as ( r t n o ~< cu fD C L 3 3 n> o n fD \u00E2\u0080\u0094 - \u00E2\u0080\u0094 \u00E2\u0080\u00A2 fD 3 \u00E2\u0080\u0094 OJ r t < < 3 i/i , \u00E2\u0080\u0094 ^ io 3 \u00E2\u0080\u00A2 \u00E2\u0080\u0094 cu 3 n -t CT \u00E2\u0080\u00A2 \u00E2\u0080\u0094 7r \u00E2\u0080\u0094 \u00E2\u0080\u00A2 * \u00E2\u0080\u0094 ^ CU fD W -< C L X I CU \u00E2\u0080\u0094 n r t fD 3 \" me z 3 fD r t C L CD 1 fD l/> 73 O S ^1 ^1 73 H CU O r+ r+ (D CU O ~ -h Z o co -! 3 \" 3 fD CU CU \u00E2\u0080\u00941 -\ -o ^\u00E2\u0080\u0094 -t X fD in in o c 1 -t vn fD \u00E2\u0080\u0094 . . . 3 \u00E2\u0080\u0094 \u00E2\u0080\u00A2 \ . 3 un 3 in \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 3 N> \u00E2\u0080\u0094> Ver t i ca l Displacement T o o ( in.) 47. drop in normal s t re s s . Therefore, the e f f e c t i v e s t ress wi th in the body of the sample remained v i r t u a l l y unchanged, f a l l i n g only gradual ly as the excess negative pore pressures d i s s i pa ted . However, due to the high permeabi l i ty of the p latens, the negative pore pressures at the s o i l -to -p laten contact d i s s ipa ted almost in s tant ly re su l t i ng in a sudden drop in e f f e c t i v e normal s t re s s . Hence, the sample was compelled to shear or ' s l i p ' at the p l a t e n - t o - s o i l in ter face where the e f f e c t i v e normal s tress and the re lated shearing res i s tance were very low. The s l ippage lasted only unt i l the negative pore pressures in the body of the sample d i s s ipa ted s u f f i c i e n t l y to reverse the balance of shearing res is tance between the platen and the estab l i shed f a i l u r e plane with in the sample. Slippage natura l l y occurred in the lower conf in ing rings because the upper conf in ing rings are deeper with a correspondingly greater contact area and f r i c t i o n a l res i s tance to s l ippage. Soi l s l ippage adjacent to the porous platens is not considered a serious problem with the UBC Ring Shear Device. However, as mentioned in a previous sec t i on , porous platens f i t t e d with metal r ibs to prevent so i l s l ippage are ava i l ab le for use should there be any doubt. A p lot of the shearing res i s tance versus cumulative shear d i s -placement fo r RS3 is presented in Figure 5.1. The res idual shearing res i s tance of about 0.05, indicated on the p l o t , corresponds to a residual f r i c t i o n angle of 3 degrees. However, a systematic experimental e r r o r , described in the d iscuss ion of test resu l t s for Haney Clay, e f f e c t i v e l y increased the measured normal stress by a constant value. The values of residual shearing res is tance and f r i c t i o n angle, as corrected for th i s systematic e r r o r , are 0.18 and 10.1 degrees, re spec t i ve l y . 48. This low f r i c t i o n angle is apparently that of the modell ing c lay used to mark the pos i t i on o f the sample. An examination of the sample a f t e r removal from the r ing shear apparatus revealed the shear plane was completely coated with a th in trans lucent layer of modell ing c l ay . This resu l t may be i n d i c a t i v e of how thin seams of weak material can s i g n i f i c a n t l y a f f e c t the strength behaviour of more competent s o i l s . 5.3 RESIDUAL STRENGTH TESTS ON HANEY CLAY 5-3-1 Soi1 Descri pt ion Haney Clay is a dark-grey, s en s i t i ve marine-deposited s i1 ty c lay . The undisturbed c lay has a f l o ccu l a ted s t ructure be l ieved to resu l t from the in teract ion of the so i l p a r t i c l e s with the s a l t so lut ions of i t s marine depos i t ional environment. However, th i s f l occu l a ted s t ructure is metastable because the clay has been sub-sequently u p l i f t e d above sea level and leached of s a l t by fresh water i n f i l t r a t i o n (Vaid and Campanella, 1977)- Due to the reduced s a l t concentrat ion, the clay p a r t i c l e s take up a more stable dispersed s t ructure upon remolding. The change from a f l occu la ted to a dispersed s t ructure is accompanied by a s i g n i f i c a n t loss in s o i l s t rength, as evidenced by the s e n s i t i v i t y of Haney Clay (Campanella and Gupta, 1969). For de ta i l ed information regarding the s t ructure and behaviour of s e n s i t i v e c l ay s , the reader is referred to Mi tche l l and Houston (1969) and Houston and Mitchel l (1969). *\u00C2\u00BB9. The s o i l was block sampled from a deposit near the town o f Haney, B r i t i s h Columbia, some 20 miles east o f Vancouver. Due to the uniformity and a v a i l a b i l i t y of the mate r i a l , Haney Clay has been studied in numerous experimental works conducted at the Un ivers i ty o f B r i t i s h Columbia over a period in excess of 15 years. Typica l c h a r a c t e r i s t i c s and propert ies for Haney Clay are as fol lows (Byrne and Aok i , 1969; Bosdet, Lum and Negussey, 1976): S p e c i f i c g rav i ty 2.8 L iqu id l im i t 44% P l a s t i c i t y index 18% Natural water content 41% Percent f i n e r than 2 microns 46 Unconfined compressive strength 1550 psf S e n s i t i v i t y 12 Maximum past pressure 7000 psf -8 Permeabi l i ty 2 x 10 cm/s 5.3.2. Ring Shear Tests TEST PROCEDURES AND RESULTS Three ring shear tests for the purpose of determining res idual strength were performed on Haney Clay: RS 2, RS 4, and RS 8. In the case of RS 2, the shear plane was cut p r i o r to shearing. RS 2 and RS 8 were mult i - s tage tests in which residual strength determinations were obtained at 4 and 10 d i f f e r e n t normal loads, re spec t i ve l y . RS 4 was terminated prematurely fo l lowing the f i r s t stage of loading when a fau l ty o - r i ng seal caused water to leak from the reservo i r into the center column of the r ing shear apparatus. 50. The test data is presented on p lots o f shearing res i s tance and v e r t i c a l displacement versus cumulative shear displacement in Figures 5.2, 5-3 and 5.4. Ve r t i c a l displacement data was not recorded during RS 2, hence, th is p lot is omitted in Figure 5-2. The tabulated resu l t s of each stage of te s t ing are presented in Tables 5.3 and 5-4. The residual strength resu l t s are a l so summarized on a Mohr p lot in Figure 5-5-DISCUSSION AND INTERPRETATION OF RESULTS The peak strength behaviour of Haney Clay during r ing shear te s t ing was markedly d i f f e r e n t in RS 2 than in RS 4 and RS 8, as shown in Figures 5-2 to 5-4. RS 2 reached a peak shearing res i s tance of less than 0.5 at a displacement of 0.03 inches, corresponding to about 4 percent s t r a i n at the center of the sample. In cont ras t , RS 4 and RS 8 reached a peak shearing res i s tance of about 0.54 at a displacement and s t r a i n of about 0.4 inches and 50 percent, re spec t i ve ly . The reduced peak strength and peak strength displacement in RS 2 is a t t r i b u t e d to the e f f e c t of p re - cu t t i ng the shear plane before t e s t i n g . Beyond the peak strength displacement, the measured or apparent shear strength dropped temporari ly below the f i n a l recorded res idual strength in RS 2 and RS 8, and to a lesser degree in RS 4. Because the residual strength is the lowest drained strength of a s o i l , i t is evident that the s o i l was not f u l l y drained during th i s period and that excess pore pressures were in f luenc ing shear strength measurements. The development of excess pore pressures during the i n i t i a l stages of te s t ing is be l ieved to be re lated to the s e n s i t i v i t y of Haney Clay. During i n i t i a l shear ing, the metastable f l o ccu l a ted s t ructure Total Normal Pressure (1bs. /sq. in. ) T 1 T 8 10 12 Shear Displacement ( in.) 125 165 CO (0 ^ 0.4 -|-0) \u00E2\u0080\u00A2 \u00E2\u0080\u0094 I O -rr. 3 01 C 3 \u00E2\u0080\u00A2 3 \u00E2\u0080\u0094 \u00E2\u0080\u00A2 - \u00E2\u0080\u0094 ' fD U l - < X I O QJ cement ay cement U J u i ui \u00E2\u0080\u00A2XJ U J OV U J o O O N M U J ro U J O I O col rV 1-ON' OO zr oo f D . 0) -1 o Ul X I sr.\u00C2\u00B0 o fD 3 fD 3 r t \u00E2\u0080\u0094 ro 3 1 I o O C O U ) o' I - r \u00E2\u0080\u0094 O U l 1^ 1\u00E2\u0080\u0094. S 3 tp ON I U l U J _ 3 U J ON ro ON cr \u00E2\u0080\u0094\u00E2\u0080\u00A2 o Ver t i ca l Displacement ( in.) o o \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 U J ro \u00E2\u0080\u0094\u00C2\u00BB o Ver t i c a l Displacement ( in.) TABLE 5.3 Test Results: RS 2 and RS 4 on Haney Clay Test Stage Normal Stress Shearing Resistance 0 ' r Shearing Rate Cumulative Displacement -4 RS 2 1 26 psi i n s u f f i c i e n t displacement- 2.9x10 in/min 2.38 inches 2 51 \" .370 20.3\u00C2\u00B0 5.6x10\" 4 \" 18.53 o -4 3 76 n .376 20.6 5.6x10 \" 19.33 o -4 4 125 \" .400 21.8 5-6x10 1 1 21.02 \" 5 165 \" .413 22.7\u00C2\u00B0 5.6x10 _ l 4 1 1 23-51 Average 0'r = 21.3\u00C2\u00B0 RS 4 1 75 \" .379 20.8\u00C2\u00B0 2.2x10~ 3 1 1 11.08 \" * i n s u f f i c i e n t displacement to achieve residual condit ion at this load. By : l i nea r regression analys i s for RS 2 and 4, 0 ' r = 23 -8\u00C2\u00B0, C r = -4.4 psi VJ1 -E-TABLE 5 . 4 Test Results: RS 8 on Haney Clay Normal Stress Shearing Resistance 0 ' r Shearing Rate Cumulative Displacement 3 1 . 5 psi 26 123 91 66 4 0 32 130 124 98 . 4 4 6 . 4 3 8 . 4 3 5 . 4 4 5 . 4 4 0 . 4 29 . 4 3 2 .451 . 4 4 2 . 4 4 7 2 4 . 0 4 \u00C2\u00B0 1 . 2 x 1 0 ~ 3 in/min 23.65\u00C2\u00B0 23-51\u00C2\u00B0 23-99\u00C2\u00B0 23-75\u00C2\u00B0 23.22\u00C2\u00B0 23.36\u00C2\u00B0 24.28\u00C2\u00B0 23.85\u00C2\u00B0 5.7x10 6.2x10 6 . 2 x 1 0 6 . 2 x 1 0 6 . 2 x 1 0 6 . 2 x 1 0 6 . 2 x 1 0 6 . 2 x 1 0 24.08 6 . 2 x 1 0 9 . 8 1 inches 1 1 . 5 9 1 6 . 0 7 1 7 . 9 8 2 0 . 0 7 2 1 . 7 3 2 2 . 6 0 2 4 . 2 9 2 6 . 0 6 2 8 . 1 1 Average 0 ' r = 2 3 - 8 By l i nea r regression analys i s for RS 8 , 0 ' r = 2 4 . 1 , C r = - 0 . 7 psi Normal Stress (1bs./sq. in.) FIGURE 5.5 Mohr Rupture Diagram: Summary of Results of Residual Strength Tests on Haney Clay It is considered that the results of RS 2 and k, i f corrected for a c a l i b r a t i o n e r r o r , would be coinc ident with the results of RS 8. ON 57. of the sen s i t i ve c lay is broken down wi th in the shear plane by remolding and is replaced by a dispersed s t ructure with greater p a r a l l e l o r i en t a t i on of c lay p a r t i c l e s . Under constant volume cond i t ions , the new dispersed s t ructure is less capable of supporting loads and part of the normal s t ress must be t rans fer red to the pore water, c reat ing excess pore pressures (Mitchel1 and Houston, 1969). However, once the s t ruc tura l change is complete with in the shear plane, the natural d i s s i pa t i on of excess pore pressures re -es tab l i shes drained condit ions throughout the sample. Excess pore pressures can a lso be generated a f t e r the shear plane is f u l l y es tab l i shed i f the sample is sheared too rapid ly to allow f u l l d i s s i pa t i on of pore pressures. This e f f e c t was observed in RS 3 where, at a displacement of about 12 inches (see Figure 5-4), the rate of shear was increased 6 f o ld producing a rapid drop in measured shear strength due to the bui ld-up of pore pressures. A subsequent reduction in the rate of shear brought about quick d i s s i -pation of the excess pressures and return to equ i l i b r i um. Following establishment of the shear plane and d i s s i pa t i on of excess pore pressures, the shearing res i s tance recorded in RS 8 became v i r t u a l l y constant regardless of normal load, as shown in Figure 5.4. This is the an t i c ipa ted resu l t based on previous exper ience, as described in Chapter 2. However, the resu l t s of RS 2, tabulated in Table 5-3> record an increase in apparent shearing res i s tance with normal s t re s s . The s ing le residual strength determination made in RS k is in c lose agreement with one of the determinations made in RS 2. 58. The true re l a t i onsh ip between the resu l t s of these r ing shear tests is best i l l u s t r a t e d by the Mohr p l o t , Figure 5-5- As shown in the f i gu re , the resu l t s of RS 8 ind icate that Haney Clay has a residual angle of f r i c t i o n of 23.8 degrees and no residual cohesion. The resu l ts of RS 2 and RS h exh ib i t the same angle of f r i c t i o n , but the data is sh i f ted to ind icate a negative cohesion in tercept . The s h i f t in cohesion intercept is evidence of a constant and systematic experimental e r ro r . The e r ro r ended fo l lowing r e c a l i b r a t i o n of the transducers at the end of RS k, i nd i ca t ing that the source of the problem was a z e r o - s h i f t e r ror in transducer c a l i b r a t i o n . When corrected for th i s e r r o r , the resu l t s of RS 2 and RS k would be co inc ident with the resu l t s of RS 8, i nd i ca t ing that Haney Clay has a residual angle of f r i c t i o n of 23.8 degrees, with no residual cohesion. 59. 5.3-3 Di rec t Shear Tests TEST PROCEDURES AND RESULTS Two s i n g l e stage d i r e c t shear t es t s f o r the purpose o f de termin ing res idua l s t reng th were performed on Haney C l a y : DS 3 and DS 5, c a r r i e d out at a p p l i e d normal s t r e s s e s o f 34 and 87 p s i , r e s -p e c t i v e l y . Both samples were p laced as und is turbed specimens, but the shear p lane was cut p r i o r to t e s t i n g in DS 5-The t e s t s c o n s i s t e d o f a s e r i e s o f a l t e r n a t i n g forward and reverse t r ave rses o f the shear box, the length o f each t r a v e r s e not u s u a l l y exceeding 0.3 i nches . The shear s t r e s s versus d isp lacement behaviour o f the s o i l in the i n d i v i d u a l t r ave rses c o n s i s t e d o f v a r i a t i o n s o f the t y p i c a l shear behaviour o f a normal ly c o n s o l i d a t e d c l a y , i l l u s t r a t e d in F igure 2.1. The tes t r e s u l t s have been summarized in p l o t s o f the u l t ima te shear ing r e s i s t a n c e determined in each t r a v e r s e of the shear box versus cumula t ive shear d i sp lacement , presented in F igures 5-6 and 5-7. V e r t i c a l d isp lacement data i s a l s o shown on these f i g u r e s . The res idua l s t reng th r e s u l t s as i n t e r p r e t e d from t h i s data are presented on the Mohr p l o t , F igure 5-5- Tabulated r e s u l t s are presented in Table 5-5-DISCUSSION AND INTERPRETATION OF RESULTS The r e s u l t s o f d i r e c t shear t e s t s on Haney Clay i n d i c a t e that there i s a reduced d isplacement to peak s t reng th when the f a i l u r e p lane is cut p r i o r to s h e a r i n g , a l s o demonstrated in r i ng shear t e s t s . DS 3, c a r r i e d out on und is turbed s o i l , and DS 5, c a r r i e d out on a p re -cu t specimen, a t t a i n e d peak s t reng th at cumula t ive d isp lacements o f 0.6 and 0.1 i nches , r e s p e c t i v e l y . Shear Displacement ( inches) FIGURE 5.6 Shear ing Res is tance v s . Cumulat ive Displacement Specimen: Haney Clay To ta l Normal Pressure = 33-9 psir Typ ica l Rate o f Shear = 1 0 x 10 i n . / m i n . * U l t ima te va lue in each recorded t rave rse DS 3 ON o ' I 1 1 1 1 1 2 3 4 5 Shear Displacement (inches) FIGURE 5.7 Shearing Resistance vs. Cumulative Displacement - DS 5 Specimen: Haney Clay Total Normal Pressure: 87 -3 p s i Typica l Rate of Shear: 7 x 10 in./min. - Ultimate value of each recorded traverse TABLE 5-5 Test Results: Direct Shear Tests on Haney Clay T e s t Normal Stress Shearing Resistance 0'r apparent Shearing Rate Cumulative Displacement DS 3 33.85 psi 5-F .350 F 19.3\u00C2\u00B0 1 x ]0~k in/min 5-3 inches R .412 R 22.4\u00C2\u00B0 1 x 10 _ Z | DS 5 87.3 11 .310 17.2\u00C2\u00B0 7 x 10~5 \" 5.3 \" F - value obtained in forward shear d i rec t i on R - value obtained in reverse shear d i rec t i on ON 63. DS 3 exh ib i ted a gradual f a l l in shearing res i s tance beyond peak strength displacement u n t i l , at a displacement of about 2.5 inches, the shearing res is tances measured in the forward and reverse d i rec t i ons d iverged, with the higher res i s tance being reg i s tered in the reverse d i r e c t i o n . This dichotomy in measured res i s tance was maintained through-out the remaining test per iod. As ind icated by the lack of divergence in the i n i t i a l test period and confirmed by subsequent examination, the divergence in res i s tance was not caused by an er ror in the c a l i b r a -t ion of the proving r ing used to measure shear fo rce . The d iverg ing res is tance phenomenon, a l so observed in tests on Hat Creek Clay, is apparently the resu l t of an i nc l i ned f a i l u r e sur face. An inc l i ned f a i l u r e surface is mechanically permiss ib le in d i rec t shear tes t ing because the freedom of movement of the upper load-ing frame includes v e r t i c a l and, to a lesser degree, rotat iona l motion. To analyze the e f f e c t of an inc l ined plane of f a i l u r e , the v e r t i c a l and hor izonta l appl ied loads have been broken down into components act ing p a r a l l e l and perpendicular to the i nc l i ned f a i l u r e surface in Figure 5.8. The resolved normal and shear forces as re lated by the f r i c t i o n angle can be descr ibed by two equations, one fo r each d i r ec t i on of t r a v e l , in two unknowns. The two unknowns, which can be determined from these equations are the angle of shear plane i n c l i n a -t ion and the angle of f r i c t i o n of the s o i l . The angle of f r i c t i o n is the residual angle provided drained residual condit ions have been es tab l i shed with in the shear plane. The equations re su l t ing from the preceding ana lys i s are presented in Figure 5.8. Toward the end of t e s t i n g , DS 3 exh ib i ted a gradual decrease in the angle of f r i c t i o n measured in the forward d i r e c t i o n and a gradual eh. Vsin0 Vcos0 H sin0 \\" H xcos0 0 -9-V \u00C2\u00BB 1 H 2 N S i n c l i n a t i o n of shear plane angle of f r i c t i o n for s o i l appl ied v e r t i c a l force appl ied hor izonta l force in forward d i r e c t i o n appl ied hor izonta l force when shear d i r ec t i on is reversed resolved forces normal to f a i l u r e plane resolved forces p a r a l l e l to f a i l u r e plane In terms of forces , the equation of s o i l strength is as fo l lows. S = N tan -6- (1) Resolve the appl ied forces shown above and subs t i tu te in equation (l) to ob ta in : (h^cos 0 - Vsin0) = (Vcos0 + H 1sin0) tan-8- (2) Reverse the d i r ec t i on of appl ied hor izonta l fo rce , resolve and subs t i tu te in equation (1) to ob ta in : (H 2cos0 + Vsin0) = (Vcos0 - h^s i n0) tan -9- (3) Combine equations (2) and (3), c o l l e c t and s imp l i f y terms, to determine the i n c l i n a t i o n of the shear plane. V(H 1 - H j tan 20 = ^ (h) V + H l H 2 The so i l f r i c t i o n angle is obtained by enter ing the determined angle of i n c l i n a t i o n in equations (2) or (3). FIGURE 5 - 8 Analys is of Direct Shear Results Influenced by an Incl ined Fa i l u re Surface 65-increase in the f r i c t i o n angle measured in the reverse d i r e c t i o n . The f i na l recorded values were about 19-3 and 22.4 degrees in the forward and reverse d i r e c t i o n s , re spec t i ve l y . By applying the ana lys i s described above, i t is ca l cu la ted that the true angle of f r i c t i o n at the end of tes t ing was 20.8 degrees, the d iverg ing recorded values re su l t ing from a shear plane i n c l i n a t i o n of 1.5 degrees. At the end of DS 3, the sample was removed and photographed. As seen from a view of the upper loading frame in Figure 5-9, the shear plane is well e s tab l i shed , h ighly po l i shed, and s1 i ckens ided. The shear plane is not f l a t but rounded, having protruded into the lower loading frame. The shape of the f a i l u r e plane may resu l t from the gradual loss of remolded s o i l through the gap between the loading frames. Lost s o i l is replaced by fresh material from the upper loading frame on ly , re su l t ing in a gradual eros ion of s o i l from the lower loading frame. The shear plane develops a rounded shape due to edge constra ints which prevent uniform so i l e ros ion. Herrman and W o l f s k i l l (1966, p. 120) encountered s im i l a r shaped shear planes in repeated d i rec t shear tests on weak c l ay - sha le s , noting that the apparent shearing res i s tance was increased by the i n c l i n a t i o n of the f a i l u r e sur face. However, as measurements were made in only one traverse d i r e c t i o n , they did not record the corresponding dec l ine in apparent shearing res i s tance in the opposite d i r e c t i o n of t r a v e l . Beyond peak strength displacement in DS 5, the shearing res i s tance f e l l to a temporary plateau ear ly in t e s t i n g , followed by a fur ther drop to a f i n a l p lateau. At the end of t e s t i n g , DS 5 exhib i ted an angle of f r i c t i o n of about 17.2 degrees with some minor sca t ter of data. FIGURE 5.9 Shear Plane at End of Tes t ing - DS 3 View of upper loading frame. Note the h ighly po l i s hed , s 1 i ckens ided sur face. The curvature of the shear plane may a l t e r the apparent shearing res i s tance of the s o i l . ON 67. 5.3.4 Review and Evaluat ion of Results The resu l t s of r ing shear tests on three samples of Haney Clay ind icate that th i s s o i l has a residual angle of f r i c t i o n of 2k degrees. The uniformity of the data obtained over a wide range of normal loads provides a strong degree of confidence in th i s determination of res idual f r i c t i o n angle. A z e r o - s h i f t c a l i b r a t i o n er ror during RS 2 and RS k produced data which, whi le f a l s e l y ind ica t ing a negative cohesion in tercept , is nevertheless in f u l l agreement with the determined angle of res idual f r i c t i o n . The resu l t s of the d i rec t shear tests c a r r i ed out on Haney Clay, DS 3 and DS 5, ind icate that Haney Clay has a residual f r i c t i o n angle of 21 and 17 degrees, re spec t i ve l y . A divergence in shearing res i s tance recorded in the forward and reverse traverse shear d i rec t ions observed in DS 5, is bel ieved to resu l t from the i n c l i n a t i o n of the shear sur face. As the shear plane i n c l i n a t i o n is smal l , in the order of I to 2 degrees, the v e r t i c a l motion caused by the i n c l i n a t i o n is masked by the downward movement due to sample losses which, in th is ser ies of t e s t s , is 1.5 to 3.0 times greater. The d iverg ing res is tance phenomena does not occur in r ing shear te s t ing because the conf in ing rings are not capable of v e r t i c a l movement or rotat iona l motion about a hor izonta l ax i s . These constra ints make shear plane i n c l i n a t i o n mechanically impermiss ible. For a va r ie ty of reasons the degree of confidence in the residual strength determinations made using the d i r ec t shear device is qu i te l im i ted . Factors in f luenc ing th i s judgement inc lude: I. S i g n i f i c a n t l y reduced tota l displacements in r e l a t i on to the large displacements obtained in the r ing shear apparatus. 68. 2. Moderately scattered data. 3- L imited number of res idual strength determinations (two) having c o n f l i c t i n g re su l t s . The strong degree of confidence in the resu l t s of r ing shear tests and the uniform nature of Haney Clay would suggest that the true res idual f r i c t i o n angle is 2k degrees, and that the values obtained in d i r ec t shear tests are too low. Barr ing errors in measurement, the source of the lowered values must be excess pore pressures wi th in the so i1 samples. The most probable source of the excess pore pressures would appear to be ingest ion of water into the shear plane during t e s t i n g . At the end of each t raverse, part of the sample is extended into the re servo i r . This mater i a l , exposed to water and carry ing no normal load, is f ree to swe l l . As shear displacement continues, the swelled so i l is drawn back into the f a i l u r e plane and again subjected to normal loading, generating excess pore pressures wi th in the swelled s o i l during the reconso l idat ion per iod. Repeated exposure of the ends of the f a i l -ure plane to water might be expected to fo s ter a gradual bu i ld -up of excess pore pressure in the sample. Other factors which might inf luence the pore pressures in the sample include the rate of shear, the rate of s o i l loss from the loading frame gap, and the length of pause between traverses - a longer pause a l lowing greater d i s s i pa t i on of pore pressures and greater swel l ing of exposed s o i l s . However, the author has found no apparent c o r r e l a t i o n in the ava i l ab le data between these parameters and the measured shear strength. 69. 5-4 RESIDUAL STRENGTH TESTS ON HAT CREEK CLAY 5.4.1 So i l Descr ipt ion Hat Creek Clay is a heav i ly overconso l idated, b r i t t l e , h ighly p l a s t i c , dark blue c lay . The s o i l was core sampled from extensive c lay deposits associated with coal measures in the Hat Creek Va l ley of B r i t i s h Columbia. A study performed at the Un ivers i ty of Western Ontario shows that the s o i l contains a high percentage of sodium montmori1 Ionite with var i ab le amounts o f carbonate, quartz and fe ldspar (Quigley, 1976). The same study presented the fo l lowing typ ica l prop-e r t i e s for the c lay : L iquid 1imit 109 to 178% P l a s t i c 1imit 34 to 39% Natural water content 35% Percent carbonate I to 68 The samples tested at UBC, contain ing a r e l a t i v e l y high percen-tage of s i l t and sand s ized p a r t i c l e s , had the fo l lowing c h a r a c t e r i s t i c s : L iqu id 1imi t 84 to 119% P l a s t i c 1imi t 27 to 28% P l a s t i c i t y index 68 to 92% Natural water content 29% Percent greater than 2 microns 40 to 50% Based on the resu l t s of a s t r a i n - c o n t r o l l e d conso l ida t ion t e s t , Hat Creek Clay has a permeabi l i ty in the order of 10 ' \u00C2\u00B0 cm per second. 70. 5.4.2 Ring Shear Tests TEST PROCEDURES AND RESULTS Three s ing le stage r ing shear tests were performed on Hat Creek Clay: RS 5, RS 6 and RS 7> c a r r i ed out at appl ied normal s tresses of 26.5, 60 and 27 p s i , r e spec t i ve l y . The test data is presented in p lots of shearing res i s tance versus displacement in Figures 5.10 to 5-12. The f i n a l r e su l t s , tabulated in Table 5.6, are p lo t ted on a Mohr diagram in Figure 5-13-To f a c i l i t a t e sample placement, the hard and b r i t t l e clay was crushed by mortar and pest le before being placed for t e s t i ng . The s o i l sample for RS 5 was crushed and placed at the natural water content. For RS 6, the so i l was crushed at the natural water content, but l i g h t l y wetted as i t was p laced. The s o i l for RS 7 was crushed, wetted and remolded unt i l i t exh ib i ted a p l a s t i c consistency before being placed. To reduce the time required for pore pressure d i s s i pa t i on at the shear plane, the shortest drainage path to the f a i l u r e plane was reduced to 1/8 inch for th is test ser ies by p lac ing a spacer beneath the lower porous p la ten. Due to the large sand f r a c t i o n in the s o i l , further reductions in the drainage path would have increased the p robab i l i t y that sand grains lodged against the platen might protrude through the f a i l u r e plane, d i s rupt ing f a i l u r e plane develop-ment and increas ing the apparent shearing res i s tance of the s o i l . During tes t ing of Hat Creek Clay, sand p a r t i c l e s from the s o i l sample had a tendency to become wedged between the conf in ing r ings , hampering adjustment of the gap. The p a r t i c l e s were dis lodged Shearing Resistance o o o o o to UJ .e- Ul ON -L u i . 3 CO rr (D 0) O r t ru o -! 3 01 o- ro \u00E2\u0080\u00A2 in un ONI o I Ul I o t o o U l o o Ve r t i c a l Displacement ( in.) o c z 73 co oo T3 3\" CD CD O CD \u00E2\u0080\u0094 -1 3 \u00E2\u0080\u0094 CD 3 3 IQ 73 CD 1/1 OJ - . rt Ul rt-O Ol -1 3 CD O CD CD 7T < O OJ -< o 01 n CD 3 CD 3 70 OO Shearing Resistance Shearing Resistance o o T V o o -p- un J L_ I co UJ o U J N> U J OJ <^ o CD 3 CD 3 rt-^ U J \u00E2\u0080\u0094 CO 3 O -C-- f O N o O N o O i un u i O CD T3 U0 P> - \u00E2\u0080\u009E ^ o o ON-I co-I CD -I o 0) o CD 3 CD 3 rt-\u00E2\u0080\u0094 K> 3 O N oo O U J U l O N o o o U l O N o Si T D 1) 1 0 I U l V e r t i c a l Displacement ( in. ) Ve r t i ca l Displacement ( in.) \u00E2\u0080\u00A2ZL Shearing Resistance Shearing Resistance o o o o o o o oo .e- u i f-o oo .t- u i ro \u00E2\u0080\u0094\u00C2\u00BB o t-o \u00E2\u0080\u0094* o Ver t i ca l Displacement ( in.) Ve r t i c a l Displacement ( in.) 'IL TABLE 5.6 Test Results : Ring Shear Tests on Hat Creek Clay Test Stage Normal Stress Shearing Resistance 0'r Shearing Rate Cumulative Displacement R S 5 1 26.5 psi .kkS 2k.0\u00C2\u00B0 5.6x10~Z' in/min 6.84 inches R S 6 1 60 \" .382 20.9\u00C2\u00B0 1.9x10_Zt \" 43.98 \" RS 7 1 27 1 1 .461 24.8\u00C2\u00B0 2.9x10 - 3 \" 30.33 -c-cr in Xi 1/1 Q) 1_ fD 20 i 10 J 10 ^ 2 4 . 8 \u00C2\u00B0 DS 4 DS 1 RS 6 \u00E2\u0080\u00941 1 1 1 1~~ 20 30 40 50 60 Normal Stress (1bs./sq. in.) DS 2 9.4\u00C2\u00B0 \"~r~ 70 FIGURE 5.13 Mohr Rupture Diagram: Summary of Results of Residual Strength Tests on Hat Creek Clay 76. by clamping the rings shut whi le shearing the sample. The problem was e l iminated by maintaining a very narrow conf in ing r ing gap. Due to the swel l ing nature of Hat Creek Clay, material lost through the conf in ing r ing gap increased in volume producing the appear-ance of excess ive sample losses. However, the actual rate of sample loss as measured by the change in sample thickness was minor. DISCUSSION AND INTERPRETATION OF RESULTS At the end of the f i r s t r ing shear test on Hat Creek Clay, RS S, the apparent f r i c t i o n angle (neglect ing poss ib le excess pore pressures) was 2k degrees. As shown in Figure 5-10, the shearing res i s tance was s t a b i l i z i n g when the test was ended such that s i g n i f i c a n t changes in shearing res i s tance would not an t i c ipa ted i f te s t ing had been extended to any reasonable displacement and durat ion. Considering that the accepted values of res idual f r i c t i o n angle for montmori1 Ionite clay ranges from k to 10 degrees (see Table 2.2), the shearing res i s tance encountered in tes t ing was higher than expected. S imi lar resu l t s were obtained during RS 6 and RS 7 which, at the end of t e s t i n g , exh ib i ted apparent res idual f r i c t i o n angles of 20.9 and 24.8 degrees, re spec t i ve l y . The high values of s o i l shearing res i s tance are a t t r i bu ted to shear plane d i s rupt ions caused by sand p a r t i c l e s in the s o i l . The d i s rupt ions to shear plane development may have been i n t e n s i f i e d by the shortened drainage path should the 1/8 inch separat ion between porous platen and shear plane have been i n s u f f i c i e n t to prevent i n t e r -act ion between sand p a r t i c l e s lodged against the platen and the shear 7 7 . plane. The inf luence of sand and s i l t content on the shearing r e s i s t -ance of several d i f f e r e n t c lays is i l l u s t r a t e d in Figure 5 -1^ . As shown in the f i g u r e , the curves for montmori1 Ionite c lay ind icate that Hat Creek Clay, having a sand and s i l t f r a c t i o n of kO to 50 percent, would be expected to exh ib i t a shearing res i s tance s i m i l a r to that o f the pure c lay . However, the continual loss o f f ines through the con-f i n i n g r ing gap during te s t i ng may have concentrated the coarser p a r t i c l e s in the shear plane, increas ing the shearing res i s tance s i gni f i cant ly . The samples appear to have been we l l -d ra ined at the end of t e s t i n g , showing l i t t l e or no va r i a t i on in measured shearing res i s tance with increas ing displacement or with changes in rate of shear. Further evidence that the samples were we l l -dra ined is provided by the s i m i l a r i t y of resu l t s obtained from RS 5, where the s o i l was placed at i t s natural water content, and RS 7, where the s o i l was wetted and remolded p r i o r to placement. Very high values of s ide f r i c t i o n ( f r i c t i o n a l forces of the loading platen and the so i l against the upper conf in ing rings) were en-countered in RS 5- In previous te s t s , the measured s ide f r i c t i o n had been in the order of 10 to 20 percent of the tota l appl ied load. In RS 5 , the s ide f r i c t i o n began at a very low value but s tead i l y increased during the te s t . When the side f r i c t i o n exceeded 30 percent of the appl ied load, the test was terminated to prevent poss ib le damage to the equipment. The high values of s ide f r i c t i o n are a t t r i bu ted to corros ion welts on the upper conf in ing r ings, but could a l so resu l t from misalignment of machine components during assembly or from granular s o i l p a r t i c l e s wedged between the conf in ing rings and the upper loading p la ten. 78. Residual Strength Envelope A. a f t e r Skempton, 1964 B e s t - f i t Relat ions f o r : B. Viennese Clay a f t e r Borowicka, 1965 C. Na - Montmori11 on i te ~~20 ' 40 ' 60 ' 80 ' ^00 Clay Fract ion (percent < 2 microns) FIGURE 5-14 Relat ion Between Clay Content and Residual Shearing Resistance for Various So i l s 79. 5.4.3 D i rect Shear Tests TEST PROCEDURES AND RESULTS Three s ing le stage d i r ec t shear tests for the purpose of determining res idual strength were performed on Hat Creek Clay: DS 1, ! DS 2, and DS 4, c a r r i ed out at appl ied normal s tresses o f 52, 78.5 and 44.5 p s i , re spec t i ve l y . To prepare the sample for DS 1, the s o i l was broken and ground with mortar and pe s t l e , and subsequently placed in l i f t s in the d i r ec t shear box. A f te r each l i f t was p laced, the loading block was l a i d on top and hammered f i rmly to compact the so i l The sample was made as deep as poss ib le to allow for l a te r conso l idat ion and loss o f sample by squeezing through the loading frame gap. The sample for DS 2 was placed in a s i m i l a r manner, except that each l i f t of s o i l was spr ink led with d i s t i l l e d water to moisten the sample. The sample for DS 4 was wetted, remolded, and s ieved to remove the medium to coarse sand f r a c t i o n . The remaining s o i l was reconsol idated under s t r a i n - c o n t r o l l e d cond i t ions , trimmed and placed in the shear box. The test resu l t s have been summarized in p lots of the ult imate shearing res i s tance determined in each traverse of the shear box versus cumulative displacement, Figures 5.15 to 5-17- V e r t i c a l displacement data is a l so shown on these f i gu res . The res idual strength resu l t s interpreted from the data are presented on a Mohr p lo t in Figure 5-13-Tabulated resu l t s are presented on Table 5-7-o cz 73 m un un \u00E2\u0080\u0094i -H CO CO -< O T3 3\" CO T J r i - fD fD 3\" CZ \u00E2\u0080\u0094 ' al O o> fD \u00E2\u0080\u0094J o \u00E2\u0080\u0094 ' \u00E2\u0080\u0094\u00E2\u0080\u00A2 Ol r i - OJ 3 \u00E2\u0080\u0094\u00E2\u0080\u00A2 ~i \u00E2\u0080\u0094. ' (D 3 3 o 3 IO O OJ 73 -! \u00E2\u0080\u00A2 - \u00E2\u0080\u0094\u00E2\u0080\u00A2 r t OJ 3 73 ui fD r t OJ fD T J fD \u00E2\u0080\u0094 ' 3: Ul \u00E2\u0080\u0094 ' < OJ \u00E2\u0080\u0094\u00E2\u0080\u00A2 Ol OI O r t Ul o \u00E2\u0080\u0094 ' - h -1 r t fD C fD o 01 3 CD OO \u00E2\u0080\u0094> O O O un 3 3 fD fD O - C L O - I OJ -i O -(D fD QJ OJ - i -1 -1 -1 fD fD o o o o 3 3 V e r t i c a l Displacement (inches) '08 1 1 \u00E2\u0080\u0094 i T r 1 2 3 5 Shear Displacement (inches) FIGURE 5.16 Shearing Resistance vs. Cumulative Displacement - DS 2 Specimen: Hat Creek Clay Total Normal Pressure: 78.4 psi.,- co Typica l Rate of Shear: 25 x 10 in./min. 7* * Ult imate value in each recorded traverse Shear Displacement (inches) FIGURE 5-17 Shearing Resistance vs. Cumulative Displacement - DS h Specimen: Hat Creek Clay Total Normal Pressure: hh.k p s i Typica l Rate of Shear: k x 10 in. /min. \u00C2\u00BB Ultimate value in each recorded traverse TABLE 5.7 Test Results: Direct Shear Tests on Hat Creek Clay Test Normal Stress Shearing Res i s tance- 0' apparent--* Shearing Rate Cumulative Displacement o -4 DS 1 51.8 psi F .166 3-h 5.0x10 in/min 3-28 inches DS 2 78.4 \" F .198 11.2\u00C2\u00B0 2.5x10 _ Z t \" S.hk DS k hh.k \" S .290 16.2\u00C2\u00B0 3-7x10\" 5 \" 3.10 F - value obtained in forward shear d i r ec t i on S - 'average' value of scattered results Tests ended prematurely due to cumulative sample losses, Residual condi t ions not achieved. 00 00 84. The shearing res i s tance versus displacement p lot for DS 1, Figure 5-15, does not show data for a rapid pre-shear ing period ca r r i ed out p r i o r to regular t e s t i ng . This sample was pre-sheared for a period of 5 consecutive test days during which the measured shear angle remained constant at 31 degrees. A f te r a three day pause in te s t ing to al low for equa l i za t i on of pore pressures, the measured shear angle f e l l to 21.7 degrees. Subsequent test resu l t s are summarized in Figure 5-15. STRAIN CONTROLLED CONSOLIDATION TEST As previous ly d iscussed, s o i l l a te r tested in DS 4 was remolded, s ieved to remove the medium to coarse sand f r a c t i o n , and reconsol idated in a s t r a in con t ro l l ed tes t . During reconso l i da t i on , free drainage was permitted at the top of the sample on ly , while pore pressure was monitored at the sample base. The basal pore pressures remained in the order of 90 to 100 percent of the appl ied s t ress throughout the test due to the extremely low permeabi l i ty of the s o i l . The appl ied s t r a i n rate was 2.4 x 10 ^ inches per minute, the lowest rate of which the test apparatus was capable. Although the assumptions used to analyze the resu l t s of the s t r a i n con t ro l l ed conso l idat ion test require that the basal pore pressure does not great ly exceed 10 percent of the appl ied stress (Byrne and Aok i , 1 969 ) , the resu l t s were nevertheless s u f f i c i e n t to demonstrate that the permeabi l i ty of Hat Creek Clay must be in the order of 10 ^ cm per second or less . This value is in agreement with that estimated by Quigley (1976). 85. DISCUSSION AND INTERPRETATION OF RESULTS As described above, DS 1 exh ib i ted high shear res i s tance over a 5 day rapid displacement preshearing period not recorded on the data p lo t s . It is l i k e l y that the o r i g i n a l dry s tate of the s o i l combined with i t s low permeabi l i ty resu l ted in the i n i t i a l te s t ing being done on e s s e n t i a l l y dry s o i l . The high shear angle measured during preshearing (31 degrees) is not unreasonable for a crushed dry c l ay . The subsequent dec l ine in the measured shearing re s i s tance , shown in Figure 5-15, would ind icate that water has reached the shear plane. The low permeabi l i ty of the s o i l appears to preclude the p o s s i b i l i t y of drainage from the porous base so ear ly in t e s t i n g . However, water may have been ingested into the shear plane as the ends of the s o i l sample were a l t e r n a t e l y thrust into the water reservo i r and drawn back into the shear plane during t e s t i n g . This mechanism, previous ly deta i l ed in the review of test resu l t s on Haney Clay, could expla in the drop in shear strength of the s o i l over a r e l a t i v e l y short period of time. At the completion of DS 1, the apparent angle of f r i c t i o n was about 9-5 degrees and decreasing. In DS 2, the apparent angle of f r i c t i o n remained approximately constant at 15 degrees throughout most of the test per iod , as shown in Figure 5-16. However, in the las t few reversals the shear angle dec l ined to about 11 degrees. In DS k, performed on reconsol idated Hat Creek Clay, the shearing res i s tance f e l l gradual ly at the s t a r t of the test before l e v e l l i n g out at an angle of f r i c t i o n of about 16 degrees. In each case, tes t ing was ended when cumulative sample loss made fur ther te s t ing imprac t i ca l . 86. As encountered in d i r ec t shear tests on Haney Clay, the tests on Hat Creek Clay general ly exh ib i ted greater shear res i s tance when tested in one d i r e c t i o n than in the other . This divergence of recorded res i s tance is a t t r i bu ted to an i n c l i n a t i o n of the f a i l u r e surface. By applying the ana lys i s summarized in Figure 5.8, i t is ca l cu la ted that the shear planes in DS 1, DS 2 and DS A had i n c l i n -at ions of about 2.2, 1.1 and 0.9 degrees, re spec t i ve l y , near the end of t e s t i n g . A l l of the d i r ec t shear tests on Hat Creek Clay exh ib i ted a genera l ly constant shearing res i s tance in the ear ly stages of t e s t i n g . In DS 1, the shearing res i s tance remained v i r t u a l l y constant throughout a 5 day rapid displacement preshearing period at a value corresponding to an apparent angle of f r i c t i o n of 31 degrees. In DS 2 and DS k, a constant value of 15 to 16 degrees was a t ta ined a f t e r a displacement of about 1.5 inches. The shearing res i s tance plateau may resu l t from shear plane d i s rupt ions caused by the high percentage of sand and s i l t in the s o i l . The removal of the coarse sand f r a c t i on in DS k, leaving the f ine sand and s i l t in the sample, appears to have had l i t t l e e f f e c t on th i s behaviour. From subsequent reductions in shearing res i s tance recorded in DS 1 and DS 2, i t would appear that the in ter ference to shear plane development suggested by the res i s tance plateau can be overcome by increased displacements. As prev ious ly d i scussed, the higher shearing res i s tance plateau recorded in DS 1 is a t t r i bu ted to the low i n i t i a l water content of the s o i l sample. 87. The resu l t s o f d i r ec t shear tests on Hat Creek Clay f a i l to adequately def ine the residual strength of the s o i l . Residual condit ions were not at ta ined in DS 1 and 2, as indicated by the decreasing shearing res i s tance at the end of te s t ing . Comparing the resu l t s of DS 4 (Figure 5.17) to DS 2 (Figure 5-16), i t appears that fur ther decreases in shearing res is tance would be expected in DS 4 i f the test had continued and that th i s test a l so was ended prematurely. Considering the low permeabi l i ty o f the s o i l , the length of the drainage path and the poss ib le e f f ec t s of water ingested into the shear plane during shearing, i t is probable that the so i l was not f u l l y drained during d i r ec t shear tests on Hat Creek Clay. 5.4.4 Review and Evaluation of Results The resu l t s of ring shear tests on Hat Creek Clay ind icate that th is s o i l has a residual angle of f r i c t i o n in the order of 21 to 25 degrees. The r ing shear samples appear to have been wel1-drained, showing l i t t l e or no va r i a t i on in measured shearing res i s tance with increased displacement or with changes in the rate of shear. However, the maintenance of a very narrow conf in ing r ing gap, which allows the clay f ines to escape but reta ins the sand f r a c t i o n , may have resul ted in a concentrat ion of the coarse f r a c t i o n in the shear plane, increas ing the shearing res i s tance of the s o i l . The d i r ec t shear tests on Hat Creek Clay were ended due to cumulative sample loss before residual condit ions could be e s tab l i shed . The rate of sample loss was very high, genera l ly in the order of 0.1 inch loss ( ve r t i c a l displacement) per inch of t r a v e l , some 15 to 60 times greater than sample losses incurred in r ing shear te s t s . It would appear that the d i rec t shear tests on Hat Creek Clay were not f u l l y drained. Considering that 10 to 20 percent of the shear plane is exposed to water at each end of the d i r ec t shear traverse and that montmori1lonit ic c lays have a low permeabi l i ty combined with a high capacity to swell when exposed to water, the potent ia l for the development of excess pore pressures by ingest ion of water into the shear plane is much greater with Hat Creek Clay than with most other s o i l s . The sand content and low permeabi l i ty of Hat Creek Clay pose formidable problems in tes t ing for residual s trength. To ensure that the v i r t u a l l y impermeable s o i l is dra ined, i t is necessary to reduce the thickness of the sample and/or the rate of displacement. As any s i g n i f i c a n t reduction in the rate of shear great ly increases the time required to achieve the large displacements needed in res idual strength te s t i ng , reduced sample thickness is the preferred opt ion. However, attempts to test thin samples of c lay contain ing sand may be f u t i l e i f sand grains lodged against the porous platens protrude through or otherwise d i srupt the shear plane. Tests may a l s o be i n f l uenced by s e l e c t i v e l oss o f f i n e s from the sample which would a l t e r the s o i l composi t ion and inc rease the measured shear ing res i s t ance . 5-5 RESIDUAL STRENGTH TESTS ON IRANIAN CLAY 5.5-1 Soi1 Descr i p t i on I ran ian Clay i s a homogeneous, l i g h t brown, c layey s i l t . The s o i l , which was chunk sampled from a Middle Eastern deser t a r e a , i s hard and b r i t t l e and has a very low na tu ra l water con ten t . Typ i ca l c h a r a c t e r i s t i c s and p r o p e r t i e s f o r I ran ian Clay are as f o l l ows (Lum and Negussey, 1977): S p e c i f i c g r a v i t y 2.78 L i q u i d 1imi t kl% P l a s t i c i t y index 18% 90. Percent s i l t 70 to 75 Percent c lay 18 to 30 Iranian Clay has a strong tendency to f l o c c u l a t e such that a suspension of the s o i l w i l l s e t t l e wi th in minutes unless mixed with a de f l occu l an t . This tendency to f l o c c u l a t e may have s i g n i f i c a n t l y reduced the apparent c lay content, tabulated above, as determined by hydrometer ana ly s i s . The agency which provided the s o i l has ind icated that the c lay port ion of the s o i l may be large ly composed of a t t a p u l g i t e . 5-5-2 Ring Shear Tests Three r ing shear tests were performed on Iranian Clay: RS 9, RS 10 and RS 11. Shearing res i s tance versus displacement p lots for these tests are presented in Figures 5-18, 5-19 and 5.20. The three samples were prepared by adding water and remolding the s o i l p r i o r to pi acement. The i n i t i a l test o f the s e r i e s , RS 9, exh ib i ted very high values of s ide f r i c t i o n on the wal ls o f the upper con f in ing r ings. Short ly a f t e r the s t a r t of t e s t i n g , the tota l load appl ied through the a i r p iston was lowered in an attempt to reduce the s ide f r i c t i o n . However, s ide f r i c t i o n remained high, in the order of 37 percent of the tota l appl ied load, and the te s t ing was ended to prevent poss ib le damage to the equipment. RS 10 was marred by f luc tuat ions in appl ied normal pressure which were caused by a f au l t in the a i r pressure regulator c o n t r o l l i n g the loading p i s ton . Due to the low permeabi l i ty of the s o i l and the short period of the pressure o s c i l l a t i o n s , undrained condit ions ex i s ted 0.7 S 0.6 c (0 ? 0.5 in \u00E2\u0080\u0094 -1 3 \u00E2\u0080\u0094 CD 3 3 tCI 73 CD \u00E2\u0080\u0094 U l -I \u00E2\u0080\u0094 QJ U l 3 r t - . cu QJ 3 3 O CD o \u00E2\u0080\u0094 < QJ U l U l T3 QJ n CD 3 CD 3 73 co Shearing Resistance o o o o O J -tr- on O N _L I 1 L Shearing Resistance o o o o oo o' oo oo CD m ui \u00E2\u0080\u00A2o CU o CD 3 CD -C-3 CO on o on on -e-on ON 31 T J co o CO zr CD QJ \u00E2\u0080\u0094' ~ J o ui I on 3 -o -i CD Ul in c -1 CD C O 3 -CD CO-QJ -I ui \u00E2\u0080\u00A2a QJ o CD 3 CD 3 r t ON T r o co ro o C O o 0 ( on 3 3 ^ O O o o Ver t i c a l Displacement ( in.) Ve r t i c a l Displacement ( in.) \u00E2\u0080\u00A2\u00C2\u00A36 94. in the shear plane region of the sample such that f l uc tua t ions in the e f f e c t i v e normal s tress were smal l . This is demonstrated by the constancy of the measured shear strength which is a funct ion of the e f f e c t i v e normal s t re s s . A p lot of tota l appl ied normal s tress and measured shear s tress versus displacement is provided in Figure 5.21. In r ing shear t e s t i n g , the test procedures are designed to create drained condit ions in the sample such that the to ta l and e f f e c t i v e normal stresses are equal. This permits c a l c u l a t i o n of shearing res i s tance, the r a t i o of the e f f e c t i v e shear s tress to the e f f e c t i v e normal s t re s s , without monitoring pore pressure. When undrained condit ions e x i s t , the e f f e c t i v e normal s tress is unknown and the parameter ca l cu la ted as shearing res i s tance is only the r a t i o of the shear s tress to the tota l normal s t re s s . This is the parameter shown on the shearing res i s tance p lot for RS 10, Figure 5-19-Despite the pressure o s c i l l a t i o n s , the e f f e c t i v e normal s tress wi th in the sample can be estimated by the average tota l normal s t res s . Using th i s approach, the resu l ts of RS 10 ind icate that Iranian Clay has a res idual angle of f r i c t i o n of about 29 degrees, measured at an average appl ied normal stress of about 62.5 p s i . The f i n a l test of the ser ies on Iranian Clay, RS 11, was hampered by a f a i l u r e in the automatic data recording system. Data was acquired by manual operat ion of the equipment, reducing the quantity of data ava i l ab le for study. However, the resu l t s of th is t e s t , p lo t ted in Figure 5.20, are cons i s tent , i nd i ca t ing that Iranian U> -O in i n OJ 70 H 60 A so A ro LQ c ro 30 H fj 20 10 H ro O / / Normal Stress Shear Stress 9 . 0 1 0 . 0 Shear Displacement (inches) 11.0 FIGURE 5 .21 Total Normal and Shear Stress vs. Displacement - RS 10 The o s c i l l a t i o n s in tota l appl ied normal stress occurred as a resu l t o f a f au l ty a i r pressure regulator. The constancy in shear s tress ind icates that, despite tota l normal stress f l u c tua t i on s , the e f f e c t stress on the shear plane remained nearly constant due to undrained condit ions within the so i l sample. 96. Clay has a residual f r i c t i o n angle of about 27.8 degrees. RS 11 was conducted at a normal s t ress of 75 p s i . The resu l ts of the ring shear tests on Iranian Clay, tabulated in Table 5.8, are shown on a Mohr diagram in Figure 5-22. These resu l t s are in c lose agreement with the accepted residual f r i c t i o n angle of a t t apu l g i t e , 30 degrees, as tabulated in Tables 2. I and 2.2. TABLE 5.8 Test Results : Ring Shear Tests on Iranian Clay Test Stage Normal Stress Shearing Resistance 0'r Shearing Rate Cumulative Displacement - 4 RS 9 1 125 psi Test incomplete 5-7x10 in/min 1.40 inches - 4 2 60 \" Test incomplete 5-7x10 \" 3-13 \" _/, RS 10 1 60 \" Scatter due to 4.3x10 11 1 4 . 4 0 \" load f luc tuat ions ( - average .555) (29\u00C2\u00B0) - 4 2 30 \" Test incomplete 4.3x10 \" 20.12 \" RS 11 1 75 \" -529 27-9\u00C2\u00B0 1-8x10 _ 3 \" 50.68 RS 1Q/ RS 27.9 29\u00C2\u00B0 \u00E2\u0080\u0094r 80 10 \u00E2\u0080\u0094r 20 30 4o 50 Normal Stress ( l b s . / sq . in . ) 60 T 70 FIGURE 5.22 Mohr Rupture Diagram: Summary of Results of Residual Strength Tests on Iranian Clay 99. 5.6 SUMMARY OF TESTING PROGRAM Ring shear tests were conducted on three c lay s o i l s having widely contras t ing p roper t ie s . Mu l t i - rever sa l d i r ec t shear tests were a l so conducted on two of the c l ays . Po s i t i ve determinations of res idual strength were obtained in r ing shear tests on Haney Clay and Iranian Clay. The resu l t s obtained in r ing shear tests on Hat Creek Clay are less d e f i n i t i v e due to the compl icat ing e f f ec t s o f sand and swel l ing c lay with in the soi1 compos i t ion. High values of s ide f r i c t i o n were encountered in some r ing shear te s t s . The problem is be l ieved to resu l t from corros ion welts ra ised on the conf in ing rings through chemical act ion of the s o i l pore f l u i d . Problems re l a t i n g to improper c a l i b r a t i o n of transducers and malfunctions of the data a cqu i s i t i on unit were a lso encountered. Despite such cons iderat ions , the ring shear device proved simple to operate and e f f i c i e n t at producing cons i s tent , ea s i l y interpreted re su l t s . No po s i t i ve determinations of res idual strength-were obtained in d i r ec t shear te s t ing . A l l d i r ec t shear resu l ts were lower than the values obtained in the ring shear dev ice, i nd i ca t ing that po s i t i ve excess pore pressure ex i s ted in the d i r ec t shear samples. In most tests the apparent shearing res is tance d i f f e r e d in forward and reverse shear d i r e c t i o n s , a va r i a t i on a t t r i bu ted to i n c l i n a t i o n of the shear plane. Inconvenient procedures and the protracted duration of the tests added to these d i f f i c u l t i e s . 100. CHAPTER VI SUMMARY AND EVALUATION RESIDUAL STRENGTH Residual strength is the lowest drained strength of a s o i l , a t ta ined at large shear displacements and unaffected by i n i t i a l s o i l s t ructure or stress h i s to ry . Many clayey s o i l s , p a r t i c u l a r l y i f overconso1idated, exh ib i t a res idual strength that is s i g n i f i c a n t l y lower than the peak value. Residual strength has important engineering app l i ca t ions in determining s t a b i l i t y of prev ious ly f a i l e d s lopes, where res idual condit ions current ly e x i s t , and in eva luat ing the long-term s t a b i l i t y of slopes in overconsol idated c l ay s , in which residual condit ions may develop through the mechanism of progress ive f a i l u r e . Clay deforms in simple shear p r i o r to reaching peak s trength. Beyond peak strength, shear displacement occurs by s l i p along the cleavage planes of adjacent p a r t i c l e s with in shear bands located in the f a i l u r e zone. Residual condit ions develop as the p a r t i c l e s in the shear bands a t t a i n p a r a l l e l alignment. The preferred cleavage mode of s o i l inf luences i t s residual s trength. 101. 5. Shear strength in c lays o r i g ina tes from inter -atomic bonding across s o l i d - t o - s o l i d contacts between s o i l p a r t i c l e s . Var iat ions in the residual strength of d i f f e r e n t minerals occurs due to var ia t ions in the strength and concentrat ion of bonding. 6. The residual strength of ac t i ve c lay minerals can be a l t e red by the inf luence of pore water chemistry on physico-chemical i n t e r p a r t i c l e forces of a t t r a c t i o n and repu l s ion. 6.2 UBC RING SHEAR DEVICE The ob jec t i ve of th is thes is project was to design and develop a p r ac t i c a l residual strength apparatus. Having considered a l t e r n a t i v e s , the r ing shear apparatus was se lected as the most p r a c t i c a l , e f f e c t i v e and adaptable tes t ing device yet devised for th i s a p p l i c a t i o n . Although the d i rec t shear and t r i a x i a l tests can be modified for res idual strength purposes, the i n a b i l i t y to obtain large uni -d i rect iona1 displacements and complicated or inconvenient test procedures severely l im i t the potent ia l value of these methods. The major features of the UBC Ring Shear Device are as fo l lows: 1. The device is capable of unl imited displacements at smoothly var i ab le rates o f shear from 3-2 inches/year to 9 inches/hour. 2. The appl ied normal s tress is smoothly var i ab le up to 200 l b . / s q . i n . Higher stresses can be obtained by increas ing the house l i ne a i r pressure suppl ied to the loading p i s ton . 3. The s o i l sample, with inner and outer rad i i o f 1.75 and 2.75 inches, re spec t i ve l y , has a var i ab le thickness up to a maximum of 0.75 inches. 102. 4. The length of the basal drainage path is va r i ab le up to a maximum of 0.25 inches. The top drainage path var ies in length with sample th ickness. 5. Sample placement is made simple and e f f i c i e n t by using the upper conf in ing rings as a mold for remolded samples and a cu t t ing unit for undisturbed samples. 6. The conf in ing r ing gap may be set at a very narrow width without i n te r f e r i n g with shear t e s t i ng . Typica l set t ings range from 0.002 to 0.004 inches. 7. Data a cqu i s i t i on units c o l l e c t and record data automat ica l ly in a form that is ea s i l y adaptable to computerized data reduct i on. 8. Only minimal superv is ion is required during t e s t i n g , a b r i e f equipment check every day or two normally being s u f f i c i e n t toensure smooth operat ion. 6.3 TESTING PROGRAM A ser ies of r ing shear tests were undertaken on three var ied c lay s o i l s as a means of eva luat ing the new dev ice. Repeatable and cons is tent resu l ts were obtained in r ing shear tests on Haney Clay and Iranian Clay, i nd i ca t ing that the residual angles of f r i c t i o n for these s o i l s are 24 and 28 to 29 degrees, re spec t i ve ly . Var iab le re su l t s , ranging from 21 to 25 degrees, were obtained in r ing shear tests on Hat Creek Clay. The va r i a t i on is a t t r i bu ted to d i s rupt ion of the shear plane by sand grains in the s o i l . This va r i a t i on may have been aggravated 103. by a gradual concentrat ion of sand-s ized p a r t i c l e s in the shear plane as f i n e r p a r t i c l e s were s e l e c t i v e l y lost through the conf in ing r ing gap. No determination of res idual strength was obtained in the d i r ec t shear apparatus. The resu l ts were incons is tent and lower than the res idual values as determined in the r ing shear dev ice, i nd i ca t ing that the tests were not f u l l y drained and that po s i t i ve pore pressures ex i s ted in the shear zone. 6.4 EVALUATION The UBC Ring Shear Device provides a simple and p r a c t i c a l method for determining residual s trength. Specimen preparat ion is quick and convenient, whi le operat ion of the device requires minimal superv i s ion . This v e r s a t i l e apparatus accomodates a wide range of sample heights , appl ied normal loads and rates of displacement. Test resu l t s are e a s i l y understood and in terpreted . The device funct ions e f f i c i e n t l y , averaging 13.5 test days per res idual strength determination during the test program. Increased e f f i c i e n c y is an t i c ipa ted as f a m i l i a r i t y is gained with tes t ing procedures and equipment. The success of the UBC Ring Shear Device in determining res idual strength is a t t r i bu ted in large measure to the natural advantages inherent in i t s mechanical con f i gura t ion . This conf igurat ion permits unl imited un id i rec t i ona l shear displacements to be appl ied to a s o i l sample of constant c ros s - sec t iona l area under near-uniform stress cond i t ions . These features provide an exce l lent laboratory model of the i n s i t u f i e l d cond i t i on . Also cont r ibut ing to the success of the device through 104. enhanced p rac t i c a l operat ion are the p rec i se l y con t ro l l ed conf in ing r ing gap and the automated tes t ing and monitoring features . While the UBC Ring Shear Device provides an e f f i c i e n f and e f f e c t i v e method for determining residual s t rength, minor modi f icat ions w i l l improve th i s c a p a b i l i t y . Problems encountered during r ing shear tes t ing include malfunctions of the data a c q u i s i t i o n unit and excess ive s ide f r i c t i o n on the upper conf in ing r ings. To avoid fur ther malfunc-t ions , the data a c q u i s i t i o n unit has been replaced with a new s e n s i t i v e and r e l i a b l e un i t . The excess ive values of s ide f r i c t i o n are be l ieved to resu l t from corros ion welts on the surface of the conf in ing r ings. It is recommended that th i s problem be e l iminated by rep lac ing the e x i s t i n g aluminum rings with co r ro s i on - re s i s t an t rings constructed of s t a i n l e s s s teel or anodized aluminum. The d i r ec t shear device proved to be an inconvenient and inadequate means of determining residual strength. The present and potent ia l performance of the d i r ec t shear device in res idual strength app l i ca t ions is l imi ted by a mechanical conf igurat ion which lacks the natural advantages of the r ing shear device. As a r e su l t , shear displacements must be reversed or repeated to accumulate large d i sp l ace -ments. The stress condit ions in the so i l sample, which s h i f t gradual ly during displacement, change abrupt ly with each reversal of shear d i r e c t i o n . These frequent d i s rupt ions in appl ied s t ress and s t r a i n produce data that is commonly sparse, contrad ic tory or d i f f i c u l t to i n te rpre t . However, with in these l im i ta t i ons s i g n i f i c a n t improvement in the performance of the d i rec t shear device may be poss ib le by 105. r e s t r i c t i n g the motion of the upper loading frame to a s ing le hor izonta l path. As present ly constructed, some v e r t i c a l , l a te ra l and rota t iona l motions of the upper loading frame occur during t e s t i n g . Mod i f i ca t ion of the device to prevent such motions, which could be accomplished by mounting the upper loading frame on a ca r r i age , would provide p rec i se control of the loading frame gap, reduce sample losses and permit te s t ing of th in samples. The shortened drainage path of th in samples would improve sample drainage, provid ing greater confidence in the test r e su l t s , and would probably reduce the duration of t e s t i n g . Re s t r i c t i n g the motion of the upper loading frame would a l so i n h i b i t shear plane i n c l i n a t i o n which hampered many of the d i r ec t shear tests c a r r i ed out during th i s study. Due to the extended duration of t e s t i n g , fur ther modi f icat ions to f u l l y automate the d i r e c t shear device would be e s sent i a l fo r con-venient and p rac t i ca l operat ion. Modi f icat ion of the d i rec t shear apparatus would improve i t s c a p a b i l i t y for determining residual s t rength. However, data would remain sparse and cumbersome to in terpret due to the incremental approach to cumulative displacements employed in th i s te s t , and the durat ion of tes t ing would remain extended as a resu l t of shear plane d i s rupt ions at each reversal of shear d i r e c t i o n . Therefore, the r ing shear device w i l l continue to provide the most p r a c t i c a l , convenient, e f f i c i e n t and e f f e c t i v e method for determining residual s t rength. 106. REFERENCES Bishop, A.W., G.E. Green, V.K. Garga, A. Andresen and J.D. Brown, 1971- A new ring shear apparatus and i t s app l i c a t i on to the measurement of residual s t rength, Geotechnique, V. 21, No. k, 273-328. Bjerrum, L., 1967. Progress ive f a i l u r e in slopes of overconso l idated p l a s t i c c lay and c lay sha les , Jour. So i l Mech. and Fdn. Div. , A .S .C .E . , V. 93, SM 5, 3-^9. Borowicka, H., 1965- The inf luence of c o l l o i d a l content on the shear strength of c l ay , Proceedings, 6th I n t ' l Conference on Soi l Mechanics and Foundation Engineering, Montreal, V. 1, 175-178. Bosdet, B., K. Lum and D. Negussey, 1976. One-dimensional conso l idat ion of Haney Clay, unpublished report on tests performed at the Univers i ty o f B r i t i s h Columbia, Vancouver. Byrne, P.M. and Y. Aok i , 1969- The S t ra in Contro l led Consol idat ion Test , So i l Mech. Series No. 9, Dept. of C i v i l Eng., Un ivers i ty of B r i t i s h Columbia, Vancouver. Campanella, R.G. and R.C. Gupta, 1969. E f fec t of Structure on Shearing Resistance of a Sens i t i ve Clay, So i l Mech. Series No. G~, Dept. of Ci vi 1 Eng. , Uni vers i ty of B r i t i s h Columbia, Vancouver. Chandler, R.J . , 1966. The measurement of residual strength in t r i a x i a l compression, Geotechnique, V. 16, No. 3, 181-186. Chattopadhyay, P.K., 1972. Residual Shear Strength of Some Pure Clay Minera ls , Ph.d. Thes i s , Un ivers i ty of A lbe r ta , Edmonton. Houston, Wi l l i am N. and James K. M i t c h e l l , 1969\u00E2\u0080\u00A2 Property i n t e r r e l a t i o n -ships in s en s i t i ve c l a y s , Jour. So i l Mech. and Fdn. 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Residual strength in conventional t r i a x i a l t e s t s , Proceedings, 7th I n t ' l . Conf. Soi l Mech., Mexico, V. 1, 433-^1-109. APPENDIX I DERIVATION OF EQUATION FOR DETERMINING tan 0' Let = uniform shear s t ress over shear plane (T* = uniform normal s tress tan 0' = t/G-r^ = ins ide radius of s o i l sample r^ = outs ide radius o f s o i l sample M = sum of moments over shear plane as measured by force transducers W = tota l normal load on shear plane 'T\" x Sample Area x Moment Arm 2 \u00C2\u00AB r t a n 0 ' ) (2Tfrdr) (r) 1 \u00E2\u0080\u00A2 J i .= 2tt "Thesis/Dissertation"@en . "10.14288/1.0062554"@en . "eng"@en . "Civil Engineering"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "The UBC ring shear device"@en . "Text"@en . "http://hdl.handle.net/2429/22214"@en .