SHEAR STRENGTH AND VOLUME CHANGE RELATIONSHIP FOR AN UNSATURATED SOIL TRINH MINH THU SCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING NANYANG TECHNOLOGICAL UNIVERSITY SINGAPORE 2006 SHEAR STRENGTH AND VOLUME CHANGE RELATIONSHIP FOR AN UNSATURATED SOIL TRINH MINH THU BEng, MSc SCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING NANYANG TECHNOLOGICAL UNIVERSITY A Thesis submitted to the Nanyang Technological University in fulfillment of the requirements for the degree of Doctor of Philosophy 2006 To my parents: Trịnh Việt Miên & Mai Thị Lan my wife: Trần Thị Thu Hơng and my children: Trịnh Nữ Anna Minh Trâm & Trịnh Minh Tân Acknowledgements ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude and sincere appreciation to my supervisor, Professor Harianto Rahardjo His unfailing interest, guidance and support will not be forgotten I am indebted to my supervisor for his patience and kindness throughout this research His care provided for me and my family is greatly acknowledged I wish to acknowledge the financial support provided by Nanyang Technological University, Singapore in the form of a research scholarship The prompt assistance given by the staff and graduate students of the School of Civil and Environmental Engineering, Nanyang Technological University are appreciated I am grateful to Prof D G Fredlund from University of Saskatchewan, Canada, Assoc Prof Leong Eng Choon, Assoc Prof Chang Ming-Fang, Assoc Prof Teh Cee Ing, Assoc Prof Chu Jian, Assoc Prof Wong Kai Sin from Nanyang Technological University, Singapore and Prof Nguyen Cong Man from Hanoi Water Resources University, Vietnam for their invaluable advice for this study Special thanks to Dr Yang Dai Quan for his valuable discussions and his reading of the theory chapter I would like to thank Mr Vincent Heng Hiang Kim and Mrs Inge Meilani for sharing their experience in conducting unsaturated soil tests Thanks also go to other geotechnical laboratory staffs, CEE, NTU, especially Mr Tan Hiap Guan Eugene, Mr Han Guan, Mrs Lee-Chua Lee Hong and Mr Phua Kok Soon from the construction laboratory, CEE, NTU I want to express my love and gratitude to my parents, Mr Trinh Viet Mien and Mrs Mai Thi Lan, for their constant encouragement throughout my life Special thanks to my wife, Mrs Tran Thi Thu Huong, and my children, Trinh Nu Anna Minh Tram and Trinh Minh Tan, for their love, understanding and constant encouragement throughout my study Finally, I am also thankful to the Ministry of Training and Education, Ministry of Agricultural and Rural Development of Vietnam, Hanoi Water Resources University, Vietnam for approving my study leave to undertake this research Acknowledgements also go to my friends who have helped me in this research programme iii Abstract ABSTRACT Shear strength of unsaturated soil is commonly obtained from a consolidated drained (CD) triaxial test However in many field situations, fill materials are compacted where the excess pore-air pressure developed during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time It can be considered that the air phase is generally under a drained condition and the water phase is under an undrained condition during compaction This condition can be simulated in a constant water content (CW) triaxial test Comparisons between the shear strength parameters obtained from the CW and the CD triaxial tests have not been extensively investigated An elasto-plastic model for unsaturated soil with the incorporation of soil-water characteristic curve (SWCC) was proposed in this study The proposed model was verified with experimental data A series of SWCC, isotropic consolidation, the CW and CD triaxial tests were conducted on statically compacted silt specimens in a triaxial cell apparatus The experimental results from SWCC tests under different net confining stresses showed that the air-entry value and the yield suction increased nonlinearly with the increase in net confining stress The results of the isotropic consolidation tests indicated that the yield stress increased with the increase in matric suction The slope of the normally consolidated line (NC), the slope of the unloading curve and the intercept of the consolidation curves at the reference stress decreased with the increase in matric suction The results indicated that the effective angles of internal friction, φ ' , and the effective cohesions, c ' , of the compacted silt as obtained from both the CW and CD tests were identical The results of the CW and CD triaxial tests indicated that the effective angle of internal friction, φ ' , and the effective cohesion, c ' , of the compacted silt were 320 and zero kPa, respectively The relationships between φ iv b Abstract and matric suction from the CW and CD triaxial tests on the compacted silt specimens were found to be non-linear The φ b angle was found to be the same as the effective angle of internal friction, φ ' (i.e., 320 ) at low matric suctions (i.e., matric suctions lower than the air-entry value) The φ b angle decreased to a magnitude as low as 120 at high matric suctions (i.e., matric suctions higher than the residual matric suction) However, the φ b angles from the CW and CD tests were different at matric suctions between the air-entry and the residual matric suction values due to the hysteretic behaviour of the soil-water characteristic curve The critical state lines at different matric suctions on the (q – p) plane were parallel with a slope of 1.28 for both the CW and CD triaxial tests, indicating the unique relationship between the deviator stress and mean net stress The results also indicated the unique relationship between the specific volume and mean net stress on the (v – p) plane for both the CW and CD triaxial tests The slope of the critical state lines on the (v – p) plane for both the CW and CD triaxial tests decreased with the increase in matric suction Reasonably good agreements between the analytical simulations based on the proposed elasto-plastic model with the incorporation of SWCC and the experimental results for the shear strength, the change in pore-water pressure and the volume change during shearing tests were obtained in this study v Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS III ABSTRACT… IV TABLE OF CONTENTS VI LIST OF TABLES XII LIST OF FIGURES XV LIST OF SYMBOLS XXIX CHAPTER INTRODUCTION 1.1 BACKGROUND 1.2 OBJECTIVES AND SCOPE OF THE RESEARCH 1.3 METHODOLOGY 1.4 OUTLINE OF THE REPORT CHAPTER LITERATURE REVIEW …7 2.1 INTRODUCTION 2.2 STRESS STATE VARIABLES 2.3 SOIL-WATER CHARACTERISTIC CURVE .8 2.4 CONSOLIDATION TESTS AND THE CONTROLLING FACTORS 2.5 VOLUME CHANGE OF UNSATURATED SOILS ………………………10 2.5.1 General……………………………………………………………………………… 10 2.5.2 Constitutive relationships………………………………………………………… 11 2.5.2.1 Soil Structure Constitutive Relationship………………………… …… 12 2.5.2.2 Water Phase Constitutive Relationship……………………… ………….16 2.6 SHEAR STRENGTH OF UNSATURATED SOILS ………………….16 vi Table of Contents 2.6.1 Shear Strength Equation…………………………………………………………… 16 2.6.2 Constant Water Content Triaxial Tests……………………………………… 21 2.6.3 Consolidated Drained (CD) Triaxial Tests……………………………………… 25 2.6.4 The Measurements of Matric Suction………………………………… ……… 28 2.6.5 Volume Change Measurements…………………………………………………… 35 2.7 REVIEW THE ELASTO-PLASTIC MODEL FOR SATURATED SOILS……………….37 2.7.1 Basic Concept of Critical State Model for Saturated Soil……………………….37 2.7.1.1 Yield Surface…………………………………………………………… 38 2.7.1.2 Critical State Parameters……………………………………… …… 40 2.7.2 Prediction of the Excess Pore-water Pressure in Normally Consolidated and Lightly Overconsolidated Saturated Soils under an Undrained Condition……………………………………………………………………………… 42 2.7.3 Prediction of the Excess Pore-water Pressure of Heavily Overconsolidated Soils…………………………………………………………… 46 2.8 REVIEW THE ELASTO PLASTIC MODEL FOR UNSATURATED SOILS ……… 48 CHAPTER THEORY 53 3.1 INTRODUCTION 53 3.2 THEORETICAL BACKGROUND FOR ELASTO-PLASTIC THEORY FOR UNSATURATED SOIL 53 3.2.1 Elastic strains 57 3.2.2 Plastic strains 58 3.2.3 Loading – collapse (LC) yield curve 59 3.2.4 Flow rules 65 3.2.5 Determination of the Mean Net Stress and the Deviator Stress at the Initial Yield Point 65 3.3 PROPOSED EQUATIONS FOR DETERMINATION OF THE MODEL PARAMETERS 69 3.4 CRITICAL STATE 73 3.5 PREDICTION OF THE CHANGE IN MATRIC SUCTION DURING CW TEST 74 vii Table of Contents CHAPTER RESEARCH PROGRAMME 80 4.1 INTRODUCTION 80 4.2 OUTLINE OF RESEARCH PROGRAMME 80 4.3 PREPARATION OF THE COMPACTED SPECIMENS AND BASIC SOIL PROPERTIES 81 4.3.1 Criteria for Preparing the Specimen 81 4.3.2 Basic Soil Properties 82 4.3.3 Static Compaction Mould 83 4.3.4 Static Compaction Process 85 4.3.5 Tests for Obtaining SWCC using Pressure Plate 86 4.4 4.4.1 TRIAXIAL SET UP AND ITS DEVELOPMENT 88 Modified Triaxial Apparatus for the Soil-water Characteristic Curve Tests (SWCC) 88 4.4.2 Modified Triaxial Apparatus for Isotropic Consolidation Tests 99 4.4.3 Modified Triaxial Apparatus for the CW and CD Triaxial Tests 100 4.5 TESTING PROCEDURE 101 4.5.1 Testing Procedure for SWCC Tests 101 4.5.2 Testing Procedure for Isotropic Consolidation Tests 103 4.5.3 Testing Procedure for Constant Water Content Tests 104 4.5.4 Testing Procedure for the CD Triaxial Tests 105 4.5.5 Final Measurement 106 4.6 TESTING PROGRAMME 106 4.6.1 SWCC Tests under Different Net Confining Stresses 106 4.6.2 Testing Programme for Isotropic Consolidation Tests 110 4.6.3 Testing Programme for Constant Water Content Tests 113 4.6.4 Testing Programme for the Consolidated Drained Tests 114 viii Table of Contents 4.7 THEORETICAL SIMULATION OF THE SHEAR STRENGTH, EXCESS PORE-WATER PRESSURE AND VOLUME CHANGE DURING SHEARING UNDER THE CW AND CD TRIAXIAL TESTS 115 CHAPTER PRESENTATION OF RESULTS 117 5.1 INTRODUCTION 117 5.2 BASIC SOIL PROPERTIES 117 5.2.1 Index Properties 117 5.2.2 Soil-Water Characteristic Curves 119 5.2.3 Isotropic Consolidation Curves 131 5.3 CONSTANT WATER CONTENT (CW) TRIAXIAL TEST RESULTS 140 5.3.1 Failure Criteria 141 5.3.2 Shear Strength Behaviours 141 5.3.3 Characteristics of the Excess Pore-water Pressure 151 5.3.4 Volume Change Behaviours during Shearing Stage 160 5.3.5 Water Content Characteristics of the Specimen at the End of the Shearing Stage 163 5.4 CONSOLIDATED DRAINED (CD) TRIAXIAL TEST RESULTS 164 5.4.1 Shear Strength Behaviours 164 5.4.2 Characteristics of the Soil Volume Changes 170 5.4.3 Water Volume Change Behaviours during Shearing Stage 173 5.5 INTERPRETATION OF THE CW AND CD TRIAXIAL TEST RESULTS USING EXTENDED MOHR-COULOMB FAILURE ENVELOPE 175 5.5.1 Failure Criteria 175 5.5.2 Constant Water Content (CW) Triaxial Tests 180 5.5.3 Consolidated Drained Triaxial (CD) Tests 192 5.5.4 Comparisons of the Shear Strength for the CW and CD Triaxial Tests 198 CHAPTER DISCUSSION OF THE RESULTS 201 6.1 INTRODUCTION 201 ix Appendix D Pore-water pressure, uw (kPa) 120 100 80 60 40 Measured Simulated 20 0 10 15 20 25 30 Axial strain, εy (%) Figure D.53 Comparison between the simulated and measured results of the change in pore-water pressure versus axial strain during shearing for the specimen CW100-300 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure D.54 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CW100-300 322 Appendix D Deviator stress, q (kPa) 1600 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure D.55 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CW150-300 Pore-water pressure, uw (kPa) 120 100 80 60 40 Measured Simulated 20 0 10 15 20 25 30 Axial strain, εy (%) Figure D.56 Comparison between the simulated and measured results of the change in pore-water pressure versus axial strain during shearing for the specimen CW150-300 323 Appendix D Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure D.57 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CW150-300 Deviator stress, q (kPa) 1600 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure D.58 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CW200-300 324 Appendix D Pore-water pressure, uw (kPa) 120 100 80 60 40 Measured Simulated 20 0 10 15 20 25 30 Axial strain, εy (%) Figure D.59 Comparison between the simulated and measured results of the change in pore-water pressure versus axial strain during shearing for the specimen CW200-300 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure D.60 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CW200-300 325 Appendix D Deviator stress, q (kPa) 1600 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure D.61 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CW250-300 Pore-water pressure, uw (kPa) 120 100 80 60 40 Measured Simulated 20 0 10 15 20 25 30 Axial strain, εy (%) Figure D.62 Comparison between the simulated and measured results of the change in pore-water pressure versus axial strain during shearing for the specimen CW250-300 326 Appendix D Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure D.63 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CW250-300 Deviator stress, q (kPa) 1600 1400 1200 1000 800 600 400 Measured Simulated 200 0 10 15 20 25 30 Axial strain, εy (%) Figure D.64 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CW300-300 327 Appendix D Pore-water pressure, uw (kPa) 200 Measured Simulated 150 100 50 0 10 15 20 25 30 Axial strain, εy (%) Figure D.65 Comparison between the simulated and measured results of the change in pore-water pressure versus axial strain during shearing for the specimen CW300-300 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure D.66 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CW300-300 328 Appendix E APPENDIX E Simulation of the CD triaxial Tests Results Using the Proposed Elasto-plastic Model with the Incorporation of SWCC 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.1 Comparison between the simulated and measured of the deviators tress versus axial strain during shearing for the specimen CD100-0 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.2 Comparison between the simulated and measured of the volumetric strain versus axial strain during shearing for the specimen CD100-0 329 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.3 Comparison between the simulated and measured results of the deviator stress versus axial strain during shearing for the specimen CD200-0 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.4 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD200-0 330 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.5 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CD100-100 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.6 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD100-100 331 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.7 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CD200-100 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.8 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD200-100 332 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.9 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CD100-200 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.10 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD100-200 333 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.11 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CD200-200 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.12 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD200-200 334 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.13 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CD100-300 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.14 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD100-300 335 Appendix E 1600 Deviator stress, q (kPa) 1400 Measured Simulated 1200 1000 800 600 400 200 0 10 15 20 25 30 Axial strain, εy (%) Figure E.15 Comparison between the simulated and measured results of the deviators tress versus axial strain during shearing for the specimen CD200-300 Volumetric strain, εv (%) 10 Measured Simulated -5 -10 10 15 20 25 30 Axial strain, εy (%) Figure E.16 Comparison between the simulated and measured results of the volumetric strain versus axial strain during shearing for the specimen CD200-300 336 ... children, Trinh Nu Anna Minh Tram and Trinh Minh Tan, for their love, understanding and constant encouragement throughout my study Finally, I am also thankful to the Ministry of Training and Education,.. .SHEAR STRENGTH AND VOLUME CHANGE RELATIONSHIP FOR AN UNSATURATED SOIL TRINH MINH THU BEng, MSc SCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING NANYANG TECHNOLOGICAL UNIVERSITY A Thesis submitted. .. strength and volume change with the assumption that contractile skin is an additional fourth phase for an unsaturated soil Contractile skin can be described as an air-water interface with pore-air and