Luận án Ph.D.: Quan hệ giữa độ bền biến dạng và thay đổi thể tích đất không饱和土 - Trinh Minh Thu

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: Trinh Viet Mien & Mai Thi Lan my wife: Fran Shi tu Fouong and my children: Trinh Hi Anna Minh Gram & Grinh Minh ân 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.

Fredlund from University of Saskatchewan, Canada, Assoc. Leong Eng Choon, Assoc. Chang Ming-Fang, Assoc. Teh Cee Ing, Assoc.

Chu Jian, Assoc. 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, ¢', 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, ¢', and the effective cohesion, c', of the compacted silt were 32° and zero kPa, respectively. The relationships between ¢ ” iv Abstract and matric suction from the CW and CD triaxial tests on the compacted silt specimens were found to be non-linear. The ¢” angle was found to be the same as the effective angle of internal friction, ¢' (i., 32°) at low matric suctions (i., matric suctions lower than the air-entry value). The ¢” angle decreased to a magnitude as low as 12° at high matric suctions (i., matric suctions higher than the residual matric suction).

However, the ó” 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 (đ — 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. Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS. GHI HH 00000000 00 II ABSTTRAC TT. 00000090009 009000 IV TABLE OF COINTTIENN TT S.

HH HH 0 000001001085VỊ LIST OF TABLES. I0 0050 XII LIST OF FIGURES. 5< 5< << 5< 5 HH 0000004000 5 XV LIST OF SYMBOLS. I1 100008 g0 XXIX CHAPTER 1 INTRODUCTION.2 OBJECTIVES AND SCOPE OF THE RESEARCH.

G1 TH TH TH HH TH 4 1. 5 CHAPTER 2 LITERATURE REV |HỀYN.1 SINSUSXO)BLOL 04 U (©), ee 7 2.2 STRESS STATE VARIABLES .3 SOIL-WATER CHARACTERISTIC CURVE .4 CONSOLIDATION TESTS AND THE CONTROLLING FACTORS.5 VOLUME CHANGE OF UNSATURATED SOILS .Ă SH SH TS KT TK TT tk kg 10 2.1 Soil Structure Constitutive RelationshIp.2 Water Phase Constitutive RelatlonshIp.6 SHEAR STRENGTH OF UNSATURATED SOILS. ---------<----«-+«-«-« LO Vi “Table of Contents 26.1 Shear Srength Equation 16 262 Constant Water Content Triaxal Tests mi 263 Consolidated Drained (CD) Triaxial Tests 25 264 The Measurement of Matric Suction. Volume Change Measurements.7 REVIEW THEELASTO-PLASTIC MODE.

FOR SATURATED SOS, ” 27.1 Basie Concept of Critical State Model for Saturated Soi a7 38 2.2 Coitical State Parameters 40 2.72 Prediction ofthe Excess Poreswater Pressure in Normally Consolidated and Lightly Overconsolidated Saturated Soils under an Undrained Condition 2 2.73 Predicion ofthe Excess Pore-water Pressure of Heavy Overconsolidated Sots.8 ReviEW THE ELASTO PLASTIC MoDEL oR UNSATURATED Som.2 THEORETICAL BACKOROUND FOR ELASTO-PLASTIC THEORY FOR UNSATURATED soi.21 Blastie strains hà 3.23 Loading ~ collapse (LC) yield carve s 3⁄24.25 Determination ofthe Mean Net Stress and the Devator Stress athe Initial Yield Point 65 33, PROPOSED EQUATIONS FOR DETERMINATION OF THE MODEL PARAMETERS.5 PREDICTION OFTHE CHANGE IN MATRIC SUCTION DURING CW TPAT,. 74 Table of Contents CHAPTER 4 RESEARCH PROGRAMME .2 OUTLINE OF RESEARCH PROGRAMME.- Ghi nH nnrn 80 4.3 PREPARATION OF THE COMPACTED SPECIMENS AND BASIC SOIL PROPERTIES "1.1 Criteria for Preparing the Specimen .3 Static Compaction MO ÏỞÏ.4 Static COMPACTION PTOC€S. «ch HH Hg 85 4.5 Tests for Obtaining SWCC using Pressure PÌÏAf€.4 TRIAXIAL SET UP AND ITS DEVELOPMENT.1 Modified Triaxial Apparatus for the Soil-water Characteristic Curve Tests 002 .2 Modified Triaxial Apparatus for Isotropic Consolidation Tests.3 Modified Triaxial Apparatus for the CW and CD Triaxial Tests. - - G c0 1 TH HH kh 101 4.1 Testing Procedure for SWCC Tests .2 Testing Procedure for Isotropic Consolidation Tesfs.3 Testing Procedure for Constant Water Content TesfS.4 Testing Procedure for the CD Triaxial T€§fS.

cv nh Tu HH HH ng 106 4.- G1 TT HH ng 106 4.1 SWCC Tests under Different Net Confining Sfr€SS€S.2 Testing Programme for Isotropic Consolidation Tesfs.3 Testing Programme for Constant Water Content TeSfS.4 Testing Programme for the Consolidated Drained Tests. 114 Vili “Table of Contents 47 THEORETICAL SIMULATION OF THE SHEAR STRENGTH, EXCESS PORE-WATER PRESSURE AND VOLUME CHANGE DURING SHEARING UNDER THE CW AND CD TRIAXIAL TESTS. us (CHAPTER 5 PRESENTATION OF RESULTS. BASIC Soul PRoPERTIES ut 5.

Soi-Water Characteristic Curve.3 Isotopic Consolidation Curves tạ 53. CONSFANTWATrECONTENTICW)TRAXIALTrSTResULTS Hô 4⁄81 Failure Criteria Hạ 532. Shear Strength Behaviours. Characteristics ofthe Excess Pore-water Pressure tị 534 Volume Change Behaviours during Shearing Stage.

160 535 Water Content Characteristics ofthe Specimen at the End ofthe Shearing Stage 163 54 CoNsoubaten DRaINep (CD) TRIAXIAL TEST RESULTS tới S41 Shear Srength Behaviours 168 54.2 Characteristics o the Soil Volume Changes.3 Water Volume Change Behaviours during Shearing Stage 3 SS INTERPRETATION OF THE CW AND CD TRIAXIAL TEST RESULTS USING EXTENDED MoutR-COULOMB FAILURE ENVELOPE vs SS. Convont Water Content (CW) Trisial Tests 180 553. Comnlidmed Drained Triasial (CD) Tests 192 554 Comparisons of the Shear Strength for the CW and CD Triasal Tens. 198 CHAPTER 6 DISCUSSION OF THE RESULTS.

INHODUCHON, 201 Table of Contents 6.2 SOIL-WATER CHARACTERISTICS CURVE .1 SWCC of the Compacted Silt Specimen at a Maximum Dry Density and an Optimum Water COntent.3 ISOTROPIC CONSOLIDATION TESTS .1 Effect of Matric Suction on the Isotropic Consolidation Curves .2 Effect of the Dry Densities on the Isotropic Consolidation Curves.4 COMBINATION OF THE YIELD CURVES IN THE ( 8 - P) PLANE.5 CRITICAL STATE CONDITION OF THE CW AND CD TRIAXIAL TESTS.1 — Critical State on (q - P) DỈAH€.2 __ Critical State on the ( V - p) ÌÏAH€.6 SIMULATION OF THE SHEARING TEST RESULTS UNDER THE CW AND CD ®9))609)2 0. SH ST TH HH Hàn nhi 226 6.2 Verification of the Proposed EQI@fÏOIS.3 Simulation of Soil Parameters for Silt Used in this Study Using the Proposed EQuAtIONS.4 Simulation of the CW Triaxial Shearing Tests Using the Proposed Model. Simulation of the CD Triaxial Shearing Tests Using the Proposed Model.7 COMPARISON BETWEEN SIMULATION AND EXPERIMENTAL RESULTS OF THE CW ,9ipI9)08:1/.1 Simulation of the CW Triaxial T€SfS. - 555cc se ssseeseeereserrseee 251 6.2 Simulation of the CD Triaxial T€SfS.ĂẶSĂS SG S Set siiiseserrsreeseree 259 CHAPTER 7 CONSLUSIONS AND RECOMENDA TIONS.c<ees oo265 7A CONCLUSIONS.

-- (G119 ng ng HH ng về 269 REFERENCES. 270 APPENDIX A CALIBRATION DATA OF MODIFIED TRIAXIAL APPARATUS FOR OBTAINING SWCC. HS rene rene heg280 Table of Contents APPENDIX B CALIBRATION DATA OF MODIFIED TRIAXIAL APPARATUS FOR ISOTROPIC CONSOLIDATION CURVES. 286 APPENDIX C CALIBRATION DATA OF MODIFIED TRIAXIAL APPARATUS FOR THE CW AND CD TESTS.289 APPENDIX D SIMULATION RESULTS OF THE CW TRIAXIAL TESTS USING THE PROPOSED ELASTO-PLASTIC MODEL WITH THE NCORPORATION 90992.

tere bene rene n ene eean eee eens eee enegs 296 APPENDIX E SIMULATION RESULTS OF THE CD TRIAXIAL TESTS USING THE PROPOSED ELASTO-PLASTIC MODEL WITH THE NCORPORATION OF SWCC. nnn rn enn nh nhàn329 XI List of Tables šT OF TABLE Table 2.1 CCharacteristies ofthe miniatute silicon diaphragm pressure transducer (after Hight, 1982) 31 Table 3.1 Soil parameters involved in the constitutive models and typical values ‘of each parameter.1 Programme for the SWCC under different net confining stresses.2 Stress conditions that were used in SWCC tests under different net ‘confining stresses.3 Programme for the isotropic consolidation tests in the triaxial apparatus under different matric suctions H2 Table 44 Initial stresses conditions that were used in the isotropic ‘consolidation tests under different matric suetions HA Table 4.5 Programme for the constant water content triaxial tests Hà Table 46 Programme for the consolidated drained triaxial tests, tá Table 5.