MATRIC SUCTION CHARACTERISTICS OF UNSATURATED RESIDUAL SOILS OF SRI LANKA This thesis was submitted to the Department of Civil Engineering of the University of Moratuwa in partial fulfillment of the requirements for the Degree ofM. Eng. in Geotechnical Engineering Thevaki Ratnajothy 074344 University of Moratuwa Department of Civil Engineering University of Moratuwa Sri Lanka August 2001 74344 DECLARATION The work included in the thesis in part or whole, has not been submitted for any other academic qualification at any institution. Signature of the Candidate Certified WJ/f Signature of the Supervisor ABSTRACT Residual soils from selected prominent landslide sites of the upcountry of Sri Lanka are tested under unsaturated conditions to evaluate pertinent unsaturated properties of the soils. The selected landslide sites are Naketiya, Kahagalla, Walhaputenna and Beragala. The tri-axial test apparatus was modified by using high air-entry disks so that pore- air pressures and pore-water pressures of a soil sample could be maintained at separate, required magnitude. Soil samples obtained from Dambulla International Stadium site were used to verify the repeatability of the test results by the modified apparatus, and the reliability of the apparatus. Soil samples obtained from each of the selected landslides sites were prepared at the maximum dry density and optimum moisture content as determined by the proctor compaction test. These were tested in triaxial conditions under five different matric suction (which is the difference between pore-air and pore-water pressure) magnitudes. Time was allowed for each sample to reach an equilibrium state under the particular matric suction applied, before traxial testing commenced. The test results are used to develop the shear strength function for each soil tested, and the soil-water characteristic curve (SWCC) for each soil over the range of matric suction (30 kPa - 200 kPa) considered in the investigation. These results are of importance in interpreting the variation of shear strength with the moisture content for these residual soils, which will have a direct bearing on stability of slopes there. i ACKNOWLEDGEMENT It is my great pleasure to express the gratitude and acknowledgement to my project supervisor Dr. U . G . A . Puswewala for his guidance and supervision extended to make this project a success. Also I would like to express my gratitude to Dr. S . A . S . Kulathilaka for his guidance and encouragement given to initiate this project. * I take this opportunity to thank Dr. T. A . Peiris who was instrumental in modifying the existing laboratory tri-axial apparatus. I express my profound thanks to Mr . K.R. Pitipanarachchi who was actively involved in developing and ensuring the modified tri-axial apparatus. Also I would like to thank Mr . D .G .S . Vithanage and Mr . D. Bandulasena for their support and assistance in carrying out tests at the laboratory. I like to convey my gratitude to Mr.H.A.S.Chandrapala, Microscopy Laboratory for his kind assistance in carrying out D T A tests and Mr . M . G . S . K . Silva, Polymer Laboratory for his kind assistance in carrying out modulus test of the membranes. * M y sincere thanks are due to Mr.P.Ariyaratne, Material Engineer, Road Development Authority for the collection of soil samples for this project. 1 would like to thank Mr . K. Sivakumar, System Analyst, Informatics (Pvt) Ltd for developing for me a simple programme to carry out the graphical analysis easily. 1 also thank Miss. S.D.P.K. Peiris for her assistance in preparing this document. I express my grateful thanks to the University of Moratuwa for providing the financial support for this project. Further I would like to extend my gratitude to the higher management of the Road * Development Authority for granting me leave in order to complete this project. Finally I extend my profound thanks to all those who had a hand either directly or indirectly for the successful completion of this project. Thevaki Ratnajothy August 2001 9 i i CONTENTS ABSTRACT i ACKNOWLEDGEMENT ii CONTENTS iii - iv List of Figures v - vi List of Tables vii List of Annexes viii 1.0 INTRODUCTION 1.1 Background of Unsaturated Soil Mechanics 1.2 Objectives of the Research 1.3 Organisation of the Research and the Thesis 1 - 4 4 - 5 5 - 6 2.0 REVIEW OF UNSATURATED SOIL MECHANICS 2.1 Unsaturated Soil as a Four Phase System 7 2.2 Stress State Variables 7 - 1 1 2.3 Soil-water Characteristic Curve (SWCC) 1 1 - 1 3 2.4 Shear Strength Function 1 3 - 1 5 2.5 Methods of Obtaining SWCC 15 3.0 EXPERIMENTAL RESEARCH PROGRAMME 3.1 Collection of Soil Samples, Classification Tests and Index Tests 1 6 - 1 7 3.2 Modification of the Triaxial Testing Apparatus 1 7 - 1 9 3.3 Triaxial Testing Programme 19-21 4.0 RESULTS AND DISCUSSION 4.1 Results of Basic Soil Tests (Compaction, Atterberg, Sieve Analysis and DTA) 22 - 30 4.2 Tri-axial Test Results for Dambulla and Discussion 30 4.3 Tri-axial Test Results and Their Interpretation for Naketiya 3 0 - 3 4 4.4 Tri-axial Test Results and Their Interpretation for Kahagalla 34 - 36 f in V 4.5 Tri-axial Test Results and Their Interpretation for Walhuputenna 36 - 39 4.6 Tri-axial Test Results and Their Interpretation for Beragala 39 - 42 4.7 Comparison with some Results Reported in Literature 42 - 45 5.0 CONCLUSIONS 5.1 Summary of Results 46 5.2 Suggestions for Future Research 46 6.0 REFERENCES 47 - 48 ANNEXES 49 - 83 ft iv V List of Figures Figure 2.1 An element of unsaturated soil with a continuous air phase 7 Figure 2.2 The stress state variables for an unsaturated soil 8 Figure 2.3 Typical soil water characteristic curve for a silty soil 12 Figure 2.4 Typical soil water characteristic curves for a sandy, ^ silty and clayey soil 13 Figure 2.5 Mohr-Coulomb failure envelope for saturated soil 14 Figure 2.6 Planer modified Mohr-Coulomb failure envelope as a planar surface for an unsaturated soil 14 Figure 3.1 The layout of the modified tri-axial apparatus 18 Figure 4.1 Particle size distribution curve for Dambulla 23 Figure 4.2 Particle size distribution curve for Naketiya 23 * Figure 4.3 Particle size distribution curve for Kahagalla 23 Figure 4.4 Particle size distribution curve for Walhuputenna 24 Figure 4.5 Particle size distribution curve for Beragala 24 Figure 4.6 DTA plot for Dambulla 25 Figure 4.7 DTA plot for Naketiya 26 Figure 4.8 DTA plot for Kahagalla 27 v Figure 4.9 DTA plot for Walhuputenna 28 Figure 4.10 DTA plot for Beragala 29 Figure 4.11 The typical DTA plot for montmorillonite 30 Figure 4.12 Deviator stress versus axial strain curves for the two Dambulla soil specimens tested under identical conditions 30 Figure 4.13 Deviator stress versus axial strain curves for Naketiya 31 Figure 4.14 Graphical analysis for Naketiya 32 ^ Figure 4.15 The relationship between matric suction and apparent cohesion for Naketiya 33 v Figure 4.16 The soil water characteristic curve for Naketiya 33 Figure 4.17 Deviator stress versus axial strain curves for Kahagalla 34 Figure 4.18 Graphical analysis for Kahagalla 35 Figure 4.19 The relationship between matric suction and apparent cohesion for Kahagalla 35 Figure 4.20 The soil water characteristic curve for Kahagalla 36 Figure 4.21 Deviator stress versus axial strain curves for Walhuputenna 37 Figure 4.22 Graphical analysis for Walhuputenna 38 Figure 4.23 The relationship between matric suction and apparent cohesion for Walhuputenna 38 Figure 4.24 The soil water characteristic curve for Walhuputenna 39 Figure 4.25 Deviator stress versus axial strain curves for Beragala 40 Figure 4.26 Graphical analysis for Beragala 41 Figure 4.27 The relationship between matric suction and apparent cohesion for Beragala 41 Figure 4.28 The soil water characteristic curve for Beragala 42 0 Figure 4.29 Total, matric and osmotic suctions for glacial till (Krahn and Fredlund, 1972) 43 Figure 4.30 Water content versus matric suction for specimen of compacted Regina clay 43 Figure 4.31 Water content versus matric suction for specimens of compacted glacial till 44 v i V List of Tables Table 2.1 Possible combinations of stress state variables for an 9 unsaturated soil Table 3.1 Set of stresses (pressures) used in the testing of each landslide soil 20 Table 4.1 Results of basic soil tests 22 Table 4.2 Summary of tri-axial test results for Naketiya 23 Table 4.3 Summary of tri-axial test results for Kahagalla 36 Table 4.4 Summary of tri-axial test results for Walhuputenna 39 Table 4.5 Summary of tri-axial test results for Beragala 42 Table 4.6 Some results of b values obtained in this research 45 Table Al-1 Correction due to the restraining effects of the rubber membranes 49 Table A2-1 Tri-axial test results for Naketiya 50 - 55 Table A2-2 Tri-axial test results for Kahagalla 5 6 - 6 1 Table A2-3 Tri-axial test results for Walhuputenna 62 - 67 Table A2-3 Tri-axial test results for Beragala 68-74 vn tld' V List of Annexes Annex 1 Calculation of correction due to the restraining effects of the membranes 49 Annex 2 Table A2-1, Table A2-2, Table A2-3 & Table A2-4 50 - 74 Annex 3 The printout of the computer programme for the graphical analysis of Naketiya 75 - 83 4 viii