Effects of matric suction and dry density on the resilient modulus of compacted clay sand mixtures

EFFECTS OF MATRIC SUCTION AND DRY DENSITY ON THE RESILIENT MODULUS OF COMPACTED CLAY-SAND MIXTURES NG TECK GUAN (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 ACKNOWLEDGEMENTS I wish to express my gratitude to my supervisor, Associate Professor Tan Siew Ann, Harry, for his kindness and understanding throughout the course of the research The most heartfelt thanks go out to the technicians in both highway laboratory and geotechnical laboratory Mr Farouk will always be fondly remembered as a hardworking and dedicated assistant with his tireless enthusiasm, even during his fasting month Sincere thanks to Mr Goh for his kind coordination, support and encouragement, without which, many things would not have been possible I am also very grateful to Madam Jamilah who has always been patient and happy to offer assistance despite her busy schedule Two very important people to thank are Mr Foo and Martin, whose technical expertise has solved numerous problems involving usage and modifications of laboratory apparatus I have been most fortunate to have many friends who have been a constant source of motivation and support Special thanks to Raymond Ong for his companionship during the difficult periods of my laboratory work His advice and encouragement is deeply appreciated To Kee Kiat and Rwe Yun who have always been ready to help in times of need, I like to express my sincere gratitude In addition, I am very happy to have known my colleagues in soft ground office, See Chia, Ma Rui, Kheng Ghee and many others, who have made my two years in NUS a memorable experience I also wish to send my warmest regards to my fellow volunteers in Tzu Chi Foundation (Singapore) It has been a truly wonderful experience for the past one year being with them Their cheerfulness and friendship has definitely added a new lease of life to my otherwise monotonous daily schedule Last but not least, I would like to thank my dearest parents for their love and support Nothing can express my eternal gratitude to them To the best of whatever I can and will do, I wish them happiness i TABLE OF CONTENTS Page Acknowledgement i Table of contents ii Summary vi Nomenclature viii List if figures ix List of tables xiii CHAPTER ONE 1.1) INTRODUCTION Background 1.1.1) The global rail revival 1.1.2) Track components 1.1.3) Resilient modulus 1.2) Objectives 1.3) Organization of thesis CHAPTER TWO LITERATURE REVIEW 2.1) Concept of resilient modulus 2.2) Resilient modulus test procedures 2.2.1) Confining pressures and load pulses 2.2.2) Standard procedures of resilient modulus test 2.2.3) Low repeatability and reproducibility of resilient modulus test 10 Factors affecting resilient modulus of cohesive soils 13 2.3.1) Stress conditions 13 2.3) ii 2.4) 2.5) 2.6) 2.3.2) Moisture content 14 2.3.3) Method of compaction 17 2.3.4) Plasticity index 17 2.3.5) Strain amplitudes 18 Correlations derived from alternative testing methods 19 2.4.1) California bearing ratio and R value 20 2.4.2) Falling weight impacting on a standard Proctor specimen 21 2.4.3) Static triaxial compression test 21 Other correlations 22 2.5.1) Hyperbolic models 22 2.5.2) Correction factors 23 2.5.3) Moisture content at constant dry density or compaction effort 24 2.5.4) Compressive strength at 1% strain 24 Scope of research 25 CHAPTER THREE EXPERIMENTAL PROGRAM AND PROCEDURES 41 3.0) Introduction 41 3.1) Material properties 41 3.1.1) Mix configurations 41 3.1.2) Compaction moisture conditions 42 3.2) Compaction tests 42 3.3) Difficulty faced during preparation of triaxial specimens 43 3.3.1) Non uniformity of density profile 43 3.3.2) Sample disturbance caused by extrusion 44 iii 3.4) 3.5) 3.6) 3.7) Single layer compaction using split mould 45 3.4.1) Two-step compaction 45 3.4.2) Lining wall of split mould with paper 46 3.4.3) Density gradient and water content profile 47 Measurement of matric suction using filter paper 47 3.5.1) Basic concept of filter paper method 48 3.5.2) Procedures of filter paper method 48 3.5.3) Precautions 49 Repeated load triaxial test 49 3.6.1) Standard test procedure for determining the resilient modulus of soils 50 3.6.2) Vertical force generation 50 3.6.3) Confining stress generation 51 3.6.4) Axial load measurement 51 3.6.5) Axial deformation measurement 52 3.6.6) Precautions 53 Unconfined compression test 53 CHAPTER FOUR RESULTS AND DISCUSSION 69 4.0) Introduction 69 4.1) Compaction curves 69 4.2) Resilient modulus test results 70 4.2.1) Resilient modulus curves for kaolinite sand mixtures 70 4.2.2) Resilient modulus cures for bentonite sand mixtures 70 4.2.3) Coefficient of variance for readings 71 4.2.4) Comparison of MR models 72 iv 4.3) 4.4) 4.5) 4.6) 4.7) 4.2.5) Representative σc and σd for analysis 72 Variation of MR with molding water and clay content 73 4.3.1) Variation of MR for kaolinite sand mixtures 73 4.3.2) Variation of MR for bentonite sand mixtures 73 Effect of suction on MR and qu 75 4.4.1) Variation of suction with clay content 75 4.4.2) Variation of unconfined compression strength with clay content 76 Summary of variation of MR and qu 76 4.5.1) Variation of MR and qu 77 4.5.2) Surface plot for MR as a function of dry density and suction 78 4.5.3) Specimens prepared at moisture content 79 Optimum mix proportions 79 4.6.1) Optimum mix for kaolinite-sand 79 4.6.2) Optimum mix for bentonite-sand 80 Strength-stiffness relationship for soil specimens 80 4.7.1) Rawang-Ipoh: Double track railway project 80 4.7.2) Importance of fabric effects on strength-stiffness relationship 81 4.7.3) Importance of suction and plasticity index on strength-stiffness relationship 82 CHAPTER FIVE REFERENCES CONCLUSION AND RECOMMENDATIONS 103 106 v SUMMARY Resilient modulus, MR, is an important parameter which characterizes the subgrade ability to withstand repetitive stresses under traffic loadings In the current research, two sandy clay materials i.e kaolinite-sand mixtures and bentonite-sand mixtures are compacted and tested in the laboratory to evaluate how their resilient modulus and unconfined compressive strength varies with different factors The factors include different clay-sand ratio, soil plasticity index and different compaction moisture conditions i.e dry, optimum and wet conditions The filter paper method is used to measure the matric suction within the compacted specimens A two step single layer compaction was employed for fabricating the test specimens It was found that the method gave a more uniform density profile within the specimen than the multi-layer compaction recommended in AASHTO-307 or a single step top down compaction method Besides dry density, matric suction developed within the specimen during compaction contributed significantly to the strength and stiffness of the soil For example, bentonite-sand specimens though less dense than kaolinite-sand specimens, are much stronger and stiffer Generally, resilient modulus and unconfined compression strength of both the mixes increase with a decrease in clay content This is despite of the drop in suction within the bentonite-sand mixtures when bentonite content decreases Through controlled experiments carried out in the laboratory on known soil material, a more distinct trend in the strength-stiffness relationship for the soil tested can be clearly observed Experiment data shows that soil compacted under each compaction moisture condition exhibit its own unique strength-stiffness relationship while no observable correlation can be found for strength and stiffness of the same soil vi compacted under different compaction moisture conditions This illustrates the importance of fabric effect when a reasonable correlation is to be found between stiffness and strength for field specimens Lastly, suction has to be taken into account when evaluating the stiffness of the soil in relation to its strength Keywords: Clay-sand ratio, compaction, dry density, plasticity index, resilient modulus, unconfined compression strength, matric suction vii NOMENCLATURE γd dry density σc confining stress σd deviator stress ε axial strain εr axial recoverable strain E modulus of elasticity Eri breakpoint resilient modulus MR resilient modulus PI plasticity index qu unconfined compression strength S matric suction w water content viii LIST OF FIGURES Figure 1.1 Simplified track granular layer and subgrade (after Li and Seliq, 1996) Figure 1.2 Figure 2: Subgrade progressive shear failure (after Li and Seliq, 1996) Figure 1.3 Excessive subgrade plastic deformation (ballast pocket) (after Li and Seliq, 1996) Figure 2.1 Strain behaviour of a specimen subjected to repeated loading (after Huang, 1993) Figure 2.2 Concept of resilient modulus of soils (after Ping et al, 2003) Figure 2.3 Repeated load wave form (after Monismith, 1989) Figure 2.4 Relationship between resilient modulus and repeated deviator stress for a silty clay (after Seed et al., 1962) Figure 2.5 Bilinear model proposed by Thompson and Robnett (1976) Figure 2.6 Relation between MR and w with constant dry density (after Li and Selig, 1994) Figure 2.7 Relation between MR and w with constant compaction effort (after Li and Selig, 1994) Figure 2.8 Resilient modulus of reconstituted silty clay as a function of stress condition (after Brown et al., 1975) Figure 2.9 Non linear stress strain behaviour of soils (after Pezo, 1991) Figure 2.10 Variation of the modulus vs log strain amplitude Figure 2.11 Variation in normalized Young’s modulus with PI for compacted subgrade soils at optimum moisture (after Kim and Stokoe, 1992) Figure 2.12 Variation of elastic threshold strain with PI of compacted subgrade soils art optimum moisture content (after Kim and Stokoe, 1992) Figure 2.13 Relationships between CBR and resilient modulus for clays (after Brown et al., 1987): (a) Keuper marl; (b) Three soils compared with empirical predictions at deviator stress of 40 kPa Figure 2.14 Equipment setup for the alternative test method device for measuring resilient modulus (after Drumm et al 1996) ix ... Effect of suction on MR and qu 75 4.4.1) Variation of suction with clay content 75 4.4.2) Variation of unconfined compression strength with clay content 76 Summary of variation of MR and qu 76... for kaolinite -sand mixtures Figure 4.9 Surface plot of variation of MR with suction and dry density for bentonite -sand mixtures Figure 4.10 Comparison of strength and resilient modulus measured... on synthetic specimens compacted in the laboratory Different mixtures of clay and sand are used to fabricate the specimens Lastly, the strength-stiffness relationship of the compacted sandy clay