Performance Evaluation of Encased Stone Column Supported Embankments with Geosynthetic Materials as Basal Reinforcement PDF

PERFORMANCE EVALUATION OF ENCASED STONE COLUMN SUPPORTED EMBANKMENTS WITH GEOSYNTHETIC MATERIALS AS BASAL REINFORCEMENT Thesis Submitted in partial fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY...

PERFORMANCE EVALUATION OF ENCASED STONE COLUMN SUPPORTED EMBANKMENTS WITH GEOSYNTHETIC MATERIALS AS BASAL REINFORCEMENT Thesis Submitted in partial fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY by VIBHOOSHA. M. P (CV16F19) DEPARTMENT OF CIVIL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA SURATHKAL, MANGALURU – 575 025 DECEMBER, 2021  PERFORMANCE EVALUATION OF ENCASED STONE COLUMN SUPPORTED EMBANKMENTS WITH GEOSYNTHETIC MATERIALS AS BASAL REINFORCEMENT Thesis Submitted in partial fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY by VIBHOOSHA. M. P (CV16F19) Under the Guidance of Dr. Sitaram Nayak Dr. Anjana Bhasi DEPARTMENT OF CIVIL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA SURATHKAL, MANGALURU – 575 025 DECEMBER, 2021  DECLARATION I hereby declare that the Research Thesis entitled “Performance Evaluation of Encased Stone Column Supported Embankments with Geosynthetic Materials as Basal Reinforcement” which is being submitted to the National Institute of Technology Karnataka, Surathkal in partial fulfilment of the requirements for the award of the Degree of Doctor of Philosophy in Civil Engineering, is a bonafide report of the research work carried out by me. The material contained in this Research Thesis has not been submitted to any University or Institution for the award of any degree. Vibhoosha. M. P. Register No. 165017CV16F19 Department of Civil Engineering Place: NITK Surathkal Date: 07-12-2021  CERTIFICATE This is to certify that the Research Thesis entitled “Performance Evaluation of Encased Stone Column Supported Embankments with Geosynthetic Materials as Basal Reinforcement” submitted by Mrs. Vibhoosha. M. P. (Register Number: 165017CV16F19) as the record of the research work carried out by her, is accepted as the Research Thesis submission in partial fulfilment of the requirements for the award of degree of Doctor of Philosophy. Dr. Sitaram Nayak Dr. Anjana Bhasi (Research Guide) (Research Guide) Professor Assistant Professor Department of Civil Engineering Department of Civil Engineering NITK Surathkal NIT Calicut (13.12.2021) Dr. B. R Jayalekshmi (Chairman-DRPC) Head of the Department Department of Civil Engineering NITK Surathkal  ACKNOWLEDGEMENT To begin with, let me thank all those who have helped me in completing this research work. Let me express my deep and sincere gratitude to my research supervisors- Dr. Sitaram Nayak, Professor, Department of Civil Engineering, NITK Surathkal, and Dr. Anjana Bhasi, Assistant Professor, Department of Civil Engineering, NIT Calicut for being constant sources of support, encouragement, and motivation. I am deeply indebted to them for their timely guidance and priceless suggestions throughout the research period. I am also grateful to their family for the love and care they have shown. I would like to thank my Research Progress Assessment Committee members- Dr. Subba Rao, Professor, Department of Applied Mechanics, and Dr. B. B. Das, Associate Professor, Department of Civil Engineering for their constructive, timely suggestions and constant support, which helped in improving the quality of my research and the thesis. My heartfelt gratitude to Dr. Katta Venkataramana- former Academic Dean, NITK Surathkal, for his moral support. I am thankful to Dr. M. B. Saidutta and Dr. A. Nithyananda Shetty- former and present Dean Academic, NITK Surathkal, for permitting me to carry out my research work at NIT Calicut. I am thankful to Dr. D.Venkat Reddy, Dr. Varghese George, Dr. K. Swaminathan - former Heads of the Civil Engineering Department and Dr. B. R Jayalekshmi, present Head of the Civil Engineering Department, all faculty members in Civil Engineering Department for the valuable discussions, all the help and support provided in the different stages of my research work. I gratefully acknowledge the help and co-operation of the non-teaching staff in the successful execution of my research and daily activities in the department. I owe my gratitude to NIT Calicut for providing the software and high- performance workstation, which played a crucial role in the timely completion of various analyses of my research work. I also express my sincere gratitude to Dr. Jose Mathew, Professor, Department of Mechanical Engineering, NIT Calicut, for permitting me to use ABAQUS software installed in the CAD-CAM center of the Advanced Manufacturing Center. I wish to thank the faculty members of the Department of Mechanical Engineering, NIT Calicut, Dr. Vineesh. K.P, Dr. Deepak Lawrence, and the Non-teaching staff- Mr. Hariharan, Mr. Sasi Kumar, Mr. Mohanlal, Mr. Abhinesh, and Mr. Sandeep for their kind help and co-operation. I gratefully acknowledge the help and co-operation of the geotechnical laboratory staff, National Institute of Technology Karnataka (NITK) - Mr. Chandrashekhar Karanth, Mr. Yateesh, and Mr. Vishwanath, in the successful execution of my experimental work. My special gratitude to Mr. Shravan Konnur and Dr. Spoorthi. S. K. for their continuous support and co-operation during my research work. I would like to thank my friends - Mr. Sree Sastha, Mr. Anoop. P, Mr. Bennet. I, Ms. Ashima J, Mrs. Geethu Thomas, Mrs. Anjana. R. Menon from NITC and Mr. Preetham. H. K., Dr. Anila Cyril, Dr. S. Anaswara, Mrs. Pooja Raj, Mrs. Nithya. R. Govind, Ms. Sreya. M. V., and Mrs. Radhika Patel from NITK for their kind help and support. I also thank all the research scholars of the Civil Engineering Department in NITK and NITC for providing a healthy research environment. I am grateful to all my teachers for showering an abundance of love and blessings. I would like to express my deep gratitude to my beloved parents Mr. V. P. Vijayan Nambiar and Mrs. Chithradevi, without whom this would have remained a dream. I am very much grateful to them for all the support and encouragement they have given me during the hard times of my research life. Special thanks to my brother Vijith. M. P., in-laws Mr. Narayanan Nair and Mrs. Sumathi, sister-in-law, Namitha, their family for their support. Special thanks to all my relatives for their love and care during this period. I express my gratitude to all who directly or indirectly contributed to my research and made this happen. Above all, I am deeply obliged to my husband, Mr. Nidhin Narayanan, for his unconditional love, constant support, and encouragement during the tough stages of my research carrier. I dedicate this thesis to him. VIBHOOSHA. M. P  ABSTRACT Lithomargic clay is extensively found along the Konkan belt in peninsular India and serves as a foundation for most structures. The reduction in strength under saturated conditions makes lithomargic clay problematic, causing many engineering problems such as uneven settlements, erosion, slope failures, and foundation problems. The effect of column configuration (i.e., equivalent number of columns with reduced diameter for the same surface area) on the performance of lithomargic clay reinforced with geogrid encased stone columns, and the basal geogrid layer was studied. The investigations were performed both experimentally through small-scale models and finite element analyses. The results were compared with the performance of lithomargic clay reinforced with ordinary and encased stone columns. A single geogrid encased stone column with a basal geogrid layer improved the load- carrying capacity of lithomargic clay by 180%. In contrast, the percentage increment in a group of three geogrid encased stone columns with a basal geogrid layer having the same surface area was 210%. It was also observed that the geogrid encasement of stone columns reduced the maximum column bulging by 38%. In comparison, geogrid encased stone columns along with basal geogrid layer reduced the bulging by 82% compared to ordinary stone columns. Geocells are a superior form of reinforcement due to their cost-effectiveness and three- dimensional confining properties. Numerical modeling of geocell is always challenging due to its three-dimensional honeycomb structure. The limitations of the equivalent composite approach (ECA) led to the recent development of full 3D numerical models, which consider geocell-infill material interaction. The present work discusses the time- dependent performance of geocell reinforced encased stone column supported embankment considering the actual 3D nature of geocells using the finite element program ABAQUS. Parametric studies were carried out to study the stress transfer mechanism, vertical deformation of the foundation soil, arching behavior, and stress- strain variation inside the geocell pockets. It is found from the analyses that with the provision of a geocell layer on top of Geosynthetic Encased Stone Columns (GESC), the stress concentration ratio improved by 47% at the end of consolidation compared to GESC alone. Also, with geocell-sand mattresses, an 80% reduction in foundation surface settlement is observed. Analysis results showed that the arching behavior is not predominant in geocell reinforced columnar structures. The proposed model's numerical results show an overestimation of stress concentration ratio and bearing capacity by ECA. The geocell-sand mattress reduced the vertical settlement of foundation soil due to the embankment construction by 80%. The vertical settlement reduction was 78% and 79% for single and two-basal geogrids, respectively. The basal geogrids and geocell-sand mattress decreased the bulging of the stone columns, and the maximum bulging was visible at a depth of 3.5 D in both cases, where D is the diameter of stone columns. 69% reduction in the lateral bulging occurred in GESC than the ordinary stone column when a single basal layer was placed. The reduction is 52% and 54% for two basal layers and geocell, respectively. When the geocell-sand mattress was placed, almost 80% of the stone column bulging occurred by the end of the embankment construction. Among the various infill materials analyzed, the aggregates were the best suited considering stress concentration ratio and vertical settlement. The mobilized tensile stress in geocell due to embankment loading was maximum for aggregates and minimum for quarry dust. Multiple layers of geosynthetic can be replaced by a single layer of geocell, considered to be a superior form of reinforcement because of the three-dimensional confinement offered to the infill material. The proposed system from this research work encased stone columns with geocells as basal reinforcement at the embankment base, serves like a Geosynthetic Reinforced Piled Embankment System (GRPES). Time-dependent analyses on encased stone columns supported basal geogrid reinforced embankment were carried out using the developed full 3D model. Compared to an unreinforced embankment, 87% reduction in lateral deformation near the toe was observed for GESC+ One basal layer at the end of consolidation, and 90% reduction in the lateral deformation was obtained when two layers of geogrids (stiffness equivalent to that of a single layer) was provided at the base. In multiple basal geogrids, the tensile force at the top layers was less compared to the bottom layers. The variation of stress reduction ratio with embankment height from different analytical methods and 3D numerical model for the encased stone column with single basal geogrid follows the same trend. Among the different design methods, Guido et al. (1987), Low et al. (1994), and Abusharar et al. (2009) significantly under-predicted the stress reduction ratio. Terzaghi’s method (1943) and BS 8006 (2010) exhibited a closer range of Stress Reduction Ratio values than that from full 3D analysis for low height embankments. These methods over-predicted the tensile force in the basal reinforcement. 3D column analysis gave lesser tensile forces compared to full 3D analyses. Guido et al. (1987) method shows good agreement with full 3D results for basal geogrid tension compared to other methods. Keywords: Stone column; Lithomargic clay; Geogrid; Geocell; Interaction; Time- dependent response; Stress concentration ratio  CONTENTS LIST OF FIGURES vi LIST OF TABLES xiii ABBREVATIONS xv SYMBOLS xvi 1 INTRODUCTION 01 1.1 GENERAL 01 1.2 GROUND MODIFICATION TECHNIQUES 01 1.3 STONE COLUMN TECHNIQUE 03 1.3.1 Load bearing Mechanism of Stone Columns 04 1.4 GEOSYNTHETIC ENCASED STONE COLUMNS (GESC) 05 1.4.1 Load Bearing Mechanism of Geosynthetic Encased Stone Columns 05 1.5 GEOCELLS 06 1.5.1 History of Geocells 07 1.5.2 Reinforcement Mechanism of Geocells 08 1.6 SCOPE AND OBJECTIVES OF THE RESEARCH WORK 10 1.7 ORGANIZATION OF THE THESIS 12 2 LITERATURE REVIEW 13 2.1 General 13 2.2 Studies on Lithomargic Clay 13 2.2.1 Installation of Geocell Mattress in the Field 13 2.2.2 Applications of Geocell 14 2.2.3 Effect of Different Parameters on the Geocell Reinforcement Performance 17 2.2.4 Design Aspects of Geocell 25 2.2.5 Experimental Studies on Geocell Reinforced Soil 30 2.2.6 Numerical Studies on Geocell Reinforced Soil 34 2.3 STUDIES ON LITHOMARGIC CLAY 39 2.4 STUDIES ON ENCASED STONE COLUMNS 40 2.4.1 Unit Cell Concept 40 i  2.4.2 Experimental Studies on Encased Stone Columns 40 2.4.3 Numerical Studies on Encased Stone Columns 43 2.5 SUMMARY 44 3 FINITE ELEMENT TECHNIQUES 47 3.1 GENERAL 47 3.2 FINITE ELEMENT METHOD (FEM) 47 3.2.1 Advantages of FEM 49 3.2.2 Disadvantages of FEM 50 3.3 FINITE ELEMENT SCHEME OF THE PRESENT STUDY 50 3.4 INTERPOLATION POLYNOMIALS (SHAPE FUNCTIONS) 51 3.5 COMPUTATION OF STRAINS 52 3.6 COMPUTATION OF STRESSES 53 3.7 CONVERGENCE CRITERIA FOR ELEMENTS 53 3.8 NUMERICAL INTEGRATION TECHNIQUES 54 3.9 CONSTITUTIVE MODELLING 55 3.9.1 Mohr-Coulomb Model 56 3.9.2 Modified Cam Clay Model (MCC) 57 3.10 FINITE ELEMENT PROGRAM USED IN THIS STUDY 60 3.11 CASE STUDY CONSIDERED 60 3.11.1 Models developed for the analyses 61 3.11.2 Constitutive models 65 3.11.3 Boundary conditions 66 3.11.4 Elements used for meshing 66 3.11.5 Contact 67 3.11.6 Methodology 67 3.12 VALIDATION OF THE DEVELOPED MODELS 68 ii 4 MATERIALS AND METHODS 71 4.1 GENERAL 71 4.2 MATERIAL PROPERTIES 71 4.2.1 Lithomargic clay 71 4.2.2 Stone aggregates 74 4.2.3 Sand 74 4.2.4 Geogrid 74 4.3 LOAD TESTS ON STONE COLUMNS 75 4.3.1 Unit cell Concept 75 4.3.2 Experimental Programme 76 4.3.3 Test Bed Preparation 76 4.3.4 Stone Column and Geogrid Installation 76 4.3.5 Experimental Set-Up and Test Procedure 77 4.4 NUMERICAL ANALYSES 80 4.4.1 Models Developed 80 4.4.2 Boundary conditions 83 4.4.3 Elements Used 83 4.4.4 Contact 84 4.4.5 Methodology 84 4.5 LOAD SETTLEMENT RESPONSE OF STONE COLUMNS 85 4.5.1 Load Tests on Single Stone Columns 85 4.5.2 Improvement Factor (IF) from Single Column Tests 87 4.5.3 Load Bearing Mechanism of Single Stone Column 88 4.5.4 Load Tests on Group of Stone Columns 89 4.5.5 Effect of Column Configuration on Load Carrying Capacity 91 4.5.6 Load Bearing Mechanism of a Group of Three Columns 93 4.5.7 Stress Concentration Ratio 94 4.5.8 Bulging Characteristics of Stone Columns 97 iii 5 NUMERICAL ANALYSES USING UNIT CELL MODELS 101 5.1 GENERAL 101 5.2 FINITE ELEMENT ANALYSES 102 5.2.1 Description of the case study considered for analyses 102 5.2.2 Constitutive models 102 5.2.3 Numerical models 104 5.2.4 Boundary conditions 106 5.2.5 Interaction 107 5.2.6 Methodology 108 5.3 LOAD TRANSFER FROM THE SOIL TO THE STONE COLUMN 108 5.3.1 Settlement-Time Response 108 5.3.2 Stress Concentration Ratio (SCR) 109 5.3.3 Soil Arching 113 5.3.4 Bulging Characteristics of Stone Columns 116 5.4 PARAMETRIC STUDIES 117 5.4.1 Effect of Stiffness of Basal Geogrid 118 5.4.2 Effect of Modular Ratio 119 5.4.3 Effect of Drainage Layer Thickness 121 5.5 EFFECT OF MULTIPLE REINFORCEMENT LAYERS ON LOAD TRANSFER 123 5.5.1 Comparison with the single layer system 123 5.6 USE OF GEOCELL SAND MATTRESS AS LOAD TRANSFER PLATFORM 124 5.6.1 Description of the Finite Element Analyses 127 5.6.2 Stress Transfer Mechanism 127 5.6.3 Settlement-Time Response 131 5.6.4 Bulging Characteristics of Stone Columns 134 5.6.5 Stress-Strain Behavior of Geocell 136 5.6.6 Effect of Stiffness of Geocell 138 5.6.7 Effect of Geocell Infill Material Properties 139 5.6.8 Comparison with Equivalent Composite Approach (ECA) 141 iv  5.7 GEOSYNTHETIC REINFORCED PILED EMBANKMENT SYSTEMS (GRPES) 145 5.7.1 Load Bearing Mechanism 146 5.7.2 Numerical Modelling 146 5.7.3 Comparison Based on Embankment Load Transfer 147 5.7.4 Summary 152 6 NUMERICAL ANALYSES USING FULL THREE DIMENSIONAL MODELS 153 6.1 GENERAL 153 6.2 NUMERICAL MODELING 153 6.3 VARIATION OF FOUNDATION SURFACE SETTLEMENT 156 6.4 EXCESS PORE PRESSURE DISTRIBUTION 159 6.5 LATERAL DEFORMATION OF THE FOUNDATION SOIL 161 6.6 TENSILE STRESSES IN THE REINFORCEMENT LAYERS 164 6.7 SOIL ARCHING 166 6.8 COMPARISON OF NUMERICAL RESULTS WITH DESIGN CODE RECOMMENDATIONS 169 6.8.1 Comparison of Stress Reduction Ratio 177 6.8.2 Variation of Geosynthetic Tension with Embankment Height 178 6.8.3 Comparison Based on Stress Concentration Ratio (SCR) 179 7 SUMMARY AND CONCLUSIONS 181 7.1 SUMMARY 181 7.2 CONCLUSIONS 182 7.3 SCOPE FOR FURTHER RESEARCH 186 REFERENCES 187 PUBLICATIONS 204 v  LIST OF FIGURES Fig. Description Page No. 1.1 Schematic diagram of geosynthetic encased stone column(Based on Zhang and Zhao 2014) 06 1.2 Confinement of soil using geocell 09 2.1 Schematic view of strip footing supported by geocell reinforced foundation bed 18 2.2 Pressure-settlement response for different width of geocell mattress (After Dash et al. 2001a) 19 2.3 Pressure-settlement response for different height of geocell mattress (After Dash et al. 2001a) 19 2.4 Various geocell configurations (a) handmade geocell diamond pattern and (b) handmade geocell chevron pattern 21 2.5 Mohr circles for both reinforced and unreinforced soil and apparent cohesion estimation (After Bathurst and Karpurapu 1993) 26 2.6 Schematic diagram of lateral resistance effect and vertical stress dispersion effect (After Zhang et al. 2010) 29 2.7 Membrane effect in the reinforcement (After Zhang et al. 2010) 30 2.8 Numerical model of single cell reinforced sand (Han et al. 2008) 35 2.9 3D model of unreinforced and reinforced foundation bed (a) unreinforced (b) geogrid reinforced (c) geocell reinforced and (d) geocell and geogrid reinforced (Hegde and Sitharam 2005(b)) 37 2.10 3D modeling of geocells as hexagonal shaped pockets (Biabani et al. 2016a) 38 2.11 Arrangement of stone columns in field (a) unit cell concept and (b) Tributary area around the stone column and the equivalent circle 41 3.1 Discretisation of the domain 48 3.2 Coupled pore pressure element 51 3.3 Mohr-Coulomb model (a) stress-strain graph and (b) yield surface in principal stress space (Ti et al. 2009) 56 vi 3.4 Yield surface of a modified Cam clay model in the q–p’ plane 58 3.5 Cross-section of GESC supported embankment (Based on Yoo and Kim 2009) 61 3.6 Arrangement of stone columns in the field (a) square layout and (b) triangular layout 62 3.7 Representation area for analyses (a) axisymmetric and (b) 3D column 63 3.8 Finite element models developed (a) axisymmetric model (b) 3D column model (c) geosynthetic encasement (3D) (d) full 3D model and (e) geogrid encased stone columns in full 3D 64 3.9 Membrane elements for reinforcement modeling (a) axisymmetric and (b) 3D column 67 3.10 Lateral deflection of GESC from the axisymmetric analysis 68 3.11 Lateral deflection of GESC from the 3D column analysis 68 3.12 Lateral deflection of GESC from the full 3D analysis 69 3.13 Hoop strain of GESC from the axisymmetric analysis 69 3.14 Hoop strain of GESC from the 3D column analysis 70 3.15 Hoop strain of GESC from the full 3D analysis 70 4.1 Sample procurement site 72 4.2 Gradation curve for lithomargic clay, sand, and stone aggregate 73 4.3 Compaction curve for lithomargic clay 73 4.4 Variation of UCS with water content 74 4.5 Installation of the stone column in the unit cell (a) single column during construction (b) encased stone column after construction with area ratio =15% (GESC) (c) placement of basal geogrid layer at the top of encased stone column (GESC+ BASAL GEOGRID) and (d) group of three encased stone columns with area ratio =15% 78 4.6 Encasing the casing pipe with geogrid material 78 4.7 Schematic diagram of the experiment set up 79 4.8 Load tests on stone columns in a unit cell 80 4.9 Finite element models developed (a) OSC (b) GESC (c) GESC with basal geogrid layer and (d) GESC group with a basal geogrid layer 82 vii 4.10 Geogrid encasement around stone columns modeled using membrane elements (M3D8R) 84 4.11 Responses of single stone columns in unit cell tank 86 4.12 Variation of Improvement Factor for single column tests 88 4.13 Column configurations for same area ratio (a) single column and (b) three columns 89 4.14 Pressure – settlement responses for a group of three columns 90 4.15 Pressure- settlement responses of single and group of three OSCs 91 4.16 Pressure- settlement responses of single and group of three GESCs 91 4.17 Pressure-settlement responses of single and group of three GESCs with a layer of geogrid at top 92 4.18 Variation of improvement factor with column configuration 93 4.19 Membrane effect-Tensile stress contours in the basal geogrid layer for a three-column group 94 4.20 Vertical stress contours in stone columns (a) GESC group and (b) GESCs + BASAL GEOGRID group 95 4.21 Variation of stress concentration ratio for single and group of three columns 95 4.22 Variation of stress concentration ratio along with the depth of the column 97 4.23 Bulging in stone columns after the load test (a) OSC (b) GESC (c) GESC+ BASAL GEOGRID and (d) GESC group 98 4.24 Lateral bulging for a single stone column 99 4.25 Lateral bulging for a group of stone columns 100 5.1 Geosynthetic reinforced encased stone column supported embankment (a) plan and (b) cross-section 103 5.2 Developed models for geogrid reinforced encased stone column supported embankment (a) axisymmetric and (b) 3D column model 107 5.3 Time-settlement graph (a) axisymmetric analysis and (b) 3D column analyses 110 viii 5.4 Variation of SCR with time (a) axisymmetric analysis and (b) 3D column analyses 112 5.5 Variation of SCR with the height of the embankment (3D column analyses) 113 5.6 Influence of embankment height on differential settlement (3D column analyses) 114 5.7 Variation of arching ratio with embankment height for GESC and GESC+ One basal layer 115 5.8 Orientation of principal stresses in the case of GESC+ One basal layer 116 5.9 Lateral bulging profile of stone column for OSC, GESC and GESC+ One basal layer 117 5.10 Effect of basal geogrid stiffness on foundation settlement 118 5.11 Effect of basal geogrid stiffness on the stress concentration ratio 119 5.12 Effect of basal geogrid stiffness on the arching ratio 119 5.13 Effect of modular ratio on foundation settlement 120 5.14 Effect of modular ratio on the stress concentration ratio 120 5.15 Effect of modular ratio on the arching ratio 121 5.16 Effect of drainage layer thickness on foundation soil settlement 122 5.17 Effect of drainage layer thickness on the stress concentration ratio 122 5.18 Effect of drainage layer thickness on lateral bulging of stone column 123 5.19 Different arrangement of reinforcement layers used in analyses (a) single reinforcement layer and (b) two reinforcement layers 124 5.20 Variation of foundation settlement with time for different basal layers 125 5.21 Variation of SCR with embankment height for different basal layers 125 5.22 Variation of differential settlement with embankment height for different basal layers 126 5.23 Variation of arching ratio with embankment height for different basal layers 126 5.24 3D column model developed (a) geocell pockets (b) single geocell pocket with infill material and (c) geogrid encased stone column 128 ix 5.25 Variation of SCR (a) with time and (b) with the height of the embankment 129 5.26 Confining effect of geocell reinforced sand 131 5.27 Load transfer mechanism in geocell-sand mattress 131 5.28 Time-settlement graph (3D column analyses) 132 5.29 Variation of excess pore water pressure with time 133 5.30 Variation of degree of consolidation with time 134 5.31 Lateral bulging profile of stone column for GESC and GESC+GEOCELL 135 5.32 Vertical stress distribution for (a) GESC and (b) GESC+GEOCELL 135 5.33 Tensile stress distribution in the geocell pockets 136 5.34 Circumferential deformation of middle geocell pocket after loading 137 5.35 Lateral strain distribution at the mid-height of geocell pocket at different embankment height 137 5.36 Effect of geocell stiffness on SCR 138 5.37 Effect of geocell stiffness on ground surface settlement 139 5.38 Effect of infill material on SCR 140 5.39 Effect of infill material on ground surface settlement 140 5.40 Lateral strain distribution at the mid-height of geocell pocket with different infill material 141 5.41 Variation of SCR with time from ECA approach and 3D numerical analyses 142 5.42 Variation of ground surface settlement with time from ECA approach and 3D numerical analyses 143 5.43 Compressive stresses distribution in the geocell soil composite layer from ECA approach 144 5.44 Compressive stresses distribution in the geocell from 3D column model 144 5.45 Load transfer mechanism in Geosynthetic Reinforced Piled Embankment Systems (GRPES) (After Lawson 2012) 145 5.46 Settlement-time response 148 5.47 Variation of degree of consolidation 148 x 5.48 Column efficacy for different embankment heights 149 5.49 Variation of SCR with embankment height 149 5.50 Variation of SCR with embankment height for different modular ratio for GESC+GEOCELL 150 5.51 Variation of arching ratio with embankment height for GRPES and GESC+GEOCELL 151 5.52 Horizontal stress contours for (a) GESC+GEOCELL and (b) GRPES 151 6.1 Full 3-dimensional models developed (a) GESC (b) GESC + One basal layer and (b) GESC+ Two basal layers 155 6.2 Foundation settlement contours at the end of consolidation for (a) unreinforced lithomargic clay and (b) GESC 157 6.3 Foundation settlement contours at the end of consolidation for (a) GESC+ One basal layer and (b) GESC+ Two basal layers 158 6.4 Variation of excess pore pressure with time at the mid-depth of lithomargic clay 159 6.5 Pore water pressure distribution at the end of construction for (a) GESC (b) GESC+ One basal layer and (c) GESC+ Two basal layers 160 6.6 Lateral displacement of foundation soil under the embankment toe at the end of consolidation 162 6.7 Lateral displacement contours for (a) unreinforced (b) GESC (c) GESC+ One basal layer and (d) GESC+ Two basal layers 163 6.8 Deflected shape of bottom geogrid (a) single layer and (b) two-layer 164 6.9 Reinforcement tension in the bottom layer along the width of the embankment (a) one layer and (b) two-layer 165 6.10 Horizontal stress contours for GESC supported embankment 166 6.11 Horizontal stress contours for (a) GESC+ One basal layer and (b) GESC + two basal layers 167 6.12 Vertical stress distribution in the embankment fill for (a) GESC+ One basal layer and (b) GESC+ Two basal layers 168 6.13 Development of arching 169 6.14 Partial soil arching 175 xi 6.15 Full soil arching 176 6.16 Variation of stress reduction ratio with embankment height 178 6.17 Double-layer coverage in BS8006 (2010) (After Lawson 2012) 179 6.18 Variation of geosynthetic tension with embankment height 179 6.19 Stress concentration ratio from different design methods 180 xii  LIST OF TABLES Table Description Page No. 2.1 Summary of optimum parameters of geocells for maximum performance 24 3.1 Unit system used in the present study 60 3.2 Material properties used in the analyses (Yoo and Kim 2009) 65 4.1 Properties of lithomargic clay 72 4.2 Properties of geogrid used for the encasement and basal reinforcement 75 4.3 Material properties used in the numerical analyses 82 4.4 Summary of load carrying capacity at different cases 86 4.5 Summary of the load carrying capacity of a group of stone columns 90 4.6 Summary of the load carrying capacity of a group of stone columns 92 4.7 Maximum Percentage of lateral bulging for different cases 99 5.1 Constitutive model parameters for lithomargic clay, stone column, and sand/fill 104 5.2 Properties of geogrid and geocell 105 5.3 Total number of elements used in the numerical model 106 5.4 Variation of foundation surface settlement with time 111 5.5 Variation of SCR for different cases 111 5.6 Variation of SCR with time (3D column analyses) 130 5.7 Variation of foundation surface settlement with time 133 5.8 Properties of various infill materials (Sand and aggregate properties based on Hegde and Sithram 2015; quarry dust properties based on Han et al. 2008) 141 xiii 5.9 Comparison between ECA and proposed 3D model of geocell 142 6.1 Total number of elements used in the full 3-dimensional model 154 6.2 Foundation surface settlement at the end of consolidation 156 6.3 Maximum lateral displacement under the embankment toe 162 xiv  ABBREVATIONS AR Arching Ratio ECA Equivalent Composite Approach FEM Finite Element Method GESC Geogrid Encased Stone Column GESC+ BASAL GEOGRID Encased Stone Columns with a Horizontal Layer of Geogrid on the Top GESC+GEOCELL Geogrid Encased Stone Column with Geocell as Basal Reinforcement GESC+ One basal layer Encased Stone Columns with Single Layer of Horizontal Geogrid on the Top GESC+ Two basal layers Encased Stone Columns with Two Layers of Horizontal Geogrid on the Top GRPES Geosynthetic Reinforced Piled Embankment Systems IF Improvement Factor MCC Modified Cam Clay Model OSC Ordinary Stone Column SCR Stress Concentration Ratio SRR Stress Reduction Ratio UCS Unconfined Compression Strength xv  SYMBOLS σ rs Radial stress acting on the stone column σ rc Radial stress of the surrounding clay R Radius of the stone column u Depth to the top of geocell layer below the footing hc Height of geocell mattress b Maximum geocell width d Pocket size of geocell B Footing width Df Embedment depth of footing b1 Width of single cell εa Axial strain of soil at failure εc Circumferential strain of soil at failure Do Initial diameter of geocell pocket (m) D1 Diameter of the sample at an axial strain of εa,(m) σ1 Normal stress (kPa) σ3 Cell pressure (kPa) Δσ3 Additional confining pressure developed (kPa) Cr Apparent cohesion (kPa) Kr Dimensionless modulus parameters of geocell-sand mattress Ku Dimensionless modulus parameters of unreinforced sand Eg Young’s modulus of geocell-reinforced sand Pa Atmospheric pressure (kPa) n Modulus exponent of unreinforced soil ps Bearing capacity of unreinforced foundation soil Prs Bearing capacity of reinforced foundation soil bed xvi ΔP1 The contribution of vertical dispersion effect on bearing capacity ΔP2 The contribution of membrane effect on bearing capacity bn Width of the uniform load ps θ Dispersion angle of geocell reinforcement T Tensile strength geosynthetic material α Horizontal angle of tensile force T [k] Element stiffness matrix {u} Nodal displacement vector {f} Nodal force vector  t +t nodal pore pressure vector vn seepage forces on the boundary. t +t [ ] The matrix governing the dissipation of pore fluid f t +t Applied load terms Nu shape functions for displacements Nπ shape functions for pore pressures {ae} Vector of nodal displacements for a particular element {ε} Strain vector [L] Interaction term between the soil, and pore fluid [B] Matrix for axisymmetric conditions [C] Constitutive matrix relating the stresses and strains E Young's modulus (kPa) ν Poisson's ratio τf Shear stress at failure xvii σf Effective normal stress c Cohesion Φ Internal friction angle of soil p‘ Mean effective stress q Shear stress p′c Preconsolidation pressure M Critical state stress ratio λ Logarithmic hardening constant for plasticity Cc Compression index κ Logarithmic bulk modulus for elastic material behavior Cs Swelling index p dευ Plastic volumetric strain increment p dεs Plastic shear strain increment dεes Elastic shear strain increment dεs Total shear strain increment dευ Total volumetric strain increment dεeυ Elastic volumetric strain increment p dευ Plastic volumetric strain increment S Center to center spacing of stone columns Re Effective radius of unit cell k Permeability(m/day) e0 Initial void ratio ao Initial yield surface size xviii φ Dilation angle D Diameter of the stone column h Depth of the stone column τcrit Critical shear stress μ Friction coefficient δ Interface friction angle Δs Foundation surface settlement H Embankment height γe Unit weight of embankment fill qs Surcharge applied on the surface of the embankment σs Stress in the soil Kp Passive earth pressure coefficient Ec Elastic modulus of stone column Es Elastic modulus of surrounding soil w Distance along the geocell width P load carried by the pile Ef Pile efficacy Kp Passive earth pressure coefficient h Horizontal stress K0 Earth pressure coefficient at rest a Width of pile cap ac Arching Coefficient S3D Stress reduction ratio xix s Center to center spacing of piles σ𝑝 Vertical stress on the column σ𝑠 Vertical stress on the geosynthetic layer P0 Uniform pressure applied on the geosynthetic layer Pc Arched vertical stress per unit length at the top of the conduit/pile Maximum vertical displacement of the foundation soil midway t between the pile caps J Tensile stiffness of the geosynthetic h1 Depth of the foundation soil  Average vertical stress per unit length at the top of the conduit/pile WTn The distributed load on the reinforcement xx xxi  CHAPTER 1 INTRODUCTION 1.1 GENERAL The ever-accelerating urbanization and industrialization always demanded the augmentation of construction processes and activities irrespective of the nature of soil present. Those areas where the subsoil conditions are notably poor pose serious challenges before the geotechnical engineers. The evolution of different techniques for improving the properties and behavior of soil arises from these challenges. The selection of methods is based on various parameters like soil type, design requirement of structures, etc. Ground modification techniques should increase subsoil strength and stress- strain modulus and a reduction in compressibility and susceptibility to liquefaction. The selection of a method depends on soil formation, soil characteristics, availability of backfill material, cost, and experience in the past. 1.2 GROUND MODIFICATION TECHNIQUES The various ground modification techniques include Replacement of problematic soils Adding various admixtures Use of geosynthetic products Dynamic compaction or vibration Accelerated consolidation Columnar systems a. Replacement of problematic soils If the depth of problematic soil is less, around 2 to 3 m, the soil is excavated and replaced by good quality soil suitable for construction. 1 b. Adding various admixtures If the depth of problematic soil is more than 3 m, treating the soil with suitable admixtures is economical. Fly ash, lime, cement, etc., are commonly used chemicals for modification. The admixtures are mixed with the soil, and the engineering properties are altered suitably. c. Use of geosynthetic products Soil reinforcement is popular worldwide because of its simplicity and economic aspects (Vidal 1969; Binquet and Lee 1975). Load-bearing elements with good tensile strength and stiffness are embedded in the soil as reinforcements. Though the soil is weak in tension, substantial tensile stress acting on the soil can be taken up by these reinforcing materials. Straws, reeds, bamboo, etc., were used as soil reinforcements in the beginning. The effective use of geosynthetic products as reinforcement has been identified since the 1970s. Geosynthetic products are usually manufactured from polymeric materials like HDPE. Different forms of reinforcement, like planar, bars, strips, etc., were effective as soil reinforcements (Jones 1996). d. Dynamic compaction or vibration This method applies to any granular soil. Here, problematic in-situ soil is densified and rearranged to a greater depth by repeated application of high-intensity impacts. It is a rapid process and results in increased shear strength and reduced permeability of the soil. e. Accelerated consolidation To construct any structure in clayey soils, it is essential to accelerate the consolidation process by reducing drainage path length. Methods like preloading, stage construction, vertical sand drains, prefabricated vertical drains, vacuum-assisted consolidation, etc., are used to achieve it, although these are highly expensive and consume time for consolidation. 2 f. Columnar systems Columnar systems such as concrete piles, timber piles, soil-cement columns, stone columns have been extensively used to support structures on problematic ground conditions. Plain concrete piles with horizontal layers of geosynthetic at the embankment base are very efficient in controlling total and differential settlements. It is a quick construction process as there is no waiting time for consolidation. For flexible and lightly loaded structures, lime columns or stone columns are ideal ground reinforcement. The stone columns are best suited for soft clay soils, peat and cohesive deposits, and silty soils. Among the different columnar systems, stone columns have been widely used to reinforce soft soils and increase the foundation soil's bearing capacity. Stone columns have been successfully applied for structures like liquid storage tanks, earthen embankments, raft foundations, etc., where a relatively large settlement can be tolerated by the structure. The stone column is preferred among other methods as it gives the advantage of reduced settlement, decreased liquefaction potential of the ground, and also accelerated consolidation settlements due to reduction in the drainage paths. Another significant merit of the stone column technique is the simplicity of its construction method. Stone columns continue to gain popularity today due to the considerable savings in cost and time that they can offer over conventional piling solutions in many circumstances. 1.3 STONE COLUMN TECHNIQUE The stone column (also called granular pile) is nothing but a vertical column element formed below the ground level with compacted and uncemented stone fragments or gravels. The technique has been used since the 1950s for improving both cohesive soils and silty sands (Barksdale and Bachus 1983). These columns considerably improve the vertical load carrying capacity and shear resistance in the soil mass. Stone column construction involves the partial replacement of unsuitable subsurface soils (usually 15 to 35 percent) with a compacted vertical column of stone that usually completely penetrates the weaker strata. The presence of the column creates a composite material of lower overall compressibility and higher shear strength than the native soil. 3 Confinement is provided by the lateral stress of the surrounding soft soil, which increases the stiffness of the stone column. Upon application of vertical stress at the ground surface, the stone and the soil move downward together, resulting in the concentration of stress within the stone column, primarily due to the column being stiffer than the soil. Stone column systems in soft, compressible soils are somewhat like pile foundations, except that pile caps, structural connections, and deep penetration into underlying firm strata are not required. Stone columns are more compressible, and when loaded, the stone columns deform by bulging into the subsoil strata and distribute the stresses at the upper portion of the soil profile rather than transferring the stresses into a deeper layer, unlike in the case of pile foundation, thereby causing the soil to support it. If installed in loose sands, Stone columns minimize the likelihood of liquefaction of these deposits due to earthquakes because they tend to dilate while shearing and dissipate the excess pore pressures generated (Mitchell and Huber 1985). The granular column materials with higher permeability accelerate the consolidation settlement and thereby minimize the post-construction settlement. Moreover, in situ stress conditions get improved due to the installation of the stone columns. Significance of stone columns in India: The method of installing stone columns (RAMMING) does not require any skilled labour. Stone column installation is economically very feasible-no high cost is required to execute the installation in the field. 1.3.1 Load bearing Mechanism of Stone Columns The load-bearing mechanism of the stone column can be explained by employing lateral bulging of columns into the surrounding soil. The stone column distributes vertical load to the soil by shear stresses at the column-soil interface and the end bearing at the bottom of the column. Due to the vertical loading, the aggregates start bulging, and significant vertical compression occurs in the column. The compression and lateral movement of aggregates together increase the stress in the surrounding soil. Thus, the passive resistance offered by the soil provides confinement for the stone column. The 4 lateral confinement is more due to the increased overburden pressure at deeper depths, and thus the bulge formation is less. The maximum bulging was visible at a depth of four times the diameter of the stone column (Greenwood 1970; Hughes and Withers 1975). 1.4 GEOSYNTHETIC ENCASED STONE COLUMNS (GESC) All traditional design of the stone column considers the undrained shear strength value Cu>15kN/m2 (Greenwood and Kirsch 1983). Hence, the soil with an undrained shear strength value of less than 15 kN/m2 demands a new technique. The problem can be solved by confining the compacted sand or gravel column in a high-modulus geosynthetic encasement. Van Impe (1985) proposed the concept of encasing the stone column by wrapping it with geotextile. Fig 1.1 shows the schematic diagram of the geosynthetic encased stone column. Among various methods of enhancing the load capacity of the stone columns, encasing the column with geosynthetic would be an ideal form since it also offers other benefits as follows (Alexiew et al. 2005), Additional lateral confinement Making the stone column act as a semi-rigid element enabling the load transfer to deeper depths. Preventing the lateral squeezing of stones into surrounding soft clays, thereby minimizing the loss of stones. Enabling a higher degree of compaction compared to the conventional stone columns. Promoting the vertical drainage function of the column by acting as a good filter. Preserving the frictional properties of the aggregates. Increasing the shear resistance of the stone column. 1.4.1 Load Bearing Mechanism of Geosynthetic Encased Stone Columns Stone columns installed in very soft clay were observed with excessive bulging due to the very low lateral confinement pressure offered by the surrounding soil. In such situations, the stone column itself derives additional confinement from the geosynthetic encasement provided. The lateral movement of aggregates causes hoop tension in the 5 geosynthetic encasement, which along with the passive resistance, offers all-around confinement to the stone column. Lateral confinement increases the stiffness of the stone column and reduces the lateral bulging by redistributing the stresses into deeper depths. The radial stress acting on the stone column, σ rs, is induced by the radial stress of the surrounding clay, σ rc, and the hoop tension, T, in the geosynthetic encasement as shown in Fig 1.1 below, i.e, σ rs = σ rc + T/R (1.1) Where R is the radius of the stone column. The second term can be viewed as the additional radial stress due to the geosynthetic encasement. Vertical load GL Lateral bulging GeosyntheticEncasement Stone column Surrounding soil Hard stratum Fig 1.1 Schematic diagram of geosynthetic encased stone column (Based on Zhang and Zhao 2014) 1.5 GEOCELLS Another popular technique in India is three-dimensional geocells as an effective soil reinforcement technique for improving soft subgrade behavior. With regards to the effectiveness, the more attractive are cellular systems owing to their 3-dimensional (3D) structure compared to planar geosynthetic reinforcements (Mhaiskar and Mandal 1996; Tafreshi et al. 2013). Geocell is a honeycomb structured polymeric cellular 6 system connected by joints (Bush et al. 1990). Combining two parts- “geo” means soil or earth, and “cell” means a cellular type of shape for infill material such as soil, the word geocell is formed. The three-dimensional honeycomb structure of geocell offers more lateral confinement to the infill soil resulting in improved load carrying capacity. This led to the widespread use of geocells for different geotechnical applications like pavements, foundations, embankments, slope protection, erosion control, etc. The geocells enclose weaker materials like soil, stones, etc., and their 3D structure provides all-around confinement. The combination of geocell and the fill material, which acts as a reinforced composite, is characterized by improved stiffness and strength to unreinforced soil. The composite system also ensures better distribution of the vertical load to a wider area by preventing lateral material spread. Fig. 1.2 shows the confinement effect of geocells on soils. Geocells exist in various dimensions, and facia colours suiting different project needs, materials used are eco-friendly and offer high strength to weight ratio and durability. Studies have shown that geocells offer an enhancement in structure reliability and life, high degree of protection for the impermeable layers, cost- effectiveness compared to other products, serves as a working platform and saves construction time, enhances the soil bearing capacity and facilitates gradual settlements and reduces lateral deformations, functions as embankment base with improved stiffness and rigidity enhancing the stability (Latha 2000; Pokharel et al. 2010; Dash and Bora 2013; Sitharam and Hegde 2013; Tafreshi et al. 2013; Hegde and Sitharam 2015a). The long term performance of the resin which is used to make the geocell and the additives added, should be appropriately tested. Also, quality control must be assured to handle, store, and install geocells in the field (IGS 2018). 1.5.1 History of Geocells For the smooth movement of military vehicles over weak subgrade, studies were carried out by U. S. Army Engineer Waterways Experiment Station on various soil reinforcement methods in the late 1970s (Webster and Watkins 1977; Webster 1979). Webster and Watkins (1977) placed different types of materials such as crushed stone, wire gabions with rock, sand confinement system, pervious polyester fabric, and coated 7 nylon membrane as base reinforcement over clay subgrade in unpaved roads and compared the rut depth after traffic loading with that oTf the unreinforced base. The studies concluded that the sand base course reinforced by isolated plastic tubes performed better than the conventional base course with crushed stones. Square-shaped grids filled with sand, called “grid cell confinement systems,” were developed after this study. Laboratory experiments (Rea and Mitchell 1978; Webster 1979) were conducted to investigate different parameters such as material, size, and shape of the grid, subgrade stiffness, sand-grid layer thickness, properties of sand, compactive effort, loading, etc. that can affect the performance of reinforced soil. Analytical formulas were developed based on the experimental results to predict the capacity of the reinforced base course (Mitchell et al. 1979) by considering different failure modes. Initially, paper and aluminum were used to make grid cells, and later Webster (1979) suggested plastic as grid material due to the many drawbacks of these materials. Polymeric materials, generally known as “geocell,” were introduced in the cellular confinement system in the 1980s. Later, materials like HDPE (high-density polyethylene), which has low-temperature flexibility, came into being (Pokharel et al. 2010; Yang et al. 2010). 1.5.2 Reinforcement Mechanism of Geocells The three-dimensional honeycomb structure of geocells confines the soil present in the pockets. The applied load will induce pressure inside each cell of the geocell. Induced pressure causes lateral movement of the confined soil, which will exert pressure on the geocell walls. Thus, deformation of the geocell membrane takes place. Due to the circumferential deformation, the stress in the geocell membrane gets mobilized, and therefore confinement pressure of soil increases (Bathurst and Karpurapu 1993). The three-dimensional confinement restricts the lateral movement of the infill soil that results in a more stable and stiffer composite structure. This triaxial state of confinement results in increased shear strength and resistance to deformation. This imparts the lateral resistance mechanism. The interlocking and frictional resistance between the surrounding soil and geocell wall also leads to higher load carrying capacity. 8  Many researchers reported that, due to the shear and bending rigidity, the behaviour of geocell soil system is similar to a foundation beam or flexible slab foundation which can carry both bending and membrane stresses (Dash et al. 2001a, 2007; Sitharam et al. 2007; Mehdipour et al. 2013). Dash et al. (2007) observed that the geocell mattress behaves as a flexural member and by increasing the thickness of the mattress deep beam behavior becomes predominant. Zhang et al. (2010) proposed three different mechanisms for geocell reinforced systems, such as lateral restrain (confinement), stress dispersion, and membrane action. This approach approximated the geocell-reinforced-soil as a ‘layer with higher flexural rigidity’. They found that the modulus and the height of the geocell contribute to the rigidity of the geocell- reinforced-soil. The interconnected pockets provide all-round confinement to in-filled soil and behaves as a semi-rigid composite slab. It redistributes the applied load to a wider area with lesser intensity to improve the load-bearing capacity of underlying soil (vertical stress dispersion effect/wide slab mechanism). The semi-rigid-slab configuration improves the performance by resisting differential settlement of concerned structure and generates membrane resistance. The deflection of geocell due to the vertical loading, generates additional tensile force transferring more load to the columnar inclusions beneath the geocell (membrane effect). Overall, the mechanism of geocell-reinforcements can be discretized as ‘confinement’, ‘membrane action’, and ‘stress distribution’. All the above three mechanisms together contribute towards the bearing capacity increment due to the placement of geocell reinforcement. Fig. 1.2 Confinement of soil using geocell 9 1.6 SCOPE AND OBJECTIVES OF THE RESEARCH WORK As more and more land becomes subject to urban and industrial development, good construction sites are difficult to find. Different techniques to improve the marginal foundation soil become a necessity. The geotechnical engineers are challenged by the presence of different problematic soils with varied engineering characteristics. In the present work, Lithomargic clay, which is widely available at the Konkan belt of peninsular India, Assam, and West Bengal, is considered for the experimental work. The road and railway embankment construction over lithomargic clay pose many engineering challenges due to reduced strength under saturated conditions. The use of columnar systems like concrete piles, stone columns, and soil-cement columns beneath the embankment can improve the bearing capacity and settlement characteristics of lithomargic clays. Among these techniques, the stone column method is preferred because of its cost-effectiveness, ease of construction, reduced consolidation time and decreased liquefaction potential of the ground. Stone columns are suited for various soils, ranging from loose sands to soft compressible clays. Applications of stone columns include support to embankments, liquid storage tanks, raft foundations, and other low-rise structures. The stone columns are popular in India because of two main reasons. Firstly, installing stone columns does not require any skilled labour-any layman to do the job. Secondly, installation is fast and economical. The suitability of the method is decided by the undrained shear strength of the surrounding soil. For very soft clays, stone columns, not being restrained by the surrounding soil, cause excessive bulging of the columns, and the soft clay particles are squeezed into the voids of the aggregates (Murugesan and Rajagopal 2009). Excessive bulging of stone columns led to the encasement of columns. Encasement of the stone column imparts additional confinement to the columns and increases the column's increased stiffness. Encasement prevents the loss of stones into the surrounding soft clay and preserves the drainage and frictional properties of the stone aggregates. To avoid damage to the geosynthetic encasement, only moderate compaction of the stone columns is carried out during installation. This, coupled with the geosynthetic strain during loading, can cause relatively large settlements. There is also the possibility of 10 shear deformations near the embankment toe, which can be critical for large height embankments. Hence, this research work proposes the concept of using encased stone column supported embankments with a horizontal layer of geosynthetic material (planar and 3- dimensional geocell) as reinforcement. This could reduce the vertical settlements and also prevent the lateral spreading of soil. Thus, the whole system may serve like a Geosynthetic Reinforced Piled Embankment System (GRPES). GRPES is a popular ground improvement technique in European countries to construct structures with strict settlement criteria. Conceptually GRPES is equivalent to a piled raft system. The present investigation is aimed to explore the time-dependent behavior of geosynthetic reinforced embankments supported on encased stone columns and compare the results with the performance of GRPES reported in the literature. Based on the literature review and identified research gap, the following objectives are proposed for the research work 1. To carry out typical laboratory model tests in the unit cell to study the following aspects, i. The behavior of a single encased stone column (with and without a horizontal layer of geogrid) installed in lithomargic clay subjected to vertical loading. ii. The behavior of a group of encased stone columns with the equivalent area as that of a single column (with and without a horizontal layer of geogrid) installed in lithomargic clay subjected to vertical loading. 2. To develop simplified numerical models (2-d axisymmetric, 3-d Column) and examine their accuracy regarding experimental data and case studies reported in the literature. 3. To investigate the time-dependent behaviour of the system modeled as a unit cell by carrying out axisymmetric and 3D column analyses. 4. To investigate the time-dependent behaviour of the overall system by carrying out full three-dimensional finite element analyses. 11  5. Interpretation of the results from experimental and numerical studies and comparison of the proposed system with the performance of Geosynthetic Reinforced Piled Embankment Systems (GRPES). 1.7 ORGANIZATION OF THE THESIS This thesis contains seven chapters. Following the introduction to the research topic in this chapter, a comprehensive literature review of the geocells, lithomargic clay and encased stone column supported embankments is made in the second chapter to understand the state-of-the-art in this area. The third chapter describes the finite element analysis procedures used for the solution of the problem. This chapter also discusses the various numerical models used in the study and the validation of the models with respect to the case studies reported in the literature. The details of various materials used in experimental investigation, the test setup, and the test procedure are included in the fourth chapter. The validation of the developed models with respect to experimental data is also given in this chapter. The fifth chapter describes the numerical investigations performed to study the time-dependent behavior of geosynthetic reinforced encased stone column supported embankments. Parametric studies using the axisymmetric and 3D Column models are discussed. The proposed system was compared with the performance of Geosynthetic Reinforced Piled Embankment Systems (GRPES) from the obtained results. In chapter six, full three-dimensional analyses of geosynthetic reinforced encased stone column supported embankment are described. Comparison of numerical results with the available design methods is also carried out. The seventh chapter summarises the entire research work performed in this thesis and lists the conclusions drawn from this research work. A brief note on the scope for further research on this topic is presented. 12  CHAPTER 2 LITERATURE REVIEW 2.1 GENERAL A comprehensive review of relevant literature on ground improvement techniques using encased stone columns and geocell has been carried out in this chapter. The research publications are classified based on experimental, numerical, and analytical studies. Geocells are a superior form of reinforcement due to their cost-effectiveness and three-dimensional confining properties. Numerical modeling of geocell is always challenging due to its three-dimensional honeycomb structure. Most of the available works of literature are based on laboratory model studies and numerical modeling using the equivalent composite approach (ECA). The critical review on the topic led to identifying the research gap and scope for current research work. 2.2 STUDIES ON GEOCELL Many researchers tried to explain the reinforcing mechanism of geocells based on their experimental and numerical studies. Predominantly, geocell reinforcements were used to support loads besides improving the performance of soft soil. 2.2.1 Installation of Geocell Mattress in the Field Bush et al. (1990) described the procedure for construction and installation of geocells in the field. Geocells are manufactured by the thermal welding of geosynthetics. It can be constructed from planar geotextiles and geogrid in the field. Different types of readymade geocells are also available. Based on the design requirements, they can be suitably selected and stretched in the ground as a geocell mattress. Before geocell mattress construction, the ground has to be cleared and leveled. After that, basal geotextile material is laid on the ground by keeping minimum overlapping distance between adjacent rolls. Over the basal layer, another geogrid sheet is placed in a transverse direction with one end stitched to the bottom layer. The 13 transverse member is rotated about the stitched end to make it vertical and temporarily tensioned with the help of timber posts. The procedure is repeated to cover the entire area. Another layer of geogrid was positioned between two transverse members, and it connected with a transverse sheet with hooked steel bars known as bodkin joints (Carroll Jr. and Curtis 1990; Simac 1990). The bodkin joints form the cellular structure, and a suitable material is filled inside the pockets. 2.2.2 Applications of Geocell Yadav et al. (2014) reviewed different applications of geocell in the field of geotechnical engineering. They reported the mechanism, field installation, and the various applications of geocell reinforcement. Geocells have been used for different structures such as embankment, foundation, reinforced wall, slope stability, and erosion control. They also mentioned the necessity of further studies to evaluate the application of geocells in other fields. Dhane et al. (2015) discussed the importance of geocells in the civil engineering field from the studies conducted by various researchers. They reported different applications and basic mechanisms of geocells and confirmed the cost-effectiveness and versatility of geocells. a. Waste Containment System Geosynthetic products in waste containment systems as liners, cover systems, leachate collection systems, cut-off wall systems, etc., became common practice (Giroud and Cazzuffi 1989; Koerner 1990; Daniel and Bowders 1996; Rowe 1998). Many researchers reported the application of geocells in waste conatinement system (Zornberg and Christopher 1999; Rawat et al. 2010; Zhao and Karim 2018). Hendricker et al. (1998) and Bouazza et al. (2002) investigated the effectiveness of geocell mattresses as a cover system for hazardous waste containment systems in southern California. They reported that using stiffness, geocells could distribute loads to a wider area, and also chemical compatibility of geocells proved its stress resistance to the waste exposure. b. Pavement and Road Construction Many researchers have stated the successful application of geocell mattresses in road construction and pavements (Rajagopal et al. 2014; Pokharel et al. 2015). The ability 14 of geocells to transfer vertical stresses to a wider area makes construction possible even over soft soil subgrade. Moreover, they raise the modulus of the layer, thereby lowering the surface deflection. The suitability of geocell in Asphalt pavement was investigated by Thakur and Han (2012) and reported their enhanced performance compared to unreinforced base layers. Emersleben and Meyer (2008) enumerated that the presence of geocell layer in the gravel base reduces the vertical stresses on the subgrade to around 30 percent of traffic load. c. Foundation In the present scenario, the construction of a foundation over weak, soft soils is highly challenging. The construction of foundations can be done either by conventional methods like piles, rafts, or improving soil properties. Also, geosynthetic materials were used to stabilize the weak soil deposits (Alawaji 2001; Basudhar et al. 2007; Sitharam and Sireesh 2004). Compared to unreinforced soil base, footing on geocell reinforced soil exhibits higher bearing capacity and reduced settlement. 3D confinement action of geocells forms a rigid composite with a higher load-bearing capacity (Latha et al. 2008; Latha et al. 2009; Latha and Somwanshi 2009; Hegde and Sitharam 2013, 2015a, 2017). Many researchers have substantiated it through laboratory model tests over different types of footings (Mandal and Gupta 1994; Dash et al. 2001a; Sitharam and Sireesh 2005, 2006; Sitharam et al. 2007; Sireesh et al. 2009; Pokharel et al. 2010; Dash 2012; Sitharam and Hegde 2013). d. Embankment Embankment construction over weaker subgrades suffers several flaws, either during pre-construction (incapability of the soils to support the construction equipment) or post-construction (excessive settlement of the weaker soil after construction). Considering the problems mentioned above, the usual remedial actions involve removing the topsoil and their replacement with stronger and stiffer material. But the method of removal and replacement is suitable only for thickness 2 to 3 m. If soft soil thickness is more, other ground improvement techniques like chemical treatment or soil reinforcement will be effective and economical. 15  The unique features of geocells, like their ability to act as a stiff, rigid base and incoming load distribution to a wider area, make it suitable for countering the inconveniences faced during the construction of embankments over soft soil. Johnson (1982), Bush et al. (1990), Zheng et al. (2009), Zhang et al. (2010), and Latha (2011) have reported the successful application of geocells in embankment construction. e. Railway Many studies were conducted on geosynthetic reinforced railway ballast (Indraratna et al. 2006, 2013, 2015; Sireesh et al. 2013; Biabani et al. 2016b). Indraratna et al. (2006) assessed the performance of geosynthetic stabilized ballast in the coastal region of Australia and confirmed the suitability and cost-effectiveness of the reinforcement. The geocell confinement of railway ballast displayed a significant reduction of the vertical deformations, which enabled low-quality material to be used as ballast. Leshchinsky and Ling (2012) verified the effectiveness of geocell reinforcement through numerical modeling. From the studies, they found out that geocells effectively confines ballast and thus reduces vertical deformation. f. Slope Protection and Erosion Control Vegetation is the usual method adopted for slope stability and erosion control. But in steep slopes and high rainfall areas, this method fails to bind the soil particles as a single entity. In such cases, geocells can be used as reinforcement (Boyle and Robertson 2007). They can retain the soil particles by retarding the surface runoff and subsequently controlling soil erosion. Many researchers studied the confining effect of geocells in soil erosion control and slope stability. Mehdipour et al. (2013) considered both bending, and membrane stresses in geocell and modeled geocell as a beam element. They found out that the main parameters of geocell reinforcement responsible for the decrease in the lateral displacements and the increased factor of safety of the slopes were bending moment and tensile strength. Geocell reinforcements mainly control the advancing of failure surfaces and reallocate the loads over a wider area and provide slope stability. 16 g. Reinforced Walls Ling et al. (2009), Chen et al. (2013), Soude et al. (2013), Latha and Manju (2016) reported the use of geocells in retaining structures. Ling et al. (2009) found out that geocell reinforced walls can resist earthquake loading to some extent. The confining effect of geocells was responsible for the performance improvement, and it also prevents the structure from collapse. h. Box Culverts Successful application of geocells in the construction of box culverts was reported by Gupta and Somnath (1994) in Bombay. A marine clay layer of 6 m was reported in the site. First tubular gabions resting on a hard mooram layer were constructed. Over the gabion layer, a geocell mattress was placed. With this arrangement, considerable improvement in the load carrying capacity of the clay bed was obtained. 2.2.3 Effect of Different Parameters on the Geocell Reinforcement Performance The influence of the various parameters on the response of Geocell reinforcement for supporting foundations and the construction of embankments was briefly reviewed in the following sections. The properties of geocells and properties of native and infill soil count for the performance of reinforcement. The effect of various geocell parameters is summarised below. i. Properties of Geocell a. Geocell dimensions Cell height and width are the two parameters that are used to express the geocell dimensions. According to Rea and Mitchell (1978), optimum footing diameter is 1.5 to 2.0 times cell width, and optimum cell height to cell width ratio was 2.25, above which considerable improvement was not observed. Based on laboratory experiments, Mitchell et al. (1979) proved that the geocell height to width ratio was between 2 to 3. Dash et al. (2001a) performed laboratory model tests on strip footings supported by geocell sand beds with additional planar reinforcement, as shown in Fig. 2.1. Poorly graded river sand and 35 x 35 mm biaxial geogrid were used to test plane strain conditions. 17  Footing B u Geocell mattress Thickness of the hc d foundation bed Basal reinforcement Sand Bed b Fig. 2.1 Schematic view of strip footing supported by geocell reinforced foundation bed (After Dash et al. 2001a) Effect of parameters such as i) height of the geocell layer (hc) and ii) placement position of planar reinforcements was studied by keeping the pocket size of geocells (d), the width of the geocell layer (b), and depth to the top of the geocell layer from the base of the footing (u) constant. The pressure-settlement response for different widths and heights of geocell mattresses are depicted in Fig. 2.2 and Fig. 2.3. Beyond (hc/ B) ratio 2 and (b/B) ratio 4, bearing capacity change is marginal, where h and b are geocell mattress height and width, B is the footing width. The test results concluded that the presence of basal geogrid under geocell mattress increases the load carrying capacity of the footing. But the effect of planar geogrid becomes marginal at large heights of geocell mattresses. Maximum performance increment was obtained for geocell height which is twice the footing width. 18  Pressure (kPa) 0 250 500 750 1000 1250 1500 0 Unreinforced 10 b/B=1 b/B = 2 Settlement, s/B (%) 20 b/B = 4 b/B = 6 30 b/B = 8 40 50 60 Fig. 2.2 Pressure-settlement response for different width of geocell mattress (After Dash et al. 2001a) Pressure (kPa) 0 300 600 900 1200 1500 1800 0 10 Unreinforced 20 hc/B=0.8 Settlement, s/B (%) hc/B = 1.6 30 hc/B = 2 hc/B=2.75 hc/B = 3.14 40 50 60 Fig. 2.3 Pressure-settlement response for different height of geocell mattress (After Dash et al. 2001a) 19  In continuation to the above studies, they varied the following parameters- formation of the geocell mattress, pocket-size of geocells (d), the height of geocell layer (hc), the width of the geocell mattress (b), depth to the top of the geocell layer below the footing (u), the relative density of soil and type of reinforcement used to form the geocell. It was observed that, though the sand filled in the cell pockets fails, the geocell mattress act as a beam due to its shear and bending rigidity and support footing. Geocell reinforcement enabled the soil to resist failures even at a settlement equal to 50% of the footing width and load as high as eight times the ultimate bearing capacity of the unreinforced sand. From Fig. 2.2 and Fig. 2.3, the maximum performance can be obtained with geocell height equal to twice the footing width, geocell layer width around four times the footing width, top of geocell mattress at a depth of 0.1B from the bottom of the footing, and by filling the geocells with denser soils. Based on the experimental works on geocell reinforced circular footing, the maximum performance of foundation in load carrying was observed for geocell layer width equal to the diameter of footing (Dash et al. 2003). The ratio of geocell height to geocell diameter, known as the aspect ratio, is a primary factor contributing to the performance of the geocell layer. A higher aspect ratio results in improved bearing capacity of geocell-supported embankments, and the improvement is less significant when the aspect ratio is greater than unity (Latha and Rajagopal 2007). Flexural strength of geocells increases with increased cell height to cell width ratio (Tang and Yang 2013). When the height of the geocell increases, the number of bodkin joint layers also increases, which in turn makes the geocell mattress a semi-rigid slab with high rigidity (Dash et al. 2001a; Dash et al. 2007). Thus, the load can be distributed to a wider area, and the overall performance of the structure improves (Hegde and Sitharam 2015a). b. Pattern of Arrangement Pokharel et al. (2010) performed lab tests on single geocell reinforced bases in the pavement. They found out that a circular-shaped one has higher stiffness and bearing capacity compared to an elliptical-shaped geocell. Chen et al. (2013) also reported that 20 the highest apparent cohesion was induced by circular-shaped geocells and lowest by hexagonal shape. Transverse and diagonal geogrids were arranged in different patterns and connected by Bodkin joints to form geocells. Chevron pattern and diamond pattern are more popular among the different patterns and are shown in Fig. 2.4. Chevron pattern was more efficient than the diamond pattern of arrangement (Dash et al. 2001b; Rai 2010). The number of joints per area is more for the Chevron pattern. Thus, the bending and shearing rigidity is more. Higher rigidity geocell pattern helps to distribute large loads uniformly to the soft foundation soils. Fig. 2.4 Various geocell configurations (a) handmade geocell diamond pattern and (b) handmade geocell chevron pattern (After Dash et al. 2003) c. Pocket Size of Geocells Though the actual shape of the geocell is triangular, pocket size is expressed in terms of equivalent diameter. To allow for axial symmetry conditions, the triangular area is transformed into a circle of the same cross-sectional area to get the equivalent diameter. The behavior of reinforced foundation beds is highly dependent on the pocket size of geocells. Rai (2010) reported that a smaller pocket size geocell gives better performance. Confinement per unit volume is more for smaller size pockets, which results in bearing capacity improvement (Hegde and Sitharam 2015b). As per Dash et al. (2003) and Rai (2010), optimum pocket size was identified as 0.8 times the footing diameter. 21 d. Properties of Geocell Material Properties of geogrid from which geocell has been formed have a major influence on the reinforced system's performance. The orientation of geogrid ribs and stiffness were some of the significant parameters. Compared to diamond openings, the square or rectangular openings geogrid give better performance improvement. Also, reinforced foundation bed bearing capacity increases with an increase in geocell elastic modulus (Hegde and Sitharam 2015b). It is explained as a higher elastic modulus of geocell material exerts higher confining pressure on infill soil, leading to bearing capacity increment. Compared to confined geocell, unconfined geocell has lower stiffness and higher ultimate load capacity (Pokharel et al. 2010). In plate load tests, the geocell that is fully embedded in the sand is referred to as confined geocell, and which is exposed to air is termed as unconfined. ii. Soil parameters a. Interface Friction angle Textured geocells were found to perform better than smooth-walled geocells as the textured surface provided a higher degree of frictional interaction between the geocell wall and the infill material. The increase in the friction angle caused only a marginal increment in the load carrying capacity for the reinforced foundation bed (Hegde and Sitharam 2015b). b. Properties of Infill Soil Granular soils are preferred over cohesive soils as geocell fill material since the confinement effect is more significant in these soils, which leads to a reduction in settlement (Latha and Rajagopal 2007). The relative density of infill material was directly affecting the bearing capacity of footing (Rai 2010; Dash et al. 2001b). Maximum efficiency of geocell can be obtained with denser infill soil. c. Embedment Depth Davarifard and Tafreshi (2015) conducted plate load tests on a multi-layered geocell reinforced bed in the field. Most of the experimental works related to geocell 22 reinforcement have been carried out for surface footings, and only a few have considered embedment depth of footing. The influence of the embedment depth on the load carrying capacity of the footing was investigated through a large-scale model test on embedded square footing. It was observed that the bearing capacity of the footing increased proportionally with an increase in embedment depth ratio (Df/B) [Df is the embedment depth; B is the footing width]. d. Properties of Native soil The properties of subsoil influence the performance of the geocell reinforced foundation. The stiffness of the foundation bed is a major factor that determines the percentage of improvement obtained through geocell reinforcement. Higher stiffness subgrade provides more support against settlement to geocell soil composite, which results in reduced membrane resistances and less improvement factor (Biswas et al. 2016). Also, geocell reinforcement is more effective in soft clay beds than sand beds (Hegde and Sitharam 2013). iii. Review of the optimum parameters of geocell for maximum performance Various researchers have reported optimum parameters of geocell mattress which gives the maximum performance and above which improvement is marginal as summarised in Table 2.1. The different parameters of the geocell mattress were expressed in terms of either footing diameter or width depends upon type footing. Properties of geocell as well as infill soil influence the performance of the reinforced foundation system. Various factors like height of the geocell mattress, width of the geocell mattress, pocket-size of geocell, the placement depth below the footing, the pattern of formation, density of infill soil, properties of geosynthetic material from which geocell has formed, etc. have discussed in this section to obtain optimum parameters for effective and economical design, and construction of geocell reinforced system. The type of construction, economy, type of subgrade, stiffness, etc., affects the quantification of improvement. Geocells can be effectively used as reinforcement both in the case of clay as well as the sand bed. 23  Table 2.1 Summary of optimum parameters of geocells for maximum performance Sl Optimum Parameters No. Reference Application hc b u Pattern Square Rea and Circular shaped 1 Mitchell 2.25b1 - - footing opening (1978) Hexagonal Mandal and 2 Strip footing 1.5B - - shaped Gupta (1994) pocket Mhaiskar and Rectangular 3 0.625B 3.4B 0 Mandal (1996) footing - Dash et al. Chevron 4 Strip footings 2B 4B 0.1B (2001b) pattern Dash et al. Circular Chevron 5 2.1D 5D 0.1D (2003) footing pattern Dash et al. Chevron 6 Strip footing 2B 4B 0.1B (2004) pattern Latha et al. Chevron 7 Strip footing 2.75B 6B 0.1B (2006) pattern Sitharam et al. Circular Chevron 8 2.4D 4.9D 0 (2007) footing pattern Sireesh et al. Circular 9 1.8D 4.9D 0.05D Chevron (2009) footing pattern Tafreshi and 10 Strip footing 1.5-2B 4.2B 0.1B Dawson (2010) - Circular Chevron 11 Rai (2010) 0.8D 6.67D 0.1D footing pattern Biswas et al. Circular Chevron 12 1.15D 6.67D 0.1D (2016) footing pattern Davarifard and Square 13 Tafreshi 0.2D 5D 0.2D footing (2015)