FINITE ELEMENT BASED DESIGN PROCEDURES

FINITE ELEMENT BASED DESIGN PROCEDURES FOR MSE/SOIL-NAIL HYBRID RETAINING WALL SYSTEMS by ABDULRAHMAN ALHABSHI, M.S.C.E. A DISSERTATION IN CIVIL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Approved

Priyantha W. Jayawickrama Chairperson of the Committee

Andrew Budek

Sanjaya Senadheera

Charles D. Newhouse

Accepted

John Borrelli Dean of the Graduate School

December, 2006

Copyright 2006, Abdulrahman Alhabshi

ACKNOWLEDGEMENTS

This study was supported by a grant from the Texas Department of Transportation (TxDOT), Project 0-5205 – Design Procedures for MSE/Soil Nail Hybrid Retaining Wall Systems. This support is very appreciated. I would like to express my sincere appreciation to Dr. Priyantha Jayawickrama for giving me the opportunity to pursue this degree under his guidance and supervision. His timely appreciation of my work was a great source of encouragement. I am very honored to know Dr. Jayawickrama and I will always treasure my association with him. I would also like to thank Dr. Andrew Budek for giving me the opportunity to participate in this project and for his valuable advice and useful suggestion during the course of my dissertation. My sincere appreciation goes to Drs. Sanjaya Senadheera and Charles Newhouse for accepting to serve in my committee. I would also like to thank my wife Ranyah who patiently reviewed this thesis and help me put it together. Finally, to my parents who have been a continuous and never ending support. Their limitless love, care and advice are much appreciated.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS................................................................................................ ii TABLE OF CONTENTS................................................................................................... iii LIST OF TABLES.............................................................................................................vii LIST OF FIGURES ........................................................................................................... ix ABSTRACT......................................................................................................................xvi INTRODUCTION .............................................................................................................. 1 1.1 Background ................................................................................................................... 1 1.2 Problem Statement ........................................................................................................ 5 1.3 Objective and Scope of Research ................................................................................. 6 LITERATURE REVIEW ................................................................................................... 8 2.1 Introduction................................................................................................................... 8 2.2 Soil Nail Walls............................................................................................................ 10 2.2.1 Historical Background ......................................................................................... 10 2.2.2 Mechanism and Behavior of Soil-nailed Structures ............................................ 11 2.2.2.1 Nail Tension...................................................................................................... 11 2.2.1.2 Shear Stress and Bending Stiffness in the Nails ............................................... 13 2.2.3 Modes of Failure of Soil-nailed Structures.......................................................... 18 2.2.3.1 Internal Failure Modes...................................................................................... 20 2.2.3.2 External Failure Modes..................................................................................... 21 2.2.4 Deformation of Soil Nail Walls ........................................................................... 22 2.2.4.1 Deformation Analysis of Soil Nail Walls ......................................................... 23 2.2.5 Comparison of Behavior between Soil Nail Walls and MSE Walls.................... 24 2.2.6 Design Methods for Soil Nail Walls.................................................................... 25 2.2.6.1 German Gravity Wall Method .......................................................................... 26 2.2.6.2 French Multicriteria Analysis ........................................................................... 28 2.2.6.2.1 Shear Resistance of the Nail .......................................................................... 29 2.2.6.2.2 Skin Friction of the Nail ................................................................................ 29 iii

2.2.6.2.3 Normal Interaction between the Soil and the Nails ....................................... 29 2.2.6.2.4 Strength of the Nail........................................................................................ 30 2.2.6.3 Kinematical Limit Analysis (FHWA 1991)...................................................... 32 2.2.6.4 FHWA 1996 Design Method............................................................................ 34 2.2.6.5 FHWA 2003 Design Method............................................................................ 36 2.2.7 Computer Design Programs for Soil Nail Walls ................................................. 36 2.2.7.1 GOLDNAIL...................................................................................................... 39 2.2.7.2 SNAIL............................................................................................................... 41 2.3 Mechanically Stabilized Earth Retaining Walls (MSE) ............................................. 42 2.3.1 Historical Background of MSE Walls ................................................................. 42 2.3.2 Types of MSE Walls............................................................................................ 42 2.3.3 Mechanism and Behavior .................................................................................... 43 2.3.3.1 Friction Load Transfer ...................................................................................... 44 2.3.3.2 Passive Resistance ............................................................................................ 46 VALIDATION OF FINITE ELEMENT ANALYSIS USING PLAXIS ......................... 47 3.1 Introduction................................................................................................................. 47 3.2 Case Studies ................................................................................................................ 49 3.3 Description of PLAXIS............................................................................................... 71 3.3.1 Mesh Elements and Density ................................................................................ 71 3.3.2 Soil Constitutive Models ..................................................................................... 76 3.4 Modeling Issues .......................................................................................................... 83 3.5 The Modeling of Finite Element Analysis Using PLAXIS ........................................ 85 3.5.1 CLOUTERRE CEBTP Wall No.1....................................................................... 85 3.5.1.1 Finite Element Modeling .................................................................................. 85 3.5.1.2 Finite Element Analysis Results:...................................................................... 86 3.5.1.2.1 Prediction of Lateral Wall Displacement....................................................... 87 3.5.1.2.2 Prediction of Nail Tensile Forces .................................................................. 88 3.5.1.3 Comparison of Slope Stability Analysis between FEM and LEM ................... 93 3.5.2 Polyclinic Wall in Seattle, WA............................................................................ 95 3.5.2.1 Finite Element Modeling .................................................................................. 95 3.5.2.2 Finite Element Analysis Results ....................................................................... 96 3.5.2.2.1 Prediction of Lateral Wall Displacement....................................................... 97 iv

3.5.2.2.2 Prediction of Nail Forces ............................................................................... 97 3.5.2.3 Comparison of Slope Stability between FEM and LEM .................................. 98 3.5.3 A-2 Wall in New Braunfels, Comal County...................................................... 105 3.5.3.1 Prediction of Wall Behavior ........................................................................... 105 3.5.4 FHWA Wall No.3 .............................................................................................. 111 3.5.4.1 Finite Element Modeling ................................................................................ 111 3.5.4.2 Finite Element Analysis Results ..................................................................... 112 3.5.4.2.1 Prediction of Lateral Wall Displacement..................................................... 112 3.5.4.2.2 Prediction of Nail Forces ............................................................................. 113 3.5.4.3 Comparison of Slope Stability between FEM and LEM ................................ 113 3.5.5 CALTRAN’s Hayward Wall ............................................................................. 119 3.5.5.1 Prediction of Wall Behavior ........................................................................... 119 3.6 Conclusions............................................................................................................... 127 3.6.1 Lessons Learned ................................................................................................ 128 3.6.1.1 Soil Nail Wall ................................................................................................. 128 3.6.1.2 MSE Wall ....................................................................................................... 129 PARAMETRIC STUDY OF HYBRID WALL SYSTEMS .......................................... 130 4.1 Parametric Study....................................................................................................... 130 4.2 The Baseline Wall..................................................................................................... 130 4.3 Description of Model ................................................................................................ 131 4.3.1 Modeling Sequence............................................................................................ 131 4.3.2 Mesh Description............................................................................................... 137 4.3.3 Modeling Elements ............................................................................................ 137 4.3.4 Material Properties............................................................................................. 137 4.4 Case Studies .............................................................................................................. 139 4.5 Analysis Results........................................................................................................ 142 4.5.1 Soil Nail Wall Height, H.................................................................................... 145 4.5.2 Vertical Spacing of Nails, SV ............................................................................. 151 4.5.3 Length of Soil Nails, L/H ratio .......................................................................... 156 4.5.4 Length of Reinforcement of MSE Wall, l/h Ratio ............................................ 165 4.5.5 Wall Setback, d .................................................................................................. 170 4.5.6 Soil Properties.................................................................................................... 175 v

4.5.7 Surface Slope, Nail Inclination and Bar size ..................................................... 188 4.6 Summary and Conclusions ....................................................................................... 210 PROPOSED DESIGN METHOD .................................................................................. 213 5.1 Introduction............................................................................................................... 213 5.2 Development of Design Charts................................................................................. 214 5.3 Development of Final Design Charts........................................................................ 225 5.4 Correlation between Lateral and Vertical Displacements ........................................ 225 5.5 Correction Factors..................................................................................................... 234 5.6 Modeling Hybrid Wall in GOLDNAIL .................................................................... 234 5.7 Example Problem...................................................................................................... 238 5.7.1 Design of Hybrid Wall System.......................................................................... 238 5.7.2 Comparison of Results....................................................................................... 243 5.8 Validation of Results for 5205 Method .................................................................... 244 5.8.1 Cases 1 through 3: Soil Nail Wall Examples..................................................... 244 5.8.2 Cases 4 through 6: Hybrid Wall Examples........................................................ 245 5.8.2 Cases 7 through 9: Effect of Varying MSE Wall Reinforcement Length ......... 246 5.9 Results Comparison between 5205-Method and PLAXIS ....................................... 253 CONCLUSIONS AND RECOMMENDATIONS ......................................................... 255 6.1 Introduction............................................................................................................... 255 6.2 Conclusions............................................................................................................... 256 6.3 Recommendations..................................................................................................... 258 REFERENCES ............................................................................................................... 261 APPENDIX A APPENDIX B APPENDIX C vi

LIST OF TABLES

3.1: Parameters input of CEBTP No.1 Wall .................................................................... 53 3.2: Input parameters for Seattle Wall ............................................................................. 58 3.3: Input parameters for Hayward Wall ......................................................................... 67 3.4: Parameters inputs of FHWA No.3 Wall ................................................................... 69 3.5a: Soil Properties for CLOUTERRE Wall No.1 ......................................................... 76 3.5b: Beam data sets parameters...................................................................................... 76 3.6: Soil data sets parameters using the HS model ......................................................... 83 3.7: Soil Properties used in PLAXIS model for CLOUTERRE Wall No.1 .................... 94 3.8a: Soil parameters used in PLAXIS model for Seattle wall...................................... 104 3.8b: Beam data sets parameters.................................................................................... 104 3.9a: Soil parameters used in PLAXIS model for A2 wall............................................ 110 3.9b: Beam data sets parameters.................................................................................... 110 3.10a: Soil parameters used in PLAXIS model for Seattle wall.................................... 118 3.10b: Beam data sets parameters.................................................................................. 118 3.10c: Geogrids data sets parameters............................................................................. 118 3.11a: Soil data sets parameters for Hayward wall....................................................... 126 3.11b: Beam data sets parameters................................................................................. 126 3.11c: Geotextile data sets parameters........................................................................... 126 4.1b: Soil data sets parameters for MSE wall ............................................................... 140 4.1c: Beam data sets parameters ................................................................................... 141 4.1d: Geotextile data sets parameters............................................................................ 141 4.2: Analysis Cases for Parametric Study...................................................................... 143 4.2: Analysis Cases for Parametric Study...................................................................... 144 4.3a: Results from parametric Study.............................................................................. 201 4.3a: Results from parametric Study (continue) ............................................................ 201 4.3a: Results from parametric Study (continue) ............................................................ 202 4.3a: Results from parametric Study (continue) ............................................................ 202 4.3a: Results from parametric Study (continue) ............................................................ 203 4.3a: Results from parametric study (with backslope but no MSE) .............................. 203 4.3b: Results from parametric Study ............................................................................. 203 4.3b: Results from parametric Study ............................................................................. 204 4.3b: Results from parametric study (continue)............................................................. 204 vii

4.3b: Results from parametric study (continue)............................................................. 205 4.3b: Results from parametric study (continue)............................................................. 205 4.3b: Results from parametric study (continue)............................................................. 206 4.3b: Results from parametric study (with backslope but no MSE) .............................. 206 4.3c: Results from parametric study .............................................................................. 206 4.3c: Results from parametric study .............................................................................. 207 4.3c: Results from parametric study (continue)............................................................. 208 4.3c: Results from parametric study (continue)............................................................. 209 4.3c: Results from parametric study (with backslope but no MSE) .............................. 209 5.1a: Correction factors.................................................................................................. 235 5.1a: Correction factors (continue) ................................................................................ 236 5.1b: Correction factors for soil nail wall with backslope instead of MSE ................... 236 5.2: Correction factors for different wall parameters..................................................... 241 5.3: Validation of design results for three soil nail walls .............................................. 248 5.4: Validation of design results for three hybrid walls................................................. 250 5.5: Effect of varying MSE reinforcement length of the design of hybrid walls........... 252 5.6: Comparison between results obtained by 5205-Method and PLAXIS predictions 254

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LIST OF FIGURES

1.1: Schematic diagram of a cross section in a hybrid wall system................................... 3 1.2: Schematic diagram of a cross section in an SMSE wall system................................. 4 1.3: Schematic diagram of a cross section at the Elbow Slide........................................... 4 1.4: Expected forces imposed by MSE wall on soil nail wall ........................................... 6 2.1: Typical construction sequences in soil nail walls....................................................... 9 2.2: Mechanism of tension mobilization in soil nail wall................................................ 12 2.3: Long nail subjected to combined loading ................................................................. 14 2.4: Elastic analysis for soil-nail interaction.................................................................... 16 2.5: Constant Modulus of lateral subgrade reaction ........................................................ 17 2.6: Principal failure modes in soil nail walls.................................................................. 18 2.7: Mode of failures in soil nail walls ............................................................................ 19 2.8: Internal failure modes ............................................................................................... 20 2.9: Deformation of soil nail walls .................................................................................. 23 2.10a: Comparison of lateral displacements between soil nail wall and MSE wall ........ 24 2.10b: Comparison of locus of maximum tension line between SN wall and MSE wall 25 2.11: German gravity wall method .................................................................................. 27 2.12: German method: Design chart for stability calculations ........................................ 28 2.13: Multicriteria slope stability analysis method .......................................................... 31 2.14: Kinematical limit analysis method: design assumptions ........................................ 33 2.15: Kinematical method: Design charts for perfectly flexible nails, N = 0 .................. 34 2.16: Tensile force distribution diagram for soil nail ...................................................... 35 2.17: Preliminary design chart for soil nail walls ............................................................ 37 2.18: Preliminary design chart for soil nail walls ............................................................ 38 2.19: Nail tension distribution used in GOLDNAIL ....................................................... 40 2.20: Transfer of frictional stress between soil and reinforcement.................................. 45 2.21: Tensile forces variation along the reinforcements .................................................. 46 3.1: Wall geometry of CEBTP Wall No.1 ....................................................................... 50 3.2: Field results from SPT and Pressuremeter tests........................................................ 50 3.3: The wall after failure by saturation of the soil from the top ..................................... 51 3.4: Instrumentation details of the grouted bars............................................................... 51 3.5: Lateral wall deformations during and after construction.......................................... 52 3.7: Tensile forces distribution along the nails ................................................................ 54 ix

3.8: Geometry of the Polyclinic wall in Seattle ............................................................... 55 3.9: Details of instrumented section of the wall .............................................................. 56 3.10: Measured wall deflection and nail loads................................................................. 57 3.11: Cross section view of A-2 wall............................................................................... 59 3.12a: Geometry and instrumentation details of A2 wall ................................................ 60 3.12b: Estimated soil parameters for A-2 wall ................................................................ 61 3.14: Lateral wall deformation at panel 5 ........................................................................ 62 3.14: Wall geometry of Hayward wall............................................................................. 64 3.15: Strain gauge data from section A-A of Hayward wall............................................ 65 3.16: Displacement measurements at section A-A of Hayward wall .............................. 66 3.17: FHWA Wall No. 3 .................................................................................................. 68 3.18: Instrumented vs. predicted data – FHWA Wall No. 3............................................ 70 3.19: Examples of typical mesh elements in PLAXIS..................................................... 72 3.20: Comparison of mesh density used to model the CLOUTERRE Wall No.1 ........... 73 3.21: Effect of mesh density on results prediction of CLOUTERRE Wall No.1 ............ 74 3.22: Effect of mesh density on prediction of distribution of nail forces ........................ 75 3.23: Effect of soil model selection on results prediction for CLOUTERRE Wall No.1 78 3.24: Effect of soil model selection on prediction of distribution of nail forces ............. 79 3.23: (a) Parameters required for the Mohr-Coulomb’s model ....................................... 81 (b) Assumed failure contour for MC model in principal stress space .............................. 81 3.24: (a) Parameters required for the Hardening Soil model ........................................... 82 3.24: (b) Assumed failure contour for HS model in principal stress space ..................... 82 3.27: Distribution of principal stresses after initial stress generating.............................. 86 3.28: Mesh model for CLOUTERRE Wall No.1............................................................. 87 3.29: Deformed mesh of the CLOUTERRE Wall No.1 .................................................. 88 3.30: Comparison between predicted and measured lateral wall displacement............... 89 3.31: Comparison of the location of maximum tensile line............................................. 90 3.32: Comparable trend of maximum nail tensile forces................................................. 91 3.33: Tensile forces distribution along soil nail (predicted vs. measured) ...................... 92 3.34: Failure surface as predicted by LEM...................................................................... 93 3.35: Developed failure surface as predicted by FEM..................................................... 94 3.36: FEM mesh for Seattle Wall .................................................................................... 96 3.37: Deformed mesh of the Seattle wall......................................................................... 97 3.38: Comparison between predicted and measured lateral wall deformation ................ 99 3.39: Comparable trend of maximum nail loads............................................................ 100 x

3.40: Locus of maximum tensile line............................................................................. 101 3.41: Tensile forces distribution along nails .................................................................. 102 3.42: Failure surface predicted by GSTABL for Seattle Wall....................................... 103 3.43: Predicted failure surface by PLAXIS for Seattle Wall ......................................... 103 3.44: Comparison between predicted and measured lateral wall deformation .............. 106 3.45: Predicted maximum nail tensile forces................................................................. 107 3.46: Predicted nail force distribution............................................................................ 108 3.47: Failure surface predicted by GSTABL for A2 wall.............................................. 109 3.48: Predicted failure surface by PLAXIS for A2 wall................................................ 109 3.49: Mesh model for FHWA No. 3 wall ...................................................................... 112 3.50: Deformed mesh of the FHWA wall No.3 ............................................................. 113 3.51: Comparison between predicted and measured lateral wall displacement............. 114 3.52: Maximum reinforcement tensile forces ................................................................ 115 3.53: Reinforcement Tensile force distribution (predicted vs. measured)..................... 116 3.54: Failure surface predicted by GSTABL for FHWA wall No.3.............................. 117 3.55: Predicted failure surface by PLAXIS for FHWA wall No.3 ................................ 117 3.56a: Deformed mesh – level backfill .......................................................................... 120 3.56b: Deformed mesh – sloped backfill ....................................................................... 120 3.57: Lateral wall deformation before and after backslope ........................................... 121 3.58: Maximum reinforcement tensile forces ................................................................ 122 3.59: Tensile forces distribution along the reinforcements............................................ 123 3.60: Slip surface as predicted by GSTABL for before and after backslope................. 124 3.61: Slip surface as predicted by PLAXIS for before and after backslope .................. 125 4.1a: Modification of the geometry of the baseline hybrid wall to optimize the wall performance .................................................................................................................... 132 4.1b: Optimization of wall performance using the modified baseline wall in terms of lateral wall deformations................................................................................................. 133 4.1c: Reduction in tensile forces in reinforcements....................................................... 134 4.1d: Increase in global factor of safety......................................................................... 135 4.2: Geometry of the baseline hybrid wall..................................................................... 136 4.3: Finite element mesh for analysis of the hybrid baseline wall................................. 138 4.4: Variables considered in parametric study............................................................... 143 4.5: Factor of safety for different heights of soil nail wall ............................................ 145 4.6: Lateral wall deformation for different heights of soil nail wall.............................. 146 4.7: Vertical wall displacements for different heights of soil nail wall ......................... 147 xi

4.8: Lateral earth pressure for different heights of soil nail wall................................... 148 4.9: Maximum tensile forces in the nails for different heights of soil nail wall ............ 149 4.10: Effect of soil nail wall heights on overall performance normalized by results from baseline wall........................................................................................................... 150 4.11: Effect of nail vertical spacing (SV) on factor of safety ......................................... 152 4.12: Effect of vertical nail spacing (SV) on lateral wall deformation........................... 152 4.13: Effect of vertical nail spacing (SV) on vertical displacement at SNW crest......... 153 4.14: Effect of vertical nail spacing (SV) on lateral earth pressure behind wall face..... 153 4.15: Effect of vertical nail spacing (SV) on maximum nail tensile forces.................... 154 4.16: Effect of vertical nail spacing (SV) on overall performance normalized by results from baseline wall ............................................................................................... 155 4.17: Effect of L/H ratio on factor of safety (Sv = 3.3ft)............................................... 157 4.18: Effect of L/H ratio on lateral wall deformation (Sv = 3.3ft)................................. 157 4.19: Effect of L/H ratio on vertical displacement (Sv = 3.3ft)..................................... 158 4.20: Effect of L/H ratio on lateral earth pressure (Sv = 3.3ft)...................................... 158 4.21: Effect of L/H on maximum nail tensile forces (Sv = 3.3ft).................................. 159 4.22: Effect of L/H ratio on overall performance normalized by results from baseline wall (Sv = 3.3ft) ............................................................................................................. 160 4.24: Effect of L/H ratio on lateral wall deformation (Sv = 4.5ft)................................. 161 4.25: Effect of L/H ratio on vertical displacement (Sv = 4.5ft)..................................... 162 4.26: Effect of L/H ratio on lateral earth pressure (Sv = 4.5ft)...................................... 162 4.27: Effect of L/H ratio on maximum nail tensile forces (Sv = 4.5ft).......................... 163 4.28: Effect of L/H ratio on overall performance normalized by results from baseline wall (Sv = 4.5ft) ............................................................................................................. 164 4.29: Effect of l/h ratio on factor of safety .................................................................... 166 4.30: Effect of l/h ratio on lateral wall deformation...................................................... 166 4.31: Effect of l/h ratio on vertical displacement .......................................................... 167 4.32: Effect of l/h ratio on lateral earth pressure ........................................................... 167 4.33: Effect of l/h ratio on maximum nail tensile forces............................................... 168 4.34: Effect of l/h ratio on overall performance normalized by results from baseline wall.................................................................................................................................. 169 4.35: Effect of wall setback on factor of safety ............................................................. 171 4.36: Effect of wall setback on lateral wall deformation ............................................... 171 4.37: Effect of wall setback on vertical displacement ................................................... 172 xii

4.38: Effect of wall setback on lateral wall pressure ..................................................... 172 4.39: Effect of wall setback on maximum nail tensile forces ........................................ 173 4.40: Effect of wall setback on overall performance normalized by results from baseline wall........................................................................................................... 174 4.41: Effect of soil cohesion (c) on factor of safety....................................................... 176 4.42: Effect of soil cohesion (c) on lateral wall deformation ........................................ 176 4.43: Effect of soil cohesion (c) vertical displacements ................................................ 177 4.44: Effect of soil cohesion (c) on lateral wall pressure............................................... 177 4.45: Effect of soil cohesion (c) on maximum nail tensile forces ................................. 178 4.46: Effect of soil cohesion (c) on overall performance normalized by results from baseline wall........................................................................................................... 179 4.47: Effect of soil friction (φ) on factor of safety......................................................... 180 4.48: Effect of soil friction (φ) on lateral wall deformation........................................... 180 4.49: Effect of soil friction (φ) on vertical displacements ............................................. 181 4.50: Effect of soil friction (φ) on lateral wall pressure................................................. 181 4.51: Effect of soil friction (φ) on maximum nail tensile forces.................................... 182 4.52: Effect of soil friction (φ) on overall performance normalized by results from baseline wall........................................................................................................... 183 4.53: Effect of soil unit weight (γ) on factor of safety................................................... 184 4.54: Effect of soil unit weight (γ) on lateral wall deformation..................................... 184 4.55: Effect of soil unit weight (γ) on vertical displacements ....................................... 185 4.56: Effect of soil unit weight (γ) on lateral earth pressure.......................................... 185 4.57: Effect of soil unit weight (γ) on maximum nail tensile forces.............................. 186 4.58: Effect of soil unit weight (γ) on overall performance normalized by results from baseline wall........................................................................................................... 187 4.59: Effect of slope surface on factor of safety ............................................................ 189 4.60: Effect of slope surface on Lateral wall deformation............................................. 189 4.61: Effect of slope surface on vertical displacements................................................. 190 4.62: Effect of slope surface on lateral wall pressure .................................................... 190 4.63: Effect of slope surface on maximum nail tensile forces....................................... 191 4.64: Effect of surface slope on overall performance normalized by results from baseline wall........................................................................................................... 192 4.65: Effect of nail inclination on factor of safety ......................................................... 193 4.66: Effect of nail inclination on lateral wall deformation........................................... 193 xiii

4.67: Effect of nail inclination on vertical displacements.............................................. 194 4.68: Effect of nail inclination on lateral wall pressure ................................................. 194 4.69: Effect of nail inclination on maximum nail tensile forces.................................... 195 4.70: Effect of nail inclination on overall performance normalized by results from baseline wall........................................................................................................... 196 4.71: Effect of bar size on factor of safety..................................................................... 197 4.72: Effect of bar size on lateral wall deformation....................................................... 197 4.73: Effect of bar size on vertical displacements ......................................................... 198 4.74: Effect of bar size on lateral wall pressure............................................................. 198 4.75: Effect of bar size on maximum nail tensile forces................................................ 199 4.76: Effect of bar size on overall performance normalized by results from baseline wall ......................................................................................................................................... 200 5.4: Design chart for tall soil nail wall with vertical spacing of 3.3ft (H = 40ft) .......... 218 5.5: Design chart for tall soil nail wall with vertical spacing of 4.5ft (H = 40ft) .......... 219 5.6: Chart for estimating maximum nail forces in short soil nail wall with vertical spacing of 3.3ft (H = 13.2ft) ........................................................................................... 220 5.7: Chart for estimating maximum nail forces in medium tall soil nail wall with vertical spacing of 3.3ft (H = 26.5ft) ........................................................................................... 221 5.8: Chart for estimating maximum nail forces in medium tall soil nail wall with vertical spacing of 4.5ft (H = 26.5ft) ........................................................................................... 222 5.9: Chart for estimating maximum nail forces in tall soil nail wall with vertical spacing of 3.3ft (H = 40ft) ........................................................................................................... 223 5.10: Chart for estimating maximum nail forces in tall soil nail wall with vertical spacing of 4.5ft (H = 40ft) ........................................................................................................... 224 5.11: Comparison of estimated factor of safety between PLAXIS and GOLDNAIL ... 226 5.12: Design chart for short soil nail wall with vertical nail spacing of 3.3ft................ 227 5.13: Design chart for medium tall soil nail wall with vertical nail spacing of 3.3ft .... 228 5.14: Design chart for medium tall soil nail wall with vertical nail spacing of 4.5ft .... 229 5.15: Design chart for tall soil nail wall with vertical nail spacing of 3.3ft................... 230 5.16: Design chart for tall soil nail wall with vertical nail spacing of 4.5ft................... 231 5.17: Correlation of between lateral and vertical wall displacement............................. 232 5.18: Prediction plot for vertical wall displacement ...................................................... 233 5.19: Effect of modeling MSE wall as vertical surcharge load ..................................... 237 5.20: Design example for hybrid wall system ............................................................... 239 5.21: Calculation of correction factors........................................................................... 241 xiv

5.22: Illustration of design steps for hybrid wall system............................................... 242 5.23: Design examples of three soil nail walls.............................................................. 247 5.24: Design examples of hybrid walls systems ............................................................ 249 5.25: Effect of varying MSE reinforcement length on the design hybrid walls ............ 251

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ABSTRACT

In recent years, many departments of transportation are working to keep pace with population growth by considering major infrastructure improvements to their highways. The successive expansion of the highway system to meet increasing demand has made extension of the right-of-way economically prohibitive. The use of earth retaining walls has allowed highway upgrades to be constructed within existing right-of-ways, consequently lowering the additional cost of acquiring separate lands. Texas Department of Transportation and other DOTs construct Hybrid MSE/Soilnail retaining wall systems to replace existing highway embankments that separate two sections of a roadway. These systems are typically used to allow for widening both sides of the road by constructing a new lane to each roadway while excluding the need to acquire additional right-of-way. The design of such systems, in particular for the soil nail wall, is done using computer programs such Goldnail and Snail. These computer codes are based on limitequilibrium methods and are typically used as design tools for conventional wall systems in which some degree of wall deflection is tolerated. They do not however, address large deflection due to significant surcharge caused by the use of excessive height of MSE wall. Moreover, these methods do not account for the additional outward thrust expected to occur at the soil nail/MSE wall interface. As a result, the requirements for designing hybrid walls systems should not only be based on stability but should also be based on wall deformation. The focus of this research study is to examine the adequacy of the current method recommended by TxDOT and to develop a design procedure for the hybrid wall systems which will address the shortcomings in the currently used methods in practice. The new performance method is based on extensive finite element analysis that will address not only the stability of the structure but also the wall deformations as well as the force transfer in the reinforcements.

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CHAPTER 1 INTRODUCTION 1.1 Background In recent years, many departments of transportation have been working to keep pace with population growth by considering major infrastructure improvements to their highways. The successive expansion of the highway system to meet increasing demand has made extension of the right-of-way economically prohibitive. The use of earth retaining walls has allowed highway upgrades to be constructed within existing right-ofways, consequently lowering the additional cost of acquiring separate lands. The primary function of earth retaining walls in highway constructions is to retrofit and maximize the use of existing space and structures. Engineers can use earth retaining walls to provide steep slopes of reinforced soil to reduce the required width for widening existing traffic lanes in constricted areas. Various types of earth retaining structures have been used successfully in the last two decades. In Texas and throughout the US, mechanically stabilized earth (MSE) retaining walls and soil nail walls are commonly incorporated into highway construction. Typical applications of such systems include but are not limited to: •

Widening within existing rights-of-way

•

Adding a lane of traffic

•

Adding a turn-around lane under a bridge abutment

•

Repairing failed slopes and retaining structures

Unlike the conventional systems that serve to retain soil behind a vertical cut, these two techniques are based on the concept of soil reinforcement that use passive inclusions in the soil mass to create a gravity structure to improve soil stability. In soil nail walls, the native undisturbed soil, adjacent to the excavation is strengthened so that it can stand unsupported at larger depths which would normally require installation of sheet piling or soldier pile bracings. This technique is composed of two major elements: a) layers of reinforcing members that are placed in predrilled holes and grouted to improve

1

the bond strength between the nail and the adjacent soil when nail stresses are mobilized and b) a shotcrete facing typically placed on the soil face which soil nails are attached into. Construction of soil nail walls is performed in vertical steps, with construction starting at the top of the excavation and proceeding down. Once an excavated level is reinforced with soil nails, first temporary and then permanent facings are applied to retain the soil. Mechanically stabilized earth walls, on the other hand, are composed of three major components: a) reinforcements which are placed in the backfill soil in unstressed condition, b) layers of granular soil backfill with drainage blankets and c) facing elements which are provided to retain fill material and to prevent slumping and erosion of steep faces. Unlike soil nail walls, MSE walls are constructed by placing reinforced fill from the “bottom up.” As the use and confidence in MSE and soil nail walls have grown, needs have emerged to use these structures in arrangements that are more sophisticated. They have been used in composite and tiered configurations for a variety of reasons such as aesthetics, stability, and construction constraints. Their use is generally dictated by severity of the grade change and availability or cost of land within a project site. For instance, MSE/Soil-nail hybrid retaining walls are now being constructed in which an MSE wall is placed on top of a soil nail wall as shown in Figure 1.1. Another possible arrangement of soil nail wall and MSE walls is the Shored MSE (SMSE). A SMSE consists of a soil nail wall, which is constructed to serve as a shoring system behind an MSE wall as shown in Figure 1.2 (Morrison, et al., 2006). A combination of the two systems, hybrid wall and SMSE, were used to stabilize an active landslide on a road section of US Highway 26-89 known as the Elbow Slide in Wyoming (Turner and Jensen, 2005). A schematic of hr cross-section of the retaining structure built at the Elbow Slide is as shown in Figure 1.3. In addition to the three preceding wall systems, multiple-tiered walls have also being constructed involving two or several levels of MSE or soil nail walls. An attempt is

2

3 Figure 1.1: Schematic diagram of a cross section in a hybrid wall system

3

currently underway at the University of Texas in Austin to evaluate and develop a design procedure for multi-tiered MSE walls (Wright, 2005).

Figure 1.2: Schematic diagram of a cross section in an SMSE wall system (Morrison et al., 2006)

Figure 1.3: Schematic diagram of a cross section at the Elbow Slide (Turner and Jensen, 2005)

4

1.2 Problem Statement The focus of this research study is to evaluate and develop a design procedure for the MSE/Soil-nail hybrid retaining wall systems. Hybrid walls are typically used to allow construction of a new road and widening of an exiting road on the slope of a hill or an embankment separating two sections of a roadway as illustrated in Figure 1.1. This system consists of two stages: an excavation to install the soil nail wall and the subsequent placement of the MSE wall top. Presently, there is no standardized procedure for designing the hybrid wall. For instance, the Texas Department of Transportation (TxDOT) currently designs both the MSE and soil nail walls to the full combined anticipated height. TxDOT also implements a design practice in which the soil nail wall is designed to the full height of the hybrid wall, and the MSE wall is designed as an independent wall with a minimum bench equivalent to 70% of its design height. Finally, another approach suggests designing each wall for a minimum L/H = 0.7 ∼1.0 where L refers to the soil nail length and H refers to the height of the soil nail wall. Other drawbacks of the current design procedure come from the fact that the current design is based on the limit-equilibrium (LE) methods using computer codes such as GOLDNAIL and SNAIL. While LE methods have proved to be economical and practical design tools for conventional wall systems in which some degree of deflection at the top of the wall can be tolerated, they do not address large deflection due to significant surcharge caused by the excessive height of the MSE wall. In GOLDNAIL, for instance, the MSE wall is currently modeled as a vertical surcharge only; this clearly does not account for the additional outward thrust as illustrated in Figure 1.4. In a hybrid wall, these deflections are expected to occur at the soil-nail/MSE wall interface; therefore, the requirement for designing the hybrid wall should not only be based on stability but should also be based on wall deformation. The aforementioned methods for designing hybrid wall systems, while proving to be reliable and construable, are overly conservative and not cost-effective. Moreover, these design methods are empirically based and have minimal or no data to support them. These design procedures do not address the deflections and the specific force transfer

5

mechanism inherent in the hybrid wall. As a result, there is a need for additional wall performance data that will allow better understanding of wall mechanisms, force transfer, and failure modes. The new design tool should be both stability and deflection control based, and should give predictions of relative deflections and preferably absolute deflection (Budek, 2004). As more data is collected, a validated design procedure can be improved, and construction of such wall systems will gain wider acceptance.

Figure 1.4: Expected forces imposed by MSE wall on soil nail wall (Budek, 2004) 1.3 Objective and Scope of Research The purpose of this research is to investigate the effect of the MSE wall surcharge on the soil nail wall during and after construction. The influence of MSE wall will be examined in terms of development of soil nail loads in the nails, lateral wall movement of the soil nail wall and earth pressure behind wall face. A design procedure will be presented for hybrid wall systems that rationally considers the following salient issues: 1. Global Stability of the hybrid wall 2. Deflection at soil nail wall crest 3. Force transfer between the bottom of the MSE wall and the top of the soil nail wall 4. Failure modes 5. Effect of construction sequence

6

A comprehensive design methodology will be proposed, which will allow for the design of cost effective and safe wall systems. The new design procedure will be performance-based and it will implicitly incorporate both stability and deflection. The design charts will rely primarily on data obtained from finite element analysis (FEA) calibrated using data from field monitored walls. The objective of this research can be summarized as follows: •

Conduct a comprehensive review of pertinent literature

•

Examine the adequacy of the design method currently used by TxDOT

•

Build a numerical model in which simulation of field conditions will be closely matched

•

Instrument and monitor selected hybrid walls under constructions

•

Calibrate the numerical model against available field measurements

•

Perform a parametric study to identify key factors that contribute the most to the overall behavior of the hybrid wall system

•

Evaluate and use results from the numerical model to formulate a finite element based design procedure for hybrid wall systems

•

Develop recommendations for design and construction methodology for future hybrid walls

7

CHAPTER 2 LITERATURE REVIEW 2.1 Introduction The concept of in situ soil reinforcement by tensile inclusion is very old. People in ancient cultures used sticks and branches for reinforcing mud houses and other religious structures. In recent years, numerous techniques of in situ reinforcement have been developed such as soil nailing and Mechanically Stabilized Earth (MSE). These two techniques are well suited to the needs of highway construction and reconstruction. They are most useful in applications such as retaining walls and bridge abutments, where they compete favorably with reinforced concrete. The application of both soil nail walls and MSE walls has been introduced 30 years ago in Europe and has gained wide acceptance in the US lately. Unlike the conventional concrete reinforced systems that serve to retain soil behind a vertical cut, these two techniques are based on the concept of using soil reinforcement as passive inclusions in the soil mass to create a gravity structure and hence improve soil stability. In soil nail walls, the native, undisturbed soil adjacent to the excavation is strengthened so that it can stand unsupported at depth, which normally requires installation of sheet piling or soldier pile bracings. This technique is composed of two major elements: a. Layers of reinforcing members that are placed in predrilled holes and grouted to improve the bond strength between the nail and the adjacent soil when nail stresses are mobilized b. A shotcrete facing placed on the soil face which soil nails are tied into. Construction is performed in vertical steps, with construction starting at the top of the excavation and proceeding down as illustrated in Figure 2.1. Once an excavated step is reinforced with soil nails, a permanent or temporary facing is then applied to retain the soil.

8

Figure 2.1: Typical construction sequences in soil nail walls (Porterfield et al., 1994)

9

MSE walls, on the other hand, are composed of three major components: a. Reinforcements which are placed in the soil; these reinforcements are initially unstressed but reinforcement forces are mobilized by subsequent deformation of the soil b. Backfill soil which is usually granular material with drainage blanket and c. Facing elements which are provided to retain fill material at the face to prevent slumping and erosion of steep faces 2.2 Soil Nail Walls 2.2.1 Historical Background Soil nailing originated in Europe in the 1960s with the introduction of the New Austrian Tunneling Method (NATM). This method emerged from rock bolting that utilized bonded steel bars and shotcrete to support tunnels. The first reported application of this technology for permanent support of retaining walls in a cut in soft rock was in France in 1961. In 1972, the French contractor Bouygues and the specialist contractor Soletanche constructed the first soil nail wall in France. The wall consisted of an 18-mhigh 70-degree cut slope in Fontainebleau sand, and was constructed as part of a railwaywidening project near Versailles. In the United States, the first documented project was in Portland, Oregon, where a soil nail wall was used during the construction of an addition to the Good Samaritan Hospital in 1976 Banerjee et al., 1998. In Europe, two major research programs to study soil nailing were undertaken; one in the late 1970s in Germany by the University of Karlsruhe and Bauer Construction, and the other in the 1980s in France through the CLOUTERRE Program. The French program consisted of a $5 million study, jointly funded by the French government and private industry, with the objective of developing a design methodology for soil nail walls. Results from testing and monitoring of six full-scale structures were used as the basis for soil nail standards in France. These standards were published in 1991, in so called the "Recommendations CLOUTERRE ". The primary finding of this research can summarized as follows (Sotcker and Riedinger, 1990): 10

a. Soil nailed structure behaves as a gravity wall b. The required length of soil nails for typical application lies within the range (0.5-0.8) of the height of the wall c. The vertical and horizontal spacings should be limited to a maximum of 1.5m, where the reinforcement ratio should be kept as 1 nail per 2.25m2 d. The earth pressure behind the wall face may be assumed to be a uniform rectangular distribution with a magnitude of 0.4 to 0.7 times the active Coulomb’s earth pressure In 1996, the U.S. Federal Highway Administration (FHWA) published its "Manual for Design and Construction of Soil Nail Walls." This manual summarizes the work in Germany, France and current U.S. practice, and serves as a guideline for soil nail design for highway works. 2.2.2 Mechanism and Behavior of Soil-nailed Structures The fundamental concept of soil nailing relies upon two possible mechanisms, both of which contributing to improve stability of soil mass: the transfer of tensile forces generated in the nails through frictional interaction between the ground and the soil nail, and the development of shear stress and bending stiffness in the nails as a result of deformation of soil mass. In additions to the aforementioned mechanisms, the soilstructure interaction between the facing and the soil helps to restrain displacement, limit decompression during and after excavation, and produce nail head load at the connection between the nail and the facing necessary to develop the force along the nails (Byrne et al., 1996). 2.2.2.1 Nail Tension Soil nailing technique results in a composite coherent mass similar to reinforced soil systems. The line of the maximum tensile forces in the nails, which usually coincide with the potential slip surface, separates the reinforced soil mass into two zones, an active zone and a resistance zone, as illustrated in Figure 2.2. The active zone is the reinforced

11

mass close to the facing where the mobilized lateral shear stresses are directed outward. This out pull action in the reinforcements results in an increase in the tension force in the nail. The resistant zone is the stable zone where the shear stresses are inward and tend to restrain the reinforcements from being pulled out.

La

Slip surface (line of maximum tension)

Figure 2.2: Mechanism of tension mobilization in soil nail wall (Byrne et al., 1996) The nails act to tie the active zone to the resistant zone. In order to achieve stability, the mobilized tensile force (Tmax) must be balanced by the effective friction along the soil-nail interface in the resistant zone behind the active block (Elias and Juran, 1991). A sufficient embedment length must also be provided into the resistance zone to prevent a pullout failure. Moreover, the pullout resistance (Tp) along with the nail head strength must be adequate to provide the required nail tension at the slip surface. The value of Tmax can be evaluated as follows:

Tmax ≤

Tp SF1

=

Where: Tp: Pullout resistance of the nail 12

D π La F1 SF1

(2.1)

SF1: Factor of safety with respect to pullout resistance (1.75 ~ 2.0) D: Diameter of drilled hole La: Embedment length in the resistance zone F1: The limit interface lateral shear stress obtained from pullout tests Jewel (1990) suggested the limit lateral shear stress or bond stress can be mobilized at the contact between the nail surface area and the surrounding soil. This bond stress is assumed to be directly proportional to the shearing resistance of the soil and is given by the following relation:

F1 = σ r′ f b tan φ ′ σ′ 0.7 ≤ r ≤ 1 σ v′

(2.2)

where: fb = Bond coefficient of the soil (skin coefficient) fb ≈ 1 for fully rough soil-grout interface and, fb ≈ (0.2 – 0.4) for smooth interface

σ'r = Normal effective stress acting on the circumference of the nail σ'v = Effective normal stress 2.2.1.2 Shear Stress and Bending Stiffness in the Nails When the active block moves relative to the resistance mass, a shear zone is developed within the soil nail wall where deformation is concentrated. The movement of the soil mass subjects the soil nails to bending moment and shear forces in addition to axial tensile forces. This interaction between the nail and the soil is resembling a laterally loaded pile, in which the movement of the soil mass loads the nails perpendicular to its long axis as illustrated in Figure 2.3.

13

Formation of hinge

Figure 2.3: Long nail subjected to combined loading (Elias and Juran, 1991) The effect of bending stiffness can be calculated using the simplified method of the coefficient of subgrade reaction, which was originally developed by Poulous and Davis (1980) for laterally loaded piles. The solution derived by the theory of Elasticity implies that at the failure surface, the bending moment in the nail is zero whereas the tension and shear forces are maximum. The non-dimensional bending moment, N can then be defined as: 2

K Dl N= h 0 γ H Sh Sv

(2.3)

where: l0 = 4

4 EI Kh D

l0 = Transfer length, E = Young’s Modulus I = Moment of Inertia D = Diameter of soil nail H = Height of the soil nail wall Sv and Sh = Vertical and horizontal spacing of the nail Kh = Modulus of lateral subgrade reaction

γ = unit weight of the soil 14

(2.4)

The transfer length defines the relative rigidity (stiffness) of the nail where the maximum bending moment is generated at a distance of (π/4) l0 from the shear surface as shown in Figure 2.4. Since the total length of nail (L) is significantly greater than three times the transfer length l0, it can therefore be considerably long (flexible nail). The horizontal subgrade reaction can be estimated using charts developed for anchored wall as shown in Figure 2.5. The bending stiffness number N for most practical structures varies from 0.1 to 1.5, which suggests behavior more closely associated with flexible reinforcement. The maximum tension forces and shear forces mobilized in the nails can be obtained using the following non-dimensional parameters:

TN =

Tmax γ H Sh Sv

(2.5)

TS =

Tc γ H Sh Sv

(2.6)

Tc can be evaluated using the following relationship: 2

⎛ Tn ⎞ ⎛ Tc ⎜⎜ ⎟⎟ + ⎜⎜ ⎝ Rn ⎠ ⎝ Rc

2

⎞ ⎟⎟ ≤ 1 ⎠

(2.7)

where: TN = Normalized axial force of soil nail TS = Normalized shear force of soil nail Tn = Axial (tensile) force acting on soil nail Tc = Shear force acting on soil nail Rn = Tensile strength of the nail Rc = Shear strength of the nail Figure 2.4 shows the elastic solution described by Schlosser (1983) and Michelle and Villet (1987) which yields the following relation for estimating the maximum bending moment in the nail.

M max =

Tc l s 4.9

15

(2.8)

where: ls = Distance between points of maximum moment

Figure 2.4: Elastic analysis for soil-nail interaction (Schlosser, 1983) For design purposes, the soil nails are assumed to provide their reinforcing action almost exclusively through the development of tensile forces associated with shear stresses mobilized at the interface between the nail and the ground. The FHWA manual for design of soil nail wall does not consider shear and bending in the nails since their benefits are still small in comparison with the minimum axial force or the mobilized nailgrout ultimate adhesion.

16

Figure 2.5: Constant Modulus of lateral subgrade reaction (Pfister et al., 1982)

17

2.2.3 Modes of Failure of Soil-nailed Structures Observations from several building sites and instrumented full-scaled soil nail walls conducted as part of the CLOUTERRE project identified three types of failure modes. As shown in Figures 2.6 and 2.7, these failure modes include: 1. External modes with potential failure surface passing entirely behind the reinforced zone 2. Internal modes that involve failure of either the nail bars or the facing or both 3. Mixed modes that involve internal failure of the reinforced zone that can extend beyond the reinforced zone of the soil mass Failure modes will depend primarily on the condition of the wall. For instance, during construction, the problems may include stability of the excavation, the excessive weight of the facing, soil variability and the presence of local water. Generally, these types of failure will mobilize bending stiffness and shear forces as illustrated by Case 2 in Figure 2.7. After construction, the concerns may include saturation of soil, unexpected surcharge, corrosion of the nail bars and soil and grout creep. Here there would likely be some settlement with mobilization of bending stiffness and shear forces along with development of shearing zone within or behind the reinforced mass (Jones, 1990).

Figure 2.6: Principal failure modes in soil nail walls (Modified after Byrne et al., 1996)

18

EXTERNAL FAILURE MODES

INTERNAL FAILURE MODES

FACING FAILURE MODES

Figure 2.7: Mode of failures in soil nail walls (Lazarte et al., 2003)

19

2.2.3.1 Internal Failure Modes The internal failure comprises three types of failure as demonstrated in Figure 2.8 and can be described as follows:

Figure 2.8: Internal failure modes (Plumelle et al., 1990) 1- Failure by nail breakage In this failure mode, the stresses in the nails have reached their maximum capacity and excessive deformation in the soil mass has occurred.

A slip surface may be

developed because of full mobilization of shear strength of the soil. Consequently, the nails are subjected not only to tensile forces but also to bending moments and shear forces. Usually, this type of failure happens suddenly with no warning. A variety of reasons may contribute and lead to this type of failure: a. Insufficient nail cross section b. Corrosion of steel bars c. Saturation of soil due to water infiltration d. Excessive surcharge on top of soil nail wall 2- Failure during excavation If the height of the soil cut is relatively large, failure can occur suddenly due to local instability and propagation to the top. This instability is caused by continuous flow of soil behind the wall facing due to successive elimination of arching effect. In order to prevent such failure, the height of the excavation at each stage should be kept less than 20

the critical height which usually ranges between 1 – 1.5 m. Another mechanism of failure in this category includes failure by piping. This failure is similar to the previous one except that the cause is different and primarily due to presence of pocket of water in the soil. During excavation, pore water pressure and water seepage; weaken the soil causing rapid and regressive failures due to soil flow. 3- Failure by pullout This failure is more frequent and happens because of inadequate shear resistance between the nail and the surrounding soil. The failure by pullout can be characterized by the following: a. Under-estimating the bond stress or pullout capacity of the nail b. Insufficient embedment length in the resistance zone c. Decrease in effective shear strength as a result of saturation of cohesive soils 4- Failure of facing Failure of the facing usually occurs due to inadequate structural design. There are three common potential failure modes of the wall facing and face-nail connection as shown by Cases h through j in Figure 2.7: a. Flexural failure when bending moment exceed the section capacity of the facing b. Punching shear failure of the facing around the nails and c. Failure of the headed-stud particularly in permanent facing. 2.2.3.2 External Failure Modes The external stability refers to the overall stability of the soil nail wall. These stability conditions are same as those associated with the performance of conventional gravity or cantilever structures. External failure can occur by:

21

a. Sliding along the base or a failure surface, under the influence of the lateral earth pressure exerted by the ground retained behind the reinforced zone b. Bearing capacity failure due to poor quality foundation soils or insufficient soil nail length. This failure is usually associated with overturning and global failure, under the compound effect of structure self-weight and lateral earth pressure c.

Overall (deep seated) slope failure of the ground in which the soil nail wall is built

2.2.4 Deformation of Soil Nail Walls The construction of soil nail walls in urban areas, in many instances, necessitate controlling wall deformation. The horizontal deformations are caused by shear and bending of the nails and by a horizontal deformation of the soil below the excavation due to lateral earth pressure. During construction of a soil nail wall, the reinforced zone tends to rotate outward about the toe of the wall. Consequently, the maximum horizontal and vertical movements occur at the top of the wall and decreases gradually toward the toe of the wall (Byrne et al., 1996). Observation of in-service walls and instrumented walls has shown that horizontal and vertical displacements at the wall crest tend to be on the same order of magnitude (CLOUTERRE, 1991). Displacements at the head of the facing depend on the following factors: 1. Rate of construction 2. Nail spacing and excavation lift height 3. Nail and soil stiffness 4. Global factor of safety 5. L/H ratio 6. Nail inclination 7. Bearing capacity of the foundation soils 8. Magnitude and location of surcharge loading

22

Soil nail walls designed with adequate factor of safety, L/H ratio and negligible surcharge loading are expected to deform in the order of 0.1% H to 0.4% H. Figure 2.9 presents CLOUTERRE recommendations for estimating wall deformations. The design parameter κ can be used to evaluate the length λ where no deformation is expected. 2.2.4.1 Deformation Analysis of Soil Nail Walls Currently, there are no rational design procedures for predicting the extent and magnitude of ground movement both horizontally and vertically. Finite element analysis conducted by Elias and Juran (1991) showed that a relationship exists between the horizontal displacement and the global factor of safety. However, no attempt has been made to use such relationship in designing of soil nail walls.

Figure 2.9: Deformation of soil nail walls (CLOUTERRE, 1991)

23

2.2.5 Comparison of Behavior between Soil Nail Walls and MSE Walls Although both wall systems use passive inclusions, the difference in their behaviors is mainly due to the method of construction used (excavation versus filling). This disparity in construction method produces not only variations in the stresses generated in the soil mass but also different displacement distribution. Both systems require a certain amount of movement to mobilize the strength of the reinforcements. In MSE walls, reinforcement strength is mobilized by compression of the fill. The stresses continue to increase on the reinforcements in the lowest portion of the wall as each additional soil lifts are placed and compacted. This places the greatest stress on the lower reinforcement strips, and results in a tendency for deformation to occur in the lower third of the wall as shown in Figure 2.9 (b). Since soil nail wall is constructed from the top down, the first row of nails will be subjected to the greatest stress. As soil is excavated at the wall face, the strength of the upper nail is mobilized as a result of the decompression or reduction in confinement of the soil as shown in Figure 2.10. In soil nail walls, the tension in the reinforcements is greatest at the top of the wall initially, and increases during excavation of subsequent soil layers (Soil Screw Manual).

(a)

(b)

Figure 2.10a: Comparison of lateral displacements between soil nail wall and MSE wall (Byrne et al., 1996)

24

(a)

(b)

Soil Nail Wall

MSE Wall

Figure 2.10b: Comparison of locus of maximum tension line between soil nail wall and MSE wall (Modified after Lazarte et al., 2003 and Christopher et al., 1990) 2.2.6 Design Methods for Soil Nail Walls The available design methods for soil nail walls may be classified into two main categories (ISSMGE-TC-17). •

Limit-equilibrium design methods or modified slope stability analyses: These approaches design the reinforced walls under failure conditions. They involve evaluation of a global safety factor while taking into account the shearing, tension, or pullout resistance of the nails crossing the potential failure surface.

•

Working-stress design methods: These methods try to estimate nail forces, such as tension, shear forces and bending generated during construction and under service loading conditions, which are then used to evaluate local stability at each level of nails.

Most of the methods that are currently used for designing soil nail walls are based on evaluating global stability of the wall against rotational failure and local stability of the reinforced soil mass at each reinforcement level. All of These methods have evolved from limit equilibrium methods (LEM) but use different definitions of safety factors and different assumptions with regard to the shape of the failure surface, mode of soil25

reinforcement interaction, and the resisting forces in the nails. In all methods, the slip surface is assumed to separate the reinforced mass into a moving block (active zone) and a stationary mass (resistance zone). The failure surface is assumed to be either bi-linear, parabolic, circular or logarithmic spiral. The methods incorporate, to some extent, the tension resistance and pullout capacity of the nails (Juran et al., 1990). The current limit-equilibrium design methods provide only a global factor of safety with respect to soil shear strength, but do not provide means of estimating the maximum tension and shear forces generated in the nails. The limitations inherent in these methods do not allow their use for evaluating local stability at each nail level or for predicting wall deformation. The following section presents a brief discussion of the different design methods and their design concepts. 2.2.6.1 German Gravity Wall Method This method was developed by Stocker et al. (1979) based on limited number of model tests subjected to substantial surcharge loads. The method uses a force-equilibrium approach with a bilinear slip surface with consideration of tension forces in the nails. The soil nail wall is designed as a gravity retaining wall, which acts to retain the soil behind it. Experimental work by Gassler (1988) showed that bi-linear failure surface is not consistent with the observed behavior of soil nail walls. His observations would rather show a failure surface that is more consistent with a circular sliding surface. However, the bi-linear failure mechanism appears to be applicable for cohesionless soil subjected to high surcharge loads. The design concept is based on the definition of an overall factor of safety described as the ratio of dissipative forces along the slip surface combined with the nail effects, divided by the external forces the system is subjected to. The general description of the force equilibrium method for global stability is shown in Figure 2.11. The minimum factor of safety is obtained by iterative procedure in which the planar angle θA at the toe, measured from the horizontal, is varied while keeping the second planar angle θB at 45° + φ/2. Figure 2.12 shows a design chart developed based on this bi-linear wedge 26

approach. The use of the chart is limited for wall with 10° face batter and 10° nail inclinations (Gassler and Gudehus, 1981). The tension force is evaluated using the normalized tension forces (µ) per unit facing surface area using the following relation.

µ=

Tm γ Sh Sv

(2.9)

where: Tm = Shear force per unit length of nail γ = Unit weight of soil Sv and Sh = Vertical and horizontal spacing of the nail The German method considers only axial forces in vertical walls. The tensile force in the nail is evaluated using the shear force per length of nail. This shear force is assumed to be constant along the length of the nail and can be obtained from pullout test. The German method recommends a global factor of safety in the range of 1.5 to 2.0 and 1.2 to 1.25 for pullout capacity.

Figure 2.11: German gravity wall method (Gassler and Gudehus, 1981)

27

Figure 2.12: German method: Design chart for stability calculations (Gassler and Gudehus, 1981) 2.2.6.2 French Multicriteria Analysis The research work in the four-year French national project “CLOUTERRE” has led to the publication of the French soil-nail design method. This method considers the contribution of both tension and shearing resistance of the nails in addition to the nail bending stiffness. Schlosser (1985) presented a Multicriteria analysis procedure for soil nailing. The overall stability is evaluated assuming a circular slip surface utilizing the simplified Bishop’s method. The method considers the mobilized lateral earth pressure on the nails and the corresponding shearing resistance developed in the nails. The method is based on the assumption that the nails behave similar to long elastic piles subjected to lateral load. The Multicriteria analysis is conducted to evaluate the factors of safety with respect to the following considerations as shown in Figure 2.13: a. Shear resistance of the soil b. Skin friction of the nails

28

c. Normal interaction between the soil and the nails d. Strength of the nail against tension, shear forces and bending moment 2.2.6.2.1 Shear Resistance of the Nail The shear resistance of the soil is calculated using the classical Mohr-Coulomb’s failure criterion as described by the following relation:

τ = c + σ v tan φ

(2.10)

Where c is the soil cohesion and φ is internal friction angle of the soil and the normal vertical stress σv = γ z. 2.2.6.2.2 Skin Friction of the Nail The skin friction, qs can be assessed by pullout test performed in the laboratory or in the field. Observations of soil nail walls by Cartier and Gigan (1983) and Schlosser (1983) concluded that the unit skin friction of the soil nail qs was independent of the soil depth. This is attributed to the fact that the reduction in the apparent coefficient of friction

µ∗ = τ σv due to decrease in dilatancy is offset by the increase in the normal stress σv; i.e. qs = µ* (z) γ z = constant Therefore, the maximum resistance of the nails is dependant on the soil-nail interaction criteria. Assuming the skin friction constant along the embedment length, the nail tensile strength, Tn is evaluated using the following relationship (CLOUTERRE, 1991). Tn ≤ q sπ D La

(2.11)

Where D is the diameter of the soil nail and La is the embedment nail length in the resistant zone. 2.2.6.2.3 Normal Interaction between the Soil and the Nails Development of the shear zone in reinforced mass results in progressive mobilization of the passive lateral earth pressure on the nails. Since this phenomena resembles interaction between laterally loaded piles and surrounding soil, the conventional p-y analysis may be used to estimate the ultimate shear forces and bending 29

moments mobilized in the nails. This analysis models the soil reaction as a series of elastoplastic springs where the lateral reaction modulus Kh is estimated from pressuremeter data or Figure 2.5 (Elias and Juran, 1991). The maximum shear force, Tc mobilized at the point of intersection between the failure surface as shown in Figure 2.13 is given by: Tc =

p D l0 2

; p < pmax

(2.12)

The maximum bending moment mobilized at the distance (π/4) l0 from point ,O is given by: 2

M max = 0.16 pD l0 < M p

(2.13)

where: p = Passive earth pressure on the nail pmax = Maximum passive resistance that can be mobilized in the soil l0 = Transfer length given by equation (2.4) Mp = Limit bending capacity of the nail 2.2.6.2.4 Strength of the Nail The nails must withstand both tension (T) and shear force (V). Assuming the nail element follows Tresca’s failure criterion (Elias and Juran, 1991): T2 V2 + 2