Laboratory Pull-Out Testing Study on Soil Nails in Compacted Completely Decomposed Granite Fill By SU Li-Jun A Thesis Submitted for the Degree of Doctor of Philosophy Department of Civil and Structural Engineering THE HONG KONG POLYTECHNIC UNIVERSITY March 2006 UMI Number: 3241089 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion.
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Box 1346 Ann Arbor, MI 48106-1346 LABORATORY PULL-OUT TESTING STUDY ON SOIL NAILS IN COMPACTED COMPLETELY DECOMPOSED GRANITE FILL SU LI-JUN Ph. THE HONG KONG POLYTECHNIC UNIVERSITY 2006 CERTIFICATE OF ORIGINALITY I hereby declare that this thesis is my own work and that, to the best of my knowledge and belief, it reproduces no material previously published or written, nor material that has been accepted for the award of any other degree of diploma, except where due acknowledgement has been made in the text. SU LI-JUN Abstract of thesis entitled LABORATORY PULL-OUT TESTING STUDY ON SOIL NAILS IN COMPACTED COMPLETELY DECOMPOSED GRANITE FILL Submitted by Su Li-Jun for the degree of Doctor of Philosophy at The Hong Kong Polytechnic University in March 2006 Soil nailing is a technique for stabilizing soil slopes and excavations by installing a large number of closely spaced passive inclusions into the in-situ soil mass. The soil nailing technique has been increasingly used worldwide since its origination in the early 1970’s because of its technical and economical advantages.
In Hong Kong, soil nailing has been commonly used to stabilize new cut and sub-standard existing slopes since the late 1980’s. The interface shear strength between a soil nail and the surrounding soil is a key parameter for design and stability assessment of the soil nailing system. However, in current practice in Hong Kong, this parameter is generally assumed to be the same as the shear strength of the soil and verified by field pull-out tests in the construction stage. Field verification tests are normally subjected to variations of the site conditions and the results are therefore scattered.
Laboratory pull-out tests have been carried out to help overcome these problems and precisely investigate the factors influencing the nail-soil interface shear strength. However, there were still some deficiencies in these tests and can be improved. A laboratory study of the pull-out shear resistance of cement grouted soil nails was therefore conducted in compacted completely decomposed granite (CDG) fill. A pull-out box with the internal dimensions of 1.6m in width and 0.83m in height was designed and constructed to carry out the pull-out tests.
An extension cylindrical chamber was provided to house an extension part of the nail and ensure that a constant 1.0m length of the test soil nail was maintained within the test box during pull-out and no cavity would be left behind the end of the test nail. A waterproof front cap was used to cover the soil nail head and prevent water leakage which made it possible to apply back pressure to saturate the testing soil in submerged tests. Comprehensive instrumentation was used and the earth pressure, suction, and pore water pressure in the soil, the deformation of the testing soil, and the pull-out force and displacement were measured. During the pull-out tests, the overburden pressure was applied before drilling to simulate the actual construction procedure of the soil nailing system.
A series of pull-out tests have been conducted using two copies of the above introduced pull-out box. The test results showed that soil stresses around the hole were largely released after drilling and recovery of the stresses due to grouting of the soil nail was minimal. The development of pull-out shear resistance was mainly derived from the constrained dilatancy of the soil. Tests in soil at different degrees of saturation showed that the peak pull-out shear resistance varies with different degrees of saturation of the soil, with higher resistances at the degrees of saturation of 50% and 75%.
Pressure grouting tests were carried out and showed that the average peak pull-out shear resistance of the soil nail increased almost linearly with the increase in grouting pressure. Numerical modeling was performed and agreements between the measured and simulated results were good. Acknowledgements I wish to express my deepest gratitude to my chief supervisor, Professor J-H. Yin, for his encouragement, support and guidance during this period of study.
It was his endless efforts and experienced guidance that made this work possible. The privilege of working with Professor Yin has appreciably influenced my professional development and perspectives. Some of the tests in the study received financial support from Civil Engineering and Development Department of The Hong Kong Special Administrative Region Government and is gratefully acknowledged. The author would like to express thanks to the Director of Civil Engineering and Development and the Head of the Geotechnical Engineering Office for the permission of the use of data from those tests which received financial support.
The improvement, setup and usage of the equipment and apparatus for the soil nail pull-out resistance studies have received valuable comments from Mr. Tony Cheung, Miss Carrie Leung, Mr. Tang, and Mr. All these comments are gratefully acknowledged.
I also wish to thank Mr. Zhou and all the technicians in the Soil Mechanics Laboratory of Department of Civil and Structural Engineering in The Hong Kong Polytechnic University for their assistance in the setup of the test apparatus and participation in some of the soil nail pull-out tests. The author wishes to express his sincere gratitude to the two examiners, Professor R. Jardine and Dr.
Zhang for their invaluable comments in their thesis examination reports and insightful questions and valuable suggestions during the oral examination. I would like to express my special thanks and admirations to my wife, Xiao Jia, for her understanding and support. I sincerely appreciate my parents and my sister for their endless encouragement and constant support. TABLE OF CONTENTS CERTIFICATE OF ORIGINALITY ABSTRACT ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES Chapter 1: INTRODUCTION 1.3 Organization of the thesis Chapter 2: LITERATURE REVIEW 2.1 The soil nailing technique 10 2.1 Characteristics of soil nailing 12 2.2 Advantages and limitations of soil nailing 14 2.3 Fields of application 15 2.4 Soils suitable for soil nailing 17 2.2 Behaviour of soil nailing 17 2.1 Soil nailing mechanism 17 2.2 Nail-soil interface shear resistance 19 2.3 Influence of bending stiffness of the nail 22 2.4 Failure modes of soil nailed structures 23 2.3 Design methods for soil nailing structures 24 2.1 The Davis method 25 2.2 The French method 26 2.3 The German method 27 2.4 The Juran method 28 2.5 Discussion on current design guides and codes 30 2.4 Factors influencing the pull-out resistance 33 2.3, Methods of installation 34 2.4 The nail surface conditions 35 2.5 Research and development 35 2.1 Large scale model tests and field monitoring 35 2.2 Laboratory testing studies 38 2.1 Laboratory pull-out tests 38 2.2 Direct shear and interface shear testing studies 41 2.4 Small scale tests 43 2.3, Numerical modeling Chapter 3: EQUIPMENT AND APPARATUS FOR PULL-OUT TESTS 3.1 Problems studied by laboratory pull-out tests 60 3.2 Numerical study on boundary effect for design of the box 62 3.3 Design and construction of two pull-out boxes 64 3.1 Investigations to be conducted using the boxes 64 3.2 Description of the pull-out box 65 3.1 Extension cylindrical chamber covering the soil nail end 67 3.2 A waterproof front cap to covering the soil nail head 68 3.
Application of back pressure for saturation of the soil 69 3.4 Measures for reducing side friction of the box 70 3.5 Instrumentation and measurements 72 3.6 Drilling machine and cement grouting tools 75 3.2 Equipment for cement grouting without and with pressure 76 3.7 Setup of the box for soil nail pull-out testing 79 3.8 Summary and conclusions 79 Chapter 4: MATERIAL PROPERTIES AND TEST PROCEDURES 4.1 Basic properties of the CDG soil 98 4.2 Determination of the shear strength of the soil 99 4.1 Conventional triaxial tests on saturated soil specimens 99 4.2 Double cell triaxial tests on unsaturated soil specimens 101 4.3 Properties of the cement grout 102 4.3 Calibration of transducers 103 4.4 Soil preparation and test procedures 105 4.2 Preparation of soil specimens for triaxial tests 106 4. Application of vertical overburden pressure 106 4.4 Hole drilling and cement grouting 107 4.5 Installation of tensiometers and/or porewater pressure transducers 107 4.6 Saturation of the test CDG soil 108 4.7 Pull-out of the nail 108 Chapter 5: INFLUENCE OF OVERBURDEN PRESSURE ON SOIL NAIL PULL-OUT BEHAVIOUR AND RESISTANCE 5.2 Stress variations during drilling and grouting 128 5.1 Stress release during drilling 128 5. Variation of earth pressure during and after grouting 130 5.3 Development of earth pressure during pull-out 131 5.4 Pull-out shear stress-displacement behaviour 133 5.5 Influence of overburden pressure on pull-out shear resistance 134 5.1 Peak pull-out shear resistance 134 5.2 Apparent coefficient of friction 135 5.6 Shear stress distribution on the nail-soil interface 136 5.7 Summary 139 Chapter 6: INFLUENCE OF DEGREE OF SATURATION ON SOIL NAIL PULL-OUT BEHAVIOUR AND RESISTANCE 6.2 Earth pressure and pore pressure responses during saturating the soil 153 6.3 Variations of earth pressure 155 6.1 Decreased earth pressure immediately after grouting 155 6.2 Variations of earth pressure 156 6.4 Failure patterns of the soil nail 157 6.1 Surface of the drillhole before and after pull-out 157 6.2 Failure surfaces of soil nails in the soil at different degrees of saturation 157 6.5 Effect of degree of saturation of the soil on pull-out behaviour and resistance 158 6.6 Summary and major findings 160 Chapter 7: EFFECT OF GROUTING PRESSURE ON SOIL NAIL PULL-OUT BEHAVIOUR AND RESISTANCE 7.2 Variations of earth pressures 177 7.1 Variations of earth pressures during drilling and pressure grouting 177 7.2 Variations of earth pressures during the whole period of testing 179 7.3 Failure patterns of the soil nail 179 7.4 Influence of grouting pressure 180 7.5 Summary and conclusions 183 Chapter 8: NUMERICAL SIMULATION OF PULL-OUT TESTS 8.2 Simulation of the shearing plane 192 8.3 Description of the finite element model 193 8.1 Mesh and boundary conditions 193 8.2 Procedure of the simulation 194 8.4 Simulation of the pull-out tests 198 8.1 Stress and strain rate contours 198 8.2 Variations of the vertical stress during the simulation 199 8.3 Influence of the overburden pressure 200 8.1 Influence of dilation angle 201 8.2 Influence of grouting pressure 202 8.6 Summary 204 Chapter 9: SUMMARY, CONCLUSIONS AND SUGGESTIONS 9.3 Recommendations and suggestions 224 REFERENCES 226 LIST OF TABLES Table 2.1 — Basic assumptions of different soil nailing design approaches 46 Table 4.1 — Properties of the CDG soil and cement grout 110 Table 4.2 — Shear strength parameters of the CDG soil 110 Table 8.1 — Material properties used in the finite element model 205 LIST OF FIGURES Figure 1.1 — 1972 Sau Mau Ping Landslide Figure 1.2 — 1972 Po Shan Road Landslide Figure 2.1 — Equipment for launched soil nails (After Myles and Bridle 47 1992) Figure 2.2 — Comparison of soil nailing, micro piles and soil dowelling 47 (After Bruce and Jewell 1986) Figure 2.3 — Contrast of the construction sequence of reinforced earth and 48 soil nailing (After Bruce and Jewell 1986) Figure 2.4 — Soil nailing mechanism (After Byrne et al.5 — Skin friction mobilization in pullout test (After Cartier and 49 Gigan 1983) Figure 2.6 — Nails subject to shear and bending (After Mitchell 1987) 49 Figure 2.