ONG Chee Wee-PhD Thesis-2009-Centrifuge Model Study of Tunnel_Soil_Pile Interaction in Soft Clay.pdf

CENTRIFUGE MODEL STUDY OFTUNNEL-SOIL-PILE INTERACTION IN SOFT CLAY ONG CHEE WEE NATIONAL UNIVERSITY OF SINGAPORE 2009 CENTRIFUGE MODEL STUDY OF TUNNEL-SOIL-PILE INTERACTION IN SOFT CLAY ONG CHEE WEE (B.Eng.(Hons),UPM, M.Sc.(Civil), NUS, P.Eng.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 DEDICATION To my dearest parents, my caring wife and my lovely twins…… i ACKNOWLEDGEMENTS It was been sweat and tears the past four years to complete my PhD and coupled with countless trials and errors; the conclusion of this thesis has finally arrived! Foremost, I would like to extend my heartfelt gratitude and thanks to a special and wonderful person, none other than my main supervisor, Prof. Leung Chun Fai. To my cosupervisor, Prof. Yong Kwet Yew, who had throughout the research period, gave me constructive comments which have stimulated the success of this thesis. I would like to thank them for their willingness to share their vast experience and guidance. In addition to this, my appreciation goes out to Prof. Chow Yean Khow for his advice in the constant students group discussion held fortnightly. Gratitude also goes out to Prof Tan Thiam Soon and A/Prof. Harry Tan Siew Ann for their advices during my qualifying examination which has propelled me to move forward towards a clearer and better direction. Prof. Tan always encouraged his students to think out of the box. One day, when I was exhausted in innovating my new centrifuge model tunnel (see Figure 3.6), I have suggested the owner of our engineering canteen, Seton Lin to turn over my model tunnel and use it as cup holder (as shown in the Figure 1 below). With this ‘turned-over’ tunnel, you can save your queuing time next time when you buy a cup of coffee or tea from NUS canteen. The innovative centrifuge model tunnel is also part of the contributions from Mr. Wong C.Y, Dr. Okky Purwana, Dr. Shen Rui Fu, Mr. Martin Loh and Ian Kit. Figure 1 The ‘turned-over’ model tunnel is now a cup holder in NUS Engineering canteen I also wish to express my sincere gratitude and thanks to Asst. Prof. Goh Siang Huat, who has been a source of endless ideas and inspiration, to Dr. Shen Ruifu, who has been advising me for my centrifuge model test and interpretation of my test results, and to Prof. K.K. Phoon, who has encouraged and supported me since I was reading my M.Sc. back in year 2002. I will never forget the wonderful time that I spent with him in organizing many conferences. Special thanks are also extended to the technical staffs of NUS Geotechnical Centrifuge Laboratory,Wong Chew Yuen, Mdm. Jamilah, Lye Heng, Shaja, John Choy, Loo Leong Huat and Adrian Tan. I would like to extend my sincere gratitude to the final year students that I had privilege of working with, i.e. Kai Yang (04/05), Eddie Hu (05/06), Ian Kit and Eliza (06/07) and Teng Hui (07/08) I also wish to credit the support of the following professionals, associates and friends for sharing their experiences and knowledge namely Prof. Andrew Palmer, Prof. I.H Wong, Dr. Dave White, Dr. Johny Cheuk, Mr. Nick Shirlaw, Mr. Ow Chun Nam, Dr. ii Jeyatharan Kumarasamy, Dr. Lin Kai Qiu, Mr. Cham Wee Meng, Mr. Lim Tuck Fang, Mr. Cheang Yew Kee, Mr. Poh Chee Kuan, Mr. Koo Chung Chong, Mr. Lee Hong Keow, Mr. Edmund Ng, Mr. Jonathan Ang, Mr. Phang Chu Mau, Mr. Chew Eik Khoon, Mr. Jimmy Chew and Mr. Lau Kim Hwa. I extend my appreciation to my many colleagues and friends who I have consulted during the course of the research, particularly Chin Hong, Ran Xia, Xie Yi, Yonggang, Xiying, San Chuin, Kheng Ghee, Kar Lu, Cheng Ti, Hung Leong, Czhia Yheaw, Haibo, Teik Lim, Okky, Dominic, William, Wang Lei, Chen Jian, Feijian, Sindhu, Ch’ng Yi, Poh Hai, Karma, Subhadeep, Khrishna, Jiangtao, Andy, Chong Hun & Ben. It was a memorable trip to Schofield Centre at Cambridge University in April/May 2007. Thank you for the kind arrangement of Prof. Robert Mair. My stay in Jesus College was wonderful and fulfilling. I would also like to thank Prof. Malcolm Bolton who has provided arrays of resource and pertinent pointers in improving my model and research. To Prof. Andrew Noel Schofield, Prof. Kenichi Soga, Dr. Stuart Haigh, Dr. I Thusyanthan, Dr. Johnson Chung, Sidney Lam, Fiona Kwok, Gui Chang Shin, Tricia Lee, Alec Marshall, Richard Laver, Hisham Mohamad, Senthil, Christopher and Dr Yueyang Zhao, all for giving me valuable advices and encouragement on my research. Thank you Geotechnical Society of Singapore for the sponsorship to Bangalore, India to attend the 6th Asian Young Geotechnical Engineers Conference (Dec 2008) in which my paper co-authored with Prof. C. F. Leung and Prof. K. Y. Yong have won the ‘Best Contribution Award’. A special acknowledgement is dedicated to Wendy and Angelia for their help and assistance in the compilation of this thesis. I would like to extend my gratitude to my parents, my wife, Shin Inn and my twins, Yee Heng and Yee Huan (born one month after I pursued my PhD), my sisters, brother-in-laws, grandparents, uncles, aunts and cousins for their never-ending love, support, tolerance and sacrifice in encouraging me to complete this research. A special mention to my brother-in-law, Moong Khai Chee for his support, guidance and technical advice ever since my undergraduate years. It is also with great pride despite the hectic schedule of my PhD research, I have also successfully passed the qualifying exams and registered myself as a Professional Engineer with Board of Engineers Malaysia in 2008; passed the Professional Engineer Fundamentals Engineering Examination, Singapore in 2007, and being registered as a Resident Engineer with Building and Construction Authority, Singapore also in 2007. To the remaining people whom I am unable to list down, please accept my sincere appreciation and thanks for the feedback, assistance, tolerance and help rendered. Last but not least, deepest appreciation and thanks to National University of Singapore for the award of this research scholarship throughout the four years period for without which this research program would not have been possible. Ong Chee Wee 28-02-2009 iii TABLE OF CONTENTS Dedication i Acknowledgements ii Table of Contents iv Summary x List of Tables xii List of Figures xiii Nomenclature xxvi CHAPTER 1 INTRODUCTION 1 1.1 Background 1 1.2 Tunnelling-Induced Soil Movements 2 1.3 Effects of Tunnelling on Piles 3 1.4 Objective and Scope of Study 4 1.5 Structure of Thesis 5 CHAPTER 2 LITERATURE REVIEW 9 2.1 Introduction 9 2.2 Techniques for Simulation of Tunnelling in Centrifuge 9 2.2.1 Simulation Technique 1 - High Density Polystyrene Foam 11 iv 4.1 Introduction 67 3.1.3 Model Pile Cap 76 v .3 Simulation Technique 3 .3 3.3.1 Principles of Geotechnical Centrifuge Modelling 67 3.3.3.3.2 Geotechnical Centrifuge Modeling 67 3.Compressed Air 12 2.3 2.3.1 Field Studies of Tunnelling-Induced Pile Responses 29 2.2.4 Simulation Technique 4 .2.3.1 Field Studies of Tunnelling-Induced Soil Movement 17 2.2 Centrifuge Model Tests of Tunnelling-Induced Soil Movement 22 2.1 Advantages of Model Tunnel 72 3.4 2.2 Instrumented Model Piles 74 3.2 NUS Geotechnical Centrifuge Facility 69 Experimental Set-Up 70 3.2.Liquid – Oil / Water 14 2.3 Predictive Methods of Tunnelling-Induced Soil Movement 24 Tunnelling-Induced Pile Responses 29 2.2 Simulation Technique 2 .2.5 2.2.4.2 Centrifuge Model Tests of Tunnelling-Induced Pile Responses 33 2.3.Mechanical Equipments 16 Tunnelling-Induced Soil Movements 17 2.2.3.1.2 Limitations of Model Tunnel 73 3.4.1 Model Tunnelling Technique 70 3.3 Predictive Methods of Tunnelling-Induced Pile Responses 36 Summary 39 CHAPTER 3 EXPERIMENTAL SET-UP AND PROCEDURE 67 3. 1 High Resolution Camera 80 3.8 Pore Pressure Transducers (PPT) 79 3.3. 2) 104 vi .1 Preparation of The Soil Sample 84 3.3.4 3.4.4.3 Installation of Model Tunnel and PPTs At 1g 86 3.1 Introduction 102 4.5 Kaolin Clay 77 3.5.7 Potentiometer 78 3.5.4.2 Lighting System 81 3.4 Post-Processing of Images 82 3.9 Non-Contact Laser Transducers 80 Image Acquisition System 80 3.4.3.4 Strong Box 77 3.2 Test Program 102 4.4.3.5 Installation of Model Pile at 1g 87 3.5.2 Pre-Consolidation Process 85 3.5.5 Assessment of Effectiveness of Image Processing System 83 Experimental Procedure 84 3.5.3 On-Board and Command Computers 81 3.3.6 Toyoura Sand 78 3.3 Tunnelling-Induced Soil Movements (Tests 1.4 Preparation Works for PIV Analysis 87 3.5.5 3.3.6 Test Procedure 88 CHAPTER 4 BASIC TESTS ON VOLUME LOSS 102 4.3. 2 Soil Surface Settlement Troughs 105 4. 4) 118 4.Effects of Pile Tip & Head Conditions (Tests 3.Effects of Pile Length (Tests 3.5.1 Induced Axial Force and Settlement 118 4.6.5 4.6 4.2.1 Test Series 4 .3.3 Test Series 3 .1 Effects of Pile Tip Conditions 151 5.Effects of Volume Loss (Tests 3.4.5.3.4.2 Test Series 2.6.1 Introduction 151 5.4 Subsurface Horizontal Soil Movements 110 4. 8) 157 5.3. 7.2 Tunnel-Soil-Piles Interaction 123 CHAPTER 5 EFFECTS OF TUNNELLING ON SINGLE PILES 151 5. 5.3 Subsurface Vertical Soil Movements 107 4.Free-Head Floating Piles (Tests 3.3.1 Tunnelling-Induced Soil Movements 122 4. 16. 10.2 Induced Bending Moment and Deflection 115 Test Series 1.4 Effects Of Distance of Pile From Tunnel 162 5. 6) 162 vii .4 4.1 Cumulative Soil Movements 104 4.3. 9.4.2 Induced Bending Moment and Deflection 119 Concluding Remarks 122 4.2 Effects of Pile Head Conditions 154 5.5 Qualitative Assessment On Excess Pore Pressure Response 111 Typical Tunnel-Soil-Piles Interactions (Test 3) 112 4.2. 13) 151 5.1 Induced Axial Force and Settlement 113 4.4. 4.6.1 Capped-Head 215 6.1 Similarities (Tunnel-Soil Interaction) 173 5.1 Tunnel-Soil Interaction 172 5.2.2 Fixed-Head 218 Pile Group Size 220 6.1 Induced Axial Force and Settlement 210 6.4 viii .2 Tunnel-Pile Interaction 5.2 Floating Pile Group 209 6.6.6. 14A. 11.4.Free-Head End-Bearing Piles (Tests 10.5.2.2 Differences (Tunnel-Pile Interaction) 175 Concluding Remarks CHAPTER 6 EFFECTS OF TUNNELLING ON PILE GROUPS 177 208 6.4. 5 And 6 167 5.3.Fixed-Head End-Bearing Piles (Tests 13.4 Comparison of Results from Test Series 4.2 Induced Bending Moment and Deflection 211 End-Bearing Pile Group 215 6.2 Test Series 5 . 14B) 166 5.2.5 Effects of Time on Pile Responses in Soft Clay 171 5.3 6.1.3.2.6.1 Similarities (Tunnel-Pile Interaction) 175 5.1.1 Introduction 208 6.1 Capped-Head 220 6.4.2 Differences (Tunnel-Soil Interaction) 174 5.6.7 174 5.6 Comparison of Soil and Single Pile Behaviours due to Inward and Outward Tunnel Deformations 172 5.6. 12) 165 5.3 Test Series 6 . 1.2 Fixed-Head 225 Pile Cap Conditions 227 6.3 Tunnel-Single Piles Interaction 283 7.1 2-Pile Group 227 6.4.6 6.1.1.2 CONCLUSIONS 281 Concluding Remarks 281 7.1 7.5.4 Tunnel-Pile Groups Interaction 286 Recommendations for Future Studies 288 REFERENCES 290 ix .5.5 6.1 Technique for Simulation of Tunnelling 282 7.2 Tunnel-Soil Interaction 283 7.1.2 6-Pile Group 229 Concluding Remarks 233 CHAPTER 7 7.6. Hence. Phase 2 of the study was conducted to study the tunnelling-induced single pile lateral and axial responses in both short. pile length and pile-to-tunnel distance were examined. The effects of factors such as volume loss. centrifuge modelling emerges as an attractive alternative option to investigate the effects of tunnellinginduced soil movement on adjacent piles. soil settlement is noted to be more dominant than lateral soil movement. It is found that the surface settlement trough in clay generally follows the Gaussian distribution curve in the short-term. In the long-term. It is challenging to carry out extensive instrumentation and monitoring in the field to observe the pile responses due to tunneling activities. The data confirmed that the empirical equation proposed by Mair et al (1993) is applicable in the prediction of the subsurface settlement troughs in clay in the short-term. pile tip and head condition. The magnitude of maximum ground surface settlement increases with time and tunnel volume loss. x . Phase 1 of the study was performed to investigate the free-field soil movements due to tunneling. In the short-term. a modeling technique was developed to simulate the inward tunnel deformation due to over-excavation of tunnel. an “Immediate Shear Zone” with large soil movement above the tunnel can be identified. In the present study. the settlement trough is wider and hence the differential settlement at the ground surface is not as serious as compared to that in the short-term.and long-term. while the zone outside the immediate shear zone is identified as “Support Zone”. Though the settlement magnitude is larger in the long-term.SUMMARY Tunnels are often constructed close to existing pile foundation in dense urban areas. pile head and toe conditions. The results reveal that soil and pile responses increase over time with long-term over short-term pile responses ratio ranging from 1. The pile-cap-pile interaction in a capped-head 6-pile group would moderate the induced pile bending moments among the piles within a pile group. clay. settlement. It was noted that tensile force and relatively large negative bending moments are induced at the pile head due to total fixity resulting in a reduction in drag load. axial force. the piles behaved like single piles standing side by side in terms of axial force and bending moment.It was found that a floating pile is mainly governed by pile settlement and a socketed pile is likely governed by its material stress when tunneling was carried out adjacent to it.32 to 2. A common trend was observed for the long-term over short-term ratio of pile responses for both single pile and pile group. regardless of pile size. For the piles in a totally fixed-head 6-pile group. bending moment. It was found that the pile group is generally beneficial as the average pile responses of a pile group due to tunneling are smaller than the average of those of single piles at the same locations. Keywords: tunnel. except that the magnitude is affected by the total number of piles in the group.4. pile. and positive bending moment at the mid-pile shaft. deflection. interaction. pile cap and pile tip condition. centrifuge modelling xi . The centrifuge model study was subsequently extended to pile groups to evaluate the effects of number of piles. It was noted that the induced pile bending moments in the middle row is smaller than that of rear row which is contrary to the induced lateral soil movements at the respective locations. 1991) Table 3. 2005) Table 4. al.2 Physical properties of Malaysian kaolin clay (After Goh.1 Scaling relation of centrifuge modeling (After Leung et at.LIST OF TABLES Table 3.1 Test program and parameters for the basic tests on volume loss Table 5.1 Test program and prototype parameters in Phase 2 study Table 6.1 Test program and prototype parameters for pile group tests xii .3 Physical properties of Toyoura sand (After Teh et. 2003) Table 3. (b) Tunnelling adjacent to pile foundation.6 Sand pouring in process during model preparation with model tunnel infilled with water (After Jacobsz..mechanical equipment of shield model machine (After Yasuhiro et al..3 Simulation technique of tunnelling .mechanical equipment of miniature shield tunneling machine (After Nomoto et al.2 Simplified tunnel lining deformation with time by simulation technique of tunnelling using high density polystyrene foam (After Ran.1 Simulation technique of tunnelling using high density polystyrene foam (After Sharma et al. 2000) Figure 2. 2003) Figure 2.8 Simulation technique of tunnelling . 1994) Figure 2.applying compressed air (After Grant & Taylor. 2006) Figure 2. 2004) Figure 2.LIST OF FIGURES Figure 1. 2002) Figure 2. 1998) Figure 2.1 Pile responses induced by tunnel construction: (a) Tunnelling under pile foundation.7 Strong-box with two openings to fix the model tunnel in place (After Jacobsz.9 Simulation technique of tunnelling . 2002) Figure 2.11 Gaussian curve approximating transverse surface settlement trough for MRT project C852. Figure 1. Singapore (After Cham.model tunnel infilled with oil (After Loganathan et al.4 Simulation technique of tunnelling . 2007) xiii .mechanical model tunnel used to simulate the tunnel volume loss by decreasing the diameter of model tunnel under 1g (After Lee and Yoo. 2002) Figure 2.2 Pile foundations supported existing buildings normally designed to resist compression load only. Figure 2. 2000) Figure 2.model tunnel infilled with water (After Jacobsz..10 Simulation technique of tunnelling . 2001 and Feng.5 Simulation technique of tunnelling .. . (b) 30mm below ground level.18 The ratio of the maximum immediate settlement to maximum longterm settlement for Shanghai Metro Tunnel No.19 Normalized post-construction surface settlement troughs due to consolidation of soft clay (After Shirlaw. 1991) Figure 2. 1989) Figure 2..26 Ground surface settlement trough over time from a typical test (After Ran. (c) 70mm below ground level. 2005) Figure 2. but stabilized after 15 days of tunnel excavation (After Emeriault et al.14 Variation in surface settlement trough width parameter with tunnel depth for tunnels in clay (After Lake et al. 1987) Figure 2. 1969) Figure 2. 1993) Figure 2.Figure 2.27 Normalized Vertical and horizontal soil movement profile at different subsurface elevations with best-fit curves: (a) 10mm below ground level. (d) 100mm below ground level. 2004) Figure 2. 1989) Figure 2.5Z to a final value in excess of 4Z in long-term (After O’Reilly et al.1991) Figure 2. 1994) Figure 2. 2004) Figure 2. 2006) Figure 2. 1995) Figure 2. (After Grant and increased xiv .17 The maximum surface settlements at Grimsby significantly over the time (After O’Reilly et al. 1992) Figure 2.15 Variation of trough width parameter K with depth for subsurface settlement profiles above tunnels in clay (After Mair et al. 2003) and clay (After Ran.13 Gaussian curve approximating transverse surface settlement trough (After Peck..16 Initial settlement trough at Grimsby increased from 1.21 Changes in Pore pressure for Shanghai Metro (After Schmidt.20 Estimated trend of excess pore pressure in normally consolidated clay surrounding the tunnel (After Schmidt. 2004) Figure 2.2 (After Zhang et al.22 Change in pore pressure measured at Thunder Bay Sewer Tunnel (Adapted from data in Ng et al.25 Comparisons of surface settlement troughs in sand (After Feng.12 Definition of parameters controlling tunnelling-induced settlement trough ( After Standing & Burland.24 Horizontal soil movement for the Singapore’s effluent outfall pipeline in tunnel (After Balasubramanian.23 Transverse movements in Toulouse subway line B were significantly increased over time. 1986) (After Shirlaw et al. 2000) Figure 2. 2004) xv .42 Variations of (a) induced pile bending moment profiles and (b) induced pile lateral deflection profiles with time in typical test (After Ran. but stabilized after 15 days of tunnel excavation (After Emeriault et al. 2000) Figure 2. 2007) Figure 2. 2005) Figure 2..39 Configuration of centrifuge tests (After Loganathan et al. 2005) Figure 2.33 Transverse movements in Toulouse subway line B were significantly increased over time. 1992) Figure 2.37 Responses of pile foundation in terms of (a) axial force and (b)bending moment for MRT North East Line Contract 704 in Singapore (After Pang et al... 2000) Comparisons of measured surface settlement and analytical solutions (After Loganathan et al.Figure 2. 2006a) Figure 2.2005) Figure 2. 1998) Figure 2... (After Cham. 2000) Figure 2. 2005) Figure 2..29 Plastic deformation mechanism for tunnels in clay (After Osman et al.38 Illustration of positions of existing instrumented piles relative to tunnels for MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore...36 Post-tunneling measurement of the development of axial force in pile P1 at Pier 20 showing the time-dependent behaviour of dragload MRT North East Line Contract 704 in Singapore (After Pang et al.28 Taylor.32 Boundary conditions of prescribed displacement (After Park.41 Tunneling-induced pile axial loads (After Loganathan et al.. 2000) Figure 2. 2005) Figure 2.31 Oval-shaped soil displacement around tunnel boundary (After Loganathan and Poulos.30 Definition of GAP parameter (After Lee et al.40 Tunneling-induced pile bending moments (After Loganathan et al.34 A piled bridge pier foundation assessed during the CTRL project (After Jacobsz et al..35 Typical section and instrumentation layout for pile-tunnel interaction study for MRT North East Line Contract 704 in Singapore (After Pang et al. 2005) Figure 2. 10 Image acquisition system Figure 3.43 Figure 2. 1999) Figure 2.9 In-flight undrained shear strength of clay Figure 3.5 Longitudinal view of model tunnel set up Figure 3. rotation and load distribution on triple pile group (After Jacobsz et al. 2005b) Figure 3.8 Model pile caps Figure 3.12 Picture captured by JAI ©CV-A2 progressive scan camera for PIV analysis xvi . 2005b) Figure 2. 2005) Figure 2...Figure 2..3 Sketch of a typical centrifuge model package (All dimensions in mm) Figure 3..11 On board set-up Figure 3.45 Settlement. 1999) Figure 2..50 Prediction of responses of pile foundation in terms of axial force and bending moment using 3D finite element analysis (After Pang et al. 2005) Figure 2..1 Schematic diagram of NUS geotechnical centrifuge Figure 3.48 Numerical analysis of pile-group responses due to tunnelling (After Loganathan et al.2 Photograph of NUS geotechnical centrifuge with the model package mounted on the platform Figure 3.4 Photograph of a typical centrifuge model package Figure 3. 2001) Figure 2.47 Tunneling-induced pile responses and Greenfield soil movement (After Chen et al. 2004) Zone of influence around tunnel in which potential for large pile settlements exists (After Jacobsz et al.6 Cross-section of model tunnel Figure 3.46 Layout of basic problem (After Chen et al.44 (a) Induced pile axial force profile and (b) pile settlement profile at 2 days in typical test (After Ran.7 Instrumented model pile (All dimensions in mm) Figure 3..49 Typical 3D finite elements mesh to back-analyse a case history on the response of pile foundation subjected to shield tunnelling (After Pang et al. 14 Evaluation of displacement vector from correlation plane..15 Experimental set-up for assessment of effectiveness of image processing system and comparison of performance of flocks and beads Figure 3.4 (b) Vectors and contour plots of soil movements after 180 days (Test 2) Figure 4.4 (d) Vectors and contour plots of soil movements after 720 days (Test 2) Figure 4.21 Set-up of the entire model package in 1g (top view) Figure 4..4 (c) Vectors and contour plots of soil movements after 360 days (Test 2) Figure 4.Figure 3.3 (d) Vectors and contour plots of soil movements after 720 days (Test 1) Figure 4. 2003) Figure 3.16 Results of assessment of effectiveness of image processing system and comparison of performance of flocks and beads Figure 3. Rn(s): (a) correlation of Rn(s).18 Estimation of ultimate settlement by Asaoka’s method (1978) Figure 3.3 (a) Vectors and contour plots of soil movements after 2 days (Test 1) Figure 4.17 Pore pressure dissipation and settlement during consolidation stage Figure 3.1 Schematic of viewing area in tunnel-soil interaction tests (all dimensions in mm) Figure 4.3 (c) Vectors and contour plots of soil movements after 360 days (Test 1) Figure 4. (After White et al.13 Image manipulation during PIV analysis.4 (a) Vectors and contour plots of soil movements after 2 days (Test 2) Figure 4.5 Surface settlement troughs over time (Test 1) xvii . (After White et al. (c) sub-pixel interpolation using cubic fit over ± 1 pixel of integer correlation.3 (b) Vectors and contour plots of soil movements after 180 days (Test 1) Figure 4. (b) highest correlation peak (integer pixel).20 The Perspex window is highly greased to ensure free movement of soil Figure 3.19 Different colours of beads were randomly embedded on the surface of clay Figure 3. 2003) Figure 3.2 Example of digital images taken during test for PIV analysis Figure 4. 18 (b) Tunnelling-induced pile bending moment (Pile BP2-E) for MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore (After Cham.8 Settlement troughs at surface. PIV) Figure 4.Figure 4.11 Comparison of ratio of iLT/iST at different depths (Tests 1 & 2) Figure 4.Test 2 Figure 4.and long-term (Test 2) Figure 4. 3% free-head floating long pile) Figure 4. 2007) Figure 4.3m depths (Test 1): (a) comparing with Mair et.17 Tunnelling-induced (a) maximum pile axial force (b) maximum pile head settlement and soil surface settlement (Test 1) (c) maximum pile bending moment (d) maximum pile head deflection and soil surface lateral movement (Test 1) Figure 4.16 Tunnelling-induced pile head settlement (Test 3) and observed freefield soil movement at pile location (Test 1.Test 1 Horizontal soil movements at different distance from tunnel centerline at 2 and 720 days .19 Tunnelling-induced pile deflection (Test 3) and free-field lateral soil movement at pile location (Test 1) xviii . 3% free-head floating long pile) Figure 4.15 (a) Tunnelling-induced pile axial force (Test 3. 5m and 10.14 Pore pressure changes due to tunnelling (Test 1) Figure 4.12 Horizontal soil movements at different distance from tunnel centerline at 2 and 720 days .3m and 9.13 Figure 4.7 Maximum surface settlements over time (Tests 1 & 2) Figure 4.6 Surface settlement troughs over time (Test 2) Figure 4.10 Distribution of inflection point ‘i’ with depth in short. 4.15 (b) Tunnelling-induced pile axial force (Pile BP1-G) for MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore (After Cham. al (1993) (b) comparing with Loganathan and Poulos (1998) Figure 4.9 Settlement troughs at surface.9m depths (Test 2): (a) comparing with Mair et. al (1993) (b) comparing with Loganathan and Poulos (1998) Figure 4. 2007) Figure 4.18 (a) Tunnelling-induced pile bending moment (Test 3. 7 and 8) Figure 5. 9. 10 and 13) Figure 5. 10 and 13) Figure 5. 7 and 8) Figure 5.24 Variation of (a) maximum pile axial force (b) pile head settlement (c) pile bending moment (d) pile head deflection with volume loss (Tests 3 and 4) Figure 4.5 Variation of (a) maximum pile axial force (b) pile head settlement (c) pile bending moment (d) pile head deflection with tip and head conditions (Tests 3. 10 and 13) Figure 5. 7 and 8) xix .9 Variation of pile head deflection and free. 9.21 Variation of pile head settlement (Tests 3 and 4) and observed freefield soil movement at pile location (Tests 1 and 2) with volume loss Figure 4.8 Variation of pile bending moment with pile length (Tests 3. 9.20 Variation of pile axial force with volume loss (Tests 3 and 4) Figure 4. 7 and 8) Figure 5.1 Pile base position investigated in the parametric studies (not to scale) Variation of pile axial force with tip condition in (a) Short-term (b) Long-term (Tests 3.6 Variation of pile axial force with pile length (Tests 3.22 Variation of pile bending moment with volume loss (Tests 3 and 4) Figure 4.field lateral soil displacement (Test 1) with pile length (Tests 3. 7 and 8) Figure 5.4 Variation of pile deflection with tip condition (a) Short-term (b) Long-term (Tests 3. 7 and 8) Figure 5.2 Figure 5.10 Variation of (a) maximum pile axial force (b) pile head settlement (c) pile bending moment (d) pile head deflection with normalized pile length over tunnel depth (Tests 3.Figure 4.23 Variation of pile deflection profiles (Tests 3 and 4) and observed free-field lateral soil movement at pile location (Tests 1 and 2) with volume loss Figure 4.3 Variation of pile bending moment with tip condition (a) Short-term (b) Long-term (Tests 3. 10 and 13) Figure 5. 9.25 Long-term to short-term ratio of pile responses for different volume losses (Tests 3 and 4) Figure 5.7 Variation of pile head settlement and soil settlement profile (Test 1) with pile length (Tests 3.11 Short pile to long pile ratio of pile responses for different pile length over tunnel depth (Tests 3. 12. 6.25 Comparison of (a) ovalisation of tunnel lining by Ran (2004). 11 and 12) Figure 5. and (b) over-cut of tunnel in the present study Figure 5.19 Variation of pile bending moment for fixed-head end bearing piles with pile-to-tunnel distance (Tests 13. 5 and 6) Figure 5. 4.22 Assessment of pile responses for different pile-to-tunnel distance (Tests 3. 9. 14A and 14B) Figure 5. 5. 5 and 6) Figure 5.18 Variation of pile deflection for free-head end bearing piles with pileto-tunnel distance (Tests 10. 5 and 6) Figure 5.12 Variation of pile axial force with pile-to-tunnel distance for freehead floating piles (Tests 3.14 Figure 5. 5 and 6) and free-field soil settlement (Test 1) with pile-to-tunnel distance Variation of pile bending moment for free-head floating piles with pile-to-tunnel distance (Tests 3.17 Variation of pile bending moment for free-head end bearing piles with pile-to-tunnel distance (Tests 10. 11 and 12) Figure 5. 5 and 6) Figure 5.Figure 5. 14B and 16) Figure 5. 2004) xx . 5 and 6) and free-field lateral soil displacement profile (Test 1) with pile-to-tunnel distance Figure 5.16 Variation of pile axial force for free-head end bearing piles with pile-to-tunnel distance (Tests 10. 14A. 5 and 6 with pile-to-tunnel distance Figure 5. 11.21 Variation of (a) maximum pile axial force (b) maximum pile head settlement and soil surface settlement (Test 1) (c) pile bending moment (d) maximum pile head deflection for Test Series 4. 8. 10.23 Lateral soil displacement profiles at different pile-to-tunnel distance (Tests 3.13 Variation of pile head settlement for free-head floating piles (Tests 3.26 Simplified tunnel lining ovalisation with time (not to scale) (Test 1) (after Ran. 7. 14A and 14B) Figure 5. 11 and 12) Figure 5.20 Variation of pile deflection for fixed-head end bearing piles with pile-to-tunnel distance (Tests 13. 13.24 Long-term to short-term ratio of pile responses for all tests (Tests 3.15 Variation of pile deflection for free-head floating piles (Tests 3. 3.6 Tunnelling-induced front pile (Test PG1) and corresponding single pile (Test 3) bending moment Figure 6. 16) Figure 6.29 Variation of maximum surface soil settlement at tunnel central line with tunnel deformation Figure 5.5 Tunnelling-induced pile bending moment (Test PG1) Figure 6.37 Long-term to short-term ratio of pile responses over time for different tunnel deformation Figure 6.27 Development of subsurface soil movements at (a) 2 days and (b) 720 days after tunnel excavation (after Ran.4 Tunnelling-induced pile head settlement (Tests PG1.31 Variation of soil deflection at pile location with tunnel deformation Figure 5.33 Variation of pile head settlement with tunnel deformation Figure 5.Figure 5.2 Tunnelling-induced front pile (Test PG1) and corresponding single pile (Test 3) axial force Tunnelling-induced rear pile (Test PG1) and corresponding single pile (Test 16) axial force Figure 6.35 Variation of tunnelling-induced maximum pile bending moment over time for different tunnel deformation Figure 5.3 Figure 6.8 Tunnelling-induced pile deflection (Tests PG1) Figure 6.28 Variation of surface soil settlement troughs with tunnel deformation Figure 5.7 Tunnelling-induced rear pile (Test PG1) and corresponding single pile (Test 16) bending moment Figure 6.1 Tunnelling-induced pile axial force (Test PG1) Figure 6.32 Variation of pile axial force with tunnel deformation Figure 5. 2004) Figure 5.36 Variation of pile deflection with tunnel deformation Figure 5.34 Variation of pile bending moment with tunnel deformation Figure 5.9 Tunnelling-induced front pile (Test PG1) and corresponding single pile (Test 3) deflection xxi .30 Variation of vertical soil settlement at pile location with tunnel deformation Figure 5. 19 Tunnelling-induced rear pile (Test PG2) and corresponding single pile (Test 11) bending moment Figure 6.22 Tunnelling-induced rear pile (Test PG2) and corresponding single pile (Test 11) deflection Figure 6.28 Tunnelling-induced pile bending moment (Test PG3) Figure 6.18 Tunnelling-induced front pile (Test PG2) and corresponding single pile (Test 10) bending moment Figure 6.21 Tunnelling-induced front pile (Test PG2) and corresponding single pile (Test 10) deflection Figure 6.11 Tunnelling-induced pile head deflection (Test PG1.12 Single pile over pile group ratio for front pile (Test 3/ PG1) Figure 6.29 Tunnelling-induced front pile (Test PG3) and corresponding single pile (Test 13) bending moment xxii .27 Tunnelling-induced front pile (Test PG3) and corresponding single pile (Test 13) axial force Figure 6.25 Single pile over pile group ratio for rear pile (Test 11/ PG2) Figure 6.10 Tunnelling-induced rear pile (Test PG1) and corresponding single pile (Test 16) deflection Figure 6.24 Single pile over pile group ratio for front pile (Test 10/ PG2) Figure 6.26 Tunnelling-induced pile axial force (Test PG3) Figure 6.Figure 6.14 Tunnelling-induced pile axial force (Test PG2) Figure 6.23 Tunnelling-induced pile head deflection in the (a) short-term (b) long-term (Tests PG2. 3 & 16) Figure 6.17 Tunnelling-induced pile bending moment (Test PG2) Figure 6. 10 & 11) Figure 6.13 Single pile over pile group ratio for rear pile (Test 16/ PG1) Figure 6.16 Tunnelling-induced rear pile (Test PG2) and corresponding single pile (Test 11) axial force Figure 6.15 Tunnelling-induced front pile (Test PG2) and corresponding single pile (Test 10) axial force Figure 6.20 Tunnelling-induced pile deflection (Test PG2) Figure 6. Figure 6.39 Tunnelling-induced pile deflection in the (a) short-term (b) longterm (Test PG4) Figure 6.31 Single pile over pile group ratio for front pile (Test 13/ PG3) Figure 6.30 Tunnelling-induced rear pile (Test PG3) and corresponding single pile (Test 14A) bending moment Figure 6.42 2-pile over 6-pile group ratio for front pile (Test PG2/PG4) Figure 6.38 Tunnelling-induced rear pile in 2-pile group (Test PG2) and corresponding middle pile in 6-pile group (Test PG4) bending moment Figure 6.47 Tunnelling-induced pile bending moment in the (a) short-term (b) long-term (Test PG5) Figure 6.40 Tunnelling-induced pile deflection (Tests PG2 and PG4) Figure 6.44 Tunnelling-induced pile axial force (Test PG5) Figure 6.36 Tunnelling-induced pile bending moment (a) in the short-term (b) in the long-term (Test PG4) Figure 6.46 Tunnelling-induced rear pile in 2-pile group (Test PG3) and corresponding middle pile in 6-pile group (Test PG5) axial force Figure 6.45 Tunnelling-induced front pile in 2-pile group (Test PG3) and corresponding front pile in 6-pile group (Test PG5) axial force Figure 6.34 Tunnelling-induced front pile in 2-pile group (Test PG2) and corresponding front pile in 6-pile group (Test PG4) axial force Figure 6.35 Tunnelling-induced rear pile in 2-pile group (Test PG2) and corresponding middle pile in 6-pile group (Test PG4) axial force Figure 6.43 2-pile over 6-pile group ratio for middle pile (Test PG2/PG4) Figure 6.37 Tunnelling-induced front pile in 2-pile group (Test PG2) and corresponding front pile in 6-pile group (Test PG4) bending moment Figure 6.33 Tunnelling-induced pile axial force (Test PG4) Figure 6.48 Tunnelling-induced front pile in 2-pile group (Test PG3) and corresponding front pile in 6-pile group (Test PG5) bending moment xxiii .41 Tunnelling-induced pile bending moment (Tests PG2 and PG4) Figure 6.32 Single pile over pile group ratio for rear pile (Test 14A/ PG3) Figure 6. 59 Tunnelling-induced middle pile in capped-head 6-pile group (Test PG4) and corresponding middle pile in fixed-head 6-pile group (Test PG5) axial force Figure 6.60 Tunnelling-induced rear pile in capped-head 6-pile group (Test PG4) and corresponding rear pile in fixed-head 6-pile group (Test PG5) axial force Figure 6.53 Tunnelling-induced rear pile in capped-head 2-pile group (Test PG2) and corresponding rear pile in fixed-head 2-pile group (Test PG3) axial force Figure 6.51 2-pile over 6-pile group ratio for middle pile (Test PG3/PG5) Figure 6.55 Tunnelling-induced rear pile in capped-head 2-pile group (Test PG2) and corresponding rear pile in fixed-head 2-pile group (Test PG3) bending moment Figure 6.50 Tunnelling-induced rear pile in 2-pile group (Test PG3) and corresponding middle pile in 6-pile group (Test PG5) bending moment 2-pile over 6-pile group ratio for front pile (Test PG3/PG5) Figure 6.57 Capped-head pile over fixed-head pile ratio (rear pile. Test PG2/PG3) Figure 6.Figure 6.61 Tunnelling-induced front pile in capped-head 6-pile group (Test PG4) and corresponding front pile in fixed-head 6-pile group (Test PG5) bending moment Figure 6.58 Tunnelling-induced front pile in capped-head 6-pile group (Test PG4) and corresponding front pile in fixed-head 6-pile group (Test PG5) axial force Figure 6.49 Figure 6.54 Tunnelling-induced front pile in capped-head 2-pile group (Test PG2) and corresponding front pile in fixed-head 2-pile group (Test PG3)bending moment Figure 6.56 Capped-head pile over fixed-head pile ratio (front pile. Test PG2/PG3) Figure 6.52 Tunnelling-induced front pile in capped-head 2-pile group (Test PG2) and corresponding front pile in fixed-head 2-pile group (Test PG3) axial force Figure 6.62 Tunnelling-induced middle pile in capped-head 6-pile group (Test PG4) and corresponding middle pile in fixed-head 6-pile group (Test PG5) bending moment xxiv . 4 & 5) xxv . middle and rear pile in the short-term (Tests PG4 and PG5) Variation of maximum bending moment for front.64 Variation of maximum bending moment for front.66 Capped-head pile over fixed-head pile ratio (front pile.65 Figure 6.69 Long-term over short-term ratio (Tests PG1. Test PG4/PG5) Figure 6. 3. Test PG4/PG5) Figure 6. Test PG4/PG5) Figure 6.Figure 6.67 Capped-head pile over fixed-head pile ratio (middle pile. middle and rear pile in the long-term (Tests PG4 and PG5) Figure 6.63 Tunnelling-induced rear pile in capped-head 6-pile group (Test PG4) and corresponding rear pile in fixed-head 6-pile group (Test PG5) bending moment Figure 6.68 Capped-head pile over fixed-head pile ratio (rear pile. 2. 8 m/s2) Gs Specific gravity H Tunnel depth i Point of inflection k Coefficient of permeability K Trough width parameter L Pile length S Soil settlement induced by tunneling Smax Maximum ground surface settlement U3D Parameter accounting for three dimensional heading effects (Gap Parameter Method) V Tunnel Volume loss X Horizontal distance between pile and tunnel centre Z Depth below ground surface Zo Depth to tunnel axis level φ’ Soil effective friction angle φcrit Critical state friction angle γ’ Effective unit weight of soil γw Unit weight of water ρ min Minimum dry density ρ max Maximum dry density xxvi .NOMENCLATURE C Tunnel cover Cu Undrained shear strength of clay Cv Coefficient of consolidation D Tunnel diameter EA Pile axial rigidity EI Pile flexural rigidity g Gravitational acceleration (9. z Ground loss with horizontal and vertical distance ε0 Ground loss ratio M Slope of critical state line in p’-q space λ Slope of isotropic compression line in p’-v space κ Slope of swelling line in p’-v space Abbreviations LL Liquid Limit LT Long-term NP Neutral Plane NSF Negative Skin Friction NUS National University of Singapore PI Plasticity Index PIV Particle Image Velocimetry PPT Pore Pressure Transducer PL Plastic Limit ST Short-term VL Volume loss xxvii .εx. the foundations may not be safe to resist the induced loads due to tunnelling. As pile foundations supporting existing buildings are generally designed to resist compression load only (Figure 1. impacts in the long-term are still not well understood. it is increasingly complex and challenging to build tunnels underground as the tunnels almost inevitably run close to or underneath some piled foundations supporting existing buildings.2). Ground movements due to tunnelling would induce additional axial (settlement and axial force) and lateral (deflection and bending moment) responses on adjacent pile foundations (Figure 1. Tunnel excavations generally cause ground settlement and deformation nearby. A 1 . many structures exist long before the tunnels are planned.Chapter 1 Introduction CHAPTER ONE INTRODUCTION 1. especially in soft clay. Although many studies have been conducted to investigate tunnel-soil-pile interactions in the short-term. At present.1). Unfortunately. 2001. et al. 1999. 2005b) to assess induced pile responses due to tunnelling.. the need for underground transportation and utility tunnels have greatly increased. As cities develop rapidly. Loganathan et al. Pang et al. tunnels are often constructed close to existing buildings due to space constraints. As such... there are very few reliable design methods (Chen at al.1 BACKGROUND In urban areas. Jacobz et al.. (O’Reilly et al. deflection.. axial and lateral loads on adjacent piles in the long-term. 2004) conducted at the National University of Singapore (NUS) also revealed that for tunnelling in clay.2 TUNNELLING-INDUCED SOIL MOVEMENTS Many research studies have been carried out to investigate tunnelling-induced soil movements. Mair et al. Loganathan et al. 2004. Ran. The results of centrifuge model tests (Ran. Peck (1969). physical modelling can be an attractive mean to study the tunnel-soil-pile interaction problem in both short-term and long-term. (1992). 2004). 2 . centrifuge tests can provide flexibility and repeatability to study tunnel-soil-pile interaction problems that could not be achieved in field tests (Mair et al. UK. 2000. 1984. several centrifuge model studies including Loganathan (2000) and Ran (2004) were conducted to examine soil movements due to tunnelling. 1. In view of the complexity of field instrumentation and monitoring. O’Reilly and New (1982). One effective way is to conduct centrifuge model tests employing artificial gravitational field to replicate the prototype stress level as experienced by the ground in the field. Under a well-controlled environment.Chapter 1 Introduction published field study covering an 11-year period of post-tunnelling monitoring for the Haycroft Relief Sewer in Grimsby. (1993) and others developed empirical formulae from field studies to predict the soil movements induced by tunnelling. Schmidt (1969).. Lake et al. In addition. 1991) in very soft clay recorded that the soil settlement in the long-term had increased significantly. thus inducing further settlement.. the ground continues to deform long after the completion of tunnelling. Haycroft Relief Sewer in Grimsby. some valuable field measurements in limited cases have been made available where the existing piles were instrumented prior to tunnel construction. However. Many researchers reported field measurement results for soil movements induced by tunnelling. 1991) and Shanghai Metro Tunnel No. Loganathan and Poulos (1998). 3 . most of these studies did not report field measurements during the post-tunnelling period despite some reports on significant long-term settlements after tunnelling in soft clay. However.Chapter 1 Introduction Besides empirical formulae and centrifuge model studies. (O’Reilly et al. Pang et al. This is because the piles usually exist long before the tunnels are constructed and it is almost impossible to install strain gauges in the existing piles to monitor the pile responses. However. (2006a) to predict the ground displacements for various shapes of tunnel deformation.3 EFFECTS OF TUNNELLING ON PILES It is generally not viable to monitor the responses of existing piles due to tunnelling. the long-term tunnel-soil interaction in soft clay clearly needs further investigation. see Selemetas et al.2. However. (Zhang et al. UK. 1. China. Jacobsz et al. (2005) and Cham (2007). (2005). Verrujit and Booker (1996). it is evident that the mechanism and calculation of tunnelling-induced soil movements in the short-term are reasonably well studied.. 2004). in particular the time effects. such analytical solutions cannot account for all aspects. see for example. Park (2005) and Osman et al.. (2005). From the above review. analytical solutions have been developed by researchers including Sagaseta (1987). However. Feng.. the wishin-place model tunnel is a simplification and idealization of a cavity contraction. and lack of predictive methods available to evaluate the effects of pile group or soil-structure interaction. the objectives of this research are:- 1. 2004 and Jacobsz et al. but with an oval-shaped GAP and well controlled volume loss to simulate the soil movements induced by tunnelling when the tunnel excavation has passed a particular section. 2003. 2005) and numerical analysis and analytical solutions (Chen et al. Two examples of major gap include limited field studies on long-term tunnel-soil-pile interaction.... 1999. The minimum volume loss that 4 ..Chapter 1 Introduction Although a good number of centrifuge model studies (Loganathan et al. More specifically.4 OBJECTIVE AND SCOPE OF STUDY The main aim of the present study is to investigate tunnel-soil-pile interaction in soft clay. 1. 2000. Ran. To develop a simulation technique of tunnel excavation associated with inward tunnel deformation pattern using centrifuge modelling technique. 2005) have been attempted to investigate the effects of tunnelling on piles. 2001 and Pang et al. the results of literature review presented in Chapter 2 reveal that there are still major gaps for a better understanding of pile responses due to tunnelling. This pattern is chosen because it is often observed in practice. The development of a qualitative and quantitative framework to assess potential impact of tunnel construction on existing piled foundations is needed to improve the current knowledge on the prediction of pile responses due to tunnelling. Loganathan et al. parametric studies were performed to evaluate the effects of various tunnel volume losses. Finally. pile distances to tunnel and pile groups. b) Chapter 3 describes the present centrifuge model set-up and experimental 5 . To study tunnelling-induced soil movement in free-field experimentally with subsequent Particle Image Velocimetry (PIV) analysis. Subsequently.5 STRUCTURE OF THESIS This thesis consists of seven chapters and the contents of subsequent chapters are briefly described as follows: a) Chapter 2 reviews the existing literature on to tunnel-soil-pile interaction. existing research studies concerning pile responses due to tunnelling are highlighted. In addition. 3. Longterm post-tunnelling pile performance was also studied. ground movements caused by different methods of tunnel excavation are examined. Longterm post-tunnelling soil movement was also studied. Firstly. The review is divided into three parts. pile tip conditions.Chapter 1 Introduction can be simulated by the present model tunnel is only 3% in order to maintain the accuracy of the experiment results. pile lengths. The vertical (axial force and settlement) and lateral (bending moment and deflection) pile responses were examined. 1.” 2. The purpose of this investigation is to examine the mechanism of tunnel-soil interaction. various methods of simulating tunnel excavation in centrifuge tests are reviewed. To study the induced pile responses due to tunnelling experimentally. The test results on the basic tests of volume loss for tunnelling on single piles are then presented in detail. c) Chapter 4 first presents the experimental results on tunnelling induced soil movements analysed by PIV.Chapter 1 Introduction procedures. A detailed description of the novel technique for simulation of tunnelling during centrifuge flight is also presented. 6 . d) Chapter 5 presents the results of parametric studies on the effects of tunnelling on single piles e) Chapter 6 presents the results on effects of tunnelling on pile groups f) Chapter 7 summarizes the main findings of the present study and proposes future works. 7 .Chapter 1 Introduction Existing Building Depth (m) 5 Induced Pile Settlement 10 Settlement Trough 15 20 25 Induced BM Tunnel Contraction 30 35 Induced Axial Loads (NTS) 1 Figure 1.1 Pile responses induced by tunnel construction: (a) Tunnelling under pile foundation. (b) Tunnelling adjacent to pile foundation. Chapter 1 Introduction Existing Building Depth (m) 5 Steel reinforcement 10 15 Existing Pile 20 25 New tunnel 30 (NTS) 1 Figure 1.2 Pile foundations supported existing buildings normally designed to resist compression load only. 8 . 1 INTRODUCTION Tunnelling would cause soil movements that would in turn induce axial and lateral loads on adjacent pile foundation. it is of great interest to understand the effects of tunnelling induced soil movements on existing piles.2 TECHNIQUES FOR SIMULATION OF TUNNELLING IN CENTRIFUGE Prior to the study of pile behaviours due to tunnelling. The methods. The soil movement pattern is directly caused by the model tunnel and affects the pile responses. (b) tunnelling-induced soil movements and (c) tunnelling-induced pile responses. However. 9 . Hence.Chapter 2 Literature Review CHAPTER TWO LITERATURE REVIEW 2. The topics investigated include: (a) model simulation technique of tunnelling. especially for long-term pile behaviours. Many studies on free-field soil movements due to tunnelling have been reported. few studies concerned the pile responses subject to soil movements caused by tunnelling. 2. effects and difficulties faced in investigating the behaviour of piles and soil due to tunnelling will be reviewed in detail in this chapter. the correct simulation of tunnel excavation plays an important role. Ng et al. Lim. 2006.Chapter 2 Literature Review Tunnel construction is three-dimension in nature and time dependent. from the field monitoring of Mass Rapid Transit North East Line Contract C704 studied by Pang (2006). shield loss due to over-cut. Moreover. In general. 3D finite elements simulations of tunnels are reported in the literature in recent years (Komiya et al. and hence the simulation of plane strain model of a long tunnel is comparable to 3D modelling in this case. Lee and Ng. tail void closure and consolidation (Shirlaw et al. Phoon et al. Lee et al. realistic three-dimensional finite element analysis has still not reached a point of development where it can be routinely used in engineering design.. Ng and Lee. 2003. the induced pile bending moment in the longitudinal direction is either equal or smaller than the induced pile bending moment in transverse direction. Moller (2006) performed a back analysis for the case of Second Heinenoord Shield Tunnel.. 2006). Lin et al. With the increasing availability of large-scale computing resources. 10 . 2006.. However.. 2002. Besides. 2002. This study clearly demonstrated that the transverse section is more critical. it is apparent that the maximum axial force developed in the pile when the Earth Pressure Balance machine (EPBM) was directly adjacent to the pile. Melis et al. 2001. Augarde and Burd. Phoon et al. as the face pressure does not much contribute to the development of the surface settlement. In order to compare the performance of 2D and 3D analyses. 2005... It is reported that the surface settlements of 2D analysis compare well with the 3D analysis. 2005.. 2006). 2004. Two common problems associated with three-dimensional analyses mean very large memory requirement and long computing times (Lee et al. 2003). 1999. the settlement due to shield tunnelling can be derived from the face loss during tunnel advancement.. . Feng et al.1 Simulation Technique 1 . Jacobsz et al. If thein a situation whenre the tunnel excavation has passed a particular section is considered. 1994. However. Loganathan. the 3D soil response around tunnel face would be important.1 shows the arrangement for the inflow of solvent into the tunnel core. 1999. 2002.. Ran et al. (2001) presented a method to simulate tunnel excavation in the centrifuge by dissolving polystyrene foam quickly using an organic solvent. Therefore. 2004) thus far considered only a wish-in-place plane strain tunnel in the simulation.. The polystyrene 11 . a prototype problem can be simulated in a scaled laboratory model hence overcoming the limitation of 1-g model. 2. this is usually referred to as a two-dimensional simulation (Taylor. if the tunnel face passes directly under a foundation.. this scenario has not been examined in this research. the vectors of the ground movement developed will be more or less in the plane perpendicular to the tunnel axis. 2003. 1998). 1996. Consequently it is reasonable to assume that a plane strain model of long tunnel section would be a good representation of tunnelling-induced soil movements. Lee and Chiang. 2002.Chapter 2 Literature Review With the availability of centrifuge modelling technique. Nevertheless. Figure 2.High Density Polystyrene Foam Sharma et al. Most of the centrifuge tests on tunnel-soil interaction that have been carried out (Bezuijen and Schrier. Hergarden et al.2. The review of various methods of simulating tunnel excavation in the centrifuge is briefly discussed here in order to gather useful information and guidance for the development of model tunnelling technique in the present study. This is probably because the development of 3D model tunnel in centrifuge is very difficult and has its limitation of boundary conditions and constraints. Figure 2. 12 . It can be seen that the lining deforms into an oval shape with tunnel spring lining protruding slightly outwards. This technique is an improvement over other methods of modelling tunnel excavation. such as reduction of air pressure supporting the tunnel lining or gradually draining heavy liquid from within the lining.2 shows the shape of the deformed tunnel lining over time obtained from a typical test. In most cases in practice. The flow of this liquid into the polystyrene foam (model tunnel) is controlled by using solenoid manifold and solvent reservoir. Ran (2004) extended Feng’s study on tunnel-soil-pile interaction to clay. The lining is left in place when the foam core has been dissolved. A limitation of this modelling technique is that this approach may not correctly simulate the actual tunnel excavation situations in the field as it can only simulate excavation cases with tunnel spring line moving outwards. the overexcavation of tunnel and the gap between the shield tunnel machine and lining would cause the tunnel spring line to move towards the tunnel resulting in inward soil movements at the tunnel spring line. which was made by wrapping a half hard brass foil around the foam core and soldering the lap joint with the help of tin solder and an electronic gun.Chapter 2 Literature Review foam core was placed tightly inside the model tunnel lining. Feng (2003) carried out a centrifuge model study to investigate tunnel-soil-pile interaction in sand. (2001) for the simulation of tunnel excavation in the centrifuge has been adopted by Feng (2003) and Ran (2004) at National University of Singapore (NUS). The technique proposed by Sharma et. al. The stiffness of the filled tunnel can approximately be made to be equivalent to that of the parent soil. Mair et al. 1976. In this modelling technique. Figure 2.2 Simulation Technique 2 .Chapter 2 Literature Review 2. To keep equilibrium between internal air pressure and the earth pressure.. Grant and Taylor (2000) and Bilotta and Taylor (2005).3 shows the schematic diagram of a typical plane strain centrifuge model tunnel using compressed air (Grant and Taylor. 1984) in which air pressure is applied to control tunnel support conditions. This support cannot be achieved by the air pressure technique. The excavation is simulated by decreasing the internal air pressure. (1991). which has to be removed during the excavation. König et al. (1999). 2000). (1991). it would be necessary to apply internal support according to the theoretical earth pressure at rest. 13 .2. This technique has subsequently been used by Chambon et al.Compressed Air One of the earlier model tunnelling technique has been developed at the University of Cambridge (Potts. Lee et al. The shape of the rubber membrane is identical with the contour of excavation. The equilibrium is kept between the internal air pressure and earth pressure during model preparation and increasing g-level. the soil mass. because air pressure remains constant along the contour of excavation whereas earth pressure varies with the change of orientation from vertical to horizontal and due to increase in vertical stress with depth. is represented in the model by a rubber membrane pressurized with air pressure. Liquid – Oil / Water The limitation of simulation of tunnelling by air pressure can be eliminated by applying fluid pressure inside the rubber membrane. The inner core of the tunnel was made of a long aluminium tube. A syringe pump was fabricated to control the volume of oil in 14 . 1981.Chapter 2 Literature Review 2. and a very thin rubber membrane was placed on top of the inner core cylinder. (König. Mair et al.3 Simulation Technique 3 . stiffness of lining as well as load concentration at the end of the tunnel lining) in the soil close to the excavation are most relevant for the investigated effect.4 shows a cross-section of the model tunnel infilled with oil developed by Loganathan et al. The cylindrical face of the assembly was then covered by a 0. A zinc chloride solution has been used to achieve a fluid with a density similar to that of the surrounding soil. Figure 2. However. This was realized in several studies related to excavation of shafts and trenches (Lade et al. A technique of decreasing tunnel diameter during centrifuge flight was adopted to model the required ground loss.. The rubber membrane was attached at both ends of the inner core cylinder by end caps which prevented any leakage of oil from between the inner core and the membrane.. 1984). and to ensure a uniform change in the tunnel diameter during the test. The stability of tunnelling procedure was represented by the equivalent surface ground loss values. This technique is well suited to study the relationship between the displacements of soil into the excavation and surface settlements or to evaluate the failure conditions. A small hole was drilled through the end cap and inner core to allow the passage of oil. (2000). soil stiffness. 1998). the technique needs to be evaluated carefully if the stress strain conditions (depending on the tunnelling technique.5-mm thick smooth-surfaced overlapping PVC spring to enhance the tunnel lining stiffness.2. During centrifuge tests. the approximately 4-mm thick annulus between the mandrel and the membrane was filled with water that could be extracted to accurately impose volume losses from 0% to about 20% on the surrounding soil. After the desired acceleration had been achieved and the model piles installed.Chapter 2 Literature Review the tunnel assembly. The recent model tunnel reported by Jacobsz (2002) was infilled with water to study the tunnelling effects on single piles in sand. The advantage of this model is that various volume losses can be simulated in one test by extracting oil slowly. Nevertheless. the solenoid valve was closed and the volume loss was imposed by extracting water slowly from the annulus around the tunnel. according to König (1998).5-m diameter tunnel at prototype scale.6. Thus. The outer diameter of the model tunnel was 60 mm. This is because the density of gas or fluid used is different from that of soil. The tunnel was connected via a solenoid valve to a standpipe in which a constant water level was maintained to automatically balance the tunnel pressure with the overburden pressure during the acceleration of the centrifuge. The model tunnel consisted of a brass mandrel surrounded by a 1-mm thick latex membrane as shown in Figures 2. representing a 4. there are indeed short-comings of the above model tunnels which control the volume loss by changing the volume of gas or fluid to simulate the volume loss. the initial stress condition of the model tunnel in this 15 .5 and 2. A pressure transducer was incorporated into the tunnel control system to monitor the pressure in the annulus of water between the latex membrane and the brass mandrel. This machine allowed the in-flight excavation of soil and also an in-flight installation of the lining. Jacobsz (2002) reported that due to the self-weight of the water in the tunnel. would also have increased the registered tunnel pressure. with the pressure near the crown 22 kPa lower and the pressure at the invert 22 kPa higher than measured. Most factors above would have resulted in the pressure being over-registered. the attempt to evaluate earth pressure acting on 16 . Moreover.7. Figure 2. In addition. which would have been difficult to extract completely.8 shows an in-flight miniature shield tunnelling machine developed by Nomoto et al. (1994). as shown in Figure 2. which would induce boundary condition and is not realistic as the middle part of the tunnel will suffer much higher stress and displacement as compared to the both ends of the tunnel when subject to high-g acceleration.2. would have affected the measured pressures.Mechanical Equipments Relatively complicated mechanical equipments have been developed to simulate the process of shield tunnelling in centrifuge. Besides. Owing to the small size of the centrifuge and the restrictions in terms of dimensions and weight of the model.4 Simulation Technique 4 . the model tunnel is fixed at both ends of the strong box. the stiffness of the tunnel membrane and the fact that the contraction of the model tunnel occurred non-uniformly. a hydrostatic pressure gradient would have existed in the tunnel. This time the whole shield tunnelling process was successfully reproduced in a centrifuge force field.Chapter 2 Literature Review simulation technique is one of the major concerns. a small amount of water trapped near the tunnel ends. 2. Chapter 2 Literature Review the tunnel lining quantitatively was not yet fulfilled. Recently. The desired volume loss was achieved when the smaller aluminum tubes were pushed through the sand box. which are able to simulate the tail void and backfill grouting in flight by contraction and expansion of shield model rings controlled by a motor. the recent model tunnels developed by Ghahremannejad at el.10. (1998) to perform centrifuge tests in both sand and clay to study the tunnelling-induced ground movement. Schmidt (1969) and subsequently many other researchers have shown that the transverse settlement trough after tunnel excavation in short-term (Cham. The tail void was also too large compared with the prototype machine (Nomoto et al. Lee and Yoo (2006) adopted another technique simulating the volume loss by decreasing the diameter of model tunnel as shown in Figure 2. 2. (2006) simulated volume loss by varying the diameter of the model tunnel along the tunnel. 1994). (2006) and Lee and Yoo (2006) can only simulate tunnel volume loss under 1g condition.11. Ghahremannejad at el. The method needs an estimate of volume loss (V) and the trough width parameter (K) to 17 .3 TUNNELLING-INDUCED SOIL MOVEMENTS 2..9. However. A plane-strain shield model machine was developed by Yasuhiro et al. see Figure 2.3.1 Field Studies of Tunnelling-Induced Soil Movement Peck (1969). 2007) can be well-described by a Gaussian distribution curve as shown in Figure 2. The shield model machine consists of steel rings and a wedge shaped shaft. 5 for tunnelling in clay (O’Reilly and New. a comprehensive summary done by Lake et al. 1993).4 to 0. Numerous field data have been collected and hence many estimates of trough width parameters have been proposed.V. 18 .14 has established the general variations of i as follows: Simple approximate relationship i = Kz o Where K = 0. However. 1981.12. The surface settlement trough. are approximated as follows: ⎛ x2 S = S max exp⎜⎜ − 2 ⎝ 2i ⎞ ⎟⎟ ⎠ V = 2π iS max (2. and K = 0.45 for sands and gravels. O’Reilly and New.13). The ground settlements are generally negligible beyond an offset of 3i from the tunnel centre line for Peck’s (1969) proposed curve (Figure 2. 1982).1) (2.25 to 0. 1982.’s (1992) field studies supported the various proposals that K can be assumed solely as 0. zo= depth to tunnel axis Lake et al.6 for clays (soft and stiff).2) where x is the offset to the tunnel vertical line and i is point of inflection. Mair et al. (1992) on tunnelling data from many countries on clay shown in Figure 2.Chapter 2 Literature Review obtain the maximum ground surface settlement (Smax) and subsequently the surface settlement profile. and volume loss.S. The trough width parameter K is relatively easy to quantify as it is largely independent of construction method and operation experience (Fujita. as defined in Figure 2. Shirlaw et al.3) 0. the trough width parameter for tunnels constructed in clay. (1993) proposed that at a depth z below the ground surface and above a tunnel depth Z0.Chapter 2 Literature Review The subsurface settlement profiles can also be reasonably approximated by a Gaussian distribution curve in the same way as surface settlements. (1994) and Shirlaw (1995). Ran et al. Shirlaw (1993) recorded examples of long-term settlements where the long-term component had the effect of increasing the 19 . Centrifuge studies by Grant and Taylor (2000) confirmed that the proposed variation of K with depth for clay by Mair et al. A comprehensive review of field data of postconstruction settlements above tunnels in soft clay has been carried out by Shirlaw (1993). (1996) irrespective of the soil conditions encountered. Similar patterns of increase in K was observed in studies by Moh et al. i. (1993) provided a good fit to the data obtained from tests within a certain range between the ground surface and tunnel axis level. (2003) observed that the Gaussian Curve can only describe the shortterm surface settlement well. However. Mair et al. (1996) and Dyer et al.325⎛⎜1 − z ⎞⎟ zo ⎠ ⎝ K= ⎛1 − z ⎞ ⎜ z o ⎟⎠ ⎝ (2. can be expressed as: i = K ( zo − z) (2.15.4) Trough width parameter is shown to increase with depth and significantly underestimated if a constant value is assumed. based on the field data collected as shown in Figure 2.175 + 0. . Hence. The maximum surface settlements over time is shown in Figure 2.2 (Zhang et al.. Mair and Taylor (1997) concluded that the long-term settlement troughs are similar to classical Gaussian curve associated with short-term settlement when positive excess pore pressure are generated during tunnelling.Chapter 2 Literature Review short-term settlement by up to a factor of 10. whereas wider long-term settlements are related to tunnel lining acting as drain and the development of steady state seepage towards the tunnel. A case history study covering an 11-year period on the Haycroft Relief Sewer at Grimsby in very soft clay soil had recorded that the initial settlement trough increased from 1. 1980. Glossop. O’Reilly et al.16. as shown in Figure 2. 1996). Similar time dependent responses can also be found in Shanghai Metro Tunnel No. 2004). particularly for tunnels in soft and compressible clay. Thus. The more typical settlement increase in the long-term is in the order of 30% to 100%. Howland. 1978. the major factors influencing the development of long-term settlements are (i) initial magnitude and distrbution of pore water pressure.18 clearly demonstrates that the long-term settlements were significant. The ratio of the maximum immediate settlement to maximum long-term settlement shown in Figure 2. generally it has been clearly shown that post-construction settlements can be significant.. and hence cannot be neglected.19 have much wider settlement troughs in the long-term. 20 . Some case histories shown in Figure 2.17 reveals that the consolidation of the disturbed settlements around the tunnel in long-term cannot be neglected.5Z to a final value in excess of 4Z. 1991 and Bowers et al.g. Similar widening of settlement trough has been reported by a number of authors (e. very limited field data are available. 21 .24 demonstrated the time dependent behavior of horizontal soil movements. while positive excess pore pressures due to shearing may result a short distance away. 2005 showed that in Toulouse subway line B. whereby two similar case histories were observed for Shanghai Metro (Schmidt. Balasubramanian (1987) reported both vertical and horizontal soil movement over almost a year time for the Singapore’s largest effluent outfall pipeline in tunnel through various types of soils. but stabilized after 15 days of tunnel excavation. For normally consolidated clays. significant zones of positive excess pore pressure can be generated even for a tunnel where unloading occurs. the transverse movements were significantly increased over time.21 and Thunder Bay Sewer Tunnel (Shirlaw et al. For lateral soil movement due to tunnelling in long-term.Chapter 2 Literature Review (ii) compressibility and permeability of the soil. The pattern of pore pressure is shown in Figure 2. The extent of the positive excess pore pressure in Thunder Bay Sewer Tunnel spanned over six tunnel diameters from the tunnel centre line. Negative excess pore pressures were noted included near the tunnel. mostly marine clay. particularly the permeability of the tunnel lining relative to the permeability of the soil. 1994) in Figure 2. and (iii) pore pressure boundary. as shown in Figure 2..22. 1989) in Figure 2.23.20. Emeriault et al. as shown by Schmidt (1989). Besides. The data shown in Figure 2. 2 Centrifuge Model Tests of Tunnelling-Induced Soil Movement With the advances of new image processing technology. the deformation of the soil propagates ‘gradually’ upwards and outwards from the tunnel cavity to the ground surface. This may explain why sinkholes on the ground surface associated with tunnelling are mainly spotted in competent soils like sand in the field. The two measured settlement troughs follow Gaussian curve well. For clay. (2001) for the simulation of ovalisation of tunnel using high density polystyrene foam.3. Feng (2003) and Ran (2004) adopted the technique proposed by Sharma et al. However. The results enable an in-depth understanding into the mechanism of ground responses associated with tunnel construction in terms of surface and subsurface soil movements.25 shows the surface settlement troughs under approximate similar volume loss in clay and sand. the deformation in sand propagates sharply and almost vertically from the tunnel to the ground surface. Figure 2. the different mechanisms suggest that the ground deformation in sand may cause more severe damages to the ground surface or structures above and nearby the tunnel. centrifuge modelling emerges to be an attractive technique to investigate the effects of tunnelling-induced soil movement on adjacent piles. it is evident that the settlement troughs are markedly different as the sand settlement trough is much narrower than that of clay.Chapter 2 Literature Review 2. such drastic settlements are less common. Therefore. 22 . However. In clay. as well as soil stresses. while in clay. the soil movements cause differential settlement spreading a wider range. This indicates different settlement propagation mechanisms in clay and sand. as reported by Ran (2004). 2. probably due to the soil condition of moderately stiff clay with less significant in long-term settlement. After the completion of tunnel excavation. the horizontal movements are not well described by assuming an average vector focus (Based on Grant and Taylor (2000). It is noted that the measured short-term surface settlement trough follows the Gaussian distribution curve fairly well. the long-term behaviour of the soil movements is not studied. Gaussian distribution curve largely underestimates the measured settlement at the far end of the ground surface. In the near surface region. the average position of the vector focus can be used to give average distribution of horizontal movement) but the agreement is very good at all other elevations. (1993) provides a good fit to their centrifuge test data.26 shows the surface settlement trough over time in clay (Ran.Chapter 2 Literature Review Figure 2. Grant and Taylor (2000) carried out a series of centrifuge tests to investigate tunnelling-induced ground movements in clay. showing that the spread of the surface settlement trough increases over time. The studies also suggested that a constant trough width regardless of volume loss is common to all of the tests and the centrifuge studies confirmed that the proposed variation of K with depth for clays by Mair et al.2. However. Furthermore. tunnelling was simulated by reducing the compressed air pressure in a model tunnel lined with a latex membrane and this caused a uniform radial contraction of tunnel. As discussed in Section 2. The incremental soil settlements become negligible after 720 days of tunnel excavation. Figure 2. However. Gaussian curve is found to be inappropriate to depict the measured longterm surface settlement troughs. The measured final trough has a somewhat wider parabolic shape than that of Gaussian curve.27 shows the profile of normalized vertical and horizontal ground movements at different subsurface elevations. the soil continues to settle with time and the rate of increase in settlement decreases with time. 23 . 2004). However. Thus.28 shows that the ground settlement troughs measured in the centrifuge tests match well with his analytical prediction (Loganathan et al.. The tunnel deformation pattern is uniform radial contraction. However. Sagaseta (1987) presented an analytical solution to predict tunnelling induced ground movements for a weightless incompressible soil by simulating ground loss around a tunnel in the form of a point sink. the method gives a wider surface settlement profile and larger horizontal movements than observed in practice. Figure 2. 1998) which is based on oval-shaped deformation.Chapter 2 Literature Review Simulation of tunnelling using liquid pressure was proposed by Loganathan et al. (2000). (2000). The tunnel is first assumed to be located within an elastic infinite medium where it collapses uniformly. the long-term effect has not been studied by Loganathan et al. Loganathan and Poulos (1998) modified Verruijt and Booker’s solution to give 24 . Solutions derived by Sagaseta (1987) are subsequently extended by Verrujit and Booker (1996) to account for compressible materials and the effects of ovalisation of the excavated tunnel boundary.3. The soil is modelled as a linear-elastic material and the solution is based on fluid mechanics concepts. complex geometries and time effects. analytical methods are mathematically limited in the efforts required to derive solutions accounting for material non-linearity.3 Predictive Methods of Tunnelling-Induced Soil Movement Although attractive as a predictive tool. relatively few analytical solutions are available. 2. (2006a) developed a kinematic plastic solution for ground movements around a shallow. Osman et al. and gave close correspondence for deep tunnels but under predicted tunnel support pressure by about 25 . unlined. Outside this zone. representing the soil as a strain-hardening plastic material. A simple power curve was used to model the stress–strain relations. the soil is assumed to be rigid. In this solution.29). tunnel embedded within an undrained clay layer. . 2006a). Osman et al. These analytical solutions for maximum surface settlements have also been validated against the centrifuge test data. and also matches the displacement field observed in centrifuge tests of tunnel failure in clay (Mair. It was also demonstrated that the shape of the surface settlement profile remained the same as the magnitude of settlement increased towards failure (Osman et al. 1979). In addition. This mechanism does not incorporate slip surfaces. Within the boundaries of the deformation zone. the pattern of deformation around the tunnel is idealised by a simple plastic deformation mechanism (see Figure 2. This solution is obtained by integrating the equilibrium equations along the tunnel centre-line from the tunnel circumference up to the ground surface and by invoking radial symmetry. (2006b) also demonstrated that an upper bound style of calculation is also capable of predicting ground displacements at any stage prior to failure. Osman et al. A simplified closedform solution is provided for the prediction of maximum surface ground settlement for the particular case of deep tunnelling. (2006a) demonstrated that the upper bound theorem applied to distributed shearing mechanism offered a reasonable assessment for collapse. the soil deforms compatibly following a Gaussian distribution.Chapter 2 Literature Review narrower settlement troughs and to account the construction effects empirically. The GAP parameter can be estimated using a theorectical method developed by Lee et al. post-construction ground responses and precise tunnelling model. 26 . and linming geometry. The ‘GAP’ parameter is defined as the magnitude of the equivalent twodimensional (2D) void formed around the tunnel due to the combined effects of the three-dimensional (3D) elastoplastic ground deformation at the tunnel face. (1983). Analytical solutions providing the most convenient way in predicting tunnelling induced ground movements. arguably accurate empirical method has certain limitations in accounting for the effect of ground conditions. a well-calibrated ‘GAP’ method was proposed by Rowe et al. 1992). Comprehensive guidelines have been provided to calculate the gap parameter (Lee et al.5) where Gp represents the difference between the cutter head and outer lining diameter while U3D and w account for 3D heading effects and workmanship quality. respectively. (1992). In view of the above. once the details of the machine support system and the soil parameters are given. overexcavation of soil around the periphery of the tunnel shield. and the physical gap that is related to the tunelling machine. construction methods. sheild.Chapter 2 Literature Review 20% for shallow tunnels (Depth to tunnel crown/ depth of tunnel axis. C/D <3). However. as GAP = Gp + U3D + w (2. The analytical solution of ground loss with horizontal and vertical distance єx.38 x 2 0.Chapter 2 Literature Review In practice. The traditional definition of the ground loss parameter is redefined as ‘equivalent ground loss parameter ε’ with respect to the gap ‘g’ parameters and incorporated in the analytical solutions. (1992) by incorporating the shape of tunnel deformation to predict tunnelling-induced undrained ground movements around a tunnel in soft ground. Hence. the radial ground deformation is not uniform since the equivalent 2D gap (tail void) around the tunnel is non-circular (e. The possible reasons for the formation of an oval-shaped gap around the tunnel are: (1) tunnel operators advance the shield at a slightly upward pitch relative to the actual design grade to avoid the diving tendency of the shield. z = ε 0 exp⎨⎢ (2. typically oval-shaped) as shown in Figure 2. as pointed out by Rowe et al. The nonlinear ground movement due to the formation of an oval-shaped gap is then modelled by adopting an exponential function to the equivalent undrained ground loss with appropriate boundary conditions.z from the tunnel centre is given as: ⎧⎡ 1.g.6) 27 . and (3) 3D elasto-plastic movement of the soil occurs at the tunnel face. Loganathan and Poulos (1998) extended the study from Lee et al. the analytical solution models the effect of non-uniform soil convergence around a deforming tunnel as shown in Figure 2.31. (1983). (2) the tunnel lining settles on the ground when the tail piece is removed.30.69 z 2 ⎤ ⎫ + ⎥⎬ 2 H 2 ⎦⎭ ⎩⎣ ( H + R ) ε x . The oval-shaped ground deformation pattern (Loganathan and Poulos. The surface and maximum subsurface settlements and the lateral displacement predicted using the proposed analytical solutions are comparable to those from the methods of Verruijt and Booker (1996) and Loganathan and Poulos (1998).C. z is the depth below ground surface and x is the lateral distance from tunnel centre-line. H is the tunnel depth. The boundary condition B.C. Park. 2003). 1998.C.-1) and three for oval-shaped deformation pattern (B. Although the method has been successfully used to back analyze some case histories in clay. 2004) is imposed as the boundary condition of the displacement at the opening to consider real non-uniform ground deformation pattern..32.Chapter 2 Literature Review where ε0 is the ground loss ratio. calculated results have to be treated with caution as certain important conditions necessary in the derivation of analytical solutions are violated and the volume loss is not conserved for undrained cases when empirical assumptions are introduced (Cheng.s-2–4). The gap parameter (Lee et al.-2 (ovalshaped) is chosen for further study to give a conservative estimation for lateral displacement. one for uniform radial deformation pattern (B. and in reasonable agreement with field observations for tunnels in 28 . are considered at the tunnel opening as shown in Figure 2. Four simple boundary conditions. Five case studies have been used to check the applicability of the proposed analytical solutions. 1992) is used to describe the displacement at the opening. The accuracy of the oval-shaped radial displacement of tunnelling has been further verified by the analytical solutions presented by Park (2005) through the case studies. The well-established Gaussian curve describing the magnitude of ground surface settlement due to tunnelling could be used as a reference frame for the assessment of pile settlement due to tunnelling.2 km of twin 8-m diameter bored tunnels using two Lovat Earth Pressure Balance (EPB) machines with tail-skin grouting. The field study took place in Channel Tunnel Rail Link (CTRL) Contract 250. 4. it can be concluded that the oval-shaped radial displacement is a better tunnelling model that provides more realistic predictions of ground movement in uniform clay. (2005) presented the results of a full-scale trial investigating the effects of tunnelling on piles.4. A comparison was made between the resulting pile head settlements due to tunnelling with the surrounding ground surface movements.4 TUNNELLING-INDUCED PILE RESPONSES 2.33): 1. The study involved the installation. Piles in Zone B settled by the same amount as the ground surface. 2. The results identified three zones of influence in which the pile settlements were correlated to the ground surface settlements (see Figure 2. The Contract involved the construction of 5. 29 . Essex. 2. Hence. Piles in Zone A settled 2-4mm more than the ground surface.Chapter 2 Literature Review uniform clay.1 Field Studies of Tunnelling-Induced Pile Responses Selemetas et al. UK. in Dagenham. loading and monitoring of four instrumented piles along the route of the twin tunnels . Piles in Zone C settled less than the ground surface. 3. (2005a) presented data from part of the MRT North East Line Contract 704 in Singapore. Jacobsz et al. (2005) and Jacobsz et al. The Terrace Gravels were grouted as a mitigation measure to increase shaft capacity at that elevation and to create a pseudo-slab beneath the pile caps. In the case of end-bearing piles. (2001). Three piled bridge pier foundation are described. Total surface settlements of 8 mm to 10 mm were observed (volume loss of 0. the strains along the length of the pile. the settlement of the superstructure was judged to be the same as the soil (Terrace Gravels) at pile toe level. both vertically and laterally (to obtain bending strain) were estimated from the ground movements with depth assuming full friction at the soil-pile interface. In the third case. where forward-thinking enabled instrumentation to be 30 . Figure 2.Chapter 2 Literature Review The study confirmed that pile head and ground surface settlement can be correlated as presented in other studies by Kaalberg et al. These were estimated and the bridge structure deemed safe for the level of movement anticipated. one with end-bearing piles and the other two friction piles. It is recommended that the pile capacities should be re-evaluated as there is potential redistribution of loads in the piles.6%) with no detrimental effects on the bridge. Pang et al.34 shows a section of the friction pile case studies where the pile toes were very close to the tunnels. No mitigation measure was implemented and no damage was sustained for the bridge. The results indicated that the piles would not be overstressed and that assuming that the pile movement is the same as that for the Free-field surface settlement is conservative. (2005) reported the case studies for the construction of the tunnels for the CTRL project in London on the effects of tunnelling on piled foundation. running parallel to the bridge could be assessed. Calculations indicate that downdrag loads were between 9 and 43% of the structural capacity of piles with peak value occurring when the face of the TBM was in line with the piles. with greater force developing in the pile nearer as might be expected.32 and 1. with maximum values. The piles are 62 m long and 1.Chapter 2 Literature Review installed in bridge pier piles so that the influence of future planned tunnels.3-m diameter EPBM tunnel.6m from the nearest piles at a depth of 21 m (to its axis). constructed in residual soils.45% only. It is reported that the range of volume loss was between 0. Correlating the developing settlements with TBM position has enabled the volume losses related to the different phases of the tunnel process to be identified. Information from the strain gauges within the piles reveals that the piles experience down-drag. Also evident is shielding of the outer pile by the inner pile between it and the tunnel (see Figure 2.35. Clear trends in bending moment distributions along the length of pile are also shown. registered as increasing axial force. The data presented related to a 6. to enable average axial loads and bending moments in transverse and longitudinal directions to be determined. The data from 2 piles of a four-pile group supporting a bridge pier are presented. at 1.37). in pairs.36. Some interesting relation between volume loss and axial force and bending moment are also presented.2 m in diameter with four sets of strain gauges installed orthogonally.37). showing increase in both quantities with volume 31 . Post-tunneling measurement of the development of axial force in pile P1 at Pier 20 showing the timedependent behavior of drag-load as indicated in Figure 2. occurring in the close vicinity of the tunnel (see Figure 2. as shown in Figure 2. The surface settlement profile due to tunnelling follows a Gaussian form with a maximum value of about 18 mm. although small. Chapter 2 Literature Review loss. It is also concluded that a volume loss up to 1.5% does not seem to have a significant effect on the piles. Cham (2007) analysed the responses of instrumented piles and ‘Free-field’ ground instrumentations to tunnelling processes such as tail void grouting, application of face pressure, tunnel advancement and thrust force in a field study involving twin tunnelling in MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore. The tunnel diameters are 6m and tunnelling was carried out with 2 Earth Pressure Balance Machines (EPBM) at a depth of 23m below the ground surface, through residual soil of Bukit Timah Granite, Kallang Formation and Old Alluvium. Tunnelling of the second tunnel commenced 4 months after the first and at the site of field study, the twin tunnels have a lateral clearance of 2.5 m. Figure 2.38 illustrates the positions of existing instrumented piles relative to tunnels. The instrumented piles are reinforced-concrete bored piles with diameter of either 600mm or 800mm, and of penetration depth ranging from 27m to 33m. At the closest points, the piles lie within 1m of the tunnel lining. It was observed that downdrag forces developed when the TBMs were directly adjacent to the piles. The maximum induced axial force due to twin tunnel advancement ranged from 7% to 72% of the pile structural capacity. In addition, piles experienced additional bending moments due to tunnelling and the induced bending moment was observed to increase with increasing volume loss. The field studies of tunnel-soil-pile interaction help to improve the current knowledge on for the prediction of pile responses due to tunnelling. Prior planning and 32 Chapter 2 Literature Review arrangements are made to instrument the pile as reported by Pang et al. (2005) are strongly recommended to study the influence of future planned tunnels to existing piles. 2.4.2 Centrifuge Model Tests of Tunnelling-Induced Pile Responses Nearly all existing field cases are limited in various aspects of measurements, which may be due to deficiencies in instrumentation planning or difficulties in collecting field data. Thus, centrifuge modelling is an alternative method to study the tunnel-soil-pile interaction problem. Loganathan et al. (2000) presented the model tunnel in-filled with oil to simulate the uniform radial contraction in centrifuge to the pile responses due to tunnel excavation in overconsolidated clay. The scope of the study focused on friction piles (single pile and a 2x2 pile group). The effects of pile tip elevation relative to tunnel axis level and volume loss on the displacements and performance of piles were investigated to study the interaction problem. The relative position of the piles in various tests is shown in Figure 2.39. Three tests were performed with the tunnel located above, at and below the pie tip level. The induced bending moment and axial force profiles at a volume loss of 1% are presented in Figures 2.40 and 2.41, respectively. It is observed that both the induced maximum bending moment and axial force occurred approximately at the tunnel spring line level in long pile cases where the pile tips were below the tunnel spring line. The maximum bending moments occurred just above the pile tips and the pile axial force increased from the pile head to the pile tip in a short pile case with pile tip at or above the tunnel spring line level. The comparison of the three tests showed that for single piles, the maximum bending 33 Chapter 2 Literature Review moment was the largest when the pile tip was located at the tunnel spring elevation, whilst the maximum axial force was the largest when the pile tip was above the tunnel spring line. It was concluded that the maximum measured bending moments vary almost linearly with ground loss values below 5%. As such, it was postulated that an elastic analysis may be performed to predict tunnelling-induced pile behaviour if the ground loss value was less than 5%. Feng (2003) performed a series of centrifuge tests to investigate the pile responses associated with a lined tunnel in dry sand at NUS and Ran (2004) extended the studies by Feng (2003) to clay. The simulation method of tunnel excavation proposed by Sharma et al. (2001) using polystyrene foam was adopted and the tunnel deformation is oval shape. Ran (2004) reported two major series of tests involving the effects of pile-to-tunnel distance and pile length, as well as on the effects of volume loss. Figures 2.42 and 2.43 present the typical pile axial and lateral responses over time. From the studies, it is demonstrated that the pile responses in clay are time-dependent. Moreover, it is evident that the induced pile responses in clay are smaller than those in sand (Feng, 2003). The limitation in the studies is that the model tunnel simulating the ovalisation of tunnel, and hence the induced pile bending moment demonstrates an opposite behaviour as most practical cases. A recent study on the influence of tunnelling on piled foundations was completed at the University of Cambridge by Jacobsz et al. (2005). The centrifuge model study (Jacobsz, 2002; Jacobsz et al., 2004) focused on tunnelling near driven piles in dense sand. Both single piles and pile groups were considered and the model was constructed at a scale of 1:75. The model tunnel comprised a 50-mm brass pipe 34 Chapter 2 Literature Review surrounded by a 1-mm thick latex rubber membrane. The 4-mm thick annulus between the pipe and membrane was filled with water which could be discharged accurately to impose volume losses from 0% to approximately 20% on the surrounding sand. It was intended to use the model tunnel to impose relatively realistic plane-strain tunnellingrelated ground movements on the surrounding ground, rather than model the progressive advance of a tunnel face and uniform radial displacement around tunnel is simulated. A number of instrumented model piles were located at various offsets and depths during the tests. A zone of influence around a tunnel was established (see Figure 2.44) in which significant base load reduction, accompanied by large pile settlements, were noted should a certain volume loss be exceeded. Piles with their bases outside the zone of influence did not suffer large settlements even at volume loss up to 10%. The stresses exerted by piles on the surrounding ground result in a subsurface settlement profile different from the Free-field situation. Pile settlements can however be approximated by the Free-field surface settlement should the pile shaft capacity not be exceeded due to volume loss. The piles in the centrifuge study possessed significant reserve (immobilized) shaft capacity. Should piles not have the reserve shaft capacity, e.g. where piles are end-bearing in sand with the shafts surrounded by soft clay, volume loss may cause more rapid settlement. Pile groups behave in a similar fashion to volume loss than individual piles (see Figure 2.45). Load transfer from one pile to another within a group only occurs once the shaft capacity of a given pile has been mobilized causing its settlement to become significant. For pile groups, these usually occurred at large volumes which are undesirable in practice. 35 Chapter 2 Literature Review In practice, the over-excavation of the tunnel and the gap between the shield tunnel machine and lining would cause the tunnel spring line to move inwards into the tunnel resulting in inward soil movements towards tunnel at the tunnel spring line. Lee et al. (1992), Loganathn and Poulos (1998) and Park (2005) had successfully proposed analytical solutions based on inward tunnel deformation or oval-shaped deformation. Hence, it is of great interest to improve the model tunnelling technique to study the pile responses due to inward tunnel deformation. It should be noted that based on St Venant’s Principle, the exact deformation shape should be immaterial if the piles are located sufficiently far away. However, in the present study, the deformation shape of the tunnel is of great importance as the piles are close to the tunnel and the soil movements would significantly affect the pile responses. 2.4.3 Predictive Methods of Tunnelling-Induced Pile Responses Chen et al. (1999) presented a simple approach to assess tunnelling induced pile responses where a two-stage uncoupled method was introduced. In the method, freefield tunnelling induced ground movements at the pile location is first approximated based on the quasi-analytical method proposed by Loganathn and Poulos (1998) . The movements were then applied to the soil elements surrounding the pile using separate numerical programs (PALLAS and PIES) to assess the lateral and vertical pile responses. The approach started with a basic problem as illustrated in Figure 2.46 where an existing single pile is situated adjacent to a tunnel under construction. The induced pile responses together with the free-field soil movement of the basic problem are shown in Figure 2.47. Subsequent parametric studies provided valuable insight into the various factors affecting pile performance, in particular the variation of maximum 36 Chapter 2 Literature Review induced bending moment and axial force with pile-to-tunnel distance and relative position of pile tip to tunnel axis level. In general, the maximum bending moment and axial force values decrease to insignificant magnitudes (less than 10% of value at X=1D) beyond a respective distance of 2D and 5D from the tunnel centre line. At a given horizontal offset from the tunnel centre line, the pile bending moment is generally the greatest when its tip is below the tunnel axis level, decreasing as the pile tip moves upwards. However, the pile lateral deflection profiles are almost identical in shape and magnitude to imposed free-field soil displacements. This is probably due to the low flexible stiffness of the pile and the homogeneous clay profile with constant Cu and Young’s modulus with depth used in the analysis. Free-field displacements are movements of the soil that occur at a distance from the pile such that the displacements are not affected by the presence of the pile. A free-field soil displacement method, in which a pile was represented by beam elements and the soil was idealized using the modulus of subgrade reaction, was proposed by Chow and Yong (1996). The magnitude of soil movement profile serves as input to the method. With this idealization, non-homogeneous soil can be easily represented. This approach requires the knowledge of pile bending stiffness, distribution of lateral soil stiffness and the correct limiting soil pressure acting on the pile with depth. Comparisons with available well-documented case histories suggest that the method gives reasonable prediction of behaviour of pile subject to lateral soil movements. Loganathan et al. (2001) incorporated the analytical solutions for tunnellinginduced ground movement (Loganathan and Poulos, 1998) into the computer programme GEPAN (Xu and Poulos, 2000) to compute the response of a 2 x 2 pile 37 Pang et al.50.49).1. As the pile responses due to tunnelling is essentially a 3D problem. the piles were gradually pushed forward in the same direction of tunnel advancement. Subsequently. The analysis back-analysed the behaviour of an instrumented pile group where the full shield tunnelling process. the front pile was found to be subjected to higher response of up to 2. was simulated (see Figure 2. over-cutting.Chapter 2 Literature Review group as shown in Figure 2. (2005b) carried out three-dimensional finite element analysis to back-analyse a case history on the response of pile foundation subjected to shield tunnelling as described in Section 2. the settlement is slightly higher that the piles in the group. In addition.48. Generally. The lateral deformation and bending moment profiles for single piles and piles in group are almost similar. the piles deflected towards the tunnel with maximum movement at the pile head level and not at the tunnel spring line. as the tunnel advanced past the pile group. 38 . The trend of pile group deformation was noted to deflect towards the tunnel transverse direction. prior to the tunnel arriving at the pile group. for single isolated piles. the axial down-drag force estimated for a single pile is about 20% higher than the down-drag force induced in a pile within the pile group. However. shield tunnel advancement. In the longitudinal direction. tail void closure and installation of lining. the ‘front’ pile has slightly higher responses than the ‘rear’ pile. Good agreement between the results from analysis and measured data was obtained which validated the FE analysis shown in Figure 2.4. including the application of face pressure. In addition.5 times compared to the rear pile. except for a small difference in bending moment at the pile cap location due to the fixity condition. the short-comings from existing literature are discussed and summarised as follows. Hence. • Opposite induced-pile bending moment was observed in centrifuge tests carried out by Ran (2004) who had simulated the ovalisation of tunnel lining.5 SUMMARY A review of the literature to-date on effects of tunnelling on adjacent pile is reported in this chapter. • Very few centrifuge studies had been carried out regarding the soil movements and pile behaviours associated with inward tunnel deformation. Centrifuge model tests emerge to be an alternative to study such long-term behaviour. centrifuge model test and predictive methods for tunnelling induced soil movements and pile responses are presented. 39 . precise tunnel deformation pattern. • Limited field studies on the long-term tunnelling-induced ground movements and pile responses. In addition. • Considerable research studies have been carried out to simulate the process of tunnels excavation in centrifuge. It should be noted that all these model tunnels could not exactly replicate the prototype tunnelling process in the field. particularly in soft clay. The techniques for simulation of tunnelling in centrifuge developed thus far have certain limitations such as maintaining initial stress-strain behaviour of soil prior to tunnelling. and the complication in implementing the excavation process during centrifuge flight. which is common in practice. an improved simulation of tunnelling technique should be developed. boundary effects of model tunnel.Chapter 2 Literature Review 2. An overview of the published field studies. extra care has to be exercised when employing the analytical solution. Hence. volume loss is not conserved for undrained cases developed by Loganathan and Poulos (1998)) when empirical assumptions are introduced. these methods have certain limitations in accounting for the effect of different ground conditions.g. construction methods. Moreover. • Certain important conditions necessary in the derivation of analytical solutions may be violated (e. 40 . (1999) are confined to free head piles and linear elastic soil model. The following chapters aim to present the detailed results of the present centrifuge model study to investigate the observations and mechanisms of tunnel-soilpile interaction addressing some key issues raised in this chapter. • Although analytical solutions provide the most convenient way in predicting tunnelling induced ground movements.Chapter 2 Literature Review • Predictive methods available do not take into account the effects of pile group or soil-structure interaction. design charts proposed by Chen et al. post-construction ground responses and precise tunnelling model. 1 Simulation technique of tunnelling using high density polystyrene foam (After Sharma et al.Chapter 2 Literature Review Brass lining Polystyrene foam Figure 2. 2001 and Feng. 2004) 41 . 2003) Figure 2.2 Simplified tunnel lining deformation with time by simulation technique of tunnelling using high density polystyrene foam (After Ran.. 3 Simulation technique of tunnelling . 2000) Figure 2.4 Simulation technique of tunnelling .. 2000) 42 .model tunnel infilled with oil (After Loganathan et al.applying compressed air (After Grant & Taylor.Chapter 2 Literature Review Figure 2. 5 Simulation technique of tunnelling .Chapter 2 Literature Review Figure 2. 2002) 43 .6 Sand pouring in process during model preparation with model tunnel infilled with water (After Jacobsz. 2002) Figure 2.model tunnel infilled with water (After Jacobsz. 1994) 44 .7 Strong-box with two openings to fix the model tunnel in place (After Jacobsz. 2002) Figure 2.8 Simulation technique of tunnelling .mechanical equipment of miniature shield tunneling machine (After Nomoto et al.Chapter 2 Literature Review Opening to fix the model tunnel Figure 2.. 9 Simulation technique of tunnelling .Chapter 2 Literature Review Figure 2.. 2006) 45 .mechanical equipment of shield model machine (After Yasuhiro et al. 1998) Figure 2.mechanical model tunnel used to simulate the tunnel volume loss by decreasing the diameter of model tunnel under 1g (After Lee and Yoo.10 Simulation technique of tunnelling . 12 Definition of parameters controlling tunnelling-induced settlement trough (After Standing & Burland.Chapter 2 Literature Review Figure 2. 2006) 46 . 2007) Figure 2.11 Gaussian curve approximating transverse surface settlement trough for MRT project C852. Singapore (After Cham. 1969) Figure 2.13 Gaussian curve approximating transverse surface settlement trough (After Peck.14 Variation in surface settlement trough width parameter with tunnel depth for tunnels in clay (After Lake et al. 1992) 47 ..Chapter 2 Literature Review Figure 2. 15 Variation of trough width parameter K with depth for subsurface settlement profiles above tunnels in clay (After Mair et al.Chapter 2 Literature Review Figure 2.. 1993) Figure 2. 1991) 48 .16 Initial settlement trough at Grimsby increased from 1.5Z to a final value in excess of 4Z in long-term (After O’Reilly et al. 17 The maximum surface settlements at Grimsby increased significantly over the time (After O’Reilly et al.18 The ratio of the maximum immediate settlement to maximum long-term settlement for Shanghai Metro Tunnel No.Chapter 2 Literature Review Figure 2..2 (After Zhang et al. 2004) 49 . 1991) Figure 2. Chapter 2 Literature Review Figure 2. 1995) Figure 2. 1989) 50 .19 Normalized post-construction surface settlement troughs due to consolidation of soft clay (After Shirlaw.20 Estimated trend of excess pore pressure in normally consolidated clay surrounding the tunnel (After Schmidt. 1989) Figure 2.21 Changes in Pore pressure for Shanghai Metro (After Schmidt.Chapter 2 Literature Review Figure 2. 1986) (After Shirlaw et al. 1994) 51 .22 Change in pore pressure measured at Thunder Bay Sewer Tunnel (Adapted from data in Ng et al. Chapter 2 Literature Review Figure 2. 1987) 52 . but stabilized after 15 days of tunnel excavation (After Emeriault et al.24 Horizontal soil movement for the Singapore’s effluent outfall pipeline in tunnel (After Balasubramanian.23 Transverse movements in Toulouse subway line B were significantly increased over time. 2005) Figure 2. 6 -0. 2004) Distance from tunnel centre-line (m) -25 -20 -15 -10 -5 0 5 10 15 20 25 Surface Settlement (mm) 0 -40 -80 2 days 30 days 180 days -120 360 days 720 days 1080 days Gaussian curve (2 days) -160 Gaussian curve (1080 days) Figure 2.25 Comparisons of surface settlement troughs in sand (After Feng.3 -0.9 -1. 2003) and clay (After Ran.2 Surface settlement trough in clay (measured) Surface Settlement Trough in Sand (measured) -1.26 Ground surface settlement trough over time from a typical test (After Ran.8 Figure 2.Chapter 2 Literature Review Distance From Tunnel Centre-Line (m) -25 -20 -15 -10 -5 0 5 10 15 20 25 0 Settlement (m) -0. 2004) 53 .5 Gaussian curve (clay) Guassian curve (sand) -1. (b) 30mm below ground level. 2000) 54 . 2000) Figure 2.Chapter 2 Literature Review Figure 2. (c) 70mm below ground level.28 Comparisons of measured surface settlement and analytical solutions (After Loganathan et al..27 Normalised Vertical and horizontal soil movement profile at different subsurface elevations with best-fit curves: (a) 10mm below ground level. (After Grant and Taylor. (d) 100mm below ground level. 30 Definition of GAP parameter (After Lee et al. 1992) 55 ..Chapter 2 Literature Review Figure 2.. 2006a) Figure 2.29 Plastic deformation mechanism for tunnels in clay (After Osman et al. 2005) 56 . 1998) Figure 2.31 Oval-shaped soil displacement around tunnel boundary (After Loganathan and Poulos.Chapter 2 Literature Review Figure 2.32 Boundary conditions of prescribed displacement (After Park. 34 A piled bridge pier foundation assessed during the CTRL project (After Jacobsz et al. 2005) Figure 2.Chapter 2 Literature Review Figure 2.. 2005) 57 . in Dagenham. UK (After Selemetas et al.33 Zone of influence due to Earth Pressure Balance (EPB) shield tunneling in London clay for Channel Tunnel Rail Link (CTRL) Contract 250. Essex.. Chapter 2 Literature Review Figure 2. 2005a) 58 ..35 Typical section and instrumentation layout for pile-tunnel interaction study for MRT North East Line Contract 704 in Singapore (After Pang et al..36 Post-tunneling measurement of the development of axial force in pile P1 at Pier 20 showing the time-dependent behaviour of drag-load MRT North East Line Contract 704 in Singapore (After Pang et al. 2005a) Figure 2. (After Cham.38 Illustration of positions of existing instrumented piles relative to tunnels for MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore. 2005a) Figure 2..37 Responses of pile foundation in terms of (a) axial force and (b) bending moment for MRT North East Line Contract 704 in Singapore (After Pang et al. 2007) 59 .Chapter 2 Literature Review Figure 2. 40 Tunneling-induced pile bending moments (After Loganathan et al.39 Configuration of centrifuge tests (After Loganathan et al.. 2000) Figure 2. 2000) 60 .Chapter 2 Literature Review Figure 2.. Chapter 2 Literature Review 0 0 -5 -5 Depth below GL (m) Depth below GL (m) Figure 2.42 Variations of (a) induced pile bending moment profiles and (b) induced pile lateral deflection profiles with time in typical test (After Ran. 2004) 61 .41 Tunneling-induced pile axial loads (After Loganathan et al. 2000) -10 -15 -20 -10 -15 -20 -25 -25 -250 -200 -150 -100 -50 Pile bending moment (kNm) 0 50 -2 0 2 4 6 8 Pile lateral deflection (mm) Figure 2.. 2005) 62 ..Chapter 2 Literature Review 0 0 -5 Depth below GL (m) Depth below GL (m) -5 -10 -15 -20 -10 -15 -20 -25 -25 0 50 100 150 Pile axial force (kN) 200 250 5.95 6 6.43 (a) Induced pile axial force profile and (b) pile settlement profile at 2 days in typical test (After Ran.05 6.44 Zone of influence around tunnel in which potential for large pile settlements exists (After Jacobsz et al.85 5.1 Pile settlement (mm) Figure 2. 2004) Figure 2.9 5. Chapter 2 Literature Review Figure 2. 2005) 63 .45 Settlement.. rotation and load distribution on triple pile group (After Jacobsz et al. 1999) Figure 2..46 Layout of basic problem (After Chen et al.Chapter 2 Literature Review Figure 2. 1999) 64 ..47 Tunneling-induced pile responses and Greenfield soil movement (After Chen et al. .. 2001) Figure 2. 2005b) 65 .48 Numerical analysis of pile-group responses due to tunnelling (After Loganathan et al.49 Typical 3D finite elements mesh to back-analyze a case history on the response of pile foundation subjected to shield tunnelling (After Pang et al.Chapter 2 Literature Review Figure 2. Chapter 2 Literature Review Figure 2.50 Prediction of responses of pile foundation in terms of axial force and bending moment using 3D finite element analysis (After Pang et al., 2005b) 66 Chapter 3 Experimental Set-up and Procedures CHAPTER THREE EXPERIMENTAL SET-UP AND PROCEDURES 3.1 INTRODUCTION This chapter presents the details of centrifuge modelling technique, experimental setup and procedures adopted in the present study. A technique of simulating tunnelling in the centrifuge is developed and described in detail in this chapter. The preparation of the strong box, soil specimens, fabrications and configurations of the instrumented model piles as well as associated equipments are then elaborated. Finally, the reducedscale model set-up and test procedure are described in detail. 3.2 GEOTECHNICAL CENTRIFUGE MODELING 3.2.1 Principles of Geotechnical Centrifuge Modelling Recently, there has been rapid development in geotechnical centrifuge modelling technology world-wide, and centrifuge testing is now commonly used to study geotechnical and geo-environmental problems. Geotechnical centrifuge modelling has also been employed to complement conventional numerical analysis and field monitoring (Schofield, 1998; Ng et al., 1998; Kimura, 1998). Each approach has its own advantages in terms of quality of result, time and cost. Particularly in cases where there are uncertainties in the applicability of a proposed design methodology, use of 67 Chapter 3 Experimental Set-up and Procedures more than one approach permits calibration of results against each other and verification of conclusions drawn. Conventional scaled physical models, in spite of their advantages of wellcontrolled soil condition and extensive data monitoring, have significant limitations in their usefulness as the fundamental mechanical behaviour of soil is highly non-linear and stress-level dependent. However, by subjecting the 1/Nth scaled model in a geotechnical centrifuge to an enhanced gravitational field N times the earth gravity, the prototype stress can be reproduced in the reduced model, and the model test results can be used to interpret the prototype behaviour in a rational manner. An important principle of centrifuge modelling is to simulate the prototype stress conditions in the model. This is done by subjecting the model components to an enhanced body force, which is provided by a centrifugal acceleration of magnitude Ng, where g is the acceleration due to the Earth gravity (i.e. 9.81 m/s2). Stress replication in an Nth scale model is achieved when the imposed "gravitational" acceleration is equal to Ng. Thus, a centrifuge is suitable for modelling stress dependent problems. Moreover, significant reduction of test time for model tests such as consolidation time can be achieved by using a reduced size model. For centrifuge model tests, model scaling laws are generally derived through dimensional analysis from the governing equations for a phenomenon, or from the principles of mechanical similarity between a model and a prototype (Schofield, 1980, Tan & Scott, 1985, Taylor, 1995a). Various commonly used scaling relations between model and prototype can be deduced as summarized by Leung et al. (1991) in Table. 3.1. It can be readily deduced from Table 3.1 that the stress level of a 30-m deep clay 68 Chapter 3 Experimental Set-up and Procedures can be correctly modelled by subjecting a 30-cm deep clay model to an elevated "gravitational" acceleration of 100g (i.e., N=100). Also, a 4-hour centrifuge modelling at 100g can correctly simulate a prototype soil settlement problem consolidated for more than 4.5 years (i.e., 4xN2 or 4x1002 hours). Substantial time reduction and hence cost savings can be achieved by adopting the centrifuge modelling technique. 3.2.2 NUS Geotechnical Centrifuge Facility Figures 3.1 and 3.2 show the NUS centrifuge facilities. The centrifuge primarily consists of a conical case, a driven shaft, and rotating arm, and two swinging platforms. It has a capacity of 40,000 g-kg and operates up to a maximum g-level of 200g, implying that the allowable payloads at 200g and 100g are 200 kg and 400 kg, respectively. The structure of the centrifuge is based on the conventional dual swing platform design. The model package is normally loaded onto one of the swing platforms with the opposing platform counter balanced by either counterweights or the other model package with identical weights. When fully spun up during test operation, the distance from the axis of rotation to the base of the platform is 1.871 m. The centrifuge is driven by a hydraulic motor delivering up to about 37 kW power. The swing platform has a working area that measures 750 mm x 700 mm and headroom of 1180 mm. A stack of electrical slip rings is mounted at the top of the rotor shaft for signals and power transmission between the centrifuge and the control room. DC voltage is transmitted through the slip rings to the transducers mounted on the centrifuge or the model package from the control room. Similarly, registered 69 Chapter 3 Experimental Set-up and Procedures signals from the transducers are then transmitted via the slip rings. The signals are first filtered by an amplifier system at 100 Hz cut-off frequency to reduce interference or signal noise pick-up through the slip rings. The amplified signals are then collected by a data acquisition system at a regular interval in the control room. Software called Dasylab © is used to process the signals whereby the signals are smoothened using a block average. Two closed circuit cameras, which are mounted on the centrifuge, enable the entire in-flight test process to be monitored in the control room. The NUS centrifuge is described in detail by Lee at al. (1991) and Lee (1992). 3.3 EXPERIMENTAL SET-UP Figures 3.3 and 3.4 show the sketch and photograph of the model package for the present study, respectively. The main features of the centrifuge model are described as follows. 3.3.1 Model Tunnelling Technique There are many modes of ground movement associated with tunnel construction. In a situation where the tunnel excavation has passed a particular section is considered, the vectors of the ground movement developed will be more or less in the plane perpendicular to the tunnel axis. Consequently it is reasonable to assume that a plane strain model of long tunnel section would be a good representation of tunnellinginduced soil movements; this is usually referred to as a two-dimensional simulation (Taylor, 1998). 70 Chapter 3 Experimental Set-up and Procedures In the present study, an innovative model tunnelling technique has been developed to simulate the inward tunnel deformation due to over-excavation. An ovalshape ground deformation pattern is imposed as the boundary condition and the gap parameter (GAP) proposed by Lee et al. (1992) is used to quantify the amount of tunnel over-cut. Loganathan & Poulos (1998) and Park (2005) evaluated that an ovalshape deformation pattern is in reasonable agreement with tunnel deformations observed in the field, as reported in Chapter 2. The longitudinal and cross section of the innovative model are shown in Figures 3.5 and 3.6, respectively. The model tunnel is made of a circular rigid outer plate and a hollow metallic circular tube of 60 mm diameter, simulating a 6-m diameter prototype tunnel at 100g. The rigid plate helps to maintain a uniform GAP for the entire model tunnel. The two radial bearings inside the model tunnel help to facilitate a smooth movement of the sliding rod and provide support to the solid aluminium sliding rods. There are nine small rods which are inserted into the respective holes of the model tunnel. A rigid circular plate is then used to encircle the model tunnel and an oval-shape GAP is created between the rigid circular plate and the point of contact of nine small rods. The whole mechanism works as such when there is a force pushing the aluminium sliding rod, the small rods will fall onto the three thinner parts of sliding rod of smaller cross-sectional area. As such, the GAP in crosssectional view will close up and this simulates the inward tunnel deformation of the oval-shape GAP. 71 the circular rigid outer plate shown in Figure 3. The percentage of volume loss has been calibrated by calculating the area of surface settlement against the GAP (see section 2.1. the hydraulic force can immediately push the sliding rod inside the model tunnel forward and the small rods will then fall onto the thinner part of the sliding rod. This causes an ‘immediate’ closure of the GAP between the tunnel linings and simulating the volume loss.1 Advantages of Model Tunnel The present model tunnel is able to simulate the precise volume loss during the process of tunnelling.6 can provide a very uniform oval-shaped of the GAP throughout the entire length of the model tunnel. as the model tunnel is mechanically controlled. This ensures that the volume loss is constant along the model tunnel.3 )created in the model tunnel at the undrained stage.3. the settlement trough is measured and the volume loss is validated.Chapter 3 Experimental Set-up and Procedures 3. Moreover. For the innovative mechanism created with the control of hydraulics system. The accuracy and repeatability of volume loss control are good. the closure of the GAP between the tunnel linings can be more effectively controlled.3. The test has been repeated and consistent test results are obtained as in each test. 72 . With a switch of hydraulics pump. The wish-inplace model tunnel is a simplification and idealization of a cavity contraction. The minimum volume loss that can be simulated by the present model tunnel is only 3% in order to maintain the accuracy of the experiment results. but with an oval-shaped GAP and well controlled volume loss to simulate the soil movements induced by tunnelling when the tunnel excavation has passed a particular section. jet grout etc.1. The accuracy of volume control will be demonstrated in Section 4. it is believed that this effect is unlikely to be significant as the test is properly conducted after the forced acceleration field is stabilized. This does not simulate the real tunnel excavation process whereby the process of tunnelling shall include excavation.3. Nevertheless. Owing to the constraints and difficulties faced in the model set-up in the centrifuge.Chapter 3 Experimental Set-up and Procedures 3. the magnitude of volume loss depends primarily on the method of tunnelling and soil conditions. the model tunnel is pre-installed at 1g instead of in-flight tunnel excavation.3.2. This means that the GAP is as small as 1-mm in model scale.2 Limitations of Model Tunnel The model tunnel is a two-dimensional plane strain model. Although the ‘rigid’ outer aluminum lining may exert some stress around lining during centrifuge spinning up. the results of the present study are still a good representation of tunnelling-induced soil movements as elaborated by Taylor (1998). Nevertheless. The three-dimensional effects of tunnelling before tunnel approaching and after tunnel passing by cannot be modelled. Few attempts have been carried out to model lower volume losses in centrifuge but the results so far are not satisfactory. Although improvements in tunnelling technology have 73 . To achieve the volume loss of 3% in 100g. a GAP parameter of 100-mm in prototype scale is needed. installation of tunnel lining. The Civil Design Criteria for Road and Rail (LTA. 6. a volume loss of 3% is simulated in the present study. depending on the tunnelling method (15% volume loss should be considered if using TBM with compressed air) (LTA. To evaluate the detrimental effect of higher volume loss.7.35 mm internal width.2 Instrumented Model Piles Two different types of instrumented model piles are used in the present study to examine the responses piles due to tunnelling.53 mm external width and 6. The final external width of each pile shaft is 12. see Figure 3. 2009) recommends the contractor to demonstrate the suitability of the selected volume loss values in relation to the values of volume loss that would occur during tunnelling.6-m diameter in marine clay are in the range of 2% to 3. The typical values for tunnels up to 6. The detailed connection principles and load-output relations were elaborated in Feng (2003). The ‘bending’ and ‘axial’ piles were connected to the strain meter with half-bridge mode and full-bridge mode.26 m in prototype scale.3.5%. The strain gauges were wired and then connected with a TDS-300 strain meter mounted on the centrifuge to form a full Wheatstone bridge circuit utilizing the dummy strain gauges provided in the strain meter to produce a temperature compensation system. 3.5% is also simulated. They were fabricated using square aluminium tubes of 9. The strain gauges were protected by a thin layer of epoxy resin for waterproofing. In view of the above.Chapter 3 Experimental Set-up and Procedures significantly reduced the volume loss due to tunnel excavation.6 mm corresponding to 1. 2009). Ten pairs of strain gauges were attached along the pile shafts to measure the bending moments along one type of pile (termed ‘bending’ pile) and axial forces along the other type of pile (termed ‘axial pile’). respectively. A very 74 . Conical tip was chosen to minimise the deviation of the piles from the vertical during installation.Chapter 3 Experimental Set-up and Procedures thin and light PVC plate with smooth and dark surface was attached to the ‘bending’ pile to facilitate reflection of laser-rays. the strain gauge outputs were then related to the calculated bending moments. A pile with a higher flexural rigidity tends to attract larger bending moments but a lower pile deflection. The corresponding strain gauge outputs were then related to the axial force. The flexural rigidity. EI. EA. the reconsolidation of the soil before simulating the tunnel excavation is deemed to be necessary in order to recover the initial stress 75 . On the other hand. It should be noted that it is not possible to correctly simulate the pile axial rigidity. Hence. EI of a prototype pile simultaneously. The calibration of the pile bending moment and axial force was conducted separately prior to the tests.97x106 kNm2 at 100g. The ‘axial’ pile was calibrated by applying incremental loads on the top of the pile resting on a digital balance. the settlement of a loaded pile will be mainly due to the compression of soil while the elastic shortening of the pile shaft is normally negligible as long as the pile axial rigidity is relatively high as in the present case. ‘Bending’ pile was calibrated by fastening the pile head with a G-clamp and hanging mass centrally at the pile tip. For example. The flexural rigidity is more crucial as the pile bending moments and lateral deflection are more sensitive. and flexural rigidity. of the model pile. the pile axial force and settlement is less sensitive to pile axial rigidity. is 3. for the purpose of measuring the pile deflection. The model piles are installed in 1g and positive excess pore water pressures are generated during installation. which is equivalent to that of a 1300-mm diameter Grade 40 concrete bored pile. the pile cap bending rigidity depends on the configuration of the piles facing the tunnel excavation. the EI of the cap is 1x 109 kNm2.24 x 108 kNm2 . Figure 3. the rotation or movement of the pile-pile cap connection can be minimized. The prototype pile cap bending rigidity for the 2 groups is 3. Pore pressure transducers are installed to monitor and ensure that the equilibrium state is achieved before tunnel excavation.3.3 Model Pile Cap The model pile cap is made of aluminium with a thickness of 25 mm or 2. This is an improved design compared to previous pile caps fabricated at NUS (Lim. It has internal dimensions of 525 mm × 200 mm × 490 mm (length ×width ×height).and 6-pile group configurations.8 shows the pile caps used in this study. 2004) as the pile caps were specifically designed to enable each pile in the group to be tightened individually by tow rows of bolts in both directions.5 m thick at 100g. 2001. One 76 . Ong.3. For the 6-pile group. the prototype EI of the pile cap is similar to the 2-pile group case. in a 6-pile group of 3x2 configurations.Chapter 3 Experimental Set-up and Procedures level of the soil and to allow the full dissipation of excess pore water pressure. Two types of pile caps were fabricated for the 2. 3. However. If the 6-pile group consists of 3 rows of 2 piles per row (2 piles x 3 rows) facing the excavation. Thus. 3.4 Strong Box The strong box is made of stainless steel alloy to contain the soil specimens. 053. Kaolin clay has critical state parameters λ of 0. plastic limit (PL) of 40 % and hence a plasticity index (PI) of 40%. N of 3.65. respectively.35 and M of 0.9 shows the measured in-flight undrained shear strength profile of the Kaolin clay used at NUS. and is consistent with that for normally consolidated clay. are 40 m2/year and 2×10-8 m/s. average κ of 0. and a specific gravity.9. all the inner walls of the strong box are heavily greased. A measuring tape is attached to the Perspex wall to provide reference co-ordinates in order to check the depth of the clay. using miniature T-bar developed by Stewart and Randolph (1991). 77 . The coefficient of consolidation Cv and permeability at pressure of 100 kPa. This would help to ensure the deformation of the model ground is under plane strain condition. 3. Gs. The effective internal friction angle. 2003). is 23o .2 (Goh. below which the shear strength increases nearly linearly with depth. The undrained shear strength profiles from the five tests are consistent and repeatable The profile indicates an over consolidated layer down to 40 mm.Chapter 3 Experimental Set-up and Procedures sidewall of the strong box is made of a 75-mm thick transparent Perspex plate. Both the front (Perspex plate) and back walls of the strong box can be removed to facilitate the installation of model tunnel and transducers during the model set-up.244. φ’. To minimize the soil/strong box friction. Figure 3.3. which allows image acquisition by a video camera mounted to the centrifuge platform. of 2.5 Kaolin Clay The physical properties of Malaysian kaolin clay used in the present study are summarized in Table 3. It has a liquid limit (LL) of 80%. 2 mm and specific gravity.3 (Teh et al.7 Potentiometers Potentiometers (model LP-50F-61) were used to measure the surface settlements and pile head settlements during the tests. The physical properties of Toyoura sand are listed in Table 3. Each 78 . The working part of the instrument consists of a resistant and a rod whose stretch can alter the resistance of the resistor and hence the output voltages.. This model has a measuring range of 50 mm and an independent linearity of ±0.2 %. the PPTs were de-aired using an electronic vacuum pump to release trapped air bubbles in the PPTs to prevent acquisition of inaccurate readings. The critical state friction angle is 32o. 2005). The minimum and maximum density of the sand is 1335 kg/m3 and 1645 kg/m3. A round plastic plate is attached to the tail end of the rod to stop it penetrating into the clay. Before the test was carried out. Gs. 3.3.8 Pore Pressure Transducers (PPT) Druck PDCR81 miniature pore pressure transducers (PPT) were used to monitor the variations in pore water pressures during the centrifuge tests. 3.Chapter 3 Experimental Set-up and Procedures 3. It has an average particle size of 0.6 Toyoura Sand The sand that underlies the clay serves as drainage channel and socket for the pile.65. of 2. The output voltages are then linearly translated to the measured distance.3.3. respectively. a direct relationship between 79 . a digital air pump and a multimeter were used to calibrate the PPTs. The relay cable connects the sensing body to the DC power supply. The calibration check was conducted by pumping air into the PPTs and recording simultaneously the air pressure as well as the PPTs output voltage readings measured by the multimeter.9 Non-Contact Laser Transducers NAIS micro laser sensor LM10 (model ANR1250) were used to measure the lateral pile head deflections during and after tunnel excavation. This model of sensors has a centre point distance (distance between sensor and target) of 50 mm and a measurable range of ±10 mm within the centre point distance.1 mm at the centre point distance.5 mm at prototype scale. the relay cable and the controller/display unit. To confirm the manufacturer’s factors. The light source comes from a laser diode and has a wavelength 685 nm and beam dimension of 0.5 μm.6 mm x 1. Hence. Calibration was carried out by securely attaching a 100-mm travel potentiometer to the sensing body of the laser sensor.3. It has a linear resolution of 0. the sensing body. The laser sensor has three main components.Chapter 3 Experimental Set-up and Procedures PPT comes with its own manufacturer’s calibration factor and this is incorporated to determine the magnitude of pore water pressure. The transducer could take up to a maximum of 10 V. namely. which translates to a linear error of 0. The transducer was connected to a multimeter so that the digital display of the voltage could be displayed. The sensing body houses the laser diode and its function is to emit laser beam upon connected to a power supply of 24V DC. The controller/display unit is used to control and set the measuring limit of the sensor. 3. Particle Image Velocimetry (PIV) is used to process the resulting images (White et al. 2003. 3. i. Zhang et al. However.10. readings outside this optimum range can still be measured by the laser sensor but to a lesser degree of accuracy. However.. 1 V per 10 mm movement of the transducer.4 IMAGE ACQUISITION SYSTEM An advanced technique of image analysis has been developed at NUS as a method of acquiring soil movement profiles from high-solution images captured in the centrifuge model tests. The output voltage reading on the laser sensor display unit varies with the displacement. The laser sensor has a specified optimum range of measurement to ensure accuracy of the reading.5 fps during tunnel excavation and at slower rate during post tunnelling. The camera’s maximum grabbing speed is 15 frames per second (fps). Each set of readings of the transducer and the laser sensor were recorded at every specified displacement intervals so that correlation between displacement and voltage could be established. Subsequently.Chapter 3 Experimental Set-up and Procedures displacement and voltage could be established. the capturing rate was set at 0. calibration is ensured to lie only within this optimum range.e. Therefore. As such. 3. 80 .4.1 High Resolution Camera JAI ©CV-A2 progressive scan camera coupled with a Tamron © lens was mounted in front of the Perspex window of the model container. 2005).. the transducer serves as an indication or a ‘ruler’ for the calibration of the laser sensor. as shown in Figure 3. as shown in Figure 3. two spot lights. in which the hard disk of the on-board computer was specially designed to provide greater resilience to physical vibration.4.10.Chapter 3 Experimental Set-up and Procedures 3. The on-board computer was remotely controlled by another commands computer in the control through a wireless connection to activate the capturing of live images of the soil movement during an in-flight centrifuge test.3 On-Board and Command Computers The progressive scan camera is connected to a computer installed on-board of the centrifuge. were each mounted at the frame arm to provide uniformly distributed lighting across the soil sample. All captured images during an inflight centrifuge test were stored in this on-board computer. each with a 50 W halogen bulb. As shown in Figure 3. which will be processed subsequently. The on-board computer is capable of sustaining high gravitational force without being damaged.4.12 displays the picture captured by the JAI ©CV-A2 progressive scan camera.2 Lighting System Lighting system is important in producing high quality images for the purpose of post processing the data. 3. All captured images could be retrieved from the on-board computer after tests.11. 81 . Figure 3. The florescent lights inside the centrifuge enclosure were turned off during the capturing of images while the centrifuge was in operation. shock and extreme temperature fluctuations. Consider one of these test patches. Zhang et al. White et al. To find the displaced location of this patch in a subsequent image. PIV operates by tracking the texture (i. The initial image is divided into a mesh of PIV test patches. Details are presented in White and Take (2002). (2005).4. (2005) confirmed that this high precision can be achieved in centrifuge conditions.v1) in image 1. Take & Bolton (2004) and Zhang et al. 2002) has been used to implement the PIV technique for post processing of the images captured during the centrifuge test.14. the spatial variation of brightness) within an image of soil through a series of images. the following operation is carried out.e. The PIV technique increases the details and precision of deformation measurements..4 Post-Processing of Images New technique of image analysis. This operation is repeated for the entire mesh of patches within the image. The location at which the highest correlation is found indicates the displaced position of the patch (u2. to produce complete trajectories of each test patch. The GeoPIV software (White and Take. (2003) and Zhang et al. The principles of PIV analysis are summarized in Figures 3. located at coordinates (u1. The correlation between the patch extracted from image 1 (time = t1) and a larger patch from the same part of image 2 (time = t2) is evaluated.. 2003. 2005) has been recently applied to geotechnical centrifuge modelling. The location of the correlation peak is established to sub-pixel precision by fitting a bicubic interpolation around the highest integer peak. 82 .Chapter 3 Experimental Set-up and Procedures 3.v2). particularly Particle Image Velocimetry (PIV) (White et al.13 and 3. and then repeated for each image within the series. it is necessary to provide an artifical texture on the clay surface as PIV technique requires random and unique textures of soil patches within the images for an accurate analysis. The set-up of the test is shown in Figure 3. Hence. Two methods of measuring the surface settlement are adopted. It is observed that the measured settlement analysed by image processing method agrees well with the direct measurement of the surface settlement using potentiometer. with an error less than 5%. while on the right handsied of the model tunnel. as well as the performance of flocks and beads as material in creating artificial textures for PIV analysis purpose. Figure 3. beads demonstrate a higher accuracy compared with the flocks. 1-mm diameter black/blue/red beads were randomly embedded on the surface of plain white clay. PIV is used to track the soil markers so that the soil movement due to tunnelling can be quantified. sand) for the application of PIV technique (Zhang et al.16 shows the settlement measured by these methods. On the left handside of the model tunnel. The first method is the direct measurement of the surface settlement using potentiometer and the second method involves measuring the soil settlement by tracking the movement of flocks and beads using the image processing method. there is not enough natural texture (e.Chapter 3 Experimental Set-up and Procedures 3.15. A typical reconsolidation stage of a centrifuge test has been conducted to assess the effectiveness of the image processing system. 2005). the exposed clay surface was sprinkled with black and gray flocks.g.5 Assessment of Effectiveness of Image Processing System The image processing technique. It 83 .4. For clay such as kaolin. Amongst these two different materials.. Chapter 3 Experimental Set-up and Procedures is probably the beads are embedded in the soil and move freely together with the soil. To prevent air trap in the clay. some of the pore water may come out from the rubber tubes. the clay sample was carefully poured into the strong box.2 in a de-airing mixer. water is poured into the strong box before the clay is poured. The clay was then pre-loaded under a pressure of 20 kPa. 3.1 Preparation of the Soil Sample The model ground was remoulded from Malaysian kaolin clay powder and water at a weight ratio of 1 to 1. Three rubber tubes are used to act as drainage for pore water of the soil under consolidation. After that. In view of this. The clay slurry was then carefully scooped into the strong box by immersing into water body to prevent air trap until it reaches a predetermined height.5 EXPERIMENTAL PROCEDURE 3. the soil container was shifted to a pneumatic loading frame. After 4 hours of mixing. which further helps to act as drainage. The underlying sand and rubber tubes together act as drainage function in this case. 84 . During the loading process.5. A thin geotextile was placed on top of the sand to separate the sand and clay. The lower part of the tubes is covered by the sand. in which a 30-mm thick sand layer is placed at the bottom. the image processing analysis can be considered a competent and reliable method to measure the soil movements in the present study. and different colours of beads are used to create unique textures. but the flocks are only spread on the surface of clay and hence influenced by the greased applied on the Perspex windows. Based on the observation. The results are shown in Figure 3. this can eliminate the uncertainty in term of long term self-weight soil consolidation in the analysis of the test data after tunnelling.2 Pre-Consolidation Process Pore pressure and soil settlement throughout the in-flight consolidation and reconsolidation were monitored by PPTs and potentiometers.18). Hence. the corresponding hydrostatic pressure at PPT can be calculated from the difference between the two measurements. as swelling of the soil sample was noted due to stress release during the set-up installation at 1g. 85 . The pore pressure and soil surface settlement appear to be stabilized after 8 hours of consolidation. a total of 6 hours are required to restore the final soil elevation during the earlier preconsolidation stage.17. The pore pressure and soil surface settlement of a consolidation test over 13 hours are recorded so as to optimise the duration of spinning and for the ease of the analysis.5.Chapter 3 Experimental Set-up and Procedures 3. whereby further analysis using Asaoka’s method (1978) depicts that the final settlement after 8 hours approaching 100% degree of consolidation (see Figure 3. Since the position of the PPT was fixed and the elevation of the free water was known from the potentiometer attached with floating ball. The same monitoring was carried out at the reconsolidation stage for every test to ensure that complete consolidation was restored. The wooden tunnel installation guide was designed in such a way that it can be used to make sure that the model tunnel is inserted into the clay perpendicularly so as to minimize any soil disturbance. Another layer of grease is again applied to the back wall to make sure that the clay’s boundary will act as free-roller support. which was secured by a small frame. was then be connected to the hydraulic hose. whereby the hydraulic force will be used to push the sliding rod inside the model tunnel so as to simulate the closure of the oval-shaped tunnel gap. the back wall of the strong box was opened. The model tunnel end cap is connected to the hydraulic hose through an aluminium tube. Along the tunnel installation guide.19. PPTs will then be used to monitor the change in pore water pressure throughout the entire experiment.4 Preparation Works for PIV Analysis After installed the model tunnel and PPTs.3 Installation of Model Tunnel and PPTs in 1g Following the completion of self-weight consolidation.8 mm wall-thickness) was inserted into the clay and excavates a cylindrical cavity through the two openings. 3.5. As shown in Figure 3. the back wall was then fixed back and the front wall was then removed. 0.5. the stainless steel tube (60 mm in diameter.Chapter 3 Experimental Set-up and Procedures 3. Similarly. The model tunnel was then subsequently inserted into the cylindrical cavity. different colours of 1-mm 86 . This will enhance the accuracy of the whole experiment. The aluminium tube. PPTs installation guide was used to ensure that the PPTs can be carefully inserted into the clay perpendicular to the soil surface. The tunnel installation guide will be fixed in position by 2 G-clamps. The pile guide is made of Perspex.5. Permanent control markers dots with known centre to centre distance were marked on the Perspex window in order to provide reference points to the subsequent image analysis by PIV. the model piles were then carefully installed at the predetermined distance at 1g. 3. Two non-contact laser transducers were used to measure the lateral deflection of the pile head.Chapter 3 Experimental Set-up and Procedures diameter beads were randomly embedded on the surface to produce an artificial texture for the subsequent analysis of PIV.21. The model package was then spun up to 100g in 10 steps at 5 minute intervals for reconsolidation 87 . The transducers were attached to a stainless steel holder mounted tightly onto the top of the container.5. The pile installation guide was designed in such a way to ensure that the model pile could be inserted vertically into the clay and this also minimizes the disturbance to the clay.6 Test Procedures The completed set-up of the entire model package is shown in Figure 3. These beads are made of light PVC so that they could move with the soil freely. The distance between the laser transducers and the bending pile is about 50-mm. Two transducers were placed on the pile head to measure the model pile settlement. 3.5 Installation of Model Pile at 1g Using the pile installation guide. The beads were pushed into the soil by the highly greased Perspex window (see Figure 3.20) of the strong box to ensure a full perfect contact and the beads can move together with the soil. All instruments were monitored regularly throughout the test.Chapter 3 Experimental Set-up and Procedures of the clay. When the sliding rod was pushed forward. 88 . the small rods lying on the sliding rod would drop onto the thinner part of the sliding rod with smaller cross-sectional area. the total pore pressure would be restored to the same state as that in the consolidation stage and the test then began when the hydraulic valve was switched on and the hydraulic force would push the sliding rob inside the mechanical tunnel forward. After about 6 hours. This caused the gap between the rigid aluminium plate and the model tunnel to close and the inward tunnel deformation at the tunnel spring line was thus simulated in this way. The centrifuge would be kept at 100g for 3 hours after the completion of tunnel excavation. The model tunnel was left in place to simulate the tunnel lining to study the post-excavation ground deformation and pile responses. Chapter 3 Experimental Set-up and Procedures Table 3.1 Scaling relation of centrifuge modeling (After Leung et at. 1991) Parameter Prototype Centrifuge Model at Ng Linear dimension 1 1/N Area dimension 1 1/N2 Volume dimension 1 1/N3 Density 1 1 Mass 1 1/N3 Acceleration 1 1/N Velocity 1 1 Displacement 1 1/N Stress 1 1 Strain 1 1 Force 1 1/N2 Time (viscous flow) 1 1 Time (seepage) 1 1/N2 Flexural rigidity 1 1/N4 Axial rigidity 1 1/N2 Bending moment 1 1/N3 89 . 0~5.2 Physical properties of Malaysian kaolin clay (After Goh.3 Physical properties of Toyoura sand (After Teh et. 2003) Property Value Liquid limit. Gs 2.5 μm Modified Cam-clay parameters: M 0. al. k 2×10-8 m/s Angle of internal friction.244 κ 0. 2005) Property Specific gravity. PL 40% Specific gravity. φ' 23o Particle size* 3.9 λ 0.2 mm Particle size. cv 40 m2/year Coefficient of permeability at 100 kPa.163 mm Minimum dry density. d10 0. d50 Value 2. φcrit 90 . ρ min 1335 kg/m3 Maximum dry density. Gs Average particle size. ρ max 1645 kg/m3 Critical state (constant volume) 32o Friction angle.35 * Manufacturer data Table 3.65 0.053 N 3.65 Coefficient of consolidation at 100 kPa. LL 80% Plastic limit.Chapter 3 Experimental Set-up and Procedures Table 3. Chapter 3 Experimental Set-up and Procedures Rotating arm (In-flight position) Drive shaft Swing platform Bearings (Static position) Conical base Figure 3.1 Schematic diagram of NUS geotechnical centrifuge Slip Rings On-board camera Strain Meter Balance Arm Counter Weight Payload Conical Base Figure 3.2 Photograph of NUS geotechnical centrifuge with the model package mounted on the platform 91 . 4 Photograph of a typical centrifuge model package 92 .3 Sketch of a typical centrifuge model package (All dimensions in mm) Potentiometers Lasers Beads Tunnel Camera Figure 3.Chapter 3 Experimental Set-up and Procedures Figure 3. Chapter 3 Experimental Set-up and Procedures Figure 3.6 Cross-section of model tunnel 93 .5 Longitudinal view of model tunnel set up GAP GAP Figure 3. Chapter 3 Experimental Set-up and Procedures 9.7 Instrumented model pile (All dimensions in mm) Figure 3.53 25 25 25 25 25 25 25 25 25 25 15 50 70 40 12.6 Strain gauge Plate to measure deflections by lasers Epoxy coating Aluminum tube End cap Figure 3.8 Model pile caps 94 . 10 Image acquisition system 95 .9 In-flight undrained shear strength of clay Lighting system Camera CCTV Figure 3.Chapter 3 Experimental Set-up and Procedures Undrained Shear Strength (kPa) 0 5 10 15 20 25 30 35 40 0 2 4 6 Depth (m) 8 10 12 14 16 18 20 22 TEST 1 TEST 2 TEST 3 TEST 4 TEST 5 24 Figure 3. Chapter 3 Experimental Set-up and Procedures Wireless system On board Imaging PC On-board Camera Strain meter Hydraulic hose for connection to tunnel Figure 3.11 On board set-up Control marker Model tunnel Texture clay Figure 3.12 Picture captured by JAI ©CV-A2 progressive scan camera for PIV analysis 96 . (After White et al. 2003) Figure 3. 2003) 97 . (c) sub-pixel interpolation using cubic fit over ± 1 pixel of integer correlation.14 Evaluation of displacement vector from correlation plane.13 Image manipulation during PIV analysis.. (After White et al. Rn(s): (a) correlation of Rn(s).. (b) highest correlation peak (integer pixel).Chapter 3 Experimental Set-up and Procedures Figure 3. Chapter 3 Experimental Set-up and Procedures LVDT LVDT Control marker Flocks Beads Figure 3.15 Experimental set-up for assessment of effectiveness of image processing system and comparison of performance of flocks and beads Potentiometer 1 -694mm Potentiometer 2 -702mm Flocks PIV -662mm Beads PIV -690mm Error=-4.5% Error=-1.5% Figure 3.16 Results of assessment of effectiveness of image processing system and comparison of performance of flocks and beads 98 Chapter 3 Experimental Set-up and Procedures 400 Total pore pressure (kPa) 300 200 100 PPT 11403 T Hydrostatic 0 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 Model time- consolidation (hours) Model time- consolidation (hours) 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 Model settlement (cm) 0 1 2 3 4 5 Figure 3.17 Pore pressure dissipation and settlement during consolidation stage 5 Settlement, Si (cm) Sult= β0 / (1-β1) 4 β0=2.4615038 3 β1=0.99925896 2 Sult= 3.325 cm 1 U=Sf /Sult= 3.325/3.325=100% 0 0 1 2 3 4 5 Settlement Si-1 (cm) Figure 3.18 Estimation of ultimate settlement by Asaoka’s method (1978) 99 Chapter 3 Experimental Set-up and Procedures Model tunnel Texture clay Figure 3.19 Different colours of beads were randomly embedded on the surface of clay Control marker Figure 3.20 The Perspex window is highly greased to ensure free movement of soil 100 Chapter 3 Experimental Set-up and Procedures Model pile Potentiometer Lasers Model tunnel Figure 3.21 Set-up of the entire model package in 1g (top view) 101 Chapter 4 Basic Test on Volume Loss CHAPTER FOUR BASIC TESTS ON VOLUME LOSS 4.1 INTRODUCTION In order to interpret the pile behaviour due to tunnelling-induced soil movement, it is important to examine the mechanism of tunnel-soil interaction. Particle Image Velocimetry (PIV) technique (White et al., 2003; Zhang et al., 2005) has been used in centrifuge model tests to obtain more accurate and detailed information on soil displacements, as described in Chapter 3. With a better understanding and results obtained in free-field experimentally in the present study, further evaluations can then be made on tunnelling-induced pile responses. 4.2 TEST PROGRAM The first half of this chapter presents the results of study on free-field soil movements due to tunnelling. The test results on the effects of tunnelling with the same volume loss on single free head piles are then presented in the second half of this chapter. The complete test program presented in this chapter is shown in Table 4.1. The centrifuge tests were performed under 100g. Unless otherwise stated, the test configurations and results are presented in prototype scale hereinafter. For all tests, the thickness of kaolin clay layer and the underlying Toyoura sand layer is 24 m and 3.5 m, respectively. The tunnel cover C (distance from ground surface to tunnel crown) and tunnel diameter D 102 Chapter 4 Basic Test on Volume Loss is 12 m and 6 m, respectively. The schematic and digital image of a typical test are shown in Figures 4.1 and 4.2, respectively. In the present study, the terminology “short-term (ST)” refers to the stage in which tunnel excavation has just been completed, under undrained condition. On the other hand, “Long-term (LT)” refers to the stage when the soil has completed consolidation due to tunnelling. After 720 days, it was observed that changes in the ground movement and pile responses are negligible. Hence the time of 720 days after tunnel excavation is taken as reaching the long-term stage. As discussed in Section 3.1.3.1.2, the magnitude of volume loss depends primarily on the method of tunnelling and soil conditions. The typical volume loss design values for tunnels of up to 6.6m diameter in marine clay are in the range of 2% to 3.5%, depending on the method of tunnelling (15% volume loss should be considered if using TBM with compressed air) (LTA, 2009). In view of the above, a volume loss of 3% (Test 1) is simulated in the present study. To evaluate the detrimental effects of higher volume loss, 6.5% (Test 2) is also simulated. Test 1 has been repeated to evaluate the repeatability and consistency of the test with and without the presence of pile. During both tests (with and without presence of pile), the beads were randomly embedded on the surface to produce an artificial texture for the subsequent analysis of PIV. Images captured in days 2, 180, 360, 540 and 720 were analyzed. It is observed that the results are consistent for both tests as long as the pile is installed far enough behind the Perspex window of the strong box. Both Tests 1 and 2 presented in this chapter refer to centrifuge model tests conducted without pile. 103 respectively. as the circumferential soil stresses increase within this zone to support the arches formed in the immediate shear zone. This leads to the observed soil movement pattern and the settlement trough at the ground surface. Figures 4.1 Cumulative Soil Movements The cumulative soil displacement vectors and contours at different times after tunnel excavation can be obtained using the PIV technique. 2006) is 20 mm and 10 mm is often set as the alert level. The 10 mm movement is selected as the bench mark because the maximum allowable settlement for shallow foundation as specified by the Civil Design Criteria for Road and Rail (LTA. For clay. In the short-term. the soil does not settle as a rigid body but gradually deforms by arching.Chapter 4 Basic Test on Volume Loss In addition. respectively. On the other hand.4 show the cumulative soil movement contours and vectors over time for both Tests 1 and 2.3(a). This zone may be identified as the ‘Immediate Shear Zone’ as the soil within this zone has likely been ‘unloaded’ due to tunnel excavation. principal soil movements are concentrated within a zone indicated in Figure 4. in these tests. 104 . For ease of comparison.3 and 4.3 TUNNELLING-INDUCED SOIL MOVEMENTS (TESTS 1 & 2) 4. The pile-to-tunnel distance and pile embedment length are kept constant as 6 m and 22 m. causing the radial stress in the immediate shear zone to be reduced due to stress relief.3. 4. Tests 3 and 4 were carried out in series 1 to investigate the behaviours of long pile (pile tip below tunnel invert) under two different tunnel volume losses. the contour of cumulative soil movement of 10 mm is highlighted as bold dash lines in the plots. the zone outside the immediate shear zone may be identified as the ‘Support Zone’. This corresponds to the respective imposed volume loss with simulated tunnel opening of approximate GAP = 100 and 200 mm (refer to definition of GAP in Section 2. 1969). This might cause the soil movements to increase in both the horizontal and vertical directions. S. The popularity of the Gaussian curve as a prediction tool for the magnitude of surface settlement due to tunnelling lies in its simplicity and efficiency.5%. i.6 show the measured surface settlement troughs over time obtained from PIV and potentiometers with volume loss of 3% and 6. This observation demonstrates that with relatively large volume loss (>3%). is given in Equation 2. The point of inflection. Based on the above findings. The surface settlement curve. It is evident that in the short-term (2 days).3. the surface settlement troughs follow a Gaussian distribution curve with a maximum ground surface settlement of 41 mm for a volume loss of 3% and 92 mm for a volume loss of 6. is 105 . It can be observed from the figures that the shear zone propagates with time and becomes wider over time due to post-tunnelling soil reconsolidation.Chapter 4 Basic Test on Volume Loss Qualitatively.4. respectively. as observed in Figures 4.2 Soil Surface Settlement Troughs The surface settlement trough along a plane transverse to the tunnel can be described by the Gaussian curve (Peck.3. the volume loss at the ground surface is close to the tunnel volume loss under such undrained condition. 4. there is further evidence that the accuracy of volume control of the model tunnel is good and reliable. it is expected that volumetric soil strain in the long-term would increase due to soil consolidation. the long-term effects of soil movement induced by tunnelling could be substantial. Figures 4.5 and 4.1.5%. As expected.3 to 4.3). Shirlaw (1993) presented case studies of long-term settlements and reported that the ground settlement due to tunnelling and the extent of settlement trough can increase significantly in the long-term in some cases. Figure 4. It should be noted that the soil has practically completed its self-weight consolidation before tunnel excavation. of the settlement trough is determined to be approximately 7.5 suggested by Mair et al.19. This value is identical to the prediction of 7.5 & 4. as illustrated in Figure 3. Hence the long-term soil movement is mainly due to stress relief of clay due to tunnel excavation and the settlement trough S are noted to become wider over time. confirming the accuracy and effectiveness of the PIV image processing technique. In the long-term.5 m for both tests.Chapter 4 Basic Test on Volume Loss determined from the settlement trough at the point when the change of gradient is zero. The remaining self-weight consolidation settlement of the soil should be very small.6. Similar good comparisons 106 . although the magnitude of maximum long-term ground settlement is larger. i. The point of inflection. using a trough width parameter k of 0. Thus it can be established that the observed settlement trough in the short-term can be reasonably predicted using existing methods.7 clearly shows that that the maximum surface settlements measured from potentiometers and derived from PIV match well. the differential settlement for a wider settlement trough is not as significant as that in the short-term. as shown in Figures 4. Gaussian distribution curve is found to be inappropriate for representing the long-term surface settlement trough with a wider parabolic shape. Nevertheless.5 m by Peck (1969). In contrast. (1993) for tunnels in clay. the ground settlement continues to increase with time. 3(a) and 4.Chapter 4 Basic Test on Volume Loss between the settlement measurements obtained by potentiometers and PIV are demonstrated in Figures 4.3 Subsurface Vertical Soil Movements The vertical soil movements can provide clues on the mechanisms associated with tunnel-soil-pile interaction.3. in comparison with existing predictive methods proposed 107 .7 demonstrates that the maximum surface settlements for both Tests 1 & 2 increase over time. 2 days). This finding implies that for tunnel in soft clay with relatively large volume loss.9 show the ST surface and subsurface settlement troughs for Tests 1 and 2.5 & 4. This large vertical deformation zone is critical and must be taken into consideration. The rate of increase in settlement is significantly reduced after a period of 360 days and becomes very small after approximately 720 days. Figures 4. Figures 4. 4. respectively.4(a) indicate that in the short-term (ST. particularly on the induced pile axial forces and settlements. The propagation of vertical soil movement trough seems to be an inverted ‘half-ripple’.8 and 4.3(b) to (e) and 4. the largest vertical soil movements are spotted in the immediate shear zone above the tunnel.4(b) to (e).6 as well. However. this zone becomes wider in the LT as shown in the contour plots over time in Figures 4. Figure 4. the surface settlements in the field should be monitored for the first 1 to 2 years after tunnelling. The maximum surface settlement often occurs at the tunnel crown for a single tunnel case. (1993) yields a better prediction as compared to the method proposed by Loganathan and Poulos (1998). the subsurface settlement profiles generally follow the prediction by Mair et al. The solution of vertical displacement around a tunnel excavation proposed by Loganathan and Poulos (1998) is given in Equation 2. (1993). Despite the above shortcoming. Mair et al. may not be accurate.7. care should be exercised when employing quasi-analytical methods to predict soil displacements due to tunnelling as certain conditions in the derivation of analytical solutions may not be valid (e. the back analysis generally validates the use of Mair et al. It should be noted that the maximum subsurface settlements measured in the experiments. (1993) and Loganathan and Poulos (1998).’s (1993) method to predict the subsurface settlements in the short-term. Hence. 1998)). i at different depths with the empirical method proposed by O’Reilly and New (1982) 108 . Figure 4. This is mainly due to the over-sizing of the tunnel end cap to prevent water seepage. In addition. The over-sized tunnel end cap greatly influenced the tracking of soil displacements which were subsequently analysed by PIV. especially when close to the tunnel. (1993) proposed that at a depth z below the ground surface. It is noted that the method proposed by Mair et al. (1993).g.Chapter 4 Basic Test on Volume Loss by Mair et al. volume loss may not be conserved (Loganathan and Poulos. and above a tunnel depth of zo. For the subsurface settlement troughs at various depths.3 and 2. the trough width parameter for tunnels constructed in clays are given by Equations 2. the influence zone predicted by Loganathan and Poulos (1998) is much greater than the measured data and prediction by Mair et al.4.10 compares the measured short-term and long-term inflection point. Zo-Z = 2i (4.1 and 4.2) In the present study.1) Zo-Z = 3i-8 (4. respectively.11 shows the ratio of iLT / iST at 109 .5 while Mair et al (1993) assumed that K varies with depth as given by Equation 2.2 for the method proposed by O’Reilly and New (1982) and Mair et al (1993).3. Hence the proposed distribution of the inflection point (i) with depth in clay in the long-term can be presented as Zo-Z = 3i-12 (4.Chapter 4 Basic Test on Volume Loss and Mair et al (1993).2 that yields convergence points lower than those in the short-term. Both methods are based on Equation 2. For the present tests in clay. a finding on the long-term behaviour is established revealing that the distribution of inflection point (i) with depth in clay can be reasonably approximated by a straight line parallel to Equation 4. The distribution of inflection point (i) with depth in clay can be simplified as Equations 4.3) It is worth examining more closely the relationship between inflection point (i) with depth in the short-term and long-term. the measured distribution of inflection point (i) with depth can be described by a straight line and the results are consistent with the prediction of Mair et al (1993).4. but O’Reilly and New (1982) assumed K=0. Figure 4. This observation is similar to the finding obtained from the centrifuge model tests conducted by Grant and Taylor (2000).4 Subsurface Horizontal Soil Movements The short-term and long-term lateral soil movements at various distances from tunnel centre-line are plotted in Figures 4.13. the horizontal soil movement caused by tunnelling diminishes with increasing distance away from the tunnel. The results from the analytical solution proposed by Loganathan and Poulos (1998) are also presented in the figures.3. the predictions by Loganathan and Poulos (1998) do not agree well with the measured data. 4. It can hence be deduced that iLT is approximately between 1. It is noted that the lateral soil movements form a bulb shape at the tunnel spring line. However. The proportion of horizontal to vertical movements at the surface is considerably greater than that at greater depths.5D from the tunnel circumference.21 to 1.e 12 m from tunnel centre-line. i. especially when the distance from the tunnel centre-line increases.29 times iST. This may be attributed to the condition that volume loss has not been conserved for undrained cases in their formulation and other factors.Chapter 4 Basic Test on Volume Loss different depths. the soil movements diminish rather rapidly in the horizontal direction and become negligible at distance of approximately 1.12 & 4. However. As expected. 110 . the soil sample is normally consolidated. To minimize the effect of reinforcement that the PPTs have on the ground.5 Qualitative Assessment on Excess Pore Pressure Responses Pore water pressure changes in the ground are monitored using pore pressure transducers (PPTs) during Test 1. This process continues until the effective stress in the ground is equivalent to the preconsolidation pressure. It is observed that additional excess pore pressure is 111 . only 2 PPTs were used. the initial pore pressure at PPT1 is lower than that at PPT2.3. As PPT1 is at a higher elevation. the pore water pressure starts to drop and stabilize. For the first 50 minutes of the test. the pore water pressure increases in 10 steps. At this state. Tunnel excavation causes stress relief on the clay surrounding the tunnel lining and thus a sharp drop in the pore water pressure is observed immediately after the tunnelling process for PPT1 which is located inside the immediate shear zone. Subsequently. the pore water pressure gradually increases over time due to dissipation of pore water pressure. Figure 4. as described in Chapter 3. Subsequently. This is because the excess pore water pressure induced by the increased acceleration field dissipates. In contrast. an opposite trend is observed for PPT 2 located outside the immediate shear zone. of which one PPT is located within the immediate shear zone and the other one is located outside the zone.14 shows the schematic location of the PPTs placed in the clay near the tunnel lining and the trend of the pore water pressure changes obtained from the PPTs throughout the test.Chapter 4 Basic Test on Volume Loss 4. This is because the acceleration of the centrifuge from 0g to 100g is divided into 10 steps with an interval of 5 minutes per step. 4 TYPICAL TUNNELLING-INDUCED PILE RESPONSES The detailed results of Test 3 are reported here as an illustrative example of test results from the beginning to the end of a typical test. Likewise. 4. This observation reiterates the importance of studying the long-term behaviour of tunnelling-induced soil movement and pile responses for tunnels with relatively large volume loss. the deflection of pile towards tunnel is taken as positive. 112 . positive lateral soil movements refer to soil movement towards the tunnel. This observation shows that the excess pore water pressure due to tunnelling has practically fully dissipated and approaches the steady state pore pressure. Bending moment inducing pile shaft curvature towards the tunnel is considered as positive. For the sign convention used in the present study. as indicated by a sharp increase in pore water pressure immediately after tunnel excavation. caused by the shearing process of the affected soil due to soil arching. downward vertical movement is regarded as positive. The above changes in the pore water pressure regime once again confirm that the behaviour of clay can be time-dependent due to low permeability of the clay sample.Chapter 4 Basic Test on Volume Loss being induced in the clay. The soil will continue to deform with time as a result of dissipation of excess pore pressures. Further observation suggests that the pore water pressure stabilizes about after two and a half hours (720 days in prototype scale) after the tunnel excavation. Lastly. 180 days. All instruments were monitored regularly throughout the entire test. both the axial and bending piles used are long floating piles (pile tip lower than tunnel invert) with free heads and tips. as shown in Figure 3. In Test 3. The four semi-conductor strain gauges are bonded on the external surface of the aluminium rod at an appropriate elevation to form a full-bridge configuration to measure the induced axial force on the pile.15(a) shows the induced pile axial force profile at 2 days. The observed trend is consistent with the field data reported by Pang et al. 4. the centrifuge would be kept at 100g for 3 hours after the completion of tunnel excavation.9. Figure 4.1 Induced Axial Force and Settlement The induced pile axial forces are directly measured from the readings of semiconductor strain gauges installed along the piles. The ultimate axial capacity of the 22-m long model pile is estimated to be 2300 kN in soft clay having an almost linearly increasing undrained shear strength profile. after which the induced axial force gradually decreases till the pile tip. It is evident that the settling soil drags the pile down and induces negative skin friction on the pile. The ultimate bending moment capacity of the pile is determined to be 3000 kNm.Chapter 4 Basic Test on Volume Loss In order to study the post-excavation ground deformation and pile responses. This is consistent with the observed downward vertical soil 113 .4. (2005a) for the MRT North East Line Contract 704 and 3D finite element analysis by Cheng (2003). 360 days and 720 days after tunnel excavation. A strain meter is used to record the strain gauge signals along the model pile. It is noted that the induced pile axial force increases with depth and reaches a maximum value approximately at the tunnel spring elevation. The soil settlement due to tunnelling would induce negative skin friction on the pile shaft and maximum negative skin friction occurs at the tunnel axis. Although the general configuration of tunnel-pile and soil condition for both cases is not identical. As non-zero value is observed near to the head of each pile. The total increment is about 90%. 4. This is an indication of the effectiveness of the de-bonding system in Contract 852. from 198 kN after 2 days to 370 kN after 720 days. the down-drag forces measured near to the pile head are negligible in Test 3. Figure 4. The neutral plane elevation becomes deeper over time.15(b) shows field data reported by Cham (2007) for MRT Circle Line Stage 3 Contract 852 in Singapore and the results are compared with those from Test 3 (Fig.16 shows the observed free-field vertical soil movement profile over time at the pile location obtained from Test 1 and the corresponding pile head 114 .17 (a) reveals that the drag load along the upper pile shaft increases with time and reaches a maximum magnitude after about 720 days. 4.7). However. the tunnelling-induced pile settlements could have resulted in some re-distribution of structural loads on the piles after tunnel advancement.Chapter 4 Basic Test on Volume Loss movement above the tunnel spring line due to soil over-cut in the process of tunnelling. This observation is consistent with that for Test 1 where the soil settlement does not increase further after 720 days (Fig.16.15(a). The readings reveal that there is a noticeable increase in maximum axial force in the longterm. . It can also be observed that a smaller increase in down-drag force acts on the upper 20m of the pile shaft. The plot of maximum pile axial force with time shown in Figure 4. Figure 4. the general induced axial force profiles are consistent for both cases. This is possible as the piles were connected by transfer beams and slab. as observed in Figure 4. The significant increase in pile settlement is likely due to the pile tip floating in the soft clay. during centrifuge experiments. the neutral plane is at a depth of about 14. However. From the subsurface soil settlement observed through the marker beads movement analyzed by PIV in Test 1 and the measured pile settlement in Test 3.2 m with subsurface soil settlement very close to the pile settlement. in the long term. Nevertheless. Two appropriate faces of the instrumented square pile were calibrated. The results also illustrate that the measured pile head settlement for Test 3 increases substantially from 6 mm in the short-term to 17 mm in the long term.Chapter 4 Basic Test on Volume Loss settlement shows the relationship between soil movement and pile settlement. These data prove that the pile responses are time-dependent.17(b).4. It is observed that the pile continues to settle after the completion of tunnel excavation until long-term ground movement has been stabilized at about 720 days after tunnel excavation. As such. the microstrain readings acquired from the data acquisition system can be readily related 115 . the pile undergoes much smaller settlement than the soil. as shown in Figure 4.2 Induced Bending Moment and Deflection The induced pile bending moments are determined from the readings of strain gauges attached along the pile shaft with the pile calibration conducted at 1g. it can be deduced that in the short-term. με (registered by the strain gauges) and the induced pile bending moment (result of hanging dead weights at the tip of the cantilevered pile) could be established. The strain gauges were connected to the strain meter with half-bridge mode and were properly calibrated so that the relationship between microstrain. the neutral plane shifts to a lower depth of 16.1 m. 4. In general.18(b) are compared to those obtained from Test 3. Figure 4. The experimental results show that in the short-term. 180 days. 4. It is noted that the bending moment profiles reported by Cham (2007) and Pang (2005) are similar to those observed in Test 3. As expected. 116 . the maximum transverse bending moment is noted to be at the tunnel axis level. The induced bending moment increases significantly by 98% to 93 kNm after 720 days.18(b) for comparison. revealing that the induced bending moment has stabilized.Chapter 4 Basic Test on Volume Loss to its corresponding bending moment from the appropriate scaling law of centrifuge modelling. It is also noted that the shape of the bending moment profile remains fairly constant over time and the maximum bending moment remains practically unchanged beyond 720 days after tunnel excavation Figure 4. the measured bending moment responses of a pile in pier 20 for C704 NEL (Pang. the bending moment at the pile head and tip are both zero as they are not restrained. It is shown that the pile bending moment increases with depth and the maximum induced bending moment toward the tunnel occurs approximately at the tunnel central axis for long piles with tips well beneath the tunnel.18(a) shows the induced pile bending moment profiles at 2 days. 360 days and 720 days after tunnelling. In addition. tunnel excavation induces a maximum bending moment of 47 kNm.3. 2005) are also included in Figure 4.17(c). The findings are consistent with the trend of soil movements induced by tunnelling as discussed in Section 4. The field measured bending moment profiles reported by Cham (2007) plotted in Fig. A comparison of pile and free-field horizontal soil displacements for the tests is shown in Figure 4. both the pile and soil move towards the tunnel. As expected. The free-field lateral soil displacements can be obtained from Test 1 whereas the pile lateral displacement profile can be obtained by integrating the bending moment profiles twice with two specified boundary conditions using the measured pile head displacements at 2 elevations for the present study. It is evident that the lateral soil movement profile has a roughly similar trend as the pile deflection profile.Chapter 4 Basic Test on Volume Loss The pile head deflection at the ground surface is obtained by geometry from the two displacement readings obtained at 2 different pile elevations above the ground. 117 .1 mm in the long-term. The lateral pile displacement is related to the free-field lateral soil movements. the magnitude of pile deflection increases with time and the maximum lateral pile deflection occurs at the pile head having a magnitude of 5 mm in the short-term and 12. showing that the pile basically deforms with the soil in a similar fashion. For Test 3.19. As the pile can be considered a rigid body. pile bending stiffness and pile-soil interaction. the magnitude of pile deflection is much smaller than that of the soil. The pile deflection profile is also ‘smoother’ than the soil movement profile due to the large pile bending rigidity. When compared with Test 3.20 compares the induced pile axial force due to tunnel excavation in Tests 3 and 4.5%.5%). It is observed that the pile axial load transfer profiles along the piles in Tests 3 and 4 are similar in trend. Figure 4. 4) In Test 4. the maximum negative skin friction is 265 kN in the short-term (2 days after excavation) and 422 kN in the long term (720 days after excavation). as compared to that of 3% for Test 3. the maximum induced axial force on the pile increases with tunnel volume loss and time. For Test 4 with a volume loss of 6. Generally. The pile-to-tunnel distance and pile length are kept constant in these tests. In the short-term. with a larger magnitude of negative skin friction for the test with a larger volume loss.1 Induced Axial Force and Settlement Figure 4.e. the pile head settlement increases significantly from 6 mm to 118 . and the measured pile head settlements from Tests 3 & 4. 4.and long-terms free-field vertical soil movement profile at the pile location obtained from PIV analysis from Tests 1 & 2. the increment in negative skin fraction is only 34% in the short-term and 14% in the long term despite an increase in volume loss of over 100% (i. the simulated tunnel opening for this model has a GAP of approximately 200 mm in prototype scale with an equivalent imposed volume loss of 6. from 3% to 6.5%.21 shows the observed short.Chapter 4 Basic Test on Volume Loss 4. The results illustrate a significant increase in pile head settlement when the volume loss increases from 3% to 6.5 TEST SERIES 1 .5%.EFFECTS OF VOLUME LOSS (TESTS 3.5. In additional.2 Induced Bending Moment and Deflection Figure 4. the soil moves towards the tunnel spring elevation. The results demonstrate that the induced pile bending moment profile has a double curvature with the moment magnitude increasing over time.22 shows the short. As such. 4. resulting in the pile bending towards the tunnel. For the induced base resistance in the short.5. 119 . The maximum induced bending moment occurs approximately at the tunnel springline.3 mm (increment of 250%) in the long-term when the volume loss increases from 3% to 6. the pile base load obtained from Test 4 is higher than that of Test 3. The moderate increment of negative skin friction as compared to drastic increment in settlement may be due to the fact that 3% volume loss has already induced substantial relative pile-soil movement for the full mobilisation of negative skin friction. Owing to tunnel over-cut. This is because the moderate increase in down drag has to be resisted by the base resistance. when vertical soil movement increases with a larger volume loss.5%.and long-terms induced pile bending moment profiles obtained from Tests 3 and 4. further ground settlement does not induce further negative skin friction significantly when volume loss increases.Chapter 4 Basic Test on Volume Loss 17 mm (increment of 183%) and drastically from 14. The trend of the bending moment profile remains fairly constant over time and the maximum bending moment remains practically unchanged after 720 days of tunnel excavation. The bending moments at the pile head and tip are zero as they are not restrained. additional pile settlement is necessary to mobilise sufficient positive shaft resistance and base resistance to maintain pile equilibrium.and long-terms.7 mm to 51. Chapter 4 Basic Test on Volume Loss In the short-term.5%. the measured pile head deflection in Test 3 is only 5 mm in the short-term and 12. while the corresponding measured pile deflection profiles are from Tests 3 and 4. Besides that. with the largest pile deflection observed at the pile head. The long term pile bending moment of 316 kNm is more than triple of that observed in Test 3. both the lateral soil movement and lateral pile deflection increase with time. As expected.23 shows the observed short.1 mm in the long-term. The maximum induced shortterm pile bending moment of 136 kNm from Test 4 is almost triple of that observed in Test 3. the pile deflection is much smaller than that of the soil.and long-terms free-field lateral soil displacement profiles at the pile location. The largest lateral soil movement occurs at the pile head location. This deflection profile is due to increasing soil stiffness with depth. Hence. Comparing the pile deflection with horizontal soil movement. the pile deflection has a smaller magnitude due to the large bending stiffness of the pile. The displacement profiles are obtained from the PIV analysis of Tests 1 and 2. The pile deflection profiles are similar for both tests and the pile moves toward the tunnel. The pile bending moment for Test 4 is larger than that of Test 3 at all times. The results reveal that the pile deflection increases with volume loss. hence induces the largest pile deflection at this elevation. the maximum induced bending moment on the pile is 47 kNm. Figure 4. and increases to 93 kNm after 720 days for Test 3. especially in the long term due to a higher volume loss and hence larger soil movements. It is evident that the maximum induced pile bending moments increase significantly when the volume loss increases from 3% to 6. The magnitude of lateral soil movement decreases with depth and so did the pile deflection. due to increase in lateral soil movement with larger volume loss and time. as compared to a much larger pile head deflection of 10 mm in the shor- 120 . Figures 4. The increment of pile deflection is significant when the volume loss increases from 3% to 6. On the other hand.24(a) to (d) show a summary of maximum pile axial force.25 shows the long-term to short-term ratio of pile responses (pile axial force. pile bending moment. Thus. it is important to keep the tunnel volume loss as small as possible in order that the long term induced pile responses are not a concern. pile head settlement.2. Nevertheless. the excessive pile movement (settlement and deflection) are the critical pile responses. it is observed that in the present floating pile condition.Chapter 4 Basic Test on Volume Loss term and 28 mm in the long-term for Test 4. especially if the pile foundation supported the existing building only designed to resist the compression load as illustrated in Figure 1. pile bending moment and pile head deflection with volume loss for Tests 3 and 4. the significant increment of bending moment under a large volume loss is detrimental to the structural integrity of the pile. pile head settlement. Figure 4.5%. pile head deflection) for the two volume losses. The results reveal that the pile responses increase over time and the ratio of all long-term/short-term (LT/ST) pile responses except axial force also increases with volume loss. 121 . A consistent trend is observed that all pile responses increase with volume loss and time. 6 CONCLUDING REMARKS 4.6. Qualitative assessment on the excess pore pressure responses has provided an understanding on the development of negative excess pore pressure in the immediate shear zone and positive excess pore pressure in the support zone. The magnitude of maximum ground surface settlement increases with time and tunnel volume loss. The data confirmed that the empirical equation proposed by Mair et al. (1993) is applicable in the prediction of the subsurface settlement troughs in clay in the short-term.Chapter 4 Basic Test on Volume Loss 4. On the other hand. 122 . Empirical equations in the short-term and long-term were proposed for the distribution of inflection point in soft clay. The surface settlement trough in clay generally follows the Gaussian distribution curve in the short-term.1 Tunnelling-Induced Soil Movement The centrifuge model tests with the application of PIV have provided useful data to examine the patterns of soil movements induced by tunnelling in soft clay. the significant soil movement zone extends much wider. soil settlement is noted to be more dominant than lateral soil movement in the long term. The settlement magnitude is larger in the long-term and the settlement trough is wider as compared to that in the short-term. In addition. In the long term. an immediate shear zone with large soil movement above the tunnel can be identified in the short-term. The main aim is to investigate the induced soil movement patterns over time. The test results shed light on the actual performance of single floating pile due to tunnelling with volume loss of 3% to 6. 123 .6. The lateral soil movement profile has a similar trend as the pile deflection profile. in this particular case. but the magnitude of the pile deflection is much smaller than that of the soil.2 Tunnel-Soil-Piles Interaction Two centrifuge model tests with different volume loss have been performed to investigate the effects of tunnelling on single free head piles.5 times when volume loss increases from 3% to 6.5%. Owing to downward soil movement. The pile deflection profile is ‘smoother’ than the soil movement profile due to the large pile rigidity.Chapter 4 Basic Test on Volume Loss 4.5%. It is found that the induced pile bending moment triples and the pile settlement and deflection increase by almost 2. negative skin friction and bending moment are induced on the pile and the magnitude of negative friction and moment increases with time and tunnel volume loss. Both the neutral plane and maximum induced pile moment take place about the tunnel centre-line elevation. Test Series 1 studies the effects of volume loss on pile performances. 5m Volume loss = 6.5% 124 . Configuration Common parameters Individual parameters Volume loss = 1 3% Kaolin clay C C = 12 m 24m D D=6m Volume loss = 2 Toyoura Sand 3.Chapter 4 Basic Tests on Volume Loss Table 4.5m 6. Configuration ‘Bending’ Pile C C = 12 m Volume loss = 3% D=6m L D Individual parameters ‘Axial’ Pile 3 Typical 4 Common parameters 24m X 2m L = 22 m X= 6 m 3.1 Test program and parameters for the basic tests on volume loss Phase 1 (Free-field soil movement) -Effects of volume loss Test No.5% Phase 2 -Effects of tunnelling on single piles Test series 1 Effects of volume loss Test No. 2 Example of digital images taken during test for PIV analysis 125 .Chapter 4 Basic Tests on Volume Loss Water 30 Kaolin Clay 120 Control marker 60 Model Tunnel 60 30 Toyoura Sand 520 Figure 4.1 Schematic of viewing area in tunnel-soil interaction tests (all dimensions in mm) 200 Texture clay Control marker Y-coordinate (pixel) 400 600 Model tunnel 800 1000 1200 200 400 600 800 X-coordinate (pixel) 1000 1200 1400 1600 Figure 4. Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 2 days 0 Depth below ground level (m) -5 -10 -15 -20 50mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 2 days 0 30 40 20 10 -5 10 -10 30 Depth below ground level (m) 20 40 10 20 -15 10 -20 (unit mm) -25 0 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4.3 (a) Vectors and contour plots of soil movements after 2 days (Test 1) 126 . 3 (b) Vectors and contour plots of soil movements after 180 days (Test 1) 127 .Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 180 days 0 Depth below ground level (m) -5 -10 -15 -20 50mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 180 days 0 20 70 90 50 80 30 40 60 30 -5 10 40 70 -10 40 60 10 40 50 20 30 -15 10 20 10 Depth below ground level (m) 30 50 20 -20 (unit mm) -25 0 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4. Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 360 days 0 Depth below ground level (m) -5 -10 -15 -20 50mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 360 days 0 90 60 50 80 0 10 30 40 -5 40 20 3 50 0 60 70 90 10 40 80 -10 70 20 50 60 30 10 40 20 10 30 -15 50 Depth below ground level (m) 30 50 20 10 10 -20 (unit mm) -25 0 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4.3 (c) Vectors and contour plots of soil movements after 360 days (Test 1) 128 . Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 720 days 0 Depth below ground level (m) -5 -10 -15 -20 50mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 720 days 0 10 0 80 70 60 11 0 90 40 30 -5 20 50 80 30 70 10 0 40 50 60 60 40 30 20 -10 50 70 30 20 40 -15 30 40 60 50 0 6 70 Depth below ground level (m) 90 20 10 10 20 10 10 -20 (unit mm) -25 0 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4.3 (d) Vectors and contour plots of soil movements after 720 days (Test 1) 129 . 4 (a) Vectors and contour plots of soil movements after 2 days (Test 2) 130 .Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 2 days 0 Depth below ground level (m) -5 -10 -15 -20 100mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 2 days 40 70 50 90 80 10 20 60 0 30 10 -5 60 20 10 0 50 40 90 80 70 Depth below ground level (m) 11 0 -10 30 10 20 60 50 -15 40 30 10 20 10 -20 (unit mm) -25 0 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4. 4 (b) Vectors and contour plots of soil movements after 180 days (Test 2) 131 .Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 180 days 0 Depth below ground level (m) -5 -10 -15 -20 100mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 180 days 0 10 40 50 90 60 14 0 70 11 0 0 12 80 0 15 0 60 13 0 -5 11 0 10 0 12 0 80 90 70 Depth below ground level (m) 60 50 -10 0 15 140 13 0 120 110 100 80 70 50 40 60 90 40 50 80 70 -15 40 60 30 50 40 30 20 30 20 20 20 -20 10 10 10 10 (unit mm) -25 0 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4. 4 (c) Vectors and contour plots of soil movements after 360 days (Test 2) 132 .Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 360 days 0 Depth below ground level (m) -5 -10 -15 -20 100mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 360 days 0 16 0 18 0 19 -5 0 13 0 16 17 0 0 18 0 15 0 14 0 14 0 15 13 0 60 870 0 90 0 10 110 17 0 12 0 0 10 0 0 12 11 0 90 0 10 90 18 0 Depth below ground level (m) 80 -10 0 13 0 14 160 12 0 11 0 0 10 90 15 0 70 80 70 60 80 12 0 -15 11 0 70 10 0 90 60 80 70 60 50 40 50 60 50 50 40 40 30 30 30 -20 20 20 20 10 -25 0 (unit mm) 5 10 15 Distance from tunnel centre-line (m) 20 25 Figure 4. 4 (d) Vectors and contour plots of soil movements after 720 days (Test 2) 133 .Chapter 4 Basic Tests on Volume Loss Vectors of soil movements after 720 days 0 Depth below ground level (m) -5 -10 -15 -20 100mm -25 0 5 10 15 20 25 Distance from tunnel centre-line (m) Contour plots for total soil movements after 720 days 0 16 17 0 0 19 0 20 21 0 18 0 0 0 15 0 14 13 0 12 0 -5 0 18 0 19 200 0 17 0 16 0 15 14 0 13 0 11 0 12 0 0 14 13 0 180 17 0 16 0 Depth below ground level (m) 11 0 -10 0 15 90 10 0 12 0 11 0 80 90 10 0 80 14 0 0 13 12 0 11 0 10 0 -15 70 90 80 70 70 60 90 80 70 60 50 60 50 50 40 40 40 40 30 -20 30 30 30 20 20 -25 0 5 10 15 Distance from tunnel centre-line (m) (unit mm) 20 25 Figure 4. 6 Surface settlement troughs over time (Test 2) 134 .Chapter 4 Basic Tests on Volume Loss Distance from tunnel centre-line (m) Distance from tunnel centre-line (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 0 26 inflection point.5 Surface settlement troughs over time (Test 1) inflection point. 'i" -80 -100 -120 -140 -160 -180 -200 -220 -240 Volume loss = 6. 'i" -40 -40 -60 -60 -80 -100 -120 -140 -160 -180 -200 -220 -240 4 6 8 10 12 14 16 18 20 22 24 26 -20 Settlement (mm) Settlement (mm) -20 2 0 0 Volume loss = 3% 2 days (from PIV) 90 days (from PIV) 180 days (from PIV) 360 days (from PIV) 720 days (from PIV) 2 days (from Potentiometer) 90 days (from Potentiometer) 180 days (from Potentiometer) 360 days (from Potentiometer) 720 days (from Potentiometer) 2 days (Gaussian curve) Figure 4.5% 2 days (from PIV) 90 days (from PIV) 180 days (from PIV) 360 days (from PIV) 720 days (from PIV) 2 days (from Potentiometer) 90 days (from Potentiometer) 180 days (from Potentiometer) 360 days (from Potentiometer) 720 days (from Potentiometer) 2 days (Gaussian curve) Figure 4. 7 Maximum surface settlements over time (Tests 1 & 2) 135 .Chapter 4 Basic Tests on Volume Loss Time (days) 0 180 360 540 720 900 1080 1260 1440 0 -20 -40 settlement (mm) -60 -80 -100 -120 -140 -160 -180 -200 -220 -240 -260 -280 -300 Test 1 (from PIV) Test 1 (from Potentiometer) Test 2 (from PIV) Test 2 (from Potentiometer) Figure 4. 3m depths (Test 1): (a) comparing with Mair et. 4. Mair et al (1993) -60 4. Loganathan & Poulos (1998) 4. Test 1 (PIV) -60 Surface settlement. Loganathan & Poulos (1998) 9. Test 1 (PIV) Surface settlement. al (1993) (b) comparing with Loganathan and Poulos (1998) 136 . Loganathan & Poulos (1998) (b) Figure 4.8 Settlement troughs at surface.3m. Test 1(PIV) 9.3m.Chapter 4 Basic Tests on Volume Loss Distance from tunnel centre-line (m) 0 2 4 6 8 10 12 14 16 18 Distance from tunnel centre-line (m) 20 22 24 26 0 0 4 6 8 10 12 14 16 -40 Surface settlement. Test 1 (PIV) 4.3m and 9.3m. Test 1 (PIV) -20 Settlement (mm) Short-term (VL=3%) -20 Settlement (mm) 2 18 20 22 24 26 0 Short-term (VL=3%) -40 Surface settlement.3m. Mail et al (1993) -100 -100 (a) 9.3m. Test 1(PIV) -80 4.3m. Mair et al (1993) -80 9.3m.3m. Test 2 (PIV) Surface settlement. Test 2 (PIV) 5m.9 Settlement troughs at surface. 5m and 10. Loganathan & Poulos (1998) 5m. Mair et al (1993) -180 10. al (1993) (b) comparing with Loganathan and Poulos (1998) 137 .9m.9m. Mair et al (1993) Short-term (VL=6. Test 2(PIV) 10. Test 2 (PIV) -140 Surface settlement. Mair et al (1993) -160 -180 -200 -220 -240 4 6 8 10 12 14 16 18 20 22 24 26 -40 Short-term (VL=6. Test 2 (PIV) -160 5m. Loganathan & Poulos (1998) Figure 4.5%) -80 -200 -220 -240 Surface settlement.9m depths (Test 2): (a) comparing with Mair et.9m.9m. Loganathn & Poulos (1998) 10. Test 2(PIV) 10.5%) -60 Settlement (mm) Settlement (mm) -60 2 -100 -120 -140 5m.Chapter 4 Basic Tests on Volume Loss Distance from tunnel centre-line (m) Distance from tunnel centre-line (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 0 0 0 -20 -20 -40 -80 -100 -120 Surface settlement. 10 Distribution of inflection point ‘i’ with depth in short.(1993) Test 1.3 1. Z(m) Test 1 (PIV) Test 2 (PIV) Figure 4.1 1 0.7 0.4 Ratio of iLT / iST 1. 2 days (ST) Test 2. 720 days (LT) Proposed long-term equation -16 -20 Figure 4. 720 days (LT) Test 2.6 0 1 2 3 4 5 6 7 8 9 10 11 12 Depth.9 0.Chapter 4 Basic Tests on Volume Loss 20 16 12 Proposed long-term equiation.5 1.11 Comparison of ratio of iLT/iST at different depths (Tests 1 & 2) 138 .6 1.and long-term (Tests 1 & 2) 1.2 1. Zo-Z = 3i-12 8 Zo-Z (m) 4 0 -4 0 1 2 3 4 5 6 i -8 7 8 9 10 11 12 (m) -12 O'Reilly and New (1982) Mair et al.8 0. 2 days (ST) Test 1. 12 Horizontal soil movements at different distance from tunnel center-line at 2 and 720 days .Chapter 4 Basic Tests on Volume Loss -40 0 -30 -20 -10 0 -40 -30 -20 -10 12m from tunnel Soil movement (mm) 0 -40 -30 -20 -10 15m from tunnel Soil movement (mm) 0 -40 -30 -20 -10 0 0 0 -5 -5 -5 -5 -5 -10 -10 -10 -10 -10 -15 Tunnel -15 -20 -20 -25 -25 -30 -30 -15 -15 Depth below GL (m) 0 Depth below GL (m) 0 Depth below GL (m) 0 Depth below GL (m) -40 -30 -20 -10 9m from tunnel Soil movement (mm) -15 -20 -20 -25 -25 -30 -30 -20 Depth below GL (m) 6m from tunnel Soil movement (mm) 4m from tunnel Soil movement (mm) -25 2 days Loganathan et al 1998 -30 720 days Figure 4.Test 1 139 . Chapter 4 Basic Tests on Volume Loss Soil movement (mm) -60 -40 -20 -80 0 -80 0 -60 -40 -20 0 -80 -60 -40 -20 0 0 -5 -5 -5 -5 -10 -10 -10 -10 -5 -10 -20 -20 0 0 Tunnel -40 0 0 -15 -60 15m from tunnel Soil movement (mm) -15 -20 -25 -25 -30 -30 -15 -20 -15 Depth below GL (m) -80 12m from tunnel Soil movement (mm) Depth below GL (m) 0 Depth below GL (m) -40 -20 Depth below GL (m) Soil movement (mm) -80 -60 9m from tunnel Soil movement (mm) -15 -20 -20 -25 -25 -30 -30 -25 Depth below GL (m) 6m from tunnel 4m from tunnel 2 days Loganathan et al 1998 -30 720 days Figure 4.Test 2 140 .13 Horizontal soil movements at different distance from tunnel center-line at 2 and 720 days . 14 Pore pressure changes due to tunnelling (Test 1) 141 .Chapter 4 Basic Tests on Volume Loss Excess pore pressure (kPa) 1)Spinning up from 1g to 100g 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 -40 -60 2) Consolidation at 100g 3)Tunnelling 4)Posttunnelling 5)Spinning down E Point 2 D (support zone) A Point 1 (shear zone) 0 500 1000 1500 2000 2500 F C B 3000 3500 4000 4500 5000 5500 6000 6500 Time (day) Point1 Point2 PPT Point 1 PPT Point 2 Figure 4. 15(b) Tunnelling-induced pile axial force (Pile BP1-G) for MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore (After Cham.Chapter 4 Basic Tests on Volume Loss Axial Force (kN) 0 100 200 300 400 500 600 0 -5 Depth (m) -10 -15 T unnel Approximate Neutral Plane -20 -25 2 days 180 days 360 days 720 days -30 Figure 4. =600mm Pile Length =31m Figure 4. 3% free-head floating long pile) Volume loss = 0.15(a) Tunnelling-induced pile axial force (Test 3.8m Pile dia. 2007) 142 .7% Tunnel ID=5. PIV) 2 days 180 days 360 days 720 days -30 Figure 4.16 Tunnelling-induced pile head settlement (Test 3) and observed free-field soil movement at pile location (Test 1.Chapter 4 Basic Tests on Volume Loss Settlement (mm) 0 20 40 60 80 100 120 0 -5 Depth (m) -10 -15 T unnel Approximate Neutral Plane -20 Pile head settlement (Test 3) -25 Free-field soil settlement (Test 1. PIV) 143 . pile bending moment (kN) (c) 100 50 0 Lateral movement (mm) 0 30 180 Free-field lateral soil movement (Test 1) (d) 25 Pile head deflection 20 15 10 5 0 0 180 360 540 720 T ime (days) 900 1080 1260 1440 Figure 4.17 Tunnelling-induced (a) maximum pile axial force (b) maximum pile head settlement and soil surface settlement (Test 1) (c) maximum pile bending moment (d) maximum pile head deflection and soil surface lateral movement (Test 1) 144 .Max. pile axial force (kN) Chapter 4 Basic Tests on Volume Loss 400 (a) 350 300 250 200 150 0 180 100 540 720 T ime (days) 900 1080 1260 1440 (b) 80 Settlement (mm) 360 60 Free-field soil surface settlement (Test 1) 40 Pile head settlement 20 0 0 180 360 540 720 T ime (days) 900 1080 1260 1440 360 540 720 T ime (days) 900 1080 1260 1440 150 Max. 18 (a) Tunnelling-induced pile bending moment (Test 3. 2007) 145 . =800mm Pile Length =33. 3% free-head floating long pile) Volume loss = 0.18 (b) Tunnelling-induced pile bending moment (Pile BP2-E) for MRT Circle Line Stage 3 (CCL3) Contract 852 in Singapore (After Cham.7% Tunnel ID=5.Chapter 4 Basic Tests on Volume Loss Bending Moment (kNm) -100 -50 0 50 100 150 200 250 300 0 -5 Depth (m) -10 -15 T unnel -20 -25 2 days 180 days 360 days 720 days -30 Figure 4.4m Figure 4.8m Pile dia. 19 Tunnelling-induced pile deflection (Test 3) and free-field lateral soil movement at pile location (Test 1) Axial Force (kN) 0 100 200 300 400 500 600 0 -5 Depth (m) -10 -15 T unnel -20 -25 ST (Test 3) LT (Test 3) ST (Test 4) LT (Test 4) -30 Figure 4.Chapter 4 Basic Tests on Volume Loss Lateral deflection (mm) 0 10 20 30 40 0 -5 Depth (m) -10 -15 T unnel -20 Free-field lateral soil movement (Test 1.20 Variation of pile axial force with volume loss (Tests 3 and 4) 146 . PIV) 2 days 180 days -25 360 days Pile deflection (Test 3) 2 days 360 days 720 days 180 days 720 days -30 Figure 4. 21 Variation of pile head settlement (Tests 3 and 4) and observed free-field soil movement at pile location (Tests 1 and 2) with volume loss 147 .Chapter 4 Basic Tests on Volume Loss Settlement (mm) 0 50 100 150 200 250 0 -5 Depth (m) -10 -15 T unnel -20 Test 3 Pile head settlement Test 4 Pile head settlement -25 Free-field soil settlement ST (Test 1) LT (Test 1) ST (Test 2) LT (Test 2) -30 Figure 4. Chapter 4 Basic Tests on Volume Loss Bending Moment (kNm) -150 -50 50 150 250 350 450 550 650 0 -5 Depth (m) -10 -15 T unnel -20 -25 ST (Test 3) LT (Test 3) ST (Test 4) LT (Test 4) -30 Figure 4.23 Variation of pile deflection profiles (Tests 3 and 4) and observed free-field lateral soil movement at pile location (Tests 1 and 2) with volume loss 148 .22 Variation of pile bending moment with volume loss (Tests 3 and 4) Lateral deflection (mm) 0 5 10 15 20 25 30 35 40 45 0 -5 Depth (m) -10 -15 T unnel Free-field lateral soil movement -20 -25 ST (Test 1) LT (Test 1) ST (Test 2) Pile deflection LT (Test 2) ST (Test 3) LT (Test 3) ST (Test 4) LT (Test 4) -30 Figure 4. 5%) (d) Figure 4.5%) (a) (b) 36 350 ST 32 ST LT LT 300 Pile head deflection (mm) Maximum pile bending moment (kNm) 400 250 200 150 100 28 24 20 16 12 8 4 50 0 0 Test 3 (VL=3%) (c) Test 4 (VL=6.5%) Test 4 (VL=6.5%) Test 3 (VL=3%) Test 4 (VL=6.24 Variation of (a) maximum pile axial force (b) pile head settlement (c) pile bending moment (d) pile head deflection with volume loss (Tests 3 and 4) 149 .Chapter 4 Basic Tests on Volume Loss 500 60 400 LT Pile head settlement (mm) Maximum pile axial force (kN) ST 300 200 100 55 ST 50 LT 45 40 35 30 25 20 15 10 5 0 0 Test 3 (VL=3%) Test 3 (VL=3%) Test 4 (VL=6. 5 Long-term effect 2 1.5 3 LT/ST ratio 2.5 1 0.5 0 Pile axial force Pile head settlement Test 3 (VL=3%) Pile bending moment Pile head deflection Test 4 (VL=6.Chapter 4 Basic Tests on Volume Loss 4 3.5%) Figure 4.25 Long-term to short-term ratio of pile responses for different volume losses (Tests 3 and 4) 150 . the results of further centrifuge model tests conducted to investigate the effects of tunnelling on single piles in clay are presented. For the test on “floating” pile (Test 3). the underlying sand thickness of 3-m is the same as that for Test 3 but the 27. 9 and 10 were performed to simulate three different pile tip conditions. a “socketed” pile in Test 9 and an “end-bearing” pile in Test 10.1 INTRODUCTION In this chapter. 9 And 10) Tests 3. For Test 10. 151 . the 22-m long pile is entirely embedded in the 24-m thick soft clay layer.1 and the pile tip positions investigated in the parametric studies are schematically illustrated in Figure 5. namely a “floating” pile in Test 3 (presented in Chapter 4).2.2 TEST SERIES 2.EFFECTS OF PILE TIP & HEAD CONDITIONS 5. the soft clay was underlain by a 8-m thick sand layer and the pile was embedded 3m into the underlying sand layer. The detailed test program and configurations in prototype scale is given in Table 5.5-m thick sand layer. The soft clay was underlain by a layer of 3.Chapter 5 Effects of Tunnelling on Single Piles CHAPTER FIVE EFFECTS OF TUNNELLING ON SINGLE PILES 5.1 Effects of Pile Tip Condition (Tests 3.1. For the test on “socketed” pile (Test 9).5 m long pile is resting on the rigid base of the container. 5. The pile heads are free. This is as expected because the pile in Tests 9 and 10 are socketed into the sand layer and hence the pile settlement is much smaller than that for a floating pile (Test 3). As such. Figures 5.2. the neutral plane for a pure end-bearing pile should be at the pile tip. the respective maximum axial force occurs approximately at the tunnel spring elevation for Test 3 but slightly lower than the tunnel spring elevation for Tests 9 and 10. The negative skin friction is highest for the end-bearing pile (Test 10) as the pile is resting on a rigid base with negligible measured pile settlement. as there will be some load transfer in the 3 m thick sand layer just above the pile tip.2. respectively. 152 .2(a) & (b) show the induced pile axial force profile of Tests 3. 1D or 6 m.term at 2 days) and in the long-term (720 days). It can be seen that the induced axial force in all tests increases downwards from the pile head. 9 and 10 at the end of tunnel excavation (short.Chapter 5 Effects of Tunnelling on Single Piles Strictly speaking this pile is not a pure “end-bearing” pile.e. The axial force is mostly transferred to the pile socket in Tests 9 and 10. the larger soil movement relative to pile settlement induces much larger drag loads on the socketed pile (Test 9). Intuitively. This is not so for Test 10 as positive skin friction is mobilised to transfer the load in the sand layer as shown in Figure 5. i. The elevation of neutral plane in the socketed pile (Test 9) is lower than that of the floating pile (Test 3). as evidenced from the steep gradient of positive skin friction shown in Figure 5. However. The neutral plane for end-bearing pile in Test 10 shifts even much lower as compared to Test 9. All tests have the same tunnel volume loss of 3% and the same pile-to-tunnel distance. respectively. The maximum bending moment is the largest among the three tests. the pile settlement for Test 9 (socketed pile) is only 2 mm in the short. 9 and 10 in the ST and LT. As a result.Chapter 5 Effects of Tunnelling on Single Piles Owing to lower part of pile socketed in stiff soil. The restraint at the pile tip would restrict pile movement and hence result in slightly larger pile bending moments.term to 17 mm in the long-term. This is probably because the longer span of pile in Test 10 is exposed to more soil movements induced by tunnelling and hence the pile length effect can be one of the factors contributing to the magnitude of induced pile bending moment. This aspect will be further examined in Section 5. Figures 5. This is due to socketing of the pile into sand. Although the volume loss for both tests is the same. the socketed pile in Test 9 exhibits a larger maximum bending moment. the settlement of the floating pile (Test 3) increases by 183% from 6 mm in the short. the pile settlement for Test 10 (end-bearing pile) is negligible. the induced bending moment on the socketed and end-bearing pile is more critical than that on the floating pile.term and 3 mm in the long term. Owing to the floating condition of the pile tip. As expected. However. the end-bearing pile (Test 10) exhibits triple curvatures profile with the maximum bending moment occurring close to the tunnel axis as well. 153 . these magnitudes are still much smaller than the observed ground surface settlement. as compared to a floating pile (Test 3). The bending moment profiles in Tests 3 and 9 share a similar trend as the maximum bending moment occurs close to the tunnel axis with double curvature. thus sufficiently restricting movement of the lower portion of the pile.3. The above observations reveal that pile settlement is more critical for a floating pile while induced negative skin friction is more critical for both socketed and end-bearing piles. On the other hand.3(a) & (b) show the variation of induced pile bending moment profiles of Tests 3. much larger deflection is noted for the floating pile. and hence causing the mid-pile shaft being bent away from the tunnel. the induced pile deflection is more critical for a floating pile as compared to socketed and end-bearing piles. This is probably due to the relatively large lateral soil movement in the immediate shear zone pushing the pile head while the pile tip is being restrained. Test 13 is modelled such that the pile head is totally fixed in position with no vertical or lateral movements allowed. but there is almost no deflection from the mid-pile shaft to the pile tip. This is because the lower part of the pile is restrained and hardly moves. with 2. From the pile deflection profile exhibited in the socketed pile. Nevertheless. In contrast.and long-term. Moreover.and long-terms.1 mm and 3 mm in the short.8 mm and 6 mm in the short.4(a) & (b) show the pile deflection profiles for Tests 3.1 mm (LT). the ‘endbearing’ pile in Test 10 is exposed to more lateral soil movements and its lower elevation of pile toe fixity point causes it to deflect more than the socketed pile in Test 9. The pile head deflection for the socketed pile (Test 9) is only 2.2. This simulates the condition where the pile cap is tied rigidly 154 . 5. the pile head deflection for the end-bearing pile (Test 10) is slightly larger than that of socketed pile. respectively. it is noted that the mid-pile shaft in end-bearing pile (Test 10) being pushed away from the tunnel. 9 and 10. respectively. This is likely due to the higher elevation of the underlaying sand layer for the socketed pile. In short. 13) Tests 10 and 13 were performed to study the effects of pile head condition. the pile head is noted to bend towards the tunnel. This is much smaller than the corresponding pile head deflection of the floating pile of 5 mm (ST) and 12.2 Effects of Pile Head Condition (Tests 10.Chapter 5 Effects of Tunnelling on Single Piles Figures 5. As such. and long-term.5-m long model pile is fully embedded into the 24-m thick soft clay layer and 3. engineers may need to evaluate the connection between the pile and the pile cap in resisting the tensile force. In a completely fixed head condition. The induced pile axial force profiles of Tests 10 and 13 are shown in Figure 5. the trade-off is that tension force would be induced near to the pile head.Chapter 5 Effects of Tunnelling on Single Piles with the ground beams. For a fixed-head pile. In both tests. It is observed that the axial load transfer profiles are similar for both the free-head pile and fixed-head pile.2. the pile is not allowed to settle. resulting in tensile force induced along the upper portion of pile. simulating an end-bearing pile. with the exception of the development of tensile force in fixedhead piles. However. Pang (2006) reported that the pile is significantly affected when the pile cap is restrained. 155 . the 27. In addition.5 m (Short-term) and 20 m (Long-term). and resting on the rigid base of the model container. The measured settlement of end-bearing pile is less than 2mm as the pile tip is rested on a rigid base.5 m thick sand layer. Tensile force is observed along the top 15 m of the pile while the maximum drag load is reduced compared to the pile without pile cap restraint. The general trend of development of tensile force is observed by Mroueh and Shahrour (2002) and Pang (2006) as well. respectively. it is noted that the reduction of maximum drag load in fixed-head pile is approximately 36% and 33% in the short. The maximum negative skin friction is observed at an elevation lower than the tunnel spring line or at approximately 17. It might be due to the load transfer curve essentially offset towards the tension side due to the total restraint at pile head (fixed-head condition) applied on the pile. Chapter 5 Effects of Tunnelling on Single Piles Figure 5. As no pile head deflection is allowed for the fixed-head pile (Test 13). see Figure 5. the effects of deflection induced by tunnelling toward the existing pile in fixed-head pile are not a major concern as compared to the free-head pile. the bending moment profile is offset towards the negative side as compared to the free-head pile. but 156 . The pile bending moment profile is similar for both cases. As expected. the measured mid-pile shaft deflection of less than 0. Since the pile deflection is negligible. This might be attributed to the fact that the pile head and toe are fixed in placed and thus the movement of the pile at mid-pile shaft could be purely due to the bending of the pile shaft due to the large rigidity of the pile. It is worth noting that the observed negative bending moment at the pile head is larger than the positive bending moment at the mid-pile shaft. where triple curvature is induced with negative bending moments at the upper and lower portions of the pile body. The large magnitude of bending moments at the pile head needs to be evaluated in practice. whilst positive pile bending moment occurs approximately at the tunnel spring line.3 shows the tunnelling-induced pile bending moment for the free.5 shows a summary of the variation of short-term and long-term pile responses with tip and head conditions. For the fixedhead pile. socketed and end-bearing piles experienced smaller induced pile settlement and deflection. Figure 5. no bending moment is induced in the free pile head which is allowed to move. while relatively large negative bending moment is induced at the fixed pile head with zero rotation and displacement.and fixed-head end-bearing piles (Tests 10 & 13).4.2mm is much smaller than that for the free-head pile. As compared to floating pile. In this section. The results of these 2 tests will be compared with the performance of the long pile in Test 3. 5. The pile tip is located approximately at the tunnel axis.76). dense sand). In addition. the piles will experience significant negative skin friction.6 m (ratio of pile length over tunnel depth.04). L/H=0. which has an embedment length of 22 m (pile tip below tunnel invert) to study the effects of pile length. 7. L/H=1. a short pile is referred to as a pile with its tip elevation at or above the tunnel axis elevation. the restriction of pile head which will cause negative bending moment and tension at the pile head (Figure 5. Particularly.g. Test 3 involves 157 .Chapter 5 Effects of Tunnelling on Single Piles larger induced pile axial force and bending moment. Thus.5(c)). it is common that a pile is rested or socketed into hard strata and engineers should assess whether the pile can resist the induced negative skin friction due to tunnelling.3 TEST SERIES 3 -EFFECTS OF PILE LENGTH (TESTS 3.5(a)). a floating pile (Test 3) will be mainly governed by pile settlement when tunnelling is carried out nearby. The pile tip is located within the immediate shear zone. pile movements (settlement and deflection) are not a major concern. However. the trade-off is the increase in pile material stress (bending moment and axial force) due to fixity.and fixed-head pile is now examined. In practice.4 m (ratio of pile length over tunnel depth. The variation of pile responses for free. in conclusion. As such. as any fixity in toe would substantially reduce the pile movement as discussed before. For end-bearing piles and piles that are socketed into stiffer material (e. 8) Test 7 was conducted using a short pile with embedment length of 11. Test 8 was conducted using a pile with embedment length of 15. which may be detrimental to the pile (Figure 5. Since the comparison is made between end-bearing piles. The pile settlement in Test 7 is significantly larger than the piles in Tests 3 and 8 whose tips are beneath the immediate shear zone. The longest pile (22 m) in Test 3 experiences the largest negative skin friction as compared with shorter piles in Tests 7 and 8. the pile tip is located far below the large soil displacement immediate shear zone. the incremental short pile settlement in the long-term over short-term is also much larger than that of the long-pile.6 shows the variation of induced pile axial force with pile length for Tests 3. 7 and 8 and the free-field vertical soil movement at the respective pile locations. Ran. 1998. as the trough width in clay is generally larger than that in sand (Rankin. This is because the pile in test 3 has the greatest pile shaft area.5 to L/H=1. The pile-to-tunnel distance. the large settlement zone in clay is noted to be wider than that in sand. The results demonstrate that the tunnelling-induced settlement on a short pile is more critical than on a long pile for both short-term and long-term.Chapter 5 Effects of Tunnelling on Single Piles a long pile case and Tests 7 and 8 involve short pile cases. However. 7 and 8. Moreover. Figure 5. The portion of the pile beneath the shear zone develops positive skin friction and end bearing to resist the down drag force.8 shows the variation of induced bending moment with pile length. The pile bending moment profile changes from double to single curvature as the pile tip moves from L/H=1.7 shows the variation of induced pile head settlement with pile length for Tests 3. In addition. 2004). These findings are consistent with the observed large settlement zone for tunnelling in sand reported by Jacobsz (2002). It is observed that the 158 . Figure 5.04 or at the tunnel axis level. Figure 5. tunnel volume loss and other parameters are kept constant in the tests. thus allowing the development of a larger negative skin friction. Chapter 5 Effects of Tunnelling on Single Piles maximum bending moment increases with pile length in both short- and long-terms. The maximum bending moment of the piles (L/H=1.04 and 0.76) occurs above the elevation of the tunnel spring line and approximately at the middle of the piles, instead of at the tunnel spring line in the case of long pile (L/H=1.5). In addition, the induced pile bending moment is greater for a longer pile (pile base located outside the immediate shear zone) than that of a shorter piles. This is due to restraint of the pile beneath the immediate shear zone with relatively small or negligible soil movement. The lateral soil movement profiles shown in Figure 5.9 provide clear evidence on the changes of induced pile bending moment. As a result, a longer pile would induce a larger bending moment as compared with a shorter pile. Figure 5.9 shows the variations of pile deflection for Tests 3, 7 and 8 and the corresponding free-field lateral soil movement profile obtained from PIV (Test 1). The pile generally deflects in a similar fashion as the free-field soil displacement profile but with much ‘smoother’ and smaller movements. The restraint in movement can be attributed to the large bending stiffness (EI) of the pile body. In contrast to the induced pile bending moment, the short pile head deflection is significantly larger than that of the long pile. In the case of long pile, its deflection is found to be the smallest among the three cases. When the pile length is reduced, the magnitude of pile deflection also increases. This is due to the fact that the long pile is partially embedded in the support zone with smaller induced soil movements while the short pile is embedded entirely in the immediate shear zone which induces relatively larger soil movements. In consequence, the long pile ‘bends’ towards the tunnel while for the short pile, it ‘shifts’ toward the tunnel with a maximum pile deflection at the pile head for both cases. 159 Chapter 5 Effects of Tunnelling on Single Piles Figure 5.10(a) shows the variation of maximum pile axial force with normalised pile length over tunnel depth (L/H=0.76, 1.04, 1.5). The results clearly show that the induced maximum pile axial force increases with pile length over tunnel depth. It is noted that the maximum axial force increases linearly with normalised pile length over tunnel depth, from 25 kN (L/H=0.76) to 92 kN (L/H= 1.04) and finally to 198 kN when the pile tip is located below the tunnel invert (L/H=1.5). However, in the long-term, the linear increment of axial force with normalised pile length over tunnel depth has a much steeper gradient than when compared with the short-term. The axial force increases from 45 kN to 92 kN and then 198 kN when the normalised pile length over tunnel depth (L/H) increases from 0.76, 1.04 to 1.5, respectively. This is because the piles whose tips are located in the zone of immediate shear zone would be intensively dragged down by the large soil movements, resulting in large pile settlement. On the other hand, piles with tips located out of the shear zone would experience less settlement as the lower part of the pile shift is socketed in stiff soil. Figure 5.10(b) shows the variation of pile head settlement with normalised pile length over tunnel depth (L/H=0.76, 1.04, 1.5). The data clearly show that when the pile tip is located within the zone of large displacements, the pile would settle excessively with the tip in the short-term with a similar magnitude to the soil displacement at the location, see Figure 5.7. Figure 5.10(c) shows the maximum induced pile bending moment with normalized pile length over tunnel depth in the short and long-terms. It is observed that the maximum bending moment of the long pile is larger than that of shorter piles due 160 Chapter 5 Effects of Tunnelling on Single Piles to the restraining effect discussed earlier in this section. Similar findings are also reported by Chen et al. (1999) and Pang (2006). Figure 5.10(d) shows the maximum pile head deflection with normalized pile length over tunnel depth in short and long-terms. The maximum induced pile head deflection of a short pile also exhibits marked increase compared with a long pile. In the short- term, the pile head deflection of the long pile (Test 3, L/H=1.5) is 5 mm and increases to 8 mm and 11 mm when the pile length over tunnel depth reduces to 1.04 and 0.76, respectively. This can be attributed to the lateral soil movement profile (Fig. 5.9) induced by tunnelling, where a long pile is restrained if the pile length extends beneath the immediate shear zone. Hence, the pile deflection is smaller when the pile length is longer. Similar trend is observed in the long term as well, in which the pile head deflection increases from 12.1 mm to 18 mm and 22 mm when the pile length over tunnel depth reduces from 1.5 to 1.04 and 0.76 respectively. To further assess the effect of pile length over tunnel depth due to tunnelling, the key pile responses are summarized in Figure 5.11. In a short pile, especially those located in the immediate shear zone, the pile structure is less vulnerable as compared to the long pile. However, there will be excessive pile movements (settlement and deflection) because of lack of anchorage of pile into the stable support zone. In this respect, a longer pile with extension of pile length into stabilised support zone tends to provide more resistance to the pile movements but will experiences larger structural stress in term of bending moment and axial force. 161 Chapter 5 Effects of Tunnelling on Single Piles The figures illustrate that a short pile is less vulnerable in terms of tunnellinginduced pile axial force and bending moment, while significant adverse effect is observed in terms of pile head settlement and deflection for short pile. This phenomenon is verified in the soil settlements and lateral soil movements shown in Figures 5.7 and 5.9. It is demonstrated that relatively large soil movements are induced within the immediate shear zone while relatively small or negligible soil movement induced in the support zone, thus when the pile base is extended into the support zone, smaller pile settlement and deflection are expected due to restraint. Paradoxically, larger axial force and bending moment are induced for longer pile caused by the same restraint. This finding provides evidence that tunnelling-induced displacements of short piles (settlement and deflection) are more critical than that of long piles. In contrast, the pile axial force and bending moment are crucial in the long pile upon tunnel excavation. This implies that the effect of pile length over tunnel depth has pronounced implications in tunnelling-induced pile responses. 5.4 EFFECTS OF DISTANCE OF PILE FROM TUNNEL 5.4.1 Test Series 4 - Free-Head Floating Piles (Tests 3, 5, 6 and 16) In Test series 4, centrifuge model tests (Tests 3, 5, 6 and 16) were performed to study the behaviours of long free-head floating piles with various pile-to-tunnels centre distances. The pile-to-tunnel distance in Tests 3, 5, 16 and 6 is 6 m (or 1D), 9 m (or 1.5D), 10m (or 1.67D) and 12 (or 2D), respectively. Other parameters are kept constant in all tests. For clarity, the results of Test 16 are not presented in Figures 5.12 162 Chapter 5 Effects of Tunnelling on Single Piles to 5.15, but the maximum pile responses of this test are presented in Figure 5.21 for comparison purpose. Figure 5.12 shows the induced pile axial force due to tunnel excavation in Tests 3, 5, 6 and 16. The test results reveal that all the induced axial load profiles are similar with the maximum values taking place slightly higher than the tunnel spring line in the short-term, with the neutral planes shifting lower over time. This implies that the axial load transfer patterns of the piles are similar irrespective of the pile-totunnel distance for a long pile. A similar trend of axial load variation with pile-totunnel distance was also reported by Ran (2004) and Mroueh et al. (1999). It is observed that the induced pile axial forces decrease with an increase in pile-to-tunnel distance. Similar steady decrease is also observed in the long-term. This may be attributed to the fact that the total contact area for piles in the immediate shear zone is reduced when the distance of pile-to-tunnel increases. As such, a shorter portion of the total pile length experiences negative skin friction as a pile is located further away from the zone of large displacements. This can be attributed to the reduced shaft contact area with the soils in the immediate shear zone when the distance of pile-totunnel increases. Figure 5.13 shows the variation of pile head settlement and the free-field vertical soil movement at the respective pile location. Similar gradual decreases in vertical soil settlement with depth and pile-to-tunnel distance are observed. The magnitude of pile head settlement also decreases with increasing pile-to-tunnel distance. This is consistent with the observed variations of pile axial forces from the three tests. Besides, the smaller magnitudes of soil settlement is expected to induce less negative skin friction (Figure 5.12) for the case in which pile tip is below the tunnel invert. Once again, the pile head settlement exhibits time-dependent behaviour and 163 Chapter 5 Effects of Tunnelling on Single Piles reaches its respective peak value after 720 days. The results suggest that the induced axial pile responses are insignificant when the pile-to-tunnel distance is larger than 2D in the present study. Figure 5.14 shows the variation of induced bending moment with pile-to-tunnel distance. Generally, the induced pile bending moment profiles are similar in all tests with the maximum bending moment occurring approximately at the tunnel spring elevation. This is consistent with the numerical predictions reported by Cheng (2003) and field measurements reported by Pang (2006). As expected, the maximum induced bending moment generally decreases with increasing pile-to-tunnel distance for both short and long-terms. Hence, it would be reasonable to assume that induced bending moment are generally small beyond a horizontal offset of 2D from the tunnel centre as magnitudes are less than 50 kNm (long-term) even with a relatively large tunnel volume loss of 3%. In addition, it is evident that regardless of pile-to-tunnel distance, the induced maximum bending moment increases for some time after the completion of tunnel excavation in all tests, exhibiting the time-dependent behaviours as described earlier. The pile responses peak at 720 days after excavation. Figure 5.15 shows the variations of pile deflection and free-field lateral soil movement profiles from these three tests. As expected, the pile deflection decreases with increasing pile-to-tunnel distance. Moreover, it is observed that the pile deflection drops rapidly from 1D to 1.5D, with a much smaller decrease from 1.5D to 2D. The pile lateral responses are best explained by the soil deflection profiles obtained from Test 1 as shown in Figure 5.15. It is noted that the lateral soil movements in the three tests increase with time and decrease with increasing distance of pile location to the tunnel, as the soil movement decreases when the pile-to-tunnel distance increases. It is also noted that at the distance 164 11 and 12 is 6 m. with the neutral plane becomes deeper over time. the lateral soil displacement is prominent at the tunnel spring elevation and surface. 11 and 12. Figure 5.2 Test Series 5 . Similar to test series 4 (effects of distance of pile from tunnel for free-head floating piles). 12) In this test series.14 and 5. for instant at a distance of 2D. This finding demonstrates that when the pile-to-tunnel distance increases. 11.16 shows the variation of induced free-head end-bearing pile axial force due to tunnel excavation with pile-to-tunnel distance for Tests 10. 11 and 12 were performed to study the effects of pile from tunnel for free-head end bearing piles. The pile-to-tunnel distance in Tests 10. However. 10 m and 14 m. 5. Other parameters are kept constant in all tests.4. the lateral soil displacement profile reveals significant soil deflection at the ground surface while the soil movement at the tunnel spring elevation becomes negligible. when the distance is large enough. a shorter portion of the pile length is inside the immediate shear zone. Tests 10. the induced axial force decreases when the pile-to-tunnel distance 165 .Free-Head End Bearing Piles (Tests 10. resulting in a smaller tunnel-pile interaction. This can be related to the soil movement pattern as observed in the immediate shear zone and thus results in the observed pile bending moment and deflection shown in Figures 5.15. the test results illustrate that all the induced axial load profiles are similar with the maximum values taking place slightly lower than the tunnel spring line in the shortterm.Chapter 5 Effects of Tunnelling on Single Piles of one tunnel diameter (1D). respectively. This implies that the axial load transfer patterns of the piles are essentially the same irrespective of pile-to-tunnel distance for an end-bearing pile. Generally. the mid-pile shaft also deflects in a similar trend.3 Test Series 6 . This may be attributed to the fact that the total contact area for piles in the immediate shear zone is lessened when the distance of pile-to-tunnel increases.4. respectively. The induced positive and negative bending moments generally decrease. The magnitude of pile head deflection decreases when the piletunnel distance increases.1. the magnitude is small and thus negligible. It should be noted that pile axial force is only investigated in Test 13 (See Figure 5. 10 m and 14 m.18 illustrates the variation of induced pile deflection profiles for Tests 10. with increase in piletunnel distance.17 shows the variation of induced free-head end-bearing pile bending moment with distance of pile from tunnel centre.Fixed-Head End Bearing Piles(Tests 13. As such.Chapter 5 Effects of Tunnelling on Single Piles increases. Figure 5. The pile-to-tunnel distance in Tests 13. Figure 5. This trend is similar to that observed in test series 4 for free-head floating piles. Other parameters are kept constant in all tests. Although the pile may undergo elastic shortening. 14A and 14B is 6 m. 14A and 14B aim to investigate the effects of pile location from tunnel for fixed-head end bearing piles. Likewise. 5. 14B) Tests 13. as noted in Section 5. as expected.4. An important feature of an end-bearing pile is that the pile would undergo minimal settlement as the pile tip is rested on very stiff ground. 14A. 11 and 12.2) but 166 . pile settlement is not a major concern. Figure 5. the pile bending moment in Tests 14A and 14B are worthy for further study as the changes are more significant and the results can also be served as the bench mark for comparison with the pile group responses presented in Chapter 6. It should be noted that for Test Series 6.21(a) shows the variation of maximum pile axial force with pile-to-tunnel distance for Test Series 4. the induced pile axial forces are observed to decrease fairly linearly with an increase in pile-to-tunnel distance for Test Series 4 and 167 . It is shown that the pile deflection is generally very small (less than 0. In all cases. the negative bending moment is larger than the positive bending moment due to the restraint at pile head.4.02 mm). 14 and 15.20 shows the pile deflection profiles which are derived from the pile bending moment profiles.4 Comparison of Results from Test Series 4. Figure 5. as the axial force profiles for pile-to-tunnel distance of 10 m and 14 m are expected to be similar to that of Test 13 but with smaller magnitudes.19 shows the variation of induced fixed-head end-bearing pile bending moment with pile distance from tunnel centre for Tests 13. there were no axial piles monitored in Tests 14A and 14B. With the test range of pile location of 6 m to 14 m from the tunnel centre. 5 and 6 Figure 5.Chapter 5 Effects of Tunnelling on Single Piles not in Tests 14A and 14B. The pile deflection profile reveals that the mid-pile shaft deflects toward the tunnel. but the magnitude decreases when the pile distance to tunnel increases. Nevertheless. 5 and 6. 5. The results indicate that both positive and negative bending moment decreases when the pile-tunnel distance increases. the most significant difference in Test Series 5 (free-head) and Test Series 6 (fixedhead) is that tension force is induced in the fixed-head pile due to the total fixed condition at the pile head.22. Similarly to pile axial force. The result seems to suggest that the pile settlement are insignificant for pile-totunnel distance larger than 2D in Test Series 4. a shorter portion of the total pile length experiences smaller negative skin friction as the pile moves away from the zone of large displacements. It is also observed that the induced axial force in Test Series 5 (end-bearing piles) is much larger than that of Test Series 4 (floating piles) because the larger soil settlement relative to pile settlement would induce a much larger axial force on the end-bearing pile. In contrast to the pile vertical responses.21(b).Chapter 5 Effects of Tunnelling on Single Piles 5.13. the pile lateral responses are different.21(c) shows the induced maximum pile bending moment with pile-to-tunnel 168 . consistent responses in the corresponding soil surface settlement (Test 1) and the maximum pile head settlement in Test Series 4 (Tests 3. 5. This may be attributed to the fact that the total contact area for piles in the immediate shear zone becomes smaller when the distance of pile-to-tunnel increases. 6 and 16) are observed from Figure 5. as illustrated in Figure 5. As such. as shown in Figure 5. It should be noted that the pile settlements of end-bearing piles in Test Series 5 and 6 are practically negligible as shown in the figure. the soil settlements within the immediate shear zone decreases. This is simply because when the pile-to-tunnel distance increases.and long-term. In addition. The soil surface and pile settlements are observed to decrease with an increase in pile-to-tunnel distance in both short. Similar steady decrease is also observed in the long-term. Figure 5. the maximum induced bending moments decrease reasonably linearly with increasing pileto-tunnel distance when the magnitude is relatively small (see for instant pile bending moment in the short-term and positive bending moment in series 6 (long-term)). the data reveals that negative bending moments are induced at pile head due to total fixity condition for Test Series 6 (fixedhead) as compared to the free-head piles in Test Series 5. it would be reasonable and safe to assume that induced bending moments are generally small beyond a horizontal offset of 2D from the tunnel centre as magnitudes are less than 50 kNm (long-term) even with a tunnel volume loss of 3%. In addition. It is evident that regardless of pile-to-tunnel distance. This is probably because the restraint at the pile toe would restrict the pile lateral movement and induce a larger bending moment. On the other hand. the trend reveals that the positive bending moment in a fixed-head pile (Test Series 6) is consistently smaller than that of the free-head pile (Test Series 5). Generally. the induced bending moments in end-bearing piles (Test Series 5) are larger than the floating piles (Test Series 4). as illustrated in Figures 5.23. The pile responses peak at 720 days after excavation. 5 and 6. since the bending moments are offset toward the negative bending moment.Chapter 5 Effects of Tunnelling on Single Piles distance in the short and long-terms for Test Series 4.22 and 5. regardless of pile-tunnel position. However. This further illustrates that the induced pile bending moments are small as the lateral soil movements are not significant when the pile-totunnel distance increases beyond 2D. Nevertheless.3. the soil within the ‘Immediate Shear Zone’ is ‘unloaded’ 169 .1. As discussed in Section 4. the bending moments decrease exponentially when the magnitude is relatively large (for example long-term bending moment in series 4. exhibiting the time-dependent behaviours described earlier. Generally. both induced maximum bending moments increase for some time after the completion of tunnel excavation in all the tests. 5 and negative bending moment in series 6). regardless of pile-tunnel distance because the lower portion of pile is restrained and cannot move.22 illustrates that the pile responses for different pile-tunnel distance is greatly influenced by the changes of the stress in both the Immediate Shear Zone and Support Zone. The observed variation of pile bending moment and deflection with pile-totunnel distance can be explained by the soil movemnt profiles obtained from Test 1. with a much smaller decrease from 1.5D. On the other hand.Chapter 5 Effects of Tunnelling on Single Piles due to tunnel excavation and gradually deforms by arching. Figure 5. it is expected that the pile head deflection for end-bearing piles (Test Series 5) is smaller than that for floating piles (Test Series 4). On the other hand. Generally.5D to 2D for both Series 4 and 5.23). it is observed that the pile deflection reduces rapidly from 1D to 1. Thus. the measured pile head deflection of Test Series 6 (fixed-head) is negligible due to totally fixed condition at pile head. when the distance is large enough. This is probably because the lateral soil movement decreases with increasing distance of pile from to the tunnel. Nevertheless. the lateral soil displacement profile 170 . However. This leads to the observed soil movement pattern which subsequently affects the pile responses as observed in this chapter. The results reveal that at locations near the tunnel.21(d) illustrates the variations of pile head deflection for the floating piles (Test Series 4) and end-bearing piles (Test Series 5). which was analysed by PIV (see Figure 5. circumferential soil stresses increase within the ‘Support Zone’ to support the arches formed in the immediate shear zone. the lateral soil displacement is prominent at the tunnel spring elevation. causing the radial stress in the immediate shear zone to be reduced due to stress relief. Figure 5. pile bending moment. As such. This study has provided strong evidence that the long-term pile behaviour needs to be considered if the volume loss due to tunnelling is large.34 to 3. It thus further illustrates that when the pile-to-tunnel distance increases. pile head settlement and pile head deflection) ranges from 1.22. It is noted that the soil movement is dominant in the vertical direction in the LT. On the other hand. the LT over ST ratio is comparatively important for pile settlement which depends on the magnitude of downward soil movement before full pile slip.24 shows a summary of the ratio of long-term to short-term pile responses for all tests presented in this chapter. The results show that the long-term to short-term pile responses (pile axial force. causing both pile bending moment and pile deflection to increase over time. a shorter portion of the pile length is inside the immediate shear zone shown in Figure 5.5. 171 . the LT over ST ratio for pile axial force is comparatively small as the axial forces might have been fully mobilized at an earlier stage. 5.5 EFFECTS OF TIME ON PILE RESPONSES IN SOFT CLAY In order to further assess the long-term pile responses. the increase in lateral soil movement is significant and thus in turn.Chapter 5 Effects of Tunnelling on Single Piles reveals significant horizontal soil movement at the ground surface while the soil movement at the tunnel spring elevation became negligible. Although the magnitude of lateral soil movement is much smaller than that of vertical soil movement. This finding is consistent with the relatively insignificant observed pile lateral responses when the pile-to-tunnel distance is more than 2D. Figure 5. In his study. 2004). there have been some reports of outward tunnel deformation in the field (George. In this section. Although outward tunnel deformation is not as commonly observed as inward tunnel deformation.26. while it is slightly larger at 3% for the inward tunnel deformation simulation in the present study.6. outward tunnel deformation was simulated (ovalisation of tunnel lining.25 and 5. Some similarities and differences can be drawn in the behaviours of soil and single piles induced by both inward and outward tunnel deformations in clay.27 shows the development of subsurface soil movements at 2 days and 720 days after tunnel excavation for the case of outward tunnel deformation. It should be noted that the volume loss for the outward tunnel deformation simulation was 2%.6 COMPARISON OF PILE BEHAVIOUR DUE TO INWARD AND OUTWARD TUNNEL DEFORMATIONS Ran (2004) carried out centrifuge model studies on the effect of tunnelling on single piles in clay. which was conducted by (Ran.Chapter 5 Effects of Tunnelling on Single Piles 5. 1991). i. 1981 and Yann and Alain. the difference between inward and outward tunnel deformation patterns and its influences to adjacent piles are investigated. Furthermore.e. as shown in Figures 5.1 Tunnel-Soil Interaction Figure 5. 5. For example. the Handbook of Plastic Pipe Institute (2003) reported that the deformation patterns of poly-material linings under service load are of horizontal-oval shape. large deformations experienced by some tunnel lining rings are encountered in some tunnelling projects. tunnel springline moves outwards). Subsurface soil movement was traced from high resolution 172 . 29 show the variation of surface soil settlement troughs with tunnel deformation over time. despite the difference in tunnel deformation. 6m or 1D from tunnel centre-line) shown in Figure 5. (2000) reported that for cases with significant tunnel ovalisation.Chapter 5 Effects of Tunnelling on Single Piles photographs of the marker beads and analyzed using Computer Program OPTIMUS instead of PIV used in this study. However.1 Similarities (Tunnel-Soil Interaction) Figures 5.6. The void created by ovalisation deformation at the tunnel crown is similar to the clearance at the upper half of the tunnel for the case of contraction deformation.5 m for both cases. 5.e. The Gaussian curve is also found to be inappropriate in depicting the measured long-term surface settlement troughs for both cases. the surface settlement trough would be narrower than the Gaussian curve. It is interesting to note that the measured short-term surface settlement trough follows the Gaussian distribution curve fairly well with the inflection point (i) at approximately 7. 173 .28 and 5. Verruijt and Booker (1996). In addition. This exhibits time-dependent behaviour of clay for both cases. it is observed that the soil continues to settle with time and the settlement rate decreases with time.1. An examination of soil settlement at the pile location (i. This finding is consistent with the observation made by Verruijt and Booker (1996).30 reveals that the soil settlement profile is fairly similar for these 2 situations. Hence the soil above the tunnel crown in both cases settles by a similar vertical distance. i.2 Tunnel-Pile Interaction In both tunnel deformation studies.e. the soil moves towards the tunnel and the immediate shear zone is defined as discussed in Section 4.31 that the soil moves in opposite directions at the pile location for these 2 situations due to differences in the deformation of the tunnel lining.Chapter 5 Effects of Tunnelling on Single Piles 5. Thus. qualitatively.5 m whereas it is slightly shorter at 22 m in the case of inward tunnel deformation.2 Differences (Tunnel-Soil Interaction) The most distinct difference in the soil behaviour between the 2 different tunnel deformations is that the soil moves intensely away from the tunnel due to spring line expansion in the case of outward tunnel deformation and a much larger deformation zone is formed. the pile-to-tunnel distance is similar. The long-term pile axial forces were not presented by Ran (2004) as the pile axial forces were measured by ‘quarter bridge’ strain gauge circuits.6. This is illustrated in Figure 5. However. The tunnel depth remains at 15 m in both cases. whereas the lateral soil movement near tunnel axis is prominent in the case of outward tunnel deformation.3 which is smaller than that of the deformation zone for the outward tunnel deformation.6. the soil movement above the tunnel crown is prominent in the case of inward tunnel deformation. 5.1. Strain gauge readings obtained with ‘quarter 174 . the magnitude is much larger in the case of outward tunnel deformation. In the case of inward tunnel deformation. the pile length in the case of outward tunnel deformation is 23. However. 6 m from the tunnel centre-line. despite the volume loss is smaller at only 2% as compared to 3% for the inward tunnel deformation case. Similar to the tunnel-soil interaction studies. respectively.2 Differences (Tunnel-Pile Interaction) It is worth noting that totally different pile bending moment profiles are induced under the two different tunnel deformation patterns. the neutral plane is found to be located approximately at the tunnel axis for both cases.32) exhibit similar profiles. whereas this does not happen for ‘full bridge’ circuits in the present study. It is observed that outward tunnel deformation causes negative maximum induced bending moment (bending away from 175 .1 Similarities (Tunnel-Pile Interaction) For both tunnel lining deformations. Hence. 5. This is because the relatively large amount of tunnel expansion at the springline for the outward tunnel deformation causes large settlement. Despite the slightly lower volume loss in the case of outward tunnel deformation.30. regardless of tunnel deformation patterns. the induced pile axial forces (Figure 5. Also.6. the pile settlement depends very much on the vertical soil movement along the pile shaft.Chapter 5 Effects of Tunnelling on Single Piles bridge’ circuits have a tendency to drift with time due to temperature changes.6. 5.33 reveals that the pile vertical settlement is also timedependent. as the piles experience negative skin friction due to settling soil around the pile shaft. Figure 5.2. In the same way.2. the pile axial forces in both cases have similar magnitudes. the volume loss in the outward tunnel deformation and inward tunnel deformation simulations is 2% and 3%. the piles continue to settle with the soil until full pile slip. As the piles are ‘floating’ in the soft clay instead of being socketed into the hard stratum for both cases. as clearly shown in Figure 5. the gap above the tunnel crown is the main cause of soil movements (Leung. These findings are consistent with the soil movements observed in Figure 5.Chapter 5 Effects of Tunnelling on Single Piles tunnel) whereas inward tunnel deformation causes positive maximum induced bending moment which bends toward tunnel (Figures 5.34 and 5. hence pushing the pile away from the tunnel. The relatively large magnitude of induced pile bending moment in the outward tunnel deformation simulation is mainly due to the large expansion of the tunnel lining at the springline. thus drawing the pile towards the tunnel. the soil moved away from the tunnel at the tunnel axis due to the protrusion of the tunnel lining at the tunnel spring line.35).36 demonstrate the pushing of the mid-pile shaft away from the tunnel due to tunnel protrusion at the tunnel spring elevation. The above comparison of pile behaviours due to inward and outward tunnel deformations illustrate that the trend of pile axial force and pile settlement behaviour and profile are essentially similar regardless of tunnel deformation pattern. 2006). However. as illustrated by the long-term to shortterm ratio of pile responses in Figure 5. in the case of inward tunnel deformation. the pile moves towards the tunnel. with the pile head deflecting more than the pile tip. 176 .. Similar to the study on deep excavations in clay at NUS (Leung et al.31. In the case of inward tunnel deformation. On the other hand. 2006). In the case of inward tunnel deformation. the induced pile bending moments. the soil moves towards the tunnel axis due to volume loss caused by excavation overcut. the clay continues to move over time after the completion of tunnel excavation. The induced pile deflection profiles shown in Figure 5. head settlement and deflection in both tunnel deformation simulations also change with time.This is because in the case of outward tunnel deformation.37. Thus. in terms of profiles and magnitude. It is noted that a floating pile is mainly governed by pile settlement when tunnelling is carried out adjacent to it.7 CONCLUDING REMARKS A total of thirteen centrifuge model tests (Tests 3 to 16) have been performed to examine the fundamental mechanisms behind the effects of tunnelling on single piles. It is noted that tensile force and relatively 177 . some opposite trends are observed in the fixed-head when compared to free-head. “socketed” pile and “end-bearing” pile were investigated to study the effects of pile tip condition. 5. Subsequently. and (3) comparison of soil and single pile behaviours due to inward and outward tunnel deformations. the pile lateral responses (bending moment and deflection) are totally opposite for both inward and outward tunnel deformations. (2) tunnelling-induced response of a single pile in relation to tunnelling-induced response of free-field ground. socketed piles are likely governed by the material stress of the pile. On the other hand.Chapter 5 Effects of Tunnelling on Single Piles the outward tunnel formation would induce higher pile responses as compared to the inward tunnel deformation under the same volume loss. Nevertheless. considerable engineering judgement and experiences are required to first determine the tunnel deformation pattern. Three different pile tip conditions. Three main items were investigated: (1) pile vertical and lateral responses in different tunnel-pile configurations (six test series). namely “floating” pile. On the other hand. the analysis of soil movements is necessary to evaluate the pile responses due to tunnelling. respectively. which in turn changes the pile behaviour significantly. On the contrary. This is due to the fact that the different tunnel deformation patterns would induce different soil movement profiles and patterns. in a case by case basis. 5 and 6 examine the effects of pile-to-tunnel distance for different pile head and tip conditions. these responses have led to the reduction in drag load and positive bending moment at the mid-pile shaft. Nevertheless. 178 . In addition. the most significant difference in Test Series 5 (free-head) and Test Series 6 (fixed-head) is that tension force is induced in the fixed-head pile due to total fixed condition at the pile head. However. consistent responses are also observed in the corresponding soil surface settlement (Test 1) and the maximum pile head settlement in Test Series 4. a longer pile with pile length in the stabilised support zone tends to provide more resistance to the pile movements but will attract more bending moment and axial force. it is observed that the pile responses decrease with increase in pile-to-tunnel distance. there will be excessive pile movements (settlement and deflection) because of lack of anchorage of pile into the stable support zone. Test Series 4. The induced pile axial forces are observed to decrease fairly linearly with increase in pile-to-tunnel distance for Test Series 4 (floating piles) and 5 (end-bearing pile) and the axial force in Test Series 5 is always much larger than that of Test Series 4 for all pile to tunnel distances. Different lengths of piles are deployed to further assess the effect of pile length over tunnel depth due to tunnelling.Chapter 5 Effects of Tunnelling on Single Piles large negative bending moments are induced at the pile head due to total fixity. In a short pile. Similarly to pile axial force. especially those located in the immediate shear zone. Generally. In this respect. the pile structural responses are less vulnerable as compared to those of long pile. but the bending moments decrease exponentially when the magnitude is relatively large. As the bending moment profile is offset toward the negative bending moment for fixed-head pile. the positive bending moment in fixed-head pile (Test Series 6) are consistently lower than that of free-head pile (Test Series 5). It can be established from the test results that induced bending moments are generally small beyond a horizontal offset of 2D from the tunnel centre.5D to 2D for both Series 4 and 5. the induced bending moments in end-bearing piles (Test Series 5) are larger than the floating piles (Test Series 4) due to the restraint at the pile toe would restrict the pile lateral movement and induce a larger bending moment. it is observed that the induced pile deflection reduces rapidly from 1D to 1. as the lower portion of the pile is restrained and cannot move. Generally. On the other hand. Some similarities and differences were drawn in the comparisons of soil and single pile behaviour in the cases of both inward (present study) and outward (Ran. The pile head deflection for end-bearing piles (Test Series 5) is smaller that of floating piles (Test Series 4).5D. 2004) tunnel deformations. with a much smaller decrease from 1. regardless of pile-tunnel distance.Chapter 5 Effects of Tunnelling on Single Piles In contrast to the pile vertical responses. Generally. the results reveal that negative bending moments are induced at pile head due to total fixity condition for Test Series 6 (fixed-head) as compared to the free-head piles in Test Series 5. This is because the lateral soil movements decrease with increasing distance of pile location to the tunnel. the pile lateral responses are different. the maximum induced bending moments decrease fairly linear with increasing pile-to-tunnel distance when the magnitude is relatively small (for instant pile bending moment in the short-term and positive bending moment in series 6. Generally. It is noted that the measured short-term surface settlement 179 . It is revealed that the pile axial force and pile settlement behaviour and profile are essentially similar regardless of tunnel deformation pattern. respectively. On the other hand. 180 . but the outward tunnel formation would induce larger pile responses as compared to the inward tunnel deformation under the same volume loss. the soil moved towards the tunnel.Chapter 5 Effects of Tunnelling on Single Piles trough follows the Gaussian distribution curve fairly well with the inflection point (i) at approximately 7. whereas in the case of inward tunnel deformation. the pile lateral responses (bending moment and deflection) are opposite in direction for both inward and outward tunnel deformations. in terms of profiles and magnitude. The most distinct difference in the soil behaviour is that the soil moves significantly away from the tunnel in the case of outward tunnel deformation.5 m for both tunnel deformation cases. 5m 181 .5m 3.Chapter 5 Effects of Tunneling on Single Piles Table 5. L=22m ‘Axial’ Pile C 3 Typical L Individual parameters 24m D 2m S ‘Bending’ Pile 3. Thickness of sand.5m.5m C = 12 m D=6m ‘Bending’ Pile X=6m ‘Axial’ Pile Volume loss = 3 % C L 24m D 10 X End-bearing free-head pile. S=3.5m ‘Bending’ Pile ‘Axial’ Pile Tie Beam 13 L C 24m D 24m X End-bearing fixed-head pile.5m ‘Axial’ Pile Socketed free-head pile. S=3.5m L=27. Pile socketed 3m in sand L=22m C 9 19m L D 3m S 8.5m L=27.1 Test program and prototype parameters in Phase 2 study Phase 2 -Effects of tunnelling on single piles Test series 2 Effects of pile tip & head conditions Test Configuration No. S=3.5m Pile embedment length. S=8.5m 3. ‘Bending’ Pile Common parameters Floating free-head pile. 6m Test series 4 Effects of distance of pile from tunnel (a) free-head floating piles Test No.5m X=6m L=11. 3 Typical Common parameters Configuration ‘Bending’ Pile Individual parameters ‘Axial’ Pile X=6m C L 5 24m D X 3.Chapter 5 Effects of Tunneling on Single Piles Test series 3 Effects of pile length for free-head piles Test No.4m Volume loss = 3 % 8 L=15.5m 16 X=9m C = 12 m D=6m L = 22 m X = 10m Volume loss = 3 % 6 X= 12 m 182 . 3 Typical Common parameters Configuration ‘Bending’ Pile Individual parameters ‘Axial’ Pile L=22m C L 24m D C = 12 m X 7 D=6m 3. 5 m X Volume loss = 3 % 14B 3. Common parameters Configuration Individual parameters 10 ‘Bending’ Pile X=6m ‘Axial’ Pile 11 C L 24m D C = 12 m X = 10 m D=6m X 3. 13 Common parameters Configuration X=6m ‘Bending’ Pile ‘Axial’ Pile Tie Beam C 14A C = 12 m D=6m L 24m D Individual parameters X = 10 m L = 27.5 m Volume loss = 3 % 12 X= 14 m ` Test series 6 Effects of distance of pile from tunnel (c) fixed-head end bearing piles Test No.5m X= 14 m 183 .Chapter 5 Effects of Tunneling on Single Piles Test series 5 Effects of distance of pile from tunnel (b) free-head end bearing piles Test No.5m L = 27. 04 D=6m Tests 3.Test 10 Fixed-head. loss=6.5 Kaolin Clay X/D=1 X/D=1. 16 Tests 6 L/H=1.Chapter 5 Effects of Tunneling on Single Piles Pile-tunnel configuration (Pile base position) Test 7 L/H=0. 9 Tests 5. except Test 4 (Vol.Test 13 Test 11 Test 14A Test 12 Test 14B Notes: Volume loss for all tests is 3%.1 Pile base position investigated in the parametric studies (not to scale) 184 .76 H=15m Tunnel Test 8 L/H=1. 4.5 X/D=2 Toyoura Sand Free-head.5%) L H X D = = = = Pile length Tunnel depth Distance between tunnel axis and centre of pile Tunnel diameter Figure 5. 5 -22.2 Variation of pile axial force with tip condition in (a) Short-term (b) Longterm (Tests 3.5 -12. fixed-head Depth (m) -7. free-head Test 10-end-bearing. free-head Test 13-end-bearing. free-head -2.5 -12. free-head Test 10-end-bearing. fixed-head Depth (m) -7. 10 and 13) 185 . free-head Test 13-end-bearing. 9.5 -27.5 -27.5 -22.5 T unnel -17.5 Test 9-socketed. free-head -2.5 T unnel -17.5 (a) Short-term Axial Force (kN) -200 0 200 400 600 800 1000 Test 3-floating.Chapter 5 Effects of Tunneling on Single Piles Axial Force (kN) -200 0 200 400 600 800 1000 Test 3-floating.5 (b) Long-term Figure 5.5 Test 9-socketed. 9.5 Depth (m) 100 150 -12. freehead Test 9-socketed.5 (b) Long-term Figure 5.5 Depth (m) 100 -12.5 -22.5 (a) Short-term Bending Moment (kNm) -200 -150 -100 -50 0 50 150 200 250 300 Test 3-floating.5 -27. free-head Test 10-end-bearing. free-head Test 13-end-bearing.5 -22. freehead Test 9-socketed. fixed-head -2. fixed-head -2.5 -7.5 -27.5 T unnel -17. freehead Test 10-end-bearing.5 T unnel -17. free-head Test 13-end-bearing.5 -7.Chapter 5 Effects of Tunneling on Single Piles Bending Moment (kNm) -200 -150 -100 -50 0 50 200 250 300 Test 3-floating. 10 and 13) 186 .3 Variation of pile bending moment with tip condition (a) Short-term (b) Long-term (Tests 3. 9.4 Variation of pile deflection with tip condition (a) Short-term (b) Long-term (Tests 3.5 Test 3-floating.5 Test 10-end-bearing.Chapter 5 Effects of Tunneling on Single Piles Lateral deflection (mm) -4 -2 0 2 4 6 8 10 12 14 -2.5 Test 3-floating. free-head Test 13-end-bearing.5 Depth (m) -7.5 (a) Short-term Lateral deflection (mm) -4 -2 0 2 4 6 8 10 12 14 -2.5 -12. free-head Test 9-socketed. free-head Test 10-end-bearing.5 -12. free-head Test 13-end-bearing.5 Depth (m) -7. free-head -22.5 T unnel -17.5 Test 9-socketed. fixed-head -27. fixed-head -27.5 (b) Long-term Figure 5.5 T unnel -17. free-head -22. 10 and 13) 187 . 10 and 13) 188 .Test 13. free-head free-head Test 10. free. free-head Test 10-end.bearing. 9. free-head free-head Test 10Test 13Test 13endendendbearing. free-head fixed-head fixed-head (at pile head) 20 LT 16 12 8 4 0 Test 3floating. socketed. bearing. bearing.bearing. freehead (a) Test 9socketed.Test 13-endbearing. free-head Test 10-end. bearing. socketed.Chapter 5 Effects of Tunneling on Single Piles 24 800 ST Pile head settlement (mm) ST 600 LT 500 400 300 200 100 0 -100 -200 Test 3Test 9floating. bearing. free-head fixed-head fixed-head (at pile head) -200 (c) Pile head deflection (mm) ST ST 150 Maximum pile bending moment (kNm) Maximum pile axial force (kN) 700 LT 12 8 4 0 Test 3floating. free. fixedhead head (b) 16 200 LT 100 50 0 -50 -100 -150 Test 3Test 9floating.Test 13-endbearing. fixedhead head (d) Figure 5. freehead Test 9socketed.5 Variation of (a) maximum pile axial force (b) pile head settlement (c) pile bending moment (d) pile head deflection with tip and head conditions (Tests 3.Test 13endendendbearing. L=22m) ST (Test 7.4m) LT (Test 7. L=22m) LT (Test 3.6m) LT (Test 8.6m) -30 Figure 5.Chapter 5 Effects of Tunneling on Single Piles Axial Force (kN) 0 100 200 300 400 500 600 700 800 0 -5 Depth (m) -10 -15 T unnel -20 ST (Test 3. L=15.4m) ST (Test 8. L=15. L=22m) LT (Test 3. Soil settlement) LT (Test 1. 7 and 8) 189 .6m) -25 -30 Figure 5. L=11. L=15.7 Variation of pile head settlement and soil settlement profile (Test 1) with pile length (Tests 3. Soil settlement) ST (Test 3. L=11.4m) LT (Test 7. L=15.6m) LT (Test 8. L=22m) ST (Test 7. L=11.6 Variation of pile axial force with pile length (Tests 3. L=11. 7 and 8) Settlement (mm) 0 20 40 60 80 100 120 0 -5 Depth (m) -10 -15 -20 -25 T unnel ST (Test 1.4m) ST (Test 8. 6m) LT (Test 8.8 Variation of pile bending moment with pile length (Tests 3. L=22m) ST (Test 7. L=22m) LT (Test 3. L=11. L=11. Soil lateral deflection) ST (Test 3.4m) LT (Test 7.4m) ST (Test 8. L=11. 7 and 8) 190 . L=15. L=15. L=15.Chapter 5 Effects of Tunneling on Single Piles Bending Moment (kNm) -100 -50 0 50 100 150 200 250 300 0 -5 Depth (m) -10 -15 T unnel -20 ST (Test 3.6m) -30 Figure 5.4m) LT (Test7.4m) ST (Test 8.6m) LT (Test 8. L=22m) LT (Test 3. Soil lateral deflection) LT (Test 1.field lateral soil displacement (Test 1) with pile length (Tests 3. L=11. 7 and 8) Lateral deflection (mm) 0 10 20 30 40 0 -5 Depth (m) -10 -15 -20 -25 T unnel ST (Test 1.6m) -25 -30 Figure 5. L=15.9 Variation of pile head deflection and free. L=22m) ST (Test 7. 5 2 Normalised pile length over tunnel depth.5 Normalised pile length over tunnel depth. 7 and 8) 191 . L/H 0. L/H (d) Figure 5.5 (a) (b) 200 30 ST Pile head deflection (mm) Maximum pile bending moment (kNm) 2 Normalised pile length over tunnel depth. L/H (c) 2 0 0 0.Chapter 5 Effects of Tunneling on Single Piles 500 80 Pile head settlement (mm) Maximum pile axial force (kN) 70 ST 400 LT 300 200 ST 60 LT 50 40 30 20 100 10 0 0 0.5 1 1.5 1 1.5 1 1. L/H 150 LT 100 25 ST 20 LT 15 10 50 5 0 0 0.5 0 2 0 Normalised pile length over tunnel depth.5 1 1.10 Variation of (a) maximum pile axial force (b) pile head settlement (c) pile bending moment (d) pile head deflection with normalized pile length over tunnel depth (Tests 3. 76) ST-Test 8 (L/H=1.11 Short pile to long pile ratio of pile responses for different pile length over tunnel depth (Tests 3.5 4 3.5 Positive effect for shorter pile 1 0. Test 3) ratio 5 4.5 3 Negative effect for shorter pile 2.04) Pile head settlement Pile head deflection LT-Test 7 (L/H=0.5 2 1.Chapter 5 Effects of Tunneling on Single Piles Short pile/Long pile (L=22m.04) Figure 5.5 0 Pile axial force Pile bending moment ST-Test 7 (L/H=0. 7 and 8) 192 .76) LT-Test 8 (L/H=1. X= 6m) ST (Test 1. X=12m) LT (Test 1. X=12m) -30 Figure 5. X=12m) -25 -30 Figure 5. X= 6m) LT (Test 3. X= 6m) ST (Test 5. X= 9m) LT (Test 5. 5 and 6) Settlement (mm) 0 20 40 60 80 100 120 0 -5 Depth (m) -10 T unnel -15 Test 3 Pile head settlement -20 Test 5 Pile head settlement Test 6 Pile head settlement -25 Free-field soil settlement ST (Test 1. X= 6m) LT (Test 1.Chapter 5 Effects of Tunneling on Single Piles Axial Force (kN) 0 100 200 300 400 500 600 0 -5 Depth (m) -10 -15 T unnel -20 ST (Test 3. X= 9m) ST (Test 6. X=12m) LT (Test 6.12 Variation of pile axial force with pile-to-tunnel distance for free-head floating piles (Tests 3. 5 and 6) and free-field soil settlement (Test 1) with pile-to-tunnel distance 193 .13 Variation of pile head settlement for free-head floating piles (Tests 3. X= 9m) LT (Test 1. X= 9m) ST (Test 1. X= 6m) -30 Figure 5.Chapter 5 Effects of Tunneling on Single Piles Bending Moment (kNm) -80 -40 0 40 80 120 160 200 240 0 -5 Depth (m) -10 -15 T unnel ST (Test 3. X=12m) Pile head deflection ST (Test 3. X= 6m) ST (Test 5.15 Variation of pile deflection for free-head floating piles (Tests 3. X= 9m) -25 LT (Test 5. X=12m) LT (Test 6. X= 12m) LT (Test 6. X= 6m) LT (Test 1 X= 6m) ST (Test 1. X= 9m) ST (Test 1. X= 9m) LT (Test 5. X=12m) -30 Figure 5. X= 9m) ST (Test 6. X= 6m) LT (Test 1. X= 9m) ST (Test 6. X=12m) ST (Test 5.14 Variation of pile bending moment for free-head floating piles with pile-totunnel distance (Tests 3. 5 and 6) Lateral deflection (mm) 0 5 10 15 20 25 30 35 0 -5 Depth (m) -10 -15 T unnel Free-field lateral soil deflection -20 -25 ST (Test 1. 5 and 6) and free-field lateral soil displacement profile (Test 1) with pile-to-tunnel distance 194 . X= 6m) -20 LT (Test 3. X= 9m) LT (Test 1. X=12m) LT (Test 3. X= 6m (ST) Test 10. X=10m (ST) Test 11.Chapter 5 Effects of Tunneling on Single Piles Axial Force (kN) 0 200 400 600 800 1000 0 -5 -10 Depth (m) -15 T unnel -20 -25 -30 Test 10. 11 and 12) 195 . X=14m (ST) Test 12. X= 6m (LT) Test 11.16 Variation of pile axial force for free-head end bearing piles with pile-totunnel distance (Tests 10. X=14m (LT) Figure 5. X=10m (LT) Test 12. X= 6m) LT (Test 10. X=10m) ST (Test 12. X=14m (ST) Test 12.Chapter 5 Effects of Tunneling on Single Piles Bending Moment (kNm) -100 -50 0 50 100 150 200 250 0 -5 Depth (m) -10 -15 T unnel -20 Test 10. X=10m (ST) Test 11.18 Variation of pile deflection for free-head end bearing piles with pile-totunnel distance (Tests 10. X=14m) LT (Test 12. 11 and 12) 196 . X= 6m (ST) Test 10. X=10m) LT (Test 11. X=14m (LT) -25 -30 Figure 5. X=14m) -30 Figure 5. X= 6m (LT) Test 11.17 Variation of pile bending moment for free-head end bearing piles with pile-to-tunnel distance (Tests 10. X= 6m) ST (Test 11. X=10m (LT) Test 12. 11 and 12) Lateral deflection (mm) -2 -1 0 1 2 3 4 5 6 7 8 0 -5 Depth (m) -10 ` -15 T unnel -20 -25 ST (Test 10. Chapter 5 Effects of Tunneling on Single Piles Bending Moment (kNm) -200 -150 -100 -50 0 50 100 150 200 0 -5 Depth (m) -10 -15 T unnel -20 -25 -30 Test 13. X= 6m (LT) Test 14A. X=14m (LT) Figure 5.19 Variation of pile bending moment for fixed-head end bearing piles with pile-to-tunnel distance (Tests 13. X=10m (LT) Test 14B. X=10m (ST) Test 14A. 14A and 14B) 197 .03 0 -5 Depth (m) -10 ` -15 T unnel -20 -25 Test 13.01 0 0.01 0. X=10m (LT) Test 14B. X= 6m (ST) Test 13. X=14m (LT) -30 Figure 5. X= 6m (LT) Test 14A.20 Variation of pile deflection for fixed-head end bearing piles with pile-totunnel distance (Tests 13. X= 6m (ST) Test 13. X=14m (ST) Test 14B. X=14m (ST) Test 14B. X=10m (ST) Test 14A.02 0. 14A and 14B) Lateral deflection (mm) -0. Test 1 40 30 20 0 0 2 4 6 8 10 12 14 10 -100 0 -200 0 Distance between tunnel centre and pile (m) 2 4 6 8 10 12 14 Distance between tunnel centre and pile (m) (a) (b) 160 Series 4 (ST) 16 120 Series 4 (LT) 14 Series 4 (ST) Series 4 (LT) 80 Series 5 (ST) 40 0 -80 -120 Series 5 (ST) Series 5 (LT) 0 -40 12 Series 6 (ST) 2 4 6 8 10 12 14 Series 6 (LT) Deflection (mm) Maximum pile bending moment (kNm) Maximum pile axial force (kN) 600 Series 4 (ST) 90 10 8 Series 6. tension (LT) 100 70 Settlement (mm) 500 Series 6 (ST) 60 Series 6 (LT) 50 Soil surface settlement (LT).21 Variation of (a) maximum pile axial force (b) maximum pile head settlement and soil surface settlement (Test 1) (c) pile bending moment (d) maximum pile head deflection for Test Series 4.Chapter 5 Effects of Tunneling on Single Piles 100 800 Series 4 (ST) 700 Series 4 (LT) Series 4 (LT) 80 Series 5 (ST) Series 5 (ST) Series 5 (LT) Series 5 (LT) 400 Series 6 (ST) 300 Series 6 (LT) 200 Series 6. head BM (LT) 0 (c) Series 6 (LT) 4 2 Distance between tunnel centre and pile (m) Series 6 (ST) 6 Series 6. tension (ST) Series 6. head BM (ST) -160 Series 5 (LT) 0 2 4 6 8 10 12 Distance between tunnel centre and pile (m) (d) Note:Test Series 4 – Free-head floating piles Test Series 5 – Free-head end-bearing piles Test Series 6 – Fixed-head end-bearing piles Figure 5. Test 1 Soil surface settlement (LT). 5 and 6 with pile-to-tunnel distance 198 14 . 22 Assessment of pile responses for different pile-to-tunnel distance (Tests 3.Chapter 5 Effects of Tunneling on Single Piles ‘Immediate Shear Zone’ Tunnel ‘Support Zone’ Kaolin Clay Toyoura Sand Figure 5. 5 and 6) 199 . 5 and 6) 6m from tunnel Soil movement (mm) 4m from tunnel Soil movement (mm) -40 0 -30 -20 -10 0 -40 -30 -20 -10 12m from tunnel Soil movement (mm) 0 -40 -30 -20 -10 15m from tunnel Soil movement (mm) 0 -40 -30 -20 -10 0 0 0 -5 -5 -5 -5 -5 -10 -10 -10 -10 -10 -15 Tunnel -15 -20 -20 -25 -25 -30 -30 -15 -15 Depth below GL (m) 0 Depth below GL (m) 0 Depth below GL (m) 0 Depth below GL (m) -40 -30 -20 -10 9m from tunnel Soil movement (mm) -15 -20 -20 -25 -25 -30 -30 -20 -25 2 days Loganathan et al 1998 -30 720 days Figure 5.23 Lateral soil displacement profiles at different pile-to-tunnel distance (Tests 3. 11.5 2 Long-term effect 1. 14A. X=10m) Test 14A (fixed-head. 7. X=6m) Test 16 (X=10m) Pile head deflection Test 5 (X=9m) Test 8 (L/H=1. X=14m) Pile bending moment Test 4 (VL=6. 8. X=6m) Test 13 (fixed-head. 9. 5. 6. 14B and 16) 200 . 10.5%) Test 7 (L/H=0. 12.5 0 Pile axial force Pile head settlement Test 3 (Typical) Test 6 (X=12m) Test 9 (Socketed) Test 12 (free-head.24 Long-term to short-term ratio of pile responses for all tests (Tests 3.5 1 0.Chapter 5 Effects of Tunneling on Single Piles 4 3.5 3 Long-term/ Short-term ratio 2. X=14m) Test 14B (fixed-head. X=10m) Figure 5.76) Test 10 (free-head. 4. 13.04) Test 11 (free-head. 26 Simplified tunnel lining ovalisation with time (not to scale) (Test 1) (after Ran. 2004) 201 . and (b) over-cut of tunnel in the present study Figure 5.Chapter 5 Effects of Tunneling on Single Piles Figure 5.25 Comparison of (a) ovalisation of tunnel lining by Ran (2004). 2004) 202 .27 Development of subsurface soil movements at (a) 2 days and (b) 720 days after tunnel excavation (after Ran.Chapter 5 Effects of Tunneling on Single Piles Distance from tunnel central line (m) 0-20 -18 -16 -14 -12 -10 -8 -6 Ground surface -4 -2 0 Depth below gr ound level (m) -5 -10 Tunnel depth 15m -15 60 ( mm) 50 40 -20 30 20 (a) 10 -25 Distance from tunnel central li ne (m) 0-20 -18 -16 -14 -12 -10 -8 -6 Ground surface -4 -2 0 Depth below ground level (m) -5 -10 1 30 (mm) -15 1 10 90 70 -20 50 30 (b) 10 -25 Figure 5. Chapter 5 Effects of Tunneling on Single Piles Distance from tunnel centre-line (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 0 -0. 720 days(VL=3%) Ran (2004).8 -0.2 inflection point. 2004.2 Sv/ Smax -0.4 -0.3 -0. 'i" -0.6 Outward tunnel deformation (Ran.9 -1 Figure 5.4 Inward tunnel deformatio (Test 3. VL=3%) -0.7 -0. 2 days (VL=3%) Test 3.1 -0.28 Variation of surface soil settlement troughs with tunnel deformation Time (days) 0 180 360 540 720 900 1080 1260 1440 0 -0. 720 days (VL=2%) Ran (2004).7 -0. VL=2%) -0. Gaussian curve (VL=3%) Test 3. 2 days.6 -0. Gaussion Curve (VL=2%) -0.1 -0.5 -0.8 Test 3.29 Variation of maximum surface soil settlement at tunnel central line with tunnel deformation 203 . 2 days (VL=2%) Ran (2004). 2 days.5 -0.3 Sv/Smax -0.9 -1 Figure 5. 2 days (VL=3%) -25 Inward tunnel deformation. 2 day s Inward tunnel def ormation. 720 days (VL=3%) Outward tunnel deformation. 720 day s -25 Outward tunnel def ormation. Test 3.31 Variation of soil deflection at pile location with tunnel deformation 204 .Chapter 5 Effects of Tunneling on Single Piles Settlement (mm) 0 20 40 60 80 100 120 140 160 180 200 0 -5 Depth (m) -10 -15 T unnel Free-field soil settlement -20 Inward tunnel def ormation. Ran (2004). 720 days (VL=2%) -30 Figure 5. Ran (2004). test 3. Test 3.30 Variation of vertical soil settlement at pile location with tunnel deformation Lateral deflection (mm) -40 -30 -20 -10 0 10 20 30 40 50 0 -5 Depth (m) -10 -15 T unnel -20 Inward tunnel deformation. 2 days (VL=2%) Outward tunnel deformation. Test 3. 2 day s -30 Figure 5. Ran (2004). 33 Variation of pile head settlement with tunnel deformation 205 .Chapter 5 Effects of Tunneling on Single Piles Axial Force (kN) 0 100 200 300 400 500 600 700 800 0 -5 Depth (m) -10 -15 T unnel Approximate Neutral Plane -20 -25 Inw ard tunnel deformation. Test 3. Ran (2004). 2 days -30 Figure 5. 720 days Outw ard tunnel deformation.2004) LT Figure 5. Test 3. 2 days Inw ard tunnel deformation.32 Variation of pile axial force with tunnel deformation Pile head settlement (mm) 30 25 20 15 10 5 0 Inw ard Tunnel Deformation (Test 3) ST Outw ard Tunnel Deformation (Ran. 2 days Outw ard tunnel deformation.35 Variation of tunnelling-induced maximum pile bending moment over time for different tunnel deformation 206 . Ran (2004). 2 days -25 Inw ard tunnel deformation. 720 days -30 Figure 5. Test 3.Chapter 5 Effects of Tunneling on Single Piles Bending Moment (kNm) -350 -250 -150 -50 50 150 250 350 0 -5 Depth (m) -10 -15 T unnel -20 Inw ard tunnel deformation. Test 3. Ran (2004). Ran (2004) Figure 5.34 Variation of pile bending moment with tunnel deformation Maximum pile bending moment (kN) 300 250 200 150 100 50 0 -50 0 -100 180 360 540 720 900 1080 1260 1440 Time (days) -150 -200 -250 -300 Inward tunnel deformation. Test 3 Outward tunnel deformation. 720 days Outw ard tunnel deformation. 5 Long-term effect 2 1.5 1 0. 2 days Ran (2004).Chapter 5 Effects of Tunneling on Single Piles Lateral deflection (mm) -5 0 5 10 15 20 0 -5 Depth (m) -10 -15 T unnel -20 Pile head deflection. 2 days Pile head deflection. 720 days -25 Ran (2004).5 3 LT/ST ratio 2. Test 3 Outward tunnel deformation. Ran (2004) Figure 5. Pile head deflection.36 Variation of pile deflection with tunnel deformation 4 3. Pile head deflection.37 Long-term to short-term ratio of pile responses over time for different tunnel deformation 207 . 720 days -30 Figure 5.5 0 Pile bending moment Pile head settlement Pile head deflection Inward tunnel deformation. Effects of tunnelling on pile groups with capped-head and fixed-head conditions. 5. Effects of time on pile group responses in soft clay.Chapter 6 Effects of Tunnelling on Pile Groups CHAPTER SIX EFFECTS OF TUNNELLING ON PILE GROUPS 6. In conjunction with the results of Tests 3 and 16 reported in the previous chapter. Effects of tunnelling on an end-bearing pile group as compared to single end-bearing pile with capped-head and fixed-head conditions. The test procedure for pile group is similar to that for single piles and the schematic plan and elevation views for the five pile group tests (Tests PG1 to 5) are shown in Table 6. 2. As piles are commonly installed in groups in practice.1. Effects of size of pile group due to tunnelling.1 INTRODUCTION The results on the effects of tunnelling on single piles presented in Chapters 4 and 5 provide valuable insights. 3. Effects of tunnelling on a floating pile group as compared to single floating pile. The study aims to address the following issues: 1. the centrifuge model study on single piles is extended to pile groups in the same soil conditions. the responses of a floating capped-head 2-pile group obtained from 208 . 4. 2 FLOATING PILE GROUP Test PG1 was carried out on capped-head floating 2-pile group with the front pile at 6 m and the rear pile at 10 m from the tunnel centreline.Tests PG3 and 5). Test PG2 investigates the responses of an end-bearing capped-head 2-pile group and the results are compared with those of a single end-bearing free-head pile obtained from Tests 10 and 11. In order to study the effects of size of pile group due to tunnelling. 6. for fixed-head . 209 . Finally. 1986). The results of Test PG1 are compared with those of single free-head floating piles Tests 3 and 16 (presented in Chapters 4 and 5).Chapter 6 Effects of Tunnelling on Pile Groups Test PG1 are compared with a single floating free-head pile. In the capped-head pile groups. the behaviours of 2-pile group and 6-pile group are compared (for capped-head .Tests PG4 and 5). for 6-pile group . the cap is connected to the individual pile heads at about 200 mm above the ground level to avoid interaction between the pile cap and the soil following the approach adopted by Bransby and Springman (1997). the effects of tunnelling on pile groups with capped-head and fixed-head conditions are investigated (for 2-pile group Tests PG2 and 3. Test PG 3 evaluates the responses of an end-bearing fixed-head 2-pile group and the results are compared with those of a single end-bearing fixed-head pile obtained from Tests 13 and 14. The centre-to-centre pile spacing is hence approximately three times pile diameter. similar to that recommended by BS8004 (BSI.Tests PG2 and 4. Unfortunately it was not possible to conduct single capped-head floating pile in the present centrifuge model setup. 1 shows the tunnelling-induced axial force for the front and rear piles in a capped-head 2-pile group.1 Induced Axial Force and Settlement Figure 6. 720 days) axial load transfer of the front pile of the 2-pile group located at 6 m from tunnel centre and that of a single free-head pile located at the same distance. As such.2 compares the short-term (ST. the 2 piles are forced to act in unison when subjected to tunnelling induced soil movement. would be moderated by the rear pile via the rigid pile cap. (2003) established that the individual pile responses are similar if the 2 piles are aligned parallel to the induced soil movement direction.2. The results reveal that the trend of axial load transfer along the front and rear piles are similar. the corresponding reduction in negative skin friction for the rear pile is 33 % (ST) and 27 % (LT). On the other hand. Figure 6. The induced axial force on the front pile. It is observed that the front pile in the pile group experiences a larger pile axial force than that of the rear pile which is further away from the tunnel. see Figure 6. 6. Such positive pile group effect is 210 . which experiences larger soil movement as presented in chapter 4. It is evident that the magnitude of the induced axial forces for capped-head pile is significantly reduced.Chapter 6 Effects of Tunnelling on Pile Groups Leung et al.3. the 2 piles are aligned perpendicular instead of parallel to the tunnel for Test PG1. Similar observation is noted when comparing the axial pile load profiles of the rear pile of the 2-pile group located at 10 m from tunnel centre and that of a single pile at the same location. The reduction of negative skin friction for the front pile is about 33 % (ST) and 28 % (LT) of that of single pile at the same location. As the front and rear piles are connected by a rigid pile cap. 2 days after tunnel of excavation) and long-term (LT. as the rear pile is being dragged by the front pile via the rigid pile cap. This can be explained by the interaction between the pile and pile cap.2 mm (short-term) and 7. the rear pile settlement becomes slightly higher than that of a single pile at the same location.Chapter 6 Effects of Tunnelling on Pile Groups consistent with the findings by Kuwabara and Poulos (1989) and Loganathan et al.2.4 compares the tunnelling-induced pile head settlement for the front and rear piles of the capped-head 2-pile group and that of corresponding single piles. This is evident by the measured pile cap deflection which will be further discussed later. It was postulated by Ong (2005) that a relaxation in the fixity of the pile cap may have occurred with the fabricated rigid pile cap used in his study of excavation-induced soil movement on piles. It is noted that since the front pile settlement is larger than that of rear pile. However. in which the front pile in the rigidly connected pile group is moderated by the rear pile and hence the pile settlement is smaller than that of the corresponding single pile. in the short-and long-terms.9 mm (long-term) for the corresponding single pile. The rear pile settlements are 2.5 shows the tunnelling-induced pile bending moment for the front and rear piles in a capped-head 2-pile group. it is worth noting that the rear pile settlement is slightly larger than that of the corresponding single pile.1 mm (long-term). Figure 6.2 Induced bending moment and deflection Figure 6. 6. (2001). However. Hence. The front pile settlement is 53% and 56% smaller than that of corresponding single piles at 6 m from tunnel centre. the pile cap has tilted slightly. respectively. a new 211 . for this study. as compared to the 2 mm (short-term) and 6. the pile-cap-pile interaction causes the front pile responses to be moderated by the rear pile via the rigid connecting pile cap. Thus. Figures 6. The results shown in Figure 6. With this new rigid pile cap. the upper bending moment profiles are different between the front and rear pile because backward dragging force is exerted on the front pile by the rear pile through the tie beam. The rear pile generates a larger negative bending moment at the pile cap level as compared to the front pile.5 demonstrate that positive and negative bending moments were induced for both the front and rear piles. (2009). The results further reveal the shadowing effects of the front pile over the rear pile from the soil movement. As before. The maximum pile bending moment occurs approximately at the tunnel spring line for the front pile while the magnitude of maximum positive or negative bending moments for the rear pile is similar.7 compare the front and rear pile responses with the corresponding free-head single piles at the same location. Although the trend of the pile bending moment profiles of 2-piles group and the corresponding free-head single piles are similar.Chapter 6 Effects of Tunnelling on Pile Groups and improved rigid pile cap has been fabricated to include double layers of bolts instead of one as used by Ong (2005).6 and 6. On the 212 . (2003) and Ong et al. since the pile cap is tilted. both front and rear piles of the capped-head pile group demonstrate negative bending moment due to the presence of pile cap while zero bending moment is recorded for single free-head pile whose head can move freely. an almost perfect fixity can be provided between the pile and the pile cap. This finding is consistent with the observation of pile group due to excavation-induced soil movements as reported by Leung et al. resulting in a smaller measured positive bending moment along the rear pile. The front pile.9 mm in the shortterm and increases continuously with time until the end of the tests with a final deflection of 3. The observed lateral pile deflection profiles are similar to those reported by Leung et al. For the rear pile. with their corresponding single piles. (2001) observed that the bending moment profiles for piles in a group and single free-head pile are almost the same. The deflection of each individual pile head is identical as the pile groups are capped.1 mm (long-term) for the 2-pile group (Test PG1) are smaller than that of a corresponding single pile at 6 m (Test 3) 213 .8. In addition. is dragged back towards the rear pile due to the connecting pile cap.9 and 6. The results illustrate that the magnitudes of the front pile deflection of 3. Figures 6. the pile deflection profiles are slightly different for the front and rear piles.2 mm (short-term) and 6. respectively.5 mm. This demonstrates that the interaction of pile-cap-pile has a significant effect on pile group for excavation and tunnelling works. (2003) on pile groups subject to excavation-induced soil movement. Since the pile cap is tilted due to differential pile settlement. considerable interaction between two piles is expected through the rigid pile cap. the pile deflection profiles for the front and rear piles in a capped-head 2-pile group (Test PG1) is different.Chapter 6 Effects of Tunnelling on Pile Groups contrary. as shown in Figure 6. The pile cap deflection is 1. Loganathan et al. Since the two piles at various distances are being capped by a rigid pile cap. the lateral soil movement on the pile is smaller but the rigid pile cap drags the rear pile deflecting towards the tunnel resulting in a different pile deflection profile.10 compare the pile deflection profile of the front and rear pile. except for a small difference at the pile cap location due to fixity condition. which is subjected to a larger soil movement. especially at the pile head. which is subjected to larger soil movements. The induced substantial bending moment at the pile head in the 2-pile group is mainly due to the restraint provided by the pile cap. which the pile tends to bend towards the tunnel due to the unloading process of tunnel excavation.12 and 6. Figure 6.4 mm (long-term). In addition.13 show the ratio of pile responses for a single pile over pile group ratio for the front and rear piles in capped-head pile and the corresponding single pile. the group effect is only beneficial in axial force and positive bending moment.Chapter 6 Effects of Tunnelling on Pile Groups from tunnel centre with pile deflection of 5 mm (short-term) and 12. the group effect is beneficial to the front pile in all aspects except for the negative bending moments. This data further suggests that pile cap plays a vital role in the pile group in resisting the lateral deflection induced by soil movement. However. However. is restrained at the pile head via the rigid pile cap and hence the upper portion of the front pile is being dragged back as oppose to the rigid body translation of pile deflection in the corresponding single pile due to free head condition. particularly bending moment. which is subjected to larger soil movement. Figures 6. for the rear pile. they are larger that that of a corresponding single pile at 10 m (Test 16) from tunnel centre with pile deflection of 2.12 provides evidence that the pile group effect is significant for the front pile. The front pile in the 2-pile group. Adverse effects are observed in the 214 . Generally. On the other hand. the deflection profile of the rear pile in the 2pile group is similar to the corresponding single pile. settlement and deflection. the results indicate that the pile deflection profile in the capped pile group is different from that of free-head single pile.1 mm (long-term). the magnitude of the deflection of the capped-head pile group is smaller than the average of the deflections of the two single piles at 6 m and 10 m from the tunnel centre.8 mm (short-term) and 5. Nonetheless. 5 m and the piles are rested on the base of the strong box to simulate end-bearing piles (see Table 6. 6.14 shows the tunnelling-induced axial force for the front piles in a capped-head end-bearing 2-pile group. 2005). the front pile would be moderated by the rear pile via the rigid pile cap. similar to that observed for the cappedhead floating 2-pile group.Chapter 6 Effects of Tunnelling on Pile Groups negative bending moment. The results show that the front pile experiences larger pile axial force than that of the rear pile.2. Figure 6. The pile group effect can be attributed to the significant pile-cap-pile interaction as the individual piles in a group are “forced” to act in unison when subject to different magnitudes of soil movement (Ong.16. As such. Unlike the floating pile group (PG1) which experiences long-term pile settlement of 7. The results of the respective free-head single piles at the same location from Tests 10 and 11 are compared with the 2 piles from Test PG2 in Figures 6. In addition.3 END-BEARING PILE GROUP 6. Although the induced axial pile profiles are similar for single piles and the 2-pile group. respectively. thus resulting in an overall positive effect for the pile group.1).5 mm 215 . the shadowing effect of the front pile on the rear pile reduces the detrimental effects experienced by the rear pile.15 and 6. the magnitude of the induced axial forces for the cappedhead piles is significantly reduced in both short-and long-terms.3.1 Capped-Head The configuration of Test PG2 is similar to that of Test PG1 except the pile length is increased from 22 m to 27.1. pile head settlement and pile head deflection. as explained in Section 6. This reduction is caused by the presence of pile cap. 8 mm for the front and rear pile. Finally.18 and 6. Secondly.19. the bending moment profile for the front and rear pile are different. whereby negative bending moments are induced at the pile upper and lower portions of the pile shaft.Chapter 6 Effects of Tunnelling on Pile Groups and 4. positive bending moment is induced near the tunnel axis and negative bending moment induced at the upper and lower parts of the pile for the pile group. while zero bending moment is recorded for the single free-head pile as the pile head can move freely without any restraint. The bending moment profiles of end-bearing single piles in Tests 10 & 11. Figure 6. especially at the upper part of the pile. the induced maximum positive bending moments are always larger than the maximum negative bending moments. However. (presented in Chapter 5) and the pile group test (Test PG2) are plotted in Figures 6. Firstly. Owing to fixity for capped pile in Test PG2. the measured pile settlement of end-bearing pile (PG2) is negligible as the pile tip is rested on very stiff ground. For the capped-head pile group. The trend of pile bending moment profiles of Test PG2 and those of the corresponding free-head single piles are similar. there is a significant difference between the capped-head pile group and free-head single pile at the pile top. the induced bending moment increases over time for both front and rear piles. both the front and rear piles experience negative bending moment due to presence of the pile cap. whilst positive pile bending moment occurs approximately at the tunnel spring line.17 shows the tunnelling-induced pile bending moment for both front and rear piles in a capped-head end-bearing 2-pile group. Three major trends have been observed in this study. The results demonstrate triple curvature in the induced bending moment. respectively. This is consistent with the capped-head floating pile 216 . The pile deflection profiles along the upper portion is similar to the cappedhead floating 2-pile group (Test PG1) due to the capping and dragging effect as explained in Section 6.21 and 6.5 mm in the long-term.2. Figure 6.1.20 demonstrate the pushing of the mid-pile shaft away from the tunnel due to underlying sand layer that restrains the pile toe movement and lateral soil movement at pile head.9 mm (short-term) and 3. which moves in rigid body translation mode. The deflection of each individual pile head is essentially identical as the pile groups are capped. The induced pile deflection is 1. X = 6 m) 217 .22 compare the pile deflection of the front and rear pile with their corresponding single piles. The induced pile deflection profiles shown in Figure 6. the profile is similar to the single pile. When compared to the corresponding single pile. the magnitude of maximum induced positive bending moment for capped-head piles is reduced by about 23% in the short-term and approximately 38 to 46% in the long-term. the pile deflection profile at the mid-pile shaft for end-bearing pile group is different from the floating pile group. Secondly. Firstly.Chapter 6 Effects of Tunnelling on Pile Groups responses in Test PG1.9 mm in the short-term and increases to 3.20 shows the tunnelling-induced pile deflection profiles for the front and rear piles in a capped-head end-bearing 2-pile group (Test PG2).2. demonstrating the positive pile group effect. Figures 6. the pile deflection profile along the upper portion for front pile is very different from the single free-head piles due to presence of the pile cap. Two main findings are observed.5 mm (long-term) is smaller than that of a corresponding single pile (Test 10. As expected. as explained in Section 5. the magnitude of the front pile head deflection for the 2-pile group (Test TG2) of 1.2. The front pile is dragged back by rear pile via rigidly connected pile cap. For the rear pile. 24 and 6.5 mm (short-term) and 3 mm (long-term) due to pile group effect (See Figure 6. the pile axial force response in Test 14A is not recorded. However. as noted in Chapter 5. The test results are compared with corresponding fixed-head single pile (Tests 13.Chapter 6 Effects of Tunnelling on Pile Groups of 2. similar to the findings for the floating 2-pile group (Test TG1). Figures 6.2 Fixed-Head Test PG3 was conducted with the pile head totally fixed in position having zero vertical or lateral movements.3. the deflection of the pile group (Test PG2) again lies in between and smaller than the average of deflections of two single piles at the same location. Nevertheless.8 mm (short-term) and 6 mm (long-term). This simulates the condition where the pile cap is tied with a rigid pile cap and very strong/stiff ground beams. The pile negative bending moment and pile head deflection are found to increase when the piles are capped in a group. 218 . except negative bending moment which is induced due to pile cap condition. The results evidently reveal that the group effect is beneficial to the front in all aspects. 6. The rear pile deflection is bigger when compared to the corresponding single pile (Test 11.25 show the single pile over pile group ratio for the front and rear piles in capped-head condition with the corresponding single pile. 14A). X = 10 m) of 1. This is consistent with the findings for the floating capped-head 2-pile group condition. On the other hand.23). positive group effects are only observed in the axial force and bending moment for the rear pile. the front pile experiences higher tensile and compression forces than that of rear pile as it is closer to the tunnel.29 and 6. Figures 6. Owing to the group and shadowing effects. as shown in Figure 6. thus the comparison of rear pile in Test PG3 with a single pile at the same location is not possible. Since Test PG3 is a 2-pile group. Similarly for pile bending moment responses (Figure 6. It is noted that pile axial force is not measured in Test 14A.26 shows the tunnelling-induced axial force for the front and rear piles of a fixed-head end-bearing 2-pile group.Chapter 6 Effects of Tunnelling on Pile Groups Figure 6. The trend of front and rear pile bending moment profiles are similar. The comparisons summarise the positive effects of the pile 219 .27. the rear pile bending moment is smaller than that of front pile as the lateral soil movement acting on the pile is much smaller. the upper pile shaft of a fixed-head end-bearing pile is subjected to tensile force while compression force is induced along the lower pile shaft. it is noted that the front pile axial force responses are smaller than that of a single pile due to the group effect of the pile.32.28). the rear pile experiences smaller bending moment. suggesting that pile-cap-pile interaction is less severe. The results indicate similar pile bending moment profiles for all cases with a smaller maximum bending moment for the 2-pile group due to shadowing effects.30 compare the front and rear pile bending moments with the corresponding single piles (Tests 13 & 14A) at the same location. As discussed in Chapter 5. When compared to single pile (Test 13).31 and 6. The single pile over pile group ratio for the front and rear piles in fixed-head condition (Test PG2) with the corresponding single piles (Tests 13 & 14A) are plotted in Figures 6. only one of each of the three pairs of was instrumented. 1x2 configuration). it is expected that a 2x2 pile group would have similar behaviour with those of a 1x2 pile group (Test PG2) except that the responses might be smaller due to greater number of piles. The minimum boundary clearance for this pile group is 12 m to the edge of the container in the direction perpendicular to the tunnel.05 to 1. 6. the reduction in pile axial tensile force and pile head bending moment are more significant than that in pile axial compression force and mid-pile shaft bending moment. 2x3 configurations) was performed and compared with capped-head endbearing 2-pile group (Test PG2.Chapter 6 Effects of Tunnelling on Pile Groups group. and 8 m to the edge of container in the direction parallel to the tunnel. Randolph and Wroth (1978) established that it is reasonable to assume a maximum shear stress at the pilesoil interface and the shear stress decreases with increasing distance from the pile. 10 m (middle) and 14 m (rear) perpendicular to the tunnel.1 Capped-Head In order to evaluate the effect of pile group size. In Test PG4. capped-head end-bearing 6-pile group (Test PG4. In general. (2003) on excavation-induced soil movement on pile group.5. two rows of piles were arranged in three columns at 6 m (front). Following the finding by Leung et al. This suggests that the restraint due to fixed-head condition is dominant. Shen (2008) reported a minimum boundary clearance of 8 m would not cause 220 . with improvement ratio of 1. It is thus decided to investigate the 2x3 configuration (Test PG4) as the effect of pile group size is expected to be more significant and hence providing further insight on pile group size effect.4. Owing to the symmetrical arrangement of the 2x3 pile group configuration.4 PILE GROUP SIZE 6. Thus the container boundary effect should not be significant for the present study. the pile axial force decreases as the magnitude of soil movement becomes smaller away from the tunnel. This is reasonable as a bigger pile group is likely to provide more shadowing effect on the piles behind the front piles and reinforcing effects to other piles in the group. As the pile settlement for end-bearing pile is negligible as presented in Chapter 5. To further compare the group effects. The reduction in the maximum drag load for the front pile of the 6-pile group (Test PG4) is about 10% to 221 . and finally to 109 kN for the rear pile.Chapter 6 Effects of Tunnelling on Pile Groups significant container boundary effect. Figure 6. the induced maximum drag load reducing from 321 kN (front pile) to 161 kN (middle pile). The observed trend appears consistent with the corresponding single piles reported in Chapter 5. the results of front and middle piles in the 6-pile group are compared with the corresponding capped-head end-bearing 2-pile group (front and rear piles) in Figures 6. The measured maximum drag load in the long-term are 510 kN (front pile). The axial forces increase over the time for all cases. The changes of the induced axial forces are mainly affected by the magnitude of the tunnelling-induced soil movement at various locations from the tunnel. followed by 261 kN (middle pile) and 186 kN (rear pile).33 shows the tunnelling-induced axial force for the front. the pile settlement of Test PG4 will not presented here.35. the piles in a bigger pile group would be subjected to lower induced axial forces.34 and 6. The results reveal that as the distance between the pile and tunnel increases. This finding suggests that pile-cap-pile interaction on the pile axial force is not significant. In the shortterm. In general. middle and rear piles in the fixed-head end-bearing 6-pile group (Test PG4). (2003).Chapter 6 Effects of Tunnelling on Pile Groups 11% while the corresponding reduction for the middle pile is slightly higher of approximately 13% to 17%. The pile-cap-pile interaction would moderate the induced pile bending moments among the piles within a pile group. respectively. The observation is consistent with finding reported by Leung et al. Since the front. the long-term pile bending moments are showing the consistent behaviours except the magnitudes are increased over time. the front pile is being dragged back while the middle and rear piles being pulled by front pile toward the tunnel through the connecting rigid pile cap. The bending moment profiles generally display triple curvatures with negative bending moments at the upper and lower portions of the pile body.36(a) and (b) show the tunnelling-induced pile bending moment for the front. middle and rear piles in a capped-head end-bearing 6-pile group. Similarly. the induced pile bending moments in the middle row is smaller than that of rear row. 222 . Figures 6. This is contrary to the induced lateral soil movements. middle and rear piles are connected via a rigid pile cap. whilst positive pile bending moments occur approximately at the tunnel spring line. in the shortand long-term. It is observed that the middle and rear piles have similar induced bending moment profiles while the front pile shows a markedly different profile. which suggesting that part of the bending moments of the pile in the middle row is transferred to the rear piles due to the interaction through pile cap. in which the corresponding lateral soil movement on the middle row of piles is larger than the corresponding lateral soil movement on the rear row of pile. as a result. For the front pile.38. Since all 6 piles at different position are capped by a rigid pile cap. middle and rear piles.7 mm (long-term). middle and rear piles in a capped-head 6-pile group (Test PG4) in the shortand long-term. Moreover.3 mm (short-term) to 2. The shape of bending moment profile in the front pile is similar for both 2-pile and 6-pile group. 6. the pile head deflection is forced to act in unison and thus the lateral deflection at pile head moves in a translation mode with the same deflection magnitude. The data demonstrated that the induced bending moment is reduced by 21% to 30% for the front row of piles and by a considerable reduction for the middle row of piles at 27% to 54% from the above finding. the pile head is dragged back towards the rear pile similar to the Test PG2 (see Fig. With more piles in a bigger pile group. The measured pile head deflection increases from 1. 223 . Furthermore. which is illustrated earlier that the part of bending moments in the middle row piles are shared by the rear row of piles. As such. respectively.37 and 6. the pile-cap-pile interaction is more significant in lateral pile responses for a bigger pile group.Chapter 6 Effects of Tunnelling on Pile Groups The comparison of bending moment responses of respective piles at the same location for the 2-pile group (Test PG2) and 6-pile group (Test PG4) are plotted in Figures 6. The data reveal that the pile deflection profiles demonstrate some differences between the front. the reduction in maximum bending moment in 6-pile group (Test PG4) as compared to 2-pile group (Test PG2) is more significant for the middle row piles than the front row piles in 6-pile group. Figures 6.40).39 (a) & (b) show the tunnelling-induced pile deflection profiles for the front. the larger pile-cap-pile interaction and shadowing of front piles over rear piles significant affect the performance of the pile group. the bending moment profiles of rear pile in 2-pile group are similar to that of middle row and rear row of piles in 6-pile group. In addition. 6.9 mm to 1. The pile head deflection reduces from 1. so would the maximum negative bending moment at the pile head.7 mm (longterm) when the pile group size increases from 2 to 6 piles. The comparison of pile head deflection for different pile groups is shown in Figure 6.3 mm (short-term) and 3.1 and 2.Chapter 6 Effects of Tunnelling on Pile Groups the middle pile in the 6-pile (PG4) group shares the same shape of the deflection profile with the rear pile in the 2-pile group (PG2) (Fig. 224 .23. This further demonstrates the positive effect of pile group increases with group size. if the pile head deflection increases.43 show the ratio of the pile group responses of the 2-pile group over 6-pile group for the respective piles at 6 m (front) and 10 m (middle piles in 6-pile group and rear pile in 2-pile group) in order to evaluate the effect of pile group size. The measured ratio of between 1. with considerably less pile cap relaxation reported by Ong (2005).41).42 and 6. It is acknowledged that the pile head deflection is directly proportional to the bending moment. In other words.5 mm to 2. Figures 6. The pile group effect is positive if the ratio exceeds one as there is a larger reduction in pile responses for the larger pile group. The consistent responses of deflection with bending moment observed in the present study show that the improved pile cap has served its intended function of providing strong pile cap fixity.1 clearly shows that a larger pile group would experience a greater positive group effect. both the induced pile tensile and compression forces reduce. i. For the front pile.44 shows the tunnelling-induced axial force for the front.Chapter 6 Effects of Tunnelling on Pile Groups 6. middle and rear piles for the fixed-head end-bearing 6-pile group (Test TG5). The results reveal that as the distance of pile from the tunnel increases. This is because a smaller soil movement is induced at location further away from the tunnel. the reduction in tensile and compression force for the middle pile is about the same. The results of front and middle piles in the 6-pile group are compared with the corresponding piles in 2-pile group (Test PG3) in Figures 6. 225 . Figure 6. the fixed-head pile group also exhibit the same positive group effect. 13% to 14% when the size of pile group increases from 2 to 6 piles.45 and 6. The configurations of Test PG5 are similar to Test PG4 except for the pile head condition. Similar to the capped-head.2 Fixed-Head Test PG5 with a fixed-head end-bearing 6-pile group (2x3 configurations) was conducted and compared with the fixed-head end-bearing 2-pile group (Test PG3. It is observed that the shapes of pile axial force profiles are similar to those of the fixed-head 2-pile group. It is also noted that the pile-cap-pile interaction in a totally fixed-head condition is not significant. the reduction in the maximum tensile force in the short-term for the 6-pile group (Test PG5) is about 25% with a smaller reduction of 14% for the maximum compression force when compared with the 2-pile group (Test PG3).4. On the other hand.46. 1x2 configurations).e. The bigger pile group is able to provide more significant reinforcing and shadowing effects. resulting an average smaller pile response due to tunnelling. the corresponding reduction of 28% and 29% is smaller for the positive bending moment. The comparison reveals that the reduction in maximum negative bending moment at the pile head is more significant than the reduction in maximum positive bending moment at mid-pile shaft for the 6-pile group.49.Chapter 6 Effects of Tunnelling on Pile Groups Figures 6. As expected. resulting in smaller bending moment on the trailing piles. whilst positive pile bending moment occur approximately at the tunnel spring line. middle and rear piles in a fixed-head end-bearing 6-pile group (Test PG5) in the short-and long-terms. A comparison of bending moment profiles of respective piles at the same location for the 2-pile group (Test PG3) and 6-pile group (Test PG5) are shown in Figures 6. The data illustrates that in the short-term. In addition. On the other hand. 226 . the front row of pile provides shadowing to the trailing middle and rear rows pile during tunnel excavation. the induced negative bending moment reduces by 34% and 41% for the front and middle piles. a bigger pile group provides more shadowing and reinforcing effect to other piles within the group. respectively. The results reveal that the absolute magnitude of the negative bending moment at the pile head is larger than the positive bending moment at mid-pile shaft suggesting a significant fixed-head effect. the induced maximum positive bending moment reduces from the front to the middle finally to the rear pile as the lateral soil movement reduces with increasing distance between the pile and tunnel. The shape of profile is similar to the fixedhead end-bearing 2-pile group (Test PG3) as well as fixed-head single piles (Tests 13. As before. respectively.48 and 6. The results reveal that the bending moment profiles display triple curvatures with negative bending moments along the upper and lower portions of the pile body. 14A & 14B).47(a) and (b) show the tunnelling-induced pile bending moment for the front. in which no vertical or lateral pile head movement is allowed.7 for bending moment. being between 1. between 1. the effect of fixity of pile cap itself has been rarely been studied before.51 show the ratio of pile responses of the 2-pile group over 6-pile group for the respective piles at 6 m (front) and 10 m (middle piles in 6-pile group and rear pile in 2-pile group).5. The most distinct difference observed is that both compression and tensile forces are induced in fixed-head pile group (PG3). 6. the behaviours of pile group heavily depend on the pile cap fixity. 227 . However. Hence.Chapter 6 Effects of Tunnelling on Pile Groups Figures 6.33 for axial force. As tensile force is induced along the upper pile shaft due to total pile cap fixity in Test PG3.02 and 1.3 and 1.5 PILE CAP CONDITIONS 6.53 show the tunnelling-induced axial force respectively for the front and rear piles of a fixed-head and capped head end-bearing 2-pile groups. Tests PG2 (capped-head) and PG3 (fixed-head) have been conducted to study the effects of tunnelling on 2-pile groups with capped-head and fixed-head condition. This is due to the total restraint provided by the pile head in fixed-head condition. Figures 6. as explained in Chapter 5. while only compression force (drag load) is induced in capped-head pile group (PG2).52 and 6. Thus. In reality. It is noted that the positive group effect is generally higher in pile bending moment than axial force.1 2-Pile Group The effect of fixity between the pile and pile cap has been studied by many researchers. there is a reduction in the maximum drag load of about 20% in the front pile and about 15% in the rear pile.50 and 6. On the other hand.57 show the induced pile responses of capped-head pile over fixedhead pile ratio for the front and rear piles.54 and 6. The induced 2-pile group bending moments in capped-head (Test PG2) are compared with those in fixed-head (Test PG3) in Figures 6. it is noted that the pile bending moment profiles for the 2 pile cap conditions are not similar. the induced negative bending moment of fixed-head pile group is much larger than that of capped-head pile group. particularly at the pile head. For the front pile.24. The capped-head pile is allowed to deflect freely and interact with other piles in a group and thus inducing a smaller negative bending moment compared to the fixed-head pile. whereby capped-head demonstrates a significant pile-cap-pile interaction while for fixed-head condition. In fixed-head condition (Test PG3). the pile-soil-pile interaction is dominant and thus suggesting that pile-cap-pile interaction is less severe as the cap is totally fixed in the position. It is also worth noting that for fixed-head. Owing to total restraint at the pile head.56 and 6.55. with the ratio of axial force of capped-head pile over fixed-head pile of 1. This is probably due to the different pile cap fixity condition. 228 . the profile is very much similar to those single fixed-head piles with the maximum bending moment occurring at the pile head. the profiles of rear pile bending moments for capped. but it is different for capped-head condition (Test PG2) due to tilting of cap resulting in a downward shift of the location of maximum bending moments. the negative bending moment induced at pile head is always larger than the positive bending moment induced at the mid-pile shaft because the fixed-head condition has provided a very rigid restraint at the pile head which dominates the lateral pile responses.Chapter 6 Effects of Tunnelling on Pile Groups Figures 6.15 to 1.and fixed-pile are similar. or reduction in maximum drag load. The figures reveal that a fixed-head is beneficial for induced pile axial force. Tests PG4 (capped-head) and PG5 (fixed-head) are compared in this section. i. Generally. the compression forces induced in the fixed-head pile (PG5) are always smaller than that of capped-head pile (PG4) with a reduction ranging from 10% to 22%.60 show the tunnelling-induced axial force for the front. i. 6.5 for front pile and 0.56 and 6.58. middle and rear piles of a fixed-head and capped head endbearing 6-pile groups. Likewise.5.06 to 1.7 for rear pile. Paradoxically. It is thus important to check the adequacy of the steel reinforcement for the different pile cap fixity.57. at about 0. Similar to 2-pile group. the maximum positive and negative bending moments of capped-head pile over fixed-head pile ratio for the front and rear piles experiencing opposite trends. 6. the negative pile bending moment ratio of capped-head pile over fixed-head pile is less than 1. which can be explained by the totally fixed-head condition. 229 . compression and tensile forces are induced in fixed-head pile (PG5). respectively. Figures 6. the induced positive pile bending moments near mid-pile shaft is beneficial to a fixed-head pile group.4 to 0.Chapter 6 Effects of Tunnelling on Pile Groups As shown in Figures 6. This indicates an adverse effect on the fixed-head pile group.59 and 6.e.e. with the cappedhead pile over fixed-head pile ratio of 1. which restraints the pile movement and thus induces the high bending moment. Nevertheless.28. it is critical at the pile head for fixed-head pile group and at the mid-pile shaft for the capped-head pile group. the trade-off for such reduction in drag load is that tensile force is induced near to the pile head due to totally fixed head condition.2 6-Pile Group The results of 6-pile group. at the pile head. part of the bending moments of the 230 . all of the middle and rear row piles (Figs.63.Chapter 6 Effects of Tunnelling on Pile Groups Unlike axial force. The front pile (Fig. for Test PG4 (capped-head 6-pile group). It is observed that since the pile cap is tilted slightly in the capped-head pile group. as discussed in Section 6.63) are noted to bend toward the tunnel (similar to the front pile in the fixed-head pile group) regardless of pile cap conditions. followed by middle and rear pile.61. as oppose to 2-pile group. It is thus expected that the magnitude of the induced pile bending moment should follow a similar trend of soil movements.and fixed-head conditions illustrates a very different behavior in transferring the induced bending movements within a bigger group of 6pile. Nevertheless. 6.61) exhibits different bending moment profiles for both capped-head and fixed-head conditions.62 and 6. similar to the comparison of 2-pile group discussed earlier. On the other hand.62 and 6. the behaviour of bending moment is much more complicated. Intuitively the induced lateral soil movement is largest at the position of front pile. the induced pile bending moments in the middle row is smaller than that of rear row.4. the comparison of capped. Despite the shape of bending moment profiles are similar in middle and rear row piles. 6.1. The results of induced 6-pile group bending moments in capped-head (Test PG4) are compared with those in fixed-head (Test PG5) in Figures 6. 6. this trend is only observed in the fixed-head 6-pile group (Test PG5) but not the capped-head 6-pile groups (Test PG4) because moderation effect is negligible for fixed-head pile group and the profiles follow those of single fixed-head pile with the magnitudes reduce with increase of tunnel-pile distance. However. the pile-cap-pile interaction would moderate the induced pile bending moments among the piles within a pile group. As a result. It is observed the upper portion of the front pile is being dragged backward in Test PG4 (capped-head) while the profile of the front pile in Test PG5 (fixed-head) is similar to those single fixed-head piles. This suggests that in a fixed-head 6-pile group (Test PG5).Chapter 6 Effects of Tunnelling on Pile Groups pile in the middle row is transmitted to the rear piles due to interaction through pile cap. It is revealed that maximum positive bending moments induced at the mid-pile shaft for capped-head (PG4) is always larger than that of fixed-head (PG5). especially when the pile group increases from 2-pile (Test PG-2) to 6-pile (Test PG4). This is consistent with the trend of observed lateral soil movement which reduces when the distance to tunnel increases. whereby the rear pile in capped- 231 . for the capped-head 6-pile group in Test PG4. The interaction among the piles in a bigger group would moderate. Nonetheless. 6.66.6 for the rear pile due to the moderating effects in capped-head pile. the front pile registered the highest induced maximum bending moment. the pile-soil-pile interaction is dominant and largely depends on the soil movement instead of pile-cap-pile interaction. Figures 6. This observation further confirms that pile-cap-pile interaction is significant in capped-head fixity.68 show the capped-head pile over fixed-head pile ratio. transfer and share the induced responses among the piles particularly for the pile bending moments.1 to 1. followed by the rear pile and finally the smallest bending moment was noted for the rear pile. thus registering a positive effect for fixed-head pile with the ratio ranging from 1. The above-mentioned behaviors can be further illustrated in Figures 6.64 & 6.65. In addition. It appears that the maximum positive and negative pile bending moments reduce almost linearly from the front row piles to the middle row piles and finally to the rear row piles in Test PG5 (fixed-head).2 for the front and middle piles and a higher ratio ranging from 1. the middle row piles are shielded in the 6-pile group and experiencing the least induced pile bending moment.9 to 2.67 and 6. 6 is observed. a pile cap tends to transmit more bending moment from the middle pile to the rear pile which would otherwise be less affected by the tunnelling-induced soil movement. all of the piles in fixed-head (Test PG5) 6-pile group behave like single piles standing side by side without direct pile-cap-pile interaction. the induced maximum negative bending moments induced near to pile head are expected to be larger in the fixed-head pile (PG5) than capped-head pile (PG4) due to the restraint enforced at the pile head. 232 . the front row piles tend to bend the most toward tunnel direction but the upper portion of the bending moment profile is being dragged back by the by middle and rear row piles via the connecting pile cap.5 to 0. it is contrary for the rear piles.Chapter 6 Effects of Tunnelling on Pile Groups head is sharing some bending moment from middle pile while rear pile in the fixedhead induced the smallest bending moment due to distance effect. From the above findings. Contrary. When the capped-head pile group is tilted due to tunnelling. However.5 to 1.75. This means that the induced negative bending moment at capped-head is larger than that in fixed-head. except that the magnitude is affected by the total number of piles because the behavior is largely governed by the pile-soil-pile interaction. Paradoxically. It is attributed to the fact that the rear pile is inducing higher bending moment than middle pile in the capped-head piles (PG4). whereby a ratio of 1. It is postulated that the rear row piles behave like ‘passive’ pile when pile cap tilts and bends toward the tunnel. Thus the ratio of capped-head over fixed-head is about 0. Thus. it is postulated that the bending moment transfer mechanism for a capped-head 6-pile group (Test PG4) is very different from the fixedhead pile. the behavior is totally different.Chapter 6 Effects of Tunnelling on Pile Groups Figure 6.34 to 3.5. thus diminishing the effects of induced soil movements acting on the piles. in which shadowing and reinforcing effects are dominant. the induced pile responses become smaller. This comparison again confirm the long-term time effects of pile responses due to tunnelling.6 CONCLUDING REMARKS This chapter presents the results of five centrifuge model tests on pile groups with different number of piles. but the trade-off is that intensive structural responses are induced in term of axial forces and bending moments at the upper shaft and pile cap.69 shows the summary of the ratio of pile responses of long-term to short-term pile group responses for all tests presented in this chapter. The data showing the longterm effects with the long-term over short-term ratio of 1. Similar plot for single pile responses have been presented in Section 5. regardless of single or group.5. This is because more efforts are required to drag or bend the entire pile group including the pile cap. When the pile toe condition changes to end-bearing (Test PG2) with a short socket. 6. In the case of a floating capped-head pile group (Test PG1).4 as oppose to the single pile long-term over short-term ratio of wider range of 1. The pile movements reduce substantially. pile cap and pile tip condition. 233 . pile cap condition or group size.32 to 2. When a pile group gets larger. the pile group is generally beneficial as the average pile group responses are smaller than the average of those of single piles at the same locations. as the head conditions play vital roles in dictating the pile responses. in which the corresponding lateral soil movement on the middle row piles is larger than the corresponding movement on the rear row piles. the position of the pile within a group demonstrates a totally different transfer mechanism in the lateral pile responses. In addition. This suggests that part of the bending moments of the middle row piles is transferred to the rear piles due to the interaction through the pile cap.Chapter 6 Effects of Tunnelling on Pile Groups The scenario becomes more complicated if different pile cap conditions are modeled. As a result. It is thus suggested that the pile-cap-pile interaction in capped-head 6-pile group (Test PG4) is less significant in the axial force as compared to bending moment and the induced pile axial forces are mainly influenced by the soil settlement and the distance between tunnel and pile. It is worth noting that the axial forces reduce when the distance between the pile and tunnel increases. capped-head piles (Test PG2 (2-pile group) & Test PG4 (6-pile group)) demonstrate significant pile-cap-pile interaction among the piles. The pile-cap-pile interaction in capped-head 6pile group (Test PG4) would moderate the induced pile bending moments among the piles within a pile group. the induced pile bending moments in the middle row is smaller than that of rear row. Generally. This is contrary to the induced lateral soil movements. as compared to that of a single pile with responses reducing with increasing distance of pile to tunnel. the fixedhead piles (Test PG3 (2-pile group) and Test PG5 (6-pile group)) behave like single piles standing side by side without direct pile-cap-pile interaction. expect that the magnitude is affected by the total number of piles because the behavior is largely governed by the pile-soil-pile interaction. 234 . Contrary. once the pile group size increases from 2 to 6 piles. 4. the results show that the fixed-head condition reduces the pile axial force. which restrains the pile movement causing a high pile bending moment. 235 . there is an increment in the pile bending moment. regardless of pile size. or in other words.32 to 2. This is due to the totally fixed-head condition. pile head and toe conditions. but with tensile forces induced along the upper pile shaft due to the total fixity at the pile cap. for the piles in fixed-head 6-pile group (Test PG5). the piles behave like single piles in term of axial force and bending moment. except that the magnitude is affected by the total number of piles. a reduction in the maximum drag load.Chapter 6 Effects of Tunnelling on Pile Groups On the other hand. Moreover. A common trend has been observed for the long-term over short-term ratio of pile responses for both single pile and pile group. In contrast. The results reveal that soil and pile responses increase over time with long-term over short-term pile responses ratio ranging from 1. 5m Fixed head Endbearing pile 6-pile group 236 .1 Test program and prototype parameters for pile group tests Test Ref.5m ‘Axial’ Pile L D ‘Bending’ Pile A A B E Tie Beam C L D Tie Beam A B E Fixed head Endbearing pile Capped head Endbearing pile 6-pile group ‘Axial’ Pile FP MP RP B C Endbearing pile 2-pile group C B C A=6m B=10m C=12m D=6m L=27.5m A B FP MP RP PG5 T u n n e l Tie Beam C Tie Beam Capped head 2-pile group A T u n n e l Floating pile 2-pile group A T u n n e l Pile head & toe condition Capped head A=6m B=10m C=14m D=6m L=27.Chapter 6 Effects of Tunneling on Pile Groups Table 6. Plan View Elevation View PG1 ‘Bending’ Pile Parameter A=6m B=10m C=12m D=6m L=22m ‘Axial’ Pile A T u n n e l FP RP C L D B PG2 ‘Bending’ Pile A B FP RP A=6m B=10m C=12m D=6m L=27.5m ‘Axial’ Pile C L D B PG3 A B ‘Bending’ Pile FP RP ‘Axial’ Pile L B PG4 T u n n e l D ‘Bending’ Pile A A=6m B=10m C=14m D=6m L=27. Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) 0 100 200 300 400 500 600 0 -5 Depth (m) -10 -15 T unnel -20 Test PG1. Rear (LT) -30 Figure 6. Front (ST) -25 Test PG1. Rear (ST) Test PG1.1 Tunnelling-induced pile axial force (Test PG1) 237 . Front (LT) Test PG1. X=6m(ST) Test 3.2 Tunnelling-induced front pile (Test PG1) and corresponding single pile (Test 3) axial force Axial Force (kN) 0 100 200 300 400 500 600 0 Distance of pile from tunnel centre = 10 m -5 Depth (m) -10 -15 -20 T unnel Test PG1.3 Tunnelling-induced rear pile (Test PG1) and corresponding single pile (Test 16) axial force 238 . Single. Rear (ST) Test PG1.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) 0 100 200 300 400 500 600 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 -15 T unnel -20 Test PG1. X=10m (ST) Test 16. Single. Single. Front (ST) Test PG1. Rear (LT) -25 Test 16. X=10m (LT) -30 Figure 6. X=6m (LT) -30 Figure 6. Single. Front (LT) -25 Test 3. 4 Tunnelling-induced pile head settlement (Tests PG1. Rear (ST) Test PG1. 2-pile group. Test 16 (X=10m) ST (2 days) 2-pile group. Front (LT) Test PG1. 3. rear. Rear (LT) -30 Figure 6.Chapter 6 Effects of Tunneling on Pile Groups Pile head settlement (mm) 20 16 12 8 4 0 Single pile.5 Tunnelling-induced pile bending moment (Test PG1) 239 . Test 3 (X=6m) Single pile. front. front. Front (ST) Test PG1. 16) Bending Moment (kNm) -100 -50 0 50 100 150 200 250 0 -5 Depth (m) -10 -15 T unnel -20 -25 Test PG1. Test PG1 (X=6m) Test PG1(X=10m) L (720 days)T Figure 6. rear. Front (LT) Test 3. X=6m (LT) -25 -30 Figure 6. Single. Rear (LT) Test 16.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -100 -50 0 50 100 150 200 250 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 -15 T unnel -20 Test PG1. Single. X=6m (ST) Test 3.6 Tunnelling-induced front pile (Test PG1) and corresponding single pile (Test 3) bending moment Bending Moment (kNm) -100 -50 0 50 100 150 200 250 0 -5 Distance of pile from tunnel centre = 10 m Depth (m) -10 -15 T unnel -20 -25 Test PG1. Single. Front (ST) Test PG1. X=10m (ST) Test 16. Rear (ST) Test PG1.7 Tunnelling-induced rear pile (Test PG1) and corresponding single pile (Test 16) bending moment 240 . X=10m (LT) -30 Figure 6. Single. Rear (LT) -30 Figure 6. Front (LT) -25 PG1. Front (ST) PG1.Chapter 6 Effects of Tunneling on Pile Groups Pile deflection (mm) 0 1 2 3 4 5 6 7 8 0 Depth (m) -5 -10 -15 -20 T unnel PG1.8 Tunnelling-induced pile deflection (Tests PG1) 241 . Rear (ST) PG1. Single. Rear (LT) -25 Test 16. Single. Front (LT) -25 Test 3. Front (ST) PG1. X=10m (LT) -30 Figure 6. X=10m (ST) Test 16.Chapter 6 Effects of Tunneling on Pile Groups Pile deflection (mm) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0 -5 Distance of pile from tunnel centre = 6 m Depth (m) -10 -15 -20 T unnel PG1.10 Tunnelling-induced rear pile (Test PG1) and corresponding single pile (Test 16) deflection 242 . Single.9 Tunnelling-induced front pile (Test PG1) and corresponding single pile (Test 3) deflection Pile deflection (mm) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0 -5 Distance of pile from tunnel centre = 10 m Depth (m) -10 -15 -20 T unnel PG1. Single. Rear (ST) PG1. X=6m (ST) Test 3. X=6m (LT) -30 Figure 6. Test 3 Single pile.Chapter 6 Effects of Tunneling on Pile Groups 16 Pile head deflection (mm) 14 12 10 8 6 4 2 0 Single pile. Test 16 ST (2 days) 2-pile group. rear. 3 & 16) 243 . front.Test PG1 LT (720 days) Figure 6.11 Tunnelling-induced pile head deflection (Test PG1. 5 1 0.12 Single pile over pile group ratio for front pile (Test 3/ PG1) Single pile/ pile group ratio 3 2.5 0 Pile axial force Pile head Pile negative Pile postive settlement bending bending moment moment ST (2 days) Pile head deflection LT (720 days) Figure 6.5 1 0.Chapter 6 Effects of Tunneling on Pile Groups Single pile/ pile group ratio 3 2.13 Single pile over pile group ratio for rear pile (Test 16/ PG1) 244 .5 Positive effect of pile group 2 1.5 Positive effect of pile group 2 1.5 0 Pile axial force Pile head settlement ST (2 days) Pile negative Pile postive bending bending moment moment Pile head deflection LT (720 days) Figure 6. Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) 0 200 400 600 800 1000 1200 0 -5 -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG2. Front (LT) Test PG2. Rear (ST) Test PG2. Front (ST) Test PG2. Rear (LT) Figure 6.14 Tunnelling-induced pile axial force (Test PG2) 245 . Front (LT) Test 10. X=6m(ST) Test 10. X=6m (LT) Figure 6.Rear (ST) Test PG2. Front (ST) Test PG2. X=10m (LT) Figure 6. X=10m(ST) Test 11. Single.16 Tunnelling-induced rear pile (Test PG2) and corresponding single pile (Test 11) axial force 246 . Rear (LT) Test 11.15 Tunnelling-induced front pile (Test PG2) and corresponding single pile (Test 10) axial force Axial Force (kN) 0 200 400 600 800 1000 1200 0 -5 Distance of pile from tunnel centre = 10 m -10 Tunnel Depth (m) -15 -20 -25 -30 Test PG2.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) 0 200 400 600 800 1000 1200 0 Distance of pile from tunnel centre = 6 m -5 -10 Tunnel Depth (m) -15 -20 -25 -30 Test PG2. Single. Single. Single. Rear (LT) -30 Figure 6. Rear (ST) Test PG2. Front (ST) Test PG2.17 Tunnelling-induced pile bending moment (Test PG2) 247 . Front (LT) -25 Test PG2.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -80 -40 0 40 80 120 160 0 -5 Depth (m) -10 -15 T unnel -20 Test PG2. Front (LT) Test 10. Single. X=10m (ST) Test 11. Rear (LT) Test 11. Front (ST) -25 Test PG2. Single. X=6m (LT) -30 Figure 6. Front (ST) -25 Test PG2. Single.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -100 -50 0 50 100 150 200 250 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 -15 T unnel -20 Test PG2. X=6m (ST) Test 10.19 Tunnelling-induced rear pile (Test PG2) and corresponding single pile (Test 11) bending moment 248 . Single. X=10m (LT) -30 Figure 6.18 Tunnelling-induced front pile (Test PG2) and corresponding single pile (Test 10) bending moment Bending Moment (kNm) -100 -50 0 50 100 150 200 250 0 Distance of pile from tunnel centre = 10 m -5 Depth (m) -10 -15 T unnel -20 Test PG2. Rear (ST) Test PG2. Front (LT) -25 Test PG2. Front (ST) Test PG2.Chapter 6 Effects of Tunneling on Pile Groups Lateral deflection (mm) -2 -1 0 1 2 3 4 5 6 0 -5 Depth (m) -10 ` -15 T unnel -20 Test PG2.20 Tunnelling-induced pile deflection (Test PG2) 249 . Rear (LT) -30 Figure 6. 21 Tunnelling-induced front pile (Test PG2) and corresponding single pile (Test 10) deflection Lateral deflection (mm) -2 -1 0 1 2 3 4 5 6 0 -5 Depth (m) Distance of pile from tunnel centre = 10 m -10 `` -15 T unnel -20 Test PG2. Single. X=6m(ST) Test 10. Single. Single.Chapter 6 Effects of Tunneling on Pile Groups Lateral deflection (mm) -2 -1 0 1 2 3 4 5 6 0 -5 Distance of pile from tunnel centre = 6 m Depth (m) -10 ` T unnel -15 -20 Test PG2. X=10m(LT) -30 Figure 6. Rear (ST) Test PG2. Front (LT) -25 Test 10. X=10m(ST) Test 11. Front (ST) Test PG2.22 Tunnelling-induced rear pile (Test PG2) and corresponding single pile (Test 11) deflection 250 . Rear (LT) -25 Test 11. X=6m(LT) -30 Figure 6. Single. 10 & 11) 251 .Test PG2 6-pile group. Test 12 (X=14m) 2-pile group. Test 10 (X= 6m) Single pile.Test PG2 6-pile group. Test 12 (X=14m) LT (b) Long-term Figure 6. Test 11 (X=10m) Single pile.Test PG4 2-pile group.Test PG4 ST (a) Short-term Pile head deflection (mm) 6 5 4 3 2 1 0 Single pile.Chapter 6 Effects of Tunneling on Pile Groups Pile head deflection (mm) 6 5 4 3 2 1 0 Single pile. Test 10 (X= 6m) Single pile. Test 11 (X=10m) Single pile.23 Tunnelling-induced pile head deflection in the (a) short-term (b) long-term (Tests PG2. 5 0 Pile axial force ST Pile negative Pile positive bending moment bending moment Pile head deflection LT Figure 6.5 Positive effect of pile group 2 1.24 Single pile over pile group ratio for front pile (Test 10/ PG2) Single pile/ pile group ratio 3 2.5 1 0.5 0 Pile axial force ST Pile negative Pile positive bending moment bending moment Pile head deflection LT Figure 6.5 1 0.Chapter 6 Effects of Tunneling on Pile Groups Single pile/ pile group ratio 3 Positive effect of pile group 2.5 2 1.25 Single pile over pile group ratio for rear pile (Test 11/ PG2) 252 . Rear (ST) Test PG3. Rear (LT) Figure 6. Front (LT) Test PG3.26 Tunnelling-induced pile axial force (Test PG3) 253 . Front (ST) Test PG3.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 700 800 0 -5 Depth (m) -10 -15 T unnel -20 -25 -30 Test PG3. 27 Tunnelling-induced front pile (Test PG3) and corresponding single pile (Test 13) axial force 254 . X=6m (LT) Figure 6. Single.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 700 800 0 Distance of pile from tunnel centre = 6 m -5 -10 T unnel Depth (m) -15 -20 -25 -30 Test Test Test Test PG3. Front (LT) 13. Single. Front (ST) PG3. X=6m (ST) 13. Front (LT) Test PG3. Front (ST) -25 Test PG3. Rear (LT) -30 Figure 6.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -200 -150 -100 -50 0 50 100 150 200 0 -5 Depth (m) -10 -15 T unnel -20 Test PG3.28 Tunnelling-induced pile bending moment (Test PG3) 255 . Rear (ST) Test PG3. 29 Tunnelling-induced front pile (Test PG3) and corresponding single pile (Test 13) bending moment Bending Moment (kNm) -200 -150 -100 -50 0 50 100 150 200 0 -5 Distance of pile from tunnel centre = 10 m Depth (m) -10 -15 T unnel -20 Test PG3. X=6m (ST) Test 13. Single.30 Tunnelling-induced rear pile (Test PG3) and corresponding single pile (Test 14A) bending moment 256 . Front (ST) Test PG3. Front (LT) Test 13. X=6m (LT) -25 -30 Figure 6. X=10m (ST) Test 14. Single.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -200 -150 -100 -50 0 50 100 150 200 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 -15 T unnel -20 Test PG3. Rear (ST) Test PG3. Rear (LT) Test 14. X=10m (LT) -25 -30 Figure 6. Single. 2 0 Pile axial tensile force Pile axial compression force ST Pile negative Pile positive bending moment bending moment LT Figure 6.4 1.32 Single pile over pile group ratio for rear pile (Test 14A/ PG3) 257 .6 1.2 1 0.4 0.4 1.Chapter 6 Effects of Tunneling on Pile Groups 2 Positive effect of pile group 1.4 0.6 1.8 1.8 0.2 1 0.6 0.8 0.8 Single pile/pile group ratio 1.2 0 Pile negative bending moment ST Pile positive bending moment LT Figure 6.31 Single pile over pile group ratio for front pile (Test 13/ PG3) 2 Positive effect of pile group Single pile/pile group ratio 1.6 0. Rear (LT) Figure 6. Middle (ST) Test PG4. Rear (ST) Test PG4.33 Tunnelling-induced pile axial force (Test PG4) 258 . Middle (LT) Test PG4.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) 0 100 200 300 400 500 600 0 -5 Depth (m) -10 -15 T unnel -20 -25 -30 Test PG4. Front (ST) Test PG4. Front (LT) Test PG4. Rear (LT) Test P4.34 Tunnelling-induced front pile in 2-pile group (Test PG2) and corresponding front pile in 6-pile group (Test PG4) axial force Axial Force (kN) 0 100 200 300 400 500 600 700 800 0 -5 Distance of pile from tunnel centre = 10 m -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG2. Front (LT) Test PG4.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) 0 100 200 300 400 500 600 700 800 0 Distance of pile from tunnel centre = 6 m -5 -10 Tunnel Depth (m) -15 -20 -25 -30 Test PG2. Front (ST) Test PG4.35 Tunnelling-induced rear pile in 2-pile group (Test PG2) and corresponding middle pile in 6-pile group (Test PG4) axial force 259 . Front (LT) Figure 6. Middle (LT) Figure 6. Middle (ST) Test PG4.Rear (ST) Test PG2. Front (ST) Test PG2. Middle (ST) -25 Test PG4. Middle (LT) Test PG4. Front (ST) Test PG4. Front (LT) -25 Test PG4.36 Tunnelling-induced pile bending moment (a) in the short-term (b) in the long-term (Test PG4) 260 . Rear (ST) -30 (a) Short-term Bending Moment (kNm) -60 -40 -20 0 20 40 60 80 100 120 0 -5 Depth (m) -10 -15 T unnel -20 Test PG4.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -60 -40 -20 0 20 40 60 80 100 0 -5 Depth (m) -10 -15 T unnel -20 Test PG4. Rear (LT) -30 (b) Long-term Figure 6. Front (LT) Test PG2. Middle (ST) -25 Test PG4. Front (LT) -30 Figure 6. Middle (LT) Test PG2. Front (ST) Test PG2. Rear (LT) -30 Figure 6.37 Tunnelling-induced front pile in 2-pile group (Test PG2) and corresponding front pile in 6-pile group (Test PG4) bending moment Bending Moment (kNm) -100 -50 0 50 100 150 200 0 -5 Distance of pile from tunnel centre = 10 m Depth (m) -10 -15 T unnel -20 Test PG4. Rear (ST) Test PG2.38 Tunnelling-induced rear pile in 2-pile group (Test PG2) and corresponding middle pile in 6-pile group (Test PG4) bending moment 261 .Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -100 -50 0 50 100 150 200 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 -15 T unnel -20 Test PG4. Front (ST) -25 Test PG4. 5 3 0 -5 Depth (m) -10 ` -15 T unnel -20 Test PG4.5 -1 -0.5 2 2. rear (LT) -30 (b) Long-term Figure 6.5 0 0. rear (ST) -30 (a) Short-term Lateral deflection (mm) -1.5 1 1. Front (ST) -25 Test PG4.5 0 0. Middle (ST) Test PG4.5 -1 -0.5 2 2. Middle (LT) Test PG4.Chapter 6 Effects of Tunneling on Pile Groups Lateral deflection (mm) -1.39 Tunnelling-induced pile deflection in the (a) short-term (b) long-term (Test PG4) 262 .5 1 1. Front (LT) -25 Test PG4.5 3 0 -5 Depth (m) -10 ` -15 T unnel -20 Test PG4. Front (ST) -20 Test PG2. Middle (LT) Distance of pile from tunnel centre = 10 m -30 Figure 6. Middle (ST) -25 Test PG4. Front (ST) Test PG4.41 Tunnelling-induced pile bending moment (Tests PG2 and PG4) 263 . Rear (ST) -20 Test PG2. Front (LT) Test PG4. Front (LT) -25 Distance of pile from tunnel centre = 6 m -30 Figure 6.40 Tunnelling-induced pile deflection (Tests PG2 and PG4) Lateral deflection (mm) -2 -1 0 1 2 3 4 0 Depth (m) -5 -10 -15 ` Tunnel Test PG2. Rear (LT) Test PG4.Chapter 6 Effects of Tunneling on Pile Groups Lateral deflection (mm) -2 -1 0 1 2 3 4 0 -5 Depth (m) -10 ` Tunnel -15 Test PG2. 8 1.6 0.4 2.2 0 Pile axial force ST Pile negative bending Pile positive bending moment moment LT Figure 6.8 0.8 Positive effect of pile group 1.2 1 c 0.8 0.Chapter 6 Effects of Tunneling on Pile Groups 2-Pile Group/ 6-Pile Group ratio 2.4 1.4 0.2 2 1.4 0.6 0.42 2-pile over 6-pile group ratio for front pile (Test PG2/PG4) 2-Pile Group/ 6-Pile Group ratio 2.43 2-pile over 6-pile group ratio for middle pile (Test PG2/PG4) 264 .4 2.4 1.2 1 0.2 0 Pile axial force ST Pile negative bending moment Pile positive bending moment LT Figure 6.2 Positive effect of pile group 2 1.6 1.6 1. Front (ST) Test PG5. Front (LT) Test PG5.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 0 -5 -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG5.44 Tunnelling-induced pile axial force (Test PG5) 265 . Middle (LT) Test PG5. Rear (ST) Test PG5. Rear (LT) Figure 6. Middle (ST) Test PG5. Middle (LT) Test PG3. Middle (ST) Test PG5.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 0 Distance of pile from tunnel centre = 6 m -5 -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG5. Front (LT) Test PG3. Rear (LT) Figure 6. Front (ST) Test PG5.45 Tunnelling-induced front pile in 2-pile group (Test PG3) and corresponding front pile in 6-pile group (Test PG5) axial force Axial Force (kN) -200 -100 0 100 200 300 400 500 600 0 -5 Distance of pile from tunnel centre = 10 m -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG5.46 Tunnelling-induced rear pile in 2-pile group (Test PG3) and corresponding middle pile in 6-pile group (Test PG5) axial force 266 . Rear (ST) Test PG3. Front (LT) Figure 6. Front (ST) Test PG3. Rear (ST) -30 (a) Short-term Bending Moment (kNm) -80 -40 0 40 80 120 160 0 -5 Depth (m) -10 -15 T unnel -20 Test PG5.47 Tunnelling-induced pile bending moment in the (a) short-term (b) longterm (Test PG5) 267 . Rear (LT) -30 (b) Long-term Figure 6. Middle (LT) Test PG5. Middle (ST) -25 Test PG5. Front (LT) -25 Test PG5. Front (ST) Test PG5.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -80 -40 0 40 80 120 160 0 -5 Depth (m) -10 -15 T unnel -20 Test PG5. Middle (ST) -25 Test PG5.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -120 -80 -40 0 40 80 120 160 200 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 -15 T unnel -20 Test PG5. Middle (LT) Test PG3.49 Tunnelling-induced rear pile in 2-pile group (Test PG3) and corresponding middle pile in 6-pile group (Test PG5) bending moment 268 . Front (ST) Test PG3. Front (ST) Test PG5.48 Tunnelling-induced front pile in 2-pile group (Test PG3) and corresponding front pile in 6-pile group (Test PG5) bending moment Bending Moment (kNm) -120 -80 -40 0 40 80 120 160 200 0 -5 Distance of pile from tunnel centre = 10 m Depth (m) -10 -15 T unnel -20 Test PG5. Front (LT) -30 Figure 6. Front (LT) -25 Test PG3. Rear (LT) -30 Figure 6. Rear (ST) Test PG3. 2 0 Pile axial tensile force Pile axial compression force ST Pile negative Pile positive bending moment bending moment LT Figure 6.4 0.6 0.2 0 Pile axial tensile force Pile axial compression force ST Pile negative Pile positive bending moment bending moment LT Figure 6.4 1.6 1.2-Pile Group/ 6-Pile Group ratio Chapter 6 Effects of Tunneling on Pile Groups Positive effect of pile group 2 1.8 0.8 0.6 0.4 0.8 1.4 1.2 1 0.6 1.8 1.50 2-pile over 6-pile group ratio for front pile (Test PG3/PG5) 2-Pile Group/ 6-Pile Group ratio 2 Positive effect of pile group 1.51 2-pile over 6-pile group ratio for middle pile (Test PG3/PG5) 269 .2 1 0. Front (LT) Test PG2. Front (LT) Figure 6. Front (ST) Test PG2.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 700 800 0 -5 Distance of pile from tunnel centre = 6 m -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG3. Front (ST) Test PG3.52 Tunnelling-induced front pile in capped-head 2-pile group (Test PG2) and corresponding front pile in fixed-head 2-pile group (Test PG3) axial force 270 . 53 Tunnelling-induced rear pile in capped-head 2-pile group (Test PG2) and corresponding rear pile in fixed-head 2-pile group (Test PG3) axial force 271 .Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 700 800 0 -5 Distance of pile from tunnel centre = 10 m -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG3. Rear (ST) Test PG3. Rear (LT) Figure 6. Rear (ST) Test PG2. Rear (LT) Test PG2. 55 Tunnelling-induced rear pile in capped-head 2-pile group (Test PG2) and corresponding rear pile in fixed-head 2-pile group (Test PG3) bending moment 272 . Rear (ST) -25 Test PG3. Front (LT) -30 Figure 6.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -150 -100 -50 0 50 100 150 200 0 Distance of pile from tunnel centre = 6 m -5 Depth (m) -10 T unnel -15 -20 Test PG3. Front (LT) -25 Test PG2. Rear (ST) Test PG2. Front (ST) Test PG2. Front (ST) Test PG3. Rear (LT) -30 Figure 6. Rear (LT) Test PG2.54 Tunnelling-induced front pile in capped-head 2-pile group (Test PG2) and corresponding front pile in fixed-head 2-pile group (Test PG3)bending moment Bending Moment (kNm) -150 -100 -50 0 50 100 150 200 0 -5 Distance of pile from tunnel centre = 10 m Depth (m) -10 -15 T unnel -20 Test PG3. 6 1.8 0.6 0.2 0 Pile compression force ST Pile negative bending moment Pile positive bending moment LT Figure 6.6 1. Test PG2/PG3) Positive effect of fixed-head pile 2 1.Capped-head pile/ Fixed-head pile Chapter 6 Effects of Tunneling on Pile Groups 2 Positive effect of fixed-head pile 1.4 1.4 0.6 0.8 1.2 1 0.57 Capped-head pile over fixed-head pile ratio (rear pile. Test PG2/PG3) 273 .56 Capped-head pile over fixed-head pile ratio (front pile.2 0 Pile compression force ST Pile negative bending Pile positive bending moment moment LT Capped-head pile/ Fixed-head pile Figure 6.4 1.8 1.2 1 0.8 0.4 0. Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 0 -5 Distance of pile from tunnel centre = 6 m -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG4. Front (LT) Test PG5. Front (ST) Test PG4. Front (ST) Test PG5.58 Tunnelling-induced front pile in capped-head 6-pile group (Test PG4) and corresponding front pile in fixed-head 6-pile group (Test PG5) axial force 274 . Front (LT) Figure 6. Rear (LT) Test PG5.59 Tunnelling-induced middle pile in capped-head 6-pile group (Test PG4) and corresponding middle pile in fixed-head 6-pile group (Test PG5) axial force Axial Force (kN) -200 -100 0 100 200 300 400 500 600 0 -5 Distance of pile from tunnel centre = 14 m -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG4. Middle (LT) Figure 6.Chapter 6 Effects of Tunneling on Pile Groups Axial Force (kN) -200 -100 0 100 200 300 400 500 600 0 Distance of pile from tunnel centre = 10 m -5 -10 Depth (m) -15 Tunnel -20 -25 -30 Test PG4. Rear (ST) Test PG4.60 Tunnelling-induced rear pile in capped-head 6-pile group (Test PG4) and corresponding rear pile in fixed-head 6-pile group (Test PG5) axial force 275 . Middle (LT) Test PG5. Middle (ST) Test PG5. Rear (LT) Figure 6. Rear (ST) Test PG5. Middle (ST) Test PG4. Front (ST) Test PG5.61 Tunnelling-induced front pile in capped-head 6-pile group (Test PG4) and corresponding front pile in fixed-head 6-pile group (Test PG5) bending moment 276 . Front (LT) Test PG5. Front (ST) -25 Test PG4.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -80 -40 0 40 80 120 160 0 -5 Distance of pile from tunnel centre = 6 m Depth (m) -10 -15 T unnel -20 Test PG4. Front (LT) -30 Figure 6. Middle (ST) Test PG5. Rear (ST) Test PG5. Middle (LT) -25 Test PG5.Chapter 6 Effects of Tunneling on Pile Groups Bending Moment (kNm) -80 -40 0 40 80 120 160 0 Distance of pile from tunnel centre = 10 m -5 Depth (m) -10 -15 T unnel -20 Test PG4. Rear (LT) -30 Figure 6.63 Tunnelling-induced rear pile in capped-head 6-pile group (Test PG4) and corresponding rear pile in fixed-head 6-pile group (Test PG5) bending moment 277 . Rear (LT) Test PG5. Rear (ST) -25 Test PG4. Middle (LT) -30 Figure 6. Middle (ST) Test PG4.62 Tunnelling-induced middle pile in capped-head 6-pile group (Test PG4) and corresponding middle pile in fixed-head 6-pile group (Test PG5) bending moment Bending Moment (kNm) -80 -40 0 40 80 120 160 0 -5 Distance of pile from tunnel centre = 14 m Depth (m) -10 -15 T unnel -20 Test PG4. 65 Variation of maximum bending moment for front. LT Fixed-head (PG5). middle and rear pile in the long-term (Tests PG4 and PG5) 278 . LT Capped-head (PG4).Chapter 6 Effects of Tunneling on Pile Groups 40 30 20 10 0 Front Middle Rear -10 -20 -30 -40 Capped-head (PG4).64 Variation of maximum bending moment for front. ST Fixed-head (PG5). ST Capped-head (PG4). LT Figure 6. ST Figure 6. LT Fixed-head (PG5). ST Fixed-head (PG5). middle and rear pile in the short-term (Tests PG4 and PG5) 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 Front Middle Rear Capped-head (PG4). 67 Capped-head pile over fixed-head pile ratio (middle pile.5 2 Positive effect of fixed-head 1.5 1 0.5 0 Pile axial tensile force Pile axial compression force ST Pile negative Pile positive bending moment bending moment LT Capped-head pile/ Fixed-head pile ratio Figure 6.5 Positive effect of fixed-head 2 1.5 0 Pile axial tensile force Pile axial compression force Pile negative Pile positive bending moment bending moment ST LT Capped-head pile/ Fixed-head pile rati Figure 6.5 2 Positive effect of fixed-head 1.Chapter 6 Effects of Tunneling on Pile Groups Capped-head pile/ Fixed-head pile ratio 3 2.5 1 0.66 Capped-head pile over fixed-head pile ratio (front pile.5 0 Pile axial tensile force ST Pile axial compression force Pile negative Pile positive bending moment bending moment LT Figure 6. Test PG4/PG5) 3 2. Test PG4/PG5) 279 . Test PG4/PG5) 3 2.68 Capped-head pile over fixed-head pile ratio (rear pile.5 1 0. rear PG5. rear PG4. middle PG4. 3. rear PG3. rear PG2. front PG4. rear Figure 6. front PG5. 4 & 5) 280 .5 1 0. front PG2. middle PG5.5 LT/ST ratio 2 Long-term effect 1.69 Long-term over short-term ratio (Tests PG1.5 0 Pile axial tensile force Pile axial compression force Pile negative Pile positive bending moment bending moment Pile head deflection PG1.Chapter 6 Effects of Tunneling on Pile Groups 2. 2. front PG1. front PG3. Tests 3 to 16 (tunnel-single pile interaction) and Tests PG1 to PG5 (tunnel-pile group interaction) . In this study. The objectives were achieved through (a) the development of a centrifuge tunnel excavation technique to simulate the inward tunnel deformation pattern commonly observed in practice (b) a centrifuge model study of tunnellinginduced soil movements in free-field analysed using Particle Image Velocimetry (PIV) technique (c) a centrifuge model study of tunnelling-induced single pile responses. pile-to-tunnel distance.were performed. The effects of factors such as volume loss. a series of centrifuge model tests were conducted to investigate the effects of tunnelling on soft clay. floating and end-bearing pile groups. single piles and pile groups in clay.Chapter 7 Conclusions CHAPTER SEVEN CONCLUSIONS 7. and (d) a centrifuge model study of tunnelling-induced pile group responses. size of pile group and pile groups with capped-head and fixed-head conditions of pile groups on pile due to tunneling were examined.Tests 1 and 2 (tunnel-soil interaction).1 CONCLUDING REMARKS The overall purpose of this research study was to investigate tunnel-soil-pile interaction in soft clay. pile tip and head condition. 281 . the observed pile behaviours are evaluated against the measured free-field soil movements due to tunnelling. In addition. A total of twenty one centrifuge model tests . pile length. However. Such simulation technique provides flexibility and reproducibility to study the tunnel-soil-pile interaction in a centrifuge model that could not be measured in field tests. 1998). The experimental results from tunnel-soil interaction presented in Chapter 4 provided clear evidence that the present model tunnel was able to simulate the precise volume loss during the tunnelling process.Chapter 7 Conclusions 7. this is usually referred to as a two-dimensional simulation (Taylor.1. in a situation where the tunnel excavation has passed a particular section. the vectors of the ground movement developed will be more or less in the plane perpendicular to the tunnel axis. Consequently it is reasonable to assume that a plane strain model of long tunnel section would be a reasonable representation of tunnelling-induced soil movements. particularly the time effects of soft clay. Moreover.1 Technique for Simulation of Tunnelling In the present centrifuge model study. Moreover. (1992) is used to quantify the amount of tunnel over-cut. Nevertheless. An oval-shape ground deformation pattern is imposed as the boundary condition and the gap parameter (GAP) proposed by Lee et al. This ensured that the volume loss was constant along the model tunnel. this model could provide a very uniform oval-shape of the GAP throughout the entire length of the model tunnel. an innovative tunnelling simulation technique was developed to simulate the inward tunnel deformation due to over-excavation commonly observed in practice. the results from Taylor (1998) and the present study showed that the two-dimensional model tunnel was able to measure the transverse responses of soils and piles. 282 . the simplified two-dimensional model tunnel was unable to simulate the complex three-dimensional effects of tunnelling before and after the passing of the tunnel boring machine (TBM). an “Immediate Shear Zone” with large soil movement above the tunnel can be identified. The magnitude of maximum ground surface settlement increases with time and tunnel volume loss. 283 . while the zone outside the immediate shear zone may be identified as the ‘Support Zone’. Empirical equations in the short-term and long-term were proposed for the distribution of inflection point in soft clay.1. In addition. The surface settlement trough in clay generally follows the Gaussian distribution curve in the short term. soil settlement was noted to be more dominant than lateral soil movement in the long term. 7. In the long term. The observed data confirmed that the empirical equation proposed by Mair et al (1993) is applicable in the prediction of the subsurface settlement troughs in clay in the shortterm.3 Tunnel-Single Piles Interaction During tunnelling. the ground around the tunnel often moves towards the tunnel opening. the significant soil movement zone extends much wider.2 Tunnel-Soil Interaction The centrifuge model tests with the application of PIV had provided a considerable body of data to examine the patterns of soil movement induced by tunnelling in soft clay.Chapter 7 Conclusions 7. The resulting ground movements induce additional axial (settlement and axial force) and lateral (deflection and bending moment) responses on adjacent pile foundations.1. The settlement magnitude is larger in the long-term and the settlement trough is wider as compared to that in the short-term. In the short-term. namely “floating” pile. For Test Series 2. In this respect.5%. Generally. However. these responses had led to the reduction in drag load and positive bending moment at the pile waist.Chapter 7 Conclusions Test Series 1 studied the effects of volume loss on pile performances. It was found that the induced pile bending moment triples and the pile settlement and deflection increase by almost 2.5 times when volume loss increases from 3% to 6. it was observed that the pile 284 . some opposite trends were observed in the fixed-head when compared to free-head. Nevertheless. a longer pile with pile length in the stabilised support zone tends to provide more resistance to the pile movements but will attract more bending moment and axial force. It is noted that tensile force and relatively large negative bending moments were induced at the pile head due to total fixity. Test Series 4. the pile structural responses (axial force and bending moment) were of less significant if compared to those of long pile. It is noted that a floating pile is mainly governed by pile settlement when tunnelling is carried out adjacent to it whereas socketed piles are likely to be governed by the material stress of the pile. three different pile tip conditions. 5 and 6 examined the effects of pile-to-tunnel distance for different pile head and tip conditions. On the other hand. especially those located in the immediate shear zone. there will be excessive pile movements (settlement and deflection) because of lack of anchorage of pile into the stable support zone. with pile base at or above the tunnel crown. in this particular case. “socketed” pile and “end-bearing” pile were investigated to study the effects of pile tip condition. In a short pile. 285 . Generally. The induced pile axial forces were observed to decrease fairly linearly with increase in pile-to-tunnel distance for Test Series 4 (floating piles) and 5 (end-bearing pile) and the axial force in Test Series 5 is always much higher than that of Test Series 4 for all pile to tunnel distance. In addition. it was observed that the pile deflection dropped rapidly from 1D to 1.5D to 2D for both Series 4 and 5. the most significant difference in Test Series 5 (free-head) and Test Series 6 (fixed-head) is that tension force is induced in the fixed-head pile due to total fixed condition at the pile head. As the bending moment profile was offset toward the negative bending moment for fixedhead pile. However. the bending moments decrease exponentially when the magnitude is relatively large. Generally. with a much smaller decrease from 1. the positive bending moment in fixed-head pile (Test Series 6) are consistently lower than that of free-head pile (Test Series 5).5D. the maximum induced bending moments decrease fairly linearly with increasing pile-to-tunnel distance when the magnitude is relatively small as in the case of pile bending moment in the short-term and positive bending moment observed in Series 6. It can be established from the test results that induced bending moments are generally small beyond a horizontal offset of 2D from the tunnel centre.Chapter 7 Conclusions responses decrease with increase in pile-to-tunnel distance. the induced bending moments in end-bearing piles (Test Series 5) are larger than the floating piles (Test Series 4) as the restraint at the pile toe would restrict the pile lateral movement and induce a larger bending moment. the results revealed that negative bending moments were induced at pile head due to total fixity condition for fixed-head piles in Test Series 6 as compared to the free-head piles in Test Series 5. The pile head deflection for endbearing piles (Test Series 5) was smaller that of floating piles (Test Series 4). On the other hand. Generally. This is because the lateral soil movements decrease with increasing distance of pile location to the tunnel. The most distinct difference in the soil behaviour was that the soil moved significantly away from the tunnel in the case of outward tunnel deformation. respectively. 7. whereas in the case of inward tunnel deformation. the pile group was generally beneficial as the average pile group responses (bending moments. axial. This is because the rigidity of a pile group provides more resistance to the tunnelling-induced soil movements.5 m for both tunnel deformation cases.1. On the other hand. 286 . 2004) tunnel deformations. It was revealed that the pile axial force and pile settlement behaviour and profile were fairly similar regardless of the deformation pattern but the outward tunnel formation would induce larger pile responses as compared to the inward tunnel deformation under the same volume loss. Some similarities and differences were drawn in the comparisons of soil and single pile behaviour in the cases of both inward (present study) and outward (Ran. as the lower portion of the pile was restrained and would not moves. the soil moved towards the tunnel. settlement and lateral deflection) are smaller than the average of those of single piles at the same locations.4 Tunnel-Pile Groups Interaction In the case of a capped-head floating pile group (Test PG1).Chapter 7 Conclusions regardless of the pile-tunnel distance. It is noted that the measured short-term surface settlement trough follows the Gaussian distribution curve fairly well with the inflection point (i) at approximately 7. the pile lateral responses (bending moment and deflection) were opposite in direction for both inward and outward tunnel deformations. in terms of profiles and magnitude. except that the magnitude was affected by the total number of piles because the behaviors was largely governed by the pile-soil-pile interaction. 287 . the fixed-head piles (Test PG3 (2-pile group) and Test PG5 (6-pile group)). as the head conditions played a vital role in dictating the pile responses. It is worth noting that the axial forces reduce when the position of the pile-tunnel increases. This suggests that part of the bending moments of the pile in the middle row was transferred to the rear piles due to the interaction through the pile cap. in which the corresponding lateral soil movement at the middle row of piles was larger than the corresponding lateral soil movement at the rear row of pile. Generally. behaved like single piles standing side by side without direct pile-cap-pile interaction. capped-head piles (Test PG2 (2-pile group) and Test PG4 (6-pile group)) demonstrate significant pile-cap-pile interaction among the piles. Thus the pile-cap-pile interaction in capped-head 6-pile group (Test PG4) has less influence on the axial force as compared to bending moment and the induced pile axial forces were mainly influenced by the soil settlement and distance effects. the position of the pile within a group demonstrated a totally different transfer mechanism in lateral pile responses. as compared to the single pile where the responses reduced consistently when the distance of pile-tunnel increases. When the pile group size increased from 2-pile to 6-pile. As a result. The pile-cap-pile interaction in capped-head 6-pile group (Test PG4) would moderate the induced pile bending moments among the piles within a pile group. On the other hand. the induced pile bending moments in the middle row was smaller than that of rear row.Chapter 7 Conclusions The scenario becomes more complicated when different pile cap conditions were modeled. This is contrary to the induced lateral soil movements. there was an increment in the pile bending moment due to the totally fixed-head condition.2 RECOMMENDATIONS FOR FUTURE STUDIES The findings from the centrifuge experiments in the present study provided the basis for the understanding of tunnel-soil-pile interaction. with recent advancement in tunnelling technology. the smallest volume loss that was modeled in the centrifuge test was 3%. 7.Chapter 7 Conclusions On the other hand. an improvement to the current model tunnel to a smaller volume loss is recommended. the piles in fixed-head 6-pile group (Test PG5) behaved like single piles in term of axial force and bending moment. To achieve this. It is also recommended that mechanical model tunnel with several small segments be developed to simulate three-dimensional tunnel excavation. In contrast. • Future work is needed to study three-dimensional tunnel excavation in order to study the longitudinal effects of tunnelling. except that the magnitude is affected by the total number of piles. the volume loss can be controlled to less than 1%. 288 . However. Hence. modification of the present two-dimensional model tunnel is needed. Some possible areas that could be explored further are discussed here: • In the present study. which restrained the pile movement that resulted in high bending moment. • The effects of soil strength on tunnelling-induced soil movement and pile responses could be further explored. and it would be interesting to study the responses of stiffer clays. 289 .Chapter 7 Conclusions • It is proposed that numerical analysis could be performed to validate the centrifuge experimental results and parametric studies could be carried out to improve the understanding of various effects of tunnelling. normally consolidated clay used was relatively soft. 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