An embedded improved soil berm in an excavation mechanisms and capacity

AN EMBEDDED IMPROVED SOIL BERM IN AN EXCAVATION – MECHANISMS AND CAPACITY ZHANG YAODONG (B.Eng, HAUT; M.Eng, ZJU) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 ACKNOWLEDGEMENTS I should like to express my deepest gratitude to my supervisors, Associate Professor Tan Thiam Soon and Associate Professor Leung Chun Fai for their constant guidance and advice throughout my study at National University of Singapore (NUS). I am particularly grateful to Prof. Tan Thiam Soon for his support and profound contribution to this thesis, and for the valuable time he spent with me discussing important issues in both academic and non-academic fields. I am grateful to Mr. Shen Ruifu, Dr. Robison and Dr. Thanadol K. for their valuable guidance and assistance during the centrifuge apparatus preparation and tests. I would also like to thank all the technical staff of the Geotechnical Engineering Laboratory and the Centrifuge Laboratory for their assistance, sharing their experience, especially to Mr. Wong Chew Yuen, Mr. Tan Lye Heng, Mr. Loo Leong Huat, Mr. Foo Hee Ann and Mdm. Puan Jamilah. I should like to thank all of my friends at the Centre for Soft Ground Engineering for their patience and support at various times over the last three and half years. I am deeply indebted to my parents and others of the family for their constant support and encouragement. Particularly to my wife who makes many sacrifices to ensure that my life is comfortable and that I can concentrate on my work. Finally, I deeply appreciate the financial assistance in the form of research scholarship and facilities provided by the National University of Singapore (NUS). ii TABLE OF CONTENTS TITLE PAGE i ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iii SUMMARY vii NOMENCLATURE ix LIST OF TABLES xi LIST OF FIGURES xii Chapter1 Introduction 1.1 Background on deep excavation 1.2 Soil improvement in deep excavation .1 1.3 Scope and objectives of study .3 1.4 Layout Chapter2 Literature Review 2.1 Introduction .8 2.2 Conventional excavation support system and its limitations 2.3 Soil stabilisation in deep excavation .10 2.4 Review on soil stabilisation in deep excavations 11 2.4.1 Field studies 12 2.4.2 Numerical studies 16 2.4.3 Experimental studies .20 2.5 Centrifuge model testing .24 2.6 Summary .27 Chapter Centrifuge Modelling 44 3.1 Introduction .44 3.2 The NUS Geotechnical Centrifuge 45 iii 3.3 Sample preparation 45 3.3.1 Soil sample preparation .45 3.3.2 Stress history of model ground .47 3.3.3 Soil-cement mixing preparation 48 3.4 Experimental setup 49 3.4.1 Model retaining wall .49 3.4.2 In-flight excavator system .50 3.4.3 Instrumentation .50 3.4.4 Data acquisition system 51 3.5 Experimental Procedure 51 3.5.1 Excavation model preparation 51 3.5.2 Excavation procedure 53 3.6 Image capturing and processing systems 54 3.6.1 Image capturing system 54 3.6.2 Image processing system 55 Chapter Application of Particle Image Velocimetry 66 4.1 Introduction .66 4.2 Principles of the Particle Image Velocimetry technique .69 4.2.1 Basic information for PIV technique 69 4.2.2 Improved approaches for DPIV image processing .71 4.3 Preliminary test – importance of texture .78 4.4 Experiments and Results .79 4.4.1 Calculation of calibration factor .80 4.4.2 Stationary errors at 1g and 100g .81 4.4.3 Calibration of movement 82 4.4.4 Calibration of a 1mm movement under 1g condition .83 4.4.5 Calibration of a 1mm movement under 100g condition .84 4.4.6 Analysis of cumulative differences .86 4.4.7 Analysis of different particle sizes and densities 87 4.5 Summary .88 iv Chapter Undrained Bearing Capacity of an Embedded Improved Soil Berm in an Excavation 107 5.1 Introduction .107 5.2 Undrained end bearing capacity of an improved berm .108 5.3 A proposed upper bound solution .109 5.4 Finite Element method (FEM) 115 5.4.1 Verification problem .117 5.4.2 Computation of N c of an embeded improved soil berm 117 5.4.3 Computation of N q of an embedded improved soil berm 119 5.4.4 Independence of N c and N q 120 5.5 Modified upper bound solution .121 5.5.1 Process and results of modification 121 5.5.2 Implications of the modified upper bound solution 124 5.6 Undrained shear resistance of an embedded improved soil berm .127 5.7 Summary .127 Chapter Behaviour of an Excavation Stabilised with Embedded Improved Soil Berm – Centrifuge Modelling 144 6.1 Introduction .144 6.2 General behaviour of an excavation stabilised by an embedded improved soil 146 6.2.1 Displacement pattern of subsoil for excavation without improved soil berm 147 6.2.2 Displacement pattern of subsoil for excavation with improved soil berm 149 6.2.3 Comparison of lateral wall movement and surface settlement .153 6.2.4 Comparison of normalised surface settlement 155 6.2.5 Performance of composite ground resistance on the passive side 158 6.2.6 Typical profiles of movements at boundaries .161 6.3 Effect of thickness of improved soil berm 163 6.4 Effect of embedment depth of the improved soil berm .164 6.5 Effect of orientation of improved soil berm 167 v 6.6 Summary of findings .170 Chapter Behaviour of an Excavation Stabilised with Embedded Improved Soil Berm – Numerical Modelling 200 7.1 Introduction .200 7.2 Finite Element Method (FEM) 201 7.2.1 CRItical State Program (CRISP) 201 7.2.2 Selection of input parameters 200 7.2.3 In situ stress states .203 7.3 Comparison of Results of FEM and Centrifuge Tests 204 7.3.1 Generated mesh, boundary conditions and construction sequence .205 7.3.2 Parameters of slip element 206 7.3.3 Comparison and discussion .207 7.3.4 Summary .212 7.4 Parametric studies 212 7.4.1 Effect of berm stiffness .212 7.4.2 Effects of berm length and width of excavation .214 7.4.3 Effect of berm thickness .219 7.4.4 Effect of embedment depth .222 7.4.5 Effect of berm orientation .226 7.5 Resistance mechanism of an excavation with an embedded improved soil berm 229 7.6 Summary of findings 233 Chapter Conclusions 276 8.1 Concluding Remarks .276 8.2 Recommendations for future studies .282 References .284 vi SUMMARY In deep excavations in soft ground, the maximum wall deflection usually occurs below the excavation level where it is not practical to install conventional steel struts. One effective solution is to improve a layer of soft soil below the formation level prior to excavation. In a wide excavation site, the use of an embedded improved soil berm provides a more economical solution to control the wall deformation. In this research, analytical, centrifuge, and numerical studies are carried out to improve the understanding of an excavation stabilised with an embedded improved soil berm. Firstly, a relatively new imaging processing technique, the Particle Image Velocimetry (PIV) was applied to centrifuge testing to measure the ground movements more accurately. Then an upper bound solution, modified with the aid of results from numerical analyses was developed to estimate the undrained end bearing capacity of an embedded improved soil berm in an excavation. This is followed by a series of centrifuge tests with different soil improvement configurations to study the mechanisms involved. Three geometry parameters are considered; namely thickness, embedment depth and orientation of the improved soil berm. Finally numerical parametric studies using the CRISP computer program are conducted to complement the results of centrifuge tests after the numerical analyses are calibrated based on the centrifuge experimental results. The centrifuge results show that the presence of an improved soil berm would influence significantly the displacement pattern of surrounding soil as a result of interaction between the berm and surrounding ground. The embedded improved soil berm displaces as a rigid body and moves horizontally and vertically as well as rotates progressively. The incremental displacement contours of the ground and improved vii soil berm obtained from PIV technique show that a composite resistance comprising passive soil resistance, end bearing resistance and interfacial shear resistance of the berm is mobilised to resist the wall movement during the excavation process. The interfacial shear resistance is usually fully mobilised earlier than the end bearing resistance during excavation process. The composite stiffness on the passive side relies on both the berm-soil shear stiffness and soil stiffness. Further numerical analyses show that the resistance mechanism of an excavation with improved soil berm is applicable to both cantilever and strutted excavations. Increase in the berm thickness is effective to reduce the wall movement by providing a larger end bearing resistance. If the treated soil volume keeps constant, an increase in berm length is more effective than an increase in berm thickness. The centrifuge tests showed that placing the berm at a higher level was more effective to reduce the cantilever type wall deflection due to the provision of a larger resisting moment as a result of a larger arm of force and berm movement. The numerical analyses show that it is important to select the embedment depth properly according to the types of wall deflections that require control. The progressive rotational movement of the berm as observed from centrifuge tests makes the berm unstable and its effectiveness to control the wall movement becomes less and less with increasing excavation depth. It is shown that provision of a downward slant coupled with mobilised horizontal berm movement help to control the rotational movement of the berm especially at later stages of excavation and consequently reduce the wall movement. Keywords: Excavation, soft soil, improved soil berm, upper bound, centrifuge, Particle Image Velocimetry viii NOMENCLATURE A Section area Ab Cross sectional area of the improved soil berm As Contact area at the top or bottom of the improved soil berm B Width of a foundation C Embedment depth of improved soil berm D Thickness of improved soil berm E Young’s modulus E External work F Collapse load K0 Earth pressure coefficient at rest Knc Value of K0 for normally consolidated soil K’ Effective bulk modulus of soil L Length of improved soil berm M Slope of critical state line in q-p’ space NC Normally consolidated clay Nc Bearing capacity factors related to soil strength Nq, Nγ Bearing capacity factors OC Over consolidated clay Qb End bearing load Qs Shaft resistance load Qu Ultimate load capacity V Velocity W Internal work X0 Position of the correlation window cu Undrained shear strength d 2-dimensional displacement vector in PIV analysis e Void ratio ecs Void ratio for critical state line at p’= kPa g Earth gravity kx Coefficient of horizontal permeability ix ky Coefficient of vertical permeability m Embedment ratio = C/D p’ Mean stress p’c Equivalent pre-consolidation pressure q Deviator stress qu Unconfined compressive strength u True displacement um Measured displacement α, β Angle variables γ Unit weight δ Slant angle of improved soil berm ε Error in PIV analysis η Stress ratio = q/p’ κ Slope of swelling line λ Slope of normal compression line λ Mobilisation factor ν Poisson’s ratio σh Total horizontal stress σv Total vertical stress σv’ Effective vertical stress σ’vmax Maximum effective vertical stress φ’ Effective friction angle of soil x Chapter Conclusions Chapter Conclusions The purpose of soil stabilisation in an excavation is to control the wall deflection and associated ground movement. In soft ground, the maximum wall deflection usually occurs below the excavation level where it is not practical to install conventional steel struts. In a wide excavation site, the use of an embedded improved soil berm provides a more economical solution to control the wall deformation. The research presented in preceding chapters is aimed at providing a better understanding of the behaviour of an excavation stabilised by embedded improved soil berm with different configurations in soft ground. In this study, a relatively new imaging processing technique, the Particle Image Velocimetry was applied to the centrifuge testing to measure the ground movements more accurately. Then an upper bound solution, modified with the aid of results from numerical analyses was developed to estimate the undrained end bearing capacity of an embedded improved soil berm during the process of excavation. Finally, important parameters concerning geometries and properties of the berm were examined in detail using both centrifuge and numerical modelling. Parameters considered were length L , stiffness E , thickness D , embedment depth C and orientation δ of the embedded improved soil berm on the passive side of an excavation. 8.1 Concluding Remarks The behaviour of an embedded improved soil berm in an excavation was studied by means of analytical methods, physical and numerical modelling. 276 Chapter Conclusions Excavation tests with different soil improvement configurations were carried out in the centrifuge and the displacements of the ground and improved soil mass was tracked by the technique Particle Image Velocimetry. Then numerical analyses were conducted to complement the results from the centrifuge tests. Based on the research work done, the following conclusions can be drawn. a) The Particle Image Velocimetry (PIV) method is a very accurate and stable method to measure the ground movement. PIV technique is a texture-based image processing method, which allows displacements of small patches to be measured to sub-pixel accuracy through statistical analysis with a certain algorithm. Even a small displacement can be accurately picked up by the PIV method which is not as easy when the traditional technique of tracking the centroid of a marker is used. In the present study, the Particle Image Velocimetry (PIV) technique has been used successfully to measure the movements of the ground and improved soil berm during the excavation process in centrifuge modelling. b) In the PIV technique, the movement between two images is obtained through the cross correlation method. Sufficient and unique textures in images are of vital importance to perform reliable and accurate analysis of cross correlation. Sand may contain sufficient inherent texture for the PIV technique to analyse the soil displacement without recourse to intrusive target markers (White et al., 2001a). However, for clay such as kaolin, there is not enough natural texture for the application of this PIV technique. The present study has shown that the use of very small particles or points marked with fine paint markers is equally effective to produce a texture for the application of the PIV technique. c) A series of results has been presented to quantify the stationary errors for centrifuge tests, and the differences between the PIV and other direct 277 Chapter Conclusions measurement technique such as dial gauges and LVDT. The results show that the PIV technique is highly reliable and also produces very stable predictions. Under 1g conditions, the difference is about 0.008 mm when comparing PIV measured values with two direct measuring techniques, while under 100g conditions, the difference is not more than 0.015 mm, a higher error mainly due to system errors such as the vibration of the camera and distortion in the container. d) The centrifuge study showed that the displacement patterns obtained from the PIV technique for an excavation with an improved soil berm are remarkably different from those for an excavation without soil improvement. The presence of an improved soil berm would influence significantly the displacement pattern of surrounding soil as a result of interaction between the berm and surrounding ground. The embedded improved soil berm displaces as a rigid body and moves horizontally and vertically as well as rotates progressively when the stiffness of the berm is beyond a threshold value. Subsequent numerical analyses showed that the threshold value is around 200 MPa. e) An upper bound failure mechanism for the improved soil berm was proposed based directly on observations from centrifuge tests. This solution is similar to the one proposed by Davis et al. (1980) for stability analysis of a plane strain tunnel. As the derived proposed upper bound solution would overestimate the end bearing capacity provided by the improved soil berm, a modified upper bound solution which combines the results of the upper bound solution and FEM analysis was then developed to improve the estimation of the undrained end bearing capacity of an improved soil berm. The modified solution shows that the undrained end bearing capacity of the berm ( qb = N c cu + γD( C + ) ) comprises D two parts; one is due to undrained shear strength cu and the other due to soil 278 Chapter Conclusions weight. From this solution, it is clear that the undrained end bearing capacity is not a constant but decreases during the excavation process. Furthermore, from the results of the end bearing capacity factor N c , it is shown that the existence of an improved soil berm will provide an additional pressure relative to the passive pressure to control the wall displacement. f) In the centrifuge tests, the incremental displacement contours of the ground and improved soil berm show that a composite resistance comprising passive soil resistance, end bearing resistance and interfacial shear resistance of the berm is mobilised to resist the wall movement during the excavation process. The interfacial shear resistance is usually fully mobilised earlier than the end bearing resistance during excavation process. The composite stiffness on the passive side relies on both the berm-soil shear stiffness and soil stiffness. At initial stages of excavation, composite stiffness of the ground on the passive side depends on both the stiffness of soil and the berm-soil interface stiffness and the berm is then very effective in controlling the wall movement. At later stages of excavation, the interfacial shear resistance has been fully mobilised and composite stiffness of the ground on the passive side relies largely on the soil stiffness. g) In the case of an improved soil raft, the fact that the incremental wall movement becomes less with increase in excavation depth showed that the resistance mechanism was totally different from that of an improved soil berm. The stiffness of the improved soil raft dominates the composite stiffness on the passive side and the improved soil raft behaved like a strut below the excavation level. The excavation process would have little effect on the stiffness of the improved soil raft. Consequently, the composite stiffness on the passive side does not change too much. 279 Chapter Conclusions h) Initial numerical analyses showed that there were two possible kinds of forces acting between the berm and the wall to control the wall movement. One is a contact load acting against the wall which comes from the mobilisation of the interfacial shear resistance and end bearing resistance. The other type is a bending moment, which occurs if a fixed connection exists between the berm and the wall. However, in centrifuge tests, it was clearly observed that separation and slippage occurred between the wall and the berm. In the numerical analyses, it was shown that only when slip elements were introduced, could the separation and slippage be captured. This separation and slippage, when allow to occur, almost eliminated the effect of the bending moment due to an artificially imposed fixed connection. This is an important point to note if finite element analysis is used in design. i) The thickness of the berm has an opposing effect on the end bearing resistance and passive soil resistance. The results from the centrifuge tests showed that a thicker berm would provide a higher end bearing resistance, but on the other hand, reduces the passive soil resistance. At later stages of excavation, the end bearing resistance became more obvious to show the improvement effect after the interfacial shear resistance was fully mobilised. Numerical analyses showed that with the current configuration in the centrifuge, increasing the berm thickness was effective in reducing wall movement especially when it is smaller than m. j) The embedment depth C of the berm would affect the horizontal movement, moment arm and the maximum available resistance of the berm in resisting wall movement. The centrifuge tests showed that placing the berm at a higher level for a cantilever excavation provided a larger resisting moment in controlling the wall rotational movement as a result of a larger moment arm and berm movement during excavation and therefore, was more effective in reducing the cantilever 280 Chapter Conclusions type wall deflection. The present study showed that it is important to select the embedment depth properly according to the types of wall deflections that required control. To control a wall kick-out, the improved soil berm could be installed at the bottom of the wall. To reduce the bulging deformation, berm could be favourably constructed at the position where the maximum bulging deformation occurs. These observations also suggest the combined use of two berms if different kinds of deflections need to be controlled. k) The progressive rotational movement of the berm as observed from centrifuge tests makes the berm unstable and its effectiveness to control the wall movement becomes less and less with increasing excavation. The study demonstrated that a downward slant berm would help to control the rotational movement of berm and consequently reduce the wall and ground movement especially at later stages of excavation. The wall movement at the location of the berm is not only necessary for the mobilisation of end bearing and shear resistance but also for control of the rotational movement of berm. Furthermore, the available end bearing and interfacial shear resistance of the downward slant berm also increase due to the increase of the average embedment depth of the berm. l) Numerical analyses showed that increasing the berm length is effective in reducing the wall movement since more interfacial shear resistance could be mobilised, but on the other hand, the larger resistance at the contact between the wall and the berm would increase the wall bending moment. The analyses also demonstrated that when the treated soil volume was kept constant, in principle increasing the berm length was more effective in reducing the wall movement than increasing the berm thickness. Furthermore, the analyses showed that the influence of the excavation width on the wall deflection of excavations with 281 Chapter Conclusions embedded improved soil berms was similar to that of braced excavations. Increase in excavation width leads to increase in the wall movement, but the improved soil berm was still effective in controlling the wall movement for wider excavations. m) Based on actual observations from centrifuge tests, numerical analyses were calibrated, for example the introduction of slip elements to capture slippage and separation, to produce a consistent set of resistance mechanisms. Subsequent numerical analyses, introducing a strut of different stiffnesses above the excavation level showed that the end bearing resistance provided by the berm is not only dependent on the berm movement but also on the overburden on the berm. When the effect of the overburden is isolated, it is found that the net mobilised end bearing is mainly related to the berm movement. This observation suggests that the resistance mechanism of an excavation with improved soil berm is applicable to both cantilever and strutted excavations. More importantly, this nearly unique dependence provides a way for incorporation into design. While this was not studied here, it is an important direction for future research. 8.2 Recommendations for future studies The present research has studied several fundamental behaviours of an excavation stabilised with an embedded improved soil berm. From the insight developed from this study, the following topics are recommended for future study: The present study showed that two or more layers of improved soil berms may be required to control different types of wall deflection in an excavation. Further efforts should be made to study the interactions of the improved soil berms and optimise the configuration of the improved soil berms. This is of great importance in 282 Chapter Conclusions the practical design. It is also established that the net end bearing resistance of an improved soil berm is mainly dependent on the berm movement. 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W (1978) “A unified approach to the soil mechanics problems of offshore foundations.” In numerical methods in offshore engineering, New York, Wiley, 361-411 291 [...]... establish the key controlling parameters of an improved soil berm and provide some guidelines for designing such a berm from the results and findings of centrifuge experiments and numerical analyses Detailed and accurate information of movement of the ground and the improved soil is of importance to understand the mechanisms involved for an excavation stabilised with improved soil Therefore, the first... 3 Chapter 1 Introduction 3) To improve the understanding of the behaviour of an excavation stabilised with an embedded improved soil berm, in particular the mechanisms involved in restraining the retaining wall; 4) To carry out numerical analyses to evaluate the mechanisms involved and conduct further parametric studies to complement the insights derived from the centrifuge experiments; and 5) To establish... understood and generally controlled by compressive modulus of the treated soil For excavations treated by embedded improved soil berms, the effectiveness is determined by the complicated interaction between the improved soil berm, the unimproved soil and the retaining wall since one end of the berm is constrained by the soft soil rather than the stiff wall or other supports The end bearing and the interfacial... original ground level in Tests WTreat, Berm- D3m and Berm- D3mR 198 Figure 6.36 Surface settlement at 2 m behind wall in Tests WTreat, BermD3m and Berm- D3mR .198 Figure 6.37 Schematic diagrams of location and typical movement pattern of an improved soil berm without inclination 199 Figure 6.38 Schematic diagrams of location and typical movement pattern of an improved soil berm with inclination... resistance between the berm and surrounding soil are the key factors to influence the effectiveness Such resistances are dependent upon the soil properties and the berm geometry As relatively few studies have been conducted, the understanding of the behaviour of an excavation stabilised by an improved soil berm is still not well established and therefore the design method for such an improved soil berm. .. movement In the second part of the study, the undrained ultimate end bearing capacity of an embedded improved soil berm was derived A modified upper bound solution which combines the results of an upper bound solution and numerical analyses was developed to estimate the undrained end bearing capacity of an embedded improved soil berm during excavation In the third part of the study, the behaviour of an excavation. .. behaviour of an embedded improved soil berm in an excavation Their studies demonstrated that behaviour of an excavation stabilised with an embedded improved soil berm is much more complicated than that with an embedded improved soil raft and more research efforts should be made on the former 2 Chapter 1 Introduction Though there were many research studies reporting the effectiveness of the soil improvement... retaining wall Owing to the large excavation area of about 150 m by 200 m, diaphragm walls were designed to be retained by soil berms and raking struts As the soil berm consisted of thick soft organic clay and marine clay, it was expected to be ineffective without improvement Thus the soil berm was treated by rows of jet grouted piles for the entire organic clay and marine clay layers and keyed into... 1 Introduction other hand, if one end of the improved soil contacts with the wall while the other end rests in the soft soil, in this thesis, this is called an embedded improved soil berm The study of such an embedded improved soil berm is the focus of this work The effectiveness of these ground improvement techniques in stabilising excavation has been proven in many successful projects worldwide and. .. of an embedded improved soil berm in an excavation Subsequently, a number of numerical parametric simulations were carried out using the finite element program known as CRISP (CRItical State Program) to complement the findings of centrifuge tests Findings from both the centrifuge tests and numerical analyses would further improve the understanding of the mechanisms of an excavation stabilised with an . location and typical movement pattern of an improved soil berm without inclination 199 Figure 6.38 Schematic diagrams of location and typical movement pattern of an improved soil berm with inclination. Horizontal and vertical soil and berm displacements at different excavation stages at elevation -8.89 m (Test Berm- D2m) 180 Figure 6.9 Incremental horizontal and vertical soil and berm displacements. movement at original ground level in Tests Berm- C6m, Berm- D2m and Berm- C10m 193 Figure 6.30 Horizontal berm movement in Tests Berm- C6m, Berm- D2m and Berm- C10m 194 Figure 6.31 Berm vertical