Mobilised mass properties of embedded improved soil raft in an excavation

MOBILISED MASS PROPERTIES OF EMBEDDED IMPROVED SOIL RAFT IN AN EXCAVATION YANG HAIBO (B.ENG, HOHAI UNIV) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE JUNE, 2009 Acknowledgements I feel most indebted to my supervisors Professor Leung Chun Fai and Professor Tan Thiam Soon for their guidance and support during my course of study at the National University of Singapore (NUS). Special thanks goes to Professor Tan Thiam Soon for his profound insights into various technical issues and utmost patience in guiding me throughout this study. I also wish to acknowledge the support and encouragement from Professor Lee Fook Hou and Dr Hong Sze Han, in particular during the final stage of my study. I would like to thank laboratory staff and fellow research students Chee Wee, Chen Jian, Czhia Yheaw, Karthi, Krishna, Kumar, Liu Yong, Ni Qing, Okky, Pang, Ryan, See Chia, Sindhu, Wang Lei, Yaodong, Yen, Yonggang, Zhao Ben among others for making my stay at NUS stimulating. Sincere thanks to my close friends, Zuduo, Wenya, Miao Xin, Jiangtao and Tang Jun, with whom I have shared both the joys and the pains in my life. To my parents I am always grateful. For many years in a small town in east China, they have relied on so little yet have given me so much to support my education, sending me all the way to postgraduate study. Of course, special thanks to my wife Xiaojing, who is of significant importance in my life. Finally, the financial support from NUS is gratefully acknowledged. iii Contents Acknowledgements Table of Contents . Summary . . . . . List of Tables . . . List of Figures . . . List of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction 1.1 Deep Excavation in Soft Soils 1.2 Embedded Improved Soil Raft 1.3 Definition of Terms . . . . . . 1.4 Objectives & Scope of Study . 1.5 Lay Out of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii iii v vii viii xiii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Literature Review 2.1 Introduction . . . . . . . . . . . . . . . . . . . . 2.2 Embedded Improved Soil Raft: Mechanism . . . 2.3 Embedded Improved Soil Raft: Mass Properties 2.3.1 Back-analyses of Field Data . . . . . . . 2.3.2 Laboratory Experiments . . . . . . . . . 2.3.3 Numerical Studies . . . . . . . . . . . . . 2.4 Factors Influencing Mobilised Mass Properties . 2.4.1 Radial Variability . . . . . . . . . . . . . 2.4.2 Defects . . . . . . . . . . . . . . . . . . . 2.4.3 Holding Piles . . . . . . . . . . . . . . . 2.5 Hertz Contact Problem . . . . . . . . . . . . . . 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 12 14 17 17 20 27 30 31 33 34 35 37 Numerical Model 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Model Verification . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Hertz Contact Problem . . . . . . . . . . . . . . . . . . . 3.2.2 Simulation of Nakagawa et al. (1996)’s Case History . . . 3.2.3 Back-Analysis of Liao and Su (2000)’s Laboratory Tests 3.3 Model Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Geometry, Material & Boundary Conditions . . . . . . . 3.3.2 Calculation of Mass Stress, Strain and Stiffness . . . . . . . . . . . . . 58 58 60 60 62 66 68 68 71 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table of Contents Mass Properties: Lateral Compression 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . 4.2 Variability in Improved Soil-cement Columns . . . 4.3 Study Approach . . . . . . . . . . . . . . . . . . . 4.4 Hertz Contact Problem . . . . . . . . . . . . . . . 4.5 Mobilised Mass Properties of Improved Soil Layer 4.5.1 One Row of Columns in Point Contact . . 4.5.2 One Row of Overlapping Columns . . . . . 4.5.3 Interaction of Overlapping and Layering . 4.5.4 Multiple Overlapping Columns . . . . . . 4.6 Summary . . . . . . . . . . . . . . . . . . . . . . iv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 86 87 90 92 93 93 95 97 99 101 Mass Properties: Lateral Compression & Basal Uplifting 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Confining Pressures . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Variation of Confining Pressures . . . . . . . . . . . . 5.2.2 Balanced Confining Pressures . . . . . . . . . . . . . 5.2.3 Uplifting Pressures . . . . . . . . . . . . . . . . . . . 5.3 Thickness of Soil Raft . . . . . . . . . . . . . . . . . . . . . 5.4 Non-Perfect Treatment . . . . . . . . . . . . . . . . . . . . . 5.4.1 Untreated Zone . . . . . . . . . . . . . . . . . . . . . 5.4.2 Further Analysis of Random Cases . . . . . . . . . . 5.5 Holding Piles . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Effects of Holding Piles . . . . . . . . . . . . . . . . . 5.5.2 Interface between Holding Piles and Soil Raft . . . . 5.5.3 Modelling of Holding Piles . . . . . . . . . . . . . . . 5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 115 118 118 119 120 125 126 126 130 132 133 135 136 138 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions & Recommendations 159 6.1 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.2 Recommendations for Future Studies . . . . . . . . . . . . . . . . 163 References 165 A Translation of Japanese Texts in Figures in Chapter 171 Summary An embedded improved soil raft is a layer of short overlapping soil-cement columns that are formed by jet grout piling or deep cement mixing which is often used to stabilise an excavation in soft soils. It is often installed below excavation formation level prior to excavation. As excavation proceeds, an embedded improved soil raft would be subjected to lateral compression from the inwards moving retaining walls. Thus, the mobilised mass properties in the lateral direction, rather than the material properties from elemental cores, are of direct importance in controlling the wall deflections and the associated ground movements. In this research, numerical simulations are employed to examine in a systematic way various influencing factors, such as layering, overlapping, combined loading of lateral compression and basal uplifting, non-perfect treatment and holding piles, which affect the mobilised mass properties of an embedded improved soil raft. The analysis starts from two soil-cement columns that are assigned with linearly elastic material model, arranged just in contact with each other and being compressed laterally. Subsequently, various assumptions in the initial model are gradually relaxed so that the model can take into account other influencing factors. They are geometry arrangement, layering, lateral compression and basal uplifting, thickness of soil raft, non-perfect treatment and holding piles. Throughout this Summary vi study, calibration and verification are carried out to check the numerical results against analytical solutions or field back-analysed data when possible. The analysis shows that when the soil-cement columns are arranged in point contact, the mobilised mass stiffness is very low. This observation is echoed by reported field back-analysis of deep excavation in soft soil that is stabilised using embedded improved soil raft. The mobilised mass stiffness can be raised by introducing some degree of overlapping among neighbouring soil-cement columns. The soil-cement columns formed in the field often have layered properties and it is shown in this study that the outer layers are more important than the inner ones in determining the mobilised mass properties. The analysis in this study shows that the uplifting pressures cause little changes in the magnitude of the mobilised mass stiffness but reduce the threshold mass strain where the mobilised mass stiffness starts to drop. The threshold mass strain is also affected by the thickness to length ratio T /L of the soil raft as well as holding piles, if present. To extend the threshold mass strain, it is necessary to increase the T /L ratio or to provide holding piles. But the beneficial effects of holding piles on extending the threshold mass strain depend on qualities of the interface zones between holding piles and soil raft. It is also shown that the impact of non-perfectly treated zones are dependent on the number of such zones and more importantly how these zones are distributed over the soil raft. vii List of Tables 2.1 2.2 2.3 3.1 3.2 Reported ratio of thickness to length (T /L) for embedded improved soil raft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variability of properties in the radial direction within a soil-cement column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of overlapping zone versus general zone within soil-cement columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 39 40 3.3 Material parameters used to simulate Hertz contact problem . . . 74 Material parameters used to simulate Nakagawa et al. (1996)’s field case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Material parameters used to simulate Liao and Su (2000)’s field case 75 4.1 Elemental material properties for the layered columns . . . . . . . 104 5.1 5.2 Elemental material properties for analysis of non-perfect columns Elemental material properties for analysis of holding piles . . . . . 142 142 viii List of Figures 1.1 1.2 1.3 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 Concepts of embedded improved soil raft and soil berm (after Zhang, 2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An embedded improved soil raft in an excavation (after Nakagawa et al., 1996) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An embedded improved soil berm in an excavation (after Khoo et al., 1997) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maximum wall deflection in a strutted excavation in soft soils (after Yong et al., 1989) . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of sheet pile wall deflections in an excavation in soft clay: ungrouted area and grouted area (after Lee and Yong, 1991) Wall bending moment profile in an excavation supported by embedded improved soil layer (after Goh, 2003) . . . . . . . . . . . . Prediction of deformed shape of embedded improved soil raft from finite element analysis (after Goh, 2003) . . . . . . . . . . . . . . Mobilisation of bearing capacity of embedded improved soil berm in an excavation (after Zhang, 2004) . . . . . . . . . . . . . . . . Supposed displacement pattern of treated soil later in an excavation (after Tanaka, 1993) . . . . . . . . . . . . . . . . . . . . . . . . . Contact arrangement of improved soil columns (after Nakagawa et al. 1996) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Displacement, bending moment and change of K value due to excavation (after Nakagawa et al., 1996) . . . . . . . . . . . . . . . . Specimen reinforced with grout columns (after Liao and Su, 2000) Stress paths on the octahedral plane (after Liao and Su, 2000) . . Normalised tangential shear modulus versus normalized shear stress for different improvement ratios (θ=120o ) (after Liao and Su, 2000) Layout patterns for reinforced soil specimens (after Liao and Tsai, 1993) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Back-analysis results of retaining wall deformations after final excavation (after Shikauchi et al., 1993), English translation see Fig. A.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Model setup for studying lateral compression of improved soil model (after Saito et al., 1994), English translation see Fig. A.2 . . . . . External load and model horizontal deformations (after Saito et al., 1994), English translation see Fig. A.3 . . . . . . . . . . . . . . . 10 11 11 41 42 42 43 43 44 44 44 45 46 46 47 47 48 48 List of Figures 2.16 Model setup and instrumentation of improved soil model under lateral compression and basal uplifting (after Ueki et al., 1995), English translation see Fig. A.4 . . . . . . . . . . . . . . . . . . . . . 2.17 Uplifting pressure and vertical deformation at the centre of the model (after Ueki et al., 1995), English translation see Fig. A.5 . 2.18 Field measurements and FEM analysis results of horizontal wall deformation (after Ogasawara et al., 1996), English translation see Fig. A.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.19 Schematic view of the excavation for approach structure (after Ayoubian and Nasri, 2004) . . . . . . . . . . . . . . . . . . . . . . . 2.20 Horizontal variability of deep mixing columns (after Kawasaki et al., 1984) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.21 Strength variation in the radial direction of the soil-cement columns (re-plotted from Kawasaki et al., 1984) . . . . . . . . . . . . . . . 2.22 Hybrid RAS-JET system used in a field trial in Singapore . . . . 2.23 Typical unconfined compressive strength profile of soil-cement columns formed by RAS-JET in a field trial, Singapore . . . . . . . . . . . 2.24 Variation of strength and modulus with distance from injection pipe (after Bader and Krizek, 1982) . . . . . . . . . . . . . . . . . . . . 2.25 Strength distribution along the radial direction (after Sakai et al. (1994)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.26 Sampling in the vertical and the horizontal directions (after Sakai et al. (1994)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.27 Wall deflections measured on two sides of a deep excavation stabilised by an embedded improved soil raft (after Shirlaw et al., 2005a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.28 Masking effect (after Morey and Campo, 1999) . . . . . . . . . . . 2.29 Results of instrumented load test on bored pile installed through a nominally 3.5m thick jet grout slab (after Shirlaw et al., 2005a) . 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Mesh, loading and boundary conditions for convergence study of Hertz contact problem . . . . . . . . . . . . . . . . . . . . . . . . Results of mesh convergence study . . . . . . . . . . . . . . . . . Numerical and analytical solutions to Hertz contact problem . . . Von Mises contours for Hertz contact problem . . . . . . . . . . . Two-dimensional photo-elastic fringe patterns (contours of principal shear stress) for contact of cylinders (after Johnson, 1985) . . Numerical model to simulate the improved soil layer reported in Nakagawa et al. (1996) . . . . . . . . . . . . . . . . . . . . . . . . Mobilised mass stiffness of the improved soil layer in Nakagawa et al. (1996) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impact of layering on the mobilised mass stiffness of the improved soil layer in Nakagawa et al. (1996) . . . . . . . . . . . . . . . . . Correlation of depth of excavation with displacement, and K value (after (Nakagawa et al., 1996)) . . . . . . . . . . . . . . . . . . . . ix 49 49 50 50 51 52 53 53 54 55 55 56 56 57 76 77 77 78 78 79 79 80 80 List of Figures 3.10 Finite element models for simulation of Liao and Su (2000)’s laboratory tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Normalised tangential shear modulus versus normalised shear stress 3.12 Normalised octahedral shear stress versus octahedral shear strain for different improvement ratios . . . . . . . . . . . . . . . . . . . 3.13 Typical stress strain relationships for cement-treated soil (after Lee at al, 2005) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Modelling of strain hardening in unconfined compressive strength test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.15 Modelling of strain softening in unconfined compressive strength test 3.16 Calculation of mass stiffness . . . . . . . . . . . . . . . . . . . . . 3.17 Increment control in simulation of UCS test in ABAQUS . . . . . 3.18 Increment control in simulation of a soil raft in two-dimensional plane strain analysis . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 5.1 Mobilised mechanics of soil-cement treated ground: (a) Embankment loading; (b) Excavation loading . . . . . . . . . . . . . . . . Analogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geometric configurations of studies conducted . . . . . . . . . . . Elemental material models: Linear elastic & elastic perfectly plastic Model and boundary conditions for simulation of Hertz contact problem and the analogy . . . . . . . . . . . . . . . . . . . . . . . Normalised mass stiffness of two columns in Hertz contact . . . . Mass behaviour of columns as arranged in the analogy . . . . . . Mass behaviour of one row of columns in point contact . . . . . . Model and boundary conditions for simulation of uniform soil-cement columns at different degrees of overlap . . . . . . . . . . . . . . . Effects of overlapping on the mass behaviour of one row of overlapping columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Von Mises stress contours for uniform soil-cement columns at different degrees of overlap . . . . . . . . . . . . . . . . . . . . . . . Mass stiffness at 0.8% mass strain with overlap parameter L . . . Interaction of overlapping and layering: increase of overlap for uniform soil-cement columns . . . . . . . . . . . . . . . . . . . . . . . Interaction of overlapping and layering: increase of stiffness ratio for two-layered soil-cement columns . . . . . . . . . . . . . . . . . Von Mises stress contours for overlapping soil-cement columns: Stiffness ratio of and 48 . . . . . . . . . . . . . . . . . . . . . . . . . Normalised initial mass stiffness for one row of overlapping columns Normalised initial mass stiffness for multiple overlapping soil-cement columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normalized initial mass stiffness for multiple overlapping soil-cement columns: Volume ratio 50% . . . . . . . . . . . . . . . . . . . . . Schematic drawings of embedded improved soil raft . . . . . . . . x 81 81 82 82 83 83 84 84 85 105 106 106 107 107 108 108 109 109 110 110 111 111 112 112 113 113 114 143 Chapter Conclusions & Recommendations 6.1 Concluding Remarks An embedded improved soil raft is a collection of short overlapping soil-cement columns that are commonly used to stabilise deep excavation in soft soils. During excavation, the embedded improved soil raft is subject to mainly lateral compression as well as uplift from the base during excavation under such loading conditions, the laterally mobilised mass properties of the embedded improved soil raft are of direct relevance to the control of wall deflections and associated ground movements. This research aimed to examine the mobilised mass properties of an embedded improved soil raft in the lateral direction. Finite element analyses have been carried out to examine various influencing factors like geometry arrangement, layering within soil-cement columns, overlapping, combined loading of lateral compression and basal uplifting, non-perfect treatment and holding piles. Finite element analyses have also been conducted to back-analyse some case histories in which the laterally mobilised mass properties in the field were reported. Some conclusions can be drawn as follows: • For one row of soil-cement columns arranged in point contact and assigned 6.1: Concluding Remarks 160 with isotropic linearly elastic material properties, the mass behaviour mobilised upon lateral compression is non-linear. The non-linearity comes from the contact between the soil-cement columns which initially is a point but grows as the columns are laterally compressed. The growth in contact helps mobilise a larger area of the improved soils and thus contributes to the increase in the mass stiffness. The non-linearity can also arise from material yielding once plasticity is introduced into the elemental material model and the stresses in the soil-cement columns reach the yielding level. The material yielding would then bring down the mass stiffness of the row of soil-cement columns. The mobilised mass stiffness in such case is less than and quantitatively only about 20% of the elemental material stiffness. Such differences between the mass properties and the elemental properties appear to account for a major part of the observed discrepancies between the initially-designed material properties of the embedded improved soil raft and those back-analysed values reported in some case histories where the soil-cement columns were arranged in point contact grid. The numerically simulated results and the reported observations from the case histories point out the important need to distinguish the mobilised mass properties from the elemental properties. • Overlapping the uniform soil-cement columns in a row helps improve the initially mobilised mass stiffness. A 30% overlap in terms of column radius in the compressive loading direction can achieve about 80% of the elemental material stiffness for a row of overlapping soil-cement columns. The impor- 6.1: Concluding Remarks 161 tance of overlap calls for better quality control in terms of column locations during the installation of the soil-cement columns in the field. • The soil-cement columns formed in the field are often not uniform and tend to have layered properties in the radial direction. It is shown in the analysis results that the outer layer plays a more important role in determining the mass properties of the improved soils. Thus, it is suggested that during quality check of an embedded improved soil raft, at least two cores less than one radius apart be taken so as to arrive at a better estimation of the mobilised mass stiffness based on the likely layering profiles. • It is found in this study that the confining pressures that an embedded improved soil raft would undergo in a typical excavation have little impact on the initially mobilised mass stiffness level. But uplifting pressures, arising from the imbalances between the confining pressures on the top and the bottom of a soil raft, can reduce the threshold mass strain where the mobilised mass stiffness starts to drop. The uplifting pressures may cause separation at the interface between soil raft and retaining walls. Such potential separation needs to be considered in modelling the soil raft, otherwise unrealistic “stable” results may be produced. • Besides uplifting pressures, the threshold mass strain where the mass stiffness starts to drop is also related to the size of a soil raft, mainly the thickness to length ratio T /L and the holding piles, if present. A thicker soil raft helps extend the threshold mass strain while such beneficial effects can also 6.1: Concluding Remarks 162 be gained by installing holding piles throughout the soil raft. In case where an embedded improved soil raft is designed to be located at very deep level in an excavation where removal of overburden can create high uplifting pressures, the ratio T /L should be increased for the soil raft to control the wall deflections and associated ground movements. For a 20 m-deep excavation, the T /L should be about 0.1488 (about m thick for a length of 20 m) if holding piles are not provided. The T /L ratio can be reduced if holding piles are installed. In addition, the effectiveness of holding piles is dependent on qualities of the interfaces between the holding piles and the soil raft which are often challenging to maintain in the field. • Two main idealisations are involved in modelling an embedded improved soil raft with holding piles using two-dimensional plane strain analysis. The first is to assume that the embedded improved soil raft is homogeneous and assign representative material properties to it and the second to smearing the holding piles into two-dimensional continuous walls. Both these idealisations are shown to cause overestimation of the threshold mass strain where the mobilised mass stiffness starts to drop. Such overestimations may be intervened by purposely reducing the equivalent material strength of the embedded improved soil raft by about 12%. But care must be taken in interpreting these results. • In the construction of an embedded improved soil raft, it is possible that some parts are left untreated. The impact of such untreated zones on the 6.2: Recommendations for Future Studies 163 mobilised mass stiffness is related to their quantities and more importantly how these zones are distributed across the soil raft. When the untreated zones are perpendicular to the loading direction (parallel to the retaining walls), they induce the most significant reduction in the mobilised mass stiffness, especially when a connected untreated zone is formed. If the untreated zones are randomly distributed, the soil raft can accommodate more such zones. These observations suggest that if one untreated zone is found in the field during quality check on the embedded improved soil raft, it is important to check the qualities of the surrounding soil-cement columns in order to better assess the impact of these untreated zones based on their likely distributions. 6.2 Recommendations for Future Studies The present research has studied the mobilised mass behaviour of an embedded improved soil raft in an excavation based on assumption that material properties of the elemental cores are linearly elastic or linearly elastic perfectly plastic. In these two material models, the behaviour of material under tensile stresses is the same as that under compressive stresses. Such behaviour is a simplification of what soil-cement mixture would behave in the field. The tensile strength of soil-cement mixture is only a fraction, typically 20% of its compressive strength (Porbaha, 2000a; Namikawa and Koseki, 2007). On the other hand, it is noted in this study that tensile stresses can develop in some parts of the soil raft, especially when uplifting pressures and holding piles are considered. It would then be of interest 6.2: Recommendations for Future Studies 164 that future studies can be carried out which distinct the compressive and the tensile behaviours of elemental material model of the soil-cement mixture. 165 References Anagnostopoulos, C.A. 2006. Physical and Mechanical Properties of Injected Sand with Latex-superplasticized Grouts. Geotechnical Testing Journal 29:1–7. Ayoubian, A., and V. Nasri. 2004. Design of Jet Grout Plugs for Base Stability and Groundwater Control. In Geotechnical Engineering for Transportation Projects, ed. M.K. Yegian and E. Kavazanjian, volume of Geotechnical Special Publication No. 126 , 1905–1914. Bader, T.A., and R.J. Krizek. 1982. Injection and Distribution of Silicate Grout in Sand. 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Appendix A Translation of Japanese Texts in Figures in Chapter Fig. A.1: Back-analysis results of retaining wall deformations after final excavation (after Shikauchi et al., 1993), original texts see Fig. 2.13 Figures: Appendix A 172 Fig. A.2: Model setup for studying lateral compression of improved soil model (after Saito et al., 1994), original texts see Fig. 2.14 Fig. A.3: External load and model horizontal deformations (after Saito et al., 1994), original texts see Fig. 2.15 Figures: Appendix A 173 Fig. A.4: Model setup and instrumentation of improved soil model under lateral comparession and basal uplifting (after Ueki et al., 1995), original texts see Fig. 2.16 Fig. A.5: Uplifting pressure and vertical deformation at the centre of the model (after Ueki et al., 1995), original texts see Fig. 2.17 Figures: Appendix A 174 Fig. A.6: Field measurements and FEM analysis results of horizontal wall deformation (after Ogasawara et al., 1996), original texts see Fig. 2.18 [...]... understandings of the mechanisms and the properties of an embedded improved soil layer in an excavation are of vital importance 1.2 Embedded Improved Soil Raft Researchers have shown that the embedded improved soil raft behaves like a strut in deep excavation in soft soils (Tanaka, 1993; Goh, 2003) and that its stiffness is a very important parameter for its performance (Goh, 2003) Since the soil raft. .. wall through inter-facial shearing and end bearing The mechanisms involved in an embedded improved soil raft and soil berm are markedly different This research focuses on the embedded improved soil raft 1.2: Embedded Improved Soil Raft 4 In the field, an embedded improved soil raft is constructed by installing short vertical soil- cement columns one by one that overlap with each other As part of typical... Scope of Study The aim of this thesis is to systematically examine the mobilised mass properties of an embedded improved soil raft in an excavation and how the known elemental properties are translated into the mass properties taking into account various in uencing factors More specifically, the objectives are to assess the effects of various in uencing factors, including geometry arrangement, overlapping,... stiffness, not the mass strength of an embedded improved soil raft 1.5 Lay Out of Thesis Chapter 2 begins with a detailed review of previous researches on the mechanisms and the mass properties of an embedded improved soil raft in an excavation It identifies the need of research to resolve the observed discrepancies between the mobilised mass properties from the back-analysis of field cases and the properties. .. 1.2: An embedded improved soil raft in an excavation (after Nakagawa et al., 1996) Fig 1.3: An embedded improved soil berm in an excavation (after Khoo et al., 1997) Chapter 2 Literature Review 2.1 Introduction This chapter presents a detailed review of research studies that have been conducted on embedded improved soil raft for supporting deep excavation in soft soils An embedded improved soil raft. .. in- situ soft soils to produce cylindrical soil- cement columns that overlap with each other and collectively form a continuous embedded improved soil raft Typical thickness and length of the embedded improved soil raft that were 2.2: Embedded Improved Soil Raft: Mechanism 14 reported in some studies are summarized in Table 2.1 Successful applications of such an embedded improved soil raft in deep excavations... region In a back analysis of the behaviour of an embedded improved soil raft in an excavation in Singapore, Pickles and Henderson (2005) showed that the backanalysed stiffness of the improved soil layer was about 70 MP a, less than 35% of the typical value obtained from unconfined compressive tests on cored samples 2.3: Embedded Improved Soil Raft: Mass Properties 19 The strength and the failure strain of. .. stiffness of the block, which is equivalent to the mobilised mass properties of an improved soil raft consisting of overlapping columns, is a key parameter and has to be above a threshold value for the method to be effective Thus the challenge is still the determination of the mobilised mass properties 2.3: Embedded Improved Soil Raft: Mass Properties 2.3 17 Embedded Improved Soil Raft: Mass Properties. .. are properties of embedded improved soil raft as a whole Since during excavation, a soil raft is mainly subjected to lateral compression from inward moving retaining wall, mass properties in this study concerns only with mobilised properties in the lateral direction The mass properties are intended to differentiate between elemental properties measured vertically from individual soil- cement columns and... result, Tanaka (1993) proposed a displacement pattern of an embedded improved soil raft in an excavation shown in Fig 2.6 The author also proposed a new way of calculating the stability factor of an excavation stabilised by an embedded improved soil raft but acknowledged “the vagueness in the determination of the strength of the treated part” The actual mobilised strength was of main concern in the stability . field back-analysis of deep excavation in soft soil that is stabilised using embedded improved soil raft. The mobilised mass stiffness can be raised by in- troducing some degree of overlapping among. through inter-facial shearing and end bearing. The mechanisms involved in an embedded improved soil raft and soil berm are markedly different. This research focuses on the embedded improved soil raft. 1.2:. installed through an embedded improved soil raft and keyed into bearing layer; • Defects refer to part of in- situ soils that may be left un -improved within an embedded improved soil raft. 1.4: Objectives