CHARACTERIZATION AND MODELING OF CEMENTTREATED SOIL COLUMN USED AS CANTILEVER EARTH RETAINING STRUCTURE SAW AY LEE NATIONAL UNIVERSITY OF SINGAPORE 2014 CHARACTERIZATION AND MODELING OF CEMENTTREATED SOIL COLUMN USED AS CANTILEVER EARTH RETAINING STRUCTURE SAW AY LEE (B. Eng. (Hons.), UTM; M. Sc., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. _________________ Saw Ay Lee January 2014 Acknowledgement First of all, I am grateful to The Almighty God for leading me through this research. Without His guidance this dissertation would not have been possible. I would like to express the deepest appreciation to my supervisors, Professor Leung Chun Fai and Associate Professor Tan Siew Ann for their patient guidance, encouragement and useful critiques of this research work. I am extremely blessed by their unconditional support. I also thank the research scholarship as well as facilities provided by the National University of Singapore to make this research a reality. Special thanks to Mr. Ang Beng Oon and Mr. Foo Hee Ann who patiently helped me in my laboratory tasks. I am also grateful to Muhammad Faizal and Dr. Xiao Huawen for their willingness to share the material and equipment during the time I conducted my experimental works. A special thank you is also extended to Mr. Ann Kee Tong and Mr. Edward Lim for their help in offering me the resources. Grateful acknowledgement is expressed to Dr. Namikawa for enlightening me on the tension-softening behavior of cement-treated soil. I would also like to thank the fellow colleagues, in particular Hartono, Kok Shien, Yang Yu, Zongrui, Junhui, Xuguang, Sun Jie, Sandi for their company through this journey. Also, I would like to thank my good friends for their support and continuing belief in me. Last but not the least, an honorable mention goes to my family for always being there for me. i Table of Contents Acknowledgement i Table of Contents …………………………………………………………………………… .ii Summary v List of Tables . vii List of Figures . ix List of Symbols xvii Abbreviations xxi Chapter Introduction 1.1 Background 1.2 Issues Related to the Use of Cement-treated Soil Columns 1.3 Objective and Scope of Study 1.4 Structure of Thesis . Chapter Literature Review 2.1 Introduction . 2.2 General Aspects of Cement-treated Soil 2.2.1 Physical Properties of Cement-Treated Soil 11 2.2.2 Mechanical Properties of Cement-Treated Soil . 13 2.3 Existing design approaches 18 2.4 Numerical Modeling on Behavior of Cement-treated Soil 23 2.4.1 2-D and 3-D Finite Element Analysis 24 2.4.2 Constitutive Models for Cement-treated Soil 26 2.5 Summary 29 Chapter Fracture Behavior of Cement-treated Singapore Marine Clay 3.1 Introduction . 46 3.2 Properties of Base Materials 47 3.2.1 Untreated Marine Clay 47 3.2.2 Ordinary Portland Cement . 48 3.3 Sample Preparation Procedure . 48 3.4 Testing Procedure and Apparatus 50 3.4.1 Uniaxial Compression Test 51 3.4.2 Split Tension Test 51 3.4.3 Three-point Bending Notched Beam Test . 52 3.5 Compressive Fracture Behavior 53 3.6 Tensile Fracture Behavior 55 3.6.1 Split Tensile Strength 56 3.6.2 Fracture Energy, Gf 58 ii 3.6.3 Tension-softening Relationship . 62 3.6.4 Parametric Studies . 67 3.6.5 Outstanding Issues . 70 3.7 Summary 74 Chapter Constitutive Model for Cement-treated Soil 4.1 Introduction . 94 4.2 Finite Element Method 94 4.3 Constitutive Models for Cement-treated Soils . 96 4.3.1 Elastic Perfectly-Plastic Tresca Model 96 4.3.2 Isotropic Model 99 4.3.3 Concrete Damage Plastic Model 100 4.4 Evaluation of Constitutive Model Prediction for Behavior of Cement-treated Toyoura Sand . 105 4.4.1 Drained Triaxial Compression Test by Namikawa (2006) 106 4.4.2 Direct Tension Test by Koseki et al. (2005) 109 4.4.3 Three-point Bending Notched Beam Test by Namikawa (2006) 112 4.5 Evaluation of Constitutive Model Prediction for Behavior of Cement-treated Singapore Marine Clay 116 4.5.1 Uniaxial Compression Test (UCT) 117 4.5.2 Three-point Bending Notched Beam Test . 119 4.6 Conclusions . 121 Chapter Numerical Approaches in Simulating Cemented Soil Mass 5.1 Introduction . 138 5.2 Weighted Average Simulation (WAS) and Real Allocation Simulation (RAS) Approaches 139 5.3 Evaluation of WAS and RAS Approaches in Simulating Cement-treated Soil Mass …………………………………………………………………………………….141 5.3.1 Test description 141 5.3.2 Numerical Analyses . 142 5.3.3 Results and Discussion 147 5.4 Study of Cement-treated Ground Improvement Pattern 151 5.4.1 Hypothetical Cases 153 5.4.2 Numerical Analyses . 155 5.4.3 Results and Discussions . 161 5.5 Conclusion . 167 Chapter Field Studies 6.1 Introduction . 192 iii 6.2 Field Case Study 1: Lateral Load Test by Babasaki et al. (1997) 193 6.2.1 Test Description . 193 6.2.2 Numerical Analyses . 195 6.2.3 Results and Discussions . 198 6.3 Field Case Study 2: Waterway Construction in Northeastern Singapore 201 6.3.1 Characteristics of Site 201 6.3.2 Construction Method . 202 6.3.3 Numerical Analyses . 204 6.3.4 Results and Discussions . 208 6.4 Field Case Study 3: Basement Construction in Central Singapore 210 6.4.1 Characteristics of Site 211 6.4.2 Construction Method . 212 6.4.3 Numerical Analyses . 213 6.4.4 Results and Discussions . 215 6.5 Conclusion . 217 Chapter Conclusions 7.1 Summary of Findings 238 7.2 Recommendations for Further Study . 242 References ………………………………………………………………………… .…….243 iv Summary Owing to presence of soft soil which covers at least one quarter of land area of Singapore, it is often necessary to improve the soft soil for various construction purposes particularly for excavation works. One such ground improvement technique is to improve the soft soil with cement to increase the in-situ strength and stiffness. The treatment may be conducted at great depth as embedded struts to support deep excavation or at shallower depth not far below ground level to support an open excavation work. However, relatively little studies had been performed to study the behavior of cement-treated wall in an open excavation. This research covers experimental studies to investigate behavior of cement-treated marine clay and numerical studies to examine the behavior of cement-treated soil columns used as a retaining system in an open cut excavation. The first objective of this study is to understand the material properties of cement-treated Singapore marine clay in terms of compression and tension behavior. A series of samples with different mix proportions and curing periods was tested by different means in the laboratory. The experimental results show that the material strength increases rapidly to a peak value and then decreases abruptly to a small value upon further straining. The tensile strength of this material is found to be 11% of its unconfined compressive strength in the cement content tested in the present study. This material becomes brittle when 20% of cement content is added and cured for 14 days. The post-peak softening of the treated clay was derived based on fracture mechanics concept. Numerical calibration analyses were carried out to evaluate the appropriateness of three available constitutive models based on laboratory test results and published data. The calibration results show that the concrete damage plasticity (CDP) model is superior to Tresca and isotropic models in simulating the behaviors of cement-treated soil in compression, tension and bending. v This thesis further examines the significance of modeling the cement-treated soil column configurations in finite element analysis. This is of importance to account for the localized overlap areas of treated soil columns and the interaction between treated and untreated soil. In practice, cement-treated soil mass with certain columnar shaped treated soil are often analyzed using weighted average simulation (WAS) approach with properties that are averaged over the treated area. In this thesis, the limitations of generalizing the properties based on comparison with the results of a published laboratory test were discussed. The analysis results show that the shortcomings of this approach can be overcome by the real allocation simulation (RAS) approach with CDP model. The established numerical approach was then adopted to study hypothetical cases of a vertical cut with three different ground improvement geometrical patterns. The study demonstrates that tensile damage in the cement-treated soil columns is the trigger for failure. The ability of this recommended numerical simulation method – RAS with CDP model is examined by back-analyzing three field case studies on cement-treated soil columns failure. The numerical approach provides a fair prediction of the ground response and failure pattern compared to field observations. vi List of Tables Table 2.1 Summary of selected E-qu relationships for cement-treated clay. Table 2.2 Summary of some Table 3.1 Basic properties of Singapore marine clay from Marine Bouvelard site. Table 3.2 Chemical composition and physical properties of Ordinary Portland Cement t – qu relationships of cemented soil. provided by supplier. Table 3.3 Mixture proportions and tests conducted for cement-treated Singapore marine clay in the present study. Table 3.4 Summary of fracture energies, Gf from BNT tests. Table 3.5 Nominal size for materials used in the present study. Table 3.6 Mixture proportions and experimental tests with addition of filter sand in clay. Table 3.7 Chemical compositions of filter sand provided by supplier. Table 3.8 Summary fracture energy Gf for cemented clay and cemented clay+sand. Table 4.1 Mixing proportions of cement-treated Toyoura sand. Table 4.2 Design parameters for Tresca model. Table 4.3 Design parameters for Isotropic model. Table 4.4 Design parameters for CDP model. Table 4.5 Summary results of three-point bending notched beam test by Namikawa (2006). Table 4.6 Tensile design parameters for CDP model in Case T2. Table 4.7 Mixing proportions of cement-treated Singapore marine clay. Table 4.8 Calibration parameters for cemented Singapore marine clay using CDP model. Table 4.9 Damage variables used in CDP model for cemented Singapore marine clay Test B1. Table 5.1 Tresca design parameters for WAS approach in Test 2. Table 5.2 CDP design parameters of lime-cement-treated soil columns for RAS approach in Test 2. Table 5.3 CDP damage variables for lime-cement-treated soil columns in Test 2. Table 5.4 Summary table of hypothetical cases. Table 5.5 Design parameters for Mohr Coulomb and Tresca models in hypothetical cases. Table 5.6 CDP model design parameters for cement-treated soil columns used in hypothetical cases. Table 5.7 CDP model damage variables for cement-treated soil columns used in hypothetical cases. Table 6.1 Model parameters for steel plate, concrete cap and untreated soil (reproduced from Namikawa et al., 2008). vii Chapter Conclusions 7.1 Summary of Findings The studies presented in preceding chapters aim to provide a better understanding of the behavior of cement-treated soil columns used as earth retaining structure in an open excavation. The findings of this study are summarized as follows. 1) Through experiments, the behaviors of cement-treated Singapore marine clay are established as follows. a) The compressive strength increases rapidly up to the peak value and then decreases abruptly to a small value upon further straining. At cement content = 20% with 14 days curing period, the treated material manifests brittle behavior with shear and cone failure observed in the test sample. The strength associated with brittleness increases with cement content and curing period. For higher cement content and longer curing period, vertical compressive crushing is dominant and columnar fracture is observed. b) Tensile strength is measured by split tension test which can be well correlated with the unconfined compressive strength as st = 0.11qu for the cement content tested in the present study. The tensile post-peak behavior is presented by a relation between stress and fracture zone deformation. Three-point bending notched beam test was employed to derive the tension-softening relation based on fracture mechanics concept. For the mix proportion used in the present study, Gf falls within a range of 2.6 N/m to 4.4 N/m. A bilinear relation is observed in the tensile stress-crack opening displacement relationship, but the relation becomes linear when the material brittleness increases. 2) The use of three-point bending notched beam test to study the tension-softening relation of cement-treated clay is a relatively new approach although it has been widely used in concrete and cement-treated sand by Namikawa (2006). Therefore, parametric studies 238 were conducted to evaluate the influences of test parameters, i.e. notch width and sample size. Test results show that there is no difference between notch width 0.7 mm and 2.5 mm due to a uniform particle size distribution around the notch tip. On the other hand, there is also no difference in the test results for 50 mm and 40 mm sample sizes. The 10 mm difference in size is insignificant compared to the overall stress transition zone which usually involved hundreds of mm according to literatures. The stability problem in the tests is closely related to the stiffness of the testing machine comparing to the brittleness of the cement-treated clay. It is observed that the stability problem is more pronounced for a more brittle specimen. 3) The comparison between Tresca, isotropic and concrete damage plasticity (CDP) models based on calibration of laboratory tests in the present study and published data shows that the third model is far superior to others in simulating the behavior of cement-treated soil: a) Classical Tresca model assumes single stiffness and perfectly-plastic failure so it cannot capture the strain hardening and softening behaviors of cement-treated soil. Although the model can be improved with a built-in tension cutoff function to limit the tensile strength, it is still incapable of modeling the strain softening part. b) Isotropic model predicts close to the uniaxial compression test measurements but not the tension failure as the yield criterion in this model allows the development of any value of tensile stress. c) In CDP model, the user-input uniaxial stress-strain relations can be employed to determine the current state of the yield surface to analyze multiaxial load cases. As a result, this model can well capture the hardening and softening behaviors of cementtreated clay and sand. The model assumes a linear loss of strength after initial cracking which is fitted well to the behavior of cement-treated clay observed in the present study. Moreover, the post-failure stress can also be defined as a tabled function of crack displacement which is demonstrated to predict the bilinear relation of cement-treated Toyoura sand well. 239 4) When weighted average simulation (WAS) approach is employed to model the cementtreated soil columns used in an open excavation, several errors can occur. These errors are identified with real allocation simulation (RAS) approach using CDP model with comparisons made to laboratory test and field cases. a) Homogenizing the mass properties only allows for a single constitutive model thus violates the underlying mechanism for treated soil and untreated soil with very different characteristics. It is shown in this study that the commonly used Tresca model fails to capture the tensile damage developed in the cement-treated soil columns as shear failure of the treated mass is predicted. b) The front columns that form along the intended line of proposed excavation are subjected to lateral load and results in a “pulling away action” at overlap areas between front columns and other tangential columns. Cracks initiate when the induced tensile stress exceeds the capacity. Therefore, regarding the treated mass as a composite material will not allow failure to happen within the mass is incorrect. c) Perfect bonding between treated soil and untreated soil assumed in WAS approach is not appropriate as the analysis result shows that untreated soil moves relatively apart from the cement-treated soil columns. 5) Hypothetical studies of a vertical cut improved by three types of ground improvement geometrical patterns (namely Grid, Tangential buttress and Double wall) show that the vulnerable part of this retaining system is the front row columns. The rear parallel row columns in Grid pattern not contribute much resistance for the retaining system. A thicker front wall (i.e. double-overlapping rows or bigger diameter of cement-treated soil column) is more effective than other ground improvement geometrical patterns as the cracks consume more energy to propagate through a longer path to cause failure. This finding is not possible to be observed in the WAS approach as it does not take the column configuration into consideration. 240 6) Back-analyses of field case studies confirmed that tensile damage is the trigger for failure initiation of cement-treated soil columns used as retaining structure in an open excavation. The horizontal tensile crack positions that appear in the front columns depend on the cantilever span and may also appear at the middle span or at the excavation level. Addition of steel members in the cement-treated soil column will shift the horizontal tensile crack positions. Nevertheless, the contribution of the steel member to the retaining wall performance highly depends on the member size. 7) The damage patterns obtained by the proposed numerical method, RAS approach with CDP model are generally consistent with the trend observed from field case studies. In this proposed numerical method, each individual treated soil column is modeled individually with assigned properties described by CDP model. CDP model captures the tensionsoftening behavior to provide a realistic simulation of failure response of cement-treated soil columns. It shows in detail the propagation of correct damage locations, stress distribution and the interaction mechanism of cement-treated soil columns and untreated soil. This is significantly meaningful as it provides a fundamentally concept for the analysis of cement-treated soil columns used as earth retaining system in an open excavation. 8) There are some design implications that can be learnt from the present study. For retaining structure with cement-treated soil columns in certain configuration, the weakest part is at the overlap area between front row columns and others. This can be improved by constructing a capping beam overtop the columns to tie them together as recommended by VERT. However, for long span cantilever front row columns, tensile crack may occur at the cantilever span. This can be overcome with a sizable column diameter to resist the induced stresses which is superior to other column configurations design. Reinforcing the front row columns with steel beam members is only effective when a considerable member size is embedded in every column. 241 7.2 Recommendations for Further Study This research studies several fundamental characterizations of cement-treated soil columns used as earth retaining structure in an open excavation. However, some improvements can be done as follows: (a) The knowledge gained from the laboratory work in this study paves the way for further development of an established laboratory test guidelines in determining the tensionsoftening relation of cement-treated clay. The currently adopted test guideline of threepoint bending notched beam test is based on RILEM’s recommendation for concrete. Although concrete and cement-treated clay both behave as quasi-brittle materials; the particle size, strength and brittleness are quite different from each other. Therefore, it would be of interest to conduct a detailed study of laboratory test guidelines in determining the tension-softening relation of cement-treated clay. (b) Since CDP model is proven to be an appropriate constitutive model in simulating the behavior of cement-treated soil, the yielding parameters can thus be studied. So far, the relevant parameters used in the present study are inferred from concrete test data. (c) To better reflect the actual cement-treated soil columns and ground performance in finite element analysis, it is recommended that the excavation be modelled in its proper sequences. 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International Journal of Civil and Environmental Engineering, 6, 106-110. 253 [...]... 6.2 Summary of case analyses based on the in-situ strength of cemented soil column Table 6.3 Model parameters for CDP model of cemented column Table 6.4 Summary table of analysis cases for case study 2 Table 6.5 Design parameters for Mohr Coulomb and Tresca models used in case study 2 Table 6.6 Design parameters for case study 3 viii List of Figures Figure 1.1 Cement- treated soil columns used in Lexington,... earlier Cement- treated soil columns of diameter 2.4 m and length 8.5 m were constructed to facilitate the 4.9 m deep open excavation The cross section of the retaining wall and site photographs are presented in Figure 1.1 On the other hand, Haque and Bryant (2011) observed substantial soil body movement behind a cement- treated soil retaining wall in Irving-Las Colinas in Texas, USA Failure of cement- treated. .. behavior of this material is not well understood As a result, this study aims to investigate the mechanism of cement- treated soil columns used as earth retaining system in an open excavation The scope of study is as follows: a) To carry out laboratory studies to investigate the compressive and tensile behaviors of cement- treated marine clay including interpretation of post-peak softening of the treated. .. effect of different constitutive models (namely elastic-perfectly plastic Tresca model, isotropic model and concrete damage plasticity model) in simulating the behavior of cement- treated soil The results are calibrated against those data obtained from different types of laboratory tests conducted on cement- treated sand and cementtreated clay c) To simulate the behavior of cement- treated soil columns... (2012) Figure 1.2 Failure case of adopting cement- treated soil columns as a retaining system for an open excavation in Northeastern Singapore Figure 1.3 Examples of soil improvement pattern Figure 2.1 Change of water content by in-situ cement treatment in Tokyo Port (after Kawasaki et al., 1978) Figure 2.2 Effect of curing time and cement content on final water content of treated clay (after Kamruzzaman,... cement- treated soil wall did happen in the field but such cases are rarely published Figure 1.2 shows failure happened in Northeastern Singapore where cement- treated soil columns were adopted as retaining system to facilitate an open excavation The front row of cement- treated soil columns collapsed when excavation reached 7 m depth 1.2 Issues Related to the Use of Cement- treated Soil Columns To optimize... material assumed in the design, as the bonding between cemented soil columns and between cemented soil column (tensile strength) and soil (cohesive strength) might be weaker than the original ground Weak bonding may cause separation of columns especially the front columns along the intended excavation line from the rest of the row resulting in toppling of these columns, making the composite mass assumption... load-displacement relations modeled by classic Tresca and CDP models Figure 6.12 Yielding zone at cemented column observed in classical Tresca model analysis Figure 6.13 Soil profile sketch for case study 2 Figure 6.14 Proposed construction supported by diaphragm wall and cement- treated soil for case study 2 Figure 6.15 On-site excavation profile for case study 2 Figure 6.16 Collapse of front row cement- treated. .. movement (m); Right: Plastic strain Figure 6.22 Ground response for Case C-R2 Left: Ground movement (m); Right: Plastic strain Figure 6.23 Ground movement and tensile damage (dt) at cement- treated soil columns for Case C-R3 Figure 6.24 Soil profile and proposed construction method for case study 3 Figure 6.25 On-site excavation profile and visible cracks at the cement- treated soil columns appeared immediate... 1.4 Structure of Thesis The outline of this thesis after this introductory chapter is presented as follows: a) Chapter 2 reviews the general aspects of cement- treated soil where the changes in physical and mechanical properties are discussed The current design procedures and methods for cement- treated soil columns used in an open excavation are also evaluated Existing numerical studies of cement- treated . CHARACTERIZATION AND MODELING OF CEMENT- TREATED SOIL COLUMN USED AS CANTILEVER EARTH RETAINING STRUCTURE SAW AY LEE NATIONAL UNIVERSITY OF SINGAPORE 2014 CHARACTERIZATION AND. interaction between treated and untreated soil. In practice, cement- treated soil mass with certain columnar shaped treated soil are often analyzed using weighted average simulation (WAS) approach. behavior of cement- treated soil columns used as a retaining system in an open cut excavation. The first objective of this study is to understand the material properties of cement- treated Singapore