VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY THIN ZAR EXPERIMENTAL AND NUMERICAL STUDIES ON BEARING CAPACITY OF GROUND IMPROVED BY SOIL CEMENT DEEP MIXING (CDM) COLUMNS MASTER’S THESIS VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY THIN ZAR EXPERIMENTAL AND NUMERICAL STUDIES ON BEARING CAPACITY OF GROUND IMPROVED BY SOIL CEMENT DEEP MIXING (CDM) COLUMNS MAJOR: CIVIL ENGINEERING CODE: 8900201.04 QTD RESEARCH SUPERVISOR: Dr. NGUYEN TIEN DUNG Hanoi, 2023 ABSTRACT The load transfer layer is often designed and constructed on top of the Cement Deep Mixing (CDM) columns. In the literature, there are a few analytical methods to evaluate stress induced on the CDM column heads, and most of them involve the use of geosynthetics embedded layers, except ALiCC method, that takes into account the effect of Shallow Mixing (SM) layer. However, this method still has some limitations in actual designs.
Point Foundation (PF) method is CDM-like method, but it has more advantages than CDM method. Although there are some initial studies on the PF method, the existing studies did not fully analyze the behavior of soil and PF columns (using a true 3D model) and did not analyze stress induced in PF columns under shallow foundations. This study focuses on two key objectives: (1) the influence of thickness and stiffness of the Shallow Mixing (SM) layer on stress induced along the column and in soil and settlement of the improved ground under 1D conditions by using numerical analysis (PLAXIS 2D) and analytical analysis; and (2) the behavior of load-settlement curves of shallow footings on PF and CDM groups of similar configurations as well as stress induced in the columns by using numerical analyses (PLAXIS 3D) and field experiments. Analysis results for the first objective indicate that, as expected, when the thickness or stiffness of the SM layer increases, the settlement of the ground decreases and the stress induced on the column head increases.
An important finding from this study is that the maximum stress induced in the CDM column is typically not on the head of the columns but at the middle depths, where soil layers are softer than the SM layer and the bearing layer at the column toes. Analysis results for the second objective indicate that when the length of the PF column head is not long enough, the effectiveness of PF columns compared with CDM columns (having the same stiffness and diameter) is not significant. The stiffness of the columns and of the SM layer influence the load-settlement curves significantly. A key finding is that the maximum stress induced in the PF column is typically near the bottom of the PF cone section.
This must be aware in practical design, as the column may be locally failed at this section due to large concentrated stress. ACKNOWLEDGEMENTS First of all, I am very grateful to my supervisor, Dr. Nguyen Tien Dung (MCE coordinator), who provided much advice for the success of this study as well as valuable experience for my career. Although he was very busy, he spent a lot of time explaining to me the key issues in geotechnical engineering and foundation engineering.
Under his valuable guidance, I got a lot of knowledge about geotechnical engineering and successfully overcame many things. This research would not have succeeded without his valuable suggestions and guidance. I would like to say many thanks to Prof. Nguyen Dinh Duc (MCE Director), Prof.
Hironori Kato (MCE co-director), Assoc. Takeda Shinichi (MCE JICA expert), and Dr. Nguyen Ngoc Vinh (MCE lecturer) for their kind supports, guidance, and recommendations in various aspects including during the lecture time and research period. Special thanks to Professor Nguyen Chau Lan, lecturer at the University of Transport and Communication, and Mr.
Hoang Duy Phuong, my senior, 3rd intake MCE student from VJU. Their explanations in PLAXIS 2D and 3D Software for numerical analysis supported me a lot in this research. Moreover, I’m thankful to Ms. Hoa Bui (MCE program assistant), Mr.
Bui Hoang Tan (MCE Lab Technical) and Ms. Pham Lan Huong (temporary program assistant). Finally, I’m very thankful to my lovely family and my best friends, who always support me in my studies and research. TABLE OF CONTENTS LIST OF TABLES.
i LIST OF FIGURES .ii LIST OF ABBREVIATIONS. General introduction of Cement Deep Mixing method. Load transfer layer. Bearing capacity of shallow footing on Head-enlarged CDM(PF) Column.
Necessity of the study. Load transfer layer. Bearing capacity of shallow footing on Head-enlarged CDM(PF) Column. Scope of the study.
Structure of thesis. Overview of cement deep mixing method. Brief view of the cement deep mixing method. Application of CDM.
Classification of CDM. Fixed type and floating type improvement. Improvement of conventional CDM method. T-shaped soil- cement column.
The Point Foundation method. Load transfer Mechanisms. Theory of analytical method. The settlement of ground improved by CDM columns under 1dimension.
Calculation of stress according to the ALiCC method. Theory of numerical method. Finite element method. Material models in PLAXIS.
The performance of research. Methodology of the first objective. Methodology of the second objective. ANALYSIS AND RESULTS OF CDM GROUPS UNDER ONE- DIMENSIONAL LOADING CONDITIONS.
A comparative study on analytical and numerical analyses. ANALYSIS AND RESULTS OF HEAD-ENLARGED CDM (PF) GROUP UNDER SHALLOW FOUNDATIONS. Introduction of SAMSE Factory project. Samse Factory phase 1.
Configuration of the PF column groups. Static load testing program on PF column groups. The geometry of PF column groups. SAMSE Factory phase 2.
Configuration of the PF groups. The geometry of PF column groups. Laboratory tests for SAMSE Factory phase 1 and phase 2. Analyses for PF groups of SAMSE Factory phase 1.
Load-settlement analysis. Analyses for PF groups of SAMSE Factory phase 2. Load-settlement analysis. Stress Induced analysis along the PF columns and CDM columns.
CONCLUSIONS AND RECOMMENDATIONS. 78 LIST OF TABLES Table 2. Typical properties of Stabilized soil (wet method). Typical Properties of Lime–Cement Stabilized Soils (Dry Method).
Input parameters for the comparative study. Input parameters for the parametric study. Input parameters for Quang Trach 1 project. Unconfined compression test and Equivalent modulus results.
Material models and parameters used for approach 1 of SAMSE phase 1. Material models and parameters used for approach 2 of SAMSE phase 1. Material models and parameters used for approach 3 of SAMSE phase 1. Material models and parameters used for SAMSE phase 2 .66 i LIST OF FIGURES Figure 1.
Ground improved by CDM columns with Load transfer layer: (a) Shallow mixing layer; (b) Geo-synthetic reinforcement LTP; (c) Geotextile layer under Embankment. Configuration of improved CDM columns: (a) T-shape column (Liu et al., 2012); (b) Point foundation (PF) (Nguyen et al. The application of CDM for on-land construction (Kitazume and Terashi, 2013). The application of CDM for marine construction (Kitazume and Terashi, 2013).
Type of column installation (Kitazume and Terashi, 2013). Type of ground improvement (a) Fixed type; (b) Floating type (Kitazume and Terashi, 2013). The T-shaped soil cement column under embankment (Song-Yu et al. Displacement of soil under TDM and CDM (Yaolin et al.
Construction of PF method. Structure of the load transfer layer from the ALiCC method (modified after ALiCC, 2006). Principle of axial symmetric unit cylinder method (Han and Gabr, 2002; Poon and Chan, 2013). Principle of 3 D unit cell method (Tan et al.
Basic idea of an elastic perfectly plastic model (Plaxis manual). Hyperbolic stress–strain relationship in primary loading for a standard dra -ined triaxial test (Schanz, 1999). The general flow chart of the research. Flow chart of the methodology for data analysis of the first objective.
Configurations of PF and CDM columns. Flow chart of the methodology for data analysis of the second objective. True 3D model for PF column group under shallow foundation in the num erical method. Ground profiles in comparative study: (a) Analytical model, (b) Numerical model.
(a) Comparison of total settlement profile; (b) Stress increment profile obt ained from analytical and numerical analyses. Improved ground of parametric study case. (a) Influence of thickness of the SM layer on settlement of the ground; (b) Influence of stiffness of the SM layer on settlement of the ground. Influence of thickness of the SM layer on: (a) stress induced on the top of the columns and clay layer, (b) stress induced along the columns and in the clay layer.
Influence of stiffness of the SM layer on: (a) stress induced on the top of the columns and clay layer, (b) stress induced along the columns and in the clay layer. Influence of improvement area ratio on settlement of the ground. Influence of improvement area ratio on: (a) stress induced on top of the col umns and clay layer, (b) stress induced along the columns and in the clay layer. The cross-sectional view of improved ground and the plan view of the sto rage yard of Quang Trach 1 Thermal Power Plant (TPP).
(a) Numerical soil domain, (b) Colour spectrum of total stress in the col umn and surrounding soil, (c) Distribution of total stress with depth in the center of CDM column and in the center soil portion. Plan view of SAMSE Factory project. Soil profiles and parameters from all five bore holes of SAMSE Factory phase 1. Soil profiles for the analysis of SAMSE Factory phase 1.
A cross-sectional view of ground improved by PF column groups for the SAMSE Factory phase 1. Plan view of three PF groups for phase 1. Shape of PF columns: Group 1 (LPF = 8. Static loading test on instrumented PF group.
Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate. Soil profile for the analysis of SAMSE Factory phase 2. A cross-sectional view of ground improved by PF column group for the SAMSE Factory phase 2. Plan view of three PF groups for phase 2.
Shape of PF columns: Group 1 (LPF = 10. Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate and concrete plate. (a) Sampling using PVC pipe. (b) Sampling using attached samplers.
The estimation of equivalent modulus of PF column from UC test result. Load settlement curves from numerical method (Approach 1) for PF groups and CDM groups and experimental static load test. Load settlement curves from numerical method (Approach 2) for PF groups and CDM groups and experimental static load test. Load settlement curves from numerical method (Approach 3) for PF groups and CDM groups and experimental static load test.
Comparative load settlement curves from numerical method (three consti tutive material model approaches) for PF groups and CDM groups and experimental static load test. Load settlement curves from numerical method for PF groups and CDM groups and experimental static load test. Load settlement curves for PF columns and CDM columns from numerical method (Optimal shape design for PF columns). Load settlement curves from numerical method for PF column and CDM column and experimental static load test.
Load settlement curves from numerical method for CDM column, PF column and experimental static load test. Load settlement curves from numerical analyses for CDM column, PF column and experimental static load test. Stress distribution result profiles along the PF column and CDM column .