(Luận văn thạc sĩ) experimental and numerical studies on bearing capacity of ground improved by soil cement deep mixing (cdm) columns

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY THIN ZAR EXPERIMENTAL AND NUMERICAL STUDIES ON BEARING CAPACITY OF GROUND IMPROVED BY SOIL CEMENT h 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 h 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 h 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 h 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 Prof 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 h 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 h LIST OF TABLES i LIST OF FIGURES ii LIST OF ABBREVIATIONS v CHAPTER INTRODUCTION 1.1 General introduction of Cement Deep Mixing method 1.2 Problem statement 1.2.1 Load transfer layer 1.2.2 Bearing capacity of shallow footing on Head-enlarged CDM(PF) Column 1.3 Necessity of the study 1.3.1 Load transfer layer 1.3.2 Bearing capacity of shallow footing on Head-enlarged CDM(PF) Column 1.4 Objectives 1.5 Scope of the study 1.6 Structure of thesis CHAPTER LITERATURE REVIEW 2.1 Overview of cement deep mixing method 2.1.1 Brief view of the cement deep mixing method 2.1.2 Application of CDM 10 2.1.3 Classification of CDM 11 2.1.4 Fixed type and floating type improvement 12 2.2 Improvement of conventional CDM method 12 2.2.1 T-shaped soil- cement column 12 2.2.2 The Point Foundation method 14 2.3 Load transfer Mechanisms 15 2.4 Theory of analytical method 16 2.4.1 The settlement of ground improved by CDM columns under 1dimension 16 2.4.2 Calculation of stress according to the ALiCC method 18 2.5 Theory of numerical method 20 2.5.1 Finite element method 20 2.5.2 Material models in PLAXIS 21 CHAPTER METHODOLOGY 25 3.1 The performance of research 25 3.1.1 Methodology of the first objective 26 3.1.2 Methodology of the second objective 28 CHAPTER ANALYSIS AND RESULTS OF CDM GROUPS UNDER ONEDIMENSIONAL LOADING CONDITIONS 31 4.1 Research Purpose 31 4.1.1 A comparative study on analytical and numerical analyses 32 4.1.2 Parametric study 34 h 4.1.3 Case study 40 CHAPTER ANALYSIS AND RESULTS OF HEAD-ENLARGED CDM (PF) GROUP UNDER SHALLOW FOUNDATIONS 43 5.1 Research Purpose 43 5.2 Project Description 43 5.2.1 Introduction of SAMSE Factory project 43 5.3 Samse Factory phase 44 5.3.1 Soil profile 45 5.3.2 Configuration of the PF column groups 47 5.3.3 Static load testing program on PF column groups 48 5.3.4 The geometry of PF column groups 49 5.4 SAMSE Factory phase 50 5.4.1 Soil profile 50 5.4.2 Configuration of the PF groups 51 5.4.3 The geometry of PF column groups 52 5.4.4 Laboratory tests for SAMSE Factory phase and phase 53 5.5 Analyses for PF groups of SAMSE Factory phase 56 5.5.1 Load-settlement analysis 57 5.6 Analyses for PF groups of SAMSE Factory phase 65 5.6.1 Load-settlement analysis 65 5.6.2 Stress Induced analysis along the PF columns and CDM columns 74 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 75 6.1 Conclusions 75 6.2 Recommendations 77 REFERENCES 78 LIST OF TABLES Table 2.1 Typical properties of Stabilized soil (wet method) .9 Table 2.2 Typical Properties of Lime–Cement Stabilized Soils (Dry Method) Table 4.1 Input parameters for the comparative study 33 Table 4.2 Input parameters for the parametric study 35 Table 4.3 Input parameters for Quang Trach project .41 Table 5.1 Unconfined compression test and Equivalent modulus results 55 Table 5.2 Material models and parameters used for approach of SAMSE phase 58 Table 5.3 Material models and parameters used for approach of SAMSE phase 60 Table 5.4 Material models and parameters used for approach of SAMSE phase 62 Table 5.5 Material models and parameters used for SAMSE phase 66 h i LIST OF FIGURES h Figure 1.1 Ground improved by CDM columns with Load transfer layer: (a) Shallow mixing layer; (b) Geo-synthetic reinforcement LTP; (c) Geotextile layer under Embankment Figure 1.2 Configuration of improved CDM columns: (a) T-shape column (Liu et al., 2012); (b) Point foundation (PF) (Nguyen et al., 2019) Figure 2.1 The application of CDM for on-land construction (Kitazume and Terashi, 2013) 10 Figure 2.2 The application of CDM for marine construction (Kitazume and Terashi, 2013) 10 Figure 2.3 Type of column installation (Kitazume and Terashi, 2013) 11 Figure 2.4 Type of ground improvement (a) Fixed type; (b) Floating type (Kitazume and Terashi, 2013) 12 Figure 2.5 The T-shaped soil cement column under embankment (Song-Yu et al.,2012) 14 Figure 2.6 Displacement of soil under TDM and CDM (Yaolin et al., 2012) 14 Figure 2.7 Construction of PF method 15 Figure 2.8 (a) Equal stress-flexible loading versus, (b) equal strain-rigid loading (mo dified after Han, 2015) 16 Figure 2.9 Structure of the load transfer layer from the ALiCC method (modified after ALiCC, 2006) 19 Figure 2.10 Principle of axial symmetric unit cylinder method (Han and Gabr, 2002; Poon and Chan, 2013) 21 Figure 2.11 Principle of D unit cell method (Tan et al., 2008) 21 Figure 2.12 Basic idea of an elastic perfectly plastic model (Plaxis manual) .22 Figure 2.13 Hyperbolic stress–strain relationship in primary loading for a standard dra -ined triaxial test (Schanz, 1999) .24 Figure 3.1 The general flow chart of the research .26 Figure 3.2 Flow chart of the methodology for data analysis of the first objective .27 Figure 3.3 Configurations of PF and CDM columns 29 Figure 3.4 Flow chart of the methodology for data analysis of the second objective 30 Figure 3.5 True 3D model for PF column group under shallow foundation in the num erical method 31 Figure 4.1 Ground profiles in comparative study: (a) Analytical model, (b) Numerical model .33 Figure 4.2 (a) Comparison of total settlement profile; (b) Stress increment profile obt ained from analytical and numerical analyses .34 Figure 4.3 Improved ground of parametric study case 35 Figure 4.4 (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 36 ii h Figure 4.5 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 .37 Figure 4.6 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 .38 Figure 4.7 Influence of improvement area ratio on settlement of the ground .38 Figure 4.8 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 39 Figure 4.9 The cross-sectional view of improved ground and the plan view of the sto rage yard of Quang Trach Thermal Power Plant (TPP) .41 Figure 4.10 (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 43 Figure 5.1 Plan view of SAMSE Factory project 44 Figure 5.2 Soil profiles and parameters from all five bore holes of SAMSE Factory phase 45 Figure 5.3 Soil profiles for the analysis of SAMSE Factory phase 47 Figure 5.4 A cross-sectional view of ground improved by PF column groups for the SAMSE Factory phase .47 Figure 5.5 Plan view of three PF groups for phase 48 Figure 5.6 Shape of PF columns: Group (LPF = 8.5m), Group (LPF = m); Group (LPF = m) 48 Figure 5.7 Static loading test on instrumented PF group 49 Figure 5.8 Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate 49 Figure 5.9 Soil profile for the analysis of SAMSE Factory phase 51 Figure 5.10 A cross-sectional view of ground improved by PF column group for the SAMSE Factory phase .51 Figure 5.11 Plan view of three PF groups for phase 52 Figure 5.12 Shape of PF columns: Group (LPF = 10.5 m), Group (LPF = 8.5 m); Group (LPF = 6.5 m) 52 Figure 5.13 Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate and concrete plate 53 Figure 5.14 (a) Sampling using PVC pipe 54 Figure 5.14 (b) Sampling using attached samplers .54 Figure 5.15 The estimation of equivalent modulus of PF column from UC test result .56 Figure 5.16 Load settlement curves from numerical method (Approach 1) for PF groups and CDM groups and experimental static load test 59 iii Results and discussions Figures 5.20 (a), (b), and (c) show a comparison of load-settlement curves for the footings on PF columns and CDM columns obtained from numerical method and static load test for group 01, group 02, and group 03 As shown in the Figure 5.20, the settlement of footings on PF columns from numerical method is similar to the settlement of the footings on PF columns from the static load test in group 01 and group 02 However, the settlement of the footings on PF columns from numerical method is not close to the settlement of the footings from static load test in group 03 Theoretically, the settlement has to be the largest in the shortest column Thus, there are some errors in the field testing program for group 03 It is clear that the numerical method can closely predict the settlement of footings on PF columns and the static load test results when using reasonable soil profiles and appropriate models for real soil conditions Axial load (kPa) Axial load (kPa) 500 1000 1500 2000 2500 3000 500 1000 1500 2000 2500 0 h Group 01 LPF = 10.5 m q = 2500 kPa 40 20 Settlement (mm) Settlement (mm) 20 40 Group 02 LPF = 8.5 m q = 2291.69 kPa 60 80 60 Static Load test PF column group CDM column group 100 Static load test PF column group CDM column group 120 80 (a) Group 01 (LPF = 10.5 m) (b) Group 02 (LPF = 8.5 m) 67 Axial load (kPa) 500 1000 1500 2000 2500 20 Settlement (mm) 40 60 Group 03 LPF = 6.5 m q = 2083.33 kPa 80 100 120 140 160 180 Static load test PF column group CDM column group 200 (c) Group 03 (LPF = 6.5 m) Figure 5.20 Load settlement curves from numerical method for PF groups and CDM groups and experimental static load test Figures 5.20 (a), (b), and (c) indicate that the settlement of footings on PF columns is slightly smaller than that on CDM columns in groups 01, 02 and 03 It can be seen that the effectiveness of PF column is not significant in all groups Thus, it is necessary to find out why the settlement of footings on PF columns is slightly smaller than that on h CDM columns.There are possible reasons why the settlement of footings on PF columns is slightly smaller than the settlement of footings on CDM columns Possible reasons and Proof In this case, the length of the PF column head is 1.0 m,  = Lh/ L = 1/10.5 = 0.09 for group 01 (LPF = 10.5 m),  = Lh/ L = 1/8.5 = 0.12 for group 02 (LPF = 8.0 m), and  = Lh/ L = 1/6.5 = 0.15 for group 03 (LPF = 6.0 m)  is the ratio of the head length and total length of PF column  value is too small (nearly 0.1) for footings on PF columns, and the diameter of the head section of PF columns is slightly larger than that of CDM columns These magnitudes can’t make a significant difference in the settlement of footings on PF columns and CDM columns In addition, the head section of PF columns for three groups is located in the upper fill layer above very soft sandy clay layer In this case, the shallow mixing layer was constructed above the PF column groups and CDM column groups The SM layer and PF/CDM column have same stiffness value (ESM = 750 MPa) In this case, the difference in stiffness between columns and 68 the soil (Ec/Es) is approximately 100 Thus, the stiffness value of SM layer and Ec/Es value is too high If Ec/Es value is much larger than 20, the column would act as a pile These reasons make an insignificant difference in the settlement of footings on PF columns and CDM columns Therefore, the  value of PF column, the stiffness value of SM layer, the stiffness ratio of the columns to soil were changed in PF column group 01 and 02 as follows (1) The length of PF column head was extended from Lh = 1.0 m, Lc = 1.0 m, and Lt = 8.5 m (i.e.,  = Lh/L = 1/10.5 = 0.09) to Lh = 6.0 m, Lc = 1.0 m, and Lt = 3.5 m (i.e.,  = 0.57) for group 01 For group 02, the length of PF column head was extended from Lh = 1.0 m, Lc = 1.0 m, and Lt = 6.5 m (i.e.,  = 1/8.5 = 0.12) to have Lh = 5.0 m, Lc = 1.0 m, and Lt = 2.5 m (i.e.,  = 0.58) (2) The stiffness ratio of the PF/CDM columns to soil was changed from Ec/Es = 100 in average to Ec/Es = 20 (3) The stiffness of the SM layer was changed from ESM = 750 MPa to ESM = 150 MPa h As shown in Figures 5.21 (a), and (b), the settlement of the footings on PF columns is smaller than (approximately 25%) that on CDM columns These results indicate that the length of PF column head should be long enough to cover the soft soil layer, and the stiffness of SM layer as well as PF/CDM column should be 10 to 20 times the stiffness of the surrounding soil layer Note that common stiffness ratio of 10 to 20 are found from many case studies (Kitazume and Terashi, 2013) Axial load (kPa) Axial load (kPa) 500 1000 1500 2000 2500 3000 40 60 80 100 120 140 1000 1500 2000 2500 20 Group 01 LPF = 10.5 m q = 2500 kPa 40 Settlement (mm) Settlement (mm) 20 500 0 Group 02 LPF = 8.5 m q = 2291.69 kPa 60 80 100 120 PF column group CDM column group 140 PF column group CDM column group 160 69 (a) Group 01 (LPF = 10.5 m) (b) Group 02 (LPF = 8.5 m) Figure 5.21 Load settlement curves for PF columns and CDM columns from numerical method (Optimal shape design for PF columns) Influence of the length of the PF column head As shown in Figure 5.22, the length of PF column head for group 01 was extended from Lh = 1.0 m, Lc = 1.0 m, and Lt = 8.5 m (i.e.,  = Lh/L = 1/10.5 = 0.09) to Lh = 2.0 m, Lc = 1.0 m, and Lt = 7.5 m (i.e.,  = 0.19); Lh = 3.0 m, Lc = 1.0 m, and Lt = 6.5 m (i.e.,  = 0.28); Lh = 4.0 m, Lc = 1.0 m, and Lt = 5.5 m (i.e.,  = 0.38); and Lh = 6.0 m, Lc = 1.0 m, and Lt = 3.5 m (i.e.,  = 0.57) For group 02, the length of PF column head was extended from Lh = 1.0 m, Lc = 1.0 m, and Lt = 6.5 m (i.e.,  = Lh/L = 1/8.5 = 0.12) to Lh = 2.0 m, Lc = 1.0 m, and Lt = 5.5 m (i.e.,  = 0.23); Lh = 3.0 m, Lc = 1.0 m, and Lt = 4.5 m (i.e.,  = 0.35); Lh = 4.0 m, Lc = 1.0 m, and Lt = 3.5 m (i.e.,  = 0.47); and Lh = 5.0 m, Lc = 1.0 m, and Lt = 2.5 m (i.e.,  = 0.58) h For group 03, the length of PF column head was extended from Lh = 1.0 m, Lc = 1.0 m, and Lt = 4.5 m (i.e.,  = Lh/L = 1/6.5 = 0.15) to Lh = 2.0 m, Lc = 1.0 m, and Lt = 3.5 m (i.e.,  = 0.31); Lh = 4.0 m, Lc = 1.0 m, and Lt = 1.5 m (i.e.,  = 0.62) Figures 5.22 (a), (b), and (c) indicate that when the length of PF column head increases, the settlement of the footings on PF columns decreases As expected, the settlement result of the footings on PF columns is smaller than (approximately 22%) that of CDM columns when  value is in the range of 0.4 to 0.6 70 Axial load (kPa) 500 1000 1500 Axial load (kPa) 2000 2500 3000 0 40 60 20 Group 01 LPF = 10.5 m q = 2500 kPa Settlement (mm) Settlement (mm) 20 Static Load test CDM column group PF column group, a = Lh/L = 1/10.5 = 0.09 PF column group, a = Lh/L = 2/10.5 = 0.19 PF column group, a = Lh/L = 3/10.5 = 0.28 PF column group, a = Lh/L = 4/10.5 = 0.38 80 500 1000 1500 2000 2500 PF column group, a = Lh/L = 6/10.5 = 0.57 40 Group 02 LPF = 8.5 m q = 2091.69 kPa 60 80 Static load test CDM column group PF column group, a = Lh/L = 1/8.5 = 0.12 100 PF column group, a = Lh/L = 3/8.5 = 0.35 120 (a) Group 01 (LPF = 10.5 m) PF column group, a = Lh/L = 2/8.5 = 0.23 PF column group, a = Lh/L = 4/8.5 = 0.47 PF column group, a = Lh/L = 5/8.5 = 0.58 (b) Group 02 (LPF = 8.5 m) Axial load (kPa) 500 1000 1500 2000 2500 Group 03 LPF = 6.5 m q = 2083.33 kPa 100 h Settlement (mm) 50 150 Static loadd test CDM column group PF column group, a = Lh/L = 1/6.5= 0.15 PF column group, a = Lh/L = 2/6.5 = 0.31 200 PF column group, a = Lh/L = 4/6.5 = 0.62 (c) Group 03 (LPF = 6.5 m) Figure 5.22 Load settlement curves from numerical method for PF column and CDM column and experimental static load test Influence of the thickness of SM layer on settlement of PF column group The 0.3 m thick of shallow mixing (SM) layer was constructed above the PF columns and CDM columns in all groups of phase The influence of the thickness of the SM layer on the settlement of the footings on PF columns was investigated by varying the thickness (tSM) from 0.2 m to 0.5 m and keeping the constant stiffness of ESM = 750 MPa 71 Axial load (kPa) Axial load (kPa) 0 100 200 300 400 500 600 700 800 900 1000 Settlement (mm) Settlement (mm) 20 20 Group 01 LPF = 10.5 m ESM = 750 MPa q = 900 kPa 40 60 Static load test CDM column group, tSM= 0.3 m PF column group, tSM= 0.3 m 80 40 Group 02 LPF = 8.5 m ESM = 750 MPa q = 825 kPa 60 80 100 PF column group, tSM = 0.2 m 100 200 300 400 500 600 700 800 900 1000 0 Static load test CDM column group, tSM = 0.3 m PF column group, tSM = 0.3 m PF column group, tSM = 0.2 m PF column group, tSM = 0.5 m 120 (a) Group 01 (LPF = 10.5 m) PF column group, tSM = 0.5 m (b) Group 02 (LPF = 8.5 m) Axial load (kPa) 100 200 300 400 500 600 700 800 900 1000 20 60 80 Group 03 LPF = 6.5 m ESM = 750 MPa q = 750 kPa 100 120 h Settlement (mm) 40 140 Static load test CDM column group, tSM = 0.3 m 160 PF column group, tSM = 0.3 m 180 PF column group, tSM = 0.2 m 200 PF column group, tSM = 0.5 m (c) Group 03 (LPF = 6.5 m) Figure 5.23 Load settlement curves from numerical method for CDM column, PF column and experimental static load test Figures 5.23 (a), (b), and (c) show the settlement results of footings on CDM columns and PF columns with the effect of various thickness of the SM layer obtained from numerical method and static load test results As expected, the settlement of footings on PF columns decreases approximately 13% to 16% with the increase in thickness of SM layer from 0.2 to 0.5 m, as shown in Figure 5.23 (a), (b), and (c) The settlement of the footings on PF columns under the 0.5 m thickness of SM layer from numerical method is very similar to the settlement of the footings on PF columns from the static load test in group 01 and group 02 72 Influence of the stiffness of SM layer on settlement of PF column group In all groups of phase 2, the stiffness value of the shallow mixing (SM) layer above the columns is 750 MPa, which is the same as the stiffness value of PF and CDM columns The influence of the stiffness of the SM layer on the settlement of the footings on PF columns was investigated by varying the stiffness (ESM) from 100 MPa to 750 MPa and keeping the constant thickness of tSM = 0.3 m Figures 5.24 (a), (b), and (c) show the settlement results of footings on CDM columns and PF columns with the effect of various stiffness of the SM layer obtained from numerical analyses and static load test results As expected, the settlement of footings on PF columns decreases (approximately 5%) with the increase in stiffness of SM layer from 100 MPa to 750 MPa, as shown in Figure 5.24 Axial load (kPa) Axial load (kPa) 0 100 200 300 400 500 600 700 800 900 1000 40 60 Static load test CDM column group, E SM = 750 MPa PF column group, E SM = 750 MPa 40 Group 02 LPF = 8.5 m tsm = 0.3 m q = 825 kPa 60 80 100 PF column group, E SM = 300 MPa PF column group, E SM = 100 MPa 120 (a) Group 01 (LPF = 10.5 m) Static load test CDM column group, Esm = 750 MPa PF column group, Esm = 750 MPa PF column group, Esm = 300 MPa PF column group, Esm = 100 MPa (b) Group 02 (LPF = 8.5 m) Axial load (kPa) 100 200 300 400 500 600 700 800 900 1000 50 Settlement (mm) 80 Settlement (mm) 20 Group 01 LPF = 10.5 m tSM = 0.3 m q = 900 kPa h Settlement (mm) 20 100 200 300 400 500 600 700 800 900 1000 0 Group 03 LPF = 6.5 m tSM = 0.3 m q = 750 kPa 100 150 Static load test CDM column group, ESM = 750 MPa PF column group, ESM = 750 MPa PF column group, ESM = 300 MPa 200 PF column group, ESM = 100 MPa 73 (c) Group 03 (LPF = 6.5 m) Figure 5.24 Load settlement curves from numerical analyses for CDM column, PF column and experimental static load test 5.6.2 Stress Induced analysis along the PF columns and CDM columns Although the strain gauge is installed along the column, the test results were not so good Therefore, the stress distribution results along the column from the field test were not performed in this research Figures 5.25 (b), (c), and (d) illustrate the total stress profiles with depth extracted at the center of the PF column and CDM column of groups 01, 02, and 03 from the numerical analysis Under the same tail diameter and same loading conditions, the maximum stress zone (peak point) of the PF column is located at the depth of around 2.6 m, very near the bottom of the PF cone section In the CDM column, the maximum stress zone is located at the depth of around 0.8 m, near the surface layer (SM layer) Actually, the peak point section is very problematic in column design If the maximum stress is over the compressive strength of the column, the columns could be broken or cracked locally due to stress concentration h Analysis results from this case study show that the total stress in the depth of 2.6 m of PF column and 0.8 m of CDM column in all groups was not over the compressive strength of the column (qu = 2600 kPa), indicating that the column would not have locally failed in this case However, the bottom of the PF cone section and the place that is near the surface layer for the CDM column should be strictly controlled due to the peak point Figure 5.25 (a) shows the location of the maximum stress zone (peak point) of the PF column and CDM column Total stress (kPa) 500 1000 1500 2000 2500 0.3 Depth (m) 2.3 4.3 Level of cone bottom = 2.3 m Group 01 LPF= 10.5 m q = 2500 kPa qu= 2600 kPa 6.3 8.3 10.3 Along PF column Along CDM column Col 56 vs Col 57 74 (a)Peak point location for CDM and PF column (b) Group 01(LPF = 10.5m) Total stress (kPa) Total stress (kPa) 500 1000 1500 2000 2500 4.3 1000 1500 2000 2500 Level of cone bottom = 2.3 m Group 02 LPF = 8.5 m q = 2292 kPa qu = 2600 kPa 2.3 Depth (m) Depth (m) 2.3 500 0.3 0.3 Level of cone bottom = 2.3 m Group 03 LPF = 6.5 m q = 2083.33 kPa qu= 2600 kPa 4.3 6.3 Along PF column Along CDM column Level of cone bottom 8.3 (c) Group 02 (LPF = 8.5 m) 6.3 Along PF column Along CDM column Level of cone bottom (d) Group 03 (LPF = 6.5 m) Figure 5.25 Stress distribution result profiles along the PF column and CDM column h CHAPTER CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions This research focused on two key objectives The first one is to study the influence of thickness and stiffness of the load transfer (SM) layer on stress induced on column heads and settlement of the improved ground for CDM groups under 1D loading conditions by using numerical and analytical analyses The second objective is to compare load-settlement curves obtained from numerical method and field load tests on the PF and CDM column groups under shallow foundations with the same stiffness 75 and same column diameter Based on the results obtained from two key objectives, some key findings from the study can be drawn as follows For the 1st objective When the thickness of the shallow mixing (SM) layer increases from 0.4 to 1.0 m, the settlement on top of the compacted layer decreases from 29.9 mm to 29.0 mm (with a difference of 0.9 mm, or approximately 3%), and the stress induced on the column head increases from 305 kPa to 367 kPa (with a difference of 62 kPa, or approximately 17%) This indicates that the increase in thickness of the SM layer has little influence on the settlement but large influence on induced stress on the column head When the stiffness of the shallow mixing (SM) layer increases from 100 MPa to 400 MPa, the settlement decreases from 30.5 to 29.9 mm (with a small gap of 0.6 mm, or approximately 2%), and induced stress increases from 268 kPa to 316 kPa (with a difference of 48 kPa, or approximately 15%) Similar to the variation of thickness, these results indicate that the increase in stiffness results h in insignificant decrease in settlement but significant increase in stress induced on the column heads The stress induced along the column is almost constant with the variation of the thickness and stiffness When the improvement area ratio (as) increases from 19.6% to 44.2%, the settlement on top of the compacted layer decreases from 30.0 to 22.0 mm (with a difference of mm, or approximately 27%), and the stress induced on head of the columns decreases from 306 kPa to 165 kPa (with a difference of 141 kPa, or approximately 46%) This behavior is attributed to the fact that, under the constant applied load and unchanged SM layer, the larger columns result in smaller induced stress on head of and along the columns An important finding from this study is that the maximum stress induced in the CDM column is typically not on head of the columns but at the middle depths where soil layers are softer than the SM and the bearing layer at the column toes This finding was consistently indicated both from typical soil profile of 76 the parametric studies and from the actual soil profile of the project case in Vietnam For the 2nd objective The load-settlement profile obtained from the numerical method with appropriate soil models, boundary conditions, and mesh refinement selected in this study is similar to that obtained from the experimental static load test Under the following conditions: (i) the length of the PF column head is not long enough to cover the soft soil layer ( = Lh/L = 0.1); (ii) PF columns and CDM columns have the same stiffness and diameter; (iii) the shallow mixing layer above the PF columns has higher stiffness; and (iv) the ratio of stiffness of PF or CDM columns over that of the surrounding soil is much larger than 20, the effectiveness of PF columns is not significant When  value is in the range of 0.4 to 0.6, the settlement of the footings on PF columns is smaller than (approximately 22%) that of CDM columns Therefore, the length of the PF column head should be long enough to cover the soft soil h layer to get the most significant effect of the PF column When the thickness of the shallow mixing layer increases from 0.2 to 0.5 m, the settlement of the footings on PF columns decreases approximately 13% to 16% However, when the stiffness of the SM layer increases from 100 MPa to 750 MPa, the settlement of the footings on PF columns decreases approximately 5% The key finding from this study is that the maximum stress zone (peak point) of the PF column is located near the level of PF cone bottom The maximum stress zone in CDM column is located near the surface layer (SM layer) Actually, the peak point sections are very problematic in column design Hence, in many cases, when using PF columns, the designer should pay attention to the end of the cone section of the PF column as the columns may be locally failed at this section due to large stress concentration 6.2 Recommendations 77 The followings are some recommendations from this study Due to the time limitations, the author did not analyze the settlement of footings on the PF column and CDM column groups under shallow foundations using the analytical method Thus, it is recommended to analyze the settlement of footings on the PF and CDM column groups using the analytical method The author did not analyze the stress induced in the PF column group using the 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