Tài liệu Analytical and numerical analyses on stiffness enhancement of ground improved by head enlarged cdm columns

VIETNAM NATIONAL UNIVESITY HANOI VIETNAM JAPAN UNIVERSITY HOANG DUY PHUONG ANALYTICAL AND NUMERICAL ANALYSES ON STIFFNESS ENHANCEMENT OF GROUND IMPROVED BY HEAD-ENLARGED CDM COLUMNS MAJOR: INFRASTRUCTURE ENGINEERING CODE: 8900201.04QTD RESEARCH SUPERVISOR Dr. NGUYEN TIEN DUNG MASTER’S THESIS Hanoi, 2020 TABLE OF CONTENTS ABSTRACT ...................................................................................................................... ACKNOWLEDGEMENTS .............................................................................................. LIST OF TABLES ............................................................................................................ LIST OF FIGURES ........................................................................................................... LIST OF ABBREVIATIONS ........................................................................................... CHAPTER I: INTRODUCTION .................................................................................... 1 1.1. General introduction of deep mixing method........................................................... 1 1.2 Necessity of research ................................................................................................. 2 1.3 Objective and Scope of research ........................................................................... 3 1.3.1 Objective of the study .......................................................................................... 3 1.3.2 Scope of the study ................................................................................................ 3 CHAPTER 2: LITERATURE REVIEW......................................................................... 4 2.1 Overview of deep mixing method ............................................................................. 4 2.1.1 Brief view of deep mixing method ....................................................................... 4 2.1.2 Application of CDM ............................................................................................ 6 2.1.3 Classification....................................................................................................... 8 2.1.4 Equipment and machine .................................................................................... 10 2.1.5 Construction procedure .................................................................................... 11 2.1.6 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 PF method .................................................................................................. 15 2.3 Theory of settlement evaluation .............................................................................. 16 2.3.1 The equivalent elastic modulus and 3D settlement of composite grounds ....... 16 2.4 Theory of numerical method ................................................................................... 18 2.4.1 Preliminaries on material modelling ................................................................ 18 2.4.2 Linear elastic model .......................................................................................... 18 2.4.3 Mohr-Coulomb model ....................................................................................... 21 2.4.4 Hardening soil model ........................................................................................ 24 2.4.5 Soft soil model ................................................................................................... 33 CHAPTER 3: METHODOLOGY ................................................................................. 35 3.1 Analysis approaches ................................................................................................ 35 3.2 Analyses using analytical method ........................................................................... 36 3.3 Analyses using numerical method ........................................................................... 38 CHAPTER 4: LABORATORY AND FIELD TEST .................................................... 40 4.1 Introduction of Samse project ................................................................................. 40 4.1.1 General information of project ...................................................................... 40 4.1.2 The PF groups ................................................................................................ 40 4.1.3 Soil profile and footing parameters ............................................................... 41 4.2 Laboratory tests for Samse project .......................................................................... 43 4.3 Static load test on PF groups ................................................................................... 47 4.3.1 The geometry and installation PF groups ...................................................... 47 4.3.2 Installing strain gauges .................................................................................. 48 4.4 Static load test on single PF column........................................................................ 49 4.4.1 Soil profile ..................................................................................................... 49 4.4.2 Footing parameters ........................................................................................ 49 CHAPTER 5: SETTLEMENT ANALYSIS AND RESULTS ..................................... 51 5.1 Settlement analyses using elastic theories ............................................................... 51 5.1.1 Verification analysis.......................................................................................... 51 5.1.2 Analyses for Ideal case and JEF case ............................................................... 52 5.1.3 Results and discussions ..................................................................................... 56 5.2 Settlement analyses using nonlinear models ........................................................... 61 5.2.1 Analyses for ideal case ...................................................................................... 61 5.2.2 Analyses for the experimental single PF column.............................................. 62 5.2.3 Analyses for PF groups at SAMSE project ....................................................... 63 CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS ................................ 71 6.1 Conclusions .......................................................................................................... 71 6.2 Limitations and suggestions ................................................................................. 72 REFERENCES .............................................................................................................. 73 APPENDIX ................................................................................................................... 76 ABSTRACT Point Foundation (PF) method is an advanced technology introduced by EXT Co. Ltd company from Korea, which has more advantages than CDM method. The shape of PF columns makes a big difference in settlement compared to conventional CDM. This study presents a comparative study on stiffness enhancement of grounds improved by Point Foundation method and by the conventional CDM method using analytical and numerical analyses. In addition, the analysis results are compared with experimental program. The stiffness enhancement is evaluated through induced settlement values under four shallow footing cases, of which one is ideally assumed and the other is an actual footing constructed. Results from both analytical analysis and numerical analysis, in which elastic models are used, indicate that in general the PF method produces a more proper stiffness distribution with depth, which in turn results in smaller settlement values. Numerical analysis results also indicate that when only soil area under the footing is improved settlement of the footing is significantly larger than that on ground improved entirely, the case of theoretical elastic soil model. This is attributed to the influence of larger horizontal displacement around the footing. Results from numerical analysis, in which inelastic model used, settlement of shallow footing on PF columns is smaller settlement of conventional CDM columns for the same ground model under certain conditions. By true 3D model of column and soil, when the load-settlement is still in relatively linear range, the settlement values from the equivalent soil model and true 3D column and soil model are relatively equal. This may suggest the equivalent soil model can be used in practice as it has been used in the elastic analyses. PF columns has been analyzed the true behavior between columns and soil (shape of PF column, interaction between column and soil), the results analysis show that when analyzing settlement of shallow footings on PF columns in soft clay, special attention should be paid to the stiffness ratio between PF column and soil. ACKNOWLEDGEMENTS I would like to express my sincere appreciation for the lecturers of Master of Infrastructure Engineering Program for their help during my undergraduate at Vietnam Japan University (VJU). First of all, I am very grateful Dr. Nguyen Tien Dung, who guided me to conduct this thesis for the part one year. He spent a lot of time telling me complicated issues in geotechnical engineering. Not about knowledge, he also taught me valuable lesson about the seriousness and carefulness in scientific research. These valuable lessons will follow me throughout the future study. I would like to acknowledge the sincere inspiration from Prof. Nguyen Dinh Duc and Prof. Hironori Kato. Their lectures covered not only specialist knowledge but also the responsibility and mission of a new generation of Vietnam. I am grateful to Dr. Phan Le Binh for his support in the last two years since I have studied at Vietnam Japan University. Thanks to him, I have learned the professional courtesy of Japanese people as well as Japanese culture. I would also like to acknowledge the staff of Vietnam Japan University, Mr. Bui Hoang Tan for their help and support. I would also like to thank Prof. Junichi Koseki, Assoc. Prof. Kenji Watanabe, Assist Prof. Hiroyuki Kyokawa as well as other members of Koseki lab, where I had 80 meaningful days internship at The University of Tokyo. It was very helpful to me. Special thanks to Associate professor Nguyen Chau Lan, lecturer at University of Transport and Communication. His explanations in geotechnical engineering helped me a lot in this study. His successful way in research encouraged me more than anything else. Thanks to Dr. Nguyen Cong Oanh (Vietnam Academic for Water Resources), he explained in detail the complex problems in finite element method for geotechnical engineering. Finally, Thanks are due to my family, who are always and support me in studies and research. LIST OF TABLES Table 2.1 Typical properties of Stabilized soil (wet method) (Modified from Elias et al. 2006) ................................................................................................................................... 5 Table 2.2 Typical Properties of Lime–Cement Stabilized Soils (Dry Method) (Modified from Elias et al. 2006)......................................................................................................... 5 Table 4.1 Unconfined compression test results ................................................................ 44 Table 4.2 Strength parameters of samples collected from the PF column. ...................... 50 Table 5.1 Input parameters for calibration analysis .......................................................... 52 Table 5.2 Input parameters for settlement analysis ......................................................... 55 Table 5.3 Input parameters for the single PF column ....................................................... 63 Table 5.4 Material model and parameter used for Samse factory project. ....................... 64 LIST OF FIGURES Figure 1.1 Configuration of improved CDM columns: (a) Point foundation (PF) (Nguyen et al. 2019a): (b) T-shape column (Liu et al. 2012) ............................................................ 2 Figure 2.1 Available ground improvement methods for different soil types (modified from Schaefer et al., 2012) .......................................................................................................... 8 Figure 2.2 Classification of deep mixing method (Kitazume & Terashi, 2013) ................ 9 Figure 2.3 Equipment of deep mixing method (DJM machine) ....................................... 10 Figure 2.4 Drilling machine (left) and Mixing shaft and blades of DJM machine (right) 11 Figure 2.5 Machine has two mixing shafts and Binder Plant for DJM method (by the courtesy of Dry Jet Mixing Association) .......................................................................... 11 Figure 2.6 Type of ground improvement (Kitazume & Terashi, 2013) ........................... 12 Figure 2.7 The T-shaped soil cement column overlain by embankment (Song-Yu at el, 2012) ................................................................................................................................. 14 Figure 2.8 Displacement of soil under TDM and SCC (Yaolin et al., 2012) ................... 14 Figure 2.9 Load- settlement curves of conventional DCM and TDM pile from physical model test (Chana Phutthananon et al, 2012) ................................................................... 15 Figure 2.10 Site construction of PF method ..................................................................... 16 Figure 2.11 Flexible rectangular loaded area.................................................................... 16 Figure 2.12 General three demensional coordinate systems and sign convention for stress ........................................................................................................................................... 19 Figure 2.13 Basic ideal of an elastic perfectly plastic model (Plaxis manual) ................. 22 Figure 2.14 The Mohr-Coulomb yield surface in the principal stress space (c=0) .......... 23 Figure 2.15 Hyperbolic stress- strain curve (Ducan & Chang, 1970) .............................. 25 Figure 2.16 Stress circles at yield (Plaxis manual) ........................................................... 26 Figure 2.17 Relationship between initial tangent modulus and confining pressure ......... 27 Figure 2.18 Unloading and reloading of silica sand under drain triaxial test consolidation (Ducan and Chang 1970) .................................................................................................. 28 Figure 2.19 Hyperbolic stress–strain relationship in primary loading for a standard drained triaxial test (Schanz, 1999) ............................................................................................... 30 Figure 2.20 Representation total yield of the HS model in principle space stress for cohesionless soil................................................................................................................ 31 Figure 2.21 Yield surface of hardening model (Schanz et al., 1999) ............................... 32 Figure 3. 1 Configuration of CDM and PF columns ........................................................ 35 Figure 4. 1 Plan view of SAMSE factory project ............................................................. 40 Figure 4. 2 Plan view of three PF groups ......................................................................... 41 Figure 4.3 Shape of PF columns: (Left) Group 1 (LPF=8.5m), (Middle) Group 2 (LPF=6 m); ........................................................................................................................................... 42 Figure 4.4 Soil profile of SAMSE factory project ............................................................ 42 Figure 4.5 Collection of mixed cement- soil samples ...................................................... 45 Figure 4.6 Secant Modulus E50 ......................................................................................... 46 Figure 4.7 Relationship between secant modulus of elasticity and unconfined compressive strength (SAMSE project) ................................................................................................ 46 Figure 4.8 Static load test on instrumented PF groups ..................................................... 47 Figure 4.9 Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate, (c) displacement sensors on steel plate and groundb...................................... 47 Figure 4.10 Strain gauge installation: (a) installation of sensors along the depth of PF, (b) setting up sensors into PF, (c) strain gauge instruments, (d) sensor in PF ....................... 48 Figure 4.11 Soil profile at Songdo site (Kim et al. 2016). ................................................ 49 Figure 4.12 Configuration of the instrumented column (Kim et al. 2016) ....................... 50 Figure 4.13 Instrumentations implemented on variable cross-section soft ground reinforced foundation (Kim et al. 2016)............................................................................................. 50 Figure 5.1 Foundation and soil domain in the numerical analysis ................................... 51 Figure 5.2 Comparison of vertical stress profiles obtained from analytical and numerical analyses ............................................................................................................................. 52 Figure 5.3 Soil profile under the examined footings (Ideal case)..................................... 53 Figure 5.4 Soil profile under the examined footings (JEF project) .................................. 54 Figure 5.5 Cross-sectional and plan views of the examined footing at JFE project......... 55 Figure 5.6 Settlement value from analytical method for Ideal case ................................. 57 Figure 5.7 Settlement value from analytical method for JEF case ................................... 58 Figure 5.8 Variation of Scorr,PF,min/Scorr,CMD ratio................................................................ 58 Figure 5.9 Settlement values from analytical and numerical analyses for Ideal case ...... 59 Figure 5.10 Settlement values from analytical and numerical analyses for JEF project .. 60 Figure 5.11 Settlement analysis from numerical method for ideal case .......................... 61 Figure 5.12 Settlement analysis from numerical method for JEF case ............................ 61 Figure 5.13 Load- settlement curves from MC model ..................................................... 62 Figure 5.14 Load settlement curves from Numerical method for PF column and conventional CDM ............................................................................................................ 63 Figure 5.15 Load settlement curves from Numerical method for PF columns and CDM columns using equivalent material (E50=150qu) ............................................................... 65 Figure 5.16 Load settlement curves from Numerical method for PF groups and CDM groups using true 3D model of PF columns and soil ........................................................ 66 Figure 5.17 Settlement profiles with depth of footings on PF and CDM columns from numerical method using equivalent material model (q = 800 kPa) .................................. 68 Figure 5.18 Load settlement curves from Numerical method for PF columns and conventional CDM columns (Optimal shape design for PF columns) ............................. 68 Figure 5.19 Variation of settlement () and effective vertical stress (v) at the toe of CDM and PF columns obtained from numerical analysis using true 3D model ........................ 69 Figure 5.20 Mohr- Coulomb failure criterion ................................................................... 76 LIST OF ABBREVIATIONS as ascc CDM c (c’) Cc Cs D Dh Df Dt eo Ei Eoed E50 Eu Eur Ec Ecomp Es E’s HCC HS L Lc Lh Lt M MC Ms NC OCR PF PI qu Improvement area ratio Improvement area ratio of conventional CDM column Cement deep mixing method Cohesion strength Compression index Swelling index Diameter of conventional soil cement column (m) Diameter of cap of HCC (m) Embedded depth (m) Diameter of tail of HCC (m) Initial void ratio Initial tangent modulus Oedometric modulus Scant elastic modulus of soil at 50 percent (kPa) Undrained elastic modulus of soil (kPa) Unloading and reloading Young’s modulus Elastic modulus of soil cement column (kPa) Elastic modulus of improved ground (kPa) Elastic modulus of soil (kPa) Young’s modulus in term of effective stress Head-enlarged soil cement column Hardening soil model Length of conventional soil cement column (m) Length of cone of PF column (m) Length of head of PF column (m) Length of tail of PF column (m) Shape factor for Cam clay ellipse/slope of critical state line Mohr Coulomb model Constrained modulus of soil (kPa) Normal consolidation Over consolidation ratio Point foundation Plasticity index (%) Unconfined compressive strength (kPa) q p’ CDM SS model Su TDM   xy z   u ur Deviator stress Mean effective stress Conventional soil cement column Soft soil model Undrained shear strength of soil (kPa) T-shaped deep mixed column Ratio of length of cap and the total length of PF column Ratio of diameter of cap of HCC and diameter of tail of PF column Horizontal stress increment (kPa) Vertical stress increment (kPa) Ratio of distance between two columns and the diameter of column Poisson ratio of soil in drained condition Poisson ratio of soil in undrained condition Poisson ratio for unloading and reloading CHAPTER I: INTRODUCTION 1.1. General introduction of deep mixing method The sustainability of the building depends largely on the foundation of the building (about from 40 to 60% value of project). Therefore, the design of the foundation is an important element in the design work. Under the development of science and technology, there are many foundation design options that are widely used. Nowadays, there are many methods for improving soft grounds such as compaction methods, vertical drains under surcharge and vacuum preloading, vibration methods, deep mixing method, and other miscellaneous methods. Among the methods, deep mixing method (DMM) is widely used as an effective method for ground reinforcement in the world. The method creates cement deep mixing (CDM) columns to increase the stiffness (i.e., to reduce settlement) and to control the stability of embankment or excavations. In general, sub-soils at different places have unique behavior under same loading condition so that finding an optimal solution that satisfies both technical and financial requirements is always an interesting question for geotechnical engineers. The DMM has been extensively used for many types of construction project, for example embankment supports, buildings, earth retaining structures, retrofit and renovation of urban infrastructures, liquefaction hazards mitigation, manmade island construction and seepage control. Deep mixing has been mostly used to improve soft cohesive soils, but it is sometimes used to reduce permeability and mitigate liquefaction of cohesionless soils. Besides the advantages, the conventional CDM column also has some limitations when it is applied to reinforce grounds under shallow foundations. Soil layers in the upper part is often weaker than that in the lower part (deeper layers), however, under shallow footings, introduced stresses are mainly distributed in the depths right below the footing. This combination of natural ground and CDM columns in cases of shallow footings is therefore ineffective. There should be a better shape of CDM columns to reinforce the ground more optimal. 1 1.2 Necessity of research Recognizing the limitations of CDM columns as stated above, EXT Co. Ltd company from Korea have recently introduced an improved type of CDM column and named it “point foundation” (PF). In principle, the PF column has three distinct parts (Fig. 1.1a): The bigger head (upper part), the transitional cone and the smaller tail. The configuration of the PF column and its construction method were introduced in Trung (2019), Nguyen et al. (2019a and 2019 b). Liu et al. (2012) developed special equipment that has foldable augers to install deep mixing (DM) columns at different diameters. Due to the shape of the columns like the letter T, they are named this DM columns T-shaped columns (Fig. 1.1b). Some researches on T-shaped soil cement column (e.g., Yi et al. 2018) showed remarkable results in settlement and lateral movement reduction compared with conventional method. P Linear layer Dh Dt Soil layer i Gravel (bed rock) Depth (m) hi Stress increment ( p) profile Soft soil layer Lt L Lc Lh Df tf tl Fill (a) (b) Figure 1.1 Configuration of improved CDM columns: (a) Point foundation (PF) (Nguyen et al. 2019a): (b) T-shape column (Liu et al. 2012) Although there were some initial researches on PF columns (i.e., Trung 2019, Nguyen et al. 2019), these studies focused on introduction of concept of the method as well as of a simple analytical method to evaluate settlement of soft ground improved by the PF columns. Much of understandings on the PF columns under shallow foundations, such as the influence or nonlinearity of soil and the influence of stiffness of PF columns to the settlement of the foundations are unfolded. It is therefore necessary to have a study to 2 further understand the behavior of PF groups under actual foundation conditions (e.g., actual soil layers, nonlinear characteristics of soil materials). 1.3 Objective and Scope of research 1.3.1 Objective of the study General objective: To evaluate the effectiveness of PF columns over that of CDM columns in reducing settlement of shallow footings on reinforced grounds using both analytical and numerical methods for the same foundation models. Specific objectives: 1. To evaluate the effectiveness when the soil layers are modelled as elastic materials using both analytical and numerical methods. 2. To evaluate the effectiveness when the soil layers are modelled as inelastic materials using numerical method. 3. To evaluate the effectiveness when the treated zone under the footing is modelled as: (i) an equivalent material; (ii) a true 3D model of PF columns and soil. 1.3.2 Scope of the study To obtain the objectives above, this study focuses on the following: 1. Evaluate settlement of shallow footings using classical theory of foundation on elastic materials. For this, settlement of shallow footings on an ideal soil profile and on an actual project soil profile is evaluated using both analytical and numerical methods. 2. Compare predicted and measured settlement values of shallow footings using nonlinear behavior of soil material. For this, settlement of shallow footings on an experimental single PF column and on experimental PF groups is evaluated using numerical method. 3. Compare predicted and measured settlement values of shallow footings on experimental PF groups, in which the treated zone is modelled as an equivalent elastic material and as a true 3D model of PF columns and soil. 3 CHAPTER 2: LITERATURE REVIEW 2.1 Overview of deep mixing method As mentioned in the objective section of this report, this study focuses on reducing the settlement of soft ground. Hence, a critical overview of soil cement column, as well as the behavior of composite ground, will be considered first then the theory of settlement evaluation will be assessed for choosing the appropriate methods for the calculation of this study. In the lieterature, there are many researches on deep mixing method (or cement deep mixed soil column), of which some typical studies are Kitazume and Terashi (2013), Bergado (1996), Rujikiatkamjorn et al. (2005), Chai and Carter (2011), Han (2015), Bruce et al. (2013), Bredenberg et al. (1999), Kirsch and Bell (2012). 2.1.1 Brief view of deep mixing method As mentioned in the introduction, deep mixing method increases the stiffness of ground by mixing in-situ soil with admixture (cement and necessary additives). Mixed soil columns created by deep mixing method has the elastic modulus at 50 percent (E50) of 75 to 1,000 times qu, where qu is the unconfined compressive strength of the column material (Kitazume & Terashi, 2013), but the value is still smaller than that of concrete pile (30,000,000 kPa). Thus, It can be considered that the work of mixed soil columns and surrounding soft soil as composited ground (not pile). Base on this assumption, most previous scholars proposed an equivalent elastic modulus of composited ground for determining the stiffness as well as deformation. The typical properties of stabilized soils are provided in Table 1.1 based on the wet method of deep mixing and Table 1.2 based on the dry method of deep mixing. The effects of different factors on the unconfined compressive strengths of stabilized soils have been discussed above. 4 Table 2.1 Typical properties of Stabilized soil (wet method) (Modified from Elias et al. 2006) Table 2.2 Typical Properties of Lime–Cement Stabilized Soils (Dry Method) (Modified from Elias et al. 2006) 5 2.1.2 Application of CDM Selection of ground improvement method should consider the following conditions: (1) structural conditions, (2) geotechnical conditions, (3) environmental constraints, (4) construction conditions, and (5) reliability and durability. Structural conditions: The structural conditions may include type, shape, and dimension of structure and footing, flexibility and ductility of structural and footing elements, type, magnitude, and distribution of loads, and performance requirements (e.g., total and differential settlements, lateral movement, and factor of safety). Geotechnical conditions: The geotechnical conditions may include geographic landscape, geologic formations, type, location, and thickness of problematic geo-material possible end-bearing stratum, age, composition, distribution of fill, and groundwater table. Soil type and particle size distribution are essential for preliminary selection of ground improvement methods as shown in Figure 2.1. This guideline is suitable for ground improvement methods for foundation support. The thickness and location of problematic geo-material are also important for the selection of ground improvement methods. For example, when a thin problematic geo-material layer exists at a shallow depth, the over excavation and replacement method is one of the most suitable and economic method. When a relatively thick loose cohesionless geo-material layer exists near ground surface, dynamic compaction and vibro-compaction methods are suitable ground improvement methods. When a relatively thick soft cohesive geo-material layer exists near ground surface, preloading and deep mixing methods may be used. When a site needs to be excavated, tieback anchors, soil nails, deep mixed columns, and jet grouted columns may be used. When a site needs to be elevated, geo-synthetic-reinforced slopes and walls can be good choices. The level of groundwater table often affects the selection of ground improvement methods. For example, when deep excavation happens in ground with a high groundwater table, deep mixed column walls may be better than soil nailed walls because they not only can retain the geo-material but also can cut off water flow. Environmental constraint: The environmental constraints may include limited vibration, noise, traffic, water pollution, deformation to existing structures, spoil, and headspace. For example, dynamic compaction induces vibration and noise, which may not be suitable in a 6 residential area. The wet method to construct stone columns by water jetting produces spoil on site, which may be troublesome for a site with limited space. Under such a condition, the dry method may be used instead. Preloading induces settlements at nearby areas, which may be detrimental to existing structures. The selection of a ground improvement method should consider the following construction conditions: (1) site condition, (2) allowed construction time, (3) availability of construction material, (4) availability of construction equipment and qualified contractor, and (5) construction cost. The selection of a ground improvement method must consider whether the site is accessible to its associated construction equipment, such as access road and headspace. Construction time is one of the most important factors for the selection of a ground improvement method. For example, preloading is a cost-effective ground improvement method to improve soft soil; however, it takes time for the soil to consolidate. The use of prefabricated vertical drains can accelerate the rate of consolidation, but sometimes it still may not meet time requirement. As a result, other accelerated ground improvement methods may be used, such as deep mixing and vibro concrete column methods. Most ground improvement methods use specific materials during construction. For example, stone columns and rammed aggregate columns use aggregate. Cement is used for deep mixing and grouting. When natural material is used, such as aggregate or sand, the cost of the material depends on the source of the material and its associated transportation distance. For example, in a mountain area, aggregate is often less expensive; therefore, stone columns or aggregate columns are often a cost-effective solution. In general, the use of locally available material results in more cost-effective ground improvement. To select a ground improvement method, engineers should gather information about possible qualified contractors and their available construction equipment. It is preferable to use a locally available qualified contractor because this will reduce the mobilization cost and the contractor is more familiar with local conditions. Construction cost is always one of the key factors that dominate the selection of a ground improvement method. The construction cost should include mobilization, installation, material, and possible disposal costs. 7 Reliability and Durability Reliability of a ground improvement method depends on several factors, such as the level of establishment, variability of geotechnical and structural conditions, variability of construction material, quality of the contractor, quality of installation, and quality control and assurance. Several researchers have exported that samples from deep mixed columns have a high variability in terms of their unconfined compressive strengths. Automatic or computer-controlled installation processes can reduce the variability of improved geo-materials. The number of well documented successful or failure case histories is also the evidence of the reliability of a specific ground improvement method. Ground improvement methods are used for temporary and permanent structures. For permanent structures, the durability of the construction material should be evaluated or considered in the design. For example, geosynthetics have creep behavior. The corrosion of steel reinforcement with time reduces its thickness. The strength of cement-stabilized soil in seawater degrades with time (Ikegami et al., 2002). Figure 2.1 Available ground improvement methods for different soil types (modified from Schaefer et al., 2012) 2.1.3 Classification The techniques most commonly employed for in-situ deep mixing in Japan can be divided into three groups: mechanical mixing by vertical rotary shafts with mixing blades at the 8 bottom end of each mixing shaft, high pressure injection mixing, and combination of the mechanical mixing and high pressure injection mixing. The various methods in these groups are classified in Figure 5.1. In the mechanical mixing techniques, binder is injected into a ground with relatively low pressure and forcibly mixed with the soil by mixing blades equipped to vertical mixing shaft(s). The binder is used either with powder form (dry method) or slurry form (wet method). The Dry Jet Mixing (DJM) method is the most common dry method of deep mixing and has usually been applied for on-land works (Dry Jet Mixing Association, 2010). Figure 2.2 Classification of deep mixing method (Kitazume & Terashi, 2013) The Cement Deep Mixing (CDM) method, the most common wet method of deep mixing, has frequently been applied for both in-water and on-land works (Cement Deep Mixing Method Association, 1999). In the high pressure injection technique, on the other hand, ground is disturbed by a high pressure jet of water and/or air, while at the same 9