INTRODUCTION
General introduction of Cement Deep Mixing method
The foundation design is crucial for the construction of structures, heavily influenced by geotechnical conditions Modern techniques address geotechnical challenges, enhancing soil foundation strength and minimizing settlements Ground improvement methods include vacuum consolidation, granular column reinforcement, geosynthetics, prefabricated vertical drains, deep mixing, premixing, and lightweight treated soil Notably, the deep mixing technique has evolved since the 1970s, significantly contributing to foundation stability and reliability.
In recent years, the Cement Deep Mixing (CDM) method has emerged as an effective solution for improving soft ground globally, addressing issues such as excessive settlement, high water content, and vibration This innovative technique enhances soil stiffness and embankment stability while also mitigating liquefaction and providing necessary lateral support However, it is important to note that under embankment loads, CDM columns may experience collapse due to external or internal failures.
In 2007, Kitazume and Maruyama noted that structural columns do not fail simultaneously; rather, they fail sequentially at different times This individual failure prevents the collective strength and stiffness of all columns from being fully utilized at once.
To enhance the workability of improved ground, a load transfer layer, such as a shallow mixing (SM) layer or a combination of aggregates and geogrid, is typically constructed atop CDM columns This design effectively distributes the load to the enhanced ground, allowing the columns to function together and improving the stability of the column system Consequently, this setup minimizes differential settlement between the columns and the surrounding soil However, the load transfer layer, often stiffer than the underlying soil, can result in the CDM columns experiencing greater stress compared to the adjacent soil.
Problem statement
A load transfer layer, such as a shallow mixing (SM) layer or a compacted aggregate layer with geosynthetics, is typically designed on top of CDM columns to effectively distribute the applied load to the ground system, ensuring balanced settlement of both the soil and the columns This process is illustrated in Figures 1.1 (a), (b), and (c), which depict ground improvement achieved through CDM columns with different load transfer layer types.
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
In structural design, the local bearing capacity of individual columns is crucial, particularly when incorporating a load transfer layer If the stress in a column surpasses its material's compressive strength, it may lead to local failure, even if the overall ground remains stable Therefore, it is essential to meticulously control the stress levels in the columns Various methods, such as the ALiCC method and British Standard, have been discussed in the literature to address these concerns.
Several methods, including BS 8006, the German method (EBGEO), the Guido method, and the Low method, have been developed to estimate stress on column heads Among these, the ALiCC method uniquely considers the stress induced on columns beneath shallow mixing layers, while the others focus on stress calculations for columns with various geosynthetic embedded layers Nonetheless, the ALiCC method has limitations in practical applications, as it does not account for the stiffness of the shallow mixing layer or the stiffness ratio between the column and the surrounding soil (E c /E s) Additionally, it only estimates the stress at the top of the column and the adjacent soil.
1.2.2 Bearing capacity of shallow footing on Head-enlarged CDM (PF) Column
Conventional Controlled Density Mix (CDM) columns face limitations when enhancing soil beneath shallow foundations, as their uniform diameter fails to address weaker layers effectively To overcome these challenges, innovative CDM column techniques have emerged, such as T-shaped columns (Liu et al., 2012) and Point foundations (PF) (Nguyen et al.).
Figure 1.2 Configuration of improved CDM columns: (a) T-shape column (Liu et al.,
2012); (b) Point foundation (PF) (Nguyen et al., 2019)
Initial studies on PF columns, including works by Nguyen (2019), Nguyen et al (2019), and Hoang (2020), have introduced the concept and assessed the performance of PF columns in improving the settlement of soft ground These studies utilized analytical methods to compare the effectiveness of PF columns with conventional CDM columns, taking into account identical volumes, soil profiles, and varying diameters.
In a study conducted by Hoang (2020), the settlement behavior of footings on prefabricated (PF) columns was assessed against conventional concrete deep mixed (CDM) columns, maintaining consistent volume, soil profiles, and column count (with three columns per group), while varying diameters The findings from the numerical analysis of both PF and CDM columns were then compared to experimental results from static load tests, highlighting the performance differences between the two column types.
Current research lacks a comprehensive analysis of soil behavior and PF columns, necessitating a true 3D model to accurately represent the interaction between PF columns and soil Additionally, there has been insufficient examination of the stress distribution along both PF and CDM columns.
Necessity of the study
The Continuous Deep Mixing (CDM) method is widely used for ground improvement projects in Vietnam, Myanmar, and beyond However, it is crucial for geotechnical engineers to thoroughly investigate the maximum stress distribution within CDM columns and the settlement behavior of the enhanced ground, considering the influence of the load transfer layer.
The current method, ALiCC, has limitations as it is only capable of estimating the stress experienced at the top of the column and the surrounding soil.
Studying the impact of the load transfer layer (SM layer) on the settlement of improved ground and the stress induced in both columns and soil is essential This analysis can be conducted through both analytical and numerical methods, allowing for the consideration of various aspects from the numerical analysis.
1.3.2 Bearing capacity of shallow footing on Head-enlarged CDM (PF) Column
Geotechnical engineers must comprehend the behavior of PF columns beneath shallow foundations, particularly regarding how the stiffness and diameter of these columns, as well as the soil's nonlinearity, affect settlement and bearing capacity Despite the global implementation of ground improvement projects by Head-enlarged CDM (PF) groups, a thorough understanding of these factors is essential for optimizing foundation performance.
To evaluate the effectiveness of PF columns in comparison to CDM columns with identical stiffness and diameter, it is essential to analyze the load-settlement behavior and stress distribution along the PF columns This can be achieved through numerical analyses using a true 3D model, complemented by experimental static load tests.
Objectives
1 To evaluate the effect of the load transfer layer on the settlement of the ground and on stress induced in the CDM columns and soil
2 To compare behavior of load-settlement curves from numerical and experimental static load tests on Heard-enlarged CDM columns group.
Scope of the study
1 Review the analytical methods and numerical method to evaluate stress induced in the columns and settlement of grounds improved by CDM columns and PF columns
2 Perform parametric numerical studies on settlement of the improved ground and stress induced on head of as well as along the CDM columns and soil under 1D conditions with different conditions of the load transfer layer
3 Simulate some static load test on groups of Heard-enlarged CDM columns under shallow foundations, which were experimentally carried out, and compare the load-settlement curves obtained from numerical analyses and the experimental program, and calculate the stress induced in CDM columns and
Structure of thesis
In this research, six chapters were performed as follows:
This chapter mentions a general introduction about the problem statement, the necessity of the research, objectives, and scope of this research
This chapter provides a comprehensive literature review on the cement deep mixing method, focusing on both analytical and numerical approaches, particularly the Finite Element Method (FEM) It explores the theoretical foundations of these methods for calculating settlement and stress induced under one-dimensional conditions.
This chapter shows the methodology flowcharts for the settlement and stress induced analysis for two key objectives of this research
Chapter 4: Analysis and Results of CDM Groups under One-Dimensional Loading Conditions
This chapter discusses the input parameters, analysis, and findings regarding the impact of the load transfer layer (SM layer) on the settlement of the enhanced ground, as well as the stress generated in the CDM columns and surrounding soil.
Chapter 5: Analysis and Results of Head-Enlarged CDM (PF) Group under Shallow Foundations
This chapter presents required information about field tests, laboratory tests, input parameters, analysis, and results for the comparison of the load-settlement curves of
This study evaluates the effectiveness of PF columns in comparison to CDM columns through a combination of numerical methods, experimental static load tests, and stress-induced analyses The findings highlight the performance differences between the two column types, providing valuable insights into their structural efficiency under load conditions.
This chapter mentions the specific conclusions and recommendations of this research.
LITERATURE REVIEW
Overview of cement deep mixing method
This research investigates the impact of the load transfer layer on stress-induced settlement in ground improved by Cement Deep Mixing (CDM) columns under one-dimensional loading conditions It also evaluates the effectiveness of Head-enlarged CDM column groups beneath shallow foundations Numerous studies have explored the deep mixing method, with notable contributions from researchers such as Bergado et al (1996), Bredenberg et al (1999), Bruce et al (2013), Chai and Carter (2011), Han (2015), Kirsch and Bell (2012), Kitazume and Terashi (2013), and Rujikiatkamjorn et al (2005).
2.1.1 Brief view of the Cement Deep Mixing method
The Cement Deep Mixing (CDM) method aims to improve ground stiffness by blending in-situ soil with cement and additives The elastic modulus of CDM columns, which ranges from 75 to 1,000 times the unconfined compressive strength (q u) of the column material, is about 50% of E 50 However, this modulus is significantly lower than that of concrete piles, which is approximately 30,000,000 kPa, leading to the classification of CDM columns as a composite ground rather than traditional piles Previous research has suggested an equivalent elastic modulus for the composite ground to assess stiffness and deformation The influence of various factors on the unconfined compressive strengths of stabilized soils is detailed in Tables 2.1 and 2.2, which outline the typical properties of stabilized soils using both wet and dry deep mixing methods.
Table 2.1 Typical properties of Stabilized soil (wet method)
Unconfined compressive strength, q u Up to 1.2 MPa for organic and very plastic clays, sludges 0.4-1.5 MPa for soft clays 0.7-2.5 MPa for medium/ hard clays 1.0-3.0 MPa for silts
1.5-5.0 MPa for fine-medium sands
Source: Modified from Elias et al., 2006
Table 2.2 Typical Properties of Lime–Cement Stabilized Soils (Dry Method)
Undrained shear strength (10-50) c u of soil (0.15-1.0 MPa)
Young’s modulus (50-200) c u of lime-cement column
Permeability (lime-cement) About the same as for in situ soil
Permeability (lime) 10-100 x in situ soil permeability
Source: Modified from Elias et al., 2006
In recent years, the Cement Deep Mixing (CDM) method has gained prominence for enhancing soft soil under embankments, with varied applications tailored to specific needs For instance, Nordic countries utilize CDM to minimize settlement, while Japan focuses on bolstering the stability of port facilities Although the underlying principle of CDM remains consistent across these regions, the applications differ significantly This versatile method is extensively employed in both terrestrial and marine construction projects.
Figure 2.1 The application of CDM for on-land construction (Kitazume and Terashi,
Figure 2.2 The application of CDM for marine construction (Kitazume and Terashi,
CDM columns are arranged in four distinct patterns: individual, block, panel or wall, and grid Individual columns are effective for reducing settlement and enhancing bearing capacity when the area replacement ratio is below 50% In contrast, a block pattern is utilized for area replacement ratios exceeding 50% to support substantial vertical and horizontal loads, improving the stability of large marine structures and preventing hazardous chemical leaching The panel or wall pattern serves multiple purposes, including acting as a curtain wall for waste containment, a seepage wall for intercepting water flow, and a retaining wall for lateral support, thus bolstering embankment stability The grid pattern, suitable for wall and block types, effectively mitigates liquefaction in sandy soils Additionally, group columns consist of isolated stabilized soil columns arranged in rectangular or triangular rows, widely used to reduce settlement and enhance the stability of low embankments and lightweight structures.
Figure 2.3 Type of column installation (Kitazume and Terashi, 2013)
The Cement Deep Mixing (CDM) method encompasses three techniques: mechanical mixing (both wet and dry methods), high-pressure injection mixing (wet method), and a combination of mechanical and high-pressure injection mixing The wet method involves mixing in situ natural soil with cement or slurries using machinery, allowing for the creation of homogeneous columns In contrast, the dry method utilizes powder binders to form columns or panels, resulting in improved soil with lower water content, reduced binder usage, and generally higher strength However, the wet method is not suitable for conditions with high water content.
2.1.4 Fixed type and floating type improvement
Cement deep-mixing (CDM) ground improvement methods are categorized into two types: fixed and floating, depending on the stiffness of the soil layers they penetrate The fixed type involves CDM columns reaching the stiff soil layer, while the floating type does not fully penetrate it, aiming to manage the equilibrium deformation of structures on soft ground Both methods are effective in minimizing excessive settlement during construction on soft ground However, in bridge construction, reaching the stiff layer with CDM columns can lead to significant differential settlement between adjacent embankments or roads and the bridge structure.
Figure 2.4 Type of ground improvement (a) Fixed type; (b) Floating type (Kitazume and Terashi, 2013)
Improvement of conventional CDM method
In 2012, Yaolin et al introduced a T-shaped deep mixing (TDM) column, which features a unique "T" design comprising a larger diameter cap at shallow depths and a smaller diameter tail at greater depths to meet stiffness requirements Research by Bergado et al (1999) demonstrated that surrounding soft soil experiences greater settlement than mixed soil columns, leading to potential instability in embankments and damage to overlying pavements While geosynthetic reinforcement or compacted granular materials can enhance stability, they may increase costs The TDM column effectively addresses differential settlement by reducing the stress concentration ratio, offering a viable solution to these challenges Figure 2.5 illustrates the T-shaped soil-cement column beneath an embankment, as noted by Song-Yu et al (2012).
Figure 2.6 illustrates the advantages of TDM by highlighting the differences in vertical and lateral displacement between TDM and CDM TDM has been effectively utilized in China to enhance soft ground conditions beneath embankments during highway construction Typically, the diameter of column caps ranges from 0.9 to 1.2 meters, while deep-depth columns have diameters between 1.3 and 2.4 meters The length of the columns varies from 11 to 25 meters (Yaolin et al., 2012).
Figure 2.5 The T-shaped soil cement column under embankment (Song-Yu et al.,2012)
(a) Vertical displacement (b) Lateral displacement Figure 2.6 Displacement of soil under TDM and CDM (Yaolin et al., 2012)
The point foundation method is an innovative ground improvement technique that enhances soil strength by combining native soils with an eco-friendly binder called "Bindearth." Introduced by EXT Company in Korea in 2012, this method has seen widespread application across the country since 2014 (Nguyen et al., 2019).
The mixed soil column method offers two key advantages: first, its funnel-like shape, comprising a head, cone, and tail, enhances structural efficiency; second, the binder used in the PF method boasts a compressive strength that is 1.5 to 2 times greater than that of conventional cement, making it a more environmentally friendly option for mixing with in-situ soils.
The patented PF method (Korea No 10-1441929, US No US 9,546,465 B2, China No CN 104411891 B) has garnered multiple certificates for its environmentally friendly practices and advanced technology from various professional organizations and ministries in Korea This innovative method is utilized for improving soft ground in various applications, including industrial structures, roads, underground constructions, earth retaining walls, and water barrier walls Notably, it serves effectively as a pile foundation for low-rise buildings, accommodating a maximum applied pressure of 300 kPa, and is ideal for transporting lightweight structures.
A PF column is structured with two primary components: the upper head and the lower tail, connected by a tapered cone The upper head, featuring a larger diameter than the tail, is designed to improve load transfer mechanisms compared to traditional CDM columns Typically, the length of the head section matches that of the cone, both measuring 1 meter The diameter of the head ranges from 1.2 to 1.4 meters, while the tail measures between 0.6 and 0.8 meters.
(a) (b) Figure 2.7 Construction of PF method
Load transfer Mechanisms
In geotechnical literature, two extreme load conditions are identified: equal stress and equal strain The equal stress condition occurs when the improved soil layer experiences ideally flexible loading, while the equal strain condition arises under ideally rigid loading The stress concentration ratio (n) is defined as the ratio of stress on the column (q c ) to that on the soil (q s ) In the equal stress scenario, n equals 1.0, resulting in greater settlement of the soil compared to the columns (S s > S c ) Conversely, in the equal strain scenario, n is greater than 1.0, leading to equal settlement of both the columns and the soil (S s = S c ) (Han, 2015).
Figure 2.8 (a) Equal stress-flexible loading versus, (b) equal strain-rigid loading
When the improved layer is under 1D equal strain condition, the strain can be calculated as follows (Han, 2015): c s z c s q q
(2.1) where 𝜀 z is vertical strain at a depth of z, M c and M s are constrained moduli of the column and the soil, respectively.
Theory of analytical method
2.4.1 The settlement of ground improved by CDM columns under 1 dimension
When CDM columns are used to improve soft ground under applied load (L, B, >>
The settlement of improved soil layers and the underlying soil can be determined using one-dimensional (1D) theory, as outlined by Day (2010) This calculation method is applicable when the width of the loading area exceeds four times the thickness of the compressive soil layer (B > 4H).
The compressive soil layer must be situated at a depth greater than twice the width of the loaded area (D > 2B) and should be positioned between two stiffer soil layers.
Under 1D loading conditions, the total settlement of the improved ground is calculated as follows: t comp untr
S S S (2.2) where S comp is the settlement of the improved layers; S untr is the settlement of the untreated soil layer (under the improved layers)
The settlement of the improved layers S comp value is evaluated as follows (Bruce et al.,
(2.3) where q is the surcharge load applied to the improved ground; h i is the thickness of sub-layer i; M comp,i is composite constrained modulus of sub-layer i
The M comp,i is evaluated as follows:
M a E a M (2.4) where a s,i is area replacement ratio (= A c,i /As oil,i ) of layer i; E c,i is equivalent elastic modulus of soil cement column at layer i; M soil,i is constrained modulus of soil layer i
The constrained modulus (M soil ) of a layer is evaluated as follows:
(1 )(1 2 ) soil soil soil soil soil
(2.5) where E soil is equivalent elastic modulus of the soil and soil is Poisson’s ratio of soil
The settlement of the untreated beneath layers S untr value is evaluated as follows
The constrained modulus of layer i, denoted as M soil,i, is defined by Equation (2.5) It is important to note that when a clay layer is present beneath the improved ground, the settlement of this clay layer should be assessed using the ultimate consolidation settlement equation, which is widely referenced in various sources.
The area replacement ratio is calculated by the following formula (Kitazume and Terashi, 2013):
(for triangular arrangement) (2.8) where d c is diameter of the soil cement column; and L 1 , L 2 is spacing between soil cement column
The elastic modulus of soil cement column is evaluated as follows (Kitazume and Terashi, 2013) c 300 u
E q (2.9) where E c is the elastic modulus of soil cement column; q u is the strength of CDM column
2.4.2 Calculation of stress according to the ALiCC method
The ALiCC method, developed by the Public Works Research Institute of Japan, utilizes CDM columns paired with a shallow mixed reinforcement layer atop the columns This innovative approach enhances load transfer efficiency, as illustrated in Figure 2.9.
Figure 2.9 Structure of the load transfer layer from the ALiCC method (modified after
The stress applied to the CDM column head (q c ) and the stress applied on the soil (q s ) between the columns are given as follows: (ALiCC, 2006)
The unit weight of the embankment soil (γ), the diameter of the CDM column (dc), the volume of the embankment acting on a single column (Vc), and the volume of the embankment affecting the soil zone between the columns (Vs) are crucial parameters that can be estimated to assess the performance of the embankment structure.
In case of low height embankment( ) tan c 2 e s d H , following equation is available to be used
90 (2.16) where s is the distance between two columns; θ is the arch angle value; H e is thickness of embankment; H sm is the thickness of the SM layer.
Theory of numerical method
Analytical solutions for evaluating ground improvement often face limitations as they overlook practical considerations Consequently, numerical methods, particularly the finite element method (FEM), are frequently employed to assess bearing capacity and settlement The effectiveness of FEM analysis heavily relies on the selection of the calculation model and input parameters, which significantly influence the resulting stress distribution on the column.
Two common methods are used to model CDM columns under applied load: (1) axisymmetric unit cell; and (2) 3D unit cell Details of the methods are presented below
In an axisymmetric unit cell model, each CDM column and its surrounding influence zone are represented as cylindrical masses The model uses the actual diameter of the CDM column, while the radius of the unit cylinder is calculated as R = 0.564s, with 's' denoting the distance between the columns, as outlined by Han and Gabr (2002) and Poon and Chan (2013).
Figure 2.10 Principle of axial symmetric unit cylinder method (Han and Gabr, 2002;
The model represents the soil surrounding a single column as a square in 3D space, as described by Tan et al (2008) At the center of the unit pier, the CDM column is depicted as a square column with a width of 0.886 times the diameter of the CDM column (d c).
Figure 2.11 Principle of 3 D unit cell method (Tan et al., 2008)
PLAXIS software offers a variety of material models, including the Linear Elastic, Mohr-Coulomb, Hardening Soil, and Soft Soil models Each model's principles, advantages, and disadvantages are detailed in the PLAXIS Manual.
The Linear Elastic model is based on Hooke's law of isotropic elasticity There are two basic elastic parameters, i.e., Young's modulus E and Poisson's ratio ν The Linear
Elastic model is not suitable for modeling soil However, it can be used to model stiff volumes in the soil, like concrete walls or intact rock formations
The Mohr-Coulomb model is a foundational linear elastic and perfectly plastic framework that serves as an initial approximation for understanding soil behavior It incorporates Hooke’s law of isotropic elasticity for the linear elastic component and employs the Mohr-Coulomb failure criterion within a non-associated plasticity context for the perfectly plastic aspect Despite its limitations in accurately capturing deformation behavior prior to material failure, the Mohr-Coulomb model is particularly effective for analyzing embankment and warehousing challenges As illustrated in Figure 2.12 of the Plaxis manual, this model represents a versatile approach to soil mechanics.
The basic principle of elastoplastic is that strains and strain rates are distinct into an elastic part and a plastic part: e p
Hook’s law is used to relate the stress rates to the elastic strain rates Hook’s law leads to:
Figure 2.12 Basic idea of an elastic perfectly plastic model (Plaxis manual)
Parameters of the Mohr-Coulomb model
The Mohr-Coulomb model requires five key parameters, which can be derived from basic sample tests and are commonly known among geotechnical engineers These parameters, along with their standard units, are essential for effective soil analysis and stability assessments.
E: Young’s modulus; v: Poison’s ratio; c: Cohesion; : Friction angle; : Dilatancy angle
Instead of using Young’s modulus as a stiffness parameter, alternative stiffness parameters can be entered These parameters and their standard units are as follows:
G: Shear modulus; E oed : Oedometer modulus Parameters can either be effective parameters or undrained parameters, depending on the selected drain type
The Hardening Soil (HS) model is an advanced elastoplastic model that accurately simulates real soil behavior, offering significant advantages over the traditional Mohr-Coulomb (MC) model Particularly in scenarios where shear forces are dominant, the HS model provides superior predictions of displacements, making it highly recommended for applications involving excavation and tunneling.
The Hardening Soil model, while sophisticated, fails to address softening caused by soil dilatancy and de-bonding effects Additionally, it does not differentiate between high stiffness at small strains and the diminished stiffness observed at engineering strain levels.
The calculation times for the Hardening Soil Model are lengthy due to the formation and decomposition of the material stiffness matrix at each calculation step As illustrated in Figure 2.13, the hyperbolic stress-strain relationship during primary loading is demonstrated in a standard drained triaxial test (Schanz, 1999).
Figure 2.13 Hyperbolic stress–strain relationship in primary loading for a standard drained triaxial test (Schanz, 1999)
Parameters of the Hardening Soil model
Hardening Soil model needs a total of seven parameters, which can be obtained from triaxial test and oedometer test These parameters and their standard units are as follows:
The E 50 reference represents the secant stiffness obtained from triaxial tests conducted at a specified reference pressure Meanwhile, E oed ref denotes the tangent stiffness derived from oedometer tests at the same reference pressure E ur reference indicates the stiffness during unloading and reloading processes Additionally, the parameter m reflects the rate of stress dependency in the stiffness behavior, while c' signifies cohesion and φ' represents the angle of internal friction.
The Soft Soil model, a Cam-Clay type model, is specifically designed for the primary compression of near normally-consolidated clay soils, making it ideal for analyzing the compression behavior of very soft soils However, it is not appropriate for excavation scenarios, as it does not effectively outperform the Mohr-Coulomb model in unloading situations.
Stress dependent stiffness (logarithmic compression behavior)
Distinction between primary loading and unloading-reloading
Memory for pre-consolidation stress
Failure behavior according to the Mohr-Coulomb criterion
Parameters of Soft Soil model
The Soft Soil model is characterized by key parameters, including the compression index and swelling index, which are essential for understanding soft soils To effectively utilize this model, five parameters must be determined: the modified compression index (λ*), the modified swelling index (κ*), effective cohesion (c), friction angle (φ), and dilatancy angle (ψ).
METHODOLOGY
The performance of research
This research aims to achieve two primary objectives: first, to analyze the impact of thickness and stiffness of the shallow mixing layer on stress at column heads and soil settlement without using geosynthetics, employing both numerical and analytical analyses via the ALiCC method under one-dimensional loading conditions Second, it seeks to compare load-settlement curves derived from numerical methods and field load tests for PF and CDM column groups beneath shallow foundations, while assessing the stress induced in both types of columns A general flow chart illustrating this research is presented in Figure 3.1.
Figure 3.1 The general flow chart of the research
3.1.1 Methodology of the first objective
In this section, a total of three cases were brought into analysis: (1) comparative study;
This article presents a parametric study and a case analysis from an actual project in Vietnam, detailed in Chapter 4 The first objective evaluates how variations in the thickness of the load transfer layer affect ground settlement, using both analytical and numerical methods to ensure the accuracy of the numerical model's input parameters Additionally, the study explores the impact of changes in the thickness and stiffness of the load transfer layer, along with the improvement area ratio, on stress distribution and settlement, employing PLAXIS 2D (V21.01) for both parametric cases and the Vietnam project Furthermore, the analytical ALiCC method is utilized to assess the influence of the load transfer layer's thickness on stress at the column head and surrounding soil Figure 3.2 illustrates the methodology for data analysis related to this objective.
Figure 3.2 Flow chart of the methodology for data analysis of the first objective
Analyses using analytical method for the first objective
The settlement of ground improved by CDM columns under 1 dimension
Under 1D loading conditions, the total settlement of the improved ground was calculated as follows: Eq (2.2) in Chapter 2 The settlement of the improved layer
S comp value was evaluated as follows: Eq (2.3) in Chapter 2 The composite constraint modulus of the layer i within the improved depth (M comp,i ) was evaluated as follows:
In Chapter 2, the constraint modulus (M soil) of a layer was assessed using Equation (2.4) The settlement value of the unimproved layers, denoted as S untr, was calculated according to Equation (2.5) To analyze the diameter and spacing, a rectangular column arrangement was employed, leading to the evaluation of the area replacement ratio as outlined in Equation (2.7).
The stress induced on ground improved by CDM columns under 1 dimension
The ALiCC method was utilized to estimate the stress on the CDM column head (q c ) and the stress on the soil (q s ) between the columns The calculations for the embankment volume affecting a single column and the volume impacting the soil zone between the columns were derived using equations (2.12), (2.13), and (2.14).
Analyses using numerical method for the first objective
This study utilized the finite element method (FEM) through PLAXIS 2D (V21.01) software for numerical analyses, employing an axisymmetric model to examine 1D conditions The CDM columns and their surrounding influence zones were represented as cylindrical masses, arranged in a square pattern The model's CDM column diameter was based on actual measurements, with the unit cylinder radius defined as R = 0.564s (Han and Gabr, 2002; Poon and Chan, 2013) The model featured horizontal fixities at the vertical boundaries and full fixities at the bottom boundary All materials in both the parametric and actual case studies were modeled using the Mohr-Coulomb framework, characterized by linear elastic-perfectly plastic behavior.
3.1.2 Methodology of the second objective
This section outlines the input parameters for footings on ground enhanced by CDM and PF columns, including soil profile, number of columns, footing dimensions, and applied load, which remain consistent across settlement and stress-induced analyses The primary distinction lies in the shape of the two column types, although their stiffness and diameter are the same Figure 3.3 illustrates the configurations of PF and CDM columns.
Figure 3.3 Configurations of PF and CDM columns
In this study, a total of two cases were analyzed: (1) three experimental PF groups (each group includes three columns) constructed at the SAMSE Factory phase 1, and
(2) three experimental PF groups (four columns in each group) constructed at the SAMSE Factory phase 2 in Ninh Binh Detailed information of these two phases is described in Chapter 5
The load-settlement analysis of the PF and CDM column groups under shallow foundation in the SAMSE Factory phase 1 case was conducted using PLAXIS 3D (V21.01) software, allowing for a comparison with experimental static load test results Three different constitutive material model approaches were utilized for the settlement analysis.
In the SAMSE Factory phase 2 project, the load-settlement behavior of PF and CDM column groups under shallow foundations was analyzed using PLAXIS 3D (V21.01) software The induced stress along the PF and CDM columns was also evaluated, with numerical analysis results compared to experimental static load test outcomes The methodology for data analysis is illustrated in Figure 3.4.
Figure 3.4 Flow chart of the methodology for data analysis of the second objective
Analyses using numerical method for the second objective
In this study, a true 3D model was utilized to analyze the behavior of soil and PF or CDM columns as separate materials using numerical methods The model accurately reflected the actual diameter of the PF and CDM columns, with results from PLAXIS 3D being influenced by input parameters, soil models, boundary conditions, and mesh refinement A shallow footing measuring 2.5 m in length and width was examined, ensuring boundaries were positioned sufficiently far to prevent result distortion, specifically three times the footing width from the center Advanced soft soil and hardening soil models were employed to analyze load-settlement behavior of PF and CDM column groups under identical stiffness and diameter conditions across both SAMSE Factory phases Additionally, the Mohr-Coulomb model was applied to the column and fill layer in both scenarios Detailed modeling information can be found in Chapter 2, with Figure 3.5 illustrating the true 3D model for the PF column group beneath the shallow foundation.
Figure 3.5 True 3D model for PF column group under shallow foundation in the numerical method
ANALYSIS AND RESULTS OF CDM GROUPS UNDER ONE-
Research Purpose
This research investigates the impact of the load transfer layer, known as the Shallow Mixing layer, on the stress experienced at the heads and along the columns of Controlled Density Mix (CDM) and the surrounding soil, as well as the settlement of the enhanced ground in CDM groups subjected to one-dimensional loading conditions.
4.1.1 A comparative study on analytical and numerical analyses
This section discusses the settlement and stress values derived from both analytical and numerical analyses under equivalent conditions The goal of this comparative study is to assess the validity of the numerical model's input parameters, including soil domain, boundary conditions, and mesh refinement After the model verification, adjustments can be made to the soil and column strength characteristics to analyze the impact of these variables.
In this comparative study, we apply key assumptions for both analytical and numerical analyses: (1) all soils and columns are treated as elastic materials; (2) the improved layer, consisting of clayey soil and CDM columns, is regarded as a composite elastic material; and (3) all layers are assumed to be homogeneous with constant parameters throughout their depth An illustration of the soil profile utilized in the analyses is presented in Figure 4.1 (a).
CDM columns and the Shallow Mixing (SM) layer are presumed to share identical physical and mechanical properties Specifically, the parameters for the columns and the clay layer include a column diameter of 0.8 meters and a column spacing of 1.6 meters, which is twice the diameter.
= 20 kN/m 3 , unconfined compressive strength, q u = 1000 kPa, elastic modulus E c 300q u = 300,000 kPa, and Poisson ratio c = 0.35 The clayey soil layer has: thickness
The finite element method (FEM) is utilized for numerical analyses through PLAXIS 2D (V21.01) software, where CDM columns are arranged in a square pattern (Figure 2.14) Subsequently, a 2D unit cylinder cell is modeled with a specified radius.
The model features a vertical boundary with horizontal fixities and a bottom boundary with full fixities, resulting in an R value of 0.564 seconds and a measurement of 0.902 meters Figure 4.1 (b) illustrates the unit cell as modeled in the program, while Table 4.1 summarizes the input parameters used for the analyses.
Figure 4.1 Ground profiles in comparative study: (a) Analytical model, (b) Numerical model
Table 4.1 Input parameters for the comparative study
Model Elastic Elastic Elastic Elastic
Type Drained Drained Drained Drained
The study varied the thickness of the SM layer between 0.4 m and 1.0 m while maintaining a constant total thickness of 1.5 m for the compacted and SM layers A comparison of surface settlement from both analytical and numerical methods, illustrated in Figure 4.2 (a), reveals that the results are closely aligned, with only a minor discrepancy of about 1.0 mm, which is expected due to the inherent limitations of FEM models compared to ideal analytical solutions Additionally, Figure 4.2 (b) presents a comparison of stress distribution in the soil layers under 1D conditions, demonstrating that the stress increment remains constant with depth, a finding that was also corroborated by the numerical analysis These results confirm the adequacy of the selected soil domain size, mesh refinement, and boundary conditions in the numerical evaluations.
Figure 4.2 (a) Comparison of total settlement profile; (b) Stress increment profile obtained from analytical and numerical analyses
Parametric studies were conducted to assess the impact of the SM layer and other factors on the settlement of improved ground and the stress experienced in the columns The same soil profile and column specifications from the comparative study were utilized, but two key distinctions were made in the numerical model: first, soil and CDM columns were treated as separate materials rather than as an equivalent material, as illustrated in Figure 4.3; second, both soil and CDM columns were modeled using the Mohr-Coulomb (MC) model for simplicity.
Figure 4.3 Improved ground of parametric study case
To assess the impact of the thickness and stiffness of the SM layer, CDM columns with a diameter of 0.8 m were arranged in a square pattern, maintaining a spacing of 2 times the column diameter (s = 2d c) between columns in the same row This configuration leads to an improvement area ratio of 19.6% Input parameters for the soil layers, columns, and the SM layer utilized in the numerical analyses are detailed in Table 4.2.
Table 4.2 Input parameters for the parametric study
Clay Layer Sand Layer CDM Column
Model MC MC MC MC
Type Drained Undrained A Drained Undrained A
Influence of thickness and stiffness of the SM layer
The study examined how the thickness of the SM layer, ranging from 0.4 m to 1.0 m while maintaining a stiffness of E SM = 300,000 kPa, affects the settlement of improved ground Results, illustrated in Figure 4.4 (a), indicate that total settlement decreases with increased thickness on both the compacted layer and the clay layer, while settlement on top of the columns remains relatively unchanged.
The study examined how the stiffness of the SM layer affects ground settlement by varying the stiffness (E SM) between 100,000 kPa and 400,000 kPa while maintaining a constant thickness of t SM = 0.4 m The results, illustrated in Figure 4.4 (b), indicate that as stiffness increases, settlement decreases at both the top of the compacted layer and the clay layer, while the settlement above the columns remains relatively constant.
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
The impact of the thickness and stiffness of the SM layer on the stress experienced at the top of the columns and the clay layer is illustrated in Figures 4.5 (a) and 4.6 (a) Additionally, Figures 4.5 (b) and 4.6 (b) demonstrate how these factors affect the stress distribution along the columns and within the clay layer Notably, Figure 4.5 (a) provides specific stress values induced on the top of both the columns and the clay layer.
The ALiCC method, as illustrated in equations (2.10) and (2.11), reveals a significant trend shown in Figure 4.5 (a): as the thickness of the SM layer rises from 0.4 m to 1.0 m, the stress at the top of the columns increases from approximately 305 kPa to 367 kPa in numerical analysis In contrast, the stress at the top of the clay layer remains relatively stable.
Top of clay layer Top of column Top of compacted layer q = 100 kPa
Stiffness of SM layer (kPa)
The analysis reveals that the stress on top of the columns is influenced by the thickness and stiffness of the surrounding SM layer Specifically, as the thickness of the SM layer increases from 0.4 m to 1.0 m, the stress rises from approximately 219 kPa to 223 kPa, according to the ALiCC method Additionally, an increase in the stiffness of the SM layer from 100,000 kPa to 400,000 kPa also results in a corresponding increase in stress on the columns.
268 kPa to 316 kPa However, the stress induced on top of the clay layer remains relatively constant of around 55 kPa
The total stress variation along the columns and surrounding soil is illustrated in Figures 4.5 (b) and 4.6 (b) Notably, the total stress within the columns exhibits two key characteristics: the thickness or stiffness of the SM layer has a minimal impact on the induced stress, and the stress profiles reveal that the induced stress is lower at the column ends, where they are anchored in stiffer materials, while it peaks in the middle section, where the columns are encased in softer soil.
ANALYSIS AND RESULTS OF HEAD-ENLARGED CDM (PF)
Research Purpose
This research compares load-settlement curves derived from numerical methods and field load tests on PF and CDM column groups under shallow foundations The study evaluates the stress induced in both PF and CDM columns, aiming to identify the most effective model for the numerical method and assess the performance of PF columns relative to CDM columns, considering their equal stiffness and diameter.
Project Description
5.2.1 Introduction of SAMSE Factory project
SAMSE Factory is situated at plot No 5 in the Cau Yen Industrial Zone of Ninh Phong Ward, Ninh Binh City, Vietnam The factory underwent two phases of ground improvement, with the layout illustrated in Figure 5.1.
Figure 5.1 Plan view of SAMSE Factory project
The SAMSE Factory project utilized the Point Foundation (PF) method to enhance the soft ground beneath shallow footings Phase 1 involved the construction of three PF columns per group, while Phase 2 included four PF columns in each group EXT Co., Ltd executed the PF construction work for this project.
Samse Factory phase 1
Prior to the commencement of construction for SAMSE Factory phase 1, five boreholes (HK 1, HK 2, HK 3, HK 4, and HK 5) were drilled to assess the site's soil conditions and gather essential geotechnical data Each borehole underwent a standard penetration test (SPT) to evaluate the ground's strength The soil profiles derived from these boreholes are illustrated in Figure 5.2, providing a comprehensive overview of the geotechnical findings for the project.
Figure 5.2 Soil profiles and parameters from all five bore holes of SAMSE Factory phase 1
Silty clay with organic matters
Silty clay with organic matters(Very Soft)
Fill materials Silty clay with organic matters(Soft)
Fill materials Silty clay with organic matters (Very Soft)
Silty clay with organic matters (Soft)
The soil profile analysis for SAMSE Factory phase 1 was conducted using data from borehole HK 1, illustrated in Figure 5.3 A cross-sectional view of the ground enhanced by PF columns for this phase is presented in Figure 5.4 The soil profile comprises five distinct layers, including fill materials from 0 to 1.6 meters and soft silty clay containing organic matter from 1.6 meters onward.
6.2 m), medium stiff silty clay (6.2 to 15 m), stiff silty clay (15 to 24.8 m), and very stiff silty clay (24.8 to 30 m) Upper layers have small SPT values (i.e., N-values are smaller than 10) The undrained shear strength of clayey soil increases along the depth, which is calculated as follows (Terzaghi and KandRalph, 1996):
5 0.22 ' u vo s (kPa) (5.1) where the value of 0.22 represents most of soft clayey soils, and the initial value s u = 5
(kPa) is considered for the affection of weathered processes at the surface of ground
The equivalent modulus of soil is estimated from undrained soil modulus using the following equation:
The undrained modulus of soil (E u) is determined by the over-consolidation ratio (OCR) and plasticity index (PI), with drained and undrained Poisson’s ratios (ν s and ν u) being key parameters in this calculation For clayey soils, the undrained modulus can be specifically calculated following the methods outlined by Das (2011).
Figure 5.3 Soil profiles for the analysis of SAMSE Factory phase 1
Figure 5.4 A cross-sectional view of ground improved by PF column groups for the SAMSE Factory phase 1
5.3.2 Configuration of the PF column groups
The plan view of the PF groups for phase 1 is shown in Figure 5.5 Each group includes three columns arranged in a triangle pattern The shallow mixing layer was
Silty clay with organic matters
The study involved the construction of 30 columns, differentiated by the length of their PF columns, as illustrated in Figure 5.6 Steel plates measuring 2.0 m x 2.0 m were utilized above the shallow mixing layer, slightly smaller than the footings' actual dimensions The specific shapes and lengths of the columns for each group are also depicted in Figure 5.6.
Figure 5.5 Plan view of three PF groups for phase 1
Figure 5.6 Shape of PF columns: Group 1 (L PF = 8.5m), Group 2 (L PF = 6 m); Group 3
5.3.3 Static load testing program on PF column groups
EXT Co., Ltd conducted the static load test (TCVN 9393:2012) for phases 1 and 2 of the SAMSE Factory, as illustrated in Figure 5.7 The test involved incrementally applying a load on a steel plate positioned above the PF column groups to obtain accurate settlement data.
Figure 5.7 Static loading test on instrumented PF group
5.3.4 The geometry of PF column groups
In the SAMSE Factory phase 1, three PF columns were constructed in each group under the tail, with a 0.15 m thick steel plate positioned on a 0.3 m thick shallow mixing layer above the columns to effectively transfer the load to the improved ground The top level of the shallow mixing layer above the columns is considered the surface ground As illustrated in Figure 5.8 (a), the dimensions of the steel plate do not fully cover all three PF columns Additionally, Figure 5.8 (b) depicts a circular hydraulic jack placed on the steel plate, which is part of the loading system used to incrementally apply pressure to the steel plate.
SAMSE Factory phase 2
Prior to commencing construction for phase 2 of the SAMSE Factory, a borehole named HK 1 was drilled to assess the site's soil conditions and gather essential geotechnical data A standard penetration test (SPT) was conducted in this borehole to determine the strength of the existing soil layers, providing a comprehensive soil profile necessary for the analysis of SAMSE Factory phase 2.
The soil profile at HK 1, depicted in Figure 5.9, consists of six distinct layers: fill materials (0 to 2.6 m), very soft sandy clay (2.6 to 10.3 m), medium stiff to stiff sandy clay (10.3 to 15.5 m), loose medium coarse sand (15.5 to 18 m), dense medium coarse sand (18 to 19 m), and medium stiff sandy clay (19 to 30 m) The undrained shear strength (s u), undrained soil modulus (E u), and equivalent modulus of clayey soil (E s) were calculated following the same methodology as in phase 1 Additionally, the equivalent modulus of sandy soil (E s) was estimated using the formula E s = 1000 N 60 (Kulhawy and Mayne, 1990) A cross-sectional view of phase 2 is illustrated in Figure 5.10.
Figure 5.9 Soil profile for the analysis of SAMSE Factory phase 2
Figure 5.10 A cross-sectional view of ground improved by PF column group for the
5.4.2 Configuration of the PF groups
In phase 2, the design features four PF column groups arranged in a square pattern, with a shallow mixing layer constructed above them As illustrated in Figure 5.11, the key distinction between these groups lies in the varying lengths of the PF columns To effectively transfer the applied pressure, steel and concrete plates of specific dimensions—1.5 m x 1.5 m x 0.15 m for the steel plate, 2.0 m x 2.0 m x 0.2 m for the first concrete plate, and 2.5 m x 2.5 m x 0.2 m for the second concrete plate—were utilized above the PF column groups, as shown in Figure 5.12.
Figure 5.11 Plan view of three PF groups for phase 2
Figure 5.12 Shape of PF columns: Group 1 (L PF = 10.5 m), Group 2 (L PF = 8.5 m);
5.4.3 The geometry of PF column groups
Figure 5.13 Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate and concrete plate
In phase 2, PF columns feature diameters of 1.2 m at the head and 0.8 m at the tail, with the top of the shallow mixing layer considered as the ground surface A 0.15 m thick steel plate and two 0.2 m thick concrete plates are positioned on a 0.3 m thick shallow mixing layer above the four PF columns to effectively transfer the applied load to the improved ground The geometry of the PF columns for this phase is illustrated in Figure 5.13 (a), while Figure 5.13 (b) depicts a circular hydraulic jack situated on the steel and concrete plates, designed to enhance the load applied to these structures.
5.4.4 Laboratory tests for SAMSE Factory phase 1 and phase 2
In the SAMSE project, two sampling methods were employed to collect soil-cement mixed samples for phases 1 and 2 The first method involved injecting a PVC pipe into the PF column immediately after mixing, allowing it to sit for 4 to 5 hours before extracting the core sample, which was then preserved in the laboratory The second method utilized attached samplers with an agitating rod during the final mixing stage, with the collected mixed soil placed in molds for laboratory preservation Subsequently, unconfined compression (UC) tests were conducted to assess the unconfined compressive strength (q u ) of the samples sourced from the construction site.
Figure 5.14 (a) Sampling using PVC pipe
Figure 5.14 (b) Sampling using attached samplers
In the study, a total of eighteen samples were prepared for the axial compression test, consisting of five groups of PVC with three samples each and one group of attached samplers The results, detailed in Table 5.1, indicate that the unconfined compressive strength of PF columns varies between 1563 and 3679 kPa, while the equivalent modulus for PF and CDM columns ranges from 271,270 to 1,280,000 kPa The average unconfined compressive strength (q u) for the samples was calculated accordingly.
The average value of q u, avg = 2600 kPa was estimated from Eq (5.4)
Table 5.1 Unconfined compression test and Equivalent modulus results
No Sampling Sample's name q u (kPa) E c (kPa)
The equivalent modulus of PF or CDM, denoted as E c, can be derived from laboratory unconfined compressive strength tests, which reflect the unconfined compressive strength (q u) Additionally, E c can be calculated using the slope of the stress-strain curve, as illustrated in Figure 5.15 A total of eighteen values of E c can be obtained from eighteen different samples.
Figure 5.15 The estimation of equivalent modulus of PF column from UC test result
The average value of equivalent modulus of PF or CDM column (E c ) from the samples was determined as follows:
The average value of E c, avg = 750,000 kPa was calculated from Eq (5.5).
Analyses for PF groups of SAMSE Factory phase 1
This section aims to see which material model approach in numerical analyses is the best to obtain close results to the measured static load test results
In phase 1 of the study, the nonlinear behavior of soil was analyzed using the finite element method (FEM) with PLAXIS 3D (V21.01) software A detailed description of the true 3D FEM model can be found in Chapter 3 Three different constitutive material model approaches were utilized for settlement analysis during this phase.
(i) Approach 1: Four materials (soft clay, medium stiff clay, stiff clay, and very stiff clay) were modelled using Hardening soil model
(ii) Approach 2: Four materials (soft clay, medium stiff clay, stiff clay, and very stiff clay) were modelled using Soft soil model
(iii) Approach 3: Soft clay and medium stiff clay layers were modelled using Soft soil model, and stiff clay and very stiff clay were modelled using Hardening soil model
In all model approaches, the materials of CDM and PF columns and fill layer were modelled using Mohr-Coulomb failure criterion
In settlement analysis, identical input parameters such as soil profile, column count, footing dimensions, column strength, and applied load for footings improved by CDM and PF columns are used The primary distinction lies in the shape and volume of the two column types, with PF columns featuring head and tail diameters of 1.2 m and 0.8 m, respectively, while CDM columns maintain a uniform diameter of 0.8 m Additionally, the dimensions of the shallow mixing layer are specified as B × L × H.
= 2.5 m 2.5 m 0.3 m, and the steel plate B L = 2.0 m 2.0 m, were used above the three columns
The settlement of PF group 01, measuring 8.5 m in length, was not conducted due to the static load test results being insufficient for alignment with any utilized constitutive model Consequently, PF groups 02 and 03 were analyzed to compare the outcomes from the numerical methods with the static load test results.
This study utilized a true 3D modeling approach, treating soil and PF or CDM columns as distinct materials Each of the four clay types—soft clay, medium stiff clay, stiff clay, and very stiff clay—was individually modeled using the Hardening Soil model, while the remaining materials were represented using the Mohr-Coulomb model.
The soil profiles for phase 1 are detailed in section 5.3.1, where the stiffness parameter (E s) and undrained shear strength (s u) for the soil layers were derived using equations (5.1) and (5.2) In the SAMSE project, a triaxial test for clayey soil layers was not conducted; hence, the Undrained B model was chosen to represent four clayey soil layers The primary parameters for the Hardening Soil (HS) model include a triaxial stiffness value (E 50 ref = E 50 = E s /1.5).
According to Kulhawy and Mayne (1990), the oedometer stiffness (E oed ref) is equivalent to E 50 ref, while the un/reloading stiffness (E ur ref) is estimated to be three times E 50 ref, based on correlations between undrained shear strength (s u) and stiffness (E s) It's noteworthy that both PF columns and the shallow mixing (SM) layer exhibit identical physical and mechanical properties Table 5.2 presents the material models and input parameters for approach 1 of the Samse phase 1.
Table 5.2 Material models and parameters used for approach 1 of SAMSE phase 1
Model MC HS HS HS HS MC Elastic
Note: MC = Mohr-Coulomb; HS = Hardening soil
Figures 5.16 (a) and (b) present a comparison of load-settlement curves for footings on PF columns and CDM columns, derived from numerical methods and static load tests for groups 02 and 03 Key insights from these results highlight the performance differences between the column types under load conditions.
The settlement of footings on PF columns is less than that on CDM columns, with the difference in settlement between numerical methods and static load tests for PF columns in groups 02 and 03 being minimal (under 20%), which is considered acceptable.
Figure 5.16 Load settlement curves from numerical method (Approach 1) for PF groups and CDM groups and experimental static load test
PF column group CDM column group
PF column group CDM column group
In Approach 2, a true 3D model was developed for the soil and PF or CDM columns, with four distinct clay materials—soft clay, medium stiff clay, stiff clay, and very stiff clay—modeled separately using the Soft Soil model, while the remaining materials were represented with the Mohr-Coulomb model The soil profiles for Phase 1 are detailed in Section 5.3.1 Key input parameters for the Soft Soil model include the saturated unit weight (γ_sat), unsaturated unit weight (γ_unsat), initial void ratio (e_int), and additional parameters such as the compression index.
The soil investigation report for phase 1 provided values for the swelling index (C_s) and recompression index (C_c) Cohesion (c') and friction angle (φ') values were estimated based on typical soil characteristics Both PF columns and the shallow mixing (SM) layer exhibit identical physical and mechanical properties Detailed material models and input parameters for approach 2 of SAMSE phase 1 are presented in Table 5.3.
Table 5.3 Material models and parameters used for approach 2 of SAMSE phase 1
Model MC SS SS SS SS MC Elastic
Note: MC = Mohr-Coulomb; SS = Soft soil
Figures 5.17 (a) and (b) illustrate the comparison of load-settlement curves for footings on PF columns versus CDM columns, derived from numerical methods and static load tests for groups 02 and 03 using a soft soil model The results indicate that footings on PF columns experience less settlement compared to those on CDM columns Additionally, the settlement values for footings on PF columns from numerical analyses closely align with those obtained from static load tests for both groups.
Figure 5.17 Load settlement curves from numerical method (Approach 2) for PF groups and CDM groups and experimental static load test
PF column group CDM column group
PF column group CDM column group
In approach 3, the modeling of soft clay and medium stiff clay layers utilized the Soft Soil model, while the Hardening Soil model was applied to stiff clay and very stiff clay layers This method ensured that both the PF columns and the shallow mixing (SM) layer shared identical physical and mechanical properties Detailed material models and input parameters for approach 3 of the SAMSE factory phase 1 are presented in Table 5.4.
Table 5.4 Materials model and parameters used for approach 3 of SAMSE phase 1
Model MC SS SS HS HS MC Elastic
Note: MC = Mohr-Coulomb; SS = Soft soil; HS = Hardening soil
Figures 5.18 (a) and (b) compare the load-settlement curves for footings on PF columns and CDM columns, derived from numerical methods and static load tests for groups 02 and 03, using both Soft Soil and Hardening Soil models The results indicate that the settlement predictions for footings on PF columns from numerical analyses closely align with those obtained from static load tests, validating the numerical method as an effective tool for predicting PF column settlement.
In this approach, the settlement of footings on PF columns is smaller than that on CDM columns under the same diameter and stiffness in both groups
Figure 5.18 Load settlement curves from numerical method (Approach 3) for PF groups and CDM groups and experimental static load test
Comparative results from three constitutive material model approaches
Figures 5.19 (a) and (b) present comparative results of load-settlement curves for the footings on PF and CDM columns obtained from numerical method and static load test
PF column group CDM column group
L PF = 6 m Soft soil and Hardening soil model
PF column group CDM column group
The study examines the performance of soft soil and hardening soil using three different constitutive material model approaches, with a focus on a length of 4 meters (L PF = 4 m) Among these approaches, Approach 1 consistently yields results that closely align with the measured static load test outcomes, as illustrated in Figure 5.19.
Figure 5.19 Comparative load settlement curves from numerical method (three constitutive material model approaches) for PF groups and CDM groups and experimental static load test
PF column group, Approach 1 CDM column group, Approach 1
PF column group, Approach 2 CDM column group, Approach 2
PF column group, Approach 3 CDM column group, Approach 3
PF column group, Approach 1 CDM column group, Approach 1
PF column group, Approach 2 CDM column group, Approach 2
PF column group, Approach 3 CDM column group, Approach 3 Group 03
Analyses for PF groups of SAMSE Factory phase 2
In phase 2, a true 3D model utilizing PLAXIS 3D (V21.01) was developed, incorporating soil and PF or CDM columns as distinct materials to analyze the nonlinear behavior of the soil The model accurately reflected the actual diameter of the PF and CDM columns, with detailed descriptions of the FEM model provided in Chapter 3.
In the analysis of settlement and stress, identical input parameters such as soil profile, number of columns, mixed column strength, footing dimensions, and applied loads were utilized for footings on ground improved by both Controlled Density Mixing (CDM) columns and Precast Foundation (PF) columns The primary distinction lies in the shapes of the two column types Various sizes of shallow mixing layers were employed, including concrete plates measuring 2.5 m × 2.5 m × 0.3 m, 2.5 m × 2.5 m × 0.2 m, 2.0 m × 2.0 m × 0.2 m, and steel plates measuring 1.5 m × 1.5 m × 0.15 m placed above the four columns In the design, PF columns featured head and tail diameters of 1.2 m and 0.8 m, respectively, while CDM columns had a uniform diameter of 0.8 m The maximum applied pressures for groups 01, 02, and 03 were recorded at 2500 kPa, 2291.69 kPa, and 2083.33 kPa, respectively.
The study utilized the Soft Soil model to simulate very soft sandy clay, medium stiff to stiff sandy clay, and medium stiff sandy clay layers, while the Hardening Soil model was applied to represent loose medium sand and dense medium sand layers.
The PF or CDM column and fill layer were analyzed using the Mohr-Coulomb model, with soil profiles for phase 2 detailed in section 5.4.1 Key parameters, including the undrained shear strength (s u), undrained soil modulus (E u), and equivalent modulus of clayey soil (E s), were calculated using specified equations The equivalent modulus for sandy soil was estimated as E s = 1000 N 60, following Kulhawy and Mayne (1990) Input parameters for the SS model, such as the compression index (C c) and swelling index (C s), were sourced from the phase 2 soil investigation report, while cohesion (c') and friction angle (φ') values were assumed based on typical soil characteristics For the HS model, triaxial density and unloading/reloading stiffness (E ur ref = 3E 50 ref) were utilized Notably, PF columns and the shallow mixing layer share similar physical and mechanical properties The 0.15 m thick steel plate was modeled with a linear elastic approach, characterized by a unit weight of 78.5 kN/m³, an elastic modulus of 10² kPa, and a Poisson ratio of 0.2 Additionally, a 0.2 m thick concrete plate was defined with a unit weight of 24 kN/m³ and corresponding elastic modulus.
E s = 30 10 6 kPa, and Poisson ratio = 0.2 This concrete plate was also modelled with Linear elastic model Table 5.5 shows material models and input parameters for SAMSE Factory phase 2
Table 5.5 Material models and parameters used for SAMSE phase 2
Note: MC = Mohr-Coulomb; SS = Soft soil; HS = Hardening soil
Medium stiff to stiff sandy clay
Model MC SS SS HS HS SS MC
Figures 5.20 (a), (b), and (c) illustrate the load-settlement curves for footings on PF and CDM columns, derived from numerical methods and static load tests across three groups The results indicate that the settlement of footings on PF columns from numerical methods aligns closely with static load test results in groups 01 and 02 However, discrepancies arise in group 03, where the numerical predictions do not match the static test results, suggesting potential errors in the field testing program The findings emphasize that the numerical method can effectively predict footings' settlement on PF columns when accurate soil profiles and models are employed.
PF column group CDM column group
Figure 5.20 Load settlement curves from numerical method for PF groups and CDM groups and experimental static load test
The analysis of Figures 5.20 (a), (b), and (c) reveals that the settlement of footings on PF columns is marginally less than that on CDM columns across groups 01, 02, and 03 This suggests that the effectiveness of PF columns is not significantly pronounced in any of the groups studied Consequently, it is crucial to investigate the underlying reasons for the observed reduced settlement of footings on PF columns compared to CDM columns.
The PF column head length is 1.0 m, with the ratio α varying for different groups: 0.09 for group 01 (L PF = 10.5 m), 0.12 for group 02 (L PF = 8.0 m), and 0.15 for group 03 (L PF = 6.0 m) This ratio, α, represents the relationship between the head length and the total length of the PF column The α values are relatively low, around 0.1, indicating that the footings on PF columns have a marginally larger head section diameter compared to CDM columns However, these differences do not significantly impact the settlement behavior of footings on PF versus CDM columns Furthermore, the head section of PF columns in all three groups is situated in the upper fill layer, positioned above a 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
PF column group CDM column group
The stiffness value of the SM layer, with an E c /E s ratio of approximately 100, indicates that the column behaves similarly to a pile when this ratio exceeds 20 Consequently, there is minimal difference in the settlement of footings supported by PF columns compared to those on 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 L h = 1.0 m, L c = 1.0 m, and L t 8.5 m (i.e., = L h /L = 1/10.5 = 0.09) to L h = 6.0 m, L c = 1.0 m, and L t = 3.5 m (i.e.,
= 0.57) for group 01 For group 02, the length of PF column head was extended from
(2) The stiffness ratio of the PF/CDM columns to soil was changed from E c /E s = 100 in average to E c /E s = 20
(3) The stiffness of the SM layer was changed from E SM = 750 MPa to E SM = 150 MPa
The settlement of footings on PF columns is approximately 25% less than that on CDM columns, as illustrated in Figures 5.21 (a) and (b) This finding suggests that the PF column head must be sufficiently long to extend over the soft soil layer, while the stiffness of the SM layer and PF/CDM columns should be 10 to 20 times greater than that of the surrounding soil This common stiffness ratio of 10 to 20 has been supported by various case studies (Kitazume and Terashi, 2013).
PF column group CDM column group
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 L h = 1.0 m, L c = 1.0 m, and L t = 8.5 m (i.e., = L h /L = 1/10.5 = 0.09) to L h = 2.0 m, L c = 1.0 m, and L t = 7.5 m (i.e., = 0.19); L h = 3.0 m, L c = 1.0 m, and L t = 6.5 m
For group 02, the length of PF column head was extended from L h = 1.0 m, L c = 1.0 m, and L t = 6.5 m (i.e., = L h /L = 1/8.5 = 0.12) to L h = 2.0 m, L c = 1.0 m, and L t = 5.5 m (i.e., = 0.23); L h = 3.0 m, L c = 1.0 m, and L t = 4.5 m (i.e., = 0.35); L h = 4.0 m, L c 1.0 m, and L t = 3.5 m (i.e., = 0.47); and L h = 5.0 m, L c = 1.0 m, and L t = 2.5 m (i.e.,
For group 03, the length of PF column head was extended from L h = 1.0 m, L c = 1.0 m, and L t = 4.5 m (i.e., = L h /L = 1/6.5 = 0.15) to L h = 2.0 m, L c = 1.0 m, and L t = 3.5 m (i.e., = 0.31); L h = 4.0 m, L c = 1.0 m, and L t = 1.5 m (i.e., = 0.62)
Figures 5.22 (a), (b), and (c) demonstrate that increasing the length of PF column heads leads to a reduction in the settlement of footings on PF columns Notably, the settlement observed for footings on PF columns is approximately 22% lower than that of CDM columns when the α value ranges from 0.4 to 0.6.
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 2 The influence of the thickness of the SM layer on the settlement of the footings on PF columns was investigated by varying the thickness (t SM ) from 0.2 m to 0.5 m and keeping the constant stiffness of E SM = 750 MPa
Static load test CDM column group
Static loadd test CDM column group
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) illustrate the settlement results of footings on CDM and PF columns, highlighting the impact of varying SM layer thickness The analysis reveals that the settlement of footings on PF columns decreases by approximately 13% to 16% as the thickness of the SM layer increases, confirming the anticipated outcomes from both numerical methods and static load tests.
The settlement behavior of footings on PF columns beneath a 0.5 m thick SM layer, as illustrated in Figure 5.23 (a), (b), and (c), closely aligns with the results obtained from static load tests conducted in groups 01 and 02, demonstrating consistency between numerical methods and empirical data.
Static load test CDM column group, t SM = 0.3 m
Static load test CDM column group, t SM = 0.3 m
Influence of the stiffness of SM layer on settlement of PF column group
In phase 2, the stiffness of the shallow mixing (SM) layer above the columns is consistently measured at 750 MPa, matching the stiffness of PF and CDM columns To assess the impact of the SM layer's stiffness on the settlement of footings on PF columns, its stiffness (E SM) was varied from 100 MPa to 750 MPa while maintaining a constant thickness of 0.3 m Numerical analyses and static load tests, illustrated in Figures 5.24 (a), (b), and (c), reveal that the settlement of footings on PF columns decreases by approximately 5% as the stiffness of the SM layer increases from 100 MPa to 750 MPa.
CDM column group, E SM = 750 MPa
PF column group, E SM = 750 MPa
PF column group, E SM = 300 MPa
PF column group, E SM = 100 MPa
Static load test CDM column group, E sm = 750 MPa
PF column group, E sm = 750 MPa
PF column group, E sm = 300 MPa
PF column group, E sm = 100 MPa
Static load test CDM column group, E SM = 750 MPa
PF column group, E SM = 750 MPa
PF column group, E SM = 300 MPa
PF column group, E SM = 100 MPa
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
Despite the installation of the strain gauge along the column, the test results were unsatisfactory, leading to the omission of stress distribution findings from the field test in this research Figures 5.25 (b), (c), and (d) present the total stress profiles at various depths for the PF and CDM columns from groups 01, 02, and 03 based on numerical analysis For identical tail diameters and loading conditions, the PF column exhibits its maximum stress zone at approximately 2.6 m depth, close to the bottom of the PF cone section, while the CDM column shows its peak stress zone around 0.8 m, near the surface layer (SM layer) This peak stress region poses significant challenges in column design, as exceeding the column's compressive strength can result in local cracking or failure due to stress concentration.
Analysis results from this case study show that the total stress in the depth of 2.6 m of