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

INTRODUCTION

General introduction of Cement Deep Mixing method

The foundation design is crucial for the successful construction of structures, heavily influenced by geotechnical conditions Modern techniques address geotechnical weaknesses, effectively reducing settlement and enhancing soil foundation strength Ground improvement methods include vacuum consolidation, granular column reinforcement, geosynthetics, prefabricated vertical drains, deep mixing, premixing, and lightweight treated soil Notably, the deep mixing method has evolved since the 1970s, providing innovative solutions for foundation stability and performance.

In recent years, the Cement Deep Mixing (CDM) method has gained popularity as an effective technique for improving soft ground globally This method addresses issues such as excessive settlement, high water content, and vibrations while enhancing soil stiffness and embankment stability Additionally, CDM is utilized to mitigate liquefaction and offer lateral ground support However, under embankment loads, CDM columns may experience collapse due to external or internal failures.

In 2007, Kitazume and Maruyama noted that columns do not fail at the same time; rather, they fail sequentially at different intervals This individual failure prevents the collective strength and stiffness of all columns from being utilized simultaneously.

To enhance the workability of improved ground, a load transfer layer—either a shallow mixing (SM) layer or a combination of aggregates and geogrid—is typically constructed atop CDM columns This layer effectively distributes the load, allowing the columns to function in unison, which boosts the stability of the column system and minimizes differential settlement between the columns and the surrounding soil (Ishikura et al., 2009) However, the stiffer load transfer layer can lead to increased stress on the CDM columns compared to the adjacent soil.

Problem statement

A load transfer layer, such as a shallow mixing layer or a compacted aggregate layer with geosynthetics, is typically designed above CDM columns to effectively distribute the applied load to the ground system This design helps balance the settlement of both the soil and the columns, ensuring stability Figures 1.1 (a), (b), and (c) illustrate the ground improvement achieved by CDM columns with different types of load transfer layers.

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 the design process, the local bearing capacity of individual columns is crucial, particularly when incorporating a load transfer layer If the stress on a column surpasses the material's compressive strength, it may experience local failure, even if the overall ground remains stable Therefore, it is essential to strictly control the stress induced in the columns Various methods, such as the ALiCC method and British Standard, have been proposed in the literature to address this issue.

Several methods, including BS 8006, the German method (EBGEO), the Guido method, and the Low method, have been proposed to estimate the stress on column heads Among these, the ALiCC method uniquely considers the stress induced on columns within a shallow mixing layer, while the others focus on stress calculations for columns with various geosynthetic embedded layers However, 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 Masonry (CDM) columns face limitations when used to enhance the ground beneath shallow foundations, as their uniform diameter does not effectively target weaker soil layers To address 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 of PF columns and assessed their effectiveness in improving the settlement of soft ground These studies utilized an analytical method to compare the performance of PF columns with conventional CDM columns, maintaining consistent volume and soil profiles while varying diameters.

Hoang (2020) conducted a numerical evaluation of footing settlements on PF columns versus conventional CDM columns, maintaining consistent volume, soil profiles, and the number of columns in groups of three, while varying diameters The findings from the numerical analysis of PF and CDM columns were subsequently compared to results from experimental static load tests.

Existing studies have not comprehensively examined the interaction between soil and PF columns, highlighting the need for a true 3D model to accurately represent this relationship Additionally, there has been insufficient analysis of the stress distribution along both PF and CDM columns.

Necessity of the study

The CDM method is widely utilized for ground improvement projects in Vietnam, Myanmar, and beyond; however, it is crucial for geotechnical engineers to thoroughly investigate the maximum stress distribution in 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 only estimates the stress on the top of the column and the surrounding soil.

Studying the impact of the load transfer layer (SM layer) on ground settlement and the stress experienced by columns and soil is essential This analysis should utilize both analytical and numerical methods, allowing for a comprehensive examination of various factors through numerical analysis.

1.3.2 Bearing capacity of shallow footing on Head-enlarged CDM (PF) Column

Ground improvement projects utilizing Head-enlarged Controlled Density Material (CDM) columns under shallow foundations are increasingly common worldwide However, it is crucial for geotechnical engineers to comprehend how these PF columns behave, particularly regarding the impact of column stiffness and diameter, as well as the soil's nonlinearity on foundation settlement and bearing capacity.

To evaluate the load-settlement behavior and stress distribution in PF columns, it is essential to conduct numerical analyses using a true 3D model alongside experimental static load tests This approach will effectively compare the performance of PF columns with CDM columns, ensuring both have identical stiffness and diameter.

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, highlighting the theoretical frameworks of both analytical and numerical methods, specifically the Finite Element Method (FEM), for calculating settlement and stress 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 ground settlement and the stress experienced by 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 article analyzes the performance of PF columns through numerical methods and experimental static load tests, focusing on the stress distribution along both PF and CDM columns The study aims to evaluate the effectiveness of PF columns in comparison to CDM columns, providing insights into their structural behavior under load.

This chapter mentions the specific conclusions and recommendations of this research h

LITERATURE REVIEW

Overview of cement deep mixing method

This research examines 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, as well as the effectiveness of head-enlarged CDM column groups beneath shallow foundations Numerous studies have been conducted on the deep mixing method, with contributions from notable 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 method (CDM) aims to improve ground stiffness by integrating in-situ soil with cement and additives The elastic modulus of CDM columns is approximately 50 percent (E 50 ) of 75 to 1,000 times the unconfined compressive strength (q u ) of the column material, yet it remains lower than that of concrete piles, which have an elastic modulus of 30,000,000 kPa Consequently, CDM columns and surrounding soft soil are classified as composite ground rather than piles Previous research has suggested an equivalent elastic modulus for the composite ground to assess both stiffness and deformation Additionally, the impact 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 h

In recent years, Cement Deep Mixing (CDM) columns have gained prominence for enhancing soft soil beneath embankments, with applications varying by region For instance, Nordic countries utilize CDM primarily to reduce settlement, while Japan focuses on improving the stability of port facilities Although the underlying principle of CDM remains consistent in both contexts, the specific applications differ significantly This versatile method is extensively used in both terrestrial and marine construction, as illustrated in Figures 2.1 and 2.2.

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 employed when the area replacement ratio is below 50%, effectively reducing settlement and enhancing bearing capacity In contrast, a block pattern is utilized for area replacement ratios exceeding 50%, providing support for significant vertical and horizontal loads, thereby 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 to intercept water flow, and a retaining wall for lateral support, as well as enhancing embankment stability The grid pattern, suitable for wall and block types, effectively mitigates liquefaction in sandy soils Lastly, group columns consist of isolated stabilized soil elements arranged in rectangular or triangular rows, widely used to minimize settlement and bolster 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, as outlined by Kitazume and Terashi (2013), encompasses three primary 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, while the dry method utilizes powdered binders to create columns or panels of enhanced soil The wet method offers the advantage of producing homogeneous columns, whereas the dry method results in improved ground with lower water content, reduced binder usage, and typically greater strength However, the wet method is not suitable for high-water-content conditions.

2.1.4 Fixed type and floating type improvement

Cement deep-mixing (CDM) ground improvement methods can be classified into two main types: fixed and floating The distinction between these types is based on the penetration of CDM columns into the underlying stiff soil layer Fixed type CDM columns extend fully to the stiff layer, while floating type columns do not completely penetrate it The floating type is designed to manage the equilibrium deformation of structures on soft ground, helping to minimize excessive settlement during construction However, in bridge construction, if CDM columns reach the stiff layer, it may lead to significant differential settlement between the adjacent embankment or road 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 consisting of a larger diameter cap at shallow depths and a smaller diameter tail at deeper levels to meet stiffness requirements Research by Bergado et al (1999) indicated that soft soil surrounding embankments experiences greater settlement than mixed soil columns, leading to instability and potential pavement damage While geosynthetic reinforcement or compacted granular materials can enhance structural stability, they may increase costs The TDM column addresses this issue by reducing the stress concentration ratio, thereby mitigating differential settlement and improving overall stability 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 comparing its vertical and lateral displacement with that of 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 for shallow columns and 1.3 to 2.4 meters for deep columns, with column lengths varying between 11 and 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 gained 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 consists of three distinct parts—head, cone, and tail—as illustrated in Figure 2.7(b) Second, the binder used in the PF method boasts a compressive strength that is 1.5 to 2 times greater than that of standard 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 certifications for its environmentally friendly techniques and advanced technology from various professional organizations and ministries in Korea This innovative method is utilized for soft ground improvement in various applications, including industrial structures, roads, underground structures, earth retaining walls, and water barrier walls Notably, it serves effectively as a pile h foundation for low-rise buildings, supporting a maximum applied pressure of 300 kPa, and is ideal for the transportation of lightweight structures.

A PF column features a unique structure comprising two primary components: the upper head and the lower tail The upper head, with a larger diameter than the lower tail, is connected to the tail by a tapered section known as the cone This innovative design enhances load transfer mechanisms compared to traditional CDM columns Typically, the length of the head section matches that of the cone section at 1 meter, with the head's diameter ranging from 1.2 to 1.4 meters and the tail's diameter between 0.6 and 0.8 meters.

(a) (b) Figure 2.7 Construction of PF method

Load transfer Mechanisms

In soil mechanics, two extreme load conditions are recognized: equal stress and equal strain The equal stress condition occurs when the improved soil layer is subjected to an ideally flexible load, resulting in a stress concentration ratio (n) of 1.0, where the soil settles more than the columns (S_s > S_c) Conversely, the equal strain condition arises under an ideally rigid load, leading to a stress concentration ratio greater than 1.0 (n > 1.0), with both the columns and soil settling equally (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 accurately 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 compressible soil layer (B > 4H).

For optimal soil stability, ensure that the depth to the top of the compressive soil layer exceeds twice the width of the loaded area, and that this compressive layer is situated 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, represented as M soil,i, is defined by Equation (2.5) It is important to note that if a clay layer is present beneath the improved ground, the settlement of this clay layer should be calculated 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 along with a shallow mixed reinforcement layer positioned above 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)

In the context of embankment design, the unit weight of the embankment soil (γ), the diameter of the column (dc), the volume of the embankment acting on a single column (Vc), and the volume of the embankment affecting the soil zone between columns (Vs) are critical parameters These values can be estimated to ensure stability and 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 often face limitations as they overlook practical aspects of improved ground conditions Consequently, numerical methods, particularly the finite element method (FEM), are frequently utilized to assess the bearing capacity and settlement of enhanced ground In FEM analysis, the selection of the calculation model and input parameters is crucial, significantly influencing the accuracy of stress distribution calculations on the columns.

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, with the unit cylinder's radius defined as R = 0.564s, where s denotes the distance between the columns (Han and Gabr, 2002; Poon and Chan, 2013).

Figure 2.10 Principle of axial symmetric unit cylinder method (Han and Gabr, 2002;

In this model, the surrounding soil of a single column is represented as a square in three-dimensional space, as described by Tan et al (2008) The central CDM column within the unit pier is modeled 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 various material models, including the Linear Elastic model, Mohr-Coulomb model, Hardening Soil model, and Soft Soil model Each model's principles, advantages, and disadvantages are detailed in the PLAXIS Manual, providing users with essential insights for effective application.

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 h

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 serves as a foundational approximation for soil behavior, combining linear elastic and perfectly plastic characteristics It utilizes Hooke’s law for its linear elastic component and the Mohr-Coulomb failure criterion within a non-associated plasticity framework for its perfectly plastic aspect This model is particularly flexible, making it suitable for analyzing embankment and warehousing issues, despite its limitations in accurately representing deformation prior to material failure For a visual representation, refer to Figure 2.12 in the Plaxis manual, which illustrates the core concept of an elastic perfectly plastic model.

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) h

Parameters of the Mohr-Coulomb model

The Mohr-Coulomb model requires five essential parameters that can be derived from basic sample tests, making them well-known among geotechnical engineers These parameters, along with their standard units, are crucial for effective analysis and application in geotechnical engineering.

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 soil behavior, particularly in conditions where shear is dominant It offers significant advantages over the Mohr-Coulomb (MC) model, especially in predicting displacements during excavation and tunnel projects Due to its superior performance, the HS model is highly recommended for applications involving complex soil interactions.

The Hardening Soil model, while advanced, fails to consider the softening effects caused by soil dilatancy and de-bonding Additionally, it does not differentiate between high stiffness at low strains and the reduced 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 Figure 2.13 illustrates the hyperbolic stress-strain relationship observed during primary loading 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 indicates the secant stiffness derived from triaxial tests conducted at a specific reference pressure The E oed reference represents the tangent stiffness obtained from oedometer tests at the same pressure level Additionally, E ur refers to the reference stiffness during unloading and reloading phases The parameter m denotes the rate of stress dependency in stiffness behavior, while c' signifies cohesion, and φ' represents the angle of internal friction.

The Soft Soil model, a Cam-Clay type framework, is specifically designed for the primary compression of near normally-consolidated clay-type soils, making it particularly effective for modeling the compression behavior of very soft soils However, it is not ideal for excavation scenarios, as it does not outperform the Mohr-Coulomb model in unloading situations Key features of the Soft Soil model include its focus on soft soil behavior and its limitations in excavation applications.

Stress dependent stiffness (logarithmic compression behavior)

Distinction between primary loading and unloading-reloading h

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 such as the compression index and swelling index, which are essential for understanding soft soils To effectively utilize this model, five parameters must be identified: 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 focuses on two main objectives: first, to analyze the effects of thickness and stiffness of the load transfer layer (Shallow Mixing) on stress at column heads and ground settlement for CDM groups under one-dimensional loading, utilizing both numerical and analytical methods (ALiCC) without geosynthetics The second objective is to compare load-settlement curves from numerical methods and field tests for PF and CDM column groups under shallow foundations, while evaluating the stress induced in PF and CDM columns A general flow chart of the 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;

The study encompasses a parametric analysis and a case study from Vietnam, detailed in Chapter 4 Initially, the impact of varying the thickness of the load transfer layer on ground settlement was assessed through both analytical and numerical methods to ensure the validity of the numerical model's input parameters Subsequently, the effects of changes in the thickness and stiffness of the load transfer layer, along with the improvement area ratio, on induced stress and settlement were explored using PLAXIS 2D (V21.01) for both the parametric and actual project cases Additionally, the relationship between the thickness of the load transfer layer and the stress on the column head and soil was examined using the ALiCC method in the parametric study Figure 3.2 illustrates the methodology for data analysis related to the first 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 of unimproved layers, denoted as S untr, was determined through Equation (2.5) To evaluate the diameter and spacing for the research, a rectangular column arrangement was utilized, and the area replacement ratio was calculated as outlined in Equation (2.7).

The stress induced on ground improved by CDM columns under 1 dimension

The ALiCC method, specifically through Equations (2.10) and (2.11), was utilized to estimate the stress exerted on the CDM column head (q c) and the stress on the soil (q s) between the columns The calculation of the embankment volume impacting a single column, as well as the volume affecting the soil zone between the columns, was derived using Equations (2.12), (2.13), or (2.14).

Analyses using numerical method for the first objective

In this study, numerical analyses were conducted using the finite element method (FEM) with PLAXIS 2D (V21.01) software, applying an axisymmetric model for 1D conditions The CDM columns and their surrounding influence zones were represented as cylindrical masses, arranged in a square pattern The model utilized the actual diameter of the CDM columns, with the unit cylinder's radius defined as R = 0.564s (Han and Gabr, 2002; Poon and Chan, 2013) The model's vertical boundaries featured horizontal fixities, while the bottom boundary was fully fixed All materials in both the parametric and actual case studies were modeled using the Mohr-Coulomb framework, exhibiting linear elastic-perfectly plastic behavior.

3.1.2 Methodology of the second objective

This section discusses the input parameters for footings supported by CDM and PF columns, including soil profile, number of columns, footing dimensions, and applied load, which remain consistent across settlement and stress analysis The primary distinction lies in the shape of the two column types, while their stiffness and diameter are equivalent The configurations of PF and CDM columns are illustrated in Figure 3.3.

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 PF and CDM column groups under shallow foundations for 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 in this settlement analysis.

In the second phase of the SAMSE Factory project, the load-settlement behavior of PF and CDM column groups under shallow foundations was analyzed using PLAXIS 3D (V21.01) software The stress distribution along the PF and CDM columns was also examined, with the numerical analysis results being compared to data obtained from experimental static load tests A flow chart illustrating the methodology for data analysis is provided 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 two-phase interactions between soil and PF or CDM columns, with the columns accurately represented by their actual diameters The numerical analyses, conducted using PLAXIS 3D, emphasized the significance of input parameters, soil models, boundary conditions, and mesh refinement on the results A shallow footing measuring 2.5 m in length and width was examined, ensuring that the boundaries were positioned sufficiently far to prevent any influence on the findings, with the distance from the footing center to the boundary set at three times the footing width Advanced soil models, including the soft soil model and hardening soil model, were employed to analyze load-settlement behavior of PF and CDM column groups under consistent stiffness and diameter conditions across both SAMSE Factory phases Additionally, the h column and fill layer were modeled using the Mohr-Coulomb model in both scenarios, with further details provided in Chapter 2 Figure 3.5 illustrates the true 3D model for the PF column group under a shallow foundation within the numerical framework.

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 by the heads and shafts of Controlled Density Material (CDM) columns, as well as the soil settlement in the improved ground under one-dimensional loading conditions Through both analytical and numerical analyses, the study aims to enhance understanding of the behavior of CDM groups in relation to these factors.

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 aim 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 confirming the model's accuracy, we can modify the strength characteristics of the soil and columns to investigate the impact of these variables.

In this comparative study, we apply specific assumptions for both analytical and numerical analyses under equivalent conditions: (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 considered homogeneous with constant parameters throughout the depth An illustration of the soil profile utilized in the analyses is depicted in Figure 4.1 (a).

The 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 using PLAXIS 2D (V21.01) software CDM columns were arranged in a square pattern, as illustrated in Figure 2.14 Subsequently, a 2D unit cylinder cell was modeled with a specified radius.

The model features a unit cell with a radius of 0.564s, translating to a diameter of 0.902 m The vertical boundaries are designed with horizontal fixities, while the bottom boundary is fully fixed A detailed summary of the input parameters used for the analyses is presented in Table 4.1, and the model is visually represented in Figure 4.1 (b).

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

In the analyses, the thickness of the SM layer varied between 0.4 m and 1.0 m, while maintaining a constant total thickness of 1.5 m for both the compacted and SM layers A comparison of surface settlement from analytical and numerical methods, depicted in Figure 4.2 (a), shows similar results with only a minor discrepancy of approximately 1.0 mm, which is expected due to the inherent limitations of the FEM model compared to ideal analytical solutions Additionally, Figure 4.2 (b) illustrates the stress distribution in soil layers under 1D conditions, revealing a consistent stress increment with depth, confirming the adequacy of the chosen soil domain size, mesh refinement, and boundary conditions in the numerical analyses.

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 columns The same soil profile and column specifications from the comparative study were utilized, but two key differences were noted in the numerical model: first, the soil and CDM columns were treated as separate materials rather than as an equivalent material, as illustrated in Figure 4.3; second, both the 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 configuration, maintaining a spacing of 2 times the column diameter between columns in the same row This arrangement yields 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 affects the settlement of improved ground, with thicknesses ranging from 0.4 m to 1.0 m while maintaining a consistent stiffness of E SM = 300,000 kPa The findings, illustrated in Figure 4.4 (a), indicate that total settlement decreases on both the compacted layer and the clay layer as thickness increases, while the settlement on top of the columns remains relatively unchanged.

The study examined how the stiffness of the SM layer affects the settlement of improved ground by varying the stiffness (E SM) from 100,000 kPa to 400,000 kPa while maintaining a constant thickness of t SM = 0.4 m Results illustrated in Figure 4.4 (b) indicate that as stiffness increases, settlement on both the compacted layer and the clay layer decreases, 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 influence 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 the columns and the clay layer.

The ALiCC method, as illustrated in equations (2.10) and (2.11), reveals a notable trend in stress distribution Figure 4.5 (a) demonstrates that as the thickness of the SM layer increases from 0.4 m to 1.0 m, the stress at the top of the columns rises from approximately 305 kPa to 367 kPa in numerical analysis In contrast, the stress experienced at the top of the clay layer remains relatively stable throughout this range of thickness.

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 at the top of the columns increases with both the thickness and stiffness of the surrounding SM layer Specifically, when the thickness of the SM layer rises from 0.4 m to 1.0 m, the induced stress on the columns increases from approximately 219 kPa to 223 kPa, as demonstrated by the ALiCC method Furthermore, as the stiffness of the SM layer escalates from 100,000 kPa to 400,000 kPa, a corresponding rise in stress at the column tops is observed, underscoring the significance of these parameters in geotechnical assessments.

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 in 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 ends of the columns, where they are embedded in stiffer materials, while it peaks in the middle section where the columns are surrounded by softer soil.

ANALYSIS AND RESULTS OF HEAD-ENLARGED CDM (PF)

Research Purpose

This research focuses on comparing 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 draw insightful conclusions regarding the most effective numerical model and the performance of PF columns in comparison to CDM columns, particularly in terms of utilizing the same stiffness and column 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, as illustrated in the plan view depicted in Figure 5.1.

Figure 5.1 Plan view of SAMSE Factory project

In the SAMSE Factory project, the Point Foundation (PF) method was utilized to enhance the soft ground beneath shallow footings Phase 1 involved the construction of three PF columns per group, while Phase 2 saw the addition of four PF columns per group The PF construction was executed by EXT Co., Ltd for the SAMSE Factory 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 strength of the existing ground Figure 5.2 illustrates the soil profiles obtained from these five boreholes, providing valuable insights for the project's development.

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 As depicted in Figure 5.4, the ground improvement for this phase utilized PF columns The phase 1 soil profile consists of 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 drained and undrained Poisson's ratios, denoted as νs and νu, respectively, are essential parameters in soil mechanics The undrained modulus of soil (Eu) is estimated based on the over-consolidation ratio (OCR) and the plasticity index (PI) According to Das (2011), the undrained modulus for clayey soils can be calculated using specific formulas that take into account these factors.

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 involves three groups differentiated by the length of PF columns, as illustrated in Figure 5.6 Steel plates measuring 2.0 m x 2.0 m were installed above the shallow mixing layer, slightly smaller than the actual footing size 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 a 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 an H 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 beneath a steel plate, which is 0.15 m thick, positioned on a 0.3 m thick shallow mixing layer to effectively transfer the load to the enhanced ground The top level of the shallow mixing layer above the columns is designated as the surface ground As illustrated in Figure 5.8 (a), the dimensions of the steel plate do not completely cover all three PF columns Additionally, Figure 5.8 (b) depicts a circular hydraulic jack placed on the steel plate within the loading system to incrementally increase the load applied to the steel plate.

SAMSE Factory phase 2

Prior to commencing construction for phase 2 of the SAMSE Factory, a borehole designated 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 detailed soil profile crucial for the analysis of the SAMSE Factory phase 2 project.

The soil profile at HK 1, illustrated 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) Key properties, including the undrained shear strength (s u), undrained soil modulus (E u), and equivalent modulus of clayey soil (E s), were calculated following the methodology used in phase 1 The equivalent modulus for sandy soil was estimated using the formula E s = 1000 N 60 (Kulhawy and Mayne, 1990) A cross-sectional view of phase 2 is presented 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 of the project, each group consists of four PF columns arranged in a square formation, with a shallow mixing layer constructed above them The layout of these PF column groups is illustrated in Figure 5.11, while Figure 5.12 highlights the key distinction between the groups, which is the varying lengths of the PF columns To effectively transfer the applied pressure, steel and concrete plates of specific dimensions were utilized: a steel plate measuring 1.5 m x 1.5 m x 0.15 m, a concrete plate measuring 2.0 m x 2.0 m x 0.2 m, and another concrete plate measuring 2.5 m x 2.5 m x 0.2 m.

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 h

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 head and tail diameters of 1.2 m and 0.8 m, respectively, 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 the 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 placed 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

Two sampling methods were utilized to collect soil-cement mixed samples for the SAMSE project 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 withdrawing it with the core sample preserved inside In the second method, attached samplers were employed with an agitating rod during the final mixing stage The collected mixed soil was then placed in molds for laboratory preservation and subjected to unconfined compression (UC) tests to assess the unconfined compressive strength (q u) of the samples from the construction site.

Figure 5.14 (a) Sampling using PVC pipe

Figure 5.14 (b) Sampling using attached samplers

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 Additionally, the equivalent modulus (E c) for both PF and CDM columns ranges from 271,270 to 1,280,000 kPa.

The average value of unconfined compressive strength (q u ) from the samples was determined as follows: h

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 column Ec is derived from the unconfined compressive strength test in the laboratory, representing the unconfined compressive strength (qu) Ec is calculated using the slope of the stress-strain curve, illustrated in Figure 5.15 A total of eighteen values of Ec can be obtained from eighteen 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 the initial phase of the study, the nonlinear behavior of soil was analyzed using the finite element method (FEM) through PLAXIS 3D (V21.01) software A detailed description of the true 3D FEM model can be found in Chapter 3 This phase involved three distinct constitutive material model approaches to assess settlement analysis effectively.

(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, the input parameters—including soil profile, column quantity, footing dimensions, column strength, and applied load—remain consistent for both CDM and PF columns The primary distinction lies in the shape and volume of these column types Specifically, PF columns feature a head diameter of 1.2 m and a tail diameter of 0.8 m, while CDM columns maintain a uniform diameter of 0.8 m Additionally, the dimensions of the shallow mixing layer are defined 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 analysis for PF group 01, with a length of 8.5 meters, could not be conducted due to the static load test results being insufficient to align with any existing constitutive models Consequently, PF groups 02 and 03 were analyzed to facilitate a comparison between the numerical method results and the static load test outcomes.

In this study, a true 3D model was developed by separately modeling soil and PF or CDM columns as distinct materials The four types of clay—soft clay, medium stiff clay, stiff clay, and very stiff clay—were each represented using the Hardening soil model, while the remaining materials were modeled with the Mohr-Coulomb approach.

In section 5.3.1, the soil profiles for phase 1 are detailed, highlighting the calculated parameters of stiffness (E s) and undrained shear strength (s u) for the soil layers using equations (5.1) and (5.2) The SAMSE project did not include triaxial testing for clayey soil layers; consequently, the Undrained B model was chosen to represent four clayey soil layers Key parameters for the Hardening Soil (HS) model were established, including the triaxial stiffness (E 50 ref = E 50 = E s / 1.5).

According to Kulhawy and Mayne (1990), the oedometer stiffness (E_oed_ref) is equivalent to the E_50_ref, while the un/reloading stiffness (E_ur_ref) is calculated as three times the E_50_ref, based on correlations with undrained shear strength (s_u) and stiffness (E_s) It is crucial to recognize that PF columns and the shallow mixing (SM) layer exhibit identical physical and mechanical properties Additionally, 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 comparative analysis 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 findings from these results highlight significant differences in performance between the two column types under varying load conditions.

The settlement of footings on PF columns is less than that on CDM columns, with numerical methods and static load tests showing minimal differences in settlement (under 20%) for both groups 02 and 03, indicating that these results are 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

In Approach 2, a true 3D model was developed for soil and PF or CDM columns, incorporating four distinct materials: soft clay, medium stiff clay, stiff clay, and very stiff clay, each modeled using the Soft Soil model The remaining materials were represented using the Mohr-Coulomb model Detailed soil profiles for Phase 1 are outlined in Section 5.3.1 Key input parameters required for the Soft Soil model include saturated unit weight (γ_sat), unsaturated unit weight (γ_unsat), initial void ratio (e_int), and additional parameters such as the compression index.

The swelling index (Cₛ) and recompression index (Cₗ) values were derived from the soil investigation report for phase 1, while cohesion (c') and friction angle (φ') were estimated based on typical soil characteristics Both PF columns and the shallow mixing (SM) layer exhibit identical physical and mechanical properties Table 5.3 presents the material models and input parameters for approach 2 of the SAMSE phase 1.

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 h

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, utilizing a Soft soil model The results indicate that footings on PF columns experience less settlement compared to those on CDM columns Additionally, the numerical analysis results for footings on PF columns align closely with the static load test outcomes for both groups.

In approach 3 of the SAMSE factory phase 1, soft clay and medium stiff clay layers were simulated using the Soft Soil model, while stiff clay and very stiff clay layers were represented with the Hardening Soil model Notably, both the PF columns and the shallow mixing (SM) layer share identical physical and mechanical properties For detailed insights, refer to Table 5.4, which outlines the material models and input parameters utilized in this approach.

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 load-settlement curves for footings on PF and CDM columns, derived from numerical methods and static load tests for groups 02 and 03, utilizing both Soft soil and Hardening soil h models The numerical analyses show that the settlement of footings on PF columns aligns closely with results from static load tests, validating the numerical method's effectiveness in 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

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

L PF = 6 m Soft soil and Hardening soil model

The study investigates the performance of three constitutive material model approaches for soft and hardening soil, 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

Analyses for PF groups of SAMSE Factory phase 2

In phase 2, a true 3D model incorporating soil and PF or CDM columns as separate materials was developed using PLAXIS 3D (V21.01) through a numerical method, accounting for the nonlinear behavior of the soil The model utilized the actual diameters of the PF and CDM columns, with a detailed description of the FEM model provided in Chapter 3.

In the analysis of settlement and stress, identical input parameters were utilized, including soil profile, number of columns, strength of mixed columns, footing dimensions, and applied load for footings on ground improved by CDM and PF columns The primary distinction lies in the shape of the two column types Various concrete and steel plate sizes were applied above the columns, specifically a shallow mixing layer of dimensions 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 1.5 m × 1.5 m × 0.15 m The design specifications indicate that PF columns have head and tail diameters of 1.2 m and 0.8 m, respectively, while CDM columns maintain 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 involved modeling various soil layers, including very soft sandy clay, medium stiff to stiff sandy clay, and medium stiff sandy clay using the Soft Soil model Additionally, loose medium sand and dense medium sand layers were represented using the Hardening Soil model.

The PF or CDM column and fill layer were analyzed using the Mohr-Coulomb model, with soil profiles detailed in section 5.4.1 Key parameters such as 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 at E s = 1000 N 60, following Kulhawy and Mayne (1990) Input parameters for the SS model, including compression index (C c) and swelling index (C s), were sourced from the phase 2 soil investigation report Cohesion (c') and friction angle (φ') values were assumed based on typical soil characteristics The HS model required triaxial density and unloading/reloading stiffness, with PF columns and shallow mixing layers exhibiting identical physical and mechanical properties A 0.15 m thick steel plate was modeled with a unit weight of 78.5 kN/m³, elastic modulus of 10² kPa, and a Poisson ratio of 0.2, while a 0.2 m thick concrete plate had a unit weight of 24 kN/m³.

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) present a comparison of 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 numerical method accurately predicts the settlement of footings on PF columns in groups 01 and 02, aligning closely with static load test outcomes 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 estimate footings' settlement on PF columns when appropriate soil profiles and models are employed.

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 minimal in these groups Therefore, it is essential to investigate the reasons behind the slightly reduced settlement of footings on PF columns compared to CDM columns.

The PF column head length is 1.0 m, with ratios (α) of 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) These α values, which represent the ratio of head length to total PF column length, indicate that the ratios are relatively small, particularly around 0.1 for footings on PF columns Furthermore, the head section diameter of PF columns is slightly larger than that of CDM columns, yet these dimensions do not significantly affect the settlement of footings on either type of column Notably, 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

The stiffness of the SM layer, with a ratio of E c /E s around 100, indicates a significantly high value When the E c /E s ratio exceeds 20, the behavior of the column resembles that of a pile Consequently, this results in minimal differences in settlement between footings supported by PF columns and 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% smaller than that on CDM columns, as illustrated in Figures 5.21 (a) and (b) This finding emphasizes the necessity for the PF column head length to adequately cover the soft soil layer Additionally, the stiffness of both the SM layer and the PF/CDM columns should be 10 to 20 times greater than that of the surrounding soil, a common stiffness ratio supported by numerous case studies (Kitazume and Terashi, 2013).

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)

Increasing the length of the PF column head leads to a reduction in the settlement of footings on PF columns Notably, the settlement of footings on PF columns is approximately 22% less 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 outcomes of footings on CDM and PF columns, highlighting the influence of varying SM layer thicknesses derived from numerical methods and static load tests Notably, the settlement of footings on PF columns decreases by approximately 13% to 16% as the thickness of the SM layer increases.

The settlement 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 both numerical methods and static load tests conducted in groups 01 and 02.

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 values of PF and CDM columns The impact of varying the stiffness (E SM) of the SM layer on the settlement of footings on PF columns was analyzed by adjusting E SM from 100 MPa to 750 MPa while maintaining a constant thickness of 0.3 m Numerical analyses and static load test results, illustrated in Figures 5.24 (a), (b), and (c), demonstrate 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

Static load test CDM column group, E sm = 750 MPa

PF column group, E sm = 750 MPa

Static load test CDM column group, E SM = 750 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 a strain gauge along the column, the test results were unsatisfactory, leading to the exclusion of stress distribution results from the field test in this research Figures 5.25 (b), (c), and (d) present the total stress profiles at varying 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 at about 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 lead to local breakage or cracking due to stress concentration.

Analysis results from this case study show that the total stress in the depth of 2.6 m of

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

This study aimed to investigate the impact of the thickness and stiffness of the load transfer (SM) layer on the stress experienced by column heads and the settlement of improved ground in Controlled Dynamic Modulus (CDM) groups under one-dimensional loading conditions through numerical and analytical analyses Additionally, it sought to compare the load-settlement curves derived from numerical methods with those obtained from field load tests.

Along PF column Along CDM column Level of cone bottom

The study reveals that with a cone bottom level of 2.3 m and a consistent column diameter, the load factor (L PF) is 6.5 m, while the pressure (q) is measured at 2083.33 kPa, and the ultimate pressure (q u) reaches 2600 kPa.

1 When the thickness of the shallow mixing (SM) layer increases from 0.4 to 1.0 m, the settlement on top of the compacted layer decreases from 29.9 mm to 29.0 mm (with a difference of 0.9 mm, or approximately 3%), and the stress induced on the column head increases from 305 kPa to 367 kPa (with a difference of 62 kPa, or approximately 17%) This indicates that the increase in thickness of the SM layer has little influence on the settlement but large influence on induced stress on the column head

2 When the stiffness of the shallow mixing (SM) layer increases from 100 MPa to

400 MPa, the settlement decreases from 30.5 to 29.9 mm (with a small gap of 0.6 mm, or approximately 2%), and induced stress increases from 268 kPa to

The findings reveal that a stiffness increase of 316 kPa, with a variation of 48 kPa (around 15%), leads to a minimal reduction in settlement but a notable rise in stress on the column heads Additionally, the stress experienced along the column remains relatively stable despite changes in thickness and stiffness.

3 When the improvement area ratio (a s ) increases from 19.6% to 44.2%, the settlement on top of the compacted layer decreases from 30.0 to 22.0 mm (with a difference of 8 mm, or approximately 27%), and the stress induced on head of the columns decreases from 306 kPa to 165 kPa (with a difference of 141 kPa, or approximately 46%) This behavior is attributed to the fact that, under the constant applied load and unchanged SM layer, the larger columns result in smaller induced stress on head of and along the columns

4 An important finding from this study is that the maximum stress induced in the CDM column is typically not on head of the columns but at the middle depths where soil layers are softer than the SM and the bearing layer at the column toes This finding was consistently indicated both from typical soil profile of h the parametric studies and from the actual soil profile of the project case in Vietnam

1 The load-settlement profile obtained from the numerical method with appropriate soil models, boundary conditions, and mesh refinement selected in this study is similar to that obtained from the experimental static load test

2 Under the following conditions: (i) the length of the PF column head is not long enough to cover the soft soil layer ( = L h /L = 0.1); (ii) PF columns and CDM columns have the same stiffness and diameter; (iii) the shallow mixing layer above the PF columns has higher stiffness; and (iv) the ratio of stiffness of PF or CDM columns over that of the surrounding soil is much larger than 20, the effectiveness of PF columns is not significant

3 When  value is in the range of 0.4 to 0.6, the settlement of the footings on PF columns is smaller than (approximately 22%) that of CDM columns Therefore, the length of the PF column head should be long enough to cover the soft soil layer to get the most significant effect of the PF column

4 When the thickness of the shallow mixing layer increases from 0.2 to 0.5 m, the settlement of the footings on PF columns decreases approximately 13% to 16% However, when the stiffness of the SM layer increases from 100 MPa to 750 MPa, the settlement of the footings on PF columns decreases approximately 5%

5 The key finding from this study is that the maximum stress zone (peak point) of the PF column is located near the level of PF cone bottom The maximum stress zone in CDM column is located near the surface layer (SM layer) Actually, the peak point sections are very problematic in column design Hence, in many cases, when using PF columns, the designer should pay attention to the end of the cone section of the PF column as the columns may be locally failed at this section due to large stress concentration.

Recommendations

The followings are some recommendations from this study

1 Due to the time limitations, the author did not analyze the settlement of footings on the PF column and CDM column groups under shallow foundations using the analytical method Thus, it is recommended to analyze the settlement of footings on the PF and CDM column groups using the analytical method

2 The author did not analyze the stress induced in the PF column group using the equivalent material model Therefore, it is necessary to analyze the stress induced in the PF column group using the equivalent material model

3 The selection of input parameters for soil and column, the stiffness ratio of the CDM column and soil, and the boundary conditions of the model need to be considered carefully in numerical analysis

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Tiêu đề	Experimental And Numerical Studies On Bearing Capacity Of Ground Improved By Soil Cement Deep Mixing (Cdm) Columns
Tác giả	Thin Zar
Người hướng dẫn	Dr. Nguyen Tien Dung
Trường học	Vietnam National University, Hanoi Vietnam Japan University
Chuyên ngành	Civil Engineering
Thể loại	master’s thesis
Năm xuất bản	2023
Thành phố	Hanoi

Định dạng
Số trang	94
Dung lượng	3,22 MB