INTRODUCTION
General introduction of deep mixing method
The sustainability of the building depends largely on the foundation of the building (about from 40 to 60% value of project) Therefore, the design of the foundation is an important element in the design work Under the development of science and technology, there are many foundation design options that are widely used
Nowadays, there are many methods for improving soft grounds such as compaction methods, vertical drains under surcharge and vacuum preloading, vibration methods, deep mixing method, and other miscellaneous methods Among the methods, deep mixing method (DMM) is widely used as an effective method for ground reinforcement in the world The method creates cement deep mixing (CDM) columns to increase the stiffness (i.e., to reduce settlement) and to control the stability of embankment or excavations In general, sub-soils at different places have unique behavior under same loading condition so that finding an optimal solution that satisfies both technical and financial requirements is always an interesting question for geotechnical engineers
The DMM has been extensively used for many types of construction project, for example embankment supports, buildings, earth retaining structures, retrofit and renovation of urban infrastructures, liquefaction hazards mitigation, manmade island construction and seepage control Deep mixing has been mostly used to improve soft cohesive soils, but it is sometimes used to reduce permeability and mitigate liquefaction of cohesionless soils
Besides the advantages, the conventional CDM column also has some limitations when it is applied to reinforce grounds under shallow foundations Soil layers in the upper part is often weaker than that in the lower part (deeper layers), however, under shallow footings, introduced stresses are mainly distributed in the depths right below the footing
This combination of natural ground and CDM columns in cases of shallow footings is therefore ineffective There should be a better shape of CDM columns to reinforce the ground more optimal.
Necessity of research
Recognizing the limitations of CDM columns as stated above, EXT Co Ltd company from Korea have recently introduced an improved type of CDM column and named it
“point foundation” (PF) In principle, the PF column has three distinct parts (Fig 1.1a):
The bigger head (upper part), the transitional cone and the smaller tail The configuration of the PF column and its construction method were introduced in Trung (2019), Nguyen et al (2019a and 2019 b)
Liu et al (2012) developed special equipment that has foldable augers to install deep mixing (DM) columns at different diameters Due to the shape of the columns like the letter T, they are named this DM columns T-shaped columns (Fig 1.1b) Some researches on T-shaped soil cement column (e.g., Yi et al 2018) showed remarkable results in settlement and lateral movement reduction compared with conventional method
(a) (b) Figure 1.1 Configuration of improved CDM columns: (a) Point foundation (PF) (Nguyen et al 2019a): (b) T-shape column (Liu et al 2012)
Although there were some initial researches on PF columns (i.e., Trung 2019, Nguyen et al 2019), these studies focused on introduction of concept of the method as well as of a simple analytical method to evaluate settlement of soft ground improved by the PF columns
Much of understandings on the PF columns under shallow foundations, such as the influence or nonlinearity of soil and the influence of stiffness of PF columns to the settlement of the foundations are unfolded It is therefore necessary to have a study to
Soft soil layer further understand the behavior of PF groups under actual foundation conditions (e.g., actual soil layers, nonlinear characteristics of soil materials).
Objective and Scope of research
To evaluate the effectiveness of PF columns over that of CDM columns in reducing settlement of shallow footings on reinforced grounds using both analytical and numerical methods for the same foundation models
1 To evaluate the effectiveness when the soil layers are modelled as elastic materials using both analytical and numerical methods
2 To evaluate the effectiveness when the soil layers are modelled as inelastic materials using numerical method
3 To evaluate the effectiveness when the treated zone under the footing is modelled as:
(i) an equivalent material; (ii) a true 3D model of PF columns and soil
To obtain the objectives above, this study focuses on the following:
1 Evaluate settlement of shallow footings using classical theory of foundation on elastic materials For this, settlement of shallow footings on an ideal soil profile and on an actual project soil profile is evaluated using both analytical and numerical methods
2 Compare predicted and measured settlement values of shallow footings using non- linear behavior of soil material For this, settlement of shallow footings on an experimental single PF column and on experimental PF groups is evaluated using numerical method
3 Compare predicted and measured settlement values of shallow footings on experimental PF groups, in which the treated zone is modelled as an equivalent elastic material and as a true 3D model of PF columns and soil.
LITERATURE REVIEW
2.1Overview of deep mixing method
As mentioned in the objective section of this report, this study focuses on reducing the settlement of soft ground Hence, a critical overview of soil cement column, as well as the behavior of composite ground, will be considered first then the theory of settlement evaluation will be assessed for choosing the appropriate methods for the calculation of this study In the lieterature, there are many researches on deep mixing method (or cement deep mixed soil column), of which some typical studies are Kitazume and Terashi (2013), Bergado (1996), Rujikiatkamjorn et al (2005), Chai and Carter (2011), Han (2015), Bruce et al (2013), Bredenberg et al (1999), Kirsch and Bell (2012)
2.1.1 Brief view of deep mixing method
As mentioned in the introduction, deep mixing method increases the stiffness of ground by mixing in-situ soil with admixture (cement and necessary additives) Mixed soil columns created by deep mixing method has the elastic modulus at 50 percent (E50) of 75 to 1,000 times qu, where qu is the unconfined compressive strength of the column material (Kitazume & Terashi, 2013), but the value is still smaller than that of concrete pile (30,000,000 kPa) Thus, It can be considered that the work of mixed soil columns and surrounding soft soil as composited ground (not pile) Base on this assumption, most previous scholars proposed an equivalent elastic modulus of composited ground for determining the stiffness as well as deformation
The typical properties of stabilized soils are provided in Table 1.1 based on the wet method of deep mixing and Table 1.2 based on the dry method of deep mixing The effects of different factors on the unconfined compressive strengths of stabilized soils have been discussed above
Table 2.1 Typical properties of Stabilized soil (wet method) (Modified from Elias et al
Table 2.2 Typical Properties of Lime–Cement Stabilized Soils (Dry Method) (Modified from Elias et al 2006)
Selection of ground improvement method should consider the following conditions: (1) structural conditions, (2) geotechnical conditions, (3) environmental constraints, (4) construction conditions, and (5) reliability and durability
Structural conditions: The structural conditions may include type, shape, and dimension of structure and footing, flexibility and ductility of structural and footing elements, type, magnitude, and distribution of loads, and performance requirements (e.g., total and differential settlements, lateral movement, and factor of safety)
Geotechnical conditions: The geotechnical conditions may include geographic landscape, geologic formations, type, location, and thickness of problematic geo-material possible end-bearing stratum, age, composition, distribution of fill, and groundwater table Soil type and particle size distribution are essential for preliminary selection of ground improvement methods as shown in Figure 2.1 This guideline is suitable for ground improvement methods for foundation support The thickness and location of problematic geo-material are also important for the selection of ground improvement methods For example, when a thin problematic geo-material layer exists at a shallow depth, the over excavation and replacement method is one of the most suitable and economic method When a relatively thick loose cohesionless geo-material layer exists near ground surface, dynamic compaction and vibro-compaction methods are suitable ground improvement methods
When a relatively thick soft cohesive geo-material layer exists near ground surface, preloading and deep mixing methods may be used When a site needs to be excavated, tieback anchors, soil nails, deep mixed columns, and jet grouted columns may be used
When a site needs to be elevated, geo-synthetic-reinforced slopes and walls can be good choices The level of groundwater table often affects the selection of ground improvement methods For example, when deep excavation happens in ground with a high groundwater table, deep mixed column walls may be better than soil nailed walls because they not only can retain the geo-material but also can cut off water flow
Environmental constraint: The environmental constraints may include limited vibration, noise, traffic, water pollution, deformation to existing structures, spoil, and headspace For example, dynamic compaction induces vibration and noise, which may not be suitable in a residential area The wet method to construct stone columns by water jetting produces spoil on site, which may be troublesome for a site with limited space Under such a condition, the dry method may be used instead Preloading induces settlements at nearby areas, which may be detrimental to existing structures
The selection of a ground improvement method should consider the following construction conditions: (1) site condition, (2) allowed construction time, (3) availability of construction material, (4) availability of construction equipment and qualified contractor, and (5) construction cost The selection of a ground improvement method must consider whether the site is accessible to its associated construction equipment, such as access road and headspace Construction time is one of the most important factors for the selection of a ground improvement method For example, preloading is a cost-effective ground improvement method to improve soft soil; however, it takes time for the soil to consolidate
The use of prefabricated vertical drains can accelerate the rate of consolidation, but sometimes it still may not meet time requirement As a result, other accelerated ground improvement methods may be used, such as deep mixing and vibro concrete column methods Most ground improvement methods use specific materials during construction
For example, stone columns and rammed aggregate columns use aggregate Cement is used for deep mixing and grouting When natural material is used, such as aggregate or sand, the cost of the material depends on the source of the material and its associated transportation distance For example, in a mountain area, aggregate is often less expensive; therefore, stone columns or aggregate columns are often a cost-effective solution In general, the use of locally available material results in more cost-effective ground improvement To select a ground improvement method, engineers should gather information about possible qualified contractors and their available construction equipment It is preferable to use a locally available qualified contractor because this will reduce the mobilization cost and the contractor is more familiar with local conditions
Construction cost is always one of the key factors that dominate the selection of a ground improvement method The construction cost should include mobilization, installation, material, and possible disposal costs
Reliability and DurabilityReliability of a ground improvement method depends on several factors, such as the level of establishment, variability of geotechnical and structural conditions, variability of construction material, quality of the contractor, quality of installation, and quality control and assurance Several researchers have exported that samples from deep mixed columns have a high variability in terms of their unconfined compressive strengths Automatic or computer-controlled installation processes can reduce the variability of improved geo-materials The number of well documented successful or failure case histories is also the evidence of the reliability of a specific ground improvement method Ground improvement methods are used for temporary and permanent structures
For permanent structures, the durability of the construction material should be evaluated or considered in the design For example, geosynthetics have creep behavior The corrosion of steel reinforcement with time reduces its thickness The strength of cement-stabilized soil in seawater degrades with time (Ikegami et al., 2002)
Figure 2.1 Available ground improvement methods for different soil types (modified from Schaefer et al., 2012) 2.1.3 Classification
The techniques most commonly employed for in-situ deep mixing in Japan can be divided into three groups: mechanical mixing by vertical rotary shafts with mixing blades at the bottom end of each mixing shaft, high pressure injection mixing, and combination of the mechanical mixing and high pressure injection mixing The various methods in these groups are classified in Figure 5.1 In the mechanical mixing techniques, binder is injected into a ground with relatively low pressure and forcibly mixed with the soil by mixing blades equipped to vertical mixing shaft(s) The binder is used either with powder form (dry method) or slurry form (wet method) The Dry Jet Mixing (DJM) method is the most common dry method of deep mixing and has usually been applied for on-land works (Dry Jet Mixing Association, 2010)
Figure 2.2 Classification of deep mixing method (Kitazume & Terashi, 2013) The Cement Deep Mixing (CDM) method, the most common wet method of deep mixing, has frequently been applied for both in-water and on-land works (Cement Deep Mixing Method Association, 1999) In the high pressure injection technique, on the other hand, ground is disturbed by a high pressure jet of water and/or air, while at the same time binder slurry is injected and mixed with the soil The combination of mechanical mixing and high pressure injection mixing exploits the features of both basic techniques
METHODOLOGY
The key objective of the study is to evaluate the effectiveness of PF columns compared to that of the CDM columns in reducing settlement of shallow footings on the grounds improved by the columns In each settlement analysis, input parameters for footings on ground improved by CDM columns and by PF columns, such as, soil profile, footing dimensions and load intensity, number of columns, are identical The key different point is that, the shape of the two column types is different but the volume of the two columns are made equal This conditions is to guarantee that, material expenses for the two column types are equal Configurations of CDM and PF columns taken into analyses are shown in Figure 3.1
Figure 3.1 Configuration of CDM and PF columns
(3.1) The volume of a CDM column is
The equal volume condition (i.e., VPF=VCDM) makes:
By setting = Lh/L, = Dh/Dt, and is angle of cone face with horizontal plane (typically
The roots of the cubic equation (i.e Dt) can be obtained that of general cubic equation (Zwillinger 2018)
In this study, a total of four footing cases were brought into analysis: (1) footing on an ideal soil profile that has a soft clay layer overlain low compressible soil bed ; (2) a typical shallow footing at Jincheon Energy Factor (JEF) (Korea), which was constructed by EXT in 2017; (3) an instrumented single PF column in Korea; (4) three experimental PF groups constructed at Samse project (Ninh Binh) Details on the cases are described in Chapters 4 and 5
The three dimensional (3D) elastic strain integration method in recommended in CFFM
(2006) is adopted herein Settlement of the deep mixed zone is calculated based on equal strain in the deep mixed ground and the adjacent untreated soil within the zone area This is equivalent using a composite modulus of the deep mixing ground and the adjacent soil
(Bruce et al, 2013) Under 3D loading conditions, uncorrected settlement at the center of a footing on an improved area can be estimated as follows Eq 2.5 in chapter 2
The corrected settlement of the at the center of the foundation can be estimated as follows
Besides soil and loading parameters, settlement of a shallow footing is also function of ,
, and =d/DCDM , where d = distance between the two CDM columns The target in this analytical analysis is to find optimal shape of PF column that results in minimum settlement value
Theoretically, the minimum settlement value can be obtained by solving the following equation system:
However, since the settlement value (S) is not a close-form equation, the theoretical approach (i.e., Eq 3.5) cannot be applied Thus, to get an approximate minimum settlement value, the following simple procedure was applied:
(i) Set = 1 , for example 1 = 2.0 and set = 1, in which 1 1.0, where = 1 for the CDM case
(ii) From a given set of 1 and 1, the value of from is varied from 1 to n (i.e., i+1 i + ) Theoretically, can vary in in the range of 0.0 1.0 In this study,
= 0.02 and the range of 0.2 0.9 were selected for analyses A settlement curve (S vs ) is obtained for a given set of 1 and 1
(iii) Step (ii) is repeated for another set of 1 and 2, where 2 = 1 + In this study
= 0.1 was selected for analyses After this step, a set of settlement curves (S vs ) for different b is obtained
(iv) Steps (i) to (iii) are repeated for different value of to examine the effect of distance between the columns
For this section, elastic settlement was analyzed for two cases: (1) ideal case; (ii) a typical footing of JEF project
3.3 Analyses using numerical method For numerical analysis, the FEM software Plaxis3D (Version 2018, licensed by Construction Informatics and Consultancy (CIC) JSC, Ministry of Construction, Vietnam) was selected as the tool to analyze settlement of shallowing footings
In this section, the nonlinear behavior of soil is taken into analysis Three constitutive models were selected for the analysis, including: (1) Mohr-Coulomb failure criterion; (2) Hardening soil model; (3) Soft soil model Details on the models were described in the literature The soil model in each cases is as follows:
(i) Ideal case: Mohr-Coulomb failure criterion (ii) Experimental single PF column: hardening soil model (iii) PF groups at Samse: Soft soil model
In all cases, the material of CDM and PF columns is modelled using Mohr-Coulomb failure criterion
Firstly, with soil material is elastic material, the numerical method used to evaluate the accuracy of the analytical method Besides, the numerical method also calculated the settlement of shallow footing when soil layer is not homogeneous
Secondly, numerical method can solve for more advanced material models The elastic- perfectly plastic soil model is used to predict the settlement of PF columns when soil materials is non-linear material
Hardening soil model and soft soil model are the most advanced for real behavior
Settlement of PF columns analyzed from these advanced models From these, analyze the results of numerical method and static load test Under the same condition of volume and soil parameters, compare settlement of PF columns and CDM columns, evaluate the effective of PF columns
The numerical method is a good solution to evaluate the effective of PF columns It also analyzes the limitations of PF columns under certain conditions.
LABORATORY AND FIELD TEST
4.1 Introduction of Samse project 4.1.1 General information of project
The PF method was applied to reinforce ground under shallow footings of Samse Factory, which is located at plot No 5, Cau Yen Industrial Zone, Ninh Phong ward, Ninh Binh city, Vietnam A plan view of the factory is shown in Fig 4.1 The PF construction work was carried out by EXT Co., Ltd
Figure 4.1 Plan view of SAMSE factory project 4.1.2 The PF groups
Before the mass PF columns construction, full-scale load tests were carried out on three
PF groups at the site to verify the design settlement values as well as to understand the behavior of treated ground The plan view of the PF groups is shown in Fig 4.2 and the shape and length of the columns in each group are shown in Fig 4.3 Each group has three columns arranged in the triangle pattern This pattern was rather uncommon for footings, but in fact it was selected to resemble the actual pattern of PF groups designed for the project The key difference from the groups is the length of the PF columns as shown in Fig 4.3 Note that due to some difficulty in finding large steel bearing places, the steel plates of 2.0 m 2.0 m were used As shown, the plates are slightly smaller than the actual size of the footings
Figure 4.2 Plan view of three PF groups 4.1.3 Soil profile and footing parameters
Before the construction work, site investigation was carried out at the site In total, five boreholes, named HK1, HK2, HK3, HK4 and HK5 were implemented for collecting geotechnical data of the 2.4 ha factory (180 × 135 m) In each borehole, the standard penetration test (SPT) was carried out to determine the strength of the ground Figure 4.4 shows soil profiles of Samse factory project
Figure 4.3 Shape of PF columns: (Left) Group 1 (LPF=8.5m), (Middle) Group 2 (LPF=6 m); (right) Group 3 (LPF=4 m)
Figure 4.4 Soil profile of SAMSE factory project
4.2 Laboratory tests for Samse project
At the Samse project, two sampling methods were used in the field to collect soil-cement mixed samples For the first method, PVC pipe was inserted into the PF column right after the completion (Fig 4.5(a)) of mixing and it was withdrawn up after 4 to 5 hours with core sample inside the pipe The tube was then brought to laboratory and preserved For the second method, special samplers were attached to the agitating rod at the final state of the mixing (Fig 4.5(b)) Mixed soil collected in the samplers is then placed in molds and preserved in the lab for the unconfined compression (UC) test later
The UC test was conducted to determine the unconfined compressive strength (qu) of samples obtained from the project Table 4.1 shows a summary of test samples done in the laboratory As shown, samples from five groups (3 samples for each group) of PVC pipe and one group of attached sampler were tested Thus, in total, 18 test samples were tested
Table 4 1 Unconfined compression test results
No Sampling Sample’s name qu (kPa) E50 (kPa)
The unconfined compressive strength (qu) of PF column material ranged from 1563 to 3679 (kPa) at 28 days This could be explained that the samples were taken from PF columns at construction site so that the quality is not as homogeneous as ones produced in laboratory
(b) Sampling using attached samplers Figure 4.5 Collection of mixed cement- soil samples The geometric average of qu from the samples was determined as follows:
(4.1) The average value of qu, avg = 2600 kPa was found from Eq (4.1)
For each sample, the secant modulus at 50% of ultimate strength (E50) was determined as illustrated in Fig 4.6 A correlation between qu and E50 obtained from the sample is shown in Fig 4.7 and is expressed as follows
As shown in the Figure, the slope E varies in a wide range from 50 to 200; Usually, E 100 is used for conservative analyses (Han et al 2015) For Samse project, a value of E 150 was selected for the analyses
Figure 4.7 Relationship between secant modulus of elasticity and unconfined
4.3 Static load test on PF groups
Static load test (TCVN 9393:2012) was carried out by EXT Co., Ltd as show in figure 4.8
An increment applied load pressing on a steel plate underlain by three PF columns performed reliable data for the verification analytical settlement evaluation on this study
Figure 4.8 Static load test on instrumented PF groups 4.3.1 The geometry and installation PF groups
Figure 4.9 Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate, (c) displacement sensors on steel plate and ground
For conducting the static load test, a group of three PF columns was constructed The diameter of head and tail of column are 1.2 and 0.8 m, respectively The level of top of column is considered as surface ground A 0.15 m thickness of steel plated placed on 03
PF columns for transferring applied load to treated ground Note that, the dimension of the plate is not fully cover all of 03 PF columns as shown in figure 4.9 (a) A circular hydraulic jack was placed on the plate, under the loading system for increasing the applied load to the plate as shown in Figure 4.9 (b) A system of 06 sensors for measuring the displacement of steel plate
Figure 4.10 Strain gauge installation: (a) installation of sensors along the depth of
PF, (b) setting up sensors into PF, (c) strain gauge instruments, (d) sensor in PF For the static loading test, each PF column was installed strain gauges immediately after finishing mixing soil column when the binder is still soft The sensors inside the PF are connected to the test instruments for collecting data of load-settlement test The installation of strain gauges is shown in Figure 4.10
4.4 Static load test on single PF column
In an experimental program, EXT carried out SLT on a single PF column The test was carried Songdo construction site, Incheon City, S Korea The test is reported in detail in Kim et al (2016) Only key information of the test is briefed in this section
Figure 4.11 shows geological profile at the site From 0 to 1.0 m is Silty sand, from 1.0 to 13.2 m is silty clay, from 13.2 to 16.3 m is silty sand and 16.3 m to 20 m is Sedentary deposit Base on SPT value, the column is in soil of medium stiffness
Figure 4.11 Soil profile at Songdo site (Kim et al 2016)
Figure 4.12 discribes configuration of the single column On the column, there are monitoring equipment for studying behavior beween column and soil Figure 4.13 shows some steps during installation of strain gauges into the PF column at the site Table 4.2 shows parameter of unconfined compression test (detail in chapter 3)
Table 4 2 Strength parameters of samples collected from the PF column
Category 0.0-0.2 m 0.4-0.5 m 0.9-1.1 m 1.7-1.9 m 3.1-3.2 m 3.4-3.5 m Modulus of elastic (MPa) 213.4 426.9 260.0 202.8 795.6 1143.8 Secant modulus E 50 (MPa) 235.2 392.6 271.4 342.7 848.3 1100.9 Uniaxial strength (kPa) 2272.6 3504.0 2468.7 3113.0 10035 7697.9
Figure 4.12 Configuration of the instrumented column (Kim et al 2016)
Figure 4.13 Instrumentations implemented on variable cross-section soft ground reinforced foundation (Kim et al 2016)
SETTLEMENT ANALYSIS AND RESULTS
5.1 Settlement analyses using elastic theories 5.1.1 Verification analysis
It is known that besides the influence of soil models and input parameters, results from numerical analyses, especially from 3D versions, depend strongly on boundary conditions and mesh refinement To minimize the influence of domain size and mesh refinement, a calibration analysis was first carried out for the elastic soil model, in which the one-fourth of footing and soil domain is shown in Figure 5.1 Table 1 shows input parameters for the calibration analysis
Figure 5.1 Foundation and soil domain in the numerical analysis The footing width (B) of 4.5 m was selected to be equal to that of the actual case analyzed later in next section Table 5.1 shows input parameters for the calibration analysis Figure 5.2 shows the normalized stress (z/p) and normalized settlement (Se/B) profiles obtained from analytical and numerical analyses It is clear that both methods result in almost identical profiles with depth with some minor deviation at the depth of large than 5B This indicates that the selected soil domain size and the mesh refinement degree are good enough for this case
Table 5 1 Input parameters for calibration analysis
Dimension factors (m, n) - 5.5, 9.0 Elastic modulus, Es kPa 8000
Figure 5.2 Comparison of vertical stress profiles obtained from analytical and numerical analyses 5.1.2 Analyses for Ideal case and JEF case
To illustrate the effective of PF columns, settlement of two shallow footing cases is brought into analysis Case 1 examines settlement of a flexible shallow footing placed on the surface of an ideal but practice-like soil profile For the idea case, a simple but practice- like soil profile was used for the analyses as shown in Figure 5.3, which consists of a 15 m thick soft clay layer overlying stiff soils For simplicity, the settlement of the underlain stiff soil is ignored The undrain shear strength (Su) of clayey soil increases linearly along the depth following an empirical equation:
Where: 0.22 represents a common factor for soft soil clayey, Suo= 5 (kPa) is an assumed strength talking into account the effects of wreathing effects at the ground surface The undrained modulus (Eu) is simply calculated as:
Eu%0Su (for NC clay with PI>50) (5.2) And the equivalent elastic modulus as:
(5.3) For the ideal case, a fully flexible square footing is selected to place on the surface of the improved soil zone (ie tf=Df=0) These conditions results in IF=IE=1.0 and therefore
Suncorr=Scorr Specific value of the footing and CDM and PF/CDM columns are also given in Table 5.2
Figure 5.3 Soil profile under the examined footings (Ideal case) JEF case
Case 2 examines settlement of a shallow footing of a real project, namely Jincheon Energy Factory (JFE), in Jincheon District, Chungcheongbuk Province, S.Korea In the JEF project, an area of 5.2 ha (158 m 338 m) was improved by a total number of 7,693 PF columns with a total length of 68,138 m The improvement work was carried out from August 1, 2016 to February 23, 2017
The soil profile under the considered footing (name F2) at JEF show in figure 5.4 As shown, the soil profile consists layers of fill materials (0 to 2.0 m), silt (2.0 to 3.0 m), loose to medium dense silty sand (3.0 to 9.0 m), medium dense to dense weathered soil (9.0 to 24.0 m) and weathered rock (24.0 m downward) The equivalent modulus of the soil (Es) was evaluated as a simple function of the normalized corrected SPT N value (Sabatini et al 2002): Es (kPa) =k(N 1 )60, where k is a correlation coefficient and varies depending soil types Following the suggestion in the manual, the coefficient was taken as: k = 400 for the first three layers, k = 700 for weathered soil layer from 10.0 m to 20.0 m, k 00 for the rest of the weathered soil layer Cross-sectional and plan views of the examined footing (named F2) at the JEF project is schematically shown in Fig 5.5
Figure 5.4 Soil profile under the examined footings (JEF project)
As shown, a square concrete footing (B B) of thickness tf was designed to place on top of the soil zone improved by 9 PF columns Specific values of the footing as well as of the PF/CDM columns are given in Table 5.2 Note that the typical diameter of CDM column (DCDM) is beforehand chosen and it acts as a required input value for the comparison
M dense to denseWeathred soil(Silty sand)Fill layer analysis The vsalues of Dh, Dt, Lh, Lt, Lc are herein varied with respect to DCMD, thus the values are note given in the table 5.2
Figure 5.5 Cross-sectional and plan views of the examined footing at JFE project
Table 5.2 Input parameters for settlement analysis
Parameter Unit Ideal case JEF case
Silt Silty sand layer Fill d d
5.1.3 Results and discussions Analytical results
For footing in each case study, the settlement calculated for two conditions of the deep mixed columns:
(1) Ground improved by CDM columns (i.e., =1)
(2) Ground improved by PF columns (i.e., >1) The literally means that all input parameters (i.e., soil properties, footing size, number of PF/CDM columns, strength of mixed columns, loading conditions, etc.) are identical for the two conditions expert the column shapes For the ideal case, the settlement was evaluated up to the depth z.0 m (end of the weathered soil layer) and for the JEF project until the depth z$.0 m (end of the weathered soil layer) In both cases, thin sublayers
h=0.1 m were used for settlement for calculated
Under the confined condition above and a give value of (d/DCDM), the value of and were varied with variation intervals of ==0.1 ( for =0.1 to 0.9 and =1.1 to 2.0) to examine settlement behavior Figure 5.6 and 5.7 show some typical settlement curve (Scorr) with respect to for ideal case and JEF project Note the horizontal line of =1 (i.e., Dh=Dt) in the figures corresponds to the settlement of the footing on CDM columns It is clear from the figures that under some certain ranges of (0.4 to 0.7), settlement values of the footing on PF columns distinctively smaller than that on the CDM columns This indicates in general configuration of PF columns under shallow footings improves the stiffness of the ground more effectively than CDM columns As show in Figure 5.7, each settlement curve ( for a given ) exists a minimum settlement value named Scorr, PF, min Figure 5.8 shows the variation of settlement ratio Scorr, PF,min/Scorr,CDM with respect to for three typical values, where Scorr,CDM is the settlement of the same footing on CDM columns It is interesting to note that, for three different typical values, the settlement ratio reaches minimums values at relatively similar values of 1.45 to 1.55 At the value around 1.5, the settlement of footing on PF columns approximately 0.9 times the settlement shallow footing on the CDM columns
Figure 5.6 Settlement value from analytical method for Ideal case
Co rre ct ed S et tle m en t, S co rr (m )
Figure 5.7 Settlement value from analytical method for JEF case
Figure 5.8 Variation of Scorr,PF,min/Scorr,CMD ratio
Elastic settlement of the same footings was evaluated using the numerical method (Plaxis 3D) with input value given in Table 5.2 The numerical analysis was carried for two improved ground conditions:
(1) The ground was equally improved entirely over the area the soil domain This is unreal but to make equal condition to be analytical analysis, which homogeneous soil layers are assumed to extend infinitely in the horizontal directions
(2) The ground was improved only under the footing area
Co rre ct ed S et tle m en t, S co rr (m )
Se ttl em en t r at io , S co rr, PF ,m in/ S co rr, CD M
Figure 5.9 and Figure 5.10 show a comparison of settlement curves for a few typical value obtained from numerical analysis (for ground condition 1) Results show in Figure 5.9 and Figure 5.10 indicate two distinctive features:
(a) Settlement curve both analytical method and numerical method show similar trends
That is, for each curve there exist a minimum settlement value at certain value in the range of 0.4 to 0.5
(b) The three typical values shown, the smallest settlement curves appears at = 1.6 for both the approaches; These two featuresimply that both numerical and analytical methods results in rather a consistent behavior
Figure 5.9 Settlement values from analytical and numerical analyses for Ideal case
Co rre ct ed S et tle m en t, S co rr (m )
Figure 5.10 Settlement values from analytical and numerical analyses for JEF project Besides the general settlement behavior as described above, the absolute settlement values from the two methods should also be noticed For the ideal case, settlement values from analytical method are larger than that from numerical method, whereas for the JEF project the values are opposite The result from the former is theoretically correct because for a finite layer thickness (herein h = 15.0 m or just 2.8B), stress profile from analytical method is larger than that from numerical method For the actual case of JEF, the settlement depends mainly on (i) actual stress increment profile below the footing; (ii) stiffness of the footing; (iii) embedment depth
The approximate analytical solutions to the three factors can produce quite different results compared with that from the 3D numerical analysis and these can be causes making the opposite results These inconsistent results need more investigation to elucidate
Figures 5.11 and 5.12 show numerical analysis results for the ground condition 2 (improved only under the footing area) for the two cases, respectively As shown, the settlement curves expose similar trend to that obtained from the ground condition 1, however the settlement magnitude is 3 to 4 times larger than that from ground condition 1
The reason is attributed to the influence of larger lateral displacement of the surrounding soil, which is not improved
Co rre ct ed S et tle m en t, S co rr (m )
The different settlement magnitude between ground conditions 1 and 2 also need more detailed studies to figure out the true scale
Figure 5.11 Settlement analysis from numerical method for ideal case
Figure 5.12 Settlement analysis from numerical method for JEF case 5.2 Settlement analyses using nonlinear models
CONCLUSIONS AND RECOMMENDATIONS
This study presents a comparative study on the effectiveness of PF columns and conventional CDM columns in reducing settlement of shallow foundations using analytical method and numerical methods The following are key conclusions drawn from the study
On settlement analysis using elastic theory:
1) Under the same ground models, settlement of the footings on PF columns is smaller than that on conventional CDM columns
2) Settlement curves (S versus ) from both analytical method and numerical method show similar trends
3) For each value, an optimal shape of PF column is obtained (i.e., giving minimum settlement) for a certain value of (, are defined in chapter 4)
When varies from 1.4 to 1.6, the optimal PF columns are obtained with varies from 0.4 to 0.6 The optimal settlement from ground improved by PF columns may be as small as 0.9 times that from ground improved by CDM columns
On settlement analyses using nonlinear materials:
1) For single columns, settlement of PF column is smaller than settlement of conventional CDM column The larger head of PF column reduces the pressure transfer to the lower portion of the column
2) Under the following conditions: (i) the ratio of stiffness of PF column (or CDM column) over that of the surrounding soil varies from 10 to 20 times; (ii) the head of the PF columns installed sufficiently deep in the soft soil layers, the settlement of footing on PF columns is generally smaller than that on CDM columns The first condition is to make sure that the equivalent material model is applicable
When the stiffness of the PF columns (or CDM columns) is much larger than the stiffness of surrounding soil layer (typically larger than 20 times), the columns would act as piles In this case, the effectiveness of PF columns is insignificant
On the behavior of true 3D column model over the equivalent material model:
1 Analyses using the true 3D model of columns and soil show that if the PF columns do not satisfy the two conditions above then the use of PF columns is not effective
2 By true 3D model of columns and soil, when the load-settlement is still in relatively linear range, the settlement values from the equivalent soil model and true 3D column and soil model are relatively equal This may suggest the equivalent soil model can be used in practice as it has been used in the elastic analyses
3 For 3D column of columns and soil, the settlement of shallow footing on PF columns is affected by soft soil layers under the floating columns In many cases, when using PF columns, pay attention to the position of columns’ toe and stiffness of PF columns
In my thesis, the research is still limited It is:
1) Not fully analyzed the behavior of soil and PF columns (true 3D model of PF column and soil)
2) In four cases, the footing is rigid foundation In case, the footing is flexible foundation, we need analyze settlement of footing on PF columns This is a problem need consider
1) Analysis ultimate bearing capacity of footing on PF columns both numerical method and analytical method by equivalent materials
2) Analysis ultimate bearing capacity of footing on PF columns both numerical method and analytical method by true 3D model of PF columns and soil.
[1] Bergado, D T (1996) Soft Ground Improvement: In Lowland and Other Environment
Deep soil mixing to reduce embankment settlement Ground Improvement
[3] Brinkgreve, R B J., E Engin, and W M Swolfs "PLAXIS 3D 2013 user manual." Plaxis bv, Delft (2013)
[4] Boussinesq, J (1885) Applications des potentiels à l’étude de l’équilibre et mouvement des solides elastiques Gauthier–Villard, Paris
[5] Bowles, J E (1977a) Foundation analysis and design
[6] Bowles, J E (1977b) Foundation analysis and design McGraw-Hill
[7] Bredenberg, H., Broms, B B., & Holm, G (1999) Dry Mix Methods for Deep Soil Stabilization
[8] Bruce, M.E.C., Berg, R.R., Collin, J.G., Filz, G.M., Terashi, M and Yang, D
.(2013) Federal Highway Administration Design Manual: Deep mixing for embankment and foundation support
[9] Chai, J., & Carter, J P (2011) Deformation Analysis in Soft Ground Improvement
[10] Das, B M (2015) Principles of Foundation Engineering Cengage Learning
[11] Das, B M., & Sobhan, K (2013) Principles of Geotechnical Engineering, SI Edition
[12] Duncan, James M., and Chin-Yung Chang "Nonlinear analysis of stress and strain in soils." Journal of Soil Mechanics & Foundations Div (1970)
[13] Fahey, M., & Carter, J (1993) A finite element study of the pressuremeter test in sand using nonlinear elastic plastic model Canadian Geotechnical Journal, 30, 348–362
[14] Han, J (2015) Principles and Practice of Ground Improvement Retrieved from Hatanaka, M., & Uchida, A (1996) Empirical Correlation between Penetration Resistance and Internal Friction Angle of Sandy Soils SOILS AND FOUNDATIONS,
[15] Helwany, S (2007) Applied Soil Mechanics with ABAQUS Applications
[16] Holtz, R (1991) Stress Distribution and Settlement of Shallow Foundations 64
[17] J Wardle, L., & A Fraser, R (1976) Numerical analysis of rafts on layered foundations Geotechnique, 26, 613–630
[18] Kim, Khi-Woong, Dong-Wook Kim, and Myoung-Su Jo "Behavior of Variable Cross-Section Soft Ground Reinforced Foundation in Soft Grounds." Journal of the Korean Geosynthetics Society 15.4 (2016): 89-96
[21] Kondner, Robert L "Hyperbolic stress-strain response: cohesive soils." Journal of the Soil Mechanics and Foundations Division 89.1 (1963): 115-143
[22] Kulhawy, Fred H., and Paul W Mayne Manual on estimating soil properties for foundation design No EPRI-EL-6800 Electric Power Research Inst., Palo Alto, CA (USA); Cornell Univ., Ithaca, NY (USA) Geotechnical Engineering Group, 1990
[23] Lambe, T William, and W Allen Marr "Stress path method." Journal of Geotechnical and Geoenvironmental Engineering 105.ASCE 14655 Proceeding
[24] Le, V H., Nguyen, D T., Nguyen, T D., & Tran, Q D (2018) Nonlinear settlement of spreadfootings on sand The Transport Journal, 86–90 Hanoi
[25] Liu, Song-Yu, et al "Field investigations on performance of T-shaped deep mixed soil cement column–supported embankments over soft ground." Journal of Geotechnical and Geoenvironmental Engineering 138.6 (2012): 718-727
[26] Mayne, P., & Poulos, H (1999) Approximate Displacement Influence Factors for Elastic Shallow Foundations Journal of Geotechnical and Geoenvironmental Engineering - J GEOTECH GEOENVIRON ENG, 125
[27] Murthy, V N S (2001) Principles of Soil Mechanics and Foundation Engineering
[28] Nguyen, D T., Nguyen, T D., Le, V H., & Hoang, D P (2019) Soft ground improvement by an improved CDM method 157-161 Vietnam – Japan Science and Technology Symposium (VJST2019)
[29] Nguyen, D T (2019) An evaluation of the effectiveness of head-enlarged soil cement columns (HCC) in ground improvement (Master thesis 2019)
[30] Nguyen, Tien Dung, Duy Phuong Hoang, Quynh Giao Tran, and Sung Gyo Chung
"Analytical and Numerical Analyses on Stiffness Enhancement of Ground Improved by Head-Enlarged CDM Columns." In Geotechnics for Sustainable Infrastructure Development, pp 579-586 Springer, Singapore, 2020
[31] Obrzud, R The hardening soil model: A practical guidebook Zace Services, 2010
[32] Phutthananon, Chana, et al "Dependence of ultimate bearing capacity and failure behavior of T-shaped deep cement mixing piles on enlarged cap shape and pile strength." Computers and Geotechnics 97 (2018): 27-41
[33] Poulos, H G., & Davis, E H (1974) Elastic solutions for soil and rock mechanics
[34] Potts, D M., Zdravković, L., Addenbrooke, T I., Higgins, K G., & Kovačević, N
(2001) Finite element analysis in geotechnical engineering: application (Vol 2)
[35] Rujikiatkamjorn, C., Indraratna, B., & Chu, P J (2005) Ground Improvement:
[36] Schanz, T., P A Vermeer, and P G Bonnier "The hardening soil model: formulation and verification." Beyond 2000 in computational geotechnics (1999):
[37] Schmertmann, J.H., Hartman, J.D and Brown, P (1978) Improved Strain Influence Factor Diagrams Journal of the Geotechnical Division, 104(No
[38] Schofield, Andrew, and Peter Wroth Critical state soil mechanics McGraw-hill,
[39] Skempton, A W (1954) The Pore-Pressure Coefficients A and B Géotechnique, 4(4), 143–147
[40] Song-Yu, L., Yan-Jun, D., Yao-Lin, Y., & J., P A (2012) Field Investigations on
28 Performance of T-Shaped Deep Mixed Soil Cement Column–Supported
[41] Embankments over Soft Ground Journal of Geotechnical and Geoenvironmental Engineering, 138(6), 718–727
[42] Enkhtur, Odgerel, et al "Evaluation of the settlement influence factors of shallow foundation by numerical analyses." KSCE Journal of Civil Engineering 17.1 (2013):
[43] Wood, David Muir Soil behaviour and critical state soil mechanics Cambridge university press, 1990
[44] Yaolin, Y., Songyu, L., Yanjun, D., Zhiduo, Z., & Guangyin, D (2012, June 18)
The T-Shaped Deep Mixed Column Application in Soft Ground Improvement
Grouting and Deep Mixing 2012, pp 389–399.65
[45] Yi, Y., S Liu, and A J Puppala "Laboratory modelling of T-shaped soil–cement column for soft ground treatment under embankment." Géotechnique 66.1 (2016):
[46] Yi, Y., Liu, S., & Puppala, A J (2018) Bearing capacity of composite foundation consisting of T-shaped soil-cement column and soft clay Transportation Geotechnics, 15, 47–56
[47] Yi, Yaolin, et al "Vertical bearing capacity behaviour of single T-shaped soil– cement column in soft ground: laboratory modelling, field test, and calculation." Acta Geotechnica 12.5 (2017): 1077-1088
[48] Zwillinger, D (2018) CRC Standard Mathematical Tables and Formulas.