Lateral Movement of Slope Stabilized with DCM Column Rows Phermphorn Boathong Doctoral student Department of Civil Engineering, Kasetsart University, Thailand e-mail:
[email protected]
Pitthaya Jamsawang Corresponding author, Lecturer, Department of Civil Engineering, King Mongkut’s University of Technology NorthBangkok, Thailand e-mail:
[email protected]
Warakorn Mairaing Associate Professor, Department of Civil Engineering Kasetsart University, Thailand
ABSTRACT This study investigates the parameters affecting the lateral movement of an excavated slope in soft Bangkok clay stabilized with deep cement mixing (DCM) columns. The parameters considered included the spacing, depth, elastic modulus and volume of the row of DCM columns. A three-dimensional finite element model (3D-FEM) was used for the analyses, with the initial calibration based on the results of full-scale field tests conducted to determine the elastic modulus of the soil. The results are presented in term of the lateral movement of soil with respect to the aforementioned parameters. Recommendations are made for selection of appropriate design parameter values to minimize the lateral movement of soil in excavated slopes in soft Bangkok clay.
KEYWORDS:
deep mixing, lateral movement, parametric study, stabilization
INTRODUCTION The key characteristics of soft clay are low strength and high compressibility. The construction of embankments over soft clay frequently leads to large lateral movement, excessive settlements and slope instability (Abusharar et al. 2009). A number of ground improvement techniques are available to address these problems, such as reinforcement with geosynthetics, pre - loading, stage construction, excavation and replacement, light-weight fill, pre-fabricated vertical drains, and reinforcement with piles (concrete piles, stone columns or deep mixing columns) and combinations of these techniques have also been used (Abusharar et al. 2009, Oliveira et al.2011). One effective ground improvement technique that is widely used for embankments on soft clay is the use of deep cement mixing (DCM) columns (Horpibulsuk et al. 2011). Deep cement mixing columns were first used in Japan and the Nordic countries in the mid-1970s, and their use then spread to China, South east Asia, and several other parts of the world (CIDT 2002). The - 1647 -
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DCM technique involves injecting cement and/or lime additives in either slurry or powdered form into the ground and mixing the additive or additives with native soil with mixing blades to form hard treated soil columns (Liu et al. 2012). DCM columns can also be used for purposes other than embankment stabilization; permanent slope stabilization is an example of another use. However, few papers related to slope stabilization using DCM columns have been published. Taesiri and Chantaranimi (2001) reported that DCM columns were effectively used to improve the stability of road embankments along canals. Watn et al. (1999) studied the strength and deformation parameters of undisturbed soil and stabilized soil for the purpose of designing a slope stabilized with DCM columns. Buathong and Mairaing (2010) studied the creep behavior of a slope stabilized with DCM columns. Priol et al. (2007) investigated the effects of various parameters on the factor of safety of a DCM-stabilized slope using three-dimensional finite element analysis. Several studies on the DCM technique conducted in recent decades have focused on laboratory testing and embankment behavior. Most laboratory studies of the DCM technique were conducted to examine the influence of the additive type or quantity on the resulting soil strength, stiffness, consolidation time and permeability (Miura et al. 2001, Horpibulsuk et al. 2004a, Rahman et al. 2011). The behavior of embankments built over DCM columns has been studied in field tests and by numerical simulation. Bergado and Lorenzo (2002) studied the differential settlement between DCM columns and the surrounding soil under embankment loading conditions. Lai et al. (2006) constructed a full-scale embankment over stiffened and unstiffened DCM columns to investigate settlement, lateral movement and excess pore water pressure in the surrounding soil. Liu et al. (2012) investigated the performance of T-shaped DCM columns supporting an embankment over soft ground. Paulo et al. (2011) conducted two-dimensional finite element analyses of embankments reinforced with DCM columns to study the influence of various factors on the settlement, vertical stress and excess pore water pressure in the surrounding soil. Han et al. (2007) use two-dimensional finite difference analysis to investigate the stresses and deformations of DCM columns used to support widening of an embankment. Huang and Han (2010) studied the time-dependent behaviour of a geosynthetics-reinforced DCM column-supported embankment using two-dimensional finite difference analysis. The existing papers related with the stabilization of excavated slopes using DCM columns do not give much attention to the lateral movement of the excavated slope. Lateral movement occurs frequently in excavated slopes in soft clay and usually contributes to failures or loss of serviceability of the excavated slope. There are several parameters that could affect the lateral movement of a slope stabilized with DCM columns, including the spacing, depth, elastic modulus and volume of the DCM columns in the slope. Due to the complexity of this problem, the effect of the parameters mentioned on the lateral movement of excavated slopes stabilized with DCM column were numerically analysed in this study using three-dimensional finite element model (3D-FEM) developed using the Plaxis 3D Foundation version 2.1 software. This study describes the two main parts of the study. The first part concerns numerical back calculation performed with field data to determine the elastic modulus of the soil considered. The second part concerns the analyses of the above-mentioned parameters. The results are presented in terms of the effects of the parameters analyzed on the lateral movement of soil and the optimum values of the parameters.
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SELECTED CASE STUDY To ensure that a reasonable value was used for the elastic modulus of the soil considered in the parametric analyses, a field study was selected for use in calibration. The field case study selected was the Suvarnabhumi Drainage Canal Project, which consisted of an excavated canal and roadways construction on soft Bangkok clay stabilized with DCM columns. Two configurations of DCM columns from the project were selected for the numerical back calculation and parametric analyses. A brief summary of the aspects of the project that pertain to this presented below. The soil and DCM column properties are presented in Fig. 1. The soft clay layer, which can be found at a depth of 15.0 m, is underlain by 4.2-m-thick medium stiff clay. Stiff clay can be encountered at a depth of 19.2 m. The undrained shear strength (Su) of the soft clay and medium stiff clay, determined from field vane shear tests, increases slightly with depth, from 5 kN/m2 at a depth of 5.5 m to 25 m kN/m2 at a depth of 15 m depth. An undrained shear strength of 132.5 kN/m2 was estimated for the stiff clay from standard penetration test (SPT) results. The values of the unconfined compressive strength (qu) and the undrained elastic modulus (Eu) of the DCM column shown in Fig. 1 were obtained from the field specimens. To ensure a factor of safety of 1.3 during the construction, the DCM column ground improvement technique was selected for this project. The canal and roadway cross-section and the configuration of the DCM columns are shown in Fig. 2. For the initial configuration of the DCM columns (Fig. 2a), a group of DCM columns (called DCM bearing columns) with 0.6 m in diameter was designed with a rectangular spacing of 1.50 m × 1.75 m to increase the bearing capacity of road embankment. Due to the low tensile strength of the DCM columns, the DCM column configuration on the canal slope consisted of seven 0.6-m-diameter for columns in a row (called as DCM column row) spaced 1.5 m apart to increase the stability of the slope. The specified compressive strengths of the DCM columns were 600 and 1000 kN/m2 for the DCM bearing columns and the columns in the DCM column row, respectively. To achieve the required compressive strength, 220 kg/m3 of cement was used. The construction sequence was divided into four stages. First, silty sand fill was used to construct a 1.2-m-thick working platform to serve as an access road and roadway. Second the DCM columns were installed. Third, the canal was excavated to a depth of 3.0 m. Forth, the road embankment was constructed to a height of 2.4 m. The construction in each stage was performed rapidly; therefore, dissipation of excess pore water pressure did not occur. A field test with the initial configuration of DCM columns (Fig. 2a) indicated that slope failure occurred when the excavation depth of the canal reached 3.0 m. Unfortunately, no instrumentation was installed during the excavation. However, excessive lateral movement was believed to be the major cause of the failure. To remedy the slope failure, additional DCM columns were added under the berm area between the DCM bearing columns and the DCM column row, as well as in front of the DCM column row (Fig. 2b). The field test was repeated with careful monitoring of the lateral soil movements in each stage of excavation, using inclinometers installed prior to the excavation of the canal. Figure 2b shows the layout of the inclinometers. The maximum lateral movement of the soil measured by inclinometer I2 after completion of the excavation was approximately 14 mm while inclinometer I1 measured a lateral movement backward during the excavation. The load induced by the operation of heavy equipment near the location of inclinometer I1 resisted lateral movement at the location of I1. Thus, the direction of lateral movement changed from forward to backward.
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PL, Wn, LL, %
Soil Profiles
0
0
50
100
Su from FVS test, kN/m2
qu of DCM column, kN/m2 Eu of DCM column, kN/m2
150 0 10 20 30 40 50 60 70 0
1000
2000
3000 0
500000
1000000
Very soft clay
Depth (m)
5
Field test For analysis
Very soft to soft clay (CH)
Soft clay
10
15
Medium to Stiiff Clay (CH)
20
PL Wn LL
Medium (CH)
Stiff Clay (CH)
Average value
Average value
Stiff Clay (CH)
Figure 1: Soil and DCM column properties.
11.00 m
1 DCM φ 0.60 m L = 8.00 m
2
+2.40 m
5.00 m
8 DCM φ 0.60 m L = 15.00 m
Roadway
24 m
+1.20 m
+0.00 m
3.5 Bearing DCM columns
CL
Canal
Canal slope
DCM columns row
+0.00 m
1
Berm
4.20 m
8 @ 1.75 m
(a)
1
-3.00 m
7 DCM φ 0.60 m L = 12.00 m
1
1 DCM φ 0.60 m L = 8.00 m 8 DCM φ 0.60 m L = 15.00 m
2
11.00 m +2.40 m
Berm
Canal slope
5.00 m
24 m 3.5
Bearing DCM columns
7 DCM φ 0.60 m L = 12.00 m 3 DCM φ 0.60 m L = 15.00 m (Additional DCM piles)
3 @1.75m I2
-3.00 m
1
4 DCM φ 0.60 m L = 8.00 m (Additional DCM piles)
4.20 m 8 @ 1.75 m
CL
Canal
+1.20 m
DCM columns row
Roadway
Inclinometer I1
(b)
Figure 2: Canal and roadway cross-section with DCM column configuration. (a) Initial DCM column configuration, (b) Remedial DCM column configuration
NUMERICAL BACK CALCULATION Numerical back calculation was performed to verify the range of the soil stiffness in term of its elastic modulus. A trial-and-error procedure was used to obtain the best fit between the measured lateral soil movement and the finite element simulation results. The back-calculated elastic modulus of the soil was then used in the parametric analyses.
Modeling and Boundary Conditions A three-dimensional finite element model implemented in the Plaxis 3D Foundation software was used in the back calculation. The typical three-dimensional mesh used for the simulation is shown in Fig. 3. The soil profile was divided into the following four layers: 5.5 m of very soft clay, 9.5 m of soft clay, 4.2 m of medium clay, and 1.6 m of stiff clay. Due to the symmetry of the problem, half of the canal was modeled with a size of 80 m x 20.8 m x 6 m. The boundary
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conditions were restraint of horizontal displacement and free vertical displacement at the side boundaries and the restraint of both horizontal and vertical displacements at the bottom boundary. Additional DCM columns
I1
I2
Bearing DCM columns
9.5 m
Soft clay
4.2 m
20.8 m
Very soft clay
5.5 m
+0.0 m
DCM columns row
+1.2 m
Medium clay
Stiff clay 1.6 m
Inclinometer 2 (I2) 80 m
Inclinometer 1 (I1)
Figure 3: Finite element mesh for numerical back calculation
Material Model and Parameters The working platform, in-situ soil and DCM columns were modeled as linearly elasticperfectly plastic materials according to the Mohr Columb failure criterion. The Mohr Columb model is considered to be a first-order approximation of real soil behavior and is highly recommended for cases in which the values of soil parameters are not known with great certainty (Brinkgreve and Swolfs, 2007). Many researchers have used the Mohr Columb model to simulate the behavior of soft clay (Hossain et al. 2006, Huang et al. 2006, Madhyannapu et al. 2006, Han et al. 2005, Chen et al. 2006). The total stress approach assuming undrained conditions (φ = 0) was used for the analyses, based on the rate of construction, the failure behavior and the available information on the soil parameters (Georgiadis and Georgiadis, 2010). Because the working platform was constructed using silty sand fill material, drained soil parameters (c′, φ′) were adopted for this material. The Su values of the soft clay and medium stiff clay were input as indicated by the short dashed line in Fig. 1, based on the increase in Su with depth. The Su of the stiff clay was assumed to be constant with depth. The undrained Poisson's ratio (νu) of the soils was assumed to be 0.495. The undrained elastic modulus values of the soils were taken to be multiples of their undrained shear strength values (Eu = αSu). To verify the range of Eu values for the soils, the α was assumed to range from 200 and 1,000 and to be constant with depth for all soils. For the DCM columns, an unconfined compressive strength qu of 1,000 kN/m2 and an undrained Young's modulus Eu of 225,000 kN/m2 were used in the analyses, as shown by the short dashed line in Fig. 1 that corresponds to the empirical relation E50 = 225qu. A tensile strength of 0.16qu was used to define the failure criterion of the DCM columns in flexural mode (Jamsawang et. al, 2011). A complete list of the parameter values used is provided in Table 1.
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Table 1: List of parameters used in numerical back calculation Very soft clay
Soft clay
Medium stiff clay
Stiff clay
DCM column
Roadway
0.0- 5.5
5.5-15.0
15.0-19.2
19.2-20.8
-
-
Model
MCM
MCM
MCM
MCM
MCM
MCM
Eu(kPa)
200-1000Su
200-1000Su
200-1000Su
200-1000Su
225000
E' = 5600
γ (kN/m2)
15.5
15.5
17
18
15
20
υ
0.495
0.495
0.495
0.495
0.33
0.33
c' (kPa)
-
-
-
-
-
5.0
φ (°)
-
-
-
-
-
30
Su (kPa)
5
7
27
132.5
500
-
Su, increment (kPa/m)
0.37
2.1
7.86
0
0
0
Material Depth (m)
Calculations For the initial conditions, the initial stresses were generated using the K0-procedure based on to Jaky's formula. The calculation stages followed the actual construction stages described in the previous section. After the generation of the initial stresses, the working platform was modeled with a height of 1.2 m. The DCM columns were then modelled as installed, and the canal was excavated to a depth of 3.0 m. The calculations were performed assuming undrained conditions because of the rapid rate of construction, which implies that excess pore water pressure was not dissipated. The back calculation process for determination of the Eu values of the soils was conducted in two steps. First, Eu values between 200 and 1,000 Su were input (constant values with depth) to verify the range of Eu. Second, the Eu value for each layer was adjusted to obtain the best fit between the field measurements and the simulation results.
Results of Back Calculation A comparison of the FEM analysis results and the measured lateral soil movement is presented in Fig. 4. The Eu/Su ratios that yielded the best fit were 300, 600, 1,000 and 1,000 for the very soft clay layer, soft clay layer, medium stiff clay and stiff clay layer, respectively. The back calculation values are consistent with those obtained using the method proposed by Duncan & Buchigani (1997), based on the plasticity index (PI) and the overconsolidation ratio (OCR). According to that method, Bangkok clay, which has an OCR value of OCR value of approximately 1-1.5 (Shibiya et al. 2003), Eu/Su = 300-600.
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Lateral Movement, mm 2
0
5
10
15
20
25
30
-150-120 -90 -60 -30 0 2
30 60
0
0
-2
-2
-4
-4
*
-6
-6
-8
-8
Soft clay 1 * Eu = 300Su
Depth, m
Depth, m
0
Lateral Movement, mm
-10 -12
-18
-10
Soft clay 2 * Eu = 600Su
-12
-14 -16
Depth of excavation
Eu for best fit
-14 Meaused Eu =200Su Eu = 400Su Eu = 600Su Eu = 1000Su Obtained Eu
-16 -18
-20
Measured Eu = 200Su Eu = 400Su Eu = 600Su Eu = 1000Su Obtained Eu
Medium clay * Eu = 1000Su Stiff clay *Eu = 1000Su
-20
(a)
(b)
Figure 4: Comparison between FEM simulations and measurement data (a) Inclinometer I2, (b) Inclinometer I1.
PARAMETRIC ANALYSES Parametric analyses were performed to study the effect of various parameters on the lateral movement of the soil and to determine optimum values of the influential parameters to use in design to minimize lateral movement during the excavation stage. The lateral movement of the soil at two positions, one close to the top of the slope and one at the middle of the slope, was calculated in the analyses (unless otherwise stated). The optimum parameter values are presented in term of ranges, beyond which an increase in the parameter values has little effect on the lateral movement of soil. To determine these rages, the lateral movement of soil at the depth of excavation was plotted with respect to each parameter. The configuration of the DCM columns shown in Fig. 2a, without additional DCM columns, was selected as the base problem. The parameters varied pertained only to the DCM column row located on the slope. The parameters analyzed included the spacing (Sr), depth (Dr), elastic modulus (Er) and volume of the DCM column row. One parameter at a time was varied with respect to the base problem to investigate the influence of that factor on the lateral movement of the slope. Table 2 presents a summary of the cases considered in the parametric analysis, except for the volume of the DCM column row, which is discussed in a later section. The soil stratification and DCM columns used in the parametric analyses are presented in Fig. 5, which also shows the lateral movement locations for considered in the analyses. The model size was 80 m x 20.8 m x 6 m. The boundary conditions were restraint of horizontal displacement and free vertical displacement at the side boundaries and restraint of both horizontal and vertical displacements at the bottom boundary. The parameter values used for the soils and
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the DCM columns in the parametric analyses are provided in Table 1. The elastic modulus values of the soil layers were obtained from the back calculation. The simulation was divided into three stages. First, the working platform was modeled with a height of 1.2 m after the generation of the initial stress conditions. Next, the DCM columns were activated. The final stage was the excavation of the canal to a depth of 3.0 m. The lateral movement was set to zero to investigate the effects of the parameters.
Table 2: Summary of parametric analysis Parameter
Dr (m)
Spacing of DCM column row (Sr)
Sr (m)
Er (MPa)
0.75 1.50∗ 3.00 6.00
Depth of DCM column row (Dr)
10.00 11.00 12.00 13.00∗ 14.50 17.10 19.20
Elastic modulus of DCM column row (Er)
75 150 225∗ 300 375 450
∗
Note: Base problem
Effect of DCM column row spacing (Sr) The design objective for DCM columns is produce a soil-column system as a composite material (Geo 11, 2011). To achieve this objective, soil arching behavior is required. Wang and Yen (1974) analytically investigated soil arching behavior for piles in rigid-plastic semi-infinite soil slopes. They concluded that there is a critical pile spacing for both sandy and clay slopes beyond which almost no arching develops. A parametric analysis was performed to determine the maximum DCM column spacing still could generate sufficient soil arching, estimated by the relative movement between the columns and the soil in between. If the DCM columns and the soil
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between the columns move nearly the same amount, the spacing can be considered to be effective in term of arching. The values of DCM column row spacing (Sr) examined were 0.75 m, 1.50 m, 3.00 m and 6.00 m.
6m
P2
Bearing DCM columns
DCM columns row
+1.2 m
9.5 m
Soft clay
4.2 m
20.8 m
Dr
Very soft clay
P1
5.5 m
+0.0 m
Sr Sr Sr Sr
Selected position for investigation of lateral soil movement
Medium clay
Stiff clay 1.6 m
Position 2 (P2) 80 m
Position 1 (P1)
Figure 5: Numerical modeling for parametric analyses To investigate the effect of spacing on the arching behavior, the lateral movement of the first DCM column of the DCM column row was compared with the movement of the soil between the columns. Figure 6 provides an illustrative comparison of two extreme cases: a very close spacing (Sr = 1.25D) and a large spacing (Sr = 10D). When the spacing is close, the soil arching is more pronounced, and the soil-column system behaves as a composite material or a group in resisting sliding of the soil. The soil and DCM column row move nearly the same amount without soil flow. It is clear that soil arching is not mobilized with the large spacing and that soil movement is concentrated between the DCM column. Based on the results shown in Fig.7, when Sr = 10D, the columns behave almost as single isolated columns, and the soil flows between them. Group behavior, in which the soil and columns move together due to pronounced aching, occurs at Sr ≤ 5D. This finding is consistent with the findings of Prakash (1962), Cox et al. (1984), Reese et al. (1992) and Liang and Zeng (2002), all of whom found that a pile spacing S ≤ 5D is required to generate group behavior. Furthermore, as the column spacing decreases, the rigidity of the soil-column system increases, so the lateral bearing capacity of the slope is improved (Yang et al. 2011). The optimum spacing of DCM columns in the DCM column row is suggested to be in range of 2.5 to 5D. A spacing S < 2.5D may not be cost-effective because soil arching can be mobilized at a larger spacing. However, both common engineering practice and numerical research related to pilestabilized slopes indicate that Sr = 4D can be thought of as the most effective arrangement (Petchgate et al. 2003, Vottipruex et al. 2001).
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Effect of DCM columns row depth (Dr) For slope stabilization with piles/columns (also referred to herein as column) the depth of column tip, measured from the natural ground level, influences the amount of lateral movement that occurs in slope. In general, the depth of the column tip is required to be greater than the critical failure surface depth to obtain sufficient passive resistances. However, increasing the column length increases the installation cost; therefore, the critical depth of the column tip beyond which the lateral movement of the slope is only slightly is sought altered. The following values of the DCM columns row depth (Dr) were investigated: Dr = 10.0 m, 11.0 m, 12.0 m, 13.0 m, 14.5 m, 15 m, 17.1 m and 19.2 m. To determine the minimum DCM columns row depth to use in first analysis, the potential failure surface depth of an unstabilized slope was first investigated. The shear strength reduction method was used to investigate the potential failure surface of an unstabilized slope. Although the exact location of the critical slip surface cannot be calculated by the shear strength reduction method, displacement zones can be used. The maximum depth of the potential failure surface was estimated approximately to be 7.0 m from the ground surface, as shown in Fig. 8. Therefore, the first DCM column row depth analyzed was 10.0 m from the ground surface, 3.0 m deeper than the maximum potential failure surface depth. Lateral soil movements at the excavation depth were plotted as shown in Fig. 9. The lateral movement of the column in the middle of the row of columns was selected for interpretation of the results. The lateral movement of the soil decreases with increasing column depth, as expected because the passive forces increase due to the generation of fixity conditions. For Dr ≤ 11.0 m, the fixity is not sufficient for the development of the DCM column row capacity, and the DCM column row exhibits rigid-body rotation without substantial flexural resistance. Fixity begins to become pronounced at Dr ≥ 12 as indicated by a substantial decrease in the lateral soil movements. However, it is advisable that the depth of the column tip be no longer than necessary because the bending moment can increase and can exceed the column’s tensile capacity. The results indicate that an increase in Dr from 10 m to 13 m has a large effect on the reduction of lateral soil movement but that further increases in Dr have little effect. The optimum Dr beyond which the lateral movement changes slightly with increasing Dr is found to be in the range of 13.0 to 15.0 m, which suggest an empirical relation for the optimum column depth as 1.9 to 2.1 times the critical slip surface depth. DM columns row
1.25D
DM columns row 1.25D DM columns row DM columns row DM columns row DM columns row DM columns row DM columns row DM columns row
1.25D
Slope direction
1.25D 1.25D 1.25D 1.25D 1.25D
(a) Close spacing Sr = 1.25D
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DM columns row
Slope direction 10.0D
DM columns row
(b) Large spacing Sr = 10.0D
Figure 6: Contours of horizontal movement at a depth of 3.0 m. (a) Close spacing Sr = 1.25D, (b) large spacing Sr = 10.0 D. (the mesh geometry has been prepared so that various DCM column row spacing could be modeled). 240
Lateral soil movement, mm
220 200
10.00D
180 160 140
Optimum spacing
120 100
5.00D
Base problem
*d = pile diameter
80 40
0
Soil in between DCM columns row
2.50D
1.25D
60
1
2
3
Sr, m
4
5
6
7
Figure 7: Lateral movement of DCM column row and soil in between at 3.0 m deep for various spacings
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0.00 m -3.00 m
Very soft clay
-5.00 m Potential failure surface
-10.00 m
Soft clay
-15.00 m -20.00 m
Figure 8: Displacement zones for potential failure surfaces before stabilization
Lateral soil movement, mm
300 Position 1 Position 2
250
Optimum value
200 Potential failure surface depth
150 100
Base problem
50
Soft clay
0
0
2
4
6
8
Medium clay
10
12
14
16
18
20
22
Dr, m
Figure 9: Lateral soil movement at a depth of3.0 m for various depths of DCM column row
Effect of DCM columns row modulus (Er) To evaluate the effect of this parameter, six values of the elastic modulus of the DCM column row (Er) were considered: 75 MPa, 150 MPa, 225 MPa, 300 MPa, 375 MPa and 450 MPa. All other parameters for the DCM column row were held constant, and the spacing was held constant at 1.50 m. It should be noted that this range of Er values includes values much higher than those reported in the literature for DCM columns. The elastic modulus of DCM column derived from field coring in soft Bangkok clay has been reported to range from 60.5 to 122 MPa (Petchgate et al. 2003). However, higher values of the elastic modulus (Eu = 225 MPa) can be found, as in the case of the base illustrated in Fig. 1. High Young's modulus values were analyzed to provide
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information that might be useful in the design of other column materials that can be used in slope stabilization, such as combinations of concrete piles and DCM (Vottipruex et al. 2001) Figure 10 shows the effect of the elastic modulus of the DCM column row (Er) on the lateral movement of the slope. As expected, the overall trend is one of the lateral soil movement with increasing elastic modulus of the DCM column row. A column spacing of 1.5 m was previously found to be sufficient to mobilize soil aching and make the soil-column system behave as a composite material. Therefore, increasing the elastic modulus can improve the lateral bearing capacity of the slope or the global stiffness of the soil-column system (Oliveira et al. 2011). Moreover, the reduction in lateral movements of the soil is significant when the elastic modulus of the DCM column row increases from 75 to 150 MPa, and tends to be constant for higher values. This means that the influence of the elastic modulus of the DCM column row only is significant when the column has low stiffness. The optimum value of Er ranges from 200 to 300 MPa; beyond this range, the effect of Er diminishes to a point at which it no longer affects the lateral movement of the soil. It should be noted that high Young's modulus for the columns may lead to failure of the columns before the soil strength between the columns is mobilized. The design concept of this type of soil-column system is that the peak shear strength of the columns is mobilized at the same time as the peak shear strength of the unstabilized soil between the columns.
Lateral soil movement, mm
300 Position 1 Position 2
250 200 Optimum Er
150 100 50 0
Base problem
0
100
200
300
400
500
600
700
800
Er, MPa
Figure 10: Lateral soil movement at a depth of 3.0 m for various moduli of DCM column row
Effect of DCM column row volume The results of the analyses described in the previous sections indicated that the optimum parameter value ranges are consistent with the parameter values used in the base problem. Therefore, to minimize the lateral movement of the slope more than in the base problem, increasing the volume of the DCM column row is proposed. In the analyses, the volume of the DCM column row was increased in the cross-sectional direction toward the slope while the spacing was held constant at 1.5 m. The effect of the volume of the DCM column row on the
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lateral movement of the slope was analyzed for four cases, excluding the base problem, as shown in Fig 11. The results shown in Fig. 12 indicate, as expected that the lateral movements of the soil decreases as the volume of the DCM column row increases. This decrease is significant for an increase in the volume of the DCM column row from that of the base problem to that of pattern 1, but the decrease in the lateral movement of the soil is slight for higher value of the DCM column row volume. Figure 12 shows that the increasing the volume of the DCM column row from that of the base problem resulted in maximum lateral movements of less than 40 mm. The main reason for this decrease is that the DCM column row is sufficiently stiff to resist sliding of the soil mass. Within the range of variation of the stabilized area, increasing the volume of the DCM column row is effective in limiting the lateral movement of the soil. The lateral movement of soil decreases significantly when the volume of the DCM columns row increases from 5.25 m3/m to 8.75 m3/m but only decreases slightly as the DCM column row increases further. Based on the results obtained for the lateral soil movement, the optimum volume of the DCM columns row, beyond which the lateral movement of the soil increase only slightly, corresponds to pattern 2.
+1.20 m
Bearing DCM columns
DCM columns row
+1.20 m
Bearing DCM columns
-13.0 m
-13.0 m
11.2 m
7.7 m
(a) Pattern 1
(b) Pattern 2 +1.20 m
+1.20 m
Bearing DCM columns
DCM columns row
Bearing DCM columns
DCM columns row
DCM columns row -13.0 m
-13.0 m 14.7 m
16.2 m
(c) Pattern 3 (d) Pattern 4 Figure 11: Analysis cases for DCM column row patterns
Lateral soil movement (mm)
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Position 1 Position 2 Base problem Pattern 1 Pattern 2 Pattern 3 Optimum column volume Pattern 4
0
5
10
15
20
25
DCM columns row volume (m3/m)
Figure 12: Lateral movement of soil at a depth of 3.0 m for various volumes of DCM column row
CONCLUSIONS In this study, the effects of parameters on the lateral movement of slopes stabilized with DCM columns were investigated. The parameters analyzed included the spacing, depth, elastic modulus and volume of the row of DCM columns located on the slope. The Suvarnabhumi Drainage Canal Project was used as a case study. Due to the complexity of the problem, the effects of the parameters were analyzed numerically using a three-dimensional finite element model (3D-FEM) implemented using the Plaxis 3D Foundation software. The simulation was performed in stage in accordance with the actual construction stages. The following conclusions can be drawn from the analysis results: 1) The lateral movement of the soil decrease as the DCM column row spacing (Sr) decreases. When the spacing is close (Sr ≤ 5D), soil arching is more pronounced and the soil-column system behaves as a strong composite material in resisting the sliding soil. When the spacing is large (Sr= 10D), the columns behave almost like an individual isolated column, and the soil flows between column. The optimum spacing (Sr) is judged to be in the range of 2.5D to 5D. 2) The DCM column row depth (Dr) has a significant influence on the lateral movement of the soil and the failure mode of the column. When the DCM column row depth (Dr ≤ 11 m) is not sufficient to provide fixity, the DCM column row exhibits rigid-body rotation without substantial flexural resistance, and the lateral movement of the soil is large. Therefore, to utilize the full capacity of the DCM column row to resist the sliding soil, the fixity condition is needed. The results show that the optimum DCM columns row depth is in the range of 1.9 to 2.1 times the critical slip surface depth. 3) The lateral soil movement decreases as the elastic modulus of the DCM column row (Er) increases. The results show that the influence of Er is only significant when the DCM column row has low stiffness. The optimum Er was found to be in the range of 200 to 300 MPa.
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4) Increasing the volume of the DCM columns row is highly effective in limiting the lateral movement of the soil and improves the factor of safety of the slope because the DCM column row is stiff enough to resist the sliding soil mass.
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