Research on Stability of Dam Substation on Inclined Soft Soil Foundation Reinforced by Pile Foundation

Abstract

For the construction of dam substations in coastal or mountainous areas, inclined soft soil foundations are very common. The unique engineering characteristics of inclined soft soil foundations can bring great difficulties to the construction of dam substations. In this paper, a pile foundation reinforcement dam slope model on an inclined soft soil foundation is established; the influence of different pile spacings, the pile length, and the soft soil foundation angle on the slope safety factor is studied; and the failure mechanism and stability of pile-supported dam slope foundation are analyzed. The research results indicate that pile foundation reinforcement can reduce the deformation of the dam slope foundation and improve stability. The pile layout has an important impact on stability, but a change in the pile spacing has little effect on the settlement surface at the bottom of the dam slope. The pile length has a significant impact on the safety of the slope within a certain range. The main stress area of the pile is 0–2 m above the pile, and its main deformation is the lateral deformation of the upper part of the pile. The research results of this article can provide parameter support and theoretical guidance for the construction of dam substations.

1. Introduction

During the construction of dam substations, different complex sites are often encountered. It is inevitable to encounter inclined soft soil foundation problems, and building a dam transformer substation on an inclined soft soil foundation is a challenging task for geotechnical engineers [1,2]. A reasonable reinforcement measure can improve the utilization rate of materials and save resources. In order to save resources and be conducive to sustainable development, more reasonable reinforcement methods need to be found. Under the influence of inclined soft soil foundations and dam self-weight, engineering problems such as the uneven settlement and excessive lateral deformation of roadbeds and slope instability are prone to occur during the construction of dam substations on sloping soft soil foundations [3]. For soft soil foundation problems, concrete piles [4,5], stone piles [6,7], deep cement mixing piles [8,9], inclined piles [10,11], and T-shaped deep cement mixing piles [12] are often used to improve the strength and stiffness of such subsoils before dam substations are constructed.

FEM, FDEM, and NMM are common methods used to analyze slope stability. Chai et al. [13] analyzed the stability of soft soil embankments improved though soil–cement columns by means of two- and three-dimensional finite element modeling. It was found that although the total settlement of the foundation decreased, the settlement rate under the embankment load increased due to the high stiffness of the column material. Zheng et al. [14] proposed a limit equilibrium model combined with the genetic algorithm (LEM–GA) by compiling a MATLAB program, which was used to predict the safety factor and failure surface of rock slopes subjected to bending and toppling failure. Phutthananon et al. [2,12] presented the results of full-scale field tests of embankments supported with wood-cored stiffened deep cement mixing (SDCM) columns and conventional DCM columns, confirmed that a less stiff material such as wood can be used as a core in SDCM columns, and evaluated the performance of a TDM pile-supported embankment (TPSE) over soft foundation subsoils using 3D numerical modeling. In addition, many scholars proposed novel processing methods. Many scholars [15] also combined FDEM and FEM to study the slope failure behavior of noncohesive media. Within each discrete element, the finite element method (FEM) formulation is embedded so that the contact forces between and deformation of these discrete elements can be more accurately predicted.

Many other scholars studied the engineering characteristics of slopes and dams on sloped soft soil foundations. Lai et al. [16,17] studied the settlement behaviors of a saturated tailings dam’s soft ground under a CFG pile composite foundation treatment, analyzed the pile stresses, and found that a CFG pile treatment is effective in reinforcing a saturated tailings dam; loading had little influence on the settlement of soil between piles. Zhou et al. [18] proposed a new scheme for strengthening embankment foundations with inclined rather than vertical PCP piles. Meanwhile, the engineering characteristics and reinforcement effect of the foundation were analyzed by establishing a finite element model. Toshinari et al. [19] investigated the external stability of DMM columns on inclined foundations by means of centrifuge model tests. Ge et al. [20] studied the working characteristics of the pile–net composite foundation during the soil filling and consolidation stage. Abe et al. [21] studied the dynamic behavior of a slope model including various inclined weak layers and compared the simulated and experimental results of the landslide displacements and response accelerations above the weak layers.

The above research results mainly analyze the pile structure. However, the damage mechanism of a slope foundation and the reinforcement effect of a pile set in an inclined soft soil foundation have rarely been studied. For different forms of inclined soft soil foundations, the engineering characteristics are not the same, as the use of a single form of reinforcement is often unable to meet the requirements. Therefore, in order to study the stability of pile foundation reinforcement on the inclined soft soil foundation of a dam substation, a pile support model of the dam slope reinforcement for an inclined soft soil foundation is established. The influencing factors of the pile spacing, pile length, and soft soil foundation inclination angle are selected. And the damage mechanism of a slope foundation, the reinforcement effect of a pile, and the stability of the pile foundation reinforcement of a dam substation are analyzed. Through the research on the stability of the pile foundation reinforcement of a dam transformer substation’s inclined soft soil foundation, a more economic and reasonable reinforcement scheme can be realized. This can save resources and is conducive to sustainable development.

2. Numerical Model Establishment

2.1. Establishment of the Model

(1)

Model overview

The construction of a dam substation over an inclined soft soil foundation can easily lead to landslides. The soft soil layer thickness, soil foundation inclination, pile spacing, and pile length are important factors influencing the stability of the slope foundation. In this paper, the model of a pile-supported reinforced dam slope over an inclined soft soil foundation is established. The specific model is shown in Figure 1. The soil is divided into three layers from top to bottom, namely, the slope fill layer, soft soil layer, and subsoil. The simplified model section form is selected as follows: the top surface width of the slope is 14.6 m, the slope height is 4 m, the side slopes are 1:1.75, and the inclination of the foundation is θ₁ = θ₂ = 6°. The thickness of soft soil layer is 2 m, and the thickness is uniform. The diameter of the pile is 0.80 m, the length is 6 m, and the elastic modulus is 30 GPa.

(2)

Model setting

In this paper, the finite element platform ABAQUS is used for modeling and calculation analysis. The detailed parameters of each part are shown in

Table 1. Physical parameters of soil layer.

Projects	Elastic Modulus (MPa)	Poisson’s Ratio	Unit Weight (kN/m3)	Cohesion (kPa)	Friction Angle (°)
Slope	100	0.35	19	20	25
Soft soil	10	0.35	18	10	0
Subsoil	250	0.25	23	\	\
Pile	30,000	0.2	\	\	\

. The model has the following boundary conditions: the top surface of the slope foundation is free, the lateral displacement of the left and right sides of the model is restricted, and the horizontal and vertical displacements of the bottom surface of the model are also restricted. Based on a large amount of survey data and related research in soft soil areas, the research results reveal a consistent pattern, and the friction coefficient between piles and soil is taken as 0.63. The Mohr–Coulomb constitutive model is used in slope filling layers and soft soil layers. It is described as an ideal elastic–plastic material. The elastic constitutive model is used in the subsoil layer and is described as an elastic semi-infinite space foundation model. In this model, the unit type of the pile and soil is the CPE4 unit. Taking the model with a pile spacing of 2.5 m as an example, there are 371 nodes and 220 grid units. The pile grid size is approximately 0.6 m, while the soil unit size is around 8 m.

2.2. Safety Factor and Deformation of Pileless Slope

The safety factor of slope foundation in the strength reduction method is defined as the ratio of the actual shear strength of the rock mass and soil to the reduced shear strength when the slope reaches critical failure. The strength reduction method in ABAQUS software is implemented by setting the soil parameters with the change in field variables, and the value of field variables is taken as the strength reduction coefficient. When the slope is damaged, the value of the field variable is identified as the safety factor. The safety factor of 4 can be considered as the safety of the slope. Therefore, in order to minimize the computation, the maximum strength reduction factor (safety factor) is set to 4 in this paper.

Studies have shown that it is feasible to solve the safety factor by using the finite element method combined with the strength reduction method in evaluating the stability of a slope [22,23]. Therefore, in this paper, the safety factor of slope foundations with and without piles is solved by the method. The settlements of the slope surface with and without piles, as well as the equal settlement surface of the slope, are analyzed. The piles are set in the inclined soft soil foundations. Details of the pile set and the finite element calculation results are shown in Figure 2 and

Table 2. Safety factor of slope with or without piles.

Projects	Pile Length/m	Pile Spacing/m	Pile Diameter/m	Safety Factor
With piles	6	4	0.8	4
Without piles	/	/	/	1.52

. According to Figure 2, when the slope foundation does not set piles, the main deformation is concentrated at the foot of the slope foundation, the maximum deformation is 1.6 m, and the safety factor is calculated to be 1.52. The maximum deformation of the slope foundation has been reduced to 5 mm, and the safety factor has been increased to 4 through the setting of piles. The influence of different pile spacing, pile length, and soft soil foundation inclination angle on the safety factor and deformation of slope foundations with piles will be studied next.

3. Analysis of Results

3.1. The Influence of Different Soft Soil Foundation Inclination Angles

The formation of soft soil layers in inclined soft soil foundations is primarily caused by transportation and accumulation, resulting in uneven thickness of the soft soil layers [5]. The safety factor and deformation of the slope foundation will be affected by the uneven thickness of soft soil layers. Therefore, the piles are set at the foot of the right slope. The piles have a length of 6 m, a distance of 4 m between each pile, and a diameter of 0.8 m, as shown in Figure 1. Three working conditions of θ₁ = θ₂, θ₁ > θ₂, and θ₁ < θ₂ are obtained by changing the inclination angle of the soft soil foundation. For the purpose of analysis, the study focused on the impact of uneven soft soil layers on the safety factor and deformation of slope foundations. In this study, θ1 was kept constant at 6°, while θ2 was varied at 5°, 6°, and 7°, respectively. The results of the calculations are shown in Figure 3 and

Table 3. Safety factor of slope with different soft soil inclination angle.

θ2	5°	6°	7°
Safety factor	4	4	3.9

. In the figure, the equal settlement surface is marked with red line segments. As shown in Figure 3, the equal settlement surface and safety factor remain almost unchanged when θ₁ = θ₂ and θ₁ > θ₂. However, when θ₁ < θ₂, the displacement of the left slope of the foundation will be reduced. When θ₁ > θ₂, the slope is more susceptible to damage when the soft soil layer is thicker. This is because the increase in thickness of the upper soft soil layer causes a change in the position of the equal settlement surface. This shows that the distribution of the slope safety factor and the equal settlement surface will be affected by changes in the foundation slope. This is because a change in the inclination angle of the foundation will result in a change in the thickness of the soft soil.

3.2. The Influence of Different Pile Spacing

The influence of pile spacing on the safety factor and deformation of slope foundations with piles is further analyzed. The geometric parameters of the slope model remain unchanged, and only the distance between piles is altered. Under the condition of a fixed pile length of 6 m and θ₁ = θ₂, when the pile spacing k is 2 m, 2.5 m, 3 m, and 4 m, respectively, the safety factor of the slope foundation and the position of the equal settlement surface are analyzed using finite element calculation. The specific layout of the piles and the calculation results are shown in Figure 4 and

Table 4. Safety factor of slope with different pile spacing.

Working Condition	Pile Length /m	Pile Diameter/m	Pile Spacing/m	Number of Piles	Safety Factor
1	6	0.8	2	10	4
2	6	0.8	2.5	9	4
3	6	0.8	3	8	4
4	6	0.8	4	6	4
5	6	0.8	3	6	3.4

. As the pile spacing increases, the maximum deformation of the slope foundation changes from 5.078 mm to 5.958 mm. This shows that the deformation of the slope foundation can be reduced by reducing the pile spacing and increasing the number of piles. From the displacement contour in Figure 4, the position of the equal settlement surface at the bottom of the slope is not greatly affected by the change in pile spacing, and it is close to the underlying layer. However, the position of the equal settlement surface of the soft soil around the slope foundation will be affected with the change in pile spacing. In the process of finite element modeling, if the piles set improperly, the slope foundation will be damaged by the lack of support. As shown in Figure 5, the displacement is mainly concentrated in the damage of the right foot of the slope, and the safety factor is 3.4. The above analysis shows that pile spacing and arrangement of piles have a significant influence on the stability of slope foundations.

3.3. The Influence of Different Pile Lengths

After studying the influence of pile spacing on the safety factor and deformation of slope foundation, the pile length was selected for the study. Only the pile lengths of the model were changed, and the pile lengths L were taken to be 3 m, 5 m, 6 m, 7 m, 8 m, 10 m, respectively, to study the effect of pile length on the safety factor and deformation of the slope foundation. The specific calculation results are shown in

Table 5. Safety factor of slope with different pile lengths.

Working Condition	Pile Length/m	Pile Diameter/m	Pile Spacing/m	Maximum Deformation/mm	Safety Factor
1	3	0.8	4	250.6	3.6
2	5	0.8	4	6.0	4
3	6	0.8	4	5.8	4
4	7	0.8	4	5.3	4
5	8	0.8	4	5.3	4
6	10	0.8	4	5.2	4

and Figure 6. Figure 6 shows that when the pile length L = 3 m, the equal settlement surface is concentrated on the left side of the slope. It is similar to the equal settlement surface of a slope foundation without piles. The equal settlement surface of models with different pile lengths is located near the bottom of the soft soil layer. With the increase in pile length, the primary deformation area of the slope foundation shifted from the right side to the upper left area and the right foot of the slope foundation. Additionally, the equal settlement surface of the soft soil surface surrounding the slope foundation experienced a significant upward movement. This shows that by adjusting the length of piles, the position of the equal settlement surface and deformation of the slope foundation and the surrounding soft soil can be effectively controlled. As a result, the stability of the most vulnerable part of the slope foundation is enhanced.

For the deformation of slope foundations, the relationship between the number of piles and the maximum deformation is shown in Figure 7. When the pile length increases from 3 m to 5 m, the maximum deformation of the slope foundation suddenly decreases from 250 mm to 5 mm. Then, with the increase in pile length, the maximum deformation of the slope foundation is almost unchanged, stabilizing at approximately 5 mm. This indicates that within a certain range, an increase in pile length has a significant effect on reducing the deformation of the slope foundation. However, after reaching a certain length, as the pile length continues to increase, the deformation of the slope foundation reaches a minimum stable value without any changes. Therefore, a reasonable design of pile length can ensure the stability of the slope foundation while saving engineering materials. In terms of safety factor, the factor of safety is 4, regardless of the length of the pile. This shows that the safety of the slope foundation can be effectively improved by the setting of piles, and reasonable pile length is needed in engineering applications.

3.4. Displacement and Stress of Piles

In order to better guide the design of engineering piles, the horizontal loading, horizontal and vertical displacements, pile shear stresses, and axial stresses are studied. The geometric parameters of the model are kept as shown in Figure 1. For the convenience of analysis, the piles are labeled as P1, P2, P3, P4, P5, and P6 from left to right. The calculation results of the horizontal load F1 are shown in Figure 8. In Figure 8, the stress change trend of all piles is a “bow” change. At the top 2 m of the pile and at the bottom, the forces are greater. At 2–4 m of the pile, P6 is subjected to larger lateral forces, P1 and P2 are the smallest, the lateral forces on the pile decreased with the downward slope, and the stress on P1 and P2 is larger when the pile is 2–5.5 m. The lateral force on the pile decreases with the upward slope, and the main force part is 0–2 m of the pile.

The displacement of piles is divided into horizontal displacement U1 and vertical displacement U2, and the two parts are analyzed separately. Figure 9 shows that the horizontal displacement of different pile positions varies greatly and has certain regularity. For the vertical displacement of different piles, the difference is at the top of each pile, while the vertical displacement of the remaining parts is 0. P1, P2, and P3 located on the slope have similar deformation law, with the maximum displacement occurring at the top of the pile. As the buried depth increases, the horizontal displacement of the pile decreases linearly, and the horizontal displacement at the bottom of the piles is approximately 0. Pile P2 is more affected because it is located below the position of maximum slope deformation, so its lateral displacement is 2 mm as the maximum lateral displacement. For pile P4 and P5, the horizontal displacement is less than 0.2 mm, so it can be considered that pile P4 and P5 are less affected by the slope soil. The larger horizontal displacements occur in pile P6, which is similar in value to pile P1. According to the above analysis, the slope foundation model is mainly composed of piles P1, P2, P3, and P6 resisting the deformation of the soil. In the actual project, a suitable number of piles and pile spacing can be set as required to improve the utilization rate of materials.

Then, the stress analysis of the pile is carried out, in which the stress of piles is divided into shear stress S1 and axial stress S2. The pile is subjected to shear stresses as shown in Figure 10a. The pile is subjected to large shear stresses mainly at depths of 0–2 m which is consistent with the displacement in Figure 9a. At 2–6 m, piles P1 and P6 are subjected to higher shear stress compared to the other piles. The pile is subjected to axial stresses as shown in Figure 10b, the axial stress is mainly concentrated in 2–4 m, and the axial stress of piles P1, P2, and P3 in this part is larger; the construction of the dam substation should focus on this part. During the construction of the project, reasonable measures can be taken to adjust the piles based on the shear and axial stresses.

4. Conclusions

In this paper, the dam slope model of a pile-supported inclined foundation is established, the influence of pile spacing, pile length, and soft soil foundation inclination on the safety factor of slope foundation is studied, and the failure mechanism and stability of the pile supported dam slope foundation are analyzed. The main conclusions are as follows:

(1)

The increase in the soft soil layer thickness will reduce the safety factor of the slope and affect the distribution of the vertical displacement equipotential surface. In the engineering design process, the influence of soft soil layers in the actual formation should be considered.

(2)

Pile foundation reinforcement of the dam slope on an inclined soft soil foundation can effectively reduce slope deformation and improve the slope safety factor. The reduction in pile spacing and the increase in piles can significantly reduce the deformation of the dam slope. Pile layout has a significant effect on the stability of slope foundations, but the change in pile spacing has no significant effect on the settlement surface of the slope bottom.

(3)

Pile length increase can significantly reduce slope deformation; when the pile length exceeds a certain range, the slope deformation reaches a very small stable value. As the pile length increases, the deformation stabilizes. Therefore, it is important to set the pile length appropriately in actual engineering design, to avoid material waste.

(4)

The pile side force is mainly distributed in the range of 0~2 m from the top of the pile, and the main deformation is the lateral deformation of the upper part of the pile. The foundation deformation is primarily resisted by the piles on both sides, and the lateral deformation in the middle position is smaller. In the actual project, it is necessary to set a reasonable number of piles and pile spacing to ensure the stability of the slope while improving the utilization of materials.