Keywords

1 Introduction

Under the action of wind load, large span transmission project of crossing tower inevitably produce additional stress and deformation. If the additional stress or deformation of pile foundation is too large, it will affect the normal use of large span project. Therefore, domestic scholars put forward new construction technology and construction methods to enhance the long-term service safety of large span projects.

Construction techniques such as high-strength prestressed pipe pile and post-grouting of foundation can not only reduce the construction period, but also improve the ability of resisting external deformation of large-span foundation [1,2,3]. In order to determine the bearing capacity of large-span pile foundation, Zhang Tianguang et al. [4, 5] found that the self-balancing test method of pile foundation can quickly and accurately determine the bearing capacity of large-span pile foundation, which provides guidance for engineering design optimization.

In order to optimize the design of pile foundation for large-span projects, many scholars used three-dimensional numerical simulation to analyze the stress deformation law of pile foundation [6,7,8,9]. Based on the quadratic programming method, Fang Xiaowu et al. [6] optimized the layout of large-span pile foundations. Through three-dimensional numerical analysis, the development depth and width of soil cracks on the side of the pile were obtained [7].

For the large span pile foundation project, the current research focuses on the construction method and bearing capacity characteristics of the foundation pile foundation. In order to limit the deformation of large span pile foundation, anti-slide pile is often constructed on the side of pile foundation, but the reinforcement effect of anti-slide pile is not clear. Through the systematic three-dimensional finite element numerical analysis, the reinforcement effect of anti-slide pile is studied, which provides guidance for the optimal design of large span pile foundation.

2 Three-Dimensional Finite Element Simulation

2.1 Engineering Situation

The Yangtze River Crossing Project of Fengcheng ~ Meili 500 kV line in Jiangsu adopts the “with-direct-direct-with-resistance” crossing mode. The length of the tensioning section is 4.055 km and the spacing distribution is 755 m-2550 m-750 m. The total height of the span tower is 385 m. The concrete filled steel tube tower is made of Q420C, and the interior is filled with C50 self-compacting concrete. The foundation adopts the cast-in-place pile scheme of the cap. The four foundations are independent and the concrete connecting beams are used between the caps. The upper layer of the foundation soil is about 10 m thick of muddy silty clay, while the lower layer is composed of fine sand, silty clay, and medium sand, without any bedrock. Three rows of anti-slide piles are arranged in front of and behind each pile foundation. In the initial design scheme, the length of anti-slide piles of the north span tower is 10.0 m and the number of piles is 360.

2.2 Calculated Operating Condition

Based on the previous study, the lateral deformation of pile foundation can be significantly reduced by installing straining beams. In this numerical calculation, the height of the straining beam is 2.0 m, and the number of straining beam supporting piles is 2. When the front and back of the anti-slide pile are 3 rows, the pile length is 0.0 m, 10.0 m, 15 m and 20 m, respectively. When the length of anti-slide pile is 10 m and 20 m, the number of anti-slide pile rows is 5, as shown in Table 1.

Table 1. Working condition of finite element calculation
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2.3 Wind Load Condition

The loads on the foundation under four kinds of strong winds are shown in Table 2. There are 6 sets of three-dimensional finite element numerical calculation grid in the evaluation and analysis of anti-slide pile effect. A total of 24 three-dimensional finite element numerical calculations are carried out considering the deformation of pile foundation under 4 kinds of wind conditions.

2.4 Finite Element Mesh and Boundary Condition

The software ABAQUS is used to analyze the deformation and stress of oversized large-span engineering pile foundation. Three-dimensional finite element grid partition USES the actual distribution of soil layer, corresponding to the stratigraphic boundary according to the actual survey line access, will be merged into thin soil property close ten layers.

The finite element mesh is divided into 519457 elements and 509692 nodes. Normal displacement constraints are applied to the sides of the formation, and three-direction displacement constraints are applied to the bottom. The coordinate system Z axis used in the finite element calculation model is vertical and upward is positive.

Table 2. Numerical calculation of wind load condition
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2.5 Constitutive Model and Soil Layer Parameters

The Mohr-Coulomb model is used to simulate the mechanical properties of the foundation soil layer and the linear elastic model is used to simulate the pile foundation and cap. The formation material parameters are set based on the geological exploration data of the north span tower foundation. The soil layer names and soil layer parameters are shown in Table 3.

Table 3. Name and parameter of foundation soil layer
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3 Influence of the Length of Anti-Slide Pile

3.1 Effect of Anti-Slide Pile Length on Three-Dimensional Deformation of Foundation

This section focuses on the analysis of the effect of the length of anti-slide pile and wind direction on the three-dimensional deformation of foundation. Four wind conditions of 0°, 45°, 60° and 90° are considered in the numerical calculation. Limited to space, only the three-dimensional deformation cloud image of the foundation under the wind condition of 90° is given.

Figure 1 shows the cloud image of foundation deformation caused by 90° wind load. With the increase of the length of cement mixing pile, the deformation of foundation decreases, but the decreasing range is limited. Under four kinds of wind loads, the deformation of foundation decreases, but the reduction range is not more than 3%.

By setting up straining beams, the four pile foundations of the large span form an integral structure, with strong overall horizontal resistance. The length of the cement mixing pile is limited, which does not form a whole with the pile foundation, and the mechanical properties of the soil within the range of the cement mixing pile are poor. Therefore, the horizontal resistance provided by cement mixing piles is limited.

Fig. 1.
figure 1

Three-dimensional deformation cloud image of foundation under 90° wind load

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3.2 Effect of Anti-Slide Pile Length on Three-Dimensional Deformation of Pile Foundation

Figure 2 shows the cloud map of pile foundation deformation caused by 90° wind load. When the length of anti-slide pile is 0 m, 10 m, 15 m and 20 m, the maximum displacement of pile foundation along the two horizontal directions (U1, U2) and vertical directions (U3) are (46.0, 45.3, 45.2, 45.1 mm), (4.84, 4.67, 4.65, 4.63 mm) and (61.1, 60.9, 60.8, 60.7 mm), respectively.

Fig. 2.
figure 2

Three-dimensional deformation cloud image of pile foundation under 90° wind load

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3.3 Effect of Anti-Slide Pile Length on Maximum Deformation of Pile Foundation

Table 4 summarizes the maximum displacement of pile foundation in three directions caused by 90° wind load under each working condition. With the increase of the length of cement mixing pile, the deformation of pile foundation is reduced, but the reduction range is limited and the reduction range is less than 2%. Cement mixing piles provide limited horizontal resistance. Under 60°, 45° and 0° wind loads, the deformation of pile foundation decreases with the increase of anti-slide pile length. The results show that the horizontal and vertical deformation of pile foundation can be reduced to a certain extent, but the reduction is less than 5%.

Table 4. Maximum deformation of pile foundation under each working condition
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4 Influence of Anti-Slide Pile Row Number

4.1 Effect of Different Anti-Slide Pile Rows on Three-Dimensional Deformation of Foundation

Table 5 summarizes the extreme values of foundation deformation under different rows of stirred piles under 90° wind load. When the length of cement mixing pile is 10 m and 20m respectively, the increase of mixing pile from 3 rows to 5 rows has little effect on foundation deformation. When the cement mixing piles are 10m and 20m respectively and the mixing piles increase from 3 rows to 5 rows under the wind load of 60°, 45° and 0°, the deformation reduction of pile foundation is very small, not more than 2%.

Table 5. Maximum foundation deformation under different working conditions
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4.2 Effect of Different Anti-Slide Pile Rows on Three-Dimensional Deformation of Pile Foundation

Table 6 summarizes the extreme deformation values of pile foundation under different rows of stirred piles under 90° wind load. When the length of cement mixing pile is 10 m and 20m respectively and the mixing pile is increased from 3 rows to 5 rows, the deformation reduction of pile foundation is not more than 3%. When the cement mixing piles are 10m and 20m respectively and the mixing piles increase from 3 rows to 5 rows under the wind load of 60°, 45° and 0°, the deformation reduction of pile foundation is very small, not more than 3%.

Table 6. Maximum deformation of pile foundation under each working condition
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5 Conclusion

The influence of anti-slide pile length and number of rows around pile foundation on the deformation of foundation and pile foundation is systematically studied with three-dimensional finite element numerical analysis aiming at the anti-slip characteristics of cement mixing piles around the north span tower foundation. Based on the numerical results, the main conclusions are as follows:

  1. (1)

    Under four kinds of wind loads, the deformation of pile foundation decreases with the increase of anti-slide pile length. It is shown that the installation of anti-slide pile can reduce the horizontal and vertical deformation of pile foundation to a certain extent, but the reduction is less than 5%.

  2. (2)

    When the length of anti-slide piles is 10 m and 20 m and the rows of anti-slide piles on both sides of the large-span foundation are increased from 3 rows to 5 rows, the horizontal and vertical deformation of pile foundation caused by wind load in four directions is reduced somewhat, but the deformation reduction of pile foundation is less than 3%.

  3. (3)

    By setting the straining beam, the four pile groups of the large span foundation form an integral structure, and its overall horizontal resistance has been strong. The length of cement mixing pile is limited. It does not form a whole with pile foundation and the mechanical properties of soil in the range of cement mixing pile are poor. Therefore, cement mixing pile provides limited horizontal resistance.