Keywords

1 Introduction

Highway in mountainous area may not only be threatened by landslide in the construction process, but also be affected by landslide in normal operation. Once the landslide occurs during the operation period, it not only damages the highway subgrade or bridge, but also threatens the safety of vehicles and pedestrians. Not only the highway landslide can cause the damage of subgrade, but also the landslide treated by anchor pile has signs of revival and deformation of anti-slide pile. For example, the landslide of Trevor-Froncysyllte highway in England slips again after one year of treatment, resulting in the inclination of anti-slide pile at most about 15° [1]; Diezma landslide of A-92 highway in Spain caused the deformation of anchorle again after 8 years of restoration and reconstruction [2]. Anti-slide pile is a common protective measure in landslide treatment engineering [3]. Anchor pile is mainly used to solve the problems of large thrust and deformation control of landslide. The deformation and failure mechanism of anchor pile [4], the internal mechanism of landslide reinforcement [5], and the design and calculation method have been studied extensively at home and abroad. The above research mainly focuses on the design and mechanical mechanism of anchor pile supporting structure, but there is little research on the supporting effect of composite structure mainly based on anchor pile in damaged highway reconstruction.

Based on the G326 highway landslide damage reconstruction as a basis, this paper studies different treatment support plan, and evaluates different support plan prevention effect.

2 General Situation of Highway Subgrade Damaged by Landslide

At about 7:00 on July 8th, 2020, a landslide occurred in G326 K41 + 300 ~ K43 + 000 section of national highway, Tianbao Formation, Shiban Village, Ganlong Town, Songtao County, Guizhou Province. The landslide is about 1100 m in length, 60 ~ 450 m in width, 330 m in height difference between front and rear edges, 2 ~ 10 m in thickness and 136 × 104 m3 in volume.After sliding, a clear sliding surface can be seen in the middle and back of the landslide, which is a light gray-green nodular marl layer with an occurrence of 260°∠26°. The sliding bed is medium-thick nodular marl of Dawan Formation of Ordovician with a small amount of thin limestone and shale in Fig. 1.

Fig. 1.
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Analysis diagram of sliding direction of landslide

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Fig. 2.
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Precipitation before landslide

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3 Simulation Study on Deformation and Failure of Landslide

According to the engineering geological conditions and investigation results of landslide, the deformation and failure process of landslide is studied by numerical simulation method. The calculation parameters are selected based on the test of indoor rock compressive strength and shear strength, and comprehensively considering regional engineering experience, as shown in Table 1.

Table 1. Mechanical parameters of landslide rock
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The deformation and failure of landslide is closely related to heavy rainfall, so the deformation process of landslide under actual rainfall conditions (Fig. 2) is simulated.

Figure 3 is a partial process diagram of landslide deformation and failure development under the actual continuous rainfall condition. At the initial stage of rainfall, the landslide deformation is small, and the main deformation is concentrated near the national G326 highway in front of the landslide (Fig. 3a); On July 8, 2020, the landslide showed obvious deformation as a whole, and the deformation amount and deformation range continued to increase (Fig. 3b).

Fig. 3.
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Cloud diagram of landslide displacement and plastic zone with rainfall process

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4 Analysis of Prevention and Control Effect of Subgrade Reconstruction in Landslide

4.1 Subgrade Reconstruction Prevention Plan

In order to restore the normal operation of National G326 Highway, the area with the most serious impact on the highway is treated by anti-slide pile + anchor cable + clearing slope + facing wall + subgrade restoration + foot wall.

4.2 Calculation Model and Parameters

According to the layout of the design scheme, a numerical calculation model was established. The physical and mechanical parameters of the rock and soil are shown in Table 1, and the parameters of the anchor cable and anti slip pile are shown in Table 2.

Table 2. Physical and mechanical parameters of supporting structure
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In FLAC 3D, cable element is used to simulate anchor cable, and solid structure element is used to simulate anti-slide pile. Mohr-Coulomb criterion is used to describe the stress-strain relationship of rock and soil.

According to the design plan, the simulation calculation of two working conditions is mainly carried out: ① Cutting + anti-slide pile plan; ② Cutting + anti-slide pile + anchor cable plan.

4.3 Governance Effect of Different Control Plans

Influence of the Combination of Cutting and Anti-slide Pile on Landslide Deformation.

On the basis of the above-mentioned cutting, a row of anti-slide piles with a spacing of 5 m is set at the landslide subgrade, totaling 20 piles. The main section is selected as the study section, and the deformation of landslide after treatment is shown in Fig. 4.

Fig. 4.
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Landslide deformation under cutting + anti-slide pile treatment

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As can be seen from Fig. 4, the displacement of landslide subgrade is reduced from 9.8 mm to 4.8 mm after the combination treatment of cutting and anti-slide pile. The displacement of front and rear edges of landslide is basically unchanged, and the anti-slide pile has a good supporting effect on landslide deformation in the treatment area.

Influence of the Combination of Cutting, Anti-slide Pile and Anchor Cable on Landslide Deformation.

The combination of cutting, anti-slide pile and anchor cable is used to support the landslide, and the stress produced at the subgrade after cutting is concentrated and dispersed to the sliding body through the anti-slide pile and anchor cable, thus improving the stress state at the subgrade and achieving the purpose of controlling the deformation of the landslide. The simulation results are shown in Fig. 5.

Fig. 5.
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Landslide deformation under cutting + anti-slide pile + anchor cable treatment

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On the basis of the above support, after adding anchor cables to the anti-slide piles at the subgrade, the displacement of the middle and rear edge of the landslide is reduced. The displacement at the subgrade is reduced from 4.8 mm to 3.1 mm under the support of cutting and anti-slide pile, and the displacement at the trailing edge of landslide is also reduced from 3.0 mm to 1.7 mm. It can be seen that the protection of anti-slide pile to landslide deformation is mainly reflected in the retaining effect of pile foundation on landslide soil, while anchor cable improves the stress state of landslide soil at the layout, so that the stress that should be concentrated in subgrade is evenly distributed on the landslide, thus reducing the deformation of landslide.

Figure 6 shows the change of anchor cable in support. It can be seen that the maximum axial force of anchor is 0.9 kN, which is mainly concentrated in the part below the elevation of the free surface, while the node displacement of anchor is evenly distributed as a whole, with the maximum node displacement of 3.0 mm. It is also verified that the anchor cable mainly improves the stress state of soil, and the stress is evenly distributed on the landslide soil.

Fig. 6.
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Stress and displacement of anchor cable supporting structure

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5 Conclusion

  1. (1)

    The landslide deformation is small at the initial stage of rainfall, mainly concentrated near the national G326 highway in front of the landslide; With the increase of rainfall, the deformation of the front part of the landslide increases continuously, and the deformation concentration area gradually appears near the right front part and the trailing edge of the landslide; On July 8th, 2020, the landslide showed obvious deformation as a whole, and the deformation amount and deformation range continued to increase, and the whole landslide was in a plastic state.

  2. (2)

    The maximum displacement of landslide under different supporting conditions is decreased, among which the maximum displacement of main sliding area and trailing edge is 9.8 mm and 3.0 mm respectively; Under the combined treatment of cutting and anti-slide pile, the maximum displacement of the main sliding area of landslide is 4.8 mm, and the maximum displacement of the trailing edge is 3.0 mm; Under the combined treatment of cutting, anti-slide pile and anchor cable, the maximum displacement of the main sliding area of landslide is 3.1 mm, and the maximum displacement of the trailing edge is 1.7 mm.

  3. (3)

    The protection of anti-slide pile to landslide deformation is mainly reflected in the retaining effect of pile foundation on landslide soil, while anchor cable improves the stress state of landslide soil at the layout, so that the stress that should be concentrated in subgrade is evenly distributed on the landslide, thus reducing the deformation of landslide. The combined treatment plan of “cutting + anti-slide pile + anchor cable” is adopted in subgrade restoration and reconstruction project, so that the overall deformation of landslide is in controllable range.