Landslide–Tunnel Interactions and Control Countermeasures under an Orthogonal System

Abstract

When a tunnel crosses a landslide orthogonally, this interaction can easily lead to instability in both the landslide and tunnel structures. Based on the relative positional relationship between a landslide and a tunnel, we studied the stress mode and deformation characteristics of the tunnel in three positional relationships: within the landslide mass, the sliding surface, and the sliding bed. The tunnel is a typical type located on a sliding surface at an engineering site, so we established a numerical model showing the intersection of the tunnel and the sliding surface. The plastic zone distribution, stress characteristics, and displacement distribution characteristics of the surrounding rock before and after tunnel excavation were studied. Based on the simulation results, we analyzed the control effect of anti-slide piles in controlling the tunnel’s deformation in the surrounding rock from four perspectives: the arrangement of anti-slide piles, the spacing between piles and tunnels, the diameter of anti-slide piles, and the depth of piles embedded in the bedrock. By analyzing the deformation law of the landslide mass and the force characteristics of the tunnel structure under the orthogonal conditions of a tunnel landslide, we provide theoretical guidance for the adoption of anti-slide piles to control instability in tunnel structures.

1. Introduction

Tunnel-crossing landslides are often encountered during tunnel construction. Excavating tunnels can displace landslides, and landslides can cause damage to the tunnel’s lining structures and lead to instability in the surrounding rock [1,2]. Therefore, it is important to study the influence of interactions between tunnels and landslides and ways to control the stability of the surrounding rock.

Tunnel excavation inevitably disturbs the original rock stress field, resulting in stress redistribution. The uneven stress distribution is more obvious when a tunnel passes through unstable areas such as landslides. Owing to the problem of tunnel alignment, many tunnels inevitably cross landslides during construction. Tunnel excavation can trigger or accelerate landslide movement, so landslides can have a great impact on tunnels [3,4]. Currently, the main methods used to study tunnel and landslide interactions are numerical simulations, theoretical analyses, and laboratory tests. Zhang et al. (2022) established three physical model tests in the laboratory to study the influences of different anti-slide pile reinforcement effects on landslides and tunnels. The results showed that after taking the pile-anchor reinforcement measures, the deformation of the landslide, the bending moment of the tunnel, and the contact stress at the interface between the tunnel and the landslide mass are all significantly reduced, further improving the operational safety of the tunnel [5]. Wu et al. (2022) focused on the parallel system of tunnel–landslide interactions and conducted model tests with three tunnel models of different burial depths. The test results indicate that the greater the burial depth of the tunnel, the smaller the impact of stress release after tunnel excavation on the slope surface above the tunnel. With the increase in burial depth, the strain of the tunnel gradually increases [6]. In order to study the influence of landslide and anti-slide piles on tunnel deformation, Li et al. (2022) established three physical model tests with different anti-slide pile spacings. The results show that with the increase in the anti-slide pile spacings, the deformation of tunnel lining and the pressure and bending moments acting on the tunnel also gradually increase. The stability of the surrounding rock can be effectively controlled by reducing the spacing of anti-slide piles [7]. Yi et al. (2016) developed a mechanical theoretical model for tunnels crossing landslides by researching deformation characteristics and stress mechanisms under various tunnel failure modes. They also analyzed interactions between landslides and summarized the factors that influence tunnel deformation and stress distribution patterns [8]. Tao et al. (2020) investigated variation trends in the shear strength parameters of weak structural planes during large deformations and landside failure from an experimental perspective. Their model tests indicate that landslide failure can be divided into four distinct stages: the soil compaction stage, the crack generation stage, the crack propagation stage, and the sliding plane transfixion stage [9]. Komu et al. (2020) and Zhang et al. (2021) used finite element simulation software to calculate and analyze the interaction mechanisms between tunnels and landslides under various factors and scenarios and validated the simulation results. They employed three-dimensional numerical analysis methods to study the relationship between the TBM tunnel excavation and landslides [10,11]. With these methods, a better understanding of the interaction effects between tunnel and landslide systems can be achieved.

Some scholars have studied tunnels and landslides separately, considering the deformations of each one independently without accounting for interaction effects between the two [12,13]. However, many other researchers have studied the interaction effects between tunnels and landslides [14,15]. Liu and Wang (2018) conducted in-depth analyses and research on the mechanisms of tunnel deformation caused by interactions in tunnel–landslide systems, using the numerical analysis method to calculate landslide deformation, stress, and tunnel deformation in detail. Their results showed that a final treatment scheme based on stress and deformation control theory can be used as a reference for similar future projects [16]. Wu et al. (2012) divided tunnel–landslide systems into three major types based on their spatial relationships: parallel, orthogonal, and oblique intersections; they studied the stress patterns for each type. By analyzing this interaction, the tunnel’s failure mode and stress can be well understood, providing a good reference value for controlling tunnel and landslide stability [17]. Similarly, Wang et al. (2014) considered the relative position of the tunnel and the landslide, analyzing an unstable landslide with repeated failures in the tunnel lining. They determined the main factors affecting the failure and deformation of the landslide and tunnel [18]. The relative positional relationship between a tunnel and a sliding surface determines the interaction mode and deformation characteristics. Zhang et al. (2021) proposed a simplified analytical method to evaluate deformation caused by landslides in existing tunnels and then used the transfer coefficient method and the limit equilibrium method to calculate the landslide pressure. They then compared their three-dimensional numerical simulation results with a simplified analytical solution, and good consistency was achieved [19]. Zhu et al. (2021) studied the causes, stress modes, and deformation characteristics of landslide collapse, looking at the interaction between tunnel landslides under an orthogonal tunnel and sliding surface system. The results showed that the tunnel–landslide system’s interaction decreases with increased distance between the two [20]. Causse et al. (2015) analyzed an unstable landslide, showing that tunnel excavation can lead to landslide instability. Excavation affects the displacement velocity of a landslide and the damage to the tunnel’s lining structure. The interaction between a tunnel and an unstable landslide is related to the tunnel’s diameter and the distance between the tunnel and the sliding surface [21]. Roberto et al. (2016) and Caterina et al. (2015) carried out long-term landslide monitoring, and they established a law of interaction in tunnel and landslide systems. Landslides after tunnel construction reduce the operational safety of tunnels [22,23]. Jiao et al. (2013) conducted a risk assessment of a tunnel crossing a landslide and analyzed the interaction between the tunnel’s structure and the local landslide’s instability [24]. This interaction can be studied through geomorphological investigations, geological explorations, monitoring activities, and numerical simulation.

The above studies show that this interaction determines the structural stability of tunnels and landslides. Based on different positional relationships between landslides and tunnels, this study analyzes their interactions for three different positions. Through theoretical analysis, a mechanical calculation model for tunnels and landslides is established. At an engineering site, using a tunnel crossing a landslide surface as an example, a numerical simulation is established to reveal the stress characteristics and displacement variation law of the tunnel; moreover, the effect of anti-slide pile reinforcement measures on its stability is discussed. By studying the stress characteristics and deformation laws of tunnels under the influence of landslides, we can provide a reference for ways to control the stability of rock surrounding tunnels and landslides.

2. Evaluation of Landslide Effects on Tunnels

2.1. Spatial Location of Landslides and Tunnels

The stability of tunnel structures affected by landslides is influenced by numerous factors, such as the angle of the sliding surface, the size of the landslide range, the relative position of the tunnel and the landslide, and the orientation of the tunnel. Different spatial relationships between the tunnel and the landslide can lead to varying effects on the tunnel, such as stress distribution, deformation, and failure characteristics. In particular, the relative position of the tunnel to the sliding plane determines the deformation pattern, stress properties, and failure mechanisms of the tunnel. Figure 1 shows three different positional relationships between a tunnel and a sliding surface.

The orthogonal relationship between a tunnel and a landslide can be divided into three geometrical cases: the tunnel is in the landslide, the tunnel intersects with the sliding surface, and the tunnel is located below the sliding surface. Their interaction can be defined as follows: the construction disturbance of the tunnel causes local deformation in the landslide. When slow deformation occurs to a certain extent, it will cause a landslide; in turn, the landslide affects the tunnel, causing cracks and deformations where the two coincide, seriously affecting the safety of the tunnel and the stability of the landslide.

2.2. Analysis of Three Positional Damage Patterns

The relative positional relationship between the tunnel and the sliding surface determines the force mechanism and deformation characteristics of the tunnel, which are the main factors determining the interaction pattern between the tunnel and the landslide. Based on this spatial relationship, the stress mode and deformation characteristics of the various types can be analyzed, and the interaction laws of the tunnel–landslide system can be determined.

(1)

The tunnel is located above the sliding surface.

When a tunnel is located above the sliding surface, the main feature of the tunnel is longitudinal bending; its positional relationship and force analysis are shown in Figure 2. Under the influence of landslide pressure, the tunnel experiences an uneven distribution of loads. As the uneven load increases, both ends of the tunnel bear significant shear force, and the middle part bears a large amount of bending moment. Under the combined action of landslide pressure and stratum resistance, the failure mode manifests as shear cracks in the tunnel crown and tension cracks in the side wall and arch foot of the tunnel; thus, the strength of the lining decreases. If there are significant in situ stresses within the landslide, the tunnel will experience further damage under the action of ground pressure. The deformation pattern of the tunnel will also become more complex.

(2)

The tunnel intersects with the sliding surface.

When a tunnel crosses the sliding surface of the landslide mass, the lining is dominated by longitudinal cracks, and the deformation on the side of the mountain is serious. The force characteristics of the tunnel are shown in Figure 3. Among them, the blue arrows represents the direction of stress around the surrounding rock, and red dash lines represents the sliding zone area. The tunnel crown and sidewalls are subject to geotechnical pressure with weak shear resistance, and shear damage can easily occur at the junction of the tunnel and the sliding surface. The bending moment of the side wall and the tunnel crown will be the largest, and the mountainside deformation will be more serious. Tunnel excavation causes the sliding surface to slide, with the lining structure on the mountainside of the tunnel bearing and transmitting the thrust from the landslide. Owing to the influence of tunnel excavation, the tunnel easily forms a loose circle, loosening the surrounding rock and further inducing instability and cracking in the tunnel.

(3)

The tunnel is located below the sliding surface.

When a tunnel is excavated below the sliding surface, its main characteristics are crown deformation and side wall deformation on the mountainside. Figure 4 shows that the tunnel crown is subjected to the combined action of rock and soil pressure and landslide disturbance pressure. The transmission of these forces is related to the surrounding rock grade, structure, and other factors. The deformation characteristics mainly manifest as the relaxation of the rock mass at the top of the tunnel, and local collapse or extrusion deformation may occur. Given the uneven load, the axial tension or shear strain of the tunnel lining structure will be greater than its yield value, the crown will bend to the tunnel, and the bottom will be compressed, so the shear deformation of the tunnel near the sliding surface is the most obvious. When the tunnel is close to the slip zone, it has a greater impact on the stability of the landslide. Thus, the surrounding rock strength of the tunnel is related to the distance of the tunnel from the sliding surface.

2.3. Tunnel and Landslide Interactions

The above analysis shows that the tunnel and landslide are both sources of disturbance and are mutually coupled. The interaction process of the two is as follows: The landslide gradually deforms and becomes unstable under various factors. The tunnel’s structure is directly affected by the landslide’s thrust, causing this structure to crack and break. Tunnel excavation relaxes deformations in the surrounding rock, reduces the strength of the surrounding rock mass, and induces landslide deformation. When the stable state of the landslide mass is broken, the landslide causes extrusion deformation in the tunnel’s lining structure in the process of sliding down.

In the current engineering investigation, when the tunnel intersects with the sliding surface, the stress on the tunnel’s lining is the most complex of the three scenarios. When the tunnel intersects with the sliding surface, the deformation at the junction of the sliding surface and the tunnel is the most obvious. At the same time, the deformation of the tunnel lining near the mountainside is significantly larger than that of the tunnel lining far from the mountainside. Therefore, it is important to study tunnel deformation characteristics and control countermeasures in orthogonal tunnel–landslide systems. The following content will focus on studying the damage conditions for when a tunnel orthogonally crosses a sliding surface.

3. Numerical Simulation of Tunnels and Landslides

A mechanical calculation model for tunnel–landslide interactions can effectively determine the structure load of tunnels. To further study these interactions, a numerical simulation model is established based on a practical project. By analyzing plastic zone failure, displacement deformation law, and stress distribution characteristics of a tunnel, effective support schemes can be proposed to ensure stable sliding, reduce tunnel deformation, and maintain the stability of the surrounding rock.

3.1. Numerical Modeling

To fully consider the influence of landslides on tunnels, the finite difference software FLAC^3D 6.0 is used to establish a three-dimensional numerical model for simulation analysis. The tunnel in the model is located on the sliding surface. The three-dimensional size of the model is 180 m × 30 m × 100 m; the height of the left side is 30 m; and the height of the right side is 100 m. The sliding mass is mainly composed of limestone and sandstone, mostly in a weak weathering state, and the cracks are highly developed. The surrounding rock grade of the tunnel is grade V, and the integrity is poor. The lining is made of C25 concrete, and the radial excavation length is 30 m. The boundary conditions of the model are fixed at the bottom of the model, and the normal constraints are taken on its front and rear boundaries and its left and right boundaries. The soil is modeled using the Mohr–Coulomb constitutive model, while the surrounding rock and supports are simulated using solid elements. The material parameters of the model are shown in

Table 1. Material parameter values.

Lithology	Modulus of Elasticity/GPa	Poisson Ratio	Density/(kg/m3)	Internal Friction Angle/°	Cohesion/MPa
Sliding mass	0.62	0.34	2000	32	0.28
Sliding zone	0.25	0.30	1800	28	0.16
Sliding bed	0.7	0.28	2100	23	0.45
Tunnel lining	27	0.24	2500	-	-

. A schematic diagram of the model is shown in Figure 5.

Determining the initial stress state is a critical step before tunnel excavation, as it affects the stress redistribution in the surrounding rock and the stability of the tunnel. Firstly, the boundary conditions of the model are defined, including the size of the model, the position and direction of the geological layer, and any possible sliding surface. The initial stress is determined by applying self-weight stress to the model. The initial stress equilibrium of the model is shown in Figure 6. Under the influence of self-weight, the vertical stress of the model is evenly distributed from top to bottom, and the whole model reaches a state of equilibrium.

3.2. Distribution of Plastic Zones in the Surrounding Rocks

After achieving initial equilibrium, the model undergoes multiple iterative processes. By progressively reducing the material strength parameters, potential sliding surfaces are identified, thereby determining the impact of the landslide on the tunnel before excavation. The distribution of plastic zone changes before and after tunnel excavation is shown in Figure 7. Before tunnel excavation, the sliding zone appears to have plastic zone damage along the tunnel direction. After the tunnel is excavated, owing to the influence of landslide and tunnel excavation, there is significant plastic zone failure in the crown and arch shoulder of the tunnel. The figure shows that the middle and the end of the landslide have entered a plastic state, and the lower part of the landslide has an obvious sliding effect.

3.3. Stress Distribution of Surrounding Rock

The vertical stress distribution of the surrounding rock after tunnel excavation is shown in Figure 8. Under the action of the mountain’s self-weight, the stress distribution is evenly distributed from top to bottom. The maximum stress appears near the right side of the mountain, reaching up to 3.2 MPa. After tunnel excavation, the disturbance caused by the upper landslide results in stress concentration on both sides of the tunnel. The stress on the right side of the tunnel is significantly greater than that on the left side, and the stress concentration is obvious. Owing to the influence of the upper right sliding zone area, the stress is mainly concentrated in the right arch waist and the tunnel crown. Therefore, the tunnel crown and haunch position on the tunnel’s near-mountainside are critical areas for controlling surrounding rock stability.

3.4. Displacement Distribution of Surrounding Rock

The displacement distribution of the surrounding rock after tunnel excavation is shown in Figure 9. After tunnel excavation, the stress is released throughout the range of the landslide, which changes its original three-dimensional stress equilibrium state and further aggravates it. From the perspective of the overall deformation law, the displacement deformation of the tunnel’s crown is greater than that of the arch bottom, and the maximum displacements are 32.5 mm and 12.3 mm, respectively. Owing to the influence of tunnel excavation, the landslide mass slides downward along the landslide surface. The disturbance stress from the landslide near the mountainside causes considerable damage to the tunnel, which leads to the large area settlement of the crown on the right side. Figure 9b shows that the maximum horizontal displacement of the surrounding rock is 22.4 mm; the subsidence phenomenon appears at the top of the tunnel near the mountainside, and the displacement deformation is obvious. There is a large displacement above the landslide and the tunnel crown, indicating that tunnel excavation exacerbates the sliding displacement changes in the landslide mass.

The displacement of the tunnel support structure is primarily characterized by the settlement of the tunnel crown on the near-mountainside. Therefore, the crown, spandrel, and arch bottom are monitored. The monitoring depth is 20 m, and the displacement change is shown in Figure 10. Owing to the influence of the landslide zone above, the crown and the right arch shoulder show large displacement changes, and the change displacement pattern manifests as follows: tunnel crown > tunnel right shoulder > tunnel left shoulder > tunnel arch bottom.

3.5. Interaction between Landslide and Tunnel

During the tunnel excavation process, the landslide is continuously disturbed, which intensifies its deformation. Spatially, the tunnel is located on the sliding surface. A penetrating plastic zone forms on the sliding surface, and the landslide’s mass slides along this plastic zone. Figure 11 shows a schematic diagram of the shear strain change after tunnel excavation. During the initial stage of excavation, the shear strain first appears near the crown on the right side of the tunnel. The increased shear strain increment means that the sliding potential along the region becomes larger. Subsequently, the maximum shear strain increment of the whole landslide mass accumulates on the sliding surface, forming a continuous maximum shear strain increment zone. Landslide failure often occurs in the section with the highest shear strain, and by analyzing the shear strain increment, the weakest points in the tunnel and landslide can be identified.

Tunnel excavation increases the shear strain increment of the shallow sliding surface, and in turn, landslide displacement greatly affects tunnel excavation. Therefore, to effectively prevent landslide and tunnel deformation, it is necessary to propose technical solutions that can stabilize landslide slip, limit tunnel deformation, and control local tunnel stress. To strengthen tunnels and prevent landslide and tunnel deformation, strong engineering support structures such as anti-slide piles are usually used.

4. Research on the Application Effect of Anti-Slide Piles

Tunnel deformation and landslide deformation are interrelated and influence each other. Currently, deformation control technology for tunnel–landslide systems is divided into two kinds of reinforcement means. This study focuses on the reinforcement effect of anti-slide piles [24,25,26]. Through a sensitivity analysis of the parameters such as the form of anti-slide piles, pile–tunnel spacing, pile diameter, and the depth of piles embedded in bedrock, we can provide theoretical guidance for controlling the stability of rock surrounding tunnels.

4.1. Stability Analysis

In our simulation, a contact element between the anti-slide pile and the soil is established, with its basic material parameters shown in

Table 2. Mechanical parameters of anti-slide pile material.

Elastic Modulus/GPa	Cross-Sectional Area/m2	Perimeter/m	Tensile Strength/MPa	Density/(kg/m3)	Poisson Ratio
40	3.14	6.28	400	2400	0.25

.

Firstly, the tunnel’s displacement deformation before and after reinforcement with the anti-slide pile is analyzed. Figure 12 shows the displacement distribution of anti-slide piles before and after reinforcement. Before reinforcement with the anti-slide pile, a maximum vertical settlement value of 27.43 mm appears above the crown support structure, and the whole sliding mass is displaced along the sliding surface of the shallow landslide. After reinforcement with the anti-slide pile, the maximum displacement is reduced by 22.1%. We inferred that anti-slide piles can effectively prevent landslide displacement and reduce tunnel crown settlement.

Figure 13 shows a bending moment distribution diagram of the anti-slide pile and a shear diagram of the tunnel’s lining. The anti-slide pile is less affected by the bending moment at the top and bottom and by the sliding zone area, and the middle part is greatly affected by the bending moment. This means that the soil pressure in the middle of the anti-slide pile is significant, which means it must bear a greater bending moment to maintain the landslide’s stability. The maximum shear stress of the tunnel’s lining appears at the arch shoulder near the mountainside (right), followed by the arch waist, and the shear stress at the crown is very small. The figure shows that, when the tunnel crosses the landslide orthogonally, the maximum shear force of the lining structure often occurs at the junction of the lining structure and the sliding surface of the landslide. Tunnel excavation accelerates the landslide, and in turn, its deformation affects the tunnel. Tunnel deformation and landslide deformation influence each other. It is important to study the application effects of anti-slide piles to control landslide stability and the rock surrounding tunnels [7].

4.2. Anti-Slide Pile Form

Figure 14 shows that anti-slide piles are divided into three types according to their role in the tunnel–landslide systems: the upper blocking (U-b) type, the lower supporting (L-s) type, and the upper block lower support (U-b and L-s) type.

An upper blocking anti-slide pile can stabilize the upper side sliding mass adjacent to the tunnel, ensuring that tunnel deformation does not occur. A lower supporting anti-slide pile can prevent the soil under the tunnel from sliding to ensure the stability of the landslide and the safety of the tunnel. When there are no signs of landslides, laying anti-slide piles under the tunnel can stabilize the mountain. The upper blocking and lower supporting anti-slide pile approach is a combination of the former two. The anti-slide pile on the upper side of the tunnel bears most of the landslide pressure to prevent the tunnel from being squeezed. The anti-slide pile under the tunnel prevents the landslide from sliding between the two rows of piles, thus eliminating the landslide’s impact on the tunnel and ensuring its safety.

Figure 15 shows tunnel deformation in its natural state and with the three anti-slide pile forms. Under the same conditions, the upper blocking and lower supporting anti-slide pile approach minimizes lining deformation in the tunnel and has the best reinforcement effect. For the arch shoulder and arch waist of the tunnel near the mountain, the control effect of the upper blocking anti-slide pile on tunnel deformation is 1.58 times that of the lower supporting anti-slide pile. The lower supporting anti-slide pile exhibits better control over the deformation of the arch shoulder and the tunnel crown on the far-mountainside than the upper blocking anti-slide pile. Unlike the natural state without anti-slide piles, the three forms reduce the deformation value of the tunnel’s lining.

4.3. Pile–Tunnel Spacing

To study displacement changes in the surrounding rock at different distances between the pile and the tunnel and record the surrounding rock’s deformation under different working conditions, a distance range is selected from 1D to 8D, where D is half of the width of the tunnel.

Figure 16 reveals that the increased distance between the pile and the tunnel leads to greater displacement and deformation in the surrounding rock, and the reinforcement effect of the under-supported anti-slide pile on the tunnel gradually weakens. Therefore, the closer the distance between the lower supporting anti-slide pile and the tunnel, the better the reinforcement effect on the lining. As the distance between the piles and the tunnel increases, the strengthening effect of the upper block lower support anti-slide piles and the upper blocking anti-slide piles on the tunnel shows a decreasing and then increasing trend. This is because the distance is too small; the reactive force generated by the anti-slide pile when resisting the sliding force is transmitted to the tunnel structure, thereby increasing tunnel displacement. When the distance between the pile and the tunnel is in the range of 4D~6D, the surrounding rock displacement is minimal. Beyond this range, the reinforcement effect of the anti-slide pile begins to weaken again. Therefore, the upper blocking and the upper block lower support anti-slide piles should be placed in the best position on the upper side of the tunnel for reinforcement. Below the tunnel, the smaller the distance between the anti-slide pile and the tunnel, the stronger the reinforcement effect.

4.4. Diameter of the Anti-Slide Pile

The diameter of the anti-slide pile is an important factor affecting its control effects and cost. This section explores the effect of different pile diameters on their control of the rock surrounding the tunnel. The pile diameter ranges from 0.5 m to 3.0 m. Figure 17 shows the displacement changes in the tunnel crown, the arch shoulder on the far side of the mountain, and the arch shoulder on the near side of the mountain.

Under the above three working conditions, the deformation of the surrounding rock at the three monitoring points gradually decreases with the increasing pile diameter. Figure 17a shows that the upper retaining anti-slide pile has a good control effect on the tunnel crown and the near-mountainside arch shoulder, and the change in pile diameter has little effect on the displacement of the far-mountainside arch shoulder. Figure 17b shows that, compared with the upper blocking anti-slide pile, the lower supporting anti-slide pile has a better control effect on the deformation of the arch shoulder on the far side of the mountain. When the pile diameter is 1.5–2 m, the deformation rate of the far-mountainside arch shoulder is the largest, and the control effect is the best. Figure 17c shows that the control effect of the upper block lower support anti-slide pile on the surrounding rock is better than that of the other two working conditions; 2.0–3.0 m is an effective range increase for the diameter of the anti-slide pile. By comparing the control effects of the different pile diameter changes on the tunnel under different working conditions, we can provide reference ideas for anti-slide piles used to control surrounding rock stability.

4.5. Depth of the Pile Embedded in Bedrock

Relevant research shows that when the sliding bed is a soft soil layer, it is advisable to use 1/3~1/2 of the length of the anti-slide pile as its embedding depth in the sliding bed. When the sliding bed is a relatively complete rock stratum, it is advisable to take 1/4 of the length of the anti-slide pile as the depth. Changing the ratio of the embedded sliding bed depth to the anti-slide pile length (1/8~1/2) affects the deformation curve of the surrounding rock relative to the pile’s embedded depth, as shown in Figure 18.

Figure 18a shows that when the ratio of the embedded depth in the sliding bed to the pile length of the anti-slide pile is between 1/6~1/4, the deformation of the surrounding rock decreases slowly, with little change in general. Relative to the tunnel crown and the right arch shoulder, the displacement of the left arch shoulder of the tunnel changes less. Figure 18b shows that when the ratio reaches 1/4, the lower supporting anti-slide pile can effectively control the displacement and deformation of the left arch shoulder of the tunnel. Figure 18c reveals that the displacement of the three monitoring points of the tunnel is significantly reduced. When the ratio is in the range of 1/4~1/2, the curve obviously changes, which proves that the control effect is strengthened. That is, when the length of the anti-slide pile is constant, the increased embedded sliding bed depth will steadily improve its deformation control effect. By carefully adjusting the length and embedded depth of the anti-slide pile, the stability of the rock surrounding the tunnel can be effectively controlled.

5. Conclusions

This study examines the interaction between tunnels and landslides. The failure modes of three different tunnel and landslide positions are analyzed, and a numerical model is established for the typical characteristics of tunnels located on landslide surfaces. We used numerical simulation software FLAC3D 6.0 to summarize the failure deformation characteristics and displacement deformation law of a tunnel under the influence of a landslide. Additionally, the control effect of anti-slide piles on the deformation of the rock surrounding the tunnel is analyzed. Our conclusions are as follows:

• Based on the relative positional relationship between the tunnel and the sliding surface, the failure modes of tunnel landslides in three different positions are summarized. These are located in the landslide mass, the sliding surface, and the sliding bed. Tensile failure and shear failure often occur at different stages of tunnel–landslide interactions. Tunnel deformation and landslide deformation exhibit mutual influences in both temporal and spatial distributions.
• A numerical model of the tunnel on the sliding surface is established. The plastic zone failure, surrounding rock stress distribution, and displacement distribution of a tunnel under the influence of a landslide are analyzed. The simulation results show that, under the influence of a landslide, there is large plastic zone damage on the tunnel crown and the spandrel of the mountainside. The arch foot positions on both sides of the tunnel are prone to stress concentration phenomena. The displacement deformation in the tunnel is as follows: crown > right shoulder > left shoulder > arch bottom.
• An optimized control scheme of anti-slide piles for controlling the stability of surrounding rock is proposed by analyzing four factors, namely, the forms of the anti-slide piles, the spacing of pile tunnels, the diameter of the piles, and the depth of piles embedded in the bedrock. The reinforcement effects of the three types are ranked from strongest to weakest as follows: the U-b and L-s > the upper blocking (U-b) > the lower supporting (L-s). The results indicate that the smaller the distance between the anti-slide pile and the tunnel, the stronger the reinforcement effect. When the length and diameter of the anti-slide pile are constant, the increased depth of the embedded sliding bed will steadily improve its deformation control effect.