## 2 Analysis of Engineering Geological Conditions
### Mechanical Modeling of Tunnel-Landslide Orthogonal System  
In Figure 4, the engineering geological section of the deep geological hazard where the tunnel crosses the landslide body in an orthogonal direction is shown. Judging from the core identification of the investigation borehole and the deployment of the deep hole displacement monitoring curve, there exists a deep creep surface, i.e., the most unfavorable sliding surface, which is located about 50 m below the tunnel on the II-II' engineering geological section. According to the angle between the sliding direction of the landslide and the longitudinal axis of the tunnel, the tunnel-landslide system can be divided into three main forms [14]: the angle of 70-90\({}^{\circ}\) is the orthogonal system, the angle of 20-70\({}^{\circ}\) is the oblique system, and the angle of 0-20\({}^{\circ}\) is the parallel system. The angle between the axis direction of a tunnel in the northwest and the main sliding direction of the upper landslide is approximately vertical, and the angle is 70-90\({}^{\circ}\). Therefore, it is approximately identified as the tunnel-landslide orthogonal system for subsequent analysis. From the plane, the tunnel-landslide orthogonal system can be divided into two forms: the tunnel passes through the interior of the landslide body and the tunnel is located outside of the landslide body (Figure 5). When passing through the landslide body, there  
Figure 4: Section of engineering geology of tunnel deep geological hazard body II-II’. Figure 3: Site map of tunnel damage caused by landslide. (**a**) Tunnel vault penetration split, (**b**) tunnel vault dropout. are also cases where the tunnel is located in different positions relative to the sliding surface in the geological section. When the landslide body is a single sliding surface, according to the relative position of the tunnel, it can be divided into three cases: the tunnel is located inside the landslide body (a), the tunnel intersects with the sliding surface (b), and the tunnel is located below the sliding surface (c) (Figure 6). The force and deformation of the tunnel-landslide system are different at different spatial locations [15,16], and according to the planar and spatial locations of the tunnel-landslide system studied in this paper, it is determined to be the tunnel-landslide orthogonal system with the tunnel located inside the landslide under the condition of a single sliding surface, and all the research herein will be carried out based on the system in the following sections. When the tunnel crosses the landslide body, the external forces acting on the tunnel are the overlying geotechnical body pressure P when the landslide is not formed, the downward sliding thrust F after the formation of the landslide, and the geotechnical body resistance K. The geotechnical body pressure P is measured by on-site measurements or by other methods, and the downward sliding force F can be determined by numerical methods or by the limit equilibrium method, as shown in Figure 7.  
When the tunnel as a whole is located inside the landslide, the tunnel can be approximated as a thin-shell hollow ground beam for analysis [17]. The top of the tunnel arch and the shoulders of the two side arches are affected by the sliding force of the landslide, while the bottom elevated arch and the bottom of the two arches are affected by the resistance of the rock and soil body or the residual sliding force of the landslide. The tunnel as a whole can be analyzed as a large deflection beam structure; the internal thrust of the landslide  
Figure 5: Schematic diagram of two cases of tunnel–landslide orthogonal system on the plane. (**a**) Intersection of tunnel and landslide, (**b**) tunnels located outside the landslide. Figure 6: Schematic diagram of the relative position of the tunnel in the tunnel–landslide orthogonal system.