## 4 Numerical Results and Discussion
### Analysis of Numerical Simulation Results of Pilot Tunnel Excavation  
#### 4.2.1 Deformation Analysis of Surface and Surrounding Rock  
Figure 7 shows the vertical deformation of the surface and surrounding rock on the central axis section of the middle pilot tunnel during the excavation of the pilot tunnel. The Y-direction was the direction of tunnel excavation, and L was the excavation distance of the middle pilot tunnel. Figure 7a-g respectively shows the vertical deformation of the surface and surrounding rock when the middle pilot tunnel was excavated to 20 m, 40 m, 60 m, 80 m, 100 m, 120 m and 140 m. After the excavation of the middle pilot tunnel, the surrounding rock had been deformed to varying degrees, the tunnel vault had subsided, and the arch bottom had uplifted. In the direction of excavation, the surface had different degrees of settlement deformation, and the surface above the tunnel excavation surface and near the fault fracture zone had large settlement deformation. With the increase of the excavation depth of the middle pilot tunnel, the maximum vertical deformation of the surface and surrounding rock continued to increase. The maximum settlement in Figure 7a-g were 3.15 mm, 3.65 mm, 3.88 mm, 7.44 mm, 29.2 mm, 31.0 mm and 31.0 mm, respectively. When the middle pilot tunnel was excavated to the distribution section of the fault fracture zone (Y = 100 m), the vertical deformation of the surface and surrounding rock increased suddenly, and the settlement of the vault increased obviously. Therefore, special attention should be paid to the change of rock and soil strain during construction in this area. Figure 6: Vertical settlement of the vault of the right tunnel. Figure 8a-e shows the horizontal deformation of the surface and surrounding rock in the cross-section at Y = 0 m, 40 m, 60 m, 80 m and 100 m after the completion of pilot tunnel excavation. The rock and soil mass around the three pilot tunnels had different degrees of horizontal displacement, which showed that the rock and soil mass at the arch waist of the pilot tunnel tended to move in the middle of the pilot tunnel. The horizontal deformation of rock and soil around the middle pilot tunnel was large, and the horizontal deformation of rock and soil around the side pilot tunnel was small. Due to the bias phenomenon caused by terrain and the influence of the excavation sequence of the pilot tunnel, the horizontal displacement on the right side of the pilot tunnel (the displacement in the negative direction of the \(X\)-axis in the figure) was generally greater than that on the left side. Therefore, in the process of pilot tunnel excavation, it was necessary to pay special attention to the change of the horizontal displacement of the surrounding rock on the middle pilot tunnel and the right side of the side pilot tunnel. Observing the section diagrams at different locations after the completion of the pilot tunnel excavation, it can be found that as the excavation sequence of the pilot tunnel excavation, the horizontal displacement of the surface and the soil mass around the middle pilot tunnel, the horizontal displacement of the surface and the soil mass  
depth increased, the horizontal displacement of the rock and soil around the pilot tunnel continued to increase. The maximum horizontal displacements of the surrounding rocks at the five cross-sections were, respectively, 6.27 mm, 6.74 mm, 6.96 mm, 10.2 mm and 29.0 mm; the horizontal displacement of the rock and soil around the pilot tunnel increased significantly near the fault fracture zone. Figure 9 shows the surface settlement during the pilot tunnel excavation, and the abscissa was the pilot tunnel excavation process expressed by the excavation distance. The excavation of the three pilot tunnels was carried out in sequence, and the excavation interval was 20 m, that was, after the excavation of the middle pilot tunnel (140 m) was completed, another 40 m side pilot tunnel needed to be excavated.