## 4 Numerical Results and Discussion
### Analysis of Numerical Simulation Results of Pilot Tunnel Excavation
#### 4.2.2 Lining Deformation Analysis  
Figure 10a-h show the vertical deformation of the lining when the middle pilot tunnel was excavated to 20 m, 40 m, 60 m, 80 m, 100 m, 120 m and 140 m, and when all the pilot tunnels were excavated. The pilot tunnel lining structure was deformed to a certain extent under the surrounding rock pressure formed by excavation and unloading, and the lining at the vault was mainly settlement deformation. The vault settlement of the middle pilot tunnel was significantly greater than that of the side pilot tunnel, and the vault settlement of the left pilot tunnel was significantly greater than that of the right pilot tunnel. Therefore, it was necessary to pay more attention to the changes in the vertical deformation of the lining structure during the construction of the main tunnel excavated first. The lining structure on the right side of the middle pilot tunnel generally had rebound uplift, which may be related to the tunnel bias pressure. Considering the small rebound value, it would mostly not affect the stability of the lining structure. Comparing the vertical deformation of the lining structure when the pilot tunnel was excavated to different positions, with the continuous advancement of the pilot tunnel excavation process, the vertical deformation of the lining continued to increase, and the maximum vertical deformation of the lining at eight locations was 0.93 mm, 0.96 mm, 1.05 mm, 1.37 mm, 1.73 mm, 2.50 mm, 2.69 mm and 3.14 mm in turn. The permissible deformation of the lining was 45 mm. In general, the vertical deformation of the lining caused by the excavation of the pilot tunnel was small, indicating that the design of the lining structure was reasonable, which was beneficial to ensure the safe and smooth construction of the pilot tunnel of the shallow buried tunnel. Figure 9: Surface settlement during pilot tunnel excavation. Figure 11 shows the vault settlement of the middle pilot tunnel during the pilot tunnel excavation. The abscissa was the excavation process of the pilot tunnel expressed by the excavation distance, and the ordinate was the vault settlement of the middle pilot tunnel. Since the sequence of pilot tunnel excavation was middle pilot tunnel--left pilot tunnel--right pilot tunnel, and the excavation interval was 20 m, that was, 40 m side pilot tunnel still needed to be excavated after the excavation of middle pilot tunnel (140 m), so the abscissa 0-140 m represented the excavation time of middle pilot tunnel and 140-180 m represented the excavation time of remaining side pilot tunnel. Y = 0 m, 10 m... 140 m indicated the distance between the vault monitoring point on the central axis of the middle pilot tunnel and the tunnel portal. The settlement of the vault of the middle pilot tunnel tunnel  
Figure 10: Vertical deformation of lining during pilot tunnel excavation (unit: m): (**a**) L = 20 m; (**b**) L = 40 m; (**c**) L = 60 m; (**d**) L = 80 m; (**e**) L = 100 m; (**f**) L = 120 m; (**g**) L = 140 m; and (**h**) The pilot tunnel excavation is completed. continued to increase with the excavation process of the pilot tunnel. When the pilot tunnel was excavated near the fault fracture zone (Y = 100 m), the vault settlement was the largest, which was 25.6 mm. Observing the change of vault settlement value at each position, the vault settlement increased sharply about 20 m in front of the excavation face, indicating that the rock and soil within 20 m behind the excavation face were greatly disturbed by the excavation. In the process of pilot tunnel excavation, special attention should be paid to its stress-strain state to prevent collapse, surge and other accidents. The vault settlement of the left and right pilot tunnels had the same variation law as that of the middle pilot tunnel. Figure 12 shows the vault settlement curve of three pilot tunnels after the completion of pilot tunnel excavation. The abscissa was the distance between the upper bench face and the portal during the excavation of each pilot tunnel.