## 4 Numerical Results and Discussion
### Analysis of Numerical Simulation Results of Main Tunnel Excavation
#### 4.3.1 Deformation Analysis of Surface and Surrounding Rock  
Figure 14 shows the vertical deformation of the surface and surrounding rock with the central axis of the left tunnel as the section during the excavation of the main tunnel. Figure 14a-g shows the excavation of the upper bench face of the left main tunnel to 20 m, 40 m, 60 m, 80 m, 100 m, 120 m and 140 m. Due to the redistribution of stress caused by the excavation unloading, the surrounding rock at the vault of the left main tunnel had subsidence deformation, the surrounding rock at the arch bottom had uplift deformation, and the surface had settlement deformation to varying degrees. With the continuous progress of the excavation of the left main tunnel, the settlement value of the tunnel vault was increasing, and the maximum values were 7.76 mm, 10.6 mm, 13.7 mm, 14.1 mm, 23.1 mm, 22.3 mm and 28.1 mm, respectively. When the upper bench face of the left main tunnel advanced near the fault fracture zone, the settlement of the tunnel vault increased by 9 mm. Therefore, during the excavation of the left main tunnel, attention should be paid to the impact caused by the sudden change of the strength of the surrounding rock near the construction interface. The geological exploration in front of the excavation face should be done well before the excavation face advances to the fault fracture zone. Figure 15 shows the horizontal deformation of the surface and surrounding rock after excavation of the main tunnel. Figure 15a-c shows the horizontal deformation of the surface and surrounding rock on the section perpendicular to the \(Y\)-axis when the excavation depth Y = 40 m, 100 m, and 130 m, in which Y = 40 m and Y = 130 m respectively represent the section at a certain distance before and after the fault fracture zone, Y = 100 m is the section through the fractured zone. Due to the excavation and unloading of the main tunnel, the surrounding rocks on both sides of the main tunnel tended to move towards the middle. The horizontal displacement of the arch foot and arch waist on the left and right sides of the left tunnel was large, and the horizontal displacement of the arch crown and arch bottom was small. The horizontal displacement of the stratum near the middle partition wall was small. The maximum horizontal displacement of Y = 40 m, 100 m and 130 m sections were 6.23 mm, 20.8 mm and 5.70 mm, respectively. It can be seen that the horizontal displacement of the stratum near the fault fracture zone was much greater than that of the stratum at other positions. Figure 16 shows the surface settlement during the excavation of the main tunnel. The abscissa was the excavation process of the main tunnel expressed by the excavation distance, and the ordinate was the surface settlement displacement. Since the excavation sequence of the main tunnel was to excavate the left tunnel first and then the right tunnel, and the excavation interval was 30 m, that was, after the excavation of the left tunnel (140 m), it still needed to excavate another 30 m of the right tunnel, so the abscissa 0-140 m represented the excavation time of the left tunnel and 140-170 m represented the excavation time of the remaining right tunnel. Y = 0 m, 10 m \(\cdots\)140 m indicated the distance between the surface settlement monitoring point above the central axis of the middle partition wall and the portal. The surface settlement at each position increased with the progress of the main tunnel excavation. Observing the interval where the settlement value suddenly increased, it can be found that when the excavation face advanced to about 20 m before the location, the surface settlement increased sharply, indicating that the rock and soil mass within 20 m behind the excavation face was greatly disturbed by the excavation.