## 4 Numerical Results and Discussion
### Stability Analysis of Large Section Tunnel under Different Excavation Sequences
#### 4.1.1 Analysis of Surface and Surrounding Rock Deformation  
In the process of tunneling with a large section, when the excavation sequences were different, the total number of excavation steps required for tunnel penetration varied, and the displacement of the surrounding rock at the same section was also different. Figure 4 shows the vertical displacement of the surrounding rock at the target section under different working conditions when excavating the target section. The vertical displacement of the surrounding rock farther away from the tunnel was distributed in layers, indicating that the meshing was more appropriate and the calculation was reliable. In the three working conditions, the vertical displacement mode of the surrounding rock was roughly the same, showing a funnel shape. The maximum vertical displacement of the tunnel surrounding rock in three conditions was 2.601, 2.615, and 2.634 cm, respectively. The comparison of the distribution range of the maximum settlement at the vault showed that Condition 1 < Condition 2 < Condition 3. That is, for the tunnel structure, Condition 1 could restrict the distribution range of the maximum settlement better, which was conducive to construction safety. During the excavation of the large section tunnel, the influence of the excavation of each rock mass part on the vertical displacement of the surrounding rock varied. Taking Condition 1 as an example, Figure 5 shows the influence of the excavation of the right pilot tunnel, the left pilot tunnel, the upper part of the middle pilot tunnel, and the lower part of the middle pilot tunnel on the vertical displacement of the surrounding rock at the target section. As shown in Figure 5a,b, when the right and left pilot tunnels were excavated, the surface settlement did not extend to the surrounding rock around the tunnel. After the excavation of the upper part of the middle pilot tunnel, the surface settlement extended to the surrounding rock of the tunnel Figure 5c. After the excavation of the lower part of the middle pilot tunnel, the vertical displacement of the surrounding rock of the target section is shown in Figure 5d, and the surface settlement reached half of the range of the middle tunnel, indicating that the deformation of the surrounding rock should be paid attention to during the excavation of the middle pilot tunnel. Figure 4: The vertical displacement of surrounding rock from excavation to target section (unit: m): (**a**) Condition 1; (**b**) Condition 2; and (**c**) Condition 3. Figure 6 shows the variation curve of the vertical surface displacement of the target section with calculation steps in various working conditions. As seen in Figure 6, after the initial ground stress balance, the surface settlement gradually increased with the advancement of the palm surface, and three stages were divided by two red vertical lines. Before the 9 th step of the calculation, the left and right pilot tunnels were excavated, whereas the middle pilot tunnel was not excavated yet, and the surface settlement increased slowly. Between the 9 th and 31 st steps of construction, the surface settlement occurred rapidly, and more than 90% of the total settlement was completed in this stage with the gradual advancement of each face of the middle pilot tunnel. After the 31 st calculation step, the ground surface settlement increased slowly with the excavation of the lower part of the middle pilot tunnel because the upper part of the rock mass excavation was completed, and the lining structure of the middle tunnel was basically completed. The final surface settlement of Conditions 1, 2, and 3 was 2.23, 2.26, and 2.32 cm, respectively. The settlement of Condition 3 was the largest, which increased by 4.04% and 2.65% compared with Conditions 1 and 2, respectively. The comparison of the three-step construction sequence could be seen as follows: Condition 3 completed the tunnel first but also the earliest to enter the surface subsidence rapid development phase. Its surface subsidence was also the largest.