## 4 Numerical Results and Discussion
### Stability Analysis of Large Section Tunnel under Different Excavation Sequences
#### 4.1.3 Stress Analysis of Lining  
In tunnel excavation, the stress characteristics of the lining were important factors that affected the bearing performance of the supporting structure. The analysis of the state of the supporting structure was helpful in judging the position of unfavorable stress and guiding construction. Figures 11 and 12 show the vertical stress of the primary support and temporary support under different conditions after tunnel penetration. The positive value in the figure is tensile stress, and the negative value is compressive stress. Figure 11: The vertical stress of the primary support in different conditions (unit: Pa): (**a**) Condition 1; (**b**) Condition 2; and (**c**) Condition 3. Figure 10: The curve of horizontal displacement of the arch waist with construction steps: (**a**) Measuring point 1; (**b**) Measuring point 2. In Figure 11, the vertical stress distribution of the primary support was similar because of the excavation process from the right pilot tunnel to the middle pilot tunnel in Conditions 1 and 2, whereas some differences in the vertical stress distribution were observed in the excavation process from the middle pilot tunnel to the left and right pilot tunnels in Condition 3. Stress concentration occurred at the vault under the three conditions, among which Conditions 1 and 2 were tensile stress concentrations. Although Condition 2 had a wider distribution range, its value was smaller than that of Condition 1. Therefore, special attention should be paid to the construction quality of concrete at the vault in the construction of Condition 1 to prevent the occurrence of cracks. The compressive stress concentration appeared at the vault under Condition 3. Due to the strong compressive capacity of concrete, the construction under Condition 3 was conducive to the stress stability of vault concrete. In the three conditions, the maximum vertical tensile stress of the primary support was 5.688, 4.563, and 4.195 MPa, and the maximum vertical compressive stress was 20.89, 22.23, and 11.59 MPa, respectively. Figure 12 shows that the temporary support at the left and right side walls of Conditions 1 and 2 distributed downward stress, whereas the middle temporary support that separates the upper and lower tunnels was squeezed by the primary support of the left and right pilot tunnels, and the upward stress was distributed. Meanwhile, due to the limitation of boundary conditions, the local stress concentration occurred at the entrance and exit of the portal in Conditions 1 and 3, and the exit portal was subjected to tension, which was an unfavorable position. In the three conditions, the maximum vertical tensile stress of the temporary support was 11.87, 11.15, and 15.35 MPa, and the maximum vertical compressive stress was 7.19, 8.21, and 12.50 MPa, respectively. Figures 13 and 14 demonstrate the horizontal stress of the primary support and temporary support under different conditions after tunnel transfixion. Figure 13 shows that the horizontal stress distribution of the primary support in Conditions 1 and 2 was consistent, and the negative horizontal stress was distributed at the vault, that is, the compressed state, which was a favorable state. In Condition 3, the positive horizontal stress was distributed at the vault in the primary support, that is, the tensile state, which was not conducive to the performance of concrete. Figure 14 depicts that in the large range of Conditions 1, 2, and 3, the temporary support mainly distributed the tensile horizontal stress, but the I-beam could play a more significant bearing role in the temporary support. Figure 12: The vertical stress of the temporary support in different conditions (unit: Pa): (**a**) Condition 1; (**b**) Condition 2; and (**c**) Condition 3. Figure 13: The horizontal stress of the primary support in different conditions (unit: Pa): (**a**) Condition 1; (**b**) Condition 2; and (**c**) Condition 3.