## 4 Numerical Results and Discussion
### Stability Analysis of Large Section Tunnel under Different Excavation Sequences
#### 4.2.2 Stress Analysis of The Second Primary Support  
Figures 17 and 18 show the vertical stress of the second primary lining and temporary support at different supporting timing. Figure 17 indicates that the difference in the vertical stress distribution between the two working conditions was mainly at the vault and the arch bottom. In Condition 2, a large tensile stress range was observed at the vault. At the arch bottom, the tension range of Condition 2 was large. In the two conditions, the maximum vertical tensile stress of the second primary lining was 3.184 and 2.238 MPa, and the maximum vertical compressive stress was 11.29 and 12.63 MPa, respectively. It can be seen in Figure 18 that the temporary support had stress concentrations at the entrance and exit of the portal and the connection position with the primary lining. It also bore the extrusion effect of the inward deformation of the primary lining as a whole. The overall safety was good because the temporary support was a steel element. In the two conditions, the maximum vertical tensile stress of the temporary support was 10.29 and 5.771 MPa, and the maximum vertical compressive stress was 6.748 and 9.145 MPa, respectively. Figure 16: The horizontal displacement of surrounding rock under various conditions from excavation to target section (unit: m): (**a**) Condition 1 and (**b**) Condition 2. Figure 19 shows the horizontal stress of the second primary lining in different support timing. The distribution difference of the horizontal stress of the second primary lining was mainly from the arch waist to the arch bottom. For the arch waist, the horizontal stress distribution of Condition 1 was relatively uniform and negative, that is, the secondary primary concrete lining of the arch waist was compressed. The horizontal stress of Condition 2 at the arch waist was smaller than that of Working Condition 1, which was beneficial to the bearing performance of the concrete. At the position of the arch bottom, the horizontal stress distribution in both conditions was positive, that is, the concrete at the position of the arch bottom was subjected to tensile stress, and the bottom tensile force in Condition 2 was greater than that in Condition 1. In terms of the overall stress of the second primary support, the supporting timing of Condition 1 was favorable for the primary support at the vault to bear pressure, whereas the supporting timing of Condition 2 enabled the primary support at the arch waist to bear less horizontal stress. Figure 17: The vertical stress of the second primary lining in different conditions (unit: Pa): (**a**) Condition 1 and (**b**) Condition 2. Figure 18: The vertical stress of the temporary support in different conditions (unit: Pa): (**a**) Condition 1 and (**b**) Condition 2. Analysis of the Influence of the Timing of Temporary Support Disassembly and Secondary Lining Support on the Stability of the Large Section Tunnel  
The secondary lining corresponds to the reinforced concrete lining applied in the inner side of the primary support, which forms a composite lining together with the primary support to reinforce the support, optimize the route drainage system, beautify the appearance, and facilitate the installation of communication, lighting, monitoring, and other facilities. In tunnel construction, the secondary lining is generally carried out after the surrounding rock and primary support deformation tends to be stable, and the best supporting timing for the secondary lining is 30-40 days after excavation, about 50 m away from the working face [34; 35]. Considering that the large section tunnel was 45 m long and complex temporary supports were arranged in the tunnel, the dismantling of the temporary supports and the timing of the secondary lining support during the progress of the section affected the stability of the tunnel section. To study the influence of the timing of temporary support disassembly and secondary lining support on the stability of the large section tunnel, two working conditions were set (Table 6).