## 3 Dynamic Change of Safety Factor
### Safety Factor of the Construction Step and Key Working Conditions  
The strength-reduction calculation was carried out for the six construction areas of the tunnel in the excavation and support states, respectively, and the change in the safety factor of the corresponding load step was obtained, as shown in Figure 7.  
The safety factor for the first construction step, referring to the excavation of area 1, was equal to 1.83. It was increased to 2.01, because of the supporting effect of the hardening lining of the excavated area 1 in the second construction step. For the third step, i.e., excavation of area 2, the safety factor was reduced to 1.55. It was increased to 1.70 due to the supporting effect of the lining in the fourth step. In the fifth construction step, i.e., excavation of area 3, the safety factor was reduced to the minimum of 1.20. Thereafter, the application of the lining raised the safety factor for the sixth step to 1.35. In the fourth cycle, the seventh step referred to excavation of area 4. At that moment, the safety factor was equal to 1.30. After applying the lining, it was increased to 1.47 in the eighth step. When it comes to the ninth step, i.e., excavation of core soil area 5, the safety factor was equal to 1.40. It was increased to 1.42 in the tenth step. Finally, the safety factor for excavation of area 6, corresponding to the eleventh step, was equal to 1.38. After the lining was made into a ring in the twelfth step, the safety factor was increased to 1.40. The trend of the change in the safety factor decreased as the area of the excavation increased. Among them, the safety factor was the smallest when excavating the No. 3 guide hole (excavation area 3), which was 1.20, so it was the key working condition in the tunnel construction process. During the excavation of the tunnel excavation areas 4, 5, and 6, the safety factor did not change much. Comparing the changes in the safety factors of a complete excavation process, it can be seen that the minimum safety factor is that when the excavation is not supported, and the initial support can improve the safety factor. For the excavation section of the  
Figure 7: Safety factor of the construction procedure. entire tunnel-group, since the initial support of the platform is closed, the excavation of this part has less impact on the safety of the tunnel when the lower part of the station floor is excavated. The excavation safety factor of the whole tunnel-group indicates that the working condition with the smallest safety factor was the excavation of area 3. Therefore, areas 1, 3, and 5 were selected for in-depth analysis. The safety factor and instability form of the selected working conditions are shown in Table 2.  
According to the research on the dynamic evolution law of the surrounding rock stability during the construction stage, the stability-safety factor of the surrounding rock changes with the construction process. It has been shown that this factor was always larger than 1.20 over the entire construction period. Notably, this is consistent with the recommendation that the stability safety factor of surrounding rock after initial support must be larger than 1.20, see, e.g., [40, 41]. The instability forms of excavation areas 1 and 5 are both partial collapse of the roof. When excavating area 3, the surrounding rock strength was reduced until failure, which occurs when the side wall of the left platform collapses. For deep-buried tunnels, local small-scale instability does not pose a major threat to the overall safety of the tunnel; however, in urban metros, local instability does not cause overall damage to the tunnel, but it leads to excessive ground settlement that is not allowed by the regulations. The magnitude of the increase in the safety factor after the support of area 3 was the largest, indicating that the initial support of the arch can significantly improve the overall stability of the tunnel.