## 3 Engineering Application
### Analysis of Tunnel Excavation Footage Based on Back Analysis Results  
Based on the back analysis results, different amounts of excavation footage of the tunnel were selected to analyze their impact on the tunnel. The optimal amounts of excavation footage under four working conditions were selected by analyzing the distribution of the plastic zone. The distribution of the plastic zone under four cyclic excavation footage conditions is shown in Figure 6.  
It can be seen from Figure 6 that the plastic zones in the four cyclic excavation footage conditions are distributed differently. Due to the reinforcement effect of the advanced small pipe, the plastic zone of the arch crown is reduced, and the plastic zone is mainly distributed on both sides of the arch foot and arch shoulder. With the increase in excavation footage, the pressure release from surrounding rock also increases, so the area of the plastic zone increases obviously. The area of the plastic zone is the largest under the condition of 2.5 m excavation footage. Considering the actual situation of the project, the excavation footage of 1.5 m should be selected for excavation. At the same time, grouting reinforcement should be strengthened on both sides of the arch foot and the arch shoulder of the tunnel to ensure construction safety.  
According to the analysis results, the excavation footage of the construction site was determined to be 1.5 m. Figure 7 shows the site construction condition of the tunnel when it is constructed according to the excavation footage of 1.5 m. In the construction process, the surrounding rock of the tunnel is relatively stable and the construction environment is safe. The monitoring data of the arch settlement change obviously in the early stage of excavation, and gradually tend to be stable in the later stage. The arch settlement value is always within the monitoring control range in the monitoring process. In the construction process, the original 1.0 m excavation footage is adjusted to 1.5 m, which effectively improves the construction efficiency.  
Figure 5: Variation in the recognized parameters.  
Figure 6: Plastic differentiation layout under various working conditions. (**a**) 1 m. (**b**) 1.5 m. (**c**) 2.0 m. (**d**) 2.5 m.  
Figure 7: Optimization of tunneling excavation footage. (**a**) Optimized results of tunnel excavation at construction site. (**b**) Field monitoring data of vault settlement.  
Figure 8 shows the results of vault settlement under different working conditions. In the simulation process, without considering the fluid-structure coupling, the arch settlement result caused by tunnel excavation is 4.26 mm, and when considering the fluid-structure coupling, the simulation result shows that the arch settlement is 5.32 mm. In the actual construction process, the maximum settlement of the vault is 5.45 mm. Therefore, when tunnel construction is carried out in water-rich areas, the numerical simulation results are closer to the actual on-site construction conditions when considering the fluid-structure coupling. Thus, the impact of groundwater on construction cannot be ignored. In the actual construction process, the corresponding water stop measures should be taken to ensure the safety of construction.