## 3 Evaluation of Reinforcement Effect of Collapse Section
### Deformation Analysis of Surrounding Rock  
After the collapsed section of the tunnel was treated, the three-step seven-step excavation method was used for construction; the time history curve of the surrounding rock deformation is shown in Figure 11. The following points are evident in Figure 11:  
1. The cumulative settlement of the tunnel vault \(X_{\text{recs}}\) was 65.1 mm, and the maximum settlement rate \(V_{\text{sr max}}\) was 12.8 mm/d, which occurred on the second day. The settlement of the vault reached 39.7 mm within 10 days before construction, accounting for 61% of the total settlement. Subsequently, the variation trend of the tunnel vault settlement could be roughly divided into three stages of rapid increase, which corresponded to the successive excavation of the upper, middle and lower steps of the tunnel. After 35 days, the vault settlement and the settlement rate tended to be stable. The horizontal convergence of the surrounding rock of the tunnel \(X_{\text{hc}}\) was 25 mm in total, and the maximum convergence rate \(V_{\text{cr max}}\) was 4.3 mm/d, which was measured on the third day after the tunnel excavation. The horizontal convergence of the surrounding rock was large in the first 10 days before the construction, reaching 13.5 mm in total and accounting for 54% of the total convergence. With the blasting excavation of the upper, middle and lower steps, the blasting impact and load release further deformed the surrounding rock and the horizontal convergence continued to increase, but the convergence rate showed an overall decreasing trend. After 35 days, the horizontal convergence and convergence rate of the surrounding rock tended to  
Figure 10: Installation of field monitoring instruments: (**a**) CL-XZ-B type steel bar dynamometer; (**b**) buried steel bar dynamometer on site.  
Figure 9: Installation of field monitoring instruments: (**a**) XYJ-4 type vibrating wire double diaphragm pressure cell; (**b**) installation of earth pressure box.  
be stable. The vault settlement and horizontal convergence were within the standard range, indicating that the treatment measures were reasonable and effective.
2. When a tunnel is excavated in the loess stratum, vault settlement is considered to be the main displacement caused by the excavation. The final vault settlement of the monitoring section reached 65.1 mm, the horizontal convergence \(X_{hc}\) was 25 mm and the vault settlement was 2.6 times the horizontal convergence. This showed that there were significant directional differences in the tunnel displacement in the loess stratum. Thus, it is very important to control vault settlement to ensure tunnel safety.
3. There were significant differences in the tunnel displacement at different excavation stages. The vault settlement of the first five parts (Figure 8), the upper step part, the middle step part and the lower step part accounted for 61%, 26.7%, 7.3% and 5% of the total settlement, respectively. The horizontal convergence of each stage accounted for 54%, 31.5%, 10.5% and 4% of the total horizontal convergence, respectively. This showed that the displacement of the tunnel was mainly affected by the excavation of the preliminary pilot pit, upper bench and middle bench. Therefore, it is necessary to limit the length of the middle and upper steps and to close the support in time to achieve better control of tunnel displacement.