## 4 Engineering Case Calculation
### On-Site Monitoring  
To verify the rationality and superiority of the optimized support measures, on-site monitoring was carried out on the surface settlement of a suburban expressway in the study section to reveal the stability of the tunnel face. A schematic layout diagram of the on-site monitoring plan is shown in Figure 16. The monitoring points JC-1-JC-9 were characteristic measuring points with an interval of 8 m, and three monitoring points were arranged between every two characteristic monitoring points. The monitoring points were arranged on the suburban highway pavement along tunnel according to a 2 m spacing. The distance between tunnel face and monitoring point was about 3 times the tunnel diameter (21-24 m). The daily excavation footage of the tunnel was about 2 m, and the next monitoring point began to enter the settlement influence range after 4 days.  
Figure 17 shows the variation trend in the cumulative displacement of different monitoring points JC-1-JC-9. It can be seen that the cumulative displacement of all monitoring points gradually increases with increasing monitoring time, and the farther from the tunnel face, the slower the cumulative displacement increases, and the smaller the maximum cumulative displacement. The settlement characteristics can be found that when the distance from tunnel face is about 3.40 times the tunnel dimensions, the cumulative deformation and change rate of monitoring point JC-1 continue to increase. The cumulative displacement reaches 34.50 mm when the tunnel face is excavated below the monitoring point, which exceeds the settlement warning value (30 mm). This has a great impact on the safety of the suburban expressway; therefore, it is necessary to stop the tunnel excavation and optimize the original plan.  
To ensure the stability of the excavated tunnel face and inhibit the settlement of suburban expressway, the optimization scheme of pipe roofing support + reserved core soil + grouting support is adopted to reinforce the tunnel face on 12 th to 16 th days, as shown in the shaded area. During the 4 days of tunnel face grouting construction, the cumulative settlement displacements of monitoring points JC-2 and JC-3 show a gradual convergence  
Figure 16: Schematic layout diagram of on-site monitoring plan.  
Figure 17: Variation trend of the cumulative displacement of different monitoring points.  
trend. After the optimization scheme is implemented, the settlement of monitoring points JC-2 and JC-3 continue to increase but the maximum cumulative settlement displacements are only 61.45% and 53.94% of that of monitoring point JC-1, and the settlement degree significantly decreases. The change trends in the monitoring points JC-4-JC-9 are essentially the same as the tunnel face continues to move forward, and the maximum cumulative displacement is roughly 15.50 mm, which is only 44.93% of monitoring point JC-1. It can be seen that the settlement deformation of monitoring points JC-2-JC-9 meets the requirements of the settlement deformation control value [34; 35], and the optimization scheme of pipe roofing support + reserved core soil + grouting support has a clear control effect on the surface deformation.