## 3 Results and Analysis  
### The Pattern of Change in the Settlement Pattern of the Arch  
Monitoring points were set up 20 cm and 45 cm away from the initial excavation face, respectively, which were used to monitor the arch and both sides of the arch haunch. Among them, the arch haunch is located above the side drifts. The monitoring results and monitoring methods are shown in Figure 5. Table 6 shows the classification results of similar models for critical excavation conditions. The analysis of Figure 5 and Table 6 shows that excavation conditions from 1 to 9 involve the excavation of the central drift, with each drift being excavated in two cycles. At this point, arch displacement begins to occur, with a concentration on the faster rate of change in the arch position. In the 12 th excavation condition, the temporary support for the diaphragm wall was removed, resulting in rapid settlement of the arch. Following the removal of the diaphragm wall, excavation of the side drift commenced. At this point, the arch haunch began to settle, and the rate of settlement of the arch top stabilized. In the 18 th excavation condition, the initial support of the side partition wall was removed, resulting in a small settlement of the arch. After further excavation of the soil below, there was once again an acceleration of arch settlement. Simultaneously, the maximum settlement of the gypsum arch is 0.3 mm. This indicates that the superior over-supporting method significantly reduces the settlement displacement of the arch during construction. The settlement values of the monitoring section at 20 cm were lower than those at 45 cm.  
Figure 5: Tunnel monitoring arrangement and monitoring results. (**a**): Settlement in the middle of the arch. (**b**): Settlement of the left arch haunch. (**c**): Settlement of the right arch haunch. (**d**): Arrangement method of cross-section monitoring points.  
Combining the displacement results, it is evident that the removal of the temporary diaphragm wall support has a more significant impact on the settlement in the middle of the arch cap. The removal of the diaphragm wall caused an incremental settlement of 17.39% of the total settlement at the top of the diaphragm wall, but the removal of the temporary side diaphragm wall had a more significant effect on the settlement of the arch haunch. The removal of the temporary diaphragm wall had a greater impact on the overall settlement of the arch cover. Therefore, on-site construction should strictly control the distance of each removal of the diaphragm wall and timely construct the backcasting section above the diaphragm wall to improve the integrity of the support structure. This can reduce the risk of settlement caused by the removal of the side diaphragm wall initial support. Furthermore, upon completion of the upper arch cover construction, the arch's foot will be positioned above the lower surrounding rock. Excavation work will then disturb the support structure of the arch feet, resulting in the settlement of the upper arch. Therefore, during the construction process, it is crucial to reinforce the surrounding rock where the arch's foot is located before excavating the main body below.