Stability Analysis of Extra-Large-Span Tunnels During Construction: A Case Study

The maximum excavation span of the large-span transition section of Badaling Great Wall station is 32.7 m, and the excavation section is as large as 494.4 m². It is rather difficult to ensure the stability of the surrounding rock during tunnel excavation, therefore, it is necessary to select appropriate construction methods. In the present study, three methods commonly used for excavating large-span tunnels including the three-bench seven-step excavation method, double side drift method, and sequential excavation method (SEM) were used in excavating the 30 m-level large-span tunnels and the results were compared. The principal stress distribution of the surrounding rock, arch settlement, and plastic zone range obtained for the three construction methods was compared using numerical calculation, thus determining the optimal construction method and proposing specific support measures. The feasibility of the construction method and the reliability of support settings were verified by monitoring the arch settlement and surrounding rock damage zone. The results from the numerical calculation and field monitoring were analyzed. The SEM was advantageous over the other two methods for the excavation of 30 m-level large-span tunnels. It is necessary to emphasize the excavation and support of the arch and the middle bench when the SEM is adopted. It was feasible to maximize the self-bearing capacity of the surrounding rock by establishing a multi-level support system with anchor cables and anchor bolts of different lengths. The study findings can provide a reference for excavating 30 m-level large-span tunnels.