Analysis of the mechanical properties of surrounding rock in large span tunnel with variable cross-section under different excavation methods

Abstract. In order to explore the impact of variable cross-section tunnel excavation on the surrounding rock, this paper takes the pump room section of the second Qingdao Jiaozhou Bay submarine tunnel as the engineering background. Using the FLAC3D finite difference software, a three-dimensional numerical simulation of the entire excavation and support process for variable cross-section tunnels with different excavation methods was conducted. The displacement field, stress field, and the variation in the forces acting on the support structure were analyzed to select a reasonable excavation method. The results show that adopting the distribution expansion excavation method combined with a three-step excavation method for large span variable cross-section tunnels can effectively control stress concentration and deformation. Compared with the direct use of the step-by-step excavation method, both the surrounding rock deformation and the force on the support structure are smaller. This study provides theoretical support and guidance for the construction design of large-span variable cross-section tunnels.