Three-dimensional collapse analysis of deep-buried tunnels in weak surrounding rock considering longitudinal collapse width and flexible support

This study performed a comprehensive three-dimensional (3D) analysis on tunnel collapse by incorporating the effects of longitudinal collapse width and flexible support characteristics of tunnel linings. The nonlinear Hoek–Brown criterion was employed for collapse analysis, and an upper limit analysis was conducted on the transverse section of the collapse body. Furthermore, the impact of flexible support systems on longitudinal collapse width was considered to enhance the accuracy of collapse predictions in a tunnel design. Comparisons between numerical simulations and theoretical analyses were performed to verify the feasibility of the proposed analytical approach. The rigid support of the structure led to inevitable active support reactions, and the predicted size of the collapse body obtained through calculation was very conservative, with a significant margin of error compared to the actual size. The collapse zone at the vault gradually extended to the sidewall and interconnected with the broken zone at the sidewall when A was greater than 0.5 or B was less than 0.65. For tunnels with σci less than 5 MPa, supporting the vault with a combination of rock bolts and anchor cables could prevent its collapse. This study offers a theoretical basis for the design of deep tunnels in weak surrounding rocks.