Evaluation of Collapse Risk and Control Strategies for Tunnel Face in Steeply Inclined Coal Seam Goafs: A Case Study

Tunnel collapses are prone to occur when crossing through coal seam goafs. This study focuses on a real engineering case of a highway tunnel in southwestern China, comprehensively discussing the spatial location characteristics of the goafs and the associated risks that possibly lead to large-scale tunnel face collapse. The widespread distribution of goafs, with a dip angle reaching up to approximately 50∘, considerably weakens the stability of the surrounding rock mass. To assess the potential extent of large deformation collapse that occurs when tunnels cross through the coal seam goafs, the generalized interpolation material point method, a variant of the regular material point method, was adopted to simulate the entire collapse process of the tunnel face. The potential failure zones of the surrounding rocks and the collapse mechanisms of the tunnel face under different excavation methods and reinforcement measures were investigated. Numerical results illustrate the combination of advanced curtain grouting measures and the bench method for excavation yields the most notable effect in controlling the large deformation of the surrounding rock. These measures were subsequently applied to the actual project. During the construction phase, the tunnel successfully crossed the coal seam goafs without any tunnel face collapses, validating the effectiveness of the proposed control strategies and the robustness of the adopted numerical methods. This case study provides valuable insights for similar engineering projects, contributing to more informed risk analysis and the design of optimized construction strategies.