Large deformation of tunnels in longwall coal mines

Large deformation of tunnels in longwall coal mines creates massive cracks in the surrounding rocks, which may cause groundwater loss and threaten the coal mine safety. Rock mechanical characterization is crucial for understanding the large deformation of underground openings. Based on the theoretical analysis and laboratory triaxial tests, this study aims to propose a strength degradation model of the rocks for a deep understanding on the large deformation of underground openings. The proposed model captures the non-linear post-peak rock strength degradation which is dependent on the plastic strains accumulated under the decreasing confining pressures. The stress-dependent elastic properties were not considered in this study. The proposed model was incorporated in a finite difference package (FLAC3D) for engineering application. The simulated surface convergence of the underground opening matches the in-mine measurements, which supports the proposed model. More importantly, the simulated crack zone after tunnel excavation agrees with the observations from borehole images. Therefore, the proposed model provides a helpful tool to evaluate the safe margin of an underground tunnel if it is close to a subsurface water-bearing formation. It also sheds lights on the underground tunnel design to prevent the adjacent groundwater from inrush.