Distinct element method based stability analysis of tunnels in a deep coal mine in China

Stability studies on two tunnels, a horseshoe-shaped and an inverted arch-shaped tunnel, in a deep coal mine in China, have been performed using the 3DEC distinct element code and presented in this study. A pseudo time-dependent support installation procedure has been developed to incorporate the stress relaxation and deformation due to time gap between excavation and support installation. The utility of incorporating stress relaxation before support installation has been explored through observation of support factors of safety and grout failures. The numerical models have been analyzed for different support configurations and unsupported situations. The reinforcement provided by the supports has been demonstrated using changes in tunnel deformations and sizes of yield zones around the tunnels, in addition to observations of support factors of safety and grout failures. Results indicate that using longer supports and floor bolting would provide higher stability for the rock masses around the tunnels. A comparison between the closure strains in the two tunnels indicates that the inverted arch tunnel may be more stable under the existing geo-mining conditions at the site. Finally, parametric studies point to the importance of cohesion and friction angle estimation for intact rock, and basic friction angle estimation for the rock mass, to ensure robust numerical results.