Three-dimensional stochastic distribution characteristics of void fraction in longwall mining-disturbed overburden

Fractures in the overburden induced by mining disturbances provide channels for heat-mass exchange between the subsurface environment and ground surface. Void fraction is an important characterization parameter to determine the difficulty of fluid flow and heat transfer. The three-dimensional stochastic distribution characteristics of the void fraction in the longwall mining-disturbed overburden are proposed based on the theoretical expressions of stratum and ground settlements and combined with a stochastic law with a normal distribution of void fractions obtained from statistical analyses from similar physical simulation experiments. The deterministic and stochastic distribution models and the discrete element numerical simulations show that the void fraction distribution is U-shaped in the cave zone and M-shaped in the bed separation zone. In the ground subsidence zone, the void fraction distribution of the horizontal fracture is M-shaped in the strike direction and an inverted U-shaped in the dip direction, and the void fraction presents an M-shaped distribution for the vertical fracture. As the depth decreases, the void fraction gradually decreases. As longwall mining progresses, the subsidence amount of stratum changes from a V-shaped into a U-shaped distribution, and the total void fraction transforms from an inverted V-shaped distribution to M-shaped distribution. The fracture distribution of the disturbed overburden forms into a fractured arch, and the fracture density gradually decreases from the foot to the top of the arch. The void fraction as a determinant parameter for stochastic distribution of permeability can provide valuable information for coal bed methane drainage, water inrush prevention, and coal fire control.