Effect of mining on shear sidewall groundwater inrush hazard caused by seepage instability of the penetrated karst collapse pillar

Mining-induced groundwater inrush, a type of serious hazard for underground coal extraction, occurs in tunnel or coal face combined with a large volume of groundwater gushing. It has an impartible relationship with geological structures such as karst collapse pillar (KCP), which is widely distributed in North China. To analyze the effect of coal mining on groundwater inrush caused by seepage instability of the penetrated KCP (PKCP), stress and seepage coupling equations are used to model the seepage rule, and a numerical FLAC3D model is conducted to determine the shear stress, damage zone, pore pressure, seepage vectors and effect of damage zone on seepage field development. The results indicate that when PKCP and the surrounding rocks show dislocation, shear failure may occur, which leads to a shear sidewall channel for groundwater inrush. As mining advances, the damage zone in the PKCP and its crushed area develop gradually, and the simulated damage zone development was consistent with observed thicknesses of crushed rocks. Under the effect of the support pressure and unloading of the coal face, an obvious seepage concentrate channel with greater hydraulic head and seepage vectors will form within the edge of the PKCP; then, the shear sidewall channel will gradually move to the other side with the coal face near and away. When the damage zones are crushed through, the connected fractures will become a strong seepage channel in PKCP, named as pipe flow, which may cause groundwater inrush.