Investigations into Mining-Induced Stress–Fracture–Seepage Field Coupling Effect Considering the Response of Key Stratum and Composite Aquifer

The effect of mining-induced stress–fracture–seepage field coupling (SFSC) is critical to the safety and productivity of the underground mining. This study investigated the spatiotemporal effect of SFSC at the Bulianta Colliery panel #31401 of the Shendong coalfield in China, characterized by ladder key stratum and composite aquifer. An integrated method including borehole and working face observation, in-site water leakage, and real-time video imaging was adopted to examine the behavior of overburden movement, water level change, and groundwater inrush. Simultaneously, the evolution of the SFSC was visualized by the FLAC3D–CFD (fluent) simulator with a developed non-Darcy model, and the dynamic characteristics of the SFSC were analyzed. The results indicated that the macro-stress shell, fractured field distributed beneath the macro-stress shell with a 48 m height difference in the lower and higher key stratum zone, and seepage field including initiation zone, acceleration zone, deceleration zone, and drainage zone were identified. The confined aquifer was affected by the fracture field with a height of 140–154 m, and groundwater invasion was triggered and developed beneath the lower key stratum. The confined aquifer was kept stable by protection of the higher key stratum by controlling the propagation of the fracture field at the height of 90–100 m. Highly permeable channels were distributed beneath the micro-stress shell as a result of the release of in-site stress, especially at the rupture region of the micro-stress shell between the lower and higher key stratum, with the maximum fluid discharge. Furthermore, the SFSC response resulted in large and medium groundwater invasion and strong variation of abutment stress peak, when creating rupture at the micro-stress shell and main roof. Finally, based on the mechanism of the SFSC, the developed optimization strategy could efficiently mediate the effect of the SFSC by lowering the mining height, increasing the advance rate, and enhancing the support resistance of the longwall mining face.