Structural evolution and instability induced shock disaster mechanism of bed separation water bearing dynamic pressure zone

Bed separation dynamic pressure water inrush is characterized by large instantaneous water inflow, strong destructiveness, and inconspicuous pre inrush signs, resulting in extremely high disaster risk and great difficulty in prevention and control. To investigate its disaster forming process and shock induced disaster mechanism, this study proposes the concept of longitudinal zoning of the overburden bed separation water bearing dynamic pressure zone (BSWBDPZ) for coal mining under overburden strata containing water filled aquifers. The formation mechanism of the BSWBDPZ is clarified for each zone, and upper dynamic pressure energy storage, middle bed separation water storage, and the lower eroded aquiclude zone are systematically identified as the decisive factors and main disaster forming conditions for bed separation dynamic pressure water inrush. A stepped composite beam contact load model is introduced to optimize the method for analyzing dynamic bed separation horizons in overburden strata, and the dynamic development horizons of overburden bed separation spaces during coal mining are calculated. Based on the theory of dynamic load induced cracking under blasting, an equivalent water decoupled blasting model for dynamic pressure transmission is constructed. The breaking energy of rock strata is converted into an equivalent dynamic load in the upper dynamic pressure energy storage zone, and the shock pressure in the middle bed separation water storage zone is solved using shock wave attenuation theory. By introducing a dynamic enhancement factor, a relational equation between dynamic strength and rock mass stress in the lower eroded aquiclude zone is established. A formula for calculating the dynamic failure radius of bed separation dynamic pressure water inrush is derived, and the shock induced disaster mechanism caused by instability of the BSWBDPZ is proposed. Taking 1121 working face of Hongliu Coal Mine as an example, the development height of the water conducting fractured zone was measured to be approximately 36 m using the underground upward borehole sectional water injection method. The dynamic development process of overburden bed separation spaces was theoretically analyzed, the shock induced disaster radius caused by instability of the BSWBDPZ was calculated, and corresponding instability induced disaster criteria were established. The evaluation results show that instability of the BSWBDPZ in 1121 working face would break through the rock strata of the lower eroded aquiclude zone, resulting in a major water inrush accident. Field detection of the overburden structure confirmed that fracturing of the upper sublayer of the coarse sandstone was the main dynamic source of the fourth water inrush. Laboratory hydrochemical analysis identified the aquifer above the coarse sandstone as the main water source of the fourth water inrush, thereby verifying the validity of the shock induced disaster mechanism caused by instability of the BSWBDPZ. The research findings provide a theoretical reference for the prevention and control of bed separation dynamic pressure water inrush disasters in mining induced overburden strata under similar conditions.