Numerical modeling of coupled stress-fracture evolution in water-resisting key strata during longwall mining

During underground coal mining, the evolution of stress and fracture development within the overlying water-resisting key strata (WRKS) significantly impacts its water-resisting properties, thereby making it a critical factor controlling the stability of groundwater seepage in coal mines. Utilizing UDEC and based on the coal-rock interburden thickness, the development height of the water-conducting fracture zone, and the theory of the “three mining-induced zones” (caved zone, fractured zone, continuous deformation zone), numerical models simulating coal seam excavation and the response of the WRKS under various relative interburden thickness conditions were constructed. These models were employed to investigate the mining-induced stress paths and fracture evolution characteristics of the WRKS at different spatial positions during coal seam advance. The results demonstrate that: The characteristic spatial distribution pattern of mining-induced stress within the WRKS follows the sequence “Initial Stress Zone—Stress Concentration Zone—Pressure Relief Zone—Stress Recovery Zone—Pressure Relief Zone—Stress Concentration Zone—Initial Stress Zone”. The maximum stress concentration factor within the stress concentration zone exhibits a negative correlation with the relative interburden thickness, while the minimum stress concentration factor within the pressure relief zone shows a positive correlation with the relative interburden thickness. The spatial extent of the fracture development zone within the WRKS exhibits a high degree of coincidence with the pressure relief zone; furthermore, the extent, duration, and fracture density of the fracture development zone all exhibit negative correlations with the relative interburden thickness. The mining-induced stress path experienced by the WRKS comprises six distinct stages: “Original Rock Stress—Stress Increase—Pressure Relief—Stabilized Pressure Relief—Stress Recovery—Asymptotically Approaching Original Rock Stress”. Concurrently, the evolution process of mining-induced fractures progresses through five stages: “Incubation—Expansion—Stabilization—Closure—Asymptotically Approaching Complete Closure”.