Investigation on the water-conducting zone evolution in weak overburden due to repeated mining of thick coal seams

Based on the II_2-1 and II₃ coal seams in China's Lingdong Coal Mine, the activation mechanism of water-conducting fractures in weak overburden under repeated mining of thick coal seams are studied by theoretical analysis, numerical simulation and field measurement in this paper. The main findings include that the key stratum theory and the critical fracturing span of the overlying rock layers were combined to determine the theoretical height of the water-conducting fracture zone in the II₃ coal seam under repeated mining disturbance as 83 m. UDEC numerical simulations revealed that as the working face advanced, fractures in the overburden above the II_2-1 and II₃ coal seams initially propagated upward following roof fracturing; after advancing a specific distance, fractures in the collapsed strata at the center of the goaf began closing and compacting; with continued mining operations, the working face gradually attained sufficient mining conditions while these mining-induced fractures entered a stable compaction stage, ultimately resulting in the water-conducting fracture zone exhibiting a characteristic distribution pattern where fractures prominently developed above the open-off cut and stopping line yet remained closed within the central goaf area, with its height stabilizing at approximately 78 m. Field measurements indicated that the actual height of the water-conducting fractures in the II₃ coal seam overburden ranged from 74.5 m to 77.1 m, averaging 75.8 m. Comprehensive results demonstrate that the height of water-conducting fracture zones in weak overburden is significantly lower than the failure height observed in conventional sandstone formations. These findings provide theoretical references for safe coal mining under water bodies.