Study on the Overburden Failure Law of Extra-Thick Coal Seam Mining Under Extremely Thick Conglomerate Strata

This study investigates the mining-induced overburden failure and the development law of the water-conducting fracture zone under key layer control during the extraction of an extra-thick coal seam (thickness ≥ 8 m) under extremely thick conglomerate strata (thickness ≥ 200 m) in the Zhaoxian Coal Mine, Binchang mining area, Shaanxi Province, China. A combined approach utilizing FLAC3D numerical simulation and ground borehole full-section resistivity monitoring was adopted. The results indicate that the primary key layer (extremely thick conglomerate) and the sub-key layer (sandy mudstone) exert a significant inhibitory and segmented control effect on fracture development. The height of the water-conducting fracture zone increases in a “step-like” pattern with working face advancement, stabilizing at 270.3 m; the Rh/m is 23.5. The overburden failure morphology evolves dynamically through stages described as “funnel shape–concave shape–inverted trapezoid shape” as mining progresses. Field resistivity monitoring results (fracture zone height of 255 m, Rh/m of 22.17) show good agreement with numerical simulations, validating the control mechanism of key layers on overburden failure. These findings provide a theoretical basis for safe mining practices and water resource protection in extra-thick coal seams overlain by extremely thick conglomerate strata.