Characteristics of overburden caving and goaf gas migration during working face mining

This study aims to reveal the characteristics of overburden caving during working face mining and gas migration in the goaf. Taking working face 5304 of Changping Coal Mine as the engineering background, a multistratum discrete element numerical model was developed in PFC to analyze the evolution of overburden caving and fracture development during mining. A numerical model of gas seepage in goaf was established in FLAC, and a fracture-density-based method for correcting overburden permeability was proposed. The FISH language was used to map parameters from the PFC fracture field to the FLAC seepage field, clarifying gas migration pathways and accumulation patterns. The findings were validated through an industrial field trial of gas drainage using high-level boreholes. The results showed that, once mining at working face 5304 had stabilized, the caving zone height was 21.8 m and the maximum fractured zone height was 74.2 m, consistent with calculations using empirical formulas in the relevant specification. The corrected permeability field closely matched the extent of mining-induced fracture development and accurately characterized overburden seepage properties. The high-efficiency gas seepage zone in the goaf was concentrated within a vertical height of 35 m above the coal seam roof. The field trial identified the lower part of the fractured zone, 25–35 m above the coal seam roof, as the optimal horizon for gas drainage using high-level boreholes, with gas drainage concentrations reaching 55% by volume. These findings provide theoretical support and an engineering reference for precise gas control in the goaf of fully mechanized caving faces and for optimizing the horizons of high-level gas drainage boreholes.