Spatio-temporal evolution of overburden fracture and a zoning model for efficient gas drainage in soft coal seam pressure relief mining: A case study from the Jiahe mining area

Soft coal seams in southern China, which typically exhibit low permeability and high stress sensitivity, pose major challenges for gas control in multi-seam mining. A key knowledge gap concerns the fracture evolution mechanisms in overlying strata and the identification of preferential gas migration pathways when protective and protected seams are successively extracted under pressure-relief conditions. To address these issues, this study integrates physical similarity simulation and 3DEC numerical modeling, taking the coal seam group mining in the Jiahe mining field, Hunan Province as the engineering background. The evolution of overburden fractures and stress distribution during pressure relief mining were systematically investigated, leading to a refined zoning model applicable for soft coal seams and an optimized layout of high‑level drainage boreholes. The results reveal that the overburden fracture development in Coal Seam V exhibits distinct stages. The penetrative fractures stabilize at 37.6 m as mining progresses, while the bed separation fractures continuously extend upward to approximately 55 m, ultimately forming an “O”‑shaped fracture ring that connects the caved and fracture zones. Taking the pressure-relief mining of Coal Seam V with an average thickness of 2.3 m as the engineering background, the results of physical similarity simulation and numerical modeling corroborate each other, yielding the caved zone heights for Coal Seam V of 8.5 m and 7.2 m, and the fractured zone heights of 37.6 m and 38.4 m, respectively. The stress concentration zones induced by re‑compaction were observed at the advancing distances of 35∼36 m and 96∼100 m. Based on the fracture evolution properties, the conventional “three vertical zones” were refined by subdividing the fracture zone into a penetrative fracture zone and a pressure relief fracture zone. Horizontally, a “four horizontal zones” model was established, comprising the rib disturbance zone, the bed separation development zone, the voussoir beam structure zone, and the goaf compaction zone, with quantitative boundaries defined by the overburden bed separation rate and the fracture angle (61∼71°). Among these, the voussoir beam structure zone was identified as the preferential pathway for gas migration. Field tests with 4 groups of high‑level boreholes demonstrated that boreholes terminated in the middle of the penetrative fracture zone (25 m above the roof) exhibited superior stability and drainage performance compared to other horizons, achieving a maximum CH 4 concentration of 86.4% and a pure gas flowrate of 2.62 m 3 /min. These findings confirm that the proposed zoning model provides critical parametric support for horizon optimization and targeted placement of high‑level boreholes under pressure relief mining conditions in soft coal seams.