Evolution law of overburden pressure relief gas enrichment area in fully mechanized caving face of three-soft coal seams and field application

Gas emissions from gas-bearing coal seams pose safety risks, especially in three-soft coal seam mines with varied fracture and gas-rich distributions. This study investigates the evolutionary patterns of overburden fractures in fully mechanized top coal caving faces of three-soft coal seams in the Weibei Coalfield by focusing on the distribution characteristics of fractures and gas-enriched areas. Accordingly, on-site monitoring, physical simulation experiments, and theoretical analysis were performed. The overburden crack area of the physical model was 80 m, according to an examination of the separation rate, displacement field, and strain field. An analysis of microseismic and borehole observations revealed that the on-site overburden crack area heights were 70 m and 75 m, respectively, which were greater than the heights calculated using empirical formulas. Considering the distribution features of the overburden crack area, FLUENT numerical simulation revealed that the optimal vertical drilling distance was in the range of 40–60 m, whereas the horizontal distance was 10–25 m. Through high-level drilling for gas extraction, gas concentrations in the upper corner and return air duct were kept within a safe range, which ensured safe working face production. These findings offer valuable insights into the development of fractures in three-soft coal seam mining and the formulation of critical parameters for gas extraction.