Fracture activation in the overburden and aquiclude stability under the mining of close-distance coal seams

BackgroundIn the Jurassic coalfield of northern Shaanxi Province, the repeated mining of close-distance coal seams exacerbates the activation of fractures in the overburden. This is prone to cause aquiclude instability, leading to groundwater loss. MethodsThis study aims to reveal the characteristics of fractures in the overburden and the aquiclude stability in the mining of close-distance coal seams. Using a method combining measurement statistics, physical simulations using similar materials, and theoretical analysis, this study proposed categorizing primary coal-rock combinations based on the water-preserved mining of close-distance coal seams in the Jurassic coalfield of northern Shaanxi. Additionally, this study analyzed the patterns of fracture activation induced by repeat mining and established the stability criterion for aquicludes under repeated mining.Results and ConclusionsThe results indicate that, based on the coal-water occurrence conditions and geological characteristics of two minable, close-distance coal seams in the upper part of the coalfield, the coal-rock combinations can be categorized into four types: thin and medium thick-thick (Type Ⅰ), thick-thin and medium thick (Type Ⅱ), thick-thick (Type Ⅲ), and thin and medium thick-thin and medium thick (Type Ⅳ) types. The characteristics of aquiclude failure caused by the development of upward and downward fractures in the overburden under the mining of the four coal-rock combinations were determined. Specifically, the upward fracture/composite mining height ratios under repeated mining generally vary from 14 to 30, significantly less than those under the mining of the single upper coal seam. Furthermore, the depths of downward fractures are 1.6 to 3.0 times the composite mining heights. The stability criterion for aquicludes was determined based on the repeat mining-induced fracture height (depth). Specifically, an aquiclude remains stable when its thickness is greater than or equal to the sum of repeat mining-induced fracture height (depth) and fracture-free thickness. Otherwise, they were unstable. The stability assessment of aquicludes in the Jurassic coalfield under repeated mining reveals that the aquicludes are in three conditions: stable-stable, stable-unstable, and unstable-unstable. These findings provide a theoretical basis for determining and controlling aquiclude stability in water-preserved mining of close-instance coal seams in the Jurassic coalfield of northern Shaanxi.