Analysis of the Evolution Law of Mining-Induced Water Flowing Fracture of Composite Hard Roof in the Border Mining Area of Inner Mongolia and Shaanxi

The mining area along the border of Inner Mongolia and Shaanxi generally contains two thick layers of confined aquifer sandstone, which form a composite hard roof. This constitutes the main hidden risk affecting the safe production of coal seams in this region.The composite hard roof is formed by two layers of thick confined water-bearing sandstone in the border mining area of Inner Mongolia and Shaanxi and is the main hidden danger affecting the safe production of coal seams. To clarify the evolution law of mining-induced water-conducting fractures under such occurrence conditions and prevent roof water damage, this paper comprehensively uses laboratory tests, theoretical analysis and UDEC numerical simulation methods to study the fracture characteristics and migration laws of high and low thick sandstones. The instability conditions of confined water-bearing sandstones in different layers are analyzed, the distribution law of water-conducting fractures is expounded, and the evolution characteristics of fractures are further quantitatively described by fractal theory. The results show that the overburden rock transport in the quarry is controlled by the composite rock beams, and the high rock beams will form a “masonry beam” structure, while the low rock beams will form a “step” structure under the influence of secondary displacement and rotation; the superposition of the initial breakage of the composite rock beam and the cycle breakage forms the “pulse” water influx characteristic and the “slow increase—sudden increase—stable” fissure development law; the change of the number of fractures is closely related to the transportation of the composite rock beams, which is divided into three stages of “slow growth-accelerated growth-periodic increase.” The fractal dimension quantitative cloud diagram further characterizes that the overall development pattern of water-conducting fissures remains unchanged under the influence of mining, and the main water-conducting channels are the fall zone and the vertical breakage fissure zones on both sides. The research results of this paper will provide a scientific basis for water control work in the working face under similar stratigraphic conditions.