Influence of thick sandstone on development of overburden mining fissures in northern Shaanxi coal mining area of Yellow River Basin and suggestions on water-preserved coal mining

The change of geological conditions of coal seam roof overburden and the development of mining fissures caused by coal mining activities are the direct causes of damage to the key underground aquifers, and also the root causes of ecological environment degradation in the mining area. As a typical geological condition, the thick sandstone developed in the overburden structure of the coal seam roof has an important impact on the development law of mining fissures in the overburden. Therefore, the influence of the thickness and location of different thick sandstones on the development morphology and height of overburden mining fractures was simulated and analyzed with the FLAC^3D numerical simulation platform based on the analysis of the geological conditions and distribution rules of the main mining coal seams in the study area, taking the roof overburden of main mining coal seam 2⁻² in Caojiatan Coal Mine of northern Shaanxi coal mining area as the geological prototype. Hence, suggestions were proposed for “water-preserved coal mining”. The results show that the thick sandstone in the roof overburden of coal seam 2⁻² in the study area is 25 m thick on average, with an average spacing of 76 m from coal seam 2⁻². Besides, the overburden mining fissures show the dynamic change in “Rectangle—L-shaped—Saddleback” characteristics for the thick sandstone 30 m away from the coal seam, “L-shaped—Inverted trapezoid—Saddleback” characteristics for the thick sandstone 70 m away from the coal seam, and “Saddleback” characteristics in the whole process for the thick sandstone 95 m away from the coal seam. The maximum development height of mining fissures in overburden decreases first and then increases with the increasing of thick sandstone horizon. The thickness of thick sandstone H≥30 m and the distance from the coal seam L>95 m, or H≥60 m and L>60 m could effectively prevent the upward development of mining fissures through the thick sandstone. In full consideration to the influence of thick sandstone on the development law of overburden mining fissures, reasonable measures were taken at appropriate spatial location and mining stage for overburden loss reduction and water conservation to realize the green mining mode of “mining while controlling, mining while protecting”. Generally, the research results could provide theoretical guidance for the coordinated development of coal mining and ecological environment protection in northern Shaanxi coal mining area of Yellow River Basin.