Research On The Influence Of Coal Seam Mining On Groundwater In Yushen Mining Area

The mining area in western China is rich in coal resources,but its ecological environment is fragile and water resources are scarce.Large-scale and high-intensity mining of coal mines will cause varying degrees of disturbance and damage to the stratigraphic structure,water resources and ecological environment in some areas.Therefore,it is of great significance to study the failure law of overlying strata and the influence degree of groundwater resources caused by coal mining activities for the rational development of coal resources and the protection of groundwater resources.In this paper,taking the mining of 2-2 coal and 3-1 coal in a mine field in Yushen mining area as an example,the hydrogeological characteristics,the development height of water diversion fracture zone and the influence degree of coal mining on groundwater are predicted in detail by means of field investigation,data collection,statistical analysis,theoretical formula calculation,numerical simulation and mathematical modeling analysis.It is expected to provide a reference basis for the development and planning of coal resources and the protection of water resources in the mining area.The hydrogeological conditions and spatial occurrence characteristics of coal seam in the study area are studied by means of comprehensive analysis and mapping,and it is determined that the lithologic structure of overlying strata in coal seam is mainly as follows:loose sand layer+soil layer+weathered bedrock layer+bedrock layer+main mining coal seam.The main aquifer is loose phreatic aquifer and weathered bedrock confined aquifer,and the main aquifer is Baode formation laterite.The bedrock is mainly the relative aquifer of sand-mudstone interlayer of Zhiluo formation and Yan'an formation,and its water-rich property is weak to extremely weak.The thickness of 2-2 coal in the main mining coal seam is 0.81m-7.54m,the average thickness is 3.65m-2.78m,and the average minable thickness is 1.85m.Through theoretical formula calculation and FLAC3D numerical simulation method,the height of water diversion fracture zone in single coal seam mining of 2-2 coal seam and double coal seam of 2-2 coal and 3-1 coal seam is predicted and studied respectively.Finally,it is determined that the height of water diversion fracture zone in single coal seam mining of 2-2 coal seam is 28 times of fracture mining ratio,and that of 2-2 coal seam and 3-1 coal double coal seam is 32 times of comprehensive mining thickness.Mapping analysis shows that the maximum height of water diversion fracture zone in single coal seam mining of 2-2 coal seam is almost the same as that of 2-2 coal seam and 3-1 coal seam,which leads to the lower section of Anding formation,and the minimum distance between the upper weathered bedrock aquifer and the bottom boundary of upper weathered bedrock aquifer is about 70m.Therefore,neither of the two mining methods will lead to direct water leakage of weathered bedrock aquifer and overlying phreatic aquifer.Using Visual Modflow groundwater numerical simulation software,the influence of coal mining on groundwater flow field is studied by constructing hydrogeological conceptual model,mathematical model and groundwater flow numerical model.The simulation results show that 2-2 coal mining in the study area has little influence on the water level of phreatic aquifer and will not cause phreatic leakage,and has a certain influence on the water level of weathered bedrock aquifer,but with the end of mining time,the decline of weathered bedrock water level slows down and tends to be stable gradually.Four indexes of weathered bedrock aquifer thickness,weathering degree,lithologic combination index,and paleo-topographic elevation are selected as the main controlling factors of water enrichment of weathered bedrock aquifer,and the water richness of weathered bedrock aquifer is predicted by random forest(RF)algorithm.Combined with the predicted results of the height of the water-conducting fracture zone of 2-2 coal,the safety prediction zone of coal seam roof cracking is carried out.On this basis,the water-rich zoning results of aquifers and the safety zoning results of coal seam roof cracking are superimposed with the help of GIS software,and then the zoning prediction of the influence of 2-2 coal mining on weathered bedrock aquifers is carried out.The prediction results show that 2-2 coal mining will not lead to the direct leakage of water in the weathered bedrock aquifer,and the weathered bedrock aquifer only produces a certain degree of seepage,and the influence degree is divided into three grades:slight,moderate and slight,and the affected light and medium light areas are mainly distributed in 11 and 12 areas,with the area of light and micro areas accounting for about 5.8%and 21.6%respectively.The extremely light and micro-affected areas are mainly distributed in 13,14 and 15 areas of the mine field,accounting for about 72.6%of the total area.