Experimental Study On Mining-induced Fracture Evolution And Roof Water Inflow Law Of Medium-deep Coal Seam In Yushen

High intensity mining of Jurassic coalfield in Yushen mining area induced structural damage of upper bedrock aquifer,leading to groundwater flowing into the working face along the mining-induced water flowing fractures,which seriously threatened the safety of mine production.Accurately grasping the development height of water flowing fractures on the working face,studying the structural damage of bedrock aquifer,the seepage filling mode of roof aquifer and the law of water inflow are the primary tasks to ensure the safety of mine production.Taking Caojiatan Coal Mine in Yushen Mining Area as an example,by means of engineering geological investigation,physical simulation experiment,numerical calculation and theoretical analysis,the experimental study on mining fracture evolution and roof water gushing law in medium and deep coal seam was carried out.The main conclusions were as follows :(1)According to the engineering geological and mining technical conditions of2-2 coal seam in Yushen mining area,five factors including coal seam burial depth,intrusive rock structure,mining method,mining height and working face slope length were selected as the main influencing factors of water flowing fractured zone.The weights and thresholds of the original ELMAN neural network were optimized by using the sparrow optimization algorithm(SSA),and the height prediction model of SSAELMAN water flowing fractured zone was established.It was predicted that the height of water flowing fracture in 122106 working face with large mining height in Caojiatan Coal Mine was 151 m,and the fracture-mining ratio was 25.18.(2)The once mining height of the working face of medium-deep thick coal seam was large.With the advancement of the working face,the height of the bedrock water flowing fractured zone increased linearly,and the roof water inflow law had obviouly periodic characteristics.Before the insufficient mining,the water flowing fractures developed only in the Yan ’ an Formation,and the pore fissure aquifer in the Yan ’ an Formation entering the mining influence range was the main water inflow source,which entered the working face in the form of roof water leaching.When the working face advanced to about 156 m,the development of water flowing fractures increased highly nonlinearly.When the mining reached full mining,the water-conducting fissures developed to the confined aquifer of Zhiluo Formation pore fissures and formed the main water-conducting channels along the goaf boundary.The pore fissure confined aquifer of Zhiluo Formation and the pore fissure confined aquifer of Yan’an Formation entering the mining influence range were the source of water inflow in the working face.The seepage flowed into the working face along the main diversion channel around the working face led to a substantial increase in the water inflow of the working face.(3)The numerical simulation results of large mining height working face in medium-deep thick coal seam showed that the seepage evolution of bedrock aquifer was a gradual process in the mining process of working face,and its evolution characteristics were closely related to the spatio-temporal evolution law of water flowing fractured zone.In the stage of insufficient mining,the seepage of roof aquifer was mainly supplied by the lateral seepage from the pore fissure aquifer of Yan’ an Formation to the goaf.After sufficient mining,the seepage of roof aquifer was mainly supplied by the lateral seepage from the pore fissure confined aquifer of Zhiluo Formation and the two sides of Yan’ an Formation to the goaf,and the vertical seepage from the weathered bedrock fissure confined aquifer to the goaf was supplemented.The working face with large mining height in medium-deep thick coal seam formed a compound water inflow mode of lateral seepage of Yan ’ an Formation bedrock aquifer and Zhiluo Formation sandstone aquifer and vertical direct recharge of weathered bedrock aquifer.(4)Based on the roof compound water inflow mode of large mining height working face in medium-deep thick coal seam,according to the theory of groundwater movement,the mathematical models of well flow mode and top vertical recharge mode of shaft wall water inflow were established.Combined with the mining conditions of thick coal seam,the water inflow of 122106 working face was comprehensively predicted to be 356m3/h,and the maximum water inflow of working face was 445m3/h,which provided reference for advanced drainage of roof aquifer,accurate prediction of water inflow and water disaster prevention and control in Caojiatan Coal Mine.