Study On Temporal And Spatial Evolution Law Of Mining Stress Of Protective Layer With Large Dip Coal Seam Group

The temporal and spatial variation law of stress in the mining of the protective layer on the large dip coal seam group is a key scientific problem,which needs to be solved urgently to ensure safe and efficient drainage of gas.In terms of this issus that safe and efficient mining of the protective layer,based on the mining project reality on the large dip coal seam group in Donglin Coal Mine,this paper studies temporal and spatial variation law of the stress on the protective layer mining by used the method of theoretical analysis,numerical simulation and field test.The main research conclusions are as follows:(1)Based on elastic thin plate theory,the primary and periodic fracture mechanical model of the basic roof of large dip coal seam is established,and its evolution and fracture mode are obtained.The results show that the basic roof fracture of large dip coal seam has the law of spatial-temporal evolution,and the spatial failure order is the middle roof,the upper roof and the lower roof.The primary fracture produces cracks in the middle and upper part of the upper surface of the long side of the roof and develops around to form the"O"type fracture.At the same time,cracks began to appear in the middle of the lower surface of the roof,which developed into an"X"type fracture along the diagonal line.When the cracks on the upper and lower surface of the roof are connected,the final fracture form"O-X"is formed.Although the final fracture form is basically the same as that of gently inclined coal seam,the generation and development of fractures are different from that of horizontal and gently inclined coal seam.Periodic fracture produces cracks in the middle of the bottom surface of the top coal wall end and develops towards the corner of the top coal wall.At the same time,cracks also occur in the middle of the upper surface of the long side of the roof and gradually develop to two short sides;When the cracks on the upper and lower surfaces are connected,the final fracture morphology"C-X"is formed.In order to verify the reliability of the theoretical analysis,the failure mode and fracture evolution law of the basic roof of large dip coal seam are analyzed through the field observation of the mining pressure of the upper protective layer.The research results are in good agreement with the theoretical analysis results.In addition,through the theoretical analysis of the floor stress distribution after the mining of the upper protective layer,it shows that it is feasible to relieve pressure and prevent outburst by the mining of the upper protective layer with large dip Angle.(2)Using FLAC^3D software,the space-time variation law of stress and deformation of roof and floor during the mining process of the protective layer on the large dip coal seam was obtained.The results indicate that the stress of coal seam and stat in the floor goes through three stages:the stress increases before mining,the stress decreases during mining and the stress recovers after mining.The deformation of floor coal and rock has undergone three processes:compression deformation,expansion deformation and deformation recovery.By the numerical simulation results and based on the pressure relief and deformation protection criteria of the protective layer mining,the relief angles of the protective layer along the upper and lower diping boundaries of the large dip Angle in Donglin Coal Mine are 70°and 80°respectively,and the relief angles along the strike direction are unevenly distributed in the direction of the coal seam inclination,with the magnitude of 56°?60°.(3)Field test through gas pressure,the evolution law of gas pressure in the protected layer along with the advance of the working face of the upper protective layer is obtained,and the variation law of gas pressure is consistent with the temporal and spatial variation law of stress.The practice of gas drainage shows that it is reasonable to optimize the parameters of gas drainage drilling based on the pressure relief range of the protected layer and the change law of pressure relief gas pressure.The best time for gas drainage is from 30m ahead of the working face to 60m behind the working face in K6 coal seam of the upper protective layer.