Study On The Effect And Mechanism Of Rock Burst Prevention By Pressure Relief Of Underlying Coal And Rock In Protective Coal Seam Mining

As a regional anti rock burst technology,protective seam pressure relief mining is more and more popularized and applied in rock burst mines.However,its pressure relief effect is difficult to test and has not formed a mature pressure relief mechanism,which can not provide enough theoretical and technical support for the on-site implementation of pressure relief and anti rock burst of underlying coal and rock mass by protective seam mining.Based on the research background of protective seam mining in Hulusu coal mine,this paper studies the protective seam of close coal seam group by comprehensive use of Matlab theoretical analysis,cyclic loading-unloading coal rock mass mechanics test,physical model test of coal-rock stress-strain evolution,numerical analysis of geological factors of protective seam mining,and on-site monitoring of pressure relief effect based on optical fiber sensing.The temporal and spatial evolution laws of stress field,strain field and displacement field of underlying coal and rock are analyzed.The variation laws of structural damage,mechanical strength and impact tendency of coal and rock under different cyclic loading and unloading conditions are explored.The sensitivity degree of interlayer spacing,mining thickness and other factors to the pressure relief effect of protective seam mining is analyzed.The application of distributed fiber sensing technology to real-time monitoring of pressure relief effect and range of field protective layer mining is carried out.The main conclusions are as follows.(1)The stress distribution law of underlying coal and rock at different depths after protective seam mining is analyzed theoretically.In the dip direction,the vertical stress of coal and rock mass is the smallest in the middle of the goaf and gradually increases to the boundary coal pillars on both sides;the horizontal stress is compressive stress below the goaf and tensile stress below the coal pillars,which gradually decreases with the increase of depth,contrary to the change trend of vertical stress.In the trending direction,the vertical stress is divided into pressurization area,relief area and recovery area.The closer the goaf is to the working face,the greater the horizontal stress is.The decrease of vertical stress is larger than that of horizontal stress.Under the condition of lower residual vertical stress,high horizontal stress has a higher compression effect on the underlying coal and rock mass,which promotes the deformation and failure of coal and rock mass and the release of high ground stress.(2)The internal relationship among cumulative damage,uniaxial compressive strength and impact tendency of coal and rock under different cyclic loading and unloading conditions is established,and the mechanism of pressure relief and impact reduction of structural damage and mechanical strength reduction of unloading coal and rock during protective seam mining is revealed.The cumulative damage of coal and rock increases with the increase of loading and unloading times and stress,and decreases with the increase of loading and unloading rate;the cumulative damage of coal and rock increases under cyclic loading and unloading,and the uniaxial compressive strength decreases;the damage of coal and rock is inversely proportional to the uniaxial compressive strength and impact tendency.The impact tendency of coal and rock weakens under cyclic loading and unloading,which is generally affected by loading and unloading stress,and significantly affected by loading and unloading times and rate.(3)The pressure relief effect of protective seam mining is significantly affected by geological and mining factors.With the increase of mining thickness,the maximum depth and degree of critical pressure relief increase,but when the mining thickness is greater than 6 m,the increase of the maximum depth of critical pressure relief gradually weakens;with the increase of interval,the degree of pressure relief decreases,the maximum depth of critical pressure relief first increases,then decreases,and then remains unchanged,the interval is about 20～30 m With the increase of face length and interlayer lithology strength,the maximum depth and degree of critical pressure relief decrease;the order of influence weight of geological and mining factors on pressure relief effect is:interlayer distance>mining thickness>interlayer lithology>face length.(4)During the mining process of protective seam,the underlying coal and rock undergoes the dynamic process of stress concentration,release and recovery,which leads to the fracture development,structural integrity damage and elastic energy release of the underlying coal and rock,creating a low stress environment for the protected seam.The vertical stress distribution curve of the protected layer shows a "U" shape.The stress concentration occurs at the opening position,and the stress decreases at the bottom.The minimum stress is 0.15 times of the original rock stress.There are two types of mining induced vertical stress changes in the protected layer:the area relatively close to the open-off cut:"low stress concentration area,pressure relief area,pressure relief not fully recovered area,pressure relief stable area";the area relatively far away from the open-off cut:"high stress concentration area,pressure relief area,pressure relief fully recovered area,pressure relief stable area".(5)The numerical simulation results show that the gangue in the goaf after the protective layer mining is uneven,which is divided into full caving compaction area and non full caving compaction area,resulting in different stress recovery state of the protected layer below the goaf.The vertical stress recovery curve of the protected layer shows a dynamic change process.When the mining range of the protected layer is small,the vertical stress recovery distribution curve of the protected layer is "U" type;when the mining range of the protected layer is large,the vertical stress recovery distribution curve gradually changes from "U" type to "W" type;when the mining range of the protected layer is large enough,the vertical stress recovery distribution curve changes from "W" type to multiple" W "type superimposed distribution.(6)The optical fiber sensing technology is used to realize the real-time monitoring of the pressure relief law and pressure relief range of the underlying coal and rock mass(strike 95.37 m,dip 128.47 m,vertical 36.94 m)in the process of protective seam mining.The optical fiber monitoring data reflects the dynamic process of stress increase and compression deformation,stress decrease and expansion deformation,stress recovery and tension deformation reduction of underlying coal and rock during the mining process of protective seam;based on the fluctuation amplitude of optical fiber strain increment to characterize the pressure relief effect,the pressure relief process is divided into three stages:the pressure relief starting stage is 40.8 m,the pressure relief active stage is 68.3 m,and the pressure relief recession stage.The results show that the strike pressure relief angle is 58.7°,the dip pressure relief angle is 63.6°,the pressure relief lag distance is 14.2 m,and the maximum vertical distance of pressure relief is 28.4 m.Based on the research of pressure relief mechanism,pressure relief influencing factors and pressure relief protection scope of protective seam mining in close distance coal seam group,the application of distributed optical fiber sensing technology in monitoring the pressure relief law and pressure relief scope of underlying coal and rock in protective seam mining is explored,which provides theoretical and technical guidance for the prevention and control of rock burst disaster in protective seam mining in Hulusu coal mine,so as to provide reference for the mining area Lay the foundation for safe and efficient development.