| The shallow coal seams in northern Shaanxi mining area is rich in reserves,and there exist multi-layer main mining coal seams generally.The strata behavior regularity,overburden failure and fracture zone development height characteristics of coal seams mining under shallow conditions are one of the important problems in the development of regional coal resources.Among them,due to the influence of repeated mining of rock mass in close coal seams mining,it is more difficult to detect the spatial structure of overburden deformation and failure,and the development of overburden failure and mining fracture is more severe.It seriously affects the ecological environment protection and the safe and green mining of coal resources in the mining area of northern Shaanxi.The study on the failure of overburden and the development of fracture zone in shallow and close coal seams mining will effectively promote the efficient development of regional coal resources.By means of theoretical analysis,numerical simulation,physical similarity simulation and optical fiber sensing technology,this paper studied the laws of overburden failure and fracture zone development in close coal seam group mining in Ningtiaota mine.Based on the mining subsidence law,the "two zones"height prediction method for shallow and close seam group mining was studied.Through physical similarity simulation,the failure process of overlying strata and the development height of fracture zone were analyzed.The distributed optical fiber is used to monitor the tensile and compressive stress state in the overlying strata of the gob in real time.Based on the variation degree of optical fiber frequency shift,the weighting of working face was characterized and compared.FLAC3D is used to study the distribution characteristics of plastic zone of overlying rock in the process of repeated mining,and the development height of fracture zone in close coal seams mining was obtained.Finally,the theoretical calculation results,simulation experimental results and numerical simulation results were compared and verified,and the development height of fracture zone in shallow and close coal seam group mining was obtained.The research shows that the key stratum structure of overlying strata is a single key stratum structure which has been mined in the upper coal seam at the end of mining.During repeated mining,the structure of the single key layer is broken,the overlying strata is shown as step sinking,the roof full-thickness shear falling,and the pressure of the working face is severe.The variation of optical fiber frequency shift accurately realizes the pressure discrimination of the working face.When the upper coal seam is mined,the first pressure step distance of the working face is 55.5m,the periodic pressure step distance is 18-31.5m.When the coal seam is mined repeatedly,the first pressure step distance of the working face is 69 m,and the periodic pressure step distance is 13.5-27m,which is basically consistent with the monitoring results of the supporting pressure of the working face.It can be used as a new method to distinguish the pressure.Through the comparative analysis of the development height of the fracture zone in the close coal seams of Ningtiaota Mine,it is concluded that the height of the fracture zone is 64.5m at the end of the upper coal seam mining,and the fracture zone develops to the surface after repeated mining.The results of theoretical calculation and similar simulation of optical fiber monitoring are basically consistent with the results of numerical simulation.The research contents of this paper can provide a theoretical basis for the development law of the three zones of the optical fiber monitoring physical similar model,and also can provide a theoretical basis for the evolution law of the mining overburden fractures in mines with similar geological conditions. |
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