Determination of the damage degree of coal rocks and study on the evolution law of permeability based on Zhaogu No. 2 Coal Mine

There are many technical problems with the mining of deep thick coal seams with thin bedrock in the mining areas in eastern China. Due to the influence of the excavation unloading effect of the working face,which is bound to cause large deformation and discontinuous failure of the overburden,the nonlinear failure of the overburden is the fundamental reason for the influx of water and sand into the working face in the thick alluvium. In order to determine the damage degree range of coal seam and overburden and the evolution law of permeability,this paper adopts the means of field measurement and numerical simulation to quantitatively characterize the damage zone of the coal wall from the distribution law of ultrasonic wave velocity and that of the overburden from the height and volume of overburden plastic zone. A seepage mode of FLAC^3D was developed,which analyzed the evolution law of permeability of overburden and goaf during retreating the longwall panel. The results show that the damage zone of the coal wall is consistent with aera of the peak front abutment pressure,about 6~7 m. The height and damage degree of the plastic zone of the overburden increase with advancing distance of the working face. When the working face advances to 130 m,the overburden reaches full mining,and the plastic zone is distributed in a saddle shape. The permeability and vertical stress of the rock mass before the failure of the impermeable layer show a negative exponential distribution. After the failure of the impermeable layer,the high permeability area shows an obvious zoning phenomenon. The permeability of the rock mass in the goaf decreases with the gradual compaction of the caved roof. The main area of water and sand seepage is determined by the distribution of a high permeability range of overburden,which provides a theoretical basis for the prevention and control of water inrush and sand burst in the roof of 14030 working face of Zhaogu No. 2 Coal Mine.