Relief Features Development Of Underlying Coal Seam After Mining The First Coal Seam In The Hexi Coalmine

In this paper, to the third mining area 3313 workface of Hexi coal mine regarded as prototype, the stress and displacement distribution of underlying coal seam were researched using FLAC3 D simulation software during mining overlying coal seam 50 m and 100 m and 200 m. By the permeability experiments, obtained penetration model at laboratory, combined with expansion and deformation features of underlying coal seam, permeability variation of underlying coal seam was got. Using COMSOL Multiphysics simulate the gas drainage radius at the situation of different drainage time and drainage drilling was optimized in underlying coal seam. The main conclusions of this paper are as follows.The coal of underlying coal seam experienced a loading-unloading process during mining overlying coal seam. With the increase of length of overlying coal seam, the previous stress concentration area begin to relief and translate into relief area. Relationship between the relief scope, extent of the relief and promote distance of overlying coal seam is positive correlation.The underlying coal seam permeability model was obtained from permeability experiment at laboratory, according to the model, in loading phase, the permeability decrease gradually. In relief phase, permeability increase rapidly. By fitting the permeability test results that we can know, under uninstall mechanics path, the connection of permeability and axial strain in loading phase is 11Bk A e-e=, in unloading phase is 22Bk A ee=.With the underlying coal seam forward, from the center to outward of the underlying coal seam, the permeability can be divided into permeability increase zone 1, permeability increase zone 2, permeability diminishing zone and the original permeability zone. The permeability of permeability increasing district 1 increased biggest and the scope also biggest. The underlying coal seam in permeability increase zone, and a wide range coal seam in permeability increase zone 1.According to the permeability distribution characteristics of the protected coal seam and the drainage radius from COMSOL Multiphysics simulation software, drainage drilling was optimized in protected coal seam.Both relief zone 1 and relief zone 2, gas drainage drilling space is 16 meters, the stress concentration zone which between relief zone 1 and original sress zone, gas drainage drilling space is 3 meters and the original sress zone is 7 meters. Gas drainage rate is more than 60% in underlying coal seam, the maximum of gas desorption index K1 is 0.25 and △h2 is 138 Pa. Maximum residual gas pressure is 0.35 MPa. The average yield is 3500 t/d and the average relative gas emission quantity is 3.8 m3/t during the recovery. Return airway gas concentration maximum is 0.7%, less than 1%, the gas gauge phenomenon has not been seen.