Effect of Pressure Relief and Permeability Increase of Bottom Layer of Protective Layer in Low Permeability Coal Seam of Gaoshan Mine

In view of the rationality of the exploitation of the protective layer to the coal seam with low permeability and high gas, the 4th and 9th coal seams in the second mining area of Gaoshan Coal Mine are taken as research objects, and the effect of pressure relief and permeability increase of the bottom layer after mining of the upper protective layer is studied. The results show that after the protective layer is mined, the pressure relief area of the bottom plate is inverted trapezoidal, the pressure relief angle is about 65°, the vertical pressure relief range is more than 50 m, the vertical stress is reduced from the original stress 7.1 MPa to about 3 MPa, and the average pressure relief value is around 3.6 MPa, the pressure relief rate is 60% on average, and the overall trend of vertical stress is “decrease-increase-stability”. The displacement change law of the protected layer is basically synchronized with the vertical stress change. As the protective layer is mined, the protective layer increases in the vertical direction. The average increase distance is about 40 mm, and the maximum displacement appears on both sides, about 53 mm. The expansion rate gradually increases from the sides to the middle and stabilizes at around 7‰. The protected layer showed obvious permeability increase in the range of 50 m to 270 m away from the open cut, the permeability increased by 70% on average, and the permeability increase range showed an inverted trapezoidal distribution.