Mechanical Analysis on Pressure Relief Principle of Underlying Coal-rock Mass with Upper Protective Seam Mining Method

On the foundation of analyzing two functions of mining protective seam, namely, relieving pressure and preventing coal and gas outburst, the mechanical model of underlying coal-rock mass was developed according to elastic force theory. In addition, the stress of any point at floor is deduced. The failure evaluation criterion of underlying coal-rock mass is also proposed. Combining the specific engineering parameters of Pingdingshan No.12 coal mine, the relation between the depth of the mining-induced fractured zone in floor and mining height has been proposed using theoretical analysis. Fracture development and stress relief law of outburst coal seam under different mining height of upper protective layer are analyzed. The studies indicate that with the increase of the depth of floor, the underlying coal-rock mass will only relieve a certain pressure and the pressure relief protection range will also narrow. Besides, the stress distribution will also develop to “V” shape form “U” shape. When the height of protective seam is designed as 2.0 m, the stress relief ratio will rise to 90% which can ensure the pressure-relief effect as well as benefit the economic and social benefits. The engineering application shows that after mining the upper protective seam of Ji-14, the gas pressure of Ji-15 outburst coal seam decreased from 1.78 MPa to 0.35 MPa with a drop of 81%, which is consistent with the theoretical calculation results of stress relief.