Study on lithologic effect of mining-induced failure mechanism of coal seam floor

In order to obtain the mining failure characteristics of coal seam floor with different lithology, the Mohr circle solution of the influence of different internal friction angles on the strength of rock mass is established, and the correlation formula between the ratio of the maximum floor failure depth at the end of the stope to the maximum floor failure depth in the goaf and the internal friction angle is constructed. Using FLAC^3D numerical simulation software, the distribution characteristics of plastic zone, maximum shear stress and elastic strain energy of coal seam floor under different lithology conditions are analyzed. From the perspective of energy accumulation and dissipation, the generalized model of mining failure mode of coal seam floor is established. Finally, the measured data of 19110 working face at Pingshuo mine area is verified. The results show that under the action of advance abutment pressure, weak rock is prone to shear failure, while the medium hard rock or hard rock without shear failure still has the possibility of further failure under the effect of mining unloading. It is found that with the increase of internal friction angle of coal seam floor rock, the maximum value of mining failure range of coal seam floor will gradually shift from the end of stope to the rear of goaf. Compared with the lithology of medium hard rock or hard rock in coal seam floor, the shear stress and elastic strain energy of coal seam floor rock mass with extremely soft rock or soft rock have obvious attenuation near the stope and transfer to the deep part, resulting in an obvious plastic zone at the end of the stope. The mining failure area of coal seam floor is divided into energy release area, energy bearing area and energy balance area. Compared with the weak or extremely weak coal seam floor rock, the distribution range of energy release area of medium hard or hard coal seam floor rock is smaller or even missing, mainly through energy bearing area and energy balance area to achieve the balance of mining disturbance energy. Through field measurement, the failure mode of coal seam floor has been verified. Under the action of mining unloading, the coal seam floor is further damaged, the lithology of the coal seam floor is fine sandstone, limestone and other hard rock mass. The coal seam floor changed from shear failure to shear-tensile deformation composite failure.