Experimental study on damage evolution and seepage characteristic of gas-bearing coal rock under true triaxial mining-induced stress

In order to reveal the permeability evolution law of gas-bearing coal under mining-induced stress, the deformation and permeability characteristics of coal under loading and unloading paths with different vertical and horizontal stress rise and fall ratios (4.75∶1, 3.50∶1, 2.25∶1) are simulated by true triaxial experiments. The research shows that the reduction of the relative unloading rate of horizontal principal stress can enhance the carrying capacity of coal and rock and increase the peak stress, a faster rate of relative unloading will cause significant changes in the horizontal strain of the coal body, leading to instability and failure of the coal body.The evolution of permeability exhibits three characteristic stages: in the initial stage, the increase of vertical stress causes the fracture to close, and the permeability remains stable or decreases; in the stage of fracture expansion, the vertical stress approaches the peak value and the permeability increases slowly; in the fracture coalescence stage, permeability rises sharply due to coal sample failure, with this phenomenon being particularly pronounced in damaged gas-bearing coal. Based on the coupling relationship between gas pressure and coal strain, a permeability prediction model with damage factor D and correction factor C was constructed.The calculation results of this model are in good agreement with the experimental data, effectively characterizing the dynamic response mechanism of permeability with respect to strain and gas pressure.