Study on mechanical deformation and permeability evolution of gas-bearing coal–rock combination under different loading and unloading paths

In deep coal mining, coal–rock combinations experience complex stress paths, significantly affecting gas migration and the risk of coal–rock–gas dynamic disasters. To elucidate the fluid–solid interaction behavior under such conditions, this study investigates the permeability evolution of coal–rock combinations through triaxial mechanical-permeability synchronous tests and numerical simulations. While mechanical deformation remains dominant, the variation in permeability is closely linked to the failure process. An energy dissipation theory is employed to quantitatively analyze the energy evolution and interlayer transfer during loading and unloading. Results show that the elastic rebound of high-strength rock after peak stress intensifies coal deformation, accelerating failure. Under constant confining pressure with loading axial pressure (CCPLAP), the damage and permeability increase more sharply compared to constant axial pressure with unloading confining pressure (CAPLCP). The energy dissipation rate is a key indicator of instability with an average of 0.51 kJ/(m3 s) under CCPLAP and surging to 37.75 kJ/(m3 s) under CAPLCP—a 74-fold increase. Additionally, permeability evolution reflects both the internal damage of coal and the flow-impeding effect of low-permeability rock. These insights contribute to understanding the coupling between mechanical failure and gas transport, offering a theoretical basis for gas-related hazard mitigation in coal mining.