Study on the influence of true triaxial principal stress loading and unloading on gas permeability characteristics of coal seams containing gangue

• Permeability evolution of coal with gangue studied under true triaxial conditions. • Intermediate principal stress reduces permeability by up to 55%. • Higher gangue volume ratio extends flow paths and lowers permeability. • A permeability model based on stress–pore pressure coupling is proposed. • Model fits well with R² > 0.96, guiding gas extraction in complex coal seams. Investigating the seepage evolution characteristics of coal containing gangue under true triaxial stress conditions is of great significance for the efficient extraction of gas in complex coal seams. In this study, a series of anisotropic stress loading and unloading experiments were conducted using a multi-functional true triaxial fluid–solid coupling testing system developed Chongqing University, China. Coal specimens with varying gangue volume ratios (0, V/20, V/30, V/50) were subjected to different stress paths to systematically evaluate the effects of the intermediate principal stress coefficient and stress path on permeability evolution. The experimental results indicate that the permeability of coal containing gangue decreases exponentially with increasing principal stress. Under three typical stress paths, the maximum permeability reduction exceeded 80%. Specifically, when the intermediate principal stress (σ₂) increased from 35 MPa to 49 MPa, the decreased by up to 55%, indicating a pronounced sensitivity of permeability to intermediate principal stress. Further analysis revealed that an increase in gangue volume ratio significantly prolongs the gas migration pathway and enhances the compression and disruption of seepage channels, thereby further reducing permeability. A theoretical permeability model was developed based on true triaxial stress and gangue pore pressure control. The model’s predictions demonstrated excellent agreement with the experimental data, with coefficients of determination (R²) exceeding 0.96, confirming its accuracy in capturing the evolution of permeability under complex stress conditions. The research results can provide theoretical support and engineering basis for the gas extraction process and drilling arrangement of coal beds with gangue development.