Research on a permeability model of coal damaged under triaxial loading and unloading

•A novel permeability model under triaxial pressure loading–unloading was developed.•The influence of effective stress and adsorption/desorption is considered in model.•The model is verified using test results of coal seepage under three conditions.•Triaxial pressure loading–unloading has a significant effect on coal permeability. It is important to research permeability models to clarify the evolution of methane seepage from coal seams and to reduce the occurrence of gas accidents. However, existing permeability models do not fully account for the influences of effective stress and adsorption/desorption on the permeability of coal rocks damaged during the three-dimensional stress loading and unloading process in coal mining. This study proposed a novel coal permeability model by more robustly considering the effects of stress and gas adsorption/desorption based on the generalized Hooke's law and Gibbs' equation. Then, a series of triaxial stress loading–unloading seepage experiments for coal rock were performed to validate the proposed model. The experimental research shows that triaxial stress loading and unloading have a significant effect on coal rock permeability. Before coal rock failure, the gas permeability shows a “V”-shaped development trend. After the deviatoric stress exceeds the compressive strength of the coal, the coal permeability increases sharply. Theoretical verification shows that the established exponential permeability model can effectively reflect not only the dynamic increase in the permeability of damaged coal under triaxial stress loading–unloading conditions but also the decrease in coal rock permeability during the increase in gas pressure with a constant effective stress. Moreover, the influence factors introduced in the permeability model can effectively reflect the influences of effective stress and gas adsorption/desorption on coal permeability under triaxial stress loading–unloading with different gas pressure conditions. These findings elucidate the variation in methane permeability of damaged coal rock under triaxial stress loading and unloading. The established permeability model can be used to carry out other numerical simulation studies to provide theoretical support for the prediction of gas permeability and efficient recovery of coalbed methane for coal mining working faces.