A permeability model for coal based on elastic and plastic deformation conditions under the interaction of hydro-mechanical effects

In engineering activities involving the mining of coal resources, gas is the main factor for causing coal and gas explosions accidents, with permeability being an important parameter in estimating gas emissions. With the steady development of the mining face, stress in front of the work face has evidently changed, that has resulted in coal damage under mining disturbance. In the underground environment, coal is in a complex environment where water, gas and external loads combine with each other, leading to changes in permeability that are often extremely complex, with effective stress being the dominant factor to changes in permeability. Therefore, coal permeability change related to damage-induced effects under different water contents should be further studied. In this study, first, the influence of water, gas adsorption and effective stress on coal fracture permeability change were considered. Further, damage variables were introduced to combine mechanical damage with water weakening damage. By quantifying changes in matrix sizes with new fracture generations, the plastic deformation in fractures was estimated. Based on a cubic model, a damaged-induced permeability model under the interaction of hydro-mechanical effects was established. From this, a triaxial seepage experiment involving the whole stress-strain process, including effective stress changes of water-bearing coal, was conducted to compare with, and to verify the results against an established model. The experimental results revealed that changes in axial strain and permeability was “S” shaped during the whole stress-strain process. In addition, during the whole stress-strain and effective stress changes process, the permeability model corresponded well with the experimental results. This demonstrated that the model could not only predict permeability change under elastic deformation, but could also describe permeability changes under plastic deformation during the damage stage. The purpose of this study was to provide a new method for predicting the permeability evolution law relating to coal mining and gas extraction.