Abstract: In order to study the mechanical behavior and permeability evolution law of coal in mining process, the mechanical and seepage experiments on raw coal under different loading and unloading conditions were carried out by using self-made thermal- hydro-mechanical (THM) coupling seepage servo testing system for gassy coal. The influence of loading and unloading stresses on coal deformation and seepage was analyzed and discussed. The relationship between dilatancy angle and plastic shear strain was obtained, and it was found that the dilatation angle changes sharply when the plastic shear strain is about 1.6%. According to the experimental phenomena and results, the structures of coal in elastic stage and yield damage stage were simplified by considering the influence of coal structure on permeability. Based on match stick model and seepage theory analysis, the influence of loading and unloading stresses on coal permeability was discussed from the perspective of strain, and two permeability models (i.e. in the elastic stage and damage stage) were established. Based on the permeability models in different stages and dilatancy angle law, the whole process of permeability model of coal was constructed. The permeability model constructed in this paper is in good agreement with the experimental results, which verifies the applicability of the expression and provides insights for the realization of coal and gas simultaneous extraction.

Key words: coal, dilatancy angle, loading-unloading, permeability model