Permeability Evolution of Intact and Fractured Coal during Progressive Deformation Subjected to True Triaxial Stresses

Coal mining is gradually moving towards a deep area of more than 1000 m. At this depth, coal seams are under extremely high stress, leading to the formation of macroscopic fractures. The effects of cleats and macroscopic fractures on permeability evolution and comparative analysis based on established permeability models between intact and fractured coal are still limited. In this study, the permeability of intact and fractured coal specimens was tested by a multi-functional test system. The permeability data were fitted and analyzed based on the established permeability models. Our results show that the permeability curve of fractured coal has an L-shaped trend different from the S-shaped trend of intact coal permeability. The macroscopic fractures increased the permeability of coal samples by 1-3 orders of magnitude. The exponential model had a better fit for the permeability of intact and fractured coal specimens than the cubic model. The mean fitting degrees based on Chen's and Yu's models were 0.973 and 0.948, respectively. The slip of the fracture surface resulted in permeability fluctuations during the progressive deformation of fractured coal. The macroscopic fractures increased the slope of permeability in the post-peak stage and reduced coal compressibility and strength.