Abstract: To describe the permeability evolution process, the strength degradation index is introduced based on Hoek-Brown strength criterion; a strain softening constitutive model with confining pressure for coal rock is established. The relationship between volumetric strain and permeability is given; and combining the strain softening model, a permeability evolution model is proposed. Then the model is implemented in FLAC. Numerical cases are simulated including the post peak strain softening behaviors of a cylinder rock sample under different confining pressures and the permeability evolution of overburden rock during mining, The results show that: (1) The effects of confining pressure on post peak strain softening mechanical behaviors and permeability evolution can be better reflected by the model proposed. (2) With the advancing of working face, more and more coal rock elements fail and the permeability also increases. Gradually the main channels of gas flow appear. (3) The model is capable of reproducing the permeability evolution process of mining coal rock, poviding a reference for the practice of simultaneous extraction of coal and gas, methane recovery and gas disaster prevention.

Key words: mined coal rock, strength degradation index, permeability evolution, confining pressure, volumetric strain, strain softening model