Study on Elastoplastic Damage Constitutive Model and Permeability Evolution Law of Gas-Bearing Coal

Original coal reservoir contains massive free and adsorbed methane, that directly affects the mechanical properties and permeability of coal with its unique dual effects, which may, thus, promote the gestation and occurrence of mine gas disasters. Mining disturbance and borehole drainage can facilitate the plastic rupture of surrounding rock mass, which would result in the gas migration law becoming more complex that, in turn, would be difficult to explain based on classical elastic deformation theory. Therefore, understanding elastoplastic deformation behavior and gas migration law relating to coal under engineering disturbance is crucial for preventing gas disasters. First, when mechanical and adsorption induced effects of gas on coal are considered, a concept of effective total pore pressure has been proposed. From which, calculation of plastic deformation in accordance with non-associated flow rule, a constitutive model that considered elastoplastic deformation (EPC Model) was established to simulate deformation and failure characteristics. Following that, a permeability jump coefficient was introduced to describe the sudden increase in permeability following damage and failure in coal, and an elastoplastic damage-permeability model (EPDP Model) was further deduced on basis of cubic law. Finally, through triaxial compression-seepage experimental data to verify the established EPC Model and EPDP Model. The results revealed that gas would weaken coal’s mechanical properties (peak strength and elastic modulus) by changing its microstructure, whose deterioration would become more obvious with an increase of gas pressure. The changes to the permeability curve were of an "S" type, which showed a good corresponding relationship with the change trends relating to whole stress–strain curve. Both the EPC Model and the EPDP Model exhibited satisfactory matching effects with experimental data. Furthermore, EPDP Model was popularized and applied, with its universality being verified by the experimental data relating to a reduction in pore pressure seepage. This research will provide a new thinking for ensuring safe and efficient production in coal mines.