Abstract: In this study, experiments were conducted on raw coal specimens by using the self-developed triaxial servo-controlled seepage equipment for thermo-hydro-mechanical coupling of coal containing methane. Experiments were carried out when the axial pressure maintained at different stress levels but the confining pressure was unloaded. We studied the effects of unloading confining pressure on deformation properties and permeability characteristics of gas-filled coal at the post-peak. The results showed that the damage variable was defined by the unrecoverable deformations of radial strain , axial strain and volumetric strain and the response of these three parameters during the process of unloading. The interval [0, 1] of damage variable was satisfied to calculate the damage quantity of coal specimen in the process of unloading. When the axial pressure remained constant and the confining pressure was unloaded, the amount of damage quantity D increased with the decrease of and the damage degree of coal specimen increased greatly. When unloading the axial pressure to different stress levels, the permeability of coal specimen increased with the growing unloading times of confining pressure. It indicated that the influence of confining pressure on permeability was more and more important when decreased. At the same time, the pores and fractures in the coal specimen were developed, expanded and extended. In addition, the permeability k hardly increased at the beginning of unloading . As continued to unload, k began to increase and the slope became growing, indicating the severe damage of coal specimen. The radius of Mohr stress circle was equivalent to an increase under this stress path of loading and unloading, and coal specimens tended to destruction causing the increasing possibility of the occurrence of secondary failure. At the higher level of , with the unloading of , the radius of Mohr stress circle increased and the supportability of coal specimens became weaker. Hence, the secondary failure easily occurred, which manifested the abrupt change of axial strain and radial strain .

Key words: confining pressure unloading, damage, damage variable expression, permeability, Mohr stress circle, secondary failure