Abstract: A series of cyclic loading and unloading tests on raw coal is conducted under the conditions of axial compression and confining pressure using self-developed servo-controlled seepage equipment for thermal-hydro-mechanical coupling of coal. The mechanical properties and permeability characteristics of coal are analyzed under different axial compressions and confining pressures. The results show that the plastic deformation degree SD increases with the increase of axial compression degree. Cumulative plastic deformation ??, cumulative amount of permeability variation ?K and cumulative volumetric strain variation ??V are defined, and some conclusions are drawn with the use of those parameters. At a relatively low level of axial loading-unloading, the cumulative plastic deformation δε3 and δε1 have the same effect on the cumulative permeability variation ?K; with the increase of ?1, ??3 is more sensitive to the change of ?K than ??1. Assuming an extreme scenario, i.e., ??V is determined only by ??1 and 2??3, the radial deformation is found more sensitive than axial deformation. The axial compression remains constant, and the axial deformation decreases linearly with the change of confining pressure. Radial deformation ?3 and ?3 also has a good linear fit. ?3 presents V-shape, and the permeability exhibits inverted V-shape. When axial load is at a low level, the loading-unloading of confining pressure does not have a significant impact on the development of plastic deformation.

Key words: plastic deformation degree, cumulative plastic deformation, sensitive, linear fitting, confining pressure, axial compression