Tests on mechanical and permeability characteristics of raw coal under complex stress paths

A series of cyclic loading and unloading tests on raw coal is conducted under the conditions of axial compression and confming 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 confming pressures. The results show that the plastic deformation degree SD increases with the increase of axial compression degree. Cumulative plastic deformation delta epsilon, cumulative amount of permeability variation delta K and cumulative volumetric strain variation delta epsilon(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 delta epsilon(3) and delta epsilon(1) have the same effect on the cumulative permeability variation delta K; with the increase of sigma(1), delta epsilon(3) is more sensitive to the change of delta K than delta epsilon(1). Assuming an extreme scenario, i.e., delta epsilon(V) is determined only by delta epsilon(1) and 2 delta epsilon(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 epsilon(3) and sigma(3) also has a good linear fit. epsilon(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.