Abstract: The cyclic loading and unloading axial compression tests of coal specimens with the pre-existing transfixion fissure were carried out by triaxial loading coal rock seepage testing device. The permeability variation of coal specimen with pre-existing fissure and the difference with permeability of intact coal specimen were analyzed. The results show that the relationship between permeability and axial compression is a negative exponential function and the stress sensitivity of permeability decreases with the increase of the loading and unloading numbers for coal with pre-existing fissure. The permeability difference of the loading stage and permeability difference of unloading stage decrease with the increase of the loading and unloading numbers. There is a positive correlation between permeability difference and stress sensitivity coefficient. There exists a significant permeability difference during the loading and unloading stage, which indicates that there is apparent loss in permeability and it decreases with the increase of the loading and unloading numbers. The permeability and stress sensitivity coefficient of coal specimens with vertical fissure are higher than those of the coal specimens with horizontal fissure and the intact coal specimens. The permeability of coal specimen with horizontal fissure is similar to that of the intact coal specimen, but the stress sensitivity coefficient is higher than that of the intact coal specimen. The permeability difference and the loss quantity of permeability of coal specimens with vertical fissure are higher than those of the coal specimens with horizontal fissure and intact coal specimens. The permeability difference of coal specimens with horizontal fissure is higher than that of the intact coal specimens, but there is slight difference in the loss quantity of permeability between them. After the cyclic loading and unloading axial compression, the loss rate of permeability is the largest for intact coal specimen, followed by the coal specimen with horizontal fissure and last is the coal specimen with vertical fissure. The recovery rate of permeability is the largest for the coal specimen with vertical fissure, followed by the coal sample with horizontal fissure, and last is the intact coal sample.

Key words: cyclic loading and unloading, axial compression, pre-existing transfixion fissure, permeability, stress sensitivity