Abstract: In this paper, to investigate the stress distribution and movement law around gas drainage boreholes, the mechanical properties of coal mass around boreholes were analyzed, a visco-elastic plastic model considering the plastic softening and dilatancy was established, and the expressions of coal stress, displacement, pore size variation and pressure-relief range were obtained. In addition, the stress distributions and pressure-relief effects around drainage boreholes in the hard and soft coal seams were comparatively analyzed, the pore size variation rules of boreholes were studied, and the reason about rapid decay of borehole drainage concentration in soft seam was explained. The results show that the coal mass around boleholes have the properties of plastic softening, dilatancy and rheological. The boreholes in soft coal seam can get better pressure-relief effect, the creep deformation is more severely, and the unstabe failure can occur in a short time, which will block the gas drainage channel, and result in rapid decline of gas drainage concentration. Meanwhile, the pore sizes of boreholes in hard coal seam are reduced somewhat, but still in stable state, and will not block the gas drainage channel. But, with the increase of ground pressure in deep minefield, the creep deformation will increase, and the unstabe failure may occur. The research results agree well with mining practice, which can provide theoretical guidance for the determination of gas drainage parameters.

Key words: gas drainage borehole, rheological property, stress distribution, movement law, plastic softening, dilatancy, soft and hard coal mass