Deformation and permeability characteristic of outburst coal under step unloading conditions

To analyze the permeability law of outburst coal under mining stress field during lower protective seam min- ing,the gas permeation experiment of coal samples was carried out by the triaxial gas-seepage test machine under the process of different ways (constant axial pressure,loading axial pressure,and unloading axial pressure) of step unloa- ding confining pressure with different rates. The results show that:The deformation of coal samples has obvious ladder shape characteristics in the experiment. When the coal samples are not destroyed,the strain increment of the coal sam- ples increases with the increase of the unloading rate of confining pressure. As the unloading of confining pressure,the deviator stress of coal samples under the unloading confining pressure with constant axial pressure and unloading confi- ning pressure with loading axial pressure increase,and its permeability shows a trend of decrease initially and then in- crease. The permeability of coal samples under unloading axial pressure and confining pressure at the same time in- creases with the unloading of confining pressure. When the change of the volume strain of coal samples is large,the variation of permeability is large. Analysis on the change of permeability of coal samples is based on the energy,and the permeability of coal samples increases exponentially with the increase of the energy dissipation rate.