Permeability evolution of unloaded coal samples at different loading rates

As coal mass is often at unloading status during mining process, it is of great significance to push on the research on permeability evolution of unloaded coal samples at different loading rates. A series of triaxial unloading experiments were conducted for initially intact coal samples using an improved rock mechanics testing system, and the permeability was continuously measured by the constant pressure differential method for methane. Permeability evolution law of unloaded coal samples and the influence mechanism of loading rates on that were studied. The results of triaxial unloading experiments indicate that the permeability of coal samples increases throughout the whole testing process without a descent stage, which is different from the permeability evolution law in conventional triaxial compression tests. The maximum permeability of unloaded coal sample, which is 4 to 18 times to its initial permeability, often appears before reaching the peak stress and increases with the decrease of axial loading rate. Stress state corresponding to the surge point of permeability of the unloaded coal samples is also discussed