Abstract: In this study, a self-developed ‘THM coupled with triaxial servo-controlled seepage apparatus for gas infiltrated coal’ was employed to study the deformation and permeability laws of sandstone. Seepage experiments were carried out at the same gas pressure and under different unloading confining pressures. Then, a seepage model was established on the basis of strain parameters of sandstone. The results showed that gas infiltrated sandstone exhibited clear brittle feature during the failure process and sudden jumps occurred among the stress, strain and flow immediately after the peak stress point. Both changes of stress-strain and permeability showed obvious periodical characteristics, which means that the gas permeability was zero during pore and micro-fissure compaction, and elastic deformation stages (I and II). During unloading and yield stage (III), new fractures were initiated and the permeability increased slightly. During unloading failure stage (IV), the stress dropped significantly, the strain increased sharply, and the gas flow also increased greatly when fractures connected with each other, which indicated that this period was dominated by the permeability. The strain, stress, and gas flow all tended to be steady after failure (V). At failure state of sandstone, the radial strain and volumetric strain were far larger than the axial strain, due to the effects of gas pressure and the Poisson. The unloading failure mode of bedding sandstone was mainly dominated with extension and shear fracture along beddings. Based on Kozeny-Carman equation and fracture flow theory, the permeability model related to strain was established. Finally, the permeation mechanism of gas infiltrated sandstone at different failure states was revealed.

Key words: mining engineering, gas, sandstone, unloading confining-pressure, permeability models