Effect of stress induced damage on gas permeability of Beishan granite

Permeability is the key factor to control long-term stability of surrounding rock of high level radioactive waste (HLW) geological repository. In view of the low permeability of the intact granite in Beishan area, the preferred site for HLW repository in Northwest China, argon was used as the fluid medium to measure the gas permeability of Beishan granite under hydrostatic pressures and cyclic loading-unloading triaxial tests. It is found that the gas permeability decreases with the increase of confining pressure and their relationship approximately conforms to the power function. Due to irreversible plastic deformation, the permeability during unloading is less than that in loading process under the same confining pressure level. In the triaxial loading-unloading test, the acoustic emission technique applied well reflects the micro-crack propagation process and quantifies the cumulative damage in granite samples. It shows that the dilatancy stress is the threshold stress of the micro cracks in rock turn from stable growth to accelerated growth. This is the physical mechanism of significant increase of permeability after reaching the dilatancy stress.