An Experimental Study of Effect of High Temperature on the Permeability Evolution and Failure Response of Granite Under Triaxial Compression

Granite is a viable host for deep nuclear waste disposal because its low permeability and high strength enable the stable and safe operation of the repository. We examined the evolution of the permeability and triaxial mechanical behaviour of granite after high-temperature treatment. First, the effect of the high temperature on the physical behaviour and permeability evolution of granite was analysed in detail. The mass, P-wave velocity, and thermal conductivity of granite decrease, but the volume increases with increasing temperature. The permeability of intact granite increases by four orders of magnitude as the cycled temperature increases from 25 to 800 degrees C. Subsequently, the effect of high temperature on the triaxial deformation and acoustic emission (AE) behaviour of granite was investigated. Under uniaxial compression at T <= 300 degrees C, the stress decreases before the peak strength is reached, corresponding to a significant AE event, which is due to the development of multiple splitting tensile fractures along the loading direction. At T >= 450 degrees C, AE event is observed once a minor stress is applied, which results from failure is controlled by thermally induced cracks. However, under triaxial compression, the temperature has little effect on the AE characteristics. The granite fails along the shear fracture plane, which becomes wider with increasing confining pressure. At T >= 600 degrees C, it is easier to form intragranular cracks and the stress quickly decreases after the peak strength is reached. The shear plane is smoother under high confining pressure. Third, the effect of high temperature on the peak strength and crack damage threshold of granite was further analysed. Generally, under uniaxial compression, the peak strength and crack damage threshold first remain relatively constant at T <= 300 degrees C, begin to decrease at T = 450 degrees C, and decrease more rapidly at T = 600 degrees C. The confining pressure notably reduces the effect of the temperature on the peak strength and crack damage threshold. Finally, the effect and mechanism of high temperature on the triaxial strength parameters of granite were further discussed. At T <= 300 degrees C, thermally induced cracks are not notable and the temperature has little effect on the strength. At 450 degrees C <= T <= 600 degrees C, thermally induced cracks are more notable and the temperature has a significant effect on the strength behaviour. Because of the thermal stress released by thermal macrocrack formation, the continuous increase in the temperature has little impact on strength.