Shear Behavior of Artificial and Natural Granite Fractures After Heating and Water-Cooling Treatment

A clear understanding of shear behaviors of granite fractures with high temperatures undergoing water-cooling treatment is important in enhanced geothermal system (EGS). Two types of fractured Gonghe granite specimens, containing either a Brazilian-induced artificial fracture and or a pre-existing natural fracture, respectively, were considered. Before direct shear tests, all specimens were suffered from slow heating and rapid water-cooling treatment, and the thermal treatment temperatures ranged from room temperature 25 degrees C (without thermal treatment) to 300 degrees C. Shear properties of natural fractures, including shear strength, shear stiffness and shear dilation, are lower than that of the artificial fractures, mainly ascribed to their lower surface roughness than the artificial ones. Better matching in the artificial fractures after thermal treatment contributes to the increase in peak shear strength. The roughness increment or reduction after thermal treatment is insignificant. The thermal treatment results in the larger sheared-off asperities damage volume after shear in the artificial and natural fractures than that without thermal treatment. There is a strong correlation between the shear stress and acoustic emission (AE) parameters in both artificial and natural fractures. The AE responses in natural fractures are weak compared to that in artificial fractures. AE events distribute uniformly near the fracture surface in artificial specimens but present dispersedly in the natural ones.