Shear behavior of intact granite under thermo-mechanical coupling and three-dimensional morphology of shear-formed fractures

The shear failure of intact rock under thermo-mechanical (TM) coupling conditions is common, such as in enhanced geothermal mining and deep mine construction. Under the effect of a continuous engineering disturbance, shear-formed fractures are prone to secondary instability, posing a severe threat to deep engineering. Although numerous studies regarding three-dimensional (3D) morphologies of fracture surfaces have been conducted, the understanding of shear-formed fractures under TM coupling conditions is limited. In this study, direct shear tests of intact granite under various TM coupling conditions were conducted, followed by 3D laser scanning tests of shear-formed fractures. Test results demonstrated that the peak shear strength of intact granite is positively correlated with the normal stress, whereas it is negatively correlated with the temperature. The internal friction angle and cohesion of intact granite significantly decrease with an increase in the temperature. The anisotropy, roughness value, and height of the asperities on the fracture surfaces are reduced as the normal stress increases, whereas their variation trends are the opposite as the temperature increases. The macroscopic failure mode of intact granite under TM coupling conditions is dominated by mixed tensile–shear and shear failures. As the normal stress increases, intragranular fractures are developed ranging from a local to a global distribution, and the macroscopic failure mode of intact granite changes from mixed tensile–shear to shear failure. Finally, 3D morphological characteristics of the asperities on the shear-formed fracture surfaces were analyzed, and a quadrangular pyramid conceptual model representing these asperities was proposed and sufficiently verified.