Shear fracturing behavior and mechanism of intact granite under thermal-mechanical coupling loading

High rock temperature is a great challenge frequently encountered during subsurface resource recovery and deep underground space utilization, and it is still unclear how the granitic rock responds to real-time high temperature upon shear loading. To better understand the shear fracture behavior and underlying processes of intact granite exposed to thermal-mechanical coupling loading, direct shear tests were conducted utilizing a newly built testing apparatus at varied normal stresses and high temperatures. Influences of different temperatures and different heating methods (real-time heating and thermal treatment) on the shear mechanical behavior were compared and discussed. Results indicate that shear stress fluctuations with some small stress drops occur as shear stress is approaching the peak strength under real-time heating, accompanied by more and earlier AE signal uprushes. This suggests that greater cracking events occur earlier during real-time heating than after thermal treatment, resulting in a lower peak shear strength. Furthermore, the peak shear strength, post-peak stress drop, and cohesion rise from room temperature (RT) to 200 °C (the peak strength increases by 8%, 5.8%, and 9.9% under normal stress of 5 MPa, 15 MPa, and 20 MPa, correspondingly), and subsequently decline from 200 °C to 400 °C. Temperature has a limited impact on shear stiffness from RT to 200 °C, but significantly reduces it from 200 °C to 400 °C, with drops of 15%, 7.9%, and 10% under normal stress of 5 MPa, 15 MPa, and 20 MPa, respectively. Moreover, the shear strength and stiffness under real-time heating are lower than those for the thermally treated specimens. The strengthening of intact granite below 200 °C upon shear is associated with loss of water and a more compacted structure, while the weakening effect of temperature on shear strength from 200 °C to 400 °C is due to the new thermal cracks and less brittle and stiff of minerals.