Effect of thermal cycles on the thermal damage, hydraulic and mechanical properties of granite

Understanding how thermal damage accumulates in granites under cyclic thermal loading is crucial for geoengineering design and stability assessment. Two types of granite with different grain sizes were subjected to multiple heating and cooling cycles at identical or increasing target temperatures using a new apparatus. Acoustic emission (AE) and thermal deformation were monitored simultaneously. Ultrasonic velocity, porosity, and permeability measurements, compression tests, and thin-section petrography were conducted to assess the thermal damage and physical and mechanical properties of the treated specimens. The results indicate that the thermal damage progression exhibits cycle-dependent decay at 300 °C, and the thermal deformation tends to be elastic. However, the uniaxial compressive strength (UCS) increases after thermal treatment, though the increment decreases with progressive cycles (from 21% to 4% for coarse-grained granite). Progressive temperature cycling induces cumulative damage amplification, manifested by tremendous thermal cracking (the total number of AE hits reaches ∼170 and 180 times that of the first cycle) and crack density escalation (the crack density reaches ∼1.6 and 2 times that of the first cycle). AE results reveal that there is a temperature-memory effect in both rock types, and show that thermal stresses are insufficient to generate new thermal damage until the prior exposed temperature is exceeded. Additionally, thermal damage is related to the microstructure of rock: coarse-grained granite with more pre-existing cracks exhibits heating-dominated damage (e.g. 94% of the AE energy was generated during heating at 300 °C), while fine-grained granite with less inherent damage is more sensitive to cooling (e.g. 71% of the AE energy was generated during cooling). Quantitative relationship among P-wave velocity, Young’s modulus, and linear crack density provide a good estimation of thermal damage, and can guide assessments of the durability of underground engineering .