Effects of high temperature and thermal cycles on fracture surface's roughness of granite: An insight on 3D morphology

The roughness of the fracture surface directly affects the strength, deformation, and permeability of the surrounding rock in deep underground engineering. Understanding the effect of high temperature and thermal cycle on the fracture surface roughness plays an important role in estimating the damage degree and stability of deep rock mass. In this paper, the variations of fracture surface roughness of granite after different heating and thermal cycles were investigated using the joint roughness coefficient method (JRC), three-dimensional (3D) roughness parameters, and fractal dimension (D), and the mechanism of damage and deterioration of granite were revealed. The experimental results show an increase in the roughness of the granite fracture surface as temperature and cycle number were incremented. The variations of JRC, height parameter, inclination parameter and area parameter with the temperature conformed to the Boltzmann's functional distribution, while the D decreased linearly as the temperature increased. Besides, the anisotropy index (Ip) of the granite fracture surface increased as the temperature increased, and the larger parameter values of roughness characterization at different temperatures were attained mainly in directions of 20°-40°, 60°-100° and 140°-160°. The fracture aperture of granite after fracture followed the Gauss distribution and the average aperture increased with increasing temperature, which increased from 0.665 mm at 25 °C to 1.058 mm at 800 °C. High temperature caused an uneven thermal expansion, water evaporation, and oxidation of minerals within the granite, which promoted the growth and expansion of microfractures, and reduced interparticle bonding strength. In particular, the damage was exacerbated by the expansion and cracking of the quartz phase transition after T > 500 °C. Thermal cycles contributed to the accumulation of this damage and further weakened the interparticle bonding forces, resulting in a significant increase in the roughness, anisotropy, and aperture of the fracture surface after five cycles.