Laboratory investigation of thermal stresses in fractured granite: Effects of fracture surface roughness and initial stress

In order to investigate the effects of fracture surface roughness and initial stress on thermal stresses generated in fractured rock masses, we conducted laboratory heating experiments on 100-mm cubic granite specimens that contained four different kinds of roughness in fractures (i.e., polished, sawcut, mated rough, and unmated rough) under different initial stresses. Thermal expansion in the vertical direction was mechanically restricted during heating, while the sample was free to expand in the horizontal direction where heating was applied through a pair of loading plates. Thermal stresses in intact rocks without discontinuities were also measured for comparison. Results revealed that the temperature distribution was nonuniform in the samples. Thermal stress was generated in the vertical direction, and it was higher in the intact rock than that in the rock masses that contained discontinuities. For the fractured rock masses, thermal stress was the highest in the polished fracture, followed by the sawcut fracture, and then the mated rough fracture, and it was the lowest in the unmated rough fracture. We found that the thermal stress has a linear correlation with the apparent equivalent Young's modulus, which characterizes the total mechanical deformation of both the granite matrix and the fracture. We also confirmed that the thermal stress level was higher for a larger initial stress in the case with the mated rough fracture. One main reason that the thermal stress varied with fracture surface roughness, fracture wall offset, and initial stress was thought to be attributed to different magnitudes of fracture stiffness, which is dominated by fracture surface contact.