Characterization of roughness and shear behavior of thermally treated granite fractures: Fid-Bau Portal

Characterization of roughness and shear behavior of thermally treated granite fractures

Meng, Fanzhen / Song, Jie / Wong, Louis Ngai Yuen / Wang, Zaiquan / Zhang, Chuanqing

Abstract Despite a large number of studies examining the effects of high temperature on the physical and mechanical properties of various rock types, few studies have addressed the effects of high temperature on the frictional behavior of rough fractures. As a benchmark of high temperature effects on rock integrity, we measure the uniaxial compression strength (UCS) and p-wave velocity (V p) of granite samples after they are heated to various temperatures as high as 500 °C and allowed to cool to room temperature. We then investigate changes in the roughness and shear behavior of thermally treated rough granite fractures using direct shear tests. The results indicate that the UCS increases by 37% at 200 °C compared with that of granite without thermal treatment, and then decreases with temperature, while the V p decreases almost linearly with increasing treatment temperature. The strengthening of rock upon heating is very likely associated with the thermal expansion of minerals, water evaporation and microcrack blunting, while the decrease in the UCS is closely related to the increase in thermal cracks. The degradation of the fracture roughness increases with normal stress, and it also tends to be greater for higher treatment temperatures. The peak shear strength, post-peak stress drop and stick-slip magnitude, which are not significantly influenced by the treatment temperature between room temperature and 300 °C, decrease with treatment temperatures above 300 °C. In particular, no post-peak stress drop occurs for the thermally treated fractures sheared under low normal stress, and the post-peak stress drop is inhibited in fractures with much higher temperature treatments and shearing under high normal stress. Our new findings contribute to a better understanding of fault slip mechanics at high temperatures in geothermal recovery and high-level radioactive waste repositories.

Highlights • Direct shear tests conducted on thermally-treated rough granite fractures. • With increasing temperature, UCS initially increases and then decreases. • P-wave velocity decreases almost linearly with increasing temperature. • Degradation of roughness increases with normal stress and temperature. • Peak shear strength and stick slip amplitude decrease drastically above 400 °C.