Evolution of cracks and permeability of granites suffering from different thermal damages

Beishan granite, the host rock of the pre-selected site of high-level radioactive waste geological repository in China, was used to study the evolution of permeability after thermal treatment under compressive condition. The main results are summarized as follows: (1) The temperature threshold existed in the range of 500℃ to 600℃. The physical and mechanical properties of granite suffered the temperature higher than the threshold changed rapidly. (2) The threshold of heating rate was identified as 5℃/min. The temperature gradient caused by hating rate higher than 5℃/min induced high thermal-stress to form cracks for standard samples. (3) The scanning electron microscope images revealed that the cracks concentrated around the grain boundary of samples subjected to temperature lower than 573℃, while the trans-granular cracks were observed in the granite samples heated to temperature higher than 573℃. The cracks in feldspar developed along the cleavage plane, so they were all smooth, however the cracks in quartz were rough due to no cleavage plane in it. (4) The permeability of samples suffering the temperature higher than 600℃ was much greater than that suffering the temperature lower than 600℃, which demonstrated the existence of temperature threshold for permeability. (5) Two typical stress-permeability curves were observed for the whole compression tests. For the specimens treated below 500℃, the stress-permeability can be characterized as three sequential stages, i.e., falling stage, maintaining low permeability during successive volume compaction, rapid growth when approaching the peak stress. However, the permeability almost decreased during the whole loading process for samples after thermal treatment, higherthan 500℃. (6) The crack volumetric strain was applied to analyze the evolution of permeability. Good linear relationship was found between the permeability and the crack volumetric strain.