Mode I fracture toughness (K?c) of frozen rocks and that of frozen-thawed rocks were obtained by using BDT and CCNBD specimens. The test temperatures ranged from +25øC to -160øC. Wet and air-dry specimens of granite and sandstone were used in order to investigate the effect of water and porosity on fracture toughness. The SEM images of the frozen-thawed rocks were also analyzed to check the density of thermal cracks. The K?c of the frozen rocks increased with a decrease in the test temperature. The ratio of increase was higher in wet condition than in dry condition and the ratio of increase for granite was higher than that for sandstone. The Kw of the frozen-thawed rocks varied within 15ø,4 from the K?c of the rocks at room temperature. Thermal cracks occurred in granite but not in sandstone. From the test results, it was inferred that water content and porosity were the most important factors affecting the behavior of rocks at low temperature.
Recently, there has been more and more interest in the storage of low temperature materials such as frozen food, LPG and LNG in underground space (Inada 1989, Lindblom & Glamheden 1997). However, there has not been much research on the thermo-mechanical behaviors of rocks exposed to low temperature. Especially a few fracture mechanical approaches have been provided (Ishzuka et al. 1984, Dwivedi et al. 2000).
At low temperature, rocks undergo tensional thermal stress and cracking will occur due to the steep temperature gradient and the difference of thermal expansion. Consequently fracture mechanical approaches are needed to explain cracking phenomena more accurately.
Thus, this paper deals with the mode I fracture toughness of two types of Korean rocks at various sub-zero temperatures, using BDT (Brazilian Disk Test, Guo et al. 1993) and CCNBD (Cracked Chevron Notched Brazilian Disk, ISRM 1995).