Mechanical and Thermal Deformation of Ghareb Borehole Core for Repository Characterization

ABSTRACT: The selection of a geologic repository for nuclear waste requires careful selection of the appropriate lithology. Evaluating the Ghareb formation for repository use, we conducted several thermo-mechanical tests of cores from two sections of the Ghareb formation from Israel. The two sections are compositionally similar but differ in observed fracture populations. A series of triaxial tests was conducted with samples from both sections to compare mechanical behavior. Thermal creep tests were also conducted to assess how increasing temperatures would promote creep in the Ghareb. The triaxial tests show that both sections of the Ghareb are mechanically weak, with low yield and failure stresses consistent with high-porosity chalks. Despite macroscopic differences along the borehole, the mechanical behavior of the two sections is fairly similar. Both exhibit large weakening induced by temperature and water, with significant time-dependent creep activated by increasing the thermal load. We propose that the existence of fractures in different sections of the Ghareb is not itself the best predictor of the mechanical behavior during repository operation, as the conditions of a repository make it much more likely that the time and water activated mechanisms of creep are likely to be dominant over brittle failure mechanisms for the lifespan of the repository.