Effect of heat exposure on anisotropic mechanical properties of host rock in the Tamusu pre-selected area for high-level radioactive waste disposal in China

As a typical sedimentary rock, clay rock with bedding structure shows clear anisotropy in its mineral composition and mechanical properties. When the clay rock is selected as the host rock for a high-level radioactive waste (HLW) geological repository, its temperature rises because of the continuous release of heat by the HLW. To examine the influence of temperature on the anisotropic mechanical properties of clay rock in the Tamusu pre-selected area for HLW repository in China, a series of direct shear tests in different directions were conducted on the Tamusu clay rock exposed to a range of temperatures. The microstructural characteristics were observed by scanning electron microscopy, and the morphology of the shear failure surfaces was analyzed with the help of 3D laser scanning. The results show that the shear strength parameters of specimens sheared perpendicular to the bedding plane were significantly higher than those of specimens sheared parallel to the bedding plane, and the former specimens exhibited a more rugged failure surface. Strength of both types of specimens increased with the increase in temperature from 30°C to 200°C because of water evaporation and mineral expansion. The mechanical anisotropy was also enhanced by heat exposure. These findings provide key information for the site characterization of the HLW repository, and a reference for the design and construction of underground engineering structures in layered clay rock.