Micromechanical behaviour of intact Opalinus Clay shale

Abstract

Nuclear waste needs to be safely stored for a long duration due to its long period of radioactivity. The Opalinus Clay shale formation is considered for storage of nuclear waste in Switzerland. The Opalinus Clay is an indurated clay shale with a well developed bedding and high anisotropy. When creating a nuclear waste repository, or any tunnel for that matter, the rock directly around is affected by disturbances caused by drilling and change in humidity. Cracks can form, which creates problems due to the possible leakage of radionuclides into the biosphere. Opalinus Clay contains a series of self-sealing properties, that can close fractures over time after their formation. This research is part of the SE-P project, a research project whose goal is to test in situ selfsealing processes. In this research, Vickers indentation tests at different humidity levels and at different directions with respect to bedding were done on two different sedimentary facies of intact Opalinus Clay to be able to measure changes in Young’s modulus and hardness. Samples were conditioned in climate-controlled boxes with saturated salt solutions. A climatic chamber with a connection to a humidity generator was built around the test-setup to minimize change of saturation during testing. Testing and data analysis were done using the method developed by Oliver and Pharr (1992). The saturation change during tests was smaller than 4% for 98% of tests. For both hardness and stiffness a decrease was found with an increase in saturation level. Also for both stiffness and hardness, the parallel values and the sandy facies values are consistently higher than the perpendicular and shaly facies, respectively. The hardness and stiffness increase with decreasing saturation is faster for the shaly facies than for the sandy facies. For the hardness, mineralogy is the most important factor while for stiffness anisotropy is the most important. The decrease in hardness is attributed to internal bond damage due to swelling, while for the stiffness the cause of a decrease with increasing saturation is suction. The higher stiffness and hardness for tests parallel to bedding are caused by an extra strain component that is formed when stress is exerted perpendicular to bedding that is caused by the closure of microcracks. The faster decrease of stiffness and hardness of the shaly facies decreases the effect of both self-sealing processes and external damages. It also increases the damage done by swelling with each desaturation and resaturation cycle in the open drift stage.