Hydro-mechanical behaviour of shallow Opalinus Clay shale

In Switzerland, Opalinus Clay shale has been selected as the host formation for radioactive waste disposal. The minimum required depth of the repository is related to the long-lasting isolation required for the disposal (1 million years). During this period, possible erosion scenarios affecting the repository need to be analysed. Opalinus Clay from shallow depths (< 70m) was sourced from a borehole in Northern Switzerland, where the formation was affected by a considerable exhumation process. This work aims to investigate the impact of the mentioned phenomenon on the hydro-mechanical behaviour of Opalinus Clay through one-dimensional consolidation and permeability measurements. Laboratory tests on intact and remoulded specimens sourced from shallow depths were performed and compared to previous studies of specimens sourced at greater depths (Mont Terri, ≈300m, and Schlattingen borehole, ≈900m). The results revealed a limited impact of the erosion phenomenon on the analysed aspects of Opalinus Clay at the laboratory scale. The compressibility and swelling indexes were shown to strongly depend on the clay-mineral content of the specimens. The impact of the diagenesis on the current void ratio of the formation was also highlighted. The mechanical response in the loading directions, perpendicular and parallel to the bedding planes, revealed a marked anisotropy in the low stress range, which reduces for high stress values.