Preliminary laboratory thermo-hydro-mechanical characterization of Opalinus clay

Argillite is being considered as a possible host rock for geological radioactive waste disposal in several countries. In the present work, Opalinus clay, taken from the Mont Terri Rock laboratory (Switzerland) was tested.A triaxial compression test was performed under saturated conditions. It was observed that preparing a specimen for triaxial test (38mm in diameter and 76mm in height) may induce certain damage on the material. In addition, when saturating the specimen under its in situ stress (2MPa), a volumetric swell of 1.2% occurred. The swelling process continued even after 200 h of saturation. Secondly, a possible effect of heating (which would be generated by the radioactive waste canister in the conditions of waste disposal) on the mechanical behavior was also tested. A soil specimen, having a suction of 39MPa was subjected to a heating/cooling cycle (temperature increased from 25 to 60°C and then decreased to 25°C) under a low confining pressure (0.1MPa). The results show that heating induced first a thermal dilation. When the temperature was maintained at 60°C, a decrease of the soil volume was observed. The subsequent cooling induced a decrease of the soil volume with the same thermal dilation coefficient obtained during the heating. The thermal behavior of Opalinus clay (thermal dilation and then contraction during heating) is then similar to that usually observed on clays. Further studies on the temperature effects are needed to clarify this behavior.