Investigation of the hydro-mechanical behaviour of a pellet/powder MX80 bentonite mixture using an infiltration column

Abstract

Non-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration.