Laboratory characterisation of the GBM bentonite of the FE experiment: insights into the thermo-hydro-mechanical behaviour and implications for the field experiment

The ‘Full-Scale Emplacement’ (FE) experiment was initiated in the Mont Terri rock laboratory in 2010 to simulate, as realistically as possible, the construction, waste emplacement and backfilling and early post-closure evolution of a high-level waste design. For this experiment, granular bentonite mixture (GBM) was adopted for the backfilling, targeting to create a homogenous and dense buffer layer. The bentonite will be subjected to various actions, including hydration from the host rock and heating from electric heater mimicking the nuclear waste. In this context, a proper characterization is fundamental for predictive analysis of the buffer behaviour and generally of the temperature and pressure development in both the buffer and the surrounding rock. The properties of the bentonite highly depend on the bentonite type, mix, density of compaction, and hydration state. To understand the barrier response and close the gap between the field results and the modelling abstractions, a thorough characterization of the bentonite is necessary. Recently, a laboratory testing campaign was conducted to characterise the GBM used in the FE experiment in terms of thermo-hydro-mechanical and two-phase flow behaviour, targeting the density, temperature, and hydration state observed in the field experiments, aiming at providing accurate data for models' calibration and field results interpretation. This contribution presents the results obtained on the water retention characterization, the thermal conductivity and the P-wave velocity measurements in a range of saturation and densities.