The closure phases of geological disposals for radioactive waste involve the construction of sealing systems utilising bentonite-based materials. These materials are selected for their low water permeability and radionuclide retention capacity. Their swelling potential, induced by the absorption of water, enables self-sealing capabilities, bridging any gaps in the sealing system and recompacting the excavation-damaged zone in the host rock. Post-closure of the facility, the host rock’s porewater hydrate the sealing systems presumably reaching water-saturation. Subsequently, metallic components presented within the galleries undergo corrosion, leading to the production of hydrogen gas. The accumulation of this gas can potentially increase gas pressure within the disposal tunnels. Sealing systems help to limit this pressure by allowing the transfer of gas along the galleries.

The present work focuses on the experimental characterisation and numerical modelling of hydromechanical-gas behaviour of compacted bricks made of a bentonite-sand mixture, in proportion of 40/60 in dry mass, considered as a candidate material for the construction of the expansive core of the sealing systems in the French concept of geological disposal.

First, Nuclear Magnetic Resonance (NMR) relaxometry experiments on relative humidity-controlled samples provided the microstructural characterisation of water mobility during wetting. Results highlighted the existence of two predominant regimes of water mobility. At drier states water is totally adsorbed to clay particles whereas at wetter states capillary and adsorbed water coexist.

Second, water transfer and hydromechanical behaviour during water imbibition experiments were characterised by the conventional methods of monitoring relative humidity and swelling pressure at different distances from the water injection front, as well as utilising Magnetic Resonance Imaging (MRI) and X-ray microtomography (µCT) imaging techniques. The combination of MRI and µCT provided data on the distribution of water mass and dry density along the sample, allowing an accurate estimation of the hydraulic conductivity of the material.

Gas injection tests on water-saturated samples of different sizes demonstrated that gas pressures below the swelling pressure of the sample could induce the entry of gas and its migration through the material. µCT images taken during the injection of gas revealed that the gas entry process takes place between sand grains where bentonite is less dense. Water displacement due to the advancement of the gas was measured downstream and captured at the grain-scale level with µCT images, suggesting that gas migration is led by a visco-capillary two-phase flow through the sand skeleton of the sample.

A modelling framework reproducing the behaviour of the bentonite-sand bricks was proposed based on experimental evidence and was implemented in the open-source finite element code Bil. For mass tranfer modelling, it considers multiphase flow with: liquid water advection, water vapour diffusion and gas advection. The water retention model is bimodal distinguishing the gas entry pressures of the sand skeleton and of the bentonite matrix. The hydromechanical coupling is based on the Barcelona Basic Model including particular extensions for bentonite-based materials. Simulation of laboratory experiments provided the calibration of model parameters for the bentonite-sand bricks.

It was concluded that the original combination of high sand proportion, while maintaining a high bentonite dry density, provides the bricks the ideal combination of hydromechanical properties with respect to the design requirements for the geological disposal of radioactive waste. These properties include low hydraulic conductivity, high swelling potential and low gas entry pressure.