Friction influence on constant volume saturation of bentonite mixed pellet-block samples, a numerical analysis

Bentonite-based materials are employed in some nuclear waste disposal concept designs to seal underground tunnels and shafts. In some cases, these barriers consist of two parts: highly compacted blocks and granular buffer material consisting of pellets. These two components have highly distinct initial properties in terms of dry density, water content, and microstructure, but, at full water saturation, those tend to homogenise. However, the simultaneous application of pellets and blocks in the same barrier section creates challenges for understanding and modelling system performance that must be tackled. Therefore, this work merges experimental and computational approaches to better understand the hydro-mechanical processes that take place throughout the interaction of the various assemblies with one another and with the environment during hydration. Hence, the role of a variety of material configurations, considering simultaneously compacted blocks and pellets, and hydration boundary conditions is analysed giving great insight about intermediate saturation states. Discrepancies between experimental measurements and model results were explained by the effect of friction after a sensitivity analysis performed with the finite element code LAGAMINE. Results from experiments and calculations were in good agreement and offer supplementary knowledge about a relevant amount of the numerous phenomena (for instance related to dry density evolution and water distribution inside the sample) taking place during initial heterogeneous bentonite samples resaturation in isochoric conditions.