Behavior of Bentonite Under Nuclear Waste Repository Conditions: Insights from a Thermo-hydro-mechanical Model Based on Hypoplasticity

Advanced modeling is fundamental for reproducing non-linear and coupled behaviors in expansive soils, especially in unsaturated and non-isothermal conditions. In this contribution, we provide an overview of the recent developments of a modeling approach based on the theory of hypoplasticity, specialized for expansive clays under a double-structure framework and enhanced with thermo-hydraulic and thermo-mechanical couplings at both structural levels. The model, plugged into a single-element driver—in turn introduced in an in-house finite-element solver—has been calibrated on element-volume experiments on MX-80 and B75 bentonites, which are candidate materials for buffer layers in deep geological repositories of radioactive waste. The so-calibrated parameters were then utilized in simulations of laboratory-scale tests, aiming at reproducing the response and homogenization of buffer layers (compacted bentonite blocks and/or pellets) under realistic combinations of hydration and prolonged heating. The relative simplicity of the hypoplastic formulation—featuring key parameters with clear physical meaning—has proven successful in model simulations, which exhibited a reasonable matching between predicted and experimental trends of saturation, volume change, and swelling pressure development under varying temperature. Within the framework of ongoing collaborative projects, further model improvements are envisaged, with the aim to offer better formulations of microstructural behaviors and improved computational efficiency.