Coupled Thermal and Hydraulic Effects in Low-Permeability Barriers: Insights from the HotBENT In-Situ Experiment

The HotBENT experiment at the Grimsel Test Site investigates the coupled thermal and hydraulic behaviour of a bentonite barrier subjected to high temperatures. The setup includes four electric heaters embedded in a bentonite buffer, with temperatures reaching up to 200 °C. Four “enhanced geosphere pressurization boreholes” using natural Grimsel groundwater support and enhance hydration of the bentonite buffer via surrounding fractured granite. This configuration induces complex interactions between drying, vapour transport, and re-saturation processes.

We present results from 3D fully coupled thermo-hydraulic simulations conducted using OpenGeoSys. The model incorporates temperature-dependent transport properties, bentonite-specific retention behaviour, and hydration from the surrounding granite. Although fractures are not explicitly represented, the surrounding rock acts as a boundary condition influencing water inflow into the bentonite.

Experimental data are used for calibration and validation. The study highlights how temperature gradients affect saturation, vapour movement, and pressure development in low-permeability media. We also discuss numerical challenges related to material nonlinearity and boundary implementation.

These results enhance our understanding of thermo-hydraulic behaviour in engineered barriers and their interaction with fractured host rocks — even when treated as continuous domains — supporting predictive modelling efforts in subsurface systems.