A numerical analysis of Thermo–Hydro–Mechanical behavior in the FE experiment at Mont Terri URL: Investigating capillary effects in bentonite on the disposal system

We investigated thermo–hydro–mechanical (T-H–M) coupled behavior observed during the full-scale heater emplacement experiment at the Mont-Terri underground research laboratory conducted in the Opalinus clay as part of the DECOVALEX-2023 Task C project. Utilizing the OGS-FLAC simulator, we created a three-dimensional model to simulate multiphase flow in the experiment, applying extended Philip and de Vries’ model and incorporating the anisotropic T–H–M properties of the Opalinus clay. The simulation, which included a ventilation process, spanned five years of heating experiments and successfully replicated the measured temperature, pore pressure, displacement, and relative humidity results in bentonite and host rock during the experiment. The analysis revealed that capillary pressure significantly influenced the pore pressure change in the host rock near the tunnel, while thermal pressurization became dominant with increasing distance. Consequently, we conducted a sensitivity analysis on a simplified model to evaluate the effect of capillary pressure on the disposal system. Capillarity is a dominant factor for the multiphase flow depending on the distance from the heat. Variations in capillary pressure were observed depending on the gas entry pressure and water retention model, indicating that the capillarity of unsaturated bentonite could inherently affect the T–H–M results within the disposal system. • This study investigates the effect of capillarity on the thermo–hydro–mechanical coupled behavior in the disposal system. • Thermo–hydro–mechanical coupled simulation results demonstrate a good agreement with the FE experiment data. • Capillarity is a dominant factor for the multiphase flow depending on the distance from the heater. • Capillarity in the unsaturated bentonite varies depending on the gas entry pressure and water retention model.