Coupled Thermo-Hydro-Mechanical Modeling of Bentonite Under High Temperature Heating and Hydration for a Bench-Scale Laboratory Experiment

Compacted bentonite is under consideration as a backfill material for engineered barrier systems in a geological repository for high-level radioactive waste. Coupled THM (thermal–hydrological–mechanical) models for laboratory tests have been effective in understanding coupled processes and improving modeling capability. While studies on bentonite behavior under heating temperatures < 100 °C have been extensively conducted, models and tests for high temperatures are relatively sparse. A bench-scale laboratory experiment at a high temperature (200 °C) was conducted, which provided 3D visualization of the density distribution via frequent X-ray CT images during the experiment, and detailed characterization of bentonite samples after the column tests was running for 1.5 years. We present here a THM model to interpret the data observed from the test, particularly focusing on the effect of the soil–water retention curve (SWRC) and the thermal conductivity function (TCF) under high temperatures. Model results provide a decent match of measured temporal and spatial changes that are the result of heating-induced desaturation, capillary-regulated hydration, swelling/shrinkage induced by local water content, temperature and pressure change, and displacement from nearby areas. The model suggests that (1) the non-linear TCF function should be chosen, (2) temperature-dependent SWRC affects the simulation results in all aspects, and (3) the linear swelling model underperforms the state surface approach in matching the displacement data. It is challenging to explain the density distribution at an early time (< 10 days) at the hydration front where increases in water content, swelling at the hydration front, and compression from nearby areas leads to high density that cannot be fully explained by the current THM model. Refinement of the mechanical model is needed in the future.