Water retention of a bentonite for deep geological radioactive waste repositories: High-temperature experiments and thermodynamic modeling

Abstract

Thermo-hydro-mechanical coupling is relevant in various natural processes and engineering applications involving clay soils. It can affect slope deformations and stability, as well as the functioning of clay barriers and energy piles. Temperature changes can alter the water retention capacity of expansive clays and, in turn, produce pressure, strength, and volume changes. In deep geological repositories, for instance, the design of bentonite buffers and the study of their interaction with the host formation must account for the heat released by radioactive decay. Here, to investigate how temperature controls the water retention capacity, vapor transfer experiments under adsorption/desorption (wetting/drying) paths were performed on the Czech B75 bentonite. The tests were conducted in a wide range of temperatures (20–80°C) and initial dry densities (0.6–1.9 g/cm3), at high total suction (4–400 MPa), without mechanical loads. The results showed a systematic loss of water retention capacity at high temperature, particularly at low suction, irrespective of the initial compaction. To predict the behavior at any temperature, a model was constructed from the Clausius–Clapeyron and the Guggenheim–Anderson–de Boer equations. It was calibrated and validated at various temperatures, also on a different bentonite (without further tuning), showing good performance. Dry density-specific calibrations did not affect the model predictions significantly, consistently with results that exclude an effect of initial compaction on water retention at high suction. The proposed model seems suitable for inclusion into thermo-hydraulic descriptions in comprehensive constitutive frameworks for expansive clays, potentially improving the understanding of some behaviors related to thermo-hydro-mechanical coupling.