Experimental investigations on thermo-hydro-mechanical properties of compacted bentonite-based backfill materials

Abstract

Study of thermo-hydro-mechanical properties of bentonite-based backfill materials is crucial for safe performances of high-level waste repositories. Column tests were conducted using a laboratory device with bentonite/sand (B/S) and bentonite/crushed granite (B/CG) mixtures (50/50 and 70/30) under close conditions of backfill materials in repositories. The device facilitated continuous measurements of hydration stress and temperature of compacted specimens under thermal and coupled thermo-hydraulic conditions. Additionally, thermal conductivity of soil mixtures was investigated using thermal probe, highlighting its dependence on temperature under constant moisture condition. Specimens for thermal conductivity tests were statically compacted at constant dry density with varying water content, and exposed to 20 °C, 40 °C, 60 °C, and 80 °C under constant volume condition. Experimental results indicated that hydration stress for both soil mixtures exhibits different swelling stages with time. With increasing sand or crushed granite content, time required for maximum hydration stress increases and impedes development of swelling. Temperature profiles were slightly higher during coupled thermo-hydraulic conditions, with B/S mixtures exhibiting higher temperatures than B/CG mixtures. Thermal conductivity increases with temperature, reaching 1.09–1.24 times for B/CG mixtures and 1.10–1.21 times for B/S mixtures at 80 °C to that at 20 °C. For a given water content, size of macropores and pore fractions was lower for B/S mixtures, thus higher thermal conductivity for B/S mixtures. Pore system for both soil mixtures improves and provides a better connectivity between particles with increasing water content. Temperature effect on thermal conductivity is significant at higher water content and temperatures due to additional latent heat transfer.