Abstract: Bentonites have been widely adopted as the buffer/backfill materials for repositories of high-level radioactive waste disposal. Characterization of the effective thermal conductivity of the buffer materials is of great importance for optimization design and safety assessments of the repositories. Using the concept of the series-parallel structural models, the porosity of soils is decomposed into two components: one in parallel and the other in series connected to the solid phase. An effective structure depends on the decomposition is introduced to describe the heat transfer process of the porous soils considering the structural connections of pores and the solid phase. On this basis, four formulas which represent different series-parallel arrangements of pore fluids (water and air mixture) are proposed to predict thermal conductivity of unsaturated soils with a comprehensive consideration of the effects of mineralogical composition, porosity and saturation. An effective model for thermal conductivity of unsaturated soil is then developed based on a linear composition of the four formulas. Parameterization of the model is discussed; and the behaviors of the model with respect to the variations in porosity and saturation are illustrated. The model is validated against experimental data of thermal conductivity of MX-80 and Gaomiaozi bentonites. The results show that the arrangement of air and water in the pores tended to be in parallel connection for unsaturated bentonites because of the fine size and wettability of the soil particles. This research may provide a helpful reference for predicting thermal conductivity of unsaturated bentonites and numerical modeling of the coupled thermo-hydro-mechanical (THM) processes in the engineered barrier systems.

Key words: unsaturated soil, coefficient of effective thermal conductivity, high-level radioactive waste disposal, bentonite