A hydraulic conductivity model for unsaturated bentonite-based materials incorporating surface enhanced vapour diffusion

The hydraulic behaviour of bentonite-based materials is crucial for assessing the performance of geological repository for high-level nuclear wastes (HLW). Unlike non-swelling soils, the soil hydraulic conductivity curve (SHCC) of bentonite-based materials upon wetting commonly presents an initial decrease followed by an increase after a certain suction. Despite decades of research, a sound theoretical description of such U-shaped SHCC remains challenging. In this study, a new model was developed by considering the surface enhanced vapour diffusion for the hydraulic conductivity in the dry branch of the U-shaped SHCC and the adsorptive-capillary flow for that in the wet branch. The corresponding differentiated vapour, adsorptive and capillary hydraulic conductivity equations over the entire suction range were established. The surface enhanced vapour diffusion model contains only two parameters defining the extent of surface enhancement relative to the traditional Fickian diffusion and their values can be easily estimated from an experimental SHCC. The adsorptive equation was derived by considering viscous water flow in the form of planar films. The capillary equation was simply the conventional macroscopic one based on the capillary pore network. The performance of the proposed model was evaluated using the experimental SHCCs of six bentonite-based materials with comparison to three existing models (VG-M, GZLH and PDI). Results show that, the proposed model can accurately predict the U-shaped SHCCs of different materials over the entire suction range, while the existing models deviate significantly from the measurements by up to six orders of magnitude. More experimental investigations are needed for further confirming the surface enhanced vapour diffusion effect.