Simplified evaluation on hydraulic conductivities of sand–bentonite mixture backfill

Abstract Sand–bentonite mixtures are planned for use as backfill materials for high-level nuclear waste disposal in Japanese project. Sand–bentonite mixtures are attracting greater attention as backfill materials because they offer properties of very low permeability and high swelling. We must investigate the hydraulic properties by experiments and evaluate quantitatively the hydraulic-conductivities of sand–bentonite mixtures to design specifications, such as dry density and bentonite content, of backfill materials. For that purpose, this study investigated hydraulic conductivities at different bentonite contents and dry densities by experimentation. In addition, we discussed the relationship between hydraulic conductivity and bentonite content from the viewpoint of bentonite swelling in backfill voids. Furthermore, this study proposed simplified evaluation for hydraulic conductivity using a parameter proposed by the author: swelling volumetric strain of montmorillonite. This evaluation method can obtain hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method can be used for designing bentonite content and compaction density to achieve very low permeability.