Influence of dry density and water salinity on the swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite-sand mixtures

Compacted Gaomiaozi bentonite-sand mixtures are regarded as attractive buffer/backfill materials for nuclear waste deep geological disposal. When the mixture blocks are emplaced, the bentonite dry density as well as the salinity of groundwater is of primary importance for their hydro-mechanical behavior. In this study, the influences of bentonite dry density and water salinity on the swelling pressure and saturated hydraulic conductivity of such mixtures are investigated using the constant volume method. Results indicated that for mixtures having a low sand content, the swelling pressure was only associated with the bentonite dry density, while the hydraulic conductivity was influenced by the presence of sand particles. Exponential relationships were noted among the final swelling pressure, the saturated hydraulic conductivity, and bentonite dry density on the case of distilled water and salt solutions infiltrating. As the salt concentration increased, the swelling pressure and hydraulic conductivity decreased and increased, respectively, and the effects became less significant as the bentonite dry density increased. The higher swelling pressure and lower hydraulic conductivity on specimens infiltrated with calcium chloride solutions were a result of cation exchange reactions and a competition between the swelling potential change caused by the calcium ions in the interlayer of smectite and electrolyte solutions. The present work can provide a reference for the design of dry density of bentonite-sand mixtures as buffer/backfill materials for deep geological disposal.