An experimental investigation on the swelling behavior of compacted B75 bentonite

Compacted bentonite is often utilized as a buffer material between the canister containing radioactive waste and the host rock in deep geological repositories. Extremely low permeability and a potential to swell and develop large swelling pressures upon hydration are desirable characteristics for such material. This study focused on the B75 bentonite, a Ca-Mg-montmorillonite-rich clay selected as a potential buffer material for the planned repository in the Czech Republic. The effects of the initial dry density, vertical load, and pore fluid salinity on the swelling behavior of the clay were investigated. One-dimensional swelling tests were performed on compacted samples prepared at various initial dry densities (1.25-1.95 g/cm(3)). Conventional oedometer tests with various hydration and stress paths were also performed. An empirically corrected diffuse double layer model proved successful in predicting the swelling pressure during the tests. Swelling under constant load, swelling pressure under constant volume, swelling upon unloading, and unconfined swelling upon hydration were compared, and a unique relationship was found between the effective stress and the dry density at saturation.