Water and soil particle movement in unsaturated bentonite with constrained and free swelling boundaries

Abstract

Bentonite is considered a barrier material in the geological disposal of high-level radioactive waste. Depending on the construction method, it is necessary to know what the behavior of the bentonite barrier will be in the wetting process during the absorption of underground water under different boundary conditions. In this study, water and soil particle movement during the wetting process of a compacted bentonite under constrained and free swelling conditions was studied experimentally and numerically. For the constrained swelling boundary condition, in which the swelling deformation was fully constrained, the distributions of the gravimetric water content (w) were measured for specimens with different dry densities (ρd), and then the water diffusivity (Dw) was obtained. It was found that Dw showed a slightly decreasing trend with an increase in ρd, while the numerical simulations showed that the difference in Dw induced by ρd was minor in terms of the evolving degree of saturation in the tested range. For the free swelling boundary condition, the distributions of w and ρd were measured for specimens with an initial ρd of 1.6 Mg/m3. To obtain Dw and the soil particle diffusivity (Ds), an existing theoretical framework, to which new concrete calculation equations had been added, was presented. Then, the performance of the framework was examined by numerical simulations to illustrate the water and soil particle movement under conditions similar to those of the experiment. It was found that the framework can describe the experimental results well, but that the accuracy of the results largely depends on the accuracy of the experimental data.

Keywords

• Bentonite
• Geological disposal
• Buffer material
• Soil particle diffusivity
• Water diffusivity