Abstract: A water infiltration apparatus is specially designed to study the unsaturated hydraulic properties of compacted bentonite-sand mixtures under unconfined condition by measuring the moisture distribution and the swelling pressure during capillary water infiltration. Experimental data show that the water diffusion coefficient is changed with the moisture content of the mixtures (dry density 1.70g/cm3, sand ratio 30%) in a U-shaped curve. It is found that, with the increasing water content, the water vapor diffusivity decreases but the liquid water diffusivity increases. A theoretical analysis is made to estimate the unsaturated hydraulic conductivity, which expresses a general increasing tendency with the increase of water content. With respect to water states, it is found that the unsaturated hydraulic conductivity of water vapor increases at low water content and then decreases, but the unsaturated hydraulic conductivity of liquid water increases continuously at the same time. After water infiltration, the compacted mixtures tended to uniform as judged by dry density. The progress of pressure monitored at different positions of the specimen is found to be closely related to water migration process. For example, the pressure at water inlet surface increased much more rapidly at earlier infiltration stage and then increased slowly. The pressure at water outlet surface, on the other hand, increased continuously. After 96 hours infiltration, the pressure at both surfaces became almost the same. The parameters derived from this experimental research, such as water diffusivity and unsaturated hydraulic conductivity, can be used for safety evaluation of buffer material to radioactive nuclides.

Key words: bentonite-sand mixtures, water infiltration test, water diffusion coefficient, unsaturated hydraulic conductivity, high level radioactive waste