Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant volume and free-swell conditions

TL;DR

This study investigates the unsaturated hydraulic conductivity of compacted sand-bentonite mixtures under different conditions, revealing microstructure influences on permeability during wetting processes relevant to radioactive waste barriers.

Contribution

It introduces in-situ infiltration experiments using resistivity probes to measure hydraulic conductivity and compares constant volume and free-swell conditions, highlighting microstructure effects.

Findings

Hydraulic conductivity decreased during infiltration under constant volume.

Under free-swell conditions, hydraulic conductivity increased as expected.

Microstructure changes during wetting influence permeability behavior.

Abstract

Highly compacted sand-bentonite mixtures are often considered as possible engineered barriers in deep high-level radioactive waste disposals. In-situ, the saturation of these barriers from their initially unsaturated state is a complex hydro-mechanical coupled process in which temperature effects also play a role. The key parameter of this process is the unsaturated hydraulic conductivity of the barrier. In this paper, isothermal infiltration experiments were conducted to determine the unsaturated hydraulic conductivity according to the instantaneous profile method. To do so, total suction changes were monitored at different locations along the soil specimen by using resistivity relative humidity probes. Three constant volume infiltration tests were conducted showing, unexpectedly, a decrease of the hydraulic conductivity during infiltration. One test performed under free-swell conditions showed the opposite and standard trend. These observations were interpreted in terms of microstructure changes during wetting, both under constant volume and free swell conditions.