EFFECT OF A HYDROPHOBIC LAYER ON THE UPWARD MOVEMENT OF WATER UNDER SURFACE-FREEZING CONDITIONS

Abstract

A water table can increase the severity of frost heave in soil by providing water for accumulation at the freezing front. Limiting flow from the water table can reduce frost heave. We hypothesized that a hydrophobic soil layer positioned between the freezing front and the water table would restrict water flow and ice accumulation at the freezing front. This hypothesis was tested in laboratory experiments using 20-cm-long soil cells designed to achieve one-dimensional flow conditions between an imposed-5 °C upper boundary and a water table at the lower cell boundary. Experiments were performed with 24 and 10 °C lower boundary temperatures. Dichlorodimethylsilane-treated soil was used to provide a 2-cm thick hydrophobic soil layer above the water table; cells with and without hydrophobic layers were used for comparison. For both lower boundary temperatures, the freezing front advanced more rapidly in the soil cells with hydrophobic layers. Final water uptake volumes corresponded to only 5% of the initial cell pore volume for cells with hydrophobic layers, but more than 20% for cells without hydrophobic layers. Although only slightly larger amounts of frost heave were observed in cells without hydrophobic layers, water contents at the freezing front indicated much greater expansion and deformation within the cells when the hydrophobic layer was absent. Overall, the hydrophobic layer reduced water uptake and ice accumulation at the freezing front.