Water freezing characteristics in granular soils: Insights from pore-scale simulations

Abstract It has been observed in both laboratory and field studies that unfrozen soil moisture content is hysteretic between freezing and thawing conditions. While a few explanations have been provided for the causes of this hysteresis, there is no general agreement in the literature on underlying causal mechanisms. In this study, we use pore-scale simulations to investigate the role that contact angle hysteresis during freezing and thawing conditions may play in the observed differences in soil moisture content during freezing and thawing conditions. We also use these pore-scale simulations to better assess the similarities between freezing and water drainage processes in pore space as such realistic assessments are very limited in the literature. Specifically, we use 3D images of natural sand packs and synthetic silicates to generate pore-scale model domains and to simulate water drainage and water freezing in the soil pore space. The simulated water drainage curve and relative permeability-saturation relations for the sand packs agrees well with measured data available in the literature for sandy soils. The simulated soil freezing curves were successfully converted to simulated water drainage curve confirming conclusions of previous studies on coarse granular soils. Water freezing simulations using different contact angles for freezing (water receding) and thawing (water advancing) showed that the hysteresis in soil freezing curves could be driven by a hysteresis in the contact angle of water-ice interface at the pore wall (i.e. grain surface). In addition, simulations of water drainage in partially frozen soils revealed that soil freezing enhances liquid water retention in pore space due to reduction of mean active pore radius and increased frequency of pore corners which agrees with the observations of published studies in laboratory and field conditions.