Electrical properties of frozen saline clay and their relationship with unfrozen water content

The unfrozen water content has a significant impact on the mechanical and thermal properties of saline soils during freezing, and their electrical properties measured by electromagnetic sensors can be a promising proxy of the unfrozen water content. This paper investigated the electrical properties and their relationship with the unfrozen water content of saline lean clay. Specimens with increasing concentration of sodium chloride were prepared for assessing their initial freezing points and unfrozen water content by the cooling test and the nuclear magnetic resonance relaxometry, respectively. In addition, the apparent dielectric permittivity and bulk electrical conductivity were measured by sensors based on frequency-domain reflectometry in the electrical property test. A significant variation was observed in both the apparent dielectric permittivity and bulk electrical conductivity of saline lean clay near the initial freezing point. They generally decreased linearly with decreasing temperatures above their initial freezing points and then dropped according to a power curve with temperatures below their initial freezing points. The bulk electrical conductivity of saline lean clay depended on salt concentration and ion conductive pathway, with the latter being dominant at temperatures below its initial freezing point. A new empirical equation was proposed for estimating unfrozen water content based on the apparent dielectric permittivity and initial salt concentration, and independent data validated its applicability of similar soils at relatively low salt concentrations. The proposed equation could be useful for the evaluation of the unfrozen water content of saline lean clay via the apparent dielectric permittivity and initial salt concentration.