Study on electrical properties of saline frozen soil and influence mechanism of unfrozen water content

Temperature and unfrozen water content are important affecting parameters in soil freezing process. This study aims to explore correlation between electrical conductivity, unfrozen water content, and temperature for silty clay under various salt content conditions and propose new theoretical relationships to predict the changes of related parameters in the freezing process. Frequency domain reflection(FDR)sensors were employed to conduct electrical performance tests on frozen soil, elucidating the response of frozen soil's electrical conductivity to unfrozen water content and temperature. The conduction mechanism of frozen soil was analyzed using SternGouy electrical double-layer theory. The analysis showed a negative correlation between soil salt content and freezing temperature. Before freezing, salt content significantly affected the crystallization pattern of salt and volumetric water content. There exists a threshold value below which no salt crystallization occurs in soil, and volumetric water content remains relatively constant, while electrical conductivity slightly decreases with decreasing temperature. When salt content exceeds the threshold value, the starting temperature of salt crystallization was influenced by the salt content, and both volumetric water content and electrical conductivity changes significantly after salt phase transition point. After freezing, both unfrozen water content and electrical conductivity significantly decrease. The remaining moisture content in soil slightly decreases with an increase in salt content. Different relevant parameter models have been proposed to describe variations in unfrozen water content and electrical conductivity with temperature after soil phase transition point. The effectiveness of different parameter-related models was validated by comparing experimental data with calculated and fitted results under various conditions. The electrical conductivity of soils with varying salt contents depends on both salt concentration of pore solution and migration pathways of ions. The primary conduction pathways include pore conduction and surface conduction, with latter gradually becoming predominant below its freezing temperature. This study provides a scientific basis for revealing the mechanism and prevention measures of freezethaw damage in the construction of subway tunnels using the artificial ground freezing (AGF) technique.