Study on the Effect of the Second Phase Transition on Water and Salt Transfer in Sodium Chloride Soil

Saline soil within cold and arid areas is characterized by frost heave and salt expansion, thereby presenting risks for the stability of engineering buildings and structures therein. Because of the deficiency in understanding ionic properties, the mechanism of saline soil deformation throughout the freezing process, especially in the second phase transition process, remains unclear. To explore the multifield coupling process in saline frozen soil, sodium chloride soil was chosen as the research object. A unidirectional freezing test was conducted to examine the impact of the second phase transition on water and salt transfer. As indicated by the results, three zones (unfrozen zone, the first phase transition zone, and the second phase transition zone) exist in the sodium chloride soil column. Water and salt accumulate at the zone boundaries as they redistribute during the freezing process. This is distinct from ice segregation in the frozen fringe of nonsaline frozen soil. Additionally, it was analyzed that the phase transition influenced water and salt transfer. The results will furnish a scientific basis for water and salt transfer in saline frozen soil.