Thermal-water-salt coupling process of unsaturated saline soil under unidirectional freezing

Salinization and desertification are closely related to water-salt migration caused by a temperature gradient. Based on the Darcy Law of unsaturated soils, the law of energy conservation and the law of mass conservation, the thermal-water-salt coupling mathematical model of unsaturated frozen saline soil was established. The model considered the latent heat of phase change, crystallization impedance, crystallization consumption and complete precipitation of solute crystallization in ice. In order to verify the rationality of the model, the unidirectional freezing test of unsaturated saline soil was carried out in an open system with no-pressure water supplement to obtain the spatial distribution of temperature, moisture and salt in the saline soil. Finally, numerical simulations are implemented with the assistance of COMSOL Multiphysics. Validation of the model is illustrated by comparisons between the simulation and experimental data. The results demonstrated that the temperature within saline soil changes with time and can be divided into three stages, namely quick freezing stage, transitional stage and stable stage. The water and salt contents in the freezing zone are layered, with peak values at the freezing front. The coupled model could reveal the heat-mass migration mechanism of unsaturated frozen saline soil and dynamically describe the freezing depth and the movement law of the freezing front, ice and salt crystal formation mechanism, and the change law of thermal conductivity and permeability coefficient.