Numerical study on the spatial–temporal distribution of solute and salt accumulation in saturated sulfate saline soil during freezing–thawing processes: Mechanism and feedback

Abstract

Knowledge of freezing–thawing (F–T) cycles of sulfate saline soil is crucial regarding the spatial–temporal distribution of solutes in the soil, soil salinization, and cold region engineering in sulfate saline soil areas. F–T tests of saturated sulfate saline soils with different salt contents, water supply conditions, and the number of F–T cycles were conducted. The coupling mechanism of water and salt migration, ice–water phase change, salt crystallization–dissolution, and deformation caused by salt expansion and frost heave of saturated sulfate saline soil during F–T processes were analyzed. A water–heat–salt–mechanics coupling mathematical model for saturated sulfate saline soil under F–T cycles is proposed using the modified salt crystallization–dissolution kinetics model. The results show that the calculated profiles of temperature, water content, concentration, and soil displacement are in good agreement with the experimental data, demonstrating that the proposed coupling model can describe the coupled water–heat–salt–mechanics process of sulfate saline soil during F–T processes. Additionally, the modified salt crystallization–dissolution kinetics model employing the FREZCHEM model can theoretically reveal the processes of salt crystal growth and dissolution. Also, the water and temperature potential gradients are the main driving force for water migrating toward the freezing front. Convection and diffusion are the dominant factors for the spatial–temporal distribution of solutes of sulfate saline soil subjected to F–T cycles, causing salt accumulation and precipitation and forming subflorescence in the frozen and unfrozen layers near the freezing front and efflorescence in the soil surface–layer at a certain number of F–T cycles. Due to the blocking effect of salt accumulation and precipitation on water and salt migration, salt expansion during freezing and residual deformation during thawing gradually stabilize with the increase of F-T cycles.