Water, Heat, Vapor Migration, and Frost Heaving Mechanism of Unsaturated Silty Clay During a Unidirectional Freezing Process

Infrastructure development in permafrost regions continues to face growing challenges from frost heaves and thaw settlement. The traditional frost heave theory considers that soil freezing is caused by the migration of liquid water in the soil; however, existing engineering practice shows that the migration of water vapor during the freezing process cannot be neglected. Based on the hydrothermal–air migration theory of unsaturated soils and their frost heave mechanism, this study established a coupled hydrothermal–air frost heave model for unsaturated silty clay under unidirectional freezing conditions. The computational model was verified through indoor modelling tests. The entire process of water vapor migration, moisture accumulation, and condensation-induced ice formation in unsaturated silty clay was comprehensively reproduced by numerical simulation. The results showed that the moisture field is redistributed during the freezing process of unsaturated soil. The increase in volumetric ice content in the frozen zone is due mainly to the migration of water vapor. Liquid water and water vapor in the unfrozen zone migrate towards the freezing edge driven by the temperature gradient, where they accumulate, leading to a decrease in the unsaturated pore space and a decrease in the equivalent vapor content. This study’s results can provide theoretical support for frost damage prevention in unsaturated silty clay in permafrost regions.