Mechanical Properties of Sandy Silt Under Freeze–Thaw Cycles and Thermo-Hydro-Mechanical Multi-Field Coupled Simulation for Typical Embankment Sustainable Risk Mitigation

Riverbank stability constitutes an essential foundation for flood-risk mitigation and eco-hydrological sustainability in cold-region river basins. Combined with water-level fluctuations induced by river ice, freeze–thaw alternations can substantially aggravate riverbank erosion, collapse and land degradation, threaten the long-term safety of riparian infrastructures, and constrain sustainable watershed management. A typical embankment along the Inner Mongolia section of the Yellow River, subjected to severe freeze–thaw conditions, was investigated in this study. Laboratory tests were performed to quantify the effects of dry density, freeze–thaw cycles, and water content on soil mechanical properties. A thermo-hydro-mechanical (THM) coupling model incorporating freeze–thaw-induced mechanical degradation, water migration, and deformation was developed and applied to simulate the stability evolution of a typical riverbank. The experimental results showed that dry density was the dominant factor affecting soil shear strength, accounting for more than 60% of the explained variance in cohesion. The soil exhibited the highest shear strength at the optimum water content, while increasing freeze–thaw cycles significantly reduced soil cohesion, particularly during the first three cycles due to rapid structural deterioration. In contrast, freeze–thaw cycles had a limited influence on the internal friction angle. By introducing the ratio between ice and unfrozen water volume contents as the key coupling variable, a phase equilibrium relationship was established to link the temperature and moisture fields. The proposed THM model reproduced the coupled evolution of temperature, moisture, stress, and displacement during freeze–thaw processes and identified the critical locations of sandy silt riverbank instability and the variations in factor of safety under different conditions. The findings advance the mechanistic understanding of freeze–thaw-related embankment failure and provide technical support for sustainable hazard assessment, resilient embankment design and risk mitigation for cold-region alluvial rivers.