<p>Horizontal freezing induces frost heave pressure in retaining structures, foundation pits, and slopes, leading to a variety of engineering challenges. A thermo-hydro-mechanical (THM) coupled model for horizontal freezing in unsaturated soils was developed. This model extends existing THM frameworks for saturated soils by incorporating vapor migration and ice segregation. The governing equations account for soil deformation, water and vapor migration, heat transfer, and the formation and growth of ice lenses. Numerical simulations were conducted using COMSOL Multiphysics, and the model was validated through comparisons with laboratory experiments using a self-developed horizontal frost heave apparatus. The impact of frost heave behavior is examined through simulations that vary the initial water content, dry density, and temperature gradients. Increased water content and higher temperature gradients exacerbate frost heave, while higher dry density slows water migration and reduces ice lens formation. The density and flux of vapor significantly influence vapor migration. Finally, the model was applied to a real foundation pit project to assess frost heave behavior under varying thermal and mechanical conditions. The results indicate that the proposed model accurately captures the development of frost heave and its interaction with soil deformation. These findings offer practical implications for geotechnical design and frost heave mitigation in cold regions.</p>