Theoretical analysis of unfrozen water content in unsaturated soil below freezing temperature is conducted

presenting a simple prediction method and its mathematical model. By considering the chemical and mechanical balance of gas-liquid phases in unsaturated soil pores

along with the soil-water characteristic curve

the soil pore volume distribution density function is derived using parameters from the Van Genuchten soil-water characteristic curve model. The relationship between the maximum pore size filled with liquid water at initial effective saturation and initial crystal pore size at a specific freezing temperature is examined to analyze ice-water phase transition characteristics in soil pores and critical effective saturation. Freezing occurs only if initial effective saturation surpasses critical crystallization saturation. A formula for predicting unfrozen water saturation in unsaturated soil below freezing temperature is provided and validated against existing experimental data.