Soil freezing characteristic curve (SFCC) relates unfrozen water content to temperature and has been extensively used as a unique curve in the numerical simulation of frost heave. SFCC is influenced by many factors, among which the most important one is soil suction. For example, at zero oC, the unfrozen water content for a soil at saturation (suction=0) will be significantly different from that for the same soil at completely dried conditions (suction=1000MKa). Consequently, even for the same soil, the obtained SFCCs will be different if the initial suctions of the soil specimens are different. In fact, the unfrozen water content inside a partially frozen soil should be a function of both temperature and suction. Consequently, a soil freezing characteristic surface (SFCS) is a more rigorous way to describe unfrozen water content in a partially frozen soil than the SFCC for the investigation of the frost heave problem.

In this study, we developed a novel SFCS for partially frozen soils. This surface delineates SFCCs under different suction states by determining the initial water content and freezing point, thus elucidating the constitutive relationship among water content, temperature, and suction. Unlike conventional SFCC studies that focus solely on unfrozen water content and temperature, the proposed SFCS provides a comprehensive understanding of soil freezing characteristics under both saturated and unsaturated conditions. The proposed SFCS was validated using experimental data involving clay, silt, and sand. This study significantly enhances our understanding of the soil freezing process and provides a theoretical constitutive surface essential for analyzing THM behaviors in frozen soils.