Influence of plasticity on unfrozen water content of frozen soils as determined by nuclear magnetic resonance

The mechanical properties of frozen soils are closely related to the unfrozen water content. It has been generally recognized that the unfrozen water content varies with plasticity of soils, while the quantitative relationship between the plastic index and the unfrozen water content is yet to be further examined. This paper takes soil samples with different plasticity as study objects, which are prepared from two typical soil materials, i.e., the Chinese ISO standard sand and bentonite. Nuclear magnetic resonance (NMR) is used to measure the unfrozen water content of soil samples under different temperatures. Test results show that there are three stages in the soil-freezing characteristic curve (SFCC), i.e., the super-cooling stage, the rapid-drop stage and the stable stage. With the increase of the plasticity index, the super-cooling stage is less significant, while the curve in the rapid-drop stage becomes gentler. In the stable stage, the unfrozen water content is barely affected by plasticity. A piecewise function consisting of a linear formula and a power formula is then proposed to describe the SFCC, where the formula parameters are functions of the plastic index. It is found that the unfrozen water content calculated by the proposed formula agrees with the measured results.