A resistivity model for testing unfrozen water content of frozen soil

Testing the unfrozen water content (θu) based on frozen soil resistivity (ρ) is considered to be a common and convenient method. However, the reliability of the theoretical model for testing unfrozen water content requires further study; hence this paper proposes a new resistivity model for testing the unfrozen water content. By considering the variable characteristics of unfrozen water in the soil freezing process and based on the three-element electrical conduction model, four types of micro unit representing different unfrozen water contents were proposed, and the resistivity model was established on the basis of microscopic analysis. The resistivity model showed that a power function can be used to describe the relationship between the soil resistivity and unfrozen water content of frozen soil. Silty clay was used as a research object. The temperature, unfrozen water content and resistivity of soil samples with different initial water content and same soil density were tested by Nuclear Magnetic Resonance (NMR) technology and resistivity testing equipment. The results of the experiments performed on frozen silty clay verified the reasonableness of the proposed model for the resistivity of frozen soils. The conductive mechanism of frozen soil was discussed, as well as the resistivity model and its in-situ application for unfrozen water testing.