Freeze-thaw impact on macropore structure of clay by 3D X-ray computed tomography

The change of soil macropores, especially fissures, due to freeze-thaw (F-T) has important implications for soil engineering properties such as hydraulic conductivity and compressibility. This paper describes a quantitative study of the soil macropore changes due to F-T by using three-dimensional (3D) X-ray Computed Tomography (CT). Saturated lean clay specimens were subjected to unidirectional F-T under different water-supply conditions for acquiring 3D X-ray CT images before and after F-T. The CT imagery data were filtered and processed for obtaining parameters such as transverse-sectional macro-porosity, macropore size, transverse-sectional fissure porosity, and longitudinal-sectional fissure orientation frequency distribution. It was found that F-T alters the transverse-sectional macro-porosity and macropore size significantly, the changes are different across the specimen height. Moreover, the changes in the transverse-sectional fissure porosity and longitudinal-sectional fissure orientation frequency distribution vary along with the specimen height. More horizontal fissures form in the unfrozen zone or near the freezing front, depending on water-supply conditions. Pore water pressure data confirm that, besides water migration, soil particles also migrate from the unfrozen zone to the freezing front during freezing with water-supply due to piping. The macropore structural changes help understand the F-T impact on soil engineering properties such as hydraulic permeability and compressibility.