Evaluation of the impact of freeze-thaw cycles on pore structure characteristics of black soil using X-ray computed tomography

Abstract

Freeze-thaw cycles extensively and intensely occur in northeast China and generate significant effects on soil pore structure. The objective of this study was to obtain three-dimensional (3D) images of soil pore structures utilizing industrial X-ray microtomography and to apply image analysis techniques to visually and quantitatively evaluate the impact of the freeze-thaw effect on pore characteristics of black soil. The soil samples were subjected to zero (T0), one (T1), five (T5), ten (T10), fifteen (T15), and twenty (T20) freeze-thaw cycles and were then scanned at a 25-μm resolution to construct pore-throat network images. The results showed that the freeze-thaw effect increased the porosity of the soil 3D microstructure and that the pore-throat networks became increasingly complex with a greater number of freeze-thaw cycles. The total porosity and macroporosity increased significantly with an increase in the number of freeze-thaw cycles (p < 0.05). The soil in the T20 treatment displayed the greatest total porosity and macroporosity, which were 247.8 and 251.1 % greater, respectively, than in the T0 treatment. In freeze-thaw processes, contraction of the solid phase is induced by thawing of ice crystals but is insufficient to counteract the displacement induced by the freezing expansion of ice crystals. Macroporosity contributes to most of the increase in total porosity between each treatment. The numbers of pore throats and paths were significantly and positively correlated (p < 0.05) with the number of macropores. The freeze-thaw effect mainly affected those pores with greater effective diameters, which may include more branch paths and throats. The findings of this study can help to improve the understanding of soil dynamic processes under freeze-thaw cycles and further enable the utilization of pore morphology in monitoring the quality of soil structure.