Soils in cold seasonally frozen regions undergo repeated freeze–thaw (F–T) cycles, during which soil moisture content, pore structure, and permeability can change substantially. Previous studies have mainly focused on the mechanical behavior of such soils, whereas few have clarified how moisture content fluctuation regulates pore-structure evolution and permeability response during F–T cycling. In this study, black soil specimens were prepared with initial moisture contents of 15%, 20%, 25%, and 30% on a dry-weight basis and were denoted as 15%-MC, 20%-MC, 25%-MC, and 30%-MC, respectively. The specimens were subjected to 0, 1, 3, 6, 9, and 12 F–T cycles. Mercury intrusion porosimetry, scanning electron microscopy image analysis, and variable-head permeability tests were used to characterize pore-structure parameters and hydraulic responses. The results showed that porosity and mean pore diameter generally increased with increasing F–T cycle number, and the magnitude of these increases depended on the initial moisture content. The 15%-MC group exhibited limited pore expansion, mainly characterized by a transition from micropores to small pores, whereas the 25%-MC and 30%-MC groups developed more mesopores and macropores. In the 30%-MC group, porosity reached its maximum after 9 F–T cycles and then decreased slightly after 12 cycles, indicating particle rearrangement or partial filling of larger pores. The permeability coefficient and cumulative infiltration also increased with increasing F–T cycle number, with more pronounced increases observed in the high-moisture groups. Tukey’s post hoc test showed that the permeability coefficients in the later F–T stages were higher than those in the early stages, particularly in the 25%-MC and 30%-MC groups. Correlation analysis and principal component regression indicated that the permeability coefficient and cumulative infiltration were positively correlated with porosity, mean pore diameter, mesopores, and macropores, but negatively correlated with micropores. Overall, the initial moisture content regulated pore-size redistribution and seepage-channel development, thereby shaping the hydraulic response of black soil under repeated F–T cycling.
Zhu et al. (Wed,) studied this question.