Effects of soil particle size and gradation on the transformation between shallow phreatic water and soil water under laboratory freezing-thawing action

The transformation between shallow phreatic water and soil water during the freezing-thawing period affects the accurate evaluation of water resources and aggravates soil salinization. In laboratory freezing-thawing experiments, constant temperature freezing at − 10, −20, and − 25 °C was conducted for 62 successive days, and natural thawing was conducted for 13 days, with a groundwater table depth of 1.0 m. The variations in the soil temperature, upward migration amount of phreatic water to soil water (UMA), and downward movement amount of soil water to phreatic water (DMA) of soil columns with particle sizes of d50 = 0.3 mm (A1), d50 = 0.75 mm (B1), d50 = 1.25 mm (C1), d50 = 1.75 mm (D1), and d50 = 2.25 mm (E1) and different particle gradations with inhomogeneity coefficients of 3.47 (A2), 4.69 (B2), 8.31 (C2), 14.67 (D2), and 19.08 (E2) were analyzed. The results showed that the soil temperature was more variable and reached stability earlier as the soil particle size or inhomogeneity coefficient increased. The cumulative UMA decreased as the soil particle size increased and the inhomogeneity coefficient decreased during the freezing process. For the constant temperature freezing at − 10, −20, and − 25 °C, the cumulative UMA and average soil particle size were fitted using an exponential function, but the effect of the soil particle size on the UMA gradually weakened as the freezing air temperature decreased. The daily DMA decreased as the soil particle size decreased, and the particle gradation improved during days 0–5 of the thawing stage, while the cumulative DMA increased as the soil particle size decreased and the inhomogeneity coefficient increased after days 9–10 during the thawing stage. The groundwater was in a negative equilibrium under freezing-thawing action, and the consumption of groundwater was greater when the average soil particle size was smaller and the inhomogeneity coefficient was larger during the freezing-thawing process. The results of this study have important significance for soil salinization prevention and the scientific evaluation of water resources.