Differential analysis and unified predictive modeling of capillary rise in sodium chloride and sodium sulfate saline soils

Capillary rise in saline soils is a key indirect factor leading to the degradation of structural performance in subgrade and similar structures. The type and salt content (SC) of the saline soil further complicate capillary rise, posing significant challenges for preventing and controlling capillary rise in subgrade and other structures. This study used sodium chloride saline soil (NCS) and sodium sulfate saline soil (NSS) as the research objects, investigating the capillary rise characteristics of saline soils and the influence of salt type and content on capillary variations. The soil suction was measured using the filter paper to aid the analysis. The results showed that adding sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) affected all three stages of capillary rise in soil. These effects included promoting elevation, gradual wane, and reverse inhibition. During the promotion stage, capillary rises rapidly, leading to significant changes in the moisture field of the subgrade and accelerating the destruction of its strength and stability. The effect of salt type on the maximum capillary height was closely related to changes in SC. Compared to NCS, NSS exhibited a more substantial capillary rise at low SC. However, as the SC increased, the inhibitory effect on capillary rise gradually intensified. According to Chinese standards, weak saline soils, traditionally considered more suitable for subgrade construction, exhibited more significant capillary rise, which requires attention in future subgrade construction. A significant correlation was observed between capillary rise and suction for both types of saline soils at different SCs. The maximum capillary height prediction model for saline soils, adjusted by soil suction, demonstrated applicability and accuracy, providing valuable insights for capillary rise prevention in subgrade construction.