Effects of initial water and salt content on permeability and microstructure of sodic-saline loessal soils

Dramatic changes in temperature and rainfall with global warming can significantly alter the moisture status of topsoil, thereby inducing soil structure degradation. However, few studies have reported the variation in permeability of saline soils during drying, which contributes to further refining the mechanism of wetting‒drying effect on soil properties. In this study, the permeability and microstructure of sodic-saline loessal soil with different initial water contents (IWCs) and salt contents (ISCs) obtained from pre-saturation and subsequent drying were explored using constant head permeability tests and SEM observations. The results show that the permeability coefficient decreases exponentially with time. The maximum permeability coefficient (K_max) of the samples decreases with decreasing IWC and ISC, while the relatively stable permeability coefficient (K_rs) is less affected. The microscopic results show that during the seepage process, the porosity and pore diameter of samples with low IWC gradually decrease, accompanied by a weakening of pore directionality and an increase in fractal dimension. In contrast, samples with high IWC show an initial increase followed by a decrease in porosity, pore diameter and pore directionality, alongside a gradual decrease in fractal dimension. The drying process promotes the formation of inter-aggregate pores and weakens aggregate stability, leading to significant microstructural disturbances in low IWC samples upon rewetting. The increase in salt content enhances particle cementation but also creates additional channels for rapid permeability. These findings carry practical implications for the prevention and control of soil erosion and engineering geohazards in saline soil regions under the impact of climate change.