Time-dependent permeability evolution in saline-affected sandy clay soils: Coupled experimental and microstructural analysis

• Time-dependent transformation of clay fabric under varying NaCl concentrations alters permeability behavior. • Short-term salt crystallization reduces permeability, while long-term salt washout increases it significantly. • Elevated electrolyte concentrations promote clay particle flocculation and microstructural reorganization. • Reduction in diffuse double layer thickness with increasing salinity affects hydraulic conductivity. • Findings support improved prediction of soil performance in saline environments for geotechnical design. Sandy clay soils are widely used in geotechnical infrastructures such as landfill liners and embankment dam cores because of their naturally low hydraulic conductivity. However, their long-term performance can be strongly influenced by pore-fluid chemistry, particularly in saline environments. This study presents a combined experimental and microstructural investigation of the time-dependent and concentration-dependent permeability evolution of sandy clay soils exposed to sodium chloride (NaCl) solutions ranging from 0 to 1.7 M. The experimental program was designed to distinguish between short-term and long-term hydraulic responses. Short-term permeability behavior was evaluated using 21-day falling-head tests, while long-term behavior was examined using an equilibrium suspension–drying cycle. This approach enabled direct comparison of two distinct evolutionary stages. Changes in Atterberg limits and mineralogical characteristics were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal a clear dual-phase hydraulic behavior. In the short term, salt crystallization and particle aggregation reduce permeability by up to approximately 70% due to partial pore clogging. In contrast, under long-term exposure, salt dissolution, halite washout, and microstructural rearrangement progressively enhance permeability, resulting in an increase of approximately 2.6 times relative to the natural soil, while the overall contrast between minimum short-term and maximum long-term permeability reaches up to approximately nine-fold depending on salinity level. SEM observations indicate initial formation of halite clusters that reduce effective porosity, followed by the development of enlarged pore channels associated with flocculation and loosening of the soil fabric. These trends are supported by XRD results, which show attenuation of clay mineral peaks during early exposure and re-emergence of crystalline phases at later stages. These coupled macro–micro observations provide a mechanistic explanation for the contrasting short-term and long-term permeability trends. The novelty of this study lies in explicitly linking time-dependent permeability evolution to corresponding microstructural transformations under controlled salinity conditions. The findings highlight the engineering significance of salinity-driven hydraulic evolution and offer practical guidance for the design, assessment, and long-term maintenance of geotechnical systems in saline-affected environments.