Study on the mechanism of water-salt migration and mechanical property evolution of sandy loess under dry–wet cycles

Under the coupled action of dry-wet cycles and salinization, loess in arid and semi-arid regions exhibits complex multi-field coupling effects involving water, salt, and mechanics, which significantly influence its engineering geological properties and trigger disasters. This study focuses on sandy loess, a material frequently associated with geological hazards, as the research object. Through systematic laboratory tests, the permeability characteristics, shear strength, and microstructural evolution of the material under cyclic salt-solution infiltration and air-drying were investigated. The results indicate that as the number of cycles increases, repeated salt dissolution and crystallization aggravate particle cementation damage and fine particle migration, leading to an increase in porosity, enhanced permeability, and a continuous decrease in cohesion. In contrast, the internal friction angle shows a non-monotonic trend of first increasing and then decreasing, which is attributed to the temporary supporting role of salt crystallization. Microstructural analysis reveals the mechanism of structural reorganization driven by water-salt migration, including the formation of clay cutans and changes in soil fabric. Furthermore, a quantitative system of macroscopic mechanical response is established based on the “mass change parameter” and “absolute degradation degree”. The research findings can provide a theoretical basis and experimental support for long-term performance evaluation of engineering structures and for geohazard risk prevention and control in sandy loess regions.