Saline soil, common in the western China, poses a significant threat to road engineering due to its salt swelling characteristics. Therefore, studying the water-salt migration patterns within saline soil subgrades and developing methods to interrupt this migration are crucial for road safety prevention and control. Based on the utilization of excavated waste soil, a new type of foamed lightweight soil based on saline soil is proposed as a subgrade separation fault in saline soil areas. Using self-developed equipment, we tested internal temperature changes, vertical displacements, and water and salt distribution after freeze-thaw cycles. The objective was to evaluate its salt insulation and swelling suppression capabilities and to explore the microstructure-based mechanisms underlying salt inhibition. Results indicate that under a temperature gradient, water and salt in the saline soil sample migrate upward, accumulating mainly in the middle and upper sections. Notably, the novel foamed lightweight soil separation fault effectively blocks water and salt migration, significantly suppressing salt swelling. Interestingly, a higher soil salt content results in a more pronounced anti-swelling effect. The porous structure of the foamed lightweight soil can not only store salt effectively, but also block salt migration, allowing salt crystallization within the soil, thereby reducing salt swelling damage.

The water-salt migration law and deformation characteristics of coarse-grained saline soils have been extensively studied and illustrated. However, owing to the influence of the chemical composition and physical properties of the soils, coarse-grained soils are prone to localized soil absorption during mixing and compaction. This type of working condition of the existing localized fine sand accumulation layers is seldom discussed in the literature. In this study, water-salt migration and deformation of natural gradation specimens and specimens with localized fine sand accumulation layers in natural gradation were monitored and detected for the field fill conditions in an airport embankment project using self-designed test equipment based on nine freeze-thaw cycle physical simulation tests at environmental temperatures ranging from-30 degrees C to 25 degrees C. Under the freeze-thaw cycle, compared with the natural gradation, the specimens with localized fine sand accumulation layers had a higher influence on water and salt migration, which indicates that the depth range of drastic changes in water and salt increased by 80% and 84%, respectively. The cumulative deformation curves under the effects of natural gradation and localized fine sand accumulation exhibited similar trends. The difference between the deformation of the natural samples and samples with localized fine sand accumulation layers was 16% when the salt content of the upper part of the roadbed was 0.3%. In addition, the cumulative vertical settlement deformation of the specimens decreased with an increase in the salt content of the upper part of the roadbed and gradually transformed into vertical uplift deformation. The results of this study provide a basis for the selection of materials for airport roadbed backfill and their application in construction in seasonally frozen areas.