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Based on the discovery of the "natural orange soil layer" in the wall foundation of the Ming Great Wall in Huangzhong County, Xining City, northwest China, this paper reveals the mechanism behind the barrier effect of this layer on the salinity deterioration through the combination of qualitative judgment and quantitative testing. Based on this mechanism, the conventional lime soil was improved by adding modified polyvinyl alcohol as the cementing agent and fly ash, thereby generating a modified cushion material. Indoor tests of freeze-thaw and temperature-humidity cycles for the modified cushion material were then conducted alongside field tests of rammed restorations in the basal sapping zone with and without the modified cushion. The test results show that the modified cushion materials have excellent durability and act as a substantial barrier to salinity deterioration. The research results provide a new approach to repairing the basal sapping of earthen sites. [ABSTRACT FROM AUTHOR]