Saline soil refers to a type of soil with excessively high salt concentration that possesses special engineering properties such as salt heaving, collapsibility, and corrosivity, and it is widely distributed in the arid and semi-arid regions of China. Dissolved salts in the soil gradually accumulate near the surface due to water evaporation or water-salt migration, forming distinct salt layers or salt crystals and causing salt heaving. After immersion in water, saline soil exhibits collapsibility due to the dissolution or erosion of salt crystals. Repeated salt heaving and collapsibility lead to distress and service safety problems in road engineering, hydraulic engineering, and building engineering in saline soil areas. Traditional methods such as replacement with high-quality fill and strengthening of retaining structures would substantially increase construction costs. Therefore, engineering construction and long-term performance assurance in saline soil areas face many challenges.

    In response, the subgrade engineering research team at the Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, proposed two resource utilization schemes for saline soil: (1) in-situ solidification and modification of saline soil using solid waste to eliminate salt heaving; (2) full utilization of saline soil to develop a saline-soil-based foamed porous isolation layer, proposing asaline soil subgradenew technology of "treating salt with salt."

    Relying on the Urumqi Ring Expressway project in Xinjiang, the research team conducted research on improvement technology for saline soil subgrade fill. By fully utilizing the widely available solid wastes such as fly ash and slag in Xinjiang and combining them with polyacrylamide, they developed an organic-inorganic combined saline soil modifier. Tests showed that fly ash and slag undergo hydration and hydrolysis reactions to generate various hydration products that fill pores and increase density; polyacrylamide, through the viscosity of the colloid itself, binds the hydration products and soil particles to form aspatial network structureand the combined solidification of the three can significantly suppress salt heaving and collapsibility of saline soil and enhance mechanical performance.

    In addition, the research team proposed a new technology of saline-soil-based foamed porous isolation layer, which fully utilizes the saline soil widely distributed along the project alignment to replace the traditionaltwo layers of geotextile and one membraneand can significantly reduce construction costs. The material consists of saline soil, cementitious material, water, and foam, and has structural characteristics such as porosity, thermal insulation, and light weight. Research results show that by setting a foamed porous isolation layer of a certain thickness at the bottom of the roadbed, good salt isolation and thermal insulation effects can be achieved. Under the action of a temperature gradient, water and salt in the saline soil subgrade migrate upward, and salt accumulates within the foamed porous isolation layer, whose porous structure also provides space for salt storage and salt heaving. In addition, due to the low thermal conductivity of the porous structure, the isolation layer has a good thermal insulation effect, further limiting the upward migration of water and salt within the saline soil, thereby achieving the goals of resource utilization of saline soil and ensuring subgrade stability. This technology has been successfully applied to the Urumqi Ring Expressway, and long-term monitoring results of the field demonstration project show good performance.

    The research results were published in journals such as and and three invention patents were granted. The research work was funded by the National Natural Science Foundation of China (42477205), the Hubei Provincial Natural Science Foundation Innovation Group Project (2023AFA019), and the Open Competition Project of the Key Laboratory of Geomechanics and Geotechnical Engineering Safety (SKLGME-JBGS2403).

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Fig. 1 Salt heaving amount of soil samples under different modifier dosages

Fig. 2Comparison of vertical deformation of specimens under water-salt migration: (a) without isolation layer; (b) with isolation layer

Fig. 3SEM images of salt storage and distribution within the isolation layer

Fig. 4 On-site casting of the saline-soil-based foamed porous isolation layer