Experimental investigation of the erodibility of soda saline-alkali soil under freeze-thaw cycle from a microscopic view

Abstract

Soda saline-alkali soil, dominated by NaHCO3, has complex engineering properties that change with external water, heat, and salt, making it prone to water erosion. By destroying soil microstructure, freeze-thaw (F-T) action can potentially enhance soil erodibility and provide water erosion material. Therefore, this work examined the freeze-thaw erosive properties and microstructural evolution mechanisms of soda saline-alkali soil. Disintegration tests and a series of microscopic tests examined the impacts of freeze-thaw cycles (N = 0, 1, 5, 10, 30, 60) and soluble salt content (η = 0.4 %, 0.8 %, 1.2 %, 1.6 %, 2.0 %) on the soil erodibility index (disintegration rate). The results demonstrated that F-T action continued to weaken the disintegration resistance of the saline soil. Moreover, this trend relatively stabilized after 10 F-T cycles.Soluble salt content had a two-phase impact on the saline soil disintegration. Increased salt content below 0.8 % improved erosion resistance, but salt content above 0.8 % accelerated the soil erosion. According to the Gouy-Chapman diffuse double layer (DDL) theory, the variation in the thickness of DDL induced by higher sodium ion contents and lower sodium ionic valence were deemed as the main causes. When the salt content increased from 0.8 % to 2.0 %, the disintegration rate increased by 49.40–71.43 %. Microscopic results showed that sequential F-T cycles decreased intra-aggregate pores (<4 μm) and increased inter-aggregate pores (>4 μm). This mechanism was attributed to the unfrozen water migration and ice segregation due to the cryo-suction process during the freezing process. Correlation analysis indicated that soil microstructure with a bimodal characteristic dominated the erodibility index. The effect of F-T action on soil erodibility was more significant than the soluble salt content, with both impacted saline soil erodibility by affecting pore distribution characteristics and soil aggregate formation, respectively. These findings may assist soda saline-alkali soil salinity prevention and control of saline soil erosion in seasonally frozen regions globally.