Effects of freeze-thaw cycles on the erodibility and microstructure of soda-saline loessal soil in Northeastern China

Soda-saline loessal soils are distributed extensively in the Songnen Plain of Northeast China. Freeze-thaw effects on soil structure and mechanical properties are becoming more frequent and intense with global warming, which further affects soil erosion in cold regions. Quantifying the effects of freeze-thaw cycles (FTCs) on the erodibility of saline soils is challenging because of the abundant soluble salt content and complex soil mechanical responses. In this study, direct shear tests, disintegration tests, and scanning electron microscopy were conducted to investigate the effects of freeze-thaw action on the erodibility indicators (shear strength and disintegration velocity) of soda-saline loessal soils and their microscopic deterioration mechanisms. Four initial moisture contents (IMCs: 8%, 13%, 18%, 23%), three soluble salt contents (SSCs: 0.5%, 1.0%, 1.5%), and six FTCs (0, 1, 3, 6, 10, 15) were considered. The results showed that successive FTCs continued to degrade the erosion resistance of the soil until it stabilized after approximately 10 FTCs. After 15 FTCs, the shear strength and cohesion decreased by 8.6–19.8% and 24.1–59.2%, respectively, while the angle of internal friction was relatively unaffected. Meanwhile, the soil disintegration velocity increased by 52.3–208.8%. During the freeze-thaw process, the water-ice phase change and the crystallization of soluble salts both led to the continuous adjustment of the microstructure of soda-saline loessal soils. It was found that microcracks within the soil structural units increased significantly, cementation between the structural units weakened continuously, pore connectivity was enhanced, and pore size homogenization increased. Correlation analysis showed that IMC was the fundamental factor affecting the soil mechanical properties, whereas SSC mainly affected the soil's resistance to disintegration. The increase in IMC and SSC exacerbated the deterioration of the soil by freeze-thaw action. This paper presents a new understanding of the erosion characteristics and micromechanisms of soda-saline loessal soils in seasonally frozen regions under freeze-thaw action.