Shear strength is one of the important mechanical properties of soil, which determines the stability and normal use of engineering buildings. Studies have shown that the shear strength of soil is not only related to its own properties, such as material composition, water content, and dry density [1
], but also related to the environment’s effect [4
]. In seasonally frozen regions, the temperature difference caused by climate change will lead to repeated freezing and thawing of pore water in the soil, which will decrease the soil strength and result in engineering damages [6
]. Therefore, the influence of freeze–thaw cycles on the shear strength of soil has always been an important research topic in seasonally frozen regions.
The Songnen Plain in northeast China distributes a large area of seasonally frozen soil with a high soluble salt content (greater than 0.3%), which belongs to saline soil [8
]. Because of the presence of salt in the pore water, the change of the structure and strength of saline soil in seasonally frozen area is more complex [9
]. On the one hand, the saline soil undergoes repeated freeze–thaw cycles under the great temperature difference. On the other hand, the salt in the soil will migrate with the change of temperature, which will lead to a change of the salt content in the soil [10
]. Under the influence of these two factors, the mechanical properties of the saline soil in seasonally frozen areas are usually poor and cause many types of engineering damage, such as road boiling, melt sinking, and the reduction of stability of the subgrade and slope. Therefore, in order to keep the sustainability of engineering construction in seasonally frozen regions, it is of great engineering significance to investigate the influence of salt content and freeze–thaw cycles on the shear strength of saline soil.
Previous studies have shown that freeze–thaw cycles will change the grain size distribution of soil, which will result in changes of the particle content in different particle groups [11
]. In addition, the salt in the soil has an agglomeration effect on the soil particles, thus changing the microstructure type of the soil [13
]. These factors will change the size, shape, and arrangement characteristics of the pores in the soil [15
]. Hu et al. [17
] studied the shear strength and microstructure characteristics of loess under dynamic impact load through a triaxial test and SEM test, and the relationship between them was analyzed. Qi et al. [18
] studied the shear strength and microstructure of silty clay and loess before and after freeze–thaw cycles through mechanical and SEM tests, and pointed out that the study of soil microstructure is an important method to explain its mechanical properties. Jiang et al. [19
] performed triaxial tests along different stress paths on undisturbed and remolded loess and observed the microstructure of soil samples before and after the mechanical tests through microscopic tests. The changing mechanism of the mechanical properties was explained through the changes in microstructure. Xu et al. [20
] conducted direct shear tests and microscopic tests on the loess samples under different dry densities, water contents, and freeze–thaw cycles, and the variations of the shear strength parameters and microstructure parameters were discussed. These studies showed that the shear strength of soil is closely related to its microstructure. By analyzing the changes in the soil microstructure, the changing mechanism of soil shear strength under different conditions can be well explained.
In recent years, researchers have conducted extensive research on the microstructure and strength of different types of soil, such as loess, laterite, silty clay, and improved soil [21
]. Some researchers also studied the effect of salt content on the microstructure and shear strength of soil [14
]. However, there are still few studies on saline soils distributed in seasonally frozen areas. Han et al. [29
] studied the influence of salt content and freeze–thaw cycles on the shear strength of soil and analyzed the microstructure of soil samples after freeze–thaw cycles by SEM tests qualitatively. Wang et al. [30
] investigated the variation law of the unconfined compressive strength of saline soil with salt content, water content, and the number of freeze–thaw cycles. Liu et al. [12
] studied the physical properties, mechanical properties, and microstructure of lime-improved saline soil under different freeze–thaw cycles. However, for the saline soil in seasonally frozen areas, the quantitative change of the microstructural parameters with salinities, freeze–thaw cycles, and the relationship between the microstructural parameters and shear strength is still unclear, so it is necessary to study their relationship, which can provide a quantitative basis for explaining the variation mechanism of the mechanical properties of saline soil from a microscopic perspective, and provide references for the symmetry between the changes of the macroscopic properties and microstructure of the saline soil in cold regions.
In this study, remolded soil samples with different salinities (0–3%) were prepared to experience different freeze–thaw cycles (0, 10, 60, 120). The unconsolidated undrained triaxial compression tests were carried out on the soil samples after freeze–thaw cycles under different confining pressures to obtain their failure strength and shear strength parameters. In addition, the microstructures of soil samples under different experimental conditions were investigated by scanning electron microscopy (SEM) tests, and the microscopic pore parameters of soil samples were extracted by Image-Pro Plus (IPP) software quantitatively. The influence of freeze–thaw cycles and salt content on the shear strength characteristics and microscopic pore parameters of the soil were discussed. Finally, the relationship between the microscopic pore parameters and failure strength of tested soil samples was fitted by principal component regression analysis.