Study on Mechanical Properties of Saline Soil and Evaluation of Influencing Factors

In cold regions, the freeze–thaw (FT) cycle is a common natural physical process. It directly affects the stability of the project by changing the mechanical properties of the saline soil. Due to the presence of salt, the effect of the freeze–thaw cycle is greater, and the effects of different salts are not the same. In recent years, the construction of various projects such as ditches and roadbeds has been carried out in saline soil areas. Therefore, to study the mechanical properties of saline soil with different salt types and contents subjected to FT conditions in cold regions, in this study, saline soil from a representative site in Nong’an was selected for the preparation of artificial specimens considering different salt types (Na2SO4, NaHCO3/Na2SO4, and NaHCO3), salt contents (0%, 0.5%, 1%, 2%, and 3%), numbers of freeze–thaw cycles (0, 10, 30, and 60), and confining pressures (100, 200, and 300 kPa); unconsolidated–undrained (UU) triaxial compression tests were carried out to study the mechanical properties including the stress–strain relationship, peak undrained shear strength (τu), cohesion (cu), and internal friction angle (φu). The results showed that the stress–strain relationship, τu and cu, basically showed a steady decline with an increase in the number freeze–thaw cycles under the condition of the same salt type and salt content, and the dynamic evolution of the soil structure was the main reason for this change. Using the Gouy–Chapman diffusion double-layer theory, the influence mechanism of the salt content on cu and φu under different numbers of freeze–thaw cycles was analyzed. There was a threshold salt content (approximately 2%) by analyzing the change law of τu and φu, and the values of different salt types were different, but the difference was not large. By comparing the properties of Na2SO4 and NaHCO3, the mechanical properties including the stress–strain relationship, τu, cu, and φu, were analyzed, and it was confirmed that the effect of Na2SO4 on the soil was greater than that of NaHCO3. According to the relationship between the change law of τu and the change law of cu and φu, it was found that the freeze–thaw cycle mainly affects τu by affecting cu, and the salt content mainly affects τu by φu. For different salt types, the freeze–thaw cycle effects and salt content are different, which in turn affects τu. Finally, the gray correlation degree was used to analyze the influence of each factor on the strength, and the order of influence of each factor on the strength was determined as the confining pressure > freeze–thaw cycle > salt type > salt content.