Effect of Freeze–Thaw–Dry–Wet Cycles on the Shear Behavior of Silty Clay Salinized in Wetting Processes

Abstract

In cold and arid saline areas, the mechanical properties of soils are usually significantly affected by some complicated conditions, especially the coupled effects of the freeze–thaw–dry–wet (F–T–D–W) cycles and soil salinization. This study experimentally investigated the effect of F–T–D–W cycles on the shear performances and microstructures of silty clay that was salinized during wetting processes. Three types of soil samples with different dry densities were designed: (1) silty clay samples without salt (Category Ⅰ); (2) silty clay samples with salt (Category Ⅱ); and (3) silty clay samples that were salinized during wetting processes (Category Ⅲ). Direct shear and scanning electron microscopy (SEM) tests were carried out, the variations in the shear strength, surface deterioration, and shear parameters (e.g., cohesion and internal friction angle) were analyzed, and the degradation mechanism was revealed. The results show that the F–T–D–W cycles and soil salinization significantly affect the shear strength of soils, especially for the samples with low dry densities. The shear strengths of soil samples with and without salt (Categories Ⅰ and Ⅱ) decrease as the F–T–D–W cycles increase. Besides, the cohesion of soil samples increases with dry density and declines with the F–T–D–W cycles due to the appearance of cracks and bond failure among soil particles. In addition, there is a threshold number of F–T–D–W cycles to significantly reduce the cohesion of soil samples, and the threshold numbers for soil samples Categories Ⅰ and Ⅱ are six and three, respectively. The repeated expansion and shrinking of soils accelerate the damage to the soil structure, which results in a decrease in cohesion and interparticle force. However, when the concentration of salt solution in soils exceeds the saturation concentration, a new denser soil skeleton is formed by the soil particles and surrounding salt crystals, which improves the shear strength of the soil samples. This study could provide deep insights into the shear performance and microstructures of silty clay exposed to F–T–D–W cycles.