Investigation of mechanical behavior and constitutive modeling of silty sand under DWFT cycles

The first phase of the North Xinjiang Water Supply Project is a seasonal water supply project. Owing to the impacts of climate and the environment, the slope of the open channel section of expansive soil has repeatedly experienced sliding damage. After silty sand was used as a replacement material for treatment, the slope still experienced sliding damage after a period of operation. Therefore, this study systematically explored the mechanical properties and physical mechanisms of silty sand under dry-wet-freeze-thaw (DWFT) cycles through direct shear, compression, SEM, electron microscopy, and triaxial tests. The research results show that: (1) As the number of DWFT cycles increases, the cohesion and internal friction angle of silty sand decrease exponentially, with maximum deterioration degrees of 4.61% and 2.52%, respectively, while the compression coefficient increases slightly. The influence of cycling on the shear and compression characteristics of silty sand is limited, and it still exhibits low compressibility. (2) Microscopic analysis indicates that the skeleton of silty sand is mainly composed of sand grains, with fine particles and clay mineral aggregates filling the pores. Cycling leads to an increase in internal pores within weakly cemented aggregates and fluctuations in microscopic porosity, but the particle skeleton remains relatively stable. (3) Macro- and micro-scale tests reveal that DWFT cycles have a minor impact on the mechanical properties of silty sand, thus it suffices to study the stress-strain relationship of uncycled samples. Triaxial tests show that silty sand exhibits a hardening stress-strain relationship with only shear contraction at low compaction degrees. As compaction increases, softening intensifies, and volumetric deformation initially involves shear contraction followed by shear dilation. With increasing confining pressure, softening diminishes, and shear contraction enhances. (4) Model validation: Parameters for the model are derived from the triaxial test results, with the stress-strain-bulk strain relationship of silty sand predicted through substitution into the model. A comparison between experimental values and model predictions indicates that the constitutive model related to sandy soil effectively simulates the stress-strain and bulk strain relationships of silty sand, adequately reflecting the variations in mechanical properties. However, several issues remain that require improvement. The research outcomes provide a scientific basis for practical engineering applications.