In order to study the effect of principal stress deflection angle on the strength and deformation characteristics of Q₂ in-situ loess under directional shear. The deformation and strength characteristics of Q₂ in-situ loess under directional shear loading were researched, and the in-situ properties in comparison with remodeled loess were analyzed. The effect of structural properties of loess on shear properties and non-coaxial properties was analyzed in comparison with remodeled loess. The test shows that due to the difference of internal structural properties of in-situ loess, when the main stress deflection angle α is between 0 and 15°, the directional angle of strain increment gradually increases with the increase of α, and the non-coaxial property increases; when the main stress deflection angle α is between 30° and 90°, the directional angle of strain increment gradually decreases with the increase of α, and the non-coaxial property decreases. The overall trend is that it first strengthens and then weakens. At the same time, because the structural properties within the in-situ loess are not destroyed before the peak strength, the total generalized strain value when reaching the damage is small, and the generalized shear strain value when reaching the peak strength is also small, usually about 2% reaching the peak strength, and then the damage occurs rapidly with larger deformation.

Based on the GDS dynamic triaxial test system, a series of dynamic characteristic tests were conducted on remolded sand-mixed soft clay with varying sand contents to systematically investigate the influence of loading frequency and sand content on plastic cumulative deformation, dynamic shear modulus, and damping ratio. The results indicate that: The development of plastic cumulative deformation exhibits a rapid initial phase followed by a decelerating phase. As the number of vibrations increases, the densification of the soil skeleton inhibits the rate of plastic deformation, while higher sand content significantly amplifies the cumulative deformation. Regardless of whether the loading frequency was below or above 1.0 Hz, the dynamic shear modulus displayed a decreasing trend with increasing sand content. For every 10% increase in sand content, the modulus reduction reached 12%~18%. During vibration, the pore pressure coefficient A continuously increased, intensifying the shear contraction effect and deteriorating soil stability. Higher sand content exacerbated this phenomenon, resulting in distinct inflection points in the dynamic shear modulus and damping ratio curves (critical sand content approximately 20%). In summary, sand content significantly influences the dynamic response mechanisms of sand-mixed soft clay by altering the soil skeleton structure and drainage pathways, providing critical insights for constructing dynamic constitutive models and guiding seismic engineering design.