Abstract: To elucidate the long-term deformation behavior of quasi-saturated subgrade soils under traffic loading, this study investigated the combined effects of initial saturation, wet-dry cycling, and principal stress axis rotation. The key factors considered in this study include the cyclic stress ratio (CSR), cyclic torsional shear ratio ( ), initial degree of saturation, number of wet-dry cycles, and cycle amplitude. Cyclic triaxial and torsional shear tests were conducted using a GDS hollow cylindrical torsional shear apparatus on remolded silty clay, which is representative of subgrade soils in a quasi-saturated state. The results demonstrate that the axial strain is significantly affected by initial saturation; higher saturation levels, which correspond to lower entrapped air content, lead to greater final axial strains. An increase in CSR significantly accelerates strain accumulation, with larger CSR increments resulting in larger strain increments. Similarly, axial strain increases in proportion to the torsional shear ratio, indicating a strong positive correlation. Furthermore, both the number and amplitude of wet-dry cycles contribute to greater cumulative deformation, with the number of cycles having a more dominant impact. These results highlight the critical role of stress path rotation and gas-liquid interactions in governing the deformation characteristics of quasi-saturated soils. The findings provide valuable insights for predicting long-term performance and designing subgrade structures subjected to repeated traffic loads, especially in regions where soils undergo seasonal moisture fluctuations.

Key words: cyclic traffic load, rotation of principal stress axes, quasi-saturated subgrade silty clay, wet-dry cycles, long-term deformation