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.