Deformation of pavement subgrade subjected to traffic loads considering multi-direction principal stress rotation

Neglecting the principal stress rotation (PSR) in multi-direction may underestimate the deformation of a geotechnical structure subjected to a complex load, such as traffic load. However, most of the existing experimental studies assume that the PSR under a traffic load exists in one direction only. In this article, several stress paths including the multi-direction PSR are first obtained under multiple traffic loads using the Finite Element Method (FEM). The dynamic shearing tests are then conducted on sand based on these stress paths considering both the uni-direction PSR and the multi-direction PSR. Comparisons are made across the stress paths, deformation, resilient modulus and shear modulus. The results indicate that, whether or not the multi-direction PSR is considered, the accumulation rate of the vertical strain gradually reduces with the increasing load cycles, and a stable status could be finally observed. However, the final vertical deformation considering the multi-direction PSR is obviously larger than that considering the uni-direction PSR under the same dynamic vertical stress. Moreover, it has been found that the ultimate shear strain in any single direction depends on the shear stress in that direction only. The complicated stress path caused by different loading frequencies at two directions of shear also increases the vertical strain, and more cycles are required to achieve a stable status. In addition, the PSR has a significant influence on resilient modulus and a slight influence on shear modulus. The results can help to improve the conventional permanent deformation prediction formulas and guide pavement subgrade design in a more conservative way.