Cyclic response of sandy subsoil layer under traffic-induced principal stress rotations: Application of bidirectional simple shear apparatus

Abstract

Cyclic response of soil layers under moving wheels includes Principal Stress Rotation (PSR), which evidently cannot be simulated with a conventional cyclic uniaxial triaxial test. The hollow cylinder apparatus (HCA) has been used as a solution to assign a more realistic stress path, taking into account the continuous PSR. However, the effective horizontal stress is also harmonically changed due to the cyclic increase in the effective vertical stress – under a horizontally constrained condition with zero lateral strain – and is inevitably overlooked in the HCA. In this study, the application of the Bidirectional Direct Simple Shear (BDSS) apparatus is scrutinized so that not only are the shear and, of course, normal stress components considered, but the expected increase in the effective horizontal stress is subsequently assigned as well. Besides, the effects of an initial shear stress component superimposed either parallel or perpendicular to the direction of moving wheels are studied. A series of cyclic simple shear experiments employing a heart-shaped stress path is conducted on a sandy soil for up to 10,000 cycles to determine the permanent deformation. An empirical model is also fitted for calculating the residual strains (i.e., vertical and shear strains). The results show the role of cyclic shear and normal stress components intensifying both shear and vertical strains. Moreover, parallel and perpendicular static shear stress components can effortlessly escalate deformation, particularly the shear strains. BDSS apparatus can be a reasonable alternative to HCA in terms of convenience in the sample preparation, real zero lateral strain condition, and assignment of static shear stress components.