Experimental identification of plastic shakedown behavior of saturated clay subjected to traffic loading with principal stress rotation

Traffic loading causes the rotation of principal stress axis and generates a heart-shaped stress path in the deviatoric stress space. Using dynamic hollow cylinder apparatus (DHCA), a series of cyclic heart-shaped and cyclic triaxial undrained tests were performed on Shanghai clay through simultaneously varying the torsional shear stress and the normal stresses. Experimental results show that the accumulated undrained responses of clay at different stress levels can be described by the shakedown approach. The permanent strain and energy dissipation were used to classify the accumulated deformation patterns, including plastic shakedown, cyclic plastic creep and ratcheting (incremental instability). For the plastic shakedown, the IPC (increment per cycle) of permanent axial strain becomes negligible and the IPC (area of hysteretic loop per cycle) of energy dissipation tends to reach a low constant level after cycling. For the cyclic plastic creep, the IPC of permanent axial strain (plastic strain increment per cycle) and the IPC of energy dissipation become nearly constant after a large number of cycles. For the ratcheting, the IPCs of strain and energy dissipation increase after a limited number of cycles. Based on these results, a new method is proposed to define the boundaries of shakedown behaviors using the effective cyclic stress ratio.