Energy-based evaluation of undrained cyclic behavior of marine soft clay under multidirectional simple shear stress paths

During the service life of marine structures, environmental loads, such as earthquakes and ocean wave storms, may exert dynamic shear stresses of different amplitudes and orientations in the foundation soil. To investigate the undrained cyclic behavior of marine soft clay under a complex shear stress state, a laboratory study was performed on remolded clay specimens using a variable-direction dynamic cyclic simple shear system, through which two mutually perpendicular shear stresses can be simultaneously applied to the specimen within the horizontal x–y plane. Stress paths of linear, circular, and elliptical patterns were obtained by adjusting the amplitude ratio (η) between the two cyclic shear stresses and applied to investigate the undrained behavior of soft clay. The undrained cyclic responses of the clay specimens were presented and then analyzed from the viewpoint of energy dissipation. A strain-softening phenomenon was observed when the cyclic effective stress path reached the failure line determined by the monotonic test. The cyclic strength of clay subjected to multidirectional cyclic shear stresses (i.e., circular and elliptical stress paths) was 0.775η times that under unidirectional shear stress (i.e., linear stress path). The coupling effects of CSR and η on the cyclic strength and accumulation of residual pore water pressure can be uniformly correlated to energy dissipation.