Experimental study on the cyclic and post-cyclic monotonic behavior of soft clay under equivalent seismic loading

The seismic behavior and post-seismic strength of soils are crucial to the seismic safety of surrounding structures. Compared with uniaxial dynamic triaxial tests, bidirectional cyclic loading better simulates the shear conditions induced by earthquakes. In this study, a series of bidirectional dynamic triaxial tests were performed on soft clay samples to investigate the dynamic behavior under equivalent seismic loading, considering the effects of both confining pressure and consolidation stress ratio. The monotonic shear behavior before and after equivalent seismic loading was also compared. The results show that the hysteretic behavior of soft clay under equivalent seismic loading exhibits both confining pressure dependency and anisotropic effects. Isotropically consolidated soft clay samples showed more pronounced stiffness degradation, whereas anisotropically consolidated samples accumulated larger deformations. The post-cyclic peak shear strength decreased under various confining pressures. However, under different consolidation stress ratios, the post-cyclic shear strength showed an increase of up to 17.6%. Following seismic loading, the excess pore water pressure decreased significantly by an average of approximately 65%. In addition, a model to predict the post-cyclic shear strength of soft clay samples was established. These findings shed light on the dynamic response and post-cyclic monotonic behavior of soft clay under equivalent seismic loading.