Experiments and elastoplastic analyses on soil disturbance of soft clay subjected to cyclic loading

Although liquefaction of sandy soils is widely recognized as a major cause of seismic damage, recent earthquakes have revealed that soft clayey grounds can also suffer significant damage. To investigate this, we conducted cyclic undrained triaxial compression/extension tests with axial strain control on undisturbed soft clay samples. The results showed that the undrained peak shear strength decreased by up to 25% with increasing number of cyclic loadings, indicating that structural degradation plays a key role. In contrast, remolded samples with a de-structured remolded sample exhibited no such strength reduction. To interpret these observations, we employed an elastoplastic constitutive model incorporating the concept of soil skeleton structure. Numerical simulations successfully reproduced the experimental trends and confirmed that the strength reduction is attributable to structural deterioration. These findings suggest that cyclic loading induces disturbance by degrading the soil structure, leading to a significant reduction in shear strength. This study highlights the importance of considering seismic damage in soft clayey sites, which is often overlooked in practice. Furthermore, the structure-based modeling approach provides a useful framework for evaluating post-earthquake performance and improving seismic design strategies for clayey ground conditions.