Elastoplastic deformation characteristics of Toyoura sand during liquefaction

In order to model the behavior of saturated sands during earthquakes based on an elastoplastic approach, large amplitude cyclic drained and undrained trixial tests are conducted on saturated Toyoura sand. Elastic properties are evaluated by unloading and reloading with extremely small strain amplitudes. It is demonstrated that elastic behavior that is measured during liquefaction can be explained by considering inherent and stress-state-induced anisotropy in modeling deformation characteristics for sands. Because the effects of aging are not involved and the stress states are mainly anisotropic, the Young's modulus measured during liquefaction is smaller than the reference value measured at the same vertical stress after some aging at isotropic stress states. Based on the reference elastic property described above and by introducing Rowe's stress-dilatancy relationships, plastic volumetric strains during liquefaction are evaluated separately for the components due to dilatancy and change of effective mean principal stress. It is found that the latter component is not zero but tends to accumulate on the swelling side as the effective mean principal stress is reduced during liquefaction. These results suggest that for proper elastoplastic modeling of liquefaction process, effects of inherent and stress-state-induced anisotropy and aging on elastic properties should be at least considered.