In this study, a cyclic viscoelastic-viscoplastic constitutive model based on the kinematic hardening rule was proposed. The proposed model, based on the generalized nonassociated flow rule, and the concept of the over consolidated boundary surface as well as the nonlinear kinematic hardening rule within the context of infinitesimal strain, was calibrated and verified using cyclic triaxial tests. In order to examine the properties of the proposed model, the element simulations considering viscoelastic effects and also viscoplastic effects were studied by both an elastic-viscoplastic model and the viscoelasticviscoplastic model based on the kinematic hardening rule. From the simulation of cyclic triaxial tests, it was found that viscoelastic behavior of clay in the small strain range is an important characteristic during motion. Both the shear modulus and the hysteretic damping ratio are strain level dependent, and these can only be explained within the framework of the viscoelastic-viscoplastic constitutive model. This study reveals that the viscoelastic-viscoplastic model can describe the damping characteristics of clay accurately at small strain levels, namely cyclic softening, while the elastic-viscoplastic model cannot do so.

INTRODUCTION

The well-known phenomenon of soil liquefaction is caused by both negative dilatancy under cyclic undrained loading conditions and upward flow of water after earthquake motion. As is well known, natural ground has a layered structure composed of sand and clay layers. Until now, many constitutive models for sand have been proposed and ground liquefaction has been studied, but most of them have been focused only on the liquefaction of the sand layer. But when we consider the response of ground composed of sand and clay layers, it can be expected that the behavior of the clay layer affects the ground motion. Up to now, elastic or elastoplastic models have been often used for clay behavior in the dynamic analysis.

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