Elastoplastic modelling for long-term cyclic stability of soft clays with consideration of structure damage

Abstract

The constitutive model of structured clays under cyclic loads is helpful to analyze the long-term stability of foundations, such as subgrade under traffic loads and foundation of an offshore gravity platform under wave loads. Responses of structured clays under cyclic loading depend on the applied cyclic stress and can be phenomenally divided into the cyclic stability, cyclic critical state and cyclic degradation. Among these three types, the cyclic stability is directly related to the long-term stability of structured soil foundations. To analyze the cyclic stability of structured soft clay, the constitutive responses under cyclic loads of different amplitudes are firstly discussed according to the shakedown theorem. Secondly, based on the modified Cam-clay model and the plastic hardening rule proposed by Li and Meissner, an elastoplastic two-surface model is herein developed to simulate the long-term responses of saturated structured soft clays under undrained cyclic loading. In the model, a special parameter for describing progressive damage of soil structure is introduced into the interpolation function of plastic modulus, which is related to the accumulated plastic deviatoric strain. The comparisons of theoretical computations with experiment data from some researchers are finally presented to demonstrate the simulation capability of this model. It is shown that the accumulated strain and the pore water pressure of soft clays under long-term undrained cyclic loading have been well simulated. Moreover, the impact of the cyclic stress ratio (CSR) on the cyclic stability is evaluated by using the empirical formula between the model parameters and the CSR. In addition, the method for calibrating the model parameters is also proved to be reasonable and feasible, thus enabling the model parameters to be determined rigorously.