Modelling of natural soft clay destruction processes using viscoplasticity theory

A viscoplastic constitutive model is proposed that can reproduce both strain rate dependence of natural soft clay behaviour and destruction processes (DPs) induced by strain or stress increments. The model tracks changes in the yield locus from the initial position (INYL), which reflects the presence of metastable structures, to the intrinsic position (IRYL), which represents structurally stable conditions, where DPs are almost complete. The model is able to simulate in oedometric conditions Bjerrum's 1967 framework, and makes a clear distinction between the vertical effective yield stress, σ′_vy, above which significant irrecoverable time-dependent strains are irreversibly activated, and the preconsolidation pressure, σ′_vp, from Casagrande's construction, helping to avoid possible sources of error in consolidation settlement predictions. Notwithstanding some simplifications, the model fits well the experimental results of oedometric tests on a natural soft clay with distinct σ′_vy and σ′_vp for which viscoplastic modelling becomes essential. The model, which the authors have implemented in the finite element code TOCHNOG, is presented in sufficient detail for application to back-analysis and solution of boundary value consolidation problems under loading conditions.