Elastic-Viscoplastic Model for Clays: Development, Validation, and Application

This paper presents an elastic-viscoplastic (EVP) constitutive model in triaxial space and general stress space for isotropic clays. The EVP model is anchored in the bounding surface theory along with the mapping rule and adopts a critical state soil mechanics framework. It incorporates creep effects, and a non-linear creep function is used in the model. The EVP deformation of clay is integrated considering a reference surface and loading surface. An image parameter is deduced to establish the image surface. The strain rate tensor of the model comprises elastic-strain-rate tensor and viscoplastic-strain-rate tensor. The model formulation is capable of accounting for composite as well as single surface ellipses. Parameters of the model can be extracted from conventional oedometer and triaxial tests. The model performance is validated by capturing the behaviours in creep test, relaxation test, strain-rate effect test, and over consolidation ratio effect test of Kaolin clay, Hong Kong Marine Deposit clay, and Fukakusa clay. The model is also implemented in a Finite Element (FE) code and used to predict the long-term performance of the Nerang Broadbeach Roadway embankment constructed in Australia. The long-term settlement prediction of this embankment is also compared with that obtained with the Modified Cam Clay (MCC) model. Pertinent details of the theoretical framework of the proposed EVP model along with its validation, FE implementation and field application are discussed in this paper.

💡 Research Summary

The paper introduces a comprehensive elastic‑viscoplastic (EVP) constitutive framework tailored for isotropic clays and demonstrates its capability to predict long‑term soil behavior more accurately than traditional models. The theoretical foundation rests on bounding‑surface plasticity combined with a mapping rule that links the current stress state to a reference surface. Two distinct surfaces are defined: a reference surface that embodies the material’s ultimate shear strength and a loading surface that follows the actual loading path. By projecting the current stress point onto the reference surface through a mathematically derived image parameter, an image surface is constructed, enabling the model to handle both single‑ellipse (single‑surface) and composite‑ellipse (multiple‑surface) configurations seamlessly.

The total strain‑rate tensor is decomposed into an elastic part, governed by conventional isotropic bulk and shear moduli, and a viscoplastic part expressed as

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