  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.
