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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 bounding surface theory along with the mapping rule and adopts a critical-state soil mechanics framework. It incorporates creep effects, and a nonlinear 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 contains an 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 behaviors in creep tests, relaxation tests, strain-rate effect tests, and overconsolidation ratio effect tests on 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 from 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. [ABSTRACT FROM AUTHOR]