Increased attention has recently been directed towards the influence of time and strain-rate effects on the behaviour of clays in one-dimensional (1-D) laboratory consolidation. The improved understanding coming from these studies must now be incorporated into improved constitutive models that can be used for analysis of foundation settlements. This paper presents a 1-D model for stepped loading using a new concept for establishing "equivalent times" during time-dependent straining. This model is then developed into a general constitutive equation for continuous loading. The model uses three parameters, λ, κ, and ψ, that can be easily found using conventional oedometer tests.The general model has been used to develop analytical solutions for creep tests, relaxation tests, constant rate of strain (CRSN) tests, and tests with constant rate of stress (CRSS). Results from three different clays have been used to examine the validity of the model. Key words: consolidation, constitutive modelling, elastic-plastic, viscous, time, creep, strain rate, relaxation.
This paper presents a new three-dimensional elastic viscoplastic (3D EVP) constitutive model for the time-dependent stressstrain behaviour of both normally consolidated and overconsolidated clays. There are two major new developments in the 3D EVP model: (i) incorporation of a nonlinear creep function with a limit for the creep volume strain under isotropic stressing conditions; (ii) adoption of a new loading surface function with a smooth shape on the π plane. Based on the "equivalent time" concept of Yin and Graham, the formulation of the new 3D EVP model is derived using the new nonlinear creep function and loading surface function. A new approach to determining all model parameters is presented. The 3D EVP model is calibrated using data from one conventional isotropic stress creep test and consolidated-undrained triaxial tests on soft Hong Kong marine deposits (HKMD). The model is verified by comparing model results with those from both normally and overconsolidated triaxial tests on HKMD and a mixture of kaolin and bentonite.Key words: creep, time-dependent, viscoplastic, equivalent time, stressstrain, constitutive model, verification, clay.
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