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A model for the behaviour of overconsolidated soil

Géotechnique (Impact Factor: 1.48). 01/1978; 28(1):1-25. DOI:10.1680/geot.1978.28.1.1
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    ABSTRACT: Based on the relationship between the current yield sur-face and the reference yield surface, a new model, called the three-dimensional unified hardening model for over-consolidated clays (the UH model), is proposed in this paper. A current yield surface is used to describe over-consolidated behaviour, and a reference yield surface to describe the yield characteristics corresponding to nor-mally consolidated clays. The UH model can model many characteristics of overconsolidated clays well, including stress–strain relationships, shear dilatancy, strain-hard-ening and softening, and stress path dependence behav-iour. The key feature of the model is the adoption of a unified hardening parameter that is independent of stress paths. Based on the SMP criterion and the corresponding transformed stress method, the proposed model can be applied conveniently to three-dimensional stress states. Compared with the Cam-clay model, the UH model requires only one additional clay parameter, the slope of the Hvorslev envelope. The validity of this new model is confirmed by data from triaxial drained and undrained compression and extension tests for clays with different overconsolidation ratios, true triaxial tests with different Lode's angles, and cyclic loading tests. Sur la base des rapports existant entre la surface élas-tique actuelle et la surface élastique de référence, un nouveau modèle, désigné modèle de durcissement tridi-mensionnel unifié pour argiles consolidées par précom-pression (modèle UH, en abrégé) est proposé dans la présente communication. On fait usage d'une surface élastique actuelle pour décrire un comportement surcon-solidé et une surface élastique de référence afin d'établir les caractéristiques élastiques correspondant normale-ment aux argiles consolidées par précompression. Le modèle UH est en mesure de modéliser un grand nombre de caractéristiques des argiles consolidées par précom-pression, y compris des rapports contrainte – déforma-tion, la dilatance au cisaillement, l'écrouissage et le radoucissement, ainsi que le comportement de dépen-dance au chemin des contraintes. La principale caractér-istique de ce modèle UH est l'adoption d'un paramètre de durcissement unifié, indépendant des chemins des contraintes. Sur la base du critère SMP et de la méthode correspondante des contraintes transformées, il est possi-ble d'appliquer de façon pratique le modèle proposé pour des contraintes tridimensionnelles. Par rapport au mod-èle Cam-clay, le modèle UH ne nécessite qu'un paramètre additionnel pour l'argile, à savoir l'inclinaison de l'enve-loppe Hvorsley. La validité de ce nouveau modèle est confirmée par des données provenant de tests de com-pression et d'extension drainé et non drainés triaxiaux pour des argiles présentant un rapport de consolidation par précompression (OCR), des tests triaxiaux vrais avec différents angles de Lode, et des tests de chargement cyclique. INTRODUCTION Naturally deposited clays are often found in different over-consolidated states. Compared with normally consolidated (NC) clays, overconsolidated (OC) clays have a lower void ratio and higher strength, and exhibit more complicated behaviours, such as shear dilatancy and strain-softening. As the overconsolidation phenomenon is found in most natural clays, the mechanical behaviour of OC clays and the constitu-tive model that describes those properties are considered to be one of the most important research topics in geotechnical engineering (Wood, 1985; Wroth & Houlsby, 1985). Although the Cam-clay model (Roscoe et al., 1963; Roscoe & Burland, 1968; Schofield & Wroth, 1968) and many other models can be used to some extent to present the behaviour of OC clays, modelling of OC clays has not been very satisfactory (Prashant, 2004). Nevertheless, much research on OC clays has been carried out since the early years in the development of soil mechanics. The work of Wroth (1971), Nadarajah
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    ABSTRACT: Based on the relationships between the Hvorslev envelope, the current yield surface and the reference yield surface, a new constitutive model for overconsolidated clays is proposed. It adopts the unified hardening parameter, to which the potential failure stress ratio and the characteristic state stress ratio are introduced. The model can describe many characteristics of overconsolidated clays, including stress-strain relationships, strain hardening and softening, stress dilatancy, and stress path dependency. Compared with the Cam-clay model, the model only requires one additional soil parameter which is the slope of the Hvorslev envelope. Comparisons with data from triaxial drained compression tests for Fujinomori clay show that the proposed model can rationally describe overconsolidated properties. In addition, the model is also used to predict the stress-strain relationship in the isotropic consolidation condition and the stress paths in the undrained triaxial compression tests.
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M. J. Pender