A Constitutive Model for Soft Clays

Vasantha Wijeyakulasuriya

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Soil Mechanics

ABSTRACT: tive Model for Soft Clays Vasantha Wijeyakulasuriya, P.Eng. Coffey Geotechnics, Toronto, Ontario, Canada ABSTRACT For deformation analyses in soft clays involving nearly monotonic loading paths, the Cambridge critical state constitutive models (CCM) have been particularly successful. These isotropic volumetric work hardening models (original Cam clay (OCC) and the modified Cam clay (MCC)) which are based on associative plasticity, invoke different empirical plastic work equations to define the shape of the respective yield curves. These models depend only on three material constants M, and . The predictive capability of these models for strain prediction varies along anisotropic compression paths and drained paths. This paper reports a model applicable for monotonic loading paths based on the simulation of an incremental stress path as being made up of an incremental undrained path and an incremental anisotropic compression path, using the same three material constants as the CCM. The shear behaviour of soft Bangkok clay is well documented in the literature. The proposed model was applied on a previously published drained stress path. Improved predictions compared to CCM were observed for the investigated test results for soft Bangkok clay. RÉSUMÉ Pour les analyses de déformation dans les argiles molles impliquant les courbes de chargement presque monotones, les modèles des états critiques ont été particulièrement réussies. Ces modèles développés au Cambridge sur écrouissage volumétrique isotrope (la Cam-Clay originale et la Cam Clay modifié) qui sont basés sur la plasticité associative invoquent équations empiriques différentes de travail de base dans la zone plastique pour définir la forme de la courbe des taux respectifs. La présente sur la simulation d'une courbe de contrainte incrémental comme étant composé d'une courbe non drainée incrémentale et une courbe de compression anisotrope incrémentale à l'aide des trois mêmes constantes matérielles comme les modèles de Cam-littérature et les valeurs des constantes matérielles sont bien établies. La modèle proposée améliorée la capacité prédictive par rapport aux modèles de Cam-Clay a été observée pour les résultats expérimentales déjà publiés pour 1 INTRODUCTION For deformation analyses in soft clays involving nearly monotonic loading paths, the Cambridge critical state constitutive models (CCM), i.e. the original Cam clay model (OCC) (Schofield and Wroth, 1968) and the modified Cam-Clay model (MCC) (Burland, 1965), have been particularly successful. These critical state models can simulate constant volume shearing at failure using an associated flow rule. These models integrate pre-consolidation, strain hardening and strain softening with the critical state phenomena in a simple manner. Historically these models can be described as the first plastic hardening models that have received wider acceptance. However, they are proven to be more successful for normally consolidated (NC) and lightly over-consolidated (LOC) regions of clay behaviour under monotonic loading conditions. Any hardening plastic model is characterised by four ingredients, namely, (a) Description of elastic behaviour (b) A yield criterion/consistency condition: definition of the current boundary in stress space to the region of elastic behaviour/ the requirement that the stress state must remain on the yield surface when plastic strains are being generated (c) A flow rule: description of the mechanism of plastic deformation (d) A hardening rule: describes the dependence of the size of the yield locus on the plastic strain. In CCM, failure is distinguished from yielding and also provide for constant volume shearing at failure through the critical state concept. In each Cam clay model, the yield locus is derived using empirical, experimentally based basic plastic work equation coupled with the use of the normality condition (associative plasticity, i.e. results in a restrictive type of material behaviour). The MCC model is widely regarded as having addressed some of the shortcomings of the OCC model, for example, with regard to the singularity of the yield surface on the isotropic axis and the over-prediction of the coefficient of earth pressure at rest, K0, of normally consolidated clays. However, both models are thermo-mechanically consistent (Puzrin, 2012). Balasubramaniam (1976) has shown that the OCC over-predicts shear strains and the MCC under-predicts strains for various stress paths, generally. This paper reports a two-staged investigation carried out with the following objectives: (a) An insight into the relative merits of the basic plastic work equations in the two CCM models (b) Based on the outcomes of stage (a), to explore the potential for a proposal for an incremental stress-strain model and to validate it against previously published data in the triaxial space.

RÉSUMÉ: stitutive Model for Soft Clays Vasantha Wijeyakulasuriya, P.Eng. Coffey Geotechnics, Toronto, Ontario, Canada ABSTRACT For deformation analyses in soft clays involving nearly monotonic loading paths, the Cambridge critical state constitutive models (CCM) have been particularly successful. These isotropic volumetric work hardening models (original Cam clay (OCC) and the modified Cam clay (MCC)) which are based on associative plasticity, invoke different empirical plastic work equations to define the shape of the respective yield curves. These models depend only on three material constants M,

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Vasantha Wijeyakulasuriya (2014) A Constitutive Model for Soft Clays in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper237,author = Vasantha Wijeyakulasuriya,title = A Constitutive Model for Soft Clays ,year = 2014}