A deviatoric softening law for predicting the deformation of soft clays subject to anisotropic consolidation

Cong Shi, Jinyuan Liu, Tareq Salloum, Laifa Cao

In the proceedings of: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Numerical Modeling

ABSTRACT: The well-known Modified Cam-Clay (MCC) family of models employ an isotropic hardening law, where the size change of yield locus is governed only by plastic volumetric strain increment or decrement. However, reviewing the incremental anisotropic consolidation tests performed on soft clays at various stress ratios reveals the presence of deviatoric softening (DS). The anisotropic consolidation tests applied with higher stress ratios tend to result in higher compressibility of the investigated clays. This indicates that the deviatoric strain can cause the yield surface to contract. To address this issue, the volumetric hardening law has been modified to incorporate soil softening owing to plastic deviatoric strain increment. This new DS law is easy to implement and requires only one additional parameter to govern the yield surface contraction. The new DS model has incorporated this DS law into the rotational anisotropic hardening law of S-CLAY1. The model has been verified through simulations of a number of incremental anisotropic consolidation tests collected from the literature. It is demonstrated that the modified softening law can provide an improved prediction of deformation of soft clays subject to the anisotropic consolidation tests under different stress ratios.

RÉSUMÉ: Les modèles bien connus de la famille MCC (Modified Cam-Clay) utilisent une loi de durcissement isotrope, selon laquelle le changement de taille du locus de rendement est régi uniquement par laugmentation ou la diminution progressive de la déformation volumétrique plastique. Cependant, l'examen des tests de consolidation anisotropes incrémentaux effectués sur des argiles molles sous différents rapports de contrainte révèle la présence d'un ramollissement déviatorique (DS). Les tests de consolidation anisotropique appliqués avec des rapports de contrainte plus élevés ont tendance à entraîner une compressibilité plus élevée des argiles étudiées. Cela indique que la déformation déviatorique provoquera la contraction de la surface de rendement. Pour remédier à ce problème, la loi de durcissement volumétrique a été modifiée pour intégrer le ramollissement des sols en raison de laugmentation de la déformation plastique du déviateur. Cette nouvelle loi dadoucissement déviatorique est facile à mettre en uvre et ne nécessite quun paramètre supplémentaire pour régir la contraction de la surface de rendement. Le nouveau modèle DS a intégré cette loi DS à la loi de durcissement anisotrope par rotation de S-CLAY1. Le modèle a été vérifié au moyen de simulations dun certain nombre de tests de consolidation anisotropes supplémentaires recueillis dans la littérature. Il est démontré que la loi de ramollissement modifiée peut fournir une meilleure prévision de la déformation des argiles molles soumises aux tests de consolidation anisotropique sous différents rapports de contrainte. 1. INTRODUCTION Hardening and softening behavior of soil refers to the yield stress increase and decrease due to accumulated plastic strains. For example, yield surface will increase as the result of the incremental plastic volumetric strain, which is usually known as volumetric hardening (Roscoe and Burland 1968). Hardening and softening can also be understood as shear strength evolution when soil is subject to shearing stress, like during triaxial tests. This behavior, linking shear strains with shear strength evolution, is known as distortional hardening or softening (Zabala and Alonso 2011, Conte et al. 2013). A pertinent example for volumetric hardening would be one-dimensional or isotropic compression tests where yield stress increases due to increased loading. The soil fails in none of the two compression tests since the stress ratios in both cases are usually lower than the failure stress ratio. Stress ratio is defined as . is the mean effective stress defined as [1] where , and are effective normal stresses at , , directions respectively. The deviatoric stress is defined to be [2]

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