Anisotropic behavior of marine clay subjected to monotonic loading

P. Balaji, S. Banerjee

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Soil and Rock Mechanics I

ABSTRACT: It is now well-understood that the isotropically consolidated clay samples do not represent true in-situ stress-strain behavior. Basically, in-situ stress condition is anisotropic in nature with same stress in radial direction and different stress in vertical direction. Here, in the present study a series of triaixial test was conducted on normally consolidated clay to study the effect of stress induced anisotropy on the response of the marine clay subjected to monotonic loading. The studies were conducted for different stress ratios. Experimental results showed that the degree of anisotropy had significant effect on shear strength and effective stress path of the soil. Based on the experimental observations, a constitutive model was developed. Finally the results computed from the proposed model were compared with test results for normally consolidated, lightly overconsolidated and heavily overconsolidated clays. For validation of highly overconsolidated clay data from published literature were used.

RÉSUMÉ: Il est maintenant bien compris que les échantillons d'argile isotrope consolidés ne représentent pas vrai comportement contrainte-déformation in situ. Fondamentalement, l'état de stress in-situ est anisotrope dans la nature avec la même contrainte dans la direction radiale et le stress différent dans le sens vertical. Ici, dans la présente étude une série de tests a été menée sur triaixial argile normalement consolidée pour étudier l'effet du stress anisotropie induite sur la réponse de l'argile marine soumise à un chargement monotone. Les études ont été menées pour différents rapports de stress. Les résultats expérimentaux ont montré que le degré d'anisotropie a eu un effet significatif sur la résistance au cisaillement et le chemin de contrainte effective du sol. Sur la base des observations expérimentales, un modèle constitutif a été développé. Enfin, les résultats calculés à partir du modèle proposé ont été comparés avec les résultats des tests pour les argiles normalement consolidées, légèrement surconsolidés et surconsolidés. Pour la validation des données d'argile très surconsolidés de littérature ont été utilisés. 1 INTRODUCTION For a last few decades, it has become inevitable for civil engineers to do construction on soft clay deposits. As a consequence, soft soil deposits are increasingly used in construction and it has therefore become indispensable for engineers to study the behavior of soft soil for better and safe design. The soft clays are mainly characterized by its low strength and designing of structures against its ultimate limit stress becomes a difficult task. In addition, soft soil undergoes large deformation and application of relatively small load produces large deformation. For safe and economic design, behavior of soft clay under static load should be studied thoroughly. Though the natural stress state of soil is anisotropic, it is often assumed as isotropic. The difficulties in simulating anisotropic stress condition and longer duration for anisotropic consolidation made the earlier researchers to focus only on isotropic stress condition. Some of the soil models such as linear elastic and plastic with mohr-coulomb failure criteria and cam-clay model helps the engineers to predict the stress-strain behavior, which is based on isotropic stress condition. But isotropic stress state does not represent a true in-situ stress-strain behavior. Constitutive model for clays has developed rapidly since then development of critical state elasto-plastic models. Many newer models have attempted to incorporate natural anisotropic behavior and other aspects, but practicing engineers are facing difficulty to use it. The paper is focused on developing a constitutive model for normally consolidated and highly overconsolidated clays under anisotropic stress condition. A model is developed by adopting non-associated flow rule with hvorslev surface as yield surface and Sclay1 as plastic potential to formulate the behavior of overconsolidated clay. 2 BACKGROUND It is well known that the natural deposition of soil is anisotropic, provided with same stress in radial direction and higher stress in axial direction. The effect of the anisotropy on undrained shear strength was first carried out by Hansen and Gibson (1949) and followed by few studies were carried out by other researchers. (Henkel & sowa 1963,Bjerrum 1973, Vaid & Campanella 1974, Donaghe & Townsend 1978, Nakase & Kamei 1983).In general, anisotropy is classified as inherent anisotropy and induced anisotropy. Inherent anisotropy is related with soil structure and it arises due to preferred orientation of soil particle formed during Ko consolidation. This inherent anisotropy is predominant in sandy deposit and heavily overconsolidated clays. Mitachi and Kitago (1976) reported the effect of anisotropic soil skeleton on stress-strain behavior of cohesive soil. The stress induced anisotropy is created by initial anisotropic stress state. This initial anisotropic stress state can be achieved by increasing both confining and axial stress (Henkel & sowa, Donaghe & Townsend and Nakase & Kamei). Henkel & sowa

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