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A model for the behaviour of overconsolidated soil

Géotechnique (Impact Factor: 1.67). 01/1978; 28(1):1-25. DOI: 10.1680/geot.1978.28.1.1
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    ABSTRACT: The features of a model for the cyclic stress-strain behaviour of cohesive soil are explained and the application of the model to estimating the dynamic compliance of footings under vertical cyclic loading is illustrated. The dynamic calculations were done with the FLAC (Fast Lagrangian Analysis of Continua) software. The model is formulated within the framework of critical state soil mechanics. However, unlike the classical critical state models, Cam clay and modified Cam clay, this model has inelastic behaviour for overconsolidated soil. The particular version of the model implemented herein is for a lightly overconsolidated soil, which deforms in an undrained manner at constant mean principal effective stress. With the addition of small-strain elastic shear behaviour the model is shown to represent well the degradation, with shear strain amplitude, in the apparent shear modulus and equivalent viscous damping ratio. The calculations herein indicate that there are two distinct aspects of shallow foundation response to cyclic loading – cyclic deformation and the accumulation of permanent settlement; the permanent settlement was found to be the more significant of the two. It was also noted that the dynamic compliance of the foundation on nonlinear soil exhibited greater variation with cyclic loading frequency than the same foundation on an elastic soil.
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    ABSTRACT: Most clays, either naturally deposited or man-made, possess a certain degree of overconsolidation owing to tamping, cyclic loading, erosion, excavation, and/or changes in groundwater tables. An easy-to-use constitutive model for overconsolidated clays is useful for relevant engineering applications. In this paper, a simple model is proposed for overconsolidated clays based on the unified-hardening (UH) model. To evaluate the potential peak stress ratio of overconsolidated clays, a parabolic Hvorslev envelope rather than a straight envelope (used in the original UH model) is adopted. The proposed parabolic Hvorslev envelope passes through the origin of the mean stress-deviatoric stress plane. It has a slope of 3 as the overconsolidation ratio (OCR) approaches infinity and intersects with the critical state line as the OCR reaches unity. This modification leads to more realistic predictions for highly overconsolidated clays than does the original UH model with a straight Hvorslev envelope and is consistent with the critical state soil mechanics in which the higher peak stress ratio in overconsolidated clays is a result of interlocking (or dilatancy) rather than cohesion. The modified UH model retains the same parameters as those in the modified Cam-clay model. Reasonable agreement between the model predictions and experimental data demonstrates that the modified model is capable of addressing the fundamental behavior of overconsolidated clays. The present model is developed for reconstituted clays with an isotropic fabric. The potential improvement of the model, taking into account anisotropy and structural effects, is discussed.
    Journal of Geotechnical and Geoenvironmental Engineering 07/2012; 138(7):860-868. DOI:10.1061/(ASCE)GT.1943-5606.0000649 · 1.47 Impact Factor