A simple four-parameter elasto-plastic model describes the non-linear volumetric behaviour of freshly deposited cohesionless soils in hydrostatic and one-dimensional compression. It expresses the tangent bulk modulus as a separable function of the current void ratio and mean effective stress using natural strains. Specimens compressed from different initial formation densities approach a unique response at high stress levels—the limiting compression curve (LCC)—which is linear in a double logarithmic void ratio-effective stress space. The model describes irrecoverable, plastic strains which develop throughout first loading and represent mechanisms ranging from particle sliding and rolling at low stresses to crushing—the principal component of deformation for LCC states. The three input parameters describing plastic deformation can be readily estimated from a hydrostatic or one-dimensional compression test loaded to high stress levels; the elastic bulk modulus requires accurate small strain measurements in unloading. The model can be further simplified at low stress levels where compressive behaviour is controlled primarily by the formation density. Input parameters for a wide range of cohesionless soils are presented from which it is possible to infer the effects of particle mineralogy, size, grading and shape on compressibility. The model gives excellent predictions of measured compressive behaviour over a wide range of stresses and densities, and provides a useful basis for the construction of hardening rules for generalized constitutive models. KEYWORDS: compressibility; constitutive relations; plasticity; sands. Un modèle élastoplastique simple à quatre paramétres permet de décrire le comportement volumique non-linéaire de sols fraîchement déposés, sous des conditions de compression hydrostatique ou unidimensionelle. Il exprime, à partir des déformations naturelles, que le module matriciel tangent est fonction de l'indice des vides courant et de la contrainte effective moyenne. Les échantillons, comprimés avec différentes valeurs initiales de den-sité, tendent vers une même réponse aux fortes contraintes—la `limiting compression curve' (LCC) —réprésentée par une droite dans le diagramme biologarithmique indice des vides-contrainte effective'. Le modéle décrit les déformations plastiques irréversibles qui se développent tout au long de la premiére mise en charge, et les mécanismes présents, depuis le glissement et la rotation des particules, sous faibles contraintes, jusqu'au broyage (mécanisme principal de déformation aux états LCC). Les trois paramétres d'entrée décrivant la déformation plastique peuvent 7eacute;tre estimés à partir d'essais de compression hydrostatiques ou unidimensionels réalisés à fortes contraintes. La determination du module élastique demande une mesure trés précise des petites déformations lors du déchargement. Ce modéle peut étre simplifié pour les faibles contraintes pour lesquelles le comportement en compression est essentiellement contrôlé par la masse volumique de la formation. L'article présente, pour une large gamme de sols pulvérulents, plusieurs paramétres d'entrée permettant d'étudier l'influence de la minéralogie, de la forme et de la granulométrie des particules sur la compressibilité. Le modéle présenté permet de prévoir avec grande précision le comportement en compression mesuré pour une large gamme de contraintes et de densités. II fournit également une base indispensable à l'établissement de lois de durcissment pour des modeles constitutifs généralisés.
This paper presents a new generalized e!ective stress model, referred to as MIT-S1, which is capable of predicting the rate independent, e!ective stress}strain}strength behaviour of uncemented soils over a wide range of con"ning pressures and densities. Freshly deposited sand specimens compressed from di!erent initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), which is linear in a double logarithmic void ratio, e, mean e!ective stress space, p. The model describes irrecoverable, plastic strains which develop throughout "rst loading using a simple four-parameter elasto-plastic model. The shear sti!ness and strength properties of sands in the LCC regime can be normalized by the e!ective con"ning pressure and hence can be uni"ed qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (VCL). At lower con"ning pressures, the model characterizes the e!ects of formation density and fabric on the shear behaviour of sands through a number of key features: (a) void ratio is treated as a separate state variable in the incrementally linearized elasto-plastic formulation: (b) kinematic hardening describing the evolution of anisotropic stress}strain properties: (c) an aperture hardening function controls dilation as a function of &formation density'; and (d) the use of a single lemniscate-shaped yield surface with non-associated #ow. These features enable the model to describe characteristic transitions from dilative to contractive shear response of sands as the con"ning pressure increases. This paper summarizes the procedures used to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capability for describing the shear behaviour of clays and sands.
Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soil's relative density and thickness, and the building's weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.
The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. Centrifuge experiments were performed to study the dominant mechanisms of seismically induced settlement of buildings with rigid mat foundations on thin deposits of liquefiable sand. The relative importance of key settlement mechanisms was evaluated by using mitigation techniques to minimize some of their respective contributions. The relative importance of settlement mechanisms was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which may be represented by the time rate of Arias intensity ͑i.e., the shaking intensity rate͒. In these experiments, installation of an independent, in-ground, perimetrical, stiff structural wall minimized deviatoric soil deformations under the building and reduced total building settlements by approximately 50%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear deformation also reduced permanent building settlements but less significantly.
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