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.

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Face-to-face and edge-to-edge free energy interactions of Wyoming Na-montmorillonite platelets were studied by calculating potential of mean force along their center to center reaction coordinate using explicit solvent (i.e., water) molecular dynamics and free energy perturbation methods. Using a series of configurations, the Gay-Berne potential was parametrized and used to examine the meso-scale aggregation and properties of platelets that are initially random oriented under isothermal-isobaric condition.Aggregates of clay was defined by geometrical analysis of face-to-face proximity of platelets with size distribution described by a log-normal function. The isotropy of the microstructure was assessed by computing a scalar order parameter. The number of platelets per aggregate and anisotropy of the microstructure both increases with platelet plan area. System becomes more ordered and aggregate size increases with increasing pressure until maximum ordered state. Further increase of pressure slides platelets relative to each other leading to smaller aggregate size. The geometrical arrangement of aggregates affects mechanical properties of the system. The elastic properties of the meso-scale aggregate assembly are reported. It is found that the elastic properties at this scale are close to the cubic systems. The elastic stiffness and 1 anisotropy of the assembly increases with the size of the platelets and the level of external pressure.

Smectites are an important group of clay minerals that experience swelling upon water adsorption. This paper uses molecular dynamics with the CLAYFF force field to simulate isothermal isobaric water adsorption of interlayer Wyoming Na-montmorillonite, a member of the smectite group. Nanoscale elastic properties of the clay-interlayer water system are calculated from the potential energy of the model system. The transverse isotropic symmetry of the elastic constant matrix was assessed by calculating Euclidean and Riemannian distance metrics. Simulated elastic constants of the clay mineral are compared with available results from acoustic and nanoindentation measurements.

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This Paper evaluates the performance of a generalized effective stress soil model for predicting the rate-independent behaviour of K" normally to moderately overconsolidated clays. The model formulation comprises three components: an elasto-plastic model for normally consolidated clay including anisotropic and strain softening behaviour, equations to describe the small strain non-linearity and hysteretic response in unloading and reloading, and bounding surface plasticity for irrecoverable, anisotropic and path-dependent behaviour of overconsolidated clays. Model complexity is controlled through the use of input parameters which can be obtained from a relatively small number of standard soil tests. The Paper illustrates the selection of model input parameters for a low plasticity, moderately sensitive clay and compares the selected values for different types of clay. Extensive comparisons with measured data from undrained shear tests performed in different modes of shearing and with overconsolidation ratios of up to 8, show that the model gives excellent predictions of conditions at maximum shear stress and describes accurately the non-linear shear stress-strain behaviour. However, it becomes less reliable at OCR > 4, where it tends to overestimate the shear strength in compression and underestimate that in extension. Further comparisons with test data from the directional shear cell demonstrate model capabilities for describing anisotropic properties of Boston blue clay due to a one-dimensional consolidation stress history. The overall high quality of the predictions reported has provided the basis for implementing the model in a general non-linear finite element analysis L'article étudie l'efficacité d'un modèle généralisé de sol, en contrainte effective, qui permet de prévoir le comportement d'argiles K0-normalement à modérément surconsolidées. Le modèle se décompose en trois parties: un modéle élasto-plastique décrivant le comportement d'argiles normalement consolidées, y compris pour des comportements anisotropes et radoucissement des déformations; des équations décrivant la réponse non linéaire, à hystérésis, de petites déformations lors de cycles déchargement-chargement; plasticité des surfaces de liaison décrivant le comportement d'argiles surconsolidées. Ce comportement non réversible et anisotrope est fonction du chemin des contraintes. La complexité du modèle est contrôlée á l'aide de paramétres d'entrée qui peuvent être obtenus á partir d'un nombre relativement réduit d'essais standards. L'article décrit la sélection de ces paramétres d'entrée pour une argile faiblement plastique et modérément sensible. Il compare également les valeurs sélectionnées pour différents types d'argiles. Des comparaisons extensives ont été réalisées avec des données mesurées au cours d'essais de cisaillement non drainés, pour différents modes de cisaillement et pour des ratios de surconsolidation pouvant atteindre une valeur de 8. Elles montrent que le modèle prédit trés bien les conditions á contrainte de cisaillement maximale et qu'il décrit précisément le comportement contrainte-déformation, non linéaire, en cisaillement. Il devient cependant moins fiable pour un OCR 4, et pour lequel la résistance au cisaillement tend á être surestimée en compression et sous-estimée en extension. D'autres comparaisons avec les données expérimentales issues de la cellule à cisaillement directionnel, montrent que le modéle permet de décrire les propriétés anisotropes, liées à un historique des contraintes de consolidation unidimensionnel, de l'argile bleue de Boston. La trés bonne qualité des prédictions permet d'envisager la transformation de ce modéle en un algorithme plus généra], non linéaire, d'éléments finis.

SUMMARYThe undrained response of cohesive soils is of paramount importance in geomechanics and it has been modelled extensively for the last 50 years. In comparison, drained behaviour of clays has received only modest attention. Drained and undrained behaviour is significantly affected by past consolidation stress history. This paper evaluates the capabilities of the MIT-S1 effective stress model, described in a companion paper, for predicting the anisotropic stress-strain-strength behaviour of clays. The paper illustrates the selection of model parameters for Lower Cromer Till, using data from standard types of laboratory tests. Comparison of model simulations with measured response for Lower Cromer Till and Boston Blue Clay illustrate model capabilities. The work focuses initially on comparisons of model predictions with measurements from undrained triaxial and plane strain tests on initially K 0 -consolidated specimens. Comparisons with measured data from undrained shear tests performed in different modes of shearing for LCT and BBC show that the model: (a) gives excellent predictions of maximum shear stress conditions and accurately describes the non-linear shear stress-strain behaviour; (b) accurately describes the anisotropic shear stress-strain-strength conditions for different consolidation stress histories; and (c) gives realistic description of mobilized friction angles, especially at large OCR's. The paper then focuses on the effects of consolidation stress history for isotropically consolidated specimens of resedimented Lower Cromer Till and Boston Blue Clay. Finally, the paper compares model predictions for drained shear tests on K 0 and isotropically consolidated specimens with overconsolidation ratios, OCR410; used to evaluate particular aspects of the critical state framework of soil behaviour. Overall, the model gives excellent predictions of the effect of initial anisotropy and overconsolidation stress history on the shear stress-strain and volumetric behaviour of clays.