Les modèles de comportement phénoménologiques développés pour les sols peinent à traduire la complexité du comportement des matériaux granulaires, essentiellement du fait de leur nature discrète. Il est usuel d’avoir recours aux analyses multi-échelle, afin de proposer des comportements macroscopiques prenant finement en compte les propriétés locales du milieu. La question de l’échelle locale a été largement étudiée et il ressort qu’une méso-échelle, définie au niveau d’arrangements locaux de particules, semble plus appropriée que l’échelle du contact entre particules pour comprendre la texture et le comportement de ce type de milieux. Dans ce cadre, ce travail de thèse se propose d’étudier la faisabilité de la définition d’une loi de comportement pour les matériaux granulaires par changement d’échelle mésoscopique-macroscopique. Un échantillon granulaire est analysé par simulation DEM (Méthode aux Eléments Discrets) d’un essai biaxial de charge et de décharge. A l’échelle mésoscopique, le milieu granulaire est subdivisé en méso-domaines dont la texture est caractérisée par deux variables : degré d’allongement et orientation par rapport à la direction de compression. Les méso-domaines ayant des caractéristiques de texture communes sont regroupés en six phases, afin de mieux discriminer leur comportement. Les variables locales sont définies : méso-texture, méso-contrainte et méso-déformation, et leurs évolutions sont analysées. Les simulations montrent que le matériau s’appuie sur les phases orientées dans la direction de compression, d’autant plus qu’elles sont allongées, pour supporter la sollicitation. Ces dernières sont capables de développer une forte dilatance et une forte anisotropie, leur permettant de récupérer une forte valeur de rapport de contraintes. A contrario les phases allongées et perpendiculaires à la direction de compression constituent des maillons faibles pour le système. Sur la base des résultats DEM, un processus de changement d’échelles -dont l’ingrédient est l’évolution des pourcentages volumiques des phases au cours de l’essai- a permis de retrouver le comportement macroscopique. Un modèle élasto-plastique à élasticité linéaire, mécanisme plastique déviatoire et écrouissage cinématique a été proposé pour modéliser le comportement de chaque phase à l’échelle mésoscopique. L’essai de charge a permis d’identifier les 8 paramètres du modèle. L’essai de décharge a permis la validation du modèle proposé. Similairement, un modèle à un paramètre est proposé, et validé, pour l’évolution du pourcentage volumique de chaque phase. Sur la base de ces modélisations, le comportement macroscopique de l’échantillon a pu être retrouvé par changement d’échelle mésoscopique-macroscopique. Cette étude ouvre la voie à la définition d’un nouveau type de modèle de comportement des matériaux granulaires basé sur l’existence de phases à l’échelle mésoscopique. / Most of the developed constitutive models for granular materials cannot capture the complexity of their behavior, due to the discrete nature of the material. The use of a multi-scale approach may help to address this issue by taking account local properties in a more precise way. It seems that the so-called meso-scale defined at the level of a cluster of particles is the relevant scale to have a better insight into the influence of the texture on the behavior of the material. In this framework, the work involved herein studies the feasibility of the definition of a constitutive law for granular materials by means of a meso-macro change of scale. A sample made of particles has been analysed using the Discrete Element Method (DEM). At the mesoscopic scale, the material is divided into meso-domains characterized by two variables: their elongation ratio and their orientation with respect to the compression direction. The meso-domains who share the same characteristics of texture define a phase. Then, six phases were defined. Some local variables such as the meso-texture, the meso-stress and the meso-strain were defined and their evolution analyzed. The simulations show that the material relies all the more on the phases oriented in the direction of compression to bear the loading as the phase holds very elongated domains. Indeed, these latter are able to develop a strong dilatancy and a strong anisotropy to withstand high stress ratios. Conversely, the phases oriented perpendicular to the direction of compression play a minor role in the global behavior of the material. On the basis of the DEM results, a change of scale process (whose key element is the volumetric percentage of a phase in the material) allowed to retrieve the global behavior of the material. An elastic-plastic model with a linear elasticity, a plastic deviatoric mechanism with a kinematic hardening was proposed to model the behavior of each phase at the mesoscopic scale. The eight parameters of the model are identified based on numerical results obtained for a loading path. The unloading path of the numerical test allowed to validate the proposed constitutive model. Similarly, a model is proposed and validated for the evolution of the volumetric percentage of each phase at the mesoscopic scale. Based on these models, the macroscopic behavior of the sample could have been derived with a mesoscopic-macroscopic change of scale technique. This study paves the way to the definition of a new class of constitutive models for granular materials, based on the existence of phases at the mesoscopic scale.

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D.Ing. / Unbound granular material has and is still being used with great success in the construction of road pavements in South Africa and many other countries around the world. Often this material is used in the main structural layers of the pavement with very little protection provided against high traffic induced stresses by way of a surface treatment or thin asphalt concrete layer. The performance of unbound granular pavement layers depend mainly on the level of densification and degree of saturation of the material in addition to the stress levels to which the layers are subjected. The main form of distress of unbound granular layers is the permanent deformation of the layer, either through the gradual deformation or rapid shear failure of the layer. Design engineers need accurate and appropriate design procedures to safeguard the road against such rapid shear failure and to ensure that the road has sufficient structural capacity to support the traffic loading over the structural design period. The recent trend in pavement design has been to move away from empirical design methods towards rational mechanistic-empirical design methods that attempt to relate cause and effect. Although a mechanistic-empirical pavement design method has been available in South Africa since the midseventies, increasing criticism has been levelled against the method recently. The models for characterising the resilient response and shear strength and estimating the structural capacity of unbound material have been of particular concern. The purpose of the research reported in this thesis was therefore to develop an improved mechanistic-empirical design model, reflecting the characteristics and behaviour of unbound granular material. The new design model consists of three components namely a resilient modulus, yield strength and plastic deformation damage model with each model including the effects of the density and moisture content of the material unbound granular where appropriate. The models were calibrated for a range of unbound materials from fine-grained sand and calcrete mixture to commercial crushed stone products using the results from static and dynamic tri-axial tests. An approximation of the suction pressure of partially saturated unbound material was introduced in the yield strength model and was validated with independent matric suction measurements on the sand and calcrete mixture. The yield strength model which is a function of the density and moisture conditions as well as the confinement pressure was calibrated for the individual materials with a high accuracy. A single plastic strain damage model was calibrated for the combined plastic strain data from all the crushed stone materials but a single model could not be calibrated for the plastic strain data of the natural gravels as these materials vary too much in terms of particle size distribution and the properties of the fines found in these materials. The formulation of the plastic strain damage model includes the density and degree of saturation of the material. A single resilient modulus model was calibrated for the combined resilient modulus data from all the materials excluding the data from a limited number of tests during which large plastic strain occurred. The resilient modulus model again ii incorporates the density, degree of saturation and the stress dependency of unbound granular material and is on an effective stress formulation for the bulk stress. Finally, the yield strength, resilient modulus and plastic strain damage models are combined in a mechanistic-empirical design model for partially saturated unbound granular material. Results from the proposed design method seem more realistic than results from the current design model and the model is not as sensitive to variation in the design inputs as the current design model is. In addition to this, the effects of the density and moisture content of the partially saturated, unbound granular material on the resilient response and performance of the material is explicitly included in the formulation of the proposed design model.

Thesis (Ph.D)--Civil and Environmental Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Santamarina, J. Carlos; Committee Member: Bachus, Robert C.; Committee Member: Burns, Susan E.; Committee Member: Cartwright, Joseph A.; Committee Member: Goldsztein, Guillermo; Committee Member: Mayne, Paul W. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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This work examines the fundamental behaviour of granular materials in packed/moving beds under simplified blast furnace conditions. Such study has a significant impact on the development of new technology such as pulverized coal injection and the performance of blast furnace operation. Experiments have shown that a number of interesting phenomena appear in blast furnace operation. These phenomena involve rich granular dynamics which currently attract strong interest from a wide scientific and engineering. However, previous work on this area, limited by the research techniques, is predominantly at large scales focusing on phenomenological descriptions, but rarely touching on the basic fundamentals governing these phenomena. A novel discrete element simulation at an individual particle level can overcome these problems. For this purpose, this work conducts a systematic study of these important phenomena, including crater formation, coke collapse, creep motion and particle percolation, by use of the discrete element method (DEM). The experiments and simulations conducted in the impact of a particle stream onto a particle bed using a 20 slot model suggest that the DEM can reproduce the experimental results well under comparative conditions. The crater size is shown to be affected by the discharging rate, discharging height and materials properties, and is related to the ratio of the input energy from the falling stream to the inertial energy from the original packing. Fundamental understanding of coke collapse based on three different configurations: batch charging, self loading and load impact have been investigated. It was found that coke collapse is a kind of continuous avalanche due to top layer particles spreading. Apparent frozen layer under rapidly flowing layer is not stationary and slowly creep motion can be detected at an arbitrary depth. The mean velocity of creep motion decays exponentially with depth. Percolation happens due to both gravity and strain. The percolation velocity under gravity is much greater than that under shear. Size ratio effect is most significant. For size ratio smaller than threshold gravity induced percolation dominate otherwise shear due to the descending of the packed bed. Additionally, this work demonstrates the value of DEM as a tool for complementing experimental observations.

Thesis (Ph. D.)--University of California, San Diego, 2000. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--Lehigh University, 2000. / Source: Dissertation Abstracts International, Volume: 60-12, Section: B, page: 6187. Adviser: Yong W. Kim.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 197-212). Also available in electronic version.