Rozprawy doktorskie na temat „Experimental Granular Mechanics”

Granular-fluid mixtures flowing down an incline in the grain-inertia regime were studied experimentally and analytically. The equations of motion are based on the kinetic theory for granular flow. The boundary conditions are formulated following two methods. The first is a simple mechanical approach based on the energy exchange at the wall. The other is based on the kinetic theory for granular flow. Solutions for dry granular flow down a chute are studied in order to compare both methods. The extension of the kinetic theory presented here, includes drag forces resulting from the interstitial fluid that cushions interparticle collisions and particle-wall collisions. Frictional stresses, produced when long term contacts are present, and fluid turbulent fluctuations are introduced in the model.
The results are compared with measurements from an experimental chute in which the inclination, the solids flow rate and fluid flow rate are all varied. The theory is found to give a good qualitative account of the observed behaviour. Finally an application of the model to the description of the mechanical behaviour of the Nevado del Ruiz 1985 debris flow is attempted.

Autour de nous, les systèmes granulaires sont omniprésents et rarement dissociés d'une phase liquide ou gazeuse. L'interaction entre les deux phases (solide et fluide) implique une dynamique d'écoulement complexe de ces systèmes couplés. Cette thèse expérimentale a pour but de caractériser la dynamique d'effondrement gravitaire instationnaire d'une colonne fluide-grains dans différentes configurations de mélange diphasique. Considérant uniquement l'effet des grains au sein du fluide, la dynamique d'écoulement d'une suspension isodense dans l'air est d'abord abordée avec une description macroscopique. En particulier, La rhéologie du fluide apparent est extraite à partir de l'évolution temporelle du front en utilisant des solutions auto-similaires, modèles de la dynamique de propagation aux temps longs. Sur la gamme du nombre de Reynolds étudiée, la dynamique d'écoulement de la suspension est décrite comme celle d'un fluide apparent Newtonien ou non-Newtonien (rhéofluidifiant, rhéoépaississant, viscoplastique) dépendant des paramètres considérés (fraction volumique, viscosité du fluide interstitiel, diamètre des grains, protocole de préparation). Afin de décrire la configuration opposée d'un massif sédimentaire pour lequel les interactions entre particules solides deviennent dominantes, la configuration d'une colonne granulaire dense saturée en fluide, i.e. où les grains sont plus lourds que le fluide porteur, est ensuite étudiée. Un travail préliminaire est consacré à la caractérisation de l'effondrement d'une colonne granulaire totalement immergée dans un fluide. Le rôle dissipatif de ce dernier sur le milieu granulaire est mis en évidence par une analyse de la dynamique d'effondrement et des caractéristiques du dépôt final. Cette caractérisation permet de classifier les régimes d'effondrement en fonction de la viscosité et de la masse volumique du fluide environnant, i.e. du nombre de Stokes et du rapport de masse volumique entre le fluide et les grains. Dans le cas triphasique, i.e. lorsque la colonne fluide-grains s'effondre dans l'air, la dynamique peut être fortement affectée par les effets capillaires à travers le nombre de Bond qui contrôle alors la mise en mouvement de la colonne initiale. Quand ces effets deviennent négligeables à l'échelle de la colonne et à celle du grain, le fluide interstitiel peut jouer un rôle moteur ou dissipatif vis-à-vis dumilieu granulaire conduisant à une longueur d'étalement plus ou moins importante en comparaison au cas sec. Le rôle du fluide interstitiel dépend essentiellement de sa viscosité modifiant, par la même occasion, la dynamique d'effondrement. Enfin, une étude préliminaire sur la dynamique d'écoulement d'une suspension non-isodense, initialement homogène, est réalisée. Cette configuration, à la transition des situations décrites précédemment, permet d'aborder le couplage de la dynamique de sédimentation des grains et celle du courant. En particulier, nous observons que la vitesse de sédimentation décroît avec l'augmentation de la fraction volumiqueinitiale en particules
On the Earth's surface, granular medias are ubiquitous and they are rarely dissociated from a liquid or a gas. The fluid-solid interaction leads to a complex flow dynamics of these coupled systems. This experimental work aims at characterizing the dynamics of the unsteady gravitydrivencollapse of a granular-fluid column within different configurations of the diphasic mixture. First, the flow dynamics of a neutrally buoyant suspension in air are characterized based on a macroscopic description of the flow. In particular, the rheological parameters of the apparent fluid have been extracted using the temporal evolution of the propagating front and self-similar solutions, models of the propagating dynamics at long times. In the considered range of the Reynolds number, the flow dynamics are described as an apparent Newtonian or non-Newtonian (shear-thinning/-thickening, viscoplastic) fluid depending on the various parameters (volume fraction, viscosity of the interstitial fluid, particle diameter, mixing protocol). In order to describe the opposite case of a sedimentary environment where particle-particle interaction becomes dominant, a second part of this work investigates the case of a fluid-saturated granular collapse, i.e. for which particles are heavier than the carrier fluid, in a dense packing configuration. For this purpose, a first part of the study is dedicated to characterize the collapse of an immersed granular column. The dissipative role of the fluid on the granular media is highlighted by an analysis of the collapse dynamics and the characteristics of the final deposit. This characterization allows to classify the regimes of the collapse depending on the viscosity and the density of the surrounding fluid, i.e. the Stokes number and the fluid-grain density ratio. In the triphasic case, i.e. when the granular-fluid column collapses in air, the dynamics may be strongly affected by capillary effects through the Bond number which controls the initial dynamics of the column. When these effects can be neglected (large Bond number) at the column and grain scales, the interstitial fluid can have a driven or a dissipative role on the granular media leading to a runout length more or less extended in comparison to the dry case. The role of the interstitial fluid depends mainly on its viscosity which also modifies the collapse dynamics. Finally, a preliminary study is realized on the flow dynamics of an initially homogeneous negative buoyant suspension column. This case, which makes the transition between the above mentioned configurations, allows to study the coupling between the settling dynamics of particles and this of the current. In particular, we observe that the settling velocity decreases with the increase of the initial volume fraction of particles

Des événements géologiques passés ont montré que les glissements de terrain, près des côtes, impliquant des volumes de quelques milliers de mètre cube à plusieurs centaines de kilomètres cube, peuvent provoquer des vagues de tsunami d’une amplitude considérable. La vague générée et l’effondrement représentent tous deux un danger important pour la population et les infrastructures situées sur ou proche des côtes. Une modélisation réaliste nécessite de tenir compte de la nature granulaire du glissement de terrain. Nous avons développé dans ce travail de thèse, une série d’expériences de laboratoire à petite échelle, pour étudier en détail le processus de génération de vague par l’effondrement d’une colonne granulaire, initialement sèche, dans l’eau. Tout un ensemble de paramètres est testé : (1) la masse granulaire impliquée (hauteur, volume, rapport d’aspect, granulométrie et densité), (2) la hauteur d’eau et (3) la configuration géométrique (plan horizontal ou plan incliné). Des expériences quasi-bidimensionnelle en canal rectangulaire, permettent d’enregistrer à la fois l’évolution temporelle de l’effondrement granulaire et celle de la surface libre de l’eau. Nous montrons que le processus de génération des vagues est piloté par la dynamique collective de l’effondrement des grains à la surface de l’eau. Nous identifions une dépendance linéaire claire entre l’amplitude relative de la vague principale et un nombre de Froude défini comme le rapport des vitesses d’avancée du front granulaire et de la vague solitaire. En particulier, l’amplitude de la vague atteint sa valeur maximale pour une profondeur d’eau intermédiaire. Le transfert d’énergie global a montré que seulement quelques pourcents de l’énergie potentielle initiale de la colonne sont transférés à la vague, issus notamment d’une perte d’énergie considérable dans l’effondrement granulaire lui-même. Enfin, nous soulignons la faible influence du diamètre et de la masse volumique des grains dans la génération de la vague. Cela suggère que la masse de l’effondrement est de faible importance par rapport à son volume. Un autre résultat intéressant est la dépendance linéaire de l’amplitude relative de la vague avec le volume immergé du dépôt final. Cette loi nous permet d’estimer l’amplitude de la vague pour des événements passés et potentiels. Malgré les échelles, géométries diverses et variées de ces événements, et l’incertitude des données, cette loi empirique provenant de notre expérience à petite échelle prédit des vagues similaires à d’autres modèles numériques ou expérimentaux
Various past geological events have shown that landslides near coastlines, involving volumes from a few thousand cubic meters to several cubic kilometers, can lead to tsunami waves with significant amplitude. The generated wave and the collapse both represent an important hazard for the population and infrastructure located on or near the coast. Realistic modeling requires considering the granular nature of landslides. Here, we developed a new set of small-scale laboratory experiments to investigate in detail the wave generated by the collapse of an aerial granular column into water. An entire set of parameters are tested: (1) the falling granular mass (height, volume, aspect ratio, grain size and density), (2) the water layer height and (3) the geometrical configuration (horizontal or inclined plane). From quasi-bidimensional experiments in a rectangular channel we record both the time evolution of the granular collapse and of the generated wave. We show that the wave generation process is driven by the collective dynamics of the granular collapse at the water free surface. We identify a clear linear dependence between the relative wave amplitude and a Froude number defined as the ratio of the granular front velocity and the solitary wave velocity. The wave amplitude reaches its maximum value at an intermediate water depth. The total energy transfer shows that only a few percent of the initial potential energy of the column is transferred to the wave, suggesting a considerable energy loss in the granular collapse itself. Finally, we highlight the low influence diameter and density of the falling grain in the generation of the wave. This suggests that the mass of the collapse is of low importance compared to its volume. Another interesting result is the linear dependence of the relative wave amplitude with the relative immersed volume of the final deposit. This allows us to estimate the wave amplitude generated by past or potential events in Nature. Despite the various scales and geometries of these natural events, and the uncertainty of the data, our empirical law, from our small-scale experiment, predicts waves similar to other numerical or experimental models

L'écoulement d'un fluide au-dessus d'un lit granulaire peut provoquer son érosion lorsque la contrainte exercée dépasse une valeur seuil. La présence d'un obstacle immergé dans l'écoulement induit une perturbation, entraînant une survitesse locale du fluide et une plus grande contrainte. L'érosion est alors localement renforcée, phénomène également appelé affouillement. Dans ce travail, nous étudions expérimentalement cette situation d'érosion complexe à l'échelle du laboratoire. Nous nous intéressons en particulier à la question du seuil d'affouillement, c'est-à-dire la vitesse d'approche du fluide minimale nécessaire à l'apparition de l'érosion au voisinage de l'obstacle. Ce faisant, nous observons que deux motifs d'érosion différents peuvent se développer au voisinage d'un obstacle : si le classique affouillement en fer à cheval au pied de l’obstacle domine aux vitesses d'écoulement élevées, on observe pour des vitesses plus faibles un nouveau motif en aval, que nous appelons affouillement en oreilles de lapin. Les seuils d'apparition de ces deux motifs d'érosion sont mesurés visuellement pour différents grains et obstacles. En complément de ces observations, nous utilisons une technique de relevé topographique par profilométrie laser. Le suivi temporel de la topographie du lit au cours du processus d'affouillement nous permet de caractériser la morphologie des deux types de motifs, et de comprendre la compétition entre leurs dynamiques d'apparition par la mesure des temps caractéristiques de formation. L'affouillement en oreilles de lapin est dans la plupart des cas perturbé dans son développement par la formation plus rapide de l'affouillement en fer à cheval. L'étude de l'écoulement, s'appuyant notamment sur des mesures de vélocimétrie par image de particules (PIV), nous renseigne sur les contraintes exercées par le fluide sur les grains, avec ou sans obstacle. Nous pouvons ainsi expliquer les valeurs de seuil d'apparition mesurées pour les deux motifs d'affouillement, qui sont associés à différentes structures de l'écoulement
Erosion occurs when a fluid flowing over a granular bed exerts a large enough shear stress. When the fluid encounters an obstacle, the modification of the flow leads to a local over speed, and thus on an increase of the shear stress in the vicinity of the obstacle. As a result, the erosion is locally enhanced and is called scouring. In this work, we investigate this complicated situation experimentally at the laboratory scale. In particular, we address the question of the scouring threshold, i.e., the minimum critical approach velocity of the fluid leading to erosion in the vicinity of the obstacle. We report the existence of two different scouring patterns: the traditional horseshoe scour at the base of the obstacle, which dominates at large flow velocities, and we also highlight another scouring pattern downstream, which is called rabbit ear scour, at moderate speeds. We determine the onset of both erosion patterns visually using different grains and obstacles. Besides, we measure the bed topography over time using a laser profilometer. By monitoring the bed topography during the scouring process, we characterize the morphology of both scouring patterns and rationalize their competitive dynamics by measuring their formation timescale. In most cases, the rabbit ear scour development is inhibited by the faster horseshoe scour growth. The characterization of the flow using Particles Image Velocimetry (PIV), provides information on the shear stress exerted by the fluid flow on the grains, with and without obstacle. We can thus rationalize the threshold values for both scouring patterns, associated with different flow structures

Inadequate knowledge of the behaviour of wet granular materials such as unsaturated soils or wet and sticky industrial bulk solids formed the basis of the current research project in which selected granular materials were experimentally characterised. The main objective was to investigate the hydro-mechanical behaviour. Specifically, the following were studied: effect of particle size, particle shape, drying-wetting cycles and void ratio on the water retention behaviour of granular materials, and effect of suction on the shear behaviour and flowability of granular materials. A total of 85 pressure plate tests, 13 triaxial compression tests with the axis translation technique used to control suction in unsaturated tests and 52 silo model tests were successfully conducted to respectively measure water retention characteristics, stress-strain and shear strength, and flowability of glass beads of high sphericity (~95% roundness) and Leighton Buzzard sand (~82% roundness). With these deliberately simple materials each considered factor was isolated and investigated at a time something hugely challenging to achieve with many unsaturated soils. Many pertinent features of hydro-mechanical behaviour observed for most soils were well captured with spherical glass beads meaning that particle-water interaction alone can produce the typical unsaturated behaviour and the particle size and shape significantly affected the behaviour. Further drying-wetting cycles did not alter the WRCs of both glass beads and sand except in the saturated capillary regime suggesting that factors other than the inert water-particle interaction through the liquid bridges are responsible for the discrepancy between the first drying-wetting cycle WRC and the subsequent drying-wetting cycles WRCs often observed in clayey soils. It’s discovered that the additional inter-particle bonding force introduced through the liquid bridges maintained by the matric suction serves to increase the stiffness, volumetric stability and shear strength of the material. The rate of increase of strength diminished with increase in matric suction. The study noted that the Beverloo law is valid for estimation of the dry mass discharge rate and that moisture alone is sufficient to maintain stable arching action depending on the hopper outlet. The study generated new knowledge in form of the effect of the material particle properties on the bulk hydro-mechanical behaviour of granular materials. An approach has also been proposed for estimating the flowability and minimum hopper outlet diameter for the wet noncohesive bulk solids.

Durant cette thèse, nous avons réalisé une étude expérimentale 2D d'un milieu granulaire dense vibré dans des conditions proches de la transition de blocage. Une des particularités de notre dispositif expérimental est le rôle joué par la gravité, qui introduit une pression de confinement variable spatialement. De plus, en combinant notre étude dynamique du milieu granulaire à une approche rhéologique, cela nous a permis de préciser le rôle des vibrations sur la mobilité de l'intrus au sein de la phase dense agitée. Dans le chapitre 3, une étude macroscopique d'agitation et de compacité du milieu a été conduite. Un des résultats majeurs est l'obtention d'une expression reliant compacité, pression et agitation, différente de ce qui est attendu de l'équation d'état associée à la théorie cinétique des gaz granulaires. Cette expression est validée pour des milieux fortement dissipatifs associés à des grandes hauteurs d'empilement. En revanche, pour des faibles hauteurs d'empilement, on retrouve bien le comportement standard des gaz granulaires. L'observation d'un comportement inverse à celui attendu pour un gaz dissipatif illustre bien le caractère non collisionel du mode de transfert énergétique. Cela suggère entre autres, un rôle important de la structure de l'empilement dans la transmission de l'énergie, particulièrement au voisinage des configurations à faibles nombres de contacts. Une question centrale est donc de comprendre l'origine de la relation obtenue en clarifiant le rôle des transports élastiques dans ces empilements à la fois désordonnés et fragiles. Par exemple, expérimentalement, il serait intéressant de déterminer comment évolue le nombre de contacts moyen par grain lorsque la compacité diminue et que l'énergie cinétique moyenne stockée dans l'empilement chute fortement. Cette caractérisation du comportement macroscopique a été complétée par une étude microscopique de la mobilité individuelle des grains. Nous avons vu qu'au sein d'un même empilement vibré pouvaient coexister plusieurs régimes (bloqué, sous diffusif, diffusif) et que cela amenait à une certaine hétérogénéité de la dynamique du milieu. Au nal, nous avons caractérisé la mobilité moyenne des grains à travers la compacité locale du milieu mais aussi en introduisant une échelle de temps qui met en évidence un comportement non local, directement relié à une mesure de l'hétérogénéité dynamique de l'ensemble de l'empilement. Enfin, en analysant les champs de déplacement des grains et leurs corrélations spatiales, nous avons pu identifier une échelle spatiale mésoscopique homogène sur l'ensemble de la cellule et correspondant à des mouvements collectifs. Il serait intéressant par la suite d'étudier les relations entre cette échelle mésoscopique et la mobilité d'un grain. À cet effet, nous avons commencé à étudier la densité des modes propres associés à la matrice de corrélation des déplacements individuels des grains. Nous avons vu que pour les phases les plus bloquées, il existe de fortes analogies avec les modes "mous" identifiés pour les systèmes élastiques désordonnés proches de la transition de blocage. Cette partie de notre travail non rapportée dans cette thèse, nécessite donc quelques précautions et un développement plus approfondis. Le dernier chapitre complète notre description du milieu granulaire par l'étude de la mobilité d'un intrus au sein de la phase vibrée dans des conditions identiques à celle du chapitre 3. La densité de cet intrus est telle qu'il coule verticalement. Nous avons montré qu'il était possible à partir des trajectoires, de déduire une relation entre un coefficient de friction effectif et la vitesse locale de l'intrus. En outre, le comportement dynamique de la chute étant très intermittent, nous avons séparé et analysé les phases de blocage et de coulée. Il émerge de cette étude un comportement pathologique associé à la dynamique de blocage qui modifie fortement les lois d'écoulement et éventuellement la rhéologie du système. Nous avons finalement pu proposer une méthode permettant de déterminer une loi effective de rhéologie pour un milieu granulaire dense vibré. Une étude paramétrique complète variant la taille de l'intrus serait nécessaire pour compléter l'étude. Néanmoins, les premiers résultats semblent indiquer un comportement rhéologique similaire de celui observé dans les milieux granulaires cisaillés, c'est à dire l'existence d'un régime rhéo-épaississant tel que la friction effective augmente avec le taux de cisaillement. Cette étude a été complétée par l'analyse des champs de déplacement au passage de l'intrus. Ceci a permis de montrer que la coulée s'effectue à travers un couple intrus-vortex qui provoque des réorganisations à grande échelle à l'intérieur du milieu. Bien que ces résultats apportent une certain éclairage expérimental sur certains aspects de la transition de blocage, ils engendrent également de nombreuses questions qui appellent à des mesures complémentaires.

La suffusion est un cas particulier d'érosion interne qui apparait dans les ouvrages hydrauliques. Elle se caractérise par le détachement et le transport des particules fines à travers les gros grains sous l'action d'un écoulement hydraulique en laissant derrière un squelette granulaire dont les caractéristiques en termes de densité et d'arrangement géométrique granulaire ont été changées. De telles modifications dans la micro-structure du sol peuvent conduire à des déformations à l'échelle macroscopique et peuvent influencer le comportement mécanique du sol. Ce travail a été consacré à l'étude du mécanisme de suffusion et à son impact sur les propriétés mécaniques des sols non-cohésifs. Pour atteindre cet objectif, deux approches ont été suivies dans ce travail: l'une numérique et l'autre expérimentale. Un modèle numérique discret a été défini pour décrire quantitativement le comportement macroscopique du sol et analyser la micro-structure des assemblages granulaires. Il est basé sur l'utilisation de particules sphériques et la résistance au roulement aux contacts, ainsi que sur une nouvelle méthode de compactage des échantillons pour simuler celle utilisée au laboratoire, et atteindre une large gamme de densités initiales. Le modèle a été validé par comparaison de résultats numériques et de données expérimentales. Ce modèle a d'abord été appliqué sur des assemblages granulaires avec des teneur en fines différentes afin d'étudier le rôle des particules fines pour une micro-structure de sol donnée, sans tenir compte d'un processus d'érosion. Il a été montré qu'il existe un pourcentage de fines, en dessous de la teneur seuil, où des particules fines peuvent commencer à participer au transfert de force, et que, si la suffusion s'initie et mobilise ces particules, ceci pourra affecter le comportement macroscopique du sol. Par la suite, une cinétique simplifiée d'extraction du grain a été proposée pour décrire le processus de suffusion. Elle est basée sur une approche de couplage fluide-solide partielle. L'intérêt de cette procédure d'extraction est que, d'une part, elle tient en compte des critères géométriques et hydrauliques complexes; d'autre part, le modèle induit un coût de calcul raisonnable. Cette procédure a été validée sur la base d'essais expérimentaux de suffusion effectués avec un dispositif nouvellement développé. Les conséquences de cette érosion sur les propriétés mécaniques des sols ont été caractérisées par la réalisation en laboratoire et la simulation d'essais de compression triaxiale. Le développement de la suffusion et ses conséquences sur le comportement macroscopique sont discutés en terme de densité et granulométrie initiale du sol, de chargement hydraulique et du type de particules érodées (actives ou inactive vis-à-vis du transfert de force)
Suffusion is a particular case of internal erosion taking place in hydraulic earth structures. It is characterized by the detachment and migration of fine particles by interstitial flow leaving behind the granular skeleton. Such modifications in the soil microstructure may lead to deformations at the macroscopic scale and may influence the mechanical behavior of the soil. This research was devoted to investigate the suffusion mechanism and its impact on the mechanical properties of cohesionless soils. To achieve this objective, two approaches were followed in this work: numerical and experimental approaches. A discrete numerical model was defined to describe quantitatively the soil macroscopic behavior and to analyze the microstructure of granular assemblies. It is based on the use of spherical particles and contact rolling resistance, as well as a new method of compaction to mimic the one used in laboratory and to reach a wide range of initial densities. The model was validated through comparison of numerical results with experimental data. This model was first applied on granular assemblies with different fines contents to study the role of fine particles with respect to a given soil microstructure, without taking into account an erosion process. It was shown that there exists a fines content, below the threshold, where fine particles may start to participate in the force transfer that if suffusion initiates and mobilizes these particles, it may affect the macroscopic behavior of the soil. Thereafter, a simplified kinetics of grain extraction was proposed to describe the suffusion process. It was based on a one-way fluid-solid coupling approach. The importance of this extraction procedure is that, on one hand, it takes into account complex geometric and hydraulic criteria; on the other hand, the model involves an affordable computational cost. This procedure was validated based on experimental tests carried out using a newly developed suffusion apparatus. Consequences of this erosion on the mechanical properties of soils were characterized from experimental and simulated triaxial compression tests. Suffusion development and its consequences on the macroscopic behavior are discussed from the results obtained in terms of the initial soil density and gradation, the hydraulic loading and the type of eroded particles (active or inactive in the force transfer)

Etude des pouzzolanes naturelles de l'ile de la reunion, comme composants de base de beton alleges, en soulignant les avantages thermiques. Proprietes chimiques, mineralogiques et physiques des granulats de pouzzolane. Adaptation d'une modelisation thermique pour les corps poreux, avec nouvelle methode de determination de la diffusivite. Description des appareillages de mesure de la conductivite et diffusivite thermiques, et presentation des resultat

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009.
Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3590. Adviser: Jonathan Freund. Includes bibliographical references (leaves 188-197) Available on microfilm from Pro Quest Information and Learning.

Refractive-Index-Matched Fluorescent Particle Image Velocimetry for Investigation of Flow in Immersed Granular Materials The ubiquity of granular materials and their importance in everyday life, industry, and nature make their study imperative for proper manipulation of the flow. As granular materials are opaque, optical imaging experiments are limited to studies at the free surface or transparent boundaries of the flow. Though techniques like magnetic resonance imaging and X-ray tomography have been used to obtain information from the interior of the flow system, these studies are limited to a small class of granular materials. Therefore, a modified yet simple optical imaging technique that can access the bulk of the flow is desirable. Our study employs refractive-index-matched fluorescent particle image velocimetry (RIM FPIV) to obtain information about the bulk of the flow. In RIM FPIV, the system of dry granular materials is made transparent by adding a suitable liquid whose refractive index is close to that of the particles (glass beads). The addition of liquid results in immersed granular materials, in which the particles are completely submerged in the liquid. Reported literature indicates that immersed granular materials exhibit shear banding and rate-independent rheology at slow shear rates, just like dry granular materials. A recent study using discrete element method (DEM) simulations and experiments reports a dilation-driven vortex in dry granular materials in the slow flow regime. However, the opacity of the granular materials restricted the experimental studies to visualizing only the free surface of the flow. By utilizing the similarities between the two media, our study on an immersed granular medium could give us insight into the flow behaviour in the bulk of dry granular systems. In this thesis, we experimentally study the flow of immersed granular materials and compare our observations with that of dry granular media. We perform experiments on slowly sheared immersed granular materials in a cylindrical Couette and present velocity profiles for the free surface using conventional PIV and the bulk of the flow using RIM FPIV. We observe broadening of the shear band in immersed granular materials and explain how the presence of fluid in interstitial spaces is responsible for such behaviour. We also report increased radial flow in the free surface of immersed granular materials due to enhancement of dilation by the fluid. In our RIM FPIV experiments, the velocity profiles in the bulk of the flow show radially inward flow close to the free surface and radially outward flow close to the bottom of the Couette cell. This observation is consistent with the simulations previously performed on a system of dry granular materials, indicating a circulating axial flow. Therefore, this study is a good starting point for experimentally validating the secondary axial flow in the bulk of both immersed and dry granular materials.

Thss thesis presents the results of an experimental programme on the static mono-tonic response of cohesive-frictional granular materials. The purpose of this experimental programme was to gain insight into the mechanical behaviour of uncemented sands, and sands with small percentages of cementation. With this objective in sight, the research involved understanding and delineating the e ects of four variables: the intermediate principal stress, stress inclination, cohesion (or cementation), and particle morphology. The hollow cylinder torsion (HCT) apparatus, which allows control over both the magnitude and direction of principal stresses, was used in this study to carry out a series of elemental tests on the model materials. The test results were analysed in a plasticity theory based framework of critical state soil mechanics. Drained and undrained HCT tests were conducted on a model angular sand to understand the combined influence of intermediate principal stress ratio (b) and principal stress inclination ( ). Sand specimens were reconstituted to a given density and confining pressure, and were sheared to large strains towards a critical state. The stresses at the critical state with varying `b' were mapped on an octahedral plane to obtain a critical state locus. The shape of this locus closely resembles a curved triangle. Also these specimens showed increased non-coaxiality between the stress and strain increment directions at lower strains. This non-coaxiality decreased significantly, and the response at the critical state was by and large coaxial. The effect of `b' and ` ' on the flow potential, phase transformation, and critical state was also investigated. At phase transformation, ` ' plays a more dominant role in determining the flow potential than `b'. The shape and size of the critical state locus remained the same immaterial of the drainage conditions. Next, small amounts of cohesion (using ordinary Portland cement) was added to this sand ensemble to study the mechanical behaviour of weakly cemented sands. The peak in the stress strain curve was used to signal the breakdown of cohesion further leading to a complete destructuring of the sand at the critical state. The response of the cemented sand changes from brittle to ductile with increase in confining pressure, while reverses with increase in density and `b'. Stress-dilatancy response for the weakly cemented materials shows the non coincidence of peak stress ratio and maximum value of dilation unlike purely frictional materials. This mismatch in peak stress ratio and maximum dilation diminishes with increase in confining pressure. The peak stress (cemented structured sand) locus and the critical state (destructured) locus were constructed on the octahedral plane from these HCT tests. The critical state locus of the cemented sand when it is completely destructured almost coincides with the critical state locus of the clean sand. Using this experimental data set, some important stress-dilatancy relationships (like Zhang and Salgado) and failure criteria (Lade's isotropic single hardening failure criteria and SMP failure criteria) were benchmarked and their prediction capabilities of such models were discussed in detail. The effect of particle morphology was also investigated in this testing programme. Rounded glass ballotini and angular quartzitic sand which occupy two extreme shapes were selected, and a series of HCT tests at different `b' values were con-ducted. A larger sized CS locus was obtained for angular particles and it encompassed the critical state locus of the spherical glass ballotini. Spherical particles exhibit a predominantly dilative behaviour, however present a lower strength at the critical state. The mobilization of strength as a result of rearrangement of angular particles and the consequent interlocking is higher. Even with contractive behaviour which is reflected in the higher values of critical state friction angle and the larger size of the yield locus for sand. Finally, a series of unconfined compression tests were performed to understand if there exists a scale separation in cohesive frictional materials. Specimens were reconstituted to a range of sizes while maintaining a constant aspect ratio and density. As the specimen size increased, the peak strength also increases, counter to an idea of a generalized continuum for all model systems. The observed secondary length scale (in addition to the continuum length scale) is obverse to the one observed in quasi-brittle materials such as concrete, rock. In order to ascertain the reason behind this phenomenon, a series of tomography studies were carried out on these contact-bound ensembles. The presence of cohesion between the grains brings about an \entanglement" between the grains, which contributes to increase in strength, with increase in the size of the sample. This in e ect bringing forth a second length scale that controls the behaviour of these cohesive frictional granular materials. This experimental data set provides quantification of various aspects of the me-chanical response of both cemented and uncemented granular materials under myriad stress conditions. This data set is also extremely useful in developing and bench-marking constitutive models and simulations.

Thss thesis presents the results of an experimental programme on the static mono-tonic response of cohesive-frictional granular materials. The purpose of this experimental programme was to gain insight into the mechanical behaviour of uncemented sands, and sands with small percentages of cementation. With this objective in sight, the research involved understanding and delineating the e ects of four variables: the intermediate principal stress, stress inclination, cohesion (or cementation), and particle morphology. The hollow cylinder torsion (HCT) apparatus, which allows control over both the magnitude and direction of principal stresses, was used in this study to carry out a series of elemental tests on the model materials. The test results were analysed in a plasticity theory based framework of critical state soil mechanics. Drained and undrained HCT tests were conducted on a model angular sand to understand the combined influence of intermediate principal stress ratio (b) and principal stress inclination ( ). Sand specimens were reconstituted to a given density and confining pressure, and were sheared to large strains towards a critical state. The stresses at the critical state with varying `b' were mapped on an octahedral plane to obtain a critical state locus. The shape of this locus closely resembles a curved triangle. Also these specimens showed increased non-coaxiality between the stress and strain increment directions at lower strains. This non-coaxiality decreased significantly, and the response at the critical state was by and large coaxial. The effect of `b' and ` ' on the flow potential, phase transformation, and critical state was also investigated. At phase transformation, ` ' plays a more dominant role in determining the flow potential than `b'. The shape and size of the critical state locus remained the same immaterial of the drainage conditions. Next, small amounts of cohesion (using ordinary Portland cement) was added to this sand ensemble to study the mechanical behaviour of weakly cemented sands. The peak in the stress strain curve was used to signal the breakdown of cohesion further leading to a complete destructuring of the sand at the critical state. The response of the cemented sand changes from brittle to ductile with increase in confining pressure, while reverses with increase in density and `b'. Stress-dilatancy response for the weakly cemented materials shows the non coincidence of peak stress ratio and maximum value of dilation unlike purely frictional materials. This mismatch in peak stress ratio and maximum dilation diminishes with increase in confining pressure. The peak stress (cemented structured sand) locus and the critical state (destructured) locus were constructed on the octahedral plane from these HCT tests. The critical state locus of the cemented sand when it is completely destructured almost coincides with the critical state locus of the clean sand. Using this experimental data set, some important stress-dilatancy relationships (like Zhang and Salgado) and failure criteria (Lade's isotropic single hardening failure criteria and SMP failure criteria) were benchmarked and their prediction capabilities of such models were discussed in detail. The effect of particle morphology was also investigated in this testing programme. Rounded glass ballotini and angular quartzitic sand which occupy two extreme shapes were selected, and a series of HCT tests at different `b' values were con-ducted. A larger sized CS locus was obtained for angular particles and it encompassed the critical state locus of the spherical glass ballotini. Spherical particles exhibit a predominantly dilative behaviour, however present a lower strength at the critical state. The mobilization of strength as a result of rearrangement of angular particles and the consequent interlocking is higher. Even with contractive behaviour which is reflected in the higher values of critical state friction angle and the larger size of the yield locus for sand. Finally, a series of unconfined compression tests were performed to understand if there exists a scale separation in cohesive frictional materials. Specimens were reconstituted to a range of sizes while maintaining a constant aspect ratio and density. As the specimen size increased, the peak strength also increases, counter to an idea of a generalized continuum for all model systems. The observed secondary length scale (in addition to the continuum length scale) is obverse to the one observed in quasi-brittle materials such as concrete, rock. In order to ascertain the reason behind this phenomenon, a series of tomography studies were carried out on these contact-bound ensembles. The presence of cohesion between the grains brings about an \entanglement" between the grains, which contributes to increase in strength, with increase in the size of the sample. This in e ect bringing forth a second length scale that controls the behaviour of these cohesive frictional granular materials. This experimental data set provides quantification of various aspects of the me-chanical response of both cemented and uncemented granular materials under myriad stress conditions. This data set is also extremely useful in developing and bench-marking constitutive models and simulations.

Tese de doutoramento em Engenharia Civil
Nesta tese estudou-se o comportamento mecânico e ambiental de materiais granulares e muito particularmente os agregados processados das escórias de aciaria produzidas na Siderurgia Nacional, com o principal objectivo de promover a sua valorização (reciclagem) como material de construção nas infraestruturas de transporte e obras geotécnicas. Neste sentido, estabeleceu-se um programa experimental para a caracterização ambiental e mecânica, quer em laboratório, quer no campo, dos agregados processados das escórias de aciaria nacionais, bem como de dois materiais naturais (Saibro Granítico e Agregado Granítico), com o propósito de se compararem as suas propriedades. Em laboratório, o principal ensaio mecanicista utilizado para determinar as propriedades de deformabilidade dos materiais granulares tem sido o triaxial cíclico, desenvolvido durante este programa de investigação. Uma câmara triaxial com 150mm de diâmetro e 300mm de altura foi instrumentada internamente para medição directa da força aplicada e dos deslocamentos no provete de ensaio. Paralelamente ao estudo sobre a valorização dos agregados processados das escórias de aciaria nacionais efectuou-se um outro, relativo à influência do índice de vazios e da granulometria nas características de deformabilidade dos materiais granulares. Esse estudo envolveu a realização de ensaios triaxiais cíclicos, em provetes com diferentes granulometrias, combinando diferentes dimensões médias das partículas (D50) e diferentes índices de vazios. Os resultados do estudo mostraram, claramente, que as equações relativas ao índice de vazios obtidas sobre areias, e normalmente utilizadas em materiais granulares com fracções mais grossas, não descrevem o comportamento destes materiais, sobretudo quando bem compactados. Com base nos resultados obtidos propôs-se para a normalização do módulo de deformabilidade em relação ao índice de vazios uma nova expressão matemática que incorpora um factor que depende da granulometria dos materiais. Os resultados mostraram ainda que o módulo de deformabilidade dos materiais granulares depende da dimensão máxima das partículas e do D50. No campo, a avaliação dos desempenhos mecânico e ambiental dos materiais efectuou-se recorrendo à construção de um trecho experimental, integrado numa estrada nacional em serviço, constituído por três secções distintas. Numa foram utilizados apenas os dois materiais naturais (no aterro e nas camadas de leito do pavimento e base), noutra foi utilizado exclusivamente agregado processado (no aterro e nas camadas de leito do pavimento e base) e na restante aplicou-se o Saibro Granítico, no aterro e na camada de leito do pavimento, e o agregado processado, na camada de base. A avaliação do desempenho mecânico e ambiental dos materiais efectuou-se durante e após a fase de construção. A avaliação do desempenho mecânico durante a fase de construção realizou-se através do controlo dos parâmetros de estado e da deformabilidade dos materiais, com recurso a diferentes ensaios. A avaliação do desempenho mecânico, após a fase de construção, efectuou-se ao longo de dois anos, quer ao nível interno das camadas do pavimento com recurso a extensómetros e do aterro através de varões extensométricos instalados durante a fase de construção, quer ao nível do comportamento global do aterro através de campanhas de nivelamento topográfico de precisão e no pavimento com recurso a ensaios com deflectómetro de impacto pesado. A avaliação do desempenho ambiental efectuou-se, ao longo de um ano, com recurso a instrumentação original em Portugal nesta aplicação: instalação de dois lisímetros. Os resultados experimentais obtidos neste estudo mostraram que os agregados processados das escórias de aciaria nacionais são inertes e apresentam um desempenho mecânico superior ao dos materiais naturais, contribuindo decisivamente para a sua valorização em infraestruturas de transporte e obras geotécnicas. Presentemente, são comercializados pela Siderurgia Nacional como um novo material de construção, com Marcação CE e marca registada com a designação de Agregado Siderúrgico Inerte para a Construção (ASIC).
This thesis studied the mechanical and environmental behaviour of granular materials and more specifically processed steel slag aggregates produced in National Iron and Steel Company, with the primary objective of promoting their use (recycling) as a building material in transport infrastructures and geotechnical work. In this context an experimental programme was established in order to environmentally and mechanically characterise, both in laboratory and in the field, the national processed steel slag aggregates, in addition to two natural materials (Granitic gravel and Granite Aggregate), with the purpose of comparing their properties. In the laboratory, the main mechanical test used to determine the deformation properties of granular materials has been the precision cyclic triaxial, developed during this research programme. A triaxial chamber 150mm in diameter and 300mm in height was internally instrumented for the direct measurement of displacement and applied force. Parallel to the study for promoting processed aggregates of national steel slags another laboratory study was carried out on the influence of void ratio and grading on the deformation characteristics of granular materials. This study involved carrying out precision cyclic triaxial tests on specimens with different gradations, combined with different particle size average and different void ratios. The results of the study clearly showed that the equations for the void ratio obtained for sand, and normally used in granular material with coarser fractions, do not describe the behaviour of these materials especially when they are well compacted. Based on the results obtained, a new mathematical expression that incorporates a factor that depends on the gradation of the material is proposed for the normalisation of the modulus of deformability in relation to the void ratio. The results also showed that this modulus in the granular material depends on the maximum size of the particles and on the particle size average. In the field, the assessment of the mechanical and environmental performance of the materials was carried out at the construction of a full scale test integrated in a national road in use which consisted of three distinct sections. In one section, only the two natural materials were used (in the embankment, capping and base layers of pavement), in another section only processed aggregate was used (in the embankment, capping and base layers of pavement) and in the remaining section Granitic Gravel was used, in the embankment and the capping layers of pavement, and processed aggregate was used in the base layer. The assessment of the mechanical and environmental performance of the materials was done during and after the construction phase. The mechanical performance during the construction phase was evaluated by controlling the state parameters and the deformability of the materials by using different tests. After the construction phase this was done throughout two years by assessing the pavement layers by using strain gauges. Furthermore embankment performance was evaluated by using strain gauge rods installed during the construction phase as well by precision topographic levelling campaigns. The assessment of pavement performance was done by using tests with a heavy falling weight deflectometer. The assessment of the environmental performance was carried out throughout one year by means of two lysimeters, instrumentation that is original in Portugal in this application. The experimental results obtained in this study show that the aggregate of national processed steel slags are inert and have a better mechanical performance than that of natural materials, decisively contributing to the promotion of these recycled materials in transport infrastructures and geotechnical works. Presently, this new construction material, with EC marking, registered as a trademark under the name of Agregado Siderúrgico Inerte para a Construção (ASIC) [Inert Steel Aggregate for Construction - ISAC], has led to an increased demand in the national market with obvious benefits for the National Iron and Steel Company.