Muitas são as pesquisas que avaliam as variáveis intervenientes no colapso dos solos: contudo, pouco tem sido publicado a respeito da influência da trajetória de ciclos de umedecimento e secagem, no comportamento colapsível do solo, e do colapso em condições de carregamento triaxial. O presente trabalho tem como objetivo estudar o colapso de um solo arenoso, típico da região centro-oeste do Estado de São Paulo, frente a tais variáveis através de ensaios edométricos convencionais, edométricos com controle de sucção e triaxiais. Dos ensaios edométricos convencionais caracterizou-se o colapso sob vários níveis de tensão, determinando assim a sobrecarga de inundação que causa máximo colapso. Nos ensaios de compressão triaxial, cada amostra foi inundada sob um determinado estado de tensão, caracterizado pela relação &#9631/&#9633. Nestes ensaios foi observado que as deformações volumétricas são dependentes da tensão confinante e da relação &#9631/&#9633, e que as deformações axiais por colapso são menores que as medidas nos ensaios edométricos convencionais. Nos ensaios edométricos com sucção controlada verificou-se que o valor inicial da sucção apresenta influência no valor do colapso total. Quanto aos ciclos de umedecimento e secagem, o solo sofreu colapso insignificante quando a sucção foi reduzida a um mesmo valor pela segunda vez. Os resultados experimentais dos ensaios edométricos com sucção controlada foram então modelados de acordo com o modelo de ALONSO et. al, (1990), e as adaptações a esse modelo propostas por FUTAI (1997) e MACHADO (1998), verificando-se a capacidade de cada formulação em representar matematicamente o comportamento colapsível do solo estudado. / There are many researches that evaluate and discuss the factors which interfere in the collapse of soils. However, not many have been published regarding the influence of the cycles of wetting in the collapsible behavior of the soil, and neither have collapse strains under triaxial load. The present work deals with the collapse behavior of a sandy soil, typical from the center-west of the State of São Paulo, considering the aforesaid aspects and using conventional oedometer, suction controlled oedometer and triaxial compression tests. In conventional oedometer tests, the influence of overburden stress on the induced wetting on collapse strains was characterized. Collapse strains were also measured in triaxial compression tests, varying the ratio of principal stress (&#9631/&#9633) when the samples are inundated. It was observed that collapse volumetric strains are dependent on confining stress and on &#9631/&#9633, and that, axial and radial strains are related to the ratio of &#9631/&#9633. The axial collapse strains observed in triaxial compression tests were shown to be lower than the ones measured in conventional oedometer tests. In controlled suction tests, the amount of collapse strains under gradual suction reduction was studied including the influence of suction reversals. It was noted that the initial suction influences the amount of collapse strains, and that, in the cycles of wetting and drying, the strains after the second cycle tended to be negligible. Experimental results from suction controlled oedometer tests were modeled through the model by ALONSO et. al. (1990) and by the adaptations to this model proposed by FUTAI (1997) and MACHADO (1998).

$aL’objectif de cette thèse est de comprendre le phénomène de suffusion (érosion interne) dans ses processus et ses conséquences sur le comportement mécanique des sols à faible cohésion par l’établissement d’une relation directe ou indirecte entre la stabilité interne de ces matériaux et leur critère de rupture. L’érosion interne vue essentiellement sous l’angle de la suffusion a été analysée par des modélisations numériques et des essais expérimentaux. Les principaux essais utilisés sont : - Ecoulements d’eau à travers un matériau granulaire dans une colonne. - Essais de cisaillement triaxial non drainés. Pour la modélisation numérique, PFC3D a été utilisé pour les simulations d’essais triaxiaux et la prise en charge du couplage fluide solide a été possible grâce à une option de type CFD (Coupling Fluid Dynamique). Ce code DEM a permis le développement d’une nouvelle méthode de génération d’échantillon granulaire à partir de sa courbe granulométrique et de sa porosité cibles. L’évolution des porosités, celle des courbes granulométriques et de l’état de stabilité interne avant et après écoulement ont été identifiées comme indicateur du niveau de suffusion. Les critères de stabilité de Kenney & Lau et de Kezdi ont permis d’apprécier la stabilité interne des échantillons. Les conséquences de la suffusion se sont manifestées par : Une diminution de l’angle de frottement interne dans le cas d’une suffusion interne; une augmentation de l’angle de frottement dans le cas d’une suffusion externe. Les comparaisons des résultats expérimentaux et numériques tant pour les écoulements que pour les essais triaxiaux ont présenté de bonnes concordances. Enfin, une proposition d’un nouveau critère de stabilité interne est établie, elle est déduite de l’analyse des relations entre le comportement mécanique et la stabilité interne, notamment la contractance du matériau. / The objective of this thesis is to understand the phenomenon of suffusion (internal erosion) in its process and its effects on the mechanical behavior of loose soil by establishing a direct or indirect relationship between the internal stability of these materials and failure criterion. Numerical modeling and experimental tests analyzed the internal erosion in terms of the suffusion. The main tests used are : - Water flows through a granular material in a column. - Undrained shear triaxial tests. For numerical modeling, PFC3D was used for simulations of triaxial tests and the treatment of solid fluid coupling was possible through a CFD type of option (Coupling Fluid Dynamics). This DEM code has allowed the development of a new method of generating granular sample from its grading curve and its target porosity. The evolutions of porosity, grading curves and the state of internal stability before and after discharge were identified as indicator of suffusion. Kenney & Lau and Kezdi stability criteria were used to assess the internal stability of samples. The consequences of suffusion were expressed by: A reduction of the internal friction angle in the case of an internal suffusion; an increase of the friction angle in the case of an external suffusion. Comparisons of experimental and numerical results for both flows and triaxial tests have shown good agreements. Finally, a proposal of a new criterion of internal stability is established, it is deducted from the analysis of the relationship between the mechanical behavior and internal stability, including the material contractance.

This thesis deals with compares the procedures carried out in the cyclical load triaxial testing device according to CSN EN 13286-7, and the procedures performed in the United States of America. In the practical part elasticity moduls of mixed recycled material are experimentally compared with mixed recycled material containing binder(connective material) and with fine-grained soil in the cyclical load triaxial testing device. Further the thesis assesses the use of mixed recycled material in the construction of a third class low-load pavement.

La nature complexe de l'argile a amené les chercheurs à s’intéresser à ses comportements mécaniques et microstructure. L'objectif de cette recherche est d'étudier l'influence des chemins de contrainte triaxiaux sur le comportement mécanique et sur les mécanismes qui se produisent au niveau microstructural. L’étude est réalisée en considérant à un niveau de contrainte donné, le phénomène de dilatance est approché suivant des chemins de contraintes différents. Dans un deuxième temps, l’étude se porte sur l’influence de la succion sur le comportement. Pour les argiles remaniées saturées, le comportement dilatant à un niveau de contrainte donné, atteint par deux chemins de contrainte différents a été finement analysé. L’influence du niveau de contrainte a été également abordée dans le cadre de cette étude. L’analyse microstructurale a été analysée à différentes échelles au moyen de différentes méthodes. Au niveau macroscopique, les résultats ont montré que, pour un niveau de contrainte donné, le comportement volumétrique est fonction du chemin de contrainte. La dilatance est activée dans le cas d’argiles surconsolidées principalement lorsque le chargement est purement déviatorique. Au niveau microscopique, les résultats mettent en évidence cinq modes d’orientation des particules qui pourraient être activés sous chargement triaxial. Les résultats révèlent que l’apparition de la dilatance dans les argiles remaniées fortement surconsolidées peut être attribuée à l'arrangement tortueux des particules d'argile, l'ouverture des micropores ainsi que la formation et l’ouverture de fissures mésoscopiques. Pour les argiles remaniées non saturées, les études sont réalisés par les essais macroscopiques et mésoscopiques sur l'argile Kaolin K13 et un mélange kaolinite-montmorillonite. L’évolution des succions a été mesurée au moyen d’hygromètres placés dans les extrémités supérieure et inférieure d’échantillon. Les résultats démontrent que la contrainte déviatorique d’argile non saturée, lorsque la succion est forte, commence par augmenter jusqu'à un sommet puis diminue pour rejoindre une valeur résiduelle. La résistance au cisaillement d’argile non saturée augmente généralement avec la contrainte moyenne et la succion. Cependant, dans nos essais à succion très élevée le développement d’une fissuration lors du séchage a conduit à perdre l’homogénéité du matériau et à réduire la résistance au cisaillement de l’éprouvette / The complex nature of clay has aroused great interest of researchers in its mechanical behaviour and microstructure. The objective of this research is to examine the influence of triaxial stress paths on the mechanical behaviour of remoulded clay and the mechanisms that occur at the microstructural level. The dilatancy phenomenon is particularly investigated considering a given stress level reached by following different stress paths. In the second part, this research focuses on the influence of suctions on the mechanical behaviour. For the saturated remoulded clay, the dilative behaviour at a given stress level, reached by following two different stress paths was analysed in detail. The influence of stress levels was also examined in this research. The microstructural analysis was conducted at different scales by means of different methods. The macroscopic results of saturated clay show that, at a given stress level, the volumetric behaviour greatly depends on the stress path. In the case of overconsolidated clay, the dilatancy is activated especially when triaxial shearing is performed following the purely deviatoric stress path. At the microscopic level, the results highlight five modes of particle orientation that could be activated under triaxial loading. The results also reveal that the appearance of dilatancy phenomenon in highly overconsolidated remoulded clay may be attributed to the tortuous arrangement of clay particles, the opening of micropores, and the formation and opening of mesocracks. For the unsaturated remoulded clay, the studies were performed through many macroscopic and mesoscopic tests on Kaolin K13 clay and a kaolinite-montmorillonite mixture. The evolution of suctions was measured using hygrometers placed near the upper and lower ends of clay specimens. The results show that, at a high suction level, the deviatoric stress of unsaturated clay increases to a peak and then decreases down to a residual value. The shear strength of unsaturated clay generally increases with increasing mean stress and suction. However, in the high suction tests, the development of cracks during drying led to the inhomogeneity of the specimens and thus resulted in relatively small shear strengths

This thesis presents the results of a series of undrained cyclic triaxial tests carried out on four lime-cement stabilized specimens and clay specimen. The shear resistance degradation rate of lime-cement column subjected to cyclic loading simulated from heavy truck was investigated based on stress-controlled test. The influence of lime and cement on the degradation rate was investigated by comparing the behavior of stabilized kaolin and unstabilized kaolin with similar initial condition. The results indicate an increase in degree of degradation as the number of loading cycles and cyclic strain increase. It is observed that the degradation index has approximately a parabolic relationship with the number of cycles. Generally adding lime and cement to the clay will increase the degradation index which means lower degree of degradation. The degradation parameter, t has a hyperbolic relationship with shear strain, but it loses its hyperbolic shape as the soil getting stronger. On the other hand, for unstabilized clay an approximate linear relationship between degradation index and number of cycles was observed and the degradation parameter has a hyperbolic shape with the increase number of cycles. It was also observed that the stronger the material was, the lesser pore pressure developed in the lime-cement stabilized clay.

Ce travail de thèse s’est attaché à investiguer, d’une part, le mécanisme d’érosion interne par suffusion dans les ouvrages hydrauliques en terre, et d’autre part, les conséquences qu’il pourrait avoir sur le comportement mécanique du sol, l’objectif principal étant d’améliorer notre compréhension de ce processus et du comportement des sols érodés. La démarche adoptée est principalement expérimentale, réalisée à différentes échelles : du macroscopique, à l’échelle du matériau, au microscopique, à l’échelle des grains constitutifs. A l’échelle macroscopique, des essais de suffusion ont été réalisés sur des échantillons de sol à l’aide d’un nouveau dispositif d’essai d’érosion, appelé perméamètre de suffusion, développé dans le cadre de cette thèse. Après érosion dans le perméamètre, les échantillons érodés sont transférés par une procédure de congélation / décongélation dans un dispositif d’essai triaxial afin d’étudier leur comportement sous chargement mécanique. Les résultats obtenus à cette échelle ont permis de donner un aperçu général du mécanisme de suffusion et de mettre en évidence son impact sur les propriétés de résistance au cisaillement du sol. A l’échelle microscopique, une analyse fine a pu être menée en réalisant un essai de suffusion des mesures in-operando de tomographie à rayons X et de la visualisation locale par techniques optiques sur un sol artificiel. La caractérisation du sol à cette échelle microstructurale a permis de mieux comprendre le processus de suffusion, notamment en mettant en évidence l’apparition et le développement de fortes hétérogénéités qui semblent avoir une influence importante sur la réponse mécanique des sols érodés / This thesis work has investigated, on the one hand, the mechanism of internal erosion by suffusion in earthen hydraulic structures, and, on the other hand, the consequences suffusion can cause on the mechanical behavior of soils, the main objective being to improve our understanding of both the suffusion process and the behavior of eroded soils. The approach adopted was mainly experimental, by a multi-scale scope: from macroscopic, at the material scale, to microscopic, at the grain scale. At the macroscopic scale, suffusion experiments were performed on gap-graded cohesionless soil samples using a newly developed testing device, called the suffusion permeameter. Then, the eroded samples are transferred by a freeze/thaw procedure to a triaxial device to study their behavior under mechanical loading. The achieved results provide a general overview of the suffusion mechanism at the sample scale and highlight its impact on soil shear resistance properties. At the microscopic scale, the suffusion mechanism has been studied by performing a suffusion test with in-operando x-ray tomography and using local visualization based on optical techniques in an artificial soil. Subsequent grain-scale analysis has provided a deeper understanding of suffusion process through the characterization of the eroded soil microstructure, particularly highlighting the occurrence and development of strong heterogeneities, which appear to play a key role in the macroscopic mechanical behavior of eroded soils

[pt] O presente estudo apresenta o comportamento dos solos reforçados com PET em pó e triturado, através de ensaios de laboratório. Foram utilizados três solos: um solo coluvionar, uma areia limpa e mal graduada e uma bentonita. Caracterização física, química e ensaios mecânicos (triaxiais CIU e cisalhamento direto) foram realizadas para cada material e misturas. O ensaio triaxial foi realizado em amostras de solo argiloso compactado com porcentagens de pó de PET de 0, 10, 20 e 30 por cento de PET triturado de 3,0 e 5,0 por cento, por seco peso de solo. Os ensaios triaxiais, em amostras de areia foram feitas a uma densidade relativa de 50 por cento e 10 por cento de teor de umidade, e com 0, 10 e 20 por cento de pó de PET, em relação ao peso seco do solo. Os ensaios de cisalhamento direto com bentonita foram feitos com porcentagens de 0 e 30 por cento de pó de PET e 3,0 e 5,0 por cento de PET triturado, por peso seco de solo. Os resultados mostraram que o teor de PET e nível de confinamento têm influência sobre o comportamento mecânico final das misturas. Com ambos os resíduos de PET, as misturas apresentam um comportamento satisfatório, aumentando ou mantendo os parâmetros de resistência ao cisalhamento semelhantes ao solo puro. Assim, para as misturas argilosos, a mistura com 30 por cento de pó de PET e a com 5 por cento de PET triturado são mais eficazes, uma vez que nelas observou-se maior melhora nos parâmetros de resistência. Para misturas de areia a inclusão PET é mais eficaz com 10 por cento de pó PET em tensões confinantes menores e umidade ótima de 10 por cento. Para misturas bentonita, a inserção de PET é mais eficaz para o PET triturado na porcentagem de 5 por cento. Portanto, o uso de resíduos de PET para o reforço do solo poderia minimizar os problemas atuais disposição do resíduo, contribuir com a redução do consumo de recursos naturais e dar um uso nobre para este material. / [en] This study presents the behavior of soils reinforced with crushed PET (Polyethylene Terephthalate) residue through experimental study. Three soils were used: a coluvionar soil, a clean and poorly graduated sand and a bentonite. Physical characterization, chemical and mechanical tests (isotropically consolidated-drained triaxial and direct shear) were performed for each material and mixtures. The triaxial test was performed on samples of clayey soil compacted within the maximum dry density and optimum moisture content with ratios of 0, 10, 20 and 30 per cent of fine crushed PET and 3,0 and 5,0 per cent of PET flakes, by dry weight of soil. The triaxial tests on sand samples were made to a relative density of 50 percent and 10 per cent of water content, and with 0, 10 and 20 percent fine crushed PET, by dry weight of soil. The direct shear tests with bentonite were made with ratios of 0 and 30 percent of fine crushed PET and 3,0 and 5,0 percent of PET flakes, by dry weight of soil. The results have shown that the PET content and level of confining stress have influence on the final mechanical behavior of the mixtures. With both residue of PET, the mixtures present a satisfactory behavior, increasing or maintaining the shear strength parameters similar to the pure soil. Thus, for the clayey mixtures, the fine crushed PET content of 30 percent and the PET flakes content of 5 percent are more effective, once they increase the strength parameters. For sandy mixtures the PET inclusions is more effective with 10 percent of fine crushed PET at lower confining stresses and the optimum content is 10 percent. For bentonite mixtures the PET inclusions is more effective for PET flakes and the optimum content is 5. Therefore, the use of PET waste for soil reinforcement could minimize the current problems of waste disposal, contribute with the reduction of consumption of natural resources and give a noble use for this material.

[pt] Este estudo apresenta o comportamento de solos reforçados com adição de pérolas de EPS (Poliestireno Expandido) através de estudo experimental. Os solos utilizados foram: um solo argiloso de origem coluvionar, uma areia limpa, mal graduada e bentonita. Foram realizados ensaios de caracterização física e de caracterização mecânica, como ensaios de compactação Proctor Normal, ensaios triaxiais consolidados isotropicamente drenados (CID) e ensaios de cisalhamento direto para buscar estabelecer padrões de comportamento que possam explicar a influência da adição de pérolas de EPS, relacionando-a com os parâmetros de resistência ao cisalhamento. Os ensaios triaxiais CID foram realizados em amostras de solo argiloso compactadas na densidade máxima seca e umidade ótima, com teores de pérolas de EPS de 0 por cento, 0,25 por cento, 0,50 por cento, 0,75 por cento e 1 por cento, em relação ao peso seco do solo e os ensaios triaxiais CID em amostras de areia foram realizados para uma densidade relativa de 50 por cento e umidade de 10 por cento, com teores de pérolas de EPS de 0 por cento, 0,50 por cento e 0,75 por cento, em relação ao peso seco do solo. Os ensaios de cisalhamento direto com bentonita foram realizados com teores de pérolas de EPS de 0 por cento, 0,50 por cento e 0,75 por cento, em relação ao peso seco do solo. Os resultados mostraram que o tipo de solo, o teor de pérolas de EPS e o nível de tensão confinante influenciam positivamente o comportamento mecânico final dos compósitos com relação aos parâmetros de resistência, porém não há uma tendência de comportamento bem definida ao analisar cada fator independentemente. Portanto, o uso de pérolas de EPS em obras geotécnicas de carregamento estático contribuiria com o menor consumo de material natural e a consequente redução dos custos de transporte e volume de material mobilizado. / [en] This study presents the behavior of soils reinforced with EPS (Expanded Polystyrene) beads through experimental study. The soils used were a coluvionar soil, a clean and barely graduated sand and bentonite. Physical characterization, Standard Proctor, consolidated drained triaxial and direct shear tests were performed to establish patterns of behavior that may explain the influence of the addition of expanded polystyrene beads, linking it with shear strength parameters. The CID triaxial was performed on samples of clayey soil compacted within the maximum dry density and optimum moisture content with expanded polystyrene beads ratios of 0 percent, 0.25 percent, 0.50 percent, 0.75 percent and 1 percent by dry weight of soil. CID triaxial tests on sand samples were made to a relative density of 50 per cent and 10 per cent of moisture content, with EPS beads ratios of 0 percent, 0.50 percent and 0.75 percent by dry weight of soil. The direct shear tests with bentonite were made with EPS beads ratios of 0 percent, 0.50 percent and 0.75 percent by dry weight of soil. The results showed that the kind of soil, the EPS content and level of confining stress level influence positively on the final mechanical behavior of the composites with respect to strength parameters, but there is no well-defined pattern of behavior to examine each factor independently. Therefore, the use of EPS beads in geotechnical works, contribute to lower consumption of natural material and the consequent reduction in transport costs and volume of mobilized material.

[pt] Os minerais evaporíticos formam as rochas e os depósitos evaporíticos. A maior parte a partir da evaporação da água por restrições de antigos oceanos. Os evaporitos hoje encontrados nos registros geológicos são de grande interesse para a indústria de mineração, como na produção de potássio em minas de carnalita e silvita, e para a exploração de petróleo, seja como rocha geradora, reservatório, selo ou elemento de estruturação do sistema petrolífero. Nesta dissertação foram analisadas amostras de evaporitos salinos de idade Aptiana oriundos da bacia de Sergipe-Alagoas, obtidos na única mina de potássio em operação no Brasil, desenvolvida atualmente pela Vale S.A.. Foi proposta uma metodologia de caracterização mineralógica e volumétrica para este tipo de material baseada em descrição macroscópica, análises de lâmina delgada, microscopia eletrônica de varredura (MEV), espectroscopia de energia dispersiva (EDS), microtomografia de raios X, análise de radioatividade natural e porosimetria a hélio. Após a caracterização dos evaporitos, foram realizados ensaios de propagação de pulso ultrassônico e ensaios de compressão triaxial, a fim de determinar os módulos elásticos do material, principal objetivo desta dissertação. Ao final foram comparados os resultados dinâmicos (ensaio ultrassônico) com os resultados estáticos (ensaio triaxial), relacionando-os com a caracterização mineralógica e volumétrica feita anteriormente. Estes módulos elásticos, como o módulo de Young, são de grande importância e de aplicação direta na engenharia de petróleo, de perfuração, de mineração e geofísica. / [en] The evaporite minerals form the evaporite rocks and deposits. The major part has been deposited from evaporation of water in ancient restricted oceans. The evaporite deposits found in geological records today are of great interest to the mining industry, such as in potassium production in carnallite and sylvite mines, and oil exploration, either as source rock, reservoir, seal or structuring element of the petroleum system. In this dissertation samples of Aptian saline evaporite age from the Sergipe-Alagoas Basin, obtained in the only potash mine in operation in Brazil, currently developed by Vale S.A. were analyzed. A methodology for mineral and volume characterization for this type of material based on macroscopic description, thin section analysis, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray microtomography, natural radioactivity analysis and helium porosimetry is proposed on this work. Following this characterization, ultrasonic pulse propagation tests and triaxial compression tests were made in order to determine the dynamic and static elastic moduli of the material, respectively, the main objective of this dissertation. At the end we compare the dynamic results (ultrasonic tests) with static results (triaxial tests) relating them to the mineralogical and volumetric characterization made earlier. These elastic modulus, such Young s modulus, are very important in petroleum engineering, drilling, mining and geophysics.

Le travail présenté dans ce mémoire a pour objectif d'aboutir à une meilleure caractérisation expérimentale du comportement non drainé du sable d'Hostun à l'échelle locale, en relation avec les différentes méthodes de reconstitution utilisées. On vise en particulier à mettre en évidence la façon dont la microstructure influence les propriétés des sols granulaires, notamment vis-à-vis de l'instabilité de liquéfaction. Le point clé de ce travail consiste en l'utilisation de capteurs piézoélectriques pour mesurer les vitesses d'onde de cisaillement au sein d'éprouvettes triaxiales reconstituées par trois techniques différentes à savoir le damage humide, la pluviation à sec et la pluviation sous eau. Les résultats obtenus mettent en évidence l'influence considérable du mode de fabrication sur le comportement mécanique du sable. Ils montrent que pour une même densité initiale, et à même pression de confinement, le comportement mécanique observé est très différent selon les trois modes de préparation. Les valeurs de vitesses d'onde de cisaillement enregistrées sont différentes d'un mode de déposition à un autre. Concernant l'anisotropie, le matériau préparé par damage humide présente un certain degré d'anisotropie inhérente, qui est toutefois minimal. En revanche, la pluviation à sec et la pluviation sous eau induisent des structures initiales plus isotropes. / This work aims to achieve a better experimental characterization of the undrained behavior of the Hostun HN31 sand at the local scale, depending on the different procedures used for sample reconstitution. We particularly highlight the influence of the microstructure on the mechanical behavior of sand especially in relation to liquefaction instability. The key point of this work consists of the use of piezoelectric transducers to measure shear wave velocities within triaxial sand specimens. These specimens have been prepared by different methods, namely: moist tamping, dry pluviation and water pluviation. The obtained results highlight the considerable influence of the deposition mode on the mechanical behavior of sand. They show that for the same initial density, and at the same confining pressure, the mechanical behavior is very different from the three preparation methods. Furthermore, the values of shear wave velocity are different from one deposition mode to another. Concerning the anisotropy, the material prepared by moist tamping presents an inherent degree of anisotropy, which it is however minimal. On the other hand, dry pluviation and water pluviation induce more isotropic initial structures. Moreover, the initial anisotropy is less important than the anisotropy induced by loading. These experimental results indicate that the initial state of a granular medium can not be described by only couple of scalar variables of initial density and confining pressure: a variable that can reflect the initial microstructure and an evolution law of this variable are necessary to model the experimental observations.