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International audience; Nowadays, raw earth construction is an area of growing interest, both for its eco-friendly characteristics and very interesting thermo-hygroscopic properties. Nevertheless, this type of construction exhibits a weakness related to cracking due to the shrinkage strains caused by desiccation. In this context, this paper presents an experimental study of restrained drying-shrinkage, carried out on a fine clayey soil. Several amounts of soil reinforcement with orderly layout patterns using vegetal flax fibers have been investigated in order to enhance soil properties. The new feature of this work is the analyzing of crack initiation and propagation by coupling the Digital Image Correlation (DIC) method with the Clay-Ring-Test (CRT) technique, using an innovative device called DIC-CRT. Suction measurements in clay samples are performed during desiccation using tensiometers. A good agreement is noticed between suction and strain evolution until cracking revealing the direct influence of suction on clay shrinkage. Through the two techniques, DIC and CRT, a relevant parameter to quantify crack intensity in materials is proposed. A significant decrease of crack ratio, by a factor of about 8, highlights the impact of soil reinforcement on the level of crack opening. Likewise, a significant decrease, by a factor of about 20, is observed in the analysis of DIC and CRT slopes, highlighting the impact of reinforcement on the kinetic of crack opening.

International audience; This work aims to investigate the micropore properties of saturated kaolin subjected to triaxial loading. Consolidated drained triaxial compression tests were conducted on both normally consolidated and overconsolidated clay specimens. In the (p′, q) plane, a given stress level was reached by following two different stress paths (i.e., the purely deviatoric stress path and the conventional constant σ′3 stress path). This approach allows observing the influence of stress path on the evolution of pore structure. The experimental approach consisted in using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) to quantify the micropore structure of the specimens after being submitted to mechanical loading. The results clearly show that the micropore structure of clay was affected by the loading conditions. Consequently, the development of induced anisotropy appeared as mainly related to the variation of pore shape and pore orientation. Meanwhile, for overconsolidated specimens featuring dilative behavior, the pores exhibited a trend to be more open (i.e., rounder) and more randomly oriented compared with the normally consolidated specimens showing contractive behavior. Even if the micropore structures deduced from the SEM-based method, on the one hand, and from MIP tests, on the other hand, concern different scales of pores, the analysis shows that both methods of investigation were quite well comparable and revealed similar variation tendencies in void ratio and pore size distribution.

Optimization and precise knowledge of the effective properties of heterogeneous materials requires the use of predictive homogenization techniques. The aim of this work is to test numerical models of homogenization with a view to predicting the elastic properties of the clay-rice straw composite. Experimental values from the literature have been compared to that from the digital models Digimat MF and Digimat FE. Note that the Digimat MF semi-analytical model gives values closer to the experimental ones. This is confirmed by the reliability study of the two models which was carried out. Indeed, up to a volume fraction value of 25%, the ratio of the values predicted by the MF model to the experimental ones are closer to unity than those obtained by the FE model. We then deduce that the MF model is more reliable in predicting the elastic moduli of the rice straw clay composite. It was also highlighted the decrease in Youngs modulus as a function of the increase in the volume fraction of rice straws. This remark is made on all models.

International audience; Optimization and precise knowledge of the effective properties of heterogeneous materials requires the use of predictive homogenization techniques. The aim of this work is to test numerical models of homogenization with a view to predicting the elastic properties of the clay-rice straw composite. Experimental values from the literature have been compared to that from the digital models Digimat MF and Digimat FE. Note that the Digimat MF semi-analytical model gives values closer to the experimental ones. This is confirmed by the reliability study of the two models which was carried out. Indeed, up to a volume fraction value of 25%, the ratio of the values predicted by the MF model to the experimental ones are closer to unity than those obtained by the FE model. We then deduce that the MF model is more reliable in predicting the elastic moduli of the rice straw clay composite. It was also highlighted the decrease in Young's modulus as a function of the increase in the volume fraction of rice straws. This remark is made on all models.

International audience; This paper studies the dilative behavior of clay during drained creep along triaxial tests under constant mean effective stress, where the volumetric strain is caused by the sole deviatoric stress variation. Tests were conducted on a saturated reconstituted clayey material by the use of a GDS stress path control apparatus. The development of the axial and volumetric strains was studied with respect to time, stress level (SL) and overconsolidation ratio (OCR). Analysis of the results ascertained that both dilatancy and contractancy phenomena could occur during creep. The magnitude of the dilatancy/contractancy during creep was guided by the test conditions (SL and OCR), which specifically governed the direction of the volumetric strain variations. The position of the stress level vis-à-vis the different volumetric domains defined by monotonic triaxial tests in the (p′–q) plane controls the evolution of the volumetric creep strain. The failure of an overconsolidated specimen could be observed for a stress level located under the maximum strength envelope, but above the critical state line in the p′–q plane, accompanied by a significant dilative behavior during creep.

International audience; This experimental work aimed to identify local mechanisms in remoulded clays under different triaxial loading conditions. The behaviour at a given stress level reached by two different stress paths, i.e. the conventional constant σ′ 3 stress path and the purely deviatoric stress path, and the behaviour at two different stress levels along a given stress path were examined. The investigation concerned both normally consolidated and overconsolidated saturated specimens of remoulded kaolin clay. The microstructural states of the clay specimens induced by different triaxial loading paths were examined at two different scales using the scanning electron microscopy and the X-ray microtomography. The results highlighted five particle orientation modes that could be activated. These modes seem to be highly dependent on the stress level, the overconsolidation ratio and the stress path. Mesocracks were found to develop within strongly overconsolidated specimens accompanied by the occurrence of dilatancy. The proposed conceptual modes provide an interesting approach to understand the mechanical behaviour of remoulded clays, especially the macroscale dilatancy phenomenon, through the possible activation of different mechanisms at micro- and mesoscales.; Cette recherche expérimentale sur le comportement des argiles vise à tenter de comprendre, et d’identifier, les mécanismes qui s’activent au niveau local en lien avec les déformations volumiques sur chargements triaxiaux. La démarche expérimentale consiste à mettre en évidence l’influence du chemin de chargement sur le comportement micro-macro. Deux différents chemins triaxiaux, l’un conventionnel à σ’ 3 constante et l’autre purement déviatoire à p’ constant, ont été menés pour atteindre le même niveau de contrainte fixé sur le plan des contraintes. Les essais ont été réalisés sur des échantillons d’argile saturée remaniée reconstituée au laboratoire, en condition normalement consolidée et surconsolidée. L’investigation microstructurale a été menée à deux échelles différentes : au moyen de la Microscopie Electronique à Balayage (MEB), et de la Microtomographie. Les résultats ont mis en évidence l’activation possible de cinq modes d’orientation privilégiée des particules argileuses. Ces modes d’orientation sont fortement dépendants du niveau de contrainte et de l’histoire du chargement. Un développement d’une microfissuration a été observé à l’échelle microscopique et mésoscopique dans les échantillons fortement surconsolidés. L’évolution de cette micro (mais aussi et méso) fissuration semble accompagner clairement le phénomène de la dilatance. Les modes conceptuels d’orientation privilégiée avancés dans ce travail, permettent de mieux comprendre le phénomène de dilatance et de mettre en lumière les mécanismes qui le gouvernent.

International audience; The main goal of this research was to try, experimentally, to identify the cause of initiation and propagation of cracks network related to desiccation. The work may provide a better understanding of the drying behavior in clayey soils. Kaolin, which is a little swelling “nearly pure” kaolinite, was studied in this context. The experimental method is based on the determination of the local two-dimensional strains and displacements fields using the softwares Vic-2D and Vic-3D on thin layers of the initially saturated clay. Using digital image correlation technique at the macroscopic level, the analyses of strain fields during drying allow to clearly identify and characterize different phenomena such as: shrinkage, stress concentration, initiation, and propagation of cracks.

International audience; Clay fine soils are characterised by an ability to change volume when subjected to suction. Soil cracking due to desiccation is a phenomenon of great importance in the environmental geotechnical engineering domain. Aiming to understand shrinkage mechanism, and conditions that lead to appearance of drying cracks, a new experimental testing coupled device digital image correlation- clay ring test (DIC-CRT) was developed to investigate fine soils shrinkage and cracking. The new feature of this paper is to link both measurement techniques, based on 2D/3D-DIC and an innovative CRT, in order to assess stress and strain in soils during desiccation. Tests were carried out on a kaolinite having a liquid limit of wL = 40%, and prepared as a slurry with w = 1.5 wL of water content. The results highlight the relevance in combining these two techniques, particularly to assess internal stresses developed in the material using CRT and link them to strains leading to cracking measured by DIC method.

International audience; This experimental work aimed to identify local mechanisms in remoulded clays under different triaxial loading conditions. The behaviour at a given stress level reached by two different stress paths, i.e. the conventional constant σ′3 stress path and the purely deviatoric stress path, and the behaviour at two different stress levels along a given stress path were examined. The investigation concerned both normally consolidated and overconsolidated saturated specimens of remoulded kaolin clay. The microstructural states of the clay specimens induced by different triaxial loading paths were examined at two different scales using the scanning electron microscopy and the X-ray microtomography. The results highlighted five particle orientation modes that could be activated. These modes seem to be highly dependent on the stress level, the overconsolidation ratio and the stress path. Mesocracks were found to develop within strongly overconsolidated specimens accompanied by the occurrence of dilatancy. The proposed conceptual modes provide an interesting approach to understand the mechanical behaviour of remoulded clays, especially the macroscale dilatancy phenomenon, through the possible activation of different mechanisms at micro- and mesoscales.

International audience; The objective of this research was to analyse the formation and propagation of cracks related to desiccation in clay and to provide a better understanding of the behaviour of two clays, a kaolinite, and a mixture of kaolinite and montmorillonite. At the macroscopic level, the strain tensors of the two clays during drying, as well as the cracking phenomenon, were studied using digital image correlation. The method is based on the determination of the local two-dimensional strains and displacements fields using the programs Vic-2D and Vic-3D. Different mechanisms of crack formation were identified: opening mode, sliding mode and tearing mode. At the end of desiccation, the cracks form a network similar to thermal fatigue or thermal shock networks. Bifurcation and coalescence of cracks can also be observed in soil. Another objective was to explore the effect of mineralogy on the behaviour of these clays. It is the reason why two different mixtures of kaolin and montmorillonite were chosen, with liquid limits ranging from 40% to 82%. The results were found to be similar in both cases, except concerning the kinetics, which is faster in kaolin.

International audience; The aim of this study is to understand the local mechanisms related to creep behavior of a typical clay under triaxial loading. The investigation concerns both normally consolidated and overconsolidated specimens of remolded kaolin clay. The macroscopic results showed that both dilatancy and contractancy could occur during creep depending mainly on the clay behavior prior to the creep test. The magnitude of the dilatancy/contractancy was controlled by the stress level, on one hand, and by the overconsolidation ratio which governed the sign of the volumetric strain variation during triaxial loading, on the other hand. At the microscopic scale, the dilatancy/contractancy phenomena were analyzed using the scanning electron microscopy (SEM). The results indicated that the microstructural evolution of the clay along mechanical loading depended on the stress history. The structural evolution during the creep phases followed the structural pattern developed under monotonic loading. The creep dilatancy phenomenon appeared strongly related to the expansion of micro pores and micro cracks within the overconsolidated clay specimens.

International audience; Desiccation cracking in drying clays is a common problem with a severe impact on the performance of clayey soils in various geotechnical engineering applications. This article highlights different methods to analyze the formation and propagation of cracks related to desiccation in clays. The research is based on an experimental study of different clays subjected to drying and a phenomenological study of cracking during free desiccation tests on thin clay layers. The approach consists, on one hand, in analyzing the characteristics of cracks with the Image-J software and, on the other hand, in determining the local two-dimensional strains and displacements fields using the digital image correlation (DIC) method. Various tests were carried out to verify the accuracy of the DIC method. Plotting the mechanical strain tensors results in a better understanding of the drying mechanisms of the studied clays. The results show that the combination of two image processing methods, associated to fracture mechanics, is a powerful tool to investigate the mechanisms of cracking in drying soils.

International audience; Series of drying tests are carried out on natural silt intended for the preparation of new raw earth concrete. This technique is different from rammed earth. The study treats the drying of various formulations of the natural silt with and without reinforcement. No binders were added to highlight the impact of reinforcement. Global and local deformation around a single crack are analysed using image processing. Adding reinforcements to natural silt reduces the percentage of cracking by a factor around 10. The suction is derived from water content corresponding to crack appearance and correlated to the ultimate resistance of the material. (C) 2015 Elsevier Ltd. All rights reserved.

International audience; The aim of this study is to analyze the influence of stress path on the dilatancy behavior of clays. Triaxial tests were conducted on saturated, remolded clay specimens. During triaxial loading, two different stress paths were considered to bring the stress level to a given point in the (p' - q) plane. After triaxial testing, scanning electron microscopy (SEM) observations and X-ray microtomography (XR-μCT) scans were performed on subsamples. The obtained images were processed using different software and thus the geometric characteristics of pores and cracks were identified. The microstructural characteristics were linked to the dilatancy phenomenon of specimens. The results indicate that dilatancy is influenced by stress path and may be attributed to the evolution of pore geometry and the presence of local open cracks in highly over-consolidated (OC) clays..

The paper addresses the issue of how physical-chemical aspects affect the mechanical behaviour of clays and how they could be integrated in the mechanical modelling. A significant influence of the high-plastic-clay fraction on the behaviour of clayey materials was experimentally highlighted, such as the compression index variation, or the strong and very marked decrease of friction angle. Microstructural investigation provides further understanding allowing to relate the observed phenomena to an activated mechanisms acting at the level of clay particles. A new approach using the Chang and Hicher micromechanical model was proposed, where physical-chemical effects, acting between clays clusters, are incorporated. Local mechanisms are introduced through repulsive and attractive forces obtained from the derivation of energy potentials.

The aim of the experimental study was to identify the local deformation properties in a clayey material which can be activated at the macroscopic ultimate state known by critical state. The approach consists of an extensive study, based on a Scanning Electron Microscope (SEM) picture analysis, of the orientation of the clay particles characterized in the last stages of triaxial loading.

The goal of this study was to analyze the relation between the behaviour of a clayey material at the macroscopic scale and its microfabric evolution. This may lead to a better understanding of macroscopic strain mechanisms especially the contractancy and dilatancy phenomena. The approach proposed in this paper is based on the study of clay particles orientation by SEM picture analysis after different phases of triaxial loading. In the initial state of the samples (one-dimensional compression), the SEM observations highlight a microstructural anisotropy with a preferential orientation of the particles normal to the loading direction. During isotropic loading, densification of the clayey matrix occurs related to a random orientation of particles indicated by the term ‘depolarization’. In the earlier stages of constant σ3 drained triaxial path on slightly overconsolidated specimens, the microstructural depolarization seems to persist inside a macroscopic domain, in which only the volumetric strains due to the isotropic part of the stress tensor evolve. Then, a rotation mechanism of the particles towards preferred directions seems to be activated. The phenomenon appears directly linked to the evolution of the deviatoric part of the stress tensor. Copyright © 2010 John Wiley & Sons, Ltd.

This paper examines the mechanical behavior of a sensitive marine clay. Various laboratory tests on intact and reconstituted samples of Guinea Gulf marine clay were performed under isotropic compression and drained triaxial compression at constant confining stresses. Microstructure analysis on intact and reconstituted samples was also carried out under different loading conditions. The effect of inter-aggregates bonding on mechanical properties is discussed. Based on experimental analysis, a new modeling method is proposed. In this approach, the clay is regarded as an assembly of aggregates of clay particles. An inter-aggregate contact law is introduced relating contact forces to aggregates relative displacements. The deformation of the assembly can be obtained by integrating the movement of the inter-aggregate contacts in all orientations. Thus, the effect of inter-aggregates bonds and debonding is considered in a direct way. The model is evaluated through comparisons between the predicted and measured results on Guinea Gulf marine clay. The evolutions of local stresses, strains, and bonds in inter-aggregates planes are discussed to explain the anisotropy induced by the applied loading. Copyright © 2010 John Wiley & Sons, Ltd.

This study strives to analyze the compressibility of deep water marine sediments originating from the Gulf of Guinea. Our approach is multi-scaled and, on the macroscopic level, based on two equations which link the overburden effective stress with the liquidity index, the Biarez & Favre equation on the one hand, and the Burland equation on the other hand (after calculation of the ICL expression as a function of the I L parameter). On the local level, the evolution of the microstructures is analysed and compared with GoG remoulded clay. Two additional observation techniques have been used, the scanning electron microscope and the mercury intrusion porosimetry.

L’objectif de cette recherche est de comprendre comment évolue la microtexture d’une matrice kaolinitique lorsque celle-ci est soumise à un chemin triaxial de révolution. L’approche consiste à réaliser un essai triaxial sur des éprouvettes du matériau saturé faiblement surconsolidé, puis à suivre l’évolution de la microstructure (orientation des particules et porosité) le long du chemin de chargement par des observations au microscope électronique à balayage. À l’état initial, le matériau est consolidé de façon uniaxiale, et les observations montrent une anisotropie microstructurale avec orientation privilégiée des particules suivant le plan horizontal perpendiculaire à la direction du chargement. Sur chemin isotrope, l’analyse montre une réorientation progressive des particules vers une isotropie structurale. Cette évolution se poursuit en début de chargement triaxial pour tendre ensuite vers une orientation privilégiée des particules selon un plan incliné en fin de chargement. L’évolution de la porosité est examinée à partir de mesures par porosimétrie au mercure, illustrées par des images seuillées de photos MEB. Celles-ci suggèrent l’existence d’une double porosité à l’état initial, qui semble se maintenir lors du chargement isotrope, puis évoluer vers une porosité unimodale, avec une légère diminution de la taille des pores, sur le chemin triaxial.

This study aims at comparing the prediction by the Biarez and Favre model as well as by the more recent Burland one, established for reconstituted normally consolidated clays submitted to oedometric loading. The former, proposed in the 1970s, uses the liquidity index IL, and while the latter introduces a parameter, Iv, which is a normalised void index based on two characteristic void ratios ( e 100 * and e 1000 * ) corresponding to the oedometric curve of σ ′ v = 100 kPa and σ ′ v = 1000 kPa . The aim of these models is to predict the compressibility parameters based on the identification of parameters represented by the Atterberg limits (wL, wP, Ip) as well as of other physical parameters such as the void ratio e or the natural water content wnat, taking into account the effective overburden pressure σ ′ v . These models, which represent the intrinsic properties of clays under compression, are compared with two experimental curves, the first one representing remoulded and reconstituted clay, and the other one a deepwater clay sediment taken from the Gulf of Guinea at a depth of 700 m.

The goal in this research was to analyze the effects of decompression and suction on the formation of cracks in a clay rock from the Andra (French National Radioactive Waste Management Agency) site at Bure (Meuse–Haute-Marne, France). The article investigates the relationship between the changes in the hydromechanical properties and the changes in microstructure and porosity. Concerning the effect of decompression, at the macroscopic scale, the study highlighted an important effect on the elastic modulus and permeability, but little effect at the microscopic scale except an evolution of mineralogy related to the oxidation of pyrite often present in layers where cracks develop. Concerning the effect of suction, at the macroscopic level, the results showed that, on drying path, the change in the properties of the material was very small, whereas, on wetting path, a large decrease in tensile strength and gas permeability was observed. At the microscopic level, observations with SEM and ESEM, and measurements with MIP, highlighted the evolution of microstructural organization as a function of suction, and the propagation and enlargement of cracks on wetting path, rather than on drying path.

International audience; The aim of this research was to characterise the elastic properties of a clayey material Kaolinite K13 and its change during a mechanical loading. An experimental test system, based on the ultrasonic propagation method, was developed. The ultrasonic tests were performed on samples recovered after oedometric loading. The experimental approach consists in investigating, through acoustic local measurements, the evolution of the elastic parameters along the oedometric path. Measurement of wave velocities at different frequencies was performed using an insertion-substitution method. Afterwards, the dynamic elastic modulus deduced from the wave velocities was compared to the static modulus obtained in the oedometric test.

International audience; This study aims to propose a new model for clay behavior by incorporating physical-chemical effects acting between clay clusters using the Chang and Hicher micromechanical approach. Local mechanisms are thus introduced through repulsive and attractive forces similar to double-layer van der Waals forces, which are obtained from the derivation of energy potentials. A specific study of local parameters and their evolution is conducted using experimental data from the mineralogy variation provoked by the variation of a remolded saturated reconstituted clayey mixture consisting of kaolinite and montmorillonite ranging from 0 to 100% montmorillonite. Using scale transition, the results of micro-macrocalculations under an isotropic loading path show a very good agreement with experimental results.

International audience; The object of this study was to understand and characterize dilatancy in clay in relation to the overconsolidation ratio. This phenomenon was investigated along a large range of loading paths, with strong emphasis placed on constant mean stress paths given that it was possible to measure the volumetric strain created by the sole deviatoric stress. Triaxial tests performed on remolded clay (Kaolinite P300) allowed us to determine three different types of behaviour for specimen submitted to a deviatoric stress: no volume change, dilatancy and contraction. In the (p', q) plane these domains are limited by the maximum strength envelope zeta, which coincides with the perfect plasticity line M for OCRs < 2. The experimental results were then compared to plastic flow theories used in the Cam Clay model and in Rowe's dilatancy theory. Finally, by measuring the plastic strain increment vectors along different stress paths in the (p', q) plane, we demonstrated that the uniqueness hypothesis of the plastic potential is not valid. The results could be explained by using two plastic strain mechanisms: a deviatoric one and an isotropic one.

International audience; This paper presents an extensive study of a variable model clayey material, made of mixtures (in different proportions) of two mineralogically different clays. The basic clays are montmorillonite (Greek clay) characterised by a high plasticity, and kaolin P300, which is a more stable clay with low plasticity. The results show a significant variation in the mechanical properties of the mixtures, especially from 35% of montmorillonite. The behaviour exhibits an increase in strength of the material on the isotropic (and also the oedometric) path, and a marked decrease in the internal friction angle down to that of the montmorillonite. Microstructural investigation provided an explanation of these phenomena through a mechanism activated at the scale of particles.

International audience

The aim of this study is to characterise along various stress paths the microstructural evolution of a deep-water marine sediment from the Gulf of Guinea (GoG). Results obtained at the macroscopic level have shown that the phenomenological behaviour of the GoG sediment is explained by the properties of remoulded samples given by the isotropic and critical state lines, and by the properties of the intact samples given by the damage evolution of the inter-aggregate bonds. Microstructural investigations conducted by mercury intrusion porosimetry and scanning electron microscopy demonstrate that the volumetric strains measured at the macroscopic level come from inter-aggregate pore variations associated with the degrading of the inter-aggregate bonds. The mechanism responsible for reorienting the particles appears to apply in the case of the remoulded sediment. As for the natural sediment, it is first necessary to increase the isotropic consolidation stress towards higher values in order to break the bonds between the aggregates before it becomes possible to activate the particle reorientation mechanism and thus approach the remoulded behaviour.

International audience; The objective of this research is to provide a better understanding of the relation between the macroscopic and microscopic behaviours of two clayey materials, a kaolinite and a mixture of kaolinite and montmorillonite. At the macroscopic scale, the approach consists of measuring the water content, void ratio and degree of saturation versus suction (s) during drying, which allows to specify the relationship between shrinkage and desaturation and highlights the characteristic phases of behaviour. At the microscopic scale, study of the orientation of the clay particles is carried out by scanning electron microscope (SEM) picture analysis under different suctions. On drying paths, the observations show an isotropy of the microfabric. The evolution of the porosity derived from mercury intrusion porosimetry tests is confirmed by SEM photograph observations.

International audience; The aim of this study is to understand and to characterise the influence of the montmorillonite fraction on the behaviour of several mixtures of kaolin and montmorillinite, ranging from pure kaolinite to pure montmorillonite. We show that beyond 35% in weight, montmorillonite is responsible for the global behaviour of the mixture. At the global level, the influence of montmorillonite becomes apparent through the fact that one-dimensional compressibility path, in the (log sigma(v)-e) plane, loses its linearity. At the local level, MIP results show that the compressibility, for any of the studied mixtures, results in variations in inter-aggregate porosity. The qualitative analysis of SEM images explains the macroscopic behaviour, showing that the more flexible particles of montmorillonite form a structured network surrounding the kaolinite particles. When the percentage of montmorillonite exceeds 35%, the role of this phase becomes major, imposing in particular the particles orientation mechanisms.

The study deals with an experimental analysis of the coupling between the macroscopic scale behavior and the local deformation properties of clayey materials, with a special focus on the induced anisotropy phenomenon. The analysis was first conducted on a remolded saturated synthetic clay called kaolin P300, then on deepwater marine clayey sediments from the Gulf of Guinea where the influence of the structure (cementation and fabric) was examined.

The oedometric path of normally consolidated unremolded clay sediments evolves, in the (log σ’v - IL) plane, toward a maximum point located above the Cc line (which characterizes the compression of material that has been remolded and reconstituted) highlighting "apparent" overconsolidation. Then, the curve tends to join the Cc line indicating the beginning of destruction (failure) of bonds between particles. This progressive rupture of bonds becomes complete when the path rejoins the remolded behavior in the high stress domain. This gradual transition, from unremolded behavior towards remolded behavior, has been investigated on marine sediments extracted from the Gulf of Guinea. Oedometric tests were performed on both natural (unremolded) clay samples and remolded ones. The study is supported by several microscopic observations with SEM (Scanning Electron Microscope) and mercury intrusion porosimetry.

The aim of the experimental study is to analyse the association between the behaviour of a clayey material at the macroscopic level and its local deformation properties. The approach is based on the study of the orientation of the clay particles by scanning electron microscopy picture analysis after different phases of triaxial loading. This leads to a better understanding and characterisation of the relation between the microfabric and the strain mechanisms (i.e. contractancy and dilatancy) observed at the macroscopic level. In the initial state (one-dimensional compression), the observations highlight the microstructural anisotropy of the slightly overconsolidated specimens with a preferential orientation of the particles normal to the loading direction. During isotropic loading, densification of the clayey matrix occurs associated with a re-orientation of the particles, leading to an increase in structural isotropy, indicated by the term ‘depolarisation'. On the triaxial path, up to 5% axial strain, depolarisation is reinforced. A rotation mechanism of the particles then seems to become activated beyond a critical threshold, directly related to the increase in the deviatoric part of the stress tensor.

L’objectif de cette etude est la caracterisation experimentale des mecanismes de deformation dans une argile surconsolidee (kaolinite P300) sur chemin triaxial axisymetrique. Un interet particulier est porte au mecanisme deviatoire du a la partie deviatoire du tenseur des contraintes. La representation dans l’espace (e, p’, q) des resultats d’essais sur chemins purement deviatoires et l’utilisation du modele Cam-clay permettent de suivre l’evolution du chemin experimental et de voir apparaitre un domaine pseudo-elastique volumique. La projection des resultats experimentaux, dans le plan (q/p’, dev /d[math]), permet de suivre precisement la loi d’ecoulement experimentale au regard des lois de Roscoe et de Rowe.

JNGG 2016- Journées Nationales de Géotechnique et de Géologie de l'Ingénieur, Nancy, FRANCE, 06-/07/2016 - 08/07/2016; L'objectif de cette recherche est d'analyser l'endommagement dans une éprouvette argileuse sous chargement triaxial à différents chemins et niveaux de contrainte. L'endommagement progressif du sol affecte les propriétés élastiques du matériau, celles-ci peuvent être approchées par la mesure des vitesses d'ondes ultrasonores que l'on sait sensibles à la variation des caractéristiques élastiques du matériau (Hall et al., 2007).Un système d'investigation expérimental original a été développé au laboratoire qui permet de mesurer la vitesse d'onde de compression dans un échantillon d'argile saturée.L'approche expérimentale consiste dans un premier temps à réaliser sur des éprouvettes d'argile remaniée saturée, normalement consolidées ou surconsolidées, différents chemins triaxiaux drainés (isotropes, s'3=constante et p'=constante). Des échantillons provenant d'éprouvettes ainsi sollicitées sont ensuite récupérés pour des essais ultrasons. Les essais ultrasons sont réalisés après déchargement mécanique de l'éprouvette. Les résultats obtenus montrent que la vitesse d'onde est influencée par le sens de la déformation volumique lors du chargement mécanique (comportement contractant ou dilatant du matériau).

International audience

Clay soil is widely distributed on the Earth’s surface, and because it is cheap and readily available, clay soil has been widely used as a building material for a very long history. Furthermore, clay can be used as not only a natural barrier in the dam cores, but also a matrix for the storage of radioactive wastes because of its retention properties. The mechanical behavior of clay materials is complex, one of the difficulties is that it is sensitive to water. During the desiccation process, clay soils undergo shrinkage, which can cause cracking. The aim of this thesis is, initially, to develop a numerical approach capable of reproducing the phenomenon of shrinkage, the distribution of water content as well as that of suction. In a second step, based on Coussy's theory for unsaturated porous media, and the mechanics of unsaturated soils, a constitutive law will be proposed to describe the behavior observed during desiccation. Finally, to reproduce the cracks distribution, based on the extended finite element method (X-FEM). The realization of numerical simulation is based on the analysis of the desiccation experiments of clay soils in laboratory. The application of the digital image correlation (DIC) technology in the desiccation experiments makes the study on the desiccation process in clay soils more accurate. The experimental results show that the clay soils will generate the theoretical shrinkage deformation caused by its own water loss in the drying path. This deformation in simulation can be related to the water content of clays through the Fredlund function. The desiccation shrinkage of clay soils has an anisotropic phenomenon. The coefficient of shrinkage ratio is used to describe this phenomenon in simulation. One of the ways to construct the constitutive of the initially saturated soft clays during drying could be in using two independent stress tensors which will enable the decomposition of total strain tensor into strain tensor due to drying shrinkage (induced part due to suction variation) and a “mechanical” strain tensor due to the total stress variation. Mechanical strain tensor can be related to total stress by using stiffness matrix. In fact, the initially saturated clay soil resistance increases during desiccation. The result of cracking in the soil under controlled suction is the result of competitions between increased soil resistance and damage caused by shrinkage during desiccation. The soil moisture crack initiation criterion will be based on soil damage and resistance. The criterion of crack propagation, meanwhile, will be based on the theory of conservation of energy. To reproduce the cracks distribution, based on X-FEM. Weibull's law will be used to consider the heterogeneous distribution of the soil. After digital model validation, applications in the geotechnical field are then considered.; L'argile est largement répandue à la surface de la Terre, et comme elle est facilement disponible, elle a été largement utilisée comme matériau de construction depuis très longtemps. L'argile peut être utilisée non seulement comme barrière naturelle dans les noyaux des barrages, mais aussi comme matrice pour le stockage pour les déchets radioactifs en raison de ses propriétés de rétention. Le comportement mécanique des matériaux argileux est complexe, une des difficultés réside dans le fait qu’il soit sensible l’eau. Durant le processus de dessiccation, les sols argileux subissent un phénomène de retrait qui peut provoquer des fissurations. L’objectif de cette thèse vise, dans un premier temps, à développer une approche numérique capable de reproduire le phénomène du retrait, ainsi que la distribution de la teneur en eau et de la succion. Dans un deuxième temps, en se basant sur la décomposition du tenseur de déformation en une partie élastique (loi de Hooke) et une partie hydrique (représentant le retrait), une méthode de calcul par éléments finis est proposée pour décrire et aider à comprendre le comportement observé durant la dessiccation. Enfin, dans le but de reproduire la distribution des fissures, la méthode basée sur des éléments finis étendus (X-FEM) est utilisée. Les simulations numériques sont basées sur l'analyse des résultats d’essais de dessiccation d’argiles de laboratoire. L'application de la technique de corrélation d'images numériques (CIN) dans les essais de dessiccation rend l'étude du processus de séchage plus précise. Les résultats expérimentaux réalisés dans des travaux antérieurs et en cours montrent que les argiles, sur le chemin de séchage, vont générer une déformation de retrait causée par la perte d'eau. En simulation cette déformation est liée à la variation de la teneur en eau des argiles grâce à la fonction de Fredlund. Sur les essais utilisés le retrait de séchage suit une loi globale anisotrope liée à la géométrie de l’échantillon d’argile testé. Le coefficient de taux de retrait, appelée aussi le coefficient d’anisotropie, est utilisé pour décrire en simulation ce phénomène. Pour construire la loi constitutive permettant d’aborder le séchage des argiles molles initialement saturées, serait d'utiliser deux tenseurs de contrainte indépendants, liés à la décomposition du tenseur de déformation totale en tenseur de déformation dû au retrait au séchage (partie induite en raison de la variation de succion) et un tenseur appelé déformation « mécanique » qui sera dû au développement de contraintes liées au blocage du processus de retrait. Le tenseur de déformation mécanique peut être lié à la contrainte totale en utilisant une la loi élastique linéaire. La résistance de sol argileux initialement saturé augmente au cours de la dessiccation. Le résultat de fissuration, dans le sol sous succion contrôlée, est le résultat de compétitions entre l’augmentation de résistance de sols et l’endommagement causé par le retrait au cours de dessiccation. Le critère d’amorçage de fissure hydrique de sol est basé sur l’endommagement et la résistance de sol. Le critère de propagation de fissure, quant à lui, est basé sur la théorie de conservation d’énergie. Pour reproduire une répartition de fissure, basé sur la méthode d’éléments finis étendu (X-FEM). La loi de Weibull est utilisée pour prendre en compte la répartition hétérogène du sol. Après la validation de modèle numérique, des applications dans le domaine géotechnique peuvent être envisagées.

With the evolution of humanity, modern constructions are proposed based on conventional materials. This is not without consequence because today the construction sector is the largest consumer of energy and the second emitter of CO2 in the world. This significantly impacts the climate of the planet. It is therefore essential today to develop materials that best respect the environment through their low greenhouse gas emission power and which consume little energy. This research is part of this process, which will contribute to the development of ecological materials for construction. We are interested in studying the elastic properties of a clay matrix composite, reinforced with vegetable rice straws. In view of the results obtained, it is shown that this material used in a traditional way in rural areas has interesting characteristics, which constitute reasons for its popularization with builders. The experimental tests in conjunction with the microstructural study made it possible to better control not only the distribution of the fibres in the matrix, but also to define the optimal formulation. The variation in the volume fraction of straws allows us to highlight the influence of straws on the earth material. Straws play the role of reinforcements in the clay-rice straw composite material (banco). They improve the tensile and compressive strength up to a threshold of 25% by volume proportion. It appeared that the Young's modulus decreases with the evolution of the volume fraction and that its greatest value is obtained when the volume fraction is 25%. Multi-scale modelling for the prediction of the elastic behaviour of the banco composite was carried out in this work, through the analytical models of Mori-Tanaka, Hashin-Strikman, the bounds of Voigt and Reuss. A numerical study was made using the Digimat software, of which the finite element module and the MF semianalytical module were used. The study of the behaviour of the material was carried out only in elasticity. Using these models, we were able to go back to the intrinsic properties of the constituents of the composite (the elastic properties of rice straws and the clay matrix, the aspect ratio of rice straws, the geometric orientation of inclusions, the volume fraction of inclusions). We have noticed that the Mori-Tanaka models and the Digimat MF model best predict the elastic behaviour of the clay-rice straw composite.; Avec l’évolution de l’humanité il est proposé des constructions modernes à base de matériaux conventionnels. Ceci n’est pas sans conséquence car de nos jours le secteur de la construction est le premier consommateur d’énergie et le deuxième émetteur de CO2 dans le monde. Ceci à un impact significatif sur le climat de la planète. Il s’avère donc indispensable de nos jours de mettre au point des matériaux qui respectent au mieux l’environnement à travers leur faible pouvoir d’émission de gaz à effet de serre et qui consomment peu d’énergie. C’est dans cette démarche que s’inscrit le présent travail de recherche qui contribuera au développement des matériaux écologiques pour la construction. Il s’intéresse à l’étude des propriétés élastiques d’un composite à matrice argileuse, renforcé avec des pailles végétales de riz. Au regard des résultats obtenus, il est montré que ce matériau utilisé de façon traditionnelle en milieu rural possède des caractéristiques intéressantes, qui constituent des raisons de sa vulgarisation auprès des constructeurs. Les essais expérimentaux en liaison avec l’étude microstructurale a permis de mieux maitriser non seulement la répartition des fibres dans la matrice, mais aussi de définir la formulation optimale du composite. La variation de la fraction volumique des pailles, nous permet de mettre en évidence l’influence des pailles sur le matériau terre. Les pailles jouent le rôle des armatures dans le matériau composite argile-pailles de riz (banco). Elles améliorent la résistance en traction et en compression jusqu’à un seuil de 25% de proportion volumique. Il est apparu que le module de Young baisse avec l’évolution de la fraction volumique et que sa plus grande valeur est obtenue lorsque la fraction volumique est de 25%. La modélisation multi-échelle en vue de la prédiction du comportement élastique du composite banco a été réalisée dans ce travail, à travers les modèles analytiques de Mori-Tanaka, Hashin-Strikman, les bornes de Voigt et Reuss. Une étude numérique a été faite à l’aide du logiciel Digimat dont le module éléments finis FE et le module semi-analytique MF ont été utilisés. L’étude du comportement du matériau a été menée uniquement en élasticité. A l’aide de ces modèles, nous avons pu remonter aux propriétés intrinsèques des constituants du composite (les propriétés élastiques des pailles de riz et de la matrice argileuse, l’aspect ratio des pailles de riz, l’orientation géométrique des inclusions végétales, la fraction volumique des inclusions). Nous avons remarqué que les modèles de Mori-Tanaka et le modèle Digimat MF prédisent au mieux le comportement élastique du composite argile-pailles de riz.

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; 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; 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