SUMMARYThis 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.

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.

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.

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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.

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International audienceThis 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

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