Mahdia Hattab

Kaolin/montmorillonite mixtures behaviour on oedometric path and microstructural variations

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 (logσ′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.

Micro–macro-experimental study of two clayey materials on drying paths

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.RésuméL’objet de cette recherche est d’offrir une meilleure compréhension du lien entre le comportement à l’échelle macroscopique et microscopique de deux matériaux argileux, une kaolinite et un mélange de kaolinite et de montmorillonite. A l’échelle macroscopique, l’approche consiste à mesurer la teneur en eau, l’indice des vides et le degré de saturation en fonction de la succion sur chemin de drainage. Ceci permet de préciser les relations entre le retrait et la désaturation et de mettre en évidence les phases caractéristiques du comportement. A l’échelle microscopique, l’étude de l’orientation des particules d’argile est réalisée par analyse d’images prises au microscope électronique à balayage (MEB). Sur chemin de séchage, les observations montrent une isotropie microstructurale du sol. L’évolution de la porosité est examinée à partir de mesures par porosimétrie au mercure, confirmées par des images seuillées de photos MEB.

Interaggregate Forces and Energy Potential Effect on Clay Deformation

AbstractThis 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.

Dilatancy Phenomenon Study in Remolded Clays – A Micro-Macro Investigation

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 (\( p^{\prime } - q \)) 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.

Identification of Local Mechanisms in Clays and Energy-Based Modelling

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.

Crack Initiation and Propagation of Clays Under Indirect Tensile Strength Test by Bending Related to the Initial Suction

The aim of this research is to analyze, by experimental approach, the impact of suction in the tensile strength and cracking phenomenon of unsaturated clayey soils samples. Through a bending tests, and a clay submitted to different level of suction (361 MPa, 110 MPa and 38 MPa), the approach consisted first in estimating the tensile strength which controls the initiation of tensile cracks. Then, using digital image correlation method, the propagation of cracks was precisely followed through the local strains development around the crack and close the crack tip.

Elastic properties in a clayey material under mechanical loading – an estimation through ultrasonic propagations

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.

Volumetric Strain Mechanisms and Induced Anisotropy Analyses in Clayey Materials

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.