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Cam-clay and hydraulic conductivity diagram relations in consolidated and sheared clay-matrices

¹ Ecole Supérieure d’Ingénieurs de Poitiers, UMR CNRS 6532 ‘HydrASA’, 40 avenue du Recteur Pineau, 86022 Poitiers cedex, ² LTHE, UMR CNRS 5564, 1209 rue de la piscine, Dom. Univ. de St Martin d’He ‘res, BP 53, 38041 Grenoble cedex, and ³ Etude Recherche Matériaux Sarl, Bât Géologie, 40 avenue du Recteur Pineau, 86022 Poitiers cedex, France

^* E-mail: Patrick.Dudoignon{at}esip.univ-poitiers.fr

(Received 2 August 2003; revised 18 January 2004)

ABSTRACT

Image analyses, carried out on thin sections made in consolidated and sheared kaolinite test pieces, allow the identification of three ‘microstructural domains’: (1) the initial isotropic matrix; (2) a partly anisotropic matrix resulting from simple particle arrangement; and (3) an anisotropic matrix resulting from rearrangement plus flattening and delamination of particles.

In order to explain the micromechanisms of the clay matrix behaviour, this paper proposes to link the ‘microstructural domains’ represented in the e vs. log p Cam-clay diagram and domains of hydraulic conductivity in the k vs. e diagram.

The hydraulic conductivities are calculated following the Kozeny-Carman relations, which take into account the micro-arrangement of particles via a tortuosity calculation. The generation of 2D images shows that the preservation of the isotropic arrangement of particles is limited by a minimum porosity value. A decrease of the porosity value below this limit can be explained only by a progressive anisotropic rearrangement of the particles.

The microtexture behaviour, induced by the superimposition of the compaction, orientation and particle flattening and delamination stages, causes an anisotropy of the hydraulic conductivity which affects (1) the interstitial water flow direction, (2) the rotation of particles itself, and (3) the damage mechanism of the clay.

RESUME: L’analyse d’image appliquée a’ l’étude pétrographique de lames minces, d’éprouvettes de kaolin consolidées et cisaillées, permet d’identifier trois types de microtexture: (1) la microtexture isotrope; (2) la microtexture partiellement anisotrope due a’ la simple rotation des particules; et (3) la microtexture anisotrope due a’ la rotation plus l’écrasement des particules.

Pour expliquer les micro-mécanismes de comportement des matrices argileuses sous contraintes, ce travail propose de faire un paralle’le entre ces domaines microtexturaux définis dans la représentation de Cam-clay (e en fonction de log p) et des domaines de conductivités hydrauliques (k) représentés dans un diagramme k en fonction de e. Les conductivités hydrauliques sont calculées a’ partir des équations de Kozeny-Carman qui permettent de prendre en compte l’arrangement des particules via le calcul de tortuosité. La génération d’images 2D montre que l’arrangement des particules est en partie gouverné par les limites inférieures de porosité propres aux différentes microtextures isotrope a’ anisotrope.

Au cours des phases de consolidation et de cisaillement, l’évolution microstructurale du matériau argileux provoque une anisotropie de k qui agit sur les écoulements d’eau interstitielle, sur la rotation même des particules et sur les mécanismes d’endommagement des matrices argileuses.

KEYWORDS: clays, consolidation, laboratory tests, microscopy, image analysis, fabric/structure of soils, hydraulic conductivity

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