Microstructural analysis of inherent and induced anisotropy in clay

Abstract

The aim of this study was to relate the mechanical behaviour of saturated clays to their structural characteristics. Two clays were studied: a kaolinite and a bentonite. The evolution of the shape, the size, the concentration and the orientation of the elements which constitute the clay structure was examined by means of scanning and transmission electron microscopes. Particular care was taken to avoid as far as possible disturbing the micro‐structure during observation. The results showed the existence of particle breakage and the creation of structural anisotropy during loading. We can conclude from this work that the mechanical behaviour to clayed materials is largely dependent on the changes which occur at the scale of the particles. Copyright © 2000 John Wiley & Sons Ltd.

Yielding and plastic behaviour of Boom clay

“…Isotropic loading up to 9 MPa rotated the yield curve in the p'-q plot, giving rise to an isotropic yield curve, in agreement with observations made by Hicher et al, (2000). In addition, the direction of the plastic strain vectors evidenced a non associated flow rule.…”

“…This option was taken because the rearrangement of soil fabric, or the plastic strains, is thought to be mainly depending on the stress direction η. This idea is supported by the microscopic observations of Hicher et al (2000) about the strong structural anisotropy due to large reorientation of particles in the direction perpendicular to the loading direction. The radial component of the plastic loading dR is given by Eq.…”

“…From the point of view of soil fabric, the initial anisotropy is related to the preferential orientation of clay particles perpendicular to the loading direction during sedimentation and also to subsequent soil diagenesis. The mechanical response of natural clays strongly depends on changes in microstructure, in particular when the initial preferential orientation is modified by further loading paths having a different orientation with respect to the initial principal stresses (Hicher et al, 2000).…”

Stress-dependent undrained shear behavior of remolded deep clay in East China

“…6 are approximately parallel to the plane. Similar observations have been reported (Hicher et al, 2000). In such a case, the electrochemical repulsive stresses due to the overlaps of the diffuse double layer of neighbor parallel clay particles will take a larger proportion of the total effective stress in the specimens with closer separation distance between clay particles (Graham et al, 1992).…”

“…When the shear strain is large enough (e.g., 10%), an obvious shear surface forms which usually intersects with the base platen in triaxial devices. Because the clay platelets near the shear surface experience intensive reorientation and tend to be parallel to the plane (Bai and Smart, 1997;Hicher et al, 2000), a great deal of intra-domain water between the clay platelets will be extruded. As a result, the pore pressure measured at the base increases quickly at an axial strain over 10% as shown in Figs.…”