Microscopic Structures in Kaolin Subjected to Direct Shear

Abstract

Synopsis A clay with near perfect particle parallelism was prepared by consolidating a slurried kaolin in a large oedometer. Shear box samples trimmed at various angles to the compression direction revealed no significant difference in drained strength. Samples with original bedding normal and parallel to the shearing direction were interrupted at various displacements and thin sections prepared after impregnation with Carbowax. The sequence of microscopic shear structures shows that in both cases the features at peak strength result from simple shear conditions and that a continuous horizontal structure appears only towards the residual stage. Intermediate structures are interpreted in terms of the kinematic restraint imposed by the configuration of the box. Detailed examination of the development of kink-bands and compression textures lead to the notion of basal plane slip as the basic mechanism in shear induced structures. Une argile ayant une orientation préférentielle quasi-parfaite a été préparée en consolidant une boue de kaolinite dans un œdomètre de grand diamètre. Des échantillons de cisaillement direct taillés suivant différentes orientations par rapport à la direction de consolidation produirent des résistantes au cisaillement comparables. Des échantillons à litage parallèle et perpendiculaire à la direction de cisaillement furent interrompus à plusieurs étages de déplacement, impregnés de Carbowax et découpés en lames minces pétrographiques. La succession des microstructures de cisaillement montre que dans les deux cas leur aspect à l'instant de la résistance maximum résulte de conditions de cisaillement simple, et qu'une structure horizontale continue n'apparait que vers la résistance résiduelle. Les structures intermédiares sont interprétées en fonction de la restriction cinématique imposée par la configuration de l'appareil à cisaillement. L'observation détaillée du développement de plis en S (kinkbands) et des textures de compression ammène à envisager le glissement suivant le clivage basique comme le mécanisme de base des structures de cisaillement.

Microstructural evidence of low‐strain, time‐transgressive subglacial deformation

“…In the final phase where strain was >10, Y shears were initiated sub-parallel to the shear zone and took up a large part of the displacement. Most other experimental results are similar (Morgenstein and Tchalenko, 1967;Tchalenko, 1968Tchalenko, , 1970Maltman, 1977), Figure 11 Examples of vertical micrographs in the sand-dominated transition zone between the till and the sub-till sediments from sites S4-S6 where strain was estimated to $3 based on fold geometry. The transition zone can be subdivided into four domains consisting of till, sand, clay and mixtures of sand and clay.…”

Discrete element simulations of direct shear specimen scale effects

“…Besides the specimen scale effects, other testing-related issues of the DSA include non-uniform stress and strain, and the associated progressive failure inside the specimen (Terzaghi & Peck, 1948;Hvorslev, 1960;Saada & Townsend, 1981). In addition, ambiguities exist in interpreting shear strength parameters owing to the fact that principal stress rotation occurs continuously until the peak state, and the failure plane may deviate from the horizontal mid-plane (Morgenstern & Tchalenko, 1967;Jewell & Wroth, 1987).…”

Observing artificially induced strain: implications for subglacial deformation

“…Unlike other publications describing shear experiments (e.g. Morgenstern and Tchalenko, 1967;Maltman, 1977Maltman, , 1987bMaltman, , 1988van den Berg, 1987;Schokking, 1998), we not only analysed clay fabrics, but also looked into changing grain arrangements in the test material.…”