Stiff sedimentary clays: geological origins and engineering properties

Abstract

This Keynote Paper † summarises and extends earlier work on the characterisation of clays, with particular reference to stiff sedimentary clays. Examples of clay characterisation are given that include the various main geological events, from sedimentation to erosion and weathering, and other phenomena such as diagenesis and brecciation. The examples demonstrate the value of plotting basic data, such as the current stress state and undrained strength, in terms of the void index. These three parameters permit an immediate, simple categorisation, in terms of geological history, of sedimentary clays. Further examples illustrate the engineering consequences of weathering of stiff clays, and the associated phenomenon of brecciation produced by the processes of cambering and valley bulging. It is emphasised that, at all stages in a clay's geological history, it is the bond strength between the soil particles, not the geological preconsolidation stress, that controls the mechanical behaviour of sedimentary clays. At all geological stages the bond strength contributes to sensitivity in terms of both stress state and strength. These sensitivities are measured against two intrinsic behaviour patterns, the intrinsic compression line and the intrinsic strength line, both of which are defined, and both of which have considerable value for the characterisation of clays. In particular, the relations between yield stress in oedometer compression and triaxial undrained strength enable the quality of laboratory test data to be assessed.La présente communication programme † résume et développe des travaux précédents sur la caractérisation des argiles en se penchant, en particulier, sur des argiles sédimentaires rigides. Des exemples de caractérisation d'argiles sont fournis : ils comprennent les principaux évènements géologiques, de la sédimentation jusqu'à l'érosion et au vieillissement, ainsi que d'autres phénomènes, comme la diagénèse et la formation de brèches. Les exemples démontrent la valeur de données de base pour les levés, comme l'état de contrainte actuel et la résistance du sol non drainé, sur le plan de l'indice de vide. Ces trois paramètres permettent d'effectuer immédiatement une simple catégorisation, sur le plan de l'histoire géologique, des argiles rigides. D'autres exemples illustrent les conséquences techniques d'érosion d'argiles rigides, et le phénomène connexe de la formation de brèches, produit par les processus du cambrage et de bombage. On souligne qu'à tous les stades de l'histoire géologique d'une argile, c'est l'intensité de liaison entre les particules de sol, et non pas les contraintes de préconsolidation géologique, qui déterminent le comportement mécanique des argiles sédimentaires. A toutes les phases géologiques, l'intensité de liaison contribue à la sensibilité sur le plan de l'état de la contrainte et de la résistance. On mesure ces sensibilités en fonction de deux configurations de comportement intrinsèques : la ligne de compression intrinsèque et la ligne de résistance intrinsèque, ces deux co...

An investigation of alterations in Zhanjiang clay properties due to atmospheric oxidation

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