Lias clay: weathering processes and their effect on shear strength

• Published in: Géotechnique
• Main Author: Chandler, R. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Thomas Telford Ltd. 1972
• Subjects: permafrost
• Online Access: http://dx.doi.org/10.1680/geot.1972.22.3.403; https://www.icevirtuallibrary.com/doi/pdf/10.1680/geot.1972.22.3.403

The processes of weathering of the Jurassic Lias clay (a clay shale) at four different sites are shown to result in a progressive change of the relationship between strength and water content, with successively shallower, more weathered layers showing higher strengths at a given water content. A weathering scheme, recognizing four weathering zones, based on the degree of oxidation, appears to correspond well with the strength-water content data. The progressive changes of fabric observed in petrological thin sections of the clay can also be related to these weathering zones. It is concluded that weathering of the Lias clay after excavation is so slow on an engineering time scale that it is probably not an important factor leading to long term slope failure. In such heavily overconsolidated clays the scatter of strength may be as much associated with small variations in water content as with the presence of fissures. A fifth site provides an example of severe disturbance of the Lias clay resulting from permafrost conditions during the Pleistocene, and it is shown that not only does this disturbance largely mask the effect that weathering has on the strength-water content relationship, but also probably results in a considerable reduction in effective stress strength parameters. On montre que les processus d'altération de l'argile du Lias Jurassique (un schiste argileux) en quatre sites différents, résultent d'un changement progressif dans la relation entre résistance et teneur en eau, au tours duquel les couches les moins profondes, plus altérées, ont des résistances plus élévées pour une teneur en eau donnée. Un schéma d'alteration, qui distingue quatre zônes d'altération en fonction de leur degré d'oxydation, semble bien correspondre aux relations de la résistance et de la teneur en eau. On peut également faire une corrélation entre les changements progressifs dans la structure, tels qu'observés sur des lames minces pétrographiques de l'argile, et ces zônes d'alteration. On en conclut que l'altération de ...