Abstract
Although usually masked by a veneer of weathered products, a discontinuity network has been observed in mudstones dated from the Lower Cambrian to the Miocene, principally in pits and quarries. Its origin is attributed to an early hydraulic fracturing of the initial muds with their drainage and progressive compaction. In contrast no natural discontinuities have been observed in the galleries of underground nuclear waste laboratories in Boom Clays at Mol, Belgium, and in Opalinus Clays at Mont Terri, Switzerland, or in deep reconnaissance borings. However, the observations were made immediately after the works. At depth, natural discontinuities are closed by the stress field. As there is no cement and with the temperatures of diagenesis being too low to allow any re-crystallization, they are sealed but not healed. They could reopen with the release of stress and/or desiccation.
Résumé
Bien qu’usuellement masquées par une couverture de produits d’altération des discontinuités de type diaclases ont été observées dans des mudstones (argiles,silts, marnes) marines du Cambrien inférieur au Miocène, principalement dans des carrières. On les attribue à la fracturation hydraulique précoce des vases originelles avec drainage et compaction progressive. En contraste aucune discontinuité naturelle n’est signalée dans les galeries des laboratoires souterrains pour étude du stockage de déchets nucléaires dans les Boom Clays à Mol, Belgique et les Opalinus Clays au Mont Terri, Suisse, ni dans les sondages de reconnaissance. Les observations ayant été faites aussitôt après les travaux les discontinuités naturelles restent probablement fermées par le champ de contraintes. En l’absence de ciment et avec des températures de diagénèse trop faibles pour permettre une recristallisation elles demeurent susceptibles de se rouvrir en cas de relâchement prolongé des contraintes et/ou dessication.
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Arnould, M. Discontinuity networks in mudstones: a geological approach. Bull Eng Geol Environ 65, 413–422 (2006). https://doi.org/10.1007/s10064-006-0060-3
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DOI: https://doi.org/10.1007/s10064-006-0060-3