Résumé
L'étude détaillée de nombreux dépôts de laves torrentielles, prélevés dans 11 bassins versants des Alpes françaises, nous a permis de définir deux familles de laves, les laves à matrice cohésive et les laves à matrice granulaire (matériau écrêtéà 20 mm). Leur typologie est précisée par des critères granulométriques discriminants relatifs aux fractions 20 à 200 μm.
Lorsque les zones sources sont clairement identifiables, l'étude des échantillons prélevés dans ces zones montre que les matériaux mobilisés présentent des caractéristiques granulométriques et géotechniques proches de celles des laves torrentielles qu'ils ont générées. Ainsi, les dépôts de laves torrentielles portent la signature de la zone source principale, tant au niveau granulométrique, que pétrophysique et géotechnique, et rendent compte d'un faible tri granulométrique pendant la propagation de la lave torrentielle.
De plus, ces études mettent en exergue l'importance des caractéristiques des matériaux potentiellement mobilisables, et notamment du critère granulométrique, sur les conditions du déclenchement des laves torrentielles.
Abstract
The paper discusses viscous and saturated flows which can propagate at high velocity in torrent channels, referred to here as debris flows. Specific morphological, textural and sedimentological criteria have been identified, on which to distinguish debris flows from other torrent deposits.
From a study of numerous debris flows sampled in 11 torrential catchment environments in the French Alps, two types of debris flows have been determined: those with a granular matrix and those with a cohesive matrix. Their typology is based mainly on the fraction of soil smaller than 20 mm although several discriminating particle size criteria have also been considered on the fraction between 20 and 200 μm. The author has selected five criteria for study, that relative to the fractions <50 μm being the most convenient.
Geotechnical tests have also been used as a complementary criterion for this typology. The values of the Atterberg limits and blue methylene tests are generally lower for the debris flows with a granular matrix (Fig. 8a and Table 5). In most situations, the plasticity index of debris flow deposits is low.
Both the soils sampled in the source areas and those mobilised as debris flows generally have particle size distributions and geotechnical characteristics similar to those of debris flow deposits reported in the literature.
The paper concludes that source materials with a granular matrix are more likely to result in debris flows (as defined here) while the source materials with a higher fine fraction are more likely to move as mud flows.
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Received: 20 July 1998 · Accepted: 20 November 1998
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Bonnet-Staub, I. Définition d'une typologie des dépôts de laves torrentielles et identification de critères granulométriques et géotechniques concernant les zones sources . Bull Eng Geol Env 57, 359–367 (1999). https://doi.org/10.1007/s100640050059
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DOI: https://doi.org/10.1007/s100640050059