Catastrophic edifice and sector failure occur commonly on stratovolcanoes worldwide and in some cases leave telltale horseshoe-shaped calderas. Many of these failures are now recognised as having resulted from large-scale landsliding. These slides often transform into debris avalanches and lahars that can devastate populations downstream of the volcano. Research on these phenomena has been directed mainly at understanding avalanche mechanics and travel distances and related socioeconomic impacts. Few investigations have examined volcanic avalanche source characteristics. The focus of this paper is to 1) describe a methodology for obtaining rock strengths that control initial failure and 2) report results of rock mass strength testing from Mount Rainier and Mount Hood. Rock mass and shear strength for fresh and hydrothermally altered rocks were obtained by 1) utilizing rock strength and structural information obtained from field studies and 2) applying rock mechanics techniques common in mining and civil engineering to the edifice region. Rock mass and intact rock strength differences greatly in excess of one order of magnitude were obtained when comparing strength behavior of fresh and completely altered volcanic rock. The recognition and determination of marked strength differences existing on the volcano edifice and flank, when combined with detailed geologic mapping, can be used to quantify volcano stability assessment and improve hazard mitigation efforts.

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The role of hydrothermal alteration in producing clay-rich rocks is discussed. Hydrothermal fluids derived from magmatic sources change rock lithologies to argillic rocks as a result of temperature, pressure and chemical effects. The grade of the argillization can vary from one in which only trace amounts of clay minerals are present to one in which there has been complete alteration into clay. Detailed geologic surface mapping and subsurface drilling are required to accurately delineate the extent and grade of alteration. Suggestions are presented for assessing the grade and amount of alteration. Two examples illustrate the effects of argillic alteration on the engineering design of excavated slopes. One example demonstrates a successful design where alteration effects were incorporated into the initial design stage. The second example illustrates where an inadequate geologic model underestimated the grade and extent of alteration, and a landslide of over four million tons of material resulted. Reappraisal of the geologic model enabled a successful mitigation and incorporation of the clay-rich lithologies to be designed, and permitted continued safe excavation at the site.

Toppling failure in the slopes of hard rock masses is a mode of movement that has only been described in the last few years. Much of the work that has appeared in the literature to date deals with the development of this mode in models. This Paper describes three field examples which came from contrasting structural settings. Each involves a different scale of mass movement; the first example comes from the much folded Upper Carboniferous of North Devon, the second from the less disturbed Carboniferous of South Wales and the third from the much altered Pre-Cambrian of the Scottish Highlands. The examples indicate that this mode of failure requires neither unusual geological conditions, nor unusual geological materials in order to develop; in fact the reverse would seem to be true. Les écoulements au sein des pentes de masses de roche dure représentent un type de mouvement qui n'est décrit que depuis quelques années. Une grande partie du travail publié à ce jour traite du développement de ce phénomène sur modèle; cet Article décrit trois exemples de chantier provenant d'ensembles structuraux contrastés. Chacun d'eux présente une échelle différente de mouvements de masse. Le Grès Carbonifère du Devon du Nord fournit avec sa couche supérieure très plissée le premier exemple, le deuxième vient du Pays de Galles dont la couche est moins remaniée et le troisième de la couche Précambrienne très altérée de la Haute Ecosse. Ces exemples indiquent que ce genre de rupture ne nécessite ni des conditions géologiques exceptionnelles, ni des matériaux géologiques exceptionnels, en fait il semblerait que ce soit l'inverse.

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