Article

The clays involved in the 1963 Vajont landslide: Genesis and geomechanical implications

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Abstract

The catastrophic Vajont landslide that occurred on 9 October 1963 caused about 2000 deaths when 270–300 Mm³ of rock and debris slid from the northern slope of Mt. Toc into the newly created artificial reservoir, displacing some huge water waves that flooded the nearby villages. The 1963 slide was an en-block remobilisation of a prehistoric rockslide lying on the southern reservoir bank. Basal sliding took place within a complex lithostratigraphic sequence of cherty–marly limestones and clay interbeds belonging to the Fonzaso Formation of Upper Jurassic age. In this work, we investigate the geological origin and provenance of the Vajont clays according to the results of mineralogical and geochemical analyses performed on several clay samples which were collected from the landslide failure surface and the same limestone sequence in the surrounding area. The sampled clayey materials contained variable amounts of clay minerals (36–96%), calcite (4–64%) and quartz (0–6%). The dominant clay minerals were illite/smectite (I/S) mixed layers with a high illite content (85–50%), which was consistent with the high percentage of K2O (1.17–5.77%). The Vajont clays included in the Fonzaso Fm. can be referred to as K-bentonites and can be interpreted as distinct deposits of volcanoclastic materials (tephra), representing the sedimentary inputs of volcanic ashes that interrupted the “normal” calcareous sedimentation during the late Oxfordian–upper Tithonian (158–145 Ma). The soft clay interbeds played a different mechanical role in the occurrence of the prehistoric rockslide and the 1963 en-block remobilisation. When referring to the ancient slope failures, distinct clay-rich layers that were concentrated within some specific lithostratigraphic intervals of the Jurassic cherty limestone sequence caused a localised decrease in the available shear strength, allowing for relative shear displacements along the dip-slope bedding planes. This caused additional shear/tensile fracturing of limestone beds and the formation of a stepped basal failure surface. During the 1963 remobilisation, the clay layers contributed to the average shear strength decrease of the debris material forming the basal shear zone of the slide, but the main triggering factor was the increase in pore pressures caused by the reservoir-induced groundwater inflow.

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... Rock slope stability problems involve both natural and artificial slopes (often referred to as "engineered slopes"), which are designed for various infrastructural works (motorways, railways, reservoirs, etc.) or mining purposes (open-pit quarries, surface and deep excavations, tunnels, underground storage chambers, etc.), and represent a key technical issue for engineering design and landslide risk evaluation. In fact, the sudden and sometimes unexpected failure of a rock slope can have considerable negative consequences in terms of economic and social losses, including damage or destruction of pre-existing structures along with injures and deaths, which in certain cases can reach huge proportions, as seen in the well-known catastrophic Vajont rockslide that occurred on 9 October 1963 (Paronuzzi and Bolla, 2012;Paronuzzi and Bolla, 2013;Paronuzzi et al., 2013aParonuzzi et al., , 2013bParonuzzi et al., 2016a;Paronuzzi et al., 2021;Bolla et al., 2020). ...
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The paper presents results from ring shear tests, carried out on two samples from the slip surface of the Vaiont landslide in north Italy. The tests were carried out at slow and fast rates of shearing in order to determine values of the coefficient of friction appropriate to static and dynamic aspects of the landslide. Both samples showed a significant loss of strength (negative rate effect), up to 60% below the slow residual strength, when sheared at rates of shearing greater than 100 mm/min. The observed loss of strength may explain the high acquired velocity of the landslide, or alternatively some combination of this and other mechanisms may have taken place. Le papier présente les résultats d'essais de cisaillement par anneau, effectués sur deux échantilIons prélevés sur la surface de cisaillement du glissement de terrain Vaiont dans le nord de l'Italie. Les essais ont été menés avec des vitesses de cisaillement lentes et rapides de manière à pouvoir déterminer les valeurs des coefficients de frottement se rapportant aux phases statique et dynamique du glissement de terrain. Les deux échantillons ont montré une perte de résistance considérable (effet de vitesse négative), allant jusqu'à 60% au-dessous de la résistance résiduelle à vitesse lente, lorsqu'ils subissaient un cisaillement à des vitesses dépassant 100 mm/ min. La perte de résistance observée peut expliquer la forte vitesse acquise par le glissement, ou il est possible qu'une combinaison de ce phénomàne et d'autres mécanismes aient eu lieu.