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In view of the length of time that has elapsed since the 1963 Vaiont landslide and the recent renewed interest in this complex phenomenon, it was decided to look back over its history through unpublished documents and pictures of that period (1958)(1959)(1960)(1961)(1962)(1963). Particular attention was paid to the geological and geomorphological features in order to explain how, since 1959, an ancient landslide has been identified on the left side of the Vaiont valley. The Vaiont landslide has been the subject of much research and discussion and as more than 30 years later many questions are still open, it is a source of stimulus for many researchers. The paper considers the main interpretative studies undertaken to date and proposes an hypothesis of what may really have happened.Résumé À cause de la longue période écoulée depuis le glissement du Vaiont en 1963, et de l'intérít renouvelé autour de ce phénoméne complexe, on a décidé de réexaminer cette longue histoire en utilisant aussi des documents inédits (photos et ecrits) de la période 1958-1963. Ici on décrit, en particulier, les aspects géologiques et géomorphologiques de la zone en question, dans le but díexpliquer comment, depuis 1959 déjà on avait décelé sur le flanc gauche de la Vallée du Vaiont un ancien glissement. La connaissance de la complexité géologique locale et des risques potentiels qu'elle présentait, ont été dans cette periode étroitement liés aux choix décidés par la Société SADE et par la Société Nationale d'Élctricité (ENEL). On a formulé ici une hypothése pour expliquer comment ce désastre a pu réellement se produire. Sur l'interprétation de ce glissement, toujours objet de recherches et de discussions, on présente ici les principaux travaux, dont la plus grande partie est basée sur l'hypothése de E. Semenza. Toutefois, à distance de presque 30 ans, beaucoup de questions sont encore sans réponses complétes, et pour cela deviennent une source d'intérêt, ce qui stimule de nombreux chercheurs.

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Landslides of the lateral spreading type, involving brittle geological units overlying ductile terrains, are a common occurrence in the sandstone and limestone plateaux of the northern Apennines of Italy. The edges of these plateaux are often the location of rapid landslide phenomena, such as rock slides, rock falls and topples. In this paper we present a back-analysis of a recent landslide (February 2014), involving the north-eastern sector of the San Leo rock slab (northern Apennines, Emilia-Romagna Region) which is a representative example of this type of phenomena. The aquifer hosted in the fractured slab, due to its relatively higher secondary permeability in comparison to the lower clayey units leads to the development of perennial and ephemeral springs at the contact between the two units. The related piping erosion phenomena, together with slope processes in the clayshales have led to the progressive undermining of the slab, eventually predisposing large-scale landslides. Stability analyses were conducted coupling Terrestrial Laser Scanning (TLS) and Distinct Element Methods (DEMs). TLS point clouds were analyzed to determine the pre-and post-failure geometry, the extension of the detachment area and the joint network characteristics. The block dimensions in the landslide deposit were mapped and used to infer the spacing of the discontinuities for insertion into the numerical model. Threedimensional distinct element simulations were conducted, with and without undermining of the rock slab. The analyses allowed an assessment of the role of the undermining, together with the presence of an almost vertical joint set, striking sub-parallel to the cliff orientation, on the development of the slope instability processes. Based on the TLS and on the numerical simulation results, an interpretation of the landslide mechanism is proposed.

Although the 1963 Vajont Slide in Italy has been extensively studied for over 50 years, its regional geological and geomorphological context has been neglected. In this paper, we use field observations and remote sensing data to elucidate the interaction between endogenic and exogenic processes that brought the north slope of Monte Toc to failure. We present the first detailed pre- and post-failure engineering geomorphology maps of the slide area. The maps delineate two main landslide blocks, several sub-blocks, compressional and extensional zones, and secondary failures in the deposit. The maps provide new insights into the kinematics, dynamics and evolution of the slide. Finally, we discuss the origin of Vajont Gorge and a prehistoric failure that occurred at the same location as the 1963 slide. We propose, as part of a newly developed multi-stage landscape evolution sequence, that the prehistoric failure was a deep-seated gravitational slope deformation (sackung) that initiated during deglaciation and\ud continued to slowly move until the catastrophic failure in 1963. We argue that the gorge was created by these deep-seated slow movements