The Vaiont landslide: re-assessment of the evidence leads to rejection of the consensus

There appears to be a clear general consensus in the literature regarding four critical issues that define the problem of the October 1963 Vaiont landslide and its behaviour that are central to the disaster: (1) the 1963 failure was a reactivation of an ancient landslide; (2) failure took place along thin clay seams (already at residual strength); (3) the sliding surface had a 'chair' shape with a (sub)horizontal base; and (4) failure was triggered by inundation of the toe of the slide mass by rising reservoir levels. The key to understanding the Vaiont landslide is the failure surface geometry, which was controlled by the structural geology. It now appears that the so-called chair structure (that was assumed to define the shape of the failure surface) does not exist, and without it, the first consensual point is untenable, and the fourth may not contain the whole truth. We have systematically re-examined the published evidence and undertaken our own new research in order to test the logical and geotechnical validity of the four elements of the consensus. Glacial processes can account for the pre-failure morphology of the landslide site; the clay seams must therefore have been at peak shear strength as there was no ancient landslide. Tectonic processes can account for the failure surface geometry, which does not have a 'chair' shape, as well as small-scale structures; and rainfall appears to have been an essential element in the initiation and development of the landslide. Our findings largely contradict the consensus position and thus form the basis of a new overarching hypothesis for the landslide that should account for all of the observed and known features, events and data.