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

Abstract

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 reexamined 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.

New, simplified and improved interpretation of the Vaiont landslide mechanics

“…Finally, the consensus position also cannot fully account for the observed patterns of movement of the landslide. Rates of ground movement correlated strongly with rainfall irrespective of reservoir level (Müller 1964;Hendron and Patton 1985) but there is no evidence of artesian groundwater below the clay layers that gave rise to the failure surface (Dykes and Bromhead 2018). Indeed, it now appears that the landslide was underdrained and probably largely devoid of significant water pressures except following periods of very heavy rainfall when transient perched water tables may have formed above the clay layers.…”

“…This is the only reasonable explanation for the observed correlation of measured movement rates with rainfall throughout 1960-1963. Limited data from three piezometers (Müller 1964;Hendron and Patton 1985;Dykes and Bromhead 2018) show that the limestones above and below the clay layers responded to rainfall and to external reservoir water very quickly. Unfortunately, there are no data that can provide any evidence for the perched water tables within the landslide mass or for the impacts of reservoir impoundment on them, but reference base-line conditions can be reasonably assumed (BPore pressure assumptions^section).…”

“…The pore water pressures acting on the failure surface at the time of failure are unknown, although they were most likely hydrostatic pressures below a transient perched water table contained entirely within the landslide mass. Because the landslide appears to have been underdrained and may therefore have been mostly devoid of significant water pressures for much of the time (Dykes and Bromhead 2018), we started by backanalysing a completely dry slope. In any case, this produces a minimum possible value for the required minimum friction angle along the slip surface on which we base our conclusions.…”

“…The underlying premise of this paper is that the failure surface of the Vaiont landslide does not have a 'chair' shape (Dykes and Bromhead 2018). The logical consequences of this are that the 1963 landslide could not have been a reactivation of an ancient landslide.…”

“…Our review of the relevant evidence, including detailed examination of the extensive collection of photographs taken by Eduardo Semenza and colleagues in the years prior to 1963(Masé et al 2004) and of the various interpretations of seismic (Caloi 1966), hydrogeological and geotechnical (Müller 1964(Müller , 1968) monitoring data (e.g. Semenza , 2010 and references therein) revealed critical early (mis)interpretations and oversights that allowed the consensus to develop and consolidate (Dykes and Bromhead 2018). We also proposed a new explanation for the geomorphological context of the landslide that can account for all of the known evidence.…”