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

New, simplified and improved interpretation of the Vaiont landslide mechanics

Both the occurrence and behaviour of the Vaiont landslide have not been satisfactorily explained previously because of difficulties arising from the assumption that the failure surface was ‘chair’ shaped. It is now known that there was no ‘chair’, which means that the 1963 landslide could not have been a reactivated ancient landslide because the residual strength of the clay interbeds would have been insufficient for stability prior to 1963. Furthermore, the moderately translational geometry reduces the influence of reservoir-induced groundwater and hence of submergence. Standard stability analyses now show that prior to 1960, the average shear strength must have significantly exceeded the peak shear strength of the clay interbeds known to have formed the majority of the failure surface. Three-dimensional stability analyses confirm these results and show that at the time of the first significant movements in 1960, the rising reservoir level had a negligible effect on the Factor of Safety. According to these results, the Vaiont landslide was most likely initiated by pore water pressures associated with transient rainfall-induced ‘perched’ groundwater above the clay layers, in combination with a smaller than hitherto assumed effect of reservoir impounding, then developed by brittle crack propagation within the clay beds, thus displaying progressive failure. Further, very heavy rainfall accelerated the process, possibly due to reservoir-induced groundwater impeding drainage of the rainwater, until the limestone beds at the northeast margin failed. With the shear strength suddenly reduced to residual throughout, the entire mass was released and was able to accelerate as observed

Steady and transient seepage in soils with non-linear permeability characteristics

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The Southwell Topple – reassessment of a very large coastal toppling failure on the Isle of Portland, UK

The Southwell Topple is a spectacular example of a toppling failure on the southeastern coastline of the Isle of Portland, on the south coast of England. Types of mass movements, which occur around almost the entire coastline of Portland and include some other much smaller but well-known topples, vary depending on local geological and topographic contexts. The ‘Southwell Landslide’ of 1734 (i.e. the Southwell Topple), differs in most respects from all the others, not least in its size. We examine the historical and geological contexts of the Southwell Topple in order to explain its origins and characteristics. The recently published bathymetric data from the DORIS project reveals the tectonic context for the landslide, particularly the frequent transform faults parallel to the southeastern coastline of Portland and the axis of the Shambles Syncline forming Portland's ‘central depression’. It appears that the Southwell Topple resulted from coast-parallel tectonic discontinuities – probably a single joint and/or transform fault – through the Portland Stone combined with preferential marine erosion of the underlying weaker Portland Sand