New, simplified and improved interpretation of the Vaiont landslide mechanics

Abstract

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. Threedimensional 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.

On the Stability of Deep‐Seated Landslides. The Cases of Vaiont (Italy) and Shuping (Three Gorges Dam, China)

“…Furthermore, a major assumption of the present work is that the landslides are creeping in a reactivation phase, allowing the material of the shear band to be considered at critical state, Journal of Geophysical Research: Earth Surface thereby deforming without any volume changes. This assumption eliminates the possibility of excess pore pressure generation inside the shear band due to volume changes, which has been shown in the literature (Garagash & Rudnicki, 2003;Iverson, 2005) to be a major mechanism that can destabilize Earth masses and landslides in particular Dykes & Bromhead, 2018a, 2018bVardoulakis, 2002b). It was adopted here because clay materials have experimentally shown to reach critical state after small amounts of accumulated shear strain, of the order of 1-2% (Tika & Hutchinson, 1999), and poses a strong limitation of the applicability of the present model to landslide events that are either a first-time event or not at critical state, or where the overburden is not rigid but deformable (Handwerger et al, 2016).…”

Hazards from lakes and reservoirs: new interpretation of the Vaiont disaster

“…This encompasses the tectonic history and structures, stratigraphy and the geotechnics of critical lithologies, hydrogeology of the entire slope, and Quaternary and Holocene geomorphology, as outlined earlier in this paper. The speed of the landslide is explained by the halving of the mean shear strength of the entire landslide as the last resistance provided by limestone beds near the (lower) eastern margin of the landslide was lost when they failed (Dykes and Bromhead 2018b), allowing the entire mass to accelerate as widely reported.…”

“…This demonstrates that the clay beds were certainly not pre-sheared (i.e. φ' = φ'peak), and by implication, the whole Vaiont landslide could not, therefore, have been a pre-existing landslide (Dykes and Bromhead 2018b). Instead it now appears to have comprised -largely if not entirely -an intact rock mass that had been deformed by tectonics but was most unlikely to have been a pre-existing landslide body.…”

“…There is a third contributory factor (CO3) which is the effect of the reservoir on the stability of the northern slope of Mt Toc. The principle of assuming minimum groundwater pressures, outlined earlier, was adopted by Dykes and Bromhead (2018b) to explore the effect of reservoir filling. In short, the reservoir water provided both support for the mountain slope on the one hand, but also a source of water that permeated into the slope and reduced the in-situ effective stresses, thus diminishing the shear strength.…”

Centrifuge Modeling and the Analysis of Ancient Landslides Subjected to Reservoir Water Level Fluctuation

“…Reservoir landslides have gradually become an important issue in engineering geology and geotechnical engineering after the occurrence of the Vajont landslide in Italy [1][2][3][4]. Reservoir-induced catastrophic landslides often cause huge losses in human life and infrastructure, such as the landslides in the Clyde reservoir [5], Revelstoke reservoir [6], Yesa Reservoir and Three Gorges Reservoir [7,8].…”