The Vajont landslide, 9th October 1963: Limit equilibrium model for slope stability analysis through the Minimum Lithostatic Deviation method

Fifty years ago the giant Vajont landslide slipped in the homonymous lake and caused a disaster. The purpose of this work is to apply classical limit-equilibrium methods as well as a the variant developed by Tinti & Manucci (2006, 2008) and called Minimum Lithostatic Deviation (MLD) method to analyse the stability of the Mt Toc flank from where the slide detached and to study the effect of the various factors influencing stability. The analysis was conducted on two profiles, one representing the west-side and one representing the east side of the slide, that were taken from the sliding surface as reconstructed by using pre-slide and post-slide topographic maps and by using suitable hypotheses from the literature on the shape of the hidden part of the surface (that is the surface that remained covered even after the slide occurrence). The analysis shows that the angle of friction is the most relevant parameter influencing the safety factor more than the material cohesion. The analysis shows further that the Vajont slide was close to instability. With data used in this paper, a drop of the basin level from 710 m a.s.l. down to 700 m a.s.l., in conditions of very high piezometric level (790 m a.s.l.) as can be produced by intense rainfall, may have the effect of drawing the safety factor below the critical line of 1 and give rise to instability.