The Influence of the Geological Model in the Stress-Strain Analysis of the 1963 Vajont Landslide

The catastrophic nature of the Vajont landslide (volume of about 300 million m(3)) that occurred on 9 October 1963 in northeastern Italy emphasises the decisive role of a good geological model on the understanding of the mechanical behaviour of a large unstable rock slope. This large rockslide is a reference case study that is very useful for understanding the decisive role of the assumed geological model on the analysis criteria adopted in the slope stability evaluation. A recent (2006-present) survey performed on the failed mass and on the detachment surface allowed us to acquire considerable new geological data on the landslide structure and on the materials involved in the 1963 slope failure. The catastrophic en-masse sliding that occurred in 1963 was effectively a reactivation of a prehistoric large rockslide, as already hypothesised by previous studies dealing with the Vajont slide, but the structure of the prehistoric landslide was different from what was previously thought. The main result of the recent geological re-examination of the 1963 Vajont landslide is the identification of a thick shear zone (40-50 m) located at the base of an overlying unstable block. The occurrence of the basal shear zone, made up of limestone angular gravel, clay lenses and displaced rock masses, permitted a rapid seepage inflow triggered by the reservoir filling and also favoured the unusual en-masse movement of the upper unstable block. In particular, two specific unfavourable geologic conditions played an important role in the 1963 catastrophic event: the high permeability of the thick shear zone and the considerably low shear strength of some very thin clay lenses. Without considering this recently acquired geological data, it is very difficult to perform hydromechanical analyses or more sophisticated numerical models capable of reproducing the catastrophic Vajont slope failure and its unexpected final en-masse movement.