The catastrophic nature of the Vajont landslide (volume of about 300 million m3) 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.
