An interdisciplinary approach for the characterisation of the clays involved in the catastrophic 1963 Vajont landslide

High-plasticity clay levels played a decisive role in the catastrophic 1963 Vajont slide, as also emphasised by Hendron & Patton (1985) in the detailed geological survey of the large upper detachment surface. However, many geological and geomechanical issues still remain open, because the geological context of these clay layers was not described in detail. This lack prevents full understanding of the clay influence on the catastrophic slope failure and on the subsequent high velocity sliding into the Vajont reservoir. A specifically-addressed geomechanical survey has been performed over the recent years (2006–2015) and many clay levels have been sampled (Paronuzzi & Bolla, 2012). On the basis of this new data, an updated engineering-geological map of the Vajont landslide has been implemented (Paronuzzi & Bolla, 2015). The clays involved in the 1963 rupture were analysed according to an interdisciplinary approach that considers lithostratigraphical, mineralogical and geotechnical aspects. This work presents the main results of this interdisciplinary characterisation. The clay samples (20) belong to three distinct units: 1) the main rupture surface involving the cherty limestone bedrock (Fonzaso Formation: Upper Jurassic); 2) the basal shear zone of the Vajont slide (angular gravel with contorted clay lenses and sheared off rock masses); and 3) the upper part of the Fonzaso Formation outcropping on the opposite valley flank. Field evidence confirms the occurrence of discontinuous clay lenses having a thickness ranging from 0.5 to 5 cm, in most cases. Clay materials exhibit variable colours: greenish grey, pale olive, pale yellow, light grey, and even white. XRD analysis revealed a rather constant mineralogical composition, including calcite, quartz and montmorillonite (19 samples), whereas plagioclase have occasionally been identified (2 samples). Montmorillonite shows both a well-crystallised structure (class A) and a poor-crystallised structure (class E), thus influencing the shear strength characteristics. Laboratory investigations on the clay samples testify to a great variability of their geotechnical properties. The clay fraction ranges from about 10% to 60%. Three main groups of clay materials can be identified on the basis of their plasticity properties: low-medium, medium and high plasticity clays. The liquid limit varies considerably, ranging from 30% up to 90%. Clay samples were also tested in the laboratory through ring shear test apparatus to investigate their shear strength. The results of the shear tests emphasize a notable variation in the characteristic residual friction angle (φres) that ranges from very low (6.7–9.7°) to considerably higher values (25.1–26.7°), thus denoting a remarkable (and previously not pointed out) geotechnical variability of the clay levels involved in the catastrophic 1963 slope failure.