Engineering geological models of the initial landslides occurred on April 30 2006, at Mount di Vezzi (Ischia Island, Italy)

On the April 30th, 2006, during the early morning, five shallow landslides were triggered on the northern side of the Mount di Vezzi (Ischia Island, Italy) that moved down slope, as very rapid flows, channelled into the hydrographical network, and reached the foot of the relief. One of the landslides stroke and destroyed a building located in correspondence of the outlet of a channel, leading to the loss of four human lives. Already by the earliest surveys, the landslides were recognised as shallow and involving loose pyroclastic soils covering a volcanic bedrock just for a depth less than one meter. Moreover the landslides appeared to be classifiable with a complex style and, according to the grain size of the depleted soils, as debris slides - debris flows with an intermediate phase of debris-avalanches. These general features make the landslides of Mount di Vezzi very similar to those that typically occur in the peri-vesuvian area on a carbonate bedrock. Owing to both these similarities, despite the different bedrock, and the interest in understanding the triggering mechanism leading to the initial debris slide, the research has been focused in the reconstruction of engineering geological models of the initiation areas. With this specific purpose, field surveys consisted of topographic, stratigraphic, penetrometric measurements as well as hydraulic testing have been carried out. Laboratory tests on the pyroclastic soils have been also executed in order to characterise their physical and index properties. Among the principal results achieved are the reconnaissance of a rotational kinematics of the initial slides and of the involvement of the surficial pyroclastic deposits, very loose and permeable, overlapping a fine ash pyroclastic soil with higher compaction grade and lower permeability. The field analyses, extended up to the characterisation of the discontinuities network in the outcrops of the volcanic bedrock located above the landslides, confirmed the hypothesis that the local increase of pore pressure up to the saturation due to a high intensity/duration rainfall as being the triggering factor. As for the landslide susceptibility assessment, the spatial localisation of the initial debris slides can be referred to a critical combination of higher pyroclastic soil thickness and slope angle, favoured by local morphological conditions and by irregularities of the interface with the bedrock. Basing on these characters, besides of the similarity in hydrogeomorphological conditions, the landslides occurred on the Monte di Vezzi can be considered analogous to those of the numerous other cases occurred along the peri-vesuvian carbonate slopes.