Instability conditions of the landslides triggered by the 2006 rainfall event in Ischia Island, Italy

Ischia is an active volcanic island located in the Tyrrhenian Sea, approximately 30 km WSW from the city of Naples in Southern Italy. On 29-30 April 2006 Ischia was hit by an intense rainfall which triggering in the Mt. Vezzi area, in the SE portion of the island, four soil slips-debris flows. The flows caused the deaths of 4 people, forced the evacuation of another 250 inhabitants and destroyed several buildings. The debris flows initiated as soil slips at the soil – fallout interface, in hollows with slope gradients ranging between 35° - 40°, and quickly transformed into flows that reached the floodplain at the base of the hill, finally coming to a stop in a low gradient road. A geotechnical characterization of the site was carried out by means of laboratory test (grain size analysis, measurement of Atterberg limits, phase relationship analysis and ring shear box) and in-situ tests (Borehole Shear Test, Constant Head Permeameter and tensiometers). In order to identify the triggering conditions of the landslides Seepage-Stability analysis and Ring Shear Test were carried out. The ring shear test is an instrumentation developed within DPRI (Disaster Prevention Research Institute) of Kyoto University with the aim of simulating the process of failure in a sample soil.