Special Lecture - The role of suction and its changes on stability of steep slopes in unsaturated granular soils

A significant part of Italian mountain areas are covered by pyroclastic deposits resting at slope angles higher than 40-50 degrees. The stability of these steep slopes in loose or poorly cemented pyroclastic materials is essentially guaranteed by the positive effects of matrix suction on shear strength until an increase in saturation (and hence a decrease in suction) is induced by seepage initiated by different processes. The Cervinara flowslide (Campania, Italy) is a typical case where rainfall infiltration increased saturation and hence led to failure of shallow pyroclastic deposits. At La Fossa Crater (Vulcano Island, north of Sicily, Italy) seepage induced by condensation of vapor produced by the degassing activity of the active volcano seems instead to be correlated to slope movements. In the paper the two case studies are examined by means of monitoring data, laboratory investigations and numerical analyses.