Postfailure Mechanics of Landslides: Laboratory Investigation of Flowslides in Pyroclastic Soils

Of all landslides, subaerial flowslides are the most dangerous. According to the literature, flowslides in saturated granular deposits are caused by static liquefaction induced by loss of stability. However, the catastrophic flowslides that occurred on steep slopes in Campania (southern Italy) in 1998, 1999, and 2005 were triggered by rainwater infiltration into shallow deposits of pyroclastic soils, which were initially unsaturated. Starting from the experience collected during the investigations of some of these landslides, the writers consider flowslide evolution as a result of a mechanical chain process characterized by saturation increase due to rainwater infiltration, mechanical degradation, volumetric collapse, and static liquefaction, culminating in complete fluidization of the soil. In the paper this chain process is investigated through wetting tests both in suction controlled triaxial apparatuses and in a well-equipped small scale slope model.