Shallow flowslides triggered by intense rainfalls on natural slopes covered by loose unsaturated pyroclastic soils

The twentieth century is notable for a number of catastrophic landslides, involving essentially granular soils of pyroclastic origin, triggered by rainfall in a wide region around the town of Napoli. In particular, on 5 May 1998 a number of large flowslides killed 160 people (Cascini et al., 1998), but many other killer landslides of the same type have occurred in the last 20 years. Also, old chronicles report catastrophic landslides mobilised in the same area in previous centuries. The problem is hence recurrent, although in more recent .years catastrophic flowslides seem to be much more frequent. There are two possible explanations for this apparent increase: (a) the rapid growth in population and land use, which is responsible for risk increase; (b) climatic changes causing concentrated and intense rainfalls. Landslide risk analysis is thus becoming the principal concem for land managers. This technical note reports some considerations regarding the mechanics of rainfall-induced flowslides in unsaturated pyroclastic soils covering steep slopes in the Neapolitan region. The proposed considerations are supported by the results of an experimental programme in progress at the Seconda Università di Napoli.