Application of a method for the assessment of volcaniclastic soil sliding susceptibility to sample areas of Campania (Southern Italy)

In this paper the results of the application of a method for the assessment of volcaniclastic soil sliding susceptibility to sample areas of Campania are presented. The method, called SL.I.DE. (from SLiding Initiation areas DEtection), is based on the concept that, for a spatially homogeneous soil cover and a spatially homogeneous triggering rainfall sequence, there exist different values of threshold slope gradient for limit equilibrium conditions, depending on the continuity and planform curvature of the soil cover. A large part of Campania, which forms an arc-shaped belt that, from N of the Mt. Somma-Vesuvius goes clockwise to S of it, within a radius of some tens of kilometres from the volcano, is extensively covered by volcaniclastic soils that derive from the alteration of airfall-sedimented ashes and pumices. The geomorphology of the steep slopes they cover is largely independent of the soils, as the latter represent a thin veneer upon carbonatic bedrock. In this part of Campania landslides within such soils are frequent, following intense and/or prolonged rainfall events. Such landslides can be classified as complex ones, occurring initially as debris slides but rapidly evolving through liquefaction into debris avalanches and/or debris flows. The consequences of debris avalanches and flows at the foot of slopes and at gully outlets have too often been catastrophic, and in recent decades the risk associated to such landslides has been growing, due primarily to increased and often uncontrolled urbanization. While the localization of such landslides depends on both the spatial distribution of some characters of the soil cover and on the spatial distribution of the triggering rainfall event, it appears reasonable to focus on the former, in order to attempt to assess the soil sliding susceptibility in the event of a triggering rainfall sequence, as it begins to appear evident that landslide-triggering rainfall can indeed occur over very limited areas. To this regard, extensive real-time monitoring of rainfall on volcaniclastic soil covered slopes has not yet been implemented in Campania, as there exist very few automatic rain gauges on the slopes, and weather radars do not exist yet. Therefore, an assessment of the soil sliding susceptibility in case of triggering rainfall proves useful, as it allows to consider also soil stabilization directly at sliding-susceptible areas within the framework of the evaluation of the options for reducing the risk related to such landslides in the studied area. In order to test the effectiveness of the SL.I.DE. method at assessing where soil slides can originate in case of triggering rainfall, it has been applied to some slope areas where such landslides have occurred either recently or repeatedly in the past, providing data regarding the position of source areas that can be considered sufficiently reliable. Comparison between actual landslides’ source areas and areas that were identified as susceptible provided good results. In addition, a comparison with the results obtained by means of other published methods on the same areas is also provided