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Ash-fall pyroclastic soil deposits covering steep carbonate slopes in the peri-Vesuvian area (southern Italy) are periodically involved in shallow landslides (about 700 events were recorded during the last three centuries, as reported by CASCINI et alii, 2008), triggered by intense and/or prolonged rainfall events, which evolve as catastrophic debris flows. In the last decades, many studies have been focused on estimating reliable relationships among the triggering of shallow landslides and the amount and duration of rainfall events, as well as the role played by antecedent soil hydrological conditions. Results of these studies have been expected to give information on temporal hazard to landslide onset to be used for setting a reliable early warning system. In this paper we present probabilistic approaches to assess rainfall thresholds triggering shallow landslides by classical empirical methods and to manage the uncertainties related to biases of data. At this scope, rainfall events related to the occurrence of debris flows along slopes of the Sarno and Lattari Mountains, known from chronicles of the last century, were analyzed by means of the empirical models of Intensity-Duration (I-D) (CAINE, 1980) and rainfall recorded in the day of the landslide occurrence (P) vs the antecedent cumulated rainfall (Pa) (CROZIER & EYLES, 1980). In order to limit and to assess uncertainties related to biases of rainfall data, a comparison with the regional probability model of high intensity rainfall, carried out in the framework of the VAPI Project (ROSSI & VILLANI, 1994) has been carried out. Moreover, rainfall data were processed by a bivariate logistic regression model resulting in the assessment of probability to landslide triggering, given an assumed rainfall event. The I-D empirical rainfall thresholds obtained by Caine model (1980) were compared to rainfall thresholds estimated by deterministic approaches (DE VITA et alii, 2013; NAPOLITANO et alii, 2016) showing a good match.

Slow-moving landslides affect several areas in the Apennine chain (Italy). They involve sedimentary rocks that are weathered in the more surficial parts. The occurrence of weathering-related clay minerals (e.g. smectites) able to trap high amounts of water is generally considered a predisponding factor for these landslides. The main goal of this research is to verify this statement, carrying out a mineralogical-geotechnical characterization of clayey rocks occurring in the Termini Nerano slow-moving landslide (Massalubrense, Italy). The mineralogical composition of soils involved in the landslide was evaluated on several drill-core samples from the studied area, by using quantitative X-ray powder diffraction analyses, calibrated through chemical bulk rock analyses. Clay separates allowed to determine the composition of the clay fraction. The results confirm that the total amount of clays, and specifically the smectite amount in mixed layers I/S, could have played a role in the landslide development. However, relationships between mineralogical composition and weathering zones in the soil have not been observed. Geotechnical analyses attested that the mechanical behavior of the material involved in the landslide depends on the amount of granulometric clay fraction in the samples, but, differently from what expected, no correlations were observed between this behavior and the measured amounts of smectite or mixed layers I/S.