The instability of Colluvial mantle in Turbidite Flysch series of the Cilento Region (Campania-Southern Italy): The November 26, 2010, Ostigliano translational slide

On November 26, 2010, heavy rainfall triggered a shallow landslide which threatened the historical center of Ostigliano (Cilento and Vallo di Diano National Park, southern Italy). Faint but diffused damages on ancient buildings led to the evacuation of 60 families and to the assessment of the reactivation hazard by a monitoring campaign. The landslide caused a very low morphological impact due to: displacement of the depleted mass, limited to less than 1 m, shallowness of the surface of rupture and translational kinematics. Geological surveys, boreholes, inclinometric measurements, dynamic penetration and geotechnical laboratory tests were carried out to characterize and monitor the mass movement. The analysis of these data allowed to reconstruct an engineering geological model of the landslide, and to characterize it as a shallow phenomenon involving the colluvial deposits formed over a turbidite flysch bedrock. Based on peak and residual shear strength tests, infinite slope stability analyses of surficial deposits were carried out, simulating different thicknesses and pore pressure conditions. Among principal results, the landslide extension was observed constrained to zones where the thickness of colluvial deposits is greater than 2 m. Moreover, instability conditions were found for residual value of shear strength only. Results of this study are proposed as a first contribute to depict this type of shallow mass movements affecting the Cilento region, whose most part is characterized by turbitide flysch series and colluvial mantled slopes. .