This paper presents results obtained from a study of geotechnical properties of some pyroclastic deposits resting on the dolomitic limestone slope of Sorrentina Peninsula. The shallow layers of pyroclastic cover suffer landslides as flowslides. These partially saturated soils, dated from volcanic activity of Campi Flegrei and Somma Vesuvio, are often remoulded and weathered to clay [Pellegrino, 1991]. In order to investigate the effect of water content on the shear strength characteristics of undisturbed samples of these pyroclastic soils, a series of direct shear tests were carried out. It was observed that a rise of saturation degree (wetting) always leads to a contraction of the sample, no matter what the applied net vertical stress. Therefore these soils are known as collapsible. Furthermore the material behaviour is work hardening, showing a clear peak at low saturation degrees and low applied vertical stresses.


It's well known that the area surrounding the city of Naples suffers instability phenomena. In particular, mud or debris flows cause a significant hazard due to both the cinematic characteristics of the motion (high rate v>1 m/s and run out distances beyond 1 km) and the increased land use of the hilly areas. These movements tend to occur on slopes that are geologically young, steep, and therefore only marginally stable. The investigated area consists dominantly of limestone of Cretaceous age, belonging to carbonate platform sediments formed between the Triassic and the Palaeocene. These soils are generally above the groundwater table in a partially saturated condition; thus they are structurally unstable and they tend to collapse and to flow because of heavy rainfall and leaks from aqueducts and sewerage. Instability phenomena are triggered by meteoric events which give rise to large variations of the degree of saturation of the involved soils.

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