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Landslides in pyroclastic soils are among the most calamitous natural phenomena causing damage to people and territory every year. The mechanisms underpinning such landslides can be controlled by multiple factors, including site conditions and features of pyroclastic soils. Spatial variations of textural characteristics and related hydro-mechanical features in the overlain pyroclastic layers can yield to relevant effects on water infiltration, with potential formation of capillary barriers at the interface between different soil layers. A better knowledge of the key triggering factors, based on engineering-geological modeling, may be carried out through a detailed field and laboratory measurements, as well as performing numerical simulations. The area surrounding Palma Campania (Napoli, Italy) is largely affected by fast-moving landslides involving pyroclastic deposits. The volcanoclastic sequence is constituted upward by (white and grey) vesicular pumices and (black) dense scoriae levels, associated with the Somma-Vesuvius "Pomici di Base" Plinian eruption (22 ka). Stratigraphic sections along the carbonate slopes of Vallone Lupici reveal failure surfaces along various pyroclastic contact layers (white/gray vesicular pumice and dark dense scoria layers). Representative samples were collected at different stratigraphic heights and analyzed to define structural and textural characteristics; the obtained data were used as input parameters to perform 2D numerical simulations to explore the propagation of hydraulic front under different conditions of water flow (flow rate) and textural features of rocks. The obtained results allow us to put constrains on capillary barriers development in function of soils characteristic and rain fall intensity. [ABSTRACT FROM AUTHOR]

In the present paper, the capillary barrier formation at the interface between soil layers, which is characterized by textural discontinuities, has been analyzed. This mechanism has been investigated by means of a finite element model of a two-layer soil stratification. The two considered formations, belonging to the pyroclastic succession of the "Pomici di Base" Plinian eruption (22 ka, Santacroce et al., 2008) of the Somma–Vesuvius volcano, are affected by shallow instability phenomena likely caused by progressive saturation during the rainfall events. This mechanism could be compatible with the formation of capillary barriers at the interface between layers of different grain size distributions during infiltration. One-dimensional infiltration into the stratified soil was parametrically simulated considering rainfall events of increasing intensity and duration. The variations in the suction and degree of saturation over time allowed for the evaluation of stability variations in the layers, which were assumed as part of stratified unsaturated infinite slopes. [ABSTRACT FROM AUTHOR]