Investigation on rainwater infiltration into layered shallow covers in pyroclastic soils and its effect on slope stability

In recent years, a number of flowslides and debris flows triggered by rainfall affected a wide mountainous area surrounding the Piana Campana (Southern Italy). The involved slopes are covered with unsaturated air-fall layered pyroclastic deposits, which stability is guaranteed by the contribution of suction to shear strength. To understand the mechanisms of infiltration and to assess the soil suction distribution in such layered deposits, infiltration experiments have been carried out in small-scale slopes. The interpretation of the results has been made with the aid of a 2D mathematical model. The results highlight that the presence of coarse-textured unsaturated pumiceous layers, interbedded between finer ashy layers, can delay the wetting front advancement, thus initially confining the infiltration process within the uppermost finer layer. A diversion of the flow from the vertical towards the slope direction occurs when the soil approaches saturation. However, whereas a high hydraulic gradient establishes across pumices, water infiltration into the deepest layer begins, and part of the water crosses the pumices, before the complete saturation of the uppermost soil profile.