An investigation of infiltration and deformation processes in layered small-scale slopes in pyroclastic soils

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 constituted by shallowunsaturated air-fall layered deposits of pyroclastic nature,which stability is guaranteed by the contribution of suction to shear strength.To understand the infiltration process and the soil suction distribution in such layered deposits, strongly affecting slope stability, infiltration tests in small-scale layered slopes reconstituted in a wellinstrumented flume have been carried out. The results highlight that the presence of a coarse-textured pumiceous layer interbedded between two finer ashy layers, in unsaturated conditions, delays the wetting front advancement, thus initially confining the infiltration process within the uppermost finer layer. However, when high hydraulic gradients establish across the pumices, water infiltration into the deepest layer starts. In sloping deposits, under high applied rainfall intensity, a seepage parallel to the slope seems to occur through the uppermost finer layer when it approaches saturation. At the same time, water infiltrates through pumices. Thus, in presence of long lasting intense rainfall, the presence of a coarse-gained layer does not necessarily impede the progressive saturation of the lowest soil layer.