Mechanical and hydraulic properties of unsaturated layered pyroclastic ashes in landslide-prone areas of Campania (Italy)

Abstract

Air-fall pyroclastic soil deposits usually display a loose fabric composed of alternating layers of ashes and pumices. Such deposits, when lying on steep slopes, represent a major geohazard due to the occurrence of landslides. This is the case of the carbonate massifs in Campania (southern Italy), a wide landslide-prone area of approximately 400 km2 covered with pyroclastic soils. In such cohesionless deposits, the additional shear strength provided by soil suction in unsaturated conditions is important for ensuring slope stability and can be jeopardized by soil wetting during rainwater infiltration. This paper provides a comprehensive view of the hydraulic and shear strength characteristics of different layers of pyroclastic deposits at different sites in Campania, revealing a broad view of their similarities and differences. To that end, some datasets from previous studies and novel data are gathered, linking the index properties, the hydraulic behavior of the soils and the contribution of suction to the shear strength of the studied materials. Two types of ashes at different positions within the stratigraphic sequence are identified: ashes interbedded between pumice layers, where landslide failure surfaces usually occur, and altered ashes in contact with the bedrock, which affects water leakage from the overlying soil profile. The former show quite uniform characteristics, and this allowed testing some predictive models for the assessment of the unsaturated shear strength of pyroclastic ashes in the absence of direct measurements. In contrast, the latter may exhibit significantly different behaviors, with great variability in hydraulic and mechanical properties.