Overview of water retention and suction stress properties of layered pyroclastic ashes in landslide prone areas of Campania, southern Italy

The loose, stratified composition of pyroclastic soil covers typically consists of layers of air-fall ashes and pumices. When these deposits are resting on steep slopes, they pose a significant geohazard due to slope instabilities. This scenario is evident in the carbonate massifs covered by pyroclastic soils in Campania (southern Italy), an extensive area of about 400 km² prone to landslides. In these porous deposits, the soil suction in unsaturated conditions plays a crucial role in enhancing the slope stability by providing additional shear strength.

This study aims to present a comprehensive overview of the hydraulic and shear strength characteristics observed in different layers of pyroclastic ashes across various sites in the Campania study area. By gathering datasets from previous studies and introducing new experimental data, the relationship between soil index, hydraulic properties and the shear strength in unsaturated conditions is examined.

The findings highlight notable differences in hydraulic properties of soils originating from the same location but belonging to different layers: ashes from intermediate layers within the soil profile, where failure surface usually occurs; weathered ashes in direct contact with the carbonate bedrock, responsible of water exchange with deeper systems. The water retention curves of intermediate ashes exhibit air entry values at approximately 4 kPa, while those in contact with the bedrock show values around 25 kPa. Conversely, soils from the same layer but from different sites exhibit relatively similar hydraulic characteristics. For example, intermediate ashes from the same layer typically display air entry values varying between 0.5 kPa and 4 kPa. The same behavior also appears regarding the influence of soil suction on the shear strength of the investigated materials, which can be estimated directly from the water retention curves.