Analysis of Shear Strength Variability of Ash-Fall Pyroclastic Soils Involved in Flow-Like Landslides

Rainfall-induced shallow landslides of flow-like type are very common in ash-fall pyroclastic soils originated from explosive activity of the Somma-Vesuvius volcano (southern Italy). Over the last few centuries, these phenomena have frequently affected pyroclastic soil-mantled slopes of mountain ranges that surround the volcano causing hundreds of casualties. Many researches have been focused on this topic, especially after the occurrence of the deadly debris flow events of May 1998, which hit Sarno Mountains causing 160 victims. Among the various aspects studied, aimed at the assessment and mapping of hazard to landslide initiation and propagation, the estimation of shear strength of ash-fall pyroclastic soils still deserves to be advanced. This is especially due to the relevant spatial variability of geotechnical properties which are controlled by complex stratigraphic settings. According to such a research focus, the present paper deals with physical and shear strength laboratory characterizations of ash-fall pyroclastic soils and the estimation of the inherent variability. A total number of 97 direct shear tests, supported by grain size and Atterberg’s limits analyses, were carried out. The high number of tests allowed to perform a statistical analysis based on quantile regression approach and aimed at considering the uncertainty related to the high variability of Mohr–Coulomb’s shear strength parameters. The results obtained show values, especially for the drained friction angle (ϕ′), generally higher than those considered in literature. Outcomes of the study and the approach proposed can be conceived as a benchmark for further analyses aimed at the assessment of hazard to initiation of this type of landslides or related physically-based rainfall thresholds.