Moisture Effects on Shear Strength and Slope Stability: Volcanic Ash Deposits from Sarno, Campanian Volcanism Region, Italy

Volcanic debris flows are large-scale, gravity-induced mass movements involving mixtures of water, mud, and volcanic sediments. These phenomena are among the most hazardous in volcanic regions, characterized by high impact forces, extended runout distances, rapid velocities, and unpredictable timing. The stability of a slope depends on the balance between driving forces (shear stress) acting parallel to the surface and resisting forces (shear strength), strictly connected to the particle size distribution, bulk density, degree of aggregation, and organic matter. Soil water content plays a critical role in this balance, influencing cohesion and internal friction, often leading to failure under lower shear stress thresholds for the same material and boundary conditions.

This study investigates the Campanian Volcanism region, an area of approximately 2,000 km² that includes over 100 towns identified as at risk. Particular attention is given to Sarno, a municipality in the western Campanian Apennines that experienced devastating rainfall-triggered debris flows on May 5–6, 1998, resulting in 160 fatalities and widespread damages. The region's deposits are composed of alternating pyroclastic layers (ashes and pumices) and colluvium overlying steep calcareous bedrock, a combination of factors that create conditions highly favourable to slope instability.

The primary aim of this research is to assess how variations in soil moisture content influence the shear strength of volcanic ash deposits, with a focus on defining the failure envelope as described by Mohr’s criterion. Laboratory analyses are conducted using an Anton Paar MCR 703e rheometer at the Chemistry Department of the University of Bari.

Ash samples collected from Sarno are subjected to controlled hydration tests, starting from a completely dry state and gradually increasing moisture content under carefully monitored conditions. Due to the specific design of the rheometer’s shear cell, the study is limited to particles passing through a 710 µm sieve. Additionally, X-ray diffractometry is employed to identify and characterize possible clay minerals in the samples, as different clay types can significantly affect soil rheological behaviour.

The findings of this study provide critical insights into the relationship between moisture content and shear strength, advancing our understanding of slope stability and the triggering mechanisms of debris flows. The obtained results will contribute to improving predictive models and informing mitigation strategies in volcanic regions.