Volumetric Water Content and Shear Failure of Volcanic Ash: Evidence From Powder Shear Cell Analyses

ABSTRACT Volcanic debris flows are among the most hazardous mass-movement processes because they are unpredictable and can rapidly attain high velocities and destructive power, causing fatalities and damage to buildings and infrastructure. Their initiation and propagation are controlled by a combination of predisposing conditions (topography, catchment morphology, geological/structural setting, etc.) and transient hydrological forcing. Water is often the key agent because it modifies the effective stress state of the soil and can favor excess pore-pressure buildup, thereby directly affecting shear strength. In this study, we investigated how increasing water content modifies the mechanical behavior of three ash-rich soils collected in the Sarno area (Campanian Volcanic Province, Southern Italy). The deposits were tested at volumetric water content values from 0 to 8 percent in 2 percent increments through undrained shear experiments performed with a rheometer under controlled conditions. The results indicate the onset of matric-suction effects associated with water-meniscus formation as the material transitions from dry to moist conditions, accompanied by an initial increase in apparent cohesion and a shift in flow behavior from a free-flowing granular response to a markedly cohesive regime. These results improve the knowledge about the influence of the moisture content on the mechanical stability of volcanic ash soils, providing constraints for predictive modeling and hazard mitigation in volcanic terrains.