Wetting-induced collapse behaviour of a natural and vegetated coarse pyroclastic soil

Abstract

Unsaturated pyroclastic soils originated by Vesuvius volcano show a collapsible behaviour upon wetting with a significant reduction in volume and rearrangement of solid skeleton. The paper investigates the role played by vegetation on wetting-induced collapse behaviour (namely, collapsibility) of reconstituted unsaturated soil specimens through two series of wetting tests in a standard oedometer. The first series of tests was performed on bare soil specimens, as to resemble the site conditions. The second group of tests was conducted on the same soil previously vegetated for 20 weeks with perennial graminae species, which are frequently used as a nature-based solution for contrasting surface erosion along slopes in different geo-environmental contexts. First, an initial small vertical net stress was applied on partially saturated specimens having similar initial saturation degree, then collapse was induced by flooding the specimens with distilled water and final vertical displacements were measured. As main outcome, soil porosity is highly reduced by the growth of grass roots. Consequently, the potential wetting collapse in the rooted soils is inhibited by low values of porosity. For similar initial soil porosity, in both bare and vegetated specimens (after root growth), a further reduction of the volumetric collapse magnitude is observed.