Influence of the basal shear zone on long-term earthflow evolution

Abstract Earthflows are intermittent flow-like slope movements, which frequently occur in stiff clay deposits. After the very first post-failure stage, during which the landslide body displays a flow style attaining displacement rates up to metres per hour, the movement gradually slows down until complete arrest, which may occur even years later. However, further surging stages may take place, during which the mass speeds up again resuming the typical flow style. Site investigations carried out in the past on some slowly moving earthflows present in a vast area of Southern Italy allowed to bring to light quite a thick basal shear zone consisting of highly remoulded soils. Thanks to the data provided by those investigations about fabric and properties of such a significant landslide part, the paper describes a hydro-mechanical scenario, in which the shear zone plays a key role on landslide evolution possibly explaining its generally high thickness and the relatively high frictional shear strength that is mobilized by the soil mass during movement.