Hydraulic characterization of an unsaturated vegetated soil: The role of plant roots and hydraulic hysteresis

Flowslides and debris flows in granular soils pose a serious threat to human life and man-made structures. Due to rainwater infiltrating into superficial unsaturated soils, rainfall is the most common triggering factor of such landslides, causing a decrease in matric suction and hence in soil shear strength. Early warning systems based on accurate analyses of groundwater response to meteorological factors are widely used to mitigate landslide risk. In such a context, the accuracy of the model adopted to calculate the groundwater field is closely related to the reliability and meaningfulness of hydraulic soil characterization. In this paper, an extensive laboratory investigation regarding the hydraulic behaviour of pyroclastic unsaturated deposits from a vegetated slope monitored on Mount Faito (Campania, Southern Italy) is presented to highlight the importance of hydraulic hysteresis and the presence of roots in shallow soils. Water retention properties and hydraulic conductivity functions were determined, focusing on a drying–wetting cycle. Tests on specimens sampled in the top ten centimetres of the soil profile were also carried out to assess the effects of plant roots on soil hydraulic properties. Inverse analyses were used to estimate the parameters of a hysteretic hydraulic model. Finally, parametric numerical analyses, carried out via a finite element code, were used to highlight the potential effects of the hydraulic characterization on the stability of sloping pyroclastic covers, including all above factors.