Root-induced changes in the hydraulic properties of a fine slope cover

Recently, the soil-vegetation-atmosphere (SVA) interaction is becoming a topic of intense scientific research within the geotechnical community, because it was recognized to potentially induce significant pore pressure variations in slopes, at both shallow and larger depths, being then responsible for weather-induced landsliding. The processes within such interaction are of different nature, and result in a chemo-thermo-hydro-mechanical transient boundary condition at the ground level, causing exchanges of liquid, gas and energy within the soil cover layer (from ground level to 3-4 metres depths). Hence, both the soil cover material and its thermo-hydro-mechanical constitutive properties, as well as the vegetation properties, become of key relevance within such processes. In this study, with reference to a full scale in-situ test where deep-rooted crop spices were seeded and farmed, the hydraulic properties of rooted clayey soil were investigated in the laboratory as well as in-situ in terms of both saturated permeability and retention properties. As for the saturated permeability, both laboratory and in-situ tests were conducted, the latter being also back-analysed. The retention states of the material were investigated by means of the filter paper technique in the laboratory and were of use for the numerical back-analyses of the Guelph-induced wetting process in the soil cover. The measurements show that roots within the vegetated zone determine both an increase of the permeability and a reduction of the retention properties of the composite material if compared to the soil in bare conditions.