Wetting–drying response of an unsaturated pyroclastic soil vegetated with long-root grass

This paper investigates the effect of a long-root grass on the hydraulic response of a partially saturated pyroclastic soil. The work is based on both long-term monitoring under atmospheric conditions and short-term wetting–drying tests, aimed to simulate rainfall/evapotranspiration during different seasons. A one-dimensional physical model was created – namely, two identical columns were filled with a pyroclastic silty sand and later equipped with tensiometers and soil moisture sensors at four depths. One column was vegetated, while the second was left bare as a control. Root growth and foliage evolution were observed for 1 year. The monitoring of the hydraulic variables highlighted the capability of vegetation to modify the retention ability of the rooted soil. During the drying tests, the final soil suction within the rooted zone was higher than in the bare soil, particularly during summer, when plant transpiration is very high. In the wetting tests, the presence of vegetation delayed the infiltration process, reducing the total amount of water infiltrating the soil and consequently the chances of rainfall causing a drop in soil suction. This paper quantifies the effects of long-root grass, here conceived as a nature-based solution viable in landslide-prone areas.