Shear strength of unsaturated soils artificially vegetated in a field test site

The effect of a long-root grass on the shear strength response of a partially saturated pyroclastic soil was investigated through a field and laboratory experimental program. Field measurements of soil water content, suction, temperature, and laboratory tests aimed to estimate the shear strength of differently rooted soils were performed. The experimental investigation was carried out on a test site located in Nocera Inferiore, (Campania region, Italy), a few kilometers far from sites of past catastrophic flow-like landslides. The experimental program was carried out on three species of Perennial graminae grass species, characterized by fine and fasciculate long roots.  

In the field, soil temperature, pH, humidity, and suction were monitored from seeding. The trends were compared with those of air temperature and humidity. Moreover, soil suction and water content trends were related to daily rainfall.

Undisturbed pyroclastic soil specimens containing roots of perennial graminae grass species were collected after 5 months from seeding and tested at natural water content in standard and suction controlled direct shear equipment. The specimens exhibited different Root Volume ratio (RV) and suction. The shear envelopes were extrapolated using Bishop formulation of effective stress, which allows to consistently consider the partially saturated condition of the soils. The experimental results outlined that the shear strength envelope of vegetated specimens moves upwards in the τ-σ’ space, but also rotates counterclockwise. In general, the cohesion intercept increases, while the effective frictional angle reduces. Moreover, the RV influence on the magnitude of friction angle and cohesion has been assessed. Densely vegetated soils undergo larger modifications of the shear strength envelop than poorly vegetate specimens.   

The authors would like to acknowledge Prati Armati S.r.l. that provided the grass species used for the tests.