Tensile strength of a compacted vegetated soil: laboratory results and reinforcement interpretation

So far, root reinforcement on soil has been primarily evaluated through direct shear and roots pull-out tests, while the effect of other stress paths and the behaviour at the soil–root¿ interface are still poorly investigated. In this regard, an apparatus with the facility to test soil and roots jointly under uniaxial extension is presented in the paper, together with its first results. Vegetated samples with Cynodon dactilon were tested after one and three months of growth. Soil exhibited a ductile response when close to saturation and a brittle one at drier states within the field capacity domain. The presence of roots increased the material’s tensile strength and enhanced its post-peak ductility. Measurements of matric suction and degree of saturation allowed interpreting the results in terms of constitutive stresses within a shear strength failure criterion for partially saturated soils. Even if plant roots critically impacted soil hydraulics, a positive strengthening effect was noticed on its mechanical behaviour. Roots mechanical and morphological features were characterised after tests. Two well-established root reinforcement models in the literature were used to interpret the results at the phenomenological scale while considering the hydro-mechanical behaviour at the soil–root interface, different root’s reinforcement mechanisms and the effect of soil’s hydro-mechanical states.The authors wish to acknowledge the support of the European Commission via the Marie Skłodowska-Curie Innovative Training Networks (ITN-ETN) project TERRE ‘Training Engineers and Researchers to Rethink geotechnical Engineering for a low carbon future’ (H2020-MSCA-ITN-2015- 675762). Furthermore, Alessandro Fraccica wishes to thank Mercedes Sondon (Universitat Politècnica de Catalunya, UPC), Luis Gandarillas, Ylenia Bianchi, Ferran Parera (support with PIV software) and Stefano Collico (support with statistical analysis). The H2020-ITN project TERRE ‘Training Engineers and Researchers to Rethink geotechnical Engineering for a low carbon future’ (H2020-MSCA-ITN-2015-675762) provided funding.Peer ReviewedPostprint (author's final draft