Scaling of the reinforcement of soil slopes by living plants in a geotechnical centrifuge

•A tree, a shrub, and a grass were all suitable for use in centrifuge tests.•Two-month-old plants were appropriate for centrifuge testing using live plants.•Strong linear relationships existed between root tensile strength and Young’s modulus.•Compromise is necessary to reconcile scaling of root depth and root mechanical reinforcement. Understanding root-reinforcement of vegetated slopes is hindered by the cost and practicality of full scale tests to explore global behaviour at the slope scale, and the idealised nature of smaller-scale testing to date that has relied on model root analogues. In this study we investigated the potential to use living plant roots in small scale experiments of slope failure that would use a geotechnical centrifuge to achieve soil stress states comparable to those in the field at homologous points. Three species (Willow, Gorse and Festulolium grass), corresponding to distinct plant groups with different root architecture and ‘woodiness’ were selected and cultivated for short periods (2 months for Willow and Festulolium grass, 3 months for Gorse). The morphologies, tensile strength and Young’s modulus of these juvenile root samples and their effects on increasing soil shear strength were then measured (via tensile tests and direct shear tests) and compared with published results of more mature field grown specimens. Our test results show that when all juvenile root samples of the three species are considered, the commonly used negative power law does not fit the data for the relationship between root tensile strength and root diameter well, resulting in very low R2 values (R2