Mechanical Reinforcement and Anchorage by Plant Roots in Soil.

The direct role of roots in reinforcing soil and providing plant anchorage is obvious, but research on the underlying mechanisms is remarkably limited. Our research has started to unravel the mechanical interactions between roots and soil, from the mechanical behaviour of individual fibrous roots to processes occurring at larger scale. Controlled laboratory studies of soil reinforced with either willow trees or barley plants found a correlation between the cross-sectional area of soil covered by roots and shear reinforcement. We measured various properties of the roots and soil to unravel the mechanisms underlying soil reinforcement by roots. Data on root distributions across shear failure surfaces and root biomechanics were used to evaluate several models of root-reinforcement. Models based on catastrophic failure of all roots at once gave poor predictions of root reinforcement. Better agreement with observed behaviour was found for a strain-based fiber-bundle-model, in which roots break progressively, with the load shared on the remaining roots at each step. This approach is being used to identify root characteristics of crops, such as stiffness, strength and architecture that could influence stabilisation by roots and plant anchorage. One example is lignin modified cereals being developed for biofuel production. Our preliminary work in the area has found disruption to lignin biosynthesis pathways has a large influence on root biomechanics. At larger scale, reinforcement by plant roots has been studied using a geotechnical centrifuge that produces full-scale conditions in a controlled, small-scale model. Tests of soil slopes planted with willow trees or controlled root analogues revealed a downward migration of the failure surface compared to fallow slopes and for the first time allowed direct measurement of the reinforcing effect of vegetation in a slope.