New modelling tools for quantification of mechanical reinforcement of soil by plant roots

Plant roots can help to stabilise riverbanks and slopes by providing additional mechanical reinforcement through tensioning of root material. This problem has typically been studied at the ultimate limit state, focussing on quantifying the peak root-reinforced soil strength. Existing models however rarely account for the gradual mobilisation of root-reinforcement associated with increasing soil displacements. Understanding these deformations is important when deformation tolerances are low, for example when constructing infrastructure embankments, or when deformations may serve as an early warning signal for slope failure.

Several new models to quantify mechanical reinforcement were developed, with varying levels of complexity. At the most basic level, fibre bundle model theory was combined with early pioneering work by Wu and Waldron to form a new fibre bundle approach that remains simple to use yet respects the physics of soil and root deformation. A second and more comprehensive analytical model was developed that can calculate reinforcements as a function of increasing soil shear displacement. This model includes key parameters such as the elasto-plastic biomechanical root behaviour, three-dimensional root orientations, root slippage and changes in the geometry of the localised shear zone in the soil. A third model comprises a full set of constitutive stress-strain relationships for rooted soil that can be used in numerical finite-element simulations. In this framework, the rooted soil is treated as a single, composite material in which the soil and root phase can each be assigned their own unique material behaviour. The composite approach simplifies model parameterisation by using independently measurable root and soil parameters, and is also powerful enough to investigate the complicated interaction between stresses and deformations in the soil skeleton and in the roots.

These models all provided good predictions of experimentally measured root reinforcements in direct shear tests. They will be useful tools both for the engineering industry, in terms of rapid quantification of root reinforcement, as well as for directing future research into the drivers of mechanical root-reinforcement.