A modified embedded beam element to improve the modelling of root–soil interfacial behaviour

Abstract: Vegetation improves slope stability through mechanical root reinforcement and root anchorage. As shallow soil slides, plant roots extending beneath a potential shear band would be subjected to either bending or tension, depending on the root orientation with respect to the direction of shearing. The roots thus provide anchorage to stabilise the soil by mobilising the root–soil interfacial properties and the root tensile or/and bending strength until the roots are broken (i.e. breakage failure) or pulled out from the soil (i.e. pull-out failure). Modelling such a complex root–soil interaction mechanism is challenging. In existing modelling techniques of root–soil interaction, the mechanical behaviour of plant roots has been modelled by solid element or embedded beam element (EBE). The former is computationally expensive (thus being rarely and hardly used to model complex root architecture systems with multi-order root components), whereas the latter assumes unrealistic rigid root–soil bonding and thus is unable to capture the root pull–out failure mode (thus typically overestimating the root reinforcement). In this study, a newly modified EBE was derived by incorporating the effects of interfacial shearing and virtually permitting roots to be failed by the pull-out mode, in addition to breakage. The performance of the modified EBE was validated against three selected case studies, and the validated model was then used for subsequent parametric analysis on the effects of root morphology on the uprooting behaviour. Our simulation results show that the root systems whose morphology and branching pattern could gain more interfacial shear resistance (e.g., oblique second-order laterals in contrast to the horizontal case) and mobilise more root internal stresses (e.g., deeper branching point between first- and second-order roots) had higher uprooting resistance. Horizontal laterals mainly mobilised their bending strength to resist the uplift, but the oblique ones mobilised more tensile strength as their orientation was more aligned with the direction of the uplift.