Numerical investigations into root vertical pull-out behavior

Abstract

Plant roots provide important soil reinforcement and improve the stability of slopes. From an engineering view, roots mechanically increase soil shear strength by transforming shear stress from soil into tensile forces of roots themselves via interface friction thus acting like soil nails. However, due to the complex spatial geometries and mechanical heterogeneities of natural root systems, more specific considerations are needed when analyzing the root reinforcement issues. According to a literature review, most studies on slope stability consider root reinforcement as an apparent cohesion by upscaling the behaviour of static individual roots. However, recent studies have shown that better predictions can be made if the progressive failure of roots is considered, thus highlighting the importance of load-displacement relations of soil-root interaction. Therefore, numerical pull-out tests considering the progressive friction interface relationship were carried out in this study to investigate the mechanism and the influences of various factors on plant root pull-out behaviour, which is of great importance to evaluating the stabilization effect of roots. In this study, the classic Coulomb friction model was adopted to simulate the interaction along the root-soil interface with the surface to surface modelling technique available in ABAQUS. The numerical investigations could be mainly divided into three parts: study of single straight root pull-out behaviour, study of the branched root segments with only first-order lateral branches (herringbone system), and study of the root segments with second-order branches (dichotomous system). Conclusions regarding the mechanism of the root pull-out process, the influence of geometry-related factors and the influence of the intrinsic factors related to mechanical properties and root-soil interaction were drawn based on the observations of the numerical pull-out tests. Progressive stick-to-slip behaviours along the root-soil interface were observed in the numerical models. Generally, the pullout resistance of roots increased with the branch depth and branch length. It was also observed that the pullout resistance had strong regression with the weight of the soil potentially lifted by the root system. The most efficient branch angle for providing pull-out resistance for the herringbone system was between 60 and 90 degrees and the most efficient branch angle for the dichotomous root system was shown to be around 45. The central symmetrical branch arrangement pattern was observed to be the most efficient in providing pull-out resistance compared with the plate symmetrical and asymmetrical patterns.