Root branching affects the mobilisation of root-reinforcement in direct shear

Abstract

The contribution of roots to the mechanical behaviour of soil has typically only been studied for the ultimate limit state. In these approaches, roots are typically modelled as straight and unbranched structures. This approach overlooks the fact that roots may have to deform significantly to mobilise their strength, a process that will be influenced by root architecture effects such as branching, amongst others. Sequential mobilisation of roots affects the peak root-reinforcement, thus differences in mobilisation are important to consider when quantifying root-reinforcement. In this paper, the effect of root branching was modelled using a large-deformation Euler-Bernoulli beam-spring model. The effect of soil was incorporated using non-linear springs, similar to p-y and t-z theory used for foundation piles. By connecting multiple beams together (i.e. applying appropriate linked boundary conditions at root connection points) the effect of branching could be analysed. A soil displacement profile corresponding with direct shear loading was then imposed and the response of the root analysed. It was shown that adding branches led to a quicker mobilisation of root-reinforcement. Branches increased the axial resistance to root displacement and changed the shape of the deformed roots. The presence of branching counteracted root slippage, and thus increased reinforcement. Larger branching densities increased this effect. This analysis demonstrated that the architecture of the root system has a strong effect on the mobilisation of root strength, which directly affects the maximum amount of reinforcement the roots will provide. Future modelling of root-reinforcement, both at the ultimate and serviceable limit state, should account for this effect.

Centrifuge modelling of root–soil interaction of laterally loaded trees under different loading conditions

“…To address this issue, reduced-scale 1g physical modelling studies in the lab (Zhang et al, 2018;Harnas et al, 2016;Mickovski et al, 2010) have previously been con-4 ducted to enhance the understanding of root-soil interaction. However, only highly simplified roots/root systems, which did not fully capture root architecture, were employed and these physical models may underestimate the push-over resistance as branching pattern has been shown in separate studies to have a large effect on root-soil interactions (Mickovski et al, 2007;Meijer et al, 2019). Additionally, the confining stresses in such 1g models did not represent field conditions (confining stress around 10 kPa) due to the reduced depths of the root analogues in the models.…”

Mechanisms of root reinforcement in soils: an experimental methodology using four-dimensional X-ray computed tomography and digital volume correlation

“…The diamond-shaped shear zone is in agreement with a previous modelling study [37] and is associated with the boundary constraints. The nonuniform shear zone thickness means that analytical modelling approaches to predict rooted soil behaviour such as [38][39][40][41], in which a constant shear zone thickness is an input parameter, need to be interpreted carefully and may require future modifications to capture this mechanism.…”

Vertical pullout tests of orchard trees for bio-inspired engineering of anchorage and foundation systems

“…Branching along the root length significantly increases pullout resistance (Schwarz et al 2011). The effect of branching is a subject of continued study and depends on both root and soil material properties that affect how branches deform and requires consideration of coupled axial and bending behavior of roots (Mickovski et al 2007, Meijer et al 2019a, Meijer et al 2019b. These studies indicate that unbranched model roots behave like piles or soil nails (Mickovski et al 2010).…”

“…It is clear, however, that the root system's ability to mobilize bearing resistances is an important contributor to its enhanced performance relative to conventional micropiles or shallow footings. Other authors have remarked on the importance of this lateral branching for engaging more soil to enhance anchorage (Dupuy et al 2005, Mickovski et al 2007, Meijer et al 2019a. The comparatively poor performance of the footing foundation indicates that self-weight is an inefficient method for developing resistance in tension relative to either the bearing or skin friction mechanisms.…”