Mechanical Behaviour of Leeward Lateral Roots During Tree Overturning

The overturning resistance of trees under lateral loads depends on the interaction between their root system and the surrounding soil, with leeward lateral roots being particularly important. This study presents a parametric investigation into the behaviour of leeward lateral roots during tree overturning using the finite element method (FEM) based on a beam-on-nonlinear-Winkler-foundation (BNWF) approach. The model efficiently simulates large root–soil deformations using non-linear p-y connectors, the properties of which were calibrated against 2D plane-strain continuum FEM simulations and validated against analytical solutions for pipeline bearing capacity (an analogous problem). Simulations varied in root diameter, length, and material properties. A critical root length was identified, beyond which further increases in length do not enhance the root’s contribution to tree moment capacity, defining an optimal root length for peak resistance. The study further demonstrates that moment capacity is profoundly more sensitive to root diameter than to length. Initial rotational stiffness, which is highly relevant to non-destructive field-based winching tests, was also found to be primarily controlled by diameter and independent of length for most practical cases. A direct comparison between leeward and windward roots under specified rotation conditions confirmed the greater mechanical contribution of leeward roots to anchorage, which is consistent with field observations.