Mechanical interactions between neighboring roots during pullout tests

Background and Aims: the quantification of root reinforcement function is important for landscape managers and engineers and it is often based on estimation of root mechanical reinforcement using models that do not consider the potential interaction between neighbouring roots. Root-soil mechanical interactions related to the root spacing and bundle geometry remain unclear including potential effects on the reliability of current models. The objective of this study is to quantify the mechanical interactions among neighbouring roots or roots networks using modelling approaches and pullout laboratory experiments. Methods: based on simple geometrical characterization of individual root geometry, we calculated dissipation patterns of frictional root-soil interfacial stresses in radial and longitudinal directions. Considering simple superposition of shear stresses within the soil matrix, we quantified characteristic root densities at which the radial mechanical interactions influence global pullout behaviour of the root bundle both for branched and unbranched roots. Laboratory pullout tests on root bundles were carried out at root spacings of 15, 35 and 105 mm. In addition, we tested effects of non-parallel (crossing) root bundle geometry. Results: we found no significant statistical differences in root pullout force for the different root spacing and crossing geometry while branching increase pullout force by 1.5 relative to unbranched roots. Moreover, the mean displacement at the pullout peak-force was 7.2% of length for unbranched roots and about 4.1% of length for branched roots. Conclusion: the study quantifies the influence of root spacing and arrangement geometry within a root bundle on its mechanical behaviour. The assumption of “non- interacting” neighbouring roots is no longer valid for root spacing less than 15 mm and root reinforcement methods. Moreover crossing roots shown a statistical difference This information is important for improved understanding root reinforcement mechanisms in steep hill slope and the interplay between anchoring failure and root bundle pullout vs. root breakage.