Large-deformation simulations of root pull-out and breakage using material point method with a multi-level grid

Tree collapse due to uprooting under extreme winds has caused enormous economic losses and human injuries, even casualties, in many coastal cities like Hong Kong. It is therefore of great interest to assess the tree uprooting resistance for evaluating the risk of typhoon-induced tree failures and improving urban tree management. However, modeling the process of tree uprooting is challenging as it involves complex large-deformation behavior within a root-soil system. Previous simulations often used finite element methods (FEM), which suffer from severe mesh distortion problems during large deformation and cannot simulate the pull-out process along the root-soil interface. In this study, a numerical model that employs the material point method (MPM) with a multi-level background grid is developed to simulate, for the first time ever, the whole large-deformation process of root pull-out, including root breakage. The developed model is firstly validated against existing centrifuge test results. Then some new insights into the root pull-out during the large deformation stage are revealed. The results show that the sticking root-soil contact model is more suitable than the frictional contact model when conducting large-deformation modeling of root pull-out. A parametric study performed shows that the root pull-out resistance increases with increasing soil dilation angle. Finally, an illustrative example is presented to demonstrate the ability of the developed model in simulating root breakage failure during pull-out.