Hierarchy-dependent shear resistance and load sharing in root system reinforced soil: Insights from 3D DEM simulations

This paper investigates the shear reinforcement mechanisms of hierarchical tree root systems using three-dimensional discrete element method (DEM) simulations. A hierarchical root model based on Bischofia javanica was developed, explicitly representing taproots, lateral roots, and fibrous roots with realistic diameter gradation, and branching topology. Direct shear simulations were performed under normal stresses of 15, 30, and 45 kPa at different shear plane depths, corresponding to root area ratios (RAR) of 0.51%, 0.72%, and 1.21%. The results show that peak shear strength increases monotonically with RAR, with the strongest reinforcement occurring when the shear plane intersects root-rich zones. The shear stress-displacement response evolves from soil dominated frictional hardening to root controlled nonlinear strengthening, followed by post peak softening governed by progressive root rupture and stress redistribution. Lateral roots dominate shear resistance, fibrous roots provide an increasingly important contribution at higher RAR and normal stress, and the taproot contributes mainly through axial tensile resistance and load transfer within the root network. Assuming a constant friction angle, the fitted linear Mohr-Coulomb failure envelopes indicate that the reinforcement effect of the hierarchical root system is reflected as an apparent cohesion increase from 4.1 kPa to 8.7 kPa relative to the unreinforced soil.