Erosion-resistance mechanisms of biopolymer-vegetation reinforced slopes under rainfall using experimental and coupled CFD-DEM methods

Rainfall-induced surface erosion poses persistent threats to the stability of highway slopes, causing significant economic losses and ecological degradation worldwide. This study investigates the combined effects of biopolymer (Xanthan gum) and vegetation (Festuca arundinacea) on rainfall-induced surface erosion through rainfall simulation experiments and CFD-DEM numerical simulations. Nine treatment combinations of biopolymer dosage (0-0.5%) and vegetation density (0-60 g·m-2) were tested under three rainfall intensities. Biopolymer and vegetation exerted opposing hydrological effects, with biopolymer increasing surface runoff by up to 17% while vegetation reduced it by 19%-42% through root-enhanced infiltration. Their combination reduced cumulative erosion by 93.1% relative to bare soil, and the relative contribution of biopolymer-vegetation interaction to erosion resistance rose from 13.9% to 24.1% with increasing rainfall intensity. Numerical simulations revealed that untreated slopes developed a positive feedback between porosity growth (0.44 to 0.58) and flow velocity increase (0.37 to 0.52 m·s⁻¹), driving progressive erosion escalation. Biopolymer bonding sustained inter-particle contact forces five times greater than untreated counterparts at stabilization, and the coordination number of the combined treatment reached 2.48 at stabilization, 34% higher than bare soil, confirming superior preservation of contact network integrity. These findings provide mechanistic insight into biopolymer-vegetation synergistic slope protection for transportation infrastructure.