Mechanical Root Reinforcement for Slope Stability: A State-of-the-Art Review and a Proposed Trait-Based Functional Root Classification System

Abstract

Vegetation has been used for slope stabilization for centuries, with conventional measures like soil nails and piles serving to complement the root reinforcement. Vegetation contributes to reinforcement through mechanical reinforcement provided by the root, and hydro-mechanical effects associated with evapotranspiration. Despite decades of research, translating root reinforcement into engineering practice remains unclear because most studies focus on specific plant species rather than on mechanically meaningful root traits, limiting comparability and predictive capability. This paper provides a comprehensive review of root characteristics and root types relevant to mechanical root reinforcement, specifically root morphology (e.g., diameter) and architecture (e.g., orientation and root area ratio (RAR) distribution). It also reviews the state-of-the-art of existing methods for quantifying mechanical root reinforcement: theoretical models, experimental methods from laboratory to field-scale, and numerical approaches based on FEM and DEM. Building on identified gaps and the need for functional trait-based root classification, a three dimensional finite element model with an explicit representation of simplified roots is developed and validated against field scale direct shear tests on vegetated soil. A systematic parametric analysis quantifies the effects of RAR, geometric arrangement (radial vs. square), structural versus non structural root systems, and root orientation relative to the shear plane. The results show that RAR alone is insufficient to uniquely characterize mechanical root reinforcement, because root systems with the same RAR can mobilize substantially different strength increases depending on the presence of dominant structural roots, their orientation relative to the shear plane, and their spatial arrangement. Based on these findings, a functional root classification is proposed that incorporates these parameters. This classification provides a more standardized and mechanically relevant basis for geotechnical analysis, numerical modeling, and vegetation-based slope stabilization design.