Positive and negative effect of herbaceous plant roots on the shear stress of shallow loess

The reinforcing mechanisms of herbaceous plant roots on the shear stress behaviour of shallow loess were explored through an analysis of root-soil mechanical interactions. Festuca arundinacea (tall fescue) was selected as a representative herbaceous species， and root biomechanical property tests， triaxial compression tests， and nuclear magnetic resonance (NMR) measurements were conducted to elucidate the underlying mechanisms. Results indicate that herbaceous roots significantly modify the shear stress behaviour of root-soil composites. First， shear strength increases with root biomass ratio (RB)， primarily due to root reinforcement and root-soil bonding， as confirmed by tensile tests on loess and roots. However， with further increases in RB， shear strength shows a weak negative correlation with RB， jointly controlled by pore structure alteration and weakening of the soil skeleton. Although the contribution of the negative mechanisms is smaller than that of the positive mechanisms， it should not be neglected under excessive root density. On this basis， a quantitative equation was proposed to identify the boundary between stages dominated by positive and negative effects， corresponding to an RB of approximately 0.531% in this study. Below this threshold， the positive effect dominates; beyond it， the negative effect becomes dominant. This study reveals the mesoscale mechanisms governing the non-monotonic evolution of shear stress in root-soil composites and provides a physically based understanding of herbaceous root effects on the shear behaviour of shallow loess.