Multiscale mechanical characteristics and slope protection effects of herbaceous roots in the loess region of the Qinghai–Tibet Plateau

Abstract Background The Xining Basin, located at the confluence of the northeastern Qinghai–Tibet Plateau and the western Loess Plateau, where extensive loess deposits are found, is densely inhabited. Major transport corridors, encompassing the Qinghai–Tibet Railway and the Lanzhou–Xinjiang High‒Speed Railway, traverse the region. The area is rendered highly susceptible to geohazards such as shallow landslides, collapses, and debris flows, especially during extreme rainfall, by its fragile geology and sparse vegetation. Posing serious threats to the local ecology, major infrastructure, and community safety, these hazards are manifestly concerning. Performing an investigation into how dominant flora enhance the stability of shallow landslides is of significance for the prevention and control of slope disasters. Methods In this study, the loess region of the Xining Basin was selected as the area of research. Three dominant herbaceous species— Elymus nutans , Elymus trachycaulus , and Elymus sibiricus —were chosen for single‒root and multiroot tensile testing. Combined with direct shear tests on root–soil composites, the enhancement effect of their root systems on soil shear strength was systematically evaluated. Results At the single‒root scale, there exists a salient increase in the tensile resistance for herbaceous roots with diameter, whereas the tensile strength manifests a decrease. Marked interspecific differences were observed: E. trachycaulus achieved the highest single‒root tensile resistance (17.10 N) and ultimate elongation rate (42.28%), whereas E. nutans exhibited the highest tensile strength (97.53 MPa). At the scale of multiroot, an increase in the number of roots increased the tensile resistance, whereas the tensile strength decreased. E. trachycaulus manifestly had the highest tensile resistance and strength in its multiroot (243.35 N, 36.43 MPa), whereas E. sibiricus had the lowest (46.14 N, 17.17 MPa). The tensile resistance and strength of multiroot were lower in measurements than in predictions on single‒root properties (14.86‒48.13%), which provides a demonstration of their mechanical behaviour entailing interactions of nonlinear complexity rather than superposition of linear simplicity. Direct shear tests on root–soil composites demonstrated that all three herbaceous species significantly enhanced soil shear strength. Compared with those of bare slopes, the cohesion ( c ) values of their root–soil composites increased by 157.99%, 104.54%, and 78.34%, respectively. Conclusion This study examined the mechanical characteristics of herbaceous roots and their slope protection mechanisms across scales from single roots to multiroot systems. These findings provide a theoretical and practical basis for the selection of vegetation and prevention of soil disasters in ecological slope protection projects.