Soil infiltration and slope stability of shrub-covered loess slopes on the northeastern Qinghai–Tibet Plateau: experimental and numerical simulation

The impact mechanism of vegetation on slope soil water infiltration and stability in the loess areas of the northeastern Qinghai-Tibet Plateau remains unclear. Understanding this mechanism is crucial for regional ecological restoration and shallow geological disaster prevention. This study investigated slopes covered by Caragana korshinskii Kom. by employing double-ring infiltration tests to explore the permeability characteristics and influencing factors of root-containing soils and to propose an appropriate infiltration model. Considering the synergistic effects of the canopy and roots, the hydrological response and stability of vegetation-covered slopes under rainfall infiltration conditions were evaluated through numerical simulation analysis. The results revealed that within the main root distribution layer (0–0.5 m), the initial and average infiltration rates and the permeability coefficient of the root–soil composite were significantly higher than those of bare land. Coarse roots with diameters of > 5 mm were the key contributors to enhancement of the infiltration capacity. The dry density, fine particle content, and initial water content of the soil around the roots were negatively correlated with the infiltration process. The Horton model effectively reproduced the infiltration process under the canopy and on bare land. The roots significantly accelerated the advance of the slope wetting front during rainfall infiltration, whereas the canopy delayed its onset and progression. The rainfall infiltration process on vegetation-covered slopes was divided into three stages: the equilibrium infiltration stage, optimal infiltration stage beneath the canopy, and secondary equilibrium stage. Vegetation enhances slope stability through coupling of the canopy and root, with an order of canopy-root mode > root mode > bare slope. Under heavy rainfall conditions, the direct contribution of canopy interception to slope stability is limited, and its primary role is to delay the occurrence of instability. During this period, the mechanical effect of roots becomes the dominant mechanism in slope protection.