Abstract: To study the effect of fissures and soil-root interstices on hydraulic and mechanical characteristics of the soil, the FEM numerical tools are used to study the influence of fissure and soil-root interstices on the infiltration during the rainfall process. Then the influence of the rainfall on the reinforcement of roots is analyzed. The zonal strength reduction method is applied to simulate the direct shear tests of the root-soil composite before and after the rainfall, considering the impacts of lateral roots’ angles. The results show that the fissures and soil-root interstices become the preferential channels for rainfall infiltration. The depth of rainfall infiltration increases with the depth of fissures. And it is mainly controlled by the vertical depth of the major root, while the influence of the inclination angle of lateral roots is small. The rainfall infiltration depth with root interstices is increased by 93.3% compared with the case without the root in the soil. The root system can significantly improve the shear strength of the soil. The shear strength of the soil with 60° lateral roots is the greatest, followed by the condition with 45°, 30° and without roots. Rainfall infiltration not only reduces the strength of the soil, but also weakens the reinforcement effect of roots. Therefore, the shear strength of soil-root composite system is greatly reduced after the rainfall, resulting in the sliding of the vegetation-covered slopes in a shallow depth.

Key words: root-soil composite, fissure, soil-root interstices, preferential flow, mechanical reinforcement, numerical simulation