Microscopic analyses of the reinforcement mechanism of plant roots in different morphologies on the stability of soil slopes under heavy rainfall

Currently, the research regarding the effects of plant roots on stabilizing earthen slopes lagged behind engineering practices and the relevant reinforcement mechanism from a microscopic perspective still remains uncertain. In this study, the stability of soil slopes reinforced by the plant roots in three different morphologies (the uniform, the upside-down triangular and the fusiform) was numerically simulated, which was verified by physical rainfall experiments. Then, computational fluid dynamics and discrete element method (CFD-DEM) coupling simulation was adopted to explore the different reinforcement mechanisms of the plant-root systems in different morphologies on the soil slope stability under heavy rainfall. It was disclosed that the roots in a uniform morphology reinforced the slope toe effectively by providing strong shear resistance in the middle and anchoring forces at the end of the root system. The roots in an upside-down triangular morphology strengthened the upper slope by virtue of its wide upper roots extending deeply into the lower soils and hence giving a full play to larger tensile forces. Compared with the previous two root systems, the roots in a fusiform morphology showed the weakest reinforcement effects due to the narrow upper roots and fewer lower roots for anchoring. During the rainfall-induced slope slip failure, the soil particles near the middle-upper part of slope featured faster sliding speed, longer sliding path and larger rolling angle than those closer to the lower slope. The existence of plant roots hindered soil particles from sliding and slowed down the development of slip surface, having the displacement area limited within the rhizosphere zone. The contact forces of roots declined first as the slip surface expanded upward and then increased slowly due to mobilization of the friction between the roots and soil particles. During the rainfall-induced slope failure, the lateral roots toward the slope interior had larger tensile forces; the longest tap-root in the middle of the root system was not always the one having the strongest tensile force, which changed over time.