Field experimental study on the influence of heavy rainfall and Chinese fir roots on landslide stability: a case study in Northern Guangdong, China

Rainfall-induced landslides are widely distributed on vegetated slopes in Northern Guangdong, China. The instability mechanism of vegetated slopes under heavy rainfall remains insufficiently understood based on field studies. This paper utilized dye infiltration tests to explore the relationship between root characteristics and seepage characteristics; and carried out slope rainfall tests to monitor rainfall infiltration processes, mechanical and hydraulic responses, and slope deformation and failure. The results showed that coarse roots (> 2 mm diameter) greatly enhanced rainfall infiltration rates and promoted homogeneous flow in shallow slope soil. Owing to the mechanical reinforcement of roots, surface erosion and internal suffusion were not evident, allowing the slope to effectively resist deformation. With continuous rainfall, water infiltrated downward along root-soil interfaces and formed preferential flow, which was mainly controlled by fine roots (< 2 mm diameter). Once infiltration reached the maximum root depth, water accumulated locally in that area to form high pore water pressure. Deformation initially occurred at the top of the slope, and the slope was prone to overall failure along the maximum root depth. These findings improve field-based understanding of failure mechanisms in vegetated slopes.