In order to understand the mechanical characteristics of tree roots and their mechanical effects on slopes, the landslide in Wuping high vegetation coverage area of Fujian province was selected as the research site, and the root tensile mechanical properties of typical tree roots in the study area were tested after classification by diameter class. Furthermore, in-situ direct shear tests of root-soil composites under different root cross-sectional area ratios (RAR) and moisture content were conducted at the landslide site, and investigations were made into the distribution characteristics of roots in the profile to explore the mechanical effects of roots on shallow landslides. The results showed as follows: (1) The tensile force of Pinus massoniana and Cunninghamia lanceolata ranged from 12.45−673.09 N in 1−7 diameter class, and the tensile force was positively correlated with the root diameter by power function; The tensile strength ranges from 7.16 MPa to 60.95 MPa, and the tensile strength is negatively correlated with the root diameter as a power function. The average tensile force and tensile strength of Cunninghamia lanceolata root were higher than those of Pinus massoniana. (2) Tree roots significantly improved the shear strength of soil, and the additional cohesion provided by roots to soil was significantly positively correlated with the shear plane RAR. The root structure of Cunninghamia lanceolata is closer to R type, and that of Pinus massoniana is VH type. Under similar RAR, Cunninghamia lanceolata roots has a better reinforcing effect on the soil than Pinus massoniana. (3) With the increase in moisture content, the shear strength of the root-soil composites of Pinus massoniana and Cunninghamia lanceolata significantly decreases, as water infiltration diminishes the additional cohesion provided by the root systems to the soil. (4) Based on the Wu model, considering the influence of moisture content on soil cohesion and additional root cohesion, an estimation model for the shear strength value of root-soil composites considering moisture content was established. Upon verification, the accuracy of this model proved to be higher than that of the Wu model, and the results were reasonable. (5) Although the root system has a reinforcement effect on shallow landslides, its contribution to the stability of shallow landslides under heavy rainfall is limited due to the influence of root distribution depth, density and water infiltration.

Due to rainfall, the soil-rock differential weathering interface of spherical weathered granite soil slopes is prone to evolve into a preferential seepage channel and undergo seepage-induced suffosion, which accelerates the deformation and instability of these slopes. However, little research has been carried out on the characteristics of seepage-induced suffosion and the migration of fine particles. Based on the unsaturated seepage theory of porous media, a numerical calculation framework is established to accurately describe the seepage-induced suffosion process at the soil-rock interface, considering the coupling relationship between the fine particle migration, suffosion initiation response and unsaturated seepage. The finite element method is used to construct a seepage-induced suffosion model for unsaturated granite residual soil under the effect of preferential flow. Based on the seepage-induced suffosion process of homogeneous soil columns, the suffosion characteristics of preferential flow under three typical soil-rock interface burial states are systematically investigated. The results show that the soil-rock interface and the matrix permeability of spherical weathered granite soil slopes are highly variable, with the wetting front forming a downward depression infiltration funnel, and the degree of depression of the wetting front becomes more pronounced as rainfall continues. The degree of fine particle loss is related to the burial state of the soil-rock interface, in which the preferential flow potential suffosion of the under-filled soil condition is the most significant, and even excess pore water pressure occurs at the interface, which is the most unfavorable to the stability of this type of slope. The research results can provide a scientific basis for accurately evaluating the stability of spherical weathered granite soil slopes under rainfall conditions.