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.

The tensile strength of roots and the friction characteristics of the root-soil interface of tree species are the indicators that play a crucial role in understanding the mechanism of soil reinforcement by roots. To calculate the effectiveness of the reinforcement of soil by tree roots based on essential influencing parameters, typical trees in the coastal region of southeastern China selected for this study were subjected to tests of the tensile mechanical properties of their roots, as well as studies on the friction characteristics of the root-soil interface and the microscopic interfaces. The results indicated that in the 1-7 diameter classes, the root tensile strength of both Pinus massoniana and Cunninghamia lanceolata was negatively correlated with the root diameter in accordance with the power function. The root tensile strength of these two trees, however, was positively correlated with the lignin content but negatively correlated with cellulose and hemicellulose contents. The shear strength at the root-soil interface and the vertical load exhibited a constitutive relationship, which followed the Mohr-Coulomb criterion. As the root diameter increased, both the cohesion and the friction coefficients at the root-soil interface gradually increased, but the growth rate stood at around 15%. The cohesion value of the root-soil interface of the two trees decreased linearly with the increase in soil moisture content within the range of 25 to 45%. At the microinterface, the root surface of C. lanceolata exhibited concave grooves and convex ridges that extended along the axial direction of roots, with their height differences increasing with the enlargement of the root diameter. The rough surface of P. massoniana roots had areas composed of polygonal meshes, with an increase observed in the mesh density with increasing root diameter.