Although the root can enhance the soil's strength, vegetation cover landslide still occurs frequently under the rainfall. To elucidate the mechanism underlying the degradation of the shear strength of root-soil composites under the influence of moisture, we investigated trees from hilly slopes in southeastern China. The tensile mechanical properties of roots were tested under varying moisture conditions.The results of previous work on the friction characteristics of the root-soil interface under different soil water content were also considered. Furthermore, large-scale direct shear tests were performed to assess the strength characteristics of root-soil composites under different root cross-sectional area ratios (RAR) and moisture contents. Based on the widely used Wu model, and incorporating the failure modes of roots in root-soil composites and the mechanism of root-soil interface friction, a root-soil composite strength degradation model was established considering the effects of moisture. Moisture significantly affected the tensile strength of fine tree roots, with the tensile strength of fine roots being lower in the saturated state than in the fresh state. In contrast, coarse roots were almost unaffected by moisture. As the moisture content increased, the additional strength provided by the roots decreased, and the root efficiency (REp) decreased significantly. The model was validated against experimental data, and the calculated results were accurate. In root-soil composites, as moisture infiltrates, the tensile strength of the roots, soil shear strength, and root-soil interface shear strength decrease to different degrees. This results in reduced resistance to deformation in the root-soil composites, leading to a decrease in its strength.

Background and AimsRoots of plants have been shown to be effective in reinforcing soils against slope failures. Two key mechanical properties in such reinforcement are the root's tensile strength (TS) and elastic modulus (EM). However, knowledge on the combined effects of root moisture content (RMC) and root diameter on these properties is scarce. The study aims to quantify these relationships for root samples of four native Australian tree (A. costata, B. integrifolia, E. reticulatus, and E. racemosa).MethodsA series of tensile tests were conducted and the root diameter at the fracture point and RMC were measured immediately after each test. Data were analysed using both univariate and multivariate analyses.ResultsBoth TS and EM declined with increasing diameter. Power-law expressions were found to describe the relationship between TS and diameter moderately well, but less so the one between TS and RMC. Multivariate analyses yielded a double power-law for TS versus diameter and RMC with a stronger fit than univariate ones. A weaker power-law was found between EM and these 2 variables. Of the four trees tested, A. costata exhibited the highest tensile strength and elastic modulus at a 1 mm diameter, while B. integrifolia yielded the lowest.ConclusionConsidering both diameter and RMC as explanatory variables of TS and EM yield better accounts of experimental data. This work contributes to a better understanding of reinforcement capacity of trees generally, as well as the specific performance of roots of four native Australian trees.