A mechanical conversion model was developed using a numerical method to correlate the shear strength increment due to root, ΔSr, with ultimate pull-out resistance, Pu, of the soil/root system. The conversion model can be applied to determine the required shear strength parameters for stability analyses of vegetated slopes. Trema orientalis (L.) Blume (India charcoal trema),a pioneer and dominant tree on the low slopeland of Taiwan, was selected as the target species of the study for the field investigation of root morphology, in-situ pull-out tests, and laboratory tensile strength tests. Using the root morphology and the strength parameters of the root material, one can develop a 2-D numerical model of the soil/root system to simulate ultimate pull-out resistance, Pu,from the pull-out test and the shear strength increment due to roots, ΔSr, of the soil/root system from direct shear tests. Eventually, a relationship function, ΔSr=f (Pu) can be proposed for practical use.
Key Words : Soil/root system, ultimate pull-out resistance, shear strength increment due to roots.