Shear strength anisotropy of rooted soils

The shear strength of rooted soils depends on the principal stress direction owing to the anisotropy in the soil structure and root system. Existing failure criteria cannot describe the strength anisotropy of rooted soils under general loading conditions because they are mainly based on the test results of direct shear. This study presents a new generalised three-dimensional anisotropic failure criterion for rooted soils. The model employs the projection of two independent microstructure fabric tensors (soil fabric and root network) on the stress tensor. Twenty-four drained triaxial compression and extension tests were carried out to measure the strength anisotropy of silty sand vegetated with vetiver grass (Chrysopogon zizanioides L.) at different overconsolidation ratios and calibrate the material parameters for the proposed criterion. Anisotropies of both cohesion and friction angle exist in rooted soil. Roots contribute mainly to the increase in cohesion (hence root cohesion) from most of the direct shear test data. Roots with predominant orientation aligning in the tensile strain direction contribute the most to soil strength. In the case of vetiver grass, which has a taproot system, their roots show the strongest reinforcement effect in the conventional triaxial extension path, in which the maximum portion of the roots is subjected to tension.