A generalised failure criterion for rooted soils

Plant roots are generally known to increase soil shear strength because of mechanical root reinforcement. Because of the anisotropic root distribution in soil, the shear strength of rooted soils is stress path-dependent, which is crucial to the engineering analysis and design of the stability of vegetated soil slopes. Existing failure criteria of rooted soils, which were mostly established based on direct-shear test results, were unable to account for the strength anisotropy of rooted soils under general loading conditions. In this presentation, we will propose and derive a new generalised 3-D anisotropic failure criterion for rooted soils. The anisotropic effects of root network and soil fabric on the shear strength of rooted soils upon various effective stress paths was captured by employing of the projection of the microstructure fabric tensors of soil and root network on stress tensors. To verify the proposed model, we will present the test results of drained compression and extension triaxial tests of saturated soil samples cultivated with deep-rooted vetiver grass (Chrysopogon zizanioides L.) under different over consolidation ratios (OCR; i.e., 1 and 3) and effective confining pressures (i.e., 50, 100 and 150 kPa). We discovered that upon compression stress path, the presence of roots increased the contractive behaviour of soil in normal consolidated samples, whereas the rooted soils were more dilative than bare soil in over consolidated samples. The presence of roots increased dilative behaviour of rooted soil following extension, irrespective to OCRs, because majority of the vetiver roots were grown vertically in a direction perpendicular to that of the major principal stress, resulting in greater mobilisation of the root tensile strength. Based on the calibrated model, we showed the anisotropies of both the cohesion and friction angle of the rooted soils. We also demonstrated why most of the studies that used the direct shear apparatus as a means of testing reported that roots affected almost exclusively the cohesion but not the friction angle. Indeed, the stress paths that direct shear followed are within section I of the deviatoric plane, where the effects of soil anisotropy on friction angle are not significant.