Résumé
Using plant roots for soil mechanical reinforcement is a common practice in the field of ecological engineering to improve slopes and river banks stability. The study of the influence of roots on the stability of forested slopes requires identifying the effect of the different root-soil interaction processes depending on soil and roots properties. For that purpose, a numerical model of rooted soils based on the Discrete Element Method was developed. The model was used to study the interaction between the roots and the soil for different loading cases and, in particular, for direct shear tests. The soil is modelled as an assembly of locally interacting spheres and the roots are modelled as deformable cylinders in the soil matrix. The model allows accounting for the root tensile loading until breakage, the root bending loading, the root-soil cohesive interaction until cohesion breakage, the root slippage associated with a frictional resistance at the root-soil interface. The study focuses on identifying the different root-soil interaction mechanisms depending on the soil type. Both frictional and cohesive soil types were used in the simulations. The effects of the roots mechanical properties - tensile, bending modulus and root-soil interfacial friction angle - and of the root number were also analyzed for the different soil types. The analysis was held both at the macroscopic scale, that is on the shear stress-strain relationships for direct shear test, and at the particles scale by analysing the spatial distribution of the contact forces magnitude, sliding contacts, and adhesion breakage for cohesive soils. The effect of the roots strongly depends on the shear strain for any soil type. The roots do not provide reinforcement but entail decrease in the shear resistance until the shear strain is large enough to induce a significant loading of the roots. For frictional soils and increasing shear strain values, the processes inducing the increase in the soil shear resistance are successively the pure tensile loading of the roots and the tensile loading combined with slippage of the root-soil interface. For cohesive soils, the pure tensile loading of the roots is followed by a progressive breakage of the cohesive root-soil links and by a complete slippage of the roots. The results show that the influence of the root number is significant if the prevailing processes are root tensile loading combined with slippage whereas it is less important if root loading is combined with progressive breakage of the cohesive links for the root configurations explored. These results illustrate the interest of the model developed to analyze the stability of rooted soils.