Anisotropic constitutive modelling of rooted soils

The mechanical behaviour of rooted soils is anisotropic. In this work, an anisotropic constitutive model for rooted soils is developed, incorporating two independent fabric tensors to represent the soil fabric and the root network. The effect of root tensile strength mobilisation on soil’s dilatancy and plastic hardening mechanism is addressed by introducing new fabric anisotropic variables (AB, AR), expressed as a joint invariant of fabric and loading direction tensors. A new root network evolution rule is proposed to capture the progressive root tensile strength mobilisation as the root orientation evolves towards the perpendicular direction of the major principal stress. The model is validated against test data. The model can predict the transition of sand’s strain-softening to hardening upon undrained triaxial extension due to the presence of roots. The model predicts lower strength and more contraction for rooted silty sands than the bare case during drained triaxial compression, as the former has a higher void ratio due to root inclusion and minimal root tensile strength mobilisation (indicated by a high AR and slow root network evolution rate). Conversely, during drained triaxial extension, the model exhibits higher shear strength and less contraction due to substantial root tensile strength mobilisation (indicated by a low AR).