An exploratory model of tree root pull-out in partly saturated soils

It is widely appreciated that plant roots enhance soil strength through two primary mechanisms: a decrease in soil saturation via root uptake and the mechanical interaction between roots and soil. It is less widely appreciated that these two mechanisms interact, and that mechanical root-soil interaction does change as saturation changes. The focus of this work, of an openly exploratory nature, is to gain insight into this aspect of the mechanical interaction between roots and soils through numerical modelling. We present a series of numerical simulations of root pullout tests, examining the effect of soil water content and root shape on pullout resistance. Numerical simulations are performed using a continuum based numerical method, the Particle Finite Element method that is able to represent the large deformations associated with pull-out. The soil is modelled as a partially saturated continuum, the constitutive response of which is described by a critical-state soil model, while roots are modelled as an elastic material. The soil-root interface is represented using a classical contact mechanics approach, where the tangential contact behaviour is described using a Coulomb law defined in terms of Bishop’s stress.