Modelling root-soil mechanical interaction considering root pull-out and breakage failure modes

Aims  In the finite element method, the mechanical behaviour of plant roots has been modelled by solid element or embedded beam element (EBE). However, the former is computationally expensive, whereas the latter is unable to capture the root pull-out failure mode. In this study, we modified the constitutive stress-strain relationship of an existing EBE to calculate uprooting resistance by considering the root-soil interfacial shearing and the strength decline as root pulls out. Methods  We introduced an elasto–softening constitutive law to describe the root–soil interface interaction and an improved damage model to capture post–peak softening behaviour in EBE. We validated the EBE against three case studies. Finally, we conducted parametric analysis to study how root geometries, morphologies and soil saturation affect the uprooting response. Results  Our new model captures the pre–peak uprooting behaviour up to the peak pull–out force (P_ul). Root systems that failed by pull–out mode always had lower P_ul than those that failed by breaking, irrespective of the root morphology. Reduction of soil effective stress following soil saturation always reduced P_ul and could change the root failure mode, depending on the anchorage length and root–soil contact surface area. Conclusions  Root–soil mechanical interaction and root failure mode, including pull–out and breakage, can now be modelled with more accuracy. We show the importance of considering soil moisture variation, which translates to variations in root reinforcement effects. The reinforcement effectiveness of deep–rooted systems can be halved, and the root failure mode can switch from breakage to pull–out, following soil saturation and reduction of soil effective stress.