Particle size effects on the axial pull-out and push-in behaviour of roots

For smaller plant roots in coarse-grained soils, large relative size of soil particles compared to roots can affect their axial resistance. Even for the larger roots of trees, particle size effects may be important, e.g. when testing 1:N scale models of tree uprooting in a geotechnical centrifuge. In this study the distinct element method (DEM) was used to investigate this problem. The sinker root of a centrifuge model tree root system under axial loading was analysed, with its simulated behaviour compared with finite element method (FEM) simulations, where the soil was modelled as a continuum and hence did not incorporate particle size effects. Both were validated against laboratory tests. Considering the same prototype size and soil particle size distribution, different scale factors/g-levels were applied to model roots, hence varying the ratio of root diameter (d(r)) to mean particle size (D-50). Even at the lower d(r)=D-50 values investigated (6-21), particle size effects on end-bearing capacity were negligible upon push-in of the root. In contrast, effects on shaft resistance (for pull-out) were significant and were interpreted by a simplified analytical model developed in this study using a combination of cavity expansion and root-particle size ratio. The absolute size of the root analogues considered was also representative of small diameter roots present in other plant species at 1:1 scale, making the analytical model also applicable to crop-lodging problems and for defining input parameters for analyses of nature-based solutions (NBS) using vegetation (e.g. for slope stabilisation).