Material stiffness, branching pattern and soil matric potential affect the pullout resistance of model root systems

  • Mickovski S
  • Bengough A
  • Bransby M
 et al. 
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Understanding of the detailed mechanisms of how roots anchor in and
reinforce soil is complicated by the variability and complexity of
both materials. This study controlled material stiffness and architecture
of root analogues, by using rubber and wood, and also employed real
willow root segments, to investigate the effect on pullout resistance
in wet and air-dry sand. The architecture of model roots included
either no laterals (tap-root) or a single pair at two different locations
(herringbone and dichotomous). During pullout tests, data on load
and displacement were recorded. These studies were combined with
Particle Image Velocimetry (PIV) image analysis of the model root-soil
system at a transparent interface during pullout to increase understanding
of mechanical interactions along the root. Model rubber roots with
small stiffness had increasing pullout resistance as the branching
and the depth of the lateral roots increased. Similarly, with the
stiff wooden root models, the models with lateral roots embedded
deeper showed greatest resistance. PIV showed that rubber model roots
mobilized their interface shear strength progressively whilst rigid
roots mobilized it equally and more rapidly over the whole root length.
Soil water suction increased the pullout resistance of the roots
by increasing the effective stress and soil strength. Separate pullout
tests conducted on willow root samples embedded in sand showed similar
behaviour to the rigid model roots. These tests also demonstrated
the effect of the root curvature and rough interface on the maximum
pullout resistance.

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