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| Titel |
An assessment of models that predict soil reinforcement by plant roots |
| VerfasserIn |
P. D. Hallett, K. W. Loades, S. Mickovski, A. G. Bengough, M. F. Bransby, M. C. R. Davies, R. Sonnenberg |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027001
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| Zusammenfassung |
| Predicting soil reinforcement by plant roots is fraught with uncertainty because of
spatio-temporal variability, the mechanical complexity of roots and soil, and the
limitations of existing models. In this study, the validity of root-reinforcement models
was tested with data from numerous controlled laboratory tests of both fibrous and
woody root systems. By using pot experiments packed with homogeneous soil, each
planted with one plant species and grown in glasshouses with controlled water and
temperature regimes, spatio-temporal variability was reduced. After direct shear
testing to compare the mechanical behaviour of planted versus unplanted samples,
the size distribution of roots crossing the failure surface was measured accurately.
Separate tensile tests on a wide range of root sizes for each test series provided
information on the scaling of root strength and stiffness, which was fitted using power-law
relationships. These data were used to assess four root-reinforcement models: (1) Wu et
al.’s (1979) root-reinforcement model, (2) Rip-Root fibre bundle model (FBM)
proposed by Pollen & Simon (2005), (3) a stress-based FBM and (4) a strain-based
FBM.
For both fibrous (barley) and woody (willow) root systems, all of the FBMs provided a
better prediction of reinforcement than Wu’s root-reinforcement model. As FBMs simulate
progressive failure of roots, they reflect reality better than the Wu model which assumes all
roots break (and contribute to increased shear strength) simultaneously. However, all of
the FBMs contain assumptions about the distribution of the applied load within
the bundle of roots and the failure criterion. The stress-based FBM assumes the
same stiffness for different sized roots, resulting in progressive failure from the
largest to smallest roots. This is not observed in testing where the smallest roots fail
first. The Rip-Root FBM predicts failure from smallest to largest roots, but the
distribution of load between different sized roots is based on unverified scaling
rules (stiffness is inversely proportional to diameter). In the strain-based FBM, both
stiffness and strength data are used to evaluate root breakage. As roots stretch across
the shear surface, the stress mobilised in individual roots depends on both their
individual stiffness and strain. Small roots being stiffer, mobilise more stress for the
same strain (or shear displacement) and therefore fail first. The strain based FBM
offers promise as a starting point to predict the reinforcement of soil by plant roots
using sound mechanical principles. Compared to other models, it provided the best
prediction of root reinforcement. Further developments are required to account
particularly for the stochastic variability of the mechanical behaviour and spatial
distribution of roots and this will be achieved by adapting advanced fibre bundle
methods.
Pollen, N., and A. Simon. 2005. Estimating the mechanical effects of riparian
vegetation on stream bank stability using a fiber bundle model. Water Resour. Res. 41:
W07025.
Wu T. H., W. P. McKinnell, and D. N. Swanston. 1979. Strength of tree roots and
landslides on Prince of Wales Island, Alaska. Can. Geotech. J. 16: 19-33. |
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