If large, woody roots penetrate beyond potential failure surfaces on slopes, mechanical stability can increase markedly when the soil is wet and prone to failure. Logging steep and unstable ground has produced disasters such as the death of more than 200 people in the Philippines in 2003 following a catastrophic landslide.
Closer to the surface of soils, a “stringy bag” of smaller or fibrous plant roots holds soil together against the stresses of water and wind. A group of soil scientists and geotechnical engineers from Dundee, Scotland have been investigating the underlying processes that control the mechanical reinforcement of soil by plant roots. The collaborative project between the Scottish Crop Research Institute and the University of Dundee was funded by the UK’s Engineering and Physical Sciences Research Council.
In a study published in the July–August issue of the Soil Science Society of America Journal, they used controlled mechanical tests on glasshouse-grown willow trees to unravel the processes involved in root reinforcement. The large amount of data collected was used to identify weaknesses in current models.
The authors performed shear tests on the root-reinforced soil. Broken versus pulled-out roots were separated on the shear-plane, and the diameters were measured. The shear strength of planted specimens compared with nonplanted specimens increased eightfold at 0.10-m shear depth, more than fourfold at 0.25-m depth, and more than double at 0.40-m depth. Separate tests measured the mechanical behavior of individual roots so that reinforcement could be modeled.
The most commonly used model, which is based on catastrophic and simultaneous failure of all roots, overpredicted reinforcement by 33% on average. Better agreement between experimental and model results was found for a fiber-bundle model. Although more commonly used to predict the performance of fiber-reinforced composites, such as the material used to manufacture tennis rackets and Formula 1 racing cars, this type of model offers considerable potential in describing and predicting the failure of root-reinforced soil. It simulates realistic conditions with roots breaking progressively from weakest to strongest with the load shared on the remaining roots at each step.
Despite this promising finding, the cross-disciplinary expertise of the researchers and the data collected allowed for some serious weaknesses in the modeling approach to be identified. In particular, the root failure mechanism was not considered properly. Roots either break or pull out from soil with numerous complex mechanical processes governing what happens. The next step for the research team will be to piece together this study with accompanying research on the bonding between roots and soil, novel geotechnical physical modeling with a large centrifuge, and further work on the biomechanics of plant roots.
By producing more realistic models of soil reinforcement by plant roots, the research could have numerous practical applications. There is growing interest in using plants to replace steel nails and concrete facings on roadside embankments because of cost and aesthetics. However, engineers have been slow to adopt the approach because predictions of mechanical stability are fraught with uncertainty. It is also not possible to predict the positive impacts of existing vegetation on soil reinforcement. Such an understanding could have major impacts on preventing landslides and soil erosion.
Adapted from S.B. Mickovski, P.D. Hallett, M. Fraser Bransby, M.C.R. Davies, R. Sonnenberg, and A.G. Bengough. 2009. Mechanical reinforcement of soil by willow roots: Impacts of root properties and root failure mechanism. Soil Sci. Soc. Am. J. 73:1276–1285. View the full article online at http://soil.scijournals.org/content/vol73/issue4