Training manual - The Vetiver Network International

Interestingly, recent research at the Agricultural Research Service of the US Department of
Agriculture (USDA) has discovered a unique fungi protein that may be primary glue (nicknamed
‘super-glue’ that holds soils together. The protein is named ‘glomalin’ , for Glomales, the scientific
name for the gruup of common root-dwelling fungin that secrete the protein through hair-like
filament called hypha.
7.1.2.3 Erosion Control and Slope Stabilization by Vetiver Grass: Unheard of by most
people up to the early 80’s until being actively promoted in the agricultural sector by Mr. Richard
Grimshaw, then of the World Bank; vetiver grass had actually been used a few centuries earlier in
India. When Indians moved overseas, vetiver was brought along with them to new localities, thus
explaining the presence of vetiver on various parts of the globe.
To assess the properties of vetiver for potential engineering usage, the author has conduced
some experiments to do just that. Four trial embankments were constructed and planted with vetiver
in August 1993 for the purpose of observing its field performance. Three months after planting, an
exceptionally heavy monsoon hit the site, which caused numerous failures along major highways
and hillside development projects. It was found that slopes planted with vetiver grass were not
significantly affected although some slopes in the vicinity failed. In mid-April 1994, an excavation
exercise was conducted to determine the rooting depth of vetiver. It was found that the massive root
networks had reached a record depth of 3.6 m after 8 mos. Of growth. Other conclusions drawn
form these trial included: vetiver could grow rapidly to form a complete hedgerow which managed
to trap wash-off soil material; from rooting depth monitoring exercise, it was evident that the roots
managed to penetrate the harder stratum (with fragments of rocks). They not only grew vertically,
but some seemed to incline themselves, following the side slope profile. From the limited trials
carried out, results appeared encouraging and promising indicating the tremendous potential of
vetiver for slope protection and stabilization work.
In response to a number of engineers’ call for more parameters for a wide range of vegetation
categories, so that they can plug them into elegant mathematical formulae in their designs, the
author and his colleague decided to carry out some experiments to include vetiver among the list of
vegetation with available parameters. One of the experiments involved tests on gain in shear
strength in soils by the presence of vetiver roots versus identical soils which are root-free. By
conducting large-scale direct shear tests at an embankment at varying depth levels, the increase in
share strength can be determined. It is also important to determine the root tensile strength
properties in the process of evaluating a plant species as a component of slope stabilization. This is
because when a plant root penetrates across a potential shear surface in a soil profile, the distortion
of the shear zone directly resists shear while the normal component increases the confining pressure
on the shear plane.
For the determination of root tensile strength, mature root specimens were sampled from 2year-old
vetiver plants grown on an embankment slope. The specimens were tested in fresh
condition, limiting the time elapsed between the sampling and testing to two hours maximum. The
un-branched and straight root samples about 15-20 cm long were connected vertically to a hanging
balance via wooden clamp at an end while the other end was fixed to a holder that was pulled down
manually until the root failed. At failure, the maximum load was monitored. The tensile strength of
root is defined as the ultimate root tensile force divided by the cross-section area of the unstressed
root (without bark, as it has weaker strength properties). It was found that the mean tensile strength
of veitver roots varies from 180 to 40 MPa for the range of root diameters 0.2-2.2 mm. The mean
tensile strength is about 75 MPa at 0.7-0.8 mm. root diameter which is the most common diameter
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