The Root of the Slope Stabilization Problem

The practice of stabilizing the soil of slopes with vegetation has occurred for a significant period of time.  The soil on slopes lacking vegetation is often shifting and otherwise moving slowly downwards by gravity, rainfall, wind, and various other factors.  Erosion that occurs may lead to further instability and massive movement of soil towards the bottom of the slope.  While the effects of this are more or less inconsequential in remote locations of forested areas, when near human populations it can cause severe damage to property, roads, and lives.  However, with the addition of vegetation, the unstable slopes become less likely to shift due to heavy rainfall or other factors of erosion.  This is because the plants’ roots take up water from the soil, preventing the accumulation of excess water, and anchor the soil so that it acts as a unit rather than individual pieces.  Although we know this basic information, one question still remains: which root characteristics provide the most stability for the soil on these unstable slopes?

Mohammed Saifuddin and Normaniza Osman set out to find the answer to this question.  In their paper, “Evaluation of hydro-mechanical properties and root architecture of plants for soul reinforcement,” they compared the roots of three different legumes.  They focused on two properties of the roots: the hydro-mechanical characteristics (water uptake) and the root architecture (underlying structure).  They determined that the species Leucaena leucocephala (of the three species studied) was the most effective for use in soil reinforcement.  This species has a long taproot, the main vertical section of the root, and lateral roots, growing from the taproot, that extend horizontally while staying close to the surface soil.  These roots have a high tensile strength (the amount of stress that can be withstood before breaking) which also provides strength to the soil, reducing the chances of movement.  The increased number of fine lateral roots and elongated root improves the uptake of water, reducing the amount left in the soil.  With this information, they concluded that L. leucocephala can be planted on unstable slopes to reduce the amount of erosion.

This picture taken from the paper (Saifuddin et al. 2014) indicates the root structures of the three species that they observed.  The first species pictured in this table has the properties that they deemed to be the best at soil reinforcement.

Link to paper: http://eds.b.ebscohost.com/ehost/detail/detail?sid=1280345c-ee2e-4fe1-b391-ab45b0013f19%40sessionmgr115&vid=0&hid=113&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=aph&AN=98388258

Even though the one species that they found to have the most desirable root system may not be the best species to grow on all unstable slopes, this study provides a set of root characteristics to look for when selecting plants for soil reinforcement.  By planting species with a long taproot system that are also native to the area in question, the number of slope failures can be decreased by a good margin.  By extension, this also means that the area can be preserved in its current state and that there would be fewer instances of destruction in human inhabited areas.

In light of this article, it may be interesting to see how trees are able to provide structure to slopes that have collapsed in the past in order to prevent further collapses.  It would also be interesting to see just how many of these plants need to be placed in an area to reduce soil movement and if there is a threshold at which there would be no more benefit in increasing the number of plants.

References

 Saifuddin, M. and O. Normaniza. 2014. Evaluation of hydro-mechanical properties and root architecture of plants for soil reinforcement. Current Science. 5:845-852