Slope failure resulting in landslide along the North Saskatchewan River in Alberta
Plant and Site Characteristics
Slope stability is a result of the interaction between physical characteristics of a site, and of the types of plants growing on it. An intervention with Soil Bioengineering techniques (such as live stakes and live fascines) typically modifies mostly just the vegetation. If broadened to include Biotechnical Stabilization techniques (such as soft gabions, or live crib walls), then the physical characteristics of the site may be altered somewhat as well.
The series of figures below are based on the work of Tsukamoto and Kusuba (1984). These illustrate how the depth of soil, and the type of bedrock beneath it effect slope stability. In the first, there is thin soil underlain by impenetrable bedrock. In this instance vegetation will have limited effect on stability because plant roots cannot get a solid attachment into the bedrock below.
The second figure shows a circumstance with a similarly thin soil, however the difference in the bedrock leads to a very different outcome. Here plants can have a major impact on slope stability as their roots penetrate into the rock, and anchor not only the plant, but the soil mass bound to them.
1) Thin soil over impenetrable bedrock _2) Thin soil over bedrock roots can penetrate
_____= slight effect on slope stability___________= major effect on slope stability
The third figure depicts a thin soil similar to the previous two, however here there is a rocky transition layer between the soil and bedrock. While this transition layer does not provide as firm a footing for the plants as does the penetrable bedrock in the second figure, it still offers significant opportunities for plants to grip on to. On a site like this plants can have a substantial effect on slope stability.
In the final figure, the soil is thick, and as a consequence plant roots do not reach the bedrock in any significant numbers. In this situation plants are unlikely to have a significant effect on slope stability. Vegetation on such a site would reduce surface erosion through previously discussed mechanisms such as interception and decreased velocity of runoff. However this would not rule out landslides as pictured above, as well as less dramatic slope failures. In fact, by increasing infiltration, vegetation could potentially decrease slope stability in wet years.
3) Thin soil with transition layer to bedrock _4) Thick soil, roots don't reach bedrock
_____= substantial effect on slope stability_____= minor effect on slope stability
Soil Bioengineering has the most to offer on slopes similar to those depicted in the second and third figures, where plant roots can significantly improve stability. Consequently assessment of a site for use of such techniques should take this into account. Biotechnical stabilization or hard engineering alternatives may be more appropriate on slopes with characteristics similar to those depicted in the first and fourth figures. On flat, or gently sloping sites, soil depth and type of bedrock are less important considerations.
Continued: Suitability of plants for biotechnical stabilization
Trees curving in order to grow straight such as these indicate slope instability and soil slumping. These trees did not germinate at the 45 º angle that the base of their trunks are now at. Past slope failure tossed them forward, but was not drastic enough to dislodge them. Since then they have put on what is called "reaction wood" to push themselves back up straight in order to compete for light.