The Mass of Vegetation
The mass of vegetation is only likely to have an influence on slope stability when larger trees are growing on the slope. A tree of 30 -50m height is likely to have a loading of approximately 100 -150kN/m2. The larger trees should be planted at the toe of the slope with a potential rotational failure as this could increase the factor of safety by 10%. However if the tree is planted at the top of the slope this could reduce the factor of safety by 10%. A suggested design vegetation envelope which shows which type of plant should be grown at each level of the slope.
Each slope stability situation should be considered independently for the vegetation involved. It is important to remember that transpiration will reduce the weight of the slope as moisture is lost. This can be significant on slopes of marginal stability.
If larger trees are removed from the toe area of a slope there will be both a reduction in soil strength due to the loss of evapotranspiration effects and a reduction in applied loading which may result in temporary suctions in clay soils which could lead to softening as the available water is drawn in to compensate for the suction forces. This is similar to the recognised softening of overconsolidated clays due to the relaxation of overburden pressures when placed in the top layers of an embankment from deep cutting.
Read more about this topic: Vegetation And Slope Stability
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