The role of vegetation in slope stability
Vegetation can improve slope stability both influencing hydrological processes that determine stability conditions
and modifying directly the soil mechanical properties. Soil hydrologic balance depends directly on vegetation for its
influence on interception, infiltration, evaporation and transpiration. Roots water absorption reduce soil water
content. Plants usually have a positive effect on mechanical properties due to reinforcement action, anchoring the
shallower soil to the deeper (Fig. 1). Roots density within the soil mass and their tensile strength contribute to
improve the capacity of the soil to resist against shear loads. The maximum tensile strength or pullout resistance of
roots, together with an assessment of roots size and distribution (Root Area Ratio), can be used to evaluate the
appropriate root reinforcement values to be used in the stability analysis of a slope.
Development of the plants root apparatus is controlled by the interaction between genetics and environment.
Roots maintain their basic characteristics, that depend on the genotype, but the same species could have deeply
different root systems for as regards root density, diameters distribution, extension and depth that can be reached
[2,3].
Laboratory data show that tensile strength generally decreases with root diameter: root strengths are lower for
large diameters and higher for small diameters [4,5]. Moreover, root tensile strength depends on the biological
components of the root: smaller diameter roots have more cellulose than larger diameter roots and therefore are
characterized by higher strength [6].
The reliable benefit of apparent cohesion is limited to shallow depths, as root distribution is mainly concentrated
within 1m from the ground surface (Fig. 4). The use of an enhanced value of the soil cohesion is appropriate for
grass and shrub areas where fine root distribution with depth is consistent and easily defined [7]. Field studies of
forested slopes [8] indicate that fine roots, 1 to 20mm in diameter, are the ones that contribute most to soil
reinforcement. Grasses, legumes and small shrubs can have a significant reinforcing effect down to depths from 0.75
to 1.5m [9]. Some researchers have attempted to compute the values of apparent cohesion due to the presence of the
roots in the ground by designing and developing in situ shear tests for different types of root systems