Vegetation and slope stability

Vegetation and slope stability are interrelated by the ability of the plant life growing on slopes to both promote and hinder the stability of the slope. The relationship is a complex combination of the type of soil, the rainfall regime, the plant species present, the slope aspect, and the steepness of the slope. Knowledge of the underlying slope stability as a function of the soil type, its age, horizon development, compaction, and other impacts is a major underlying aspect of understanding how vegetation can alter the stability of the slope. There are four major ways in which vegetation influences slope stability: wind throwing, the removal of water, mass of vegetation (surcharge), and mechanical reinforcement of roots. Vegetation and slope stability are interrelated by the ability of the plant life growing on slopes to both promote and hinder the stability of the slope. The relationship is a complex combination of the type of soil, the rainfall regime, the plant species present, the slope aspect, and the steepness of the slope. Knowledge of the underlying slope stability as a function of the soil type, its age, horizon development, compaction, and other impacts is a major underlying aspect of understanding how vegetation can alter the stability of the slope. There are four major ways in which vegetation influences slope stability: wind throwing, the removal of water, mass of vegetation (surcharge), and mechanical reinforcement of roots. Wind throw is the toppling of a tree due to the force of the wind, this exposes the root plate and adjacent soil beneath the tree and influences slope stability. Wind throw is factor when considering one tree on a slope, however it is of lesser importance when considering general slope stability for a body of trees as the wind forces involved represent a smaller percentage of the potential disturbing forces and the trees which are in the centre of the group will be sheltered by those on the outside. Vegetation influences slope stability by removing water through transpiration. Transpiration is the vaporisation of liquid water contained in plant tissue and the vapour removal to the air. Water is drawn up from the roots and transported through the plant up to the leaves. The major effect of transpiration is the reduction of soil pore water pressures which counteracts the loss of strength which occurs through wetting, this is most readily seen as a loss of moisture around trees. However it is not easy to rely on tree and shrub roots to remove water from slopes and consequently help ensure slope stability. The ability to transpire in wet conditions is severely reduced and therefore any increase in soil strength previously gained in evaporation and transpiration will be lost or significantly reduced, consequently the effects of transpiration cannot be taken into account at these times. However it can be assumed that the chance of slope failure following saturation by storm event or periods of extended rainfall will be lessened as a result of transpiration. Moreover, although changes in moisture content will affect the undrained shear strength, the effective shear stress parameters as commonly used in routine slope stability analysis are not directly influenced by changing moisture content, although the water pressures (suctions) used in the analysis will change. It is important to note that desiccation cracks can potentially be extended by vegetation in dry weather promoting the deeper penetration of water to a potential slip plane and increased water pressure into the soil during the wet periods. Nevertheless, these cracks will be filled by roots growing deeper into the soil as they follow the path of least resistance. Studies in Malaysia have shown that there is a significant relationship between root length density, soil water content and ultimately slope stability. Slopes that had high root density (due to dense vegetation on the surface) were less likely to undergo slope failure. This is because a high root length density results in low soil water content which in turn results in an increase in shear strength and a decrease in soil permeability. It is suggested that root length density and soil water level could be used as indicators of slope stability and possibly could be used to predict future slope failure. Transpiration is accentuated when the vegetation has an extensive root system and rapid transpiration continues throughout winter. The removal of water is also affected by the shading provided by vegetation. Shading helps prevent the desiccation of the soils which results in shrinkage and cracking allowing the deep penetration of rain water. Plants need to have a high leaf to root ratio and have the ability to persist through hot summer months in order to provide effective shading of the soils. 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 100–150 kN/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.