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, and mechanical reinforcement of roots.
Wind throwing 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. Removal of water
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 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 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 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. 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.
Mechanical reinforcement of roots Roots reinforce the soil through growing
across failure planes, root columns acting as piles, and through limiting
surface erosion. = Root growth across failure planes =
When roots grow across the plane of potential failure there is an increase
in shear strength by binding particles. The roots anchor the unstable surficial
soil into the deeper stable layers or bedrock. This most readily occurs when
there is rapid deep growth of roots which last for more than two years.
However it is important to note that the strength exerted by roots generally only
extends down to 1m while most failures occur between 1.2 – 1.5m soil depth.
= Root reinforcement model = The root reinforced earth root model is
the result of the root elongation across a potential slip plane which produces a
tensile root force which is transferred to the soil by cohesive and frictional
contacts between the root and the soil. = Tensile root strength contribution and
pull out resistance = The pull out resistance of a root is the
measured resistance of root structure to be pulled out of the ground and is
likely to be only a little less than the measured tensile strength of the root
which is the roots resistance to breaking as measured in the laboratory.
In the cases where there is no pull out data available the tensile strength data
maybe used as a rough guide to the maximum pull out resistance available.
The tensile root strength of a range of diameters over a range of species has
been tested in the laboratory and has been found to be approximately 5 –
60MN/m2. In order for the root to actually enhance slope stability the
root must have sufficient embedment and adhesion with the soil. The way that
roots interact with the soil is intricate but for engineering purposes
the available force contributions may be measured with in situ pull out tests.
= Root morphology and modes of failure = The root length and the type of root
branching effects the way that root failure occurs Three different modes of
failure have been identified in hawthorn roots which relate to the root soil
relationship which is shown in the shape of the roots and the shape of the
failure curve. Roots which have no branches tend to fail in tension and
pull straight out of the ground with minimal resistance. Roots which have
multiple branches generally fail in stages as each branch breaks inside the
soil. These roots can then separated into two different groups; 1) those that
initially reach their maximum peak force and then maintain a high force that
progressively decreases as the root branches fail after significant strain
and 2) those that break with increasingly applied force. In a number
of tests considerable adhesion between a segment of the root and the soil can be
measured prior to the root eventually slipping out of the soil mass.
= Type A failure = Roots that do not have branches
generally fail in tension and pull straight out of the ground with only
minimal resistance. The root reaches its maximum pullout resistance then rapidly
fails at a weak point. The root easily slips out of the soil due to the gradual
tapering which means that as the root is pulled out it is moving through a space
that is larger than its diameter which consequently has no further bonds or
interaction with the surrounding soil. = Type B failure =
Type B failure occurs when branched roots initially reach their maximum peak
resistance then sustain a high resistance which slowly reduces as the
branches of the roots fail after significant strain. In some tests
considerable adhesion between a section of the root and the soil mass can be
measured before the root eventually slips out. Forked roots require a
greater force to be pulled out as the cavity above the fork is thinner than
the root which is trying to move through the cavity, this can then result in
deformation of the soil as the root moves through the soil.
= Type C failure = Roots that have multiple branches or
forked branches also can undergo tensile failure but predominantly fail in stages
as each branch breaks within the soil. These roots break with increasingly
applied force in stages in the form of stepped peaks corresponding to the
progressive breaking of roots of greater diameters. The root progressively
releases its bonds with the soil until final tensile failure.
In some cases when the root has a sinusoidal shape with many small
rootlets along its length the root reaches its maximum pull out resistance
on straightening and then breaks at the weakest point, however at this point the
root is not pulled out of the soil as it adheres and interacts with the soil
producing a residual strength. If pulling was stopped at this point, the
root would give increased strength to the soil. However, if the root is
completely pulled out of the ground then there is no further interaction with
soil and therefore no increase in soil strength is provided.
= Factors which affect root pull out resistance =
Studies have shown that the pull out resistance of hawthorn and oak roots are
affected by intra species differences, inter-species variations and root size
in a similar as way as root tensile strength varies. In the pull out test
the applied force acting on the root acts across a larger root area, which
involves multiple branches, longer lengths) than the short length of root
used in tensile strength tests. In pull out test the root is likely to fail at
weak points such as branching points, nodes or damaged areas.
The studies also showed that there is a positive correlation between maximum
root pull out resistance and root diameter for hawthorn and oat root.
Smaller diameter roots had a lower pull out resistance or breaking force than
the larger diameter roots. = Root columns acting as piles =
Trees and root columns can prevent shallow mass movement through acting as
piles when there is buttressing and soil arching through a woody deep root system
which has multiple sinker roots with embedded stems and laterals.
= Limitation of surface erosion = Vegetation can also be used to control
water erosion by limiting surface processes such as sheet wash and
overland flow. Vegetation can provide a considerable contribution to the
stability of slope through enhancing soil cohesion. This cohesion is
dependent upon the morphological characteristics of root systems and the
tensile strength of single roots. There is considerable evidence of fine
roots resisting surface erosion. The role of fine roots in general slope
stability is not fully understood. It is thought that the fine roots help keep
the surface soil together and prevent surface erosion. The fine root network
may have an apparent enhanced cohesion which is comparable to geosynthetic mesh
elements. The limitation of surface erosion processes is particularly
apparent in areas of shrub and grass where the fine root distribution is
consistent and clearly defined, however cohesion is generally limited to the top
1m of soil. References
Sources British Broadcasting Corporation 2007,
Biology, viewed 10 June 2007, www.bbc.co.ukgcsebitesizebi05006.gif
Greenwood, J.; Norris, J. & Wint, J. 2007, ‘Discussion: Assessing the
contribution of vegetation to slope stability’, Proceedings of the
Institution of Civil Engineers, vol. 160, no. 1, pp. 51–53.
INTBAU 2007, International network for traditional building, architecture and
urbanism, viewed 2 June 2007, www.intbau.orgScarano/scarano3.580.jpg
Selby, M. 1993, Hillslope materials and processes, Oxford University Press,
Oxford, Great Britain. Watson, A. & Marden, M. 2004, Root
tensile strength as an indicator of the performance of indigenous riparian
plants – how do they rank?, Landcare Research, Lincoln, NZ.