Engineering Advantages of Vegetation on Slope Stabilization

There are various conventional methods used to improve stability of slope and surface erosion. They all have merits and demerits, but the use of vegetation has many advantages such as root does not corrode, they are self-repairing, regenerating and environmental friendly. This discipline has gained a global recognition for a long time and has been addressed as a new entity, “Ecological Engineering” which is defined as the design of sustainable ecosystem that integrates human society with its natural environment for the benefit of both. This paper considers the potential engineering influences of vegetation and how it can be characterized on site within a geotechnical framework for stability assessment. To gain more understanding on its soil root interaction and effects on slope stabilization, the mechanical and hydrological effect of vegetation would be combined and their overall effect on slope stabilization and slope stability analyses would be evaluated. The results obtained for the Vetiver Grass and Lime Tree will be considered. In overall the results show the considerable improvement in the slope stability by applying vegetation on finite slope depending to their location on slope. The results also indicate that Vetiver Grass can cause significant improvement in the slope stability compare to the Lime Tree even when it is located at the crest of slope due to its roots geometry and lower weight.


INTRODUCTION
The influence of vegetation can be hydrological and mechanical factors, beneficial or adverse to slope stabili hydrological and mechanical parameters @ IJTSRD | Available Online @ www.ijtsrd.com | Special Issue Publication | November 2018 There are various conventional methods used to improve stability of slope and surface erosion. They all have merits and demerits, but the use of vegetation has many advantages such as root does not corrode, repairing, regenerating and mental friendly. This discipline has gained a global recognition for a long time and has been addressed as a new entity, "Ecological Engineering" which is defined as the design of sustainable ecosystem that integrates human society with its ment for the benefit of both. This paper considers the potential engineering influences of vegetation and how it can be characterized on site within a geotechnical framework for stability assessment. To gain more understanding on its soiland effects on slope stabilization, the mechanical and hydrological effect of vegetation would be combined and their overall effect on slope stabilization and slope stability analyses would be evaluated. The results obtained for the Vetiver Grass Tree will be considered. In overall the results show the considerable improvement in the slope stability by applying vegetation on finite slope depending to their location on slope. The results also indicate that Vetiver Grass can cause significant ment in the slope stability compare to the Lime Tree even when it is located at the crest of slope due to its roots geometry and lower weight.
Lime Tree, slope divided into which can be ility [1]. The s reflecting the effect of vegetation in stab additional effective cohesion; slice due to the vegetation; a force by the roots present on wind force; possible chang strength due to moisture rem and changes in pore water pres have been further explained geotechnical framework. Roo determining factor when eval vegetation on slope stability functions that the plant may bioengineering system in support, anchor, drain rei depending upon the type of bioengineering, the nature characteristics. The importa slope stabilization and su is enormous.

Slopes
Slopes may be man-made as highways and rail-roads, e containment of water, land industrial and other developm and other water conduits and t Slopes may also be naturally stream banks. At all location not level, there are forces movements of the soil from points. The significant impor the component of gravity, wh of the probable motion. Also well recognized, is the force several forces produce shear soil mass and a movement shearing resistance on every positive failure surface throughout the mass is sufficiently larger than the shearing stress.

Cause and mechanism of slope failure
The causes of major failure of slopes are the insufficient control of surface water and the presence of local weaknesses, discontinuity, sheet jointing. Adoption of deficient geological or hydrogeological modes of slope design is the most important factor of major failures in engineered slopes. Another problem associated with the large slides is adverse groundwater conditions undetected during design and construction stage [2].
Minor slope failures are caused by surface water, the mechanism mainly involves concentrated surface runoff leading to erosion and water ingress during intense rain, inadequate maintenance generally takes the form of blocked or cracked drainage channel, and inadequate attention to proper detailing. Another cause of minor failure in slope is local weakness in the ground mass, most in soil cuts and rock cuts are associated with the presence of local weakness of weak geological material and adverse ground water build up of local transient perched water table.
Common failure mechanism of a fill slopes are flow slides due to inadequate compaction, washout and sliding and those for soil cut slopes are washout and sliding [3].
The second mechanism of failure is liquefaction which is the sudden collapse of metastable soil structure within a loose soil mass in a slope when it is subjected to a high degree of saturation under sustained shear stresses, resulting in a significant reduction of soil shear strength and leading to a flow slide type of failure which is a special case of sliding failure.
The third mechanism is washout which is detachment of part of the soil mass induced by the scouring action of running surface water. reinforcement and identified a series of empirical and physical based relationship between root development and soil strength. Even low root density can provide substantial increase in shear strength and the magnitude additional apparent cohesion varies with the distribution of the roots within the soil and with the tensile strength of the individual roots. [4,5].
Root reinforcement is a function of root strength, interface friction between root and soil and the distribution of root within the soil and root-reinforced soil is more able to resist continued deformation without loss of residual strength than soil alone [5]. The magnitude of the mechanical reinforcing effect of vegetation is a function of the following root Properties: density, tensile strength, tensile modulus, length/diameter ratio, surface roughness, alignment; straightness and angularity and orientation to the direction of principal strains [1].

Root area ration
The ability of a tree to reinforce soil will depend, not only in the depth to which its root systems extend but also on the total cross-sectional area of its roots at the given depth [6, 7, 8 and 9].

Root tensile strength
Nilaweera and Nutalaya [10] pointed out that the pullout resistance of a tree is generally controlled by its root strength and morphological characteristics and the pull-out resistance of the tree increased with root length distribution and the depth of root penetration.

Anchorage, arching and buttressing
The taproot and the sinker roots of many tree species penetrate into the deeper soil layers and anchor them against down-slope movement.

Surcharging
Surcharge is the effect of the additional weight on a slope resulting from the presence of vegetation. Surcharge could have adverse effects, although it can be beneficial depending on the slope geometry, the distribution of vegetation cover and the properties of the soil. Wind loading is particular relevant when considering the stability of individual trees, but is of lesser significant for general slope stability, where the wind forces involved represent a much smaller proportion of the potential disturbing forces, and trees within a cluster (stand) are sheltered to some extend by those at the edge.

Hydrological effect of vegetation 1.4.1. Rainfall interception
Vegetation intercepts a proportion of the incoming rainfall, part of which is stored on the leaves and stems of the plants and is returned to the atmosphere by evaporation. Thus, interception decreases the rate and volume of rainfall reaching the ground surface.

Surface water runoff
The combination of surface roughness, infiltration and interception, surface water runoff from the vegetated areas is much less than that of bare soil.

Infiltration
Vegetation increases the permeability and infiltration of the upper soil layers due to roots, pipes or holes where the roots have decayed, increased surface roughness.

Evaporation and transpiration
Hydrological effects involves the removal of soil water by evaporation through vegetation, which lead to an increase in soil suction or reduction in porewater pressure, hence an increase in the shear strength [11]. Therefore, vegetation affects slope stability hydrologically by extracting soil moisture through transpiration. Apart from increasing the strength of soil by reducing its moisture content, evaporation by plant reduces the weight of the soil mass.

Materials and methods
The study has been carried out within the United Kingdom and its environs as this study is the continuous of Rees and Ali [11 and 12] works with temperate climate with plentiful rainfall all year round. The plant used for the research will be limited to the mature lime tree (Tilia) and Vetiver Grass. The transpiration rate, weight; root geometry of these plants shall be used. Mechanical properties of Boulder clay soil would be considered. Analyzing the factor of safety of vegetation on finite slopes will be done by using SLIP4EX computer program.
The equations used in the SLIP4EX spreadsheet are derived from the basic limit equilibrium stability equation [13]: By resolving forces to determine N΄, the full stability equation based on effective forces will be obtained [14].
The simple mathematical form of the Greenwood stability equations with the Factor of Safety simply expressed by a summation of restoring and disturbing moments or forces makes the inclusion of additional forces due to ground reinforcement, anchors or vegetation effects relatively straightforward.
It is not straightforward to add these additional forces in the Bishop and other "sophisticated" published solutions where the global factor of safety is applied to the shear strength parameters for each slice of the analysis resulting in some unrealistic force scenarios for the slices where anchor and reinforcement loads are applied [15]. The General equation 2 is adapted for inclusion of the vegetation effects, reinforcement and hydrological changes, Figure 2, as follows [16, 17 and 18]: But in the SLIP4EX changes in ground water table due to vegetation are included however the changes in ground water table employed in their work were taken directly from piezometer readingsno numerical simulation of this process was involved. The study was based on the effective stress approach and as such is valid only for saturated soils. In this study, the stability of an unsaturated soil slope is considered in relation to soil suction created by the plant water-uptake process. These changes primarily affect the matric suction component only. Hence matric suction is considered in this study by adding equation [4] into the SLIP4EX [19].
International Journal of Trend in Scientific Research and Deve @ IJTSRD | Available Online @ www.ijtsrd.com  Table 1, the use of lime tree the slope has shown an increase of Factor of Safety (FOS) and an increase the middle of slope. However, is has a when the lime tree was used at the crest the difference of 2.75%. On the other ha Vetiver Grass at the toe, middle and at t slope has indicated increased percent especially at the middle of the slope, b increase of 3%.