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Ootacamund, known as Ooty is frequently subjected to numerous landslides and gravitational movements.
The aim of the study is to select the suitable plants which increase the stability of slopes. It is well understood that vegetation influences slope stability in two ways: through hydrological and mechanical effects. Hydrological effects involve the removal the removal of soil water by evapo-transpiration through vegetation, which lead to an increase in soil suction or a reduction in pore-water pressure, hence an increase in the soil shear strength. The density of the roots within the soil mass and root tensile strength contribute to the ability of the soils to resist shear stress. The site analysis was carried out in three main aspects which are topographic, climatic and botanical. According to the local environmental conditions suitable species were selected. The increase in shear strength of those species according to the soil type was carried out.
Slope instability is one of the major problems in geotechnical engineering where disasters, like loss of life and property, do occur. A natural slope is different from an embankment or a man-made slope in that the effects of vegetation and soil variability play an important role in their stability. The one method of soil formation is the vegetation growth on the rock. In contrast the vegetation act as the soil binder. This function
depends on the properties of the soil and the characteristics of the plant. Mainly this function depends on the root characteristics of the particular species.
SLOPE STABILITY AND ROOT STRENGTH
There are two types of slips or mass movements that characterize stability problems. Slips or
slides on slopes fall into 2 categories: deep seated and shallow seated .Deep-seated problem is
geotechnical or geological in nature. It can only be addressed taking into account slope geometry, soil
strength, climatic condition, groundwater characteristics, etc. and can be ascertained by slope stability
PLANT GROWTH VERSUS ENGINEERING
From the past researchers dense over compacted soils tend to have low hydraulic conductivities, Poor aeration and high resistance to root penetration. As slope increased that is above 30ÌŠ deep rooted vegetation like shrub lowering the landslide hazard. The gain in soil stability due to plant roots is exclusively based on the increase of the angle of internal friction F¢ from 34.3° to 39.4° without any noticeable change in apparent cohesion c'. The soil-reinforcing effects of the plants are likely also to be attributable to the higher stability of unplanted soil with a higher dry unit weight, which in this case corresponded to an increase from g 15.5 kN/m3 to 19 kN/m3.
The study area covers a part of Ooty town in the Nilgiris district, a mountainous terrain in the north-western part of Tamil Nadu, India. The Ootacamund landform has gentle mounds with a thick soil cover (Parthasarathy and Vaidyanathan, 1974). Most part of the Nilgiris is deeply weathered and at some places thick soil cover of up to 40 m is found. The soil available are derived from rocks, having an admixture of organic matter and capable of supporting vegetation. In this study, 2:1 homogenous slope with a height of about 10 metres is used to investigate the effects of vegetation on the stability of a slope
Consolidated-untrained triaxial compression tests were conducted to obtain the data of cohesion and angle of internal friction.The field density can be easily obtained from the core cutter method. The average density of 3 samples are taken as field density. To predict the suitable plant these characteristics are needed.
Table1. Soil Properties in site 1 - kagguchi (Nilgiris)
Angle of internal friction F¢
(From RMD-Chennai) Table 2 .Climatic conditions in Nilgiris
25°C - 10°C
21°C - 5°C
Species selection is based mainly on the micro climatic condisions. The root tensile strength of the species also play an vital role in the selection of the species.Root pull-out tests were used to both characterize interface properties and find the forces required to pull-out roots embedded in soil. The tests involved pulling out pre-embedded or grown plant roots from the soil and measuring the pull-out force with increasing displacement. This has direct relevance to the additional reinforcing root forces 'anchoring' beyond the failure surface during slope or shears failure.
The available root force acting on the base of each slice, T is calculated by the equation,
st = T = Trd * L
Trd = Design root force per square metre of the soil.
L = Length of the slip surface.
Trd = Tru / F.O.S
Tru = Ultimate root force per square metre across the slip plane.
F.O.S = Factor of safety. Normally factor of safty for this problem is 8. (Green wood et al 2003)
Tru = N x ( P D2) x TP
TP = Pull out resistance.
N = Average number of roots of the plant.
Table 3. Diameter and tensile strength of root of various herbs Cheng et al ( 2003) and Ke et al.(2003)
Avg diam. of roots (mm)
Avg tensile strength (MPa)
Common Centipede grass
Camellia sinensis (Ooty tea)
2.00 to 4.00
The roors of the plants can be treated as elastic-reinforced elements and a function of the tensile strength of the roots can be added directly to the Mohr-Coloumb equation for failure criteria.The stability of the slope is considered as a curve. From the probable of failure, case Toe failure is considered. Wu et al. (1979) incorporated the effects of vegetation in slope stability analysis by using conventional limit equilibrium method. In limit equilibrium methods, the shear strength of the soil along a potential slip surface is assumed to be fully mobilised at the point of failure. The Mohr-Coulomb equation is used to describe the shear strength of the soil:
τ = c′ + (s - u) tan φ¢
By incorporating the effect of root reinforcement,
The equation becomes:
τ = (c′ + cR) + (σ - u) tan φ′
The apparent root cohesion (cR) is incorporated in their infinite slope analysis and found an increase in the factor of safety (F.O.S) for some slopes. The results indicated that tree roots improved the stability of forested slopes.
cR = 1.2 x st x Ar
Ar = root area ratio defined as the area of the root crossing the shear plane divided by the total
cross- sectional area of the shear plane.
This equation assumes that the reinforcement from vegetation is limited by the tensile strength of the roots and that all roots are just about to break when peak shear strength is mobilized: both assumptions are questionable.
From the previous case studies (regional climate (15ÌŠ C-30ÌŠC)) the species vetiver grass and camellia sinesis (root properties obtained from the cultivator of Ooty) are well matches with the climatic conditions of Ooty.
The average tensile strength of Vetiver grass = 85.10 MPa.
The area ratio Ar= åniai / A
ni = The number of roots in class i.
ai = the mean cross sectional area of root in size class i
From the past research and study, it is well known that the area ratio increases with the increase of shear strength. This matches with the modified Mohr- Coloumb failure theory in the influence of vegetation.
Ar = 0.98 %
The shear strength increase = 0.5 Ksf
The vetiver grass may be planted at the toe and base of the slope.
SELECTION OF SPECIES FOR THE MIDDLE OF THE SLOPE:
Deep rooted trees should be planted at the maximum shear zone. Eucolyptus trees are well suited for the climatic conditions of Ooty but the root soil interaction property is questionable. The tree normally grows more than 60m.so the self weight of the tree is increase due course of time. Pinus trees are also noticeable in this locality but the mean tensile root strength of the tree is not so high.
Mean Tensile Strength (MPa)
Though all the above pinus species are well suited for the Ootagamund Climate Pinus ponderosa is only visible in this area. The tensile force is very low .So pinus desiflora may be suggested to plant in the mid way of the slope.
It is known that the shear strength of the soil is increased by the plantation of askhus ( vetiver grass )
Fig. 4 Failure surface Mitigation on slope
From the slip circle it is well known that the high potential land slide is in the mid way. According to that deep rooted plants should be planted in in the middle. The shrubs or grass type vegetation may be Planted in the toe and the base of the slope. As mentioned in the introduction, vetiver roots are very strong with high mean tensile strength of 75 MPa or approximately 1/6th of strength of mild steel.
When the dense and massive root networks act in unison, they resemble the behavior of soil nails normally used in civil engineering works. With its innate power to penetrate through hardpans or rocky layers, the action of vetiver roots is analogically likened to 'livingsoil nails' by the author (Hengchaovanich, 1998).
Decrease in root diameter from 5 to 2 mm can result in a doubling or even tripling of tensile strength.Clearly finer roots can contribute significantly to soil reinforcement and shear strength increase.Finer roots have the advantage of not only higher strengths but also superior pullout resistance because they have higher specific surface areas than larger roots at equalent area ratios. Though pinus species are well suited for the Ootagamund Climate Pinus ponderosa is only visible in this area. The tensile force of Pinus ponderosa is very low .So pinus desiflora may be suggested to plant in the mid way of the slope.
Instead of identifying the dangerous trees suitable species may be planted by suitable analysis. To achieve these, soil properties should be predicted at the suitable intervals.
In the study area the various parameters were collected from the site such as slope, cohesion angle of internal friction, average density of the slope. Micro and Macro climatic conditions are also important factors. Afforestation of the barren land prohibition of deforestation for any other activities are very essential for Controlling landslide activities in the study area. Askhus ( vetiver grass ) may be planted in the slopes to decrease the possibilities of the land slides. Camellia sinensis (Ooty tea) also act as the soil binder which is suitable for the micro climatic conditions of Ooty but routine maintenance and pesticide are needed for this plant. Though pinus species are well suited for the Ootagamund Climate Pinus ponderosa is only visible in this area. The tensile force of Pinus ponderosa is very low .So pinus desiflora may be suggested to plant in the mid way of the slope.
To achieve the remarkable mile stone in the field application of this eco
engineering, interdisciplinary research is needed in the field of Meteorology , Botany, soil engineering is needed.
FOR FURTHER RESEARCH AND DEVELOPEMENT
The root tensile strength shall be locally conducted in the field for the various species
which suits for the local environmental conditions. The slope analysis shall be carried out using Janbus method of slope analysis using finite element or finite difference method of analysis. To carry out analysis using Janbus method Piezo metric line, properties of soil in various layers are needed. Root decay and strength loss shall be also taken into account to calculate the increase in shear strength.
Janbus method of slope analysis for influence of vegetation
The general equation: (Jambus method of slope analysis)
F = å [C¢l + ( W COS a - U l (U2-U1) sin a ) tanF ] / å W sin a
Including the influence of vegetation (Green wood)
F = å [ (C¢+CV) l + ( W + WS) COS a - (U+ÑU)l (U2-ÑU2V)-(U2-ÑU2V)sin a-DW sin(a-b)+Tsinq ) tanF ]
å[ (W + Wv) Sin a+ DWsin(a-b)+T cosq]