Engineering characterization of Vetiver system for shallow slope - - PowerPoint PPT Presentation

Engineering characterization of Vetiver system for shallow slope stabilization

Apiniti Jotisankasa

• T. Sirirattanachat, C. Rattana-areekul,
• K. Mahannopkul, J. Sopharat*

Kasetsart University, *Prince of Songkla University

Outline of presentation

• Introduction - Slope failure and erosion problems

– Vegetation and slope stability – concerns for limitation of using vetiver grass for steep slope

• Research methodology and background theory on

seepage and strength

• Mechanics - Root tensile strength and soil

cohesion due to root reinforcement

• Hydraulics - Permeability and soil-water retention

curve of root-reinforced soils

• Numerical analysis – rain infiltration & slope

stability interaction

• Conclusions

Problem statement

• Slope erosion/Slope failure in Thailand, related to heavy rainfall

Source: Department of Highways

Vetiver grass system for erosion prevention and shallow stabilization

• Chrysopogon zizanioides or formerly

known as Vetiveria zizanioides

• Traditionally planted as hedgerows

parallel to the slope contour

• Of very dense fine vertical root system

that penetrates as deep as 3-4 meter in some applications

• Effective for shallow slope stabilization,

reduction of runoff erosive energy and sediment trap (Hengchaovanich, 1998, Truong et al., 2008)

อ.ทองผาภูมิ จ.กาญจนบุรี

Photo Courtesy of Dr. Weerachai Na-Nakorn Implemented for erosion control and slope stabilization along highways

อ.ทองผาภูมิ จ.กาญจนบุรี

Implemented for erosion control and slope stabilization along highways Photo Courtesy of Dr. Weerachai Na-Nakorn

Photo Courtesy of Dr. Weerachai Na-Nakorn

Photo Courtesy of Dr. Weerachai Na-Nakorn

Various aspects of influence of vegetation on slope stability

• Canopy interception of rainfall and

evapotranspiration will reduce pore water pressure

• Root fibers reinforcement increases soil shear

strength.

• Conventionally, vegetation-covered and root-

permeated ground reported to be of higher permeability and infiltration rate (Styczen & Morgan, 1995).

• However, Rahardjo et al. (2014) suggested that the

Vetiver grass tended to act as slope covers to minimize the infiltration of rainwater into slopes.

(Coppin & Richard, 1990) Higher infiltration- Higher pore water pressure = Reduced stability Still unresolved issues

• In 2011, H.M. the king Bhumibol of Thailand, suggested practitioners to exercise

certain caution when applying Vetiver on steep slopes and encouraged researchers to investigate into this aspect.

• Aim at revisiting engineering characters of vetiver- benefit, limitation and

adverse effect)

Theory & Assumptions

• Unsaturated seepage- permeability and moisture are

function of positive & negative pore water pressure

∂ 𝜖𝑦 𝑙𝑦 𝜖ℎ 𝜖𝑦 + ∂ 𝜖𝑧 𝑙𝑧 𝜖ℎ 𝜖𝑧 + 𝑅 = 𝑛𝑥 𝜖𝑣𝑥 𝜖𝑢

• Shear strength (considering root

reinforcement and suction)) -

τ = 𝑑𝑠 + 𝑑′ + 𝜏𝑜 tan 𝜚′ − 𝑣𝑥 tan 𝜚𝑐 Root reinforcement Pore water pressure - affected by infiltration (not considering transpiration) Soil-water retention curve Permeability

q

x

Z

Intact root Shear zone Deformed root

st st

t t

𝑑𝑠 = 𝑢𝑆 𝐵𝑆 𝐵 (sin 𝜄 + cos 𝜄 tan 𝜚′)

Root cohesion Mobilised tensile stress in roots Root area ratio Root cohesion is mainly a function of

• Root fiber tensile

strength

• Root percentage

(Root area ratio)

• Root distribution can

change, develop or degrades with time and maintenance conditions. Orientation- (mainly constant) Wu et al., 1979

Infinite slope model

F =

𝑑𝑠+𝑑′ +(𝛿∙𝑨∙𝑑𝑝𝑡 2𝛾) tan 𝜚′ −𝑣𝑥 tan 𝜚′′ 𝛿∙𝑨∙sin 𝛾∙cos 𝛾

Suitable for shallow slide Slope stability expressed as factor of safety, F = Resistance/Destabilizing force

Research approach

Numerical modelling slope stability, rainfall- infiltration, run-off, scenario analysis Laboratory investigation Root cohesion, Soil permeability, Soil-water retention curve,

• f root-reinforced sample

Field observation Actual root distribution (Root area ratio) Pullout-capacity/Field direct shear test

Empirical knowledge/ experience from practitioners

FIELD LAB NUMERCIAL SIMULATION EMPIRICAL EXPERIENCE

Root tensile strength

• Early work by Henchaovanich & Nilaweera (1996)

and Hengchaovanich (1998) found that tensile strength, TR, of Vetiver root ≈ 1/6 of the strength of steel bar. (Fully grown vetiver, 2 years

• ld)
• Subsequent works on lab-grown vetiver

(Sungwornpatansakul & Rajani, 2006, Voottipruex & Sungwornpatansakul, 2003) found less value of TR

• Most of earlier tests conducted on stress-

controlled conditions (not strain-controlled)

Current studies

• Strain-controlled condition of tension test
• Abrupt failure is avoided and more reliable

stress-strain relationship obtained

Root-fiber tensile strength

• Better gripping method and

strain-controlled test condition yields in higher fiber strength than previously obtained

• Growing conditions and age

significantly controls root strength more than the sub- species.

Direct shear tests on vetiver reinforced specimen

• Large direct shear tests on clayey

sand (14 cm in diameter)

• Transparent acrylic tube as

sample holder (For investigating the root distribution)

• Test in soaked condition

Normal force

Shear force Shear force Vetiver roots reinforce specimen

Large direct shear test result

• Root cohesion as function
• f root biomass
• Early work on soil at natural

moisture content (Hengchaovanich, 1998) a = 6-10 kPa/kg/m3,

𝑑𝑠 = 𝑙

𝐵𝑆 𝐵

= 𝑏 ∙ 𝜍𝑆

• Current study, a = 1.9 kPa/kg/m3 for soaked sample (1/3 of early work

unsoaked sample)

• Moiture condition plays important role in vetiver grass stabilization

effect (Stabilization effect can be significantly reduced)

Permeability of root-reinforced soils

• To investigate the influence of root on soils’ permeabilty
• Three major soil types were used for tests, namely clayey

Sand (SC), low plasticity Silty soil (ML), and high-plasticity Clay (CH), commonly found in Thailand

• Vetiver was planted in specimens for various duration (less

than 8 months) before permeability test

Experimental set-up

Calculation of root percentage

• ‘Side root area ratio’ and ‘biomass root per volume’

Influence of vetiver root percentage

• n hydraulic conductivity (k) of soil

SC – sandy clay ML- Low plastic Silt CH- Fat clay

• The overall influence
• f roots in this study

seems to decrease the permeability of the CH and ML soils

• nce fully grown (Due

to root penetration into soil macro void

• As for SC soils,

however, the trend is still not clear, (both decreasing and increasing effect) Only applies to young vetiver less than 8 months (for older vetivers, root degradation may have influence)

Field observation of root degradation

• Minirhizotron system has been used, for
• bserving fine roots intersecting the surface of a

transparent tube buried in the soil (a non- destructive method)

• Useful for studying changing conditions of roots

Field observation of vetiver roots

• Field site on top of 45o degree slope

in Surathani, South Thailand, (Sandy soil)

• Before and after photos of vetiver

grass that disappeared from the slope due to invasion from native species

Before After Minirhizotron

Oct 2014

Before (With vetiver) After (Vetiver disappeared) cm cm

Root pattern

• After the Vetiver disappeared and its roots decayed, the root area

ratio decreased significantly leading to loss in root cohesion and decreased factor of safety.

• This emphasizes the importance of frequent maintenance of the VS in

practice in order to sustain long-term slope stability.

• How does this increased void potentially affect infiltration and

stability of slopes?

Before (With vetiver) After (Without vetiver)

Numerical analysis of rain infiltration into slope with/without vetiver

Objectives

• To explore both advantage and potential risk of

vetiver grass on slopes by way of numerical modeling.

• The Finite Element Method was used to analyze

infiltration of rain into slope

• Limit-equilibrium method for slope stability

calculation

• 2 hypothetical slopes with gradient of about 27o

and 60o. For both cases, the slopes were modelled with and without vetiver row in order to compare the effects of vetiver on stability.

Two typical slopes

• Natural slopes (26 degree) with/without rows of

vetiver grass

• Rock cut slope (60 degree) with/without rows of

vetiver grass

• Rainfall simulation (initial state = 300 mm/month,

transient = 3.6 mm/hr = 86.4 mm/day)

• Assume constant rainfall for 2 days (total rain =170

mm) and investigate the change of pore water pressure and factor of safety

Soil properties in the analysis

M a te ria l 𝛿𝑡𝑏𝑢 ,

k N /m

3

𝑑′+ 𝑑𝑠,

k P a

𝜚′ 𝜚𝑐 𝛿𝑛𝑝𝑗𝑡𝑢 , k N /m

3

T

• p

s

• il 1

7 2 2 .8 1 7 .6 1 3 .9 1

6 .5 [1 2

• 1

4 ]

S

• il1

1 8 .5 2 3 2 2 7 .7 1 8

[1 2

• 1

4 ]

S

• il2

1 8 .7

2

3 2 2 7 .7 1 8

[1 2

• 1

4 ]

V e tiv e r ro

• t z
• n

e 1 8 .5 1 1 3 2 2 7 .7 1 8

[4

• 5

, 1 2

• 1

4 ]

Permeability of root zone is assumed to be 2 times permeability of no-root zone (more permeability root zone or effect of decayed roots considered) Root cohesion,Cr, of 20 kPa assumed.

Natural slopes (26 degree) with/without rows of vetiver grass

Initial condition from steady state analysis

• Contour of pore water pressure (kPa)
• (time= 0 hr) Average infiltration of 300

mm/month for case 1

• 5
• 5
• 5

5 10 1 5 20 2 5 30 35 4

Pore water pressure distribution at different times for slope with vetiver grass rows

• As rainwater infiltrated the ground,

the pore water pressure increased more dramatically at the middle and upper part of the slope, while at the toe, the pore water pressure remained practically unchanged.

Comparison between pore waterpressure in slopes with vetiver rows and without vetiver rows (at 12 hours time = 43 mm of rain)

• There was only very slight difference between

the two cases.

• Except at the top part of slope, for slope with

vetiver rows, the root zone appeared to conduct some water to a greater depth

• All in all, there is not much significant

difference between the pore water pressure of 26o slopes with or without vetiver.

TOP LOWER MIDDLE

Natural slope 26 degree

• Limit Equilibrium slope stability analysis carried out based on pwp

from transient seepage analysis

• The slope without vetiver grass appeared to fail (FS=1) when the

total rainfall reached about 120-170 mm

• The increased cohesion due to roots (cr) more than offsets the

higher permeability of root zone that induce greater infiltration into slopes, for the case of 26.6o slope

No adverse effect of vegetation on stability for 26.6o slope, only beneficial

Rock cut slope (60 degree) with/without rows of vetiver grass

• 10 m high slope (2 m high step) vetiver planted on each bench

2 m Vetiver rows

Pore water pressure variation After 24 hours = 84 mm

• With vetiver hedgerows on slope, groundwater can infiltrate to a greater depth

through the assumed more permeable root zone, resulting in higher pore water pressure in the slope.

• Without the vetiver rows, part of the rainfall would not permeate the ground and

tend to become runoff.

With vetiver Without vetiver vetiver No vetiver

0.969

With vetiver after 48 hours 172 mm of rain

Failure surface (FS=0.969) of the slope with vetiver rows, after 48 hours of rain (172 mm). The failure surface extended deeper than the root zone of the vetiver

Weathered rock slope 60 degree

• Factor of safety for the 60oslope with permeable root

zone is about 10% lower than the slope without root zone due to the increased pore water pressure induced from increased infiltration through the root zone.

Conclusions

• There was concern that vetiver grass might pose adverse effects
• n slope stability in some circumstances, due to higher infiltration

induced by root.

• In this study, a new technique of root observation in the field,

combined with laboratory test and numerical simulation, helps practitioners to better understand the engineering characteristic of the VS, both mechanical and hydraulic.

• For slopes steeper than 60 degree, the effect of vetiver root

degradation, leading to higher infiltration into slope, could pose some theoretical risk of increased instability (though only 10%).

• Nevertheless, for usual cases of slope being about 30 degree, the

vetiver grass appeared to be effective in slope stabilization, despite higher infiltration for cases of decayed root.

• The new technique of root observation could also be used in any

future bio-engineering projects as a quality assurance tool

Acknowledgements

The authors gratefully acknowledge the financial support and encouragement by

• Chaipattana Foundation,
• Sustainable Energy Foundation,
• Kasetsart University Research and Development

Institute (KURDI), Thailand.

• Office of the Royal Development Projects Board
• Ban-natum community

Thank you very much for your attentions