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
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) Source: Department of Highways
Implemented for erosion control and slope stabilization along highways อ . ทองผาภูมิ จ . กาญจนบุรี Photo Courtesy of Dr. Weerachai Na-Nakorn
Implemented for erosion control and slope stabilization along highways อ . ทองผาภูมิ จ . กาญจนบุรี 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 (Coppin & Richard, 1990) • In 2011, H.M. the king Bhumibol of Thailand, suggested practitioners to exercise certain caution when applying Vetiver on steep slopes and encouraged evapotranspiration will reduce pore water pressure researchers to investigate into this aspect. • Root fibers reinforcement increases soil shear • Aim at revisiting engineering characters of vetiver- benefit, limitation and adverse effect) strength. • Conventionally, vegetation-covered and root- permeated ground reported to be of higher Higher infiltration- Higher pore permeability and infiltration rate (Styczen & water pressure = Reduced stability Morgan, 1995). • However, Rahardjo et al. (2014) suggested that the Still unresolved issues Vetiver grass tended to act as slope covers to minimize the infiltration of rainwater into slopes.
Theory & Assumptions • Unsaturated seepage - permeability and moisture are function of positive & negative pore water pressure ∂ 𝜖ℎ ∂ 𝜖ℎ 𝜖𝑣 𝑥 𝜖𝑦 𝑙 𝑦 𝜖𝑦 + 𝜖𝑧 𝑙 𝑧 𝜖𝑧 + 𝑅 = 𝑛 𝑥 𝜖𝑢 Soil-water retention curve Permeability • Shear strength ( considering root reinforcement and suction) ) - τ = 𝑑 𝑠 + 𝑑 ′ + 𝜏 𝑜 tan 𝜚 ′ − 𝑣 𝑥 tan 𝜚 𝑐 Pore water pressure - affected Root reinforcement by infiltration (not considering transpiration)
Deformed Root cohesion is mainly t x root a function of - Root fiber tensile q strength Shear s t - Root percentage zone Z s t (Root area ratio) - Root distribution can t change, develop or degrades with time and Root area ratio Intact root maintenance conditions. Wu et al., 1979 𝑑 𝑠 = 𝑢 𝑆 𝐵 𝑆 𝐵 (sin 𝜄 + cos 𝜄 tan 𝜚′ ) Orientation- Root cohesion Mobilised tensile stress (mainly in roots constant)
Infinite slope model Suitable for shallow slide Slope stability expressed as factor of safety, F = Resistance/Destabilizing force 𝑑 𝑠 + 𝑑 ′ +( 𝛿∙𝑨∙𝑑𝑝𝑡 2 𝛾 ) tan 𝜚 ′ −𝑣 𝑥 tan 𝜚 ′′ F = 𝛿∙𝑨∙ sin 𝛾∙ cos 𝛾
Research approach FIELD Field observation Actual root distribution (Root area ratio) Pullout-capacity/Field direct shear test Empirical knowledge/ experience from practitioners EMPIRICAL Laboratory investigation Numerical modelling Root cohesion, EXPERIENCE LAB slope stability, rainfall- Soil permeability, infiltration, run-off, Soil-water retention curve, scenario analysis NUMERCIAL of root-reinforced sample SIMULATION
Root tensile strength • Early work by Henchaovanich & Nilaweera (1996) and Hengchaovanich (1998) found that tensile strength, T R , of Vetiver root ≈ 1/6 of the strength of steel bar . (Fully grown vetiver, 2 years old) • Subsequent works on lab-grown vetiver (Sungwornpatansakul & Rajani, 2006, Voottipruex & Sungwornpatansakul, 2003) found less value of T R • 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 𝑑 𝑠 = 𝑙 𝐵 𝑆 = 𝑏 ∙ 𝜍 𝑆 𝐵 • Root cohesion as function of root biomass • Early work on soil at natural moisture content (Hengchaovanich, 1998) a = 6-10 kPa/kg/m 3 , • Current study, a = 1.9 kPa/kg/m 3 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 on hydraulic conductivity (k) of soil • The overall influence of roots in this study seems to decrease the permeability of the CH and ML soils once fully grown (Due to root penetration SC – sandy clay into soil macro void • As for SC soils , ML- Low plastic Silt however, the trend is CH- Fat clay 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 observing 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 45 o 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 Minirhizotron Before After
Oct 2014
Root pattern cm cm After (Vetiver disappeared) Before (With vetiver)
After (Without vetiver) Before (With vetiver) • 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?
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 27 o and 60 o . 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
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