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www.eng.chula.ac.th Laboratory Investigation of Laboratory Investigation of Laboratory Investigation of Laboratory Investigation of Laboratory Investigation of Vetiver Root Reinforcement for Slope Reinforcement for Slope Reinforcement for Slope Reinforcement for Slope Protection Presented by Dr. Dr. Boonrat Lohwongwatana Dr. Dr. Dr. Dr. Faculty of Engineering, Chulalongkorn University Co Co Co Co- Co authors: Co Co Co Suched Likitlersuang Sirintra Vanno and and and Soamshine Boonyananta Chulalongkorn University The Sixth International Conference on Vetiver Vetiver (ICV-6), 6), 6), Danang, , , Vietnam, 5 – 8 May 2015

www.eng.chula.ac.th Mode of failure (Skepmton, 1953) Flows Slides Slumps D/L = 0.5 – 3% 5 – 10% 5 15 15 – 30% Slope before Failure Total Length (L) Degree of Rotation Max. Depth Slope after Failure (D) Failure Angle Toe Foot

www.eng.chula.ac.th Live pole Potential deep-seated failure Potential shallow failure Self-regenerative and sustainable Grass (almost maintenance free) Diverse vegetation Grass root Soil nail Live pole ( root of small tree/shrub)

www.eng.chula.ac.th Two effects of vegetation on soil slope [1] Hydrology - Plant roots may increase subsoil permeability at the same time the vegetation will intercept rainfall and transpire water, eventually leading to lower water pressures (i.e., higher suctions) in the slope. [2] Mechanical - The presence of the roots will lead to reinforcement in the penetrated regions.

www.eng.chula.ac.th Root Root- Root soil-water interaction Evapotranspiration Rainfall Solar energy Evaporation Infiltration Atmosphere Plant Soil Water uptake Complex plant-soil-atmospheric interaction (Greenwood et al., 2004; Blight, 2005; Pollen-Bankhead & Simon, 2010)

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www.eng.chula.ac.th Unsaturated soil mechanics • Soil water characteristic curve (SWCC) => matric suction (u => matric suction (u => matric suction (u a – u w ) vs. degree of saturation => matric suction (u => matric suction (u   • Shear strength:          c tan u u tan tan f nf a w b f Net normal stress

www.eng.chula.ac.th Root structures Fan C.C. and Chen Y.W. (2010) (a) Linden hibiscus (H-type); (b) Japanese Mallotus (VH-type); (c) Chinese tallow tree (V-type); (d) ironwood (VH-type); (e) white popinac (R-type)

www.eng.chula.ac.th Shear plane – root area ratio aD    b Wu et al. (1979): Shear reinforcement was ri i calculated from the sum of the forces required to break each individual, crossing the shear plane by where 1.2 is a correction factor for root orientation. 1.2 n    c A r ri i A  i 1 shear plane

www.eng.chula.ac.th Objectives • To understand the mechanism of vetiver root reinforcement for slope protection Overall strength = Soil + Water + Root system + Interface • To evaluate the shear strength contribution of the vetiver root for soil slope • To demonstrate the role of vetiver root for slope stabilization

www.eng.chula.ac.th Vetiver grass – the Royal initiatives • The Chaipattana Foundation • Office of the Royal Development Royal Development Royal Development Projects Board (RDPB) • Land Development Department (LDD) • Corporate social responsibility of Corporate social responsibility of PTT Public company limited.

www.eng.chula.ac.th Chulalongkorn University Team • Prof. Suched Likitlersuang Department of Civil Engineering, Department of Civil Engineering, Faculty of Engineering Faculty of Engineering Faculty of Engineering Faculty of Engineering Faculty of Engineering Faculty of Engineering • Dr. Boonrat Lohwongwatana Department of Metallurgical Engineering, Department of Metallurgical Engineering, Faculty of Engineering Faculty of Engineering Faculty of Engineering Faculty of Engineering Faculty of Engineering Faculty of Engineering • Assist. Prof. Assist. Prof. Assist. Prof. Assist. Prof. Assist. Prof. Sirintra Vanno Department of Landscape Architecture, Department of Landscape Architecture, Faculty of Architecture Faculty of Architecture Faculty of Architecture Faculty of Architecture Faculty of Architecture Faculty of Architecture • Dr. Soamshine Boonyananta Department of Art, Music and Dance Education, Department of Art, Music and Dance Education, Faculty of Education

www.eng.chula.ac.th Sample preparation Mr. Adithep Vangbunkong Lowland Highland (Master student, Department of Civil Engineering)

www.eng.chula.ac.th Root observation The average growth rate of vetiver roots = 30 cm/month (1 cm/day)

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www.eng.chula.ac.th Image processing Image processing – root area ratio 6 months lowland vetiver – 3.36% 6 months highland vetiver – 4.56%

www.eng.chula.ac.th Direct shear tests

www.eng.chula.ac.th Large direct shear test

www.eng.chula.ac.th Results of direct shear tests Shear strength Increasing in Test Specimen parameters cohesion (kPa) c = 6.8 kPa;  = 22.8 o Bare soil - 4 months old single c = 7.7 kPa;  = 29.7 o Standard direct 0.9 vetiver low land shear test 4 months old single c = 13.7 kPa;  = 28.8 o 5.9 vetiver high land c = 2.5 kPa;  = 21.8 o Bare soil - 6 months old group c = 5.1 kPa;  = 28.4 o Large direct 2.6 vetiver low land shear test 6 months old group c = 8.5 kPa;  = 29.2 o 6.0 vetiver high land

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Study of vetiver root 6) Long end fine tip surfaces 7) 3 rd -order branch 5) Thick 2 nd -order branch 1) 1 st -order branch 2) Thin 2 nd -order branch 4) Knotted fine tip 3) Fine tip 8) Root cap

www.eng.chula.ac.th 1) 1 st -order branch

www.eng.chula.ac.th 1) 1 st -order branch

www.eng.chula.ac.th 1) 1 st - 2) Thin 2 nd -order order branch branch

www.eng.chula.ac.th 2) Thin 2 nd -order branch

www.eng.chula.ac.th 2) Thin 2 nd -order branch

www.eng.chula.ac.th 3) Fiber pullout

www.eng.chula.ac.th 4) Knotted fine tip

www.eng.chula.ac.th 5) Thick 2 nd - order branch

www.eng.chula.ac.th 7) Third-order branch

www.eng.chula.ac.th Vetiver root tip

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Adhesion at the Interface Microscopic scale effect is important

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www.eng.chula.ac.th Soil planted roots A1_Elbow A1_Shaft A2_Spindles A4_Cut section A2_Spindles2 A1_Root tip lower down A1_Root tip A3_EggShape Caught

www.eng.chula.ac.th Sand planted roots B1_Shaft B1_B2 Overlap B2_and Sand? B3_ B2_and Sand? B3_ B1_Tip next to B2 B2_and Sand?

www.eng.chula.ac.th Root Caps A1_Root tip B1_Tip next to B2 A1_Root tip lower down Elongated cells ready to expand. •

www.eng.chula.ac.th Sample A vs B Comparison: Particles A3_EggShape Caught B1_Shaft B2_and Sand? A3_EggShape Caught 2x Zoom We observe both soil and sand particles • embedded in the roots Particles can be differentiated from roots • based on surface morphology

www.eng.chula.ac.th Root Hairs and their spindles B3_ A2_Spindles2 A2_Spindles B3_ Filamentous tip growth is much more • extensive in sample A (soil) Some root hairs are visible in sample B • (and previous plug sample), but are significantly shorter and sparser

Sample F: Comparison of EM vs. LM Root segment, pulled apart and surrounded by soil F – LM Segment 1 (uncoated) (root F – EM details clearly visibly in EM) Fractur e surface F – LM (coated) Limited depth of focus

Sample A fracture surface Fracture sruface Root hairs (invisible in LM)

www.eng.chula.ac.th Tensile delamination - root

www.eng.chula.ac.th Core fiber pull out

www.eng.chula.ac.th Findings Root hairs are important • anchorage for Vetiver grass. Dense filamentous structures • are substantially more developed in the soil-planted sample Vasculature is well-defined in • cross-sectional samples. Future study of fractured • roots and failure modes. A4_Cut section zoomed 2x

Mechanics of roots and root hairs

www.eng.chula.ac.th Electron microscope observation Root tip of Root tip of vetiver Root tip of Root tip of Root tip of vetiver Root tip of vetiver grass. vetiver vetiver Deformed root Width of the micrograph is Width of the micrograph is Width of the micrograph is Width of the micrograph is x approximately 100 micron.  b The root hairs are of the order of micron level and their Shear z T R interfacial area is contributing T R zone q significantly to the friction due to their increased surface area.  b High density of root hairs. Intact root Low magnification image. Interface friction between soil and root between soil and root between soil and root between soil and root between soil and root between soil and root between soil and root between soil and root between soil and root This mechanism provides ( Gray & Gray Gray & & & & Sotir , 1996) adhesion between root and soil during shear which could be directly linked cohesion term in Mohr-Coulomb failure criterion framework.