Laboratory Investigation of Vetiver er Root Reinforcement for Slope Protection Presented by Dr. Boonrat Lohwongwatana Faculty of Engineering, g, Chulalongkornorn University Co- o-authors: Suched Likitlersuang Sirintra Vanno and Soamshine Boonyananta Chulalongkornn University The Sixth International Conference on Vetiverer (ICV-6), Danang, Vietnam, 5 8 May 2015
Mode of failure (Skepmton, 1953) Flows Slides Slumps D/L = 0.5 3% 5 10% 15 30% Total Length (L) Slope before Failure Degree of Rotation Toe Slope after Failure Foot Failure Angle Max. Depth (D)
Live pole Potential deep-seated failure Potential shallow failure Self-regenerative Grass and sustainable (almost maintenance free) Diverse vegetation Grass root Soil nail Live pole (root of small tree/shrub)
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.
Root-soil il-water interaction Evapotranspiration Rainfall Solar energy Infiltration Evaporation Plant Atmosphere Soil Water uptake Complex plant-soil-atmospheric interaction (Greenwood et al., 2004; Blight, 2005; Pollen-Bankhead & Simon, 2010)
Unsaturated soil mechanics Soil water characteristic curve (SWCC) => matric suction (u a u w ) vs. degree of saturation Shear strength: f c nf tan u u a u w tan b f Net normal stress
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)
Shear plane root area ratio Wu et al. (1979): Shear reinforcement was 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. c 1.2 n A r ri i Ashear plane i 1 ad ri i b
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
Vetiverer grass the Royal initiatives The Chaipattana Foundation Office of the Royal Development Projects Board (RDPB) Land Development Department (LDD) Corporate social responsibility of Corporate social responsibility PTT Public company limited.
Chulalongkorn University Team Prof. Suched Likitlersuang Department of Civil Engineering, Faculty of Engineering Dr. Boonrat Lohwongwatana Department of Metallurgical Engineering, Faculty of Engineering ering Assist. Prof. Sirintra Vanno Department of Landscape Architecture, Faculty of Architecture Dr. Soamshine Boonyananta Department of Art, Music and Dance Education, Faculty of Education
Sample preparation Lowland Highland Mr. Adithep Vangbunkong (Master student, Department of Civil Engineering)
Root observation The average growth rate of vetiver roots = 30 cm/month (1 cm/day)
Image processing root area ratio 6 months lowland vetiverer 3.36% 6 months highland vetiverer 4.56%
Direct shear tests
Large direct shear test
Results of direct shear tests Test Standard direct shear test Large direct shear test Specimen Shear strength Increasing in parameters cohesion (kpa) Bare soil c = 6.8 kpa; = 22.8 o - 4 months old single vetiver low land c = 7.7 kpa; = 29.7 o 0.9 4 months old single vetiver high land c = 13.7 kpa; = 28.8 o 5.9 Bare soil c = 2.5 kpa; = 21.8 o - 6 months old group vetiver low land c = 5.1 kpa; = 28.4 o 2.6 6 months old group vetiver high land c = 8.5 kpa; = 29.2 o 6.0
Study of vetiver root surfaces 6) Long end fine tip 1) 1 st -order branch 7) 3 rd -order branch 2) Thin 2 nd -order branch 5) Thick 2 nd -order branch 3) Fine tip 4) Knotted fine tip 8) Root cap
1) 1 st -order branch
1) 1 st -order branch
1) 1 st - order branch 2) Thin 2 nd -order branch
2) Thin 2 nd -order branch
2) Thin 2 nd -order branch
3) Fiber pullout
4) Knotted fine tip
5) Thick 2 nd - order branch
7) Third-order branch
Vetiver root tip
Adhesion at the Interface Microscopic scale effect is important
Soil planted roots A1_Elbow A1_Shaft A2_Spindles A4_Cut section A1_Root tip lower down A1_Root tip A2_Spindles2 A3_EggShape Caught
Sand planted roots B1_Shaft B1_B2 Overlap B2_and Sand? B3_ B1_Tip next to B2 B2_and Sand? B2_and Sand? B3_
Root Caps A1_Root tip A1_Root tip lower down B1_Tip next to B2 Elongated cells ready to expand.
Sample A vs B Comparison: Particles A3_EggShape Caught B2_and Sand? B1_Shaft 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
Root Hairs and their spindles B3_ A2_Spindles A2_Spindles2 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 (uncoated) Fractur e surface F EM Segment 1 (root details clearly visibly in EM) F LM (coated) Limited depth of focus
Sample A fracture surface Fracture sruface Root hairs (invisible in LM)
Tensile delamination - root
Core fiber pull out
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
Electron microscope observation z q x Deformed root b T R T R b Shear zone Root tip of vetiver er grass. Width of the micrograph is approximately 1000 micron. The root hairs are of the order of micron level and their interfacial area is contributing significantly to the friction due to their increased surface area. Intact root Interface friction between en soil and root (Gray & Sotir tir, 1996) High density of root hairs. Low magnification image. This mechanism provides adhesion between root and soil during shear which could be directly linked cohesion term in Mohr-Coulomb failure criterion framework.
Acknowledgement The AUN/SEED-Net (JICA) The Chaipattana Foundation Office of the Royal Development Projects Board The Sustainable Energy Foundation - PTT Co., LTD Dr. Songkiert Tansamrit and P Yai PTT Co., LTD Dr. Pitayakon Limtong from LDD Mr. Atichart Ruksajitr The Chaipattana Foundation