Assessment of Vetiver Grass Root Reinforcement in Strengthening the Soil

Abstract

Vegetation has been used globally for centuries to control soil erosion on slopes. It is widely recognized that vegetation contributes to slope stability, but to an unquantifiable degree. Vetiver grass (Chrysopogon zizanioides) is being utilized to reduce the soil erosion and strengthen the slopes. Vetiver roots are massive, fine-structured, and grow fast and deep. With an average tensile strength of 85 MPa, which approximates to one-sixth of that of mild steel, Vetiver root is compared to as ‘living soil nail’. Roots of trees and other vegetation increase the shear strength of soil by root reinforcement. The difference between shear strength values of root-permeated soil and root-free soil sheared under the same conditions gives the shear strength increase caused by the roots. This paper presents the investigation on increase in shear strength of soil by Vetiver roots. Using a custom made large-scale in situ direct shear test apparatus, in situ direct shear tests were conducted at different depths on a soil plot with one-year-old Vetiver grass planted at 0.15 m spacing in an equilateral triangular pattern. The tests prove that the Vetiver grass roots increase the shear strength of soil by up to 139% at 0.15 m depth and up to 47% at 0.75 m depth.