Laboratory and modelling investigation of root-reinforced system for slope stabilisation

Most natural slope failures are induced by seepage and/or rainfall. Soil bioengineering is an environmentally friendly method which employs vegetation to reinforce the soil in sloping terrain. The vegetation can contribute to slope stability in two ways, mechanical and hydrological. This paper demonstrates the effect of a vegetation root matrix on a soil slope and focuses on mechanical reinforcement using an example of vetiver grass. Vetiver grass (Vetiveria nemoralis A. Camus) specimens, grown for under a year, were used in this study. The investigation programme includes root observations, direct shear tests and centrifuge model tests. The growing rate of the vetiver roots and the root area ratios were observed during the tests. The cohesion and angle of internal friction of root-reinforced soils were determined from a standard direct shear apparatus and a large direct shear apparatus. A series of centrifuge tests was carried out to demonstrate the effect of vegetation on seepage- and rainfall-induced slope failures. The results indicate that the vetiver roots showed rapid growth within a year and that the shear strength of the root-reinforced soil was significantly increased by the bundle of roots. The results also reveal that the bundle of root fibres in the centrifuge model tests helped to reduce the deformation of the soil slope due to instability by increasing the shear strength of the slope.