Slope Bioengineering

The Vetiver system (VS) has proven to be very effective in mitigating erosion and shallow slope instability, provided it is applied correctly. Nevertheless, quantitative studies of how much contribution the VS really provides to slope stabilization in the field are still relatively scarce. This study outlines an approach for such purposes, including laboratory tests, field observation as well as numerical analysis. Laboratory tests include root & soil mechanics characterization, such as tensile strength, as well as direct-shear testing and permeability tests of root-reinforced soil sample. This, together with field observation of root quantity by means of mini-rhizotron, enables quantification of root-area ratio and the root cohesion that exists in actual slopes. It can be seen from a test site in Surathani, South Thailand, that after the Vetiver was invaded by the local species and its roots decayed, the root area ratio decreased significantly leading to loss in root cohesion and decreased factor of safety. This thus emphasizes the importance of frequent maintenance of the Vetiver in practice in order to sustain long-term slope stability. Also based on studies using numerical simulation, the VS appeared to have mainly beneficial effects for slope of 26o gradient (1V:2H). However, when applying the VS on very steep slope (>60o, 2V:1H), practitioner should exercise certain cautions, especially for the case of easily degradable rock such as claystone, since there could be theoretical adverse effect of increased infiltration through root zone, amount to 10% reduction in factor of safety.