The study of the effects of vegetation on slope stabilization for landslide prevention in Thailand

Landslide causes major disturbance to natural ecosystem in tropical montane forest. The main factor that causes slope failure or landslide is climate triggered which is intense or prolonged rainfall and seepage. During the past few decades, rain-induced landslides in Thailand occur more frequent causing mortality and economic loss. The consequences of shallow landslide at depth 0.5-3.0 m are the most destructive type of landslide in Thailand. Existing countermeasures for landslide prevention and mitigation are mostly geotechnical engineering structure, and they are considered to be costly and might not be compatible with the soft ground of eroding soil. Therefore, the use of bioengineering method to prevent shallow landslide has been alternatively used for providing long-term stability of slope. One of the factors controlling landslide is vegetation cover. Most commonly used soil bio-engineering technique in Thailand and other Southeast Asian countries is the use of Vetiver) Chrysopogon zizanioides) due to its cost-effectiveness, environmental-friendliness, complex root structure, and high mechanical strength. However, the propagation rate of Vetiver is too low which produces insufficient amount of Vetiver for the use in landslide-prone area. The objective of this study is to survey indigenous plant species which have high mechanical strength and similar root characteristics to Vetiver in order to further promote to use as soil bioengineering structure. Three candidate species: Thysanolaena maxima, Calamus viminalis, and Sida acuta were selected for studying root morphology and mechanical strength. Thysanolaena maxima performed best in root tensile strength, while Sida acuta has the deepest rooting distance. Due to its long life span and economic value, Thysanolaena maxima was selected for the testing on the improvement of slope stability. It was found that there was 19.23 kN/m2 increase in soil shear strength in root-permeated soil compared to bare soil in non-slide site, while there was 2.21 kN/m2 decrease in slide site. The study also found that there was no significant different in soil permeability of three soil samples. The other engineering method should be employed together with soil bioengineering in order to improve drainage in landslide-prone area.