Stability Assessment of a Bio-Engineered Slope in Western Thailand K. Rajamanthri Department of Civil Engineering, Kasetsart University, Bangkok, Thailand A. Jotisankasa Department of Civil Engineering, Kasetsart University, Bangkok, Thailand ABSTRACT: This paper aims to investigate the influence of vetiver grass (Chrysopogon zizanioides) on bio- engineered soil slope in Thailand. Slope stability and seepage analysis have been conducted to study the be- havior of the vegetated slope using the data obtained from samples collected from the site. Results of the la- boratory experiments are incorporated into numerical models to evaluate the response of the slope under dif- ferent climatic conditions. In this study, finite element approach was used to analyze the infiltration behavior of the slope and limit equilibrium method was used for slope stability assessment. Further a parametric study was carried out to identify the most effective location of the slope to have vegetation in order to enhance the stability. When slope area was covered by vegetation, it maintained the highest safety margin and it was about 33 % increase compared to the bare slope. Further, after the four-day infiltration event the slope with its entire slope surface area protected with vegetation possessed the highest safety margins and the percentage variation of the safety margins was observed to be only 24%. Keywords: slope stabilization, vetiver, root zone, root reinforcement, slope stability, infiltration analysis 1 INTRODUCTION Rain induced shallow slope instabilities and soil erosion caused by runoff are frequent geotechnical problems that the engineers have to confront with especially in tropical climatic regions (Jotisankasa et al., 2014; Rahardjo et al., 2017; Rajamanthri et al., 2021). Thus, geotechnical experts have devel- oped different structural measures to rectify such problems. Nowadays, use of soil bioengineering approaches for slope rectification are becoming popular due to its comparatively low cost, aesthetic beauty it adds to the slope, minimum disturbance to the slope, less maintenance in the long term, as well as sustainability when compared with com- monly used structural mitigation methods (Wu et al., 1979; Coppin and Richards, 1990; Jotisankasa et al., 2014). When selecting vegetation for slope rectifica- tion measures due consideration should be paid to their properties as they have a direct impact on the slope-behaviour in both short and long run (Coppin and Richards, 1990). As a result in Thailand exten- sive researches have been conducted on Vetiver grass (Chrysopogon zizanioides) to understand its different impacts on soil properties, when used as a slope stabilization and erosion control measure (Jotisankasa et al., 2014; Nguyen et al., 2018; Ra- jamanthri et al., 2021). Normally vetiver planted as hedgerows along slope contours and vetiver system has wide range of applications in soil bio engineer- ing such as soil water conservation, slope stabiliza- tion by root reinforcement, erosion control and sediment trap since vetiver roots penetrate as deep as 3-4 meter. (Truong et al., 2008). However, con- tribution of vetiver grass towards different soil bio engineering applications is essentially a function of its properties, namely plant age and root biomass (Leung et al., 2018; Rajamanthri et al., 2021). In this study, effectiveness of vetiver grass in maintaining the stability of a slope has been inves- tigated. Further an infiltration analysis has been coupled with a stability analysis to study the re- sponse of bio-engineered slope to an infiltration event, with incorporating the soil properties ob- tained from undisturbed samples collected from the site. Numerical analysis was further extended to study the spatial distribution of the vegetation in a slope in enhancing stability. 2 STUDY AREA AND SOIL PROFILE The study area is located in Kanchanaburi prov- ince, in Western Thailand (Fig. 1). The slope of in- terest (Ithong 1) suffered from a mass movement and erosion in 2006. This slope was rectified using geosynthetic reinforced soil (GRS) wall together with vetiver vegetation (Fig. 2) nearly two years ago. Accordingly, the plant age of the vetiver at the