GEOTECHNICAL INVESTIGATION AND STABILITY ANALYSIS OF LANDSLIDE USING NUMERICAL AND LIMIT EQUILIBRIUM METHODS: (A CASE STUDY OF FUDALE, GARASSE WOREDA, GAMO ZONE, SOUTHERN ETHIOPIA)

Abstract

Landslides are natural disasters that happen when rocks, debris, or soil along a slope, causing harm to human life and the environment around them. Hilly regions of South Ethiopia pose a significant danger to communities and infrastructure. In 2023, Fudale, Garasse, experienced a landslide due to intense rainfall, leading to fatalities, property destruction, and disruption of livelihoods. Therefore, this research was aimed at analyzing the geotechnical conditions and stability factors associated with the landslides in the Fudale area. The study primarily aimed to investigate the geotechnical conditions that contribute to landslide occurrences and identify the key influencing factors. Slope stability analysis was conducted using FEM and LEM through Plaxis 2D and GeoStudio (SLOPE/W) software, respectively. Additionally, the research sought to propose remedial measures to mitigate future landslide risks in the affected area. To fulfill these objectives, both fieldwork and experimental methods were used. Soil samples were collected from the crest and toe of the slope at depths of 1.5-3 meters, following ASTM procedures to test their index and engineering properties, with five test pits analyzed in the laboratory. The findings from laboratory and field tests indicated that the predominant soil types in the research area were fine-grained, comprising 37.60–55.96% clay and silt, 4.50–7.38% sand, and 0.10–0.80% gravel, which are particularly susceptible to landslides when saturated. The Fudale landslide, triggered by factors such as rainfall, slope aspect, curvature, and slope angle, occurred. Triaxial test results demonstrated that cohesion and internal friction angles ranged from 28.02 to 57.60 kPa and 9.0 to 16.94 degrees, respectively. The calculated factor of safety (FOS) values are 0.972 (by Plaxis 2D) and 0.946 (by GeoStudio (SLOPE/W)) for slopes in natural conditions, respectively. This shows the slopes are unstable. A modified slope angle (gentle slope) using GeoStudio (SLOPE/W) is suggested, achieving a factor of safety (FOS) of 1.688. This FOS for the modified slope angle is greater than that of the natural slope, mainly as a result of the impact of the steep gradient present in the natural slope. The proposed remedial measures for the Fudale landslide include geometry modification, surface drainage, afforestation rehabilitation, and engineering structures for damaged areas.