Numerical stability analysis and geotechnical investigation of landslide prone area (the case of Gechi district, Western Ethiopia)

Abstract

The primary route linking Tobba, Gechi and Beddele has experienced landslides at three locations, causing significant disruption to traffic. The juniper trees and electric poles have suffered damage, and the extension lines are at risk. This investigation aims to identify the geotechnical conditions, soil type, and their role in landslide initiation, slope stability analysis, and measures to reduce the risk of landslides. The study employed fieldwork, laboratory analysis, and software analysis. The fieldwork involved measuring the slope geometry and landslide magnitude, as well as geophysical resistivity to determine the soil type and thickness of the weak zone. Soil samples were taken and tested for grain size analysis, Atterberg limit, water content, unit weight of soil, specific gravity, and shear strength parameters, following ASTM procedures. Plaxis v 8.6 software was used to supplement the slope stability analysis. The results indicate that the soil in the study area is classified as MH- elastic silt and elastic silt with sand (S1BH3 and S2BH1) behaving as sand. The fine-grained soil constitutes ≥50% of the soil, which significantly influences the soil properties. The numerical value of the factor of safety (FOS) from slope stability analysis decreases as the phreatic level increases. Electrical resistivity profiling indicated that there is a change in soil matrix and grainsize distribution. The dry unit weight varies from 6.28KN/m3 to 43.23KN/m3 and the slope angle varies from 14.09° to 14.62°. The cohesion of the soil varies from 40.71KN/m2 to 67.02KN/m2 and the angle of internal friction varies from 15.12° to 26.62°. The landslides were triggered by heavy rainfall, and the primary factors affecting slope stability are soil type and characteristics, surface and groundwater, and slope steepness. To maintain the slope's stability, it is recommended to modify the slope geometry, construct sheet piles and retaining walls, provide adequate drainage structures, and afforest the area with long-rooted grass and trees.