GEOTECHNICAL INVESTIGATION AND STABILITY ANALYSIS OF LANDSLIDE USING NUMERICAL METHODS: THE CASE OF CHIRA TOWN, SOUTH-WESTERN ETHIOPIA

Abstract

Landslide is a natural disaster in which earth mass materials like rock, debris, or earth move down a slope and cause damage to living things, as well as the natural environment. In 2020, the landslide occurred in Chira town following heavy rainfall. As a result, Residential houses, internal roads of the towns, pipelines, and natural environment were damaged following the land subsidence of the study area. Therefore, this research aimed to conduct geotechnical conditions and stability analysis of landslide occurred in Chira Town. The main objective of the study was include investigation on geotechnical conditions and their role in landslide occurrence, identification of the main triggering factors, slope stability analysis by using Plaxis 2D and Slide software's, and propose remedial measures to safeguard future failure in the study area. To achieve the objective of this study, experimental and analytical methods were conducted. To determine subsurface condition, Geophysical test was conducted. Soil samples were taken from the crest and toe of the affected and unaffected slope area at different depths and test on index and engineering properties of soils was conducted following the ASTM procedure. Three slopes were analysed by plaxis 2D and slide softwares on affected and unaffected slopes. Based on laboratory tests and geophysical investigation, the soil types of the study area were dominated by fine-grained soils (Clay &silt) and they initiate the occurrence of landslide since they are weaker soil when contact with water. Rainfall is the major triggering factor and from prediction rate of causal factors, the soil type, land use, elevation, distance to stream, slope, aspect and curvature were a reason for the occurrence of Chira landslide by 22.03%, 18.89%, 15.75%, 15.46%, 10.87%, 9.7%, and 7.5% respectively. FS varied between 1.34 to 1.95 (by plaxis) and 1.35 to 1.93 (by slide) for affected slopes and 0.66 to 0.98 (by plaxis) and 0.67 to 0.97 (by slide) in saturated and dry state respectively. Hence, they are considered as unstable slopes. FS for unaffected slope varied between 1.83 to 3.18 (by plaxis) and 2.02 to 3.17 (by slide), which is stable. The LE overstimate the FS from 5% - 20% over FE analysis in dry and partially saturated conditions respectively. Groundwater increament decreases the FS by 42.45% - 50.75% (by plaxis) and 35.94% - 49.93% (by slide) software's. Surface drainage, planting vetiver vegitation, and proper landuse management were the proposed remedial measures for Chira town landslide.