A LANDSLIDE RECONNAISSANCE ALONG A PORTION OF THE FELEGESELAM CHIDA-AMEYA ROAD AND A SLOPE STABILITY ANALYSIS OF THE METOSO KEBELE LANDSLIDE, JIMMA ZONE

Abstract

Landslides, which commonly occur in Ethiopia's central, S-SW, and N-NW highland regions, have recently emerged as serious geo-hazard hazards that have a significant impact on life, infrastructure, and the natural environment. This is because the area contains complex geological and geomorphological settings [1] . This research focuses on landslides that happened in the south and southwestern regions along portions of the low volume road between Felegeselam and Ameya-Chida during 2019-20. The study aimed to give more emphasis on assessment of landslides along the portion of the road existed and investigating long-term stability conditions of natural slope failure occurred in Metoso kebele. Thus, assessment of the cases of landslides conducted by preliminary investigation and reconnaissance survey on the study area and applying detail investigation such as field tests (i.e. geophysical resistivity) and geotechnical laboratory tests(as per ASTM & AASHTO standard) for evaluation of long-term stability conditions of natural slope failure occurred in Metoso kebele. Stability analysis was done by finite element method using plaxis-2D version 20 software. According to the results of a field study conducted in the area, 18 different cases of landslides are occurred during 2019-20. Most of the landslide types are characterized by both active and reactivated state of activity, shallow in depth commonly slide (earth & debris), debris Flow and debris fall and most of them occurred during heavy rain season (i.e. August, July and June). Among the main causes of the problem are the presence of unstable soil deposits or adverse lithology in the area, difficult geomorphology and high groundwater availability. Additionally, the presence of prolonged and heavy rainfall in the region, Improper road geometric design ( poor road route selection, drainage system and steepness cut slope), abundance of springs, aquifers and high fluctuation of ground water table, the incorrect cultivation style or land use and deforestation are all considered the primary triggers for the issue. The landslide that occurred in Metoso kebele was subject to a detail investigation and stability analysis, and the results revealed, the following: The natural moisture content ranges between 34.84 and 46.84%. Grain size analysis indicates that the study area is composed of fine gravel, coarse sand, medium sand, fine sand, and clay-silt in a range between 0.42–6.9 %; 0.99–1.72 %; 3.23–4.97 %; 8.63–13.75 %; and 69.22–84.63 %, respectively. From the Atterberg limits, the liquid limit, plastic limit and plasticity index (PI) of the soils is in the range of 50%-58%, 26%-39% &14% -32% respectively. The free swell index and linear shrinkage of soil is in the range of 19.23–28% & 6.16–10.2 respectively. According to V the AASHTO soil classification system, the soil particle distribution characterized by fine grained soil, medium to high plasticity silty-clay (A-7-5(36)) and inorganic clays of high plasticity soil(A-7-5(25)) and has a poor subgrade rating. The unconsolidated undrained tri axial test showed that the soil had cohesion and angle of internal friction of the range of 14.51 to 45.08 kN/m2 and 5.61 and 14.29o respectively. The coefficients of permeability of the sol vary between 10-4 to 10-7 cm/sec that indicates low permeability. Results obtained from geophysical resistivity analysis showed that the presence of residual soil or unconsolidated soil deposit, accumulation of aquifers and existence of ground water table at shallow depth. Evaluation of the slope stability analysis performed using the Mohr-Coulomb model approach showed that the factors of safety and deformation are 0.552 and 6.69 m, respectively due to static loading with rainfall infiltration. This indicates that the study area long-term slope stability condition is within failure due to the presence of steep cut slope and extreme alteration of surface & subsurface water. Lastly, remedial measures such as providing adequate and compatible types of drainage system on the natural slope in order to reduce water infiltration and seepage; modification of geometric design by best route selection & applying stable cut slope, application of slope stabilization methods (mechanically Stabilized Earth Embankments and afforestation) can overcome such geo-hazard.