ASSESSMENT OF SLOPE STABILITY USING NUMERICAL METHOD: A CASE OF WEYIZA - WULUKODHE ALONG THE MORKA-GIRCHA-CHENCHA ROAD PROJECT

Abstract

Slope instability is one of the world's most common natural geohazards. Slope failures are very common in steep and rugged terrain, resulting in fatalities and infrastructure damage. This study examines the assessment, identification of the cause, and implementation of corrective measures for slope failure. It also looks at how stable the existing slope is along the access route from Weyiza to Hulukodhe on the Morcha-Gircha-Chencha road project in the Southern Ethiopia regional state, where slope instability is common. For this, a detailed field investigation, which included in-situ soil testing, soil sampling, and measuring slope geometry, was carried out. For the slope stability analysis, the Rocscience SLIDE V6 software used the limit equilibrium method to find the factor of safety. In both natural and fully saturated conditions, the software employs Bishop's, Janbu's, Spencer's, GLE's, and Morgenstern-Price methods and conducts the slope analysis with and without seismic load applications. AASHTO classified the slope-forming soils as A-2-7, A-7-5, and A-7-6, while USCS categorized them as CH, MH, and SM. In general, the soil in the slope sections was mostly silt and clay, mixed with some gravel, and had moderate to high plasticity. The main factors contributing to the instability of the slope sections under investigation were the nature of the soil, the presence of excessive slope angles, and the moisture fluctuation effect. The sensitivity analysis revealed that the slope's stability in the study area was mostly dependent on cohesion. In natural conditions, the safety factor ranges from 0.592 for slope section FS 43+740 to 1.162 for slope section ES 44+420, according to the numerical analysis. It also ranges from 0.347 for slope section FS 40+100 to 0.983 for slope section ES 44+420 in saturated conditions, as well as when there is a dynamic load on both soil conditions. The safety factor in dynamic conditions (taking into account the seismic stress) is marginally lower than the safety factor under static conditions.The researcher has proposed and reanalyzed remedial measures for slope sections that have failed, aiming to improve the factor of safety by reducing the slope angle and reinforcing with soil nails. When the researcher provided 10 m-long soil nails with 0.5 m x 1.0 m spacing, the factor of safety increased from 0.347, the lowest value among all FoS, to 1.411, and the slope angle decreased from 600 to 260 to 1.397. Therefore, for this study, the suggested mitigation measures include reducing the slope angle and installing soil nails for the designated slope failure-prone area as well as for the failed slope parts following reconstruction.