EVALUATION OF SLOPE STABILITY CONDITIONS BY NUMERICAL ANALYSIS, A CASE OF OTOLO-SAWLA ASPHALT ROAD PROJECT, SOUTHERN PART OF ETHIOPIA

Abstract

Slope instability related issues in engineered, as well as natural slopes, are common challenges to geotechnical enginers. Due to the slope instability, so many associated risks arise around the cut slope so that it is advisable to analyze the slope stability ahead of excavation. Once slope failure occurs, traffic flow gets obstructed and which ultimately limits the road serviceability level impacting the economy. This thesis presents the investigation, analysis, cause identification, and remedial measures for failed slope and checks stability of the existing slope. Seven cut slope related instability locations were selected along with Otolo-Sawla Asphalt Road Project located in the Southern part of Ethiopia. In this thesis, disturbed samples were taken from a test pit dug in the slope section by hand excavation. In the case of existing slope locations, three samples were taken based on the slope angle and the material forming the slope through field observations. Laboratory tests were conducted to determine the index properties and engineering properties of the soil-forming the selected slope sections. The main objective of this thesis was to investigate slope stability conditions of the Otolo-Sawla asphalt road project using a numerical method considering selected slopes at varied geological and geographical locations. A suggestion of remedial measures to stop further failure of the slope section can be analyzed by modeling with commercial software PLAXIS 2D. Under this thesis, the factor of safety for the identified slope failure was determined using Finite Element software (PLAXIS 2D) based on the strength reduction approach. Based on the analysis the causes for failed slopes were identified as poor slope material, excessive slope height, and high groundwater levels. In this thesis the order of sensitivity of factors affecting the slope stability obtained as φ>H>c>γ >k >E>μ. A stability check for the existing slopes was carried out and was found slightly stable. On the results from the analysis, the failed slope sections should be reconstructed with a slope angle less than 50% and an adequate drainage system should be provided. Existing slopes near the failed slopes are prone to failure because of similar soil properties and groundwater influence. The provision of geosynthetic for adequate drainage has been recommended as a remedial solution to avoid failure of the existing slopes near the failed slopes.