Slope Stability Analysis Along Selected Steep Sections Of Dibibisa Ridge Of Adama Town Using Limit Equilibrium And Finite Element Methods, Central Ethiopia

Abstract

Adama town is one of the rapidly growing towns in the country, and its population is growing at a staggering rate as well. The town is bounded by the two ridges (one of whom is Dibibisa ridge) and are both striking nearly in the NNE-SSW direction. Moreover, urbanization and population growth of the town is leading to the construction of infrastructures such as roads and residential buildings near this relatively steeply sloped ridges. However, these ridges in particular, the Dibibisa Ridge regularly have slope instability issues that endanger surrounding infrastructure projects and human life. As a result, the goal of this study is to examine slope instability issues along selected sections of the Dibibisa ridge in Adama town. Detailed fieldwork which includes discontinuity surveying, in-situ strength test, mapping, sampling, and laboratory testing was carried out to achieve the intended objective. Based on a detailed field work a total of five critical slope sections (i.e. two structural controlled rock slopes and three soil slope sections) were identified during field work and analyzed using kinematic(dips), finite element (Phase), and limit equilibrium (Slide2D,rocplane & swedge) methods of stability analysis. The rock mass rating showed that the rock of the study area ranges from fair to good quality. Under kinematic analysis, the two rock slope sections are admissible to two planar and one wedge mode of failure. All three critical soil slope sections are unstable at all anticipated conditions under both limit equilibrium and finite element method of slope stability analysis. From the results of slope modification analysis the factor of safety increases as the width of benches as well as number of benches increases and in the slope section (D1S4) the factor of safety was increased to 1.222 as two benches of 5m wide used under slide 2D on which was initially unstable with the factor of safety 0.267, Similarly as 3 benches of the same width used under slide 2D the factor of safety of the slope section reaches 1.219. At the slope section (D1S2) flattening of slope angle from initial 45⁰ to 35⁰, 28⁰, 25⁰ and 18⁰ increases the factor of safety of the slope from initial 0.284 to 0.77, 0.89, 1.022 and 1.151 respectively under slide 2D analysis. At slope section (D2S1) flattening of slope angle from initial 46⁰ to 35⁰, 25⁰, 23⁰, and 20⁰ increases the factor of safety from initial 0.412 to 0.684, 0.920, 1.02, and 1.315 respectively. As indicated from the analysis of the results, the identified slope sections are susceptible to failure under actual field scenario under different projected conditions. Hence, this study recommended Benching method to be adopted as economical mitigating measure for soil slope sections.