Slope Stability Analysis along the Critical Sections of Woliso to Wonchi Lake Road, Central Ethiopia

Abstract

Slope instabilities are a major geological hazard that frequently leads to the failure of various civil engineering structures such as roads and open-pit mines. The road which connects Woliso town to one of the main tourist attraction in Ethiopia (Wonchi Lake) is also frequently affected by slope instabilities. Hence, this study is aimed at identifying and analyzing critical slope sections along this road by using Kinematic, Limit Equilibrium Method (LEM) and Finite Element Method (FEM). The input parameters for the analysis were determined through field investigation methods such discontinuity surveying, in-situ Schmidt hammer test, and sampling and via laboratory testing of samples. From field investigation, three critical structural controlled and six critical non-structural controlled slope sections were identified. Schmidt hammer rebound test conducted on the study area revealed that Uniaxial Compressive Strength (UCS) value ranges from 42.17MPa to 136.49MPa and 16.7 MPa to 28.9 MPa for rocks of structural and non-structural controlled slope sections, respectively. The stability analysis of structural controlled slope sections is first done via kinematic analysis and the results showed that RSS1 is unstable for planar mode of failure whilst RSS2 and RSS3 became unstable for wedge mode of failure. Further stability analysis of planar mode of failure in terms of Factor of Safety (FOS) using LEM-based Rocplane 2.0 software revealed that RSS1 is unstable under all anticipated conditions. Similar analysis for wedge mode of failure using Swedge 2.0 software also revealed that RSS2 is unstable only under dynamic saturated condition while RSS3 is unstable under all conditions due to intersection of JS1 and JS3, and stable under all conditions due intersection of JS2 and JS3. Similarly, stability analysis using both LEM based Slide 6.0 and FEM-based Phase 2.0 software have also shown that all six non structural controlled slope sections are unstable under saturated conditions. This is illustrated that rainfall/or saturation is the major factor contributing to slope failure in the study area. Moreover, the FOS obtained from LEM-based Slide 6.0 software and SRF obtained from FEM-based Phase 2.0 software are in a very close agreement which validated slope instability problems in the study area. Depending on the analysis of results, a total bolt capacity of 1960 and 4000 tons were designed for RSS1 to stabilize planar failure due to JS1 and JS3 respectively. Similarly, rock bolts with a total capacity of 3750 ton and shotcrete with a shear strength of 1600 t/m2 at RSS2, and rock bolt with a total capacity of 31200 ton and shotcrete with a shear strength of 2950 t/m2 at RSS3 were designed to stabilize unstable wedge. However, considering the budgets of bolts and the current stage of this road construction, removing unstable rock wedges is recommended. Moreover, for non-structural controlled sections, it is designed to reduce upper slope profile of SSS2 to less than 23.190 and the middle slope profile to less than 33.690 , and top slope profiles of SSS4 and SSS5 to an angle less than 63.430 and 27.220 , respectively. Moreover, surface and subsurface drainage systems are also recommended to in these slope sections for better control of water effects.