Geotechnical Characterization and Slope Stability Assessment along selected section of Tercha-Chida Road, Southern Ethiopia

Abstract

Slope stability assessment is essential for the safety and sustainable development of mining, civil, and environmental engineering projects worldwide. It is also important for the long term survivability of existing and excavated slopes. Geotechnical characterization and Slope stability analysis has been carried out to evaluate the potential for failure of the cut slope. The study area was along a road section between Tercha in the Dawro zone and Chida in Konta woreda. The main objective of the study is a geotechnical characterization of soil and rock mass along the selected road sections, assessing the potential failure by determining the factor of safety of the critical slope sections and proposing the remedial measure. Along the road sections of Tercha in the Dawro zone and Chida in Konta, eight sections have been selected. Out of that six of them are rock slopes and the other two slopes are soil slope. On the rock slope, detailed joint mapping was carried out and the result of the discontinuity survey indicates that the dominant joint orientation is sub-horizontal to vertical. The subsurface conditions of the critical slope section were studied through six boreholes and seismic refraction data obtained from ECDSWC; accordingly, the selected slope sections were classified into three geological formations namely; (1) top soils (a pyroclastic deposit with 10m thickness (2) weathered and fractured rock (20m thickness) and (3) bed rock. The disturbed and remolded sample was obtained from 2 pit excavations. The clayey gravel soil is brown with a specific gravity of 2.68, Natural water content of 3.6%, Plastic limit of 31.93, and a liquid limit value of 51.983. the optimum moisture content, which was 22.1%, and maximum dry density with the value of 1.58g/cm3. the direct shear test result shows the cohesion and friction angle 12.2 and 30.5˚ values respectively. Similarly, all rock units observed in the critical slope sections including Tuff, Ignimbrite, Rhyolite, Basalt, and unwelded tuffs are identified and their respective point load strength, dry and saturated density and slake durability were determined. The critical slope section analysis was carried out using limit equilibrium, finite element, and kinematic analysis methods. Two structurally controlled rock slope failures have been identified and each has a planar and a toppling failure. The effect of saturation, material properties and loading conditions significantly affect the stability of the slope. Benching and Constructing pressure Berms at the toe and the replacement of slipped material with free-draining material is important to increase the stability of the slopes.