Slope Stability And Water Tightness Assessment Of Gololcha Dam Site, West Hararghe Zone, Eastern Ethiopia.

Abstract

The economy of Ethiopia is mainly dependent on agriculture. Water is very essential for agriculture that sustains the development of the country. Gololcha dam is an earth and rock fill dam, which is under construction on Kurkura River in Eastern Ethiopia for the purpose of irrigation. For the dam to meet the intended primary purpose, its safety must be determined and the reservoir must store enough amount of water for the long period of time. This study aimed to conduct detailed geological, geophysical, surface and subsurface geotechnical investigation to characterize the engineering geological properties in the view of slope stability and water tightness. To achieve the objective of the study, geological and engineering geological mapping, discontinuity surveying, digging test-pit, permeability testing, sampling and laboratory testing have been carried out. A critical slope stability analysis on the right abutment (RAS 1 and RAS 2) was also carried out. The area is covered with volcanic rock (Basalt) and soil deposit (alluvial and residual) both at the dam site and reservoir area. The exposed basaltic rock is highly fractured and fragmented, which falls into very poor to good rock quality according RQD value obtained from boreholes. RMR and GSI were used to identify the quality of rock mass and the result revealed that the rock in the study area is classified as fair. The strength of the rock of the study area ranges from medium to high strength. The soil found in the study area was dominated by sandy and silty soil of low to high plasticity with plastic index ranges from 9.72% to 20.95%. The right abutment RAS 1 slope section is non-structural controlled failure which is unstable during saturated condition with FOS = 0.85 and stable during dry condition. Whereas RAS 2 slope section is structurally controlled with one planar mode of failure and two wedge mode of failure. The analysis for RAS2 slope section was conducted by Rocplane and Swedge software for planar and wedge mode of failure respectively. From analysis result RAS2 is unstable under saturate conditions and stable under dry condition. The permeability of surface rock ranges from 4.08*10-3m/s to 1m/s and permeability from boreholes ranges from 4.68*10-5 cm/s to 1.29*10-2 cm/s. Based on permeability result, excessive seepage problem was expected from abutments and reservoir rim. Whereas, the reservoir area covered by soil is almost watertight. Finally, this study recommended proper remedial measures such as provides sufficient drain hole, remove loose material from the slope, rock bolt and anchor for stabilization of structural controlled slope section, shotcrete and reducing slope angle for stabilization of non-structural controlled slope section and grouting and clay blankets to control leakage problems.