Engineering Geological Investigation of Katar Dam Foundation and Abutment Slope Stability Analysis, Arsi Zone, Oromia, Ethiopia

Abstract

The primary pillar of the Ethiopian economy in terms of income, employment, and export earnings generation is agriculture. About 80 percent of the population live in rural, agriculture-dependent areas but the majority of the population is still impoverished and faces food insecurity. The Katar dam site is a 35m height rainwater-harvesting hydraulic structure implemented by the Ethiopian government in order to enhance irrigation and bolster food security in the region. Despite this, the dam site has engineering geological challenges due to the presence of weak, extensively fractured, and weathered ignimbrite rock, as well as weak unwelded to welded tuff rock mass near the dam base and abutments. Engineering geological investigation of the dam foundation and abutments on the dam site's rocks and soils addressed these challenges. To achieve these objectives, geological mapping, sampling and laboratory analysis of samples, discontinuity surveying, in-situ strength test analysis, core drilling, geophysical investigation, in-situ permeability test, and slope stability study were carried out. From geological mapping, the principal lithological unit of the research region were residual soil deposits, ignimbrite rock, welded tuff rock unit and unwelded tuff unit with varied degree of fracturing and weathering. At the dam site, intact rock strength test indicated that the uniaxial compressive strength value ranges from 8.97MPa to 22.87MPa and the UCS estimated from the in-situ strength test shows a range of 24.15MPa to 33.88MPa showing the rock mass constituting dam site have very low to low strength rock mass. At the dam site, the subsurface rock quality designation ranged from very poor to excellent in terms of rock mass quality. According to the packer test results, there should be significant leakage at the intake conduit borehole and right abutment due to hydraulic conductivity with representative Lu values of ranging from 9.08-12.07 and 2.14- 13.23, respectively. The majority of the soils in the reservoir and dam site were clayey silt and sandy silty soils, with hydraulic conductivity values indicating low to impermeable soil mass and plasticity index values ranging from 6.4% to 31.9% with low to high and 24% with high plasticity of soil. The allowable load-bearing capability of the rock formations at the dam's abutment and intake conduit varies between 0.02MPa and 16.11MPa, and between 0.36MPa and 2.3MPa, respectively. The slope sections were unstable under fully saturated static and dynamic conditions, according to the slope stability analysis. This research recommends curtain grouting for the permeable rock layer, further detailed study of soils at the reservoir area, additional evaluation of design and implementation of remedial measures such as rock bolts, and intensive and adequate dam foundation design.