Engineering Geological Investigation of the Katar Dam Foundation and Abutment Slope Stability Analysis, Southeast Ethiopia

Agriculture is vital to the Ethiopian economy, providing income, jobs, and exports. Despite this, Ethiopia is actively implementing irrigation projects through construction large number of micro dams to increase food security. Katar Dam is one of the micro dams under construction which aims to enhance irrigation and food security. However, the dam site is challenged by engineering geological problems due to the presence of weak, fractured ignimbrite and unwelded tuff rock units in this area. To recommend an appropriate remedial solution for the dam foundation and abutments, geological mapping, geophysical survey, core drilling, geological logging, and in situ and laboratory tests were carried out. Geological mapping revealed the main lithological units in the study area, including residual soil deposits, ignimbrite rock, welded and unwelded tuff rock units with varying degrees of fracturing and weathering. Results showed the dam site rock mass rating was of fair quality, with RMR scores of 55 and 59 for the right and left abutments, respectively. GSI described the rock as blocky, while uniaxial compressive strength (UCS) tests indicated a very low to low strength, with intact rock strength ranging from 8.97 to 22.87 MPa. Subsurface rock quality varied greatly, and packer tests indicated potential leakage due to high hydraulic conductivity. Soil analysis at the dam and reservoir sites found mostly clayey silt and sandy silty soils with low permeability and varying plasticity. Rock formations load-bearing capacity was also low, suggesting instability under static and dynamic conditions. To mitigate these issues, the study recommends curtain grouting for permeable layers and a robust dam foundation design. These measures aim to ensure the dam structural integrity and provide the intended services.