Geological and Geotechnical Assessment of the Bargun Dam Site of Somali Region, Ethiopia

Abstract

The lack of detailed geological and geotechnical investigations at the proposed Bargun Dam site, which is located in Ethiopia’s Shebelle River Basin, poses significant risks to design integrity, safety, and long-term performance. This study presents a comprehensive geological and geotechnical evaluation aimed at ensuring a safe and sustainable dam design. Field investigations, including geological mapping, test pitting, and soil and rock sampling, were followed by laboratory analyses to determine the physical and mechanical properties of foundation materials. Rock mass quality was assessed using the Rock Mass Rating (RMR), Rock Quality Index (Q), Rock Mass Index (RMi), and Hoek–Brown criteria. Slope stability was analyzed kinematically, while bearing capacity (qu) and permeability (Ks) were estimated using empirical correlations applicable to data-scarce environments. The results indicated RMR, Q, RMi, qu, and Ks values of 46–59, 1.06, 0.48, 0.55–23.22 MPa, and 2.14×10⁻²–3.7×10⁻⁵ cm/s, respectively. The average rock mass deformation modulus ranged from 5.03 to 9.64 GPa, suggesting a potential risk of differential settlement. Based on the kinematic analysis the left abutment slope is susceptible to oblique toppling, requiring slope modification. However, the site’s low seismicity and manageable sediment conditions are favorable for dam construction. Overall, detailed subsurface exploration to identify concealed cavities and targeted grouting are recommended to mitigate leakage and stability risks, ensuring the dam’s long-term safety and functionality.