Integrated Geological, Geophysical, and Geotechnical Site Characterization of Ajima Dam Site, Amhara Region, Ethiopia: Implications for Seepage and Foundation Stability

Abstract The Ajima Dam, a 48m-high earth fill structure in the Amhara Regional State, Ethiopia, was investigated using an integrated approach combining geological mapping, borehole drilling (19 boreholes, 633m total), geophysical surveys (VES, seismic refraction), and geotechnical testing. This study provides the first comprehensive characterization of the site, identifying a critical load stress mismatch and complex geological setting. The allowable bearing capacity of the upper 5m (< 1.1 MPa) is insufficient for the applied dam load (1.154 MPa), indicating a factor of safety < 1.0 that requires excavation. Seepage analysis reveals high permeability in the valley floor (Lugeon values up to 75 LU, equivalent to 7.5×10⁻⁶ m/s) and left abutment (66% of tests > 1 LU), while the right abutment shows low permeability (76% of tests < 1 LU). Rock mass quality improves with depth, RQD ranges from 25–50% (poor) in weathered zones to 90–100% (excellent) in fresh ignimbrite below 35m, UCS values vary from 1.6 MPa (tuff) to 99.2 MPa (basalt). Seismic hazard assessment places the site in Zone IV (PGA 0.20g), with site amplification potential due to velocity contrasts (Vs < 1000 m/s in overburden vs. >3500 m/s in bedrock). Critical geological hazards include faults crossing the dam axis (N86°W), caves in laterite soils, and pervious zones (k = 10⁻⁵ m/s) requiring grouting. Excavation of the upper 5-6m of valley floor materials, 30m-deep curtain grouting with 2–3 rows at 1.5-2.0m spacing, upstream impervious blanket extending 30m from the dam toe, and seismic resistant design modifications are some of the recommendation part to minimize the dam risk.