CHARACTERIZATION OF SITE FOR WATER PLANT BUILDING FOUNDATION USING RESISTIVITY METHODS ; A CASE OF EAST WALLAGA ZONE, GUTO GIDA WOREDA, HARO SORGA AREA

ABSTRACT This study presents a comprehensive geophysical evaluation of a proposed water treatment plant site in Ethiopia’s East Wallaga Zone, integrating Vertical Electrical Sounding (VES) and Electrical Resistivity Tomography (ERT) techniques to assess subsurface conditions across the 10,800 m² area. The investigation, comprising 12 VES stations and 4 ERT profiles, revealed resistivity variations from 1.26 Ω•m (clay-rich zones) to over 1000 Ω•m (intact bedrock), delineating a four-layer subsurface structure. The uppermost layer consists of ~5 m thick dry top soil (1.26–35 Ω•m), followed by ~19.4 m of weathered basalt (35–250 Ω•m), underlain by ~119 m of partially fractured basalt (250–800 Ω•m), and extending into a ~97 m thick competent basalt bedrock (>1000 Ω•m). The combined geophysical approach, validated with borehole data, effectively identified optimal foundation zones in the high-resistivity bedrock while flagging problematic clay and weathered zones requiring soil stabilization or deep foundations. This integrated methodology not only provides site-specific engineering solutions but also establishes a cost-effective, replicable framework for infrastructure development in similar volcanic terrains, demonstrating the value of multidisciplinary geophysics for sustainable construction in geologically complex regions. The study’s technical rigor and practical applicability make it a valuable reference for future projects in developing countries facing comparable geotechnical challenges.