Integrated Geotechnical and Geoelectrical Characterization for Foundation and Earthing Design of Power Infrastructure in the Niger Delta: A Case Study of Ebubu, Rivers State, Nigeria

Abstract

Reliable development of power infrastructure in the Niger Delta requires simultaneous understanding of the mechanical behaviour and electrical properties of the subsurface. This study presents an integrated geotechnical and geoelectrical characterization of the Ebubu power-infrastructure site, Eleme Local Government Area, Rivers State, Nigeria, undertaken to establish foundation competence and provide subsurface electrical-resistivity information relevant to grounding and lightning-protection design. The investigation combined twelve boreholes drilled to 15 m, twelve cone penetration tests (CPT), standard penetration testing (SPT), laboratory classification, undrained triaxial and one-dimensional consolidation testing, and twelve Schlumberger vertical electrical soundings (VES) with electrode spreads extending to 90 m. The geotechnical investigation identified a laterally consistent sequence comprising soft-to-firm silty clay to approximately 6.5 m, firm sandy clay between approximately 6.5 and 9.5 m, and loose-to-medium dense medium-to-coarse sand below. The near-surface clay exhibited moderate plasticity and compressibility, with an average bulk unit weight of 18.6 kN/m³ and design undrained cohesion of approximately 60 kPa. Groundwater was encountered at approximately 9.4 m depth. For shallow foundations founded at about 1.5 m, an allowable bearing pressure of approximately 100 kPa was considered appropriate, while the predicted settlement under a net loading of 100 kPa was approximately 50 mm, with about 97% expected within 1.5 years. Apparent resistivity measurements ranged from 282 to 2,468 Ωm across the investigated electrode-spacing range, revealing substantial electrical heterogeneity and increasing apparent resistivity responses with depth of investigation at most sounding locations. Integration of the geotechnical and geoelectrical datasets demonstrates that the mechanical foundation environment and electrical grounding environment are controlled by different but interacting subsurface attributes, particularly lithology, moisture condition, clay content, density and groundwater. The study demonstrates the value of integrated ground characterization for power infrastructure in deltaic environments and provides a transferable framework for combining foundation engineering and earthing considerations within a single site-investigation programme.