Geophysical Site Investigation Practice for Foundation Design in Abuja

This study examines the practice of geophysical site investigation as applied to building foundation design in Abuja, Federal Capital Territory, North Central Nigeria. Abuja's rapid infrastructural development on geologically variable terrain including basement complex, alluvial deposits, and lateritic soils creates significant challenges for foundation engineering. Geophysical investigation methods including seismic refraction, MASW, and ERT offer cost-effective supplements to conventional geotechnical boring that can detect subsurface variability at spatial resolutions unachievable by boring alone. This study reviews 22 geophysical site investigation reports produced for various Abuja development projects from 2019 to 2023, obtained from engineering firms and developers. Report coverage, methodology selection, data quality, and utilisation in foundation design are assessed. Site visits to 10 completed projects compare actual foundation performance with predicted subsurface conditions. Key informant interviews with structural engineers using geophysical reports supplement the documentary analysis. Findings reveal that MASW is the most widely deployed method in Abuja site investigations, used in 16 of 22 reports, providing shear wave velocity profiles correlated with N-values from SPT borings. However, only 58 percent of reports include seismic refraction to complement MASW, creating gaps in bedrock depth estimation for deep foundation design. Foundation performance assessment shows good correlation between MASW Vs30 values and observed settlement behaviour at 8 of 10 visited sites. The study concludes that combined MASW-seismic refraction investigation is the most appropriate geophysical package for Abuja foundation design and recommends minimum geophysical investigation requirements in the Abuja building permit process.

Keywords: geophysical site investigation, foundation design, Abuja, MASW, seismic refraction.