Geophysical Investigation of Foundation Condition of a Site in Agbura Town Bayelsa State Using Electrical Resistivity Method

Abstract

This study presents a geophysical investigation of the foundation conditions at a proposed construction site in Agbura Town, Bayelsa State, Nigeria, using the electrical resistivity method. The research is motivated by the increasing incidence of building failures in Nigeria, often attributed to inadequate understanding of subsurface conditions and neglect of pre-construction site investigations. The study area lies within the Niger Delta, characterized by soft, water-saturated, and compressible alluvial deposits that pose significant challenges to foundation stability. Vertical Electrical Sounding (VES) employing the Schlumberger array configuration was used to acquire subsurface resistivity data at selected locations. For interpretation, the IPI2Win software was employed. The data were processed and interpreted to delineate subsurface lithology, identify stratification, and determine the depth to competent layers suitable for foundation placement. The results enabled the identification of geoelectric layers, including topsoil, clayey formations, and more competent sandy strata at depth. Findings reveal the presence of weak, low-resistivity clayey and water-saturated zones in the near surface, which are unsuitable for supporting heavy structures due to their high compressibility and low bearing capacity. Conversely, relatively higher resistivity zones corresponding to sandy materials were identified at greater depths, indicating more competent layers for foundation support. The interpreted VES results revealed resistivity values ranging from 1.61 to 22,430 Ωm, indicating a subsurface sequence composed of conductive clayey materials, sandy/lateritic topsoil, weathered or fractured basement, and fresh basement rock. The low-resistivity zones, ranging from 1.61 to 15.3 Ωm, were interpreted as clayey or saturated clayey layers, while the higher-resistivity sandy/lateritic materials ranged from 99.4 to 575 Ωm. Fresh basement was identified by very high resistivity values between 6,133 and 22,430 Ωm. The study demonstrates that electrical resistivity methods provide a reliable, cost-effective, and non-invasive approach for evaluating subsurface conditions. It emphasizes the importance of integrating geophysical investigations into engineering site assessments to mitigate risks of structural failure. The results offer valuable insights for safe and sustainable foundation design in flood-prone and geologically complex environments like Agbura. It is recommended that geophysical investigations be integrated into pre-construction planning to minimize structural failure risks in the Niger Delta region.