Application of geophysical and geotechnical techniques for foundation studies in part of Isheri Oshun, Lagos, South-western Nigeria

This study combines geophysical and geotechnical methods to assess the subsurface conditions for a church building located in Isheri Oshun, Southwestern Nigeria. We conducted six Vertical Electrical Soundings (VES) and three 2D Electrical Resistivity Tomography (ERT) profiles, complemented by eight Cone Penetration Tests (CPT) and four Standard Penetration Tests (SPT) to enhance the geophysical findings. The VES and 2D-ERT results indicated a topsoil layer with low resistivity (< 40 Ωm), suggesting moist, clayey, or silty soils at depths ranging from 0 to 2.5 m. Beneath this, we identified a high-resistivity zone (> 300 Ωm) between 2.5 and 7 m, interpreted as robust lateritic formations. The outcomes from the CPT and SPT supported the geophysical data, highlighting a layer of very stiff, silty lateritic sandy clay with a hard pan found between depths of 2.25 to 4 m. A correlation analysis between the 2D ERT data and SPT N-values generally revealed a positive relationship, indicating that higher resistivity values correspond to higher N-values and vice versa. However, some deeper zones exhibited low correlations, suggesting that core materials in their normal state may not always reflect accurate resistivity values—areas with high resistivity could exist under challenging conditions. This highlights the need to consider site-specific factors when interpreting subsurface information. Despite the CPT reaching a maximum depth of only 1.25 m, the allowable bearing pressures ranged from 98.1 to 359.7 kN/m², with no groundwater detected. The integrated data interpretation confirmed that surface layers within the top 2.2 m are unsuitable for the proposed church construction due to their low strength, while the deeper layers (2.25 to 4 m) exhibit favourable geotechnical characteristics. Given that the incompetent soil extends beyond 2 m, we recommend a raft foundation. This type of foundation effectively distributes the building’s load over a wide area, alleviating soil pressure and minimizing potential differential settlement issues.