Geotechnical challenges in crystalline basement terrain: mapping subsurface conditions with electrical resistivity tomography in Ado-Ekiti, Nigeria

Subsoil investigation constitutes a critical step in the planning and execution of any construction project. It is a prerequisite for the design and construction of safe, stable, and sustainable structures. Subsoil investigation using Electrical Resistivity Tomography (ERT) was carried out in Ado-Ekiti, Nigeria to assess the integrity of foundation soils/near-subsurface geomaterials. The study area is underlain by the crystalline basement terrain of southwestern Nigeria. The study delineated low resistivity zones having as low as 63 Ωm at a depth of about 10 m, localized pockets of clay intercalated by lateritic soil, a stretch of geomaterials of high resistivity values over 10000 Ωm observed at a depth of 6 m, and a structural feature diagnostic of fractured zone with intense weathering at depths stretching beyond 25 m. The presence of an underground water channel within the fractured basement rock is significant. The geological variations along the traverses confirm the heterogeneity of the basement complex rocks, even over short distances. This is crucial for foundation design. A gross assumption of uniformity could be hazardous to the stability of the structure. Geophysics remains a very fundamental tool that can be applied in civil engineering work. Use of integrated geophysical methods would reduce ambiguities and enhance site characterization for construction purposes.