Localized Features in Subsoil Investigation and Implications for Foundation Works: Case Study From Ilora, Southwestern Nigeria

Subsoil investigations involving geoelectric depth sounding, boring and sampling with in situ and laboratory tests were carried out at an interchange location in a typical basement complex terrain of Ilora, Southwestern Nigeria with the objective of detailing the subsurface characteristics and appropriate foundation. The geotechnical investigations utilized 2 Numbers exploratory boreholes while geoelectric depth soundings were carried out at seven (7) points along the traverse using the Schlumberger array. An allowable bearing capacity of 800 kN/m2 was observed for the bedrock revealed at 4.00 and 6.95 m, respectively. The geoelectric depth sounding delineated the bedrock (of resistivity 3000 Ωm) occurring at varying depths of 4.00 to 12.00 m with structural features diagnostic of faulting (resistivity of 1082 / 1282 Ωm) and clayey saprolite of significant  thickness (resistivity of 58 / 66 Ωm; thickness of 12 m) localized within about 45 m span. Integration of the data sets indicated that the drilling programme could not decipher the localized features which could be hazardous to the stability of the structure despite the good correlation observed at the points of investigation. This study underlines the need to complement traditional geotechnical testing with geophysical exploration methods for optimal results in subsoil investigation.   Keywords: bedrock, foundation failures, geotechnical investigation, saprolite, standard penetration tes