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/m 2 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.

Vertical electrical sounding, well inventory and physicochemical analysis were conducted to evaluate soil corrosivity and aquifer protective capacity of overburden units in the basement complex terrain of Ado-Ekiti, southwestern Nigeria. The topsoil is composed of slightly corrosive materials at the eastern, southern and northeastern flanks and the central portion with resistivity values ranging from 60 to 180 Ωm. Moderately corrosive/slightly corrosive materials (with resistivity values of 10 < ρ < 60 Ωm) constitute the second layer around the eastern, southern and north-eastern flanks. Pockets of areas in the northwestern, southeastern, eastern and central parts of the metropolis are practically non-corrosive with resistivity values in excess of 200 Ωm. Zones of good, moderate, weak and poor overburden protective capacity were delineated, with longitudinal conductance (S) values of 0.7 < S < 4.9, 0.2 < S < 0.69, 0.1 < S < 0.19 and S < 0.1 mhos, respectively. On a regional consideration, 23.31%, 18.80% and 57.9% of the study area is characterised by overburden materials of poor, weak and moderate protective capacity, respectively. Only 6.02% of the area indicates good overburden protective capacity.

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