Application of Vertical Electric Soundings for Foundation Investigation in a Basement Complex Terrain: A case Study of Ijebu Igbo, Southwestern Nigeria

摘要

Vertical Electrical sounding (VES) has been used to study the subsurface lithology of a site for a proposed building in Ijebu Igbo,Southwestern,Nigeria with a view to recommending the appropriate depth to which the foundation could be sited.The area extent of the site was estimated to be 2500m2.Seventeen (17) VES stations were established along three profiles.The Schlumberger electrode array was employed with current electrode spacing AB/2 of 100m.Data collected was plotted and curved matched.The result of the curved matching was used as an input for the computer iteration software.The subsurface layers resistivity,thickness and depth were calculated quantitatively.The results revealed three to four geoelectric layers.These are inferred as topsoil,sandy layer,clayey horizon and basement.The basement rocks were either fresh or weathered.The resistivity of the top soil ranges from 166.7 to 1027.7 flm with depth range of 0.6-1.2m whereas the resistivity of the fresh basement ranges from 1460.7 to 3255.8.ohm-m while that of the weathered basement ranges from 302.8- 752.8ohm-m.Clayey horizon is predominant in the entire area with varying depth of between 3.8m and 10.2m.The presence of clay in the clayey horizon are at varying depth is of concern,this is not too good as engineering materials and need to be further analyzed so as to determine the moisture contents,percentage passing to determine its clay content.The fresh basement at the central part of the site could withstand stress as there was no evidence of structural geological features such as faults which can aid subsidence.This study has provided preliminary information that could guide further studies and foundation design of the proposed building.