Determination of Sub-Soil Surface Layers Suitable for Site Foundation in Bowen University Iwo, Osun State

Geo-electrical methods offer a fast, cheap and cost effective method of evaluating competency of soils for building foundations. Electrical investigation of sub-surface soils in Bowen University, Iwo, Osun State, Nigeria, was undertaken in order to acquires a VES data of the subsurface soil, construct geological section of the subsurface, delineate the possible geological structural beneath the study area and to categorize the area into different subsoil competence zones which will assist to prevent the structural failure and collapse of building in the future. Seven vertical electrical sounding stations were surveyed for the site investigation. Schlumberger array was used for the VES with maximum current electrode separation (XY/2) of 75m and readings of the resistance observed from the Terrameter were converted to apparent resistivity. The data obtained was presented as geoelectric sounding curve and geoelectric section shows the subsurface layer resistivity and thickness while geoelectric sounding curve were obtained by plotting apparent resistivity value against electrode spacing on a tracing paper superimpose on log graph sheet. All were iterated on the computer with a software program called WinResist version 1.0 The results revealed three to four geo-electric sections within the study area which comprises topsoil, sandy clays/possibly gravel, lateritic layers and fresh basement which are between the depth of 1.1m to 20.9m below the earth surface. VES 1, 2, 4 and 6 of the study areas shows the lowest resistivity values that suggest high clay content or possible fracture which have impact on the competence and reliability of the soil for construction and building development. Therefore, the topsoil has to be excavated beyond the depth of 4.8m for the choice of shallow foundation that will be suitable for any engineering work and building development in the study area,