Geoelectrical investigation of foundation failure in Akowonjo, Ogbomoso, Nigeria

Abstract

An electrical resistivity investigation was carried out in Akowonjo, Ogbomoso, Nigeria with the aim to obtain various lithological units and subsurface geological features within the study area. Fifty-four sounding points adopting Schlumberger array, with 100 m as the maximum value for half-current electrode spacing were occupied in the study area. Partial curve matching technique and automated platform (known as WinResist) were used to interpret the obtained data qualitatively and quantitatively. The resulting final layers parameters involving resistivity and thickness of each layer were used to map the two-dimensional geoelectric variations beneath the study area. The curve types obtained from iterations of WinResist, which is based on 1-D forward modelling approach include H, KH, HA and QH. Four subsurface layers were identified. These layers are topsoil, laterite, weathered/partly weathered layer and fractured/fresh bedrock. The topsoil had resistivity and thickness values ranging from 76 – 1858 Ωm and 0.4 – 4 m respectively. The lateritic layer lies under the topsoil with resistivity and depth values ranging from 649 – 2021 Ωm and 0.7 – 1.9 m, followed by a weathered/partly weathered layer with resistivity and depth values ranging from 17 – 880 Ωm and 1.9 – 25.2 m, respectively. The fourth layer is the fractured/fresh bedrock which depict varying resistivity from 260 - 33385 Ωm. It is revealed that thick clayey materials dominate the first and second subsurface layers within the study area. This is known to be the major cause of cracks experienced in the buildings in Akowonjo community, Ogbomoso, Nigeria. It is recommended that further foundations for civil engineering works in the study should be extended beyond the zone of water content fluctuation.