Electrical resistivity determination of subsurface layers, subsoil competence and soil corrosivity at and engineering site location in Akungba-Akoko, southwestern Nigeria
A geophysical investigation involving the electrical resistivity method was carried out at a site located in the eastern part of Akungba-Akoko, southwestern Nigeria. The aim of the investigation was to characterize the site according to subsurface lithologic layering, subsoil competence and soil corrosivity, which may affect the location and maintenance of engineering constructions that may be built on the site.
A total of 1063 vertical electrical sounding (VES) stations were established within the site along 58 traverse, but only 949 out of the established stations were eventually occupied for sounding as some of the stations fell on bare rocks or in inaccessible areas. Raw apparent resistivity values for three theoretical depth levels (0.5 m, 3 m and 8 m) were plotted for all the VES stations, contoured and then interpreted qualitatively in terms of soil competence. The methods of partial curve matching and 1-D computer iteration were also used to quantitatively interpret the VES curves that were generated. The quantitative interpretation results (layer resistivities and thicknesses) obtained for the topsoil were used to determine the subsurface lithologic layering and to characterize the soil corrosivity.
Three main subsurface layers-the topsoil (0-3.9 m thick and 30-5500 ohm-m in resistivity), the weathered layer (0-44.6 m thick and 9-781 ohm-m in resistivity) and the partially weathered/fractured/fresh bedrock (270 to ∞ ohm-m in resistivity)-make up the subsurface. The distribution of apparent resistivity values at different depth levels and the topsoil resistivity values suggested that competent soils (≥ 100 ohm-m) with slightly to practically non-corrosive (≥ 60 0hm-m) tendencies underlie the site. The study concluded that the characteristics of the earth materials in the site would be favourable to normal engineering structures/materials that may be located on it.
Keywords: resistivity, subsurface layering, competence, corrosivity, engineering site
Ife Journal of Science Vol. 8 (2) 2006: pp. 159-177