Geoelectric assessment as an aid to geotechnical investigation at a proposed residential development site in Ilubirin, Lagos, Southwestern Nigeria

Abstract

The study focuses on the application of electrical resistivity methods as a guide for a detailed geotechnical investigation due to the inhomogeneity of the soil materials cum inability of the drilling beyond 38 m from the three initially drilled geotechnical boreholes which did not allow for proper foundation decisions and thus necessitated the research. Two-dimensional (2D) electrical resistivity data were acquired using the Wenner array along ten (10) traverses of about 500 m long, and forty-four (44) vertical electrical sounding (VES) data were acquired along the various traverses using the Schlumberger array with a maximum spread of 620 m. The VES data were interpreted using partial curve matching technique and one-dimensional (1D) computer iteration using WinRESIST software. The 2D dataset were processed using DIPROWin software. The inversion of the 2D resistivity data was constraint by the VES results and available borehole data. The results of the interpretation of the electrical resistivity data reveal that the lithological units underlying the study area compose of clay/peat, clayey sand, sandy clay, and sand. The results of the interpretation of the electrical resistivity data guided by the range of resistivity (ρ) values and knowledge of the geology of the area reveal that the lithological units underlying the study area are composed of peat (ρ < 10 Ωm), clay (10 > ρ < 100 Ωm), clayey sand (100 > ρ < 200 Ωm), sandy clay (200 > ρ < 300 Ωm), and sand (ρ > 300 Ωm). The various layers are intercalated with each other, and thickness values vary from one location to another up to a maximum depth of 70 m. The VES results show that the clayey material underlies the sand-filled topsoil and there are indications of competent sand layers at depth beneath these clayey layers in the study area. The 2D pseudosections reflect that the different lithological units are intercalated with varying thicknesses across the study area. Thus, the study reveals that the subsoil within the study area is quite inhomogeneous and great care and expertise is required for developing the site. Anonymously low-resistivity areas delineated along each of the traverses are locations recommended for targeted geotechnical investigation prior to construction.