GEOTECHNICAL AND GEOPHYSICAL EVALUATION OF SUBSOILS IN IKATE AREA, SOUTHWESTERN NIGERIA: IMPLICATIONS ON FOUNDATION INTEGRITY AND CORROSIVITY

Abstract

Geophysical and geotechnical methods remain one of the most effective approaches for the evaluation of subsoil properties prior to engineering construction. This is important in the delineation of soil sequence and estimation of soil parameters necessary for ensuring sufficient supports for structures. This study therefore presents the results of subsoil integrity and corrosivity probability evaluation using Vertical Electrical Sounding (VES) and Standard Penetration (SPT) in Ikate Area, Lagos, Nigeria. Data were acquired in eight (8) VES positions using Pasi terameter deploying the Schlumberger array system along four traverses while SPT-N values were obtained at every 1.5m within six (6) boreholes drilled to 30.0m depth in the study area. The combined results of VES and SPT delineate four to five geologic units corresponding to topsoil, different shades of clays, peat, sandy/clayey sand and sand. The low resistivity values of the clayey, peaty, and sandy layers especially along traverse four was used to rate them as highly or extremely susceptible to corrosion. The presence of highly compressible organic clay and peat in the second layer precludes the adoption of shallow foundation in the medium dense sandy layer that made up the topsoil (0.0 – 2.50m). Foundation analysis carried out with N-values shows that pile installed within the clayed sand/sand clay and sand layers encountered between 12.0 – 27.m with diameter range of 300 – 600mm could mobilize ultimate and allowable loads ranges of [702.9 – 5012.4KN] and [234.3 – 1670.8KN]. The correlations of resistivity with N-values returned high to weak positive linear relationships suggesting that resistivity values may not be effective in estimation of the strength and stiffness of subsoil. Conclusively, the study demonstrated the complimentary role of both VES and SPT as an effective geoengineering characterization tool.