Abstract

The high spate of differential settling and/or sudden collapse of buildings in Lagos has necessitated a holistic approach to foundation investigation prior to the erection of such structures as a panacea to environmental and engineering tragedy of our time, which we try to address in this work. In this paper, a 2D electrical resistivity tomography (ERT) and geotechnical investigation were carried out at Ogudu Estate in Lagos, Nigeria. The field geometry was made up of eight traverses; each measuring 315 m. 64 electrodes were deployed with inter-electrode spacing of 5 m, to cover the study area. Three cone penetrometer test (CPT) and a standard penetration test (SPT) were conducted to identify the depth to competent layer as a constraint for ERT survey. The CPT and SPT tests shows that the subsurface around the area is composed of materials of very low shear strength (< 5 kg/cm2) interpreted as peat/clay at near surface to a depth of 8.2 m (being the maximum depth probed by the CPT test) and 25 m for the SPT. This agrees with the ERT result as the peat/clay was delineated to a depth of 25 m under the second half of most of the traverses which have high proximity to the SPT test hole. However, the inimical clay/peat layer was mapped to a deeper depth of 50 m under the first half of some of the traverses, alluding to heterogeneity of the subsurface layer in the study area. Generally, the overall depth to competent layer that could support a sizeable engineering structure is confined to the second half of the surveyed area north east (NE) portion as indicated in almost all the profiles at deeper depth mapped by ERT. Thus, the study shows that foundation investigation need be complemented with geophysical survey. By this way, where the engineering soil tests terminates geophysical survey could continue and could be a veritable tool to decipher deeper subsurface structures inimical to engineering construction.

Key words: Foundation investigation, engineering structures, tomography, competent layer.