Geophysical and geotechnical evaluation of foundation conditions of a site in Ago-Iwoye area, southwestern Nigeria

摘要

Geophysical and geotechnical studies were carried out to investigate the foundation conditions of a proposed structure in Ago-Iwoye area, southwestern Nigeria. Ago-Iwoye area is underlain mainly by porphyroblastic granite gneiss, biotite granite gneiss, biotite hornblende gneiss, porphygilitic granite pegmatite and aplite. Vertical electrical sounding (VES) revealed three different lithological layers, with apparent resistivities of 261?m, 85.?m and 170.2m respectively. These represent the lateritic horizon, underlain by a layer of wet plastic clay and the third layer is a gravelly sandy horizon, occurring at a depth of about 3.7m. The horizontal profiling on the other hand revealed a subsurface that is laterally inhomogeneous, with the variation in resistivities being very slight along 2nd profile to very high along 3rd and 4th profiles. The cone penetration tests (CPT) also revealed that the lithology of the soil vary from point to point across the study area, with cone resistance of 5-28 Kg/cm~2 obtained for a depth of 0.00- 0.20m, 10-250 Kg/cm~2 for a depth of 0.20m-2.00m and 40-250Kg/cm~2 for a depth of 2.00m to 4.00m. The results obtained from this study have emphasized the usefulness of geophysical methods in complimenting geotechnical studies in establishing variation in lithology accompanied by variation in the allowable bearing pressure of foundation soils.