Engineering Geophysical Correlation of Foundation Soils in a Basement Complex Terrain; A Case Study of O.A.U Campus, South Western Nigeria

Abstract

An extension to the existing Geology Department Building, Obafemi Awolowo University Ile-Ife, Nigeria is under construction. The construction site which covers an area of about 3000 m² is located west of the existing Building within the University Campus. A case study is presented here, in order to attempt a correlation between the near surface geology of the site for the extension and the existing Geology Department building. By this correlation, appropriate suggestion on the integrity of the structure being built on the new site can be predicted and recommendations made accordingly. The study was accomplished by the combined use of the vertical electrical resistivity sounding (VES) and the seismic refraction techniques. The subsurface layer delineated are the topsoil, clayey sand/laterite, weathered layer and the weathered/fractured/fresh basement with resistivity and thickness ranging from 103 - 155 Ohm-m and 0 - 1.3 m , 209 - 305 Ohm-m and 1.0 - 3.1 m, 96 - 208 Ohm-m and 3.3 - 14.4 m and 350 - ∞ Ohm-m respectively with depth to rock head between 4.0 - 15.0 m. The resistivity values of the layers, as well as velocities of seismic wave, showed great similarity and one could say that there is little risk attached to the site under construction. The only likely problem is that of water seepage into the foundation floor, as indicated by basement depression from VES and fractured layer from the seismic refraction method, which could be mitigated by designing a channel way for water to flow out or sealing off the fracture and anticipated water flow path with grouted cement. The latter was eventually incorporated into the foundation design of the new building. The near surface geology of the study area is found to be favourable to the structure being put on it and there is practically no risk attached to the integrity of the building.