Geophysical and Geotechnical Investigations of the Subsurface for Construction Purposes at Federal College of Education Osiele, Abeokuta, Southwestern Nigeria

Knowledge of the surface and subsurface structures are vital for construction procedures. Integrated geophysical and geotechnical methods were applied to image the subsurface for evaluation of the stratigraphy and the competency of each geoelectric layer for construction purposes at the Federal College of Education Osiele, Abeokuta South-western Nigeria. Nine Vertical Electrical Sounding (VES) using Schlumberger electrode configurations were conducted over a current electrode spacing of 200 m. Multi-channel Analysis Surface Wave (MASW) was the seismic technique used for the geotechnical analysis of the study area and laboratory analysis was performed to investigate, Atterberg limit, particle size distribution, compaction test, specific gravity, and California bearing ratio. Results from the geophysical investigation revealed four to five geoelectric layers: topsoil, clayey sand, sandy clay, laterite, and fresh basement. The MASW results have s-wave velocities range of 40–500 m/s and analysis showed four layers.  The laboratory analysis revealed that all the ten traverses have specific gravity, which is out of limit except sample 8 which is 2.80, plastic index and Atterberg limits of liquid were within permissible values of 12% and 35% respectively except samples 9 and 10, and California Bearing Ratio within specified limits. Geophysical and geotechnical investigation of the subsurface carried out in the study area for construction purposes revealed that the foundation of a heavy structure should be targeted at around 20 m into the subsurface.