Use of Engineering Geophysics to Investigate Gbongon Flyover Bridge Foundation, Gbongon, Southwest Nigeria

Engineering geophysical investigation was conducted to understudy the structural and lithological characteristics of subsurface materials with the aim of determining the appropriate location, competent layer and depth of core holes required for geotechnical analysis at a proposed site for a flyover bridge at Gbongon using electrical resistivity method (Sounding and Profiling). The different subsurface lithologies were delineated and depth to the top of the different layers was determined. Approximate thickness of the different subsurface layers and depth to the top of the basement or competent layers were established. The lateral or horizontal continuity of the subsurface layers was also delineated. Three different subsurface lithologic units were established from the sounding curves namely; lateritic topsoil, clayey-sand and, fresh basement. The topsoil is relatively thin along the traverse with average resistivity and thickness values of 45Ωm and 4.0m respectively, and is predominantly lateritic clay. Clayey-sand was encountered at shallow depths of 4.0meters on the average in all locations and the average resistivity and thickness values for the Clayey-sand are, 56Ωm, and 5.0m respectively. Basement is the fresh bedrock and is the last layer. It is relatively shallow in the study area and the average resistivity and depth values to the top of basement are, 1302Ωm, and 9.0m respectively. The resistivity values are high because of its crystalline nature. The overburden is assumed to include all materials above the presumably fresh basement which is regarded as the competent layer in this case. The depth to the bedrock varies from 7.0 to 10.0m and the average depth to the bedrock is 9.0m.