Geophysical Evaluation of Foundation Soils in a Basement Complex Terrain: A Case Study of LAUTECH Campus, Ogbomoso, Southwestern Nigeria

摘要

Electrical resistivity surveys were carried out at the eastern part of LAUTECH campus, Ogbomoso, southwestern Nigeria in order to investigate the subsoil foundation characteristics and thus provide subsurface information that will guide the design of foundation of engineering structures at the location.Forty-one (41) VES stations were occupied at the study area using the Schlumberger electrode configuration with electrode spacing varied from 1m to 100m. Interpretation of data was done by initial partial curve matching followed by computer-aided iterative technique. The layer parameters were used to generate geoelectric sections while the geographic coordinates and resistivity values were used to produce the isopach map of the overburden and the bedrock relief map.The interpretation of the geoelectrical data revealed that the study area consists of three to four layers defined as the topsoil, clay/sandy clay layer, and weathered/fresh bedrock. The resistivity and thickness of the topsoil range from 51 to 1887.1Ωm and 0.3 to 3.2m respectively. The resistivity of the weathered layer varies between 13.4 and 415Ωm and is 2.0 to 17.2m thick. The layer comprises a mixture of clay and sandy clay. Bedrock varies between 109 and 70380Ωm while depth to the bedrock ranges from 2.1 to 19.6m. The isopach map of the overburden and the bedrock relief map show that the study area is underlain by relatively thin ( 10m) as well as bedrock ridges and depressions. Areas with thin overburden are associated with high bedrock relief (>350m) while those with thick overburden are associated with bedrock depression (<350m).The subsurface of the study area can support deep foundations, except at VES 28 where the overburden thickness is less than 3m. The areas underlain by sandy clay and bedrock ridges are expected to be more competent for safe foundation. The clay layer will be good foundation materials if the clay has activity less than 0.75 indicative of low swell-shrink potential. The results of this study will serve as useful guide in the choice of sites for engineering structures and also reduce the number of geotechnical probes for foundation studies and hence reduce the overall cost of investigation.