Engineering Geophysical Investigation of an Earth Dam Site across River Kiyan, Southwestern Nigeria

Geoelectrical and geotechnical surveys were conducted across river Kiyan, in Ibapon, Southwestern Nigeria to characterize the subsurface and assess the suitability of the location for siting an earth dam. Vertical electrical sounding (VES) and 2D resistivity profiling were conducted along the dam axis and three parallel axes upstream. The geotechnical investigation involved in situ Standard Penetration and Permeability tests performed on subsoils in boreholes drilled along the dam axis, and laboratory tests conducted on eight (8) soil samples collected from test pits dug along the parallel axes. The VES data were interpreted by partial curve matching and 1D computer iteration to obtain the layer parameters which were used to construct the geoelectric sections. The 2D resistivity data were interpreted using 2D resistivity inversion procedure to generate the 2D resistivity sections. The geotechnical tests were conducted and interpreted in accordance with the procedures set out in American Society for Test Materials (ASTM D1586) and British Standards (BS.1377). The results of the resistivity surveys along the dam axis revealed sandy topsoil, clay/sandy clay saprolite and weathered/fractured/fresh bedrock. The topsoil is suitable material for the caprock over the dam embankment while saprolite is fairly good-to-good materials for core of the dam embankment. The subsoils are medium-to-very dense, fairly impervious with medium plasticity. The study area is suitable for site an earth dam provided appropriate remedial measures are applied to the weak/fractured zones beneath the dam axis, and the pervious zones upstream.