Integrated Geophysical, Geotechnical And Stream Sediment Investigation For A Dam Project Across River Agbara, Southwestern Nigeria

Proper site selection is crucial for constructing civil engineering projects, such as a dam, that would have a long-term sustainable benefits to mankind. A combined geophysical, geotechnical and sediment analysis were carried out at a site along Agbara River channel in Afin-Akoko southwestern Nigeria, with the aim of determining the conditions, potentials, suitability of the subsurface geology, environmental appraisal and river flow assessment on the site for construction of a sustainable dam. This dam is considered important for addressing the current severe water supply challenge in the area. Geophysical survey, involving Schlumberger vertical electrical sounding (VES), and geotechnical investigation of nine soil samples excavated from different trial pits at the depth of 1.5 - 3 m, were carried out using standard procedures. Five river sediment samples collected from the river were analyzed for environmental appraisal related to concentration of heavy minerals. Topsoil resistivity variation (57 – 1,339 Ωm) and thickness (0.9 – 15.5 m) reflects the degree of soil compaction and anthropogenic activities in the area. Northeastern and southwestern parts of the area characterized by stable zones of competent bedrock with thin overburden are suggested suitable for the proposed dam construction. Results of soil types showed that 77.8 % of the tested soils are good foundation materials (A-2-6 to A-2-7) with maximum dry density above 1,956 kg/m3 and optimum moisture content (OMC) below 14.5 %. The unconfined compressive strength (UCS) of the soil (230.1 – 320.3 kN/m2) indicating the soils’ very stiff consistency is attributed to its resistance to volume change under load, capable of withstanding embankment weight, thus, protecting the dam from erosion and flood. The soils’ resistance to shear failure guarantees firm bond between the projected dam and subsoil foundation to avert washouts and piping. Coefficients of volume compressibility and consolidation of the soils show no tendency of differential settlement. Coefficient of permeability, within 3.96 × 10-6 - 2.40 × 10-5 cm/sec, indicate impervious nature of the subsoil guiding against seepage. Results indicate that River Agbara sediments are mineralogically sub-mature to mature, suggesting mixed acid igneous and high grade metamorphic rock source. The heavy minerals are moderately stable to ultra-stable indicating an oxygen-rich (oxic) depositional environment. Dam foundation design and construction is recommended on properly compacted soil across Agbara River with priority for compacted spillway and rock lining for sustainability. Earth fill dam with consistent maintenance is encouraged for its optimal usage and proposed benefits to the society.