Geoelectrical Assessment of a Proposed Dam Site around Ehuhe area of Oji River, Southeastern Nigeria.

Abstract

The current study involves applying the Vertical Electrical Sounding (VES) by using the symmetrical Schlumberger configuration to evaluate the suitability of the soil underlying Ehuhe area of Oji River Southeastern Nigeria for dam construction. The survey was conducted along the proposed dam axis (about 477 m long) and its vicinity with the view to determining the presence or otherwise of deleterious matter along the dam axis down to a depth of 35m below the surface, evaluate the geo-structural setting of the concealed bedrock, the fracture pattern and possible dam seepage along the dam axis and its banks as well as give suggestions on any other measures/studies that may be needed for a proper engineering design. Twelve (VES) sounding stations at between 40m (VES 1 – VES 10) and 60m (VES 10 – VES 12 and VES 2 – VES 11) were located and fully occupied along the dam axis. The data obtained were subjected to 1-D inversion algorithm to determine the layer parameters. The results show that the subsurface is remarkably inhomogeneous in geological composition. The geoelectric section revealed three (3) lithologic units defined by top loose dry sandstone, hard but jointed iron oxide-cemented sandstone and porous sandstone, with shales being totally absent. Resistivity values range from 9200-66100, 1440-161000 and 555-21300 ohm-m in the topsoil, ironstone and porous sandstone respectively. Depth to each layer varies from 0.5-12.0 m in the topsoil, 3.6-22.1m in the ironstone rich zone and greater than 40m in the porous sandstone region. No structures that will be of deleterious effect were observed in all the VES points down to about 40m. From the results and field observations it is concluded that the proposed dam axis is underlain by high sandy/ arenaceous matter usually brought in from both Ehuhe and Udi areas with high porosity and potentially rife for great infiltration. It is expected that high bearing capacity shall characterize the rock units. However the absence of shales means high infiltration in a sandy environment. Efforts should be made to provide some suitable blanket materials for the dam and mechanism for desilting when the dam is put in place.

Keywords: Geoelectric Section, Geometric factor, Lithology, Oji River, Seepage.