Geophysical Investigation of Damsite in a Sedimentary Terrain: A Case Study

摘要

Geophysical investigation was carried out at a site in Okada town near Benin City, southwsetern Nigeria, in order to evaluate the suitability of the area for a damsite. The survey was conducted along the proposed dam axis (about 850 m long) and its vicinity by using Vertical Electrical Sounding to unravel the subsurface profile, which in turn determines if there would be any subsurface lithological variation (s) that might lead to structural failure in the dam foundation. To achieve the above objectives, six sounding stations were located and fully occupied along the dam axis while 2 VES were conducted close to 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 four to 5 lithologic units defined by the topsoil, lateritic hard pan, dry sandy formation and various combinations of sandy clay, clayey sand and sand as constituting the subsurface of the proposed damsite. Resistivity values range from 1422-6895, 1201-10525, 1820-3606 and 105-9270 m in the topsoil, lateritic hard pan, dry sand formation and the clayey sand/sandy clay/sand layer(s), respectively. Layer thicknesses vary from 0.5-2.0 m in the topsoil, 1.4-8.2 and 6.5-31.2 m in the lateritic hard pan and dry sandy formation, respectively. Based on these results, it is concluded that the subsurface material in the study area is highly competent to withstand the load of the proposed dam. However, resistivity values <200 m at depths below about 31.7 m indicate high porosity, high clayey sand content and high degree of saturation, which are indicators of soil conditions requiring serious consideration in the design of the dam.