AN INVESTIGATION OF SUBSURFACE STRUCTURES FOR CONSTRUCTION PURPOSE IN MAKURDI, USING ELECTRICAL RESISTIVITY SURVEY
An electrical resistivity survey was carried out on site proposed for the construction of state secretariat office complex in Makurdi, Benue state, using vertical electrical sounding (VES) with Shlumberger electrode configuration. The soundings, which comprised of fifteen VES was done at three different locations to ascertain the suitability of the subsurface formations at the site for construction and to determine a suitable depth to which the foundation of the proposed buildings should be dug in order to avoid structural failure. The technique utilized pairs of current and potential electrodes inserted into the ground. By measuring the voltage between the potential electrodes, the apparent resistivity of the subsurface was determined. About 165 resistivity readings was taken to generate a 1-D profile of the subsurface. The result obtained showed that the study area is extensively underlained by shale, which is expansive soil and can cause failure to buildings and roads in the area. The largest spacing between the current electrodes WAS 80meters, thus the maximum depth of investigation is 27meters. To maintain a measurable potential difference, the potential electrode spacing was varied from an initial value of 0.5meters to 2.5meters. Between the surface and the depth investigated, the following formations were delineated: loam, sand, clay, laterite and sandstone. The lateritic layer to a depth of 1.2m-3.0m was found to be suitable for the foundation of the buildings.