Geoelectrical Investigation of a Failed Portion of Ajaokuta-Itobe Road, North-Central Nigeria

A Geoelectrical investigation was carried out along a segment of the AjaokutaItobe highway in Kogi State, North-Central Nigeria with the aim of establishing the cause(s) of the failure of that portion of the road. Vertical Electrical Sounding (VES) involving the Schlumberger array and 2-D imaging using the dipoledipole configuration were both utilized along a 200m traverse established parallel to the road covering both the stable and failed portions. Four subsurface geologic layers were inferred from the result of the 2D imagine. A topsoil with resistivity values ranging from 42Ωm to 84.4Ωm, weathered layer with very low resistivity values from 14.7Ωm to 38Ωm, fractured basement (Resistivity varying between 92.1Ωm to 227Ωm) and the fresh basement with resistivity value as high as 2440Ωm. The curve types identified from the interpretation of the Vertical electrical Sounding were HA, AA, HA, and A type, with the number of layers delineated by the geoelectric section being four (4). The topsoil (sandy clay) for the first three (3) VES points have resistivity values of 54Ωm, 27Ωm and 50Ωm with thickness of 0.9,m 0.9m and 0.7m respectively. For the fourth VES point, the topsoil has been replaced by the weathered layer with resistivity value of 42Ωm and thickness of 6.7m. The second layer (clay) has resistivity values ranging from 10Ωm to 45Ωm and its thickness is in the range of 2 to 7m. The third layer delineated with resistivity values of 57Ωm, 182Ωm, 74Ωm and 406Ωm (which is the weathered basement) is very thick at VES point 1(36m) but almost disappears at VES point 3. Its thickness at VES points 2 and 4 are 10m and 4.4m respectively. The fourth layer having resistivity values between 406Ωm and infinity represents the fresh basement rock. The causes of the failure in the studied highway may be attributed to the presence of low resistivity materials (clay) at the topsoil/sub-grade and the probable presence of near-surface linear features such as faults, fractures and lithologcal contacts beneath the failing portion of the highway.