VERY LOW FREQUENCY ELECTROMAGNETIC GEOPHYSICAL SURVEY ALONG FAILED SECTIONS OF SANGO-OTA-IDIROKO HIGHWAY AND SELECTED FEEDER ROADS IN OTA, SOUTHWEST NIGERIA

Abstract

Very low frequency electromagnetic (VLF-EM) survey was conducted along failed segments of some roads in Ota, southwest Nigeria to reveal the geologic dispositions of subsurface material underlying the road pavements. Geonics EM-16 VLF was employed for data acquisition across a total distance of about 2.6 km. Fourteen traverses were positioned along failed road segments with one control traverse arranged along stable road segments. Real and quadrature field data components were processed via Fraser and Karous-Hjelt filtering to suppress noise and enhance signal strength. Positive peaks of filtered real component on Fraser plots depict the presence of conductive subsurface materials. Good correlation exists between positive peaks on Fraser plots and conductive zones on 2-D current density at several lateral distances and depths across all the traverses. Delineated VLF anomalies represent weak zones that might pose serious threat to road stability. These conductive anomalies are presumably composed of clayey bodies (laterite/lateritic soil) in the subgrade soil used for road construction or unexcavated host materials in the subsurface. Changing volume of clayey materials under varying climatic conditions is a major cause of distress to engineering structures. Clayey materials by virtue of their pore characteristics (high porosity and low permeability) tends to undergo alternating series of expansion and contraction under different climatic conditions. These seasonal dispositions would eventually manifest as cracks and potholes on road pavements. Uneven distribution of different adjacent subsurface lithologies contributes to differential settlement, which is a possible catalyst of road failure.