Remote Sensing and Integrated Geophysical Investigations of Unstable Sections of Ibadan-Iwo-Osogbo Highway, Southwestern Nigeria

Abstract

High records of road accidents due to bad roads and failure of other engineering structures globally have necessitated this study. Geophysical investigation was carried out on six failed sections and two stable ones along the Ibadan-Iwo-Osogbo highway to examine the geological factors responsible for the highway failure in the area. A Landsat ETM+ (Enhanced Thematic Mapper Plus) imagery of the study area and its environs was acquired and processed for lineament analyses. Magnetic and Very Low Frequency Electromagnetic (VLF-EM) measurements were taken at a 5 m interval along each traverse established parallel to road pavements. The electrical resistivity method involving Schlumberger Vertical Electrical Sounding (VES) and 2-D imaging using the dipole–dipole array were utilized. Lineaments were identified across failed localities. Lateral magnetic variations in the near-surface, geologic materials characterized the study area. The generated 2-D VLF-EM models showed conductive zones corresponding to the fractured zones or conductive clay materials within the basement rocks. Subgrade soils below the highway pavement along the failed sections are typical of incompetent clayey and sandy clay/clayey sand formation with resistivity values between 20 and 475 Ωm, while subgrade soils beneath the stable sections have moderate to high resistivity values of 196–616 Ωm. The 2-D subsurface resistivity structures across the failed sections identified low resistivity, water absorbing clay, and lithological contacts. Water absorbing clay-enriched subgrade soils, identified in near-surface linear features, are major geologic factors, along with a poor drainage network, causing the highway failure. Remote sensing and geophysical investigations of highways should be carried out before construction to effectively complement routine geotechnical studies.