SUBSURFACE STRUCTURES AND THE EFFECT ON FLEXIBLE PAVEMENT PERFORMANCE: THE SHAGAMU-BENIN EXPRESSWAY, SOUTHWESTERN NIGERIA, AS CASE STUDY.

Abstract

Geoelectric and electromagnetic methods were conducted on failed highway pavement segments at Ilisan  and Ore  localities along Shagamu-Benin expressway. The objective was to determine the cause(s) of perpetual pavement failure along segment of the highway which have always defied rehabilitation.

Twenty schlumberger Vertical Electrical Soundings (VES) were carried out with the electrode spacing (AB/2) varying from 1 m to 75 m with the total spread of 150 m. The VES data were presented as sounding curves and interpreted by partial curve matching technique and computer assisted 1-D forward modeling.  The interpretation results of the VES were presented as geoelectric sections. 2-D resistivity imaging using Dipole-dipole array with electrode spacing (a) = 5 m and expansion factor n = 1 – 5 was conducted along the road segments and along the same profile with the VES. The 2-D resistivity imaging data were inverted using the Dipro software. VLF-EM inline profiling with station interval of 5 m was carried out to confirm the geologic features depicted by the VES and 2-D resistivity imaging. The VLF-EM results were presented as profiles.

The geoelectric sections identified the lateritic sand of resistivity values of 1230 ohm-m and an average thickness of about 2.2 m as the subgrade materials at Ilisan locality and partly weathered basement with resistivity values ranging from 283 ohm-m to 7848 ohm-m as the subgrade materials at Ore locality.  The subgrade materials at both localities are adjudged competent to sustain the stability of the highway pavement. However, the 2-D resistivity imaging and VLF-EM identified low resistivity zones (<100 ohm-m), which diagnostic the presence of buried stream channel and shear zone suspected to be a fault beneath the subgrade materials at Ilisian and Ore localities respectively. These features may have been responsible for the flexible pavement failure always experienced at the investigated highway segments.

Keywords: Subsurface Structure, Subgrade Materials, VES, Resistivity and VLF-EM.