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.
Highlights
The economy of any nation depends on the quality of her mode of transportation which involves movement of people and goods from one location to another
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
Summary
The economy of any nation depends on the quality of her mode of transportation which involves movement of people and goods from one location to another. In countries where the development of these transportation infrastructures has followed a rational, coordinated and harmonized path, economic growth normally received a big boost (Beesley, 1973). The quality and extent of road network determine the mobility of people, goods and services within the country. The problem of failed road pavement has made it difficult, expensive and more arduous to move products and services from producer to consumers. This often leads to loss of human life, man-hours and high cost of goods and services. Road pavement failure can be attributed to engineering properties of subgrade materials, geology of the road route , hydrology/hydrogeology and geomorphology of the area traversed and usage factor (Ajayi, 1987;. Meshida,1981; Madedor, 1993; Arumala and Akpokoje, 1987 and Teme et al, 1987)
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