Geophysical Investigation of Ilesha-Owena Highway Failure, in the Basement Complex Area of Southwestern Nigeria
The aim of this research was to detail the subsurface geoelectric sequence, map the subsurface structural features within the sub-grade soil, delineate the bedrock relief and determine the resistivity type curves that characterize the failed and stable segments of the Ilesha-Owena Highway as a means of establishing the cause(s) of the road pavement failure. The investigation involved the electrical resistivity, magnetic and Very Low Frequency Electromagnetic (VLF-EM) geophysical methods. The electrical resistivity survey utilised the Schlumberger Vertical Electrical Sounding (VES) and dipole-dipole horizontal profiling. Magnetic and VLF-EM measurements were made at 5 m intervals along each of the segments while YES measurements were made at interval of 25 m. The electrical resistivity data were quantitatively interpreted by a computer-iteration technique involving the RESIST and Dippro cm programmes while the KHFILT software was used to process the VLF-EM data which together with the magnetic profiles were qualitatively interpreted. The results showed that the inverted VLF-EM anomaly along the stable road segment mapped a generally resistive subsurface. The magnetic profile along the segm6nt was relatively flat indicating homogeneous subsurface devoid of any major structural feature. Only one fairly conductive zone beneath the failed segments 1 and II respectively, typical of a linear structure, was delineated by the VLF-EM anomaly while two and four magnetic anomalies typical of thin/thick dykes were correspondingly delineated along the two segments. Four major subsurface layers —the lateritic/sandy/sandy clay/clayey sand topsoil; the lateritic layer; the weathered basement and the presumably fresh bedrock were delineated beneath the stable and failed segment II while three subsurface layers- the sandy/sandy clay/clayey sand topsoil, the clayey weathered basement and the presumably fresh bedrock were obtained beneath failed segment I. The geoelectric sections revealed uneven bedrock topography beneath the three segments. The resistivity histogram plot and 2-D resistivity structure showed that the stable segment was characterized by high resistivity laterite to a depth of up to 6 m while the failed segments I and II displayed low resistivity clay and sandy clay topsoil/sub-grade soil. Also narrow deeply weathered zones typical of linear features located within the basement bedrock were delineated beneath failed segments I and II. These features correlated with magnetic derived linear features. It was concluded that the causes of highway pavement failure in a typical basement terrain identified from this study included poor drainage, presence of near surface linear features within the sub-grade soil, clayey nature of topsoil/sub-grade soil below the highway pavement and excessive cut into near surface low resistivity water absorbing clay enriched substratum.