A geophysical assessment for engineering performance of subgrade soils: a case study of the Ago-Iwoye/Ilishan road, South-Western Nigeria

Abstract

Geophysical application of electrical resistivity method has been employed to differentiate subsurface target of interest from the encompassing strata along the Ago-Iwoye/Ilishan road in South-western Nigeria. However, a geological model which links variation in lithological characteristics of subgrade soils to the engineering performance of the road is yet to be a research subject. This research is a special geotechnical application of geophysics for revealing the subsurface disposition and predicting the stability state of subgrade materials. Field assessment of rock types, and their relationship was followed by electrical resistivity measurement along the road. Schlumberger and dipole – dipole arrays were adopted for fifty-eight (58) Vertical Electrical Sounding (VES) and 2-D resistivity imaging involving Dipole-Dipole profiling technique was adopted in the sedimentary segment. Field data were inverted to 2-D resistivity structures inferentially delineated contour colorations using ‘DIPRO for Windows’ software. Geo-electric layers revealed Sedimentary Basin (SB) between Ilishan and Irolu, between Irolu and Ijesha-Ijebu, Transitional Zone (TR) with abrupt contrast along the plane of an unconformity and crystalline Basement Complex (BC) towards Ago-Iwoye. The 2-D resistivity structures across the different geological terrains revealed with 50Ωm - 3289Ωm at SB,233Ωm - 8028Ωm at TZ and 8Ωm - 3009Ωm at BC. From geotechnical view, subgrade materials underlying the SB and TZ segments can be adjudged competent to sand and clayey sand units with minor occurrences of incompetent clay and sandy clay lenses. The occurrence of subsurface discontinuities at the BC end of TZof the road would adversely affect the pavement stability.