Integrated geophysical investigation of the Igbara-Oke – Igbaraodo road pavement failure in Ondo/Ekiti state, southwestern Nigeria

Abstract

Integrated geophysical investigation, involving the electrical resistivity and magnetic methods was carried out along the Igbara-Oke – Igbara-Odo roadway in Ondo/Ekiti State. This was with a view to investigating the cause(s) of the persistent pavement failure recorded over the years along the road, in spite of repeated rehabilitation works. The resistivity survey utilized 1-D Vertical Electrical Sounding (VES) with the Schlumberger array and 2-D imaging with the dipole-dipole array while profiling technique was adopted for the magnetic survey. Two (2) stable and two (2) unstable segments were identified for the geophysical investigation. Total field magnetic data were acquired along the identified segments of the road at 5 m intervals. The data were corrected for diurnal variation and offset and presented as residual magnetic profiles. Both qualitative visual inspection and quantitative (2-D modeling) interpretations were applied to the resulting residual magnetic profiles. The 2-D dipole-dipole profiling data were acquired along the magnetic traverses. The 2-D data were inverted into 2-D resistivity structures and were used to constrain the location of the nineteen (19) VES stations. The VES curves were interpreted quantitatively and the results were used to generate geoelectric sections. The magnetic profiles, 2-D magnetic models, 2-D dipole-dipole images and the geoelectic sections along the stable segments were characterized generally by relatively high resistivity (> 151 Ωm) weathered layer; significantly thick (>1.5 m) lateritic subbase; A and KH type curves with increasing layer resistivity and hence competence with depth, within the upper two layers and a subsurface devoid of geological structures (faults, fracture or joints). Along the failed segments, relatively thin (< 1.5 m) or non existing lateritic subgrade; low resistivity weathered layer(< 151 ohm-m);H type curve starting with decreasing resistivity typical of incompetent substratum and subsurface geological structures suspected to be faults, fractures, joints or lithologic boundaries characterized these segments. This study concluded that failures along the investigated roadway were caused by clayey subsoils with relatively low resistivity (< 151 ohm-m); differential settlement on road segment cut into saprolite (decomposed rock rich in clay) and presence of geological structures such as lithologic boundaries, faults and fractured zones.

Keywords: Geophysical Investigation, Pavement Failure, Subsurface Competence, Roadway.