The causes of road failure along Ago-Iwoye, Ishara Road, South-Western Nigeria was investigated by studying some of the geophysical properties of subgrade soils underlying the stable and unstable sections of the road. Appropriate geophysical survey methods employed along Ago-Iwoye, Ishara Road within longitude 003⁰51’E-003⁰54’E and latitude 006⁰56’N-006⁰58’N located in South-Western Nigeria.
Vertical Electrical Sounding (6 VES, 3 per locations on profiles of 100m) technique using Schlumberger electrode array and Constant Separation Traverse (2 CST) using Wenner electrode array were employed on unstable and stable locations of the road. The data was interpreted using a manual curve matching technique and followed up by iteration using computer assisted software. The VES established four geologic layers which are: top sol (154.7Ωm-980.2Ωm), sand/lateritic sand (96Ωm-991.1 Ωm), weathered basement (159.3-3051 Ωm), and fresh/fractured basement (320 Ωm-3077.8 Ωm). The CST revealed 3 layers with resistivity ranges 138 Ωm-435Ωm in the topsoil, 147 Ωm-788 Ωm in the weathered basement comprising laterite, and 89.4Ωm -2457Ωm comprising of weathered basement, fractured/fresh basement.
This study shows that low resistivity is peculiar to failed segments of the road due to high moisture and clay in the subgrade soils and thus leads to road failure. Other factors that can be attributed to the road failures is differential settlement of subgrade clay, poor design etc. It is hereby recommended that hydraulic sand fills be applied to areas that are incompetent and road embankments on the road. Drainage systems should be included in future reconstruction of the road.