Integrated geophysical investigation of subsurface conditions influencing road failure along the Lusada–Ketu–Ado Odo Road, Ogun State, Southwestern Nigeria

The Lusada–Ketu–Ado Owie Road, which links Crawford University, Lusada Market, and Ado Owie town, has experienced significant structural degradation, reflective of the growing trend of road failures across Nigeria. This study employs an integrated geophysical approach using both one-dimensional Vertical Electrical Sounding (VES) and two-dimensional Electrical Resistivity Tomography (ERT) to investigate subsurface conditions contributing to the road's failure. The Wenner array configuration was utilized with an inter-electrode spacing of 5 m across four ERT profiles, and six levels of data acquisition per profile. VES data were also acquired at each profile location, with one site (VES 3/Profile 3) serving as a control due to its satisfactory pavement performance. Resistivity values varied significantly across profiles: 52.3–3183 Ωm in Profile 1, 136–1522 Ωm in Profile 2, 252–6362 Ωm in Profile 3, and 198–25595 Ωm in Profile 4. Low-resistivity zones (less than 126 Ωm) detected across all profiles indicate the presence of water-saturated clayey materials, known for their high porosity, low permeability, and poor load-bearing capacity—factors that lead to swelling, differential settlement, and eventual road failure. In contrast, high-resistivity zones (>1800 Ωm) suggest compacted, competent subsurface materials conducive to long-term pavement stability. The comparison between 1D and 2D results highlights the limitations of using VES alone, as it fails to capture lateral heterogeneities and discontinuities evident in the 2D profiles. This integrated geophysical approach proves crucial for identifying critical failure zones and informing targeted engineering interventions. Recommendations include the excavation of unstable clay-rich soils, replacement with suitable materials, and implementation of robust drainage systems. The findings underscore the value of combining geophysical techniques for sustainable road design, rehabilitation, and maintenance in geologically complex environments.