Geophysical and Geotechnical Characterization of Subgrade Soils Associated with Flexible Pavement Failure in Parts of Ogun State, Nigeria

Background and Objective: Road pavement failure is a persistent problem along many highways in Nigeria, largely due to inadequate consideration of subsurface geotechnical and geological conditions prior to construction. This study integrates geophysical and geotechnical approaches to investigate the causes of recurring pavement failure along the Mowe-Ofada Road, Southwestern Nigeria, within the Dahomey Basin underlain by the Paleocene Eocene Ewekoro Formation. Materials and Methods: Field investigations comprised reconnaissance survey, two-dimensional electrical resistivity imaging, and vertical electrical soundings using dipole dipole and Schlumberger arrays. These were complemented by geotechnical tests, including particle size distribution, Atterberg limits, compaction, and California Bearing Ratio, conducted on soils from failed road sections. Subsurface characterization revealed multiple layers dominated by clay and clay-rich materials, with extensive low-resistivity zones indicative of moisture-laden and weak subgrade conditions. Results: Geotechnical results indicate high plasticity, fine-grained textures, and moderate bearing capacity, reflecting poor load-support characteristics and high susceptibility to moisture-induced deformation. Compaction characteristics further classify the soils as only marginally suitable for subgrade applications. Conclusion: The study establishes that clay-dominated subgrade, high moisture retention, and weak geoelectric zones are the principal contributors to pavement failure along the Mowe-Ofada Road. Excavation of incompetent materials and installation of effective drainage systems are recommended. The findings demonstrate the effectiveness of integrated geophysical and geotechnical methods for reliable pavement failure assessment and improved road foundation design.