Integrated Electrical Resistivity Tomography and Geotechnical Assessment of Road Failure Susceptibility along the Umuahia–Bende–Ohafia Road, Southeastern Nigeria

Recurrent failure of the Umuahia–Bende–Ohafia road in Abia State, southeastern Nigeria, has imposed significant socio-economic costs, yet the subsurface causes of failure at specific points along the corridor have not been systematically characterised. This study integrates two-dimensional Electrical Resistivity Tomography (ERT) with geotechnical laboratory testing to evaluate the causes and spatial distribution of road-failure susceptibility along the ~60 km corridor. Twenty 2-D ERT profiles (Wenner array, 5 m electrode spacing, 105 m spread, RES2DINV inversion) were acquired at representative points, complemented by soil sampling and testing for natural moisture content, Atterberg limits, particle-size distribution, bulk density, compaction (OMC/MDD), California Bearing Ratio (CBR) and shear strength. Resistivity values across the profiles ranged from <1 Ω·m to >20,000 Ω·m and were used, together with published resistivity–lithology relations for the Bende–Ameki Formation, to delineate clay, silt, sand and lateritic/gravel horizons beneath the road. Natural moisture content (3.3–50%), plasticity index (5–25%) and percentage fines (13–74% passing 0.075 mm) varied systematically with the resistivity signature at each site. Sites underlain by thick, low-resistivity (<100 Ω·m) clay-rich horizons with high natural moisture content and plasticity index (e.g., Isieke Ibeku and Ohafia LGA Secretariat) coincide with observed severe pavement failure, whereas sites with thin, low-plasticity, well-drained lateritic or sandy horizons remain structurally sound. On this basis the road was classified into high-, moderate- and low-vulnerability sections. No evidence of faulting was observed on any profile, indicating that failure is controlled by near-surface lithological and hydrogeological factors rather than tectonic structures. The results demonstrate that ERT, calibrated against limited geotechnical ground-truth, can be used cost-effectively to map road-failure susceptibility and to guide targeted subgrade improvement and drainage interventions before and after road construction.