Engineering Geological Baseline Condition Assessment of Sobe–Owan Bridge Substructure Using Integrated Non-destructive Geophysical Techniques and In-Situ Penetration Testing

In Nigeria, bridge maintenance and monitoring programmes rarely include post construction subsurface geological material assessment. The geological material evaluation of the bridge site, even after construction, would assist in the identification of concrete spalling, lithologies delineation, developed fractures, groundwater level variation, scouring activities, and soil settlement. On this basis, the Sobe–Owan bridge site was investigated based on elevated vibration intensity recorded in previous studies. The study utilized integrated geophysical methods, stream water physical parameter measurement, and cone penetration testing. The result of the geological succession revealed alluvium sand and red sand (topsoil), clay alluvium sand mixture (i.e., subsoil, with an angle of friction of 40.86° and cohesion of 384.8 kPa), shale, and sandstone (basal rock). The depths to the basal rock vary from 20.7 to 38.4 m of irregular topography. Degree of scouring activity is at near surface (less than 5 m), with the occurrence of cross-cutting linear structures signified degree of sedimentation, water saturation, and scouring. The scouring action occurs notably within the vicinity of the foundation piers. The stream water’s high velocity during flow and physical quality showed that the water is capable of causing damage/spalling to the concrete due to favourable values of some parameters during the seasons. The minimum depth to pier/pile cap was found to be between 3.30 and 6.25, which implies there could be more stages of pier/pile cap within the subsurface system, since the minimum depth to basal sandstone is 20.7 m. This study concludes that no serious geological hazard has been identified, except near-surface scouring zones, which are normal for bridges over stream channels, although weakened/distressed rebar within the correct and or concrete might not be underrated.