Pre-foundation geophysical investigation of a site for structural development in Oka, Nigeria

ABSTRACT Frequency of structural failure globally has necessitated geophysical investigation of subsurface geology of a site for engineering construction works. Combined very low frequency electromagnetic (VLF-EM) and electrical resistivity methods were used to provide detailed information on subsoil profile for documentation and references for durable and sustainable construction works. Thirteen traverses were established from which geophysical data were acquired. Major conductive geological interfaces suspected to be faults/fractured zones were identified from the plots of VLF-EM data. These points serve as 50 sounding stations further investigated using Schlumberger electrode array with vertical electrical sounding technique and electrical resistivity imaging on selected four traverses of the site. The acquired data were processed, inverted and interpreted. VLF-EM 2-D inverted models revealed conductive zones at some locations suggesting incompetent zones, responsible for structural instability. Saturated clayey subsoil and uneven bedrock topography with depressions at some points could cause differential settling which has negative impact on engineering structures. Structural failure may arise from existence of concealed geological structures, deep weathering/fractured bedrock, heterogeneous and structurally deformed (F1–F16) subsurface geological setting. Thus, classified unstable sections are considered priority in structural design and construction to mitigate unforeseen challenges. Deep foundations in form of piers and piles are encouraged to avert structural failure.