Geotechnical Investigation and Foundation Design Recommendations for Multi-Storey Buildings in Benin City, Edo State

Benin City in Edo State, within the South South geopolitical zone, is experiencing rapid urban densification with increasing demand for multi-storey residential and commercial buildings. Adequate geotechnical investigation and foundation design are critical to preventing the differential settlement and structural distress that have been observed in several recently constructed buildings in the city. This study conducts geotechnical investigations at six selected sites across Benin City, employing standard penetration tests, plate load tests, and laboratory analysis of disturbed and undisturbed soil samples to characterise the subsurface conditions. Soil profiles at all sites revealed a variable stratification of lateritic fill, medium-dense sandy clay, and fine-grained alluvial deposits at varying depths. Foundation design recommendations were developed for hypothetical five-storey residential buildings at each site, considering strip foundation, raft foundation, and bored pile options based on the computed safe bearing capacities and allowable settlement limits. Results indicate that two of the six sites, located within the seasonally inundated Ikpoba River corridor, are unsuitable for shallow foundations due to compressible organic clay layers at depths of 1.5 to 3.0 metres, necessitating bored pile foundations socketed into competent bearing stratum at 8 to 12 metres depth. The remaining four sites were found suitable for raft foundations with appropriate edge beam reinforcement. The study highlights the risk of omitting site-specific geotechnical investigation and recommends its mandatory inclusion in the Edo State building permit application process. Keywords: geotechnical investigation, foundation design, Benin City, multi-storey buildings, soil characterisation.