Authors: Olagunju, K.T., Olobaniyi, S.B., Oyedele, K.F. and Adefuwa, A.
Abstract
Geotechnical frameworks and calculations rely on adequate characterisation of the subsurface soil units. Boring tests and their distribution provide, not only geotechnical parameters but also to reveal geotechnical units and lateral extent, which are particularly important for safe and cost-effective foundation design. This study presents two cases of geotechnical site investigation involving boring tests in two areas of Lagos metropolises that raised concerns about uncertainties in subsurface characterisation caused by the inadequacy of field techniques employed. This study revealed the limitation of geotechnical designs at A and B, largely due to the uncertainty of subsurface characterisation induced by inadequate quality and quantities of field tests. Using data from geotechnical boring
(percussion drilling) and resistivity survey (2D tomography and VES), the occurrence of unusual ironstone was established between 9 to 15m depth at Site A, which prevented further drilling with a percussion rig. However, this ironstone occurrence is non-lateral as the resistivity obtained in 2D profiles ranges from 15 Ωm – 74 Ωm from the surface to 35 m, suggestive of Clay and Sand. Thereby complicating geotechnical design. Notably, one boring is grossly inadequate for characterising the complexity of the geology at Site A. Similarly, a forensic investigation at
Site B indicates the inadequacy of one boring test since the correlation of results from three new borings revealed a complex geology which was missed using only one. Installed piles on this site were mostly seated on Organic Clay between 15 and 30m depth. Evidence presented in these case studies underscores the importance of comprehensive subsurface characterisation to enhance design decisions for geotechnical recommendations.