Subsurface Integrity Assessments of a Proposed Plaza Building at Oniru Lekki, Lagos, South-western Nigeria, Using Geoelectrical and Geotechnical Methods of Investigations

Subsurface integrity assessment for building foundation using geophysical and geotechnical methods was carried out at Oniru, Eti- Osa, Lagos State. A total of twenty-five Vertical Electrical Sounding (VES), five 2-D Resistivity Imaging Survey traverses and two borings with Standard Penetration Test (SPT) were carried out at the study site. The acquired data were processed quantitatively using partial curve matching and computer iteration technique to generate the geoelectric sections, the 2-D model and the SPT logs. The VES results revealed five to six geologic units corresponding to topsoil, clayey sand, sandy clay, clay/peat, and sand while the 2-D resistivity structure corresponds with the VES result. The borehole log reveals sand layers having an N-value of 22-30 which is indicative of medium-dense, brown (medium to fine) sand with occasional gravels from depth 1.50 - 7.50m. The borehole log information correlates well with the VES/2-D result. For the sand layer which connotes dense, grey (medium to fine) sand with occasional gravels from a depth 7.50 – 15.75m with N-value ranging from 26–30m. The study analysis shows that the proposed building could be placed on the dense sand at depth 7.50 – 15.75m, this, however, should depend on the proposed load, length, and breadth of the proposed building. This study has provided useful information about the subsurface condition for engineering structure and zone suitable for the proposed foundation.