Engineering geophysical evaluation of suspected weak foundation soils at National Theatre Iganmu, Lagos, Nigeria

Foundation geotechnical research studies in semi-onshore environment are well known in literature. However, a study which incorporates geophysical with in-situ engineering site investigations around a structure undergoing differential settlement is uncommon. This research was carried out around the National Theatre area, Iganmu, Lagos, Nigeria. It was meant to determine the nature of the lithological layers, thicknesses and depth to competent level for the performance of subsoil systems under static loading. A total of ten (10) Cone Penetrometer Test (CPT), and twenty (20) Vertical Electrical Soundings (VES) were acquired along four (4) geotechnical boreholes. Four (4) traverses were also acquired with a maximum spread length of 200 m. The VES data were processed with curve matching and subjected to computer iteration techniques, the result obtained were presented as 1D resistivity profile. The horizontal profiling was also processed and presented as 2-D resistivity imaging. The study area was underlain by four distinct lithological layers. These were represented as topsoil, clayey sand/sandy clay, clay and clayey sand/clay. Electrical resistivity profiling and imaging clearly revealed the inhomogeneity nature of the subsoils, while the geotechnical presented soils with poor bearing values of predictably considerable settlement potential within the depth of 1 to 7 m. However, appreciable bearing values were prominent between depths of 8m and 10m around the structure. Adopting a pile foundation for high column load is recommended from depth of 8m during rehabilitation and upgrading of the structure.