Subsurface exploration for soil geotechnical properties: Implications for infrastructure design and construction in Victoria Island, Lagos, Nigeria

Abstract

An exploration of subsurface soil properties has been conducted at a proposed construction site in the aquatic city of Lagos, Nigeria, through the integration of geotechnical and geophysical methods. The aim was to characterise the soil properties and assess their implications for the proposed multi-storey infrastructure. Standard Penetration Tests (6), Dutch Cone Penetrometer Tests (14), and dipole-dipole and pole-dipole electrical resistivity imaging were conducted at the site. The topsoil, consisting of sandy lateritic clay (10–5595 Ωm), extends from 0 to 4 m and grades into dense sand at other locations, reaching a depth of 5.50m. The topsoil exhibits high shear strength (≤150 kg/m²), an allowable bearing pressure of 54 kN/m² to 85 kN/m², and a safety factor of 2.5. A 3- to 6-m-thick layer of soft, amorphous clay/peaty clay (0–11 Ωm) lies beneath the topsoil. Grey, medium to very dense sand (0–2387 Ωm), containing infrequent gravel, is found beneath this layer down to 30 m depth. Within the peat/peaty clay, zones containing freshwater show high resistivity responses (83–2380 Ωm), whereas clayey sand with saline water exhibits lower resistivity (0–40 Ωm). The total ground-bearing pressure for the proposed structure is estimated at 225 kN/m². Precast piles reaching 18 m depth, with diameters and safe working loads of 600 mm/1140 kN, 800 mm/2010 kN, or 1000 mm/3150 kN, are recommended for the multi-storey development. This study is novel because it reduces the ambiguity and high cost typically associated with subsurface explorations that preclude the combination of geotechnical and geophysical methods.