2D Geophysics and in-situ Geotechnical Exploration of an Engineering-Challenged Site in Southwestern Nigeria

Authors: Oyedele KF, Adegbola RB, Adebisi NO, Ogunlana FO

DOI Info: N/A

ABSTRACT

Failure of structures due to inadequate foundation investigation is imminent at Westwood Development Estate, Sangotedo area of Lagos. This calls for an integrated method for site investigation of the underlying geological medium in the area. In this study, geophysical and geotechnical approaches were to define the number of subsurface soil layers, their thicknesses and established the depth to competent layers. Voltages of electrical field induced by the distant grounded current electrodes were measured and interpreted based on the apparent electrical resistivity (ρ_app) values as the function of depth. The geotechnical method was done by pushing an instrumented cone and a sample tube into the ground. Cone penetration resistance (qc) and tube’s number of blows at 150 mm to various depths were recorded. From both methods and borehole logs, topsoil has ρ_app values range of 283-1128 ?m with qc and respective allowable bearing capacity (ABC) ranges of 2 - 40 kg/cm² and 529 – 10584 kN/m². The second layer (loose fine to coarse grained sand) had ρ_app range of 226 – 2544 ?m and qc (2 - 40 kg/cm²) with ABC range of 10584 – 31752 kN/m² respectively. The third and fourth layers composed mainly of medium dense fine to coarse grained sand. The top layer to a depth of 1.4 m may be inimical to foundation of any structure, and require excavation. Moderate structures such as bungalows and a storey building can be accommodated at 3.0 m depth. However, pile foundations are recommended at greater depth for medium to high rise structures.

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