Combination of Geophysical and Geotechnical Methods for Foundation Studies at Ejirin, Epe, Lagos, Nigeria

Authors: Adeogun OY, Adeoti L, Ogunlana BA, Ishola KS, Oyeniran TA, Alli SA

DOI Info: N/A

ABSTRACT

The neglect of near surface investigation by populace and government of Nigeria prior to engineering construction has caused incessant collapse of buildings, loss of lives and properties. This informed the use of combined geophysical and geotechnical methods to reveal the nature and competence of the subsurface strata prior to construction at Ejirin/Epe, Lagos, Nigeria. The two geophysical techniques used were 2-D electrical imaging and vertical electrical sounding (VES) while the geotechnical method adopted was the cone penetration test (CPT). The data were acquired along five (5) traverses and a borehole data was used to constrain the data interpretation. The 2-D, VES and CPT data were processed with Dipro, WinResist and Microsoft Excel software respectively. The 2-D and VES results revealed topsoil with resistivity of 91.6 – 1928.8 Ωm, clayey sand with resistivity of 350.5 – 1094.4 Ωm, lateritic clayey sand with resistivity of 1045.9 – 4920 Ωm, sand with resistivity of 341.3 – 1252.2 Ωm and sandstone with resistivity of 4125.9 – 11028.3 Ωm. The sand/sandstone of resistivity ranging 341.3 – 11028.3 Ωm and thickness ranging 19 – 80 m delineated at the fourth and fifth geo-electric layers represent the competent layers due to their resistive and suspected consolidated nature. The CPT was only able to delineate two layers with values of cone resistance ranging from 10 – 150 kg/cm² indicative of soft clay/ clayey sand and lateritic clayey sand to depth of 5 m. Hence, a deep foundation by piling with a minimum pile length of 45 - 50 m is required for the proposed multi-storey building.

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