By: GI Alaminiokuma and JI Omigie

Keywords: Bonny Island, Seismic refraction, dynamic elastics parameters, engineering foundation properties

Abstract: The geotechnical properties of soils for civil engineering constructions in Nyuene, Bonny Island were evaluated using seismic refraction survey. A 3-layer soil structure was delineated: topmost layer, 6.9 m thick; second layer, 17.1 m thick and consolidated bedrock layer extending to great depths. Results across these layers respectively reveal that Vp/Vs has averages of 1.29, 1.50 and 1.78 indicating fluid-saturated top layers; Poisson’s ratio has averages of -0.72, 0.05 and 0.29; Shear modulus has averages of 2.10 x108 Nm-2, 5.20 x108 Nm-2 and 8.09 x108 Nm-2; Young’s modulus has averages of 0.67 x108 Nm-2, 10.80 x108 Nm-2 and 20.80 x108 Nm-2; Bulk modulus has averages of 0.60 x108 Nm-2, 4.62 x108 Nm-2 and 16.7 x108 Nm-2. Furthermore, the ultimate bearing capacity averages at 464.5 kNm-2, 764.6 kNm-2 and 995.8 kNm-2 respectively across the layers and the allowable bearing pressure averages at 116.1 kNm-2, 191.2 kNm-2 and 248.9 kNm-2 respectively. The trend of the dynamic elastic parameters respectively indicates increase in compressive soil strength, increase in the capacity to withstand shear stress and increase in compaction. These values show that the consolidated layer can withstand more pressure and bear more load than the other layers. The properties of this layer fall within the standard criteria for civil engineering constructions. For the construction of high-rise buildings/skyscrapers and bridges in the study area, 24m of topsoil should be reinforced by piling. However, for other non-high-rise buildings/structures, about 7.0 m of topsoil should be excavated for proper siting of foundations due to the tidal flooding and land subsidence in Bonny Island and environs. This method should be combined with Cone Penetrometer Test at early stage of any construction work to properly determine the soil depth and geotechnical properties to locate foundations. This will forestall the problems of cracks in foundations and buildings collapse.

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