Abstract
The near-surface anisotropic properties within the South-Western Niger Delta were investigated to determine their impacts on geotechnical constructions. Uphole data analyses reveal a weathered layer that is thickest in the NW, E and SE with an average of 4.2 m and thinnest in the North-Central with a value of 3.8 m; a sub-weathered layer that is thickest in the NW and SW with an average of 32.1 m and thinnest in the SE with a value of 19.0 m and an underlying consolidated layer with great thickness. Vp/Vs values of 1.16265, 1.16279 and 1.16278 were computed for the layers, respectively. Similarly, Poisson’s ratio of -0.9214, -0.9201 and -0.9202 were obtained for the layers respectively. Shear modulus has an average of 4.01 × 108 Nm-2 in the weathered layer, 26.0 × 108 Nm-2 in the sub-weathered layer, and 49.98 × 108 Nm-2 in the consolidated layer. Young’s modulus has an average of 0.63 × 108, 4.11 × 108 and 7.98 × 108 Nm-2 in the layers, respectively. Bulk modulus has an average of 0.74 × 108 Nm-2, 4.62 × 108 Nm-2 and 0.94 × 108 Nm-2 in the layers, respectively. The ultimate bearing capacity, qf has an average of 804.9, 2241.0 and 3349.8 kNm-2 in the layers, respectively. The allowable bearing pressure, qa of the soil has an average of 201.2, 560.2 and 837.4 kNm-2 in the layers, respectively. Core samples collected to a depth of 66 m from 12 wells revealed an admixture of sand, sandy-clay, clayey-sand and dominant clay units. These quantitative results correlate with the standard geotechnical values for clay. The large deposit of about 30 m of clay delineated in this research may act as expansive soils which portends danger for foundations of buildings located in this area and must be avoided. Key words: Anisotropy, uphole, near-surface, clay, geotechnical, Niger Delta.
Highlights
The near-surface refers to the uppermost 50 to 100 m of the earth’s subsurface (Sheriff, 2002)
It is made up of weathered low velocity materials that are usually fluidsaturated and unconsolidated. Physical properties such as thickness, lithology, density, seismic signal traveltime, velocity and attenuation effects among others within this zone may rapidly change laterally and/or vertically. This variation in the physical properties of a medium depending on the direction in which they are measured is known as anisotropy (Sheriff, 2002; Allaby, 2008)
Uphole locations UH 4, UH 5, UH 8, UH 9, UH 10 and UH 11 were moved to new interception points (IP) as their original shot points fell on mud flat or directly inside creeks with high water levels
Summary
The near-surface refers to the uppermost 50 to 100 m of the earth’s subsurface (Sheriff, 2002) It is made up of weathered low velocity materials that are usually fluidsaturated and unconsolidated. Physical properties such as thickness, lithology, density, seismic signal traveltime, velocity and attenuation effects among others within this zone may rapidly change laterally and/or vertically. This variation in the physical properties of a medium depending on the direction in which they are measured is known as anisotropy (Sheriff, 2002; Allaby, 2008).
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