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.