V3I3P11

Textural Classification of Borehole Sediments of Bonny Island, Coastal Niger Delta, Nigeria: Implication of Depositional environment and geotechnical evaluation

Sunday H. C.¹, Okujagu D. C.², Osayande A. D.^3*

Abstract

The textural properties of subsurface sediments of three boreholes (BH1, BH 2, BH 3, 0-14 m depth) on Bonny Island, coastal Niger Delta, Rivers State, Nigeria were objectively studied in order to explain depositional conditions and evaluate geotechnical consideration. Standard sieve analysis was applied to thirty samples and the statistical data of the grain-size (mean (Mz), sorting (σ 1 ), skewness (Sk1 ) and kurtosis (KG)) were obtained with Folk and Ward (1957) graphical method. The material that serves as the major textural fraction in all boreholes is sand. Mean grain sizes vary between 1.8 – and 3.2 () medium – very fine sand), sorting values 0.6-1.2 -moderately well-sorted to moderately sorted), skewness is largely positive ( +0.08 to +0.35), and the kurtosis is meso -leptokurtic (KG = 0.90 -1.40). All boreholes of interest represented by an upward-fining trend indicate successive weakening of energy to the present-day depositional surface. A coarse-sand bed at 911 m in the BH3 that is covered in with coarse sand is seen as a palaeotidal channel or a beach-ridge deposition. The coastal-fluvial facies of the Benin Formation is given a sediment as a part of a barrier-lagoon complex. Geotechnically, the deeper sandy formations have intermediate permeability and sufficient bearing strength worth structural anchorage, but since near-surface silty units have low permeability, ground enhancement is necessary during the structural loading.

Keywords:

Grain-size analysis; textural classification; borehole sediments; Bonny Island; Niger Delta; Benin Formation; depositional environment; geotechnical engineering