Journal of Earth Sciences and Geotechnical Engineering

Engineering Geology of the Niger Delta

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• Abstract

  The Niger delta, an area of roughly 30,000km2, is experiencing physical development challenges due to poor soil conditions, a situation which has escalated project cost and as a consequence impeded development. This study has characterized the geomorphic sub-environments of the Niger Delta using geotechnical and geophysical indicators to define basic parameters for economical and preliminary front end engineering design. The geomorphic sub-environments, each with its distinctive mode of formation and geologic history exhibited characteristic geotechnical and geophysical properties. This study explores the variations in topography, groundwater level and geo-resistivity of the shallow horizon (0-10m) as well as geotechnical properties and their variation across the Niger delta region. Both geophysical and geotechnical field testing were mostly at 1km interval and along linear traverses stretching some 178km in the west and 190km in the east. The geotechnical properties investigated included undrained strength (derived from triaxial tests, CPT, Vane shear and Pocket penetrometer), natural moisture content, unit weight, and Atterberg limits. In general terms, the topography shows a gradual decent towards the coastal limit of the Coastal plain sediments, where it meets the Mangrove swamp sediments. The lower Niger flood plain sub-environment is distinctively lower in elevation compared to the coastal plain sand but higher than both the Mangrove swamp and Beach ridge. Results show that the geotechnical properties are a reflection of the geomorphic subenvironment. The water table is season dependent and varies across the Niger delta from 0m at the coast to 10m at the northern end. Apparent resistivity also varies across the geomorphic units with the largest values recorded in the Coastal plain sands. The mangrove swamp, apart from exhibiting the lowest apparent resistivity values shows a distinctive increase with depth between the 0-3m and that of 0-10m depth profiles because of the highly heterogeneous composition within this soil horizon in this area. Undrained strength values derived from CPT, Triaxial, vane shear and pocket penetrometer, all similarly showed characteristic variations across the geomorphic units, with the mangrove soils as the weakest (0- 20kPa) with a high probability of sudden increase in firmness or soil progression to sandy units between 3m and 10m. The increase in undrained strength with depth between 0-10m, in the other geomorphic units on the other was attributed largely to normal consolidation with overburden pressure playing a leading role. Results of consolidation settlement analysis show the mangrove swamp as the most compressible with settlements of up to 180mm, followed by flood plain sediments and then the coastal plain sands in the more elevated areas for an incremental vertical load of 30kPa. Similarities in the patterns of spatial variability in geotechnical properties across the geomorphic units in both eastern and western segments of the Niger delta have been determined.