Investigation of Geotechnical Engineering Properties of Subsurface Soils in Bonny Island, Eastern Niger Delta

The purpose of the investigation was to evaluate the subsurface lithologies as well as to examine the properties of the soils in the area, especially with respect to their strengths and bearing capacities, with a view to determining the suitability or otherwise of the subsoil as bearing media for a shallow foundation system. Borings for subsurface exploration in the area were made at three (3) well spaced locations with a hand auger to a maximum depth of 30m each below the existing ground level and the execution of two (2) Cone Penetrometer Tests (CPTs) to maximum depth or refusal. The study is aimed at determining the suitability of the location prior to the design and construction of foundations in the area. Results of the study reveal that BH 1 is characterized by soft, dark grey peat with a thickness of 2.0m. This layer of peat is underlain to 9.0m by medium dense, dark grey fine sand and in turn overlies medium dense to dense, grey fine to medium sand to the terminal depth at 30.0m, and becoming light grey at 24.0m. In BH 2, the topsoil is soft, dark grey peat to 0.50m. Underlying the above layer of peat to 1.0m is loose, light brown fine sand with plant ischolar_mains. From 1.0m to 3.0m is a layer of soft, grey sandy silt which is underlain by medium dense, grey fine sand of 3.0m thickness. From 6.0m to 9.0m is dark grey, fine sand with intercalation of silty clay at 9.0m and from 10.0m to 22.0m is medium dense, grey fine to medium sand. From 23.0m to the terminal depth at 30.0m is medium dense to dense, brown fine to medium sand. The study shows that the topsoil in borehole 3 is similar to borehole 2. However, underlying the peat in BH 3 is brown fine sand to 1.0m and underlying the fine sand to 3.0m is medium dense, light brown fine sand. From 3.0m to 14.0m is medium to dense, light grey fine sand which in turn is underlain by medium dense to dense, fine to medium sand of 8.0m thickness. From 22.0m to the terminal depth at 30.0m is medium dense to dense, grey fine medium sand. Generally, the sand layers in all the boreholes exhibited high SPT values at deeper depths. The results obtained from this study have emphasized the usefulness of geotechnical studies in establishing variation in lithology accompanied by variation in the allowable bearing pressure of foundation soils.