Nature Environment and Pollution Technology

H. O. Nwankwoala ¹

Affiliations
1 Department of Geology, University of Port Harcourt, NG

Abstract

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.

Keywords

Subsurface Soils, Lithology, Geotechnical Properties, Niger Delta.

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P. O. Youdeowei ¹, H. O. Nwankwoala ²

Affiliations
1 Department of Environmental Management, Institute of Geosciences and Space Technology, Rivers State University of Science and Technology, P.M.B. 5080, Nkpolu-Oroworukwo, Port Harcourt, NG
2 Department of Geology, University of Port Harcourt, P.M.B. 5323, Choba, Port Harcourt, NG

Abstract

In this study, a total of six (6) boreholes were sunk with rotary drilling equipment to depths of about 60 ft (about 18.30m) for BH1, BH2 and BH3. Boreholes 4, 5 and 6 were drilled to 13.60m, 12.60m and 12.20m, respectively. The measured static water levels fall between 4.45m, 3.75m, 0.94m, 6.30m, 6.10m and 6.20m for BH 1, BH 2, BH 3, BH 4, BH 5 and BH 6, respectively. All the boreholes are overlain by a reddish brown, fine grained, silty clay overburden of 3.00m to 4.60m in thickness. This is followed by a thinner stratum of dark brown, fine-medium grained, clayey sand in BH4 and BH5, which is absent in BH6. Another layer of silty clay underlies this formation in both the boreholes with a 0.9m thickness of clayey sand in BH4 preceding the sand and gravelly sand aquifer. There is less measure of uniformity in lithostratigraphic correlation between BH6 and the other two boreholes located within proximate vicinity. Below the overlying silty clay soil, BH6 records a rhythmic succession of loose sand and medium to very coarse, and moderately plastic, gravelly silty clay intercalations, before the aquifer (gravelly sand) is encountered. With respect to the area, the subsurface drainage pattern is multi-directional from north-east to the south-west and vice versa, with the migration being more inclined west-wards towards the location of BH3, which records the lowest depth to water table. Leachates introduced at a given point in the area will migrate as pollution plumes along these flow paths.

Keywords

Obite Gas Plant, Geological Baseline Study, Boreholes, Overburden.

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