AN INTEGRATED GEOTECHNICAL INVESTIGATION FOR STRUCTURAL FOUNDATION SUITABILITY ACROSS NIGER RIVER, NORTH CENTRAL NIGERIA

Abstract

- The need to assess the suitability of foundation soil for oil and gas installations, such as pipeline, stations, and all other ancillary facilities, necessitated this study across the Niger River of north central Nigeria. A total of 17 boreholes and cone penetration tests were conducted. Samples were recovered from the boreholes and tested in the laboratory to obtain soil parameters. The investigation revealed that the subsurface is essentially composed of FINE SAND/SAND and CLAY. The FINE SAND is largely the superficial soil except in BH1 where CLAY is the superficial soil. The FINE SAND is loose at the top becoming medium dense and clayey at the base as it grades into CLAY. It has a greyish colour, with moisture content between 6% and 29% and bulk unit weight between 15.17 kN/m3 and 21.76 kN/m3 . The CLAY underlies the FINE/SAND and SAND. It is generally greyish in colour, with moisture content between 12% and 29% and bulk unit weight between 15.17 kN/m3 and 21.30 kN/m3 and an average strength of 156 kN/m2 except where it is the surficial soil and soft (strength < 25 kN/m2 ). Oversize materials were not encountered during the investigation and as such shall not pose any risk to horizontal directional drilling operation. However, on the Island, within the FINE SAND, bed of gravel occurs in BH8 and BH9. In terms of plasticity, due to saturation, the soils are not expected to swell. Nevertheless, it is advisable to add polymers to the drilling mud to prevent transfer of water from the drilling fluid into the soil as a precaution for clayey soils at the entry and exit points and on the Island. Most of the soil contains fines less than 30% and therefore, specific measures may be employed to improve borehole stability during drilling and installation.