Subsurface Investigation Using Electrical Resistivity and Standard Penetration Test as Guide for Gas Pipeline Installation in Lekki Peninsula, Lagos

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Adeoti, L
Ishola, K.S
Adesanya, O
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Electronic Journal of Geotechnical Engineering
The study focuses on the assessment of corrosives probability and cathodic protection potentials of the subsurface layers using Vertical Electrical Sounding (VES) and Standard Penetration Tests (SPT) with a view to ascertaining the suitability of the subsoil information for the installation of gas pipelines. A total of fifteen (15) VES points were carried out using Schlumberger electrode array along three traverses. NSPT -values were obtained at every 1.5m within the four boreholes drilling within the study area. The geo-electric sections produced from the interpreted data reveal three to five geo-electric layers which correspond to top soil, sand, and saline sand. A sand formation with very high resistivity indicates that this layer is not likely to cause corrosion of the buried pipelines as such it gives very good cathodic protection to the pipelines buried within this layer. The saline sand layer, a geo-material is associated with low resistivity which might be due to salt water intrusion, has very high potentiality to corrosion and very poor cathodic protection. It implies that any pipeline buried within this layer would be highly prone to corrosion. The results of the four borehole logs correspond with the depth soundings technique. The study underlines that the results of the NSPT – values of soil samples fall between 15 and 50 which are indicative of medium to dense sand. These values further reveal the competence of the soil strength for engineering purposes. However, the analysis shows the soil type in each layer within geo-electric section is not consistent because it oscillates between sand and saline sand due to their differences in resistivity. Thus, the proposed pipeline infrastructure /facilities to be installed within the study area due to systems design or other factors should be adequately supported with good cathodic protection systems. This study has shown the effectiveness of carrying out geophysical and geotechnical studies prior to burying of pipelines.
Schlumberger array , Geo-electrical layer , Corrosive , Cathodic protection , pipelines , NSPT –values
Adeoti, L., Ishola, K. S., & Adesanya, O. (2013). Subsurface investigation using electrical resistivity and standard penetration test as guide for gas pipeline installation in Lekki Peninsula, Lagos. Electronic Journal of Geotechnical Engineering, 18, 2791-2804.