Geotechnical Assessment of Subsoil Conditions for Foundation Design of the Overhead Bridge along Olu Obasanjo Road, Port Harcourt, Rivers State, Nigeria

Abstract

Aim: This study aimed to characterize the subsoil geotechnical conditions along Olu Obasanjo Road in Port Harcourt, Rivers State, Nigeria, and to evaluate their implications for the foundation design of a proposed overhead bridge. Study Design: The work was based on an integrated field and laboratory geotechnical investigation, combining rotary borehole drilling, in‑situ testing, and laboratory analysis of soil and groundwater samples to derive design parameters for deep‑pile foundations. Place and Duration of Study: The investigation was carried out at a proposed flyover site in Port Harcourt, within the eastern Niger Delta region of Nigeria. Borehole drilling, CPT testing, sampling, and associated laboratory analyses were completed during a single site‑investigation campaign over a period of about three months in the project’s foundation design phase. Methodology: Two exploratory boreholes were drilled to depths of about 35.0 m using a GM‑600 rotary drilling rig equipped with Standard Penetration Test (SPT) tools, and a series of cone penetration tests (CPT) were performed to refusal depth. Disturbed and relatively undisturbed soil samples were collected for laboratory testing. Index properties (natural moisture content, Atterberg limits, grain‑size distribution, bulk density, and specific gravity), shear‑strength parameters from unconsolidated–undrained triaxial tests, and one‑dimensional consolidation characteristics were determined in accordance with relevant BS and ASTM procedures. Groundwater levels and basic chemical parameters (pH, soluble chloride, and sulphate) were also measured. Allowable axial compression capacities of single piles were evaluated using static pile design formulations based on the interpreted stratigraphy and in‑situ test results for conventional bored piles and continuous flight auger (CFA) piles. Results: The subsurface stratigraphy to about 30 m depth consists of an upper reddish lateritic clayey soil overlying a loose to medium‑dense reddish grey fine‑grained sand layer between roughly 10.5 and 25.5 m, underlain by deeper dense greyish fine to coarse sands. Average index properties for representative samples include a liquid limit (LL) of about 55%, a plastic limit (PL) of about 15%, and a natural moisture content (Wn) of about 22%. Measured SPT N‑values increase from about 7 at around 13.5 m depth to about 37 at 30.0 m, while CPT cone resistance in the loose to medium‑dense sand layer ranges from approximately 4 to 15 MPa between about 10.5 and 25.5 m, indicating a transition from medium‑dense to dense conditions. Using these parameters, allowable axial bearing capacities of 800 mm diameter single piles were estimated at about 795 kN and 1,065 kN for bored piles with lengths of 16 m and 20 m, and about 1,035 kN and 1,460 kN for CFA piles of the same diameters and lengths. Groundwater was encountered at an average depth of about 13.0 m below ground level. Conclusion: The established stratigraphy, strength parameters, and computed pile capacities indicate that, despite the presence of loose to medium‑dense sands overlying deeper dense sand horizons, the subsoil at the site can safely support deep piled foundations with tip levels between about 16 and 20 m when designed using appropriate CFA or bored pile systems. The findings highlight the importance of detailed site‑specific geotechnical investigations for flyover projects in the heterogeneous, weakly consolidated sediments of the Niger Delta, in order to achieve safe and economical foundation design.