Subsurface Characterization and Bearing Capacity Assessment of Selected Coastal Sand in Lagos Southwestern Nigeria

The geotechnical response of sediments and geo-earth materials within the coastal area presents a significant encounter during foundation process. This challenge is well pronounced to the building engineers handling construction process in rapidly urbanizing coastal regions. This study presents the bearing capacity of coastal plain sands in the Okun Ajah area of Lagos, Nigeria. For the purpose of the study, an integrated approach of Cone Penetration Test (CPT), Standard Penetration Test (SPT), and Multichannel Analysis of Surface Waves (MASW), was employed to depict the detailed subsurface information. Shear wave velocity (Vs), rigidity modulus (G), SPT blow counts (N-values), and cone resistance (qt) were utilized to determine the mechanical properties of the subsurface materials. Unconsolidated and low shear strength materials in form of loose to saturated loose sands were delineated between the depth values of 2.50 m and 18.00 m with corresponding Vs values ranging from 120 to 240 m/s and rigidity modulus values between 18 and 103 MN/m². This was corroborated by low cone resistance (qt) values of 10 – 15 kg/cm² and low SPT N-values of 4 – 6 illustrated within the depth values of 2.50 – 8.00 m depth interval. The findings establish that the study area is underlain by geo-earth materials that are vulnerable to settlement and bearing capacity failure. Based on this, it is expected that foundation design for engineering structures in this area should consider resilience, stability, and long-term performance.