Geochemistry

This study examined the sedimentological and geochemical characteristics of problematic soils within the Lagos segment of the Dahomey Basin, Southwestern Nigeria, to understand their composition, depositional setting, and engineering implications. Eight borehole samples (BH1–BH8) were analyzed for particle size distribution, water content, density, and plasticity characteristics, while representative samples (L2 and L5) underwent X-ray diffraction (XRD) analysis. The particle size distribution results showed that the soils are predominantly fine- grained comprising of clayey or silty materials to fine grained sand with mean grain sizes between 3.30ϕ and 6.83ϕ. Sorting values between 3.54 and 4.86ϕ classified them as very poorly sorted sediments deposited under fluctuating, low-energy coastal conditions. Water content ranged from 23.8 % to 36.9 %, density from 1.77 to 1.88 g/cm³, and void ratios from
0.65 to 0.81, indicating moist, moderately compacted soils with high porosity. The liquid limit and plasticity index values, ranging from 37.7–58.2 % and 21.3–32.6 % respectively, revealed high plasticity and significant swelling potential. XRD results confirmed montmorillonite (41–42 %) as the dominant mineral, alongside quartz (33–39 %) and minor actinolite or albite. These properties collectively explain the soils’ poor strength, high compressibility, and moisture sensitivity. The study concludes that the Lagos coastal soils are expansive and structurally unstable, requiring stabilization, effective drainage, or deep foundations to support safe and durable engineering development within the Dahomey Basin