Assessment of Stratified Saturated Subsoils for Foundations Design in Part of Southwestern Nigeria

Various studies on geotechnical properties of soils, which underlain the part of the coastal area of Nigeria are yet to account for characteristics governing design and analysis of foundations. This study aims at elucidating consistency in local soil properties with respect to strength, and consolidation parameters for foundations design strategy. Field testing include 12 Cone Penetrometer Tests (CPT) and 15 number borings in Standard Penetration Tests (SPT). Disturbed and undisturbed soil sampling were done at various depths. All laboratory tests (natural moisture content, Atterberg limits, grain size analysis, undrain triaxial and oedometer consolidation) were in accordance with the British Standard 1377, BS (1990). Patterns of the penetration resistance-depth curves show that stratified soils dominate a group of foundation soils in the area. Particle size is in the order of distribution gravel clay silt sand, with 10% to 30% by weight of fines. Coefficient of permeability values (1.0 × 10^–10 ≤ k ≤ 4.7 × 10^–8 m/s) revealed slow water movement with water content (15%–25%), and degree of water saturation range of 79%–94% which confirms partial saturation. Consistency limits showed that the soils have medium to very high plasticity index. They further confirm Illite and kaolinite as the major clay minerals in the soils. Allowable bearing pressures and coefficient of compressibility ranges are 242.77–384.91 kN/m^2 and 0.030–0.965 m2/MN respectively. Despite the water and organic content, the soils can be non-problematic due low compressibility and high strength characteristics at sampling depth range of 10 and 22 m.