SETTLEMENT RISK ASSESSMENT OF TROPICAL BLACK CLAYS FOR SHALLOW FOUNDATIONS IN NORTH-CENTRAL NIGERIA

Abstract

Tropical black clays are problematic foundation materials in humid tropical environments due to their compressibility, moisture sensitivity, and stress-history effects. This study evaluated the shear strength, compressibility, and settlement susceptibility of tropical black clays from Gwagwalada, Gidigidi, Sunti, and Zungeru in North-Central Nigeria. Laboratory investigations included index property determination, soil classification, triaxial compression, and one-dimensional consolidation testing to determine cohesion (c), friction angle (φ), compression index (C_c), preconsolidation pressure (σ′_p), and coefficient of consolidation (C_v). The soils were predominantly classified as CL, CL–CH, and CH and exhibited moderate expansion potential. Consolidation results indicated moderate to high compressibility (C_c=0.167–0.221) and variable stress-history conditions (σ′_p=72–157kPa). A comparative assessment framework based on the interaction of C_c and σ′_p was developed and complemented by a preliminary Settlement Risk Index (SRI). Computed SRI values (0.00141–0.00261kPa⁻¹) classified the soils into moderate, high, and very high settlement-susceptibility categories, with Gidigidi deposit exhibiting the highest relative risk. The results indicate that settlement susceptibility is controlled by compressibility and stress history, while drainage characteristics influence consolidation rate. The proposed framework provides a practical basis for comparative assessment of tropical black clay deposits, although further validation using conventional settlement analyses and field-performance data is recommended.