EMPIRICAL CORRELATION BETWEEN CBR AND INDEX PROPERTIES OF TROPICAL RESIDUAL SOILS

Tropical residual soils, widely found in developing regions, exhibit diverse engineering behaviors due to variations in mineralogy, particle size distribution, and degree of weathering. This study investigates the empirical correlation between California Bearing Ratio (CBR) and index properties—specific gravity, grain size distribution, and Atterberg limits—of tropical residual soils in Niger State, Nigeria. A total of 45 disturbed samples were collected from five locations: Gidan Kwano, Lapai Gwari, Brighter, Gidan Waya, and Gidan Mangoro. Laboratory tests were conducted to determine index and compaction properties, followed by CBR tests under unsoaked and soaked conditions. Statistical summaries revealed wide variation in soil behavior, with plasticity index ranging from 4.73% to 23.63% and maximum dry density from 1.94 to 2.25 g/cm³. The CBR values also varied significantly, with Brighter recording the highest unsoaked CBR of 109.84% and Gidan Kwano the lowest at 15.76%. Using multilinear regression analysis, strong correlations were established between CBR and selected index and properties, with R² values exceeding 0.90 in most cases. These findings affirm the feasibility of using index properties for rapid prediction of CBR, providing a cost-effective tool for pavement subgrade evaluation, especially in resource-constrained settings.