Reliability Evaluation of Optimum Moisture Content of Tropical Black Clay Treated with Locust Bean Waste Ash as Road Pavement Sub-base Material

A FORTRAN based first-order reliability program (FORM) was used to assess the suitability of tropical black clay (known as black cotton soil) treated with locust bean waste ash (LBWA) mixtures as road pavement sub-base material. Soil was treated with up to 10% LBWA and compacted with British Standard light, BSL; West African Standard (WAS) and British Standard Heavy (BSH), energy. Laboratory results were used to generate a regression model for the reliability analysis using optimum moisture content of compaction (OMC) as dependent variable and the relevant soil parameters; LBWA; gravel content (GR); specific gravity (Gs); plasticity index (PI); sand content (Sa) and percentage fine (PF) as independent variables to compute reliability/safety indices. The results indicate that for the laboratory-based model, reliability index is sensitive to variability in all the soil parameters with the exception of PF which have constant safety index values. Results showed that locust bean waste ash, LBWA; GR; Gs, PI and Sa are greatly influenced by the coefficient of variation and therefore must be strictly controlled in LBWA treated black cotton soil for use as sub-base material in road pavement. Safety indices of −1.29 to −0.58, −0.041 to −0.415 and 1.33 to −0.224 were recorded for the effect of OMC on the reliability indices of BSL, WAS and BSH compaction energy respectively. Stochastically, results obtained did not produce acceptable safety index value of 1.0 as recommended by the Nordic Committee on Building Regulations. Therefore, more potent additives such as cement or lime are recommended to model optimum moisture content of compaction for black cotton soil–LBWA mixtures for use as sub-base material for road pavement.