Effect of Fly Ash on Atterberg Limits and Specific Gravity of Chikoko Soil in Niger Delta, Nigeria

Chikoko soil, a highly plastic, organic-rich deltaic clay that underlies large parts of the Niger Delta, presents recurring foundation and pavement problems because of its high liquid limit and plasticity index. This study quantified the effect of Fly ash (FA), a Class F pozzolanic industrial by-product, on the consistency limits and specific gravity of Chikoko soil. FA was blended with the natural Chikoko soil at 0, 5, 10, 15 and 20% by dry weight of the soil, and each mixture was tested in accordance with ASTM D4318-17 (Atterberg limits) and standard specific gravity procedures. Increasing FA content produced a consistent reduction in liquid limit (LL) from 39.0% to 24.0% (−38.5%), a reduction in plasticity index (PI) from 12.0% to 3.8% (−68.3%, with a minimum of 3.0% at 15% FA), and a decline in specific gravity (Gs) from 2.65 to 2.55 (−3.8%). Regression analysis of variance (ANOVA) confirmed the trends were statistically significant for LL (F (1,3) = 66.67, R² = 0.957, p < 0.01), PI (F (1,3) = 24.56, R² = 0.891, p < 0.05) and Gs (F (1,3) = 625.0, R² = 0.995, p < 0.001), while the plastic limit (PL) showed a weaker, non-monotonic response (F (1,3) = 5.53, R² = 0.648, p > 0.05), peaking at 29.6% at 5% FA before declining to 20.1% at 20% FA. These results demonstrate that FA is an effective, low-cost stabilizing agent that measurably reduces the plasticity and moisture affinity of Chikoko clay, supporting its use as a sustainable subgrade and embankment stabilizer in the Niger Delta.