Highway subgrades in the Niger Delta region experience wide seasonal swings in moisture, and this study examines how such swings alter the stiffness that pavement designers rely on. Soil samples typical of the lateritic and sandy clay subgrades found across Delta State were compacted at moisture contents spanning four percentage points below to six above the optimum, then tested for resilient modulus under repeated load triaxial conditions consistent with published protocols. Stiffness fell sharply once moisture rose past the optimum point, with the sandy clay losing proportionally more of its modulus than the lateritic material at the same moisture excursion. A regression relationship linking resilient modulus to moisture deviation and dry density gave a coefficient of determination above 0.85 for both soil types. The results support drainage focused and moisture controlled compaction practice for Delta State roads and give a locally grounded input for mechanistic empirical pavement design in the region.

Keywords: compaction moisture, Delta State, lateritic soil, pavement design, resilient modulus, subgrade stiffness.