Geotechnical Characterization and Deformation Susceptibility Assessment of Lateritic Subgrade Materials along Six Road Corridors in Ondo and Edo States, Southern Nigeria

Residual soils formed by in-situ weathering are widely used as pavement subgrade in humid tropical regions, yet the reliability of plasticity-based classification for anticipating subgrade competence across contrasting weathering regimes remains poorly quantified outside site-specific case studies.

This study examines ninety-five subgrade soil samples collected along six road corridors totalling approximately 180 km in Ondo and Edo States, southern Nigeria, forming two geographically separate corridor networks that span basement-complex (schist belt) and coastal-plain sedimentary terrains.

Particle size distribution, Atterberg limits, compaction characteristics and California Bearing Ratio were determined in accordance with BS 1377:1990, using British Standard heavy compaction, and soils were classified using the AASHTO system.

Liquid limit ranged from 15.2 to 65.2%, plasticity index from 7.3 to 45.1%, maximum dry density from 1.13 to 2.43 g/cm³, and four-day soaked CBR from 1 to 47%.

Thirty-six samples (38%) classified as A-7-6, with a mean soaked CBR of 5.9%, and 61 of 95 samples (64%) recorded soaked CBR below the 10% benchmark adopted here for subgrade assessment.

Soaked CBR differed significantly among AASHTO groups (Kruskal–Wallis H = 23.3, p < 0.001), although the ordering was partial rather than monotonic, with adjacent groups statistically indistinguishable.

Plasticity index showed a stronger bivariate correlation with soaked CBR (r = - 0.49) than the clay-size fraction (r = +0.39), the latter being positive and sensitive to influential observations.

The results indicate that plasticity indices, rather than grain-size composition alone, provide the more dependable first-order screening of subgrade risk in this terrain.