Geotechnical Evaluation of Subgrade Soils and their Implications for Road Construction: A Case Study of the Oru-Ijebu Igbo Road, Southwest, Nigeria

This study presents a geotechnical evaluation of subgrade soils along Oru-Ijebu Igbo Road, Southwestern Nigeria, with the aim of determining their suitability for road construction. A total of twenty-six (26) samples, including thirteen (13) disturbed and thirteen (13) undisturbed, were collected at depths between 0.4 to 1.0 meters from both failed and stable sections of the road. The samples underwent various tests following the British Standard code of practice, including natural moisture content, grain size distribution, specific gravity, Atterberg limits, California Bearing Ratio (CBR), compaction, and shear strength (Triaxial). The results revealed moisture content values between 6.6% and 10.5%, indicating that the soils require low moisture for optimal compaction. Grain size analysis showed that fine grains constituted 21.0% to 67.0%, while coarse grains ranged from 1.0% to 27.0%, indicating poor subgrade quality due to the high fine grain composition. Specific gravity values ranged from 2.59 to 2.62, with 8 out of 13 samples meeting the standard, suggesting notable laterization. Atterberg’s limits indicated low clay content with liquid limits between 42.0% to 44.0% and plasticity indices between 27.1%, 29.0%, classifying the soils as having intermediate, plasticity.  Maximum Dry Density (MDD) ranged from 1770 kg/m³ to 1887 kg/m³, while Optimum Moisture Content (OMC) ranged from 10.0% to 16.0%, suggesting the soils require further compaction. CBR values for unsoaked samples ranged from 43% to 84%, indicating good subgrade quality, except for samples 1, 3, 4, 5, 9, 11, and 12. Cohesive strength values were moderate, ranging from 65.0 kN/m² to 80.0 kN/m², with an internal friction angle of 11º to 15º. Based on the AASHTO classification, 7 samples were rated as fair to poor (A-7-5), and 6 samples were rated as excellent to good (A-2-7). Ultimately, only six (6) samples met the Nigerian Federal Ministry of Works and Housing (FMWH) standards for road construction, emphasizing the importance of thorough geotechnical investigation. Results indicate significant variability in soil properties along the Road align, with some sections exhibiting low bearing capacity, high plasticity and suboptimal compaction behavior-conditions that pose challenges to pavement performance and long term structural stability.