Assessment of Soil Index Properties, Asphalt Thickness and Drainage Design as Factors in Pavement Distress: A Case Study of Agbaje–Ijokodo Residential Road, Southwestern Nigeria

Abstract

Road infrastructure plays a crucial role in economic growth, mobility, and social development, yet premature failures remain a recurring challenge in many developing regions. This study investigates the geotechnical properties of subgrade soils, pavement asphalt thickness, and drainage conditions along the Agbaje - Ijokodo residential road in Ibadan North Local Government Area, Oyo State, Nigeria, with the aim of identifying the underlying causes of early pavement deterioration. Fieldwork involved reconnaissance surveys, visual inspections, measurement of asphalt thickness at 26 failed locations, and collection of disturbed soil samples for laboratory analysis. Tests conducted included grain size distribution, Atterberg limits, compaction, California Bearing Ratio (CBR), and consolidation. Results revealed that the soils are predominantly fine-grained, with fine content (38.2 - 67.7%) exceeding the Federal Ministry of Works and Housing (FMWH) limit of 35%. Most samples exhibited high liquid limits (>35%), high plasticity indices, and linear shrinkage above the 8% threshold, indicating poor engineering quality. Natural moisture contents (17–40%) frequently surpassed the FMWH limit of 16%, while unsoaked CBR values (13–15%) fell short of the 30% requirement for sub-base materials, rendering the soils suitable only as subgrade support. Compaction tests yielded high optimum moisture contents (17.7–18.6%), and consolidation results confirmed medium to high compressibility, further suggesting settlement risk and moisture sensitivity. Asphalt pavement thicknesses were grossly inadequate, ranging from 0.8 to 1.7 in. (average 1.1 in.), below the FMWH/AASHTO minimum of 2.0–2.5 in. for residential roads. Additionally, drainage systems were poorly designed, undersized, or blocked, with some sections lacking drainage entirely. The findings attribute premature failure of the Agbaje - Ijokodo road to the combined effects of weak soil properties, insufficient asphalt thickness, and inadequate drainage infrastructure. Sustainable rehabilitation will require soil stabilization, improved drainage design, and reconstruction with pavement layers that meet established engineering standards.