Effects of Rainfall Erosivity and Soil Erodibility on Road Networks in Ota, Ogun State, Nigeria: Laboratory Geotechnics and GIS-Based USLE Assessment

Abstract

Road infrastructure in rapidly urbanising humid-tropical settings is increasingly exposed to water erosion, yet the joint influence of rainfall erosivity (the capacity of rainfall to erode) and soil erodibility (the susceptibility of soil to erosion) on road performance is seldom quantified in southwestern Nigeria. This study combined laboratory geotechnical testing with GIS-based soil-loss modelling to evaluate these controls for roads in Ota, Ogun State, using the Bells University of Technology road corridor as the detailed case. Disturbed soil was tested for natural moisture content, Atterberg limits, particle-size distribution and standard Proctor compaction (ASTM D2216, D4318, D6913 and D698) and classified under AASHTO M 145; erodibility, erosivity and annual soil loss were mapped in ArcGIS with the Universal Soil Loss Equation (USLE). The soil had a natural moisture content of 8.95%, a liquid limit of 31.1%, a plastic limit of 18.7% and a plasticity index of 12.4%. Maximum dry density was 1,659 kg m⁻³ at an optimum moisture content of 14.5%, and density fell sharply on the wet side of optimum. Dry sieving returned only about 1% passing 0.075 mm, probably an underestimate for this aggregated tropical soil, so the AASHTO class A-2-6 is provisional. The corridor lies on the Ndasin soil series; rainfall erosivity moved from the moderate class in 2011–2020 to the high class in 2021–2024; and mapped soil loss fell in the low and moderate classes (upper class limits of about 16.5–22 t ha⁻¹ yr⁻¹; state-wide maximum about 24.6 t ha⁻¹ yr⁻¹), well above commonly cited tolerable rates. Plastic fines, a narrow compaction-moisture window, poor drainage and unprotected slopes make the corridor vulnerable to progressive surface loss. Drainage maintenance, slope step-downs, moisture-controlled compaction and soil stabilisation are recommended.