Subgrade quality evaluation of Gbongan – Ode Omu highway, Southwestern Nigeria

Abstract

Incessant failure of the Gbongan – Ode Omu highway flexible pavement calls for investigation of the responsible factors. Soil samples from the stable (S) and unstable (U) sections of the highway were evaluated so as to determine their suitability as highway subgrade soils. X-ray diffractometry (XRD) was used to investigate the mineral composition while X-ray fluorescence (XRF) was used to access their geochemistry. Geotechnical properties investigated include specific gravity, grainsize distribution, consistency limits, moisture dry density relationship, soaked and unsoaked California bearing ratio (CBR), shear strength, permeability, and consolidation. Results showed that the schist derived lateritic soil (S) is made up of 26% iron oxide (goethite and hematite), 21% quartz, 16% amphibole, 15% kaolinite and 15 % epidote. The banded gneiss derived lateritic soil (U) soil contains 54% quartz and 21 % kaolinite while, feldspars and mica make up the remaining 25% of the soil. All the soil samples rated as fair to poor subgrade according to AASHTO classification. The S soils classify as low plasticity soils – CL while most of the U soils classify as high plasticity soils – CH. Soaked and unsoaked CBR ranged between 9% and 18% and 18% and 23% respectively for the U soils. It ranged between 16% and 23% and 19% to 28 % for the S soils. All the soil samples are rich in fines with low compressibility and permeability. The presence of significant of iron oxides in the S soil is believed to impact their physical properties.