Contributions of Engineering Geological Properties of Subgrade Soils to Premature Failure of Major Highways in Southwestern Nigeria: A Case Study of Akure-Ikere Ekiti Highway

: Geotechnical analyses were carried out to examine the contributions of engineering geological properties of subgrade soils to the failure of the Akure-Ikere Ekiti road, Southwestern Nigeria. Field observations revealed that the road is in a very poor state of serious deformation and disrepair as most parts of the road alignment have failed. The alignment of the studied road is predominantly underlain by Granite, Charnockites, and Migmatites. Laboratory tests results showed that the natural moisture content ranges from 10.98 to 21.4%, liquid limit from 22.8 to 47.7%, plastic limit from 19.2 to 24 .6, plasticity index 3.6 to 26.3%. The grain size analysis revealed that the amount of fines ranges from 15.9 to 49%. Others are linear shrinkage, between 1.4 and 10%, free swell between 25 and 46%, maximum dry density from 1593 to 2016 kg/m, and CBR between 5 and 48%. The specific gravity ranges from 2.64 to 2.74. With reference to AASHTO classification, 5% of the samples was classified as A-4, 15% classified as A-2-4, 40% classified as A-6, while 40% classified as A-7-6. The dominance of fair-to-good California bearing ratio, fair to good maximum dry density, high linear shrinkage and A-7-6, A-6, and A-2-4 soil groups have combined to give fair-to-good geotechnical properties to the studied soils. Generally, the fair to good geotechnical properties of soil of the road under study is an indication that the contribution of subgrade soil to the failure of the highway is negligible. The total breakdown of the road can be traced to substandard engineering specifications which are complemented by a poor drainage system.