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Economic importance of road has increased enormously over the last decades in Nigeria and to a large extent, they are complementary to other modes of transportation. However, the deterioration of a road begins soon after construction. Some of the roads in Nigeria do not last up to 5 years before failure becomes imminent. Road failure has been defined in different ways by various authors. According to [14], a road stretch is said to have failed when a part or all the parts of the road surface or any part of the road structural section or the entire road prism or pavement at any point along the highway is deformed and rendered un-motorable. Highway failure occurs when the road no longer performs its traditional function of providing continuous smooth surface for minimal frictional movement of vehicles. When such failure occurs before the anticipated design life, it is rightfully described as premature [5]. Highway premature failure occurs Abstract Mineralogical, geochemical and geotechnical analyses were carried out to evaluate the contributions of geological and geotechnical properties of subgrade soils to the incidence of failure of the Ipele-Isua road, southwestern Nigeria. Field observations showed that the road is in a very poor state of disrepair as major portions of the road alignment have failed. The road alignment is underlain in places by Quartz Schist, Banded Gneiss and Granite Gneiss. The results of the laboratory tests revealed that the natural moisture content ranges from 6.5 to 23.3%, liquid limit from 20.1 to 55.1%, plasticity index 5.2 to 22.2%. The grain size analysis showed that the amount of fines ranges from 13 to 60.8%. Others are linear shrinkage between 2.9 and 11%, free swell between − 16.67 and 41.94%, maximum dry density from 1520 to 1792.1 kg/m 3 and CBR between 2 and 17%. The specific gravity ranges from 2.51 to 2.63. The major clay minerals present are kaolinite and dickite with dominance in 50% of the samples. Major oxides present are SiO 2 , Al 2 O 3 , Fe 2 O 3 , K 2 O. Na 2 O, MgO and CaO are generally less than 1%. Based on AASHTO classification, 19% of the samples are classified as A-2-4, 33% classified as A-2-6 while 44% classified as A-7. Dominance of low California bearing ratio, low maximum dry density, high linear shrinkage and A-7 soil group expectedly contributed to poor geotechnical properties of the subgrade. Absence of drainage is suspected to have worsened the failure as exhibited on the road.

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