Engineering Geology of Failed Sections of Isua – Idoani Road Southwestern Nigeria

Abstract

There is an increasing incidence of road failures in Southwestern Nigeria Many asphalt roads have failed sections soon after construction and in any case rarely reach their design life before major repair is required.Numerous studies of failed sections of roads in Southwestern Nigeria have suggested the link between underlying geology and road failures.Engineering geological investigations of failed sections of Isua-Idoani road in Ondo State, Southwestern Nigeria has been were conducted with a view to determining the influence of the geology to the incidence of road failure along the stretch of the alignment. The geological mapping of the road alignment reveals that the road stretch was founded on two different lithologies, which are quartz-schist and granite-gneiss. The natural moisture content of the subgrade soils ranges from 6.5 to 19.6%, liquid limit from 24.1 to 55.1%, plastic limit from 9.1 to 21.2, plasticity index 5.2 to 21.2%. The grain size analysis shows that the amount of fines ranges from 29.2% to 60.8%. The linear shrinkage between 5 and 11.4%, free swell between -16.67 and 41.94%, maximum dry density between 1520 Kg/m³ and 1780Kg/m³.The results of the study show that the lithology is not the dominant reason for the failure of the road sections studied. Similar failures occur on stretches founded on the quartz-schist as well as granite gneiss (see Adeyemi 1995 and Jegede 1998). The poor geotechnical properties of the subgrade soils as indicated by the low maximum dry density; high linear shrinkage, high liquid limit values, high amount of fines and particularly poor drainage is the greater culprit in these cases.

The results of the study suggest that closer attention need to be placed on the groundwater regimes of streams and rivers valleys along roads.

Keywords: Engineering Geology, Road pavement failure, Geotechnical properties, Southwestern Nigeria