EFFECT OF CALIFORNIA BEARING RATIO OF SOILS USED AS SUBGRADE MATERIALS ON THE BENIN-AKURE ROAD, NIGERIA

Abstract

An investigation was carried out to assess the suitability of soils brought in for use as subgrade materials for the Benin- Sobe axis of the Benin – Akure road by testing the bearing capacity using the California Bearing Ratio (CBR)test. The road passes through four soil parent materials: - Coastal Plain Sand- CPS, Imo-Shale (IS), BendeAmaki (BA) and the False Bedded or Basement Complex (FB).Soil samples were collected from four profile pits based on the parent materials. Soil samples were also collected about 50 cm from the tarred road representing materials brought in for the subgrade. The soils were analyzed for physicochemical properties and the California Bearing Ratio. Analyses were carried out on soils collected from each section of the parent materials.Results obtained showed that there were more road failures in the Imo Shale,BendeAmeki and to a less extent in the Coastal Plains segment of the road, and least in the False Bedded parent material section.CBR for unsoaked soils showed BendeAmaki having highest values (14.6 and 16.18) for the 2.5 and 5 mm penetration test; however values were not significantly different from the other parent material sections ( p> 0.05).The Imo Shale soils were poorly drained and are seasonally floodedand had the least CBR values (4.64, 5.2 for the 2.5 and 5 mm readings for the top and bottom penetration test. Coastal Plain Sands and BendeAmeki values were not significantly different from the Imo shale but values were higher.Soaking the soil did not significantly affect the soil strength for the False Bedded segment of the road; hence the failure rate was lowest.The False Bedded had higher CBR values for the soaked test, indicating that the parent material through which the road is built had significant effect on the failure rates