Geotechnical Properties of Subgrade Soils along Sections of the Ibadan–Ife Expressway, South-Western Nigeria

Abstract

Subgrade soils beneath sections of Ibadan-Ife highway were studied with a view to identifying factor(s) responsible for the stability or failure of sections of the expressway.Bulk soil samples were collected from four stable locations on the Ibadan bound carriageway while four samples were collected from unstable locations on the Ife bound way. Specific gravity, grain size distribution, liquid limit, plastic limit, linear shrinkage, California Bearing Ratio (CBR) and Unconfined Compressive Strength (UCS) of the soils compacted at West African and Modified American Association of State Highway and Transportation Officials (AASHTO) levels were determined.The soils are essentially well graded with those from stable locations having between 25.42 & 56.89% amount of fines and 29.92 & 83.00% amount of fines in those from unstable locations. Variation in the degree of laterization of soil samples resulted in significant variation in the amount of fines in subgrades. Soils from the stable locations are predominantly medium plasticity soils while those in the unstable locations possess higher plasticity. Three soils from stable locations and two from unstable sections are good to fair subgrade soils while those from two unstable sections and one from stable location are poor subgrade/subbase materials based on the AASHTO classification system. The studied soils gave better compaction characteristics at modified AASHTO than at the West African level, with an Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) of 10.7-15.4 and 1850-2037KN/m2 and 9.60 -14.0 and 1810-2076 KN/m2, respectively for those from unstable and stable locations. Soaking of the soils compacted at both compaction levels resulted in over 60% reductions in strength measured in terms of CBR. Similarly at both levels of compaction, curing led to significant increase in the UCS, although, Modified AASHTO level generally gave higher values of UCS. Field observation showed that groundwater levels are generally higher in the vicinity of unstable locations than stable ones. The observed nature of the pavement is thus due mainly to the weakening influence of groundwater on subgrade soils.