Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol.4, No.23, 2014 67 Geotechnical Properties of Subgrade Soils along Sections of the Ibadan–Ife Expressway, South-Western Nigeria Adeyemi, G.O 1 Oloruntola, M.O 2 Adeleye, A.O 3 1.Department of Geology, University of Ibadan 2.Department of Geosciences, University of Lagos, Lagos 3.Formerly of Department of Geology, University of Ibadan All correspondence to oloruntolamoroof@yahoo.com 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/m 2 and 9.60 -14.0 and 1810-2076 KN/m 2 , 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. Introduction Over the years, significant efforts have been made in Nigeria in the development of roads and highways infrastructures, with the construction of over 20,000 km of road network nationwide. While little efforts have been made in the development of rail networks and waterways, the roads and highways have been undergoing continuous developments. However, large sections of these roads and highways fail shortly after construction and/or reconstruction. The roads are continuously reconstructed or rehabilitated without any adequate effort to identify factor(s) responsible for their continuous failure, in order to provide long lasting solution to the incessant highway failure. Apart from the huge costs expended on the reconstruction and/or rehabilitation of these roads, failed portion have continued to serve as death traps to people through fatal accidents As noted by Gidigasu (1974), highway failure is a common phenomenon in the tropics, mostly occurring in form of rutting, pitting and waviness. The major contributing factors to highway failure in the tropics include misuse, over usage and poor construction (Ajayi, 1985). A number of studies by workers such as Adegoke-Anthony and Agada (1982), Adeniyi (1984), Anowai (1986), Teme and Anowai (1986), Olusola and Oloruntola (1998) have identified causes of road failure in Nigeria to include geotechnical factors, excessive haulage loads, poor construction, poor/inadequate drainage. However, studies by Gidigasu (1972), Mesida (1985), Ajayi, (1985), and Adeyemi (1992) have identified geotechnical factors as the cause of most road failures in the country. In many cases, subgrade and subbase materials on which the pavements are placed rarely meet the highway sub-grade/sub-base specifications. These situations are due largely to the fact that most times, road and highways are constructed without any, or sometimes with inadequate, geotechnical studies of the soil along the highways alignments and borrow pits, where materials for construction are won. A ride along the Ibadan-Ife highway will show that the older Ibadan bound carriage way is more stable than the newer Ife bound carriage way. This study is thus aimed at evaluating the geotechnical properties of the sub-grade materials beneath some sections of the highway, with a view to determining the factors responsible for the failure of some sections of the highway. The Ibadan-Ife highway (Fig. 1) is an important continuation of the Lagos-Ibadan highway, which aids movement of goods and services from Lagos and adjoining towns and cities to Northern part of Nigeria. The highway was upgraded to a dual carriage way quite recently but as typical of most highways in Nigeria, some sections of the highway have failed, just a few years