Abstract
Influence of spatial variation in subgrade properties on the integrity of highway pavement design is the burden of this research. Interruption, delay and discomfort in transportation flow that characterize roads across the globe due to poor pavement conditions are pathetic experiences. This work is an attempt to investigate the influence of subgrade variation on the design thicknesses of highway flexible pavements, using subgrade properties and traffic characteristics encountered along the Enugu-Onitsha expressway. Soil samples collected from selected sections of the expressway were subjected to geotechnical laboratory tests like particles size distribution, Atterberg’s limits, compaction and California bearing ratio in order to analyse their properties. The results generated from these three samples from different stations across the road showed that the soil materials along the Enugu-Onitsha expressway vary significantly. Sample from Nkpor, Awkuzu and Kwata stations contain 15%, 45% and 80% fines (silt and clay) respectively, likewise their MDD and OMC are 1852KN/m3, 1980KN/m3 and 1788KN/m3; 11.7%, 11%, and 14.6% respectively. The 48 hour soaked CBR values of soil samples from Nkpor and Awkuzu stations are 8% and 9% respectively while Kwata station is 2.3%. The plasticity index and liquid limit results were also 11%; 31% and 9%; 27.4% for Awkuzu and Kwata samples while Nkpor sample in a non-plastic soil. Based on ASSHTO classification, the soils fall within A-2-4, A-6 and A-4 categories which depicted a significant variation in properties, fair to bad. Based on the design outputs for pavement thicknesses, it was concluded that designing a length of highway with a single CBR value will be uneconomical for areas with significantly higher surface modulus as well as providing insufficient thicknesses for areas with lower actual CBR than the observed. Sectional pavement design at not more than 1km intervals was recommended.
Highlights
Over the years, significant efforts have been made in Nigeria towards the development of highway infrastructures, but despite the efforts the undue failure of road pavements at quite an alarming rate remains characteristic of most Nigerian roads
Kwata junction region is almost direct reverse of Nkpor soil sample because it contains about 80% fines and 20% sand, which entails that the soil is of high clay content
The Nkpor soil sample was classified as an A-2 (A-2-4) indicating that the sample is non-plastic silty or clayey sand which could be rated as good to fair subgrade/fill materials
Summary
Significant efforts have been made in Nigeria towards the development of highway infrastructures, but despite the efforts the undue failure of road pavements at quite an alarming rate remains characteristic of most Nigerian roads. Different stakeholders have been worried about the causes and possible remedies to this menace. It has been observed that not all the sections of the road pavements fail at the same time. Some sections of the road pavements still stand and some have outlasted their design life. From the Civil Engineer’s perspective, this scenario could be as a result of one or combination of some of these; carelessness workmanship, poor supervision and quality control, inadequate drainage provisions, ignorance of regular maintenance programs, insufficient initial soil exploration, among others. The influence of bad roads on traffic flow, travelers comfort, and environmental condition of an area cannot be over emphasized.
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