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This study researched into how new and existing highways in Nigeria can be designed for construction or reconstruction to check under design and premature distress and accommodate the growing numbers of heavy vehicles. It presents an analysis of the determination of the thickness of flexible pavement for Lagos- Ibadan Expressway as designed using the CBR method, the AASHTO method, the Asphalt Institute’s method and the Overseas Road Note 31 method and a comparative analysis to show the inadequacies of the existing CBR method in use in Nigeria.
The study involved the determination of traffic volume on Lagos- Ibadan Expressway, axle load measurement of the vehicles and material testing and evaluation to determine the different inputs to be used for the various design methods.
The study uses traffic volume studies with 2009 as the baseline year. The results show that the Average Annual Daily Traffic (AADT) on the expressway was 37,996 vpd with about 19,369 vpd in the heavier direction. Heavy duty vehicles constitute an average of 18.41% of the ADT with about 3,642vpd in the heavier direction.
The axle load measurement showed that most of the heavy vehicles were overloaded resulting in high vehicle damage factors of 11.12 and 9.39 for the Ibadan bound (northbound) and Lagos bound (southbound) vehicles respectively. The average daily ESAL was determined to be 34,199 in the heavier direction with a projected cumulative value of 117,138,497 over a design period of 10 years at a 4% growth rate.
Subgrade CBR was found to be 19%, sub base CBR was found to be 22% and aggregate CBR assumed to be 80%.
These values were used to determine the pavement thickness using the different afore mentioned methods and thicknesses were comparatively analyzed. It was concluded that the CBR method should be discontinued as pavement design method in Nigeria in favour of the other methods: AASHTO, Asphalt Institute and Overseas Road Note 31 methods.
The portion of the highway most obvious to the motorist is the road pavement, the surface of a road on which vehicles will travel. Natural ground cannot support the wheel loads of vehicles- particularly when wet. The road pavement provides traction for vehicles to travel as well as transfer normal stresses from the vehicle to the underlying soils (sub-grade). The pavement reduces the stresses on the subgrade to such a level that the subgrade does not deform under the action of traffic. At the same time, the pavement layers themselves need to be strong enough to tolerate stresses and strains to which the layer is exposed (Rolt, 2004).
Pavement design gives the specification of the various layers that make up the pavement in terms of their thickness and constituent materials.
1.1.1 Methods of Pavement Design
Methods of pavement design can be subdivided into two main groups:
1. Methods derived purely from empirical studies of pavement performance;
2. Methods which make use of the calculated stresses and strains within the pavement (theory), together with studies of the effect of these stresses and strains on the pavement materials (mechanistic behaviour). These are usually called ‘mechanistic methods’, ‘theoretical methods’, or, simply ‘analytical methods’
The two methods are complimentary. Empirical methods require some theoretical understanding to help extend them to different conditions, while mechanistic methods require empirical information for calibration. Neither method is ideal on its own, but the combination of the two provides a competent basis for design (Rolt, 2004).
Only a small percentage of the area of the surface of a road needs to show distress for the road to be considered unacceptable by roads users. It is therefore the weakest part or the extreme tail of the statistical distribution of strength which is important in design (TRL (2003): Overseas Road Note 31 (ORN31)).
1.2 PROBLEM ANALYSIS
In Nigeria today, it is no longer a surprise to drive along dilapidated road pavements. Many roads, instead of providing safe passage to destinations, have long become death traps. This situation is on the increase and fast becoming the order of the day.
However good the pavement design, the condition of the road will slowly deteriorate with time and traffic. The long-term behaviour of the road will also depend on the maintenance that is undertaken. The aim is to design the road to carry traffic satisfactorily for a safe period of time without needing major structural maintenance (Rolt, 2004).
An inquiry into the situation of the nature and condition of road pavements in Nigeria has shown that many of these roads have failed to perform optimally during their design life. They have witnessed frequent deterioration and failures along significant stretches, rendering many sections of the road impassable few years after completion. These pavement failures may be attributed to several factors, which include geotechnical properties of the soil, topography and drainage, climate, depth to water table, geology, and design (under design)and construction materials (Osadebe and Omange, 2005). Abam et al (2000) attributed many of these road failures to the use of foreign standards, particularly the British Standard (BS) and the American Standard (ASTM) that have no regard for local environmental and ecological peculiarities.
Also the roads in Nigeria at the time of design were not intended to be the main travel route for heavy duty vehicles carrying goods from one axis of the country to the other, which have significant damaging effects on the road. The increasing percentage of heavies can only be a reflection of the ineffective railway system in Nigeria.
1.3 AIM AND OBJECTIVES OF THE RESEARCH
The degenerate condition of the roads in Nigeria is unacceptable, and obvious steps have to be taken to revert this. This study aims to provide insight for pavement design engineers into the particular pavement design procedure(s) appropriate for road design in Nigeria.
The specific objectives of this study are to:
1. Review the prevailing pavement design method(s) in Nigeria as well as various alternative design methods available.
2. Investigate the trend of growth of vehicles on the Lagos- Ibadan expressway.
3. Make comparative assessment of the prevailing design method and the alternative methods and recommend an appropriate pavement design procedure for Nigeria.
1.4 RESEARCH QUESTION
In the absence of an adequate railway system in Nigeria, how can the pavement thickness of new and existing highways in Nigeria be determined for construction or reconstruction to check under design and premature distress and accommodate the growing numbers of heavy vehicles?
1.5 SCOPE OF THE STUDY
This study reviews design procedures for new flexible pavements with the view to identifying the most desirable procedure(s) to be used for the design for construction and reconstruction of new and existing Nigerian roads. The study is limited to research on a major federal highway: Lagos-Ibadan expressway.
1.6 SIGNIFICANCE OF THE STUDY
This study is significant in the sense that it will bring to light method(s) to be used for pavement design in Nigeria after careful consideration of all the local environmental and ecological factors to ensure the pavements live out their expected lifetime without underperforming.
1.7 DEFINITION OF TERMS
New Pavement is a pavement structure placed on a prepared sub-grade. It applies to new highway construction, to a relocated highway, or to the new part of a widened highway.
Pavement Reconstruction Reconstructed pavement or full depth reconstruction results when an existing pavement structure is completely removed to the sub-grade and replaced with a new pavement structure. This type of work is needed when the existing pavement has deteriorated to such a weakened condition that it cannot be salvaged with corrective action. The type and extent of pavement distress will determine when pavement reconstruction is necessary.
Sub-grade The undisturbed virgin substrate or embankment material which the pavement structure is placed upon.
Design Life The terminal pavement condition at which partial reconstruction or a major overlay would be necessary.
Bearing Ratio The load required to produce a certain penetration using a standard piston in a soil, expressed as a percentage of the load required to force the piston the same depth in a selected crushed stone. Bearing Ratio values are normally determined using the California Bearing Ratio (CBR) test method.
Design Bearing Ratio (DBR) The selected bearing ratio used to design the pavement. It is based on a statistical evaluation of the CBR test results on the soil samples.
Soil Support Value (SSV) – An index of the relative ability of a soil or stone to support the applied traffic loads. It is specifically used for the pavement design method in the AASHTO Interim Guide for Design of Pavement Structures. The soil support value of the sub-grade is related to its CBR (DBR).
Structural Number (SN) A measure of the structural strength of the pavement section based on the type and thickness of each layer within the pavement structure.
Layer Coefficient The relative structural value of each pavement layer per inch of thickness. It is multiplied by the layer thickness to provide the contributing SN for each pavement layer.
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