1.0 INTRODUCTION 

Road pavement construction has experienced many significant problems mainly in sub-base and base design and construction over the years in this country (Nigeria).

Roads in Osun State have a very high volume of traffic which comprises of many type of commercial vehicles, cars and trucks. Roads have been constructed for more than 2000years. Even the earliest engineers practiced drainage of the pavement’s sub-surface layers in order to keep the highway functioning. Nowadays, many billions of Naira are spent on road construction and maintenance each year, and road transport in the primary method of mobility for Nigeria people or goods, Transport constitutes about 10% of the gross national product of the Nigerian union and more than 80% of the total transport of people and goods is provided y the infrastructure. Investment in road construction and maintenance in Nigeria is therefore at a very high level and any improvement can have a significant effect on the Nigeria economy. Not only will well maintained roads and highway contribute economically, for they are also essential for social development of Nigeria. On the other, the deterioration of roads can have serious consequence for the safety and comfort of roads user.

In many countries premature break-down especially of low volume asphalt roads, is an accelerating problem increasing traffic and traffic roads demand higher performance levels from pavement if premature rutting and cracking is to be avoided. At the same time climate change means that highway are likely to be exposed to more rain and to greater thermally induced cracking that can lead the run-off into the structure. Once in the pavement or embankment, plays a primary role in giving shorter service life and in increasing the needs of rehabilitation measures. A further problem that is becoming of more and more concern 1is the possibility that the road is a source of contaminants that are having an undesirable effect on the hydro1ological environment. Two sources are possible.

(i) Contaminant in runoff from rainwater or snowmelt that flows over the top `of the pavement and than soaks into the ground.

(ii) Leaching of contaminant from the roads construction materials and earthworks.

             According to E.J Yoder and M. w Witczak (1975), the foundation of the roads which comprises of both sub-base and base courses should be well compacted and to make them free from water. Most design techniques strength test made on sub-grade samples that is in soaked or nearly saturated condition. This has been saturated practice for many years since it is assumed that the saturated case is the controlling one. On the other hand, it is known that even under certain desert condition water accumulate in sub-base and base course under a pavement and the moisture content of the soil can become high.

The in – place moisture content or ground water in sub-base and base course under flexible pavement is influenced by amount of rainfall and high-rise of water table.

In roads, two type of pavement are considered for use in roads construction.

I. Rigid pavements 

II. Flexible pavements

A flexible pavement : As defined by AASHO is a pavement structure which distributes loads to the sub- grade that is, the foundation of the roads and this relies on the bituminous aggregate  interlock, particle friction and collation for stability, the strength of the supporting foundation, the quality materials and the construction procedure.

Rigid pavement: it can be as a pavement structure which distributes loads evenly to the sub- grade i. e foundation having one course, a Portland cement concrete slab of relatively high binding resistance. The foundation of the roads which comprises of the structure of a concrete pavement slab should be designed for the loads, in order to render year of service. The factors below should be considered

1. The amount, type and weight of present anticipated traffics

2.        The depth o ground water should be considered.

3.        The strength and quality of concrete to be used.

4.        Supporting power and character of the sub- grade.

5.        The climate of the region (Osun State) in which the pavement is to construct. 

Also for a road to be constructed for this purpose, certain criteria have to be considered.

I. Traffic density and the vehicle pattern.

II. The economic importance of the roads

III. Topography of the area.

IV. Weather duration.

1.1 SCOPE OF THE STUDY

The study focus on various source by which base and sub- base cause of road pavement get in contact with water and proffer solution to curtail the ingress of water in to the road pavement structure so that the road pavement structure at Iree – Ibokun in Osun state as a case study well serve beneficial community up to the expected optional level.

1.2 AIMS AND OBJECTIVE

The primary aim of the project is to investigate the primary cause of roads pavement failure on the structural stability of sub- base and base course of typical flexible pavement at Iree- Ibokun road in Osun state.

The primary objectives of the project are basically on 

I. To identify, investigate and control contaminants leaching from soils natural aggregate and by- productivity and moisture condition.  

II. To examine the geotechnical properties of the materials 

2.2.1 LIMITATION OF THE STUDY

The limitation to this research work is due to.

1. Economic situation of the country which lead to inadequate finance.  

2. Time there is limitation period of time to carry out the project.