- Contributed
A helicopter view of a quarry established by Bouygues Travaux Publics to serve the Highway 2000 project.
MARL IS used extensively in Jamaica for different aspects of civil engineering and general construction works, and has become the material of first choice for road construction projects.
The road construction sector is one of the largest consumer of aggregates.
Marl, as defined by the World Book Encyclopaedia, is a type of rock that consists of almost equal amounts of calcium carbonate and clay, and is formed by the layered accumulation of other rocks and mineral fragments.
The structure of a road consists essentially of a sub-grade, a sub-base, a road base and a wearing surface usually of bituminous material (asphalt).
The sub-grade is the bottom of the road structure on which the pavement layers are constructed. If the sub-grade is made of weak soil, then the thickness of the road structure is usually greater in order to prevent overstressing.
The sub-base rests directly on the sub-grade and is usually the largest portion of the volume of the road structure, ranging from a minimum of 150 mm (6 inches) to as much as 225 mm (9 inches), but this could be as much as 750 mm (2 feet 6 inches) if capping is used.
The sub-base is required to perform at least four functions:
- It provides a structural layer which distributes loads to the sub-grade
- It provides a working platform for construction traffic and a base for accepting the road base and bituminous layers
- It acts as a protective layer and works with the bituminous layer to help keep the sub-grade from the effects of weather.
- It may provide a drainage layer to remove water from the pavement
The marl fill serves both as the foundation of the road (sub-base) as well as the main structure (road base), which together are responsible for supporting the wearing surface and ultimately the traffic loads.
MARL IN ROAD CONSTRUCTION
The use of any engineering material must be based on a thorough understanding of the nature and characteristics of that material. Otherwise it will be difficult if not impossible to obtain the desired results.
Marl can contain as much as 50 per cent clay, which places severe limits on its integrity when exposed to moisture. In the construction process, the carriage way is first excavated and the sub-grade rolled - the latter is not always done - then marl is applied in layers and compacted in the presence of moisture to the required thickness.
The thickness of the marl is important, and so is the degree of compaction, as well as the weight of the roller that is used, as these will determine the structural capacity of the road in terms of the safe traffic load that it can support without failure.
Of equal importance, the road should always be constructed in such a way that water neither settles on the roadway nor runs for any significant distance along its surface.
After compaction the surface must be sealed and the final overlay of the roadway applied. The thickness and composition of this final surface finish, whether bituminous or cement based, is important as this will be exposed to traffic and must withstand daily wear and tear as well as wide variations in weather conditions.
The overlay as well as all edges of the roadway must be so treated that seepage of moisture into the marl sub-base is completely prevented. If significant seepage occurs, then the marl will begin to show signs of expansion and failure will occur. After that it will only be a matter of time before the roadway begins to break up and eventually disintegrate.
The failure of Jamaican roads is generally blamed on marl. However, this is due in part to a misunderstanding of the role that marl should play in the overall construction of the road. The real problem is in the construction process, that is how the marl is applied; while a related issue is the quality of the marl, which can be controlled for by testing the material before it is accepted for use.
A major problem is that some quarries do not have the approval and/or license of the Jamaica Bureau of Standards. In addition the consistency of marl varies widely even from the same quarry and in the absence of on-site equipment that can make these assessment quickly and accurately, the problem is bound to continue.
Marl does have several shortcomings, but it is still the most viable material for road construction in Jamaica. It is therefore in the country's interest to develop the technology for maximising its use and minimising its abuse.
Contributed by Alvin Savage, civil engineer and member of the Jamaica Institution of Engineers.
