River mining: alternative sources of aggregates. (CR/03/095N).
The availability of aggregate sources varies from place to place within a country, depending on the geology. There are two main sources of aggregates in Jamaica; sand and gravel from alluvial deposits, especially from the active river channels, and limestone. The latter dominates in the north and west of Jamaica and the major producers of sand and gravel are in the south and east of the island. River sand is often in short supply and is generally the only available source of natural fine aggregate, an essential component of concrete and other building products.
There is particular concern about the environmental effects of instream sand and gravel mining and thus there is a need to know to what extent alternative materials can augment or replace sand and gravel from this source. In Jamaica, the alternative sources include, marine sand and gravel, manufactured sand, river terrace deposits and recycled aggregates.
There has not been any production of marine aggregates in Jamaica but the environmental concerns of onshore extraction and land-use pressures require marine sources to be seriously investigated. Offshore sources are thought to exist off the mouth of the Yallahs river in Saint Thomas, Jamaica, but have not been investigated in detail. However, a range of resource assessment, environmental impact, production and investment issues need to be addressed prior to any development.
Fine aggregate can be manufactured by crushing and processing hard rocks such as limestone and sandstone to produce fine grained (sand-sized) material. The degree to which such crushed rock sand can replace natural sand varies with rock type, the degree of quarry processing used and end-use. Nevertheless, in many parts of the world such material is a major source of fine aggregate. In Jamaica, 'stone dust' is produced as a by-product of limestone quarrying and is used, principally, in concrete block making and in asphalt products. Jamaica has large resources of rocks that can be crushed to produce manufactured sand, but improvement in quarry crushing and processing plant is required to consistently produce manufactured sand of acceptable quality.
In Jamaica, substantial resources of sand and gravel occur within the floodplain and terrace deposits and beneath the agricultural lands of the major river valleys. These deposits are not currently worked for sand and gravel aggregates; extraction is largely restricted to the river channel deposits. Development of the floodplain and terrace deposits in Jamaica will ease the pressure on mining of river channel deposits, reducing the impacts of river extraction. Nevertheless, there are environmental and other impacts associated with the working of river terrace deposits. If these resources are to be developed in future, a range of studies will be required, including exploration surveys, hydrogeological investigations and environmental and social impact assessments.
Many mineral wastes fulfil the technical requirements to substitute for primary aggregates and many governments are now encouraging greater use of mineral wastes as aggregates. In Jamaica, the degree of recycling of mineral waste materials into secondary aggregates is very small, although some construction and demolition waste is used as fill.
In conclusion, there are several potential sources of alternative aggregate materials in Jamaica. There are, however, large resources of natural primary aggregate materials, although their extraction creates considerable environmental problems. The planning and management of aggregate resources must be based on the consideration of all possible sources. Therefore, the viability of alternative materials needs to be considered when formulating aggregate resource management plans.
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