sand and gravel resources

sand and gravel resources Sand and gravel are geological materials of great commercial value that tend to be taken for granted. They are familiar as piles of material in the builders' yard, and their significance is easily overlooked. If they ever think about such materials, most people probably regard them as boring, and certainly unexciting, and overlook their significance for modern life. Gravels are, however, important to the aggregates and construction industries for making concrete and for road-building. They are also used, to a more limited extent, for railway ballast. Sand is in demand as an abrasive, for mortar, for making glass, as a filter medium in the treatment of water supplies and sewage, and in foundries to make moulds for casting.

Materials such as sand and gravel should ideally be close to the places where they are to be used. Needless to say, this is not always the case; and where it is, there is inevitably competition between extraction and the use of land for other purposes. There is an imbalance between demand and supply of sand and gravel in many countries. The main sources of gravel are unconsolidated superficial or drift deposits of Quaternary age. These may be of fluviatile, glacial, periglacial, lacustrine, aeolian, or marine origin. The estuaries of the River Thames and the River Trent provide large quantities of sand and gravel in the United Kingdom. Elsewhere, beaches and marine sands are used. Marine aggregates are an important source in northern Europe, the United States, and Japan, although some would question whether a relatively scarce resource such as shingle should be used for aggregate when crushed rock would be equally suitable.

Unconsolidated materials such as sands and gravels are usually extracted using mechanical excavators, shovels, loaders, floating cranes, or dredgers. Marine sands and gravels are excavated from the sea bed by dredgers.

After excavation, the material is sorted into various grades according to size, using vibrating screens or by hydraulic methods, scrubbed to remove adhering particles of silt or clay, and then treated to remove water.

In concrete, the role of gravel, as for any other aggregate material, is to provide bulk and strength. The quality of the material is therefore important. Sampling and testing are essential to establish its mechanical and other properties before use. Low porosity is, for example, required for road paving and for concrete. Chemical properties can also be important; surface spalling and serious staining can result if only one particle in about three thousand contains a significant amount of iron sulphide. Opaline silica, if present, can cause expansion and cracking. Sands are subjected to comparable tests.

K. Vala Ragnarsdottir and and B. Wilcock

Bibliography

Manning, D. A. C. (1995) Introduction to industrial minerals. Chapman and Hall, London.