The Webster dictionary provides a simple definition for gravel as loose, rounded, fragments of rock. Geologically, and more precisely, gravel is defined as a natural accumulation of rounded rock fragments which consists mostly of particles larger than sand (greater than 2 mm in diameter), but which often naturally occurs as a mixture with sand and silt. Commercially though, the definition is more functionally demanding and requires that the aggregate be composed of clean, properly shaped and sized particles that are tough, durable and sound, and that retain their uniform physical and chemical properties over long periods of time in the uses in which they are applied — uses that range from ornamental landscape aggregate, filter medium, construction fill to component of concrete and asphalt. The sand and finer fractions of rock particles recovered from gravel deposits have their own commercial uses. In our daily lives we see hundreds of different applications for gravel and sand, each having its own special requirements for size, appearance/color, hardness, shape, durability, inertness, etc.
Significant sources of sand and gravel include the deposits of unconsolidated sediments present in the flood plains of rivers. These are known as alluvial gravels. Walk along a stream bank and see for yourself what these river sediments look like and how the grain sizes of the rock and sand mixture ranges from tiny particles of silt to boulders.
Typically deposits of alluvial gravel are mined using surface extraction methodology utilizing power shovels and front end loaders. The choice of excavating equipment is based on whether the gravel is mined dry or wet. Pits of varying shape, size and depth are excavated during the process of extraction. During the mining process excess water may have to be pumped from the pits and when extraction is completed the pits may fill with water derived from the surrounding aquifer.
In dry pits the excavated mixture of sand and gravel is loaded into trucks or onto conveyor belts for transport to the processing plant where it is cleaned and sized, and prepared for transport to customers by either truck or train.
Many processing plants produce a variety of size mixtures in order to meet the needs of diverse end-uses and customers. To achieve this, the gravel and sand mixture brought to the plant from the quarry pits might be washed to remove fine particles and organic matter and then classified into various particle sizes by passing the feed through a series of vibrating screens. Some facilities mechanically crush the larger sized materials that are recovered.
For background on the current gravel mining site plan near Lyons, see our page on Martin Marietta’s Special Use Permit.