Courtesy Wake Stone Corp.-- This is a view of an operating crushed stone (aggregate) quarry that is located near Raleigh, NC. Shown here are many of the elements of aggregate quarries. Granitic material is being mined. This crushed stone operation is located near an Interstate so the crushed stone product can easily be trucked to road construction sites and to concrete plants.
In the foreground diesel-powered 50-ton-capacity haul trucks are loaded by diesel-powered hydraulic shovels with 7.5 cubic yard bucket capacity. A drill is positioned on a 40-foot high bench. The drill is preparing holes to hold explosives which will break the rock into pieces that can be loaded into the trucks and taken to the primary crusher. The drill utilizes a compressed air powered 'hammer' in the bottom of the hole. This down-hole type drill helps minimize noise to the surrounding area.
Rock that is larger than can be accepted by the primary crushed is being broken by the crawler-mounted hydraulic machine in the upper left corner of the image. A water truck sprays water on rock that was recently broken by the explosives. The water spray helps reduce dust.
Portions of the conveyer assembly is shown in the upper right. This conveyer assembly carries rock to the sizing plant located on the surface just out of view to the upper right corner. Rock up to 5 feet by 4 feet x 4 feet are crushed to not more than 12 inches in diameter by the primary jaw crusher. This material is further reduced in size by the secondary cone crusher. The primary crushing plant is now located in the pit just out of view in this image.
CRUSHED STONE (Aggregate) overview
In 2000, North Carolina was ranked eighth in the United States in crushed stone production. Nearly 70,000,000 metric tons of crushed stone were produced with a value of about $491 million. Click here for a graph showing the dollar value of crushed stone and construction aggregates in North Carolina for the period 1970 - 2000. Click here for a similar graph but for the amount in thousands of metric tons.
Crushed stone, from operations like this, is used to make concrete and asphalt roads, concrete for homes and office buildings, and is used in a wide variety of erosion control projects. Aggregate operations are relatively long lived and are vital to the economic health of communities.
Mines are permitted before production begins. A comprehensive reclamation plan is part of the permitting.
Aggregates are construction materials of crushed stone, sand and gravel. About 10 tons of aggregates are required annually for each North Carolina citizen. A typical residential subdivision requires about 300 tons of aggregate per home.
The single largest market for aggregates is road and street construction, including base and asphalt paving for highways, parking lots and other pavements. One mile of typical 2-lane asphalt road with aggregate base requires about 25,000 tons. Other large markets are portland cement concrete for bridges, pavements and building structures, riprap and erosion control stone, and railroad ballast.
Approximately 50 percent of all aggregate is used for publicly funded construction projects¿i.e., highways, water and sewer systems, public buildings, airports and other county and municipal public works projects.
Crushed stone makes up 85 percent of aggregate production; construction sand and gravel, about 15 percent.
North Carolina is the eighth largest crushed stone producing state in the U.S. Aggregate is produced from about 135 crushed stone quarries and about 500 sand and gravel sites throughout the state. Crushed stone, sand and gravel plants account for 85 percent of all permitted mining operations. There is aggregate mining in 80 of North Carolina¿s 100 counties.
The average production life of a crushed stone quarry is 40 to 50 years or more. Sand and gravel deposits are typically worked out in much shorter time.
The aggregates industry in North Carolina employs just over 3,000 people. The average number of employees at an aggregate production site is about 20.
The year-end average price of aggregate was $6.80 per ton at the plant site. Of all North Carolina mineral products, construction aggregates represent 84 percent of total quantity and 69 percent of total value.
Over 90 percent of all aggregate is moved by dump truck. Most aggregate is used within 40 miles of where it is produced. Some aggregate is moved by railroad, and a small amount is moved by barge off the Carolina coast.
Because aggregate is a heavy, low cost per ton product, haul distance largely controls the price of aggregate. Truck haul costs approximately 12 to 15 cents per ton-mile. Because of the high cost of transportation, virtually all aggregate produced in North Carolina is used in North Carolina.
The North Carolina Mining Act, passed in 1971, and its regulations require any aggregate mining operation of one acre or more to apply for and receive North Carolina mining permit. The permit is issued for 10 years, but can be withdrawn if all requirements of the permit are not complied with. Many other local and state permits and environmental regulations control the operation of aggregate plants. All sites must be reclaimed before the reclamation bond will be released at the conclusion of mining.
Once reclaimed, crushed stone quarries are desirable as water reservoirs or recreational lakes, often with residential subdivisions nearby. Sand and gravel sites, also, are useful as wildlife habitat, agricultural fields or as lakes for a variety of uses, including groundwater recharge. Wetlands are often created as a result of mining. The industry is very sensitive and responsive to its need to be a good steward of the environment and a good neighbor in the community.
A ready supply of crushed stone, sand and gravel is necessary to support future economic development. The biggest concern facing the aggregates industry in coming years is obtaining zoning favorable to the extraction of aggregates. Near urban areas where construction materials are critically needed, it is most important to allow appropriate zoning and the necessary permits to assure a continued supply of aggregates.