Crushed Stone in NC

Crushed Stone/Aggregate Overview

Aggregate is the technical name for crushed stone, sand, and gravel used in construction. It's used to build homes and offices, roads, bridges, tunnels, sidewalks, and runways. It is an essential material for building our world's infrastructure because of it's strength and stability. All U.S. states, with the exception of Delaware, produce crushed stone.

Photograph Courtesy Wake Stone Corp. 

Process

The photo above is a view of an operating crushed stone (aggregate) quarry that is located near Raleigh, NC. 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.

Mines are permitted before production begins. A comprehensive reclamation plan is part of the permitting. Additional information about aggregate is found through the North Carolina Aggregate Association.

Uses of Aggregate

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.

Production

Crushed stone makes up 85 percent of aggregate production; construction sand and gravel, about 15 percent.

North Carolina is the sixth 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.

Transportation

Over 90 percent of all aggregate is moved by dump truck. Most aggregate is used within 50 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.

Permitting/Environment

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. You can find more information about mine permitting through the DEQ's Mining Program.

DEQ Mining Program

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.