Quarrying
Quarrying is a widely used method for extracting stone and involves an open-pit process to obtain various materials. The techniques applied in quarrying depend on factors such as the physical properties of the stone, its intended applications, and the proximity to populated areas. Generally, quarrying involves the removal of overburden—waste rock, soil, and vegetation—to access the desired stone, which is then excavated through methods like blasting, cutting, or drilling. Dimension stone, which is often used in construction and ornamental purposes, is typically extracted with minimal blasting, employing techniques like drilling and broaching or rock sawing.
Crushed stone, another significant product of quarrying, is primarily used in construction and road building. This process usually involves blasting to break the rock into smaller fragments, followed by further processing. While traditional quarrying is common, some operations face community opposition due to environmental impacts, including noise and dust, as well as changes to the landscape. Additionally, underground mining has become a more prevalent alternative, enabling year-round operations that can mitigate some of the disruptions associated with surface quarrying. Overall, quarrying is essential for sourcing materials used in various industries, while also raising important considerations regarding environmental and community impacts.
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Quarrying
Quarrying is the open-pit mining process whereby dimension stone and crushed stone are commonly obtained.
Background
Quarrying is the common method for mining stone. This open-pit process may use a variety of techniques to obtain the desired material, depending on the physical characteristics of the material being removed, its intended use, the size and shape of the deposit, the quarry’s proximity to inhabited areas, and other considerations. In general, quarrying involves stripping away the overburden—whatever waste rock, soil, and vegetation interfere with access to the deposit—then excavating, cutting, or blasting so that the desired stone blocks or fragments can be removed. Quarrying may involve entering an outcrop from the side or creating an excavation sunk below ground level. The latter type of excavation, called a pit quarry, may extend hundreds of meters deep. Benches, or ledges, are cut into the rock mass to provide access to progressively deeper levels.
Dimension Stone
Dimension stone, rock that is cut to a particular shape and size, is used for construction and ornamentation. Granite, limestone, dolostone, marble, sandstone, and slate are among the rocks most commonly used as dimension stone. Little blasting is used in dimension-stone quarrying, particularly where the stone in question is soft and easily shattered, although light powder charges may be set off in deep drill holes in order to separate blocks of stone from the rock mass. Blocks can also be broken off from the parent rock through drilling and broaching, a process in which a series of deep, closely spaced holes is drilled into the rock along a straight line. A broaching tool is used to break the “webs” between the holes, creating a channel cut. A similar method is drilling and wedging, which involves drilling a line of holes and hammering wedges into the holes to split off a block of stone. The Incas and ancient Egyptians are thought to have quarried their building stones in this manner.
Another method of quarrying is rock sawing, which, while generally more expensive than other processes, is also more efficient, as it wastes little material and produces a smooth-surfaced cut. Various combinations of diamond saws, wire saws, diamond-wire saws, and chain saws may be used. Jet piercing, a rapid method that cuts a channel several centimeters wide, uses a high-temperature flame combined with a stream of water to disintegrate and clear away rock. Wherever possible, splitting and cutting operations take advantage of natural joint and cleavage planes in the rock. Once blocks are separated from the parent rock, they are sawed or split into smaller blocks and removed for final cutting and finishing, often at a processing plant on the quarry site.
Crushed Stone
Crushed stone is used chiefly in construction and road building. Raw materials for industry, agriculture, and metallurgy (notably limestone and dolomite) may be obtained as a by-product of crushed-stone quarrying, although the more high-purity materials are often mined from underground works. While limestone and dolostone are the most common sources of commercial crushed stone, granite, diabase, sandstone, and marble are also frequently used. The rocks are initially broken by blasting, which shatters rock into fragments that may be further reduced in crushing machines during subsequent processing. Explosives are detonated in holes drilled into the rock face in a pattern designed to produce maximum fracturing. Tricone rotary drills, long-hole percussion drills, and churn drills are commonly used to create the holes. Dynamite is still employed at some smaller operations, although low-cost ammonium nitrate fuel oil mixtures are typically used in medium to large operations. Shattered rock is removed from the quarry and transported to a processing facility, where it is sorted and reduced to marketable sizes. Primary crushing may be performed at the quarry.
Where quarries extend below the water table, plumbing systems may be installed to pump water from the excavation. This solution is not always practical: In Florida, for instance, high water tables and extremely porous limestone deposits make it difficult economically to dewater some quarries. In such cases, a dragline is used to retrieve broken stone from under the water after blasting.
While crushed stone is still commonly obtained from quarries, underground mining of this material has become increasingly prevalent. Crushed-stone quarries tend to meet with considerable community resistance because they alter the scenery, interfere with alternate land uses, and generate noise, dust, and blast vibrations. Quarrying operations are also subject to closure because of severe weather conditions, while underground mines can operate year-round. In some deep-cut quarries underground mines have evolved, with tunnels leading off from the quarry.
Bibliography
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National Research Council. Sand and Gravel Mining, and Quarrying and Blasting for Crushed Stone and Other Construction Minerals: A Working Paper Prepared for the Committee on Surface Mining and Reclamation, Board on Mineral and Energy Resources, Commission on Natural Resources, National Research Council. Washington, D.C.: National Academy of Sciences, 1980.
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U.S. Geological Survey.