The concept of quarrying has been around since the dawn of civilization, with many different working techniques having been developed.
A quarry is typically defined as being a surface excavation for the production of rock as the principal product. As such, it differs from an open cast, open-cut, surface or open-pit mine in that the rock itself is the valuable commodity, rather than a specific mineral within the rock mass.
Quarrying is used mainly in the production of construction and building materials, such as solid stone or crushed rock for aggregates, or for raw materials for processes such as cement manufacture. As a technique, quarrying is normally only used where raw materials of adequate quality and size cannot be obtained economically by other means. Since natural sand and gravel are not always readily available, for example, a large proportion of the world's annual output of aggregates is produced by quarrying and processing rock.
Rock has been quarried for the construction of buildings and monuments since before recorded history. There is certainly evidence for stone production in ancient times from numerous sites around the world, although it is unclear at which stage formal quarries began to emerge, rather than people just making use of naturally occurring boulders.
Within recorded history, we have a better understanding of some of the techniques used in quarry operations. In the Egyptian dynastic period, for instance, construction of the earlier pyramids depended mainly on limestone that was quarried using copper hand tools, while later pyramids, although built mainly of mud brick, still used quarried stone for facing. Granite was also a major building material, especially for detail work, and was quarried directly from the bedrock at locations close to some of the country's largest monuments. Ancient Egyptian stoneworkers cut trenches all the way around the blocks of granite that they wished to extract, isolating them from the bedrock, with the rock then being broken free using massive wooden levers.
The Romans also quarried rock on a vast scale for their construction projects, both for buildings and monuments. A wider range of rock types was produced, including fine marble, which was used for artwork such as sculptures and in public architecture. The Romans used quarry hammers to isolate the blocks they wanted, then used metal wedges to pry them free from the bedrock.
In the absence of modern technology such as drills and explosives, rock-splitting required the use of other approaches, such as wedging or fire setting. The use of wetted wooden wedges inserted in natural cracks or in shallow holes, with the swelling of the wood causing the rock to split, dates back to ancient times, as does fire-setting. Producing the stone needed for the construction of Great Zimbabwe, for example, is believed to have relied on firesetting to split building blocks from natural granite outcrops, making use of the observation that the local rock weathers naturally along planes of weakness that could be exploited in this way.
Development of most modern rock quarries involves stripping the overlying soil and weathered rock to get to the hard rock underneath. This is then worked in a 'bench' system, removing the rock in layers that can be returned to year after year as the quarry is developed. The quarry becomes deeper with each subsequent bench, with stepped benches reaching up to the original surface.
With few exceptions, modern quarries rely on drilling and blasting to fragment the rock, which is then loaded onto offhighway trucks or belt conveyors for transport to a processing plant of some sort. Loading is usually done with wheel loaders or excavators, which combine adequate loading capacity with maneuverability. This allows them to move from area to area within the quarry, as needed. Where blasting results in the formation of large boulders that are too big for the loading equipment to handle, secondary breaking will be needed, either by drilling and blasting individual boulders or by using excavator-mounted hydraulic hammers to break them. The exceptions to drill-and-blast include the production of dimension stone, where the demand is for large pieces of rock rather than fragmented material for further processing.
The production of dimension stone, which is covered on a separate page, involves carefully splitting large blocks of raw stone away from the quarry face, using wedges or diamond-impregnated wire saws. Another exception is where the rock is soft enough to be ripped, using a large dozer or a ripper tine mounted on a hydraulic excavator, with the dozer then being used to push the broken rock onto a hopper or mobile crusher which feeds a belt conveyor system. Quarry design depends on a number of factors including the pre-existing topography, intended output, infrastructure and environmental footprint. In general, however, quarries can be grouped into five principal types – hillside, hilltop, valley bottom, coastal and combined – as illustrated below.
Bench blasting is the most widely used method of production blasting in quarrying, strip mining and construction excavation. This involves drilling inclined, vertical or horizontal blastholes in single- or multiple-row patterns to depths ranging from a few meters to 30 m or more, depending on the desired bench height. Where the excavation is shallow, less than 6 m (19.7 feet), one level may suffice. In deep excavations, a series of low benches, offset from level to level, are recommended for operational convenience. The bench height is often two-to-five-times the burden distance, while the ratio between the burden and the spacing is typically between 1:1.25 and 1:2.
The material must have a certain strength and hardness, and the crushed particles must acquire a defined shape, quite often with a rough surface. Consequently, soft sedimentary rocks and material that breaks into flat, flaky pieces are often unacceptable as raw materials for aggregate. On the other hand, igneous rocks such as granite and basalt, as well as highly metamorphosed rocks such as gneiss, are well-suited to aggregate production.
Since drilling is a critical part of the quarry production process, the best planning, figuring, calculations and explosives are worthless if the area to be blasted is not drilled properly and responsibly. Basically, if the drilling goes bad and is off pattern, the entire blasting operation will fail. Drilling in any surface mining or quarrying environment invariably follows a pattern that has been designed to take into account natural parameters of the rock including hardness and strength, the presence of planes of weakness such as faults or fracturing, and the degree of fragmentation needed in the blasted product. The drill pattern will be designed according to hole spacing (along the bench) and burden (distance from the front free face) for a given hole diameter, and thus stipulate the amount of explosives needed for each charged hole.
Generally, a less powerful drill rig that produces small diameter holes will have to drill on a closer pattern than a machine driving a larger-diameter bit. Drilling is normally done using heavy-duty Down-The-Hole (DTH) and tophammer drill rigs. In Epiroc's case, these rigs can be equipped with the Hole Navigation System (HNS) which gives operators the ability to drill parallel holes with precision and complete drill plan accuracy.
The drilling and blasting sequence is shown schematically in the illustration above. If the drill rig operator is instructed to remain on a specific pattern, he must do so and not alter it unless authorized. The operator must also keep the blaster-in-charge informed of any changes in the rock while drilled, or indeed any mistakes, so that the blaster can make any necessary adjustments to the charge. The drill rig operator should tell the blaster about fractures or other abnormalities in the rock, changes in the strata and sand or mud seams in the rock, so that explosives can be loaded in the hole with these factors taken into consideration.
The operator must also inform the blaster-in-charge of any 'short' holes – holes that are not drilled to the expected or planned depth. In other words, the driller is the blaster's eyes on the ground and, as such, can make or break a blasting operation. This information can also be extracted from the quality log available on SmartROC drill rigs. Quarry operators commonly design fragmentation blasts for safety, economy, ease of use at the primary crusher, and even public relations, but they often forget about quality.
The blast layout must be properly engineered, documented and adhered to for maximum consistency. Varying the blast pattern may mean changes in the product size across the operation. Smaller shot rock, resulting in less crushing at the secondary and tertiary stages, may mean less improvement through crushing, so the mineral quality and/or physical properties of the product may be affected. Conversely, it is important to remember that size-reduction through crushing becomes more expensive as the material being crushed gets smaller, so in some respects it can be beneficial to reduce the crushing duty by increasing the initial fragmentation at the quarry face. There is also the question of transport, since loaders, trucks and belt conveyors will have a maximum rock-size constraint, above which boulders will need expensive secondary breaking.
There are various factors to be considered when trying to achieve optimum efficiency and overall economy from quarrying operations. The difference between product revenues and the costs of production must be maximized. It should be noted that crushing, screening and storage represent almost half of the costs, whereas drilling represents less than 15%. More often than not, the crushing operation is a bottleneck in the overall work cycle. It is sometimes the case that extra expenditure in drilling and blasting might be the only way to assure free flow through the crusher and full capacity in the plant, which improves the operation's economics. Achieving an even fragmentation and the creation of smooth benches will also have a positive effect on loading and transport equipment.
In the 1980s, a trend among rock producers shifted attention away from large-diameter holes, which produced more boulders and more fines, in favor of medium-size (89–165 mm; 3.5–6.5 in) holes. In addition, limiting the blast size reduces micro cracking, and hence the production of fines.
The opposite situation occurs in open-pit mining where the generation of fines is favorable, as these will pass through the mill with a minimum of costs.