Dimension stone is one of the most sustainable materials used in construction, cladding, paving and other applications. Dating back to the Moai statues on Easter Island, its popularity is now rising again and, with the right equipment, quarrying is more affordable than ever.
Over the last 20 years, the global production of dimension stone has grown rapidly, especially for building projects where architects are making increasing use of the wide variety of colors, textures and finishes that natural stone can offer. And, while the use of stone is growing, our ability to cut and process hard rock more efficiently has led to a vast increase in the types and colors of material being supplied to the market.
In addition to its use in construction, dimension stone is also needed for monumental masonry, as the raw material for sculpture, monuments and tombstones. Today, seven countries – China, India, Turkey, Iran, Italy, Brazil and Spain – account for around two thirds of the world's output of dimension stone. In general, there is a trend towards using stone whenever this is economically possible, in effect marking a return to traditional practices.
As a consequence of this rising demand, there is definitely an opportunity to develop and expand the dimension stone industry (DSI) worldwide. While the prospects are looking favorable, development of the DSI will often depend on local factors such as location, quality and suitability of stone deposits, and the availability of funding to develop or expand appropriately sized quarries, as well as logistical issues such as the provision of adequate transport infrastructure to link stone producers with customers.
Dimension stone is the name given to natural rock that has been quarried and shaped to certain dimensions or specifications for use in building and construction, and in the production of sculpture, monuments and memorials. In essence, the term refers to any stone that is capable of being quarried in large blocks and subsequently processed into slabs, blocks, tiles or flagstones. In practice, there is a somewhat grey area between classical dimension stone, which is largely ornamental in use, and sized natural construction materials, where the physical characteristics of rocks are used to produce regularly shaped building stone.
From a historical perspective, dimension stone production and use goes back a long time with, for instance, Mesolithic and Neolithic monuments in Europe, the Middle East and elsewhere clearly having been built using shaped stone. Classical Greek and Roman architecture features high levels of skill in dimension stone use. For example, the Romans discovered and exploited the world's only known source of purple porphyry rock for use in decorative columns for their Temple of Jupiter. This task involved quarrying the stone in column-sized pieces in the Red Sea Hills of eastern Egypt, then transporting them over land and sea to Rome. Elements of the Great Pyramid were also sourced from great distances before being hewn to precise dimensions, while other examples of high-quality stone masonry can be found on every continent.
The most commonly used commercial stones today include marble, granite, slate and sandstone, all of which can be found in a vast array of visual and physical properties. This is by no means an exhaustive list, however, with rocks such as limestone, basalt, gabbro, travertine and tufa also widely used where their properties are suitable. Volcanic in origin, tufa is essentially a very soft rock, but is easy to work with. Examples of its use range from a construction and cladding material on many buildings in Armenia to the giant moai statues of Easter Island. The main qualities of dimension stone that determine its popularity and use include its color, patterns and texture, its durability and the consistency of supply. Different markets demand different quality characteristics.
Dimension stone is quarried by cutting, or separating by some other means, large blocks of stone from the natural rock mass. The size of individual block produced depends on a number of factors, including the homogeneity of the rock itself, the ability of the quarry operator to handle the rough stone, and the required end use for the stone once it has been shaped. A typical block size might be in the order of 6 m3 (200 ft3), which would relate to a block weight of 10–18 t, depending on the density. The way an individual quarry is operated can vary enormously.
The physical characteristics of the rock mass (how homogeneous it is, and whether there are defined lines of weakness such as regular fracturing or lamination), the size of both the resource and the market for its products, and the financial resources of the operator, all play a role in deciding the quarry design and capacity. In a large-scale operation, the first stage in production is to loosen individual blocks that may contain thousands of cubic meters of material, from quarry benches 10 m or more in height.
Conversely, a small-scale quarry may have a very limited output, produce raw blocks weighing 5–10 t, and have a lower bench height that is suited to available production technology. The overall concept is the same, however: to produce raw blocks that can then be processed into a higher-value product. Looked at in this way, the raw block is a valuable asset in its own right, and has to be handled carefully – small, irregular fragmented blocks are less marketable than large ones. As a result, high-value blocks are treated gently.
For example, some operators use a 'pillow' of soil or sand to support newly loosened raw stone blocks while they are being handled in the quarry. For harder material such as granite or other intrusive rocks, blocks are usually split away from the quarry face by drilling a line of closely spaced, accurately aligned holes, then inserting wedges and shims (sometimes called plugs and feathers) into them. Driving the wedges into the holes sequentially causes the rock to crack along the line of the holes, thus allowing the block to be pried free. Softer rock, such as marble, can be cut with diamond-impregnated wire saws, while blocks of soft (but not crystalline) limestone are often cut out using mechanical saws. Large volumes of materials like slate can be loosened by the careful use of low-energy explosives such as black powder, placed in pre-drilled holes along a quarry bench.
The aim here, of course, is to loosen the raw stone sufficiently without fragmenting it, which would render it useless for roofing-slate or monument manufacture. Small amounts of explosives can also be used to free blocks of harder material from the quarry bench floor. The stone blocks are moved from the quarry to the processing plant using large frontend loaders and flatbed trucks. The rough blocks can then be stored at the quarry as inventory or taken immediately for processing at a fabrication plant; these are often integrated with the quarrying operation, or are located nearby to reduce transport costs.
Raw blocks are put through a series of processing steps, depending upon the end product required. This usually involves the use of wet cutting into precisely dimensioned blocks or thin slabs with diamond-impregnated wires or circular saws, followed – if required – by polishing or honing.
The thickness of individual slabs again depends on the end use, with architectural cladding or commercial paving demanding a thicker section than, for instance, material destined for use as domestic interior floor or wall tiling. Individual quarries are often quite small operations that supply local demand. In addition, dimension stone companies sometimes have several quarries for different stone types or colors that operate intermittently, depending upon the demand for a particular stone. Unusable stone rubble is crushed and sold as construction aggregate.
Flexibility and drilling precision are key factors in selecting appropriate equipment for producing DSI raw materials. The need for flexibility results from typical DSI quarrying practice, with individual blocks being selected on the basis of their suitability for the intended end use, and the probability that the block will remain intact while being won from the quarry face. As a result, drilling equipment has to be very mobile and maneuverable, while at the same time providing the level of drilling accuracy needed in terms of hole straightness, alignment and parallelism to produce accurate break-lines for high quality block production.
Hole diameters used for DSI drilling are typically less than 45 mm, given that the purpose of the holes is to provide a line of weakness within the rock mass for splitting, rather than to hold explosives, as in conventional quarrying. However, holes also need to be closely spaced, so that crack propagation between them is enhanced once the wedging process begins. In addition, hole deviation must be minimized in order to achieve as clean a break as possible. This in turn places constraints on the depth of hole that can be achieved with one-pass drilling at small diameters, with the use of extension drill steel more likely to result in increasing deviation with depth unless strict control systems are in place.
Depending on the intended end use for the raw block, it is perfectly feasible to use conventional drill rigs, such as Epiroc's FlexiROC T15 R hydraulic rig, which has a single boom and provides a high level of maneuverability both for the machine itself and for positioning the boom. Its application is somewhat limited, of course, since it can only drill one hole at a time, and the boom and feed inclination have to be reset accurately for each hole in sequence. The next step in providing an answer to this type of problem is to progress to the custom-designed, specialist DSI drill systems that make up Epiroc's range, such as the SpeedROC 1F. Similarly equipped with a single boom, and also completely self-contained, this differs from the more general purpose FlexiROC T15 R in that its boom carries a guide frame that allows sideways movement of the tophammer drill feed.
Because of this, the machine can drill a 3.5 m long series of holes from one set-up on the bench, reducing down-time and ensuring drilling accuracy to hole depths of up to 2.4 m in one pass, or 9 m using extension rods. Being able to drill for longer during a shift means higher productivity, of course, with the SpeedROC 1F capable of drilling up to 400 linear meters a day.
Achieving yet higher productivity still requires the use of more than one drill feed on a rig, as found on Epiroc's SpeedROC 2F and SpeedROC 3F. A larger machine than the SpeedROC 1F, this carries two and three separate hydraulic rock drill feeds on its 4 m wide guide frame, giving the potential for drilling up to 1 000 linear meters a day. Single-pass hole drilling of up to 4 m is possible, while its maximum hole depth reach is again 9 m. The extending boom allows the drilling of parallel rows of holes along a bench from a single set-up, with the rig having a maximum surface coverage of nearly 260 m2 without being moved. The hole depth needed in any particular applications depends on the size of block being won, and there are circumstances where drilling and splitting is simply not achievable, either because the block would be too big, or the rock is not competent enough to withstand the forces imposed during splitting. In this case, and assuming that the rock is not too hard or abrasive, sawing using diamond impregnated wire can be a solution.
The SpeedROC 1F, SpeedROC 2F, SpeedROC 2FA and SpeedROC 3F can also be used for secondary work on blocks that have been extracted from a bench, either by drilling rows of holes across the block or by cutting through it again. In each case, the aim is to maximize the yield from a block while minimizing the amount of waste generated, with small-diameter holes and thin wires going a long way to achieving this.