Operators can choose from a greater range of bolting options and equipment than ever—the challenge is to identify the best solution for local rock conditions
By Simon Walker, European Editor
In May, E&MJ took a close-up look at shotcreting technology, which is increasingly becoming a preferred means of ground support in appropriate situations. However, shotcrete is rarely used alone, especially where long-term support is needed, with rock bolts usually forming an integral part of a ground-control package.
As with shotcreting, rockbolting technology has moved on over the years, with standard point-anchor bolts having been superseded by more sophisticated and long-lasting systems. That is not to say, of course, that no place remains for the simple mechanical-shell system: far from it, in fact, with its benefits of easy installation and low cost often outweighing its disadvantages where the rock conditions are suitable. Similarly, simple spun-in resin-bonded bolts have their own specific applications, especially where the need for support is shorter-term, as in open stoping.
The fundamental principle behind rockbolting remains the same, however. The concept of reinforcing the rock mass so it holds itself together is as valid now as it was when rockbolting was first introduced, perhaps in the 1890s. Some sources credit the idea of using metal pins secured with wedges to reinforce rock masses as dating from Roman times, although it is difficult to correlate this with the drilling technology available then. Almost certainly, coal mines in Silesia (now in southern Poland but then German territory) were early users of rockbolting, as reported in a paper presented in 1918.
In the United States, coal mines were also quick to see the advantages of self-supported rock over passive timber and steel arches, while the hard-rock sector seems to have been slower on the uptake. One of the first major U.S. users of the concept was the innovative St. Joe in the Missouri Lead Belt; rockbolting was in use in some of its operations during the 1920s and 1930s, with W. W. Weigel reporting on the practice and its success in creating self-supporting beams above the haulages in a landmark article published in the May 1943 edition of E&MJ.1
Even by the 1950s, though, U.S. hard-rock mining remained reticent to abandon its traditional timber for thin steel pins. In a paper presented to the second U.S. symposium on rock mechanics in April 1957,2 Howard Schmuck of Colorado Fuel and Iron Corp. noted that of the 3 million bolts being installed each month in U.S. mines, only about 3% were being used in the non-coal sector.
“Rock bolting in metal mines has not taken over to the extent that it has in coal mines, but the past four years has seen their use in these mines increase very rapidly until today there are six times as many bolts used per month as at the beginning of 1953,” Schmuck said. “Originally, most rock bolting in metal mines was done in development headings and haulageways, but their use in stoping operations is now gaining rapidly.”
And that was just the start of it. Today, it would be hard to find an underground mine where rockbolting is not the norm; except in specialized situations, timbering is mostly history.
Bolt Technology Developments
As any textbook on the topic will say, there are essentially three different types of rockbolt available: mechanical, grouted and friction. Each has its advantages and disadvantages, in terms of technical capabilities and durability as well as cost implications. By way of background, a brief outline of each system follows.
Mechanical—The earliest, and hence the simplest, system to come into widespread use. Slot-and-wedge bolts came to be replaced by expansion shells. The big advantage of mechanical-type bolts was their speed of installation at a time when passive support systems were still widely used; drill the hole, insert the bolt and surface plate, tighten the nut and that was that. Grouting with cement-based materials could be used as a secondary means of both protecting the bolt steel from corrosion and increasing the bond length. However, this simplicity came at the cost of integrity, especially where the rock mass being bolted was liable to spall. Everyone will have seen the situation where redundant bolts stick out, providing no support because the rock carrying the plate has fallen away.
Grouted—Recognizing that point anchoring had this major disadvantage, the concept of fully grouting the bolt inside the whole hole came into play in the 1960s. Initial uptake was project-specific, mainly because of the length of time needed for cement-based grout to set and cure sufficiently for tension then to be applied to the bolt. The solution, already gaining in popularity 10 years later, was resin, which cured faster and was simpler to handle but was more expensive. Mass-injectable resins came to be replaced by encapsulated products, with resins of different curing times being used to anchor the bolt in the hole and to provide steel-to-rock bonding along the remainder of the bolt length.
Friction—Typified by the Split Set and Swellex systems, friction bolts rely on full-length contact between the bolt and rock to provide the support required. The major advantage of both types is their ease of installation, with the drill being used to force the Split Set tube into place while Swellex relies on high-pressure water to expand its steel tube against the hole sides.
Which system is most appropriate for an individual operation very much depends on local parameters. The location (haulage sidewall or stope back, for instance), the competence of the in-situ rock mass, the presence of clearly defined jointing or lamination, and the length of time for which active support is required are all factors that have to be considered. Economics will also play a part here, since it is pointless to invest in a high-cost, long-term option if only a short-term solution is needed. It goes without saying the reverse is also true: inadequate initial support can lead to high long-term costs.
Perhaps the most significant recent developments in bolt technology have been systems that can recognize and compensate for sudden rock-mass movements such as rockbursts. Atlas Copco’s Rockex is one such, while from Australia, Garford’s Dynamic design of yielding bolt is aimed at a similar market. These are, of course, designed for a very specific market segment, where rock-mass stresses and stress-release patterns are such that controlled bolt deformation can provide both a safety mechanism and continued active support even after destressing has occurred.
System Design
From its early days, the design of rockbolt-based roof support often relied on rules of thumb that had been found to be satisfactory in certain situations. Typical examples might include:
• The Mont Blanc tunnel rule, which states the length of a rock bolt should be one-half to one-third the heading width;
• Bieniawski’s rule that the bolt-length to bolt-spacing ratio is acceptable between 1.2:1 and 1.5:1 in mining; and
• The finding that a mechanical rock bolt installed 30° off square to the rock face may provide only 25% of the support tension produced by a comparable bolt drilled straight into the rock, unless a spherical washer is used.
However, while rules such as these may have been adequate in the past, better understanding of how rock masses react during mining has meant support systems can be designed much more specifically. In a paper presented at the 2008 AIMS conference,3 J. Ran and R. Sharon outlined how Barrick Gold designs bolting support at its underground operations.
Since 2005, each of the company’s underground mines has had a ground-control management plan in place, they said, noting the “installation of ground support elements, and/or support systems, is an important component of ground control and should be effectively managed in a systematic and comprehensive manner. This plan covers all geotechnical aspects including data collection, ground support design, standards, procedures, quality control programs, training and mine design issues.”
Looking at the types of ground support used in Barrick’s mines, the selection criteria must consider the following:
• Demand conditions, such as requirements of different excavations and expected environmental conditions, including significant mining-induced stress change, or excessive corrosion;
• Performance characteristics of support elements such as rigidity, strength, resistance to corrosion and susceptibility to repeated high vibration levels; and
• Operational factors, including skills of the workforce, available equipment, equipment compatibility and maintenance, and local supply of support products.”
Put another way, there is no one-size-fits-all approach to rockbolt support design. Each operation has to be considered individually, such that the company uses a wide range of bolt types throughout its mines, and sometimes within individual mines, as conditions require. For example, resin-grouted rebar bolts are commonly used in its North American mines, but less frequently in Australia, where the use of friction bolts is preferred. Some of its Australian mines also grout their friction bolts in place to increase the bond strength, and one of them installs Garford Dynamic bolts to help control the effects of rock-mass seismicity.
Barrick’s U.S. mines are also significant Swellex users, with plastic-coated Swellex bolts being used in its Nevada operations where corrosion is an issue. Cement-grouted cable bolting is widely used at intersections and to support wide openings.
Ran and Sharon noted “traditional design approaches have been applied successfully to operations that are relatively simple and experience good ground conditions. The rock mass classification approach has been widely utilized for support design in various ground conditions, but has limitations in providing sound guidance when conditions become increasingly challenging. Sophisticated numerical models can provide additional guidance for controlling complex excavation shapes in a poor quality and/or structurally complex rock mass, but they do require a high degree of skill and experience to provide reliable results.
“Advancements made in rock mechanics practice have led to improvements in the principles and methodology for support design and selection. However, every mine has a unique mining environment, and the ground response to support is complex and often not fully understood. As a result, the application of ground support varies over a wide range, and support design and selection remain largely experience-based,” they said.
Specialized Systems
Mines developed in high-stress rock environments present particular challenges in terms of their support requirements. Rockbursts are by no means solely a deep-mine phenomenon, but reflect the local rock stress regime. Being able to retain a level of support after a high-energy stress release means that less damage may occur to the mine infrastructure, as well as reducing the amount of repair work that will be needed.
Atlas Copco’s Roofex system, introduced in late 2008, is designed to provide support in new, deep underground excavations in poor quality rock or in areas prone to seismic events. The Roofex bolt comprises a steel bar inside a smooth plastic sheath that is fixed inside the hole with cement or resin grout. An energy absorber allows the bolt to extend outward under load, while maintaining its load capacity, thereby allowing it to absorb both sudden displacements and gradual yielding deformation within the rock mass being supported. The displacement capacity can be pre-selected during manufacturing, Atlas Copco notes, so bolts can be designed for specific stress environments.
From Australia, Garford’s Dynamic solid bolt also features energy-absorption capability. The system is supplied as a solid bolt with a dynamic device, polyethylene tube, resin-mixing device, slot pin nut and dome ball attached. If a seismic event occurs, the bolt can move through the dynamic device, enabling it to absorb the energy and remain intact. The polyethylene sleeve allows the bolt to slip through the dynamic device which, since it is mechanical, means that energy absorption can be repeated if the stress builds up again.
In a paper presented at the 10th Underground Operators’ Conference, held in Tasmania in 2008, R. Varden et al described the use of the Garford Dynamic bolt at the Kanowna Belle gold mine in Western Australia.4 Owned by Barrick Gold since its takeover of Placer Dome in 2006, Kanowna Belle is a deep, open-stoping operation set within a high-stress environment.
The increasing risk of seismicity as the mine gets deeper led to an evaluation of alternative support systems that could handle destressing events better than the friction-bolt methods that had been used previously. The selection of the resin-bonded Garford bolt was based on a number of factors, including its compatibility with existing development-drilling equipment, which meant that the same set-up could be used for both face drilling and support.
Testing of the system was undertaken at the Western Australian School of Mines’ dynamic test facility, with the Garford bolt subsequently being introduced as part of the initial development support method. Not every situation was successful, the authors reported, with problems being encountered where attempts were made to install the bolts in ground that had already been fractured by previous seismic activity. Nonetheless, they concluded, “Kanowna Belle has successfully implemented a well-tested, one-pass dynamically capable support system as part of the development cycle at the mine.”
Rockbolting is not, of course, just about the steel and how it is fixed. Drilling the hole is a critical part of the operation, and one that can incur a significant proportion of the total bolt-installation cost if the equipment proves to be unsuitable for the rock conditions.
Utah, USA-based Brady Mining offers one solution, at least for operations where haulages or production headings are being driven limestone, shale, potash, rock salt or similar sedimentary rocks. The company claims its polycrystalline diamond (PCD) cutter bits, designed specifically for rockbolting operations, can drill up to 300 times longer than conventional tungsten carbide-tipped bits, while substantially reducing dust generation and noise levels.
The company’s managing director, Russ Myers, explained to E&MJ how its PCD bits differ from tungsten carbide bits in operation. “Rotation quickly blunts a tungsten-carbide bit so over time more energy has to be used to force the drill rods into the hole,” he said. “PCD is different in that the bit remains sharp for much longer; as the outer layer of crystals are worn off, new sharp crystals are exposed beneath them.”
Acknowledging coal remains Brady’s major market, Myers added that the company is expanding its range into mines working in other bedded types of deposit. “We have eliminated percussion drilling for rockbolting in U.S. limestone operations, and are currently testing our bit technology in European metal-mining conditions,” he said.
Brady claims rotary drilling with its PCD bits offers major advantages over using conventional bits in terms of productivity, in both wet- and dry-drilling applications. Fewer bit changes are needed, while drill-maintenance requirements are reduced and drill-rod life is extended.
Recent Success Stories
Polish-based Mine Master produces a four-model range of roofbolting rigs, which have been designed in conjunction with the American specialist in this field, J.H. Fletcher. Mine Master reported last year that it had won a further contract from one of Estonia’s oil-shale producers, bringing the total number of its Roof Master 1.7 rigs operating in the country to six.
Mine Master claims the machines, operating in 2.6–2.7-m-high entries under highly variable roof conditions, have been installing around 100, 2.2-m-long bolts per shift. The company also reported having won a second order from the Altynken gold operation in Kyrgyzstan for a Roof Master 1.7 rig, which is equipped with a Fletcher telescopic boom and heavy-duty drill.
Fletcher itself, long recognized for its expertise in bolting equipment for the coal industry, expanded into the industrial-mineral and hard-rock sectors at the start of the 2000s. Today, the company produces two series of rockbolting rigs aimed at this market: the 3000 Series, for which all drilling and bolt-insertion operations are done remotely from the operator’s cab; and the 3100 Series, which have a separate basket station for the operator to install the resin and bolts once the holes have been drilled remotely.
Both series have a high-lift capability, with the 3000 designed to operate in opening heights of up to 35 ft (10.7 m) while the 3140 can reach even higher—up to 40 ft (12.2 m). The 3100 series are also equipped so that the operator can inch the entire machine from position to position from the basket, without having to drop back down to the main cab to do so.
From South Africa, Sandvik reported last year that contractor Murray and Roberts Cementation had been successfully using one of its DS310 roofbolting rigs to install support during development work at Hotazel Manganese Mines’ operations in the Northern Cape. The project included driving three tunnels through a worked-out section of the Wessels mine to access new underground reserves.
Formerly designated the Robolt 5-126 XL, the DS310 is a one-man operated, compact modular rockbolter designed to install all the most common types of rockbolts in small- and medium-sized headings, Sandvik said. The rock conditions at Wessels are particularly severe, with the roof strata consisting of banded ironstone. Although bit-life was highly reduced as a result, the machine was installing an average of 60, 5-m-long bolts per shift, the company noted, giving 95% availability and 81% utilization.
Although not prone to rockbursts, massive evaporite deposits such as salt and potash present other rock-mass stability problems, such as long-term yielding and convergence. Atlas Copco trialed its Roofex system at Iberpotash’s Vilafruns potash mine in Spain under these conditions, and was able to show that it can handle gradual yielding issues that conventional rockbolts were unable to withstand.
Working at a depth of between 600 and 900 m, Vilafruns is a continuous-miner, room-and-pillar operation producing rooms 7–8 m wide and up to 5.5 m high. The Roofex trials began in 2007, with Atlas Copco reporting the following year that while conventional resin bolts were being overstrained by convergence movements of 50–60 mm over four to six weeks, the Roofex bolts were able to accommodate these rock movements and continued to provide support.
Speed and Security
Since speed is the economic key to installing roof support in today’s mechanized mines, the clear trend is towards cutting the number of separate operations needed. In this context, friction bolts have a clear advantage, since the same machine is used to drill the hole and press the bolt in to its final position. By contrast, cement-grouted bolts or cables involve a much more complex installation process. Resin-bonded bolts offer a compromise, but resin capsules must be installed in the correct sequence and mixed thoroughly for the bolt to be fully effective. Interestingly, in their AIMS paper, Ran and Sharon noted that because of problems encountered with the proper mixing of resin cartridges, there has been renewed interest in the use of pumpable resin.
Little wonder, then, the world’s leading bolting-rig manufacturers have all focused on developing machines that can minimize the number of installation stages for a given bolting system. In this context, hard-rock mining presents different challenges to the coal sector, not least in terms of typical operating heights for installing roof support.
Rockbolting technology has come a long way. Operators have a greater range of options available. What matters above all is the system selected does the job it is intended to do: provide long-term safety and security to the people working below.
References
1. Weigel, W.W., “Channel Irons for Roof Control,” E&MJ; 144 (5); May 1943.
2. Schmuck, H. K., “Theory And Practice Of Rock Bolting,” 2nd U.S. Symposium on Rock Mechanics, April 21–24, 1957, Golden, Colorado.
3. Ran, J. and Sharon, R., “Underground Support Applications at Barrick Gold,” 6th International Symposium on Rock-bolting in Mining and Injection Tech-nology and Roadway Support Systems, May 14–15, 2008, Aachen, Germany.
4. Varden, R., et al, “Development and Implementation of the Garford Dynamic Bolt at the Kanowna Belle Mine,” 10th Underground Operators’ Conference, April 14–16, 2008, Launceston, Tas-mania, Australia.
