Rockbolting Technology Keeps Mines Safe and Secure
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
As with shotcreting, rockbolting technology has moved on over the years, with standard point-anchor bolts having been superseded by more sophisticated and long-lasting systems.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 resinbonded 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 selfsupported 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. hardrock 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 noncoal 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 fulllength
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 stressrelease 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 rockboltbased
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 onehalf
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-fitsall 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.
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.
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, onepass 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.”
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 drillmaintenance requirements are reduced and drill-rod life is extended.
Recent Success Stories
Polish-based Mine Master produces a fourmodel
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, cementgrouted
bolts or cables involve a much
more complex installation process. Resinbonded
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 Rockbolting
in Mining and Injection Technology
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, Tasmania,
Australia.