Support in Many Forms
Maintaining roof-support integrity is essential in any mine. E&MJlooks at today’s support options from the expert supplier’s viewpoint.
By Simon Walker, European Editor
Rockbolting concepts have also been extended to include cable bolts, once the exclusive domain of major civil engineer-ing projects such as caverns but now increasingly becoming commonplace as an in-depth support method for, say, a weak hangingwall in open stoping or cut-and-fill mining. Mesh screen is also much more widely used than before, acting as a catch-all for spalling rock and as rein-forcement for shotcrete. However, the major disadvantage here is that mesh is time-consuming, and hence expensive, to install properly, as well as being awkward to handle. One potential answer seems to be thin, sprayed-on membranes which, while still at a relatively early stage along the acceptance path—in hard rock min-ing, at least—can provide an equivalent capability in terms of spall protection, yet at significantly lower labor input.
Rather than looking once again at the various systems that are currently on offer in technical terms, E&MJ asked some of the world’s leading suppliers of rockbolts, shotcreting admixtures and TSL membranes for their views on how these systems have been developed in recent years, where they are heading, and what challenges are on the horizon for ground control in mining. Here are some of their responses.
The Major Advances
E&MJ asked what major advances have been achieved in ground-control technol-ogy in hard rock mining over the past 10 to 15 years. According to Lars Hage, technical manager for sprayed concrete at the Switzerland-based producer of construction and mining concrete admix-tures, BASF, the first and foremost point is that there has been a vast improve-ment in the attitude and approach to safety in underground environments. The biggest advance in terms of ground con-trol technology, he said, has been the move from dry- to wet-mix sprayed con-crete, with developments in the concrete industry meaning that it is now possible to produce wet-mix sprayed concrete with compressive strengths of more than 60 MPa at 28 days.
Hage went on to explain that the advent of modern admixtures applied to wet-mix sprayed concrete has reduced problems such as high rebound and dust levels, poor compaction and loss of structural performance significantly, by enabling the placed concrete to be ini-tially plastic in nature. It has also increased output capacity considerably, he added. The introduction of alkali-free set accelerators in particular has allowed for long-term durability in both rock support and tunnel linings, as well as providing an environmentally friendly and non-caustic solution, as opposed to their predecessors, sodium silicate and aluminate.
Furthermore, the addition of steel or synthetic fibers to the sprayed concrete mix has led to improved flexural and shear toughness, and resistance to impact. This is particularly the case in areas of high deformation, or where crack control is needed.
New advances in spraying manipula-tors have made the task of setting the optimal nozzle angle and distance even easier, thanks to technology that auto-matically controls the spraying distance, speed and angle of the spraying jet to the surface. “The mechanization of all procedures that has taken place in the mining industry in the last 20 years has gone hand-in-hand with the develop-ment of sprayed-concrete technology, leading to higher and more efficient out-put,” Hage said.
From the perspective of a major inter-national rockbolting systems supplier, with its North American headquarters in Utah, Dywidag Systems International (DSI) made reference to the U.S. hard rock market. Here, it said, major advances have included the utilization of inflatable expansion bolts and cable-bolt technologies, along with coordinated FLAC and other ground control/geologic analysis methods. These have helped to improve the overall system of supports, with other ground-control stabilization technologies working as a system.
With reference to Thin Spray-on Liner (TSL) technology, Rob Hawker, technolo-gy manager at Minova in Australia, told E&MJthat no TSL has yet been success-fully implemented as a mesh-replace-ment system for in-cycle use in hard rock mines. The company’s principal product in this area, the cement modified poly-mer Tekflex, has been widely used in coal applications to help control rock degra-dation through scaling, spalling and weathering, providing a flexible, high ten-sile-strength lining that can accommo-date stresses associated with rock move-ment as well as acting as a barrier to moisture degradation.
Most of the development work Minova has undertaken over this period has been focused on improving its current range for special applications such as escape-shaft liners or preventing weathering, Hawker added. Looking ahead, he said the company has continued to invest in TSL research and has several new systems that it hopes to commercialize within the next two years. These systems are focused on improving the cure times and the application process, he said.
While Minova’s focus has been on cement-based TSLs, Canadian company Spray On Plastics has developed a two-part polyurea product, RockWeb. Technical specialist Tyler Macpherson’s view is that the biggest advance in TSL technology has been the ability to use liquid RHSF (reactive, high-strength, flexible) TSLs to cover irregular or broken surfaces. These systems provide the absolute highest tensile strengths avail-able, he stated, can cure without the need for, or interference from moisture, and develop their strength much faster than other types of systems—in hours compared with days.
“Most significant is the webbing tech-nology that allows us to do this,” he added. “We have also done considerable equipment development that has made application quick and easy, and we have created bonding systems for excellent adhesion to rock and concrete. We have also developed state-of-the-art equip-ment that makes mobilization fast and simple: no different from hooking up a drill jumbo or bolter,” Macpherson said.
DSI North America claims leadership in inflatable expansion bolt and coating technologies, and is working on expand-ing further the ground conditions for which inflatable expansion bolts can be used. The company told E&MJ it has also developed a major role in the supply of hard rock cable-bolt and resin technolo-gies, while its engineering staff has extensive experience in helping its cus-tomers with geologic analysis and product development.
BASF’s Hage noted that the compa-ny has made significant contributions in advancing ground-support technology, with a comprehensive range of chemi-cals and equipment for wet-mix sprayed concrete in particular. Its R&D team is continuing to develop solutions through expertise in cement chemistry, concrete technology and material science, he added.
In practice, of course, ground-support systems are only as good as the standard of installation or application. A poorly designed support system will not per-form well in the long term, and may not even address immediate safety issues such as spalling where the surrounding rock mass has been damaged during excavation, or where inherent geotechnical features bring the potential for local-ized failures. Such concerns have led to the increased use of shotcrete and mesh, with roofbolts providing the com-pressive arch needed for durable self-support. E&MJ asked each of the respondents what challenges they see in installing modern roof-support systems, and how these can be overcome.
DSI: All underground mining opera-tions see ground control as a major expense, so partnering with DSI allows the operator to have a business partner with industry-leading expertise, innova-tive products, and superior service and support with experienced professionals. These elements are strategic to allow underground mining and tunnelling oper-ations to be safe and productive.
Harker: “In relation to TSLs, chal-lenges include the ease of application and adaption to the current equipment available in mines and controlling the quality of the application. A design thick-ness of 5 mm is very hard to achieve con-sistently on a rough rock surface with no gaps in the spray.”
Macpherson: “We see a lot of work rehabbing broken shotcrete. Damaged shotcrete is very difficult to fix by over-coating because the cracks propagate through the new shotcrete. Also, in places like electrical substations or refuges, the equipment inside cannot be removed for shotcreting so it difficult to repair because of rebound cleanup and ‘collateral damage’ to infrastructure. In some cases, a shotcrete nozzleman can-not even get access because of the bulk of his hoses.”
Hage looked at a different perspec-tive: “The major challenges to the indus-try are establishing enough qualified pro-fessionals to meet industry demand, and the involvement of individual people and companies in the industry who don’t understand the business of ground sup-port. This leads to inferior solutions being offered, and inadequate training of their representatives and industry personnel. The solution would be to introduce indus-try-accepted training packages and ensure the certification of nozzlemen.”
With mines getting deeper, rock conditions will inevitably become more challenging. E&MJ sought the respondents’ views on the key areas of ground-control technology that they envisage will be able to help mining companies operate safely at greater depths.
Hage: “Mines getting deeper will lead to more heat and deformation. To tackle these challenges, more developed and flexible mechanization will be necessary, as well as improved logistics. Optional technologies such as fast-setting sprayable polymer membranes provide effective surface support and protection against weathering of rock and strata, which are important for the safety and efficiency of a mine.”
He went on to quote one of his col-leagues, Warren Mahoney, market man-ager for ground support at BASF Australia. “Faster setting sprayed con-crete accelerators in the initial 1- to 2-hour period after spraying, and real-time thickness control during the application of sprayed concrete will help mining. More consideration should also be given to the use of resin injection for very badly fractured ground. This technology has an extremely fast setting time for bonding strata with minimal downtime,” Mahoney said.
Macpherson: “Generally, operations interested in using RockWeb are experi-encing unravelling ground or high conver-gence. Though it cannot stop the conver-gence (due to its flexibility) when applied underneath a stiffer support like bolts and screen, the unravelling of broken ground can be reduced as the smaller pieces cannot be dislodged from between key blocks.” He added that while this can also be achieved using shotcrete, shot-crete tends to break in squeezing ground, allowing the unravelling to continue.
Macpherson went on to look at logis-tics issues. “As mining operations reach greater and greater depths, the chal-lenges of moving materials like shotcrete increase. RockWeb was used in a cone sump application at a depth of 9,300 feet because the mine was unable to move shotcrete to the level via two shafts fast enough to complete the job econom-ically or on time. The RockWeb materials and equipment were sent to the level in two cage runs,” he said.
Minova: Harker agreed, “The key is to develop technology that reduces han-dling, both in terms of transportation to the mine heading but also in terms of site application. Minova’s development focus is on technologies that will lend themselves to automated installation.”
DSI: DSI has been engaged in the development of a full dynamic bolting product line for the past three years. These products include dynamic steel bolts/resin assist, dynamic cable/resin assist, and dynamic mesh that have all been tested and rated. This work has been completed in Canada and Australia to prepare products that will allow mining companies to work safely and productively at greater depths.
Each of the respondents also addressed the ever-present question of finding sufficient skilled labor to install their systems safely and effectively. From Minova’s perspective, Harker told E&MJ that the company does not just focus on the development of products, but has a systems approach to development. “With this in mind, we aim to provide the tools needed for ease of installation, regard-less of labor force skills,” he said.
“Having enough skilled personnel will always be an issue, particularly in boom-ing economies,” said Hage. “Provided there is a good level of skilled supervi-sion and training incentives in place, this will go a long way to support ade-quate levels of conformity when installing these systems."
Training is clearly critical here, and DSI pointed out that it purposely hires experienced mining professionals to aid its customer base with ground-control installation training issues. Its engineer-ing and R&D staff work closely with its customers to design products and systems that are ‘miner-friendly’ for installation and usage, the company added.
For Spray On Plastics, Macpherson considered that finding good installers was unlikely to be a challenge, outside of simply finding good miners. “We provide training and on-site technical support, as well as highly experienced contract workers where companies don’t feel they have the workforce to apply the material themselves,” he said. “We also rent and lease equipment for single projects so that mines do not have to troubleshoot or maintain it on their own.”
With TSL technology being developed as an alternative, or add-on, to shotcreting, E&MJ asked the three companies involved in this area for some ideas on how the costs and long-term durability of the two systems compare.
“The costs and long-term durability of the two technologies are not really com-parable, as membranes are complemen-tary to sprayed concrete,” said Mahoney from BASF Australia. “There is a big dif-ference in performance between these two systems. Sprayed concrete is pre-dominately used for structural ground support and in some cases just to seal rock from weathering, whereas sprayable membranes are not to be used for struc-tural support. Instead, their principal purpose is for sealing rock against weath-ering or for surface support to bind together scatty ground to reduce the inci-dence of small rocks from falling out. Due to the speed of application for mem-branes, this can make them very cost-effective against sprayed concrete when used for the correct purpose.”
“Applied costs are similar between 4 in. of plain shotcrete and 4 mm of RockWeb, but the peripheral costs make RockWeb attractive,” stated Macpherson. “Application times and logistical costs are reduced dramatically. RockWeb reduces conveyance volumes by 25 times when compared to dry shotcrete, freeing up valuable cage time. Post-application ventilation times are also reduced.”
He went on to point out that when applied to the intrados of shotcrete lin-ers, RockWeb can increase the flexural strength of the shotcrete by up to 15%. “The application of the high-tensile layer to the inside spreads tension away from the shotcrete and helps to delay break-age, he added. “When breakage occurs, the residual strength of the shotcrete is improved dramatically as the joints open less, and less material falls out. It is the best choice for protecting shotcrete in large infrastructure areas and high-backs like crushers, internal shafts or shops where small shotcrete ejections can cause serious bodily harm.”
Harker agreed: “Shotcrete has a proven track record in underground environments and civil works, so its durability is well-understood. Any future TSL system will be tested to ensure comparative durability in any of these working environments. The challenge for operations is that as they need to mine deeper, transport costs play a key part in system-installation costs. This is where TSLs have a major advantage, with only one-tenth of the materials-handling needed.”
What becomes clear from this discus-sion is that in the future, deeper mines will become increasingly reliant on a combination of techniques for their ground-support needs. Each of the sys-tems outlined here has its own place, strengths and shortcomings. With rock conditions varying from heading to head-ing, as well as from level to level, mines will need to make site-specific choices; there is no ‘one-size-fits-all’ answer when personnel safety is involved.
Rockbolting in one of its many forms will doubtlessly continue to form the front line, either alone or together with meshing. Cable-bolting extends this capability deeper into the rock mass. Shotcreting—either plain or reinforced— requires a better-developed delivery infrastructure, but offers long-term sta-bility so long as ground movements are limited in extent. TSLs are quick to install and offer good spall resistance, but do not have the shotcrete’s structural capacity. Each operation must find its own optimum support solution.
Resistive unrolling keeps the untensioned grid under better control during
Inventing a Sticky Solution
Wire mesh has long been used to provide coverage of roofs and sidewalls, catching spalled rock before it falls and—if it is installed correctly and pinned in place with rockbolts—provid-ing additional support to the rock mass. However, as anyone will know, who has ever had the challenge of handling it, and trying to ensure that the mesh conforms closely to the rock surface, it is not the world’s easiest material to work with. As a result, all too often the job is not done properly, and the support provided is not as good as it could be.
Improved accuracy and speed of installation are just two of the benefits to be gained by using a synthetic support grid instead of wire mesh, which can be heavy and unyielding, says U.S.-based Convenience Products. The company has teamed up with Tensar International Corp., manufacturer of TriAx TX Geogrid, an extruded and stretched polymeric grid that can be used in roof and sidewall installations.
Made from lightweight, high-strength, corrosion-resistant polymers, Tensar reports that TriAx TX is equivalent to 10-gauge weldmesh, coming in rolls that can be installed using roof-bolters, miner-bolters or jumbos. “Polymeric rolled grids are much safer and easier to handle, transport and install than metal screens,” said the company’s sector manager, John Steffenino. “For years, everyone has tried to install rolled mesh without the use of holding brackets.”
What was needed, however, was a method of holding the grid in its rolled configuration with just the right amount of resistance to allow it to be pulled apart when the roll was unwound during installation, according to Steffenino, who approached Convenience Products to see if its Touch ‘n Seal Mine Foam would provide an answer.
This led to the development of a patent-pending process known as ‘resistive unrolling,’ the companies report. The Mine Foam is strong enough to prevent rolls of grid material from unwinding under their own weight, yet allows them to be pulled apart in a controlled manner while keeping the remainder of the roll intact.
|The Responders in Profile
DSI has a long history in ground control and is among the leading ground-con-trol systems providers in Australia and North America. In the U.S. market, DSI Underground Systems claims to be the leader in hard rock product sales, with a range that includes steel, resin, cables, composites, standing steel sup-ports, backfills and other widely used ground-control methods.
BASF has a strong foundation in concrete technology, and was one of the early proponents of sprayed concrete. Through collaboration with the mining industry, this is now widely accepted as a very successful means of ground sup-port. The company’s ground-consolida-tion interests today encompass the injection of fractured strata, cavity fill-ing, water-ingress control, and supply-ing high-performance grouts for rock anchors.
Minova’s TSL range consists of two product types. The traditional Tekflex type is a flexible TSL system that is available as either a two-component liq-uid/powder or a single component dry-sprayed version. The second type, Capcem, is a rigid TSL material that is extensively used for rock weathering pro-tection in South African mines.
Spray On Plastics, with 20 years’ experience in the development of TSLs, has patented what it claims is a unique way of spraying polyurea coatings by cre-ating polymer strings or ‘webs.’ It says that these plastic fibers reach and stretch around obstructions before melt-ing back together to form a resilient structural membrane. It now has two RockWeb products, one for general areas like drifts and pillars, and the other designed for remote spraying in large boreholes—developed as an emergency support system for the Chilean mine res-cue in 2010 in the event of unravelling in the boreholes.