New Approaches Offer Better Ergonomics and Quality for Roof Bolters – undergound mining
Byline: STEVE FISCOR/EDITORIAL DIRECTOR – MINING
The most difficult job found underground is roof bolting. Pinning the top is not only hard physical labor, it’s an acquired art. Bolting machine operators tune into the sound of the roof and drilling speed knowing that it predicts the conditions that lie ahead for the miner section.
Similar to everyone else, roof bolters have seen their workplace improve steadily. The dust from the drilling process has been directed away from the machines. Virtually every machine in operation today has an automated temporary roof support (ATRS) system (See the Cover photo) that holds the roof hydraulically. With these improvements, their jobs, however, would never be considered easy. The miners still have to manhandle bolts and whatever accessories they might be installing. Meanwhile, they have to be ever mindful of the geology, the pace of production, and the quality of their work.
Most of the recent advancements in roof-bolting technology would be considered evolutionary and one of the best sources for information in the area of roof support is West Virginia University’s (WVU) Ground Control Conference, which is held annually during August in Morgantown, W.Va. Each year, the conference publishes more than 40 papers presented by professionals who debate ground-control techniques with a passion.
Among the many areas discussed this year, two technologies stood out as having potential to change the work environment for the roof bolter. The first, a field proven system, demonstrates how a newly developed material-handling system from Fletcher Mining Equipment could decrease operator fatigue, especially during tough bolting conditions. When a miner section encounters bad geology or wet conditions, the last thing a mine operator wants is a miner injured while installing systems designed to protect others.
Although it’s still in the prototype stage, the second concept, the Eclipse System, uses an offset bolt head to improve resin mixing in the hole. Its announcement shook the foundation of conventional resin-bolting theory and has already been debated through the pages of Coal Age (Operating Ideas, August 2002, page 18; and Letters, November 2002, page 8). One sure-fire way to get the crowd at the WVU conference excited is to debate resin mixing and its relationship to the annulus – the difference of diameters between the borehole and the bolt.
The miner sections depend heavily on roof-bolting crews. Without roof support, the face does not advance. Clearly any advantage afforded to these crews enhances the mine’s safety record and overall production.
SURFACE CONTROLS AND SCREENING ERGONOMICS
Anyone who has spent an extended amount of time at the face has been struck by falling rock. If that person was a roof bolter, the chances are even greater. According to data from the Mine Safety and Health Administration (MSHA), from 1995-1999, an average of 700 injuries per year were reported, including one or two fatalities, that resulted from roof rock falls. Almost all of these injuries were caused by relatively small pieces of rock from the immediate roof, not a major collapse.
Health and safety professionals refer to this sort of roof failure as skin or surface failure – where the fall does not extend more than a few inches into the roof. The supports used to prevent these falls are called surface controls. They vary from large bearing plates to steel straps or header boards, and steel or plastic screen (mesh). Each control has its own application and effectiveness, which depends on the roof geology, mine conditions, and the life expectancy for the entry in which it was installed.
Sometimes roof geology is so poor that nothing short of full coverage will provide the best protection. In such adverse conditions, roof screen provides the best protection. A number of factors, however, prevent mines from readily embracing the use of screening, including material costs, installation time, and ergonomic risks. Handling screen can be difficult. The concern is that the use of the roof screen may increase the likelihood of strains and sprains.
Ordinarily, roof-bolting machines are not well-equipped to hold screen and associated supplies. In addition to storage capacity, screen handling is also a challenge for the operators. The hand loading of roof screen onto the machine occurs quite often during uneven and muddy floor conditions. Steel screen can be cumbersome. A 16- x 5-foot (ft) of 8-gauge steel screen weighs approximately 30 pounds (lb). Overhead lifts and awkward positions, along with lifting, pulling, and twisting movements, have negative ergonomic affects. The screening does, however, protect the roof bolter during the drilling process because it’s placed against the roof when the ATRS is raised.
Researchers looked at four case studies that ranged from a scoop dropping off a bundle of screen and supplies in the section that would have to be manhandled, to a new system developed by Fletcher where materials were loaded into pods on the surface that were subsequently loaded onto the roof bolter by remote control at the face.
The worst-case scenario had an average of 1.72 minutes to place the sheet of screen on the ATRS and slide it into position for every 4 ft of advance. For a bolting advance of 120 ft, screening time would add 52 minutes per shift. Because of damage during transport, the operators had to jerk the sheets apart and the bolter had difficulty keeping the screen from sliding around on the ATRS.
In the best-case scenario that used a walk-thru bolter without pods, it took 0.73 minutes to load each sheet onto the roof bolter. It took 0.72 minutes to place the screen onto the ATRS. The total average time per 4-ft advance was 1.45 minutes. For a bolting advance of 120 ft per shift, the screening cost the crew an additional 39 minutes. The walk-thru bolter made material-handling efforts better. Ergonomic risks still exist with awkward position, pulling and overhead lifting.
The case study that involved the new Fletcher bolter with the pods – a model CHDDR walk-thru bolter equipped with a separate screen tray that minimizes the potential for screen damage – posted better results as far as the time consumed to install screening. For a 120-ft advance, the average roof screening time was calculated to be 25 minutes. A comparison between the case studies showed that it took three men approximately 13 minutes to hand load a shift’s worth of supplies onto the bolter. With the material-handling system, it took two men about 6 minutes.
More importantly, however, the bolting supplies are not hand loaded onto the bolter while underground, but are loaded by vendors in the pods above ground and the pods are brought underground. The stress/strain is reduced in handling the screen because the operator can pull off the screen at a more comfortable level. The screen has less chance of getting damaged because the bundles are handled less and do not get damaged during transport. Roof bolters do not have to struggle retrieving materials. Operator fatigue is reduced.
Screening obviously is a great way to offer roof-skin protection. The ergonomics involved in setting the screen presents problems and the new Fletcher material-handling system has potential to alleviate these drawbacks. This frees more time for the roof-bolting machine operator to concentrate more fully on drilling and spinning those bolts into place.
ECLIPSE OFFSET MIXES BETTER
The majority of the roof bolts installed in the United States are 5/8-inch deformed rebar, fully grouted into a 1-inch diameter borehole with polyester resin. Commonly referred to as B-series or No. 5 bar, this system has proven to be quite effective in mines with above-average roof conditions. One of the more critical variables affecting resin-bolt performance is the size of resin annulus. According to Fosroc, a company that manufactures resin cartridges, the optimum annulus has been proven to be 1/4-inch. This means that it effectively mixes the two resin components and incorporates the plastic film packaging within the cure resin. As the annulus size increases, mixing becomes less efficient.
Glove fingering – when the plastic film is forced against the borehole wall and interferes with the mechanical interlock of the resin and the rock mass – has been found on B-series, 3/8-inch annulus bolts within fallen coal mine roof and has been identified by more than one observer as the cause of the fall.
Fosroc evaluated many ways to enhance mixing of B-series resin systems. One initiative proposed to offset the shaft and use the enlarged path of rotation to increase turbulence in the hole. To set a benchmark, the difference in alignment between the bolt head and the central axis (the offset) for No. 5 bolts purchased from five bolt manufacturers was measured and it averaged 0.035 inches.
Lab testing correlated the degree of glove fingering with resin formulation and the bolt head to shaft axis alignment. According to the research, glove fingering and inhomogeneity in the cured resin was evident with the standard No. 5 rebar, B-series bolts and standard resin, but virtually eliminated with the offset bolts and specially designed Eclipse polyester resin. Glove fingering occurred over 45% of the grouted length of the standard B-series bolts (head offset less than 0.035 inches), and less than 13% of the 0.125-inch offset bolts.
Sample Eclipse bolts were cut perpendicular to the long axis of the shaft. The quality mixing, full-resin contact, and perfect incorporation of the plastic film within the consistent homogenous resin mass is evident. The researchers said that despite the intentional wobble produced by the offset bolt, no voids were found at either the rock or bolt-resin interface.
MSHA tested 10 bolts. Of those, seven bolts were tested with a 10[thorn] wedge under the head with the offset facing the thicker end of the wedge and three with the offset facing the thinner end of the wedge. Yield and ultimate loads averaged 20,600 lb and 29,800 lb, respectively. All of the bolts met ASTM minimum strength requirements.
MSHA technical support issued a letter report that stated that Excel Mining Systems has taken the precautions to ensure that the offset heads are manufactured in a way that does not adversely affect bolt strength. In a fully grouted application, loading at the bolt head would be low and the test results would indicate that the potential benefits in reducing gloving with No. 5 rebar could be beneficial to the industry.
Several 9-inch encapsulation pull tests were conducted in 1-, 1-1/8-, and 1-1/4-inch holes. The Fosroc Eclipse slowest resin was pre-mixed and placed in the boreholes. The bar was inserted through the mixed resin and rotated three times. Two centered and two Eclipse tests per hole size demonstrated the effect of centering on pullout strength, with No. 5 grade 60 rebar bolts. The threaded end of the rebar was pulled to 10 tons maximum load, exceeding the 9.2-ton expected yield load of the bar. None of these tests resulted in resin failure.
The newly formulated Eclipse B-series resin, when combined with a 1/8-inch offset between the bolt head and the shaft axis, reduced the glove fingering 70%. The Eclipse system allows the 3/8-inch B-series annulus to achieve the positive mixing characteristics associated with the 1/4-inch optimal annulus systems, with no reduction in structural bolt performances.
Over time, the Fosroc Eclipse system will gain more acceptance. Meanwhile, the improvements that Fletcher continues to incorporate into its bolting equipment only means better working conditions for the miners.
REFERENCES
Campoli, Alan A., Peter S. Mills, and Kerry Dever, Eclipse System Improves Resin Anchored Rebar Bolting, 21st International Conference on Ground Control in Mining, Aug. 6-8, 2002, pp. 126-130.
Robertson, Susan and Gregory Hinshaw, Roof Screening: Best Practices and Roof Bolting Machines, International Conference on Ground Control in Mining, Aug. 6-8, 2002, pp. 189-194.
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