By Steve Fiscor, Editor-in-Chief
Most western underground coal mines use wire mesh as a form of secondary roof support. It’s a well-established practice for high seams and several mines use mesh on the ribs as an extra precaution to prevent the coal from sloughing. Over the years, mines have implemented good systems for handling wire mesh. A prime example is the material handling systems developed by Fletcher for its line of roof bolters, which helps miners install wire mesh.
Wire mesh is relatively inexpensive. The miners know how to handle it and where to buy it, but there are some downsides to wire mesh as well. It’s conductive and corrosive. If it’s damaged, protruding wires can injure unsuspecting miners. The wire mesh panels are hard to handle and the bundles are extremely heavy. The miners pick up the screen panel, carry it onto the bolter, and place it on the TRS. The awkward nature of handling the panels poses potential bending, slipping, tripping and lifting type injuries.
At Oxbow Mining’s Elk Creek mine, a longwall operation located near Paonia, Colo., meshing the ribs with steel wire has been difficult. Getting the mesh up on the rib, positioning it properly and holding it in place can be a challenge. “Years ago, when Oxbow first started mining, the miners did not mesh the ribs, except in places where they thought they needed it, around the feeder-breakers, power centers, and where the miner operator stood when he made the cut,” said Jens Lange, production superintendent, Oxbow. “As we mined deeper into the mountain, that policy changed and we moved to 100% rib meshing.”
When the bolter goes into a cut, the crew likes to advance the vent tube, which is on the left side of the entry (opposite the miner operator), to get it as close to the bolter canopy as possible. “That puts an individual between the machine and the rib,” Lange said. “We kept asking ourselves, ‘How could we do that better?’”
About four or five years ago, Oxbow began to explore other options and they eventually approached Fletcher and asked them what they thought. Since that initial conversation, Oxbow has worked on three systems with Fletcher and Huesker, a company that manufacturers synthetic mesh (or grids). As it is with trial and error, each system improved progressively. “Oxbow was motivated to find a new method,” said Bill Kendall, western U.S. sales manager, Fletcher. “They were concerned about safety, yet very cooperative—willing to try anything. They had already tried several ideas on their own.” Working with Huesker, Oxbow and Fletcher devised a concept to use synthetic mesh.
Tom Bailey, marketing manager-mining, Huesker, remembers sketching the initial concept on Kendall’s kitchen table after dinner a few years ago. “We wanted to keep it as simple as possible,” Bailey said. “He told me what Fletcher’s bolting machines could do and I explained to him what we could do with the grids. We were able to meet in the middle.”
What they envisioned was a handling system to carry, dispense, and tension rolls of mesh. Mounted on the roof bolter, it would pay out mesh as the bolter trammed forward. As the mesh is unrolled and bolted to the roof, the fan-folded ends automatically unfurl for use as rib mesh. Fletcher designed new mechanical attachments for its bolters. Huesker manufactured custom rolls of grid for each cut. And, Oxbow agreed to test the prototype underground. The current protoype testing began five months ago and it proved successful—the system was easy to work with; it did tension the mesh properly; and the sides did unfold as the machine trammed forward.
The beautiful thing about this new process is that, once the first row of bolts is installed, everything is automatic, Lange explained. The miners tram the machine forward, install the roof bolts and the rib bolts, and repeat the process. “This system works well for us because it allows us to mesh the ribs as we bolt the roof,” Lange said. “When the miners advance the vent tube, the rib is fully supported.”
Meshing the Ribs
Synthetic mesh is lightweight, nonconductive, and non-corrosive. The material cost is double, but Oxbow believes the benefits of its use would outweigh the added costs. When the bolters are working with the system, they are fully enclosed by the mesh, which protects them from smaller nuisance rock falls. The unknowns are how well it performs long term and how it performs compared to wire mesh once it becomes fully loaded with longwall abutment pressure.
The synthetic grids are 16.4-ft wide at the factory. “We roll them out side-by-side overlapping a foot or whatever we need to meet the entry width,” Bailey said. “Then we fan-fold the sides to a specified seam height. Once they are folded in, we roll it into 38-ft long sections for Oxbow and cut it and tie it.” When the rolls of mesh arrive at the face at Oxbow, they measure 20-ft wide and 14 inches in diameter, and weigh 75 lb. To date, Oxbow has installed 55 rolls of mesh.
From a material handling standpoint, the miners handle one 75-lb roll of mesh and they are finished for the entire cut. A single 16-ft steel wire panel weighs 68 lb. A 35-ft cut requires seven wire mesh panels on the roof and six on the ribs. Normally two people are required to handle one piece of wire mesh. “We would drag the wire mesh behind the machine,” Lange said. “Every row, both bolter operators or a bolter operator and a third man would get off the machine and carry a piece of wire mesh up on it. We have a slight 5º dip to the mine that presents a slipping hazard, especially in crosscuts. We no longer have those problems.
“The other thing is the pieces of rock that can fall where the wire roof and rib mesh meet,” Lange said. “We have the ability to eliminate this by pressing the mesh right into the corner. We were also worried about turning corners, but this does seem to work better than the wire mesh. It’s just a lot easier to use.
“Oftentimes we cut coal 18- to 20-ft wide,” Lange said. “Obviously 16-ft mesh doesn’t go all the way to the rib. We wanted full coverage rib-to-rib and up in the corners.” Oxbow tried working with 4-ft supplementary sheets and panels that would interlock. Nothing really worked well.
“What we have been able to do is develop these grids specifically for this purpose,” Bailey said. “What we are trying to do is provide enough roof support without putting too much into the ribs—improving the safety of the entire system—while trying to save the mine some money.”
Oxbow is using the Huesker 200SP5 grids with built-in strength bands. “In 2002 we started using the strength bands to replace the wire ropes used previously,” Bailey said. “The strength bands are integrated into the grids. When these grids are seamed together, the strength bands fall within a foot or two of the roof-rib intersection so that the miners can hang vent tubes and J-hooks for cabling from them.”
The mesh can be custom designed for each mine’s roof control plan. Theoretically, the grid has a tensile strength equivalent to 10-gauge wire mesh and Oxbow’s roof control plan calls for 8-gauge wire mesh. The major concern, however, is the toughness of the synthetic grid. As the longwall approaches and passes, and it begins to load up, will it tear? “We have to use felt pads between the bearing plates and the synthetic grids to keep the plates from cutting the mesh,” Lange said.
“As we mine into deeper cover, we have to keep people out between the rib and the bolter,” said Terry Hayes, safety director, Oxbow. “If it sloughs and they get caught between the rib and the bolter, they could be seriously injured. Also, when the longwall comes through, we pull all of the wire mesh down. That means that a miner has to remove the conventional bolts holding the wire mesh with an impact wrench. That can be somewhat hazardous. With the Huesker grids, it might be easier for us to remove. When the longwall comes through in November, we will know for sure.”
Retrofitting the Bolters
The retrofit for the bolter consists of a simple roll holder which pins to the TRS. The existing articulating arms and roof pads are replaced with new arms and rounded pads to allow the mesh to slide over the TRS without snagging. The new arms include hydraulically extending tubes to support the mesh close to the roof-rib intersection. The extending tubes also include flexible polyurethane ends which can grip the mesh for tensioning both side-to-side and front-to-rear. It’s a relatively low cost ($25,000) upgrade.
A 35-ft roll is hand-loaded into the dispenser before the bolter enters the cut. It is then manually pulled over the TRS and anchored with existing mesh clamps. The bolter trams into the cut and raises the TRS near the roof. The side tubes are then extended hydraulically to tension the mesh and the TRS is then raised to the roof. The mesh is then bolted to the roof.
After the first row of roof and or rib bolts is installed, the TRS is lowered approximately 1 ft. The bolter is trammed forward. The end snubbers on the ATRS create enough drag to automatically tension the mesh in both directions. There is no need for additional man-handling. The ATRS is set against the roof and the bolts are installed
“Basically, we designed a mechanism to carry the rolls on the front of the TRS and a set of cylinders to tension the mesh,” Kendall said. “Once you install the first row of bolts, then as you tram forward, tensioning the mesh and rib mesh deployment is automatic.”
The mesh is stiff, we can hang our vent tubes and utilities from it. As the miners become more familiar with the system, it would become a faster process,” Lange said. “So far, the time to bolt a cut is about equal to wire mesh, but the potential is there to do it faster. We have not noticed any decrease in development rates.”
The initial testing has been completed at Oxbow and it showed that the system is technically feasible. “The system works well and we are refining it to make it a little more user friendly,” Kendall said. “The original concept was to make rib meshing easier and safer. One of the off-shoots is that it’s making roof meshing and rib meshing easier and safer. It’s a system that we can retrofit to 90% of the Fletcher bolters and it can be used in heights as low as 6 ft.”
Originally, this was a ventilation issue that turned into a ground control issue, Lange explained. “We saw the potential problem of men between the machine and the rib and the motivating factor was to keep the rib fully supported while they advance the ventilation tubing,” Lange said. “There may be some issues when the longwall come through. We will have to wait and see.”
Several other western mines have already expressed an interest in the system and they too are waiting to see what happens when the Oxbow longwall hits it. That should happen around Thanksgiving.