By Steve Fiscor, Editor-in-Chief
The old refrain is becoming all too common in the underground coal mining sector: All the good coal is gone. In addition to a decent mining height, another characteristic of good coal is a thick solid sandstone roof. Anymore, those conditions are hard to find as well. So, the easy coal disappears and the mines are cutting coal from seams with rock partings and weak roofs, knowing the ribs that once stood may not be standing in the future.
As mines venture into more difficult conditions, they will require better tools to get the job done. Several of the suppliers who provide ground control products for the mines are developing products and services to meet those needs. The days of just installing 6-ft fully-grouted bolts on 5-ft centers and calling it good seem to be dwindling. Today several different variations of cable bolts—a method of ground control traditionally reserved for hardrock operations—are a reality in U.S. coal mines. Over time weak roof conditions usually require secondary support. These systems are usually cumbersome to handle and install. But, ingenuity has prevailed in several instances.
Overlay the more difficult mining conditions on a coal market where prices have fallen flat, and a lot of the mines have been forced to cut back and they are relying more on the suppliers to not only maintain consistent supply and quality, but to also provide technical assistance with roof control plans.
Jennmar Solves Problems
For years, Jennmar has been making arches and square sets. More recently the company has developed an impact resistant lagging. When a mine suffers a roof fall, miners will construct a canopy of five or six arches on a skid and they will push it into the fall cavity. The Mine Safety and Health Administration (MSHA) requires the mines to backfill the cavities. This can be quite expensive, anywhere from $50,000 to $100,000 depending on the size of the cavity.
This new type of impact-resistant lagging is wood lagging that has been encased in a combination of recycled rubber and plastic. It will absorb and redistribute the energy from falling rock in the cavity. It’s also water proof. “We developed a special testing machine at the Jennmar plant in Virginia where we dropped a 1,000-lb piece of steel on different lagging panels,” said John Stankus, president of Keystone Mining Services, the engineering affiliate company of Jennmar. “MSHA has seen the samples and they like what they see.”
Stankus believes this will be an important invention for the mining industry. Fall cavities are difficult to bolt. The mines often use foam or place cribbing with bags of sand and rock dust above the arches in the fall cavity to absorb the impact energy. To solve this problem, the Jennmar engineers decided to pursued a whole new theory, impulse momentum.
“The idea is that, when a rock falls, we wanted to maximize the length of the impact, or the duration of the impact,” Stankus said. “We want the material to absorb the energy and not fail. So it needs to have the flexural strength to give, but not fail. This new system should be able to solve the problem at a fraction of the cost of backfilling with foam.” Last month, Jennmar finalized the design of the lagging panel. The company is working with a firm in Texas that is manufacturing the panels and it has ordered 1,000 pieces. Several mines in Midwest have already agreed to test them.
Jennmar recently formed a new company called JennChem that specializes in pumpable cribs and polyeurathane injection, and the company plans to have a new mine seal available in May. “JennChem will offer PUR injection systems, pumpable cribs, or J-Cribs, and two new pumpable seals that meet the 120 psi criteria,” Stankus said. “These are tools that will help the miners operate safely and effectively.”
Jennmar is also developing a couple of new types of tensionable cable bolts. “We now have two types of cable bolts,” Stankus said. “Jennmar’s post-tensioned cable bolt is installed like any cable bolt and then, as a secondary step, a hydraulic unit applies tension to the system. That’s an effective bolt that applies a lot of tension to the roof. In difficult conditions, such as roof sag or convergence especially in a longwall headgate, this bolt will put a lot of tension to the roof for additional protection.”
The Instal cable bolt system has been on the market for a while and the company has been working on a number of improvements. Jennmar is developing a new style Instal cable bolt with a new expansion shell that will be a significant improvement.
“The system would be installed just like a regular Instal bolt,” Stankus said. “The bolter operator drills the hole, puts the resin cartridge in the hole and torques it up. It’s that easy. We are also developing a new type of cable bolt that can be tensioned off the bolting machine.” Last year, Jennmar finished the development of a low-profile cable bolting system for low-coal applications. It involves a new type of cable bolt housing and head that gives it a lower profile, which keeps people from bumping their heads on the system.
The company’s Twin-Lok resin has two speeds of resin in one cartridge. When installing fully-grouted threaded rebar, miners will sometimes place a cartridge of fast resin in first and then follow up with a second cartridge of slower setting resin. A mistake could be costly. The Twin-Lok cartridge is color-coded so the miners know which end to put in the hole first.
Another new product Jennmar is marketing is the PH Roof and Rib Channel invented by a mine engineer who designed the Panther mine in West Virginia and more recently the Mach mine in Illinois. “Jennmar purchased the patent rights from him,” Stankus said. “The patent is still being processed.”
The PH channel fits into the corner where the roof and rib meet. Miners place a bolt through it and it controls cutter roof and spalling. If the mine is using wire mesh, it does a good job of keeping the wire mesh intact. A lot of mines with horizontal stress in the Midwest have a tendency to develop cutter roof and this helps support those types of roof conditions. Jennmar has a couple of different versions of the PH Roof & Rib Channel.
Last year Jennmar was also designing a lot of slopes as new mines accessed reserves. The company’s Stress Geologic and Support Design System, or SGS system, was a spin off from that engineering work. “We can apply the SGS system to other types of ground control designs,” Stankus said. “It’s a new methodology involving some fairly complex computer modeling techniques. This is a technical service that we offer to the mines. It analyzes geology and stress to determine appropriate support design.”
Jennmar’s Australian division is developing a self-drilling bolt. “We will have two different types of self-drilling bolts that we will introduce to the market later this year,” Stankus said. “What separates these bolts from previous self-drilling designs is cost and ease of installation. The self-drilling bolts that are available now are expensive and installing the resin is difficult.”
Fletcher to Develop Low-Profile, Crawl-Through Bolters
Over the years, Fletcher Mining Equip-ment has built a lot of bolting machines for the coal business. More recently the company has noticed a trend toward walk-through bolting machines. “The company’s number one seller for years was the Roof Ranger with outside controls,” said Ben Hardman, associate sales manager, Fletcher Mining Equipment. “It seems like more and more, however, the roof and rib exposure is forcing the operators inside the machines.” The walk-through DDRs and the HDDR roof bolters now represent more than one-half of new machine sales for Fletcher.
Allaying some of the claims of traditionalists, Hardman said that mine after mine claims that bolters working in walk-through machines put as many bolts up in a shift as the miners operating Roof Rangers. “The material handling through the walkway might be a bit of a hindrance, but as far as operating the machine, it is just as fast as the Roof Ranger,” Hardman said.
“Preventing injuries from roof or rib hazards far outweighs any production considerations,” said Tim Burgess, vice president-engineering, Fletcher Mining Equipment. “We are also starting to see coal companies specify these machines in areas that would traditionally have the outside controls, such as low seams in Virginia.”
For low-seam applications, the Fletcher engineers have been working on new bolter designs to make it easier for the operators to navigate the walkway. It will be a lower machine with a wider walkway. “With these lower seam heights, the walk-through chassis is more of a crawl-through chassis,” Burgess said. “The idea would be to increase the walkway width to accommodate crawling miners. Some of the early walk-through machines had a 19-ich width. That’s simply not wide enough to work on your knees. We are looking at 26- to 27-inch walkways on these lower-profile machines.”
Another big design change on the other walk-through machines is the rib-access booms. The rib-access boom debuted at MINExpo 2008 and from the show the unit went straight to work at Bridger Coal Co. “They just ordered another machine,” Hardman said. “Since then, every machine that ships to western U.S. mines has this boom specified. We have since changed out four sets at a longwall mine in Illinois. It’s growing and it gives the operators better access to handle all of the rib support material.”
When the miners are setting the angle pieces in the corner or trusses, these booms make the job safer and easier. “We had instructed the operators to use a resin inserter, but it never fails that someone would climb out on the mast,” Hardman said. “That is the worst possible place for them to be.”
The rib-access boom will soon become standard for walk-through bolters. “The mines in the East are using the HDDRs for the reach, not necessarily the rib support capabilities,” Hardman said. “As soon as they start doing it, they will be wanting to convert to this style machine.” Fletcher also has multiple solutions to address roof skin issues and rocker pad deflectors.
A bit of a departure from traditional roof bolting but well within the arena of ground control, Fletcher engineers have developed a machine that dramatically increases worker safety during mine slope beam installations. It eliminates much of the need for heavy lifting and awkward reaching that can result in injury.
In the past, workers hefted large beam components into position manually, holding them stationary until they could be bolted together, Hardman explained. After several beams had been set, they were fastened together using connector bars and corrugated metal panels. Finally, grout or concrete would be pumped around the metal to make a sealed structure. To manage these operations, miners had to climb around the structures with tools and materials, maneuvering them into position. “We wanted to eliminate as many hazards from these activities as possible,” Hardman said.
With the new machine, beams can be pre-assembled, trammed to the installation site and lifted into position by the operator, located in the tram deck, under a canopy. Next, workers can move to a powered platform while installing connectors and panels, and pumping the sealer. Exposure to injury is decreased, while productivity is improved.
The machine is built on a small face drill chassis, and uses a small face drill boom. The boom swings, lifts and extends. Two 360˚ rotary actuators ensure accurate beam positioning. The platform lifts, tilts and extends so workers can be stable. A walk-over deck design allows easy access from platform to tram deck.
Miner Bolter Designed for Safety and Productivity
Joy Mining Machinery has developed the Joy 14ED25 miner bolter with a rugged sump frame mechanism allowing simultaneous cut and bolt operations. The machine has been specifically designed for the Pittsburgh No. 8 seam and other thin- to medium-seam applications (6 to 10 ft) with good roof conditions. Two AFX drill feed units, positioned behind the miner’s cutterhead, are used to simultaneously insert two 6-ft bolts while the machine is cutting its 52-inch advance cycle. Rib drills are also available for simultaneous rib drilling for one or two rib bolts on each side.
The machine has been extensively proven underground with six machines currently in operation, and several more 14ED25 machines on confirmed orders. Operating results have been impressive, according to Joy, with the machines achieving monthly averages of 60- to 100-ft of advance per 8-hour operating shift. Peak advance rates of more than 300 ft in a single shift have been achieved on several occasions.
The 14ED25 miner bolter has been beefed up to out last other miner bolters operating in U.S. mines. Reliability on this machine has also been impressive with the six operating machines achieving from 90% to 99% monthly availability. The design aim was to achieve up to three times the operating life of standard miner bolters prior to the first major rebuild, and early indications after more than 10 months of operations are extremely positive, according to Joy.
Safety is a primary concern for Joy and the 14ED25 is equipped with roof canopies and rib protection shields, as well as incorporating good operator working locations, which are ergonomically designed to protect the operators and make their jobs easier. Also, by offering increased drivage rates and semi-mechanized operations, the number of operators in the working area is reduced.
DSI Focuses on the Details
More recently, DSI Underground Systems has been leveraging products developed internally “The Met-Tech Tandem Anchor, resin-assisted expansion shell is a prime example,” said Roland Walker, engineering director, DSI Underground Systems. “For years there has been an issue of the expansion shells spinning when the miners have been trying to tighten them up. That makes the mines wonder if they really are providing any support. Met-Tech, a company DSI acquired, came up with a propriety patented design resin assisted shell that alleviated the problem and it gave the mines the installation success they were looking for.”
Another example, Walker offered, was the resin-tensioned rebar, where the miners spin the bolt one way to mix the resin and then spin the other way to tighten the nut. “For years, there was a problem of the nut running into the thread and the nut was stripping,” Walker said. “Met-Tech invented a small stop nut to improve the system. We are focusing on the details these days.”
As far as polyester resins for the mining industry, development activity was fairly stagnant for years. “Resin grouted systems were invented in the 1970s and refined in the 1980s,” said Phil Gramlich, business development manager-resin products, DSI Underground Systems. “There was a period of time where R&D for resins leveled off, it seemed as though most of the major technical challenges had been met. But today, it’s safe to say we have multiple R&D projects to enhance speed, strength, shelf-life, installation qualities, as well as manufacturing capabilities.
In the last couple of years, there has been an increased demand for more exotic packaging schemes and 2-speed resins, Gramlich explained. “Non-box packaging eliminates cardboard waste underground,” Gramlich said. “We are working on several improvements in the way no-box packaging is put together. There was limited capacity from a manufacturing standpoint and we’re taking steps to enhance our capabilities in that area.”
DSI was the first in North America to offer 2-speed resin, Gramlich explained. “The advantage with 2-speed resin is building tensioned beams as the bolts are installed,” Gramlich said. “It lends to ease of handling and minimizes the stock that has to be maintained on site. We are working on enhancing that product line.”
DSI is also developing a fully-grouted cable bolt. Historically, cable bolts in the U.S. have been installed partially grouted. A typical 12-ft cable bolt would have 4 ft of grout. “There has been certain situations where the cable needed to be fully encapsulated, which is a design challenge for polyester resins,” Gramlich said. “We have over come that design challenge and we now offer a product that will work in that application.”
In addition to bolts and resin, DSI Underground Systems has the market leader, American Commercial, in slope support and design, along with specialty steel products for mining and tunneling with three U.S. manufacturing locations, and decades of experience in the North America marketplace.
The latest offering for DSI Under-ground Systems is the technical services and support group. Dr. Jinsheng Chen, rock mechanics manager, along with Tom Blevins, P.E., mine support manager, oversees the support and services aspects of the DSI Mine Engineering and Support Group. “When a mine has roof control problems, they call and expect an answer whether they are product-related or not,” Dr. Chen said. “We are working hard to meet those expectations.”
DSI now has the capacity to numerically model a roof control system based on different rock properties, in-situ stress fields, and bolting systems, Dr. Chen explained. “The system allows us to better understand how different bolting systems, bolt length and spacing affect ground control,” Dr. Chen said. “Numerical modeling is the key to performing a detailed prediction of stress, deformation, and support loads for evaluating the stability of mine structures.”
Dr. Chen said DSI Underground Systems was recently approached by a coal mine suffering from roof falls. The roof geology was a thinly laminated shale/sandy shale and the roof conditions were weak. They had been using a 5-ft fully-grouted (passive) bolt. “We went underground and looked at the roof falls,” Dr. Chen said. “For that rock type, they really needed a clamping force of an active bolt to increase the shear strength of weak bedding planes of the lower roof strata. If you do not have a strong member within the bolting horizon, you have to develop the beam building affect. After we collected the
information and applied the numerical modeling, we found the mine needed to use a different type of primary ground support system. They moved from a passive fully grouted bolting system to an active bolting system and all indications are that the change in product is helping control the roof for the mine.”
Looking toward the future, ground control engineers believe the industry should investigate more corrosion resistant products. They are also looking at resin and rebar systems that interact with the natural geology to give superior strength and the flexibility to deal with more adverse conditions. Perhaps there is a cost effective solution to help new mines extend the life and strength of their primary ground support system.