By Steve Fiscor, Editor-in-Chief
A continuous miner operator might load a record amount of shuttle cars in a shift, but the section doesn’t advance until the roof is bolted. Just like all of the other jobs underground, when the conditions are good, the work is relatively easy. Ground conditions often change and the miners have to deal with those conditions.
Roof bolting is hard work, where miners often have to lift cumbersome pieces of heavy metal into awkward positions. When conditions change, and the miners are out of their element, safety can become a concern. High roof conditions can present a problem for people who usually work with low profile machines and vice versa.
Better engineered designs and improved technology are making the job safer and more productive. The folks that build roof bolters are constantly improving their machines. Likewise, the companies that manufacture bolts and resin cartridges are always looking at ways to improve their products. At the end of the day, it’s all about operator “buy-in.” The only thing more frustrating to a roof bolter than trying to install a rib bolt with limited protection is to find you have a box of limp resin cartridges.
Improving Roof Bolting Machines
In addition to its recently announced HV 32 roof drill for harder rock, J.H. Fletcher has now modified its lower profile DDR roof bolter with the Rib Access design, which has become very popular with its other models operating in higher seam heights. The HV 32 drill adds a percussive action to the rock drill giving it better penetration for harder rock. “Really, it’s a rock drill,” said Ben Hardman, sales and marketing, J.H. Fletcher Co. “It’s mostly geared toward limestone mines, but several coal operators have found a use for them. One unit is running in a Maryland coal mine with a hard roof rock. Other coal operators are using them on slope driving applications.”
The big difference in comparing roof drilling between coal and hard rock applications would be the head room. A coal drill would have to be low-profile. “We could not place a standard 2.5- to 3-ft rock drill hammer on a coal drill,” said Ward Morrison, industrial mineral engineering manager, Fletcher. “The diameter or the breadth of the drill is not an issue for coal mines, but the length is. This new HV 32 unit has been built in a much more manageable 20-in. size. This unit is short and fat, while most rock drills are long and thin.”
The hydraulically-driven unit is similar to most rock drills except for its shape. “The controls are also very similar to what we have on standard coal machines,” Morrison said. “One lever-operated valve is used for each action: feed, impact and rotation. It’s a very simple change for a coal operator to move from rotation to percussive action.” The rotation and impact section bolt together and that feature makes the device easier to maintain.
Fletcher’s success with the Rib Access HDDR boom in western U.S. applications is well-documented. “They have become a standard design feature now,” Hardman said. “With the lower seam heights in the East and the more difficult mining conditions, the walk-thru DDR has become one of our top sellers. This lower profile machine has a non-lifting boom. One of the problems operators encountered was materials handling. Installing the bolts and plates on the rib and brow was a little difficult. The Rib Access design on the walk through DDR was the obvious next step to protect coal miners.”
Four Rib Access walk-thru DDRs are currently operating in the field. “These machines have been operating for four to six months,” Hardman said. “That number will increase greatly soon. A number of customers have machines on order and, once they see the Rib Access walk-thru DDR, they will more likely convert those machines.”
The Rib Access feature protects the miners from rib sloughage and it also has some ergonomic features with respect to material handling. With a cover plate height of 36 in., the double-boom DDR has the lowest chassis for a walk-thru machine. Operating height is a little higher (50 in.) and up to 8 ft. The Rib Access DDR can be equipped with either crawler pads or rubber tires.
“As far as tramming from one entry to another, it moves as quickly as any of the other machines,” said Dave Morris, senior product manager, Fletcher. “The tramming function can be controlled remotely. The remote operation is becoming more important. When the machines are fully loaded with bolts, resin, plates, etc., with a lower chassis, visibility can be limited. The remote tramming feature allows the operator a lot more freedom. Instead of climbing into the deck, they can stand back from the machine and see their surroundings well.”
The roof bolters still have to climb through the middle of the machine to set the ATRS and perform their primary duties. The position controls are operated from the mid-point of the machine. The drilling capacity remains the same, but safety and productivity improves from a materials handling aspect while bolting the rib.
“The object was to provide full protection for the miners at all times,” Morris said. “This was a crawl-thru chassis. By repackaging the motor-pump configuration, we made the walkway wider.”
Fletcher will display one of these machines at MINExpo 2012 with an air curtain system mounted on it. “The air curtain is a centrifugal blower mounted on the chassis that filters the air and ventilates through the canopy,” Hardman said. “The operator always has filtered air. We made this improvement first on a roof Ranger. This will be a first for a walk-thru DDR. So far, the mines that use it, love it.” Operator buy-in can make or break a new project, Hardman explained. “In this case, they loved the unit,” Hardman said.
Supporting Heavier Loads
While not much has changed recently in the area of fully grouted rebar bolts (passive bolts), many of the vendors marketing roof control systems have announced new products as far as active systems, such as tensionable roof bolts and cable bolts for primary and secondary support. As Waverly McFarland, vice president of marketing for Minova explained, the cable bolt’s ability to resist shear forces caused by horizontal roof movements and large downward deformations has made them a very viable solution for heavy ground conditions.
Minova’s Tensionable Cable Bolt (TCB) system offers the same versatility of traditional cable bolts while providing the ability to apply active tension that results in roof compression in the immediate roof. “Using existing bolting equipment, the TCB provides top-to-bottom tension from the bearing plate to the anchorage zone,” McFarland said. “Depending on the mining cycle and application specifics, these bolts can be installed for both primary and/or secondary support.”
The use of the TCB System reduces the requirements for additional secondary support since the high-capacity cables are able to maintain more volumes of fractured and separated roof material. In addition, TCBs can be installed in adequate lengths to intersect competent roof suspension materials or help form competent roof beams when installed with sufficient resin grout. “As part of our ongoing continuous improvement focus, we have added strand indentations to our cable bolt configurations,” McFarland said. “These indented strands impart higher anchoring capacity to the cable bolt, increased support stiffness, and offer the option of eliminating the traditional Garford Bulb from the bolt design.”
Minova’s Very Low Insertion Force (VLIF) resin is a key component of its TCB system. “The VLIF resin cartridges make it easier to insert the cable bolt into a fully grouted environment while offering a fast setting upper half for quick anchoring and a slower setting lower portion to lock in the bolt tension and roof compression,” McFarland said.
The more frequent requests that Minova hears from the field is: ease of installation and increased holding capacity for challenging ground conditions, especially with rib support and control. “Our FiRep GRP [Glass Fiber Reinforced Plastics] bolts are an excellent option for these rib applications,” McFarland said. “With a continuous thread, these bolts can be trimmed to specific lengths as needed. They offer a high ultimate load and maximum bondage with all grouting material. The bolts can be easily cut using standard mining equipment.” The low weight relative to steel makes them easier to transport and install.
A Stiffer Resin Cartridge
Echoing those sentiments, Renn Oler, engineering director for DSI Underground, mentioned that he and his team are having similar success with passive and active systems. In addition to its composite bolt (Mateen GFRP) and the Tandem Anchor bolts, DSI will soon introduce a tensionable cable bolt. Several longwall mines are using the Tenax, Tencate polymer mesh roof support system.
“The composite bolt is particularly interesting, not only because of its high tensile strength, which is double or triple the strength of the steel, but it is also non-sparking,” Oler said. “DSI has one of the strongest nuts on the market. That will only increase the strength of the cable bolt or any of the other active systems.
The DSI resin assisted Nos. 5, 6 and 7 Tandem Anchor bolts use a design that emphasizes the slit in its shell and chamfer on the nut. As the tandem anchor is rotated and loaded, the plug pushes the shell down over the chamfered nut. Without the silt and chamfered nut, the shell shatters due to the extreme forces the plug applies to the leaves.
Over the years roof bolters have complained that resin cartridges will go limp after long periods of time. “Polyester resin cartridges have to be treated differently than other mine consumables due to the shelf-life of the product,” said Oler. “Mine operators are routinely told to rotate their inventory and avoid storage locations where polyester resin products are exposed to heat, or direct sunlight. Even with these precautions, some resin products have to be scrapped due to shelf-life and heat exposure issues.”
DSI Underground Systems has been working diligently to solve the problems associated with the shelf-life polyester resin cartridges “We recently launched FASLOC Extreme line of polyester resin products,” Oler said. “The Extreme product will stay firm for an extended period. It’s a stiffer package. Our cartridges are now lasting much more than six months.”
Steel Supports for Slopes and Roof Falls
When DSI merged with American Commercial, it gained immense experience in steel supports for mining and tunneling operations, such as steel arches, supports, liner plates and lattice girders, etc. The crossover application for this technology for coal operations is in either new slope construction (or rehabilitating existing slopes), or supporting large underground expanses, such as turnouts or roof falls. “We engineer steel structures to support underground operations,” said Ron Smith, vice president, engineering, DSI Underground. “We design the steel configurations to support the really large openings at the bottom of the slope.
“We also specialize in portal canopies, which are very useful in protecting portal construction from a dangerous highwall,” Smith said. “In zones where it’s impossible to stabilize the roof, such as major roof falls, miners can erect the arches with high-strength lagging on a movable platform in a safe area and then advance their way back into an area safely.”
The biggest change Smith has seen in his nearly 40-year career is the improving industry standards for steel. “We used to build everything with an A36 steel and Grade 50 was priced at a premium,” Smith said. “Now, with all of the advances in steel production, we can use a higher grade steel that has 50 ksi for the same price. More loads can be supported with the same section.” Mining conditions are site specific and DSI can custom fabricate these sections for each mine, while keeping the same structural integrity.
The arches within these canopies carry the static load. The space between the arches is normally filled with wood lagging because it’s usually inexpensive and easy to install. “Wood has limited strength,” Smith said. “In a potential high impact area, a mine would want to use the safest, most economical design, which would be arches set on 3-ft centers. In some roof fall situations, coal operators have placed fill above the canopies to lessen the impact.”
Most modern slopes use a compartmentalized scenario, with men and materials transported on the bottom and a conveyor in a top compartment. “That’s a great opportunity for our horseshoe arches because you can place a deck beam at spring line and divide those two areas,” Smith said. “In a side-by-side configuration, the mine would want to use a square set because of the shorter height and longer span. The straight beam, however, does not offer the same strength as the arch.” A design compromise would use a flatter arch, which has a little less rise to it. That design retains some of the strength of the curved or bent beam, Smith explained, while getting away from a straight beam which requires a much heavier section.”