Underground drilling for aggregate is more commonplace than is suggested by all the surface quarries seen alongside highways. Some 114 underground room-and-pillar aggregate mines (mostly limestone) operate in the U.S., primarily in an area roughly bounded by eastern Nebraska, central Pennsylvania, northern Georgia and southern Missouri. Each of those mines annually produce on average 1.2 to 1.5 million tons of aggregate. The number of these mines could grow. On the West Coast, for example, California may turn to underground drilling as its surface quarries play out.
All this subterranean activity and potential new underground mining has equipment manufacturers and aggregate producers working together to keep drilling machines and mines in productive harmony.
Typical room-and-pillar mine
Underground mines are opened by producers either by horizontally drilling from an adjacent quarry or by sinking a shaft or incline to a below-ground deposit. Various tunneling machines can be used to burrow toward a deposit, but once a floor level is reached, a suite of mining equipment takes over.
A typical mine utilizes at least one single-boom drill and one or more two-boom jumbo drills. One or two rock support drills also will be on the floor to bolt weaker areas of the mine “back,” or ceiling, which is held up by pillars. Those pillars can be 50 feet in diameter and range up to 35 feet in height. The need for back bolting varies widely: An estimated 10 percent of mine operations must have all overhead material bolted together to stabilize it, while some mines need no bolting at all.
Other equipment commonly found in the underground rooms include aggregate loaders and trucks to convey material to the surface, as well as auxiliary operating equipment. The question an aggregate producer faces in opening up a new mine is, which drilling machines on the market will meet the needs of my operation, and what criteria should I use in selecting them?
Selecting a drill rig
Logical first considerations for a buyer are the mechanical and hydraulic features that will maximize production. Tramming speed is one such feature. That is, how easily can a particular 25-ton drill rig “tram,” or move itself, from a drilled rock face to the next open face to be worked. None of these machines move quickly—generally less than 10 mph. Yet nimbleness and speed are relative. Every non-drilling minute reduces a shift’s total production, so one or two mph makes a difference.
A producer should consider the speed with which a drill rig can position its boom or booms for drilling, because the efficiency of this set-up process also helps determine a shift’s output. After aligning itself for drilling, a drilling rig’s rock penetration and hole-flushing rates are the rest of the story. Drilling is accomplished with rotary percussion drills that are simultaneously pressed into a hole as they swiftly rotate and are forcefully tapped for extra rock breakage. Air and mist are constantly injected through the drill bit to flush out shattered material.
“This is happening at a very high speed, of course,” says Rick Robinson, account manager for Sandvik USA and a 20-year veteran of the industry. The most common feed lengths (comprised of drill rod, coupling, and bit) are 16 and 18 ft with finished holes ranging from 14.5 to 16.5 ft deep. “From the time you position a drill and start drilling, a hole will be drilled in anywhere from 50 seconds to two and a half minutes depending upon the hardness of the rock.” In short, quick is good and quicker even better.
One boom or two?
Another choice facing a producer is between single-boom drill rigs and two-boom rigs. The choice is not as obvious as it might seem. Robinson says that when a mine has 7 to 10 open faces to drill each shift, a pair of single boom drills can make fast work of it. A double-boom rig also will do the job, he says, “but because of other factors, production of a two-boom rig vs. a single-boom drill is not precisely 2-to-1. Typically, the two-boom rig will drill 1.5 to 1.7 times what a single-boom rig can drill. It depends on the operator and the hardness of the rock.”
A Sandvik jumbo single-boom model introduced at ConExpo, the DT912D, is representative of self-contained mobile units on the market. The articulated jumbo unit is 56 ft long when carrying its longest boom, weighs more than 26 tons and is four-wheel-driven. Powered by a 277-hp Cummins engine, the rig is a diesel-hydraulic self-contained unit, which means it operates its boom under diesel power rather than from electricity. Therefore, there’s no trailing electrical cord.
The rig’s Tier 4 Final engine has significantly reduced diesel particulate emissions compared to previous models, which somewhat offsets the environmental advantage of electric power. At the same time, fuel consumption has been reduced to about 5 gallons per hour. The drill rig also has an onboard 264-gallon water tank for water-only suppression of drilling dust. With no water lines to supply it, the rig is more mobile.
Drilling tools themselves make a difference in production. Example: The new Sandvik model drills rock 55 percent faster than two key competitors in the market, according to Robinson. “So there is added efficiency from how quickly a feed length can drill a hole. Yet just as important is the overall life of the tool. If you can only drill 10 holes before having to change a bit, your down time eats away at your drilling time. We want to offer a producer tools that are at the top end of drill life.”
Other considerations in choosing an underground drill rig are less about a machine’s mechanics and more about operator well-being. The U.S. Mine Safety and Health Administration (MSHA) focuses on worker environment and safety issues, things like ergonomics for long-term physical comfort, reduction of physical hazards for an operator, and reduced noise and dust emissions. As recently as 10 to 15 years ago, many manufacturers placed little emphasis on these safety and ergonomic issues. Today, most manufacturers dutifully comply with MSHA regulations—and some conscientiously exceed the standards.
“It is important that the buyers of drill rigs spend as much money as they need to but no more,” Robinson says, and one bottom line is that machinery that exceeds MSHA standards costs more. “What we find is that if customers don’t ask the right questions, they don’t realize the overall value that’s being offered for the money. It is up to us who are selling our products to make sure the full value of a rig is made apparent to each customer.”
Operator safety issues will remain an overriding concern so long as underground machines are operated from a driver’s seat. That may not always be the case. Robinson knows of no underground aggregate mine in the U.S. that is operated solely with autonomous drilling rigs. However, the superior drilling accuracy and productivity of computerized, telematics-controlled rigs foreshadows their arrival in greater numbers in underground aggregate operations. “I do foresee that becoming more and more common.”
This article was contributed by Sandvik.