Underground drilling for aggregate is more commonplace than is usually recommended by all the surface quarries seen alongside highways. Some 114 underground room-and-pillar aggregate mines (mostly limestone) operate within the U.S., primarily in a neighborhood roughly bounded by eastern Nebraska, central Pennsylvania, northern Georgia, and southern Missouri.
Each of those mines annually produces, on average, 1.2 to 1.5 million tons of aggregate. The number of these mines could grow.
Typical room-and-pillar mine
A typical mine utilizes a minimum of one single-boom drill and one or more two-boom jumbo drills. One or two rock support drills also are going to be on the ground to bolt weaker areas of the mine “back,” or ceiling, which is delayed 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. In contrast, some mines need no bolting at all.
The question an aggregate producer faces in opening a replacement mine is, which drilling machines on the market will meet the requirements of my operation, and what criteria should be in selecting them and of which directional drilling companies should I use?
Selecting a Drill Rig
Tramming speed is a feature. That is, how easily can a particular 25-ton drill rig “tram,” or move, from a drilled rock face to the next open face to be worked. None of those machines move quickly—generally but ten mph. Yet nimbleness and speed are relative.
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 remainders 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 drilling bit to flush out shattered material.
“This is happening at a very high speed,” says Rick Robinson, account manager for Sandvik USA and a 20-year veteran of the industry. The most common feed lengths (consisting of drill steel, coupling, and bit) are 16 and 18 ft with finished holes starting from 14.5 to 16.5 ft deep.
One boom or two?
A different decision confronting a manufacturer is among single-boom drill rigs and two-boom rigs. The choice is not as obvious as it might seem. When a mine has 7 to 10 clear faces to drill every time, a set of single boom drills can do it quickly.
A double-boom rig also will do the work, he says, “but due to other factors, production of a two-boom rig vs. a single-boom drill is not precisely 2-to-1. Normally, the two-boom rig will penetrate 1.5 to 1.7 times something a single-boom rig could penetrate. 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 may be a diesel-hydraulic self-contained unit, which suggests it operates its boom under diesel power instead of 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 drilling rig also has an onboard 264-gallon cistern for water-only suppression of drilling dust.
With no water lines to provide it, the rig is more mobile. Drilling tools make a difference in production. For example, The new Sandvik model drills rock 55 percent faster than two key competitors within the market, consistent with Robinson. “So there’s 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 ten holes before changing a bit, your downtime eats away at your drilling time. We want to offer producer tools that are at the top end of drill life.”
Other considerations in choosing an underground drilling rig are less than a few machines’ mechanics and more about operator well-being. The U.S. Mine Safety And Health Administration 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.