Retreat mining: Risky business?

FOLLOWING an August roof fall incident at Stillhouse Mining in Kentucky in which two men died, retreat mining has again taken center stage.
Retreat mining: Risky business? Retreat mining: Risky business? Retreat mining: Risky business? Retreat mining: Risky business? Retreat mining: Risky business?

 

Donna Schmidt

Published in the December 2005 American Longwall Magazine

 

Russell Cole, 39, the mine foreman, and Brandon Wilder, 23, were fatally injured after a section of roof 20ft wide, 20ft long and 11ft high collapsed the evening of August 3.

 

Wilder's body was recovered about seven hours after the incident while Cole’s remains were located by search dogs on August 7.

 

The accident, which followed two retreat mining fatalities in Kentucky last year, was similar to many others of its type in that it was caused by a roof fall.

 

Some 10% of US underground coal production comes from retreat mining, but it accounts for as much as a quarter of underground fatalities, making this mining method an area of focus for safety advances.

 

American Longwall spoke to Chris Mark, chief of the Rock Mechanics Section for the National Institute for Occupational Safety and Health (NIOSH) at its Pittsburgh Research Laboratory.

 

Mark gave some insight on the advantages of retreat mining, trends in the sector, and what his group and others are doing to make pillar recovery safer for workers.

 

Compared with a longwall operation, according to Mark, the lifespan of a room and pillar operation that uses retreat mining is generally significantly less.

 

“If you’ve got a 5 million ton reserve, it might be there for 5 or 10 years – it could be there less,” he said, and added that many retreat mining operations work two sections, but rarely more than two.

 

Often, the operations are uncomplicated to set up and quick to mine because of their location and the method used.

 

“They’ll move around quite a bit,” Mark said.

 

“Even if it’s the same block of coal, they may work out one area and then start another mine right around the corner to have easier access to it.

 

“Most of these mines are drift mines – right in the side of a mountain rather than down a slope or shaft – so they’re very easy to set up.”

 

The very nature of retreat mining lends itself to an unfavorable safety situation, and NIOSH has worked diligently to define those dangers for operations. The Pillar Recovery Risk Factor Checklist that Mark developed in a 2003 study with NIOSH colleagues Frank Chase and Deno Pappas addresses local stability, global stability, and other risk factors. The more checklist questions that can be answered with a “yes”, the lower the overall risk.

 

One technology Mark highlighted was mobile roof supports.

 

“Mobile roof supports look like longwall shields mounted on bulldozer tracks,” he said.

 

“But unlike a longwall, where you’re under the shield for safety, that’s not what these do at all. You’re not supposed to be under them.”

 

In an area where pillars have been recovered, Mark explained, gravity will eventually take over and cause the area not supported to cave in. Mobile roof supports are sometimes also called “breaker line supports” for that very reason – their job is to break off a roof fall as it moves out of the pillared area and into an intersection as well as to hold up areas of broken roof.

 

Mark noted that roof support technology had come a long way over time.

 

“In the [past], they used to set timbers – in fact, they still do in about half the mines,” he said.

 

“Guys have to go right up to the edge of the gob to set timber posts.

 

“Mobile roof supports got rid of the posts ... but that’s still a very dangerous location to be in.

 

“Roof bolts are still the main protection against roof falls on retreat sections. Often the best way to reduce the risk is to install longer and stronger roof bolts.”

 

So, why choose retreat mining?

 

“There’s really only three ways to get product from an underground mine,” Mark said. “One is longwall, one is room and pillar with retreat mining and one is room and pillar without retreat mining.”

 

Many factors, including shape of the coal reserve, elevation of the seam and location (such as a high ridge) can decide for an operation whether retreat mining is the best option.

 

Space and potential tonnage are the biggest predictors for retreat mining versus longwall mining, said Mark.

 

“A longwall works best in something like the Pittsburgh seam or the Herrin No. 6 seam, where you just have miles of flat, thick, uniform coal. You want to have 50 million tons in one big block of coal ... in order to justify a longwall.”

 

While using a longwall to extract smaller blocks of coal is technically feasible in some cases, he added, in many cases it does not make financial sense for an owner.

 

Also, while there are many room and pillar operations throughout the US, many incorporate retreat mining to increase the amount of product they can take from the reserve area they are working from.

 

“When they develop, they’re taking out maybe 40% of the reserve, maybe not even that much,” Mark said.

 

“So the only way to make it pay is to bring your extraction ratio up.”

 

Mark noted that the value of the seam’s coal, which is steadily increasing in the current coal boom, is also a vital factor, as operations obviously want to get as much coal from an area as possible.

 

With the risk inherent in the retreat mining method, industry trends show a push towards safety rather than phasing out the method altogether.

 

Safety is the key to the future of retreat mining, said Mark.

 

“I’d say it is definitely working towards safety. The economics are pushing things very heavily in that direction right now.

 

“There are fewer and fewer really high-quality reserves in...click here to read on.

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