Ahead, probe of Utah mine cave-in
US investigators are likely to examine the growing – some say perilous – use of 'retreat mining.'
As the three-week search for six men missing in a Utah coal mine enters a last-ditch-effort phase, the federal investigation into what caused the catastrophic cave-in is likely to begin soon and to include a close look at mining practices in this seismically volatile region.
Like the investigation by the Mine Safety and Health Administration (MSHA) after the 2006 Sago mine accident in West Virginia, this probe at the Crandall Canyon Mine will determine if anything could have prevented the cave-in and could lay the groundwork for industry improvements that reduce the likelihood of similar tragedies in the future.
The investigation will be crucial for the nine other coal mines operating in the same vicinity as the Crandall mine. At 1,500 to 2,000 feet below the surface, they are the deepest coal mines in the United States. All are susceptible to "bumps," the mining-induced seismic activity that apparently caused the Aug. 6 cave-in.
Assessing the risks from such susceptibility becomes increasingly critical as more US mines are subject to two or three rounds of mining. In such operations, coal pillars left during the initial mining period – which support the weight of the mountain above – are being removed as miners finish their extraction work, allowing the mine cavity to refill. Such was the case at Crandall Canyon, officials say.
"MSHA has to look at these practices and say, 'if we're going to do this, we have to up the ante in terms of safety recommendations, and say there are certain limitations [on the extent of this kind of mining],' " says J. Davitt McAteer, former head of MSHA and now vice president at Wheeling Jesuit University in West Virginia.
Over the weekend, rescue workers drilled a sixth bore hole at the mine to try to gain some information about the missing men, but it yielded no definitive results. Owner Bob Murray and government officials agreed to try one more location "that may provide conclusive evidence of the fate of the six trapped miners," said MSHA assistant secretary Richard Stickler, in a statement late on Aug. 26. That work, impeded Monday by bad weather, was set to resume Tuesday.
Utah's richest seam of coal swings in the shape of a canted "L" from Colorado westward, under the Wasatch Plateau and dropping down south into Emery County – where Crandall Canyon Mine is located.
This area has been mined for the past 30 years, and that activity has caused the mountainous region to routinely "bump," or readjust, to redistribute the mountains' massive vertical weight above the cavities created by long-wall mining. In long-wall mining, the process used in most Utah mines, machines called continuous miners run forward along a tunnel, chewing up the coal along the face of the tunnel and spitting it into a mine car waiting behind.
Mining engineers help mine owners map out the tunnels and decide which pillars of coal need to be left behind and how many man-made pillars, or jacks, need to be installed to support the weight of the mountain.
Bumps that result from mining and redistribution of a mountain's weight routinely register on the University of Utah's seismology charts. Since seismologists began tracking this activity in 1978, they had recorded 19,122 seismic events as of Aug. 9 in the Wasatch Plateau-Book Cliffs coal mining region. For two years preceding the Aug. 6 cave-in, they recorded 152 seismic events within 1.9 miles of the damaged part of Crandall Canyon Mine.
Seismic activity is not necessarily a predictor of a catastrophic event. For example, the magnitude 1.6 event on Aug. 16 that killed three rescuers and injured six others "would have been in the realm of the size of many thousands of events we've recorded without having any direct impact in the mine," says Walter Arabasz, director of the University of Utah Seismograph Stations. The reading on Aug. 6 was 3.9.
That's because most seismic events don't occur right where miners are working, unlike the one Aug. 16, which happened near the mine opening where rescuers were valiantly digging their way in.
"Seismic monitoring might be diagnostic of stress conditions in the mine, but it's only one piece of information available to a mining engineer," says Dr. Arabasz. For example, he says, information about mining activity at the time of the seismic event, including where coal extraction is taking place, would complement data from seismologists and possibly lead to better predictions about future stress points in the mines.
MSHA officials have indicated that so-called retreat mining, in which the barrier pillars are removed, was taking place at the Crandall Canyon Mine. The agency no doubt will look at the application and approval process that allowed the practice.
Those documents are ordinarily posted on MSHA's website, but they're not there now. Officials say the documents aren't publicly available at this time. Experts who've had access to these documents say Crandall Canyon Mine officers requested permission on June 3 to engage in retreat mining, and that MSHA granted it June 15.
"The circumstance in that mine that eventually became troublesome was a decision to remove elements of that barrier pillar," says Arabasz.
"The last mine [owner] left that barrier wall for a reason," McAteer says. "This remining really raises safety and health considerations that weren't there when the original mining was done." Secondary mining is increasing, he says, because "the good stuff is gone. So now they are trying to pick up the scraps."
Murray Energy Group bought Crandall Canyon Mine a year ago, according to MSHA documents. It also owns two other mines, officials say.
Other mines in the region may be prone to similar accidents "depending on how much coal they want to get out and how deep they are," says civil engineer Stephen Dmytriw, who worked for MSHA for 23 years and now teaches at the Colorado School of Mines in Golden.
"Those companies would submit a plan to MSHA – a ground control plan and a ventilation plan," Mr. Dmytriw says, and MSHA would approve the plan or not.
The investigation is likely to take months, at least. But many experts say the MSHA needs to act quickly to ensure that similar accidents do not occur at other mines operating in the region.