Stress Causes Killer Mine Bumps
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By SETH BORENSTEIN and JENNIFER TALHELM
The Associated Press
Friday, August 17, 2007; 7:19 PM
WASHINGTON -- It's a benign name for something that explodes like a
hand grenade. Mine "bumps" shoot high-speed coal and rock at anyone in
the way as support pillars buckle.
Three rescue workers at the collapsed Utah mine were killed Thursday
by a bump in a tunnel as they tried to reach six trapped miners. The
coal mine's location, depth and mining practice made it prone to
bumps, according to mining experts and more than a dozen federal
reports.
It's an issue of stress.
Bumps occur when too much stress builds on mine support structures.
Often the mine's roof and floor are strong, so what gives under the
pressure is the coal, usually on the support pillars. It can cause the
floor and roof to buckle, too.
R. Gunnar Gurtunca, director of a government lab that researches
mines, compared a bump to a hand grenade.
"If you throw a hand grenade, it explodes small pieces around it, and
if you get hit by it, you die," said Gurtunca of the Pittsburgh
Research Lab at the National Institute for Occupational Safety and
Health. "The same thing with pillars."
Gurtunca said bump deaths have dropped off dramatically in the past 15
years. Between 1930 and 1995, 166 bumps in U.S. coal mines killed 78
people, according to a study by NIOSH.
On Thursday night, as rescue workers slowly burrowed in at the
Crandall Canyon mine, pressure from the 2,000 feet of mountain above
pushed on the coal pillars and "when that energy gets released it's
like an explosion," federal Mine Safety and Health Administration
director Richard Stickler said Friday in Utah.
The bump was so strong it measured a magnitude 1.6, according to the
University of Utah.
"It's really just a dynamic failure of the coal," said West Virginia
University mining engineering professor Keith Heasley.
Given the situation at Crandall Canyon, the bump could not be
considered unexpected:
_ For decades mining studies have shown that bumps are more prevalent
in the West and at deep levels underground. "The severity and
frequency of bumps increase with depth," said a 1988 Bureau of Mines
study.
_ For more than 40 years, federal researchers have highlighted two
counties in Utah, including where Crandall Canyon is located, as prone
to bumps. In Utah, 17 "bump events" caused 24 deaths and 14 injuries
between 1957 and 1994, according to the NIOSH. That's far more than
any other state.
"It doesn't come as a surprise to me," said J. Davitt McAteer, who was
director of MSHA during the Clinton administration. "We have had bumps
out there for a long time."
_ A significant bump occurred at the mine last year, according to an
April memo from a Crandall Canyon consultant.
_ The 1988 mine study said that bumps are three times more frequent in
room-and-pillar mines, such as the section of Crandall Canyon where
the collapses occurred, where miners leave pillars of coal to hold up
the roof.
_ And in mines that do retreat mining, a type of room-and-pillar
mining that involves pulling the last bit of coal out from pillars as
work is finished, bumps occur even more frequently. Retreat mining
intensifies the stress that causes bumps, according to a 1995 NIOSH
analysis.
MSHA officials have said they approved a retreat mining plan for
Crandall Canyon and an internal company memo indicates it was
practiced at the mine. However, mine owner Robert Murray has said
retreat mining could not have caused the Aug. 6 accident that trapped
the six miners.
The specific roof and other conditions at Crandall Canyon are so
unstable that some companies would opt to leave coal behind rather
than retreat mine, said Larry Grayson, who worked in coal mining for
nine years until 1984 and is now a professor of energy and mineral
engineering at Pennsylvania State University.
http://www.washingtonpost.com/wp-dyn/content/article/2007/08/17/AR2007081701846.html