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. 

Underground Mining: Room and Pillar Mining

This drawing depicts the room and pillar method of underground mining. Most underground coal is mined by the room and pillar method, whereby rooms are cut into the coal bed leaving a series of pillars, or columns of coal, to help support the mine roof and control the flow of air. Generally, rooms are 20-30 feet wide and the pillars up to 100 feet wide. As mining advances, a grid-like pattern of rooms and pillars is formed. When mining advances to the end of a panel or the property line, retreat mining begins. In retreat mining, the workers mine as much coal as possible from the remaining pillars until the roof falls in. When retreat mining is completed, the mined area is abandoned.

There are two types of room and pillar mining--conventional mining and continuous mining. Conventional mining is the oldest method and accounts for only about 12% of underground coal output. In conventional mining, the coal seam is cut, drilled, blasted and then loaded into cars. Continuous mining is the most prevalent form of underground mining, accounting for 56% of total underground production. In continuous mining, a machine known as a continuous miner cuts the coal from the mining face, obviating the need for drilling and blasting.

Underground Mining: Coal Miners at Work

http://www.umwa.org/mining/ugmine.shtml

Salt Lake City, Utah

Crandall Canyon Mine timeline

Day 1: Monday, Aug. 6

• A seismic event registering 3.9 on the Richter scale is recorded by the University of Utah Seismograph Stations in the area of the Crandall Canyon Mine at 2:48 a.m.

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