Figure 1.—Lightning can ignite methane in sealed areas of mines.

No. 489 May 2001

Reducing the Danger of Explosions

in Sealed Areas (Gobs) in Mines

Objective

To identify techniques to reduce the probability of gob gas

explosions.

Background

In the last 6 years, seven explosions of methane and/or coal dust

occurred within worked-out, sealed areas (gobs) of underground

U.S. coal mines. These explosions, believed to have been started by

lightning, destroyed many mine seals and caused much damage

external to the sealed area. Three of these gob gas explosions

occurred in one mine in Alabama over a 3-year period. Fortunately,

the explosion forces and the toxic gases that vented from the sealed

gob area did not cause fatalities or injuries, but they destroyed

several large seals. If miners had been inspecting or working near

these areas, the potential for serious injury and/or death would have

been high. The United Mine Workers of America and the Mine

Safety and Health Administration (MSHA) requested that the

National Institute for Occupational Safety and Health (NIOSH) help

identify the mechanisms for lightning penetration into the gob and

recommend ways to reduce the probability of occurrence of future

explosions from such lightning penetrations into sealed areas of

underground coal mines.

Because three of the explosions occurred in a single mine, special

attention was focused on these particular explosions. The first

occurred in April 1994 in a sealed area, which enclosed about 1.35

square miles of abandoned workings (gob). This explosion

destroyed 3 of the 38 seals that surrounded the gob. These seals

were less than 2,000 ft away from three 4.5-in-diam steel-cased test

wells that extended from the surface into the mine entry. At the time

of the explosion, the National Lightning Detection Network (NLDN)

verified 12 lightning strikes within 10 miles of the mine, including

several above the gob. After the 1994 explosion, the damaged seals

were rebuilt using an acceptable seal design capable of withstanding

a 20-psi explosion, as required by 30 CFR 75.335. On January 26,

1996, a second gob explosion destroyed five more cementitious

pumpable seals less than 2,000 ft away from those destroyed in 1994

and even closer to the steel-cased wells. The NLDN verified

72 lightning strikes in the area of this second gob explosion.

Compressive strength analyses of fragments from the destroyed seals

showed that strengths ranged from 11 to 138 psi, with an average of

83 psi. This is over 100 psi below the minimum 200-psi compressive

strength requirement for cementitious pumpable seals. These seals

were again replaced. On July 9, 1997, the third and most violent

explosion occurred in the same vicinity of the last two explosions.

Three more cementitious pumpable seals were destroyed, including

one newer seal that exceeded the minimum 200-psi compressive

strength requirement. The NLDN verified 695 lightning strikes above

the mine during the time in which the explosion occurred. MSHA's
accident investigation report indicated that the maximum pressure

(propagating forces) of this explosion exceeded 20 psi. The current

seal construction regulations, which relate to all suitable underground seal designs, assume that an explosion occurring in the gob

will not be stronger than 20-psi pressure. However, if a large

flammable gas volume exists in the gob, the resulting explosion

pressure can be >20 psi.

Approach

Two conditions are necessary for an explosion to occur: (1) a

fuel-air mixture with a fuel concentration in the flammable

(explosible) range and (2) an effective source of ignition for that

mixture. If the source of ignition, in this case presumed to be the

lightning, cannot be eliminated, then the only alternative is to

eliminate flammable concentrations or reduce the volume of the

flammable mixture present in the sealed area. If the gob atmosphere

contains methane concentrations greatly above the upper flammable

limit of 15% methane, it will be unaffected by lightning or by other

potential sources that might exist in the gob. Other ignition sources

could include the sudden discharge of old batteries, roof falls, and

spontaneous combustion. After the third explosion in the same gob

area in the Alabama mine, all parties agreed to pressure balance the

sealed gob area in order to reduce the leakage of air into the gob and

therefore to increase the concentration of methane in the gob to well

above the upper flammable limit. By reducing the average

differential pressure from 3 in to about 0.46 in of water gauge across

the gob, the average volumetric air leakage into the sealed area was

reduced by a factor of 2.5. This reduction in seal leakage greatly

reduced the probability of formation of a large volume of flammable

methane-air mixture in the gob.

Results

The sealed gob area was pressure balanced in July 1997, and the

methane concentrations in the gob were monitored by MSHA

through February 2000. The gas samples from behind the seals

indicated methane levels >20%. This is well above the upper

flammable limit. During this same period, several severe storms,

accompanied by lightning, passed over the gob area without

apparently triggering a gob explosion. One storm was so severe that

an imbedded tornado passed over the mine, ripping the doors from

the ventilation fan without disturbing the gob. To date, it seems that

the combination of constructing suitable seals that met the

requirements of the CFR, coupled with pressure balancing of the

sealed gob, helped reduce gas leakage and minimized the formation

of a flammable methane-air mixture in the gob.

Recommendations

Minimizing pressure differentials across seals should be considered an essential part of the overall strategy for sealing gobs.

Reducing the pressure differential reduces the air leakage through the

seal and thus reduces the formation of large flammable methane-air

volumes in the gob. Any wires or metal conductors, including steelcased wells, that connect the surface and the gob area should be

removed. These contribute to the transfer of energy into the mine. If

possible, nonconductive well casings should be used. In addition, old

batteries, which are another potential ignition source, should not be

left behind in the gob. During the sealing process, adequate rock dust

(80% incombustible) should be used both inside and outside of the

sealed areas to reduce the contribution of coal dust to a methane

explosion.

For More Information

For more information on these findings, contact Michael J. Sapko

or Kenneth L. Cashdollar, NIOSH Pittsburgh Research Laboratory,

P.O. Box 18070, Cochrans Mill Rd., Pittsburgh, PA 15236-0070,

phone: (412) 386-6619 or (412) 386-6753, fax: (412) 386-6718,

e-mail: msapko@cdc.gov or kcashdollar@cdc.gov. In addition,

"MSHA Accident Investigation Report, Non-Injury Explosion, Oak

Grove Mine, July 9, 1997" by Doniece Scott and Clete R. Stephan

describes the various pressure-balancing techniques for all seals and

other safety aspects that need to be addressed when using this

approach.

To receive additional information about occupational safety

and health problems, call 1-800-35-NIOSH (1-800-356-4674),

or visit the NIOSH Web site at www.cdc.gov/niosh

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety

and Health.