Mine Safety and Health Administration
Contact: (703) 235-1452
February 8, 1996
MSHA CONCLUDES LOSS OF PILLAR STRENGTH LED TO MINE ACCIDENT
A report released today by the Labor Department's Mine Safety and Health Administration (MSHA) concludes that loss of pillar strength due to rock material weaknesses and stresses in the mine triggered a massive pillar collapse last February at Solvay Minerals Inc., Mine in Sweetwater County, Wyo. A pillar is an area of unmined ore that supports the overlying layer of rock.
According to the MSHA report, officials of Solvay have initiated various measures to prevent recurrence of the pillar failure. "Solvay has taken steps to revise the mine plan and to redesign the mine so that this type of incident does not happen again," said J. Davitt McAteer, assistant labor secretary for mine safety and health.
No federal regulations were violated, so Solvay was not cited. As a result of unique geological conditions that contributed to the accident at Solvay last year and a collapse at a Retsof, N.Y., salt mine in March 1994, MSHA has begun a study of ground-control practices in selected underground metal and non-metal mines. "The study will look at current ground-control practices and safety standards to determine what improvements are needed," said McAteer. "MSHA will draw on input from industry, labor and academia."
Two of the 54 miners working underground on Feb. 3, 1995, became trapped when the entire southwest section of the trona mine experienced severe pillar failure. The damaged section of the mine measured 3,000 by 7,000 feet.
Daniel Jereb, 28, a shuttle car operator, was rescued 33 hours after the pillar collapse. Thirteen hours later, rescue teams discovered Michael Anderson, 26, also a shuttle car operator. Anderson died during transport to the hospital; his death was attributed to asphyxiation. The other 52 miners escaped almost immediately after the incident with no serious injuries.
Members of several mine rescue teams were on site throughout the ordeal. "The efforts of these men and women were extraordinary," said McAteer. "They endured for nearly 48 hours to recover the two trapped miners. Their efforts underscore, once again, the value of mine rescue teams."
A separate MSHA technical evaluation report on the accident recommended that oxygen-generating, self-contained self-rescuers (SCSRs) be made available to all underground personnel.
We are encouraging companies to voluntarily comply with this recommendation," said McAteer. "We have initiated a review of the rules requiring underground placement of these devices. We believe that SCSRs can be of such considerable help that this mine and all underground mines should purchase and deploy them. This would be a fitting tribute to the memory of Mike Anderson."
Initially there was public speculation that, because of its intensity, the pillar collapse had been caused by an earthquake; however, investigators did not find this to be true.
Severe floor heaving, rib slabbing and roof falls occurred in the mine's working areas and entries. While the event lasted only 5 or 6 seconds, ground falls and rumblings continued for several hours thereafter.
An air blast resulting from the pillar collapse caused stoppings throughout the south areas of the mine to be blown out. Dense clouds of dust and high concentrations of gases were liberated. Air flow in the production shaft was reversed for about 17 minutes, totally disrupting the mine's ventilation system. Roadways, although littered with debris, were generally intact and passable.
While loss of pillar strength over a period of time due to rock material weaknesses and stresses has been deemed by MSHA officials as the cause of the large-scale pillar failure, the report identifies other possible causes that were considered:
- -- Failure of the large barrier pillar, which overloaded
the nearby panel pillars and caused their rapid deterioration.
- -- Sudden failure of overburden strata above the southwest
panels. Failure of this pressure arch led to overload and
destruction of the panel pillars throughout the
southwest section of the mine.
- -- A large gas pocket or gas reservoir collapsed underneath
part of the southwest panels. Again, pillars were overloaded, and
pillar failure occurred throughout the southwest part of the mine.
To prevent recurrence of the collapse, Solvay officials have initiated the following measures:
- -- Retreat rooms have been shortened in length to provide
larger barriers between panels.
- -- Crosscut centers have been increased from 55 feet to 65
- -- The overall panel extraction rate has been decreased.
- -- A 250-foot barrier pillar will be left in the southeast
part of the mine.
Copies of the complete accident report and the technical evaluation report are available by calling one of the following MSHA offices: Green River, Wyo., (307) 875-6300; Salt Lake City (801) 975-3450; Denver (303) 231-5465.