Just before 8:00 pm local time, a powerful burst of rock exploded in an area where at least 20 miners were working in the Lucky Friday mine. All the miners escaped to safety but several remain hospitalized. This silver mine in the Coeur d’Alene region has been the site of numerous serious incidents this year, including two fatalities. The first on April 15, 2011, a cave-in at 6,000 feet that took the life of Larry “Pete” Marek, 53. The second, the November 17, 2011 incident that took the life of 26 year old Brandon Gray. Relevant to yesterday’s catastrophic event, the mine operator reported on November 16 to the Mine Safety and Health Administration (MSHA):
“We experienced a rock burst on the 5900 mainline during our scheduled blasting time. The rock burst measured over a 2.8 Richter. The rock burst caused some damage in the 5900 54 Ramp as well.” (5900 refers to 5,900 feet, or more than one mile, underground.)
The vice president of investor relations for the mine operator Hecla mining, said the firm believed the December 14 incident was related to “seismic activity” and was not mining related. It will take a few weeks to sort out the evidence, but the company’s assertion is not necessarily consistent with what mining engineers and geologists have observed at this mine and other deep mines where pressures build up in the strata. The seismic activity, which may register 2 to 4 on the Richter scale, may be induced by intense mining itself.
Geologists and mining engineers abroad have studied for years the physical conditions in deep underground mines that create the environment for rock bursts. The Poles and South Africans are some of the world’s experts on the means to reduce the risk of rock bursts.
In the U.S., the former US Bureau of Mines (now part of CDC’s NIOSH) focus some of their research at the agency’s Spokane (WA) Research Laboratory on catastrophic mine failures and means to prevent them. They’ve written at least a dozen papers about rockbursts, many of them using evidence from the Lucky Friday mine. A 1998 paper, for example, discusses how rockburst risks can be forecast:
“Recognition of the role of particular geologic features in the spatial distribution of rock burst hazards provides an opportunity for anticipating, rather than only reacting to, a changing level of rock burst hazard.”
Another paper from 1998, entitled “Classification of large seismic events at Lucky Friday Mine,” the authors indicate
“the study provides a foundation for the design of measures to reduce further the potential for damage arising from these events and to assess whether changes in mining activity aimed at reducing one type of rockburst will increase the incidence of other types.”
A paper from 2002 dissected a large rock burst at the Lucky Friday, demonstrating
“…conclusively that damage was caused by a splitting and buckling failure mechanism initiated by separation of rock into layers along steeply dipping metamorphic shear foliation subparallel to the affected crosscut. The thin tablular layers then buckled into the crosscut from both sides.”
The researchers concluded:
“Rigorous control of the burst by these layers suggests practical measures for avoiding bursts in crosscuts in the future by repositioning and reorienting openings so that splitting and buckling are reduced or precluded.”
A 1995 paper reported:
“Rock-burst damage and related ground support problems may be reduced by (1) planning development openings so they cut bedding, faults, and joints at angles greater than 50 degrees, (2) giving extra attention to ground support in situations where unfavorable geometries cannot be avoided, and (3) destressing or eliminating pillars.”
There are other NIOSH papers here, here, and here, including a 2002 paper entitled “Sixty years of rockbursting in the Coeur d’Alene district of northern Idaho: lessons learned and remaining issues,” and another on the “Role of fault slip on mechanisms of rock burst damage, Lucky Friday Mine, Idaho” in which the authors discuss rockburst prevention that combines compressible fill and destressing.
With this degree of attention on rockbursts and prevention of them at the Lucky Friday mine, I have to wonder how the mine operator and government researchers integrated the findings of these studies into the mine’s engineering and day-to-day operations? Were these simply interesting academic exercises funded by taxpayers, or public health interventions that were implemented and evaluated?