DEPARTMENT OF LABOR
MINE SAFETY AND HEALTH ADMINISTRATION
Metal and Nonmetal Mine Safety and Health
Report of Investigation
Underground Metal Mine
Double Fatal Fall of Ground Accident
June 5, 2001
Coeur Silver Valley Inc.
Wallace, Shoshone County, Idaho
I.D. No. 10-00082
Stephen A. Cain
Supervisory Mine Safety and Health Inspector
Randy W. Horn
Mine Safety and Health Inspector
Mine Safety and Health Specialist
George J. Karabin
Supervisory Civil Engineer
Originating Office - Mine Safety and Health Administration
2060 Peabody Rd., Suite 610
Vacaville, CA 95687
Lee D. Ratliff, District Manager
On June 5, 2001, Perry N. Stack, and Wayne L. Brenner, production miners, were fatally injured by a fall of ground while installing roof bolts in the 186 stope between the 4300 and 4600 levels. The fall of ground was estimated to be 8- to 11-feet-wide by 10-to 12-feet-long, and filled the work area with 6 to 8 feet of material.
The accident occurred as a result of a rockburst caused by stress that exceeded the strength of the rock in the 186 stope.
Perry N. Stack had a total of 21 years mining experience, with 18 months at this mine. Wayne L. Brenner had 10 years mining experience, with 45 days at this mine. Both miners had received training in accordance with 30 CFR Part 48.
The Galena Mine, an underground silver mine, owned and operated by Coeur Silver Valley Inc., a subsidiary of Coeur d'Alene Mine Corp., was located two miles west of Wallace, in Shoshone County, Idaho. The principal officials were Michael G. Lee, vice-president and general manager; David G. Turcotte, mine manager; and Daniel J. Peterson, safety director.
The mine normally operated three, 8-hour-shifts a day, five days a week. Total employment was 225 persons. Of these employees, 203 worked in the underground portion of the mine with the remainder working at the surface mill and office.
Access to the mine was through two vertical shafts. The mine used the cut and fill mining method, with sand backfill, to extract ore from high-grade silver-copper vein deposits. A flotation mill produced a silver rich concentrate, which was sold to third-party smelters in the United States and Canada.
The last regular inspection of this operation was completed on February 5, 2001.
DESCRIPTION OF ACCIDENT
On the day of the accident, Wayne Brenner and Perry Stack (victims) reported for work at 7:00 a.m., their normal starting time. Brenner was filling in for Stack's usual partner who was on vacation at the time. They were assigned their normal underground work duties that consisted of rock bolting the walls and back of the 46-186E stope.
Work progressed normally until about 12:41 p.m. when a rockburst occurred, releasing a large amount of material that covered both miners.
Previously installed instrumentation located underground and on the surface detected the rockburst and triangulated the position of the event close to the 186 working area. Mine management immediately attempted to contact all personnel underground. Unable to contact the two miners working in the 186 stope, miners attempted to locate the victims. Once they observed the extent of the ground fall, they realized that the two miners were buried and began rescue attempts. Due to the constricted work area, it took approximately six hours to find and recover the first victim. The second victim was recovered two hours later.
INVESTIGATION OF THE ACCIDENT
MSHA was notified of the accident at 1:00 p.m., on June 5, 2001, by a telephone call to Collin Galloway, supervisory mine safety and health inspector. An investigation was started that same day. An order was issued pursuant to Section 103(k) of the Mine Act to ensure the safety of miners. MSHA conducted an inspection of the accident site, interviewed miners, and reviewed appropriate training and work procedure documents. MSHA conducted the investigation with the assistance of mine management and employees. The miners' representative participated in the investigation.
� Mineral production was by an overhand cut-and-fill method. Veins were accessed through horizontal crosscuts and development drifts driven perpendicular to the vertical shafts. The development drifts represented the main mine levels within the veins and were typically spaced at 300-foot vertical intervals. Timbered raises, following the vein, were driven roughly vertically between adjacent levels. Stopes, normally about 100 feet in length, were excavated horizontally between successive vertical levels. Crosscuts in waste rock were generally developed 8 feet to 10 feet wide and roughly 10 feet in height. Drifts, within the veins, varied from 5 feet to over 14 feet in width and 8 feet to 10 feet in height. Stopes were typically mined 8 feet high and 5-6 feet wide, although overbreak or the extraction of wider mineralized zones might result in stope widths of over 11 feet.
� The stoping process involved five steps: drilling, blasting, barring down loose rock and bolting, mucking, and sandfilling. The back of the stope was drilled with 8-foot long holes, loaded with explosives, and shot. After blasting, miners worked off the muck pile to scale and support the ground with rock bolts. Both drilling and bolting were performed using jackleg drills. The broken ore was then loaded out with a slusher, which drew muck to the open raise where it fell through an ore pass to a lower collection point. When mucking was complete, sandfill slurry (composed of waste tailings) was then pumped into the stope to support the walls and raise the floor to begin another cycle.
� Ground support in newly developed openings was provided with rock bolts and timber on an as-needed basis. Rock bolts, which were usually the primary ground support, included 4-foot and 6-foot long mechanical bolts and Split Sets. In firm ground, such as crosscuts developed in waste rock, 5/8-inch diameter mechanical bolts were used. In weaker ground, typically encountered in the stopes, Split Set stabilizers were preferred. Both the mechanical bolts and Split Sets were installed with 12-inch by 16-inch plywood boards (7/8-inch-thick) and 6-inch by 6-inch steel bearing plates, and placed on 2-foot to 3-foot centers. Timber sets and stulls were used in conjunction with the rock bolts as ground conditions dictated.
� Rockbursts occur when a volume of rock is strained beyond its elastic limit and the accompanying failure is of such nature that accumulated energy is released instantaneously. The size of failures that occur are largely dependent on the strength of the rockburst and its proximity to the ground openings. Steeply dipping ore veins occur in hard, brittle quartzites and argillites. Concentrated stress fields occur at depths near structural discontinuities in these host rocks. At depths greater than 2,000 feet, rocks of these types are known to burst. As stopes progress toward the drifts above, stress is concentrated in the vertical ore pillar of the stope and the drift above. When these stresses exceed the strength of the rock, rockbursts occur. Rockbursts have also been experienced while advancing development drifts.
� On December 23, 1998, three rockbursts occurred at the Galena Mine in one day and one of those bursts was located in the 46-186 stope (injuring one miner). Over the last two years, (excluding the June 5, 2001, accident) the mine had experienced five reportable rockbursts including one in the 46-186 stope on March 19, 1999.
� The Galena Mine implemented a rockburst control plan outlining mining procedures to reduce the occurrence of rockbursts. These included the stair-step staggering of stopes in the same vein, single shifting of stopes, sandfilling the mined out stopes, and destressing.
� Mine management determined that bursts occur when the pillar approached a vertical dimension of 30 to 40 feet. Actions to destress the pillar began when the vertical dimension approached 50 to 60 feet. Destressing consisted of drilling and blasting a fan shaped pattern in the back and upper corners of the ribs. Timber supports and rockbolts had been of minimal value in preventing damage from rockbursts other than those of very minor magnitude.
� A typical destress round combined an 8-foot backstope (the series of blast holes drilled in the back) with two holes drilled upward, 15 feet to 20 feet deep, into the back and hanging wall or rib on 5-foot to 10-foot centers. The destress round was timed with millisecond delays which first detonated the backstope and secondly detonated the long holes. Different variations of destressing were used depending on conditions.
� A microseismic monitoring system at the Galena mine used an array of 43 geophones located throughout the mine that comprised two distinct sub-systems installed during different decades. A computer on the 4600 level controlled the wide area network (WAN), that consisted of 30 geophones and covered most of the active mine workings. A computer located on the 3700 level controlled the local area network (LAN), which monitored the 98 stope on the 4000 level and was made up of 13 geophones. These computers relayed information to the surface, through fiberoptic cables, where the data was processed. The processing computer located events by triangulation of information received from at least three geophones. The microseismic monitoring system was separate from a seismograph recording station located on the surface.
� Seismic event locations were not routinely plotted on mine maps, but the events were recorded, and posted daily on a bulletin board in tabular form showing magnitude, 3-D coordinates (easting, northing and elevation) and the stope or area affected (an occurrence located within a 350-foot radius). The accuracy of a specific event location was generally considered to be 50 feet or less, depending on the magnitude of the event and the number of geophones sensing it. Mine management indicated that information from the microseismic system had never predicted a rockburst at the Galena Mine. However, the system had been used successfully, in conjunction with observation, to assess the effect of the destressing program and to identify mine areas showing marked increases or decreases in microseismic activity. This led to the withdrawal of personnel on several occasions.
� The fatal rockburst accident occurred in the 46-186E stope between the 4300 and 4600 levels (Appendix E). The stope extended approximately 95 feet to the northeast and east-northeast from the 186 raise, was about 8 feet high, and varied in width from 5 feet to 11 feet. At the time of the accident, the thickness of the sill pillar between the 46-186E stope and the 4300 level above had been reduced to about 34 feet. Total overburden above the stope was estimated at 5,485 feet.
� The 46-186E stope was developed in the steeply dipped 184 vein, which struck northeast and dominantly dipped steeply southeast. In the 186 raise, the vein widened at the 4300 level and dipped steeply to the northwest and thinned and splayed out with depth to the 4600 level. The local northwest dip placed the hanging wall to the northwest and the footwall to the southeast, which was the opposite orientation of the vein's northeast and southwest extensions. By convention at the Galena mine, the southeast wall was considered to be the hanging wall and the northwest wall was considered to be the footwall, reflecting the dominantly southeast dip of the 184 vein.
� Wall rock in the 46-186E stope was steeply dipping, thinly bedded grayish-green quartzite and metamorphosed alternating beds of light and dark siltstone. Iron-stained quartzite and siltite dominated the northwest wall. Both the back and southeast wall were characterized by quartzite and siltite breccia (containing silver and copper minerals) cemented with quartz and carbonate. The 46-186E stope was observed to be intersected by various orientations of geologic planar features, resulting in a very blocky stope wall surface (Appendix F).
� During 1999, 186 East & West were being mined out together. In November of 1999, 186 West Stope was advanced to relieve potential stress created by the intersection with the 178 vein. The 186 West stope was completed and backfilled by February 2001 at which time mining resumed in the 186 East stope.
� At the 4600 level, the 46-186E stope was initially "straight," but as mining progressed upward, the 184 vein twisted and the 46-186E stope assumed a "dog-legged" shape. The turn to the east created a "point" in the hanging wall (southeast wall) that appeared to protrude over the mined-out and sand-filled area below (Appendix C). Mine records were not clear on when the "dog-leg" in the 46-186E stope occurred, but it reportedly existed when the i-cut was made and the sill pillar was 58-feet thick.
� Destressing in the area of the 186 stope had been extensive. The use of destressing holes was determined in a three-fold process using miner input, supervisor inspection, and discussion during weekly meetings. Based on this input, a decision was made regarding number, location, and directional placement of holes. Destress holes were drilled down from the 4300 level running the length of the 186 stope in 1998. Additional destressing rounds were shot with each successive cut in the 46-186E stope since March 2001 (three cuts).
� Ground support was provided with Split Set stabilizers (staggered 4-foot and 6-foot lengths with 6-inch by 6-inch bearing plates and 12-inch by 16-inch by 7/8-inch plywood boards). Reportedly, the two victims were installing rock bolts with a jack leg drill near the turn in the stope (approximately 50 feet from the 186 raise) at the time of the rockburst.
� The rockburst occurred at 12:42:01 p.m., on June 5, 2001, as indicated by Company records of microseismic activity, and the seismic monitoring station on the surface. The event timing was further corroborated by the seismic network of the Montana Bureau of Mines and Geology, which indicated a 2.0 Richter event in the Wallace area at 12:41:55 p.m. Eight additional microseismic events were detected inside a 350-foot radius of the 186 raise within 40 minutes of the burst.
� Damage caused by the rockburst was severe. Accounts of mine personnel who participated in recovery operations indicate that the 46-186E stope was virtually full of fragmented rock (wall to wall and floor to back) beyond a point 15-20 feet northeast of the 186 raise, and sloped toward the 186 raise, from there at about a 45E angle. After substantial material was removed with the slusher, it was estimated that the southeast rib failed to a depth of 5-6 feet near the "dogleg" as 11-foot-long stulls were required for support, and that the back had failed to a height of 6 to 8 feet adjacent to that location. The extent of damage beyond the turn could not be assessed as the stope was reported full of rubble (to the back) at that point and the material was not removed.
� The event also caused damage to the 186 raise and the overlying 4300 level (34 feet above). Loose rock and fractured walls were observed at virtually every landing in the 186 raise from the 4300 level to the 46-186E stope. Damage to the 4300 level was in the form of fractured walls, dislodged rock, and broken timber supports as observed from the 186 raise looking northeast.
� Access to the 46-186E stope was limited during the investigation to an area within 15 feet to 20 feet of the 186 raise. Subsequent to the recovery effort, additional burst damage occurred in the hanging wall, and propagated 5 feet to 6 feet further toward the 186 raise. Several "thumps" and "pops" were audible as the investigation proceeded.
� Appendix D represents a composite estimate of damage to the 46-186E stope based on miner accounts and laser range finder measurements made during the investigation. The stope height from the sand floor to the undamaged back near the 186 raise was 15 feet, and nearly 23 feet at the peak of the cavity in the back. The distance from the raise timber to the start of the back cavity and initial hanging wall failure was 19 feet, 50 feet to the Split Set fixtures hanging from the back, and 55 feet to the back wall of the stope at the turn. The total weight of rock displaced by the rockburst was estimated to be in excess of 275 tons.
The accident was caused by excessive stress that built up in the pillars of the stope and exceeded the strength of the rock.
Order No. 7965959 was issued on June 5, 2001, under the provisions of Section 103(k) of the Mine Act:
A rockburst occurred on the 186 stope on the 4600 level entrapping two miners. This order is issued to assure the safety of persons at this operation until the mine or affected area can be returned to normal mining operations as determined by an authorized representative of the secretary. The operator shall obtain approval from an authorized representative for all actions to recover persons, equipment and /or restore operations in the affected area.This order was terminated on June 7, 2001, after it was determined that normal mining operations could resume.
A. Persons participating in the investigation (See Below)
B. Persons interviewed (See Below)
C. Plan View Location of 46-186E Stope Below 4300 Level
D. Details of 46-186E Accident Site - 6/5/01
E. Plan View Overlay of 4300 Level, 4600 Level and 46-186E Stope
F. Observed Geology of 46-186E Accident Site
Related Fatal Alert Bulletin:
Persons Participating in the Investigation
Coeur Silver Valley
Michael G. Lee .......... vice president & general managerMiners' Representatives
David G. Turcotte .......... mine manager
Daniel J. Peterson .......... safety engineer
Richard Legualt .......... miners' representativeMine Safety and Health Administration
David Gray .......... miners' representative
Steven A. Cain .......... supervisory mine safety and health inspector Randy W. Horn .......... mine safety and health inspector
George Karabin .......... supervisory civil engineer
Sandin Phillipon .......... mining geologist
Isabel Williams .......... mine safety and health specialist
Coeur Silver Valley
Michael G. Lee .......... vice president & general manager
David G. Turcotte .......... mine manager
Daniel J. Peterson .......... safety engineer
Steve Latina .......... mine supervisor
Steve Knoll .......... engineer technician
Richard Legualt .......... production miner
Leroy Ballensky .......... production miner
Dan Marek .......... production miner
Bill Christman .......... production miner