17th CONFERENCE ON GROUND CONTROL IN MINING

Interaction of Large Events with Excavation

The link between a mining-induced seismic event and
excavation at a particular location is not always clear. However,
it is reasonable to expect that if an excavation truly triggers a
seismic event, that event should be located in a region that under-
goes a nontrivial change in stress field as a result of excavation.
Only three of the 29 large events were located within 10 m (30 ft)
(three diameters) of the ramp face, where ramp excavation could
conceivably play a role in triggering these events. Damage to the
ramp was reported with two of these events. First-motion
information from the third event was characteristic of other events
occurring at much greater distances from the ramp face, sug-
gesting that the event was independent of the ramp and was only
coincidentally located near the ramp face. The two damaging
events, on the other hand, displayed normal first-motion patterns
consistent with movement of rock into the ramp. Moreover, since
both of these events were located beneath the mining horizon, the
ramps were the only voids that could be closed by normal
movement. These two events and, possibly, the unlocated event
that damaged the 06 pillar, are the only cases in which ramp
excavation appeared to trigger a large seismic event.

The relationship between large seismic events and mining can
be examined by plotting the location of these events relative to
the mining front (figure 4A). Each type of event appears to
maintain a fairly constant position with respect to the mining
front, and the first-motion patterns reported for these sets of
events are consistent with the influence of mining-induced
stresses (figure 4B-D). The deepest events were located at the
southern end of the central shear zone, and relieved concentrated
horizontal stresses below the 06 stope. The next major set of
events, right-lateral events along the Hook Fault, was driven by
relief of east-west horizontal stresses by mining in the 06 and 07
stopes and concentration of north-south stresses under the lagging
05 stope. Concentration of stress in the dipping beds was relieved
through vertical shearing (figure 5), and vertical stress on bedding
planes above the stope floor was relieved by normal movement
toward the stope.

DATA SET 2: EVENTS AT THE FACE

‘A second set of events was formed from the large number of
nondamaging seismic events detected during development of the
ramps. Events were considered only if they occurred within a
distance of 30 m (100 ft) of the development face. Detection by
the seismograph was defined as the minimum seismic energy
threshold. The rationale for examining these events was twofold.
First, although these events did not cause damage, many occurred
in similar locations and with similar levels of seismic energy

Figure 4.—Location of major groups of large events
relative to mining face. A, Longitudinal section; B, deep
events responding to east-west abutment stress; Cc,
intermediate events responding to leading 06 and 07 stopes;
D, uppermost events relieving stress in beds through
vertical shear and vertical stresses by normal slip.

321

Vertical
shear

Hook right lateral
24m
Central shear zone
-24.m to -67 mM

LEGEND
D Down
U Up

N_ Normal 05
Lithopillar
o

Sh


17th CONFERENCE ON GROUND CONTRCL IN MINING

were developed at favorable orientations to structure in strong
vitreous quartzite.

Nearly all of the large events examined in this study were
driven by mining of the Lucky Friday vein. The relatively simple
mining geometry of the underhand longwall front combined with
a fairly complex geologic setting to drive a rich mix of seismic
events and, potentially, rock bursts. The influence of geologic
features on seismicity is clearly displayed in first-motion patterns
that show slip on a number of faults, bedding planes, and fracture
zones. This influence is also apparent in the bounding of seismic
activity by shear zones, faults, and subunit boundaries. Mining
clearly concentrates the stress available to drive many of these
events, but there is a marked difference in the seismic response to
the concentration of stress between the two limbs of the mine.
One explanation suggested by earlier in situ stress studies is that
the level of premining stress differs through the rock mass. Thus,
the highly stressed, and stress concentrated, southern limb is
seismically active, while the quieter east limb lacks high levels of
premining stress, and the deep footwall lacks a mechanism for
concentration of premining stresses. Further work is needed to
discern how varying intial stress fields can be reasonably included
in an assessment of the spatial distribution of rock burst risk.

The lack of damage from large events reflects well on the
ground control measures that have been applied. Outside the 07
area, most damage was recorded at the unsupported face or in
immediately adjacent ribs, often before or during installation of
support. The 06 pillar events are a case in point. The first event
caused significant damage to the face, but three subsequent events
were successfully contained by the support system, It also
appears that injuies tended to occur when an area of intense rock
bursting was first entered and that changes in practices were
successful in preventing subsequent injuries in cach of these
areas.

The sources of information available to this study, including
conventional seismic monitoring systems, geologic mapping,
damage observations, and good background information on rock
mass properties and in situ stress, were sufficient to identify
geologic controls that spatially concentrate seismic activity and
rock bursting. Recognition of the role of particular geologic
features in the spatial distribution of rock burst hazards provides
an opportunity for anticipating, rather than reacting to, a changing
level of rock burst hazard. However, much work will be required
to make good on this promise.

ACKNOWLEDGMENTS

This study was made possible by a large database of seismic
information, damage reports, mine records, and geologic data.
The cooperation, openness, and assistance of Lucky Friday Mine
staff, particularly Clyde Pepin, was invaluable and is gratefully
acknowledged. This effort benefited from the efforts of a large
number of our colleagues, particularly Ted Williams, NIOSH, and
Dr. Ken Sprenke, University of Idaho. Valuable assistance in
developing figures and visualizing mine geometry, geology, and
density of development-induced seismicity was provided by Ken

324

Strunk, Greg Wood, Danielle Haraldson, and Paul Peterson. The
assistance of Priscilla Wopat in editing and preparing this
manuscript is also gratefully acknowledged.

REFERENCES

Blake, W., and D. Cuvelier. Developing Reinforcement
Requirements for Rockburst Conditions at Hecla’s Lucky Friday
Mine. In Rockbursts and Seismicity in Mines, Proceedings of the
2nd International Symposium on Rockbursts and Seismicity in
Mines, ed. by C. Fairhurst (Minneapolis, MN, June 8-10, 1988).
Balkema, 1990, pp. 401-406.

Blake, W., and D. Cuvelier. Rock Support Requirements in a
Rockburst Prone Environment: Hecla Mining Company's Lucky
Friday Mine. In Rock Support in Mining and Underground
Construction: Proceedings of the International Symposium on
Rock Support, ed. by P. K.Kaiser and D. R. McCreath (Sudbury,
ON, June 16-19, 1992). Balkema, 1992, pp. 665-674.

Dolph, Jerry. Luck Ran out for Old John at the Lucky Friday
Mine, Coeur d’Alene Press, Mine Tailings, Vol. 90, No. 8, Feb.
22, 1993.

Girard, J. M., T. J. McMahon, W. Blake and T. J. Williams.
Installation of PC-Bascd Seismic Monitoring Systems with
Examples from the Homestake, Sunshine, and Lucky Friday
Mines. Jn Proceedings: Mechanics and Mitigation of Violent
Failure in Coal and Hard-Rock Mines, ed. by H. Maleki, P. F.
Wopat, R. C. Repsher, and R. J. Tuchman. U.S. Bur, Mines
Spec. Publ. 95-01, 1995, pp. 303-312.

Jenkins, F. M., T. J. Williams, and C. J, Wideman. Analysis of
Four Rockbursts in the Lucky Friday Mine, Mullan, Idaho, USA,
In International Deep Mining Conference: Technical Challenges
in Deep Level Mining, ed. by D. A. J. Ross-Watt and P. D. K.
Robinson (Johannesburg, Sept. 17-21, 1990). S. Afr. Inst. of
Min. and Metall., Johannesburg, S. Afr., 1990, pp. 1201-1212.

Kaiser, P. K., D. R. McCreath and D, D. Tannant. Rockburst
Support. In Canadian Rockburst Research Program 1990-1995:
A Comprehensive Summary of Five Years of Collaborative
Research on Rockbursting in Hardrock Mines. Vol 2. CAMIRO
Mining Division, Sudbury, ON, 1998, p. 3.

Lourence, P. B., S. J. Jung and K. F. Sprenke. Source
Mechanisms at the Lucky Friday Mine: Initial Results from the
North Idaho Seismic Network. Jn Rockbursts and Seismicity in
Mines, Proceedings of Third International Symposium on
Rockbursts and Seimsicity in Mines, ed. by C. Young (Kingston,
Ontario). Balkema, 1993, pp. 217-222.

Poad, M. E., G. Johnson, J. K. Whyatt and J, R. Hoskins.
Underhand Longwall Program at Lucky Friday Mine, Mullan, ID.
In Proceedings: Mechanics and Mitigation of Violent Failure in
Coal and Hard-Rock Mines, ed, by H. Maleki, P. F. Wopat, R. C.
Repsher, and R. J. Tuchman. U.S. Bur. Mines Spec. Publ. 95-01,
1995, pp. 335-346.


17th CONFERENCE ON GROUND CONTROL IN MINING

LEGEND
Central shear zone event €9 05 ramp normal slip event, no damage
07 ramp normal slip event <> 06 ramp pillar event

Hook Fault slip event & Vertical slip event

*Op-o

05 ramp normal slip event
with damage

Figure 3.—Locations of large events assigned to various
groups.

showed remarkable consistency in first-motion attributes. The
discrepancy between systems can be interpreted as showing
vertical shear on a plane of weakness with net deformations that
significantly close (implode) the stope. The vertical relative
movement (northern block down) of these events is the same as
the offset observed between walls of the Lucky Friday vein. This
sense of movement also matches the first-motion pattern observed
in a series of very large (2.5+ M,) vertical shear scismic events
(Whyatt et al., 1996b) that occurred on the vein.

Central Shear Zone Events: This group of five seismic
events occurred in a cluster near the vein at the southern margin
of the central shear zone between the Hook and Offset faults. The
group appears to be bounded to the north by the northernmost
vertical shear zone. Macroseismic first motions for these events
were primarily dilational, and the one event captured by the NISN
appeared as an implosion. However, compressional first motions
measured on a few macroseismic system geophones suggest left-
lateral first motions for three of these events. Left-lateral
movement is consistent with closure of the vein. Morcover, these
movements are consistent with the stress field measured directly
above these events on the 5300 level (Whyatt et al., 1995d).
However, this sense of movement (and stresses measured at the
§300 level) is inconsistent with the northwest orientation of the
regional stress field. Vertical slip on a plane parallel to the vein
is indicated for another event, and a confused pattern was
developed for the fifth,

320

Hook Fault Slip Events: This group of five scismic events
occurred over a period of 60 days and is the first evidence that the
Hook Fault, which is nearly coincident with the axial plane of the
Hook Anticline, plays a role in the generation of seisrnic events.
The macroseismic system and NISN both reported first-motion
patterns dominated by right-lateral motion. The first event is
interpreted as having occurred on the Hook Fault between the two
vertical shear zones, while all four subsequent events were
concentrated around the intersection of the Hook Fault with the
northernmost shear zone.

05 Ramp Normal Slip Events: This group of four events
occurred throughout the study period in two separate arcas of the
05 ramp. First-motion information for these events was some-
what sparse, but generally indicated normal slip on bedding
planes, which is a well-established mechanism for seismic events
in the southern limb of the mine (Whyatt et al., 1996b). Spatially,
the first two events, each of which damaged the ramp, occurred
between the up and down ramps midway between the vertical
shear zones. The second set of two events occurred at the same
location near the intersection of the ramp with the northern-most
vertical shear zone within three weeks of each other and did not
damage the ramp.

06 Ramp Pillar Normal Slip Events: The first event in this
group damaged the development face. Repair required removal
of 100 mt of broken rock. While the magnitude of this event was
measured, the seismic systems failed to provide a location or first-
motion pattern. Three additional events occurred close to the
first during the following 5 months. The events were located in
a single 6 m (20 ft) diameter cluster in the vicinity of damage
from the first event. This cluster was located just to the north of
the northernmost vertical shear zone and just above the pillar
between 5480-06 up and down ramps. The macroseismic system
showed normal slip first-motion patterns, but not on any known
plane of weakness. NISN reported implosional first-motion
patterns for these same events, suggesting some reduction in
excavated volume. The repeated occurrence of these events at the
same location in the 06 ramp pillar suggests that some feature, or
set of features, probably including the ramp pillar, contributed to
this concentration of activity. It also suggests that the hazard
posed by the first of these events at the unsupported face was
fully controlled by ground support measures installed prior to the
subsequent large events.

07 Ramp Normal Slip Events: This group of two events
occurred during a 10-day period. The events were located above
the 5480-07 ramp and above the mining horizon. The in-mine
macroseismic system generated a first-motion pattern showing
normal slip on bedding planes for the first event, but missed the
second. The regional NISN network reported an implosional
first-motion pattern for the first event, but the pattern for the
second event was confused. This discrepancy between near-field
and far-field readings is similar to that observed in other sets of
events and likely reflects initiation of movement along bedding
combined with significant closure of the 07 stope. The proximity
of event locations to the ramp suggests that the ramp might have
had some influence on event initiation, even though the first
motions appear to be unrelated to the ramp.


147th CONFERENCE ON GROUND CONTROL IN MINING

Fracture zones

Hard vitreous quate
subunit

Figure 5.—Schematic showing the sense of
vertical slip, stratigraphy, and stope in a
vertical section.

to those that did. Some of these may have been caused by
mechanisms that have little potential for damage, but some, at
least, would have resulted in damage if a less robust ground
support system had been installed. Second, the rate at which
these events were generated is likely to be indicative of local rock
mass conditions.

The large number of these events precluded analysis on a
case-by-case basis. Instead, the overall rate of occurrence of these
events was compared to progress in developing the ramp system.
The position and advance of each ramp face was determined for
each of the 36 months encompassed by this study. The number
of small scismic events within 30 m (100 ft) of the face during
that month was ascertained and then normalized to provide the
number of events per 30 m (100 ft) of advance. A plot of these
results (figure 6) shows two major zones of development-induced
seismic activity. The first and westernmost of the zones lies
entirely within a hard lithologic subunit and is cut off rather
dramatically at the Hook Fault. A similar but more gradual cutoff
is evident near the vein at the subunit boundary. The second and
more centrally located zone is not as clearly constrained.

DATA SET 3: ROCK BURSTS

Data set 3 was defined for events that reportedly damaged a
ramp face, rib, or back within the 5400 and 5480 sublevels. The
set consists of 20 events that injured four miners and required
removal of 750 mt (830 st) of rock during repair of the ramp back,
rib, or face (at 10 to 100 mt per burst). Rib damage, reported in
11 of these 13 events, predominated. Back damage was reported
in three events, and face damage in two events. Burst damage to
the floor was reported for one event. The majority of these events
are best described as strain bursts, and many appear to have been
driven by buckling of slabs in the ramp rib or face.

Events
per 30m
== 018
= 6-10
mm 11-15
wm 16-20

- gm >20

oO

Figure 6.—Density of small seismic events

Aneyewitness account of a smal]-magnitude rock burst in the

_ Ol ramp on May 20, 1993, illustrates this mechanism. The burst

322

occurred as two miners were drilling bolt holes in the rib to install
bolts and chain-link fencing. One miner reported that his jackleg
drill seemed to leap toward the rib, as if the drill stee] had broken
(it had not). Ln the next instant, the rib burst almost from floor to
back along a 5-m (15-ft) section of the ramp to a depth of about
1 m (3 ft). A miner was bruised but not seriously injured, being
protected somewhat by the fencing hanging between them and the
burst.

An exarnination of the rib at the ends of the burst cavity
revealed many closely spaced, joint-like fractures at a slight angle
to the surface of the rib. These fractures had not been recorded
during mapping of the rib several days earlier. Whether or not
they were preexisting structures, their prominence after the burst
suggests they at least opened as the burst occurred. The miner's
perception that his drill leaped toward the rib suggests that quasi-
static dilation of the rib occurred as a prelude to the burst. The
movement was likely caused by fracturing and bending of rock
layers defined by fractures prior to buckling of these slabs into the
ramp.

A number of similar accounts have been gathered from other
rock bursts. These accounts share the following observations:

X_ Sudden, intense fracturing of intact rock into coffee-cup size
or smaller rubble.

X A loud, instantaneous report. The report is described as
resembling an exploding charge or a sonic boom. For nearby
observers, the sound seemed to originate at the immediate
site of the burst. It seems likely that instantaneous fracturing

of rock at the rock burst site is responsible for the sound.



17th CONFERENCE ON GROUND CONTROL IN MINING

Whyatt, J. K., T. J. Williams, and W. G. Pariseau. Trial
Underhand Longwall Stope Instrumentation and Model
Calibration at the Lucky Friday Mine, Mullan, Idaho, USA. In
Rock Mechanics. Proceedings of the 33rd Symposium, ed. by
JR. Tillerson and W. R. Wawersik (Santa Fe, NM, June 3-5,
1992). Balkema, 1992, pp. 511-519. —

Whyatt, J. K. and M. J. Beus. In Situ Stress at the Lucky Friday —
Mine (In Four Parts): 1. Reanalysis of Overcore Measurements
from 4250 Level. U. S. Bur. Mines RI 9532, 1995a, 26 pp.

Whyatt, J. K., F. M. Jenkins, and M. K. Larson. In Situ Stress at
the Lucky Friday Mine (In Four Parts): 2. Analysis of Overcore
Measurements from 5300 Level. U. S. Bur. Mines RI 9560,
1995b, 34 pp.

Whyatt, J. K., M. J. Beus, and M. K. Larson. In Situ Stress at the
Lucky Friday Mine (In Four Parts): 3. Reanalysis of Overcore
Measurement from the Star Mine. U. S. Bur. Mines RI 9567,
1995c, 30 pp.

Whyatt, J. K., T. J. Williams and W. Blake. In Situ Stress at the
Lucky Friday Mine (In Four Parts); 4, Characterization of Mine
In Situ Stress Field. U.S. Bur. Mines RI 9582, 1995d, 26 pp.

Whyatt, J. K., B. G. White and J. C. Johnson. Strength and
Deformation Properties of Belt Strata, Coeur d'Alene Mining ~
District, ID. U.S. Bur. Mines RI 9619, 1996a, 65 pp.

Whyatt, J., T. J. Williams, W. Blake, K. Sprenke and C.

Wideman. Mining-Induced Seismicity at the Lucky Friday Mine. ~
Seismic Events, M>2.5, 1989-1994. U.S. Dept of Energy,

Lawrence Livermore National Laboratory, UCRL-CR-125538, —
Sept., 1996b, 165 pp.

Whyatt, J., and B. White. A Survey of Rock Bursting and
Seismicity during Ramp Development, Lucky Friday Mine, =
Mullan, Idaho. (In progress)

Williams, T. J ., C. J. Wideman, K. F. Sprenke, J. M. Girard, and

T. L. Nichols. Comparison of Data from In-Mine Rock Burst a
Monitoring Systems and North Idaho Seismic Network, Lucky
Friday Mine, Mullan, Idaho. Jn Proceedings: Mechanics and
Mitigation of Violent Failure in Coal and Hard-Rock Mines, ed.
by H. Maleki, P. F. Wopat, R. C. Repsher, and R. J. Tuchman.
U.S. Bur. Mines Spec. Publ. 95-01, 1995, pp. 265-281.

Wilson, Ed. Building an Effective Safety Program-The Lucky
Friday Experience. Evolution of a Managed Safety Program.
| Presentation at the Pacific Northwest Metals and Mining
‘Conference, Spokane, WA, May 1997.
i
i

325 a


17th CONFERENCE ON GROUND CONTROL IN MINING

Rock Bursting and Seismicity During Ramp Development,
Lucky Friday Mine, Mullan, !daho

J. K. Whyatt, Mining Engineer
B. G. White, Geologist

National Institute for Occupational Safety and Health
Spokane Research Laboratory
Spokane, WA

ABSTRACT

A comprehensive survey of mine seismicity and rock bursting
during development of two sublevels at the Lucky Friday Mine,
Mullan, ID, USA, was conducted to better define rock failure
mechanisms and sources of ground control hazards. Survey data
included rock burst damage reports, seismic event locations and
magnitudes and, for the most energetic events, first-motion
information, Several subsets of this database, including large
seismic events, rock bursts, and microseismic activity near the
face, were analyzed. Elements of the large-event and rock burst
data sets were nearly independent. That is, there was no
relationship between the risk posed by a seismic event and its
energy, although such a relationship is well established for the
mine as a whole. All data sets showed that certain geologic
features appear to control the spatial distribution of events and the
spatial distribution of rock burst risk. The data also suggest that,
within the scope of this study, the greatest risk of injury occurs
when a pocket of heightened rock burst risk is first encountered
and that this risk is controlled when miners adapt their practices
to these conditions, 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, 2
changing level of rock burst hazard. However, much work will
be required to make good on this promise.

INTRODUCTION

Ground falls can be attributed to a number of failure
mechanisms. Of these, rock bursting! is an unusually hazardous
failure mechanism. Persistent mining-induced seismicity is a
feature of many deep mines in the United States, and events with
Richter magnitudes of 2 to 4 are not unusual. Catastrophic rock

‘A rock burst is defined as damage to an excavation that
occurs in a sudden or violent manner and is associated with a
seismic event (Kaiser et al.,1998).

317

bursts may also occur in mines with sparse records of mining-
induced seismicity. The 5.2-Richter-magnitude event at the
Solvay Mine in which three-quarters of a square mile of the mine
suddenly collapsed is a recent example.

Despite a long history of rock burst research, much remains
to be learned. Substantial efforts have been made to understand
rock burst mechanisms, alter mine designs to reduce rock
bursting, improve the effectiveness of ground support, and
anticipate (and even predict) the occurrence of rock bursts.
However, the occurrence of any particular mining-induced
seismic event is still a surprise, and the consequences of a
particular event are difficult to ascertain without inspection of
affected areas of a mine.

This National Institute for Occupational Safety and Health
(NIOSH) study was designed to address the hypothesis that
geologic structures, independently and in conjunction with
mining-induced stresses, control the spatial distribution of rock
burst hazards. This hypothesis implies that the level of hazard
can be anticipated, providing an opportunity to control and/or
avoid the hazard. This hypothesis was suggested by recent
studies that found variations in the premining stress field that
were associated with particular geologic structures (Whyalt et al.,
199Sd), and that these stresses promote some types of the largest
(Richter magnitude greater than 2.5) rock bursts (Whyatt et al.,
1996b).

This study examines ramp development history in a portion
of the Lucky Friday Mine for possible links between (1) geologic
structures and (2) patterns of rock burst hazards and mining-
induced seismicity. The authors examined a large population of
seismic events, defined groups of events with similar charact-
eristics, and explored the essential, repeatable characteristics of
these groups. This approach serves to provide a balanced picture
of overall seismic activity while de-emphasizing the effect of
errors, data deficiencies, and random aspects of individual events.

fe


147th CONFERENCE ON GROUND CONTROL IN MINING

X Rock rubble expelled into the mine opening before miners
could react. Initial expulsion of rubble is followed by a brief
interval in which rock burst debris continues to be distributed
about the burst site. Miners involved in these bursts are not
generally knocked down by the ejected rock. Instead, they
are engulfed by a fluid-like flow of rock debris. Miners have
been partially or completely buried while standing erect.
Their legs have been extensively bruised but not broken from
the impacts of individual rock fragments.

‘A dense cloud of dust that immediately fills the air. Near a
major burst, dust may be so dense for several minutes that
miners have the impression that their lamps have gone out
(Dolph, 1993).

An air pressure shock wave or “air blast” that travels through
the mine. The initial air blast shock may be followed by a
period of burst-induced wind that varies from very brief to
sustained, depending on the change in excavated volume
induced by the burst.

X A conceve cavity that narrows with depth, Weak discon-
tinuitics often form the back of the cavity, which is revealed
when rubblized rock is removed. Damage reports examined
in this study reported cavities ranging from 30 to 120cm (1
to 4 ft) deep.

Rock bursts, like events in the first two sets, occurred in a
few clusters involving only a small portion of the ramp system.
Figure 7 illustrates the locations of these clusters and locations
where these bursts resulted in injuries. The location of injuries
within the clusters of rock bursts suggest that the level of hazard
was influenced by sudden onsets of rock bursting that may have
surprised miners. Adding destress holes to the round and
adjusting support measures were probably responsible for the lack
of subsequent injuries in these clusters, but detailed records of
these measures were not obtained.

Figure 7 also indicates which rock bursts could be linked to
records of a particular seismic event. This link could not be
forged for a majority of reported rock bursts, including all bursts
reported in the 07 ramp, and 3 of the 4 reported in the 06 ramp.
Establishing a link between damage and a seismic event can be
difficult, particularly when the event has little seismic energy or
occurs with blasting. The time at which the ramp is damaged can
be difficult to ascertain if a section of ramp is idle when the
damage occurs and the seismic event is not unusually energetic.
Finally, it is also possible that some of these rock bursts,
particular those with minor amounts of reported damage, might
have occurred independently of a seismic events and, thus, be
more appropriately described as falls of ground. However, there
was no correlation between amount of damage and seismic
magnitude within this data set. In fact, many of the most
damaging bursts could not be linked with a seismic event while
while one of the least damaging bursts was included in the set of
large seismic events.

Most of the rock burst clusters fell within the footwall of the
southern limb of the Lucky Friday vein where both large and
small seismic events were concentrated. However, bursting in the

323

LEGEND

QO Rockburst

© Pockburst with injury

Sersenttabe seismic event

Figure 7.—Locations of rock bursts and rock burst injuries.

07 ramp was a notable exception. The scarcity of both large
seismic events (data set #1) and seismic events in general (data
set #2) in the 07 area suggests that stress levels were lower than
in burst-prone zones to the south. Steeply dipping bedding and a
steeply dipping fault both cross the 07 ramp at low angles and
appear to have severely weakened ramp ribs. These bursts were
largely eliminated from the 07 ramp in subsequent sublevels by
reorienting to cut bedding at high angles and repositioning to
avoid the fault.

DISCUSSION

This study was designed to examine rock burst hazards
encountered during development of just over a mile of ramp
within the 5400 and 5480 sublevels of the Lucky Friday Mine.

The study examined 29 large seismic events, hundreds of
microseismic events, and 20 rock bursts. Repair of damage
caused by these bursts required removal of 750 mt (830 st) of
broken rock. Eyewitness accounts were used to build a composite
description of a typical rock burst. These accounts indicate that
most of these bursts were caused by buckling of rock plates
defined by preexisting or stress-induced discontinuities within the
ramp rib or face. These bursts can best be described as strain
bursts. They generated low levels of seismic energy and did not
meet energy threshold criteria useful in other areas of the mine for
identifying potentially damaging events, However, the spatial
distribution of large and small seismic events did correlate with
rock bursting in some areas. The 07 ramp in exception and
may reflect a difference in burst conditions. The 07 ramp
bursting occurred largely as a result of location and orientation
relative to weak bedding planes and a fault. Intense seismicity
and rock bursting were correlated in areas that were highly
stressed, as shown by the 5300 level stress measurement, and



17th CONFERENCE ON GROUND CONTROL IN MINING

An intense scismic response to mining; exceptional
cooperation by the mine operator; and a wealth of geomechanical,
operational, and seismic data made the Lucky Friday Mine a
particularly useful laboratory for this study. The Lucky Friday is
considered to be the most seismically active mine in the Coeur
d'Alene Mining District and among the most seismically active
(in terms of seismic energy per ton of ore mined) mines in North
America (Jenkins et al., 1990). Whyatt et al. (1996b) found that
events with local magnitudes greater than 2.5 otcurred, on the
average, at a rate of one every 2 months over a 6-year period.
This.study addresses only a portion of the seismicity that occurred
from “mid-1990 through mid-1993 and focuses on seismicity
associated with development of the 5400 and 5480 sublevels.
The selection of study dates and sublevels was based primarily on
the relative quality and availability of seismic, in situ stress,
geologic, and damage data.

This report begins with an overview of Lucky Friday Mine
geology and its geomechanical setting, then discusses develop-
ment and screening of the seismic event data set. Whyatt and
White (in progress) provide detailed data on individual events.
Various groups of seismic events are identified, evaluated for
degree of rock burst hazard, and investigated for links to various
geologic features and mining progress. Finally, the study
discusses the potential for integrating these results into planning
future development openings in rock-burst-prone mines.

LUCKY FRIDAY MINE: BACKGROUND

The Lucky Friday Mine is located in the castern portion of the
Coeur d’ Alene Mining District of northern Idaho. The mine used
a traditional overhand cut-and-fill method until a temporary
closure of the mine in 1986. A mechanized underhand longwall
mining method, which eliminated burst-prone pillars, was
introduced when the mine was reopened. A sublevel ramp system
was developed to provide rubber-tired vehicles with access to
these stopes. The 5400 sublevel was the first to use the current
ramp configuration. The change in mining method was supported
by a cooperative research effort by Hecla Mining Co., the U.S.
Bureau of Mines (USBM), and the University of Idaho (Poad et
al., 1995; Whyatt et al., 1992). The underhand cut-and-fill
method is proving to be a much safer way to mine in rock-burst-
prone ground (Poad et al., 1995). Ground control systems were
also substantially improved during this period (Blake and
Cuvellier, 1990, 1992). These measures contributed to a
remarkable six-fold reduction in Mine Safety and Health
Administration (MSHA) reportable accidents at the Lucky Friday
(Wilson, 1997).

Geology

The Lucky Friday vein forms an S-shape in plan view and
extends horizontally about 460 m (1,500 ft). Splits off this vein
extend potential stope length to over 610 m (2,000 ft). The vein
is composed primarily of galena, with lesser amounts of quartz,
siderite, and sphalerite. The vein ranges from 0.6 to 9 m (2 to 30
ft) wide, but averages about 1.5 m (5 ft) wide. The Revett
Formation, which has hosted most historic production in the
Coeur d’Alene Mining District, forms the wall rock. Because the

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vein itself dips steeply (70°to 90°) to the south and east, it comes
into contact with progressively older rocks with depth (figure 1B),
Presently, mining is active within the quartzitic lower member of
the Revett Formation under 1.5 km (1 mile) of overburden.

Numerous faults and folds are apparent, and many of these
intersect the vein structure. The most pronounced faults are the
North and South Contro] faults that delineate the ends of the
Lucky Friday vein. The most pronounced fold, the Hook
Anticline, divides the mine into southern and eastern limbs. A
minor fault approximately traces the axial plane of this fold. The
tock mass surrounding the vein is made up of thick vitreous and
sericitic quartzite beds with soft interbeds of argillite generally
less than 5 cm (2 in) thick, These beds have been grouped into
15- to 46-m (50- to 150-ft) thick subunits of predominantly hard,
brittle, vitreous quartzite and relatively soft, plastic argillite and
sericitic quartzite (figure 1A). An in-depth study of rock and rock
mass properties was recently conducted for each of these three
rock types and their various combinations as found in formations
and stratigraphic subunits of the Coeur d’ Alene district (Whyatt
et al., 1996a).

A 0 15 30

LEGEND
Scale, m WRB Hard subunit
— 4450 level
ibe
Lower Revett HERE Sot subunit
Formation
0 100 200
Lat

Scale, m

N
Lucky Friday*,,
in Hook Fault

-~ 5500 level

5300 level

Figure 1.—Stratigraphy (A) and structure (8), Lucky Friday
Mine.

In Situ Stress

The in situ stress regime at the Lucky Friday Mine was the
subject of a recent in-depth investigation (Whyatt et al., 1995a-
1995d). Considerable variation in the in situ stress field was
observed among overcore cells and sites. Unusual stress
conditions, including a rotation of horizontal principal stresses
and a vertical stress component double that of overburden
loading, were measured at a site on the 5300 level of the mine
directly above the ramp sublevels examined in this study. Stress



17th CONFERENCE ON GROUND CONTROL IN MINING

conditions were confirmed by unusually intense and concentrated
seismicity and rock bursting encountered during subsequent
excavation in this area. Raisebore breakouts observed in a
number of locations from the 5100 through the 5700 levels of the
mine were mapped to establish bounds to the spatial extent of the
stress field rotation. These results are plotted on a map of the
5300 level on the basis of lithology (figure 2). Breakouts in softer
subunits showed the maximum horizontal principal stress orient-
ed uniformly to the northwest. Breakouts and stress measure-
ments in the harder subunits showed similar results, except in a
service raise on the 5400 level and at the overcore measurement
site on the 5300 level, both of which are adjacent to the Offset
Fault. Thus, this stress perturbation appears to be limited in
extent, but likely extends into a portion of the study area along the
Offset Fault.

45°

38/Offset

Fault \

Figure 2.—Orientation of horizontal maximum principal
stress at various locations projected onto 5300 level plan.
A, Locations in relatively soft subunits; B, locations in
relatively stiff subunits.

Seismic Monitoring Systems

Seismic activity was monitored by three seismic networks
during the study period. The oldest of these systems, operated
since 1973, is an analog microseismic system that determines the
location of seismic events and includes a surface seismograph for
estimating relative magnitudes of events. The development of
powerful digital systems based on the personal computer (PC)
provided an opportunity to supplement the microseismic system
with full-waveform monitoring systems. The mine-wide digital
seismic monitoring system (macroseismic system) was installed
in 1989 (Girard et al., 1995), and the district-wide North Idaho
Seismic Network (NISN) began operation in March 1992
(Lourence et al., 1993). Williams et al. (1995) provide a detailed
summary of system components, geophone networks, and the
types and format of data produced by these systems.

Data Screening and Sorting

‘The present study was based on a database containing records
provided by the USBM, the University of Idaho, and Hecla Min-
ing Co. Data was collected for events that occurred from mid-
1990 through the first half of 1993. These data included records
from all three seismic systems discussed above, rock burst
damage reports, and the progress of stope and development

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headings. Reports included, wherever possible, eye-witness
accounts and on-site inspection of rock burst damage. This
database grew to include information on several thousand events.
Detailed event records and seismic first-motion data are too
voluminous to be included here, but are available in a related
paper (Whyatt and White, in progress).

The database was screened using a number of criteria to
define sets of events of interest to this study. Data on events
occurring outside the physical or temporal limits of this study
were eliminated, and then the remaining data were sorted into
three sets of events for further analysis. The first of these sets
consists of events with the largest seismic magnitudes located
within 91 m (300 ft) of a development face. A second set was
defined to include events of all magnitudes but required a location
within 30 m (100 ft) of a development face. A third set was
defined to contain the 20 events with reported damage. These
data sets are examined in the following sections.

DATA SET 1: LARGE SEISMIC EVENTS

The seismic energy released by large mining-induced seismic
events has always attracted attention. These events have been
associated with many large-scale failures, like the 5.2-magnitude
event produced by the collapse of a portion of the Solvay Mine in
Wyoming. As such, they are generally perceived as posing a
great risk to miners, The seismic signals from these events can be
detected at great distances, providing seismologists and ground
control engincers with abundant information on event energy,
location, and mechanism that is often not available for smaller
events. Thus, the largest events, defined as events registering 30
mm or more on the surface seismograph trace (approximately 0.5
M,), were chosen for the first group of events addressed in this
study. The mine has found the 30-mm threshold to be useful for
identifying the events most likely to damage the mine. Thirty
millimeters roughly corresponds to the threshold for collection of
digital seismic records as well. This set was also limited to
events occurring within 91 m (300 ft) of a development face.
Large events occurring farther than 91 m from the ramp face were
considered unlikely to damage the ramp, and their occurrence
would be unrelated to ramp progress.

Groups of Large Events

Twenty-nine large events were sorted into a number of groups
with common characteristics, including location, activated
geologic feature, and failure mechanism (figure 3). Damage to
mine workings was reported for only six of these events, but for
three of the six, the relationship proved to be only coincidental.
Overall, these events accounted for surprisingly little damage and
appear to be part of the rock mass response to mining rather than
to development.

Vertical Slip Events: Macroseismic first-motion patterns
showed that this sct of six seismic events was generated by dip-
slip movement, north block down, on two cast-west-striking
vertical fracture zones in the footwall of the 05 and 06 stopes.
The district-wide NISN consistently detected implosional first
motions, These events occurred over a single 45-day period and