Figure 1. Plan view of Lucky Friday Mine, 5500 sublevel

Role of fault slip on mechanisms of rock burst damage, Lucky Friday Mine, Idaho,

USA

B. G. WHITE & J. K. WHYATT

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

ABSTRACT: Various methods of monitoring slip movements on bedding planes, as well as examination of rock burst damage in stopes,

suggest that many rock bursts in the Lucky Friday Mine are closely associated with these movements. Slip displacements along bedding

simultaneously reduce the physical dimensions of stopes and increase compressive stress along stope margins. Such changes, in turn,

contribute directly to sudden failures of rock and cemented sandfill surrounding stopes. We believe that the reduction of rock burst hazards

must be based on a clear understanding of the basic mechanisms involved. The current work emphasizes the mechanical role of wall rock

movement and seismicity in generating conditions that promote rock burst damage. Based on our interpretations of these mechanisms,

we briefly address several hypothetical practices that could influence rock burst hazards in similar mining situations. The overall goal

of this research is to advance the NIOSH mission of improving the health and safety of the nation’s workers.

1 INTRODUCTION

The Coeur d’Alene Mining District in northern Idaho is the second

largest silver-mining district in the world as well as a leading U.S.

producer of lead and zinc. At recent mining depths of nearly 2,000

m, Hecla Mining Co.’s Lucky Friday Mine has been one of the

most active mines in North America in terms of seismic energy per

tonne of ore mined (Jenkins et al. 1990). 

Here, as elsewhere, a major challenge is to understand the

contributions of mining-induced movements and seismicity in wall

rocks to localized rock burst damage. As considered in this paper,

“rock bursts” are violent outbursts of broken rock, and “damage”

refers to damage to openings involving rock and rock support

structures. During a rock burst, hundreds of tonnes of pulverized

rock may be expelled into openings. A sudden, sharp sound, a

seismic signal, an air blast, and a dense cloud of dust are typical

indicators that a burst has occurred. In most cases, elastically

driven buckling of layered rock caused by high local stress is a

plausible explanation for actual damage (White et al. 1995; Maleki

and White 1997), but most mining-induced seismicity requires a

different explanation. 

To better evaluate these factors, researchers at the Spokane

Research Laboratory of the National Institute for Occupational

Safety and Health (NIOSH) examined many rock burst sites at the

Lucky Friday Mine to identify the mechanical behavior of the rock

during rock bursts. In examining these sites, we attempted to

distinguish clearly between the mechanisms of local rock burst

damage and those that cause seismicity unassociated with

immediate damage. In particular, we looked for evidence of

movement in wall rocks that might explain the damage observed,

and we tried to identify relationships among movement, seismicity,

and damage. We consider that the existence of high ground

stresses at burst sites prior to these events is characteristic, and that

damage is ultimately caused by this high stress.

Development openings that coincide with zones of intensified

mining-induced seismicity provide a unique opportunity to identify

wall rock movements that are the source of seismicity. This paper

focuses on the more frequent rock bursts that affect stopes. Ribs or

rib-abutment junctures are most commonly damaged, but

abutments and backs are occasionally affected also. In the

underhand stopes of the Lucky Friday Mine, damage to overhead

cemented sandfill is often linked to rock bursts. 

We believe that the reduction of rock burst hazards must be

based on a clear understanding of the basic mechanisms involved.

The current work emphasizes the mechanical role of wall rock

movement and incidental seismicity in generating conditions that

promote rock bursts.

2 GEOLOGY AND IN SITU STRESS

The Lucky Friday vein (Figure 1) is hosted by slightly metamorphosed Precambrian sedimentary strata. Early mapping of the

stratigraphy by Hecla geologists showed that rock bursts were

prevalent in the quartzitic lithologies that characterize the upper

and lower members of the Revett Formation, but were uncommon

in the argillitic middle Revett member. The burst-prone strata are

dominated by strong, thick-bedded, vitreous quartzite, but thin

laminations in the quartzite create planes of weakness that may

greatly alter the failure characteristics of this rock. In addition,

thin, weak interbeds of soft argillite separate the quartzite beds at

intervals of 0.5 to 5 m. Argillite interbeds have commonly been

sheared by tectonism, forming a weak clay gouge. Widespread
Figure 2. Rock burst damage in the 5750-05 stope. Note that both ribs, as well as the

overhead sandfill, were damaged in this event. 

core drilling in the vicinity of the vein demonstrates that wall rocks

have been abundantly, but irregularly, fractured by tectonism.

These fractures are believed to aggravate rock burst problems in the

stopes. On a larger scale (tens to hundreds of meters), beds are

tightly folded about steep axes, and these folds, in turn, are

truncated by the steeply dipping vein. A series of subparallel faults

intersect the vein and also form bounding faults at its ends. Strata

generally dip into the vein from the footwall side and away from

the vein into the hanging wall. This structural complexity has

greatly hindered understanding of the processes that cause damage.

The Lucky Friday vein is composed primarily of massive galena

with lesser amounts of quartz, siderite, and other sulfides. Width

of the vein ranges from 0.5 to 8 m, but averages about 1 m. The

northeasterly striking part of the vein is approximately vertical,

while the more westerly trending portions dip steeply southward.

Narrow galena stringers commonly parallel the main vein. These

stringers evidently are planes of weakness, as they often form

extensive portions of stope walls.

Various investigators (see Whyatt et al. 1995 for a critical

review) have consistently identified the direction of greatest in situ

stress at mines throughout the district as nearly horizontal and

trending west to northwest. At the Lucky Friday Mine, the greatest

in situ stress is nearly normal to the northeast-trending part of the

vein and oblique to the west-trending part. The direction of

greatest stress is also approximately parallel to the set of faults that

intersect the vein and bound its ends. 

3 DAMAGE

Rock burst damage may affect ribs, the cemented sandfill, or the

abutment, and may involve several of these locations in the same

event (Figure 2). A common pattern of damage is one that affects

the lower footwall rib and adjacent abutment and the diametrically

opposite upper hanging wall rib adjacent to the sandfill. In these

cases, damage in the hanging wall frequently extends a short

distance up along the side of the sandfill. Such diametrically

symmetrical damage is frequently seen at the mine and resembles

the pattern formed by dog-eared breakouts in holes drilled in highly

stressed ground. Failure resulting from elevated stress normal to

the plane containing the damage is suggested. However, since

cemented sand is relatively weak, the damage to wall rock abutting

the sandfill requires extensive fracture development unrelated to

the immediate burst event. Other distinctive types of damage

involve floor heave across the entire width of the abutment and

failures of sandfill. Although the softness of sandfill probably

precludes violent failure, the appearance of sandfill failures

resembles those caused by rock bursts, which leads us to include

sandfill failures as “rock burst damage.”

Significant damage to drift walls is also common. Corners

between the ramp crosscut and stope are particularly vulnerable,

and minor damage at the contact between ribs and sandfill at this

location is frequent. Persistent raveling of rock in the upper part

of the footwall near the sandfill-rib contact suggests that intense

fracturing often occurs here, most likely during mining of the

previous cut, when such sites were probably subjected to high

stress. 

3.1 Inferred damage mechanisms

A characteristic of burst sites both in development openings and in

stopes is that the rock has a generally tabular character, with the

layers lying approximately parallel to the affected surface. The

layers may be mining-induced fractures, bedding planes, or other

preexisting structures. Localized stress fractures and buckling of

rock layers (Maleki and White 1997) containing such fractures

suggests that burst damage commonly results from shortening of

layered rock to a point where the rock fails suddenly. Confirmation

of the essential role of rock layers in burst damage was obtained

when new development openings in a burst-prone part of the mine

were realigned with respect to the strike of steeply dipping beds.

When openings were driven directly across bedding instead of

nearly parallel to bedding, rock bursting ended.

Layered rock at burst sites at the Lucky Friday Mine typically

resembles the layered appearance of rock behind the face of long-
Figure 3. Representative cross sections. (A) Part of

Lucky Friday vein showing distribution of seismicity. Data

from 17 cuts are superimposed about a common stope

position. (B) South African gold reefs. Reef and data are

rotated 90° from horizontal for more direct comparison

with Lucky Friday. (Joughin and Jager, 1983). 

wall stopes in South African reef mines (e.g. Joughin & Jager

1983:Figure 1), where rock bursts are a persistent hazard. In the

reef mines, failure of layered rock has been identified as a

fundamental mechanism of rock burst damage. However, in

contrast to Joughin & Jager, who concluded that the expulsion of

layered rock in bursts resulted from interleaving of rock layers by

compressional movement, we prefer the view that it is buckling of

such layers that causes fracturing and expulsion (e.g. White et al.

1995; Maleki & White 1997). We note that interleaving of rock

layers would necessarily result in the appearance of buckling, as

layers double up and slip past each other in the central part of the

affected volume of rock. Perhaps both phenomena have a role in

creating rock burst damage.

3.2 Cause of damage

Violent, compressive failures of rock in bursts require the existence

of stress sufficiently high to initiate failure, as well as a sustained

application of stress in the form of “following load,” such as may

be provided by a soft loading system (e.g. Hedley, 1992).

Evidence of elevated compressive stress is provided by flat-dipping

extension fractures and thrust faults that form in cemented sandfill

and the abutment and in diametrically opposite patterns of damage,

as described earlier. The existence of high stress may also be

inferred from documented shortening of stope dimensions and from

the occasional development of mining-induced folds seen in stope

walls.

Evidence of progressive or sudden reductions in the physical

dimensions of stopes suggests that rock burst damage results, in

particular, from shortening along stope margins. Significant

shortening necessarily causes increased stress in the affected

material and may lead to failure. At the same time, the violence of

such a failure (hence, a rock burst) may also require that the rock

or cemented sandfill have been under compression prior to failure

so that initial resistance to failure enabled the storage of elastic

strain energy in the surrounding rock and resulting rapid movement

toward the burst. As an alternative, bedding slip may at first have

been prevented by some asperity within the slip plane.

4 MINING-INDUCED WALL ROCK MOVEMENTS

4.1 Seismicity

An extensive seismic monitoring system installed within the mine

records seismic events and identifies source locations.

Approximately 90% of all seismicity originates near the active

stopes (Figure 3A). The largest of these events is believed

primarily to reflect slip on bedding planes (Whyatt & White 1998).

It is notable that the remaining 10% of the seismic events, which

mainly originate outside of this zone, includes events with the

greatest magnitudes and accounts for about 90% of all seismic

energy released at the mine.1

 The highest magnitude events lie

close to known major faults and are interpreted as indicating wall

rock movement toward the mined-out part of the vein (Figure 1).

The larger seismic events cause major damage primarily where

stopes and development openings are intersected by a major slip

plane (Whyatt et al. 1997). This suggests a minimal role for

seismic impulses in the generation of rock bursts and probably a

greater role for wall rock movements over time. 

The distribution of seismicity about Lucky Friday stopes

resembles that in longwall stopes in South African gold mines (e.g.

Joughin & Jager 1983), but with one major difference (Figure 3B).

In contrast to seismicity in the gold mines, which occurs

symmetrically about stopes, Lucky Friday seismicity is highly

asymmetric and most abundant on the footwall side. This

coincides with the marked asymmetry of the bedding structure of

Lucky Friday wall rocks and emphasizes the role of bedding plane

slip in the generation of seismicity. 

4.2 Structures caused by mining-induced movement 

Several types of recurrently seen structures are believed to

indicate mining-induced wall rock movement. These structures

include mining-induced thrust faults or shear ruptures; disrupted,

mining-induced extension fracture zones; and bedding plane faults

containing a distinctive, soft white gouge. The presence of the

white gouge (Figure 4) in structures that are believed, for other

reasons, to have been caused by mining and the absence of the

white gouge at locations away from the vein suggest that the gouge

originated from mining-induced slip. 

1

T.J.Williams, SRL-NIOSH, unpublished data collected during field

work 1988-1995.
Figure 7. Inferred mining-induced thrust fault or shear

rupture offsetting Lucky Friday vein. Extension fractures lie

about 30° to fault.

Figure 4. Soft, white gouge due to slip along argillite

interbed

Figure 5. Mining-induced extension fractures in stope

walls.

Figure 6. Disrupted zone formed by shearing within 4-cmwide zone of closely spaced mining-induced extension

fractures in stope walls.

Flat-dipping extension fractures are common in the stope walls

and in the advancing stope face (Figure 5). Concentration of these

fractures in the upper one-third of mining faces and their

persistence up to 10 m into the ribs confirm that they are caused by

mining and that they form in the immediate abutment. These

fractures are sometimes densely concentrated within zones 5 to 10

cm thick. Thin, tabular rock layers within these zones are

occasionally broken, rotated, and disrupted (Figure 6). Evidence

of slip has been established by offsets of veins up to several tens of

centimeters. Thus, these extension fracture zones may evolve into

flat-dipping thrust faults. 

Flat-to-moderate-dipping thrust fault shear ruptures seen in stope

faces offset the vein by as much as 10 to 20 cm (Figure 7). Like

the mining-induced bedding plane faults, these crosscutting shear

ruptures also contain finely granulated white gouge. Indirect

evidence that the thrust faults result from mining is seen in a

consistent angle of about 30° between the faults and adjacent

mining-induced extension fractures, even where the extension

fractures locally possess greater-than-normal dips. The shear

ruptures may reflect a recurrent fault plane solution obtained from

seismic data. However, correlation of structures with specific

seismic events, as has been done for some similar structures in

South Africa (Gay & Ortlepp 1979), has not been possible. 

4.3 Identification and measurement of wall rock movement

Movements taking place in wall rocks in response to mining have

been documented or inferred by various investigators through the

use of strain gages and pressure cells, closure measurements, the

appearance of fractures in shotcrete, offsets in artificial markers

(paint marks and reflective reference lines), calculations of seismic

source locations and fault plane solutions, repeated leveling and

surveying traverses, and modeling. However, efforts to interpret

and document movements over time have been frustrated because

of raveling of wall rock, disturbance of reference points,

localization of slip, and transitory access to active parts of the

mine. Thus, the practical difficulties in obtaining definitive data

over time at the Lucky Friday are great, and useful data are scarce.

In some cases, the simplest, most expedient methods have been the

most effective, even though results are more qualitative than

quantitative.
Figure 8. Fractures formed in shotcrete because of mininginduced movement along bedding. (A) Extension fracture

overlying argillite interbed. Extension is toward mined-out part of

vein to the right. (B) Fracture parallel to bedding showing normal

slip. Note wider gap at irregularity near top of photo.

While fault plane solutions have provided imporatant insights

into many large seismic events, they have not significantly aided

the understanding of rock bursts at the Lucky Friday. In a detailed,

year-long study of 1750 seismic events (Jung et al. 1995), 50%

were found to be all-dilatational. Such events are typically small

and so abundant they are not recorded during routine operations.

Thirty-six percent of the solutions were ambiguous, and the

remaining 14% were a mix of strike-slip, normal, low-angle thrust,

and a few oblique-reverse, with no obvious, systematic relationships to damage. It is also evident that the largest and bestdocumented seismic events are not necessarily associated with the

greatest damage, while significant damage may result from small

seismic events (Whyatt et al. 1997). Finally, as described in this

paper, damage in the stopes appears to be only indirectly related to

some major seismic events. 

Slip on northwesterly striking faults has been inferred from

localized clusters of seismic events and from visible evidence of

differential squeezing of abutments and cemented sandfill. The

largest seismic events, up to Richter magnitude 4, have occurred

along the large faults that bound the ends of the vein. When rock

bursts damage stopes during these larger seismic events, the

damage is not notably different from damage associated with

smaller seismic events and is not necessarily more severe.

However, following particularly large seismic events, minor

amounts of shakedown may be found widely distributed about the

mine, suggesting that the large seismic events may promote damage

by triggering smaller, localized events. 

Closure measurements in stopes and development openings have

been made by numerous investigators and Lucky Friday Mine

personnel. Much of this work is unpublished. Gradual closure and

step increases in closure that may or may not correspond to

recorded seismic events have been identified (Hsiung et al. 1992a,

b; Whyatt et al. 1992; Williams et al. 1992). Reported

measurements have commonly been in the range of 3 to 25 cm

during the 6- to 8-week period required to mine a single cut. These

measurements are consistent with physical evidence of squeezing

seen in the overhead sandfill.

The more definitive observations have often been the simplest.

These include progressive fracture development in shotcrete and

offsets in reference marks painted across bedding planes. However,

movements recorded by these means are rather qualitative, because

it has never been possible to obtain thorough coverage within

stopes or along the length of individual ramps for extended periods

of time.

Shear and dilational movements in wall rock are most obviously

shown by fractures that began developing in shotcrete soon after it

was applied in access ramps (Figure 8). The two most common

movements involve dilation and dip-parallel slip on sheared, gougy

argillite interbeds that separate thick vitreous quartzite beds.

Dilation is seen more frequently than slip. The direction of dilation

is consistently toward the mined-out part of the vein, and gaps 1 to

2 cm wide may be seen along bedding planes. The most common

direction of slip is normal (hanging wall down), although reverse

movements have occasionally been identified. Net slips of 1 to 2

cm are common. In addition to fractures that parallel bedding, en

echelon fractures may cross bedding at about 45° and also identify

dip-parallel slip. 

Near the vein, where ramp sections are too short lived to warrant

application of shotcrete, other indicators of movement have been

used. Marks painted across gougy argillite interbeds have shown

progressive normal (hanging wall down) slip along bedding planes

up to about 10 cm over a period of 1 month, and rapid movement

up to at least 15 cm is thought to have occurred during an

individual rock burst event (Figure 9). It is notable that slips of

these amounts have occasionally involved bedding planes that

intersect the abutment. Thus, it is apparent that normal bedding

plane faults sometimes pass into the unmined vein.

The most extensive, longest lasting, and detailed closure and

strain gage measurements have never differentiated among

movements resulting from slip or dilation along bedding or

movement caused by dilation of mining-induced extension

fractures. In these studies, the data can be interpreted as indicating

both dilational and slip movements on bedding planes, although

dilation of mining-induced fractures and buckling-type deformation

may also have been involved. Most data of this type suggest that

all displacement associated with closure increases toward the vein.

The various mining-induced fractures, bedding plane slips, and

shear ruptures seen near Lucky Friday stopes are summarized in

Figure 10.

Estimates of the expected distribution of seismicity in Lucky

Friday wall rocks caused by slip on bedding (Board 1994:Figure

11) suggest a preponderance of reverse faulting. However, the displacement documented in access ramps indicates a preponderance
Figure 9. Offset paint marks resulting from bedding plane

slip. (A) Mark offset by gradual slip during 6-week period.

(B) Offset paint marks and deformed chain link fencing

developed during August 28, 1998, rock burst. 

of normal slip. Furthermore, seismicity is concentrated in the

footwall of the stope, where normal slip is most frequently

identified, in an elongate pattern that extends up-dip and down-dip

(Figure 11). 2

 The predicted distribution of events resulting from

reverse faulting along bedding is not apparent. Thus, normal

faulting on bedding appears to reflect the dominant seismic slip

mechanism near the stope.

The movements identified above can be compared to common

types of failure that occur in the walls of highway cuts and open-pit

mines. Strata that dip toward open cuts, as in the footwall of the

Lucky Friday vein, are vulnerable to slippage, and buckling

sometimes occurs in the lower part of slipping layers. Layers that

dip away from cut walls, as in the hanging wall of an vein, are

displaced by toppling, although toppling in the mine would be

greatly constrained by the cemented sandfill and the narrow width

of the mined-out vein. While slip has been frequently documented,

toppling is not evident at active operating levels. However, it is

reasonable to expect toppling at higher mine elevations, in parts of

the mine no longer accessible. At present operating levels, the

effects of toppling may be manifested primarily as a concentration

of vertical stresses in the hanging wall sides of stopes. 

Although a comparison of wall rock movements at the mine with

those in walls of open cuts and pits is illustrative, it is likely that a

major component of closure is elastic, the result of stress relief.

Since dilation is present along bedding planes, it is likely that the

direction of greatest stress relief in each individual cut is

approximately parallel to bedding dip. Stress relief is also likely to

extend a considerable distance up-dip—possibly 100 m or more to

the up-dip extent of seismicity—so that the net bedding-parallel

component of closure from stress relief may be somewhat large.

We presume that the dip-parallel component of stress relief is also

the major contributor to rock burst energy at some burst locations

in stopes.

4.4 Examples of bedding plane slip events 

A rock burst extensively damaged the 5750-05 stope on August 28,

1998 (Figure 2), in a Richter-magnitude 3.1 seismic event. Rock

was expelled from both ribs, and the overhead cemented sandfill

failed to an average depth of 1 m for a distance of about 25 m along

the length of the stope. As is common, the event occurred during

blasting. Bedding slip believed to have taken place during this

event could be documented by offsets in marks painted across

exposed argillite interbeds in the access ramp. A 2-cm slip was

found 30 m from the stope, and a 13-cm slip (Figure 9B) was found

27 m from the stope. Possibly even greater slip took place, but this

can not be confirmed because some rock on one side of the larger

slip detached from the rib and may have moved independently of

the substrate. Evidence of slip could not be seen on any other

argillite interbeds, but many paint marks were destroyed by

raveling during the event. Thus, the minimum net slip in the event

was 15 cm. 

Eastward and downward, slip planes were projected to the

location of the 5840-06 stope, which had been mined deeper than

the 5750-05. The 5840-06 stope had become temporarily

inactivated and was (and remains) inaccessible, while the 5850-05

stope continued to be advanced. It seems likely that slip on these

planes was controlled primarily by the presence of the 5840-06

stope, but that the slip planes projected westward so that they

intersected the 5750-05 area deep in the abutment (Figure 12).

Based on these observations, minimum dimensions of the slip zone

are estimated as being 90 m long by 30 m wide along dip. We

presume that most of the seismic energy originated from slip on

these two bedding planes. 

Damage to the stope in this event included apparent compression

of the sandfill to failure. Damage that involved ribs could also be

attributed to vertical shortening and buckling of the affected rock

mass, as discussed in the following section. However, no bedding

slip could be documented in or near the stope that could have

caused such shortening, as all reference points were disturbed. 

Several months earlier, a paint mark across bedding near the

5850-06 stope was progressively offset for a total of 18 cm during

the 6 weeks required to complete a cut. The bedding slip plane was

seen in the access ramp 4 m from the vein, indicating that the slip

plane intersected the abutment. In the next cut, this slip plane cut

across the vein and offset it about 25 cm. No seismicity could be

specifically related to the progressive slip.

2

The cross section displaying seismicity about a Lucky Friday stope

shows low density close to the stope. A systematic error that causes

event loca-tions to be calculated as originating too far from the stope is

suspected.
Figure 10. Comparison of structures at Lucky Friday Mine and Witwatersrand gold reef. (A)

Cross section of Lucky Friday vein identifying structures activated or caused by mining; (B)

comparable structures about Witwatersrand gold reef (rotated 90° from horizontal [Joughin &

Jager 1983]).

4.5 Interpretation of effect of bedding slip on rock bursts

Progressive movements in the footwall of the Lucky Friday vein

generally cause a reduction in the physical dimensions of the stope.

Compression of sandfill is a necessary and obvious consequence of

downward bedding slip. However, slip on bedding planes that

intersect or pass near the stope may reduce stope dimensions in 

various other

ways (Figure 13). Except for bursts that affect the hanging wall

rib, shortening of stope margins primarily results from normal

(down-dip) slip on bedding planes. The nature of the resulting

damage depends partly on the elevation at which a particular slip

plane intercepts a stope. If the slip plane intercepts the rib, only the

sandfill is likely to be affected (Figure 13A). However, bedding

plane slip that fails to pass through relatively 
Figure 11. Actual seismicity around Lucky Friday stopes and

comparison to distribution predicted by Board, 1994 (gray

area). Dashed line represents orientation of bedding.

Figure 12. Schematic representation of slip zone from

August 28, 1998, event, as seen in 05 ramp and projected to

06 stope.

Figure 13. Interpretations of rock bursts in stopes caused by

movement within the surrounding wall rock. (A) Sandfill

damage caused by slip on bedding plane that intersects foot

wall rib; (B) failure in footwall rib resulting from slip on

bedding plane that does not truncate rock layers in rib formed

by galena stringers; (C) damage to foot wall rib-abutment and

diagonally opposite corner; (D) abutment damage caused by

slip on mining-induced extension fractures; (E) damage to

hanging wall rib attributed to high stress resulting from

toppling of inclined strata. 

tabular rock layers formed between a stope and weak, galena-filled

stringers may cause shortening and failure of the rock while also

compressing the sandfill (Figure 13B). Bedding slip that intersects

the underlying abutment may concentrate stress at the abutmentfootwall junction and fracture quartzite along its internal

laminations, forming layers that may burst by buckling (Figure

13C). However, the diametrically opposite corner, at the juncture

of the hanging wall rib and the sandfill, probably fails as a result of

the sandfill exerting pressure on previously fractured rock.

The high horizontal component of in situ stress documented at

the mine may be enough to cause mining-induced extension

fractures and shear ruptures in the abutment and for slip to take

place on these fractures without associated bedding plane slip.

However, bedding slip could also promote formation of fractures

and thereby contribute to abutment bursts (Figure 13D). 

Movement associated with bursts that affect the hanging wall rib

has not been documented. We speculate that these bursts are

promoted by high vertical stress in the hanging wall resulting from

toppling of overlying strata toward the mined-out vein (Figure

13E). 

5 COMPARISON TO WITWATERSRAND 

The geometries of preexisting and mining-induced structures in

Lucky Friday wall rocks bear striking resemblances to structures

found in Witwatersrand gold mines as discussed by Joughin &

Jager (1983), Pretorius (1966), and Adams et al. (1981) (Figure

10). The obvious differences are that the Lucky Friday vein is

subvertical rather than gently dipping and cuts across strata instead

of being stratabound. In each case, the direction of greatest in situ

stress is nearly normal to the vein. However, some of the structures

act very differently.
The mining-induced extension fractures that form in the

abutment of the Lucky Friday vein and that are implicated in

abutment bursts resemble the fractures most closely associated with

face bursts in South African mines. In both cases, stope closure

and consequent shortening of the affected stope margin apparently

increases stress and promotes formation of these fractures. Slip

identified along mining-induced extension fracture zones in the

Lucky Friday duplicates slip along fractures seen after rock bursts

in South African stopes (Joughin & Jager 1983). Shear ruptures at

the Lucky Friday are virtually identical in disposition and geometry

to those found in the South African mines. 

In the Lucky Friday Mine, bedding in the footwall approximates

a common type of mining-induced extension fracture documented

by Joughin & Jager (1983) in South Africa (structure 3, Figure

10b). Although dilation occurs in both settings, shear is present

only at the Lucky Friday Mine. Unlike in South African mines,

where bursts mainly involve the face (Joughin & Jager 1983),

bursts in Lucky Friday stopes affect all stope margins and several

corners. However, the various planar discontinuities parallel to the

affected margins in the Lucky Friday typically play a role in rock

bursts similar to that of the mining-induced fractures parallel to the

South African stope faces. 

Planes of weakness represented by galena stringers in wall rocks

parallel to the Lucky Friday vein duplicate the geometry of bedding

planes in the South African mines. At the Lucky Friday, these

planes are directly involved in compressive rock burst failures,

whereas bedding in South African mines apparently contributes

primarily to roof falls. 

6 REDUCTION OF ROCK BURST HAZARDS

A limited range of measures may be considered for reducing rock

burst hazards that result from shortening of stope dimensions as a

result of slip on bedding. Unfortunately, some measures may

aggravate other problems. Several hypothetical measures are

mentioned here for the purpose of illustrating how an

understanding of the mechanisms involved could be translated into

practical methods of rock burst prevention. 

Given the strong role of bedding slip in promoting bursts, a

mining method that limits the ability of rock to slip along bedding

could be considered. Mining by end-slicing the vein is a

possibility, although it is likely to cause the steep, northwesttrending faults to assume a greater role in rock bursts. Using the

current mining method, dip-parallel stabilizing pillars could limit

bedding slip, but would eliminate some ore and might also cause

burst problems associated with the pillars. Increasing the strength

of sandfill, decreasing its compressibility, or increasing the

percentage of fill may not be able to halt bedding slip and its effects

on the stope, since stress release parallel to bedding dip as a result

of mining may be concentrated at the immediate stope. Increasing

the compressibility of the sandfill through installation of a layer of

compressible foam within the sand may help maintain its integrity.

However, if the sand actually does help reduce down-dip bedding

slip, compressibility would diminish this capability. Destressing

the abutment may reduce most problems resulting from bedding

slip, but may also increase compression of the sandfill. A

combination of compressible fill and destressing the abutment may

be a reasonable compromise.

7 SUMMARY AND DISCUSSION

We have identified movement taking place in Lucky Friday wall

rocks in response to mining and have interpreted mechanisms by

which these movements lead to rock bursts. Our results suggest

that movement in wall rock alters conditions in the stope in a way

that promotes bursts. Specifically, slip movement in wall rocks

causes shortening at the margins of stopes, and the shortening, in

turn, increases stress and stress-induced failure. 

We conclude that most seismicity originating distal from damage

sites is only incidental to bursting. However, incidental seismicity

identifies slip movements that have the capability of changing

stress conditions that may result in bursts. Hence, while seismicity

resulting from slip on bedding planes is not in itself hazardous,

such seismicity is associated with the development of hazardous

stress levels around stopes. 

Based on these conclusions, we suggest that a clear distinction

should be drawn between incidental seismicity and rock burst

damage. Incidental seismicity primarily indicates that wall rocks

are moving in response to mining and identifies generalized regions

where such movements are taking place. However, definitive

indicators of these movements and identification and mapping of

active geologic or mining-induced structures are necessary to infer

how incidental seismicity may be related to rock bursts. 

The current research is guided by the conviction that reduction

of rock burst hazards in underground metal mines can be best

accomplished if the mechanical causes of rock bursts are clearly

understood. The descriptions and interpretations provided here

suggest plausible alternatives for reducing this hazard, thus

advancing NIOSH’s mission of improving the health and safety of

the nation’s workers.

8 ACKNOWLEDGMENTS

We gratefully acknowledge the cooperation and assistance of Hecla

Mining Co., owner and operator of the Lucky Friday Mine, for

making this research possible. Through these efforts, the NIOSH

Spokane Research Laboratory continues the mission of the former

U.S. Bureau of Mines in improving the safety of miners. We

would also like to thank Clyde Peppin and Doug Bayer of Hecla

Mining Co. and Wilson Blake, consultant, for their thoughtful

reviews of this paper

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