60 YEARS OF ROCKBURSTING IN THE COEUR D’ALENE DISTRICT OF NORTHERN IDAHO, USA: 

LESSONS LEARNED AND REMAINING ISSUES

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

 W. Blake, Hayden Lake, ID, USA

T. Williams and B. White, Spokane Research Laboratory, National Institute for Occupational Safety and Health, 

Spokane, WA, USA

ABSTRACT

Sixty years of rockbursting in the Coeur d’Alene district

has taught painful lessons and led to a number of practical

advances in controlling rockburst hazards. This paper summarizes those lessons, concentrating on practical measures

that have been successfully adopted to reduce hazards.

These lessons are explained in the context of district mining

history and current understanding of rockburst phenomena.

Overall, the paper provides the practicing mine engineer

with an appreciation of rockburst hazards and an overview

of practical measures that can be used to control these

hazards in the context of Coeur d’Alene district experience.

INTRODUCTION

The phenomena of rockbursts in the Coeur d’Alene

district were first reported as “air blasts” in the early 1900's.

The first fatality to be described as an air blast in the press

occurred in the 1600-level stope of the Greenhill-Cleveland

Mine in 1914. Rockbursts did not become a severe

operational problem until the 1940's, as mining followed

veins into deeper and more quartzitic rock. In the past 60

years, the district has suffered 22 rockburst fatalities in five

different mines.

Rockburst research in the district began in earnest during

the 1940’s with the work of Dr. Leonard Obert and Dr. Phil

Shenon, among others. Considerable progress in controlling

rockburst hazards has been made in the intervening 60 years,

thanks to a variety of efforts undertaken by mining

companies, universities, and government agencies. Unfortunately, guidance on practical measures that reduce rockburst hazards are often difficult to find in a literature that

tends to focus on seismology. 

This paper is an attempt to gather and organize the best of

this practical knowledge for the use of practicing engineers

in rockbursting and potentially rockbursting mines. It

begins with a short introduction to the Coeur d’Alene district

followed by three main sections that present practical

measures for controlling rockburst hazards. The first section

addresses the relationship between seismicity and

rockbursting. The second reviews tactical measures for

controlling rockburst hazards that can be instituted locally

on short notice. These measures include ground support,

destressing, and changes in mining rate. The final section

presents strategic methods for controlling rockburst hazards,

including design of mining methods and sequences that

minimize hazards. 

This paper was written as part of an effort to reduce

rockburst hazards that has been undertaken by the Office for

Mine Safety and Health Research of the National Institute

for Occupational Safety and Health (NIOSH). These efforts

include a longstanding cooperative research program in the

Coeur d’Alene district, as well as work with other rockbursting, and potentially rockbursting, mines.

Selection, explanation, and evaluation of rockburst safety

measures that have been used in the Coeur d’Alene district

required considerable judgment on the part of the authors.

Given the complexity of the rockburst safety issue, there will

likely be differences of opinion and experience. Where such

differences occur, the authors invite criticism and

discussion. Honest debate of these issues can only improve

our methods for controlling rockburst hazards.

ROCKBURSTS AND MINING-INDUCED

SEISMICITY

The term “rockburst” has been defined in an impressive

variety of ways. From a practical standpoint, the regulatory

definition (30 CFR 57.3000) is most relevant to U.S. mines.

This definition is
A sudden and violent failure of overstressed rock resulting in

the instantaneous release of large amounts of accumulated

energy.

This definition is somewhat vague as to whether a failure

must result in actual damage to a mine or even a real hazard

to miners in order to be classified as a rockburst. The

definition of an MSHA-reportable rockburst is more explicit

(30 CFR 57.3461). A rockburst must be reported to MSHA

if it 

1. Causes persons to be withdrawn,

2. Impairs ventilation,

3. Impedes passage, or

4. Disrupts mining activity for more than 1 hour.

In other words, there must be damage to the mine,

sufficient injury to a person for that person to be withdrawn

from a work area, or sufficient concern over the safety of

miners to either withdraw miners or disrupt mining activity

for more than 1 hour.

The MSHA definition requires a sudden or violent failure,

implying that a seismic event is produced. Indeed, there has

been a regrettable tendency to equate seismic events to

rockbursts and to use the latter term for all mining-induced

seismic events. Seismic events are created by unstable

deformation processes—including fracturing of brittle rock

and stick-slip sliding on discontinuities of all scales—that

release a pulse of seismic energy. Most seismic events pose

no hazard to miners, and only a small minority have the

potential for harm or damage to mine openings. 

Kaiser et al. (1998) explicitly considered the relationship

between seismic events and rockbursts in their similar but

simpler and more expansive definition for a rockburst. Their

definition simply states that a rockburst is a seismic event

that is associated with damage to a mine opening. This

definition encompasses two key aspects of rockburst

damage. First, damage may be caused by creation of a

seismic event and/or by seismic shaking of a mine opening.

Second, the deformation mechanism responsible for a

seismic event must, directly or indirectly, overcome the

structural capacity of an underground opening in order for

a rockburst to be said to occur. Thus, the difference between

a rockbursting and nonrockbursting mine is as much a

characteristic of the mining system as it is of the mining

environment.

Likewise, the difference between a mine experiencing

falls of ground and a mine experiencing rockbursts lies in

whether seismic events are produced, since seismic events

are indicative of the “instantaneous release of large amounts

of accumulated energy” required by the MSHA definition.

In the absence of eyewitness accounts, it is often difficult to

tell if a fall of ground has been accompanied by a seismic

event—i.e., whether it is a rockburst as well. Seismic

monitoring systems are often useful for determining whether

falls of ground are indeed rockbursts, and small portable

systems are readily available. While these systems cannot

differentiate damaging seismic events from harmless

seismicity, they do provide event time and location reports

that can be cross-checked with underground damage reports.

Coeur d’Alene District Seismicity

Mining in the Coeur d’Alene district typically produces

considerable levels of seismic activity. Much of this

seismicity is a harmless part of the rock mass adjusting to

mining. Long-term plots of seismic energy versus tons

mined are usually linear within similar geologic regimes.

However, large contrasts in the level of seismicity produced

per ton mined are generally observed between geologic

formations, generally related to the “sandiness” or

proportion of quartz in the rock. Seismicity also varies with

in situ stress, which has been shown to vary between

geologic formations and structures (Whyatt, 2000) as well

as with depth.

Microseismic monitoring with sensitive listening equipment was developed by the U.S. Bureau of Mines (USBM)

in the late 1930's and tested at the Sunshine Mine in 1941

after a double fatality. While this experiment did detect

microseismic activity, systematic seismic monitoring was

not attempted until the 1960's, when the USBM initiated

seismic monitoring at the Star and Galena mines. These

early seismic arrays were often focused on individual sill

pillars, since sill pillar bursting was the first type of rockburst to be recognized and was, for a time, thought to be the

only type.

Monitoring of mine seismicity showed that seismicity

reflects both local geologic structures and changing stress

conditions, some of which were associated with rockbursts.

However, efforts to predict specific rockbursts based on

patterns of seismic activity have largely failed. Seismic

systems have also provided invaluable, real-time information on the location of major seismic events or rockbursts.

Mine staff use this information to check on crews most

likely to be affected and can often begin rescue operations

within minutes of a major rockburst.

Further development of seismic systems in the 1980’s

(Girard et al., 1995) led to the capture of digital seismic

waveform records. These records can be used to discern the

mechanism, including the direction of slip, that caused a

particular seismic event. 

Coeur d’Alene District Rockbursts

Coeur d’Alene rockbursts can be divided into three major

types—strain bursts, pillar bursts, and slip bursts—

primarily by how they relate to mining activities. That is,

strain bursts depend on geology of the immediate perimeter

of the opening, pillar bursts depend on pillar design and

geology, and slip bursts depend on regional changes in the
Figure 1.—Fracture zone typically encountered around deep

excavations in the Coeur d’Alene diatrict (after White and Whyatt,

1999).

state of stress on faults. These types also differ in the type

and distribution of resulting damage and the level of seismic

energy accompanying a damaging rockburst. All three types

are dangerous, but to different degrees in different mines

(table 1). Overall, pillar and strain bursts have proven to be

the most hazardous, while slip bursts have the largest

seismic magnitudes.

Table 1—Fatalities at Coeur d’Alene district mines by type of

rockburst.

Mine Strain Pillar Slip 

Sunshine . . . . . . . . . . . . . 3 2

Star/Morning . . . . . . . . . 3 1

Galena . . . . . . . . . . . . . . 2 6

Lucky Friday . . . . . . . . . . 2 2

Coeur ............... 1

TOTAL . . . . . . . . . . . . . . 9 11 2

Strain burst. Tannant et al. (1996) defines strain bursts as

“rapid bulking of a rock mass due to unstable (dynamic)

fracturing in the vicinity of an opening.” Mining depths

typical of the Coeur d’Alene district result in stresses in rock

around the perimeter of deep openings that exceed strength.

This rock fractures, shifting stresses to better-confined rock

deeper in the rock mass. The resulting fracture zones have

been mapped in South Africa and, more recently, in the

Coeur d’Alène district (figure 1). 

Eye-witness accounts of strain bursts in the Coeur d’Alène

district usually include a number of the following

observations (Whyatt and White 1998).

• Sudden, intense fracturing of intact rock into coffee-cupsize or smaller rubble.

• A loud, instantaneous report. The report is described as

resembling an exploding charge or a sonic boom. For

nearby observers, the sound seems to originate at the

immediate site of the burst.

• Violent expulsion of rock rubble into mine openings

before miners can react. Initial expulsion of rubble is

followed by a brief period in which debris continues to be

distributed about the burst site. Miners involved in these

bursts are not generally knocked down by the ejected rock,

but are engulfed by a fluid-like flow of rock debris. Miners

have been partially or completely buried while standing

erect, their legs extensively bruised but not broken. The

greatest risk of fatal injury is suffocation, primarily from

pressure on the chest.

• A dense cloud of dust that immediately fills the air. A

major strain burst may create dust so dense that miners have

the impression that their lamps have gone out.

• An air pressure shock wave or “air blast” that travels

through the mine. The initial air blast may be followed by a

sustained closure-induced wind if there is a significant

change in excavation volume.

• A concave cavity that narrows with depth into the rib. A

planar fracture, fault or bedding plane often forms the back

of the cavity. Cavities 30 to 200 cm deep are commonly

reported for 3- by 3-m drifts.

Some excavation-induced fractures are created during

blasting, followed by further fracture development that

usually decays with time after blasting. However, fracturing

may become unstable at any time, creating a seismic event

and, if damage is induced, a strain burst. The energy

required for unstable fracturing can be supplied by a number

of mechanisms, including shattering of brittle rock, buckling

of rock plates, and/or sliding along discontinuities. Buckling

of rock layers or plates that are defined by geology and/or

stress-induced fracturing is often evident in the Coeur

d’Alene district. Fairhurst and Cook (1966) have shown how

buckling of a plate in the rib of an opening releases both

strain energy stored in and around the plate and gravitational

potential energy (through subsidence of rock above the

opening). Roof subsidence, and hence available gravitational

potential energy, can be increased further by the presence of

discontinuities (figure 2).

Locations that support strain bursts make up only a small

portion of the district rock mass at current mining depths.

For instance, a recent study of strain bursting during

development of a deep ramp system found that less than

10% of the system was affected by strain bursting (Whyatt

and White, 1998). However, strain bursts that did occur

pulverized as much as 100 tons of rock in ramp ribs.

Relative to damage, strain bursts are generally associated

with much smaller seismic events than other rockburst types.

Locations prone to strain bursts are often marked by

structural weaknesses that define plates parallel to the

opening perimeter, strong brittle rock, and/or unusually high

stress levels. 
Figure 2.—Buckling of rock plates is driven by release

of gravitational and elastic potential energy (A). A weak

discontinuity can dramatically increase the amount of

energy released, resulting in a more hazardous

rockburst (B).

Pillar burst. A pillar burst is an unstable pillar failure that,

like a strain burst, is caused by unstable movement resulting

from some combination of fracturing, sliding, and buckling.

This movement will generally extend deeper into the rock

mass than is the case with strain bursts and often involves

the core or foundation of a pillar. As such, there is often a

loss of pillar load-carrying capacity that can liberate a

considerable amount of energy. Despite their similarity to

other types of bursts, pillar bursts are considered separately

because of their direct dependence on mine layout, mine

sequence, and mining method. 

In the Coeur d’Alene district, pillars are most commonly

created by mining a vein from multiple levels simultaneously in such a way that mining progresses toward previously

mined areas. The vein between an advancing stope and

previously mined ground is called a “sill” pillar. Typically,

mining will create an array of sill pillars that are gradually

reduced in size and eventually removed. 

Board and Fairhurst (1983) reported that rockbursting in

district overhand stopes typically began as the sill pillar was

reduced to 18-20 m ( and peaked at 12-15 m). Pillar bursts

are particularly likely to be triggered if a sill pillar with these

dimensions is cut into two smaller pillars by excavation of

an “I-drift” (drift cut on the vein) or is intersected by a

crosscut or ramp. 

Pillar bursts typically cause damage and closure in

adjacent excavations, particularly the associated stope and

nearby haulage drifts. The seismic event is the result, rather

than the cause, of damage to the load-carrying capacity of

the pillar and to immediately adjacent openings. However,

shaking of surrounding openings can also cause damage,

particularly where ground is weak, poorly supported, and/or

loaded close to capacity

The potential for pillar bursting is best managed through

mine planning, particularly mining method, pillar geometry,

pillar load, backfilling practices, mining rate, and preconditioning. For instance, pillar bursts can be eliminated with

a longwall mining method which does not create pillars.

However, other types of rockbursts may still occur. 

Slip burst. Slip bursts are defined both by mechanism (stickslip shear movement on a discontinuity) and the regional

nature of driving forces. These bursts are less likely to be

triggered by a particular blast and are more likely to occur

during a shift. Stick-slip sliding can also occur as part of a

burst in a pillar or the immediate skin of an opening in more

direct response to mining. However, these bursts are best

considered as pillar or strain bursts, respectively, since they

respond similarly to burst control measures. 

Slip occurs when the ratio of shear to normal (effective)

stress along the fault plane reaches a critical value, the

coefficient of friction (tangent of the friction angle). In most

cases, mining activity causes slip by removing normal stress,

although some local intensification of shear stress may also

occur. Changes in stress along a fault are often linked to

mine activities by time-dependent deformation processes.

These time-dependent processes can act over long periods

of time, regardless of continued mining. For instance, a

number of sizable seismic events (almost certainly caused by

slip) occurred over a period of several months after mining

was halted at the district’s Galena Mine. The largest of

these, a 3.0-magnitude event, occurred nearly 300 days after

mining had ceased (Kranz and Estey, 1996). 

The stick-slip mechanism can produce seismic events with

significant seismic energy and has been linked to the largest

seismic events in the district. For instance, Whyatt et al.

(1997) studied large seismic events (2.5 to 4.2 Ml

) at the

Lucky Friday Mine over a recent 6-year period and showed

that all were slip events. Moreover, these events were caused

by repeated movement along five separate structures (figure

3). Generally, intense damage was often observed where

slip planes crossed excavations. Shaking damage was found

near the event and in areas where rock was poorly

supported, well fractured, and/or unusually weak.
Figure 3.—Plan view of 5100 level, Lucky Friday Mine, showing typical slip movements that produced

large-magnitude slip bursts (after Whyatt et al. 1997).

Summary

Summarizing, several lessons have been learned about

seismicity and rockbursting in the Coeur d’Alene district. 

! Most seismicity is a normal, safe, and desired rock mass

response to mining. Only a small minority of seismic events

constitute a rockburst hazard. The size of this minority

depends on local rock mass conditions and mine practices.

! Individual rockbursts (and seismic events) cannot be

predicted. However, changes in mining-induced seismicity

can provide insight into changes in rock mass conditions and

geologic structures, which can affect the likelihood of a

rockburst.

! Three types of rockbursts are active in the district—

strain, pillar, and slip. Each type presents a unique hazard

and must be considered separately in both evaluating the

level of rockburst hazard and designing protective measures.

TACTICAL MEASURES

Measures that can be taken locally and at short notice in

response to a heightened level of rockburst hazard can be

described as tactical measures. By contrast, strategic

measures are those that must be integrated into mine design

and long-term planning. Tactical measures that have proven

successful in the Coeur d’Alene district include ground

control systems, destress blasting, and manipulation of the

rate of mining.

Ground Support

Ground support measures in rockbursting ground are

designed to suppress damage where possible. Where damage

is unavoidable, ground support should serve to contain

damage, which preserves access and prevents burial of

miners. Conventional rock bolts and timber sets respond

best to static loads and are poorly suited for this task. They

often fail under the dynamic loads exerted by a rockburst.

The addition of energy-absorbing, yielding supports and a

flexible surface covering improves prevention and containment of rockburst damage, particularly damage from seismic

shaking. These measures confine and knit together

fragments of rock within the fracture zones. A well-knit

fracture zone will deform without failure of supporting

elements or a fall of ground while exerting confining

pressure on the rock deeper in the excavation wall. 

A typical support configuration in rockbursting ground

consists of a combination of Split-Set and resin-grouted

Dywidag bolts along with chain link mesh (figure 4).

Vulnerable points such as intersections can be reinforced

with cable lacing. Steel-fiber-reinforced shotcrete is often

used in highly stressed areas where small strain bursts occur

during drilling for bolting and blasting. Shotcrete is also

useful in suppressing damage from seismic shaking. These

measures are described in detail by Blake and Cuvelier

(1990, 1992).
Figure 4.—Typical support scheme employed in rockbursting

ground.

Blasting

Control of rockburst hazards with blasting practices

started with the recognition that roughly 75% of district

rockbursts occur with, or in the hour following, a blast.1

Thus, blasts are often restricted to the end of a shift,

particularly in areas where multiple headings are being

worked simultaneously. Blast timing is sometimes used to

accentuate, rather than limit, seismic shocks from rounds in

an attempt to trigger rockbursts that might otherwise occur

during a shift. In some cases, such a blast will sufficiently

soften the fractured rock adjacent the opening to prevent a

rockburst. In others, the rockburst will occur, but occur with

the blast instead of during the shift.

Destressing is an extension of this procedure designed to

fracture a highly stressed portion of the rock mass. It is an

extension in the sense that additional holes are drilled into

ground that is to be fractured by blasting, but not pulled.

The objective is to induce a crushing, rather than a bursting,

failure mode in the rock. Deformation occurring as part of

this crushing failure will cause a shift in stresses from the

destressed rock to other areas that can carry it more safely.

Destressing can be pursued on a range of scales, from a

portion of a future rib to an entire pillar. 

When the rock at the face of a stope or heading is

“popping” or “bumping” during drilling, face destressing is

normally carried out to eliminate this hazard. Two or more

holes are drilled ahead of the face and/or are fanned out in

the walls. The bottom half of each hole is loaded with

explosives and shot early in the round, so that the holes will

not “pull” muck. This type of destress blasting fractures the

ground around the new face, thereby preventing the ground

from “working” at the face. The addition of destress holes to

development and stope rounds has proven quite useful in

reducing strain bursts. 

Volley-fired backstope rounds are sometimes combined

with destress holes. A portion of such a round does pull,

allowing for continued production. The shock provided by

the volley-fired round, coupled with firing of destress holes,

is meant to control the rockburst hazard by triggering

incipient rockbursts while the destress holes also serve to

prevent further bursting in the immediate back of the stope.

When this approach does trigger a rockburst, the damage is

rarely “controlled” and is often very extensive, although it

is safe as miners are evacuated at blasting time. However,

sill pillars will retain a solid core susceptible to bursting

until blastholes can be drilled through the entire intact core

of the pillar.

Sill pillar destressing by drilling and blasting a single row

of holes along the vein was first attempted by a joint USBMASARCO research project at the Galena Mine in the late

1960’s. Sill pillar destressing quickly became the principal

strategy to control pillar bursting and has been routinely

carried out at the Galena, Star, and Lucky Friday mines in

sill pillars mined to less than 15 m in height. The most

effective destress blasts used large-diameter holes (greater

than 100 mm) and a hole spacing of 3 m or less. They are

loaded with high explosive to within about 4 m of the collar.

However, there are some disadvantages to pillar destress

blasts. First, guidelines for design of a destress blast are

scarce, so considerable experimentation can be required.

Second, there is the operational problem. The length of time

and effort to drill destress holes results in a significant loss

of production with no guarantee of success. Hence, there is

a tendency is to delay sill pillar destressing until the pillar is

relatively small and thus very highly stressed. This can result

in “bumping” and bursting during the drilling of destress

holes, which in one case caused a fatality. Finally, it is

difficult to assess whether a mass destress blast has

completely eliminated the prospect of bursting in a sill

pillar. Injuries, including fatal injuries, have occurred as a

result of rockbursts in supposedly destressed sill pillars.

Moreover, a failed destress blast may actually increase the

level of hazard.

1

Bill McLaughlin's study of Galena, Star, and Lucky Friday

bursting versus blasting and hazard time from 1973-1978

showed that, at the Galena Mine, 79% of bursts occurred with

blasting or between shifts. Similar results were obtained for the

Star (68%) and Lucky Friday mines (74%) during this period.

McLaughlin also found a similar level (68%) at the Lucky

Friday Mine from 1982-1985.
Figure 5.—Typical plot of number of microseismic events occurring

by day of the week in a mine working a 5-day week. 

Overall, destress blasting has proven to be an effective

method for reducing hazards from strain bursting, but it is

often problematic when applied to pillar bursts.

Mining Rate

One of the oldest techniques for controlling rockburst

hazard is changing the rate of advance. Faces will often be

shut down when unusual seismic activity is observed and

then restarted after seismic activity quiets down. Miners on

a second shift have frequently been kept out of their stopes

until the heightened seismic activity from a first-shift blast

decays to the “background” rate for that stope. Slowing

down the mining rate increases the time available for the

rock to deform inelastically (“work”), which transfers stress

away from the perimeter of the excavation. Generally, a

faster mining rate will produce more seismicity per ton

mined than a slower rate.

The importance of time is also evident in the distribution

of seismic activity (figure 5) and fatal accidents during the

work week (table 2). Generally, activity increases early in

the week, reaches a plateau mid-week, and then falls over

the weekend. Time effects on seismicity and rockbursting

are not well understood, and reasons for the predominance

of fatalities on Wednesday (and to a lesser degree, Friday)

are not apparent.

Table 2.—Fatal rockburst accidents by mine and day of week.

Mine Sun Mon Tue Wed Thu Fri

Sunshine . . . . . . . . 3 1 1

Star/ Morning . . . . 4

Galena . . . . . . . . . 2 1 1 4

Lucky Friday . . . . . 1 3

Coeur .......... 1

TOTAL 3 11 2 6

Summary

In summary, a number of tactical measures have been

used successfully to reduce rockburst hazards. Tactical

lessons that have been learned include—

! Support systems that absorb energy and deform without

breaking provide the best support in rockburst-prone

ground. Even where these systems suffer damage, they are

often able to limit falls of ground and permit access where

other systems fail completely.

! Destress blasting of rock, particularly highly stressed

brittle rock, immediately surrounding an excavation can

reduce rockburst hazards. Destress holes can be efficiently

integrated into conventional rounds. Destress blasting of

entire sill pillars is more problematic, but can improve

conditions.

! Slowing the rate of extraction will often reduce the

amount of seismicity in relation to tonnage mined and may

actually prevent bursting under some conditions. 

STRATEGIC METHODS

Measures that must be planned in advance, must be

applied to large portions of a mine, and/or are relatively

inflexible over time are described as strategic methods for

rockburst hazards control. By contrast, tactical measures are

those that can be taken locally, and at short notice, in

response to a heightened level of rockburst hazard. Strategic

measures are inevitably based on judgments about the

relative level of rockburst risk inherent in alternative mine

designs. 

These judgments are based primarily on experience, some

of which has been codified in criteria like the energy release

rate and excess shear stress. These methods have progressed

significantly over the past 60 years, but continue to have

important limitations. Where these methods have been

applied in the district, they have largely served to confirm

and explain old rules that have proven to be of value. These

rules can be found in various internal mining company

documents dating from the 1950’s, and some can be traced

back South African literature of the 1920’s. These rules are

listed at the end of this section. 

Generally, these old rules seek to avoid creation of large

voids and small pillars, particularly in the vicinity of burstprone geologic features. Where burst-prone geologic
features are encountered, these locations should be mined

and filled first while the extraction ratio—and the level of

mining-induced stress—is low. Similar reasoning applies in

the case of multiple veins where the vein is a burst-prone

structure. For instance, mining the hanging wall vein first in

an overhand stope removes this structure and the

accompanying stress from the hanging wall of following

stopes. Application of these rules to the Coeur d’Alene

district requires changes in mining method and learning to

recognize geologic features that contribute to rockburst risk.

Mining Methods

Backfilling of stopes was the first fully implemented

adaptation of Couer d’Alene mining methods to

rockbursting conditions, with a variety of open stoping

methods giving way to overhand cut-and-fill mining by

1940. Backfill improved conditions in a number of ways,

including limiting the amount of open ground and hence the

severity of air blasts produced by rockbursts. 

Work on stope sequencing began with the observation that

a pillar burst was more likely to affect multiple adjacent

stopes (on strike) when sill pillars of equal size were

maintained. This observation was confirmed and explained

by one of the first applications of numerical modeling (on an

analog computer) and the energy release rate (Board and

Crouch, 1997). As a result, single, flat-backed, cut-and-fill

mining fronts at both the Star and Lucky Friday mines were

changed to a center lead stope geometry. The Galena Mine

adopted a stair-stepped (east end leading) sequence for its

largest vein.

By the mid-1980’s, sill pillar bursting at the Lucky Friday

had become severe, with three fatalities in 3 years. As a

result, the Lucky Friday converted to an underhand cut-andfill mining method that did not create pillars. Underhand

mining also provided for an engineered back consisting of

reinforced cemented fill. The fill back has proven to hold

up well under both gradual stope closure and dynamic loads

from nearby seismic events. While continuing to be the most

seismically active mine in the district, the Lucky Friday has

had an outstanding safety record since switching to

underhand longwall mining.

Geologic Features

Recognition of geologic features that contribute to

rockburst hazards is a key part of formulating strategic

measures for minimizing these hazards. Experience in the

Coeur d’Alene district indicates that particular rock types

and various kinds of discontinuities exert a strong influence

on rockburst hazards. 

The geologic structure of the district is complex. At least

five major periods of tectonic deformation have taken place,

resulting in the Belt strata being highly folded and faulted.

The high tectonic stresses responsible for the complex

structure have also left most district mines with unusually

high horizontal stress fields. Veins are generally located in

linear bands along fractures and fault zones and consist of

highly variable proportions of sphalerite, galena, and

argentiferous tetrahedrite in a gangue dominated by either

quartz or siderite. White (1998) provides a more comprehensive treatment of district geology.

Coeur d'Alene district veins lie within a regional sequence

of Precambrian metasediments referred to as the Belt

Supergroup. Belt strata are characterized by thick, uniform

sequences of slightly metamorphosed and predominantly

fine-grained sediments with varying proportions of quartz

and argillite. Veins of the mining district dip steeply,

typically cut through strata, and are most economic in

quartzitic strata, which is also the host rock for most of the

significant rockbursts that have occurred. 

In situ stress, as well as rockburst hazards, have been

found to vary with geology in the Coeur d’Alene district.

Whyatt (2000) has shown that in situ stress levels in

quartzitic strata tend to be higher than in softer rocks at

similar depths. Moreover, the most intense stress conditions

are found in quartzitic strata that lie within the zone of

silicification around quartz veins. These intense stresses and

the brittle nature of silicified rocks contribute to increases in

rockburst hazards. In fact, the degree of rockburst hazard

experienced during mining of adjacent quartz and siderite

veins has been found to vary significantly (Whyatt et al.,

2000).

The connection between rockbursting and geology was

made following the first reported rockburst, which occurred

at the Sunshine Mine in April 1939 on the 2500 level. This

burst was associated with the mining front leaving the

relatively soft St. Regis Formation and entering the harder,

more massive quartzites of the upper member of the Revett

Formation. In April 1956, the first rockburst was reported at

the Galena Mine on the 2400 level, also in the upper

member of the Revett Formation, as initial mining on the

newly discovered Silver vein created small pillars. 

In the case of the Lucky Friday, the vein became

economic—and burst-prone—as it entered the upper Revett

below the 2000 level. This bursting continued with depth

until about the 3250 level, when the vein entered the softer

middle Revett Formation. A renewal of serious bursting

began again in 1983 as the vein entered the harder lower

Revett at about the 4450 level. At the Star Mine, thinbedded sections of lower Revett were not burst prone, but

thicker quartzite beds were. 

Strain bursts often occur when mine openings intersect a

dike or a more massive section of quartzite beds, usually

during development. Strain bursts are particularly sensitive

to the orientation of discontinuities around the perimeter of

an opening, since these discontinuities define plates that can

buckle into the opening. Sets of discontinuities (bedding
planes, joint sets) should be intersected at as high an angle

as possible.

The largest seismic events at both the Star and the Lucky

Friday have all occurred out in the walls on faults, primarily

in the hanging wall. These large events appear to be

associated with the overall geometry of mine excavation

rather than day-to-day mining. Individual faults should also

be cut at high angles, for several reasons. First, as faults

increase in scale, the scale of the associated gouge zone also

increases. High-angle intersections minimize the amount of

the gouge zone that must be supported. Second, gouge can

define strain-burst-prone plates where an opening enters and

leaves a fault zone. Third, slip bursts have been shown to

cause the greatest damage at the intersections between a

fault and mine opening (Whyatt et al., 1997). Some of the

most devastating bursts in the Coeur d’Alene district have

occurred when the vein becomes a seismically active fault.

Thus, high-angle intersections limit exposure to slip-burst

damage. 

Summary

In summary, a number of strategic guidelines have been

used successfully to reduce rockburst hazards in the Coeur

d’Alene district. These guidelines include a number of old

rules.

! A properly planned sequence of stoping for the whole

ore body should be adopted and followed as closely as

possible.

! The merging of large excavations at depth should be

avoided.

! Pillars should be eliminated or reduced to a minimum.

! Parallel veins should be stoped singly, the hanging wall

vein first (footwall vein first if underhand mining).

! Where veins branch, stoping should begin at the

intersection and then progress away from the intersection

one branch at a time.

! Where possible, stoping should proceed away from a

fault or other plane of weakness.

! Mined-out areas should be filled, and filling should

proceed concurrently with extraction and be kept as close to

the face as possible.

A few additional guidelines can be gleaned from Coeur

d’Alene district experience.

! Underhand longwall mining is a practical, economical

mining method that can reduce rockburst hazards.

! Openings in rockburst-prone ground should cut weak

discontinuities and slip-prone faults at high angles.

! Rockburst hazards vary greatly with geology and are

greatest in hard quartzitic strata, particularly in rock altered

by silicification around quartz veins. 

DISCUSSION

A great deal has been learned about rockburst phenomena

during the 60 years of mining in the Coeur d'Alene district.

It is now recognized that there are different types of

rockbursts with different causal mechanisms and that these

mechanisms are affected by mining decisions in different

ways. The hard and brittle quartzites of the Revett

Formation have been identified as the host rock for almost

all rockbursts, and silicified rock within the Revett

Formation has been shown to be particularly hazardous.

District experience has also shown that many general rules

for reducing rockburst hazards proposed early in the

twentieth century are valid and worthy of continued

application. Indeed, some of the significant advances in

district mining have occurred as cost-effective methods for

applying these rules have been developed.

District experience has suggested a number of additional

rules that the authors have attempted to describe. These

descriptions are a first attempt and should be improved by

continued discussion and debate within the mining

community, which we invite. The rules are offered as an

attempt to summarize district experience with rockbursting

as a guide for ongoing mine operations within the district

and to apply these hard-won lessons to other mining

districts.

While a great deal has been learned about rockbursts and

developing rockburst control methods and procedures, it is

clear that many mysteries remain. Our understanding of the

causative mechanisms of rockbursts needs to be improved,

taking into account that each burst is both mine and site

specific. The element of time in these mechanisms is poorly

understood, despite the fact that many mines have long used

mining rate to control rockburst hazards. Our knowledge of

rockburst damage mechanisms is limited and needs to be

improved so that we can design more effective ground

support. Finally, we need to improve our ability to anticipate

rockburst problems likely to arise from various combinations of geologic setting, mining plans, and various tactical

measures.

This paper is an attempt to summarize progress gained

from the experience and efforts of a community of miners

and mining professionals. The scope of this attempt almost

guarantees that important facets of district experience and

practice have been neglected or misrepresented. Clarifications, corrections, and extensions of this work are most

welcome, for these lessons have been too hard-won—in

suffering and in lives lost—to get wrong, ignore, or forget.

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