MECHANICS OF A LARGE, STRAIN-TYPE ROCK BURST AND

DESIGN FOR PREVENTION

Brian G. White, Theodore J. Williams, and Jeffrey K. Whyatt

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

ABSTRACT. Detailed mapping and examination of the site of a large rock burst at the Lucky Friday Mine, Mullan, ID, demonstrated

conclusively that damage was caused by a splitting and buckling failure mechanism initiated by separation of rock into layers along

steeply dipping metamorphic shear foliation subparallel to the affected crosscut. The thin tablular layers then buckled into the crosscut

from both sides. Rigorous control of the burst by these layers suggests practical measures for avoiding bursts in crosscuts in the future

by repositioning and reorienting openings so that splitting and buckling are reduced or precluded. 

1. INTRODUCTION

Many of the lessons that have been learned about rock

bursts in the Coeur d’Alene district of northern Idaho

were recently summarized by Whyatt et al. [1]. However, a comprehensive understanding of rock burst

damage mechanisms has remained elusive. 

Fairhurst and Cook [2] considered stress-induced

buckling of rock layers as a rock burst mechanism, but

most researchers have limited this mechanismto fairly

minor events. White et al. [3], White and Whyatt [4,

5], Maleki and White [6], and Whyatt and White [7]

described field examples of large rock bursts and

other rib deformation that they inferred to have resulted from buckling. The evidence for one large burst

described as caused by buckling, the Craig drift rock

burst (Strathcona Mine of Falconbridge, Ltd., in

Ontario, Canada), is suggestive but not compelling

(see [8]). Many studies simply sidestep consideration

of damage mechanisms or ascribe damage to a seismic

impulse or to crushing, with no specific damage

mechanism specified. 

We have noted many examples of time-dependant

buckling in soft, argillitic strata at district mines, so it

is clear that this deformation mechanism is common.

We have also discerned a strong tendency for rock

bursts to damage crosscuts and ramps driven subparallel to steeply dipping bedding and faults. However, actual data confirming that buckling was

involved in these bursts have been limited. The site of

an extensive rock burst (Richter magnitude 2.4) that

took place on December 27, 2000, at the Lucky Friday

Mine provides unusually compelling evidence for a

buckling failure mechanism.

A case study of this rock burst was conducted as part

of a project to reduce rock burst hazards being undertaken by the Spokane Research Laboratory (SRL) of

the National Institute for Occupational Safety and

Health (NIOSH). These efforts are made possible

through a longstanding cooperative research relationship between the SRL and Hecla Mining Co.’s Lucky

Friday Mine, Mullan, ID.

2. GROUND CONDITIONS

The Lucky Friday ore body is a narrow, steeply

dipping galena-quartz vein that has been mined over

a vertical extent of 1500 m and to depths of nearly

2000 m. The vein is currently mined from successively developed sublevels, each of which enables development of a number of up- and down-ramps to the

vein (fig. 1) [9].

Crosscuts to 06 stopes are driven southeastward from

a primary sublevel haulage (fig. 1), which is the best

practical location and orientation in the area for

avoiding a known major fault. The 06 crosscuts are

developed at the same elevation as the highly stressed

abutment. Rock bursts affecting 06 crosscuts and

ramps have been a recurrent problem, with major
Figure 1.—Map of typical sublevel showing 06 crosscut,

central shear zone, and folded quartzite.

damage usually involving the ribs and lesser damage

affecting the back, floor, or corners.

The minimum standard ground support used in crosscuts includes 5-by-5-cm, 9-gauge chain link fencing

that is pinned to the back with Dywidag

1

 and Split-Set

rock bolts. Across the back, three rows of 1.8-m-long

Dywidag bolts are installed along with 0.9-m-long

Split-Set bolts at a maximum bolt spacing of 0.9 m.

The walls are screened and bolted with 1.2-m SplitSet bolts on 1.2-m centers to within 0.9 m of the floor.

In crosscuts, a 5-cm-thick layer of steel-fiber-reinforced shotcrete is also standard. 

3. GEOLOGY

Strata at the burst site are massive sericitic quartzite

beds (fig. 2) that sometimes split into thin layers along

bedding laminations, evidently because of high stress

along bedding and the absence of confining pressure.

The 6020 crosscut approximately follows the axial

plane of a prominent, nearly overturned anticline that

plunges about 30° at this location, resulting in the

relatively low bedding dips seen along part of the

crosscut (figs. 1 and 2). Steep dips are mapped on the

north limb of the fold.

The burst site lies within a structural zone referred to

as the “central shear zone,” which is defined by

steeply dipping, metamorphic shear foliation and an

anastomosing network of minor faults (fig. 1). It is

evident that foliation creates important planes of

weakness, as the rock often splits into layers parallel

to this fabric. Most faults in the central shear zone

have little or no gouge, but one fault containing

several centimeters of gouge forms the effective

northern limit of the damage zone. The strike of the

central shear zone structures traverses the crosscut at

a slight angle in its most extensively damaged part,

but changes to a more northwesterly direction toward

the west, where damage diminishes.

No in situ stress data are available from near the burst

site, but the orientation of stresses can be reasonably

inferred. The direction of greatest stress in quartzite

strata at all mines in the district is subhorizontal and

usually trends northwest, with a magnitude approximately twice that of the least and intermediate stresses

[10]. At the site, the direction of least stress is probably oriented toward the mined-out part of the vein,

which is grossly upward and southeastward. Thus, it

is likely that the direction of greatest stress plunges

moderately toward the abutment, approximating the

plunge of the local folds, and roughly parallel to

structures of the central shear zone.

4. OBSERVED DAMAGE 

The rock burst expelled approximately 320 tons of

broken rock from both ribs, approximately doubling

the width of the access (fig. 2). Additional rock came

down from the back. After the displaced rock had

been removed and new ground support installed, an

extensive cavity was exposed on each side of the

crosscut. A series of cross sections along the crosscut

(figs. 3A through 3E) shows that the damage along

opposite sides, both major and minor, is diagonally

opposed and centered at different elevations on the

respective sides.

Where the burst cavities are most extensive and deepest, they coincide with a location where the back

exceeded design height by 1 to 2 m. However, it was

not clear whether the added height resulted from

overbreak or rock burst damage. In either case, it is

likely that conditions in the back created an effectively

greater height at the time of the burst, for inspection

showed that bedding exposed in the back was intensely split and loosened along bedding plane laminations,

conditions that probably caused the ground to cave.

1Mention of specific products and manufacturers does not

imply endorsement by the National Institute for Occupational Safety and Health.
Figure 2.—Map of burst site.

West of the major damage, minor breakouts and

buckled shotcrete persisted along the upper part of the

south rib, whereas spalling affected the lower north

rib. Thus, the locations of minor burst damage were

diagonally opposed, just as the more extensively damaged parts of the crosscut.

Immediately west of the south-side cavity, inspection

of in-place Split-Set bolts showed that they were bent

by shearing displacements at depth. Elsewhere, a

single broken and bent Split-Set bolt remained in

place. These examples serve to emphasize a common

observation at burst sites: That a large percentage of

the rock bolts involved in bursts–typically about

half–have been bent as a result of shearing among

rock layers. 

The most prominent geologic structures seen in the

ribs were steeply dipping separation planes parallel to

the northwest-trending central shear zone (fig. 2).

These structures split the rock into 5- to 20-cm-thick

layers. The layers were conspicuous at the edges of

the burst cavities, where they formed extensive planar

surfaces that also tended to form the deeper limits of

the cavities (figs. 2 and 3), but were poorly developed

across the central part of the back. This contrasts with

separations along bedding plane laminations, which,

while prominent in the back, were not present at all in

the ribs. In each case, the direction of splitting was at

a low angle to the respective surfaces and close

enough to the predicted orientations of mininginduced fractures that splitting along inherent planes

of weakness due to high stress was likely.

All damage, including the relatively trivial spalling

and shotcrete buckling at the west ends of the affected

area, was confined to a 7- to 8-m-thick zone parallel

to central shear zone structures. This damage also
Figure 3.—Cross sections across burst site. Locations are shown in Figure 2.

extended deepest and highest where the back was

highest. The burst cavity on the south side developed

in layers that included all the minor damage to the

west on both sides of the crosscut. However, the

northern portion of the 7- to 8-m-thick unit trends

eastward along the center of the crosscut, although it

could not be mapped further because of temporary use

of the ramp for waste storage. 

6. STRESS AND MOVEMENT INDICATORS

The directions and relative magnitudes of ground

movements involved in the burst were identified by a

burst-created fold preserved deep in one of the burst

cavities and by bent Split-Set bolts. The fold is defined by intensely broken foliation layers that wrap

discontinuously about an axis (fig. 3B, 3C, and 3D).

The fold plunges northwest, with its plunge locally

measured at 22° as viewed along the side of the fold

where the broken layers dip approximately vertically.

The amount of shortening represented by the fold can

be roughly estimated from the amount of curvature

and locally is at least 0.1 to 0.2 m, which identifies

significant vertical convergence. 

The fold developed above a planar substrate formed by a basal foliation separation plane or minor fault (fig. 3C and

3D). However, the fold appears to die

out in the direction of plunge, as planar

layers ultimately cover the layers involved in the fold (fig. 3A). This gives

the fold a conical shape. These geometries require that slippage took place on

a series of basal shear planes, with

movements converging toward the center of the fold from one side or the

other, or both. The displacement must

die out along strike over a distance of 5

to 15 m from the original rib (as estimated from plan view). Since the fold

axis plunges at a low angle to the northwest, compressive shortening along a

line raking steeply southeast is indicated.

The second indicator of movement is

provided by bent Split-Set bolts that

remained relatively intact in a surviving

part of the original south rib (fig. 2).

Inspection of the interiors of the bolts

showed that the collars had moved

several centimeters upward and northwestward relative to the deeper part of the bolts. Bending was

concentrated at individual foliation separation planes

intersected by the bolts, as verified by projection

along a plane, while the outer layer of rock moved

relatively diagonally upward toward the elevation of

the deeper part of the burst cavity. The bolts were bent

within a plane about normal to the fold axes on the

north side of the crosscut. Hence, movements on both

sides were consistent with slightly diagonal displacement of rock layers toward the central parts of the

burst cavities. We note that interlayer slip with displacement of outer layers toward the axial plane of the

fold is characteristic of concentric-style folding,

which buckling represents. Thus, the slip displacement expressed by the bent bolts may have resulted

from incipient buckling, although no conspicuous

buckling was observed. 

7. INTERPRETATION 

If a seismic impulse had been involved in breaking

and heaving rock from the burst cavities, the intensely

broken rock that defines the fold would certainly have

also been ejected, or at least become so disrupted as to
Figure 4.—Schematic of buckled, broken rock layers

responsible for damage.

be unrecognizable as a fold. Only compressive shortening in the plane of the shears, accompanied by

slippage along a basal shear plane, can account for the

fold (fig. 4). If crushing were instantaneous, which is

implicit, bulking of broken rock would have briefly

created confining pressure within the interior of the

broken mass, enabling the fold to develop without

being ejected. Damage to both ribs was accompanied

by displacement toward the central part of each cavity

and is reasonably explained by buckling and breakage

of the rock layers. Damage to diagonally opposed

parts of the opening, frequently seen with large rock

bursts in crosscuts, also argues for the burst resulting

from compressive stress. 

As is well known, the critical load required to cause

buckling varies directly with the square of the slenderness ratio (ratio of thickness to length). Hence, the

coincidence of the most extensive damage with the

site where the back had exceeded design height also

supports the interpretation that the damage resulted

from buckling of foliation layers, since the slenderness ratio would have been significantly reduced at

this location. This may also help explain preservation

of the western portion of the south rib where the bent

rock bolts were found, as well as the general decrease

in damage westward.

8. BREAKOUT GEOMETRY

Fairhurst and Cook [2] described how layers parallel

to a free surface fail through buckling. In the present

case, where the involved layers are slightly inclined to

the walls of the opening in both strike and dip, the

geometries that would result from buckling can still

be estimated and compared against what was actually

seen. Fairhurst and Cook [2] emphasized that cavities

formed by buckling narrow with depth as a result of

an increase in “normal stress” at the ends of the

layers. Where layers dip at about 75°, as in the current

example, the limits of buckling would be inclined

accordingly, and breakouts would appear as in figure

4, with one side being the inverted mirror image of the

other. This is a good representation of what was

actually seen at the site.

On the side of the crosscut where layers dip toward

the opening, the innermost layers would have become

the most tightly folded (fig. 4). However, since the

strike of the layers was also oblique to the crosscut,

the outermost layers would have assumed the position

of the innermost layers at some distance into the

section. Consequently, the fold would be conical and

plunge into the section, just as is seen at the burst site.

Thus, the geometries predicted to be produced strictly

by buckling are identical to those actually seen and

provide additional support for the interpretation that

the breakouts and the fold resulted from buckling

caused by compression along a line raking steeply

southwest.

Using the same reasoning, if the crosscut crossed

foliation from its footwall side, the axis of folding

would be constrained to plunge toward the vein. Compressive shortening would be along a line that extends

toward the previously developed ramps andmined-out

part of the vein. Since the stress in this direction

would likely be reduced, buckling would be inhibited,

and bursting would either involve less energy or

would not occur at all. Of course, buckling would also

be precluded if openings were at a large enough angle

to the strike of the layers.

It is possible that the burst would not have taken place

if the back had not been excessively vulnerable. Since

the excessive height was apparently caused by back

failures resulting from splitting of strata along bedding laminations, placement of 06 crosscuts a short

distance to the north, where bedding dips steeply,

would have eliminated this situation. 

9. SEISMIC DATA

The location of the seismic source, as refined by

WilsonBlake (personal communication, 2002),leaves

little doubt that it originated very near or at the site of

the damage. Assuming all the energy involved in the
burst originated as strain energy, we used the reported

local Richter magnitude and a reasonable assumption

of strain density based on (1) an overcore test at a

separate site at the Lucky Friday where a rock burst

subsequently took place and (2) laboratory measurements of elastic rock properties to calculate the

volume of rock necessary to supply this amount of

strain energy (see[11] for details). It is in the range of

2450 to 4900 m3

, depending on seismic efficiency.

This is equivalent to a volume of rock measuring 8 by

33 by 9.3 to 18.6 m and is roughly 25 to 50 times the

volume of the rock ejected in the burst. Thus, the

burst cannot be accounted for by strain energy

contained in the broken rock. The additional energy

was probably liberated by sliding on shear planes

bounding the affected rock, which released elastic as

well as gravitational potential energy. We note that

slippage along faults is commonly apparent in

magnitude 2.5 or greater events at the mine and that

pure strain energy bursts larger than magnitude 1.0 are

rare. 

10. SUMMARY AND DISCUSSION

Our observations provide a reasonable basis for concluding that this burst resulted from buckling of

structurally layered rock in response to high local

stress and an unfavorable orientation of these layers

with respect to a mine crosscut. The seismic energy

was supplied by release of stored strain energy from

broken rock combined with fault slip movement.

Although Fairhurst and Cook [2] apparently considered that buckling is a common mechanism by which

rock fails, there have been surprisingly few descriptions of large failures involving buckling. They emphasized that the fundamental mode of mininginduced deformation begins with creation of fractures

parallel to free surfaces and formation of slabs that

deform further by buckling. However, we have

demonstrated that geologic flaws in the form of foliation and bedding promote development of tabular

layers that may buckle when the layers are slightly

oblique to affected surfaces. This recognition is vital

because it establishes the possibility of positioning

and orienting openings so as to reduce or eliminate

rock bursts under some circumstances.

A full understanding of such features provides a basis

for optimizing the location and orientation of mine

openings and matching the density of ground support

and amount of detressing to the degree of rock burst

hazard. Such efforts must be comprehensive. That is,

hazards from all types of rock bursts must be

understood, as well as the relationship of geologic

structures and mining-induced stress changes. Otherwise, efforts to reduce hazards from a particular

source or type of rock burst might inadvertently lead

to increases from another type.

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of experience with rock bursts in the Coeur d’Alene Mining

District. Presentation at Soc. of Min. Eng. annual meeting,

Denver, CO, Feb. 27-Mar. 1. Available from author.

2. Fairhurst, C., and N.G.W. Cook. 1966. The phenomenon of

rock splitting parallel to the direction of maximum compression in the neighborhood of a surface. In Proceedings of the

First Congress on the International Society of Rock

Mechanics (Lisbon, Portugal, Sept. 25-Oct. 1, 1966). Lisbon,

Portugal: Nat. Lab. of Civil Eng., V. 1, pp. 687-692.

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factors in rock bursts in the Coeur d’Alene Mining District:

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