variables, the number of variables was reduced by combining some variables into new ones. In
addition, the cause-and-effect (9) structure in the data was identified, helping to tailor the
procedures for multiple regression analysis using forward stepwise inclusion of dependent
variables. The new variables were as follows:

Pgqratio Ratio of maximum principal horizontal stress (P) to minimum stress (Q).
Strenrce Ratio of uniaxial compressive strength of the roof to the coal.
Jointrf Joint spacing times roof beam thickness divided by mining height.

Gradyield Ratio of roof and floor yield characteristics to stress gradient.
Panelwd Ratio of panel width to depth.
Youngre Ratio of Young’s modulus of the roof to the seam.

Table 4 presents the bivariate correlation coefficients between the variable “damage” and selected
geologic and geometric variables. Energy (M,), face factor of safety, stress gradient, pillar factor
of safety, joint spacing, and uniaxial compressive strength of roof to coal were the most
significant. Other variables were poorly correlated with damage, including the ratio of P to Q,
pillar width, and Young’s modulus of roof to coal.

Table 4,—Bivariate correlation coefficients between damage and selected other variables

Significant variables’ Coefficient Insignificant variables Coefficient
Damage ................. 1 Pillar width 0.1
Energy .---... eee eee eee 0.65 Ratio of P to 0.1
Gradyield ...............6 0.57 Young’s modulus of roof to coal oe 0.07
Jointrf . 2.2... ee eee eee 0.52
Pillar factor of safety ........ -0.44
Uniaxial strength of roof to coal 0.36
Face factor of safety -0.33
No. interacting seams . 0.33
Panel width to depth -0.31

Mining method ............ 0.26
* Two-tailed tests

Multiple Linear Regression Analysis

There are two methods used by engineers and researchers as tools to help predict conditions in the
future: statistical and computational. Starfield and Cundall (10) identify rock mechanics prob-
lems as “data-limited,” that is, one seldom knows enough about a rock mass to use computational
models unambiguously. Statistical methods, on the other hand, are uniquely capable of being
applied where there are good data but a limited understanding of certain natural phenomena, such
as coal bumps

In this study, the authors combined the strength of both methods to identify important variables
and to develop predictive capabilities. Computational methods have been used to assess the influ-
ence of a combination of geometric variables into single variables, such as pillar factor of safety and


Original drift outline

Figure 6.—Mechanism of failure from strain
burst. Shaded areas identify inferred source of
elastic strain energy, or following load, that
drives bursting. Bending of rock slabs is exagger-
ated; brittle deformation would be expected be-
fore bending reaches the extent shown.

Original drift outline

Figure 7.—Mechanism of failure from fault-
bounded strain burst. Bedding or fault plane
physically separates rock mass involved in burst
from adjacent portions of rock mass. Slip along
this plane enables a relatively large amount of
elastic strain energy to be supplied to a relatively
small volume of rock. Compare with figure 6.


* — Stress gradient and yield characteristics. Mining toward areas of high stress creates a poten-
tial for coal bumps, while localized yielding roof and floor conditions encourage gradual fail-
ure, which reduces the amount of damage.

Table 5.—Standardized regression coefficients and statistical significance

Variable Standardized coefficient T-significance
Energy ........-..00. 0.28 0.049
Pillar factor of safety .... -0.34 0.011
Method .......-.....4 0.26 0.064
Gradyield ............ -0.55 0.0004
Constant............. NA 0.234

NA Not applicable.

ROLE OF PREEXISTING STRUCTURE IN VIOLENT FAILURE IN HARD-ROCK
MINES (COEUR D’ALENE MINING DISTRICT)

Despite obvious differences in host rocks and mine layouts, violent failures in coal and hard-rock
mines have important things in common. However, the settings for rock bursts in mines of the
Coeur d'Alene district are diverse, often making it difficult to identify specific mechanisms
involved in violent failures. As a result, geologic factors affecting bursts in hard-rock mines have
not been studied as intensively as they have in coal mines. Much of the data concerning
mechanisms of damage is observational. In addition, all workers do not agree on the extent or
nature of the mechanisms involved. Efforts by the mines to minimize rock burst damage often
involve trial and error, and the effectiveness of these efforts is often difficult to measure.

Major mines of the Coeur d'Alene Mining District are the Bunker Hill, Sunshine, Star, Galena,
and Lucky Friday, but only the Lucky Friday, Sunshine, and Galena currently operate on a large
scale. In all cases, steeply dipping veins containing lead, zinc, and silver have been mined, Re-
cent operating depths extend to as much as 2,000 m beneath the surface. Most mining has involv-
ed overhand cut-and-fill techniques, but recently, the Lucky Friday Mine has changed to an under-
hand cut-and-fill technique in which cemented sand is used as backfill. Rock bursts have been
most frequent at the Lucky Friday Mine, but have also affected the Sunshine and Galena mines.

Host rocks of Coeur d'Alene ore bodies are slightly metamorphosed Precambrian sedimentary
strata (11). The thick-bedded quartzitic strata of the district are often thinly laminated and
separated from each other by thin interbeds of argillite. These bedding features appear to greatly
influence the mechanical response of quartzite and its role in rock bursts and other ground control
problems (12-13).

A complex tectonic history in the Coeur d'Alene district includes formation of large-scale folds
that created steep bedding dips on the scale of individual mines. The ore bodies postdate these
fold episodes. In addition, postmineralization faulting has been intense. Younger tectonism
caused extensive shearing along the steeply dipping argillite interbeds and caused locally extensive
fracturing of quartzite.


released energy. This was very useful for increasing goodness-of-fit and enhancing multiple regres-
sion coefficients. Statistical methods were used to identify significant variables, to build confidence
intervals, and so forth.

The multilinear regression procedure consisted of entering the independent variables one at a time
into an equation using a forward selection methodology (9). In this method, the variable having
the largest correlation with the dependant variable is entered into the equation. If a variable fails
to meet entry requirements, it is not included in the equation. If it does meet the criteria, the
second variable with the highest partial correlation will be selected and tested for entering into the
equation. This procedure is very desirable when there is a cause-and-effect structure among the
variables. An example of the cause-and-effect relationship is shown when a greater depth reduces
pillar factor of safety, contributes to an accumulation of energy, and ultimately results in greater
damage. Using the above procedures, any hidden relationship between depth and pillar factor of
safety, energy, and damage is evaluated and taken into account during each step of the analysis.

Several geomechanical variables (table 3)

nan Q 20 T ob T
were initially used as dependent vari- :
ables. The “damage” variable, how- 2 SP _
ever, resulted in the highest multiple re- Wy 1.9 . o. 4
gression coefficient. The multiple corre- a Ose ~~. |
lation coefficient (R), which is a measure O L . °
of goodness-of-fit, for the last step was N ° 7
0.87. QO -05 be 7
ac ee
1.0 4
The assumptions of linear regression an- 2 ask ° e
alysis were tested and found to be valid fag ° j ° f
by an analysis of variance, F-statistics, 0 -2-0

a ; "2 4 0 1 2
and a plot of standardized residuals (fig- REGRESSION STANDARDIZED RESIDUAL

ure 4) (9). Residual plots did not indi- .
cate the need for inclusion of nonlinear F'9U"* 4.” Standardized seatterplot for dependent variable,

terms because there was no special pat-
tern in the residuals.

Important Variables Contributing to Bump-Prone Conditions
Based on an examination of standardized regression coefficients (table 5), the following variables
best explain variations in damage and thus statistically have the most significant influence on coal

bump potential.

* Energy release. This variable includes the effects of the mechanical properties of the roof and
coal, depth, stress field, and joint density, and thus directly relates to damage.

* Method. Mining method has a bearing on coal bump potential. The room-and-pillar method
is associated with a higher degree of damage than is longwall mining.

* Pillar factor of safety. Gate pillar geometry contributes directly to the severity of damage.


Figure 5.—Rib failures at the Lucky Friday Mine. Bedding is near-vertical and parallel to the opening. Both
ribs were equally damaged. A, Typical burst damage. Note undamaged back and rock bolts and mesh
displaced from ribs. 8, Burst-modified shape of lateral from original rectangular cross section. Damage
affected upper left and lower right ribs. Bedding dips steeply to the left.


GEOTECHNICAL FACTORS INFLUENCING VIOLENT FAILURE IN U.S. MINES

Hamid Maleki
Maleki Technologies, Inc., Spokane, WA, USA

Brian White
Spokane Research Center, National Institute for Occupational Safety and Health
Spokane, WA, USA

ABSTRACT

Sudden, violent failures of rock around mine openings influence access, ventilation, and safety
in both hard-rock and coal mines. To develop predictive tools for assessing the potential for
violent failure, the authors initiated a comprehensive study using (1) multiple linear regression and
numerical modeling analyses of geological and mining conditions at 25 sites to identify the most
significant factors contributing to stress bumps in coal mines and (2) investigations of the
geological and mining factors contributing to rock bursts in the Coeur d’ Alene Mining District of
northern Idaho, particularly the influence of preexisting structures on rock bursts.

Twenty-five factors were initially considered in the study of coal bumps. The most important
variables were identified as (1) mechanical properties of strata, (2) gate road geometry and/or gate
pillar factors of safety, (3) roof beam thickness, joint spacing, and stiffness characteristics, and
(4) stress gradients associated with previous mining and anomalous geologic conditions.

In the Coeur d'Alene district, burst damage related to preexisting structures results primarily from
(1) sudden buckling and crushing of rock layers where surfaces of development and production
openings lie at low angles to bedding, faults, or other planar structures and (2) disturbance of
loose ground caused by seismic fault-slip on preexisting faults or previously sheared bedding
planes that intersect veins near pillar-stope margins.

INTRODUCTION

Coal bumps and rock bursts are defined as sudden and violent failures that occur near openings
and that are of such a magnitude that they expel large amounts of material into the opening. Such
violent failures are not only a safety concern in U.S. mines but have also affected safety and
resource recovery in other countries, including Canada, Germany, England, Poland, France,
Mexico, China, India, Norway, and South Africa.

The geomechanics of conditions leading to violent failure are similar in both hard-rock and coal
mines in that stresses exceed the strength of the rock mass near a mining excavation or at a
geologic discontinuity. In coal mines, excavation takes place in near-horizontal seams under near-
vertical maximum principal stress fields. Most geologic discontinuities are horizontal but are
intersected by near-vertical cleats and joints. In burst-prone hard-rock mines of the Coeur d’ Alene
Mining District of northern Idaho, where much of the work on rock bursts has been done,
maximum principal stresses are horizontal; however, geologic discontinuities tend to be near-


The most extensive damage associated with large fault-slip events occurs where slip surfaces
intersect mine openings. The usual descriptions of actual damage tend to involve rock being
heaved from the surfaces of openings or loose ground being shaken down, but triggering of local
strain bursts is often suspected.

Fault-slip bursts at the Lucky Friday Mine have often been associated with pillar or stope margins.
This suggests that stoping may increase shear stress on preexisting faults and argillite interbeds
or may promote slip by decreasing normal stress on fault planes. Such slippage is commonly
interpreted as closing mined-out stopes.

Much low-level seismicity and shotcrete fracturing in Lucky Friday development ramps appear
to reflect nearly continuous movements along bedding planes that dip steeply toward the vein.
These fault-slip movements apparently represent progressive accommodation of the wall rock to
mining and usually are not large enough to cause damage.

An August 1994 seismic event at the Lucky Friday Mine (figure 8) has been interpreted as result-
ing from strike-slip movement when highly stressed wall rock slipped along a preexisting fault
(13). Interpretation is based, in part, on observations of locally intensified squeeze of sandfill in
stopes and inferred fracturing and buckling-type heave of the unmined vein along mining-induced
fractures. Relatively minor failure of loose, broken ground affected various ribs near the event.

Noy
“tt Con,
tre
SN Fauy

Moderate sand “SS
squeeze and

interpreted sill
fracturing -_ao~
- a ~
= 7 ~
Ss y, \
_—- FS Fault . “ \
T= \
| . j
Interpreted
fault slip \ Approximate ~/
N \ hypocenter >
\ . 7
Extensive sand wa
~~ squeeze nd
ote and interpreted sill
crushing
o 5 10
a a)
Scale, m

Figure 8.—Richter-magnitude 4 event centered eastward and above the stope in the Lucky Friday Mine
Squeezed sand and locally fractured and heaved the sill. Damage was most severe south of the F3 Fault.
This event probably resulted from strike-slip movements on known faults, which tended to close the stope.


Applying Fairhurst and Cook’s hypothesis to rock bursts in the Coeur d’Alene district means
that—

1. Less stress is required to buckle preexisting layers of rock than is needed to split massive,
unfractured rock of the same composition. Thus, unbolted, very thinly layered strata may be
deformed by buckling under relatively modest levels of stress loading. Closely spaced cleavage
and joints are also locally involved in rock bursts for this reason.

2. Faults and bedding planes physically isolate buckling layers from the underlying rock mass.
Slip along these planes is unimpeded by bonding to the adjacent rock mass (figure 7). This
probably enables elastic strain energy from a large volume of a surrounding rock to supply
following load to the deforming layers. White and others (73) defined strain bursts influenced by
faults in this manner as fault-bounded strain bursts. Repair of rock burst damage often reveals
that the rock burst cavity terminates at depth against a such a fault or bedding plane, thus
identifying this type of strain burst.

As noted, extensive strain bursts and fault-bounded strain bursts often affect horizontal openings
such as crosscuts and drifts. However, steeply inclined linear openings, such as shafts, raises, and
raisebore holes are particularly susceptible to buckling failure. Because faults and strata in district
mines generally dip steeply, openings intersect structures at low angles, a condition that favors
buckling of rock layers into the opening. In addition, the common west-northwesterly strikes of
steeply dipping strata and faults approximate the direction of greatest in situ stress (15). Thus,
the direction of greatest stress and following load potential also coincides with the direction of
rock layering. At the Lucky Friday Mine, ore passes where these relationships are evident have
been sites of especially bothersome ground control problems.

The strong relationship of strain bursts to structures that nearly parallel the surfaces of mine
openings suggests that bursts can be avoided if openings are aligned so that they cross geologic
structures at a large angle. This has been successfully tested in a part of the Lucky Friday Mine
that had been plagued with large strain bursts involving the ribs (figure 5A). Ramps on the
sublevels where strain bursts were occurring were found to have been driven nearly parallel to
steep bedding. When access ramps were driven nearly perpendicular to these structures, rock
bursts affecting the ribs ended in this part of the mine.

Fault-Slip Bursts

Fault-slip bursts represent damage that results from slip on a fault plane, possibly at some distance
from a damage site. Hypocenters of these events frequently approximate the locations of known
faults. Physical evidence for fault slip is rarely observed directly, but such events have been
documented in the district by distinctive seismic signatures and through interpretations of burst
damage and local geology (12, 16).

Despite the large amount of energy that is often released in fault-slip events, actual damage is
often impressively minor. Damage is thought most commonly to affect ground that is already
highly fractured, usually as a result of prior tectonic deformation. Common direct results of fault-
slip events are squeeze of sandfill, fracturing and possible heave of sills, and localized collapses.


All district production is closely associated with the quartzite, which commonly forms the
immediate wall rock of vein ore bodies and development openings. This rock type is also most
frequently involved in violent ground failures. Violent ground failures in district mines are
classified as either strain bursts or fault-slip bursts (3).

Strain Bursts

Strain bursts result from concentrated stress near mine openings. These bursts typically affect de-
velopment openings such as crosscuts, raises, and initial cuts in overhand stopes. Shafts and raise
boreholes are also sites for this type of rock burst damage. Although strain bursts are commonly
thought to represent relatively surficial failures, pillar bursts and more extensive violent bursts that
disrupt rock to a depth of several meters beneath the affected surfaces are also considered strain
bursts (13). These more extensive strain bursts are responsible for most of the violent damage that
occurs in district mines. The most characteristic strain bursts involving wall rock cause damage
to ribs, the junctions of ribs and the back, or the junction of the floor and the back (figure 5).
Strain bursts that affect ribs are generally thought to characterize mining districts with’ high
vertical stress loading, rather than horizontal loading, which is the case in the Coeur d'Alene
district. This observation particularly emphasizes the fact that geologic factors other than in situ
stress often influence rock bursts.

The sedimentary origin of the host rock assures that the wall rock in all mines in the district is
divided into layers. Even thick, relatively homogeneous quartzite beds are internally layered on
a fine scale and have often undergone partial mechanical delamination as a result of tectonism.
Wall rock and ribs are thus inherently separated into steeply dipping slabs of variable thickness.

In addition to nearly ubiquitous, steeply dipping sedimentary layers, steeply dipping, gouge-filled
faults are common in all mines of the district. Where these structures are subparallel to ribs and
lie a short distance behind the surface of a rib, they form narrow, steeply dipping slabs that are
frequently involved in strain bursts.

Fairhurst and Cook (14) suggested that strain bursting involves instantaneous buckling of slabby
rock. Examination of rock burst damage at district mines supports this concept. Preexisting
structures that divide the rock mass into layers promote strain-type rock bursts when these struc-
tures approximately parallel the surfaces of openings. High ground stresses parallel to the wall
rock layers and to these surfaces apparently cause the slabs to buckle to the point of brittle failure.
Once brittle fracture is initiated, stored elastic strain energy contained in the surrounding rock
mass instantly and thoroughly crushes the buckled slabs.

The major distinction between large and small strain bursts is thought to lie in the relative
capability of the surrounding wall rock to supply the elastic strain energy, or "following load,"
that drives the burst. Surficial strain bursts may involve only one slab. An extensive strain burst
involves many slabs that buckle essentially simultaneously (figure 6), releasing stored elastic
energy from a substantial volume of the surrounding rock mass.


12 T T T T T T

Standard deviation=134
|_ Mean=500 4
N=25

FREQUENCY

200 300 400 500 600 700 800 900
DEPTH, m

Figure 1.—Frequency diagram for depth

wf) TO TT T_T
Standard deviation=0.5
Mean=3

N=22

T T T T T T
Standard deviation=10
Mean=19

N=23,

FREQUENCY

4

FREQUENCY

n

0 12 18 24 30 36 42 48
4.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25
RICHTER MAGNITUDE, M4 PILLAR WIDTH, m

Figure 2.—Frequency diagram for released energy Figure 3.—Frequency diagram for pillar width
where 0 indicates insignificant yielding in the roof and floor and 2 indicates favorable, gradual
yielding.

© Stress gradients. Stress gradients varied from 0 to 2, depending on whether or not mining
proceeded toward an area of high stress (resulting either from previous mining or rapid

changes in topography) and/or abnormal geologic conditions such as occasionally found near
faults or grabens.

Bivariate Correlations and Data Reduction

Based on preliminary bivariate correlations among all geologic, geometric, and geomechanical


Table 1.—Statistical summary of geologic variables.

Variable Mean Stan. dev. Range No. of cases
Joint sets 1.4 0.6 1-3 25
Cleat sets 1.8 0.4 1-2 25
Inseam partings . . 1 0.9 0-3 21
Joint spacing, m bee 6.7 5.5 1.5-15 24
Rock Quality Designation ......... 77 18 50-100 15
Depth,m ......... 0.000 ee 500 134 275-820 25
Roof beam thickness, m vee 4.3 3.3 1,5-12.2 25
Young’s modulus, coal, MPa see 3310 830 2410-4620 25
Young’s modulus, roof and floor, MP 20,700 6900 6900-33,000 25
Uniaxial strength, MPa........... 22 5.2 13.8-32 25
Uniaxial strength, roof and floor, MPa 100 23.8 55-150 25
Max. horizontal stress, MPa . . 13 7.6 0.7-26 25
Interacting seams . 1.2 0.4 1-3 25
Local yield characteristics ........ 0.8 0-2 25

Table 2.—Statistical summary of geometric variables.

Variable Mean Standard deviation Range No. of cases
Pillar width, m..........6. 19 10 9-42 23
Pillar height, m . 2.5 0.3 1.7-3 25
Entry span, m bee 5.8 0.3 5.5-6 25
Barrier pillar width,m....... 50 27 15-73 6
Face width,m ........... 167 40 61-244 25
Mining method ........... 1.2 0.4 1-2 25
Stress gradient ........... 0.9 0.6 0-2 25
Table 3.—Statistical summary of geomechanical variables
Variable Mean Standard deviation Range No. of cases
Pillar factor of safety 0.8 0.3 0.5-1.4 23
Face factor of safety . 0.9 0.2 0.6-1.5 22
Energy (M,)........00002 eee 3 0.5 2-4 22
Damage .......-2.0002 esses 1.4 1 0-3 25

4 times the total seam thickness in the immediate mine roof. Although there is some evidence
that massive upper strata have contributed to coal bumps in some mines (8), their influence
was not directly evaluated in this study because of the lack of geological and mechanical
property data.

¢ Local yield characteristics. Local yield characteristics of immediate roof and floor strata in-
fluence coal pillar failure and the severity of coal bumps. This factor varied from 0 to 2,


vertical. In this study, the authors used the wealth of geotechnical data from coal mines in four
states and hard-rock mines in the Coeur d’ Alene Mining District to address the significant factors
contributing to violent failure.

FACTORS INFLUENCING COAL BUMPS

In an attempt to identify the most significant factors that contribute to coal bumps, the authors
analyzed geologic, geotechnical, and in-mine monitoring data from 25 sites in 6 room-and-pillar
coal mines and 19 longwall mines in Colorado, Utah, Virginia, and Kentucky. Both compu-
tational and statistical techniques were used in the analyses, and both violent and nonviolent fail-
ures were studied. The scope of these studies involved—

¢ Obtaining mechanical property values for roof, floor, and coal seams through laboratory tests
of samples of near-seam strata. The in situ strength of coal seams was estimated using proce-
dures suggested by Maleki (1).

¢ Calculating both maximum and minimum secondary horizontal stresses using overcoring stress
measurements from one to three boreholes (2).

* Calculating pillar and face factors of safety in individual case studies using both two- and
three-dimensional, boundary-element techniques (3-5). Results were compared with field data
when such data were available.

* Calculating energy release from a potential seismic event using both boundary-element
modeling and analytical formulations as suggested by Wu and Karafakis (6) to estimate energy
accumulation in both roof and coal, and energy release (7) in terms of Richter magnitude (M,,
using the following formula:

1.5 M, =a X log (B) - 11.8, qd)
where E = total accumulated energy in roof and seam, ergs,
and A = coefficient depending on joint density,

* Assessing the severity of coal bumps using a damage rating developed by and based on the
authors’ observations of physical damage to face equipment and/or injury to personnel, as well
as observations by other researchers as cited in the literature. Damage levels were arbitrarily
assigned a ranking between 0 and 3. Level 1 signifies interruptions in mining operations while
level 3 signifies damage to both face equipment and injuries to mine personnel.

The first step of the analyses involved the identification of 25 geologic, geometric, and geome-
chanical variables that might have had the potential to contribute to coal bump occurrence (tables
1, 2, and 3). Typical frequency histograms are presented in figures 1, 2, and 3 and show that
these case studies provided good coverage of the variables.

° Roof beam thickness. Roof beam thicknesses varied between 1.5 and 12 m. The thickness
chosen for evaluation was the strongest beam of the near-seam strata located between 1 and


10.

14,

15.

16.

Zipf, R. K., Jr. Stress Analysis in Coal Mines with MULSIM/NL. Paper in Proceedings
of the 89th Meeting of the Rocky Mountain Coal Mining Institute (Beaver Creek, CO, 1993).
Rocky Mt. Coal Min. Instit., Lakewood, CO, 1993, pp. 38-43.

Wu, X., and M. G. Karfakis. An Analysis of Strain Energy Accumulation Around Long-
wall Panels Under Strong Roofs. Paper in Proceedings, 5th Conference on Ground Control
for Midwestern U.S. Coal Mines, ed. by Y. P. Chugh and G. A. Beasley (Collinsville, IL,
June 1994). Dept. of Min. Eng., S. IL Univ., 1994, pp. 230-253.

McGarr, A. Some Applications of Seismic Source Mechanism Studies to Assessing
Underground Hazard. Paper in Rockbursts and Seismicity in Mines, ed. by N. C. Gay and
E. H. Wainwright (Proc. of Symp. on Seismicity in Mines, Johannesburg, S. Afr., 1982).
S. Afr. Inst. Min. Metall., Symp. Series 6, 1984, pp. 199-208.

Maleki, H. An Analysis of Violent Failure in U.S. Coal Mines: Case Studies. Paper in
Proceedings: Mechanics and Mitigation of Violent Failure in Coal and Hard-Rock Mines,
ed. by H. Maleki, P. F. Wopat, R. C. Repshar, and R. J. Tuchman. U.S. Bur. Mines
Spec. Publ. 01-95, 1995, pp. 5-25.

SPSS. Statistical Packages for the Social Sciences, Chicago, IL, 1995.

Starfield, A. M., and P. A. Cundall. Toward a Methodology for Rock Mechanics Model-
ing. Inter. J. Rock Mech. Min. Sci. & Geomech. Abst., v. 25, no. 3, 1988, pp. 99-106.

Hobbs, S. W., A. B. Griggs, R. E. Wallace, and A. B. Campbell. Geology of the Coeur d'Alene
Mining District, Shoshone County, Idaho. U.S. Geol. Surv. Prof. Paper 478, 1965, 139 pp.

Scott, D. F. Geologic Investigations Near an Underhand Cut-and-Fill Stope, Lucky Friday
Mine, Mullan, ID. U.S. Bur. Mines Rep. Invest. 9470, 1993, 21 pp.

White, B. G., J. K. Whyatt, and D. F. Scott. Geologic Factors in Rock Bursts in the Coeur
d'Alene Mining District: Structure. Paper in Proceedings: Mechanics and Mitigation of
Violent Failure in Coal and Hard-Rock Mines, ed. by H. Maleki, P. F. Wopat, R. C.
Repshar, and R. J. Tuchman. U. S. Bur. Mines Spec. Publ. 01-95, 1995, pp. 217-230.

Fairhurst, C., and N. G. W. Cook. The Phenomenon of Rock Splitting Parallel to the Di-
tection of Maximum Compression in the Neighborhood of a Surface. Paper in Proceedings
of the First Congress on the International Society of Rock Mechanics (Lisbon, Portugal,
Sept. 25-Oct. 1, 1966). V. 1, Nat. Lab. of Civil Eng., Lisbon, Portugal, 1966, pp. 687-
692.

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

Williams, T. J., C. J. Wideman, and D. F. Scott. Case History of a Slip-Type Rockburst.
PAGEOPH, v. 139, No. 3/4, 1992, pp. 627-637.