The study also provided indirect evidence about which
geologic factors do affect pillar strength. It does not seem likely
that the in situ strength of U.S. coal seams really is uniform,
because there is quite a bit of variability in the in situ data.
However, the strength of the intact coal, which is what is
measured in a laboratory test, is apparently largely irrelevant to
the pillar strength. Instead, it seems likely that variations in seam
strength are due to large-scale geologic features like bedding
planes, clay bands, and rock partings. Such features probably
determine the amount of confinement which can be generated
within the pillar, and therefore the load-bearing capacity of the
pillar core,

COAL SPECIMEN STRENGTH, psi

01,000 2,000 3,000 4,000 5,000 6,000_7,01
3.0) ee
25 ° ° i |
Bo
2.0 3 24 ed
rr) Bae
o 3 °
215 § og W
z ie ae a
Sao} ror? 2d
Key a
L eSuccess |
0.5) "Failure
a es es as
° 714212838429

COAL SPECIMEN STRENGTH, MPe

Figure 4. ARMPS SF compared with coal specimen
Strength,

MASSIVE PILLAR COLLAPSES

Most of the pillar failures included in the ARMPS data base
are “squeezes” in which the section converged over hours, days or
even weeks, Another important subset are 15 massive pillar
collapses (Mark et al., 1997b). These occurred when undersized
pillars failed and rapidly shed their load to adjacent pillars, which
in tum failed. The consequences of such chain reaction-like
failures typically include a powerful, destructive, and hazardous
airblast.

Data collected at 12 massive collapse sites revealed that the
ARMPS SF was less that 1.5 in every case, and was less that 1.2
in 81% of the cases (figure 5). What really distinguished the
sudden collapses from the slow squeezes, however, was the
pillar’s wh ratio, Every massive pillar collapse involved slender
pillars whose w/h was less than 3. Laboratory tests have shown
slender coal specimens typically have little residual strength,
which means that they shed almost their entire load when they fail
(figure 6). As the specimens become more squat, their residual
strength increases, reducing the potential for a rapid domino
failure. The mechanism of massive collapses has been replicated
in a numerical model (Zipf and Mark, 1997).

Two alternative strategies were proposed to prevent massive
pillar collapses:

© Prevention: With the prevention approach, the panel pillars
are designed so that collapse is highly unlikely. This can be
accomplished by increasing either the SF of the pillars, or
their wh ratio.

25 =
Collapse °
2 O Squeeze °
T|  oSuccess af ane)
me)
°
fe
154 ° e 80. ° g Po
oa A fon)
g oO o
z ° ° Eg
< if $ o ato
e, a
« ¢ a is]
05+ oo a
0 + + + +
0.00 1.00 2.00 3.00 4.00 5.00
Width-To-Height Ratio

Figure 5. ARMPS SF and pillar width-to-height ratio for pillar collapses and other case histories.



Salamon, M. D. G., 1989, Significance of Strata Control to the
Safety and Efficiency of Mining, Paper in Proceedings of the 8*
International Strata Control Conference, Dusseldorf, FRG, 9 pp.

Salamon, M. D. G. and H. Wagner, 1985, Practical
Experiences in the Design of Coal Pillars. Paper in Proceedings
of the 21* International Conference of the Safety in Mines
Research Institutes, Sydney, Australia, pp. 3-10.

Salamon, M. D. G., 1992, Strength and Stability of Coal
Pillars, Paper in Proceedings, Workshop on Coal Pillar Mechanics
and Design, BuMines IC 9315, pp. 94-121.

Su, D. W.H., and G. J. Hasenfus, 1996, Practical Coal Pillar
Design Considerations Based on Numerical Modeling, Presented
at 15th International Conference on Ground Control in Mining,
Golden, CO, (handout).

Su, D. W. H,, and G. J, Hasenfus, 1997, Effects of In-Seam
and Near-Seam Conditions on Coal Pillar Strength, Presented at
16th Intemational Conference on Ground Control in Mining,
Morgantown, WV, (handout),

Zipf, R. K., 1992, MULSIM/NL Application and
Practitioner’s Manual, BuMines IC 9322, 48 pp.

Zipf K, and C. Mark,1997, Design Methods to Control
Violent Pillar Failures in Room and Pillar Mines. Transactions
IMM. Vol. 106, Sept-Dec, pp. A124-A132,

APPENDIX. THE MARK-BIENIAWSKI
RECTANGULAR PILLAR STRENGTH FORMULA

A major drawback of most of the classic pillar strength
formulas is that they only apply to square pillars. They
underestimate the strength of rectangular pillars that contain
proportionately much more core area. The concept of the "stress
gradient” provides the link which allows classic formulas to be
extended to rectangular pillars. The stress gradient defines the
vertical stress within the pillar at maximum load as a function of
the distance from the nearest rib.

Although classic formulas do not explicitly consider the effect
of internal pillar mechanics, it is apparent that they imply a non-
uniform stress distribution because of the w/h effect. A derivation
of the implied stress gradients was published by Mark and
lannacchione (1992). For example, the Bieniawski formula:

S, = S, (0.64 + 0.36 wh)
implies a stress gradient of:
S, = S, (0.64 + 2.16 x/h)
where: S, = Pillar strength
S, = In situ coal strength

S, = Vertical pillar stress
x = Distance from pillar rib

When this stress gradient is integrated over the load bearing
area of a rectangular pillar, the Mark-Bieniawski pillar strength
formula is obtained:

S, = S, (0.64 + 0.54 w/h - 0.18 (w/Lh)
Where L = pillar length. The approach is illustrated in

figure A-I, and described in more detail by Mark and Chase
(1997).

Pillar’s load-
bearing capacity

RECTANGULAR PILLAR

L. Pillar length

W Pillar width
EQUIVALENT

SECTION OF
STRIP PILLAR

SQUARE PILLAR

Figure A-1. Stress distribution concept used in the Mark-
Bieniawski pillar strength formula.



variable, to check if the pillar strength formula could be
improved.

Depth of Cover (H): H was included as an independent
variable primarily to check the loading formulation.

The entry width and the primary support were included as before

‘The statistical analysis showed that the ALPS (R) SF and the
CMRR still correctly predicted 85% of the outcome, including
94% of the failures. None of the other three new variables would
be included even at the 50% confidence level (a 90% confidence
level would be required for a covariate to be considered
statistically significant). Figure 2 shows the distribution of the
case histories, and the revised design equation:

ALPS (R) SF= 2.0 - 0.016 CMRR.

Since 1987, ALPS has become the most widely-used pillar
design method in the U.S. The ALPS-CMRR method directly
addresses gate entry performance, and makes U.S. longwall
experience available to mine planners in a practical form.
Tailgate blockages are far less common today than they were 10
years ago, and ALPS can surely claim some of the credit.

PILLAR DESIGN FOR RETREAT MINING
The classical empirical pillar strength formulas were all
developed for room and pillar mining. However, none ever
attempted to consider the abutment loads that occur during pillar
recovery operations. The abutment load formulas used in ALPS
provided a means to rectify that shortcoming.

The Analysis of Retreat Mining Pillar Stability (ARMPS)
employs the same basic constructs as ALPS, adapted to more
complex and varied mining geometries (Mark and Chase, 1997).
The abutment load formulas have been adapted to three
dimensions, to account for the presence of barrier pillars and
previously-extracted panels. The Mark-Bieniawski pillar strength
formula is used to estimate pillar strength. Features such as
varied entry spacings, angled crosscuts, and slab cuts in the

barrier can all be modeled.

To evaluate the validity of ARMPS, more than 200 retreat
mining case histories have been obtained from field visits
throughout the U.S. Variables included in the statistical analysis
included the w/h, CMRR, UCS, and H. When the entire data set
was evaluated, it was found that 77% of the outcomes could be
correctly predicted simply by setting the ARMPS SF to 1.46.
Including either the depth or the w/h increased the r slightly,
without improving the accuracy (figure 3). The depth and the w/h
ratio were strongly correlated with each other within the data set.

The accuracy improved when the data set was divided into
two parts. One group included only cases where cover was
shallow (H less then 200 m (650 ft)) and where the pillars were
not squat (w/h was less than 8). For this group, when ARMPS =
15 it successfully predicted 83% of the outcomes. However, for
the deep cover/squat pillar group, only 58% of the cases were
correctly predicted at ARMPS SF=0.93. No other variables could
be included in either group at the 90% confidence level. The
conclusion seems to be that ARMPS works quite well at shallow
depth and moderate w/h ratios. It is of much less value for squat
pillars at greater depth.

COAL STRENGTH TESTING AND PILLAR DESIGN

The classic pillar strength methods considered laboratory
uniaxial compressive strength (UCS) tests an integral part of
pillar design. However, neither the longwall or the room-and-
pillar case history analyses described above found UCS to be of
any help in pillar design. An earlier study (Mark and Barton,
1996) had reached a similar conclusion. That study explored
DUCS, the largest and most complete data base of the uniaxial
compressive strength of coal ever assembled. It found that the
“size effect” varies dramatically from seam to seam, depending on
the coal cleat structure. More importantly, there was no
correlation between the UCS and the in situ pillar strength
documented in case histories of failed pillars (figure 4). The
conclusion was that pillar design was more reliable when a
uniform coal strength was used.

3
© Satisfactory
25 Unsatisfactory
2 °
o
a
2£ .
a 18 oy
«
< = ." °
1 1} r "4 °
8 °
a .
os . o =
0
‘650
0 500 1000 1500 2000
Depth of Cover (ft)

Figure 3. ARMPS SF and depth of over 200 retreat mining case histories.



© Containment: In this approach, high-extraction is practiced
within individual compartments that are separated by barriers.
The small pillars may collapse within a compartment, but
because the compartment size is limited, the consequences are
not great. The barriers may be true barrier pillars, or they
may be rows of development pillars that are not split on
retreat. The containment approach has been likened to the
use of compartments on a submarine.

Design charts have been developed for each approach,
considering the width of the panel, the seam thickness, and the
depth of cover (Mark et al., 19976).

USE OF NUMERICAL MODELING IN PILLAR DESIGN

It is beyond the scope of this paper to discuss in detail the
contributions made by numerical modeling to pillar design.
However, two efforts are particularly worthy of note.

Su and Hasenfus (1996,1997) have employed finite element
models (FEM) to explore the effect of various geologic conditions.
They have shown, for example, that a rock parting may increase
the pillar strength, while a clay parting can reduce it. A weak
floor can reduce the pillar strength by as much as $0%, All of
these effects are minimal for slender pillars, but become much
more pronounced once the whh exceeds 5. The models also
indicated that varying the coal strength had almost no effect on
pillar strength. [t is remarkable that despite the differences in
technique, many of the conclusions from Su and Hasenfus’ FEM
parallel those derived from empirical back-analysis of case history
data.

Displacement-discontinuity (DD) methods have been used for
many years to estimate pillar loadings in complex and multiple-
seam geometries. When it was introduced in 1992, the DD
program MULSIMINL represented a significant advance because
it incorporated realistic pillar and gob mechanics (Zipf, 1992).
The next-generation DD model, called LAMODEL, was recently
developed by Heasley and Salamon (1996a, 1996b). LAMODEL
simulates the overburden as a stack of layers separated by
frictionless interfaces, thus providing a realistic suppleness to the
overburden that was not possible with older DD models. Input
pillar strength parameters are typically derived from empirical
formulas, using the stress gradient approach described in the
Appendix (Heasley, 1998; Karabin and Evanto, 1994), Thus the
modeled pillar strengths are closely linked to real-world behavior,
while LAMODEL’s analytical mechanics allow it to accurately
analyze complex mining situations including multiple seams,
random pillar layouts and/or variable topography. The result is a
powerful synthesis of empirical and numerical approaches to
pillar design.

STRESS, MPa

0 60 100 150 200 250
STRAIN, 10°

Figure 6. Complete stress-strain curves for coal
specimens (after December 1986).

CONCLUSIONS

Recent years have seen significant advances in the state-of
the-art in coal pillar design. From a practical standpoint, the
development of reliable empirical methods like ALPS and
ARMPS has been particularly valuable. They have been widely
accepted throughout the mining community because they have
been verified by extensive data bases of real-world case histories,
and because they have been readily available in user-friendly
computerized formats. The DD models MULSIM/NL and
LAMODEL have been another important success story.

The research has led to some other important conclusions,
including:

© Laboratory testing of small coal samples, particularly uniaxial
compressive strength tests, are not useful for predicting pillar
strength;

© The strength becomes more difficult to predict as the pillar
becomes more squat;

© The wh ratio is important for predicting not just the pillar
strength, but the mode of failure, and;

© Many ground control problems must be considered from the
standpoint of entry stability, where pillar behavior is just one
component.

Furthermore, old concepts of pillar “failure” have given way
to anew paradigm that identifies three broad categories of pillar
behavior:



THE STATE-OF-THE-ART IN COAL PILLAR DESIGN

C. Mark

NIOSH

Pittsburgh, PA.

ABSTRACT

By 1980, the U.S. mining community had reached a broad
consensus regarding coal pillar design. The pillar load could be
estimated from tributary area theory, and the pillar strength from
empirical formulas and laboratory coal strength testing. Then the
growth of longwall mining required new thinking. Recently,
powerful design methods have emerged from analysis of large
data bases of real-world pillar successes and failures. These
include the Analysis of Retreat Mining Pillar Stability (ARMPS),
the Analysis of Longwall Pillar Stability (ALPS), the Mark-
Bieniawski rectangular pillar strength formula, and guidelines for
preventing massive pillar collapses. Sophisticated numerical
models have also helped transform the pillar design landscape.
In the process, our understanding of pillar mechanics has been
greatly enriched. A new paradigm divides pillar failure into three
categories:

- Slender pillars (w/h<3.0), which are subject to sudden
collapse;

= Squat pillars (w/h>10), which are dominated by entry
failure (rib, roof, or floor) and coal bumps, and;

- Intermediate, in which pillar squeezes seem to be the
most common failure mode.

INTRODUCTION

The science of pillar design in the U.S. goes back nearly a
century. One early pioneer noted that "to mine without adequate
pillar support will result, sooner or later, in a squeeze; the
inherent effects of which are crushing of the pillars, caving of the
roof, and heaving of the bottom" (Bunting, 1911). Various pillar
design formulas were proposed in the early days, based upon
laboratory testing, full-scale pillar testing, and back-analysis of in-
mine case histories (see Mark and Barton, 1996). They were
developed for an industry that relied almost exclusively on room-
and-pillar mining at relatively shallow depth.

The energy crisis of the 1970's and 1980's saw a revival of
interest in coal pillar design. A number of ambitious field studies

were undertaken, many of them funded or conducted by the U.S.
Bureau of Mines. By 1980, the "classic" pillar design
methodology had fully matured. It consisted of three steps:

1, Estimating the pillar load using tributary area theory;

2. Estimating the pillar strength using a pillar strength
formula, and;

3. Calculating the pillar "safety factor" (SF).

Several formulas were available for step 2. Each estimated
the pillar strength as a function of two variables, the pillar’s
width-to-height ratio (w/h) and the coal seam strength determined
from laboratory testing (Bieniawski, 1984).

The growth of longwall mining exposed some serious
shortcomings in this classic pillar design methodology. Most
obvious was the need to go beyond tributary area and consider the
abutment loads brought about by full-extraction mining. More
serious was that longwall mining raised the issue of what
constituted pillar "failure." The classic approach contended that
“pillars will fail when the applied load reaches the compressive
strength of the pillars" and that "the load-bearing capacity of the
pillar reduces to zero the moment the ultimate strength is
exceeded” (Bieniawski, 1992). This model was clearly
inappropriate for the squat (large w/h) pillars used in longwall
mining. When longwall pillars “failed,” their load-bearing
capacity did not disappear. Rather, the gate roads became
unservicable.

Arthur Wilson of the British National Coal Board was the first
to take a radically different approach to pillar design. His analytic
method treated the pillar as a complex structure, with a non-
uniform stress gradient, a build-up of confinement around a high-
stress core, and progressive pillar failure. Although his
mathematics were seriously flawed (Mark, 1987; Salamon, 1992),
Wilson's basic concepts are now broadly accepted and underlie
nearly all modern numerical models.

By 1990, the number of pillar strength formulas and
numerical models had proliferated, but their predictions for squat



© Slender pillars (w/h<3), which have little residual strength
and are prone to massive collapse when used over a large
area;

© Intermediate pillars (4<w/h<8), where “squeezes” are the
dominant failure mode in room-and-pillar mining, and where
empirical pillar strength formulas seem to be reasonably
accurate, and;

© Squat pillars (w/h>10), which can carry very large loads and
are strain-hardening, and which are dominated by entry failure
(roof, rib, and floor) and by coal bumps.

Certainly, more work remains before the age-old questions of
pillar design is finally solved. In particular, there is much more
to learn about the mechanics of squat pillars and roof-pillar-floor
interactions. Currently, there is no accepted way to determine the
frictional characteristics of the contacts, bedding planes, and
partings that are so crucial to pillar strength. It is similarly
difficult to characterize the bearing capacity of the floor. Simple,
meaningful field techniques for estimating these properties will
be necessary for further progress with either numerical or
empirical techniques.’ Indeed, the cross-pollination between the
numerical and empirical methods that has characterized the recent
past can be expected to bear further fruit in the future.

REFERENCES

Barczak, T. M, 1992, The History and Future of Longwall
Mining in the United States, BuMines IC 9316, 26 pp.

Bicniawski, Z. T, 1984, Rock Mechanics Design in Mining
and Tunneling, A. A. Balkema, 272 pp.

Bieniawski, Z. T, 1992, A Method Revisited: Coal Pillar
Strength Formula Base on Field Investigations, Proceedings,
Workshop on Coal Pillar Mechanics and Design, BuMines IC
9315, pp. 158-165.

Bunting, D, 1991, Chamber Pillars in Deep Anthracite Mines.
Trans. AIME, vol. 42, pp. 236-245.

Heasley, Keith A., and Miklos D. G. Salamon, 1996a, “A
New Laminated Displacement-Discontinuity Program,”
Proceedings of the Second North American Rock Mechanics
Symposium, Montreal, Quebec, Canada, June 19-21, p. 1879-
1886.

Heasley, Keith A., and Miklos D. G. Salamon, 1996b, “A
New Laminated Displacement-Discontinuity Program:
Fundamental Behavior,” Proceedings of the Fifteenth
International Conference on Ground Control in Mining, August
13-15. p. 111-125.

Heasley, Keith A. 1998, "Numerical Modeling of Coal Mines
with a Laminated Displacement-Discontinuity Code", Ph.D.
dissertation, Colorado Schoo! of Mines, in progress.

Tannacchione, A. T., and C. Mark, 1990, Possible Mechanism
for Surface Vibrations Near Maxwell Hill, West Virginia, Bull.
Assoc. Eng. Geol., vol. 27, no. 3, pp. 341-353.

Karabin, G. J., and M. A. Evanto, 1994, Experience with the
Boundary Element Method of Numerical Modeling as a Tool to
Resolve Complex Ground Control Problems, Paper in
Proceedings, 13 Conference on Ground Control in Mining,
Morgantown, WV, pp. 201-213.

Mark, C, 1987, Analysis of Longwall Pillar Stability, Ph.D.
‘Thesis, PA State Univ., University Park, PA, 414 pp.

Mark, C, 1992, Analysis of Longwall Pillar Stability: An
Update, Paper in the Proceedings of the Workshop on Coal Pillar
Mechanics and Design, USBM IC 9315, pp. 238-249.

Mark, C, 1990, Pillar Design Methods for Longwall Mining,
USBM IC 9247, 53 pp.

Mark C, and T. M. Barton, 1996, The Uniaxial Compressive
Strength of Coal: Should it be Used to Design Pillars?,
Proceedings, 15th International Conference on Ground Control in
Mining, Golden, CO, August 13-15.

Mark, C., and F. Chase, 1997, Analysis of Retreat Mining
Pillar Stability, Paper in New Technology for Ground Control in
Retreat Mining, NIOSH IC 9446, pp. 17-34.

Mark, C., F. Chase, and R. K. Zipf, Jr, 1997b, Preventing
Massive Pillar Collapses in Coal Mines, Paper in New
Technology for Ground Control in Retreat Mining, NIOSH IC
9446, pp. 35-48.

Mark, C., F. E. Chase, and G. M. Molinda 1994, Design of
Longwall Gate Entry Systems Using Roof Classification, Paper in
New Technology for Longwall Ground Control: Proceedings of the
USBM Technology Transfer Seminar, USBM SP 94-01, pp. 5-18.

Mark, C. and A. T. fannacchione,1992, Coal Pillar Mechanics:
Theoretical Models and Field Measurements Compared, Paper in
the Proceedings of the Workshop on Coal Pillar Mechanics and
Design, USBM IC 9315, pp. 78-93.

Mark, C. and G, M. Molinda, 1996, Rating Coal Mine Roof
Strength from Exploratory Drill Core, Proceedings, 15th
International Conference on Ground Control in Mining, Golden,
CO, August 13-15, pp. 415-428.

Molinda, G. and C. Mark, 1994, The Coal Mine Roof Rating
(CMRR)--A Practical Rock Mass Classification for Coal Mines,
USBM IC 9387, 83 pp.

MSHA, 1993, Coal Mine Fatal Accident Investigation Report,
Fatal Coal Mine Roof Fall at South East Coal Mine.

MSHA, 1996, Coal Mine Fatal Accident Investigation Report,
Fatal Coal Mine Burst at Harlan-Cumberland C-2 Mine.



pillars varied widely. One study compared 10 formulas, and
found that some predicted that pillar strength would increase
exponentially as the w/h ratio increased, others predicted it would
tend towards a maximum limiting value, and still others predicted
an intermediate, linear increase (figure 1). Stress measurements
from 34 coal pillars were also analyzed, but were no help in
narrowing the field (Mark and lannacchione, 1992).

.———
Epliea frmias
se Samon ture
———— Holland-Gaddy

- Bieniawski
I sheorey
= — Obert Duvall :

4r Analytic formulas 7
=== Wison t
+ Barron fi
+1 Salamon “Squat” t
Numerical Models
Hsuing-Peng
— Kripakov J

NORMALIZED PILLAR STRENGTH

5 7 9 100-1315
PILLAR WIDTH-TO-HEIGHT RATIO

Figure 1, Comparison of pillar strength predictions
from various formulas.

The need for material properties is another Achilles heel
shared by both the classic formulas and numerical methods.
Relating laboratory uniaxial compressive strength tests to full-size
coal pillars has been the subject of controversy since the tum of
the century. Numerical models compound the problem by
requiring friction angles and post-failure properties in addition to
the in situ coal strength. It is difficult to have faith in the
accuracy of even the most sophisticated numerical models when
critical material properties and structural features are either
guessed or ignored,

Fortunately, another approach is available to help solve
complex ground control problems. The empirical, or statistical,
approach relies instead on the scientific interpretation of actual
mining experience. Hundreds of longwall and room-and-pillar

panels are mined each year, and each one is a full-scale test of a
pillar design. Once data has been collected on enough of these
case histories, statistical techniques can be used to determine
those combinations of factors most likely to result in pillar failure.

The empirical approach is widely used in other fields, such as
medicine, where the scientific understanding of the physical
problem is incomplete, but a large quantity of data is available.
Because the solutions are so firmly linked to reality, they are
particularly well suited for solving practical problems like pillar
design. Perhaps the best example of the approach is the Salamon
and Munro pillar strength formula, which has been so convincing
it has been used to design more than one million South African
pillars (Salamon and Wagner, 1985).

Effective use of the empirical technique requires, as Salamon
(1989) pointed out, “a reasonably clear understanding of the
physical phenomenon in question.” Without prudent
simplification, the complexity of the problem will overwhelm the
method’s ability to discern relationships between the important
variables. But a key advantage is that critical variables may be
included even if they are difficult to measure directly, through the
use of “rating scales.”

This paper will focus on a number of practical pillar design
methods that have been developed through the use of the
empirical technique.

Recent Examples of Pillar Failure

Given today's high-production, advanced technology
underground coal industry, do pillar failures still happen?
Unfortunately, the answer is yes. Some recent examples:

Massive Collapses: In 1992, miners were splitting pillars at
a southern West Virginia mine when the fenders in a 2.3 ha
area suddenly collapsed. The miners were knocked to floor by
the resulting air blast, and 103 ventilation stoppings were
destroyed. At least 12 similar events have occurred in recent
years, miraculously without a fatality (Mark et al., 1997b).

Pillar Squeezes: At a Kentucky coal mine, pillars were being
extracted in the main entries under 270 m of cover. The
pillars began to crush in response to the vertical load,
resulting in a roof fall that killed two miners (MSHA, 1993).
This incident is an extreme example of hazardous conditions
that can be associated with slow pillar failure. At least 45
recent instances of pillar squeezes in room-and-pillar mines
have been identified (Mark and Chase, 1997).

Longwall Tailgate Blockages: In 1984, 26 miners at the
Wilberg Mine in Utah could not escape a deadly fire because
ofa tailgate roof fall. Similar blockages were common in the
1980's, and 50 cases have been documented (Mark, 1992).

Pillar Bumps: Extracting the initial lift from a standing
pillar at a deep, eastern Kentucky operation resulted in a
bump that killed two miners (MSHA, 1996). Bumps are not
confined to pillars, however--another fatal bump occurred at
a Utah longwall face just days later.



90

80 ag

70+
60 =

CMRR

50 rT]
404

304

20

© Satisfactory
m Unsatisfactory

0.20 0.70

1.20 1.70 2.20

ALPS(R) SF
Figure 2. ALPS (R) SF and CMRR for longwall tailgate performance case histories.

Multiple seam interactions: Some studies indicate that most
remaining coal reserves will experience multiple seam
interactions. At a West Virginia mine where four seams had
been previously extracted, one fatality occurred when the roof,
collapsed without warning beneath a remnant barrier pillar.

Abandoned mine subsidence: As suburban development
expands into historic coal mining areas, unplanned subsidence
has become an important issue. In one case, water caused
floor failure in a mined-out section of an active mine, causing
$1 million in damage to overlying structures. In another,
residents above 50 yr old workings were disturbed by
seismicity emanating from collapsing pillars (lannacchione
and Mark, 1989).

DESIGN OF LONGWALL GATE ENTRY SYSTEMS

In the fifteen years after 1972 the number of U.S. longwall
faces grew from 32 to 118 (Barczak, 1992). The new technology
created a host of operational and safety problems, including the
maintenance of stable travelways on the tailgate side.
Researchers initially viewed gate entry ground control primarily
as a pillar design issue. The clear correlation between larger
pillars and improved conditions that had been established by trial-
and-error at many mines supported this approach.

Comparing longwall pillars to traditional coal pillars, the most
obvious difference is the abutment loading. The major
contribution of the original Analysis of Longwall Pillar Stability
(ALPS) was a formula for estimating the longwall pillar load,
based on numerous underground measurements (Mark, 1990).

It became clear, however, that tailgate stability required more
than good pillar design. Other factors, such as roof quality and
artificial support, were clearly important. Attacking this complex
problem with analytical or numerical models would have been
extremely difficult. On the other hand, the problem was ideal for
the empirical approach.

Data were collected from approximately 55% of all U.S.
longwall mines, selected to represent a geographic and geologic
cross-section of the U.S. longwall experience. A total of 64 case
histories were classified as “satisfactory” or "unsatisfactory" based
on the conditions in the tailgate (Mark et al., 1994).

Each case history was described by the ALPS stability factor
(SF), entry width, primary support rating, and the Coal Mine Roof
Rating (CMRR). The CMRR then weighs the importance of all
the geologic factors that determine roof competence, and
combines them into a single rating on a scale from 0 to 100
(Molinda and Mark, 1994; Mark and Molinda, 1996).

Multi variate statistical analysis showed that when the roof is
strong, smaller pillars can safely be used. For example, when the
CMRR is 75, the an ALPS stability factor (SF) of 0.7 is adequate.
‘When the CMRR drops to 35, the ALPS SF must be increased to
1.3. Significant correlations were also found between the CMRR_
and both entry width and the level of primary support (Mark et
al., 1994).

The ALPS data base was recently revisited, with several new
variables added. These included:

Rectangular pillar strength formula: All the SF were
recalculated with the Mark-Bieniawski formula (see the
Appendix to this paper) substituted for the original
Bieniawski formula. The new result is designated as the
ALPS (R) SF.

Uniaxial compressive strength: Nearly 4,000 laboratory tests
were compiled from the literature into the Database of
Uniaxial Coal Strength, or DUCS (Mark and Barton, 1996).
From these data, typical seam strength values were obtained
for 60 U.S. coalbeds.

Width-to height ratio (w/h): The w/h of the largest pillar in
the gate entry system was included as an independent