REFERENCES

Bryan A, Bryan JG, Fouche J [1966]. Some problems of strata control and
support in pillar workings. Min Eng (London) 123:238-254.

Chase FE, Zipf RK, Mark C [1994]. The massive collapse of coal pillars -
case histories from the United States. In: Proceedings of the 13th Conference on
Ground Control in Mining. Morgantown, WV: West Virginia University,
pp. 69-80.

Das MN [1986]. Influence of width/beight ratio on postfaiture behavior of
coal. Intl J Min Geol Eng 4:79-87.

Ferriter RL, Zipf RK, Ropchan DM, Davidson J [1996]. Report of technical
investigation underground nonmetal mine collapse accident, Solvay Mine. U.S.
Department of Labor, Mine Safety and Health Administration.

Madden BJ {1991]. A re-assessment of coal pillar design. J South Afr Inst
Min Metall 9/(1)-27-37.

Maleki H [1995]. An analysis of violent failure in U.S. coal mines—case
studies. In: Maleki H, Wopat PF, Repsher RC, Tuchman RJ, eds. Proceedings:
Mechanics and Mitigation of Violent Failure in Coal and Hard-Rock Mines.
Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, SP 01-95,
pp. 5-25.

Mark C, Chase FE [1997]. Analysis of retreat mining pillar stability
(ARMPS). In: Mark C, Tuchman RJ, comp. Proceedings: New Technology for
Ground Control in Retreat Mining. Pittsburgh, PA: U.S. Department of Health

and Human Services, Public Health Service, Centers for Disease Control,
National Institute for Occupational Safety and Health, IC 9446.

Ropchan D [1991]. Ground support evahuation—Belina No. 2 Mine ID No.
42-01280. Deaver, CO: U.S. Department of Labor, Mine Safety and Health
Administration.

Salamon MDG [1970]. Stability, instability, and design of pillar workings.
Intl J Rock Mech Min Sci 7:613-631.

University of New South Wales School of Mines [1994].
performance. Strata Control for Coal Mine Design Apr5.

Zipf RK Jr. [1992]. Analysis of stable and unstable pillar failure using a
Jocal mine stiffness method. In: lannacchione AT, Mark C, Repsher RC,
Tuchman RJ, Jones CC, comp. Proceedings of the Workshop on Coal Pillar
‘Mechanics and Design. Pittsburgh, PA: U.S. Department of the Interior, Bureau
of Mines, IC 9315, pp. 128-143.

Zipf RK [1996]. Analysis and design methods to control cascading pillar
failure in room-and-pillar mines. In: Bieniawski ZT, ed. Milestones in Rock
Engineering, Balkema, pp. 225-264.

Zipf RK, Mark C [1996]. Design methods to contro! violet pillar failures
in room-and-pillar mines. In: Proceedings of the 15th Conference on Ground
Control in Mining. Golden, CO: Colorado Schoo! of Mines, pp. 571-585.

Pillar


LEGEND N

Collapsed area

ie) 60m
() 200ft —
Scale Ova fab

_JaoanaGa
WIDOCGHO000

39

DUO
=pa0000 qaogu0Dgv4o0000
TragnogobGgoaooouanvoI000

OoOv000D00DN

Figure 3.—Falled small-center development workings at Mine D.

FLOOR RECOVERY (MINE G)

Mine G is located in Utah and was extracting the 8-m (25-
ft) thick Lower O'Connor Seam {Ropchan 1991]. There were
previous workings in the Upper O'Connor above Mine G,
separated by 18-23 m (60-80 ft) of overburden. The total
overburden above the collapsed area was about 170 m (550
fd).

Room-and-pillar workings were advanced 2.4 m (8 ft) high
on 18-m (60-ft) centers. The panel was developed nine entries
wide and 535 m (1,740 ft) long. The pillars were not ex-
tracted on retreat, but an additional 3 m (10 ft) was removed
from the floor, leaving 5.4-m (18-ft) high remnants. Mining
the floor coal decreased the w/h ratio of the pillars from 5 to
2.2 and reduced their strength by about 45%.

The collapse occurred when the section was within two
crosscuts of being completely retreated. The force of the
airblast hurled three miners for distances of 12-30 m (40-100
ft), causing one severe head laceration. A 2-ton shop car was
blown through a stopping. There was extensive damage to
ventilation structures; concrete blocks from stoppings were
scattered up to 30 m (100 ft). The main mine fan was stalled,
and airflow in the mine was temporarily reversed. There was

some speculation that a north-south trending fault that
bordered the panel may have contributed to the collapse.

SUMMARY OF CASE HISTORIES

Table 1 summarizes the mining dimensions of 13 examples
of massive pillar collapses in U.S. coal mines. All occurred
during the 1980's and 1990's, and all happened suddenly or
without significant warning. Most resulted in airblasts and
damage to the ventilation system.

Analysis of the data reveals some important similarities.
First, the ARMPS SF was less than 1.5 in every case and less
than 1.2 in 81% of the cases. This implies that the pillars were
not sized to carry the full overburden load. Pillar failures are
not unusual; however, most are slow and nonviolent. What
apparently distinguishes the sudden collapses from the slow
squeezes is the pillar’s wh ratio. Every massive pillar collapse
involved slender pillars with a w/h ratio of less than 3.
Another common characteristic of the collapses is that the
overburden was judged to be relatively strong in every case.
Finally, the collapsed areas were all at least 1.6 ha (4 acres),
and the minimum dimension of a collapsed panel suffering
major damage was 110 m (350 ft).


might be prudent to increase the SF to 2.0. The SF can be
increased by increasing the pillar width, decreasing the
extraction ratio, or both. These two design criteria have been
combined to develop guidelines for small-center mining.
Figure 7 was developed assuming square pillars with an SF of
2.0 or a wh ratio of 4.0.

‘When using 6-m (20-ft) wide entries, the minimum
suggested pillar sizes are increased by about 6%. Also note
that these design criteria are only for controlling massive pillar
collapses. At greater depths, pillar sizes may need to be
increased beyond a w/h ratio of 4 to maintain an adequate SF.
The failure of pillars with a wh ratio greater than 4 should be
a slow squeeze rather than a sudden collapse.

PILLAR SPLITTING: A CONTAINMENT
APPROACH

Fenders left from pillar-splitting operations have failed at
even shallow depths. For example, 3- by 12-m (10- by 40-ft)
fenders in a 3-m (10-ft) seam have an SF of 1.5 at only 55 m
(180 ft) of cover. The potential for a destructive massive
collapse can be reduced by limiting the size of the gob area.
To separate the gob areas, rows of unsplit development pillars
can be left as barriers. This strategy is based on two
assumptions:

¢ By limiting the span above the mined-out area, a
bridging failure of the strong overburden is less likely.

¢ By minimizing the size of the potential collapsed area,
any airblast resulting from a collapse would be less powerful.

Table 1 shows that no major collapses have been
documented in which the gob area was less than 1.5 ha (4
acres). In the five cases where the gob area was between 1.5
and 1.9 ha (4 and 5 acres), about 60% of the incidents resulted
in major damage. Additionally, no damaging incidents
occurred when the minimum dimension of the mined-out area
was less than 100 m (350 ft). Using these data, acceptable
dimensions of a pillar-splitting operation might be a maximum
area of 1.2 ha (3.2 acres), with a minimum dimension of less
than 90 m (300 ft). For example:

« Assuming 18- by 18-m (60- by 60-ft) centers in a nine-
entry system with four rows split, the mined-out area would

have a minimum dimension of 72 m (240 ft) and an area of
about 1.1 ha (3 acres), as shown in figures 8A and 8B.

* Assuming the same pillar size in a six-entry system with
five rows split, the minimum dimension would be 90 m (300
ft) and the area would be about 1 ha (2.5 acres), as shown in
figures 8C and 8D.

The next question is: how many unsplit rows should be left
between these mined-out areas? The goal is to leave enough
of a “barrier” so that the failure of one gob area does not
initiate failure in adjacent areas. ARMPS was used to evaluate
the loading on unsplit pillars between two mined-out areas.
The program was modified so that two “front” gobs could be
applied to the unsplit pillars. The analyses were run with
abutment angles of 90°, which assumes that none of the load
is carried by the gob, but instead is transferred to the barriers.

In the first set of analyses, two rows of full-sized pillars
were used as the barrier. An ARMPS SF of 1.5 was deemed
necessary to prevent the collapse of one gob area triggering
the collapse of an adjacent area. Three rows of pillars were
used in the second set of analyses; the SF was reduced to 1.0
because of the greater stiffness of the barrier. Pillars on 18-
by 18-m (60- by 60-ft) centers were used in all cases.

Other parameters that were varied included the number of
tows thal were split (three, four, and five), the entry width (5.5
and 6 m (18 and 20 ft)), the seam height (2, 2.5, and 3 m (6,
8, and 10 ft), and the number of entries in the section (five,
seven, and nine). The results are presented in figure 9, which
shows the suggested maximum depth of cover for each
combination of parameters. In general, considering 55-m
(18-ft) entries in a 2.5-m (8-ft) seam, it appears that two rows
of unsplit pillars are an adequate barrier at depths less than
about 300 ft and that three rows are acceptable to about 170 m
(550 ft) of cover.

Barriers must also be left between extracted panels. These
can be unsplit development pillars or solid coal. If unsplit
development pillars are used, the analysis in figure 9 should
apply. For solid coal barriers, figure 10 shows the suggested
widths, using the same loading assumptions. For a 2.5-m (8-
ft) seam, a 17-m (55-ft) solid barrier appears to be appropriate
at 75 m (250 ft) of cover, and 23 m (75 ft) might be needed at
120 m (400 ft).


4l

MECHANICS OF MASSIVE PILLAR COLLAPSES

A conceptual model of a massive pillar collapse can be
described as follows. Undersized, regularly spaced remnant
pillars help the stiff and competent roof to bridge a relatively
wide span. A pressure arch is created, with much of the
overburden load being transferred by the stiff roof to the
barrier pillars surrounding the extraction area. Within the
pressure arch, the pillars are shielded from the full weight of
the overburden. Eventually, any one of a number of mech-
anisms may cause the pressure arch to break down:

+ The extraction area becomes so large that it exceeds the
bridging capacity of the roof.

* Mining approaches a fault or other discontinuity.

+ The roof weakens over time.

* The remnant pillars weaken over time.

Once the pressure arch breaks down and additional
overburden load is shifted to the pillars, their structural
characteristics are such that a sudden, massive collapse can
occur. Slender pillars have little residual strength and shed
Joad rapidly as they fail. When one fails, the weight it
transfers can overload adjacent pillars, and a rapid “domino”
failure of adjacent pillars can ensue. Pillars that are more
squat retain most of their load even after failure. Such pillars
will squeeze slowly, rather than collapse.

Laboratory tests have shown that the residual strength of
coal specimens depends on their w/h ratio [Das 1986].
Specimens with a wh ratio of less than 3 typically have little
residual strength, which means that they shed almost their
entire load when they fail (figure 4). As the specimens

160
9.0
120 4
wh=7.7
LJ
oa 4
=
® 80 4
Ww
c
=
b J
wh=4.5
“0 a an
Nn 82
L L lL l
to) 50. 100 150 200 250
STRAIN, 1073

Figure 4.—Complete stress-strain curves for indian coal
‘specimens, showing increasing residual strength with increasing
wh ratio (after Das [1986}).

become more squat, their residual strength increases. Once
the w/h ratio reaches 8-10, the specimens become "strain-
hardening,” which means that they never shed load, and
sudden collapse is impossible.

Figure 5 summarizes available postfailure modulus data for
large in situ coal specimens and full-scale coal pillars. The
dashed line indicates a conservative envelope for these limited
in situ data. In general, the laboratory postfailure moduli
exceed the large-scale test values.

The importance of the postfailure stiffness is further
explained by the theory of local mine stiffness, first proposed
by Salamon [1970] and discussed by Zipf [1992, 1996]. The
theory states that if the pillar’s postfailure modulus (Kg) is less
than the stiffness of the mine roof (the local mine stiffness, or
K,,), the failure is stable and gradual (figure 6B). If K, ex-
ceeds K,,, on the other hand, the failure is sudden and violent
(figure 6A). The local mine stiffness depends on the modulus
of the immediate roof; floor and pillar materials; and the
layout of pillars, mine openings, and barrier pillars. The post-
failure stiffness, Kp, depends on the w/h ratio of the coal pillar,
as shown in figure 5. Using a boundary-element method pro-
gram similar to the USBM's MULSIM/NL program, it is
possible to simulate both massive pillar collapses and stable,
progressive pillar failures [Zipf 1996]. The behavior of com-
puter simulations changes depending on whether the model
satisfies or violates the local mine stiffness stability criterion.

4,000 7] ooo
e
2,000F- 4
pd
s 5 BEA
roe
= -2,000F- Be . 4
ao te
3 i
5 sop 1° ° © 4
8 '
s 7°
w -6,000h- ¢ 4
2
3 e
i -g,000/- 4
=
3 Z
-10,000F- 4
20,000}, ° * ° 4
-20,000},° $
-30,000f-e 4
-40,000 f+» ® 4
-s0000Le Lu ty
° 2 4 6 8 10 12 14

WIDTH/HEIGHT RATIO
Figure 5.—Postfailure modulus of coal pillars, in situ coal
specimens, and laboratory samples. Darkened circles represent
laboratory tests, remaining symbols represent in situ tests [Chase
et al. 1994].


‘a Hep OU seywOIPUl US¥Q—3LON
TORUS TOTIN HOE AG OOO) OST AG COT Wee TZT TRATES HEATH (OOH OED )
— (008 Aq 008) os! Aq ozs (sr) eb rd - - AM
“Ain{ul | ‘eBewrep soe; (oz9't Aq 08) O6y Aq Ost (yet) ez 24 (ov Aqor)zsAqz, = (oss)ens =n
‘Ww {002 4q aoe) $12 Aq 06 oz uz (eeAqa zAqz = (osz)ez = HO
(008 Aq 022) Sz Aq ze (vou) 29 to (00e) 16 AM
ew (og6 Aq 008) 062 Aq Ovz en) we apt (ovAqor)zrAqe = (008) 16 «SAM
e272 {oe Aq of) 6 Aq6
“sBujddoys 2¢ {ows Aq ose) 091 Aq 001 28) {ozAqoz)9Aq9 = (Szz)69 AM
“TeUUIN {009 Aq ose) O81 Aq 001 oo'e {oe Aqos)6Aq6e = (Sze) 668 =
‘aBujddoys £01 {00s Aq os») ost Aq ont ea foyAqoizrAqe = (s61)09 AM
kd (ov Aq 02) 21 Aa9
“aBurddors o2 (009 Aq 009) 081 Aq 081 coe {oe Aqoe)eAqe = (08) S8. «SAM 8
“eBurddoisoy = (008 Aq age) ost Aq 001 ory (Gvz)sz AM “2a
00%
"WhO [Nem Uv, ‘sBuiddoys ze - - 001 (opz)ez = AM
{00s Aq 005) 091 Aq Os) (4) €% $0") (S22) @ AM
yriquye wo. eBeweg (w) ws ‘ezjs esdeyjoo ones u/m dS Sdwuy (u) wy ‘ez eid bithed ang Aojsyy 88D

SOU [209 Uj Sesde!/09 se))\d CASEEW—'L C1QUL


45

35

20

15
10

18 ft Wide Entries
ARNPS SF = 2.0 ee er

——Seam Ht. = 6 ft

—=—Seam Ht. = 8 ft

-e- Seam Ht. = 10 ft

45—
40+
35 +
30 +

MINIMUM SQUARE PILLAR WIDTH, ft
°

25 +
20 +
15 4
10 +
54

20 ft Wide Entries
ARMPS SF = 2.0 _ercc

+ + + +
100 200 300 400 500
DEPTH OF COVER, ft

Figure 7.—Suggested minimum square pillar size to avoid massive pillar collapse. A, 5.5-m (18-ft) entries; B, 6-m (20-ft)

entries.

43


PREVENTING MASSIVE PILLAR COLLAPSES
IN COAL MINES

By Christopher Mark, Ph.D," Frank E. Chase,” and R. Karl Zipf, Jr., Ph.D.*

35

ABSTRACT

A massive pillar collapse occurs when undersized pillars fail and rapidly shed their load to adjacent pillars,
which in turn fail. The consequences of these chain-reaction failures can be catastrophic. One effect of a
massive pillar collapse can be a powerful, destructive, and potentially hazardous airblast. Thirteen recent
massive pillar collapses have been documented in West Virginia, Ohio, Utah, and Colorado. Data collected
at the failure sites indicate that all of the massive collapses occurred where the pillar width-to-height (w/h) ratio
was 3.0 or less and where the Analysis of Retreat Mining Pillar Stability Factor was less than 1.5. The unique
structural characteristics of these pillar systems apparently result in sudden, massive pillar failures, rather than
the more common slow “squeezes.” The field data, combined with theoretical analysis, provide the basis for
two partial-extraction design approaches to control massive pillar collapses. These are the containment
approach and the prevention approach; practical examples are provided of each.

'Mining engineer, Pittsburgh Research Center, National Institute for Occupational Safety and Health, Pittsburgh, PA.
ist, Pittsburgh Research Center, National Institute for Occupational Safety and Health, Pittsburgh, PA.
Lecturer, Department of Mining and Metallurgical Engineering, University of Queensland, Brisbane, Queensland, Australia.


A Entry Width = 18; 2 unsplit rows as Barrier

9
8
7
5 + +
250 300 350 400 451

200

oa

0

B Entry Width = 18; 3 unsplit rows as Barrier

NUMBER OF PANEL ENTRIES

+ + + t
400 450 500 550 600 650 700 750 800
ALLOWABLE DEPTH OF COVER, ft
KEY
—-h = 6'; 3 Rows Split -e-h=6'; 4 Rows Split -»-h=6'; 5 Rows Split
—~-h = 8; 3 Rows Split -—h =8'; 4 Rows Split -e-h =8'; 5 Rows Split
—-—h = 10'; 3 Rows Split —h = 10"; 4 Rows Split —h = 10°; 5 Rows Split

Figure 9.—A, suggested maximum depth for two rows of unspiit pillars as barrier between gob areas, 5.5-m (18-f) entry, 18-
by 18-m (60- by 60-f) pillars. B, suggested maximum depth for three rows of unsplit pillars as barrier between gob areas, same
entry and pillar sizes.


zz iziziziziziz
Oocooooo
Oooooo0o0o
ages asa

SHHSR eS
oooououo

Zizizi|

L)N

IO OW
1 OCS

N NOON

y

N N

N

NNNNNOON

N Wt
N No

NN!

NN f

OOUCU

AZIZAZI ZAZA
ooo0cooo
nooooooo
ooooooog
Zizizizizizizie)
Baaeee ee
ae

ADOOOOoo
Oooooooo
oooooooo
Biziziziziz ii

Zizizizi2
Doooo
Ooooo0
ooooo

Zizizizlz

45

Figure 8.—Possible pillar-splitting plan for airblast control. A, nine-entry system, two rows of unsplit pillars for
barrier. B, nine-entry system, three rows of unsplit pillars for barrier. C, six-entry system, two rows of unspitt pillars
for barrier. D, six-entry system, three rows of unsplit pillars for barrier.


42

A Unstable failure
"soft" load system

LOAD

B Stable failure
"stiff" load system

CONVERGENCE

Figure 6.—tilustration of the local mine stiffness concept. A, local mine stiffness (K,) is less than postfailure stiffness of the pillar (K,),
resulting in unstable faiture. B, local mine stiffness (K,) exceeds the pillar’s postfailure stiffness (K,), resutting in slow and stable failure.

DESIGN APPROACHES TO CONTROL MASSIVE PILLAR COLLAPSE

In coal mining, small-center mining and partial pillaring
are methods to achieve high extraction without full pillar
recovery. Both leave significant remnant pillars in the mined-
out areas. For example, mining on 15-m (50-ft) centers using
6-m (20-ft) entries leaves about 35% of the coal in 9- by 9-m
(30- by 30-ft) pillars. Splitting pillars developed on 18- by
18-m (60- by 60-ft) centers leaves about 22% of the coal.
Both techniques can be adapted to avoid massive pillar col-
lapses following the strategies of prevention or containment.

In 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 w/h ratio. In
the containment approach, high extraction is practiced within
individual compartments that are separated by barriers. The
small pillars may collapse within a compartment; however,
because the compartment size is limited, the consequences are
not significant. The barriers may be true barrier pillars, or
they may be rows of development pillars that are not split on
tetreat. The containment approach has been likened to the use
of compartments on a submarine.

Full extraction can be another strategy to avoid massive
pillar collapses. Mining all of the coal removes the support to

the main roof, thereby limiting the potential width of the
pressure arch. Although some “first falls” behind longwalls
and other full-extraction systems have been destructive, they
generally involve areas smaller than massive pillar collapses.

SMALL-CENTER MINING: A PREVENTION
APPROACH

Square pillars are generally used in small-center mining.
Table 1 indicates that three collapses involved 9-m (30-ft)
square pillars, and one involved 12-m (40-ft) square pillars.
Square pillars may be designed to be collapse-resistant in two
ways. The first is to increase their w/h ratio. Because no
collapses have been documented in which the w/h ratio was
greater than 3.0, a design whh ratio of 4.0 is suggested to
provide an adequate margin of safety.

Pillar collapses may also be avoided by maintaining a
sufficiently high SF. The ARMPS case history data base
{Mark and Chase 1997] suggests that normally an ARMPS SF
of 1.5 is sufficient to limit the probability of pillar failure.
Where slender pillars are being employed and their failure
may result in a massive collapse rather than a slow squeeze, it


38

semilaminated sandstone with a CMRR of 64; the floor was
composed of an extremely firm sandstone. Coalbed cleating
was nonexistent. All roadways in the mine were 6 m (20 ft)
wide and were driven on 18-m (60-ft) centers in the relevant
area.

In 1992, the operator was splitting pillars in the panel
shown in figure 2. After the 6-m (20-ft) wide split, two 3- by
12-m (10- by 40-ft) fenders with an SF of 0.94-1.15 remained.
When the operator began to mine the pillar row outby the last
row split (figure 2), a massive collapse of the fenders in the
gobbed-out area initiated. The roof bolter operator on the
section indicated that he and his coworkers were knocked to
the floor by the resulting airblast, and 103 stoppings were
destroyed. The pillars where the collapse terminated had an
SF of 1.97. Overburden in the collapsed area ranged from 53
to 66 m (175 to 215 ft).

A subsequent pillar collapse occurred at Mine C, ap-
parently triggered by time deterioration and front abutment
pressures generated by full pillar extraction. Roadways in the
collapsed area were driven on 15-m (50-ft) centers, and 91
pillars with an SF of 1.08 failed. Pillars with an SF of 1.69
halted the collapse. These roadways were driven on 18-m
(60-ft) centers. No stoppings were damaged, and the over-
burden in the area was 99 m (325 ft).

Mine C was visited in February 1994 to observe diagonal
pillar splitting, which is not a common practice. Roadways
were driven on 15-m (50-ft) centers, and the pillar splits were
5 m (16 ft) wide. The extraction percentage was 86%. The
triangular remnant stumps were observed to routinely crush
out after finishing the pillar row, and the roof caved im-
mediately inby the breakers. The breakers and wedges

showed no weight. Where the first pillar collapse occurred in
Mine C using the traditional 6-m (20-ft) wide split through a
12- by 12-m (40- by 40-ft) pillar, 78% of the coal was
extracted. This 8% increase in resource recovery, coupled
with a less stable triangular stump with a smaller perimeter,
probably explains why the roof caves more readily than in
traditional pillar splitting.

SMALL-CENTER MINING (MINE D)

Mine D is located in Mingo County, WV, and is extracting
the 3.4-m (11-ft) thick Dorothy Coalbed. The roof consisted
of 76 cm (2.5 ft) of laminated fossiliferous shale and 7 cm (3
in) of rider coal, and 25 m (80 ft) of cross-bedded sandstone
was observed in the highwall. The roof had a CMRR of 81.
Below the noncleated coalbed was 1.5 m (5 ft) of sandy shale
and 28 m (91 ft) of sandstone. All roadways in the mine were
6 m (20 ft) wide.

In 1992, ninety-four 6- by 6-m (20- by 20-ft) pillars with
an SF of 1.15 and thirty-two 9- by 9-m (30- by 30-ft) pillars
with an SF of 1.45 failed. As shown in figure 3, the pillar
failures occurred in a panel driven off the mains. The
resultant airblast blew out 37 stoppings. The only other
stopping in the mine had a hole in it. Some of these stoppings
were as far away as 244 m (800 ft) from the perimeter of the
collapse. In one stopping, it was determined that some of its
14-kg (30-Ib) cinder blocks had been hurled 152 m (500 ft).
Fortunately, the occurrence was on an idle shift, and no one
was in the mine. The collapse was halted by pillars in the
main entries, which were 12- by 12-m (40- by 40-ft) and had
an SF of 3.33. Cover over the collapsed area was 69 m (225

Figure 2—Location of split-piltar collapse at Mine C.


v
LEGEND fe) 60m
7 oe
= | Collapsed area ° 200Ft 0 a } O O

37

oqo0ngo0og00O0L

oods00oenT,

ce

BL JOWaGdds wd

ofy/oddowa

i)
0
joOooooooo

QO
a)
QO
QO
O

ooonvogo

OOOUVG

OooooOoOvooRvU

ODOO0VU
OOUU!
OUOO{

0
OQ
QO
0
O
Og
a)
O
D
0
q
Q
Q

Figure 1.—Failed split-pillar workings In Mine A.

been rock dusting ribs immediately outby the area that
collapsed. The airblast blew out 26 cinder block stoppings
and the fan house weak wall, which closed the mine for days.

As was the case in many of the other collapses that were
studied, a number of fenders near the edge of the collapse did
not fail. There are two possible explanations for this: (1) The
collapse might terminate as soon as the competent roof units
were able to bridge the span, or (2) the collapse might termi-
nate where the fenders were shielded from the full load by the
adjacent abutment. In the second case, the 12- by 12-m (40-
by 40-ft) pillars with an SF of 2.33 may have provided a hinge
line, which allowed the roof to cantilever over the first several
rows of fenders.

An earlier collapse had occurred at Mine A in partially
pillared workings under very similar conditions. Damage was

limited to blown out stoppings, and no one was injured.
Complete documentation of this case was unavailable.

After the second collapse, the practice of pillar splitting
was reexamined at the mine. Several sets of mobile roof
supports were purchased, and retreat mining continued with
full pillar extraction. Most recently, some pillar splitting has
been conducted, with rows of unsplit pillars left as barriers to
isolate retreated areas.

PILLAR SPLITTING/ABUTMENT LOAD
OVERRIDE (MINE C)

Mine C is located in Logan County, WV, and is extracting
the 3-m (10-ft) thick Dorothy Coalbed. The immediate and
main roof throughout the mine is composed of a fine-grained,


36

INTRODUCTION

Massive pillar collapses in room-and-pillar mines have also
been labeled “cascading pillar failures,” “domino-type
failures,” or “pillar runs.” In this type of failure, when one
pillar collapses, the load that it carried transfers rapidly to its
neighbors, causing them to fail, and so forth. This failure
mechanism can lead to the rapid collapse of very large mine
areas. In mild cases, only a few tens of pillars might fail;
however, in extreme cases, hundreds, even thousands, of
pillars can collapse.

Massive pillar collapses can have catastrophic effects ona
mine. Sometimes these effects pose a greater safety risk than
the underlying ground control problem. Usually, the collapse
induces a devastating airblast due to the displacement of air
from the collapsed area. An airblast can totally disrupt the
ventilation system at a mine by destroying ventilation
stoppings, seals, and fan housings. Flying debris can seriously

injure or kill mining personnel. The collapse might also
fracture a large volume of rock in the pillars and immediate
roof and floor. In coal and other gassy mines, this frag-
mentation can lead to the sudden release of large quantities of
methane gas into the mine atmosphere, creating an explosion
hazard. Finally, a massive pillar collapse can release
significant seismic energy that may be experienced on the
surface as a small earthquake.

Fortunately, not all pillar failures are sudden, massive
collapses. Most are slow “squeezes” that develop over days to
weeks, and because of their slow progress, do not pose as
great a danger to mining personnel. A central goal of the
research described in this paper was to identify the physical
characteristics that distinguish sudden collapses from other
pillar failures.

CASE HISTORIES

The most infamous massive pillar collapse in history
occurred in 1960 at Coalbrook North Colliery in South Africa.
Thousands of 12- by 12- by 4.2-m (40- by 40- by 14-ft) pillars
collapsed over a 305-ha (750-acre) area in 5 min, killing 437
miners {Bryan et al. 1966]. Numerous other, smaller collapses
have been reported in South Africa since then [Madden 1991].
In Australia, the New South Wales Joint Coal Board reported
eight massive pillar collapses between 1990 and 1993
[University of New South Wales School of Mines 1994].

Massive collapses have also occurred in metal and
nonmetal mines. Zipf and Mark [1996] documented six
examples from lead-zinc, copper, silica, and salt mines. The
largest occurred at a Wyoming trona mine in 1995, where
160 ha (400 acres) of 4- by 29- by 6-m (13- by 95- by 19-ft)
fenders collapsed, resulting in a Richter magnitude 5.3
earthquake and one fatality underground [Ferriter et al. 1996].
The ventilation system at the mine was heavily damaged, and
an estimated 1 million m? (30 million ft’) of methane was
liberated on the day of the collapse. Methane release levels
did not return to normal until 3 months later [Ferriter et al.
1996].

In 1992, the former U.S. Bureau of Mines (USBM) was
asked to investigate a massive pillar collapse and resultant
destructive airblast that had occurred in a coal mine in Mingo
County, WV. Subsequent investigations found 12 other
examples, which were documented by field investigations
[Chase et al. 1994]. Geotechnical evaluations examined the
competency of the immediate roof, as well as that of the main
roof and its susceptibility to caving. The Analysis of Retreat
Mining Pillar Stability (ARMPS) program [Mark and Chase

1997] was used to determine the pillar stability factors (SF).
Four examples that illustrate different mining methods and
effects are described in detail below.

PILLAR SPLITTING (MINE A)

Mine A is located in Mingo County, WV, and is extracting
the 2.9-m (9.5-ft) thick Coalburg Coalbed. A 28-m (90-ft)
thick massive sandstone unit with a compressive strength of
83 MPa (12,000 psi) formed the roof above the collapsed area.
The Coal Mine Roof Rating (CMRR) of the immediate roof
was calculated to be 74. Below the noncleated coalbed is
10.5 m (34 ft) of competent sandy shale and sandstone units.
All roadways were 6 m (20 ft) wide.

In 1991, the panel shown in figure 1 was developed. All
roadways were driven on 18-m (60-ft) centers and were under
85 m (275 ft) of cover. After the panel was completed, partial
pillar recovery was begun. A 6-m (20-ft) wide split was
mined through the middle of each pillar, and two 3- by 12-m
(10- by 40-ft) fenders with an ARMPS SF of 0.75 remained.
Because of the competency of the roof and the support
provided by the regularly spaced uniform fenders, no caving
occurred while the panel was being retreat mined. Three
weeks after the panel had been abandoned, an area measuring
approximately 140 by 155 m (450 by 500 ft) containing 107
fenders collapsed. Miners on a nearby section were knocked
to the floor by the resultant airblast. One miner was bounced
off of a steel rail and required 26 stitches to his head.
Fortunately, no miners were near the collapse. However, if
the failure had occurred 15 min later, two miners would have


100
90
KEY
80 ——6' Seam
70 -=8' Seam
60 10° Seam

REQUIRED BARRIER PILLAR WIDTH, ft
8

Required Barrier Pillar Width vs Depth of Cover

47

40
30
20
10
0 —- + + —t 4
0 100 200 300 400 500

DEPTH OF COVER, ft
Figure 10.—Suggested solid coal barrier width between two areas where pillars have been split.

CONCLUSIONS

The potential for massive pillar collapses should always be
considered when designing room-and-pillar mining opera-
tions. A collapse can occur when one pillar fails suddenly,
overstresses its neighbors, causing them to fail, and so forth,
in very rapid succession. Very large mining areas can collapse
via this mechanism within seconds with little or no warning.
The collapse itself can pose serious danger to nearby mining
personnel. Additionally, the collapse can induce a violent
airblast that disrupts or destroys the ventilation system.
Further critical danger to miners exists if the mine atmosphere
becomes explosive or contaminated as a result of the pillar
collapse.

Research has found that massive collapses in coal mines
have the following common characteristics:

Slender pillars (w/h ratio less than 3.0).
Low SF (less than 1.5).
Competent roof strata.
Collapsed area greater than 1.6 ha (4 acres).
* Minimum dimension of the collapsed areas greater than
110 m (350 ft).

Two alternative strategies may be successful in preventing
massive pillar collapses. For small-center mining, prevention
may be applied by increasing either the wh ratio or the SF.
Containment is appropriate for pillar splitting and requires
leaving barriers or rows of unsplit pillars to limit the area of
potential collapses. A final strategy is to go to full pillar
extraction. By removing the support provided by the remnant
fenders left during traditional pillar splitting, the bridging
capacity of the roof should be substantially reduced.

Finally, it is important to note that the massive pillar col-
lapses discussed in this paper are not to be confused with coal
bumps or rock bursts. Although the outcomes may appear
similar, the underlying mechanics are entirely different.
Bumps are sudden, violent failures that occur near coal mine
entries and expel large amounts of coal and rock into the
excavation [Maleki 1995]. They occur at great depth, affect
pillars (and longwall panels) with large w/h ratios, and are
often associated with mining-induced seismicity. The design
recommendations discussed here for massive pillar collapses
do not apply to coal bump control.