(Total = 82)

27 (33%)

16 (20%)

39 (47%)

10 (12%)

17 (21%)

Other US

Other 

WV

Pillaring 

Fatals

WV 

Pillaring

Other US 

Pillaring

Figure 1. Ground Fall Fatalities in the U.S., 1992-2001.

EVALUATION OF PILLAR RECOVERY IN SOUTHERN WEST VIRGINIA

Christopher Mark, Chief, Rock Mechanics Section

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

George Karabin, Supervisory Civil Engineer

Joseph C. Zelanko, Mining Engineer

M. Terry Hoch, Chief, Roof Control

Mine Safety and Health Administration

Pittsburgh Safety and Health Technology Center

Pittsburgh, Pennsylvania USA

Frank Chase, Research Geologist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

Pillar recovery continues to be a significant ground control

hazard. During the past decade, 10 miners were killed during pillar

extraction operations in southern West Virginia. Studies conducted

during the past decade have identified a number of “risk factors” that

can be used to evaluate pillar extraction plans:

• Cut sequence

• Final stump

• Timber or Mobile Roof Supports 

• Roof bolting

• Intersection span

• Depth of cover

• Roof quality

• Age of workings

For each of these factors, rock mechanics science suggests which

alternative would be expected to be more risky. For example,

numerical models were used to evaluate different cut sequences, and

indicated that less roof convergence occurred with the outside lift

method than with the Christmas tree in the particular environment

simulated. For many of the risk factors, accident statistics confirm

the science. One finding was that currently almost 70% of the retreat

coal in southern WV is being mined with MRS. In contrast, timber

supports were used in 70% of the past decade’s pillaring fatalities.

This paper discusses each of the risk factors in turn, presents the

relevant accident statistics, and shows how the risk factors can be

combined to estimate the overall hazard. It also addresses the use of

pillar design to minimize the risk of global stability hazards including

squeezes, massive collapses, and bumps. 

ACKNOWLEDGMENTS

The authors would like to thank the Roof Control Specialists of

MSHA District 4 for providing information on the current status of

pillar recovery in southern West Virginia, and Mr. Deno Pappas of

the NIOSH, Pittsburgh Research Laboratory for his invaluable data

analysis.

INTRODUCTION

During the year 2001, nine roof fall fatalities occurred in the U.S.

Of the nine, three occurred during pillar recovery operations.

Unfortunately, 2001 was not an unusual year. A NIOSH report

issued in 1997 found that pillar recovery accounted for about 10% of

all U.S. underground coal production, but was associated with about

25% of the roof and rib fatalities between 1989-96 (1). Figure 1

shows during the decade 1992-2001, there were a total of 82 roof fall

fatalities in U.S. coal mines. Of these, 27 (33%) occurred during

pillar recovery operations1

. Six of the incidents (all outside West

Virginia) resulted in double fatalities. 

This paper focuses on southern West Virginia, the coalfield with

the greatest number of pillar recovery operations. Southern West

Virginia was also chosen because the most complete set of data was

available there. Lessons drawn from the southern West Virginia

experience should be helpful to pillar recovery operations throughout

the U.S. A nationwide study of pillar recovery is currently underway.

1

These statistics actually underestimate the number of deaths

associated with pillar recovery. In two instances, one in Utah and one

in West Virginia, miners were killed by shuttle cars as they attempted

to flee premature roof collapses. Both fatalities were classified as

“machinery” accidents.
During the past several years, the regulatory agencies and many

mine operators in southern West Virginia have been very pro-active

in implementing new safety technology to reduce the roof fall risk

during pillar recovery. For example, the greatest concentration of

Mobile Roof Supports in the U.S. is in southern West Virginia.

However, the purpose of this paper is not to highlight any specific

innovation or regulatory action, or to make comparisons between

mining regions. Rather, it focuses on the technical ground control

aspects of pillar recovery.

Pillar recovery is a complex process, and a wide variety of mining

techniques are used to accomplish it. It seems evident that certain

pillar recovery techniques, or certain aspects of the pillar recovery

process, may be riskier than others. The goal of this paper is to

isolate the most significant hazards associated with pillar recovery, so

that the overall level of risk can be reduced.

PILLAR RECOVERY DEMOGRAPHICS AND

ACCIDENT RATES

As part of this study, MSHA Roof Control Specialists from

District 4 in Southern West Virginia were asked to provide

information on pillar recovery practices in each of the mines they

inspected. The data included whether the mine extracted pillars, what

pillar recovery method they most commonly employed, whether the

pushout was recovered, and whether the mine used Mobile Roof

Supports.

The information was then linked with the MSHA accident and

employment database (2) for the year 2000 (table 1). In all,

information was available on mines that produced 47.6 million tons,

or 83% of the total underground production in District 4 during 2000.

Room-and-pillar mines that practiced pillar recovery accounted for

60% of the 47.6 million tons (three longwall mines in District 4 also

engage in pillar recovery). Assuming that pillar recovery typically

accounts for about one-third of the production at these room-andpillar mines, then about 20% of the District 4 underground

production comes from pillar recovery. 

Table 1. Demographics of pillar recovery in southern West

Virginia in 2000

Mine Grouping

Summed

hours

(thousands)

Summed

tons

(millions)

Tons/hr

Ground fall

injuries/

200 Khrs

Longwall Mines 2,940 20.16 13.88 1.50

Room-and-Pillar, 

 Non-Retreat 1,961 8.06 4.11 1.84

Room-and-Pillar, 

 Retreat 5,710 34.09 5.97 2.35

ALL MINES 12,512 66.11 5.28 2.14

R&P Retreat, With 

 MRS 3,853 21.13 19.09 2.91

R&P Retreat, 

 Without MRS 1,858 12.96 13.45 2.40

The 1997 NIOSH report found that nationwide, the roof fall risk

to miners during pillar recovery was about 3 times that of miners on

development. Since 1991, 38% (10/26) of all fatal underground

accidents in District 4 were the result of unplanned roof falls during

retreat mining. With 38% of the fatalities associated with about 20%

of the production, mathematically a coal miner on a pillar recovery

section in Southern West Virginia was 2.5 times more likely to be

fatally injured in a roof fall than a miner on an advancing section.

The same NIOSH report found that the roof/rib non-fatal injury

rate was generally lower in pillar recovery mines than in other roomand-pillar mines. In District 4, for example, the retreat mine rate was

1.37 while the non-retreat rate was 1.64 (1). In 2000, however, the

retreat mine roof/rib injury rate was 2.75, while the rate at other roomand-pillar mines was 1.84.

ROCK MECHANICS OF COAL PILLAR RECOVERY

When pillars are first developed, they must carry the entire weight

of the overburden. If they are adequately sized, a new equilibrium is

established, and ground control consists primarily of securing the

immediate roof above the entry. 

Pillar removal disturbs the equilibrium and creates an inherently

unstable situation. Man-made supports cannot carry the full weight

of the overburden. The roof is subjected to new stresses and

deformations. The ground will cave in, the only question is when.

Ground control means keeping the roof up until the miners have

completed their work and left the area.

Prior to 1988, 30 CFR 75 drew a distinction between “full” and

“partial” pillar recovery. Full pillaring was defined as extraction that

allows total caving of the main roof, while partial pillaring left

sufficient coal in place to support the main roof. However, many

pillar plans fall between these two extremes, and the distinction can

be blurred and confusing. This language was removed when the roof

control regulations were revised and implemented in 1988.

A different approach is proposed in this paper. The goal is to

achieve a safe and stable working area for the miners involved in

pillar extraction, whatever pillar recovery method is used. This is

achieved by minimizing the “risk factors” associated with different

elements of the pillar recovery process. Risk factors are divided in

two main groups:

• Global Stability: Prevention of section-wide pillar failure.

• Local Stability: Prevention of roof falls in the working area.

GLOBAL STABILITY RISK FACTORS

Proper pillar design is the key to ensuring global stability. There

are three main types of pillar failure, each of which requires its own

approach.

Pillar Squeezes

Squeezes occur when the pillars are too small to carry the loads

applied to them. As the loads are gradually transferred, the adjacent

pillars in turn fail. The results can include closure of the entries,

severe rib spalling, floor heave, and roof failure. The process may

take hours or days, and can cause an entire panel to be abandoned.

The Analysis of Retreat Mining Pillar Stability (ARMPS)

program can be used to help size pillars to carry both development

and abutment loads (3). ARMPS has been calibrated by back-analysis
0

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000

Depth of Cover (ft)

ARMPS SF

Satisfactory

Unsatisfactory

Figure 2. Suggested ARMPS Stability Factors, based on an

expanded case history data base (3).

Figure 3. Common cut sequences used in the eastern U.S. 

Top: Christmas tree. Middle: Outside lift. Bottom: Three cut.

of hundreds of pillar recovery case histories. The database has

recently been expanded to include more deep-cover cases, and new

design guidelines have been proposed (see figure 2 (3)).

Massive Collapses

Massive collapses are pillar failures that take place rapidly and

involve large areas. One effect can be a powerful, destructive

airblast. Of fourteen massive collapses that have been documented

since 1980, all but two have occurred in southern West Virginia.

They have caused several injuries but, miraculously, no fatalities.

Data collected at the failure sites indicate that all the massive

collapses have occurred where the pillar width-to-height (w/h) ratio

was 3.0 or less, and the ARMPS SF was less than 1.5. Such

conditions occur most often in worked-out areas where pillars have

been split. Guidelines for preventing or containing massive collapses

have been published (4). These guidelines have been largely

implemented in southern West Virginia since 1998, and no

documented massive collapses have occurred since then.

Pillar Bumps

Bumps are sudden outbursts of coal and rock that occur when

stresses in a coal pillar cause it to rupture without warning, sending

coal and rock flying with explosive force. Of the 172 bumps included

in the NIOSH coal bump database that extends back to 1950, 54

(31%) occurred in southern West Virginia. The most recent incident

there was a double fatality in the Beckley seam nearly 10 years ago.

Pillar retreat mining or barrier splitting accounted for 50% of the

bumps in the nationwide database. Nearly 95% of the bumps

occurred at depths greater than 1,000 ft (5).

Research has shown that bumps are much less likely when barrier

pillars isolate each new panel from the abutment loads transferred

from nearby gob areas. At depths of greater than 1,000 ft, Chase et

al. (3) suggest that properly designed barriers can enhance pillar line

stability. Special extraction techniques, such as the thin pillar

method, can also be helpful.

LOCAL STABILITY: PRIMARY RISK FACTORS

Global stability is a necessary, but not sufficient, condition for

creating a safe working area. Local stability depends on a number of

factors, of which the following four are most critical.

Cut Sequence

By far the most popular methods of pillar recovery used today are

those that require no additional roof bolting during retreat. A study

of representative roof control plans from mines nationwide found that

50% used some form of Christmas tree sequence, 42% used outside

lifts, and 23% used some type of three-cut plan (figure 3). In contrast,

split-and-fender and pocket-and-wing plans were in place at only 19%

and 8% of the mines, respectively (7). Note the total percentage is

greater than 100% because many mines use more than one sequence.

The information provided by the MSHA Roof Control Specialists

indicates that the Christmas tree method is nearly universal for highextraction pillar recovery in southern West Virginia. With the

Christmas tree method accounting for at least 90% of high extraction

pillar recovery, it is not statistically significant that 9 of the 10 pillar

mining fatalities between 1992-2001 occurred on Christmas tree

sections. In 7 of these incidents, the miners were in compliance with

the approved roof control plan when the fatality occurred.

Outside of West Virginia, just 2 of the 11 fatal incidents occurred

where a full Christmas tree plan was being used. In several other

incidents, however, the mines employed 3-cut plans with lifts taken

left and right from the same entry. There was only one fatality where

an outside lift plan was used. Data on the prevalence of the different

pillaring methods outside of southern West Virginia is not yet

available, so the significance of these numbers cannot be determined.
(a) Full Model - 540’ x 810’, 400’ depth

(b) Christmas Tree Sequence - 21’ entries

57’ x 90’ centers

(c) Split & Wing Sequence - 21’ entries

75’ x 90’ centers

(d) Pocket & Fender Sequence - 21’ entries

57’ x 90’ centers

(e) Outside Lift Sequence - 21’ entries

57’ x 90’ centers

1 2

4

6

8

3

5

7

9 10

5 1

6 2

7

8

3

4

3 2

6 5

1

4

2

3

4

5

1

(a) Full Model - 540’ x 810’, 400’ depth

(b) Christmas Tree Sequence - 21’ entries

57’ x 90’ centers

(c) Split & Wing Sequence - 21’ entries

75’ x 90’ centers

(d) Pocket & Fender Sequence - 21’ entries

57’ x 90’ centers

(e) Outside Lift Sequence - 21’ entries

57’ x 90’ centers

1 2

4

6

8

3

5

7

9 10

1 2

4

6

8

3

5

7

9 10

5 1

6 2

7

8

3

4

5 1

6 2

7

8

3

4

3 2

6 5

1

4

3 2

6 5

1

4

2

3

4

5

1

2

3

4

5

1

Figure 4. Computer model of retreat cut sequences.

From a rock mechanics standpoint, it makes sense to compare the

Christmas tree to the outside lift method. Christmas tree plans are

characterized by cuts being taken from both the left and the right

pillars, while with outside lifts only one pillar is mined at a time2

.

Comparing just these two methods, the Christmas tree plan would be

expected to be more risky than outside lifts because:

• Wider unsupported spans are mined;

• More time is spent at the same location (to complete both the

right and left lifts), and;

• The operator of the remote controlled continuous miner (CM)

may stand in a non-optimum location for either the left or the

right lifts (see section below on “Operator Positioning”).

The basic advantage of the outside lift plan is that operator always

has a solid pillar at his back. It also has some disadvantages,

however:

• It can’t be used in wide pillars without leaving large remnant

fenders of coal (and wide pillars may be required to meet

global stability requirements in thick seams or under deep

cover), and;

• It usually employs deeper cuts, making the CM more difficult

to extract if it is trapped while extracting a lift by a roof fall

or rib roll.

To provide some further insight into the influence of the cut

sequence on ground stability, the boundary element numerical model

(BESOL) was used to compare four common pillar recovery plans in

an identical mining environment (a 400-ft depth of cover and a 5-ft

seam height). The mining methods evaluated were the Christmas

Tree, Split and Wing, Pocket and Fender and Outside Lift. The

particular pillar/opening geometries, cut sequences and timber

supports (placed during each cut) used in each model were based on

actual plans used by mines in southern West Virginia. Figure 4

depicts the general model geometry and the cut sequences used to

simulate each of the pillaring plans.

Figure 5 shows convergence contours for each of the four mining

methods after roughly one-third of the coal has been extracted. The

0.2-foot convergence level has been highlighted for reference

purposes. The convergence data generated represents gross

movement of the main roof/floor and higher levels would be

indicative of increased potential for a roof fall.

• Split & Wing – Because of the substantial yielding of the

narrow fenders, the 0.2-foot convergence contour engulfs the

entire split and extends well into the intersection outby where

the lifts are being taken.

• Pocket & Fender – The 0.2-foot contour level engulfs the

entire work area and extends down the entry to a point just

short of the intersection.

• Christmas Tree – The 0.2-foot convergence contour extends

outby the last cut into the work area of the next cut.

• Outside Lift – The 0.2-foot level remains within the last cut

taken.

2

The three-cut plans that are popular in some coalfields come in many

varieties. Some resemble the outside lift, others employ left and right cuts like

the Christmas tree. The percentage of coal recovered also varies widely,

depending on the initial size of the pillar and the width and depth of the cuts.

Because of the wide variety, the term “three cut” is not very descriptive, and

specific plans must be evaluated on an individual basis.
Xmas Tree Xmas Tree

Pocket & Fender Pocket & Fender

Split & Wing Split & Wing

Outside Lift Outside Lift Outside Lift

Convergence (ft)

Xmas Tree Xmas Tree

Pocket & Fender Pocket & Fender

Split & Wing Split & Wing

Outside Lift Outside Lift Outside Lift

Convergence (ft)

Figure 5. Roof convergence contours after several cuts. The 0.2 ft cut contour is highlighted in white.

In this particular scenario, the outside lift method appeared most

likely to result in stable ground conditions. In general, the models

indicate that high stress develops in the fender(s) being mined, that

properly sized fenders withstand the stresses developed, and that

undersized fenders yield prematurely - allowing gob pressures to

override them and cause elevated convergence in the work area.

Final Stump or Pushout

The final stump is a critical element in roof control during pillar

recovery. While in place, it helps protect the active intersection,

which is generally the weakest link because of its wide span. Once

the stump is removed, or is made too small to provide support, the

intersection may become unstable, like a chair with one leg removed.

Between 1992 and 2001, 6 of the 21 nationwide pillar recovery

fatalities occurred during extraction of the pushout or last lift. All but

one of these incidents occurred in West Virginia. In other words,

50% of the West Virginia fatals involved the last lift. Since the final

lift accounts for far less than 50% of the total time required to recover

a pillar, this is clearly a very high-risk activity.

Traditionally, miners have been reluctant to leave the final stump

because they were concerned that stumps in the gob would inhibit

caving and cause a squeeze. Recent experience seems to indicate that

fears about leaving stumps might have been exaggerated. In most

cases, it appears that the optimum pillar extraction plan may be one

that purposely leaves a final stump sized to provide roof support

without inhibiting caving. Guidelines for sizing the final stump were

recently published (6), and are summarized in table 2.

Table 2. Guidelines for sizing the final stump (7)

Seam Height (ft) Stump size (ft)*

4 8.5

6 9.5

8 10

12 10.5

*Cut-to-corner distance (see figure 6).

For a stump to perform its function, it must not be cut any smaller

than needed. Plans that specify a set number of lifts can result in

undersized stumps if actual pillar dimensions are smaller than

expected. A better practice is to specify the cut-to-corner distance

(figure 6). Foremen can use spray paint to mark the stump

dimensions on the rib as a guide to the CM operator.

Using the outside lift sequence for illustration, the numerical

model shows what can happen as mining approaches the last lift.

While maximum intersection convergence occurs when the pushout

is mined (all coal support is removed) a critical increase in

intersection convergence, with a corresponding potential for roof

failure, can occur earlier in the sequence if the outby end of the pillar

totally yields. Figure 7 shows that once cut 4 is taken, the final stump

yields and the 0.2-ft convergence level suddenly propagates well into

the intersection. Subsequent mining of the pushout (cut 5) would take

place beneath roof that might already be unstable. Under the right

circumstances, this condition could be expected with any pillar

recovery cut sequence.
Figure 6. Cut-to-corner distances for the final stump (6).

Convergence (ft)

Cut 3 Cut 3

Cut 5 Cut 5

Cut 4 Cut 4

Convergence (ft)

Cut 3 Cut 3

Cut 5 Cut 5

Cut 4 Cut 4

Figure 7. Roof convergence as mining approaches the final stump.

The 0.2 ft cut contour is highlighted in white.

Mobile Roof Supports vs. Timbers

Traditionally, timber posts provided supplemental support for

pillar recovery. More than 100 roadway, turn, and breaker posts can

be required to extract a single pillar. As supports, timber posts have

a number of disadvantages:

• Setting them exposes miners to roof falls. During the past

decade, two miners have been killed while setting posts, both

of them in West Virginia;

• They have limited load-bearing capacity. A typical 6-inch

diameter hardwood post can carry about 50 tons, but most

actual posts have flaws and are even weaker;

• They have limited convergence range. Wood posts can break

after only 1-2 inches of roof-to-floor convergence, and their

post-failure strength is almost nil, and;

• Their weight and bulk result in material handling injuries,

particularly in high coal.

For all of these reasons, both MSHA and NIOSH have advocated

the use of Mobile Roof Supports (MRS) for pillar recovery. MRS are

shield-type support units mounted on crawler tracks (figure 8). They

were first employed in West Virginia in 1988, and more than 100

units were in use in the U.S. by 1997 (7). The advantages of MRS are

that they:

• Are operated remotely, at some distance from the pillar line;

• Have a support capacity of 600 of 800 tons per unit, and are

employed in pairs or sets of four;

• Can maintain their load even if the roof moves downward

more than a foot, and;
Figure 8. A Mobile Roof Support (MRS).

0 5 10 15 20 25 30

29-36

37-44

45-52

53-60

61-68

69-76

77-100

101-120

SEAM HT. RANGE (IN)

NUMBER OF MINES

YES MRS

NO MRS

Figure 9. Distribution of MRS by seam height in

southern West Virginia.

• Eliminate most material handling.

Two disadvantages are their cost and the resulting necessity to

recover them if they are trapped by a rock fall.

The statistics now seem to justify the enthusiasm for MRS. In the

past 10 years, three of the 10 West Virginia pillar recovery fatalities

occurred where MRS were being used3

. Our study found that in

2001, MRS mines accounted for 69% of all the manhours in southern

West Virginia retreat room-and-pillar mines. Extrapolating

backward, a conservative assumption is that between 45 and 50% of

the pillar recovery manhours since 1992 were on MRS sections.

Using these data, it appears that a miner on a timber section has been

about two times as likely to be fatally injured than a miner protected

by MRS.

Using MRS can be a highly effective means of reducing the risk

of pillar recovery. However, they must be employed properly. The

pillaring plan should show the proper location for MRS during each

lift, and the plan should be carefully followed. If the pushout is

recovered, four MRS should be used, and at least two of them should

be located directly in the intersection. MRS should always be moved

in pairs, one canopy length at a time, so that they can support each

other. 

One disadvantage of MRS is that their operating range is limited

to seams thicker than approximately 42 inches. Figure 9 shows that

in southern West Virginia, the vast majority of mines in seams thicker

than 52 inches already use MRS. But of the 54 mines who reported

a seam height of 52 inches or less, only 7 were using MRS. In these

thin seam mines, a timber plan that requires an adequate number of

posts installed at the proper times and in the proper locations is

essential.

Roof Bolting

Longwall mine operators recognize that headgate and tailgate

entries will subjected to abutment loads during retreat mining, and

will therefore require extra roof bolting. Unfortunately, pillar

recovery sections have sometimes been considered “short term,” and

therefore candidates for a lower density of roof support. In fact,

increasing the roof bolt support in many cases can be the simplest

way to reduce the risk of roof falls during pillar recovery.

The failure of roof bolt systems has been a factor in a number of

recent pillaring roof fall fatalities, including:

• Broken roof bolts, sheared by roof movement, were found in

three incidents (two of them in WV);

• Missing heads and plates, cut off by the CM, were found in

two incidents (one in WV), and;

• Bolts were too short and missed their normal anchorage in

sandstone when the underlying shale thickened in one WV

incident.

There is no widely accepted method for designing roof bolt

patterns for retreat mining, though the Analysis of Roof Bolt Systems

(ARBS) method can be a good starting point (8). In general,

depending on the roof strata and other factors, the effectiveness of

roof bolt systems for pillaring can be improved by using:

• Longer bolts that build a thicker beam or anchor in better

quality roof;

• Stronger bolts, using larger diameter rod or higher grade

steel, that are less likely to break from rock movement, 

• Extra intersection support such as cable bolts, and/or;

• Point anchor resin-assisted bolts that can provide warning of

high loads (while fully grouted bolts may break along their

lengths without warning).

Another advantage of supplemental roof bolt support for pillar

recovery is that bolts can be installed well outby the pillar line, before

the ground is affected by the high stress environment.

OTHER RISK FACTORS

Roof Geology

Weak rocks like shale, mudstone, and coal, are more likely to be

fractured and damaged by abutment stresses on the pillar line. They

are also more likely to contain slips, slickensides, horsebacks and

other discontinuities that have contributed to many pillar line

fatalities.

Weak roof normally requires a higher level of roof bolting.

Leaving a final stump for roof support is also more critical where the

roof is weak. Every effort should be made to identify major

3 discontinuities before mining and apply supplemental support. It may The MRS were only implicated in the fatality in one of these instances. In

the other two cases, broken roof bolts were considered the primary cause.
Figure 10. Pillar point created by mining

with continuous haulage.

be necessary in some cases to avoid pillaring certain areas where

hazardous roof features are known or suspected.

Intersection Span

Intersections are the Achilles heel of coal mine ground control.

Research has shown that an intersection is 8-10 times more likely to

collapse than an equivalent length of entry or crosscut. Even a

seemingly small increase in the intersection span can greatly reduce

stability, because the rock load is proportional to the cube of the span

(10). Intersection hazards are most acute where the roof is weak. 

Maintaining stable intersections is essential to safe pillar

recovery. This can be accomplished by:

• Minimizing the entry width;

• Reducing the number and depth of turnouts;

• Using longer and/or stronger bolts in the intersections;

• Leaving an adequate final stump, and;

• Installing extra standing support (MRS or roadway posts) in

the intersection if the final stump is extracted.

Depth of Cover

Greater depth means higher stress, both vertical and horizontal.

During the past decade, approximately 30% of the pillar recovery

fatalities have occurred in the relatively small number of mines where

the depth of cover exceeds 750 ft. It seems that because global

stability is harder to achieve at depth, the roof is more likely to be

unstable. Proper pillar design is critical to successful mining at deep

cover, but deep cover also magnifies the importance of all the other

risk factors.

Multiple Seam Interactions

Many U.S. coal reserves occur where previous mining has been

conducted above or below. The problem is particularly acute in the

Central Appalachian coalfields, which includes southern West

Virginia. Localized high stress zones can occur either above or

below old works, and subsidence can damage the roof hundreds of

feet above abandoned gob areas. In recent years, at least two pillar

line fatalities appear to have been influenced by multiple seam

interactions. Zones of potential interactions should be carefully

mapped in the planning stage, and pillar recovery should be avoided

where severe interactions are anticipated.

Recovery of Older Pillars

In many mines, pillars in old workings constitute substantial coal

reserves. Such pillars can present an attractive target for extraction.

Unfortunately, in many cases those pillars were not designed with

pillar recovery in mind. Their dimensions may be inappropriate or

irregular, and entry and intersection spans may be too wide. Most

importantly, the roof bolting may be inadequate, and the roof rock

may have degraded over time. Supplemental bolting is often

required, particularly in intersections, to prepare old works for pillar

recovery.

Non-Uniform Pillar Dimensions

Pillar recovery is safest when a routine can be developed and

strictly followed. Developing panels with uniformly sized pillars,

which facilitates a controlled and orderly extraction procedure, is

strongly recommended. Where pillars are different sizes, whether by

design or because of poor mining practice, “improvisation” is often

necessary. In such cases, plans that call for a fixed number of lifts can

result in a final stump that is too small. Requiring specific minimum

cut-to-corner distances can help ensure that a properly sized final

stump is left in place.

Odd-sized pillars can also result in oversized intersection spans.

Pre-mining surveys should be completed to identify such hazards, and

resupport may be necessary.

Continuous Haulage

Continuous haulage systems can result in improved productivity,

particularly in thin seam operations. Unfortunately, they have several

disadvantages for pillar recovery. In normal operations, the haulage

system works out of the center entry intersection. The pillars must be

retreated from both sides towards the middle, resulting in a pillar

point (figure 10). Also, the center entry is often mined wider to

accommodate the equipment, and the center entry intersections are

particularly vulnerable to roof falls. Finally, the haulage system is

more difficult to withdraw quickly if a hazard develops.

One partial solution was developed by a West Virginia mine after

a fatality. An extra bridge was added to the haulage system, which

then allowed it to be worked from the outby intersection. Then the

entire row of pillars could be worked from right to left, eliminating

the pillar point. It is also helpful to flatten the angles out as much as

possible.

Operator Positioning

In more than half of the recent pillar recovery fatalities in West

Virginia the victim was the CM operator or helper. According to

MSHA’s Program Policy Manual, “Investigation of a few of these

[fatal roof fall accidents that occurred during pillar recovery

operations] revealed that miners were occupying work locations inby

the mining machine while coal was being mined or loaded. This

practice should be discouraged, recognizing that recently mined coal

pillars reduce the amount of support in these areas.” With regard to

30 CFR 75.221, Roof Control Plan Information, the Policy Manual

states that “work procedures and location of miners while coal is

being mined or loaded should be incorporated into the roof control
plan as part of the description of the mining system utilized during

pillar recovery.”

The pillar line is a dangerous place, and miners should never

congregate there. Ideally, the operators should be outby the threeway intersection created by the lift at all times. Training and

retraining may be necessary to prevent bad habits from developing.

CONCLUSIONS

Pillar recovery continues to be one of the most hazardous

activities in underground mining. Global stability, achieved through

proper pillar design, is a necessary prerequisite for safe pillar

recovery. Local stability means preventing roof falls in the working

area. It is achieved by minimizing the “risk factors” described in this

paper. 

The Risk Factor Checklist (Appendix) can be used to identify

potential problem issues for specific pillar plans. The more questions

on it that can be answered with a “yes,” the less risky the plan is

likely to be. The checklist does not weight the individual risk factors,

nor is it necessarily a comprehensive list. It is simply a tool to help

mine planners evaluate the overall level of risk, and possible ways to

reduce the risk.

The Roof Control Plan is essential to every underground coal

mine, but nowhere is it more important than in pillar recovery.

Pillaring leaves little tolerance for error, and mistakes can be deadly.

Roof Control Plans must be carefully drawn up to address the sitespecific conditions, and then carefully implemented and followed.

Both miners and foreman involved in pillar extraction should be

trained to know and understand the plan.

REFERENCES

1. Mark C., McCall, F.E. and Pappas, D,M. A Statistical Overview

of Retreat Mining of Coal Pillars in the United States.

Proceedings, New Technology for Coal Mine Ground Control in

Retreat Mining, NIOSH IC 9446, 1997, pp. 2-16.

2. MSHA. Quarterly employment and Coal Production:

Accident/Injuries/Illnesses Reported to MSHA Under 30CFR

Part 50. Denver, CO, U.S. Dept. of Labor, MSHA, Office of

Injury and Employment Information, 2000.

3. Mark, C. and Chase, F.C. Analysis of Retreat Mining Pillar

Stability (ARMPS). Proceedings, New Technology for Coal

Mine Ground Control in Retreat Mining, NIOSH IC 9446, 1997,

pp. 17-34.

4. Chase, F.C., Mark, C. and Heasley, K.A. Deep Cover Pillar

Extraction in the U.S. Proceedings, 21st International Conference

on Ground Control in Mining, Morgantown, WV, 2002, in press.

5. Mark, C, Chase, F.C. and Zipf, R.K. Preventing Massive Pillar

Collapses in Coal Mines. Proceedings, New Technology for

Coal Mine Ground Control in Retreat Mining, NIOSH IC 9446,

1997, pp. 35-48.

6. Iannacchione, A.T. and Zelanko, J.C. Occurrence and

Remediation of Coal Mine Bumps: A Historical Review.

Proceedings, Mechanics and Mitigation of Violent Failures in

Coal and Hard Rock Mines, USBM SP 01-95,1995, pp. 27-68.

7. Mark, C. and Zelanko,J.C. Sizing of Final Stumps for Safer Pillar

Extraction. Proceedings, 20th Intl. Conf. On Ground Control in

Mining, Morgantown, WV, 2001, pp. 59-66.

8. Chase, F.C., McComas, A., Mark, C. and Goble, C.D. Retreat

Mining with Mobile Roof Supports. Proceedings, New

Technology for Coal Mine Ground Control in Retreat Mining,

NIOSH IC 9446, 1997, pp. 74-88.

9. Mark, C. Analysis of Roof Bolt Systems. Proceedings, 20th Intl.

Conf. On Ground Control in Mining, Morgantown, WV, 2001,

pp. 218-225.

10. Molinda, G.M., Mark C., Bauer, E.R, Babich, D.R. and

Pappas, D.M. Factors Influencing Intersection Stability in U.S.

Coal Mines. Proceedings, 17th International Conference on

Ground Control, 1998, pp. 267-275.

APPENDIX. - PILLAR RECOVERY RISK FACTOR

CHECKLIST

Local Stability Risk Factors (Primary)

• Cut sequence: Is an outside lift sequence being used?

• Final stump: Is an adequate final stump consistently being

left in place?

• Support: Are Mobile Roof Supports being used?

• Roof bolts: Is extra roof support used in intersections? 

Global Stability Risk Factors

• Pillar Design: Is the ARMPS SF adequate to prevent a

squeeze?

• Collapse Prevention: If the ARMPS SF<2.0 and the pillar

w/h<4.0, either on advance or in the worked-out area, have

steps been taken to prevent a massive pillar collapse?

• Barrier Pillar Design: If the depth of cover is greater than

1000 ft, are stable barrier pillars (SF>1.5 to 2.5) being used

to separate the panels?

Other Risk Factors 

• Roof geology: Is the roof at least moderate in strength?

• Intersection span: Have entry widths and turnouts been

minimized?

• Multiple seam interactions: None anticipated?

• Depth of cover: Less than 650 ft?

• Block size: Are the blocks uniform in size?

• Age of workings: Is the development less than 1 year old? 

• Continuous haulage: None?

Note: The Risk Factor Checklist can be used to identify potential

problem issues for specific pillar plans. The more questions on it that

can be answered with a “yes,” the less risky the plan is likely to be.

The checklist does not weight the individual issues, nor is it

necessarily a comprehensive list. It is simply a tool to help mine

planners evaluate the overall level of risk, and possible ways to reduce

the risk.