Mark, C.

 

 

 


 

Deep Cover Pillar Recovery in the US 

Chris Mark, Principal Research Engineer
 

NIOSH Pittsburgh Research Laboratory
 

Pittsburgh, PA
 

ABSTRACT 

In the wake of the Crandall Canyon mine disaster, the U.S. 

Congress asked the National Institute for Occupational Health and 

Safety (NIOSH) to study the safety of deep cover pillar recovery 

operations in the US. The legislation defined “deep cover” to 

be greater than 1,500 ft, but NIOSH is also evaluating mines at 

depths greater than 1,000 ft when multiple seam interactions are 

encountered. By these definitions, there about 35 active mines that 

engage in deep cover pillar recovery, in the states of UT, CO, VA, 

WV, and KY. To date, NIOSH has documented ground conditions 

and ground control experience at nearly all of them. This paper 

provides an overview of current deep cover pillar recovery practice. 

Specific ground control issues that are discussed include: 

•		Pillar recovery sequences, 

•		Pillar and barrier pillar design, 

•		Coal bumps, 

•		Thick seam pillar extraction, and 

•		Multiple seam interactions. 

INTRODUCTION 

On August 6, 2007, a violent coal bump occurred at the Crandall 

Canyon mine near Price, Utah. Six miners working in the South 

Barrier section of the mine were presumed trapped. Ten days later, 

three rescuers were killed in a second bump. Underground rescue 

efforts were suspended, and the original six miners were presumed 

to have been fatally injured. 

At the time of the incident, the Crandall Canyon miners were 

engaged in retreat mining in the South Barrier section. With cover 

that exceeded 2,200 ft at its deepest point, these were some of the 

deepest pillar retreat operations ever attempted in the US. 

Crandall Canyon was the nation’s fourth coal mine disaster in 

less than two years. It therefore generated intense interest in the 

mining community and the public at large. In response to the 

disaster, Congress directed that NIOSH conduct “a study of the 

recovery of coal pillars through retreat room and pillar mining 

practices in underground coal mines at depths greater than 1,500 

ft.” They further directed that the study include analyses of: 

•		Conditions under which retreat mining is used, including 

conditions relating to seam thickness, depth of cover, strength 

of the mine roof, floor, and pillars, and the susceptibility of the 

mine to seismic activity, and; 

•		Procedures used to ensure miner safety during retreat mining. 

NIOSH will submit a report to Congress on December 31, 2009, 

containing the results of the study. The report will also include 

recommendations to enhance the safety of miners on retreat mining 

sections and recommendations for future research. 

The study is being conducted in collaboration with the University 

of Utah (U of UT) and West Virginia University (WVU). At WVU, 

a team led by Prof. Keith Heasley is investigating pillar design for 

deep cover retreat mining, focusing on the LaModel numerical 

modeling program. The U of UT effort, led by Prof. Kim McCarter 

and Prof. Walter Arabasz, is concentrating on the application of 

seismic monitoring to reduce the bump risk. The results of these 

projects will be reported in detail in the final report to Congress, 

but they will not be discussed here. 

The purpose of this paper is to provide an update on the NIOSH 

study’s progress, and to report some preliminary findings. The 

study builds upon past NIOSH retreat mining research in the areas 

of pillar design, deep cover, and roof fall prevention (Mark and 

Tuchman, 1997; Chase et al., 2002; Mark and Zelanko, 2005). 

To conduct the current study, NIOSH partnered with 16 different 

coal companies to obtain information on the industry’s experience 

with deep cover pillar recovery. NIOSH researchers conducted 

underground investigations at 18 mines located in UT, CO, WV, 

VA, and KY, and they collected data from 17 more. These mines 

have included nearly every active mine that has recovered pillars 

at depths exceeding 1,500 ft, and the majority of mines with 

experience between 1,000 and 1,500 ft. In all, more than 200 

retreat mining case histories have been documented and added 

to the Analysis of Retreat Mining Pillar Stability (ARMPS) data 

base. Roof control professionals at the MSHA have also provided 

much valuable information. The study has focused particularly on 

bump control, and included discussions in Germany with German 

rock burst specialists. It has also made extensive use of the MSHA 

accident and injury data base. 

 
 
 

 

DEMOGRAPHICS OF DEEP COVER PILLAR RECOVERY 

The first task was to determine how many deep cover pillar 

recovery mines there are, and where they are located. Depth of 

cover is not a parameter that mines routinely report to either MSHA 

or the Department of Energy (DOE). Fortunately, in early 2008, the 

MSHA Roof Control Supervisors developed lists of the deep cover 

mines in their Districts, and this information was generously shared 

with NIOSH. 

Figure 1 shows that of the approximately 42,000 underground 

coal miners in the US1

1 Calculated from the number of underground worker hours reported to MSHA, 

assuming that 2000 hours equals one worker-year. 

, less than 3%, or 1,200, work at mines that 

recover pillars at depths greater than 1,500 ft. An additional 3,900 

work at retreat mines with depths between 1,000 and 1,500 ft. Past 

studies have indicated that only about 1/3 of the hours at a retreat 

mine are actually spent in pillar recovery operations (Mark et al., 

1997), and the current study found that at a typical deep cover 

retreat mine, at least 50% of the pillar extraction is conducted 

beneath shallow cover. Therefore, it seems likely that total 

exposure of miners to pillar recovery operations at depths greater 

than 1,500 ft is less than 1⁄2 of one percent of all hours worked 

underground. 

 

14,000,000 

Underground Hours Worked 12,000,000 

10,000,000 

8,000,000 

6,000,000 

4,000,000 

2,000,000 

0 

Deep Cover >1,500 ft 

Deep Cover >1,000 ft 

Not Deep Cover 

Southern VA NEast KY SEast KY West 

WV 

Figure 1. Location of deep cover pillar recovery operations in 

the U.S., 2007. 

Figure 1 also shows that the greatest concentration of deep cover 

retreat mining is in southeastern KY, specifically in Harlan County. 

The next largest grouping is in adjacent Wise County, VA. Active 

mines with pillar recovery experience at depths greater than 1,000 

ft are located throughout eastern KY and southern WV. In the 

West, there are two, deep cover room-and-pillar retreat mines, one 

in UT and the other in CO. One western longwall mine (in addition 

to Crandall Canyon) has also recovered pillars prior to abandoning 

a worked-out seam. There is apparently no deep cover pillar 

recovery at all in the Northern Appalachian, Illinois, or Alabama 

coal regions. 

GEOLOGY AND MINING CONDITIONS 

Previous studies have found that roof rocks in the Central 

Appalachian, Utah, and Central Colorado coalfields are relatively 

hard and strong (Rusnak and Mark, 2000; Mark, 2007). 

Observations in the deep cover retreat mines conform to this trend. 

The lowest CMRR values measured were approximately 45, which 

is considered “intermediate strength” roof. Most of the mines 

had CMRR values in the 50’s, and some were as high as 75. As 

evidence of the relatively benign conditions, more than 80% of the 

deep cover mines employ 4- or 5-ft fully grouted bolts as primary 

support, despite their depth. MSHA statistics also indicate that the 

rate of unplanned roof falls is about 25% lower in this group of 

mines than in other room-and-pillar mines. 

The overburden is also relatively competent, typically consisting 

largely of thick sandstones and siltstones. The floor rocks tend to 

be firm, and groundwater is seldom a major issue. 

On the other hand, multiple seam interactions are an important 

concern at a big majority of these operations. Almost 80% 

encounter workings less than 200 ft above or below their active 

mining. 

GROUND CONTROL SAFETY IN DEEP COVER 

PILLAR OPERATIONS
 

Pillar recovery accounts for no more than 10% of the coal mined 

underground, yet it has historically been associated with more than 

25% of all ground fall fatalities (Mark et al., 2003). Within retreat 

mining, deep cover has long been identified as a “risk factor” 

(Mark et al., 1997). During the past 15 years, there have been 24 

fatal incidents during retreat mining (Figure 2), and six of those 

have occurred at depths greater than 1,000 ft (see Table 1). The data 

shown in Figure 1 indicates that only about 10% of retreat mining 

is conducted at depths exceeding 1,000 ft (assuming that half of 

the pillars recovered at the deep cover mines are actually beneath 

shallower cover.) Since 10% of the exposure was associated with 

25% of the fatal ground fall incidents during this period, the risk to 

deep cover miners was significantly greater than that faced by other 

retreat miners, which was already elevated relative to the ground 

fall risk to underground coal miners in general. Moreover, 4 of the 

6 deep cover incidents resulted in multiple fatalities. 

1996199719981999200020012002200320042005200620072008 

0 

1 

2 

3 

4 

Fatal Incidents 

Year 

Figure 2. Trends in retreat mining fatal incidents, 1996-2008. 

Despite these grim statistics, the accident trends do seem to 

indicate that some progress is being made in the area of retreat mine 

safety. Prior to 2003, an average of two fatal incidents occurred 
 

 

 

 

each year during pillar recovery operations. During the past six 

years, there have been just five incidents (figure 2). A key cause of 

the change has been the widespread adoption of safer retreat mining 

techniques and technology. A concerted effort by MSHA and 

NIOSH (Mark and Zelanko, 2005; Mark et al., 2003) promoted the 

following three steps to safer pillar recovery: 

• Global stability through proper pillar design; 

• Local stability with proper roof support, and; 

• Worker safety through proper section management. 

Table 1. U.S. deep cover pillar (depth >1,000 ft) retreat mining fatal incidents --1996-2008. 

Year Cause Mine State No. 

Fatalities Link 

1996 Pillar 

Bump C-2 KY 2 www.msha.gov/FATALS/1996/FAB96C34.HTM 

1998 Roof fall Lightfoot No. 2 WV 1 www.msha.gov/FATALS/1998/FAB98C02.HTM 

1998 Roof fall Darby Fork 

No. 1 KY 1 www.msha.gov/FATALS/1998/FAB98C24.HTM 

2005 Roof fall Stillhouse #1 KY 2 www.msha.gov/FATALS/2005/FAB05c1112.asp 

2007 Roof fall Cucumber WV 2 www.msha.gov/FATALS/2007/FAB07c0203.asp 

2007 Pillar 

Bump 

Crandall 

Canyon UT 9 http://www.msha.gov/FATALS/2007/CrandallCanyon/ CrandallCanyonreport.asp 

Total 17 

The Crandall Canyon incident, like the double bump fatality at 

the C-2 Mine in Kentucky 11 years earlier, was a clear example of 

global instability caused by improper pillar design. The MSHA 

Fatality Investigation Report concluded that the design at Crandall 

Canyon was “destined to fail” because the remaining production 

and barrier pillars were too small to carry the overburden load 

(Gates et al., 2008). 

Global stability is a necessary, but not sufficient, condition 

for creating a safe working area. Proper roof support is required 

to maintain local stability. The final pillar stump (sometimes 

called the “pushout”) provides critical roof support during pillar 

recovery. Traditionally, miners tried to extract all the coal during 

pillar recovery, and many fatalities occurred during the mining of 

the final stump. Research has now shown that the optimum pillar 

extraction plan leaves a final stump that is engineered to provide 

roof support without inhibiting caving (Mark and Zelanko, 2001). 

Most roof control plans that are now in use do not allow the mining 

of the pushout, and specify a cut-to-corner distance that ensures that 

the stump is properly sized. 

One striking feature of the pillar recovery fatalities is that in 

nearly every case, the victim was beneath bolted roof. In many 

cases, bolt failure was itself implicated in the fatality. Sheared and 

broken 5/8-in, fully-grouted rebar bolts contributed to three of the 

four deep cover roof fall fatal incidents listed in Table 1 (Lightfoot, 

Darby Fork, and Cucumber). Increasingly, mines are using longer 

and/or stronger bolts to support areas that will be retreat mined. 

In addition, cable bolts or other special bolts are employed in 

intersections, which are the most hazardous locations for miners 

during pillar recovery. 

Traditionally, timber posts provided supplemental support for 

pillar recovery, but they have many disadvantages. Mobile Roof 

Supports (MRS) provide better ground control, and they can 

be set remotely, away from the dangers of the pillar line. Today, 

perhaps 50% of all retreat coal is mined with MRS, primarily in the 

thicker seams. Unfortunately, several of the recent victims in pillar 

recovery fatalities have been MRS operators that were standing 

unnecessarily in unsafe, inby locations. These incidents have 

underlined the third factor, effective section management. Careful 

planning of the production process, good supervision, and training 

and retraining are necessary to prevent bad habits from developing. 

Most rock fall injuries underground, more than 400 per year, 

are caused by relatively small pieces of rock falling from between 

supports. Analysis of the MSHA accident and injury statistics 

shows that miners in deep cover retreat operations are not at 

significantly greater risk of rock fall injury than other room-andpillar miners. One part of the explanation may be that roof 

bolting, which is a significant source of rock fall injuries, is seldom 

employed during retreat mining. 

Rib falls do seem to be a serious problem at deep cover pillar 

recovery mines. During 2007, fully one-third of all the rib fall 

injuries in the entire US underground coal industry occurred in 

the small group of deep cover retreat mines. Rib falls have killed 

15 mineworkers since 1995, including three at deep cover pillar 

recovery mines (though none of the incidents occurred during 

retreat mining operations). The two main factors that lead to an 

increased risk of rib falls are thicker coal seams and higher stress 

levels. For example, analysis of the 15 fatal rib fall incidents 

revealed that two-thirds occurred at depths exceeding 900 ft, and 

the mining height was at least seven ft in every case. Rib bolting 

can be highly effective in reducing the risk of rib falls. It is 

significant that apparently none of the 15 U.S. fatality sites were 

ever rib bolted (Mark et al., 2009). 

DEEP COVER RETREAT MINING METHODS 

Most room and pillar mines in the US employ continuous 

mining machines with shuttle car haulage, and this holds for 26 of 

the 30 active deep cover retreat mines that were visited or whose 

Roof Control Plans (RCPs) were reviewed for this study. The 
 

C 

B A 

Figure 3. Methods for recovering coal pillars. A) Left-right. B) 

Outside lift. C) Split and fender. 

exceptions were four KY operations, primarily in thinner seams, 

that employed continuous haulage. 

Development panels are typically driven five to nine entries 

wide, with pillars recovered on retreat. Among this group of mines, 

it is very unusual to widen the panel by driving rooms on retreat. 

Barrier pillars are almost always left between panels, for both 

ventilation and ground control. Most of the mines do take slab cuts 

from the barriers during retreat, but they do not completely remove 

the barriers. 

In most cases the pillars in a row are recovered in sequence, 

beginning with the pillar nearest the previous panel gob. The mines 

that employ continuous haulage typically prefer to “close in the 

center,” taking their last lifts from the belt entry. 

By far the most popular methods of pillar recovery used today 

are those that require no additional roof bolting during retreat. Most 

plans can be classified as either “left-right,” (also called Christmas 

tree mining or twinning) in which lifts are taken on both sides of 

the entry, or as “outside lift,” in which cuts are taken on just one 

side (see Figure 3). “Split and fender” plans can be used when the 

pillars are so large that they can’t be fully recovered by lifts taken 

from the entries. The roof in the splits must be bolted, however, 

which is undesirable from both the worker safety and operational 

standpoints. 

A NIOSH study (Mark et al., 2003) estimated that, nationally, 

about 60% of retreat mining used left-right techniques, 35% used 

outside lifts, and only 5% some form of split and fender. Among 

the deep cover retreat mines, left-right plans were even more 

popular, used by 28 of the 30 mines. Again, the exceptions were 

two of the continuous haulage mines that employed outside lifts. In 

spite of the fact that deeper cover usually means larger pillars, none 

of the mines routinely employed split and fender techniques. Leftright plans were used to extract pillars as large as 75 by 75 ft (95 by 

95 ft centers), even though in this configuration a 20-ft-wide fender 

of coal had to be left down the center of the pillar because the lifts 

could not reach the pillar center. 

Three of the mines, all in the east, are mining two seams 

simultaneously. When both seams are mined on advance, the 

entries are 12-15 ft high. To minimize rib control issues, one of the 

mines extracts only the top seam and the parting on development. 

The floor in the entries and both seams in the pillars are then 

recovered during retreat. This floor mining sequence has the 

additional advantage that smaller pillars can be used, because of the 

reduced width-to-height ratios for the intact pillars outby the pillar 

line. 

The deep cover mines have adopted many of the local stability 

“best practices” that NIOSH and MSHA have advocated (Mark 

and Zelanko, 2004). Of the 28 mines using left-right extraction 

sequences, only 4 extract the final pushout. At the others, an 

engineered stump, usually measuring 8 by 8 ft or 10 by 10 ft, 

is left as a roof support. Six of the mines do not even take any 

lifts from the crosscut. Only four of the mines rely solely on 

timber for standing support, and the remainder all employ MRS. 

All but four routinely install extra supplemental support in the 

intersections where retreat mining is planned. The extra support 

typically consists of a pattern of 4 to 6 cable bolts or resin-assisted 

mechanical shell bolts, 8 to 12 ft long. 

PILLAR DESIGN 

The MSHA report on Crandall Canyon (Gates et al., 2008) 

emphasized the role of the flawed pillar design in the disaster. In 

the report’s words, the “pillar dimensions were not compatible with 

the deep overburden and high abutment loading that existed in the 

South Barrier section,” and as a result the “stress level exceeded 

the strength of a pillar or group of pillars near the pillar line, and 

that local failure initiated a rapid and widespread collapse that 

propagated outby through the large area of similar sized pillars.” 

The report documented how the two pillar design software 

packages used to develop the design, ARMPS and LaModel, were 

both employed improperly, resulting in the flawed design. In the 

wake of Crandall Canyon, MSHA published a Product Information 

Bulletin (MSHA, 2008a) and a Procedure Instruction Letter 

(MSHA, 2008b) on ARMPS to help ensure that pillars are designed 

properly. 

ARMPS, for Analysis of Retreat Mining Pillar Stability, was 

originally developed by NIOSH in the mid 1990’s (Mark and 

Chase, 1997.) ARMPS estimates the magnitude of the loads that 

develop during the various stages of the retreat mining process, 

and calculates a “stability factor” (SF) by comparing the loads to 

the estimated load bearing capacity of the pillars that must carry 

them. The power of ARMPS is not derived from the accuracy of 

its calculations, but rather from the large data base of case histories 

that it has been calibrated against. Statistical analysis has been 

used to propose design guidelines that do the best job of separating 

the “successful” case histories from those that were “unsuccessful.” 

A case history is considered a success when an entire panel 

was recovered without significant ground control incident. The 

unsuccessful cases include: 

•		Squeezes, which are non-violent pillar failures that may take 

hours, days, or even weeks to develop; 
•		Collapses, which occur when large areas supported by slender 

pillars (w/h < 4) fail almost simultaneously, resulting in an 

airblast, and; 

•		Bumps, which are sudden, violent failures of one or more 

highly stressed pillars. 

The original ARMPS data base consisted of approximately 150 

case histories, representing a broad range of cover depths. Analysis 

indicated that when the depth of cover was less than 650 ft, a SF of 

about 1.5 was a reasonable starting point. However, for deep cover 

cases, two conclusions were drawn: 

•		Many panels with a SF well below 1.5 were successful, and; 

•		No single SF was able to separate the successful from the 

unsuccessful cases. 

Accordingly, a follow-up study was conducted which focused 

on deep cover pillar recovery (Chase et al., 2002). During this 

study, an additional 100 case histories were collected from mines in 

Central Appalachia and the West where the depth of cover exceeded 

750 ft. The analysis indicated that squeezes were the most likely 

failure mode when the depth of cover was less than 1,250 ft, but 

bumps predominated in the deeper cover cases. Design guidelines, 

including suggestions for barrier pillars to isolate active panels from 

nearby gobs in bump prone ground, were also proposed (Figure 4). 

ARMPS SF Weak and Intermediate Roof Strength Strong Roof 

650 ft ≤ H ≤ 1,250 ft 1.5 - [H-650] / 1000 1.4 - [H-650] / 1000 

1,250 ft ≤ H ≤ 2,000 ft 0.9 0.8 

Barrier Pillar SF 

H > 1,000 ft ≥ 2.0 ≥ 1.5 * 

≥ 2.0 ** 

* Nonbump prone ground 

** Bump prone ground 

The ARMPS case history data base, showing the suggested ARMPS SF for design. 

ARMPS SF 3.0 

2.5 

2.0 

1.5 

1.0 

0.5 

0.0 

Satisfactory 

Unsatisfactory 

0 200 400 600 800 1000 1200 

Depth of Cover (ft) 

1400 1600 1800 2000 

Figure 

2002). 

4. ARMPS deep cover design criteria (Chase et al., 

 

The current deep cover study has added another 200 cases to the 

ARMPS data base. Additional cases have also been added from the 

Analysis of Multiple Seam Stability (AMSS) data base (Mark et al., 

2007), and there are now nearly 800 cases in all. Figure 5 shows 

that the trends in the expanded database are very similar to those 

found in 2002. For example: 

•		Of the 518 successful cases, 298 (57%) met or exceeded 

the suggested design criteria, while 220 did not. For the 

unsuccessful cases, only 20 (17%) met the design criteria, 

while 100 (including all but one of the bump and massive 

collapse cases) did not. 

•		Of those 56 multi-panel retreat cases that employed barrier 

pillars with a BP SF>2.0, just 10% were failures. For the 72 

cases where the BP SF< 2.0, 47% were failures. 

•		At depths greater than 1,500 ft, 16 of the 20 failures are 

bumps, while only 4 were squeezes. Between 1,000 and 1,500 

ft the proportions are nearly reversed, with 24 squeezes and 4 

bumps. 

At the time of this writing, further analysis of the ARMPS case 

history data base is ongoing. The goal is to determine if further 

refinements to the ARMPS design criteria, or to ARMPS itself, 

could result in improved predictions of pillar design performance. 

Additional parameters in the data base that will be investigated 

include: 

•		Seam Hardgrove Index (HGI), 

•		Uniaxial compressive strength, 

•		Mine location (state, county), 

•		Coal seam, 

•		Roof quality (CMRR), 

•		Panel width, and; 

•		Panel width-to-depth ratio. 

Preliminary analyses have indicated that narrow panels at 

depth may be successful with lower ARMPS SF than wider panels 

provided they employ adequate barrier pillars. The implication 

of this finding is that a narrow panel may create a “pressure arch” 

which transfers some of the tributary area development load, and 

subsequent abutment loads, from the production pillars to the 

barriers. Numerical modeling is also being employed to investigate 

this possibility. If both the analytical and empirical lines of 

research ultimately confirm the pressure arch approach, it may be 

incorporated into an adjusted ARMPS loading model. 

BUMP CONTROL IN DEEP COVER RETREAT MINES 

Bumps have long been among the most feared hazards in deep 

retreat mines. As long ago as 1935, Rice described bumps in 

the coal mines of Harlan County, KY and Wise County, VA. A 

comprehensive data base of 172 bump events compiled in 1995 

indicated that more than 80% of the bumps reported by room-andpillar mines occurred during the process of pillar or barrier pillar 

recovery (Iannacchione and Zelanko, 1995). 

Unfortunately, despite decades of research, the sources and 

mechanics of bumps are imperfectly understood, and the means to 

predict and control them remain elusive. Coal bumps share these 

characteristics with both bursts in hard rock mines and natural 

earthquakes. 

Some valuable generalizations can be made, however. First 

and foremost, high stress is a universal feature of bump prone 

conditions. Deep cover is the primary source of high stress, but 

stress levels can be further increased by retreat mining abutment 

loads or multiple seam interactions. 

Most bumps are also associated with mining activity. In a highly 

stressed coal pillar, the greatest stress is not generally right at the 

edge of the pillar, but rather some distance from the rib. The 

situation is stable because of the confinement provided by the 
coal nearer the rib. When that coal is extracted, the confinement is 

removed, and the highly stressed coal can fail suddenly and 

violently. Pillar splitting, which removes the coal that is confining 

the most highly stressed pillar core, is an activity that is particularly 

prone to cause bumping. ARMPS SF 

2.50
 

2.00 

1.50 

1.00 

0.50 

0.00 

0 500 

Success 

"Squeeze" 

"Collapse/Bump"

1000 1500 2000 2500 

 Depth of Cover (ft) 

Figure 5. The 2009 ARMPS case history data base, showing the Chase et al., (2002) design criteria. 

Coal bumps themselves generate seismic events, though they are 

usually small ones. But seam level bumps can also be triggered 

by seismic events that originate elsewhere. Full extraction mining 

causes much seismicity that originates when stored energy is 

released by rock failure occurring up to several hundred feet 

above (or below) the seam. The failures include sudden roof-tofloor convergence in the worked out areas, shear slip motion on 

rock fractures in the overburden, or some combination of these 

two mechanisms (Pankow et al., 2008). Mining activity can also 

reactivate pre-existing faults, which in turn can result in a bump 

(Swanson, 2008). Only a tiny fraction of seismic events induced 

by mining result in coal bumps, however. For example, German 

seismic monitoring stations recorded 4,623 mining-induced 

events over one ten year period, but only 18 coal bumps occurred 

during that same period (Brauner, 1994). In fact, even very large 

events may have little impact on the mine if they occur well above 

the workings, such as the magnitude 4.2 event that was located 

approximately 500 ft above the Willow Creek mine but did not 

result in a coal bump at the mining horizon (Ellenberger et al., 

2001). On the other hand, an event that is too small to register on 

a regional seismic network can result in a serious injury if a miner 

is in the wrong place at the wrong time. 

Geologic factors also contribute to bump proneness. The 

presence of strong, massive sandstone near the seam has often been 

noted where bumps have occurred (Iannacchione and Zelanko, 

1995). On the other hand, coal seam characteristics do not seem 

to play an important role. Iannacchione and Zelanko (1995) noted 

that bumps have occurred in at least 25 different US coalbeds, 

varying from strong, blocky seams to the very friable Pocahontas 

No. 3 and No. 4 seams. Laboratory studies conducted by Babcock 

and Bickle (1985) showed that most coals can fail violently if 

they are highly stressed and the confinement is suddenly reduced. 

Extensive German laboratory studies using large-scale specimens 

have also concluded that nearly all types of coal, with the exception 

of anthracites, can burst. In their experiments, coal seams ranging 

in unconfined compressive strength from 700 to 7,000 psi have all 

been shown to be bump prone (Brauner, 1994). 

Table 2 lists the coal bump events that have occurred in US 

room and pillar mines from 1983-2008. Most of the events were 

identified by conducting a search of the narratives in the MSHA 

accident and injury database, using key words like “bump” and 

“bounce.” The narratives were carefully screened to remove all but 

those cases that clearly referred to a violent ejection of coal from 

the rib. Most of the events were relatively small, and were only 

reported to MSHA because they happened to result in an injury. 

A second group of events resulted in extensive damage to at least 

several pillars, and many of these have been further documented 

by additional sources. By far the largest of this second group was 

the disaster at Crandall Canyon, and the three other fatal incidents 

listed in Table 2 were also multi-pillar bumps. 
 

Table 2 shows that documented bumps have occurred at just 

18 room-and-pillar mines during the past quarter of a century2

. 

Table 2. Bump events in U.S. room and pillar mines, 1983-2008. 

Years State Seam No. 

Events 

Multi- Pillar 

Events Fatalities Injuries Depths (ft) Source 

1996 CO D 1 1 0 0 1,560 

2007 CO B 1 1 0 0 1,400 Maleki (2009); Swanson 

(2008) 

1995-2001 CO D 9 0 4 1,550-2,200 

1990, 1996 KY Creech 2 1 2 3 1,300 Foutch et al., (1996) 

2002-2003 KY Darby 4 0 4 1,500-1,900 Newman (2008) 

1998 KY Kellioka 1 0 0 1600 

1989 KY Harlan 1 0 1 

1999-2002 KY Darby 3 3 0 0 1,400 Newman (2002) 

2002 KY Harlan 1 0 0 

1995, 2007 UT Hiawatha 5 2 9 1 1,600, 2,100 Gates et al., (2008) 

1983-1991 UT Hiawatha 11 2 0 0 

1989, 2004 UT Centenial 2 0 2 1,100 

1986-1993 UT Rock 

Canyon 7 2 0 6 1,800 Boler et al., (1995); Maleki (1995) 

1987 UT Hiawatha 3 1 0 3 1,600 

1993 WV Beckley 2 1 0 2 1,150 

1984 WV No. 2 Gas 2 1 1 1 800 Campoli et al., (1987) 

1984 WV Chilton 1 1 0 5 750 Campoli et al., (1987) 

1983-1985 WV Pocahontas 

No. 4 5 1 1 4 

2006 WV Powellton 1 1 0 0 1,100 Gauna and Phillipson 

(2008) 

2 The table also contains one bump that affected multiple pillars on a mains development 

at a Colorado longwall. It is worth noting that the MSHA data base also indicates that 

development sections at almost every longwall mine in Utah and the North Fork Valley 

of Colorado have experienced injuries caused by small bumps.

Almost all of these mines have been located in Utah, the North 

Fork Valley of Colorado, or Harlan County, KY. There have also 

been four bumps scattered around southern WV. 

The relatively small number of mines with bump experience 

is striking, as is the relatively small number of bumps at each of 

them. Even “bump-prone” mines often apparently work for years 

without encountering a bump. In many cases, there appears to be 

a tendency for bumps to cluster in distinct areas, indicating that 

geologic controls are important. Some mines have established “red 

zones” within which no pillar recovery is conducted. For example, 

one mine in Harlan County with a history of bumps has defined a 

red zone wherever the depth of cover exceeds 1,550 ft and a 5-ftthick sandstone is located within 4.25 ft of the roofline. 

It is also significant that most of the multi-pillar bumps occurred 

in mines that have also had small, injury bumps. It seems that 

small bumps may be a valuable indicator of the potential for larger 

bumps. Whyatt (2008) showed that in western longwall mines, an 

increased frequency of small bumps has often foreshadowed a 

 larger event. 

Most of the multi-pillar bumps, including all of those that 

resulted in fatalities, can be attributed to poor mine design or 

mining practices. In most instances, the barrier pillars were either 

too small or were being extracted on retreat. In some of these cases, 

the bumps were apparently triggered by pillar splitting. Therefore, 

it seems likely that many of the most dangerous events might be 

avoided through proper pillar design and the application of the “red 

zone” concept (Iannacchione and Tadolini, 2008). 

Unfortunately, not every multi-pillar bump is so easily classified. 

Three of the mines listed in Table 2 have experienced bumps in 

areas that would not have been considered high risk. For example, 

the incident described by Gauna and Phillipson (2008) occurred 

in seam that had never had a bump, on a development section, 

beneath old works that had not been retreat mined. Yet this same 

mine has extensive bump-free experience recovering pillars under 

similar depths of cover beneath a variety of highly stressed upper 

seam remnants. The incidents described by Newman (2002) 

similarly occurred during development beneath first workings 

that would not have been considered high risk, and the incident in 

CO mine described by Swanson et al., (2008) and Maleki (2009) 

was a similar scenario. In that last instance, the heaviest damage 

was centered not at the development faces, but in area of steeply 

dipping faults more than 500 ft outby. Fortunately, none of these 

events resulted in injury to personnel. 
 

 

CONCLUSIONS 

Pillar recovery operations at depths exceeding 1,500 ft are 

currently conducted in mines located in southeastern KY and 

western VA, together with two western mines. These operations 

account for about 2% of all underground hours, but only a small 

fraction of those hours are actually spent in deep cover retreat 

mining. About three times as many miners scattered across the 

central Appalachian coalfields are working in mines that recover 

pillars at depths exceeding 1,000 ft. 

Deep cover pillar recovery operations have had a worse-thanaverage ground fall safety record over the past 15 years. Currently, 

however, most deep cover mines are employing best practices in 

their retreat mining methods, including installing extra roof support, 

leaving the final stump, and using MRS. Rib control remains a 

concern at some operations because of high stresses and tall pillars. 

More than 200 new retreat mining case histories have been 

added to the NIOSH ARMPS data base. Preliminary analysis 

indicates that the current guidelines (Chase et al., 2002) are still 

appropriate. Ongoing analysis is investigating whether including 

other parameters or adjusting the loading model could result in an 

improved pillar design methodology. 

Although documented bumps have occurred at only about 20 

room-and-pillar mines during the past 20 years, they remain a 

significant concern. In mines that are considered bump prone, 

application of “red zones” and adherence to pillar design best 

practices should significantly reduce the risk. Large, multi-pillar 

events appear to present the greatest hazard. Unfortunately, several 

of these have occurred in recent years where the risk would have 

been considered relatively low. These events underscore the need 

for continued vigilance and research. 

DISCLAIMER 

The findings and conclusions in this report have not been 

formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent agency 

determination or policy. 

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