DEEP COVER PILLAR EXTRACTION IN THE U.S. COALFIELDS
 

Frank E. Chase, Research Geologist
 

Christopher Mark, Chief, Rock Mechanics Section
 

National Institute for Occupational Safety and Health
 

Pittsburgh Research Laboratory
 

Pittsburgh, Pennsylvania USA
 

Keith A. Heasley, Associate Professor
 

West Virginia University
 

Morgantown, West Virginia USA
 

ABSTRACT 

Deep cover retreat mining (overburden in excess of 750 ft) is an 

important emerging issue which will intensify in the future as the 

more easily mined shallow seam reserves are depleted. Analysis of 

Mine Safety and Health Administration (MSHA) statistics indicates 

that deep cover pillar recovery accounts for a disproportionate share 

of the underground coal mine roof/rib fall fatalities and injuries. Past 

research has shown that previously recommended Analysis of Retreat 

Mining Pillar Stability (ARMPS) stability factors (SF’s) may be 

excessive for deep cover pillar design. The objectives of this study 

were to evaluate the various methods and strategies by which panels, 

production pillars and barrier pillars are developed and extracted 

under deep cover, and to develop appropriate design guidelines. In 

the course of the research, 29 mines in 7 states were investigated to 

collect panel design case histories. At each mine site, underground 

geotechnical data were collected on the pillar line in order to 

document roof rock, coalbed and floor conditions. The analyses 

indicated that squeezes were the most likely failure mode where the 

depth was less than 1,250 ft, but bumps predominated in the deeper 

cover cases. Immediate roof rock quality, the ARMPS SF’s, and 

barrier pillar stability factors were all found to be important 

parameters in determining the outcomes of the case histories. Design 

guidelines, including suggestions for barrier pillars to isolate active 

panels from nearby gobs in bump prone ground, are also proposed. 

INTRODUCTION 

Over the past 2 decades, retreat mining has gained a disparaging 

reputation in terms of safety. Since 1978, approximately 25% of the 

roof/rib fall fatalities have occurred during pillar recovery operations. 

However, retreat mining only accounts for about 10% of the total 

U.S. underground coal production (1). Since 1997, deep cover 

(overburden in excess of 750 ft) pillaring operations have accounted 

for 40% of the fatalities which have occurred during pillar recovery. 

To put this in perspective, Mine Safety and Health Administration 

(MSHA) Roof Control Specialists from across the country were 

surveyed and 48 deep cover pillaring operations were identified. In 

addition, comparative evaluations conducted by National Institute for 

Occupational Safety and Health (NIOSH) personnel of MSHA data 

(2) determined that ground fall incidence rates were 27% higher for 

deep cover retreat mining operations as compared to all other roomand-pillar mines. 

Realizing that deep cover pillar recovery was an important 

emerging issue which will intensify in the future as mines are forced 

to go deeper, NIOSH investigators began examining the situation in 

1997. Because there were relatively few prior research efforts in the 

area of ground control for deep cover pillar extraction, NIOSH 

personnel went to the coalfields to document the actual experiences 

of the operators. The underlying premise was that information 

gathered by documenting the trial-and-error/success panel design 

refinement processes of several mining operations should yield 

valuable design guidelines and strategies. This in the same research 

methodology that proved successful in generating and validating the 

Analysis of Retreat Mining Pillar Stability (ARMPS) computer 

program (3) which today is widely used to size pillars for retreat 

mining. 

Analyses of approximately 150 case histories in the original 

ARMPS database found that where the depth of cover is less than 

750 ft, a Stability Factor (SF) of about 1.5 is normally a reasonable 

starting point. However, for the deep cover cases two conclusions 

were drawn (3, 4): 

• Many panels with a SF less than 1.5 were successful, but; 

• No single SF seemed to be an appropriate design criterion. 

The goal of this study was to develop appropriate criteria for 

applying ARMPS to size pillars for deep cover, and determine what 

other significant factors should be considered in design. In order to 

accomplish this objective, 97 panel design case histories were 

gathered at 29 mines located in the following states: CO, KY, PA, 

TN, UT, VA, and WV. Underground geotechnical data on the 

immediate roof rock, coalbed, and floor conditions were collected for 

each case history. Due to the fact that limited core hole data was 

available at several mines in the immediate vicinity of the case 

history, the main roof rock’s composition, strength, and caving 

characteristics could not be considered. Obviously, this was 

unfortunate because the characteristics of the main roof can play an 

important role in determining the outcome of a particular design, for 

example, the likelihood of a bump occurring. Also, excluded from 

the data base were any panels which were over- or undermined. 

During this investigation, careful attention was also paid to 

documenting the various methods and strategies by which panels, 
 

 

 

 

production pillars and barrier pillars were developed and extracted to 

determine the current state-of-the-art. In order to select mine sites 

representative of the deep cover population, the opinions of several 

Roof Control Specialists from the Mine Safety and Health 

Administration (MSHA) and State Department of Mines personnel 

throughout the country were solicited. 

GROUND CONTROL CONCERNS 

Hazards associated with pillar extraction tend to intensify with 

depth. Pillar failures, including both bumps and squeezes, are 

generally more severe at depth and are evidence of a highly stressed 

environment. Bumps are sudden violent pillar failures where the coal 

is expelled into the workings. Documented bumps in the deep cover 

database have caused fatalities, serious injuries, personnel 

entrapments, and/or equipment damage. Many of these events shook 

the surface facilities and adjacent mine workings. As compared to 

shallow cover pillar extraction, there is an audible increase in coal 

pillar popping and roof thumping and bouncing at greater depths. 

Squeezes (also called rides or pillar runs) are nonviolent gradual 

pillar failures that cause noticeable coal sloughage and roof-to-floor 

convergence. It may take hours, days, or even weeks for a section to 

squeeze. As the pillars steadily fail, the overlying strata settle and the 

roof may break. Some squeezes which have occurred during idle 

shifts have resulted in equipment entrapments. Also, extensive 

portions of panels and mains have been abandoned due to squeezes. 

Other effects of a deep cover high stress regime can include 

excessive roof falls, pillar spalling, and floor heave. Failed panel 

design case histories attributed to roof falls were documented under 

both weak and competent immediate roof strata (Appendix 1). When 

mining under weak roof, the structural integrity of the rock may be 

sufficient enough to withstand development stresses; however, the 

strata may fail later when subjected to retreat mining induced 

abutment stresses, as was the case in a Colorado mine visited. 

Conversely, the beam building ability of a strong immediate and main 

roof rock units may inhibit caving. This can generate inordinate 

pillar line stresses, which, in turn, can produce severe pillar 

sloughage and floor heave. As the size of the worked-out area 

expands, the bridging capability of the roof may be exceeded and it 

caves. The result can be a powerful and potentially hazardous air 

blast. A sudden failure of a massive roof unit can also produce a 

hazardous “feather edge”which can override the breakers into the 

workings. The feather edge fracture has a conchoidal appearance, 

and is essentially a brittle failure phenomenon. Feather edge failures 

have been responsible for several fatalities in Australia (5), and at 

least one pillar line fatality in the U.S. 

Horizontal stress magnitudes also tend to increase with depth. 

Roof potting on development, cutters, and long running roof falls are 

all problems associated with horizontal stress. Horizontal stress may 

also be concentrated around the gob areas created by retreat mining. 

Some mines have experimented with stress control techniques like 

“advance-and-relieve” mining to improve conditions in operations 

subjected to high horizontal stresses (6, 7). 

In thicker coalbeds, overstressed pillars are prone to severe 

spalling and pose a serious threat to underground miners. Since 1995, 

rib roll fatalities have averaged more than one per year. In high coal, 

miners almost always indicate that one needs to pay more attention 

to the ribs than to the roof. Highly cleated coalbeds are particularly 

hazardous because these planes of weakness can define huge vertical 

slabs of coal which can roll over without warning (figure 1). Some 

mines experiencing cleat related rib rolls have been compelled to 

orient entries 45° to the face cleat to maintain safer travelways in both 

entries and crosscuts. However, this orientation can cause the cleat 

to segment the pillar corners into large triangular columns of coal 

which tend to fail into the intersections. After experiencing these 

various conditions, some operators have opted to drive entries at a 

low angle (25-30°) with respect to face cleat in an attempt to 

minimize rib sloughage problems. 

Figure 1. Vertical coal pillar slabs associated 

with face cleat. 

Floor failure can also be a deep cover operational issue. More 

typically, a competent roof tends to punch overstressed pillars into a 

weaker floor units causing heave in the roadways. Heave can be so 

extreme that equipment is not left in the working faces during idle 

shifts for fear of entrapment. Instances where it was necessary to use 

the continuous miners to regrade roadways for equipment clearance 

into the faces have also been documented. In one mine visited in 

southern West Virginia, approximately 4.5 ft of heave was observed 

just outby the pillar line in the 9 ft thick Beckley Coalbed (figure 2). 

Figure 2. Excessive pillar line floor heave. 
 

 

 

 

PANEL DESIGN 

Coal mine operators have employed different production panel 

design philosophies under deep cover. One strategy employed is to 

develop a wide section (9 or more entries) the entire length of the 

panel on advance, and then recover the pillars on retreat. With this 

approach, large production pillars are developed with the intent that 

they, and the adjacent barrier pillar(s), should be able to withstand all 

anticipated loading conditions encountered during panel advance and 

retreat. One drawback to this full panel advance and retreat method 

is that at greater depths, the production pillars can become too wide 

to be fully extracted with single pass pillaring techniques. Most 

operators indicate that once the entry centers exceed 80 ft and leaving 

significant stumps is undesirable, pillar splitting before extraction 

becomes the only alternative if the pillars are to be fully extracted. 

Pillar splitting is generally not desired because it requires numerous 

place changes and roof bolting. In a thick coal high stress regime, rib 

rolls pose a serious threat to bolter operators and splitting is generally 

avoided. When conducting full panel advance and retreat, some 

operators slab cut the barrier pillar(s) as they pull the section back. 

An alternative approach is the panel advance and rooming out on 

retreat method (figure 3). With this method, a narrow panel (4 or 5 

entries) is advanced, leaving a large barrier between the section and 

the previous panel gob. On retreat, rooms are driven into the barrier, 

and then these and the panel production pillars are recovered all the 

way across the section. This technique is a modified version of the 

Old Ben method (8) which was used in Illinois in the 1960's and 

1970's. One advantage of the panel advance and rooming method is 

that if problems are encountered on retreat, development into the 

barrier can be halted and a few rows of production pillars can be left 

intact so as to contain or isolate the problems inby. 

Figure 3. Panel advance and rooming out on 

retreat mining method. 

The “thin-pillar” technique is a variant of the panel advance and 

rooming method which has been used for bump control (9). With 

this approach, both development entries and rooms are driven on 

narrow centers to create pillars that are designed to yield as they are 

developed. The goal is to have the minimum amount of ground 

opened up at any time. However, extremely serious problems can 

arise if the pillar sizes, extraction sequence, timing, etc., are not 

designed and executed properly. If pillars are too large to yield yet 

too small to withstand the applied loadings, they can be prone to 

squeezes or bumps. 

Barrier pillars are an essential element in deep cover retreat mine 

design. Traditionally, barrier pillars have been employed to isolate 

active panels from adjacent gobs as a stress control technique. As the 

cover deepens, it becomes more important to isolate the active panel 

from side abutment loads transferred from the adjacent mined out 

workings by employing barriers pillars. An important design issue 

is just how wide the final remnant or inby barrier pillar (after rooming 

and/or slabbing) should be (figure 3). This topic is a critical and life 

threatening design concern in highly stressed environments because 

of the historically high occurrence of bump incidences during partial 

and full barrier pillar extraction (10). Campoli et al. (11) proposed 

just such a design method for sizing barrier pillars under deep cover. 

In the example he provided, no barrier was needed when the cover 

was less than 1,000 ft, but then the suggested barrier pillar width 

ranged from 150 to 240 ft as the cover increased from 1,200 to 

2,200 ft. It should be noted that leaving large remnant barrier pillars 

can cause loads to transfer to seams above and below. Therefore, 

when mines are in multiple seam configurations, pillar load transfer 

should be anticipated. Pillar load transfer can cause various ground 

control problems (12), including bumps (10). 

PILLAR EXTRACTION METHODS 

Deep cover operators practice both full and partial production and 

barrier pillar recovery during panel retreat. An operator’s rationale 

for electing one extraction method over another is usually based on 

factors including: equipment and timber availability and cost, pillar 

size, coalbed thickness, roof competency, and local custom. 

Approximately two thirds of the panels in the data base were 

extracted using either the Christmas tree or split and fender extraction 

methods (13). Of the two techniques, Christmas treeing is usually the 

one most favored by operators because it does not require place 

changes and bolting. Another extraction method practiced to fully 

recover large pillars is the pocket and wing procedure (13) which also 

requires place changes and bolting. Some operators indicated that if 

large pillars require splitting, that the split and fender method is 

preferred because if minimizes gob exposure as compared to the 

pocket and wing technique. In five panel designs studied, the outside 

lift method was used. In order to fully extract a pillar using this 

process with 40 ft extended cut lengths, the section needs to be driven 

up on narrow centers (60 ft or less). 

The most commonly cited reasons for opting for partial pillar 

recovery were safety and/or productivity. Some operators indicated 

that the roof rock in their mines was weak and sometimes fell 

prematurely on the pillar line. They also felt that the remnant stumps 

which remained after pillar recovery acted like coal cribs and 

provided just enough load bearing capacity to support the roof during 

the extraction process. Partial pillar recovery also reduces the number 

of turn posts required to extract a pillar. The sacrificed coal is 

justified based on safety and/or economics. In high coal, setting posts 

weighing 175 pounds or more requires three miners. One miner has 

to climb a step ladder which in itself can be hazardous. In addition, 

because far fewer posts are set during partial pillar recovery, miners 

minimize their exposure to rib rolls in high coal. Economically, 

setting posts is expensive and reduces production time. This is 
 

 

especially true in western mines where there is a scarcity of 

inexpensive hardwoods. In some of the 12 ft plus thick western 

reserves, some operators notion of retreat mining is only to mine the 

floor coal. To combat the posting issues, several operators have 

turned to mobile roof support usage (figure 4). 

Figure 4. Full pillar extraction using mobile roof supports. 

One of the more favored partial pillar recovery techniques is 

pillar splitting. Most typically the pillars are designed on narrow 

entry centers (60 ft or less) and crosscut centers are usually 100 ft or 

less. On retreat, from one to three extended cut lifts (splits) are taken 

from the entry or crosscut. Another popular partial pillar recovery 

method is slabbing, where successive adjacent lifts are removed from 

a pillar leaving a significant saw toothed remnant stump. These lifts 

are usually taken from the entry. If lifts are also taken from the 

crosscut, this technique is referred to as “L” slabbing (figure 5). 

When practicing partial pillar recovery under competent roof rock 

which does not cave, the possibility of a massive remnant pillar 

collapse occurring in the mined out workings is a distinct possibility. 

These events should be considered and preventive measurements 

taken because both the roof fall and the resultant air blasts can be life 

threatening and devastating (14). 

Figure 5. “L” slabbing on a super section using mobile roof 

supports. 

Most typically, pillars developed by mining into the barrier are 

extracted in the same manner as are the production pillars in the 

panel. One noticeable exception is a variation of the wongawilli 

technique (15) employed by a few southern WV mines. With this 

method, four rooms, up to 200 ft long, are driven on 50 ft centers into 

the barrier. The 30 ft pillars are then extracted by taking consecutive 

lifts as shown in figure 6. 

PREVIOUS PANEL GOB 

BARRIER PILLAR 

0 150 

1 2 

SCALE, FT 

3 

4 

5 

6 

7 

8 

Figure 6. Barrier pillar development and extraction using a
 

modified wongawilli technique.
 

DESCRIPTION OF THE DATA BASE 

During this investigation, 97 panel design case histories were 

gathered at 29 mines located in 7 states. At each mine, underground 

geotechnical data on the immediate roof rock quality, coalbed, and 

floor conditions were collected. Careful attention was also paid to 

documenting the various methods and strategies by which panels, 

production pillars and barrier pillars were developed and extracted. 

The following parameters were determined for each case history: 

•	 Roof Quality was evaluated using the Coal Mine Roof Rating 

(CMRR) system (16). The case histories were categorized as 

having weak (CMRR <45), intermediate (45<CMRR<65), 

and strong (CMRR>65) immediate roof rock conditions; 

•	 Panel Advance Width; 

•	 Panel Retreat Width (the panel advance width, plus rooms 

driven into and/or slab cuts taken from the barrier pillar(s) on 

retreat); 

•	 ARMPS SF using the normal default valves for in situ coal 

strength and the active mining zone; 

•	 Barrier Pillar SF determined using the ARMPS computer 

program, and; 

•	 Outcome, either success, squeeze, bump, or panel 

abandonment due to excessive roof falls. 

When examining the data base (figures 7-9), it was readily 

apparent that there were only a handful of weak immediate roof rock 

cases. A total of 8 weak immediate roof rock cases were collected, 

and half of those were failures. In addition, the deepest successful 

weak roof rock case history occurred at approximately 850 ft. Given 

the fact that 60% of the deep cover mines were investigated during 

this study, the authors contend that the scarcity of weak roof rock case 

histories is indicative of the deep cover mine population, and does not 
signify a data base quirk. Quite simply, based on past experiences, 

operators have determined that it is not feasible to mine under weak 

roof conditions in a deep cover, high stress regime. As for the 

remaining case histories in the data base, they were fairly evenly 

divided between “intermediate” and “strong” roof rock categories. 

5 

SE 

SUCCESS 

CA

S 

4 SQUEEZE 

STORIE

 OF 

BUMP 3

RE

ROOF FAILURE 

NUMB

HI

2 

1

750 1000 1250 1500 1750 2000 

DEPTH OF COVER 

Figure 7. Deep cover weak roof rock data base. 

20 

SE

SUCCESS 

AC

ES 

SQUEEZE 15 

F 

RI

 O

BUMP 

ERMB

HISTO10 ROOF FAILURE 

NU

5 

750 1000 1250 1500 1750 2000 

DEPTH OF COVER 

Figure 8. Deep cover intermediate strength roof rock data base. 

14 

SUCCESS 

E 

12 

SQUEEZE

 CAS

HISTORIES 10 BUMP 

F NUMBER O

8 ROOF FAILURE 

6 

4 

2 

750 1000 1250 1500 1750 2000 

DEPTH OF COVER 

Figure 9. Deep cover strong roof rock data base. 

The data base includes 16 bump and 14 squeeze failures. It 

should be noted that a majority of the squeezes (70%) occurred in the 

intermediate roof strength category, while 76% of the bumps 

happened under strong immediate roof rock conditions. Figures 7-9 

also show that when the depth of cover was less than 1,250 ft, most 

of the failed cases were squeezes. As for the immediate floor quality 

in the squeeze data base, 6 cases occurred where the floor was weak 

and 3 cases had an intermediate floor strength. Surprisingly, five 

squeezes happened in panels which had a strong immediate floor. In 

general, the bumps occurred under deeper cover and in wider panels 

as shown in Appendix 1. In the bump data base, it is important to 

note that in 64% of the cases barrier pillars were not employed to 

isolate active panels from adjacent side gobs. 

The use of barrier pillars also varied with depth. In the cases that 

were shallower than 1,300 ft, only 40% of the active panels were 

separated from adjacent gobs by barrier pillars. Deeper than 1,300 ft, 

68% of the panels used barrier pillars. Only 27% of the strong roof 

cases used barrier pillars, compared with 62% for the weak and 

intermediate cases. When the mines which were operating under 

strong roof did use barriers, the SF’s were often lower. 

Another interesting observation was that all 21 ARMPS Loading 

Condition 2 case histories (3) were successful. In Loading Condition 

2, side abutment load transfer does not occur because the adjacent 

panels (if any have been driven) have not been retreat mined. 

Therefore, the program considers these areas as being unmined coal 

or, infinitely large barrier pillars. 

DATA ANALYSES 

Figure 10 compares the ARMPS SF’s, depth of cover and 

outcomes for approximately 250 shallow, moderate and deep cover 

panel design case histories. Analyses indicate that an ARMPS SF of 

1.5 or greater is appropriate where the depth of cover is less than 

650 ft. As the cover increases from 650 to1,250 ft, there seems to be 

a decreasing trend in SF’s for both the successful and the unsuccessful 

cases. However, deeper than 1,250 ft, there does not seem to be any 

clear trend. These observations, combined with the fact that the most 

common failure mechanism shifts from a squeeze to a bump at 

approximately 1,250 ft of cover, seems to justify separating the data 

into two groups by depth. Logistic regression was used to analyze the 

two groups. The failures were weighted as two in order to balance the 

data. Because of the small number of weak immediate roof rock 

cases, they were added to the intermediate strength roof rock 

category. 

3 

Satisfactory 

2.5 

Unsatisfactory 

2 

ARMPS SF 1.5 

1 

0.5 

0 

0 500 1000 1500 2000 

Depth of Cover (ft) 

Figure 10. ARMPS case history data base. 
1.5

When considering the cover (H) group ranging from 650 to 

1,250 ft, the only two variables which were significant at the 0.15 

level were the immediate roof rock quality and the ARMPS SF. The 

analyses also confirmed that the necessary ARMPS SF could be 

reduced if the immediate roof is strong. For the deepest cover (H 

>1,250 ft) grouping, the only two significant variables at the 0.15 

level were the immediate roof rock quality and the barrier pillar 

stability factor. Again, strong immediate roof permitted a reduction 

in the suggested SF. Figure 11 compares the ARMPS SF, barrier 

pillar SF and the outcomes for the 57 case histories where the depth 

of cover was 1,000 ft or greater. As shown in figure 11, out of 12 

cases, only one failure occurred when the ARMPS SF was greater 

than 0.8 and the barrier pillar SF was greater than 2.0. Conversely, 

30 case histories had an ARMPS SF less than 0.8 and a barrier pillar 

SF less than 2.0, and 60% of these cases were failed designs. Of 

these 18 failed designs, 13 were bump events. In addition, every 

bump case history collected had a barrier pillar SF of less than 1.9. 

Based on these analyses, conservative design guidelines are proposed 

in Table 1. It should be noted that when examining figures 10 and 

11, there are numerous successful case histories with stability factors 

less than those suggested in Table 1. Therefore, the 

recommendations proposed in Table 1 should be considered as first 

approximation design guidelines which should be tempered with other 

cite specific variables deemed relevant based on past experiences and 

sound engineering judgement. Finally, regression analyses also 

indicated that narrower panels reduced the required SF, but only at 

the 0.25 significance level. 

OUTBY BARRIER PILLAR STABILITY FACTOR 6 

5 

4 

3 

2 

1 

0 

STRONG ROOF ROCK SUCCESS 

STRONG ROOF ROCK FAILURE 

INTERMEDIATE OR WEAK ROOF ROCK SUCCESS 

INTERMEDIATE OR WEAK ROOF ROCK FAILURE 

0 0.5 1

ARMPS STABILITY FACTOR 

Figure 11. ARMPS and barrier pillar stability factors for the 

overburden exceeding 1,000 ft. data base. 

Table 1. Pillar design considerations 

Immediate roof rock 

quality 

Weak and 

intermediate roof 

strength 

Strong roof 

ARMPS SF 

650 ft < H # 1,250 ft 15 . − 





H − 650 



 14 . − 





H − 650 



 1000 1000 

1,250 < H # 2,000 ft 0.9 0.8 

Barrier pillar SF 

H > 1,000 ft $2.0 $1.51 

$2.02 

1

Nonbump prone ground 

2

Bump prone ground 

DISCUSSION 

One of the rationales for this research endeavor was the 

observation that lower ARMPS stability factors may be successfully 

employed when mining at deeper cover. There are two plausible 

explanations for this: 

•	 The actual pillar strengths of the larger pillars used at depth 

are greater than that predicted by Mark-Bieniawski formula 

used in ARMPS, or; 

•	 The actual pillar loadings are less than ARMPS predicts. 

Recent research indicates that the immediate roof strength may be 

related to pillar strength for squat pillars (large w/h ratios). For 

example, data collected by Gale (17) indicates a wide range in 

measured strengths for pillars having the same width-to-height ratio. 

He attributed these strength differences to pillar confinement or, lack 

thereof. Gale concluded that strong immediate roof rock units with 

high shear strength can generate greater pillar confinement which 

increases the pillars strength. 

Pillar loading may be affected by both the geology and the depth 

of cover. Where dealing with strong roof members at depth, the beam 

forming ability of stiffer immediate and main roof rock units may 

more readily transfer and equally distribute the mining induced loads 

to nearby abutments and barrier pillars. Conversely, where mining 

under weaker roof, one would expect the load transfer to be more 

problematic. Using field stress measurements collected in some of 

the deeper Australian coal mines, Colwell et al. (18) back-calculated 

lower abutment angles than the 21° default angle which ARMPS uses. 

In fact, it was noted that: “the abutment angles calculated for the two 

deepest mines, are the smallest of any in the database, 5.9 and 8.5°.” 

An examination of the Australian database also indicates that for the 

most part, an abutment angle of 21° is reasonable for the generally 

shallow supercritical panels (panel depth to panel width ratio less than 

approximately 1.3). For the normally deeper, subcritical panels 

which have higher depth-to-width ratios (H/P), lower abutment angles 

are warranted. 

In another relevant article, Heasley (19) using LAMODEL 

suggests that the constant abutment angle concept employed by 

ARMPS probably over predicts the amount of abutment load as the 

depth of cover increases. Heasley thought it unreasonable that the 

gob loading remain constant after H/P exceeds 1.3. Heasley contends 

that “if the overburden displacement is considered to be linearly 

proportional to the depth, and the gob material is strain-hardening, 

then the gob should support an increasing percentage of load as the 

panel gets deeper.” He also suggested that some type of systematic 

abutment angle reduction with increased depth might be more 

realistic. 

In order to examine Heasley’s suppositions, the SF’s for the 

database were recalculated using adjusted abutment angles backcalculated from the laminated overburden model with a constant 

lamination thickness. As was expected, there was marked increase in 
 

 

calculated SF’s for subcritical panels as the depth of cover increased. 

However, no apparent correlations between the adjusted SF’s and 

panel performance could be established. A more concentrated effort 

in this endeavor is warranted if the pillar mechanics of deep cover 

recovery is to be fully understood. 

CONCLUSIONS 

1.	 Ground control problems associated with pillar extraction 

generally intensify with increased depth. Conditions responsible 

for failed panel design case histories documented during this 

investigation include: bumps, squeezes, or excessive roof falls 

which caused large portions of, or entire panels to be abandoned. 

2.	 Past research suggests that under shallow to moderate cover, an 

ARMPS SF of 1.5 seems to be appropriate. The data collected 

during this investigation indicates that where the depth cover 

exceeds 650 ft, lower ARMPS SF’s can be successfully 

employed. In the overburden range between 650 and 1,250 ft, 

immediate roof rock quality and ARMPS SF were determined to 

be the significant variables. Greater than 1,250 ft, roof rock 

quality and barrier pillar design were concluded to be the 

significant variables. 

3.	 Currently, deep cover operators are more likely to employ barrier 

pillars where the depth of cover exceeds 1,300 ft; however, their 

usage is not as widespread as one would anticipate. The data 

collected during this investigation substantiates the utility of 

barrier pillars to isolate active panels from nearby gobs where the 

depth of cover exceeds 1,000 ft. This is especially true in highly 

stressed, bump prone ground conditions. 

4.	 Analyses of the database indicates that roof rock quality is an 

integral component in the panel design process. ARMPS SF’s 

for production and barrier pillars can be lower when the 

immediate roof is strong (CMRR>65). Conversely, under 

weaker roof conditions, operators should consider advancing 

narrower panels and deploying larger barrier pillars to isolate the 

active working from adjacent gob areas. 

5.	 The data suggests that squeezes are the predominate failure mode 

in mines operating at moderate depths with intermediate strength 

immediate roof rock conditions. However, bumps typically 

occur at greater depth and under stronger roof rock units. 

6.	 A conservative approach to panel design for deep cover pillar 

recovery is to advance a narrow panel which is separated from 

the adjacent gob with a large barrier pillar. On retreat, rooms can 

be driven into the barrier pillar to extract a portion of it. In bump 

prone ground conditions, past experiences and sound engineering 

judgement should be employed when determining how wide the 

final or inby barrier pillar should be so as to isolate the workings 

from adjacent gobs. Information collected during this 

investigation indicates that when the barrier pillar SF was greater 

than 1.9, no bumps occurred. 

7.	 This investigation confirmed that there is a decreasing trend in 

satisfactory ARMPS SF’s as the depth of cover increases. It is 

possible, as other researchers have postulated, that ARMPS’s 

constant abutment angle concept over predicts the abutment 

loads and underestimates the gob loading in subcritical panel 

designs. In this case, some type of systematic abutment angle 

reduction with increased depth might be warranted. However, a 

greater understanding of deep cover pillar mechanics is necessary 

to calibrate this reduction and this topic warrants future research 

efforts. 

REFERENCES 

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

of the Retreat Mining of Coal Pillars in the U.S. Proceeding, 

New Technology for Ground Control in Retreat Mining, U.S. 

Department of the Interior, Bureau of Mines, IC 9446, 1997, 

pp. 2-16. 

2.	 MSHA. Quarterly Employment and Coal Production: Accident/ 

Injuries/Illnesses Reported to MSHA under 20 CFR Part 50. U.S. 

Department of Labor, Mine Safety and Health Administration, 

Office of Injury and Employment Information, Denver, CO, 

1995-1999. 

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

Stability (ARMPS). Proceedings, New Technology for Ground 

Control in Retreat Mining, U. S. Department of the Interior, 

Bureau of Mines, Pittsburgh, PA, IC 9446, 1997, pp. 17-34. 

4.	 Mark, C. Empirical Methods for Coal Pillar Design. 

Proceedings, Second International Workshop on Coal Pillar 

Mechanics and Design, NIOSH IC 9448, 1999, pp. 145-154. 

5.	 The University of New South Wales, School of Mines. Strata 

Control for Coal Mine Design. Strata Control Newsletter, Issue 

No. 1, April,1993, 8 pp. 

6.	 Chase, F.E. Mark, C., Mucho, T.P., Campbell, P.L. and 

Holbrook, D.A. The Advance and Relieve Method: A Horizontal 

Stress Control Technique. Proceedings, 18th International 

Conference on Ground Control in Mining, Morgantown, WV, 

1999, pp. 300-308. 

7.	 Dolinar, D.R., Mucho, T.P., Oyler, D.C. and Pablic, J. Utilizing 

the Advance and Relieve Method to Reduce Horizontal Stress 

Affects on the Mine Roof, A Case Study. Proceedings, 19th 

International Conference on Ground Control in Mining, 

Morgantown, WV, 2000, pp. 137-148. 

8.	 The University of New South Wales, School of Mines. Strata 

Control for Coal Mine Design. Strata Control Newsletter, Issue 

No. 2, August 1993, 8 pp. 

9.	 Mucho, T.P., Barton, T.M. and Compton, C.S. Room and Pillar 

Mining in Bump Prone Conditions and Thin Pillar Mining as a 

Bump Mitigation Technique. U. S. Department of the Interior, 

Bureau of Mines, Pittsburgh, PA, RI 9489, 1993, 13 pp. 

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

Remediation of Coal Mine Bumps: A Historical Review. 

Proceedings, Mechanics and Mitigation of Violent Failure in Coal 

and Hard Rock Mines, U. S. Department of the Interior, Bureau 

of Mines, Pittsburgh, PA, Special Publication 01-95, 1995, pp. 

27-68. 

11. Campoli, A.A., Mucho, T.P. and Zipf, R.K. Bump Control 

Design Protocol for Room and Pillar Coal Mining. Proceedings, 
 

Mechanics and Mitigation of Violent Failure in Coal and Hard 

Rock Mines, U. S. Department of the Interior, Bureau of Mines, 

Pittsburgh, PA, Special Publication 01-95, 1995, pp. 181-199. 

12. Haycock, C. and Zhou, Y. Multiple-Seam Mining–A State-ofthe-Art Review. Proceedings, 11th International Conference on 

Ground Control in Mining, The University of Wollongong, 

WSW, 1990. 

13. Kauffman, P.W., Hawkins, S.A. and Thompson, R.R. Room and 

Pillar Retreat Mining (A Manual for the Coal Industry. U.S. 

Department of the Interior, Bureau of Mines, IC 8849, 1981, 

228 pp. 

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

Collapses in Coal Mines. Proceedings, New Technology for 

Ground Control in Retreat Mining, U.S. Department of the 

Interior, Bureau of Mines, Pittsburgh, PA, IC 9446, 1997, pp. 35­

49 

15. Shepherd, J. and Chaturvedula, V.K.. Strata Mechanics of Pillar 

Extraction Goaf Edges. Proceedings,11th International 

Conference on Ground Control in Mining, The University of 

Wollongong, WSW, 1992, pp. 13-21. 

16. Molinda, G.M. and Mark, C. Coal Mine Roof Rating (CMRR): 

A Practical Rock Mass Rating for Coal Mines. U.S. Department 

of the Interior, Bureau of Mines, IC 9387, 1994, 83 pp. 

17. Gale, W. J. Experience of Field Measurement and Computer 

Simulation Methods of Pillar Design. Proceedings, 2nd 

International Workshop on Coal Pillar Mechanics and Design, 

NIOSH Publication No. 99-114, IC 9448, 1999, pp. 49-61. 

18. Colwell, M., Frith, R. and Mark, C. Analysis of Longwall 

Tailgate Serviceability (ALTS): A Chain Pillar Design 

Methodology for Australian Conditions. Proceedings, 2nd 

International Workshop on Coal Pillar Mechanics and Design, 

NIOSH Publication No. 99-114, IC 9448, 1999, pp. 33-49. 

19. Heasley, K.A. The Forgotten Denominator; Pillar Loading. 

Proceedings, 4th North American Rock Mechanics Symp., Pacific 

Rocks 2000, Seattle, WA, July 31-Aug 3, 2000, pp. 457-464. 
Inby Panel Panel Pillar Ext ARMPS Outby Outby Inby Name Case H h w/h LC BP advance retreat Roof Floor Ext. meth. Comments Size pct SF BP width BP SF BP SF 

 

width width width 

CO Mine A 1 1,560 9 50 x 110 40 5.5 3 0.6 0 0 0 0 230 230 W I L Slab Miners entrapped by coal 

pillar bump. 

CO Mine B 1 800 11.5 65 x 70 41 5.6 2 0.95 - - - - 780 780 I I 2-1/2 OL from Satisfactory design. 

entry & floor coal 

CO Mine C 1 850 9 60 x 100 38 6.7 2 1.28 - - - - 320 630 W I OL Satisfactory design. 

CO Mine C 2 850 9 50 x 110 40 5.6 2 1.2 - - - - 370 470 W I OL Satisfactory design. 

CO Mine D 1 750 7 42 x 100 41 6 4 0.95 0 0 0 0 260 730 I I Partial OL Satisfactory design. 

0 0 0 0 

CO Mine D 2 750 7 42 x 100 41 6 3 1.21 0 0 0 0 340 500 I I Partial OL Satisfactory design. 

CO Mine D 3 800 7 32 x 82 48 4.6 3 0.76 0 0 0 0 220 495 I I Partial OL Satisfactory design. 

CO Mine D 4 950 7 42 x 100 41 6 3 0.94 50 1.1 50 0.56 335 500 I I Partial OL Several rows of pillars lost 

due to excessive loading 

under deepest panel cover. 

CO Mine D 5 1,100 7 42 x 100 41 6 4 0.59 0 0 0 0 260 730 I I Partial OL Three rows of pillars lost 

0 0 0 0 due to heave and sloughage 

under deepest panel cover. 

CO Mine E 1 1,250 8.5 30 x 80 52 3.5 4 0.36 135 2.37 135 1.61 170 270 S S Double split & Excessive heave and floor 

0 0 0 0 floor coal bumps caused panel to be 

removal abandoned. 

CO Mine E 2 1,250 8.5 30 x 80 52 3.5 3 0.39 0 0 0 0 170 270 S S Single & double Satisfactory design. 

split and floor 

coal removal 

CO Mine E 3 1,250 8.5 30 x 80 52 3.5 3 0.36 0 0 0 0 170 370 S S Double split & Satisfactory design. 

floor coal 

removal 

CO Mine E 4 1,250 8.5 30 x 80 52 3.5 3 0.36 0 0 0 0 170 370 S S Double split & Satisfactory design. 

floor coal 

removal 

CO Mine E 5 1,700 8.5 30 x 80 52 3.5 4 0.22 130 1.8 130 1.0 170 370 S S Single & double Overstressed pillars next to 

0 0 0 0 split & floor coal the barrier were abandoned 

removal due to severe bumping. 

CO Mine E 6 1,750 8.5 30 x 80 52 3.5 3 0.3 0 0 0 0 170 370 S S Double split & Satisfactory design. 

floor coal 

removal 

CO Mine E 6 2,000 8.5 30 x 80 52 3.5 2 0.39 - - - - 170 170 S S Single split and Satisfactory design. 

floor coal 

removal 

Appendix 1.--Deep cover case history data base 

6 
 Inby Panel Panel Pillar Ext ARMPS Outby Outby Inby Name Case H h w/h LC BP advance retreat Roof Floor Ext. meth. Comments Size pct SF BP width BP SF BP SF width width width 

CO Mine E 7 2,000 8.5 30 x 80 52 3.5 4 0.16 125 1.5 125 .71 170 270 S S Double split & Down dip pillars by barrier 

0 0 0 0 floor coal pillar and side gob bumped. 

removal 

CO Mine E 8 2,000 8.5 30 x 80 52 3.5 4 0.22 125 1.6 125 1.0 170 270 S S Single split & Richter 3.7 bump event 

0 0 0 0 floor coal shook surface facilities 

removal 

CO Mine F 1 882 6.3 60 x 60 44 9.5 3 1.11 0 0 0 0 400 400 I I S&F Satisfactory design. 

CO Mine F 2 889 11 50 x 60 46 4.5 4 0.57 60 0.99 60 .49 520 680 I W S&F Satisfactory design. 

238 4.57 238 4.84 

CO Mine F 3 961 5 50 x 60 46 10 3 1.14 65 1.97 65 1 280 280 I W S&F Satisfactory design. CO Mine F 4 961 5 50 x 50 49 10 3 0.89 0 0 0 0 420 420 I W S&F Lost 3 rows of pillars due to 

excessive pressures. 

CO Mine F 5 1,250 5 40 x 80 47 8 3 0.75 95 2.13 65 0.71 240 270 W W Xmas Roof falls over-rode the 

breakers and the section was abandoned. 

KY Mine A 1 878 8 61 x 61 42 7.6 4 0.58 0 0 0 0 340 340 S S S&F Excessive pressures in 

0 0 0 0 pillar point caused 3 rows 

of pillars to be lost. 

KY Mine A 2 1,166 7 61 x 61 42 8.7 3 0.75 29 0.71 29 0.18 340 570 I S S&F Lost 6 rows of pillars in 

squeeze. 

KY Mine A 3 1,193 7.2 61 x 61 42 8.5 3 0.89 160 2.5 160 2.2 340 560 S S S&F Satisfactory design. 

KY Mine A 4 1,235 6.7 61 x 61 42 8.7 3 0.74 0 0 0 0 340 590 S S S&F Severe bump fatally injured 

2 miners in pillar point. 

KY Mine A 5 1,290 7 41 x 51 50 5.9 4 0.26 0 0 0 0 260 260 S S S&F Moderate coal pillar bump 

0 0 0 0 pushed the continuous 

miner out of the lift. 

KY Mine A 6 1,366 5 61 x 61 42 12.2 3 0.72 0 0 0 0 340 980 S S S&F Lost 4 rows of pillars due to 

excessive pressures. 

KY Mine A 7 1,489 8 61 x 81 38 7.6 3 0.71 140 1.7 130 1.1 340 450 S S S&F Satisfactory design. KY Mine A 8 1,630 7 61 x 81 38 8.7 3 0.83 140 2.21 140 1.31 500 500 S S S&F Satisfactory design. 

KY Mine B 1 1,300 4.5 36 x 61 50 8 3 0.69 110 2.56 65 0.78 350 380 I S Xmas Satisfactory design. 

KY Mine B 2 1,600 5 61 x 74.8 40 12.2 3 1.05 175 3.13 140 1.8 340 370 S S Xmas Satisfactory design. 

KY Mine B 3 1,700 5 51 x 61 44 10.2 3 0.47 50 0.6 20 0.04 625 655 S S Xmas Satisfactory design. 

KY Mine B 4 1,850 5 66 x 74.8 39 13.2 3 0.82 150 2.37 120 1.08 360 390 I S Xmas Satisfactory design. KY Mine B 5 1,950 5 66 x 71 39 13.2 3 0.72 150 2.21 115 0.92 360 390 I S Xmas Satisfactory design. 

KY Mine C 1 800 4.2 23 x 50 64 5.5 3 0.57 35 1.32 10 0.07 335 365 W W OL Lost 14 rows of pillars in 

squeeze. 

Appendix 1.--Deep cover case history data base 

7 
 

7 

Inby Panel Panel Pillar Ext ARMPS Outby Outby Inby Name Case H h w/h LC BP advance retreat Roof Floor Ext. meth. Comments Size pct SF BP width BP SF BP SF width width width 

KY Mine D 1 1,000 10 50 x 50 49 5 4 0.34 0 0 0 0 370 370 I S S&F While retreating bottle 

0 0 0 0 necked mains, numerous 

fenders were lost due to excessive loading. 

KY Mine E 1 775 6 35x60 53 5.8 3 0.84 80 2.24 40 0.53 355 440 I I Xmas Satisfactory design. 

KY Mine E 2 800 5.6 35x60 52 6.3 2 1.12 0 0 0 0 345 410 S I Xmas Satisfactory design. 

KY Mine E 3 800 6 35x60 53 5.8 3 0.81 80 2.16 40 0.51 360 440 I I Xmas Satisfactory design. 

KY Mine E 4 800 4.3 70x70 41 16.3 3 2.43 150 5.9 120 4.6 380 440 I I Xmas Satisfactory design. 

KY Mine E 5 1400 5.5 35x50 56 6.4 3 0.42 100 1.83 70 0.61 350 410 I I Xmas Satisfactory design. 

KY Mine E 6 1400 4.3 35x65 52 8.1 3 0.57 90 1.99 60 0.56 355 420 I I Xmas Satisfactory design. KY Mine E 7 1425 4.3 70x60 43 14 3 1.25 220 5.25 190 4.16 380 410 I I Xmas Satisfactory design. 

KY Mine E 8 1500 4.3 35x60 44 8.1 3 0.84 90 1.78 50 0.36 355 440 I I Xmas Moderate bump caused face 

equipment damage. 

KY Mine E 9 1600 4.3 60x65 44 14 3 0.75 75 1.4 45 0.27 370 435 I I Xmas Severe bump pushed 

continuous miner back 15 feet out of the lift and broke the frame. 

KY Mine E 10 1700 7.4 70x50 44 6.8 3 0.45 0 0 0 0 375 395 I I Xmas Moderate bump events 

caused several pillars to be abandoned. 

KY Mine E 11 1900 4.3 60x70 43 14 3 0.94 170 3.15 140 1.6 320 380 I I Xmas Satisfactory design. 

KY Mine F 1 764 5.7 35 x 60 52 6.1 2 1.26 - - - - 350 240 S S OL Satisfactory design. 

PA Mine A 1 806 7.2 60 x 60 39 8.3 4 1.13 0 0 0 0 710 710 W W P&W Lost 115 pillars overnight 

squeeze, 

PA Mine A 2 853 7.2 70 x 80 34 9.7 2 2.71 - - - - 539 539 W W P&W Satisfactory design 

TN Mine A 1 1,000 2.5 40 x 35 61 14 3 0.87 35 1.58 0 0 260 290 I W OL Majority of panel lost due 

to squeeze. 

TN Mine A 2 1,026 2.5 35 x 30 66 12 2 0.98 - - - - 240 270 I W OL Satisfactory design. 

TN Mine A 3 1,026 2.5 35 x 30 66 12 3 1.2 180 9.87 180 14.7 240 240 I W None Squeezed caused 2,200 ft of 

mains to be abandoned. 

UT Mine A 1 1,200 8.4 65 x 65 0.42 7.7 2 0.86 - - - - 350 440 l I L Slab Satisfactory design. 

UT Mine B 1 1,000 7 60 x 60 0.44 8.6 3 0.72 0 0 0 0 180 340 S W Partial Xmas Satisfactory design. 

UT Mine B 2 1,100 9 70 x 80 0.38 7.8 3 0.91 215 2.59 115 1.17 380 410 S I S&F Satisfactory design. 

UT Mine B 3 1,100 7 60 x 60 0.44 8.6 3 0.82 0 0 0 0 590 590 S W Partial Xmas Satisfactory design. 

UT Mine B 4 1,200 7.5 60 x 60 0.44 8 3 0.55 0 0 0 0 420 420 S W Partial Xmas Moderate squeeze occurred 

at pillar point. 

UT Mine B 5 1,200 9 80 x 70 0.38 7.8 2 0.93 - - - - 400 600 S I S&F Satisfactory design. 

Appendix 1.--Deep cover case history data base 
 Inby Panel Panel Pillar Ext ARMPS Outby Outby Inby Name Case H h w/h LC BP advance retreat Roof Floor Ext. meth. Comments Size pct SF BP width BP SF BP SF width width width 

UT Mine B 6 1,200 5.5 60 x 60 0.44 10.9 2 1.25 - - - - 340 340 S W S&F Satisfactory design. UT Mine B 7 1,500 9 60 x 80 0.4 6.7 4 0.43 60 0.61 60 0.28 350 380 I I L Slab Satisfactory design. 

350 4.34 300 4.36 

UT Mine B 8 1,600 9 70 x 80 0.38 7.8 2 0.82 - - - - 380 410 I I L Slab Satisfactory design. 

UT Mine C 1 800 7.5 63 x 63 0.42 8.4 3 1.03 0 0 0 0 415 415 S S S&F Satisfactory design. 

UT Mine C 2 800 7.5 63 x 63 0.42 8.4 3 0.93 0 0 0 0 350 350 S S S&F Satisfactory design. 

UT Mine C 3 800 8.2 63 x 63 0.42 7.7 4 0.68 0 0 0 0 350 350 S S S&F Excessive bumping caused 

panel abandonment. Panel located in ridge nose. 

UT Mine C 4 1,000 8 63 x 63 0.42 7.9 3 0.77 0 0 0 0 350 350 S S S&F Satisfactory design. 

UT Mine C 5 1,000 8 63 x 63 0.42 7.9 2 0.93 - - - - 1,100 1,100 S S S&F Satisfactory design. 

UT Mine C 6 1,200 6.6 63 x 63 0.42 9.5 3 0.67 0 0 0 0 250 250 S S S&F Satisfactory design. 

UT Mine D 1 1,500 9 60 x 60 44 6.7 3 0.61 145 1.88 145 1.48 500 415 S S S&F Three rows of pillars 

bumped. 

UT Mine D 2 1,650 9 60 x 60 44 6.7 3 0.5 105 1.29 105 0.88 500 420 S S S&F A Richter 3.6 bump event 

occurred when 7 rows of pillars failed violently. 

UT Mine D 3 2,000 9 60 x 60 44 6.7 2 0.5 - - - - 500 425 S S S&F Satisfactory design. 

VA Mine A 1 1,700 5.5 60 x 60 44 10.9 3 0.61 0 0 0 0 340 340 S S S&F Moderate coal pillar bump. 

VA Mine B 1 790 5.5 35 x 50 55 6.4 2 1.15 - - - - 240 240 W W 2 Cut Satisfactory design. 

WV Mine A 1 970 5.5 40 x 60 50 7.3 2 1.06 - - - - 440 470 I W Xmas Satisfactory design. 

WV Mine A 2 1,054 5 60 x 60 44 12 2 1.38 - - - - 500 530 I W Xmas Satisfactory design. 

WV Mine B 1 750 8 50 x 70 44 5 2 1.17 - - - - 580 580 S S S&F Satisfactory design. 

WV Mine B 2 750 8 50 x 70 44 5 3 1.04 59 1.53 59 0.84 580 580 S S S&F Stable LC2 development 

pillars protecting mains 

failed after adjacent panel was pillared. 

WV Mine B 3 800 7 50 x 70 44 7.1 3 0.95 0 0 0 0 440 440 S S S&F Satisfactory design. WV Mine B 4 900 6 50 x 70 44 8.3 3 0.98 45 1.15 45 0.48 470 470 S S S&F Satisfactory design. WV Mine B 5 1,000 6 70 x 70 40 11.7 3 1.07 0 0 0 0 560 560 S S S&F Satisfactory design. 

WV Mine C 1 1100 6 55 x 77 44 9.2 3 0.94 80 1.7 45 0.38 300 360 I S Xmas Lost 4 rows of pillars in 

squeeze. 

WV Mine C 2 1100 6 50 x 69 46 8.3 3 0.72 70 1.2 35 0.21 310 370 I S Xmas Satisfactory design. WV Mine C 3 1100 6.5 50 x 79 43 7.7 2 0.98 - - - - 650 650 I S Xmas Satisfactory design. 

WV Mine D 1 750 7.5 40 x 80 47 5.3 4 0.84 80 1.99 50 .71 315 505 I I Xmas Satisfactory design. 

0 0 0 0 

WV Mine D 2 750 7.5 50 x 70 44 6.7 3 1.05 45 1.29 45 0.59 360 360 I I 2 cut Satisfactory design. 

Appendix 1.--Deep cover case history data base 

9 
 Inby Panel Panel Pillar Ext ARMPS Outby Outby Inby Name Case H h w/h LC BP advance retreat Roof Floor Ext. meth. Comments Size pct SF BP width BP SF BP SF width width width 

WV Mine D 3 750 4.7 35 x 80 49 7.4 4 0.98 60 2.15 30 .42 300 465 I S Xmas Satisfactory design. 

0 0 0 0 

WV Mine D 4 750 7.5 40 x 60 50 5.3 2 1.09 - - - - 320 350 I I Xmas Satisfactory design. 

WV Mine D 5 900 4.7 35 x 70 51 7.4 3 1.04 0 0 0 0 300 465 I S Xmas Satisfactory design. 

WV Mine D 6 900 7.5 40 x 80 47 5.3 3 0.83 0 0 0 0 500 675 I I Xmas Satisfactory design. 

WV Mine D 7 950 7.5 40 x 60 50 5.3 3 0.71 60 1.59 60 0.83 560 560 I I Xmas Heavily loaded outby 

workings caused panel to be abandoned. 

WV Mine E 1 850 9 50 x 50 49 7.1 4 0.43 0 0 0 0 370 370 S W Xmas Pillar point roof fall had 

0 0 0 0 continuous miner buried for 

2 weeks. Excessive heave. 

WV Mine E 2 900 7 50 x 50 49 7.1 3 0.67 0 0 0 0 440 620 S I Xmas Satisfactory design. 

WV Mine E 3 1,150 6 55 x 70 43 9.2 4 0.6 43 0.89 43 0.32 330 490 S S Split & Xmas Pillar point bump caused 

0 0 0 0 lost time injury. 

WV Mine F 1 825 4.5 50 x 50 49 11.1 4 0.91 0 0 0 0 440 440 S S P&W Moderate bump occurred in 

0 0 0 0 0 pillar point. 

WV Mine G 1 850 4.5 60 x 40 50 8.9 2 1.24 - - - - 660 660 S I Xmas Satisfactory design. 

Legend: 

Appendix 1.--Deep cover case history data base 

BP - barrier pillar Ext pct - extraction percentage on advance h - mining height H - overburden I - intermediate rock strength LC - loading condition OL - outside lift P&W - pocket and wing S - strong rock strength S&F - split and fender SF - stability factor W - weak rock strength w/h - width-to-height ratio Xmas - Christmas tree