SIZING OF FINAL STUMPS FOR SAFER PILLAR EXTRACTION
 

Christopher Mark, Chief, Rock Mechanics Section
 

NIOSH, Pittsburgh Research Laboratory
 

Pittsburgh, PA
 

Joseph C. Zelanko, Mining Engineer
 

PS&HTC
 

Mine Safety and Health Administration
 

Pittsburgh, PA
 

ABSTRACT 

Pillar recovery continuestobe one of the more hazardous activities 

in underground coal mining. Safety requires that the roof above the 

intersection remain stable until after the pillar has been extracted. 

Artificialsupports (timbers and Mobile Roof Supports) are essential to 

roof stability, but so is the final remnant stump or pushout. Traditional 

mining practices usually called for the complete extraction of the final 

stump, but the recent trend (both in the U.S. and internationally) seems 

to be towards mining plans that leave a remnant stump. 

For this study, a sample of roof control plans from the Mine Health 

and Safety Administration (MSHA) Coal Districts were analyzed to 

determine current pillar recovery practices. Both full- and partialextraction plans were included. Special attention was paid to whether 

plans require that a final stump be left, and whether requirements 

regarding the dimensions of the remnant stump are included. Foreign 

experience with final stumps is also summarized. 

It seems that the risk of major pillar falls can often be reduced by 

leaving final stumps that are large enough to protect the intersection, 

but small enough that they do not inhibit the caving of the gob. Because 

finalstumps are often irregular in shape, a new approach for estimating 

their strength is described. Analyses were conducted to assess the 

effect of seamheight and depth of cover on the potentialvariationin the 

size of remnant stumps. 

INTRODUCTION 

Although rarely in the spotlight, pillar recovery continues to be an 

important segment of the U.S. coal industry. Technology developed 

during the last 15 years, particularly mobile roof supports (MRS) and 

extended cuts, have improved both safety and productivity. Pillar 

recovery appeals particularly to room-and-pillar mines that cannot 

achieve acceptable coal recovery from development mining only. 

Today, less than 10% of underground coal comes from the second 

mining of pillars (1). 

1983198419851986198719881989199019911992199319941995199619971998199920002001 

0 

1 

2 

3 

4 

5 

6 

7 

No. of Incidents 

Non-Final Lift Incidents 

Final Lift Incidents 

Year 

Figure 1. Fatal accidents occurring during pillar recovery operations, 1983-2001. Data current as of June 1, 2001. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In the past, pillar recovery has been associated with 

disproportionate rates of roof fall fatalities (1). Between 1980 and 

1997, nearly 25% of all roof fall fatals occurred on pillar sections. 

More than one-third of these took place during the mining of the final 

stump (or the last lift or pushout). The last five years, however, have 

seen a marked reduction in the number of pillar recovery fatalities 

(figure 1). More widespread use of MRS for roadway support can 

undoubtedly take some of the credit for the improvement. However, 

the authors believe that another important factor has been that more 

mines are using cut sequences that leave a final stump rather than 

trying to extract the entire pillar. 

The goals of this paper are to document the role of final stumps 

in pillar recovery, and to help mine operators use them to reduce the 

risk of serious accidents. The paper begins with a discussion of the 

mechanics of pillar recovery. It then looks at recent trends in pillar 

recovery strategies, using representative Roof Control Plans from all 

the pertinent Districts. International experience with final stumps is 

also cited. Next,it analyzes MSHA FatalAccident Reportsinvolving 

finalstumps from the past 18 years. The size of the rock fall, the size 

of the final stump, the mining sequence, and other key variables are 

evaluated. Finally, issues involved in sizing final stumps are discussed, 

and placed in the context of an overall pillar recovery strategy. 

MECHANICS OF PILLAR EXTRACTION 

Full pillar extraction has always involved a basic contradiction. 

On the one hand, the ground needs to cave to minimize the loads on 

the active pillar line. But on the other, the caving must not occur until 

after the miners have safely left the area. 

The final stump is a critical element in this delicate balancing act 

(figure 2). While it is there, the stump 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. 

Figure 2. Typical retreat mining geometry, illustrating 

terminology used in this paper 

Traditionally, however, miners have been reluctant to leave the 

final stump because they were concerned that caving might be 

inhibited. The conventional wisdom was that unless the pillars were 

totally extracted, the stumps left in the gob would provide too much 

support and would cause the rock to hang up. The extra “pressure” 

would then be thrown on the outby pillars, resulting in a “ride” or 

“squeeze” (2, 3). Final stumps were only to be abandoned when poor 

roof conditions made their recovery too risky. This meant that the 

foreman and mining crew often had to make a difficult decision on a 

pillar-by-pillar basis, and a wrong choice could prove disastrous. 

Recent experience seems to indicate that the fears about leaving 

stumps may have been exaggerated. Even 10 years ago several large 

companies were routinely leaving the final stump for safety reasons. 

Throughout the 1990's more mines adopted 3-cut and other mining 

sequences that left significant stumps of coal in the gob. Yet the 

incidence of squeezes does not seem to have noticeably increased. 

Better pillar design and faster mining rates may have helped. But it also 

seems that small stumps may not provide as much overburden support 

as had been thought. In many cases, it appears an optimum pillar 

extraction plan may be one that purposely leaves a final stump sized to 

provide roof support without inhibiting caving. 

REGULATORY ISSUES 

Several elements of 30 CFR 75, Mandatory Safety StandardsUnderground Coal(4) deal specifically with pillar recovery operations. 

According to ¶75.220(a)(1), “Each mine operator shall develop and 

follow a roof control plan, approved by the District Manager, that is 

suitable to the prevailing geologicalconditions,and the mining systemto 

be used at the mine.” The roof control plan must contain technical 

information regarding the method of pillar recovery and the sequence 

of mining pillars (¶75.221(a)(8)). 

Pillar recovery criteria and specific criteria to be considered in the 

approval of the pillar recovery portion of the plan are identified in two 

sections of the standard. Section ¶75.207 discusses the use of breaker 

and turn posts, and the use of a single, 16 ft wide roadway to access the 

final stump. Section ¶75.222(d) addresses pillar dimensions, the width 

of splits and lifts, and the spacing of posts. These criteria generally 

reflect successful experience at a majority of mines. Some provisions 

were included as criteria rather than mandatory safety standards 

because MSHA recognized that individual criteria may not be 

appropriate for all mines due to the various geologic conditions 

encountered (5). The regulations have always purposely left the 

DistrictManager considerable discretion to account for localconditions 

and new technologies (3, 6). 

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 and minimized the possibility of 

undue forces overriding the working places. 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. 

PILLAR RECOVERY PRACTICE IN THE U.S. IN 2001 

The popularityof room-and-pillar retreatmining variessubstantially 

across the U.S. Currently, pillar recovery provisions are included in 

approved roof control plans in ten of eleven Coal Mine Safety and 

Health Districts nationwide. Pillar recovery operations are most 

common in minesin the centraland southern Appalachians (Districts 4, 

5, 6, and 7). Western mines (District 9) and those in the northern 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Appalachians (Districts 2 and 3) have a greater abundance of 

reserves which can be longwalled. In these areas, longwall retreat 

has become much more common than room-and-pillar retreat for high 

recovery mining. One operator currently practices mechanized pillar 

recovery in the anthracite region of northeast PA (District 1) and 

several operators perform limited secondary mining (pillar splitting or 

perimeter mining) in the Illinois Basin (District 8). No pillar recovery 

operations are active in District 10 (Western Kentucky) and District 

11 (Alabama) at this time. 

For this evaluation, representative roof control plans were 

obtained from each pertinent MSHA district. Many mining companies 

maintain pillar recovery provisions in their roof control plans despite 

the fact that they do not rely on it as a primary production method. All 

26 of the mines selected for this study were chosen to reflect 

operations which were actively engaged in pillar retreat mining. The 

provisions were evaluated in terms of pillar extraction method (e.g., 

pillar dimensions and cut sequence) with particular focus on criteria 

associated with pillar remnants. 

By far the most popular methods of pillar recovery utilizedat the 

subject mines were those which required no additional roof bolting 

during retreat. Fifty percent of the mines incorporate some form of 

Christmas tree (figure 3), 42% use outside lifts,and another 23% use 

a three-cut or some other form of unbolted slab cut. In contrast, split 

and fender and pocket and wing methods were used at 19% and 8% 

of the mines, respectively. 

Of 26 roof control plans considered, nine contained provisions for 

a single pillar recovery plan. The remaining 17 plans each 

incorporatedseveraldifferent recoveryplans. An individual mine may 

maintain multiple recovery plans for a variety of reasons. For 

example, outside lift plans are typically only used when the pillars are 

less than 40 ft wide. If deeper cover requires increased pillar 

dimensions, the mine may use Christmas tree or split and fender 

methods to achieve similar recoveries. Several plans might also be 

necessary to accommodate changing roof conditions or to respond to 

equipment problems (like an inoperable MRS unit). In other cases it 

appears that multiple plans were needed to accommodate various 

equipment types (e.g., one section uses shuttle cars, another uses 

continuous haulage). At other mines, different plans were developed 

for various support types (e.g., timber versus mobile roof supports). 

Figure 3. Pillar extraction mining methods; (a)
 

Christmas tree, (B) outside lift and (C) three cut 
 

Analysis of the plans suggests that remnant pillar sizing varies 

considerably among room and pillar retreat mines. Thirteen plans 

(from eight mines) could be considered partial pillar plans; these 

operations usually involve only pillar splitting or minimal slabbing. 

Twenty-four plans (from nine mines) contain provisions for taking the 

pushout while 38 plans (from 18 mines) leave the final stump. Of 

those 38 plans, 32 (from 14 mines) specify dimensions of the final 

stump; 11 mines have plans that call for minimum dimensions of at 

least 8 ft, while 8 have plans that call for minimums of less than 8 ft. 

In many instances, the finaldimensions of remnants including the 

pushout are not stated explicitly in the pillar recovery provisions of the 

roof control plan but are constrained nonetheless by the cut sequence 

and pillar dimensions. These implicit dimensions can be effective 

under usual circumstances but can easily be influenced by changes 

during the miningprocess. For example, even relatively small changes 

in fender thickness or angle of attack could compounda in successive 

cuts and result in an undersized pushout. Specified minimum 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

dimensions may be preferable since they help ensure a prescribed 

minimum amount of support for the final lift. 

It is interesting tocompare current pillarrecovery plans with those 

used in the past. During the late 1970's, MSHA roof control plans 

were evaluated as part of a U.S. Bureau of Mines (USBM) contract 

study of pillar extraction techniques (7). The results were later 

summarized in USBM information Circular 8849, “Room and Pillar 

Retreat Mining, A Manual for the Coal Industry” (6). The study 

found that about 38% of all U.S. mines employed room and pillar 

retreatmethods at thattime andthese mines accounted for about 19% 

of the total U.S. underground production (7). 

The report divided the various retreat mining methods into the 

broad headings “full” and “partial” recovery. Of the full retreat 

operations, the primary recovery methods were split and fender 

(73%), pocket and wing (17.2%), and outside lifts (6.2 %). The main 

partial recovery methods were pillar splits (81%) and outside lifts 

(15.2%). 

Clearly, pillar recovery techniques have changed dramatically 

during the past 20 years. The most obvious change is that few of 

today’s plans require roof bolting while the pillars are being extracted. 

The widespread adoption of remote control operation of continuous 

mining machines, whichmakes it possible to mine “extended cuts,” is 

responsible for this change. 

MRS (or breaker line supports) are another technological 

development that has influenced recovery methods. Since their 

commercial introduction in 1988, the number of mobile roof supports 

in use in the U.S. has grown to about 50 sets (8). Other technologies 

such as continuous haulage systems, also have influenced pillar 

recovery methods. 

INTERNATIONAL PILLAR RECOVERY PRACTICE 

In South Africa, pillar recovery is the primary method of retreat 

mining. As in the U.S., miners there traditionally attempted total 

extraction of every pillar (9, 10). However, when the mining industry 

shifted from the thin seams in KwaZulu-Natal to the thicker seams in 

Mpumalanga and the Free State, total extraction was found to be 

unfeasible. As an alternative, the concept of “high extraction” was 

developed. With high extraction, final “stooks” (stumps) of predetermined size are left in planned locations. The goal is to leave 

stumps that are large enough to support the intersection, but small 

enough to fail once they are in the gob (11). Typically, the final stumps 

measure 13 by 20 ft in a 13 ft high seam (12). 

In Australia,where seams have always been thick, generalpractice 

has been to leave the final “stook x.” In recent discussions with 

Australian pillar recovery specialists, the following general guidelines 

emerged: 

•	 In weak top, the minimum stook x dimension is about 6 ft. 

Since the pillars are typically 30 ft wide, and the inby edge of 

the final stump measures about 12 ft, the total area is about 

270 ft2. In fractured ground, the minimum dimension may be 

increased to 8-10 ft. 

•	 In strong top, no final stump is needed under normal 

conditions. However, 6 ft stumps will be left before the first 

fall or whenever caving is not occurring regularly. 

Under deeper cover (1,000 ft in the South Coast mines for 

example), stumps as large as 750 ft2 may be left. However, these are 

expected to crush out in the gob. In general, hard strong sandstones 

that bridge for long periods are considered the most difficult conditions. 

Table 1. Pillar extraction fatalities occurring during mining of last lift or final stump 

Year State Seam No. of fatals Mining 

method1 Roof type Seam 

height, ft

 stump area, 

ft2 

Size of fall, 

tons Fall type 2 

1998 CO Upper D 2 OL siltstone 9 140 1950 1 

1996 WV Beckley 1 CT shale 7 150 1000 1 

1995 WV Coalburg 1 CT sandstone 8 65 38000 2 

1993 WV Poc. No. 3 1 CT shale 5 70 600 3 

1992 WV U. Dorothy 1 CT sandstone 7 100 2200 1 

1991 WV Hernshaw 1 CT shale 4.5 40 1000 1 

1990 KY Creech 1 - shale 3 20 2000 1 

1990 KY Highsplint 1 S&W shale 5 200 5000 1 

1990 PA Pittsburgh 1 S&W sandstone 10 180 ? ? 

1989 KY U. Hignite 3 CT ? (slip) 5 120 450 3 

1987 WV Dorothy 1 S&T shale 7 140 3200 1 

1986 KY “B” 1 S&W sandstone 3.5 100 170 3 

1986 VA U. Banner 1 S&W shale 5 40 4000 1 

1985 WV Coalburg 1 shale 6 175 200 3 

1985 WV Gilbert 2 S&W shale 4 ? 12500 2 

1984 VA L. Banner 1 S&W sandstone 3.5 0 2000 1 

1983 WV Coalburg 1 shale 6 20 1700 1 

1Mining Method: OL=Outside Lift; CT=Christmas Tree; S&W=Split and Wing 

2Fall Type: 1=Intersection Falls; 2=First Falls; 3=Small Falls. 
ANALYSIS OF PILLAR RECOVERY FATALITIES 

Roof falls that occurred during pillar extraction have claimed the 

lives of 67 miners since 1982. Of these, 21 were killed in 17 separate 

incidents involving the final stump or last lift. Details of these final 

stump fatalities are contained in table 1. 

The finalstump fatalities can be dividedintothree groups according 

to the type of roof fall involved: 

Intersection collapses: These 10 incidents are the ones that most 

clearly mighthave been prevented by properly sizedfinalstumps. They 

involvedroof falls of 1,000 to 5,000 tons. In some cases, the final stump 

may have been too small, or it was in the process of being extracted. 

In other cases, the finalstump appears to have been in the wrong place, 

resulting in an excessive span. 

First falls:First falls are a specialcase of intersection falls. There 

have been two since 1982, and each involved a very large volume of 

rock. Stronger final stumps probably could not have prevented such 

falls from occurring, but they might have been able to break them off 

inby the active intersection. 

Small falls: In 5 of the incidents, the size of the fatal rock fall 

weighed lessthan1,000tons. Such small loads clearly did not cause the 

failure of the final stump. Indeed, they are similar to the typical fatal 

roof falls that occur earlier in the retreat mining process. Such falls are 

caused by the stress redistributions and deformations that accompany 

pillar recovery. A stump that is too small might indirectly contribute to 

such falls by allowing too much deformation to occur. 

The size of the finalstump at the time of the roof fall was evaluated 

from the figures included with the reports. These figures are often not 

to scale,so the dimensions must be considered as estimates. In 5 of the 

10 intersection collapse cases, the stump was less than 50 ft 2

. In three 

cases, it was about 150 ft2

, and in one case it was 200 ft2. The incident 

with the largest finalstump also involved the largest volume of rock in 

the fall, 5,000 tons. 

Some other characteristics of the fatal final stump roof falls are: 

Roof quality: Most of the fatal roof falls occurred where the roof 

was shale or other weak rock. Surface cracks near the outcrop were 

a factor in four of the incidents, but the most recent one was in 1990. 

Horsebacks are mentioned in several incidents. 

Mining method: The Christmas tree method has been 

disproportionately represented in recent years. Christmas tree mining 

can result in wider spans, particularly if mining is conducted too close 

to the active intersection on both sides of the entry. The outside lift 

method was only involved in one fatal incident. 

Roof support:Only one of the finalstump fatalitiesinvolvedMRS. 

That case was also a first fall in whicha relatively smallstumpwas left. 

Perhaps most important, the MRS were not placed in the intersection 

where they could have done the most good (13). 

SIZING THE FINAL STUMP 

It seems that under many circumstances, a properly sized final 

stump could reduce the risk of a hazardous premature roof fall. The 

purpose of this section is to provide information that can assist mine 

operators in sizing final stumps that are: 

•	 Large enough to provide effective support to the roof above the 

intersection, but; 

•	 Small enough that it does not prevent the main roof from 

caving. 

The basic elements of any pillar design problem are the applied 

load and the pillar strength (or more precisely, the pillar load-bearing 

capacity). The final stump problem is unique because of the geometry 

of both the load and the stump itself. 

The loading applied to a final stump is much less than the full 

overburden loading that a typical pillar carries. Calculated capacities 

clearly are smaller than tributary area load, indicating that these pillars 

must have shedmuchoftheir load(including transferred abutment load) 

to adjacent unmined pillars. However, the capacity provided by the 

remnant may still be sufficient to afford substantial support for the 

adjacent intersection. From the fatality report data presented in table 

1, it appears that an approximate design load might be about 4,000 tons. 

Mark - Bieniawski 

Vertical Stress at Ultimate Strength ( psi )

0.1 

Distance from rib ( x/h ) 

500 

0.2 0.3 0.4 0.5 

1500 

1000 

A. 

B. 

Stress Distribution 

Figure 4. (A) Pillar stress distribution (cross-section to the pillar 

core). (B) Plan view of the stress distribution . 
The load-bearingcapacity depends on the strength of the stumpand 

its load-bearing area. Traditional pillar strength formulas cannot be 

used directly because of the odd shape of the stump. However, the 

“method of slices” (figure 4) provides an approximation of the load 

bearing capacity, provided two assumptions are made (14): 

•	 The strength of any pillar element is a function of its distance 

from the nearest pillar rib, and; 

•	 The strength function is known. 

For example, the Mark-Bieniawski stress function is: 

Fv = S1 [0.64 + 2.16 (x/h)] 

Where: 

Fv = Pillar stress function 

S1 = In situ coal strength 

x = Distance from the nearest pillar rib 

h = Pillar height 

The load-bearing capacity of eachslice is determinedby multiplying 

the area of the slice by its verticalstress at ultimate strength. The total 

load-bearing capacity of the stump is the sum of the load-bearing 

capacity of the slices. 

The area of a final stump depends on cut-to-corner distances L1 

and L2, and on the cut angles N1, N2 (figure 5). A spreadsheet program 

was prepared that calculates the stump area for any combination of 

these parameters. 

Figure 5. Determination of the load-bearing area 

of the final stump. 

Figure 6 shows the load bearing capacities of stumps for a variety 

of seam heights and cut-to-corner distances. It assumes that L1= L2, 

and N1 = N2 = 75 deg. It appears that stump capacity is very sensitive 

to the cut-to-corner distance, and less sensitive to the seam height. 

Stump Capacity (tons) 

35000 

30000 

25000 

20000 

15000 

10000 

5000 

0 

4 6 8 10 12 14 

4-ft Seam 

12-ft Seam 

8-ft Seam 

Cut-to-Corner Distance (ft) 

Figure 6. Load-bearing capacity of final stump for a variety of 

seam heights, assuming that L1=L2=L, N1=N2=75 deg., and that 

the stump has not yielded. An SF of 2.5 is suggested to account 

for yielding of the stump. 

When the cut-to-corner distances are not equal (L1... L2), then a 

reasonable approximation is: 

L eff = L 1 * L2 

This approximation is valid for 0.67 < L1/L2 < 1.5. When the L1/L2 

ratio is outside that range, one of the cuts is very close to the 

intersection and the performance of the stump has probably been 

compromised. 

The next issue is the appropriate Stability Factor (SF), which is 

more precisely a capacity-to-load ratio. Unfortunately, there is not 

enough field data to determine an appropriate SF statistically. From the 

fatality reports, it appears that an SF of at least 2.5 might be 

appropriate. Two factors may contribute to the relatively high SF: 

•	 Final stumps usually consist of yielded coal. The MarkBieniawski formula assumes that the pillar is initially intact, but 

the corners of a pillar are likely to yield as the pillar is 

extracted. The residual strength of the slender remnant stump 

is generally considerably less than the original strength (15). 

•	 Final stumps generally have w/h ratios in the 1-2 range. 

The ARMPS data base contains few successful case histories 

from pillars so slender (16). Recent research suggests that 

traditional pillar strength formulas (like Mark-Bieniawski) may 

overestimate the strength of very slender pillars (17, 18). 

Finally, the ability of the final stumps to resist gob caving should be 

evaluated. The SF of the stumps once they are isolated in the gob can 

be estimated by comparing their load-bearing capacity to the 

overburden load originally carried by the complete pillar. Figure 7 

showsthe SF as a function of depth for severalcommon entry centers. 

Van der Merwe (11) suggests that stumps with SF<0.3 will be too 

weak to inhibit caving. These final stumps meet his criterion once the 

depth of cover exceeds 100-150 ft. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SF for Overburden Load 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

Stability Factor 

8000 ton Stumps 

12000 ton Stumps 

0 100 200 300 400 500 600 700 

Depth of Cover 

Figure 7. Capacity-to-load ratios for full overburden load of final 

stumps isolated in the gob, assuming stumps have not yielded. 

COMPARISON WITH ROADWAY SUPPORTS 

Since the purpose of final stumps is to serve as roof support, it is 

worthwhile to compare them to the other types of roadway support 

used in pillar recovery. Figure 6 shows that a typical 10 ft by 10 ft 

stump would have an estimated load-bearing capacity of about 10,000 

tons if it had not yielded. For purposes of comparison, this value should 

be reduced bythe SFof 2.5, giving an effective stump capacity of about 

4,000 tons. 

Wood posts have traditionally been used as roadway supports. A 

typical 6-in-diameter hardwood post can carry approximately 50 tons. 

If 12 such posts are used to support an active intersection, the total 

capacity is about 600 tons. The value of an effective final stump to 

supplement timber supports is obvious. 

A single mobile roof support, on the other hand, is usually capable 

of carrying 600 tons. Four MRS are used in a typical pillar recovery 

operation. Their total capacity is, therefore, almost as great as a 10 ft 

final stump. The greater capacity, and better stiffness characteristics 

(13),of MRScomparedtotimbersis probably a big part of the excellent 

safety record they have established during the past decade. 

CONCLUSIONS 

Recent U.S. coal price increases will likely result in more roomand-pillar operations due to the relatively low capital investment and 

broad applicability of these systems. Undoubtedly, some of these 

operations will utilize pillar recovery methods to optimize resource 

recovery and/or mining productivity. As this segment of the industry 

expands, appropriate design and training will be imperative to avoid an 

associated increase in mining accidents. Lack of available skilled 

workers is a particular concern at the moment since the coal industry 

is facing a labor shortage in some areas (19, 20). As indicated in World 

Coal (21), “Pillar recovery requires a highly disciplined management 

approach, with the needtocarefully follow a detailedtechnicalschedule 

to ensure that safety is maintained.” 

This paper has defined a “high extraction” approach to pillar 

recovery. Its basic principle is that a final stump is left in place to 

support the active intersection. The stump is more like a roof support 

than a pillar, which is why its size does not need to increase with depth. 

Guidelines have been provided to help size the final stump to carry the 

maximum anticipated rock load above the intersection. The stumps are 

also too small to seriously inhibit caving, at least at depths in excess of 

150 ft. In general, the engineered final stumps require that no more 

than 5-10% of the original coal be left in place. 

From a practical standpoint, the technique is simple to implement. 

Once the proper cut-to-corner distance has been selected, the foremen 

can use spray paint to mark the pillars in advance of mining. Explicit 

minimumdimensions and this type of visualmarker should helpmaintain 

a minimum level of support for the adjacent intersection. However, 

these activitiesshould be viewed as establishinga guideline;they should 

not deter miners from leaving larger stumps when necessary based on 

the conditions at hand. Ground conditions on the retreat section must 

be gauged to ensure that the minimum prescribed dimension is 

adequate. 

Proper use of final stumps is just one element in safe pillar mining. 

Wherever possible, mobile roof supports should be used because of 

their superior support capacity. Other safety tips are listed on the “Best 

Practices” card available at the MSHA website (22). 

During the past 5 years, the mining community seems to have 

largely succeeded in eliminating fatal roof falls associated with the 

mining of the last lift or final stump. It is hoped that the information 

presented in this paper can help to permanently make such incidents 

relics of the past. 

REFERENCES 

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

ofRetreatMining of CoalPillarsin the UnitedStates. Proceedings, 

New Technology for Ground Control in Retreat Mining, NIOSH 

Information Circular 9446, Pittsburgh, PA, 1997, pp. 2-16. 

2.	 Stahl, R.W. Extracting Final Stump in Pillars and Pillar Lifts with 

Continuous Miners. BuMines RI 5631, 1960, 13 pp. 

3.	 Montague, P.G. Pillar Recovery. Proceedings, American Mining 

Congress, Chicago, IL, Am. Min. Congr., Washington, DC, 1988, 

pp. 531-548. 

4.	 U.S. Code of Federal Regulations (1998). Title 30–Mineral 

Resources: Chapter I–Mine Safety and Health Administration, 

Department of Labor; Subchapter O–CoalMine Safety and Health; 

Part 75–Mandatory Safety Standards–Underground Coal Mines. 

5.	 Federal Register, Vol. 53, No. 17, Wednesday, January 27, 1988, 

“Rules and Regulations,” pp. 2354-2374. 

6.	 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 Information Circular 

8849, 1981, 228 pp. 

7.	 Kauffman, Peter W. and Steven A. Hawkins, “Operational 

Techniques for Room and Pillar Retreat Mining - Final Report,” 

USBM Contract No. HO282021, March 1980, 76 pp. 

8.	 Howe, Larry, International Sales Manager, J.H. Fletcher & Co., 

personal communication, 2001. 

9.	 Galvin, J. M. Review of Overseas Pillar Extraction Techniques -

South Africa. University of New South Wales, School of Mines, 

Strata Control for Coal Mine Design, Progress Report 4/93, 1993, 

152 pp. 

10. Beukes, J. S. Considerations in the Design and Layout of Pillar and 

Rib Pillar Extraction Panels. Proceedings SANGORM Symp: 

Advances in Rock Mechanics in Underground Coal Mining,1989, 

pp. 105-111. 

11. Van der Merwe, J.N. and Olivier, F.T. The Development of a 

Crush-PillarSystemfor High Extraction CoalMining. Proceedings 

MASSIMIN 92, Johannesburg, SAIMM,1992, pp. 403-407. 

12. Mark C. Ground Control in South African Coal Mines–A U.S. 

Perspective. Proceedings, 18th International Conference on 

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