Behavior of a coal pillar prone to burst in the southern Appalachian Basin of the United States 

Anthony Iannacchione 

US Department of the Interior, Bureau of Mines, Pittsburg, Pa., USA 

ABSTRACT: One of the more pressing engineering problems for deep longwall mines in the Southern Appalachian 

Basin is to design longwall systems that will eliminate catastrophic coal pillar failure from the working 

environment. The purpose of this study is to quantify the behavior of a particular size abutment pillar prone 

to burst or bump. Because the 24.4-111 (80-ft) square abutment study pillar is within the pillar size commonly 

used in this region, understanding the behavior of this pillar should aid in future efforts to design optimum 

mining strategies. This research has attempted to construct the pillar stress and roof-to-floor convergence 

profiles in order to evaluate the ultimate strength, deformation modulus, visco-elastic deformation, and 

violent failure characteristics of the pillar during longwall mining. This information reflects the 

overriding influence of the local geologic characteristics on bursts, provides the field measurements needed 

to verify concurrent modeling studies, and suggest some basic mechanisms associated with the coal pillar 

bursts discussed in this paper. 

1 INTRODUCTION 

Owing to the complex interaction of full-extraction 

techniques, overburden greater than 300-rn ( 1000-f t) , and unique geological characteristics, the Southern 

Appalachian Basin has had a long history of coal 

pillar burst problems. One of the earliest detailed 

reports on these occurrences was compiled by Rice 

(1935). This report identified numerous sites of 

fatal coal pillar bursts in Harlan County, Kentucky, 

and Wise County, Virginia. In one 4-month period 

eight miners were killed and a number injured by 

coal bursts . Holland and Thomas (1 954) examined 177 instances 

of pillar bursts, most of which were from the 

Southern Appalachian Basin, and found that the 

primary cause was unfavorable mining practices in 

abutment areas. Talman and Schroder ( 1958) emphasized the influence of very stiff overlaying strata 

on the occurrence of bursts in two different coalbeds in southern West Virginia, thereby confirming 

the importance of local geologic characteristics. 

Comprehensive laboratory and field data have been 

reported by Crouch and Fairhurst (1973) and Wang 

et a1 . ( 1976) on the properties of the Pocahontas 113 

and /14 Coalbeds in West Virginia and Virginia. 

Campoli et al. (1987) documented the geologic, 

mining, and engineering parameters at five sites in 

West Virginia and Virginia where recent miner 

fatali ties were associated with coal bursts. 

Since most of these studies identified burst 

occurrences while slabbing and splitting pillars 

adjacent to large gobs during room and pillar 

mining, the potential for bursts in conjunction with 

longwall mining was anticipated. For this reason, 

the U.S. Bureau of Mines has been cooperating on 

joint field investigations with mine operators in 

Buchanan County, Virginia. These studies are 

obtaining the basic data necessary to optimize 

engineering designs in the hope of minimizing 

potential problems in the future. 

2 GEOLOGIC CHARACTERISTICS OF THE STUDY SITE 

Geological examinations and in-mine core drilling of 

the gate entries containing the study area have 

identified a 10-m thick, stiff siltstone (18 to 

30 GPa Young's modulus) overlaying the coalbed 

(Figure 1). This unit is in turn overlain by a 

Figure 1. Stratigraphic section within the study 

area. 

x W 

massive 40-m-thick quartz areni te sandstone. The 

coaibea fioor consists of an equally stiff siltstone 

at least 10 m thick (Figure 1 ) . Underground 

observations have indicated a persistent absence of 

prominent roof and floor fractures or joints. The 

main roof, dominated by the thick quartz arenite 

sandstone, is exceedingly difficult to break and 

complicates the use of general abutment load 

calculations, such as those identified by Mark 

(1 987) , to account for the extreme loads measured 

within the study area (see average stress 

calculations later in this paper). 

.......... ............ ............. .............. 

.............. 

.............. 

............... 

............... 

.............. 

3 PROPERTIES OF THE 24.4-M-SQUARE POCAHONTAS 113 COAL 

PILLAR 

I- C '.'..' V) 0 ............................ 

............. 

............... V) 

L member 

Pocahontas No, 3 

Siltstone member 

Pocahontas No. 2 

Coatbed 

Perhaps the best way to begin an analysis of the 

behavior of pillars is to compare the 24.4-m (80-ft) 

square pillar with other pillars by using empirical 

pillar design formulae. This analysis will help to 

highlight problems with pillars having large widthto-height (W/H) ratios in burst-prone environments. 

This analysis will also emphasize the inability of 

the gate entry system to support the abutment loads 

at this site. 
In determining a factor of safety for coal pillars 

1.78-• (70-in) in height within the study area, it 

is first necessary to calculate the pillar loads 

generated during development mining. With an 

average overburden of 564 m (1 850 ft) , average 

development loads of 21 .9 MPa (31 80 psi) were calculated using the tributary area theory. 

Empirical equations developed by Wang et al. 

(1976) and Bieniawski (1984) were used to determine 

a range of the ultimate strengths for the abutment 

pillar. Wang's equations were used in this analysis 

because they were based on actual in situ field test 

for the Pocahontas 63 Coalbed in West Virginia. The 

ultimate strength of a pillar is achieved when the 

pillar has reached its highest average stress. Once 

this point is achieve&, the average pillar stress 

drops with a corresponding increase in pillar 

deformation, indicating the beginning of failure. 

The ultimate strength of the abutment coal pillars 

was calculated at 19.5 MPa (2833 psi), assuming the 

strength for a 1.68-01 (66-in) coal cube (Wang 

et al., 1976) of 5.2 MPa (758 psi). The W/H ratio 

(24.4-m / 1.78-10 (80-ft / 5.83-ft)) of this pillar 

is 13.7. Using an alternate equation by Bieniawski, 

the ultimate strength of the abutment pillar was 

calculated at 28.4 MPa (4124 psi). It can be seen from these two values that the abutment pillars 

could have a safety factor of 0.9 or 1.3 after the 

development of the gate road entry system. This 

range of safety factors is of interest considering 

that Holland (1964) indicated that a pillar will 

probably become virtually indestructible when the 

W/H ratio exceeds 10. Yet, as is later shown, this 

pillar was able to withstand far higher stresses 

during longwall mining and eventually was destructible. Clearly. the empirical design 

formulae underestimate the strength of pillars with 

high W/H ratios. 

Using an empirical method developed by Mark and 

Bieniawski (1 986) called Analysis of Longwall Pillar 

Stability (ALPS), an accurate distribution of 

stresses within pillars in response to abutment 

loading can be determined. ALPS has the capability 

of estimating loads applied to the gate pillars, 

determining the pillar strength and load-bearing 

capacity. and calculating a "stability factorn for 

the entire gate entry design. The stability factor 

for the gate entry configuration at the study site 

was found to be less than 0.5. This estimate 

indicates sufficient loads were present to evaluate 

pillar behavior at and beyond its ultimate strength. 

4 INSTRUMENTATION AND MINING METHOD 

Evaluation of the behavior of the abutment pillar 

was accomplished by measuring pressure changes 

within and strata deformations around the coal. 

These observations were performed as the area was 

loaded during the extraction of the longwall panels 

on either side of the gate entries. A total of two 

abutment pillars and four yield pillars were 

instrumentea with hydraulic coal cells in this study 

(Figure 2). Approximately 70 convergence stations were installed throughout the study area at the 

intersections and midpoints of every entry. Only a portion of the entire data set is discussed in this 

paper. 

Throughout the length of the gate entries 

encompassing the study area, the abutment pillars were flanked on either side by 9.1-m (30-ft) yield 

pillars (Figure 2). With the arrival of the 

longwall faoe, these yield pillars crushed and 

ceased to be significant load bearing members. This 

statement is based on the performance of six coal 

cells placed across the width of 4 yield pillars and 

their adjacent convergence stations. The principal 

function of the yield pillar after failure was to serve as a protective barrier between a bursting 

Coal cell 

* Convergence statlon 

Plltor studied in thls 

a Other instrumented 

plllars In the study 

(All dinensions In 

meters) 

Figure 2. Layout of the mine around the abutment 

pillar and the location of the instruments used in 

this analysis. 

abutment pillar and the longwall face. The 183-111 

(600-ft) wide longwall panels on either side of the 

gate road entries were the seventh and eighth in a series of east-west oriented mining areas 

(Figure 2). 

The instruments used in this evaluation Were 5-cm 

(2-in) diameter coal cells called Borehole Platen 

Flatjack (BPF) and convergence anchors measured with 

portable rods. The coal cell is a modified 

stainless steel hydraulic flatjack inserted between 

two aluminum platens (Bauer et al. 1985). The cells 

were set, three in one hole, at or near 17.2 MPa 

(2500 psi) and oriented to sense stress changes in 

the vertical direction. All 18 coal cells were set 

from 6 closely spaced drillholes (3 drillholes on 

either side of the pillar) across the width of the 

pillar. Convergence stations were set 0.91 m (3 ft) 

apart along this same trend in the entries on either 

side of the pillar. 

All of the instruments were in place, and 

monitoring began when the longwall face of panel A 

was -263-m (-863-ft) away from the instrumented 

abutment pillar. (Note that - indicates in towards 

the face, and + out towards the mouth of the 

section.) Figure 2 shows the placement of the coal 

cell, array B, within the pillar and the roof-tofloor convergence arrays A and C in the entries with 

respect to the longwall panels (A and 0). Because 

the hydraulic pressures measured from the coal cells 

represent an unrealistic value of pillar stress and 

because it was the purpose of this paper to analyze 

the average pillar stress and load, corrections to 

the readings of the coal cells were applied. The 

corrections applied to these pressure changes, 

herein referred to as cell calibration factor (CCF), 

have been the focal point of an exhausted laboratory 

and numerical modeling studies at the Bureau 

(Heasley 1989). The approach used in his analysis was to combine the analytical solution for the 

displacements around a borehole and the analytical 

solution for the borehole-platen interaction with 
the empirical stiffness behavior of the flatjack. 

The resulting model of the BPF response has been 

verified using measured laboratory data with very 

good results. The CCF for each instrument was 

calculated based upon the individual fluid volume, 

length of tubing and coal properties. The stress 

values reported herewithin represent cell readings 

corrected by the above procedure. 

5 PILLAR BEHAVIOR DURING LONGWALL MINING 

Pillar stress change and strata convergence were 

analyzed to determine the ultimate strength and 

failure characteristics of the abutment pillar. 

These characteristics are important in determining 

the fundamental mechanism responsible for a burst 

and will be utilized in the future designs of mining 

systems for the Southern Appalachian Coal Basin. 

Assessment of the performance of the abutment pillar 

was achieved by examining the profiles of vertical 

stress distribution and the average stress across 

the pillar, and by examining the convergence in the 

adjacent entries. It should be noted that the 

average stress calculations were based on measuring 

the area under the profiles of the vertical stress 

distribution (Figures 3 and 4) and dividing this 

number by the pillar width. 

0 55 55 29.9 29 9 35.4 

DISTANCE FROM EDGE OF CONVERGENCE ARRAY A, m 

Figure 3. Detailed change in stress and convergence 

profiles from instrumented arrays A, B, and C during 

longwall face passage of panel A. 

1 140r t ARRAY A 

Figure 4. Detailed change in stress and convergence 

profiles from instrumented arrays A, B, and C during 

longwall face advance of panel B. 

lZ0- 

E 

'j 100- 

0 z W 

80- W 

z 60- 

- z 

40- 

z a 

5 20- 

0 

5.1 Detailed Stress and Convergence Profiles 

$+ Dlstarce of KEY longwall face 

fron lnstrunented array, n 

--- 

- 

-1340 -646 

- 

-1195 - - - -278 

-%O 

In an attempt to better understand the distinct behavioral stages associated with the abutment pillar, 

detailed profiles of the individual stress and convergence changes are presented in Figures 3 and 4. 

Figure 3 shows the change in stress and convergence 

profiles from instrumented arrays A, B, and C 

(Figure 2) during the face passage of the panel A 

when the pillar load was rapidly increasing. The 

peak abutment stress first developed along the 

pillar edge and then migrated inward to about 3 to 

3.7 m (9.8 to 12 ft). The coal from this peak stress 

zone to the outside edge of the pillar was highly 

fractured and is referred to as the "yield zone." 

The stresses in the pillar core were initially lower 

than the peak abutment stresses but rose dramatically as the ultimate strength of the pillar was 

approached. This pattern of behavior is very similar 

to Wagner1 s (1 974) in situ test in South African 

coal mines using a "uniform deformation" loading 

system of coal pillars with a W/H ratio of 2. It is 

also interesting to note that the magnitudes of the 

peak abutment stresses remained constant after the 

initial rise suggesting the yield criteria proposed 

by Barron (1 984) might apply to this case study. 

Convergence measurements during this period of 

increased pillar load from array C (Figure 3) 

consistently measured a uniform closure rate across 

the 5.5-m (18-ft) span. It is clear that little or 

no roof bending took place and that the roof and 

floor rock were apparently acting as stiff plates. 

This conclusion is supported by the stress 

distribution profiles themselves and by the other 

convergence data located throughout the gate road 

entries. In array A, the roof-to-floor convergence 

profiles dip towards the yield pillar. Clearly the 

strata around the yield pillar and adjacent entries 

were breaking and rotating toward the newly formed 

gob in panel A. Observations of small fractures in 

the siltstone floor rock at the abutment pillar 

edge, separating the gate road into two distinctive 

loading environments, may possibly explain the 

uniform loading observed within the abutment pillar. 

It is assumed that only uniform convergence of the 

roof and floor over the abutment pillar could have 

produced symmetric stress profiles. 

During the end of panel A face advancement [ +I91 -m 

(+627-ft) to +232-m (+761-ft)}, little or no change 

in the stress profiles was observed (Figure 4) , suggesting that pillar loads were nearly constant. 

However, beyond this point, peak abutment stresses 

remain virtually constant on the gob side of the 

pillar while stresses in the pillar core and the 

solid side of the pillar reduce significantly. It 

is difficult to rationally explain why the drop in 

pillar core str-evses was ii~t accoiiipanied by a droi; 

in the peak abutment stresses. Possibly, this trend 

is not "real7' owing to the low number of operating 

coal cells; however, two horizontally oriented 

pressure cells placed 3-m (10-ft) and 9.1-m (30-ft) 

into the abutment pillar continued to measure 

significant horizontal conf inement until the pillar 

began to burst at face position -156-m (-513-ft). 

0 5,5 5.5 29.9 29.9 35.4 

DISTANCE FROM EDGE OF CONVERGENCE ARRAY A, m 

5.2 Change in Average Stress and Convergence 

The average stress of the pillar was calculated at 

different longwall face positions from Figures 3 

and 4. Calculation of the average stress is quite 

straightforward because of the high concentration of 

coal cells across the pillar. Note that the stress 

profiles are fairly symmetrical during the passage 

of the longwall face in panel A with the exception 

of face position +148-m (+485-ft). Calculation of 

average stress during the advance of the longwall 

face in panel B became more difficult as many of the 

cells on the right side of the pillar indicated zero 

stress. These zero stress conditions could have 
Occurrence - oC polior bursTs 

start of 

I Stakda pllar Fo~lurv 

-174 -37+42 +L9l -1340 -960 -278 0 

-P66 -98 0 +148+232 -1195 -646 -155 

DISTANCE OF LONGVALL FACE FROM INSTRUMENTED PILLAR, n 

Figure 5. Average change in pillar stress and roofto- f loor convergence at various face positions of 

longwall panels A and B. 

been a result of cell failure or local coal pillar 

failure. The statement that the ultimate strength 

of the pillar was exceeded and stable failure began 

was base upon the overall average stress 

calculations across the pillar, where clearly a drop 

in the stress level of the pillar core occurs. These 

values have been calculated as a means of defining 

the change in the behavior of the pillar from an 

unfailed to a failed state (Figure 5). 

The abutment pillar was first affected by the 

advance of panel A (Figure 3 and 5) when the 

longwall face was about -1 50-m (-492-ft) away. As 

the face advanced, the stress and convergence both 

increased rapidly until panel A was mined out. 

When the longwall face in panel B was started 

(-1 460-m (-4790-ft) I, the average stress apparently 

began to decrease and the convergence continued at a 

slow but steady rate (Figure 5). Decreasing average 

stress continued until the face was at -960-m 

(-3150-ft), at which point the decrease in average 

stress moderated slightly. At the same time, roofto-floor convergence continued at approximately the 

same rate. 

There is only one obvious reason for the unloading 

of the pillar due to mining activity in and around 

panel B. From face position +191-m (+627-ft) to 

+232-m (+761-ft) , the pillar was subjected to 

approximately constant loading with continuous 

deformation. This could have represented some type 

of visco-elastic behavior. From face position -1460- 

m (-4790-ft) to -278-m (-912-ft), the coal underwent 

stable pillar failure. During this stable failure, 

the pillar's load capacity reduced dramatically with 

continuing deformation. From face position -232-m 

(-91 2-f t) onward, the coal underwent a series of 

small burst. 

The process described above, where pillar failure 

occurred during a stable loading period, may be 

analogous to creep failure in a laboratory test 

specimen as discussed by Goodman (1980); Figure 6 

illustrates this principle where the failure 

occurred when a test sample, loaded to some critical 

stress level just below the ultimate strength, 

creeps until the accumulated strain intersects the 

falling part of the complete stress-strain curve. 

In applying this phenomena to coal pillars, the term 

visco-elastic deformation may be more appropriate. 

5.3 Analysis of Pillar Deformation Modulus 

Assuming that the roof - to- f loor convergence measured 

adjacent to the instrumented pillar (Figure 2) were 

indicative of the pillar deformation, it is possible 

to plot the change in average pillar stress against 

Ultimate 

stre,ngth Point 

STRAIN 

Figure 6. Creep in relation to the complete stressstrain curve (Goodman 1980). 

Figure 7. In situ stresss-strain curves for the 

Pocahontas /I3 Coalbed as the pillar was loaded to 

failure. 

50 - 

the estimated pillar strain. Pillar strain was 

calculated by assuming the average pillar 

deformation was similar to the average entry 

deformation from array C (Figures 3 and 4) and 

dividing this average by the original coalbed 

height of 1 .78-m (70-in). In fact the true pillar 

strain would be less than the average entry roof-tofloor convergence. However, for this analysis the 

shape of the curve would not change significantly if 

the average entry strain were multiplied by a 

fraction. Figure 7 shows the changes in slopes of 

the pillar stress-strain relationship as the coal is 

loaded to failure. This is very similar to the 

approach used by Wang et al. (1976) to evaluate the 

deformation modulus of a pillar in the Pocahontas //3 

Coalbed of southern West Virginia. 

During the face passage of panel A, the slope of 

the stress-strain curve gradually decreases 

(Figure 7). This decrease is most pronounced after 

the longwall face passes the instrumented array. 

d 

a 

40 -- 

vi 

~1 

Ultimate strength 

Face next 4 (lS1 ?=-- to instrunents _.. 7 

(0 n) 0 h Start of 

W 

W 

u 

0 ,02 .04 .06 

CHANGE IN STRAIN, pm/m 
This apparently indicates an slight inelastic pillar 

response. The relatively higher pillar modulus as 

the longwall face is approaching the instrumented 

array may be related to the very rapid loading 

experienced during this time. With the completion 

of panel A and the beginning of panel B, the curve 

becomes almost flat, suggesting visco-elastic 

deformation under almost constant load. Beyond this 

point the slope becomes negative, indicating the 

pillar was in the post ultimate strength failure 

regime. This would appear to support the assumption 

that pillar failure occurred when the face of 

panel B was at or about -1460-m (-4790-ft) away from 

the pillar. 

Analysis of the slopes of the stress-strain curve 

prior to the face passage of panel A (Figure 7) 

indicates a pillar deformation modulus of 

approximately 2.2 CPa (320,000 psi). Pillar 

deformation modulus calculations from data 

presented by Wang et al. (1976) averaged about 

2.38 CPa (340,000 psi) and ranged between 2.07 and 

2.76 CPa (300,000 and 400,000 psi). Wang et al. 

(1976) also determined that the average modulus from 

laboratory tests of this same coal to be 2.45 CPa 

(356,000 psi). The reader is advised to remember 

that these calculations are based upon entry 

deformations and not pillar deformations and that 

the modulus of the pillar is constantly changing in 

response to the proportions of yielded to solid 

(elastic to nonelastic) coal during pillar loading. 

6 CHARACTERISTICS OF PILLAR BURSTS 

The pillar began to bursts in a series of small 

events at a panel B face position of approximately 

-278-m (-911-ft). Unfortunately, by the time the 

longwall face of panel B had reached -156-m 

(-51 3-ft) from the abutment pillar, all but one of 

the coal cells had failed. The subsequent pillar 

behavior can therefore only be reported as based on 

visual observations. When the face of panel B had 

moved to within -156-m (-513-ft) of the pillar, the 

entire rib of the abutment pillar had sloughed off 

into the entry. Shortly after this, one side of the 

abutment pillar burst into the crosscut, partially 

closing it off (Figure 8). At this point, many of 

the other pillars between the instrumented abutment 

pillar and the longwall face were at least partially 

disintegrated. Additional bursting of the abutment 

pillar occurred at a face distance of -113-111 

(-371 -ft). During this burst the crosscut closest 

to the face was totally closed off, while the other 

crosscut was partially closed off. Total 

disintegration of the pillar probably occurred at a 

face distance of -45-m (-1 48-ft). No further 

observation were made beyond this point. 

Clearly, the bursts were initiated well after the 

pillar had passed its peak load bearing capacity. 

By the time the face was -278-m (-91 1-ft) away, 

approximately one-third of the load gained during 

panel extraction may have been shed from the pillar. 

Perhaps the most interesting observation made in 

this study (to the best of our knowledge the 

phenomenon has not been observed elsewhere) is the 

possibility that the pillars began to fail in a 

stable manner and did not burst for a considerable 

time after this failure began. Several scenarios 

that might explain the mechani sm are postulated 

below. 

7 MECHANISM FOR PILLAR BURSTS 

Several hypothesis may be employed in speculating on 

the possible mechanism responsible for the coal 

pillar bursts discussed in this study. Certainly, 

the ability of coal to store high levels of elastic 

strain energy which can be suddenly released in the 

form of bursts was suggested by Holland and Thomas 

(1 954) and has been greatly expanded on by many 

other researches. In the case of the pillar studied 

in this paper, it is difficult to know if any 

significant change in the levels of e l a s t i c s t r a i n 

energy occurred during the mining of the panel B, 

since the average stress appeared to decrease and 

the pillar deformation characteristics did not 

change until bursting began. 

Another possible explanation for the coal bursts 

may be the sudden release in constraint betueen the 

coal pillar and the surrounding strata (e . g., 

slippage between the roof and/or floor and coalbed). 

Babcock and Bickel (1984) from the study of coal 

samples from 15 different mines, found that stress 

alone can produce bursts if constraint of the pillar 

was suddenly lost. This mechanism could be 

responsible for the bursts in this study, but actual 

existence of a change in t h e p i l l a r constraint was 

not readily apparent. 

A third hypothesis can be suggested as a possible 

mechanism that seems to fit the characteristics of 

this sfte well, As stated earlier, field data 

indicate that prior to bursting, the pillar was 

already following its post peak deformation curve. 

Salamon (1970) has demonstrated that, for a test 

specimen, equilibrium is maintained throughout the 

post peak force-displacement curve (the slope of 

which is called stiffness) up to the point at which 

the stiffness of the loading device becomes tangent 

to the curve (Figure 9). Salamon indicated that at 

Stiffiiess of 

Loading device 

Point at which 

unstable failure 

occurs 

Post peak 

Figure 8. View of the crosscut entry inby the 

abutment pillar when the longwall face from panel B 

was 159 m away. 

Figure 9. Criterion for test sample instability in 

terms of load-displacement relationships between the 

sample and the loading device (Salamon 1970). 
this point in the test equilibrium would be 

interrupted by the dynamic yielding or violent 

failu~e of the specimen. (Equilibrium prevails only 

as long as the machine stiffness is greater than 

the sample stiffness.) Similarly, a pillar loaded 

beyond its ultimate strength, and following the 

downward portion of the post-peak curve, would 

continue to shed load and def orm in a "stable" or 

steady manner until the post-peak curve intercepted 

the local mine stiffness curve. From this point 

onward, the stiffness of the pillar is greater than 

the stiffness of the mine, resulting in dynamic or 

unstable yielding of the coal pillar. 

A fourth potential mechanism would be related to a 

dynamic stress wave generated from the abrupt 

failure of a large mass of rock or to the violent 

failure of an adjacent pillar. A sharp, 

instantaneous stress pulse applied to a highly 

stressed pillar could cause it to burst. 

Unfortunately, there does not appear to be any 

current way to prove or disprove any of the above 

mechanism with the field data available. However, 

these mechanisms supply useful insight into 

understanding the behavior of the pillar which is 

imperative if engineering designs are to 

successfully eliminate this hazard from our mines. 

8 SUMMARY AND CONCLUSIONS 

Using data gathered from borehole pressure cells and 

roof - tcr convergence instrumentation, the behavior of 

a 24.4-m (80-ft) coal pillar prone to burst was 

investigated. The important characteristics of this 

behavior can be summarized as follows: 

* As noted in many other studies, certain 

geological characteristics of the site must be 

present for a burst problem to occur. In this 

particular study, a moderately stiff siltstone 

overlain by an extremely stiff quartz arenite 

sandstone virtually free of fractures or joints 

appears to be a major factor influencing the 

occurrence of bursts. * Empirical pillar strength and gate entry design 

formulations indicate the abutment pillar and the 

entry system in general had low safety "factors." 

These formulations provide evidence that the 

observed pillar should have been sufficiently loaded 

to fail prior to the passage of longwall panel B. 

* The in situ pillar deformation modulus was 

estimated to be at least 2.2 GPa (320.000 psi) just 

prior to the passage of the longwall face in 

panel A. * Post-peak pillar failure occurred not as the 

coal was loaded to its ultimate strength, but 

possibly after a short period of visco-elastic 

deformation. The pillar was thought to strain under 

constant load until it reached the downward side of 

the complete stress-strain curve, at which time 

pillar failure was initiated. 

* Coal bursts did not occur until after the pillar 

had experienced significant steady disintegration or 

failure. * Possible explanations for mechanism responsible 

for the pillar bursts observed in this study are 

excessive elastic levels of strain energy in the 

pillar core, a change in pillar constraint at the 

coalbed interfaces, the sudden existence of a 

condition where the pillar stiffness is in excess of 

the surrounding mine stiffness, and the 

instantaneous application of a dynamic stress pulse. 

All of these mechanism may have promoted violent 

failure of the pillar. 

will reduce the occurrence of bursts in coal mines. 

The author is extremely grateful to all of his 

colleagues at the Pittsburgh and Denver Research 

Centers and to Dr. Barron a visiting professor from 

the University of Alberta who have provided many 

stimulating discussions on the subject and have 

shown great patience in the process. 

REFERENCES 

Babcock, C.O. and D.L. Bickel 1984. Constraint - 

The Missing Variable in the Coal Burst Problem. 

Proc. 25th U.S. Symp. on Rock Mech., Evanston, IL, 

pp. 639-647. 

Barron, K. 1984. An Analytical Approach to the 

Design of Coal Pillars. CIM Bulletin, August, 

PP. 37-44. 

Bauer, E.R., G.J. Chekan, and J.L. Hill, Jr. 1985. 

A Borehole Instrument for Measuring Mining-Induced , 

Pressure Changes in Underground Coal Mines. Proc. 

26th U.S. Symp. on Rock Mech., Rapid City, SD, 

pp. 1075-1 084. 

Bieniawski, Z.T. 1984. Rock Mechanics Design in 

Mining and Tunneling. A.A. Balkema, Rotterdam, 

Netherlands, pp. 197-209. 

Campoli, A.A., C.A. Kertis, and C.A. Goode 1987. 

Coal Mine Bumps: Five Case Studies in the Eastern 

U.S. U.S. Bureau of Mines IC 9149. 34 pp. 

Crouch, S.L. and C. Fairhurst 1973. The Mechanics 

of Coal Mine Bumps and the Interaction Between 

Coal Pillars, Mine Roof, and Floor, U.S. Bureau of 

Mines Contract Report H0101778, Feb., 88 pp. 

Goodman, R.E. 1980. Introduction to Rock Mechanics. 

John Wiley & Sons, New York, pp. 74-75. 

Heasley, K.A. 1989. Understanding the Hydraulic 

Pressure Cell. Proc. 30th U.S. Symp. on Rock 

Mech., Morgantown, WV, in press. 

Holland, C.T. 1964. The Strength of Coal in Mine 

Pillars. Proc. of the 6th Symp. on Rock Mech., 

Rolla, MO, pp. 450-466. 

Holland, C.T. and E. Thomas 1954. Coal-Mine Bumps: 

Some Aspects of Occurrence, Cause, and Control. 

U.S. Bureau of Mines Bulletin 535, 37 pp. 

Mark, C. 1987. Analysis of Longwall Pillar 

Stability. PhD Thesis, Pennsylvania State 

University, State College, PA, pp. 8-24. 

Mark, C. and Z.T. Bieniawski 1986. An Empirical 

Method for the Design of Chain Pillars for 

Longwall Mining. Proc. 27th U.S. Symp. on Rock 

Mech., Tuscaloosa, AL, pp. 415-422. 

Rice, G.S. 1934. Bumps in Coal Mines of the 

Cumberland Field, Kentucky and Virginia--Causes 

and Remedy. U.S. Bureau of Mines RI 3267, 36 pp. 

Salamon, M.D.G. 1970. Stability. Instability and 

Design of Pillar Workings. Int. J. Rock Mech. 

Min. Sci., Vol. 7, pp. 613-631. 

Talman, W.G., and J.L. Schroder, Jr. 1958. Control 

of Mountain Bumps in the Pocahontas #4 Seam. 

Transactions of the A.I.M.E., pp. 888-891. 

Wagner, H. Determination of the Complete LoadDeformation Characteristics of Coal Pillars. 

Proc. 3rd. Int. Cong. on Rock Mech., ISRM, Denver, 

pp. 1076-1 081 . Wang, F.D., W.A. Skelly, and J. Wolgamott 1976. In 

Situ Coal Pillar Strength Study. U.S. Bureau of 

Mines Contract Report tH0242022, 110 pp. 

9 ACKNOWLEDGEMENTS 

The work described in this paper is part of an 

extensive investigation carried out by the 

U.S. Bureau of Mines to design mining system that