THE EVOLUTION OF INTELLIGENT COAL 

PILLAR DESIGN: 1981-2006 

Christopher Mark, Chief, Rock Mechanics Section 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 

ABSTRACT 

The first International Conference on Ground Control in Mining 

opened with the topic of pillar design. Two classic papers were 

presented, one by Bieniwski and the other by Wilson. 

Unfortunately, the two methods were so radically different from 

each other that it was nearly impossible to reconcile them. Adding 

to the confusion were the many other pillar strength formulas (such 

as the Salamon-Munro, the Holland-Gaddy, and the Obert-Duvall, 

just to name a few) that were also available. Little wonder that 

discussions of pillar design in those days often ended with 

anguished cries of “but which formula is the right one?” 

The past 25 years have seen substantial progress in the science 

of coal pillar design. Indeed, one testament to the improvement is 

the relative scarcity of papers on the topic at recent Conferences. 

Two factors have been largely responsible for the progress that has 

been made. The first has been the collection of large data bases of 

actual case histories of pillar performance in a variety of settings, 

from shallow room-and-pillar mines through deep cover longwalls. 

These have made possible the development of empirical design 

procedures that are closely linked to real world experience. The 

second important factor is the development of sophisticated 

computer models that can accurately simulate pillar behavior and 

roof/pillar/floor interactions. Together, these two lines of research 

have led to a new understanding of pillar mechanics that identifies 

three modes of pillar failure: 

•	 Sudden, massive collapse, accompanied by airblast, for 

slender pillars (width/height<4) 

•	 Squeezing, or slow, non-violent failure, for most room and 

pillar applications (4<w/h<10) 

•	 Entry failure or bumps for deep cover and longwall 

applications (w/h>10) 

It is particularly satisfying that the insights gained from numerical 

models broadly support those obtained from the empirical studies. 

While far less controversial than in the past, pillar design 

problems continue to arise. One recent example is pillar design for 

highwall mining. NIOSH has just released a software package, 

called ARMPS-HWM, which employs a number of modern pillar 

design concepts. Since highwall mining web pillars are long and 

slender, the greatest danger is that of a sudden collapse. ARMPSHWM suggests two possible prevention strategies, one which 

concentrates on the SF of the webs, and the other which creates a 

“pressure arch” using properly sized barrier pillars. 

The paper will close with a discussion of some current needs in 

coal pillar design, including: 

•	 Updating older empirical methods, such as ALPS, where 

changes in technology (new types of roof support, more 

demanding ventilation requirements, faster retreat rates) may 

have made some of the original case histories obsolete. 

•	 Methods for determining site-specific coal strengths, 

focusing on bedding plane strength and other factors that 

may effect confinement, as input for both empirical and 

numerical design. 

•	 Improved methods for evaluating coal pillar performance for 

environmental issues, such as surface subsidence and 

hydrologic impacts, which consider such factors as depth, 

w/h ratio, water immersion/drainage, and time dependent 

seam strength. 

ACKNOWLEDGEMENT (AND APOLOGY) 

The topic of pillar design is one of the most important in the 

field of coal mine ground control, and the substantial progress that 

has been made has been due to the collective effort of many 

researchers and practitioners. In a brief overview like this one, it 

was only possible to mention a few of those who have made 

important contributions. As an apology to the many whose 

valuable work I was unable to include, I can only say that you are 

in very good company. 

INTRODUCTION 

Babcock et al. (1981), writing in their survey paper for the First 

Conference on Ground Control in Mining, traced the science of 

pillar design all the way back to Coulomb in 1773. During the 

ensuing century, a variety of researchers tested rock specimens of a 

variety of sizes and shapes. However, it was not until 1911 that 

Bunting (1991) proposed the first true pillar design method for coal 

mines. Bunting described the necessity for pillar design this way: 

"To mine without adequate pillar support will result, sooner or later, 

in a squeeze; the inherent effects of which are crushing of the 

pillars, caving of the roof, and heaving of the bottom." 
In developing his formula, Bunting and his collaborators tested 

the strength of coal specimens in the laboratory and conducted 

back-analysis of full-scale pillar failures (“squeezes”) underground. 

Using essentially the same approach, a number of pillar design 

formulas were developed during the next 70 years around the world. 

These “classic” methods consisted of three steps: 

1. Estimating the pillar load using tributary area theory; 

2. Estimating the pillar strength using a pillar strength formula, 

and; 

3. Calculating the pillar "safety factor" (SF). 

Step 1, estimating the load, was fairly straight forward for an 

industry that relied almost exclusively on room-and-pillar mining at 

relatively shallow depth. The tributary area estimate was 

considered sufficient, though it was recognized that in narrow 

panels the pillars near the edges might not experience the full load. 

More complex were the issues associated with pillar strength. 

The two big issues were the “size effect” and the “shape effect.” 

The size effect was most prominent in the laboratory, where coal 

strength testing showed that larger specimens were much weaker 

than small ones. The shape effect referred to the observation that 

slender (low width-to-height ratio) pillars were weaker than ones 

that were more squat. 

As the number of classic formulas proliferated, so did the 

arguments. Should the shape effect be represented as a straight line, 

or as an exponential equation? Was there such a thing as a 

“critical” specimen size? Could a “universal” formula even exist, 

or did each one have its own place? These issues were discussed at 

length in a number of survey papers that were a persistent theme in 

those days (Babcock et al., 1981; Logie and Matheson, 1983; 

Hustrulid, 1976). 

In some respects, Bieniawski represented the culmination of the 

classic approach to pillar design. In his paper at the First 

Conference in Morgantown, Bieniawski clearly described the 

issues involved in pillar design, and the advantages and 

shortcomings of the available methods. He then outlined a logical, 

step-by-step approach to sizing coal pillars. Indeed, Bieniawski’s 

work has provided a firm foundation upon which many of the 

developments of the past 25 years have been built. 

However, the First Conference also contained a paper that 

described a radically new and different approach to pillar design. 

Arthur Wilson (1972, 1981) of the British National Coal Board had 

first proposed his “hypothesis concerning pillar stability” in 1972, 

but by 1981 he had expanded and refined it considerably. His 

frame of reference was deep longwall mining, where very large 

pillars were routinely employed. Here, the goal of pillar design 

was not to prevent a pillar collapse, but rather to ensure the 

serviceability of the gate entries. 

Wilson’s first problem was the need to go beyond tributary area 

and consider the abutment loads brought about by full-extraction 

mining. His concept of the “load balance,” whereby the reduction 

of load in the gob equals the excess load carried by the chain pillars, 

allowed the first serious quantification of abutment loads. 

More fundamental were Wilson’s innovations in defining pillar 

strength. In contrast to the empirical formulas, where “strength” 

was simply the failure load divided by the pillar area, Wilson 

treated the pillar as a complex structure, with non-uniform stresses 

throughout. His key insight was that the “shape effect” is caused 

by the build-up of confining stress within the pillar, which creates a 

high-strength “core” in the pillar center. While Wilson’s 

mathematics contained some serious flaws (Mark, 1987; Salamon, 

1992), his basic concepts are unchallenged today and underlie 

virtually all numerical models (Gale, 1996). 

For many First Conference participants, however, it was pretty 

difficult to see how Bieniawski’s approach could ever be reconciled 

with Wilson’s. While they both purported to address pillar design, 

the input parameters, mathematical formulas, and (most 

importantly) the predicted pillar sizes seemed to be radically 

different. 

In 1992, the situation seemed, if anything, to have become more 

confused. In that year the U.S. Bureau of Mines sponsored the first 

Workshop on Coal Pillar Mechanics and Design (Iannacchione et 

al., 1992), which featured 22 different papers from leading 

practitioners from around the world. Nearly every paper described 

a different approach, and these were approximately evenly split 

between empirical, analytical, and numerical methods. Their 

predictions for pillar strength varied widely, however, even in their 

trend. Some predicted that pillar strength would increase 

exponentially as the w/h ratio increased, others predicted it would 

tend towards a maximum limiting value, and still others predicted 

an intermediate, linear increase (figure 1). Stress measurements 

from 34 coal pillars were also analyzed, but were no help in 

narrowing the field (Mark and Iannacchione, 1992). 

Figure 1. Comparison of pillar size predictions from selected 

pillar design formulas. 
Moreover, the Workshop participants could not even seem to 

reach agreement on something as fundamental as what constituted 

pillar "failure." The classic approach contended that "pillars will 

fail when the applied load reaches the compressive strength of the 

pillars" and that "the load-bearing capacity of the pillar reduces to 

zero the moment the ultimate strength is exceeded" (Bieniawski, 

1992). In this view, which was represented most strongly by the 

South African experience, the only true failures were those in 

which the panel width was very wide compared to the depth, and 

subsidence could actually be confirmed on the surface. Pillars with 

w/h ratios greater than about 10 were considered “indestructible” 

(Wagner, 1992). 

At the other extreme were those whose experience was framed 

by longwall mining. These experts had seen plenty of examples 

where pillars with w/h ratios well in excess of 10 had proved too 

small and resulted in poor ground conditions. Obviously such 

squat pillars had not “failed” in the classic sense that their loadbearing capacity had disappeared. Yet they had failed to perform 

their ground control function. In many of these cases, conditions 

improved when the pillars were made larger. Clearly pillar design 

was still essential to maintaining gate road stability. 

Observing the discussion, an outsider might have been forgiven 

for thinking that he had happened across a modern-day Tower of 

Babel. There were at least three groups, the empiricists, the 

modelers, and the theoreticians, each apparently speaking their own 

language. Even within each group there were bitter disputes. 

Yet just seven years later, by the time of the second Workshop 

on Coal Pillar Mechanics and Design (Mark et al, 1999), a rough 

consensus had been reached on a unified theory of coal pillar 

mechanics. What had happened? 

THE NEW PILLAR MECHANICS PARADIGM 

The explanation can be summarized by another ancient parable, 

the one about the three blind scholars and the elephant. Each 

explored a different part of the elephant—one the trunk, another an 

ear, the third a leg. Based on his own observations, each one felt 

that he could describe the elephant, yet their descriptions were so 

different from one another that they could find no common ground. 

Only when they put all their observations together, however, could 

they get a true picture of the beast. 

 

The answer in this case was that while all coal pillars are made 

of the same basic material, not just their strength but their behavior 

can vary dramatically depending on their shape. In fact, three 

broad categories of pillar behavior and failure mode can be 

identified, each defined by an approximate range of width-to-height 

ratios (Mark, 1999): 

!	 Slender pillars, whose w/h ratios are less than about 3 or 4. 

When these pillars are loaded to their maximum capacity, 

they fail completely, shedding nearly their entire load. 

When large numbers of slender pillars are used over a large 

area, the failure of a single pillar can set off a chain reaction, 

resulting in a sudden, massive collapse accompanied by a 

powerful airblast. 

!	 Intermediate pillars are those whose w/h ratios fall 

between about 4 and 8. These pillars do not shed their 

entire load when they fail, but neither can they accept any 

more load. Instead, they deform until flexure of the 

overburden transfers some weight away from them. The 

result is typically a non-violent pillar “squeeze,” which may 

take place over hours, days, or even weeks. The large roofto-floor closures that can accompany squeezes can cause 

hazardous ground conditions and entrap equipment. 

!	 Squat pillars are those with w/h ratios that exceed 10. 

These pillars can carry very large loads, and may even be 

strain-hardening (meaning that they may never actually shed 

load, but just may become more deformable once they 

“fail.”). None the less, the pillar design may fail because 

excessive stress is applied to the roof, rib, or floor, or 

because the coal bumps. Moreover, the strength of squat 

pillars can vary considerably depending upon the presence 

of soft partings, weak roof or floor interfaces, and other 

geologic factors. 

Although derived from laboratory data, figure 2 illustrates how 

the post-failure behavior and the residual strength of coal pillars 

changes with their shape (Das, 1986). 

Figure 2. Effect of width-to-height ratio on the behavior of 

coal pillars. 

What was the evidence for this new model of pillar mechanics? 

In essence, two largely separate lines of research had converged 

upon very similar conclusions. One source was a new generation 

of empirical studies, the other sophisticated numerical modeling. 

EVIDENCE FROM MODERN EMPIRICAL METHODS 

Prior to 1990, most classic pillar design methods had been 

derived from curve-fitting to coal strength data obtained from 

laboratory or in situ testing. The most notable exception was 

Salamon and Munro’s formula, which was based entirely on 

statistical analysis of 98 unfailed and 27 collapsed pillar panels in 

South Africa. Salamon and Munro had developed their formula 

following the sudden, disastrous 1960 pillar collapse at the 

Coalbrook Colliery in which 437 lives were lost (Wagner, 1992). 
Salamon’s approach, that of using case histories involving fullscale pillars from actual mines, has a lot to recommend it. With 

such real-world data, it is not necessary to fully understand the 

mechanics, though a “reasonably clear understanding of the 

phenomenon in question” (Salamon, 1989) is needed to guide both 

the data collection and the statistical analysis. Moreover, the 

design equation that results from the analysis is generally simple, 

realistic, and thoroughly verifiable. In essence, it makes the past 

experience of a broad segment of the industry available to mine 

planners in a practical form. 

The Analysis of Longwall Pillar Stability (ALPS) was the first 

modern pillar design method to employ a large case history data 

base like Salamon’s (Mark, 1990; Mark, 1992). While the original 

ALPS research focused on defining longwall abutment loads using 

stress measurements, the real crux was identifying the proper SF to 

use for design. The case history data, obtained from a broad crosssection of mines across the U.S., showed that both successes and 

failures, defined in terms of tailgate serviceability, occurred over a 

wide range of pillar SFs. Clearly other factors—like the strength of 

the roof—were involved. 

This observation fit well with studies conducted as early as the 

1960's that had concluded that "whether or not the stress [from an 

extracted longwall panel] will influence a roadway depends more 

on the strength of the rocks which surround the roadway itself than 

on the width of the intervening pillar" (Carr and Wilson, 1982). 

Yet the variety and complexity of geologic environments had 

defied effective measurement, making it difficult to incorporate 

rock strength into design. The Coal Mine Roof Rating (CMRR) 

overcame this obstacle by providing a quantitative measure of the 

structural competence of coal mine roof (Molinda and Mark, 1994). 

When the CMRR was included in the analysis, ALPS could 

successfully predict the outcome in 85% of the case histories 

(figure 3). The analysis indicated that under very strong roof, the 

SF could be as low as 0.7, while under weak roof, an SF of 1.3 was 

required (Mark et al., 1994). 

80 

90 

Unsatisfactory 

Satisfactory 

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

ALPS SF 

CMRR 70 

60 

50 

40 

30 

20 

Figure 3. The ALPS case history data base and design
 

formula, showing the effect of roof quality as measured 

by the CMRR. 

Building on the success of the ALPS method, the research that 

culminated in the Analysis of Retreat Mining Pillar Stability 

(ARMPS) program employed an even larger case history data base. 

Here, most of the failures (unsatisfactory designs) involved pillar 

squeezes. For much of the data base, an SF of 1.5 seemed to 

separate the successful designs from the unsatisfactory ones (Mark 

and Chase, 1997). There were two interesting exceptions, however. 

At one extreme, the ARMPS data base included case histories 

of 12 massive pillar collapses, each of which had occurred so 

suddenly that they generated powerful airblasts (Mark et al., 1997). 

Like the more common squeezes, the collapses all involved cases 

where the ARMPS SF was less than 1.5. What really distinguished 

the sudden collapses from the slow squeezes, however, was the 

pillar=s w/h ratio. Every massive pillar collapse involved slender 

pillars whose w/h was 3 or less (figure 4). Subsequently, it was 

noted that all of Salamon’s South African collapse cases also 

involved pillars with w/h less than 4. Apparently, these types of 

failures form a separate class, distinct from the squeezes that are 

more common in the U.S. 

0 

0 1 2 3 4 5 

Width-to-Height Ratio 

Figure 4. Pillar collapse case histories from the US: ARMPS 

SF and width-to-height ratio. 

2.5 

Collapse 

Squeeze 2 

Success 

ARMPS SF 1.5 

1 

0.5 

The other anomaly occurred with the cases where the depth of 

cover exceeded 750 ft. In this group, both successes and failures 

occurred with SF that were well under 1.5, and it was much more 

difficult to separate them. A later study (Chase et al., 2002) added 

nearly 100 more deep-cover cases to the data base. Most of the 

failures were still squeezes, but bumps became more common at 

greater depth and with stronger roof. The study concluded that the 

apparent pillar strength for these squat, deep cover pillars was more 

variable than it was for the typically more slender, shallower pillar 

cases (figure 5). Roof quality was found to be significant; as was 

the use of barrier pillars (which no doubt reduced the applied load). 

Figure 5. The ARMPS case history data base and design 

formula showing the effect of increasing depth of cover. 
EVIDENCE FROM NUMERICAL MODELING 

The ability of numerical models to contribute in a meaningful 

way to the understanding of coal pillar mechanics depended upon 

the development of computer codes that included (Gale, 2005): 

•	 Post-failure simulation of the “strain softening” process. 

•	 Simultaneous assessment of shear, tensile and bedding plane 

failure within the material, together with the effect of joints 

and structural weakness. 

•	 Adequate simulation of the material properties and stress 

distribution within the ground. 

•	 Ability to simulate failure of strata above and below the 

pillars, and to simulate the correct stress path within the 

pillar system. 

Also, it was essential to validate the model results with 

extensive field monitoring programs. 

Su and Hasenfus (1999) employed finite element models (FEM) 

to explore the effect of various geologic conditions on pillar 

strength. They found that a rock parting may increase the pillar 

strength, while a clay parting could reduce it. A weak floor could 

reduce the pillar strength by as much as 50%. All of these effects 

were minimal for slender pillars, but became much more 

pronounced once the w/h exceeded 5 (figure 6). The models also 

indicated that varying the uniaxial coal strength had almost no 

effect on pillar strength. Field measurements of pillar strength, 

though limited in extent, supported the modeling results. 

Figure 6. Results of numerical simulation of coal pillar 

mechanics (Su and Hasenfus, 1999). 

Gale (1996, 1998) reported on stress measurements and FLAC 

models conducted during coal pillar design investigations in 

numerous mines. Gale concluded that a key advantage of 

numerical modeling is that it is possible to look beyond the pillar 

itself, and explore the interactions between the coal and the 

surrounding rock. He observed that pillar strengths seemed to fall 

into two groups: 

•	 Strong roof and floor rock where confinement was easily 

generated within the pillar, and; 

•	 Weak rock or bedding planes, which could fail either in 

compression or shear, and which limited the confinement 

that could be developed within the pillar, and thus limited 

the strength of the pillar system. 

As a first pass, Gale suggested that the strength of the first 

group could be approximated by the Bieniwski formula using a 

coal strength of about 900 psi, while the second group would 

require a coal strength of about 600 psi (see figure 7). These pillar 

strength estimates are not very different from those obtained from 

ALPS if a range of SFs from 1.0 to 1.5 is employed. 

Figure 7. Results of numerical simulation of coal pillar 

mechanics (Gale, 1996). 

Gale (2005) emphasized that the strength of a typical squat 

pillar system can be impacted by three main factors: 

•	 The presence of weak materials or bedding planes within the 

pillar, at the roof and floor interfaces, or in the immediate 

roof or floor; 

•	 A change in the stress field, particularly a reduction in the 

horizontal (confining) stress, such as that which can occur 

adjacent to a longwall gob, and; 

•	 The ability of the pillar to minimize roadway deformation. 

Gale (2005) also observed that “in small pillars (w/h <4 or 5) 

the ability to develop confinement in the pillars is less, and as such 

the post-failure load capacity of the system is low. The effect 

occurs irrespective of the strength of surrounding strata, and is also 

increasingly dependent upon actual coal properties.” 

One other development helped break down the walls between 

the empirical and the analytic and numerical methods. Mark and 

Iannacchione (1992) showed that each empirical formula actually 

implies a non-uniform stress distribution that can be calculated 

explicitly, so long as some reasonable assumptions are made. 

These implied stress distributions can be compared directly to a 

stress distribution obtained in a numerical model or a Wilson-type 

formula. The stress distribution implied by the Bieniawski formula 

was used to develop the Mark-Bieniawski formula for rectangular 

pillars (Mark and Chase, 1997). It has also been used to derive 

strength parameters for use in boundary element models (Heasley 

and Chekan, 1999; Karabin and Evanto, 1994). Thus the modeler 

can have confidence that the modeled pillar strengths are closely 
PLANNING BARRIER PILLAR PLACEMENT

Close multiple seam failure

linked to real-world behavior, while the model’s analytical 

mechanics allow it to accurately analyze complex mining situations 

including multiple seams, random pillar layouts and/or variable 

topography. The result is a powerful synthesis of empirical and 

numerical approaches to pillar design. 

PILLAR DESIGN FOR HIGHWALL MINING 

The new pillar mechanics paradigm is not in itself a method for 

pillar design. Rather, it provides a framework within which 

solutions to specific pillar design problems can be developed. One 

recent example is the new NIOSH program for highwall mining, 

called ARMPS-HWM. 

Highwall mining now accounts for perhaps 4% of U.S. coal 

production, and that percentage seems to be growing rapidly (Zipf 

and Mark, 2005). During highwall mining, overburden support is 

provided by web and barrier pillars (figure 8). Unfortunately, there 

have been a number of major pillar failures, both in the U.S. and in 

Australia (Zipf, 1999; Shen and Duncan-Fama, 2001). These have 

resulted in large rockfalls from the highwall, and in many cases 

have trapped the continuous mining machines underground 

(figure 9).

– 

Figure 8. Typical highwall mining web and barrier pillars. 

Figure 9. Consequences of the collapse of highwall mining 

pillars. 

Development of the ARMPS-HWM methodology involved a 

series of steps, each of which was informed by the lessons that have 

been learned about pillar design. The first step was to define the 

likely failure mode. Because highwall mining usually takes place 

under relatively light cover, the web pillars are typically slender 

(w/h<3). The pillar mechanics model suggests that the failure of 

such pillars could take the form of a sudden, massive collapse, and 

indeed most highwall mining pillar failures have been of this nature. 

Based on the experience of underground room-and-pillar 

mining, two alternative strategies have been developed to prevent 

massive pillar collapses (Mark et al., 1997): 

! Prevention: With the prevention approach, the panel pillars 

are designed so that collapse is highly unlikely. This can be 

accomplished by increasing either the SF of the pillars, or 

their w/h ratio. 

! Containment: In this approach, high-extraction is practiced 

within individual compartments that are separated by 

barriers. The key to the success of the containment approach 

is to limit the compartment to a width that is too narrow to 

collapse, or is at least narrow enough that the consequences 

of a collapse are manageable. The containment approach has 

been likened to the use of compartments on a submarine. 

The next step was therefore to select the appropriate panel width 

(distance between barriers) for the two design strategies. The 

potential for a collapse involving slender pillars depends upon the 

load-bearing capacity of the pillars and the local mine stiffness 

(LMS). The LMS in turn depends upon the width of the panel 

(Zipf, 1999). Based on case histories of pillar collapses 

underground (Mark et al., 1997), a maximum panel width of 200 ft 

was determined for the containment approach. In many cases, 200 

ft between barriers translates into about 10 holes. ARMPS-HWM 

also suggests that the maximum number of holes between barriers 

be limited to 20 even when the prevention approach is being used, 

to minimize the consequences of any potential failure. 

Next, it was necessary to define the load applied to, and the 

strength of, the web pillars. Tributary area provided a simple 

estimate of the load. The strength prediction would have been 

more complicated 25 years ago, because the webs are long strip 

pillars rather than the square pillars assumed by the traditional 

pillar strength formulas. Fortunately, the Mark-Bieniawski formula 

now provides a convenient way to estimate the strength of strip 

pillars (or indeed any rectangular or parallelogram shaped pillars). 

A critical part of the analysis was to select the minimum 

suggested SF for the web pillars. ARMPS-HWM again called upon 

the experience with slender pillars in underground room-and-pillar 

mining, where an SF=2.0 has been suggested when using the 

prevention approach (Mark et al., 1997). However, since the 

potential consequences of a highwall mining collapse are not as 

severe as those that could be associated with an underground 

collapse, a minimum suggested SF of 1.6 was recommended. 

For the containment approach, the barriers are assumed to be 

close enough together that they will shield the webs from the full 

overburden load. Rather than attempting to adjust the estimated 

load, however, the same effect was achieved by reducing the 

required SF. Therefore, a minimum suggested web SF of 1.3 was 

selected. 

With either the containment or the prevention approaches, it 

was recommended that the web w/h ratio be maintained at 1.0 or
Web pillar SF 

1.6 When the panel width (excluding the barrier) 

exceeds approximately 200 ft (60 m) 

1.3 When the panel width (excluding the barrier) 

is less than approximately 200 ft (60 m) 

Barrier SF 

2.0 When the barrier’s width-to-height ratio < 4.0 

1.5 When the barrier’s width-to-height ratio >= 

4.0 

Overall SF 

2.0 Applicable to all conditions 

higher, to help maintain web pillar integrity. In addition, there is 

very little data available on coal pillars with w/h ratios that are less 

than 1.0, so it is unclear whether the pillar strength formulas are 

still valid. Recent studies in underground stone mines have found 

that the strength of very slender pillars can be highly variable 

because they are subject to different failure processes than 

traditional pillars (Esterhuizen, 2006). 

The barrier pillars are integral part of the highwall mining pillar 

system. The Mark-Bieniawski formula can again be used for the 

pillar strength, but the loading estimate was a little more 

complicated. ARMPS-HWM assumed a “worst case” where the 

webs on either side of the barrier might have failed. The residual 

strength of the failed webs would probably be very low, but would 

not be less than the gob that is modeled in ALPS and ARMPS. 

Therefore the same default abutment angle of 21 degrees is used to 

define the barrier pillar load in ARMPS-HWM (figure 10). 

Loads on Barrier Pillars 

Wide Panel/ 

Shallow Cover 

Barrier 

Narrow Panel/
 

Deep Cover 

Barrier 

Figure 10. Barrier pillar loadings used in ARMPS-HWM. 

The w/h ratio of the barrier was another concern. If the 

potential failure is to be limited to a single compartment, it is 

essential that the barrier pillar itself not collapse. The likelihood of 

collapse is minimized when the barrier is not a slender pillar itself. 

Therefore the suggested minimum SF for the barrier is 2.0 when 

the barrier’s w/h is less than 4.0, and 1.5 when the w/h is greater 

than 4.0. 

Table 1 summarizes the design criteria used in ARMPS-HWM. 

In addition to the SFs for the webs and the barriers, ARMPS-HWM 

also calculates an overall SF for the pillar system consisting of one 

barrier and one panel of webs. The suggested minimum SF for the 

system is 2.0. 

Table 1. ARMPS-HWM design guidelines. 

ARMPS-HWM SUGGESTED MINIMUM SF 

The final step was to verify ARMPS-HWM using real-world 

case history data. The analysis could not be based on collapse 

experience, because the available collapse data was considered 

insufficient. However, NIOSH was able to collect data from more 

than 3000 successful highwall mining holes mined in southern 

West Virginia during a recent three-year period (Zipf and Mark, 

2005). Each case was analyzed to determine the maximum depth 

of cover, the web thickness, the number of holes between barrier 

pillars, and the barrier width. Some of the results are shown in 

figure 11. Based on these analyses, it appears that ARMPS-HWM 

does provide a reasonable first approximation of minimum 

suggested pillar widths. 

600 

SF=1.6 

Wide Panels 

500 

Cover Depth (ft) Depth of Cover (ft) 400 

300 

200 

100 

0
 

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 

ARMPS HWM Stability Factor 

600 

SF=1.5 

500 

400 

300 

200 

100 

0 

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 

Barrier Stability Factor (SF) 

Figure 11. Comparison of ARMPS HWM design equations to 

the data base of successful HWM case histories in central 

Appalachia. (a) Web pillars. (b) Barrier pillars. 

NEW FRONTIERS IN PILLAR DESIGN 

Underground coal mining continues to evolve, and pillar design 

must keep up. Three areas of current interest are discussed below. 

Updating Older Empirical Formulas 

Empirical design methods draw their strength from being 

closely connected to actual mining experience. But what happens 

when that experience changes over time? The ALPS method, for 

example, is based on longwall mining case histories from the 

1980’s and early 1990’s. A lot has changed since those early days 
that have affected gate road stability and design requirements, 

including: 

•	 Improved tailgate support: Nearly every ALPS case history 

employed wood cribs for secondary support in the tailgate. 

Today, wood cribs have been almost entirely replaced by 

concrete, engineered wood, or cable supports. The greater 

strength and stiffness of these new roof supports may have 

reduced the amount of pillar support necessary for stability. 

•	 Ventilation requirements: The simple U-ventilation system 

was used in most of the ALPS case histories. This meant 

that the only ventilation requirement was to keep the tailgate 

open outby the longwall face for return air. Currently, most 

U.S. longwalls bring some fresh air up the tailgate, and there 

are usually some expectations that the center entry will 

remain available for airflow. These new requirements may 

place an extra burden on the pillar system. 

•	 Increased extraction rates and panel dimensions: Tailgate 

stability problems are much more likely to develop when the 

full tailgate abutment load sits in one place for a long period 

of time. Since longwalls now mine coal much more rapidly 

than they did 15 years ago, and they are subject to far fewer 

mechanical delays, it should be easier to “run away” from 

stability problems before they get out of control. On the 

other hand, wider faces mean that each pass takes longer to 

complete, and longer panels subject future tailgates to the 

side abutment load for longer periods of time. 

For these reasons, it probably makes sense to revisit ALPS with 

an updated case history data base, one that reflects the longwall 

experience of the past 15 years. A new study would also benefit 

from the powerful analytical tools that have been developed during 

that time, including Support Technology Optimization Program 

(STOP) for evaluating tailgate support, Analysis of Roof Bolt 

Systems (ARBS) for rating primary support, and Analysis of 

Horizontal Stress in Mines (AHSM) for measuring the impact of 

horizontal stress on tailgate stability. 

Seam Specific Coal Strengths 

The issue of coal strength has bedeviled pillar design from the 

beginning. The “classic” approach was to test the uniaxial 

compressive strength (UCS) of small specimens in the laboratory, 

and then apply a “size effect” reduction factor to obtain an estimate 

of the in situ strength. This approach was thoroughly discredited 

by a comprehensive study, involving 4000 individual UCS test 

results from over 60 coal seams, which found that there was no 

correlation between the laboratory UCS and actual pillar strength 

(Mark and Barton, 1996). It seems that laboratory tests actually 

measure the degree of cleating in the specimen, but that cleat 

density has little relationship to pillar strength. 

Mark and Barton’s study also confirmed that the design 

formulas were far more successful in predicting performance when 

a uniform strength of 900 psi was employed. Studies conducted in 

South Africa and Australia have also found that a uniform coal 

strength worked reasonably well in pillar design formulas (Galvin 

et al., 1999). 

While it is fortuitous that a uniform coal strength is sufficient 

for many pillar design problems, it is hardly satisfactory. Mark and 

Barton’s study did not prove that all coal seams actually are the 

same strength, it only showed that laboratory testing was no help in 

identifying the differences that surely exist. Recent South African 

studies have indeed focused on determining seam-specific strengths 

through back calculation (Salamon et al., 2006), and they have 

concluded that there are significant variations between the 

coalfields. 

In obtaining a solution, it will probably be necessary to divide 

the problem into two parts. For pillars whose w/h ratio is less than 

about 4 or 5, the in situ UCS may be an important contributing 

factor to overall pillar strength (Gale, 2005). But since laboratory 

UCS tests don’t correlate with in situ strength, and in situ testing is 

too expensive, could rock mass classification help? Some tentative 

efforts in this direction were too early to be informed by more 

recent understanding of pillar mechanics (Kalamaras and 

Bieniawski, 1993; Trueman et al., 1992). A new attempt would 

have to focus on the presence of softer and harder layers of coal 

that can be found within a pillar, as well as partings consisting of 

rock, bone coal, and clay. For U.S. applications, it would also be 

necessary to consider the effect of the rock “cap” that is created 

when extra height is mined above thin seams. 

The strength of squat pillars, in contrast, is determined almost 

exclusively by the confinement that can be generated within them. 

The confinement in turn is determined by the strength of the 

bedding planes within the pillar, roof and floor contacts, and even 

weak bedding planes in the immediate roof and floor. Developing 

simpler techniques for evaluating the strength of these contacts, and 

implementing them in pillar design, presents a significant challenge. 

Protecting Infrastructure and the Environment 

Pillar design is becoming increasingly important to protect 

surface structures, crop land, gas wells, surface streams, and 

groundwater supplies. “Guaranteeing” long-term stability is a 

tricky proposition, however. 

One issue is the long-term strength of the pillars themselves. A 

South African study showed that the rate of pillar scaling (or 

sloughing) increased with the height of the pillars, but decreased 

over time (van der Merwe, 1998). It is not clear whether these 

results were applicable outside the Vaal Basin, however. In the 

U.S., one study found that as clay partings in the ribs weathered 

over time, their strength decreased substantially (Biswas et al., 

1999). On the other hand, the rate of deterioration of the partings 

(towards the center of the pillar) decreased as the pillars aged, and 

the coal itself was hardly affected by weathering. Long term pillar 

failure has also been associated with floor failure (Chugh and Trent, 

1996) and with inundation of sealed workings (Grgic et al., 2006). 

The long-term strength of coal pillars is only one aspect of the 

problem, however. A more significant issue may be related to the 

potential consequences of pillar “failure.” Many subsidence 

regulations were developed when most coal mining involved room 

and pillar extraction of “thick” seams (>6 ft) at shallow cover 

(Unrug et al., 2001). Under such conditions, a pillar squeeze or 

collapse would be expected to result in severe damage to the 

surface. Any “failure” of the squat pillars employed today surely 

causes much less deformation at the seam level, and the resulting 

ground strains are further mitigated by the greater depths of cover. 

It seems that there may be plenty of room to update subsidence 

rules of thumb by eliminating antiquated concepts based on 

extraction ratios and laboratory UCS testing, and replacing them 

with more scientific ones that employ w/h ratios, stability factors, 

and calculated surface strains. 

One reason that long term pillar strength remains so poorly 

understood, at least in the U.S., is that there has never been a 
thorough, scientific, empirical study of pillar failures in abandoned 

mines. There should be plenty of raw material, in the form of case 

histories in the files of state subsidence regulatory agencies. The 

few studies that have been published have used only a small 

fraction of the case histories that should be available (Marino, 

1989). Since subsidence does not pose a hazard to today’s working 

miners, however, government safety research funds are not 

available for such a study. 

CONCLUSIONS 

Recent years have seen significant advances in the state-of-theart in coal pillar design. From a practical standpoint, the 

development of reliable empirical methods like ALPS and ARMPS 

has been particularly valuable. They have been widely accepted 

throughout the mining community because they have been verified 

by extensive data bases of real-world case histories, and because 

they have been readily available in user-friendly computerized 

formats. The tremendous advances in numerical modeling have 

been another important success story. 

The research has led to some other important conclusions, 

including: 

! Laboratory testing of small coal samples, particularly 

uniaxial compressive strength tests, are not useful for 

predicting pillar strength; 

! The strength becomes more difficult to predict as the pillar 

becomes more squat; 

! The w/h ratio is important for predicting not just the pillar 

strength, but the mode of failure, and; 

! Many ground control problems must be considered from the 

standpoint of entry stability, where pillar behavior is just 

one component. 

Certainly, more work remains before the age-old questions of 

pillar design is finally solved. In particular, there is much more to 

learn about the mechanics of squat pillars and roof-pillar-floor 

interactions. Currently, there is no accepted way to determine the 

frictional characteristics of the contacts, bedding planes, and 

partings that are so crucial to pillar strength. It is similarly difficult 

to characterize the bearing capacity of the floor. Simple, 

meaningful field techniques for estimating these properties will be 

necessary for further progress with either numerical or empirical 

techniques. Indeed, the cross-pollination between the numerical 

and empirical methods that has characterized the recent past can be 

expected to bear further fruit in the future. 

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