TOWARD PILLAR DESIGN TO PREVENT COLLAPSE OF ROOM-AND-PILLAR MINES

R. Karl Zipf, Jr.

Chief, Catastrophic Failure Detection and Prevention Branch

NIOSH – Spokane Research Laboratory

Spokane, WA 99207

509-354-8090

rzipf@cdc.gov
ABSTRACT

In some instances, extensive room-and-pillar workings

can collapse with little warning and pose a serious risk to

underground miners. Traditional strength-based pillar

design methods applicable to coal or hard-rock mines use a

factor of safety defined as pillar strength divided by pillar

stress. Factor of stability, defined as local mine stiffness

divided by post-failure pillar stiffness, may offer a way to

design room-and-pillar mines and eliminate collapses.

Three alternative design approaches to decreasing the risk of

large-scale catastrophic collapses are described: the

containment approach, the prevention approach, and the

full-extraction approach. Until good data on the post-failure

behavior of pillars become available, the containment and

full-extraction options are the safest. The limitations in our

ability to evaluate both the stability of old workings and the

long-term performance of room-and-pillar mines are

described.

INTRODUCTION

Room-and-pillar mining accounts for a significant

portion of the total mineral production in the United States

As shown in table 1, well in excess of $6 billion worth of

mineral commodities are produced each year by this method.

A substantial portion ($3.55 billion) of coal production still

comes from room-and-pillar mining. Metallic minerals

valued at about $1 billion, plus nonmetallic minerals valued

well in excess of $1 billion, are also produced via room-andpillar mining. A significant ($600 million) and growing

portion of stone and aggregate production uses room-andpillar mining. In addition, many other mineral commodities

not noted in this table (talc, iron, copper) are or have been

produced in the United States using the room-and-pillar

technique.

The objective of this paper is to show mine layouts for

selected coal, metal, and nonmetal mines that have

experienced large-scale, catastrophic pillar collapse. Basic

pillar mechanics are reviewed along with the important

factors that govern stability. Finally, alternative design

approaches are discussed that decrease the risk of

catastrophic collapse. Research issues related to collapse of

room-and-pillar mines are summarized.

If the strength of a pillar in a room-and-pillar mine is

exceeded, it will fail, and the load that it carried will be

transferred to neighboring pillars. The additional load on

these pillars may lead to their failure. This mechanism of

pillar failure, load transfer, and continuing pillar failure can

lead to the rapid collapse of very large areas of a mine. In

some cases, only a few tens of pillars might fail; however,

in extreme cases, hundreds, even thousands, of pillars can

fail. This kind of failure has many names—progressive

pillar failure, massive pillar collapse, domino-type failure,

or pillar run. Swanson and Boler (1995) coined the term

“cascading pillar failure,” or CPF, to describe these rapid

pillar collapses. A recent review by Zipf (in press) provides

some documentation on 21 instances of large-scale pillar

collapses in room-and-pillar mines, mainly in the United

States

CPF can have catastrophic effects on a mine, and

sometimes these effects pose a greater health and safety risk

than the underlying ground control problem. Usually, the

CPF induces a devastating airblast caused by displacement

of air during the collapse. An airblast can totally disrupt the

ventilation system at a mine by destroying ventilation

stoppings, seals, and fan housings. Flying debris can

seriously injure or kill mining personnel. CPF might also

fracture large volumes of rock in the pillars and the

immediate roof and floor, leading to the sudden release of

large quantities of methane into the mine atmosphere and

possibly a methane explosion.

Table 1 – Value and production by room-and-pillar mining in the United States

Commodity Total U.S. production, Room-and-pillar mining Typical

extraction, 

tons Percentage Tonnage Value, million U.S.

dollars 

%

Coal1 1,014,000,000 20 202,000,000 $3,550 60

Lead2 493,000 90 444,000 $432 75

Zinc2 722,000 60 433,000 $491 75

Soda ash2 10,100,000 80 8,000,000 $664 65

Potash2 1,300,000 100 1,300,000 50

Salt2 40,800,000 60 32,000,000 50

Gypsum2 19,000,000 50 9,000,000 $66 75

Stone and aggregate3 1,200,000,000 10 120,000,000 $600 75

1

 National Mining Association, Coal Data, 1999. 2 U.S. Geological Survey, National Mineral Surveys, 1998. 3 National Stone

Association, The Aggregate Handbook, 1996.
Figure 1.—Mine layout for coal mine collapse 1 (Chase et al. 1994)

Many case histories exist of CPF in coal mines. The

most infamous example is the Coalbrooke Colliery in South

Africa where 437 miners perished when 2 km2 of the mine

collapsed within a few minutes on January 21, 1960 (Bryan

et al., 1964). During the 1990’s, U.S. coal mines had at

least eight of these failures (Zipf, in press). Recent reports

from India (Sheorey et al., 1995) and Australia (Galvin,

1992) indicate that catastrophic pillar collapses and the

associated airblasts have caused problems there as well.

Chase et al. (1994) documented a massive pillar collapse.

Figure 1 shows the mine layout and collapse area. Seam

thickness was about 3 m. Using 6-m-wide rooms, a system

of 12-m2

 pillars with a width-height ratio of 4 was

developed on advance. On retreat, these pillars were split

down the middle with a 6-m-wide room to leave two 3-mwide fenders having a width-height ratio of 1. The mine had

split about nine rows of pillars with this method when a CPF

occurred, causing seven rows of fenders to fail. The airblast

destroyed 26 stoppings and caused one injury. The

extraction ratio in the failed area was about 78%.

A recent Mine Safety and Health Administration

(MSHA) report (Richmond, 1998) describes another

massive ground failure in a panel in the process of being

secondarily mined by pillar splitting. Figure 2 shows the

mine layout and collapse area. Seam thickness was about 2

m. The collapse area measured approximately 180 by 460

m and occurred in a panel adjacent to a set of mains. Pillars

in the mains were 12 by 18 m, and panel pillars were 9 by

18 m. The panel pillars were mined with a 6-m-wide split,

leaving 6- by 9-m stumps having a width-height ratio of

about 3. An array of approximately 9 pillars by 14 pillars

collapsed. The airblast destroyed 23 stoppings plus the

weakest wall of the fan house. Fortunately, there were no

injuries. The extraction ratio in the failed area was about

70%.

Catastrophic pillar failures have also happened in many

metal mines. At least four examples have occurred in the

United States since 1972 (Zipf, in press). Dismuke et al.

(1994) describe a large pillar collapse in a major section of

a room-and-pillar base metal mine. Figure 3 shows the

collapse area. The failure began in four centrally located

pillars and spread rapidly to include almost 100 pillars.

Pillar width was 8.5 m, and room width was 9.7 m. Pillar

height was about 12 m for a width-height ratio of 0.70. The

extraction ratio was about 78%. Damage caused by the

airblast from the collapse was minor, and no one was

injured.

The largest and most devastating examples of CPF have

occurred in nonmetal mines. During the 1990’s, U.S.

nonmetal mines had at least five such failures (Zipf, in 
Figure 2.—Mine layout

for coal mine collapse 2 (Richmond 1998)

Figure 3.—Mine layout for metal mine collapse (Dismuke et al. 1994)
press). Swanson and Boler (1995) and Zipf and Swanson

(1999) describe the mine geometry and aftermath of a

collapse in a major evaporite mine. Figure 4 shows the

layout for this mine. The bed mined had a thickness of

about 2.85 m. During advance mining, a system of 12-mwide chain pillars using 4.3-m-wide rooms was developed

off a set of mains. At the same time, the rooms were mined

down one side, leaving long, narrow, 3.8-m-wide panel

pillars having a width-height ratio of 1.33. A small pillar

about 7.6 m wide with a width-height ratio of about 2.66

was left between panels. On retreat, additional rooms were

mined on the other side of the chain pillars. The mine

achieved an overall extraction of about 60%; extraction

within a panel was a little more than 70%.

Mines experiencing a cascading pillar failure generally

exhibit the following characteristics.

(1) Extraction ratios are usually more than 60%. A high

extraction ratio will put pillar stress close to peak strength

and provide ample expansion room for the failed pillar

material.

(2) Width-height ratios of pillars are always less than 3 for

coal mine failures, usually much less than 1 in metal-mine

failures, and less than about 2 for nonmetal mine failures.

A low width-height ratio ensures that the failed pillar

material can easily expand into the surrounding openings

and that the failed pillar will have little residual load-bearing

capacity.

(3) The number of pillars across the panel width is always

at least five and usually more than 10, which typically

ensures that pillars have reached their full tributary area

load. Minimum panel widths for CPF are at least 80 m.

(4) Substantial barrier pillars with width-height ratios

more than 10 are absent from the mine layout.

(5) Although CPF seems more prevalent in shallow mines

less than 100 m deep, this may be only a reflection of the

prevalence of shallow room-and-pillar coal mines.

TRADITIONAL STRENGTH-BASED PILLAR

DESIGN METHODS

Traditional strength-based pillar design first requires an

estimate of pillar stress and then an estimate of pillar

strength. The factor of safety for the pillar is then evaluated

as pillar strength divided by pillar stress. An acceptable

safety factor depends on the tolerable risk of failure. A

safety factor of 2 is typical for pillars in main development

headings or panels during advance mining. Safety factors of

1.1 to 1.3 are typical for panel pillars after retreat mining.

Safety factors much less than 1 are possible within panels

where pillar failure is the intent.

Estimating pillar stress first requires an estimate of in

situ vertical stress as FV = (z, where ( is the unit weight of

rock and z is depth to the mining horizon.

The tributary area method (Farmer, 1992) then provides

a first-order estimate of average pillar stress. The tributary

area approach makes many simplifying assumptions: the

mined area must be extensive, all pillars should have the

same dimensions, and pillars at the edge of a panel should

have the same stresses as those in the middle. 

The method ignores the deformation properties of the

surrounding rock mass relative to pillar rock along with any

rock failure, such as pillar yielding and associated stress redistribution. In actuality, pillars at the center of a panel have

higher stress levels than pillars at the edge of a panel.

Quasi-three-dimensional, boundary-element programs such

as ExamineTAB (2000), MULSIM/NL (Zipf, 1992a, 1992b)

and LAMODEL (Heasley, 1997, 1998) can provide

reasonable estimates of pillar stresses across a panel or

within an individual pillar.

Over the past several decades, a large amount of rock

mechanics literature has addressed pillar strength in both

coal and metal/nonmetal mines (Obert and Duvall, 1967;

Hoek and Brown, 1980; Bieniawski, 1992; Mark and

Iannacchione, 1992; Brady and Brown, 1993; Mark, 1999).

Much of this work is empirical and has addressed two

issues—the size effect whereby rock strength decreases as

specimen size increases and the shape effect whereby rock

strength increases as the width-height ratio increases.

Classic empirical pillar strength formulas usually follow

one of two general forms.

σ σ P S a b

W

H

= +

⎛

⎝

⎜ ⎞

⎠

⎟ '

or

σ

α

P K β

W

H =

Pillar strength formulas by Obert and Duvall (1967) and

Bieniawski (1968) follow the first form, whereas formulas

by Salamon and Munro (1967) and Holland (1964) follow

the second. In these forms, size effect is accounted for

directly via the unit pillar strength σS’ or the rock constant

K. σS’ is the strength of a cubical pillar (width-height = 1)

at or above the critical size, and K is a constant

characteristic of the pillar rock. For most U.S. coal, Mark

(1999) recom-mends 6.2 MPa for σS’. In the Bieniawski

formula, the constants a and b are 0.64 and 0.36,

respectively.

Traditional strength-based pillar design methods can

determine panel pillar size directly. Operational considera-
Figure 4.—Mine layout for evaporate mine collapse (MSHA 1996). A, Overall layout for part of mine; B,

layout of southwest panels; C, details of typical panel
tions such as equipment and productivity requirements

frequently set the panel width, and usually it is set large.

Barrier pillar width will then depend on panel width. Based

on strength considerations alone, a narrow panel will require

a narrow barrier pillar and a wide panel will require a wide

barrier pillar. Rock mechanics factors such as the strength

and deformability of the rock mass do not into enter panel

width determination with the traditional strength-based

design methods. Other rock mechanics considerations are

needed to determine maximum panel width rationally along

with panel pillar and barrier pillar sizes.

STABILITY-CRITERION-BASED PILLAR DESIGN

METHODS

Understanding CPF requires more than just understanding pillar strength and applied pillar stress of the traditional

strength-based pillar design method using factor of safety.

The underlying mechanics of CPF are more complex. The

nature of the pillar failure process depends on the relative

magnitude of certain mechanical properties of the rock mass

and the pillar. 

A rock specimen loaded in a laboratory test frame is

analogous to a mine pillar loaded by the surrounding rock

mass. Based on this analogy, Salamon (1970) developed a

stability criterion that determines whether the failure process

occurs in a stable, nonviolent or in an unstable, violent

manner. Figure 5 illustrates the criterion. Stable, nonviolent

failure occurs when

K K LMS P >

and unstable, violent failure occurs when

K K LMS P <

where is local mine stiffness and is post- KLMS KP

failure stiffness at any point along the load-convergence

curve of the pillar.

In the traditional strength-based pillar design method,

pillars are sized using a factor of safety, which is defined as

pillar strength divided by pillar stress. Stability-criterionbased pillar design methods also consider a factor of

stability, which is defined as the local mine stiffness (KLMS)

divided by post-failure pillar stiffness (KP). Considering the

factor of stability leads to three different approaches to

controlling large collapses in room-and-pillar mines:

containment, prevention, and full extraction.

Containment

In the containment approach, shown in Figure 6, an array

of panel pillars that violate the local mine stiffness stability

criterion and can therefore fail in an unstable, violent

manner if their strength criterion is exceeded are surrounded

or “contained” by barrier pillars. The panel pillars have a

factor of safety greater than 1, but the factor of stability is

less than 1. The primary function of barrier pillars is to limit

potential failure to just one panel. Barrier pillars have a

high width-to-height ratio, typically greater than about 10,

and contain panel pillars with a low width-to-height ratio,

typically in the 0.5 to 2 range. It is a noncaving room-andpillar method in that panel pillars are not meant to fail

during retreat mining. Applying the containment approach

does not explicitly require evaluation of the factor of

stability, hence calculation of local mine stiffness, and good

data on the post-failure stiffness of pillars are not required.

Figure 5.—Unstable violent failure versus stable nonviolent failure. A, Unstable failure in a soft loading system. Loading machine

stiffness is less than post-failure stiffness. B, Stable failure in a stiff loading system. Loading machine stiffness is greater than postfailure stiffness. (Swanson and Boler, 1995)
Figure 6.—Containment approach to room-and-pillar layout. A, Mine layout. Pillar failure is compartmentalized. B, Stability

condition is such that pillars with low width-height ratios violate the local mine stiffness stability criterion, and a panel can fail

violently. Adequate barrier pillars that restrict the spread of unstable failure surround them. Extraction for layout shown is 59%.

Prevention

In contrast to the containment approach, the prevention

approach shown in Figure 7 “prevents” CPF from ever

occurring by using panel pillars that satisfy both the local

mine stiffness stability criterion and a strength criterion.

The factor of safety and the factor of stability for the panel

pillars are both greater than 1. Therefore, panel pillars

cannot fail violently, and CPF is a physical impossibility.

Strictly speaking, this approach may not need barrier pillars

to ensure overall stability against CPF; however, their use is

still advisable. To satisfy the local mine stiffness stability

criterion, the panel pillars will usually have high widthheight ratios (greater than about 3 or 4) and high strength

safety factors as well (greater than 2). Another approach to

increase local mine stiffness and satisfy the stability

criterion is to limit panel width with properly spaced and

sized barrier pillars. Applying the prevention approach

requires evalua-tion of the factor of stability, calculation of

local mine stiffness, and good data on the post-failure

stiffness of pillars.

Full-Extraction Mining

The full-extraction approach shown in Figure 8 avoids

the possibility of CPF altogether by ensuring total closure of

the opening and full surface subsidence on completion of

retreat mining. This approach does not require barrier

pillars for overall panel stability; however, they are needed

to isolate extraction areas and protect mains and bleeders.

The factor of safety for the panel pillar remnants is much

less than 1 to force them to fail immediately after retreat

mining.

CONCLUSIONS AND RECOMMENDATIONS

CPF is a potential problem faced by all room-and-pillar

mining operations. CPF occurs when one pillar fails

suddenly, which then overstresses the neighboring pillars,

causing them to fail in very rapid succession. Within

seconds, very large mining areas can collapse while giving

little or no warning. The collapse itself poses danger to

miners. In addition, the collapse can induce a violent

airblast that disrupts or destroys the ventilation system.

Additional hazards to miners exist if the mine atmosphere

becomes explosive as a result of a collapse. At least 17
Figure 7.—Prevention approach. A, Mine layout. Unstable pillar failure is impossible because panel pillars with higher width-height

ratios or a combination of panel and barrier pillars satisfy the local mine stiffness stability criterion. B, CPF is physically impossible.

Extraction for layout shown is 63%.

collapses have occurred in the past 20 years in U.S. roomand-pillar mines (Zipf, in press).

Traditional strength-based design methods using a factor

of safety are not sufficient to eliminate the possibility of

CPF in room-and-pillar mines, and the number of

documented collapses in the United States alone provides

mute testimony to that statement. Pillar arrays with large

average strength safety factors can fail in a CPF if just a few

pillars in the array begin to fail. Pillars with large strengthbased safety factors (for example 1.5) still have a finite

probability of failure, and if the number of pillars in an array

is large, failure somewhere in the array can become a near

certainty, so that failure could in turn initiate CPF.

Advanced rock mechanics considerations using the local

mine stiffness stability criterion and a factor of stability are

needed to design room-and-pillar mines that control CPF.

The mechanics of CPF are well understood. Strainsoftening behavior is the essential mechanical characteristic

of pillars that fail rapidly via this mechanism. Pillars that

exhibit strain-softening behavior can undergo a rapid

decrease in load-bearing capacity upon reaching their

ultimate strength. The strain-softening behavior of pillars

depends on both inherent material properties and geometry.

Pillars with a low width-height ratio exhibit a greater degree

of strain-softening behavior than pillars with a higher widthheight ratio and typically elastic-plastic or strain-hardening

material behaviors.

While the principles behind CPF are fairly well

understood, there are significant gaps in our ability to

evaluate the stability of existing room-and-pillar mines and

assess the performance of alternative mine layouts. The

limitations stem from a lack of

(1) Field data on the complete stress-strain behavior of

full-scale mine pillars,

(2) Material property data under dynamic loading

conditions, and

(3) Simple assessment techniques to delineate and monitor

how stresses are transferred among panel pillars, barrier

pillars, and solid abutments.

Such rock mechanics knowledge is critical for utilizing

the design approaches suggested. Large mine collapses can

pose enormous safety hazards to miners and room-and-pillar

mining operations. Of the three alternative design

approaches described to decrease the risk of large-scale

catastrophic collapses, containment and full extraction
Figure 8.—Full extraction approach. A, Mine layout. Failure of pillar remnants along with overburden occurs immediately after pillar

extraction. B, Stability condition. Retreat mining must ensure development of sufficiently weak remnant pillars. Extraction for

layout shown is 67%.

options are the safest approaches to apply until good data on

the post-failure behavior of pillars become available. Then,

the prevention approach based on an evaluation of the factor

of stability with the local mine stiffness stability criterion

may enable safe room-and-pillar mining with higher

extraction.

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