The Unpredictable Life Cycle of a Coal Pillar 

Stephen C. Tadolini, Chief, Rock Safety Engineering Branch 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA

Peter (Yunqing) Zhang, Sr. Geotechnical Engineer

Pennsylvania Services Corporation (affiliate of Foundation Coal) 

Waynesburg, PA

ABSTRACT 

A unique circumstance created by monitoring a pre-driven 

longwall recovery room permitted measuring the stresses of a coal 

pillar throughout its entire life cycle in less than a week. A fender 

pillar, created in approximately the middle of a longwall panel at a 

depth of 650 ft, transformed from a solid barrier pillar - to a 

yielding pillar - to a residual pillar as 3 ft slices were methodically 

removed with the longwall shearer. The complete transformation, 

or life cycle, took place in less than 12 hours. The stresses were 

quickly transferred from the pillar onto the standing pumpable 

concrete supports and into the outby pillars. Roof to floor closure 

measurements, combined with the timing of the pillar behavior, 

provides a detailed look at the uncontrollable convergence of 

underground mine openings. Pillars remain the most important 

form of “primary support” and understanding these life cycles is 

vital for safe and efficient mine design, in both room and pillar and 

longwall panel extractions. 

INTRODUCTION 

Pillar design and stability analysis for coal mines has continued 

since Vicat, in 1833, provided a simple equation for the strength of 

a rectangular shaped specimen that was loaded in compression. In 

1867, Bauschinger produced pillar design equations that assembled 

much of the work that had been completed to that time. In 1911, 

Bunting, utilizing the tributary area method, was the first to 

indicate that the pillar size should be increased proportionately with 

the depth of mining and the thickness of the coal seam. 

Pillar stability analysis in coal mines is a complex undertaking 

because of the variable stages of loading and the changes in stress 

during critical times. The stability requirements of a pillar also 

vary, ranging from long-term applications such as bleeder and main 

entry stability or short-term applications, the premise for this paper, 

such as a fender pillar created during the extraction of a longwall 

panel being terminated in a pre-driven recovery room. 

Irrespective of the methodology used to design coal pillars 

their primary purpose is to serve as the primary support system 

between the immediate roof and floor. Previous research into the 

examination of the fender pillar indicated that it was a critical, if 

not one of the most critical, components of the recovery room 

design. A 3-dimensional finite element model was used to simulate 

the global ground reaction response to the rock behavior around the 

longwall panel while the submodel zoomed into a specific section 

of the room to provide the detailed response (Tadolini, et al., 2002). 

The results of this modeling exercise indicated that the fender pillar 

created by extracting the longwall panel, behaved most closely to 

that of a traditional yield pillar. Yield pillars are defined as a pillar 

that yields or fails upon isolation from the coal seam or yields 

during the longwall development cycle. The yield pillar allows a 

general lowering of the roof and subsequent transfer of overburden 

load onto the neighboring structures. This concept is used 

primarily in deep mining conditions, to improve ground control 

conditions, reducing the effects of high stresses which can cause 

subsequent coal mine bumps or bounces (Tadolini and Haramy, 

1992). 

As part of a comprehensive pre-driven recovery room design 

and evaluation, a fender pillar was instrumented to evaluate its 

loading behavior during its entire life-cycle. Additionally, the 

amount and timing of the room convergence was recorded to assist 

with understanding the fundamental behavior of the pillars 

designed to support the roof in a pre-driven recovery room. The 

ultimate goal of this research work is to more accurately estimate 

the NIOSH developed ground reaction curve for coal mining at 

different depths and geological settings and determine the effect, if 

any, of standing and intrinsic support systems. 

Yield Pillar Theory 

Under normal loading conditions, coal demonstrates a strainsoftening behavior under both uniaxial and triaxial compression 

(figure 1). When a coal pillar is stressed, fractures around the 

perimeter will occur along the line from point A to B. The onset of 

fracture does not mean that the pillar has reached peak strength 

(failed). If the pillar is large enough to sustain the loads applied 

throughout the mining cycle without reaching peak strength, this 

pillar is termed a "stiff” or "large" pillar. For the region up to the 

peak strength (B), the coal behaves elastically and elastic-plastic 

after yield begins. After the pillar peak strength has been exceeded, 

the pillar will still have some load-bearing capacity or residual 

strength, and enter a phase often called strain-softening. The yield 

stress is the stress at which permanent deformation first appears. 

After the peak strength has been exceeded and the pillar continues 

to yield, permanent deformation begins to take place and any 

additional loading may result in excessive entry deformation. As 
Figure 1. Idealized coal behavior. 

stress continues to increase due to stress redistribution caused by 

adjacent panel mining, the pillar can no longer fulfill the support 

function, and entry closure may occur. 

Mining at great depths often results in ground control problems 

caused by high stress concentrations. These problems have 

increased the interest in the yield-pillar concept as a means to 

redistribute stresses onto the adjacent panels and away from the 

gateroads. Several theories were developed for designing yield 

pillars in underground longwall mines. Various cases of yieldpillar response to loading were observed in underground coal mines. 

Numerous chain (yield) pillar designs were investigated to 

determine what, if any, site-specific conditions may contribute to in 

situ behavior. Babcock (1985) and Peng (1985) have utilized 

laboratory studies and numerical modeling allied with field studies, 

respectively, to investigate the effects of different roof and floor 

materials on pillar strength. These studies indicate that relative 

roof-to-coal and floor-to-coal properties influence both pillar 

stability and failure mode. As variable near-seam lithology, 

physical properties, seam thickness, and overburden depths are 

more often the rule rather than the exception, yield-pillar design 

concepts that incorporate some of these factors were investigated. 

The yield-pillar theories that most closely resemble the behavior of 

a fender pillar in a pre-driven recovery room are Wilson's (1972, 

1977, and 1981) confined core approach. 

Wilson's confined core theory (1972) proposes that two regions 

exist within a typical pillar: a confined core region and a yield zone. 

The yield zone is located along the pillar ribs and surrounds the 

confined core found in the pillar interior, as shown in figure 2, inset 

A. Coal strength ranges from nearly zero at the ribs to full strength 

in the confined core. The highest vertical stress between the core 

and the yield zone is termed the peak abutment stress, σv. The 

stresses in the confined core, σm, are the average stresses prior to 

mining and are not influenced by the excavation of the entries or 

the superimposed loads from panel mining. Wilson defines the 

yield pillar width, W, as 2Xb; that is, when the two yield zones 

meet, the coal strength in the entire pillar is exceeded by the loads 

imposed, as illustrated in figure 2, inset B. The major difference in 

this application is that the fender pillar can be as long as the 

longwall panel itself, up to 1,400 ft. However, the idealized coal 

behavior and yield zones can be analyzed to create the three 

defined regions or phases of coal mine pillar loading and the effect 

that they may have, if any, on the stability of a recovery room. 

Figure 2. Yield zone confined core zone. 

Effective applications of yield pillars are fairly complex rock 

mechanics problems, which can’t be completely explained by these, 

simplified coal behavior diagrams. There are essential 

requirements to fulfill to expect a successful yield pillar application 

and these are also extremely important in pre-driven longwall 

recovery rooms. These requirements come from the need for the 

load shedding mechanism to be successfully accomplished, either 

onto the standing and intrinsic supports, longwall shields, or outby 

pillars. Load shedding can take place if the following requirements 

are satisfied: 

♦ There is nearby load-bearing area of unmined coal, standing 

or intrinsic supports, longwall shields to sustain the 

transferred loads. 

♦ The roof and floor are sufficiently competent to facilitate the 

load transfer without a debilitating roof fall (termed room 

collapse in our case) or floor heave. 

♦ The stiffness of the surrounding rock mass is sufficiently 

high to ensure that the equilibrium of the immediate and 

main roofs remain stable during and after the “load shedding 

and transfer” process. 

If one or more of these criteria is not satisfied, the pillar will 

collapse suddenly in an uncontrollable manner and the entire 

recovery room can be lost for the equipment removal. 

In this specific case the panel was intersected by 8 observation 

chutes driven 20 ft deep. Observation chutes are mined into the 

longwall coal panel to ensure that the face equipment enters the 

recovery room on the correct mining horizon. If the face intersects 

the recover room high, the steel intrinsic supports will be hit with 

the shearer, which could result in major roof and equipment 

damage. Likewise, it the face comes in below the correct mining 

horizon, a brow would remain where the inby edge of the recovery 

room intersects the pillars, which would be unsupported and 

difficult to safely retain. The pillar of interest was 130 ft wide and 

became isolated when the 20 ft observation chute was intersected 

by the longwall face. Figure 3 shows the entire recovery room and 

the detail of the C chute used for this study. 
C Chute

A B C D E F G H

20 ft

10 ft

Panel BPC

Convergence 

measurements

Tailgate Headgate

second entry

third entry

recovery line

#1

#4

#3

#2

C Chute 

A B C D E F G H

20 ft 

10 ft 

Panel BPC 

Convergence 

measurements 

Tailgate Headgate 

second entry 

third entry 

recovery line 

#1 

#4 

#3 

#2 

Figure 3. The entire pre-driven recovery room with the details of the instrumented C chute test. 

FIELD INVESTIGATION 

The field test designed to specifically evaluate the fender pillar 

behavior was only a small portion of the field investigation 

designed to examine the entire pre-driven recovery room designed 

and used for the B4 panel in Emerald Coal Resources, LP’s 

Emerald mine (Barczak et al., 2007). Emerald Coal Resources, LP 

is an affiliate of Foundation Coal Corporation. A 16-ft recovery 

room was designed and supported, with intrinsic and standing 

supports, to handle both the front abutment pressure and the 

inherently weak immediate roof conditions. The instrumentation 

package to examine the pillar behavior was designed and installed 

to record the vertical stresses and stress changes using borehole 

pressure cell (BPC) data and roof-to-floor closure using Serata Rate 

Closure Meters. Figure 3 shows the portion of the recovery room 

used for the pillar performance evaluation. 

Borehole Pressure Cells (BPC’s) 

Borehole Pressure Cells (BPC’s) have been used to measure 

changes in pillar stresses for over 40 years. They consist of a 

hydraulic flatjack encapsulated in grout that is inserted into a 

borehole drilled with a precision bit, to control the diameter and 

amount of fluid required to pressurize the cell. The cells are 

pressurized, as completed for this study, to the predetermined 

vertical stress based on overburden depth. The cells were fitted 

with pressure transducers to enable a data acquisition system to 

continuously record the data from remote locations. The 

instruments were not installed to determine or analyze absolute 

stresses, but to provide a reasonable approximation of the pressure 

changes that were being experienced during the creation and 

ultimate failure of the fender pillar (yield pillar). Long-term 

inelastic adjustments between the BPC’s and the coal seam may 

obscure or exaggerate actual stress measurements and changes. All 

the inadequacies of these instruments are recognized. However, the 

instruments do provide a reasonable means to observe changes in 

the pillar stress distributions during this investigation and are useful 

in monitoring and “visualizing” the progression of pillar yielding as 

the fender pillar is extracted. The use of BPC’s to monitor the 

changes in stress distribution provides an understanding of sitespecific behaviors and overall pillar stability. Haramy and 

Kneisley published a complete description of the equipment, 

technique, and theories in 1991. Figure 4 shows a photograph of 

the pressure transducers being calibrated using a dead-weight 

testing unit. 

Figure 4. Borehole pressure cell transducers being calibrated in 

the laboratory using a dead-weight testing unit. 
Serata Rate Closure Meters

Serata Rate Closure Meters were originally designed and

fabricated to measure and calculate roof-to-floor closure rates

during pillar retreat mining operations. The instruments can be

moved quickly and safely and is accurate to 0.0001-inches with a

total travel of 6-inches. All the closure meters were calibrated prior 

to installation in the laboratory and the programs used in the data 

loggers reflected the specific equations determined for each 

instrument. 

Fender Pillar Behavior

The BPC’s were installed and operational when the face was

still approximately 2,950 ft (about 2 months) from entering the 

recovery room. Figure 5 shows the vertical pillar stress from 1,500

ft until entry into the recovery. The stresses started to increase 

when the face was about 500 ft away. Based on an overburden 

depth of 650 ft, this results in a (Fd/D) of 0.77, where Fd is the face 

distance and D is the overburden depth. This was 18 days before 

the face finally entered the recovery room. Figure 6 shows the last 

250 ft of the fender pillar behavior that took place in the last 

30 hours of mining activity. The peak stress (7,500 psi) was 

realized when the fender pillar was 21 ft wide. The longwall 

shearer removed two additional slices that left a 15 ft wide pillar. 

The stress dropped to 5,800 psi. The stress continued to shed from 

the pillar and only residual loads were visible when the pillar was 

estimated to be 12 ft wide. It is interesting to note that the stresses 

increased rapidly, realizing the peak, when a total of 9 ft was 

removed from the fender pillar in less than 4 hours. 

8000 

7000 

Roof and Floor Behavior

Roof to floor closure measurements were recorded as the face

approached at the locations indicated on figure 3. A photograph 

looking into recovery chute C is shown in figure 7. Note the 

closure meter no. 2 positioned in the center of the recovery chute

and parallel to the outby abutment pillar edge. 

Figure 7. Looking inby the C chute. The Serata closure meter

is installed and recording roof-to-floor closure. 

The total closure is shown in figure 8 and some important 

Vertical pillar stress, psig 6000 

5000 

4000 

3000 

2000 

1000 

0 

1400 1200 1000 800 600 400 200 

Distance to recovery room, ft 

times are indicated with series points. In general the room

remained stable when the longwall face was approximately 130 ft 

from entering the pre-driven recovery room, this correlates well

with the vertical stress being measured in the fender pillar. When 

the longwall face is approximately 75 ft inby the recovery room,

the loads on the fender pillar begin to increase dramatically. This is

also reflected in the closure measurements that increased by an 

average of nearly 100 percent. The maximum closure 

measurements that could be safely determined were recorded when 0 the longwall face was 24 ft from entering the pre-driven recovery

room. Contrary to typical roof and closure behavior, the maximum 

Figure 5. Fender pillar borehole pressure cell response when the 

longwall was 1,500 ft inby the pre-driven recovery room. 

8000

7000

measurements, about 1.75-inches, were recorded at a distances 12.5

and 21 ft from the abutment pillar edge. The closure measured

with no. 1, established in the center of the recovery chute parallel to

the outby fender pillar edge, was 1.5 inches. Figure 9 shows the

floor area around the no. 3 closure meter. Floor cracks, the result 

of localized floor heave, pushed the middle of the entry upward 

Vertical pillar stress, psig .6000 toward the main roof. This partially explains the higher recorded

closure measurements. Visual observations recorded in the 

immediate area also indicated that when the standing pumpable

5000 

4000 

concrete supports began to yield and the loads transferred onto the 3000 outby abutment pillars, rib spalling occurred on both sides of the 

2000 pillars which increased the effective room span. This often results 

1000 in roof sag in this type of immediate roof strata...

0

250 200 150 100 50 0 

Distance to recovery room, ft 

Figure 6. Borehole pressure cell response when the longwall 

face was 250 ft inby the pre-driven recovery room. 
 . 2.0 and intrinsic supports. Data indicated that this large stress transfer 

0.0 

0.4 

0.8 

1.2 

1.6 

250 225 200 175 150 125 100 75 50 25 0

Distance to Recovery Room, ft 

#1 

#2 

#3 

#4 

Figure 8. Total roof-to-floor closure as the longwall face 

Total Closure, in approached the recovery room. 

could occur in about 12 hours with normal longwall mining. One 

of the key components, to safely transfer that stress, is the behavior 

of the fender pillar. Some may argue that the behavior of the 

fender pillar has very little to do with the success or failure of the 

pre-driven longwall recovery room. Once the fender pillar begins 

to yield the uncontrollable convergence will continue to deform the 

coal fender and drive the failure system until the pillar is 

completely removed. Furthermore, load shedding will always take 

place and the loads will always have to be transferred. The key to 

success or failure is the amount of displacement that takes place in 

the entry and weather or not the roof can sustain those deformations. 

When the fender pillar yields and ultimately fails, there are load 

transfers to both the outby abutment pillars and onto the longwall 

shields. There is nothing that can be done with standing or intrinsic 

supports to prevent this transfer. The fender pillar is being 

Figure 9. Floor heave located approximately in the middle of 

recovery chute C. 

GENERAL DISCUSSION 

Coal pillars are essentially left between the immediate roof and 

floor to support areas to enable miners and machinery to move 

about the deposit and safely mine the seams. Ground control 

engineers constantly struggle with the balance of extracting the 

maximum amount of the coal reserve to remain economically 

viable but to ensure the safety of the worker. In a longwall 

recovery, the coal that is left is insignificant to the total production 

and safety for the worker and equipment becomes the primary 

concern. The outby abutment pillars, by design, are large and can 

adequately protect the entries when the forward abutment loads are 

transferred from the fender pillar, longwall shields, and standing 

removed as a support element from the system and the remaining 

components must accept the additional loads. 

The detail of the fender pillar loading for the last 50 ft is shown 

in figure 10. When the face is about 21 ft from entering the 

recovery room the inby side is yielded which forms an isolated 

pillar. The first part of this curve can be termed stiff because the 

load continues to increase. When the pillar reaches the peak, again 

at the 21 ft distance, the load begins to drop. This soft zone, by 

visually examining the data, continues until the pillar is about 15 ft 

wide. The load begins to drop off rapidly, as additional slices are 

made with the shearer, completely failing when the face is about 

11 ft. The unloading of the pillar, from the peak strength until the 

load drops completely to zero, can be defined as the residual 

strength of the pillar and can been seen in the field data. 

Vertical pillar stress, psig . 8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 

Peak Strength 

Residual Strength 

 Stiff Soft Failed

50 45 40 35 30 25 20 15 10 5 0 

Distance to recovery room, ft 

Figure 10. The behavioral zones and vertical stress 

measurements for the remaining 50 ft of the fender pillar. 

Pillar Stress Values 

The state of stress in a coal pillar can be expressed in two 

distinct parts, the volumetric and the deviatoric. The BPC response 

to the radial stress acting upon its core is a function of the initial 

setting pressure and the physical properties of the host material, in 

this case coal. Exhaustive laboratory tests conducted by Babcock 

determined that a ratio of one-third is realistic to compare cell 

pressures with actual rock or pillar stresses (Babcock, 1994). 

Considering the peak stress of the BPC installed in the fender pillar 

was measured to be 7,500 psi, this factor reduced the actual pillar 

stress to 2,500 psi, this value appears realistic. The traditional 

pillar formulas, developed primarily for square pillars, can be 

routinely applied to determine the theoretical stress. However the 

measured stress occurred when the length of the pillar (in this case 

130 ft), peaked at 21 ft (when the pillar was nearly isolated by the 
20 ft observation chutes), which resulted in an unrealistically high 

result. The BPC results can, without argument, be used to record 

the distances from the recovery room that the coal pillar realizes 

additional loads, peak loads, and ultimately yields. 

Throughout the investigation, visual observations were 

recorded that included pillar spalling, roof cracks, geological 

anomalies, and floor heave. This information, coupled with the 

instrumentation data, was valuable in interpreting the ground 

behavior and the transition of the fender pillar, essentially a barrier 

pillar, to a yielding pillar and ultimately a residual pillar. Visual 

observations and shield loading has been used in two previous 

recoveries to determine the width of the fender pillar before it was 

transformed into a residual pillar. The observations were difficult 

to interpret and provided only a “rough” estimate. The use of the 

BPC provided a reliable distance for additional numerical modeling 

and subsequent design calculations. Figure 11 shows a photograph 

of visual observations being recorded prior to the longwall 

equipment entry into the pre-driven recovery room. 

Figure 11. Visual observations were made and recorded 

routinely throughout the investigation. 

CONCLUSIONS 

The life cycle of a coal pillar, isolated at the end of a retreating 

longwall panel, was documented in an underground mine with 650 

ft of overburden. The pillar was transformed from a solid barrier 

pillar, to a yielding pillar, to a residual pillar in less than 12 hours. 

Pillar design and stability analysis have intrigued ground control 

engineers since Vicat provided a simple equation in 1833. The 

metamorphosis of this instrumented coal pillar confirmed the 

relationship of overburden stress transfer and subsequent loading of 

the structures left to safely remove portions of the horizontal coal 

seam. 

The behavior of the fender pillar adjacent to a pre-driven 

longwall recovery room is unique in that it performs the function of 

supporting the immediate room, successfully transferring the stress 

through the adjacent standing concrete and intrinsic supports and 

onto the outby abutment pillars. The “performance” of this pillar is 

critical to this stress transfer as standing and intrinsic support 

capacities can be increased or decreased depending on the 

confidence the ground control engineer has on the behavior and 

load carrying capacity of the pillar. In this investigation, the peak 

loads were realized when the pillar was 21 ft wide. The pillar 

maintained 75% of the peak load after an additional 6 ft of the 

pillar was removed with the shearer and remained in the soft, and 

transitioned to a failed state when an additional 3-4 ft was mined 

from the back side of the pillar. At that time the pillar was about 

12 ft wide. The pillar, having served its useful purpose, was 

pushed over with the shearer on the next longwall passes and 

shipped unceremoniously out of the mine. The stresses previously 

carried by the pillar were simultaneously transferred to the concrete 

standing supports, intrinsic supports, longwall shields, and onto the 

outby abutment pillars that were designed to accept the additional 

loads. The deformations recorded in the pre-driven recovery room 

entry and the chute never exceeded 2-inches and the equipment was 

safely removed and installed to extract the next longwall panel. 

The data obtained from this investigation can now be used to 

calibrate 3-dimensional finite element models where different 

support scenarios can safely be evaluated for the next recovery 

room area. 

NIOSH and Pennsylvania Services Corporation are continuing 

to examine pre-driven longwall recovery rooms to better 

understand ground reaction behaviors and provide engineering 

designs that will provide the safest ways to move a longwall 

equipment system under diverse and potentially hazardous mining 

conditions. 

ACKNOWLEDGEMENTS 

The authors would like to acknowledge the NIOSH staff that 

provided the technical assistance in fabrication, installing, 

monitoring, and interpreting the instrumentation package designed 

for this study - Dave Oyler, Craig Compton, Ted Klemetti, John 

Marshall, Tim Batchler, and Tom Barczak. Additionally we would 

like to recognize the assistance provided by the Management and 

Operations Staff of Foundation Coal affiliates, particularly J. D. 

Pekar and his crew, who provided the logistics and mine support 

for the investigation. 

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