Assessment of stable and failed pillars in 


underground limestone mines
 

g.s. esterhuizen, d.r. dolinar and 

j.l. ellenberger 

G.S. Esterhuizen, D.R. Dolinar and J.L. Ellenberger, 

members sMe are senior research engineer, lead research engineer 

and lead research scientist, respectively, with the national institute 

for occupational safety and health (niosh), Pittsburgh, Pa.

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Introduction 

Underground limestone mines 

in the United States make use of the 

room-and-pillar method of mining 

to extract the relatively flat-lying 

limestone deposits in the Eastern 

and Midwestern regions of the United States. Limestone pillars are required to support the overburden 

and to provide a safe, stable working 

environment for mining personnel 

and equipment. Unstable pillars can 

result in rock falls from the pillar ribs and can lead to the 

collapse of the roof if one or more pillars should fail. Fallof-ground injuries from the roof and pillar ribs accounted 

for about 15 percent of lost workdays in underground 

limestone mines from 1997 to 2006 (Mine Safety and 

Health Administration, 2007). Limestone pillars of vari

n 

c

ous sizes and shapes are used to support the overburde

rocks in U.S. limestone mines with varying degrees of su

cess (Iannacchione, 1999; Esterhuizen et al., 2006). 

This paper provides an assessment of the performance 

of these pillars as observed at 98 different locations in 34 

mines in the Eastern and Midwestern regions of the Unit

ed States. The results are presented in a stability chart 

that indicates the failed and stable pillars observed as 

well as the modes of observed instability. Data from one 

nonproducing mine in Ohio was added to the database, 

owing to its great depth of workings and reported stable 

Abstract 

Pillars in underground limestone mines are required to 

support the overburden and provide a safe, stable working 

environment for mining personnel and equipment. Pillar 

stability was assessed by the National Institute for Occupational Safety and Health in underground limestone mines 

in the Eastern and Midwestern United States. It was found 

that current mine layouts have been successful in providing 

support to the overburden,while a small number of isolated 

pillar failures were observed.The stable pillar layouts and 

failed pillars were plotted on a chart that demonstrates the 

relationship between pillar width-to-height ratio and pillar stress. The results show that pillar failures occurred at 

the lower range of width-to-height ratios and can occur at 

relatively low stress levels. Zones indicating the potential 

risk of pillar instability are shown on the chart, based on 

hazards associated with the onset of rib spalling, large 

angular discontinuities and unconfined pillars. The chart 

can assist limestone mine planners to evaluate potential 

instability in current or new pillar layouts that are similar 

to those included on the stability chart. 

pillar conditions (Bauer et al., 2005). 

Newly designed or current pillar layouts can be compared to the current 

experience by plotting them on the 

chart. An example is given in which 

a typical pillar layout is assessed using the chart data. 

Observations of pillar stability 

Pillars were assessed at underground limestone mines located in 

Illinois, Indiana, Iowa, Kentucky, 

Maryland, Missouri, Ohio, Pennsyl

vania, Tennessee and West Virginia. The pillar stability 

was evaluated at more than one location at each mine to 

capture the variety of geological and mining conditions. 

Two criteria were used to assess the success of the pillars. 

The first criterion is the requirement for regional stability, 

which is defined as the need for a pillar system to success

fully support the overburden. If a pillar system fails in 

providing regional support, a wide area collapse is likely 

to occur with associated surface subsidence. The second 

criterion is the requirement for local pillar stability, de

fined as stable pillar ribs. Rib conditions were assessed 

by noting the presence of rib spalling, stress fractures or 

open cracks. 

The presence of geological features such as joints, 

slips, faults and weak bedding planes was also recorded. 

Where unstable conditions or failed pillars were ob

served, the likely factors contributing to pillar instability 

were noted. In addition, pillar and room dimensions were 

measured and information on the depth of cover, extent 

of floor benching and other mining parameters collected. 

Mine maps were obtained so that the pillar loads could 

be calculated using the tributary area method or numerical models. 

Rock samples were collected to determine the uni

axial compressive strength (UCS) of the rocks. The UCS 

results were grouped into three categories based on the 

average strength obtained at the individual mine sites 

and are shown in Table 1. It can be seen that there is a 

considerable variation in the strength of the limestone 

being mined.This variation in strength was accounted for 

in the assessment of pillar stability. Table 2 summarizes 

the dimensions and cover depth of the pillar layouts that 

were investigated. 

The results of the survey showed that all the mine 

sites visited were successful in terms of the criterion for 

regional stability. No case of wide area collapse or signs of 

large areas of overloaded pillars were observed. However, a number of isolated failed pillars in otherwise stable 

layouts were observed. These isolated failed pillars are 
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likely to be the lowest strength members of the pillars 

in a layout, and do not represent the average strength of 

the pillars. It was further observed that most of the failed 

pillars had been stable on initial mining but became unstable when the height was increased by floor benching. 

The key parameters describing the unstable pillars are 

summarized in Table 3, and examples of the failed pillars 

are shown in Figs. 1 through 4. It can be seen that stress 

related rib slabbing and spalling is a feature in three of 

the four failed pillars shown. In addition, the weaken

ing effects of angular discontinuities or thin weak bands 

within the pillars are clearly demonstrated. 

The criterion for local stability was assessed by con

sidering all cases where rib spalling or slabbing was 

observed. Figure 5 shows an example of rib spalling at 

approximately 900 ft of cover. It was found that rib spalling can initiate when the average pillar stress exceeds 

about 11 percent to 12 percent of the UCS. In some cases, 

it becomes necessary to install rib support, such as screen 

and bolts, to secure the rib walls. However, not all of the 

pillars that exceeded the 11 percent to 12 percent stress 

ratio showed signs of rib spalling.Therefore, 11 percent to 

12 percent stress level should be interpreted as the lower 

limit for the onset of rib spalling. An assessment of the 

results further showed that instability caused by jointing 

or geological structures was not directly related to the 

pillar stress but was simply a function of the presence or 

absence of unfavorable geology. 

Figure 1	 

Remaining stump of a collapsed pillar in an abandoned 

area. Thin weak beds in the pillar and moist conditions 

are thought to have contributed to the failure. The widthto-height ratio was 0.82, and the average pillar stress 

was about 11 percent of the UCS. 

Figure 2 

Pillar that has an original width-to-height ratio of 1.7 

failed by progressive spalling. Thin weak beds are 

thought to have contributed to the failure. The average 

pillar stress was about 11 percent of the UCS prior to 

failure. 

Development of pillar stability chart 

All data collected on pillar performance were collated 

in a spreadsheet, and a pillar stability chart was devel

oped in which the pillar stress, normalized by the UCS, 

was plotted against the width-to-height ratio of the pil

lars, as shown in Fig. 6. The pillar dimensions used in this 

chart are the minimum pillar width and maximum pillar 

height. This means that the smaller width of a rectangu

lar pillar and the maximum height of partially benched 

pillars were used in creating the chart. Where possible, 

dimensions are based on actual underground measure

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ments. However, in the case of the failed pillars, it was not 

always possible to measure the original pillar dimensions 

and these were obtained from mine maps showing the 

original dimensions. The chart shows both stable pillar 

layouts and the individual failed pillars. The diameter of 

each stable layout data point is related to the number 

of pillars in the layout, and varies from tens of pillars to 

more than 1,000 pillars in some of the cases. The failed 

pillar data points, on the other hand, each represent only 

a single pillar. 

A number of lines have been added to the chart to high

light potential failure mechanisms, while the background 

shading indicates the relative risk of pillar instability. Each 

of these lines and shaded zones are discussed below: 

•	 Limit of experience: A curve that bounds the case

histories was drawn to indicate the limit of current 

limestone pillar experience. It can be seen that the 

lower bound width-to-height ratio is about 0.25 at 

a low stress-to-strength ratio. The upper limit of 

stress is about 25 percent of the UCS. This curve 

simply bounds the current experience and does not 

represent a pillar strength equation. 

•	 Unconfined pillar core: Studies of stress distribu

tions within pillars (Lunder and Pakalnis, 1997; 

Esterhuizen, 2006) show that when the width-toheight ratio decreases below a value of about 0.8, 

the core of the pillar becomes unconfined. Because 

much of the strength of a pillar is derived from the 

confined core, the lack of confinement can result in 

reduced pillar strength. Notably, about 75 percent 

of the observed failed pillars had width-to-height 

ratios of less than 0.8.A review of failed pillar data 

in hard rock mines (Lunder and Pakalnis,1997; Mar

tin and Maybee, 2000; Esterhuizen, 2006) showed 

that pillar strength is highly variable at these low 

width-to-height ratios. Factors that can contribute 

to the variable strength include the sensitivity 

of slender pillars to the presence of unfavorable 

geological structures and the phenomenon of rock 

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splitting parallel to the applied load in low confinement (Diederichs, 2002). Splitting and slabbing of 

limestone was observed at stress levels that are only 

less than 12 percent of the UCS of the rock (Iannacchione,1999),which indicates that pillar strength 

might be significantly reduced in the absence of 

confinement. 

Figure 3	 

Partially benched pillar failing under elevated stresses 

at the edge of bench mining. Typical hourglass formation 

indicating overloaded pillar. Width-to-height ratio was 

0.44 based on full benching height and average pillar 

stress was about 12 percent of the UCS. 

Figure 4 

Partially benched pillar that failed along two angular 

discontinuities. Width-to-height ratio is 0.58 based on 

full benching height and average pillar stress is about 4 

percent of the UCS. 

•	 The onset of spalling:As discussed earlier,spalling

and slabbing of pillar ribs was observed to start 

when the average pillar stress was around 11 per

cent to 12 percent of the UCS.Spalling represents an 

additional rock-fall hazard that must be addressed 

when mining. Limited spalling can typically be addressed by repeated scaling of pillar ribs. In cases 

of severe spalling or slabbing of a pillar ribs, it has 

been necessary to support the ribs with screen and 

rock bolts where long-term access is required. 

•	 Angular discontinuity hazard: Angular disconti

nuities that are exposed on both sides of a pillar 

can cause a significant reduction in the strength of 

a pillar. Figure 7 shows a pillar that was severely 

compromised by an angular discontinuity. It was 

necessary to install long rock bolts right through the 

pillar to prevent it from failing. During the survey 

of pillar conditions, large angular discontinuities 

were observed in several mines, but did not appear 

to be wide

spread within 

the limestone

f o r m a t i o n s

currently being

mined. When

the width-toheight ratio of

a pillar is less

than 1.5, a discontinuity with

a dip of 30° can

b e e x p o s e d

on both sides

of the pillar.

When designing pillars in this zone, the potential 

for encountering unfavorable angular discontinuities should be considered. 

The shaded zones in Fig. 6 indicate the relative risk 

of instability based on a qualitative assessment of the 

likelihood and consequences of pillar instability. Darker 

shading indicates a relatively higher risk. The following 

four risk zones are indicated: 

•	 Very low risk: No failed pillars or stress related rib

instability was found in this zone. Pillar layouts that 

plot within this zone can, therefore, be expected to 

have a very low risk of failure. 

•	 Low risk: A few isolated cases of pillar instability

were observed within this zone. Pillars should be 

designed with due consideration of the potential 

for rib spalling and pillar weakening by large discontinuities. 

•	 Moderate risk:Two moderate-risk zones are indicated. 

Pillars that plot near the upper part of the experience 

curve are subject to increasing stress and there is limited experience with pillar performance at those stress 

magnitudes. Slender pillars with width-to-height ratios 

of less than 0.8 are vulnerable to the effects of large 

angular discontinuities and the lack of confinement. 

Table 1 

Uniaxial compressive strength of limestone rocks collected at mine sites. 

Average, Range, Samples Representative 

Group psi psi tested limestone formations 

Lower strength 12,800 6,400-20,800 50 Burlington, salem, 

 Galena-Plattesville 

Medium strength 19,600 11,900-30,000 100 Camp Nelson, Monteagle, 

 Plattin, Vanport, 

 Upper Newman, Chickamauga 

High strength 31,800 22,000-43,700 32 Loyalhanna, Tyrone 
Figure 5 

Example of rib slabbing when the average pillar stress 

exceeds about 11 percent of the UCS. 

Several failed pillars were observed within this zone. 

•	 High risk: Pillars within the high-risk zone are 

potentially exposed to a combination of increasing 

stresses, rib spalling,large discontinuity hazards and 

the lack of confinement. In addition, pillars that fall 

outside the limit of current experience are considered to represent an elevated risk. Several of the 


single failed pillars fall within the high-risk zone.
 

It can be seen that many of the stable pillar layouts 

shown in the chart fall within one or more of the shaded 

zones. These zones do not imply that the pillar layouts 

are necessarily unstable, but rather indicate the types 

of stability hazards that might be encountered and that 

would need to be accounted for in the design and during 

the course of mining. 

Table 2 

Summary of mining dimensions and cover depth of

mines included in study. 

Dimension Average Minimum Maximum 

Pillar width (ft) 43.0 15.0 70.5 

Pillar height (ft) 36.5 15.8 124.6 

Width-to-height ratio 1.41 0.29 3.52 

Cover depth (ft) 385 75 2,200 

Table 3 

Summary of observed failed pillars. 

Width- Average 

Pillar Pillar to- pillar Rock 

width, height, height, stress, strength, 

Case ft ft ratio psi psi Factors contributing to pillar failure 

1 35 60 0.58 1,305 31,175 Partially benched pillar, contains angular discontinuities 

2 35 60 0.58 1,363 31,175 Partially benched pillar, contains angular discontinuities 

3 35 60 0.58 1,494 31,175 Partially benched pillar, contains angular discontinuities 

4 50 90 0.56 1,827 22,185 Pillar fully benched to 90-ft height reduced width-to-height ratio 

5 35 60 0.58 1,856 31,175 Benched pillar, contains angular discontinuities 

6 40 90 0.44 2,494 21,750 Partly benched pillar 

7 28 52 0.54 2,494 21,750 Large steep dipping discontinuity and elevated stress ahead of benching 

8 40 90 0.44 2,509 21,750 Partly benched pillar 

9 26 32 0.81 2,755 23,200 Thin weak beds in limestone, pillar undersized causing elevated stress 

10 42 24 1.73 2,525 23,200 Thin weak beds in pillar causing progressive spalling 

11 41 50 0.82 2,583 23,200 Thin weak beds in pillar and moist conditions, pillar collapsed 

12 20 40 0.49 2,755 23,200 Benched pillar is undersized causing elevated stresses 

13 22 40 0.54 2,900 23,200 Benched pillar is undersized causing elevated stresses 

14 12 28 0.43 3,495 31,175 Undersized pillar 

15 27 30 0.90 3,625 23,200 Thin weak beds in pillar caused progressive slabbing 

16 18 24 0.75 3,915 23,200 Undersized pillar subject to elevated stress 

17 40 52 0.77 1,220 23,900 Partially benched pillar, contains angular discontinuities 

18 40 52 0.77 1,100 23,900 Partially benched pillar, contains angular discontinuities 

Example of applying the stability chart to assess

a pillar layout 

Consider a new limestone mine in the Midwestern 

United States that will use 40-ft-wide square pillars on 

80-ft centers with 40-ft wide headings and crosscuts. The 

pillar height on development will be 28 ft. Benching of an 

additional 28 ft of floor stone is planned, which will result 

in an ultimate pillar height of 56 ft. The initial width-toheight ratio will, therefore, be 1.43, reducing to 0.71 after 

floor benching. The UCS of the rock is 140 MPa (20,000 

psi). If the maximum depth of cover is 300 ft, the average 

pillar stress in square pillars can be estimated using the 

tributary area method

 (1)

where 

σp is the average pillar stress in pounds per square

inch,

h is the depth of cover, 

w is the pillar width and

b is the room width in feet.

The average pillar stress for the example is calculated 

to be 9.10 MPa (1,320 psi), which is 6.6 percent of the 
UCS. This point plots within the “Low Risk” zone in Fig. 

6. The pillars should be free of stress related rib spalling 

during initial mining but unfavorable angular disconti

nuities can compromise their strength. A geotechnical 

investigation might be warranted to confirm or refute 

the presence of such structures. The chart further shows 

that after bench mining to a height of 56 ft, the pillars 

will plot within the “Moderate Risk” zone because the 

pillar cores will become unconfined in addition to being 

vulnerable to large angular discontinuities. It might be 

prudent to reduce the bench height to 20 ft for example, 

which will increase the benched pillar width-to-height 

ratio to 0.83. This will reduce the potential for instability 

of the benched pillars. 

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Figure 6 

Limestone pillar stability chart showing stable pillar layouts and single failed pillars. Potential failure mechanisms and 

relative risk of pillar instability are indicated. 

Conclusions 

The assessment of pillar conditions in U.S. limestone 

mines has shown that: 

•	 pillars in current mining operations have been suc

cessful in providing support to the overburden and 

preventing wide area pillar collapse and associated 

surface subsidence; 

•	 pillar rib instability in the form of spalling or stress

related slabbing appears to initiate when the aver

age pillar stress exceeds about 11 percent of the 

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uniaxial compressive strength of the rock; and 

•	 a small number of isolated failed pillars were ob

served in otherwise stable layouts. 

Factors contributing to the isolated pillar failures included unfavorably oriented geological structures and 

elevated stress levels in undersized pillars. Stress related 

pillar failures were associated with progressive slabbing 

and spalling of pillar ribs. Unfavorable geology, such as 

angular discontinuities in the pillars and thin weak beds 

in the limestone formation, contributed to failure in some 

cases. Notably, about 75 percent of the observed failed 

pillars have width-to-height ratios of less than 0.8. 

A pillar stability chart developed from the data shows 

that a limiting curve can be drawn that encloses the ob

served cases. The limiting curve does not represent a 

pillar strength equation, but simply bounds the current 

pillar experience. The potential failure mechanisms and 

relative risk of pillar instability have been indicated on 

the chart, based on observations and analysis of pillar 

performance. The chart and the indicated stability zones 

can assist mine designers in anticipating stability issues 

in their pillar layouts and implementing appropriate risk 

mitigation measures. 

As a cautionary note, the stability chart is based 

on observations of limestone mine pillars in the Mid

western and Eastern United States and should not be 

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used to assess pillar stability in cases outside of this 

geographic area. In addition, the validity of the chart is 

limited to pillar cases that have similar dimensions, rock 

strength and depth of cover as those used to develop 

the chart. 

Figure 7 

Example of a pillar that is bisected by a large angular 

discontinuity. 

References 

Mine Safety & Health Administration, 2007,Webpage: www.msha. 

gov/stats. 

Esterhuizen G.S.,Iannacchione,A.T., Ellenberger, J.L., and Dolinar, 

D.R., 2006, “Pillar stability issues based on a survey of pillar perfor

mance in underground limestone mines,” 25th International Conference 

on Ground Control in Mining, Morgantown, WV, pp. 354-361. 

Iannacchione, A.T., 1999, “Analysis of pillar design practices and 

techniques for U.S. limestone mines,” Trans. Inst. Min. Metall., Sect. A: 

Min. Industry, Vol. 108, September-December, pp. A152-A160. 

Bauer, S.J., Munson, D.E., Hardy, M.P., Barrix, J., and McGunegle, 

B., 2005, “In situ stress measurements and their implications in a deep 

Ohio mine,” 40th U.S. Rock Mechanics Symposium,Anchorage,Alaska, 

June 25-29, Paper 05-804. 

Lunder, P.J., and Pakalnis, R., 1997, “Determining the strength of 

hard rock mine pillars,” Bull. Can. Inst. Min. Metall., Vol. 90, pp. 51-55. 

Esterhuizen, G.S., 2006, “Evaluation of the strength of slender pillars,” Trans. Soc. Min. Metal. Explor., Vol. 320, pp. 69-76. 

Martin, C.D., and Maybee, W.G., 2000, “The strength of hard rock 

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pillars,” Int. Jnl. Rock Mechanics & Min. Sci., Vol. 37, pp. 1239-1246. 

Diederichs, M.S., 2002, “Stress induced damage accumulation and 

implications for hard rock engineering,” NARMS-TAC 2002, Hammah 

et al., eds., University of Toronto, pp. 3-12.