25th International Conference on Ground Control in Mining 

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ABSTRACT 

 A survey of pillar conditions was carried out at 21 operating 

limestone mines that use the room-and-pillar method. The 

surveyed pillars were all located in rock that was classified as 

“Good” to “Very Good” using the RMR rock mass classification 

system. The average pillar width was 14.5m and the width to 

height ratio varied between 0.53 and 3.52. Pillar conditions were 

rated using a visual rating system that accounts for the effect of 

geological structures as well as stress related fracturing. 

 Nine cases of pillar instability were recorded. Pillar instabilities 

were typically limited to individual pillars or small groups of pillars 

within an otherwise stable layout. Local geological structures such 

as inclined discontinuities or weak bedding planes contributed to 

instability in seven of nine cases that were observed. Elevated 

pillar stresses contributed to instability in the two remaining cases. 

All the unstable pillars observed had width-to-height ratios of less 

than 1.5. 

 It is concluded that pillar instability is most likely to be caused 

by unfavorable geological structures in pillars with width to height 

ratios of less than 1.5. Stress related instability such as rib spalling 

becomes more prevalent when the average pillar stress approaches 

approximately 20 MPa. 

INTRODUCTION 

 Underground limestone mines in the United States make use of 

variations of the room-and-pillar method to extract limestone 

formations that are generally flat lying, but may dip up to 35°. 

Pillar stability is one of the requisites for safe working conditions in 

a room-and-pillar mine. 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. Fall of ground accidents were the cause of 

36% of fatalities and 12% of all injury related lost work days in 

underground limestone mines in the decade of 1996 to 2005 (Mine 

Safety and Health Administration, 2006). 

 Current research at the National Institute for Occupational 

Safety and Health (NIOSH) has the objective to reduce ground fall 

accidents in limestone mines through safe pillar and roof span 

design. As part of this research a survey of pillar stability in 

limestone mines was conducted in the Eastern United States. The 

rock mass properties, mining dimensions, pillar performance and 

roof conditions were recorded at each site. This paper summarizes 

the data collected, describes examples of pillar instability and 

presents a discussion of the likely causes of instability. 

FIELD SURVEY OF PILLAR PERFORMANCE 

Mining Areas and Geological Setting

 Data collection visits were made to 21 different underground 

stone mine sites in the Central and Eastern United States. The data 

were collected at mine sites and grouped in the following four 

limestone mining areas: 

a. Northern Appalachian Area: The Northern Appalachian 

limestone mining area includes Pennsylvania and Northern 

West Virginia. Three limestone deposits found in this area 

were examined, locally known as the Loyalhanna, 

Greenbrier, and Vanport limestone. The oldest is the 

Loyalhanna limestone. It is of Mississippian age and is 

generally flat-lying, cross bedded, and very silaceous 

(Adams, 1970). The Greenbrier limestone is an extensive 

unit deposited during the Mid-Mississippian and occurs 

above the Loyalhanna. The uppermost limestone in this area 

is the Vanport limestone. The Vanport limestone is a 

member of the Allegheny group of Pennsylvanian age and is 

generally flat-lying (Iannacchione and Coyle, 2002). 

b. Southern Appalachian Area: The Southern Appalachian 

limestone mining area consists of Southern West Virginia, 

Western Virginia, Eastern Tennessee, and Eastern Kentucky. 

Economic horizons include limestone within the 

Chickamauga Group and the Monteagle limestone formation. 

The steeper dipping Orodovician Chickamauga Group is 

mined at dips of up to 35 degrees. The Monteagle is a gently 

dipping Upper Mississippian limestone which is mined on 

more than one horizon, (Brann and Freas, 2003). 

c. Cincinnati Arch Area: The Cincinnati Arch limestone 

mining area is essentially associated with an anticlinal 

(upward or high in the center) fold that brings Ordovician 

limestones and dolomites near the surface, (Hansen, 1977). 

The axis of the Cincinnati Arch extends southward from 

Ohio, east of Cincinnati to Tennessee. The limestone 

formations are gently dipping. Economic horizons include 

the Camp Nelson and the Tyrone. 

PILLAR STABILITY ISSUES BASED ON A SURVEY OF PILLAR 

PERFORMANCE IN UNDERGROUND LIMESTONE MINES 

Gabriel S. Esterhuizen, Sr. Research Fellow 

Anthony T. Iannacchione, Acting Geotechnical Engineering Section Chief

John L. Ellenberger, Research Geophysicist 

Dennis R. Dolinar, Mining Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 
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d. Illinois Basin Area: The Illinois Basin limestone mining area 

extends from western Indiana to Illinois and south to 

Kentucky. The area primarily contains Silurian to 

Mississippian sediments. Dolomite and limestones are the 

dominant lithology, followed by shale, sandstone, chert, 

anhydrite, and coal. Economic horizons are located in the 

Galena-Platteville group. The formations are relatively flat 

lying and undisturbed. High quality limestone is produced 

from crystalline reef formations, (Swann, 2006; U.S. 

Geological Survey, 2006). 

Data Collected 

 Data were collected on rock strength, jointing and other 

geological structures, pillar dimensions, pillar performance and 

roof stability. Between two and five data sets were collected at 

each mine site. A data set attempts to describe the general 

performance of the pillar layout at each location by inspecting an 

area of approximately 100 by 100 m (300 x 300 ft). It was found 

that individual unstable pillars can occur within an otherwise stable 

layout. In such cases, the unstable pillars were recorded separately 

and the likely causes of instability noted. The following data were 

collected: 

1. Rock mass data, including rock strength, joint frequency, 

joint conditions and groundwater condition. The data was 

collected in accordance with the requirements of the RMR 

system of rock mass classification (Bieniawski, 1989). Since 

drill core was not available at any of the sites, the rock 

quality designation (RQD) could not be obtained. The joint 

frequency approach, proposed by Laubscher (1990), was 

used to obtain the combined joint spacing and RQD rating. 

The joint frequency was measured in two orthogonal 

windows, each 3 m wide by 1.8 m high. Rock strength was 

based on field estimates, supplemented by uniaxial 

compressive strength (UCS) test data. The presence of 

continuous geological structures in each area was observed 

and their orientations and spacing recorded. Continuous 

structures were defined as any structure that exceeded 10m 

in dip or strike continuity. These types of structures 

typically extended from the roof to the floor of a pillar or 

extended across the width of a room. 

2. Pillar conditions were assessed using a visual rating system 

broadly based on similar classification systems developed by 

Lane et al. (1999), Siefert et al. (2003), Krauland and Soder 

(1987), Lunder (1994) and Pritchard and Hedley (1993). 

The ratings were divided into two categories; the first is the 

Pillar Stress Rating, which describes the effects of stress 

related fracture through intact rock, shown in table 1. The 

second is the Geological Structure Rating, which describes 

geological effects on the pillar shape, such as joint defined 

block release, block sliding along continuous geological 

structures or squeezing along weak bands, shown in table 2. 

It was observed that pillars with condition ratings of 3 and 

below can typically be made safe by regular scaling 

procedures and may require occasional rib bolting or screen. 

Mine operators will typically barricade off areas that contain 

pillars with condition ratings of 4 and above. Alternatively, 

extensive support systems are installed to preserve the 

integrity of these pillars. 

3. Pillar and room dimensions were measured and information 

on the depth of cover, extent of floor benching and other 

mining parameters collected. The data allowed the average 

pillar loads to be calculated using the tributary area method. 

Table 1. Pillar Stress Rating 

Rating Sketch Description 

1 

None 

No stress related fracturing or 

spalling observed. Joint or blast 

related damage may exist. 

2 

Minor 

Minor slabs or spalling, fractures 

through intact rock at corners, 

pillar corners and walls may be 

concave, does not typically 

deteriorate after initial mining 

and scaling. 

3 

Moderate

Slabbing, onion-skin, fractures 

more than 1m long, joints 

opened, corner damage, pillars 

may need re-scaling after initial 

development. Original square 

pillar shape maintained. 

4 

Severe 

Spalling to hourglass shape. 

Open cracks in pillar more than 

1m long, debris around pillar, 

original square shape of pillar no 

longer visible, saw tooth slabs on 

ribs 

5 

Very 

Severe 

Formation of large open cracks, 

extreme hourglass. Pillar likely 

lost most of its residual strength. 

Table 2. Geological Structure Rating 

Rating Sketch Description 

1 

None 

 Less than 0.3 m (1 ft) of joint 

related fallout during blasting. 

Blast damage may exist. 

2 

Minor 

 Pillar shape affected by 0.3- 1 m 

(1-3 ft). Some joint or bedding 

fallout during blasting, may form 

step at bedding planes. No or little 

further fallout after initial scaling. 

3 

Moderate

 Pillar shape affected by 1-3 m (3-

10 ft). Joint or bedding controlled 

fallout. Fallout can continue after 

initial mining and scaling. 

4 

Severe 

 Large block fallout >3 m (>10 ft). 

Pillar shape compromised by large 

block extrusion or block sliding on 

steep plane. Falls continue after 

initial mining and scaling. 

5 

Very 

Severe 

 Pillar bisected by through-going 

structure dipping at more than 35 

degrees. Potential or actual loss of 

top half of pillar. Pillar strength 

depends on discontinuity strength. 
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Rock Mass Characteristics 

 The results of the rock mass classification data are summarized 

in table 3. The table is based on field classifications carried out at 

66 different locations in the underground mines visited. The 

resulting average RMR value is 78 which classifies the rock mass 

as “Good” quality. The range of RMR values is tight; all the values 

are within 18% of the average value. The minimum RMR value of 

64 was observed in a mine in the Cincinnati Arch area where 

bedding planes contained weak infill and damp conditions were 

observed. The maximum RMR of 89 was observed in the Illinois 

Basin area, where the rock mass contained few joints and was 

generally undisturbed. 

 Continuous geological structures that have strike continuity in 

excess of 10 m were observed at 32 of the 66 sites logged. These 

structures were typically steeply dipping features with an average 

dip of 82 degrees and seldom had any visible vertical displacement 

along them. Shallow dipping thrust fault structures, dipping at less 

than 30 degrees, were also observed. The frequency of continuous 

structures varied between 1 and 30 structures per 100 m, with a 

calculated average value of 8.2 per 100 m. 

Dimensions of Room and Pillar Layouts 

 The pillars were designed to be square in 52 of the areas 

recorded. The remaining 14 sites had rectangular pillars. Pillar 

width is defined as the minimum horizontal pillar dimension 

measured underground during the surveys. The recorded pillar and 

room dimensions are presented in table 4. The percent extraction 

figure is based on the actual measurements made at the 

underground sites, and not the planned extraction ratio. 

 The data shows that pillars are initially developed at an average 

width to height ratio of 1.73 and then benched to a width to height 

ratio of 0.98. Many of the instabilities observed during the study 

were related to pillars that were initially stable during development, 

but became unstable during or after benching. 

Table 4. Dimensions of room-and-pillar mining layouts 

Parameter Average Minimum Maximum

Pillar height – development (m) 7.9 4.8 10.7 

Pillar height – benched (m) 17.6 9.9 38.0 

Pillar width (m) 14.5 5.1 28.6 

Pillar width to height ratio – 

development 1.73 0.93 3.52 

Pillar width to height ratio – 

benched pillar 0.98 0.53 2.35 

Room width (m) 13.4 9.1 16.8 

Depth of cover (m) 130 23 533 

Local Percent extraction (%) 72 54 84 

Pillar Conditions 

 The pillar survey did not reveal any cases in which an entire 

array of pillars had collapsed. The distributions of observed pillar 

conditions are presented in figure 1, based on the Pillar Stress 

Rating (PSR) in table 1 and the Geological Structure Rating (GSR) 

in table 2. Note that the ratings refer to the general condition of the 

pillars in a layout, and not the condition of an individual pillar. 

 Figure 1a shows that 85% of all layouts visited had PSR values 

of 1 or 2, implying “None” to “Minor” stress related damage. Only 

15% of cases were rated as “Moderate”. The relationship between 

the average pillar load in a layout and the PSR is shown in figure 2. 

The pillar stress was determined by the tributary area method, 

based on measurements of pillar dimensions and the depth of cover. 

The results show that stress related fracturing can become 

“moderate” as the average pillar stress approaches and exceeds 

20 MPa. It was observed that mine operators installed rib support 

in some of the pillars rated as “moderate” to maintain safe working 

conditions. 

 Regarding geological structure effects, figure 1b shows that 

20% of pillar layouts visited had GSR rating of 3, implying 

“Moderate” pillar damage caused by geological structures. These 

were caused by a variety of structures, including weak bedding 

Table 3. Summary of rock mass classification data 

Range of Ratings 

Parameter Descriptive observations Maximum 

Possible Rating

Average 

Rating Minimum Maximum 

Intact rock strength Strong to very strong rock, UCS typically 80-

150 MPa 15 14.6 11 15 

Joint frequency Average frequency is 1.72 joints per m. Range is 

typically 1 to 5 joints per m. 40 23.3 14 30 

Joint roughness Rough to slightly rough joint surfaces, bedding 

joints generally very rough 6 4.5 2 6 

Joint infill thickness Generally no infill, isolated cases of calcite 

filling observed. 6 5.5 1 6 

Joint fill strength If present, soft calcite infill, occasionally clayey 6 5.7 3 6 

Joint weathering Joint walls generally unweathered, hard rock 6 6.0 4 5 

Joint length Joint continuity typically in 1-3 m range 6 3.9 1 6 

Groundwater Typically dry, occasional damp conditions 

observed 15 15 12 15 

RMR 100 78 64 89 
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planes, inclined discontinuities and joints that extended from the 

top to the bottom of a pillar. In the remaining 80% of layouts the 

geological structure only had a minor effect or no effect on pillar 

stability. 

EXAMPLES OF UNSTABLE PILLARS 

 Unstable pillars were observed at nine locations, where a single 

pillar or a small group of pillars was unstable within an otherwise 

stable layout. Since the unstable pillars did not reflect the typical 

conditions in a layout, they were individually documented, and are 

summarized in table 5. The unstable pillars represent a very small 

percentage of the total number of pillars in limestone mines. 

 Table 5 shows that the unstable pillars all had width to height 

ratios of less than 1.5. Geological structures or weak bedding 

planes were associated with seven of the nine unstable cases. Only 

two cases were related to stress spalling and fracturing. 

 More detailed descriptions of cases 1, 2, 4, 5, 7 and 8 in table 5 

are discussed below. 

Table 5. Summary of observed unstable pillars 

Case Width to 

height ratio 

Tributary area 

stress (MPa) Factors contributing to instability Consequences 

1 0.56 14.4 Inclined discontinuities, pillar was in process of being 

benched. 

Benching activities halted in this area 

2 0.59 14.4 Inclined discontinuities, pillar was in process of being 

benched. 

Benching activities halted in this area 

3 1.44 6.23 Shallow dipping fault structure caused extrusion of large 

blocks from pillar. 

Development halted and resumed beyond 

fault 

4 0.55 3.44 Inclined structure bisects the pillar, pillar benched. Pillar bolted and reinforced to prevent 

failure 

5 0.44 15.0 Pillar load causing hour-glassing and spalling in pillars at 

edge of benching. 

Pillars in disused part of mine 

6 0.92 19.38 Continuous joint resulted in unstable rib during benching. Benching halted and resumed beyond 

this pillar 

7 0.73 Not valid Weak bedding infill materials and relatively high pillar 

load, pillar at edge of benching. 

Pillar in disused part of mine 

8 0.93 14.7 Weak bedding infill materials and relatively high pillar 

load, benched pillar (collapsed). 

Pillar in disused part of mine 

9 0.92 13.1 Pillar load causing hour-glassing and spalling, pillar 

benched. 

Pillar in disused part of mine 

Figure 2. Relationship between average pillar stress, calculated

by the tributary area method, and the Pillar Stress Rating 

R2 = 0.6546

0.0

1.0

2.0

3.0

4.0

0.00 10.00 20.00 30.00 40.00

Average Pillar Stress (MPa)

Pillar Stress Rating 

Figure 1. Distribution of the Pillar Stress Rating and the 

Geological Structure Rating for pillar layouts (refer to tables 1 

and 2). 

0%

10%

20%

30%

40%

50%

60%

1 23456

(a) Pillar Stress Rating Frequency . 

0%

10%

20%

30%

40%

50%

60%

123456

(b) Geological Structure Rating Frequency . 
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Mine A: Northern Appalachian Area 

 Pillar instability during bench mining of the floor occurred at 

several instances in a Northern Appalachian mine in the 

Loyalhanna Limestone. The pillars were observed to have a higher 

occurrence of continuous geological structures, in the form of 

through-going angular bedding joints, than elsewhere in the country 

(Iannacchione and Coyle, 2002). These bedding joints had a 

dramatic effect on pillar strength when they were oriented at angles 

that permitted these structures to daylight on either side of the pillar 

(Iannacchione, 1999). The average rock mass properties at this 

mine are summarized in table 6, showing an RMR value of 78, 

which is described as “Good Rock”. 

Table 6. Average Rock Mass Rating at Mine A 

Parameter Value Rating

Intact rock 

strength 

>200 MPa 15 

Joint Frequency 1.6 joints/m 23 

Joint Condition Slightly rough to rough joints with no 

infill, poorly developed bedding 

planes, joint walls unaltered 

25 

Groundwater Dry 15 

 Total: 78 

 The pillars in the layout were square, 10.4 m (34 ft) wide, and 

8.2 m (27 ft) high on development. The room width was 13.4 to 

14.6 m (44 to 48 ft). Floor benching increased the pillar height to 

18.6 m (61 ft), which reduced the width to height ratio from 1.3 to 

0.56 in one case, (Case 1 in table 5). The height was 17.6 m (58 ft) 

in the second case (Case 2 in table 5). The depth of cover was 

approximately 91 m (300 ft) in the areas where instabilities 

occurred. The average pillar stress in this layout is approximately 

14.4 MPa (2,080 psi). 

 Figure 3 shows two pillars that became unstable during 

benching. The large angular discontinuities prominent in the 

photographs did not open or extend until benching occurred. The 

structures are often very difficult to see on development, when the 

pillar width-to-height ratios are typically greater than 1.0. They are 

most likely to be recognized when benching occurs and greater 

pillar heights expose more structures and additional bench blasting 

loosens the rock associated with these discontinuities. In both 

cases, benching could not be completed around the pillars for fear 

of total pillar collapse. 

Mine B: Cincinnati Arch Area 

 A single collapsed pillar within a layout of otherwise stable 

pillars was observed at a limestone mine within the Cincinnati Arch 

area, (Case 8 in table 5). A photograph of the failed pillar is 

presented in figure 4, which shows that a portion of the roof at the 

pillar location had failed. The pillars in this layout are square in 

plan and had been initially developed at a height of 7.0 m (23 ft) 

and a width of 13.7 m (45 ft). The planned room width was 9.1 m 

(30 ft) and the depth of cover is approximately 204 m (670 ft). 

Benching had been carried out to a height of approximately 14.6 m 

(48 ft), reducing the planned width-to-height ratio of the pillars 

from 1.95 to 0.93. The average rock mass rating in the vicinity of 

the collapsed pillar was 64 units, shown in table 7. 

Figure 4. Stump of failed pillar at Mine B, showing roof 

failure at location of pillar and limited spalling of adjacent 

pillars (Case 8 in table 5). 

Figure 3. Partially benched pillars that became unstable during 

benching at Mine A (Cases 1 and 2 in table 5). Figure 3a 

shows failure along two discontinuities while figure 3b shows 

evidence of multiple discontinuities. 

 (b) 

 (a) 
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Table 7. Rock Mass Rating parameters in vicinity of failed 

pillar, Mine B 

Parameter Value Rating 

Intact rock strength 120 – 150 MPa 12 

Joint Frequency 2.8-4.3 joints/m 18 

Joint Condition 

Slightly rough to rough joints 

with no infill, bedding joints 

contain 5mm weak fill, joint 

walls unaltered 

24 

Groundwater Damp 10 

Total: 64 

 Of particular importance is the presence of bedding joints 

spaced between 0.3m and 1m (1 – 3.3 ft) apart. Some of the 

bedding joints contain a soft white infill material, up to 12 mm 

(0.5 inch) in thickness. The area was otherwise free of continuous 

geological structures. The rock mass in the area was damp. 

 The area had been mined more than 5 years before the pillar 

failure occurred and had been used to dispose of fine limestone 

byproduct by hydraulic placement. The progression of pillar 

instability and ultimate failure was not observed by the mine staff, 

since the pillar was located in a disused area of the mine. The 

surrounding pillars are generally stable, some signs of instability 

were observed in the form of minor rib spalling in the adjacent 

pillars, seen to the right of figure 4. The roof span between the 

remaining pillars was about 32 m (105 ft). It is not known whether 

the roof failure was simultaneous with the pillar collapse or not. 

The average pillar stress in this layout is 14.7 MPa (2,100 psi), 

based on tributary area calculations using specific weight of 

overburden of 2.6. 

 The cause of the failure is thought to be a combination of 

several factors including the weak bands in the pillar, the relatively 

large overburden load and the moist conditions in the area of the 

failure. Pillar instability observed elsewhere in the mine showed 

that progressive spalling of the pillar ribs is bounded by the weak 

bedding layers, as shown in figure 5, (Case 7 in table 5). These 

weak bedding layers are not present at all locations in the mine. 

The fact that only a single pillar failed indicates that the 

surrounding pillars are sufficiently strong to carry the additional 

load that was transferred from the failed pillar. The failed pillar 

and unstable pillars at this mine represent a very small fraction of 

the total number of otherwise stable pillars in the mine. 

Mine C: Southern Appalachian Area 

 Brittle spalling resulting in hourglass formation at relatively low 

stress was observed at a mine in the southern Appalachian area, 

case 5 in table 5. In the area of concern, the pillars were square 

with side dimensions varying between about 12.2 and 15.2 m (40-

50 ft) and were initially benched to 15.8 m (52 ft) high. Further 

benching was partially carried out, which increased the pillar height 

up to 27 m (90 ft). The room width was measured to be 16.4 m 

(53 ft), and the depth of cover is 140 m (464 ft). The width to 

height ratio of the benched pillars was as low as 0.44 in the 27 m 

(90 ft) high area and 0.77 in the 15.8 m (52 ft) high area. 

 The limestone is a strong rock mass with an average UCS of 

150 MPa (22,000 psi). Jointing is near vertical with an average 

spacing of about 50 cm (1.6 ft). Joint surfaces are rough, and the 

joint continuity is less than 3 m (10 ft). Bedding joints are poorly 

developed and did not appear to affect the pillar stability. The rock 

mass rating for this area was determined to be 78 units as shown in 

table 8. 

Table 8. Rock Mass Rating parameters in vicinity of unstable 

pillar, Mine C 

Parameter Value Rating 

Intact rock strength 150 MPa 14 

Joint Frequency 2.0 joints/m 21 

Joint Condition Rough joints with no infill, poorly 

developed bedding, joint walls 

unaltered 

28 

Groundwater Dry 15 

 Total: 78 

 The pillars were about 15 years old but benching had been 

carried out less than 5 years ago. A number of pillars were 

reported to be progressively spalling to the current hourglass shape, 

as shown in figure 6. Inspection of the pillars revealed that open 

vertical fractures or joints existed in the pillar ribs. Evidence of 

fracture through intact rock and slab formation was seen. 

Columnar fragments of rock about 2 to 3 m long were scattered 

about the pillars, as seen in the foreground. The presence of nearvertical open fractures and open joints seems to confirm that stress 

related failure is taking place in these pillars. The effect of 

benching and the irregular pillar sizes in this area are thought to 

have resulted in local pillar stresses in excess of the 15 MPa 

(2,200 psi) that the tributary area method would predict. 

Figure 5. Pillar instability observed in Mine B, showing 

the influence of weak bedding layers (Case 7 in table 5).
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Mine D: Southern Appalachian Area 

 Pillar instability was caused at this mine by the presence of a 

single continuous geological structure that intersected one pillar, 

shown in figure 7 (Case 4 in table 5). The limestone formation dips 

at about 30 degrees and is mined using a room and pillar layout. In 

the area where the unstable pillar is located, the roof of the 

workings was excavated to a 30 degree bedding plane, while the 

floor was mined in horizontal steps. The general rock mass 

classification in the area was found to be 76, shown in table 9. 

Table 9. Rock Mass Rating parameters in vicinity of unstable 

pillar, Mine D 

Parameter Value Rating 

Intact rock strength 120 - 150 MPa 12 

Joint Frequency 1.4 joints/m 24 

Joint Condition 

Rough, discontinuous with no 

infill, poorly developed 

bedding planes 

25 

Groundwater Dry 15 

Total: 76 

 The unstable pillar was located near the outcrop of the mine 

workings, under about 80m (260 ft) of cover. The pillar 

dimensions in the area are 15.3 m x 15.3 m (50 ft) and the room 

width is about 13 m (43 ft) The pillar height is 29 m (95 ft) 

resulting in a width to height ratio of 0.55. The average pillar stress 

is calculated as 3.4 MPa (500 psi), assuming tributary loading. 

 Special support measures were installed to prevent deterioration 

and failure of the pillar. Holes were drilled through the pillar and 

long thread-bars were installed to provide interior reinforcement 

and provide active resistance to shearing along the inclined 

discontinuity. Additional bolting was installed to preserve the 

pillar ribs. The support measures have been successful and the 

pillar has remained stable for more than 30 years. 

SUMMARY AND CONCLUSIONS 

 Nine cases of pillar instability were recorded at 21 underground 

limestone mine operations in the Eastern United States. Pillar 

instabilities were limited to a single pillar or a small group of 

pillars within otherwise stable layouts. An assessment of the 

unstable pillars showed that: 

• Local geological structures, such as inclined discontinuities 

or weak bedding layers, were identified as the main 

contributing factor to pillar instability in seven of the nine 

cases. Progressive spalling and fracturing caused by 

elevated pillar loads contributed to the instability of the 

remaining two cases. 

• Geological structure related instability can occur at very low 

pillar loads, as low as 3.4 MPa (500 psi) in one observed 

case. 

• Evidence of stress induced spalling and fracturing was found 

as the average pillar stress increased beyond about 20 MPa 

(2,900 psi). Some of the pillars in this stress range needed 

rib support to maintain safe working conditions. 

• The unstable pillars all had width-to-height ratios of less 

than 1.5. Eight of the nine cases of instability were related 

to benching of the floor, which reduced the width to height 

ratio of the pillars. Pillars that were previously stable 

became unstable during benching. 

 The study highlights that unfavorable geologic structures can 

cause pillar instability in limestone mines at any depth of cover. 

Pillars with width to height ratios of less than 1.5 are particularly 

prone to this type of instability. The results also show that the 

prevalence of rib instability can be expected to increase as mines 

become deeper and stress related fracturing increases. 

 This study forms part of a NIOSH research project that has the 

objective to reduce ground fall accidents in limestone mines and 

Figure 6. An unstable pillar at the edge of benching 

operations at Mine C, showing the effect of brittle failure and 

spalling resulting in an hourglass shape (Case 5 in table 5). 

Figure 7. Pillar instability caused by a continuous geological 

structure cutting through the pillar at Mine D. Thread-bar bolts 

that were installed right through the pillar are visible (Case 4 in 

table 5). 
25th International Conference on Ground Control in Mining 

361 

will be incorporated into design guidelines for safe pillar layout 

design. 

ACKNOWLEDGEMENTS 

 The authors would like to thank the staff of the mining 

operations visited for their willingness to show us both their 

successful pillar layouts as well as the less successful cases. 

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K.N., John Wiley and Sons, New York, pp. 83-100. 

Bieniawski, Z.T. (1989). Engineering Rock Mass Classifications. 

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Brann, R.W. and Freas, R.C. (2003). Multiple Level Room and 

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Hansen, M.C. (1977). The Geology of Ohio--the Ordovician Ohio 

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