Phillipson, S.

31st International Conference on Ground Control in Mining 

Massive Pillar Collapse: A Room–and–Pillar Marble Mine Case Study 

Sandin E. Phillipson, Geologist 

Roof Control Division MSHA 

Pittsburgh, PA 

ABSTRACT 

The Mine Safety and Health Administration’s Roof Control 

Division responded to a massive ground failure at an underground 

marble mine in northern Georgia. The ground failure occurred 

in a benched area that had been abandoned in the early 1990s. It 

involved the failure of an estimated 19 pillars, in conjunction with 

a massive roof fall of an undetermined extent that was at least 60 ft 

high. Pillars in the benched area had been mined nominally 40 x 

40 feet to a height of 50 feet ft, with 40-ft mining widths, although 

it seems likely from measurements and accounts from mining 

personnel that pillars were even smaller than intended. Average 

overburden depth was 500 ft, and the rock mass was characterized 

by three sets of intersecting geological discontinuities that not 

only defined very blocky conditions that were conducive to pillar 

spalling and volume loss, but they were also adversely oriented in 

such a way as to cut diagonally through pillars at a high angle. 

The recently released S-Pillar software from NIOSH was used 

to determine pillar safety factors in the failure area and indicated 

that pillars were undersized for the conditions. S-Pillar was used 

to assess pillar stability in active areas to evaluate the likelihood 

of experiencing similar collapses in the future. S-Pillar correctly 

identified the pillars in the bench area as having been at risk for 

instability, in light of their ultimate failure. A two-dimensional 

finite element model was used to evaluate the possible stages 

of failure and to assess the likelihood of continued failure in 

the benched area that might affect more distal mine openings. 

Although stone mines are generally characterized as having more 

forgiving conditions than those found in softer sedimentary rocks, 

the failure shares some characteristics with domino-style massive 

pillar collapses experienced in coal and trona mines. First, the 

minimum dimension of the benched area approached 350 ft; 

second, the pillars defined by benching had small width-to-height 

ratios (0.8), and; third, the pillar failure area was slightly greater 

than 4 acres. 

The massive pillar collapse and associated roof fall illustrate the 

importance of incorporating pillars that have been properly sized 

for depth and for the final bench height. Furthermore, the designed 

dimensions must be adhered to during the actual mining process. 

Finally, the importance of understanding the geological conditions 

and their effect on the rock mass is critical. 

INTRODUCTION 

On September 21, 2011 at 3:45 a.m., employees on the surface 

at an underground marble mine located in northern Georgia felt 

what they thought was a shot being fired underground. Shortly 

thereafter, it was found that a portable toilet inside the mine and 

a tag-in board outside the portal had been knocked over by an 

apparent air blast. Further investigation revealed that a massive 

roof fall had occurred in the benched area of a portion of the mine 

that had been abandoned in the early 1990s. Fortunately, the 

massive collapse resulted in no injuries. Following notification 

of the Mine Safety and Health Administration (MSHA), personnel 

from the Southeastern Metal and Nonmetal Safety and Health 

District conducted a traverse around the perimeter of the failure 

area and determined that up to 19 pillars had collapsed amidst a 

massive roof fall. Although such domino-style pillar collapses 

have happened in coalmines (Mark, Chase, and Zipf, 1997) and 

trona mines (Swanson and Boler, 1995), the extent of failure that 

occurred at the study mine is almost without precedent in stone 

mines (Dismuke, Forsyth, and Stewart, 1994), which are commonly 

characterized by relatively forgiving environments due to high 

rock strengths. The height of the roof fall was later confirmed 

by drilling from an upper level to be at least 60 ft, although the 

maximum height and extent of the roof fall is unknown. The pillar 

failure encompassed a rectangular area 300 ft in width by 600 ft in 

length. The width may be considered to be larger if an additional 

row of pillars that is exhibiting signs of failure is included. 

Because of the large pillar failure area, the possibility that 

failure might continue to propagate into active portions of the 

mine and the possibility that similar failure might be encountered 

in newer areas of the mine that had been planned to incorporate 

benching, MSHA Technical Support conducted an evaluation of 

the failure mechanism and ground conditions in various parts of 

the mine. Measurements of dimensions, strata orientation, and 

structural discontinuities were obtained for incorporation into the 

NIOSH S-Pillar software program (NIOSH 2011), as well as a 

staged finite element model using Phase2

 (Rocscience, Inc. 2011). 

These models were used to assess the mechanism and progression 

of the failure, evaluate pillar stability in old and new workings, 

and evaluate the likelihood of continued failure in old and new 

areas. This case study validates the use of S-Pillar to evaluate 

stability in underground stone mines, highlights the importance 

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31st International Conference on Ground Control in Mining 

of understanding the effects of structural discontinuities on pillar 

stability, and reinforces the need to adhere to the designed pillar 

dimensions to maintain stability. 

BACKGROUND 

The subject mine is one of several underground room-andpillar mines that extract marble in an area approximately 50 miles 

north of Atlanta, Georgia (Figure 1). The mines are developed 

in the Cambrian-aged Murphy Marble, a formation within the 

Murphy Belt, which predominantly contains marble and schist 

with lesser amounts of quartzite and meta-conglomerate. These 

metasediments were originally deposited in a stable region 

as conglomerate, sandstone, shale, and calcic and dolomitic 

limestone (Graham 1969). Following deposition in the Cambrian, 

deformation occurred near the end of the Ordovician (as a result 

of the Taconic Orogeny) and again at the end of the Pennsylvanian 

(as a result of the Alleghanian Orogeny) (Graham 1969). The 

Murphy Belt, representing a tightly folded, overturned syncline, 

trends northeast from northern Georgia into southwestern North 

Carolina and includes, from oldest to youngest, the Nantahala, 

Brass town, Murphy, Marble Hill Hornblende Schist, and Andrews 

Formations (Fairley 1969). Later interpretations of the stratigraphy 

differ by including quartzite units (Tusquitee Quartzite and Nottely 

Quartzite) of formation rank in the sequence (Power and Forrest 

1973; Groszos 1986). Although the sequence of lithologic units 

is agreed upon by previous studies, the assignation of those units 

to formal formations has been confusing and inconsistent (Power 

1976). The Murphy Marble is interpreted by Power and Reade 

(1973) and Groszos (1986) to represent a carbonate bank. Near 

the study mine, Fairley (1969) and Power (1976) interpreted the 

Murphy belt to have been folded into an overturned syncline, and 

the marble interlaces with and passes laterally into the schist of the 

Andrews and Brass town Formations (Power and Reade 1973). 

Figure 1. Location map showing position of study mine relative 

to the extent of the Murphy Marble in northern Georgia and 

southwestern North Carolina. 

The subject mine is located in the informally named Whitestone 

Belt, one of three discontinuous pockets of marble that occur 

within the Murphy Belt (Power 1976). The Whitestone Belt is 

overlain by the quartz-biotite schist of the Brass town Formation, 

substantiating interpretations that the marble occurs on the 

overturned limb of a syncline (Power 1976). The marble is divided 

into an upper (actually lower in the stratigraphic sequence because 

of overturning), medium- to fine-grained dolomitic marble and a 

lower, coarse-grained calcite marble. The dolomitic marble is in 

contact with the overlying quartz-biotite schist, which hosts many 

thin layers of talc-phlogopite schist (Power 1976). The main body 

of the unit is massive, fine-grained dolomite marble that is 500–600 

ft thick but may have been repeated due to faulting. Along the 

two-mile outcrop of the Whitestone Belt, eight random room-andpillar mines had been developed by 1976, using lateral mining 

dimensions of 38–64 ft (Power 1976). Bench heights in older 

workings reached 150 ft, but, by 1976, ultimate mining heights had 

been reduced to 65 ft (Power 1976). 

The strike and dip of the formation in the northern Georgia 

marble mining district is variable, but marble banding along the 

Whitestone Belt generally trends N 05° W and dips toward the 

east from as little as less than 10° to as much as 30°. This local 

northerly trend is significantly different from neighboring legs of 

the Murphy Belt, which trend northeast and southeast, respectively. 

Where the marble bands dip less than 10°, mine workings 

appear very similar to a standard limestone mine that would be 

characterized by a relatively horizontal roof horizon. Where the 

dip increases, mining levels at this and other northern Georgia 

marble mines are arranged in a series of horizontal stair-steps that 

may partially overlap as the levels follow the formation to depth. 

In these examples, although mining levels remain horizontal, the 

roof horizon is commonly characterized by a saw-toothed profile, 

defined by intersecting partings and joints that define hanging 

wedges. Because of the propensity for these wedges to drop out 

of the roof despite scaling, at least one area marble mine performs 

routine pattern bolting to ensure that the wedges are supported. 

Bolting is not, however, performed at the study mine. 

The mine extracts marble for its calcium carbonate content by 

the regular room-and-pillar mining method on multiple levels, 

generally stepping gently downward toward the east. The mine 

is characterized by a prominent “dog leg” where the marble 

formation changes strike and dip, associated with a change in 

ground conditions and mining layout (Figure 2). Initial mining, 

characterized broadly as random room-and-pillar, was conducted 

on the 1,070-ft level, which represents the elevation above sea 

level at outcrop in the valley. In the northern half of the mine 

(that area north of the hinge point in the dog leg), regular roomand-pillar mining was conducted on the 920-ft level and the 880-

ft level, stepping down toward the east with the dip of the marble. 

This area had been abandoned in the early 1990s, although ramps 

are being developed to the 720-ft level to continue to follow the 

formation down-dip. In the southern half of the mine, regular 

room-and-pillar mining was conducted on the 890-ft and 820-ft 

levels, which are superimposed because of the very low dip angle 

of the formation, representing the current active mining horizons. 

The collapse occurred in the 920-ft level, which had been mined 

in the 1980s and subsequently benched in the central area in the 

late 1980s to early 1990s. The original mining height was planned 

for 25 ft, and the ultimate benched height was 50 ft. Mining 

dimensions are defined by pillars designed 40 x 40 ft with 40-ft 

mining widths. Actual mining widths were generally measured as 

42–43 ft.

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31st International Conference on Ground Control in Mining 

Table 1. Input values used to assess pillar stability in the 

collapse area with S-Pillar at the study mine. 

Parameter Value 

Pillars Square 

Heading Centers 80 ft 

Heading Width 40 ft 

Crosscut Width 40 ft 

Development Height 25 ft 

Benched Height 50 ft 

Maximum Cover Depth 500 ft 

Unconfined Compressive Strength 15,000 psi 

Large Discontinuities? Yes, spaced 30 ft, dip 60° 

Figure 2. General mine map, showing extent of pillar failure 

area (red) in relation to the rest of the mine. Prominent “dog 

leg” represents change in strike and dip of the marble deposit, 

and coincides with a change in ground conditions. 

METHODOLOGY 

Measurements of the strike and dip of structural discontinuities, 

as well as their spacing, were collected in different parts of the 

mine using a standard Brunton hand transit and tape measure. 

Measurements of mining dimensions were obtained in the mine 

using a laser range meter and compared to planned dimensions 

portrayed on digital mine maps. Observations were conducted at 

the margin of the collapse area and in the active mining area. The 

condition of the roof and pillars was assessed in various parts of 

the mine. These values were used for performing evaluations of 

pillar stability with the NIOSH S-Pillar program (Table 1) and used 

for input parameters in the Phase2 (Rocscience, Inc. 2011) finite 

element modeling program (Tables 2 and 3). Fifteen thousand psi 

represents the default value for unconfined compressive strength 

in S-Pillar, a program based on case studies of limestone mines. 

A marble mine in the same formation, within approximately five 

miles of the study mine, reported that core samples of the deposit 

exhibited UCS values of 12,000 psi, considered low for the marble 

as a whole. Thus, the 15,000-psi value is considered reasonable 

and may, perhaps, even be generous. Input values in Phase2

 are 

based on published values for a range of samples since actual 

geotechnical data were not available for this specific mine.

Pillar safety factors were determined with S-Pillar for the 

collapse area and compared to the current active mining area. The 

sequence of mining, benching, and pillar collapse was represented 

in Phase2 in a series of staged excavations. Pillars were removed 

in the model to simulate their failure. The eastern-most pillar was 

removed first, based on recollections of mine personnel who stated 

that those pillars exhibited sloughing shortly after benching and 

on their portrayal on mine maps as being noticeably undersized 

compared to the neighboring pillars. 

OBSERVATIONS 

Observations in the collapse area were conducted at the 

southern and western margin of the benched area (Figure 3). At 

the southwestern margin of the benched area, the west sides 

of the pillars are 25 ft high where benching ceased but are 50 ft 

high on the east side, where they have been exposed by benching 

or where a ramp had been driven onto the 920-ft level (Figure 4). 

The roof of the drift, even at the margin of the collapse area, was 

extensively damaged with what appeared to be recent small falls 

of rock, accompanied by pillar sloughing, based on the bright, 

fresh surfaces exposed in contrast to the dark soot coloration on 

areas that had remained intact for a long period of time. In this 

area, the contorted marble beds strike approximately N 05° W and 

dip up to 20° E. Prominent, iron-stained joints strike parallel to 

the marble but dip perpendicularly to marble banding (N 10° W, 

63° W). A second, less prominent joint set strikes N 65° W and is 

oriented vertically. 

The roof fall material consisted of very large, angular blocks and 

slabs (6-10 feet long) that were piled up above the 50-ft high roof 

line, obscuring any pillars that might have remained in the interior 

of the fall rubble (Figure 5). The lip of a very large fall cavity 

was visible in the roof, although the top and inside surfaces could 

not be discerned because of condensation in the air that obscured 

long-distance vision. The large blocks were jagged-edged and 

did not generally exhibit flat surfaces that would be indicative of 

unraveling along joints. Instead, the blocks’ jagged edges were 

suggestive of failure by bending stress. 

Farther west from the fall, on the unbenched portion of the 920-

ft level, pillars were observed to respond to additional abutment 

loading resulting from the massive collapse. All pillars in these 

rows over a distance of eight crosscuts were characterized by 

prominent, dipping joints that strike parallel to marble banding but 

dip diagonally through the pillars from corner to corner (Figures 6 

and 7). The pillars exhibited freshly spalled surfaces along the 

joint traces, and the joints exhibited open apertures even in the 

center of the pillar. Some joints appeared to have a milled, rubblelike appearance that is suggestive of shearing. The damage 

observed in the 25-ft-high pillars is suggestive of additional 

abutment loading resulting from the adjacent, massive collapse. 

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31st International Conference on Ground Control in Mining 

Table 2. Input values for rock properties used to assess 

collapse area with Phase2 at the study mine. 

Parameter Value 

Field stress and body force; 

Initial Element Loading 1:1 ratio of horizontal to 

vertical stress 

Poisson’s Ratio 0.3 

Young’s Modulus 432,000,000 psf 

Failure Criterion Mohr Coulomb 

Material Type Plastic 

Tensile Strength 345,000 psf 

Peak Friction Angle 35° 

Peak Cohesion 400,000 psf 

Dilation Angle 10° 

Residual Friction Angle 30° 

Residual Cohesion 40,000 psf 

Residual Tensile Strength 28,800 psf 

Unit Weight 165 pcf 

Table 3. Input values to describe geological discontinuities 

affecting the rock mass for evaluating collapse area with 

Parameter Value 

Marble Banding Partings 

Slip Criterion Mohr Coulomb 

Tensile Strength 0 psf 

Peak Cohesion 23,000 psf 

Peak Friction Angle 30° 

Normal Stiffness 600,000 psf/ft 

Shear Stiffness 600,000 psf/ft 

N 05° W, 65° W Joints 

Slip Criterion Mohr Coulomb 

Tensile Strength 0 psf 

Peak Cohesion 15,000 psf 

Peak Friction Angle 28° 

Normal Stiffness 400,000 psf 

Shear Stiffness 400,000 psf 

N 05° W, 90° Joints 

Slip Criterion Mohr Coulomb 

Tensile Strength 0 psf 

Peak Cohesion 23,000 psf 

Peak Friction Angle 30° 

Normal Stiffness 600,000 psf/ft 

Shear Stiffness 600,000 psf/ft 

Phase2 at the study mine. Figure 3. Map of collapse area, showing observation traverse 

(heavy purple line) and damaged pillars (red = believed to 

have failed; orange = taking weight, exhibiting degradation). 

Red lines represent prominent, iron-stained joints; black lines 

represent joints. Collapse was localized to the benched area of 

the 920-ft level, shown in light blue. 

Figure 4. The row of pillars at the margin of the benched area is 

characterized by extreme degradation, with the eastern portions 

(facing the benched side) 50 ft tall and exhibiting failure along 

pre-existing geological structures. 

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31st International Conference on Ground Control in Mining 

Figure 5. Roof fall debris is composed of very large blocks. The 

photo vantage point is approximately 40 ft from the rubble. 

Figure 6. Open-aperture joint, which strikes N 10° W and dips 

63° W, is located in the center of pillar II28. This orientation of 

joints typically runs diagonally from corner to corner in the JJ, 

II, and HH pillars. White outline represents close-up shown in 

Figure 7. 

Further assessment was performed in adjacent areas of the 920-ft 

level where active haulage ways were defined and in the southern 

area of the mine where active mining was being conducted, in order 

to ascertain whether widespread collapse should be a prevailing 

concern in this mine. Observations in those areas indicated that 

the quality of the ground was much better than in the collapse 

area. There appear to be two main reasons for this, both of which 

are reflections of the geological conditions. First, the set of joints 

that strikes N 05° W and dips approximately 65° W, thereby 

dissecting pillars diagonally, is not present south of the collapse 

area. Therefore, it appears that this joint set exerts a particularly 

deleterious effect on ground stability when present. This 

diagonally crosscutting joint set is not only in the most unfavorable 

orientation for pillar stability but is also generally characterized by 

Figure 7. Close-up of Figure 6, showing open aperture along joint 

that penetrates deeply into the pillar. This feature contrasts with 

the irregular, localized nature of small fractures that represent 

blast damage on the pillar skin. 

heavy iron staining, indicating open apertures that easily transmit 

water and offer little to no cohesion between opposing blocks 

of rock. Second, a change in dip, from up to 20° in the collapse 

area to less than 10° in the southern, active area results in a more 

stable roof horizon that is not characterized by a sawtooth profile 

of intersecting joints. It is interpreted that the sawtooth profile 

makes it easier for progressive raveling to occur, especially since 

the mine does not install rock bolts. Although the southern area 

hosts abundant, closely spaced (10–24 in.) vertical joints that strike 

N 65° W, they do not exhibit open apertures, and pillars exhibited 

virtually no evidence of spalling (Figure 8). 

Figure 8. Typical condition of pillars on the 820-ft and 890-ft 

levels, showing an absence of open-aperture joints or evidence 

of raveling. 

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31st International Conference on Ground Control in Mining 

DISCUSSION AND ANALYSIS 

S-Pillar 

The 920-ft level collapse area shares several similarities with the 

circumstances of domino-style cascading collapses in coalmines. 

These types of collapses are commonly characterized by wide 

panels (350 ft) that encompass more than 4 acres. (The study 

mine’s collapse area encompassed 4.1 acres.) In these collapses, 

tall, slender pillars (those with a small width-to-height ratio) had 

been developed (Mark, Chase, and Zipf 1997). It is interpreted that 

those were finely balanced systems and that the failure of only a 

single pillar was enough to shed load onto surrounding pillars, 

precipitating a cascading, rapid failure. Research performed by 

NIOSH on stone mine pillar performance identified two main 

factors in predicting pillar instability: 1) width-to-height ratio of 

0.8 or less and 2) safety factors of 1.8 or less (Esterhuizen, Dolinar, 

and Ellenberger 2008; Esterhuizen, Dolinar, and Ellenberger 2011; 

Esterhuizen, Dolinar, Ellenberger, and Prosser 2011) (Figure 9). 

Pillar stability was further influenced by the spacing and orientation 

of planar geologic discontinuities. Pillars in the failed 920-ft level 

were 50 ft high and 40 x 40 ft, defining a width-to-height ratio of 

0.8, cut by high-angle joints that further degraded the strength of 

pillars that only had a safety factor of 1.1 as calculated by S-Pillar. 

Figure 9. Analysis of pillar design on the 920-ft level using 

NIOSH S-Pillar software for limestone mines. Benched pillars 

on 920-ft level are solidly outside the acceptable design category, 

with a safety factor calculated by S-Pillar of only 1.1. 

Mine personnel stated that the M row of pillars in the benched 

area had exhibited signs of degradation shortly after benching, 

prompting the abandonment of the area in the late 1980s or early 

1990s. Other mine personnel suggested that the back horizon 

may have encroached upon or even reached the contact with the 

overlying quartz-biotite schist which, especially in combination 

with the previously reported talc layers, could have represented a 

large potential delamination plane. The mine map portrays the M 

pillars as smaller than the nominal 40 x 40 ft dimensions, which 

results in an even lower width-to-height ratio and lower safety 

factors. It is, therefore, interpreted that failure initiated in the M 

row and that, as those pillars failed, their loads were transferred 

onto the adjacent, meta-stable benched pillars, which failed in 

succession. Because the area had been abandoned for many years, 

the relative timing of pillar failure and roof failure is unknown. 

Measurements in adjacent areas indicated that mining widths were 

generally several feet larger than the nominally planned 40 ft, 

resulting in corresponding pillar dimensions several feet smaller 

than 40 x 40 ft. Thus, the pillars are likely to have actual width-toheight ratios and safety factors that are even smaller than would be 

indicated for the planned dimensions. 

Conditions in the southern, active mining areas appear much 

better than in the collapse area. The marble exhibits a very lowangle dip so that it is nearly horizontal, greatly reducing the 

occurrence of wedges in the back. Further, the N 10° W, 65°W 

joints that represent planes of preferential weakness and negatively 

influence pillar stability in the north are absent. The 890-ft level is 

not intended to ever be benched because of the 43-ft sill thickness 

to the underlying 820-ft level. NIOSH S-Pillar software indicates 

that for the current 25-ft development height, pillars should display 

acceptable width-to-height ratio (1.5) and safety factor (2.39) 

values. The lower, 820-ft level, is intended to be benched. Even 

though ground conditions appeared good during the underground 

evaluation, the S-Pillar analysis indicates that, when benching is 

performed, the resulting pillars will be on the margin of acceptable 

design (Figure 10). The S-Pillar software illustrates the importance 

of pillar dimensions used in the pillar strength calculations to 

determine the pillar safety factor. The software indicates that a 40 

x 40 x 50-ft pillar is on the margin of acceptable stability, and the 

mine must recognize the importance of adhering to the designed 

dimensions. Despite the intention to mine pillars 40 x 40 ft, with 

40-ft mining widths, the widths measured were generally 42–43 ft. 

The S-Pillar analysis indicates that this loss of pillar volume would 

be sufficient to transition the resulting benched pillars out of the 

acceptable design field, as the pillar safety factor is reduced from 

1.87 to 1.62. 

Figure 10. NIOSH S-Pillar analysis results for benching on the 

820-ft level, showing marginal but still acceptable stability. 

Finite Element 

Observations suggest that the JJ, II, and HH pillars (labeled in 

the following diagrams) are being subjected to excessive loading. 

The JJ pillars, which are adjacent to the 50-ft benched area, could 

fail. Thus, large-scale collapses could continue in the northern 

area as remaining pillars are, in effect, removed. A finite element 

modeling program was used to assess the possible progression of 

failure, and the possible consequences of continued failure (Figures 

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31st International Conference on Ground Control in Mining 

11a-f). The two-dimensional model, which treats square pillars as 

strip pillars, incorporated major geologic discontinuities, including 

partings along dipping marble beds (inclined 20°), vertical joints, 

and 65° dipping joints that dissected pillars as ubiquitous joints. 

In the model, strength factors (equivalent to a safety factor for 

model elements but calculated differently from the safety factors 

discussed in terms of S-Pillar) for elements in the K, M, and N 

pillars were reduced to less than 1 immediately upon benching the 

920-ft level (Figure 11b). It should be recognized that the model

results should be interpreted only as stability trends, rather than

absolute values. At this stage, the model suggests that both the

M and K pillars are affected by elements with safety factors less

than 1.

Figure 11a. Distribution of safety factors with development 

only on the 880-ft and 920-ft levels. Pillar rows are annotated as 

referred to in the text. Box in upper right portrays orientations 

of geologic discontinuities incorporated in the model, based on 

field observations.

Figure 11b. Distribution of safety factors following benching of 

the 920-ft level to 50-ft height. Note the significant reduction of

element safety factors in benched pillars compared to Figure 11a. 

However, the M pillar was chosen as the first pillar row to fail 

based on smaller-than-planned dimensions on the mine map, and 

statements from mine personnel indicating that the M pillars had 

been subject to constant raveling, interpreted as a sign of taking 

weight, possibly from cantilevering beneath the quartz-biotite 

schist delamination horizon. Following the loss of the M pillar, 

interpreted as the most likely to have failed in the benched area, 

elements in the L pillar also exhibited safety factors of less than 1, 

and safety factors began to exhibit a marked decrease in the roof 

of the resulting unsupported area (Figure 11c). With the loss of 

the L pillar, safety factors in the K pillar were drastically reduced, 

and safety factor values less than 1 propagated high into the roof, 

indicating the onset of the large roof fall that occurred (Figure 11d). 

As failure progressed to include the loss of the K pillar, safety 

factor values in the JJ pillar were reduced to less than 1, and two 

arcs of low-value safety factors were shown developing high into 

the roof, similar to a pair of closing pincers (Figure 11e). Figure 

11e represents the observed state of the ground at the time of the 

investigation, with a large roof fall having developed above the 

failed pillar area, the JJ (and likely also the N) row of pillars taking 

weight, and a large reduction in safety factor values for elements 

in the II pillars. The model results tend to be substantiated by 

the presence of open joints and freshly spalled surfaces observed 

on the JJ, II, and HH pillars. It should be recognized that pillars 

need not have toppled over or collapsed completely in order to 

represent a state of failure. Instead, it is more likely that the pillars 

would have gradually crushed out, deforming after a style of strainsoftening as incremental movement occurred along numerous 

joint planes. Most of the pillars likely retained some low level 

of residual strength and could even exist as remnants among the 

roof fall debris. However, the pillars are considered to have failed 

when that residual strength was no longer equal to the axial stress 

imparted from the weight of the overlying rock mass. With the 

expected continued destabilization and eventual failure of the JJ 

pillar, the model indicates the potential for an enlargement of the 

ground collapse area in the roof so that, even a secondary escape 

way, prudently abandoned, would be encompassed by safety factor 

values less than 1, despite being located 100 ft above the roof of the 

collapse area (Figure 11f). 

Figure 11c. Distribution of safety factors following the loss of the 

M pillar, interpreted to have been the first to fail based on mine

personnel accounts. 

Figure 11d. Distribution of safety factors following the loss of 

the L pillar. 

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CONCLUSIONS 

The MSHA Technical Support, Roof Control Division conducted 

an assessment of ground conditions at the study mine in order to 

better understand the causes of a massive collapse in an abandoned 

area and to determine whether the active mining levels might be 

subject to the same potential for collapse. Underground 

observations indicate the geological conditions are different in 

Figure 11e. Distribution of safety factors following the loss of the 

K pillar. The distribution of low-value safety factors generally 

mirrors the shape of the large roof fall cavity that was observed 

during the underground investigation. Note the low safety 

factors affecting the JJ pillar, which is substantiated by field

observations as indicated in Figure 4. 

Figure 11f. Distribution of safety factors following potential, 

but likely, loss of the JJ pillar. With the loss of the JJ pillar, the 

potential ground collapse area expands to encompass the former 

secondary escapeway, located approximately 100 ft above the 

920-ft level. Safety factor values in the II pillar have been greatly 

reduced, as substantiated by field observations as indicated in

Figures 6 and 7. 

the abandoned, collapsed area compared to the active areas. At 

this time, the potential for pillar failure in the active portions of 

the mine, on the 820-ft and 890-ft levels is low. This is mainly 

because of more favorable geological conditions characterized 

by an absence of the N 05° W, 65° W joints and a moderation of 

the marble dip to less than 10° that resulted in elimination of the 

heavily jointed, sawtooth roof profile.

Analysis of the collapse and active areas appears to validate the 

effectiveness of the NIOSH recently released S-Pillar software for 

evaluating pillar stability in stone mines. Although the case study 

database is based on limestone mines, the marble in this instance 

appears to have similar enough properties that actual failures 

occurred as would have been indicated by very low S-Pillar safety 

factors. Because S-Pillar has essentially been calibrated to the 

site-specific conditions at this mine by being able to back-analyze 

an actual collapse, S-Pillar results should be carefully considered 

when designing bench heights in future mining areas. 

It must also be noted that actual mined dimensions should be the 

same as designed dimensions. Mining widths of 43 ft, resulting 

from blasting over break of 1.5 ft on each of the four pillar sides, 

translates into a pillar that is 37 x 37 ft rather than 40 x 40 ft. 

This is 14% smaller than intended. If pillars are already designed 

to meet only the minimum stability value, to allow maximum 

recovery, this 14% could represent the difference between stability 

and failure when combined with other unexpected circumstances 

that may adversely affect pillar quality. In addition to adhering to 

designed dimensions, the effect of depth and associated axial load 

must also be recognized when following a dipping horizon. A 

traditional design that has been used for many years could be found 

to be inadequate at greater depths. 

Finally, the importance of understanding the effect of geological 

discontinuities cannot be overstated. Rock strength may mean little 

when the rock mass is characterized by several intersecting sets of 

closely spaced joints, particularly when joints are arrayed to cut 

diagonally through pillars. 

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