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Massive Pillar Collapses in U.S. Underground Limestone Mines: 2015–2021

Gregory M. Rumbaugh, Christopher Mark, and Todd Kostecki

Mine Safety and Health Administration

ABSTRACT

From 2015 to 2021, five massive pillar collapses occurred at four underground stone mines in the eastern 

United States. These events resulted in powerful airblasts that damaged mine infrastructure and mobile 

equipment, seriously injured miners, and disrupted underground travelways. Each of these pillar collapses 

propagated through the overlying strata, causing a subsidence basin on the surface. Pillar collapses are 

particularly hazardous for miners because they can occur with little warning and can affect miners that are far

from the pillar collapse area. The following case studies show that certain factors may increase the likelihood 

of a pillar collapse. For instance, each of the five events involved the collapse of at least twelve benched pillars 

with width-to-height ratios (w/h) of 0.8 or less. This study elaborates on these factors and proposes a 

framework to assist mine operators where they identify similar hazards.

INTRODUCTION

A massive pillar collapse, also known as a cascading pillar failure, domino failure, or pillar run, occurs when an 

array of pillars, fail in quick succession (Mark et al. 1997). The collapsing ground and resulting airblast are 

hazardous to miners underground and on surface, and experience demonstrates they can occur with little overt 

warning (Mark and Rumbaugh 2022). This is especially the case for large underground limestone operations.

Currently, there are 100 active underground stone mines operating in 19 states as shown in Figure 1. Eightysix of these mines extract crushed, broken limestone, while the remaining extract various crushed or 

dimensional stones. Underground stone mines can have a long mine life. For instance, stone mines can have

areas mined over 10 years prior, with some having areas over 100 years old. Many of these legacy workings 

contain pillars that were not developed with modern pillar design methods, and have the added issues of 

weathering and age-related deterioration.

Figure 1. Active underground stone mines in the U.S. Metal mines, such as those extracting zinc, are 

not included.

A variety of engineering methods and modern design tools are available to guide the design of new pillars and 

evaluate the stability of existing stone pillars. One of the most widely used stone pillar design programs in the 

KY.

21%

PA.

12%

IL.

11%

IA.

8%

IN.

8%

MO.

7%

GA.

6%

TN.

6%

Remaining 

States (11)

21%

n = 100
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United States is the S-Pillar software that NIOSH developed (Esterhuizen et al. 2011). S-Pillar derives its 

empirical database from studies at dozens of U.S. underground stone mines and is applicable for tabular, single 

seam, mining conditions with simplified pillar layouts. This software establishes two design criteria for pillar 

stability; the pillar should have a minimum Factor of Safety (FOS) of 1.8 and a minimum w/h (width-to-height 

ratio) of 0.8.

While limestone is typically a very strong rock, deposits contain joints and other geologic features that will 

impact pillar strength, especially slender pillars, where w/h is less than 0.8. Development of slender pillars 

occurs during secondary mining when bench mining the floor. Slender pillars are also sensitive to variations in 

pillar width that blast damage and overbreak can cause.

Since 2015, five massive pillar collapses occurred at four underground stone mines in the eastern United States. 

In three of the four mines, the pillar collapses occurred in legacy workings and were at least 15 years old. The 

pillar collapse areas in all five of the case studies contained pillars that did not meet the S-Pillar criteria.

CASE STUDIES

Mine A – April 2015, Pennsylvania

At approximately 5 AM, three underground miners were about to enter the portal to begin their shift when they 

heard what sounded like the firing of shots underground. An airblast then exited the portal with enough force 

to throw the miners and their equipment approximately 50 ft, injuring all three miners. Two of the miners were 

unconscious and required helicopter transport to the hospital. Significant subsidence occurred at the surface 

above the mine, displacing an area over seven acres (Figure 2). It was determined that these events were the 

result of a collapse of up to 35 pillars (Figure 3). Fortunately, no other miners were underground at the time of 

the pillar collapse and no other injuries occurred.

Figure 2. Aerial view of Mine A subsidence

The mine had operated for approximately 25 years. Mine maps showed that the collapsed pillars were some of 

the first pillars in the mine, and the mine operator benched the area approximately 15 years prior to the pillar 

collapse. Although the benched areas were inactive legacy workings, a travelway bisected the pillar collapse 

and served as an escapeway for miners.
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The operator’s state mining permit defined the rooms and pillars as 35-ft square on 80-ft centers. The limestone 

averages 60 ft in thickness, with mining planned in two lifts (i.e., initial development and benching). The height 

of the initial development was 23 ft. Secondary bench mining removed an additional 30 ft from the floor, 

resulting in a total planned mining height of 53 ft. Observations at the perimeter of the pillar collapse showed 

the actual mining dimensions deviated from planned. The centers measured ≈80 ft, but the average pillar width 

was only 31 ft, resulting in an as-mined dimension ≈10% less than planned. In addition, the mining height in 

the benched area measured 58 ft. It is estimated that the as-mined dimensions, reduced the w/h from 0.66 to 

0.54 (Esterhuizen et al. 2019). 

The standard deviation of the pillar widths around the perimeter of the fall was 4.5 ft (Esterhuizen et al. 2019). 

Therefore, a similar pillar dimension variability likely existed in the pillar collapse area. Using the S-Pillar 

software, the FOS of the pillar collapse was slightly above 1.0 (Esterhuizen and Murphy 2010). The size of the 

benched area was approximately seven acres, some of which had been backfilled with fine crusher waste 

(Esterhuizen et al. 2019). As shown in Figure 3, the mine operator benched most of the pillar collapse area, 

apart from the unbenched travelway bisecting the pillar collapse area.

A prominent joint set, 500 ft wide in plan view, exists through the eastern half of the pillar collapse. The joint 

spacing was from 10 to 16 ft and dipped from 60 to 80 ̊. A low angle thrust fault along the southeast edge of the 

pillar collapse zone intersected the joint set. NIOSH conducted simulations after the pillar collapse and found 

that the large discontinuities coupled with the w/h = 0.54, resulted in a pillar strength lower than what could 

be predicted by established hard-rock pillar strength equations (Esterhuizen et al. 2019). 

Adapted from Esterhuizen et al. 2019

Figure 3. Pillar collapse periphery at Mine A
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Mine B – October 2020, Pennsylvania

On October 6th, 2020, approximately 25 pillars collapsed at a southwestern Pennsylvania limestone mine. This 

pillar collapse registered as a 2.9 magnitude event on the U.S. Geological Survey’s seismic network. See Figure 

4a for an image of the resulting subsidence and Figure 4b for an image of the pillar collapse perimeter.

Figure 4. (a) Subsidence from collapsed workings and (b) an underground perspective of the 

rubblized periphery

The pillar collapse occurred in a benched region of the underground room-and-pillar works, with a depth of 

overburden of ≈160 ft (Figure 5). Development mining occurred five to six years prior to the pillar collapse, at 

a mining height of 25 to 30 ft, or 50% of the total height of the Loyalhanna Limestone Bed in the area of the 

collapse. Bench mining of the lower portion of the Loyalhanna was ongoing and conducted just four days prior 

to the pillar collapse. It was not evident if bench mining began recently or had been on-going since 

development. The full bench height ranged from 45.9 to 48.0 ft. Mudstones, shales, and siltstones overlay the 

mining horizon and operations left 8 ft of limestone in the roof to provide a stable back; however reliable data 

via drilling logs were not available to attest to the main and immediate roof thickness and composition in the 

vicinity of the collapse. It should be noted that while the immediate and main roof can be a contributing factor 

to pillar instability, the focus of these case studies was primarily on the generalized pillar geometry.

A contributing factor to the pillar collapse was the as-mined pillar widths being smaller than the planned pillar 

widths of 40 ft, which created unacceptable w/h values. Field measurements (n = 32) around the border of the 

pillar collapse indicated that ≈90% of the measured pillars were less than 40 ft and ≈13% were less than 30 ft. 

Along the periphery of the pillar collapse area, the average width was ≈35 ft with some as low as 25 ft, resulting 

in w/h of 0.53 (25.3 ft/48.0 ft). The mine plan showed the smallest width of a pillar inside the pillar collapse 

area as 20.6 ft, resulting in an even lower (estimated) w/h = 0.43. At the boundary of the pillar collapse area,

where the pillar run ceased, and using a conservative pillar height of 45.9 ft, ≈44% of periphery pillars were 

below a w/h of 0.76.

The description of the notably complex structural geology of the Loyalhanna bed was consistent with previous 

ground-control work (Iannacchione and Coyle 2002). Specifically, nearly 375 measurements from mines within

the Loyalhanna indicate two distinct joint sets. The first joint set exhibits mostly vertical jointing in the 

direction of N47 ̊W, and the second joint set is 90 ̊ offset at N48 ̊E at varying dips. The former set strikes across, 

and the latter set is nearly parallel to the Chestnut Ridge. Observed after the pillar collapse, were four joint sets 

(one longitudinal, one transverse, and two conjugates). The longitudinal joint set had strikes at N35 ̊E to N45 ̊E 

with variable spacing ranging from 10 to 20 ft and local joint clusters (i.e., three to six) at spacings from 5 to 10

ft. The transverse joint set had strikes measured at 7 to 16 ̊ offset from N47 ̊W, at N54 ̊W to N63 ̊W. These joints 
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were non-persistent with dips to the southwest. Two conjugate sets were steeply dipping and displayed strikes 

at the upper and lower bounds of the northwest quadrant.

Figure 5. Pillar collapse periphery at Mine B

Mine C – November 2020 and July 2021, Pennsylvania

During the pre-dawn hours in November 2020, a surface miner returned to his worksite on a spoil pile where 

he had parked a brand-new excavator, with only 10 machine-hours on it, at the conclusion of his last shift. 

Surprised by not seeing the excavator, the miner stopped and exited his truck. Instead of finding the excavator, 

he realized that he was standing near the edge of the surface depression shown in Figure 6, and narrowly 

avoided driving into the sinkhole. The excavator is engulfed in the sinkhole and has not been found.

The sinkhole overlays room-and-pillar workings in the Valentine Limestone, which are part of an active mine 

owned and operated by a different company than the surface operation. The underground workings 

experienced a pillar collapse during the previous night. What occurred that morning would be the first of many 

sinkholes that would appear in the spoil pile area and adjacent properties. The cause of these sinkholes was 

the collapse of approximately 12 pillars, comprising six acres. In July 2021, a second pillar collapse occurred in 

the same panel, resulting in more subsidence and an airblast that caused damage to mine-infrastructure, 

including communication beacons, ventilation brattices, escapeway signage, haulage mirrors, and barricades.

The area of the second collapse involved approximately 20 pillars, covering nine acres. The surface subsidence, 

which was adjacent to an abandoned and an inhabited residence, manifested within 48 hours of the pillar 

collapse. This subsidence feature was approximately 800 ft south of the November collapse, 5 to 25 ft deep and 

comprising ≈3.5 acres of trees and a pond.
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Figure 6. A 260 ft diameter by 100 ft deep sinkhole associated with the pillar collapse at Mine C

The July 2021 pillar collapse registered as two distinct seismic events on the regional Pennsylvania State 

Seismic Network of 1.4 magnitude at 3:45 AM and 2.6 magnitude at 4:00 AM local time. Nearly half of the 36 

acres of benched area collapsed during these two events (Figure 7) and were the first failure of its kind since 

the mine began operations around 1950.

Figure 7. Pillar collapse regions at Mine C
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The operator initially developed the panel that collapsed between 1998 and 2001, at a height of ≈30 ft. Benching 

occurred from 2002 to 2006, which increased the total mining height to 72 ft. The pillars were diamond-shaped, 

with north-south oriented headings developed 53 ̊ from the east-west headings. The seam dips from 10 to 15 ̊

to the southeast and center-to-center spacing between headings was 115 ft. The planned pillar widths were 65 

ft. According to the mine operator, blasting during benching and spalling of the pillars over time may have

reduced the pillar width to 55 ft. The depth of cover ranged from 350 to 650 ft.

The w/h of the pillars was ≈0.76 and analysis showed a FOS of ≈1.4 for the collapsed panel; both values were 

less than those that S-Pillar suggests (Esterhuizen and Murphy 2010). The w/h was calculated using the 

minimum width of the pillar. Moreover, there are several unaccounted factors in the software’s calculations, 

particularly those associated with the dip of the seam, impact of adjacent face development, adjacent surface 

mine, overlying spoil pile, complex geology, and the angled headings and crosscuts. Most notable is that the 

seam dips from 10 to 15 ̊ and the S-Pillar database only includes flat-lying formations. A steeply dipping seam, 

such as this one, would further reduce stability of the pillars; therefore, the actual FOS is likely less than the SPillar calculated value of 1.4.

The mine operator had been developing the faces immediately east of the pillar collapse area for a few weeks 

prior to the pillar collapse. Miners had been traveling along the perimeter of the pillar collapse area during that 

time. The development work in the eastern faces likely influenced the pillar collapse as the overburden load 

would re-distribute and likely increase the stress on the benched pillars in the pillar collapse area.

An active surface mine was also extracting stone adjacent to the panel from the same formation. This surface 

mine had been regularly blasting immediately adjacent to the pillar collapse area for the 13 months preceding 

the pillar collapse. A seismograph in the northwest corner of the panel detected ground vibrations from surface 

and underground blasting, however, whether pillar stability was affected, or unaffected, by blasting cannot be 

made with a high level of confidence at this time. Nonetheless, the awareness of these adjacent activities is 

important.

Finally, the primary spoil disposal area for the adjacent surface mine was over the northern half of the panel. 

Satellite imagery shows initial construction of the spoil pile in the early 1990s. Over the past 30 years, the spoil 

height appears to have increased by 200 ft above the initial ground elevation, creating an increasing surcharge 

load on the pillars which may have contributed to their failure.

Mine D – August 2021, Tennessee

In August 2021, a foreman started his examination shortly before 5 AM. While traveling on a roadway adjacent 

to benched legacy workings, he heard falling rock. Over the course of the next hour, the foreman and other 

miners heard what they described as “large,” “loud,” and “continuous” falls of rock coming from the area. During 

this same time, one of the haul truck operators discovered recently spalled material from a pillar rib. The 

material was blocking the main haul road out of the production area adjacent to these benched legacy workings. 

This spalled material was in the same location where material had just been scaled from the rib and cleaned up 

the prior day. The foreman then evacuated all personnel from the mine.

The miners tagged out and gathered at the mine office, near the portals of the mine. Suddenly a pillar collapse 

occurred. The miners reported feeling a pressure pulse or wave, heard rumbling from the portals, then watched

an airblast with a velocity between 120 and 180 mph exit the portals. The airblast damaged nearby 

infrastructure and buildings and resulted in a sinkhole that encompassed an area of approximately nine acres 

as shown in Figure 8. Fortunately, no injuries occurred. The pillar collapse registered as a 3.1 magnitude seismic 

event. The pillar spalling and the resulting rock that miners noticed on the roadway on the day before the pillar 

collapse was an indication of the changing stability conditions. Miners cleaned up the material and scaled the 

pillar but observed fresh fallen material on the morning of the pillar collapse. During the months following the 
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pillar collapse, miners did not go underground until the mine operator remotely examined the mine with 

autonomous mapping drones and Wi-Fi controlled ground robots and determined that the mine was stable.

Figure 8. Sinkhole associated with the pillar collapse at Mine D and the associated underground plan 

view

The exploration program indicated that approximately 40 pillars, developed 70 years prior, collapsed across 

an area of 14 acres. Extracted over the next 35 years were two floor benches making the total mining height 

between 80 and 90 ft. The depth of cover was approximately 400 ft. The pillar widths were 36 ft minimum and 

averaged 53 ft. These widths suggest that the minimum w/h ranged from 0.40 to 0.45 for the pillar collapse 

area.

The collapsed pillars had been the subject of a pillar stability study in 2005 which included field measurements 

and photographs as shown in Figure 9. As were the circumstances in two of the other cases, nearby 

development mining had recently taken place. Since 2012, development headings had extracted a large barrier 

of intact limestone directly north of the area that collapsed. The presence of through-going joints in the pillars 

and karst features may have also been factors in the pillar collapse.

Figure 9. Pillars involved in the massive pillar collapse of Mine D (Photo credit: Dr. G.S. Esterhuizen)
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DISCUSSION

Many active underground mines contain legacy workings. Evidenced by these case studies, legacy workings, 

particularly benched areas where the w/h does not satisfy modern design recommendations, may be prone to 

pillar collapse. These cases demonstrate the need for a risk management approach to assess the likelihood and 

consequences of pillar collapses in similar workings.

A proposed qualitative approach for assessing these legacy areas consists of the following steps. First, a mine 

operator identifies an area of standing pillars and evaluates the likelihood of a collapse of the pillars, based on 

the factors such as those in the Massive Pillar Collapse Likelihood Matrix (Table 1). This matrix helps define the 

geotechnical and mining geometric data necessary to conduct the pillar collapse assessment. The assigned 

factors are high, moderate, or low in rating. A high rating suggests an increased potential for a pillar collapse

and a low rating suggests a decreased potential. All factors should be considered, even though there is no 

standard method for combining the individual factor ratings at present. Rather, a mine operator can assess the 

overall likelihood of a pillar collapse based on an evaluation of all the factors in the matrix.

Table 1. Massive pillar collapse likelihood matrix

Low Moderate High

Pillar stability Meets all applicable 

design criteria

Does not meet applicable 

design criteria

w/h (pillar system 

average) w/h>1.0 0.8<w/h<1.0 w/h<0.8

Pillar dimension 

variability

All pillars approximately 

equal size

A few pillars smaller than 

average

Many pillars smaller than 

the average 

Spanning potential of 

panel/benched area

Strong overburden/deep 

cover/narrow pillar array

Moderate strength 

overburden/moderate 

cover/moderate pillar array 

width

Weak overburden/shallow 

cover/wide pillar array

Size of benched area Small Moderate Large

Major geologic features None If a fault, karst, or other major geologic feature is present, 

assess its potential contribution to the collapse likelihood

Soft floor None Possible, but minimal evidence 

of pillar distress

Thick weak floor causing 

pillar dilation

Weak bands in the 

pillars None Possible, but minimal evidence 

of pillar distress

Thick, weak band causing 

pillar dilation

The first two factors, pillar stability and w/h, are the most critical. Assessment of pillar stability occurs with 

software such as S-Pillar or other widely applied and successful pillar design methods. The analysis must 

consider the depth of cover, extraction ratio, actual pillar dimensions, limestone strength, and the presence of 

joints or other geologic features. Since mine maps can be inaccurate, especially in legacy areas, verifying the 

actual pillar dimensions is important. The results can then be compared to the design method’s criteria. Some 

design methods have limitations or restrictions for their use, such as ranges of mining geometries, rock 

strengths, or other geologic characteristics. The w/h is the second component of the pillar analysis. A pillar 

array with an average w/h less than 0.8 is a concern because the strength of slender limestone pillars can be 

difficult to accurately quantify. The variability of pillar dimensions is also important because if some pillars 

within the array are significantly smaller than the others then these smaller pillars can fail first, transfer stress 

to neighboring pillars, and subsequently trigger a pillar collapse.
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The tributary area theory assumes that a pillar carries its full share of the overburden weight. Only a few pillars 

in the center of the array may carry the full tributary area load, while the pillars near the edges of the panel 

may be shielded due to arching from barrier to barrier. When the pillar array is narrow, the cover is deep, 

and/or the overburden is strong, a pressure arch can transfer load so that pillars carry less than the full 

tributary area load; this can decrease the likelihood of a pillar collapse. Conversely, when the overburden is 

weak, the cover is shallow, and/or the panel is wide, a pressure arch may not transfer load and the pillars may

carry the full tributary area load; this can increase the likelihood of a pillar collapse. Therefore, pressure arching

directly affects pillar collapse potential.

Research has also identified several geologic factors related to the likelihood of a collapse. A major geologic 

feature, such as a fault or karst, can weaken the overburden and/or decrease the strength of pillars within an 

array. A soft floor can also reduce pillar strength by foundation/shear failure (heave or punch) resulting in

pillar slabbing and roof deterioration. Clay layers or other weak banding can affect pillar stability. Rib dilation 

and slabbing increases the likelihood of collapse by reducing a pillar’s strength and its load bearing area

(Esterhuizen et al. 2011). The operator should note the presence of karsts within the mining units and look for 

karstic subsidence features and water features on the surface overlying these areas.

The next step of the risk management process is to evaluate the consequences of the potential pillar collapse. 

The Massive Pillar Collapse Consequence Matrix (Table 2) can serve as a guide to conduct the evaluation, 

considering the potential hazards that would be present in different locations in the mine, and the exposure of 

miners to those hazards. For areas where the combination of likelihood and consequence is great enough, the 

mine operator can take measures to mitigate the risk (Mark 2021).

Table 2. Sample massive pillar collapse consequence matrix

Location of Miners Conditions/Hazards Consequence Miners Exposed, # Exposure Freq.

Working within pillar 

collapse area, engaged in 

active benching operations

Massive rock fall, no 

warning Death

Working or traveling in 

travelways or haul roads 

directly adjacent to pillar 

collapse area

High air velocities, flying 

debris, small rock falls

Death or severe 

injury

Working or traveling 

directly above a pillar 

collapse area

Sudden development of a 

surface sinkhole

Death or severe 

injury

Working or traveling in 

high velocity air pathways 

leading from pillar 

collapse area to portals

Diminishing air velocities 

depending on number of 

pathways and distance 

from pillar collapse

Injury

Other locations in the mine Damage to ventilation 

controls or egress routes Indirect hazards

If a pillar region has an elevated likelihood of pillar collapse, control techniques could reduce the consequences

of a potential pillar collapse. For example, a mine operator can reduce exposure to a pillar collapse and airblast 

by relocating travelways and other mine infrastructure to areas of lower potential. This method might be 

particularly appropriate for infrastructure or travelways located directly adjacent to a potential collapse area.

Another control is to establish airblast-attenuation barriers, such as bulkheads, or rock barriers, as a precaution 

to protect miners and infrastructure. Properly designed barriers are those where the bulkhead material cannot 
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become airborne. Backfilling a pillar array, with fines or blasted material, could reduce the intensity of an 

airblast, because it reduces the void volume. Backfilling may provide some confinement to the pillars, however, 

one of the areas described within this paper was partially backfilled prior to the pillar collapse.

A monitoring program could also serve as a valuable tool to complement the pillar collapse management

program. Trained mining crews can observe warning signs, such as rock noise emanating from pillars, or new 

rib spalling via pillar condition surveys. Monitoring programs could include real-time measurement such as 

micro-seismic monitoring which could potentially provide early warning of pillar failure. The monitoring 

program would include a record keeping system accompanied with management processes developed to 

ensure that warning signs receive appropriate and expeditious responses. Technologies, such as LiDARequipped drones, could aid in the program and provide mine operators the capability to gather pillar 

dimensions for the pillar collapse assessment with high resolution and accuracy. Periodic surface inspections 

can identify subsidence, karst features, and changes in water features that may serve as precursor indicators 

of pillar collapse and can be easily implemented with current drone capabilities.

SUMMARY

There is clearly a need to evaluate the stability of active and legacy workings. Three of the four mines in the 

case studies had travelways adjacent to legacy workings that collapsed. Two of the mines had active faces 

adjacent to the legacy workings that collapsed. Some commonalities emerged from the five pillar collapse cases 

(Table 3). The depths were relatively shallow, ranging from 160 ft to 600 ft. Each pillar collapse created surface 

subsidence and registered on regional seismic networks. The pillar collapse areas included 12 to 40 failed 

pillars and encompassed 5 to 15 acres. The collapsed pillars were in benched areas where the mine operator 

extracted the floor after initial development, which created slender pillars with total heights ranging from 50

to 80 ft and w/h ranging from 0.45 to 0.76. These ratios are below the 0.8 minimum value recommended by 

NIOSH, and demonstrate the need to maintain dimensions that meet, or exceed, recommended design values 

(i.e., FOS) and criteria.

Table 3. Characteristics of the five pillar collapse areas

Mine A Mine B Mine C Mine D

Pillar collapse date Apr. 2015 Oct. 2020 Nov. 2020 Jul. 2021 Aug. 2021

Overburden, ft. 200 160 500 600 400

Bench height, ft. 58 50 72 72 80

w/h (min.) 0.54 0.63 0.76 0.76 0.45

Pillars 35 25 12 20 40

Pillar collapse area, ac. 7 5 6 9 14

The five recent pillar collapses represent an alarming number, considering there are only about 100 limestone 

mines operating in the United States. For planning purposes, existing underground mines may appear 

attractive to access reserves without requiring new permitting or logistical challenges. However, these mines 

may contain areas of legacy pillars developed without modern pillar stability evaluation techniques. Although 

assessment of legacy areas is not routine, legacy areas can still be a significant hazard to miners. Mine operators 

should carefully evaluate active mining areas, particularly when they are benching. Fortunately, no fatalities 

occurred during these recent pillar collapse events; however, three miners did sustain serious injuries and

many miners narrowly avoided fatal or serious injuries.

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