The mobile equipment used in underground coal mines is not
typically equipped with seat belts, and there is little or no foam
padding on the seats or the underside of the overhead canopy.
Another factor that may contribute to injuries caused by powered
haulage in low seams is the operator's restricted field of vision.
Due to the low profile design of equipment used in thin seams,
the operator is often in an almost fully reclined posture while
driving mobile equipment.

Machinery

The rates of nonfatal days lost injuries involving machin-
ery incidents are higher in low and medium seams than in high
seams. This trend persists for incidents involving roof bolting
operations, which accounted for 71 percent of machinery non-
fatal incidents and 4 of the 16 fatal machinery incidents (see
Figure 1). All 4 fatal incidents occurred in low or medium seams.
Additional information about injuries involving roof bolting op-
erations can be obtained by looking at the actual source of the
injury. For nonfatal days lost injuries, the specific source of
the injury is coded as the mining machine itself at twice the
rate in low seams as in high seams. All four fatalities resulted
from crushing injuries involving the roof bolting machine. Turin
et al.®) contains several recommendations about the prevention
of fatalities involving roof bolters. An additional sourcé of non-
fatal injuries is roof bolts. The rate of injuries involving roof
bolts is four times higher in low seams than in high seams. As
noted by Grayson et al.,” one reason that injuries involving
bolts are more prevalent in low coal seams may be that miners

frequently must bend and unbend roof bolts to install them into
the roof.

Incidents involving continuous mining machines accounted
for only 16 percent of nonfatal days lost injuries but 70 percent
of the fatal injuries. Seam height does not appear to be related to
the rate of injuries associated with continuous mining machines,

Handling Materials

The highest rate of nonfatal days lost injuries is associated
with the “handling materials” injury category. Most of these
injuries involve musculoskeletal sprains or strains, particularly
of the back. The rate of injuries does not appear to vary much
with seam height. The highest rate is found in the medium-seam.
height category. Medium seams also have the highest rates of
back injuries overall, the highest rates of injuries resulting in
strains or sprains, and the highest rates of injuries resulting from
all types of over-exertion (lifting objects, pulling or pushing
objects, and wielding or throwing objects). This may reflect the
fact that miners in 43- to 60-inch seams cannot stand fully erect
when lifting and transporting materials, and they may experience
difficulty in using their leg muscles to do as much of the lifting.
Additionally, these miners are often stressing their backs with
continual stooping and duck walking.

Nonpowered Hand Tools

Rates of nonfatal days lost injuries resulting from incidents
involving nonpowered hand tools do not vary substantially with
changes in seam height.

FIGURE 1
Coal miner operating roof bolter machine in a low-seam mine.


TABLE I
Nonfatal days lost injuries in small underground bituminous coal mines by seam height and type of
incident, 1992-1996

Seam height
Number of injuries Injuries/100 FTE
Low Medium High Low Medium High
‘Type of incident 42" 43”-60" 261” <42" 43"-60" 261"

Roof falls 470 441 263 1.45 1.82 2.03
Rib falls 34 48 56 10 .20 43
Powered haulage 708 389 147 2.18 1.61 1.14

Personnel carriers 160 15 20 49 31 AS

Load-haul-dump 268 101 32 83 42 25

Conveyors 102 41 ll 31 17 09

Shuttle cars 112 141 70 34 58 54
Machinery 552 409 162 1.70 1.69 1,25

Roof bolters 409. «294 108 1.26 1.21 83

Continuous miners 75 3 27 23 30 21
Handling materials 1102 938 433 3.39 3.87 3.35
Nonpowered hand tools 283 187 104 87 77 80
Slip or fall of person 203 223 204 62 92 1.58
Stepping or kneeling 101 33 20 31 14 1S

on an object
All other 167 102 46 51 42 36
Totals 3620 2770 1435 11.14 11.09

11.44

Rib Falls

The sides or walls of the tunnels created by extracting the
coal are referred to as “ribs.” As expected, the rate of nonfa-
tal days lost injuries caused by material falling from the ribs
increases substantially as mining height (and the height of the
rib) increases. The two fatalities resulting from falls of rib both
occurred in high-seam mines.

Powered Haulage

‘The rate of nonfatal days lost injuries associated with powered
haulage decreases substantially as seam height increases. This’
trend is particularly evident for nonfatal days lost injuries in-
volving personnel carriers (such as man trips), load-haul-dumps
(mostly scoops), and conveyors (includes mobile bridges). One
reason the rate of injuries involving shuttle cars is actually lower
in thin-seam mines may be that proportionately fewer shuttle
cars are used in low seam mines as compared to medium- or
high-seam mines. Continuous haulage systems and battery pow-
ered scoops tend to be used in place of shuttle cars in thin-seam
conditions. In spite of the tendency to use alternatives to shut-
tle cars in thin-seam conditions, fatalities involving this type of
equipment are more prevalent in lower seams. All seven fatalities
involving shuttle cars occurred in low- and medium-seam mines.

Based on their microanalysis of injuries to West Virginia’s
small mine shuttle car operators during 1993, Grayson et al.”
state that “the real problem with accidents occurring to shuttle

car operators is in small mines.... All aspects of performing
this job are a problem, but the largest impact with respect to
lost workdays is in operation of the vehicle. Most of the prob-
lem lies in tramming the machine, which is a seam thickness
phenomenon.”

A consistently decreasing trend is not observed across seam
height for rates of fatalities involving various types of powered
haulage equipment. However, the lowest rates usually occur in
high seams. In fact, of the 20 miners killed in powered haulage
incidents, only two were in high seams.

A contributing factor to the higher rates of injuries observed
in low and medium seams involving powered haulage incidents
is the low clearance of the mine roof. As the mining height de-
creases, the mine roof, as well as installed roof bolts and plates,
become additional sources of injury to the miner. Necks, backs,
heads, and shoulders are the body parts most frequently injured
when miners contact the mine roof. Of the 210 incidents in
which the source of injury was reported to be the mine roof,
32 percent involved load-haul-dumps, 19 percent involved man
trips, and 9 percent involved shuttle cars. According to the injury
narratives, these miners were injured when the haulage equip-
ment they were operating ran over a bump or hole or rock on
the mine bottom, causing them to be thrown into the roof. In
medium-seam heights miners are similarly injured when they
are thrown up into the protective canopy on the equipment they
are operating.


Slips and Falls
As expected, the rates of nonfatal days lost injuries due to
slips or falls increases substantially as seam height increases.

Stepping or Kneeling on Object

‘The rates of nonfatal days lost injuries due to stepping or
kneeling on an object are substantially higher in low seams than
in either medium or high seams. The activity of the miner is usu-
ally described as crawling or kneeling at the time of the injury.
As expected, rates of injuries to miners while crawling or kneel-
ing were strongly related to seam height. The rate is 10 times
higher in low seams than in high seams. About 75 percent of
these injuries were to the knee. Articles on miners’ knee disor-
ders have been published in the research and medical literature
for many years.) Miners in low-seam conditions often work
‘on hard and uneven surfaces, pushing their joints to extreme
ranges of motion with static stress. Prolonged kneeling can ad-
versely affect the skin, bursae, and knee joints, resulting in cuts
and scrapes, bursitis, laxity of the knee joint, or torn menisci.
As expected, when the rates of knee injuries for different seam
height categories were compared, substantial differences were
found. Overall, rates of knee injuries decreased as seam height
increased. About 23 percent of the knee injuries in low seams
were the result of crawling or working on the knees for extended
periods of time. This percentage dropped to 9 percent in medium
seams and less than 5 percent in high seams, Grayson et al.”
report that, in comparison to large mines, miners in small mines
experience a much greater number of lost workdays because of
problems associated with crawling, including cysts from wear-
ing knee pads and damage to nerves in the hands.

All other

Of the 25 fatalities due to all other causes, 14 happened in
Jow seams, one in medium, and 10 in high seams. Methane gas
explosions caused 13 fatalities. Seven others were caused by
electrocution. Very few of the total NFDL injuries fall in the “all
other” category—less than | percent.

CONCLUSIONS

For mine operators and government agencies to receive the
greatest return on their investments in miners’ safety, itis impor-
tant that they have a good understanding of the types of hazards
faced by workers at different types of mines. This study shows
that the types of factors posing the greatest threat to coal miners’
safety vary substantially with seam height. The findings from
this study should help guide mine safety professionals who are
seeking to improve safety at thin-seam mining operations.

As the working height decreases to the extent that a miner
must stoop, duck walk, or crawl, the miner’s vision, posture,
and mobility become increasingly restricted. The physical de-
mands placed on the miner generally increase as seam height
is reduced. Additionally, as mining height decreases, the mine

roof increasingly becomes an additional source of injury to the
miner. Although equipment is sized to fit in thinner coal seams,
the height restriction makes it more difficult to use protective
canopies. It also puts limitations on the placement of operator
compartments, which affects what and how much an operator
can see while in the cab. Lower mining heights can also make
tasks more difficult to perform, such as requiring a roof bolter
operator the added effort of bending and unbending roof bolts
to insert them into the mine roof.

A safety concern at medium-seam operations is posture. Min-
ers working at these heights cannot lift using the traditionally
advocated safe lifting procedures based on unrestricted height.
Laboratory studies of the effects of lifting materials using differ-
ent postures (¢.g., kneeling, stooping, etc.) suggest that working
heights in the 48- to 72-inch range are more stressful on the back
than working heights which require a person to remain kneeling
or which allow a person to stand."

Reduced mobility is another concern in lower seam heights.
Miners who must duck walk or crawl from place to place within
their work area may be moving more slowly and with greater
expenditure of energy than miners who are free to walk upright.
Miners’ decreased ease of movement and restricted view of the
roof are two factors which may contribute to the finding that
miners are twice as likely to be killed by falls of unsupported
roof in seams below 60 inches as compared to higher seams.

It appears that visibility, limits to mobility, and posture re-
strictions may all affect miners’ safety. However, further inves-
tigations are needed to gain a better understanding of the ex-
act processes by which low-seam height contributes to injuries,
and to identify better strategies for injury prevention in this very
challenging work environment. This study strongly suggests that
seam height is an important factor in understanding the reasons
for injuries at small mines. However, further research is needed
to determine what other factors may also be responsible for the
differences.

ACKNOWLEDGMENT
‘The authors thank Larry Layne for his many excellent sug-
gestions on earlier drafts of this manuscript.

REFERENCES

1, National Research Council: Toward Safer Underground Coal
Mines. National Research Council, Washington, DC (1982).

2. National Academy of Sciences: Fatalities in Small Underground

Coal Mines. Contract No. J0100145, USBM Open File Report No.

124-83, National Academy of Sciences, Washington, DC (1983).

. Winn, G-L.; Grayson, R.L.; Elliott, G.D.: The Problem of Small-
Mine Safety in West Virginia. Prof Safety 39(5):41-44 (1994).

4, Peters, R.HL; Fotta, B.: Statistical Profile of Accidents at Small
Underground Coal Mines, USBM Special Publication 18-94. In:
Improving Safety at Small Underground Mines, R.H. Peters, Ed.
pp. 5-14, NIOSH, Pittsburgh, PA (1994).

. Fotta, B.; Mallet, L.G.: Effect of Mining Height on Injury Rates in
U.S. Underground Nonlongwall Bituminous Coal Mines, National

»


The Influence of Seam Height on Lost-Time Injury
and Fatality Rates at Small Underground Bituminous

Coal Mines

Robert H. Peters, Barbara Fotta, and Launa G. Mallett
National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory,

Pittsburgh, Pennsylvania

Due to variations in the thickness of U.S. coal seams,
there is great variability in the height of the roof where un-
derground miners work, Restrictions imposed by low seam
heights have important safety consequences, As the height
of their workplace decreases, miners must stoop, duck walk,
or crawl, and their vision, posture, and mobility become in-
creasingly restricted. Low seam height also places impor-
tant restrictions on the design of mobile equipment and
other mining machinery. Using the employment and injury
data reported to the Mine Safety and Health Administration
(MSHA) from 1990 to 1996, small underground bituminous
coal mines with less than 50 employees were stratified by
average coal seam height according to the following cate-
gories: low (<42”), medium (43”-60"), and high (>61”).
Injury rates for both nonfatal days lost and fatality cases
were examined by seam height and leading type of injury
incidents. The leading types of incidents associated with fa-
talities were roof falls and powered haulage equipment. In
comparison to high-seam mines, miners working in low or
medium seams are at higher risk of being killed by pow-
ered haulage equipment, roof bolting machines, and falls
of unsupported roof. The leading types of incidents asso-
ciated with nonfatal injuries were handling materials and
powered haulage. As mining height decreases, miners are
at increasingly higher risk of having a nonfatal injury from
incidents involving roof bolting machines, load-haul-dump
equipment, personnel carriers, and powered haulage con-
veyors. As mining height increases, miners are at increas-
ingly higher risk of having a nonfatal injury from slips and
falls and incidents involving shuttle cars and roof and rib
falls. Knee injuries are a particularly severe problem in low-
seam mines. The rate of injuries to miners while crawling or
kneeling is 10 times higher in low seams than in high seams.

Seam height can vary from as low as 20 inches to a height
of 12 feet or more. Seam height effects not only mining method
and equipment choices, but also the posture, mobility, and vision
of the worker. The relationship between seam height and injury
frequency is confounded by the effects of both mine size and
mining method. Mines operating in thinner seams of coal (less
than 43 inches high) tend to have fewer employees and are more
likely to use continuous or conventional mining versus longwall
mining methods.)

During the past three decades, several researchers have re-
ported that small underground coal mines experienced signifi-
cantly higher fatality rates than larger mines.'~* The National
Academy of Sciences (NAS) found that, during the period
1978-1980, the fatality rate for mines with 50 or fewer employ-
‘ees was about three times that of mines with over 250 employees,
and almost twice that of mines with 51 to 250 employees. The
NAS researchers examined the influence of several potential
reasons for the strong correlation between mine size and fatality
rates, including company ownership, union status, length of time
the mine was in operation, and seam thickness. They concluded
that none of these factors could account for the large disparity.

Of the 90 million hours worked by underground miners at bi-
tuminous coal mines in 1996, approximately half were worked
in mines with an average seam height of five feet or less. In ad-
dition, 94 percent of mines operating in seams of 3.5 feet or less
employed fewer than 50 people. Thin-seam or low coal mines
are located almost exclusively within the southem Appalachian
coal fields. And, in fact, 96 percent of small thin-seam mining
operations are located in three states: Kentucky, West Virginia,
and Virginia. The proportion of total U.S. underground bitumi-
nous coal being produced from thin-seam mines appears to have
remained constant during recent years. However, as thick seams
of underground coal are depleted, one might expect to eventu-
ally see a decline in the average height of the U.S. coal seams
being mined,


2

2

Institute for Occupational Safety and Health Information Circular
Report 9447. NIOSH, Pittsburgh, PA (1997).

Peters, R.H.; Fotta, B.: A Review of Recent Data Concerning Acci-
dents Caused by Falls of Unsupported Roof. In: Proceedings of the
25th Annual Institute on Coal Mine Safety and Health Research,
pp. 129-137. Virginia Polytechnic Institute and State University,
Blacksburg, VA (1994).

Grayson, R.L.; Althouse, R.C.; Winn, G.L.; Klishis, MJ.: A New
Injury Analysis Methodology for Developing Prioritized Work-
place Intervention Strategies. App! Occup Environ Hyg 13(1):41~
52.(1998).

8. Turin, .C.: Human Factors Analysis of Roof Bolting Hazards in
Underground Coal Mines. USBM Report of Investigations 9568,
NIOSH, Pittsburgh, PA (1995).

9. Bruchal, L.: Occupational Knee Disorders: An Overview. In: Ad-
vances in Industrial Ergonomics and Safety VI, A. Bittner and
P. Champney, Eds., pp. 89-93. Taylor and Francis, Philadelphia
(1995).

10, Gallagher, S.; Hamrich C.; Comelius, K.M.; Redfern, M.: The Ef-

fects of Vertical Space Restriction on the Movement Experienced
by the Lumbar Spine. In: Proceedings of 1997 American Industrial
Hygiene Conference, Dallas, TX, p. 19 (1997).


The objective of this study is to identify and quantify the
kinds of injuries associated with working in thin-seam mining
operations. One advantage of using more recent data is that a
much higher proportion of mines now report their seam height
to the Mine Safety and Health Administration (MSHA). The
NAS researchers reported that 78 percent of mines with 20 or
fewer employees and 59 percent of mines with 21 to 50 em-
ployees failed to report seam heights during the period of their
study (1975-1980). In the present study, information about seam.
height was available for 99 percent of all active U.S. coal mines.

METHODS

‘The data used for this study are a subset of the data collected
annually by MSHA and recorded in its employment and accident
databases for the U.S. coal industry. Data for bituminous coal
were included, while anthracite was excluded due to differences
in mineral stratification and mining methods utilized. The non-
fatal days lost (NFDL) injuries were examined for a five-year
period, 1992-1996. NFDL injuries were defined as those that re-
sulted in a permanent disability and/or in days away from work
or days of restricted work activity. Since the number of fatalities
per year is much smaller than the number of nonfatal injuries,
a seven-year period (1990-1996) was used in computations of
fatality rates. We only included injuries and fatalities that oc-
curred at underground locations. Injuries to workers at surface
facilities such as preparation plants, shops, or offices were not
included,

To reduce the confounding effects of mining method and mine
size on injury rates, mines using longwall mining methods and
mines employing an average of 50 or more underground work-
ers were identified and excluded from analysis. This does not
entirely eliminate the confounding effects of mining method.
Although we were unable to identify all mines using conven-
tional mining methods, we assume that there are relatively few
of them, Using information from the injury database, we know
that 9 percent of the injuries occurring in low-seam mines were
associated with conventional mining methods. The correspond-
ing percentages for medium- and high-seam mines were 3 per-
cent and 1 percent, respectively. However, limitations to the
MSHA employment database did not permit calculation of the
corresponding number of hours worked at conventional mines.

During the most recent year we examined (1996), 704 small,
underground, nonlongwall bituminous coal mines were in oper-
ation. Our calculation of nonfatal days lost injury rates, covering
the period 1992-1996, is based on data from 1793 mines. Our
calculation of fatality rates, covering the period 1990-1996, is
based on data from 2461 mines. Mines were categorized by
their average seam height as low (<42”), medium (43”-60"), or
high (>61”). Mines with missing or unrealistic seam heights
(e.g., seam height of one inch) were individually examined
and corrections were made based on valid seam heights for the
mine from previous and/or subsequent years, When such correc-
tions could not be made, the mines were excluded from further

analyses. The excluded mines accounted for less than 1 percent
of underground exposure hours,

The method of acquiring seam height information differed
depending on whether the injury under consideration was fatal
or nonfatal. MSHA conducts detailed on-site investigations of
all fatal incidents in the mining industry. Information about the
actual height of the mine roof at the scene of the fatality is nearly
always included in MSHA''s fatal accident investigation reports.
This was the source used to place the fatality cases into a seam
height category. However, the information about the height of
the mine roof at the location of nonfatal injuries is somewhat
less precise. MSHA does not usually conduct on-site investi-
gations of nonfatal injuries, and does not ask mine operators to
record this information on their injury report. Instead, it requests
mine operators to report their mine’s average seam height. Con-
sequently, we used the average seam height reported to MSHA.
during the year that the injury occurred, as a proxy measure of
the approximate height of the roof at the location of an injury.
The seam height at various locations within an underground coal
mine can be somewhat variable. However, previous analyses)
established that the mine’s reported average seam height serves
as an adequate proxy to the actual height of the mine’s roof at
locations where miners were injured,

Injury rates were computed using the reported total number
of hours worked at underground locations as the measure of
worker exposure for each mine. NFDL injury rates were cal-
culated per 100 full-time equivalents (FTE). MSHA reports in-
jury rates based on 200,000 hours of work, which is equivalent
to 100 FTE. The fatality rate was based on 100,000 FTE, i.e.,
200 million hours. Each FTE represents 2000 hours, assuming
40 hours of work/week for 50 weeks/year. The formulas were
as follows:

1, NFDL Injury Rate = (Number of lost-time injuries/sum of
hrs) * (200,000 hrs)

2. Fatal Injury Rate = (Number of fatalities/sum of hrs) * (2 x
10° hrs)

For this study, a decision was made to calculate fatality rates
including cells with a small number of cases, e. 1,or2cases.
Because reporting of incidents to MSHA is required by law, this ~
surveillance system captures all fatalities with a sensitivity of
100 percent. However, one must be cautious in interpretation of
rates based on these numbers due to instability. Therefore, both
the number of incidents and rates are presented in the results.

RESULTS

During the seven-year period, 1990-1996, 117 miners were
fatally injured at small nonlongwall underground bituminous
coal mines. Table I presents fatality rates broken down by type
of incident and by seam height category. The overall fatality rate
was 109.6 per 100,000 FTE. The three leading types of incidents
responsible for fatalities are: 1) roof falls, 2) powered haulage,
and 3) machinery.


TABLE I
Fatalities in small underground bituminous coal mines by seam height and type of incident, 1990-1996

Seam height
Number of fatalities Fatalities/100,000 FTE

Low Medium High Low Medium High
‘Type of incident <42" 43"-60" 261" <42" 43"-60" 261”
Roof falls 23 24 7 46.0 64.1 36.3
Supported roof 10 16 5 20.0 42.7 25.9
Unsupported roof 13 8 2 26.0 214 10.4
Rib falls 0 0 2 0 0 10.4
Powered haulage 8 10 2 16.0 26.7 10.4

Personnel carriers 1 2 0 20 53 0

Load-haul-dumps 2 3 0 40 8.0 0
Conveyors 1 2 2 2.0 53 10.4

Shuttle cars 4 3 0 8.0 8.0 0
Machinery 7 5 4 140 13.4 20.7

Roof bolters 3 1 0 6.0 27 0
Continuous miners 4 3 4 8.0 8.0 20.7
All other 14 1 10 28.0 2.7 51.8
Totals 52 40 25 104.1 106.8 129.6

Table II presents NFDL injury rates broken down by type of
incident and by seam height category. During the five-year pe-
riod, 1992-1996, there were a total of 7825 underground NFDL
injuries resulting in lost time or days of restricted activity. The
overall NFDL injury rate was 11.24 per 100 FTE. The six leading
types of incidents responsible for these injuries are: 1) handling
materials, 2) machinery, 3) powered haulage, 4) slip or fall of
person, 5) roof falls, and 6) nonpowered hand tools.

Overall Rates

The overall fatality rate is highest for high-seam mines. How-
ever, one must be cautious in interpreting the overall fatality rate
findings because they are based on a relatively small number
of incidents. An explosion at the South Mountain mine, which
killed eight miners, had a large impact on the fatality rate for
high-seam mines. If this multiple-fatality disaster at a high-seam
mine had not happened, medium-seam mines would have had
the highest fatality rate (104.2), and the rates for low- and high-
seam mines would have been nearly identical (76.0 and 77.8,
respectively). The overall rates of NFDL injuries for each seam
height category are nearly identical.

When we broke the injury data down by both seam height
category and type of incident, some important differences in the
types of factors primarily responsible for miners’ injuries and
fatalities became apparent.

Roof Falls
Rates of nonfatal days lost injuries caused by roof falls in-
crease as seam height increases. Conversely, the rate of fatalities

caused by roof falls is lowest in seam heights above 60 inches.
One reason fatality rates are higher at lower seam heights may
be that MSHA regulations permit mines operating in heights of
less than 42 inches to apply for exemption from the requirement
that all underground mobile equipment have overhead canopies
to protect the operator. Of the 54 miners killed by roof falls in
small mines during 1990-1996, seven were operating equipment
without a canopy.

Twenty-three of these 54 miners (43%) were under unsup-
ported roof, and 31 were under supported roof. (The miners
killed by falls of unsupported roof had all gone beyond the last
tow of roof bolts at the time the roof fell on them.) In com-
parison to seams above 60 inches, the rate of fatalities caused
by falls of unsupported roof in lower seams is twice as high.
A variety of potential explanations have been suggested to ac-
count for this difference. One factor that may be contributing
to this difference in fatality rates is that, when one must crawl to
move about, it may be tempting to take shortcuts through areas
of unsupported roof. Another factor that may contribute to the
higher rate of fatalities caused by falls of unsupported roof in
lower seams is that it is more difficult for miners to get a good
look at the roof when crawling or stoop walking as opposed
to standing in an upright posture. There appears to be no rela-
tion to seam height for fatalities caused by falls of supported
roof. Based on their analysis of injuries to West Virginia’s roof
bolter operators caused by roof falls, Grayson et al.” noted that
the amount of lost time per injury was much greater at small
mines than at large mines (97.2 days vs. 13.8 days per injury).
Grayson et al. attributed this difference to better equipment in
large mines.