Table 1. Number of Mines Using Observational and Monitoring Techniques

Observation or | Sound- | Obser- | Wedge | Borescope } Scratch | Extensom | Miners ] Guardian | Telltale
monitor type | ing | vation hole tool eter Helper | Angel
Number of 9 u 5 6 4 2 7 4 1
Mines

To date, observational techniques are used more than
monitoring techniques and accounted for 31 of the 48 mines or
65 % of the total. Drilling observational holes to detect
conditions in the roof was the single most used technique with
11 of the 48 mines or 23 % of the total. The data also show
that 11 of the 48 mines or 23 % of the total used or have
available commercial mechanical monitors (Miners Helper or
Guardian Angel). This relatively low number of overall usage,
as well as the fact that 46 % of the mines use no additional
monitoring methods, suggests apparent limitation to utilizing
these methods. A comprehensive ground control plan not only
includes the basic observational, visual and hands-on.
components, but also uses supplemental observational and
monitoring techniques and regularly reads, analyzes and
displays information gained from these efforts. When this type
of information is logged or mapped it provides a documented
history of ground conditions. This information can be analyzed
and prepared by either consulting firms or with in-house
expertise. The availability of this information at the time of a
major ground fall or when unstable geologic conditions are
encountered is extremely useful in deciding a course of action
or alteration of the mining plan. Mines that follow these
practices and that promote open communication and
participation from everyone at the site are the mines with the
most pro-active approaches towards ground control safety.

NIOSH’S ROOF MONITORING SAFETY SYSTEM.
While there have been considerable advancements in roof

monitoring, existing instruments have several limitations which
are minimizing the impact of this technology on underground

‘stone mining. These limitations include: 1) difficulty in taking
readings in high roof or back areas, 2) exposure to dangerous
ground while reading the monitors, 3) complexity in making
repetitive readings, 4) problems with making multiple
measurements horizons with multiple anchors, 5) difficulty in
seeing waming devices in the dusty and foggy production face
areas, and 6) expense of most commercial monitors.

A new generation Roof Monitoring Safety System (RMSS)
developed by NIOSH improves on the existing methods for
determining roof stability. This electro-mechanical roof
monitor includes the following features: 1) can be fabricated in
most standard mine shops, 2)-is relatively inexpensive [single
point RMSS costs less than $40/unit to fabricate in-house], 3)
reduces potential damage from face blast because of the in-hole
positioning [this is true only of the single point RMSS}, 4)
compatible with existing commercial data measurement
devices, 5) capability for monitor reading at ground level
[allows miner to take readings away from potentially unstable
roof conditions] 6) remotely monitored with either multimeter
(Figure 8) or data acquisition systems (Figure 8), and 7) can
accommodate as many as six anchor points [this is truc only of
the multipoint RMSS]. See Appendix A for more information
on fabrication, installation, and data analysis of the single point
RMSS.

Perhaps the greatest benefit of this new monitor is that the
miner does not have to make readings in areas of questionable
roof stability. After all, these instruments are most often
deployed in areas were roof rock instabilities are suspected.
Figure 9 illustrates the advantage of the electro-mechanical
RMSS over other existing techniques.

Figure 7 - Photograph of the commercially available Miners Helper, the Guardian Angel, and the
RMT’s Dual Height Telltale roof mechanical extensometer monitoring devices.


Figure 13 - Overall view of RMSS components.

Figure 14 - Top-anchor being placed into
insertion tool.

POTENTIOMETER
LARGE WASHER (3/8")
ASSEMBLY

SCREW (B= 32 x 1”)

NUT (8-32") (2 EA.)

CABLE RETENTION SLOT

RET (1/8") (5 EA)
STABILIZER LEAF (2 EA.)
POTENTIOMETER GUARD
ALUMINUM TUBING (3/8 OD 1/47 1D)
ALL THREAD (1/4 x 11/2")

NUT 1/4")

ANCHOR WITH CABLE

= BOTTOM OF ANGLE ASSEMBLY
= TOP OF ANGLE ASSEMBLY



A SITE SPECIFIC EXAMPLE OF A PRO-ACTIVE.
GROUND CONTROL PLAN,

To further illustrate how information from roof monitors can
help make safety decisions about the stability of mine roofs a
site specific field test was performed and is discuss below. In
1996 the immediate roof at an operating stone mine began to
fail approximately 90 m (300 ft) from the end of a previous
directionally controlled roof fall. This original failure took on
the appearance of a series of low-angled shear planes cutting or
ripping the rock. The orientation of these planes was
perpendicular to the orientation of the local horizontal stress
field. Seven deflection monitors were quickly placed along the
projected failure trend. Two of seven monitors used the 20-
anchor point sonic probe: the other five were 3-anchor point
prototypes of the RMSS.

Data collected from three of these monitors are shown in
Figure 12. Monitor No. 7 collected deflection measurements
for almost 70 days prior to total roof collapse. During this
time, the roof deflected in three distinct phases. The first phase
was marked by a slow but steady deflection in the lower roof
beam. At approximately 40 days there was a sudden increase
in the deflection of the beam. The third phase indicated that
the beam deflection rate lessened, but ended in total roof
failure. A total of approximately 5 cm (2 in) of roof deflection
occurred prior to roof collapse. Data from monitor No. 3
showed a much different trend. Unlike monitor No. 7, this
instrument was placed close to an existing failure; therefore,
significant beam deflection could have already occurred. The
area began to “cut or rip” on July 26, rapidly extending the
zone of failed roof. The roof associated with monitor No. 3
went from stable to unstable in a matter of 5 hours.

Monitor No. 4 was purposely placed slightly away from the
main failure trend. The magnitude of deflection measured from
this instrument was 1/10 the magnitude of the instruments
within the failure trend. However, these measurements did
show that beam bending and associated shearing extended
significant lateral distances on the order of 6 m (20 fi) from the
fall’s edge and 12 m (40 ft) from the center of the fall. This
monitor also showed that while deflection was initiated in the
lowest beam, beam separations quickly moved much higher in
the roof,

This information proved extremely valuable to the mine.
Several pro-active ground control strategies were implemented
as a result of supplemental roof monitoring. Unstable roof
areas were identified and personnel were restricted from
entering. A new roof support plan was initiated that prohibited
the progressive failure of the various roof beams. Roof
monitoring was initiated in other areas of questionable
stability. All of these actions had an extremely positive effect
on the miners and regulators working at the site.

—e—0.2 mbeam
60 | —o—0.4 mbeam
—2—0.6 m beam

Maximum Beam Deflection, mm

fo) 10 20 30
Room Width, m.

Figure 11 - Maximum deflection from gravity loading for
various limestone beam geometries.

PROBLEMS, CRITICAL ISSUES, AND THE NEED
FOR COOPERATION

RMSS technology has been utilized at 6 underground stone
mines. Approximately 20 monitors have been fabricated,
installed and are being read on a regular basis. These
preliminary tests have revealed the following procedural
questions: 1) how often should monitors be read, 2) where
should monitors be placed within the entry, and 3) how should
the data be analyzed? They have also revealed the importance
of solving associated rock mechanics issues, such as: 1) what
are critical deflection rates, 2) what geologic factors influence
deflection magnitudes prior to a roof fall, and 3) at what
locations in the roof does the failure initiate? The answers to
these questions can provide the basis for
recommendations/guidelines and can be used effectively and
efficiently to improve the safety conditions for underground
stone miners.

While much has been learned, more knowledge
will be needed to help answer the question “when should
monitoring techniques be used?” Because performance results
are heavily dependent upon site specific conditions related to
geologic, stress, and mining conditions, all monitoring data
must be calibrated for site specific conditions. The solution to
this problem demands a major research effort founded on a
common goal and good communication. The best way to
achieve this goal is to have Industry, labor, and government
working together to gain the required data and knowledge.

SUMMARY AND CONCLUSIONS

This research is intended to serve as a catalyst to develop
better engineering tools and strategies that will improve safety
by better understanding roof behavior. An understanding of the
complex behavior associated with roof instabilities is expected
to provide a method for developing the “safest” decisions in


Figure 8 - Single-point RMSS with multimeter and multi-
point RMSS with data acquisition.

Metal rod Flag drope
Micrometer

Multimeter

Figure 9 - Comparison of three roof monitoring
techniques.

HOW DO ROOF MONITORS ASSESS STRATA
STABILITY?

Every opening made in rock causes a redistribution of
stresses in the adjacent strata, In bedded or layered
sedimentary rock, the roof beams deflect into the opening
immediately after excavation (Figure 10). A decay in the rate
of deflection occurs as the stresses redistribute in response to
the opening. Eventually, the beam deflection stabilizes
(Figure 10). In most strata this initial deflection takes place
very quickly. For example, NIOSH researchers have installed
monitors in the face area a few days after a production blast
and have not been able to detect this period of decelerating
deflection. As time passes, these roof beams are subjected to
natural and man-made processes such as dramatic humidity
variations from changing environmental conditions, chemical
alterations from strata water, tectonic stresses, additional
mining adjacent to the opening, and heat from machinery.
‘These processes initiate a period of instability which often
continues until failure occurs. Generally, the rate of deflection
increases steadily through this period. The entire length of this
period can be minutes, days, or even years. For example, it is

not uncommon for the immediate roof beam to fail sometime
between the blast and the time when miners first re-enter the
face, in most cases a few hours. These beams are found draped
over the rubble stone pile produced from the production blast.
It is also not uncommon for beam failure to occur months or
even years after the excavation has been formed. This period
of quiet, between the decelerating and accelerating beam
deflection periods, is the environmental condition which all
miners desire to work under.

Roof beam deflection due to gravity loading can be estimated
using standard formulas for deflection of beams or plates. For
example, the maximum deflection (d) of a clamped. elastic
beam is given by

where y is the unit density of the limestone, E is the Modulus
of Elasticity, L is the length of the beam in meters, and t is the
thickness in meters. Figure 11 shows the effect varying beam
lengths and thickness have on the maximum deflection of a
23.5 kN/m* (150 lb/ft), 41.4 GPa (6 Mpsi) fully intact beam
of limestone. This deflection is, in general, very small and can
take place quickly after an opening is excavated or much later
as weathering processes aid in forming new, thinner beams.
Tectonic processes can produce additional roof beam deflection
by axially loading roof beams. Two-dimensional models have
demonstrated how different levels of horizontal stresses may
cause up to several centimeters of deflection prior to failure of
roof beams (Iannacchione et. al, 1998). Vertical loading of roof
beams from overlying or adjacent mining can load roof beams
which in turn can accelerate deflection. In all of these
examples, roof beam deflection is viewed as a precursor of
failure and, if recognized early, could result in pro-active
control solutions.

Typical roof failure pattern

=

8

3

& | Room mined Roof fails

g

& i

< il Beam deflection begins

gs 7

3 \ ¢

/

a Sak a

SSS
Time

Figure 10 - Typical cycles experienced by failing stone roof
beams. Note the periods of decelerating, quiet, and
accelerating deflection rate.


REFERENCES:

Anon, “Guidance on the use of rockbolts to support
roadways in coal mines, HSE Books, 1996, 35 pp.

Altounyan, P.F.R., DN. Bigby, K.G. Hurt, and H.V. Peake,
“Instrumentation and Procedures for Routine Monitoring of
Reinforced Mine Roadways to Prevent Falls of Ground, 2% Int.
Conf. Of Safety in Mines Research Inst., 1997, New Delhi,
India, pp. 759-766.

Iannacchione, A.T., D. R. Dolinar, L. J. Prosser, T. E.
Marshall, D. C. Oyler, and C. $. Compton, “Controlling Roof,
Beam Failures from High Horizontal Stresses in Underground
Stone Mines”, 17* Int, Conf. On Ground Control in Mining,
Morgantown, WV, Aug. 4-6, 1998, pp, 102-112.

Tannacchione, A.T., L. J. Prosser, T.E. Marshall, C.S.
Compton, D. R. Dolinar, D. C. Oyler, and D.M. Pappas, “An
Instruction Booklet for the Remote Monitoring Safety System
(RMSS),” National Institute for Occupational Safety and
Health Internal Publication, Dec. 10, 1997, 21 pp.

Parker, J., “How Convergence Measurements Can Save
Money”, Engineering and Mining Journal, August, 1973, pp.
92-97

APPENDIX A - SINGLE POINT RMSS FABRICATION,
INSTALLATION, AND DATA ANALYSIS

An instruction booklet providing detailed information on how
to fabricate, assemble, install, operate. and analyze data of the
single point RMSS has been prepared (Iannacchione, et al.,
1997). An advantage of the RMSS is that it can be fabricated
from components available at most local suppliers (Figure 13).
Parts include a potentiometer, spur gear, rack, stainless steel
cable, 2-conductor electrical wire, washers, screws, nuts, angle
assembly, 2-conductor electrical wire, cable and ring crimp
connector, spring, rivets, metal banding, and aluminum tubing.
It is also worth noting that the top-anchor can be inserted into
the monitoring holes with insertion tools made of pipe 3/4 inch
galvanized pipe or schedule 80 PVC with a 3/8 inch wide by
5/8 inch deep notch cut into the end of the rod (Figure 14).

Monitor installation takes about 20 minutes and requires an
open 2-inch hole drilled into the roof to a depth of 14 feet or
more. An electrical cable from monitor to a multimeter is
installed with enough length so that readings can be made
safely while on the floor. To read the RMSS, a multimeter or a
data acquisition unit with high resolution and accuracy is
required. The minimum meter resolution and accuracy
acceptable for use with the RMSS are 1 ohm and 0.2%,
respectively. The instruction manual provides examples of data
sheets and graphs to record and analyze beam deflection with
time. The two graphs consist of a Resistance Conversion Chart
(Converts multimeter resistance readings to beam deflection)
and a beam deflection graph (plot of deflection versus time)


PREVENTING INJURIES CAUSED BY UNRECOGNIZED STONE MINE ROOF BEAM FAILURES
WITH A PRO-ACTIVE ROOF CONTROL PLAN

A.T. Jannacchione
L.J. Prosser
R. Grau
D.C. Oyler
D.R. Dolinar
T.E. Marshall
C.S. Compton
NIOSH
Pittsburgh, PA


concert with the existing mining practice. Developing a pro-
active roof control plan allows for a quick and timely response
and ensures that every response is the one that is most
appropriate in relation to existing conditions. Important
characteristics associated with a pro-active roof control strategy
are:

1. Visual and hands-on roof condition information is the basis
of any roof control strategy,

2. Supplemental observational and monitoring techniques
provide additional useful information,

3. Regularly recorded and charted information from basic and
supplemental monitoring, placed on mine maps and shared
with miners, fosters the development of a pro-active roof
control plan.

There are currently many useful monitoring techniques
available to underground stone mines. However, many of these
techniques have certain operational problems and often lack
adequate information to apply them at local mine sites. To help
address these problems and to provide a better means of
collecting and sharing roof deflection data, NIOSH has
developed the RMSS. The RMSS has the following
advantages: 1) local fabrication, 2) inexpensive, 3) placed in
boreholes protected from blast damage, 4) read remotely, and
5) compatible with many kinds of data acquisition systems.
Although a first step has been made here through the
presentation of the RMSS, it is envisioned that improvements
are possible and indeed likely. For example, both the single-
and multiple-point RMSS can be incorporated into a mine-wide
monitoring system. It is our hope that improvements to
fabrication procedures/components and development of

Deflection, cm

Days

computer software to assist in managing the large streams of
data associated with mine-wide monitoring scenarios can be
made.

Because hazardous roof beam deflection is dependent on
site-specific geologic, stress, and mining characteristics; any
roof monitoring technique must be calibrated for local
conditions. The only way this can occur is if industry, mine
workers, and government work together to gain the required
data and knowledge. The use of the RMSS as well as other
observational and monitoring techniques by the underground
stone mines is expected to enhance miners’ understanding of

roof behavior and provide a tool for pro-active intervention
when hazardous ground conditions exist. Knowledge gained
through shared experiences will aid in developing innovative
engineering techniques that will mitigate falls of ground and
reduce the potential for injuries to mine workers.

os Monitor No.4
4
& 04 _
H B Monitor No.3 a
H o8 a, /
& 02 } | July 28
ot on I
1
° °
° Days 120 Hours 12

Figure 12. - Roof behavior associated with a large roof fall caused by high horizontal stresses monitor No. 7 was a 3-anchor

point prototype of the RMSS and monitor No. 4 was a 20-anchor point sonic probe.


in height. roof-to-floor convergence monitors are difficult to
install, maintain, and analyze. Roof and rib extensometers
have enjoyed wider use than convergence monitors, however,
they are difficult to read because of their location on the roof
line or back.

In its simplest form, roof and rib extensometer monitoring
can be accomplished with a scratch tool (Figure 6). This device
can detect separations and provide an indication of loose rock
layers or roof beam deflection. Information on the location and
size of the separation can be marked on the roof and used to
assess potential future roof degradation.

Figure 6 - Photograph of a
scratch tool.

Extensometers permanently installed in drill holes have been
used in underground mines to detect ground fall hazards for
many years. Sonic probe extensometers have been widely used
in the US, UK, and Australia. This commercial system allows
for 20 permanent anchors up to a 6m (20ft) height. The probe
is temporarily inserted when measurements are made. Home
made mechanical extensometers have consisted of a top and
bottom anchor, steel wire or rigid tubing, and some kind of
micrometer or dial gauge. These devices have been used for
decades in metal mines in Michigan, Missouri, and Idaho. For
example, in the Missouri lead belt district a deflection rate of
(0.007 in/month) is considered a good warning of strata failure.
Extensometers have been used as real time hazard warming
devices. Parker (1973) discussed an ingenious method to alert
miners of strata movement by adding a warning light to an
extensometer.

The most common commercially available mechanical
extensometer monitoring devices are the Miners Helper, the
Guardian Angel, and the Dual Height Telltale (Figure 7).
‘These monitors generally have one or two anchors points which
measure the overall separation of rock layers in the immediate

roof. If roof deflection is detected by the Miners Helper and
the Guardian Angel a reflecting flag drops from the roof line
signaling the potential for imminent roof failure, In some cases
this information has been used to indicate a need to add roof
support, remove roof rock, or danger off affected areas.

Coal mines have used telltales for decades to warn miners of
strata movement. The telltale is a rigid bar, possibly a roof
bolt, anchored into the roof. A small section of rod protruding
from the borehole is covered with three bands of reflective
tape. The portion of the bar closest to the roof is generally
green, followed downward by yellow and then red. The idea is
that as the roof deflects downward the roof line can easily be
see to move through the green, yellow and finally red tape
zones. Recently Bay Tech has produced an electronic telltale
In the United Kingdom, coal mines use telltales every 20 m (65
) with action bands from 0.4 to 2 em (0.2 to 1 in). Between
1990 and 1995 falls of ground were reduced from 267 to 6,
partially because of the use of telltales (Altounyan et. al, 1997)

OVERVIEW OF GROUND CONTROL
PLANNING METHODS

Generally, underground mines use observational techniques,
primarily visual inspection as a means of determining roof
stability. Through the processes of blasting, drilling, and
scaling additional knowledge related to roof conditions is
gained. For example, a driller preparing to bolt notices a
sudden increase in the penetration rate, realizes that possibly a
gap or clay seam was encountered. Much of this “hands-on
information provides an overview of the general conditions
related to roof stability, However, this base of knowledge can
quickly deteriorate if that hands-on experience is lost through
changes in employment or other circumstances. Many mines
supplement visual inspections and knowledge gained through
hands-on experience with various types of observational and
monitoring techniques. Based on observations of and
discussions with personnel at 48 mines, 49 observational and
monitoring techniques beyond basic visual inspections were
used at 26 different mines (Table I). This suggests that
additional information is beneficial or needed to solve some
classes of ground control problems.


6 4
26% no 2%
5
10% 4 3
2% 48%

Figure 3 - Severity of roof, face, and rib fall
accidents from 1990 to 1996.

If dangerous roof beam deflection values are known, they can
be used as a clear indicator of roof instability (Parker, 1973).
Undoubtably there is a significant potential for roof monitoring
to assist miners and mining operations in gathering quantitative
as well as qualitative information. But for this technology to be
fully implemented several technical problems must be
overcome: 1) the factors which influence the variability in roof
beam deflection and failure must be identified , 2) a simple,
inexpensive method to monitor dangerous levels of roof beam
deflection must be produced, and 3) guidelines for evaluating
the output of these monitoring devices must be established.

CURRENT ROOF MONITORING TECHNOLOGY

Observing and monitoring rock deformations provides
information for making critical mining decisions and potentially
acts as a waming of impending hazardous conditions

Room Width, ft

16.4 32.8 49.2 65.6
25

20

Frequency

s 10 15 20
Room Width, m

Figure 4 - Histogram of room widths for 65 underground
stone mines.

Traditionally miners have sounded the rock, listening for the
drummy sounds that signal loose rock. ‘The act of drilling,
exploration, roof bolt, or blast holes can provide much
information about the rock. If separations are present, the drill
will often accelerate through these zones. Dust and water from

Bedding (dashed)

Bedding plane separation

Bedding (dashed) —_Joints (doshdot)

Figure 5 - Two different modes of roof rock
failure that are common in underground stone
mines.

adjacent fractures or drillholes indicates the occurrence of
hidden rock fractures. Borescopes and borehole cameras have
been used to observe fracture characteristics and roof lithology
with increasing frequency as the quality and cost of these
devices improves. Wedges inserted into angular fractures or
horizontal bedding planes along the roof and ribs of the mine
have been used to observe the movement of rock masses, i.c. if
the wedge falls out, the rock mass is moving.

Observational techniques can be extended by monitoring the
movement of the mine roof on some kind of regular basis.
Monitors can be divided into two basic types: 1) roof-to-floor
convergence monitors, and 2) roof and rib extensometer
monitors. Because most stone mine development rooms
average 7.1 m (23 ft) and bench rooms average 16.4 m (54 ft)


ABSTRACT

Unrecognized roof beam failures resulted in 69% of the falls
of ground injuries occurring in underground U.S. stone mines
from 1990 to 1996. Field investigations at 45 underground
stone mines suggest that excessive beam sag or deflection is the
principal unstable roof behavior characteristics associated with
miner injuries. Laminated roof structures, common in most
underground stone mines, allow the immediate roof to separate
into distinct beams. The timely determination of roof beam
deflection could provide stone miners with advanced
knowledge of potentially hazardous roof. Existing methods to
assess roof conditions consist of observational and monitoring
techniques. Observational techniques include open drillholes,
sounding, wedging, and borescopes. Monitoring techniques
include scratch tools, extensometers, telltales, Miners Helpers,
and Guardian Angels. Miners have difficulty utilizing these
techniques because roof beams are located an average of 8 m
(26.5 ft) above the miner’s head in environments that are dark,
dusty and noisy and these deflections are measured in
millimeters of movement. To help mitigate the potential for
falls of ground injuries to underground stone miners, the
National Institute for Occupational Safety and Health (NIOSH)
has developed the Roof Monitoring Safety System (RMSS) to
aid in monitoring dangerous levels of roof beam deflection. If
enough site-specific information is collected and placed on
highly visible maps, individual mining operations could
establish general guidelines for responding to specific roof
beam deflection occurrences. This would help in implementing
a pro-active, comprehensive roof control plan for reducing falls
of ground injuries,

BACKGROUND

From 1990 to 1996, 16 states reported 92 injuries from falls
of roof, rib or face in the more than 90 underground stone
mines within the United States (Figure 1). Missouri,
Pennsylvania, and Kentucky accounted for 48 % of the total
number of injuries (Figure 2). Of this total, 11 miners have
been fatally injured. Of the 11 fatalities 10 or 91 % were
associated with unrecognized loose or failed rock within the
roof beam. Additionally, unrecognized roof beam failures
resulted in 69 % of the falls of ground injuries occurring in
underground U.S. stone mines from 1990 to 1996. While this
number is not large in magnitude, a work force of fewer than
2,000 miners makes for a very high fatal accident rate. The
severity of the typical fall of ground injury is, in general, very
high. Approximately 62 % of all roof, face, and rib fall injuries
were designated by the Mine Safety and Health Administration
(MSHA) as some kind of lost time accident. MSHA assigns
each accident a severity value from 1 to 6. A I represents a
fatality, a 2 a permanent disability, and 3 a lost time accident
(Figure 3).

INTRODUCTION
The majority of underground stone mines operate in
relatively thick limestone formations. Intact (solid) pieces of
limestone are generally very strong, often having compressive
strengths of 207 MPa (30,000 psi) and tensile strengths of 14

Reference to specific products does not imply endorsement by
NIOSH

20 4
18
16 +
14
12

10
mmm Total

Frequency

—— Fatal

oN BD

1990
1991
1992
1993
1994
1995
1996

Years

Figure 1 - Roof, face, and rib fall injuries in the
underground stone industry.

MPa (2000 psi). Persistent horizontal bedding planes typically
cause limestone roof rock members to separate into beams
ranging from 0.15 m (0.5 ft) to 1 m (3 ft) thick which span
large rooms, ranging from 6.1 m (20 ft) to 18.3 m (60 ft), and
averaging 13.1 m (43 ft) in width Figure 4). The deformation
characteristic that affects roof beam stability is excessive
deflection (Figure 5). If deflection or bending of the roof
beams becomes excessive, roof failure can occur leading to
injuries to miners. However, these beams can contain vertical
and sub-vertical disco ities (vertical joints or fractures and
sub-vertical cross-beds planes) which sometimes affect local
roof beam strength producing wedge or prism shaped failures
(Figure 5).

mp IN mo
KS ox IL WV SD 18%
1A
cA
PA
TN 16%
ny GA VA KY
8% 10% 14%

Figure 2 - Underground stone roof, face, and rib fall
injuries by state from 1990 to 1996.