map by the operator and verified underground by reading the
markings on bolt heads. At the same time, immediate roof
lithology is verified and intersection spans are measured.
Standard procedures are followed for intersection corner
location and the measurement of 3-way, turnouts, and odd
intersection geometries. It is critical at intersection falls that the
intersection span is measured, if possible. Eight intersections
around the fall are measured to determine if any trend existed
toward overspanned intersections in proximity to the fall. It is
also important to measure spans in successfully supported
ground in order to contrast these with failed ground.

Mine workings are then partitioned by roof bolt type and
length, CMRR, intersection span, and other influencing
variables like depth of cover and area beneath stream valleys.
Drivages are then measured for these blocked areas in order to
normalize their impact (Table 1).

TABLE 1 EXAMPLE OF DATABASE.

Current Database Results

The following are summary data from the current database.
From 12 mines, over their entire existence, a total of 639 roof
falls were tabulated. Roof fall rates were calculated per 10,000
ft of drivage for each mine (Fig. 8). The fall rate for
intersections was also calculated. The mines are located in
most of the major coal basins in the country and include small
mines, large mines, room and pillar operations, and
longwalls. The final data set will contain about 30 mines and
will cover a range of poor roof to strong roof.

Mine | cmRR | Bott | Tension | Grout Bott poty | Row — | Entry | Intersection | Depth | Hergmt | Roof Fat
Length Capacity | Row | Spacing | Wisin | spaniny | (Ave) | ttn Rater
Kips) oy
1 45 6 yes |None | 18 4 5 19 65 350 | 5.5 5.69
2 = 44L 5 No | Full 18 4 5 19 59 350 | 5.5 1.74
3. 441 6 Yes | Partial | 18 4 5 19 59 350 | 5.5 | 0.29

*Roof falls per 10,000 ft of drivage

Roof fall rates (falls per 10,000 linear foot of drivage) are then
calculated from the reported falls in each area. In the end, each
blocked area of the mine which constitutes some combination
of the above variables is assigned a roof fall rate (Fig. 7). In
this way the contribution of each variable to the roof fall rate
can be calculated and converted into the regression equation.

Eo) OT Tomar ean AT nk

# owe

Figure 7. Example of partitioning of mine with overlapping
geologic and bolt variables.

ee

Figure 8. Total and Intersection fall rates for each of the mines
in the database.

The roof falls which make up our mine sampling are
classified by location in intersections or entry segment (the
mined entry between intersections). Of the falls now included
in the database, 59% now occur in intersections.

In order to compare the stability of intersections vs. entry
segments, the falls were calculated as a percentage of total
mined places fallen. Figure 9 shows that intersections were 2.5
times as likely to fall as entries. Individually in the 12 mines,
the ratio of failed intersections to failed entries ranged from 1.5
to 3.3.


Mining method has also played a role in intersection stability
at this mine. Where a miner-bolter was used to drive
development sections there was a dramatic improvement in roof
conditions. No falls have occurred in development by miner-
bolter. If the roof fall rate in these areas were the same as that
of areas driven by place-change miners, 10 falls would have
been expected. The improvement may have been due to the
smaller entry width and intersection span resulting from the
miner-bolter, or to quicker roof support installation,’ or a
combination of both.

CONCLUSIONS,

By using the roof stability experience of the U.S. coal
mining industry, a model of the parameters which influence
roof failure is being developed. Through a system of monitoring
standup time of roof falls, and sorting out anomalous causes of
falls (abutment loading, unusual geology, etc.), roof falls are
counted and roof fall rates are assigned to partitioned areas of
the mine. Then roof geology is determined, intersection spans
are measured, and roof bolts are documented for each partition.

MSHA and NIOSH roof fall data has shown that
intersections are, on a foot by foot basis, about 8-10 times more
likely to fall than entries, Three variables appear to have the
most influence on roof fall rates:

© Coal Mine Roof Rating - For the weakest group of roof
rocks (CMRR<= 45), roof fall rates increase for
increasing sum-of-the-diagonals intersection span,
indicating that weak rocks have some limit of stable span.
Variability in this correlation is due to increased support
in at-risk intersections. It is critical to accurately define
the roof fall rates for CMRR in the weakest roof rocks in
order to anticipate instability.

© Intersection Span - For the entire range of CMRR in the
sample population, intersection span increases with
increasing CMRR, confirming that stronger rocks can
support bigger spans. It is the transition zones between.
weak, moderate, and strong CMRR ranges which have to
be better defined. Case studies have indicated that
replacing 3-way intersections with 4-way intersections
may not reduce the total number of roof falls.

© Roof Support - Individual case histories show various
roof fall rates attached to different support. At one mine
supplemental support was necessary in numerous
intersections where the span exceeded 67 ft. At another
mine loss of bolt tension in 55% of the tested bolts may
have contributed to the extremely high roof fall rate. At
another mine, roof falls were dramatically reduced by the
use of 8 ft point anchored, resin-assisted bolts. These
types of individual results will be more meaningful when
analyzed relative to the whole database.

‘The effects of mining method (miner-bolter vs place change
mining and practice (intersection development, poor roof bolt

installation) also have an impact on intersection stability. When
complete, the database will contain data sets on 30-35 U.S. coal
mines and a multivariate analysis will be used to determine the
influence of each variable on the ultimate stability of an
intersection.

FUTURE RESEARCH

We will continue to collect field data from coal mines and
expand our data base to include 30-35 mines, With that data as
input, a multivariate analysis will be applied, The end goal will
be io develop a design model for the safe excavation and
support of intersections.

REFERENCES,

1. Blevins, C.T., and Dopp, D. Ground Control
Experiences in a High Horizontal Stress Field at Inland
Steel Coal Mine No. 2. Paper in Proceedings of the

Fourth Conference on Ground Control in Mining,
Morgantown, WV, July 22-24, 1985, pp. 227-233.

2. Hanna, K. and Conover, D. Design of Coal Mine Entry
Intersections. Soc. Min. Eng, AIME Preprint No. 88-
39, 1988, 11 pp.

3. Ingram, D.K. and Chase, F.E. Effects of Ancient Stream
Channel Deposits on Mine Roof Stability: A Case Study.
US.BM. RI9092, 1987, 33pp.

4. Gercek, H. Stability of Intersections in Room and Pillar
Coal Mining. Ph.D. Thesis, Pennsylvania State Univ.,
1982, 186 pp.

5. Unal, E, Development of Design Guidelines and Roof
Control Standards for Coal Mine Roofs. Ph.D. Thesis,
Penn State, 1983, 355 pp.

6 Peng, SS. Roof Falls in Underground Coal Mines.
Dept. of Mining Engineering, WVU, Morgantown, WV,
Technical Report No. TR 80-4, September, 1980, 44 pp.

7. Mark, C., Molinda, GM., Schissler, AP., and Wuest, WJ.
Evaluating Roof Control in Underground Coal Mines
With the Coal Mine Roof Rating. Proceedings of the
13th International Conference on Ground Control in
Mining, West Virginia University, Morgantown, WV,
August 2-4, 1994, pp. 252-260.

8. Hanna, K. and D.P. Conover. Design of Coal Mine
Entry Intersections. Paper at SME Annual Meeting,
Phoenix, AZ, Jan. 25-28, 1988.


Figure 5 shows the fall rate for mines by seam height. The
seams with the highest fall rate are >106 in, Figure 6 shows the
distribution of roof fall rates by mine size (no. of underground
employees). Small mines with less than 20 employees and
mines with 51-150 employees had the highest fall rates.

ROOF FALL RATES BY SEAM HEIGHT FOR U.S.
COAL MINES -1996

B43
2 uss
E
 sovs
= 7680
F
& so-s0s
ci

>106

o 02 4 08 O8 1 12 we
ROOF FALLS/10,000 FT DRIVAGE

Figure 5. Roof fall rate for U.S. coal mines by seam height -
1996.

ROOF FALL RATES BY MINE SIZE FOR U.S.
COAL MINES -1996

120

21-80

51-150

151-250

UIG MINE EMPLOYEES

>260

0 04 02 03 04 805 Os

ROOF FALLS/10,000 FT DRIVAGE

Figure 6. Roof fall rate for U.S. coal mines by mine size -
1996.

FIELD STUDY OF INTERSECTION INSTABILITY,

There are a number of factors which affect the stability of
coal mine roof in general and, intersections specifically. These
factors include:

. Roof geology (CMRR)

. Entry span

. Artificial support

. Horizontal stress

. Time between mining and support installation
. Abutment

aueauene

In order to study these factors, we have constructed a
database of ground control variables from coal mines
throughout the U.S. This database currently contains 12 mines
and uses roof falls as the failure criteria, When the mine survey
is complete the analysis will use a multivariate statistical
approach to consider influence factors on roof falls and
determine a design equation for intersection stability. Through
experience the first three variables listed above have been
shown to contribute most to the fall of roof. The other
variables are important but are usually more site specific. In
this project attention is being focused on roof geology as the
key to intersection stability. In the past, empirical studies have
been hampered by the large number of geologic variables which
were recorded and applied to the model (e.g. uniaxial strength,
RQD, thickness of the weakest bed, location of the weakest
bed). Generally the influence of geology was divided between
all the variables and not one of the variables exhibited a
significant influence. The Coal Mine Roof Rating (CMRR)
should resolve this problem (/5).

In gathering roof stability data for each mine, standard
procedures have been established to ensure data integrity.
Extensive interviews with operators are conducted to identify
the roof problems, establish the roof geology, and document
support. From several failure criteria, the roof fall rate has
been selected as the outcome variable which will be used to
judge the influence of other mining and geologic variables.
‘Non-injury roof falls are defined according to MSHA (16) as
meeting any of the following criteria:

© Falls above the roof bolt anchorage.

© Falls which impair ventilation.

© Falls which impede passage of persons.

© Falls which cause miners to be withdrawn from the area
affected.

© Falls which disrupt regular mining activities.

Falls of unsupported ground in the face area which do not
trap equipment are normally not reported. Areas within the
mine that are affected by abutment loading (longwall tailgates,
pillar lines, multiple seam interaction zones) are excluded from
analysis, as are zones of anomalous geology (eg. major faults).
The distribution of standup time of supported ground is being
determined, and data base will include only falls occurring less
than 18 months after development. This is done so that roof
falls that are time dependent and occur in outby areas are not
given the same priority as falls near the face which expose more
miners to hazard. At one mine near Providence, KY 50% of
the falls occurred within one month of development, while only
10% of the falls occurred more than 12 months after
development (17).

Roof geology is also broadly sampled by selecting mines for
study which represent the full working range of CMRR. Within
each mine enough roof exposures are observed to adequately
define each of the significant roof types. Most often, this
means visiting any and all roof falls that are accessible.
CMRR verification is also done from core logs of holes on the
property when possible, Roof bolts are identified on the mine


Probability of Intersection Failure|

j
i
I

aay

Figure 9. Comparison of intersection and entry failure rates.

The relative stability of the type of intersection (3-way vs.
4-way) was also investigated using the preliminary data from
the 12 mine database, The percentage of 3-way and 4-way
intersections was tabulated. Then the percentage of 3-way falls
and 4-way falls was determined. Figure 10 shows the
proportion of 3-way vs. 4-way falls in comparison to the
percentage of all of these types of intersections, 42% of all
intersections were 3-ways, but only 36% of all falls in
intersections were in 3-ways. 58% of all intersections were 4-
ways, but 64% of all falls in intersections were in 4-ways.
Proportionately there was a higher fall rate in 4-ways than 3-
ways. In this data set then, a 4-way intersection is 1.28 times
as likely as a 3-way to fall. However, since it would normally
take two 3-way intersections to replace one 4-way, the remedy
of replacing 4-ways with 3- ways is not likely to reduce the
number of roof falls. It would take a fall rate of at least twice
as high for 4-ways to consider replacing them with 3-ways. A
mine that systematically replaced 4-ways with 3-ways would be
expected to see a 56% increase in total number of falls.

3-way fails vs 4-way falls

Fal 1% Dey wtereectore
Om Sm tte
2 Any arcs
peered

Figure 10. Failure rate of 3-way vs. 4-way intersections.

Coal Mine Roof Rating

Past experience at individual mines demonstrates a
correlation between CMRR and roof fall rate. (11). Roof fall
rate decreased as the CMRR increased, The CMRR for the
database ranges from 32 (extremely weak) to 75 (very
competent). Figure 11 shows significant variation in fall rate
with individual CMRR, indicating that other factors also
influence the fall rate, Individual mines show variable degrees
of correlation between CMRR and roof fall rate. At one mine
in Mingo Co., WV the roof fall rate for a weak shale (CMRR
= 40) was 3.4 per 10,000 ft of drivage, while the roof fall rate
for a firm shale or stackrock (CMRR = 50) was 1.0
per 10,000 ft of drivage. Both rock types were bolted with 48
in fully grouted bolts.

2

Root Fel Rate (tte per 10,000 fof drvaga)

° —s—_#_. 8

ry ”

Figure 11. Relationship of CMRR to roof fall rate.

A relationship was found between CMRR and intersection
span (Fig. 12). Generally, as CMRR increased in the 12 mines
in the database, the average intersection span (sum-of-the-
diagonals) increases. Factors such as roof support and local
mining practice influence this relationship. This may indicate
that operators know from experience that stronger roof is better
able to support a larger span and this is reflected in their mining
widths.

2 Fy ry 0 0 70 0
‘CMRR:

Figure 12. Relationship of CMRR to intersection span.


10.

11.

Unal, E. Empirical Approach to Calculate Rock Loads
in Coal Mine Roadways. Proceedings Fifth Conf, On
Ground Control in Mining, West Virginia University,
Morgantown, WV, June 11-13, 1986, pp. 234-241.

Pothini, B.R. and H. von Schonfeldt. Roof Fall
Prediction at the Island Creek Coal Co. Proceedings of
the Ist International Symposium on Stability in Coal
Mining, Vancouver, 1978, pp. 215-227.

Mark, C., Watts, R., Marshall, T.E., and M.O.
Serbousek. Using Computerized Mine-Wide Monitoring
for Ground Control. Proceedings of the SME Annual
Meeting, Albuquerque, New Mexico, February 14-17,
1994.

Maleki, HN. and JR. McVey. Detection of Roof
Instability by Monitoring the Rate of Roof Movement,
BuMines RI 9170, 1988, 12pp.

15.

16.

17.

Altounyan, PFR., Bigby, DN., Hurt, KG., and Peake,
HV. Instrumentation and Procedures for Routine
Monitoring of Reinforced Mine Roadways to Prevent
Falls of Ground. Paper in Proceedings of the 27"
International Conference of Safety in Mines Research
Institutes, New Delhi, India, 1997, pp. 759-766.

MSHA Accident Employment Database (1996).
Molinda, G.M. and Mark, C. Coal Mine Roof Rating

(CMRR): A Practical Rock Mass Classification for Coal
Mines. U.S.B.M. Information Circular 9387, 1994, 83

PP.
CER 30, Part 75.223, 1989, pg. 502.

Available from G. Molinda


Factors Influencing Intersection Stability in U.S. Coal Mines

Gregory Molinda, Research Geologist
Christopher Mark, Acting Chief, Disaster Prevention & Response Branch
Eric Bauer, Mining Engineer
Daniel Babich, Mining Engineer
Deno Pappas, Civil Engineer

National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

ABSTRACT

Groundfalls are much more likely to occur in coal mine
intersections than in entries. NIOSH is using the experience of
U.S. coal mines to determine the factors which influence
intersection instability and provide guidelines for the safe
excavation and support of intersections. Detailed field
investigations have resulted in a database of U.S. coal mines
containing 12 mines and 639 roof falls so far. By using the
roof fall rate as the outcome variable, correlations between roof
geology (CMRR), intersection span, and roof support have
been established. Case studies have indicated that replacing 3-
way intersections with 4-way intersections may not reduce the
total number of roof falls. Additionally, the size of intersection
spans tend to decrease with lower (weaker) CMRR. Protocols
have been established for the collections of roof bolt
parameters. The performance of individual roof bolts can now
be tracked with roof fall rate.

BACKGROUND

In underground coal mines, tens of thousands of intersections
are driven each year. Intersections represent exposed ground
which can span 25-50 ft, well over the normal width of an
entry. When pillar corners are rounded to make travel easier,
and when pillar spall (especially in high seams) exposes or
undermines more roof, the larger spans can easily become
hazardous. In 1996, there were a total of 2,824 unplanned roof
falls from 892 underground coal mines. These falls resulted in
711 injuries and 8 fatalities. Of the roof falls with known
locations, approximately 71% occurred in intersections.
Considering that intersections account for only 20-25 % of the
total drivage, on a foot by foot basis, roof falls are 8-10 times
more likely to occur in an intersection than an equivalent length
of entry. An estimate of number of intersections driven per
year reveals the exposure hazard to miners. Ata rate of 1,500
intersections driven per million tons mined and 230 million

development tons in 1995, approximately 350,000 intersections
are driven per year in U.S. coal mines.

INTRODUCTION

The increased likelihood of failure in intersections has been
documented in the past. These studies have primarily focused
on specific localized problems leading to failure, including
horizontal stress and geologic discontinuities (transition zones
related to paleochannels, faults) (/,2,3). Numerical modeling
studies have confirmed that intersections are less stable than
entries, and that 4-way intersections are less stable than 3-way
intersections. Intersection instability was found to be
dependent on rock quality and the ratio of horizontal stress to
vertical stress (4,5). Considering that intersections are at much
more risk of failure than entries, it is surprising that more
attention is not given to engineering special support for
intersections. Even mines with demonstrated intersection
problems often do not install more support during development
for these areas, preferring to rely on supplemental support
when problems occur. Other secondary factors contributing to
intersection failure include the quality of bolt installation (long
unsupported times before bolting, oversized holes,
fingergloving, etc.), and turnout frequency and location (2,6)).

Oversized intersection span has been identified as a cause of
failure (6,7,8). Suggestions for controlling overspanned
intersections and turnouts include longer bolts in the
intersection comers near the ribline (8), limiting turnouts to
corners not at critical angles to the principal horizontal stress
direction, and sequencing crosscuts to avoid turnouts
altogether.


Intersection Span

The average intersection spans were calculated for each
combination of variables comprising each mine partition and
represented as one row in the database. Figure 13 shows the
correlation between intersection span and roof fall rate for all
CMRR values of 45 and below. One mine with a wide
intersection span (74 ft) and a lower roof fall rate (2.78) can be
explained because heavy steel sets may be supporting failed
roof and contributing to a lower roof fall rate. The CMRR
values represent the weakest roof. The weakest roof rocks are
the most sensitive to large spans and have the highest roof fall
Tates.

i.

a. .

i 3

i‘ .

fT .
oo

8
a

eS
Intersection Span (8)

Figure 13. Relationship of intersection span to roof fall rate for
weak rocks.

A number of ways to minimize intersection spans have been.
proposed. In one Pennsylvania mine, roof conditions
deteriorated in wide intersection spans (one diagonal measured
‘52 ft) when a track chute entered a panel and also intersected a
crosscut. By dropping one crosscut and staggering another, the
span was reduced. Reducing the number and location of
turnouts should result in reduced intersection spans. The
practice of driving crossouts straight across the developed
entries, where possible, as well as supporting at-risk
intersections before they become intersections, will stabilize
spans (8).

Roof Bolting

There were numerous bolt types, lengths and spacings used
in the twelve mines. These ranged from 4 ft to 12 ft lengths,
tensioned and untensioned bolts, and ungrouted, partially
grouted, and full-column grout. Bolt spacing varied from 3-5
bolts per row and 4-6 ft row spacing.

Poor roof bolt installation may be able to account for some
of the variation in the roof fall rate regression model. This
topic includes the following: long lag times before bolting,
overdrilled holes, loss of tension, loss of resin due to lack of
containment, etc. This type of data can be important but very
difficult to obtain. In one mine we found a high rate of tension
bleed off (55% of bolts tested had lost tension), Also a
significant number of bolts observed in roof falls were “finger-
gloved,” indicating the resin was not mixed properly. Long
bolting lag times are more damaging in laminated rock with a
CMRR in the 40's or below. Data shows that 50-80% of the
total deformation and load changes occur within a few hours or
days after initial excavation (8).

Case History

At one western Pennsylvania mine in the Pittsburgh seam 55
intersection diagonals were measured. The measured data
considers intersection spans in fallen areas and non-fallen areas.
The falls observed in this mine were cleaned up and accessible.
The most significant observation was that for 83% of the roof
falls, the sum-of-the-diagonals exceeded 70 ft (Fig. 14). In
contrast, none of the non-fallen bolted intersections had a span
exceeding 70 ft. As the span increases so does the amount of
support. Supplemental support was necessary in 10
intersections where the average span was 67.1 ft. Although
none of these were fallen, they required additional support to
stabilize them. The mean span for the roof falls at this mine
is only 11% greater than the mean for the stable intersections,
but the estimated rock load is 43% greater. The CMRR for this
mine is 40, typical of the Pittsburgh seam. The correlation of
increased span with more roof falls is an indication that a sum-
of-the-diagonals of 70 ft or less is necessary for stable roof in
this geology.

4- Way Intersection Diagonals

+ Roo! falls
+ Non-Fallen, Supplemental Support

4 Non-Fallen, Roof botts only

Figure 14. Intersection spans for roof bolts, supplemental
support, and fallen roof areas at an eastern coal mine.


For any given geology, there are spans which will induce
entry failure. The key is defining the allowable span based on
roof strength and roof reinforcement, and not to exceed it.
Individual roof control plans specify maximum spans which
can vary considerably between and within MSHA districts.
Some roof control plans limit the sum-of-the-diagonals to 60-
70 ft (Figure 1), specify the number, location, and size of
turnouts in the intersection, or restrict turnouts to specific
entries.

D, + Dge Intersection span

oO
D2
Turnout —~
Figure 1.  Sum-of-the diagonals intersection span
measurement.

Rock Load and Roof Stress

Intersections may easily become overspanned if care is not
taken when mining. When the roof rock is strong, small increases
in span probably will not compromise the stability, but where the
roof is weak, small increases in span can add significant load to
the bolted interval above the intersection. If the geology is
uniform, the rock-load height can be expected to be proportional
to the span and to the roof rock quality, according to the equation
proposed by Unal (9):

100 - RMR.

h,=
_ 100

ye (1)

where _h, = Rock-load height, ft;
RMR = Rock Mass Rating, which is equivalent to
the CMRR (/5)

w. = Entry width, ft.

This equation implies that the rock-load height above an
intersection of competent roof, with an CMRR near 100, would
be very small, while an intersection with incompetent roof
(CMRR near 0) would have a rock-load height approximately
equal to the span. Assuming that K can be substituted for the
geologic adjustment factor, (100 - CMRR / 100), equation 1 can
be rewritten as:

1 (KY we Q)

thus the rock-load height is proportional to the span.

Assuming that the rock load above the intersection can be
represented as a pyramid (fig. 2), the volume of rock and rock
load over the intersection can be estimated by the equation:

ow.) (w,) (A,
y= 2 679)

5 % G)

where R, = Rock load above intersection, Ib
V, = Volume of rock above intersection, ft;
Y = Unit weight of rock, pef;
w, = Entry width, ft; and
w, = Crosscut width, ft.

Assuming w,= Ww, in most instances, then the the rock load is
proportional to the cube of the span, as shown in equation (4)
where w, and K(w,) have been substituted for w, and h,
respectively. If it is assumed that the rock load above an
intersection can be represented by a cube (instead of a
pyramid), the load is still proportional to the cube of w,
equation (5).

_ Kew?

R, 7

Y @)

(wo) ¥- 6)

Judging from the shape of roof fall cavities, the truth usually
lies somewhere in between these two idealizations. In either

cura s40

Figure 2. Comparison of rock loads above the intersection.

case, a small increase in entry width (and therefore increase in
intersection span) results in a significant increase in the rock
load. As figure 2 shows, increasing entry span by just two ft can
increase the intersection load by nearly 40%.


Where the geology is not uniform, and roof falls are truncated
by an overlying self-supporting strong bed, the rock load height
may be constant regardless of the intersection span. In this
case, the rock load increases in proportion with the square of
the span.

Roof Monitoring

Roof strain monitoring has been employed to predict
imminent intersection failure. Roof monitoring and the use of
early warning devices to predict intersection stability is not a
widespread practice in U.S. coal mines. Roof monitoring
programs have been implemented at a few mines, with
generally successful results. One example was the convergence
monitoring at over 600 stations in the Virginia mines of Island
Creek Coal Company in the 1970's (/0). It was found that
closure rates in excess of 0.1 in/d indicate instability.

The monitoring program was thought to have prevented
“numerous” falls while it was in operation (11). Maleki and
McVey (12) monitored roof sag at 139 stations in 2 westem
U.S. coal mines. At one mine, sag rates less than 0.01 in/d
denoted stability, while sag rates of 0.025 in/d was the critical
sag rate that usually indicated a roof fall would occur within 8
days, At the other mine, 0.032 in/d was found to be the critical
sag rate.

The use of early waming devices is a requirement in UK
coal mines. Falls of ground have been dramatically reduced in
British coal mines since the advent of roof bolting and
monitoring in 1991 (73). “Action levels” of roof strain are
established by roof instrumentation, and monitors are placed at
maximum intervals of 65 ft (20m) of entry. Inby monitors are
read every shift and recordings are made on a weekly or
monthly basis. Any movement exceeding | in. signals the need
for some remedial action, generally the installation of
supplemental support. This “action level” will vary with
geology and support and should be customized to each mine
roof situation. These monitors cost $25-100 depending on
numbers of anchors and ability to be attached to data recording
systems.

NIOSH INTERSECTION FAILURE PROJECT

The NIOSH office of Mine Safety and Health at Pittsburgh
Reasearch Laboratory has an ongoing project to study the
factors which affect the stability of intersections. Once these
factors have been identified and evaluated, the goal is to
estimate the probability of failure in intersections and provide
engineering guidelines for safe design and support of
intersections.

The approach is to use roof stability experience of U.S. coal
mines in order to determine allowable intersection spans based
‘on geology, and to prescribe support for that span.

In order to compare the relative frequency of roof falls in
coal mines, a roof fall rate was calculated for all U.S.
underground coal mines using data from 1996 (/4). Only non-
injury roof falls reported to MSHA were considered in the
analysis. The roof fall rate in each of the mines was calculated
based on the estimated amount of entry drivage. The entry
drivage was estimated from the reported production, adjusted
for seam height, and whether the mine employed a longwall.
The actual seam height was used in the conversion. As an
example, in two mines with an equal number of roof falls and
equal production, the mine with the higher seam would have a
higher roof fall rate because it would have less drivage and less
exposed roof. Development tons in longwall mines were
estimated to be 1/4 of total annual tons. Therefore, the fall rate
for longwall mines was multiplied by 4. Figure 3 shows the
distribution of mines by total roof fall rate. Fifty-six percent
of the mines reported no roof falls in 1996. Seventy-five
percent of the mines had fall rates of .3 falls/10,000 ft of
drivage or less (almost all of these mines had <10 falls in
1996). 48 mines reported 10 falls or more in 1996. While most
of the mines had low fall rates, almost 50% of the falls came
from mines representing only 5% of the production. This
means that a large proportion of the reported falls are coming
from relatively few mines. Figure 4 shows the roof fall rate
(falls/f of drivage) for intersection falls only. [t mirrors the rate
for all falls.

ROOF FALL RATE FOR U.S. COAL MINES -1996

56%

TIMBER OF WINES
288 8 8

Fy

0 ,0008-3 3006-6 6006-9 9006-12 >1.2
ROOF FALLS/10,000 FT OF DRIVAGE.

Figure 3. Roof fall rate for U.S. coal mines - 1996.

INTERSECTION ROOF FALL RATE FOR U.S.
COAL MINES -1996

as

Figure 4. Intersection roof fall rate for U.S. coal mines - 1996.