AUSTRALIAN CASE HISTORIES

New South Wales (NSW) Mine "A": Recovery rooms
were used on 12 panels in 3 different coalbeds at this mine
(Simpson et al., 1991). On the earlier panels in the Fassifern
coalbed 1.8 or 2.1 m (6 or 7 ft) resin bolts, “w" straps and mesh
were used for support. Standing support, consisting of two rows
of timber props was used only on the first panel. The rooms were
4.8 m (16 ft) wide, except the first, which was only 4.2 m (14 ft)
wide. A panel was also recovered in the shallower Great
Northern coalbed using the same configuration. When the first
recovery room was planned for the deeper Young Wallsend
coalbed, three rows of 10 m (33 ft) fully grouted cable bolts were
added, (at least near the headgate, the published report is unclear
on whether cable bolts were used across the entire room) and a
row of 1.8 m (6 ft) diameter standing supports known as Big
Bags were installed on 5 m (16 ft) centers near the outby rib.

The mine management attributed the success of the recovery
rooms to a large extent to the presence of a soft claystone floor
which allowed the panel fenders to be slowly punched into the
floor, with manageable floor heave, and avoiding fender yield
‘They reported that the fenders typically did not yield until the last
few meters and in some cases did not yield at all. In the deeper
Young Wallsend coalbed, this mechanism was not relied upon,
the room was heavily supported by both secondary and standing
support, even though the shale floor there was also soft. The
sandstone or conglomerate immediate roof above the Fassifern,
Great Northern and Young Wallsend probably also contributed
to the success of the recovery rooms at this mine. The mine
preferred to keep the tailgate back, as much as 8 m (26 ft) in the
Young Wallsend coalbed, so that if ground control problems
‘occurred, they would be less likely to affect the entire panel

NSW Mine "B": Four rooms were mined into, the first
being a full face recovery room 6.5 m (21 ft) wide and the next
three being narrow (3.5 m or 12 ft) full face entries driven for
ventilation. The recovery room failed, causing the shields to go
‘on the solid and several months were required to recover them
The room was supported by a standard primary bolting pattern,
with spot cable bolting only in areas considered critical, such as
gate road and chute intersections, No standing support was used.
The motor on the shearer ranging arm broke down when the face
was just a few meters from the room and the face remained idle
for an extended period. The fender failed when the face was 2m
(6 ft) from the room. High water flows from the gob and gas
flows into the tailgate entry were noted in the course of the mine
through and interpreted as the effect of the failure and subsequent
weighting of an overlying sandstone aquifer and the opening of
fractures in failed rock to allow gas flow.

‘The remaining mine throughs were supported by cable bolts,
props and glue injection into the panel fender. Primary support
consisted of 2.4 m (8 ft) bolts and “‘w” straps on a 0.8 m (2.5)
spacing, 10 m (33 ft) cable bolts on a density of 3 cables/2 m of
entry and 3 rows of 150 mm (6 in) props on 0.8 m(2.5 ft)centers,
over the entire length of the entry. These mine throughs were
successful, although there was no necessity to remove the shields.

NSW Mine A recovery room was used to
successfully recover a single longwall at this mine. No specific
information is available on the supports or on the panel geometry,
but it is known that significant secondary support in the form of
cable bolts and timber props was used

NSW Mine "D": Recovery rooms were used on two
short, 200 m (660 ft) wide panels at this mine. The immediate
roof in both cases is the massive, competent Coalcliff Sandstone.
Both mine throughs were considered completely successful. No
information is available on the type of support used in the rooms
in either case. Recovery rooms were not used on subsequent
panels because the roof lithology changed from competent
sandstone to shale and because later panels were also significantly
longer reducing the importance of rapid face moves.

NSW Mine "E": No information is available except that
several panels were recovered, the panels were narrow, about
50 m (164 ft) and they were successful.

Queensland: A single 5.2 m (17 ft) wide recovery room
was attempted at this mine (Klenowski et al., 1990). Primary
roof support consisted of five rows of 2.1 m (7 ft) resin bolts on
1.5 m(5 ft) spacings with “w” straps. The inby and outby bolts
were angled over the fender and barrier pillar. Two rows of 8 m
(26 ft) cable bolts were also used on 4 m (13 ft) centers, Grouted
1.8 m (6 ft) fiberglass rib bolts were installed in the fender and in.
the barrier pillar. Standing support was only planned for use on
anas required basis. The design was based upon the results of an
instrumented 15 m (50 ft) stub entry

Convergence was noted to begin to accelerate when the
fender was 6 m (20 ft) wide. When the panel was 5 m (16 ft)
from the room the fender failed and the shields began to
continuously yield. Just prior to entering the room a maximum
‘of 0.42 m (17 in) of convergence took place at one place on the
face where the shearer was hung up under the canopy of a shield
and typical convergence may have been 0.26 m (10 in). Less
convergence took pla the recovery room. When the shields
entered the room the mining height was greater and the available
hydraulic fluid was insufficient to set them (due to fluid losses
from continuous yielding of the shields). A delay took place until
the hydraulic reservoir was refilled. The convergence continued
after the room was entered and eventually it was necessary to set
timber props, with a total of 392 props finally being used, Of the
440 shield legs on the face, 104 were found to have failed after
the mine through due to malfunction of the gas yield valves. One
conclusion arrived at by the mining company was that had the
shields not failed, the roof convergence would not have been so
large and fewer timber props would have been required
However, itis possible that the high rates of convergence caused
the damage to the shields

After-the-fact, back analysis indicated that a “tensile failure"
extended upward to a rider seam located 16 m (50 ft) above the
German Creek Seam (figure 2). Convergence at the face
occurred much more rapidly at the face than in the recovery
room, indicating that "the pivot point of the failing roof beam was
cover the barrier pillar" (Klenowski et al., 1990)


Wynne, T., J.C, Stankus, S. S. Peng and C. T. Holland,
“Design and Implementation of Roof Control Systems for
a Longwall Full Face Recovery Room and Chutes at Mettiki
Mine,” Proceedings, Longwall USA. Maclean Hunter
Presentations, Inc., Aurora, CO, June 8-10, 1993, 148-
158 pp.

Chen, J., M. Mishra, S. Cario, and J. DeMichiei, “Longwall
Mining-Through the Backfilled In-Panel Entries at Cyprus
Emerald Mine,” Proceedings, 16th International Conference
on Ground Control in Mining. WV Univ., Morgantown,
WY, August 5-7, 1997, 1-8 pp.

Simpson, J., P. Neal, and R. Frith, The Use of Pre-Driven
Roadways for Longwall Recoveries. Coal Journal
Australia’s Journal of Coal Mining Technology and
Research, v. 33, 1991, pp 33-41

van der Merwe, J. N., “Strata Control Aspects of the
Backfilling of a Dyke Ahead of a Longwall Face
Underneath Incomplete Pillar Extraction at Sigma Colliery,”
Proceedings, Regional Conference for Africa of the
International Society for Rock Mechanics South African
National Group, Marshalltown, South African National
Group on Rock Mechanics, 3-4 November, 1988, pp. 135-
142.

van der Merwe, J. N., “Strata Control Considerations and
Experience Gained at the Collieries of Sasol Coal,”
Proceedings, Sangorm Symposium: Advances in Rock
Mechanics in Underground Coal Mining, South African
National Group, September 12, 1989, pp. 23-35.

15.

16.

van der Merwe, J. N., (ITASCA Africa). Personal
Communication, April 1998.
Bookshar, W. B., P. Simpson, A. A. Campoli,

M.R. Amick, and F. Stafford III. “Cutable, Variable Yield,
Cement Cribbing Successfully Support 84 Mine Longwall
Cut Through Entries,” Preprint Longwall USA, 9-11 June,
1998, pp. 69-89.

. Pulse, R. (1990). Ground Stability Evaluation Around

Collapsed Longwall Face, Memo to B. D. Owens, MSHA
file #6017, May 30, 2 pp.

. Ropchan, D. (1990). Longwall Recovery Room Roof

Failure, Weston, CO. Memo to J. Demichiei, MSHA file
46017, May 22, 4 pp.

Klenowski, G., R. N. Philips, and B. Ward (1990). Strata
Control Techniques for Chock Salvage, German Creek
Mines, Queensland. Bowen Basin Symposium,
pp. 114-122.

Hendon, G. Gate Road Pillar Extraction Experience at Jim
Walter Resources (1998). Proc. 17" Intl. Conf On Ground
Control in Mining. Morgantown, WY, in press.

Stansbury, B. Personal Communications, April 1998.

. Smyth, J. Personal Communications, April 1998

Wynne, T. Personal Communication, April 1998.


intrinsic support will not resist the failure because the support is
internal to the detached block. A strong main roof can help the
overburden to bridge across the room and face resulting in more
favorable face and room conditions

CHARACTERISTICS OF PRE-DRIVEN ROOMS

In this study, every effort was made to obtain information
ona variety of descriptive characteristics of all the case histories.
The results are summarized in Table 1. The goal was to identify
those characteristics which correlated with the failed case
histories. Statistical techniques, including Pearson correlation
and logistic regression, aided the analysis.

Immediate Roof: Descriptions of the roof geology were
usually contained in the literature, and were quantified using the
Coal Mine Roof Rating (CMRR). In almost every instance, the
published description was supplemented by CMRR data collected
by the fourth author in past visits to the mines.

A very strong correlation between the CMRR and weighting
failure was found. All of the six weighting failures occurred
where the roof was relatively weak (CMRR<S0). The correlation
is less evident for roof falls (figure 3).

Main Roof: Insufficient data were available on main roof

° ‘Swophing Faire
23 Roof Fal Fane
in Osucessts
Bs | ft —raoer _
gn»

18
i:

5

o| mm 17

ps Oo 6 9S 92 6 DB OH
CHR

Figure 3 Histogram of CMR versus mine through occurrences. All
‘weighting type failures took place with CMRReSD, while root fll
{failures took place over a wide CMRR range

geology for analysis. The authors believe, however, that the main
oof geology should be very important to weighting failures. It
seems likely that the CMRR is substituting for a characterization
of the main roof in many instances.

Floor: Soft floor was reported in 14 of the mine throughs,
but these included three of the weighting failures. Under some
conditions where the thin, heavily loaded fender is likely to punch
into the floor, the potential for failure could be increased,
although there were also successful cases where soft floor was
credited with delaying fender yield and contributing to the
success of the recovery rooms.

Overburden Depth: The case histories cover a very wide
range of cover depths. Roof falls were somewhat more likely to
‘occur at greater depth, but not weighting failures (figure 4).

x Ld
3 0
Eos | avieaiina
3 20 |_Jea Root Pan Faire}
8
Sis
gz 10

st

°

to 20~«SODSSCOSCwDC700
Cverburden, m

Figure 4, Histogram of overburden versus mine through
‘occurrence. Weighting failures were not strongly correlated to
room depth

Seam Thickness: No correlation was found between seam
thickness and either type of failure, It is worth noting that while
there is less potential for roof weighting when the seam is thin,
the tolerance for convergence is also usually much less.

Mining Rate: A slow mining rate seems to be a strong
predictor of both types of failures. Twenty mine throughs were
associated with reports of slower than normal mining, either
because they were meshing, or because a breakdown occurred
near the recovery room. Four of the weighting incidents, and two
of the roof falls, were in this group. However, it may be that
slow mining rates are more likely to be reported when a failure
occurred. The rate may actually have been slow in many of the
reported successes

Room Width: There seems to be a little correlation
between room width and difficulties. The very widest recovery
rooms, the 11 m (36 ft) rooms at the Maryland mine, were
trouble free. However, it seems reasonable that overall stability
will improve as the room width decreases, all other factors being
equal.

Room Length: It might be expected that overburden
weightings would occur more often in longer rooms. No such
correlation is apparent in these data, however (figure 5). Two of
the severe weightings (one in South Africa and the other in
Alabama) occurred on faces that were less than 110 m (350 ft)
wide.

Weighting Faire]
‘Root Fall Faure
DSuccessful

inn

so 100 «180,200 280 300350
Room Length, m

Figure 5.- Histogram of room length versus mine through
occurrence

Number of Occurrences
cosas RsRsh

Shield Capacity: Some correlation was observed between
shield capacity and roof falls, but heavier shields apparently have
not helped prevent weighting failures (figure 6)


Figure 2.-Conceptual model of a weighting type room failure,
based on observations and measurements at the Queensland
recovery room failure (after Klenowski et al., 1990).

SOUTH AFRICAN CASE HISTORIES

South Africa Mine "A": Six mine throughs were
attempted, five to mine through dikes and an experimental
recovery of part of a face (van der Merwe, 1988; 1989). The
dike mine throughs used either mat packs (rafts of timbers wired
together and laid one on top of the other) or filled entries, except
for the third mine through where only timber props were used.
For the first and second mine throughs mat packs were used, for
the third timber props, for the fourth a back fill material
consisting of cement, plaster and sand, and sifted ash as well as
timber props, and for the fifth a mix of coal fines and other
unspecified materials used to improve the flow characteristics and
strength of the mix.

All of the mine throughs except the third were successful.
In that case, the room was entered 0.3 m (1 fl) too low and the
recovery room eventually collapsed before the dike could be
blasted away to allow advance into the room.

The sixth mine through was a recovery room test and only
covered a portion of the panel. During the mine through the
conveyor belt broke and the face sat idle for 8 hours when the
face was 3 m(10 ft) from the room. Apparent water entry from
a joint in the fender softened the floor rock and caused the fender
to punch into the floor. The floor under the face conveyor
heaved and left the face conveyor and shearer too high to allow
the shields to be advanced, The recovery room roof also
converged until it was impossible to enter, forcing shield recovery
in place 3 m(10 ft) from the recovery room, a “lengthy process”

‘After the event, the conclusion was that “in longwalling, a
deviation from standard practice should only be considered if the
potential benefits outweigh the potential negative consequences
of failure, longwalling is not a very tolerant mining method."

South Africa Mine "B": A room was mined across a
longwall face to remove a dike. The roof rock included a 30 m
(100 ft) sandstone approximately 30 m (100 ft) above the
coalbed. In order to reduce the risks of the mine through the

panel width was reduced some 50 m (160 ft) before reaching the
room, from the normal 200 to 150 m (660 to 490 ft). No
information is available on the support types used, except that
mine management felt that if the technique was used in the future,
standing support would be used. The reasons given were to give
better resistance to abutment loading and to control the dike,
pieces of which tended to fall out while entering the room

FAILURE MECHANISMS,

In the vast majority of cases, longwalls have been
successfully mined into pre-driven rooms. Of the 130 cases, only
11 were apparently complete failures. However, the costs
associated with these failures, and the hazards they created, were
very substantial

The failures can be divided into two categories. The first
includes five cases where the problems were due to roof falls
occurring in front of the shields. The second group consists of
cases involving severe shield weightings accompanied by major
convergence

Roof falls occurred in two situations as the longwalls
approached the pre-driven rooms. At the face, failures developed
in the unsupported roof span between the shield tips and the
remnant fender (the portion of the longwall panel between the
shields and the room) because of an increased span resulting from
extensive coal yielding in the fender. In this type of failure, there
‘was insufficient secondary support in position to prevent the roof
fall. Roof failures in the recovery room developed because of the
high stress and deformation environment and the large span from
the shield tips to the abutment pillar once the fender was mined
out. Often a substantial portion of the room was involved with
this type of failure.

Weighting type failures resulting in the most severe ground
conditions were due to main roof action caused by the failure of
a weak roof. These types of failures were accompanied by
significant room and face convergence where shields were loaded
to yield, often becoming iron bound. The development of a new
cave break at the edge of the abutment pillar resulted in the loss
of the immediate roof cantilever as a supporting element (figures
1 and 2). In the six cases where the weighting failure occurred,
accelerated rates of convergence leading to failure initiated when
the fenders were 3 m (10 ft) or less in width. Up until then, the
remnant panel was providing substantial support to the main roof.

Where insufficient standing support is present in the room,
the shields may be called upon to control most of the weight of
the new caving block that extends across the recovery room,
doubling the roof span they previously supported. As shown in
figure 2, the failure may extend up higher into the main roof
because of a wider pressure arch causing a further increase in the
shield load. Further, the shields are not in good position to
handle this new load distribution. As the detached block moves,
the main roof is affected. If the main roof is not strong enough
to bridge over the detached block it will subside, progressively
adding to the loads on the face area. With this type of failure,


6.0

a
a)
5 8 O Successful
£40 o @ Weighting Failure
zg ARoof Fall Failure
a im
= 30
& in)
ed
#20
= o
s Qo
* 40 vat

o Oo
ol $e 08 ng :
30 40 50 60 70 80 90
CMRR

Figure 10.-CMRR versus Standing Support Density, for successful mine throughs and cases of weighting and roof fall
type failures, All weighting failures took place with CMRR<S0 and standing support density <0.5 MPa.

CONCLUSIONS

As the vast majority of cases attest (92%), longwall mining
into pre-driven rooms can be safely accomplished with adequate
support for the conditions. However, severe ground control
problems from weighting can be encountered where there is weak
roof and little or no standing support. Although it appears of
little value in preventing weighting failures, intrinsic roof support
is of significant value in preventing roof falls. In the cases of
weighting failures, the failure involves the main roof and the
establishment of a new caving break ahead of the face along the
abutment pillar. The costs associated with these failures and the
hazards created are substantial. However, even with a weak roof,
(CMRR=40), a standing support density of 1.0 MPa (145 psi)
appeared sufficient to prevent or control failure and allow the
longwall to enter or pass through the rooms. With stronger roof
(higher CMRR values) the standing support density required
appeared to decrease. This is an observation and should not be
considered a recommendation to eliminate standing support solely
‘on the basis of measured CMRR values. Other factors that
contribute to this type of failure include soft floors that can result
in pillar punching and slow mining rates.

With a stronger roof, where the weighting type failure does
not occur, much less standing support or intrinsic roof
reinforcement alone have been successful, However, even under
these conditions roof falls occur either in the unsupported area
between the shields and the fender or in the room itself. In the
cases where the falls occur in the rooms, they can be controlled
by increased roof reinforcement or by the addition of standing
support

REFERENCES

Bauer, E., J. Listak, M. Berdine, W. Bookshar, D. Raab,
and T. Mucho, "Longwall Recovery Utilizing the Open
Entry Method and Various Cement-Concrete Supports,”
Proceedings, 7th Intemational Conference on Ground
Control in Mining. WV Univ., Morgantown, WV, 1988,
30-42 pp.

Bauer, E. R., J. M. Listak, and M. Berdine. Assessment of
Experimental Longwall Recovery Rooms for Increasing
Productivity and Expediting Equipment Removal
Operations. USBM RI 9248, 1989, 20 pp.

Bauer, E. R. and J. M. Listak, "Productivity and Equipment
Removal Enhancement Using Predriven Longwall Recovery
Rooms," Paper in the Proceedings of the 1989 Multinational
Conference on Mine Planning and Design , Lexington, KY,
May 23-26, 1989, pp. 119-124

Listak, J. M. and E. R. Bauer, “Front Abutment Effects on
Supplemental Support in Predriven Longwall Equipment
Recovery Rooms,” Proceedings, 30th US Symposium on
Rock Mechanics. A. A. Balkema, Brookfield, VT, June 19-
22, 1989, 809-816 pp.

Wynne, T. , J.C. Stankus, S. Guo, and S. S. Peng, “Design,
Monitoring and Evaluation of a Pre-Driven Longwall
Recovery Room”, Proceedings, 12th International
Conference on Ground Control in Mining. WV Univ.,
Morgantown, WV, August 3-5, 1993, 205-216 pp.


International Experience with Longwall Mining into Pre-driven Rooms

David Oyler, Mechanical Engineer
National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

Russell Frith, Principal
Strata Engineering (Australia) Pty. Ltd.
AUSTRALIA

Dennis Dolinar, Mining Engineer
Christopher Mark, Acting Chief, Disaster Prevention and Response Branch
National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

ABSTRACT

Unusual circumstances may require that a longwall retreat
into or through a previously driven room. The operation can be
completed successfully, but there have been a number of
spectacular failures which exposed miners to serious roof fall
hazards. To help determine what factors contribute to such
failures, the National Institute for Occupational Safety and Health
(NIOSH) compiled a comprehensive international database of
130 case histories. The cases include five failures where major
rock falls occurred in front of the shields, and six even more
serious failures involving major overburden weighting. This
suggests two room failure mechanisms. The first is a roof fall
type failure caused by loading of the immediate roof at the face
as the fender narrows. The second is a weighting type failure
caused by the inability of the roof to bridge the recovery room
and face area, and affecting rock well above the immediate roof.
The data indicate that the roof fall type of failure is less likely
when intensive roof reinforcement (bolts, cables, and trusses) is
employed together with higher-capacity shields, The overburden
weighting failures, in contrast, occurred when the roof was weak
and little standing support was used.

Acknowledgments
The authors would like to thank Thomas M. Barczak of the

NIOSH, Pittsburgh Research Laboratory, for providing the data
required for determining standing support load capacities.

INTRODUCTION

Although not standard industry practice, pre-driven longwall
recovery rooms and cross panel entries have been used in a
number of mines for various reasons, including:

= to speed up recovery of the longwall upon completion of
a panel;

— with "super" longwali panels, to mine through entries
that were driven mid-panel to facilitate ventilation and
escape;

- — toextract old barrier pillars that may include crosscuts or
crossing entries, or;

= when a geologic feature, such as a dike or a fault, has
been removed prior to longwall mining.

Experience has shown, however, that there are serious
ground control risks with the procedure. Several spectacular
failures have occurred, where rock falls or severe weighting
pressures on the shields required weeks or even months to work
through. During these incidents, miners were exposed to
extremely hazardous conditions while working in very confined
spaces

To help prevent such failures in the future, NIOSH has
compiled a comprehensive database of all known examples of
longwalls mined into or through pre-driven rooms. A total of
130 case histories have been collected from 17 mines in the U.S.,
Australia, and South Africa, covering the period from the mid-
1980's through 1997. The data were obtained primarily from the
literature, supplemented by personal communications and
experience where applicable. For each case history, every effort
was made to obtain information on the geology, the dimensions
of the pre-driven room, the support installed, and the results.
Table 1 shows the complete database. Some further details on
the mines and case histories is provided in the paragraphs that
follow.


»
.3 Wipe Fale
g GRoof Fall Fature
i» Siew
be
g 0
go
i 0
2
22 in
at a
300 350 400 450 S00 550 600 450 700 750 600 50 900.950
Shield Capacity, mee tons

Figure 6.-Histogram of shield capacity versus mine through occurrence

Roof Reinforcement: Roof reinforcement includes all
intrinsic supports elements, such as roof bolts, cable bolts, and
trusses. The quantitative rating is obtained by determining the
load capacity of each element per unit area of roof supported by
the element and multiplied by the length of the element. This
Reinforcement Density Index (RDI) has the units of MPa-m.
Where several types of support were used, the ratings for
individual supports are summed.

Heavy roof reinforcement was apparently successful in
reducing the incidence of roof fall type failures. Roof
reinforcement was apparently not effective in preventing
weightings (figure 7)

‘mWeighting Fature|
BRoof Fall Faire
o: ‘

Number of Occurrences
osB esses

—
025 050 075 1.00 125 150 175 700
ROOF REINFORCEMENT DENSITY INDEX (RDI), MPa"

Figure 7, Histogram of Roof Reinforcement Density Index (RDI) versus
‘mine through occurrence

Standing Support: A characteristic of every one of the
weighting failures is the lack of standing support (figure 8). In
two mines, after a severe weighting failure developed in a room
without standing support, adjacent rooms were mined
successfully with standing support. These two cases indicate that
standing support can be the difference between success and
failure. ‘The importance of standing support also lends credence
to the proposed mechanism of weighting failure.

Face-Room Angle: Because of the limited data on cases
where the room was at an angle to the face, no detailed analysis
was conducted on this parameter. However, the mine throughs
in the documented cases were successful. As noted earlier, some
mines have entered the pre-driven rooms at small angles with the
tailgate lagging 6-10 m (20-30 ft) behind the headgate to limit the
extent of ground control problems. The evidence that a narrow
fender still provides significant support suggests that this
approach may be valid.

‘WWeighting Failure
fool Fail Failure LY
OSuccessti__||

ri te, =

ds to 15 20 23 30 35 40 a5 90 95 6
handing Support Density, MPa

Figure 8.-Histogram of standing support versus mine through occurrence. No

‘occurrences of weighting falures were observed when the standing support

density was greater than 0.5 MPa

Number of Occurrences.
cSBBSEBSSE

MULTIVARIATE ANALYSES

Logistic regression was employed to obtain insight into
possible design guidelines. ‘Three analyses were performed,
analyzing the roof falls, the weightings, and the combined set of
failures. Mining rate and weak floor were not included in the
analysis because the data were judged incomplete

Looking at roof falls alone, the two most important
variables were shield capacity and the total intrinsic support (RDI
or reinforcement density index). It appears that as the shield
capacity increases, the required standing support decreases, and
vice versa (figure 9). However, the relationship explains only half
of the roof failures

= 200 1
3 1.80 o (0 Suncessfl
= 10 0 Weighting Falure
2 vo ‘Roof Fal Failure
ig

# 400 cr

= 080 oO

pe ==

s& $i
= om —F 1
eZ 000

3 0 4 50 HD GO GD 7 7D BH BD
@ ‘Shield Capacity, rrefric tons:

Figure 9-Stield capacity versus Roof Renforoerrent Density Index, for
‘suecessfu rrine throughs and cases of roof fll and weighting type falures,

For weighting failures, the CMRR and standing support are the
two important variables (figure 10). A highly significant
relationship indicates that when the CMRR was greater than 30,
little standing support was necessary. For CMRR=40, the
successful cases used a standing support density of at least
1.0 MPa (145 psi). For CMRR values in the range of 45 to 50,
standing support densities as low as 0.5 MPa (73 psi) appeared
sufficient to prevent or control weighting failures. However, the
cost of standing support is small compared to the cost of a
weighting room failure, and the observations made from figure 10
should not be taken as a recommendation to eliminate standing

support.


WONGWALE PANEL =) HG

Figure 1, Typical physical behavior of a pre-driven room under weighting type roof failure. From underground
observation of the Colorado recovery room failure, Note the fracture observed to develop at the front edge of

the abutment pillar. (From Pulse, 1990)

were used. The first room employed 1.2 by 1.8m (4 by 6 ft)
flyash concrete piers. The piers were designed to “replace the
load-bearing capacity of the coal” by providing a support
resistance of 5.5 MPa (800 psi). To reduce costs, fibercrete cribs
with 0.3 m (1 ft) wood cap blocks were used on the next room.
Again, the cribs were placed to give a support density of 5.5 MPa
(800 psi). The concrete cribs were hard on the shearer and
stageloader, however, and so the last two panels employed
poured concrete cylinders, The concrete was pumped from the
surface to fill 1 m (3 ft) diameter cardboard tubes. The top 0.2 to
0.3 m(8 to 12 in) above the concrete cylinders was wedged with
wood. The support density was reduced on the fourth panel
following good results from the third. No ground control
problems were encountered in any of the recovery rooms,

Pennsylvania Mine "B": Three different techniques have
been used at this Pittsburgh seam operation. Little information
is available on the results from the first three 4.9 m (16 ft) wide
recovery rooms, The mining company considered the rooms
successful, but did not have full confidence in the technique
because of floor heave and the failure of some of the donut cribs
and several later panels were recovered short of prepared rooms,
using conventional techniques.

More recently, a set of three entries were driven across a
longwall panel so that the panel could be lengthened (Chen et al.,
1997). The entries were filled with a low strength cement-flyash
mixture, There were few ground control problems in mining
through the entries, although the wood and steel left in the rooms
caused equipment problems.

A full-face pre-driven recovery room was less fortunate, No
standing support was used, though the roof was heavily
reinforced with 2.4 m (8 ft) mechanically-anchored resin-assisted
bolts, 3.7 m (12 ft) cable bolts, mesh, and channels. The mining
rate averaged more than 15 m/d (50 fi/day) as the panel
approached the room. When the face was 3 m (10 ft) away, the
room began to deteriorate and a number of shields were

constantly on yield. The face entered the recovery room at the
headgate and tailgate areas, but the roof converged to the floor
‘over much of the entire mid-panel section. It took several weeks
to get through the collapsed roof, with remedial efforts including
the use of polyurethane grout and the installation of cribs

‘The face was then advanced to a second entry that had been
mined at the same time as the original recovery room. This room
‘was supported by two rows of donut cribs, a row of 0.76 m
G0 in) wooden cribs, and cable bolts. The longwall recovery was
successful, although the donut cribs were heavily damaged and a
large amount of convergence took place.

Pennsylvania Mine "C"; A three-entry system was driven
across one panel to allow it to be extended beyond an adjacent,
shorter panel. The entries were driven at a 30 degree angle to the
panel, each supported by a single row of 1.2 m (4 ft) diameter
poured cement cribs on approximately 2.4 m (8 ft) centers
(Bookshar et al., 1998). The body of the cribs was made up of
astiffhigh strength concrete, and the cribs were then topped with
a plastic bag approximately 0.3 m (1 ft) thick and filled with a
yielding proprietary cement. The mine throughs were successful,
with the largest measured deformations less than 80 mm (3 in) in
the tailgate.

West Virginia: Eleven faces have been recovered using full-
face recovery rooms at this mine (Smyth, 1998). The first two
used some standing support, but most have used just roof bolts
and cable bolts for reinforcement. The coal is quite thin, leaving
little room for convergence. While some shields have been stuck,
most face recoveries have been conducted without incident


U.S. CASE HISTORIES

Alabama Mine "A": This mine has gained considerable
experience with mining through pre-driven rooms in recent years
(Hendon, 1998). Successful mine-throughs include:

= 18 crosscuts extracted with a 75 m (250 ft) wide
longwall face;

= two “probe entries” driven across a 120 m (400 ft) wide
face; and,

= a number of 42 m (140 ft) crosscuts inside the same
120 m (400 ft) face.

In each of these cases, the face entered the pre-driven room
at an angle, generally about 7 degrees. Relatively little additional
bolting was used to reinforce the roof, which was usually
competent siltstone. Standing support consisted of, at most, a
single row of fiber cribs on 6 m (20 ft) centers,

There was also one notable failure. At the “pull-out
crosscut” of the same 120 m (400 ft) face, a "massive roof fall"
occurred at mid-face which required 2 weeks to clean up. This
was the most heavily supported of any of the mined through
entries, with a double row of propsetters installed on 1.5 m (5 ft)
centers. However, the other difference was that the wall
approached the pull-out crosscut much more slowly to facilitate
meshing. It was concluded that “substantial standing support was
needed at the pull-out point where the face retreat rate was
reduced significantly" (Hendon, 1998).

Alabama Mine " A longwall was used to extract a
110 m (350 ft) barrier pillar which was crossed at right angles by
a set of four main entries (Hendon, 1998). The first entry was
supported by double rows of propsetters, but a "massive squeeze"
developed as the last coal was removed from the fender. The
shield canopies were forced onto the face conveyor and 1 month
was required to get the longwall moving again. The remaining
three entries were reinforced with double rows of fiber cribs and
propsetters, and were extracted without incident. The roof
consisted of 1.5 m (5 ft) of mudstone and coal, overlain by
competent siltstone.

Alabama Mine "C": Partial recovery rooms, 45-60 m
(150-200 ft) long, have been used for many years at this mine
(Stansbury, 1998). These have been located either in the middle
or near the gate ends of the panels. Relatively light roof
reinforcement, and no standing support, has been sufficient in the
partial recovery rooms

When a recent panel was extended through three pre-
existing entries, the decision was made to fill them with a
0.7 MPa (100 psi) cellular concrete, There were no strata contro!
incidents, but other complications made the experience
unsatisfactory,

Most recently, cable trusses and concrete pilasters were
employed in a full-face recovery room. The coal fender punched
into the floor, there was significant shield convergence, and
numerous pilasters crushed out, but the face was recovered on
schedule. The pilasters were built of solid concrete blocks with

approximately 15% wood.

Colorado: On one of the early longwall panels at this mine,
a decision was made to recover the longwall from a sub-main
entry. The room was supported by a single row of square
fibercrete cribs topped with 0.15 m (6 in) wood blocks (Ropchan,
1990), Roof reinforcement included 2.4 m (8 ft) fully grouted
rebar cn 0.65 m (2 ft) centers with chain link fence. The
immediate roof consisted of a weak, highly slickensided shale
about 3-4 m (10-12 ft) thick, overlain by a series of weak
siltstones, sandstones, and shales. The shale-siltstone floor was
also weak

When the panel fender was between 1-2 m (3-6 ft) wide, the
face advance stopped for 6 hours because the pan line was stuck.
The roof began to converge rapidly as the fender crushed, and
many shields yielded with several becoming iron bound. The
pillars in the recovery room punched into the roof with heavy rib
spalling and cutter roof failure. All the fiber cribs failed either by
splitting or crushing with many showing a hour glass failure
configuration. The recovery room was then heavily reinforced
with wood cribs, though convergence continued.

The subsequent investigation concluded that the roof had
broken at the pillar line with the rock mass moving toward the
face as shown in figure 1 (Pulse, 1990). Investigators noted a
“tensile fracture" which resulted in the “roof moving toward the
gob.” With the roof beam apparently pivoting around the pillar
rib, the largest roof movements were experienced at the face, and
a significant amount of the abutment load had shifted onto the
shields. Under such conditions the face could not move the last
few feet into the room.

Recovery of the longwall was accomplished where the face
had been halted short of the recovery room. The remnant fender
was then mined out with a continuous miner after the roof was
grouted and heavily supported. Approximately 2 months were
required to recover the longwall face.

Maryland: This mine has used 23 recovery rooms, 6 of
which were 11 m (36 ft) wide. The only failure was one of the
standard rooms where roof falls necessitated 2 weeks of remedial
action. The extra support in this case was a row of concrete cribs
on the inby side of the recovery room and a row of wooden cribs
onthe outby side. The damage occurred when the face was 11 m
(35 ft) from the room. Mining rates were slow because of wire
meshing activity.

The wide recovery rooms were designed so that the face
would not have to slow down for meshing (Wynne et al., 1993).
The room was developed and supported in two passes.
Supplemental support included eight rows of concrete donut
cribs, three rows of truss bolts, and two rows of 5 m (16 ft),
25 mm (1 in) diameter roof bolts. All the wide rooms were
reportedly mined without serious incident (Wynne, 1998)

Pennsylvania Mine "A": A total of four recovery rooms
were successful at this Pittsburgh seam mine (Bauer et al., 1988:
Bauer et al., 1989). Three different types of concrete supports


Table 1.-Data base of parameters used in analyzing the performance of longwall mine throughs of pre-driven rooms.

No. Of | Soft |Depth Seam | Panel | Room | Shield | RDI‘ |Standing| Slow | Out
Country | State |Reference'| Rooms |Floor| _m_|CMRR| Ht, m | Width, m|Length, m] Capacity’ | MPam |Sup. MPa Mining®|come‘
US PA | 1,2,3,4 4 N | 150 | 40 | 2.4 244 61 454 0.37 5.6 N 4
US. PA | 1,2,3,4 1 N | 150 | 40 | 2.4 183 183 454 0.43 5.6 N 1
US PA | 1,2,3,4 1 N_ | 150 | 40 | 24 183 183 454 0.37 41 N 1
US PA _| 1,2,3,4 1 N_ | 150 | 40 | 2.4 183. 183 454 0.37 2.8 N 1
Us PA 12 3 N_ | 168 | 40 | 29 270 270 794 0.22 15 N 1
US. PA 7 3 N | 210} 40 | 22 250 250 635 0.53 18 N 1
Us. PA 7 3 N [210 | 40 | 22 305 305 635 0.29 4.8 N 1
US PA 1 N_| 220} 40 | 22 305 305 635 0.88 0.0 N 3
us PA 4 N_ | 220 | 40 | 22 305 305, 635 0.72 18 Y 1
us MD | 56,19 16 N_ | 190 | 40 | 26 229 229 599 0.33 12 Y 1
US MD | 5,6,19. 1 N_| 190 | 40 | 26 229 229 599 0.33 1.2 Y 2
US MD | 56,19 6 N_ | 190 | 40 | 26 229 229 599 0.66. 46 N 1
US co | 13,14 1 Y_| 140 | 35 | 21 168 168 590 0.62 0.3 ¥ 3
Aus__|NSW. 8 4 Y_| 90 | 60 | 3.1 200 200 590 0.64 01 N 1
Aus {NSW 8 6 Y_ | 90 | 60 | 31 200 200 §90 0.75 0.0 N 4
Aus __|NSW 8 1 Y | 50 | 82 | 34 200 200 590 0.75 0.0 N 4
Aus __|NSW 8 4 N_ | 290 | 50 3 200 200 590 1.83 04 1
Aus _|NSW 4 :
Aus |NSW. 4 N | 275 | 45 3 150 150 617 0.00 Y 3
Aus__|NSW 3 N_ | 275 | 45 3 150 150 617 0.93 0.1 N 4
Aus__| QLD 15 1 N_| 190 | 50 | 24 200 200 726 0.76 0.1 N 3
Aus __|NSW 2 400 | 70 200 907 1
Aus several 50 363. 1
S. Africa 4 200 150 1
S. Africa 9,140,114 4 N 200 200 2
S. Africa 9,10,14 4 N | 125 | 50 200 200 327 3.5 N 1
S. Africa 10,44 1 Y | 70 | 35 3 200 100 327 0.55 0.0 Y 3
us WV 18 1 N | 305} 50 | 15 244 244 530 0.55 0.1 N 4
US Ww 18 4 N_| 305 | 50 | 1.5 244 244 590 0.42 0.1 N 41
US Wy 18 6 N_}| 305 | 50 | 1.5 244 244 530 0.42 0.0 N 1
Us WwW 18 6 N_ | 305 | 50 | 1.5 244 244 590 0.42 0.0 N 1
US AL 17 3 N | 366 | 67 | 21 265 265 726 0.13 0.7 N 1
US AL 17 3 N | 366 | 67 | 21 265 265 726 0.414 0.3 N 1
us AL 17 12 N_ | 366 | 67 | 2.1 265 53. 726 0.41 0.0 N 1
US. AL 16 1 Y_ | 610 | 47 | 23 107 107 590 0.15 0.2 N 3
US AL 16 3 Y_ [610 | 47 | 23 107 107 590 0.15 0.7 N 1
US AL 16 1 N_ | 610 | 57.5 | 2.5 76 76 590 0.15 0.0 N 2
US AL 16 1 N_ | 610 |57.5| 2.5 76 76 590 0.15 01 N 2
US AL 16 16 N_ | 610 |57.5| 3.0 76 76 590 0.29 0.1 N 1
US AL 16 2 N_ | 610 | 68 3.0 122 67 590 0.09 0.0 N 1
US AL 16 6 N | 610 | 68 | 3.0 122 67 590 0.32 0.0 N 1
us AL 16 41 N | 610} 68 | 3.0 122 67 590 0.32 02 Y 2

‘References, Refers to the numbers of the references at the end of this paper.
4Soft Floor. Y=Soft, N=Normal or not noted as soft by the original source

*Metric tons.

“RDI. Reinforcement Density Index. The product of the support capacity and the support length, divided by the tributary area affected by the support and
‘summed for all support types. In the case of trusses the length of one anchor is used. The index does not apply to standing supports.
‘Slow Mining. Y=Slow mining. N=Normal or rapid mining or rate unknown.
“Outcome. 1=Successful outcome, 2=Failure due to face break or roof fail, 3=Failure due to major overburden weighting