10

experienced by C1, the cribs were never in danger of fail-
ure. Since the ultimate strength of steel-fiber-reinforced
cribs ranges from 800 to 1,000 kips (70), C1 (the crib that
experienced the greatest amount of loading) reached less
than 8 pet of its ultimate strength. Therefore, the loads
experienced in the angled crosscut were probably never a
threat to entry stability. Visual observation also revealed
that the cribs remained stable until they were mined into.
Crushing of wooden header blocks between the roof and
cribs was seen to occur, but the cribs showed no sign of
weakening. The shearing machine was able to mine into
the entry and mine out the cribs with little change to its
normal shearing pattern.

Table 1.—Average load on donut cribs in angled
‘erosscut, pounds

Face position, ft c1 c2 c3 C4
Minus 40 23,691 *13,311—:16,923
Minus 35 24,819 14,100 18,051
Minus 30 29,331 © 14,667 20,307
Minus 25 29,895 14,667 24,255
Minus 20 34,971 15,231 15,947
Minus 15 40,614 18,051 32,151
Minus 10 47,946 © 19,743 92,715
Minus 5 43,998 23,127, 36,099
Minus 3 47,382 27,639 43,998
i) oO 9 i

800 F
seeneee C4

600

400

AVERAGE STRESS CHANGE, psi

200

ie)
-200

-150
FACE POSITION, ft

-100 -50 °

Figure 14.—Average stress change in donut cribs,

CONCLUSIONS

Information obtained from this study indicates that gate
road chain pillars can be mined successfully during panel
retreat. Recovery of chain pillars can increase resource
recovery, increase productivity, and lessen the long-term
detrimental effects of surface subsidence. There have
been few successful attempts at chain pillar recovery
because of the difficulty in controlling the front abutment
pressure ahead of the face.

From an operations perspective, this method was con-
sidered a success by mine personnel because the roof,
pillars, and entries remained stable and face advance
progressed at a normal rate as the panel and the chain
pillars were mined.

From a research perspective, the Bureau sought to
determine if this method of longwall retreat mining
created abnormal and/or uncontrollable loading conditions

in the headgate that would make pillars and entries un-
stable, thereby precluding the mining of the chain pillars.
Pillar and supplemental support behavior, both recorded
and observed, indicates that, for this panel’s layout and
conditions, roof, entries, pillars, and supplemental support
remained stable. The data revealed no unusual loading
conditions that would suggest that extracting the chain
pillars is detrimental to safety.

This study has shown that, through the use of angled
crosscuts between the chain pillars, along with proper sup-
port techniques in the entries and crosscuts, safe mining of
chain pillars was accomplished under the conditions at this
site. However, because of the headgate’s importance to
the entire longwall operation, extreme caution should be
the rule; an unsuccessful attempt to recover chain pillars
could prove to be dangerous as well as very costly.


LONGWALL RETREAT OF GATE ROAD PILLARS

By J. M. Listak’ and E. R. Bauer’

ABSTRACT

This report describes a U.S. Bureau of Mines investigation into the removal of gate road chain pillars
in conjunction with longwall retreat mining. The research objective was to determine, through evalua-
tions of the loads imposed on the gate road pillars and supplemental support clements in the entries,
how the pillars, supplemental support, and entries were affected by the longwall abutment pressure. To
achieve this goal, vibrating wire stressmeters and hydraulic flatjack pressure cells were installed in the
support elements to monitor stress change caused by abutment pressure as a function of longwall face
advance. The findings documented in this report are the result of information collected from instru-
mentation and observation at the mine.

Pillar and support loading measurements and stability observations revealed that the roof and the sup-
port elements remained stable during panel retreat, while the face progressed at a normal rate of ad-
vance. The ability to safely and efficiently mine chain pillars during panel retreat increases resource
recovery and can provide several other benefits described in this report.

1Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


affected the stability of the area, vibrating wire stress-
meters (VWS’s) were installed prior to longwall retreat in
the pillar to be mined and in the two pillars adjacent to
the pillar to be mined. The stressmeters were positioned
across the pillars in order to provide a history of pillar
loading as a function of longwall face advance. Figure 6
shows an exploded view of the instrumented area relative
to the entire longwall panel layout.

The donut cribs were instrumented with hydraulic pres-
sure cells (flatjack cells) to determine the effects of
abutment load transfer to the crosscuts and intersections
as the longwall face approached. Figures 7 and 8 illus-
trate the arrangement of pressure cells in the donut cribs.
The leads of the VWS’s and pressure cells were extended
to a central-readout station at a safe location and moni-
tored by mine and Bureau personnel as the longwall face
approached the instrumented area, as shown in figure 9.

Tailgate

iaaiseanaiewanaaenasne
I!

son fi

—

Direction of
mining

Row of
pillars to be mined

NliDlOooo!

Close visual inspection of the pillars, entries, and entry
intersections was also maintained by section supervisors as
mining progressed into the pillar removal area.

FIELD DATA ANALYSIS
Pillar Response

The VWS’s were installed in the gate road pillars after
development and prior to longwall retreat. Therefore,
data in this report represent the stress change from devel-
opment loading rather than absolute stress. As previously
stated, stressmeters were installed across the width of the
headgate pillars in order to characterize stress changes due
to abutment pressure as a function of longwall face ad-
vance. Baseline stressmeter data taken prior to mining,

LEGEND
FJC  Flatjack cell

VWS. Vibrating wire
stressmeter

FUC 10, 11, 12
FIC 7.895) I VWs 12
3 vws 11
56 € at MWS!
Readout FUC 12,3
\Fe VWS 8

VWs 9 a:

all
ws 7
‘ aii wws"3"
p : vws 2
E vws {

Track entry
ena
° 50
—_ ae
aa —___Seole, ft

Figure 6.—Detalled area of instrumentation site.


(5) reported that several panels were extracted by this
method and at times, serious headgate problems devel-
oped. The 60-ft pillar size was increased to 100 ft in a few
panels, but in 1980 the practice was stopped. The com-
pany decided that the overall benefits realized were not
worth the ground control risks involved with the method,
and the belt was moved adjacent to the longwall panel.

Another failed attempt at chain pillar recovery took
place at a mine in West Virginia in August 1989. The
pillars that were to be mined were located in a four-entry
tailgate system. Conventional rectangular chain pillars
were used for gate road support. After mining several of
the pillars the procedure was abandoned because of poor
ground conditions that developed as the face neared the
crosscuts. Front abutment loads coupled with the residual
gob loads from the previously mined-out panel created
uncontrollable conditions, causing pillar deterioration and
roof failure in the crosscuts, even with the use of supple-
mental support.

To cite another example, Mid-Continent Resources’
Dutch Creek No. 2 mine in Colorado successfully mined

through previously driven entries (6). The success was
attributed to steel-fiber-reinforced concrete cribbing in the
entries and crosscuts and their alignment. The No. 2 mine
was 500 ft above the Dutch Creek No. 1 mine, and the
need to realign a panel in the upper mine with a panel in
the lower mine made it necessary to mine through pre-
viously driven entries. The entries and crosscuts mined
through were aligned at 30° and 60° angles to the longwall
face. Although there is no evidence to prove that this was
a major factor that contributed to the success of the
experiment, mine management believed that, in addition to
the concrete cribbing, the angled crosscuts provided a
favorable condition for roof control.

Two primary goals of the U.S. Bureau of Mines in
researching underground coal mining are to reduce the
cost of mining coal and to ensure the health and safety of
miners. This study seeks to help achieve these goals by
evaluating entry stability and the response of support
elements while mining a row of chain pillars in a longwall
headgate.

CHAIN PILLAR RECOVERY CONCEPT

Figure 14 shows a typical longwall panel layout utilizing
a three-entry gate road system. The longwall equipment
is assembled in the setup entry and only the panel is
extracted during retreat. Figure 1B illustrates an extended
panel with the shearer removing one row of headgate
chain pillars during panel retreat. The only additional
requirement for mining the pillars is the extension of the
face conveyor and roof supports into the headgate. Ideal-
ly, recovery of both headgate chain pillars would offer
complete resource recovery and eliminate the long-term
surface subsidence effects. However, the nature of gob
development in mined-out areas requires that at least one
row of pillars remain as the tailgate for the next panel. If
both pillars are extracted, no tailgate would exist for the
next panel. Figure 2 shows that one row of pillars is
required to act as the succeeding panel’s tailgate.

After the next panel is extracted, the gob flushing
around the remaining pillar would close the remaining
tailgate entry and, depending upon the amount of supple-
mental support, could offer lateral confinement that would
provide strength to the pillar. The subsidence trough
would still remain over the longwall panel, but pillar
degradation and the subsequent threat of subsidence would
be reduced.

Redistribution of stress as the panel is mined may
jeopardize the success of extracting chain pillars. The
potential exists for ground control problems to arise when
the chain pillar is nearly mined out and the face nears the
crosscut between it and the succeeding pillar. As the pillar
decreases in size, it loses its capacity to carry load. As a

result, abutment loads, advancing parallel to and ahead of
the longwall face, will create instability around the pillar,
particularly in the four-way intersections between the two
rows of gate road pillars. Since the headgate area pro-
vides access to the longwall panel and the location of the
stage loader and face monitoring equipment, removing the
headgate chain pillars could be a risky endeavor. To al-
leviate problems due to abutment loading, some form of
supplemental support is required to maintain entry stabil-
ity. Consequently, additional support is an added expense,
and dealing with supplemental support when each crosscut
is mined into could be problematic.

Gate road pillar layout can play an important role in
lessening the effects of abutment loads and subsequently
the amount of supplemental support required, Since the
front abutment travels ahead of and parallel to the face,
conventionally developed crosscuts, also parallel to the
face, will experience the full effect of the abutment load.
In addition, abutment loads around the large exposed roof
areas in the four-way intersections could also prove to be
a problem. However, by using an angled crosscut, the
pillar offset will reduce the area of exposed roof relative
to the parallel advance of the front abutment and offer
support to front abutment loads as the face approaches
and passes through the crosscuts. In addition, the four-
way intersections would be eliminated, and the remaining
three-way intersections would require less supplemental
support. Figure 3 shows how the use of angled crosscuts
could help control the front abutment loads.


INTRODUCTION

The future of underground coal mining is dependent
upon highly productive longwall systems. Innovative meth-
ods of increasing longwall productivity are continually
being sought by industry and research organizations. An
obvious trend that has transpired over the last several
years to improve productivity is the increase of panel
dimensions. Larger panels have become increasingly pop-
ular because they provide increased resource recovery, less
gate road development, fewer face equipment moves, in-
creased productivity, and mitigation of long-term surface
damage due to differential subsidence. In the last
10 years, panel width has increased more than 43 pct from
an average of 495 ft to more than 707 ft (1)2 Although
ground control considerations and electrical and hydraulic
limitations of longwall equipment continue to dictate
longwall face width, technical advances in these areas are
allowing for wider faces. Similarly, panel lengths are also
increasing but are usually governed by the extent of coal
reserves and/or the mine layout relative to surface
features (i.¢., buildings, highways, topography, etc.).

A longwall panel’s boundary is formed by its headgate
and tailgate entries, which in turn define the panel’s
dimensions. Since a new panel must be completely devel-
oped before longwall face equipment can begin panel
retreat, development of gate road entries often requires
utilizing several production units (continuous miners) in
order to maintain uninterrupted production from the long-
wall. Still, continuous miner development is slow com-
pared with retreat of the panel and proves to be a hin-
drance to an otherwise efficient system, thus making
longwall mining dependent upon the efficiency of con-
tinuous miner sections. Furthermore, the advance rate of
continuous miners is a function of the number of gate road
entries employed. Accordingly, advance rates of con-
tinuous miners suffer when more gate road entries are
developed. The majority of longwall mine layouts in the
US. employ three- or four-entry gate roads because of
ventilation and escapeway requirements. Of the 96 active
longwall faces surveyed in 1990 (2), 40 pct used 3 entries,
34 pet used 4 entries, 23 pct are unknown, and 3 pet used
other configurations for their gate road systems.

Although productivity advances have been introduced to
continuous mining by means of satellite miners and ex-
tended cut operations, technological advances in long-
walling are outpacing these improvements and continue to
be developed and readily adopted by operators. This
technology has enabled longwall panel extraction rates to
more than double over the past 5 years, putting additional
pressure on gate road development sections.

Italic numbers in parentheses refer to items in the list of references
at the end of this report.

Since it is likely that continuous mining practices will
remain relatively unchanged for many years, other meth-
ods of increasing longwall production must be pursued.
One such method, which would increase resource recovery,
provide additional time for continuous miners to develop
gate road entries, and lessen the long-term damaging
effects of surface subsidence, would be to extend the long-
wall face equipment into the headgate in order to mine
one or more rows of the gate road chain pillars during
retreat of the panel. This would require some additional
face equipment; however, the benefits could be significant.
For example, coal recovery on a 600- by 5,000-ft face that
has a mining height of 6 ft and has a three-gate road entry
system using square pillars on 100-ft centers (assume 20-ft-
wide entries) can be increased about 10 pet by recovering
one row of gate road pillars along with the panel. Fur-
thermore, recovering one row of chain pillars effectively
increases the panel’s dimensions and, subsequently, the
panel’s life, thus providing additional time for the devel-
opment of the next panel by continuous miners, An added
benefit to mining a row of gate road pillars is that, for
some mines, belt removal and relocation would not be
necessary. During continuous miner development in a
three-entry system, it is desirable to locate the belt in the
center entry for improved ventilation and shorter shuttle
car haulage distances. For these reasons, the belt is often
located on the center entry but later moved so that the
longwall belt can be situated adjacent to the panel.
However, in a three-entry system, if the pillar nearest the
panel is mined out with the panel, the belt would already
be located in the proper entry.

Recovery of gate road chain pillars is not a new idea.
In 1976, Simpson applied for and received a patent for
headgate chain pillar recovery (3). Simpson describes the
technique of “pump packing” for control of abutment pres-
sure. Pump packing involves mixing broken coal with a
binder such as bentonite and water to form a thixotropic
material. Used by the British to construct packwalls, this
material flows under pressure and solidifies rapidly when
the pressure is relieved. The pumped material provides
homogencous roof support ahead of the longwall face yet
is low enough in strength to be easily mined by the shear-
ing machine. Hearnshaw and Gallaher (4) discuss a simi-
lar approach using pneumatic stowing of gob material for
ground control.

There have also been a few operators that have tried
this method. In Illinois, Old Ben Coal Co. developed a
three-entry system with the belt located in the middle
entry and extracted the 60-ft chain pillar between the belt
and intake entries as the panel was retreated. Cavinder


FIELD INVESTIGATION

MINE SITE

A study was conducted by BethEnergy Mines Inc. and
the Bureau to evaluate entry stability and the response of
support elements during the mining of a row of chain pil-
lars in a longwall headgate. The mine is located within
the Appalachian Plateau province of southwestern Penn-
sylvania, Topographic relief in the area does not exceed
350 ft and dips are generally less than 4°, Mining takes
place in the Pittsburgh Coalbed, which lies stratigraphically
within the Pennsylvania age coal-bearing strata of the
Monongahela Group.

The immediate roof was composed of approximately
4 ft of gray shale overlain by thin members of coal and
carbonaceous shale. Upper members were composed pri-
marily of gray sandy shales.

The floor rock was composed predominantly of fire
clay. Floor heave was not a problem in the study area.
Geologic features such as clastic dikes, kettlebottoms, and
jointing were managed by the mine’s roof control plan.

The study area consisted of one longwall panel and its
associated headgate. The average depth of cover over
the panel was 600 ft. The panel under investigation was
2,200 ft long and 900 ft wide (including the recovered
pillars). Pillar recovery was planned on this panel because
of the panel’s unusually short length (2,200 ft). Another
reason to recover a pillar from this headgate was the inor-
dinate number of entries that were developed. Because of
ventilation and supply considerations, a six-entry, five-pillar
headgate system was employed as the headgate. A 100- by
100-ft abutment pillar was flanked on one side by two 20-
by 100-ft pillars. Two more pillars were located on the
opposite side and adjacent to the panel to be mined: a 20-
by 100-ft pillar and a 40- by 100-ft pillar to be retreated
with the panel. The crosscuts between the chain pillars to
be retreated were mined at 40° angles from the panel's
length. Since it was not known how this method of panel
retreat would affect the headgate area, the panel was laid
out such that the beginning of the panel could be mined
conventionally (ic., not mining the pillars). When the
face had advanced approximately 550 ft, the panel’s width
narrowed to 840 ft, thus allowing the pillars to be ex-
tracted with the panel. Figure 4 illustrates the complete
longwall panel layout and shows the row of pillars that
were mined in conjunction with the retreat of the panel,

To guard against headgate roof falls in the crosscuts
and intersections, donut cribs were installed in the angled
crosscuts between the pillars that were to be mined, The
donut cribs were constructed using 22-in-diam, 3-in-thick
reinforced concrete disks that are hollow in the center

(donuts). To facilitate load transfer and even distribution
of load, 1/8-in-thick plywood disks were inserted between
every four courses as the cribs were built. Timber block-
ing between the last course and the roof, as shown in fig-
ure 5, also provided for yielding in the cribs.

No additional supplemental support was used in the
entry between the mined chain pillar and the longwall
panel (i.e, adjacent to the longwall panel). However,
hydraulic supports were used in the headgate area around
the stage loader.

INSTRUMENTATION SITE

Roof falls in the headgate area would pose a threat to
Jongwall face personnel and equipment and were therefore
the most important aspect of pillar recovery to be ad-
dressed. To determine how the removal of these pillars

Tailgate

[| Panel i
. cama
O Row of mining im
| pillars to be mined
ee
ie eee ==)

Headgate
° 400 —

Bae
Figure 4.—Layout of study panel.

Timber blocking

Donut “me

Fiber inserts

TORTI STII —_

Figure 5.—Typical donut crlb construction In angled crosscut,


Longwall Retreat of Gate Road Pillars

By J. M. Listak and E. R. Bauer



Direction of mii

Readout
station

Scale, ft

LEGEND

FJC  Flatjack cell
VWS Vibrating wire stressmeter
P Instrumented pillar

Figure 10.—Stressmeter and flatjack location in pillars and donut cribs.

incomplete, instrument recording was terminated because
of wire damage from rib sloughage). The steep rise in
stress change in the pillar was not accompanied by any
problems on the longwall face or in the gate entries. As
shown in figures 11B and 11C, stress change increases in
pillar P2 were gradual with the exception of VWS 4. VWS
4 was located 5 ft from the rib in pillar P2 and was the
stressmeter located next to the mined pillar. Peak stress
change and subsequent unloading was experienced in VWS
4 when the face was about 30 ft away from this set of
stressmeters, indicating that pillar failure occurred in the
form of rib sloughage or yielding. However, if rib slough-
age occurred, it is likely that the stressmeter’s lead wire
would have been severed, rendering it unreadable. VWS
9, located 5 ft from the rib in pillar P2 approximately 40 ft
outby VWS 4, did not exhibit the same behavior. The
increase in stress change continued at a gradual rate until
the face had passed the instrument. This may be ac-
counted for by the stressmeter’s proximity to the donut
cribs. Stressmeters located in pillar P3 (figure 114)
showed very little increase in stress change.

Figure 12 illustrates VWS 4 through 6 as stress profiles,
at various face positions, of the 20-ft pillar (P2) adjacent
to the mined-out pillar. There is a distinct difference in
pillar behavior shown during the 30 ft of face advance that
is presented in the graph. When the face was 20 ft from
the instrument site, loading on the outer edge of the pillar
nearest to the panel was more than two times that of the
pillar’s core and opposite edge. This is the distribution of
stress that is normally associated with an abutment pillar.
However, as the face drew nearer to the instrumentation
site, loading on the outer edge nearest to the panel contin-
ually decreased while the pillar’s core loading continually
increased. When the face had progressed to 10 ft past the
instrumentation site, stress redistribution in the pillar has
caused the core to be loaded higher than the outer edges,
and thus the pillar took on the characteristics of a yield
pillar.

To further characterize pillar and entry behavior in
response to longwall retreat of the chain pillars, com-
parisons were made between data collected during this
study and pillar and front abutment information from


Figure 7.—Donut crib shown with installed hydraulic flatjack.

Figure 8.—Hydraullc flatjack arrangement in donut crib.

and subsequent data taken with respect to longwall face
advance, provided information corresponding to vertical
stress. The three instrumented pillars, designated as P1,
P2, and P3, are shown in figure 10. Pillar P1 is the pil-
lar that was mined out with the longwall panel, and it was
monitored to determine how the front abutment affects
the stability of the pillar and the entries around it. Pil-
lars P2 and P3 were also monitored to determine the
effects of stress transfer as pillar P1 was mined. The

Figure 9.—Instrument readout station.

abutment-yield? pillar arrangement used in the study area
was the same configuration that had been proven to offer
good ground conditions on previous panels. An earlier
Bureau study concluded that average pillar loads were
lower, and they appeared to stabilize when an abutment-
yield configuration was utilized (8).

Figures 114 through 11D illustrate the progression of
stress increase in the three pillars as the longwall face ap-
proached the instrumentation site. Four graphs are shown
for the three pillars because stressmeters 4 through 6 and
7 through 9 were located in the same pillar. Figure 11D
shows the abutment pressure ahead of the face in pillar
Pi. Initially, three stressmeters were installed in this
pillar; however, the lead wire of VWS 10 was cut by rib
sloughage and is not presented on the graph. The graph
shows the gradual increase of stress, beginning when the
face was approximately 200 ft ahead of the instruments.
When the face was approximately 25 ft away, a rapid
increase in stress change occurred before VWS 11 and 12
were lost (in this case, and others, where data are

3The term "abutment-yield” is used for descriptive purposes to des-
ignate the existence of two adjacent pillars, one large and one small.
The smaller 20-ft pillar is referred to as the yield pillar even though it
does not exhibit yield pillar behavior. Theoretically, a true yielding pil-
lar (based on Wilson's (7) equations) at this mine has a width of about
10 ft.


a es
ae wieadasts sca

Pye eee a i 7 aT “1 ro

200

Scale, ft Se el

Figure 1.—Conventional and mined headgate-chain pillar panel layouts. A, Only panel is mined; B, headgate chain pillars are also
mined.

Next panel to
be mined

Gob flushing

x) EX]

Figure 2.Gob development behind longwall face. Figure 3.—Chain pillar recovery using angled crosscuts.


kip

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT
foot Ib pound
inch pet percent

1,000 pounds psi _ pound per square inch



su

REFERENCES

1. Merritt, P. C. As Time Changes, So Do Longwalls. Coal, v. 92,
No. 2, Feb. 1991, pp. 40-41.

2. ___. Longwall Census. Coal, v. 92, No. 2, Feb. 1991, pp. 42-49.

3. Simpson, T. L. Longwall Mining With Chain Pillar Recovery.
USS. Pat. 3,999,804, Dec. 28, 1976.

4. Heamshaw, G., and G. Gallaher. Chain Pillar Recovery During
Longwall Panel Retreat Using Pneumatic Stowing To Provide Roof
Support, Paper in Proceedings of the Second International Conference
on Innovative Mining Systems, ed. by L. W. Saperstein. PA State Univ.,
University Park, PA, 1986, pp. 45-47.

5. Cavinder, M. Longwall at Old Ben in Southern Illinois. Paper in
Proceedings of the Illinois Mining Institute (Springfield, IL, Oct. 13-14,
1983). IL Min. Inst., 1983, pp. 22-34.

6. Turnipseed, M. J. Longwall Advance Through Existing Mine
Openings. Paper in Proceedings from American Mining Congress Coal
Convention (Pittsburgh, PA, 1986). Am. Min. Congr., 1986, pp. 1-5.

7. Wilson, A. H. An Hypothesis Concerning Pillar Stability. Min.
Eng, v. 131, No. 141, 1972, pp. 409-417,

8. Listak, J. M,, J. C. Zelanko, and T. M. Barton. Effects of Various
Longwall Chain Pillar Configurations on Gate Road Stability. BuMines
RI 9184, 1988, 17 pp.

9. Listak, J. M., and E. R. Bauer. Optimization of Supplemental
Support for Open-Entry Longwall Equipment Recovery. Paper in
Longwall USA 1989 Conference Papers. Am. Min. Congr. Coal Conf.
and Longwall USA Exhib. and Conf,, Pittsburgh, PA, 1989, pp. 19-27.

10. Smith, R. (Commercial Pantex Sika). Private communication,
1991; available upon request from J. M. Listak, BuMines, Pittsburgh, PA.



CONTENTS

Abstract .......
Introduction .. .
Chain pillar recovery concept
Field investigation . . . .
Mine site
Instrumentation site.
Field data analysis
Pillar response . .
Donut crib response
Conclusions
References .

ILLUSTRATIONS

Conventional and mined headgate-chain pillar Panel tayouts bees
Gob development behind longwall face .

Chain pillar recovery using angled crossents tee
Layout of study panel beeeeee
Typical donut crib construction in angled erosscut .
Detailed area of instrumentation site
Donut crib shown with installed hydraulic flatjack .
Hydraulic flatjack arrangement in donut crib . .
Instrument readout station ..... see eeneeses
Stressmeter and flatjack location in pillars and donut cribs
Progression of stress change increases in pillars P1, P2, and P3
Stress profile across pillar P2 ............ 002 cee eee
Location of instrumented donut cribs in angled crosscuts .
}. Average stress change in donut cribs ............+0eeeeeee

PERE Sper aHayye

TABLE

1, Average load on donut cribs in angled crosscut .......... 0000 eec eee eeeeeeeee beeeeee

Page

RE ecanuuuenre

Boowmiarnsad4uusee


Report of Investigations 9444

Longwall Retreat of Gate Road Pillars

By J. M. Listak and E. R. Bauer

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
T S Ary, Director


previous studies conducted at this mine (8-9). The pre-
vious studies show that stress change behavior in the
abutment and yield pillar in the study panel was similar to
the behavior exhibited by pillars in this study during
normal longwall face retreat. When compared with data
from previous studies, there was no discernable difference
that would suggest abnormal behavior in the pillars.

Analysis of the stressmeter information from this site
did not reveal any conditions that could be interpreted as
out of the ordinary for this mine. Although visual obser-
vation showed that some rib sloughage and pillar degrada-
tion occurred, these occurrences were not unlike the usual
longwall gate road entry incidents that transpire during
Jongwall retreat.

Donut Crib Response

Hydraulic flatjack cells were located around the perim-
eter of cach of the- donut cribs to determine the load

500 T T T
P3 A
°
1,000 T T T
P2 B
KEY
= VWs 4 |
evws 5
soot =. VWSE
i
oe
a
4000 1 r 1
2 Cc
x
oO
n
8
4]
ec
z
5
D
1,000 q
a VWS 12
500 7
°
-200 “150 “100 “50 ° 50

Figure 11.—Progression of stress change Increases in pillars
P1, P2, and P3.

FACE POSITION, ft

experienced by the cribs in the angled crosscut and the
headgate intersection. The cribs were designated as C1,
C2, C3, and C4 and were located in the angled crosscut
from the belt entry to the entry adjacent to the longwall
panel as shown in figure 13. The disparity of readings
obtained from the three flatjack cells in each crib indicated
that the cribs experienced uneven loading. The uneven
loading could be attributed to irregular contact area
between the roof or floor, nonvertical crib attitude, or the
effects of wedging at the roof. Therefore, the readings
from the three cells were averaged to obtain one reading
for each crib. Figure 14 shows the average stress change
in the donut cribs relative to longwall face position. The
readings were also converted to pounds to obtain the load
on the cribs, The loading on the donut cribs relative to
the last 40 ft of face advance before the cribs were mined
out is summarized in table 1. The highest loading con-
dition, 68,817 Ib or 688 kips, occurred in crib C1. This
was expected because of its proximity to the intersection
of the belt entry and because of its location relative to
pillars P1 and P2. Although the highest loading was

900 Ro
KEY
800 Face position, f1
2-20 0-5
B 70r +15 x0
& e-10 910
ug 600 4
2
% soof 4
&
& aoof- 4
a
W 300+ 4
5 pool 4
10a, 0, 0, x9 2.00% 0 QT
7
o 24 6 8 1 2 4 6 18 20
VWS POSITION ACROSS PILLAR WIDTH, ft
Figure 12.—Stress profile across
Panel SS _Ditestion of mining

Donut crib plan cq

© qo
cl
Belt entry

rar

Figure 13.—Location of instrumented donut cribs in angled

crosscuts.