3 psi
ow
t
1

=
TT
4

Joi
0 20 40 60 80
INTO RIB,

DISTANCE ft

VERTICAL PRESSURE, 10’

FACE DISTANCE-DEPTH RATIO

0.45 0.36 0.27 0.18 0.09 0
“3 4 1 T T 7

o KEY
oS Cell Depth, ft

~ 3h © B10

Wi x B4 20

3 A BS 30

a a B2 40

= ot ° BI 63

= |=

5 |=

a 1k

g

z

20 n n 1 1

-500 -400 ~300 -200 ~100 0

Figure 16.—Forward abutment pressure at headgate site B.
Gap in pressure data occurred during relocation of instrument
monitoring station.

During panel 1 mining, coal ejected from the face
necessitated the wearing of protective clothing. Using the
stress site data and a method proposed by Lu (12-13) to
convert cell pressures to absolute ground stresses, it
appears that the mining-induced stresses are insufficient to
induce significant fracturing of the face. Figure 19 shows
a Mohr’s circle generated from conversion of cell pres-
sures to ground stresses versus failure envelopes developed
from testing of the Wadge coal seam. While the cell T25
vertical pressure reading is low compared with that of the
adjacent cells (figs. 15, 18) and could modify the ground
stress calculations, the calculation of nonfailure agrees with
other measurements that indicate that a significant yield
zone did not occur.

Horizontally oriented BPC’s were not installed with the
other cells at this site. Since Lu’s technique requires

15

T T T
KEY
2 Site A
4h 4 Site B 4
a © Site H
rs ® Site T
gab |
a
2
i
“24 4
L L Ll L
° 200 400 600 800 7,000

DEPTH INTO PANEL RIB, ft

Figure 17.—Stress profiles of panel rib at 0-ft face distance.

FACE DISTANCE-DEPTH RATIO

ul

2

BZ 0.45 0.36 0.27 0.18 0.09 0
B 2 T T T T

=

22

gS, KEY

ae © 125 BPV

g x 126 CPC

2 4 128 BPH

= 0 \ f 1 1

= 500 400-300 200-100 0
2

FACE DISTANCE, ft

Figure 18.—Forward abutment prossure of stress site at site
T. (BPV and BPH designate borehole pressure cells oriented to
measure vertical and horizontal pressures, respectively.)

horizontal pressure cell readings for each vertical cell,
calculation of ground stress for the other tailgate rib cells
was not made.

Yield Pillar Behavior

The panel 1 three-entry headgate included a 35-ft-wide
yield pillar adjacent to the panel and an 80-ft-wide abut-
ment pillar. The yield pillar was sized to yield with panel
1 mining. The abutment pillar was sized to withstand first
panel mining-induced loading yet yield with panel 2 min-
ing. Yield pillar studies included evaluation of pressure


Crosscut 75
7 19 Ti
123 7251728 129 T31 21 13 +5 Tio Ti7 115 113
724 126 «130 T22 «Tt 12 Ti 114 T12
wr [
a Vertical BPC
= Horizontal BPC
° PC
0 10 20 30 Crosscut 73 [
Scale, ft

Figure 8.—Instrumentation detall of site T.


affect yield pillar sizing. This approach was also used to
determine a recommended pillar size, a pillar where maxi-
mum core pressure equals tributary pressure, by solving
iteratively

o = keF09 109 (2),
2m

2. 3
and (¥_) - 6545 (asa (|,
2m 2m 2m

where

@

o, = tributary pressure.

Figure 21 shows the recommended yield pillar widths for
the rigid and yielding end conditions. At the in-mine 9-ft
pillar height and 0.30 Poisson’s ratio, the recommended
pillar width is about 26 ft. Figure 21 illustrates a com-
parison between the methods. Under yielding roof and
floor conditions, the methods calculated from the prop-
erties used nearly identical maximum yield pillar widths.
Both indicate that not only should the 35-ft-wide pillars
yield, but that they are near critical width. The rigid con-

19

assumption of rigid roof and floor does not represent in-
mine conditions.

Summarizing yield pillar behavior with the proviso that
poor response by two BPC’s makes firm conclusions more
difficult, it appears that (1) the 35-ft-wide pillars yielded
following face passage; (2) the pillars are probably near
critical width and could possibly be reduced in width to
induce, if wanted, earlier yielding; and (3) the presented
design techniques accurately estimate required pillar
widths. However, they are sensitive to coal properties and
may require site-specific testing to “fine tune" the design.

Load Transfer Mechanisms

Load transfer between chain pillars and from the chain
pillars onto panel 2 were investigated to determine panel
effects on the future panel 2 tailgate, to determine abut-
ment pillar loading resulting from yielding of the smaller
chain pillar, to assess roof behavior, and to correlate, if
possible, observed pillar loading to the suggested main
roof action.

Site H readings during panel 1 mining are summarized
in figure 22. The profiles were generated to show mining-

dition pillar widths should be less than 20 ft; however, the induced pressures during distinct phases: the forward
6
A B
sk KEY |. KEY 4
0-400 ft 4 400 ft
° 0 © 600 ft
a 350 ft ° 800 ft
ath A 400 ft 4 L 5 1,000 ft 4
.
wh
é 7 AS | 1
&
g
2 -
IE 4 - 4
8 CC ze eel Lees ee] Leos
Longwall pane! 2 Pillar abutment Yield pillar Longwai! panel 2 Pillar abutment Yield pillar

Figure 22.—Stress profiles at headgate site H.


deg
ft
in

m

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

degree pei pound (force) per cubic inch
foot pet percent
inch psi pound (force) per square inch

meter ton/ft? ton per square foot



16

4.5 1
B 30h 4
*
e
a
a
w
a
&
2 15h 4
&
1
0 Ny 1.5 Nh 3.0

NORMAL STRESS (Ny.Np), 10° psi

Figure 19,—Stress state from cell readings at 0-ft face dis-
tance. Slope of failure envelope corresponds to a friction angle
of 36°.

cell data and a comparison between in-mine and predicted
behavior.

Two yield pillars, sites H and B at crosscuts 75 and 40,
respectively, were monitored to compare design versus in-
mine behavior. Pillar pressures versus face distances are
shown in figure 20. Despite poor cell response from one
BPC installed in each pillar, the observed loading histories
suggest similar pillar responses.

The site H pillar indicated increased loading with ap-
proach of the face and reached maximum pressure when
the face was 300 ft outby. Maximum vertical pressure
(5,200 psi) was reached on the rib nearest panel 1 and
decreased with distance into the pillar. Between 300 and
500 ft, both ribs crushed out and the pillar retained a
stressed core, Cell H17 responded poorly, despite repairs
and repressurization, However, based on H18 and H19
data, the pillar appeared to respond elastically until
yielding and a central, stressed core remained.

Pillar B (fig, 20), also indicated an elastic response to
mining, prior to crushing out. Poor cell response (B7),
prevented determination of whether the pillar also contains
a stressed core. Although two BPC’s responded poorly,
the pillars generally indicate similar behavior. Cells H19
and B8, nearest panel 1, show their highest stresses at face

distances of 250 to 300 ft, and both pillars yielded by a
distance of 500 ft.

A previous study at another western U.S. coal mine ob-
served that very local conditions, e.g., variations in roof,
floor, and coal properties, influenced pillar yielding. Two
design approaches were compared to the in-mine design,
sized to yield with panel 1 mining. Results using Wilson’s
(14-15) expressions for both rigid roof and floor and for
yielding roof and floor conditions (equations 1 and 2,
respectively) were calculated as follows and are shown in

figure 214:

=2m 4
w 2eh is} @
and
a
We aye
‘leur
where W = yield pillar width, ft,

8
0

pillar height, ft,

4 = overburden pressure, ft,

P= Li - | . & art ye,
vk k

p = artificial edge constraint,

PB = failed coal uniaxial strength, 1 ton/ft?,
_ Lssing
~ T-sing’

and @ = friction angle, deg.

Figure 21 indicates the required yield pillar widths for
both the rigid and the yielding roof and floor conditions,
Since the roof consisted primarily of a black silty shale that
deteriorated with exposure to air and required mesh for
supplemental support and since the immediate floor did
not contain massive, stiff strata, the yielding roof-floor con-
dition most accurately simulates in-mine conditions, The
yielding condition curve indicates that the 35-ft-wide yield
pillar nearly matches the size required from the Wilson
(14-15) method. Pillar pressure data suggest the existence


22

Vdd

300

$ 2s . :
* 2 =o ey
rs} 2 ‘a,
i | | i l |
T T T T T T
2
Ze
s ge228
7 2 = ~ 5S 3 YY yy L
se
2 ~e UNZE
| | i | WAH 7
fe £ #@ 8 &@ © &

30d “FONVHO JUNSSIYd 1130 JOVUIAY

~100

headgate sites H and B. A, Face distances from -400 ft (face inby) to 500 ft (face

Figure 24.—Mean pressure Increases at

outby); B, outby face distances to 900 ft.


18

20

YIELD PILLAR WIDTH, ft

10
KEY
A Rigid .
° Yield Wilson (14-15)

0 | 1

F Chen (16-17)

————— a
A Critical rigid |
© Critical yield

© Recommended rigid

; o Recommended yield

7 8 9

10
PILLAR HEIGHT, ft

7 8 9 10

Figure 21.—Comparison of estimated yield pillar width, Wilson (A) versus Karmis-Chen (16-17) (B) approach.

of a stressed pillar core. Since the actual and required pil-
lar widths are so close, it appears that the 35-ft-wide pillar
may represent a critical or maximum yield pillar width.

Since one pillar indicated a stressed central core, a yield
pillar design method proposed by Karmis (16) and Chen
(17) was investigated to determine critical width. This
technique includes friction angle, Poisson’s ratio, and pillar
geometry functions to size both a critical yield pillar width
(one containing a central stressed core), and a "recom-
mended’ width (one having a peak stress equal to the trib-
utary pressure).

Using the following expressions, the same friction angle
(35.6), and a Poisson’s ratio range of 0.30 to 0.40, maxi-
mum or critical yield pillar and recommended widths were
determined (fig. 21):

i -48f, (3)

1-1] yhe 1075
W =2m)9.61+cos |= cos] 72° -
F [ass

where W_ = yield pillar width, in,
m = pillar height, in,
7 = overburden density, pci,
h = depth, in,
fk) = Kt,
and = f(u)_ = -0.028 + 0.057 » + 0.17 y?, where

» = Poisson’s ratio.

Figure 21 indicates that at the lower Poisson’s value:
(0.30) and corresponding to Wilson’s (14-15) yielding roof
and floor conditions, yielding should occur. The 0.40 ratio
case, corresponding to the rigid condition, indicates that
smaller pillars would be required, and as seen using the
previous approach, property variations can significantly


Main roof caving was investigated not only to character-
ize subsurface failures but also to correlate, if possible,
roof action with observed pillar behavior, especially load
transfers between the headgate chain pillars and between
the pillars and the adjacent panel 2. Since the TDR bore-
hole, surface subsidence line B, and the underground
instrumentation sites H and T (fig. 4), were not superim-
posed, the following assumption was made. Since surface
subsidence results from subsurface caving, similar surface
subsidence development curves should reflect similar sub-
surface caving. To test this assumption, subsidence
development curves were plotted for three stations located
over the panel 1 axis: SM 82, closest to the TDR bore-
hole; SM 79, in line with the subsurface instrumentation
sites; and SM 100, part of survey line B (fig. 4). Sub-
sidence development curves versus outby face distance and
elapsed time are shown in figure 13. Included with this
figure is the subsidence development curve for SM 106,
part of the survey line closest to the outby sites A and B.
Figure 13 indicates a very close agreement between the
three inby monuments—SM 79, SM 82, and SM 100—and
with SM 106 above the outby instrument stations. Since
the subsidence development curves agreed so closely, it
was concluded that the following roof action occurred
(fig. 12). With retreat of panel 1 (curves 1 and 2), arching
over the cavity and surface subsidence occur. With failure
of the Twentymile Sandstone (curve 3) and the overlying
strata (curve 4), the arch fails and surface subsidence
accelerates. Continued subsidence occurred as the gob
consolidated (curve 5).

100,

13

PRESSURE MEASUREMENTS

Mining-induced pressures were measured using encap-
sulated BPC’s installed in panel 1, the headgate and tail-
gate chain pillars, the adjacent panel, and in the solid
instrumentation sites immediately north of the tailgate
(fig. 3).

Pressure cell data analysis included (1) characterization
of the forward abutment, (2) in situ yield pillar behavior
and comparison to design techniques, and (3) yield pillar-
abutment pillar load transfer.

Forward Abutment Studies

The vertical component of the forward abutment was
measured using BPC’s installed in the panel 1 headgate
and tailgate ribs. Figures 14 through 16 show the meas-
ured pressures versus face distance. The tailgate rib also
included a stress site consisting of two BPC’s, one to
measure vertical pressure and the other to measure hori-
zontal pressure, and one cylindrical pressure cell (CPC) to
measure radial pressures.

Figures 14 through 16 include inserts of pressure pro-
files into the panel 1 rib at a 0-ft face distance. For com-
parison, these profiles are combined in figure 17. Meas-
urements generally agree with previous western U.S. coal
mine studies in that forward abutment pressures begin
when the face approaches to within 1/4 to 1/3 ft of the
depth, and at approximately 0.1 ft of the depth, there are
accelerated pressure increases (8-11). However, unlike

qgost a, T+ epee Ee
on ala y ¢ 20
Ps of
ZT ot 4 Fe 4
. * 4 KEY
3 © SM 82
a ° 2 SM 79
@ x 4 L & SM 100 4
© SH 106
z |3 2
3 xl 2 J La 4
a a
° °
A Xl
1 L 1 1 1
t) 1 2 3 + 5 0 100 200

FACE DISTANCE OUTBY SUBSIDENCE MONUMENTS, 10° ft

TIME OUTBY SUBSIDENCE MONUMENTS, days

Figure 13.Subsidence development versus face distance and time.


Solid coal .

Peer
ees

Sls stusalclai/a(velfaisadiseciae a

Faut zone Panel 1
ESS oa SOOERSASEDEES ECDBSSASERS SOCOCBBBEBOBSSOSBSSTERG|
Bi 2 iMPax 8 ie
hen mez z

6 etree
LEGEND
A. BK, TS, 6 hstrument a , 9 160-300
+ TOR borehole Scale, m
° MPBX borehole

Figure 3.—Mine layout and study site locations, headgate sites H and B and tailgate sites Tand A. (MPBX = multiple position bore-
hole extensometer.)

i U
Solid coal As

BER EEEEEEEES RESO ty

Ke[ela mv/an/a/e/s/a/a[a]ss]a(a] SEES SSCA

° :
LEGEND
Inst t sit
A, BH, T & Instrument sites 3 ‘60 300
+ TDR borehole Le
{0} MPBX borehole Scale, m
. Subsidence monument

Figure 4.—Instrumentation location detail of panel 1, surface and subsurface. (MPBX = multiple
position borehole extensometer.)


6. Dowding, C. H., M. B. Su, and K. M, O'Connor. Measurement of
Rock Mass Deformation With Grouted Coaxial Antenna Cables. Rock
Mech. and Rock Eng, v. 22, No. 1, 1989, pp. 1-23.

7. O'Connor, K.M., and T. Zimmerly. Application of Time Domain
Reflectometry to Ground Control in Potash Mining, Paper in
Proceedings of Tenth International Conference on Ground Control in
Mining, ed. by S. S. Peng (June 10-12, 1991). Dep. Min. Eng., Coll.
Miner. and Energy Resour, WV Univ., Morgantown, WV, 1991,
pp. 115-121.

8. Haramy, K. Y., and R. O. Kneisley. Comparative Study of West-
ern U.S. Longwall Panel Entry Systems. Paper in Proceedings of 30th
US. Rock Mechanics Symposium, ed. by A. W. Khair (June 19-22, 1989).
Balkema, 1989, pp. 125-132.

9. DeMarco, M. J,, J. R. Koehler, and P, H. Lu. Characterization of
Chain Pillar Stability in a Deep Western Coal Mine—Case Study. Soc.
Min. Eng. AIME preprint 88-76, 1988, 11 pp.

10. Haramy, K. Y., R. O. Kneisley, and J.P. McDonnell. Longwall
Face Bursts and Inadequate Caving: A Case Study. Paper in Proceed-
ings of Sixth International Conference on Ground Control in Mining, ed.
by S. S. Peng (June 9-11, 1987). Dep. Min. Eng., Coll. Miner. and
Energy Resour., WV Univ., Morgantown, WV, 1987, pp. 18-31.

11, Haramy, K. Y., and R. O. Kneisley. Yield Pillars for Stress
Control in Longwall Mines. Int. J. Min. and Geol. Eng,, v. 8, 1990,
pp. 287-304.

25

12, Lu, P. H. Mining-Induced Stress Measurements with Hydraulic
Borehole Pressure Cells. Paper in Proceedings of Twenty-Fifth Sympo-
sium on Rock Mechanics (Northwestern Univ., Evanston, IL, June 25-27,
1984). Soc. Min. Eng. AIME, Littleton, CO, 1984, pp. 204-211.

13. ___. A New Method of Rock Stress Measurement With Hy-
draulic Borehole Pressure Cells. Paper in Proceedings of International
‘Symposium on Rock Stress and Rock Stress Measurements, Stockholm,
Sweden (Sept. 1-3, 1986). CENTEK Publ, Lulea, Sweden, 1986, pp. 237-
mS,

14, Wilson, A. H. Research Into the Determination of Pillar Size:
‘An Hypothesis Concerning Pillar Stability. Min. Eng. (London), v. 131,
1972, pp. 409-417
. The Stability of Underground Workings in the Soft Rocks
of the Coal Measures. Ph.D. Thesis, Univ. Nottingham, United King-
dom, 1980, 204 pp.
16. Karmis, M., and G. Chen, An Investigation Into Yield Pillar
Behavior and Design Considerations. Paper in Proceedings of the
Multinational Conference on Mine Planning and Design, ed. by G. T.

‘Lineberry and K. F. Unrug (May 23-26, 1989). Univ. KY, Lexington,

KY, 1989, pp. 13-20.

17. Chen, G., and M. Karmis. Computer Modeling of Yield Pillar
Behavior Using Post-Failure Criteria. Paper in Proceedings of the
Seventh International Conference on Ground Control in Mining, ed. by
S. S. Peng (Aug. 3-5, 1988). Dep. Min. Eng., Coll. Miner. and Energy
Resour., WV Univ., Morgantown, WV, 1988, pp. 116-125.


INTRODUCTION

A geotechnical study was conducted at a western U.S.
coal mine to assess large-scale strata response during the
extraction of the initial longwall panel. This study included
surface and subsurface studies in conjunction with an in-
mine demonstration of an automated, mine-wide ground
control monitoring system. Details regarding the installa-
tion and operation of the monitoring system are presented
in reports by Hanna, Conover, and Haramy (J-3)3

Surface subsidence and subsurface measurements to
characterize main roof caving and pressure measurements
to describe pillar and panel response to longwall mining
were used in the geotechnical investigations. Of special

concern were caving of the mine roof, since both the im- ‘

mediate and main roof contained competent sandstone
units, and the in-mine behavior of the headgate chain

pillars, The three-entry headgate system included one
yield pillar, adjacent to the active panel and designed to
yield with first panel mining, and a larger abutment pillar,
sized to support mining-induced panel 1 loading yet yield
with second panel mining. The headgate chain pillars
were designed not only to provide support during mining
of the Wadge Seam but also, through controlled yielding
and resultant load transfer, to reduce potential stress-
induced problems in the subadjacent Wolf Creek Seams,
approximately 135 ft below the active seam and tentatively
planned for future mining (4).

This study was begun prior to the startup of panel 1;
ground control measurements have been monitored since
mining commenced.

MINE DESCRIPTION

The mine, located approximately 20 miles southwest of
Steamboat Springs, CO, was opened in 1983 and, following
a 1988 decision, converted from room-and-pillar to long-
wall mining. The first panel, under approximately 1,050 ft
of cover, was developed in the lowest portion of the mine
to facilitate drainage and pumping of ground water to the
surface. Panel 1, approximately 620 ft wide by 7,200 ft
long, retreated from west to east and was oriented at
approximately 45° to the major and minor joint sets to
facilitate roof caving (4).

Panel 1 included three-entry gate roads. The panel 1
tailgate utilized 80-ft-square pillars on 100-ft centers;
the headgate used a 35-ft-wide yield pillar and an 80-ft-
wide abutment pillar, both 80 ft long. The headgate
chain pillars were sized to increase coal recovery, to pro-
vide first and second panel mining entry stability, and to
minimize stress concentrations in the subadjacent Wolf
Creek Seams.

GENERALIZED STRATIGRAPHY, STRUCTURE, AND STRESS FIELDS

The mine is located on the south flank of a structural
basin, the southeastern most extension of the Washakie-
Sand Wash Basin of northwestern Colorado and south-
central Wyoming. The coal measures approximately 300 ft
thick, occurs in the Mount Harris Member of the Upper
Cretaceous Williams Fork Formation, and is bounded by
two massive sandstone beds: the Trout Creek and the
Twentymile (fig. 1). The coalbed being mined, the Wadge
Seam, is 8.5 to 10 ft thick. The immediate roof consists
primarily of mudstone and sandstone sequences. Of par-
ticular interest, because of potential roof caving problems,
is an up to 25-ft-thick sandstone bed, approximately 12 to
15 ft above the coalbed. The immediate floor consists of
bone coal, carbonaceous mudstones, channel sandstones,

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

and thin coals (4). The Wolf Creek Seams (135 ft below)
are being considered for future mining.

The geologic structure underwent two deformational
periods. The first period produced predominantly large
north-trending folds and associated strike-slip and reverse
faults. The second period produced generally northwest-
striking normal faults (fig. 2). Structurally, the seam is
regular, strikes N. 50° E., and dips N. 7° W., except near
the Oak Creek anticline where the strike is N. 70° E. and
the dip is 8° to the north, The topography parallels the
dip, and the overburden is constant over the longwall area,
1,060 to 1,100 ft thick. The seam exhibits prominent cleat-
ing; the face cleat is consistently oriented N. 45° to 70° W.
and dips both southwest and northeast. The less well-
defined butt cleat roughly parallels the secondary roof joint
set, striking at N. 40° to 60° E. The major joint system
strikes at N. 35° to 70° W., with one set dipping at 65° to


LARGE-SCALE STRATA RESPONSE TO LONGWALL
MINING: A CASE STUDY

By R. O. Kneisley’ and K. Y. Haramy?

ABSTRACT

This U.S. Bureau of Mines report summarizes a study of large-scale strata response to longwall min-
ing at a western U.S. coal mine. This study utilized surface and subsurface measurements, geologic
mapping, in situ stress measurements, and pressure cell readings to characterize strata behavior. Pre-
liminary analysis of surface subsidence and time-domain reflectometry (TDR) was used to determine
a suggested main roof caving sequence. Coal ejected from the face apparently resulted from brittle
failures that occurred because of lack of significant yield zone development. The combination of a
strong coal with pronounced directional behavior, low overburden pressures, a good caving roof, and
a high-production environment that minimized time-dependent loading apparently reduced yielding of
the longwall face. The panel 1 headgate-entry design appears adequate; however, the abutment pillar
and adjacent panel tailgate rib are highly stressed and may contribute to problems during second panel
mining. The small chain pillar yielded after passage of the face, but the pillar width may be near
maximum since evidence of a stressed core exists.

\Mining engineer.
Supervisory mining engineer.
Denver Research Center, U.S. Bureau of Mines, Denver, CO.


14

a
020 4060-80
DISTANCE INTO RIB, ft

VERTICAL PRESSURE, 10° psi
T
1

FACE DISTANCE-DEPTH RATIO

0.45 0.36 0.27 0.18 0.09 0
4 T T T T
. KEY
z Cell Depth, ft
bo H23 10
~ 20
a
3
&
g
&
2
0 L n L 1
500 400 =300 =200 100 0

FACE DISTANCE, ft
Figure 14.Forward abutment pressure at headgate site H.

previous studies that indicate, through rapid vertical pres-
sure losses, yielding at about 0.01 ft depth into the face, no
evidence for development of a significant yield zone at this
mine exists. For sites H and T, only the cells at the panel
edges (those installed at 10 ft inside the rib) indicate
yielding at the 0-ft face distance. The other site H and T
cells continued to increase and, as shown in figures 14
through 16 inserts, the panels depict an essentially elastic
stress distribution. The outby sites A and B indicated
continued pressure increases and did not show yielding,
even for the 10-ft-depth cells (fig. 17).

Figure 17 summarizes the "final" pressure readings ver-
sus cell depth into the panel 1 rib, While differences exist
between these final pressure profiles, they agree that the
longwall panel, to a depth of 100 ft within the ribs,
retained vertical pressure and did not yield as the face
approached. Yielding, where observed, was limited to
approximately 10 ft inside the panel rib. The data indicate
some differences between the inby sites H and T and the
outby sites A and B. The peak pressures at sites H and T

~
a

a

o
T
L

n
wn
T

a eT
0 20 40 60 80 100
DISTANCE INTO RIB, ft

VERTICAL PRESSURE, 10° psi

FACE DISTANCE-DEPTH RATIO
0.36 0.27 0.18 0.09 0
T T

0.45
5

T T

KEY
Cell Depth, ft

10° psi
we =
T

is

FORWARD ABUTMENT PRESSURE ,

1 L 1
~300 -200 -100 0
FACE DISTANCE, ft

0 L
-500 ~400

Figure 15.—Forward abutment pressure at tailgate site T.

exceeded those at sites A and B; however, these higher
pressures probably result from the fact that the 10-ft-depth
cells yielded and pressure redistribution within the face
occurred.

Forward abutment effects were also monitored from a
three-cell stress site installed in the panel 1 tailgate rib.
Measurements from the stress site, consisting of one CPC
and two BPC’s, are shown in figure 18. The stress site
data also indicate the absence of a significant yield zone
within the face; while the horizontal cell (T28) shows a
pressure drop just prior to cutting by the shearer, the
vertical cell (T25) continued to increase until the BPC was
cut.


SITE GEOLOGY AND PHYSICAL PROPERTIES

Included in this investigation was a geological study of
the four underground instrumentation sites (figs. 3-4), and
the retrieval of core for physical properties testing (ta-
bles 1-2). The roof strata were sampled from 10 bore-
holes drilled during site instrumentation; stratigraphic
columns for only the 2 headgate sites, middle entry roofs,
are shown in figure 5. The immediate roof, a black silty
shale varying in thickness from 1 to 3 ft, deteriorates with
exposure to air, separates along thin bedding planes, and
fractures into small blocks. Overlying the immediate roof
are four sandstone beds, designated as “A” through "D" in
ascending order. The A sandstone did not appear as a
distinct unit in the panel 1 boreholes. The C sandstone,
the thickest and most consistent unit, is present at the top
of three of the boreholes. The upper sandstone unit, D,
was not intersected in any of the boreholes.

Table 1.Summary of physical properties tests

Num-

Geological material and Mini- Maxi- Mean ber of
physical property mum = mum sam-
ples

Black shale:
Compressive strength ..... psi..
interbedded siltstone, black shal
Compressive strength ..... psi... 11,611 22,032 "Es}
32

1,441 ND ND

Young's modulus (E) ...10° psi 174 496 259

Poisson's ratio (y 0.14 0.46 0.33,
Interbedded sandstor

black shale:

Compressive strength ..... psi...

Young's modulus (E) ...10° ps

Poisson's ratio (u) ..
Sandstone:

Compressive strength ..... psi...

Young's modulus (E) ...10° psi

Poisson's ratio seeteese
Pyritic black sh

Compressive strength ..... psi... 13,704 17,762 ee

2

1.68 6.95 2.69)
019 0.45 0.34,

6,679 23,826 15,661
40

193 689 3.21

7,933 30,819 14,939)
26
0.12 0.48 0.31,

Young’s modulus (E) ...10° psi 2.42 3,08 2.75)
Poisson's ratio (u) .. 0.30 047 0.36,

ND No data.

A northwest-trending normal fault intersected panel 1
between crosscuts 35 and 36 in the headgate and between

crosscuts 39 and 40 in the tailgate. In panel 1, the fault
strikes at N. 26° W. and dips near vertical. The fault
planes are less than 10 ft apart, and the coal seam is offset
6 ft. In addition to movement along fault planes, move-
ment has occurred parallel to bedding planes in the roof,
as evidenced by slickensides. The slickensides can be ob-
served along a bedding plane, approximately 2 in above
the coalbed, and indicate nonuniform direction of move-
ment. Bedding plane movement is thought to have re-
sulted from folding, Shear movement has created gouge
zones and displacement within the Wadge coal seam; zone

_ widths vary from less than 1 in to up to 10 ft, and the

length varies from a few feet to 100 ft. Two shear zones
intersect the headgate near sites H and B.

Table 2.—Summary of tensile strength tests

Geologic material Mean value, Number
psi of samples

Black shale ... ND ND
Interbedded siltstone, black shale 765 1
interbedded siltstone, sandston

black shale . 557 19
Sandstone 718 10
Pyritic black shale 579 6

ND No data.

The coalbed exhibits a prominent face cleat, striking
northwest and dipping near vertical. The less well-defined
butt cleat trends northeast, with a vertical dip varying 6°.
Cleat spacing varied from 6 in to 2 ft, except in the shear
zones where spacing decreased to less than 1 in. Cleat
direction roughly parallels joint orientations and may in-
dicate that local uniform stresses may have induced the
formation of both the joints and cleats within the mine
area. Rib behavior is apparently influenced by the cleat
orientation. The north-south ribs, longwall face, and pillar
crosscut ribs remained relatively intact and free standing,
while the east-west ribs, those forming the panel ribs and
pillar entry ribs, exhibited a greater degree of sloughage.
While measurements of cleat shear strength were not
made, directionally related yield zone development in-
dicates that differences between the face and butt cleat
strengths exist.


20

abutment phase, -400 to 0 ft; side-abutment phase, to
approximately +400 ft; and a postmining phase, to 1,000
ft. The side-abutment phase was divided to investigate any
abutment pillar loading resulting from yielding of the
smaller, adjacent chain pillar.

Forward abutment effects were mainly confined to the
yield pillar and to the panel-side half of the larger chain
pillar. Smaller (up to 100 psi) increases were measured on
the panel 2 tailgate rib; these occurred on the outer 40 ft
of the panel. Forward abutment extent decreased with
distance from panel, and the panel entry system responded
elastically to panel 1 mining. Average yield pillar pressure
(mean of the BPC readings) increased from about 1,100 to
1,900 psi, a 73-pet increase in mean pressure. Abutment
pillar pressure increased from 1,375 to 1,700 psi, a 23-pet
rise over premining average BPC pressure. Panel 2
increases were on the order of 6 pet.

The side-abutment phase encompasses pressure changes
measured between the 0-ft and +400-ft face distances.
During this period, maximum yield pillar pressures oc-
curred prior to failure. Abutment pillar pressure in-
creases, due to main roof loading and load transfer from
the yield pillar, and the panel 2 tailgate rib-pressure
increases continued. Figure 22 includes two pressure
profile lines, the maximum yield pillar pressure at about
350 ft and a postyield pressure profile at 400 ft. The
preyield profile (350 ft) continued to show an elastic
response to panel 1 mining. Maximum pressure occurred
on the small pillar and decreased toward panel 2. Average
small pillar BPC pressure increased to about 3,875 psi, an
approximate 2,700-psi mean pressure increase (250 pet)
over premining pressure. The abutment pillar continued
to respond elastically, showing a mean BPC pressure of
2,200 psi, an 825-psi average cell pressure increase, or
100 pet over premining levels. The panel 2 tailgate
showed minimal loading, an approximate 200-psi, 18 pet-
increase over premining levels.

From 350 to 400 ft, the small yield pillar failed and
apparently transferred load onto the adjacent abutment
pillar and the panel 2 tailgate rib. During this period,
yield pillar cell readings decreased drastically; pressures
decreased about 4,000 psi on the rib nearest panel 1 and
to 0 psi on the farthest rib. Core pressure dropped to
3,200 psi, but eventually stabilized. Average yield pillar
cell readings dropped from a maximum 3,875 to 1,650 psi,
but were still higher than the premining level (1,100 psi).
Mean abutment pillar stresses increased to 2,830 psi, a
600-psi increase, and show loading of the pillar core.
Small increases (less than 200 psi) continued to
accumulate on the panel 2 rib.

With retreat of the face (fig. 22), yield pillar core
pressure stabilized at about 2,900 psi. Between 400 and
600 ft, the abutment pillar began to show evidence of rib
yielding. Readings inside the rib closest to the yield pillar

indicate a 900-psi drop that was accompanied by pressure
increases in the abutment pillar core. Except for minor
stress changes between the 800- and 1,000-ft distance
readings, abutment pillar stresses evidently stabilized at
800 ft. The panel 2 rib continued to experience pressure
increases; a maximum pressure of 2,300 psi was measured
at 20 ft inside the rib.

The headgate site B data are summarized in figure 23.
As seen at site H, the forward abutment induced an elastic
response to panel 1 mining. Pressure increases were
greatest on the small pillar and decreased with distance
from panel 1, Small chain pillar average cell pressure
increased to 2,540 psi, a 95-pet increase over the pre-
mining average (1,300 psi). Abutment pillar pressure

‘ increased to 1,830 from 1,630 psi, a pressure change in-

crease of 12 pct. Panel 2 tailgate rib loading was minimal,
a pressure change increase of 5 pet. The site B small
chain pillar yielded with face passage; however, unlike the
site H pillar, the postpeak stress phase lasted from about
250 to 550 ft. During this maximum pressure phase, yield
pillar cell pressures approached 7,000 psi just before
failing. With small pillar yielding, significant pressure
increases occurred on the abutment pillar; the average cell
pressure increased by approximately 800 psi, to 3,100 psi.
Average pillar cell pressure change at this time was about
1,500 psi, a 90-pet increase over premining average cell
pressure. With face passage to about 900 ft outby site B,
pillar loading stabilized. From 560 to 900 ft, pressure
changes were generally less than 100 psi. The abutment
pillar still exhibited very high rib stress (approximately
8,000 psi) and remained elastic. Panel 2 continued to
indicate high rib stress at 20 ft (2,800 psi). Figure 24
summarizes the pressure change percentages resulting
from the four phases: forward abutment, peak small pillar
pressure, postyielding, and postmining.

Figure 24 indicates good agreement for average cell
pressure changes between sites H and B, especially prior
to small chain pillar yielding, Previously discussed poor
cell response from two BPC’s may account for the larger
decrease in the site B yield pillar. Prior to yielding, the
sites exhibit very close agreement between average cell
pressure increases and the distinct mining phases: the
panel tailgate increases were 16 and 18 pct, abutment pil-
lar increases were 60 and 42 pet, and yield pillar increases
were 252 and 270 pet.

The previously presented analysis of main roof caving
action determined that several distinct caving phases
occurred (figs. 11-13), When the face was approximately
150 ft outby, the TDR data indicated that failure of the
Twentymile Sandstone had occurred and that complete
collapse of the cavity occurred when the face was about
350 ft outby. Additional retreat of the face resulted in
continued subsidence, probably due to compaction of the
gob.


: REPORT OF INVESTIGATIONS/1992

Large-Scale Strata Response to Longwall
Mining: A Case Study

By R. O. Kneisley and K. Y. Haramy

BUREAU OF MINES |



| Crosscut 75 |

Mt H2 H3 4 HS Ho H1O Hit Hi2 Hi3} H17 H18 H19) H23_H24 H25 H26 27

LEGEND
4 Vertical BPC

Crosscut 73

0 10 20 30
[on or
Scale, ft

Figure 6.—Instrumentation detail of site H.


— 20 ft
End of hole

I
Ss
a3 ‘HLdad

— 5 ft

— Oft

KEY

Black shale

Interbedded siltstone ond
black shale

Interbedded sandstone, siltstone,
and black shale

Sandstone

Pyritic black shale

Figure 5.—Stratigraphic column of headgate sites of middle-entry roofs.

DATA ANALYSIS

The evaluation of strata response to panel 1 mining
included characterization of main roof caving from TDR
readings and surface subsidence measurements and
mining-induced pressure changes from hydraulic pressure
cells installed at the instrumentation sites shown in fig-
ures 3 and 4, Of particular interest were the behavior of
the approximately 200-ft-thick Twentymile Sandstone, 700
to 750 ft above the coalbed, and the in-mine performance
of the headgate chain pillars. Also included were analyses
of mining-induced load transfers.

Underground instrumentation emphasized the use of
encapsulated borehole pressure cells (BPC’s) installed at
four sites to monitor mining-induced pressures in the pan-
el and adjacent chain pillars. Additional BPC’s were in-
stalled in the adjacent panel and in the immediately north,
solid coal to investigate load transfer; horizontal stresses
were monitored from cells installed in the immediate roof.
Figures 6 and 7 show detailed instrumentation layouts for
headgate sites H and B, respectively, and figures 8 and 9
show the tailgate sites, T and A, respectively.

Roof caving was monitored via TDR surveys of a co-
axial cable installed from the surface in a 1,050-ft-deep
borehole located over the instrumented panel. Location of
the TDR borehole and surface subsidence survey lines are
shown in figure 4.

MAIN ROOF CAVING ACTION

Main roof caving was monitored from TDR surveys of
a coaxial cable cemented into an approximately 1,050-ft-
deep borehole over panel 1; figure 10 shows a typical TDR
survey with a roof break. Recent developments in TDR
signature interpretation include differentiation between
tensile and shear failures (6). Tensile failure is indicated
by a small but extended voltage reflection trough prior to
the large, positive circuit reflection. Shearing, on the other
hand, produces a distinctive negative voltage reflection
spike prior to the large, positive open circuit reflection (7).
A previous study over a partially pillared, room-and-pillar
section at the mine determined that caving conditions were


85° SW. and the complementary set dipping at 80° to 90°
SE. (4). The in situ field was determined using the over-
coring technique (4-5). Figure 2 shows the test locations
and the magnitude and direction of the maximum and
minimum secondary stresses. Measurements from tests
performed in the main entries and closer to the portal
indicated the existence of excessive horizontal stress; that
is, stress levels exceeding values expected from Poisson’s
effect. Subsequently, the entries were developed at ap-
proximately 45° to both the geologic structure and to the
major principal stress direction (4). While these earlier
test results indicated a generally northwest-trending stress
direction, two test sets performed closer to panel 1 show
a nearly east-west stress direction. Whether this apparent
rotation of stress direction is due to influence of the fault
zone, more accurately represents the panel 1 stress state,
or results from other factors is unknown. Maleki (4) sug-
gests that the mine roof stress distribution is controlled
more by bed thickness than by elastic modulus. Possible
slippage along slickensided mudstone-sandstone interfaces
moderates stresses in the weaker roof zones and subse-
quently concentrates stresses in the more intact roof zones.
The resulting stress redistribution is thought to contribute
to minor cutter roof at the pillar corners (4).

Normal faults,

Lewis Shate

Twentymile Sandstone

Tongue of Lewis Shale

Wadge coal sean

Wolf Creek coal seam

Trout Creek Sandstone

0 200 400
uit J

Scale, ft

560 psi
th]
1,260 psi
673 psi
N84°w
| 875 psi
Ky Root
N N57°W ym
aN
N ‘786 psi
h Roof 200 psi
. 52° ’
600 psi
Roos-and-pi! lar Pit wall pit floor

workings

Figure 2.Geologic structure and in situ stress fields.

Figure 1.—Generalized stratigraphic column.


VERTICAL PRESSURE, 10° psi

7

6 T T T T T “Tv T T T T T
A
5b KEY 4
Cell Depth, ft
4b OHI7 =10 4
AHI8 = :19

28

3

2

i

0

7 T T T T T T T

B

6F KEY 4
Cell Depth, ft

Sr © B6 10 4
4 87 17

4b 28 4

1 L n L L L

0
-500 -400 -300 -200 -100 0 100 200 300 400 500 60
_FACE DISTANCE, ft _

Figure 20.—Vertical pressure readings in yleld pillars. A, Site H (crosscut 75); B, site B (crosscut 40).


two rigid pillars, and a headgate with both a rigid and
yielding chain pillar is shown in figures 25 and 26. Both
the headgate and tailgate entry systems were instrumented
prior to longwall mining, and the tailgate entries were not
subjected to any previous longwall mining influence,
Panel 1 mining induced significant loading on the tail-
gate rib of panel 2, but did not transfer significant load

23

pillars. The tailgate chain pillars responded elastically and
essentially isolated any panel 2 mining influence. The
headgate abutment pillar pressures exceeded those meas-
ured in the tailgate. Higher pillar pressures were at-
tributed to elastic stress increases because of being closer
to panel 1 than its panel 2 counterpart and stress transfer
resulting from yielding of the small, adjacent chain pillar

onto the solid coal adjacent to the panel 1 tailgate chain (1).

= yl 4 KEY | ob 4

2 4 0 ft

7 © 550 ft

& SP 17

2 ob L

a L
ao 0] [ooo oo] [oo «oo ° ecco] [ooo oo] [pod
Solid Tailgate chain pillars Ponel 2 Headgate chain pillars

Figure 25.—Panel 1 mining effect on tailgate and headgate pressure distribution at sites T (A) and H (6).

A 0 ft
© 550 ft

=o,

3

VERTICAL PRESSURE, 10° psi
o

pid

°

oe eo] Loo

23] 00

Doooy

Ze |

ooo oo]

Solid

Tailgate our pillars

Panel 2

Headgate chain pillars

Figure 26.—Panel 1 mining effect on tailgate and headgate pressure distribution at sites A (A) and B (6).


CONTENTS

Introduction
Mine description ............. ccc eee
Generalized stratigraphy, structure, and stress fields
Site geology and physical properties
Data analysis ...
Main roof caving action .
Pressure measurements .
Forward abutment studies ..
Yield pillar behavior .......
Load transfer mechanisms
Summary and conclusions . .
References ...........05

Generalized stratigraphic column
Geologic structure and in situ stress fields .
Mine layout and study site locations ......
Instrumentation location detail of panel 1. .
Stratigraphic column of headgate sites of middle-entry roofs .
Instrumentation detail of site H
Instrumentation detail of siteB ...
Instrumentation detail of siteT ...
Instrumentation detail of site A .......
10. Typical TDR survey with tensile roof break
11. Main roof caving of panel 1 versus face distance and time
12. Suggested sequence of main roof caving .......... .
13. Subsidence development versus face distance and time .
14. Forward abutment pressure at headgate site H .
15. Forward abutment pressure at tailgate site T .
16. Forward abutment pressure at headgate site B
17. Stress profiles of panel rib at 0-ft face distance
18. Forward abutment pressure of stress site at site T
19. Stress state from cell readings at 0-ft face distance
20. Vertical pressure readings in yield pillars
21. Comparison of estimated yield pillar width, Wilson versus Karmis-Chen srr
22, Stress profiles at headgate site H
23, Stress profiles at headgate site B .
24.
25,
26.

PRENAWAYwNE

. Mean pressure increases at headgate sites H andB ...
. Panel 1 mining effect on tailgate and headgate pressure distribution at sites T and
. Panel 1 mining effect on tailgate and headgate pressure distribution at sites A and B....

TABLES

an

Summary of physical properties tests
Summary of tensile strength tests

Nn


10

| Crosscut 44
AMS ADS AS AQT AIS AT Aig) At B
tr bie ote po
AG ANB M6 Ae a2
Key | Crosscut 43 [
4 Verticel BPC
Q Horizonte! BPC
Oo
° 10 » x | Crosscut 42 ]
Seale, ft

Figure 9.Instrumentation detail of site A.

Ay we AMS

[Ae AS Ato



12

SUBSIDENCE MONUMENT NUMBERS
57 58 59 60 61 99 100

SUBSIDENCE, ft
Gt is)

4

0
2 150 Shale
g 300
. Sandstone
5 450
zB 600
a
8 Shale
5 750
oe
& 900

Interbedded shale
1,050 and sandstone
' STIS AYES

Panel 2 : Panel 1
Figure 12.—Suggested sequence of main roof caving. Curves 1 and 2 represent roof movement due to initial caving as the face
passed; curve 3 results from failure of the main roof sandstone unit; curves 4 and 5 occur as the upper roof units subside onto the
consolidating gob.


Library of Congress Cataloging in Publication Data:

Kneisley, R. O. (Richard O.)
Large-scale strata response to longwall mining : a case study / by R.O. Kneisley
and KY. Haramy.

p. cm. — (Report of investigations / Bureau of Mines; 9427)
Includes bibliographical references (p. 24).
1. Rock mechanics—Colorado—Steamboat Springs Region. 2. Longwall mining—
Colorado—Steamboat Springs Region. 3. Time-domain analysis. I. Haramy,

Khamis Y. Il. Title. III. Series: Report of investigations (United States. Bureau
of Mines); 9427.

TN23.U43 [TN317] 622 s—dc20 —[622’.334] 92-10295 CIP



21

8
A B
a 4 L 4
Key
6+ ig -400 tt 1 b 4
° oft
a 250 ft
a 550 ft
. sk H L Key 4
s © 560 ft
oy © 600 ft
" 2 800 ft
i aL 1H L 6 900 ft 4
4
8 Lt L 4
2b “/ + L 4
sh _| 4
re eee] Lee ee TL > ny ie |
Longwa!! panel 2 Pillar abutment Yield pillar Longwal! pane! 2 Pillar obutwent Yield pillar

Figure 23.—Stress profiles at headgate site B. A Face distances from -400 ft (face inby) to 500 ft (face outby); B, outby face

distances to 1,000 ft.

The site H pressure profiles (fig. 22) indicate that
maximum yield pillar loading occurred at approximately
the same face distance (350 ft) that cavity collapse
occurred. This maximum loading may indicate the dis-
tance at which side-abutment effects extend behind the
face. Postfailure pressure profiles suggest that the abut-
ment pillar and panel 2 tailgate rib experienced stress
increases because of yielding of the small chain pillar
(fig. 22). Passage of the face from 400 to 1,000 ft resulted
in small increases on the panel 2 rib, but significant pres-
sure increases on the abutment pillar. Abutment pillar in-
creases apparently resulted from continued yielding of the
small chain pillar and stress redistributions, resulting from
yielding of the abutment pillar rib (fig. 22). Included in
the panel 2 study is a gob reloading site. While this pre-
liminary analysis suggests that side-abutment effects extend

to about 350 to 400 ft behind the face and that pillar
yielding increases stress on the abutment pillar and on the
panel 2 tailgate rib, firm conclusions regarding pillar stress
changes and load transfers between the pillars and the gob
will have to await the panel 2 gob pressure results.

In summary, the load transfer analysis suggests that (1)
maximum stress levels and main roof caving occur “simul-
taneously;" (2) this distance represents the side-abutment
extent; (3) small chain pillar yielding causes stress buildups
on both the abutment pillar and panel 2 tailgate; (4) the
tailgate rib is under increased stress because of panel 1
mining and maximum pressure increases occur at 10 to
20 ft inside the rib; and (5) the abutment pillar and
tailgate rib were highly stressed for panel 2 mining,

A comparison between the longwall-induced pressures
on the two-entry configurations, a tailgate supported by


Report of Investigations 9427

Large-Scale Strata Response to Longwall
Mining: A Case Study

By R. O. Kneisley and K. Y. Haramy

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

BUREAU OF MINES
T S Ary, Director


SUMMARY AND CONCLUSIONS

This study provided a unique opportunity to assess
large-scale strata response to first panel longwall mining,
As the instrumentation sites were designed to measure
large-scale behavior, detailed analysis of pillar stresses,
intersection stability, and immediate roof behavior were
not performed. Panel 2 mining past the most recently
installed sites, one including a gob reloading station and
the other in the fault area, should improve understanding
of any roof action-load transfer mechanisms and determine
whether the present abutment pillar size functions as
designed. Results of this study suggest that the large-scale
strata response at this mine is controlled by a relatively
shallow overburden with good caving characteristics, and
a strong coal with a well-defined face cleat.

Pressure measurements generally agreed with previous
western U.S. coal mine observations that forward abut-
ment effects begin at a distance about one-quarter to one-
third the depth ahead of the face, and that significant
increases begin when the face is about 0.1 ft depth away.
Unlike other studies, however, no evidence exists for a
significant yield zone at the face. The shallow overburden
(1,050 ft) and good caving characteristics of the main roof
apparently do not generate the stresses necessary to cause
significant yielding of the coal face. Another possible
contributing factor is the high face retreat rate (40 to 60 ft
per day) that minimized any time-dependent loading.

Yield pillar studies determined that the present pillar
width did not function as designed; panel 1 mining induced
yielding, but not until the face retreated outby. Mixed
evidence suggested that the yield pillars are near critical
width, contained a stressed core that later stabilized,
yielded without observed bumping, and the widths could
be reduced if the immediate roof can withstand additional
deflection. Two yield pillar design techniques indicated
that in-mine behavior could be simulated. However,
these methods are very sensitive to material properties,

and it is suggested that “fine tuning" a yield pillar size
should include, if possible, site-specific measurements and
properties.

Entry system pressure redistribution (load transfer)
resulted from the panel 1 side-abutment loading and small
chain pillar yielding, Maximum yield pillar loading oc-
curred at approximately the outby face distance at which
the main roof cavity collapsed. Subsequent failure of the
yield pillar increased pressure on the abutment pillar and
panel 2 tailgate rib, especially up to 20 ft within the rib.
Continued loading of the panel 2 tailgate rib and adjacent

_ abutment pillar apparently resulted from continued small

pillar yielding and observed yielding of the abutment pillar
rib closest to panel 1.

Panel 2 conditions are difficult to predict, but the
results indicate some potential areas of concern, Panel 2
tailgate stresses, especially within 20 ft of the rib, may
contribute to face bump severity. The abutment pillar is
highly stressed, and second panel mining could increase
the likelihood of pillar bumping. Actual conditions will
dictate the final pillar sizes, but this analysis indicates that
the present headgate design is adequate for first panel
mining. This study did not include a detailed roof behav-
ior site, and it may be that the final entry design will de-
pend on maximizing the stability of the lower 10 ft of slick-
ensided, easily fractured immediate roof. Narrower pillar
widths have the advantages of earlier stress transfers and
potentially lower tailgate stresses if the transferred stresses
are sufficient to induce panel 2 tailgate rib yielding. The
disadvantages include larger stress transfers onto the panel
2 tailgate if yielding of this rib does not occur and, pos-
sibly, increased roof problems due to increased deflection.
Wider pillars should reduce load transfer onto the tailgate
rib, but they create higher stress zones on the pillar edges
and could contribute to roof or rib failures, especially at
the pillar corners.

REFERENCES

1, Hanna, K, K. Y. Haramy, and T. Ritzel. Automated Longwall
Mining for Improved Health Safety and Productivity at the Foidel Creek
Mine. Soc. Min. Eng. AIME preprint 91-165, 1991, 8 pp.

2. Conover, D. P., K. Hanna, and'T. L. Muldoon. Mine-Wide Mon-
itoring Applications in Ground Control Research. Paper in Proceedings
of the Ninth International Conference on Ground Control in Mining, ed.
byS.S. Peng (June 4-7, 1990). Dep. Min. Eng., Coll. Miner. and Energy
Resour., WV Univ., Morgantown, WV, 1990, pp. 135-141.

3. Haramy, K. Y., W. C. Smith, and R. O. Kneisley. Automated
Ground Control Monitoring. Paper in Proceedings of the Third Con-
ference on Ground Control Problems in the Illinois Coal Basin, ed. by

Y. P. Chugh (Aug, 8-10, 1990). Dep. Min. Eng., South IL Univ., Car-
bondale, IL, 1990, pp. 227-235.

4. Malki, H., M. Hardy, and R. Tifft. Geomechanical Mine Design
of the Foidel Creek Mine, Paper in Proceedings of the Seventh Inter-
national Conference on Ground Control in Mining, ed. by S. S. Peng
(Aug, 3-5, 1988). Dep. Min. Eng., Coll. Miner. and Energy Resour. WV
Univ., Morgantown, WV, 1988, pp. 137-148.

5. Bickel, D. L, and D. A. Donato. In Situ Horizontal Stress
Determinations in the Yampa Coalfield, Northwestern Colorado, Bu-
Mines RI 9149, 1988, 43 pp.


B21 822 B23 B24 B25
$4

KEY

a Vertical BPC

0

10 20 30
Scale, ft

Crosscut 41

B16 B17 B18 §=— B19 B20 B8 87 BE
fl ra eae
Crosscut 40

Figure 7.Instrumentation detail of site B.

85 B4 B3 B2

BI


favorable. The approximately 10-ft-thick C sandstone unit,
10 to 15 ft above the coalbed, caved readily, and the main
roof, with additional pillaring, collapsed. Based on these
TDR surveys, this previous study indicated that early face
support loads, corresponding to an 80-ft-thick roof, could
be expected (4).

Data from the TDR surveys over panel 1 indicated that
roof movement began almost immediately after passage of
the face and was essentially completed within 2 weeks
after mining (fig. 11). Six distinct caving events were
observed. Events 1 through 3, at depths of 720, 640, and
540 ft, within the Lewis Shale began caving soon after face
passage. This lower roof caving phase was completed
within 6 days after face passage, a face distance of about
130 ft. The fourth distinct cave, 240 ft deep, on day 7
marked the failure of the 150- to 200-ft-thick Twentymile
Sandstone. This failure occurred when the face had
retreated approximately 150 ft outby. Total retreat at this
point was about 600 ft. Subsequent failures in the over-
lying shale members occurred until the face retreated ap-
proximately 350 ft outby the TDR borehole; readings were
terminated at this time. Included with figure 12 are sub-
sidence data from surface monument SM 82, closest to the
TDR borehole. With retreat of the face, subsidence accel-
erated as the cave zone ascended, especially after the fail-
ure of the Twentymile Sandstone and the overlying strata.
During the surface subsidence monitoring period shown in
figure 12, a maximum subsidence of 2.98 ft was measured;
during panel 1 mining, SM 82 subsided over 4 ft.

0
200 4
400 :
= 600k 4
& Roof
° break
800 4
1,000 4
1,200
POSITIVE VOLTAGE
REFLECTION

Figure 10.—Typical TDR survey with tensile roof break.

GT T T

200

fe

SN
7

‘SUBSIDENCE AND CAVING DEVELOPMENT, ft

N
N
\ 4
N ‘
600}- \
4 ‘ 42
\
KEY ‘
O TR \
800/- A Subsidence \
Twentymi le Sandstone \
4 1 1 1 1 WN 3
L
1,000, ito ats 300400 10 5 20 %

FACE DISTANCE OUTBY TDR, ft

TIME QUTBY TOR, days

Figure 11.—Main roof caving of panel 1 versus face distance and time.

11

‘SURFACE SUBSIDENCE, ft