Figure 6. Pian view of stress distribution around pillaring
and cave with reference to a N 70° E direction of the
maximum horizontal stress

Figure 7 shows the original extraction plan. The
plan called for the pillar to be extracted from a room
developed from the R1 entry. This original panel
layout had crosscuts on 18.3 m centers with the
Jength of the extracted pillar of approximately 30 m.
The room length and extent of extraction into the
barrier was controlled by the reach of the continuous
haulage system which was 79 m.

Figure 7. Original pillar plan used in E15 panel

A cut back entry at the end of the room was mined
and supported to complete the pillar. This allowed
for adequate ventilation and complete pillar
extraction. Once the cut back was completed, the
mining sequence was to first cut into the barrier and
then extract the pillar. The pillar was mined from the
barrier toward the R1 entry. Only a smail pillar
remnant about 3.0 to 4.6 m wide was left along the

R11 entry next to the cave. With this system, a cave
quickly developed that was about 30 to 37 m wide
and that extended from the pillar remnant to the
barrier. At several intersections the cave did extend
into the R1 entry. However, with this plan, the cut
back entry was very difficult to mine because the
entry was subjected to severe roof damage from the
horizontal stress concentration. Figure 8 shows the
stress concentration that developed around the cut
back entry. After 6 pillars were mined using the first
plan, a second pillaring plan was developed to
address this issue.
N

High stress.
concentration

Figure 8. Forward stress abutment and concentration with
respect to original pllaring plan

Second Pillar Pian

The second pillar plan eliminated the cut back
entry, substituting a notch cut into the pillar parallel
to the R1 entry from the outby room prior to the pillar
extraction (figure 9). This notch allowed for a cut
through of the pillar during the early stages of
pillaring for ventilation and allowed for the full pillar
width to be mined. Normally, the notch would be
bolted with only 2 rows of bolts inby the rib line.
With this second pillaring plan, the horizontal stress
appeared to be concentrated near the notch and on
the inby half of the crosscut where stress damage in
the form of cutters still occurred (figure 10).
However, the intervening coal pillar between the
notch and the crosscut reduced the combined
affects of the two openings on the stress
concentration. During pillar extraction, the cuts that
mined into the notch, cuts 4 and 5 resulted in the
roof caving from the notch inby to the barrier. After
this cave, there were few problems encountered with
the extraction of the remainder of the pillar.


To rehabilitate the damaged roof, cribs, posts and
cable bolts were used as supplemental support in
the R1 entry and across the panel along the narrow
zone of damage. Similar, though not as severe
damage was seen outby the cave in E17. Again
supplemental support was used to control the stress
damage. In E17 the damage approached but did
not reach the mains.

Stress Reduction Zone

Stress relief began when the cave was across from
the test site and was competed to a large extent
when the cave was about 150 m inby. After the
cave passed, the stress change measurements
showed that there was a significant stress reduction
in the shale below the sandstone in the direction of
the cave that ranged from 5.5 to 8.3 MPa (figure 15).
Based on the original stresses measured in the
shale, this would represent between a 40 and 60%
reduction in the horizontal stress. At the 0.6 m level,
there was stress relief in both directions, though the
reduction parallel to the cave was only about 2 MPa.
Even in the sandstone at a depth of 6.1 m, there
was a Stress reduction of 8.3 MPa that represents
about a 20% reduction in the horizontal stress.
Further, evidence of the stress reduction is
by the fact that no roof falls occurred in
£15, E17, and E19 panels adjacent to the caves
during development of about 1,600 m of panel
length.
Although there are no quantitative measurements
as to the extent of the stress reduction zone that
. there are a number of qualitative
indicators that can be used to approximate the
distance. In both the E15 and E17 panels, there
was no increase in damage or any roof falls in the
L2 entry as compared to other entries. Because of
the lack of damage or any change in the level of
damage which occurred in the L2 entry, it appears
that some stress relief was occurring in that entry, a
distance of 67 to 76 m from the cave. Further, in the
13 panel, the panel adjacent to E15, almost 33% of
the L2 entry suffered stress damage including roof
falls. Another indication of the extent of stress relief
was that the stress damage in E17 was limited to the
R1 entry and pillar crosscut outby the E15 pillaring.
Little damage was seen inby where the E15
"advance and relieve” pillaring had begun. The
distance from the cave in E15 to the R1 entry in E17
‘was about 150 m. During retreat mining of panel
£13, in early December, test site instrumentation
monitored 0.7 to 1.4 MPa of stress change as the
pillar line passed. The distance from the E13 cave
to the test site was about 120 m. Therefore the
extent of the reduction zone appears to be between
75 to 150 m, sufficient to cover the panel width
(figure 16).

Observations in the E19 panel provide further
evidence that “advance and relieve” and retreat
mining generated some stress reduction in an
adjacent panel. The E19 pane! had little or no stress
damage while the cave lagged behind the pillaring.
This may indicate that the horizontal stress had
been reduced to some extent in E19 prior to mining
by both the “advance and relieve” cave and the
retreat mining cave in E17 that could have stress
shadowed a portion of the E19 panel. A similar
situation developed in the E17 panel where a
section of the panel appeared to be in a stress
shadow created by the “advance and relieve” cave
in E15. However, for the “advance and relieve”
sections in the three panels, only the last portion of
the E19 panel was in a stress shadow developed by
a retreat cave. Further, the use of the “advance and
relieve” method in the E17 and E19 panels assured
that the stress relieve zone would cover the entire

Because of the cave orientation with respect to the
stress field, the faces were not relieved immediately
but fell into the stress shadow 1 or 2 crosscuts
behind the face (figure 16). In much of the
development in the 3 panels, shielding the faces
was not critical since immediate stress damage did
not occur in the advanced workings. When this
damage did occur at the faces such as at the end of
E15, the damage was limited because the roof was
exposed to the high stresses for onty a short time.
Once behind the cave, the openings suffered no
further roof damage.

Cave Height and Stress Relief

Because the cave height in part controls the size of
the stress relief zone, it is important to establish the
geometric relationship between the cave height and
stress reduction zone. Obviously, no direct
measurements could be made on the cave height.
However, to obtain an estimate of the cave height,
measurements on the height of a long fall that had
occurred in the in the N 30° W direction were made.
The fall height from the top of the mined opening
was about 9.1 to 9.8 m resulting in a total opening
height between 10.7 to 11.3 m. This was assumed
to be the height of the cave in the “advance and
relieve” section. The height of the cave will be
limited by the bulking of the caved material and not
by the width of the cave as long as the cave width is
Sufficient to develop the failure (Bell, 1975). The
height of the cave will usually be between 2 to 10
times the mining height with the higher heights
achieved in weaker shales (Kendorski, 1993).

The width of the stress relief zone from the cave
appears to be between 75 to 150 m. With a cave
height of only 10.7 to 11.3 m this stress reduction
zone was 7 to 14 times the cave height. Further, the


Floor Heave

With the start of “advance and relieve” pillaring in
both panels E15 and E17, significant floor heave
developed. For these panels, the floor geology
consisted of 0.3 m to 0.6 m of shale underlain by 1.2
to 1.5 m of fireclay while the overburden averaged
210 m. Figure 12 shows typical floor heave that
occurred. Systematic roof to floor convergence
measurements were made in the intersections of the
belt, L1 and L2 entries along the length of each
panel.

“zg

Figure 12. Floor heave experienced in E15 panel

a
IN

CONVERGENCE, cm

o 20 40 «+60 80 100 120 140 160
TIME, days
Figure 13. Floor heave rates across E17 panel

The maximum floor heave occurred in both panels
during the first 245 m of "advance and relieve”
pillaring. In E15 the average heave rate in this panel
section was 0.6 cm/day, while in E17 the average
rate was 0.96 cm/day. These rates were maintained
for periods of up to 40 to 60 days before leveling off.
In places, 50 to 75 cm of floor heave resulted. The
floor heave occurred across the panel width and was
not confined to the area adjacent to the cave.
Figure 13 shows the pattern of floor heave over time
across the E17 panel 60 m inby the start of pillaring.
The accelerated rate of floor heave shown in figure
13 after 115 days is the result of retreat mining in the

panel. The variable initial rates between 20 and 30
days may reflect the failure of the hard shale layer.

Because of the floor heave, an evaluation was
made of the pillar stability and the pillar loads using
Analysis of Retreat Mining Pillar Stability (ARMPS)
(Mark and Chase, 1997). In the panels, the stability
factor of the pillars between the belt and R1 entry
was 1.16 while the average stress on these pillars
were 20 MPa. For these pillars, the crosscut
spacing was 18.3 m and entry spacing was 21.3 m.
The entry spacing for the pillar between the belt and
the L1 entry was 24.4 m.

The heave necessitated removal of the floor in
sections of the panels in both the belt and the L1
travel way outby the face. Because of the floor
heave problem, the pillaring plan and pillar and
panel design in the E19 panel were altered with the
third pillar plan discussed below.

Third Pillar Plan

To address the floor heave issue, a third pillaring
plan and a new panel design were developed and
implemented in panel E19. The size of the pillars in
the panel were increased by extending the distance
between crosscuts to 21.3 m. Instead of a pillar
remnant as was left in E15 and E17, a pillar was
now left between the cave and the R1 entry that was
on average 12.2 m wide. Figure 14 shows this third
pillaring plan. A double notch was used to assure
that a cut through was made during pillaring for both
ventilation and caving. Besides the change in pillar
dimensions, the width of the caved zone was
decreased to about 24.4 to 27.4 m.

Notch

yp,
j
;

Rt o 10
Ld

Figure 14. Pillaring plan in E19 that was changed because
of floor heave


Essentially, when the pillar was now mined, a
portion of the horizontal stress concentration was
now over a. section of the pillar to be extracted.
However, instead of being a problem for
development, the stress concentration now assisted
in generating a cave. Not completely outlining the
pillar by development mining at the notch was
another essential feature of this design. This
minimized or eliminated the potential for a roof fall at
this location that could develop and adversely affect
pillaring and face development. Later, the second
plan was adjusted slightly where a bigger pillar
remnant with an average width of 9 m was left along
the R1 entry. With the larger remnant, the R1 entry
was kept open, where previously the roof had caved
in some of the intersections.

Scale, m
° 10
—)

Figure 9. Second pillaring plan used in E15 and E17 panels

The second pillaring plan was used for 16
additional pillars in the E15 panel and for the
adjacent E17 panel where 25 pillars were mined
(figure 11). With E17, pillaring began just 75 m inby
the mains and 200 m outby the start of pillaring in
E15. Again, a pillar and a standing room were left
outby the cave.

Figure 10. Forward stress abutment and concentration
with respect to second pillaring plan

sc
‘4
rer)
Seal taeed
e me pr Mote
a) recto
Wider,

Figure 11. Plan view of panels E15, E17 and E19 showing
location of test site .


Forward Horizontal Stress Abutment

With cave initiation, a horizontal stress abutment
was created in front of the cave (figures 6 and 8).
The damage in the cut back entry which required a
pillar plan modification previously mentioned was
one indication of this abutment. Even with the
second pillaring plan, the outer half of the room used
to extract the pillar in front of the cave sustained
stress damage. However, the location of this
damage was generally predictable though requiring
at times longer bolts to be installed. Also, these
areas were subjected to these stress conditions for
only a short period of time which minimized the
damage that could be done.

This forward horizontal stress abutment was seen
by the increase in stresses as the cave front
approached and passed the test site. At 37 m from
the cave, stress increases ranged from 1.4 to 5.5
MPa. This stress increase began when the cave
was 60 m outby the test site. At the 0.6 m level, the
roof conditions did not allow this added stress to be
sustained which quickly dropped to show a stress
reduction. At other depths, the stress concentration
just sub parallel to the cave remained higher than
the original levels.

Some roof damage did occur in the faces inby the
cave, especially in the R1 entry where a cutter would
normally develop from the crosscut at the cave front
and progress inby toward the face of R1. About
74% of R1 received some stress damage in E15.
This again was an indication of the forward
horizontal stress abutment.

For the first 300 m of “advance and relieve”
pillaring in E15, this stress damage was largely
confined to the R1 entry. However, in the last 90 m
of the pillared section of the E15 panel, cutters and
guttering were seen in the other three entries. This
damage occurred both at the face and outby the
face in portions of the L2 entry that were not yet in
the stress shadow. After the “advance and relieve”
pillaring was halted to allow for connecting entries to
an adjacent panel to be mined, this damage
continued inby for the rest of the panel (a distance of
another 120 m). in this section of the panel,
guttering and severe roof damage occurred mainly
in the faces. A roof fall did occur in the belt entry 30
m inby the cave. Both the E11 and E13 panels that
were adjacent to E15 were terminated at the edge of
this zone because of severe roof damage and roof
falls. However, no roof falls occurred in the entries
adjacent to the cave and the stress damage did not
appear to get any worse. No roof control problems
occurred during retreat mining in E15.

Rear Horizontal Stress Abutment

Roof damage in the entries and crosscuts in the
panels just outby the cave was probably caused by
the rear horizontal stress abutment (figure 16). This
abutment did not advance with the mining, but
remained stationary. In E15, the damage consisted
of cutters that developed in the R1 entry from the
cave outby for a distance of approximately 60 m.
This stress damage was parallel to the length of the
cave. Stress damage also occurred in the crosscut
just outby the cave. A cutter ran across the belt
entry then into the crosscut from the belt entry to the
1 and L2 entries. This was a narrow zone about 3
m to 5 m wide of fairly severe roof damage that
crossed the entire panel at a right angle (figure 17).
Because the rear abutment was stationary, the
damage from this abutment appeared to be more
severe than that resulting from the forward
abutment. The damage parallel to the cave in the
R1 entry was probably caused by the concentration
of the maximum horizontal stress. However, the
stress damage across the panel may have in part
been caused by the minimum horizontal stress.

TOU.

YYYYY Yes,

Figure 16. “Advance and relieve” panel with cave showing
the forward and rear abutments and stress reduction zone

Figure 17. Rear abutment stress damage in L1 entry


“ADVANCE AND RELIEVE” MINING, A METHOD TO MITIGATE THE AFFECTS OF HIGH HORIZONTAL
STRESS ON THE MINE ROOF

D. R. Dolinar, T. P. Mucho, and D. C. Oyler
Nail Inst for Occuptn! Sfty and Health
Pittsburgh, PA, USA

J. Pablic
Marion Center, PA, USA

ABSTRACT

At a mine in central PA, high horizontal stresses
have caused tong running roof falls resulting in
hazardous conditions for the miners and the
premature abandonment of panels. Because of
these conditions, the mine requested assistance of
the National Institute for Occupational Safety and
Health (NIOSH) in utilizing the “advance and relieve”
method to mitigate the affects of horizontal stress on
the mine roof. “Advance and relieve” mining
involves the removal of a pillar or a portion of a pillar
during development, thereby creating a cave along
one side of the panel. Following cave development,
the horizontal stress is relieved over the workings
adjacent to the cave in the active panel. The mine
has used this system in three panels with a
significant reduction in stress damage and no large
roof falls.

INTRODUCTION

When ground control problems are caused by

horizontal stress, there are several approaches that
can be used to combat the conditions that develop.
One is to relieve or reduce the stress so that it
causes less damage to the rock in the immediate
vicinity of the mine openings. Horizontal stress relief
can be accomplished by intentionally caving the roof
that then in theory creates a stress shadow or a
zone of reduced stress. Other mine openings driven
in this stress shadow may suffer less damage. The
geometry and location of the reduced stress zone
depends on a number of factors including the cave
height and cave orientation with respect to the
horizontal stress direction.

Longwall mining generates a cave with an
accompanying stress shadow (Mark et al., 1998 and
Mucho and Mark, 1994). However gateroad

development is usually too far from the cave to be
affected by the reduced stress. There are also
zones around the longwall cave where stress
concentrations are generated from the horizontal
stress (Mark et al., 1998, Dolinar et al., 1996 and
Sue and Hasenfus, 1996). [In room and pillar
operations, retreat mining that develops a cave will
also create horizontal stress relief zones. However,
unless properly sequenced, development mining
may not be protected by the cave. In the worst
case, development could be through zones of
horizontal stress concentrations generated by the
cave, thus increasing the likelihood of encountering
stress related problems.

A stress shadow can also be created by using a
sacrificial opening or entry driven into the stress field
in advance of the adjacent entries. With the failure
of this opening, the following adjacent entries should
be protected from the effects of high horizontal
‘stress. However, it is difficult to mine and maintain a
sacrificial opening due to slower development and
substantial supplemental support, requirements. A
modification of the sacrificial entry concept was
conducted as an experiment at a mine in West
Virginia. In this experiment, the center entry of a 3
entry gateroad was mined in an arched configuration
to a height 4.1 m (Aggson 1988 and Aggson and
Mouyard, 1988). The entry was then lined with steel
sets and lagging. The roof subsequently failed and
created an opening with an effective height of 7.6 m
above the floor. The extent of the stress relief zone
was at least 25 m. This extensive stress relief zone
could not be explained by the rock acting as a
continuous, elastic, homogenous material.
However, underground observations and numerical
modeling, showed that slip on horizontal bedding
planes was a significant contributor in forming such
a large stress relief zone.


the 10" Intemational Conference on Ground Control
in Mining, Morgantown, WV, pp. 15-25.

Mark, C., and Chase, F. E., 1997, “Analysis of
Retreat Mining Pillar Stability (ARMPS)", Pittsburgh,
PA: U. S. Department of the Interior, Bureau of
Mines, IC 9446 Proceedings of the New Technology
for Ground Control in Retreat Mining, pp. 17-34.

Mark, C., Mucho, T. P., and Dolinar, D. R., January
1998, "Horizontal Stress and Longwall Headgate
Ground Control", Min Eng, pp. 61-68.

Mucho, T. P., and Mark,C., 1994, "Determining
Horizontal Stress Direction Using the Stress

Mapping Technique", Proceeding of the 13”
Intemational Conference on Ground Control in
Mining, Morgantown, WV, pp. 277-289.

Su, W. H., and Hasenfus, G. J., 1995, "Regional
Horizontal Stress and its Effect on Longwail Mining
in the Northem Appalachian Coalfield’, Proceeding
of the 14” Intemational Conference on Ground
Control in Mining, Morgantown, pp. 39-45.

Worotnicki, G., and Witopn, R. J., 1976, "Triaxial
"Hollow Inclusion” Gauges for Determination of Rock
Stress In-situ", Proceedings of the ISRM Sypm.
Investigation of Stresses in Rock, Advances in
Stress Measurement, Inst. of Eng Pub. No. 4.


extent of the stress relief can not be explained by
the response of a continuous, homogeneous, elastic
material. This implies that much of the stress relief
must come from movement and slippage along
bedding planes or other discontinuities (Aggson,
1988 and Aggson and Mouyard, 1988).

DESIGN CONSIDERATIONS FOR “ADVANCE
AND RELIEVE” MINING

Based on this investigation, there are a number of
ground control issues that must be considered when
applying the “advance and relieve mining method.
These issues include:

* Direction of horizontal stress- the maximum
horizontal stress at the mine was nearly
perpendicular to the panels. Therefore, the
right side of the panels were pillared to
maximize protection to development but the
faces were not stress relieved immediately.

. Roof geology- the immediate roof at the mine
consisted of a laminated shale that was
susceptible to stress damage.

« Extent of stress relief zone- the stress relief
zone extended across the width of the panel
for a distance between 75 to 150 m or
between 7 to 14 times the cave height.

° Forward horizontal stress abutment- a
pillaring plan was developed that could mine
into and advance this horizontal stress
concentration. For the successful plans the
pillar was not completely outlined by
development.

° Rear horizontal stress abutment-this stress
concentration was stationary and resulted in
locally severe stress damage that required
supplemental support and a_ sufficient
distance between the cave and the mains.

e Pillar layout and cut sequence- the extracted
pillar and cut sequence were across the cave
front. This minimized the amount of pillaring
in the stress concentration zone.

. Floor heave/ pillar size- with the weak floor,
significant amounts of floor heave occurred
because of the exposure time. However, a
pillar and panel design was developed that
significantly reduced the floor heave.

CONCLUSIONS
The “advance and relieve" mining system was an
effective way to minimize stress damage and to
prevent roof falls caused by high horizontal stresses.
With this system a significant reduction in the
horizontal stress in the panel behind the cave was.

achieved. However, to successfully apply this
mining method requires that a number of ground
control issues be addressed.

REFERENCES

Aggson, J.R., 1978. "Coal Mine Floor Heave in the
Beckley Coalbed An Analysis”, Pittsburgh, PA: U. S.
Department of the Interior, Bureau of Mines RI 8274.

Aggson, J. R., 1988,"Geomechanical Evaluation of
a Coal Mine Arched Entry", Soc of Min Eng.,
Littleton CO, 7 pp.

Aggson, J. R, 1988, “Mouyard DP.
Geomechanical Evaluation of a Coal Mine Arched
Entry", International Journal of Min and Geophy
Eng., pp. 185-193.

Barish, K., 1980, "Truss Bolting On-Cycle in Jane
Mine Lower Freeport Seam", ‘st International
Conference on Ground Control in Mining,
Morgantown, WV, pp. 1-10.

Bauer, E. R., 1990, "Cutter Roof Failure: Six Case
Studies in Northern Appalachian Coal Basin",
Pittsburgh, PA: U. S. Department of the Interior,
Bureau of Mines IC 9266.

Bell, F. G., 1975, "Site Investigations in Areas on
Mining Subsidence", Newnes-Butterworths, London,
168 pp.

Bickel, B. L., 1993, "Rock Stress Determination
from Overcoring-An Overview’, Pittsburgh, PA: U. S.
Department of the Interior, Bureau of Mines, Bulletin
694.

Chase, F.E., Mark, C., Mucho, T, P., Campbell, P.
L., and Holbrook, D. A., 1999, "The Advance and
Relieve Mining Method: A Horizontal Stress Controi
Technique", Proceeding of the 18” Intemational
Conference on Ground Control in Mining,
Morgantown, WV, pp. 300-308.

Dotinar, D. R., Tadolini, S. C., and Blackwell, D. V.,
1996, "High Horizontal Movement in Longwall Gate
Roads Controlled by Cable Support Systems”,
Proceedings of the 15” intemational Conference on
Ground Control in Mining, Golden, CO, pp. 497-509.

Enever, J. R., Wold, M. B., and Crawford, G. R.,
1990, "Hydraulic Fracturing for Rapid Low Cost
Stress Measurements in Underground Mines”, The
AUSIMM Proceedings, Parkville, Aus., pp. 19-24.

Kendorski, F. S., 1993, "Effect of High-Extraction
Coal Mining on Surface Subsidence and Ground
Water’, Proceeding of the 12” Intemational
Conference on Ground Control in Mining,
Morgantown, WV, pp 412-425.

Krupa, E. D., and Khair, W. A., 1991, “Assessment
of Underground Structural Design", Proceeding of


Using the ARMPS analysis, the pillar between the
R1 and belt now had a stability factor of 2.1, with an
average vertical stress of 12.4 MPa. The pillar
adjacent to the cave had a stability factor of 1.7 with
an average vertical stress of 15.2 MPa. The larger
pillar along the cave kept the R1 entry open for
ventilation. Although floor heave still occurred in
19, the average rate of floor heave was only 0.30
cmiday. This rate can be compared to that in panel
£21 a panel adjacent to E19 , which was at the
same depth and had essentially the same pillar and
panel design as E19 except that instead of pillaring
on advance, an R2 entry was developed. The floor
heave rate averaged 0.23 cmiday across the E21
panel. Essentially, the floor heave in E19 was only
slightly more than that expected from a panel with
no “advance and relieve” mining.

With a weak floor, floor heave must be considered
when adopting the "advance and relieve” method
because the openings will be subjected to an
increased vertical abutment from the cave for the life
of the panel. This additional time allowed for
increased floor heave. Further, this floor heave

well outby the face and was worse at the
start of the panel.

in the E19 panel during "advance and relieve”
pillaring, no stress damage was observed even at
the faces. Also, caving was not as complete as
before and lagged further outby the pillaring.
Typically the cave stayed behind the pillar line by
about 3 fo 5 crosscuts. Therefore, inby the cave, a
large mined out area 27 m wide by to 60 to 90 m
long remained open. After the first 6 pillars were
extracted, the final 18 pillars in the panel when
mined were behind the retreat cave in panel E17.

Extracted Pillar Layout and Cut Sequence
Direction

Besides the pillar plan developed to deal with the
forward abutment, the layout and direction of the
extracted pillar with respect to the cave and the
stress concentration generated by the forward
abutment also appears to be important to the
successful extraction of the pillar. In this case the
long dimension of the pillar was across the cave
front as was the direction of the cut sequence.

This pillar layout and cut sequence allowed for the
high stress concentration to be intersected only in
the cuts toward the back portion of the pillar furthest
from the R1 entry, cuts 4 and 5. After cuts,
there appeared to be less stress in the roof. If the
cave initiated as usual after these cuts, this would
further reduce the stress in the roof for the remaining
pillar cuts. Therefore, for the cuts after 4 and 5, the
roof may not have been subjected to the highest
horizontal stresses or full abutment stress thus

making extraction of the rest of the pillar easier
(figure 10). Also, with this pillar design, the high
stress concentration was kept in the outer portion of
the pillar extraction room and away from the rest of
the panel.

HORIZONTAL STRESS ABUTMENTS AND
REDUCTION ZONES

“Advance and Relieve” Stress Monitoring

The horizontal stress changes that occurred from
the “advance and relieve” mining in E17 were
measured by instruments placed at a test site in the
L2 entry of £15 panel. Figure 11 shows the location
of the test site with respect to E17 where the
distance from the cave to the instrumentation was
37 m. Two types of instrumentation were used, the
USBM 3 component borehole deformation gage and
the CSIRO hollow inclusion cell (Bickel, 1993 and
Worotnicki and Witopn, 1976). The borehole
deformation gages were located at depths of 0.6,1.5,
3.1, and 4.6 m and the CSIRO cells at 1.5 and 6.1m
in the roof. The 6.1 m cell was in the sandstone roof
unit.

Figure 15 shows the stress changes with time as
the panel approached, then passed the test site for
the gages at 0.6 and 3.1m. The cave front position
is also indicated on the graphs. On this figure, the
largest stress reduction is AP and the smallest AQ.
The orientation of the stress changes are given as
the azimuth to the smallest siress reduction
component AQ. A minus value represents a stress
reduction while a positive value represents a stress
increase. Although the data is not shown, at the 6.1
m depth there was a stress reduction of 8.3 MPa
after the cave had passed the test site with an
azimuth direction of 233 .

‘STRESS CHANGES. Mpa

ST aN is wae we Me am

Figure 15. Horizontal stress changes measured at the test
site. (A) 0.6 m depth and (b) 0.3 m depth. The azimuth
direction Is to 40.


both hydraulic fracturing and overcoring with the
USBM 3 component borehole deformation gage
(Bickel, 1993 and Enever et al, 1990). The hydraulic
fracturing was done with a minifrac system from an
underground borehole in the sandstone above the
coal at a depth of 7.9 m in E24 panel on the south
side of the E mains (Su and Hasenfus, 1995). The
horizontal stresses in the sandstone from this
measurement were the maximum horizontal stress
P= 44.2 MPa from N 87° E and the minimum
horizontal stress Q= 26.7 MPa from N 3° W.

Overcoring stress relief measurements were made
at a site in E mains just outby the entrance to E15
panel. Measurements were attempted at depths
from 4.6 m to 7.0 m. Only one measurement in the
shale was successful. The measurement in the
shale was at a depth of 5.2 m and the calculated
stresses were P = 13.6 MPa from N 78 W and Q =
11.8 MPa from N 12° E. An elastic modulus of 23.4
GPa was used to determine the stress. In the
sandstone, the core disked during overcoring.

Clearly a substantial stress was measured by

hydraulic fracturing in the sandstone that is further
indicated by the core disking (Aggson, 1978). The
measured horizontal stress in the shale is much
higher than the expected vertical stress of 5 MPa.
Surprisingly, the measurements in the shale found
that the horizontal stress was nearly uniform with
maximum stress only 10% greater than the minimum
stress. However, the directional nature of the roof
falls and the mapped stress damage features
indicates the existence of a more highly anisotropic
stress field. Therefore, based on all available
information, the maximum horizontal stress direction
was assumed to be N 70° E for design purposes.

THE “ADVANCE AND RELIEVE" METHOD

Because of the extent of roof falis and stress
damage, the mine decided to try "advance and
retieve” mining. This mining system would be used
on 3 panels on the north side of E mains, panels
£15, E17, and E19 (figure 1). The “advance and
relieve” method involved the extraction of a pillar
and a portion of the barrier on one side of the panel
during development. Figure 5 shows a portion of the
E15 panel with the right side of the panel being
pillared. The pillar and barrier were mined just outby
the faces. With the pillar extraction, a cave was.
created along the right side of the panel. In this
panel layout, the faces were driven 1 to 2 crosscuts
ahead of the cave.

LIU LIS Li

YY UY YY

Lr

uy Ae Lod

Belt

Figure 5. Plan view of E15 panel showing “advance and
relieve” pitaring

Panel Layout and Pillar Plan

The panels and cave were in the N 30°
W direction and therefore at a 100° angle or nearly
perpendicular to the maximum horizontal stress from
N 70° E. With this panel orientation, the maximum
zone of protection that should occur in front of the
cave would develop when the pillaring was
conducted on the right side of the panel. However,
the advancing faces would still not be stress relieved
because the stress shadow lagged one or two
crosscuts behind. At the Tanoma Mine, because the
roof falls and much of the stress damage usually
occurred several days after mining, stress relief of
the faces was not critical. Figure 6 shows a plan
view of panel E15 with the assumed stress pattern
that should develop around the cave in two
dimensions.

in E15, the “advance and relieve" mining did not
begin until after the panel had been advanced 275
m. At the Tanoma Mine for a 5 entry panel, entries
were designated from left to right as L2, L1, belt, R1
and R2. The R1 entry was adjacent to the cave
(figure 5). The first pillar that was outlined by the
“advance and relieve" system was not mined. This
allowed for the modification of the panel layout from
the original 5 entry to the 4 entry configuration with
extension of mining into the barrier.


10/-
sek =- | Shale
| BESSS9 sandstone
feet
ui
2
Pies Shale
pay
2
0

Figure 2. Roof geology in the E mains and panels
Ground Control Problems

In the E mains and panels, the mine has
experienced directional roof control problems that
are related to horizontal stress. The stress damage
includes long running roof falls and cutters. Roof
falls as long as 610 m and up to 9.1 to 10.7 m high
have occurred generally in the northwest or
southeast direction (figure 1). About 95% of the
fallen roof is in the N 30° W orientation. Figure 3
shows the amount of fallen roof normalized by the
drivage in a given direction. In the N 30° W direction
(150° azimuth), over 5% of the drivage has failed
while sightly over 2% of the drivage has fallen in the
N 40° W direction (140° azimuth). In the other
mining directions, less than 1% of the roof has
fallen. These roof falls have occurred on both
development and during retreat mining where falls
occur just outby the cave. When the roof falls
occurred during retreat mining the panels or portions
of the panels would often be abandoned.

Mapping of the stress damage also indicates the
directional nature of the roof problem by
documenting more subtle roof damage in addition to
roof falls (Mucho and Mark, 1994). This stress
damage is in the form of cutters or gutters (figure 4 ).
In the panels, most of the stress damage again
occurred in the N 30° W entries. In panel E13 about
23% of the drivage had stress damage in the form of
cutters or guttering. Ninety-five percent of this
damage was in the N 30° W entries with 45% of the
drivage in this direction suffering stress damage.

Although much of the stress damage occurred
outby the faces after the roof had been supported,
occasionally, damage occurred during face advance
or prior to installation of the primary support.
However, the majority of the roof falls occurred from

a few days to several weeks after an opening was
mined. About 50% of the roof falls in the N 30 W
direction occurred in the belt entry whose width was
usually wider than the other entries.

ROOF FALL LENGTH, pct

igure 3. Direction of failed roof normalized by drivage in a
given direction

Figure 4. Stress induced cutter roof failure

Direction and Magnitude of the Horizontal Stress

Initially, to determine the direction of the maximum
horizontal stress, the pattern of the roof falls was
used. The highest percentage of roof falls occurred
in the N 30° W direction (figure 3). This would
indicate a maximum horizontal stress from N 60° E.
However because there was no drivage between N
30° W and north, the actual maximum stress
direction could be more toward the east. Therefore,
the maximum stress direction could fall between N
60° E to N 80 E. It was assumed that the maximum
stress could not be any closer to the east-west
direction or to the N 50° E direction because of the
limited amount of falls perpendicular to those
directions. Therefore, a N 70° E direction was
selected as the direction of the maximum horizontal
‘stress for the design and evaluation of the "advance
and relieve" mining method.

To further define the maximum horizontal stress
orientation and determine the stress magnitude, in
situ stress measurements were also made using


Another approach that can be utilized especially in
room and pillar mining is to create a cave adjacent
to a panel or set of entries during development. This
mining system is called the "advance and relieve”
method. The first documented case of using the
“advance and relieve" mining method in the U.S.
was at the Sargent Hollow Mine, Wise County, VA
(Chase et al, 1999). The mine had experienced
poor roof conditions and directional roof falis
associated with high horizontal stress and roof
geology that was susceptible to stress damage. The
mine used the system on one pane! and on a portion
of a second panel successfully. The investigation
reached 4 main conclusions: (1) the roof control
problems associated with high horizontal stress at
this mine could be significantly reduced by "advance
and relieve” mining, (2) a knowledge of the stress
direction is critical when determining a panel
orientation and a pillaring plan, (3) the degree of
stress relief and the extent of the stress relief zones
is a three dimensional problem, and (4) the face
areas will be more quickly relieved the closer they
are to the pillared out workings, however, this will
reduce the safety zone that protects the working
faces from the cave.

The present study was conducted at the Tanoma
Mine, Tanoma Mining Company, Inc., a room-and-
pillar coal operation that was experiencing roof
cutters and long running roof falls caused by
horizontal stress. Because these roof conditions
created hazards for the miners, and caused several
productions panels to be abandoned prematurely,
the mine requested assistance from the National
Institute for Occupational Safety and Health (NIOSH)
in applying the "advance and relieve” method to
reduce the affects of the horizontal stress during
panel development. This paper presents information
on the application of the "advance and relieve"
system at the mine and discusses the important
ground control issues related to the method.

TANOMA MINE

The Tanoma Mine, is located about 16 km
northeast of Indiana, PA. For this study, the area of
interest is the E mains and panels that are shown in
plan view in figure 1. To date, 15 panels have been
driven south off the mains, generally in the S 30° E
direction, and 7 panels have been driven on the
north side of the mains in the N 30° W direction. To
develop the panels, a continuous haulage system is
used with crosscuts driven at 60° angles off the
entries. Panels are developed using either a 4 or 5
entry system. In the panels, crosscuts are on 18.3
m centers and entries are on 24.4 m centers. The
entry width is 5.5 m. The primary roof support is
either a 1.5 m, #5 or a 1.8 m, #6 fully grouted rebar
installed on a 1.2 m row spacing with 4 bolts per

tow. Supplemental support consists of 2.4 m, #6
fully grouted rebar and 1.52 cm and 1.78 cm
diameter cable bolts with lengths ranging from 3.7 to
6.1 m. In the mains, a shuttle car system is used for
haulage and the entries and crosscuts are driven at
90E angles. The overburden depth over the E main
section ranges from 180 to 270 m. Several mines in
the area have been noted for directional ground
control problems related to the horizontal stress
(Barish, 1980, Bauer, 1990 and Krupa and Khair,
1991).

Figure 1. Plan view of E mains and panels showing long
falls.

Geology

The Tanoma Mine is in the Lower Kittanning or B
seam and has a coal height of 1.2 m. The geology
of the immediate roof in the E mains and panels
consists of a gray or black shale to a depth of about
6.1 m. This shale is overlain by a competent
sandstone that is generally 0.6 to 0.9 m thick. In
areas, the shale will become silty especially above a
depth of 4.6 m in the roof and near the sandstone
unit. However, at times the sandstone may be
within 3.0 to 4.6 m or closer to the seam. Figure 2
shows a typical geologic section of the mine roof.