UTILIZING THE "ADVANCE AND RELIEVE" METHOD

TO REDUCE HORIZONTAL STRESS AFFECTS ON

THE MINE ROOF, A CASE STUDY

Dennis R. Dolinar, Mining Engineer

Thomas P. Mucho, Branch Chief

David C. Oyler, Mechanical Engineer

National Institute for Occupational Safety and Health

Pittsburgh, PA

Jim Pablic, Mine Superintendent

Tanoma Mining Co.

Marion Center, PA

ABSTRACT

A room and pillar coal operation in central Pennsylvania was

experiencing roof cutters and long running roof falls caused by

high horizontal stresses. The roof conditions created hazards for

the miners, and caused several production 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" mining to reduce the affects of the

horizontal stress during panel development. 

The "advance and relieve" plan that was developed called for

removing a pillar on one side of the panel as it was being

advanced, thus creating a cave. The caving then relieved a

portion of the horizontal stress across the panel. Because the

horizontal stress direction is key to the success of this method,

stress mapping as well as mining experience was used to establish

the direction of the horizontal stress. Subsequent field

measurements provided an estimate of the stress magnitude.

Three panels were mined using this technique, and good roof

conditions were achieved in all these panels.

Instrumentation was used to monitor the stress changes in the

roof created by the cave. Stress relief of over 1,000 psi was

measured 120 ft from the cave and to depths of 20 ft into the roof.

The lateral extent of the stress relief zone across the panels

appears to be about 400 to 500 ft. 

This case history has provided much insight into the practical

application of the "advance and relieve" method discussed in this

paper. 

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 such approach is to relieve or

reduce the horizontal stress magnitude. With a reduction of the

horizontal stress, less damage should be done to the rock in the

immediate vicinity of the mine openings. Horizontal stress relief

can be accomplished by caving or by the failure of an entry or an

opening that then creates a zone of reduced stress or a stress

shadow. Other mine openings driven into this stress shadow will

see less horizontal stress and therefore suffer less damage. The

geometry and location of the reduced stress zone will depend in

part on the height and the orientation of the failure or cave with

respect to the horizontal stress direction. 

Longwall mining generates a cave with an accompanying

zone of horizontal stress relief (1, 2). However development

mining is usually too far from the cave to be in the stress shadow.

There are also zones around the cave where stress concentrations

are generated from the horizontal stress (1, 3, 4). 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

or worse, could be through zones of horizontal stress

concentrations generated by the cave thus increasing the chance

and severity of 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

entries could be protected from the affects of high horizontal

stress. However, mining and maintaining a sacrificial opening

can be difficult 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 13.5 ft

(5, 6). The entry was then lined with steel sets and lagging. The

roof subsequently failed and created an opening with an effective

height of 25 ft above the floor. The extent of the stress relief zone

was at least 80 ft. 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 horizontal bedding planes were a

significant contributor in forming such a large stress relief zone.
North

E13

E Mains

E15

E17

E24

LEGEND

0 500 1,000

Scale, ft

Roof falls

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

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 (7). The mine had

experienced poor roof conditions and directional roof falls

associated with high horizontal stress and roof geology that was

susceptible to stress damage. The mine used the system on one

panel and on a portion of a second panel successfully. From this

investigation, there were 4 main conclusions including: (1) the

roof control problems associated with high horizontal stress could

be significantly reduced by the "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 production 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" mining 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. 
30

25

20

15

10

5

0

Shale

Shale

Sandstone

DISTANCE, (ft)

Figure 2. Roof geology in the E-mains

and panels

0

0

1

2

3

4

5

6

30 60 90 120 150

AZIMUTH, deg

ROOF FALL LENGTH, pct

N30 Wo

Figure 3. Direction of failed roof normalized by

drivage in a given direction

TANOMA MINE

The Tanoma Mine, is located about 10 miles 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 60 ft centers and

entries are on 80 ft centers. The entry width is 18 ft. The primary

roof support is 5 ft, # 5, or 6 ft, #6 fully grouted rebar with 4 ft

row spacings and 4 bolts per row. Supplemental support consists

of 8 ft, #6 fully grouted rebar and 0.6 and 0.7 in diameter cable

bolts with lengths ranging from 12 to 20 ft. In the mains, a shuttle

car system is used for haulage and the entries and crosscuts are

driven at 90° angles. The overburden depth over the E main

section ranges from 600 to 900 ft. Several mines in the area have

been noted for directional ground control problems related to the

horizontal stress (8, 9, 10).

Geology

The Tanoma Mine is in the Lower Kittanning or B seam and

has a coal height of 46 in. The geology of the immediate roof in

the E mains and panels consists of a gray or black shale to a depth

of about 20 ft. This shale is overlain by a competent sandstone

that is generally 2 to 3 ft thick. In areas, the shale will become

silty especially above a depth of 15 feet in the roof and near the

sandstone. However, at times the sandstone may be within 10 to

15 ft or closer to the seam. Figure 2 shows a typical geologic

section of the mine roof.

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 2,000 ft and up to 30 to 35 ft high

have occurred generally in the northwest or southeast direction

(figure 1). About 95 pct 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,

over 5 pct of the drivage has failed while a little over 2 pct of the

drivage has fallen in the N 40° W direction. In the other mining

directions, less than 1 pct 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. A little over 40 pct of the

drivage is in the N 30° W direction while about 20 pct of the

drivage is in both the east-west and N 30° E direction. 

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 (2). This stress damage is in the

form of cutters and gutters (figure 4 ). In the panels, most of the

stress damage again occurred in the N 30° W entries. In one

panel (E13) about 23 pct of the drivage had stress damage in the

form of cutters and guttering. Ninety-five percent of this damage

was in the N 30° W entries with 45 pct of the drivage in this

direction suffering stress damage. 

Although much of the stress damage occurred after the roof

was supported outby the faces, occasionally, damage occurred

during face advance or prior to installation of the primary

support. However, the commencement of the roof falls varied

from a few days to several weeks after an opening was mined.

About 50 pct of the roof falls in the N 30° W direction occurred

in the belt entry whose width is usually wider than the other

entries.
Figure 4. Stress induced cutter roof failure

L2

L1

Belt

R1

N

N70 Eo

100

Scale, ft

0

Figure 5. Plan view of E15 panel showing

"advance and relieve" pillaring

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 occurs in the N 30° W direction

(figure 3). This would indicate a maximum horizontal stress from

N 60° E. However because there is no drivage between N 30° W

and north, the actual maximum stress direction could be more

toward the north. Therefore, the maximum stress direction could

fall between N 60° E to N 80° E. It is 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 both hydraulic fracturing and overcoring

with the USBM 3 component borehole deformation gage (11, 12).

The hydraulic fracturing was done with a minifrac system from an

underground borehole in the roof (4). These measurements were

made in the sandstone at a depth of 26 ft in E 24 panel on the

south side of the E mains. The horizontal stresses in the

sandstone from this measurement were the maximum horizontal

stress, P= 6,410 psi from N 87° E, and the minimum horizontal

stress, Q= 3,870 psi 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 15 to 23 ft. Only one measurement in

the shale was successful. The measurement in the shale was at a

depth of 17 ft and the calculated stresses were P = 1,975 psi from

N 78° W and Q = 1,710 psi from N 12° E. An elastic modulus of

3.4 million psi are used to determine the stress. In the sandstone,

the core disced during overcoring.

Clearly a substantial stress was measured in the sandstone that

is further indicated by the core discing (13). The magnitude of the

horizontal stress in the shale is much higher than the vertical stress

that could be expected at a depth of 700 ft while the ratio of the

maximum to minimum stress is only 1.1. However, without

confirming measurements, these results may just reflect a local

stress condition. Because of the directional nature of the roof falls

and the mapped stress damage features, the maximum horizontal

stress was assumed to be N 70° E for design purposes.

THE "ADVANCE AND RELIEVE" METHOD

Because of the extent of roof falls and stress damage, the mine

decided to try "advance and relieve" mining. This mining system

would be used on 3 panels on the north side of E mains, panels E15,

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. 

Panel Layout and Pillar Plan

The panels and cave were developed 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 direction,

to maximize the zone of protection that could occur in front of the

cave, pillaring was conducted on the right side of the panel.
N

Stress

concentration

Stress

reduction

Stress

concentration

Figure 6. Plan view of stress distribution around pillaring and

cave in panel with reference to a N 70° E direction of the

maximum horizontal stress

6

6

5

5

4

4

3

3

2

2

1

1

7

7

8

8

9

9

10

10

12

12

13

13

14

14

15

15

16

16

17

17

11

11

R-1

Room

1

Legend

 Cut sequence

 Posts

0 40

Scale, ft

Figure 7. Original pillaring plan used in E15 panel

P

P

N

N70 E

o

Cave

17

16

15

13

12

14

11

10

9

8

7

1

2

3

5 4

6 High stress

concentration

Forward

abutment

Figure 8. Forward stress abutment and concentration

with respect to the original pillaring plan

However, with this panel layout, the faces would still not be stress

relieved because the stress shadow lagged one or two crosscuts

behind the faces. Figure 6 shows a plan view of panel E15 with

the assumed stress pattern that develops around the cave in two

dimensions. Because the roof falls and much of the stress damage

did not occur immediately, stress relief of the faces was not

critical. 

In E15, the "advance and relieve" mining did not begin until

after the panel had been advanced 900 ft. During panel

development, the two left side entries are designated L1 and L2

with L2 adjacent to the barrier , the belt is in the center while the

first entry to the right of the belt is the R1 entry. 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.

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 60 ft centers

with the length of the extracted pillar of approximately 100 ft.

The room length and extent of extraction into the barrier was

controlled by the reach of the continuous haulage system which

was 258 ft. 

A cut back entry at the end of the room was mined and

supported to outline 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

then extract the pillar. The pillar was then mined from the barrier

toward the R1 entry. Only a small pillar stump about 10 to 15 ft

wide was left along the R1 entry next to the cave. With this

system, a cave quickly developed that was about 100 to 120 ft

wide that extended from the pillar stump 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 this plan, a second pillaring plan was

developed to address this issue.

Second Pillar Plan

The second pillar plan that was adopted and used for the

remainder of panel E15 and the adjacent panel E17 is shown in

figure 9. Instead of making a cut back entry, a notch parallel to the
1

2

3

7

8

9

10

11

12

13

14

Cave

Forward

abutment

High stress

concentration

P

P

N

N70 E

o

5

6

4

Figure 10 Forward horizontal stress abutment and

concentration with respect to the second pillaring plan

12

11

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

10

10

11

13

14

12

1

1

R1

Room

Notch

14 13

1

Legend

 Cut sequence

 Posts

0 40

Scale, ft

Figure 9. Second pillaring plan used in

E15 and E17 panels

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

location of test site

R1 entry was cut into the pillar from the outby room prior to the

pillar extraction. 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. Figure 10 shows the stress

concentration developed around the notch. 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.

After this second plan was adopted, another adjustment was

made where a bigger pillar stump with an average width of 30 ft was

left along the R1 entry. With the larger stump, the R1 entry was

kept open, where previously the roof had caved in some of the

intersections. 

This new pillaring plan was now used for the rest of panel E15

encompassing 16 pillars and the adjacent E17 panel where 25 pillars

were mined (figure 11). With E17, pillaring began just 250 ft inby

the mains and 650 ft outby the start of pillaring in E15. Again, a

pillar and a standing room were left outby the cave.

Floor Heave

With the start of "advance and relieve" pillaring in both panels

E15 and E17, significant floor heave developed. Over the panels

the floor consisted of 1 to 2 ft of shale underlain by 4 to 5 ft of

fireclay while the overburden averaged 700 ft. 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.
Figure 12. Floor heave experienced in E15 panel.

0

5

10

15

20

25

30

35

20 40 60 80 100 120 140 160

CONVERGENCE, in

TIME, d

L2

L1

B KEY

Figure 13. Floor heave rates across E17 panel

7

8

9

10

11

12

13

14

4

5

6

1

2

3

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Notch

Room

R1

1

Legend

 Cut sequence

 Posts

0 40

Scale, ft

Figure 14. Pillaring plan in E19 that was

changed because of floor heave

The maximum floor heave occurred in both panels during the

first 800 ft of "advance and relieve" pillaring. In E15 the average

heave rate in this panel section was 0.24 in/day, while in E17 the

average rate was 0.38 in/day. These rates were maintained for

periods of up to 40 to 60 days before leveling off. In places, 20

to 30 in of floor heave resulted. This 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 200 ft 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) (16). 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 2,900 psi. For these

pillars, the crosscut spacing was 60 ft and entry spacing was 70 ft.

The entry spacing for the pillar between the belt and the L1 entry

was 80 ft.

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 new design

discussed below. 

Panel E19

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 70 ft. Instead of a pillar stump as was

left in E15 and E17, a pillar was now left between the cave and the

R1 entry that was on average 40 ft 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 for caving.

Besides the change in pillar dimensions, the width of the caved zone

was decreased to about 80 to 90 ft.

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

1,800 psi. The pillar adjacent to the cave had a stability factor of

1.7 with an average vertical stress of 2,200 psi. The larger pillar
10/17 11/14 11/28 12/12 12/26 1/9 1/23 2/6 2/20 3/5 10/31 3/19 4/2

Date

AZIMUTH, deg

STRESS CHANGES, psi

0

90

180

270

360

0

0 400 800 1,200

-1,400

-1,400

90 -1,000

-1,000

180 -600

-600

270 -200

-200

200 360

200

A

B

Direction

Cave position, ft

KEY

 Q

P

Figure 15. Horizontal stress changes measured at the test site. 

(a) 2-ft depth and (b) 10-ft depth. The azimuth direction is to

)Q

along the cave prevented the R1 entry from falling so that the

entry remained open for ventilation. Although floor heave still

occurred in E19, the average rate of floor heave was only 0.12

in/day. 

In the panel during "advance and relieve" pillaring no stress

damage was observed. 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 to 5 crosscuts. Therefore,

inby the cave, a large mined out area 90 ft wide by to 200 to 300

ft long remained open. 

HORIZONTAL STRESS ABUTMENTS AND

REDUCTION ZONES

Monitoring of Stress Changes from the Cave

The horizontal stress changes that occurred from the pillaring

in E17 were measured by instruments placed at a test site in the

L2 entry of E15 panel. Figure 11 shows the location of the test

site with respect to E17 where the distance from the cave to the

instrumentation was 120 ft. Two types of instrumentation were

used, the USBM 3 component borehole deformation gage and the

CSIRO hollow inclusion cell (11, 14). The borehole deformation

gages were located at depths of 2, 5, 10, and 15 ft and the CSIRO

cells at 5 and 20 ft in the roof. The 20 ft 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 2 and 10 ft.

The cave front position is also indicated on the graphs. On this

figure, the largest stress reduction is )P and the smallest )Q. The

orientation of the stress changes are given as the azimuth to the

smallest stress reduction component )Q. A minus value

represents a stress reduction while a positive value represents a

stress increase. Although the data is not shown, at the 20 ft depth

there was a stress reduction of 1,200 psi after the cave had passed

the test site with an azimuth direction of 233°.

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 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

suffered stress damage. 

This forward horizontal stress abutment can be seen by the

increase in stresses as the cave front approaches and passes the

test site. At 120 ft from the cave, stress increases ranged from 200

to 800 psi. This stress increase began when the cave was 200 ft

outby the test site. At the 2 ft level, the roof could not sustain this

added stress which quickly dropped to show a stress reduction.

At other depths, the stress concentration although dropping from

a peak remained higher than the original stress conditions in the

direction just sub parallel to the cave. 

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 pct of R1 received some stress damage in E15.

This again was a sign of the forward horizontal stress abutment. 

For the first 1,000 ft of "advance and relieve" pillaring in E15,

this stress damage was largely confined to the R1 entry. However,

in the last 300 ft 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, this damage continued inby for the rest of the

panel (a distance of another 400 ft). In this section, guttering and

severe roof damage occurred mainly in the faces. A roof fall did

occur in the belt entry 100 ft 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. This abutment did not advance with the mining, but

essentially remained stationary. In E15, the damage consisted of

cutters developed in the R1 entry from the cave outby for a distance

of approximately 200 ft. This stress damage was parallel to the

length of the cave.
Figure 16. Rear abutment stress damage in the L1 entry

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 L1 and L2 entries. This was a narrow

zone about 10 to 15 ft wide of fairly severe roof damage that

crossed the entire panel at a right angle (figure 16). This damage

zone ran both in the N 30° E and east-west crosscuts with an

orientation sub parallel to the direction of the maximum

horizontal stress.

To migrate the damage, cribs and posts were set as

supplemental support along with cable bolts 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 with the start of the cave 200 ft inby, the damage

approached but did not reach the mains. 

In E19, to moderate the effects of the rear abutment, a caving

chamber was mined just outby the area where the pillaring began

(figure 11). This chamber involved mining into the barrier from

the R2 entry for a distance of 100 ft prior to the start of the

pillaring. This created a chamber that was 100 ft long and 50 ft

wide. This chamber caved very readily as did the initial 100 to

200 ft of the pillared section. The "advance and relief pillaring

then began just inby this caving chamber with the cave in the

chamber initiating the cave in the pillared section. There was still

outby damage in the R1 entry for a distance of about 50 ft while

the roof fell in the crosscut between the caving chamber and the

R1 entry. However, the damage did not extend across the panel

as occurred in E15.

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

500 ft inby. With the cave passing, there was significant stress

reduction in the shale below the sandstone in the direction of the

cave that ranged from 800 to 1,200 psi. Based on the stresses

measured in the shale, this would represent between a 40 and

60 pct reduction in the applied horizontal stress. At the 2 ft level,

there was stress relief in both directions though the reduction

parallel to the cave was only about 300 psi. Even in the sandstone

at a depth of 20 ft, there was a stress reduction of 1,200 psi that

represents about a 20 pct reduction in the horizontal stress.

Further evidence of the stress reduction is suggested by the fact

that no roof falls occurred in E15, E17, and E19 panels adjacent to

the caves during development about 4,000 ft of panel length.

During retreat mining in the panels, no roof falls or roof control

problems were encountered indicating that the workings were stress

relieved. 

Although there are no quantitative measurements as to the

extent of the stress reduction zone that developed, 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 of damage

which occurred in the L2 entry, it appears that some stress relief is

occurring in that entry, a distance of 220 to 250 ft from the cave.

Further, in E13 panel, the panel adjacent to E15, almost 33 pct of

the L2 entry suffered stress damage including roof falls. Another

indication of the extent of stress relief is 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 500 ft. During retreat mining of

panel E13, in early December, test site instrumentation monitored

100 to 200 psi of stress change as the pillar line past. The distance

from the E13 cave to the test site was about 400 ft. 

Cave Height

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 N 30° W direction were made.

The fall height from the top of the mined opening was about 30 to

32 ft resulting in a total opening height between 35 to 37 ft. This is

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 (15). 

DISCUSSION

Stress Reduction Zone

Clearly there is a directional roof control problem caused by

the high horizontal stress at the mine with a N 70° E direction for

the maximum horizontal stress being assumed. With the "advance

and relief" method and a panel and cave direction of N 30° W , the

angle of the horizontal stress with respect to the cave was 100° or

nearly perpendicular to the maximum horizontal stress. This

created a stress reduction zone that protected or shadowed most of

the panel behind the cave from the affects of the maximum

horizontal stress. Figure 17 shows the stress reduction zone

generated by the cave.
N

N70 E

o

100

Scale, ft

0

Cave

P

P

Rear abutment

Forward abutment

 Stress

reduction

 zone

Legend

 Stress Damage

 Coal

Figure 17. "Advance and relieve" panel with cave showing the

forward and rear abutments and stress reduction zone

A significant stress reduction was achieved with caving that

ranged from 800 to1,200 psi at a distance of 120 ft from the cave.

This level of stress relief may represent a reduction of up to 50

pct of the maximum horizontal stress. More stress relief would

occur closer to the cave while the amount of stress relief will

diminish with distance. Underground observations indicate that

once the openings were in the stress shadow, minimal stress

damage occurred to the mine roof. Further, no roof falls occurred

in the "advance and relieve" sections yet there was nearly 18,000

ft of drivage shadowed by the caves in three panels. This allowed

for retreat mining in the stress relieved sections with no roof

control problems or roof falls.

The width of the stress relief zone from the cave appears to

be between 250 to 500 ft. With a cave height of only 35 to 37 ft

this stress reduction zone was 7 to 14 times the cave height.

Further, the 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 (5, 6).

The extent of any stress relief zone will determine the width of the

panel or mining that can be effectively protected. 

During this investigation, 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. 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 only a short time. Once behind the cave, the openings

suffered no further roof damage. 

With the no roof falls during retreat mining, the stress

concentration that usually developed in front of the pillaring

appears to be absent or greatly reduced by the stress relief generated

on development. Prior to "advance and relieve" mining, sections of

panels could not be mined during retreat mining because of roof

falls caused be the stress concentration generated by the retreat

cave.

Forward and Rear Horizontal Stress Abutments

and Pillaring Plans

As the stress reduction zone was created, both forward and rear

horizontal stress abutments were developed. Figure 17 shows the

location of these stress abutments with respect to the cave. The

forward stress abutment results from the concentration of the

horizontal stress in front of the cave. Because of this stress

concentration, localized roof damage occurred in the faces adjacent

and forward of the cave. 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. 

Because of the forward horizontal stress abutment, adjustments

to the original pillaring plan had to be made. The first pillaring plan

was not successful because of the difficulty in trying to mine an

entry across and into the stress concentration (figure 8).

With the second plan, the ground control problems resulting

from the stress concentration were minimized. The stress

concentration was still on the inby half of the crosscut and in the

notch where stress damage in the form of cutters and guttering 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. When the barrier and pillar

were now mined, a portion of the horizontal stress concentration

was now over a section of the pillar to be extracted. This

concentration was located near the notch and was seen between cut

sequence 4 and 5 with the roof cave initiating when these cuts were

taken. However, instead of being a problem for development, the

stress concentration now assisted in generating a cave. Once the

cave occurred at this stress concentration point, there were few

problems with the mining of the rest of the pillar.

A rear horizontal stress abutment was also created that was

similar to the forward abutment except that this rear abutment was

stationary (figure 17). Essentially, the stress concentration once

fully developed was not limited in time in causing stress damage.

As a result, the damage of the rear abutment appeared to be more

severe than that resulting from the forward abutment. Two zones of

damage developed, one outby and parallel to the cave in the R1

entry and the other zone a narrow band that ran across the panel,

perpendicular to the cave. 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. 

Supplemental support in the form of cribs, posts, cable bolts

and longer primary support was required in both zones to control

the damage. Because of the outby damage parallel to the cave,

when designing the panel, a sufficient distance must be left

between the start of the cave and the mains to prevent damage to

the mains. In E17 a distance of 250 ft appeared to suffice. This

distance allowed the damage and the stress concentration to

dissipate before reaching the mains. The caving chamber in E19

seems to have reduced the rear abutment stress damage.

Apparently, the rear abutment stress was shifted into the barrier

pillar between the end of the chamber and the mains.

Cave Height and Width

The panel width that can be protected is dependent on the

extent of the stress relief zone and this is in part a function of the

caving height. However, the cave height is to a large degree

controlled by material bulking and not by the cave width (15).

Usually, the cave will reach a height that is 2 to 10 times the

mining height with the higher heights achieved in weaker shales

(17). The cave width is only important in that this dimension

must be sufficient to allow the full cave height to be achieved

through the failure of the roof. In the case of Tanoma Mine, a

20 ft wide entry was able to generate the full cave height,

however a larger width is required to consistently initiate and

propagate the cave. A width of 100 to 120 ft appeared to be

adequate for generating a cave that reached a maximum height in

two of the panels. In E19 panel, a caving chamber 50 ft wide

readily caved but in the pillared section a span of 80 ft did not

cave easily. The difference may be the result of the caving

chamber being affected by the full magnitude of the horizontal

stress, while the cave in the "advance and relieve" section was in

a stress shadow generated by the pillaring both on advance and on

retreat from both E15 and E17 panels.

Floor Heave

Whether the floor heave is the result of the vertical or

horizontal stress is not known for certain. However, with the

significant change in floor heave rates in E19 where the cave size

was reduced, the pillar size increased and another pillar added,

suggests that the floor heave may have been caused by the vertical

stress. In this case, the third pillar plan and panel design reduced

the floor heave rate by at least 50 pct.

With a weak floor, floor heave must be considered when

adopting the "advance and relieve" method because the openings

will be subjected to the floor heave for extended periods of time.

This additional time will allow for more floor heave to occur.

Further, this floor heave developed well outby the face and was

worse at the start of the panel. This is just the opposite of what

occurs during retreat mining where the largest amount of floor

heave will develop between 2 to 3 crosscuts outby the pillar line

where the exposure time limits 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 both on development and on retreat. With this

system a significant reduction in the horizontal stress in the panel

behind the cave was achieved. However, a knowledge of the

horizontal stress direction is important to understand what stress

will be reduced and where the location of the stress concentration

zones will be in the panel. In this case, the maximum horizontal

stress was nearly perpendicular to the panel and cave and resulted

in the stress relief zone encompassing most of the panel behind the

cave. Because of the orientation of the stress field with respect to

the cave, stress relief did not occur at the faces but occurred one or

two crosscuts behind the face as the cave was advanced. Since, roof

falls and severe stress damage usually did not occur immediately,

this situation was acceptable. 

With the creation of a stress reduction zone, forward and rear

horizontal stress abutments were also generated. The forward

abutment which moved with mining required the development of a

pillaring plan that could be used in the high stress concentration

zone. There was also some increased damage in the workings

adjacent to and ahead of the cave. The rear horizontal stress

abutment was stationary and therefore resulted in more severe stress

damage necessitating the use of supplemental support. This damage

was localized and in predictable locations.

The extent of the stress relief zone was 7 and 14 times the cave

height. The development of such a large zone of reduced stress

must rely on the movement along bedding planes and other

discontinuities and not on the elastic response of the rock mass.

The cave height and therefore the extent of the stress relief zone will

be limited to a large degree by the bulking of the caved material and

not necessarily by the cave width. The success of this technique

depends on the extended dimensions of the stress relief zone that

can shadow the entire panel width and even beyond to adjacent

development. 

With a weak floor, floor heave may occur and therefore must

be considered. The panel adjacent to the cave, especially at the start

of the pillaring, will be subjected to the increased vertical abutment

stress for an extended period of time. This results in the

development of a large amount of floor heave that can impact

mining. Retreat mining will usually not have the same effect

because of the limited exposure time. However, a modified

pillaring plan and panel design was able to reduce the heave rate

significantly. 
REFERENCES

1. Mark C, Mucho TP, Dolinar DR. Horizontal Stress and

Longwall Headgate Ground Control. Min Eng, Jan. 1998,

pp. 61-68.

2. Mucho TP, Mark C. Determining Horizontal Stress

Direction Using the Stress Mapping Technique. Proceeding

of the 13th International Conference on Ground Control in

Mining. Morgantown, WV, 1994, pp. 277-289.

3. Dolinar DR, Tadolini SC, Blackwell DV. High Horizontal

Movement in Longwall Gate Roads Controlled by Cable

Support Systems. Proceeding of the 15th International

Conference on Ground Control in Mining. Golden, CO,

1996, pp. 497-509.

4. Su WH, Hasenfus GJ. Regional Horizontal Stress and its

Effect on Longwall Mining in the Northern Appalachian

Coal field. Proceeding of the 14th International Conference

on Ground Control in Mining. Morgantown, 1995, pp. 39-

45.

5. Aggson JR. Geomechanical Evaluation of a Coal Mine

Arched Entry. Soc of Min Eng. Littleton CO, 1988, 7 pp.

6. Aggson JR Mouyard DP. Geomechanical Evaluation of a

Coal Mine Arched Entry. Inter Journal of Min and Geophy

Eng., 1988, pp. 185-193.

7. Chase FE, Mark C, Mucho TP, Campbell PL, Holbrook DA.

The Advance and Relieve Mining Method: A Horizontal

Stress Control Technique. Proceeding of the 18th

International Conference on Ground Control in Mining.

Morgantown, WV, 1999, pp. 300-308.

8. Barish K. Truss Bolting On-Cycle in Jane Mine Lower

Freeport Seam. 1st International Conference on Ground

Control in Mining. Morgantown, WV, 1980, pp. 1-10.

9. Bauer ER. Cutter Roof Failure: Six Case Studies in Northern

Appalachian Coal Basin. Pittsburgh, PA: U. S. Department of

the Interior, Bureau of Mines IC 9266, 1990.

10. Krupa ED, Khair WA. Assessment of Underground Structural

Design. Proceeding of the 10th International Conference on

Ground Control in Mining. Morgantown, WV, 1991, pp. 15-

25.

11. Bickel BL. Rock Stress Determination from Overcoring-An

Overview. Pittsburgh, PA: U. S. Department of the Interior,

Bureau of Mines, Bulletin 694, 1993.

12. Enever JR, Wold MB, Crawford GR. Hydraulic Fracturing for

Rapid Low Cost Stress Measurements in Underground Mines.

The AUSIMM Proceedings. Parkville Aus., 1990, pp. 19-24.

13. Aggson JR. Coal Mine Floor Heave in the Beckley Coalbed

An Analysis. Pittsburgh, PA: U. S. Department of the Interior,

Bureau of Mines RI 8274, 1978.

14. Worotnicki G, Wltopn RJ. Triaxial "Hollow Inclusion"

Gauges for Determination of Rock Stress In-situ. Proc. ISRM

Syp Investigation of Stresses in Rock, Advances in Stress

Measurement: Inst. of Eng Pub. No. 4, 1976,.

15. Bell FG. Site Investigations in Areas on Mining Subsidence.

Newnes-Butterworths, London, 1975, 168 pp.

16. Mark C, Chase FE. Analysis of Retreat Mining Pillar Stability

(ARMPS). Pittsburgh, PA: U. S. Department of the Interior,

Bureau of Mines, IC 9446 Proceedings: New Technology for

Ground Control in Retreat Mining, 1997, pp. 17-34.

17. Kendorski FS. Effect of High-Extraction Coal Mining on

Surface Subsidence and Ground Water. Proceeding of the 13th

International Conference on Ground Control in Mining.

Morgantown, WV, 1993, pp 412-425.