Evaluation of Support and Ground Response as Longwall Face Advances

into and Widens Pre-Driven Recovery Room

Thomas M. Barczak, Senior Research Engineer 

Stephen C. Tadolini, Chief, Rock Safety Engineering Branch 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA

Peter (Yunqing) Zhang, Sr. Geotechnical Engineer

Pennsylvania Services Corporation (affiliate of Foundation Coal) 

Waynesburg, PA

ABSTRACT 

A cooperative study was conducted with Emerald Coal 

Resources, L.P., an affiliate of Foundation Coal Corporation, and 

the National Institute for Occupational Safety and Health (NIOSH) 

to evaluate the effectiveness of (partially) pre-driven longwall 

recovery rooms supported with pumpable roof supports. This 

paper evaluates the load transfer mechanics associated with the 

advancement of the longwall face into the pre-driven recovery 

room and the widening of the room to recover the longwall shields. 

The results show that the yielding of the panel fender produced 

uncontrollable convergence that caused yielding of the shields and 

stiff pumpable roof supports when the face was approximately 10 ft 

from the recovery room. The shields continued to yield until the 

face advanced into the recovery room. Convergence in the predriven recovery room typically reached 6-8 in once the pumpable 

supports yielded and shed load through a declining residual load 

capacity prior to being cut out by the longwall shearer. Roof 

deformations occurred beyond the 32-ft horizon with total 

deformation ranging from 2 to 4 in, indicating that standing support 

was necessary to help control the span as the longwall face 

advanced into the pre-driven recovery room. Stability improved as 

the pumpable supports were cut out, thereby reducing the span 

between the shields and the outby pillars. In the end, the shields 

were successfully recovered under stable ground conditions. 

However, a disconcerting discovery was the load shedding of the 

outby pillars as the recovery room was mined into. It is postulated 

that this may be due to the progression of the rear “abutment” 

moving toward the recovery room as the panel fender and standing 

support in the recovery room yield and shed load. This behavior 

was not expected with a relatively narrow room that is fully 

supported with standing support. If this mechanism is indeed 

occurring, the width of the room and performance of the standing 

support becomes even more critical. Premature failure of the 

support can lead to excessive convergence on the shield line that 

cannot be controlled even with modern shields, resulting in roof 

instabilities that can lead to catastrophic results. 

INTRODUCTION 

Pre-driven recovery rooms provide opportunity to reduce the 

longwall move time by having a place where the mining equipment 

will be removed, prepared (supported) before the longwall 

approaches the end of the panel. In cases such as the Emerald mine, 

the concept can also provide improved ground control. The weak 

shale roof in the Pittsburgh seam can be difficult to control in high 

stress environments such as those created when the longwall face is 

recovered. Screen and fabric mesh are typically installed to help 

control the immediate roof and prevent gob flushing into the face 

area as the shields are recovered. Problems have been experienced 

in the past with installing this mesh during the recovery operation 

as the damaged immediate roof would loosen and create cavities 

that impeded support advance during the final stages of the panel 

extraction and development of the recovery room. A pre-driven 

recovery room alleviates these problems by allowing the mesh and 

other support elements to be installed under conditions that are 

more benign and less hazardous to the mine workers. 

However, the pre-driven recovery room must be adequately 

supported to ensure success. Failure to provide adequate ground 

control will create an extremely hazardous environment and can 

lead to catastrophic results with injuries to mine workers and loss 

of mining equipment. History has shown that it is very difficult to 

provide adequate support without the use of standing support to 

help control the large span of roof as the room is mined into (Oyler 

et al., 1998 and 2001). Since the longwall shields must advance far 

enough into the room to allow removal, a requirement for the 

standing supports is that they must be cut by the longwall shearer 

so they can be easily removed as part of this process. The yielding 

of the last section of the longwall panel due to abutment stresses 

produces a component of convergence that is uncontrollable, 

requiring the standing supports to be able to yield accordingly and 

maintain support capability until the shields are fully into the room. 

Here too, history has shown that supports that are too stiff and nonyielding, despite their high capacity, fail prematurely and endanger 

ground control (Tadolini et al., 2002). 

These requirements have caused Emerald to utilize a pumpable 

roof support system for the recovery application in combination 

with roof bolts, cable bolts, slings, meshing, and straps. Although 

the intrinsic support is necessary to build an immediate roof beam 

that helps support the main roof, movement above the bolted 

horizon is common, and the standing support can be the most 

critical component of the support system. The pumpable support 

used in this case was a 30-in-diameter cementitious support in 

which the grout is pumped into an empty, reinforced fabric bag that 

is hung from the roof of the recovery room. The bag provides a 

containment form for filling the support, but more importantly, 
provides confinement of the cementitious grout to supply a 

sustainable residual support capacity after the grout is fractured and 

the peak support loading is exceeded due to the yielding of the 

longwall panel fender. This capability to provide residual loading 

is advantageous, particularly compared to conventional concrete 

cribs which do not have a useful residual load capacity. However, 

the pumpable supports are very stiff, and unlike conventional 

concrete cribs, do not use wood to soften their response. Therefore, 

their residual load performance is the key to their effectiveness in 

this application. One of the objectives of this study was to acquire 

additional data on loading response of the pumpable supports 

during the panel fender yielding and advancement of the face into 

the recovery room. 

Although other studies of pre-driven longwall recovery room 

operations have been conducted, questions regarding the loading 

mechanics and support design requirements remained unanswered 

as evidenced by premature support failures (Tadolini et al., 2002), 

roof falls, and loss of shields (Oyler et al., 1998), while other 

operations were successful (Wynne et al., 1993). One local roof fall 

and excessive roof sag occurred at the completion of the previous 

(B3) panel at this mine using this same recovery room design and 

support plan with only minor differences. 

The objective of this study is to develop a better understanding 

of the loading mechanics associated with pre-driven longwall 

recovery room operations so that optimum support techniques can 

be applied and more effective support systems can be designed to 

reduce the risk of failure. The level of instrumentation used in this 

study was more extensive than previous studies and provides rare 

information to fulfill this objective. 

SITE DESCRIPTION AND RECOVERY ROOM 

INSTRUMENTATION PROFILE 

The approach utilized at the Emerald mine is a hybrid design in 

that the recovery room is only partially (50 pct) pre-driven. A 16-ft 

wide entry at the end of the panel is pre-driven and supported long 

before the face approaches. After the face mines into the predriven room, the room is widened an additional 16 ft to provide 

sufficient room for the recovery of the longwall equipment. 

The B4 longwall panel was 1,450 ft wide. The depth of cover 

averaged 600 ft. Figure 1 shows the roof lithology from two 

coreholes near the B4 recovery area. The immediate roof consists 

of clay shale, silty shale and rider coal, which is common to the 

Pittsburgh seam providing a relatively weak immediate roof 

structure. The area is overlain by 10 and 22-ft thick limestone 

members and 11 and 25-ft thick shale layers as shown in figure 1. 

Figure 2 shows the support design for the recovery room. The 

recovery room was supported with 8-ft, grade 75, combination 

bolts. Combination bolts were also angled over the longwall panel 

and the outby pillar ribs to provide additional support as the face 

cuts into and then widens the recovery room. Eight-ft cable bolts 

on 4-ft spacing were installed 2 ft from the panel rib to help support 

the roof when the first row of cribs were cut out, and 12-ft cable 

bolts on 4-ft spacing were installed 2 ft from the pillar rib to 

reinforce the roof from possible shear/tension fractures. The roof 

was glued with polyurethane resin (PUR) on the outby (pillar) side 

to strengthen the roof in the area adjacent to the pumpable cribs. 

The mine chose not to use PUR on the inby side, as was done on 

the previous panel, to facilitate caving of the immediate roof once 

the shields entered the room. 

Three rows of 30-in-diameter pumpable roof supports were 

utilized in the recovery room as shown in figure 2. These were 

Calcium-Sulfo-Aluminate (CSA) grout filled supports that are 

identical to the supports used for the longwall tailgate except the 

spiral metal wire in the bag was replaced with a fiberglass wire that 

could be more easily cut by the longwall shearer. The performance 

characteristics of this support, as measured in the laboratory 

through full-scale testing in the NIOSH Mine Roof Simulator and 

during the longwall recovery in this study are shown in figure 3. 

As seen, the pumpable support is very stiff with peak load 

occurring at less than 0.5 in of displacement. The support sheds 

load during yielding as the bag acts to provide confinement and 

maintain a residual loading capacity. 

D29 

Figure 1. Geological columns in the longwall recovery area. 

565 

570 

575 

580 

585 

590 

595 

600 

605 

610 

615 

620 

625 

630 

635 

claystone

Thick 2.56 

560 

limestone 

Thick 22.37 

Carbonaceous clay shale

Thick 0.50 gray clay shale

Thick 1.40 

gray clay shale

Thick 5.80 

gray clay shale

Thick 10.74 

carbonaceous clay shale Thick 0.75 

carbonaceous clay shale

Thick 1.05 

coal undifferentiated 

Thick 1.87 

coal undifferentiated Thick 0.21 

gray clay shale Thick 1.58 coal undifferentiated 

Thick 0.22 carbonaceous clay shale Thick 0.76 coal undifferentiated Thick 0.76 gray clay shale 

Thick .88 coal undifferentiated Thick 6.14 

gray clay shale Thick 3.04 

claystone

Thick 2.80 

shale Thick 4.80 

725 

730 

735 

740 

745 

750 

755 

760 

765 

770 

775 

780 

785 

790 

claystone

Thick 9.75 

720 

shale 

Thick 11.50 

shale 

Thick 25.25 

C312 

gray clay shale 

Thick 5.37 

coal undifferentiated 

Thick 0.71 

carbonaceous clay shale 

Thick 0.04 

coal undifferentiated 

Thick 6.27 

claystone

Thick 0.13 

DEPTH 793.00 

limestone 

Thick 10.50 

carbonaceous clay shale 

Thick 8.08 

gray clay shale 

Thick 0.48 

limestone 

Thick 1.50 
.

1.5' 1.0' 4.5'

ire mesh 30'' crib on 6 spaci 30'n' cg rib on 6 spaci 30'n' cg rib

30 ' crib

30 ' crib

30 crib 

1.5'

400 

450 

500 

Longwall Face 

8' combination bolt 

on 4' spacing 

20 deg 

Recovery Area 

2.0' 

8' combination bolt on 4' spacing 

0.6" x 8' cable bolt on 4' spacing 

Steel strap with w ' ' on 6' spacing 

2.0' 

' 

' 

'' 

8' combination bolt 

with steel strap 

12' cable bolt 

8' combination bolt 

4.0' 

8' cable bolt 

450 

Longwall Face

8' combination bolt

on 4' spacing

20 deg

Recovery Area

2.0'

8' combination bolt on 4' spacing

0.6" x 8' cable bolt on 4' spacing

Steel strap with w

1.5' 1.0' 4.5' 

ire mesh 

30'' crib on 6' spacing30'' crib on 6' spacing

30'' crib on 6' spacing 

30 ' crib 

30 ' crib 

30 crib 

1.5' 

8' combination bolt on 4' spacing 

8' combination bolt on 4' spacing 

0.6" x 12' cable bolt on 4' spacing 

2.0'

'

'

''

8' combination bolt

with steel strap

12' cable bolt

8' combination bolt

4.0'

8' cable bolt

450

16 ft 

350 

300 

250 

200 

150 

100 

50 

0 

Longwall Recovery Room - Chute C - Center Row 

MRS Full-Scale Laboratory Test 

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 

VERTICAL DISPLACEMENT, inches 

Figure 3. Comparison of the pumpable support full-scale 

laboratory tests and performance measured in the longwall 

recovery room. 

Figure 2. Support design for the longwall recovery room. 

A diagram depicting the approaching longwall panel, predriven 

VERTICAL LOAD, kips 

recovery room and outby pillar configuration is shown in figure 4. 

The instrumentation for this study was clustered in areas near 

Chutes C, F, and G and included: 

•  Shield leg pressures – Leg pressures were monitored by th

DBT pressure transducers installed on each of the longwall 

shield legs. Data was collected every 10 seconds. 

•  Pumpable support loading – Specially-made hydrauli

(water-filled) metal bladders, sized to fit the area of the 

pumpable support, were instrumented with pressure 

transducers and calibrated in the NIOSH Mine Roof 

Simulator to measure pumpable support load development. 

•  Support displacement – Wire-pull displacement transducer

were mounted to the side of the pumpable support to 

measure support displacement. 
Right Pillar BPC

@ 26 ft depth

Addit onal convergence

measurements

Room is widened

C Chute

F I

40-ft 3 points Extensometer ho e

10-ft Ang ed observa on ho es (one ho e a 45 degree

and one ho e at 80 degree)

25-ft Magnetic Extensometer ho e

25 ft scope ho es

6 ft from the recovery

•  Sonic probes were used to measure roof deformation up t

24 ft into the roof. 

•  Three-point mechanical extensometers attached t

displacement transducers were used to measure roof 

deformation relative to the 26, 32, and 38 ft roof horizons. 

•  Convergence rods with displacement measurin

transducers were used to measure roof-to-floor 

convergence in the recovery room and in the outby C chute 

adjacent to the stress measurement locations. 

•  Borehole Pressure Cells were used to measure panel an

outby pillar stress development in the C chute area. Figure 

4 depicts the location of the cells. One cell was installed in 

the panel 10-ft from the recovery room to measure yielding 

of the panel fender as the longwall face approached, two 

cells were installed in the outby pillars at a depth of 10-ft 

and two other cells 20-ft from the from the outby edge of 

the recovery room to measure load transfer from the panel 

to the outby pillars. Two more cells were installed 26-ft 

from the outby edge of the recovery room in the adjacent 

pillar of the C chute location, again to measure load 

transfer from the longwall panel as the fender yielded. 

20 ft Panel BPC 20 ft anel

10 ft

SUPPORT AND GROUND RESPONSE 

DURING MINING PHASES

An evaluation of the support and ground response is made 

according to the progression of the mining process in the following 

order: (1) mining of the longwall panel to observe front abutment 

effects prior to yielding of the panel fender, (2) extraction of the 

remaining 25 ft of the panel to evaluate yielding of the panel fender, 

(3) removal of the standing supports and advancement through the 

pre-driven recovery room, and (4) widening of the pre-driven 

recovery room. 

Mining of the Longwall Panel Prior 

to Fender Yielding 

The effects of the front abutment pressure began to manifest in 

the recovery room with increased pumpable support loading when 

the face was approximately 100 ft away. The increase in loading 

rate remained very gradual until the face was approximately 25 ft 

from the recovery room. Overall, the three instrumented pumpable 

supports increased in load by 90 kips (panel side support), 63 kips 

(middle row support), and 77 kips (pillar side support) during this 

period, representing 21%, 23%, and 35% of the observed peak 

10 ft Left PillarBPC’s 

@ 10 ft depth 

@ 20 ft depth ’s 

Location of support load and 

convergence measurements 

i

 to this location 

Roof deformation 

measurements 

10 ft 

A B C D E F G H

’s 

’s 

A B C D E G H 

second entry 

third entry 

recovery line 

- l 

l ti l l t 

l 

l 

- l 

A B C D E F G H

10 ft Left PillarBPC

@ 10 ft depth

@ 20 ft depth Right PillarBPC 

@ 26 ft depth 

Addit onal convergence 

measurements 

Room is widened 

C Chute 

I 

Left Pillar BPC's 

Location of support load and 

convergence measurements

i

 to this location

Roof deformation 

measurements

10 ft

loading on the supports. 

An interesting discovery was revealed concerning the 

development of the pumpable support loading in response to the 

panel mining. The pumpable support loads were well correlated to 

the shield cycles even when the face was 100 ft away from the 

recovery room. The support loading increased rapidly as the 

shearer passed the face location adjacent to the support location in 

the recovery room, with very little load increase during the 

remainder of the shield cycle although the leg pressures were 

increasing significantly throughout the shield cycle. As expected, 

the shields are responding to the immediate roof and the main roof 

Tailgate Headgate

Figure 4. Diagram showing instrumentation in the longwall activity while the recovery room supports at this juncture are seeing recovery room. 

75 ft FACE POSIT 50 ft IONS 

7,000 

Shield cycle 

Shield pressures 

Pumpable 

support load 

Pillar side 

pumpable 

support 

Middle row 

pumpable support 

Pumpable support next to panel 

Shearer passage 

85 

6,000 75 

PUMPABLE SUPPORT LOAD, kips 

SHI

ELD L

E

G PRE

S

SURE, psi 

5,000 65 

4,000 55 

3,000 45 

2,000 35 

1,000 25 

0 15 

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0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 

DATE AND TIME 

Figure 5. Correlation between pumpable support load and shield cycle showing load increase during shearer passage. 
20 ft

10 ft

Longwall

face

C Chute

10 ft

10 ft

20 ft

10 ft

Longwall

face

C Chute

10 ft

10 ft

75 ft FACE POSITIONS 50 ft 

7,000 

20 ft 

10 ft 

Longwall 

face 

C Chute 

10 ft 

10 ft 

Shield cycle 

Shield 

Panel BPC 

pressure 

Pillar (10-ft depth) 

Shearer passage 

900 

800 

6,000

SHIELD LEG PRESSURE, psi 

700 

5,000 

600 

4,000 500 

3,000 400 

300 

BPC PRESSURE, psi 

2,000 

200 

1,000 100 

0 

12/15/06 

0:00 

12/15/06 

1:00 

12/15/06 

2:00 

12/15/06 

3:00 

12/15/06 

4:00 

12/15/06 

5:00 

12/15/06 

6:00 

DATE AND TIME 

Figure 6. Correlation between borehole pressure cells and shield cycles. 

0 

12/15/06 12/15/06 12/15/06 

7:00 8:00 9:00 

the main roof activity. Figure 5 shows an example for shield 

number 175 (adjacent to Chute C in the recovery room) as the face 

advanced from a location 75 away to 50 ft away the recovery room. 

As the face advanced closer than the 50 ft location, the pumpable 

supports began to increase in loading during the full shield cycle, 

but the majority of the load increase continued to be consistent with 

the shearer passage and a step-type function in the load 

development continued. 

The Borehole Pressure Cells (BPCs) monitoring the panel and 

pillar stress followed a similar pattern, although not as distinct as 

the support loads (see figure 6). Part of this was due to the much 

75 ft FACE POSITIONS 50 ft

2,100 

slower sampling rate of the BPC data (10 minutes compared to 15 

seconds). Similar increases in stress were measured in the panel 

BPC (positioned 10 ft from the leading or inby edge of the recovery 

room as shown in figure 4) and the pillar BPCs (positioned 10 ft 

from the trailing or outby edge of the recovery room) as the face 

advanced to the 50-ft location. However, as the face continued to 

advance toward the recovery room, the panel BPC increased at a 

much faster rate than the pillar BPCs as shown in figure 7. This 

suggests that the abutment load was focused on the panel and not 

transferred across the recovery room to outby pillars during this 

phase of the mining process. 

FACE POSITIONS 25 ft 

BPC PRESSURE, psi 

1,800 

1,500 

1,200 

900 

600 

300 

0 

20 ft 

10 ft 

Longwall 

face 

C Chute 

10 ft 

10 ft 

Panel BPC 

Pillar (10-ft depth) 

BPCs 

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0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 

DATE AND TIME 

Figure 7. Stress on the panel BPC began to increase at a faster rate than the pillar BPC as the face advanced closer to the recovery room. 
FACE POSITION, ft from recovery 

25 ft 22 ft 19 ft 16 ft 13 ft 10 ft 7 ft 4 ft 

9,000 450 

8,000 400 

7,000 

6,000 

Panel BPC 

Pillar side 

pumpable support 

Middle row 

pumpable support 

Pumpable support next to panel 

Pumpable support 

Shield pressures 

loading increases 

during entire shield 

cycle 

Pumpable support 

loading increases mostly 

during shearer passage 

Yielding of 

shields and 

pumpable 

supports 

300 

5,000 250 

350 

SHIELD LEG PRESSURE, psi 

4,000 200 

3,000 150 

SUPPORT LOAD, kips 

2,000 100 

1,000 50 

0 0 

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18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 

DATE AND TIME 

Figure 8. When the longwall face was approximately 16 ft inby the recovery room, the pumpable support loading increased during 

the full shield cycle. 

Yielding of the Panel Fender as Face Approached 

the Recovery Room 

As the face advanced within the 25-ft location from the 

recovery room, the ground and support responses began to change 

dramatically, again revealing several interesting observations. Prior 

to this, the abutment effects were almost totally caused by the 

shearer passage. Now, more localized behavior, most likely 

involving the immediate roof activity, was causing the support 

loads and panel stress to increase during the full shield cycle (see 

figure 8). The pumpable supports, panel BPC, and longwall shields 

all yield during this period as the load capacity of the panel fender 

is exceeded. The sequence of events that describe the load 

mechanics are as follows: 

1. Panel BPC yields – The BPC in the longwall panel located 

10 ft from the leading (inby) edge of recovery room 

measuring change in stress as the longwall face approached 

began to increase in loading rate when the face was 

approximately 25 ft away from the recovery room or 15 ft 

from the BPC position. The pressure in the cell increased 

from 2,100 psi at the 25-ft location to approximately 5,300 

psi when the face was 16 ft from the recovery room or just 

6 ft from the BPC. During the next shield cycle, the change 

in pressure reached a maximum of 7,500 psi, and then 

dropped to about 5,750 psi at the end of the shield cycle. At 

the time when the cell was cut out by the shearer, the cell 

pressure was approximately 4,200 psi. 

The shield leg pressures provide further insight into the 

loading mechanisms associated with the panel fender 

yielding. The panel stress measurements are located 

adjacent to the C Chute location and shield number 175 

(see figure 4), which is about 1,000 ft from the headgate on 

this 1,450 ft-long face. For bidirectional cutting, the shield 

cycles are likely to alternate between short and long cycles 

since the support is located away from the center of the 

panel where the shield cycle time differential would be 

minimized. As shown in figure 9, the panel fender yielded 

during a long cycle (114-minute duration) as the shearer 

had to move to the headgate and back to complete the 

loading cycle. It was somewhat surprising to observe that 

the peak stress occurred not when the shearer passed by this 

location, but when the shearer was over 1,000 ft away at the 

headgate, particularly given the close proximity of the 

longwall face to the stress cell (approximately 6 ft away). 

This could be the result of the time dependent properties of 

the coal, in which the highly stressed coal will continue to 

deform (strain) even under static load conditions. The 

unloading rate is also nearly the same as the loading rate 

during this shield cycle. The detail of the loading is 

magnificent as exemplified by the temporary stop in load 

shedding when the shearer passes back in front of the 

pressure cell (see figure 9). 

One question is whether the yielding of the BPC cell 

located 10 ft from the edge of the recovery room indicates 

that the remainder of the panel fender has also yielded. 

Here, too, the shield data provides valuable information. 

Analysis of the shield data suggests that the panel fender 

has not yet yielded. The shield data (see figure 8) shows 

that on the cut immediately following the BPC yielding, the 

shield loading behavior is similar to that observed prior to 

the yielding of the BPC, while on the next cut, the shield 

load increases quickly after the shield is set and reaches 

yield load without the shearer being in the immediate 

vicinity. 
 .

2. Pillar fender BPCs shed load – The stress cells installed in These two cells peak and then gradually shed load during 

the outby pillar rib (10 ft from the edge of the recovery the shearer passage on the shield cycle following the peak 

room) also increased in concert with the panel stress cell. loading of the panel cell (figure 10). Since the stress levels 

This indicates that some of the abutment stress is on the pillar cells were only 25% of the level that caused 

transferring across the recovery room, but the magnitude of yielding of the panel cell, it is unlikely that the coal pillar at 

the stress change is much less in the pillar than in the panel. the cell location was actually yielding due to loading 

FACE POSITION, ft from recovery room 

28 ft 25 ft 22 ft 19 ft 16 ft 13 ft 10 ft 

8,000 

Shearer position 

Panel BPC 

Shield cycle 

Shearer at headgate 

Shearer at BPC location

250 

Headgate SH

E

ARE

R PO

SITION, shield # 

Tailgate 

7,000 

6,000 

5,000 

4,000 

3,000 

225 

200 

175 

150 

125 

100 

BPC

75

 PRESSURE, psi 

2,000 

50 

1,000 25 

0 0 

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18:00 18:30 19:00 19:30 20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00 

DATE AND TIME 

Figure 9. Yielding of panel BPC occurred when the longwall shearer was approximately 1,000 ft away from the cell location. 

FACE POSITION, ft from recovery room 

28 ft 25 ft 22 ft 19 ft 16 ft 13 ft 10 ft 7 ft 4 ft 

8,000 450 

6,000 

Shield 

pressures 

Panel BPC 

Pillar (10-ft depth) 

BPCs 

Load shedding of 

Pillar BPCs 

Load shedding of 

panel BPC 

Shield cycle 

Shield yielding 

Load shedding of 

pumpable supports 

350 

7,000 400 

SHIELD LEG AND BPC PRESSURE, psi 

PUMPABLE SUPPORT LOAD, kips 

5,000 300 

4,000 250 

3,000 200 

2,000 150 

1,000 100 

0 50 

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18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 

DATE AND TIME 

Figure 10. Evaluation of load shedding associated with pumpable supports and BPCs measuring panel and pillar stress. 
a

beyond the coal strength during this load shedding event. 

However, some rib spalling did occur to a depth of 2-3 ft. 

3. Yielding of the shields and pumpable roof supports – The 

pumpable roof supports in the recovery room continue to 

develop load as the panel and pillar BPCs shed load as 

previously described. The pumpable supports yield two 

shield cycles after the panel BPC yields and one cycle after 

the pillar BPC sheds load (see figure 10), putting the yield 

of the supports at a face position of approximately 10 ft. 

The shields also yield during this cycle. Again, the nature 

of the shield loading provides insight into the load 

mechanics. After being set, the shield-loading rate is 

significantly higher than on previous shield cycles and the 

shields quickly reach their rated capacity and begin to yield. 

They continue to yield even though the shearer is moving 

away from the shields toward the headgate in a long shield 

cycle. The shields continue to yield for the next two cycles 

as the face mines into the recovery room. This behavior 

suggests that the panel fender yielded as the shearer cut was 

reducing the face position from the 10 ft location to 

approximately 7 ft. 

The yielding of the pumpable supports trigger roof movements 

and convergence in the recovery room before the face even gets 

there. The pumpable roof supports are very stiff supports, capable 

of developing over 400 kips (200 tons) of load capacity in less than 

0.5 in of displacement. Prior to the yielding of the pumpable 

supports, the roof remained intact as a composite structural unit 

without additional separation among rock layers. As the pumpable 

supports began to rapidly increase load while the panel fender was 

yielding (see figure 10), the data indicates that roof was moving 

slightly “upward” as shown in figure 11. This apparent “upward” 

movement is most likely compression of the immediate roof 

structure or closing of preexisting fractures along bedding planes or 

elsewhere within the first 26 ft of roof. Since the support requires 

compression to build load, it is also possible that the floor ahead of 

the panel fender in the recovery room was also moving upward 

(heaving) as the panel fender yielded. 

Another performance characteristic of the pumpable support is 

once the peak loading is exceeded by fracturing of the cementitious 

grout, the support also sheds considerable load very quickly with 

little additional displacement. This rapid relaxation of the support 

load immediately caused a change in the roof activity in which 

separations and deformations of the roof structure begin to rapidly 

occur as shown in figure 12. At the C chute located approximately 

1,000 ft from the headgate, the roof structure above 26 ft remained 

largely intact with only minor separations (approximately 0.1 in) 

occurring above the 26-ft horizon. Similar roof responses were 

measured at the G chute location (approximately 450 ft from the 

headgate), however at the F chute, roof deformations occurred 

beyond the 32-ft horizon. 

The roof-to-floor convergence also increased dramatically with 

the yielding of the pumpable supports. By the time the face mined 

into the recovery room, the convergence measured from the middle 

row support displacement reached 5 in, while the roof deformation 

at this time was approximately 1.2 in. This indicates that about 

75% of the convergence was from floor heave and/or main roof 

movement beyond the 38-ft horizon during the panel fender 

yielding. Figure 13 shows the relative ground movement in the 

recovery room and in the C chute area. 

Removal of Standing Supports and Advancement 

through the Pre-Driven Recovery Room 

The next phase of the mining operation was to cut out the 

pumpable supports and advance the face to the pillar line to prepare 

for widening of the face. During this phase, no coal was being 

mined, however since the pumpable supports were helping to 

control the roof span and deformation process, the removal of the 

FACE POSITION, ft from recovery room 

36ft 33 ft 31 ft 28 ft 25 ft 22 ft 19 ft 16 ft 13 ft 10 ft 7 ft 4 ft 

8,000 -0.500 

7,000 

Panel BPC 

Pillar (10-ft depth) 

BPCs 

Roof deformation 

from extensometers 

Dilation 

Compression 

26 ft anchor 

32 ft anchor 

38 ft anchor 

-0.375 

6,000 -0.250 

BPC PRESSURE, psi 

ROOF MOVEMENT, in 

5,000 -0.125 

4,000 0.000 

3,000 0.125 

2,000 0.250 

1,000 0.375 

0 

12/15/06 

12:00 

12/15/06 

14:00 

12/15/06 

16:00 

12/15/06 

18:00 

12/15/06 

20:00 

12/15/06 

22:00 

12/16/06 

00:00 

12/16/06 

02:00 

12/16/06 

04:00 

12/16/06 

06:00 

0.500 

DATE AND TIME 

Figure 11. Roof initially is compressed during yielding of panel fender and then begins to deform downward into the recovery room. 
 .

.

FACE POSITION, ft from recovery room 

25 ft 22 ft 19 ft 16 ft 13 ft 10 ft 7 ft 4 ft 1 ft -2 ft 

CONVERGENCE AND R

OOF M

OVEMENTS, in

-1.0 

0.0 

1.0 

2.0 

3.0 

4.0 

5.0 

6.0 

12/15/06 18:00 12/15/06 22:00 12/16/06 2:00 12/16/06 6:00 12/16/06 10:00 12/16/06 14:00 12/16/06 18:00 

DATE AND TIME 

. 

0 

80 

160 

240 

320 

400 

480 

560 

PUMPABLE SUPPORT LOAD, kips 

Pumpable support 

loading Pumpable support displacement 

(convergence)

 Roof deformation (26, 32, 38 ft 

horizons from roof) 

Figure 12. Roof deformations and convergence profile during pumpable support loading and failure. 

FACE POSITION, ft from recovery room 

31 28 22 16 10 4 1 -2 -5 -8 -11 -14 

-0.50 

0.00 

0.50 

1.00 

1.50 

2.00 

2.50 

ROOF D

EFORMA

TION, in 

-1.00 

0.25 

1.50 

2.75 

4.00 

5.25 

6.50 

RECOVERY ROO

M CONVERG

ENCE, in 

Roof Deformation 

C Chute Crosscut 

Roof Deformation 

C Chute Recovery Room Near 

middle row of supports 

Recovery Room 

Convergence 

(Panel side support) 

Recovery Room 

Convergence 

(Middle support row) 

Recovery Room 

Convergence in C chute 

2 ft from 

recovery 

room 

20 ft from 

recovery 

room 

12/15/06 12/15/06 12/15/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/17/06 12/17/06 12/17/06 12/17/06 

12:00 16:00 20:00 0:00 4:00 8:00 12:00 16:00 20:00 0:00 4:00 8:00 12:00 

DATE AND TIME 

Figure 13. Convergence and roof deformation profiles in C Chute area of recovery room. 

supports can be critical to the stability of the recovery room. At the locations from the edge of the recovery room, remained fairly 

C chute location, the shield loading decreased as the supports were constant during this process. The roof continued to deform slightly 

extracted and the face advanced toward the pillar line (see figure in the chute as measured by a three-point extensometer in the center 

14). As the first row of supports was being extracted, the shield of the entry located 16 ft from the outby edge of the pre-driven 

loading remained high but below the yield rating. As the second recovery room, but the deformations were nearly an order of 

row of pumpable supports was being extracted, the shield loading magnitude less than that seen in the recovery room. An additional 

did not change much after being set and this condition remained 1 in of closure occurred for a distance of about 20 ft in the C chute 

throughout the advancement through the pre-driven recovery room. crosscut as measured by electronic closure meters measuring roofThe pillar stress, as measured by the cells at both the 10-ft and 20-ft to-floor displacement (see figure 13). 
<

TWAP SP Adjusted (Avg 10)

<

S175 Site # 3 (C) - Center Row Site # 3 (C) Pillar Ser es10

The time-weighted average shield pressure varied across the 

longwall face during the phase of mining in which the supports 

were cut out and the face was advanced through the pre-driven 

recovery room. This is shown in figure 15, which plots the change 

in shield loading (difference between the time-weighted average 

pressure and the setting pressure), as the last row of supports were 

being extracted and the face advanced to the pillar line. The shield 

loading at some locations was considerably higher than that 

observed at the three instrumentation sites. One example is shown 

in figure 16, where shield number 185 yielded on all cycles during 

the extraction of the pumpable supports and advancement through 

the pre-driven recovery room compared to shield 175 which had 

little change in loading during and following the extraction of the 

last row of supports. There was no obvious visual physical 

difference in the recovery room condition. Without 

instrumentation, it is difficult to determine with certainty what 

causes this difference in behavior. The persistent yielding of the 

shields indicates there was considerably more closure at these 

locations. This could be caused by: (1) longer cantilever of main 

roof indicative of periodic weighting, (2) less control of the 

immediate roof structure allowing deformation to greater heights 

into the roof, or (3) greater yielding of the pillar due to the 

abutment stress. 

Widening of the Pre-Driven Recovery Room 

The next phase of the recovery process was to widen the room 

by extracting an additional 16 ft of the coal pillars to create 

sufficient room to recover the longwall equipment. This phase 

7,000 

- i 

100 - 25 ft 25 - 0 ft Mine Through Room Widen Room 

Removal of 

first support Removal of 2nd 

and 3rd support 

Pumpable 

support load 

448 

6,000 384 

SHIELD LEG PRESSURES, psi 

5,000 320 

4,000 256 

PUMPABLE SUPPORT LOAD, kips

3,000 192 

2,000 128 

1,000 64 

0 0 

12/14/06 12/14/06 12/15/06 12/15/06 12/15/06 12/15/06 12/16/06 12/16/06 12/16/06 12/16/06 12/17/06 12/17/06 12/17/06 12/17/06 12/18/06 12/18/06 

15:00 21:00 3:00 9:00 15:00 21:00 3:00 9:00 15:00 21:00 3:00 9:00 15:00 21:00 3:00 9:00 

DATE AND TIME 

Figure 14. Shield loading decreased as the pumpable supports were extracted from the longwall recovery room. 

-

1,400 

Chute C Chute D Chute E Chute F Chute G Chute H Chute I 

1,200 

TWAP-SET PRESSURE, psi 

1,000 

800 

600 

400 

200 

0 

-200 

220 200 180 160 140 120 100 80 60 40 20 

SHIELD NUMBER 

Figure 15. Shield loading was low but did vary across the face during and after the extraction of the pumpable supports. 
- 

.

 .

-20 ft -23 ft -26 ft -29 ft

FACE POSITION, ft from recovery room

requires a variation in the typical longwall mining process in that pressure periodically during the shield cycle. This behavior 

extensions are included to the face conveyor advance rams to allow indicated that the immediate area is largely destressed. What 

advancement of the shearer and cutting of the last portion of the would cause this? One possibility is that the front abutment 

pillar without advancement of the supports. The mesh was loading on the pillars has dissipated or has been pushed well ahead 

deployed and exposed roof was bolted after each cut to provide a of the face area. 

fully supported room for recovery. The shields developed little, if 

any, loading during the extraction of the pillars to widen the room. The BPCs located in the pillar at depths of 10 ft and 20 ft from 

On several occasions, the shields struggled to maintain set pressure, the recovery room again document the impact of the shearer 

with the positive set control feature activating to re-supply pump passage on the loading mechanics. As the face cuts to the 10 ft 

FACE POSITIONS, ft from recovery room 

10 ft 7 ft 4 ft 1 ft -2 ft -5 ft -8ft -11 ft 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

SHIELD LEG PRESSURE, psi

Shield 175 

Shield 185 

No increase in 

loading on 

shield 175 

Shield 185 yielding throughout recovery 

room 

12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/16/06 12/17/06 

0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 

DATE AND TIME 

Figure 16. Comparison of two shields, one with high shield loading and one with low shield loading, during and after pumpable 

support removal. 

FACE POSITION, ft from recovery room 

-20 ft -23 ft -26 ft -29 ft 

800 

850 

900 

950 

1,000 

1,050 

1,100 

1,150 

1,200 

1,250 

PILLAR B

PC PRESSUR

E, psi 

0 

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

4,000 

4,500 

5,000 

SHIELD LEG PRESSUR

E, psi 

Longwall 

face 

C Chute 

20 10' ft 

Shield 

positive set 

activity 

Pillar BPCs located 20 ft 

from recovery room 

Shield cycle 

12/17/06 12/17/06 12/17/06 12/17/06 12/17/06 12/17/06 12/17/06 12/17/06 12/18/06 12/18/06 12/18/06 

9:00 11:00 13:00 15:00 17:00 19:00 21:00 23:00 1:00 3:00 5:00 

DATE AND TIME 

Figure 17. Outby pillar pressure cells show effects of shearer advance as pillar is extracted to widen room. 
location, the measured pressure change in the cells located there 

spiked to 2,852 psi and then shed load rapidly as the cell and/or 

contact condition is damaged from the shearer activity. As seen in 

figure 17, the pillar stress measured at the 20 ft locations increased 

with each shearer passage and remained fairly constant once the 

shearer cut was made until the next mining cycle. Overall, the load 

increments were much smaller than that observed with the panel 

fender yielding. These pillars do not yield. The convergence 

measured from the electronic convergence meters also continued to 

increase during the pillar extraction at about the same rate as when 

the pre-driven room was mined through (see figure 13). As the 

extraction approached the location of the convergence rods, the 

convergence rate increased significantly as expected. 

DISCUSSION OF RESULTS AND UNEXPECTED 

DISCOVERIES 

1. Panel fender pressures – The BPC in the panel recorded a 

maximum pressure change of nearly 7,500 psi. This was 

higher than expected given the overburden and proximity 

of the BPC to the panel edge at the time the peak stress was 

measured. The depth of cover was approximately 600 ft at 

this location. Assuming an abutment stress of 3-5 times the 

insitu stress, this provides an abutment stress of 1,800 to 

3,000 psi. Furthermore, the cell was only about 6 ft from 

the longwall face when this pressure was recorded. As 

such, the confinement provided by the yield zone was 

relatively small, and it was not expected that it would be 

sufficient to develop stress readings that were 2.5-4.2 times 

the abutment stress. Mark and Iannacchione (1992) 

indicated that BPC instruments tend to read significantly 

higher (by as much as a factor of 2) than vibrating wire 

instruments which correlate better to pillar strength 

formulas. This could account for the apparently high 

reading, although applying a reduction factor would also 

result in very low stress on the pillar side. It should also be 

noted that none of the measurements in Mark and 

Iannacchione’s study were in fender pillar applications. 

Measurement of panel stress change during advancement 

into a full-width pre-driven recovery room in a previous 

study at this mine were typically around 2,500 psi (Oyler et 

al., 2001), again with vibrating wire stress gages. 

2. Difference in panel stress and pillar stress – The expected 

load transfer mechanics is for the front abutment stress to 

transfer from the panel to the outby pillars as the panel 

fender yields and is no longer able to sustain load. The 

pillar cells located 10 ft from the recovery room did show 

stress increase as the panel fender yielded, but the stress 

change of about 1,000 psi was much smaller than that seen 

in the panel cell which increased by nearly 7,500 psi. It 

was expected that the stress changes would be of similar 

magnitude to account for the load balance that must occur. 

Numerical modeling by Zhang and measurements taken in 

previous longwall studies support this expectation (Zhang 

et al., 2006 and Oyler et al., 2001). If the panel 

measurement is correct and the stress development in the 

pillars was indeed much smaller, this would suggest that 

the load was distributed over a greater area to reduce the 

stress. The measured pillar stress was similar to that 

measured in previous studies (Oyler et al., 2001). 

3. Shedding of pillar stress – The increase in pillar stress, as 

measured by the first set of cells 10 ft from the pillar edge, 

was followed by a near equal shedding of stress in a 

relatively short time frame (one shield loading cycle) 

following the yielding of the panel fender. Since the same 

cells increase in pressure as the pillars are mined to widen 

the recovery room, the load shedding is not due to yielding 

of the coal. So what caused the pillar stress to decrease? It 

was postulated that the drop in stress is caused by a 

decrease in load caused by a reduction in span. Since the 

panel fender is no longer capable sustaining load, it implies 

that the rear "abutment" moves forward closer to the shield 

line causing the reduced span and drop in load. The lower 

load then explains the drop in pillar stress. This load 

shedding behavior has been observed in previous longwall 

recovery rooms which were not stable (Oyler et al., 2001). 

When the panel fender yields or abruptly looses it load 

carrying capacity, the abutment must transfer across the 

recovery room onto the pillars. This is a shock to the 

overall system which apparently produces significant 

changes in the loading mechanisms. 

4. Lack of shield loading during widening of recovery room – 

The shield loading did not increase above the set pressure 

during the widening of the recovery an additional 16 ft by 

extraction of the pillars. In many areas of the face, the 

shield pressure development was small once the pumpable 

supports were extracted. Again, this indicates that the face 

area was destressed and the abutment pressures were 

dissipated. This supports the rear abutment hypothesis 

postulated above. 

CONCLUSIONS 

Front abutment pressures increase as the longwall face 

advances toward the recovery room. As the panel advances toward 

the recovery room, this pressure causes stress increase in the panel, 

the pumpable roof supports, and the outby pillars suggesting that 

the load is bridging across the recovery room prior to the yielding 

of the panel fender. These effects begin to occur when the face is 

approximately 100 ft away and gradually increase until the face is 

about 25 ft away, at which time the loading rates increase 

significantly. When the face is close enough such that strength of 

the panel fender is exceeded to the point where it can no longer 

support load, which in this case was about 10 ft, the yielding of the 

panel fender triggers a cascading sequence of events, which cause 

yielding of the shields and pumpable supports. The yielding of the 

pumpable supports in the recovery room then triggers another 

sequence of events in which the roof in the recovery room above 

the supports deforms downward and closure of the recovery room 

increases dramatically. 

The shedding of the pillar stress is a significant discovery with 

potentially major consequences. The surprising event in this 

process was that the load shedding of the panel fender also 

coincided with stress shedding of the outby pillar stress (to a depth 

of at least 10 ft) and this occurred just prior to the yielding of the 

pumpable supports. First, the stress shedding of the pillar in itself 

is surprising, but if this is going to occur, the expectation is that the 

pumpable supports would yield prior to the stress shedding of the 

pillar stress. It is postulated that the cause of the stress shedding of 

the pillar may be due to advancement of the rear abutment toward 

the recovery room, thereby reducing the span that the main roof is 

bridging from the gob over the recovery room. The effect of this 

would be a reduction in main roof loading, thereby accounting for 

the reduction in pillar stress. While this may initially seem like a 
good thing, the advancement of the rear abutment stress would be 

caused by a large of amount of convergence close to or in the 

recovery room. Although the outby pillars may shed load, they are 

not yielding. Therefore, the convergence profile would be 

described as minimal at the pillar area and increasing quickly 

through the recovery room toward the panel (rear abutment). This 

bending of the strata can produce high tensile stresses in the 

immediate roof beam that can cause failure at the maximum 

bending moment somewhere over the outby pillars. If this stress 

would cause failure of the main roof beam, the weighting of this 

coupled with the large convergence could lead to catastrophic 

weighting and roof failure. 

Obviously, this did not occur in this case. The room was stable 

and the supports were recovered without incident. Nonetheless, if 

this mechanism is indeed the process that is occurring, several 

significant design implications are revealed. 

•  Standing support is critical to achieving successful stabilit

of recovery rooms, even in partially pre-driven recovery 

rooms such as these. The support must be able to control 

the immediate roof since it fails or separates above the 

bolted horizon, even with the application of cable bolts. 

Without them, the roof must span from the outby pillars to 

the longwall shields. 

•  Premature yielding of the standing supports can be ver

detrimental to the roof control process, particularly with 

supports that shed load upon yielding, as is the case with the 

pumpable supports used in this study. It appears that the 

standing supports can help to delay the process of the roof 

deformation and load shedding of the pillars, essentially by 

helping to control the roof span. Premature yielding would 

increase this span and place greater demand on the shields, 

but since the shields are already being pushed into persistent 

yielding during most of the shield cycle when the panel 

fender yields, there is little if any additional work that the 

shields can provide. 

•  The pre-yield capacity of the pumpable supports is not th

issue; they would yield regardless of their capacity (within 

the design capability of these supports systems). The 

critical design parameter for the pumpable support is the 

residual load capacity and their ability to sustain residual 

loading until the longwall shields are fully advanced into the 

recovery room. The pumpable cribs used in this study are 

very stiff supports, reaching yield load at less than 0.5 in of 

convergence, shedding load rapidly after the cementititious 

grout strength is exceeded. 

•  Higher capacity shields are not likely to make muc

difference. The yielding of the panel fender transfers load 

beyond the capacity of any modern shield. However, poorly 

maintained shields, ones that have leaking leg cylinders, are 

likely to cause excessive convergence that can lead to 

failure of the arching capability of the main roof and 

collapse of the recovery room. 

In conclusion, an understanding of the loading mechanisms 

associated with longwall recovery using pre-driven rooms is critical. 

While safer from a roof support installation perspective, the 

concept of using a pre-driven room increases the risk of the 

equipment recovery, and a better understanding of the loading 

mechanisms can help to ensure that catastrophic weighting failures 

will not occur. Some surprising results were revealed in this study, 

particularly the load shedding of the pillar stress prior to the 

advancement of the longwall face into the recovery room and the 

largely destressed condition that existed during the widening of the 

room. It is recommended that additional studies be conducted to 

further evaluate the loading mechanics associated with pre-driven 

longwall recovery rooms. Numerical modeling by Zhang et al. 

(2006) did not show the load shedding of the pillar stress, 

indicating that the models are not yet fully simulating the loading 

conditions. Additional numerical modeling studies, perhaps using 

FLAC (Itasca, 2001) that can more accurately simulate the post 

failure rock behavior should be constructed to further study this 

problem. In addition, efforts to improve the loading characteristics 

of the pumpable supports should continue. Reducing the stiffness 

and/or load shedding as well as increasing the yield displacement 

and residual loading capacity should be pursued if this support is to 

continue to be used for longwall recovery rooms. 

REFERENCES 

Itasca (2001). User’s Guide. 

Mark, C. and Iannacchione, A.T. (1992). Coal Pillar Mechanics: 

Theoretical Models and Field Measurements Compared. 

Proceedings of the Workshop on Coal Pillar Mechanics and Design, 

NIOSH Information Circular 9315, pp 78-93. 

Oyler, D.C., Frith, R.C., Dolinar, D.R. and Mark, C. (1998). 

International Experience with Longwall Mining into Pre-Driven 

Rooms. Proceedings of 17th International Conference on Ground 

Control in Mining, Morgantown, WV, pp 44-53. 

Oyler, D.C., Mark, C., Dolinar, D.R. and Frith, R.C., (2001). A 

Study of Ground Control Effects of Mining Longwall Faces into 

Pre-Driven Longwall Recovery Room. Geotechnical and 

Geological Engineering 19:137-168. 

Tadolini, S.C., Zhang, Y. and Peng, S. (2002). Pre-driven 

Experimental Longwall Recovery Room Under Weak Roof 

Conditions–Design, Implementation, and Evaluation. Proceedings 

of 21st International Conference on Ground Control in Mining, 

Morgantown, WV, pp 1-10. 

Wynne, T., John, S., Guo, S. and Peng, S.S. (1993). Design, 

Monitoring, and Evaluation of a Pre-driven Longwall Recovery 

Room. Proceedings of 12th International Conference on Ground 

Control in Mining, Morgantown, WV, pp 205-216. 

Zhang, P., Mishra, M., Trackemas, J., Zeglen, E., Huff, C., Peng, 

S.S. and Chen, J. (2006). Pre-Driven Longwall Recovery Room 

Under Weak Roof Conditions–Design, Evaluation, and Monitoring. 

Proceedings of 25th International Conference on Ground Control in 

Mining, Morgantown, WV, pp 221-228.