ROCK MASS BEHAVIOR AND SUPPORT RESPONSE 


IN A LONGWALL PANEL PRE-DRIVEN RECOVERY ROOM 


Dr. Stephen C. Tadolini 


Dr. Thomas M. Barczak 


National Institute for Occupational Safety and Health 


An underground investigation was designed and implemented by the National Institute for 

Occupational Safety and Health (NIOSH) to evaluate the effectiveness of pre-driven 

longwall recovery rooms supported with pumpable concrete roof supports. The pumpable 

roof supports were instrumented with hydraulic pressure cells to measure support loading. 

Displacement of the support was also measured to assess the support reaction to ground 

movements. Movement up to 12.5 m (40 ft) into the roof was measured with sonic probes 

and multipoint extensometers. Borehole pressure cells were installed in the panel fender and 

outby pillars to evaluate load transfer. Shield loading was also assessed to determine the 

impact of the panel fender yielding on face loading. 

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 3.12 m (10 ft) from the recovery room. The shields continued to yield until 

the face advanced into the recovery room. Convergence in the pre-driven recovery room 

typically reached 15-20 cm (6-8 inches) 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 10 m (32 ft) horizon with total deformation ranging 

from five to 10 cm (2 to 4 inches), 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. 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 instabilities that 

can lead to catastrophic results. Understanding 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 is providing valuable information for the 

refinement of the NIOSH Ground Reaction Curve for standing and intrinsic roof support 

system design. 

1.1 Introduction 

Pre-driven recovery rooms provide opportunity to reduce the longwall move time by having 

a place where the mining equipment can be removed in a prepared (supported) entry before 

the longwall approaches the end of the panel. In such cases, 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 predriven 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 recovery 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 non-yielding, despite their 

high capacity, fail prematurely and endanger ground control (Tadolini et al., 2002). 

These requirements have caused the mine to utilize a pumpable roof support system for the 

recovery application in combination with roof bolts, cable bolts, 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 0.8 m (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, don’t 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 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. 

2.1 Site Description and Recovery Room Instrumentation Profile 

The approach utilized for the recovery plan is slightly different because the design considers 

an entry only partially (50 pct) pre-driven. To complete this, a 5-m (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 pre-driven room, the room is widened an additional 5-m (16 ft) to provide 

sufficient room for the recovery of the longwall equipment. 

The B4 longwall panel was 453 m (1,450 ft) wide. The depth of cover averaged 187 m (600 

ft). 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 3.1 and 7.8-m (10 and 22-ft) thick limestone members and 3.4 and 7.8-m (11 

and 25-ft) thick shale layers. 

Figure 1 shows the support design for the recovery room. The recovery room was supported 

with 2.5-m (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 cut into and 

then widened the recovery room. Two and a half meter (8-ft) cable bolts on 1.25-m (4-ft) 

spacing were installed 62.5 cm (2 ft) from the panel rib to help support the roof when the 

first row of cribs were cut out, and 4.8-m (12-ft) cable bolts on 1.25-m (4-ft) spacing were 

installed 62.5 cm (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 0.8-m (30-in) diameter pumpable roof supports were utilized in the recovery 

room as shown in figure 1. 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, indicate the 

pumpable support is very stiff with peak load occurring at less than 12.7 mm (0.5 in) of 

displacement. The support sheds load rapidly during yielding as the bag acts to provide 

confinement and maintain a residual loading capacity. 
 

Scale, Meters 

2 

Top View 

0 

2.5 m combination bolt 

installed at 20o angle 

Steel straps and 

wire mesh 

2.5 m combination 

bolt installed at 45o 

angle 

Side View 15 mm x 2.5 m cable 

bolt (1.25 m centers) 

15 mm x 3.75 m 

cable bolts (1.25 m 

centers) 

2.5 m combination 

bolts (1.75 m centers) 

.8 m diameter crib 

20o angle 45o angle 

RECOVERY AREA	 LONGWALL FACE 

Figure 1. Support design for the longwall recovery room. 

A diagram depicting the approaching longwall panel, predriven recovery room and outby 

pillar configuration is shown in figure 2. 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 the DBT pressure 

transducers installed on each of the longwall shield legs. Data were collected every 

10 seconds. 

•	 Pumpable support loading Specially-made hydraulic (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 transducers were mounted to the 

side of the pumpable support to measure support displacement. 

•	 Sonic probes were used to measure roof deformation up to 7.5 m (24 ft) into the 

roof. 
Panel BPC 6 m

3 m

Roof deformation Location of crib loads and measurements convergence measurements

3 m 3 m Additional convergence 

measurements 

Left PillarBPC’s ddi ional

@ 3 m depth 

Right PillarBPC’s

Room dened to this location Room is widened to this location i

@ 6 m depth Tailgate C Chute Right PillarBPC’s Headgate

 

 

 @ 8 m depth 

 

A B C D E F G H

reco ery line

second entry

third entry

Figure 2. Diagram showing instrumentation in the longwall recovery room. 

•	 Three-point mechanical extensometers attached to displacement transducers were 

used to measure roof deformation relative to the 8.1, 10, 11.9 m (26, 32, and 38 ft) 

roof horizons. 

•	 Convergence rods with displacement measuring 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 (BPCs) were used to measure panel and outby pillar stress 

development in the C chute area. Figure 2 depicts the location of the cells. One cell 

was installed in the panel 3.1-m (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 3.1-m (10-ft) and two other cells 6.2-m 

(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 8.1-m (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. 

3.1 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 7.8 m 

(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.
SHIELD LEG PRESSURE, MPa

PUMPABLE SUPPORT LOAD, kN 

23 m FACE POSITIONS 16 m

Shield cycle 

Shield pressures 

Pumpable 

support load 

Pillar side 

pumpable 

support 

Middle row 

pumpable support 

Pumpable support next to panel 

Shearer passage 

48

41

35

28

21

14

7

378

334

289

245

200

156

111

67

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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 3. Correlation between pumpable support load and #175 shield cycles showing load 

increase during shearer passage. 

3.1.1 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 30.2 m (100 ft) away. 

The increase in loading rate remained very gradual until the face was approximately 7.8 m 

(25 ft) from the recovery room. Overall, the three instrumented pumpable supports increased 

in load by 400 kN (90 kips) (panel side support), 278 kN (63 kips) (middle row support), 

and 342 kN (77 kips) (pillar side support) during this period, representing 21%, 23%, and 

35% of the observed peak 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 31.2 m (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 activity while the recovery room supports at this juncture are seeing the main roof 

activity. Figure 3 shows an example for shield number 175 (adjacent to Chute C in the 

recovery room) as the face advanced from a location 23.4 m (75 ft) away to 15.6 m (50 ft) 

away the recovery room. As the face advanced closer than the 15.6 m (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. 
Longwall

face

C Chut

23 m FACE POSITIONS 16 m 

SHIELD LEG PRESSURE MPa 

0 

BPC PRESSURE, MPa 

e

Longwall 

face 

C Chute 

Shield cycle 

Shield 

pressures 

Pillar ( depth) BPCs 

Panel BPC 

Shearer passage 

48

41

35

28

21

14

7

6.2

5.5

4.8

4.1

3.4

2.7

2.0

1.3

0.6

3 m 

3 m 

3 m 

6 m 

3 m 

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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 4. Correlation between borehole pressure cells and shield cycles. 

3.1.2 Yielding of the Panel Fender as Face Approached the Recovery Room 

As the face advanced within the 7.8-m (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 (figure 

4). 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: 

•	 Panel BPC yields The BPC in the longwall panel, located 3.1 m (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 7.8 m (25 ft) away from the recovery room or 4.7 m (15 ft) from the 

BPC position (figure 5). The pressure in the cell increased from 14.5 MPa (2,100 

psi) at the 7.8-m (25-ft) location to approximately 36.5 MPa (5,300 psi) when the 

face was 5 m (16 ft) from the recovery room or just 1.9 m (6 ft) from the BPC. 

During the next shield cycle, the change in pressure reached a maximum of 51.7 

MPa (7,500 psi), and then dropped to about 39.6 MPa (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 28.9 MPa (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 2), 

which is about 312.5 m (1,000 ft) from the headgate on this 453 m (1,450 ft) long 
FACE POSITION, m from recovery 

7.6 m 6.7 m 5.8 m 4.9 m 4.0 m 3.0 m 2.1 m 1.2 m 

SHIELD LEG PRESSURE, MPa 

0

SUPPORT LOAD, kN 

Panel BPC 

Pillar side 

pumpable support 

Middle row 

pumpable 

Pumpable support next to panel 

Shield pressures 

Pumpable support 

loading increases 

during entire shield 

cycle 

Pumpable support 

loading increases mostly 

during shearer passage 

Yielding of 

shields and 

pumpable 

supports 

62

55

48

41

34

27

20

13

6

2,000

1,770

1,550

1,330

1,110

890

670

450

220

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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 5. When the longwall face was approximately 5 m (16 ft) inby the recovery room, 

the pumpable support loading increased during the full shield cycle. 

t

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 5, 

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 BPC stress occurred not when the 

shearer passed by this location, but when the shearer was over 312.5 m (1,000 ft) 

away at the headgate, particularly given the close proximity of the longwall face to 

the stress cell (approximately 1.9 m (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 as 

the shearer passes back in front of the pressure cell (see figure 6). 

One question is whether the yielding of the BPC cell located in the panel, 3.1 m 

(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 5) 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. The panel fender may not be completely yielded but the steady 
225

250 55

48

6

FACE POSITION, m from recovery room 

8.5 m 7.6 m 6.7 m 5.8 m 4.9 m 4.0 m 3.9 

Shearer position 

Panel BPC 

Shield cycle 

Shearer at headgate 

Shearer at BPC location 

50

25

Headgate SHEARER POSITION, shield # Tailgate

41

34

27

20

13

200

175

150

125

100

75

0

BPC PRESSURE, MPa 

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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 6. Yielding of panel BPC occurred when the longwall shearer was approximately 

302 m (1,000 ft) away from the cell location. 

decline in the BPC pressure (before being cut) and the rapid increase in shield 

pressure may suggest that the fender continues to yield during the time when the 

shearer has moved away from this zone. 

•	 Pillar fender BPCs shed load The stress cells installed in the outby pillar rib (3.1 

m (10 ft) from the edge of the recovery room) also increased in concert with the 

panel stress cell. This indicates that some of the abutment stress is transferring 

across the recovery room, but the magnitude of the stress change is much less in 

the pillar than in the panel. These two cells peak and then gradually shed load 

during the shearer passage on the shield cycle following the peak loading of the 

panel cell. Since the stress levels on the pillar cells were only 25% of the level that 

caused yielding of the panel cell, it is unlikely that the coal pillar at the cell 

location was actually yielding due to loading beyond the coal strength during this 

load shedding event. However, some rib spalling did occur to a depth of 0.6-0.9 m 

(2-3 ft). 

•	 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 (shown in figure 5). The pumpable supports yield two 

shield cycles after the panel BPC yields and one cycle after the pillar BPC shed 

load, putting the yield of the supports at a face position of approximately 3.1 m (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. 
t t

 

FACE POSITION, m from recovery room 

11.3 m 10.4 m 9.4 m 8.5 m 7.6 m 6.7 m 5.8 m 4.9 m 4.0 m 3.0 m 2.1 m 1.2 m 

12 55

Panel BPC 

Roof deformation 

on 

from 

si ompres C

7.9 m 

anchor 

ation 

Pillar 3 m depth) 

Dil

BPCs 9.8 m anc

11.6 m an

hor 

chor 

9 48

6 41

BPC PRESSURE, MPa

ROOF MOVEMENT, mm

3 34

0 27

3 20

6

9

13

6

12

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12:00 14:00 16:00 18:00 20:00 22:00 00:00 02:00 04:00 06:00

DATE AND TIME 

Figure 7. Roof initially is compressed during yielding of panel fender and then 

begins to deform downward into the recovery room. 

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 3.1 m (10 ft) location to approximately 

2.2 m (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 1,775 kN (200 tons) of load capacity in less than 12.7 

mm (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, the data indicate that the roof was moving slightly “upward”, as shown in figure 7. 

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 8.1 m (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. 

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 12.7 cm (5 in), while the roof 

deformation at this time was approximately 3.7 cm (1.2 in). This indicates that about 75% of 
the convergence was from floor heave and/or main roof movement beyond the 11.9-m (38­

ft) horizon during the panel fender yielding. 

3.1.3 	 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 supports can be critical to the stability of the 

recovery room. At the C chute location, the shield loading decreased as the supports were 

extracted and the face advanced toward the pillar line (see figure 8). As the first row of 

supports was being extracted, the shield loading remained high but below the yield rating. 

As the second row of pumpable supports was being extracted, the shield loading did not 

change much after being set and this condition remained throughout the advancement 

through the pre-driven recovery room. The pillar stress, as measured by the cells at both the 

3.1- and 6.2-m (10- and 20-ft) locations from the edge of the recovery room, remained fairly 

constant during this process. The roof continued to deform slightly in the chute as measured 

by a three-point extensometer in the center of the entry located 5 m (16 ft) from the outby 

edge of the pre-driven recovery room, but the deformations were nearly an order of 

magnitude less than that seen in the recovery room. An additional 25.4 mm (1 in) of closure 

occurred for a distance of about 6.2 m (20 ft) in the C chute crosscut as measured by 

electronic closure meters measuring roof-to-floor displacement. 

 

0

SHIELD LEG PRESSURES, MPa 

0

PUMPABLE SUPPORT LOAD, kN 

30.5 - 7.6 m 7.6- 0 m Mine Through Room Widen Room 

Removal of 

first support 

Removal of 2nd 

and 3rd support 

48

41

34

27

20

13

6

1,995

1,710

1,425

1,140

2855

570

285

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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 8. Shield loading decreased as the pumpable supports were extracted from the 

longwall recovery room. 

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 predriven recovery room. The shield loading at some locations was considerably higher than 

that observed at the three instrumentation sites. For example, shield number 185 yielded on 

all cycles during the extraction of the pumpable supports and advancement through the predriven 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. 

3.1.4 Widening of the Pre-Driven Recovery Room 

The next phase of the recovery process was to widen the room by extracting an additional 5 

m (16 ft) of the coal pillars to create sufficient room to recover the longwall equipment. This 

phase requires a variation in the typical longwall mining process in that extensions are 

included to the face conveyor advance rams to allow advancement of the shearer and cutting 

of the last portion of the pillar without advancement of the supports. The mesh was deployed 

and exposed roof was bolted after each cut to provide a fully supported room for recovery. 

The shields developed little, if any, loading during the extraction of the pillars to widen the 

room. On several occasions, the shields struggled to maintain set pressure, with the positive 

set control feature activating to re-supply pump pressure periodically during the shield cycle 

(figure 8). This behavior indicated that the immediate area is largely destressed. What would 

cause this? One possibility is that the front abutment loading on the pillars has dissipated or 

has been pushed well ahead of the face area. 

The BPCs located in the pillar at depths of 3.1 and 6.2 m (10 and 20 ft) from the recovery 

room again document the impact of the shearer passage on the loading mechanics. As the 

face cuts to the 3.1 m (10 ft) location, the measured pressure change in the cells located 

there spiked to 19.7 MPa (2,860 psi) and then shed load rapidly as the cell and/or contact 

condition is damaged from the shearer activity. As recorded, the pillar stress measured at the 

6.2 m (20 ft) location 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. As the extraction approached the location of the convergence rods, the 

convergence rate increased significantly as expected. 

4.1 Discussion of Results - Unexpected Discoveries 

•	 Panel fender pressures The BPC in the panel recorded a maximum pressure 

change of nearly 51.7 MPa (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 187 m (600 ft) at this 

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

an abutment stress of 12.4 to 20.7 MPa (1,800 to 3,000 psi). Furthermore, the cell 
was only about 1.9 m (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 17.2 MPa (2,500 psi) (Oyler et al., 2001), again with 

vibrating wire stress gages. 

•	 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 3.1 

m (10 ft) from the recovery room did show stress increase as the panel fender 

yielded, but the stress change of about 6.9 MPa (1,000 psi) was much smaller than 

that seen in the panel cell which increased by nearly 51.7 MPa (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). 

•	 Shedding of pillar stress The increase in pillar stress, as measured by the first set 

of cells 3.1 m (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 loses 

its 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. Also an important consideration is 

the pillar BPCs behavior is similar to the shields at this time, exhibiting lower 

loads. This implies that they are being more affected by the immediate roof as 

compared to the main roof during further inby mining. 
•	 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 5 m (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. 

5.1 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 31.2 m (100 ft) away and gradually increase until the 

face is about 7.8 m (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 3.1 m (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 3.1 

m (10 ft)) and this occurred just prior to the yielding of the pumpable supports. First, the 

stress shedding of the pillar 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. It may also be just coincidental in this 

specific case, the periodic weighting interval coincidentally occurred when the face entered 

the recovery room. The effect of this stress rotation would be a reduction in main roof 

loading, which would account 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 stability 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 very 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 the 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 12.7 mm (0.5 

in) of convergence, shedding load rapidly after the cementititious grout strength 

is exceeded. 

•	 Higher capacity shields are not likely to make much 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. 
Disclaimer 

The findings and conclusions in this report have not been formally disseminated by the 

National Institute for Occupational Safety and Health and should not be construed to 

represent any agency determination or policy. 

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