1

DESIGN PARAMETERS OF ROOF SUPPORT SYSTEMS 

FOR PRE-DRIVEN LONGWALL RECOVERY ROOMS 

S. C. Tadolini 

T. M. Barczak 

NIOSH 

Pittsburgh, PA

ABSTRACT 

 Reduction in the recovery time required and 

improvements in safety during a longwall face move have 

compelled coal mine operators to examine and use predriven longwall recovery rooms. The recovery entry is 

developed at the longwall stop position and supported 

ahead of time so that the required combinations of standing 

and internal supports can be installed before the longwall 

face approaches. A calibrated three-dimensional finite 

element model, developed in conjunction with an 

underground test area, provided the critical components 

and design principals for a comprehensive parametric 

study. The study indicates that vertical, horizontal, and 

shear stresses concentrated on the outby abutment pillar 

corner of the recovery room resulting in deformations in 

the immediate roof above the primary roof support 

anchorage level. The results of the numerical model and 

field site observations indicated that the combination of the 

internal bolting system and the standing concrete supports 

are critical for the successful and safe extraction of the 

longwall equipment. Cable supports installed at and angle 

over the pillars can help stabilize the disturbed zone above 

the adjacent fender and outby pillars, however; standing 

supports are required in most sedimentary roof conditions 

to increase stability and assure a safe longwall extraction. 

INTRODUCTION 

 Moving longwall equipment from the end of the 

current panel into the next “start or set-up” room has 

always been an operation that requires thorough 

engineering and planning. Conventional methods require 

that a pre-determined location be established and 

preparations be made between 12 to 14 shearer cuts from 

the extraction point. After each cut is taken, welded wire 

mesh or chain link material is placed above the shield 

canopy so that the mesh overlies the shields to be 

extracted at the longwall stop position. The selected 

material is usually reinforced with steel wire ropes that 

run the width of the longwall panel. The last 8 to10 ft are 

mined and the shields are not advanced. This area is 

prepared for shield extraction by installing wire mesh or 

chain-link material against the mine roof and pinned with 

bolts. In most cases, either hand-held drilling equipment 

or specialized single boom bolters designed for this 

application are used. The meshing and bolting cycle 

times can vary from operation to operation and are 

usually a function of the immediate roof condition. 

Experience in the longwall recovery areas of this 

particular field site has shown that the weak roof must be 

heavily reinforced with wire mesh and long bolts to 

control the area during the tear-down and extraction 

process. 

 Recent improvements to the longwall extraction 

process have been made with the introduction of nylon 

woven geotextile materials. The material is usually 

provided in one pre-assembled sheet that permits 

coverage of the entire longwall roof area. Briefly, the 

fabric is brought into the mine in a roll and is transported 

down the longwall face on top of the conveyor pan. The 

material is hung up on the shield tips and is unrolled so it 

will unwind behind the shields as the face continues to 

advance, usually 25 to 40 ft. At the longwall stop 

position the material is bolted to the roof using the same 

equipment and techniques utilized in a traditional 

extraction. While these preparations are necessary to 

minimize the hazards of roof falls and prevent failed 

material from falling into the position of the extracted 

shield, they can also slow the advance rates, impede 

production, and, more importantly, do not eliminate the 

hazards associated with applying the wire-mesh or 

geotextile materials during the shield recovery process. 

When the face withdrawal begins, additional 

reinforcements or support such as roof bolts, cribbing, Ibeams, props, and even polyurethane injection may be 

required to stabilize the entry for safe shield removal. 

The area is restricted, which often makes the movement 

and installation of these materials cumbersome and 

potentially hazardous. 
2

 As an alternative to this described method, mines have 

investigated and utilized pre-driven recovery rooms for 

longwall face moves. This entry is developed and 

supported ahead of time so that the required combination 

of standing and internal support and reinforcement can be 

installed before the longwall face approaches. The major 

benefit of this approach is that the supports are placed 

under traditional development mining conditions, 

installed with an automated temporary roof support 

(ATRS) roof bolting system. The ATRS limits the 

exposure of the operators to the hazards associated with 

roof bolts installed with hand-held equipment and narrow 

profile line bolters under unsupported and highly stressed 

roof. Pre-supported recovery rooms would also permit all 

of the required supports and reinforcement to be installed 

prior to the front abutment pressures, which can create 

production problems and support delays using traditional 

longwall equipment recovery methods. 

 To investigate the feasibility of a pre-driven longwall 

recovery room, a pre-driven room was mined and 

supported between two existing recovery chutes as shown 

in Figure 1. The recovery room, driven 170 ft long and 16 

ft wide, was supported with several types of internal roof 

reinforcements and high-capacity roof-to-floor cuttable 

concrete cribs. Instrumentation was installed to monitor 

the loading and stiffness characteristics of the cribs and 

differential sag-stations were used to help evaluate the 

roof behavior. Additionally, understanding the actual 

field response of the room and supports was 

supplemented with 3-dimensional finite element models. 

This paper describes the design process, support 

methodology, monitoring, and numerical results and 

presents specific suggestions and recommendations based 

on this research work. 

RECOVERY ROOM TEST SITE AND GEOLOGY 

 The 1,000-ft-wide longwall face is located in the 

Herrin No. 6 coal seam in Illinois. A pre-driven recovery 

room was driven between the inby ends of the no. 4 and 

no. 5 recovery chutes, as shown in Figure 1. The five 

recovery chutes were developed months prior to the 

mining, reinforcement, and support of the recovery room. 

The recovery chutes are traditionally used as multiple 

access ways to remove face equipment during the 

longwall extraction. The longwall chutes and the 

recovery entry were mined 16 ft wide. The overburden 

depth was 312 ft and coal seam was 7.5 ft thick. The 

pillar centers in the area are shown to be 150 ft long and 

65 ft wide. The recovery chutes were driven 120 ft into 

the longwall block on approximately 150 ft centers. The 

immediate roof consists of laminated shale that is 6 ft 

thick, overlain by gray shale of various thicknesses. A 

weak layer of claystone in the immediate floor ranged 

from 1 to 1.5 ft thick with an underlying competent 

limestone 5 ft thick. In general, this roof is considered 

extremely weak with calculated Coal Mine Roof Rating 

(CMRR) values that range from 40 to 45 (Molinda and 

Mark, 1994). Another consideration, and one of the 

reasons this area and method were selected, was the 

presence of a series of clay dikes that intersect the coal 

seam and interrupted the roof and floor members. These 

features are not new to this operation and the areas 

adjacent to the dikes were generally found to be 

extremely weak and unconsolidated. Operations felt that 

the safest way to extract the longwall equipment from this 

area, limiting personnel exposure to the hazards, would be 

LONGWALL PANEL

RECOVERY ROOM

CLAYDIKES

Figure 1. Plan view of longwall recovery area. 

Recovery room between chutes no. 4 

and no. 5. 
3

to use a pre-driven recovery room. The clay dike zones 

would be supported with additional bolts and surface 

control measures prior to the forward abutment stresses 

created by the approaching longwall. 

FINITE ELEMENT RESULTS 

 The finite element models for longwall mining 

encompass a large area because they must include the 

adjacent gob zones from the previously mined panels, the 

entry and pillar systems, and the moving longwall face. 

The only viable solution to obtain realistic results was to 

use 3-dimensional simulation. Due to the large size of the 

area of interest, the finite element modeling must be 

portioned into two steps, the global model and the submodels. The global model is used to investigate the 

overall rock behavior around the longwall panel while the 

sub-model focuses on a particular section to allow 

detailed analyses. ABAQUS, used for this study, uses a 

sub-model linked with the global model by computed 

displacements to reduce the model size for the local 

response analysis. A cross-section of the sub-model is 

shown in Figure 2. To conduct this investigation the 

modeling consisted of three steps: 

1. A global model was used to determine the magnitude 

and changes in the abutment pressures as the longwall 

face advanced toward the proposed recovery room. 

The goal was to determine the worst conditions the 

recovery room would be subjected to during the 

extraction. 

2. A sub-model is extracted for the panel at the center of 

the global model. Two rows of concrete cribs, one at 

the center and the other against the inby rib of the 

proposed recovery room are “installed”. 

3. The sub-model is used to simulate the cement crib 

loading, stress distribution around the recovery room, 

and the yielding of both the immediate roof and floor 

under the loading conditions to which the recovery 

room may be subjected. 

 Based on the finite element modeling results and 

engineering analyses, a complete reinforcement and 

support system design for the proposed recovery room 

was recommended. 

The Global and Sub-model

 The dimensions of the global model were 1,080 ft 

wide by 630 ft long and 350 ft high consisting of one-half 

the region from the previously mined panel and one half 

the region from the unmined panel. The global model 

consists of more than 105,000 elements run at 1/6 step per 

increment, requiring up to 20 hours of computer run time. 

The abutment pressures generated as a result of longwall 

mining were simulated using a special gob model (Morsy 

and Peng, 2001). All of the materials, coal, roof, and 

floor, were assumed to behave linearly-elastic until they 

reached yield as defined by the Mohr Coulomb failure 

criteria. Beyond the yield point the materials were 

considered to behave perfectly plastic. 

 The changes on the abutment pressures were 

investigated for three longwall face locations, that is, 

when the face was 60, 18, and 12 ft from the recovery 

room. The global model revealed that when the face was 

12 ft from the inby rib of the recovery room, the 12 ft coal 

pillar fender yielded completely. This was defined as the 

worst possible scenario for the internal and external 

reinforcement and support systems. The peak abutment 

pressures of 1,200 to 1,500 psi were 3.5 to 4.4 times the 

estimated original vertical stress as a function of 

overburden (1.1 psi/ft x 312 ft). As expected, the 

abutment pressures at the center of the longwall face were 

larger than those calculated at both ends of the panel. 

 A three-dimensional sub-model, 168 ft long by 9 ft 

thick and 350 ft high, was extracted from the global 

model to represent the recovery room. As shown in 

Figure 2, two rows of 4-ft diameter concrete cribs were 

installed and a uniformly distributed load of 166 psi was 

applied to roof to simulate the powered shield support. A 

crib capacity of 900 tons was simulated to determine the 

abutment pressures applied as the longwall approached. 

The portion of the abutment pressures transferred to crib 

A, located close to the oncoming face, was 480 psi and 

500 psi for crib B, which was located in the center of the 

recovery entry. The calculated maximum roof deflections 

above and adjacent to the cribs were approximately 

2.8 inches. 

Figure 2. Cross-section of the 3-dimensional submodel. 

12' yield zone

gob

outby pillar shield pressure

concrete cribs

A B
4

REINFORCEMENT AND SUPPORT SYSTEM 

DESIGN 

 The result of the 3-dimensional finite element results 

provided the necessary criteria for consideration for the 

final reinforcement and support system design for the 

proposed recovery room. The main considerations were: 

1. The immediate roof above the recovery room, 

consisting of a 6-ft thick laminated shale, yields during 

the initial development of the recovery room. 

2. When the longwall face is 12 ft from entering the 

recovery room, the 12-ft coal block or fender pillar 

had yielded. 

3. The immediate roof continues to yield to a depth of 15 

ft and the outby barrier pillar yields to a depth of 4 ft 

upon the arrival of the large forward abutment 

pressures. 

4. The immediate floor is claystone that yields on the 

outby side of the recovery room, but not under the 

concrete cribs. 

5. The roof in the outby portion of the recovery room 

away from the cribs, over the abutment pillars, could 

yield and fail as the longwall panel approaches. 

Roof and Rib Bolting

 Based on the five major points obtained from the 

modeling, combined with strata engineering experience, a 

roof and rib bolting plan was designed and implemented 

as shown in Figure 3. Two-piece, 9 ft long, highly 

tensioned resin-assisted bolts were specified to confine 

and maintain the weight of the laminated shale in the 

entire recovery room. The capacity of the bolts, 46,200 

lbs or 23.1 tons, was nearly double the required design 

load of 12 tons. The majority of the bolts were installed 

with a 12-inch-square, 3⁄4-inch-thick plywood board and 

8-inch-square bearing plates. To enhance the support 

capacity and provide the required suspension component, 

should a separation occur above the primary bolting 

system, three, 0.6-inch-diameter, 15-ft-long cable bolts 

with a 30 ton capacity, were installed through a roof 

channel as secondary reinforcement between the rows of 

primary bolts. The primary bolts and secondary cables 

were installed on a 4-ft row spacing. To help establish 

and maintain surface control, the entire roof was screened 

with 8-gage welded wire mesh. This operation’s 

experience with prior longwall removals in the weak 

laminated shale roof dictated that full screening and a 

dense roof bolt pattern be utilized to control the 

immediate roof. 

 Additionally, two cables were installed as close as 

possible to the inby and outby side ribs of the recovery 

room. The outby side rib cable bolts were inclined at 30 

degree angles from vertical to support the weak laminated 

shale roof adjacent and above the outby pillar. The model 

indicated that the roof in this zone could yield and fail as 

the longwall panel approached the area. The inby side 

cable bolts, angled at approximately 60-degrees from 

vertical, over the inby side rib of the shields to be 

recovered were installed to prevent shear failure in front 

of the longwall shields due to the yielding in this zone. 

These cable bolts would also help prevent the falls that 

have traditionally occurred immediately behind the 

longwall shields as they were removed. 

 The outby pillar rib was reinforced from the roof to the 

floor with welded wire mesh that was bolted with a fullcolumn, resin-grouted, 4-ft-long, 3⁄4-inch rebar bolts. The 

bolts were placed in conjunction with 12-inch-square, 

3⁄4-inch-thick, plywood boards and 8-inch-square bearing 

plates. The bolts were placed on about 5-ft staggered row 

spacing at the top and bottom of the mesh panels. The 

inby panel rib of the recovery room was reinforced with 

1-inch-diameter wooden dowels anchored with resin. The 

dowels were 4-ft-long, and 12-inch square plywood 

boards served as the bearing plates. Wooden dowels were 

used for this application so they could be easily cut by the 

shearer and transported on the coal conveyor system. 

 For comparison and evaluation purposes, the 

calculated roof reinforcement of all of the installed 

reinforcement systems, the roof and cable bolts, was 

determined using the method described by Oyler et al., 

1998. This quantitative rating is calculated by 

determining the load capacity of each support per unit 

area of coverage, multiplied by the length of the system. 

The Reinforcement Density Index (RDI) has the units of 

Figure 3. Roof and rib reinforcement system 

application. 

15' ANGLED CABLE BOLT

9' DOUBLE LOCK BOLTS

LONGWALL FACE

MINING DIRECTION

LAMINATED

SHALE

GRAY

SHALE

16' WIDE RECOVERY ROOM

WIRE

MESH CONCRETE CRIBS

15' CABLE BOLT

RIB PINS

DOWEL PINS
5

psi·ft. Because two-types of support were used, the 

supports were summed. The calculated RDI for the 

recovery room was 487 psi·ft. Upon review of previous 

recovery room data, no roof fall failures (29 cases) were 

reported when the RDI was greater than 218 psi·ft (Oyler 

et al., 1998). The index calculated for this case does not 

take into account the additional surface and rib control 

achieved by using welded wire mesh, roof channels, rib 

bolts, and wooden dowels. 

Standing Support

 The standing support selected for the project was a 48-

inch diameter Minova1

 cuttable crib. To ease the 

construction process and minimize interference to coal 

production, the crib and bag materials were supplied in 

traditional concrete trucks and pumped to the site through 

a 3.5-inch diameter pipe. The steel pipe was dropped 

down the 5.5-inch casing that was placed in the original 

8-inch diameter borehole, allowing enough room for a 

communications line that was critical to the pouring 

process. The cribs were poured in two lifts into a 5-mil 

thick plastic bag supported by reusable fiberglass forms. 

Two rows of staggered cribs were placed on 10-ft crib 

spacings. The first row was placed about 1-ft from the 

inby side rib of the recovery room while the second row 

was placed in the center of the entry. To provide some 

additional yielding capability, roof contact was 

established by filling 4-ft square woven nylon bags or 

“pillows” filled with a pliable cementitous grout. The 

pliable grout was supplied from the surface down the 

3.5-inch diameter pipe and the bags filled until they were 

in contact with the mine roof. The final height of the 

cribs ranged between 81 and 88 inches while the bag 

thickness varied between 8 and 14 inches. The 

engineering specifications for the concrete materials were 

3,500 psi for the crib and 800 psi for the bags. Based on 

the strength of the 34 cribs with a diameter of 48 inches 

and the dimensions of the recovery room (16 ft wide and 

170 ft long), the total standing support capacity being 

used to minimize the mechanisms of a weighting failure 

distributed across the room was calculated to be 550 psi. 

The plan view of the crib locations and internal roof 

support system are shown in Figure 4. 

 

1

Mention of any company name or product does not 

constitute endorsement by the National Institute for 

Occupational Safety and Health. 

Instrumentation

 An instrumentation package was selected that would 

help evaluate the performance of the cribs and the 

primary and secondary roof support systems. Ten 

differential sag-stations were installed across the room 

and in the recovery chutes to monitor roof deformation. 

The differential sag-station anchors were located at the 6-, 

10-, and 15.5-ft depths into the mine roof. Roof-to-floor 

closure stations were established at the same locations so 

that the floor stability could be evaluated. Six cribs 

spaced along the length of the recovery room were fitted 

with flat-jack load cells placed between the top of the 

concrete crib and the bag so that actual crib loads could 

be measured. To complement the crib loading behavior, 

metal datum pins were driven into the concrete cribs and 

bags so that the actual crib displacements could be 

determined. The crib support stiffness, an indication of 

performance and a key component for evaluation, could 

be determined by measurement of the crib load and 

displacement. Figure 5 shows the specific cribs that were 

instrumented and the cross-section A-A illustrates the 

approximate measurement configuration. Individual 

Figure 4. Plan view of the standing support and 

internal roof reinforcement system.

CEMENT CRIBS

9' x 7/8" DIA. DOUBLE LOCK BOLTS

J-CHANNEL OR MONSTER MAT

WITH 15' CABLE BOLT 

LEGEND 

16' WIDE ENTRY

EXISTING RECOVERY CHUTE

PREVIOUSLY BOLTED

EXISTING RECOVERY CHUTE

PREVIOUSLY BOLTED

CLAYDIKE

15' ANGLED CABLE BOLT
6

shield loading was also monitored from the on-board 

computer at the stage loader area. The loads were 

transmitted by the data acquisition system transducers 

installed on the longwall shield legs. The shields had a 

working capacity of 650 tons. Figure 6 shows a 

photograph of a small zone of completed ribs, roof 

reinforcement, and instrumentation. 

TEST SITE RESULTS 

 The area had been driven, reinforced, and the crib 

pouring took place 39 days or 1,100 ft before the longwall 

face was to enter the area. This allowed for proper curing 

of the concrete materials. The instrumentation package 

was completed with the installation of the differential sagstations when the longwall face was 980 ft inby the 

recover room. The data were read and evaluated on a 

weekly basis until the face was within 200 ft, and then the 

data were read daily. When the face was within 100 ft, 

the test instrumentation was read every shift. For the final 

50 ft, personnel continuously monitored the instrumentation and documented roof, pillar, and crib behavior until 

the longwall safely entered the recovery room. The actual 

roof-to-floor closure was limited to a maximum of 

1.50 inches when the longwall face was approximately 

16 ft from entering the room. The differential sag-station 

located in the same area recorded that only 0.40 inches of 

the movement was attributed to roof separations and the 

remaining 1.10 inches was the result of floor heave. 

 The greatest measured crib load was approximately 

1,000,000 lbs. This load, coupled with the crib 

displacements, results in a stiffness calculation for the 

concrete crib and the bag of 1.69 x 106

 lb/inch. This 

value is significant because it represents an indication of 

the resistance to load. While this stiffness helped 

minimize roof closure and movement, the stiff material 

Figure 6. Photograph of completed cribs, roof 

supports, and instrumentation. 

Figure 5. The specific cribs that were 

instrumented and the sag-station and 

closure locations (left). Cross-section 

of the instrumented crib areas 

(above).
7

properties may have caused the cribs to fail in a brittle 

mode as shown in Figure 7. 

 While a traditional bolting system probably would 

have little value in preventing a weighting failure, it does 

provide significant confinement and prevents roof falls in 

weak roof conditions. When supplemented with the 

longer high-capacity cables, especially between the inby 

side rib of the recovery room and the shield tips, it 

appeared that only minor loading was transferred from the 

roof to the cribs. The walk or travel way between the 

center cribs and the outby side rib remained open and no 

falls occurred along the entire length. The roof yield zone 

identified in the model was supported with the primary 

bolts and the angled cables. Based on the differential sagstation data it appeared that the cables were successful in 

suspending the lower roof members bound by the 

tensioned roof bolts. The total roof bolting system, with a 

calculated reinforcement index of 487 psi·ft was effective 

but highly conservative. The number and lengths of the 

bolts and cables were more than adequate in this case. 

For future applications, the bolting system should be 

evaluated for specific roof strengths as well as tectonic 

and gravitational forces. The support on the inby and 

outby ribs of the recovery room proved to be extremely 

successful in controlling rib spalling and sloughage 

throughout the mining process. The rib support systems 

provide confinement to the outby pillar and the inby panel 

fender, which may have increased the residual pillar 

strength and minimized the respective pillar yield zones. 

POST LONGWALL RECOVERY ANALYSIS 

 An additional benefit of a 3-dimensional finite element 

model is the ability to calibrate the sub-model with actual 

field data to assist with a parametric or “what if” behavior 

scenarios. These can include altering the crib spacing, 

over-burden depths, and crib stiffness. The behavior of 

the pre-driven recovery was analyzed with no cribs and 

compared to the cribs placed on the 10-ft staggered 

spacing. The horizontal, vertical, and shear stresses were 

determined and resolved into displacements, plastic 

deformation, and subsequent bolt and crib load. The 

complete analysis is presented in Tadolini, 2003. 

Sub-Model Results with Bolts Only-No Cribs Installed

 The first series of models examine the behavior of the 

pre-driven longwall recovery room with no standing 

concrete cribs installed. The resistance to closure and 

loading is only provided by the primary resin-assisted 

tension bolts and the cable bolts. The resin-assisted bolts 

are installed with 9 tons (18,000 lbs) of pre-tension and the 

entry has supplementary cables installed between the rows 

with 1 ton (2,000 lbs) of active bearing plate load. The 

cross-section of the sub-model and the plan view of the 

bolting support plan are presented in Figures 8 and 9 (both 

Figures show the location of the cribs for the next phase of 

the analysis). The longwall is moving into the recovery 

room from right to left and the longwall shields are exerting 

166 psi on the roof immediately above the contact area. 

Figure 7. Brittle crib failure prior to the longwall 

entering the recovery room. 

Figure 8. Cross-section of the recovery room 

entry. Note shield pressure and crib 

locations. 

longwall face

166 psi

crib

bag
8

 As the longwall approaches the test area where the 

fender pillar is 16 ft wide, the bolt loads were determined 

and the roof movement at the center of the 16-ft wide 

recovery room was calculated. The computer model 

determined a roof movement or displacement of 3.00 

inches. The individual bolt and cable loads are presented 

in Table 1. 

Table 1. Individual bolt loading in pre-driven entry 

with no cribs. The longwall panel is 16 ft 

from entering the entry.

Bolt 

No. 

Row 1 

Tension 

Bolt Load 

(tons) 

Row 2 

Cable 

Bolt 

Load 

(tons) 

Row 3 

Tension 

Bolt Load 

(tons) 

Row 4 

Cable 

Bolt 

Load 

(tons) 

1 23.2 17.9 23.2 17.9 

2 23.4 13.5 23.4 13.5 

3 14.4 8.9 14.4 8.9 

4 9.4 3.2 9.4 3.2 

5 -0.8 -0.8 

Stress Analysis: The 3-dimensional finite element model 

was used to determine the vertical, horizontal and shear 

stresses of the recovery room entry as the forward 

abutment load materializes on the roof and pillars that 

form the pre-driven recovery room. As expected, based 

on the concentration of the horizontal and vertical 

stresses, the highest shear stresses are located in the 

corner adjacent to the outby pillar. These shear stresses 

are capable of causing yield in the corner area that can 

extend into the immediate roof. The shear stresses in the 

roof area of the tensioned bolts in row 1 and the cable 

bolts in row 2 are shown in Figures 10 and 11. The 

maximum shear stress, approximately 400 psi, is exerted 

near the outby pillar edge. 

Resin assisted 

bolts

cable bolts

row 1

row 2

row 3

row 4

Resin assisted bolt (7/8-inch diameter, 

9 ft long, 6 tons installed tension).

Cable bolt (15 ft long, 30 ton 

capacity, 1 ton installed tension).

crib

1

2 1

2

3

3

4

4

5

Outby

Pillar

Longwall

Face

Mining Direction

1

2 1

3 2

4 3

5 4

Figure 9. Plan view of the bolt locations and crosssection of the applied bolting systems. 

Figure 10. Shear stress distributions across row 1, resin-assisted tensioned bolts, over the 

recovery room. No cribs installed. 

-15 -10 -5 20 25

5

10

15

Height, ft

-500

-700
9

Plastic Strain Deformation: The final phase of this 

analysis on the behavior of the pre-driven recovery room 

is probably the most revealing with respect to the 

permanent roof damage and the areas where falls can 

initiate and propagate. The stresses exerted on the roof 

are transformed to plastic deformation and contoured for 

analysis. Figure 12 illustrates the plastic strain in the roof 

for the area reinforced with the tensioned roof bolts. The 

plastic strains indicated highlight and area where 

permanent roof failure has propagated through the roof to 

a depth of 14 ft. As shown, this tensile roof failure is well 

above the 9 ft primary bolt length. 

Sub-Model Results with Bolts and Concrete Cribs on a 

10-ft Row Spacing

 A sub-model was constructed to evaluate both the crib 

and bolt support behavior of the pre-driven longwall 

recovery room. This was the actual spacing used in the 

field study and the case used to calibrate the model. The 

concrete cribs were placed in a staggered row spacing of 

10 ft as previously shown in Figures 3 and 9. The order of 

support placement was that the bolts were installed first 

and then the cribs poured. A crib can be placed 

underneath an intrinsic bolt support reinforcement which 

affects the individual bolt loads as shown in Figure 9.

-15 -10 -5 0 20 25

5

10

15

Height, ft

-10

-2

0

00

-400

-500

-300

-300

Figure 11. Shear stress distributions across row 2, cable bolts, over the recovery 

room. No cribs installed. 

20 25

5

10

15

9 ft Support Height

Height, ft

0.001

Figure 12. Plastic Strain distributions across row 1, resin-assisted tensioned bolts, 

over the recovery room. No cribs installed. 
10

Table 2. Individual bolt loading in pre-driven entry 

with concrete cribs on a 10-ft row spacing. 

The longwall panel is 16 ft away. 

Bolt 

No. 

Row 1 

Tension 

Bolt Load 

(tons) 

Row 2 

Cable 

Bolt Load 

(tons)

Row 3 

Tension 

Bolt Load 

(tons) 

Row 4 

Cable 

Bolt 

Load 

(tons) 

1 12.8 10.0 11.2 9.6 

2 11.3 2.1 4.1 0.2 

3 7.5 0.8 3.8 -9.8 

4 -7.4 -6.9 6.9 0.5 

5 -0.8 -0.8 

Note: Negative values indicate compressive bolt loads. 

 As the longwall approaches the test area, where the 

pillar fender is 16 ft wide, the bolt loads were determined 

and the roof movement realized at the center of the 16 ft 

wide recovery room was calculated. The roof displacement 

was 2.20 inches. The individual crib load was 960 tons. 

The individual bolt and cable loads are presented in 

Table 2. 

 As shown, the maximum loads of the primary tension 

bolts occur in the row nearest the outby abutment pillar. 

The maximum tension bolt load is 12.8 tons (25,600 lbs). 

The cable loading is also reduced dramatically by placing 

the concrete cribs into the support scenario. The highest 

load is 10 tons (20,000 lbs). As the longwall face pillar 

fender is mined, the reduced strength of the pillar the 

standing concrete crib supports help prevent roof 

displacements. When no cribs were installed and support 

was solely provided by the tension and cable bolts, the 

room closure was greater, 3.00 inches. When the concrete 

cribs are installed and the fender pillar is reaching the 

yielding state, the closure is reduced to 1.6 inches. This 

large reduction is attributed to more load being carried by 

the crib system which reduced roof movement and 

subsequent separation. The plastic strains in the roof, when 

cribs were installed, were also analyzed to provide insight 

into prospective roof damage. 

Plastic Strain Deformation: The plastic strain deformation was the final step of the analysis on the behavior of 

the pre-driven recovery room with concrete cribs installed 

on 10-ft row spacings. The stresses previously described 

were converted to plastic deformation and revealed the 

permanent “damage” that took place in the immediate and 

main roof structures. The plastic strain in areas of bolt 

rows 1 and 2 are shown in Figure 13 and 14, respectively. 

 As shown, a yielding zone developed in the immediate 

roof near the outby barrier pillar. It is interesting to note 

that the yielding across the roof areas reinforced with 

cable bolts is smaller than the zones reinforced with the 

tensioned bolts. This is because the cable bolts near the 

outby pillar accept the shear forces, which reduces the 

yield zone. Figure 13 indicates that small amounts of 

plastic strain are experienced slightly above the primary 

bolt horizon. The cable supports were designed to accept 

the additional loading of roof separations in the primary 

support horizon, these loads do not approach the design 

Figure 13. Plastic strain distributions across row 1, resin-assisted tensioned bolts, over the 

recovery room. Concrete cribs installed on 10-ft row spacing. 

-15 -10 -5 20 25

5

10

15

Height, ft

0.008

.014
11

capacity of the cable systems if only vertical loads are 

applied. As the roof continues to move, the cribs have the 

capacity to accept additional loads and a redistribution of 

loads will take place. The area next to the outby abutment 

pillar appears to be a critical zone, even if standing 

concrete cribs are installed for support. The individual 

bolt loads are well within design capacities but the 

formation of plastic strains above the primary supports 

suggest that cable bolts may be critical for maintaining 

roof stability until the shields have safely entered the area. 

MECHANISMS OF POTENTIAL FAILURE FOR 

PRE-DRIVEN LONGWALL RECOVERY ROOMS 

 As the longwall face approached the pre-driven 

recovery room, the front abutment loads are exerted over 

the fender pillar and onto the bolt system and standing 

supports in the recovery room. Under these conditions, 

the inby fender pillar would yield and the depth or 

distance of the yield zone would be dependent on the 

magnitudes of the abutment loads. As the inby fender 

pillar totally yields, the residual strength of the pillar is 

decreased to a fraction of the original intact strength. The 

intact strength is dependent on the width, height, and coal 

strength properties. 

 When the inby fender pillar is in a critical state of 

yielding, it is subjected to considerable front abutment 

pressures. As the inby fender pillar begins to yield and is 

reduced to 10% of the original intact pillar strength, the 

high abutment pressures jump over the recovery room and 

apply the forces to the outby abutment pillar, which can 

cause abutment pillar yielding. 

 As the inby fender pillar totally yields and is 

continually removed with the longwall shearer, the 

support capacity approaches zero and the remaining 

abutment pressure must transfer to the internal bolting 

support systems, the concrete cribs, and the outby 

abutment pillar. At that time, if the load on the cribs 

exceeds the crib capacity, they would fail and increase the 

likelihood that the roof could collapse into the entry. 

 The worst possible condition for a longwall equipment 

recovery occurs if the inby fender pillar collapses after 

yielding ahead of the longwall shields and the equipment 

is unable to enter the pre-driven room. When this occurs, 

the immediate roof above the inby fender pillar is largely 

unsupported and a roof failure onto the cribs and in front 

of the shields is likely. If the internal bolting 

reinforcement and any standing supports (concrete cribs, 

wooden cribs, props, etc.) collapse, the outby abutment 

pillar would be subjected to high loading and a new cave 

line can be formed near the outby pillar line by the 

increased shear stress concentrations and subsequent 

plastic deformations. The entire immediate roof could 

collapse making the equipment extraction very dangerous 

and nearly impossible. 

EFFECT OF PRIMARY AND SECONDARY 

REINFORCEMENTS AND STANDING SUPPORTS 

ON THE STABILITY OF THE RECOVERY ROOM 

 As previously described, the primary roof bolt support 

reinforcement system is used to maintain the stability of 

the pre-driven longwall recovery room before the 

longwall approaches the area. Depending on the number 

and spacing of the bolts, the support density helps to 

maintain the local stability of the pre-driven recovery 

room during the initial mining process and secondary 

support system installations. As shown, some of these 

supports may end up above a concrete or wooden crib 

support and can be in compression once the roof 

10

15

-15 -10 -5 20 25

5

Height, ft

0.14

Figure 14. Plastic strain distributions across row 2, cable bolts, over the recovery 

room. Concrete cribs installed on 10-ft row spacing. 
12

movement develops that loads the cribs. These bolts 

serve no role in the final reinforcement scenario unless 

the cribs directly underneath should collapse and fail. 

The secondary intrinsic reinforcements, in this case cable 

bolts, are used to increase the reinforcement density to 

help resist roof movements and redistribute abutment 

pressures. 

 From the analyses presented, two areas of the roof are 

critical to assuring the stability of the pre-driven recovery 

room; the roof areas above the inby fender pillar and the 

outby abutment pillar. Because the roof yield zones have 

the potential to extend into the roof beyond the anchorage 

horizon of the primary bolts, cable bolts, or longer 

combination bolting systems should be used in the outby 

side of the recovery room to reinforce these areas. The 

cables provide reinforcement and are anchored high 

enough into the roof to be effective in the cases analyzed. 

The cable bolts installed vertically in the middle of the 

pre-driven recovery room provided only minimal support 

because much of the load was carried by the concrete 

cribs. This may have been a function of the installation 

tension and subsequent bearing plate loads. 

CONCLUSIONS & RECOMMENDATIONS 

 The moving of longwall equipment from the 

completion of a current panel to the next set-up or start 

room remains a critical operation from both a safety and 

productivity perspective. Pre-driven recovery rooms can 

allow the safe and rapid retrieval of the longwall 

equipment, but history indicates that the support systems 

used must be carefully engineered, designed, and applied. 

 The pre-driven recovery room loading mechanism is 

complex. As the longwall face approaches the recovery 

room, the size of the fender pillar is reduced. The front 

abutment pressures are applied to the fender pillar, the 

recovery room roof, and the outby barrier pillar. As the 

longwall panel fender becomes thinner, the peak strength 

of the coal is reached and yield of the fender pillar begins. 

The abutment pressures, previously carried by the panel 

fender, transfer to the support systems installed in the predriven recovery room and onto the outby barrier pillar 

system. As the panel fender strength is reduced to a low 

residual strength and then eliminated completely by 

extraction, the abutment forces must transfer to the 

support system installed in the pre-driven recovery room 

and the outby abutment pillar system. The transfer phase 

of the large abutment loads can occur rapidly, loading the 

standing support and bolting systems. The designs of 

these two components are critical for safe and efficient 

longwall equipment retrievals. 

 A field test area was established to evaluate a predriven longwall recovery room. A 3-dimensional finite 

element model ABAQUS was used to examine the 

stresses, deformations, and yield zones that may be 

encountered during the longwall mining extraction. A 

conservative design was applied at the test site based on 

the results of the model, an understanding of the weak 

roof and how it behaved under load, and a critical 

technical review of previous case studies. 

 Two rows of 48-inch diameter concrete cribs were 

poured in place with a concrete fill design specification of 

3,500 psi. As described, several of the concrete cribs 

experienced brittle type failures caused by uneven loading 

near the crib edges. Even in this partially failed mode, the 

remaining strength of the cribs was sufficient to allow the 

safe entry of the longwall equipment system into the predriven recovery room. 

 Based on the instrumentation data obtained from the 

pre-driven recovery room and the field observations, the 

3-dimensional ABAQUS model was calibrated. The 

calibrated model was used to evaluate the bolting and crib 

support systems. The loading of the bolts and cables near 

the outby pillar also confirmed the behavior observed in 

the preliminary test area and referenced in the literature. 

 As the longwall reduces the width and strength of the 

inby fender pillar, the immediate roof can start to deflect 

down and toward the edge of the failing fender pillar. 

This action creates tensile stresses and plastic deformation 

in the zone of roof near the outby barrier that loaded the 

primary bolt and cable system. In the test area, the crib 

systems were intentionally installed near the inby fender 

pillar and inside the centerline of the entry to restrict this 

rotational movement. However, as evidenced by the 3-

dimensional finite element modeling and subsequent 

analyses, the bolting systems not directly reinforcing the 

roof near a crib may be subjected to higher loads and 

required to perform more of the work. Secondary cable 

bolts installed directly underneath or near cribs 

contributed very little toward maintaining stability. 

During the design process, the bolting system used must 

consider two distinct loading conditions; the immediate 

roof loads realized during the initial development and the 

heavy abutment forces applied to the supports during the 

longwall entry into the pre-driven recovery room. If the 

cribs are too stiff and experience brittle failure before the 

longwall panel can safely enter the pre-driven room, the 

bolting support system, particularly the longer highcapacity cable bolts, may play a vital role in maintaining 

the immediate roof and help resist main roof caving. 
13

 The results of this research can be summarized by the 

following suggestions for the initial design of a pre-driven 

longwall recovery room: 

9 Both standing cribs and cable bolts should be used in 

conjunction with the primary bolting system to support 

pre-driven recovery rooms. 

9 The row spacings of the concrete cribs are dependent 

on entry room width and overburden depth. Initial 

trials should be conservative to prevent weighting type 

failures. If two rows are selected, concrete cribs 

should be placed near the fender pillar and the outby 

abutment pillar. If only one row is selected, it should 

be placed near the fender pillar. These are the areas 

identified as highly susceptible to shear failures in the 

immediate roof and the zone critical for safe entry into 

the recovery room. The cribs also enhance pillar 

stability by providing confinement to the pillars as 

sloughed coal piles off and around the cribs. 

9 To supplement the crib supports, vertical cable 

supports should be installed between crib locations. 

The length should be long enough to extend beyond 

the predicted yield depth of the immediate roof. 

9 Angled cable bolts should be installed into the roof 

above the inby fender pillar and the outby abutment 

pillar to help support the immediate roof in the event 

of shear failures. This is particularly important if the 

immediate roof is classified as weak. 

9 It should not be necessary to install secondary cable 

supports or high-capacity roof bolts in locations where 

cribs will be placed. Under normal extraction 

conditions the standing support will alleviate the 

loading upon the roof and cable bolts. The caveat 

being if the cribs are not properly designed and fail 

before the longwall shields have entered the area, the 

existing secondary cable supports will be subjected to 

high loading. In this scenario, secondary cable bolts 

where cribs are placed could be beneficial. 

 Pre-driven longwall recovery rooms can be safely 

designed and used to extract longwall equipment. 

Combinations of intrinsic bolting systems and standing 

support must be used to cope with the potentials of roof 

fall and weighting type failures. This research effort 

concludes that the primary bolting system should be 

designed to minimize the potential of roof fall failures 

during the development cycle. The cribs and strategically 

placed cable bolts are a critical component to reduce the 

potential of weighting failures that can lead to a massive 

collapse of the roof before the longwall can safely enter 

the area. These support components have a symbiotic 

relationship and assist each other throughout various 

stages of the mining process. Both are therefore prudent 

for a safe and successful longwall equipment recovery. 

ACKNOWLEDGEMENTS 

 The authors would like to thank Dr. Yunqing Zhang 

for the assistance provided with the numerical modeling 

study presented in this paper. 

REFERENCES 

 Molinda, G. M. and Mark, C., 1994, “Coal Mine Roof 

Rating (CMRR): A Practical Rock Mass Classification 

for Coal Mines,” U.S. Bureau of Mines IC 9387, 83 pp. 

 Morsy, K. M. and Peng, S. S., 2002, “Evaluation of a 

Mine Panel Failure Using the Local Mine Stiffness 

Criteria - A Case Study,” Transactions, Vol. 312, pp. 8-

19. 

 Oyler, D., Frith, D., Dolinar, D. R., and Mark, C., 

1998, “International Experience with Longwall Mining 

into Pre-driven Rooms,” Proceedings 17th International 

Conference on Ground in Mining, Morgantown, WV, 

Aug 4-6, pp. 44-53. 

 Tadolini, S. C., 2003, “Ground Control Support 

Considerations for Pre-Driven Longwall Recovery 

Rooms, “ Ph. D. Dissertation submitted to College of 

Engineering and Minerals Resources, West Virginia 

University, Morgantown, WV, 163 pp.