1

PERFORMANCE OF ROOF SUPPORT UNDER HIGH 

STRESS IN A U.S. COAL MINE 

David Oyler and Christopher Mark 

NIOSH 

Pittsburgh, PA 

Winton Gale 

Strata Control Technologies 

Wollongong, NSW, Australia 

Jinsheng Chen 

RAG Pennsylvania 

Waynesburg, PA

ABSTRACT 

 The National Institute for Occupational Safety and 

Health’s (NIOSH) Pittsburgh Research Laboratory (PRL), 

RAG Pennsylvania and Strata Control Technologies of 

Australia have collaborated to conduct an extensive study 

of roof bolt strata interaction at the Emerald mine in 

Southwestern Pennsylvania. The primary goal of the 

project was to obtain detailed data on the interaction 

between the mine roof and the support elements for use in 

modeling studies. The study site was a longwall tailgate 

subjected to high horizontal stress. Three arrays of 

instruments were installed at the site, one in the tailgate 

entry and two in an adjacent crosscut. Pumpable concrete 

cribs were present in the tailgate array, and cable bolts 

were installed in one of the crosscut arrays. The 

instruments included mechanical and sonic extensometers 

for measuring roof movement, instrumented roof bolts, 

and three-dimensional roof stress cells. The study was 

ultimately successful in determining the magnitude of the 

horizontal stress concentration, the height of roof failure 

and the roof failure sequence, and the loading history of 

the primary roof supports. 

ACKNOWLEDGMENTS 

 The authors would like to express their appreciation to 

Chester Huff, Senior Mining Engineer, RAG Emerald 

Resources, Craig Compton, Mining Engineering 

Technician, NIOSH, and all of the other RAG and 

NIOSH personnel who contributed to the project. 

INTRODUCTION 

 Nearly 1,500 roof falls occur each year in U.S. 

underground coal mines, creating serious safety hazards 

and operational impacts. The cost of the support installed 

to prevent roof falls approaches $1 billion annually. 

Improving the effectiveness of roof support is a major 

goal of the NIOSH mine safety research program. 

 Although field studies have provided important 

insights into the performance of roof supports and their 

interaction with the mine roof (Mark et al., 2000; Signer, 

2000), few U.S. studies have combined detailed 

extensometer and bolt load data with systematic 

measurements of roof stress. This complete suite of data 

is essential for validation of numerical models (Gale and 

Tarrant, 1997). 

 In late 2002 the NIOSH Pittsburgh Research Lab 

entered into a research partnership with RAG 

Pennsylvania and Strata Control Technologies of 

Australia to conduct a baseline study of roof support 

performance and roof behavior in a U.S. coal mine. The 

site was the tailgate of the 11 North longwall at the 

Emerald mine, located in Greene County, Pennsylvania 

(figure 1). The site was chosen because it was anticipated 

that the extension of 11 North beyond the start line of 10 

North would result in a significant horizontal stress 

concentration (Mark et al., 1998). From past experience 

at Emerald mine, a horizontal stress window like the one 

created by 11 North could be expected to cause the 

collapse of the crosscut and severe damage to the tailgate, 

providing a unique opportunity to gather data from the 

complete progression of the roof failure from initial 

development through final collapse of the entry. 

 In addition to providing baseline data for validation of 

numerical models, the study had the following specific 

objectives: 

• To quantify the magnitude of the stress increase 

associated with the horizontal stress concentration; 

• To determine the height of roof failure and the 

sequence of the roof failure process; 
2 

• To evaluate the effectiveness of the roof bolts installed 

as primary supports, in particular with regard to their 

length, load bearing capacity and failure modes, and; 

• To investigate the interaction between the primary 

support and two types of secondary support. 

 The purpose of this paper is to present the most 

significant results from the field study. The numerical 

modeling program is currently underway, and its findings 

will be reported in future publications. 

STUDY SITE 

 The Emerald mine operates in the Pittsburgh coalbed, 

mining the main bench and about 0.3 m (1 ft) of roof 

shale. Typical mined heights range from 2.1 to 2.4 m (7 

to 8 ft), with entry widths of 4.9 m (16 ft). 

 A geologic column of the mine roof obtained from a 

vertical core hole drilled at the site is shown in figure 2. 

The roof may be roughly divided into three units: 

• A sequence of coals and weak, slickensided shales in 

the lowest 2.7 m (9 ft); 

• A slightly stronger claystone sequence from 2.7 to 

5.4 m (9 to 18 ft), and; 

• A significantly stronger limestone above 5.4 m (18 ft). 

 The in-mine coreholes barely reached the limestone, 

so its thickness and strength were estimated from a nearby 

surface corehole. The low uniaxial compressive strength 

and RQD for the bolted horizon results in an estimated 

Coal Mine Roof Rating (CMRR) of 37 (Mark et al. 2002). 

 No pre-mining in situ stress measurements were made 

as part of this study. However, nearby measurements 

indicate that the maximum horizontal stress conforms to 

the regional orientation of approximately N70E. The 

magnitude of the maximum stress varies with the stiffness 

of the rock, but is typically on the order of 11 MPa 

(1,600 psi) for rock with a Young’s modulus of 20 GPa 

(3x106

 psi), (Dolinar, 2003). The depth of cover above 

the site is 200 m (650 ft). 

 The primary roof supports used throughout the study 

area were 22 mm (7/8 in) diameter, 2.4 m (8 ft) long, 

grade 75, two-piece combination bolts installed with 

1.2 m (4 ft) of resin grout in a 35 mm (1-3/8-in) hole. The 

yield load for these bolts is about 19 tonnes (21 tons), and 

their ultimate load tested in the lab at 28 tonnes (31 tons). 

The bolts were installed three per row, with rows on 1.2 

Figure 2. Composite core log from the study site, 

showing rock physical properties. Data from the 

vertical corehole drilled in the study crosscut and 

from a surface corehole. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Redstone 

Limestone

Claystone

Gray RQD 0%

Shale

Claystone/

Fireclay RQD 40%

Bone RQD 27%

Rider RQD 29%

Coal

 RQD 0%

Coal

Depth, m

5.0

5.5

Fireclay RQD 36%

w/thin UCS 24.4 MPa

Ls, slips, UCS 36.4 MPa

& clay RQD 50%

veins UCS 17.7 MPa

Black RQD 89%

shale UCS 31.3 MPa

w/coal streaks

Figure 1. Emerald mine and study site.
3

m (4 ft) centers. The two outside bolts in each row were 

installed from the continuous miner, together with a roof 

channel, and the middle bolts were installed later by the 

center bolter. 

 Three arrays of instruments were installed at the site 

(figure 3). Array T was located in the tailgate entry and 

arrays B and C in an adjacent crosscut. Crosscuts at 

Emerald Mine typically suffer more damage on 

development because they are oriented less favorably than 

the gate entries relative to the maximum in situ horizontal 

stress. 

DOOR

H2 H3

H6 H7

H5

H4

H1

N

Pumpable Cribs

 Surface

 Corehole

(overburden 198 m)

Vertical 

Corehole

HI Cells

B array

C array

T array Scale, m

0 5

-5

-35-30-25-20-15-10

05

1015

20

11 North Panel

Stopping

Figure 3. Site map. Distances shown are meters from 

the T array. 

 Supplemental supports were installed at two of the 

arrays. At crosscut array C, nine rows of three cable bolts 

each were installed between the rows of combination 

bolts. Cable bolts are not typically required in the 

Emerald mine gateroads, but were installed at this 

location to obtain data on their behavior. The cable bolts 

were 3.6 m (12 ft) long, 15 mm (0.6-in) diameter, and 

partially grouted with 1.2 m (4 ft) of resin. In the tailgate, 

a single row of 0.76 m (30 in) diameter pumpable 

concrete cribs were set prior to the start of the 10 North 

longwall. 

INSTRUMENTATION 

 Instruments were deployed to measure roof 

movements, support loads, and roof stress changes. The 

most detailed roof movement data were provided by 

multi-point sonic extensometers, with magnetic anchors 

located approximately every 0.3 m (1 ft) to a height of 

5.8 m (19 ft) above the roof line. However, the sonic 

extensometers could only be read manually, and it was 

anticipated that deteriorating roof conditions would 

eventually make it unsafe to continue reading them. 

Therefore, they were supplemented by mechanical, three 

point extensometers that were monitored remotely. The 

anchors of the mechanical extensometers were located at 

2.1 m (7 ft), 3.6 m (12 ft) and 5.8 m (19 ft). Since the 

data from the mechanical extensometers generally agreed 

with the sonic probes, they will not be reported here. 

 Loads on the lower, 1.2 m (4 ft) long, ungrouted 

portions of the combination roof bolts were measured 

using three types of devices: 

• Strain gauges installed inside the bolts using the 

technique developed by Dr. Hani Mitri at McGill 

University (Mitri et al., 2001); 

• Hydraulic U-cells with a strain gauge pressure 

transducer calibrated to convert cell pressure to load in 

tonnes, and; 

• Commercially available strain gauge load cells. 

 The strain gauged combination bolts were prepared at 

McGill using roof bolts originally obtained from Emerald 

mine. Holes were drilled into the head of each bolt to 

accept a single strain gauge, near the bolt head, and 

electrical connectors. The bolts were individually 

calibrated to 13 tonnes (14 tons) by NIOSH and found to 

have a highly linear load-to-strain-gauge-signal response 

in the elastic range. NIOSH also tested combination bolts 

to determine the post-yield load-deformation relationship, 

so that estimates could be made of the bolt loads beyond 

yield. During the study the strain gauged bolts performed 

well, with no failures. All the bolt loads reported in this 

paper are from the strain gauged bolts. 

 Although U-cells have been used successfully in past 

studies (Mark et al., 1998), they had a high rate of failure 

in this study. Of the nine U-cells deployed, two failed 

due to bursting or leaks during installation, and at least 

two others had poor performance due to leaks induced 

during the installation process. Moreover, recent 

laboratory studies have shown that the U-cell stiffness is 

approximately the same as that of the bolts, producing a 

bolt-cell system with about one-half the stiffness of a 

normal bolt. This reduced system stiffness caused the six 

bolts installed with U-cells to reach significantly lower 

loads than the strain gauged bolts. For these reasons the 

U-cell data obtained from the combination roof bolts will 

not be presented here. 

 U-cells were also used to monitor the loads on three of 

the cable bolts, but only one provided useful 

measurements. Loads on two pumped-in-place cement 

cribs were measured using hydraulic flatjacks, but 

instrument failures again resulted in the loss of useful 

data. Fortunately, detailed data on the behavior of 
4

pumped-in-place cement cribs at the Emerald mine are 

available in a report by Barczak et al. (2003). 

 Horizontal stress changes were measured by seven 

CSIRO Hollow Inclusion (HI) cells that were installed in 

the roof above the gateroad pillar inby and adjacent to the 

crosscut site. Each HI cell consisted of an array of 12 

strain gauges arranged to allow three dimensional 

determination of stress changes. The cells were grouted 

in place using an epoxy designed specifically for HI cells 

to provide coupling between the rock and the gauges in 

the cell. Seven cells were installed with a 100% success 

rate using procedures developed by Dr. Jan Nemcik of 

SCT. Before installation, the holes were undercored to 

identify appropriate setting zones and to obtain samples 

for physical property testing. 

 The strain data obtained from the HI cells were 

reduced using software developed by the U.S. Bureau of 

Mines (Larson, 1992). For presentation purposes, the 

stress changes have been transformed into principal 

stresses perpendicular to the crosscut. The absolute 

principal stresses are not presented in this report. 

 SCT has observed that it can require up to several 

months for the glue of HI cells to fully cure, and during 

this time glue creep and water uptake can affect the 

readings. While glue creep may have been a factor in the 

early readings, fortunately the 11 North mine by took 

place more than six months after the cells were installed. 

 Figure 4 is a cross section showing the instrumentation 

in each array. During initial entry development by the 

miner-bolter, two strain-gauged and two U-celled bolts 

were installed on adjacent channels at each array. The 

center bolter later installed the third strain-gauged bolt 

and a commercial load cell. The center bolter also drilled 

holes for one sonic and two mechanical extensometers at 

each array, and these were installed later. At the C array, 

the center-bolter installed 9 rows of three cable bolts each. 

U-cells were installed on the row closest to the C array. 

When the pumped-in-place cement cribs were installed in 

the tailgate, approximately one month after entry 

development, the two T array flatjacks were deployed. 

 The stress cells were installed in a fan pattern as 

shown in figure 5. Cell H7 was installed towards the top 

of the lower shale unit, while cell H6 was placed in the 

bottom of the limestone. The other five cells were 

installed to provide information on the stress distribution 

within the upper claystone unit. 

RESULTS 

 The study was conducted in three phases. The first 

was development, which took place early in October of 

2002. The sonic extensometers and instrumented roof 

bolts were installed at this time. Two HI cells and half of 

the mechanical extensometers were installed in lateNovember, and the remaining instruments were installed 

in early-December. Roof conditions in the gateroad 

entries during development were generally good, although 

roof cutters developed in crosscuts outby the study 

crosscut, near the 10 North start-up room. 

Coal Coal

0 5 ft

0 2 m

Strain gauged bolts

Sonic

Extensometer

Mechanical

Extensometer

Figure 4. Typical instrument array. The C site 

included 9 rows of three cable bolts, and one row 

instrumented with U-cells in addition to the 

instruments shown. 

Figure 5. Section view of HI cell locations. 

H1 H2 H3 H4

H5

H6 

H7 

HI Cell Locations

Scale, m

Vertical Corehole

Angled Corehole

0 2 Inby rib

Study crosscut

Outby rib

Section view N77.5°E
5 Copyright © 2004 by SME 

 The second phase commenced with the start up of the 

10 North longwall in mid-December. Although the 10 

North set-up room was approximately 100 m (300 ft) 

from the study site, conditions noticeably deteriorated in 

the crosscut as the longwall moved away. This was 

attributed to development of the horizontal stress 

concentration at the corner of the 10 North gob. More 

severe changes occurred closer to the 10 North start-up 

room, including the collapse of a crosscut adjacent to the 

start-up room. Unfortunately, the instruments could not 

be connected to the continuous data loggers until midJanuary, so complete data (particularly from the stress 

cells) are not available for this period. 

 The third phase was the mine-by of the 11 North 

longwall, which started up in mid-May from a set-up 

room approximately 960 m (3146 ft) inby the study site. 

The 11 North longwall passed the study site on July 20 

and 21. Monitoring of some instruments continued for a 

few more days, until the longwall face was approximately 

60 m (200 ft) outby. 

 Development: The roof bolt loads at installation were 

typically 3 to 8 tonnes (3 to 9 tons). The loads slowly 

increased before stabilizing about a month later (figure 6). 

Bolts in the B array, the array nearest the middle (track) 

entry of the three entry gateroad system, saw the highest 

loads, between 10 and 15 tonnes (11 and 17 tons), and the 

C and T array bolt loads were in the range of 5 to 11 

tonnes (5 to 12 tons). Initial extensometer readings were 

made several days after development, so the data do not 

include the initial roof sag, but by mid-November the roof 

at all three sites had stabilized with less than 5 mm (0.2 

in) total deformation (figure 7). However, the maximum 

height of roof movement was about 3.3 m (11 ft), well 

above the top of the bolting horizon at 2.4 m (8 ft). Two 

HI cells (H5 and H7) were installed on November 21, 

with initial readings on November 24. Despite the “glue 

creep” effect, the stress changes between November 24 

and December 10 appear to be less than 2 MPa (300 psi). 

Prior to the start up of the 10 North longwall the roof 

appeared to have stabilized, with no significant rock 

failure, although the T array sonic extensometer indicated 

strain levels close to 1% in the first 0.3 m (1 ft) of the roof 

skin. 

 10 North Longwall Start-Up: The first readings 

after the start of 10 North were made on January 7, 2003, 

after the longwall had advanced approximately 300 m 

(1,000 ft). During that time the two HI cells showed that 

roof stress increases on the order of 3.5 MPa (500 psi) had 

occurred, and there were increases in bolt load and roof 

movement at the B and C arrays. At the T array bolt 

loads were unchanged and there was only a slight increase 

in roof sag, primarily within the bolted horizon. These 

observations are consistent with the development of a 

horizontal stress concentration around the 10 North gob. 

 The maximum height of roof deformation at the B and 

C arrays increased during this 10 North start-up period, to 

3.9-4.2 m (13-14 ft), with total deformations of 16 and 10 

mm (0.6 and 0.4 in), respectively (figure 7). Bolt loads at 

the B array increased by 3-5 tonnes (3-6 tons) to 12-22 

tonnes (13-24 tons), with the highest readings indicating 

that bolts were reaching yield (figure 6). At the C array 

bolt load increases were somewhat less, to 8-15 tonnes (9-

17 tons). 

 There was no nearby mining activity between midJanuary and early-April, but the HI cells (now all 7) 

continued to indicate increases in stress. The rate of 

increase gradually decreased through April, but never 

completely stabilized (figure 8), implying that the breakup 

of the roof near the start up room of the 10 North panel 

may have caused a gradual transfer of stress to the 

vicinity of the study site. Figure 9 shows the stress 

Laboratory yield of combination bolts was 19 tonnes, ultimate load 28

tonnes. Sensor readings over 19 tonnes (yield) have been estimated

by assuming a post yield stiffness 1/15 the elastic stiffness.

Loads, tonnes

25

20

15

Time scale change

Figure 7. Profile of roof movement in the study 

crosscut at the B array sonic extensometer, from 

December 2002 through the last reading on 

July 20, 2003, with roof lithology. Face distances 

shown are in meters, with the face inby the 

extensometer. Depths are referenced to the roof 

line. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

12/10/2002

1/08/2003

4/29/2003

7/15 (face: +143)

7/17 (face: +90)

7/18 (face: +66)

7/19 (face: +39)

7/20 (face: +26)

Sonic Extensometer

 (B array)

Depth, m

5.0

5.5

Displacement, mm

0.0 10 20 30 40 50

Top of cable bolts

Bolt tops
6

changes that had been measured by the end of April. The 

major principal stresses are all compressive, and range in 

magnitude up to 20 MPa (2,900 psi). The orientations of 

the principal stress increases imply that the crosscut roof 

yielded or “softened,” forcing the stress higher above the 

crosscut. 

 Further evidence of roof softening during this period is 

apparent in the measurements of bolt loads and roof 

movements. By the end of April the maximum height of 

deformation at both the B and C arrays was 4.2 m (14 ft), 

and the maximum deformations at the B and C 

extensometers were 25 and 12 mm (1 and 0.5 in), 

respectively. Maximum roof strains in excess of 1.5% 

were measured at two points in the B extensometer, while 

the C extensometer had one location where the strain 

exceeded 1%. The T array continued to be nearly stable 

during this period, except at the roof skin. 

 Mine-by of the 11 North Panel: Definitive changes 

in roof stress resulting from the 11 North longwall began 

to be observed in all HI cells by late-June, when the face 

was approximately 180 m (600 ft) inby the T array. After 

the face reached 15 to 20 m (50 to 70 ft) inby the cells the 

rate of stress change greatly accelerated. In most cases 

the stresses increased as the face approached, typically by 

about 7 MPa (1000 psi). The direction of the maximum 

principal stress increases continued to be in a generally 

sub-horizontal orientation, directed around the softened 

roof of the crosscut (figure 10). In the last HI cell 

readings (figure 11), large stress changes continued to 

take place, with increases in the vertical stress 

components and relief (reduction) of the horizontal 

components. 

 Both of the sonic extensometers in the crosscut 

recorded significant roof movement as the face 

approached. Total deformations in each increased by 

about 20 mm (0.8 in) and the height of roof movement 

reached 4.9 m (16 ft). Most of this deformation occurred 

during the last 23 m (75 ft) of advance before the face 

reached the crosscut, which is when the extensometers 

were abandoned due to safety considerations. In contrast, 

the T array remained fairly stable until the face was 7 m 

(23 ft) from the array. Then total movements of more 

than 60 mm (2.5 in) were measured before the instrument 

was destroyed as the face moved past it. The final 

deformation profile looked very similar to the two 

crosscut profiles, including a sharp “knee” just below the 

top of the bolts and another high-strain zone at about 

3.5 m (13 ft). 

Figure 8. HI cell stress changes from cell H4 located 4.0 m above the roof and 7.6 m into the gateroad 

pillar. Vertical and normal stresses parallel and perpendicular to the face are shown. The shear 

stresses are not shown. Negative values are compressive changes. 

Vertical Stress

Stress changes with respect to 12/10/2002 initial reading.

118 82 51 21 21 3 -1 -21 -43 -68

7/13 7/14 7/15 7/16 7/17 7/18 7/19 7/20 7/21 7/22 7/23 7/24

-2611

-2211

-1811

-1411

-1011

-611

-211

189

Stress change, psi

Face distance from T array, m

Time scale change

11 North startup

959

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

11/30 12/28 1/25 2/22 3/22 4/19 5/17 6/14 7/12

Stress change, MPa (- is compressive)

Normal Stress to Face

Normal Stress to Face
7

 The largest bolt loads were measured at the B array 

where all the bolts were in yield by the time the face was 

40 m (130 ft) inby the array. At the C array bolt loads 

were considerably less. One C array bolt went into yield 

suddenly when the face was 105 m (340 ft) inby, and the 

center bolt yielded almost as the face passed. The cable 

bolts reached their maximum recorded loads at this time, 

apparently below yield. After the face passed the study 

crosscut the mechanical extensometers indicated that roof 

deformations large enough to load a cable bolt well into 

yield (more than 50 mm) took place below the tops of the 

cable bolts. 

 Bolt loads at the T array remained low, between 10 

and 18 tonnes (11 and 20 tons), until the face was within 

3 m (10 ft) of the array. Only the middle bolt ever went 

into yield before data from the bolts were lost as they 

went behind the shields. 

 As the face passed by the site, nearly every HI cell 

measured an immediate horizontal stress reduction of 

about 10 MPa (1,500 psi), indicating that caving of the 

longwall panel immediate roof resulted in stress relief. In 

general, the horizontal stress parallel with the longwall 

face decreased somewhat more than that perpendicular to 

the face. The exception was H6, which was set at the 

bottom of the limestone member. In H6 the horizontal 

component parallel with the face continued to increase 

until the face was 15 m (50 ft) past the cell, probably due 

to delayed caving of the limestone member. Even then, 

only the stress parallel with the face was relieved, while a 

high level of horizontal stress continued to be carried 

above the crosscut. The delayed stress relief parallel with 

the face at H6 indicated that the limestone remained intact 

above the gateroads for some time after the lower rock 

units caved. 

 In contrast, the vertical stresses continued to increase 

until measurements ceased. On average, the vertical 

stress increase due to the mining of 11 North was about 

13 MPa (1800 psi). 

Figure 9. Principal stress changes through 

April 9, 2003 in the plane perpendicular to the 

study crosscut, with respect to initial HI cell 

readings (See figure 3). Stress changes with 

arrows are tensile or stress relief, stresses without 

arrows are compressive. 

 Estimated 

softened roof

 zone

0 2

Scale, m

10

Stress scale, MPa

0

Study Crosscut

H7

H1 H2 H3 H4

H5

H6

Figure 10. Principal stress changes through 0330 

on July 21 in the plane perpendicular to the study 

crosscut, with respect to initial HI cell readings. 

The face is 14 m (47 ft) outby the T array (See 

figure 3). Stress changes with arrows are tensile 

or stress relief, stresses without arrows are 

compressive. 

 Estimated 

softened roof

 zone

0 2

Scale, m

10

Stress scale, MPa

0

Study Crosscut

H7

H1 H2 H3 H4

H5

H6

Figure 11. Principal stress changes through 0000 on 

July 23 (except H4 and H5, for which data were 

available only through July 22) in the plane 

perpendicular to the study crosscut, with respect to 

initial HI cell readings. The face is 58 m (190 ft) 

outby the T array (See figure 3). Stress changes with 

arrows are tensile or stress relief, stresses without 

arrows are compressive. 

H1

H2 H3 H4

H5

H6

H7

0 2

Scale, m

10

Stress scale, MPa

0

Study Crosscut

 Estimated 

softened roof

 zone
8

 Bolt loads at the crosscut arrays reached a maximum 

shortly after the face had passed, and then began to 

decrease. Of the 17 instrumented combination bolts, only 

2 maintained their load, while 12 shed load gradually, and 

3 lost load suddenly. The reduced loads may have been 

associated with the horizontal stress relief that occurred 

when the face passed, or to failure of the grouted portion 

of the bolts. 

CONCLUSIONS 

 The most surprising result of the study was that the 

roof conditions at the study site were not as severe as 

experienced in past “stress windows” at the Emerald 

mine. Instead, at the time the data loggers were removed, 

with the face 58 m (189 ft) outby the study site, the 

crosscut, though observably damaged, was still standing. 

Even the tailgate entry generally stayed open for some 

distance into the gob. During a subsequent visit to the 11 

North tailgate well outby the study site, it appeared that 

the typical vertical abutment stress caused more gateroad 

damage than had the horizontal stress window. 

 Despite the moderate ground conditions, significant 

stress changes were measured. The total horizontal stress 

increase, confirmed by the stress relief that occurred as 

the longwall passed, indicates that the mining-induced 

stress concentration approximately doubled the original in 

situ stress. The orientation of the stress increases 

indicates that the stresses predominantly passed over, and 

not through, the softened (yielded) roof immediately 

above the crosscut. 

 The sonic extensometers clearly showed that from 

initial development the conditions in the crosscut were 

more severe than in the tailgate entry. This was expected 

because the crosscut was oriented nearly perpendicular to 

the regional maximum principal horizontal stress. The 

greatest deformations were measured at the B array. This 

could be due to the B array’s proximity to the 10 North 

gob and the track entry, or because of the additional 

support provided by the cable bolts at the C array, or 

because the stopping near the C array provided some 

support (Oyler et al., 2001), or to a combination of these 

effects. 

 While the three arrays differed in the timing and 

magnitude of the roof deformation, it was significant that 

the deformation process followed a broadly similar 

pattern at all three locations. In all three instances 

maximum height of movement was approximately 5 m 

(16 ft), and significant roof strains occurred both near the 

top of the combination bolts (at 2.4 m) and approximately 

1.0 m (3 ft) above them. 

 The roof bolt loading pattern was consistent with the 

extensometer data, with the greatest loads at the B array 

and the least at the T array. The measurements indicated 

that the roof supports in use at the mine, that is the 

combination bolts and pumpable cribs, appear to be 

sufficient to maintain roof control under the observed 

conditions. Following development, the loads appeared 

to normally be below the yield capacity of most of the 

bolts, and roof movements were largely limited to the 

bolted horizon. During longwall mining, some of the 

bolts in the tailgate reached yield loads as the face 

approached, implying that the secondary supports 

(pumpable cribs) could have been necessary to provide 

adequate roof support. 

 Unfortunately, little reliable load data were collected 

from the pumpable cribs and the cable bolts. However, 

data from a pressure gauge on one pumpable crib and the 

limited cable bolt load data combined with extensometer 

data give some indications of the loads on both of these 

types of supports. The limited data available suggest that 

neither the cribs nor the cable bolts developed significant 

loads before the face passed. However, with nearly 

50 mm (2 in) of roof movement, it seems likely that some 

load eventually developed on both the cables and the 

cribs. Indeed, one explanation for the lower bolt loads at 

the C array is that the cables assumed some of the load. 

Recent field and numerical model studies (Tadolini and 

Barczak, 2003; Barczak et al., 2003) both indicate that 

supplemental supports probably reduced the loads that 

would have otherwise have been applied to the roof bolts. 

 In summary, the study was successful in providing a 

substantial set of baseline data from a U.S. coal mine, 

incorporating stress change, roof deformation, and 

support loads. Although different results would be 

expected under different geological conditions, this data 

set will provide a solid foundation for calibrating 

international state-of-the-art numerical models for U.S. 

conditions. Ultimately, it will help make these 

sophisticated design tools available to U.S. mine planners. 

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9

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