ANALYSIS OF ROOF BOLT SYSTEMS

Christopher Mark, Chief, Rock Mechanics Section

Gregory M. Molinda, Research Geologist

Dennis R. Dolinar, Mining Engineer

NIOSH, Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

Despite more than half a century of experience with roof bolting,

no design method has received wide acceptance. To begin to

improve this situation, National Institute for Occupational Safety and

Health (NIOSH) conducted a statistical study of roof bolt

performance at a number of mines throughout the U.S. Case histories

were collected from 37 mines with a variety of roof bolt types and

patterns in a wide range of geologic environments. Performance was

measured in terms of the number of roof falls that occurred per

10,000 ft of drivage. The study found that roof falls are rare when

the roof is strong and the stress is low, even with light roof bolting

patterns. The focus of this paper is on the more difficult conditions,

where the roof is weaker and/or the stress is higher.

Analysis of the results led to guidelines that can be used to make

preliminary estimates of the required bolt length, capacity, and

pattern. The guidelines are based on the depth of cover (which

correlates with stress) and the roof quality (measured by the Coal

Mine Roof Rating (CMRR), and the intersection span. Another

contribution is a formula for estimating the horizontal stress level in

the eastern U.S. coalfields as a function of the depth of cover. The

design guidelines are currently being implemented into a computer

program called Analysis of Roof Bolt Systems (ARBS).

INTRODUCTION

Roof bolts are the first line of defense protecting mineworkers

from the hazards of ground falls. Because roof bolts utilize the

inherent strength of the rock mass, they have many advantages when

compared with earlier standing support systems. Due to of their

central importance, roof bolts have received more research attention

than any other ground control topic, with the possible exception of

coal pillars. 

Roof bolt design consists in specifying the proper bolt type,

capacity, length, and pattern for a particular roof rock, stress level,

and application. The interactions between these variables are very

complex, and our understanding of their mechanics remains

imperfect. Numerous roof bolt design methods have been proposed

over the years, but none has gained widespread acceptance by the

coal mining industry (1). This is unfortunate, because more than

1,500 roof falls occur each year in U.S. coal mines (2). 

To help develop scientific guidelines for selecting roof bolt

systems, the National Institute for Occupational Safety and Health

(NIOSH) conducted a study of roof fall rates at 37 U.S. mines (3).

The study evaluated five different roof bolt variables: Length,

tension, grout length, capacity, and pattern. Roof spans and the Coal

Mine Roof Rating (CMRR) were also measured. Stress levels could

not be measured directly, but the depth of cover was used as a standin for stress. The outcome variable, which measured the success of

the roof support system, was the number of Mine Safety and Health

Administration (MSHA)-reportable roof falls that occurred per

10,000 ft of drivage.

In all, nearly 100 case histories were collected. Areas that were

affected by longwall mining, multiple seam interactions, or major

faults were excluded, as were roof falls that took place more than 18

months after development. Details of the data collection procedures

and the case history data base have been published elsewhere (2).

Analysis of the entire data set led to some preliminary roof bolt

design guidelines (1). This paper will focus on the subset of more

difficult conditions, those with weaker roof and higher stress. The

first step is to define those difficult conditions, based on the

mechanism of roof bolt support.

ROOF GEOLOGY AND THE REINFORCEMENT

MECHANISM OF ROOF BOLTS

The principle objective of roof bolting is to help the rock mass

support itself. Some researchers have ascribed different support

mechanisms to different types of roof bolts. For example,

mechanical bolts were originally thought to work in suspension,

while resin bolts primarily built beams (4). Others have described

the beam-building mechanism of tensioned bolts, and the frictional

support of fully grouted bolts (5). 

It seems, however, that the reinforcement mechanism is actually

dictated to the bolts by the ground, rather than the other way around.

Four mechanisms can be identified, depending on the geology and

the stress regime:

• Skin Control: In strong, massive roof that is essentially selfsupporting, cracks, joints, crossbeds, or slickensides can

create occasional hazardous loose rock at the skin of the
 A B

 C D

Figure 1. Roof bolt support mechanisms: (A) simple skin support, (B) suspension,

(C) beam building, and (D) supplemental support in failing roof.

opening (figure 1a). In this environment, the function of the

bolts is to prevent local rock falls, not to prevent a major

collapse. A pattern of relatively light, short roof bolts is

usually sufficient. Skin control is also an important

secondary function of roof bolts in weaker ground.

• Suspension: In many mines, a stronger unit that is largely

self-supporting overlies a weak immediate roof layer

(figure 1b). In these circumstances, roof bolts act to suspend

the weaker layer. Experience has shown that roof bolts are

extremely efficient in the suspension mode (6, 7, 8), though

suspension becomes more difficult if the weak layer is more

than 3 ft thick. The Coal Mine Roof Rating (CMRR)

somewhat quantifies this effect through the Strong Bed

Adjustment (9). The traditional dead-weight loading design

approach is generally appropriate for suspension applications

(1).

• Beam Building: Where no self-supporting bed is within

reach, the bolts must tie the roof together to create a “beam”

(figure 1c). The bolts act by maintaining friction on bedding

planes, keying together blocks of fractured rock, and

controlling the dilation of failed roof layers (5, 10). In

general, roof bolts have to work much harder in beam

building than in suspension, and higher densities of support

are required. However, it is these applications (and those in

the next category) that have been most troublesome for

design.

• Supplemental Support: Where the roof is extremely weak,

and/or the stress extremely high, roof bolts may not be able

to prevent roof failure from progressing beyond a reasonable

anchorage horizon (figure 1d). In these cases, cable bolts,

cable trusses, or standing support may be necessary to carry

the dead-weight load of the broken roof, and the roof bolts

act primarily to prevent unraveling of the immediate roof

(11).
Figure 2. Roof fall rates for different CMRR values

0

500

1000

1500

2000

2500

0 1000 2000 3000 4000 5000

Measured Horizontal Stress (psi)

Depth of Cover (ft)

Measured Stress

Log Regression

Linear Regression

Figure 3. Horizontal stress in eastern U.S. coal mines. Actual

stress measurements from Mark and Mucho (9).

These broad categories can be seen in the data collected in this

study. Figure 2 shows the roof fall rates for all the case histories,

divided into three groups:

• Failures (more than 1.5 roof falls per 10,000 ft of drivage)

• Intermediate (the roof fall rate is between 0.4 and 1.5 falls

per 10,000 ft)

• Successes (the roof fall rate is less than 0.4 falls per 10,000 ft)

It is clear that mines with weaker roof were much more likely to

encounter roof falls. When the CMRR was less than 50, 29 cases fell

in the Failure category, while there were just 16 Successes. When the

CMRR was 50 or greater, the proportions were more than reversed,

with Successes outnumbering Failures by 6-to-1. 

HORIZONTAL STRESS AND DEPTH OF COVER

Geology is not the only factor that determines the reinforcement

mechanism, however. The transition between suspension and beam

building depends heavily on the level of stress. The same roof bed

that is self-supporting in a low stress environment may require

substantially more reinforcement when subjected to higher stresses.

Worldwide research over the past 20 years has shown that the

horizontal stress is usually two or three times larger than the vertical

stress. In addition, the horizontal stress attacks the roof directly,

while the pillars carry most of the vertical stress. 

In U.S. coal mines, horizontal stresses are rarely measured

underground. However, a comprehensive data base collected by

Mark and Mucho (12) shows a strong correlation between increasing

depth and greater levels of maximum horizontal stress in the eastern

U.S. (figure 3). Two regression equations are shown, linear and

logarithmic:

FH = 1.23 H + 1306 (1a)

FH = 2250 log10 (H) - 4075 (1b)

Where: FH = In situ horizontal stress (psi)

 H = Depth of cover (ft)

They both predict very similar values for the range of depths in

the case histories. However, the logarithmic equation is preferred

because other research suggests that the horizontal stress gradient

tends to decrease at greater depths (13). In the western U.S., the

horizontal stress is generally about equal to the vertical, but it is

highly variable.

For the study described in this paper, it was impossible to

measure the horizontal stress at any of the sites. However, based on

the data shown in figure 3, the assumption will be made that the

depth of cover can be used to estimate the stress level. 

In figure 4, the case histories are plotted against the CMRR and

the depth of cover. A discriminant line is shown separating the data

into two groups:

CMRR = 9 + 17 (log10 H) (2)

Roof falls are rare above the line, and this is the strong roof

and/or low stress regime where roof bolts are apparently working by

Skin Control or Suspension. Below the line roof falls were much

more common, and roof support was clearly more difficult. This is

the weak roof/high stress regime where the bolts apparently have to

work by Beam Building or with Supplemental Support. The

remainder of this paper will address roof support design for this

regime.
20

30

40

50

60

70

80

90

100 300 500 700 900 1100 1300

Depth of Cover (ft)

CMRR

Successes

Failures

Intermediate

discriminant

Figure 4. Support mechanisms in practice. Above the

discriminant line is the skin support/suspension regime; below the

line is the beam building/supplemental support regime.

Figure 6. Paired cases of long and short bolts showing benefits

of long bolts in decreasing roof fall rates.

INTERSECTION SPAN

After the CMRR and the depth of cover, the roof span is the next

most important factor affecting roof stability. In coal mines, the

greatest spans are encountered in intersections. Approximately 70%

of all roof falls occur in intersections, though they account for just

20-25% of the total drivage (14). Clearly, roof falls are much more

likely in wider spans. Any successful roof bolt design methodology

must consider the span effect. 

Figure 5 plots the case history spans (reported as the average of

the sum-of-the-diagonals) against the CMRR. The best equation

separating the Successful cases from the Failures is also shown as:

IsG = 20 + 0.26 (CMRR) (3)

Where IsG = Suggested Intersection Span (average of the sum-of-thediagonals, ft). Surprisingly, the correlation was not improved when

either the depth of cover or the seam thickness were included.

BOLT LENGTH

An original goal of the study was to determine which of the bolt

design parameters (length, tension, pattern, capacity) were most

important. Unfortunately, statistical analysis of the entire data set did

not reveal any significant correlations (3). However, bolt length was

confirmed as a critical parameter in a separate analysis that used

“paired data” from individual mines.

From the large data set, 13 pairs of case histories were extracted

where two different lengths of roof bolts were used at the same

mine. For a pair to be selected, the roof bolt lengths had to differ by

at least one ft, and at least one of the roof fall rates had to be greater

than zero. Figure 6 shows that in 11 of the 13 cases, the roof fall

rate was less with the longer bolt. The average reduction in the roof

fall rate was 65%. The greater effectiveness of the longer bolts was

statistically significant at the 98% confidence level. Four of the

pairs mixed shorter, pre-tensioned bolts with longer, fully-grouted

bolts. In three of these pairs, the longer bolts had the lower roof fall

rate. More details on the study can be found in Molinda et al. (3).

Building upon these results, an equation was developed to guide

the selection of the proper bolt length (LB). Equation (4)

incorporates the major factors that should affect the required bolt

length, namely the span, the stress level, and the roof quality: 

LB = (Is/13) (log10 H) ((100-CMRR)/100)1.5 (4)

Where Is = the actual intersection span (average of the sum-of-thediagonals, ft).

The roof quality term in equation (4), ((100-CMRR)/100)1.5 is

based on the relationship originally proposed by Unal (15), but it has

been adjusted to magnify the effect of the weak roof.

Figure 7 compares the bolt lengths predicted by equation (4)

with the actual bolt lengths in the case histories. For the successful

cases, the mean bolt length was very close to the equation’s

prediction. In the unsuccessful cases, on the other hand, the mean

bolt length was 8 in too short. Moreover, of the 13 cases when the

actual bolt length was shorter by 1.5 ft or more than the length

recommended by equation (3), 77% were failures. These data

further confirm the importance of proper bolt length in preventing

ground falls.

It should be noted, however, that good anchorage is necessary

to ensure that the entire length of a fully grouted bolt is working. In

weak rock, the anchorage factor of a typical 13-ton capacity No. 6

25.00

30.00

35.00

40.00

45.00

50.00

55.00

60.00

25.0 27.0 29.0 31.0 33.0 35.0 37.0 39.0

Intersection Span (ft)

CMRR

Successes

Failures

Figure 5. Effect of roof span on roof fall rate. Above

the line spans are appropriate, below the line

spans are generally too wide.
0

2

4

6

8

10

12

-2 -1 0 1 2

Actual bolt too short by: (ft)

Number of Cases

Successes

Failures

Figure 7. Comparison between the actual bolt lengths from the

case histories with the bolt length predicted by equation (4). 

When the X-axis value is positive, the actual bolt length was

shorter than the equation (4) prediction.

25

30

35

40

45

50

55

60

2 4 6 8 10 12 14 16

PRSUP

CMRR

Successes

Failures

Discriminant

Figure 8. Effect of ARBS on roof fall rate. Above the

discriminant line the ARBS was generally adequate,

below the line it generally was not.

rebar bolt may be less than 0.5 tons per grouted inch (5), meaning

that the upper 26 in will pull out before that portion of the bolt

achieves its yield load. In other words, a 6-ft bolt with a low

anchorage factor might provide less support than a 5-ft bolt with a

good anchorage factor. Smooth holes, a large annulus, and/or poor

resin quality can also reduce the anchorage factor and the effective

bolt length. If the anchorage is questionable, short encapsulation pull

tests should be used to identify the problem and make adjustments (1,

16).

PATTERN AND CAPACITY OF ROOF BOLT SYSTEMS

The intensity of support provided by a roof bolt system is

determined by:

• The load-bearing capacity of the individual bolts;

• The density of the bolt pattern, and;

• The length of the bolts.

The capacity of the roof bolt is normally determined by the

diameter and the grade of the steel. One obvious advantage of

stronger bolts is that they can carry more broken rock. They can also

produce more confinement in the rock and accept higher pre-tension

loads. The capacity of a bolt can also depend on the anchorage,

however, as noted above.

The density of a roof bolt pattern depends on the number of bolts

per row and the spacing between rows. In the U.S., support density

does not vary much. With the advent of dual-head roof bolting

machines, four bolts per row have become the near universal

standard. Bolt spacing is limited by law to 5 ft, but is seldom less

than 4 ft. With entries varying in width from about 15 to 20 ft, bolt

densities range from one bolt per 12 to 25 ft2

.

To measure the bolt intensity, a summary variable, ARBS, was

developed that includes all three factors:

ARBS = (Lb) (Nb) (C) (5)

 (Sb) (We) 

Where Lb= Length of the bolt (ft)

Nb = Number of bolts per row,

C = Bolt capacity (1000's of pounds, (or kips))

Sb = Spacing between rows of bolts (ft),

We = Entry width (ft)

Figure 8 shows that the necessary ARBS increases as the CMRR

decreases. The prediction becomes even better if the depth of cover

is included in the equation:

ARBSG = (5.7 log10 H) - 0.35 CMRR + 6.5 (6)

Where ARBSG is the suggested value of ARBS for the given CMRR

and depth of cover.

VERIFICATION

Figure 9 shows the performance of the design methodology

represented by equations 2 through 6. All of the 71 case histories

that fell below the line in figure 4 are plotted. The two axes are:

• The difference between the suggested ARBSG and the actual

ARBS, and;

• The the difference between the suggested IsG and the actual

Is.

Therefore, a positive value on the x-axis means that the span was too

large. A negative value on the y axis means that the ARBS was

inadequate.

The lower right-hand quadrant of the graph contains 19 cases in

which the equations predict that both the span was too large and the

bolt intensity was too low. Of these, 16 were in fact failures, and

only one was a success.

The upper left-hand quadrant contains 18 cases in which the

equations predict that both the span and the bolt intensity were

adequate. Of these, 9 were successes and 4 were failures.

Therefore, for these two groups of cases, the design equations

predicted the actual outcome 83% of the time.

For the case histories in the other two quadrants, one of the

parameters was within the guidelines, while the other was not. It

seems reasonable that if the actual span were less than the suggested
span, then it might be possible to reduce the required ARBS.

Similarly, if the span was larger than recommended, a higher ARBS

might compensate for it.

The discriminant equation shown on the graph reflects an

adjustment to the suggested ARBS as follows:

ARBSGadj = ARBSG - 0.3 [IsG - Is] (7)

Equation (7) correctly predicted 76% of the successful and failed case

histories. Only 6 successes were mis-classified, and all but one of

these was very near the line. Of the 8 mis-classified failures, 5 also

fell close to the line. The 3 further away represent mines that

installed high densities of support and still had many roof falls. They

serve as reminders that roof bolt design remains an imperfect science

and there may be regimes where roof bolts may not be adequate.

Nevertheless, the overall results are sufficiently promising that the

following step-by-step guidelines can be proposed.

ANALYSIS OF ROOF BOLT STABILITY (ARBS)

1. Evaluate the Geology. The CMRR should be determined either

through underground observation or from exploratory drill core.

Zones of markedly different CMRR should be delineated. If the

thickness of individual beds varies within the bolted horizon, this

effect should be noted. Special features, such as faults or major

geologic transition zones, should be treated separately. 

2. Evaluate the Stress. It is unusual for stress measurements to be

available, so the design procedures use the depth of cover as a

rough estimator. One warning is that the case histories were

purposely chosen to be away from stress intensifiers like retreat

mining or multiple seam interactions. Where such intensifiers

are expected, it might be prudent to increase the “effective

depth” used in the equations.

3. Determine the Roof Bolt Reinforcement Mechanism using

equation (2). If the actual CMRR exceeds the one calculated by

equation (2), then the support mechanism is probably skin

control and/or suspension, and the traditional, dead-weight

loading design technique is probably appropriate. Otherwise,

the support mechanism is probably “beam building,” and the

ARBS technique may be used (continue with the steps 4 through

8.)

4. Determine the Intersection Span. Equation (3) should be used

to determine the recommended maximum span. Where

available, actual measurements of the diagonal spans should be

made, and the real values used in the equations.

5. Determine the Bolt Length using equation (4), and rounding to

the nearest half-ft.

6. Determine Required Roof Bolt Intensity. The design equation is

based on equations 6 and 7:

ARBSG = (SF) (0.3 (IsG - Is)) [(5.7 log10 H) - 

(0.35 CMRR) + 6.5] (8)

-10.00

-7.50

-5.00

-2.50

.00

2.50

5.00

-8.00 -6.00 -4.00 -2.00 .00 2.00 4.00 6.00

ARBS Difference

S

pan Difference (ft)

Successes

Failures

Discriminan

Few Roof Falls

Many Roof 

Falls

Figure 9. Verification of the design equations (3), (6), and (7). The case histories are plotted according to the difference between the actual

ARBS and the calculated ARBS (X-axis), and between the actual span and the calculated

span (Y-axis). The discriminant line is equation (7)
ARBS for 6 ft Bolts

0

5

10

15

20

25

3 3.5 4 4.5 5

Bolt Yield Capacity (tons)

ARBS = 4

ARBS=7

ARBS=10

ARBS=13

Figure 10. ARBS design charts relating the bolt spacing to bolt

yield capacity. Based on 4 bolts per row and 18 ft wide entries. 

(A) Bolt length = 4 ft and (B) Bolt length = 6 ft

0

5

10

15

20

25

3 3.5 4 4.5 5

Bolt Spacing (ft)

Bolt Yield Capacity (tons)

ARBS = 4

ARBS=7

ARBS=10

Where SF= Stability Factor (1.2 recommended)

The minimum recommended ARBS is approximately 3.0. 

7. Determine the Capacity and the Bolt Pattern. Equation (5) is

used to determine the remaining design variables. The bolt

length was selected in step 5, and normally the entry width and

the number of bolts per row are also fixed. The capacity and the

row spacing the remaining variables, and they can be adjusted to

achieve the suggested ARBS. Figures 10a and 10b may be useful

in this process. 

8. Select Skin Support: Plates, header, mats, or mesh should be

specified to ensure that loose rock between the bolts does not

pose a hazard.

CONCLUSIONS

This paper has presented some first steps towards providing a

scientific basis for roof bolt design. The design approach explicitly

considers the most important factors that determine the performance

of a roof bolt system. The two key input parameters are the roof

quality and the stress level, and the outputs include suggested values

for the intersection span, the bolt length, the bolt capacity and the bolt

pattern. Each of the elements in the design technique is supported by

extensive case history data.

A crucial finding was the importance of the depth of cover to

roof bolt performance. The evidence clearly supports the

proposition that in situ horizontal stresses tend to increase with

depth, and make the job of roof support more difficult. An explicit

connection between the depth of cover and the stress level is

provided. Further research is necessary to refine the relationship.

It should also be noted that the design methodology was derived

from the case histories based on the risk of roof falls in intersections.

In some circumstances, it may be possible to reduce the level of

support between intersections. On the other hand, the case histories

were all from areas that were unaffected by retreat mining. Where

elevated stresses are expected, the stress level could be adjusted by

increasing the “effective depth” in the equations.

The field data also indicated that in very weak roof, it may be

difficult to eliminate roof falls using typical U.S. roof bolt patterns.

When the CMRR was less than 40 at shallow cover, and less than

45-50 at deeper cover, high roof fall rates could be encountered even

with relativley high roof bolt densities. Faced with these conditions,

special mining plans or routine supplemental support might have to

be considered.

Much more remains to be learned to further improve the

efficiency of roof bolt design. One important question is the

interaction between primary roof bolts and the various supplemental

supports that may be installed. Supplemental supports vary greatly

in their stiffness and other characteristics, and they may not always

be compatible with the primary support. The quality of roof bolt

installation remains a critical issue. The effect of poor load transfer

on resin bolts was discussed above, but there are many other ways

in which the capability of a bolt may be defeated by improper

installation (17). Installing supports with pre-tension may be one

way to increase their efficiency, but a truly scientific study of the

value of pre-tension remains to be published. With nearly 100

million roof bolts installed each year, the mining community has

much to gain from continued roof bolting research.

REFERENCES

1. Mark, C. Design of Roof Bolt Systems. In Mark, C.,

Dolinar, D.R., Tuchman, R., eds. New Technology for Coal

Mine Roof Support, Proceedings, NIOSH Open Industry

Briefing, NIOSH IC 9453, 2000, pp. 111-132.

2. Molinda, G.M., Mark C. and Dolinar, D.R. Assessing Coal

Mine Roof Stability Through Roof Fall Analysis. In Mark,

C., Dolinar, D.R., Tuchman, R., eds. New Technology for

Coal Mine Roof Support, Proceedings, NIOSH Open Industry

Briefing, NIOSH IC 9453, 2000, pp. 53-72.

3. Pappas, D.M., Bauer, E.R. and Mark, C. Roof and Rib Fall

Accidents and Statistics: A Recent Profile. In Mark, C.,

Dolinar, D.R., Tuchman, R., eds. New Technology for Coal

Mine Roof Support, Proceedings, NIOSH Open Industry

Briefing, NIOSH IC 9453, 2000, pp. 3-22.

4. Gerdeen, J.C., Snyder, V.W., Viegelahn, G.L. and Parker, J..

Design Criteria for Roof Bolting Plans Usign Fully Grouted

Non-Tensioned Bolts to Reinforce Bedded Mine Roof. U.S.

Department of the Interior, Bureau of Mines, OFR 46-80,

Vols. III-IV, 1979.
5. Peng, S.S. Roof Bolting Adds Stability to Weak Strata. Coal

Age Magazine, December 1998, pp. 32-38.

6. Conway, C.C. Roof Support with Suspension Rods. Min.

Cong. Jour., June 1948, pp. 32-37.

7. Damberger, H.H., Nelson, W.J. and Krausse, H.F. Effect of

Geology on Roof Stability in Room-and-Pillar Mines in the

Herrin (No. 6) Coal of Illinois. Proceedings, 1st Conference on

Ground Control Problems in the Illinois Coal Basin,

Carbondale, IL, Southern Illinois University at Carbondale,

1980, pp. 14-32.

8. Mark, C., Molinda, G.M., Schissler, A.P. and Wuest, W.J.

Evaluating Roof Control in Underground Coal Mines with the

Coal Mine Roof Rating. In: Peng, S.S., ed. Proceedings, 13th

International Conference on Ground Control in Mining,

Morgantown, WV, University of West Virginia, 1994, pp. 252-

260. 

9. Molinda, G.M. and Mark, C. Coal Mine Roof Rating

(CMRR): A Practical Rock Mass Rating for Coal Mines.

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

Mines, IC 9387, 1994, 83 pp.

10. Gale, W.J., Fabjanczyk, M.W. and Guy, R.J. Optimization of

Reinforcement Design of Coal Mine Roadways. In: Aziz, N.I.,

Peng, S., eds. Proceedings, 11th International Conference on

Ground Control in Mining, Wollongong, New South Wales,

Australia: University of Wollongong, 1992, pp. 272-279.

11. Scott, J.J. Roof Bolting in Mining-USA. Proceeding, Norsk

Jord-ogFjellteknisk Forbuns, Oslo, Norway, November 27,

1992, 13 pp.

12. Mark, C. and Mucho, T.P. Longwall Mine Design for

Control of Horizontal Stress. Proceedings, New Technology

for Longwall Ground Control Technology Transfer Seminar,

U.S. Department of the Interior, Bureau of Mines, SP 94-01,

1994, pp. 53-76.

13. Bigby, D.N., Cassie, J.W. and Ledger, A.R. Absolute Stress

and Stress Change Measurements in British Coal Measures.

Proceedings, ISRM Symposium, British Geotechnical

Society, Eurock ‘92, 1992, pp. 390-395.

14. Molinda, G.M., Mark, C., Bauer, E.R., Babich, D.R. and

Pappas, D.M. Factors Influencing Intersection Stability in

U.S. Coal Mines. In: Peng SS, ed. Proceedings, 17th

International Conference on Ground Control in Mining.

Morgantown, WV, University of West Virginia, 1998, pp.

267-275. 

15. Unal, E. Empirical Approach to Calculate Rock Loads in

Coal Mine Roadways. In: Peng SS, ed. Proceedings, 5th

International Conference on Ground Control in Mining.

Morgantown, WV, University of West Virginia, 1984, pp.

232-246.

16. Karabin, G.J. and Debevic, W.L. Comparative Evaluation of

Conventional and Resin Bolting System. MESA IR 1033,

1976.

17. Mazzoni, R.A., Karabin, G.J. and Cybukski, J.A. A Troubleshooting Guide for Roof Support Systems. U.S. Department

of Labor, Mine Safety and Health Administration, IR 1237,

1996, 101 pp.