NEW DEVELOPMENTS WITH THE COAL MINE ROOF RATING

Christopher Mark, Chief, Rock Mechanics Section

Gregory M. Molinda, Research Geologist

Timothy M. Barton, Mining Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

The Coal Mine Roof Rating (CMRR) was first presented at this

Conference nine years ago. Since its introduction, the CMRR has

been incorporated into many aspects of mine planning, including

longwall pillar design, roof support selection, feasibility studies, and

extended cut evaluation. It has also become truly international, with

involvement in mine designs and funded research projects in South

Africa, Canada, and Australia.

The purpose of this paper is to bring the mining community up to

date with recent improvements and applications of the CMRR. The

most important new development is a streamlined process for

determining the CMRR from exploratory drill core. Just three types

of information are now required:

• Fracture spacing (or RQD)

• Uniaxial compressive strength (or axial Point Load Testing)

• Diametral Point Load Testing

The moisture sensitivity adjustment has also been changed, and

new research has related the immersion test to slake durability.

Most recently, the CMRR has been implemented in a computer

program, which can be obtained from NIOSH free of charge. The

program facilitates calculation of the CMRR from either underground

or drillcore data. Values from many locations can be saved in a single

file, and an interface with Autocad allows CMRR contour plots to be

integrated into mine planning.

INTRODUCTION

Roof falls continue to be the greatest single hazard faced by

underground coal miners. One reason is that mines are not built of

manmade materials like steel or concrete, but rather of rock, just as

nature made it. The structural integrity of a coal mine’s roof is greatly

affected by natural weaknesses, including bedding planes, fractures,

and small faults. The engineering properties of rock cannot be

specified in advance, and can vary widely from mine to mine and even

within individual mines.

Engineers require quantitative data on the strength of rock masses

for design. Traditional geologic reports contain valuable descriptive

information but few engineering properties. Laboratory tests, on the

other hand, are inadequate because the strength of a small specimen

is only indirectly related to the strength of the rock mass.

The Coal Mine Roof Rating (CMRR) was developed nearly 10

years ago to try to fill this gap (1, 2). The CMRR weighs the

geotechnical factors that contribute to roof competence, and combines

them on single rating scale of zero to 100. The CMRR integrates

years of research into geologic hazards in coal mining with the

worldwide experience with rock mass classification systems. To

verify the procedure, field data were collected from nearly 100 mines

in every major coalfield in the U.S.

The CMRR makes it possible to compare ground control

experience from different coalfields, even when the geological

conditions are very different. This has allowed the collection of large

case history databases, which have been the basis of design

procedures to solve a wide variety of ground control problems. 

Two recent developments should facilitate the integration of the

CMRR into geologic exploration and mine design: 

• The procedures for collecting CMRR data from drill core have

been greatly simplified, and;

• A computer program that speeds calculation and interfaces with

mine mapping software is now available.

MODIFIED PROCEDURES FOR DRILL CORE

The CMRR can be determined from underground exposures such

as roof falls and overcasts, or from exploratory drill core. In either

case, the main parameters measured are:

• The uniaxial compressive strength (UCS) of the intact rock;

• The intensity (spacing and persistence) of discontinuities such

as bedding planes and slickensides;

• The shear strength (cohesion and roughness) of discontinuities,

and;

• The moisture sensitivity of the rock.

The CMRR is calculated in a two-step process. First, the mine roof

is divided into lithologic/structural units, and Unit Ratings are

determined for each. Then the CMRR is determined by combining
Figure 1. Diametral and Axial Point Load Tests.

SHALE

0

5000

10000

15000

20000

25000

30000

0 200 400 600 800 1000 1200

POINT LOAD INDEX (Is (50), psi)

UNIAXIAL COMPRESSIVE STRENGTH (UCS,psi)

Figure 2. Relationship between Axial PLT and UCS tests for

shale (3).

Figure 3. CMRR rating scale for Axial Point Load or UCS tests.

the Unit Ratings and applying appropriate adjustment factors. The

second step is the same regardless of whether the Unit Ratings were

from data collected underground or from core.

The procedures for gathering data and calculating the CMRR from

underground exposures have remained essentially unchanged since

they were first proposed in 1993. Procedures to determine Unit

Ratings from drill core were originally presented by Mark and

Molinda in 1996. These have now been streamlined and updated

based on recent research. 

The new drill core equation is:

Unit Rating = UCS Rating + Discontinuity Rating.

Where:

• The UCS may be determined either by traditional laboratory

tests or from Axial Point Load Tests, and;

• The Discontinuity Rating is the lower of the Diametral PLT

Rating or the Discontinuity Spacing Rating. 

Unconfined Compressive Strength Rating

Laboratory testing is generally considered the standard method of

determining the UCS. Unfortunately, laboratory tests are expensive

because the samples must be carefully prepared. The variability in the

results is also high, with the standard deviation typically about onethird of the mean for coal measure rocks (3).

As an alternative, the CMRR recommends the Point Load Test

(PLT) for drill core. The PLT has been accepted in geotechnical

practice for nearly 30 years (4). An advantage of the PLT is that

numerous tests can be performed, because the procedures are simple

and inexpensive because minimal sample preparation is required. The

apparatus is also inexpensive and portable. The International Society

for Rock Mechanics (5) has developed standard procedures for testing

and data reduction.

Another advantage of the PLT is that both diametral and axial tests

can be performed on core. In a diametral test, the load is applied

parallel to bedding (figure 1). The diametral test is therefore an

indirect measure of the lateral strength, or bedding plane shear

strength.

The axial PLT is used to measure the UCS. The Point Load Index

(Is50) is converted to UCS by the following equation:

UCS = K (Is50) (1)

Where K is the Conversion Factor. Mark and Molinda (6)

originally adopted the K values proposed by Vallejo (7) for the

CMRR, which were K=12.5 for clay-rich rocks, and K=17.4 for silty

and sandy rocks. Vallejo’s study involved a relatively small sample

base, however. 

To better determine the appropriate value of K for use in the

CMRR, a comprehensive study involving more than 10,000 tests of

coal measure rocks from 6 states was conducted (6). The study found

that K=21 worked for the entire range of rock types and geographic

regions (figure 2). The study also found that the variability of the

PLT measurements, as measured by the standard deviation, was no

greater than for UCS tests.

Figure 3 shows the UCS/Axial PLT rating scale used in the CMRR

program.

Discontinuity Spacing Rating

Most standard geotechnical core logging procedures include some

measure of the natural breaks in the core. The two most commonly

employed are the fracture spacing and the RQD. Fracture spacing is

easily determined by counting the core breaks in a particular unit, and
100

DSR = 10.5ln(RQD) - 11.6

Discontinuity Spacing Rating (DSR)

20

50RQD

20 25 30 35 40 45

DSR = 5.64ln(FS) + 24

Fracture Spacing (FS - inches)

10

50

2

1

5

100

20

50

Figure 4. CMRR rating scale for Fracture Spacing or RQD.

Diametral PLT Ratings

0

10

20

30

40

50

60

70

0 100 200 300 400

Diametral PLT Strength (Is(50))

Diametral Rating

y=0.144x+20

y=0.104x+27.5

when PLT <34.7 rating =25

when PLT>312.5 

rating = 60

Figure 5. CMRR rating scale for Diametral Point Load tests.

then dividing by the thickness of the unit. The RQD is obtained by

dividing combined length of core pieces that are greater than four

inches in length by the full length of the core run.

Both measures have their advocates in the geotechnical

community. Priest and Hudson (8) suggested that the two can be

related by the following formula:

RQD = 100 e-0.1L (0.1L+1) (2)

Where L=number of discontinuities per meter.

As input, the CMRR uses both the RQD and the fracture spacing.

When the fracture spacing is greater than about 1 ft, the RQD is not

very sensitive, so the fracture spacing is used directly. At the other

extreme, when the core is highly broken or lost, the RQD appears to

be the better measure. Either measure may be used in the intermediate

range. 

The program uses the following equations to calculate the

Discontinuity Spacing Rating (DSR) of core from RQD and the

fracture spacing. The equations were derived from the original

CMRR rating tables.

DSR = 10.5 ln (RQD) - 11.6 (3a)

or

 = 5.64 ln (fracture spacing, inches) + 24 (3b)

The minimum value of the DSR is 20, and the maximum is 48 (see

figure 4).

Diametral PLT Rating

One problem with both the fracture spacing and the RQD is that

they actually measure the strength of discontinuities as well as their

spacing. Weak discontinuities may break apart during drilling, while

strong ones might withstand the rigors of the drilling process.

The problem is particularly acute with layered rocks such as shales.

Such rocks may be recovered intact, with RQD=100, yet their lateral

strength may be one-sixth of their axial (6). Since the most severe

loading applied to coal mine roof is normally lateral, caused by

horizontal stress, bedding plane shear strength is a critical parameter.

Unfortunately, bedding plane shear strength is almost never tested

directly in the US. The diametral PLT is a convenient index test that

may be used as a substitute. Because the precise relationship between

bedding plane shear strength and the PLT is not known, and since it

seems unlikely that the same K-factor used to convert the axial test to

the UCS would apply, the new CMRR uses the Point Load Index

(IS50) directly. The Diametral PLT rating values were derived from

the original CMRR tables and the data presented by Mark and

Molinda (6), and are shown in figure 5.

If the diametral test results show that the rock fabric or laminations

are low strength, it would be illogical to give the rock high marks for

discontinuity spacing. Therefore, the Discontinuity Rating is the

lower of the Diametral PLT Rating or the Discontinuity Spacing

Rating.

The Unit Rating is simply the Discontinuity Rating plus the

UCS Rating.

OTHER MODIFICATIONS TO THE CMRR

Moisture Sensitivity Deduction

Moisture sensitivity can affect roof stability in several ways. The

rock itself may be weakened, or may slake or slough. In extreme

cases, rock may disintegrate completely and turn to mud when

exposed to groundwater. Clay minerals can also expand, causing

swelling pressures in the roof.

In the original CMRR, the maximum deduction for moisture

sensitivity was 25 points. In practice, this deduction proved to be too

large. The new maximum deduction is 15 points. An adjusted

Immersion Test data sheet is shown in figure 6. The moisture

sensitivity ratings are then determined using table 1. If immersion test

results are not available, moisture sensitivity can sometimes be

estimated visually in underground exposures.

Table 1. Moisture Sensitivity Ratings

Moisture Sensitivity Immersion Index Rating

Not Sensitive 0-1 0

Slightly Sensitive 2-4 -3

Moderately Sensitive 5-9 -7

Severely Sensitive >9 -15

Note: Apply to Unit Rating only when the roof is damp or

water is leaking through the bolted internal.
IMMERSION TEST

Mine Date 

Unit No. Tester 

Sample Description (Lithology, bedding, etc.):

 

 Immersion Breakability

Observation Rating Observation

Rating

Appearance of Water

Clear = 0 No Change 0

Misty = -1 Small Change-2

Cloudy = -3 Large Change -6

Talus Formation Breakability Index 

None = 0

Minor = -1

Major = -3 

Cracking of Sample

None = 0

Minor/Random = -1

Major/Preferred Orientation =-3 

Specimen Breakdown = -9

Total Immersion Index 

 

 

Procedure for Immersion Test

1. Select sample(s) - ~ hand sized.

2. Test for hand breakability.

3. Rinse specimen (to remove surface dirt, dust, etc.).

4. Immerse in water for 1 hour.

Figure 6. Data sheet for the Immersion Test.

Usually, some time is required for contact with humid mine air to

affect rock strength. In short-term applications, therefore, it may

not be appropriate to apply the moisture sensitivity deduction.

The CMRR program now reports both the Unit Rating and the

CMRR with and without the moisture sensitivity deduction.

Relationship Between Immersion and Slake Durability Tests

The CMRR employs the simple immersion test to measure

moisture sensitivity. While numerous other tests have been

proposed, the closest thing to a standard moisture sensitivity index

is probably the Slake Durability Test (SDT). Hoek (4)

recommended the SDT as a basic geomechanical test, ISRM

standard procedures have been developed for it, and it is an integral

part of Bieniawski’s Rock Mass Rating (RMR).

The SDT is intended for use in establishing the rate of breakdown

in a rock mass in which stability is suspected to vary with time. To

perform the test, 10 lumps of rock, each weighing about 0.1 lbs, are

oven dried, weighed, and then rotated through a water bath for 10

minutes. The repeated wetting and drying, together with the mild

abrasion that takes place during the test, causes moisture sensitive

rocks to break down. The slake durability index is the final dry

weight of the sample expressed as a percentage of the original dry

weight (4).

Research was conducted to explore the relationship between the

SDT and the immersion test. Rock samples were collected

underground from a variety of mine settings, carefully wrapped to

maintain in situ moisture content, and tested in the laboratory. A

total of 96 tests were run on 16 distinct rock types from 9 mines.
Figure 7. Comparison of the Slake Durability and

Immersion Tests.

5

10

15

20

25

30

5 10 15 20 25 30

UCS/Axial PLT Rating

Ball Peen Rating

Figure 8. Comparison of Axial Point Load and Ball Peen Tests.

The results are shown in Figure 7. Table 2 indicates how the

results from either test can be used for input to the CMRR:

Table 2. Moisture Sensitivity Classes from Immersion and

Slake Durability Tests

Moisture Sensitivity

Class

Immersion

Index

Slake Durability

Index

Not Sensitive 0-1 100-98

Slightly Sensitive 2-4 98-92

Moderately Sensitive 5-9 92-80

Severely Sensitive >9 <80

From the testing conducted to date, there is a good correlation

between the two tests for the Not Sensitive and Slightly Sensitive

classes. The correlation is less reliable for distinguishing

“moderately sensitive” rocks from “severely sensitive” rocks.

Relationship Between Ball Peen Hammer Test and the

UCS/Axial PLT 

The Ball Peen Hammer Test, originally proposed by Williamson

(9), has been the CMRR standard test for underground data

collection. Mark and Molinda (4) compared results for both tests,

and found a good correlation. In that comparison, however, the PLT

results were converted to UCS using the Vallejo conversion factors.

Figure 8 shows the comparison between the two tests, using an

expanded data set and converting the PLT data to UCS with K=21.

In 17 of the total of 21 pairs (or 81% of the cases), the difference

between the two measurements was 4 points or less. To account for

the changed K, the original Williamson rock classes have been

slightly adjusted, as shown in table 3:

Table 3. Approximate UCS Ranges from Ball Peen Hammer

Tests

Ball Peen

Hammer Class

Williamson UCS

Range (psi)

CMRR UCS Range

(psi)

Molds <1,000 <2,000

Craters 1,000-3,000 2,000-5,000

Dents 3,000-8,000 5,000-10,000

Pits 8,000-15,000 10,000-17,000

Rebounds >15,000 >17,000

Strong Bed Adjustment

One of the most important concepts in the CMRR is that the

strongest bed within the bolted interval often determines the

performance of mine roof. The strong bed adjustment (SBADJ) in

the CMRR depends upon:

• The Strong Bed Difference (SBD), which is the difference

between the strong bed’s Unit Rating and the thicknessweighted average of all the Unit Ratings within the bolted

interval;

• The thickness of the strong bed (THSB, ft), and;

• The thickness of the weak rock suspended from the strong bed

(THWR, ft).

In the original CMRR, the SBADJ was determined using a table.

For improved accuracy and to facilitate implementation of the table

in the computer program, equation (4) was derived using multiple

regression: 

SBADJ = [(0.22 SBD*THSB) –2.5] * [1 – (0.1 (THWR - 1.7))] (4)

The SBADJ ranges from 0 up to 90% of the SBD. Other rules

that apply are that the maximum THSB that can be entered into the

equation is 4 ft, and the allowable range of the THWR is 1.7-8.5 ft.

The THSB must also be at least 1 ft, because experience has shown

that thinner units cannot be counted on to reinforce the roof, and

may actually weaken it because they can concentrate horizontal

stress.

THE CMRR COMPUTER PROGRAM 

The CMRR program is designed to facilitate the entry, storage,

and processing of field data. Either core or underground data can

be entered, and calculations are updated instantly when a change is

made. This allows the user to vary parameters, such as the bolt

length, to see their effect on the final CMRR.

Figure 9 shows the underground data entry screen. Drop-down

menus are used to enter the data for each of the parameters. In the

core data screen (figure 10), the user has the option of entering PLT

test data, and having the program automatically determine the mean

UCS and diametral Is(50). Otherwise, the user can enter the mean

strength values directly. 
Figure 9. Underground data entry screen from

the CMRR Program.

Figure 10. Drill core data screen from the CMRR Program.

An important feature of the new program is a built-in interface

with Autocad. Data from up to 200 locations can be entered and

saved in a single file, along with their location coordinates. The

program can create a file for export that includes both the calculated

CMRR values and the locations. A CMRR layer can then be created

in Autocad for use in mine planning. 

RECENT APPLICATIONS OF THE CMRR

During the past 8 years a number of mine planning design tools

have been based on the CMRR. The first, and perhaps the best

known, was its incorporation into the ALPS pillar design program

(10). A large database of longwall case histories was collected from

throughout the US, and subjected to statistical analysis. The results

showed that when the roof was strong (CMRR>65), longwall chain

pillars with an ALPS SF as low as 0.7 could provide satisfactory

tailgate conditions. On the other hand, when the roof was weak

(CMRR<45), the ALPS SF might need to be as high as 1.3. 

Some more recent examples are described below.

Longwall Tailgate Design (Australia)

ALPS was the starting point for an Australian Coal Industry

Research Project ACARP) whose goal was to develop an Australian

chain pillar design methodology (11). The project aimed to

calibrate ALPS for the different geotechnical and mine layouts used

in Australia. Ultimately, case history data were collected from 60%

of Australian longwall mines.

The study found strong relationships between the CMRR, the

tailgate SF, and the installed level of primary support. Design

equations were developed that reflected these trends. The final

product, called the Analysis of Longwall Tailgate Serviceability

(ALTS), was implemented in a computer program and has become

widely used in Australia.

Roof Bolt Selection

To help develop scientific guidelines for selecting roof bolt

systems, NIOSH conducted a study of roof fall rates at 37 U.S.

mines (12, 13). The study evaluated five different roof bolt

variables, including length, tension, grout length, capacity, and

pattern. Roof spans and the CMRR were also measured.

Performance was measured in terms of the number of roof falls that

occurred per 10,000 ft of drivage.

The study found that the depth of cover (which correlates with

stress) and the roof quality (measured by the Coal Mine Roof

Rating (CMRR)) were the most important parameters in

determining roof bolting requirements. Intersection span was also

critical. The study’s findings led to guidelines that can be used to

select the proper span, bolt lengths, and bolt capacity based on the

CMRR. The results have been implemented into a computer

program called Analysis of Roof Bolt Systems (ARBS).

Longwall Mining through Open Entries and Recovery Rooms

Unusual circumstances may require that a longwall retreat into

or through a previously driven room. The operation is usually

completed successfully, but there have been a number of spectacular

failures. To help determine what factors contribute to such failures,

an international data base of 131 case histories was compiled (14).

The study found that the CMRR and the density of standing

support were the two most important parameters in predicting

severe weighting-type failures. These failures only occurred when

the CMRR was less than 55, and when the support density was less

than 70 psi. When the CMRR was 40 or less, all the successful

cases employed a standing support density of at least 145 psi.

Roof Fall Evaluations (South Africa)

The CMRR featured prominently in an important research

project sponsored by the Safety in Mine Research Advisory

Committee (SIMRAC) and other leading industry, labor, and

government organizations in South Africa. The goal of the project

was to investigate the causes of fatal roof failures in South African

coal mines. A team of recognized experts visited a broad spectrum

of mines and collected data at 182 roof fall sites. The study found

that roof falls were more likely where the roof was less competent

in terms of the CMRR. Another finding was that the CMRR
20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5 0.6

Primary Support Rating (PSUP)

Coal Mine Roof Rating (CMRR)

USA Case Histories

S. Afr. Case Histories

South African

Mines

Australian

Mines

Figure 11. Relationship between the CMRR and roof bolt

density in the US, Australia, and South Africa.

correlated well with roadway widths. Based on data presented by

Mark (15), (see figure 11), the study also concluded “in South

African coal mines, less support is used for comparable roof

conditions than either the USA or Australia. This supports previous

conclusions that in South African coal mines, the density of

supports needs to be increased” (16). 

Another SIMRAC study found the CMRR easy to use and robust

enough to adequately describe the roof conditions at most South

African collieries (17). It took less than four hours for a trained

geologist to become competent with the method. The results seemed

more reasonable than those obtained from the RMR, which tended

to overrate ground conditions by at least one class (20 points) due

to its lack of sensitivity to the characteristics of bedded strata. Some

improvements were suggested for the CMRR, including adjustments

for joint orientation, blasting, and horizontal stress. A follow-on

SIMRAC project is currently underway.

Baseline Comparison of Ground Conditions (Canada)

The underground coal industry of Canada is small and

geographically dispersed. To assist the mines in maintaining worldclass safety standards, CANMET established the Underground Coal

Mine Safety Research Consortium. One of the Consortium’s first

projects was aimed at establishing a “best practice” baseline for

conducting geological and geomechanical assessments and applying

the findings to geotechnical design.

The CMRR was found to be particularly valuable in the

assessment (18). It allowed the Canadian underground mines to be

compared with each other and with international benchmarks.

Based on the CMRR, many ground control safety technologies

developed in the US were found to have direct application to the

Canadian mines.

Other Applications

• Extended cuts can collapse prematurely in weak roof. Data

collected from 36 mines found that when the CMRR was

greater than 55, extended cuts were nearly always routine, but

when the CMRR was less than 37, they were almost never

taken (19).

• Tailgate support guidelines incorporating the CMRR have

been included in the STOP program (20).

• Input for numerical models have been derived from the

CMRR (21).

• Multiple seam mine design guidelines have been developed

that incorporate the CMRR (22).

• Hazard analysis and mapping has been based on the CMRR

(23).

CONCLUSIONS

Roof geology is central to almost every aspect of ground control.

The CMRR makes it possible to quantify roof geology so that it can

be included in mine planning decisions. Worldwide experience has

shown that the CMRR is a reliable, meaningful, and repeatable

measure of roof quality. 

A wide variety of design tools that are based on the CMRR have

now been developed. They address a broad range of ground

control issues, and rely upon large databases of actual mining case

histories. Without the CMRR, it would not have been possible to

capture this invaluable experience base. 

The new core procedures and computer program further expand

the potential of the CMRR. It is now possible to routinely collect

CMRR data during geologic exploration or from underground

mapping, complete the calculations, and integrate the results into

mine mapping software. Foreknowledge of conditions means

better mine planning and fewer unexpected hazards underground.

REFERENCES

1. Molinda, G. and C. Mark. The Coal Mine Roof Rating

(CMRR)–A Practical Rock Mass Classification for Coal Mines.

Proceedings, 12th International Conference on Ground Control

in Mining, Morgantown, WV, 1993, pp. 92-104. 

2. Molinda, G. and C. Mark. The Coal Mine Roof Rating

(CMRR)–A Practical Rock Mass Classification for Coal Mines.

USBM IC 9387, 1994, 83 pp. 

3. Rusnak, J. and Mark, C. Using the Point Load Test to

Determine the Uniaxial Compressive Strength of Coal Measure

Rock. Proceedings, 19th International Conference on Ground

Control in Mining, Morgantown, WV, 2000, pp. 362-371.

4. Hoek, E. Rock Mechanics Laboratory Testing in the Context

of a Consulting Engineering Organization. Int. J. Rock Mech.

and Mng. Sci., Vol. 22, 1977, pp. 93-101.

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Conference on Ground Control, Golden, CO, 1996, pp. 415-

428. 

7. Vallejo, L.E., Welsh, R.A. and Robinson, M.K. Correlation

Between Unconfined Compressive and Point Load Strengths

for Appalachian Rocks. Proceedings, 30th U.S. Symposium on

Rock Mechanics, 1989, pp. 461-468.
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18.

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in Mining, Morgantown, WV, 1999, pp. 282-290.

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New Technology for Coal Mine Roof Support, NIOSH Open

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Experience with Longwall Mining into Pre-Driven Rooms.

Proceedings 17th International Conference on Ground Control,

1998, pp. 44-53. 

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Perspective. Proceedings, 18th International Conference on

Ground Control in Mining, Morgantown, WV, 1999, pp. 186-

193.

16. Van der Merwe, J.N. In situ Investigation into the Causes of

Falls of Roof in South African Collieries. Proceedings, 20th

International Conference on Ground Control in Mining,

Morgantown, WV, 2001,pp. 105-118.

17. Butcher, R.J. Application of the Coal Mine Roof Rating

System in South African Collieries. Proceedings, 20th

International Conference on Ground Control in Mining,

Morgantown, WV, 2001, pp. 317-321.

18. Forgeron, S., Mark, C. and Forrester, DJ. Standardization of

Geological and Geomechanical Assessment at Underground

Coal Mines in Canada. CIM Bulletin, July, 2001, pp. 83-90.

19. Mark, C. Application of the Coal Mine Roof Rating (CMRR)

to Extended Cuts. Mining Engineering, April, 1999, pp. 52-56.

20. Barczak, TM. Optimizing Secondary Roof Support with the

NIOSH Support Technology Optimization Program (STOP).

Proceedings, New Technology for Coal Mine Roof Support,

NIOSH Open Industry Briefing, NIOSH IC 9453, 2000, pp.

151-164.

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