314 MINING INDUSTRY ANNUAL REVIEW

W

and

D. DEBASIS

Using the coal mine roof rating (CMRR) to

assess roof stability in U.S. coal mines

 

The stability of any underground opening is, in large part, a

function of the strength of the rock mass which surrounds it.

The Coal Mine Roof Rating (CMRR) has been developed to

quantify the defects in the rock mass and compile a strength

value which can be used for engineering design. The CMRR

has been applied to a number of ground stability problems,

including chain pillar design, roof bolt selection, hazard

assessment, intersection design, and numerical modeling.

The CMRR procedure and some of these applications are

described in this paper. The CMRR will soon be available in

a Visual Basic computer program, allowing easy integration

into exploration programs and standard roof fall

assessments.

Introduction

hile overall safety in U.S. coal mines has

improved dramatically in the last 50 years,

fatality rates continue to exceed other major

industrial sectors (fig. 1). Fatalities due to ground falls make

up a significant portion of this rate. In 1998 there were 790

injuries and 13 fatalities due to falls of roof. These were

reported from 827 underground mines producing 380 million

tonnes of coal. In 1998, of 2,617 reportable falls, 30%

resulted in injury or fatality (MSHA, 1998).

Longwall mining continues to be much safer than room

and pillar mining. While longwall production is approaching

the levels of room and pillar mining, the roof fall rate for

room and pillar mining is significantly higher (Dolinar and

Bhatt, 2000) (fig. 2). For this reason a number of the

National Institute for Occupational Safety and Health

(NIOSH) ground control research studies have concentrated

on this area. Intersections are significantly more likely to fall

than non-intersections. In 1996 over 71% of reported roof

falls with known locations occurred in intersections, making

them 8-10 times as likely to fall as other locations.

In the U.S., all unsupported roof is considered hazardous

and it is illegal to travel under unsupported roof. Therefore,

 

Mr. Gregory M. Molinda, Research Geologist and Dr. Christopher Mark,

Chief, Rock Mechanics Section, National Institute for Occupational Safety

and Health, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania,

USA and Dr. D. Debasis, Professor, Mining Engineering, Southern Illinois

University, Carbondale, Illinois, USA

it is the failure of supported roof which contributes to the

vast majority of injuries. A number of factors contribute to

the failure of supported ground. These include: overspanned

intersections, insufficient support, excess horizontal stress,

multiple seam abutment loading, and undersized pillars. An

element in many of these failures is weak geology. The

accurate assessment of the strength of the rock mass is

critical to the stability of the opening. Coal mine operators

have always made an assessment of roof rock quality on

some level. Usually the information is qualitative and

obtained from roof exposure due to mining, or less reliably,

from drill hole data. The information may be presented as a

hazard map. Historically, accounts of mine roof geology have

been highly descriptive and required interpretation for use in

engineering design and support selection. More quantitative

efforts at determining the roof strength have included rock

tests like uniaxial compressive strength, the Brazillian

indirect tensile test, direct shear, or triaxial tests. These tests

suffer from small sample size but, more importantly, fail to

measure the real rock weaknesses. Existing rock mass

classifications have also been applied with limited success

(RQD, RMR, URCS, Q system) (Molinda and Mark, 1994.).

The CMRR, developed by the Bureau of Mines in 1994,

is now widely used for a variety of purposes including roof

hazard assessment, chain pillar design, and stress modeling.

The underlying philosophy of the CMRR is that it is not the

strength of the intact rock fabric which makes a stable rock

mass, but the defects or discontinuities which weaken or

destroy the roof beam (Molinda and Mark, 1994). The

CMRR is intended to evaluate the roof discontinuities which

most contribute to the weakness and failure of the roof mass.

The emphasis is on weak bedding planes, slickensides, joints,

and laminations. As a result, the geologic origin of a

discontinuity is less important than its engineering

characteristics. The CMRR is designed to evaluate the

inherent strength of the bolted interval and returns a number

from 0-100, with 100 being absolutely solid roof. 

The CMRR is a two-part system. First, the Unit Rating of

each rock member in the bolted interval is determined by

evaluating the discontinuities in the rock with simple field

tests (fig. 3). A chisel is struck parallel to bedding to

determine the tensile strength on bedding. A ball peen
JOURNAL OF MINES, METALS & FUELS, 2001 315

Fig. 3. Components of the coal mine roof rating (CMRR).

Comparison of Fatality Incident Rates

0

10

20

30

40

50

60

70

1995 1996 1997 1998 1999

Year

Fatalities/100,000 Workers

All Underground Mining Ground Falls

All US Manufacturing All US Construction

Fig. 1. Fatality rates of selected industries as compared to underground

mining.

Room and pillar

Longwall

9

8

7

6

5

4

3

2

1

0

1988 1992 1996 1989 1993

YEAR

1990 1994 1998 1991 1995 1999 1997

Y= -0.0445x +7.31

R = 0.0892 2

Y= -0.1748 +3.22

R = 0.7711 2

Fig. 2. Comparison of longwall and roo and pillar injury rates.

for the spacing and frequency of bedding planes, joint sets,

slickensides, and other discontinuities, with lower point

values representing weak, closely spaced slickensides or

laminations. Points are deducted for moisture sensitivity and

disturbed bedding would likely have a Unit Rating = 29 (fig.

4). At the other end of the scale, a massive, crystalline,

sandstone with faint, widely-spaced, cross bedding would

have a Unit Rating = 77 (fig. 4).

The second part of the CMRR is the determination of the

Roof Rating. The thickness-weighted average of the Unit

Ratings is determined, and adjustments are made for Unit

associations, including an addition of points for the presence

of a strong bed in the bolted roof. One of the most important

concepts incorporated into the CMRR is that of the strong

bed. Through many years of experience with roof bolting

throughout U.S., coalfields mine operators have found that

the overall structural competence of bolted roof is very often

determined by the quality of the most competent bed within

the bolted interval. This fact was early recognized with

mechanical bolts. Regulations of the U.S. Mine Safety and

Health Administration at 30 CFR 75.204(f)(1) require that

“roof bolts that provide support by suspending the roof from

overlying stronger strata shall be long enough to anchor at

least 0.3 m into the stronger strata”. In recognition of this

importance an addition of points is applied when a strong

roof unit at least 0.3m thick is present in the roof beam. The

points increase with thicker, stronger units. Other features of

the roof rating include a deduction for weak Unit contacts,

and a deduction for groundwater inflow.

The working range of the CMRR is 25-100. When the

CMRR is less than 25, the roof usually collapses

immediately upon mining.

Originally designed to use roof falls and overcasts as

sources of data input, the CMRR has since been adapted to

be calculated from drill core (Mark and Molinda, 1996). This

fulfills a need for calculating the CMRR in advance of

mining for mine planning purposes. In the calculation for

core both the compressive strength of the rock matrix and

tensile strength of the bedding and discontinuity surfaces are

measured by the point load test (IRSM, 1985) This proven

rock test has been adapted to measure both axial and

diametral (bedding) strength. A new conversion factor from

point load index of strength (Is(50)) to uniaxial compressive

strength (UCS) has been determined from a large data base

recently provided by a large U.S. coal

company (Rusnak and Mark, 2000). 

Applications of the CMRR

NIOSH has made extensive use of the

CMRR as a research tool. In ground

control investigations ranging from chain

pillar design to intersection sizing, the

CMRR has helped in developing

remedies for roof failure. The CMRR has

been most useful in comparing roof

geologies from different regions. Figure
316 ,MINING INDUSTRY ANNUAL REVIEW

24

28

20

16

12

8

4

0

Weak

 45

Moderate

45 to 65

Strong

 65

COAL MINE ROOF RATING (CMRR)

NUMBER OF CASES

N o rth e rn A p p a lachia n Illinois

C olo ra d o , W yo m in g

N e w M e xic o

U ta h

C oal basin

S o u th e rn A p p a lachia n

W a rrio r

Fig. 5. Geographic distribution of CMRR measurements in the U.S.

Fig. 4. Rock units with ferm number and rock strength

(1 mega pascal = 145 pounds/ft2

Fig. 6. Example of a CMRR map in one of the study mines.

5 shows the distribution of CMRR’s collected from around

the U.S. (Mark et al., 1994). The data reflects what is

generally thought about roof strength: weaker, soft rocks,

susceptible to horizontal stress are found in the northern

Appalachian basin, and stronger, more massive sandrocks

found in the southern Appalachian basin and in the Uinta

basin in Utah. The interior basins in the U.S. generally have

more carbonate sequences as they alternate between marine

and terrestrial environments.

HAZARD MAPS

For many years, operators have been creating hazard maps

which are usually contoured maps of drill hole data

indicating the presence, thickness, or location above the seam

of some hazardous rock type. For a clearer and more

objective presentation, CMRR values can be and contoured

in the same way. Figure 6 shows a CMRR map of a mine in

eastern Ohio. Most of the mine roof is a strong sandstone

(CMRR=76), but there are “shale pods” occurring in the roof

which cause bad top. The worst is a lag deposit on the margin

of a paleochannel which has caused numerous roof falls and

very uneven roof due to failure before bolting and between

bolts (CMRR=28). With practice the operator can begin to

identify a potentially weak roof, which may be prone to

different types of failure. Some roof may be unable to

tolerate any sag at all or may be susceptible to horizontal

stress. By correctly defining these zones and identifying their

strength, via the CMRR, secondary support can be applied in

an efficient way.

In a study at a mine in the Illinois basin, the typical roof

consists of 1.0 m of shale (Unit Rating = 35) overlain by

0.4m of limestone (Unit Rating = 100). The CMRR of this

sequence = 55. The roof fall rate = 0.27 roof falls/hectare of

supported roof (Fig. 7). When the limestone thins to less than

0.3 m the CMRR drops to 44, and the roof fall rate increases

to 1.53 roof falls/hectare (Fig. 8). The mine’s bolting plan
317

Fig. 7. Roof geology and its effect on roof falls in an Illinois coal

mine 

(1 acre = 0.405 hectare).

Limestone < 1 ft. Thick

Roof Falls

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Limestone Thickness

Fig. 8. Map of an Illinois mine showing limestone thickness and roof falls

(1 acre = 0.405 hectare and 1 metre = 3.28 ft).

PRSUP 

L N C

14.5 S W

b b

b e

= • •

• •

called for bolt anchorage at least 0.6 m above the limestone

with a minimum 1.8 m long bolt when the limestone thinned

to less than 0.3 m. Even with this plan the roof fall rate in the

thin limestone was much higher than in the normal thicker

limestone. One explanation is found in the presence of clay

veins in the roof. When a large clay vein severed the

limestone beam, limestone less than 0.3 m provided little

residual support and failed more frequently.

EXTENDED CUTS:

Extended cuts (cuts greater than 6 m in length) are

commonly used with remote control continuous miners.

Extended cuts can greatly increase productivity, but they

have been associated with a number of fatal roof fall

accidents. When extended cuts are attempted in weak roof,

the roof may collapse before it can be bolted, causing

hazardous conditions. To help predict where conditions may

not be suitable for extended cuts, data on the CMRR and

extended cut experience were collected at 36 mines in 7

states (Mark, 1999). It was found that when the CMRR was

greater than 55, deep cuts were routine

in nearly every case. When the CMRR

was less than 37, extended cuts were

almost never taken. Between 38 and 55,

extended cuts were feasible sometimes

but not others. The data also show that

extended cuts are less likely to be stable

if either the entry span or the depth of

cover increases (Fig. 9a and 9b).

ROOF BOLT SELECTION:

In an empirical study of 37 coal

mines conducted throughout the U.S., a

number of geotechnical parameters

were documented in an effort to explain

most of the moderate to high roof fall rate cases are

concentrated below CMRR#50. All cases of CMRR#30

show moderate to high roof fall rates, and high roof fall rates

are rare for cases of CMRR$60. Intersection spans were also

measured and figure 11 shows the relationship between

intersection span and CMRR for high, moderate, and low

roof fall rates. For intersections with CMRR$50 and sized

according to the equation line there were no cases of high

roof fall rates. This line can be used to indicate whether

smaller spans might be helpful in relieving the incidence of

roof falls.

Roof bolting is mandatory for all coal mine excavations

in the U.S. In the area of roof bolt selection, the CMRR is

also useful. While no single bolt parameter showed a strong

relationship with roof fall rate, a combination of variables

expressed as an index of support density and capacity did.

The following index variable was developed to represent 5

different bolt

parameters.

where:

Lb=Length of the bolt (m)

Nb=Number of bolts per row

C=Capacity (kN)

Sb=Spacing between rows of bolts (m)

We=Entry width (m)

Equations were also developed from the field data which

show the relationship between CMRR and bolt density and

capacity as represented by PRSUP.

The suggested value of PRSUP is determined as:

PRSUP=15.5-0.23 CMRR (low cover)

PRSUP=17.8-0.23 CMRR (high and moderate cover)
318 MINING INDUSTRY ANNUAL REVIEW

Fig. 9a. Relationship between CMRR and entry width for

extended cur roof data.

Fig. 9b. Relationship between CMRR and depth of cover for

extended cut roof data.

CMRR VS Intersection Span

40

50

60

70

80

0 20 40 60 80 100

CMRR

Intersection Span, ft

Low Fall Rate

Moderate Fall Rate

High Fall Rate

Span =27 + 0.66*CMRR

Span =35 + 0.66*CMRR

Fig. 11. Relationship between CMRR, intersection span,

and roof fall rate (1 metre = 3.28 ft).

Fig. 10. Relationship between the CMRR and roof fall rate

(1 metre = 3.28 ft).

that they are based on. Bolt length selection also incorporates

the CMRR (Mark, 2000)

LONGWALL PILLAR DESIGN:

The design of chain pillars for longwall mining has come

a long way since the 1970's. Tailgate failures, once common,

are now rare. Most of the gateroads in the U.S. are now

designed using the Analysis of Longwall Pillar Stability

(ALPS) procedure. As with other ground control issues, it is

apparent that the strength or weakness of the roof rock will

contribute to the stability of the opening. Today the roof rock

strength as measured by the CMRR is fully integrated into

the ALPS procedure to arrive at a safety factor which varies

with the strength of the roof rock. Data was collected from

44 longwall mines producing 62 individual gateroad case

histories for evaluation. The success or failure of each case

was determined and discriminate analysis was used to

determine which geotechnical variables were significant

predictors of success or failure of the tailgate. Two variables,

the ALPS safety factor and the CMRR were the most

significant predictors. Figure 13 shows the relationship

between CMRR and the ALPS safety factor. The success or

failure of each tailgate case history was successfully

predicted in 82% of the cases. That is, successful cases are

well separated from failures based on the following

regression equation:

ALPS SFR = 1.76 - 0.014 CMRR

This equation can be used in design. Suggested guidelines

for three different roof conditions are summarized in Table

1 (Mark, 1994).

TUNNEL DESIGN:

Wherever geologic description of the roof is important a

CMRR can be calculated to lend clarity and better indicate
JOURNAL OF MINES, METALS & FUELS, 2001 319

Deeper Cover

25.00

35.00

45.00

55.00

65.00

75.00

85.00

2.00 4.00 6.00 8.00 10.00 12.00 14.00

PRSUP

CMRR

Design

Equation

No Roof Falls

Few Roof Falls

Many Roof Falls

Shallow Cover

25.00

35.00

45.00

55.00

65.00

75.00

85.00

2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

PRSUP

CMRR

Design 

Equation

No Roof Falls

Few Roof Falls

Many Roof Falls

Figure 12a. Design equations for selecting bolt pattern and capacity. 

The field data used in the derivation of the formulas are shown along

with the original ”discriminate equations” (dotted line). Deeper cover

(depth>120m).

90

80

70

60

50

40

20

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Lower bound of borderline region

ALPS STABILITY FACTOR

COAL MINE ROOF RATING

Discriminant equation

Design equation

Satisfactory cases

KEY

Unsatisfactory cases

Fig. 13. Relationship between the CMRR and ALPS stability factor

in U.S. tailgates.

Fig. 14. Output of the CMRR visual basic program

(1 metre = 3.28 ft and 1 inch = 2.54 cm).

approximately 3.2 km away (Rusnak, 1998). The life of the

tunnel was expected to be 15 years. CMRR values calculated

from boreholes indicated that the 0.7 m of rock above the

seam was weak (CMRR 38-41), but this unit was overlain by

6 m of massive sandy shale (CMRR=75). The decision was

made to take down this weak rock and bolt the roof in the

massive shale. The operator considered this analysis of roof

stability with the CMRR to have been successful and

enhanced the value of their exploration core data.

OTHER APPLICATIONS:

Some other areas where the CMRR has been applied to

ground control problems include:

1. Federal regulators (MSHA) have used the CMRR to

describe the excessive strength of the immediate

sandstone which contributed to a fatality in a “first fall”

pillaring accident.

2. The CMRR has been used to describe the rock mass

characteristics of roof for input in numerical models

(Karabin, 1994).

3. The CMRR has been used to identify roof which may be

at risk in the design of yielding pillars (DeMarco, 1994).

4. The CMRR has been used in the evaluation of massive

pillar collapses (Chase, 1994).

(CMRR  35)(CMRR  55)(CMRR  75)

Suggested ALPS SF 1.3 1.0 0.7

Entry width, m 4.3 5.8 6.2

PRSUP 12 9 6
320 MINING INDUSTRY ANNUAL REVIEW

multiple seam mine design (Luo et al., 1997).

7. The CMRR has been used in hazard analysis and mapping

(Wuest et al., 1996).

8. The CMRR has been used in tailgate support selection

(Harwood et al., 1996).

9. The CMRR has been used in feasibility studies

(Beerkircher, 1994).

CMRR computer code

The CMRR has been written to a computer code for easy

application. The Visual Basic 6.0 code can be used with

Windows 95, 98, and NT operating systems. It contains

separate data screens for use with traditional underground

exposure and also for data input from core. A complete help

file also explains the CMRR philosophy and procedure as

well as the individual data inputs. The output can be in

ASCII text format for input to standard spreadsheet packages

or graphically as a lithologic roof log with Unit Ratings and

the CMRR attached. Figure 14 shows the graphical output.

Summary

The inherent strength of the coal measure rock mass is

critical to the stability of mined openings. But unlike

construction materials, natural rock has a continuous

variability broken by structural and depositional defects. As

a result, the quantitative assessment of the strength of the

rock mass in the roof of coal mine openings has been

difficult and descriptive at best. The CMRR has been

developed to use simple field tests and observation to

quantify the defects in the rock mass and translate them into

an engineering value for mine opening design and support

selection.

The CMRR has been used extensively to compare the

strength of rock masses located in different regions and coal

basins. This is necessary in property assessment, hazard

mapping, and a number of economic studies including

mining height and reject. By understanding even the general

breakdown of CMRR values into strength, (CMRR = 0-45

weak, 45-65 moderate, >65 strong) the operator can apply a

number of other analyses including support requirements,

mine layout, and opening design. 

Developed from observation of roof falls underground,

the CMRR can now be calculated by using point load testing

on core. This method provides an inexpensive alternative to

extensive load frame testing. The CMRR will soon be

available as an executable Visual Basic program and will be

easily integrated into any standard drill exploration program.

References

1. Beerkircher, M.D.. Monterey Coal Company’s

Longwall Project. Paper in the Proceeding of the IL

United States. Paper in the Proceeding of the 13th

International Conference on Ground Control in

Mining, August 2-4, 1994, pp. 69-80.

3. Colwell, M., Frith, R. and Mark, C. Calibration of the

Analysis of Longwall Pillar Stability (ALPS) for

Australian Conditions. Paper in the Proceedings of

the 18th International Conference on Ground Control

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

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Determine the Uniaxial Compressive Strength of

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