ABSTRACT

The Pittsburgh Research Laboratory of the National Institute 

for Occupational Safety and Health (NIOSH) endeavors to provide 

national and world leadership in the prevention of work-related 

illness, injury, and death. This is accomplished by using a 

scientific approach to gather information, assemble and create 

knowledge, then translate the knowledge and results into products 

and services. These products and services are then delivered to all 

personnel who can effect prevention. Obviously, the development 

and use of instruments to monitor mine roof conditions provides an 

opportunity to execute that mission.

In the late 1990’s, based on observations at a number of

underground limestone mines, only a few used roof monitors. 

Subsequently, NIOSH introduced the Roof Monitoring Safety

System (RMSS) as an aid to roof inspection beyond traditional 

visual and sounding techniques. Initial expectations were that by 

gaining roof movement information, ultimately roof sag rates could 

provide indications of eventual failure.

During this investigation, RMSSs were installed in 13

underground limestone mines in six states and this paper analyzes 

the data obtained at three of these mines. This paper also provides 

an overview of the system, data, analysis, and applications relevant 

to the RMSS.

INTRODUCTION

Control of roof and rib rock under ideal conditions, where 

competent rock exists free of high stresses, can be accomplished by 

following some straightforward procedures for sizing room and 

pillars (1). The integrity of a mine structure is greatly affected by 

the natural weaknesses or discontinuities that disrupt the continuity 

of the roof and rib. Geologic discontinuities can originate while the 

material is being deposited by sedimentary or intrusive processes, 

or later when it is being subjected to tectonic forces (2).

Where natural weaknesses and discontinuities exist, roof

monitors can provide additional information related to roof rock 

stability. Further, by obtaining measurements of rock movement 

and rates over time, the operator has information on the roof

stability of the opening. This information can be used in the event 

of a potential problem to add support to the mine roof or remove 

equipment. 

For example, at a mine in West Virginia the Miner’s Helper1, 

another type of monitor, was used to determine roof stability along 

a main haulage road. At one station a significant increase in the 

rate of roof movement was measured, at another only some

movement occurred, and at a third no movement was detected. 

This was very helpful information, as the extent of the unstable 

area was identified. Upon further investigation, it was found that a 

significant separation had developed at the 5-ft (1.6-m) bedding 

plane and the area was determined unsafe for use. In the meantime 

the haul road was re-routed around this area (3).

Despite the value demonstrated by the above example, a

relatively low number of mines use roof monitors. The sound of 

breaking or cracking rocks and visual inspection are often relied 

upon to indicate unstable ground conditions. In some cases, roof 

bolt holes can be examined for gaps or cracks with a scratch tool. 

If separations are detected a monitor can be installed to determine if 

the separation may expand. An additional safety component of the 

RMSS is that the readings from this instrument can be taken at 

ground level or further away from an unstable area by adding wire 

cable. In December of 1997, NIOSH introduced the RMSS at an 

Underground Stone Safety Seminar at Evansville, Indiana, USA. 

After the seminar, nine mine operators indicated an interest in using 

the RMSS. Installation of the prototype monitor began in 1998. 

During the next three years, approximately 60 monitors were 

installed at 13 mines in six states (Table 1).

Table 1. Roof Monitoring Safety System Installations

1998-2000.

State Number of Mines Number of Monitors

Illinois 1 4

Indiana 2 4

Iowa 1 2

Kentucky 5 15

Pennsylvania 3 35

Tennessee 1 *

*Mine operator built and installed own monitors based on 

RMSS design.

 

1Mention of any company name or product does not constitute endorsement 

by the National Institute for Occupational Safety and Health.

CONSIDERATIONS FOR USING ROOF MONITORS IN UNDERGROUND LIMESTONE 

MINES IN THE USA

L. J. Prosser, Research Geologist

Thomas E. Marshall, Engineering Technician

Stephen C. Tadolini, Supervisory Mining Engineer

Anthony T. Iannacchione, PhD., Civil Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Cecil Banta, Vice President Operations

Liter’s, Inc.

Louisville, Kentucky USA
The basic premise for the RMSS was to provide a simple design 

combined with easy installation and use. Figure 1 shows the 

original design used during initial field trials. The open structure of 

the monitor created a problem with moisture and water. 

Installation also proved to be somewhat cumbersome. A revised 

enclosed design corrected both shortcomings and is shown in 

figure 2. These modifications provided simple construction and 

easy installation; two key criteria for use in the underground mine 

environment.

BACKGROUND

Where and why are the essential questions when contemplating 

monitor usage. Obviously, in order to obtain data on rates, the 

RMSS or any roof monitor must be installed in strata that move or 

have some degree of instability. Monitor locations for some of the 

13 mines in the study resulted from suspicion of unstable

conditions. At other mines, RMSS usage reflected an interest in 

developing a ground control plan, collecting roof movement

information for long term planning, and general safety and

informational purposes. 

In a massive roof beam devoid of discontinuities, no movement 

would be expected (figure 3). If a discontinuity such as a wedge 

exists, and a monitor is not installed within the rock affected by it, 

the monitor will not show movement prior to failure (figure 4). 

Conditions most conducive to the use of monitors for collecting 

roof movement data are in layered strata of roof rock as shown in 

figure 5. Installation of the RMSS in the middle of an intersection 

under these conditions presents the best opportunity to capture roof 

movement data. The middle of the intersection is commonly 

regarded as the area most prone to roof sag.

Figure 1. Initial design of RMSS and volt meter.

Figure 2. Revised design of RMSS.

Figure 3. Monitor installation in massive roof rock.

Figure 4. Monitor installation in wedge failure vicinity.

Figure 5. Layered strata.
The RMSS requires a 2-in (5-cm) diameter hole extending 

typically about 13 ft (3.9 m) into the roof. The RMSS anchoring 

system, however, can be adjusted to accommodate any depth hole. 

In most installations, two 6 ft (1.8 m) lengths of setting rods are 

attached to the approximate 1 ft (0.3 m) RMSS. The anchorage 

material is strips of metal banding sized to firmly secure the top 

setting rod and the RMSS itself at the bottom of the hole. Roof 

movement between the anchorage points at the top and bottom of 

the hole is detected through a potentiometer attached to a rack and 

spur gear connected to the setting rods. Movement, measured in 

ohms, is converted to inches as determined by prior calibration. A 

conductive cable is extended from the monitor across the mine roof 

and down the rib to the readout device at eye level. If the 

installation site is located at an active face, the RMSS can be 

modified by attaching jack and plug combination to the wire. The 

wire can then be unplugged during blasting and easily reconnected 

afterwards. Readings are taken with an ohm meter (4).

If dangerous roof beam deflection values are known, they can 

be used as a clear indicator of roof instability. Therefore, observing 

and monitoring sag is an important ground control measurement 

tool. Research has indicated that the rate of roof movement may 

often be more meaningful than the specific magnitude of movement 

(5). Vertical stresses within limestone mines are roughly

equivalent to the weight of the overburden (1.0 psi/ft of depth). 

Local conditions attributed to discontinuities or faults and in

mountainous areas may somewhat affect the vertical stress but in 

general the above approximation has been found to hold true. The 

horizontal stress component can vary from the Poisson’s ratio 

component of the vertical stress to many times this value,

depending on the geologic factors and the topography of the region 

(6).

RMSS INSTALLATION PROCESS AND INSTRUCTIONS

Typical mine site installations of RMSSs by NIOSH personnel 

were conducted in the following manner (figure 6):

ß Mine operator selects sites for installation.

ß Installation holes are drilled in roof to appropriate depth.

ß Bucket\Basket truck with an operator is provided to obtain 

access to the roof.

ß RMSS is inserted in hole and cable is attached and extended 

along roof and down the rib.

Once the installation hole is drilled, the RMSS can be in place 

and operating in less than 20 minutes. A book for logging the roof 

movement or stability is provided to the operator. The book

contains information on how the RMSS works, a schematic of the 

components, an RMSS parts list, a data collection (ohms readings) 

sheet, and a table converting ohms to inches of movement. Also 

provided is the Roof Monitor (RMSS) General “Rules of Thumb” 

as shown in figure 7.

INSTALLATION OUTCOMES

As reported at the 19th International Conference on Ground 

Control in Mining (7), related below is a communication about a 

roof fall that occurred and was recorded by an installed RMSS.

“The mine foreman called today at about 9:00 a.m. and said 

that the monitor (RMSS) at 39-A was showing movement that 

amounted to 0.255 inch (0.648 cm) over two days. The roof 

where the monitor was located did not look bad to the eye 

prior to the roof fall according to the mine foreman. He said 

he would danger off the area and work at some other faces 

since he could hear the roof working now and the monitor 

(RMSS) was showing movement. At 12:40 p.m. the area 

caved in taking the monitor (RMSS) with it.” (Note: the 

associated data will be presented in the Data Analysis section).

Obviously, this was a best-case scenario in RMSS utilization, in 

that the use of the RMSS allowed an unstable area to be identified. 

At other mines, however, the results were less than ideal, both in 

terms of roof movement information and operators incorporation of 

the RMSS into a ground control plan. Detailed below is a case in 

which the concept of the RMSS was not understood or utilized as 

intended. An RMSS was installed on May 31, 2000, and the initial 

reading was 670 ohms. The next reading was taken on June 20th 

some 20 days later, and an increase to 832 ohms was recorded. 

That increase in ohms equates to nearly 0.75 in (1.9 cm) in roof 

movement. Such a reading would suggest caution and require that 

frequent readings be taken. However, the next reading was not 

Figure 6. Installation of RMSS.

Figure 7. Roof Monitor (RMSS): General “Rules of Thumb.”
taken until July 30th, 40 days since significant movement was 

recorded. The reading on July 30th was 797 ohms, or a decrease 

from the previous reading by 35 ohms. At this point, it should have 

been realized that a problem existed with the voltmeter or

instrument. From this time, the recorded readings were taken on 

average three times a month and remained at 793 ohms for about 

nine months. A possible explanation for this is that how the 

monitor worked was not fully understood. A contributing factor 

could also be that ground conditions were generally stable thus 

limiting the interest in utilizing the RMSS as a detection tool.

The outcomes from the other 11 mines could be characterized 

as somewhere in between the two cases described above. At four 

mines, interest after installation failed to materialize, management 

changes also occurred and after approximately six months these 

installations were abandoned. During this time, based on the few 

readings received, no roof movement occurred. At two other

RMSS installation sites, the underground operations were

converted to surface mines. At one of these mines, the RMSSs 

effectively showed roof movement, although only for a short 

duration because of the conversion to a surface operation. Moisture 

problems related to the original open design instrument as

previously discussed negated valid data collection at another site 

and geologic conditions of the wedge failure nature as shown in 

figure 4, existed at another site, which was also shortly abandoned. 

At two other mines, the use of the monitor or the monitor itself 

was modified for a specific application. In one case the RMSS was 

used to determine if roof rock stability was affected by mechanical 

scaling with an impact hammer. The test showed that little or no 

movement resulted from this method of scaling under the particular 

conditions at this mine (7). In the other instance of adapted RMSS 

usage, an operator needed to monitor the stability of the immediate 

roof of a haulage way in a benched area with ceiling heights of 

100 ft (33 m). As previously stated, the RMSS requires a 2-in 

(5-cm) diameter hole for insertion; however, only roof bolt holes of 

a smaller diameter were available and it was not possible to drill 

new holes at that height. Therefore, the operator attached a 2-in 

(5-cm) diameter collar at the roof line and was able to modify the 

top anchor of the setting rod to accommodate the smaller diameter 

bolt hole. The readings from these monitors were initially stable. 

However, because the monitors used at this mine were of the 

original open design, moisture became a problem. 

Another mine provides insights relative to the usefulness of 

monitors without actually detecting roof movement. In this case no 

roof movement provided useful information to the operator. 

Installations at a mine operated by Liter’s Inc., Louisville, KY, 

showed a long-term application of monitoring despite apparent 

stable roof conditions. In 1998, four RMSSs were installed at the 

Rock Springs Mine located near Louisville, the highest populated 

and largest city in the state of Kentucky. This mine began 

underground operations about a decade before this time. In 2000, 

the company began planning for expansion that would necessitate 

tunneling underneath a major interstate highway system. By that 

time, housing in the area had also expanded and encroached upon 

the mining operation. During the permitting process a major

concern of all parties was blasting, with citizens primarily

concerned about potential structural damage to their homes.

One of the numerous items presented at the public hearings and 

in the permit documentation was the roof movement readings from 

the RMSS installations at the Rock Springs Mine. In this case, 

because the readings showed no movement, the operator was able 

to show a history of roof stability. As shown in figures, 8 and 9, 

the readings were recorded monthly by the mine worker and 

communicated to NIOSH and graphed on approximately a

quarterly basis.

Another element of this situation worth noting is that during 

this time three different Mine Superintendents were responsible for 

recording and transmitting the monitor data. In addition, the mine 

expansion permit also required monitoring the two, 395-ft (132-m) 

tunnels that opened the reserves on the other side of the highway 

(figure 10). Installation of that system was completed in March 

2003. Each tunnel has three RMSSs deployed that are tied to a data 

recording system with the capability to provide hourly readings of 

the roof conditions in the tunnels (figure 11). 

Figure 8. RMSS readings sheet from Rock Springs Mine.

Figure 9. RMSS movement at Rock Springs Mine.

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0 100 200 300 400 500 600 700 800 900

Days

Movement (in)

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0.21

0.25

Movement (cm)
DATA ANALYSIS

Of the 13 mines in the study, three mines provided sufficient 

data for assessment. The first, the Rock Springs Mine, used the 

RMSS readings to show that a pattern and history of stable 

conditions existed underground over a prolonged period of time. 

The other two appeared to exhibit elevated levels of horizontal 

stresses that caused unstable ground conditions.

Another Liter’s, Inc. mine, Big Bend, Meade County, KY, also 

utilized RMSSs. This mine began development in August of 1999 

and unstable roof and ground conditions were in evidence by 

January of 2000. Aware of the monitors used at the Rock Springs 

Mine, Big Bend’s mine personnel requested assistance and RMSS 

installations. Two RMSSs were installed and both of these 

monitors, shown as RMSS #1 and RMSS #2 in figure 12, were 

located in the high roof area, the original roof line. 

Some early movement of these monitors most likely resulted 

from blasting or face advancement because of the proximity of the 

instruments to the working faces (8). Another RMSS (RMSS #3) 

was installed in August as the mine advanced and showed

significant movement (figure 13). That area was bermed off in 

September, due to a roof fall located in the proximity of this RMSS 

#3. Also during this time, monitor readings were taken on a daily 

basis as would be expected under the circumstances. 

Conditions at Big Bend continued to deteriorate and attempts to 

mine at other roof horizons failed to substantially improve the 

situation. On November 3, 2000, because of safety considerations, 

Liter’s decided to surface mine the reserve and abandoned

underground mining operations. Figures 14 and 15 illustrate the 

now exposed roof of the former underground mine after blasting. 

Figures 16 and 17 provide a close-up view of what had been the 

mine roof rock. Underground observations relating to the geology 

combined with the roof movement measured with the RMSSs were 

apparent once these exposures were reviewed.

Figure 10. Map showing existing mine workings with 

projections.

Figure 11. Underground tunnel in Rock Springs Mine.

Figure 14. Big Bend Mine portal.

Figure 13. Roof movement at RMSS #3.

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Days

Movement (in)

-0.13

0.00

0.13

0.25

0.38

0.51

0.64

Movemnt (cm)

Figure 12. Mine development and RMSS locations.
At Big Bend, the RMSSs proved to be a useful tool in

measuring roof movement, as related in the following

correspondence received from the Mine Manager at Big Bend on 

March 23, 2001.

“I just wanted to thank you and NIOSH for your help with 

the problems we had in our mine. The roof monitors you 

installed were a big help in keeping our crew safe. The 

information you gave us on horizontal stress and other 

geological situations helped us to determine that it was 

unsafe to continue underground mining. We have closed 

the underground and have started surface mining. I 

sincerely appreciate your help.”

As discussed earlier, the best-case scenario for RMSS use as a 

safety tool occurred when the RMSS allowed an unstable area of 

mine roof to be identified. Additional information on roof

movement measurements were also obtained at this same mine. 

Because of unstable ground conditions, attributed to high levels of 

horizontal stress, and the successful use of the RMSS this mine was 

extremely interested in utilizing more RMSSs. During the course 

of studies at this mine, more than 30 instruments were installed 

with the first RMSS installed in 1998. 

 

RMSS utilization at this mine is reflected in roof fall data from 

MSHA during the 1998-2000 period. During that time, the 13 

mines that installed RMSSs showed a total of 16 reports classified 

as “fall of roof or back” (9). The mine that installed more than 30 

RMSSs accounted for eight of these reported ground falls. 

Conversely, the other mines in the study, that were characterized 

with interest that failed to materialize after installations, were listed 

as having only two of the falls of roof or back during this time. 

The one mine described as “not fully understanding or utilizing the 

RMSS concept” showed no falls of roof or back in this data system. 

Therefore, it appears the RMSS was used more in mines with less 

stable roof conditions.

Another aspect of beneficial deployment of monitors is that 

there is strength in numbers. The higher the number of monitors 

installed, the greater the coverage of roof area with potential to 

provide information on instabilities. As in the case with the Rock 

Springs Mine, this operation also established responsibility and a 

process for reading the monitors.

ROOF MOVEMENT

A breakdown of the 30 RMSS installations, as related to roof 

movement, is as follows: 3 significant, 2 moderate, and 25 low or 

no movement. Significant movements are characterized as reaching 

failure, moderate, as continuing to move or progress, and low, as no 

or some movement but subsequently stabilizing. Figure 18 shows 

an area of the mine where all three conditions occurred. 

The following discussion details the history of two RMSSs that 

showed significant movement to illustrate the complexity of

defining critical rates of movement. One of the RMSS locations, 

that had roof failure, showed significant movement during a short 

period of time. RMSS #19 was installed on October 22, 1999, and 

the initial reading was 2,143 ohms. During the next 50 days, eight 

Figure 15. Exposed roof rock at Big Bend Mine.

Figure 16. Big Bend roof rock.

Figure 17. Close-up of roof rock.

Figure 18. Mine map with RMSS locations.
readings were recorded and movement occurred, as indicated by 

the increase in ohms to 2,156 or approximately 0.06 in (0.15 cm). 

In the next five days, readings were taken daily with movement 

progressing rapidly to 2,225 ohms, an increase equivalent to nearly 

0.3 in (0.79 cm). Prior to failure in this area, a smaller roof fall 

occurred that disabled the monitor. Because of the deteriorating 

conditions no repair was attempted. Figure 19 shows the rapid 

progression of movement that preceded the eventual failure. 

RMSS #21 was installed on November 11, 1999, the initial 

reading was 2,224 ohms. In contrast to RMSS #19, this monitor 

increased to 2,236 ohms or approximately 0.06 in (0.15 cm) in 235 

days; during this time 31 readings were taken. In the next 45 days 

(seven readings) another 0.06 in (0.15 cm) of movement occurred. 

RMSS #21 reached 0.3 in (0.79 cm) of movement, the point at 

which RMSS #19 was approaching failure, in mid-September of 

2000, a period of 340 days. During the next five months another 

1.0 in (2.5 cm) of movement was recorded. Interestingly, during 

the next six months the rate of movement subsided, as movement 

for that time period was 0.18 in (0.95 cm). Failure finally occurred 

after a total of nearly 2 in (5.0 cm) of movement (figure 20).

The difference in failure rates and amounts of movement 

between these two RMSS monitoring sites apparently relates to the 

existing conditions relative to the locations. At RMSS #21, a roof 

fall had occurred approximately one intersection (40 ft -13.3 m) 

from this installation and the previous history of roof fall patterns 

indicated the direction of continuation would approach that of

RMSS #21. The earlier fall apparently provided some relief of 

stresses, resulting in a more gradual progression to failure than 

observed at RMSS #19. At that location at the time of installation 

relatively stable conditions existed, unlike the area that affected 

RMSS #21.

At RMSS #27, another unique pattern of roof movement can be 

observed. At this site, failure has not occurred despite more than 

2.25 in (5.68 cm) of roof movement. Two periods of rapid 

progression of movement have occurred followed by periods of 

gradual but increasing movement (figure 21). The variety of rates 

and amount of movement before failure illustrate the complexity of 

determining when a roof fall will occur even when using a monitor. 

However, the rates of movement provide the operator with useful 

information to address the change in ground conditions. The rates 

coupled with past mining experiences and knowledge can be used 

in decision-making regarding changes in roof support, direction of 

mining, or berming-off and unsafe area. The RMSS, or any 

instrumentation, is best utilized in conjunction with additional

ground control information. 

SUMMARY

Unless monitor readings become a dedicated or obligated

responsibility or ground conditions become problematic the interest 

in monitoring is likely to wane over time. In two instances the 

RMSS may have served to assist in the prevention of injury or 

death to mine workers in the underground limestone industry. Roof 

monitors such as the RMSS can provide valuable information on 

roof conditions if installed, read, and evaluated with due diligence. 

As with any tool, proper use and understanding are essential to 

receive the maximum benefit of that implementation. Those mines 

and operators who recognize that simple yet imperative fact gain 

the greatest benefit from RMSS utilization.

REFERENCES

1. Obert, L. and Duvall, W.I. Rock Mechanics and Design of 

Structures in Rock. John Wiley and Sons, Inc., New York, NY, 

1967, pp. 63-68.

2. Mark, C. and Iannacchione, A.T. Best Practices to Mitigate 

Injuries and Fatalities from Rock Falls. Proceedings, ThirtyFirst Annual Institute On Mining Health, Safety and Research, 

2000, pp. 115-130.

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1.50

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1.27

2.54

3.81

5.08

6.35

Movement (cm)

Figure 20. Roof movement at RMSS # 21.

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1.50

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2.50

0 100 200 300 400 500 600 700 800 900 1000 1100

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0

1.27

2.54

3.81

5.08

6.35

Movement (cm)

Figure 21. Roof movement at RMSS #27.

Figure 19. Roof movement RMSS #19.

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0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 10 20 30 40 50 60

Days

Movement (in)

0.00

0.13

0.25

0.38

0.51

0.64

0.76

0.89

1.02

Movement (cm)
3. Shaffer, D. and Petersen, G. Practical Rock Mechanics for 

Practical Miners. Aggregates Manager, January 2000, Vol. 4, 

No. 10, pp. 26-31.

4. Iannacchione, A.T., Prosser, L.J., Marshall, T.E., Compton,

C.S., Dolinar, D.R., Oyler, D.C., and Pappas, D.M. An

Instruction Booklet for the Roof Monitoring Safety System 

(RMSS). National Institute for Occupational Health and Safety 

Internal Publication, Dec. 10,1997, 21 pp.

5. Ivor Hawkes Associates. Measurement of Mine Roof

Movement. Final Report prepared for United States

Department of the Interior, Bureau of Mines, Contract Number 

HO366033, Development of Multiposition Roof Sag Indicator, 

July 1978, 135 pp.

6. Marshall, T.E., Prosser, L.J., Iannacchione, A.T., and Dunn, M. 

Roof Monitoring in Limestone Mines-Experience with the 

Roof Monitoring Safety System (RMSS). Proceedings, 19th 

International Conference on Ground Control in Mining, Aug. 

2000, pp. 185-191. 

7. Iannacchione, A.T., Mucho, T.P., and Prosser, L.J. Controlling 

Roof and Ribs in Underground Mines. Aggregates Manager, 

August/September 1996, Vol. 1, No. 5, pp. 33-37.

8. Mucho, T.M. and Dolinar, D.R. Report of Investigation of 

Ground Control Issues at Liter’s Big Bend Quarry, National 

Institute for Occupational Health and Safety, Personal

Communication, October 6, 2000.

9. U.S. Department of Labor, Mine Safety and Health

Administration, www.msha.gov, data Retrieval System, 

As developed by PIER.