Roof Monitoring in Limestone-Experience with the

Roof Monitoring Safety System (RMSS)

Thomas E. Marshall, Engineering Technician

L. J. Prosser, Research Physical Scientist

Anthony T. Iannacchione, PhD, Civil Engineer

National Institute for Occupational Safety and Health

Pittsburgh, PA, USA

Michael Dunn, General Superintendent

Conco Western Stone Company

North Aurora, IL, USA

ABSTRACT

During the past few years, the Pittsburgh Research

Laboratory of the National Institute for Occupational Safety

and Health (NIOSH) examined and characterized conditions at

a majority of the underground stone mines in the United States.

Observations at these mines revealed a limited degree of roof

monitoring beyond visual inspection and sounding. When

monitors are used, they typically require the miner to measure

movement at the roof. If conditions are unstable, the miner

may be in a hazardous situation while recording data. Based

on this scenario, researchers surmised that a simple,

inexpensive monitor with the capability of recording data at a

distance from the mine roof would be a safer way to gain the

information. Additionally, more widespread use of monitors

could potentially lead to better understanding of roof

movement in general.

A monitor to meet this need was designed, tested, and

subsequently improved as experience was gained in its’ use at

a number of underground stone mines. The Roof Monitoring

Safety System (RMSS) can provide an initial indication of

movement in roof beams. By understanding and measuring

roof movement in underground mines, the potential for injuries

and fatalities to mine workers from falls of ground can be

reduced. Also, officials at a mine with a history of data are

better prepared to make a decision on remedial actions in the

event of ground falls. This paper will outline the evolution of

the RMSS and how it can be used in a comprehensive proactive ground control safety program. Also included is a case

history describing how the RMSS was used in an evaluation of

the effectiveness of a mechanical impact scaling machine at an

operating limestone mine. 

INTRODUCTION

Falls of roof and rib rock are responsible for a high

number of mining injuries and fatalities. Visual inspections

and the sound of breaking or cracking rock are typically the

most relied upon indicators of unstable roof conditions. In

some mines, roof bolt holes are checked for separations or gaps

with a scratch tool providing further information about the

stability of the roof. Using monitors such as the Roof

Monitoring Safety System (RMSS) can confirm roof

movement that was surmised only by eye or ear, or potentially

detect movement that was missed by these means. An added

safety feature of the RMSS is the capability of locating the

measurement station at a distance from where the monitor is

placed in the roof.

It must also be noted that stone, as well as other industrial

minerals, are the basic raw materials used in construction.

Continuing economic growth and highway building in

particular have resulted in record level demand for stone.

Constraints to surface mining and the availability of resources

are increasing the development of new underground stone

mines. These new mines, in many cases will be employing

new and inexperienced miners. The knowledge and ability of

experienced miners that rely on sight and sound for detection

of unstable ground conditions may not exist. 

MONITOR USAGE

Control of roof and rib rock under ideal conditions where

competent rock exists free of high stresses, can be

accomplished by following some straight forward procedures

for sizing rooms and pillars (1). However, in most mining

operations and as is the case in the underground stone mining

sector, ideal conditions are seldom present (2). Generally,

underground mines use "hands-on" information to provide a

daily assessment of the general picture related to roof

conditions. However, this type of knowledge can be lost

through changes in employment or other circumstances.

Many mines supplement visual inspections and knowledge

gained through hands-on experience with various types of

observational and monitoring techniques. Based on

observations and discussions with personnel at 48 mines, 49

observational and monitoring techniques beyond basic visual

inspections were used at 26 different mines (table 1) (3). This

suggests that additional information is useful in maintaining

a ground control program.
Figure 1. Photograph of the commercially available Miners

Helper, the Guardian Angel, and the RMT’s Dual Height

Telltale roof mechanical extensometer monitoring devices.

Table 1. Number of mines using observational and

monitoring techniques.

Observation or monitor type Number of

mines

Sounding ............................ 9

Observation hole . . . . . . . . . . . . . . . . . . . . . . 11

Wedge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Borescope . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Scratch tool . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Extensometer . . . . . . . . . . . . . . . . . . . . . . . . . 2

Miner's Helper . . . . . . . . . . . . . . . . . . . . . . . . 7

Guardian Angel . . . . . . . . . . . . . . . . . . . . . . . 4

Telltale .............................. 1

To date, observational techniques are used more than

monitoring techniques. The data shows that in 14 instances

mines used or have available commercial mechanical monitors

such as the Miner's Helper, Guardian Angel or Telltale (figure

1).

The scratch tool and wedge are all used as a means of

detecting rock movement. The wedge and scratch tool are

simple devices that typically can be fabricated from materials

used at a mine site. Wedges or spikes are driven into the roof

and painted or marked so as if they move it can be noticed. A

scratch tool is inserted into a roof bolt or observation hole.

The end of the scratch tool is bent so that as it is pushed in and

out of the hole a crack or separation can, in a sense, be felt.

The commercially available mechanical monitors are more

sophisticated. The Miner's Helper or Guardian Angel are

similar in that if roof movement occurs a reflecting flag drops

from the roof line. The Telltale is used extensively in coal

mines in the United Kingdom. The Telltale is a rigid bar

anchored in the roof and slightly protruding from the borehole

at the roof line. The protruding end is covered with different

colored bands of reflective tape each indicating more

pronounced movement. 

Recently, at a mine in West Virginia the Miner's Helper

was used to monitor stability in a main haulage road. At one

station a significant increase in the rate of roof movement was

measured, at another some movement occurred and at third no

movement was detected. This was very helpful information, as

the extent of the bad area was identified. Upon further

investigation, it was found that a significant separation had

developed at the 5 ft (1.52 m) bedding plane and it was in an

intersection aligned with the prominent jointing, meeting all

the criteria for another roof fall. It was determined the area

was unsafe to use and, if it did not fall on its own in a

reasonable amount of time, it would be shot down. In the

meantime, the haul road was re-routed around this area (4).

Despite this one example, a relatively low number of

mines use monitors. A comprehensive ground control plan

not only includes the basic observational, visual and hands-on

components, but also uses supplemental observational and

monitoring techniques, and regularly reads, analyzes and

displays information gained from these efforts. When this

type of information is logged or mapped it provides a

documented history of ground conditions. This information

can be analyzed and prepared by either consulting firms or

with in-house expertise. The availability of this information

at the time of a major ground fall or when unstable geologic

conditions are encountered is extremely useful in deciding a

course of action or alteration of the mining plan. Mines that

follow these practices and that promote open communication

and participation from everyone at the site are the mines with

the most pro-active approaches towards ground control safety

(3).

DEVELOPMENT OF THE RMSS

The RMSS was introduced at a National Institute for

Occupational Safety and Health (NIOSH) "Safety Seminar for

Underground Stone Mines" in Evansville, IN, USA in

December 1997. During the first half of 1998, monitors were

installed at five mines–one in Illinois, three in Kentucky and

one in Pennsylvania. From the initial introduction of the

RMSS it was anticipated that improvements and modifications

would occur, assuming that once miners and operators were

exposed to the monitor suggestions would evolve. In one

case, the RMSS was used to determine if an impact hammer

used on a mechanical scaler was causing or increasing

instability of roof rock. In another instance, an operator with

high ceilings, as a result of benching needed to monitor roof

stability along a haulage way. The RMSS requires a 2-in

(5.08-cm) diameter hole for installation, however only roof

bolt holes of a smaller diameter were available and it was

impossible to drill new holes at that height. The operator

attached a 2-in (5.08-cm) collar at the roof line and was able

to install the RMSS. These examples illustrate the potential

for even further enhancements of this tool.

Also during the first 6 months of RMSS use it became

apparent from field testing that exposure to water was a

problem. Monitors installed at "wet" mines were not

functioning. The spur gear became locked-up thus precluding

movement. As a result of this field testing the RMSS was

revamped and the inner workings were enclosed in plastic

pipe. At the same time, the anchoring method was also

improved and simplified to facilitate installation.
 

Figure 2. RMSS, cable and read-out box.

Figure 3. Overview of the Remote Monitoring

Safety System (RMSS) components.

HOW THE RMSS WORKS

(and when it did)

The new RMSS, cable and read-out box are shown in

figure 2. The read out box has a green light that stays lit until

battery replacement is needed. The red light is set to come on

to indicate movement. When the red light comes on can be

adjusted for increments of movement as low as 0.001 in

(0.003 cm). It is important to recognize that the RMSS red

light should not be considered as a primary or only means of

determining the stability of roof or rib. It is intended to serve

as a supplement to existing methods or procedures for

determining the stability of conditions.

An overview of the RMSS components is shown in figure

3. When rock begins to move, shift or separate, the setting

rod is affected. The setting rod which is anchored in the top

of the hole moves the potentiometer as the rack and spur gear

react to the movement. Movement of rock layers within the

mine roof is measured relative to a fixed-point calibration at

the monitor housing. The housing contains a potentiometer,

roller switch, plastic rack, and spur gear. Movements are

detected by the transfer of electromotive forces through the

rack of the spur gear, which is attached to a 5,000-ohm

potentiometer. Movement is precisely measured by

comparing the output of the potentiometer to a control level

calibration (set with a screw). Cable is attached to stereo

plugs and extended from the roof to ground level. If the

RMSS is located at an active face the cable can be unplugged

during blasting and reconnected afterwards and readings can

be taken. The readings are taken with a ohm meter. The

monitor requires a 2-in (5.08-cm) diameter hole extending 12

to 18 ft (3.6 to 5.4 m) into the roof.

Related below is the communication that occurred

pertaining to a ground fall at a mine with RMSS’s installed.

“The mine foreman called today at about 9:00 am and said

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

amounted to 0.255 in (0.648 cm) over two days. The Red

warning light started to flash as soon as the movement

occurred. 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 and the

monitor (RMSS) showing movement. At 12:40 pm the area

caved in taking the monitor (RMSS) with it. This monitor

(RMSS) had a data logger attached to it which was in

working order and recorded data.”
In this particular case, the RMSS performed as intended. It

should be noted that sound and sight also contributed to the

decisions made during this process. As in any comprehensive

ground control program all elements of information are used

and this time were instrumental in preventing a possible

injury. Subsequent to this event another fall occurred. This

time, however the results were the reverse. A RMSS located

near this fall area was slightly too far away and the red light

indicator did not trigger. It is important to remember that the

RMSS or any monitor will not show movement if the rock

layer it is anchored in does not move. Fortunately, the sound

of breaking rock and recent history of falls in this section of

the mine provided sufficient warning and the area was inactive

at the time of the fall. Again the key to a comprehensive

ground control program is combining all elements of

information. 

DATA ANALYSIS

To read the RMSS, a multimeter is required. Multimeters

come in a variety of price ranges depending on resolution and

accuracy and built-in functions. The most important factors in

choosing a meter for this application are the meter resolution

and accuracy. Resolution (or precision) is the ability of the

meter to resolve a change in resistance; accuracy indicates how

close the measurement is to the true resistance. Generally,

meter resolution is better than meter accuracy, but fortunately

the resolution is more important to making a measurement,

provided that the same meter (or meters) is used to make all

the measurements from a single RMSS. Several meters

between 100 and 350 dollars were purchased and, as might be

expected, the resolution and accuracy of the meters were

generally proportional to their price. The low-cost meterhad

resolutions and accuracies in the range of 2 ohms and 0.5%,

respectively; the highest priced meters sampled had

resolutions and accuracies in the range of 0.1 ohms and

0.07%, respectively. A resolution of 1 ohm represents the

 ability to resolve approximately 0.004 in (0.01 cm) of

movement.

The best resolution and accuracy are usually found in the

portion of the meter range above 1,000 ohms and below meter

midrange; this can vary, however, with meter type, and meter

specifications should be carefully read so that instruments can

be set to take advantage of the meter’s most accurate range.

The minimum meter resolution and accuracy acceptable for

use with the RMSS are 1 ohm and 0.2%, respectively. This

means that the minimum cost for an acceptable meter for

RMSS use can be expected to be about $200.

To measure and record sag, two graphs and a data sheet

must be kept. These are:

1. Data Sheet - Record of readings of the monitor, with times

and dates (figure 4).

2. Resistance Conversion Chart - Converts multimeter

resistance readings to sag (figure 5).

3. Sag Graph - Plot of sag versus time (figure 6).

Careful detailed records are essential for effective use of

the roof monitor. The following information should be

included on the Resistance Conversion Chart and on the Data

Sheet:

• Monitor identification number.

• Monitor location.

• Date and time of monitor installation.

• Model and serial number of the multimeter used to

make the measurement.

In addition all other data collected such as gaps detected with

a scratch tool and other observations about conditions should

be included with these records.

Instrument No. ____ Installation date __________ Depth of anchor ___

Mine ____________ Multimeter type __________

Location ____________________________ ________________________________________

Date Resistance Sag, inches Date Resistance Sag, inches

Figure 4. Data sheet.
Figure 6. Sag graph.

Initial Resistance Measurements

Proper resistance measurements must also be recorded on

the Resistance Conversion Chart immediately after the monitor

is installed. The following readings are required:

• Monitor resistance - before the wire(s) are connected.

• Total resistance - after the wire(s) are connected,

including wires and monitor.

• Wire resistance - computed from the difference of the

total and monitor resistances.

To begin data collection, connect multimeter to wire leads

on RMSS to read monitor resistance. Then connect wires to

the monitor and connect the multimeter to the end of the wires

to obtain the total resistance.

Initial Chart Layout

Before the Resistance Conversion Chart can be used to

convert resistance to sag, it must be scaled for each individual

monitor. The procedure is as follows:

• Write the total resistance from the first monitor reading

at the left side of the horizontal axis of the chart (at the

point marked A). For this demonstration, use 1,204

ohms.

• Write in the resistances as shown on the horizontal axis

label. Each major tick mark on the chart represents

10 ohms; each minor tick mark represents 1 ohm for a

5,000-ohm potentiometer.

If the initial total resistance is 1,204 ohms, the large divisions

on the horizontal axis will read 1,204, 1,214, 1,224, etc.

Conversion of Resistance to Sag

This process converts resistance readings to measurements

of movement or sag of roof.

Resistance Conversion Chart

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Resistance, ohms (major ticks are 10 ohm, minor ticks are 1 ohm)

Enter initial sensor + cable resistance at A. Then Scale x-axis.

Sag, inches

A__

Sensor ID#: 

Date Installed___________________________

Initial sensor resistance (ohms): ___________

Enter cable resistance & changes here:

 Date Cable R (ohms) Meter MDL & S/N

 

 

This sheet is for 500 ohm, 10 turn potentiometers. 

Calibration factor 254.648 ohms/inch

Figure 5. Resistance conversion chart.
1

Figure 8. Pick-type scaler.

Figure 7. Hammer-type scaler.

1. The multimeter is connected to the monitor wires and the

resistance is then read.

2. On the data sheet, record the time, date, multimeter model

and serial number, and the resistance reading.

3. The resistance readings are converted to a sag by finding

the resistance on the horizontal axis of the Resistance

Conversion Chart, moving up the chart perpendicular to

the horizontal axis to the diagonal line, and then moving

left to the vertical axis. The number on the vertical axis is

the sag in inches.

4. Enter the sag on the data sheet.

5. Plot the sag on the sag versus time graph.

NOTE: If the wire is shortened, added to, or replaced, the

change should be noted, a measurement of the new cable

resistance made, and the new resistance recorded on the

Resistance Conversion Chart. Wire resistance may be

determined by shorting the two cable wires together at the

monitor and reading the resistance of those two wires at the

other end of the cable. The new wire resistance is the

resistance of both wires (5).

MECHANICAL SCALING EXPERIMENT

As mentioned previously, an operator used the RMSS to

devise an experiment to determine the affect on roof

conditions when using an mechanical impact scaler. A

summary of that test is outlined below.

Mechanical scaling has dramatically improved

underground mine safety over the last two decades. Two types

of mechanical scalers include a prying pick design and a

hydraulic impact hammer. Their effectiveness is high

although some operators say they are easy to "mine" with or

may do more roof damage by creating cracks and causing falls.

To help assess any potential harm to the roof, a test was

undertaken using the NIOSH developed RMSS and a hydraulic

impact hammer. This procedure allowed the collection of realtime rock movement data during the scaling operation. The

experiment was designed to quantify rock movement induced

by a mechanical impact hammer used for scaling in an

underground mine. A RMSS was installed in a 2-in (5.08-cm)

diameter vertical drill hole 14 ft (4.2 m) deep. A newly

blasted face requires some degree of scaling to stabilized the

area before mining can continue. This can be done by hand

with a scaling bar or by mechanical means with an impact

hammer or a pick, shown in figures 7 and 8. The pick type

machine uses leverage to pry loose rock from the roof. The

chassis is rubber-tire-mounted for easy mobility. These

machines are often used as "battering rams" where loose rock

is difficult to pry free. Where the pick is applied is the only

area where rock falls, making more frequent setups than with

a hydraulic hammer whose energy usually causes loose rocks

to fall in and around the vicinity of the scaling.

The scaling hydraulic hammer is identical to those used to

break boulders in quarries or in crusher feeders, only a smaller

scale, usually. These hammers can be mounted on track

excavators or rubber tired units when used underground.

Hammer differences, other than internal design, reflect either

a high-energy low frequency (blows per minute) or lower

energy higher frequency breaking philosophy. These hammers

are rated by class relative to ft-pounds; classes range from 175

to 11,000 ft-pounds.

This experiment was conducted in an underground dolomite

mine at a depth of 257 ft (78.3 m) below the surface with a

1,500-ft-pound class hammer. A RMSS was installed in the
Figure 9. Monitor location.

center of a 36-ft (10.8-m) wide drift in a 2-in (5.08-cm) hole

drilled 14 ft (4.2 m) into the roof. This roof acts as a sill

between two mine levels, having a thickness of 26 ft (7.9 m).

Figure 9 shows the monitor’s location relative to 5-ft (1.5-m)

mechanical anchor roof bolts at 5- to 6-ft (1.5- to 1.8-m)

spacing. The figure also shows the bedding planes intercepted

by these bolts.

Twenty-four test points were marked on the roof emanating

radially from the RMSS at about 5-ft (1.5-m) centers (figure

9). While the hammer was hitting each point, ohms were read

on the RMSS. These data were plotted for each location

relative to the initial RMSS reading and a contour map based

on the ohms read was generated. The increase in ohms, used

to measure rock movement was so low, however, that rock

movement was negligible. The conversion rate is 254.648

ohms to about 1 in (2.54 cm) of movement. The maximum

ohms read was 0.0004 during the test.

As expected, the roof movement increased as the

hammering grew closer to the RMSS. In this location, roof

bolts had no positive effect, as rock movement was nearly

uniform independent of bolt location. The final reading at

steady state (no hammering) was 1.1804 KS 0.0002, or

0.2 ohms less than initial. This could be due to the connection

of the ohm meter to the cable during readout. Also when

reading such low level resistance some slight deviation is not

uncommon. It is also possible but not likely that the RMSS

slipped slightly in the hole; however, this small difference has

little impact on the results. (Slippage has not occurred at other

field sites even a year after installation.) Finally, the roof was

inspected closely for cracks; none were found.

It seems that a properly sized hammer can scale a mine roof

with minimal adverse impact. For completeness, the test

should be repeated with a much larger hammer.

SUMMARY

The RMSS is intended to serve as an engineering tool that

will improve safety by better understanding of roof behavior.

An understanding of the complex behavior associated with roof

and rib instabilities is expected to provide a method for

developing the “safest” decisions in concert with the existing

mining practice. This proactive strategy allows for a quick and

timely response in relation to existing conditions.

A critical point to realize is that hazardous roof beam sag is

dependent on site- specific geologic, stress, and mining

characteristics. Therefore, use of the RMSS or any type of

monitor must be suited for local conditions. There are also

many technical issues that need to be further addressed, such

as: (1) what are critical sag rates, (2) how much sag occurs

prior to a roof fall, (3) at which locations in the roof does the

failure occur, (4) how often should monitors be read, (5) where

should monitors be placed within the entry, (6) when should

monitors be used, and (7) how should the monitor data be

analyzed? If enough site-specific information is collected, then

general guidelines can be established and a meaningful safety

tool will have been developed. The only way this can occur is

if industry, mine workers, and Government work together to

gain the required data and knowledge.

The NIOSH's Pittsburgh Research Laboratory will attempt,

as resources allow, to assist the end users in utilizing this

technology. 

REFERENCES

1. Obert L, Duvall WI. Rock Mechanics and the Design of

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

NY, 1967, pp. 638.

2. Parker J. Everything Goes Underground Eventually.

Aggregates Manager, June 1996, pp. 30-35.

3. Iannacchione AT, Prosser LJ, Grau III RH, Oyler DC,

Dolinar DR, Marshall TE, Compton CS. Preventing

Injuries Caused by Unrecognized Stone Mine Roof Beam

Failure with a Pro-Active Roof Control Plan. SME

preprint 99-87. Littleton, CO: Society for Mining,

Metallurgy, and Exploration, Inc., 1999, 9 pp.

4. Aggman. Operations Section, January 2000.

Http://www.aggman.com/pages/agg%2 00100/

Operationa0100.html., 2000.

5. Iannacchione AT, Prosser LJ, Marshall TE, Compton CS,

Dolinar DR, Oyler DC, Pappas DM. An Instruction

Booklet for the Remote Monitoring Safety System

(RMSS). National Institute for Occupational Safety and

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