FIG.1

Roof. face and rib-fall injuries in the underground stone

The factors that influence the variability in roof

beam deflection and failure must be identified.

A simple, inexpensive method to monitor dangerous

levels of roof beam deflection must be produced.

Guidelines for evaluating the output of these monitoring devices must be established.

Current roof-monitoring technology

Observing and monitoring rock deformations provide information for making critical mining decisions.

Traditionally, miners have sounded the rock, listening for

the drummy sounds that signal loose rock. The act of FIG.2

Underground stone roof, face and rib-fall injuries by state

from 1990 to 1996.

A. T. lannacchione, member SMll.J. Prosser, R. Grau, D.C. Oyler, D.R.

Dolinar, T .E. Marshall and C.S. Compton are deputy director, research physical

scientists, mining engineer, mechanical engineer, mining engineer, engineering technician and engineering technician, respectively, with the US National Institute for Occupational Safety and Health, Pittsburgh Research Center, Cochrans Mill Road, Pittsburgh, PA

152J6.

F rom 1990 to 1996, 16 states reported 92 injuries

from falls of roof, rib or face in the more than 90

underground stone mines in the United States (Fig.

1). Missouri, Pennsylvania and Kentucky accounted for

48% of the total number of injuries (Fig.2). Of this total, 11 miners were killed. Of the 11 fatalities, 10, or 91 %,

were associated with unrecognized loose or failed rock

within the roof beam. Additionally, unrecognized roof

beam failures resulted in a majority of the falls of ground

injuries occurring in underground US stone mines from

1990 to 1996.

This number is not large in magnitude. But a work

force of fewer than 2,000 miners makes for a high fatalaccident rate. The severity of the typical fall-of-ground

injury is, in general, very high. About three-fifths of all

roof; face and rib fall injuries were designated by the US

Mine Safety and Health Administration (MSHA) as

Inaustry.

30 r

some kind of lost time accident. MSHA assigns each accident a severity value from one to six. A one represents

a fatality, a two, a permanent disability and three, a lost

time accident (Fig. 3).

Most underground stone mines operate in thick

limestone formations. Intact (solid) pieces of limestone

are generally strong. They often have compressive

strengths of 207 MPa (30,000 psi) and tensile strengths of

14 MPa (2,000 psi). Persistent horizontal bedding planes

typically cause limestone roof rock members to separate

into beams ranging from 0.15 to 1 m (0.5 to 3 ft) thick

that span large rooms, ranging from 6.1 to 18.3 m (20 to

60 ft) and averaging 13.1 m (43 ft) wide (Fig. 4).

The deformation characteristic that affects roof

beam stability is excessive deflection (Fig.5). If deflection, or bending, of the roof beams becomes excessive,

roof failure can occur, leading to injuries to miners. However, these beams can contain vertical and subvertical

discontinuities (vertical joints or fractures and

subvertical cross-bed planes) that sometimes affect local

roof-beam strength, producing wedge or prism shaped

failures (Fig. 5).

If dangerous roof beam deflection values are known,

they can be used as a clear indicator of roof instability

(Parker,1973). There is a potential for roof monitoring

to assist mining operations in gathering quantitative and

qualitative information. But, for this technology to be

implemented, several technical problems must be overcome:
Table 1

Number of mines using observational and monitoring techniques.

potential for imminent roof failure. In some cases, this

information has been used to indicate a need to add roof

support, remove roof rock or danger off affected areas.

Coal mines have used telltales for decades to warn

miners of strata movement. The telltale is a rigid bar -

possibly a roof bolt -anchored into the roof. A small

section of rod protruding from the borehole is covered

with three bands of reflective tape. The portion of the

bar closest to the roof is generally green, followed downward by yellow and then red. The idea is that, as the roof

deflects downward, the roof line can easily be seen to

move through the green, yellow and red tape zones.

Recently, Bay Tech has produced an electronic telltale. In the United Kingdom, coal mines use telltales

every 20 m (65 ft) with action bands from 0.4 to 2 cm (0.2

to 1 in). Between 1990 and 1995, falls of ground were

reduced from 267 to six, partially due to the use of telltales (Altounyan et. al, 1997).

FIG.3

Severity of roof. face and rib-fall accidents from 1990 to

1996.

FIG.4

Histogram of room widths for 65 underground stone mines.

25

20

15

drilling exploration, roof bolts or blastholes can provide

much information about the rock. If separations are

present, the drill will often accelerate through these

zones. Dust and water from adjacent fractures or drill

holes indicate the occurrence of hidden rock fractures.

Borescopes and borehole cameras have been used to

observe fracture characteristics and roof lithology.

Wedges inserted into angular fractures or horizontal

bedding planes along the roof and ribs of the mine have

been used to observe the movement of rock masses -if

the wedge falls out, the rock mass is moving.

Observational techniques can be extended by regularly monitoring the movement of the mine roof. Monitors can be divided into two basic types: roof-to-floor

convergence monitors, and roof and rib extensometer

monitors. Most stone mine development rooms average

7.1 m (23 ft) and bench rooms average 16.4 m (54 ft) in

height, roof-to-floor. So convergence monitors are difficult to install, maintain and analyze. Roof and rib

extcnsometers are more widely used than convergence

monitors. However, they are difficult to read because of

their location on the roof line or back.

In its simplest form, roof and rib extensometer

monitoring can be accomplished with a scratch tool (Fig.

6). This device can detect separations and provide an

indication of loose rock layers or roof beam deflection.

Information on the location and size of the separation

can be marked on the roof and used to assess potential,

future roof degradation.

For many years, extensometers permanently installed in drill holes have been used in underground

mines to detect ground fall hazards. Sonic probe extensometers have been widely used in the United States, the

United Kingdom and Australia. This commercial system

allows for 20 permanent anchors up to a 6-m (20-ft)

height. The probe is temporarily inserted when measurements are made.

Homemade mechanical extensometers have consisted of a top and bottom anchor, steel wire or rigid tubing, and some kind of micrometer or dial gauge. These

devices have been used for decades in metal mines in

Michigan, Missouri and Idaho. For example, in the Missouri lead belt district, a deflection rate of 0.17 mm/

month (0.007 in./month) is considered a good warning of

strata failure. Extensometers have been used as realtime hazard warning devices. Parker (1973) discussed an

ingenious method to alert miners of strata movement by

adding a warning light to an extensometer.

Some common commercially available mechanical

extensometer monitoring devices are the Miners Helper,

the Guardian Angel and the Dual Height telltale (reference to a specific product does not imply endorsement

by N ational Institute for Occupational Safety and

Health (NIOSH) (Fig.7). These monitors have one or

two anchor points that measure the overall separation of

rock layers in the immediate roof. If roof deflection is

detected by the Miners Helper and the GuardiallAngel,

a reflecting flag drops from the roof line, signaling the

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Observation or Sounding Observation Wedge Borescope Scratch Extensometer Miners Guardian Telltale

monitor type hole tool Helper Angel

Number of mines 9 11 5 6 4 2 7 4
FIG. 5

Two different modes of roof-rock failure that are common

in underground stone mines.

Overview of ground-control planning methods

Generally, underground mines use observational

techniques, primarily visual inspection, to determine roof

stability. Additional knowledge related to roof stabiliby

is gained through blasting, drilling and scaling. For example, a driller preparing to bolt notices a sudden increase in the penetration rate. He realizes that possibly

a gap or clay seam was encountered.

Much of this hands-on information provides an

overview of the general conditions related to roof stability. However, this base of knowledge can quickly deteriorate if that hands-on experience is lost through

changes in employment or other circumstances.

Many mines supplement visual inspections and

knowledge gained through hands-on experience with

various types of 9bservational 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 mines (Table 1). This suggests that additional information is beneficial or needed to solve some classes of

ground control problems.

To date, observational techniques are used more

than monitoring techniques. Observational techniques

accounted for 31 of the 48 mines, or 65% of the total.

Drilling observational holes to detect conditions in the

roof was the most used technique with 11, or 23%, of the

48 mines. The data also show that 11 of the 48 mines

used or have available commercial mechanical monitors

(Miners Helper or Guardian Angel). This relatively low

number of overall usage and the fact that 46% of the

mines use no additional monitoring methods suggest apparent limitations to using these methods.

A comprehensive ground-control plan includes the

basic visual and hands-on components. But it also uses

supplemental observational and monitoring techniques.

And it 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 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 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 proactive approaches towards ground control safety.

FIG. 6

NIOSH's roof-monitoring safety system

There have been considerable advancements in roof

monitoring. But existing instruments have severallimitations that minimize the impact of this technology on

underground stone mining. These limitations include:

Difficulty in taking readings in high roof or back areas.

Exposure to dangerous ground while reading the

monitors.

Complexity in making repetitive readings.

Problems with making measurements within multiple roof horizons.

Difficulty in seeing warning devices in the dusty and

foggy production face areas.

Bedding plane separation Bedding (dashed)

:::::::::::=:~-::::~::::::::=: ::::= \ Main roof ""---~-

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FIG.7 FIGS. SA AND B

Photograph of a commercially available Miners Helper, the

Guardian Angel and the RMT's Dual Height Telltale roof

mechanical extensometer monitoring devices.

Single-point RMSS with multimeter and multipoint RMSS

with data acquisition.

Expense of some commercial monitors.

NIOSH developed a new generation Roof-Monitoring Safety System (RMSS). It improves the existing

methods for determining roof stability. This electromechanical roof monitor includes several features.

.

.

.

It can be fabricated in most standard mine shops.

It is relatively inexpensive (single-point RMSS costs

less than $40/unit to fabricate in-house ).

It reduces potential damage from face blast because

of the in-hole positioning (this is true only of the

single point RMSS, Fig. Sa).

It has the capability for monitor reading at ground

level (allows the miner to take readings away from

potentially unstable roof conditions).

It can be remotely read with a multimeter or commercial data-logging devices.

It can accommodate as many as six anchor points

( this is true only of the multipoint RMSS, Fig. Sb ).

natural, geologic and mining processes. These include

dramatic humidity variations from changing environmental conditions, chemical alterations from strata water, tectonic stresses, additional mining adjacent to the

opening and heat from machinery. These processes initiate a period of instability that sometimes continues

until failure occurs.

In some mining conditions, the immediate roof beam

may fail sometime between the blast and the time when

miners first re-enter the face. These beams are found

draped over the rubble stonepile produced from the production blast. In other mines, roof beam failure will periodically occur days, months or even years after the

excavation has been formed. The time between the decelerating and the accelerating beam-deflection generally represents a period of stable roof conditions.

Roof beam deflection due to gravity loading can be

estimated using standard formulas for deflection of

Perhaps the greatest benefit of this new monitor is

that the miner does not have to make readings in areas

of questionable roof stability. After all, these instruments are most often deployed in areas where roof rock

instabilities are suspected. Figure 9 illustrates the advantage of the electromechanical RMSS over other existing

techniques.

How do roof monitors assess strata stability?

Every opening made in rock causes a redistribution

of stresses in the adjacent strata. In bedded or layered

sedimentary rock, the roof beams deflect into the opening immediately after excavation (Fig. 10). A decay in

the rate of deflection occurs as the stresses redistribute

in response to the opening.

Eventually, the beam deflection stabilizes (Fig. 10).

In most strata, this initial deflection takes place quickly.

For example, NIOSH researchers have installed monitors in the face area a few days after a production blast

and have not been able to detect this period of decelerating deflection.

As time passes, these roof beams are subjected to
FIG.9

Comparison of three roof-monitoring techniques.

beams or plates. Figure 11 shows the effect varying beam

lengths and thickness have on the maximum deflection

of a 23.5 kN/m3 (150 lb/cu ft), 41.4 GPa (6 million psi)

fully intact beam of limestone. This deflection is small

and can take place quickly after an opening is excavated.

Or it can take place much later as weathering processes

aid in forming new, thinner beams.

Tectonic processes can produce additional roofbeam deflection by axially loading roof beams. Two-dimensional models have demonstrated how different

levels of horizontal stresses may cause up to several centimeters of deflection before failure of roof beams

(Iannacchione et al., 1998).

Vertical loading of roof beams from overlying or

adjacent mining can load roof beams that, in turn, can

accelerate deflection. In all of these examples, roof

beam deflection is viewed as a precursor of failure and,

if recognized early, could result in pro-active control solutions.

FIG. 1 O

Typical cycles experienced by failing stone roof beams.

Note the periods of decelerating, quiet and accelerating

deflection rate.

Typical roof failure pattern

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FIG. 11

Maximum deflection from gravity loading for various limestone beam geometries.

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Site-specific example

of a proactive ground-control plan

A site-specific field test was performed to illustrate

how information from roof monitors can help make

safety decisions about the stability of mine roofs. In

1996, the immediate roof at an operating stone mine began to fail about 90 m (300 ft) from the end of a previous

directionally controlled roof fall.

This original failure took on the appearance of a series of low-angled shear planes cutting or ripping the

rock. The orientation of these planes was perpendicular

to the orientation of the local horizontal-stress field.

Seven deflection monitors were placed along the projected failure trend. Two of the seven monitors used the

20-anchor point sonic probe. The other five were

three-anchor point prototypes of the RMSS.

Data collected from three of these monitors are

shown in Fig. 12. Monitor No.7 collected deflection measurements for almost 70 days before total roof collapse.

During this time, the roof deflected in three distinct

phases. The first phase was marked by a slow but steady

deflection in the lower roof beam. At about 40 days,

there was a sudden increase in the deflection of the

beam. The third phase indicated that the beam deflection rate lessened but ended in total roof failure. About

50 mm (2 in. ) of roof deflection occurred before roof

collapse.

Data from monitor No.3 showed a much different

trend. Unlike monitor No.7, this instrument was placed

close to an existing failure. Therefore, significant beam

deflection could have already occurred. The area began

to cut, or rip, on July 26, rapidly extending the zone of

failed roof. The roof associated with monitor No.3 went

from stable to unstable in five hours.

Monitor No.4 was purposely placed slightly away

from the main failure trend. The magnitude of deflection

measured from this instrument was one-tenth of that

from the instruments within the failure trend. However,

these measurements did show that beam bending and

associated shearing extended significant lateral distances

on the order of 6 m (20 ft) from the fall's edge arid 12 m

( 40 ft) from the center of the fall. This monitor also

showed that, while deflection was initiated in the lowest

beam, beam separations quickly moved much higher in

the roof.

10

0

0 10 20 30
This information proved valuable to the mine. Several proactive

ground-control strategies were

implemented as a result of supplemental roof monitoring. Unstable

roof areas were identified and personnel were restricted from entering. A new roof-support plan was

initiated. It prevented the progressive failure of the various roof

beams. Roof monitoring was initiated in other areas of questionable

stability.

FIG. 12

Roof behavior associated with a large roof fall caused by high horizontal

stresses. Monitor No.7 was a three-anchor point prototype of the RMSS and

Monitor No.4 was a 20-anchor point sonic probe.

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Critical issues

and the need for cooperation

RMSS technology has the potential to provide information that

could be used to solve associated

rock mechanics issues, such as:

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maps and shared with miners, fosters the develop.

ment of a proactive roof-control plan.

What are critical deflection

rates?

What geologic factors influence

deflection magnitudes prior to a o

roof fall.

At what locations in the roof does the failure initiate?

The answers to these questions can provide the basis for recommendations and guidelines. And they can

be used effectively and efficiently to improve the safety

conditions for underground stone miners.

While much has been learned, more knowledge will

be needed to help know when monitoring techniques

should be used. Because performance results depend

heavily on site-specific conditions related to geologic,

stress and mining conditions, all monitoring data must be

calibrated for site-specific conditions. The solution to

this problem requires research efforts founded on a common goal and good communication. The best way to

achieve this is to have industry, labor and government

working together to gain the required data and knowledge.

Conclusions

This research is intended to serve as a catalyst to

develop better engineering tools and strategies that will

improve safety by better understanding roof behavior.

An understanding of the complex behavior associated

with roof instabilities is expected to provide a method

for developing the safest decisions in concert with existing mining practices. Developing a proactive roof control plan allows for a quick and timely response. And it

ensures that every response is the one that is most appropriate in relation to existing conditions. Here are

some of the important characteristics associated with a

proactive roof control strategy:

There are many useful monitoring techniques available to underground stone mines. However, many of

them have operational problems and often lack adequate information to apply them at local mine sites.

To help address these problems and to provide a

better means of collecting and sharing roof deflection

data, NIOSH has developed the RMSS. The RMSS has

several advantages. It is inexpensive and can be fabricated locally. It can be placed in boreholes protected

from blast damage. It can be read remotely. And it is

compatible with many kinds of data acquisition systems.

A first step has been made here through the presentation of the RMSS. But improvements are possible and

indeed likely. For example, both the single- and multiple-point RMSS can be incorporated into a minewide

monitoring system. It is hoped that improvements can

be made to fabrication procedures/components and the

development of computer software to assist in managing

the large streams of data associated with minewide

monitoring scenarios.

Hazardous roof-beam deflection depends on sitespecific geologic, stress and mining characteristics. So

any roof-monitoring technique must be calibrated for local conditions. This can occur only if industry, mine

workers and government work together to gain the required data and knowledge.

The use of the RMSS and other observational and

monitoring techniques by the underground stone mines

is expected to enhance miners' understanding of roof

behavior and provide a tool for proactive intervention

when hazardous ground conditions exist.

Knowledge gained through shared experiences will

aid in developing innovative engineering techniques that

will mitigate falls of ground and reduce the potential for

injuries to mine workers. (References are available from

the authors.) .

Visual and hands-on roof condition information is

the basis of any roof control strategy.

Supplemental observational and monitoring techniques provide additional useful information.

Regularly recorded and charted information from

basic and supplemental monitoring, placed on mine