Roof Stability Issues in Underground Limestone Mines in the United States 

Gabriel S. Esterhuizen, Senior Research Engineer 

Dennis R. Dolinar, Lead Research Engineer
 

John L. Ellenberger, Lead Research Scientist
 

Leonard J. Prosser, Research Scientist
 

Anthony T. Iannacchione, Principal Research Engineer
 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 

ABSTRACT 

The room-and-pillar mining method is used extensively in 

underground limestone mines in the Eastern and Midwestern U.S. 

The rock mass is typically a near-horizontal, bedded deposit at 

relatively shallow depth. A survey of 34 mines was conducted in 

which data on roof spans, rock mass properties and support 

practices were collected as part of a National Institute for 

Occupational Safety and Health research project into ground 

control in limestone mines. The average room width is 13.5 m 

(44 ft), accommodating large mechanized equipment. The results 

show that the immediate roof beam can consist of many individual 

layers and can contain several near-vertical joints. In spite of the 

room widths, the roof is naturally stable in many mines. Observed 

roof instabilities can generally be related to excessive horizontal 

stresses or unfavorable geological structures that cause block 

fallout or beam failure of the bedded roof rocks. The survey 

showed that the rock mass quality of the limestone does not vary 

significantly throughout the Eastern and Midwestern U.S. Roof 

conditions and the need for support were found to be closely related 

to the thickness and competence of the first layer of limestone in 

the roof. The findings highlight the need to identify local 

geological structures and the roof beam characteristics so that 

support alternatives can be evaluated. Monitoring and 

observational technologies that are available for identifying 

potentially unstable roof are presented, which include: roof 

characterization using the Rock Fall Risk Index, automated logging 

of roofbolt hole drilling, microseismic monitoring and 

displacement monitoring. 

INTRODUCTION 

Underground extraction of the relatively flat-lying limestone 

deposits in the Eastern and Midwestern U.S. is carried out by the 

room-and-pillar method. High productivity and low mining costs 

are achieved by utilizing large mechanized equipment in rooms that 

have an average width of 13.5 m (44 ft). The immediate roof is 

typically a strong limestone bed that may be reinforced by rock 

bolts. In about one third of the mines, limestone in the floor is 

extracted by benching in-between the pillars, which requires the 

roof to remain stable during this secondary extraction phase. Falls 

of ground from the roof and pillar ribs can be a significant safety 

hazard, which is exacerbated by the height of the workings. The 

National Institute for Occupational Safety and Health (NIOSH) is 

investigating roof stability in limestone mines with the objective to 

develop guidelines for designing stable roof spans. This paper 

summarizes and evaluates the results of a survey of rock mass and 

roof conditions that were carried out in 34 underground limestone 

mining operations in the Eastern and Midwestern U.S. Methods of 

assessing potentially unstable roof conditions are discussed. 

SURVEY OF GEOTECHNICAL PARAMETERS AND 

ROOF CONDITIONS 

Data Collection 

Data were collected on rock strength, jointing and other 

geological structures, room and pillar dimensions, roof stability and 

pillar performance. Categorizing roof stability is a challenge, 

because roof conditions can only be subjectively assessed. In 

bedded strata, it is relatively easy to identify small-scale 

instabilities if the roof horizon is formed by a smooth bedding 

plane, because the voids left by rock fragments that fell out will be 

readily visible. When the roof line is formed by blasting through 

intact rock or by a rough natural surface, it is not a simple matter to 

identify potential or past instabilities. In addition, evidence that a 

rock fragment dislodged from the roof does not necessarily imply 

that the fragment posed a safety hazard; it may have been removed 

during blasting or subsequent scaling operations. Between two and 

five data sets were collected at various locations at each mine site. 

A data set describes the stability of the roof and pillars in an area of 

approximately 100 by 100 m (300 x 300 ft). The following 

information was recorded: 

1.	 Rock mass data was collected in accordance with the 

requirements of the Rock Mass Rating (RMR) system of 

rock mass classification (Bieniawski, 1989). This included 

rock strength, joint frequency, joint conditions and 

groundwater information. Since drill core was not available 

at any of the sites, the rock quality designation (RQD) could 

not be obtained. The joint frequency approach, proposed by 

Laubscher (1990), was used to obtain the combined joint 

spacing and RQD rating. The joint frequency was measured 

in two orthogonal windows, each 3 m (10 ft) wide by 1.8 m 

(6 ft) high. Rock strength was based on uniaxial 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

compressive strength (UCS) tests conducted in the 

laboratory on samples collected at the mine sites. 

2.	 It was observed that large discontinuities that extend across 

a room can have a significant effect on roof stability. These 

discontinuities are often widely spaced and may not be well 

represented in the 3 m (10 ft) wide mapping windows used 

in this study. Large, widely spaced discontinuities were 

therefore identified and described separately. 

3.	 Information on the roof composition was obtained from the 

mine personnel. These data included aspects such as the 

type and thickness of the roof beds, the location of 

prominent bedding planes and any other factors that impact 

roof stability under operational conditions. 

4.	 The condition of the roof was visually assessed. Observed 

roof instabilities were categorized by their size and are 

called rock falls or roof falls. Rock falls are smaller than 

about 1 m (3 ft) in length and typically fall out of the roof as 

single fragments. Roof falls can extend across the full 

width of a room and usually consist of multiple rock 

fragments. Table 1 shows the sub-categories used to 

describe rock falls and roof falls. These sub-categories are 

related to the causes of the failures. It is recognized that a 

roof fall can be the culmination of several factors, however, 

in most cases it is possible to identify a dominant factor that 

contributed to the roof failure. 

5.	 Pillar and room dimensions were measured and information 

on the depth of cover, blasting procedures, roof support type 

and spacing, and other mining parameters were collected. 

Table 1. Types of roof instabilities. 

Type Description 

Rock Falls Isolated rock fragments less than about 1 m (3 ft) 

in length. 

Slabs - Thin slabs caused by weathering or stress spalling, 

less than about 30 cm (1 ft) in length and about 25 

mm (1 inch) thick. 

Blocks - Blocky rock fragments caused by the intersection of 

joint planes, blasting factures, bedding or stress 

fractures. 

Beams - Stepped roof or brow formed by fall propagating to 

bedding plane. 

Roof Falls Falls larger than about 1 m (3 ft) in length 

typically consisting of multiple rock fragments. 

Blocks - Large discontinuities and joints associated with fall. 

Beams - Bedded layers in the roof fail under gravity loading. 

Stress - Horizontal stress-related shearing and buckling of 

roof beds. 

Caving - Fall caused by progressive spalling or blocky roof or 

weathering of weak strata. 

Geological Setting 

The limestone mines included in this study are concentrated in 

the Interior Plains and the Appalachian Highlands physiographic 

regions (U.S. Geological Survey, 2007). Twenty four of the mines 

fell within the Interior Plains region and are located in Illinois, 

Iowa, Kentucky, Missouri and Indiana. The Appalachian 

Highlands region includes the remaining ten mines in this study 

which are located in Pennsylvania, Maryland, West Virginia, 

Virginia and Tennessee. 

Limestone deposits included in the study that are located in the 

Interior Plains region are generally flat-lying or only gently dipping 

and include rocks ranging across most of the Paleozoic Era 

Ordovician Age (5 x 108 years) to the Pennsylvanian Age (2.7 x 108 

years). The Ordovician Age in this region includes the economic 

horizons of the Camp Nelson and the Tyrone limestones. The 

Mississippian Age includes the very silicieous and cross bedded 

Loyalhanna as well as the Greenbrier and the Monteagle. The 

Monteagle is a gently dipping Upper Mississippian limestone 

which is mined on more than one horizon (Brann and Freas, 2003). 

In the Pennsylvanian Age, the Vanport member of the Allegheny 

Group is mined (Iannacchione and Coyle, 2002). 

Overall, the limestone rocks encountered in the Appalachian 

Highlands region are similar in age to those found in the Interior 

Plains region. They differ in that they have been transformed 

through mountain building processes to consist of elongated belts 

of folded and faulted sedimentary rocks. Mines that were visited 

operate in the Middle Ordovician Five Oaks formation, the Vanport 

and Loyalhanna mentioned earlier, and the steeper dipping 

Monteagle formation which is mined at dips of up to 35 degrees. 

Horizontal Stress Field 

Stress measurements and field observations have shown that the 

horizontal stresses in the Appalachian Highlands and Interior Plains 

regions can be much higher than the overburden stress. Horizontal 

stresses have been measured in limestone mines (Iannacchione et 

al., 2003) and in many of the area’s coal mines (Mark and Mucho, 

1994). Research has shown that the horizontal stress may be 

explained by the effect of plate tectonics (Dolinar, 2003; 

Iannacchione et al., 2002). Tectonic loading is related to the 

movement of the North American plate as it is pushed away from 

the Mid-Atlantic ridge. A constant strain field of between 0.45 and 

0.90 millistrains is associated with the tectonic loading, which 

induces higher horizontal stresses in the stiff limestone strata. The 

induced stress magnitude is not necessarily related to the cover 

depth for depths encountered in limestone mining operations, but 

rather to the stiffness of the strata. Horizontal stresses are not 

necessarily present in all the limestone formations because local 

features such as outcropping and folding may have relieved the 

stresses over geological time (Iannacchione et al., 2003; 

Iannacchione and Coyle, 2002). 

A review of horizontal stress measurements in limestone and 

dolomite formations in the Eastern and Midwestern U.S. and 

Eastern Canada has shown that the maximum horizontal stress can 

vary between 7.6 MPa (1,100 psi) and 26 MPa (3,800 psi) up to 

depths of 300 m (1,000 ft). Limited information is available at 

greater depths. The orientation of the maximum horizontal stress is 

between N60°E and N90°E in 80% of the sites. This agrees with 

the regional tectonic stress orientation as indicated by the World 

Stress Map Project (2007). The minimum horizontal stress is 

approximately equal to the vertical stress. 

The horizontal stress can cause beams within the roof to buckle 

and fail in shear (Iannacchione et al., 2003). Failure can initiate as 

guttering in one corner of an excavation, called “cutter roof” in coal 

mines, and propagate to a large-scale roof fall (Esterhuizen and 

Iannacchione, 2004). Falls related to horizontal stress typically line 

up in the direction perpendicular to the regional maximum 
 

 

 

 

 

 

 

 

 

 

 

Figure 1. Typical oval shaped roof fall associated with 

horizontal stress in the roof. 

horizontal stress and are oval shaped when seen in plan view, see 

figure 1. Careful observation of the roof falls and other signs of 

excessive stress can assist in identifying the orientation of the 

maximum horizontal stress (Mark and Mucho, 1994). 

ROCK MASS CHARACTERISTICS 

Uniaxial Compressive Strength 

Cores were drilled from rock samples collected at 20 of the 

mines visited and tested in the laboratory for uniaxial compressive 

strength and elastic properties. The average rock strength was 

computed for each mine site. The results show that 68% of the 

average rock strength values lie in the range of 120 to 180 MPa 

(17,400 to 26,100 psi), with minimum and maximum values of 70 

and 302 MPa, respectively (10,100 and 43,800 psi). The highest 

strengths were found in the siliceous Loyalhanna and the Tyrone 

formations, while the lowest strength was found in the Monteagle 

formation in the Interior Plains region. 

Jointing and Bedding 

Discontinuities within the limestone formations were 

subdivided into bedding related discontinuities and joints. All the 

sites visited contained one or more sets of joints. The average joint 

spacing was 0.4 m (15 in) and the trace length typically in the range 

of 1 to 3 m (3.3 to 10.8 ft). Joints are typically rough with no 

infilling or weathering. Isolated cases containing soft calcitic or 

clayey infill were observed. Large joints that extend from roof to 

floor or across the width of an excavation were observed in about 

40% of the locations visited. These are discussed in greater detail 

below. 

Bedding layers do not always form a discontinuity in the 

limestone. It was found that the trace length of bedding 

discontinuities was greater than that of the joint sets. Bedding 

discontinuities typically had very rough surfaces. Isolated cases of 

bedding joints with calcite infill were observed. The average 

spacing of bedding discontinuities is 0.9 m (3 ft) with trace lengths 

typically in the 3 to 10 m (10 to 30 ft) range with about 30% of the 

cases extending greater than 30 m (100 ft). Bedding discontinuities 

are often used to establish a stable roof line. It was found that 36% 

of the underground locations visited made use of a local bedding 

plane as the roof line. 

Occasionally, bedding discontinuities were observed within the 

pillar ribs that extend over several hundred meters with relatively 

thick weak clayey or calcite infill. Such bedding discontinuities are 

expected to have a significant effect on roof stability if they occur 

within the immediate roof of an excavation. Since such 

discontinuities are not visible when they are above the roof line, 

data on their presence is limited. Further investigation and analysis 

of the effect of such weak layers in the roof is underway. 

Large Joints 

It was found that large widely-spaced joints exist at about 43% 

of the underground sites visited. The average spacing of the large 

joints was 12 m (40 ft) with a minimum of 1 m (3 ft) and maximum 

of about 100 m (330 ft). The data collection approach used in this 

study did not identify spacings of larger than 100 m (330 ft). The 

dip of these discontinuities typically fell in the range of 70 – 90 

degrees, with isolated cases in the range of 30 – 70 degrees. Large 

discontinuities dipping shallower than 30 degrees were categorized 

as bedding related features. The large discontinuities can contain 

soft infill materials but the fill material is seldom more than 5 mm 

(0.2 in) in thickness. 

Rock Mass Rating 

In all cases the data collection for rock mass rating was carried 

out approximately 2 m (6 ft) from the floor of the mining horizon. 

The rating results therefore do not describe the detail of the rock 

layer in the immediate roof, but rather represent the typical rock 

mass conditions at the site. The rock mass rating is presented in 

terms of the RMR, which classifies the rock mass on a scale of 0 to 

100, with higher numbers indicating stronger rock masses. The 

RMR values were found to fall in a narrow range, and the ratings 

for the immediate roof are not expected to be significantly different 

from the remainder of the formation. Figure 2 shows the 

distribution of RMR values obtained. It can be seen that the values 

range between 60 and 90, which lays within the “Good” to “Very 

Good” quality categories. 

Very Poor Poor Fair Good Very Good 

10 

20 

30 

40 

50 

60 

70 

Frequency 

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 

Rock Mass Rating (RMR) 

0 

Figure 2. Distribution of Rock Mass Rating (RMR) values in 

underground limestone mines. 
 

 

 

 

 

 

 

EXCAVATION STABILITY 

Mining Dimensions and Roof Reinforcement 

Measurements were made of the room width and the diagonal 

span across four-way intersections. In addition, other dimensions 

describing the room and pillar layout were taken. Table 2 provides 

a summary of the results. It can be seen that room widths vary 

between 9.1 m (30 ft) and 16.8 m (55 ft). Sixty eight percent of the 

room widths fall in the 12-15 m (40-50 ft) range. The diagonal 

span measured at four-way intersections averaged 21.7 m (70 ft). 

Table 2. Summary of mining dimensions. 

Average, Minimum, Maximum, Dimension m (ft) m (ft) m (ft) 

Mining height 11.6 (38.1) 4.8 (15.7) 38.0 (124.7) 

Room width 13.5 (44.3) 9.1 (29.9) 16.8 (55.1) 

Intersection diagonal 21.7 (71.2) 29.6 (97.1) 16.1 (52.8) 

Pillar width 13.8 (45.3) 4.6 (15.1) 28.6 (93.8) 

Roof reinforcement is carried out through the use of rock bolts 

of various types. Regular reinforcement by pattern bolting or 

irregularly spaced bolts was observed at 46% of the locations 

visited. The remaining 54% of sites were naturally stable or 

occasionally used roof bolts to support the roof in isolated areas. 

Fully-grouted bolts are most commonly used, with friction bolts 

and mechanical anchor bolts being less prevalent. Bolt lengths 

vary from 0.9 m (3 ft) to 2.4 m (8 ft) with 1.8 and 2.4 m (6 and 8 ft) 

long bolts making up 67% of the bolts in use. Bolt spacing of 

1.5 m (5 ft) and 1.8 m (6 ft) are the two most commonly observed 

spacings, and the maximum bolt spacing was 2.4 m (8 ft). In 

extreme situations, cable bolts and sealant injection have been used 

to stabilize the roof. These are considered special applications and 

were not included in the study. 

Roof Instabilities 

All but four of the 34 mines visited had experienced some form 

of small-scale rock falls or larger roof falls. The results of the 

survey of roof conditions are presented below, in accordance with 

the categories shown in table 1. The frequency of smaller scale 

rock falls was as follows: 

1.	 Slabs: 11% of the total roof area observed. 

2.	 Blocks: 6% of the total roof area observed. 

3.	 Beams: 11% of the total roof area observed. 

In the remaining 72% of roof area observed, the roof was stable 

with no sign of current or past instability, which includes the roof at 

the four mines that did not show any signs of instability. Figure 3 

shows an example of a 13-m-wide, naturally stable excavation with 

excellent roof conditions. Most of the above listed instabilities are 

addressed by scaling, rockbolting or screen installation as part of 

the normal support and rehabilitation activities. 

In addition to small scale rock falls, large roof falls were 

observed at 19 of the 30 mines that experienced small scale roof 

instability. The large falls made up a very small percentage of the 

exposed roof in the mines; many of the mines only had a single 

instance of a large roof fall. Roof falls were categorized by 

identifying the most significant factor contributing to each fall, as 

Figure 3. Naturally stable 13 m (44 ft) wide roof spans in a 

limestone mine in the Central Plains region. 

shown in table 1. A summary of these factors and the relative 

frequency of occurrence of each are presented below: 

1.	 Stress: Horizontal stress was assessed to be the main 

contributing factor in 36% of all roof falls observed. These 

falls are equally likely to occur in shallow or deep cover. 

A roof fall related to stress-induced damage was observed 

at a depth of as little as 50 m (150 ft) in one case. 

2.	 Beams: The beam of limestone between the roof line and 

some overlying weak band or parting plane failed in 28% 

of all roof falls observed. 

3.	 Blocks: Large discontinuities extending across the full 

width of a room contributed to 21% of the roof falls. 

4.	 Caving: The remaining 15% of the roof falls was attributed 

to collapse of weak shale exposed in erosion channels or 

progressive failure of weak roof rocks. 

Although the large roof falls only make up a small percentage 

of the total roof exposure, their potential impact on safety and mine 

operations can be very significant. Most cases of large roof falls 

required barricading-off or abandonment of the affected entry. 

When large roof falls occur in critical excavation areas, the repair 

can be very costly. Figure 4 shows a case where extensive support 

was required to rehabilitate a large roof fall. 

Roof Beam Stability 

The stability of excavations in bedded deposits is closely tied to 

the composition of the first beam of rock in the roof. An 

assessment of the data collected showed that 25 of 34 mines were 

attempting to maintain a specific thickness of limestone beam in 

the immediate roof. A constant thickness of roof beam is achieved 

either by probe drilling to determine the thickness of the roof beam 

or by following a known parting plane or marker horizon. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4. Repair of a large roof fall in which horizontal stress is 

thought to have contributed to the failure. 

The average roof beam thickness in mines that were able to 

mine without regular support was 2.25 m (7.4 ft), while the average 

beam thickness in the mines that were using regular roof support 

was 1.3 m (4.3 ft). Several of the mines that used regular support 

do so to alleviate the effects of horizontal stress, which is not 

related to beam thickness. If these mines are removed from the 

data, the average beam thickness in mines that use regular support 

drops to 0.8 m (2.6 ft). These results seem to indicate that mines 

with a relatively thin beam of limestone in the immediate roof are 

more likely to encounter unstable roof and regular roof bolting 

becomes necessary. 

The beam thickness is obviously not the only factor to consider 

when deciding on roof reinforcement. Other aspects such as roof 

jointing, bedding breaks, blast damage, groundwater and horizontal 

stress can contribute to roof instability resulting in the need for rock 

bolt support. 

Horizontal Stress Issues 

This study showed that horizontal-stress-related roof instability 

can occur at any depth of cover. This is not unexpected, given that 

the horizontal stresses are caused by tectonic compression of the 

limestone layers, which is not related to the depth of typical 

limestone mines. Observations show that the tectonic stresses in 

limestone formations that outcrop may have been released over 

geologic time by relaxation towards the outcrop (Iannacchione et 

al., 2002). Consequently, outcropping mines can have highly 

variable horizontal stress magnitudes which depends on the amount 

of relaxation that occurred and the distance from the outcrop. 

Operations that experience horizontal-stress-related roof 

instability can consider changing their layouts so that they are 

favorably oriented relative to the maximum horizontal stress. This 

can be achieved by laying out the main development direction 

parallel to the maximum horizontal stress and minimizing the 

amount of cross-cut development (Parker, 1973). The direction of 

the maximum horizontal stress can be determined by various stress 

measurement techniques or can be inferred from stress-related roof 

failures (Mark and Mucho, 1994). Modifying a layout in this 

manner will not necessarily eradicate all stress-related problems, 

but has been shown to considerably reduce these problems 

(Kuhnhein and Ramer, 2004). 

Rock Mass Classification and Roof Stability 

The RMR rock mass classification carried out as part of this 

study showed poor correlation to the roof spans used in the various 

mining operations. In addition, the rock classification could not 

discriminate between cases of stable and unstable roof spans. 

Application of other classification methods such as Barton’s Qsystem (Barton et al.,1976) and the Modified Matthews Stability 

Graph method (Potvin, 1988 and Nickson, 1992) also showed poor 

correlation. This can be explained as follows: 

1.	 The limestone rock mass is relatively strong and the rock 

mass strength does not dictate excavation spans. In 

addition, the rock mass classification values and excavation 

spans both fall in a narrow range, which precludes the 

identification of a trend. 

2.	 The stability of the roof in bedded limestone rocks is 

closely tied to the characteristics of the immediate roof 

beam. Beam stability considerations are not well 

represented in the rock mass classification methods 

assessed. 

It is evident that an alternative roof classification method is 

required for limestone mines. The method should clearly 

differentiate between potentially stable and unstable roof conditions. 

The results of the survey indicate that such a method should be 

focused on the immediate roof beam and consider its thickness and 

composition, horizontal stress and the presence of large joints. 

Comparison of the Physiographic Regions 

An evaluation was made of the collected data to determine 

whether differences exist in rock conditions and roof stability 

between the Appalachian Highlands and Interior Plains 

physiographic regions. The evaluation showed that the regions are 

very similar in terms of rock mass strength as expressed by the 

RMR values. The average uniaxial compressive strength of the 

rocks in the Appalachian Highlands region appears to be slightly 

higher, but insufficient data is available to determine the level of 

statistical significance. The average room width in the 

Appalachian Highlands is 13.7 m (45.0 ft) and in the Interior Plains 

it is 13.5 m (44.2 ft), which indicates that the rock conditions must 

be similar, allowing similar excavation dimensions to be developed. 

Roof bolting is used in about 50% of the mines in both regions, 

again confirming that rock conditions are similar. Roof bolt 

spacing and lengths were not significantly different in the two 

regions. Horizontal-stress-related roof stability issues were also 

equally prevalent in the two regions. 

ROOF ASSESSMENT AND MONITORING
 

ALTERNATIVES 

Ground control in limestone mines is complicated by the fact 

that excavations are large and small rock falls can have significant 

consequences. Changes in the rock mass, such as the thickness and 

competence of the immediate roof beam, the presence of large 

discontinuities and horizontal stress can all contribute to instability. 

Many of these important factors are not readily observable during 

routine examinations. NIOSH therefore advocates a systematic 
 

 

 

 

 

 

 

 

 

 

 

 

 

approach to identifying and mitigating potential roof instability 

through a program of assessment and monitoring. 

Roof assessment techniques that are well established include 

roof sounding during scaling operations, borescope inspection of 

holes drilled into the roof, using a scratch tool for locating voids in 

the roof and observation of geological anomalies. Three 

technologies that can supplement these techniques are: a) roof fall 

hazard assessment through the Roof Fall Risk Index (RFRI), b) 

automated drill recording, and c) roof displacement monitoring. 

These technologies can provide the information necessary to 

develop a significant component of a comprehensive ground 

control plan. 

Roof Fall Hazard Assessment 

NIOSH developed the Roof Fall Risk Index (RFRI) as a tool for 

systematically identifying roof fall hazards in operating mines 

(Iannacchione et al., 2007). The RFRI focuses on the character and 

intensity of defects caused by a wide range of local geologic, 

mining and stress factors and is equated directly to changing roof 

conditions causing roof fall hazards. A significant range of defects 

found at underground stone mines are classified into 10 categories 

(known as defect categories), each of which is assigned an 

assessment value. The total rating varies from 0 to 100, with 

values approaching 0 representing safer roof conditions, while an 

RFRI approaching 100 represents a serious roof fall hazard. 

The RFRI is a hazard assessment technique that can be used as 

both a training and communication tool. This technique requires 

that roof fall hazards be mapped and the spatial distribution within 

the underground workplace determined. The RFRI strives to assess 

roof conditions over large, continuous areas, with fewer timeconsuming measurements than are used in many existing rock mass 

classification systems. This produces a more comprehensive 

assessment of changing roof conditions than was previously 

possible. Examples of the application of this technique are 

available in Iannacchione et al. (2006). 

Automated Drill Recording 

Characterization of the immediate roof through automated drill 

recording is a recent development introduced by J.H. Fletcher and 

Co. 1 located in Huntington, WV. Measurements of force, velocity 

and operational characteristics during roof or face drilling are used 

to determine geotechnical properties of the rock mass. Of 

particular interest to underground stone mines is the presence and 

location within the roof rock of bedding separations, weak beds, or 

fracture systems. Knowing the presence and location of these 

discontinuities allows for the development of a roof support 

strategy that considers a proper bolting system such as bolt length 

and bolting horizon. 

Collins et al. (2004) reported on a field test of the automated 

drill recording system in an underground limestone mine. For this 

test, five holes were drilled at various locations in the mine. The 

holes were subsequently bore-scoped with a video camera for 

comparison purposes. In each test hole, the voids identified by the 

roof mapping system closely resembled the information obtained 

from the bore-scoped data. 

During the past few years, the system has been improved and 

upgraded. The accuracy of this roof characterization technology 

1

Reference to a specific product does not imply endorsement by NIOSH. 

was assessed at 3 underground limestone mines in 2007 with 

J. H. Fletcher and Co. providing the roof mapping data and NIOSH 

conducting the bore-scoping of the same holes. At each mine, 5 to 

8 holes were used in the comparisons. In all cases, large strata 

separations or weak beds were readily shown with the mapping 

system and were clearly visible on the bore-scoped video. 

Roof Displacement Monitoring 

Roof displacement monitors used in conjunction with a 

comprehensive ground control plan can be an effective tool in 

providing essential information on a mine’s roof stability. 

Monitoring in most cases should be considered a long-term activity 

to be most productive and effective. Prior to monitoring, locations 

of highest concern should be identified based on observation 

regarding geologic conditions using a system such as the above 

mentioned RFRI. In addition, mine areas of constant exposure to 

the mine worker, such as main travel ways, maintenance and repair 

areas and underground crusher stations are locations suited for 

proactive monitoring (Prosser et al., 2003). 

The NIOSH experience with roof monitors encompassed the 

installation of more than 100 Roof Safety Monitoring System 

(RMSS) units at 21 underground limestone mines (Marshall et al., 

2000). At 85% of the monitor sites no roof movement was reported 

or recorded. In four of the mines, roof movement or instability was 

recorded that resulted in timely responses to potentially unstable 

roof. At one mine with excessive horizontal stress, approximately 

33% of the monitoring sites, showed roof movement, with some 

cases showing more than 10 cm (4 in) of roof deflection, which 

aided in decisions concerning ground control and worker safety. 

One U.S. company, that appears to have monitors suited for 

underground limestone applications is Simplified Mine Instruments, 

located in Iron River, MI. Four different types of instruments are 

available which provide increasing capability to measure roof 

movement depending on conditions and circumstances: a) the basic 

or “watchdog” model drops a reflective rod into view when a preset amount of movement has occurred; b) the “Miner’s Helper” can 

be used to measure the roof movement directly; c) the “Remote 

Miner’s Helper” adds the ability to measure at a distance from the 

monitoring station if the location is difficult to access and allows 

for readings to be taken electronically; and d) a three-point 

extensometer is available to measure movement at 3 locations 

within a single borehole. 

Microseismic Monitoring 

Rock under stress emits audible microseismic emissions at a 

rate which increases with increasing stress level. The rate of 

emission generally increases prior to and during major fracturing of 

the rock. If sufficient fracturing occurs in the rocks in the roof of 

an excavation, the rock begins to deflect into the opening. 

Microseismic monitoring system can detect the microseismic 

events and calculate the source of the event. Early research by the 

U.S. Bureau of Mines (Obert, 1945) identified the potential for 

using microseismicity as an indicator of mine stability. NIOSH 

research (Iannacchione et al., 2004a) examined the relationship 

between microseismic activity and the corresponding roof 

deflection associated with two large roof falls in a limestone mine. 

The results showed a connection between roof rock instabilities and 

trends in microseismic activity. In addition, microseismic activity 

was sometimes shown to occur prior to roof deflection, 

demonstrating that rock fracturing precedes roof deflection. The 

results suggest that microseismic monitoring combined with roof 
 

 

 

 

 

 

 

 

 

 

deflection monitoring could be used to better determine the relative 

stability of local roof rock conditions. Application of this 

technology can form the basis for assessing roof stability and 

planning appropriate ground control strategies (Iannacchione et al., 

2004b). 

SUMMARY AND CONCLUSIONS 

The study of roof conditions in limestone mines in the Eastern 

and Midwestern U.S. has shown that: 

•	 Rock mass conditions in the limestone formations are 

relatively uniform throughout the Appalachian and Interior 

Plains regions. Mining dimensions, roof support practices 

and roof instabilities do not vary significantly between the 

regions. 

•	 Roof stability is closely related to the thickness and 

composition of the layer of rock in the immediate roof of 

the workings. Mines that did not use roof reinforcement 

generally had thicker limestone beds in the immediate roof. 

•	 Large roof falls were observed at 19 of 34 mines visited, 

in many cases at just a single location. 

•	 The main factors contributing to large roof falls are: 

horizontal stress, large joints and insufficient thickness of 

the immediate roof beam. 

•	 Horizontal-stress-related roof instability can occur at any 

depth of cover. 

•	 The reviewed rock mass classification schemes do not 

sufficiently characterize the immediate roof beam to be 

useful for roof span or support design in limestone mines. 

•	 Technologies such as the RFRI method of roof hazard 

assessment, automated drill recording, displacement 

monitoring and microseismic monitoring can assist mine 

personnel in identifying and monitoring roof conditions as 

part of a comprehensive ground control management plan. 

The results presented in this paper forms part of a NIOSH 

research project aimed at providing roof span and pillar design 

guidelines for limestone mines. The authors wish to thank the staff 

of the various mining operations visited for their willingness to 

share their experiences and their time to accompany us during the 

underground examinations. 

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