Ellenberger, J.

 

 

A Roof Quality Index for Stone Mines Using Borescope Logging 

John Ellenberger, Lead Research Scientist
 

NIOSH Pittsburgh Research Laboratory
 

Pittsburgh, PA
 

ABSTRACT 

The National Institute for Occupational Safety and Health 

(NIOSH) has collected pillar and span data at 34 stone mines 

in 10 states to develop pillar and roof span design guidelines for 

underground stone mines. Pillar, floor and roof conditions each 

contribute significantly to the overall mine stability. The roof 

may be stable for a century, or it may appear to be stable but fail 

with little warning. Clearly, an assessment of the mine roof is 

critical to the safety of these operations. However, up until now, 

the assessment of the mine roof came primarily from exposures in 

roof falls or portal areas. This provides only limited information, 

and a more proactive approach is needed to assess changing mine 

conditions. 

A separate effort was undertaken to examine the roof structure 

with the use of a borescope. Numerous physical and geologic roof 

structures were observed with a stratascope/borehole video camera. 

The objective of this work was to develop a technique of assigning 

numerical values to features seen within the mine roof and apply 

those values to formulate a roof stability evaluation. Such features 

as changes in color, grain size, and general lithologies are readily 

observed. Minor structures in the rock, fossils and fractures are 

also readily identified. Each feature may have a variable and sitespecific impact on roof quality. 

A number of mines were visited and a series of holes were 

borescoped in each mine. The features identified in the roof were 

classified and potential for roof damage related to each feature was 

assessed. A process is presented that relates the structures and their 

locations to provide a numerical value, or roof quality index that 

can be used to assess roof stability. 

INTRODUCTION 

The National Institute for Occupational Safety and Health 

(NIOSH) has collected data at 34 underground stone mines to study 

factors that influence pillar and roof span stability. The data from 

the 34 mines has been studied with the objective of developing 

guidelines for designing stable pillars and roof spans (Esterhuizen 

2007a). 

Only surface features were observed during the initial phase of 

data collection at the 34 mines. Natural fractures such as joints, 

induced damage, such as blast damage in the pillars or the mine 

roof, and pillar load damage such as weak band loading or pillar 

spalling were all observed during data collection (Esterhuizen 

(2007b). Examination of mine roof using a borescope presented the 

opportunity to observe features that were not visible from the drift. 

BACKGROUND 

Underground stone mines pose numerous workplace safety 

problems. A major concern for all underground mines is the 

integrity of the mine roof and its potential to fall, whether it simply 

creates an obstacle, damages equipment, or causes harm to a 

worker. Engineers and geologists have developed rating systems for 

well over 100 years that attempt to use numerical values to improve 

workplace safety by predicting ground instability, stability. Ritter 

(1879) is generally acknowledged as the first to attempt a formal 

empirical approach to tunnel design to be used to establish support 

requirements. Further efforts to evaluate a rock mass have been 

developed since Ritter’s work because there is a pronounced need 

to tie the nature of the rock mass to engineering design. Currently, 

RQD as developed by Deere (1964, 1968) is commonly used during 

field logging of core to provide an assessment of potential rock 

quality. RQD is essentially a measure of the percentage of core 

that has a length component that is at least double its diameter, thus 

assessing the fractures within the rock structure. Other techniques 

of classifying rock masses, such as RMR, incorporate joint spacing, 

character of the joint surfaces, and orientation of joints relative to 

the load direction (Barton, 1974; Bieniawski, 1976, 1979; Hoek, 

1995). 

Efforts to use data collected with a borescope is now appearing 

in the literature. Malkowski et al., demonstrated the utility of an 

Endoscopic Rock Mass Factor (ERMF) in Polish underground 

coal mines (2008). Rock masses are rated across a range of six 

classes varying from intact rock (Class I) to a completely crushed 

rock mass (Class VI). Equipment is readily available to examine 

and record features in mine roof (Locotos, 2009). The equipment 

available today is relatively low in cost and easy to operate. Data 

collected from scoping of mine roof can also be an additional aid 

in supplementing a Roof Fall Risk Index (RFRI) (Iannacchione, 
 

 

 

2007). After conditions have been mapped, they can be reviewed 

to assess changes. 

EQUIPMENT AND PROCEDURES 

The borescope and recording instrumentation fits neatly in a 

single carrying case, and a set of rods that attach to the camera are 

carried separately. The borescope is cylindrical and small enough 

to operate in a 1-in (25 mm) borehole (Figure 1). The camera 

attaches to a small distribution box and the video is seen on a screen 

and an audio narrative through a microphone can also be provided. 

The recording unit is an MP3 recorder and the files are saved on a 

digital card. The rods that are used to move the camera about in the 

borehole are indexed to provide the depth to the camera location. 

Files are readily transferred to a personal computer. Viewing the 

videos usually requires the installation of a CODEC1. 

1 Mention of company name or product does not constitute endorsement by the National 

Institute for Occupational Safety and Health. 

Logs can 

then be generated from the video files. The video files are easily 

stored, and generally require less that 40 MB of storage space for 

an entire hole. In view of this file size, a 2 GB card will store the 

information from about 50 holes. 

Figure 1. Borescope in carrying case. A) Camera, B) Battery, C) 

On/Off power distribution, D) Microphone, E) Video Monitor, 

and F) MP3 Recorder. 

In some cases, the borehole wall will be covered with debris 

from the drilling process, and it is necessary to clean the hole. A 

garden sprayer equipped with a brush or roller can be used to clean 

the wall of the borehole. 

FEATURES OBSERVED 

NIOSH scoped roof holes at 13 stone mines in six states. A 

total of 71 holes were logged, consisting of over 700 feet of hole. 

Considering the overall amount of hole length examined, the 

features observed on a regular basis were relatively few. Stylolites, 

partings, contacts and cracks (later split into closed or open cracks) 

constituted nearly all of the features observed. 

The most common feature or discontinuity observed was a 

stylolite. Stylolites seen in the borescope are visible as a dark gray 

to black jagged discontinuity. The filling is often clay, iron oxide, 

or carbon. Stylolites develop after the deposition of the limestone 

and result from compaction and pressure solution during the life of 

the rock (Figure 2a). 

The second most common feature observed is a carbon-filled 

parting. Carbon partings are observed as a black trace or bedding 

plane in the borescope (Figure 2b). The filling in the parting may 

actually be carbonaceous or dark colored clay and is representative 

of a bedding plane. Partings, by their nature, tend to have lower 

cohesion with the rock above and below them than a continuous 

bed of rock, and by their nature would represent a weaker plane 

than stylolites. 

The third type of feature observed was a lithologic change. 

The exposed roof in all the mines visited was limestone. In some 

cases, the rock observed along the entire hole was limestone, and 

no change of lithology was exposed. In other cases, the limestone 

was found to be overlain by shale or sandstone. An example of a 

lithologic contact is shown in Figure 2c. 

The final feature observed while borescoping was a crack. 

A crack can be either a failure within a bed, or a separation 

where contrasting units meet, and may be of geologic origin or 

mining induced. Most of the cracks observed in this study were 

encountered in locations that one would consider to be natural 

planes of weakness such as limestone to shale contacts or bedding 

planes that would be contributed to mining. For purposes of this 

work, cracks are further distinguished as “closed” if the aperture 

is less than 1/32 in (1 mm) or “open” if the aperture exceeds 1/32 

in (1mm) (see Figure 2d). The immediate concern is the extent of 

the crack. That is, does it occur over a large area, such that the 

drift span can be compromised or is the damage confined to a few 

square feet, thereby reducing the probability of contributing to roof 

failure. This is addressed by assigning a more significant weight 

to open cracks than closed cracks. Figure 3 shows the frequency 

distribution of features encountered in the 71 holes logged during 

this study. 

2a) Stylolite 2b) Parting 

2c) Lithologic Change 2d) Crack 

Figure 2. Photographs of features encountered while borescoping. 

2a) Stylolite, 2b) Carbon parting, 2c) Lithologic change, and 2d 

Open Crack. 

RATING AND ASSIGNMENT 

This work is directed toward the development of a rating system 

that is mine specific, i.e., the numerical rating of a given hole at one 

mine is not intended to be related to a different mine, but should 
 

relate to other holes at the same mine. Characteristic features of 

the mine roof for a given mine are identified and assigned to zones 

and weighted to give the resultant numerical value. Each borehole 

is reviewed individually. The example presented in Figure 4 shows 

several typical holes. Each feature is assigned a frequency value 

and a zone weight value. The values are multiplied together, and 

added to give zone weights and the zones are added together to 

yield total hole values. These factors will be discussed in more 

detail below. 

 

Percent

70 

60 

50 

40 

30 

20 

10 

0 

% Total Holes Affected by 

Discontinuity 

Stylolites Partings Contacts Closed Open Cracks 

Cracks 

Discontinuity Type 

Figure 3. Relative frequency of discontinuities observed in 71 

holes. 

General discussions with mine personnel have resulted in the 

assumption that the features encountered in the roof tend to have 

less impact on roof control when they are encountered more than 4 

ft from the roof horizon (Esterhuizen 2007b). The roof zone rating 

is included to take into account the position of the feature in the 

roof. A higher weighting value is generally assigned to zones that 

are closest to the exposed roof. The thickness of each zone can 

be adjusted to fit the specific site, as well as the number of zones. 

Three or four zones are recommended with a higher weighting 

value assigned to the immediate roof. There may be exceptions to 

the normal weighting profile such as a situation where mechanical 

bolts are being installed in which case, the zone including the 

anchorage horizon may be weighted more heavily. 

The first step in developing a mine specific rating system is to 

examine several holes that can be assumed to be representative of 

the mine roof. The holes should be spread across the mine and 

located to include the variety of conditions that may be expected 

throughout the mine. Frequency values are established by 

identifying the features that are seen in a given zone of the roof and 

assigning a numerical value to the feature for that zone. 

The feature weight allows for characteristics to be given more 

detailed consideration with regard to the zone of the roof where 

they are encountered. For example, a lithologic change in the roof 

that is encountered in the first foot or two of the roof may cause 

greater concern than if it is encountered at a depth of 7 ft. By 

separating and adjusting the characteristic weight in each zone, the 

changes in anticipated impact are easily recognized (see Figure 5). 

When features that pose concerns are identified in a hole, the 

weighting of severity and location will produce a number that is 

notably different from holes with no such feature. The higher value 

should immediately alert the operator to do further examination to 

establish the extent of the concerns and conduct appropriate actions 

to compensate for the condition. 

Although the rating presented is intended as a template that 

should be altered to provide emphasis on features of concern at a 

specific mine, it was applied to the holes from all the mines in the 

study to establish variability. The rating values were found to range 

from a low of zero to a high of 326. Figure 6 shows the distribution 

of rating values. Cracks and open cracks that were encountered in 

the first two to four ft (0.7-1.3m) were weighted more heavily when 

cracks were encountered close to the immediate roof resulting in 

high rating values. Although roof rating index values of less than 

100 would appear to carry reduced concern of failure, any abrupt 

relative change on an area may indicate the need for further 

scrutiny. In a situation where borescoping is being done routinely, 

it would be desirable to drill additional holes to establish the extent 

of an open crack or other feature that results in an increase from the 

surrounding ratings. Support or travel patterns may then be altered 

if warranted. 

MAPPING

Data collected from borescoping can readily be stored on a 

computer and designated on a map to provide a stability assessment. 

An example of a mine map showing borescope weighted values is 

shown in Figure 7. Note that one of the values is unusually high. 

Since the values on the map relate to a rating and assignment like 

the one described earlier, the high value should immediately draw 

attention. A further assessment with additional holes would then 

conducted to establish the extent of the area influenced by the 

feature that produced the high rating. As the amount of data on the 

map increases, it becomes possible to contour values and analyze 

trends. 

COST AND BENEFIT 

The cost of borescoping by a mine operator can be highly 

variable and is largely a function of the cost of the borescope, the 

frequency of use, and the level of record maintenance. Several 

quality borescopes are available, and a high quality instrument with 

recording capabilities can be acquired for under $5,000. After the 

purchase of equipment, the cost to an operation will be driven by 

the amount of data desired, and whether records of examinations 

are accumulated. While one operator may elect to scope a single 

hole at each intersection to confirm roof horizon control, and 

not maintain written records as long as results confirm expected 

conditions, a different operator may elect to examine a hole from 

each round and record the data in a CAD program for future 

reference. 

The borescopes are relatively easy to use and may be operated 

with little training. The unit used in this study simply requires that 

the camera, microphone and MP3 player be switched on, and the 

MP31 player set to “record.” An engineer or geologist should be 

able to recognize features in the roof. Since there are relatively 

few features encountered in the boreholes, virtually any interested 

party can be trained to operate the borescope. If borescoping is not 

performed as part of the mining cycle, more time will probably be 

spent in moving from hole to hole than is involved in the scoping 

effort. The examination time for a single hole is likely to take 

about 10 to 15 minutes. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Date Date Date Date Date 

No.: 1 No.: 2 No.: 3 No.: 4 No.: 5 

Level: Level: Level: Level: Level: 

Loc: Loc: Loc: Loc: Loc: 

Rating 61 54 12 138 81 

Oper. JLE Oper. JLE Oper. JLE Oper. JLE Oper. JLE 

X 

X X Sandy SH 

8' Sandy SH 8' SHALE 8' 8' 8' Sandy SH 

Sandy SH SHALE Sandy SH 

Sandy SH SHALE X Sandy SH 

Sandy SH Sandy SH SHALE Sandy SH 

7' Sandy SH 7' Sandstone 7' SHALE 7' 7' Sandy SH 

SHALE SHALE SHALE Sandy SH 

SHALE SHALE SHALE Sandy SH 

SHALE SHALE SHALE Sandy SH 

6' Sandstone 6' Sandy SH 6' SHALE 6' 6' SHALE 

SHALE Sandy SH SHALE SHALE 

SHALE Sandstone SHALE SHALE 

SHALE SHALE SHALE SHALE 

5' SHALE 5' SHALE 5' SHALE 5' 5' SHALE 

SHALE SHALE SHALE SHALE 

SHALE SHALE SHALE SHALE 

SHALE SHALE SHALE SHALE 

4' SHALE 4' SHALE 4' SHALE 4' 4' SHALE 

SHALE SHALE SHALE 

Sty SHALE SHALE SHALE 

SHALE SHALE 

3' Sty 3' 3' 3' 3' SHALE 

Sty Open Crack 

Sty Sty X 

Sty Open Crack 

2' 2' 2' 2' 2' 

Sty Sty Sty 

Closed Crack 

Sty Sty 

1' 1' 1' 1' Sty 1' Sty 

Sty 

Sty 

Figure 4. Logs of hole showing significant features and locations. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Zone 

Characteristic 

(Char) Frequency 

Char 

Weight 

Zone 

weight 

Item 

Value 

A (0-2') Stylolite 1 10 0 

A Lith change 3 10 0 

A Parting 2 10 0 

A Crack <1/32 6 10 0 

A Crack >1/32 10 10 0 

A Other 10 0 

B (2-4') Stylolite 1 6 0 

B Lith change 2 6 0 

B Parting 1 6 0 

B Crack <1/32 5 6 0 

Crack >1/32 8 6 0 

B Other 6 0 

C (4-6') Stylolite 1 3 0 

C Lith change 1 3 0 

C Parting 1 3 0 

C Crack <1/32 4 3 0 

Crack >1/32 7 3 0 

C Other 3 0 

D (6-td) Stylolite 1 1 0 

D Lith change 1 1 0 

D Parting 1 1 0 

D Crack <1/32 2 1 0 

Crack >1/32 5 1 0 

D Other 1 0 

Composite hole rating 0 

Figure 5. Sample rating and assignment table. The total rating 

value will appear when frequency values identified in the hole 

are entered. 

 

0 

5 

10 

15 

20 

25 

30 

35 

40 

Percent

Percent of Holes by Borescope 

Rating 

0-25 26-50 51-75 76-100 100+ 

Borescope Rating 

Figure 6. Distribution of borescope ratings for 71 boreholes. 

Figure 7. Mine map showing borescoped hole locations and roof 

index ratings. 

CONCLUSIONS 

A number of features were observed in this study during 

borescoping. However, not all features contribute significantly to 

roof weakening. For example, although stylolites were encountered 

frequently, they rarely created a plane of weakness that threatened 

ground stability. Contacts, or lithologic changes, can be significant, 

but generally have less effect as their distance into the roof 

increases. Partings represent inherent planes of weakness. Cracks 

in the roof are a cause for concern and as a result should be given 

increased weight in the evaluation. Closed cracks tend to be local 

while open cracks are more likely to involve a more extensive area. 

The following conclusions are supported by this study of stone 

mine roof: 

1.	 borescoping results in improved recognition of general roof 

conditions, 

2.	 the level of security by ‘knowing the conditions’ is improved 

compared to the uncertainty of assuming what is there, 

3. borescoping can aid in roof horizon control, 

4. changes in mine roof can be mapped and reviewed, and 

5. the thickness of the main roof beam can be confirmed. 

When there is a question about the integrity of a stone mine roof, 

borescoping is an excellent tool to establish the nature of the roof 

and its stability. The equipment is easy to use, and can be used as 

much or as little as needed. Each mine will tend to have its own 

characteristic roof features, so that recognition training can be 

undertaken with a short learning curve. It is recommended that site 

specific ratings be used to provide emphasis on roof features that 

are problematic at any given mine. 

Ultimately, NIOSH plans to incorporate the borescoping 

assessment into a Roof Fall Risk Assessment procedure that will be 

used to assist in span design for underground stone mines. 

DISCLAIMER 

The findings and conclusions in this report have not been 

formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent agency 

determination or policy. 

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Barton, N.R., Lien, R., Lunde, J., (1974). Engineering 

Classification of Rock Masses for the Design of Tunnel 

Support. Rock Mechanics and Rock Engineering 6:189-239. 

Bieniawski, Z.T., (1976). Rock Mass Classification in Rock 

Engineering. Proceedings of the Symposium on Exploration 

for Rock Engineering, Johannesburg, pp.97-106. 

Bieniawski, Z.T., (1979). The Geomechanics Classification in Rock 

Engineering Applications. Proceedings of the 4th International 

Congress on Rock Mechanics, Montreux, 1979, pp.41-48. 
 

Deere, D.U., (1964). Technical Description of Rock Cores for 

Engineering Purposes. Rock Mechanics and Geology. Volume 

1, Number 1, pp. 17-22. 

Deere, D.U., (1968). Geologic Considerations. In: Rock 

Mechanics in Engineering Practice. Ed. K.G. Stagg and O.C. 

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in Underground Limestone Mines in the United States. 

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F.M. Locotos Co, Inc., (2008). Mine and Tunnel Roof Control 

Products. Retrieved November 18, 2009, from website http:// 

www.fmlocotos.com/Scope_Package.pdf 

Malkowski, P., Niedbalski, Z., Majcherczyk, T., (2008). Endoscopic 

method of rock mass quality evaluation – new experiences. 

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2nd U.S.-Canada Rock Mechanics Symposium, San Francisco, 

CA, USA. 

Ritter, W., (1879). Die Statik der Tunnelgewolbe. Berlin Springer.