EVALUATION OF MINE SEALS USING GROUND PENETRATING RADAR 

William D. Monaghan 

Michael A. Trevits 

Michael J. Sapko 

NIOSH, Pittsburgh Research Laboratory 

Pittsburgh, PA 

Abstract

Mine seals are used extensively in underground coal mines to segregate mined-out areas and to 

isolate fire zones or underground areas that are susceptible to spontaneous combustion. Over the years, 

30,000 mine seals have been erected in underground coal mines in the United States. Mine seals, along 

with rock dusting and ventilation, represent the fundamental means of preventing underground coal 

mine explosions. In this study the National Institute for Occupational Safety and Health (NIOSH) used 

ground penetrating radar (GPR) technology to determine if each of three test mine seals were uniformly 

constructed. Three mine seals representing different construction methods and materials (cast-in-place 

foamed cement; solid block, polyurethane foam and gravel; and wire mesh, rebar and gunite) were 

erected at the NIOSH Lake Lynn Laboratory’s underground mine as part of an on-going research 

program. The seals were imaged using GPR with 400-, 500-, 900-, and 1,000-MHz antennas. We found 

that variations in uniformity existed in the material for the cast-in-place foamed cement seal. Variations 

in uniformity also existed in the seal made from solid block, polyurethane foam and gravel. The postprocessed radar records correlated closely with recovered core samples (areas of gravel and 

polyurethane foam, and areas containing only polyurethane foam). Finally, the radar records of the seal 

made of wire mesh, rebar and gunite showed the various components. The results of this work suggest 

that GPR could be used as a tool to evaluate mine seal characteristics and construction uniformity. 

Introduction 

 Over the years, 30,000 seals have been erected in underground coal mines in the United States. 

Seals, along with rock dusting and ventilation, constitute the dominant portion of the last line of defense 

against underground coal mine explosions. Explosion-resistant underground mine ventilation seals 

serve several functions including protection from propagating explosions and separation of air between 

active and worked out mine areas (Weiss, 1993). Seals are also used to isolate active fire zones or areas 

susceptible to spontaneous combustion. (Sapko, 2003). 

 During the 1990s, seven documented explosions of methane and/or coal dust occurred within 

sealed areas of underground U.S. coal mines (Hurren, 1993; Scott, 1996). These explosions, believed to 

be initiated by lightning strikes on the surface, destroyed several seals and caused considerable damage 

in the active underground mine workings. Fortunately, these explosions did not cause fatalities or 

injuries. However, the potential for a disaster exists, emphasizing the need for explosion-resistant seals 

that can perform under various mining and environmental conditions (Sapko, 2003). In order for seals 

to perform as expected, their as-built condition must conform to the engineering design specifications. 

 Title 30, Part 75.335 of the U.S. Code of Federal Regulations (CFR) states that abandoned 

areas of underground coal mines must be either ventilated or isolated from active workings through the 
use of seals capable of withstanding a static horizontal pressure of 20 psi (Code of Federal Regulations, 

2004). To effectively isolate areas within a mine, a seal should be designed to control the methane gas 

and air exchange between the sealed and open areas so as to prevent toxic and/or flammable gases from 

entering the active workings. A seal must also be capable of preventing an explosion from propagating 

into or out of the sealed area. 

 NIOSH has evaluated the use of a chamber technique for pressure loading full-size seals using 

compressed air, water, or confined gas explosions (Sapko, 2003). The end-product of this research 

effort is the development of alternative methodologies to better characterize the strength properties of 

mine seals and their ultimate interaction within the surrounding mine strata (NIOSH, 2002). Currently, 

no reliable method exists to inspect or confirm the condition of an erected mine seal underground 

without drilling or dismantling a portion of the seal and jeopardizing its structural integrity. 

 Ground penetrating radar (GPR) has been used in agricultural, archeological, construction, 

environmental, forensic, geological, groundwater, military, and mining applications (Olhoeft, 2004)1. 

 

1

 Additional information on GPR can be found the Internet using search terms such as “ground penetrating radar”, “GPR”, 

“georadar”, “ground probing radar”, “subsurface radar” etc. 

 In 

some cases, GPR is used as a non-invasive method to examine the as-built conditions of structures. A 

GPR system generates an electromagnetic pulse that is transmitted into the ground with an antenna that 

is moved along the surface, generally at a uniform speed and direction. Whenever there is a change in 

the dielectric constant of the subsurface material, a portion of the pulse energy is reflected back to the 

surface and is detected by the receiving antenna. This reflected pulse provides information about 

changes in electrical properties of the subsurface material. The recorded two-way travel time is 

determined by measuring the time interval between the start of the transmit pulse and start of the 

received reflected signal. The amplitude of the reflected signal is influenced by the material type, size 

and geometry of the target, the signal attenuation characteristics of the material, and the total distance 

that the pulse has to travel. 

 In this study, GPR was used as a non-invasive means of evaluating mine seal construction 

uniformity. The equipment used was a GSSI SIR® System 2 (SIR-2) Model No. DC-2 control unit built 

by Geophysical Survey Systems, Inc.2

 

2

Mention of a specific product or trade name does not imply endorsement by NIOSH. 

The SIR-2 is a lightweight, portable, general-purpose radar 

system and is available as an intrinsically safe unit for use in the underground coal mine environment 

(figure 1). 

Figure 1.: Engineer using the SIR-2 unit.

The reflected pulse is processed by the control unit 

and the data are displayed on the monitor and stored on an 

internal hard disk. The output display can be (1) a single 

wiggle trace (analogous to an oscilloscope trace), (2) a 

waterfall plot of the wiggle traces, or (3) a multicolored 

line scan in which the reflected signal amplitudes are 

represented by various colors according to a user-selected 

color look-up table. During this study, the dielectric 

constant was adjusted in the SIR-2 unit until the arrival 

time of the reflected signal (expressed in term of depth) 

approximated the thickness of the mine seal. As a result of 

the tests, a dielectric constant of 4.5 was used for Mine Seal 

Nos. 1 and 2 and a dielectric constant 6.0 was used for 
Mine Seal No. 3. The GPR records generated during this study were analyzed using GSSI’s Radar Data 

Analyzer for Windows (RADAN) version 6.02

. This package allows the user to operate in the Windows 

environment with application-specific modules (GSSI, 2004). 

Objective and Approach 

The objective of our work was to determine if GPR could be used to detect changes in the 

composition of three Mine Safety & Health Administration approved type of mine seals that could 

potentially indicate areas of structural weakness. We approached this problem by using four shielded 

antennas whose frequency spectra produced pulses centered at near 400-, 500-, 900-, and 1,000-MHz. 

These antennas were selected because they could provide the required depth of penetration with the 

necessary resolution, are relatively lightweight, and could be easily positioned and moved along the face 

of the seals. 

The mine seals were constructed in the NIOSH Lake Lynn Experimental Mine (LLEM). The 

Lake Lynn Laboratory (LLL) is a unique research facility, located about 50 miles southeast of 

Pittsburgh, Pennsylvania, that is designed to provide a full-scale mining environment for the testing and 

evaluation of mine health and safety technologies. The LLL occupies more than 400 acres and is 

composed of surface test and training areas and an underground mine (figure 2). 

Figure 2.: Photo of the Lake Lynn Laboratory. Various 

test areas are shown (buildings and open areas). The red 

circle shows the main entrance to the underground mine. 

The LLEM was built at an 

abandoned commercial limestone 

quarry where underground entries 49-

ft wide by 33-ft high (old workings) 

were developed when surface mining 

ceased in the late 1960's. Later, under 

the auspices of the U.S. Bureau of 

Mines, 7,545 ft of new underground 

development was constructed using 

19.7-ft wide by 6.6-ft high entries 

(new workings). The new workings 

can be configured to simulate modernday coal mining scenarios; including 

room-and-pillar and longwall mining 

layouts (NIOSH 1999, Triebsch, 1990) 

(figure 3). 

Two large-scale underground chambers were constructed in the mine to conduct pneumatic, 

hydrostatic, or explosion pressure loading of mine seals. The large chamber measures 29.9-ft wide by 

16.1-ft high by 10.2-ft deep with a maximum cross-sectional area of 481.4 ft2

. The smaller of the two 

chambers is 20-ft wide by 8-ft high by 10.2 ft deep and can accommodate a seal design with a crosssectional area up to 160 ft2 (Sapko, 2001). 
Figure 3.: Layout of the Lake Lynn Laboratory Mine. 

Mine Seal No. 1 

 Mine Seal No. 1, a cast-in-place foamed cement seal, was constructed in the large chamber in the 

LLEM and building the form was the first step in the construction process. The vertical components of 

the front and back portions of the form were built from 6.5-in by 5-in posts that extended from the mine 

floor to roof. A post was placed at both ribs and additional posts were spaced between the rib posts on 

35-in centers. The void space behind the posts in the chamber was 8-ft on the left side and 6-ft on the 

right side. Next, 8-in wide by 1-in thick boards 

were placed in a horizontal position and were nailed 

to each post (figure 4). 

Figure 4.: Photo of inside of Mine Seal No. 1 

showing the wooden form. 

The boards were placed at 

the bottom of the form and an 8-in horizontal space 

was left between each row of boards. This 

procedure was followed on the front and back walls 

of the form. Fiberglass insulation was then packed 

into any void space on the mine roof, ribs, and floor 

areas to minimize slurry leakage around the form. 

A 48.5-in space was left by design between the 

front and back walls of the form for the mine seal. 

Mine brattice cloth was placed on the inside area of 

the form, was overlapped on the mine roof and rib 

areas, and was secured with straps to form a tight 

seal. Eye bolts were then attached to each post 

through the brattice cloth and were tied together 

with 1/8-in galvanized steel cable that was extended 

between the front and back of the form (figure 5). 

The eye bolts were spaced approximately 32-in apart on each post and were used throughout the vertical 

extent of the form. The eye bolts and cable served to tie the front and back walls of the form together 

and assisted in keeping the walls parallel and straight. 
Figure 5.: Photo of inside of mine seal 

No. 1 before foamed cement was added. 

The mine seal was constructed in a series of 

three lifts over a three-day period. The seal mixture 

used was made of 18 gallons of foam concentrate, 

approximately 436 gallons of water and 989 bags of 

Quikrete2

 cement. The water and foam concentrate was 

mixed together and was added to the cement at a rate of 

8.5 gpm. The resulting slurry was then pumped into the 

form. The first lift was poured to a height of 72.5-in 

above the mine floor and was left to set overnight. The 

next day, before the second lift was poured, it was 

observed that a hard surface had formed at the top of 

the lift. The second lift was poured to a height of 77.5-

in (or 150-in above the mine floor) using the same 

mixing procedure and was also allowed to set overnight. As had been observed the previous day, it was 

noticed that a hard surface had formed at the top of the second lift. The third lift was constructed using 

the same procedure as the first two lifts to fill the remaining 43.2-in space to the mine roof. The overall 

dimensions of the resulting mine seal were 29.9-ft wide by 16.1-ft tall by 4-ft thick. 

The seal material was cured for 33 days before the GPR evaluation was performed. In order to 

gain direct access to the face of the seal, three vertical sections were cut into the wooden form and 

brattice cloth. The three vertical sections were approximately 8-in wide and exposed the seal material 

from the mine floor to roof. Three vertical GPR survey lines were positioned along these sections 

shown in figure 6. 

Figure 6.: Photo of mine seal No. 1 and 

location of GPR survey lines. 

The vertical sections were located at 4-ft from the left rib (line 1), about 12-ft from 

the right rib near the middle of the seal (line 2), and 4-ft from the right rib (line 3). Each vertical section 

was subdivided into one-foot segments for use as reference points during the GPR evaluation. 

GPR Evaluation 

The GPR survey of each section was conducted in 

two parts because of the excessive height of the seal. The 

initial portion of each GPR survey line extended from the 

mine floor area to a height of 8-ft and the second portion 

extended from a height of 8-ft to the mine roof (16.1-ft). 

Since the thickness of the mine seal was known, it was 

decided to use 400-, 500-, and 900-MHz antennas to 

image the seal. After a short series of tests, it was 

determined that the 900-MHz antenna could not penetrate 

the entire thickness of the mine seal and was not used 

further. Also, surveys of the lower section of the mine 

seal were only conducted with the 400-MHz antenna 

because of technical problems with the antenna. The 

setup parameters used for each survey using the 400-and 

500-MHz antennas are shown in table 1. 

A total of 25 surveys were made of the mine seal. Each survey was made by placing the antenna 

against the seal face and then moving downward at a constant velocity. Reference points were placed 
into the record corresponding to the 1-ft marks on the mine seal. In this manner, it was easy to associate 

any observed anomalies with the specific location on the mine seal. A representative post-processed 

radar record and the associated interpretations are shown in figure 7. The vertical scale in the figure 

represents the height above the mine floor and the horizontal scale represents depth into the seal. The 

reflected pulse energy is shown in terms of a color scale, with hot colors (yellow, white) being the 

higher level of reflected pulse energy and colder colors (brown, black) the lower levels. Areas in the 

record shown with similar colors should be interpreted as similar levels of reflected pulse energy. In the 

radar record, the top of the lift is visible along with the back of the seal. As discussed earlier, the top of 

a lift represents a boundary in the seal. Figure 8 shows the positions of all observed anomalies from the 

GPR surveys of the mine seal. As shown in figure 8, a total of 12 anomalies were observed in surveys 

where the 500-MHz antenna was used and four anomalies were observed in surveys where the 400-MHz 

antenna was used. Of the anomalies observed, 11 were located near the top of a lift and were either 

related to the hard surface that had formed at the top of the lift or areas where the seal components could 

have been mixed at different ratios. The five remaining anomalies could be related to the construction 

practice of the mine seal (use of eyebolts and galvanized wire). 

Table 1.: SIR 2 set-up used for GPR survey of Mine Seal No. 1. 

Parameter 400-MHz Antenna 500-MHz Antenna 

Data collection mode Continuous Continuous 

Range, ns 25 25 

Samples per scan 512 512 

Resolution, bits 16 16 

Number of gain points 5 6 

Vertical high pass filter, MHz 60 50 

Vertical low pass filter, MHz 800 1,000 

Scans per second 32 32 

Horizontal Smoothing, scans 4 4 

Transmit Rate, KHz 64 64 

Dielectric Constant 4.5 4.5 

Figure 7.: Interpreted radar record (Line 2) for 

Mine Seal No. 1 using a 400-MHz antenna. 
Figure 8.: Location of observed anomalies from radar 

scans of mine seal No. 1. 

Mine Seal No. 2 

Mine seal No. 2, a solid block, 

polyurethane foam, and gravel seal, was 

erected in the small chamber in the LLEM. 

The seal was constructed 8-ft high by 20-ft 

wide and about 2.5-ft thick. The seal was 

not hitched into the mine roof, rib or floor 

areas. The seal was built using two parallel 

walls of solid blocks with a 17.5-in open 

space between the walls. The walls were 

constructed with dry-stacked solid blocks 

(each measuring 6-in by 8-in by 16-in) and 

was capped by 2-in by 8-in by 16-in blocks 

and wedges at the mine roof (figure 9). 

Figure 9.: Photo of solid block walls for Mine Seal No. 2.

The open space between the block walls was 

filled in a series of lifts using gravel and 

polyurethane foam. The polyurethane 

material was poured into the gravel bed and was supposed to infiltrate the gravel bed before it began to 

foam. By design, as the resin turned to foam, it was supposed to pick-up and disperse gravel in the 

resulting matrix. For each lift, gravel was placed to a depth of 10- to 12-in. Polyurethane resin was then 

poured over the gravel bed at a rate of about 2.5 gpm and the resulting foam rose to a height of about 10-

in above the level of the gravel bed (figure 10). The core of the seal was constructed in 7 lifts over a 

5.5-day period and any remaining open space above the lifts was then closed to the mine roof using 

injected polyurethane foam. In all, the seal was built using 292 blocks, 10,350 lbs of limestone and 

2,000 lbs of polyurethane resin. Once the inner portions of the seal were built, the exposed face of the 

seal was coated with B-Bond2

 to a maximum thickness of 0.25-in. 

GPR Evaluation 

GPR was used to determine if the 

gravel had been evenly dispersed

throughout the mine seal. For the radar 

survey, two parallel vertical GPR survey 

lines were drawn on the mine seal face 

and were positioned about 6 ft from each 

mine rib (figure 11). Reference points 

(labeled A-F), spaced about 18-in apart, 

were then placed on the seal and were 

added to the radar records for

comparative purposes. Each survey was 

made by placing the antenna against the 

seal face and then slowly moving

downward at a constant velocity. The

radar survey of the seal was made using 

1,000-MHz antenna because this antenna 
provided the necessary depth of penetration and the desired level of resolution. A total of 52 surveys 

were made of the mine seal. Table 2 shows the setup parameters used for each survey. 

 

Table 2.: SIR 2 set-up used for GPR survey of Mine Seal No. 2. 

Parameter 1,000-MHz Antenna 

Data collection mode Continuous 

Range, ns 16 

Samples per scan 512 

Resolution, bits 8 

Number of gain points 4 

Vertical high pass filter, MHz 250 

Vertical low pass filter, MHz 2000 

Scans per second 32 

Horizontal Smoothing, scans 4 

Transmit Rate, KHz 64 

Dielectric Constant 6.0 

Figure 10.: Photo of inner core of Mine Seal 

No.2 during addition of polyurethane resin. 

Figure 11.: Photo of Mine Seal No. 2 showing 

GPR survey lines. 

Figures 12 and 13 show two representative post-processed radar records and their associated 

interpretations. Reflected pulse energy is shown in terms of a grayscale, with near-white being the 

highest level of reflected pulse energy and near-black the lowest. Areas in the record shown with 

similar gray tones should be interpreted as similar levels of reflected pulse energy. The vertical scale 

shown in the figures corresponds to the reference points (labeled A-F) shown in figure 11. 

In the radar records (figures 12 and 13), it is possible to see both the front and back block walls. 

It was hypothesized that the area between the block walls showed varying amounts of reflected pulse 

energy because of the concentration of gravel in the polyurethane foam was most likely not consistent 
throughout the seal. To confirm this hypothesis, several large coreholes (6- by 18-in) were drilled into 

the seal after the GPR study was completed (figure 14). For this work, the block face of the seal was 

removed to facilitate core recovery. Figure 15 shows an example of a recovered core sample. This 

sample clearly shows a lift boundary and a detailed analysis of this core sample showed that the gravel 

was not evenly dispersed in the polyurethane foam. This confirms the hypothesis made from the radar 

records. 

Figure 12.: Interpreted radar record for Mine Seal No. 2, Line 1. 

Figure 13.: Interpreted radar record for Mine Seal No. 2, Line 2. 
Figure 14.: Photo of Mine Seal No. 2 showing location of coreholes. 

Figure 15.: Photo of recovered core sample from Mine Seal No. 2. 

 Refer to figure Nos. 13 and 14. 

Mine Seal No. 3 

Mine Seal No. 3 was erected in the small chamber in the LLEM. The seal was constructed 8-ft 

high by 20-ft wide and 11-in thick and was built using Stayfoam2

 panel backing and Insteel 3D2 wire 

mesh panels. The panels were anchored to the mine roof rib and floors with No. 8 reinforcing bar 

(rebar) that was drilled and set into the exposed strata. Across the front of the seal, the segments of 

rebar that were anchored into the roof and floor strata were affixed with steel strapping to vertical 

sections of rebar and the wire mesh panels (figure 16). The entire face of the seal was then sprayed with 

Riregrete2

 gunite to complete the construction. 
Figure 16.: Photo of Mine Seal No. 3 showing the mesh 

panels and reinforcing bars. 

GPR Evaluation 

GPR was used to determine if the metal construction components could be resolved. For the 

radar survey, three perpendicular horizontal lines (lines 1-3) were positioned on the seal with the first 

line located about 6-in below the mine roof interface and the remaining two lines spaced about 24-in 

apart. In addition, two vertical lines (lines 4-5) were positioned approximately 4-ft from each mine rib. 

Reference points, spaced 24-in apart, were added to each survey line and these points were also added to 

the radar records (figure 17). 

Figure 17.: Photo showing Mine Seal No. 3 with GPR survey 

lines. 

Figures 18 and 19 show 

representative post-processed 

radar records of Mine Seal 

No. 3 and their associated 

interpretations. The reflected 

pulse energy is shown in terms 

of a color scale, with hot colors 

(yellow and bright green) being 

the higher level of reflected 

pulse energy and colder colors 

(brown and black) the lower 

levels. Areas in the record 

shown with similar colors 

should be interpreted as similar 

levels of reflected pulse energy. 

The vertical scale shown in the 

figure corresponds to the 

 Each survey was made by placing the antenna against the seal face and 

then moving along the line at a constant velocity. The radar survey of the seal were made using 1,000-

MHz antenna because this antenna could provide the necessary depth of penetration and the desired 

level of resolution. A total of 22 surveys were made of the mine seal and table 3 shows the setup 

parameters used for each survey. 
reference points shown in figure 17. In the radar records, two layers of wire mesh can be seen. The 

Stayfoam2

 panel however cannot be discerned from the back of the second wire mesh panel most likely 

because it was attached to the panel. This also explains the higher energy reflections seen in the deeper 

portions of the radar records (shown as back mesh areas in the records) because the Stayfoam2 panel had 

a closely spaced mesh and thus a larger reflective surface. 

Table 3.: SIR 2 set-up used for GPR survey of Mine Seal No. 3. 

Parameter 1.2-GHz Antenna 

Data collection mode Continuous 

Range, ns 20 

Samples per scan 1024 

Resolution, bits 16 

Number of gain points 4 

Vertical high pass filter, MHz 1,000 

Vertical low pass filter, MHz 3,000 

Scans per second 32 

Horizontal Smoothing, scans 4 

Transmit Rate, KHz 64 

Dielectric Constant 6.0 

Figure 18.: Interpreted radar record for Mine Seal No. 3, Line 1.
Figure 19.: Interpreted radar record for Mine Seal No. 3, Line 3. 

Summary and Conclusions 

In this study, GPR technology was used to determine if mine seal construction materials could be 

non-invasively inspected in place. Three mine seals that were built of widely different materials were 

evaluated at the NIOSH Lake Lynn Laboratory Experimental Mine. An analysis of the resulting radar 

records for each seal shows that GPR can indeed be used to delineate mine seal components. The results 

of this work suggests that mine seals could be evaluated using GPR to determine if the seal was 

constructed as designed with all seal components in place. The study implies that GPR can also be used 

to identify material uniformity and anomalies within a seal that could indicate areas of structural 

weakness. Finally, it is suggested that additional research should be conducted to determine if GPR can 

be used to assess mine seal integrity and possibly delineate the effects of long-term exposure to 

underground mine conditions (internal fracturing or cracking caused by roof-to-floor convergence which 

may not be determined by visual inspection of the face of the seal). 

References

Code of Federal Regulations, 2004, Title 30–Mineral Resources, Chapter I–Mine Safety and Health 

Administration Department of Labor, Part 75–Mandatory Safety Standards–Underground Coal Mines, 

Subpart D–Ventilation Sec. 75.335 Construction of Seals. 

Geophysical Survey Systems, Inc. (GSSI), 2004, RADAN 6 Users Manual, MN43-171, p. 135. 
Hurren W.E., G. N. Tuggle, and F. I. McGruder. Mary Lee No. 1 Mine, Accident Investigation Report 

(Underground Coal Mine) U.S. Labor Dept. MSHA, August 22, 1993. 

National Institute for Occupational Safety and Health (NIOSH), 1999, Lake Lynn Laboratory (flyer), 

Pittsburgh, PA: U.S. Department of Health and Human Services, Public Health Service, Centers for 

Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) 

Publication No. 99-149, 1 p.

National Institute for Occupational Safety and Health, 2002, A compendium of NIOSH Mining 

Research 2002, Washington, DC: U.S. Department of Health and Human Services, Public Health 

Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and 

Health, DHHS (NIOSH), Publication No. 2002-110, p. 73. 

Olhoeft, G.R., 2004, Ground Penetrating Radar, Home Page, November 26, 2004 , 

http://www.g-p-r.com.

Sapko M.J. and Weiss E.S., Evaluation of new methods and facilities to test explosion-resistant seals. 

Proceedings of the 29th International Conference of Safety in Mines Research Institutes, Katowice, 

Poland, October 8-11, 2001, Vol. 1, pp. 157-166. 

Sapko, M. J., Weiss, E. S., Harteis, S. P. Alternative Methodologies for Evaluating Explosion-Resistant 

Mine Ventilation Seals. Proccedings of the 30th International Conference of Safety in Mines Research 

Institutes, South African Institute of Mining and Metallurgy, October 2003, pp. 615-640. 

Scott, D. S., E. L. Checca., C. R. Stephen, and M. J. Schultz., Oak Grove Mine, I.D. No. 01-00851, 

Accident Investigation Report, U.S. Labor Dept. MSHA, January 29,1996. 

Triebsch, G.F. and M.J. Sapko, 1990, Lake Lynn Laboratory: A State-of-the-Art Mining Research 

Laboratory, in Proceedings of the International Symposium on Unique Underground Structures, 

June 12-15, 1990, Denver, CO, Chapter 75, pp. 75-1 to 75-21.

Weiss, E. S., Perry, J. W., Stephan, C.R., 1993, Strength and Leakage Evaluations for Coal Mine Seals, 

Proceedings of the 25th International Conference of Safety in Mines Research Institutes, September 13-

17, Pretoria, South Africa, pp. 149-161.