1

 

REMOTELY INSTALLED MINE SEALS FOR MINE FIRE CONTROL 

Michael A. Trevits NIOSH, Pittsburgh, PA 

Alex C. Smith, NIOSH, Pittsburgh, PA 

Thomas A. Gray, GAI Consultants, Inc., Homestead, PA 

Lynn M. Crayne, Howard Concrete Pumping Co., Cuddy, PA 

Phil Glogowski, GAI Consultants, Inc., Homestead, PA 

Abstract 

Mine fires constitute one of the greatest threats to the 

health and safety of those working in the underground 

environment and each event has the potential for 

disastrous consequences. Of the major mine fires and 

thermal events that have occurred in the United States in 

the last 6 years, it is estimated that remotely installed 

seals could have been used in 63% of the events to control 

fire growth or to aid in fire suppression work. The 

National Institute for Occupational Safety and Health 

(NIOSH) is conducting full-scale tests at the NIOSH Lake 

Lynn Experimental Mine to evaluate and improve remote 

mine seal construction technology. The main focus of 

this work is to develop reliable technology that will 

completely close the mine opening from floor-to-roof and 

rib-to-rib. This paper presents the results of remote seal 

installations using grout-based materials. 

Disclaimer: The findings and conclusions in this report 

are those of the authors and do not necessarily represent 

the views of NIOSH. 

Introduction 

 Remotely installed mine seals have become an 

important component of the mine fire-fighting control and 

suppression arsenal. Of the mine major fires and thermal 

events that have occurred in the United States in last 

6 years, it is estimated that remotely installed seals could 

have been used in 63% of the events to control the fire or 

to aid in fire suppression work. 

 Remotely installed mine seals are utilized when 

direct underground access to the mine fire area is 

impossible or too dangerous. The seals are typically used 

to isolate the fire area and limit the inflow of oxygen. 

Once an area is sealed, the fire can be more readily 

controlled or suppressed by flooding the area behind the 

seals with water, gas-enhanced foam, inert gas, silt or 

other material. 

 Underground observations of remotely installed mine 

seals suggest that currently available commercial 

technology often does not achieve the goal of fully 

closing the mine opening (figure 1). If the mine seals do 

not mostly close the opening, then oxygen inflow cannot 

be controlled which can lead to growth of the mine fire. 

The seals that mostly close the mine opening however 

may be used to restrict and control the amount of air and 

inert gas that passes in or out of a fire area. 

Figure 1. Remotely installed mine seal that did not close 

the mine void (Urosek, 2005). 
2 

 The need to evaluate, improve and develop new 

technology to remotely construct mine seals was 

identified jointly by National Institute for Occupational 

Safety and Health (NIOSH) and the Mine Safety and 

Health Administration (MSHA) in 2001. This need 

resulted in a NIOSH research project (NIOSH, 2001). In 

addition, MSHA agreed to serve as a technical consultant 

in this effort. The first phase of the work involved the 

qualitative review of existing technology used to remotely 

construct mine seals. The review included materials used 

to construct mine seals, including cement and 

polyurethane foam, and an analysis of the available 

material mixing technologies (surface versus downhole 

mixing technologies) (Trevits and Urosek, 2002). The second phase of the work involved the remote 

construction of mine seals. The research was conducted 

at the NIOSH Lake Lynn Experimental Mine (LLEM) 

located approximately 60 miles southeast of Pittsburgh, 

Pennsylvania. The LLEM is a world-class, highly 

sophisticated underground facility where large-scale 

explosion trials and mine fire research is conducted 

(NIOSH, 1999) (figure 2). As a part of a prior research 

study, a 6-in diameter cased borehole was drilled and 

completed in the first cross-cut between the B and C 

Drifts of LLEM. It was determined that this mine area 

and the accompanying borehole was suitable for the seal 

construction work (figure 3). The thickness of the 

overburden in the area of the borehole is 197 ft. The 

cross-cut in the mine measured 19 ft wide, 40 ft long and 

7 ft high, with a mine floor slope gradient of 1.13 percent. 

A second borehole, located about 30 ft away, was 

available for viewing the mine seal installation using a 

downhole video camera. 

Figure 2. Layout map of the Lake Lynn Laboratory 

Experimental Mine. 

Howard Concrete Pumping Company (Howard)1

 of 

Cuddy, Pennsylvania and GAI Consultants, Inc., (GAI)1

of Homestead, Pennsylvania served as research partners 

with NIOSH in this effort. This paper describes the 

development of novel grout-based technology, evaluation 

of the materials used, construction practices, and followup testing. 

Figure 3. Underground layout of the seal construction 

site. 

 The objective of this research effort was to develop a 

specialty grout product and a method for placing the 

product through a borehole into a mine opening to build a 

mine seal. There were several additional factors that were 

included in the engineering design process. These factors 

are listed as follows: 

 • The methodology developed must be quickly 

deployable (within a few days). 

 • The mine seal must be rapidly installed (within a 

day or so). 

 • The mine seal material used must be locally 

available. 

 • The mine seal must be made of non-combustible 

material. 

 • The grout material used must allow placement in 

a free space without excessive flow if the mine is 

open and unobstructed and have flowable 

characteristics should the mine opening contain 

roof fall debris, cribbing, posts, and equipment 

and conveyor structures. 

 • The grout and the installation technology should 

facilitate full mine roof-to-floor and rib-to-rib 

closure. 

 • The seal must be strong and withstand the force of 

a methane gas explosion of about 20 psi. 

 

1

 Mention of a specific product or company name does 

not imply endorsement by NIOSH. 
3 

Seal Material Placement Technology 

 A model mine opening was constructed at Howard’s 

facility for testing and direct observation of the 

performance of the downhole and surface equipment. 

The model mine opening consisted of a small excavation 

in a hillside. The roof of the model mine was made using 

crane mats so a drill rig could be located over the mine 

void to hold the pipe for the downhole equipment 

(figure 4). Two series of tests were performed at the 

model mine along with an initial test at the LLEM before 

the final seal material delivery technology and seal grout 

mixture was developed. Changes were made to the 

cement content, admixtures and additive ratios to improve 

stickiness, time-of-set and application uniformity. 

Laboratory work was also conducted to improve the grout 

blends by modifying admixtures and additive ratios. 

After each test, modifications were made to the materials 

and equipment. 

Figure 4. View of the model mine. 

The final technique developed included a specialized 

directional elbow for directional placement of bulk fill 

material (figure 5) and a proprietary spray nozzle for 

material to address the remaining open areas in the mine 

void (figure 6). The spray nozzle required the use of two 

strings of pipe (one inside of the other) to convey two 

streams of material to the nozzle. The spray nozzle 

permitted the blending of the two-part grout accelerator 

mix with sufficient air velocity to transport the grout to 

the mine roof-and-rib areas. The bulk grout was pumped 

to the borehole using a positive displacement pump and 

compressed air. The sprayed grout was moved to the 

borehole using a conventional grout pump and 

compressed air. A detailed report on the development of 

this technology is presented by Gray et al (2004). 

Figure 5. Elbow for bulk fill placement. 

Figure 6. Spray nozzle. 

Seal Material Development 

During the initial work, it was decided that the first 

material to be placed into the mine would be a bulk fill 

material designed to occupy most of the open space in the 

mine void. The bulk fill material was comprised of a 

mixture of fly ash, Portland cement, and 2A (3/4-in 

minus) crushed limestone aggregate. A conventional 

concrete admixture was used to accelerate the set of the 

grout. The material was blended to achieve a pumpable 

mixture that had adequate strength and rapid setting 

properties. Fly ash was added to produce a mix that could 

be pumped to the borehole, travel down the borehole 

without segregation and provide a moderate degree of 

flowability. Once the material was in-place, the aggregate 

would provide sufficient shear resistance for the grout to 

be somewhat immobile until the mix set. Typical initial 

set time for this mixture could be achieved in 15 to 

20 minutes and would support foot traffic in 30 to 

45 minutes. 
4 

A second proprietary material was designed to fill 

any remaining open space above the bulk fill especially 

along the problematic roof-rib line areas. This material 

consisted of a two-part grout blend that was developed 

using a novel combination of materials procured from 

Master Builder’s Concrete Products Laboratory1

 in 

Cleveland, Ohio. The grout was generally a mixture of 

ASTM Class-F fly ash and Portland cement. The initial 

testing of the grout indicated that a conventional shotcrete 

accelerator would not produce sufficient stiffening in the 

desired time frame. Additionally, it did not exhibit 

suitable rheological and hardening properties required for 

the grout application. Further testing determined that a 

specially modified proprietary mixture was more effective 

in providing the desired grout characteristics than the 

conventional admixtures. In general, the proprietary 

mixture is made up of two parts. Part A improves the 

pumping characteristics and provides a reaction platform 

for Part B and is mixed with the grout before it is pumped 

into the borehole. Part B creates an immediate stiffening 

of the grout. Part B is added to the grout mixture at the 

spray nozzle (positioned at the mine level) using the 

stream of air that also transports the grout to the mine 

roof-and-rib surface. Other additives accelerate the 

hydration process and facilitate rapid strength 

development. The water content of the mix was also 

adjusted to improve the stiffening properties of the grout 

and produce the required stickiness for the grout spray to 

adhere to the mine roof-and-rib areas. 

As the work on the seal material development 

progressed, it became apparent that the uniform, 

consistent blending of the constituents in the sprayed 

grout was critical to the grout performance. The final 

portion of the grout mix design work focused on a 

sensitivity study that identified the grout’s reaction to 

deviations in the blending process. It was concluded that 

it would be necessary to finely meter the ingredients in 

the grout mix to achieve the desired performance. After a 

series of field and laboratory tests, adjustments were 

made to the equipment used to control material feed and a 

significant improvement of the material mix was achieved 

by the Howard Concrete and GAI team. It was also 

believed that sufficient latitude for field adjustments 

existed in the material design to account for changing 

conditions that might be encountered in the field. 

Mine Seal No. 1 

 The equipment used for this work included a 

volumetric mixer batch plant, cement storage silo, water 

tanks, pumps, air compressor, a drill rig, and 

miscellaneous support equipment such as trucks and 

loaders. Initial operations included calibrating the batch 

plant so that a uniform flow of bulk material could be 

mixed to produce a rate of approximately 30 yd3

 per hour. 

Placement of the bulk fill for seal No. 1 was initiated 

using a mixture composed of 2A crushed limestone 

aggregate, fly ash and cement. This mixture was pumped 

into the mine opening using a string of casing. Bulk fill 

was pumped over different time intervals with a pause 

between intervals to allow the in-place grout to stiffen and 

begin to set. This process was used to control the extent 

of lateral material flow out of the cross-cut areas. The 

pumping time and the pause intervals were determined by 

visual observation via a downhole video camera. The 

installation of this seal was not designed to be a “blind” 

operation so in-mine to surface communication was also 

facilitated through the use of a mine pager phone system. 

Pumping was terminated after approximately 112 yd3

 of 

material had been placed into the cross-cut (figure 7). 

Underground examination revealed that the mine opening 

had not been completely sealed (open spaces were 

observed at the mine roof-and-rib areas) and some of the 

bulk fill material had flowed into the adjacent mine areas. 

Figure 7. Underground view of bulk fill material for seal 

No. 1. 

 A dual string of drill pipe and casing affixed with the 

spray nozzle was then placed into the borehole in 

preparation for the second part of the mine seal 

construction. Unfortunately, after only a few minutes of 

pumping, a critical hose failed on the surface and the 

pumping operation was terminated. Underground 

examination of the sprayed areas indicated that the spray 

mixture did not stick to the mine rib areas and flowed 

away. Also, since minimal space (about 12 inches) 

between the bulk fill and the bottom of the borehole was 

available, it was decide to remove 18 inches of bulk fill 

material below the bottom of the borehole to provide 

sufficient space for follow-up backfilling work. 

 The disappointing results of the spray nozzle 

application indicated that additional work was needed to 

further refine the material mix components before the 

spray nozzle was used again. In the interim, after 

reviewing the progress made during the placement of the 
5 

bulk fill, it was also decided to fit the end of the casing 

string of pipe with an elbow (refer to the section on Seal 

Material Placement Technology above) to provide a 

means of directionally controlling the placement of grout 

material (refer to figure 5). It was also thought that this 

elbow configuration could facilitate roof-rib closure with 

the bulk fill material. 

 After some additional laboratory and design work, 

the newly designed elbow was lowered into the mine 

opening from the surface borehole. It was thought that 

use of this design might achieve full mine void closure 

thus eliminating the need for the spray nozzle. Once the 

elbow was positioned in place, pumping of the seal 

material began using a 2A limestone aggregate, fly ash 

and cement mixture. Compressed air was added to the 

flow stream to facilitate movement of the material 

towards the mine rib areas. Seal material was pumped 

into select locations along the mine rib areas in an attempt 

to fill the mine opening. Pumping was terminated after 

approximately 100 yd3 of material had been placed into 

the cross-cut and after the elbow became plugged.

 Underground examination revealed that the mine 

opening had not been completely sealed, some of the 

material had flowed beyond the cross-cut and into the 

adjacent mine areas. The area directly below the borehole 

and in the immediate vicinity of the elbow had been 

completely sealed to the mine roof. Several unsuccessful 

attempts were made to dislodge the plug in the elbow and 

it was ultimately decided to terminate the construction of 

mine seal No. 1. In general, before the elbow became 

plugged, significant progress had been made towards 

filling the mine opening. A subsequent meeting with 

Howard/GAI team revealed that additional design work 

was necessary before installation of seal No. 2 could 

begin. Later, mine seal No. 1 was removed.

Mine Seal No. 2 

Pumping of the first part of seal No. 2 (bulk material) 

began using a sand, fly ash and cement mixture. This 

material was pumped into the mine opening using the 

directional elbow. The bulk material was pumped in a 

series of lifts to fill most of the mine opening. Pumping 

was terminated after approximately 55 yd3

 of material had 

been placed into the cross-cut. It should be noted that that 

communication with underground personnel was allowed 

to orient the directional elbow and complete the 

construction of the first part of the seal. Underground 

examination revealed that seal material was placed to 

within 1.5 ft of the mine roof below the borehole and 

within 2.5 to 3 ft of the mine roof near the rib areas 

(figure 8). 

Figure 8. View of bulk fill placement for seal No. 2. 

It was decided to remove 6 in of material at the top of 

the seal to allow sufficient room to test the capability of 

the spray nozzle. For this part of the seal installation, the 

raw material was brought to the site using “Redi-Mix” 

trucks. This equipment worked well with the small 

volume batch plant used for this work. After conducting 

a 10 yd3

 surface test of the seal mixture (fly ash, cement 

and accelerators), a dual string of drill pipe and casing 

affixed with the spray nozzle was then placed into the 

borehole in preparation for the second part of the seal 

construction. Once the nozzle penetrated the mine 

opening, seal material was sprayed in a back-and-forth 

motion along the mine rib areas to fill in the gaps. 

Interaction between observers underground and engineers 

on the surface ensured that the nozzle was aimed in the 

proper direction. Good mine roof-and-rib contact was 

made with the sprayed material. The problematic corner 

areas at the mine roof-rib intersection were filled before 

the grout began to build up and migrate towards the spray 

nozzle (figure 9). 

Figure 9. Underground view of spray nozzle during seal 

No. 2 construction. 
6 

Filling of the remaining area near the borehole was 

accomplished by lowering the spray nozzle into the wet 

material and then rotating the operating spray nozzle 

through a 360 degree arc. Eventually, the material builtup around the nozzle and closed the mine opening 

(figure 10). In all, a total of 22.5 yd3

 of sprayed material 

was used to close the mine opening. An underground 

examination showed that the mine seal material (both 

bulk and sprayed material) was sprayed about 12 ft from 

the borehole towards the B-Drift and only about 9 ft from 

the borehole towards the C-Drift (this reduced distance 

was due to the slope of the mine floor). The final shape 

of the seal approximated a truncated pyramid whose base 

measured 19 ft wide (the width of the cross cut) by 21 ft 

deep and whose top measured 19 ft wide (the width of the 

cross cut) by 3 to 5 ft deep. Later, mine seal No. 2 was 

removed. 

Figure 10. Underground view of mine seal No. 2 from the 

B-Drift. 

Mine Seal No. 3 

The design concept for seal No. 3 called for using 

only the spray nozzle and eliminating the bulk component 

of the fill. Furthermore, this seal installation was to be 

conducted without direct communication with observers 

in the underground mine. The only means of observing 

the progress of the work was through the nearby 

observation borehole using a downhole camera. The 

material mix was also altered somewhat from that used 

for seal No. 2 as the water component was slightly 

reduced. This change would facilitate an increase in the 

amount of Part B in the mix and would increase the 

stiffness of the material. 

As discussed earlier, underground information 

showing the orientation of the spray nozzle and extent of 

the seal construction was limited to observations made 

with a borehole video camera that was installed in the 

second borehole located about 30 ft away. All material 

used was brought to the site in “Redi-Mix” trucks and the 

various components were added to the mix using a small 

batch plant. Installation of the seal was initiated using the 

spray nozzle rotating through a 360 degree arc. 

Installation progressed smoothly and the material throw 

distance was about 20 ft on the B-Drift side and about 15 

to 18 ft on the C-Drift Side. The difference in throw 

distance is attributed spray pressure and the slope of the 

mine floor. Spraying of the seal material continued along 

the 360 degree arc until it was decided by the engineers 

on the surface to only spray the C-Drift side. It was later 

disclosed that this approach was used to limit the size of 

the seal to approximately one-half the area of the crosscut area yet still allow for a sufficiently sized seal. Work 

was terminated for the day due to closure of the local 

cement plant after only 35 yd3

 had been placed into the 

mine opening. 

Spraying of seal material resumed the next day and 

seal material was sprayed along a 70 degree arc across the 

upslope, C-drift side of the cross-cut. Pumping continued 

until about 40 yd3

 of material had been placed into the 

mine void. Pumping was terminated when it was 

determined that seal material had rolled back onto and 

enveloped the spray nozzle and this material could not be 

removed or moved away using the nozzle. In addition, it 

was thought by engineers on the surface that underground 

visibility had diminished significantly (due to water vapor 

and fog accumulation in the mine) as observed through 

the downhole camera. Later it was determined that a 

gasket in the downhole camera had failed, causing a 

build-up of water that obscured the lens and ultimately 

caused the camera to become unusable. 

An underground examination of the seal void showed 

that the mine void appeared closed on one side of the 

borehole along the cross-cut, but a significant hole 

remained on the other side of the borehole (figure 11). 

The Howard/GAI team later concluded that the full 

capability of the spray nozzle had yet to be tested. 

Therefore, it was thought that another attempt should be 

made to build a seal (called seal No. 3-A) using the spray 

nozzle in the down slope area of the cross-cut towards the 

B-Drift and that viewing of the progress of construction 

might be easier because this operation would take place 

about 20 ft closer to the observation borehole. Some of 

the material from Seal No. 3 was removed from the area 

of the borehole and along the ribs to allow the spray 

nozzle unobstructed movement and to permit seal 

material to be sprayed the maximum distance from the 

borehole. 

A fixed-position video camera was located below the 

second borehole because the downhole camera was 

damaged as noted previously. This camera would provide 

the same function as the downhole camera without 

compromising the in-mine communication restriction 

placed on this experiment. This camera was not moved or 
7 

rotated during construction work and was positioned to 

provide a view across the total width of the cross-cut. 

Figure 11. Underground view of mine seal No. 3 from the 

B-Drift. 

The material mix was altered somewhat from that used 

for seal No. 3 as the water component was again slightly 

reduced. This change would increase the stiffness of the 

material to minimize material flow away from the 

borehole on the down slope side of the cross-cut. The 

construction of seal No. 3-A began by rotating the spray 

nozzle back and forth through a 70 to 80 degree arc. The 

spray material was thrown a maximum distance from 20 

to 22 ft from the borehole although most of the material 

seemed to be fall along an arc from 8 to 10 ft from the 

borehole. Pumping continued until about 37 yd3

 had been 

placed in the mine void when it was determined from the 

video camera image that the material had been placed to 

within a few inches of the mine roof (figure 12). The 

resulting mine seal was a large bowl-shaped structure 

extending about 8 to 10 ft from the borehole. The 

addition of accelerator (Part B) to the spray was then 

stopped and grout was permitted to flow from the spray 

nozzle to help infill any remaining voids in the mass of 

the seal. Pumping was terminated after about 3 yd3

 of this 

material had been pumped and a total of 40 yd3

 was 

pumped to construct this seal. 

Figure 12. Underground view of mine seal No. 3-A from 

the B-Drift. 

Unfortunately the observations made using the video 

camera did not agree with the actual conditions in the 

mine void. The mine roof near the area of the borehole 

had been broken upward on the B-Drift side and this 

irregularity was obscured by the general slope of the mine 

roof towards the video camera location. Although the 

video images showed the front top (from the B-Drift side) 

of the seal to be at or near the mine roof, in fact, the seal 

was nearly 18 inches from the mine roof along an arc 

about 8 to 10 ft from the borehole (figure 13). However, 

upon closer inspection inside the bowl-shaped structure, it 

was observed that seal material was placed to within 4 to 

6 ft, radially, from the borehole and was at the mine roof 

level completely across the mine opening (figure 14). 

Figure 13. Underground view of mine seal No. 3-A from 

the B-Drift (note dotted line shows outline of the seal). 

Figure 14. View of mine seal material inside of the bowlshaped structure of Mine seal No. 3-A (close to injection 

borehole). 
8 

Seal Strength Tests 

Unconfined uniaxial compressive strength tests were 

conducted on 3-in diameter cylinder samples that were 

collected during seal construction. Samples were 

collected underground as the material was being placed in 

the mine void. The results of the tests are shown in 

figures 15 and 16. The marked difference between the 

uniaxial compressive strength of the bulk material used 

for seal No. 1 as compared to seal No. 2 is most likely the 

sand component used in seal No. 2. With respect to the 

sprayed material, the uniaxial compressive strength of the 

bulk fill material is substantially higher than that of the 

sprayed fill material. The reason for the lower uniaxial 

compressive strength of the sprayed material is that the 

sprayed material does not contain sand or aggregate and 

most likely had air bubbles trapped in the mixture from 

the mine seal material placement process. 

Figure 15. Results of unconfined uniaxial compression 

tests on bulk seal material samples. 

Figure 16. Results of unconfined uniaxial compression 

tests on spray seal material samples. 

Unconfined compressive tests were conducted after 

1, 2, 3, 5, 7, 14 and 28 days on samples collected from the 

sprayed material used to construct seal No. 3-A. The 

results of these tests showed that the material achieves 

significant strength quickly and given sufficient seal 

thickness could, in all likelihood, withstand the force of a 

mine explosion shortly after installation. Also, note that 

in figure 16, that there is an overall increase in 

compressive strength from one seal to another. This is a 

result of alteration of the grout mix components as 

discussed earlier. 

 Although the major thrust of this research effort was 

aimed at development of material mixes and mine seal 

construction techniques, the benefits of constructing the 

seal at the LLEM included the option of testing the seal’s 

ability to confine mine air and also to withstand the forces 

of a mine explosion. Air leakage tests were conducted on 

seal Nos. 2 and 3-A by building a frame on one side of the 

mine seal and covering the frame with brattice cloth. 

Next an opening was made in the brattice cloth the size of 

an anemometer to facilitate air velocity measurements. 

Once this work was completed, air flow in the mine was 

adjusted to produce a desired differential pressure and the 

air leakage through the seal was measured. The results of 

the air leakage tests are shown in Table 1. 

Table 1. Results of air leakage tests. 

Parameter Seal 

No. 2 

Seal 

No. 3-A1

Seal 

No. 3-A2

Differential 

Pressure, 

 inches of 

water gage 

0.52 1.05 1.52 0.8 1.5 0.85 1.5 2.25 

Air 

Leakage 

Rate, 

ft3

/min 

252 322 426 296 409 221 305 365 

1

Several holes were observed in rib-roof areas remaining 

from seal No. 3. 

2

Test performed after polyurethane foam was used to fill 

holes observed during initial test. 

Prior to conducting the air leakage tests, several holes 

(on the order of about 1-in diameter) were observed in 

seal No. 2 near the mine roof area. Therefore, the air 

leakage values shown in the table were not totally 

unexpected. During the initial test of seal No. 3-A several 

holes were observed in the rib-roof areas. The holes were 

created during the installation of seal No. 3 and the 

material left in place from the remnants of seal No. 3. 

The holes were filled with polyurethane foam and the test 

was conducted again. The results of the second test on 

Seal No. 3-A showed the air leakage rates were reduced 

after the polyurethane foam was applied. To determine 

where the seal leaked air, a fog machine was used to 

create smoke and was placed on the upwind side of the 
9 

seal. Air pressure on that upwind side of the seal was 

increased to force the smoke through the seal. The smoke 

was observed at the mine roof areas on the downwind side 

of the seal. This observation was significant because it 

suggests that the seal material may not have been sprayed 

long enough to completely close the mine void or that the 

method used to complete the seal (as described earlier) 

eroded some of the sprayed seal material and created the 

holes. Also, it is important to note that leaks were not 

detected in the body of the seal, along the floor or rib 

areas.

 An explosion test was conducted on mine seal No. 2. 

The mine seal withstood a pressure of 18 psi with no 

visible signs of damage. To conduct the explosion test, a 

known quantity of methane gas was injected in the end of 

the C-Drift near the cross-cut where the seal No. 2 was 

installed. This area was temporarily closed with a frame 

and brattice cloth to confine the gas. The gas was diluted 

with air to achieve an explosive concentration. The gas 

was then ignited producing an explosion. An explosion 

test on seal No. 3 was not conducted because it was 

assumed that the seal was of significant thickness and 

strength and would withstand the force of a methane gas 

explosion (of about 20 psi). 

Conclusions and Recommendations 

 The overall objective of the work was to determine if 

an underground mine seal could be constructed remotely 

from the ground surface. This objective was achieved as 

two seals were successfully built through a borehole and 

the seals were confined to the cross-cut of the mine 

opening. The technology used to build the seal was tested 

and an appropriate material mix design was developed for 

both bulk and sprayed seal material. The results of 

follow-up testing (including compressive strength tests on 

seal samples, air leakage and explosion tests) showed that 

strong and robust seals were constructed as required in the 

design process. The issue of air-leakage may not be 

significant because the leakage rates were considered to 

be relatively small. In some cases, a certain amount of air 

leakage can be acceptable if the exchange of mine air and 

fire suppression agents (water, gas, foam, etc) into or out 

of the mine fire zone is manageable from the surface. 

However, if significant quantities of mine air can freely 

move across the area where mine seals have been 

remotely installed and where fire suppression agents 

cannot be contained as desired, the remotely installed seal 

most likely contains large holes and the installation is a 

failure. It is thought that air leakage may be further 

minimized by spraying the face of the seal with grout 

material using the spray nozzle from the observation 

borehole after completion of the downhole seal 

installation. This application should be tested in the 

future. 

 Results of the work to date suggest that this remote 

seal construction system may have merit for isolating a 

mine fire. This technique however does require 

additional trials to increase operator experience and 

overall familiarity with the technology. 

 One of the fundamental keys to successful in-mine 

seal construction using this technology is the ability to 

directly observe the progress of the work and that a blind 

seal installation (installation without an observation 

borehole) is most likely impossible. The only means of 

observing the in-mine construction may be via a nearby 

borehole that is equipped with a downhole video camera 

unless a camera can be directly affixed to the spray 

nozzle. Our experience suggests that conventional 

downhole camera lighting systems have a limited 

horizontal range of penetration (about 30 to 40 ft). Also, 

fog is created in the mine void as the seal material begins 

to set-up and this fog can significantly obscure the mine 

seal and limit the ability observe the work from a nearby 

borehole. This problem may become even more acute 

should the mine void be filled with smoke. It is suggested 

that future research be conducted using a downhole laser 

or a radar imaging device that offers real-time imaging 

and data processing with the capability of penetrating 

smoke, dust or fog. 

 A 6-in diameter, 197 ft deep, cased borehole was used 

during the trials at LLEM and the downhole equipment 

was designed to meet this need. The issue of working 

with this equipment at deeper depths should be evaluated 

as these conditions will be most likely encountered in the 

field. 

 Finally, it is suggested that this technology should be 

further tested with the construction of a mine seal in an 

entry that contains debris (roof fall material) and mine 

structures (possibly cribbing, track, or conveyor 

structures). This approach will test the ability of the seal 

material to flow around obstructions and still form a seal 

while closely matching the conditions most likely found 

in an underground mine. 

Acknowledgments 

 The authors would like to recognize John E. Urosek 

and Clete R. Stephan, MSHA, for their input and support. 

Special thanks are also made to Eric S. Weiss and the 

NIOSH staff at the LLEM facility for their 

professionalism, dedication, and assistance in the conduct 

of this research effort. We would also like to recognize 

Charles D. Campbell, MSHA, Samuel P. Harteis, NIOSH 

and Roger McHugh, Duquesne Light for their expertise in 

providing the technical review of this paper 
10 

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