Evaluation of remotely installed mine seals for mine fire control

M.A. Trevits

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania

T.A. Gray & P. Glogowski

GAI Consultants, Inc., Homestead, Pennsylvania

L.M. Crayne

Howard Concrete Pumping Company, Cuddy, Pennsylvania

ABSTRACT: Of the 19 major mine fire events (including thermal events) that have occurred in the last 6 years

(2000–2005), it is estimated that remotely constructed mine seals could have been used at 65% of the events to

control or suppress the fire. Underground observations of seals that have been remotely installed during mine

fire events show that they often do not fully achieve mine roof-to-floor and rib-to-rib closure. Unfortunately, the

inability to reliably close the mine void has limited or precluded the regular use of this technology. The National

Institute for Occupational Safety and Health is conducting full-scale underground experiments at the Lake Lynn

Experimental Mine to identify and remedy existing remote sealing technology shortcomings, to develop novel

technologies, and to transfer the new or improved technologies to the mining industry. This paper will discuss

the remote mine seal testing program and will provide the results of the in-mine experiments.

Disclaimer:The findings and conclusionsin thisreport are those ofthe authors and do not necessarily represent

the views of the National Institute for Occupational Safety and Health.

1 INTRODUCTION

It has been reported that from 1990–1999 there were

81 coalmine firesin the United States(DeRosa, 2004).

Moreover, in the last 6 years, 19 major mine fires

and thermal events have occurred in underground coal

mines in the United States. On average, about three

events have occurred each year with a maximum of

five events over this time period. These statistics suggest that mine fires are occurring with alarming frequency. It is estimated that remotely constructed mine

seals could have been used at 65% of the events to

control or suppress the fire.

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 partner 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) (Trevits and Urosek, 2002).

The second phase of the work involves the remote

construction of mine seals. The research was conducted at NIOSH’s Lake Lynn Experimental Mine

(LLEM) located approximately 60 miles southeast of

Pittsburgh, PA. The LLEM is a world-class, highly

sophisticated underground facility where large-scale

explosion trials and mine fire research is conducted

(NIOSH, 1999) (figure 1).

Figure 1. Layout of the Lake Lynn Experimental Mine.
Howard Concrete Pumping Company (Howard) of

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

of Monroeville, Pennsylvania served as research partners with NIOSH in this effort. This paper describes

the development of novel grout-based technology for

remote mine seal construction, evaluation of the materials used, construction practices, and follow-up testing. An in-depth discussion of this work is described

by Gray et al. (2004).

2 SEAL DESIGN CONSIDERATIONS

The objective of this work was to develop a specialty

grout product and a method for placing the product

through a borehole into a mine opening to form 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 capable of

being deployed quickly.

• The mine seal must be rapidly installed.

• The seal material used must be locally available.

•	 The grout material must be of a consistency to allow

placement in a free space without excessive flow if

the mine is open and unobstructed but must also

be capable of filling a mine opening containing

roof fall debris, cribbing, equipment or conveyor

structures.

•	 The grout and the methods of application must

facilitate mine roof-to-floor and rib-to-rib closure.

•	 The seal must be strong enough to withstand the

force of a mine explosion (up to 20 psi).

Previously, a 6-in diameter cased borehole was completed in the first cross-cut between the B and C Drifts

of the LLEM and it was determined that this borehole was suitable for the seal construction research

(figure 2).

Figure 2. Underground layout of the seal construction site.

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. Testing conditions for this technology was not

designed as a “blind test” so in-mine to surface communication was facilitated through the use of a mine

pager phone system.

3IN-MINE MATERIAL PLACEMENT

TECHNOLOGY

Priorto installing themine seal atLLEM, amodelmine

opening was constructed at Howard’s facility for testing and direct observation of the performance of the

downhole nozzle and pumping equipment. The model

mine opening consisted of a small excavation in a hillside. The roof of the model mine was formed using

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

void to hold the pipe for the downhole equipment.

Two series of tests were performed at the model mine

along with an initial test at the LLEM before the final

placement strategy and grout mixture was developed.

Changes were made to the cement content, admixtures

and additive ratios to improve stickiness, time-of-set

and application uniformity. Some 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, equipment

and equipment usage.

The final technique developed included a specialized directional elbow for directional placement of

bulk fill material (figure 3) and a spray nozzle for

material to address the remaining open areas in the

mine void (figure 4).

Figure 3. Directional elbow for bulk fill placement.

The specialized 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 while allowing sufficient air velocity to
4 GROUT MATERIAL

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.

 

Figure 4. Spray nozzle.

Constructing an effective mine seal through a single

borehole is a difficult engineering challenge.The grout

mixture cannot be too fluid or it will merely flow away

from the borehole. If the grout mixture is too stiff, it

will tend to build quickly forming a mound at the bottom of the borehole and will not flow and fill the mine

roof-rib areas. It was decided that the first material

to be placed in the mine would be a bulk fill material designed to fill most of the open space. This was

also the least costly component of the fill material and

would help to lower the overall cost of the seal. The

bulk fill material used a mixture consisting 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.The

amount of fly ash added was sufficient to produce a

mix that could be pumped to the borehole, travel down

the borehole withoutsegregation and provide a moderate degree of flowability. Once the grout was in-place,

the aggregate would provide sufficientshearresistance

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.

The second material to be used to fill any remaining open space above the bulk fill along the roof-rib

line was a two-part grout blend that was developed

with the assistance of Master Builder’s Concrete Products Laboratory in Cleveland, Ohio. The basic grout

was to be a blend 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 propertiesrequired for the grout

application. Further testing determined that Master

Builder’s TCC system was more effective in providing the desired grout characteristics than conventional

admixtures. The Master Builder TCC System 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 is a liquid, high-performance

shotcrete and grout accelerator that reacts with Part A

to create an immediate stiffening of the grout. Part B

is added to the grout mixture using 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. The reaction between the Part A and Part B

admixtures essentially provides the initial stiffening

through a flocculation process that is unrelated to the

chemical hydration ofthe cement productsin the grout.

Therefore, a concrete accelerator was also added at the

nozzle to accelerate the hydration process. The addition ofthe accelerator along with the cement content of

the grout facilitated rapid strength development of the

in-place grout spray. To improve the stiffening properties of the grout and produce the required stickiness

for the grout spray to adhere to the mine roof-andrib areas, the water content of the mix was adjusted

while retaining the fluidity and pumpability of the mix

through the addition of a high-range water-reducing

additive.

As the work on the seal material development

progressed, it became apparent that the uniform, consistent blending ofthe constituentsin 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 very finely meter the ingredients in

the grout mix to achieve the desired performance from

the sprayed grout. 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 GAI engineers.

5 MINE SEAL CONSTRUCTION

Pumping ofthe first part ofthe remote seal(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 the mine most of the opening.

Pumping wasterminated 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 base. 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 5).

Figure 5. View of bulk fill placement for mine seal.

It was decided to remove 6-in of material near the

top of the seal to allow sufficient room to test the capability of the spray nozzle. 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

6-in diameter 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 6).

Figure 6. Use ofspray nozzle duringmine seal construction.

Filling of the remaining area near the borehole was

accomplished by lowering the spray nozzle into the

wet material below the nozzle and then rotating the

operating spray nozzle through a 360 degree arc. Eventually, the material built-up around the nozzle and

closed the mine opening (figure 7). In all, a total of

22.5 yd3 ofsprayed material was used to close the mine

opening.An underground examination showed that the

mine seal material (both bulk and sprayed material)

had flowed about 12 ft from the borehole towards the

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

the C-Drift.The 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.

Figure 7. View of completed mine seal.

6 MATERIAL AND MINE SEAL TESTS

Unconfined compressive tests were conducted on 3-in

diameter cylinder samples (cylinder area – 7.07 in2)

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 Table 1.The compressive strength of the bulk

fill material is substantially higher than that of the

sprayed fill material. The reason for the lower compressive strength of the sprayed material is that the

sprayed mix does not contain sand and had air bubbles

trapped in the mixture from the mine seal material

placement process.

Although the majorthrust ofthisresearch 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 by building a frame on one side of the mine

seal and covering that 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.Air

leakage tests were conducted on the mine seal and the

results are shown in Table 2 (Weiss, 2003).

Prior to conducting the air leakage tests, several

holes (on the order of about 1 inch diameter) were

observed in the seal near the mine roof area.Therefore,

the air leakage values observed in the table were not

totally unexpected.

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 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. The mine seal withstood a static load pressure of 18 psifromthe explosion

with no visible signs of damage (Weiss, 2003).

Table 1. Results of compressive strength tests on cylinder

samples.

Bulk fill material Spray fill material

Compressive Age, days Compressive

Age, days strength, psi strength, psi

9 2403 5 230

28 3818 7 270

– – 28 468

– – 56 765

Table 2. Results of air-leakage tests on mine seal.

Differential pressure across

the seal, inches of water gage 0.52 1.05 1.52

Air-leakage rate, ft3/min 252 322 426

7 RESEARCH FINDINGS AND

RECOMMENDATIONS

The overall objective of the work was to determine if

a mine seal could be constructed remotely from the

ground surface. This objective was achieved as a seal

was successfully built through a borehole and was

confined to the cross-cut of the mine opening. The

technology used to build the seal was tested and

the correct material mix design was developed. The

results of follow-up testing showed that a strong and

robust seal was constructed as required in the design

constraints. The issue of air-leakage can be addressed

by slowing the rotation of the spray nozzle to allow

for a more substantial build-up of seal material. As

an additional remedy, it may be also be possible to

insert the spray nozzle into the observation borehole

and spray the entire face of the seal to close and fill

any remaining holes.

Results of the work to date suggest that this remote

seal construction system may have merit for isolating

a mine fire.Thistechnique however doesrequire additional trials since considerable communication with

the subsurface personnel was needed to achieve rib-torib and roof-to-floor closure. One of the fundamental

keys to successful in-mine construction is the ability to directly observe the progress of construction.

Because this was a research and demonstration project,

communication between the surface operation and the

underground seal location was permitted. This will

not be the case when a mine fire occurs. Additional

research is therefore planned to further refine the construction method. A mine seal should be constructed

at the LLEM without voice communication with the

surface. The only means of observing the progress

of construction should be via the nearby borehole

equipped with a downhole video camera with sufficient resolution capabilities and lighting. Experience

gained during this work also suggests that a downhole

laser or radar imaging device should be developed that

offers real-time imaging and is capable of penetrating

smoke, dust or the fog that forms in the mine opening

as the seal material begins to set.

A 6-in 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

in smaller diameter boreholes should be addressed

along with the fact that deeper overburden depths will

undoubtedly be encountered. Perhaps an additional

spray nozzle should be constructed to facilitate remote

seal construction in small-diameter boreholes.

Finally, it is suggested that this technology should

be further evaluated through construction of a mine

seal at LLEM in a mine entry that is obstructed with

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 Urosek and

Clete Stephan, MSHA, for their input and support

throughout the design and conduct of this research

project. Special thanks are also made to Alex Smith,

Eric Weiss and the NIOSH staff at the LLL facility for
their professionalism, dedication, and assistance in the

conduct of this research effort.

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DeRosa, M.I. 2004. Analysis of Mine Fires for all U.S.

Underground and Surface Coal Mining Categories: 1990–

1999. NIOSH Information Circular 9470.

Gray, T.A., Trevits, M.A., Crayne, L.M., and Gloglowski. P.,

2004. Demonstration of Remote Mine Seal Construction.

2004 SME Annual Meeting Denver, CO February 23–25

Preprint No. 04–194.

National Institute for Occupational Safety and Health. 2001.

A Compendium of NIOSH Mining Research 2002. DHHS

(NIOSH) Publication No. 2002–110.

National Institute for Occupational Safety and Health. 1999.

Lake Lynn Laboratory (flyer), DHHS (NIOSH) Publication No. 99–149.

Trevits, M.A. and Urosek, J.E. 2002. Technology for Remote

Mine Seal Construction. 2002 SME Annual Meeting

Phoenix, AZ February 25–27 Preprint No. 02-185.

Weiss, E.S. 2003. Supervisory Mining Engineer, Disaster

Prevention and Response Branch, NIOSH. Personal Communication October 23.

Weiss E.S. 2003. Supervisory Mining Engineer, Disaster

Prevention and Response Branch, NIOSH. Personal Communication December 10.