ABSTRACT

The National Institute for Occupational Safety and Health

(NIOSH), Pittsburgh Research Laboratory (PRL), in collaboration with

the Mine Safety and Health Administration (MSHA), the mining

industry, and seal manufacturers recently conducted a series of fullscale experiments within the underground experimental mine at PRL’s

Lake Lynn Laboratory. The purpose of the experiments was to

evaluate the explosion-resistant characteristics of several new seal

designs for rapid deployment during mine emergencies. These seals

can be deployed in 6 to 10 hr and are capable of withstanding

explosion overpressures in excess of 140 kPa (20 psi). These novel

seal designs use available mine materials, do not require

conventional rib hitching, and, most importantly, can substantially

reduce exposure time for coal miners during sealing and mine

recovery operations.

INTRODUCTION

The probability of a mine fire occurring in the U.S. is low, but

should one occur the local fire area must be controlled rapidly, safely,

and efficiently. Mine fires that are not controlled within the first 2 hr

generally require sealing at a cost of hundreds of thousands of dollars

a day for up to several weeks of active fire fighting. Time is most

important when constructing seals and miners may be placed at great

risk during construction. Even when mine fires are successfully

sealed, experience has shown that there is a high probability of

explosions occurring within 72 hr of sealing, therefore a seal should

be capable of withstanding explosion overpressure shortly after

construction. Controlling a fire by reducing the exchange of oxygen

requires surrounding the fire area quickly with barriers capable of

withstanding moderate-strength explosions as the contained

atmosphere transitions from the fuel-lean to fuel-rich condition. Once

the fire becomes established, the chances for successful in-mine

sealing decrease rapidly with each hour that passes.

A priori planning for sealing is paramount to successfully

controlling an underground fire and for rapidly constructing a seal

during mine recovery. Rapid sealing of a mine section should be part

of normal mine planning and layout. In the event of a fire, having

developed sealing strategies can significantly improve miner safety

and reduce the loss of time and dollars. The published works by D.

Mitchell (1971 and 1990) provide important guidelines for sealing fire

areas that should be considered when developing specific mine

strategies. The location of the seal is as important as the quality of

the seal. Seals should be located first in areas where the least

number of seals are needed and the sealed area should be large

enough for hot, combustible gases to expand without endangering the

miners who are building the seals. The bottom, ribs, and roof should

be firm and above potential flood levels. Seals should be constructed

in a level area, preferably below the elevation of the fire, and placed

in areas where the roof is sufficiently supported. Storing sealing

materials at key locations prior to the occurrence of a fire can

significantly minimize construction delays and greatly reduce the

burden on the miners who would be required to move and place these

materials at the sealing location while wearing self-contained

breathing apparatus. Also, communication with the surface should be

maintained to all sealing areas, and the miners constructing the seals

should be able to retreat swiftly to safety. 

If the decision is made to seal a section of a mine, the quicker the

seals are built the less exposure to miners. As part of an effective

1

DESIGNS FOR RAPID IN-SITU SEALING

M. Sapko

E. Weiss

Natl Inst for Occuptnl Sfty & Health

Pittsburgh, PA

J. Trackemas

RAG American Coal Co.

Waynesburg, PA

C. Stephan

Mine Sfty & Health Admn

Pittsburgh, PA
sealing operation, materials should be readily available at the mine,

should require minimum time for construction, should minimize air

leakage into and out of the fire area, should not crush out with

roof/floor convergence, and should be capable of withstanding

explosion overpressures that frequently occur behind fire seals. 30

CFR 75.335 (1997) requires a seal to “withstand a static horizontal

pressure of 20 pounds per square inch (140 kPa).” Construction of

the standard-type solid-concrete-block seal with floor and rib hitching

as defined in the CFR requires considerable time.

The strength of the standard-type solid-concrete-block seal is due

primarily to an arching action that takes place within the thickness of

the seal, which applies lateral thrust to the coal ribs. However,

strength increase due to arching action between the mine roof and

floor is not realized in most cases due to inadequate coupling

between the top of the seal and the mine roof. During construction of

the standard-type solid-concrete-block seal, it is difficult to uniformly

load or completely fill the gap between the top of the seal and the

mine roof with mortar; thus the effectiveness of vertical arching

becomes critical. In the field, most of the standard-type block seal

strength comes from the rib-to-rib arching action. An alternative

design concept is based on improving the arching action by providing

better coupling between the seal and the mine roof, which can be

done by pre-loading the seal with pressurized grout bags. 

To address these issues, the following organizations participated

in a joint research effort: PRL; Strata Products, Inc.; RAG, American

Coal Company; FOMO Products, Inc.; Burrell Mining Products

International, Inc.; and HeiTech Corporation. The project's purpose

was to evaluate the strength characteristics and air-leakage

resistance of a pre-loaded wood crib seal design; a light-weight,

cementitious Omega block* seal design; and a design consisting of a

series of grout-filled bags. These seals were specifically designed for

rapid construction and quick setting as compared to the more

standard method of constructing a mortared concrete block seal

design hitched or keyed into the mine ribs and floor. Standard seal

evaluations within the Lake Lynn Experimental Mine (LLEM) require

that the seal be allowed to cure a minimum of 28 days before

subjecting the seal to the required 140 kPa explosion pressure. Given

the time restraint of mine fire scenarios, these rapid seal designs were

engineered to be capable of withstanding a 140 kPa explosion

pressure 24 hr after construction. 

This report discusses the construction techniques, testing

methods, and explosion performance data for the seal designs under

consideration for use during rapid sealing operations, or for general

use in areas with some roof-to-floor convergence. 

EXPERIMENTAL MINE AND TEST PROCEDURES

Mine Explosion Tests

All of the mine explosion characteristics and air-leakage tests on

the various seal designs were conducted at the LLEM (Mattes et al.

1983; Sapko et al. 1987; Triebsch and Sapko 1990). The LLEM is

located approximately 80 km southeast of Pittsburgh, near

Fairchance, PA. The LLEM is one of the world's foremost mining

laboratories for conducting large-scale health and safety research.

This laboratory is unique in that it can simulate current U.S. coal mine

geometries for a variety of mining scenarios, including multiple-entry,

room and pillar mining, and longwall mining. The dimensions of the

drifts and crosscuts are typical of modern U.S. geometries for coal

mine entries and range from 5.5 to 6.0 m wide and approximately 2 m

high. 

Figure 1 shows an expanded view of the seal test area in the

multiple entry section of the LLEM. All of the seals and stoppings

were constructed in the crosscuts between the B and C drifts. The

nominal dimensions of these crosscuts are approximately 2-m high by

6-m wide. 

Prior to each explosion test, a 60-ton hydraulically operated,

track-mounted, concrete and steel bulkhead was positioned across E

drift to contain the explosion pressures in C drift. For a typical

evaluation test on a seal design for use in a U.S. coal mine, 19 m3

(661 ft3) of natural gas (~97% CH4) was injected into the closed end

of C drift. A plastic diaphragm was used to contain the natural gas

and air mixture within the first 14.3 m of the entry, resulting in a ~210-

m3 gas ignition zone. An electric fan with an explosion-proof motor

housing was used to mix the natural gas with the air in the ignition

zone. A sample line within the ignition zone was used to continuously

monitor the gas concentrations using an infrared analyzer. In

addition, samples were collected in evacuated test tubes and sent to

the PRL analytical laboratory for more accurate analyses using gas

chromatography (GC). The GC analyses verified the infrared

analyzer readings of ~9% of methane in air. Three electrically

activated matches, in a triple-point configuration equally spaced

across the face (closed end) of the entry, were used to ignite the

flammable natural gas and air mixture. Barrels filled with water were

located in the ignition zone to act as turbulence generators to achieve

the projected 140 kPa pressure pulse. The pressure pulse generated

by the ignition of this methane-air zone generally resulted in static

pressures ranging from ~150 kPa (~22 psig) at crosscut X-1, 129kPa

(~19 psig) X-2 to ~115 kPa (~17 psig) at X-3 the most outby seal. 

To ensure that all of the seal designs would undergo at least a 140

kPa explosion pressure pulse, a small amount of coal dust was used

for several of these tests in addition to the natural gas ignition zone.

The coal dust was loaded onto shelves that were suspended from the

mine roof on 3-m increments starting at 13 m from the closed end

(near the end of the natural gas ignition zone). When ignited, this coal

dust increased the average explosion overpressure from ~140 kPa

(20 psi) for the natural gas ignition zone itself to185 kPa (26.5 psi) for

the hybrid natural gas/coal dust ignition zone. 

Instrumentation

Each drift has ten data-gathering stations inset in the rib wall.

Each data-gathering station houses a strain gauge pressure

transducer and an optical sensor to detect the flame arrival. The

2

*Reference to specific products is for informational purposes and

does not imply endorsement by NIOSH.

Figure 1 - Seal test area in the Lake Lynn Experimental Mine
pressure transducer is perpendicular to the entry length and therefore

measures the static pressure generated by the explosion. The

transducers were rated at 0-690 kPa (0-100 psia), with 0-5 V output,

infinite resolution, and response time less than 1 ms. The flame

sensors used silicon phototransistors, with a response time on the

order of microseconds. These phototransistors were positioned back

from the front window of the flame sensors to limit the field of view and

precisely indicate arrival of the leading edge of the flame at each

station. Pressure transducers (0-410 kPa or 0-60 psia) were installed

in the face of each seal to measure the actual pressure loading.

Linear variable differential transformers (LVDTs) were used to

measure displacement of the mid-point of the back side of each seal

during pressure loading. The LVDT was attached to the back (B-drift

side) of a seal via fishing line to protect the sensor from being

destroyed in case of seal catastrophic rupture. The LVDTs provide a

reliable method for precision measurement of linear displacement in

the direction of the wall movement, perpendicular to the plane of the

seal. The LVDT measures up to ± 8 cm (3 in) of bi-directional seal

movement. The direction of displacement is indicated by the sign of

the output voltage. The LVDT calibration is verified by varying the

position of the fishing line at the seal by pre-determined distances and

measuring the corresponding output voltage. The main body of each

LVDT was attached to a steel frame located on the B-drift rib and

connected to the seal via a 2.7- kg (6 lb) test fishing line. The springloaded LVDT maintains tension on the line.

The data gathered during the explosion tests were relayed from

each of the data-gathering stations to an underground instrument

room off C-drift and then to an outside control building. A high-speed,

64-channel, PC-based computer data acquisition system (DAS) was

used to collect and analyze the data. This system collected the

sensor data at a rate of 1,500 samples/sec over a 5-sec period. The

data were then processed using LabView and Excel software and

outputted in graphic and tabular form (discussed in the "Explosion and

Air-Leakage Test Results" section). The reported data were averaged

over 10 ms (15-point smoothing).

Air-Leakage Determinations

An important factor to be considered for any seal design is its

impermeability, or its ability to minimize air leakage from one side of

the seal to the other. Measurements of the air leakages across the

seals were conducted before and after each of the explosion tests. A

wooden framework with brattice cloth or curtain was erected across C

drift outby the last seal position. This curtain effectively blocked the

ventilation flow, which resulted in a pressurized area on the C-drift

side of the seal. By increasing the speed of the four-level LLEM main

ventilation fan while in the blowing mode, the resultant pressure

exerted on the seals increased from approximately 0.25 kPa (1-in

H2O) for the lowest fan speed setting to nearly 1.0 kPa (3.7-in H2O)

for the highest fan speed setting.

On the B-drift side of each seal design, a diaphragm of brattice

with a 465-cm2 center opening was installed across each crosscut. A

vane anemometer was used to monitor the air flow through the

opening on the diaphragm to determine the leakage rates through the

seal. During these air-leakage tests, a pressure gauge was attached

to a copper tube on the B-drift side to monitor the differential pressure

across the seal.

As the ventilation fan speed was increased, the pressures and the

air flows through each seal were recorded. Based on data (Stephan

1990a; Greninger et al. 1991) previously collected during the

evaluation program with solid-concrete-block and cementitious foam

seals, U.S. guidelines for acceptable air-leakage rates through seals

were developed for the LLEM seal evaluation programs. The airleakage rates through the seals during both pre- and post-explosion

leakage tests were evaluated against these established guidelines.

Acceptable air-leakage rates are as follows: for pressure differentials

up to 0.25 kPa (1-in H2O), air-leakage through the seal must not

exceed 2.8 m3/min (100 cfm). For pressure differentials over 0.75

kPa (3-in H2O), air leakage must not exceed 7.1 m3/min (250 cfm).

The flow rate was calculated from the linear air speed measured by

the vane anemometer and the area of the opening through the

brattice behind each seal.

The following two sections discuss the construction process and

the performance testing of these seals when subjected to a pressure

wave produced by a methane and coal dust explosion.

SEAL CONSTRUCTION

Wood Seal Pre-Loaded with Grout Bags

Wood crib type seals are generally used in deeper coal mines that

experience excessively high roof and/or floor convergence, which

results in premature and, at times, catastrophic failure of more

traditional-type seal designs. However, previous LLEM evaluations

(Weiss et al. 1993) have determined that wood crib seals cannot

withstand a 140 kPa pressure pulse prior to convergence loading on

the seal without instituting labor-intensive methods to strengthen the

seal design. During LLEM explosion evaluations, the use of

pressurized grout bags in conjunction with the use of an easily applied

adhesive along the wood crib joints has been effectively

demonstrated to provide several advantages when constructing

underground coal mine seals. One advantage is the time required for

seal construction compared with the standard-type solid-concreteblock seal and other mortared block seal designs. With the

construction materials located at the site, it requires approximately 7

hours for two miners to stack and glue the wood cribs, about 1.5 hours

to fill the packsetter bags, and about 45 minutes to foam and coat

both sides. By comparison, two miners require about 60 to 70 hrs to

complete a mortared standard-type concrete block design.

Additionally, wood crib seals are near full strength within 24 hr of

completion and do not require the 28-day cure period of mortared

block seals. This quick construction and cure time is particularly

beneficial when installing seals to isolate a fire zone and/or a gob area

prone to spontaneous combustion. 

The use of hardwood cribbing reduces materials handling

requirements, which may further reduce injuries that are typically

associated with handling the smaller, yet heavier, standard-type solidconcrete-block. The hardwood cribbing timbers, 15- by 13- by 76-cm

(6- by 5- by 30-in long), are commonly used for roof support for many

eastern mines. Finally, the wood cribs are dimensionally consistent

throughout and allow for easy construction with interlayer glueing.

Figure 2 shows a schematic and figure 3 is a photo of completed

wood seal, which was placed in crosscut 1. The 15- by 6-cm (6- by

2.5-in) half timbers were used to overlap the vertical seams. Two 13

mm wide by 76-cm long (0.5-in wide by 30-in long) beads of HandiStick adhesive were applied to each timber between rows and two 13-

mm wide beads were applied to the vertical sides of each piece.

Approximately one 1-L (32-oz) can of Handi-Stick adhesive provided

two courses of wood crib coverage. The glue starts to set within 3 min

and cures to full strength in 24 hr. During the seal construction at

LLEM, the mine temperature dropped to 4 °C (40 °F), making it

difficult to keep the glue warm during application. For optimal

performance, the glue should be stored and used at temperatures

above 10 °C (50 °F). 

The packsetter bags, as manufactured by Strata Products Inc. of

Marietta, Georgia, were similar in design to the bags used during a

previous seal evaluation program (Weiss et al. 2002). The

dimensions of the packsetter bags can vary depending on the seal

3E
design thickness and construction techniques. Twelve 1.2-m by 1.4-

m (48 in by 55-in) packsetter bags (working dimensions 1.2 m by 1.2

m) were used along the seal-rib and seal-roof interface to lock the

seal into place and to further compress the glued joints. One

polyurethane foam pack (‘Silent Seal’ as manufactured by Fomo

Products, Inc. of Norton, Ohio) was used to coat one side of the seal

perimeter- ~17 kg (37 lb) per foam pack. The Strata Mine sealant

(manufactured by Strata Mine Services, Inc. of Richland, Virginia)

consists of a latex-based cementitious product with nylon

reinforcement fibers was used to coat the faces of the seal. The

sealant is packaged in 19-kg (42-lb) pails and is generally applied by

hand; personnel wear protective rubber gloves when applying the

sealant. The product manufacturer’s recommendation for the use of

the Silent Seal foam and Handi-stick adhesive were followed during

seal construction.

A modified grout pump powered by the hydraulic take off from the

mine’s battery scoop was used to facilitate the packsetter bag filling

process. As an alternative method for filling the bags where a battery

powered scoop or compressed air supply may not be available, the

bags can be filled using a hand-pump unit. The packsetter grout is a

specially formulated Portland cement- based mixture that is blended

and packaged for Strata Products, Inc. by Quickrete in Virginia. One

of the key components of the grout is calcium aluminate, which

decreases curing times and increases the compressive strengths

compared with conventional Portland cements. The compressive

strength of the packsetter grout is 2.5 MPa (362 psi) after 24 hr, 3.0

MPa (435 psi) after 7 days, and 4.0 MPa (580 psi) after 28 days. This

grout is a high-yield grout which requires significant amounts of water

compared to conventional cements. Approximately 55 L (14.5 gal) of

water is required per 23-kg (50-lb) bag of packsetter grout. The

packsetter bag is designed to contain the entire amount of water with

no seepage to meet the maximum specification of 2% free water after

the mixing with the grout is complete. The grout is also classified as

a non-shrink grout, which specifies a less than 1% shrinkage during

the cure period; this is a critical specification required when using the

grout in a pre-stressing operation. 

In the LLEM test, the packsetter bags were filled with grout to an

internal pressure of 350 kPa (50 psi) for the seal in crosscut 1. The

packsetter bags along the mine roof were injected first (starting at the

center and working toward the ribs) followed by the rib bag closest to

the mine floor on each side of the seal. The remaining rib bags were

then filled in no particular order. When injected with grout, the

packsetter bags overlapped both sides of the wood crib wall a

minimum of 8 cm or 3 in.

Upon completion, sealant was applied to selected perimeter

areas on both sides of the seal. Foam was used at the interface

between the bags and the mine roof to fill any gaps. The mine sealant

was then applied by hand to the back side (B drift) of the wood crib

seal and then covered with brattice curtain. Several pieces of 2.5 cm

by 15 cm (1-in by 6-in.) hardwood boards were nailed over the brattice

to the non-explosion side of the wood crib seal. About 30 cm (12 in)

of the rib around the perimeter of the brattice was coated with foam;

the foam was used to adhere the brattice to the rib/roof/floor

perimeter. The front and back of the seal were then sprayed with

Strata sealant to cover the brattice/foam interface and any exposed

foam (figure 4). A construction time of approximately 12 hr or 60

4

Figure 2- Wood seal design with packsetter bags 

Figure 3- Pre-loaded wood crib seal

Figure 4- Coating wood seal with Strata sealant
worker-hr was required. Since this was a prototype seal design and

modifications to the construction process were required, it is

anticipated that the construction time would decrease for future seal

installations. 

Omega Low Density Block Seal

The ~1-m (40 in) thick Omega block design, schematic shown in

figure 5, was 5.8 m wide by 2.1 m high (19 ft wide by 6.8 ft high).

Approximately 264 Omega blocks, measuring 20- by 40- by 60-cm (8-

by 16- by 24-in), were used with an average block weight of 20.3 kg

(44.7 lb). Unlike the previously evaluated Omega block seal designs

(Stephan 1990b; Weiss et al. 1993), no pilaster was used and no

hitching was required on the ribs and floor with this rapid seal design.

The block course was alternated to stagger joints from front to back

and left to right (figure 5 and 6). About 26 bags of Quickrete Blocbond high-strength fiber mortar was used to fully mortar the joints and

as sealant on both sides of the seal. The low viscosity Bloc-bond was

applied to all block-to-block interfaces to a mortar joint thickness of

about 6 mm (0.25 in). The 6-cm gap between the last course and the

mine roof was filled with 2.5-cm by 20 cm long (1-in by 8 in) rough cut

boards aligned lengthwise from rib to rib. One row of these boards

was placed in the middle of the top seal course with two rows of

additional boards place symmetrically on each side of the center row,

with the lengthwise board edges flush with the inby and outby side of

the seal. Each row of wood was wedged on about 30-cm (12-in)

centers and the gap between the wedges and board rows filled with

Bloc-bond. A 0.6-cm (0.25 in) thick coating of Bloc-bond was then

applied to both faces of the seal. Seal construction was completed in

9.5 hr and 28.5 worker hours. The Bloc-bond achieves 2000 psi

compressive strength within the first 24 hr. 

HeiTech Column Bag Pumpable Seal

The HeiTech pumpable bags that were used in this study are

primarily used for ground support in longwall mining. They provide

improvement in ground support capability as well as reduce material

handling. The pumping site for multiple seals can be located in

excess of 10,000 ft away and on the surface. For a surface pumping

station, a minimum of a 4-in diameter borehole is required to allow 3-

cm (1.25in) PVC lines to convey the accelerator and cement slurry.

This remote pumping location is especially beneficial when several

seals are required and where the handling of material is difficult. The

pumpable bag seal design (figure 7) was constructed by positioning

six 76-cm (30-in) diameter cylindrically shaped column bags (with

sewn in reinforcement rings or bands spiraling around the

circumference of the bags) equally across the crosscut. Each bag

was held in place to the mine roof using four PVC adjustable pogo

sticks; nylon straps were used to secure the pogo sticks in place

during the grout injection process to ensure that the pogo sticks would

not bow. These column bags were separated approximately 12 to 15

cm (5 to 6 in) with the end bags approximately 10 cm (4 in) from each

rib. No hitching was required with this seal. The material used within

each bag was a two-component cementitious grout. Equal quantities

of accelerator (90 bags of PacBent 120 Accelerator - M-PB20-Acc)

and cement (90 bags of Blue Circle Special Cement Pacset 140

Cementitious - M-PS30-Cem manufactured by Rockfast Mining

Products) were used to fill the column bags. The average bag weight

for both the accelerator and cement product was 25 kg (55 lb). The

bags were grout-injected using a 2.15:1 powder to water ratio; i.e.,

100 kg (220 lb) of Accelerator/Cement mix to 212 L (56 gal) of water.

A total of 2245 kg (4950 lb) of PacBent accelerator powder and an

equal amount of the Pacset cement powder were used with

approximately 4770 L (1260 gal) of water. Based on the powder to

water ratio used during this construction, HeiTech estimated the

compressive strength of the grout to be in the 41 to 55 MPa (600 to

800 psi) range. Subsequent analyses of 6 batch samples showed an

average compressive strength of 41.2 ± 4.3 MPa (597± 63 psi). Four

mixers were used during the grout injection process--two for each

powder. An Edeco Mindeb single-action pump was used to inject the

grout components into the bags. The pumping distance was

approximately 60 m (200 ft). This single-action pump injected ~4 L (1

gal) of the accelerator slurry on the first cycle followed by ~4 L of the

cement slurry on the second cycle; these components were then left

to mix within the bag.

Each of the 6 bags was initially filled with 30 cm (12in) of grout;

this alternating filling process was repeated until each bag was filled

to the mine roof. One bag without the reinforcement bands was then

inserted between each filled column bag and between the rib and the

adjacent column bag; a total of 7 of these bags were required (figure

7 and 8). Tie-wire was spiral-wrapped around two adjacent filled

column bags to provide a means of preventing the unfilled bag

between from bulging out too much on one side or the other during

the grout injection process. The tie-wire was cut and removed before

testing. A construction time of approximately 10 hr or 50 worker-hr

was required. Since this was a prototype seal design and

5

Figure 5- Omega block design

Figure 6 - Omega seal construction
modifications to the construction process were required, it is

anticipated that the construction time would decrease for future seal

installations. The Silent Seal foam was used along the seal perimeter

and between the bags on the B-drift side to minimize air leakages.

EXPLOSION AND AIR-LEAKAGE TEST RESULTS

Air leakage rates through the seals during both pre-and post

explosion leakage tests were evaluated against guidelines

established by MSHA. For pressure differentials up to 0.25 kPa (1-in

H2O), air leakage through the seal must not exceed 2.8 m3/min (100

cfm). For pressure differentials over 0.75 kPa (3-in H2O), air leakage

must not exceed 7.1 m3/min (250 cfm).

The pre-explosion air leakage rates (table 1) through each of the

three seal designs were within the acceptable guidelines.

Wood Seal Pre-Loaded with Grout Bags

The pressure and LVDT displacement data measured during the

LLEM test #396 on the pre-loaded wood crib seal are shown in figure

9. Within 0.45 sec, the pressure on the seal rose to about 150 kPa

(22 psig) and the center of the seal showed a permanent center

displacement of ~2 cm (0.75 in). The wood crib seal design with the

packsetter bags survived the explosion with no significance evidence

of any outward damage. Portions of the perimeter sealant on each

side of the seal at the packsetter bag and seal/roof interface were also

dislodged during the explosion. 

Post-explosion air-leakage measurements showed that this wood

crib seal design with the packsetter bags maintained minimal

leakages (2.1 m3/min at 0.17 kPa or 73 cfm at 0.7 in H2O as listed in

table 2) well within the acceptable rates; therefore, this design would

continue to serve its intended function to limit air movement into and

out of a seal area.

The pre-loaded wood crib seal was also subjected to a second

slightly stronger explosion (LLEM test #399). Within 0.5 sec, the

pressure on the seal rose to about 155 kPa (22.5 psig) and the center

of the seal showed an additional displacement of 3.3 cm (1.3 in) for a

total displacement of 5 cm (2 in) for both explosions. Following the

second explosion, the air-leakage rate across the seal increased to

3.8 m3/min at 0.2 kPa (135 cfm at 0.8 in H2O); however, the airleakage guidelines were not applied since this was the second

explosion test against the seal. 

Omega Low Density Block Seal

The pressure data measured during the LLEM test #404 on the

Omega block seal are shown in figure 10. The LVDT failed to function

during the test. Within 0.2 sec, the gauge pressure on the seal rose

from zero to about 180 kPa (26 psig). Post-explosion observations of

the Omega seal revealed little evidence of any outward damage.

Post-explosion air-leakage measurements showed that the Omega

block design maintained minimal leakages (0.3 m3/min at 0.25 kPa or

12 cfm at 1.0 in H2O as listed in table 2) and was still well within the

acceptable limits for these evaluations. 

6

Table 1. Air-leakage measurements before the first explosion

test

Seal Type Air-Leakage Rates a, m3/min (cfm), 

at pressure differential, kPa (in H2O)a

Wood seal with 1.1 (39) at 0.17 (0.7)

Pre-Loaded Grout Bags

Omega Low Density 0.3 (10) at 0.25 (1.0)

Block Seal

HeiTech Column Bag 1.4 (50) at 0.25 (1.0)

Pumpable Sea

a Acceptable Guildelines 2.8 m3/min (<100 cfm) at 0.25 kPa (1.0 in

of H2O).

Table 2. Air-leakage measurements after the explosion. 

Seal Type Air-Leakage Rates a, m3/min (cfm), at

pressure differential, kPa (in H2O)a

Wood seal with 2.1 (73) at 0.17 (0.7)

Pre-Loaded Grout Bags

Omega Low Density 0.3 (12) at 0.25 (1.0)

Block Seal

HeiTech Column Bag 1.5 (53) at 0.25 (1.0)

Pumpable Seal

a Acceptable Guidelines 2.8 m3/min (<100 cfm) at 0.25 kPa (1.0 in

of H2O).

Figure 7 - HeiTech pumpable bag design

Figure 8 - Completed HeiTech seal
HeiTech Column Bag Pumpable Seal

Figure 11 shows the HeiTech seal pressure loading history and

centerline displacement from LLEM test #404. Within about 0.4 sec,

the pressure on the seal rose to about 170 kPa (25 psig) and the

center of the seal showed a permanent displacement of nearly 4 cm

(1.5 in). Even though the seal shifted 4 cm, the post-explosion

leakage (1.5 m3/min at 0.25 kPa or 53 cfm at 1.0 in H2O) remained

within acceptable limits.

CONCLUSIONS

This research effort was designed primarily to determine the

strength characteristics of the three seal designs for use in rapid

sealing operations during a mine emergency or recovery situation.

The program objective was to determine the ability of newly

constructed seal designs to withstand a pressure pulse of at least 140

kPa (20 psig) while still maintaining significant resistance to air

leakage within 24 hours after construction. The wood seal utilizing

the quick-setting grout-filled packsetter bags, the Omega low density

block seal without hitching, and the HeiTech design with a series of

interlocking pumpable grout bags can be constructed in less than 10

hours and all withstood 140 kPa (20 psig) explosion pressure. 

These seal designs use existing ground support and stopping

materials, require minimum power and compressed air for

construction, do not require conventional rib hitching, and, most

importantly, can reduce exposure time for coal miners during sealing

and mine recovery operations. 

ACKNOWLEDGMENTS

The authors thank RAG, American Coal Company in

Waynesburg, PA, for providing the seal construction materials and

labor for the wood crib seal; Ed Barbour, sales and marketing

manager, and Joe Atkins, application technician, of Strata Products,

Inc., Marietta, GA, for their contributions to the construction of the

seals and the installation of the packsetter bags; Michael Seagal of

FOMO Products, Inc., Norton, OH, for providing the Handi-Stick

adhesive and glue gun to the project; Charles Lash, technical services

representative, of Burrell Mining Products International, Inc., New

Kensington, PA, for providing the Omega block and Bloc-bond; Joe

Bower, northern region manager, of HeiTech, Morgantown, WV, for

providing the seal construction materials and labor required for the

pumpable bag seal design.

The authors also thank the following Pittsburgh Research

Laboratory personnel who played a key role in the experimental

setup, instrumentation, and data collection activities: physical science

technicians Cindy A. Hollerich, Frank A. Karnack, Donald D. Sellers,

and William A. Slivensky; electronic technicians Kenneth W. Jackson

and Richard A. Thomas.

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7

Figure 9 - Pressure and displacement histories recorded on the

wood seal

Figure 10- Pressure and displacement histories recorded on the

Omega block seal

Figure 11- Pressure and displacement histories recorded on the

HeiTech seal
explosions: Bruceton Experimental Mine versus the Lake Lynn Mine.

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