UNITED STATES DEPARTMENT OF ENERGY 

PITTSBURGH RESEARCH CENTER 
Cover photo: Application of a cernentitious surface coating to 

the face of a low-density cernentitious block seal design within 

the Lake Lynn Experimental Mine. This seal design was 

evaluated, against a full-scale mine explosion, for its strength 

and air leakage characteristics. (Cover and inside photos by 

William A. Slivensky, Pittsburgh Research Center.) 
Report of Investigations 9634 

Evaluation of Polymer Construction Material 

and Water Trap Designs for Underground 

Coal Mine Seals 

By Eric S. Weiss, William A. Slivensky, Mark J. Schultz, 

Clete R. Stephan, and Kenneth W. Jackson 

UNITED STATES DEPARTMENT OF ENERGY 

PITTSBURGH RESEARCH CENTER 
International Standard Serial Number 

ISSN 1066-5552 
pressurepulse ............................................................................. 4 

Pressurized entry for leakage determination rates across the seals ..................................... 5 

Conceptual drawing of the composite polymer seal design ........................................... 6 

Injection of the polymer to the dry aggregate layer between the block walls ............................. 6 

Completion of the polymer and aggregate core near the mine roof ..................................... 6 

Final injection of the polymer between the concrete block walls to ensure closure to the mine roof ........... 6 

Condition of the 46-cm-thick polymer and aggregate core seal design. which withstood a 269-kPa explosion 

pressurepulse ............................................................................. 9 

Injection of the 9 1 -kg/ml-density polymer-only core ............................................... 9 

Condition of the 91-kg/m3-density polymer-only core seal following the 141-kPa level explosion ............ 9 

Inlection of the cellular concrete slurry between the wood and brattice cloth form walls .................... I I 

Condition of the low-density cementitious block seal following a 138-kPa explosion pressure pulse .......... I? 

Three water trap design openings installed through a solid-concrete-block seal ........................... 14 

Typical 15-cm-diam PVC water pipe trap showing the 15-cm-long extension ............................ 15 

TABLES 

Guidelines for air leakage through a seal ......................................................... 5 

Summary of seal air leakage rates as a function of pressure differential ................................. 7 

Summary of test conditions and results for polymer seal designs ...................................... 8 

Summary of test conditions and results for the 1.2.m.thick cellular concrete seal designs ................... 12 

Summary of test conditions and results for low-density cementitious block seal designs .................... 13 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

h hour pct percent 

min minute s second 

Factors for Conversion to U.S. Customary Units 

- -- 

To convert from- To- Multiply bycentimeter 

square centimeter 

kilogram 

kilogram per cubic meter 

kilometer 

kilo pascal 

kilopascal 

liter 

meter 

square meter 

cubic meter 

cubic meter per second 

milligram per liter 

metric ton 

degree Celsius 

inch 

square inch 

pound 

pound per cubic foot 

mile 

pound (force) per square inch, 

gauge 

inch of water (pressure) 

gallon 

foot 

square foot 

cubic foot 

cubic foot per minute 

ounce per cubic foot 

short ton 

degree Fahrenheit 

4 

0.2642 

3.2808 

10.764 

35.3147 

0.5886 

0.001 

1.1023 

1.8 and add 32 

Mention of any company name or product does not constitute endorsement by the U.S. Department of Energy Pittsburgh 

Research Center. 

Visit our Mining Health and Safety Research site on the Internet, accessible via http://www.usbm.gov. 
EVALUATION OF POLYMER CONSTRUCTION MATERIAL AND WATER 

TRAP DESIGNS FOR UNDERGROUND COAL MINE SEALS 

By Eric S. Weiss,' William A. Sli~ensky,~ Mark J. S~hultz,~ Clete R. Ste~han,~ and Kenneth W. Jackson5 

ABSTRACT 

The Pittsburgh Research Center (PRC)~ and the Mine Safety and Health Administration (MSHA) are 

participating in a research program to evaluate the strength characteristics and air leakage resistance of seals 

and water trap designs for use in underground coal mines. This program is being conducted at PRC's Lake 

Lynn Laboratory near Fairchance, Fayette County, PA. 

Seals designed with a 40-cm-thick polymer (polyurethane) and aggregate core between two dry-stacked (no 

mortar) concrete block walls (coated on outby sides) withstood a 138-kPa pressure pulse while maintaining 

acceptable air leakage rates. Similar seal designs utilizing a 97-cm-thick, 91-kg/m7-density, polymer-only core 

did not survive; however, a 5 1 -cm-thick, 203-kg/mi-density, polymer-only core seal successfully withstood 

the explosion pressures. 

Evaluations of cellular concrete seal designs have shown that a two-pour slurry injection technique did not 

adversely affect the strength of the 1.2-m-thick seals when subjected to a 138-kPa pressure pulse. Two seal 

designs utilizing low-density cementitious block have also been evaluated with successful results. 

In addition to seal strength tests, various 15- and 30-cm-diam U-shaped water trap pipes and a water trough 

design were evaluated under explosion conditions and were shown to be inadequate when evaluated under 

worst-case conditions. Simple modifications to the various water trap designs were successfu1 in preventing 

the passage of explosion flame through the trap. 

'~ining engineer, Pittsburgh Research Center, U.S. Department of Energy, Pittsburgh, PA. 

'Physical science technician. Pittsburgh Research Center, U.S. Department of Energy, Pittsburgh, PA. 

'Mining engineer, Ventilation Div.. Mine Safety and Health Administration, Pittsburgh, PA. 

4~rincipal mining engineer, Ventilation Div., Mine Safety and Health Administration, Pittsburgh, PA. 

'~lectronics technician, Pittsburgh Research Center, U.S. Department of Energy, Pittsburgh, PA. 

'This work originated under the U.S. Bureau of Mines prior to transferring to the U.S. Department of Energy on April 4, 1996. 
INTRODUCTION 

During the normal course of underground coal mining, it 

sometimes becomes necessary to install seals to isolate 

abandoned or worked-out areas of a mine. This practice 

eliminates the need to ventilate those areas. Seals may also be 

uscd to isolate fire zones or areas susceptible to spontaneous 

combustion. To effectively isolate areas within a mine, a seal 

should- 

* Minimize leakage between the sealed area and the 

active mine workings so as to prevent toxic and/or 

flammable gases from entering the active workings; 

Be capable of prevcnting an explosion initiated on one 

side from propagating to the other side; and 

Continue its intended function for 1 h when sub-jected 

to fire conditions. 

Titlc 30, Part 75.335 of the Code of Federal Regulations 

(CFR) (117 requires a seal to "withstand a static horizontal 

pressure of 20 pounds per square inch [I38 kPa]." 

PRC and MSHA are jointly investigating the capability of 

various seal construction materials and designs to meet or 

exceed the requirements of the CFR. This work is in support 

of PRC's Disaster Prevention research program to improve 

safety for underground mine workers. Previous research (2) 

indicates that it would be unlikely for overpressures exceeding 

138 kPa to occur very far from the explosion origin provided 

that the area on either side of the seal contained sufficient 

incombustible and minimal coal dust accumulations. This is 

the first full-scale test program to evaluate seal designs in 

entry geometries similar to those of current U.S. underground 

coal mines. Previous evaluation of seal designs were conducted in the smaller entries of PRC's Experimental Mine at 

Bruceton, PA (2). The 2.7-m-wide by 1 .$-m-high entries of 

the Bruceton Experimental Mine were typical geometries for 

U.S. mines in the early 1900's when the Experimental Mine 

was first developed. However, technological advances in 

ventilation, roof support, and mining machinery have resulted 

in increased entry sizes. 

PRC's seal research program had previously addressed, 

through explosion testing at the Lake Lynn Experimental 

Mine (LLEM), the integrity of solid-concrete-block seals (3), 

low-density cementitious block seals (4), cementitious foam 

seals (3, 5), and wood block seals (6). Various other 

alternative seal designs have more recently been constructed and testcd under this seal research program. This effort 

7~talic numbers in parentheses refer to items in the list of references at the 

included several series of seal designs utilizing polymer, 

cellular concrcte, and low-density cementitious block construction materials. 

A growing concern within the mining industry and MSHA 

centered on the design and effectiveness of water trap devices. 

30 CFR 75.335(c) states that "(1) a corrosion-resistant water 

pipe or pipes shall be installed in seals at the low poicts of the 

area being sealed and at all other locations necessary when 

water accumulation within the sealed area is possible; and (2) 

each water pipe shall have a water trap installed on the outby 

side of the seal." (I) These U-shaped traps are designed to 

allow the passage of water through an explosion-resistant seal 

so as to prevent the buildup of a large impoundment of water 

against the seal. The trap design must be able to prevent the 

passage of mine gases through the trap. 

The research program focused on whether typical water 

trap designs could successfully prevent the passage of an 

explosion flame through the trap, even during periods when 

there may be minimal liquid in the trap. The primary concern, 

therefore, was whether a 138-kPa explosion pressure pulse 

could forcibly remove the water from the trap, thus enabling 

the explosion flame to propagate through the trap and ignite 

a flammable methane (CH,) atmosphere on the active side of 

the seal. Various types and sizes of water traps were installed 

and evaluated under full-scale conditions within the LLEM. 

The overall ob-jective of this research is to determine 

whether seals constructed from various materials and designs 

can withstand a 138-kPa CH,-air explosion without losing 

their structural integrity. The seal must not only be physically 

strong, but also minimize leakages. A safety and cost benefit 

will also result from these evaluations in that many of these 

new seal designs require less material and worker-hours to 

install than the standard solid-concrete-block seal. 

Full-scale research into the development of explosion- and 

leakage-rcsistant seals assists MSHA in setting adequate 

standards and provides useful information to the industry for 

the improvement of mining safety. As new seal construction 

materials and designs become available, performance data 

from full-scale explosion tests are needed to evaluate their 

strength characteristics. Additionally, air leakage determinations are essential for all seal types. The LLEM can be used 

to provide these data. MSHA has requested technical 

assistance in this area. 

This report discusses the construction techniques, testing 

methods, and test data collected for these most recent series of 

explosion-resistant seal and water trap designs for underground coal mines. 

end of this report. 
EXPERIMENTAL PROCEDURE 

MINE EXPLOSION TESTS 

All of the explosion and air leakage determination tests on 

the various seal designs were conducted in the LLEM (7-8). 

Lake Lynn Laboratory is one of' the world's foremost facilities 

for conducting large-scale mining health and safety research. 

The LLEM 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. 

Figure 1 shows a plan view of the LLEM. The underground 

entries consist of approximately 7,620 m of workings developed 

in the mid- 1960's for the commercial extraction of limestone 

and 2,286 m of entries developed in 1980-8 1. These more 

recent entries are depicted in figure 1 as drifts A, B, C, and D, 

each of which is 520 m long and closed at the inby end, and 

drift E which is 122 m long and connects C and D drifts. 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 2 shows an expanded view of the seal test area. CH, 

was injected into the closed end of C drift. A plastic diaphragm 

was used to contain the 10 pct CH,-air mixture within the first 

14.3 m of the entry (an approximate 1 90-m3 zone). Electric 

matches, located at the closed end of the entry, were used to 

ignite the flammable CH,-air mixture. Before the ignition of 

the CH,, a 60-t, track-mounted, concrete and steel bulkhead 

was positioned across E drift to contain the explosion pressures 

in C drift. Barrels filled with water were located in the gas zone 

to act as turbulence generators to achieve the 138-kPa pressure 

pulse. 

All of the seals were constructed in the crosscuts between the 

B and C drifts; these crosscuts are approximately 2 m high by 

5.8 m wide. The average cross-sectional area of the crosscuts 

is 11.6 m2. 

INSTRUMENTATION 

Each drift has 10 environmentally controlled data-gathering 

stations that house the instruments (shown in figures 1 and 2). 

Each data-gathering station houses a pressure transducer to 

measure the static pressure generated by the explosion and an 

optical sensor to detect the flame travel. A variety of other 

Surface quarry -7 

Figure 1 .-Plan of the Lake Lynn Experimental Mine. 
the ignition source). Previous explosion studies conducted at 

the LLEM showed that the explosion pressure pulse decayed 

less rapidly with distance in the larger, more typically sized 

LLEM entries than in smaller entries presumably because of the 

smaller surface-to-volume ratio (9). The pressure pulses 

exerted on each seal were measured by interpolation of the data 

from the nearest C drift pressure transducers both inby and 

outby the crosscut position. An additional pressure transducer 

was installed on the C drift (explosion side) face of the seal in 

crosscut No. 1. The pressure data recorded from this transducer 

correlated well (less than 7-kPa difference) with the pressure 

data obtained through interpolation. 

An important measure of the damaging potential of the 

explosion pressure pulse is the pressure impulse, which is the 

time integral of the pressure profile or the area under the 

pressure-time curve. The destructive forces of the explosion 

blast wave depend on both the maximum peak overpressure and 

the impulse (9): Under current evaluation criteria, a seal design 

need only withstand a minimum static pressure pulse of 138 kPa 

while maintaining its air leakage resistance; impulse requirements have yet to be defined. For this reason, seal designs were 

frequently subjected to higher level explosion pulses in the 

LLEM as a means to evaluate the various seal designs against 

higher impulse loadings. 

I I I I 1 

0 1 2 

TIME AFTER IGNITION, s 

Figure 3.-Typical computer-generated static pressure transducer traces in C drift following a 138-kPa level explosion pressure 

pulse. 

AIR LEAKAGE DETERMINATIONS 

An important factor to be considered for any seal design is 

its air impermeability, or its ability to minimize leakage from 

one side of the seal to the other. Measurements of the air 

leakages across the seals were conducted before and after each 
these maximum acceptable air leakage rates in cubic meters per 

second as a function of pressure differential in kilopascals. For 

pressure differentials up to 0.25 kPa, air leakage through the 

seal must not exceed 0.05 &Is; for pressure differentials over 

0.75 kPa, air leakage must not exceed 0.12 m3/s. The differential pressure was measured using the copper tubing 

through the seal. The flow rate was calculated from the linear 

air speed measured by the vane anemometer and the area of the 

opening through the brattice cloth behind each seal. 

a postexplosion visual inspection of that seal revealed sub

stantial structural damage) was considered not to meet the 

minimum standards as specified in the CFR for an underground 

coal mine seal and therefore failed. Postexplosion air leakage 

tests were not performed on seals that exhibited significant 

damage in terms of large cracks and/or block displacement. 

Seals that withstood the pressure pulse with little or no outward 

signs of damage were tested for air leakage resistance. 

TEST RESULTS 

POLYMER FOAM SEALS 

Numerous seal designs utilizing a polymer (polyurethane 

foam) material as part of the seal construction process were 

tested in the LLEM. This polymer seal design concept was 

developed and patented by MICON, Glassport, PA. The 

polymer grout is comprised of two chemical components: 

polyisocyanate and polyol resin. Each of these components was 

contained in separate 208-L drums. The liquid components 

were injected at a 1:1 ratio using a two-component polymer 

pump and blended together with an in-line static mixer at the 

injection end of the hose. When using the polymer components 

(as when using any material), all safety precautions on the 

proper storage, transportation, and handling must be followed, 

including wearing of appropriate personal protective equipment 

as specified in the materials safety data sheets. 

Test Series 1 

The first series of seals, installed in March 1993, was a 

composite structural design consisting of two dry-stacked (no 

mortar) hollow-core, concrete block walls with a polymer and 

limestone aggregate (No. 57 stone; stone size ranged from 1.0- 

to 1.9-cm core) (figure 5). Seal locations in crosscut Nos. 2,4, 
between the block walls. 
layer of new aggregate was stowed in place as soon as the 

polymer was applied to the previous aggregate layer without 

waiting for the lower lift to fully expand. This caused some 

horizontal movement of the expanding polymer instead of the 

desired vertical rise, resulting in an outward bulge of the 

partially constructed front concrete block wall. This movement 

of the front block wall did not adversely affect the strength of 

these seals because additional aggregate and polymer were 

required to be used within the expanded seal core volume, thus 

providing additional mass to the final seal design. However, 

during subsequent test series with the polymer seals in the 

LLEM, hydraulic props were utilized to anchor large surface 

area support devices (such as wood pallets) against the partially 

constructed front seal wall to prevent any horizontal movement 

of that wall due to the expansion of the polymer (see figure 8). 

In addition, each of the polymer-saturated aggregate lifts was 

given sufficient time (generally 10-15 min) to fully expand 

before the next lift was initiated. 

During actual coal mine installations, multiple seals are 

constructed simultaneously. Following the injection of the 

polymer onto the dry aggregate layer at one seal location, the 

polymer would then be applied to the dry aggregate layers on 

numerous adjacent seals. Therefore, before work would resume 

on the f'irst seal, sufficient time would elapse to allow the 

polymer-saturated aggregate layer from that first seal to fully 

expand before the addition of the next aggregate layer. 

Before subjecting the seals of the first test series to the 

explosion overpressures, air leakage tests were conducted on 

these seals. As listed in table 2, the leakage rates across the 

polymer and aggregate core seal designs 1, 2, and 3 were within 

the air leakage guidelines established for this program as listed 

in table I. The seals were then subjected to a pressure pulse that 

was generated from the ignition of a CH,-air zone at the closed 

end of the mine entry. 

Postexplosion observations showed that the seals survived 

the 145-kPa pressure pulse with no outward signs of damage 

(see table 3, seals 1-3). Postexplosion air leakage tests showed 

that each of these polymer and aggregate core seal designs 

maintained acceptable leakage resistance (see table 2, seals 1-3). 

A second explosion test (test 2 in table 3 for seals 1-3) was 

conducted to evaluate the strength characteristics of these same 

three polymer and aggregate core seal designs when subjected 

to a higher level pressure pulse. The same polymer seals had 

already withstood the 145-kPa pressure pulse (more than the 

required minimum of 138 kPa) while maintaining acceptable air 

leakage resistances. The stronger pressure pulse was achieved 

by using a zone of pulverized coal dust in addition to the CH, 

z,one. The coal dust was loaded on shelves suspended from the 

mine roof. The shelves were positioned every 3 m starting just 

inby the 14.3-m-long gas zone and extended 78 m along the 

entry. When suspended in the mine atmosphere, the coal dust 

reached a concentration of 100 mg/L; this assumed a complete 

and uniform distribution of the coal dust throughout the entry 

before the flame arrival. The pressure and flame from the 

ignition of the CH, zone dispersed and ignited the coal dust, 

which resulted in the development of a pressure pulse that 

ranged from 221 kPa at seal 1 to 276 kPa at seal 3. This 

stronger pressure pulse completely destroyed seals 1 and 3. 

The 46-cm-thick polymer and aggregate core seal (seal 2), 

as shown in figure 9, withstood the higher pressure pulse (269 

kPa). Very little damage occurred to this seal, except for the 

displacement of the front face of many of the hollow-core 

concrete blocks on both sides of the seal. However, the inside 

block faces of these hollow-core blocks were still intact and 

maintained the isolation of the polymer and aggregate core from 

the mine environment. The sealant around the perimeter of the 

seal was also intact. Postexplosion leakage determinations on 

this surviving seal 2 in crosscut No. 4 could not be conducted 

because the seals in crosscut Nos. 1-3 were destroyed. 

Table 2.-Summary of seal air leakage rates as a function of pressure differential 

(air leakage rates, m3/s) 

Preexplosion pressure differential Postexplosion pressure differential 

Seal type Seal No. Outcome 

0.25 kPa 1.1 kPa 0.25 kPa 1.1 kPa 

Polymerlstone ............ 1 0.02 0.06 0.03 0.06 Pass. 

2 .03 .09 .04 .12 Pass. 

3 .03 .08 .04 .10 Pass. 

Polymer only ............. 4 .05 .13 .12 .25 Fail. 

5 0 <.01 NAP NAP Fail. 

6 .ll .25 NAP NAP Fail. 

7 .01 .03 NAP NAP Fail. 

Polymerlstone ............ 8 0 0 0 '0 Pass. 

9 0 0 0 ' .02 Pass. 

10 0 0 0 l.03 Pass. 

Polymer only ............. 11 0 0 0 ' .02 Pass. 

Cellular concrete .......... 1 0 .02 0 .02 Pass. 

2 0 .05 .02 .07 Pass. 

3 0 .03 .O 1 .04 Pass. 

4 03 .08 .04 .12 Pass. 

Low-density block ......... 1 0 .O 1 0 .01 Pass. 

2 0 .02 0 .02 Pass. 

r]Ap Not applicable. 

0.96-kPa pressure differential. 
Table 3.-Summary of test conditions and results for polymer seal designs 

Overall seal Designed core Amount Amount Average core Explos

Test series No. seal Crosscut Block thickness, thickness, polymer, stonefill, density, and date No. No. tY Pe cm cm kg kg kg/& Test 1 

1 (Mar. 1993) . . . . 1 2 HC 117 76 1,045 - - 145 kPa; 

passed. 

- 145 kPa; 

passed. 

- 145 kPa; 

passed. 

2 (Oct. 1993) . . . . 4 1 HC 135 97 1,360 0 9 1 145 kPa; 

failed. 

5 2 LDF 137 97 1,315 0 9 1 141 kPa; 

failed. 

6 3 None 97 97 1,315 0 9 1 138 kPa; 

failed. 

7 4 LDF 109 69 1,000 0 9 1 131 kPa; 

failed. 

3 (Mar. 1995) . . . . 8 2 S 7 1 41 838 2,585 734 136 kPa; 

passed. 

9 3 HC 7 1 41 825 2,336 64 1 134 kPa; 

passed. 

10 4 S 76 46 763 2,721 571 128 kPa; 

passed. 

passed. 

NAP Not applicable. 

HC Hollow-core concrete block. LDF Lowdensity foam block. S Solid-concrete block. 

Expansion of polymer caused block wall bowing to nearly 51 cm in center. 

Polymer expansion increased core thickness to 41 cm. 

Expansion of polymer increased core thickness to 11 2 cm in center section of seal. 

Expansion of polymer increased core thickness to 76 cm in center top sections of seal. 

NOTE.-Dash indicates no data available 
seals utilized dry-stacked (no mortar) block walls (hollow-core 

concrete blocks for seal 4 in crosscut No. I; low-density 

cementitious blocks for seal 5 located in crosscut No. 2) with a 

97-cm-thick core of the polymer. The injection of the polymer 

between the two low-density cementitious block walls of seal 5 

in crosscut No. 2 is shown in figure 10. Seal 6 in crosscut No. 

3 consisted only of the 97-cm-thick polymer core; the block 

walls were removed after the polymer core was injected. This 

seal design was not intended for use in coal mines, but only as 

a means to evaluate the capability of the polymer itself to 

withstand the pressure pulse. Seal 7 in crosscut No. 4 consisted 

of the dry-stacked (no mortar) low-density cementitious blocks 

(average weight of 21 kg) with a 69-cm-thick core of the 

polymer. Upon completion of the injection of the polymer 

cores, the exterior wall faces of each of the seals were coated. 

Following construction, these polymer-only core seals were 

air leakage tested, then subjected to the explosion pressure 

pulse. The air leakage rate measured across seals 5 and 7 ftll 

well within the established guidelines (see table 2). However, 

seals 4 and 6 exceeded these air leakage guidelines at both 

the lower and higher pressure differentials. Postexplosion 

observations of the seals following the CH, ignition revealed 

that polymer-only core seals 5, 6, and 7 were completely 

destroyed by the pressure pulse, which ranged from 145 kPa at 

seal 4 in crosscut No. 1 to 13 1 kPa at seal 7 in crosscut No. 4 

(table 3 and figure 11). Postexplosion air leakage rates, as 

Figure 11 .--Condition of the 91-kg/m3-density polymer-only core 

seal following the 141-kPa level explosion. 

measured at the surviving polymer-only core seal 4 in crosscut 

No. 1, were 0.12 m3/s at a 0.25-kPa pressure differential, or 

over twice the required maximum leakage rate (see table 2, seal 

4). The failure of the polymer-only core seal designs 5, 6, and 

7 seemed to result from a combination of factors. It appeared 

that the polymer could not properly bond to the solid strata 

because of the presence of a very fine layer of dust particles that 

were present on the solid strata before the initial polymer 

injection. These dust particles bonded with much of the 

polymer and prevented direct polymer-to-strata contact, which, 

in turn, negatively impacted the adhesion characteristics of the 

polymer to the solid strata. The 91-kg/m7-density polymer was 

also adversely affected by the surface moisture present on the 

solid strata, even though these surfaces appeared dry before the 

polymer injection. Observations of the postexplosion core 

showed that the surface moisture reacted with the liquid 

polymer during the initial injection period and significantly 

reduced the density of that initial polymer layer at the polymer 

and mine strata interface. Another contributing factor to these 
In March 1995, a third series of polymer seal designs (table 

3; seals 8,9, 10, and 1 1) was tested in the LLEM to determine 

the minimum quantity of aggregate required. During the first 

test series (seals 1-3) with the polymer and aggregate core seals 

in March 1993, it had been very difficult to quantify the 

amounts of aggregate used because the material was delivered 

and handled in bulk form. For this third series, the aggregate 

was dried and packaged in plastic-wrapped 22-kg bags to 

eliminate the excessive surface moisture effects on the polymer 

expansion process, which was experienced when using bulk 

aggregate. This also allowed for increased accuracy in 

measuring the amount of aggregate used per lift and simplified 

the material handling effort by eliminating the need to hand 

shovel the aggregate into the core. 

Two of the polymer and aggregate core seals evaluated 

during this third series were similar in design, except that the 

dry-stacked (no mortar) walls of seal 8 in crosscut No. 2 

consisted of solid-concrete block (average 22 kg each) and seal 

9 in crosscut No. 3 utilized hollow-core concrete block. Both 

designs maintained a nearly 41-cm-thick core of polymer (1 60- 

kg/m3 design density) and aggregate. The core density was 

designed to be 560 kg/m3, consisting of an aggregate density of 

456 kg/& and a final polymer density of 104 kg/m3. 

polymer-only core between two solid-concrete-block walls. 

The solid-concrete blocks were dry-stacked (no mortar) with 

staggered vertical joints and installed with the 15-cm side of the 

block up. Approximately 480 blocks were used in this seal, 

which included a 41- by 41-cm center pilaster designed into 

each wall (similar to seal 10). Twenty-three lifts of a 160- 

kg/&-density polymer8 was injected between the block walls 

into the 6.8-m3 volume (6.1-m-wide by 2.2-m-high entry). A 

total of 1,225 L, or 1,385 kg, of polymer was required to 

complete the core, resulting in a final core density of 203 kg/m3. 

The total mass for polymer-only core seal 11 was approximately 12,304 kg. The exterior wall faces were then 

coated. 

During the preexplosion air leakage evaluations for this third 

test series, no leakage was detected through any of the seals 

against pressure differentials across each seal of up to 0.96 kPa 

(see table 2, seals 8-1 1). Following test 1 of the third test series, 

all four seal designs survived the pressure pulse (from 136 kPa 

at seal 8 in crosscut No. 2 to 1 17 kPa at seal 1 1 in crosscut No. 

5) with no outward sign of damage (see table 3; seals 8-1 1). 

Postexplosion leakage rates (see table 2, seals 8-1 1) were also 

well within the accepted guidelines. The explosion pressure 

pulse for test 1 was lower than the required 138-kPa minimum 

Approximately 738 L, or 838 kg, of the liquid polymer was 'This was the design density; actual density was approximately 203 kg/m7 

injected into the core volume, and 2,585 kg of aggregate was bad 0, samples taken during the core injection. 
On average, a 10- to 15-cm-high aggregate layer saturated 

with the polymer typically experienced an additional vertical 

expansion of 13 to 20 cm within the relatively dry conditions 

(40-60 pct relative humidity) of the LLEM. Therefore, a fully 

expanded polymer and aggregate lift ranged from 23 to 35 cm 

high. During this polymer expansion process, a vertical movement of the aggregate also occurred and resulted in a 

distribution of the aggregate within the bottom 20 to 25 cm of 

the fully expanded lift. Generally, as was documented during 

the LLEM installation process, any increased height in the total 

lift beyond the first 20 to 25 cm consisted of polymer only (no 

aggregate). Subsequent observations of each of the core cross 

sections following removal of the seals from the mine verified 

these findings. Based on thc outcome of these full-scale 

evaluations within the LLEM, the polymer composite seal 

designs, which have successfully met the requirements, have 

been approved for use in U.S. underground coal mines. 

CELLULAR CONCRETE SEALS 

Cellular concrete is a material similar to cementitious foam, 

which successfully withstood explosion pressures within the 

To ensure that the slurry completely filled to the mine roof, two 

similar-sized polyvinyl chloride (PVC) bleeder ports with 

Figure 12.-Injection of the cellular concrete slurry between the 

wood and brattice cloth form walls. 
30 days to cure (time period required for material to approach 

maximum strength characteristics), then subjected to air leakage 

and explosion testing. The preexplosion air leakage data for 

each seal were well within the established benchmark standards 

for these tests (see table 2). All four seal designs withstood 

the required explosion pressure pulse with no 

vertical fractures on both sides of the seal. Seal I, which 

utilized the woven steel mesh within the seal body, revealed no 

such fracturing and only very minor surface cracking. It must 

be noted, however, that all four seal designs withstood the 

required minimum 138-kPa pressure pulse. 

Table 4.-Summary of test conditions and results for the 1.2-m-thick cellular concrete seal designs 

Average Explosion test outcome 

Crosscut and Average height Reinforcement compressive 

seal No. of first pour, m strength. 

kPa Test 1 Test 2 

Yes 1,675 145 kPa; 327 kPa; 

passed. passed. 

Yes 1,390 148 kPa; 297 kPa; 

passed. failed. 

No 1,870 138 kPa; 31 0 kPa; 

passed. marginal.' 

No 2,075 124 kPa; 328 kPa; 

passed. failed. 

' Seal withstood the explosion pressure pulse, but sustained numerous vertical fractures on both sides of the seal. 
bolts were grouted into the floor and ribs. In operating mines, 

keying of the seal is achieved by hitching or trenching to a 

depth of at least 10 cm into the solid strata of the ribs and floor 

before erecting the seal. The simulated keying, as used in the 

LLEM, has been permitted in place of the standard keying 

techniques and is used in somc coal mines with hard, sandstone 

bottoms. 

The design for both low-density block seals was essentially 

the same except for the size of the center-support pilaster. Each 

seal wall was 61 cm thick with a center interlocking pilaster. A 

1.2- by 1.2-m pilaster was incorporated into the seal 1 design; 

seal 2 was constructed with a 1.8-m-wide by 1.4-m-thick 

planking and wedging to the main seal wall were conducted 

first, then the pilasters were planked and wedged in a similar 

manner. 

Preexplosion air leakage rates through each seal were 

negligible for pressure differentials up to 0.75 kPa (table 2). 

Observations of the seals following the 138- to 148-kPa level 

explosion pressure pulse revealed no outward damage to either 

design except for very slight hairline cracks in the surface 

coating (figure 13 and table 5). Subsequent air leakage data 

showed that each seal maintained acceptable leakage resistance 

characteristics (see table 2). 

Table 5.-Summary of test conditions and results for low-density cementitious block seal designs 

Crosscut and Seal wall Pilaster width1 

thickness, thickness, No. block Explosion test seal No. crn crn outcome 

1 ................ 6 1 ' 12211 22 168 Passed; 148 kPa. 

2 ................ 6 1 '1 8311 42 192 Passed; 138 kPa. 

'Coated on both inby and outby face. 

'Coated only on inby face. 
partition area, on each side of the seal wall, extended 41 cm 

from the seal wall and was 81 cm long (distance between the 

pilasters) and 30 cm deep. This trap design is referred to as a 

water trough. Each of these trap designs was tested separately. 

Figure 14.-Three water trap design openings installed through 

a solid-concrete-block seal. 

The results of the water trap evaluation showed that the three 

trap designs typically used in coal mines were inadequate when 

tested under worst-case conditions within the LLEM. The 138- 

kPa pressure pulse, as generated from the CH, ignition on the 

C-drift side of the seal, removed all of the water from each of 

the three trap designs (tested separately) and, in several 

instances, allowed passage of the explosion flame through the 

trap. During one test using the 15-cm-diam water pipe trap, the 

explosion pressure pulse pushed the water out of the trap, 

enabling the explosion flame to then travel through the trap and 

ignite he CH, zone surrounding the trap on the opposite side ol' 

the seal, as shown by optical sensor data and vidco camera. 

During one test with the water trough and three duplicate tests 

with the 30-cm-diam water pipe trap designs, sufficient watcr 

was dispersed around the trap opening on the B-drift side of the 

seal to cool and quench any flame that traveled through the trap. 

This quenching effcct resulted in the inability to ignite thc 

surrounding flammable CH,-air atmosphere. Even though no 

ignitions occurred, in a worst-case, dry-mine, or zero-flow 

scenario, the water trap designs could not rerill after the 

explosion and would thus allow the passage of flammable 

andlor toxic gases from one side of the seal to the other. 
modification to the water trough design. An additional course 

of block was installed around the water trough area on the 

active (B-drift) side of the seal. The installation of this 

additional course effectively raised the water level approximately 15 cm above the original water trough on the other 

(C-drift) sealed side of the seal. When subsequently evaluated 

under explosion conditions, this modified water trough trap 

design prevented the passage of flame through the trap. 

Postexplosion observations showed that the water level within 

the trough on the B-drift side of the seal was to the top of the 

trough. By simply adding one additional course of block to the 

active side of the water trough, this modified trap design not 

only prevented the passage of flame, but also maintained a full 

water level within the trough, thereby preventing the passage of 

gases and/or flame through the trap. 

Explosion tests were then conducted against a slightly 

modified 15-cm-diam PVC water pipe trap. The end of the 

PVC trap was extended vertically on the active side of the seal 

to enable the water level behind the seal to raise an additional 

15 cm (see figure 15). The water level behind the seal was such 

that the entire 15-cm-diam trap opening through the seal was 

just below the water level. This had an outcome similar to that 

the seal to prevent higher water levels behind the seal. 

Depending on the pipe trap diameter, this may require some 

digging into the active side mine floor to accommodate the 

U-shaped pipe trap. 

These simple modifications to the trap designs allowed the 

water level in the sealed area to raise to a level just above the 

trap opening through the seal, resulting in the prevention of the 

explosion flame from propagating through the trap while, at the 

same time, maintaining a full water level within the trap. These 

trap design modifications will provide increased explosion 

protection to existing water trap installations on seals that are 

currently producing water. However, these modifications do 

not address the protection concerns for either new or existing 

seal installations where there is zero or low water flow 

conditions. Under these low or zero water flow conditions, the 

water traps (both existing and modified versions) will be 

colnpletely emptied of water following the explosion and, in 

some cases, fail to prevent propagation of the explosion flame 

through the trap. The focus of our research program is 

currently on designing and evaluating water trap designs that 

can prevent explosion flame propagation while maintaining full 

water levels within the trap following the explosion. 
CONCLUSIONS 

Four types of construction materials were evaluated in the 

LLEM as design alternatives to the standard solid-concreteblock design typically used for underground coal mine seals. 

Each design was evaluated under full-scale mine explosion 

conditions for its capability to withstand the required pressure 

pulse of at least 138 kPa while maintaining resistance to air 

leakage. 

Several seal designs were evaluated utilizing two dry-stacked 

(no mortar) walls consisting of either concrete block or lowdensity cementitious block with polyurethane foam (with and 

without added stone aggregate) cores between the walls. The 

seal designs with the 41 -, 46-, and 76-cm-thick polymer foam 

and aggregate cores sandwiched between two dry-stacked, 

hollow-core or solid-concrete-block walls survived the 138-kPa 

pressure pulse while maintaining acceptable air leakage rates, 

thus meeting the requirements. Four seal designs constructed 

with 97-cm-thick cores consisting of 9 1 -kg/m3-density polymer 

foam (no stone aggregate) did not survive the explosion 

overpressures. However, a seal constructed with a 5 1 -cm-thick, 

203-kg/m"density polymer foam core (no stone aggregate) 

between the two dry-stacked solid-concrete-block walls 

survived the explosion pressure pulse and subsequent air 

leakage tests. These tests showed that moisture had caused 

bonding problems between the polymer and the aggregate and 

between the polymer and the mine strata. To reduce the adverse 

effect of moisture on the polymer, dry-bagged aggregate is now 

used. The seal area must also be properly prepared, which 

entails removal of all excessive dust, standing water, and 

moisture accumulations. In addition, it is recommended that a 

high-density polymer be applied initially to the mine strata to 

reduce the adverse effects of moisture on the polymer density. 

Full-scale explosion testing of cellular concrete seal designs 

showed that the two-pour construction technique utilized during 

the injection of the slurry between the 1.2-m-spaced forms did 

not adversely affect the strength of the seals. Two additional 

low-density cementitious foam block seal designs constructed 

with an alternative bonding mortar withstood the required 

pressure pulse while maintaining acceptable air leakage rates. 

Evaluations of several U-shaped water trap pipes and a water 

trough design typically used in coal mines showed that they did 

not prevent the passage of the explosion flame and/or gases 

from one side of a trap to the other following an explosion 

pressure pulse. Simple modifications, such as an extension pipe 

on the pipe traps, which can be retrofitted to the various trap 

designs, have been successfully demonstrated under full-scale 

explosion conditions to prevent the passage of flame while, at 

the same time, maintaining an adequate level of water within 

the trap. 

Based on the most recent tests conducted in the LLEM, 

alternative seal construction materials are now being used 

regularly in U.S. underground coal mines. PRC7s seal research 

program at Lake Lynn Laboratory will continue to evaluate, 

under full-scale conditions, innovative seal designs and 

improved water trap devices. Many of these seal designs will 

provide increased safety for underground mine workers by 

reducing materials handling and associated injuries, decreased 

construction times when installing seals under hazardous fire or 

potential explosion conditions, and/or enhanced overall seal 

performance in terms of strength characteristics, air leakage 

resistance, and better durability in high-convergence areas. 

ACKNOWLEDGMENTS 

The authors thank Jack M. Teatino, Donald D. Sellers, Frank their research support work required in constructing and 

A. Karnack, and Samuel E. Angelo (miner mechanics), and Paul evaluating the various seal designs. 

K. Stefko and Paul D. Kolarosky (mine foremen) of PRC for 

REFERENCES 

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