EVALUATION OF NEW METHODS AND FACILITIES TO TEST EXPLOSION 

RESISTANT SEALS 

Michael J. Sapko and Eric S. Weiss 

Pittsburgh Research Laboratory, 

National Institute for Occupational Safety and Health 

Pittsburgh, PA 15236 U.S.A. 

ABSTRACT 

The Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational 

Safety and Health (NIOSH) has conducted full-scale explosion experiments to evaluate 

the strength characteristics of several seal designs for isolating worked out areas in 

underground coal mines. The PRL is currently pursuing the development of improved 

seal evaluation methods and facilities based on pneumatic and hydrostatic testing that 

would lend themselves to in-situ seal testing in an operating mine. Two chambers were 

constructed and instrumented within PRL’s Lake Lynn Experimental Mine (LLEM) for 

the evaluation of seals. Two solid-concrete-block seals, 13.5 and 40.3 square meter 

cross-section and 406 mm thick with a central pilaster, were constructed in both the 

small and large chambers and tested to failure with a series of methane explosions 

producing loading pressures ranging from 386 to 688 kPa. 

A preliminary size-scaling relationship is presented for estimating the ultimate 

strength of masonry block seals built with rib and floor hitching. Testing seal designs in 

the chambers shows promise as an alterative evaluation method to that of full-scale 

explosion testing within the LLEM. 

INTRODUCTION 

Since 1993, seven documented explosions of methane and/or coal dust occurred within 

sealed areas of underground U.S. coal mines. These explosions, believed to be initiated 

by lighting strikes on the surface, destroyed numerous seals and caused considerable 

damage in the active areas of the mines. Fortunately, these explosions did not cause 

fatalities or injuries. One explosion that shows strong evidence of being caused by 
lightning occurred at the Mary Lee No. 1 mine in Alabama on August 22, 1993 (Hurren 

et al., 1993). From all indications, the explosion occurred at 4:40 pm. At the same 

time, the National Lightning Detection Network (NLDN) observed a 21.7 kA lightning 

flash in the vicinity of a fan shaft that vented the area where the explosion occurred. 

Subsequent investigations at the fan shaft indicated electrification of the 4.5 m high 

metal vent pipe extending from the concrete shaft cap; this cap was destroyed during the 

explosion. 

Three explosions occurred in the gob at the Oak Grove Mine, Alabama (Scott et al., 

1996). In each instance, severe thunderstorms and lightning occurred above the sealed 

gob. With each of these events, the NLDN documented scores of lightning flashes 

within the vicinity of a 114 mm diameter steel cased hydro-geological test hole 

connecting a surface pumping station to the gob. Several 1.2 m thick pumpable 

cementitious seals were destroyed, including one that was rebuilt after a previous gob 

explosion. Analysis of the fractured seal material showed compressive strengths less 

than that required by the Mine Safety and Health Administration (MSHA), except the 

rebuilt seal which was stronger than required. These gob explosions, initiated by 

lightning, forcefully demonstrate the need for adequate seals and sealing procedures to 

protect mine workers against explosions. 

Title 30, Part 75.335 of the Code of Federal Regulations (CFR) (1995) states that 

abandoned areas of a mine must be either ventilated or isolated from active workings 

through the use of seals capable of withstanding a static horizontal pressure rise of 20 

psi (138 kPa). Seals are also used to isolate fire zones or areas susceptible to 

spontaneous combustion. To effectively isolate areas within a mine, a seal should be 

designed to control the methane and air exchange between the sealed and open areas so 

as to prevent toxic and/or flammable gases from entering the active workings. A seal 

must be capable of preventing an explosion from propagating into or out of the sealed 

area. Early U.S. Bureau of Mines (USBM) research indicated 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 (Mitchell, 1971). Pressure balancing across the seals 

plays a key role in seal deployment strategies by minimizing the exchange of gases and 

limiting the resulting volume of flammable gas in the gob. 
In the early 1930’s, the USBM conducted a series of tests and found that restraining 

the edges of a seal caused a dramatic increase in the seal strength to a much higher level 

than predicted by plate theory (Rice et al., 1930, 1931). Full-scale explosion 

experiments also showed concrete walls that were recessed into the roof, ribs and floor, 

and had a thickness to width ratio of at least 0.1, resisted much higher pressures than the 

theoretical design pressure. These results showed that recessing the ends of the concrete 

wall into the surrounding strata allows the wall to act as a flat arch. This arching 

behavior transmits a lateral thrust to the strata, which then acts as a buttress to prevent 

seal movement. Several efforts have been made to explain the arching behavior through 

various static design models. However, blast analysis of masonry and concrete 

structural elements has traditionally been more of an art than a science. It has been 

difficult estimating structural loads due to detonations and deflagrations and predicting 

load deformation behavior, especially for masonry walls. Early U.S. research on the 

response of walls to blast loads was conducted during World War II by the National 

Defense Committee (1946) and then later refined methods were developed to consider 

the load-time history and structural parameters such as material strengths and support 

conditions. The Department of Army, Navy, and Air Force published an important 

document entitled “Structures to Resist the Effects of Accidental Explosions,” 

TM 5-1300, (1990) which is useful for predicting the ultimate strength of masonry and 

concrete walls. As a continuation of this work, the U.S. Army Research and 

Development Center, (Slawson, 1995) developed a single degree of freedom (SDOF) 

computer code referred to as the Wall Analysis Code (WAC). This code, which is 

discussed later in the paper, was used to examine the response of solid block masonry 

seals exposed to pressures produced from the combustion of pre-mixed methane-air 

concentrations. 

Many countries, including the U.S., Australia, France, Germany, Poland, and 

China, have pursued research for developing and evaluating explosion-resistant 

structures for sealing sections of underground mines. Since the early 1990's, the PRL 

and MSHA have been jointly investigating the ability of various existing and new seal 

designs to meet or exceed the requirements of the CFR. Before any new seal can be 

deemed suitable by MSHA for use in underground coal mines, the seal design is 

generally required to undergo full-scale performance testing at PRL’s LLEM (Triebsch 
and Sapko, 1990). A long-term seal research program conducted at the LLEM has 

resulted in the development of revised standards under CFR Title 30, Part 75 and the 

subsequent development of many new alternative, innovative seal designs that are now 

being used within the U.S. mining industry (Weiss et al., 1996). 

Evaluating seals by full-scale mine explosion testing makes it difficult to determine 

the precise conditions for seal failure and, although not required by CFR Title 30, 

difficult to obtain the strength safety factor for the seal. As a alternative method to the 

full-scale mine explosion evaluations in the coal mine sized entries of the LLEM, 

NIOSH constructed two test chambers within a seldom used area of the original, highroof workings of the LLEM. These chambers will allow for the evaluation of seal 

designs against static pressure loadings and will allow for the development of geometric 

size-scaling guidelines for use when installing seals in mine entries in excess of 2.4 m 

high and/or 6.1 m wide. This paper provides a brief overview of the ongoing study to 

evaluate the use of these chambers for pressure loading of full-scale seal designs using 

compressed air, water, or internal gas explosions. 

TEST CHAMBERS 

In 1998, PRL (Sapko et al., 1999) constructed two large-scale underground chambers 

within the original workings of the LLEM to conduct pneumatic, hydrostatic, or 

explosion pressure loading of candidate seals. Figure 1 is a schematic of the large 

chamber. The chamber dimensions are 9.1 m wide by 4.6 m high by 3.1 m deep with a 

maximum cross sectional area of 42 m2. The smaller of the two chambers is 6.1 m wide 

by 2.4 m high by 3.1 m deep and can accommodate a seal design with a cross sectional 

area up to 15 m2

. This smaller chamber is similar in cross sectional area to the crosscuts 

in the multiple entry section of the LLEM where the seal designs have traditionally been 

evaluated using the full-scale methane and/or coal dust explosions. 
Figure 1. Large test chamber for pressure loading of seals with water, compressed air, 

and with the combustion of confined concentrations of methane-air 

Both chambers are connected via remote controlled air valves to two diesel driven 

air compressors which provide 28 m3

/min (1000 cfm) of air. The air compressors were 

used to conduct the pre- and post-explosion leakage measurements. The seal is 

pressurized to about 12.7 cm of water and then as the air leaks out the pressure decay is 

recorded. The rate of pressure decay is then converted to an average volumetric flow 

rate. Parallel leakage tests were conducted to compare the pressure decay method with 

the conventional steady state method used during the multiple entry seal evaluations. 

The conventional method involved measuring, with an anemometer, the air that passes 

through the seal and a 0.1 m2 opening in a nearly air-tight brattice curtain installed 

between the seal and the anemometer while maintaining a constant differential pressure 

across the seal. The two leakage methods compared favorably. The air compressors 

were also used to slowly pressure load the seal up to 140 kPa depending on the leakage 

rates through the seal. 

Internal explosions of methane-air were used to characterize the ultimate failure 

strength of the seal. To achieve these methane-air ignitions, each chamber is equipped 

with methane and oxygen injection systems. The oxygen and methane are supplied by 
compressed gas cylinders. A pre-determined amount of methane is metered into the 

chamber behind the seal and thoroughly mixed with air using an explosion-proof fan 

located within the sealed area of the chamber. The fan generates an air flow within this 

area of 85 m3/min. Uniformity of pre-test gas concentrations were determined by 

drawing gas through tubing and into an on-line infrared methane analyzer and a para 

magnetic oxygen analyzer. Samples were also collected in evacuated glass tubes for 

subsequent analysis by gas chromatography. The flammable gas mixture was ignited at 

the center of the combustible volume by a 0.5 s electrical discharge from a 30 kv 

luminous tube transformer across a 3.2 mm spark plug gap. 

The two chambers are equipped with internal 0-1.4 MPa (0-200 psia) strain gage 

pressure transducers (1000 Hz) for measuring the internal explosion pressure history. 

Three spring-loaded linear variable displacement transducers (LVDT) were mounted 

around a 90E bend outside the chamber exit and connected to the test seal via 

lightweight nylon line. This mounting system protected the LVDTs from any 

projectiles that might occur. One LVDT was connected at the exact center (mid-height 

and mid-width) of the seal. A second LVDT was connected at a 1/4-height and midwidth point. A third LVDT was connected at the 3/4-height and mid-width point. As 

the seal is pressure loaded, the seal displaces outward and the LVDTs measure this 

displacement by generating an output signal of ~68 mv/mm. Data was recorded at 2000 

samples/s per channel with a WINDAQ PC-based data acquisition system (DAS). 

CHAMBER PNEUMATIC TESTS 

The standard-type solid-concrete-block seal design was chosen for the initial evaluation 

since this design was extensively evaluated for several years in the PRL’s Bruceton 

Experimental Mine (BEM) and in the LLEM. This standard-type seal was used to form 

the basis for the current CFR Title 30, Part 75.335. Of the solid-concrete-block seals 

tested in the experimental mines, only the standard-type 406-mm-thick seal with 

staggered and fully mortared joints, a center pilaster, floor and rib keying, and wedged 

at the roof successfully withstood the required pressure pulse. Figure 2 is a schematic 

of the standard-type solid-concrete-block seal. This same seal design was installed in 

both the small and large chambers. Seal keying in the chambers was accomplished by 

butting the seal against two 0.4 by 0.8 m solid-concrete-block rib support columns; 
these columns were positioned to contact the 0.3 m wide steel H beams. Solid-concreteblocks and mortar were placed between the base of the seal and the steel beam to 

simulate simple floor keying by buttressing the seal against the steel. Both sides of each 

seal were coated with a waterproofing sealant to help minimize air leakage. Each seal 

was allowed to cure for 28 days before testing. Before and after each seal test, the airleakage across the seal was measured. Each seal was initially pressure loaded through 

the use of the twin air compressors. The air pressure behind the seal increased from 0 to 

144 kPa (0-20.9 psig) in 290 s and then decayed to 14 kPa (2 psig) approximately 600 s 

after the air supply was discontinued, as shown in figure 3. Due to excessive air leakage 

at the higher pressures and limited compressor capacity, the maximum pressure 

obtained behind the seal in the large chamber was 76 kPa (11 psi). For comparison, 

figure 3 also shows the explosion pressure history measured at the standard-type seal 

constructed within a crosscut in the multiple entry section of the mine. The methane 

explosion in the open-ended drift produces a much more rapid dynamic loading of the 

seal. The seal’s initial resistance to failure during the multiple entry explosion is 

primarily governed by the reaction of the seal’s inertial mass to rapid pressure loading 

whereas, the resistance to failure from the long term pressure loading is primarily 

controlled by the strength properties of the seal. 

Pilaster center 

Transverse pattern 

of laying block 

Figure 2. Standard-type solid-concrete-block seal design with center 

pilaster 
6 

Time, s
 

0 1 2 3 4 5
 

Pressure, kPa 

160
 

140
 

120
 

100
 

80
 

60
 

40
 

20
 

0 

-20 0 100 200 300 400 500 600 

Bottom Scale 

Top Scale 

Time, s 

Figure 3. Comparison of standard-type seal pressure loading at 138 kPa with 

compressed air in the small chamber to that of a methane explosion in the 

multiple entries 

CHAMBER EXPLOSION TESTS 

In addition to the pneumatic tests, these standard-type solid-concrete-block seal designs 

were exposed to several internal ignitions of methane. Five such ignitions or explosions 

were conducted in the small chamber, exposing the standard-type seal with a pilaster, 

seal C1, to increasing pressures. Four explosions were conducted with the similar seal 

design, seal C2, without a pilaster. The standard-type seal design with a center pilaster 

in the large chamber, seal L1, was explosion tested to failure. A summary of the seal 

type, seal dimensions, and explosion test data are shown in table 1. 
 

Table 1. Summary of small and large chamber test results for the solid block seal with and 

without pilaster. 

Explosion Test 1 2 3 4 5 6 7 8 9 10 

Seal1 

C1 C1 C1 C1 C1 C2 C2 C2 C2 L1 

Width, m 5.14 5.14 5.14 5.14 5.14 5.14 5.14 5.14 5.14 8.53 

Height, m 2.62 .62 .62 2.62 2.62 2.62 2.62 2.62 2.62 4.72 

Thickness, m 0.4 0.4 0.4 0.4 0.4 0.42

 0.42 0.42 0.42 0.4 

Mid line movement 

at Max P, mm 1.52 - 2.03 3.3 +76.2 - - - - +76.2 

Max Pressure, P, 

kPa 390 22 651 49 688 18 24 38 669 221 

Result P P P P F P P P F F 

1

20 x 15 x 40 cm (nominal 8 x 6 x 16 inch) solid concrete block. Average compressive strength 

of block =16.56 +/- 0.69 MPa (2400 +/-100 psi) 

2

No pilaster. 

P=Passed post-explosion leakage 

F= Failed - catastrophic rupture 

The standard-type seal (C1 seal with a center pilaster) withstood four constant 

volume explosions before it ruptured at a peak static pressure of 688 kPa. After test 3, 

hairline cracks were visible primarily along the central mortar joints. The postexplosion leakage increased to about 2.7 m3

/min (97 cfm) at 0.25 kPa (1 in water gauge) 

which was still within the acceptable limits. For the fourth test, approximately 6 m3 

(210 ft3

) of oxygen was injected into the chamber followed by the methane injection, 

resulting in a near stoichiometric mixture. The pressure profile produced from the 

combustion of this oxygen-enriched mixture is shown in figure 4. As expected, the 

combustion was much more rapid as compared to the test without the oxygen injection 

and generated a peak static pressure of 688 kPa at 0.34 s after ignition. Prior to the 

fourth test, the standard-type seal design had never been destroyed during full-scale 

BEM or LLEM mine explosion tests which generated overpressures up to 317 kPa. 

Seal C2, without the center pilaster, ruptured during the fourth explosion (test 9) at 

a peak static pressure of 669 kPa or ~20 kPa below the failure pressure of seal C1 with 

the center pilaster. Under these test conditions, both seals provide a margin of safety of 

about 4.8 to 5 times the CFR requirement. 
Chamber Pressure, kPa 

800 

700 

600 

500 

400 

300 

200 

100 

0 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Center Displacement, mm 

0 0.1 0.2 0.3 0.4 

Time After Ignition, s 

Figure 4. Static pressure profile during test 5 that resulted in the failure of 

the standard-type solid-concrete-block seal design in the small chamber 

The second series of tests was conducted in the large chamber with the standardtype, solid-concrete-block design. The main seal wall was 406 mm thick with a 813 m 

thick center pilaster; the cross sectional area of the seal was 40.26 m2

. Pneumatic 

testing of the large seal was not as successful as with the smaller seal due to the larger 

seal surface area. With the two compressors operating at full capacity, the maximum 

attainable air pressure behind the large chamber seal was 71 kPa. At this air pressure, 

the LVDT indicated a maximum deflection of 2.5 mm. Post-test leakage produced an 

acceptable 0.7 m3

/min (25 cfm) air leakage at 0.25 kPa water gauge. No visible 

indication of surface cracks in the horizontal or vertical mortar joints was evident. The 

pneumatic testing studies indicated that, in most cases, the post-test leakage was less 

than the pre-test leakage measurements. It appears that, as the seal flexes from pressure 

loading, the fine dust kicked up by the injection of compressed air enter the small 

orifices and plugs them. Future studies will continue to better identify and characterize 

this effect. 

After the pneumatic tests, methane was injected into the chamber and, when mixed 

with the air, produced an ~5.7% methane-air atmosphere. The flammable gas mixture 

was then ignited. The resulting pressure history is shown in figure 5. Based on the 

timing of the LVDT data, the maximum pressure peak reached 221 kPa in about 13 s at 
which time the seal ruptured. The displacement of the centerline of the seal as a 

function of pressure loading is shown in figure 6. The seal flexed nearly linearly with 

the pressure loading to a maximum displacement of about 11 mm at a peak static 

pressure loading of 207 kPa (30 psig), at which point, the seal ruptured and displaced 

outward. 

0 

50 

100 

150 

200 

250 

Chamber Pressure, kPa 

0 

50 

100 

150 

200 

250 

Center Displacement , mm 

0 2 4 6 8 10 12 14 

Time After Ignition, s 

Figure 5. Static pressure profile during test 10 that resulted in the failure of the 

standard-type solid-concrete-block seal design in the large chamber 

250 

Chamber Pressure, kPa 200 

150 

100 

50 

0 

0 20 40 60 

Center Displacement, mm 

Figure 6. LVDT displacement data as a function of pressure loading on the 

standard-type solid-concrete-block seal during test 10 in the large chamber 
 

 

 

SIZE-SCALING OF SEALS 

Mine geometries can often vary within the mining horizon and from mine to mine 

throughout the U.S. mining industry. Entry height, requiring seals, can range from a 

low of 0.8 m to a high of 4.5 m. Major geologic faults or roof fall areas may require 

even larger seals. Geometric size-scaling relationships are needed to simply and 

reliably scale explosion performance results gathered in nominal 2.1 m high by 6.1 m 

wide entries to other dimensions while maintaining explosion isolation at the required 

138 kPa level. The U.S. Army Research and development Center developed a SDOF 

computer code WAC for determining the ultimate strength of reinforced and nonreinforced masonry concrete walls. This code provides engineers with a useful tool to 

calculate the response of typical walls subjected to various blast loads. The WAC, 

calculates the resistance function (load-deflection) of a wall given construction details 

such as dimensions, material properties, and support conditions. The SDOF method, 

models the response of a structural element as a spring-mass system. The effective 

mass of a SDOF method is based on the deformed shape of the wall and loading 

distribution. The spring stiffness describes the resistance of the responding element to 

deformation due to the applied loading. The resistance function may be linear, bilinear 

(elastic-perfectly plastic), or multi-linear. The code calculates the actual SDOF 

equivalent loads given the explosion pressure history and solves the equation of motion 

to determine the response time history of a critical central point on the wall. 

A parametric study was conducted using the WAC to develop simplified sizescaling relationships for predicting the ultimate strength of a seal and then comparing 

these predictions with recent LLEM chamber data. The key scaling parameters 

considered for the initial study were seal width, seal height, compressive strength of the 

concrete or masonry units, material density, wall thickness, and the rib and floor 

hitching. The WAC predictions for ultimate strength were regressed and the best fit is 

identified as an arching theory. Figure 7 is a plot of this arching prediction model in 

terms of structure thickness, width, and material compressive strength as a function of 

failure pressure. The WAC output was correlated with a simplified formula for the 

arching action in transverse laterally loaded masonry wall panels (Anderson, 1984 ) as: 
P=k*Fc*(T/W)2

; 

where P is the predicted ultimate failure pressure in kPa, Fc is the material compressive 

strength of the block in kPa, T and W are the thickness and width respectively of the 

seal in m, and k is the slope. Also shown in figure 7 are the experimental results from 

testing the 406 mm thick standard-type solid-concrete-block seal in the small chamber 

and a similar seal in the large chamber, C1 and L1, respectively. Pressure at Seal Failure, kPa

900 

800 

700 

600 

500 

400 

300 

200 

100 

0 

Model Prediction Small Chamber 

Large Chamber 

0 10 20 30 40 50 60 70 80 90 100 110 120 

Fc*(T/W)2 

Figure 7. Comparison SDOF model predictions for ultimate seal strength with 

experimental failure data for the standard-type solid-concrete-block seals in both 

the large and small chambers 

 The failure pressure 

for both the large and small standard-type seal with pilaster agree well with the SDOF 

code predictions and the simplified best fit approximation of: 

P=0.35 * Fc*(T/W)2 

to the SDOF code. Although the agreement with experimental data is good, this 

preliminary approximation for ultimate seal strength should be used with caution for 

predicting the ultimate strength. The accuracy of the prediction relies on quality 

masonry construction, close contact between the seal and the rib abutments, and the 

abutment thrusts must be higher than the values to cause crushing of the masonry 

(17.25 MPa) under arching action. These results suggest that the arching theory 

approximation to the SDOF computer code predictions provides a reasonable method of 

approximating ultimate arching strengths with rigid abutments. Research continues to 

refine these relationships to include abutment strength and thicker seal designs with and 

without hitching. 
SUMMARY 

Before MSHA will deem a seal design suitable for use in underground coal mines, the 

design has to be evaluated and, in many instances, undergo full-scale explosion testing. 

These new chambers show promise as a method for pressure testing of mine seals. The 

chambers may provide a means for evaluating hydrostatic effects, extend the duration of 

the pressure loading, facilitate testing to failure to determine the seal’s strength safety 

factor, and facilitate testing the strength and leakage characteristics of seals used to 

impound water. 

These preliminary results indicate that the explosion evaluation of seals using the 

chamber approach show promise as an alternative to the present full-scale explosion 

evaluation method. The large and small chambers will help facilitate the development 

of new, stronger seal designs, the development of geometric size-scaling relationships, 

the validation of in-situ seal performance for safe use underground, and replace the 

expensive full-scale performance testing at the LLEM. 

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Anderson, C. (1984). “Arching Action in Transverse Laterally Loaded Masonry 

Walls.” The Structural Engineer, Vol 62B, No. 1, March, 22 pp. 

Hurren W.E., Tuggle, G.N. and McGruder, F. I. (1993). “Mary Lee No. 1 Mine,” 

Accident Investigation Report (Underground Coal Mine), U.S. Labor Dept. MSHA, 

August 22. 

National Defense Research Committee (1946). “Effects of Impact and Explosions.” 

Summary Technical Report of Division 2, Washington, D.C. 

Joint Departments of Army, Navy, and the Air Force (1990). “Structures to Resist the 

Effects of Accidental Explosions,” 

TM 5-1300/NAVFAC P397/AFR 88-22, November. 

Mitchell, D. (1971). “Explosion-Proof Bulkheads: Present Practices.” U.S. Department 

of the Interior, Bureau of Mines RI 7581, 16 pp. 

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Department of the Interior, Bureau of Mines RI 3036, 11 pp. 
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Slawson, R.T. (1995). “Wall Response to Airblast Loads: The Wall Analysis Code 

(WAC).” U.S. Army Engineering and Development Center, Vicksburg, MS. 

Scott, D.S., Checca, E. L., Stephen, C. R. and Schultz, M. J. (1996). “Oak Grove Mine, 

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Triebsch, G. and Sapko, M.J. (1990). “Lake Lynn Laboratory: A State-of-the-Art 

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75-21. 

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Safety and Health Administration, Department of Labor; Subchapter O–Coal Mine 

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Underground Coal Mine Seals.” U.S. Department of the Interior, Bureau of Mines, 

USBM RI 9634, 16 pp.