good ventilation constitute the dominant portion of America's 

line of defence against underground coal mine explosions. 

Without reliable seals, a great number of miner's lives could be 

in jeopardy. Within the last ten years, seven documented 

explosions of methane and/or coal dust occurred within sealed 

areas of underground US coal niines (Hurren, Tuggle and 

McGruder, 1993; Scott ef al, 1996). These explosions, believed 

to be initiated by lightning strikes on the surface, destroyed 

numerous seals and caused considerable damage external to the 

j(g)

A plastic diaphragm contained the methane-air mixture within 

the first 14 m. A fan, with an explosion-proof motor housing, 

mixed the methane and air. The ignition of the nine to ten per 

cent methane-air zone generated a peak pressure pulse of 

approximately 140 kPa as the explosion propagated down the 

entry. 'This peak pressure pulse, measured at the wall 

perpendicular to the direction of propagation, remained relatively 

constant throughout the length of the seal test zone in C-drift. 

sealed area. Fortunately, these explosions did nit cause fatalities 

or injuries. 'The potential for a life-threatening disaster exists 

however, emphasising the need for explosion resistant seals that 

can perform under various mining conditions. 

Title 30, Part 75.335 of the Code of Federal Regulations (30 

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 slnric horizontal pressure rise of 

20 psi (1 38 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 control the methane and air 

exchange between the sealed and open areas so as to prevent 

toxic andlor 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 US Bureau of 

Mines (USBM) research indicated that it would be unlikely for ~ata-galhering slatlon 

overpressures exceeding 138 kPa to occur very far from the 

1' 

I. National Institute for Occupational Safety and Health, Pittsburgh 

Research Laboratory, PO Box 18070, Pittsburgh PA 15236, USA. FIG 1 - Seal test area in the LLEM. 
Test chamber approach 

Two large-scale underground chambers were constructed within 

the LLEM to conduct pneumatic, hydrostatic, or explosion 

pressure loading of candidate seals. Figure 2 is a schematic of the 

chamber design. The large chamber dimensions are 9.1 m wide 

by 4.6 m high by 3.1 m deep with a maximum cross-sectional 

I Support steel CH,- Air or water pressure loading 

to simulate I-- 

L 

Two - Chambers 

Smaller- 6.7 m wide X 2.6 m high --vL.. .? 

Larger - 9 m wlde X 5 m high - 

FIG 2 - Large test chamber for pressure loading of seals with 

water, compressed air and with the combustion of confined 

concentrations of methane-air. 

p() 

a 90 degree bend outside the chamber and connected to the test 

seal via lightweight, near zero stretch (fishing) line. This 

mounting system protected the expensive LVDTs from flying 

seal fragments. One LVDT was connected at the exact centre 

(mid-height and mid-width) of the seal. A second LVDT was 

connected at the 114-height and mid-width point. A third LVDT 

was connected at the 314-height and mid-width point. As the seal 

was pressure loaded, the seal displaced outward and the LVDTs 

measure this displacement by generating an output signal of 

approximately 65.6 mV/mm. Data were recorded at 2000 

samples per second, per channel, with a WINDAQ PC-based data 

acquisition system. 

Chamber pre- and post-test leakage 

Although many of the standard seal designs appeared to be 

mostly intact after the confined explosion within the chamber, 

some seals were later shown to be unable to properly limit the 

exchange of air from one side to the other. The conventional 

method, for measuring air leakage through seals explosion tested 

within the LLEM, involves measuring, with an anemometer, the 

air that passes through the seal and through a 465 cm2 window in 

a nearly air-tight brattice curtain installed downstream of the 

seal, while maintaining a constant differential pressure across the 

seal (Greninger et al, 1991). 

Air leakage from the chamber was determined by recording 

chamber pressure decay, as air leaked through the seal. 

Compressed air was used to initially raise the pressure to about 

1.2 kPa and when stopped, the pressure in the chamber behind 

the seal began to decay. Results from the pressure decay method 

were compared with the conventional window method. While 
significantly with seal designs using composite type or more 

elastic type materials that relax after loading. For these 

situations, the air pressure decay approach would be the method 

of choice for determining air leakage. 

Water leakage, m3/min 

FIG 3 - Comparison of post test air leakage with maximum 

water flow. 

Types of seals tested 

Several of the seal designs evaluated previously in C-drift and in 

this chamberstudy are shown in Figure 4. These seal designs, 

with the test designations in parentheses, include the 

standard-type, solid-concrete-block seal (Cl, C2, C6, and LI), 

the pumpable cementitious plug design (C3, C7, and L2), the 

chambers for most of the comparison studies. 

The three other seal designs (Figure 4) were also performance 

tested using the chamber approach. Exposing these seals to 

methane-air explosions, within the large and small chambers, 

allowed for the determination of the ultimate failure pressure and 

provided data for developing geometric size-scaling 

relationships. The determination of the ultimate failure pressure 

is not a performance requirement of 30 CFR, but such data will 

provide seal manufacturers and mine operators with an estimate 

of design safety factors for a particular seal design. 

Water pressure loading 

Hydrostatic (water loading) tests were also conducted on two 

standard-type, solid-concrete-block seal designs. One seal, with a 

5.5 m wide by 2.4 m high unsupported span between the centre 

pilaster and each rib, was located in the small chamber within the 

LLEM. The other seal, having a similar 5.4 m wide by 2.4 m 

high unsupported span, was located in a 'butt' entry of the 

SRCM. The chambers within the LLEM are constructed in a 

solid unyielding limestone formation, while the 'butt' entry in the 

SRCM is within the Pittsburgh coal seam. 

Figure 5 is a schematic of the finished seal as constructed in 

the SRCM. Hitching of the seal required the removal of about 

0.25 m of crushed limestone from the mine floor to expose the 

solid coal base. This crushed limestone was used to control water 

accumulations in the SRCM. A 0.15 m thick by 0.6 m wide 

concrete footer (approximately 21 MPa compressive strength) 

was const~vcted on the solid coal floor to provide a base Tor the 

seal. A 0.5 m wide by 0.15 m deep channel was cut vertically 

into both ribs to provide hitching. The small gap between the 

mine roof and the top of the seal was filled with Quikcrete 
FIG 5 - Schematic of the SRCM used for water loading the 

standard-type seal design with pilaster. 

0 

0 500 1000 1500 2000 2500 3000 3500 4000 4500 

Time, s 

FIG 6 - Water pressure history for the standard-type seal design tested in the small LLEM chamber. 

C7-70 / 6.46 ! 2:66 _ 1 -_ 1.22 5 

Gunitelrebar steel mesh seal 

0.28 I 
Since available water supplies were insufficient for determining 

the ultimate failure pressure, internal chamber explosions of 

methane-air mixtures were used to produce over pressures up to 

690 kPa, suficient for loading all test seals to ultimate failure. 

These explosion tests also provided data to compare the response 

of the same seal design to both dynamic (rapid pressure buildup) 

and static (slow pressure buildup) loading. 

CHAMBER EXPLOSION TESTS 

To determine the ultimate failure pressure of various seal 

designs, methane-oxygen mixtures were injected into the void 

volume behind the various types of seals and then ignited in the 

centre of the confined chamber. Obviously. this type of 

evaluation test using methane is intended only for controlled 

experimental research and not intended for use in coal mines. 

Shown in Table 2 is a summary of seal dimensions and 

explosion test results. Several explosions of varying intensity 

were conducted in the small chamber against the standard-type, 

solid-concrete-block seal with a centre pilaster (seal CI in 

Table 2). Four explosions were also conducted against a 

modified standard seal design (seal C2 in Table 2) that did not 

include a centre pilaster. In addition to the small chamber tests, a 

standard-type seal design with a centre pilaster was tested to 

failure in the large chamber (seal LI in Table 2). To evaluate the 

size-scaling issues, this large chamber seal was constructed to 

the same thickness as the small chamber seal. 

DNR Did not rupture 

R Ruptured 

1 Pilaster 16 inches wide by 32 inches deep 

2 Pilaster 72 inches wide by 32 inches deep * 20 x 15 x 40 cm solid concrete block. Average block 

compressive strength is 17.9 f 0.69 MPa. 

The standard-type seal C1, withstood four constant volume 

explosions before it ~ptured at a peak static pressure of 688 kPa. 

The first four tests (Cl-5, C1-8, CI-9, and C1-10 in Table 2) 

subjected the seal to pressure loadings ranging from 390 kPa to 

651 kPa. It was only after the C1-9 test (65 1 kPa) that hairline 

cracks were visible along the central mortar joints. The 

post-explosion leakage rates did increase to about 2.7 m3/min at 

0.25 kPa, which was still within the acceptable limits. To further 

increase the pressure loadings for the C1-l l test, -6 m3 (210 ft3) 

of oxygen was first injected into the chamber followed by the 

methane resulting in a near stoichiometric ratio of 13 per cent 

methane mixture. 
100 

0 

0 5 10 15 20 

Time, s 

FIG 8 - Chamber explosion pressure histories recorded during 

testing of standard seal C1 in the small chamber to failure. 

0 0.5 1 1.5 2 

Displacement, mm 

FIG 9 - Standard seal deflection as a function pressure loading. 

The remains of seal C1 after rupture are shown in Figure 10. 

The centre of the seal was blown out while part (152 mm thick 

by 406 mm wide) of the pilaster on the explosion side remains. 

The almost conical-shaped perimeter shear pattern is visible in 

explosion at a peak static pressure of 669 kPa, or -20 kPa below 

the failure pressure of seal C1 with the centre pilaster. Both seals 

provide a margin of safety of about 4.8 to five times the CFR 

requirement when restrained between 34.5 MPa concrete floor 

and 113 MPa limestone roof. Unless the mine site roof-floor 

conditions are the same as these experimental conditions 

(134.5 MPa), this same margin of safety may not be realised 

Also, in an underground mine, post construction floor to rod 

convergence may add significant compression stress to the seal. 

This pre-stressed condition may reduce the strength of the seal, 

since less pressure would be required to increase the stress in the 

masonry to the point, where it crushes during the arching action. 

A mixture of 5.7 per cent methane-air was ignited in the large 

chamber with a 41 cm thick standard seal. The chamber pressure 

rose rapidly to about 222 kPa in 13 s and then rapidly decayed tb 

zero as the combustion gases vented through fractures, which 

formed as the seal began to break up and displace outward. The 

midpoint displacement of the seal as a function of pressure 

loading IS shown in Figure 11. The midpoint of the seal displaced' 

linearly with increasing explos~on pressure up to about 12 mm 

deflection at -207 kPa. The chamber pressure continued tcr 

increase to about 222 kPa, remaining relatively constant, until the 

midpoint of the seal displaced about 71 mm the maximum rang!; 

of the LVDT. At 71 mm, chamber gases rapidly vented througH 

the fractured seal and the chamber pressure rapidly dropped tcY 

zero. In this example, the ultimate failure pressure for this sear 

was selected 207 kPa, where the seal abil~ty to resist pressud 

fails rapidly. This approach was used to determine the ultimat& 

failure pressure for each seal and provided necessary data trr 

develop the following size-scaling relationships. 
In the early 1930s. 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, Greenwald, and Howarth, 1930; 

Rice et al, 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. Their 

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 behaviour transmits a lateral thrust to the strata, which 

then act as a buttress to prevent seal movement. Several efforts 

have been made to explain the arching behaviour through various 

static design models. 

This is a convenient formula to use to compare the effect of 

parameters using the same value P,. P, is defined as the limiting 

arching thrust that induces a uniform stress over a depth of 

2 t (0.5 - elt). 

Combining Equations 2 and 3, one obtains the following 

expression for the ultimate load used for size-scaling data from 

this study. 

TABLE 3 

Comparison of results from the proposed new chamber test method with current full-scale explosion test method. 

Seal type 

Standard seal CI 

Standard seal without pilaster C2 

Cementitious plug seal C3 

Steel mesh and gunite PR- I 

Low density block C4 

- 

LLEM 138 kPa 

explosion loading 

Passed 

Passed 

Passed 

Passed 

Passed 

Small chamber 138 kPa 

water or air loading 

Passed 

Passed 

Passed 

Passed 

Passed 

- 

Ultimate strength (kPa) 

690 

538 

220 

>214 

152 
in Figure 13 are the experimental results from the chamber testing 

of the pumpable cementitious seals, for both large and small 

chambers, constructed with material with an average compressive 

strength of 1725 kPa. Similarly, the same approach was applied to 

the plug seal data. Although the plug seal data is more limited, the 

best straight line fit produces an n = 0.76. 

Results from these studies with the standard seal design 

suggests an ultimate strength size-scaling relationship of 

w = 1.3 f, (t/L)' for seals 241 cm thick and seal height L between 

SUMMARY 

Before MSHA will deem a seal design suitable for use in 

underground coal mines, the design has to be evaluated and, in 

most instances, undergo explosion testing within the LLEM. 

Results from this study indicate that a static design analysis 

coupled with an in situ hydrostatic approach shows promise as an 

alternative method for performance testing seals that would be 

consistent with the intent of the 30 CFR, 20 psi (138 kPa) static 

FIG 12 - Size-scaling of the ultimate seal strength for the standard seals 240 cm thick using arching theory. 
Kenneth Helfrich, co-op student from Georgetown University; and 

Joseph Sabo, Paul Stefko (foreman), and Jack Teatino of the 

SRCM. The authors also acknowledge James Addis, John Glad 

(lead), Timothy Glad and James Rabon mechanical-technician 

contract personnel for Akima for their significant efforts in the 

seal construction and cleanup. 

REFERENCES 

Anderson, C, 1984. Arching action in transverse laterally loaded masonry 

walls, The Stnrctural Engineer, 62B(l):22. 

British Standards Institution, 1978. BS 5628, Code of practice for the 

structural use of masonry: part I; un-reinforced walls (British 

Standards Institution: London). 

Greninger, N B, Weiss, E S, Luzik, S J and Stephan, C R, 1991. 

Evaluation of solid-block and cementitious foam seals, USBM RJ 

9382, 13 p. 

Hurren, W E, Tuggle, G N and McGruder, F 1, 1993. Mary Lee No I 

Mine, Accident investigation report, underground coal mine, August 

(US Labor Dept MSHA). 

Massachusetts Institute of Technology, 1954. Behavior of walls panels 

under static and dynamic loads 11 (Department of Civil and Sanitary 

Engineering). 

McDowell, E L, McKee, K E and Sevin. E, 1956. Arching action theory 

of masonry walls, Proceedings of the American Society of Civil 

Engineers, Paper 915, Journal of the Structural Division, 

82(ST2):915-1-915-18. 

Chapter I - Mine Safety and Health Administration, Department of 

Labor; Subchapter 0 - Coal Mine Safety and Health; Part 75 - Mandatory Safety Standards - Undewound Coal Mines. 

Weiss, E S, Cashdollar, K L, Mutton, I V S, Kohli, D R and Slivensky, W 

A, 1999. Evaluation of reinforced cementitious seals, Pittsburgh, PA: 

US Department of Health and Human Services, National Institute for 

Occupational Safety and Health, R19647. 

Weiss, E S, Cashdollar, K L and Sapko, M J, 2002. Evaluation of 

explosion-resistant seals, stoppings, and overcast for ventilation 

control in underground coal mining, Pittsburgh, PA: US Department 

of Health and Human Services, National Institute for Occupational 

Safety and Health, RI 9659. 

Weiss, E S, Greninger, N B. Stephan, C R and Lipscomb, J R, 1993. 

Strength characteristics and air-leakage determinations for alternative 

mine seal designs, USBM R19477,21 p. 

Weiss, E S, Slivensky, W A, Schultz, M J and Stephan, C R, 1997. 

Evaluation of water trap designs and alternative mine seal 

construction materials, in Proceedirzgs 27th International Conference 

of Safety in Mines Research Institutes, New Delhi, India, Vol 2, 

pp 973-981 (Oxford and IBH Publishing Company). 

Weiss, E S, Slivensky, W A, Schultz, M J, Stephan, C and Jackson, K W, 

1996. Evaluation of polymer conshuction material and water trap 

designs for underground cod mine seals, USBM RI 9634, 16 p.