1 INTRODUCTION 

Permanent stoppings are utilized to control and direct the ventilation air flow through underground 

coal mines to dilute and render harmless methane, 

entrained coal dust, and other contaminants at the 

working face and other areas of the mine. The 

Ventilation controls section (§ 75.333) of Title 30 

Code of Federal Regulations (30CFR) requires that 

permanent stoppings be built and maintained between intake and return air courses beginning at 

the third connecting crosscut outby the working 

face, and separate other air courses and direct air 

as specified. To perform the intended function and 

meet the requirements of 30 CFR § 75.333, permanent stoppings are to be constructed in a traditionally accepted method and of materials that 

have been demonstrated to perform adequately or 

in a method and of materials that have been tested 

and shown to have a minimum strength equal to or 

greater than the traditionally accepted in-mine controls. A few examples of traditionally accepted 

(Federal Register Preamble 1996) stopping construction methods are as follows: 1) 20-cm and 

15-cm (8-in and 6-in) concrete block (both hollowcore and solid) with mortared joints; 2) 20-cm and 

15-cm concrete blocks, dry-stacked and coated on 

one or both sides with a strength enhancing sealant 

suitable for dry-stacked stoppings; and 3) steel 

stoppings (minimum 20-gauge) with seams and perimeter sealed with a suitable mine sealant. 

 Unlike mine ventilation seal structures 

(30CFR §75.335; Greninger et al. 1991, Mitchell 

1971, Weiss et al. 2002) that are commonly used to 

isolate unused sections of a mine, stoppings are not 

intended to withstand even moderate explosion 

overpressures. Unfortunately, mine explosions do 

occur. Depending on the location and severity, 

explosions can result in fatalities and injuries to 

underground mining personnel, and cause considerable damage underground to equipment and 

structures. MSHA personnel conduct investigations of these explosion accidents to determine the 

root cause(s) as a means to mitigate or eliminate 

future occurrences. As part of these postexplosion investigations, the location and condition of underground structures and debris are 

mapped. This information assists in determining 

the strength and the direction of the forces of the 

explosion. 

 The research evaluations at the LLEM involved various full-scale stoppings subjected to 

known overpressures generated from methane 

and/or coal dust explosions. Since the construction 

details and explosion pressure forces were known 

for these tests, the post-explosion observations and 

data will be useful for future investigations of mine 

explosion accidents. 

Explosion evaluation of mine ventilation stoppings

E. S. Weiss, K. L. Cashdollar, & S. P. Harteis 

U.S. Department of Health and Human Services, Centers for Disease Control, National Institute for Occupational Safety and Health, Pittsburgh, PA, USA 

G. J. Shemon, D. A. Beiter, & J. E. Urosek 

U.S. Department of Labor, Mine Safety and Health Administration, Pittsburgh, PA, USA 

ABSTRACT: The National Institute for Occupational Safety and Health (NIOSH) and the Mine Safety and 

Health Administration (MSHA) conducted joint research to evaluate explosion blast effects on mine ventilation stoppings at the NIOSH Lake Lynn Experimental Mine (LLEM). After mine explosion accidents, 

MSHA conducts investigations to determine the cause(s) as a means to mitigate or eliminate future occurrences. As part of these post-explosion investigations, the condition of underground stoppings, including the 

debris from damaged stoppings, are documented as evidence of the strength and the direction of the explosion 

forces. The LLEM data showed that a total pressure of 52 kPa (7.6 psi) destroyed the solid concrete block 

stopping, ~36 kPa (~5.2 psi) destroyed the hollow-core concrete block stopping, and 9 kPa (1.3 psi) destroyed 

the steel panel stopping. These results will assist investigators in determining the explosion forces that destroy or damage stoppings during actual coal mine explosions. 
Figure 1. Plan view of the Lake Lynn Experimental Mine showing multiple entry area and stopping locations 

2 EXPERIMENTAL MINE AND TEST 

PROCEDURES 

The explosion evaluation tests on the various permanent stoppings were conducted within the experimental mine at Lake Lynn Laboratory (Mattes 

et al. 1983, Triebsch & Sapko 1990). Lake Lynn 

Laboratory is located about 80 km (50 miles) 

southeast of Pittsburgh, near Fairchance, Fayette 

County, PA, and occupies more than 1.6 km2

 (400 

acres) at a former limestone mine. 

The LLEM consists of nearly 2,300 m (7500 ft) 

of underground mine entries developed for mining 

research. Figure 1 shows an expanded view of the 

stopping test area in the multiple-entry section of 

the LLEM. The faces, or closed ends, of A-, B-, 

and C-drifts are at the right of the figure. The 

three drifts are each about 520 m (1700-ft) long 

and extend far to the left of the portion shown in 

the figure. The stoppings were built in the crosscuts between B- and C-drifts, and the test explosions were ignited in C-drift. The permanent stoppings were constructed in crosscut 4 (X-4) at 

108 m (355 ft), X-5 at 138 m (452 ft), X-6 at 

168 m (550 ft), or X-7 at 198 m (649 ft) from the 

face of C-drift. Each of the crosscuts is approximately 6.1 m wide by 2.2 m wide (20.0 ft wide by 

7.3 ft high) with a cross-sectional area of about 

13.4 m2

 (145 ft2). Explosion-resistant seals from a 

previous study were located in X-1 through X-3. 

Before each explosion test, a 60-t pneumatically 

operated, track-mounted, concrete and steel bulkhead was positioned across E-drift to contain the 

initial explosion pressures within C-drift. 

Most of the tests utilized a ~9% methane-air 

concentration within a 3-m-deep by 3.7-m-wide 

(10-ft-deep by 12-ft-wide) ignition zone (~27 m3

or ~955 ft3

) contained in the C-drift face area with 

a clear plastic diaphragm (Fig. 1). An explosionproof fan mixed the natural gas and air prior to ignition. Electrically activated matches located at 

the face (closed end) or outby the face within the 

gas ignition zone, depending on the explosion 

overpressure desired, were used to ignite the 

flammable natural gas and air mixtures. In some 

of the tests, shelves of coal dust were located 

within a distance of 3 to 12 m (10 to 40 ft) from 

the face as a means to increase the explosion overpressures. 

Pressure transducers were mounted in every 

data gathering panel in A-, B-, and C-drifts. The 

pressure transducers of particular interest for these 

stopping evaluations were along the C-drift rib at 

93, 123, 153, 182, and 231 m (304, 403, 501, 598, 

and 757 ft) from the face at the data gathering 

panel positions shown in Figure 1. These rib transducers were perpendicular to the direction of the 

propagating explosion, and therefore measured 

static pressures. There was also a pressure transducer mounted on the C-drift side of each of the 

stoppings. These transducers faced the explosion 

forces propagating into the crosscut, and measured 

the total pressure (static plus dynamic) at the stoppings. The static pressure is the pressure that is 

exerted in all directions; the dynamic pressure is 

the pressure associated with the explosion wind or 

gas flow. Attached at the center (mid-height and 

mid-width) of the B-drift side of each stopping was 

a linear variable differential transducer (LVDT) 

that measured the movement of the center of the 

stopping during each explosion. A high-speed, 

PC-based computer data acquisition system collected the data from the various instruments at a 

sampling rate of 1500 per second. The reported 

data were averaged over 10 ms. 

3 STOPPING CONSTRUCTION 

3.1 Concrete block stoppings 

Four hollow-core concrete block stoppings were 

constructed in X-4 through X-7 between C- and 

B-drifts at the LLEM (Fig. 1). Each stopping was 

located approximately 1.5 m (5 ft) toward B-drift 
from the midpoint of the crosscut (approximately 

7 m or 23 ft deep into the crosscut, as measured 

from C-drift). The blocks were 3-core concrete 

blocks, with nominal dimensions of 15-cm by 

20-cm by 40-cm (6-in by 8-in by 16-in). The uniaxial compressive strength of the block material 

was 13 MPa (1900 psi). A concrete foundation 

was installed along the width of each crosscut, and 

a small amount of mortar was used under the first 

course of block for leveling purposes. The remaining blocks were dry-stacked (no mortar between 

the block joints) with staggered joints. Wood 

wedges were used to tighten each block course at 

the mine ribs. Wood header boards and wedges 

were used between the top block course and the 

mine roof to tighten the structure. An approximately 6-mm (1⁄4-in) thick coating of an approved 

sealant was applied to both sides of the stoppings 

as shown in Figure 2. 

After the completion of the explosion test 

evaluation and the removal of the first set of stoppings, additional concrete block stoppings were 

constructed in X-4 and X-5 using solid concrete 

blocks, with nominal dimensions of 15-cm by 20-

cm by 40-cm (6-in by 8-in by 16-in). These stoppings were constructed in the same manner as the 

previously evaluated hollow-core concrete block 

stoppings. 

Figure 2. Miner applying sealant to concrete block stopping

3.2 Steel panel stoppings 

Jack Kennedy Steel Stoppings manufactured by 

Jack Kennedy Metal Products and Buildings, Inc., 

(hereinafter referred to as the steel panel stoppings) 

were constructed within the LLEM as per the written Instruction Guide provided by the manufacturer. (Disclaimer: Mention of any company or 

product does not constitute endorsement by 

NIOSH or MSHA). These stoppings consisted of a 

series of 30-cm wide by 183-cm high by 5-cm 

thick (12-in wide by 72-in high by 2-in thick) vertical telescoping steel panels (formed from 20 gage 

galvanized steel sheeting) that could be lengthened 

or shortened to accommodate roof heights between 

1.8 and 3.1 m (72 and 120 in). 

The stoppings were constructed on level concrete foundations in X-6 and X-7. Since the 

LLEM installation required the use of panel 

heights in excess of 1.5 m (5 ft), three rows of steel 

angle bars, extending horizontally from one rib to 

the other, were required as per the manufacturer’s 

Instruction Guide. Figure 3 shows the nearly completed steel panel stopping in X-6 as shown from 

the B-drift side. The steel angles were positioned 

into small holes that were cut into each rib. The 

angle bars were located at 0.6, 1.0, and 1.7 m (22, 

39, and 68 in) from the mine floor in accordance 

with the manufacturer’s specifications. 

Figure 3. Nearly completed Kennedy steel stopping showing 

 the back side of the panels attached to the horizontal steel 

angle bars 

To start the stopping installation, the first steel 

panel was installed near the center of the crosscut. 

As part of the installation, a piece of rigid foam 

was manually inserted at the top and bottom of 

each panel to provide a better seal with the roof 

and floor. Then, a specially-designed installation 

jack was positioned within the panel’s top and bottom grooves and used to exert a roof-to-floor pressure according to manufacturer’s specification to 

temporarily hold the stopping panel in place. A 

wire twist clamp was fastened around the horizontal rib-to-rib angle, inserted into the inside flanges 

of the panel, and then tightened. Six clamps were 

used on the panel to attach it to the three horizontal 

angles. The temporary jack was then removed 

from the panel. The steel angles and clamps maintain compression and keep the panels aligned. 

After the installation of the center panel, a second panel was then installed against the outby rib, 

jacked into place, and secured with twist clamps. 

This process was repeated until all of the panels 

were installed across the crosscut. The panels 

were alternated such that the intersection of the 

telescoping panel sections offset each other. This 
was accomplished by turning every other panel upside down. Lapover panels were used to cover any 

gaps between panels, and side extensions were 

used to cover any large gaps between the panels 

and the mine rib. A polyurethane sealant was applied to the perimeter on both sides of the stopping 

and between all of the vertical panel joints on the 

smooth, or closed panel side (which is the high 

ventilation pressure side in an actual coal mine installation). All of the steel stoppings were constructed such that the high ventilation pressure side 

of each stopping was on the C-drift side, which 

was the side that would be directly impacted by the 

explosion forces. 

4 EXPLOSION TEST RESULTS 

4.1 Hollow-core concrete block stoppings 

The four hollow-core block stoppings were evaluated in a series of LLEM explosion tests (#427-

#434). In test #428, the hollow-core concrete 

block stoppings in X-4 and X-5 were destroyed by 

the explosion. The hollow-core concrete block 

stoppings in X-6 and X-7 showed little damage 

from this explosion. The pressure traces associated with the stopping in X-4 are shown in Figure 

4. The pressure measured by the transducer in 

front of the stopping is listed as “X-4C, 108 m” 

and is shown as the solid black curve in the figure. 

The maximum measured pressure value was 

36 kPa (5.2 psi). The pressure traces from the two 

transducers in the C-drift rib at 92.7 and 122.8 m 

(304 and 403 ft) were adjusted in time so that the 

peaks would match that of the transducer in X-4. 

The interpolated (weighted average) static pressure 

from these two transducers was 25 kPa (3.6 psi) at 

the X-4 stopping location. The B-drift pressure 

behind the stopping was near zero until after the 

stopping was destroyed. 

Figure 4. Pressure traces for the block stopping in X-4 during 

LLEM test #428 

The debris field from the destroyed X-4 stopping is shown in Figure 5. The plan view debris 

map is shown in Figure 6, with whole or partial 

blocks shown as solid squares and pieces of blocks 

shown as small x’s. The original stopping location 

between C- and B-drifts is shown by the double 

horizontal line in the crosscut near the bottom of 

the figure. Note that these pressure traces, photograph, and debris map for the X-4 hollow-core 

concrete block stopping are representative of the 

type of data collected on each of the stoppings during the three evaluation programs described in this 

paper. The interpolated static pressure was 23 kPa 

(3.4 psi) at the X-5 stopping that was also destroyed during test #428. The LVDT data showed 

that both the X-4 and X-5 stoppings moved more 

than 8 cm (3-in or the maximum displacement that 

can be measured by the sensor) as they were destroyed. 

Figure 5 – Debris from block stopping in X-4 after LLEM 

test #428, looking toward B-drift 

Figure 6. Plan view debris map for the X-4 stopping after 

LLEM test #428, with original stopping location shown by 

double line near bottom of figure 
The pressure data recorded for the intact hollow-core concrete block stopping in X-6 showed 

an interpolated static pressure of 22 kPa (3.2 psi); 

the LVDT showed almost no movement of this 

stopping. At the intact stopping in X-7, the total 

pressure was 23 kPa (3.4 psi), the interpolated 

static pressure was 20 kPa (2.9 psi), and the LVDT 

showed a movement of less than 0.5 cm (0.2 in). 

4.2 Steel panel stoppings

The two steel panel stoppings in X-6 and X-7 were 

evaluated during a series of two LLEM explosion 

tests (#457-#458). During test #457, there was little or no observable damage to either of the steel 

panel stoppings. Both the total and static pressures 

were ~6 kPa (~0.8 psi) at the steel panel stoppings 

in X-6 and X-7. The pressures in B-drift behind 

the stoppings were negligible. The LVDT showed 

~1 cm (~0.4-in) maximum displacement for the 

two steel panel stoppings, but returned close to its 

original position after the explosion. 

During test #458, the steel panel stoppings 

in X-6 and X-7 were destroyed by the explosion. 

The maximum total pressure measured at X-6 was 

9 kPa (1.3 psi). The interpolated static pressure 

was 8 kPa (1.2 psi) at the X-6 stopping location. 

The B-drift pressure behind the stopping in X-6 

was near zero until after the stopping was destroyed. The debris from the destroyed X-6 steel 

panel stopping is shown in Figure 7. The stopping 

in X-6 was displaced from its original position as a 

unit. 

Figure 7 – Debris from the X-6 steel panel stopping after 

LLEM test #458, looking toward B-drift 

Similar pressure data were collected for the 

steel panel stopping in X-7. The total pressure at 

the stopping in X-7 was 9 kPa (1.25 psi), and the 

interpolated static pressure was 8 kPa (1.2 psi). 

After the test, one side of the X-7 stopping was 

still partially attached to the outby rib, but the remainder of the stopping was on the floor. 

4.3 Solid concrete block stoppings 

The two solid-concrete-block stoppings in X-4 and 

X-5 were evaluated during a series of seven explosion tests (#457-#463). During test #460 (fourth 

explosion test against these stoppings), the total 

pressure was 46 kPa (6.7 psi) and the static pressure was 31 kPa (4.4 psi) at the X-4 solid-block 

stopping. At the X-5 solid-concrete-block stopping, the total pressure was 33 kPa (4.7 psi) and 

the static pressure was 26 kPa (3.8 psi). The 

LVDT’s showed movements of about 3 cm (1.2 in) 

for the X-4 stopping and about 2 cm (0.8 in) for 

the X-5 stopping. These maximum LVDT displacements were recorded ~0.02 s after the occurrence of the peak total pressures on each stopping. 

After the explosion test, both stoppings were essentially intact. However, pronounced cracking 

was evident on the X-4 stopping between the first 

and second block course, with a 1.3- to 2.5-cm (1⁄2- 

to 1-in) total displacement of the entire second 

block course toward B-drift. Additional horizontal 

cracking above the entire top block course was 

noted, and a hairline crack extended nearly rib-torib across the centerline of the stopping. For the 

X-5 stopping, a pronounced crack was observed 

between the first and second block courses with a 

0.6 cm (1⁄4-in) displacement of the entire second 

course toward B-drift. 

During test #461, the pressures were not 

higher than during test #460, and no significant 

additional damage was observed. During test 

#462, the total pressure was 52 kPa (7.6 psi) and 

the static pressure was 39 kPa (5.7 psi) at the X-4 

solid concrete block stopping. This stopping was 

essentially destroyed. Most of the stopping blocks 

were scattered to and beyond the B-drift intersection. At the X-5 block stopping, the total pressure 

was 46 kPa (6.7 psi) and the static pressure was 38 

kPa (5.5 psi). The stopping withstood the pressure 

pulse. The X-5 stopping exhibited a more pronounced horizontal crack between the first and 

second block courses, with an approximately 1.3 

cm (1⁄2-in) displacement of the entire second block 

course toward B-drift. A new hairline crack extending from the right center of the stopping to the 

outby floor corner was also evident. At the center 

of the X-5 stopping, the LVDT showed movement 

of slightly over 2.3 cm (0.9 in) during the explosion, but returned close to its original position after 

the explosion. 

 A summary of the explosion pressure data for 

the various stoppings is shown in Tables 1 and 2. 

Note that for all of these stopping evaluations, the 

failure pressures are based on the construction 

conditions and explosion tests within the nonyielding limestone strata at the LLEM. Stopping 

strengths in mines may vary from these LLEM results when the stoppings are constructed in a coal 
 

 Table 1 – Static Pressures Necessary to Destroy Stoppings 

Stopping Type Static Pressure at which 

stopping survived 

Static Pressure at which 

stopping was destroyed 

 kPa psi kPa psi 

Kennedy Metal 6 0.8 8 1.2 

Hollow-Concrete-Block 22 3.2 23 3.4 

Solid-Concrete-Block 38 5.5 39 5.7 

 Table 2 – Total Pressures Necessary to Destroy Stoppings 

Stopping Type Total Pressure at which 

stopping survived 

Total Pressure at which 

stopping was destroyed 

 kPa psi kPa psi 

Kennedy Metal 6 0.8 9 1.3 

Hollow-Concrete-Block ~23 ~3.4 ~36 ~5.2 

Solid-Concrete-Block 46 6.7 52 7.6 

seam and if subjected to roof convergence and/or 

floor heave. 

5 CONCLUSIONS 

NIOSH and MSHA conducted full-scale evaluation studies to determine the explosion pressures 

required to destroy typical coal mine ventilation 

stoppings at the LLEM. In summary, a static pressure of 39 kPa (5.7 psi) and a higher total pressure 

of 52 kPa (7.6 psi) destroyed a typical solid concrete block stopping as installed within the LLEM. 

In comparison, a static pressure of 23 kPa (3.4 psi) 

and a higher total pressure of ~36 kPa (~5.2 psi) 

destroyed a typical hollow-core concrete block 

stopping. A static pressure of 8 kPa (1.2 psi) and a 

slightly higher total pressure of 9 kPa (1.3 psi) destroyed the typical steel panel stopping. The results from the recent LLEM evaluations will assist 

investigators in more accurately determining the 

explosion forces that destroy or damage stoppings 

during actual coal mine explosion accidents. 

6 DISCLAIMER 

The findings and conclusions in this report are 

those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health. 

7 REFERENCES 

CFR. Code of Federal Regulations. Washington, DC: U.S. 

Government Printing Office, Office of the Federal Register. 

Federal Register Preamble, Rules and Regulations, Vol. 61, 

No. 18, pg. 9783, 1996. 

Greninger, N.B., Weiss E.S., Lusik S.J., & Stephan C.R. 

1991. Evaluation of solid-block and cementitious foam 

seals. Pittsburgh, PA: U.S. Department of the Interior, 

Bureau of Mines, RI 9382. 

Mattes, R.H., Bacho A., & Wade L.V. 1983. Lake Lynn 

Laboratory: construction, physical description, and capability. Pittsburgh, PA: U.S. Department of the Interior, 

Bureau of Mines, IC 8911. 

Mitchell D.W. 1971. Explosion-proof bulkheads: present 

practices. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 7581. 

Triebsch G.F. & Sapko M.J. 1990. Lake Lynn Laboratory: a 

state-of-the-art mining research laboratory. In: Proceedings of the International Symposium on Unique Underground Structures. Golden, CO: Colorado School of 

Mines, Vol. 2, pp. 75-1 to 75-21. 

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

of explosion-resistant seals, stoppings, and overcast for 

ventilation control in underground coal mining. Pittsburgh, PA: U.S. Department of Health and Human Services, Public Health Services, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, DHHS (NIOSH) Publication No. 

2003-104, RI 9659