Strengthening Existing 20-psi 

Mine Ventilation Seals 

With Carbon Fiber-Reinforced 

Polymer Reinforcement 

RI 9673 

REPORT OF INVESTIGATIONS/2008 

Department of Health and Human Services

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
Report of Investigations 9673 

Strengthening Existing 20-psi Mine Ventilation Seals 

With Carbon Fiber-Reinforced Polymer Reinforcement 

By Eric S. Weiss and Samuel P. Harteis, P.E. 

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health
 

Pittsburgh Research Laboratory 

Pittsburgh, PA 

January 2008 
This document is in the public domain and may be freely copied 

or reprinted. 

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DHHS (NIOSH) Publication No. 2008–106 

January 2008 

SAFER • HEALTHIER • PEOPLETM 
CONTENTS
 

Page 

Abstract............................................................................................................................................1 

Introduction......................................................................................................................................2 

Experimental mine and test procedures ...........................................................................................3 

Mine explosion tests ................................................................................................................3 

 Instrumentation ........................................................................................................................5 

Air leakage determinations ......................................................................................................8 

Seal construction..............................................................................................................................9 

 Cementitious pumpable foam seal...........................................................................................9 

Low-density block seal ..........................................................................................................16 

Carbon fiber-reinforced polymer reinforcement technique ...........................................................20 

 Cementitious pumpable foam seal.........................................................................................20 

Low-density block seal ..........................................................................................................25 

Explosion test results .....................................................................................................................28 

Conclusions....................................................................................................................................34 

Acknowledgments..........................................................................................................................34 

References...........................................................................................................................................35 

Appendix.—Summary tables of pressure data for LLEM explosion tests ....................................37 

ILLUSTRATIONS 

1. Plan view of the Lake Lynn experimental mine (LLEM).........................................................4 

2. Seal test area in the LLEM........................................................................................................4 

3. Front view and side view of housings and mounts for the pressure transducers on the 

 explosion (or C-drift) side of the low-density block seal in crosscut 3 ...............................7 

4. LVDT mounted behind seal to measure movement of the seal during an explosion ...............7 

5. Schematic of setup for seal leakage test ...................................................................................8 

6. Outby form wall for the pumpable cementitious foam seal....................................................10 

7. View between the two form walls showing the injection and bleeder pipes, the flexible 

 metal straps holding the overlapped brattice tight to the strata, and the steel cable ties 

 between the inby and outby form wall posts .....................................................................11 

8. Electric-powered screw auger for the cement feed and the heated water and foam tank.......12 

9. Pumping of cementitious foam slurry between form walls ....................................................12 

10. Digital scale and bucket used to calculate the density of the cementitious foam slurry.........13 

11. Connection of the slurry injection hose for filling the top third of the seal............................14 

12. Temperature measurements during the initial 7-day cure period for the pumpable 

 cementitious foam seal at C–320 .......................................................................................15 

13. Full wet-bed construction on all horizontal and vertical block joints.....................................17 

14. Schematic of the 24-in-thick, low-density block seal with center pilaster .............................18 

15. Installing the wood planking on the top course of block........................................................18 

16. Wood wedges driven perpendicular between the mine roof and wood planking...................19 

17. Completed 24-in-thick, low-density block seal as seen from the C-drift (explosion) side.....19 

18. Injection of the two-component foam behind the rigid urethane board that was secured 

between the form wall posts with lag screws.....................................................................21 
 

 

CONTENTS—Continued 

Page 

19. Construction of the solid-concrete-block wall.....................................................................22 

20. Injection of the two-component foam through the block to fill the 1-in gap between the

 seal and concrete block wall ...........................................................................................22 

21. Applying the resin saturant coat over the CFRP base wrap.................................................23 

22. Cutting the woven CFRP base wrap to match the contour of the mine rib and roof...........24 

23. Completed CFRP reinforcement retrofit to the C–320 pumpable cementitious foam seal 

 and the LVDT displacement sensors mounted to posts and attached to seal..................24 

24. Positioning the rigid urethane board between the first course of concrete block and 

the seal ............................................................................................................................26 

25. Injection of the two-component polyurethane foam at the perimeter and behind the rigid

 urethane board.....................................................................................................................26 

26. Completed block wall to the right of the pilaster and the installed rigid urethane board 

to the left .........................................................................................................................27 

27. Applying the primer resin with a paint roller to the concrete block and pilaster.................27 

28. Completed CFRP reinforcement retrofit to the crosscut 3 low-density block seal..............28 

29. Test setup for the mine seal retrofit evaluations in the LLEM ............................................29 

30. Pressures and LVDT displacements at the crosscut 3 seal during test 1 (LLEM test 508)....30 

31. Pressures and LVDT displacements at the C–320 seal during test 1 (LLEM test 508).......31 

32. Pressures and LVDT displacements at the C–320 seal during test 2 (LLEM test 509).......32 

33. Pressures and LVDT displacements at the crosscut 3 seal during test 2 (LLEM test 509)....33 

TABLES 

1. Air leakage measurements before the first explosion test (LLEM test 508) .......................29 

2. Air leakage measurements after the first explosion test (LLEM test 508) ..........................30 

3. Air leakage measurements after the second explosion test (LLEM test 509)......................33 

A–1. Maximum wall and seal pressures during test 1 (LLEM test 508) ......................................37 

A–2. Maximum wall and seal pressures during test 2 (LLEM test 509) ......................................38 
 

ACRONYMS AND ABBREVIATIONS USED IN THIS REPORT 

ASTM American Society for Testing and Materials 

CFR Code of Federal Regulations 

CFRP carbon fiber-reinforced polymer 

DG data-gathering 

LVDT linear variable displacement transducer 

MSHA Mine Safety and Health Administration 

NIOSH National Institute for Occupational Safety and Health 

NMA National Mining Association 

LLEM Lake Lynn Experimental Mine 

PRL Pittsburgh Research Laboratory (NIOSH) 
 

 

 

 

 

 

 

 

 

 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cfm 

 ft

 ft2 

 ft3 

gal 

hr 

 Hz

 in

 in H2O 

J

 lb 

 lb/ft3 

m

 min

 ms

 psi 

sec 

 °C

 °F 

 cubic foot per minute 

 foot

square foot 

cubic foot 

gallon 

hour 

 hertz

 inch

inch of water 

 joule

 pound 

pound per cubic foot 

 meter

 minute

 millisecond

 pound-force per square inch 

second 

 degree Celsius

 degree Fahrenheit 
 

STRENGTHENING EXISTING 20-psi MINE VENTILATION SEALS 

WITH CARBON FIBER-REINFORCED POLYMER REINFORCEMENT 

By Eric S. Weiss1 and Samuel P. Harteis, P.E.2 

ABSTRACT 

The National Mining Association and the Mine Safety and Health Administration 

requested that the National Institute for Occupational Safety and Health (NIOSH) conduct fullscale evaluations of a recently developed carbon fiber-reinforced polymer (CFRP) reinforcement 

technique for upgrading existing mine ventilation seals to withstand an explosion pressure of 

50 psi or greater. The evaluation was conducted in the NIOSH Pittsburgh Research Laboratory’s 

Lake Lynn Experimental Mine (LLEM) near Fairchance, PA. The purpose of the evaluation was 

to determine the blast resistance of the CFRP reinforcement technique, designed to increase the 

strength of two types of existing 20-psi-rated in situ mine ventilation seals. 

Two candidate ventilation seals were selected to represent the 20-psi alternative seal 

designs typical of those used in mines prior to July 2006. Installed within the LLEM were 

(1) a 4-ft-thick pumpable cementitious foam seal using material with a designed 150- to 200-psi 

average compressive strength at 28 days and (2) a 24-in-thick, low-density cementitious block 

seal with a center pilaster and keyed to the floor and ribs. The materials necessary for the CFRP 

reinforcement technique were then installed on the outby (active) side of each seal, and the seals 

were subjected to explosion overpressures. Both seals with the CFRP reinforcement retrofit 

successfully withstood explosion pressures of 60 psi or greater during the full-scale evaluations 

in the LLEM. 

1

Manager (Mining Engineer) – Lake Lynn Laboratory Section. 

2

Research Engineer (Mining Engineer). 

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA. 
 

 

 

INTRODUCTION 

Over one-half of all U.S. underground coal mines, which employ more than 70% of the 

U.S. underground miners, use ventilation seals to isolate the active sections of a mine from the 

abandoned areas. An underground explosion within the sealed area puts miners at risk when 

ventilation seals fail. In January 2006, the Sago Mine explosion disaster in West Virginia 

claimed the lives of 12 coal miners. This explosion, which was initiated within the sealed area of 

the mine, destroyed the mine ventilation seals that protect the miners from underground hazards. 

As a consequence, the Mine Safety and Health Administration (MSHA) now requires all new 

seal construction to withstand 50-psi or higher explosion pressures [72 Fed. Reg.3 28795]. 

There are nearly 14,000 mine seals installed in U.S. coal mines, and nearly all were 

designed and installed to the 20-psi standard. A priority of the National Institute for Occupational Safety and Health’s (NIOSH) Pittsburgh Research Laboratory (PRL) is to investigate 

methods to increase the strength of existing 20-psi seals. The National Mining Association 

(NMA) and MSHA have identified several possible solutions that have the potential to significantly increase the pressure rating of the seals. NIOSH, MSHA, and NMA representatives met 

with the developer of a carbon fiber-reinforced polymer (CFRP) reinforcement technique, and 

NIOSH agreed to evaluate the method. The NMA, MSHA, and NIOSH then agreed on a test 

plan. The evaluation was conducted in NIOSH’s Lake Lynn Experimental Mine (LLEM) during 

March-April 2007. 

The CFRP reinforcement technique was designed4

 to strengthen a 20-psi-rated seal to 

resist the head-on explosion pressure from a methane ignition equal to 85 psi, but the seal retrofit 

was still only considered a 50-psi design. Designing the retrofit to withstand up to an 85-psi 

reflected pressure pulse generated from a head-on explosion better accounts for the variability in 

the strength of the existing seal, the variability in the properties and strength of the retrofit 

materials, and the variability in the geology of the underground coal mine strata. The CFRP 

reinforcement technique was first reviewed by MSHA and NIOSH for technical merit prior to 

conducting the LLEM evaluations. Similarly, all future seal designs and strength-enhancing 

retrofit designs for existing 20-psi-rated seals need to undergo a thorough review of the structural 

engineering design of the seal as provided by the manufacturer, including any potential health 

and safety issues associated with the selection and installation of the seal design materials, 

proper construction techniques, entry dimensions, and the anchoring and interactions to the coal 

mine strata at the proposed seal location. The large-scale LLEM evaluations would then 

complement the structural engineering designs and provide more accurate information for 

evaluating future seal designs and for calibrating structural design models. 

This report discusses the construction techniques, testing methods, and explosion test 

data collected during the CFRP reinforcement seal retrofit evaluations in 2007. There are several 

seal reinforcement techniques that have been used in mines, including the construction of 

independent stronger seals in front of the existing 20-psi-rated seals or constructing a solidconcrete-block seal in front of the existing seal and then infilling the void between the seals with 

concrete. There are several other proposed reinforcement techniques that include the application 

of materials to the existing 20-psi-rated seals in a manner similar to the CFRP reinforcement 

technique. The CFRP reinforcement technique discussed in this report was the first such upgrade 

3

Federal Register. See Fed. Reg. in references. 4

The fiber-reinforced construction method was designed by Ravi Kanitkar, Senior Structural Engineer, Crosby 

Group, Redwood City, CA. 

2
 
evaluation to existing 20-psi-rated seals conducted by NIOSH in the LLEM. NIOSH does not 

endorse one method over another method, but reports methods that have successfully withstood 

the required explosion pressures. 

EXPERIMENTAL MINE AND TEST PROCEDURES 

Mine Explosion Tests 

The full-scale explosion tests were conducted in the LLEM [Mattes et al. 1983; Triebsch 

and Sapko 1990]. The LLEM is part of PRL’s Lake Lynn Laboratory, which is located about 

50 miles southeast of Pittsburgh near Fairchance, Fayette County, PA. It is one of the world’s 

foremost mining laboratories for conducting large-scale underground and surface safety and 

health research. Figure 1 shows a plan view of the LLEM. 

The LLEM is a former limestone mine in which five drifts (horizontal passageways in a 

mine) were developed in 1979–1980 to simulate the geometries of modern U.S. coal mines. The 

mine has four parallel drifts: A, B, C, and D. D-drift is a 1,600-ft-long single entry that can be 

separated from E-drift by a movable bulkhead door. In order to simulate room-and-pillar workings, drifts A, B, and C can be used, and these drifts may be separated from E-drift by a movable 

bulkhead door located near the C- and E-drift intersection (Figure 1). The A-, B- and C-drifts are 

each approximately 1,600 ft long, with seven crosscuts at the inby end. C- and D-drifts are 

connected by E-drift, a 500-ft-long entry that simulates a longwall face. 

The explosion tests were conducted in the C-drift entry area. The entry and crosscuts are 

approximately 20 ft wide by about 61⁄2 ft high, with cross-sectional areas of 130–140 ft2

. Figure 2 

is a closeup plan view of the seal test area in the multiple-entry area of the LLEM. In this 

example, there are seals in the first four crosscuts from the face or closed end of C-drift. Note 

that, at the LLEM, the first crosscut is the one nearest the face (closed end). The flammable 

natural gas-air volume (ignition zone) is limited to the 47-ft-long butt entry located inby 

crosscut 1 in C-drift. The ignition zone is contained by a plastic diaphragm attached to a wooden 

perimeter framework near the outby end of the 47-ft-long butt entry. The bulkhead door is closed 

between C-drift and E-drift before the test. 

For an explosion test, the natural gas was injected into the ignition zone, and an electric 

fan with an explosion-proof motor housing was used to mix the natural gas with the air in the 

ignition zone. Sample lines within the ignition zone were used to draw samples to an infrared 

analyzer for continuous monitoring of the methane concentrations. The flammable natural gas-air 

volume was ignited using a triple-point ignition source. This ignition source consists of three sets 

of two 100-J electric matches that are equally spaced at midheight across the closed end (face) 

and ignited at the same time. The resultant explosion pressure travels out C-drift. Figure 2 

represents the unconfined, side-on explosion pressure loading method that was requested by 

MSHA for use in evaluating the strength of seal designs within the LLEM from 1990 through 

2005 [Stephan 1990a,b; Greninger et al. 1991; Weiss et al. 1993a,b,c; 1996; 1997; 1999; 2002]. 

The ignition of the approximately 10% methane-in-air concentration within the 47-ft-long 

butt entry of C-drift typically generated unconfined, side-on explosion pressures at the crosscut 

seal locations ranging from approximately 20 to 25 psi at the various seal locations. Typically, 

shelves of pulverized coal dust located outby the gas zone would be added to generate higher 

side-on explosion pressures against the seals, although this increased side-on explosion pressure 

loading could have also been accomplished by expanding the volume of the gas zone. Igniting 

3
 
increasingly larger volumes of flammable methane-air will result in increasingly higher 

explosion pressures, although this is dependent on other factors such as the location of the 

ignition source within the flammable volume [Sapko et al. 1987; Zipf et al. 2007]. 

Figure 1.—Plan view of the Lake Lynn Experimental Mine (LLEM). 

Figure 2.—Seal test area in the LLEM. 

4
 
 

The 1992 revisions to the 30 CFR5 75.335 sealing regulations were based on the general 

consensus that explosions were most likely to originate from the active working sections, and 

any subsequent explosion pressures acting on the seals would be more representative of a side-on 

loading that would not exceed 20 psi. Based on the conclusions of the MSHA mine explosion 

investigation reports in the 1990s and early 2000s, any seals that were destroyed and/or damaged 

due to explosions that originated from within the sealed areas were primarily a result of 

inadequate seal construction due to poor quality-control issues, and the explosion pressures were 

estimated to be less than 20 psi. Since the Sago Mine disaster in 2006 and the subsequent MSHA 

investigation [Gates et al. 2007], MSHA has issued new seal strength requirements, and the 

NIOSH evaluations within the LLEM are now focused on the effects of a confined, head-on 

explosion pressure on seal performance. 

During the evaluation of the CFRP reinforcement technique, one retrofitted alternative 

20-psi-rated seal was evaluated using a confined, head-on (or face-on) explosion pressure 

loading method. A second retrofitted alternative seal was constructed in an open crosscut and 

thereby subjected to a confined, side-on explosion pressure loading (a sweeping pressure pulse). 

Two seals from previous evaluations remained in crosscuts 1 and 2 between B- and C-drifts, and 

they served to block those open crosscuts. One of the retrofitted alternative seals was installed 

across C-drift between crosscuts 3 and 4 to accomplish the desired confined, head-on explosion 

pressure loading method. The second retrofitted alternative seal was installed in crosscut 3 and 

subjected to a confined side-on explosion pressure loading. This confined head-on evaluation 

method will be further discussed in the “Explosion Test Results” section later in this report. 

Instrumentation 

Each LLEM drift has 10 data-gathering (DG) stations inset in the rib wall at the locations 

shown in Figures 1–2. Each DG station houses a strain gauge transducer to measure the explosion pressure and an optical sensor to detect the flame arrival. The explosion pressure is dynamic 

in nature and has two components: an omnidirectional pressure component (the pressure that is 

exerted in all directions and is measured perpendicular to the gas flow) and a wind or velocity 

pressure component (pressure due to gas flow). The total explosion pressure is the sum of the 

omnidirectional pressure and the wind or velocity pressure. The transducers in the DG stations in 

the wall measure the omnidirectional pressure. All of the explosion pressures reported in this 

publication are overpressures or gauge pressures (pressures above local atmospheric pressure) 

rather than absolute pressures. 

Nagy [1981, p. 58] referred to the omnidirectional pressure as the “static pressure” to 

differentiate it from the “dynamic pressure,” or velocity component. However, the omnidirectional pressure is not actually “static” as it does vary with time during the explosion. This 

terminology by Nagy had been used in previous U.S. Bureau of Mines and NIOSH reports, but it 

was confusing to many readers and will no longer be used. 

The pressure transducers of particular interest for these seal evaluations were installed 

along the C-drift rib at 234, 304, and 403 ft from the face at the DG station positions shown in 

Figures 1–2. The pressure data collected in all of the DG stations in B- and C-drifts are listed in 

the Appendix (Tables A–1 and A–2). There were also pressure transducers mounted on the inby 

side of the seal that was installed across C-drift and on the C-drift side of the seals in the 

5

Code of Federal Regulations. See CFR in references. 

5
 
________________________ 

crosscuts. One or two of the transducers faced the explosion forces and measured the total 

explosion pressures at the seals. Other transducers were oriented perpendicular to the oncoming 

explosion and measured the omnidirectional pressure. The velocity (or wind) pressure would act 

on objects in an open entry as the explosion travels down the entry. When the velocity (or wind) 

pressure reaches a solid surface, such as a seal across the entry or crosscut, the velocity would go 

to zero, and the total explosion pressure would be approximately the same as the omnidirectional 

pressure at that location. In addition, when the pressure pulse reaches a seal, it is reflected, and 

the resulting total reflected pressure can be about twice the incoming pressure pulse value. 

Figure 3 shows the heavy steel housings and mounts for the pressure transducers on the 

explosion (or C-drift) side of the low-density block seal in crosscut 3. Similar housing and 

mounts are used at the other seal locations. For these tests (LLEM tests 508–509) involving the 

CFRP reinforcement technique for upgrading the strength of seals, there were pressure 

transducers placed in front of each of the two candidate seals. For the 24-in-thick, low-density 

block seal in crosscut 3, two transducers were mounted horizontally to face the incoming 

pressure wave—one mounted within a steel housing on a steel post anchored near the center of 

the seal and one embedded flush into the seal near the center. The crosscut 3 seal also had one 

transducer mounted vertically on both the inby and outby crosscut ribs. A vertically mounted 

transducer on the center post was not used in crosscut 3 during these evaluations (although the 

housing on the steel post is shown in Figure 3). The original intent of the transducers mounted 

vertically on both the inby and outby crosscut ribs was to document the pressures exerted on the 

seal as a function of time to better understand the nonuniform pressure loading (a sweeping 

pulse) on a crosscut seal. However, it must be noted that the peak pressures recorded on these 

transducers were significantly influenced by the reflected and/or focused pressure pulses 

occurring at these locations. When the original outward traveling explosion pressure pulse 

interacted with the crosscut seal, the transducer located on the outby seal-rib interface measured 

a higher pressure compared to the transducers located at the center of the seal and the inby sealrib interface. A similar situation was experienced at the transducer located on the inby seal-rib 

interface when the inwardly traveling reflected pressure pulse (resulting when the head-on 

explosion encountered the seal across C-drift) passed the crosscut seal. 

For the 4-ft-thick pumpable cementitious foam seal across C-drift, two transducers were 

mounted horizontally to face the incoming (head-on) pressure wave—one mounted to a steel 

post anchored near the center of the seal and one embedded flush into the seal near the center. 

The seal across C-drift also had two transducers mounted perpendicular to the gas flow— 

one mounted vertically to the steel post anchored near the center of the seal and one mounted on 

the left outby rib next to the seal. 

Other instruments may also be installed at various locations in the LLEM during an 

explosion test. Behind each seal were linear variable displacement transducers6

 (LVDTs), as 

shown in Figure 4. The LVDT measures the movement of the seal during an explosion [Weiss 

et al. 1999, pp. 5–6]. An LVDT was mounted midheight and midwidth on the outby face 

(nonexplosion side) of each seal. For the crosscut 3 seal, LVDTs were also mounted midheight 

and one-quarter distance from each rib. For the seal across C-drift, an additional LVDT was 

mounted midheight and one-quarter distance from the right inby rib. Also shown in Figure 4 are 

the horizontal and vertical breakwires used to measure the approximate time of seal failure. 

6

Also referred to as “linear variable differential transformers.” 

6
 
 

 

Figure 3.—Front view (left) and side view (right) of housings and mounts for the pressure transducers 

on the explosion (or C-drift) side of the low-density block seal in crosscut 3. 

Figure 4.—LVDT mounted behind seal to measure movement of the seal during an explosion. 

The vertical yellow wires to the left of the sensor post and the horizontal blue wires above the LVDT 

anchoring plate on the seal are the breakwires. 

7
 
During the explosion tests, a high-speed, PC-based National Instruments Corp. data 

acquisition system collected the data from the various instruments at a sampling rate of 1,500 

samples per second. The reported data were smoothed (or averaged) over 15 points, or 10 ms. 

A second Kinetic Systems, Inc., data acquisition system collected the data at a sampling rate of 

5,000 samples per second (51-point smoothing). The data are smoothed to eliminate the high 

spike transient noise signals, but smoothing can reduce the real pressure data and thereby reduce 

the maximum reported peak pressure. However, the raw, unsmoothed data are also analyzed for 

the pressure transducers of particular importance. 

Air Leakage Determinations 

Measurements of the air leakages through the seals were taken before and after each of 

the explosion tests. To measure the leakage through the seal, the bulkhead door at D-drift is first 

closed to allow for the pressurizing of the entry area inby the C-drift seal locations. The volume 

inby the seals is pressurized by operating the fan at the different speed settings in the blowing 

mode (pushing air into the mine). Since the airflow in C-drift is blocked by the seals and the 

D-drift bulkhead door is closed, the pressure differential across the seals increases for each of the 

four fan settings. Typically, the pressure differential across the seals ranges from 0.8 in H2O at 

the lowest fan setting to upwards of 4.5 in H2O at the highest (fourth) fan setting. A copper tube, 

with a pressure gauge on the outby side of the seal, extends through each seal as a means to 

measure this pressure differential. A relatively airtight wood framework with a brattice curtain 

overlay is installed on the outby (nonexplosion) side of each seal. A circular hole, the size of the 

anemometer, is cut in the brattice curtain to measure the air leakage through the seal at the 

varying pressure differentials across the seal (Figure 5). For additional details on the air leakage 

measurement technique used during the LLEM evaluations, refer to Weiss et al. [2002, pp. 6–7]. 

A seal must exhibit minimal air leakage rates before and after each explosion test, and 

these rates must meet the MSHA-established guidelines for seal evaluations within the LLEM. 

For pressure differentials up to 1 in H2O, air leakage must not exceed 100 cfm. For pressure 

differentials greater than 3 in H2O, air leakage must be less than 250 cfm. 

Figure 5.—Schematic of setup for seal leakage test. 

8
 
SEAL CONSTRUCTION 

Seals installed from a previous evaluation program in 2006 were left in place in crosscuts 

1 and 2. The seal in crosscut 1 was a 16-in-thick, solid-concrete-block seal with a 16-in by 32-in 

interlocked center pilaster and simulated keying (hitching) to the ribs and floor. The seal in 

crosscut 2 was a 40-in-thick, low-density block seal without rib and floor keying (unhitched). 

Both of these seals were subjected to seven previous explosion tests. To ensure that the crosscut 

2 seal did not fail during this current evaluation, 6-in by 6-in by 1⁄2-in thick steel angle (American 

Society for Testing and Materials (ASTM) A–36) was installed on the B-drift (nonexplosion) 

side of the seal across the roof and floor to buttress the seal. The angle was secured to the 

concrete floor and limestone roof using 1-in-diam, 9- and 12-in-long anchor bolts (Kwik Bolt III 

manufactured by Hilti Inc., Tulsa, OK) on less than 18-in centers. A vertical center angle was 

also secured in place behind the seal. All gaps between the steel angle and roof and the steel 

angle and seal were filled with high-strength mortar (BlocBond, product No. 1225–51, a fiberreinforced surface bonding cement manufactured by Quikrete Co., Atlanta, GA; 50-lb average 

weight per bag). The candidate seals for evaluating the CFRP reinforcement technique were 

constructed in crosscut 3 and across C-drift between crosscuts 3 and 4 at a location 320 ft from 

the closed end (C–320). 

Cementitious Pumpable Foam Seal 

A 48-in-thick pumpable cementitious foam seal using material with a designed 150-psi 

average compressive strength at 28 days was installed across C-drift at a distance of 320 ft from 

the closed-end ignition zone (location “C–320”) (see Figure 2). The purpose of placing a seal 

using a material with a lower average compressive strength than normal was to account for the 

variability and potential quality-control issues that can be associated with the construction of all 

types of pumpable cementitious foam seals in mines. To be conservative, MSHA wanted the seal 

retrofit to be conducted on a structure that was representative of a lower-strength seal. The seal 

manufacturer was requested to place a seal with an approximate average compressive strength 

equal to 150 psi. The seal was constructed by personnel from the R. G. Johnson Co., Washington, 

PA. These types of seal designs (although using materials with higher compressive strengths) 

were originally evaluated against the unconfined, side-on explosion overpressures in the early 

1990s [Greninger et al. 1991; Weiss et al. 1993a; 1996] and represent flat-wall designs. Additionally, the seal design for this evaluation was constructed of material such that its overall 

strength would minimally survive the previous 20 psi-level explosion overpressure requirement. 

The pumpable cementitious foam seal is a plug-type seal; this seal is not keyed into the ribs or 

floor. The average dimensions in C-drift at the seal location are 18.7 ft wide by 7.3 ft high. The 

seal was constructed on the 8-in-thick reinforced concrete floor in C-drift. 

The inby and outby form walls each consisted of six 6-in by 4-in untreated hardwood 

posts cut to various lengths to fit the entry height (Figure 6). A post was positioned approximately 8 in from each rib with an approximately 42-in center-to-center spacing between each 

post across the entry. Wood wedges were used between the top of each post and the mine roof to 

tighten each post. The inby and outby form walls were spaced to provide an interior seal thickness of 48 in. Rough-cut hardwood boards (8 in wide by 1 in thick) were attached with nails to 

the interior side of the vertical wood posts on each form wall. The spacing between these 

horizontal crossboards was approximately 8 in. The boards were installed as close as possible 

9
 
against the roof, floor, and ribs. Brattice cloth (“Dura-Skrim” manufactured by Raven Industries, 

Inc., Sioux Falls, SD, MSHA approval No. 07–BA040001–0) was then attached to the interior of 

each form wall, overlapping approximately 4 in on the roof and ribs. Flexible metal straps (1 in 

wide) were attached to the roof, ribs, and posts to secure the overlapped brattice cloth in place 

and minimize leakage of the cementitious slurry during pumping (Figure 7). The completed form 

wall is shown in Figure 6. Eye bolts were attached to the inside of each vertical post at three 

heights (approximately one-quarter, one-half, and three-quarter heights). A 1⁄8-in-diam galvanized steel cable (seven-strand cable with seven wires per strand) was secured to the eye bolts on 

the opposing vertical posts on the inby and outby form walls (Figure 7). These cables serve as 

form ties and prevent the outward displacement of the form walls during the pumping of the 

cementitious slurry. There were six rows of cable ties across the entry (one cable for each 

inby/outby post set) between the form walls at each of the three heights. Fiberglass insulation 

was used along the ribs and floor to minimize cementitious slurry leakage between the brattice 

cloth liner and strata. Additionally, polyurethane foam (Hilti CF810 Crack & Joint Pro Foam 

manufactured by Hilti, Inc., Tulsa, OK) was applied at the roof of the inby form wall to 

minimize leakages during the slurry injection process. Construction of the form walls required 

21⁄2 days to complete, with three miners working 8-hr shifts, or 60 worker-hours. 

Figure 6.—Outby form wall for the pumpable cementitious foam seal. 

10
 
 Figure 7.—View between the two form walls showing the injection and bleeder pipes, the flexible 

metal straps holding the overlapped brattice tight to the strata, and the steel cable ties between the 

inby and outby form wall posts. 

The center opening in the outby form wall (Figure 6) is initially used to pump the slurry 

between the form walls. As the slurry height increases between the form walls, brattice cloth and 

crossboards are used to incrementally close and seal the center opening. The slurry injection for 

the top third of the seal is accomplished through a center injection port (Figure 6). Bleeder pipes 

are used as an indicator as to when the slurry has reached the mine roof. The injection port and 

bleeder pipes are installed through one of the upper crossboards. Internal pipes connected to the 

injection port and bleeder pipes are installed to the highest roof points between the two form 

walls to ensure complete filling of the cementitious slurry to the mine roof (Figure 7). The 

bleeders are capped during the pumping operation when the slurry begins to flow through the 

bleeder pipes. A 200-gal tank, containing a mixture of 120 gal of water and 5 gal of foam 

concentrate, was pressurized to 65–80 psi. Cement (Saylor’s Type I portland cement manufactured by Essroc Cement Corp., Nazareth, PA) was loaded into an electric-powered screw 

auger and mixed with the water and foam mixture (Figure 8). The slurry mixture was then 

pumped between the form walls (Figure 9). The weight of a bucket filled with the cementitious 

foam slurry was used to calculate the density of the slurry (Figure 10). Based on this calculated 

density, the feed rates of the cement auger and the water/foam mixture could then be adjusted to 

provide the desired density for the slurry that would then be pumped between the form walls. 

11
 
 Figure 8.—Electric-powered screw auger (center of photo) for the cement feed and the 

heated water and foam tank (left side of photo). 

Figure 9.—Pumping of cementitious foam slurry between form walls. 

12
 
Figure 10.—Digital scale and bucket used to calculate the density of the cementitious foam slurry. 

The first attempt of constructing the seal with a lower-than-usual density cementitious 

foam slurry material (150-psi average) resulted in the collapse of the foam structure at some 

point after the first-day pour and before the second-day pour. Several reasons can explain this 

seal collapse. The average compressive strength of the material was significantly lower than 

requested for this seal. During the slurry-filling process, the material compressive strength is 

based on the weight of the material samples taken before and during the pours. The unit weight 

bucket used to hold the sample and the electronic scales used to weigh the bucket samples were 

rarely cleaned of the waste material that flowed over the bucket and/or spilled onto the scales. 

The extra weight of the sample material may have adversely affected the calculated unit weight 

of the sample and the subsequent adjustments made to the mix. The cold mine water used for the 

mix water may have affected the initial set time for the material. A significant amount of bleed 

water was also observed on the concrete floor of the LLEM during the slurry-filling process. 

Bleed water will migrate downward toward the mine floor since the aerated material is less dense 

than the bleed water. This may have resulted in voids formed within the central portion of the 

seal at or near the floor. Samples obtained from the seal placements revealed that voids formed at 

the bottom of the sample containers since the aerated concrete will tend to float on the bleed 

water. For the material placement within the seal, more bleed water would accumulate in the 

central portion of the seal since this is the location where the seal material was discharged from 

the hose. The voids forming within or at the bottom of the seal in combination with a slow set 

time (due to the cold mine water), low material strength, and an excessively high first pour with 

the lower-strength material are plausible explanations for why the central portion of the seal 

collapsed. After the collapse, the outby form wall was disassembled and the first-day pour 

material was removed. The outby form wall was then rebuilt. 

13
 
During the second attempt of constructing the seal, the density of the cementitious foam 

slurry material was slightly increased (with the goal still being to achieve a final cured material 

with <200-psi average compressive strength), the height of each day’s pour was significantly 

reduced to prevent the material collapse, and the mine water was preheated (the heated water 

temperature ranged from 66 °F to 84 °F, with an average of 75 °F based on periodic readings of 

the outlet water temperature gauge). On day 1, a 38-min pumping time resulted in a slurry height 

of 31 in. On day 2, a 71-min pumping time increased the total slurry height to 68 in between the 

form walls. Day 3 required a 32-min pumping time through the center injection port (Figure 11) 

to complete the closure to the mine roof. Twice during this final injection period, the slurry was 

forced between the form walls under pressure to ensure that all of the voids were filled. The 

slurry-pumping operations required three miners a total of ~2.3 hr (or 7 worker-hours) to 

complete the seal. 

Figure 11.—Connection of the slurry injection hose for filling the top third of the seal. 

Samples of the cementitious foam slurry for the second seal were randomly taken during 

the pumping operations by R. G. Johnson Co., Inc. (tested by Professional Service Industries, 

Inc., Pittsburgh, PA) and Transco Products, Inc., Chicago, IL (tested by Construction Technology Laboratories, Skokie, IL). The samples collected by R. G. Johnson were contained in 

Styrofoam four-cylinder sample packs (capped with a Styrofoam lid) and averaged 143-psi 

compressive strength (individual sample results ranged from 100 to 190 psi) after an average 36­

day cure period. The samples collected by Transco Products were contained in individual airtight 

and watertight capped plastic sample containers and averaged 241-psi compressive strength 

(individual sample results ranged from 190 to 290 psi) after an average 41-day cure period. The 

14
 
 

________________________ 

 

collected samples and compressive strength tests for both sets of samples did not meet all of the 

ASTM testing standards, which may account for the differences in the test results. Also, the 

additional handling7

 of the Transco Products samples may have reduced the air content in the 

samples, resulting in higher compressive strengths. The differences between the required 

strength and the actual strength further demonstrates the potential quality-control issues with 

continuous-feed volume batching of the cementitious foam material in controlling the slurry mix 

and strength of the material. 

To document the heat release during the cementitious foam curing process, a series of 

thermocouples were installed between the form walls at one-fourth, one-half, and three-quarter 

height near the center of the seal. Figure 12 is a graph showing the results of internal temperature 

measurements every 5 min during the initial 7-day cure period for the cementitious foam seal. 

Within the first approximately 10 hr of cure time for each of the three cementitious foam slurry 

pours, the internal temperatures as measured by the thermocouples increased to a high temperature ranging between 88 and 98 °C (190 and 208 °F). After this initial 10-hr cure time, the 

internal temperatures of the curing cementitious foam material steadily decreased over time. 

Seven days after the first pour, the temperatures on all three thermocouples ranged between 

40 and 50 °C (104 and 122 °F), while 21 days after the first pour, the temperatures on all three 

thermocouples were ~18 °C (~64 °F). 

0 

20 

40 

60 

80 

100 

120 

Degree C

Top 1/3 TC 

Middle TC 

Bottom 1/3 TC 

0:00:005:17:40

10:35:2015:53:0021:10:4026:28:2031:46:0037:03:4042:21:2047:39:0052:56:4058:14:2063:32:0068:49:4074:07:2079:25:0084:42:3990:00:1995:17:59

100:35:39105:53:19111:10:59116:28:39121:46:19127:03:59132:21:39137:39:19142:56:59148:14:39153:32:19158:49:59 

Time hr:min:sec 

Figure 12.—Temperature measurements during the initial 7-day cure period for the pumpable cementitious 

foam seal at C–320. (TC = thermocouple.) 

7

The slurry material was transferred from the bucket into a plastic bag, the plastic sample molds were filled from 

the bag, then the samples were agitated with a rod. 

15
 
Low-Density Block Seal 

A 24-in-thick, low-density block seal with a 48-in by 48-in center pilaster and simulated 

keying of the ribs and floor (using the 6-in steel angle) was installed in crosscut 3 between 

B- and C-drifts of the LLEM (Figure 2). This seal design was originally evaluated against 

unconfined, side-on explosion overpressures during LLEM evaluations in 1994 [Weiss et al. 

1996] and represents the 20-psi pilaster-type alternate seals used in coal mines. The average 

dimensions in crosscut 3 at the seal location are 18.8 ft wide by 6.75 ft high. 

The seal was constructed approximately 6–7 ft into the crosscut (as measured from the 

C-drift side) on a small concrete foundation (leveling pad) that tapered from 0- to 4-in thick 

(as measured from C- to B-drifts) on top of an 8-in-thick reinforced concrete floor. The roof and 

floor slope in crosscut 3 is approximately 6% as measured from C-drift to B-drift and approximately 5% rib to rib. This pad is leveled only along the C- to B-drift plane to provide a level 

surface for the first course of block. As a result, the use of the leveling pad could affect the angle 

of interface between the top of the seal and the mine roof, which may provide a means for the 

sloping mine roof to confine and resist seal movement (to an unknown degree) at the roof-seal 

interface during the explosion tests that originate in C-drift compared to any resistance obtained 

had the seal been constructed within a level entry or an entry that sloped in the opposite direction 

(where the roof would “open up” in the direction that the seal would displace). Similar issues are 

associated with a seal constructed in an actual coal mine since the first course of blocks is still 

generally installed on a level surface to facilitate the wall construction. 

The entire crosscut (roof, ribs, and floor) was dampened with a fine spray of water just 

prior to seal construction. Each of the individual 8-in by 16-in by 24-in Omega 384 low-density 

blocks (manufactured by Burrell Mining Products in Bluefield, WV) was wet-brushed (using a 

coarse bristle brush dipped in water) to remove any loose material to ensure good bonding with 

the BlocBond mortar. BlocBond, as properly mixed according to the manufacturer’s recommendations, was applied approximately 1⁄4 in thick to the floor as each block was laid. Approximately 127 full Omega blocks and 44 custom-cut Omega blocks were used to complete this seal. 

Average weight of the blocks was 45.7 lb. Construction of the odd-numbered courses (first, third, 

fifth, seventh, and ninth) each consisted of ~17 Omega blocks (16-in dimension parallel to the 

longitudinal axis of C-drift). Six of these blocks were offset a half-block at the center of the 

crosscut (three toward C-drift and three toward B-drift) to provide the 48-in by 48-in pilaster. 

The even-numbered courses (2nd, 4th, 6th, 8th, and 10th) each consisted of ~14 blocks (16-in 

dimension parallel with C-drift), with four additional blocks each cut to 12 in by 24 in (used for 

the pilaster extensions on either side of the seal). The vertical block joints of the odd-numbered 

courses offset those vertical block joints of the even-numbered courses (Figure 13). The completed seal required 10 courses of block. All of the blocks used for the 10th (final) course were 

cut and mortared into place so as to leave an approximately 2-in gap between these blocks and 

the mine roof. 

Figure 14 illustrates the alternating courses and staggered block joints. The construction 

techniques for this seal included staggering the vertical block joints a minimum of 4 in (onequarter the length of the block), maintaining at least a 1⁄4-in BlocBond mortar joint (block-toblock interfaces and block-to-strata interfaces, including the floor and ribs) and providing a 1⁄4-in 

coating of BlocBond on both faces (sides) of the seal. Approximately 56 bags of properly mixed 

BlocBond were used as joint mortar and sealant. To tighten the seal with wood wedges at the 

mine roof without damaging the Omega block, one layer of 12-in-wide by 1-in-thick hardwood 

16
 
 

planks was run lengthwise (rib to rib) from each side of the seal on the top course of block of the 

main seal wall (Figure 15) and on the 12-in by 48-in pilaster extension on each side of the seal. 

The plank joints were staggered between the front and back planks. The planking was set in a 

thin layer of BlocBond. From each side of the seal, wood wedges were then driven perpendicular 

to these planks between the mine roof and planks as a means to tighten the entire seal structure. 

The wedges were driven on 6- to 12-in centers (Figure 16). The main wall planking was positioned and wedged first, then the two cut blocks used for the top course pilaster extension were 

mortared into place on each side of the seal. After these blocks were installed, a 12-in-long by 

48-in-long wood plank was positioned and wedged over this pilaster extension on each side of 

the seal. BlocBond was then used to completely fill all remaining gaps between the wedges and 

mine roof. 

Lengths of 6-in by 6-in by 1⁄2-in thick steel angle (ASTM A–36) were then installed on 

each rib and along the floor (against the main seal wall and the pilaster) on both sides of the seal. 

The steel angle was secured to the ribs and floor using 1-in-diam by 9-in (or 12-in) long anchor 

bolts (Hilti Kwik Bolt III). The maximum bolt spacing was 18 in. All gaps between the steel 

angle and ribs and the steel angle and seal were filled with BlocBond. Figure 17 shows the 

completed 24-in-thick, low-density block seal in crosscut 3 as seen from the C-drift (explosion) 

side. Seal construction required approximately 9 hr (or 36 worker-hours) to complete. This did 

not include the time required to mobilize the construction equipment and materials at the site. 

Figure 13.—Full wet-bed construction on all horizontal and vertical block joints. The vertical block joints 

are staggered. 

17
 
Figure 14.—Schematic of the 24-in-thick, low-density block seal with center pilaster. 

Figure 15.—Installing the wood planking on the top course of block. 

18
 
Figure 16.—Wood wedges driven perpendicular between the mine roof and wood planking. 

Figure 17.—Completed 24-in-thick, low-density block seal as seen from the C-drift (explosion) side. 

19
 
CARBON FIBER-REINFORCED POLYMER REINFORCEMENT TECHNIQUE 

In early April 2007, following the required curing time for the two constructed alterative 

20-psi-rated seals, the CFRP reinforcement retrofit (BlastSeal by Transco Mine Services Co., 

Chicago, IL) was installed on the outby side of each seal. The outby side refers to the accessible 

side of the seal in an actual coal mine. For the cementitious pumpable foam seal, the materials 

necessary to complete the CFRP reinforcement retrofit were installed on the outby side of the 

C-drift seal. For the low-density block seal in crosscut 3, the materials were installed on the 

B-drift side. The 50-psi reinforcement upgrades, installed by the manufacturer, were structurally 

designed to withstand up to an 85-psi reflected pressure generated from a head-on explosion 

pressure pulse. The CFRP reinforcement retrofit technique was similar for both seals. This 

reinforcement retrofit was installed by personnel from Sustainable Construction Group, LLC, 

of Chicago, IL, and Transco Mine Services Co. 

Cementitious Pumpable Foam Seal 

The materials necessary for the CFRP reinforcement technique were first installed on the 

48-in-thick cementitious foam seal located across C-drift at C–320. The exposed sections of 

brattice, on the outby side of the cementitious seal, were removed. Any cured cementitious foam 

material that extended beyond the wooden crossboards of the outby framework was chipped 

away to provide a more uniform roof-to-floor surface. 

The 4-in-thick rigid, high-density urethane board (Precision Board PBHT–15, a 4-ft by 

8-ft by 4-in thick high-temperature tooling board, 15-lb/ft3

 density, manufactured by Coastal 

Enterprises, Orange, CA) was installed between the existing 6-in by 4-in wood framework posts 

of the seal and attached directly to the wooden crossboards with lag screws and 2-in fender 

washers or 2.5-in metal pin plates. Each section of the rigid urethane board was cut and trimmed 

to fit between the upright wood posts of the seal framework. The rigid urethane boards were also 

cut in half, with the bottom half attached first to facilitate construction. The following steps were 

taken to secure the rigid urethane board to the seal: 

(1) Removal of the dust and debris from the mine floor and ribs; 

(2) Application of beads of liquid nails to the back of the rigid urethane board; 

(3) Spray application of a two-component, 3-lb/ft3

-density polyurethane foam (Touch ‘n 

Seal Foam Kit 120 manufactured by Convenience Products, Fenton, MO) to the concrete mine floor between the wood posts of the seal framework; 

(4) Positioning of the bottom section of the rigid urethane board into the high-density 

foam at the floor; 

(5) Securing of the rigid urethane board to the seal by installing lag screws and 2-in 

fender washers (whose large outside diameter better distributes the forces applied 

when tightening) or 2.5-in metal pin plates through the rigid urethane board and into 

the wood crossboards of the seal framework; 

(6) Application of beads of liquid nails to the back of the top section of the rigid 

urethane board; 

(7) Application of the two-component foam on the top of the bottom section of rigid 

urethane board with the top section of rigid urethane board then positioned in place; 

(8) Securing of the top section of rigid urethane board to the seal using the lag screws 

and 2-in fender washers or 2.5-in metal pin plates through the rigid urethane board 

and into the wood crossboards of the seal framework (Figures 18–19). 

20
 
 

Holes were drilled through the rigid urethane board at random locations approximately 12–18 in 

apart. The two-component foam was injected through the holes, beginning at the bottom, to fill 

the uneven areas between the urethane board and existing seal surface (Figure 18). The excess 

two-component foam that expanded out from between the rigid urethane boards, the wood posts, 

and the roof/floor was then trimmed flush and removed. 

After the rigid urethane board was secured to the seal, the first course of 8-in-wide by 

6-in-high by 16-in-long solid-concrete block was laid wet using BlocBond. A full bed of 

BlocBond was applied to the concrete floor, the vertical joints between blocks, and the gaps 

between the block and ribs. One 50-lb bag of BlocBond was used for this course. (Note that 

BlocBond was used only on the first course). This first course of block was offset approximately 

1 in outby the wood framework posts of the cementitious seal. The concrete blocks were laid 

with the 8-in block dimension down, and three beads of a one-component polyurethane adhesive 

(Touch ‘n Seal Mine Block Mortar manufactured by Convenience Products, Fenton, MO) were 

used on all of the joints, with the block joints staggered (Figure 19). After completion of every 

two to three block courses, the two-component, high-density foam was injected into the 

approximately 1-in-wide gap between the seal and block wall. As the block wall approached the 

mine roof, holes were drilled through a few of the solid-concrete blocks to continue the twocomponent, high-density foam injection in the 1-in-wide gap between the seal and block wall 

(Figure 20). A total of 15 courses of block were required to complete the wall (206 full blocks 

plus 21 partial blocks). In several areas, pieces of concrete block and/or the rigid urethane board 

were cut to size to complete the closure of the course to the mine rib. The two-component foam 

was then injected into this area to fill all gaps. Wood wedges were used to tighten the block wall 

to the mine roof. The two-component foam was again used to fill the gaps between the rigid 

urethane board and concrete block wall. 

Figure 18.—Injection of the two-component foam behind the rigid urethane board that was 

secured between the form wall posts with lag screws. 

21
 
 

 

Figure 19.—Construction of the solid-concrete-block wall. A one-component 

foam adhesive was used on all of the block joints. 

Figure 20.—Injection of the two-component foam through the block to fill the 

1-in gap between the seal and concrete block wall. 

After completion of the wall, 1 gal of BlastSeal Primer Resin was mixed and painted on 

the wall using a paint roller. The primer resin was a two-component resin (parts A and B) mixed 

to a 2A:1B ratio. The primer resin was allowed to cure for 45 min to achieve the proper 

tackiness. The cure time will vary depending on the temperature and humidity conditions. 

BlastSeal Tack Resin was then applied on top of the primer resin using a paint roller and 

smoothed with trowels. The tack resin was a two-component resin (parts A and B) mixed to a 

1A:1B ratio. The tack resin was applied only in a sufficiently large area to accommodate the 

22
 
 

application of the 51-in-wide woven CFRP reinforcement wrap. A total of 4.5 gal of tack resin 

was used on this seal. Using scissors, the 51-in-wide bidirectional woven CFRP reinforcement 

BlastSeal base wrap was then cut to size and installed from roof to floor at the center of the block 

wall. BlastSeal Saturant Resin was then applied to the CFRP base wrap. The saturant resin was a 

two-component resin (parts A and B) mixed to a 2A:1B ratio. Paint rollers were used to apply the 

saturant resin coat over the wrap while, at the same time, the wrap was pressed into the tack coat 

to remove any trapped air bubbles behind the wrap. Additional panels of base wrap were 

installed in a similar manner with a 6-in overlap to the previous panel wrap until the entire face 

of the block wall was covered (Figure 21). Figure 22 shows the woven CFRP base wrap being 

cut to match the contour of the mine rib and roof. A total of six panels of the bidirectional CFRP 

base wrap were required to cover the concrete block wall. A total of 3.5 gal of the saturant resin 

was used to coat the base wrap. A second unidirectional woven CFRP BlastSeal finish wrap was 

then applied over the base wrap so that the high-strength carbon fibers were installed in the 

vertical direction. The finish wrap was coated with the saturant resin. Each panel of the finish 

wrap was centered roof to floor over the panel joints of the base wrap. A total of 4 gal of the 

saturant resin was used to coat the unidirectional finish wrap. The wraps did not overlap on the 

roof, ribs, or floor. 

Two 9-ft-long sections of 6-in by 6-in by 1⁄2-in thick steel angle (ASTM A–36) were then 

anchored on 12-in centers across the roof and similarly across the floor using 1-in-diam by 12-in 

long Hilti Kwik Bolt III anchor bolts (for coal mine applications, different bolts would be 

required to provide equivalent anchorage strength). All gaps between the steel angle and roof and 

the steel angle and seal were filled with the high-strength mortar (BlocBond). Figure 23 shows 

the C–320 pumpable cementitious foam seal with the completed CFRP reinforcement retrofit. 

The average mine temperature was ~59 °F with an average relative humidity of ~83% during the 

time required to install the CFRP reinforcement retrofit at the C–320 seal. 

Figure 21.—Applying the resin saturant coat over the CFRP base wrap. 

The light gray undercoating on the concrete block is the resin tack. 

23
 
 

 

Figure 22.—Cutting the woven CFRP base wrap to match the contour of the mine rib 

and roof. 

Figure 23.—Completed CFRP reinforcement retrofit to the C–320 pumpable cementitious 

foam seal, and the LVDT displacement sensors mounted to posts and attached to seal. 

24
 
Low-Density Block Seal 

The CFRP reinforcement retrofit for the 24-in-thick, low-density block seal with a center 

pilaster (48 in by 48 in) located in crosscut 3 was constructed in a manner similar to that of the 

cementitious pumpable seal. The 4-in-thick, rigid, high-density urethane board was installed on 

each side of the seal’s center pilaster and attached directly to the seal. The following steps were 

taken to secure the rigid urethane board to the seal: 

(1) Removal of the dust and debris from the mine floor and ribs; 

(2) Cutting and positioning of the rigid urethane board for as tight a fit as possible to the 

roof and floor (Figure 24); 

(3) Securing of the rigid urethane board to the seal by positioning 2-in by 4-in boards 

onto the rigid urethane board and wedging it in place against a roof jack. 

Holes were then drilled through the rigid urethane board for injection of the two-component 

foam (Touch ‘n Seal Foam Kit 120). The foam was used to fill, until refusal, the voids between 

the seal and the back of the rigid urethane boards (Figure 25). The two-component foam 

expanded from between the rigid urethane boards, sides, and the roof/floor, which held the board 

in place when the 2×4 bracing was removed. The excess foam was then trimmed flush with 

adjoining surfaces. 

After the rigid urethane board was secured to the seal with the two-component, highdensity foam, the concrete block walls on each side of the center pilaster were constructed in the 

same manner as for the seal across C-drift. A total of 13 full courses of block were required to 

complete the wall (136 full blocks plus 26 partial blocks) on either side of the pilaster. For the 

top course of each block wall, half (4-in by 8-in by 16-in solid) and cap (2-in by 8-in by 16-in 

solid) blocks were used to provide a tighter fit to the mine roof. Wood wedges were used to 

tighten the block wall to the mine roof. Wedges were temporarily used at the ribs to tighten each 

block course, but were later removed after the one-component foam adhesive (Touch ‘n Seal 

Mine Block Mortar) between the blocks cured (Figure 26). The two-component foam was then 

injected to fill all gaps between the block and at the perimeter. 

The woven CFRP base and finish wraps were installed in a similar manner as the C–320 

cementitious pumpable seal. After completion of the wall sections, 1.5 gal of primer resin was 

mixed and painted on the pilaster and two concrete block walls using a paint roller (Figure 27). 

The tack resin was then applied on top of the primer resin using a paint roller and smoothed with 

trowels. A total of 4.5 gal of tack resin was used on this seal. The 51-in-wide bidirectional woven 

CFRP base wrap was then cut to size and installed from roof to floor starting at the center of the 

pilaster. The saturant resin was then applied to the CFRP base wrap. Additional panels of base 

wrap were installed in a similar manner with a 6-in overlap to the previous panel wrap until the 

entire face of the pilaster and two concrete block walls were covered. A total of five panels of the 

base wrap were required to cover the pilaster and concrete block walls. A total of 3.5 gal of the 

saturant resin was used to coat the base wrap. A second unidirectional woven CFRP finish wrap 

was then applied over the base wrap so that the high-strength carbon fibers were also installed in 

the vertical direction. The finish wrap was coated with the saturant resin. Each panel of the finish 

wrap was centered roof to floor over the panel joints of the base wrap. A total of 4 gal of the 

saturant resin was used to coat the finish wrap. The wraps did not overlap on the roof, ribs, or 

floor. Two 9-ft-long sections of 6-in by 6-in by 1⁄2-in thick steel angle (ASTM A–36) were then 

anchored on 12-in centers across the roof and similarly across the floor (shorter angle pieces 

25
 
 

 

were used on the floor since the 4-ft-wide pilaster already had bolted steel angle to simulate floor 

hitching) using 1-in-diam by 12-in-long Kwik Bolt III anchor bolts. All gaps between the steel 

angle and roof and the steel angle and seal were filled with the high-strength mortar (BlocBond). 

Figure 28 shows the completed CFRP reinforcement retrofit application to the crosscut 3 seal. 

The average mine temperature was ~60 °F with an average relative humidity of ~71% during the 

retrofit installation on the low-density block seal in crosscut 3. 

Approximately 16 hr were required to complete the retrofit upgrade to the pumpable 

cementitious foam seal and about 12 hr for the low-density block seal. However, it is anticipated 

that as personnel become more familiar with the construction techniques, the total upgrade 

installation time will require 8–10 hr per seal. 

Figure 24.—Positioning the rigid urethane board between the first 

course of concrete block and the seal. 

Figure 25.—Injection of the two-component polyurethane foam at the 

perimeter and behind the rigid urethane board. 

26
 
 Figure 26.—Completed block wall to the right of the pilaster and the installed rigid urethane 

board to the left. 

Figure 27.—Applying the primer resin with a paint roller to the concrete block and pilaster. 

27
 
Figure 28.—Completed CFRP reinforcement retrofit to the crosscut 3 low-density block seal. 

EXPLOSION TEST RESULTS 

Preexplosion air leakage measurements on the seals showed no detectable leakages at 

either seal at pressure differentials up to 4.3 in H2O (Table 1). Figure 29 is a schematic of the test 

setup for the CFRP reinforcement retrofit mine seal evaluations in the LLEM. The difference 

between the test setup shown in Figure 29 and that shown in Figure 2 is that a seal is now 

installed across C-drift between crosscuts 3 and 4, thereby confining the explosion pressures. For 

the first explosion test (LLEM test 508), a gas ignition zone was confined by a plastic diaphragm 

across C-drift at 47 ft from the face (Figures 2 and 29). About 661 ft3

 of natural gas was injected 

into the ignition zone to produce a mixture of approximately 10% methane-in-air. The flammable 

gas was ignited using the triple-point ignition source. The pressure pulse propagated out C-drift 

past the seals in crosscuts 1, 2, and 3. This test method is essentially the same as that used during 

the LLEM seal tests during the 1990s and early 2000s [Stephan 1990a,b; Greninger et al. 1991; 

Weiss et al. 1993a,b,c; 1996; 1997; 1999; 2002]. However in this test, the explosion was confined by the seal located across C-drift at 320 ft, and the pressures were built up higher than in 

the past tests. The seals in the crosscuts experienced the side-on sweeping explosion pressure 

pulse, and the seal in C-drift experienced the head-on explosion pressure pulse. 

28
 
HB 

 

 

 

 

Table 1.—Air leakage measurements before the first explosion test 

(LLEM test 508) 

Air leakage rates, cfm, 

at pressure differential of— Location 0.8 1.4 2.3 4.3 

in H2O in H2O in H2O in H2O 

Retrofitted seal in crosscut 3................... 0 0 0 0 

Retrofitted seal across C-drift (C–320) ... 0 0 0 0 

C-Drift 

B-Drift 

A-Drift 

CB LDB LDBwP 

CB – Concrete block seal with pilaster 

LDB – Buttressed 40-in-thick, low-density block seal 

LDBwP – Retrofitted 24-in-thick, low-density block seal with center pilaster 

PS – Retrofitted 48-in-thick pumpable cementitious foam seal 

PS 

Figure 29.—Test setup for the mine seal retrofit evaluations in the LLEM. 

The first explosion test generated an initial outgoing side-on (or sweeping) explosion 

pressure of approximately 28 psi (at ~0.58 sec) at the retrofitted low-density block seal in 

crosscut 3 (Figure 30). The head-on explosion at the retrofitted cementitious pumpable foam seal 

at C–320 generated a reflected peak explosion pressure of 61 psi (Figure 31). As can be seen in 

Figure 30, a subsequent 36-psi peak side-on explosion pressure at ~0.68 sec was generated at the 

retrofitted low-density block seal in crosscut 3 due to the reflected pressure pulse from the 

C–320 seal. These explosion pressure data are for horizontally mounted transducers at the middle 

front of the seals, and the data are averaged over 10 ms. Both seals with the CFRP reinforcement 

retrofit survived the explosion. Additional details of the pressure data generated during test 1 

(LLEM test 508) are presented in Table A–1 in the Appendix. 

29
 
 

 

 

 

 

Behind each seal was an LVDT mounted to the posts, as shown in Figure 4. The LVDT 

measures the movement of the seal during the explosion. The C–320 seal exhibited a permanent 

displacement of approximately 1⁄2 in (represented by curves at the top of Figure 31). The crosscut 

3 seal exhibited a permanent displacement of approximately 0.04 in (represented by curves at the 

top of Figure 30). 

Postexplosion air leakage measurements were then collected on each seal. The crosscut 3 

seal had no detectable leakage. The C–320 seal exhibited only negligible leakage when subjected 

to pressure differentials up to 4.1 in H2O (Table 2). 

Table 2.—Air leakage measurements after the first explosion test 

(LLEM test 508) 

Air leakage rates, cfm, 

at pressure differential of— Location 0.8 1.4 2.2 4.1 

in H2O in H2O in H2O in H2O 

Retrofitted seal in crosscut 3................... 0 0 0 0 

Retrofitted seal across C-drift (C–320) ... 0 0 0 0 

Pressure, psi 

70 0.08 

60 0.00 

50 X-3, 256 ft, embedded -0.08 

15-pnt smoothed NI data X-3, 256 ft, center, horizontal 

X-3 LVDT-R, outby 

40 -0.16 X-3 LVDT, center
 

X-3 LVDT-L, inby
 

30 -0.24 

20 -0.32 

Transmetrics pressure 10 -0.40 

transducers at X-3 seal 

0 -0.48 

0.45 0.50 0.55 0.60 0.65 0.70 0.75 

Time, sec 

Figure 30.—Pressures and LVDT displacements at the crosscut 3 seal during test 1 (LLEM test 508). 

LVDT, in

30
 
80

90

-16

-14

-12

-10

-8

-6

-4

-2

0 

10 

20 

30 

40 

50 

60 

70 

Pressure, psi 

0 

2 

LVDT, in

 C-drift pressure, center, horizontal

 C-drift pressure, center, vertical

 C-drift LVDT, center

 C-drift LVDT, right 

0.50 0.55 0.60 0.65 0.70 0.75 0.80 

Time, sec 

Figure 31.—Pressures and LVDT displacements at the C–320 seal during test 1 (LLEM test 508). 

A second explosion test (LLEM test 509) was then conducted to generate a higher 

explosion pressure at the retrofitted low-density block seal in crosscut 3. A longer 71-ft gas 

ignition zone was used at the face of C-drift. In this test, the plastic diaphragm used to confine 

the gas mixture was located just outby crosscut 1. The 71-ft ignition zone was filled with 

1,265 ft3

 of natural gas to give a mixture of ~10% methane-in-air. Although this zone was only 

~50% longer than the ignition zone for the first test (LLEM test 508), the flammable gas volume 

was ~90% greater. This was due to the additional volume in crosscut 1 between the seal and the 

bulkhead door leading to E-drift, as shown in Figures 2 and 29. Additionally, 8 lb of pulverized 

coal dust was loaded on shelves suspended from the mine roof at four locations outby the end of 

the ignition zone. These shelves were located at 77, 87, 97, and 107 ft from the closed end. The 

flammable methane-air zone was ignited using the triple-point ignition source at the face of 

C-drift, and the explosion pressure pulse propagated out C-drift past the seals in crosscuts 1 

through 3 to the seal in C-drift. The seals in the crosscuts experienced the side-on explosion 

pressures, and the seal in C-drift experienced the head-on explosion pressure. Because the 

flammable gas zone was much larger and 32 lb of pulverized coal dust was used, the resulting 

explosion pressures were much higher than those in the first test. To achieve the desired 

explosion pressure at the retrofitted low-density block seal in crosscut 3 during the second test, 

the explosion pressures at the retrofitted C–320 seal would be well in excess of the 85-psi design 

strength of the CFRP reinforcement retrofit as installed on the pumpable cementitious foam seal. 

The total explosion pressure at the C–320 seal was ~132 psi (reflected pressure resulting 

from the head-on explosion) as recorded at the center of the seal, and the seal was destroyed 

as expected (Figure 32). The low-density block seal with the CFRP reinforcement retrofit in 

crosscut 3 survived the explosion. The initial outgoing pressure wave generated a side-on 

31
 
200

220

-66

-60

-54

-48

-42

-36

-30

-24

-18

 -12

 

sweeping pressure pulse of approximately 50 psi (at ~0.49 sec) at the retrofitted low-density 

block seal in crosscut 3 (Figure 33). The cables to the pressure transducers at crosscut 3 were 

severed before the peak reflected pressure traveling back inby from the C–320 seal could be 

recorded. The estimated side-on explosion pressure at the crosscut 3 retrofitted seal was greater 

than 60 psi, based on the data from the pressure transducer in a nearby DG station located on the 

mine rib inby the seal location (C–234 ft, wall pressure trace shown in Figure 33). Additional 

details of the pressure data generated during test 2 (LLEM test 509) are presented in Table A–2 

in the Appendix. 

Based on the LVDT sensor data, the crosscut 3 retrofitted seal exhibited a permanent 

displacement of ~2 in (represented by curves at the top of Figure 33). As shown in Table 3, 

postexplosion air leakage measurements revealed that the crosscut 3 retrofitted seal maintained 

acceptable air leakage resistance well within the MSHA-established guidelines for the seal 

evaluations conducted in the LLEM. 

0.48 0.49 0.50 0.51 0.52 0.53 0.54 0.55 0.56 

Time, sec 

C-drift, center, horizontal 

LVDT, center, mid-height 

LVDT, in 

0 

180 -6 

160 

140 

120 

100 

80 

60 

40 

20 

0 

-20 

Pressure, psi 

Figure 32.—Pressures and LVDT displacements at the C–320 seal during test 2 (LLEM test 509). 

32
 

6 
80

90

-24

-21

-18

-15

-12

-9

-6

-3

 

 

 

 

 

 

Pressure, psi 

3 

0 

70 

 X-3 pressure, 256 ft, center 

C-234ft, wall pressure 

X-3 LVDT, right-outby

X-3 LVDT, left-inby 

 X-3 LVDT, center 

cable 

pressure transducer at seal stopped 

working after first peak, due to severed 

60 

50 

40 

30 

20 

10 

0 

0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 

Time, sec 

Figure 33.—Pressures and LVDT displacements at the crosscut 3 seal during test 2 (LLEM test 509). 

Table 3.—Air leakage measurements after the second explosion test 

(LLEM test 509) 

Air leakage rates, cfm, 

Location at pressure differential of— 

0.55 1.0 1.8 3.7 

in H2O in H2O in H2O in H2O 

Buttressed seal in crosscut 2.................... 19 28 37 56 

Retrofitted seal in crosscut 3 .................... 24 35 47 70 

Retrofitted seal across C-drift (C–320) ..... Seal destroyed. 

LVDT, in

33
 
CONCLUSIONS 

Following the Sago Mine disaster in West Virginia in early 2006, MSHA issued an 

emergency temporary standard requiring new mine ventilation seal designs to withstand 

50-psi or higher explosion pressures. The vast majority of the approximately 14,000 mine 

ventilation seals that had been installed in U.S. coal mines prior to July 2006 were designed to 

the previous requirement under 30 CFR 75.335 that alternative seals had to withstand a static 

horizontal pressure of 20 psi. These existing 20-psi-rated alternative seals in mines may now be 

required to be strength-enhanced to meet the new 50-psi or higher explosion pressure ratings 

depending on the location of the seal within the mine, the type of construction materials used, 

poor quality control during seal construction, the condition of the seal, and/or the concentration 

of methane behind the seal. In response to this need, NIOSH participated in a cooperative 

research project with the NMA and MSHA to evaluate an innovative method to strengthen the in 

situ 20-psi mine ventilation seal designs. 

A carbon fiber-reinforced polymer (CFRP) reinforcement technique for upgrading 

existing 20-psi mine ventilation seals was successfully evaluated in the LLEM. The CFRP 

reinforcement retrofit withstood total explosion pressures of 60 psi or greater during full-scale 

evaluations in the LLEM. The LLEM tests should be construed as being conducted in ideal, 

controlled conditions in solid, unyielding strata. The actual results that may be expected in 

various coal mine environments may be somewhat different. 

Several variations of the CFRP reinforcement technique for upgrading existing 20-psirated seals have been recently deemed suitable by MSHA for use in underground coal mines, 

including two 120-psi upgrade designs. Several other upgrade designs are currently in the 

technical review process and under development by the manufacturer. 

NIOSH will continue to evaluate new, innovative retrofit techniques for addressing the 

strength adequacy issues associated with existing seals in coal mines. Strengthening existing 

20-psi mine ventilation seals to withstand 50-psi or greater explosion pressures will result in a 

significant reduction in the failure rates of these seals, thereby enhancing the safety of miners in 

the active areas of underground coal mines. 

ACKNOWLEDGMENTS 

The authors thank Adam K. P. Brown, Project Manager and Safety Director, Transco 

Mine Services Co., for taking the initiative, in the wake of the Sago and Darby Mine No. 1 

disasters in 2006, to develop a partnership for adapting the CFRP reinforcement techniques used 

in other industries to the underground coal mining industry. Together, the partnership members 

designed a strength-enhancing upgrade for existing mine ventilation seals that better withstands 

the explosion overpressures resulting from ignitions within the sealed area, thereby improving 

the safety of underground coal miners. We thank the following personnel associated with this 

partnership for their roles in the design of the CFRP reinforcement technique, materials, and/or 

installations within the LLEM: Edward J. Wolbert, President/CEO, and J. Michael Sollie of 

Transco Products, Inc.; George A. Mathes, P.E., CEO, First Defense, LLC, Phoenix, AZ; 

Ravi Kanitkar, Senior Structural Engineer, P.E., Associate, Crosby Group, Redwood City, CA; 

Jim Butler, President, HJ3 Composite Technologies, Tucson, AZ; John O’Leary, Vice President 

Special Projects, Coatings Unlimited, Inc., St. Louis, MO; and Timothy R. Ervin, Mark Luhn, 

Michael P. Boner, and Joshua W. G. Hamm of Sustainable Construction Group, LLC, St. Louis, 

MO. 

34
 
The authors also thank the NMA and its members Bruce Watzman, Vice President, 

Safety and Health, Washington, DC; Philip Molesky of CONSOL Energy, Inc., Scenery Hill, 

PA; John Gallick of Foundation Coal Co., Waynesburg, PA; and Doug Conaway of Arch Coal, 

Inc., St. Louis, MO, for their support and guidance and for providing funding for the materials 

required to install the seal retrofits. In addition, we thank the following MSHA personnel for 

their technical guidance and support during this program: Richard A. Allwes, Darren J. Blank, 

Michael C. Superfesky, and Carol L. Tasillo, Civil Engineers; Terence M. Taylor, Senior Civil 

Engineer; and Kelvin K. Wu, Ph.D., Chief (now retired), Mine Waste and Geotechnical Engineering Division, Technical Support, Pittsburgh, PA. 

The authors acknowledge the following mechanical technicians with Ki Corp. (a NIOSH 

contractor) for the construction of the low-density block seal, assistance in the underground 

materials handling, and the extensive cleanup operations: James D. Addis, Timothy W. Glad, 

James R. Rabon, and Bernard T. Lambie. 

In addition, the authors acknowledge the following NIOSH–PRL personnel without 

whose contributions this program could not have been accomplished: Kenneth L. Cashdollar, 

Principal Research Scientist, for data analyses and photography; Kenneth W. Jackson, Electronics Technician, for sensor calibrations and installations and for his participation in the 

testing, data collection, and analyses; Cynthia A. Hollerich, Physical Science Technician, for 

written and photographic documentation of the seal constructions; Richard A. Thomas, 

Electronics Technician, and John Soles, Physical Science Technician, for installing thermocouples and documenting the temperature data during the cure period for the pumpable 

cementitious foam seal; and William A. Slivensky, Frank A. Karnack, and Donald D. Sellers, 

Physical Science Technicians, for their extensive participation in the installation of the sensors 

and mounting equipment, explosion and air leakage testing, and construction monitoring. 

All photographs in this report were taken by Cynthia A. Hollerich, Kenneth L. Cashdollar, and Eric S. Weiss of NIOSH–PRL. 

REFERENCES 

72 Fed. Reg. 28795 [2007]. Mine Safety and Health Administration, 30 CFR part 75: 

sealing of abandoned areas; emergency temporary standard. 

CFR. Code of federal regulations. Washington DC: U.S. Government Printing Office, 

Office of the Federal Register. 

Gates RA, Phillips RL, Urosek JE, Stephan CR, Stoltz RT, Swentosky DJ, Harris GW, 

O’Donnell JR Jr., Dresch RA [2007]. Report of investigation, fatal underground coal mine 

explosion, January 2, 2006. Sago mine, Wolf Run Mining Company, Tallmansville, Upshur 

County, West Virginia, ID No. 46–08791. Arlington, VA: U.S. Department of Labor, Mine 

Safety and Health Administration. 

Greninger NB, Weiss ES, Luzik SJ, Stephan CR [1991]. Evaluation of solid-block and 

cementitious foam Seals. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, 

RI 9382. NTIS No. PB 92–152115. 

Mattes RH, Bacho A, Wade LV [1983]. Lake Lynn Laboratory: construction, physical 

description, and capability. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, 

IC 8911. NTIS No. PB 83–197103. 

Nagy J [1981]. The explosion hazard in mining. Pittsburgh, PA: U.S. Department of 

Labor, Mine Safety and Health Administration, IR 1119. 

35
 
Sapko MJ, Weiss ES, Watson RW [1987]. Size scaling of gas explosions: Bruceton 

experimental mine versus the Lake Lynn mine. Pittsburgh, PA: U.S. Department of Interior, 

Bureau of Mines, RI 9136. NTIS No. PB 88–230248. 

Stephan CR [1990a]. Construction of seals in underground coal mines. Pittsburgh, PA: 

U.S. Department of Labor, Mine Safety and Health Administration, Industrial Safety Division 

(ISD) report No. 06–213–90, August 1, 1990. 

Stephan CR [1990b]. Omega 384 block as a seal construction material. Pittsburgh, PA: 

U.S. Department of Labor, Mine Safety and Health Administration, Industrial Safety Division 

(ISD) report No. 10–318–90, November 14, 1990. 

Triebsch G, Sapko MJ [1990]. Lake Lynn Laboratory: a state-of-the-art mining research 

laboratory. In: Proceedings of the International Symposium on Unique Underground Structures. 

Vol. 2. Golden, CO: Colorado School of Mines, pp. 75–1 to 75–21. 

Weiss ES, Greninger NB, Perry JW, Stephan CR [1993a]. Strength and leakage evaluations for coal mine seals. In: Proceedings of the 25th International Conference of Safety in 

Mines Research Institutes (Pretoria, South Africa, September 13–17, 1993), Conference Papers 

for Day One, pp. 149-161. 

Weiss ES, Greninger NB, Slivensky WA, Stephan CR [1993b]. Evaluation of alternative 

seal designs for coal mines. In: Proceedings of the Sixth U.S. Mine Ventilation Symposium (Salt 

Lake City, UT, June 21–23, 1993). Chapter 97. Littleton, CO: Society for Mining, Metallurgy, 

and Exploration, Inc., pp. 635–640. 

Weiss ES, Greninger NB, Stephan CR, Lipscomb JR [1993c]. Strength characteristics 

and air-leakage determinations for alternative mine seal designs. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 9477. NTIS No. PB94111275. 

Weiss ES, Slivensky WA, Schultz MJ, Stephan CR, Jackson KW [1996]. Evaluation of 

polymer construction material and water trap designs for underground coal mine seals. Pittsburgh, PA: U.S. Department of Energy, RI 9634. NTIS No. PB96–123392. 

Weiss ES, Slivensky WA, Schultz MJ, Stephan CR [1997]. Evaluation of water trap 

designs and alternative mine seal construction materials. In: Dhar BB, Bhowmick BC, eds. 

Proceedings of the 27th International Conference of Safety in Mines Research Institutes (New 

Delhi, India, February 20–22, 1997). Vol. II. New Delhi, India: Oxford & IBH Publishing Co. 

Pvt. Ltd., pp. 973–981. 

Weiss ES, Cashdollar KL, Mutton IVS, Kohli DR, Slivensky WA [1999]. Evaluation of 

reinforced cementitious seals. Pittsburgh, PA: U.S. Department of Health and Human Services, 

Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 99–136, RI 9647. 

Weiss ES, Cashdollar KL, Sapko MJ [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 Service, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) 

Publication No. 2003–104, RI 9659. 

Zipf RK Jr., Sapko MJ, Brune JF [2007]. Explosion pressure design criteria for new seals 

in U.S. coal mines. Pittsburgh, PA: U.S. Department of Health and Human Services, Public 

Health Service, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, DHHS (NIOSH) Publication No. 2007–144, IC 9500. 

36
 
APPENDIX.—SUMMARY TABLES OF PRESSURE DATA FOR 

LLEM EXPLOSION TESTS 

Table A–1.—Maximum wall and seal pressures during test 1 (LLEM test 508) 

WALL PRESSURES 

B-drift C-drift 

Distance Pressure Distance Pressure 

ft psi ft psi 

10 0.03 13 27.6 

108 0.04 84 25.4 

158 0.05 134 23.2 

211 — 184 23.0 

257 0.04 234 30.8 

329 0.04 304 50.1 

427 0.04 403 0.03 

526 0.04 501 0.04 

626 0.01 598 0.03 

782 0.02 757 0.03 

SEAL PRESSURES 

Distance Location ft Seal type Pressure 

psi 

Crosscut 1 59 16-in-thick solid-concrete-block with 16-in by 32-in pilaster 

Crosscut 2 156 40-in-thick, low-density block with floor and roof keying 

(buttressed) 

Crosscut 3 246 24-in-thick, low-density block with 48-in by 48-in pilaster 

(retrofitted) 

Seal at C–320 320 4-ft-thick pumpable cementitious foam (retrofitted) 

1

29.1 

1

26.4 

1

36.3 

2

60.6 

1

Side-on or sweeping pressure pulse.

2

Reflected pressure resulting from a head-on explosion.
 

Pressures in C-drift and at seal locations are listed to nearest 0.1 psi. 

Data sampled at 1,500 Hz, with 15-point smoothing (averaged over 10 ms). 

Wall pressure data are for the rib-mounted transducers located within the DG stations. 

Seal pressure data are for horizontally mounted transducers at the middle front of the seals. 

37
 
Table A–2.—Maximum wall and seal pressures during test 2 (LLEM test 509) 

WALL PRESSURES 

B-drift C-drift 

Distance Pressure Distance Pressure 

ft psi ft psi 

10 5.0 13 50.4 

108 4.4 84 42.9 

158 5.0 134 42.7 

211 — 184 57.5 

257 2.6 234 58.4 

329 2.5 304 91.5 

427 2.6 403 2.9 

526 2.4 501 2.7 

626 2.3 598 3.0 

782 2.4 757 2.5 

SEAL PRESSURES 

Distance Location ft Seal type Pressure 

psi 

Crosscut 1 59 16-in-thick solid-concrete-block with 16-in by 32-in pilaster 

Crosscut 2 156 40-in-thick, low-density block with floor and roof keying 

(buttressed) 

Crosscut 3 256 24-in-thick, low-density block with 48-in by 48-in pilaster 

(retrofitted) 

Seal at C–320 320 4-ft-thick pumpable cementitious foam (retrofitted) 

1

44 

1

72 

1

>60 

2

132 

1

Side-on or sweeping pressure pulse.

2

Reflected pressure resulting from a head-on explosion. Seal destroyed during explosion.
 

Pressures in C-drift and at seal locations are listed to nearest 0.1 psi. 

Data sampled at 1,500 Hz, with 15-point smoothing (averaged over 10 ms). 

Wall pressure data are for the rib-mounted transducers located within the DG stations. 

Seal pressure data are for the horizontally mounted transducers at the middle front of the seals. 

38
 
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