Scale, m 

LEGEND - Data-gathering station 

BUREAU OF M' 
U.S. Department of the Interior 

Mission Statement 

As the Nation's principal conservation agency, the Department of 

the Interior has responsibility for most of our nationally-owned 

public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, 

wildlife, and biological diversity; preserving the environmental 

and cultural values of our national parks and historical places; and 

providing for the enjoyment of life through outdoor recreation. 

The Department assesses our energy and mineral resources and 

works to ensure that their development is in the best interests of 

all our people by encouraging stewardship and citizen participation in their care. The Department also hasa major responsibility 

for American Indian reservation communities and for people who 

live in island territories under U.S. administration. 

Cover Photographs: Top, U.S. Bureau of Mines Lake Lynn 

experimental mine. Mine seals constructed in the crosscuts 

between B and C drifts; Bottom, Damage to typical woodblock seal when subjected to 20-psig pressure pulse. 
Report of Investigations 9477 

Strength Characteristics and Air-Leakage 

Determinations for Alternative Mine 

Seal Designs 

By E. S. Weiss, N. B. Greninger, C. R. Stephan, 

and J. R. Lipscomb 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Bruce Babbitt, Secretary 

BUREAU OF MINES 
Library of Congress Cataloging in Publication Data: 

Strength characteristics and air-leakage determinations for alternative mine seal 

designs / by E.S. Weiss . . . [et al.]. 

p. cm. - (Report of investigations; 9477) 

Supt. of Docs. no.: I 28.23: 199:9477. 

1. Concrete mine stoppings (Mining)-Airtightness-Testing. 2. Cement composites-Testing. 3. Foamed materials-Testing. 4. Wood-Testing. 5. Strength 

of materials. 6. Coal mines and mining. I. Series: Report of investigations (United 

States. Bureau of Mines); 9477. 

TN23.U43 [TN304] 622 s--dc20 [622'.4] 93-25429 CIP 
CONTENTS 

Page 

Abstract ........................................................................... Introduction ........................................................................ Background ...................................................................... Purpose ......................................................................... Acknowledgments .................................................................... Experimental procedure ............................................................... Mine explosion tests ................................................................ Air-leakage determinations ........................................................... Summary of previous test results ......................................................... Solid-concrete-block seals ............................................................ Cementitious foam seals-Test series 1 .................................................. Omega 384 foam-block seals .......................................................... Discussion of recent test results ......................................................... Cementitious foam seals-Test series 2 .................................................. Wood-blockseals .................................................................. Testseriesl ................................................................... Testseries2 ................................................................... Testseries3 ................................................................... Conclusions ........................................................................ References ......................................................................... 

ILLUSTRATIONS 

Lake Lynn experimental mine ....................................................... Seal test area in the LLEM ......................................................... Computer data from 20-psig pressure pulse in C drift ...................................... Pressurized entry for leakage-determination rates across the seals ............................. Curtain configuration for leakage tests through test seal .................................... Monitoring of air flow through test seal ................................................ Standard.type. solid-concrete-block seal ................................................ Wooden framework for cementitious foam seal ........................................... Brattice liner attached to framework used to contain the cementitious foam slurry ................. Typical wood-block and rock-dust seal design ............................................ Rock dust between timber rows ...................................................... Wedging of wood blocks to mine roof .................................................. Nailed-wood-block seal design 1 ...................................................... Damage to typical wood-block seal when subjected to 20-psig pressure pulse ..................... Nailed-wood-block seal design 2 ...................................................... Staggered wood-block joints ......................................................... Wood-block and rock-dust seal design 2 ................................................ 

TABLES 

MSHA-established tentative guidelines for air leakage through a seal .......................... 6 

Summary of test conditions and results for previously tested solid-concrete-block seal designs in the 

LLEM ....................................................................... 8 

Summary of test conditions and results for previously tested cementitious foam and low-density foamblock seal designs in the LLEM ..................................................... 9 

Summary of test conditions and results for recent 4-ft-thick cementitious foam seal designs .......... 12 

Summary of test conditions and results for the wood-block convergence seal designs ............... 20 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

degree Fahrenheit lb pound 

foot lb/ft3 pound per cubic foot 

square foot ~ct percent 

cubic foot per minute psi pound (force) per square inch 

hour PS~ pound (force) per square inch, gauge 

inch s second 

inch of water (pressure) 

Disclaimer of Liability 

Reference to specific products in this report does not imply endorsement by the U.S. Bureau of Mines. 
STRENGTH CHARACTERISTICS AND AIR-LEAKAGE DETERMINATIONS 

FOR ALTERNATIVE MINE SEAL DESIGNS 

By E. S. ~eiss,' N. B. ~renin~er,~ C. R. ~te~han,~ and J. R. ~ipscornb~ 

ABSTRACT 

The U.S. Bureau of Mines and the U.S. Mine Safety and Health Administration (MSHA) are 

participating jointly in a research program to evaluate the strength characteristics and air-leakage 

resistance of various proposed seal designs for use in underground coal mines. The full-scale seals were 

constructed in the USBM's experimental mine at the Lake Lynn Laboratory, air-leakage tested, then 

subjected to pressure pulses of 20 psig or greater. 

In experiments prior to this study, seven seal designs using solid-concrete blocks were tested. Only 

the standard-type seal passed the explosion and air-leakage criteria. Tests also were performed on four 

seals constructed with low-density foam blocks. All four of these seal designs withstood the pressure 

pulse. In more recent studies, nine cementitious foam seal designs of varying thicknesses and densities 

were investigated. Six of the nine designs successfully survived the explosion overpressures. Six woodblock convergence seals also have been tested. The typical 3-ft-thick, wood-block seal design currently 

used in many coal mines did not maintain its integrity, in the absence of convergence forces, following 

the explosion test. Five modified wood-block seals successfully withstood the 20-psig pressure pulse. 

Based on these tests, three alternative seal construction materials, cementitious foam, low-density 

foam block, and wood, have been approved by MSHA for use in underground coal mines. 

 inin in^ engineer. 

2~hemical engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 

3~rincipal mining engineer, Industrial Safety Division, U.S. Mine Safety and Health Administration, Pittsburgh, PA. 

4~hysical science technician, Pittsburgh Research Center. 
INTRODUCTION 

BACKGROUND 

During the normal course of underground coal mining, 

it sometimes becomes necessary to seal off abandoned 

areas to eliminate the need to ventilate them. Seals also 

are used to isolate fire zones or areas susceptible to 

spontaneous combustion. Therefore, the mine seals must 

be capable of isolating these areas of the mine from the 

active workings. To isolate areas within a mine effectively, 

a seal should 

control the gas-air exchanges between the sealed and 

open areas to prevent toxic and/or flammable gases from 

entering active workings, 

be capable of preventing an explosion initiated on 

one side from propagating to the other side, and 

continue its intended function for 1 h when subjected to a fire test incorporating a specific (ASTM E119-88) 

time-temperature heat input, or its equivalent (I).' 

Title 30, Part 75.335 of the U.S. Code of Federal Regulations (CFR) requires a seal to ". . . withstand a static 

horizontal pressure of 20 pounds per square inch . . . ." If 

a seal includes ". . . the use of timbers, the timbers also 

shall be coated on all accessible surfaces with flameretardant material having a flame spread index of 25 or 

less, as tested under ASTM E162-87." These revised 

regulations, effective November 16, 1992, were based, in 

part, on the results of the research presented in this report. Cementitious foams, like concrete, are incombustible 

and consist entirely of inorganic material that does not 

burn. Heavy timber of 4411 or greater thickness provides 

2 h of fire resistance. 

The U.S. Bureau of Mines and MSHA jointly are investigating the ability of various existing and new alternative seal materials and designs to meet or exceed the 

requirements of the CFR. This research is part of the 

USBM's overall mission to reduce mine accidents and improve working conditions in mines. Early USBM research 

(2) indicated that it would be unlikely for overpressures 

exceeding 20 psig to occur very far from the origin of the 

explosion provided that the area on either side of the seal 

contained sufficient incombustible, and minimal coal dust, 

accumulations. This is the first full-scale test program to 

evaluate seal designs in entry geometries similar to those 

of current U.S. coal mines. 

The seal research program previously has addressed, 

through testing at the USBM's Lake Lynn Experimental 

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

- - 

'1talic numbers in parentheses refer to items in the list of references 

at the end of this report. 

(3), low-density foam-block seals (4), and an initial test 

series with cementitious foam seals (3, 5). A brief summary of these published data on the construction techniques, preexplosion and postexplosion leakage measurements, and the effects of the explosion on these seals is 

presented in this paper. 

At the request of MSHA, several additional seal designs 

have been constructed and tested under the USBM's sealdevelopment investigation. This effort included a second 

series of cementitious foam seals and three series of tests 

on wood-block convergence seals. The cementitious foam 

seals, in this second test series, were all 4 ft thick with 

compressive strength designs varying from 100 to 200 psi. 

In addition to evaluating the strength characteristics of 

the cementitious foam seals, the effect of longer pumping distances for the cementitious foam slurries also was 

evaluated. 

Wood-block seals are used commonly in deeper mining 

operations to offset the problems associated with roof, 

floor, and/or rib convergence and movement resulting 

from overburden stresses. When this strata movement is 

exerted on the standard-type, concrete-block seal, the seal 

fails because of the stiffness characteristics of the block. 

Wood-block seals are less stiff with better deformation 

characteristics and they will yield or compress in response 

to strata movement. Experimental mine tests were conducted to evaluate the ability of wood-block seals to 

withstand the 20-psig pressure pulse while maintaining 

acceptable air-leakage rates. The wood-block seals were 

not subjected to any convergence forces other than those 

obtained from the use of wooden wedges at the roof and 

ribs prior to the explosion tests. The installation methods, 

leakage determinations, and explosion results associated 

with the cementitious foam and wood-block seals are 

presented in this report. 

PURPOSE 

The objective of this research is to determine whether 

seals constructed from various materials and designs can 

withstand a 20-psig methane-air explosion without losing 

their structural integrity. Not only must the seal be 

physically strong, but it also must effectively control gas-air 

exchanges between sealed and open areas. A safety and 

cost benefit also may result from these evaluations in that 

some of these new seal designs require fewer worker-hours 

and less materials handling to install than the standardtype, solid-concrete-block seal. 

Full-scale explosion-proof seal research provides input 

to MSHA for setting adequate standards and useful information to industry for the improvement of mining safety 
and economics. The Mine Safety and Health Administra- designs become available, MSHA needs performance data 

tion has issued new safety standards for underground coal from full-scale dynamic tests to evaluate the strength charmine ventilation that became enforceable regulations in acteristics and air-leakage determinations with these new 

November 1992. These new regulations address the con- seal types. The LLEM can be used to provide this data, 

struction of seals. As new seal construction materials and and MSHA has requested USBM assistance in this area. 

ACKNOWLEDGMENTS 

Kenneth Jackson, electronics technician, and William 

Slivensky, physical science technician, both with the 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA, who assisted in the arrangement of the instrumentation for the tests, monitored the construction of the 

seals, assisted in conducting the tests, and performed the 

initial evaluation of the conditions of the seals following the explosion. The research support work required 

in the construction and leakage testing of the seals was 

coordinated through John Perry, supervisor of SSI Services, Inc., at the Lake Lynn Laboratory, Fairchance, PA. 

The authors also acknowledge the following MSHA personnel for their involvement in the design of the seals 

and in the monitoring of the construction of these seals: 

Stephen Sawyer, chief, Industrial Safety Division; Steven 

Luzik, supervisory chemical engineer; and William Hoffman, physical scientist, of the Industrial Safety Division, 

Bruceton, PA. 

EXPERIMENTAL PROCEDURE 

MINE EXPLOSION TESTS 

All of the explosion and air-leakage determination tests 

on the various seal designs were conducted at the USBM's 

LLEM (6-7) located near Fairchance, PA. The LLEM is 

unique in that it is the only research facility in the world 

that can simulate current U.S. coal mine geometries for a 

variety of mining scenarios, including multiple-entry room 

and pillar mining and longwall mining. 

Figure 1 shows a plan view of the LLEM. The underground workings originally were used to extract limestone. 

In the late 1970's, the Bureau developed the experimental 

mine adjacent to these old workings. Drifts A, B, C, and 

D are 1,700 ft long and closed at the inby end. Drift E is 

500 ft long and connects C and D drifts. The dimensions 

of the drifts and crosscuts are typical of modern U.S. 

geometries for mine entries and range from 18 to 20 ft 

wide and approximately 6 to 7 ft high. Each drift has 10 

environmentally controlled data-gathering stations housing 

the instruments. Each data-gathering station houses a 

pressure transducer to measure the static pressure generated by the explosion, and an optical sensor to detect the 

flame travel. 

Figure 2 shows an expanded view of the seal test area. 

Methane gas was injected into the closed end of C drift. 

A plastic diaphragm was used to contain the 10-pct 

methane-air mixture within the first 47 ft of the entry. 

Electric matches located at the face in three locations were 

used to ignite the flammable methane-air mixture. Prior 

to the ignition of the methane gas, a concrete-steel 

bulkhead was positioned across E drift to contain the 

explosion pressures in C drift. Barrels filled with water 

were located in the gas zone to act as turbulence generators to achieve the 20-psig pressure pulse. To generate 

higher pressures, pulverized coal dust was placed on foam 

shelves that were located at 10-ft intervals outby the gas 

zone along the mine roof. To achieve pressures of 25,30, 

and + 35 psi, the number of shelf locations was increased 

to extend the coal dust loading to 70, 100, and 260 ft, 

respectively, from the face. 

All of the seals were constructed in the crosscuts 

between the B and C drifts. These crosscuts are approximately 6 to 7 ft high by 20 ft wide. The average crosssectional area of the crosscuts is 125 ft2. Generally, the 

seals were located in these crosscuts at a distance of 

approximately 5 to 8 ft from C drift. The pressure pulses 

exerted on each seal were measured by interpolation of 

the data from the nearest pressure transducers both inby 

and outby the crosscut position. Figure 3 shows a typical 

pressure trace generated from the LLEM computers. 

AIR-LEAKAGE DETERMINATIONS 

An important factor to be considered for any seal 

design is its impermeability, or its ability to prevent or 

reduce the exchange of gases from one side of the seal to 

the other. Measurements of the air leakages across the 

seals were conducted before and after each of the explosion tests. For these air-leakage tests, the D-drift bulkhead door (see figure 1) was closed to direct all of the 

ventilation flow to the seal locations in C drift. A stopping 

of double-brattice curtain was erected across C drift outby 
Data-gathering station 

I Crosscut 

Figure 2.Se.I test area in the LLEM. 
Figure 4.4ressurized entry for leakage-detenninatlon rates across the soals. 
Figure S.-Curtain configuration for leakage tests through test seal. 

Figure 6.--Monitoring of air flow through test seal. 

foam seals, MSHA personnel have developed tentative 

guidelines for acceptable air-leakage rates through a seal 

for this seal-testing program. The air-leakage rates 

through the subsequent seals during both preexplosion and 

postexplosion leakage tests were judged against these 

MSHA-established tentative guidelines. Table 1 shows 

these maximum acceptable air leakage rates, in cubic foot 

per minute, as a function of pressure differential, in inch 

of water. For pressure differentials up to 2 in H20, air 

leakage through the seal should not exceed 150 ft3/min. 

For pressure differentials over 3 in H,O, air leakage 

should be less than 250 ft3/min. 

Table 1 .--MSHA-established tentative guldeiines 

for air leakage through a seal 

Pressure differential, Air leakage thmugh 

in H20 seals, #/min 

Up to 1.0 ............ 5100 

Up to2.0 ............ 5150 

Up to3.0 ............ 5200 

More than 3.0 ......... 5250 
Not to scale 

Figure 7.--Standard-type, solid-concrete-block seal. 
with floor keying, also failed structurally. All of the thinwall, mortared seals failed structurally, even when constructed with a pilaster and floor keying. Table 2 

summarizes the test conditions and results for these solidconcrete-block seal designs. The use of coatings (118-inthick coatings of Austin and Associates' Fibercoat) on the 

seals did not strengthen the seals significantly; however, 

coating did help to minimize leakage through high-strength 

seals. The standard-type seal, keyed into both floor and 

ribs, withstood overpressures up to 42 psig without signs of 

failure. After five tests with explosion pressures >20 psig, 

the leakage through the standard-type seal was measured 

at only 87 ft3/min at a pressure differential of 1 in H20, an 

acceptable leakage rate. 

CEMENTITIOUS FOAM SEALS-TEST SERIES 1 

Five cementitious foam seal designs were constructed'in 

the crosscuts between B and C drifts of the LLEM to 

evaluate their strength characteristics and air-leakage 

resistance when subjected to a pressure pulse of 20 psig 

(3, 5). Tekseal cementitious foam (manufactured by Celtite Corp.,) was the material used in the construction of 

these seals. A simple wooden lattice structure, consisting 

of upright posts wedged to the roof and floor and horizontal crossboards with brattice material attached to 

the inside, was used as a form and liner to contain the 

cementitious foam slurry for each of the seals.6 No simulated keying was used with the cementitious foam seal 

designs. The slurry, consisting of the Tekseal powder, 

water, and air, was injected into the forms by means of a 

pump and hose assembly. A multiple-point injection technique was adopted that permitted the final slurry mixture 

to be injected, under slight pressure, near the top of the 

structure at three ports to ensure complete filling to the 

mine roof. 

The cementitious foam seals varied in thickness and 

design compressive strength. The compressive strength of 

the seal material varied according to its density, which 

was affected by the amount of Tekseal powder used per 

cubic yard of seal. Two of the seals had design compressive strengths of approximately 200 psi (density of about 

46 Ib/ft3), with thicknesses of 8 ft (crosscut 2) and 4 ft 

(crosscut 3). Another seal with a design compressive 

strength of 100 psi (density of approximately 30 lb/ft3) was 

4 ft thick (crosscut 4). The final two seals had compressive strengths of 50 psi (density of about 23 lb/ft3) with 

thicknesses of 8 ft (crosscut 5) and 4 ft (crosscut 6). A 

standard-type, solid-concrete-block seal remained in crosscut 1 from the previous test series. 

6~dditional bracing is needed on both sides of the seal to prevent 

collapse from the lateral forces exerted on the form during the injection 

of the cementitious foam slurry. 

Table 2.--Summary of test conditions and results for previously tested solid-concrete-block seal designs in the LLEM 

h'k~xin~um Postexplosion air-leakage 20-psig test 

Design Thickness, ft Crosscut overPressurel rates,' f?/min outcome2 

~sig 1 .O in H20 4.0 in H20 

Standard-type seal, wet-wall, 1.3 1 22 87 94 Passed. 

plaster, rib and floor keying. 

Standard-type seal except no 1.3 2 2 1 N Ap NAP Failed. 

floor keying. 

Wetwall, plaster, rib and floor .7 3 19 NAp N Ap Do. 

keying, coating on inby side. 

Wetwall, plaster, rib and floor .7 4 315 NAP NAP Do. 

keying, coating on outby side. 

Standard-type seal except no 1.3 2 17 N Ap NAP Marginal at c20- 

pilaster. psig pressure. 

Drywall, pilaster, rib and floor .7 3 18 NAP N Ap Failed. 

keying, coating on both sides. 

Standard-type seal except dry 1.3 2 20 NAp NAP Do. 

wall. 

NAp Not applicable. 

 r re explosion air-leakage tests were not conducted. Postexplosion leakage tests were not performed against seals that exhibited 

significant damage in terms of block removal and/or large gaping cracks. 

2~efer to reference 3 for detailed information. 

3Approximate pressure lower because of the venting through the opening formed when the seal in crosscut 3 failed. 
Following a curing period that exceeded 30 days, the Explosion tests have shown that 200-psi compressive 

cementitious foam seals were subjected to a nominal 20- strength, 4- and 8-ft-thick cementitious foam seals erected 

psig explosion pulse on the C-drift side. The pressure was in a 125-ft2 crosscut can structurally withstand a U)-psig 

approximately 29 psig at the fnst cementitious foam seal Pressure pulse, and subsequently, maintain acceptable airnear crosscut 2 and 21 to 22 psig at the seals in crosscuts leakage resistance based on ~ostex~losion air-leakage 

3, 4, and 5; and decreased to about 13 psig at the farthest measurements. Cementitious seal designs, regardless of 

outby seal in crosscut 6. Both of the 2m-psi-strength seals the cementitious f~am's have been 

survived the explosion and exhibited insignificant air- by MSHA for use in underground coal mines if the seals 

have compressive strength equal to or greater than 200 psi, leakage rates at pressure to 4.25 in and if they are constructed ue those tested in the LLEM. after the explosion. The 4-ft thick, 100-psi-strength seal 

displayed a series of cracks on both sides of the seal. 

Subsequent air-leakage tests showed that the seal was 

still maintaining acceptable air-leakage values, but because of the size and number of cracks, MSHA personnel 

considered the seal's performance marginal. Additional 

testing on the 100-psi-strength seal design needed to be 

conducted for final determination. The two 50-psi-strength 

seals failed under the explosion pressures. The 84- 

thick, 50-psi seal displayed severe fractures that extended 

through the entire seal. The 4-ft-thick, 50-psi seal was 

totally destroyed by the explosion. Table 3 summarizes 

the test conditions and results for these cementitious foam 

seal designs. 

OMEGA 384 FOAM-BLOCK SEALS 

Four seal designs were constructed in the crosscuts 

between B and C drift at the LLEM using Omega 384 

low-density block, as manufactured by Burrell Mining 

Products, Inc. (4). Omega 384 is a glass-fiber, reinforced, 

lightweight block (16 by 24 by 8 in with a density of 

23.9 lb/ft3) that is impervious to water and air leakage to 

pressure differentials of up to 8.0 in H20. A cementitious, 

fiberglass-reinforced bonding agent, Burrell Bond, was 

used in conjunction with the Omega 384 block (for mortar 

at block joints and for surface coatings) and was allowed 

to cure for at least 28 days. 

Table 3.--Summary of test conditions and results for prevlously tested cementitious foam 

and low-density foam-block seal designs In the LLEM 

Maximum Postexplosion air-leakage 20-psig test 

Design Thickness, ft Crosscut OverPresSure, rates,' f?/min outcome2 

~sig 1 .O in H,O 4.0 in H,O 

CEMENTITIOUS FOAM 

Compressive strength, psi: 

200 .............. 8 2 29 0 31 Passed. 

4 3 22 52 114 Do. 

100 .............. 4 4 22 47 114 Marginal. 

50 ............... 8 5 21 1 80 420 Failed. 

4 6 13 NAP NAP Do. 

LOW-DENSIN FOAM BLOCK^ 

Pilaster: 

2 ................ 2.7 2 20 2 1 52 Passed. 

2 3 2 1 1 40 294 Marginal. 

1 ................ 2 4 20 39 87 Passed. 

2 5 19 63 139 Do. 

NAp Not applicable. 

 reex explosion air-leakage tests were not conducted. Postexplosion leakage tests were not performed against seals 

that exhibited significant damage in terms of block removal and/or large gaping cracks. 

2~efer to references 3 and 4 for detailed information. 

3~esign compressive strengths were slightly different than actual strengths; for example, the seal in crosscut 4 was 

designed for 100 psi, but sample testing indicated an actual strength of 78 psi. 

4~1 of these seals utilized mortared joints (wetwall), coating on both sides, keying at the ribs and floor, and staggered block design. 
Simulated keying (hitching) on the floor and ribs using 

a 6- by 6- by 112-in-thick steel angle secured with 24-inlong by l-in-diam case-hardened steel bolts on 18-in ccnters was applied to all of the seal designs. These bolts 

were grouted into the floor and ribs. All of the block 

joints were staggered and mortared in each design. A surface bonding mortar of at least 0.25-in thickness was applied to the inby and outby faces of each seal. Each seal 

was wedged approximately 0.5 to 1 ft on top against the 

mine roof. Three of the seals *ere 24 in thick and one 

(located in crosscut 2) was 32 in thick. The seals located 

in crosscuts 2 and 3 each had two pilasters (48 in thick by 

48 in wide and located approximately one-third of the distance in from each rib); the seal in crosscut 5 had only one 

pilaster (centered) of similar dimensions. The seal in 

crosscut 4 had one large pilaster 56 in thick by 72 in 

wide located at the center of the seal. A standard-type, 

solid-concrcte-block seal, installed previously, was in 

crosscut 1 (4). 

The four Omega 384 block designs were subjected to an 

explosion that exerted a pressure pulse of approximately 

20 psig on each seal. Each of the four seal designs survived the test. Air-leakage measurements were then taken 

across each of the seals to determine the seals' air-leakage 

resistance characteristic. The air-leakage rates across the 

seals in crosscuts 2, 4, and 5 fell well within the MSHAestablished tentative guidelines (less than 250 ft3/min) for 

pressure differentials up to 4.0 in H,O. The leakage 

across the seal located in crosscut 3 exceeded these guidelines. Table 3 summarizes the test conditions and results 

for these low-density foam-block designs. 

Testing has confirmed that the Omega 384 seal designs 

meet the requirements of 30 CFR Part 75.335 (2) if constructed in the same manner as the seals in the LLEM (4). 

DISCUSSION OF RECENT TEST RESULTS 

CEMENTITIOUS FOAM SEALS-TEST SERIES 2 

Five additional seals were installed in the crosscuts between B and C drifts of the LLEM as part of a second test 

series to evaluate cementitious foam as a seal construction 

material. All of the seals were 4 ft thick and installed in 

the crosscuts with an average cross-sectional area of 

approximate:y 125 ft2. The designed strengths and 

thicknesses of two of the five seals, constructed with 

Celtite Tekseal, were based on the successful test results 

from the first test series (3, 5). Celtite also had improved 

on its cementitious foam dry-powder formulation, and the 

remaining three designs incorporated the Tekseal I1 

material. 

The wooden framework with attached brattice cloth 

used to contain the cementitious slurry was similar to that 

used during the first test series (3). No simulated keying 

with a steel angle was used with these seal designs. 

Figures 8 and 9 show two phases of the lumber and brattice framework construction. A copper tube was positioned through the middle of each seal (fig. 9), prior to the 

slurry injection, for monitoring the pressure differential 

across each seal during the leakage tests. Thermocouples 

were attached to the tubing to monitor the reaction temperatures of the cementitious slurry during the curing 

period. 

The seals installed in the first two outby crosscuts were 

constructed with the original Tekseal material. The dry 

Tekseal powder (packaged in 45-lb bags) was added to the 

hopper, auger-fed into a bin where it was mixed with 

water, aerated, and pumped into the seal forms. The 

LLEM's water supply at the time of construction ranged 

in temperature from 45" to 50" F. The design compressive 

strengths of the seals located in crosscut 1 and crosscut 2 

were 200 and 140 psi, respectively. The slurry used for 

these seals was pumped through 400 ft of hose. 

The other three seals were constructed with Tekseal I1 

with designed compressive strengths of 200 psi (crosscut 3), 150 psi (crosscut 4), and 100 psi (crosscut 5). The 

water used in the slurries for these seals was heated to a 

temperature range of 60" to 64" F. The increased water 

temperature was achieved by heating the mine's water supply. The water was passed through a bank of large electric 

heaters prior to going into the mixing pump. The elevated 

water temperature hastened the initial curing period required of the Tekseal I1 slurry, and served to harden and 

improve the compressive strengths of the seals. The improved characteristics of the Tekseal I1 formulation resulted in a reduction in the amount of dry powder (fewer bags 

per cubic yard of seal) required to obtain the desired 

compressive strengths as compared with that of the original Tekseal powder (approximate density of 30 to 38 lb/ 

ft3 for Tekseal I1 as compared with 46 Ib/ft3 for the 

original Tekseal to achieve comparable 200-psi-design 

compressive strength seals). The slurry was pumped 

through 850 ft of hose during the construction of the 

100-psi-designed compressive strength seal in crosscut 5. 

This distance would be typical of actual underground seal 

installation. 

Compressive strength measurements were made on 

90 samples, 3 in diam by 6 in long, that were collected 

during the slurry injection period from the five cementitious foam seals. Personnel from MSHA collected and 

tested the samples. Eighteen samples were collected from 
Figure 9.-4rattlce liner attached to framework used to contain the cementltlous foam slurry. Note copper tubing 

through the middle of the seal. 
each seal; six each from the bottom, middle, and top sections of the seal. Half of the samples were allowed to 

cure underground under the same temperature and humidity conditions as that for the seals; the rest were removed 

to a surface laboratory. Compressive strength tests were 

conducted on the samples at three time increments; 2, 4, 

and 6 weeks after seal completion. The compressive 

strength test results show, as expected, that the strengths 

increase as the curing time increases. The tests conducted 

on the samples after 6 weeks of curing, which corresponds 

to the time of the first explosion test conducted on the 

seals, generally resulted in average compressive strengths 

(table 4) well above the design strengths for both the 

samples cured in and out of the mine for all of the seals. 

Thermocouples were installed in the first three outby 

seals. The explosion pressures generated at the seal 

locations during each of the mine tests increased, respectively, as follows: 16 to 19, 21 to 26, 26 to 30, and 30 to 

35 psi during the final test. Following each of the first 

three explosion tests, minor damage was noted on several 

of the seals. This damage consisted of burned brattice 

cloth and removal of some of the upright wooden posts 

that had been part of the original form designed to hold 

the cementitious foam slurry. All five of the seals withstood the pressures exerted on them during the first three 

explosion tests. Air-leakage tests following each of these 

explosions showed that the air leakages were still within 

the established tentative guidelines (4). During the final 

explosion test, which generated pressures up to 35 psi, the 

seal in crosscut 5 (Tekseal 11, 100-psi strength) was comseals to measure the exothermic reaction temperatures of pletely destroyed. Upon testing, the seal in crosscut 4 

the curing Tekseal and Tekseal I1 materials. The tempera- (Tekseal 11, 150-psi strength) exhibited excessive airtures, as recorded from the thermocouples imbedded in leakage rates; at a pressure differential of 4.3 in H20, its 

the center of the seals, for the first two outby seals (con- leakage was 618 ft3/min. The seals in the first three crossstructed with the original Tekseal) peaked at about 145" F cuts essentially were undamaged and had acceptable airapproximately 1.5 days after the slurry injection. The leakage rates. At a differential pressure of 4.3 in H20 the 

temperature for the seals constructed with the Tekseal I1 leakage was 21 ft3/min for the crosscut 1 seal, 90 ft3/min 

peaked at 140" F after 7 days. The temperature from for the crosscut 2 seal, and 92 ft3/min for the crosscut 3 

thermocouples imbedded 6 in inside each face of each 

seal ranged from 5" to 20" F lower than the center 

temperatures. 

Air-leakage rates were measured at various pressure 

differentials for each seal before the explosion tests. With 

a pressure differential of approximately 4.4 in H20, the 

leakage rates across each of the seals starting from the 

face were 21, 35, 99, 80, and 83 ft3/min, respectively. All 

of these values were well within the MSHA-established 

tkntative guidelines shown in table 1. 

Four explosion tests were conducted in C drift to 

evaluate the performance of the five cementitious foam 

seal. Table 4 summarizes the test conditions and results 

for these cementitious foam seal designs. 

All five of the cementitious foam seals, as constructed 

in the 125 ft2 crosscuts during this second test series, 

survived the 20-psi explosion test, and subsequently, maintained negligible air-leakage rates. It must be noted, 

however, that the seals described in this report were approved by MSHA for use in underground coal mines only 

if constructed in a similar size and manner as in this study. 

Seals erected in larger than 125 ft2 crosscuts may need 

to be either thicker and/or have greater compressive 

strength. 

Table 4.--Summary of test conditions and results for recent 4-ft-thick cementitious foam seal designs 

-- 

Design Actual Maximum Air-leakage rates, f?/min 20-psig 

Crosscut compressive compressive overpressure, Preexplosion Postexplosion test 

strength, psi strength,' psi 

1 ...... 200 208 26 21 2 1 21 21 passed .3 

2 ...... 1 40 157 25 2 1 35 21 60 Do. 

3 ...... 2~ 376 22 37 99 3 1 85 Do. 

4 ...... 21 50 219 22 26 80 52 152 Do. 

5 ...... 21 00 168 21 30 83 61 154 ~arginal? 

'~ctual compressive strength values were based on an average value for 18 samples collected and laboratory tested for each seal. 

2~ekseal IIused for this seal. 

3~egree of damage - none. 

4~egree of damage - marginal. Minor damage, hairline cracks appearing to extend through seal. 
The seals were constructed in the LLEM under conditions analogous to those that may be encountered during 

seal construction in actual underground coal mining operations. However, measures must be taken to prepare the 

seal site, and additional bracing of the framework is recommended. All of the manufacturer's specifications on 

site preparation, form installation, and slurry injection 

must be carefully followed. As with the installation of any 

seal design, all loose material on the roof, ribs, and floor 

must be removed down to competent strata. All accumulated water in the seal area must be removed or absorbed 

with cement powder prior to the injection of the foamed 

slurry. In one mine there was difficulty in maintaining the 

framework in that the upright posts were kicking out at 

the bottom during the slurry injection period. To overcome this problem, it is recommended that roof bolts or 

tie wires be secured near the floor, at half height, and at 

the roof between two upright posts on opposite sides of 

the seal to reduce the tendency of those posts to move. 

This method should be used, if necessary, at a minimum 

of three locations across the width of the seal. Additional 

angle braces also can be installed laterally across the seal 

to strengthen the framework. To maximize closure at the 

roof and ribs, the framework-brattice must be erected in 

such a manner as to eliminate slurry leakages during the 

injection process. This may require stuffing brattice 

material or rags in the gaps between the roof and framework. When the gaps are plugged, the slurry can be injected within the framework through the injection ports 

near the roof under a slightly positive pressure. When 

injecting the fmal slurry material into the form under 

pressure, complete closure to the ribs and roof can be 

obtained. 

The proximity of cribbing and other obstructions near 

the area being sealed could result in injury to personnel in 

the event of a form collapse. For additional safety protection for the personnel injecting the slurry, it is recommended that extension pipes be attached to the injection 

hose to remove the personnel from the immediate vicinity 

of the seal. 

After the cementitious foam has cured, it is advisable 

that the wood framework and brattice be removed on the 

outby side to enable inspections of the seal. Observations 

can be made as to the integrity of the final seal with 

respect to complete contact with the ribs and roof. Any 

gaps should be filled immediately with additional cementitious foam or its equivalent. A suitable sealant (8) should 

be applied to the accessible surfaces of the seal to reduce 

drying. The seal then can be checked periodically for 

structural integrity (stress cracks) and impermeability 

(resistance to air leakages) and sealant can be reapplied if 

necessary. 

WOOD-BLOCK SEALS 

Wood-block seals currently are used in many mines 

that experience high convergence forces. These high convergence forces exceed the compressive strength of the 

standard-type, concrete-block seals, rendering these seals 

ineffective. Various wood-block (also referred to as cribblock) seal designs have been evaluated during this program. These tests determined strength and air-leakage 

resistance characteristics of the seals relative to the established guidelines of Title 30, Part 75.335 of the CFR. 

This research also focused on methods to strengthen existing wood-block seals to minimize the labor and costs associated with replacing inadequate seal designs. Once constructed, each seal was subjected to a 20-psig explosion 

pressure pulse. All of the seals were tested without any 

convergence forces (except that from wedging) that may 

be exerted on the seals from the roof, floor, and/or ribs 

under actual mine conditions. The tests with the woodblock seal designs were conducted in the absence of convergence forces since the extent and magnitude of these 

forces can not be predicted or guaranteed at every sealing 

location. These convergence forces also may take a while 

to build up and act on a newly erected seal. Furthermore, 

there is a compelling need to know the strength of an explosion that a newly installed crib-block seal can withstand. 

Standard crib blocks with nominal dimensions of 6 by 

6 by 36 in long were used during construction. These 

hardwood crib blocks weigh, on average, 30 lb each (density of about 44 lb/ft3). Approximately 38 crib blocks were 

used per row per seal. Approximately 15 rows of crib 

blocks were required to complete each of the seals. 

Therefore, 570 to 590 crib blocks were needed for each 

seal. As was the case with the solid-concrete-block (3, 5) 

and Omega block (4) seal designs, keying was simulated by 

bolting a 6- by 6- by l/Zin-thick steel angle to the floor 

and ribs. The bolts were grouted into the ribs and floor. 

Inadvertently, 314-in-diam steel bolts were used instead of 

the required 1-in-dim case-hardened steel bolts to secure 

the steel angle for all of the wood-block seal designs. This 

resulted in the failure of the restraint mechanism designed 

to simulate hitching. 

Test Series 1 

A wooden crib-block and rock-dust seal design typically 

used now in many underground coal mines was tested. 

This crib-block seal was installed in crosscut 2 between B 

and C drifts (fig. 10). In this design, the mine floor was 

first leveled, and then a layer of rock dust was placed on 

the mine floor to start construction. Wooden crib blocks 

(6 by 6 by 36 in long) were installed lengthwise, parallel to 
.in - thick 

dust layer 

Figure 10.-Typical wood-block and rock-dust seal design. 
Figure 12.-Wedging of wood blocks to mine roof. 

Following the curing period and the second leak test, 

the seals were subjected to a gas explosion generated at 

the face of C drift. A postexplosion inspection of the 

nailed-crib-block seal in crosscut 1 revealed that the seal 

was displaced en masse approximately 1 ft in the direction 

of B drift. Note that it was later determined that the 

314-in-diam steel bolts were not of adequate strength to 

secure the steel-angle simulated hitching. The seal had 

been subjected to a pressure wave of 23 psi, as measured 

with nearby static pressure transducers. The plywood 

facings on this seal received minor damage along their 

peripheries. The leakage test on this seal showed, as 

expected, that excessive amounts of air were passing 

through the damaged seal. At 2.2 in H,O, the air leaking 

through the seal was measured at 1,650 ft3/min. This 

As can be seen in figure 16, each of these crib blocks was 

positioned so that all of the block joints were staggered. 

The center crib blocks were installed lengthwise, parallel 

to the crosscut ribs, in the same manner as was the entire 

original seal in the first test series. For this seal, the crib 

blocks were secured together using 20-penny screw-type 

nails. In addition to nailing the crib blocks verticallv to " 

the lower course, each block also was nailed horizontally 

to the adjacent block. In the staggered block sections of 

the seal, this generally resulted in a full block being 

secured with nails to the surrounding blocks at eight locations. The blocks in the top row near the roof were all 

positioned lengthwise, parallel to the ribs, to facilitate nailing. Plywood sheeting (5/8 in thick) was inserted behind 

the steel angle keying on the floor and ribs and attached 

to both faces of the seal using 16-penny common nails. 

These nails were installed on 6-in centers. The Celtite 10- 

14 Airtight sealant was applied to the perimeter and 

plywood joints on both sides of the seal. 

The crib-block and rock-dust seal in crosscut 2 was 

erected in the same manner as it was during the first test 

series except that 3/4-in-thick plywood sheeting was inserted behind the floor and rib keying and secured to both 

faces of the seal with 16-penny screw-type nails on 6-in 
Figure 13.-Nailed-wood-block seal design 1. 

Figure 14.-Damage to typical wood-block seal when subjected to 20-psig pressure pulse. 
Figure 1 J.-Nalled-wood-block seal deslgn 2. 

Figure 16.4taggered wood-black joints. 
rock dust layer 

Figure 17.-Wood-block and rock-dust seal design 2. 
anchor the extra bracing to the floor angle were sheared 

of!' on the modified nailed-crib-block seal, and all four of 

these bolts were sheared off on the crib-block and rockdust seal in crosscut 2. The plywood sheets, in some 

areas, pulled slightly away from the crib block, as evidenced by the fact that the nails were extracted from the 

block and that there were scrape marks on the mine roof. 

The modified nailed-crib-block seal in crosscut 1 flexed 

approximately 8 in at the top center of the seal on the Bdrift side. On the C-drift side, the seal moved en masse 

about 1.5 in toward B drift. The crib-block and rock-dust 

seal in crosscut 2 showed very little evidence of movement 

of the plywood. On the B-drift side, the plywood flexed 

away from the explosion approximately 3 in at the top 

center of the seal; no evidence of seal movement was 

evident on the explosion side of the seal (C drift). All of 

the nails used to attach the plywood to the crib block 

appeared to have held tightly. Note that this seal utilized 

16-penny screw-type nails to secure the plywood to the crib 

block; whereas, the modified nailed-crib-block seal in 

crosscut 1 used 16-penny common nails to attach the plywood. Both seal designs survived a greater than 20-psi 

explosion force. 

Both seal designs failed to pass the postexplosion airleakage tests. For the modified nailed-crib-block seal in 

crosscut 1, an average of 1,370 ft3/min of air was passing 

through the seal at a pressure differential of 1.3 in H20. 

For the crib-block and rock-dust seal design in crosscut 2, 

an average of 1,005 ft3/min of air was leaking across the 

seal at a pressure differential of 1.3 in H20. Excessive 

quantities of air were passing through the seals; therefore, 

the seals no longer met the MSHA-established tentative 

guidelines for air-leakage resistance. However, it was later 

the judgement of MSHA that this excessive leakage was 

due primarily to the failure of the restraint mechanism 

(inadequacy of the 3/4-in-diam bolts used to hold the 

steel-angle simulated hitching). 

Test Series 3 

The seals from the second test series were judged to be 

of adequate strength to withstand the 20-psig pressure 

pulse. During the third test series, emphasis was placed 

on developing a method of applying sealant to the seal 

designs to meet or exceed air-leakage requirements following an explosion. 

The seal design constructed in crosscut 1 was very 

similar to the original nailed-crib-block seal design in 

crosscut 1 from the first test series (fig. 13). All of the 

crib blocks were installed lengthwise, parallel to the ribs 

and wedged tightly to the roof and ribs. Two 20-penny 

screw-type nails were used to secure each block to the 

adjacent block. One important difference was that for the 

new seal design the crib blocks were nailed both horizontally and vertically to the adjacent block, instead of just 

in the vertical plane as was the case in the first test series. 

Another change was that the plywood sheeting was inset 

behind the steel-angle keying for the new seal design. Plywood sheeting was not placed in this manner for the seal 

in the first test series. Both sides of the seal were covered 

with 5/8-in-thick plywood sheets. Another change for this 

design was the manner in which the sealant was applied to 

the seal. Both sides of the seal were given a 1/2-in-thick 

coating using the Celtite 10-14 sealant prior to the installation of the plywood sheeting. It was believed that a 

more cohesive bond could be achieved between the block 

and the block-mine interfaces to limit sufficiently the 

amount of air leakage across the seal following the 20-psig 

explosion. The plywood sheeting was set behind the simulated hitching and attached to the crib block using 16- 

penny screw-type nails on 6-in centers. 

The seal design constructed in crosscut 2 (fig. 17) was 

similar to the crib-block and rock-dust seal that was tested 

in the same crosscut in the second test series. However, 

in this case the sealant was applied fully to both faces 

prior to attaching the 3/4-in-thick plywood sheets. Four 

days after the application of the sealant, the plywood facings were set behind the steel angle keying and attached to 

the crib block using 16-penny screw-type nails on 6-in 

centers. 

Measurements of the air leakage through the two seals 

were conducted prior to the 20-psig level explosion test. 

The modified nailed-crib-block seal design in crosscut 1 

exhibited negligible air leakage across the seal. At 4.5 in 

H20, the air leakage was 86 ft3/min. The crib-block and 

rock-dust seal design in crosscut 2 displayed an air leakage 

of only 67 ft3/min at 4.5 in H20. Both of these values are 

well below the MSHA-established tentative guideline of a 

maximum air-leakage rate of 250 ft3/min for water gauges 

exceeding 3.0 in H20. 

The ignition of the 47-ft long, 10-pct methane-air zone 

at the closed end of C drift generated a pressure pulse of 

approximately 20 psig on each seal. A post-test inspection 

of the seals revealed some structural damage. Crib-block, 

plywood-facing, and steel-angle (simulated keying) displacements occurred in both seals. A subsequent airleakage test showed that both seals exhibited significant 

air-leakage rates. At 0.9 in H20, each seal exhibited air 

leakage in excess of 1,000 ft3/min. For water gauges up to 

1 in H,O, the maximum allowable air leakage should be 

100 ft3/min. Table 5 summarizes the test conditions and 

results for the wood-block convergence seal designs in this 

third test series, as well as those tested in the previous two 

series. 
Table 5.--Summary of test conditions and results for the wood-block convergence seal designs 

Maximum Air-leakage rates,' f?/min 20-psig 

Seal type Crosscut overpressure, Preexplosion ~ostexplosion~ test 

Test series 1: 

Nailed-wood-block seal2 . . 1 23 0 27 1,066 NAP passed .6 

Typical wood-block and , 2 22 0 58 NAP NAP ~ailed.~ 

rock-dust seal.3 

Test series 2: 

Modified nailed-wood- 1 23 7 45 1,370 NAP passed .6 

block seal.4 

Modified wood-block and 2 21 72 167 1,005 N Ap Do. 

rockdust seal.' 

Test series 3: 

Modified nailed-wood- 1 20 40 86 1,155 N Ap Do. 

block 

Modified wood-block and 2 19.5 24 67 1,039 NAP Do. 

rockdust seal .' 

NAp Not applicable. 

'~ostexplosion air-leakage tests were not performed against seals that exhibited significant damage or those that exceeded the MSHAestablished tentative guidelines at the lower level pressure differentials. 

'see figure 13. 

3~ee figure 10. 

4~ee figure 15. 

'see figure 17. 

'MSHA judged that the seal passed with minor damage and the excessive postexplosion air-leakage rates were due primarily to the 

failure of the restraint mechanism (simulated keying). 

7~ailed. Totally destroyed. 

The results of the explosion tests against the woodblock designs have shown that the 314-in-diam bolts 

(thread-bolt) that were used were not adequate, in most 

cases, to prevent the movement of the steel angle. Casehardened, solid-steel rods with 1 in dim were used successfully to secure the steel angle during the testing of the 

solid-concrete-block and Omega block seal designs. This 

bolting arrangement was of sufficient strength to secure 

the steel angle for all of the explosion tests with pressure 

pulses ranging up to 42 psig. Therefore, if this procedure 

is ever to be used in underground coal mines in place of 

hitching into the solid, 24-in long by 1-in-dim casehardened steel rods (imbedded 18 in) on 18-in centers are 

to be used. 

Without being subjected to convergence forces from the 

roof, ribs, and/or floor, all but one of the 6 tested cribblock seal designs were considered of adequate design 

strength to withstand a 20-psig pressure pulse. The one 

design that failed to withstand the pressure pulse was the 

typical crib-block and rock-dust seal design that is used 

currently in many mines. All of the wood-block seal 

designs failed to meet the tentative criteria for postexplosion impermeability in the absence of mine convergence forces. However, it is the judgement of MSHA that 

all of the modified wood-block convergence seal designs 

did meet the strength requirements of the CFR and would 

have exhibited acceptable postexplosion leakage rates if 

the restraint mechanism had not failed. Additional research is still needed to finalize a preexplosion and postexplosion air-leakage criteria. Research will also continue 

in the design and full-scale testing of convergence seals 

that can withstand a 20-psig pressure pulse in the absence 

of convergence forces from the roof and/or ribs. 

CONCLUSIONS 

Four types of seal construction materials were explosion while still maintaining those features that provide resistand air-leakage tested in the LLEM. The tests were de- ance to air leakage. 

signed primarily to determine the strength characteristics As first noted in previous reports,. seven seal designs 

of the seal designs. Each seal design was evaluated as to using solid-concrete blocks were tested in the LLEM. 

its ability to withstand a pressure pulse of at least 20 psig Only the standard-type seal passed the explosion and 
1. American Society for Testingand Materials. Standard Method for 

Fire Tests of Building Construction and Materials. E119-80, in 1992 Annual Book of ASIU Standards: Volume. 04.07, Building Seals and Sealants; Fire Standards; Building Constructions. Philadelphia, PA, 1992, 

pp. 331-351. 

2. Mitchell, D. W. Explosion-Proof Bulkheads. Present Practices. 

BuMines RI 7581, 1971, 16 pp. 

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

Evaluation of Solid-Block and Cementitious Foam Seals. BuMines 

RI 9382, 1991, 13 pp. 

4. Stephan, C R Omega 384 Block as a Seal Construction Material. 

MSHA ISD Report No. 10-318-90, Nov. 14, 1990, 15 pp. 

5. Stephan, C. R Construction of Seals in Underground Coal Mines. 

MSHA ISD Report No. 06-213-90, Aug. 1, 1990, 35 pp. 

6. Mattes, R H., A. Bacho, and L. V. Wade. Lake Lynn Laboratory: 

Construction, Physical Description, and Capability. BuMines IC 8911, 

1983,40 pp. 

7. Triebsch, G., and M. J. Sapko. Lake Lynn Laboratory: A Stateof-the-Art Mining Research Facility. Paper in Proceedings of International Symposium on Unique Underground Structures (Denver, CO, 

June 12-15, 1990). CSM Press, v. 2, 1990, pp. 75-1 to 75-21. 

8. Sawyer, S. G. Mortar for Use in the Construction of ConcreteBlock Stopping and Seals in Underground Mines. MSHA ISD Report 

No. 02-174-92, June 22, 1992, 13 pp. 
LOYER