Improving stopping donstrudtion 

to minimise leakage 

Roy H. Grau III, Andrew L. Mazzella and Anu L. Martikainen 

Office of Mine Safety and Health Research (OMSHR) 

National Institute for Occupational Safety and Health (NIOSH) 

Pittsburgh 

Abstract 

The proper sealing of stoppings is an important step in 

reducing leakage from the intake to the return airways. 

Leakage and the subsequent loss of ventilation resulting 

from improperly sealed stoppings can lead to unhealthy and 

unsafe working conditions. The research presented in this 

paper investigates the total leakage of a stopping, including 

air leakage through the stopping, at the stopping perimeter, 

and through the coalbed. The study also examines sealing 

considerations for stoppings that are constructed under roof 

control screen, the effects that wooden wedges had on 

inhibiting efficient application of polyurethane foam sealant, 

and airflow leakage through the surrounding coal. The work 

involved building a stopping in a dead end room of the 

NIOSH Safety Research Coal Mine and then pressurising 

the room using compressed air. Stopping leakage was 

evaluated by measuring air pressure loss in the enclosed 

room due to the air leakage. Part of the research utilises a 

diluted soap solution that was applied to the stopping and 

the surrounding coal to detect air leakage signified by 

bubble formations. The results show that stopping leakage 

can be minimised with proper sealing. 

Introduction 

Stoppings are the primary ventilation control system used 

by mine operators to course ventilation air through a mine. 

Since stoppings separate fresh intake air from the return air 

that may contain dust, contaminants, or potentially 

explosive methane mixtures, it is important to keep leakage 

to a minimum through stoppings to keep personnel both 

healthy and safe. The number of stoppings being 

constructed each year requires the mining industry to 

devote considerable resources and time for construction 

and maintenance of these structures. Knowledge of 

efficient stopping construction benefits all mineworkers. 

Prior research concerning stopping leakage focused on 

stopping construction aspects such as block types, 

construction techniques, sealant products, and sealing 

methods. The tests included both laboratory and in-mine 

tests. Generally, laboratory studies included pressurising 

areas that were enclosed by a stopping built exclusively for 

testing. In-mine tests often simply averaged leakage rates 

from several stoppings because of the difficulty in 

determining the actual leakage from a single stopping in a 

working coal mine. 

The research presented in this report was conducted on 

one stopping that was built at the NIOSH Safety Research 

Coal Mine in Bruceton, PA. It differs from previous research 

in that the stopping was constructed underneath steel and 

fiber roof control screen, which is finding increasing 

application in underground mines to control minor falls of 

the roof. Two other areas the research investigated were 

the effects that wooden wedges had on inhibiting efficient 

application of polyurethane foam sealant and 

understanding airflow leakage through the surrounding 

coal. Understanding leakage quantities through the 

coalbed and through or around the stopping will help mine 

ventilation planners to better identify possible leakage 

paths and to quantify their flows for ventilation analyses. 

Since the tests involved sealing gaps in or around the 

stoppings, the information found is also transferrable to 

stoppings that are built underneath uneven roof. 

Background 

Some of the earliest work on stopping leakage was 

performed by Peascod and Keane (1955), who claimed 

that the air quantity losses through stoppings are greatest 

in the outby portion of the intake where there is the greatest 

pressure differential, accounting for about 75% of the total 

loss. Further stopping leakage research was performed by 

Holland and Skewes (1962), who found that the leakage 

through a mortared joint stopping was reduced 10-15% by 

applying a mortar or paint coating. A dual coating of mortar 

combined with paint reduced the leakage rate to 2-3% of 

the original rate by comparison to a stopping having no 

coating. These findings showed that a cement coat or 

painting significantly decreased leakage and that the 

stopping block material itself was secondary to coating in 

preventing leakage. 

Further stopping research was performed by Kawenski et 

al. (1965), who pressure tested stoppings composed of 

blocks of slag, cinder, and gravel as well as polyurethane 

and mortar sealants. Similar to what Holland and Kewes 

(1962) determined, Kawenski found that the seal of the 

stopping to the ribs, roof, and floor - not the block type 

itself - was the most important variable in reducing leakage. 
However, it appears that minimal work in either research 

cited above addressed leakage at the perimeter of the 

stopping in real-life conditions. 

Timko and Thimons (1983) evaluated various stopping 

construction methods to determine the best ways to 

reduce leakage. Their study found that the construction 

method significantly affected leakage and concluded that 

multiple layers of sealants, as well as sealants containing 

fiber (reinforced), increased air leakage resistance. 

Applying the sealant using a brush rather than a trowel 

resulted in less leakage. Reducing leakage around the "

perimeter of the stopping was best achieved by keying"

the stopping blocks into the ribs, top, and bottom of the 

entry. 

Although leakage through stoppings was reduced by the 

above enhanced installation techniques, all stoppings 

leaked to various degrees. Timko and Thimons noted in 

their study that stopping leakage was reduced an average 

of 33% (25-36% range) after six months due to rock dust 

plugging small voids, but that leakage increased 40% after 

two years due to stopping damage from ground 

movement. Leakage rates exceeding 0.3 m3/s (600 cfm) 

were found in new stoppings where loose rib rock was not 

removed before stopping construction. Lastly, leakage was 

minimised where the corner interface of the stopping with 

the rib or roof was a smooth patchwork extending several 

inches from the stopping onto the rib, almost eliminating air 

leakage in the stopping perimeter. 

Gandy (1998) measured the resistance across four 

stopping types in five ventilation surveys at continuous 

miner sections. He found stoppings that were covered with 

TYVEK ®, a blanket type material manufactured by 

Dupont, produced significant reductions in leakage. 

Alternatives to block stoppings, such as those constructed 

of steel sheets, are available. Steel stoppings are generally 

assembled from prefabricated panels, which are pre-sized 

to the dimensions of the mine entry. Steel, or Kennedy 

stoppings, are advantageous because of quick assembly 

time and ease of material handling. Oswald et al. (2008) 

performed a study investigating and comparing the 

resistances of block stoppings and Kennedy stoppings. 

This research found that in all cases, the leakage 

resistances of block stoppings were higher than those of 

Kennedy stoppings. Furthermore, in stoppings of average 

condition, block stoppings had a 40% higher air resistance 

than Kennedy stoppings. However it should be noted that 

leakage through steel stoppings such as the Kennedy 

stopping can be reduced by sealing the joints using a tape 

suitable for that purpose. A study performed by Burke 

(2003) found that block size uniformity is crucial in creating 

a stable stopping, while blocks of non-uniform size 

promote stress concentrations in the stopping. These 

stresses can result in fracturing and failure of the stoppings 

with subsequent leakage. Batchler and Barczak (2008) and 

Oyler et al. (2001) found that horizontal foam strips laid 

between rows in concrete block stoppings, designed to 

yield under load, actually increased the stopping damage, 

with the yield strips promoting unequal loading, resulting in 

leakage cracks. 

A final consideration for reducing leakage through 

stoppings is the condition of the ribs and roof, which 

includes the presence of roof control screening. Roof 

screening is becoming more common in coal mines to 

prevent falls of rock. Robertson and Hinshaw (2002) found 

that roof screen is effective in reducing falls of smaller rock 

from the immediate roof. No studies have been performed 

determining if roof screen hinders adequate sealing 

between the roof and the stopping. Certainly by 

observation, stoppings built under roof screening have the 

potential to increase air leakage. 

Stopping construction 

The prototype stopping was constructed in the NIOSH 

Safety Research Coal Mine in a dead-ended entry using 

materials and procedures commonly used in underground 

coal mines for stoppings as shown in Figure 1. 

Figure 1. Test setup 

The stopping was built approximately 1.2 m (4 ft) from the 

end of the coal face, creating a small room that was 

enclosed by the stopping, the face of the mine, and the 

coal ribs. The entry dimensions were approximately 4.3 m 

(14 ft) wide x 2.0 m (6.5 ft) high. The stopping was 

constructed using Omega blocks, which is a low-density 

product 352 kg/m3 (22 lbs/ft3) composed of a mixture of 

Portland cement, fly ash, and other substances. The block 

dimensions are 0.61 m x 0.41 m x 0.2 m (24 in x 16 in x 8 

in). These blocks were dry stacked with the 0.61-m x 

0.41-m (24-in x 16-in) side down and the stopping was not 

keyed into the rib or floor. This arrangement is typical of 

that used in a coal mine as the larger base area increases 

the stopping's stability. The stopping was installed with a 

man-door that was set at a slight angle allowing it to 

automatically close due to gravity. 

In order to simulate leakage and mine ventilation 
pressures, compressed air was injected into the enclosed 

room at varying rates until a maximum pressure of 1245 Pa 

(5 in WG) was reached inside the room. The pressure was 

measured using a Viatran pressure transducer, Model 

2746CBCCH. The compressed air was measured using a 

Sip 200-DC24-Air Sage Prime thermal mass gas flow meter. 

A total of four tests were performed where a predetermined 

sealant application was applied on the stopping after each 

test. With the tests performed in this manner, it was 

possible to determine the impact each additional sealant 

application had on leakage. 

Results 

The first pressure-leakage test, Test 1, was performed 

without sealant applied to the stopping. As compressed air 

was introduced into the enclosed room, the numerous 

leakage paths prevented air pressure to develop. During 

subsequent pressure-leakage tests, the stopping was 

progressively sealed more thoroughly. In preparation for 

Test 2, B-Bond MS Mine Sealant was applied to the 

high-pressure side of the stopping and along the stopping 

perimeter. B-Bond Mine Sealant, manufactured by 

Quickrete, is a surface bonding cement containing a blend 

of Portland cement, fiberglass fibers, and special additives. 

After a drying period of four weeks, Test 2 was performed 

with the room being pressurised to 1245 Pa (5 in WG) with 

a flow of 0.472 m3/sec (1,000 cfm). During pressurisation, 

the relationship between pressure and leakage was linear 

as shown in Figure 2. 

Figure 2. Airflow leakage at different pressures for stoppings 

tested 

Leakages were easily detected by 

feeling the airflow with the hand and by sight using smoke 

from ventilation smoke tubes as shown in Figure 3.

Figure 3. Smoke leaking through the stopping perimeter at 

the roof for Test 2 

 The 

leakage was located primarily at the top and sides of the 

stopping and around the man-door. 

Preparation for the third pressure-leakage test (Test 3) 

included adding polyurethane foam sealant to the perimeter of the high-pressure side of the stopping as shown in 

Figure 4 and around the man-door. 

Figure 4. Polyurethane foam sealing stopping at roof on 

pressurised stopping side for Test 3 

These improvements 

resulted in a leakage reduction of 62% to 0.177 m3/sec 

(375 cfm) at 1245 Pa (5 in WG). The pressure increase followed the relationship shown in Figure 2. 

Although airflow leakage was significantly reduced during 

Test 3, leakage was still evident even though the stopping 

was sealed with polyurethane foam. Ventilation tube smoke 

released on the high-pressure side readily passed between 

the roof and foam, showing that the interface between the 

stopping and the roof provided minimal resistance to 

airflow leakage. Although the foam appeared to thoroughly 

seal the stopping perimeter, the foam was covering the 

stopping and roof control screen only superficially, which 

did not create an adequate seal. This test showed that 

there were two hindrances to effectual sealing of the 

stopping: the presence of roof control screen and the 

presence of wooden wedges that are typically used to 

keep the stopping in place. Both the roof control screen 
and the wooden wedges blocked the penetration of the 

polyurethane foam and prevented the foam from filling 

voids between the blocks and the roof or ribs. The wooden 

wedges were positioned in the normal manner with the 

wedges extended out past the plane of the stopping wall. 

Nevertheless, the placement of the wedges in this manner 

prevented the foam from being sandwiched between the 

stopping block and the roof and allowed only a superficial 

foam coating. 

Preparations for Test 4 consisted of removing the 

polyurethane foam at the roof perimeter by hand scraping 

and removing some of the wedges. The foam was then 

reapplied between the stopping block and the roof, rib, and 

roof control screen material. This foam application differed 

from the previous test in that, rather than superficially 

spraying the stopping, the wand applicator tip was inserted 

in any cracks or leakage paths, allowing the foam to flow 

from inside to outside the crack. The surface area of the 

low-pressure side of the stopping also was sealed with 

B-Bond. These changes produced a dramatic reduction in 

stopping leakage. Figure 2 shows the leakage function 

being nearly linear for Test 4. A pressure of 1245 Pa (5 in 

WG) pressure was achieved in the sealed room at a leakage flow of 0.019 m3/sec (40 cfm). This result was 

obviously more favourable than that for the first test, which 

had 0.472 m3/sec (1,000 cfm) leakage at the same 

pressure. 

In order to determine the benefit of sealing the stoppings 

and to relate the test results with those for other types of 

stoppings, resistance factors for the stoppings performed in 

Tests 2 to 4 are compared to those found by Oswald 

(2008) and are detailed in Figure 5. 

Figure 5. Resistance factors for various stopping types 

The chart shows 

varying resistance factors compared to stopping quality 

and provides ratings for Kennedy (steel) stoppings and 

concrete block stoppings on a scale of poor, average, and 

excellent based on observation criteria reported by Oswald. 

Stoppings rated in poor condition have several cracks that 

allow significant leakage and are in need of repair. Average 

stoppings are the most common stopping condition found 

in underground coal mines and would be stoppings that 

although not near perfect in both condition and 

construction, are adequate in their application in the mine. 

Stoppings in excellent condition have high air resistance 

and have minimal leakage; generally, these stoppings are 

those that have been coated with a sealant. The stopping 

assessed in Test 1 was not considered in this evaluation 

due to its high leakage quantity. The stopping in Test 2 fell 

into the poor category and had slightly lower resistance 

than both the block stopping and Kennedy stopping. The 

stopping in Test 3 fell into the average stopping category, 

as it had a resistance lower than the block stopping but 

higher than the Kennedy stopping. The stopping in Test 4 

fell into the excellent category; and had a resistance nearly 

50% greater than the block stopping with excellent 

condition, and 140% greater than the Kennedy stopping. 

The results of the current stopping tests can also be 

compared with the results found by Timko and Thimons 

(1983) from stoppings assessed in underground coal 

mines. The authors found that a hollow core stopping that 

was brushed with reinforced mortar on one side and with 

the stopping keyed into the floor and rib gave excellent 

leakage results of 0.023 m3/s (50 cfm) for 9.29 m2 (100 

sq ft) of stopping surface at 620 Pa (1 in WG). This was 

comparable with the best stopping in Test 4, which leaked 

0.025 m3/s (52 cfm) after normalising the test results to an 

equivalent size and pressure. This result is noteworthy 

because a stopping that is keyed into the rib and floor 

requires more time for construction than simply applying 

polyurethane foam. 

Normalised leakages for the poor and average stoppings 

from Tests 2 and 3 were also compared to those values 

obtained by Timko and Thimons. Normalised leakages for 

the poor and average stoppings were 0.097 m3/s (205 cfm) 

and 0.087 m3/s (185 cfm), respectively. These values 

exceeded the 0.066 m3/s (140 cfm) reported by Timko and 

Thimons for a new conventional hollow block stopping 

coated with mortar on one side. The poor and average 

stoppings leaked less than the 0.109 m3/s (230 cfm) 

reported for the same stopping studied by Timko and 

Thimons after a two-year period. One significant difference 

between the test stopping and the Timko and Thimons 

stopping was that the test stopping had a door while the 

stopping studied by Timko and Thimons did not. 

Soap solution bubbles test 

Visual observation of leakage through stoppings gives a 

better understanding of leakage paths, particularly those 

through the strata surrounding the stopping. Also, smoke 

can be difficult to discern at low leakage quantities, as 

evidenced in Test 4. To further understand leakage 

pathways, after Test 4 was completed, the stopping was left 

intact and a watery soap mixture was sprayed onto the 
surrounding strata and on the low-pressure side of the 

stopping. As in the previous tests, compressed air was 

injected into the room. A photographic record was 

developed of bubbles being formed on the stopping's 

surface and surrounding strata. This was performed at 

pressures of 375 Pa, 750 Pa, and 1120 Pa (1.5 in, 3.0 in, 

and 4.5 in WG). 

Bubbles started forming at the lowest pressure, 375 Pa (1.5 

in WG), and as would be expected, bubble size, growth 

rate, and quantity increased with pressure. The bulk of the 

bubbles formed at the stopping perimeter, primarily at the 

roof, but also along both ribs. No leakage was observed at 

the floor. The access door, which was sealed with a rubber 

gasket and foam, leaked at all pressures. The air leakage 

through the coal rib appeared to be minimal, and bubbles 

were only present at 1120 Pa (4.5 in WG) and located 

about 0.3 m (1 ft) outby the stopping. At one point, a very 

small bubble appeared approximately 0.91 m (3 ft) outby 

the stopping on the right rib. 

Figure 6. Soap bubbles on stopping and at the right rib at 

1245 Pa (4.5 in) WG 

Stopping construction rules-of-thumb 

This study has added to the knowledge developed through 

the years detailing the best stopping construction methods 

to minimise leakage. Combining past studies and the 

results from this study, rules-of-thumb that minimise 

leakage for constructing block stoppings and, where 

applicable, for steel stoppings are provided. 

• Block type is much less important than the sealing 

method for preventing leakage. 

• Remove broken weathered rib material before 

constructing the stopping and, if possible, key the 

stopping into the floor and ribs. 

• Apply reinforced sealant on both high and low-pressure 

surface areas. The sealant is better applied using a brush 

rather than a trowel. 

• Apply a reinforced mortar on the perimeter several inches 

onto the rib to form a cove between the stopping and the 

rib or roof. 

• Past studies have shown that laying horizontal foam 

strips between rows in block stoppings may actually 

promote stopping damage by creating unequal loading 

of the stopping. 

Apply polyurethane foam with the wand tip inserted into 

cracks rather than applying it superficially. Polyurethane 

foam can be effective in reducing leakage when the 

stopping is built under uneven roof. 

Apply polyurethane foam before inserting wooden stability 

wedges in the stopping. Wedges should be inserted as the 

foam is drying. 

Conclusions 

These tests showed that using a normal visual inspection 

to determine the adequateness of a sealant or 

polyurethane foam at inhibiting air leakage through a 

stopping is unreliable. During these tests, a coal mine room 

was enclosed by a stopping that was sealed with B-Bond 

MS Mine Sealant and polyurethane foam. The 

polyurethane foam was applied in a spraying manner on 

the stopping and the room was pressurised using 

compressed air. Smoke from ventilation smoke tubes 

revealed many leakage paths around the foam. 

Although these results are limited by the tests being 

performed at one location, with only a few scenarios 

including one brand of foam and sealant, the results do 

show some significant findings. The superficial application 

of the polyurethane foam provided poor sealing, 

particularly with highly irregular surfaces such as roof 

support screen material or where wooden wedges blocked 

the penetration of the foam. A subsequent test showed that 

a better approach was to apply the polyurethane foam by 

inserting the pressure wand in cracks or leakage paths as 

far as possible, applying the foam and then inserting the 

wedges while the foam was still wet. 

A water-soap solution was used to visually detect air 

leakages. Based on this solution being sprayed on the 

stopping and coalbed, the air leakage through the coalbed 

was minimal compared to the leakage through or around 

the stopping. The test revealed only a couple of bubbles on 

the coalbed and only at the highest pressure of 1120 Pa 

(4.5 in WG). The soap bubble tests revealed that the 

stopping face leaked at random locations but most leakage 

occurred at the stopping perimeter. Leakage occurred at all 

pressures even though both sides of the stopping were 

coated with B-Bond Mine Sealant. 

As would be expected, leakage differences increase 

significantly as pressures increase, and the differences in 

leakage between the best-sealed stopping and the other 

stoppings were significant. This would indicate that more 

diligent effort in stopping sealing is necessary where higher 

pressures are present. 
Disclaimer 

The findings and conclusions in this report are those of the 

authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. 

Mention of any company name, product, or software does 

not constitute endorsement by NIOSH. 

References 

Batchler, T.J., and Barczak, T.M. 2008. Impact of 

Deformable Materials and Convergence on the Transverse 

Load Capacity of Mine Ventilation Stoppings, 12th 

U.S./North American Mine Ventilation Symposium, Reno 

NV, June 9-11, 2008 - Wallace (ed), pp 239-244, ISBN 978-

0-615-20009-5. 

Burke, L.M. 2003. Numerical Modeling for Increased 

Understanding of the Behavior and Performance of Coal 

Mine Stoppings. Thesis submitted to the Faculty of the 

Virginia Polytechnic Institute and State University, 

December, 114 pp. 

Gandy, J.R. 1998. Determination of Resistance Across 

Development Section Return Stoppings. A Thesis submitted to the Faculty of Department of Mining Engineering. 

College of Engineering and Mineral Resources, West 

Virginia University, December, 116 pp. 

Holland, C.T., and Skewes, W.J. 1962. Investigating 

Construction Materials and Methods for Stoppings in Coal 

Mine Ventilation Systems. Transactions of the American 

Institute of Mining, Metallurgical and Petroleum Engineers, 

Society of Mining Engineers, Volume 220. pp 21-30. 

Kawenski, E.M., Mitchell, D.W., Bercik, G.R., and Frances, 

A. 1965. Stoppings for Ventilating Coal Mines. BuMines RI 

6710, 20 pp. 

Oswald, N., Prosser, B., and Ruckman, R. 2008. Measured 

values of coal mine stopping resistance. 12th U.S./North 

American Mine Ventilation Symposium 2008 - Wallace (ed), 

ISBN 978-0-615-20009-5. 

Oyler, D.C., Hasenfus, G., and Molinda, G.M. 2001. Load 

and Deflection Response of Ventilation Stoppings to 

Longwall Abutment Loading: A Case Study. Proceedings 

of the 20th International Ground Control in Mining 

Conference. Morgantown, WV, pp. 34-41. 

Peascod, W., and Keane, A., 1955. The Effect of Leakage 

on Mine Ventilation. Colliery Engineering, Vol. 32, pp. 207-

211. 

Robertson, S.B., and Hinshaw, G.E. 2002. Roof Screening 

Best Practice and Roof Bolting Machine. Proceedings of 

the 21st International Conference on Ground Control in 

Mining. Morgantown, WV: West Virginia, pp. 189-194. 

Timko, R.J., and Thimons, E.D. 1983. New Techniques for 

Reducing Stopping Leakage. BuMines IC 8949, 15 pp.