..^ " "^ a"*- »<^ .* ./ %.. %^^%^/ , -j:^ ^^ - - f° ■^ " ""^ o 4v , ^°-n^.. r . 1 « < s^^ V .* .-fSfe-. %,/ ,^.. *,^^^* ,y ■•' .^^ ^--^^^ iO^ .i-.°' "> o^ -^..^ ^'"' "- /\*^;.%\. .c°V>;j^^V ./,-^;.'X .c^V^^"^. '» .^-^ ' ;^V \/^^\/ V^^*/ \'^^\/ V^^*/ \/^^\/ -^^^^ \^K*' ^/\. °-'%^*' ■^^^^ \^K** Z'^^. '■®^*\'3^'^^"^^ ''•-^K*\/\. -»? ?;* <,h' 4°- ^o ^5.°-%. - > « ■ft' ^. "^ V^<.\ c°\^^^'^o /,^^^X ^^^:^B.'> y«-^k-\ .^°^;^'> • A^ •^.<^^^ ^^^^^' -it. "'-• ^.K^ :• «0 V ..i:i^'. ci, Co ip^ > IW-' /''** '•"•■ **'■'** '-W" /% •.^•- **% -J -% ■■ ^"^W^-- y.-^i-X "^'-m--^ /•■•^\ ■O. a"* » , n ' • "* '> • .0* *-- ■■ c.~ .1 ^' • A* V*.: '*..*' - / .-a:.-:..'^. .v\*::^^*.\. y.'A-i:./^. .v^^.;;.^,.^ y,.--./^,^ v^\,^ %.<^" • \./ - \<^^ - :» -■AT Vv • \./ • \.^" - '. '^6* 'bV C«S 4P V\ "^0^ ■rAo^ ^* aV "^^ . '• ♦ ^ " <* .. o^ ••-»" Jp 6* ■.%. / • • • Ay y«-^i'\ ^°^:r^'> .^<-^^A co\'^^^"°o ;* <^.^^ O- * • » " 'bV'' i^- ** «"V *w* •■' - •^ A*- '^./ .* ^-./ ^ "■> jP-7j •^tff 'V. " • ' <^^ i^-^^. '''i V-^.i^i^'* 'G>* iO'"' ..1V-. *> O'^^.iJnL'* '^'■ ■ . » • ^^-^^^ V ^0* •; .'J^'- °- •^^^^<^ **^ ^,^> ■ X/ •■^ <»i,^*' .V*==^1EV- '%,e.'i ■^ r," ' * '<5>^ <, ♦'T'.. .••* . ^•/ <^^/^'^\/ ^.*^-\**\ "o^-^^-/ ^^/^-'^ v**'-att\''^/ -'MM: %.** ••^\v^*".-^t ■%.**■* J^-i^ \^ __ _ _^ ^J^rS sri/jii "^ V -p^^^ IC 8949 Bureau of Mines Information Circular/1983 New Techniques for Reducing Stopping Leakage By Robert J. Timko and Edward D. Thimons UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 8949 // New Techniques for Reducing Stopping Leakage By Robert J. Timko and Edward D. Thimons UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Horton, Director ^w^^^ o. Library of Congress Cataloging in Publication Data: Timko, Robert J New techniques for reducing stopping leakage. (Bureau of Mines information circular ; 8949) Includes bibliographical references. Supt. of Docs, no.: I 28.27:89'19. 1. Concrete stoppings (Mining)— Airtightness. 1. Thimons, Kdward D. II. Title. III. Series: Information circular (United States. Bureau of Mines) ; 8949. ~~T1:^^&5tU4- [TN304] 622s [622'.42'028J 83-600226 CONTENTS Page Abstract 1 Introduction 2 Background 3 Construction techniques 4 Brush-on application of mortar 4 Area preparation 5 Air leakage tests 8 Procedures 8 Initial test 9 Floor grout tests 9 Six-month test 10 One-year test 10 Maintenance 13 Discussion of results 15 ILLUSTRATIONS 1 . Sealed concrete block stopping 2 2. Stoppings separating parallel entries 3 3. Apparatus for testing joint strength of block sample 5 4. Pouring grout into block to help seal floor beneath stopping 6 5. Average time required to construct stoppings 7 6. Average quantity of mortar required to construct stoppings 7 7. Test setup for creating pressure differential across stopping 8 8. Air leakage evidenced by drop in SFg concentration over time 9 9. Air leakage through newly constructed stoppings 10 10. Air leakage through stoppings after 6 months 11 11. Air leakage through stoppings after 1 yr 11 12. Mortar-filled trough used to seal floor and stopping face 12 13. Brushing flexible sealant onto stopping face 14 14. Air leakage through stoppings after maintenance 15 TABLE 1 . Joint strength of block samples 4 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT cfm cubic foot per minute lb pound ft foot min minute ft2 square foot ppb part per billion ft3 cubic foot psi pound per square inch in inch yr year in3 cubic inch NEW TECHNIQUES FOR REDUCING STOPPING LEAKAGE By Robert J, Timko and Edward D. Thimons ABSTRACT Because of leakage through and around permanent stoppings in under- ground mines, more air must be forced into a mine than would otherwise be required for ventilation. As power costs increase, costs resulting from air leakage add increasingly to the operating costs of mining. The Bureau of Mines evaluated four different stopping construction techniques based on the ideas that (1) airtightness could be enhanced by brushing rather than troweling on mortar sealant and that (2) modi- fied mortars (mortars containing glass fibers and other additives for increased strength and adhesion) would improve sealing performance. Air leakage tests comparing conventional to modified stoppings were done when the stoppings were built, after 6 months, after 1 yr, and following simple maintenance. It was found that stoppings can be built and maintained better if — The area where a stopping is to be built is properly prepared. Stoppings are constructed using the techniques devised by the Bureau. Stoppings are periodically examined for leaks. Stoppings found to be leaking, especially at the perim- eters, are resealed. ^Physical scientist. ^Supervisory physical scientist. Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. INTRODUCTION Ventilation is required in underground mines to dilute and remove hazardous dust and gases , and to provide a sufficient quantity of fresh air for those working underground. Air is made to move by cre- ating a pressure differential between two points. This differential is usually ac- complished by a fan, operating as either a positive (blowing) or negative (ex- hausting) air displacement device. In room-and-pillar type mining, air is directed to the working sections by in- stalling walls called stoppings (fig. 1). Stoppings are usually constructed of con- crete block and sealed with mortar. They are placed in crosscuts between parallel entries and separate fresh incoming air from dust- or gas-laden exhaust air (fig. 2). In many mines , less than 50% of the to- tal ventilation air entering the mine ac- tually reaches the working faces. 3 Most air bypasses its planned course by leak- ing through or around imperfections in stoppings. When this happens, the fan must work harder to induce more air to compensate for the loss in order to de- liver the required quantity to the face area. Since the cost of power needed to operate the large fans that pull or push air through a mine is increasing, the ^Ramani, R. V. , R. Stef anko, and G. W. Luxbacher. Advancement of Mine Ventila- tion Network Analysis From Art to Science (contract H0 133040 , PA State Univ.). Volume 4: Sensitivity of Leakage and Friction Factors. BuMines OFR 123(4)-78, 1977, 115 pp. FIGURE 1. - Sealed concrete block stopping. <1- -< KEY 1=1 Stoppings ► Intake air d — < Return air Beit Tracl< FIGURE 2. = Stoppings separating parallel entries. Bureau contracted for research to evalu- ate ways to reduce this waste of energy through better stopping construction and sealing techniques. The results of this research are presented in this report. BACKGROUND The Bureau entered into a contract with Mine Safety Appliance Research, Inc. (MSAR) to examine the problem of air leakage through stoppings and means for alleviating it. This investigation was carried out in coal mines , but the re- sults are applicable to any kind of un- derground mine in which concrete blocks are used for stoppings. An initial survey was done to determine what sealants were being used and how they were applied. Troweling on mortar, especially modified mortar, was the usual method for sealing stoppings in coal mines. Some problems were found with troweling mortar sealants on the vertical stopping surfaces. It was discovered that vertical surfaces could be more thoroughly coated by brushing on rather than troweling on the mortar sealant. Most commercially available mortar seal- ants can easily be made compatible for brushing. Laboratory examinations of the flexural strengths of several brands of modified mortar showed that (1) the performance of different brands of modified mortar did not vary appreciably and (2) the brush- on technique was the key to increased joint strength. Based on these findings, several stoppings were constructed in an underground mine where they subsequently underwent a 1-yr evaluation. Each stop- ping was sealed by the brush-on tech- nique, but it was found that more than just mortar application was required to ensure good long-term performance. In particular, it was necessary to prepare the surface where the stopping contacted the floor, ribs, and roof. This included removing broken surface material, con- structing some type of floor seal, and sealing the ribs and roof. Maintenance was also found to be necessary because of flexing or crushing of stoppings by com- pressive loads. Resealing techniques were developed that enable mine personnel to reduce air leakage without the expend- iture of much time or expense. CONSTRUCTION TECHNIQUES BRUSH-ON APPLICATION OF MORTAR The brush-on technique evolved after investigators visited several mines and watched laborers using trowels to mortar- seal stoppings. This appeared to require considerable skill and experience. With- out experience, the trowel proved to be a difficult instrument to use because con- siderable manipulation is required to transfer mortar from the mixing apparatus to the surface being coated. At times it seemed that more mortar was dropped than reached the stopping face. By substituting brushes for trowels, laborers could more easily apply mortar. Tests with several types of brushes showed that they needed to be stiff enough to hold mortar, yet flexible enough to spread the mortar across a masonry surface. A plastic-bristle whitewash brush performed best and was used in all subsequent experiments. In preliminary laboratory tests to evaluate the brush-on method, block sam- ples consisting of two half concrete blocks and one whole concrete block were prepared by brush-coating both faces only to simulate a dry-stacked assembly, or by brush-coating the faces and joints to simulate wet-wall construction. The samples were then placed on a labora- tory instrument (fig. 3) to determine joint strength. The results are shown in table 1. For both dry-stacked and wet-wall stop- ping construction, troweling is consid- ered the standard mortar application technique. Dry-stacked stoppings are TABLE 1. - Joint strength of block samples, psi Application method and mortar Average force to break Aligned joints Misaligned joints DRY-STACKED SAMPLES Troweled on 1 face: B-Bond. Brushed on both faces: B-Bond Genstar mine sealant mix. . Thoro System products: With glass fibers Without glass fibers.... 3,247 3,267 3,306 3,407 3,287 3,213 3,433 3,667 2,853 2,160 WET- WALL SAMPLES Troweled on joints: Sakrete...., Brushed on joints and both faces; B-Bond , Genstar mine sealant mix , Thoro System products: With glass fibers , Without glass fibers... , 1,650 4,167 4,387 4,800 4,213 1,640 3,300 3,653 NT 3,553 NT Not tested. Load cell Upper support member Concrete block test specimen Lower support member FIGURE 3. - Apparatus for testing joint strength of block sample. usually mortared on both faces only, while both faces as well as the joints of wet-wall stoppings are mortared. The most commonly used mortars are B-Bond mortar (for dry-stacked stoppings) and Sakrete mortar (for wet-wall stoppings). Table 1 shows how resultant joint strengths compared when these (troweled- on) mortars and other (brushed-on) mor- tars were used. Permission was obtained to construct stoppings using the brush-on technique in Rochester and Pittsburgh Coal Co.'s Urling No. 1 coal mine in western Penn- sylvania. Conventional stoppings used in this mine typically were built by dry- stacking hollow concrete blocks and trow- eling mortar on one stopping face. There was little area preparation to ensure airtightness. The investigators' initial modification to conventional stoppings was the substitution of the brush-on technique for troweling. Stoppings con- structed in this manner are referred to in this report as quick-build stoppings. As expected, brushing on glass-fiber- enhanced mortar was much easier than troweling. Laborers were able to seal locations around the stopping perimeter that were impossible to coat with trowels. AREA PREPARATION A chronic problem with conventional and quick-build stoppings is that after a time the perimeters leak air. To reduce or alleviate this leakage, research was concentrated on preparing the surfaces where stoppings contact the floor, ribs, and roof. Two methods of floor sealing were eval- uated. In the first, a trough was dug into the floor. Dry modified mortar was poured into the trench and mixed with wa- ter to form a footer. The first concrete block course was then laid on the still- wet mortar. The second method entailed no floor preparation. The first concrete block course was laid on broken floor material, and water glass (sodium silicate) was poured into the holes in the hollow- core concrete blocks (fig. 4). The water glass then spread into the broken mate- rial beneath the stopping. As the pres- sure differential across the stopping increased, air was forced beneath it, solidifying the water glass and forming a barrier against airflow. In order to successfully form a seal, the liquid had to penetrate deep enough to create a high resistance across the entire stopping base. The technique used to seal the ribs was similar to that used to seal the floor. Before beginning stopping construction, a mattock, pry-bar, or similar tool was used to create a channel in both ribs. Stopping blocks were then "keyed" into the channels with mortar, forcing the mortar into any void between the stopping block and the rib. This formed a solid structure from rib to rib. When the stopping was completed, a mortar cove was created by again forcing mortar into the rib corners , then feathering it out onto the stopping and rib surfaces. To seal the roof, wood wedges were first forced into the void above the stopping. These wedges were parallel to the stopping face and approximately FIGURE 4. = Pouring grout into block to help seal floor beneath stopping. 1 in. back from the face. Mortar was then brushed into the 1 in. void, creat- ing a rounded cove. Excess mortar was feathered out onto the roof and stopping face. As previously mentioned, leakage was primarily around stoppings. In an effort to control perimeter leakage, the return- side perimeters of several stoppings were coated. To do this, one laborer remained on the return side during construction. The laborer brush-coated the return pe- rimeter and then squeezed through an opening that had been left in one upper corner. The opening was then closed from the intake side. Thus, the return perim- eter was also sealed except for one upper corner. In addition to quick-build stoppings three other types of stoppings with dif- ferent levels of complexity were built. Universal stoppings were dry-stacked and had a footer, keyed ribs, sealed roof, and one face sealed with one coat of mod- ified mortar. High-pressure stoppings were made of hollow-core wet-laid con- crete blocks; had a footer, keyed ribs, a sealed roof, and two brush coats of modified mortar on the face. In addition the return perimeter was sealed. Hybrid stoppings were made in the same way as high-pressure stoppings, except that solid-core blocks were used instead of hollow-core blocks. Hybrid stoppings were thought to combine the best quali- ties of all the other modified stoppings. Stopping sizes averaged approximately for each stopping type and the average h-l/1 ft high by 18 ft wide. Figures 5 quantity of mortar, in 50-lb bags, re- and 6 show the average construction time quired to seal each type. Conven- Quick- Universal High tional build pressure FIGURE 5. - Average time required to construct stoppings. Hybrid Conven- Quick- Universal High Hybrid tional build pressure FIGURE 6. - Average quantity of mortar required to construct stoppings. AIR LEAKAGE TESTS PROCEDURES In the tests described below, there was a sufficient pressure differential across the stoppings to quantify the air leak- age. However, when the differential across the stoppings is less than 0.1 in w.g., it must be increased by a fan-and- parachute setup placed on the intake side of the stopping (fig. 7) before the leak- age can be measured. The parachute is fastened to roof bolts, then inflated with a fan. As the inflation pressure increases, a differential is created across the stoppings. For these tests the f an-and-parachute setup was not necessary. Stopping air leakage rates are mea- sured using a tracer gas, sulfur hexa- fluoride (SFg), which is a nontoxic, col- orless, odorless, and chemically and thermally stable gas. It is easily dis- pensed in air and has found widespread acceptance as a means for measuring mine ventilation.'* '^Drivas, P. J., P. G. Simmonds, and F. H. Shair. Experimental Characteriza- tion of Ventilation Systems in Buildings. Environ. Sci. Tech., v. 6, No. 7, 1972, pp. 609-614. The sampling technique for all stopping examinations, except the floor grout, was to enclose a volume of air on the return side of the stopping. For the conven- tional stoppings, this was done by erect- ing a brattice curtain an arbitrary dis- tance from each stopping. The modified stoppings were built in the same cross- cuts and on the intake sides of several other conventional stoppings , hence the conventional stoppings enclosed the vol- ume. Sulfur hexafluoride was released between the new and conventional stop- pings. The volume between the stoppings and the brattice curtains was then calcu- lated and recorded. After the SFg was released and per- mitted to diffuse in the volume, one end of a 0.25-in-ID tube was placed in the volume; the other end was connected to a sampling pump and remained in the return. At predetermined intervals, a sample was drawn by inserting a Luer device with an attached 90%-evacuated, 0.61-in^ test tube into a hypodermic needle that had been pierced through the sampling tube wall. Any leakage through the stoppings resulted in a drop in SFg concentration over time. Data reduction involved injecting a 0.0061-in^ sample of SFg into a gas /^fc^^^ Leak ^ — WTube / \ V-'--'^^^ ^--"^^ ^^ y^ / fsFe^ L, 11^^//'^^/^^ Sample J\:-^y(x Fan V.^1/ L».=.= Parachute stopping Test stopping Curtain FIGURE 7. - Test setup for creating pressure differential across stopping. chromatograph. A concentration was de- termined and graphed with respect to time (fig. 8). The air leakage rate, Q, was calculated as follows: Ti-i2 where Q = air leakage rate, (1) C] = SFg concentration at time Tj , ppb, and V = stopping-to-curtain volume (ft^). To make results from all stoppings com- parable, actual air leakage rates were converted to cubic feet per minute per hundred square feet of stopping area per inch water gauge pressure differential. A chronological discussion of each leak- age rate study follows. INITIAL TEST C2 = SFg concentration at time T2 , ppb. 1,000 100- Q. < a: o o o CO 20 30 TIME,nnin FIGURE 8, - Air leakage evidenced by drop in SF concentration over time. Conventional and modified stoppings were examined for airtightness with- in 1 month of construction. As expected, the conventional stoppings were leak- ing more than the modified stoppings (fig. 9). By simply brushing on the modified mortar instead of applying it with a trowel, air leakage rates through the stoppings were reduced by more than 50%. For all the extra time and materials necessary to con- struct the hybrid stoppings, they were only slightly more airtight than the more easily constructed universal stoppings. The most impressive result of this test was the airtightness of the high-pressure stoppings. Leakage rates through high- pressure stoppings averaged 93% lower than leakage rates through conventional stoppings. Comparison of the high- pressure and hybrid stopping results showed that nothing is gained by using solid-core rather than hollow-core con- crete block. FLOOR GROUT TESTS The pourable floor grout tests were performed on four stoppings already built just inby the intake air shaft. A brat- tice curtain was erected on the intake side of the stoppings, and an initial SFg test was run. The face and the stopping perimeter (except at the floor) were recoated, and another SFg test was run. The floor grout was poured into a trough at the base of each stop- ping, and another air leakage test was performed. 10 Conven- tional Quick- build Universal High pressure Hybrid FIGURE 9. = Air leakage through newly constructed stoppings. These tests attempted to show the im- provement in airtightness after each re- sealing technique. In two of four stop- pings there was improvement, but in two others, leakage increased after the floor grout was applied. The only explanation is that when the trench was opened in front of each stopping, voids were cre- ated in two of the stoppings that the pourable grout was not able to close. More research needs to be done on this technique. SIX-MONTH TEST Because various problems prohibited completion of the airtightness tests for conventional stoppings, all effort was concentrated on remeasuring the modified stoppings and comparing the 6-month re- sults with the initial leakage rates. All four types of modified stoppings showed a decrease in air leakage af- ter 6 months (fig. 10). Air leakrates decreased — 27% through the quick-build stoppings. 34% through the hybrid stoppings. 37% through the universal stoppings. 40% through the high-pressure stoppings. Structurally, all stoppings appeared sound with only fine hairline cracks evi- dent at some perimeters. To explain why leakage rates around modified stoppings decreased with time, it was assumed that any uncoated holes remaining in the blocks or joints were very small and that dust entrained in air passing into the imperfections tended to plug these holes. This appeared reason- able because dust tracks were found sur- rounding several small holes in a stop- ping face. ONE- YEAR TEST During the second 6 months, air leakage through all modified stoppings increased dramatically. Since the conventional stoppings lacked a 6-month test, the leakage increase from them spanned 1 yr. If, however, the conventional stoppings performed similarly to the modified ones, the increase in leakage from them was 11 < £ o Conven- tional Quick- Universal High build pressure Hybrid FIGURE 10. - Air leakage through stoppings after 6 months. also more dramatic during the second 6-month period. Comparing figure 11 to figure 10, the largest increase in leakage rates oc- 400 350 300- V 250 o o S. 200 150 100- 50- 1 1 1 1 1 1 1 - 1 — V//A Conven- tional Quick- build Universal High pressure Hybrid FIGURE 11,- Air leakage through stoppings offer 1 yr. curred with the quick-build stoppings; air leakage rose slightly less than 400%. Conversely, hybrid stopping leakage in- creased the least, about 125%. The increase in stopping leakage was attributed primarily to the low humid- ity effect of winter. Low humidity tends to shrink and crack mortar, as well as the wooden wedges used to secure stoppings against the roof. Cracks were visible at most stopping perimeters, especially along the stopping-roof junc- tions. Leakage was audible around sev- eral stoppings. Because of good roof and floor con- ditions, stoppings did not appear to be under compression. In fact, some stop- pings had separated slightly from the roof and ribs. Due to the low humidity or possibly even freeze-thaw cycles, it appeared that the mortar and wood wedges at the stopping-roof junctions had shrunk enough to cause the stopping to move slightly away from the roof, creating a gap for air. In addition, ribs had spalled due to atmospheric conditions, causing more air leakage through them. Because this degeneration could not be tolerated, the following maintenance plan was devised. 12 ^ FIGURE 12. - Moriar-filled trough used to seal floor and stopping face. 13 MAINTENANCE Simple techniques were devised to re- duce or eliminate air leakage in older stoppings. The basic premise was to keep the maintenance as simple as possible, thereby minimizing the time required to complete the work. Unless there is ac- tive roof, rib or floor movement and the stopping face has fractured, most leakage through older stoppings will be at the perimeters. Recoating the roof and ribs created few problems. By the time maintenance was necessary, a pressure differential ex- isted across the stoppings. The extent of audible perimeter leakage determined how far out from the stopping perimeters the sealant coating would be spread. Most stoppings repaired required an 8- to 12-in-wide coating. The sealant was brushed on the stopping surface, then feathered onto the roof and ribs. Air leakage was also pronounced along the floor. This was most evident with the quick-build stoppings , which have no footer. Floor material adjacent to all stoppings was badly fractured, making it impossible to seal leaks by simply brushing-on sealant. A trench a few inches deep was cleared at the base of several quick-build and universal stoppings. Dry modified mortar was poured into the trough, followed by a liquid latex and water solution (fig. 12) and the mortar and solution were thoroughly mixed to a honey-like consistency. Not only did this fill and seal holes beneath the stoppings, but the material was also scooped from the trough and brush-coated onto the perimeters and any holes in the stopping faces. Since stopping maintenance should en- sure airtightness over a long period (e.g., 1 yr or more), it was thought important to apply a face sealant that would remain airtight. Flexible sealants appear to be better suited for long-term air barriers. There are several flexible sealants being marketed, but because of restrictions placed on their use under- ground, not many are suitable for mine stopping work. A semi-liquid acrylic latex loaded with glass fibers was chosen for face maintenance. This sealant was brushed on two stopping faces (fig. 13). Its short-term performance was compar- able to mortar based sealants. Similar sealants packaged in ready-to-use pails may be better designed for rapid mainte- nance since they require no time for mix- ing. In addition, since it is a latex- based sealant, it should remain flexible when cured, possibly eliminating the need for continuous maintenance. RESEALING TESTS Stopping leakage test results after performing maintenance are shown in fig- ure 14. Comparison with the original leakage rate tests (fig. 9) shows the quick-build and universal stoppings to be at least as airtight as when new. The time spent resealing stoppings var- ied with the stopping type. Since quick- build stoppings leaked more, more time had to be spent maintaining them. The total time necessary to reseal each quick-build stopping averaged two worker- hours. The other stopping types took slightly less time. 14 FIGURE 13. - Brushing flexible sealant onto stopping face. 15 Conven- tional Quick- Universal High build pressure Hybrid FIGURE 14. = Air leakage through stoppings after maintenance. DISCUSSION OF RESULTS Three techniques designed to reduce air leakage through and around concrete block stoppings were examined. Two techniques, brush-coating mortar and area prepara- tion, were designed specifically for new stoppings. The third, recoating stopping faces and perimeters, was used to help return older stoppings to the low leakage rates achieved when they were new. The new techniques were used to build stoppings in a coal mine. Leakage tests were performed on modified stoppings as well as those conventionally found in the mine. Initial, 6-month and 1-yr tests showed the modified stoppings to be much more airtight. After 1-yr, all modified stoppings ex- hibited an increase in air leakage, though they still remained more airtight than the conventional stoppings. Simple and inexpensive resealing techniques were performed on several stoppings. All had airtightness values as good or better than the amount measured when they were new. These approaches were examined in an effort to simplify stopping construction and reduce air leakage. Their only con- straint is that the new techniques be no more expensive than conventional methods of construction. Research has shown that airtightness and longevity are enhanced when stoppings are properly built using the Bureau's techniques. It is important for the mining industry to realize that air leakage through and around stoppings requires more air to be induced than is really necessary. As power costs increase, the additional air required means higher operating costs and thus more overhead. INT.-BU.OF MINES, PGH., PA. 27124 $631 ''''°'*' %.^" f^^z "^-^/^ .^^"t %.^" ^^ .,. ^-^ ""'- %.„.' /ii^-. -'-.Z ."^is- %.„.^ /^i^-. ■^^„.*''* .•:^Ei--^.„.' .-^i^-. ■'^„.^ «• ^"^^^ \mm: «5.^^^. vVA * 5? "^* -y^i*'^/ >^ "^ --^isBS?.* **' "^^^ %^^%^>^.* >^ -^^ ^3^ '» . • • *^ y o»"*. ^^ <> ♦■'TVi' .0 ^* <•>" p^.-^."^^ O . 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