TN295 
 
y .••■'• 
 
 
 ^ c*> . r^ •* "A^ ^ 
 
 •^0^ 
 
 
 ^^0^ 
 
 
 .^" ">. -r^^*?'** .'y V 
 
 
 '^oK 
 
 
 
 .^'\ 
 
 
 
 ./■\ 
 
 
 v^. %-^^^* . '^'^ '"^ . ^^^W* .<^^ V- 
 
 '^^o^ 
 
 
 - ^oV^ 
 
 * ^ 
 
 
 
 
 .5.">-s 
 
 
 
 >^'\ °-^^^ /\ \W/ /% l^/ ..% 
 
 t» ■> 
 
 o_ * 
 
 o > ._gi 
 
 
 
 
 .*^\ 
 
 
 
 
 v-^^ 
 
 ^o9 
 
 
 .^ ^^^ 
 
 A" <^ • o ,. ' , V^ 
 
 .*^% 
 
 
 
 
 
 .4 O. 
 

 •^b 
 
 
 ■vr> 
 
 
 -•'y\ V^^^*'/' ■'\/^"^\/ V'^^'^0^ 
 
 \''-^-\<V V-^^^O^^ \.-.^'\'v" "V, -^^!^^' 0^^' '^^o^'^o^.^-' ^ ^ 
 
 :^^"'^. 
 
 
 
 V.^^ 
 
 ^^>. 
 
 ? .♦^'"^. 
 
 
 
 
 ^-^c 
 
 vV^ 
 
 
 
 ^•\/ %-*^-/ V^^-^*,/ V^\.o' \^^\/ -o 
 
 
 
 *bv^ 
 
 4O VS ' ' 
 
IC ^°^^ 
 
 Bureau of Mines Information Circular/1985 
 
 Recognizing Signs of Excessive Loading 
 in Underground Coal Mines 
 
 By Eric R. Bauer 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 75 
 
 A^;NES 75TH AV*^ 
 
Information Circular 9017 
 
 Recognizing Signs of Excessive Loading 
 in Underground Coal Mines 
 
 By Eric R. Bauer 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Donald Paul Model, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 
^^ .n\^ 
 
 f.^ 
 
 B 
 
 (\0^ 
 
 Library of Congress Cataloging in Publication Data: 
 
 Bauer, Eric R 
 
 Recognizing signs of excessive loading in underground coal mines. 
 
 (Information circular / United States Department of the Interior, Bu- 
 reau of Mines ; 9017) 
 
 Bibliography: p. 29. 
 
 Includes index. 
 
 Supt. of Docs, no.: I 28.27:9017. 
 
 1. Mine roof control. I. Title. II. Series: Information circular 
 (United States. Bureau of Mines) ; 9017. 
 
 TN295.U4 [TN2881 622s [622\8l 84-600355 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 1 
 
 Acknowledgments 2 
 
 Explanation of excessive loading 2 
 
 Signs of excessive loading. 2 
 
 Mine roof supports 2 
 
 Timbers and posts .' 2 
 
 Cribs 2 
 
 Steel beams 2 
 
 Wood beams 3 
 
 Metal straps • 3 
 
 Roof bolts 3 
 
 Roof bolt plates 3 
 
 Cap boards 3 
 
 Cap wedges 16 
 
 Header boards 16 
 
 Yieldable steel legs 16 
 
 Chocks and shields 18 
 
 Mine installations 18 
 
 Overcasts, brattices, and block structures 20 
 
 Sealants 20 
 
 Mine openings 20 
 
 Roof 22 
 
 Ribs 22 
 
 Floor 22 
 
 Conclusions 28 
 
 References 29 
 
 Appendix. — Glossary 30 
 
 ILLUSTRATIONS 
 
 1. Mine areas subject to excessive loading and associated signs of excessive 
 
 loading 3 
 
 2. Splitting wood timber 4 
 
 3. Several bent (or buckling) wood timbers 5 
 
 4 . Cracked wood timber 6 
 
 5. Crushing wood crib, which also appears to be bending because of excessive 
 
 vertical loading 7 
 
 6 . Crushing and cracking concrete crib 8 
 
 7. Steel beam bending at beam end 9 
 
 8 . Steel beam twisting at the supported end 9 
 
 9. Splitting or cracking wood beam 10 
 
 10. Wood beam bending at its midspan 10 
 
 1 1 . Crushing wood beam. 11 
 
 12. Sheared metal strap. 11 
 
 13. Bolt that has sheared or failed in tension because of excessive loading... 12 
 
 14. Bending roof bolt plate 12 
 
 15. Stress (strain) patterns on roof bolt plate as a result of excessive 
 
 loading 13 
 
 16. Roof bolt plate squeezing into a header board 13 
 
 17. Splitting or cracking cap board 14 
 
11 
 
 ILLUSTRATIONS— Continued 
 
 Page 
 
 18. Crushing cap board 15 
 
 19. Split cap wedge and "brooming" of a timber 16 
 
 20. Crushing cap wedges 17 
 
 21. Cracked cap wedge 17 
 
 22. Split or cracked header board 17 
 
 23 . Bending header board 18 
 
 24. Yield (crushing) of a yieldable steel leg 19 
 
 25. Longwall support "going on the solid" (total collapse of the legs) because 
 
 of excessive loading 19 
 
 26. Longwall support punching into the roof and floor 19 
 
 27. Cracking overcast 20 
 
 28. Crushing block brattice 21 
 
 29. Rib sealant that is cracking 22 
 
 30. Rib sealant that is spalling 23 
 
 3 1 . Roof being crushed by wooden support members 23 
 
 32. Roof crack caused by excessive loading 24 
 
 33. Shearing roof 24 
 
 34. Roof sagging because of excessive loading 25 
 
 35. Crushing rib 25 
 
 36. Cracked rib 26 
 
 37. Spalling rib 27 
 
 38. Heaving and cracking mine floor 28 
 
RECOGNIZING SIGNS OF EXCESSIVE LOADING IN UNDERGROUND COAL MINES 
 
 By Eric R. Bauer 
 
 ABSTRACT 
 
 This Bureau of Mines publication describes and illustrates the physi- 
 cal signs of excessive loading on mine roof supports, installations, 
 and openings. It is designed to assist mine personnel in recognizing 
 potentially hazardous ground conditions, by emphasizing the preliminary 
 effects of developing unstable ground. Increased worker awareness can 
 be expected to reduce the injuries and fatalities resulting from falls 
 of roof, rib, and face. 
 
 INTRODUCTION 
 
 Accidental falls of roof, rib, and face continue to be a leading 
 cause of injuries and fatalities in underground mines. Statistics show 
 that for the period of January 1978 through June 1984, underground coal 
 mine fatalities due to ground control problems averaged 44.2 pet of all 
 underground fatalities (1) ,^ This substantial percentage is a result 
 of many factors but indicates that the ability of miners to identify 
 developing hazardous ground conditions needs to improve before a reduc- 
 tion in fatalities, injuries, and productivity loss can occur. 
 
 The present publication examines the effects of excessive loading on 
 mine roof supports, installations, and openings. Support items such as 
 roof bolts and plates, steel beams, posts' and timbers, cribs, over- 
 casts, and stoppings are examined. Conditions of roof, ribs and floor 
 that suggest excessive loading are considered. Numerous photographs, 
 taken in operating underground coal mines by Bureau of Mines personnel, 
 supplement the text by providing actual examples of the conditions 
 discussed. 
 
 This publication is intended for mine operators and safety managers, 
 and for use by training specialists in increasing the ability of exper- 
 ienced miners to recognize hazardous roof conditions and in instructing 
 new miners in the necessary techniques. 
 
 ^Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
 ^Underlined numbers in parentheses refer to items in the list of references preced- 
 ing the appendix. 
 
ACKNOWLEDGMENTS 
 
 The author wishes to thank the manage- 
 ment and personnel of the following mines 
 for their cooperation and permission to 
 take photographs in their mines: Warwick 
 No. 3 Mine, Greensboro, PA, Duquesne 
 
 Light Co.; Kitt No. 1 Mine, Philippi, WV, 
 Kitt Energy Corp.; Greenwich Colleries, 
 Ebensburgh, PA, Pennsylvania Mines Corp.; 
 Grove No. 1 Mine, Jennerstown, PA, G. M. 
 and W. Coal Co. 
 
 EXPLANATION OF EXCESSIVE LOADING 
 
 Load (weight) on mine roof supports, 
 installations, and openings occurs nor- 
 mally as the seam is extracted, owing to 
 the weight of the immediate roof , main 
 roof , and overburden that acts on the re- 
 maining support members. This loading 
 normally occurs in the vertical plane 
 but may be transferred to the horizontal 
 plane. Ordinarily, the in-place supports 
 (pillars, roof, and artificial supports) 
 adequately support the load and no physi- 
 cal signs of excessive loading appear. 
 
 However, loading may exceed the support 
 capabilities of the in-place supports.-^ 
 Such excessive loading produces harmful 
 effects that can be detected by visual 
 observation. The miner must learn to 
 recognize these effects as warning signs 
 of potential roof falls, rib rolls, floor 
 heave, etc. While excessive loading does 
 not always lead to complete failure, it 
 is a warning symptom that a serious 
 hazard is developing and thus must be 
 investigated. 
 
 SIGNS OF EXCESSIVE LOADING 
 
 The signs of excessive loading take 
 many forms and involve numerous items and 
 areas underground. These include mine 
 roof supports (timbers and posts, cribs, 
 wood and steel beams, cap boards and 
 wedges, header boards, yleldable legs, 
 and chocks or shields), mine installa- 
 tions (overcasts, brattices, block struc- 
 tures, and sealants), and mine open- 
 ings (roof, ribs, and floor). The signs 
 include splitting, bending, twisting, 
 crushing, cracking, spalling, stressing, 
 shearing, sagging, heaving, and squeez- 
 
 ing. 
 
 Figure 1 lists the areas affected 
 
 and the associated signs of excessive 
 loading, 
 
 MINE ROOF SUPPORTS 
 
 Mine roof supports are the physical re- 
 inforcements installed to provide artifi- 
 cial support of the exposed mine roof. 
 Excessive loading can deform them and de- 
 crease their supporting capability. De- 
 formations and loss of support action may 
 
 — 
 
 -"For a more complete explanation of 
 roof loading, support, and/or falls, see 
 references 2-5. 
 
 be gradual or sudden, depending on the 
 type of support and the amount and rate 
 of loading. In either case, visual signs 
 of excessive loading usually appear prior 
 to complete support failure. 
 
 Timbers and Posts 
 
 Excessive loading of wood timbers and 
 posts can be recognized by splitting, 
 bending (also known as buckling) , crack- 
 ing, and punching into the roof and/or 
 floor (figs. 2-4). 
 
 Cribs 
 
 Wood or concrete cribs reveal exces- 
 sive loading by bending or crushing 
 (figs. 5-6). 
 
 Steel Beams 
 
 Steel beams (I-beams or track rails) 
 are placed in contact with the roof and 
 held in position by wood posts or steel 
 legs at the beam ends. Bending at the 
 midspan and/or beam ends and twisting at 
 
 ^See appendix for definition of terms. 
 
Mine areas 
 
 subject to 
 
 excessive loading 
 
 Signs of excessive loading 
 
 llililiiilililfill 
 
 Roof supports 
 
 Timbers (posts) 
 
 •;•! 1 <•; i i > i 1 
 
 Cribs 
 
 :•; !•! 1 1 ; 1 i ; 
 
 Steel beams 
 
 \m\m\ 1 ; ! ! ; 1 1 
 
 Wood beams 
 
 •;•! '•I^! ! 1 1 ; 1 
 
 Metal straps 
 
 !•! 1 1 1 1 ;•; 1 1 
 
 Roof bolts 
 
 1 > ! 1 : !•' 1 < 
 
 Bolt plates 
 
 !•! ! < i !•! 1 1 !• 
 
 Cap boards 
 
 •i ' ;••! ! 1 I 1 1 
 
 Cap wedges 
 
 •i ; \m\m\ ' i i I i 
 
 Header boards 
 
 •!•; \m\m\ ! 1 ' 1 1 
 
 Yieldable legs 
 
 !•!•!•! 1 ; 1 ! 1 
 
 Chocks, shields 
 
 1 1 !•! : 1 1 i ' ' 
 
 Installations 
 
 Overcasts 
 
 I l«l«l ! 1 i 1 1 
 
 Brattices 
 
 i 1 >i«i i i { 1 1 
 
 Block structures 
 
 1 i !•!•! 1 ! 1 < 
 
 Sealants 
 
 1 1 1 i^i^i 1 ! I 
 
 Openings 
 
 Roof 
 
 ! !•!• 1 !••! 1 
 
 Ribs 
 
 1 !•;•!•> 1 1 : ! 
 
 Floor 
 
 i 1 i !•! 1 1 : l«l 
 
 KEY 
 # Areas affected and associated signs 
 of excessive loading 
 
 FIGURE 1. - Mine areas subject to excessive loading 
 and associated signs of excessive loading. 
 
 the beam ends are the signs of excessive 
 loading (figs. 7-8). They may also indi- 
 cate poor design practices , which accen- 
 tuate the amount of loading. 
 
 Wood Beams 
 
 Wood beams provide support and are used 
 in the same manner as steel beams. They 
 show excessive loading by splitting or 
 cracking, bending, and crushing (figs. 9- 
 11). Defects in the beams may cause 
 failure at low loads; thus, this must be 
 considered prior to assuming that the 
 failure indicates excessive loading. 
 
 Metal Straps 
 
 Metal straps, also known as bacon 
 skins, steel headers, and steel mats, are 
 bolted to the roof or rib and are de- 
 signed to keep material from falling out 
 around the bolts. They react to exces- 
 sive loads by bending and shearing. The 
 bending is difficult to detect because of 
 the bends created during installation 
 of the strap over uneven roof and ribs. 
 Figure 12 shows an example of a sheared 
 strap due to excessive loading. 
 
 Roof Bolts 
 
 Roof bolts are the primary method of 
 artificial support employed in mines to- 
 day and have a long history of successful 
 use. Many problems can result from ex- 
 cessive loading of roof bolts and roof 
 trusses, most of which are unobservable 
 and cannot be photographed. These in- 
 clude anchor slippage, bending within the 
 hole, stressing, loss of bonding, and 
 torque loss or gain. The only detectable 
 sign is the shearing or failure in ten- 
 sion of the bolt or truss, most readily 
 observable when it occurs at or near the 
 head of the bolt or in the cross link of 
 a truss (fig. 13). 
 
 Roof Bolt Plates 
 
 Roof bolt plates show excessive loading 
 by bending, stressing, and squeezing. 
 Bending is seen as the downward movement 
 of the plate corners (fig. 14). Stress 
 patterns, which develop as the load in- 
 creases, can sometimes be seen in the 
 rock dust or rust that accumulates on the 
 plate (fig. 15). Squeezing is the in- 
 denting of the plate into the roof or 
 wood header board (fig. 16). 
 
 Cap Boards 
 
 Wood cap boards are 
 the roof or floor and 
 ports (timbers, posts, 
 beams, etc.) to provide 
 ing surface and better support. They in- 
 dicate excessive loading by splitting 
 or cracking and crushing (figs. 17-18). 
 
 placed between 
 
 artificial sup- 
 
 , cribs, steel 
 
 a larger bear- 
 
FIGURE 2. - Splitting wood timber. Timbers adjacent were installed at a later date. 
 

 
 FIGURE 3. - Several bent (or buckling) wood timbers. 
 
~1 
 
 ^ 
 
 
 ...^S^^MHHd 
 
 i 
 
 siW * 
 
 , 
 
 i 
 
 
 ^ 
 
 ^J^nHMM 
 
 
 ..^■j ^ - ^ 
 
 ^ 
 
 r 
 
 p' .-^ 
 
 
 
 ,S|^H 
 
 V 
 
 
 / 
 
 r 
 
 ^ ,r#-# 
 
 ^M^^H 
 
 ^Kg^g 
 
 
 il^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^lP 
 
 ■ iTimmH^I 
 
 ^^^^^^^^^^Hk.^^%^ 
 
 l^H 
 
 Ji;lI^iillBBBi 
 
 ^^^^^Hpos-' 
 
 ^ « ^^^^^^^^^^^19^^* '<^^p^^^^ 
 
 
 
 
 
 
 1 i ^'^- 
 
 ^^ ^ , ■ ^ 
 
 F 1 j^S?"^ 
 
 ^ .^m 
 
 Hj^K. ^^ -'^ 
 
 
 Mi^ ^ ^||Mfl[^9HMittH|^^^^^^ ^^^V^^^^^^^^^H^^HH^B^^ 
 
 -i 
 
 
 J 
 
 
 FIGURE 4. - Cracked wood timber. 
 
FIGURE 5. - Crushing wood crib, which also appears to be bending because of excessive vertical 
 loading. 
 
FIGURE 6. - Crushing and cracking concrete crib. The effects are due to the combination of exces- 
 sive loading and differential settlement between the two sides. 
 
•pl*Slll,,l^|.|ll illllin,,^ 
 
 FIGURE 7. - Steel beam bending at beam end. 
 
 FIGURE 8. - Steel beam twisting at the supported end. This may also occur when a poorly designed 
 beam, improperly installed, is subjected to small loads. 
 
10 
 
 
 "^'™''**W«WS(c 
 
 FIGURE 9. - Splitting or cracking wood beam. 
 
 FIGURE 10. - Wood beam bending at its midspan. Lateral loads, as well as vertical loads, may be 
 the cause of this bending. 
 
11 
 
 FIGURE 11. - Crushing wood bee 
 
 m. 
 
 FIGURE Vl. - Sheared metal strap. 
 
12 
 
 FIGURE 13. - Bolt that has sheared or failed in tension because of excessive loading. 
 
 FIGURE 14.- Bending roof bolt plate. Although this is an indication of excessive loading, the plate may 
 be underdesigned for this application, resulting in a false indication of the loading present. 
 
13 
 
 FIGURE 15. - Stress (strain) patterns on roof bolt plate as a result of excessive loading. 
 
 FIGURE 16. - Roof bolt plate squeezing into a header board. 
 
14 
 
 FIGURE 17. - Splitting or cracking cap board. Since tliis is a thick block thot would tend to 
 fail at lower loads, this may not be a sign of excessive loading. 
 
15 
 
 FIGURE 18. - Crushing cap board. Some crushing may be desirable to allow support-strata 
 interaction; however, the amount shown is sufficient to indicate excessive loading. 
 
16 
 
 Defects in the wood may lead to premature 
 failure at lower loads, which must be 
 considered before determining the loads 
 responsible for the failure. 
 
 Cap Wedges 
 
 support. They will split or crack (fig. 
 22), bend (fig. 23), and crush (fig. 16) 
 when subjected to excessive loads. As 
 before, the presence of defects must be 
 considered before the final analysis is 
 completed. 
 
 Cap wedges show signs of excessive 
 loading sooner than the supports with 
 which they are used. This is because of 
 the relative thinness of the wedges and 
 the preloading (pressure) on the wedges 
 at installation. Excessive loading is 
 seen as splitting, crushing, and cracking 
 (figs. 19-21), Again, defects as well as 
 the material strength may be a factor in 
 determining the loading that will cause 
 failure. 
 
 Header Boards 
 
 Header boards are installed between the 
 roof or rib and bolt plate to provide 
 increased bearing surface and better 
 
 Yieldable Steel Legs 
 
 Yieldable steel legs are used as sup- 
 port members for steel beams and in con- 
 juction with arch assemblies. They yield 
 at a sliding friction slip joint when 
 their design friction limit is exceeded. 
 Increased loading is indicated by either 
 nonadjacent alignment marks (made on the 
 legs at the time of installation) or by 
 bare spots where the slip joint couplings 
 were previously located. This increased 
 loading is an indication of possible fu- 
 ture excessive loads. Figure 24 shows 
 the amount of yield (crushing) at a slip 
 joint. 
 
 FIGURE 19. - Split cap wedge and "brooming" of o timber. 
 
17 
 
 FIGURE 20. - Crushing cap wedges. These are 
 the initial indicators of future excessive loading. 
 
 FIGURE 21. - Crocked cop wedge. This is on 
 early indication of future excessive loading. 
 
 FIGURE 22. - Split or cracked header board. 
 
18 
 
 FIGURE 23. - Bending header board. 
 
 The legs may also twist or bend, usu- 
 ally after they have yielded as far as 
 their design limit allows. By this time, 
 roof conditions would have deteriorated 
 so much that travel in the affected area 
 would be prohibited; thus, no photographs 
 of this condition are included. 
 
 Chocks and Shields 
 
 Chocks and shields provide roof support 
 during longwall mining. They show exces- 
 sive loading by crushing (also known as 
 collapsing). This involves either "going 
 on the solid" (total collapse of legs) 
 (fig. 25) or punching into a soft floor 
 or roof (fig. 26). 
 
 MINE INSTALLATIONS 
 
 A second area where evidence of exces- 
 sive loading is found is mine installa- 
 tions, such as overcasts, brattices, 
 block structures, and sealants. Exces- 
 sive loading appears primarily as crush- 
 ing or cracking; sealants are prone to 
 spalling as a result of excessive loading 
 on mine pillars. 
 
 As in the case of mine roof supports, 
 the effects on mine installations develop 
 either slowly or rapidly, depending on 
 the type of installation and the amount 
 and rate of loading. The effects are 
 usually observable prior to complete 
 structural failure of the installation. 
 
19 
 
 S]^ 
 
 ^^^^^mm^^^^^^^^^^^'>^^^^^^^^^^'^^^^^^^^^^z^^m. 
 
 FIGURE 24. - Yield (crushing) of a yieldable steel 
 leg. 
 
 B 
 
 FIGURE 25. - Longwall support "going on the sol- 
 id (total col lapse of the legs) because of excessive 
 loading. A, Front leg collapse because of roof frac- 
 ture at longwall face; B, rear legcollapse because of 
 failure of roof to cave properly. 
 
 FIGURE 26. - Longwall support punching into the roof and floor. 
 
20 
 
 Overcasts, Brattices, 
 and Block Structures 
 
 Overcasts are used to Isolate intake 
 air from return air. Block brattices 
 (stoppings, bulkheads, seals, etc.) are 
 placed in crosscuts to separate air cur- 
 rents and force the air ahead until it 
 reaches the desired points (6, pp. 212- 
 213). They are also used to isolate 
 abandoned workings from active workings. 
 Block structures include pump houses, 
 transformer stations, retaining walls, 
 battery charging stations, underground 
 shops, and other cement block structures. 
 
 Overcasts, brattices, and block struc- 
 tures show excessive loading by crushing 
 or cracking of the cement block and mor- 
 tar from which they are constructed. An 
 example of a cracking overcast is shown 
 in figure 27, and a crushing brattice is 
 shown in figure 28. 
 
 Sealants 
 
 Mine sealants are applied to mine roof, 
 ribs, and block structures to minimize 
 the effect of the wet-dry cycle of ven- 
 tilating air and to strengthen the area 
 sprayed. They reveal excessive loads 
 by cracking and spalling (figs. 29-30). 
 Cracking and spalling may also be a re- 
 sult of improper application of the seal- 
 ant, which would cause premature failure 
 at lower loads. 
 
 MINE OPENINGS 
 
 The final area that must be examined 
 for signs of excessive loading are the 
 mine openings. These include the roof, 
 rib, and floor. They can crush, crack, 
 shear, sag, spall, or heave when loaded 
 beyond their ultimate strength. De- 
 terioration of mine openings is the 
 
 FIGURE 27. - Cracking overcast. 
 
21 
 
 4 
 
 ■*%>„ 
 
 ^^ 
 
 FIGURE 28. • Crushing block brattice. 
 
22 
 
 "beginning of the end," since it is usu- 
 ally irreversible and will progress until 
 access to the area becomes unsafe or 
 impossible. 
 
 Roof 
 
 Mine roof can show excessive loading in 
 several ways. The most detectable signs 
 are crushing, cracking, shearing, and 
 sagging (figs. 31-34). Of these, sagging 
 is the most difficult to detect without 
 using sophisticated instrumentation. 
 
 Ribs 
 
 For mine ribs , signs of excessive load- 
 ing are crushing, cracking, and spalling 
 (figs. 35-37). 
 
 Floor 
 
 Mine floor shows excessive loading by 
 cracking and heaving, which usually occur 
 simultaneously (fig. 38). 
 
 FIGURE 29. - Rib sealant that is cracking. 
 
23 
 
 FIGURE 30. - Rib sealant that is spalling. 
 
 FIGURE 31. - Roof being crushed by wooden support members. 
 
24 
 
 FIGURE 32. - Roof crock caused by excessive loading. 
 
 FIGURE 33. - Shearing roof. 
 
25 
 
 Roof bolts 
 
 Not to scale 
 
 FIGURE 34. - Roof sagging because of excessive loading. 
 
 FIGURE 35. - Crushing rib. 
 
26 
 
 FIGURE 36. - Cracked rib. 
 
27 
 
 FIGURE 37. - Spoiling rib. 
 
28 
 
 .iilljissasia 
 
 ' mimm:m«ms^ — ::=i, . 
 
 
 
 
 > 
 
 ^> 
 
 ^ 
 
 -**' 
 
 X. 
 
 FIGURE 38. - Heaving and cracking mine floor. 
 
 CONCLUSIONS 
 
 Many safety areas must be addressed by 
 government and industry in the effort to 
 reduce injuries and fatalities due to 
 roof, rib, and face falls, A most impor- 
 tant area is the training of miners to 
 routinely look for and recognize the 
 signs of hazardous ground conditions, in- 
 cluding excessive loading on supports, 
 installations, and openings in under- 
 ground mines. This publication is de- 
 signed for use by mine safety instructors 
 engaged in such training and as a refer- 
 ence guide for mine personnel. 
 
 It must also be noted that what appears 
 on the surface to be excessive loading 
 may be the result of defects in the sup- 
 port members. Load measuring devices are 
 
 available with which to supplement the 
 visual observations, and their use may be 
 necessary before a final determination of 
 the cause and amount of loading can be 
 made. 
 
 Finally, roof, ribs, and support mem- 
 bers can fail suddenly without revealing 
 any evidence of overloading. Although 
 this is the exception rather than the 
 rule, an area must not be assumed to be 
 completely safe just because no visual 
 signs of excessive loading are present. 
 All miners must be continuously aware of 
 their surroundings and any changes that 
 occur, no matter how small the changes 
 may be. 
 
REFERENCES 
 
 29 
 
 1. Mine Safety and Health Administra- 
 tion (Dep. Labor). Mine Injuries and 
 Worktlme, Quarterly. Closeout Edition, 
 1978-1982, p. 17; Closeout Edition, 1983, 
 p. 18; Jan. -June, 1984, p, 17. 
 
 2. Forbes, J. J., E. Thomas, and A. J. 
 Barry. Questions and Answers on Roof 
 Support in Bituminous-Coal Mines. U.S. 
 Dep. Interior Handbook, 1951, 90 pp. 
 
 5. Caudle, R. D. Mine Roof Stability. 
 Paper in Ground Control Aspects of Coal 
 Mine Design. Proceedings: Bureau of 
 Mines Technology Transfer Seminar, Lex- 
 ington, Ky. , March 6, 1973. BuMines IC 
 8630, 1974, pp. 79-85. 
 
 6. Cassidy, S. M. (ed.). Elements of 
 Practical Coal Mining. Soc. Min. Eng. 
 AIME, 1973, 614 pp. 
 
 3. Mine Enforcement and Safety Admini- 
 stration (now Mine Safety and Health Ad- 
 ministration (Dep. Labor)). Programmed 
 Instruction Workbook No. 3, 1976, 111 pp. 
 
 4. Morgan, T. A. Coal Mine Roof Prob- 
 lems . Paper in Ground Control Aspects of 
 Coal Mine Design. Proceedings: Bureau 
 of Mines Technology Transfer Seminar, 
 Lexington, Ky. , March 6, 1973. BuMines 
 IC 8630, 1974, pp. 56-61. 
 
 7. Woolf, H. B. (ed.). Webster's New 
 Collegiate Dictionary. G. & C. Merriam 
 Co., 1979, 1532 pp. 
 
 8. Thrush, P. W. A Dictionary of Min- 
 ing, Mineral, and Related Terms. BuMines 
 Spec. Publ., 1968, 1269 pp. 
 
30 
 
 APPENDIX. —GLOSSARY 1 
 
 Bending — The turning or forcing of a 
 support member from a straight shape 
 to a curved. Sometimes referred to as 
 buckling. 
 
 Cracking — The breaking, splitting, or 
 snapping apart of a support member such 
 that a fissure (crack) appears. 
 
 Crushing — The squeezing or forcing by 
 pressure of a support member so as to al- 
 ter it into a mass or destroy it. The 
 failure of a material by compression. 
 
 Heaving — The upward lifting of the mine 
 floor. The rising of the bottom after 
 removal of the coal. 
 
 Sagging — The drooping, sinking, set- 
 tling, or bowing of the mine roof, usu- 
 ally maximum at the center of the span. 
 Curving downward in the middle, usually 
 as a result of improper loading or 
 supporting. 
 
 Shearing — The cracking of the mine roof 
 because of stress resulting from applied 
 forces that cause two contiguous roof 
 parts to slide relative to each other. 
 
 ^Definitions are taken or adapted from 
 references 7 and 8, which appear in the 
 list preceding the appendix. 
 
 The separation of a roof bolt into two 
 distinct pieces because of stress. The 
 cutting of a metal strap by a roof bolt 
 or roof bolt plate. 
 
 Spalling — The breaking off of chips , 
 scales, or slabs of mine ribs, sealants, 
 or roof. Flaking off of rock, mineral, 
 or metal from its surface. 
 
 Splitting — The lengthwise dividing of a 
 mine support, usually along the grain or 
 seam. 
 
 Squeezing — The forcing or thrusting by 
 compression (imbedding) of a roof bolt 
 plate into a header board or mine roof 
 because of the exertion of pressure on 
 opposite sides. The effect of the clo- 
 sure of an entry on supports. Sometimes 
 referred to as punching. 
 
 Stressing — The formation of stress pat- 
 terns on roof bolt plates because of the 
 force exerted when the plate presses 
 on, pulls on, or pushes against another 
 structure. The resistance of a body to 
 compressional, tensional, or torsional 
 force. 
 
 Twisting — The turning or shape changing 
 of a support member under torsion and as- 
 suming of a partially spiral shape. 
 
 t^U.S. CPO: 1985-505-019/20,038 
 
 INT.-BU.OF MINES, PGH., PA. 27951 
 
xaO 
 
D DD 
 
 n 
 
 > 
 
 -1 
 
 3 
 
 CI 
 
 73 
 
 o 
 
 Q_ 
 
 o 
 
 Q 
 
 o 
 
 n 
 
 n 
 n 
 
 Q. 
 
 n 
 
 3 
 
 -1 
 
 3 
 O 
 
 c 
 
 3 
 
 -♦■ 
 
 w 
 
 o 
 
 Q_ 
 
 J 
 
 
 
 tfl 
 
 c 
 
 
 
 Q 
 
 
 •^ 
 
 ^ 
 
 
 O 
 
 10 
 
 n 
 
 
 
 tn 
 
 
 
 
 3- 
 
 3 
 
 T3 
 
 3- 
 
 in 
 
 3 
 
 Q 
 
 Q 
 
 
 -*■ 
 
 a 
 
 O- 
 
 3 
 
 
 tt 
 
 O 
 
 3 
 
 <a 
 
 ZT^ 
 
 O 
 
 -^ 
 
 Q_ 
 
 n 
 
 n 
 
 3 
 
 in 
 
 n 
 
 01 
 
 Q 
 
 
 (Q 
 
 O 
 
 n 
 
 • 
 
 —t' 
 
 
 
 
 2. 
 
 
 o 
 
 
 
 
 
 
 Q_ 
 
 -0 
 
 — 
 
 n 
 
 3 
 
 < 
 
 
 • 
 
 n 
 
 (/I 
 
 m 
 
 
 
 
 Q 
 
 
 o 
 
 3- 
 
 
 
 U) 
 
 
 < 
 
 
 
 
 n 
 
 
 n 
 
 (/) 
 
 
 ^ -n 
 
 Sq 
 
 3) > 
 
 •D r- 
 I- 
 
 2J CO 
 
 2 
 
 H 
 -4 
 Cfl 
 CD 
 
 Oo c 
 
 rn ^ C 
 
 7 CD 
 
 # mo 
 
 < m 2 
 > z m 
 
 2 C CO 
 
 O 
 
 m 
 
 Tl 
 
 > 
 
 ii 
 
 _ m 
 
 H 
 
 m 
 
 o 
 
 33 
 
 m 
 D 
 
 c 
 > 
 
 I— 
 
 O 
 
 T3 
 TJ 
 O 
 30 
 
 H 
 C 
 Z 
 
 H 
 -< 
 
 m 
 
 TJ 
 
 n 
 O 
 
 -< 
 m 
 
 33 
 
 in 
 
 Is 
 
 2 C) 
 - m f" 
 Z 2 > 
 
 i o o 
 o) T -n 
 

 ,-lo<. - 
 
 J 
 
 
 
 
 
 
 
 -Ao^ 
 
 
 ^^•n.^ 
 
 
 ^^/ -A- %/ A- \/ -'Mi' \y'A' *--/ 
 
 "^^ ^^^ ^'i 
 
 '^0^ 
 
 ^-lo*. 
 
 ^^0^ 
 
 
 'b\? 
 
 V.^^^ ;^fe\ \^/ -'^^'■° \-&^ ^^^^' \^^ •*^^'° %^^^ °'^%'°^ "^v^/ -^^ 
 
 5^^^^.. °.¥S^; ^^^\ ^^K*" ^^^"^^^ "-^^^^ '^^^\ %^^*° ^.^^'^^^ °-%W** '?)^^% ^* 
 
 ' G^ "^ * 
 
 
 ^^^^^^ ;^fe\ \^/ »^^^''° %^^^ ''^fe*' "^vpc,/ 
 
 .^ ... 
 
 
 
 
 'oK 
 
 ''^^'1^% ''^'^ '* fo^ ^''^^^' 
 
 rAQ' 
 
 cCl°-<v 
 
 
 .^9^ . 
 
 
 <. ^'T'. s^ ,G^ ^ -o.»» A <^. ♦?^T* .G^ ^ 'o..- A 
 
 .•b^o 
 
 J> ^^ -.^^^v^/ >" "^ 
 
 . aV ^^ . 
 
 u ♦^ »i 
 
 
 ° A^ 
 
 
 
 
 
 
 
 ^^-^K 
 
 • A v^ : ^ 
 
 
o ^ 
 
 
 ■ u 
 
 3Pv% 
 
 
 o « ' .^ 
 
 ^*/i^ii;iA >^..^;:>- ./Vi-^A. .co^.^%"°. A^^y.^^,X. ^ """ 
 
 '. '-^^.^ ov^^^e^-- ^r.^^ :m^r^. '^^^ ^^^-. ^o'f :M^^^ ^^^0^ i 
 
 '^o^ 
 
 ^oV 
 
 .0' 
 
 V^^\/ %'^''\<'' \'^^\/ \.^^^/ ^^,'^^\/ %^-.- , 
 
 .%^ y.^fc"-^. ./^:4.-i:.X .y.^^«k^"^ 
 
 
 
 
 <0 ►«?''^,^^* ^0 -7- 
 
 
 
 .0' .!*^'* ".> 
 
 ^^. 
 
 
 
 V^!^\/ %'^-"/ *^,'-3^\/ %'W,-'J> v-.!^\/ %■••■„■ 
 
 r- 
 
 V-i 
 
 * ■ 
 
 0^ .o-JL"*. "^ "^ 
 
 ■V.<ir ^'^ 
 
 ^..^'-. <t 
 
 >-o^ 
 
 
 o -^ 
 
 
 
 '>o^ 
 
 'bV 
 
 ^°--<^. 
 
 "V.^^ 
 
 
 WERT ,, , 
 
 BOOKBINOiNG || "^ 
 
 CriOiville. Pa 
 utv-^ug 1935 
 
 '. '-n.-o^ f 
 
LIBRARY OF CONGRESS 
 
 002 955 975 2