r^" > ' %'''^^'^'\^' '^^^'^T^'*,/ ^^^'^^^o^ «^ ■'^ — n V, * » « O « . < • • • '^ v" • -^^^ ^^ A>:^A. .cO^.^;:>o ./Vi,;^A ^^^ --" ' '- °o v^\:r.°'.\ y^^^'% ^^^:S'\. A^"^^^^^ -^^ <^^ v-;^ ^ . o '^^.^ ov^^^mn^-" ^o>^ ;^^.'' -^^0^ r^'^M'; -^o-f '^^^^-'^ '-^--^ ■ "-^^0^ o, ^ A *j U '-^^0^ ^ \* 'y N* ' ,0^^ \. *.-T^' A 0^ ^o '^ -^^0^ r- <. 'o . * ^-^0,*' .• .*^% i0^.vL!nL'* '^^ o N " <^ v*^^ ,.^^^^. ■^o^..9^ ,0 s^^^ 0^ o-v.v /\^-;^,\ /.'^> °o ^M.^i o,^^^'-^ ^^^^ :^^^- '-^^0^ r^'^S'; -^o^ ^°^ ^°-v^. °^ ^--' / ., V^^ y "°^^-'/ '^^,'^^\/ "°^'^»^/ 'V^^rtTo^^^' *S* v\ (\\\ sSiP //ti ^. r* ^ ^°-;^. '^o^ 't.-o^ .^^ '^^' .^'». '^^ ^^ •: %/ ■»: X/ .'»•- %.** /MM' \-/ A-- ^-^.*" •^'- '^--^^ .* -,... ^._ y,c^..\ c°*,.^%>o ,/.^^.\ '° -ii^i' °- / •■■• ■•| »- o. .^ .K ^-1°^ 0^ IC 9066 Bureau of Mines information Circular/1986 Longwall Roof Support Technology in the Eighties A State-of-the-Art Report By Ernest A. Curth and Jeffrey M. Listak UNITED STATES DEPARTMENT OF THE INTERIOR -'4^ "^^^ ' ^*^ / '*'^ Information Circular 9066 Longwall Roof Support Technology in the Eighties A State-of-the-Art Report By Ernest A. Curth and Jeffrey M. Listak UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Model, Secretary BUREAU OF MINES Robert C. Norton, Director f1 0( c\0 b^ Library of Congress Cataloging in Publication Data: Curth, Ernest A Longwall roof support technology in the eighties. (Information circular ; 9066) Bibliography: p. 35-37. Supt. of Docs, no.: I 28.27: 9066. 1. Ground control (Mining). 2. Longwall mining, l. Listak, Jef- frey M. II. Title. III. Series: Information circular (United States. Bureau of Mines) ; 9066. TN295.U4 [TN288] 622s [622'. 28] 85-600288 CONTENTS Page Abstract 1 Introduction 2 Acknowledgments 2 Estimates of roof support loads 3 Longwall support load prediction 3 Statutory standards 4 Alternative analysis of roof support adequacy,.... 5 Estimates of roof loads In terms of the geometry of various shield types 6 Methods of Improving ground control on longwall faces 12 Reduction of shield weight and simplification of functions 17 Shield design and testing. 19 Dust control and ventilation 20 The role of the Bureau of Mines In development of longwall roof support technology 21 Demonstration of shield-type longwall roof supports 22 Demonstration of longwall mining 23 Longwall mining In steeply dipping seams 26 Longwall mining of thick seams 28 Single-pass mining 28 Multlllf t working 29 Sublevel caving 31 Thin-seam longwall mining 31 Conclusions and summary 32 References 33 Appendix A. — National Coal Board Mining Department Instruction PI/1982/6: The use of powered supports on longwall faces 38 Appendix B. — Specifications for Thyssen RHS 12/30 shield 40 Appendix C. — Manufacturers 42 ILLUSTRATIONS 1. Concept of roof block and forces 3 2 . Shield diagrams 6 3 . Lemnlscate tracks 7 4. Lemnlscate shield 7 5 . Double-telescoping leg 8 6. Four-leg shield 10 7. Roof shield 10 8. Chock shield 11 9. V-type shield U 10. Load distribution on a canopy 13 11. Upswept sliding canopy extension 13 12. Piano key selector 15 13. Bi-dl rotary valve, 15 14. Remote batch control 16 15. Floor pressure through different bases 16 16. Effect of extension ratio on yield load 18 17. Equalization of support force for shields with inclined legs 18 18. Equalization of tensional force in link bars by substituting hydraulic cylinders 19 19. Schematic of testing bending and torsional loads in the structure 20 20. Side shield 21 ii ILLUSTRATIONS — Continued Page 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 1. 2. 3. Effect of dust seals , Caliper shield 320 HSL , Leraniscate shield 18/30 , Lemniscate shield 12/30 , Snowmass shield , Aligning shield in pitching strata , Tensional in-f ace anchorage , Two-leg high-seam shield , Two-bench mining method , Multilif t system Chock shield , Mid-Continent shield X-type shield Six-leg shield Plow face schematic , Plow face TABLES Longwall-ground-control-related accident experience 1977-83 Evolution of roof cover and load density by face support systems. Comparison between early and modern shields 21 22 24 25 27 27 28 29 30 30 30 31 33 33 34 34 2 12 17 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT cm centimeter m3 cubic meter Gmt billion metric tons (gigaton) mm millimeter kg kilogram MPa megapascal kN kilonewton m/s meter per second kN/m2 kilonewton per square meter (kilopascal) mt metric ton mt/m^ metric ton per square meter kPa kilopascal mt/m^ metric ton per cubic meter L liter N/cm2 newton per square centimeter L/min liter per minute pet percent m meter s second m2 square meter LONGWALL ROOF SUPPORT TECHNOLOGY IN THE EIGHTIES A State-of-the-Art Report By Ernest A. Curth ^ and Jeffrey M. Listak ' ABSTRACT It took only 9 years from the first appearance of roof shields on the U.S. longwall mining scene to the present predominance of shield faces. An apparent consequence is the welcome downward trend in accidents re- lated to failures in longwall ground control. The report addresses load prediction, the effects of shield geometry on support loads, factors contributing to ground control, and related techniques. The Bureau of Mines took an active part in the evolution of longwall mining, including developing one of the first shield faces in 1975, the first lemniscate- type shields in 1976, the Mine Roof Simulator completed in 1980, the first shields in steeply pitching coalbeds in 1981, and the first multi- lift working of a thick coalbed in 1982. The information offered in this state-of-the-art report will assist in establishing criteria for roof support selection. Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. INTRODUCTION In terms of roof support technology , the introduction of shields in the seven- ties and eighties was a major step for- ward. Table 1 indicates the predominance of shield faces on the U.S. longwall min- ing scene and its apparent consequence, a welcome downward trend of ground-control- related accidents. The rapid progress of longwall technol- ogy began in 1975 and is highlighted by a development leading from the caliper shield to the modern lemniscate type. Shields have been gaining preference over chocks since their advent to longwall mining in 1975. Safety and productivity factors favoring shields over chocks are — 1. A sheltered working space requiring minimum cleanup work. 2. Structural stability that allows advancing without delay even with brush- ing roof contact and, by controlling all lateral loads , removing the requirement for cumbersome leg restraint and restora- tion devices. 3. Wide hydraulic range of mining height. The development of roof shields in the Federal Republic of Germany preceded and paralleled their rapid adoption by U.S. miners. In 1983, 87 pet of the total production in Germany came from shield faces (1).^ In Great Britain, the Na- tional Coal Board introduced the first shield support system in 1977 to initiate Advanced Mining Technology (ATM) and Heavy Duty Mechanization (HDM) programs; as of mid-1982, 44 shield faces were operating, or 7.5 pet of a total of 581 faces (7). TABLE 1 . - Longwall-ground-control- related accident experience, 1977-83 Census of Shield Ground- longwall Total Shield faces. control- faces faces faces pet of total related accidents 1 1977 (2). 77 15 19 143 1978 (2) (2) (2) 59 1979 (3). 91 40 43 60 1980 (4). 105 57 54 89 1981 (2) (2) (2) 73 1982 (5). 112 93 83 87 1983 (6). 118 99 84 71 ^Source: Health and Safety Analysis Center, Mine Safety and Health Admini- stration, U.S. Department of Labor. 2Census not available in 1978 and 1981. The Bureau of Mines has taken a leading part in the introduction of shields to the U.S. mining industry through technol- ogy transfer seminars, studies, and par- ticipation in several longwall demonstra- tions including one of the first shield faces in 1975 (8) and the first shields with lemniscate gear in 1976 ( 9_) . The first part of this report presents load predictions, estimates of loads in terms of the geometry of various types of shields, and factors contributing to ground control and related technology. The second part highlights the scope and variety of the Bureau's role in the prog- ress of longwall mining technology. The objective of this state-of-the-art report is to assist mine operators in select- ing a roof support system fitting site- specific strata conditions. ACKNOWLEDGMENTS The authors are indebted to the rep- resentatives of Dowty Corp, , Warren- dale, PA; Heintzmann Corp,, Lebanon, VA; Hemscheidt America, Pittsburgh, PA; Min- ing Progress, Ine, , Charleston, WV; and ^Underlined numbers in parentheses re- fer to items in the list of references preceding the appendixes . Thyssen Mining Equipment Division, Mar- ion, IL, for their cooperation in the preparation of this report and gratefully acknowledge the permission granted by Gluckauf , Essen, Federal Republic of Ger- many, to use figures 17, 18, 19, 26, and 27; and by Bergbauforschung GmbH, Essen, Federal Republic of Germany, to use fig- ures 3, 8, 15, 16, and 21. ESTIMATES OF ROOF SUPPORT LOADS LONGWALL SUPPORT LOAD PREDICTION A premlning Investigation includes core drilling to provide data for isopachs of various strata intervals and the over- burden thickness. Physical properties of rock are determined either from the cores (10) or directly by geophysical logging to obtain indicators of rock mass behavior. Where the underground site is acces- sible, roof and floor bearing-capacity tests can be carried out to determine contact area requirements for roof sup- ports and cutting pattern (9). For exam- ple, a soft underclay that fails at 210 N/cm^ must be covered by leaving a layer of coal thick enough to prevent sink- ing of the supports. Conversely, soft, friable roof material may be kept from spalling by leaving a layer of roof coal. Several methods have been conceived to arrive at an estimate of the mean load density required to support the roof of a projected longwall site: 1. Barry ( 11 ) proposed that the needed mean load density at yield can be esti- mated by considering strata separation and cantilever action in a stiff strata. 2. Wilson ( 12 ) conceived the immedi- ate roof above a longwall face as a free block that must be supported (fig. 1). W^jJUVL Jll=K\'~\A>J>^K^ ' ~=J. TTxrrs'r^m The face break is assumed caving angle along a line takes place. Rock quality, jointing of the roof strat angle. A caving angle of tive of a very friable roof dicates friable strata, medium firm, and 45° and firm and very firm rock. The load density (R) in me square meter is calculated to occur at a where caving bedding, and a affect this 0° is indica- rock; 15° in- 30° indicates 60° indicate respectively, trie tons per by (13) W R = TT- (L + H tan a), zr where W=LxHxcxd weight of free face block, mt , c = roof support centers, m, d = density of roof material, mt/ m^ , L = span from face to canopy rear end, m, r = length from face to the loca- tion of the resultant roof support force, m. and H = caving height, m, a = caving angle, degrees. Wilson assumed that the caving rock occu- pies 1.5 times the volume of the virgin strata and thus the caving height to the bridging strata is H = 2h, twice the extraction height h. The bridging beds form a beam supported on the coal face and the compacted gob, Wilson also in- troduced a corrective equation for in- clined seams: R = -( sin6 tan6 + cos6 j mt/m2. where and FIGURE 1. - Concept of roof block and forces. 6 = inclination of coalbed, degrees tan6 = 0,4 friction coefficient between rock beds. 3. Wade (L4) of more friable 1.25 times the thus accounting traction height, lowing formula (MLD): assumed an immediate roof nature that occupies only volume of virgin strata, for four times the ex- He developed the fol- for mean load density MLD = (1 + I Cp) X 4dh mt/m^, n=l where h = mining height, m, d = density of roof rock, mt/m^ , and Cp = magnification coefficients. Magnification coefficients are identified as hanging of immediate roof, local face activity, bridging of immediate roof pri- or to first fall, main roof weight, and extended downtime. Determination of load density, proposed by Barry, has found application for most U.S. longwall designs, including Bureau of Mines-sponsored demonstrations such as the shields in the Kaiser Steel Mines and the Old Ben project in Illinois. Wil- son's method is mostly used in the United Kingdom and formed the basis for recently issued instructions of statutory effect (appendix A). Wade's concept is the most conservative one; it involves significant factors of magnification and therefore leads to high estimates. STATUTORY STANDARDS Guidelines for the approval of caving longwall operations were issued in the Federal Republic of Germany in 1966. The required minimum load density is consid- ered a function of extraction height in coalbeds under 18° of pitch. A correc- tive equation is introduced for pitches exceeding 18°. The required load den- sity (A) is calculated by the following formula (15) : A=1.6x2x2.5M=8M mt/m2, where M = coalbed thickness, m, 2.5 = density of strata, mt/m^ , 1.6 = a safety factor, and 2 = a factor allowing for cav- ing of roof strata to a height twice the thickness of the coalbed. For pitching coalbeds the formula becomes (15) A = (5 + 0.15 E) M, where E = pitch, grad (1 grad = 0.9°) The above formulas were conceived for the frame and chock supports of the six- ties. With the advent of shields in the seventies, manufacturers and mine opera- tors chose to raise the required mini- mum load density to 15 M mt/m^ from the statutory 8 M mt/m^ to account for the inclination of shield legs resulting in a mechanical disadvantage (15). Mandatory directives issued by the Federal Republic of Germany ( 16 ) in 1977 address — 1. Dimensions of the travelway, which shall not be less than 0.6 m in width and 70 pet of the extracted thickness in height. 2. Control of a roof support from its neighbor, 3. Deadman controls, 4. Dust control by water sprays and dust seals between units. 5. Powered face sprags for extraction in excess of 2.4 m. 6. Examination and approval of each prototype at the material testing center maintained by Nordrhein-Westf alen State, In the United Kingdom the National Coal Board issued Guideline Pl/1982/6 titled "The Use of Powered Supports on Longwall Faces" in 1982 (appendix A), The new statute supersedes earlier (1966) safety standards that called for a maximum prop- free front of 2 m and includes — 1. Setting and yield load densities in different longwall face zones. 2. Maximum support centers. 3. Efficient hydraulic system. 4. Span of canopy tip to face in dif- ferent extraction heights. 5. Powered forepoles in coal higher than 2.5 m and for a web deeper than 0.8 m. 6. Powered face sprags in coal higher than 2.3 m. The instructions list statutory minimum load densities in different face zones. The statute reflects Wilson's thoughts on determining load densities (17) . To maintain the immediate strata intact, the support must carry the weight of a free block of immediate strata extended to a height of 2H, where H is the extracted coalbed thickness. Thus, load density for setting (Ag) — As = 2 H X 2.5 = 5 H mt/m2 where 2.5 = strata density, mt/m^ and H = extraction height, m. As supports are lowered and advanced, each adjacent support has to bear an ex- tra load of 1.5 times the original load: 1.5 X 5 H= 7.5 H mt/m2 The yield valve is designed to maintain maximum support resistance without damage to the structure. The worst condition, occurring after the shearer pass and be- fore the support is advanced, may last for a considerable time, so a safety fac- tor of 2 is applied. Thus, load density at yield (Ay) is — Ay = 5 H X 1.5 X 2 = 15 H mt/m2. Statutory minimum load densities are listed for different face zones that des- ignate face portions of advancing long- walls, most common in European coal min- ing. The face line zone is flanked by two buttress zones that form the bound- aries against the pack holes and often feature roof supports of greater strength than those along the face line. Pack zone supports secure the pack building activities. They frequently are equipped with rear canopy extensions and extra legs. The roadhead zone supports ex- tend over the gateroads, where face-end- forming activities are concentrated in a congested space. Since packwall and roadhead supports are located at the face ends , it is thought that roof behavior requires face end supports to be of only two-thirds the yield load density of the face line zone supports, or 10 H mt/m^. In the United States, Mandatory Safety Standard CFR 75.201-3 calls for approval of the roof support system of the long- wall on an individual basis. A roof con- trol plan must be submitted to the District Manager of the Mine Safety and Health Administration. The plan usually includes number, type, and capacity of the roof supports, and the method of the recovery at the termination of a panel. ALTERNATIVE ANALYSIS OF ROOF SUPPORT ADEQUACY British and German statutory standards determining load densities as a function of extracted seam height are, of course, arbitrary. A load density of 20 times seam thickness for a mining height of 1 m, or 20 mt/m^, may not be adequate at all, but a value of 10 times thickness in a 3-m coalbed, or 30 mt/m^, may be fully satisfactory (18) . Rather than mining height, the German research center in Es- sen targets the area occupied by cavities greater than 0.3 m in height in an ob- served roof area to provide estimates of roof control adequacy. The information is derived from data collected on numer- ous faces through the Essen center's longwall face surveyance method and sta- tistical evaluation (19) . These data are processed by an analysis of variance with a view of obtaining a criterion for roof control through the interaction of following parameters: 1. Thickness of shale overlying the coalbed, less or greater than 2 m. 2. Rock pressure, MPa, as calculated (20). 3. Mean load density of roof support, kPa. A. Mean span canopy to face, m. The result is an estimate of the percent- age of the area of cavities higher than 0.3 m in a mapped roof area. Roof con- trol is more than adequate if such cavi- ties occupy less than 10 pet of the roof area under observation. ESTIMATES OF ROOF LOADS IN TERMS OF THE GEOMETRY OF VARIOUS SHIELD TYPES Figure 2A is a schematic of a two- leg caliper shield of the type installed in New Mexico in 1975 (8^). The canopy tip describes a circular arc when the shield is raised or lowered. Hence, the critical span between canopy tip and face widens with increasing height of the shield unless compensated by an extension. When yielding, caliper shields develop a horizontal thrust to- ward the face and an increase in vertical load because the friction between canopy and roof rock must be overcome. The friction coefficient between steel and rock is estimated to be 0.3 = tan 16.7°. The two legs are arranged between gob shield and base so that the support force is introduced into the hinge be- tween canopy and gob shield. The me- chanical disadvantage due to the shield geometry is a function of the extraction height and reduces the force acting on the canopy hinge. The efficiency of this caliper system is quite low, and the force available at the canopy is re- duced to 50 pet of the leg force in some cases. Support force and mean load density can be estimated by following formulas: A = _ 2 T e cosy A = 2 S e cosy setting, cosy = 1 ^(/i=0) A, Caliper 9, Lemniscate FIGURE 2. - Shield diagrams. A = A.U ^ Aui ^ _ 2 T e cosy where and 2 S e cosy fg C M 2 T e cosy f 3 C M 2 T e cosy fn C M at yield point, cosy = 1 when yielding, cosy = 0.96 setting, cosy = 1 at yield point, cosy = 1 when yielding, cosy = 0.96 S = setting load per one leg, kN, T = yield load per one leg, kN, 0.3 = friction coefficient be- tween steel and rock, y = tan "10.3 = 16.7°, A = vertical component of roof support force in the can- opy hinge, kN, C = length of canopy plus span of canopy tip to face, m, M = shield spacing, m. Ay, = mean load density, kN/m^, e, fg, fn ^re measured for each working height, m. Figure IB is a schematic of the early lemniscate shield installed in Illinois in 1976 (Jl.) • The canopy tip of a lem- nlscate-type shield moves up and down in a nearly straight line that is the cen- ter part of a lemniscate or figure-eight curve; therefore, the span between canopy tip and face barely changes. Figure 3 shows the relationship between the lem- niscate curve described by the canopy hinge point, the moving tracks of the pivotal link bars, and the pole where the t • 0) 1 i^ o> N 'Ap'-.--?- c "*^ o 5 (T / 1 / / l\ 1 \ 1 \ I \ \ \ FIGURE 4o - Lemniscate shield. FIGURE 3. - Lemniscate tracks. A = 2 T e yielding elongations of the link bars intersect. The solid lines show the range of action. The support force remains nearly uni- form over the operating range of the lem- niscate shield, while support forces in caliper shields increase with the min- ing height. The structures of caliper shields must be dimensioned accordingly to resist the greatest load and may be- come very heavy. Higher efficiency of the system, uniform support force, and a nearly equal span of roof exposure in terms of the mining height are the advan- tages of the lemniscate shields versus the caliper shields. The- two legs of this early lemniscate- type shield are arranged between the gob shield and the base so that the support force is introduced into the hinge be- tween the canopy and the gob shield, A mechanical disadvantage in terms of the extraction height reduces the force that acts on the canopy hinge. Support force and mean load density can be estimated by following formulas: . 2S e ^^, K = f — c~M setting A = 2 T e « fc C M yielding A = 2 S e setting where S = setting load per one leg, kN, T = yield load per one leg, kN, A = roof support force at the canopy hinge, kN, C = length of roof bar plus span of canopy tip to face, m, M = shield spacing, m, A^, = mean load density, kN/m^ , and e, fg are measured for each work- ing height, m. Figure 4 is the schematic of a mod- ern lemniscate shield introduced in Illi- nois in 1978 that has become the standard two-leg shield design In the United States and abroad (21) » The legs are In- serted directly Into the canopy, and the gob Is jointed to the canopy rear end to eliminate the dead corner where debris can accumulate and foul the canopy move- ment. A large ram stabilizes this joint and provides an adequate load at the tip of the canopy. Double-telescoping legs provide the re- quired range of shield height. The dif- ferent support forces in each stage of such a leg can be equalized either by an internal yield valve in the bottom of the piston of the smaller stage or by design- ing equal piston areas for each stage (fig. 5). Efficiency of this design is greatly Improved over that of previous ones. The support force available at the canopy Is 90 pet of the leg force and greater. Double-telescoping legs are standing nearly straight with a minimum of incli- nation within the working height range. The resultant magnitude of the support force, its location on the canopy, tip load, breakoff load, and mean load den- sity are estimated by the following for- mulas (fig. 4) : Ax = S s + N n A (x + a) = S e S (e - s) - N n a = N n + S s A K A_ (x + c) FIGURE 5. - Double-telescoping leg. A = resultant support force, kN, D = A - K A =^ « CM where S = force of two legs, kN, N = force of stabilizing ram, kN, X = resultant distance from joint, m, L = length of canopy, m, K = tip load, kN, D = breakoff load, kN, C = length of canopy plus span of canopy tip to face, M = shield spacing, m, Aj, = mean load density, kN/m^ , Given: Tip load Canopy length Force of two legs Canopy length plus , span of canopy tip to face Shield spacing K = L = S = C = M = and e, s, n, a are measured for each working height, m. An example of evaluation of resultant force magnitude and location and mean load density follows: 50 kN 2.7 m 2 X 1,600 kN 2.7 + 0.3 m before the shearer pass 1.5 m 0.3 m 0.8 m 2.42 m 2.82 m Results: Stabilizing ram moment N n = K (L - c) = 50 x 2.4 = 120 kNm S (e - s) - N n c = s = a = e = Resultant support force A = Resultant distance from x = j oint Mean load density before A,, = cutting Figure 6 is the schematic of a four- leg shield with two rear legs supporting the gob shield and thus stabilizing the joint. These shields provide high tip loads and achieve a more even floor load- ing than two-leg shields. They are used in several mines in Ohio, Pennsylva- nia, and West Virginia (fig. 7). Support force, tip load, breakoff load, and mean load density are evaluated as fol- lows (22): 2 X 1,600 (2.82 - 0.8) - 120 „ ,^^ ,„ 2742 = ^'^^^ ^^ Nn+Ss 120 +2x 1,600 x 0.8 , ^„ 1 = ?: — ttt:^ = i.UZ m A 2,620 A 2,620 t._„ ,.^/ 2 C^ = 3 X 1.5 = ^^2 kN/m2 A]: c = S^ s Ai (b + c) = Si e c = s b ei - s 1 ^1 Ai = _ ^1 Si s S2 eg ^ A2 b ^ Aj c A^ + A2 A = Si ei H- S2 eg a + b 10 T = A (a + d) D = A - T A =-A- ^ CM FIGURE 6. - Four-leg shield. where Si = force of two front legs, kN, S2 = force of two rear legs, kN, Ai = support force resultant for front legs, kN, A2 = support force resultant at the joint, kN, a = resultant distance from joint, m, A = resultant magnitude, kN, T = tip load, kN, D = breakoff load, kN, L = length of canopy, m, C = length of canopy plus span of canopy tip to face, m. FIGURE 7. - Roof shield. 11 and M = shield spacing, m, A,, = mean load density, kN/m^, ej, 62, s, b are measured for each working height, m. Ai Ci + A. Figure 8 Is a schematic of a four-leg shield where all four legs support the canopy. A version of this type, the chock shield with straight legs, is the preference in National Coal Board mines. Table 2 shows that the chock shield, ap- plying immediate forward support in the one-web-back mode, clearly stands out as the roof support with the most complete cover and highest load density. In the United States V-type shields are used in low coal in West Virginia and in some thick coalbeds in the West (fig. 9). Support force resultant magnitude and location and mean load density are esti- mated by the following formulas (22) (fig. 8): ~ Ci = Si b ii - s Co = eo - s Ai = _ Si si A2=^ a = A = _ '^l Ai + A2 Si ei + S2 ^2 a + b Aw A C M FIGURE 8.- Chock shield. FIGURE 9. - V-type shield. 12 where Sj = force of two front legs, kN, S2 = force of two rear legs, kN, Aj = resultant support force for front legs, kN, A2 = resultant support force for rear legs , kN , and a = resultant distance from joint, m, A = resultant support force, kN, ej, e2 , Sj, S2 , b are measured for each working height, m. METHODS OF IMPROVING GROUND CONTROL ON LONGWALL FACES The following requirements are critical to achieving and maintaining ground con- trol on longwall faces: 1. Minimizing support delays. 2. Reducing the span of canopy tip to face. 3. Adequate support force. Means to meet these requirements are op- eration mode, canopy extensions, advance with brushing contact, hydraulic supply, hydraulic control function, and floor control. The one-web-back mode provides a conve- nient traveling space and immediate for- ward support by advancing the roof sup- ports at once after the shearer has cut by and before the conveyor is moved up. However, the entire shield can be brought up closer to the face by operating in the up-to-the-conveyor or conventional mode. The conveyor is pushed ahead first after the shearer pass, and the shield follows. Roof support, close to the face, can then be applied without delay if canopies are equipped with powered extensions. The canopy of a shield designed to op- erate in the one-web-back mode must be longer than that of a conventionally ad- vancing shield by at least the width of the cutting web, and there is a trend toward increasing canopy length to ac- commodate wider webs, larger conveyors, shearer haulage racks , and even pipes to carry material for placing packwalls in the gateroads. However, a canopy should be designed to maintain a stable roof contact during shield advance by having a ratio of its front portion to its rear portion of no greater than 2:1. Front and rear por- tion are related to the locus of the sup- port force resultant. Canopies with long front portions have poor roof contact. The extent of unsupported roof is widened TABLE 2. - Evolution of roof cover and load density by face support systems Face support system^ Yield load, mt Area, m2 Cover, m" Cover, pet of area Density at yield, mt/m^ Before cutting: 4-leg chock 6-leg shielded chock...., 2-leg shield (IFS) , 4-leg chock shield 4-leg chock shield (IFS), After cutting: 4-leg chock 6-leg shielded chock,,,. 2-leg shield (IFS) 4-leg chock shield 4-leg chock shield (IFS) 100 260 325 450 450 100 260 325 450 450 3,0 4,8 5.1 5.1 6.0 3.5 5.5 5.1 6.25 6.0 0.7 4.0 4.35 4.35 5.74 ,74 4,32 4,35 4.85 5.74 23,3 83.3 85.5 85.5 96.0 21,1 78,5 85,51 77,5 96,0 33,3 54,2 63.7 88.2 75.0 28.6 47.3 63.7 72.0 75,0 ^IFS = immediate forward support. Source: Lewis (7, p. 13J . 13 because the front of the canopy does not touch the roof. Canopy extensions can provide the de- sired roof contact. Figure 10 shows that a canopy divided by a ratio of 2.6:1 contacts the roof 0.4 m behind the canopy tip according to tests conducted at Es- sen research facility. Federal Republic 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 CANOPY LENGTH, m 400 UJ q: / 0.9-1.8 m \^ UJ / >^'^""^**^ \ ^20 1 X ^\ \ ~ o f^r ^V \ U- Jr ^^ \ fe Extension ratio \ 1 to 2 "^ \ o g: 15 V. ID \ CO Extension ratio in 1 to 3 1 1 1 1 1 1.8 1.6 1.4 1.2 1.0 0.8 0.6 SHIELD HEIGHT(h), m FIGURE 16. - Effect of extension ratio on yield load. four-leg shields for two-leg shields will result in a weight saving of 18.5 pet. Considerable inclination of legs is re- quired in thin-coalbed mining with the consequence that the support force great- ly increases in terms of mining height. Figure 17 shows an increase from 1,400 kN to 2,400 kN when the shield is raised from 0.6 to 1.4 m. However, the struc- ture must be dimensioned to accept the 1.4 1.8 0.6 1.0 SHIELD HEIGHT, m FIGURE 17. - Equalization of support force for shields with inclined legs. maximum force. Equalization to a me- dian force of 1,900 kN will result in a lighter structure. The equalization will be achieved by adjusting the leg pressure in terms of the shield height or the in- clination of the rear link bars. A shield with straight double- or even triple-telescoping legs is not free from lateral forces exerted by the motion of the roof strata from the face to the gob and the base movement during setting. Figure 18 shows the increase of forces in the front link bars of such a shield with an extension range from 2.2 to 6m and considering a friction coefficient of 0.3 between canopy and roof rock. The dif- ference in tensional load is very high and can be equalized by substituting hy- draulic cylinders for the front link bars. Controlling the force in the hy- draulic cylinders by a yield valve limits bending moments in gob shield and base and thus results in a lighter structure. In conclusion, weight reduction is achievable by reducing lateral forces parallel to the strata by means of hy- draulic link bars and regulation of the leg pressure to limit peak loads. However, it appears that weight reduction 19 and design simplification are conflicting complexity demands. Weight savings lead to more Germany. according to the tests in SHIELD DESIGN AND TESTING Manufacturers use CADD (computer-aided design drafting) (29-30) . The programs include assessment of the foundation pressure, linkage and pin design, and fi- nite stress analyses to generate dimen- sions of structural members and amount of weld. After solving technical problems, microcomputers produce technical specif- ications, outline drawings, and prices needed for quotations, thus alleviating strain on technical resources and deliv- ering tenders to customers in minimum t ime , In the United Kingdom, Testing Proce- dures for Powered Support were first is- sued in 1966 and updated recently. The HSE (Health and Safety Executive) Tests include (16) — 1. Strength and performance. 2. Stability. 3. Possibility of abrupt failure of components. Fh=0.3 Fab Fh limited if Fl= constant 120 Hydraulic link Yield valve O 100 if) a: -I 60 LlI O 40 20 KEY F/^t) Support force Fh Horizontal force Fi_ Link bar force F^ol F^-0.5 F^^ F|_= constant I 4 6 8 10 RANGE, m FIGURE 18. - Equalization of tensional force in link bars by substituting hydraulic cylinders. 4. Practical design. 5. Reliability. Tests are performed in load-reactant frames at the Bretby testing facility of the Mining Research and Development Establishment. The roof supports are exposed to thousands of cycles between setting and yield load plus 12 pet. Com- mercial approval will be granted to manu- facturers after successful laboratory testing, underground trials, and National Coal Board technical approval. In the Federal Republic of Germany, statutory examination and approval of each prototype roof support is carried out in the Material Testing Center of Nordrhein-Westfalen State at Dortmund, The objective of testing is to assure that minimum safety standards are met (31) . Testing used to be limited to structural members such as legs, can- opies, and bases. However, with the ad- vent of shields it became necessary to subject whole units to dynamic tests, and a four-column testing machine was in- stalled with a maximum opening of 4.8 m. The press produces 6,300 kN vertically and 2,000 kN hotizontally (fig. 19). The test program includes reliability of hy- draulic controls. Special yield valves designed to shed load quickly in the event of a rockburst and thus prevent structural damage to legs are also tested in the press, which can retract the upper platen at a velocity of 0.5 m/s. The test facility at the Essen research center is mining research and development related and offers the opportunity to manufacturers to test their designs un- der simulated underground conditions. A large test frame was built in 1963. Whole roof support units can be subjected to convergence and lateral thrust through the movement of an inclined top platen against a floor platen in opposite directions (19). 20 4- KEY Gages / through 5 iR J*M 4 '^5 SIDE VIEW BASE, BENDING Not to scale I BASE, TORSION H I '/3 -bearing near toe PLAN VIEWS FROM /I FIGURE 19. - Schematic of testing bending and tor- sional loads in the structure. In 1975 a 5,900-kN press was completed to accommodate an urgent need for testing shields that came on the German mining scene in increasing numbers. The tests include forces exerted on roof and floor, the support resistance, bending moments in the structural members, penetration of the base into the floor, reliability of shield structure under load cycling, function and operating mode of the canopy, angular positioning of canopy and gob shield, width and height of travelway, behavior on uneven ground, and sideways mobility of the double-acting advancing ram. In the United States the Bureau of Mines Mine Roof Simulator at the Pitts- burgh Research Center is the most power- ful research facility of its kind in the world. It is a computer-controlled elec- trohydraulic press with a maximum opening of 4.9 m. It can simultaneously exert 13,300 kN vertically and 7,100 kN lat- erally to simulate strata conditions to which roof supports could be exposed (32- 33 ) . The Mine Roof Simulator can play a role in domestic fabrication of roof sup- ports to find flaws in design so that manufacturers may know where to direct efforts for improvement. The Bureau of Mines is collecting field data from instrijmentation to measure leg and canopy cylinder pressures by trans- ducers and strain in lemniscate link bars by strain gages. These data are used to determine the resultant load vector on a shield by its parameters: magnitude, lo- cation on canopy, and inclination. These parameters provide design information to assess internal and external forces. Analysis of field data can be achieved by feeding the data into the computer of the Mine Roof Simulator to obtain a simulated load profile. The objective of this ef- fort is to advance understanding of sup- port behavior as a first step in improv- ing the design (34). DUST CONTROL AND VENTILATION Automatic sprays may be mounted on the canopies to control dust originating from the gob during shield advance, and the working space is sealed against dust by sideplates fitted between the shields and kept tight by springs and hydraulic rams (fig. 20) . Figure 21 is a chart indicat- ing the experience with several types of dust seal arrangement in the Federal Re- public of Germany. The sideplates are also used to steer and align adjacent shields against each other. This is most 21 DUST SEAL TYPE FIGURE 20. - Side shield. important In undulating ground and in pitching strata. High air velocities in the restricted longwall cross-sectional area can cause Laid upon Underslung SHIELD ALIGNMENT Up to 18* pitch Undulating roof Most frequent design Little funnel effect Strong funnel effect FIGURE 21. - Effect of dust seals. physical discomfort to miners and initi- ate aerodynamic dust entrainment. Re- search in dust elutriation carried out in England by the National Coal Board indi- cated that the threshold velocity lies at about 135 m/min, above which the aerody- namic entrainment increases very rapidly. The ventilation cross-sectional area (Q) is a function of mining height ac- cording to an empirical formula developed in Europe (35) : Q = 3.75 (M - 0.3) for chocks and four-leg shields, m^, Q = 3 (M - 0.3) for two-leg shields, m^, or where M = mining height, m. Obviously, in terms of face ventilation, faces with two-leg shields are at a dis- advantage relative to those with chocks or four-leg shields. THE ROLE OF THE BUREAU OF MINES IN DEVELOPMENT OF LONGWALL ROOF SUPPORT TECHNOLOGY The Bureau of Mines has been an ini- 1980 tiator of longwall roof support technol- ogy, and the following milestones mark the Bureau's contribution to its rapid progress: 1981 1975 One of first two shield faces in the United States installed in the York Canyon Mine, New Mexico. 1982 1976 The first lemniscate-type shields installed in the Old Ben No. 24 Mine, Illinois. Subsequently the company purchased 10 more longwall faces for operation in the Illinois division. The Mine Roof Simulator at Bruce- ton, PA, and related field data acquisition, as described under "Shield Design and Testing." The first longwall in a steeply pitching coalbed in Snowmass Mine, Colorado. The top slice in a multilift ex- traction of a thick coalbed. This is an advancing face, unique in the United States, in the Dutch Creek No, 2 Mine, Colorado, 22 A thin-seam mining project was shortlived and terminated owing to unsuitable strata conditions. DEMONSTRATION OF SHIELD-TYPE LONGWALL ROOF SUPPORTS The Bureau of Mines entered into a cost-sharing agreement with Kaiser Steel in 1974 to demonstrate longwall mining with shield supports in the Mesa Verde coal basin in New Mexico. At Kaiser's York Canyon Mine, ongoing longwall mining with chocks had met with severe roof con- ditions. Wire mesh had to be stretched over the canopies to contain debris drop- ping out between the roof supports. Pro- ductivity under these circumstances was very low. The Hemscheidt^ 320 HSL caliper-type shields selected by Kaiser and Bureau en- gineers in 1974 proved to be most effec- tive in sheltering the working place from debris and dust. The shield face that began operation in 1975 was one of the first longwall faces supported by shields in the United States, The shields pro- vided adequate roof control for the ex- traction of three panels and were then transferred to Kaiser's Sunnyside Mine in Utah, where in 1982, the shield face at- tained a world record longwall production of 18,497 mt of raw coal in 24 h. Today caliper shields are an obsolete design and are no longer fabricated. Figure 22 is a schematic of the se- lected two-leg caliper shield. The ver- tical extension ranges from 1,50 m to 3.5 m, of which half is hydraulic and half is by mechanical extension members applied to the legs. The 2^n-wide exten- sion range is achieved by having two po- sitions of the legs on the base and also two positions for the joint between the gob shield and the base. The legs are single telescoping. The shield weighs 9 mt and is 4.3 m long. The powerpack supplies a hydrau- lic working pressure of 34 MPa, and the shield is designed to yield at 41 MPa. ■^Reference to specific products does not imply endorsement by the Bureau of Mines . FIGURE 22. - Caliper shield 320 HSL. Thus, the working pressure equals 83 pet of the yield pressure to maintain a strong thrust against the roof strata im- mediately after exposure and to prevent bed separation. A 0,5-m pushout cantilever compensates for the widening span between canopy tip and face due to the circular motion of the canopy tip of a caliper shield when the shield is raised. The two legs are arranged between the gob shield and the base so that the sup- port face is introduced into the hinge between canopy and gob shield. The force in the two legs can exert a thrust of 2,620 kN and yield a load of 3,140 kN, The mechanical disadvantage due to the shield geometry is a function of the ex- traction height and reduces the force that acts on the canopy hinge. The shield is operating in the one-web-back mode to provide a convenient travelway. Mean load density at the yield point ranges from 345 to 505 kPa over the ef- fective extraction range from 1.70 to 3.35 m. The canopy is designed to maintain a stable roof contact during the shield advance by the action of two rams that swing it against the roof and by having a ratio of front portion to rear portion of canopy of no greater than 2. The canopy tip exerts a load of 49 kN at yield. The divided base is self-cleaning and the double-acting advancing ram, acting through reversed linkage, is mounted in an inclined position to counteract the 23 tendency of the shield to dig into the floor. Also the shield is designed so that the toe of the base remains ahead of the projection of the canopy hinge joint. The shields were first controlled di- rectly. However, during the first face- to-face move the controls were changed to adjacent operation to protect the shield setter. The shield can be advanced while automatically maintaining a soft roof contact to keep the roof intact after ex- posure and wipe debris off the canopy. Dust originating from the gob during shield advance is controlled by automated sprays mounted on the canopies. The working space is sealed by sideplates fitted between the shields and maintained by springs and hydraulic cylinders. DEMONSTRATION OF LONGWALL MINING The Illinois coal basin holds signifi- cant reserves and is one of the important coal producing provinces in the United States. Most underground coal mines in Illinois use room-and-pillar methods with the result that the average recovery of coal approximates only 50 pet, roof con- trol is difficult, and productivity needs to be improved. After previous attempts at longwalling in Illinois using chock-type supports had failed, the Bureau of Mines entered into a cooperative agreement with Old Ben Coal Co. in 1975 to demonstrate longwall min- ing in the Herrin No. 6 Coalbed, As a result of a premining review. Mine No. 24 near Benton, IL, became the demonstra- tion site, and a roof support system was specified (9^). Extraction of the first longwall panel indicated that the roof could be controlled by shield-type sup- ports. Consequently Old Ben purchased equipment for 10 additional longwall faces start-ing in 1978. Longwall mining technology greatly benefitted from Old Ben's expertise. The Thyssen RHS 18/30 shields selected by Old Ben and Bureau engineers in 1975 were the first lemniscate-type shields in the United States. The premining study recommended a mean load density at yield of 862 kPa in the closed position and a minimum of 0.15 m of floor coal to remain intact, owing to a soft underclay that fails at 210 N/cm^ when wet. Figure 23 includes a side view of the shield in the one-web-back position; the shearer and conveyor are also sketched to indicate relative position. Vertical travel ranges from 1.8 to 3 m in one stroke without the use of extension mem- bers so that cavities can be supported quickly. Owing to the lemniscate gear, the span between canopy tip and face re- mains 0.3 m over the entire hydraulic range to provide a safe clearance between the rotating shearer drums and the canopy tip. The 5-m-long shield, weighing 15.5 mt , was the heaviest roof support in the United States in 1975. The powerpack supplies a hydraulic working pressure of 30 MPa, and the shield was designed to yield at 38 MPa. Thus, the working pres- sure equals 80 pet of the yield pressure. The two legs are arranged between the gob shield and the base so that the sup- port force is introduced into the hinge between the canopy and gob shield. Ex- treme force is concentrated in the legs that together can exert a thrust of 4,600 kN and yield a load of 5,830 kN. A mechanical disadvantage due to shield geometry is a function of the extraction height and reduces the force that acts on the canopy hinge. At 2.15-m extraction, the shield can exert a thrust of 3,600 kN and sustain a load of 4,550 kN. Canopy extensions are of the articu- lated type, called flipper. They are 0.6 m long to accommodate a 0.6-m cutting web and, when extended, allow a one-web-back operation of the shield. However, the entire shield can be brought 0.6 m closer to the face and operate in the up-to-the- conveyor mode by lowering the flipper. In the closeup position the shield is rated to yield at a mean load density of 1,200 kPa before the shearer pass and 960 kPa after the shearer has cut by. By design, the canopy maintains a sta- ble roof contact during the shield ad- vance because a ram pushes it against the roof, and the ratio of its front portion to its rear portion is not greater than 2. The tip of the canopy extansion sus- tains a load of 36 kN. 24 ■'rr-".'^.-v. '^^^^^^^^^^^^^^^w^^^^^^m^;^w^^^^ SIDE VIEW FRONT VIEW -^^ PARTIAL TOP VIEW OF BASE FIGURE 23. - Lemniscate shield 18/30. The base is divided and self-cleaning. Mounting the double-acting advancing ram in an inclined position corrects the tendency of the shield to bury into the floor. Owing to reverse linkage, the full force of the ram piston end is ap- plied to advance the shield. The shield is controlled from its neighbor to protect the shield setter. The shield can be advanced while automat- ically preserving a soft roof contact to keep the roof intact after exposure and wipe debris off the canopy. Automated sprays mounted on the canopy and seals by sideplates fitted between the shields and maintained by springs and hydraulic rams provide control of dust originating from the gob during shield advance. The Thyssen RHS 18/30 shield, when se- lected in 1975, was one of the best roof supports available and provided adequate roof control and dependable operation (21) . The experience gained on the first longwall faces in Mine No. 24 led to the concept of an improved shield type, the Thyssen RHS 12/30 and the Hemscheidt G-520-12/28 (fig, 24), Improvements in the shield geometry added up to a more efficient introduction of force from the legs into the canopy. At an extraction height of 2.1 m, the 1975 shield trans- mits 78 pet of the axial leg force into the roof, v/hile the modern shield exerts 96 pet of the leg force. The following improvements were made: 1, The legs are inserted directly into the canopy, 2, The rear end of the canopy is jointed to the gob shield to eliminate the dead corner where debris could accu- mulate, impeding canopy movement, 3, The lemniscate links are straight, eliminating expensive fabrication. The rear links no longer need the "clamshell" protective plate, 4, The shields are collapsible to 1.2 m of height for movement under low clear- ance. Double-telescoping legs provide a range of 1.3 to 3 m of vertical travel. 25 FIGURE 24. - Lemniscate shield 12/30. 5. The cutting web increased from 0.6 to 0.75 m to improve productivity, and therefore, the flippers were lengthened to 0.75 m. 6. The flippers can be lifted as much as 20° up into the roof to provide an up- swept canopy extension for close roof contact in adverse conditions. 7. When the flippers are retracted and folded under the canopy, clearance between them and the shearer body is increased. 8. A large ram stabilizes the joint between the canopy rear end and the gob shield. With a force of 651 kN the ram provides a load of 72 kN at the tip of the extended flipper. 9. The gob shield is shorter and lighter. 10. The mean floor pressure is only 183 N/cm^ because the effective floor contact area is large and the resultant roof load transmitted to the floor is lo- cated more than 1 m behind the toe of the base. 11. Three shields in each maingate are equipped with "pendulum plates," articu- lated bases that aid in overcoming soft or uneven floor and obstacles. These three shields have longer canopies to protect the conveyor drive (36). 12. The modern shield weighs 13 rat, 2.5 mt less than the early lemniscate shield, owing to its lighter structure as a result of improved geometry. The lower weight and the collapsed height of 1.2 m greatly facilitate installation, recov- ery, and transportation of the shield and reduce cost of face-to-face moves ac- cordingly. With flippers retracted the shield fits a 4.75-m-long shaft cage deck and can easily be turned around in the restricted space of a recovery area. 13. A single pressure hydraulic system simplifies hosing of the shields and maintenance. Specifications of the latest shields ac- quired in 1981 are listed in appendix B. Similar shield types with or without can- opy extensions have become the standard 26 for modern two-leg shields in extractions ranging from 1.5 to 3m in the United States and abroad. LONGWALL MINING IN STEEPLY DIPPING SEAMS Mobile room-and-pillar mining equipment works efficiently in strata pitching less than 10°. The maximum pitch rubber-tired equipment can negotiate is 22°. Also ground control problems in pitching coal- beds become intense and miners' safety critical owing to adverse pillar loading and lateral shifting of the overlying strata. Resource recovery becomes unac- ceptably low. Therefore, in Europe, mod- ern mechanized longwall methods using shields were adapted to strata pitching more than 22° and up to 60°. Mechaniza- tion of very steep coalbeds pitching more than 60° has been attempted by forming a diagonal front. In the United States pitching coalbeds occur in the West and in the Pennsylvania anthracite province. In Colorado alone, the reserve base of coal recoverable in pitching coalbeds under less than 900 m of overburden is estimated to amount to 1.4 Gmt (27). The Bureau of Mines recognized the need of introducing modern longwall mining methods to steeply pitching coalbeds to enhance worker safety and health, produc- tivity, and recovery of the valuable re- source, and entered into a cooperative agreement with Snowmass Coal Co. in 1979 to demonstrate longwall mining a coalbed 2.1 m in height and pitching 30°. For roof support on a shearer face, Snowmass selected Hemscheidt two-leg lemniscate-type shields. The legs sup- port the canopy, and each leg yields at 1,560 kN. The shield can be set with force of 2,490 kN and is rated to yield at a mean load density of 718 kPa at the extracted mining height (fig. 25) . The legs are double-telescoping and double- acting, and the shield can extend from 1.5 to 3.4 m. The shields are used in an up-to-the-conveyor bidirectional opera- tion mode and, therefore, are equipped with 0.75-m pushout cantilevers and face sprags to provide immediate support to the face after the shearer pass and to prevent spalling of the face coal. The cantilever is pushed out of the profile of the canopy and thus can be extended, while the shield is set against the roof. The shields are grouped in three-shield sets, which the manufacturer called Troika with the Russian three-horse sled in mind (38). The Troika sets are self- advancing independently from the face conveyor, using a 4.5-m-long floor beam to accomplish the advance. The center shield of each Troika set is rigidly at- tached to the floor beam. (After the center shield is lowered from the roof, the floor beam is advanced by the double- acting rams of the two outer shields and pulls the center shield a step forward. The center shield is then set against the roof and holds the floor beam in the for- ward position. The downdip shield is lowered next, pulled up to the beam by its own double-acting ram, and set. Low- ering, advancing, and setting the updip shield complete the move.) An entire Toika set can be steered uphill or down- hill by the action of aligning rams that maneuver the center shield laterally. The floor beam transmits this movement to the flanking shields, turning the set. Each Troika set is controlled from the lower shield of the upslope unit. The face conveyor is pushed by double- acting rams connected to the topmost shield of a Troika set but not attached to the conveyor. The conveyor cannot be retracted. An in-face anchorage system holds the conveyor in place to overcome downhill creep due to its own weight and the impetus of shearer travel. The an- chorage system consists of rams attached to every other Troika set. The rams are hitched to the conveyor by 381- by 89-mm chains. The anchorage must be released for each Troika advance. For uphill mo- tion of the conveyor, all anchorage rams along the face are pressurized. Downhill motion is initiated by releasing as many as half of all the anchorage rams . The maingate end Troika is operated in the one-web-back mode to accommodate the conveyor drive. The shield line does not extend into the tailgate, which is sup- ported by fiber-reinforced concrete crib- bing and single hydraulic props. 27 FIGURE 25. ~ Snowmass shield. In the Federal Republic of Germany, steeply pitching coalbeds form 15 pet of the total reserve base, but only 9 pet of the production comes from this category. The Troika principle, originally devel- oped in Germany and used on the Snowmass face in the United States , has been aban- doned (39). Each shield is connected to the conveyor by its double-acting ram as in flat seams because the 4.5-m Troika floor beam did not accommodate well to adverse floor conditions and accumula- tions of debris. Also keeping the face straight and controlling the roof when traversing strata discontinuities such as faults proved to be impractical with the Troika approach. Figure 26 is a schematic of a shield with three side rams to adjust alignment of the units. In-face anchorage of the panline is achieved by cylinders hitched to the conveyor to apply tension to the system (fig. 27). p 1 Aligning ^ [ forces SIDE VIEW PLAN VIEW FIGURE 26. - Aligning shield in pitching strata. 28 FIGURE 27. - Tensional in-face anchorage. LONGWALL MINING OF THICK SEAMS Three-fourths of the U.S. coal reserve base lies west of the Missippi River. There are important reserves of bitumi- nous and subbituminous coal in Colorado, New Mexico, Utah, and Wyoming that can only be mined by deep methods. Accord- ing to estimates identified reserves in the 3- to 6-m range within these States amount to 29 Gmt of bituminous and 70 Gmt of subbituminous coal (40). A thick coalbed is defined as one that falls beyond the range in which produc- tivity can be achieved with room-and- pillar mining methods. The threshold lies at approximately 3.5 m of thickness. Wherever mining of thick western coalbeds in the 3.5- to 5.5-m range and pitching less than 20° is being attempted, con- tinuous or conventional room-and-pillar methods seldom extract more than 2.5 m of mining height in a bottom slice; there is an irretrievable loss of more than 70 pet of the resource, entailing the hazard of spontaneous combustion. Therefore, the development of mining methods that provide a safe and efficient operation, as well as optimal recovery of the valua- ble resource of low-sulfur fuel with ac- ceptable environmental hazard, is in the national interest. In Europe and Japan, thick coalbeds are extracted by mechanized longwall mining, and extraction methods fall into the fol- lowing categories: 1. The full-face or single-pass system with extraction of the full seam thick- ness in one single lift. Such mining in a 4.5-m coalbed takes place in the east- ern portion of the Ruhr District, and mining of a 5.5-m seam is scheduled for the near future. The longwall roof is supported by shields with wide vertical ranges and operating in a one-web-back mode. The coal is extracted with double- drum shearers designed to operate in thick coal. In the United States a long- wall setup capable of extracting coal to a height of 4 m in a single pass has been operating in Wyoming since 1981. 2. The multilift system in ascending or descending order for an extraction in excess of 6 m. Ascending slicing, which requires hydraulic or pneumatic stowing of the mined-out lift, is practiced in Poland. Mining of lifts in descending order calls for preparation of an artifi- cial mat or leaving a parting for roof formation between slices and is performed in Japan, Hungary, England at Daw Mill Colliery, and the United States at Mid- Continent's Dutch Creek Mine (41) . 3. The sublevel caving or draw system in 6- to 15-m coal, which is practiced at Blanzy in southern France, at Velenje in Yugoslavia, and in Hungary. Single-Pass Mining Application of the single-pass system was addressed by the Department of Energy through a contract, "Assessment of the Single Pass Thick Seam Longwall Mining Method," awarded to Ketron, Inc., in 1979. The technical approach to this study included the type of roof support to be installed (39). Compared with the other thick-seam min- ing methods, the single-pass system has a potential of higher productivity and im- proved coal recovery. Simplified face formation without artificial roof or floor provides a faster rate of face ad- vance. Gateroad maintenance is easier. Problems arising from high liberation of methane are more controllable. Improved coal recovery diminishes the hazard of spontaneous combustion. Potential disad- vantages of the single-pass system are bulky, and costly roof supports and 29 spalling of coal from the face. Per- sonnel protection and disposal of large pieces of coal or rock must be part of the design. Figure 28 shows a two-leg shield that can be extended from 2.2 to 6m with triple-telescoping legs and hydraulic lemniscate linkage bars. Substituting hydraulic cylinders and yield control valves for lemniscate bars accomplishes limitations of lateral loads in an ef- fort to reduce the weight of the struc- ture. The shields are equipped with pushout cantilevers and face sprags. Such shields are installed in the West- falen Mine in the Ruhr District. The shields are to be used in a one- web-back two-bench mining method (fig. 29). By first cutting the top bench and advancing the roof support and then ex- tracting the bottom bench with the face sprags set against the top coal, slough- ing of the face coal can be reduced to manageable proportions. Most thick-seam longwalls in Germany are operated on the advance by either driving the gateroads ahead or profiling them with the face. Multilift Working In the United Kingdom the 7-m Warwick- shire Thick Coalbed at Daw Mill Colliery near Coventry is extracted in two slices, the top one on the advance and the bottom one on the retreat (42) (fig. 30). The top slice face is 250 m long, while the .•# FIGURE 28. - Two-leg high-seam shield. 30 '////////A ^//{/////A ^ u \^=M u u z^ Roof support Top coal cut 1st Start support advances, bottom 1 web back bench still present ////////// y///////// u u Conveyor advance '77?/, Face sprags set before cutting bottom bench FIGURE 29. - Two-bench mining method. Mudstone --^^^^r^--.^^=T^.^^ Coal Advancing face in fop secfion ' ^ Refreofing face ^ in bottonn secfion y r- Nof fo scale SEAM SECTION Z±3 PLAN VIEW n/t/// ////>/ /ii -T-in, SECTION FIGURE 30. - Muitilift syster FIGURE 31.- Chock shield. bottom slice face, to be mined later on retreat, will be shortened so that the bottom gateroads will be placed in the destressed strata within the envelope of the pressure arch generated by the top slice extraction. Currently the top slice is 3.6 m in height, and the roof is supported by chock shields, which are equipped with face sprags; the shields weigh 12.5 mt each and are installed on 1.5-m centers (fig. 31). Mining plans call for a 4.5-m extraction in the future. In the United States, Mid-Continent's Dutch Creek Mine longwall is operated on the same principle as the Warwick- shire face at Daw Mill in the United Kingdom, In 1979 the U.S. Government en- tered into a cooperative agreement with Mid-Continent Resources to demonstrate extraction of the 8.5-m Coal Basin Seam in two slices (43). Currently the top slice face extracts 3 m of coal on the advance. The 245-^-long face was the longest in the United States in 1982. There are 162 Hemscheidt 2-leg shields installed on 1.5-m centers at Dutch Creek (fig. 32). The shields extend from 2.4 to 3.6 m, and the double-acting advancing rams provide a 1-m-deep web. The two double-acting double-telescoping legs yield a load of 455 mt. The shields are equipped with pushout cantilevers that are extendable from to 1.5 m under full load. This feature, unique in shield de- sign, is intended to provide control of the roof immediately after the shearer pass under the most adverse conditions. With a friction coefficient of 0.3 be- tween rock and steel and 0.1 between steel parts, or a total friction coeffi- cient of 0.4, it takes a 1,320-kN ram to extend the cantilever under full load (41). 31 FIGURE 32. - Mid-Continent shield. The canopy extension offers the options of one-web-back or up-to-the-conveyor modes. Immediate forward support can be provided either by pushing out the canti- lever or by pulling the whole shield up to the conveyor. A face sprag can be extended from the pushout cantilever to prevent spalling of the face coal. The Mid-Continent multilift demonstra- tion takes place in an outburst-prone coalbed, and a number of shield legs were damaged when a violent coalburst occurred in the floor in 1983. Therefore, special rapid-load-shedding valves had to be sub- stituted for the yield valves that came with the shields. The sudden impact of a roof rapidly sinking with a velocity of 1.5 to 2.5 m/s releases energies that cannot be sustained by the legs. The interior hy- draulic pressure rises rapidly and far exceeds the design pressure so that the legs burst open before the ordinary yield valves can shed the load. Burst- proof props were first developed for the South African gold mines, which have been plagued by rockbursts ever since they have been in operation. In the special Mid-Continent valves, the ordinary yield valve of a roof support leg is supple- mented by a coalburst valve. If the in- terior pressure in a leg exceeds the yield pressure, Belleville springs will be compressed, causing large ports to open and release the hydraulic fluid quickly. Sublevel Caving Sublevel caving as practiced in France has not found any application in the United States or Canada. Recent reports describe four-leg roof shields developed by the French that accommodate two ex- traction and haulage systems (44) . How- ever, ground control may be problematic even in favorable strata conditions. Coal recovery may be far from complete, and the remaining coal is prone to spon- taneous combustion, requiring nitrogen infusion. THIN-SEAM LONGWALL MINING An estimated 44 Gmt, or 29 pet, of the coal reserve base to a depth of 300 m in 32 the Eastern United States falls into the 0.7- to 1.1 -m range (45), but a much smaller proportion of production (10.8 pet in 1975) comes from this range. Of- ten not recovered as the consequence of selective mining, this coal is a source that will become increasingly important. Mining thin coalbeds in sequence from top to bottom in the strata profile makes use of what resources are available and ex- tends the life of mines that age economi- cally as extraction moves away from open- ings. Another advantage of such planned mining is a better control of rock mass behavior and methane emission. At pres- ent, all active thin-seam longwalls are located in the Eastern Coal Province, and a number of operations have been sus- pended owing to the erosion of the market for metallurgical coal. The disproportion between production from and available reserves in thin seams is also typical for Europe. For in- stance, in the Federal Republic of Ger- many 50 pet of coal reserves occur in less than 1.30 m of thickness, but only 11 pet of the total production comes from this category ( 46 ) . Shields designed for thin-seam coalbeds must meet enough of the following re- quirements to provide a satisfactory tradeoff (47) : 1. They must be collapsible to 0.55 m for transportation in low entries. 2. They must have a wide hydraulic range of mining height with double- telescoping legs. 3. They must have a capacity. Note that large support a mechanical disadvantage, caused by the inclination of the legs, reduces the vertical force exerted against the roof, particularly in low mining height. 4. To control a fragile roof canopy, extensions cannot be used owing to space limitation in 1-m and less extraction. Therefore, an up-to-the-conveyor mining mode is adopted to minimize roof expo- sure. This mode is often applied in con- nection with crawl pans. 5. Crawl pans should not be narrower than 0.6 m so that injured persons on stretchers can quickly be moved out of the face area. A travelway width of 0.60 m is statutory in the Federal Republic of Germany. 6. To follow floor variation, the bases often are equipped with pendulum- type skids. 7. A divided base through which de- bris accumulated in the travelway can be passed into the gob is desirable. Several types of shields were de- signed to meet these desirable objec- tives , including — 1. Two-leg shields. 2. Four- and three-leg shields with legs arranged in an X-fashion with one or two legs supporting the gob shield and thus stabilizing the joint between gob shield and canopy (fig. 33). 3. Six-leg shields to combine a safe and convenient travelway with a mini- mum of propf ree front (fig. 34) . These shields are used on shearer faces, while the other two categories often operate in connection with plows (figs. 35-36). CONCLUSIONS AND SUMMARY Shields were introduced to U.S. long- wall mining in 1975, and in only 9 years their innovative development has greatly advanced technology. A more favorable accident experience has been a welcome additional benefit. Mine operators who introduce longwall mining must select roof supports to op- timize performance under site-specific strata conditions and mine design re- quirements. Their decision making will be assisted by the concepts of load 33 •V;.4=^l.,l FIGURE 33. - X-type shield. FIGURE 34.- Six-leg shield. 34 FIGURE 35. - Plow face schematic. FIGURE 36. - Plow face. 35 prediction, factors contributing to ground control, and related technology described in this report. Information presented herein on recent developments in advancing roof support technology should assist in the development of spec- ifications to be issued to prospective bidders; such specifications address de- sign of roof support components, hydrau- lic capabilities, and related technology and safety factors. Properties of a two- leg shield that was selected through such selection criteria in 1981 are listed in appendix B, Currently, most roof support structures come from the United Kingdom and the Federal Republic of Germany; appendix C lists only one domestic producer. How- ever, a trend to improve hydraulic com- ponents and fabricate and assemble the structures here in the United States is noticeable. The high cost of shipping entire units from Europe, including in- surance and custom duties that amount to 10 to 15 pet of the unit price, is a fac- tor in favor of domestic assembly. REFERENCES 1. Kundel, H. Die Strebtechnik in Deutschen Steinkohlenbergbau in Jahre 1983 (Longwall Face Technology in the German Coal Mining Industry in 1983) . Gluckauf, V. 120, No. 11, 1984, pp. 669- 685. 2. Gross, M. A. 1977 Census of Long- wall Installations, Off the Wall, Huwood-Irwin, Aug, 1978, 6 pp. 3. . (Dep, Energy). Private communication, 1980; available upon re- quest from E. A. Curth, BuMines , Pitts- burgh, PA. 4. . Census of Longwall Instal- lations. Coal Age, v. 85, Dec. 1980, pp. 89-101. 5. Sprouls , M. Longwall Census '82. Coal Min. and Proc. , v. 19, Dec. 1982, pp. 43-47, 50-53, 56-69. 6. . Longwall Census '83. Coal Min. and Proc, v. 20, Dec, 1983, pp,'49- 51, 7, Lewis, S., and L. R. Stace. Strata Substitution and Reinforcement Techniques in the United Kingdom. Paper in Pro- ceedings of the Seventh International Strata Conference (INIEX, Liege, Belgium, Sept. 1982). INIEX, Liege, Belgium, 1982, 18 pp. 8. Lawrence, R, G. , and R. King. Demonstration of Shield-Type Longwall Supports at York Canyon Mine of Kaiser Steel Corporation (U.S. Dep. Energy con- tract DE-ACO 1-7 4ET 12530, Kaiser Steel Corp.). Apr, 1980, 193 pp,; NTIS DOE/ET/ 12530-Tl, 9. Curth, E. Safety Aspects of Long- wall Mining in the Illinois Coal Basin. BuMines IC 8776, 1978, 37 pp. 10, Moebs , N, , and E, Curth, Geologic and Ground Control Aspects of an Experi- mental Shortwall Operation in the Up- per Ohio Valley, BuMines RI 8112, 1976, 30 pp, 11, Barry, A, J,, 0. B, Nair, and J, S, Miller, Specifications for Se- lected Hydraulic-Powered Roof Supports With a Method To Estimate Support Re- quirements for Longwalls. BuMines IC 8424, 1969, 15 pp, 12, Wilson, A, H, Support Load Re- quirements on Longwall Faces, Min, Eng, (London), v, 134, June 1975, pp. 470- 488. 13, Peacock, A. Design of Shield Sup- ports for the U.S. Mining Industry. Pa- per in Proceedings of the First Annual Conference on Ground Control in Mining (WV Univ., Morgantown, WV, July 27-29, 1981). WVUniv., Morgantown, WV, 1981, pp. 174-185. 36 14. Wade, L. V. Longwall Support Load Prediction From Geologic Information, Pres. at Soc. Min, Eng. AIME Fall Meet- ing, Denver, CO, Sept. 1-3, 1976. Soc. Min. Eng. AIME Preprint 76-1-308, 14 pp. 15. Jacobi, 0. Praxis der Gebirgsbe- herrschung (Practice of Ground Control). Gluckauf, Essen, 1976, 494 pp. 16. Gluckauf. Richtlinien Fur die Bauartzulassung von Schreitausbau (Guide- lines for the Approval of Powered Roof Support). V. 113, No. 29, 1977, p. 1015. 17. Stokes, H. Modern Powered Sup- ports. Min. Eng. (London), v. 143, Aug. 1983, pp. 51-58. 18. Herwig, H. Die Wirkung des Geb- irgsdrucks auf den Hangendzustand im Streb (The Effect of Ground Stresses on Roof Conditions on a Longwall). Gluck- auf, V. 117, No. 21, 1981, pp. 1419- 1423. 19. Curth, E. Coal Mining Techniques in the Federal Republic of Germany — 1971. BuMines IC 8645, 1974, 52 pp. 20. Everling, C. , and A. Meyer. Ein Gebirgsdruck-Rechenmodell als Planung- shilfe (A Mathematical Model for Ground Stress Distribution). Gluckauf Res. Rep,, V. 33, 1972, pp. 81-88. 21. Janes, J. A Demonstration of Longwall Mining (contract J0333949, Old Ben Coal Co.). BuMines OFR 86(2)-85, Nov. 1983, 105 pp. 22. Ratz, B. W, Proceedings of Long- wall Conference for Illinois Operations, West City, IL, 1979, 18 pp.; available upon request from E. A. Curth, BuMines, Pittsburgh, PA. 23. . Wege zum Verbessern der Hangendbeherrschung im Gewinnungsf eld (Methods of Improving Roof Control on the Longwall Face), Gluckauf, v, 120, No, 3, 1984, pp, 128-132. 24. Brezovec, D. Martinka Avoids Headgate Dust, Coal Age, v, 85, Dec, 1980, pp, 104-110, 25. Buschmann, H, E, Elektrohydrau- lische Ausbausteuerungen bei der Bergbau AG, Niederrhein (Electrohydraulic Roof Support Controls at Niederrhein Mining Co,), Gluckauf, v, 120, No, 3, 1984, pp, 135-140, 26, Irresberger, H, Schildausbau ein- facher, leichter? (Shield Support Sim- pler, Lighter?) Gluckauf, v, 118, No. 18, 1982, pp. 927-933. 27. Hahn, L. Stand und Kunftige Ent- wicklung des Strebausbaus (Status and Fu- ture Development of Longwall Roof Sup- port) . Gluckauf, V. 120, No. 3, 1984, pp. 157-162. 28. Ratz, B. W. Die Weiterentwicklung des Schildausbaus in den 80er Jahren (Re- cent Development of Shield Support in the Eighties). Gluckauf, v. 119, No, 19, 1983, pp, 925-929, 29, Richardson, F, J, Application of CADD in the Mining Industry, Min, Eng, (London), v, 143, Dec. 1983, pp. 303-308. 30, Allen, A, D, Modern Roof Support and the Effect of the Mining Department Instruction on the Use of Powered Roof Supports on Longwall Faces, Min, Tech- nol,, V, 65, Aug, 1983, pp, 309-312, 31, Herms , W, Baumusterprufungen des Grubenausbaus im Zulassungsverfahren (Prototype Testing of Roof Support for Approval), Gluckauf, v. 120, No. 2, 1984, pp. 84-90. 32. Barczak, T. , and C. Goode. Con- siderations in the Design of Longwall Mining Systems. Ch. in State-of-the-Art of Ground Control and Mine Subsidence, ed. by Y, Chugh and M, Karmis, Soc, Min, Eng, AIME, 1982, pp, 39-50, 33, Carson, R, , P, Yavorsky, T. Barc- zak, and Fuad Maayeh, State-of-the-Art Testing of Powered Roof Support, Paper in Proceedings of the Second Conference on Ground Control in Mining (WV Univ, , Morgantown, WV, July 19-21, 1982), WV Univ,, Morgantown, WV, 1982, pp, 64-69. 34. Barczak, T. , and R. Carson. Tech- nique To Measure Resultant Load Vector on Shield Supports. Ch. in Rock Mechanics in Productivity and Protection, ed. by C. Dowding and M. Singh (Proc. 25th Symp. on Rock Mechanics, Northwestern Univ., Evanston, IL, June 25-27, 1984). Soc. Min. Eng. AIME, 1984, pp. 667-679. 35. Fussel, W. , and F, Portge, Beh- errschung des Ausgasung durch Wetter- technische Zuschnitt des Abbaubetriebes (Methane Control by Planning the Face 37 Ventilation), Gluckauf , v. 112, No. 20, 1976, pp. 1172-1174. 36. Cavinder, M. Longwall Results in the Illinois Coal Basin. Min, Congr. J, , V. 68, Mar. 1982, pp. 37-40. 37. Reynolds, J. First North American Longwall in Pitching Seams Proven Feasi- ble. Min. Eng. (Littleton, CO), v. 35, Dec. 1983, pp. 1615-1618. 38. Wisecarver, D. , and J. Greenlee. Steep Seam Longwall. Ch, in Longwall- Shortwall Mining, State-of-the-Art , ed. by R. V. Ramani. Soc. Min. Eng. AIME, 1981, pp. 211-215. 39. Scheidat, L. Erfahrung rait Schil- dausbau in der geneigten Lagerung auf Erin (Experience With Shield Supports in Steeply Pitching Strata at Erin Mine) . Gluckauf, V. 120, No. 3, 1984, pp. 147- 149. 40. Adam, R. , and W. Douglas. Assess- ment of the Single Pass Thick Seam Long- wall Mining Method (U.S. Dep. Energy con- tract DE-AC01-79ET14246, Ketron, Inc.). 1982, 192 pp.; NTIS DE 8300 1401. 41. Goode, C, J. Jaspal, and T. Barc- zak. Support Selection for the Multilift Mining Method. Paper in Proceedings of the First Annual Conference on Ground Control in Mining (WV Univ. , Morgantown, WV, July 27-29, 1981). WV Univ., Morgan- town, WV, 1981, pp. 186-200. 42. Drake, D. , and A. McCarthy. Re- view of Ten Feet Extraction at Daw Mill Colliery. Inst. Min. Eng,, Mar. 1978, 42 pp.; available upon request from E. A. Curth, BuMines, Pittsburgh, PA. 43. Bourquin, B. J., and J. S. Jaspal. Mid-Continent Has Early Success With the Longest Longwall Face Ever Operated in the U.S. Min. Eng. (Littleton, CO), v. 36, Jan. 1984, pp. 48-52. 44. Benech, M. Ein Hochleistungsstreb mit Abziehen der Hangendkohle in der Re- viergesellschaf t Blanzy (A Highly Produc- tive Longwall Face Extracting Coal by the Sublevel Method in the Blanzy Mining Dis- trict) . Gluckauf, V. 118, No. 13, 1982, pp. 646-649. 45. U.S. Bureau of Mines. The Reserve Base of Bituminous Coal and Anthracite for Underground Mining in the Eastern United States. BuMines IC 8655, 1974, 428 pp. 46. Bergmann, M. , and H. Kundel. Die Tatigkeit des Arbeitskreises "Geringmach- tige Floze" in den Jahren 1978 bis 1982 (Activity of the Task Force on Thin Seams During 1978-82). Gluckauf, v. 119, No. 6, 1983, pp. 287-291. 47. Curth, E. Longwall Mining of Thin Seams. Paper in Proceedings of the First Annual Conference of Ground Control in Mining (WV Univ., Morgantown, WV , July 27-29, 1981). WV Univ., Morgantown, WV, 1981, pp. 239-259. 38 APPENDIX A. — NATIONAL COAL BOARD MINING DEPARTMENT INSTRUCTION PI/ 1982/ 6: THE USE OF POWERED SUPPORTS ON LONQJALL FACES 1. To ensure the effectiveness of sup- ports on longwall faces, this instruction and the associated Notes of Guidance lay down criteria to be taken into consid- eration when the coal face is being designed, 2. Where powered supports approved for the purpose of Regulation 16(2) of the Coal and Other Mines (Support) Regula- tions 1966 are intended for use on long- wall faces planned to commence operation after 1 January 1983 systematic support systems shall be designed and provided in accordance with the standards laid down in this instruction, 3. For the purpose of this instruction the face working shall comprise four zones of operation, namely: (a) the face line zone. (b) the buttress zone. (c) the pack zone. (d) the roadhead zone. 4. Every face design shall separately specify for each of the zones the de- signed setting resistance and the de- signed yield resistance to be offered by the powered support system which shall not be less than the values given in the following table: Zone Resistance, mt/m^ Face Buttress. Pack Roadhead. 15.0 H 15.0 H 10.0 H 10.0 H H = designed extracted height, m. ^Or 7.5 mt/m2 , whichever is greater. ^Or 15.0 mt/m , whichever is greater. 5. The designed distance between the centres of adjacent supports shall be stated and shall not normally exceed 1.5 m. Provided, however, that where it is necessary to set props and bars between adjacent powered supports this dis- tance may be increased to accommodate them. 6. In the roadhead zone where the pow- ered supports have been designed for that zone the distance between adjacent pow- ered supports shall not exceed that laid down by the manufacturer, 7. Every face design shall specify the hydraulic supply system to the powered supports to achieve the designed setting resistance, 8. Where the designed extracted height is less than 2,5 m the designed distance between the tip of the powered support roof beam and the coal face before normal cutting shall not exceed 0,4 m. Excep- tionally, and for the time being, where compliance with this requirement is im- practicable, the designed distance be- tween the tip of the powered support roof beam and the coal face may be increased by a distance not exceeding 0,1 m. 9. Where the designed extracted height is 2,5 m or greater, the designed dis- tance between the tip of the powered sup- port roof beam and the coal face before normal cutting shall not exceed 0,5 m. 10. Powered forepoles shall be provided on all powered supports, unless they are of an immediate forward support design, which are intended for use: (a) where the designed extracted height is 2.5 m or greater; or (b) where the designed depth of web exceeds 0.8 m. The forepoles shall be extended systemat- ically to provide support behind the coal getting machine so that the extended tip of each is not more than 0.1 m behind the centre of the roof. 11. On all powered supports intended for use where the designed clearance between the top race of the armoured face con- veyor and the roof at the place exceeds 2.3 m powered face sprags shall be pro- vided, and the Manager's Support Rules shall specify the system to be adopted for their use. 39 12. Where an immediate forward support system is used, the supports shall be ad- vanced as close as practicable behind the coal getting machine and normally this distance shall not exceed 10.0 m. 13. The Manager's Support Rules shall include provision for the systematic sup- port of that part of every face where maintenance on coal getting machines or other work is required to be carried out in advance of the powered supports. 14. Relaxation from this instruction or any part thereof may be granted only by the Director-General of Mining. 15. Those to whom this instruction is distributed are reminded that it is their responsibility to bring its provisions to the notice of any members of their staff, not included in the distribution list, who are concerned with complying with this instruction and Notes of Guidance or taking action on it. 40 APPENDIX B. — SPECIFICATIONS FOR THYSSEN RHS 12/30 SHIELD Roof load characteristics: Leg capacity (LC) kN.. 4,780 Vertical roof load at yield (VRL) , kN; Minimum at 1.2-m height 3,647 Maximum at 2.6-^ height 4,697 Roof load efficiency (VRL/LC) , pet: Minimum 76 Maximum 98 Mean load density at yield, 1 web back before cut: Web cm. . 76 Minimum roof area at 1.2-m height , m^,, 5.14 Mean load density .kPa. . 709 Maximum roof area at 2.6-m height m^,, 5.20 Mean load density kPa. . 903 Base (divided type) : Bearing length m. . 2.30 Overall length m. . 2.40 Width of each skid m. . 0.55 Effective bearing area m^.. 2.31 Average floor pressure , N/cm^, . 183 Maximum floor pressure N/cm^. . 310.5 Gob shield (1 piece): Length m. . 2.29 Width m. . 1.43 Hinge arrangement Lemniscate Canopy: Length without flipper extension m, , 2.40 Length of flipper m. . 0.76 Total length m. . 3. 16 Width m. . 1.43 Ratio of fore part to rear part, related to resultant location 2.2 Span of canopy tip to coal face m,, 0.30 Flipper: Angle of uplift deg. . +20 Tip load when horizontal kN. . 71 Cylinders 2 Force of 2 cylinders at 37 MPa kN.. 351 Canopy cylinder: Number 1 Set: Piston at 37 MPa kN. . 651 Retract: Rod at 37 MPa kN.. 363 Yield: Piston at 45 MPa kN. . 792 Advancing cylinder (reverse linkage) : Force, kN: Piston (shield pull) 351 Rod (conveyor push) 163 Effective stroke m. . 0.76 Leg (double-telescoping, double-action) : Length, m: Closed 1.14 Extended 2.76 41 Leg — Cont inued Piston area, cm^ : 1st stage 531 2d stage 531 Operating pressure in both stages: Set MPa. . 37 Yield MPa. . 45 Set-yield ratio pet. . 82 Force, kN: Set 1,966 Yield 2, 390 Surface finish ym. . 4 Plating material Nickel Plating thickness ytn. . 30-50 Steel quality of structural components (canopy, gob shield, base, lemniscate) , MPa: Links 345 Legs 490 Pins alloy : 50 VCrVi^ 980 Structure Box welded Side sealing (1 side, plates on canopy and gob shield): Springs 4 Cylinders 4 Force of 1 cylinder: push kN.. 120 Travelway width, m: At base 0.55 1.14 m above base 0.60 Hydraulics: Diameter, mm: Pressure line 25 Re turn 1 ine 31 Fluid (oil-in-water) pet, . 2-5 Operating pressure MPa. . 37 Adjacent control Full flow valve Steck-o coupling General information: Weight mt . . 13 Range: Closed , m. . 1.2 Extended , m. . 3 Overall length, flipper retracted m. . 4.65 Overall width m. . 1.43 Width with side plates extended m.. 1.68 Automatic contact advance under 9.6-kPa pressure against the roof, water sprays, pressure indicator on each leg, mounting plates for light fixtures. 42 APPENDIC C. — MANUFACTURERS Home Office Babcock Roof Supports Ltd. Aidan House, Tynegate Precinct Sunderland Rd, , Gateshead Tyne & Wear NE8 3HY, United Kingdom U.S. Representation Huwood-Irwin Co, P.O. Box 409 Irwin, PA 15642 Tel. (412) 863-5000 Bochumer Eisenhutte-Heintzmann GmbH POB 101029 D-4630 Bochum 1 Federal Republic of Germany Dowty Mining Equipment Ltd, Ashchurch Gloucestershire GL20 8JR, United Kingdom Heintzmann Corp, P.O. Box 1027 Lebanon, VA 24266 Tel. (703) 889-5533 Dowty Corp. 177 Thorn Hill Rd. Thorn Hill Industrial Park Warrendale, PA 15086 Tel. (412) 776-3693 Gullick Dobs on Ltd. P.O. Box 12 Ince , Wigan Greater Manchester WNI 3DD United Kingdom Hemscheidt Maschinenf abrik Bornberg 97-103 D-5600 Wuppertal 1 Federal Republic of Germany Joy Manufacturing Co. Oliver Bldg. Pittsburgh, PA 15222, U.S.A. Klockner-Becorit PF 209 D-4350 Recklinghausen Federal Republic of Germany Gfullick Dobson, Inc. 603 Parkway View Dr. Pittsburgh, PA 15205 Tel. (412) 787-5852 Heimscheidt America Corp, 252 Parkwest Dr. P.O. Box 500 Pittsburgh, PA 15230 Tel. (412) 787-7130 U.S. producer Kloeckner-Becorit North America, Inc. 790 Manor Oak Two 1910 Cochran Rd. Pittsburgh, PA 15220 Tel. (412) 344-8200 Machinoexport 35, Mosf ilmovskaya ul. 117330 Moscow, U.S.S.R. None Marrel Mines - Bennes Marrel S.A. ZI St-Etienne Boutheon BP 56-42160 Andrezieux Boutheon, France None 43 Home Office Material de Fond et d' Industrie rue de Chsmps de Mars 57202, Sarreguemlnes , France Mitsui Mllke Machine Co. Ltd. 2, Asahl-Machl, Omuta-Clty Fukuoka, Japan Thyssen Bergbautechnlk PF 281144 D-4100 2 Dulsburg Federal Republic of Germany Westfalla Lunen D-4670 Lunen Federal Republic of Germany U.S. Representation None Long Alrdox Co. Robinson Plaza III, Suite 320 Rte. 60, Robinson Township Pittsburgh, PA 15205 Tel. (412) 787-8292 Thyssen Mining Equipment Dlv. of Thyssen, Inc. Stanley Bldg. , Suite 302 Marlon, IL 62959 Tel. (618) 997-5328 Mining Progress, Inc. 300 Boulevard Tower Charleston, WV 25301 Tel, (304) 343-5593 U.S. Department of the Interior Bureau of Mines— Prod, and Distr. Cochrans Mill Road P.O. Box 18070 Pittsburgh. Pa. 15236 AN EQUAL OPPORTUNITY EMPLOYER OFFICIAL BUSINESS PENALTY FOR PRIVATE USE. S300 I I Do not wish to receive this material, please remove from your mailing list* I I Address chonge* Please correct as indicated* H 153 86 .^'\ A^ -^ "WW* J> % 'Mis* ^y -^ .•iq. ^0 - ' ° <>■'->' A<''' V^^'v^ V''-^^%c,^ \-^^^^.^'^ ^< ■. .^^°- ■ " *^°- .^^ ^ '::^' A ^'^ .V v*'--'^°' ^^^^^:^'\^^'' "V^^^^'Z "V*3^\/ -o '^.'%o'^ ^^^^^-^-y'' » cSf^Wv.vtv'^.. O 5>,;*r^'\/ -o^^^^^o'^ V^^V V'^'^^^' V---^^^ ^. Co. ^<=.-' ''? a"^ '. o ^^°<. 0^ 0°"". -^o .^^ •^' ■.<> 0# %^ ..^^^.- A »V.-. % .0*".-.. -^o •• -^ -^^r^-^ oV^^^u^" ^^^^^ '-'^m^^\ ^^^s '^0^ 'bv ^^-o;-o'\/ %'^'Uo^ \^^\/ "o^^^-^^^o^ \'^^-\/ ^c.''??^"^\o^ '^^ .:2^ -'/l^^ ' '-^^ ^4 ^oV" HECKMAN BINDERY INC. €^JUN 86 W N. MANCHESTER, ^ INDIANA 46962 * .^^ J* (- ,