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BUREAU OF MINES 
 INFORMATION CIRCULAR/1989 
 
 J<0 
 
 Practical Considerations in Longwall 
 Face and Gate Road Support 
 Selection and Utilization 
 
 By Thomas M. Barczak, David E. Schwemmer, 
 and Carol L. Tasillo 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Information Circular 9217 
 
 Practical Considerations in Longwall 
 Face and Gate Road Support 
 Selection and Utilization 
 
 By Thomas M. Barczak, David E. Schwemmer, 
 and Carol L. Tasillo 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Manuel J. Lujan, Jr., Secretary 
 
 BUREAU OF MINES 
 T S Ary, Director 
 
TVla5)s 
 no.12J7 
 
 Library of Congress Cataloging in Publication Data: 
 
 Barczak, Thomas M. 
 
 Practical considerations in Iongwall face and gate road support selection and 
 utilization. 
 
 (Bureau of Mines information circular; 9217) 
 
 Bibliography: p. 22. 
 
 Supt. of Docs. 4io.: I 28.27:9217. 
 
 1. Mine roof control. 2. Longwall mining— Safety measures. I. Schwemmer, 
 David E. II. Tasillo, Carol L. III. Title. IV. Series: Information circular (United 
 States. Bureau of Mines); 9217. 
 
 TN295.U4 
 
 [TN288] 
 
 622 s [622'.334] 
 
 88-600440 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Research perspective 2 
 
 Development of practical considerations 4 
 
 Shield support considerations 5 
 
 Support selection and design considerations 5 
 
 Shield type 5 
 
 Component constructions 5 
 
 Support capacity 8 
 
 Performance acceptance testing 8 
 
 Active versus passive load application 9 
 
 Static versus dynamic testing 10 
 
 Critical load evaluations 10 
 
 Operational considerations 12 
 
 In situ performance monitoring 13 
 
 Crib support considerations 14 
 
 Support selection and application 14 
 
 Construction 16 
 
 Failure observations 18 
 
 Laboratory testing and analysis 19 
 
 Conclusions 21 
 
 Bibliography 22 
 
 ILLUSTRATIONS 
 
 1. Mine roof support optimization 3 
 
 2. Organization scheme for shield configurations 4 
 
 3. Organization scheme for gate road supports 5 
 
 4. Location of support resultant resistance 6 
 
 5. Comparison of solid base and split base shield designs 7 
 
 6. Split caving shield design 7 
 
 7. 2:1 canopy ratio 7 
 
 8. Total load determinations on shield supports 8 
 
 9. Shield leg mechanics considerations 9 
 
 10. Friction effects on performance testing of shield supports 10 
 
 11. Most critical contact configuration for two-leg shield supports 10 
 
 12. Critical load conditions for shield supports 11 
 
 13. Direction of displacement loading for shield performance tests 11 
 
 14. Friction-free shield tests 11 
 
 15. Horizontal shield constraint 12 
 
 16. Shield-setting pressure considerations 12 
 
 17. Setting shield with tip raised 13 
 
 18. Advancing support under partial contact with roof strata 13 
 
 19. Measurement of shield loading 14 
 
 20. Wood and concrete crib strength and stiffness 15 
 
 21. Wood crib subjected to horizontal displacement 15 
 
 22. Wood crib stiffness as a function of crib height 16 
 
 23. Construction of wood cribs with overhanging ends 16 
 
 24. Unstable parallelogram wood crib configuration 16 
 
 25. Improving wood crib strength by adding additional blocks per layer 17 
 
ILLUSTRATIONS-Continued 
 
 Page 
 
 26. Instability in wood crib resulting from weak crib member 17 
 
 27. Composite wood-concrete crib structures 18 
 
 28. Load transfer disks used in concrete crib constructions 18 
 
 29. Typical failure in concrete crib support 19 
 
 30. Explosive failure of concrete crib support constructed with load transfer disks 20 
 
 31. Convergence rate effect on wood crib support resistance 20 
 
 32. End effect considerations in crib testing 20 
 
 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 
 
 ft 
 
 foot kip/in 2 
 
 kip per square inch 
 
 in 
 
 inch pet 
 
 percent 
 
 in/min 
 
 inch per minute psi 
 
 pounds per square inch 
 
 kip/in 
 
 kip per inch 
 
 
PRACTICAL CONSIDERATIONS IN LONGWALL FACE AND 
 GATE ROAD SUPPORT SELECTION AND UTILIZATION 
 
 By Thomas M. Barczak, 1 David E. Schwemmer, 2 and Carol L. Tasillo 3 
 
 ABSTRACT 
 
 The U.S. Bureau of Mines has been conducting research to optimize the design and utilization of mine 
 roof support systems. An objective of these efforts is to evaluate the mechanical and structural responses 
 of various mine roof support systems under simulated load conditions in the Bureau's mine roof 
 simulator. Underground studies are also made to evaluate the in situ behavior of support structures. 
 The purpose of this report is to document practical applications for longwall face and gate road supports 
 that have resulted from these studies. This report is not intended to be an all-inclusive manual on every 
 aspect of support utilization, but it does provide a comprehensive assessment of practical considerations 
 relating to the mechanical and structural behavior of these support systems. Forty-six recommendations 
 are made to provide assistance to mine operators in the testing, selection, and utilization of longwall face 
 and gate road supports. Many of these recommendations offer innovative solutions to everyday problems 
 faced by mining personnel in the control of ground in longwall mining. 
 
 'Research physicist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
 Structural engineer, Boeing Services International, Pittsburgh, PA. 
 3 Senior project engineer, Boeing Services International. 
 
INTRODUCTION 
 
 The Bureau of Mines has been conducting research to 
 optimize the design and utilization of underground mine 
 roof support systems. Much of this research was con- 
 ducted in the Bureau's mine roof simulator and was in- 
 tended to develop fundamental relationships concerning 
 the mechanics and structural responses of various mine 
 roof support systems under simulated load conditions. 
 Several publications are available that extensively docu- 
 ment these studies (see "Bibliography" section). These 
 publications describe the parameters under consideration 
 and test results for each area of investigation. Each report 
 contributes to understanding the engineering mechanics 
 involved in roof support systems. The reports contain 
 detailed theoretical and analytical analyses of support be- 
 havior and do not concentrate on practical applications. 
 The purpose of this report is to document the practical 
 applications that have resulted from these various studies. 
 
 The scope of this report is limited to longwall face and 
 gate road supports, specifically shields and various forms 
 of wood and concrete cribbing. References are made to 
 pillar considerations, but this report is concerned primarily 
 with artificial supports. It is not intended to be an all- 
 inclusive manual on every aspect of support utilization, but 
 
 represents a fairly comprehensive assessment of the prac- 
 tical considerations relating to mechanical and structural 
 behavior of these support systems. Concise recommenda- 
 tions are made for testing, selection, application, construc- 
 tion, and operation of these supports. 
 
 Proper ground control is essential to the safety of 
 underground coal mining. Significant improvements have 
 been made in longwall ground control during the past de- 
 cade, but ground control problems still persist and remain 
 a limiting factor in longwall mining productivity. Many 
 articles and a few books 4 have been written to assist coal 
 operators in ground control techniques. Much is to be 
 learned from these publications and from the experience 
 gained from mining personnel as they labor to achieve 
 success. This report is not intended to be a reiteration of 
 this literature or a synopsis of operator experiences. 
 Rather, it is intended to advance the state-of-the-art of 
 longwall ground control by contributing new information 
 as an outgrowth of the Bureau's research in support be- 
 havior and strata interaction studies. However, to main- 
 tain a comprehensive discussion on the subject of longwall 
 face and gate road support considerations, some informa- 
 tion of common knowledge is included. 
 
 RESEARCH PERSPECTIVE 
 
 Before discussing practical considerations, a brief over- 
 view of the Bureau's research program is presented to pro- 
 vide some background for the practical considerations de- 
 veloped in this report. Figure 1 shows the overall technical 
 approach pursued in the mine roof support optimization 
 research. This research combines studies of support 
 mechanics with rock mechanics investigations to ensure 
 optimum compatibility when considering the interaction of 
 the support structure with the surrounding strata in re- 
 sponse to various loading conditions. As the diagram in 
 figure 1 implies, ground control considerations must in- 
 volve evaluations of both the support structure and the 
 geological environment. 
 
 The research to date has primarily been in the area of 
 support mechanics, using the Bureau's mine roof simulator 
 to simulate controlled loading of wood and concrete crib- 
 bing and longwall shields. Areas of evaluation for shield 
 supports included stability, stiffness characteristics, load 
 transfer mechanics, and critical load considerations. 
 Parameters under investigation were type of shield (two- 
 leg versus four-leg), height, setting pressure, contact 
 
 configuration, direction of applied displacement (vertical 
 versus horizontal), and horizontal constraint. Areas of 
 evaluation for gate road crib support structures included 
 stability, load-displacement relationships, failure mechan- 
 ics, material considerations, and geometric considerations. 
 Study parameters for wood cribs included load rate, con- 
 figuration (contact area), height, wood type, moisture con- 
 tent, and horizontal displacement. Compressive strength, 
 configuration (solid versus open), square versus round ge- 
 ometry, and height were evaluated for concrete cribs. The 
 type, amount, and location of wood in composite wood- 
 concrete structures was also investigated in these studies. 
 This research represents a 5-year effort initiated in 
 1983. Future research will investigate other parameters 
 that affect support response, such as gob loading on shield 
 supports and interaction effects in multiple crib support 
 utilization. In addition, technologies developed in the lab- 
 oratory will be employed and evaluated underground. 
 
 4 Peng, S. S. Coal Mine Ground Control. Wiley, 1978, 450 pp. 
 Peng, S. S., and H. S. Chiang. Longwall Mining. Wiley, 1984, 708 pp. 
 
I 
 
 Support 
 mechanics 
 
 I 
 
 I I 
 
 I Rock | 
 
 i mechanics \ 
 
 &■ j 
 
 • Load transfer 
 
 • Stability 
 
 • Structural integrity 
 
 • Weight of 
 unstable strata 
 
 • Convergence 
 
 !• 1 
 
 Interaction | 
 | support and strata g 
 
 Vtet*z^.4S^^a*,te*^^t^ 
 
 Load conditions 
 Support response 
 
 Support 
 selection 
 
 ^r^WI^WW»«tff.lWW4 l J'WI^ 
 
 Optimization 
 
 AiVttiaXAMSiiiV&XfwAKftili 
 
 'M&KWW&teWW T WMmq 
 
 Optimum 
 
 compatibility I 
 
 Support 
 design 
 
 Maximize \ 
 
 efficiency | 
 
 5 Engineer support to geological conditions J 
 
 Figure 1.— Mine roof support optimization. 
 
DEVELOPMENT OF PRACTICAL CONSIDERATIONS 
 
 From the research perspective presented in the previous 
 section, it is seen that there are several parameters in- 
 volved in mechanical and structural analyses of roof sup- 
 port behavior. To relate these to practical considerations 
 of ground control in longwall mining, a scheme is devel- 
 oped to organize this information in specific areas of prac- 
 tical interest. Figure 2 depicts an organization scheme for 
 shield considerations and figure 3 reveals a similar organ- 
 ization for gate road crib supports. 
 
 Practical considerations for shield supports are 
 discussed in terms of (1) support selection and design con- 
 siderations, (2) performance acceptance testing, (3) opera- 
 tional considerations, and (4) in situ performance monitor- 
 ing. Topics of discussion for each of these areas are 
 shown in figure 2 and include such things as type of shield, 
 component constructions, setting pressure recom- 
 mendations, methods of testing, and failure assessments. 
 Because crib supports are passive supports, operational 
 
 considerations are not an issue. However, as passive sup- 
 ports, cribs are less adaptive than shields to changing load 
 conditions, and failure is more an area of concern for crib 
 supports. Hence, selection of the proper crib support for 
 specific conditions is critical. Practical considerations for 
 crib supports are discussed in terms of (1) support selec- 
 tion and application, (2) construction, (3) failure observa- 
 tions, and (4) laboratory testing and analysis. 
 
 The format for presenting practical considerations is to 
 identify concise and definite recommendations for applica- 
 tion or evaluation of longwall face and gate road support 
 systems in the control of ground in longwall mining. 
 Accompanying most recommendations is an illustration to 
 graphically describe the issue under consideration. Follow- 
 ing each recommendation is a brief technical explanation 
 to support the recommendation. The 46 recommendations 
 are numbered consecutively through the report. 
 
 Shield support consideration 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Sele< 
 
 ar 
 
 des 
 
 ;tion 
 
 id 
 
 ign 
 
 
 Perfor 
 test 
 
 mance 
 ing 
 
 
 Oper 
 
 ation 
 
 
 In situ monitoring 
 
 • Type of 
 
 shield 
 
 
 • Active 
 
 versus 
 
 
 • Setting 
 
 pressure 
 
 
 • Shield loading 
 
 
 
 passive testing 
 
 
 
 
 
 • Component 
 
 
 • Static versus 
 
 
 • Setting procedures 
 
 
 • Structural failures 
 
 constructions 
 
 
 dynamic testing 
 
 
 
 
 
 • Support capacity 
 
 
 • Critical load 
 evaluations 
 
 
 • Advance procedures 
 
 
 • Analysis methods 
 
 
 
 
 
 
 • Yield pressure 
 
 
 
 Figure 2.— Organization scheme for shield configurations. 
 
Crib support consideration 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Selection 
 
 and 
 
 
 Construction 
 
 
 Failure 
 
 
 Laboratory 
 
 applic 
 
 ation 
 
 
 
 1 — -J 
 
 
 observations 
 
 
 tesl 
 
 Mng 
 
 • Concrete 
 wood 
 
 versus 
 crib 
 
 
 • Concrete 
 wood 
 
 ' versus 
 
 
 • Expected failure 
 modes 
 
 
 • Size effects 
 
 • Strata conditions 
 
 
 • Geometries 
 
 
 • Signs of failure 
 
 
 • Rate effects 
 
 • Seam height 
 
 
 • Stiffness 
 
 
 • Potential danger 
 
 
 • Machine stiffness 
 
 • Convergence 
 
 
 • Strength 
 
 
 
 
 
 • Sustained loading 
 
 Figure 3.— Organization scheme for gate road supports. 
 
 SHIELD SUPPORT CONSIDERATIONS 
 
 SUPPORT SELECTION AND 
 DESIGN CONSIDERATIONS 
 
 Topics of discussion in support selection and design 
 considerations are shield type, component constructions, 
 and support capacity considerations. This section provides 
 information on when to use what support, how to evaluate 
 required support capacity, and what to consider and avoid 
 in specific shield designs. 
 
 Shield Type 
 
 1. Use four-leg shield designs in conditions where can- 
 tilevered strata is likely and two-leg shields in less com- 
 petent strata conditions (fig. 4). 
 
 Four-leg shields have a larger capacity potential than 
 two-leg shields because of the larger number of leg cylin- 
 ders, and are therefore better able to support larger loads 
 caused by cantilevered strata. Four-leg shields also pro- 
 vide a resultant resistance force closer to the gob than 
 two-leg shields. This reduces required support capacity to 
 maintain moment equilibrium, improves support stability, 
 and may help induce breakage of the strata to alleviate the 
 
 cantilevered strata condition. In contrast, two-leg shields 
 provide a resultant resistance force closer to the face, and 
 they are more effective in controlling less stable strata 
 prone to cavity formation in the face area. In addition, 
 two-leg shields are likely to have a shorter canopy length 
 and produce less cycling of roof strata. 
 
 Component Constructions 
 
 2. Use solid base configurations only in soft bottom con- 
 ditions where bearing pressures of split base designs 
 exceed the strength of the floor strata (fig. 5). 
 
 Structurally, solid bases are more stable because they 
 provide an out-of-plane stiffness. The larger contact area 
 provided by the solid design provides a lower bearing 
 pressure on floor strata, making solid base designs more 
 suitable in soft bottom conditions. Despite these structural 
 advantages, the trend has been toward split base configura- 
 tions. Split base configurations more easily accommodate 
 floor irregularities during support advancement and have 
 sufficient flexural stiffness to adequately distribute loads 
 for most floor strata. 
 
LongwallWIj 
 
 face 
 
 
 ^mm^m^w^iw^^^^^^^m^^T^^m^ 
 
 Support response 
 
 1 
 
 Long wall tM 
 
 face 
 
 Resultant 
 force / 'F^ 
 
 Four- leg 
 shield 
 
 ; #\w?^ 
 
 #W^J^AW^^^^ 
 
 Figure 4.— Location of support resultant resistance. 
 
 3. Avoid split caving shield designs (fig. 6). 
 
 Split caving shields are utilized to permit out-of-plane 
 rotation of the canopy to enable the canopy to better 
 maintain contact with irregular roof strata. Split caving 
 shield designs reduce out-of-plane shield stiffness, and the 
 shield is less able to resist out-of-plane (lateral) loading. 
 The result of this design is often an unstable configuration. 
 Nearly all installations using the split caving shield design 
 have experienced stability problems. 
 
 4. Solid canopy designs provide the most effective roof 
 control. 
 
 Articulated canopy designs are becoming obsolete. 
 Canopy articulation generally reduces shield stability. 
 Flipper arrangements at canopy ends prevent debris from 
 falling but probably do little to control roof behavior or 
 inhibit failure. Loads generated in canopy extensions must 
 be transmitted through the canopy structure, and therefore 
 add to the strength requirements of the canopy design. 
 They should be avoided except in very friable roof 
 
 conditions where added protection is required in the un- 
 supported span between the coal face and the canopy tip. 
 
 5. The requirement of a 2:1 canopy ratio is only a rule of 
 thumb and should not be a primary design consideration 
 (fig- 7). 
 
 The 2:1 canopy ratio relates the location of the resultant 
 support resistance from the canopy tip to the distance 
 from the resultant force to the canopy-caving shield hinge. 
 It is intended to ensure an adequate active force over the 
 full length of the canopy, particularly at the canopy tip. 
 The effectiveness of the canopy to maintain adequate roof 
 pressure along its full length is significantly dependent 
 upon the flexural stiffness of the canopy structure. As the 
 flexural stiffness decreases, less tip loading is likely. 
 Another significant consideration that violates the 2:1 re- 
 quirement is the shape of the canopy. Some canopies are 
 shaped concavely upward to ensure tip loading. Canopy 
 structures that promote initial contact at the tip should not 
 be steadfastly evaluated by the 2:1 rule. 
 
Figure 5.-Comparison of solid base and split base shield designs. 
 
 Split 
 caving shield 
 
 Canopy ratio = jj- 
 
 •H-«— fa- 
 
 Figure 6.— Split caving shield design. 
 
 Figure 7.-2:1 canopy ratio. 
 
6. Be cautious of lemniscate link designs that promote 
 bending of the link structure. 
 
 Some two-leg shield supports use "curved" link designs 
 where the line of action between the pin holes is not con- 
 sistent with the centroid of the member. This is usually 
 done to provide adequate clearance at collapsed heights. 
 These designs impose eccentric loading causing potentially 
 large stresses due to bending, which must be considered in 
 the design. One support tested in the simulator reached 
 material yield in the link at much less than rated support 
 capacity. With the exception of pin friction, links need 
 only be designed for axial loads. 
 
 Support Capacity 
 
 7. Support capacity determinations should consider support 
 stiffness and face convergence (fig. 8). 
 
 Historically, the most common analytical method to 
 determine support capacities has been to evaluate caving 
 characteristics using some form of bulking factor formula- 
 tion to estimate a rock mass that must be maintained in 
 equilibrium by the support resistance. Estimation of cav- 
 ing characteristics is difficult and is generally not an accu- 
 rate means to assess support capacity. Support resistance 
 
 required to maintain equilibrium of the rock mass as eval- 
 uated in these or other methods should be used to assess 
 required setting forces and not yield loads. The total load 
 on the support will be the sum of the setting force and the 
 reactive load due to convergence, which is a function of 
 the support stiffness. Hence, support capacity determina- 
 tions should be based upon face convergence and support 
 stiffness. Hsiung proposes a methodology using equivalent 
 support-strata stiffness and face convergence to size shield 
 supports. 5 The Bureau is currently developing a similar 
 methodology using shield stiffness and face convergence 
 parameter considerations. 
 
 PERFORMANCE ACCEPTANCE TESTING 
 
 This section provides information on how to test shields 
 and evaluate their performance in the laboratory. Topics 
 for discussion include issues of active versus passive 
 testing, static versus dynamic testing, and critical load 
 evaluations. 
 
 5 Hsiung, S. M., Y. M. Jiang, and S. S. Peng. Method of Selecting 
 Suitable Types of Shield Supports at Longwall Faces. Paper in Pro- 
 ceedings of the Seventh International Conference on Ground Control 
 in Mining. WV Univ. 1988, pp. 161-168. 
 
 Setting load (S) = W 
 
 Convergence (S) 
 
 K = Shield stiffness 
 
 Convergence load (R)= K-*8 
 
 Total load (F)= Setting load (S)+ convergence load (R) 
 
 F= W + K*8 
 
 Figure 8.— Total load determinations on shield supports. 
 
Active Versus Passive Load Application 
 
 8. Performance testing on shields with double telescoping 
 leg cylinders by passive load application, where leg pres- 
 sures are used to generate shield loading by reactions 
 against a static frame, should be conducted at less than 
 full first stage leg extensions to be compatible with in 
 mine behavior and performance testing by active load 
 application (fig. 9). 
 
 Because of leg mechanics, the effective area of the leg 
 cylinder changes for leg convergence in comparison to leg 
 extension when the first stage is fully extended. Tests con- 
 ducted in the Bureau's mine roof simulator indicate that 
 leg force is reduced by 50 pet on some shields when the 
 first stage is fully extended and pressurized to produce leg 
 extension. This reduction in leg force will provide lower 
 stresses in the support structure, and will result in errone- 
 ous evaluations of the support's structural integrity. This 
 in turn may jeopardize the safety of the miners in the 
 event that critical loading was ignored by not providing the 
 maximum leg force. 
 
 9. Leg pressures should be higher than in-service yield 
 pressure for passive load application (testing in static 
 frames) to achieve equivalent results at yield pressure 
 under active load application (testing in active frames) 
 (fig. 10). 
 
 When the shield is developing reactions to applied dis- 
 placements that cause compression of the leg cylinders, 
 the friction in the leg cylinder and the caving shield-lem- 
 niscate joints acts to help resist the applied displacement 
 and therefore increases support capacity. Conversely, in a 
 static frame where the leg cylinders are pressurized there- 
 by causing leg extensions, the friction opposes the leg 
 forces and caving shield-lemniscate assembly stiffness caus- 
 ing a reduction in support resistance. Friction effects are 
 likely to be on the order of 3 to 5 pet of the total support 
 resistance, but can reach 10 pet under certain load condi- 
 tions. The friction effects can be evaluated by examination 
 of the hysteresis in load-unload responses during testing. 
 
 LEG EXTENSION 
 
 F 
 
 LEG CONVERGENCE 
 
 TTTT 
 0|A 2 
 
 Leg force (F) =O t A 2 
 
 A, 
 
 A 2 
 0~|. 2 
 
 KEY 
 Leg force 
 Trapped fluid 
 Pressure application 
 Area of 1st stage 
 Area of 2nd stage 
 Cylinder pressure 
 
 Leg force (F)=0| A t 
 
 Figure 9.— Shield leg mechanics considerations. 
 
10 
 
 Static frame testing 
 
 U I ) ) I I I / / / A 
 
 KEY 
 
 »• Leg force (L) 
 
 " Frictional force (f) 
 
 Active load application ^ — 1> Support resistance (F) 
 
 f 
 
 <h> l / I I / I / / / ) / I / / l_ 3 <| 
 
 F =L+ f 
 
 Figure 10.— Friction effects on performance testing of shield 
 supports. 
 
 Figure 11— Most critical contact configuration for two-leg 
 shield supports. 
 
 minimize costs. This practice is acceptable, but tests 
 should be prioritized with the most critical load conditions 
 evaluated last. Otherwise, the worst load case may pro- 
 duce component stresses that damage weaker components 
 and mask the effect of less critical load conditions. Test 
 results at the completion of each test series should be 
 analyzed so that effects of each load case are better 
 understood. 
 
 Static Versus Dynamic Testing 
 
 10. Static tests should be used to evaluate strength of mate- 
 rial considerations in component designs and cyclic tests 
 used to evaluate material fatigue and weld integrity. 
 
 All support structures should first be analyzed under 
 static load conditions to eliminate any load rate effects and 
 to provide an understanding of load transfer within the 
 structure. Fatigue studies are also an important considera- 
 tion and should be conducted under all load conditions 
 representative of underground conditions. Cycling should 
 be done over the full range of support loads, ideally from 
 zero to yield load. Conditions that produce a change in 
 stress from tension to compression or vice versa are gen- 
 erally more critical than conditions that maintain the same 
 stress state. Cycle rate may also have a significant influ- 
 ence on fatigue failure. Generally, lower frequency load 
 application (load rate per cycle) is more critical than 
 higher frequency load application. 
 
 11. Cumulative cycle tests should be prioritized and the 
 most critical load condition conducted last. 
 
 Generally, all tests are conducted on one support to 
 observe cumulative effects of all load conditions and to 
 
 Critical Load Evaluations 
 
 12. The most critical contact configuration for two-leg shield 
 supports is standing the support on the toe of the base 
 (fig- U). 
 
 Simply supporting the base on its toe with any canopy 
 contact configuration causes maximum loading in the base 
 member and lemniscate links. In this configuration the 
 base is subjected to maximum bending moment as the rear 
 link acts in tension to pull the rear of the base upward 
 while the leg force pushes it down. The lemniscate links 
 also are subjected to maximum loading because they must 
 act to offset leg forces to provide equilibrium of the base 
 to maintain the base-on-toe configuration. 
 
 13. Specific canopy and base contact configurations should 
 be designed to evaluate critical loading in each compo- 
 nent of the support structure (fig. 12). 
 
 A critical contact configuration for one component may 
 not be critical for another component. Critical canopy 
 contact configurations are largely independent of base 
 contact and critical base contact configurations are in- 
 sensitive to canopy contacts. Contact configurations that 
 should be considered in critical load testing of shield sup- 
 ports are illustrated in figure 12. 
 
11 
 
 SYMMETRIC CONTACT CONFIGURATIONS 
 
 
 V 
 
 Full contact 
 
 Canopy bending 
 
 "tcdlcj 
 
 Base bending 
 
 
 Base-on-toe 
 
 Base -on-rear 
 
 Leg socket 
 
 11/ 
 
 KEY 
 
 X Critical load member 
 V Contact 
 
 UNSYMMETRIC CONTACT CONFIGURATIONS 
 
 Canopy 
 
 ^3 
 
 up 
 
 ^3 fc£S 
 
 + + 
 
 
 + + 
 
 SS3 ^ 
 
 
 K 
 
 + 
 
 
 + 
 
 Base 
 
 Toe^ 
 
 + 
 
 KEY 
 ESS Contact 
 + Leg connection 
 
 14. 
 
 Figure 12.— Critical load conditions for shield supports. 
 
 Performance testing of shield supports should be con- 
 ducted by applying or inducing both vertical and hori- 
 zontal displacements to the support structure (fig. 13). 
 
 The caving shield-lemniscate assembly has very little 
 vertical stiffness (0-30 kips/in) and will not be significantly 
 loaded by vertical displacements (roof-to-floor conver- 
 gence). Horizontally, the caving shield-lemniscate assem- 
 bly is quite stiff (400-900 kips/in), and large loads can be 
 generated, provided horizontal freedom in the pin joints is 
 overcome by the horizontal displacement or from horizon- 
 tal constrainment during setting. Vertical displacements 
 are likely to produce larger stresses in the canopy and 
 base, since bending is the primary loading mechanism for 
 these components. Horizontal displacements will add to 
 the bending of these members. 
 
 15. Friction-free or zero horizontal load tests should be 
 conducted to evaluate two-leg shield stability and link 
 loading (fig. 14). 
 
 When sufficient friction exists between the canopy and 
 base strata interfaces, frictional forces are developed which 
 
 VERTICAL 
 DISPLACEMENT 
 
 FACE- TO -WASTE 
 HORIZONTAL DISPLACEMENT 
 
 WASTE -TO -FACE 
 HORIZONTAL DISPLACEMENT 
 
 Figure 1 3— Direction of displacement loading for shield 
 performance tests. 
 
 / /_/_/ / /_/_ Z I 'CL t ~ r Z T . 
 
 ■■■ •» 
 
 • ♦• •• 
 
 KEY 
 — ^ Resultant force 
 
 O Restraint 
 #"•"• Roller 
 — c=» Shield displacement 
 
 Figure 14.— Friction-free shield tests. 
 
 oppose the horizontal component of the leg forces. This 
 directs the line of action of the applied resultant force 
 away from the toe of the base, thereby aiding in support 
 stability and reducing peak pressure distribution on the 
 support base in the toe area. The frictional forces also 
 minimize link loading because horizontal displacement of 
 the canopy-caving shield joint is minimized. In the 
 absence of these frictional forces, stability must be pro- 
 vided by the caving shield-lemniscate assembly. Therefore, 
 
12 
 
 friction-free tests are considered to be a critical load con- 
 dition for two-leg shield supports. These tests can be con- 
 ducted by allowing the load applying platens of a biaxial 
 active test frame to displace freely in the horizontal direc- 
 tion or by placing rollers or bars on top of the shield can- 
 opy or under the base in a static frame test. 
 
 16. Shields should be horizontally constrained during set- 
 ting and load application to remove freedom in the pin 
 joints during critical load testing (fig. 15). 
 
 Tests conducted in the Bureau's simulator revealed that 
 considerable freedom exists in the numerous joints of a 
 shield structure, and that loads will not be developed in the 
 caving shield-lemniscate assembly unless the shield is prop- 
 erly constrained to remove this freedom. Some contact 
 configurations, such as base-on-toe contact, require con- 
 strainment to maintain stability of the configuration, while 
 others, such as full canopy and base contact, can be main- 
 tained without constrainment. It is recommended that the 
 canopy be displaced horizontally to remove pin freedom 
 prior to the test and that link activity be monitored as an 
 indication of load generation in the caving shield-lem- 
 niscate assembly during critical load testing of shield 
 supports. 
 
 OPERATIONAL CONSIDERATIONS 
 
 17. Use higher setting pressures for high shield heights where 
 the lower stage of the leg cylinder is fully extended 
 (fig. 16). 
 
 Shield setting force is provided by the vertical compo- 
 nent of the leg force generated by the applied setting pres- 
 sure. Despite the more efficient geometry consideration 
 at high shield heights, the setting force can be considerably 
 smaller than that provided at lower shield heights where 
 the leg is operating at more of an inclined angle from the 
 plane of the canopy. Reductions in setting force will occur 
 when the lower stage of the leg cylinder is fully extended. 
 The reason for this reduction in setting force is that the 
 effective area of the leg cylinder is smaller when in this 
 configuration. At high shield heights, when the first stage 
 is fully extended against the mechanical stops of the leg 
 cylinder casing, setting force equals the product of the 
 applied setting pressure (measured in the lower stage) and 
 the leg area of the upper stage. When the lower stage is 
 not fully extended, the pressure in the upper cylinder ex- 
 ceeds the lower cylinder pressure in proportion to the ratio 
 of the areas and results in a larger setting force. Tests on 
 several shields in the Bureau's simulator have shown re- 
 ductions in setting forces of up to 50 pet when the lower 
 stage is fully extended. 
 
 18. Setting pressure should be based upon face convergence 
 and shield stiffness (fig. 8). 
 
 Optimum setting pressure is the m inimum pressure that 
 provides stability and equilibrium of the strata. The goal 
 is not to prevent convergence, but to provide a setting force 
 which is compatible with shield stiffness so that the load 
 generation from the resulting convergence is consistent 
 with the yield load capability of the support. Hsiung 
 promotes a similar concept employing the stiffness rela- 
 tionship of the support-strata system and measured face 
 convergence. 6 Shield stiffness characteristics for various 
 
 ^ork cited in footnote 5. 
 
 —4 
 
 A 0.05 F, 
 
 0.95F 
 
 0.98R 
 
 UNCONSTRAINED SHIELD SETTING 
 
 \ 0.55 R, 
 
 KEY 
 
 ► F v Vertical force 
 ^F H Horizontal force 
 t-* Constraint 
 
 CONSTRAINED SHIELD SETTING 
 
 Figure 1 5.— Horizontal shield constraint 
 
 Figure 1 6.— Shield-setting pressure considerations. 
 
13 
 
 setting pressures and shield heights have been evaluated 
 by the Bureau under controlled loading in the mine roof 
 simulator. Stiffness characteristics should be made avail- 
 able by support manufacturers to operators for analysis 
 when purchasing supports. 
 
 19. Avoid setting shield with canopy tip raised up (fig. 17). 
 
 All critical load configurations should be avoided if pos- 
 sible. Generally, the operator has little control over the 
 contact configuration established from setting the support, 
 but the operator can control the attitude of the canopy, 
 and setting the support with the canopy tip raised can be 
 avoided. Setting the support with the tip up usually results 
 in the support standing on the toe of the base which, as 
 described earlier, is the worst load case for two-leg shield 
 supports. Setting the support with the canopy tip raised 
 also produces maximum bending in the canopy structure. 
 
 20. Advance the support under partial contact with the roof 
 (fig- 18). 
 
 To the extent practically possible, shields should be 
 advanced under partial contact with the roof. This will 
 ensure setting the shield in a constrained configuration, 
 which removes freedom in the pin joints and increases 
 support capacity and stability. In a constrained configura- 
 tion, the caving shield-lemniscate assembly will develop 
 vertical and horizontal reactions that act to increase sup- 
 port resistance against both vertical and horizontal dis- 
 placements. The constrained configuration will also result 
 in a greater setting force for the same leg pressure. 
 
 IN SITU PERFORMANCE MONITORING 
 
 21. Monitor leg pressures to provide an overall indication of 
 shield loading but recognize the limitations of leg pres- 
 sure measurements in shield load analysis (fig. 19). 
 
 The easiest and best overall indicator of shield loading 
 is an assessment of leg pressures. Vertical loading applied 
 to the roof support by strata weight or convergence can be 
 reasonably estimated from measurement of leg pressures. 
 Vertical resistance provided by the caving-shield-lemniscate 
 assembly will be a source of error in determination of 
 vertical support loading by leg pressure measurements, but 
 this error is likely to be less than 5 pet of the total vertical 
 force. The area and pressure of the lower stage of the leg 
 cylinder should be used to determine leg force. Horizontal 
 forces cannot be accurately determined from leg pressure 
 measurements. The Bureau has developed several tech- 
 niques to measure horizontal loading. Horizontal loading 
 should be measured if support stability or structural fail- 
 ures are in question. 
 
 22. Inspect welds for signs of fatigue failures. 
 
 Welds should inspected during each face move to assess 
 fatigue loading of shield structures. Visual inspection 
 should be adequate as a first means of inspection. Ob- 
 served or expected problem areas can be treated with dye 
 penetrant for further evaluation. The base appears to be 
 the member that often fails first, but all members should 
 be inspected. Particular attention should be paid to the 
 leg socket area of the base member. Leg sockets are fre- 
 quently cast members that are more difficult to weld, and 
 the leg socket typically represents an area of stress concen- 
 tration in most shields. 
 
 23. Be aware in making evaluations of structural problems 
 that failed components alter load transferring 
 mechanisms and can obscure the cause of the problem. 
 
 Observed failure of one component does not necessarily 
 indicate the source of the problem. When supports are in 
 service underground, it is often very difficult to physically 
 examine all of the support structure. This can result in 
 misinterpretation of problem areas. For example, several 
 cases have been brought to the Bureau's attention where 
 
 . _ ~^^ __ Roof strata ~ — — -^ 
 
 WW' 
 
 Figure 17.— Setting shield with tip raised. 
 
 R00f — _ 
 
 Friction with roof strata 
 
 ^Lg^SLj^^^ w^---^.-^ 
 
 Floor "" 
 
 Figure 18.— Advancing support under partial contact with roof 
 strata. 
 
14 
 
 ELASTIC MODEL 
 
 Measure shield 
 displacements 8 V .8 H 
 
 Determine shield stiffness 
 
 F v = K, 8 V + K 
 
 F h= 
 
 K, 8, 
 
 2 °h 
 + K 4 8 h 
 
 INSTRUMENTED HINGE PIN 
 
 Mec 
 
 isure 
 
 leg pressure 
 
 and 
 
 pin forces 
 
 V 
 
 L v + 
 
 p v 
 
 F H = 
 
 L v + 
 
 P H 
 
 P v Instrumented 
 t pin 
 
 ©-P, 
 
 LINK STRAIN MODEL 
 
 Measure leg pressures 
 and frontor rear link strain 
 
 
 L*V L + 
 L*H L + 
 
 F*V F 
 F*H F 
 
 v L . 
 
 v F . h l , 
 
 Hp = Geometric 
 configuration 
 coefficients 
 
 Link 
 ♦* strain. 
 
 (L) ' 
 
 Leg pressure 
 
 Figure 19.— Measurement of shield loading. 
 
 mine personnel observed structural failure of the caving 
 shield near the lemniscate pins, when the real cause of the 
 problem was failure of the base structure that was unno- 
 ticeable while the support was in service. Mistakes can 
 also be made in overstrengthening a component as a re- 
 sult of a failure. Overstrengthening of one component 
 could alter the load transfer and result in subsequent fail- 
 ure of another component. Careful consideration should 
 be given in making major modifications to the structural 
 design of the support. Ideally any modification should be 
 laboratory tested prior to returning the component to field 
 service. 
 
 24. Consideration should be given to fracture mechanics of 
 shields constructed of high-strength steel. 
 
 Structural problems with shield supports are not likely 
 to be catastrophic. Most longwall support structures are 
 constructed of relatively mild steel (40-60 kips/in 2 yield 
 strength) with good ductility, which allows considerable 
 deformation before failure. Some support components, 
 canopies in particular, are constructed of high strength 
 steel, 100 kips/in 2 yield strength or greater. These steels 
 are significantly less ductile and are more likely to fracture 
 when loads exceed the elastic design strength. High- 
 strength steels are also more difficult to weld and may be 
 more susceptible to fatigue failures than components 
 constructed of milder steel. Cracks in any steel structure 
 represent a potential loss of structural integrity and 
 constitute a potential safety hazard, but any crack in a 
 high-strength steel construction should be perceived as a 
 sign of imminent danger. 
 
 CRIB SUPPORT CONSIDERATIONS 
 
 This section describes wood and concrete crib behaviors 
 and their application as gate road supports in longwall 
 mining. Topics of discussion include selection criteria, 
 construction considerations, and analysis of crib supports 
 by controlled testing in the laboratory. 
 
 SUPPORT SELECTION AND APPLICATION 
 
 25. Use concrete cribs where high strength is required and 
 wood cribs where concrete stiffness is not compatible 
 with expected convergence (fig. 20). 
 
 A goal in crib support selection for any application is to 
 design a structure with a stiffness that is compatible with 
 the expected convergence. Reinforced concrete cribs are 
 stiff structures that normally fail at vertical convergence of 
 2 to 3 pet of the height of the crib, whereas wood cribs are 
 very flexible structures and are capable of deforming ver- 
 tically 35 to 40 pet of their height in response to a converg- 
 ing mine roof. Concrete crib stiffness can be reduced to 
 provide more yield capability by reducing the strength of 
 the concrete or by incorporating wood within the support 
 
 structure. These issues are further discussed in the "Con- 
 struction" section. 
 
 26. Adjust crib stiffness and density across entry widths to 
 be compatible with convergence profiles. 
 
 Depending on gate road pillar design and abutment 
 loading, convergence is likely to vary across the width of a 
 longwall entry. Minimum convergence can be expected to 
 occur near the panel. Crib stiffness can be modified (by 
 the application of wood volume in concrete cribs) to be 
 compatible with convergence at the location of the crib 
 support. 
 
 27. Do not use steel-fiber-reinforced concrete cribs in appli- 
 cations where the shearer is required to cut through the 
 cribs. 
 
 Crib supports are used in recovery room operations to 
 provide additional support during face moves. Concrete 
 supports are sometimes used in place of wood supports 
 because they have high strength and the shearer can cut 
 
15 
 
 through them during the recovery operation. The steel 
 fibers in reinforced concrete cribs tend to hold the con- 
 crete together as the shearer cuts the crib. Large uncut 
 pieces jam the face conveyor and stageloader. Therefore, 
 nonreinforced concrete is preferred for these applications. 
 Because nonreinforced concrete can fail violently, it is 
 suggested that the crib be surrounded by something to 
 contain the material during failure. One mine poured 
 nonreinforced concrete underground in tubes to form 
 small pillars that effectively controlled the ground during 
 longwall recovery. 
 
 28. Use concrete cribs at crib heights where wood cribs 
 experience buckling failure. 
 
 Concrete is more uniform and hence provides for better 
 stability in crib constructions than wood, which has incon- 
 sistent material properties. Stable concrete crib structures 
 in excess of 30 ft in height (with cross-sectional areas com- 
 parable to 6-ft-high wood crib constructions) have been 
 reported. 
 
 29. Use wood cribs in areas of high horizontal displacement 
 (fig. 21). 
 
 Wood cribs are able to deform sufficiently to remain 
 stable against horizontal displacements (displacements 
 applied perpendicular to the longitudinal axis of the sup- 
 port). Tests conducted in the Bureau's simulator have 
 shown wood cribs greater than 6 ft in height can withstand 
 12 in of horizontal displacement with 24 in of vertical dis- 
 placement and remain stable without significant loss of 
 support capacity. This capability decreases as the cribs are 
 reduced in height. At shorter heights, the cribs become 
 stiffer and are less able to deform. The shortest wood crib 
 tested was 50 in. It remained stable but exhibited a 35-pct 
 reduction in support resistance (vertical load capacity) at 
 12 in of horizontal displacement. 
 
 30. Increase the density of crib supports as the seam height 
 increases (fig. 22). 
 
 The stiffness of wood crib structures increases nonlin- 
 early as the height of the structure is reduced, resulting in 
 larger load reactions per unit convergence. As an example 
 of the effect of height on wood crib resistance, a 110-in- 
 high square crib had 35 pet less support resistance than a 
 50-in-high crib at 10 in of displacement and 60 pet less 
 resistance at 20 in of displacement. 
 
 Figure 20.— Wood and concrete crib strength and stiffness. 
 
 Figure 21.— Wood crib subjected to horizontal displacement 
 
16 
 
 CONSTRUCTION 
 
 31. Wood cribs should be constructed with minimum 6-in 
 overhang on the ends of the blocks (beyond contact area 
 between block layers) to enhance stability (fig. 23). 
 
 Overhanging the ends of the crib blocks causes the 
 blocks to interlock as they deform from convergence, pro- 
 viding a rotational restraint between blocks that significant- 
 ly improves stability. Weaker crib blocks have a tendency 
 to roll as they fail and cause instability in the crib struc- 
 ture. The rotational restraint provided by overhanging 
 ends helps to prevent this action. Horizontal displace- 
 ments further aggravate block rotation and crib instability. 
 Overhanging the ends of the blocks is an effective means 
 of improving stability against horizontal displacements. 
 
 32. Wood cribs should be constructed in square geometries 
 (fig. 24). 
 
 The strength of wood cribs is proportional to the prin- 
 cipal moment of inertia of the crib structure. Wood cribs 
 that have the same principal minimum moment of inertia 
 will have the same load carrying capability. Therefore, 
 although a rectangular crib has a larger maximum moment 
 of inertia than a square crib equal in width to the rect- 
 angular crib, both have the same minimum moment of 
 inertia and will have the same load carrying capability. 
 Because the square crib consumes less material and space, 
 it is the more optimum configuration. 
 
 800 
 
 600 
 
 o 
 
 O 400 
 
 
 KEY 
 Nominal height 
 
 50 in 
 
 60 in 
 
 80 in 
 
 1 10 in 
 
 200 
 
 6 8 10 12 14 16 18 
 VERTICAL DISPLACEMENT, in 
 
 22 
 
 Figure 22.— Wood crib stiffness as a function of crib height. 
 
 Figure 23.— Construction of wood cribs with overhanging ends. Figure 24.— Unstable parallelogram wood crib configuration. 
 
17 
 
 33. Use additional blocks per layer to improve the strength 
 of wood crib supports (fig. 25). 
 
 Experiments were conducted by the Bureau to see if 
 increasing interblock contact area by changing the con- 
 struction of wood cribs from square to parallelogram 
 geometries would increase the strength of the wood crib. 
 Results showed that only a minimal increase in strength 
 was achieved, with a reduction in stability and support 
 resistance at large displacements (greater than 10 in). It 
 was concluded that square geometries are a better overall 
 configuration, and that increases in strength should be 
 provided by adding additional blocks to crib layers. 
 
 34. Wood cribs should be constructed from the same type 
 of wood (fig. 26). 
 
 A primary failure mechanism for wood crib supports is 
 instability, generally caused by differential compression or 
 rotation of individual crib blocks caused by variation in 
 their material properties. Construction of wood cribs from 
 the same material type will minimize localized differences 
 in crib deflections and enhance stability. 
 
 35. Incorporate wood in concrete crib constructions to re- 
 duce the stiffness and enhance the yield capability of 
 the structure (fig. 27). 
 
 If additional strength and yield capability are desired, 
 layers of wood should be added between the layers of con- 
 crete. The wood layers act to provide a more uniform 
 load distribution on the concrete elements and provide 
 constraint against lateral expansion. Tests have shown that 
 thin sheets of plyscore or plywood added between each 
 
 layer of concrete will increase the strength of the crib by 
 a factor of 2 to 3. If additional yield capability without 
 additional strength is required, wood additions should be 
 concentrated at the top or bottom of the structure. The 
 increase in yield and reduced stiffness of these composite 
 configurations will be proportional to the amount of wood 
 incorporated in the structure. The incorporation of wood 
 in concrete cribs to improve strength or yield capability is 
 effective for both circular and rectangular crib geometries. 
 
 36. Use lower strength concrete to reduce the strength of 
 concrete crib supports. 
 
 If the reactions developed in concrete cribs exceed the 
 bearing strength of the roof or floor strata and wood cribs 
 are incapable of providing the desired resistance, use a 
 lower strength concrete in the concrete crib design. 
 
 "Weak crib 
 ■ element 
 
 Figure 25.— Improving wood crib strength by adding additional 
 blocks per layer. 
 
 Figure 26.— Instability in wood crib resulting from weak crib 
 member. 
 
18 
 
 High-strength concrete (7,000-8,000 psi) is typically used in 
 concrete crib construction to achieve maximum strength. 
 A specific strength concrete can be used to provide a crib 
 that is most compatible with the conditions in which the 
 crib is to be employed. 
 
 37. Concrete cribs should be constructed to minimize non- 
 uniform loading (fig. 28). 
 
 Effort should be made to ensure as uniform load as 
 possible on concrete crib supports. Eccentric loading or 
 concentrated loads (point loads) can significantly reduce 
 the strength of the crib. Use wood wedges and crib blocks 
 to maintain symmetric loading at roof and floor interfaces 
 and remove any debris between layers during crib con- 
 struction. Ideally, concrete cribs should be constructed on 
 stable floor and loose rubble should be removed prior to 
 crib construction. Additionally, wood (load transfer disks) 
 can be incorporated between layers of blocks to provide 
 more uniformly distributed loading on each block. 
 
 FAILURE OBSERVATIONS 
 
 38. Wood cribs provide visual and audible signs of loading 
 and rarely fail catastrophically. 
 
 Crackling or popping noises are frequently heard as 
 wood cribs take load. Maximum out-of-plane deflection 
 will occur in the middle of the wood crib structure (typical 
 of column buckling). If large horizontal displacement of 
 
 Figure 27.— Composite wood-concrete crib structures. 
 
 Figure 28— Load transfer disks used in concrete crib 
 constructions. 
 
19 
 
 the roof relative to the floor occurs, the crib profile will 
 resemble more of an S-shape. 
 
 39. Watch for individual crib blocks that have rotated rela- 
 tive to the cross section of the crib structure (fig. 26). 
 
 If one or more crib blocks rotate out-of-plane to the 
 cross section of the crib structure, the crib could become 
 unstable and kick out under load. This condition usually 
 occurs with a weak or deteriorated crib block, in crib con- 
 structions where the ends were not overhung, or in condi- 
 tions where the roof is displacing horizontally relative to 
 the floor. 
 
 40. If concrete cribs are crushing out, their stiffness is prob- 
 ably incompatible with the ground convergence. 
 
 Properly designed concrete cribs should not fail. Con- 
 crete has more than adequate strength to support gravity 
 loading of unstable strata in nearly all applications. If they 
 are failing, it is more than likely from excessive conver- 
 gence that is incompatible with the material stiffness. 
 Methods to reduce stiffness and improve yield capability 
 have already been discussed. 
 
 LABORATORY TESTING AND ANALYSIS 
 
 43. Wood and concrete cribs tested in the laboratory will 
 exhibit greater strength than cribs utilized underground if 
 the laboratory convergence rate is greater than the under- 
 ground convergence rate (fig. 31). 
 
 Both wood and concrete have load rate dependencies 
 that affect crib strength. Tests conducted on wood cribs by 
 the Bureau indicated a 30-pct increase in wood crib resis- 
 tance for convergence rates of 0.1 in/min or greater com- 
 pared to a convergence rate of 0.005 in/min. Load rate 
 studies have not been made on full-scale concrete cribs, 
 but laboratory tests on concrete test cylinders have shown 
 similar load rate dependencies. 
 
 44. End effects can significantly affect postyield behavior of 
 concrete cribs (fig. 32). 
 
 Frictional forces developed along interface boundaries 
 between cribs and test machine platens or the under- 
 ground environment affect the distribution of stresses in 
 the specimen and are referred to as end effects. Tests on 
 
 41. Concrete cribs do not provide signs of loading until fail- 
 ure is imminent (fig. 29). 
 
 Crack initiation in cementitous materials occurs at the 
 microscopic level at about 30 pet of ultimate load, but 
 macroscopic crack growth is not visually observable until 
 95 to 100 pet of the ultimate strength is reached. Many 
 times concrete cribs will fail with little or no warning. 
 Generally, crack formation will initiate near the middle of 
 the crib structure and progress upward and downward. 
 Shear type failures are common for solid crib constructions 
 from rectangular blocks or circular disks. Open cribs con- 
 structed of rectangular blocks will generally fail at inside 
 contact boundaries, which are areas of high stress concen- 
 trations. The incorporation of wood layers appears to 
 change the failure mechanics of concrete cribs. Composite 
 wood-concrete cribs show vertical crack propagation, which 
 is indicative of tensile failure from uniaxial stress fields. 
 
 42. Composite wood-concrete crib structures have higher 
 energy absorption capability and increased potential for 
 explosive failure (fig. 30). 
 
 Wood provides for more uniform loading and acts to 
 provide a confinement to lateral expansion. This enhances 
 the observed strength by allowing the crib to generate 
 pressures closer to the unconfined compressive strength of 
 the concrete, but is also disadvantageous in terms of 
 energy absorption. Concrete cribs dissipate energy by 
 crack growth and plastic deformation. More uniform 
 loading and constraint to lateral expansion causes more 
 strain energy to develop and less crack growth, which can 
 result in highly explosive failures. 
 
 Figure 29.— Typical failure in concrete crib support. 
 
— ^^mtm^^m^ 
 
 20 
 
 Figure 30.— Explosive failure of concrete crib support constructed with load transfer disks. 
 
 300 
 
 250 
 
 - 200 
 
 < 
 
 CO 
 UJ 
 
 ai 
 
 99 
 ce 
 
 150 
 
 100- 
 
 0.02 004 0O6 0O8 
 
 CONVERGENCE RATE, in/min 
 
 010 
 
 Figure 31.— Convergence rate effect on wood crib support 
 resistance. 
 
 Triaxial state 
 of stress 
 
 Uniaxial state 
 of stress 
 
 Triaxial state 
 of stress 
 
 END ZONE 
 CENTRAL ZONE 
 END ZONE 
 
 Figure 32.— End effect considerations in crib testing. 
 
21 
 
 small-scale concrete cylinders have shown that specimen 
 behavior during loading (prior to reaching ultimate 
 strength) is largely independent of end effects, but behav- 
 ior after reaching ultimate strength (postyield behavior) is 
 significantly dependent on frictional boundary considera- 
 tions. Laboratory test results indicate that postyield load 
 carrying capability is significantly reduced if the frictional 
 restraint is reduced. 
 
 energy that is available, the faster a crack will grow. 
 Ideally, a stiff testing machine is desirable so that energy 
 is not built up in the test frame and released to the speci- 
 men during testing, as this would accelerate crack growth 
 and reduce crib strength. 
 
 46. Sustained loading will reduce the strength of concrete 
 cribs. 
 
 45. Stiffness of the test machine can affect concrete crib 
 performance. 
 
 Crack propagation in concrete specimens is dependent 
 upon available (potential) energy. Generally, the more 
 
 Sustained loadings of greater than 70 pet of the ultimate 
 compressive strength have been shown to reduce the 
 strength of concrete specimens. Full testing of concrete 
 cribs has not been attempted, but it is assumed a similar 
 relationship exists. 
 
 CONCLUSIONS 
 
 Forty-six recommendations or practical considerations 
 have been presented to assist in the selection and utiliza- 
 tion of longwall face and gate road supports. Many of 
 these are a direct outgrowth of the Bureau's ground con- 
 trol research in support mechanics, and many offer innova- 
 tive solutions to everyday problems faced by mining per- 
 sonnel in the control of ground in longwall mining. 
 Several of these considerations have application outside of 
 longwall mining as well. 
 
 In all of these considerations, one basic theme is em- 
 phasized; roof supports that are most compatible with the 
 conditions in which they are to be employed should be 
 selected. Pursuant to this theme are some fundamental 
 concepts that are extremely important and often over- 
 looked or unrecognized by mining personnel. These are 
 highlighted as follows and are the foundation of the prac- 
 tical considerations presented. 
 
 o The control of ground in any mining operation involves 
 a rather complex interaction between support elements 
 and the strata environment. Both the environment and 
 the support behavior and their interaction must be con- 
 sidered in any analysis of ground control. 
 
 o A "bigger, the better" attitude in support selection is 
 not a good ground control practice. Too much support 
 resistance can be just as detrimental and catastrophic as 
 too little support. 
 
 o Observed support loading is not a supreme indication 
 of required support capacity. Passive support elements 
 (shields and crib supports) react loads in response to 
 displacements imposed by the strata in proportion the 
 stiffness and mechanical properties of the support 
 structure. Simply because a support structure fails or 
 yields, it does not necessarily mean that larger support 
 capacity is required. A smaller capacity support with 
 less stiffness or setting force may perform well where 
 larger capacity supports have failed. 
 
 o Initial conditions are an important consideration in 
 analysis of support behavior and subsequent ground 
 control. As much consideration should be given to 
 shield setting pressures as yield loads. Setting pressures 
 should not simply be set at the maximum available 
 pump pressure. 
 
 o Boundary conditions are also important considerations 
 in support analysis. Not only is contact configuration 
 important when evaluating longwall roof support struc- 
 tures, but so are the direction of applied displacement 
 (vertical or horizontal) and constrainment to these dis- 
 placements. Lack of consideration of these parameters 
 can lead to erroneous evaluations of the integrity of the 
 support. 
 
22 
 
 BIBLIOGRAPHY 
 
 Barczak, T. M. Impact of Horizontal Load on Shield Supports. 
 Paper in Proceedings of the Fourth Conference on Ground Control in 
 Mining, ed. by S. S. Peng and J. H. Kelly. WV Univ., 1985, pp. 50-57. 
 
 . Optimization of Longwall Supports. Pres. at Soc. Min. Eng. 
 
 AIME Fall 1987 Meeting, Pittsburgh, PA, Oct. 12-13, 1987, 25 pp.; 
 available upon request from T. M. Barczak, BuMines, Pittsburgh, PA. 
 
 . An Overview of the Bureau's Mine Roof Support Studies 
 
 Research Program. Pres. at 1985 AMC Coal Convention, Pittsburgh, 
 PA, May 12-15, 1985; 13 pp.; available upon request from T. M. 
 Barczak, BuMines, Pittsburgh, PA. 
 
 . Resultant Load Vector Studies on Shield Supports. Paper 
 
 in Proceedings of the 21st International Conference of Safety in Mines 
 Research Institute, Sydney, Australia, Oct. 15-21, 1985, 5 pp. 
 
 . Rigid-Body and Elastic Solutions to Shield Mechanics. 
 
 BuMines RI 9144, 1987, 20 pp. 
 
 . State-of-the-Art Testing and Analysis of Mine Roof Support 
 
 Systems. Paper in Eastern Coal Mine Geomechanics. Proceedings: 
 Bureau of Mines Technology Transfer Seminar, Pittsburgh, PA, 
 November 19, 1986. BuMines IC 9137, 1987, pp. 3-11. 
 
 Barczak, T. M., and W. S. Burton. Assessment of Longwall Roof 
 Behavior and Support Loading by Linear Elastic Modeling of the 
 Support Structure. BuMines RI 9081, 1987, 7 pp. 
 
 . The Significance of Specimen Stiffness and Post Yield 
 
 Characteristics on Passive Roof Support Design. Paper in Proceedings 
 of the Sixth International Conference on Ground Control in Mining, 
 Morgantown, WV, June 9-11, 1987. WV Univ., 1987, pp. 219-226. 
 
 . Three-Dimensional Shield Mechanics. BuMines RI 9091, 
 
 1987, 23 pp. 
 
 Barczak, T. M., and R C. Garson. Shield Mechanics and Resultant 
 Load Vector Studies. BuMines RI 9027, 1986, 43 pp. 
 
 Barczak, T. M., and R C. Garson. A Technique to Measure Resul- 
 tant Load Vector on Shield Supports. Ch. 68 in Rock Mechanics in 
 Productivity, Protection (25th Symp. on Rock Mechanics, Chicago, IL, 
 June 25-27, 1984). Soc. Min. Eng. AIME, 1984, pp. 667-679.2 
 
 Barczak, T. M., and C. A. Goode. Considerations in the Design of 
 Longwall Mining Systems. Published in Proceedings for State-of-the- 
 Art Ground Control in Longwall Mining and Mine Subsidence 
 (Honolulu, HI, Sept. 4-5, 1982). Soc. Min. Eng. AIME, 1982, pp. 39- 
 50. 
 
 Barczak, T. M., and S. J. Kravits. Shield-Loading Studies at an 
 Eastern Appalachian Minesite. BuMines RI 9098, 1987, 81 pp. 
 
 Barczak, T. M., and D. E. Schwemmer. Critical-Load Studies of a 
 Shield Support. BuMines RI 9141, 1987, 15 pp. 
 
 . Effect of Load Rate on Wood Crib Behavior. BuMines 
 
 RI 9161, 1988, 11 pp. 
 
 . Stiffness Characteristics of Longwall Shields. BuMines 
 
 RI 9154, 1988, 14 pp. 
 
 Barczak, T. M., and C. L. Tasillo. Factors Affecting Strength and 
 Stability of Wood Cribbing: Height, Configuration, and Horizontal 
 Displacement. BuMines RI 9168, 1988, 23 pp. 
 
 Barczak, T. M., and P. M. Yavorsky. State-of-the-Art Testing of 
 Powered Roof Support Systems. Paper in Proceedings, Second 
 Conference on Ground Control in Mining, ed. by S. S. Peng. WV 
 Univ., 1982, 64-77 pp. 
 
 Goode, C. A., T. M. Barczak, and J. Jaspal. Support Selection for 
 the Multilift Mining Method. Paper in Proceedings, First Annual 
 Conference on Ground Control in Mining, ed. by S. S. Peng. WV 
 Univ., 1981, pp. 186-200. 
 
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