°o **^°* a°° V * %^ :«: W -M& %/ •*'* Sfe *- «" • 4* ** • %. "••.•'" A ' 0y V ^* %/ •*£&-' \/ #»; %/ •»'•- V •'«£ S<* ••' ^o* . ^ V °*. *•• « ♦ r ^ ^ ** ** •j^&'- ^ «^ •>« . <■' ' . *^0 A o » " " ^ >-6* °^. ••••' A -•K2* • A V «^ C ♦' s^ ^ » o ***** ••£&• ^ -'^fe: ****** **\ f^ ** v \ '*m. : /\ : W? : ** v ' f v t -> ^* sat. V"> .1-,- •> v^.V'" ♦«♦ -i- a - •> " •"* •^^ r ^d ^ s o ^ 0c v "*»^ •bv' *b& Ai&k% £*£ttk°» /^^^o / .^ BUREAU OF MINES INFORMATION CIRCULAR/1989 Safety Evaluations of Longwall Roof Supports By Thomas M. Barczak UNITED STATES DEPARTMENT OF THE INTERIOR Mission: As the Nation's principal conservation agency, the Department of the Interior has respon- sibility for most of our nationally-owned public lands and natural and cultural resources. This includes fostering wise use of our land and water resources, protecting our fish and wildlife, pre- serving the environmental and cultural values of our national parks and historical places, and pro- viding for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also promotes the goals of the Take Pride in America campaign by encouraging stewardship and citizen responsibil- ityforthe public landsand promoting citizen par- ticipation in their care. The Department also has a major responsibility for American Indian reser- vation communities and for people who live in Island Territories under U.S. Administration. C^Ju/M^, &*ruw /fa**; Information Circular 9221 Safety Evaluations of Longwall Roof Supports By Thomas M. Barczak UNITED STATES DEPARTMENT OF THE INTERIOR Manuel J. Lujan, Jr., Secretary BUREAU OF MINES T S Ary, Director 1\\ ^ 5 Library of Congress Cataloging in Publication Data: Barczak, Thomas M. Safety evaluations of longwall roof supports. (Bureau of Mines information circular, 9221) Bibliography: p. 17 Supt. of Docs, no.: I 28.27:9221. 1. Mine roof control-Evaluation. 2. Longwall mining. I. Title. II. Series: Information circular (United States. Bureau of Mines); 9221. TN295.U4 [TN288] 622 s [622\334] 89-600003 CONTENTS Page Abstract 1 Introduction 2 Support design and mechanics 4 Failure mechanisms 8 Inadequate support capacity 9 Structural failure 9 Base failures 10 Canopy failures 10 Failures of caving shield and links 11 Leg cylinder failures 11 Instability 11 Safety precautions 12 Support capacity considerations 12 Structural failure and stability considerations 13 Hypothetical failures 13 Case studies of support failures 14 Future research efforts 17 Conclusions 17 References 17 ILLUSTRATIONS 1. Bureau's mine roof simulator 3 2. Two-dimensional diagram of shield support 4 3. Three-dimensional representation of longwall shield 4 4. Shield component construction 5 5. Vertical and horizontal loads acting on shield support 6 6. Gob loading on caving shield 7 7. Load conditions (displacements) imposed on shield support 8 8. Lateral loading caused by uneven roof conditions along face 8 9. Conditions that produce maximum loading in support components 8 10. Shield supports going solid from inadequate capacity 9 11. Relationship between strata convergence and support setting force 12 12. Failure of leg cylinder casing 14 13. Split caving shield design that failed from instability 14 14. Permanent deformation of base structure 14 15. Example of leg socket failure 15 16. Base failure and subsequent modification 16 17. Failure of canopy structure from high stresses in canopy hinge 16 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT ft foot pet percent in inch psi pound per square inch ksi kip per square inch SAFETY EVALUATIONS OF LONGWALL ROOF SUPPORTS By Thomas M. Barczak 1 ABSTRACT State-of-the-art longwall roof supports provide effective strata control, but failures of these support systems still occur. To identify failure mechanisms and the impact these failures have on the safety of the support system, the U.S. Bureau of Mines has been conducting research on shield mechanics in the Bureau's unique mine roof simulator, as well as field studies of in situ support loading and investigations of longwall failures. Three types of failures are discussed: (1) inadequate support capacity, (2) structural failure, and (3) instability. Hypothetical situations with proposed courses of action and case studies of actual longwall failures are described. This information is intended to assist the Mine Safety and Health Administration (MSHA) and industry mining personnel in safety evaluations of longwall roof support systems. Research physicist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. INTRODUCTION Longwall roof supports are robust structures that are designed to provide effective ground control during the extraction of a number of longwall panels. A typical life expectancy for these support systems is 7 to 10 longwall panels, which constitute 14,000 to 20,000 operating cycles, assuming a 30-in cut per shearer pass in panels 5,000 ft in length. Failure of the support system can be attributed to one of three things: (1) inadequate support capacity, where support resistance is insufficient to prevent strata con- vergence beyond the hydraulic yield of the leg cylinders; (2) structural failure caused by either fatigue or compo- nent loading beyond its design strength; or (3) instability caused by structural failure of support components, exces- sive wear in the pin joints of the structure, or load condi- tions incompatible with the support design. Many of these failures develop gradually and usually do not present an immediate safety hazard in their early stages of development. However, as the failure progresses to the point where the structural integrity of the support is compromised, the capability of the support to provide effective ground control and safety to the miners is endangered. Such circumstances could result in cata- strophic failure, with potential loss of life. Further, many incipient failures are difficult to detect while the support is in service underground. Typically, supports are not rou- tinely inspected for failure, and problems are most often detected during advanced stages, long after failure has begun. As part of its program to improve mining safety, the Bureau of Mines has been conducting research on shield mechanics for the past 5 years, in the Bureau's unique mine roof simulator shown in figure 1. These nondestructive tests provide a basic understanding of sup- port responses and load transfer mechanics for various load conditions. In addition, the Bureau has conducted several tests in cooperation with participating coal operators and support manufacturers to duplicate observed in-service support failures in the simulator. Field observations of support failures by the Bureau have also provided experiences that contribute to these discussions of safety evaluations of longwall roof supports. The objec- tives of this information circular are to identify the signs of failure, describe the mechanisms that cause support failure, and provide some recommendations for courses of action pertaining to continued operation of support systems that are in the process of failure. Longwall mining in the United States is now at the point where many of the existing face support systems are approaching the end of their service lives. Many mining companies are trying to extend the life of their equipment because of financial hardships associated with current difficult market conditions in the coal mining industry. These developments make it likely that the number of longwall support equipment failures will grow in the next 3 to 5 years. Mine operators and MSHA inspectors must make difficult judgments as to the safety of longwall faces. These people do an excellent job of protecting the American miners, and it is hoped that this information circular will assist them in this goal by pointing out what to look for and what to expect for the types of support failures likely to be encountered underground. It should also make mine personnel more aware of the potential hazards associated with longwall roof support and assist them in the initial selection of equipment. Figure 1. -Bureau's mine roof simulator. SUPPORT DESIGN AND MECHANICS Before support failures are discussed, a brief summary of support design requirements and support mechanics is presented. A more detailed presentation of this material can be found in various other Bureau publications {1-4). } The objectives here are to provide only a basic under- standing of why the support is designed as it is, how it Italic numbers in parentheses refer to items in the list of references at the end of this report. Canopy Leg cylind \ \ capsule ' / •) link Base Figure 2. -Two-dimensional diagram of shield support. transfers loads imposed by the strata, and what conditions cause maximum loading of the support components. This discussion will be limited to shield designs with lemniscate linkage in the caving shield. All shields purchased within the past 10 years, both two-leg and four-leg designs, utilize the lemniscate link design to maintain a constant tip-to- face span throughout the operating range of the support. Approximately 70 pet of existing longwall faces employ two-leg shield supports. A two-dimensional diagram identifying the components of a two-legged longwall shield is shown in figure 2. The canopy and caving shield span the width of the support in the third dimension, and there are both left-side and right- side leg cylinders, front links, and rear links, as depicted in figure 3. Most bases are also of a split configuration, incorporating a left side and a right side. Canopies and bases are constructed as stiffened plates, with a top and bottom plate separated by internal stiff- eners. The plate structure provides bending strength while the stiffeners transfer shear. Bending is the primary loading mechanism for canopy and base components. Links are primarily designed for axial loading and are generally constructed as open or solid box sections. The caving shield is also a stiffened plate design, with bending as the Figure 3.-Three-dimensional representation of longwall shield. primary loading mechanism. These component construc- tions are illustrated in figure 4. Reference to these com- ponent constructions will be made when discussing support failures. Shield supports are designed to provide resistance to both vertical and horizontal loading imposed by the strata, as illustrated in figure 5. In addition, shield supports employ a caving shield to prevent gob debris from entering the face area; hence, the structure must be designed for loads imposed by the gob as well. Figure 6 depicts gob loading on the caving shield. In short, the shield is designed to provide resistance to (1) roof-to-floor displace- ments imposed by the weight and convergence of the over- burden strata, (2) face-to-waste horizontal displacements caused by displacement of the strata in the face area toward the gob, and (3) waste-to-face loading (displace- ments) imposed by gob loading on the caving shield and internal forces developed from horizontal components of the leg reactions. A two-dimensional representation of these three load conditions is illustrated in figure 7. The shield must also be designed with sufficient out-of-plane (in reference to the two-dimensional diagram depicted in Canopy Caving shield PLAN G3 II ll ll ll !• ii i H ll H II II H ll II ii II II II II H ii II II II II II i' 'i I i! PLAN SIDE ELEVATION ii "O ■^ S) SIDE ELEVATION Lemniscate links 1 ! ! 1 ! 1 Centerline — *- i : : ! ! 1 Base ) ii ii(p)i; I . . ii .1 i i I. PLAN PLAN Base SIDE ELEVATION SIDE ELEVATION Figure 4. -Shield component construction. figure 2) stiffness to resist lateral loading parallel to the face. In level seams, strata loading parallel to the face is unlikely, but lateral loading can be caused by uneven roof conditions along the face line, in which the canopy is not set parallel to the base (fig. 8). Pitching seams with face advance along a strike are more likely than nonpitching seams to have a lateral load component acting on the shields. The support contacts the roof and floor strata via the canopy and base, respectively. Loads are transferred between these components through either the leg cylinders or the caving shield-link assembly. Resistance to vertical loading is provided primarily (95 pet) by the leg cylinders, while resistance to horizontal loading is provided by both the leg cylinders and the caving shield-link assembly. The distribution of horizontal load between the leg cylinders and caving shield-link assembly depends on the degree of translational freedom in the numerous pin joints of the structure. As the support ages and these joints wear, the effectiveness of the caving shield-link assembly to resist horizontal loading decreases. Conditions that produce maximum loading of each sup- port component are summarized in figure 9. Overall, the worst condition for two-leg shield supports is standing on the toe of the base. This produces maximum stresses in the base structure, lemniscate links, and caving shield. Vertical shield loading Horizontal shield loading Vertical shield loading Figure 5.-Vertical and horizontal loads acting on shield support Maximum stresses in the canopy are developed when tip loading is a maximum, which occurs when the canopy is set with the tip in the air or under two-point contact at the ends of the canopy. Evidence of high tip loading can be high pressure in the canopy capsule or rotation of the canopy tip about the leg connection toward the floor. Leg pressure is an indication of overall shield loading and, obviously, leg loading. Leg pressure increases with vertical displacements and face-to-waste horizontal displacements. Gob loading on caving shield Figure 6. -Gob loading on caving shield. LU-LfJJ^ Vertical dispacement Increasing load No load Conopy set parallel to base Canopy not set parallel to base Figure 8. -Lateral loading caused by uneven roof conditions along face. CONTACT CONFIGURATION Face -to waste Increasing Increasing horizontal displacement load load Waste-to- face \ Increasing horizontal displacement \ load Decreasing load Figure 7. -Load conditions (displacements) imposed on shield support. EXPECTED SUPPORT RESPONSE Full canopy and base contact. Use as control standard. Maximum canopy stress due to bending. Potential large stresses in base due to bending with increased link loading. Worst case configuration for two- leg shield. Maximum stresses in base and lemniscate links. High link stresses as links must maintain stability of base structure. Maximum shear stresses in leg socket base connection Figure 9. -Conditions that produce maximum loading in support components. FAILURE MECHANISMS "Failure" is used in this discussion in the context of of the miner. Failure does not necessarily imply complete support performance that differs from design intentions. loss of support capability. Three types of support failures Failure causes diminished performance, which potentially are discussed: (1) inadequate capacity, (2) structural renders the support ineffective or hazardous to the safety failure, and (3) instability. INADEQUATE SUPPORT CAPACITY When support resistance is insufficient to prevent convergence of the strata beyond the hydraulic yield capa- bility of the leg cylinders, supports are said to "go solid," as illustrated in figure 10. In this configuration, the supports usually cannot be advanced and often are abandoned in place. With the trend toward higher capacity supports, abandonment of support systems because of inadequate capacity is now a relatively rare occurrence. Assessment of support capacity can best be made by observing leg pressures. Obviously, if leg pressures never reach yield pressure, support capacity is more than adequate. Loading of supports to yield load should not structurally damage the support, and occasional yielding of legs during a support cycle should not be judged as an indication of inadequate support capacity. As long as the support maintains effective ground control, occasional yielding is acceptable. Concern should be raised when yielding becomes persis- tent and more frequent as the mining cycle progresses. If this occurs on isolated supports, it is likely that there is simply a mechanical problem with those particular sup- ports. The greater concern occurs when the majority of the face supports yield excessively. It is this situation that indicates inadequate support capacity. Support (hydraulic) yielding produces a sound that can be heard by a person standing near the support. The sound is produced by hydraulic fluid passing through the yield valve. There may also be some mechanical noise associated with the change in geometry of the structure. Yielding is a means for the support to relieve load. In normal operation, the support yields slightly for a second or two and then recovers and continues to provide resis- tance to strata loading. Supports that fail to recover quickly probably have defective yield valves. STRUCTURAL FAILURE Structural failure can be attributed to either fatigue or component loading beyond its design strength. Fatigue occurs from repeated loading. Under the action of cyclic loads, cracks can be initiated as a result of cyclic plastic deformation. Even if the nominal stresses are well below the elastic limit, locally the stresses may be above yield because of stress concentrations at inclusions or mechani- cal notches. Consequently, plastic deformation occurs locally on a microscale, but it is insufficient to be described in engineering terms using strength of materials concepts (5). Mechanically, fatigue causes crack formation and promotes crack growth as the number of loading cycles increases. When a crack grows large enough (reaches Figure 10. -Shield supports going solid from inadequate capacity. 10 critical crack length), crack growth becomes unstable and failure (fracture) occurs. Fatigue failures generally form in weldments because of stress concentrations associated with material discontinuities or inherent flaws in pour welds. Fatigue failures develop gradually, but a fracture that results in loss of load-carrying capability can be sudden, being caused by one more application of load. Therefore, fatigue failures can be difficult to detect and can be potentially catastrophic. Structural failures from loading beyond design strength also develop gradually, but they are easier to detect than fatigue failures. The primary failure mechanism is general yielding, in which cumulative deformations are sufficiently widespread to threaten the structural integrity or designed function of a component. Failure (fracture) by static loading is unlikely since longwall roof supports are gen- erally constructed of mild steel, which exhibits good duc- tility. This means that the member will plastically deform or develop plastic hinges to effectively transfer the load before reaching ultimate strength and rupturing. For example, canopy or base sections may permanently deform (bend) an inch or more during plastic deformation. The general yielding observed in these members may not des- ignate areas of maximum stress. Maximum stresses gener- ally develop around holes, such as pin clevises, or in areas where the geometry of structure changes drastically. Deformation in these areas is usually difficult to detect since it is confined to a small area. These localized stress concentrations may not affect the overall structural integ- rity of the component. Conditions that induce maximum loading in support components were identified in figure 9. Base Failures Base failures seem to be the most prevalent type of failure and usually occur from fatigue after the support has been in operation for several panels of extraction. A common failure mechanism is for the leg socket casting to break away from the base structure. Formation of this failure is difficult to detect while the support is in service, as the leg socket is housed deep inside the base structure and this area usually is full of debris. Once the leg socket breaks loose, the support quickly becomes inoperable. The bottom plates of the base have insufficient strength to withstand the leg forces, and the leg cylinder rips the base apart by tearing off the bottom plate. Failure of the base structure (plates) can also occur without failure of the leg sockets. The probable failure mechanism is bending of the base. This is more likely to occur in minesites that have very strong immediate floor strata. In these hard floor conditions, the shearer may leave steps in the floor, as it is difficult to maintain a constant height of extraction from cut to cut. The base structure is then simply supported in two locations and is flexed as loading is applied. Repeated flexure causes the base to deform (plastically) or promotes fracture from fatigue, which eventually results in failure of the base structure. In softer floor conditions, the strata deform to provide a fuller contact to the base, which alleviates much of the bending and reduces the risk of failure. Standing the support on the toe of the base can also result in damage of the base structure. This configuration causes maximum stresses in the toe region, and the base deforms (bends) usually where the cross section is a minimum in the section of the base forward of the leg connection. Internally, the base structure is constructed with stiffeners that hold the top and bottom plates apart to form a beam arrangement that gives the base its bending and shear strength. Cases have been reported in which these stiffeners were not properly welded in place or the dimension tolerances were not within specifications. In these cases, the stiffeners broke loose and the base structures collapsed. This problem appears to be largely a matter of quality control, but it is critical to support safety. Since the stiffeners are hidden inside the base structure, it is virtually impossible (excluding X-ray or ultrasonic inspection) to see these deficiencies prior to failure. Canopy Failures Canopy structures are constructed of stiffened top and bottom plates similar to those of base structures, and hence, they are susceptible to bending-induced failures as well. Structurally, canopies are less stiff than bases, making them more susceptible to failure from bending than base structures. However, while permanent deforma- tion of the canopy is a fairly common occurrence, destruc- tion of canopies appears to be less frequent than observed destruction of bases. This suggests that canopies are less often subjected to critical bending. Three reasons why canopies might avoid critical loading are (1) immediate roof strata are usually partially fractured, and full contact with the canopy is more easily obtained, which minimizes bending moment; (2) tip loading on the canopy is usually smaller than toe loading on the base, since the resultant force is more likely to be located near the toe of the base than near the tip of the canopy, and (3) the canopy surface area is larger than the base area, allowing the canopy to distribute load more efficiently. Another common deformation of the canopy is "wrinkling" of the top plate between the internal stiffeners. This is probably due to concentrated loading at locations between the stiffeners but might also be an indication of failure of the weld that holds the stiffeners in place. If the stiffeners are not secure, the plate may buckle from forces developed within the plane of the plate thickness. 11 Failures of Caving Shield And Links Link members have become considerably more robust in shield designs of the past 10 years, and failures have been substantially reduced. Since the caving shield-link assembly has very little vertical load capacity (stiffness), links are not highly stressed for most load conditions. Almost all link failures can be attributed to conditions or operating practices that promote standing the support on the toe of the base or conditions that cause large horizon- tal displacement of the canopy relative to the base. Failure of the structure is most likely to occur in the region near the pin hole located on each end of the mem- ber. The failure mechanism is most likely crack formation somewhere on the circumference of the hole from local- ized high stress development. The pin holes elongate from continued wear and contact with the higher strength link pins. This results in point loading of the pins and high stress development at the contact areas. These failures are difficult to detect since this area is obscured from view by the caving shield clevis. Although link failures are rare, they can be catastrophic, since the links provide horizontal stability to the support structure. Likewise, caving shield failures are fairly rare but are more likely to occur than link failures. While links are designed primarily for axial loading only, shield mechanics indicate the primary loading mechanisms for the caving shield are bending and torsion. Maximum stresses and failure are most likely to occur in the clevis areas, where pins connect the link members to the caving shield. Some general yielding by bending deformation of the caving shield structure may also occur. Leg Cylinder Failures Assuming the face area is sufficiently stable to prevent violent outbursts of energy (bumps), it is unlikely that leg cylinders will experience structural failure since they are designed to control loading by hydraulically yielding at specified pressures. The most common failure associated with leg cylinders is seal leakage. Evidence of seal leakage is a gradual drop in pressure after the support is set against the roof. Another potential failure mechanism for hydraulic leg cylinders is for the yield valves to malfunction, allowing leg pressure to increase beyond design levels. Usually, the excessive pressure causes seal leakage, so that it is unlikely that sufficient pressure would develop to rupture the cylin- der casing. Supports utilized in bump-prone areas should use high- volume yield valves to allow quick discharge of fluid in order to prevent excessive development of pressure in the leg cylinders. Dynamic loading of supports without the proper yield valves can result in structural damage to the leg cylinders. INSTABILITY The shield is designed to provide stability against vertical, horizontal, and lateral loading. Because of the geometry of the structure, its weakest capability is against lateral (parallel to the face) loading. However, lateral loading is likely to be the smallest component of loading as the strata are least inclined to displace in a lateral direc- tion. Likewise, shield mechanics do not promote internal force developments in the lateral direction. In addition, lateral stability is enhanced by adjacent shields. Assuming there is no structural damage, it is unlikely for state-of-the-art shields to become unstable for vertical, horizontal, or lateral loading regardless of the canopy and base contact configuration. Some configurations produce temporary instability that causes the canopy or base to move slightly, but this movement is sufficient to adjust the contact configuration to a more stable configuration. Standing the support on the toe of the base, particularly with gob loading on the caving shield, is the most unstable configuration for two-leg shield supports. Obviously, when there is structural damage of any kind, the internal stability of the support structure is com- promised. Situations such as those previously described, where components are destroyed or their structural integrity is threatened, can easily render the support unstable. In addition to the base failures previously described, another failure mechanism that leads to dan- gerous instability of the shield is failure of the link pins. The link pins can be subjected to high shear forces from twisting of the caving shield promoted by unsymmetric loading or by large reactions developed from horizontal displacements of the canopy relative to the base. If the link pins in either the front or rear links fail, the shield would collapse under its own weight (geometric instability). Failure of the leg cylinders would also constitute a stability problem, but as indicated, leg cylinders are unlikely to fail from structural damage. Leg cylinders are also protected by internal check valves that prevent pressure loss in the event of rupture of the hydraulic feed line. Designed yielding of the leg cylinder does not promote shield instability. 12 SAFETY PRECAUTIONS This information circular attempts to provide generic evaluations of support safety. However, assessment of support failures is site specific and the magnitude of the problem must be considered in making judgments about support safety. The information provided here can help identify problems and suggest solutions, but the final judgment must be made with considerations of the severity of the problem, number of supports affected, past perfor- mance history, face conditions at the time of failure, and overall situation at the minesite. Finally, the best policy is always to correct problems as soon as they occur. How- ever, since this is often impractical and at times impos- sible, these generic recommendations regarding safety precautions for problem support systems are made with the realization that they are not applicable for all circumstances. SUPPORT CAPACITY CONSIDERATIONS When excessive yielding occurs on most of the face sup- ports because of inadequate capacity, some steps should be taken to reduce support loading. Operationally, there are several courses of action. First, it is generally a good practice to accelerate face advance (mining rate) as much as possible during times of excessive support loading. To some extent, subsidence of the overburden is time dependent, and accelerated advance rates may reduce loading caused by time-dependent failure of intermediate strata. Second, strata behavior and subsequent support loading are dependent upon support-setting loads. Gen- erally, there is an inverse relationship between strata convergence and support-setting forces, as shown in fig- ure 11, where face convergence decreases with increased setting forces. Since support loading after being set increases with increased convergence in proportion to the stiffness of the support structure, reducing convergence by increasing the setting force (leg pressure) reduces overall support loading. However, the total support load is the sum of setting load and subsequent load due to conver- gence. Therefore, increasing the setting load is effective in reducing overall support load only if the reduction in load subsequent to support setting is greater at the higher setting force than the increase in setting load. Conversely, it may be possible to reduce total support load by reducing the setting force if strata convergence is small and setting forces are high. In most circumstances, exces- sive support yielding occurs when support resistance is inadequate to effectively control the strata, and an increase in setting pressure probably is the most effective means of control, but the relationship between total load and setting force should be understood when making judgments of optimum setting forces. Questions are often raised concerning situations where one leg of a support is inactive or operates with diminished resistance. Obviously, this reduces the capacity of the support, but usually the support will remain functional and stable. From a load distribution viewpoint, loads will transfer down the side of the structure with the active leg. It is unlikely the leg pressure in the remaining leg will dou- ble, since additional loading imposed by the strata will be shared by adjacent supports. No large increase in com- ponent stress development (normalized to leg pressure) is likely, although there may be some increase in component stresses due to lateral loading if full canopy contact cannot be achieved at setting, producing unsymmetric load devel- opment. Stress development in this configuration should not exceed the design strength of the structure or pose any danger to the structural integrity of the support. Improper leg behavior due to seal leakage or malfunc- tioning control systems, when isolated to one or a few supports and when at least one leg (in two-leg shields) is functioning properly, should be corrected but usually does not pose an immediate safety danger. Generally, repairs can be delayed until scheduled maintenance periods. Improper leg behavior in isolated supports, in which all legs are not performing to design standards but provide at least 50 pet of design capacity, also usually does not pose an immediate safety hazard, but the problem should be corrected at the earliest convenient time. Finally, dimin- ished leg behavior on the majority of the face supports that results in less effective ground control should be corrected immediately. Z) < Ld O LU O cr UJ > o o SETTING LOAD (P s ) Figure 11. -Relationship between strata convergence and support setting force. 13 STRUCTURAL FAILURE AND STABILITY CONSIDERATIONS As previously indicated, components on most supports are constructed of mild steel (40 to 60 ksi yield stress) that is capable of large plastic deformation without fracturing. Hence, canopies and bases that are permanently deformed but do not show any signs of fracture along welds or elsewhere generally do not pose an immediate safety hazard. Yet, the significance of deformed members must be understood. While permanent deformation does not lower the yield strength of the material, members that are deformed by stressing beyond the elastic limit of the material (plastic deformation) maintain residual stresses after being unloaded. The residual stresses make the stress-strain curve more nonlinear so that the material will strain more for a given load increment, which means that it then takes less loading to further deform the member. Essentially, the member is weaker once it is deformed and gradually becomes weaker and weaker as the residual stresses build up from continued plastic deformation by loading beyond the material's yield stress. Therefore, while deformed members may not constitute an immediate safety hazard, they should be a warning of potential prob- lems that eventually may seriously threaten safety. Most higher strength steel constructions are used in canopies for support applications in low seams. These constructions use steels of 80 to 100 ksi yield stress, which exhibit much more brittle failure than the more commonly used mild steel constructions. Members constructed of these high-strength steels are much more susceptible to fracture from static loading beyond the elastic range. Therefore, deformation and crack formation are much more critical to support safety for components constructed of high-strength steels. Crack formation in any part of the support structure is a sign of potentially imminent danger. The crack indicates that the steel has failed. Whether this crack will propagate to cause destruction of a support component depends on many things, most notably, the ability of the member to effectively redistribute loading within itself. This is what makes judgments of support safety in these situations difficult. In any event, cracks in a support structure should be closely monitored, and as soon as the structural integrity of the support component is threatened, the support should be taken out of service. Support resistance is provided primarily by the hydraulic legs. Hence, it is unlikely that the support will show any signs of diminished load-carrying capability (support resistance) while structural problems to other components are developing. The issue concerning these structural problems is more one of stability than of support resis- tance. An unstable support is more dangerous to the safety of the miners than one that goes solid from inadequate capacity. For example, failure of the link pins would do little to affect the capacity of the support, but as indicated previously, the support would collapse under its own weight from instability. HYPOTHETICAL FAILURES To help put these issues into perspective, some specific recommendations of safety precaution are described for hypothetical structural problems that are most likely to be encountered. Again, these are generic solutions to site- specific problems that require careful consideration. It should also be understood that shield supports have very little redundancy in their design; all components must function to provide a stable and safe support. While most components are paired (left- and right-side member), it is generally not wise because of this apparent redundancy to permit continued operation when only one component has failed. Finally, it is better to try to determine the cause of failure before supports are repaired and put back in service. Fixing one problem may create another by transferring loads to other parts of the structure. 1. Low leg pressure in one leg cylinder .-Assuming the support continues to function and provide effective strata control, the operator can continue mining while investi- gating the cause of diminished leg resistance. If the problem can be corrected on routine maintenance shift, it should be. If it cannot be fixed on routine maintenance (for example, if the problem is determined to be seal leakage), the operator should continue mining until a convenient opportunity presents itself to change out the leg. If strata control does not diminish, operation can continue to panel completion, but leg reactions and support behavior should be closely monitored. 2. Wrinkling of canopy top plate.-The operator should monitor periodically for crack formation in the plate structure and continue to operate until crack formation appears. After crack formation begins, the operator should change out canopies at earliest convenience; panel completion is usually acceptable. 3. Bent canopy— The operator should continue to operate while monitoring for crack formation. The operator should inspect weldments and look for signs of failure where the tapered section of canopy begins and look for any indication that internal stiffeners are broken loose, such as large deformations running down the length of the canopy. If structural integrity of canopy seems threatened (that is, crack development and progression), the canopies should be changed out as soon as possible. 14 4. Leg socket broken loose from base structure. - Operations should be stopped and the base unit replaced immediately. This is a very serious problem. 5. Significant crack growth in top or bottom plate weldments of base structure. -The, mine operator should replace the base units with stiffer design at first scheduled maintenance period. Obviously, if failure has resulted in destruction of the base unit, it must be replaced immediately. 6. Sheared link pin -Operations should be stopped immediately and the pins replaced. The operator should examine for damage of adjoining clevises and repair or reinforce clevises as necessary. CASE STUDIES OF SUPPORT FAILURES A few case studies of support failure are described in an attempt to put the issues of longwall support safety into perspective. The names of minesites and support manu- facturers are withheld so as not to disclose any confiden- tial information. 1. Leg cylinder casing failure. -Figure 12 shows one of the rare occurrences of leg casing failure. A section of the casing ruptured and blew out while the leg cylinder was under high pressure. Luckily, no one was injured. The cause of failure was attributed to damage done by welding of the hose inlet lug on the cylinder casing. 2. Unstable shield design. -An entire face of four-leg shields was lost in 1988 because of stability problems. Split caving shield Figure 13.— Split caving shield design that failed from instability. Figure 12. -Failure of leg cylinder casing. Figure 14. -Permanent deformation of base structure. 15 These shields featured a split caving shield design as illustrated in figure 13. No apparent structural damage was observed prior to failure. The split caving shield reduces out-of-plane stiffness of the structure, which reduces the support's resistance to lateral (parallel to the face) loading. This design, coupled with wear in the pin joints, was the probable cause of failure. The entire face was abandoned in place. 3. Bent base structure -An example of general yielding of a component by loading beyond its elastic design strength is shown in figure 14, where the toe region of a base structure is permanently deformed. Notice the rela- tively smaller cross section of the base, which provided inadequate bending strength. Standing the support on the toe of the base was identified as the load condition that produced the failure. The failure eventually rendered the support inoperable, and the base units had to be replaced. 4. Leg socket failure. -An example of leg socket failure in the base structure of a two-leg support is shown in figure 15. The figure shows dye penetrant application to highlight the failure, which occurred in the weldments. These bases were removed and replaced in the middle of panel extraction, causing several weeks of downtime. 5. Base stiffener failure. -Figure 16 shows a modification that was made to stiffen a base structure in the region between the front and rear link pin and to provide out-of- plane stiffness to the base structure. An interesting point concerning this failure was that the first sign of failure noticed underground was in the caving shield clevis area. The weakness in the base unit caused the base to bend, which in turn caused excessive link loading. This is an indication of how failure of one component can affect the loading of another component. 6. Structural failure of canopy. -Figure 17 shows failure (crack formation) of a canopy structure near the canopy hinge, from loading beyond its design strength. Also shown in the figure is a finite element analysis of stress concentrations in this section. This is a good example of how development of localized high stresses in areas around holes causes failure. The hinge area was strengthened by adding a reinforcement plate in front of it. Figure 15.-Example of leg socket failure. 16 Area of modification Figure 16. -Base failure and subsequent modification. SIDE VIEW *C _fo' KEY b'.v.y.l < 40,000 psi 1 ■ 40,000 - 70,000 psi M > 70,000 psi «---^-4=°="-i Ttnxr j; i_j^ _H |l_ -ti- ll II E) L-l-L-J j. i — ighest principal stress Finite element model stress prediction Not to scale Figure 17. -Failure of canopy structure from high stresses in canopy hinge. 17 FUTURE RESEARCH EFFORTS The Bureau proposes to conduct controlled destructive testing of various shield supports in the mine roof simulator. The primary goal will be to assess the impact of failure on the safety of the support system. More specifically, the objectives of this research will be to identify conditions that cause failure and the effect of the failure on the load transfer mechanics of the support. Once this is understood, better judgments can be made as to the safety of support systems that have developed structural and mechanical problems. In addition, the Bureau is continuing current nonde- structive testing efforts to further enhance the knowledge of support mechanics. The intent of this research is to improve support selection and design and to further improvements in performance testing techniques. These efforts will minimize the risk of support failure with current supports and provide engineering guidelines to eliminate current failure mechanisms in future support designs. CONCLUSIONS Longwall mining has improved the safety and increased the productivity of underground coal mining. Ground control is an important consideration in longwall mining. State-of-the-art powered roof supports usually provide effective ground control, but failures of roof support structures still occur. These failures lower productivity and endanger the safety of the miners. This information circular provides insight into the causes of these failures and their impact on the capability of the support to continue to provide safety to the miners. The Bureau's research as presented in this report should provide assistance to MSHA and industry personnel in conducting safety evaluations of longwall roof support systems and should make mine managers and engineers more aware of the problems and consequences of support failures. REFERENCES 1. Barczak, T. M., and D. E. Schwemmer. Horizontal and Vertical Load Transferring Mechanisms in Longwall Roof Supports. BuMines RI 9188, 1988, 24 pp. 2. . Two-Leg Longwall Shield Mechanics. BuMines RI 9220, 1989, 34 pp. 3. Barczak, T. M., D. E. Schwemmer, and C. L. Tasillo. Practical Considerations in Longwall Face and Gate Road Support Selection and Utilization. BuMines IC 9217, 1989, 22 pp. 4. Barczak, T. M. Research on Shield Supports Using the Bureau's Mine Roof Simulator. Paper in MinTech '89. Sterling Publ., Mar. 1989. 5. Brock, D. Elementary Engineering Fracture Mechanics. 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