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By Rudiger Baltz

The sudden failure of bodies of rock is a serious problem commonly experienced in the international mining industry. These explosive releases of energy are referred-to as ?bumps?, ?outbursts? and ?rockbursts?. Rockbursts can occur at such a rapid speed that they present a direct threat to safety in large parts of the mine workings. The build-up of excess stress within the body of rock can be attributed to different causes. In shallow workings, for example, sudden and violent failure of the roof strata can be caused by leaving insufficient pillar width when practising room-and-pillar mining. At much greater working depths, as it is the case in the German coal industry, rockbursts are generally attributable to high levels of rock pressure. This can even result in coal outbursts when driving individual roadways. More than thirty years ago the German coal industry suffered from numerous rockburst incidents which caused many deaths and serious injuries. This led directly to the establishment of the Rockburst Prevention Centre, which was created at the instigation of the mining companies and the chief mines inspectorate. The consistent logging and analysis of all rockburst incidents has led to the development of a system that is still used as a basis for preventing underground rockbursts and minimising the risk of such phenomena occurring. Working closely with the mining authorities and mining companies the accredited experts of the Rockburst Prevention Centre have played a key role in helping to develop and implement a range of methods for the early detection and prevention of rockbursts.

Jan 1, 2008

By Gregory E. Smith

"Tensionable headed roof bolt systems are common in the U.S. mining industry. In the last 15 years or so, they have made a comeback as a hybrid system where a mechanical anchor is used in conjunction with resin. When such a system is used, it is common to use a spherical washer to comply with the requirements of 30 CFR Part 75.204(f)(8). This standard requires an angle-compensating device be used in installations where the bearing plate is offset from the roof bolt axis by an angle greater than five degrees. This paper evaluates system performance of four different traditional angle-compensating device/plate combinations including a new patented extruded plate/extruded washer system that improves overall load capacity and stretch.INTRODUCTIONA large western U.S. underground coal mine was using a fully grouted headed rebar system as their primary support. They had developed their component specifications over many years of working with suppliers to improve and optimize these components to integrate well as a system. As shown in Figure 1, the components included a patented 7 /8"" -diameter Grade 60 headed rebar bolt with a flange capable of full bar load (Walker, 2009) and a patented 8 in x 8 in Grade 3 extruded hole bearing plate manufactured by DieMatic Corporation.The mine decided to pursue a fully grouted tensionable system using a mechanical anchor as the tensioning device. Their primary goal was to eliminate the need for the roof bolt operator to secure (hold) the bolt during resin set time. An additional benefit was the increased initial tension on the bolt. To comply with the requirements of 30 CFR Part 75.204(f)(8), regarding tensioned roof bolts, they were required to include an angle-compensating device because a share of the bolt population would be installed at angles greater than five degrees from the perpendicular to the bearing plate. Also, to improve cycle time and torque/tension consistency, the mine desired to have the washers and anchors assembled at the factory. This required the use of a bearing plate with a center hole large enough to accommodate the 1-3/8-in-diameter mechanical anchor to pass though it during assembly by the roof bolt operator. The mine wanted to continue to use an extruded hole bearing plate in the tensionable system, and EMS was asked to propose a plate and angle-compensating device."

Jan 1, 2016

By Stephen R. Iverson, Jeffrey C. Johnson, Donovan J. Benton, Michael J. Raffaldi, Lewis A. Martin

"The NIOSH ground control safety research program at Spokane, Washington, is exploring applications of photogrammetry to rock mass and support monitoring. This paper describes two ways photogrammetric techniques are being used. First, photogrammetric data of laboratory testing is being used to correlate energy input and support deformation. This information can be used to infer remaining support toughness after ground deformation events. This technique is also demonstrated in a field application. Second, field photogrammetric data is compared to crackmeter data from a deep underground mine. Accuracies were found to average 8 mm, but have produced results within 0.2 mm of true displacement, as measured by crackmeters. Application of these techniques consists of monitoring overall fault activity by monitoring multiple points around the crackmeter. A case study is provided in which a crackmeter is clearly shown to have provided insufficient information regarding overall fault ground deformation. Photogrammetry is proving to be a useful ground monitoring tool due to its unobtrusiveness and ease of use.INTRODUCTIONNIOSH Mine Safety ResearchPhotogrammetry systems have been implemented by the National Institute for Occupational Safety and Health (NIOSH) as part of its ground control research to improve mine safety. Conventional monitoring of ground movement has typically focused on movement of a few discrete points that are marked or anchored to instruments. Loss of a designated point or anchor, or the ability to identify a stable point in an unstable area, have often frustrated monitoring efforts. NIOSH researchers are using photogrammetry to conduct full-field measurements of rock surfaces underground. Periodic measurements of the entire surface of a ramp allow patterns of displacement to be observed. Full-field measurements provide much more insight into ground behavior than point measurements.BackgroundPrevious work by Benton, et al. (2014) discussed laboratory calibration and verification testing of two photogrammetry systems being used by NIOSH researchers. Both systems utilize stereoscopic image pairing techniques for photogrammetric reconstruction. This paper discusses how a laboratory system was used to conduct volumetric analyses of testing of shotcrete panels with varying types of reinforcement. The laboratory system was found to be accurate to within 2 mm. Additionally, a field photogrammetry system was found to produce linear measurements within 1 mm of known lengths in laboratory conditions. This system is currently being used at a deep underground mine to observe ground support conditions and deformations that have occurred. This paper discusses comparison of preliminary volumetric measurements of rib deformation to laboratory tests, as well as comparison of the field system against a crackmeter installed across a fault surface where it intersects the ramp."

Jan 1, 2015

By Brian Daniels, James Farroni

"Draw rock (small rock that forms/falls when bolting the roof and/or rib) is the cause of numerous injuries annually. These injuries result in lost work hours and low morale. The report below explains how ""Mine Safe Draw Rock Shields"" (Pans) are used in an underground operation. This report will chronicle the application of these pans in a Southern West Virginia coal mine along with testing data that measures their effectiveness. Since the introduction of these pans in 2002 the amount of reportable accidents relating to draw rock has decreased by 80%. The report below reinforces that perimeter contact is critical to the performance of these pans. In addition the strength of these products at the perimeter will determine their effectiveness. The results of the testing data outlined in this paper, along with the physical records at the study mine, confirm that the use of pans has significantly reduced draw rock falls at this facility.BACKGROUNDSupplemental support is defined as ground control that is used in addition to required support in an underground mine. Some examples of supplemental support are:• Pans• Mesh/Geogrid• Mats• Half Headers• Boards• ChannelA typical roof condition that warrants the use of pans is when loose small rock is dislodged while bolting. If the top is very scaly (loose, thin rock) additional supplemental support can be used in conjunction with pans such as mesh or geogrid (See Figure 1). Pans will provide protection where used and approximately 12 in (250 mm) out from perimeter. This is achieved by contact with a surface which will put scale in tension up to 12 in (250 mm) from perimeter of pan.INTRODUCTIONThe ease of applying a roof support system is critical to effectively operating a deep mine. Figure 2 shows how a pan is installed in conjunction with a roof bolt and bearing plate. The Mine Safety and Health Administration (MSHA) requires that supplemental support cannot decrease the performance of required support. The goal of using all supplemental support is to provide additional safety for the miner which equates to increased productivity.The ability of skin control products to contact a surface is the single most critical characteristic (Figure 3 and Figure 4). If the material does not contact the surface it cannot be effective. Therefore, the flexibility, design and proper installation of the products are critical in determining their effectiveness. Figure 5 shows the proper installation of the roof pan.To reiterate, all skin control must be applied in conjunction with a roof bolt and ASTM approved bearing plate as shown in Figure 4."

Jan 1, 2010

By Rob Thomas

Strata management is a critical and now necessary requirement in the New South Wales underground coal mining industry. As a formal and structured process, strata management allows ground support and mining methods to be tailored to the anticipated and encountered geotechnical environment in a controlled manner. In its application, strata management therefore enables operators to identify and control geotechnical hazards and the risks they pose to the mining operation, including risks to personnel safety and mine production. The paper considers recent developments in strata management in the New South Wales underground coal mining industry. Points covered include the statutory requirements stipulated in the Coal Mine Health and Safety Regulations, Geotechnical Hazard Plans, tiered support plans, the Authority to Mine concept, Trigger Action Response Plans and monitoring. Several successful and unsuccessful case studies are included which make specific reference to roadway development and longwall retreat and in doing so, the unique demands imposed by these two different aspects of the mining operation on strata management.

Jan 1, 2007

By Darin Cotton, Tim Burgess, Tim Martin

"As the leading US supplier of roof control drills, J. H. Fletcher and Company® has encountered an extensive set of roof strata challenges and has overcome various drilling issues by varying machine parameters and working with customers and tool manufacturers to develop solutions. Every mine wants to drill and install roof bolts as fast as possible with maximum safety at the lowest possible installation cost for their conditions. In most circumstances, the intuitive method of higher speeds and pressures can actually have a detrimental effect on the roof drill safety and performance.This paper will outline recent tests carried out at several different coal mines in the Eastern United States. The tests focused on optimizing the drilling parameters of feed pressure, feed rate, rotation speed, and drill torque.The drill feed, rotation speed, and rotation torque, as well as the type of bit selected, have a definite impact on the performance of the roof drill including drilling speed, safety, and tool life. The drilling process can be improved while also improving operator safety and reducing bolt installation costs by maintaining the appropriate roof drill settings for the specific strata conditions.The advantages of improving the drilling process includes increased bit life, increased drill steel life, reduced hanging drill steels, reduced clogging of the vacuum system, decreased whipping drill steel potential, reduced noise generation, and increased cutting size which reduces fine dust generation and dust collection system servicing.INTRODUCTIONIn recent times, the economic pressures that coal producers are facing have created an increased emphasis on controlling operating costs. Several coal operators have pointed out their dissatisfaction with low bit life, and there is a renewed interest in improving tool life and machine performance as part of an overall cost control strategy. Operators are looking for ways to reduce tooling cost, maintenance costs, and drill downtime without decreasing drill performance. This renewed interest in reducing operating costs, along with the ongoing desire to improve safety, created a perfect environment to take a fresh look at the existing fleet of machines to see what could be accomplished. The goal was to take a practical approach to reviewing machine parameters to determine what changes could be made. The emphasis was on changes that would be feasible to roll out to an entire fleet of machines in a short period of time with minimal capital expenditure. The thought process was that significant improvements could be made by simply tuning up the machine parameters to provide a more efficient and safer bolting process."

Jan 1, 2015

By Eric C. Poeck, Ryan Garvey, Ugur Ozbay, Kun Zhang

"A coal bump is characterized by the sudden failure of one or more pillars and an associated release of kinetic energy. Although the geologic conditions surrounding coal bumps are often similar, their occurrence and magnitude are difficult to predict. This paper presents the development of an approach to assess the potential for coal bumps in room and pillar mines through the use of energy concepts and special consideration of the interface properties between the coal and overlying rock. Back analyses were performed on the widespread collapse of a room and pillar mine with an associated 3.9 local magnitude seismic event. Pillar and mine-scale models were constructed using the distinct element method to simulate the mining stages leading up to the collapse. A variety of loading conditions, material properties, and interface properties were evaluated for their effect on unstable failure of single pillars, and the mine-scale model was used to study the evolution of failure during progressive mining. The extent of unstable failure was quantified in these models by calculating the total kinetic energy released during each mining stage. Through the parametric analysis of single pillar models, it was found that softening parameters in either the coal or the coal/ rock interface can individually facilitate unstable failure of pillars, but the combination of the two in a single model produced a much higher release of energy during failure. The results of the minescale model further illustrated the trend as the combination of softening parameters in the coal and coal/rock interface produced a series of failures that most accurately reflected the collapse event at the mine. The method of room and pillar stability analysis demonstrated in this study effectively explores the potential for large coal bumps.IntroductionThe analysis of coal pillar behavior is challenging because natural variability in geologic conditions requires estimation of material strengths and predicted loads for a given mining situation. Large width-to-height (w/h) ratio pillars and deep overburden present an additional challenge, as the failure behavior of such pillars is more complex and may involve unstable, sudden collapse. Empirical methods have improved the success of coal pillar design for a wide range of conditions, but, for deep cover and squat pillar (w/h > 10) geometries, pillar performance predictions are not as accurate (Mark, 2000). As with any empirical approach, more back analysis and the consideration of new variables may help improve understanding and accuracy of future designs."

Jan 1, 2015

By Andrzej Walentek

This paper presents the results of numerical modeling of the fractured zone around longwall gateroads. The numerical computations have been based on the results of underground measurements of the fractured zone around longwall gateroads, which were performed for various geological and mining conditions and measured with the use of the endoscopic method with a digital camera. Having results of the underground investigations, trials of duplicating them by the Finite Element Method using software PHASE and the Hoek-Brown failure criterion were performed. In order that the fractured zone predicted by the model-based computations closely simulate that measured underground, some selected parameters in the Hoek-Brown failure criterion were modified based on the earlier determined relations used for prediction of the roof-to-floor convergence in longwall gateroads. As a result, the model-based computations produce results very close to those obtained from the underground measurements. Determination of the fractured-rock zone around the mine roads is of great importance to mining practice, especially in planning and designing supports for longwall gateroads. The influence of the front abutment pressure causes the fractured rock zone around the roads to increase and results in an increase of support load. Moreover, proper determination of the fractured rock zone is one of basic elements in the process of planning roof bolting for mine workings.

Jan 1, 2009

By Hongwei Zhang, Sen Yang, Shimin Liu

"This laboratory study investigates the damage evolution of sandstone specimens under two types of cyclic loading conditions through monitoring and analyzing the variations of elastic moduli and ultrasonic velocities. We adopted the rigidity degradation method for evaluating the damage accumulation through the rock dynamic ultrasonic velocity measurements. Compared to the ultrasonic test, the method of rigidity degradation shows deficiencies in the detection of damage development. The cyclic loading within the elastic limit led to a stiffening trend for the loading process but a softening trend for unloading process. The method of rigidity degradation was unable to describe the damage accumulation in this situation, while P-wave velocity clearly reflected the damage development. Moreover, a simplified model is established for interpreting the behaviors of the modulus variations and ultrasonic velocity. The cyclic loading within elastic limit enhances the anisotropy of rock, which causes a denser packing pattern in the direction of axial stress. The persistent damage accumulation under the cycling with an increasing upper limit is the consequence of the tensile failure in the microstructure of sandstone. INTRODUCTION The damage accumulation due to dynamic (cyclic) loading is highly detrimental in many engineering structures, such as mine openings, petroleum and natural gas boreholes, tunnels, rock foundations, and underground chambers. The sources of cyclic or repetitive loadings can be roughly divided into two types: (1) periodic operation, including drilling, blasting, and mining; (2) random vibration, including earthquakes and traffic loads. The repetitive loading-unloading process opens and closes the micro-pores and micro-cracks within the rock, further inducing the growth of cracks. As expected, the accumulating damage becomes a potential factor to trigger the rapid and violent failures of large-scale engineering rock structure."

Jan 1, 2017

By Jurgen te Kook

State-of-the-art for subsidence calculation are stochastical models. Although these models were successfully used all over the world they have their limitations due to the fact that they are not based on mechanical laws. For example the models did not take the rock mass structure and the rock mass characteristics into account. Therefore in a research program the skills of geomechanical numerical models for subsidence calculation were investigated. A case study shows that numerical models which are in principle based on the elastic theory are suitable for subsidence prediction. Due to the principles of these models the results are not sufficient in the case of multiple seam extraction. More extended geomechanical numerical models like FLAC and UDEC were investigated in further steps of the research work. Up to now large area geomechanical models require unrealistic long calculation times if the level of detail is similar to that of small scale calculations. Due to this it was necessary to implement a simplified description of the rock mass structure and characteristics. Different case studies show that this approach is successfull for subsidence calculation by numerical geomechanical models even for multiple seam extraction.

Jan 1, 2008

By William G. Pariseau, Mark K. Larson, Douglas R. Tesarik, Heather E. Lawson

"This contribution describes development and application of a user-friendly finite element program, UT3PC, to address three important problems in underground coal mine design: (1) safety of main entries, (2) barrier pillar size needed for entry protection, and (3) safety of bleeder entries during the advance of an adjacent longwall panel. While the finite element method is by far the most popular engineering design tool of the digital age, widespread use by the mining community has been impeded by the relatively high cost of and the need for lengthy specialized training in numerical methods. Implementation of UT3PC overcomes these impediments in three easy steps: First, a material properties file is prepared for the considered site. Next, mesh generation is automatic through an interactive process. A third and last step is simply execution of the program. Examples using data from several western coal mines illustrate the ease of using the application for analysis of main entries, barrier pillars, and bleeder entry safety.INTRODUCTIONRational design of safe underground entries and crosscuts, barrier pillars, and bleeder entries begins with a site-specific analysis of stress. An analysis of stress identifies the critical regions of high-stress concentration where yielding is likely and additional or more focused ground control measures are needed. This analysis also identifies regions where stress concentration is low, and risk mitigation is less urgent. Displacements induced by mining, including roof sag, floor heave, and pillar squeeze, are also of importance to stability. The popular finite element method is well-suited for such analysis and has been in use for studying coal mine engineering problems since the late 1960s (Dahl, 1969). The method has been taught in undergraduate engineering curricula for many years and offers an alternative to existing design procedures based mostly on rules of thumb, empirical methods based on mine data, and predigital-age concepts of strata mechanics. However, despite the versatility of the finite element method, usage has been limited, mainly because of the cost of the software and the required training in numerical methods.The program UT3PC (Pariseau, 2017) has evolved from early two-dimensional versions in the era of mainframe computers. These early versions were still quite limited using only a few hundred elements that required overnight turnaround times. Recent versions of UT3PC are convenient and efficient desktop programs that use several million elements. With the advent of personal computers in the mid-1980s and subsequent increase in computer speed, expanded memory, and lower costs, applications for mining increased in three-way cooperative projects among industry, the former U.S. Bureau of Mines, and academia. The Spokane Research Center of the U.S. Bureau of Mines, in cooperation with the University of Utah, developed a two-dimensional desktop finite element program, UT2PC, available to the mining community in 1991. This was a short course offered at the 1991 SME Annual Meeting in Denver, CO."

Jan 1, 2017

By Stephen C. Tadolini, Anthony T. Iannacchione

"Coal burst represented a major hazard for some U.S. mining operations. This paper provides an historical review of the coal burst hazards, identifies the fundamental geological factors associated with these events, and discusses mechanisms that can be used to avoid their occurrences. Coal burst are not common in most underground mines. Their occurrence almost always has such dramatic consequences to a mining operation that changes in practice are required. Fundamental factors influencing coal burst events include strong strata, abnormal strata caving, elevated stresses, critical size pillars and the lack of sufficiently sized barrier pillars during extraction. These factors interact to produce excessive stress, seismic shock and loss of confinement mechanisms. Over the 90 years of dealing with these hazards, many novel prevention controls have been developed including novel mine designs and extraction sequences, most of which are site specific in their application. Without an accurate assessment of the fundamental factors that influence coal burst and knowledge of their mechanisms of occurrence, control techniques may be misapplied and risk inadequately mitigated. INTRODUCTIONCoal burst1are violent failures of ribs, roof or floor in underground mines. They are known to occur in complex ways and often under unique sets of conditions. This has made them extremely difficult to anticipate and control. Assessing coal burst risk requires engineers, managers, and safety professionals to recognize the fundamental factors responsible for these events. This helps practitioners recognize the hazard potential and understand the context in which various controls and barriers should be used. Only then can coal burst events be anticipated andappropriate mitigation techniques deployed.Over the last 90 years, a few coal burst events have been anticipated but most have eluded prediction. The authors examined the historical record (Appendix A) and determined that an inadequate understanding of the initiating geologic factors and their associated mechanisms of occurrence have contributed to the poor predictive capability. The historic record also indicates that control techniques have typically been site specific in their application."

Jan 1, 2015

By Thomas A. Cook

The effectiveness of tensioned roof bolt systems has been well researched and documented over the years (Stankus, 1990, 1991, 1997; Guo, 1997; Denton, 2000; Su, 2007). Higher and more consistent installed tension has been shown to improve the beaming effect and associated roof stability especially in highly-stressed, weak, laminated roof strata. To accomplish higher and more consistent bolt-installed tensions, various types of anti-friction washers have been developed and applied to many types of tensioned roof bolt systems. In addition to increasing bolt-installed tension, these washers also provide a lubricating effect that minimizes sparking during the bolt torqueing cycle. The most popular and widely used anti-friction washer configuration consists of a plastic washer that is used in conjunction with a standard hardened steel washer. This washer configuration has performed well in the field in terms of increasing the bolt-installed tension (or plate load) and the related tension/torque ratio (the amount of tension achieved per one ft-lb of applied torque). However, the installed tension does not always maintain consistency. One reason for this inconsistency is that the plastic anti-friction washer sometimes dislodges and shreds or tears before the bolt torqueing cycle is complete. This condition prevents the plastic washer from acting as an anti-friction medium for the entire torqueing cycle. Jennmar Corporation and Keystone Mining Services, LLC, developed the ?TT? washer in early 2012. The ?TT? anti-friction washer is comprised of two zinc-coated (galvanized) hardened steel washers that are joined together with an exclusive polymer. The ?TT? washer has been extensively tested and applied in the Sewickley, Pittsburgh, and Indiana #5 coal seams. The test results indicate an average increase in installed load and tension/torque ratio of 20%. More importantly, the installed load consistency improved, with a significant reduction in the standard deviation of values. This paper will detail the development and testing of the ?TT? anti-friction washer with case studies of actual mine applications.

Jan 1, 2014

By Gennaro G. Marino

As part of the development of a shopping center in West Virginia, a significant bench had to be cut into a mountainside that was over 700 ft. high. As a result, a steeply stepped, over 400-ft.- high wall was created. The high wall exposed old abandoned room-and-pillar coal workings at the toe of the cut. Because of exposure to the elements, the mine openings began to severely deteriorate with significant potential of undermining of the rock cut face and causing a large rock fall(s). Exacerbating this wall problem was a set of fairly planar sub-parallel and sub-vertical joints creating, in essence, thin rock slabs. Also present were stress relief fractures that were observed in the exposed pillars. Given the proximity of the project building to this high rock face, mitigation of the rock fall potential was deemed necessary. In order to prevent continued damage and deterioration of these exposed mine entries, the rubblized openings were filled with bulkhead grout and pressure injected with grout. The total amount of grout installed in these openings was about 3,945 CY. Also, a rock drape was attached to the entire face to mitigate small rock falls from natural causes. There also existed another level of mining below the ground surface and under an existing building at a depth of about 160 ft. Under a portion of the building, the pillars in these old works were second mined, and the building seemed to have been already affected by limited subsidence above this area. Also, calculated pillar safety factors in this high extraction section of the mine were too low. Consequently, these old works were stabilized by saturation grouting. An indication of overall roof distress was the significant fracturing encountered in the bedrock overburden during the drilling of the injection holes and the volume of grout placed in these holes. The total grout installed in the high extraction section of the lower mine was about 6,660 CY. This paper describes the investigation and the grouting techniques employed at the rock cut face and under the existing building to mitigate continued damage to avoid future subsidence problems at the face and the building premises.

Jan 1, 2013

By Ken W. Mills

The ground movements associated with underground coal mining and, in particular, longwall mining, are recognised to include horizontal subsidence movements, but the mechanics of the processes that cause these horizontal movements are not well understood. Over the last two decades, three-dimensional subsidence monitoring has become routine in Australia and has provided a wealth of measurements of horizontal movements caused by mining subsidence. These measurements and other sub-surface observations allow the processes that cause mining-induced horizontal movements to be inferred and, subsequently, verified. In this paper, the mechanics of the processes that cause horizontal movements, particularly those in sloping topography, are described and discussed on the basis of field observations. There are several processes recognised to generate horizontal subsidence movements. In flat terrain, systematic horizontal movements cause the surface to move initially toward the newly created goaf and, subsequently, in the direction of mining. Tectonic energy stored as horizontal stress is released by mining, and, when the horizontal stresses are high, the magnitude of this horizontal stress relief movement is large enough to be perceptible for some kilometres from the panel. In sloping terrain, there is an additional component of horizontal movement that occurs in a down slope direction. This movement, sometimes referred to as valley closure movement, has a magnitude that is typically much greater than systematic or stress relief movements.

Jan 1, 2014

By Michael Gauna, Christopher Mark

"For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines. Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by a roof fall than other coal miners. Since 2007, however, there has been just one fatal roof fall on a pillar line. This paper describes the process that resulted in this historic achievement. It covers both the key research findings and the ways in which those insights, beginning in the early 2000s, were implemented in mining practice.One key finding was that safe pillar recovery requires both global and local stability. Global stability is addressed primarily through proper pillar design, and became a major focus after the 2007 Crandall Canyon mine disaster. But the most significant improvements resulted from detailed studies that showed that local stability, defined as roof control in the immediate work area, could be achieved with three interventions:• Leaving an engineered final stump, rather than extracting the entire pillar;• Enhancing roof bolt support, particularly in intersections, and• Increasing the use of Mobile Roof Supports (MRS)A final component was an emphasis on better management of pillar recovery operations. This included a focus on worker positioning, as well as on the pillar and lift sequences, MRS operations, and hazard identification. As retreat mines have incorporated these elements into their Roof Control Plans, it has become clear that pillar recovery is not ""inherently unsafe.""The paper concludes with a discussion of the challenges that remain, including the problems of rib falls and coal bursts.INTRODUCTIONPillar recovery has always been an integral part of underground coal mining in the US. When room-and-pillar methods are employed, large blocks of coal in the form of pillars are initially left in place to support the weight of the overburden. Unless these pillars are subsequently recovered, the coal they contain will never be mined."

Jan 1, 2016

By Steve Tadolini

Underground roof falls can have devastating effects on the safety of operations personnel, and equipment, and seriously impact production. Immediate steps are usually taken to danger off and secure the area while clean-up efforts are designed and implemented. A critical step, often overlooked due to production demand, is the determination of - why the roof fall occurred so that additional occurrences can be avoided in future developments or what measures can be taken to identify areas that have similar ?symptoms?? Forensic engineering includes the investigation of materials, products, structures or components that fail or do not operate/function as intended. The authors define forensics in ground control engineering as the study of evidence discovered while examining underground roof falls by applying expert observational techniques and established mining engineering principles to determine the probable cause, hypothesize solutions, and design engineering safeguards to prevent reoccurrences. Simply stated, the primary steps in the forensics of underground roof fall techniques are: ? Assess regional and local conditions prior to the event. ? Assess post-event conditions. ? Hypothesize plausible ways in which the pre-event conditions ? became the post-event conditions.Search for evidence that either denies or supports the various ? hypotheses.Apply engineering skill to relate the various facts and evidence into a cohesive scenario of how the event may have occurred. This paper details the proposed technique by utilizing a roof fall case study that required the examination of five primary factors: mine design parameters, excavation geology, method of excavation, method of support, and mining cycle times.

Jan 1, 2009

By Bill A. Kyslinger

"In underground mining, machine design is predominantly dictated by mine conditions and individual customer desires. In partnership with Foresight Energy, J. H. Fletcher & Company was tasked to design and manufacture a new roof bolting machine with six independent drilling apparatus on board capable of drilling and bolting the roof and ribs with material handling. This is the first Fletcher® six-head machine to be designed and built for the mines in the United States. The machine is intended to work in the Illinois Coal Basin.The objective was to produce a machine capable of drilling and installing six bolts simultaneously with a limited number of operators. The goal of the mine is to decrease the time to bolt a cut to improve the safety level of the current roof bolting method, improve efficiency and to improve the bottom line cost of entry development. The customer wanted four mast feed drills at the front of the machine dedicated to installing roof bolts and then another two mast feed drills behind them dedicated to rib bolts. This dictated the requirement of latched controls, which would allow the operator to start drilling one hole and then latch the controls to be able to move on to the next. To accomplish this, we developed a unique hydraulic latch circuit to maintain operator safety and compliance. Guarding was added to prevent an operator from coming into accidental contact with a latched drill.The result of the design is a machine with a single platform and six independent masts with drillheads: four masts strictly for drilling and installing roof bolts on the front of the platform and two masts on the back of the platform for rib bolts. The feed and rotation controls at each operator's station include a latch control for drilling. The latch control will allow the operator to lock in the feed and rotation once the hole has been collared, allowing the operator to move on to the next hole. The rear of the machine is equipped with material handling. There are two material pods with winch controls as well as a mesh rack that includes mesh lift, mesh tilt, and mesh sump.The six-head roof bolter allows fewer operators to drill and install roof and rib bolts, which in tum lowers the miners' roof exposure per cut. This design reduces the operator's exposure from the inherent pinch points and rotary hazards once he has engaged the latch drilling. Therefore, the machine will help to decrease the time to bolt a cut, improving productivity while enhancing the ability to operate the machine safely."

Jan 1, 2016

By Naj Aziz

The shear behaviour of reinforced rock joints is investigated for the bolt-grout-rock interaction and for failure mechanism. The effectiveness of the bolt application under lateral and axial loading conditions within surrounding materials is investigated in different medium strength. Double shearing testing of bolts were studied in concrete blocks of 20, 40, 50 and 100 MPa strengths, subjected to different pretension loads of 0, 5, 10, 20, 50 and 80 KN respectively. The experimental study was complemented with three-dimensional numerical analysis. Parameters examined include: shear resistance, shear displacement, induced strains and stresses during bolt bending process and its ultimate failure across the sheared joint planes. The conclusions drawn from the study were; the level of bolt resistance to shearing was influenced by the bolt profile configuration, the strength of the rock or medium influenced the level of load generated on the bolt, and the increase in bolt pretension has contributed to the increased shearing load of the bolted medium.

Jan 1, 2007

By Stephen C. Tadolini, Minova USA Inc., Peter S. Mills, David R. Burkhard

"Fiber-reinforced spayed concrete has been used for several years in civil and tunneling operations. Research conducted to reduce cure times and increase compressive and flexural strengths resulted in the development of Tekcrete Fast®, a cementitious product capable of obtaining 41 MPa (6,000 psi) compressive strength and 8 MPa (1,200 psi) flexural strength in only 3 hours and reaching 7 day strengths of 62 MPa and 11.7 MPa (9,000 and 1,700 psi), respectively. A single bag product that uses conventional shotcrete and gunite application systems make it a natural crossover product for mining applications. The discovery of incredible adhesion properties and high resistance to chloride permeability helps ensure long-term stability and increases the ease of application. Project results from Disaster City® in Texas and the application for rehabilitating a coal mine belt entry are presented. The case study illustrates the effectiveness of the product in stabilizing a coal mine beltway and adjacent cross-cuts that were subjected to progressive sloughage due to humidity and cyclical loading. INTRODUCTION Original research produced a shotcrete formulation that extends beyond stabilization and can be used to restore the structural integrity of damaged infrastructure. The primary challenge was to develop a shotcrete or gunite that had structural strength within a few minutes to prevent parasitic loading, allowing ingress into damaged structures minutes after applying the mix formulation. This also applies to mining conditions where rapid support of progressive failures can minimize failure. An additional benefit was discovering that the product had extremely high adhesion properties in addition to high flexural strengths. The approach to product design was the development of “single bag” dry mixes that are entirely self-contained, can be rapidly transported anywhere in the field or underground, and can be deployed using simple equipment. Dry mix eliminates issues relevant to high shear water mixing because water is added in the form of a mist at the point of delivery; thus a low water-to-cement ratio can be achieved. However, since there is no onsite powder mixing, the product must have a high degree of inherent homogeneity. Minova offers a commercial version of this material, Tekcrete Fast®, a product that has been successfully deployed in the mining industry and is currently being used in civil, construction, and tunneling operations. The material can rapidly stabilize lose ground in metal, nonmetal, and coal mines. This paper described the technology development, product testing results, and applications and describes a case study to stabilize a coal mine beltway."

Jan 1, 2017