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By Bill Kyslinger

In underground mining, machine design is predominantly dictated by mine conditions and individual customer desires. In partnership with Boart Longyear Poland and KGHM, J. H. Fletcher & Company was tasked to design and manufacture a single-boom, single operator drill module for installation of 1.2 meter expansion bolt in a 1.6 meter height mine. The objective was to produce a machine to provide ergonomic improvements for the operator while working in the mine environment. This dictated the requirement for a small easily controlled, maintenance friendly design with a remote operator?s control station. The selected design method was to use starter and finisher drill steels. This combination, requiring a number of mechanized machine motions, provides the ability to drill the required depth in the specified mine height. To accomplish the task from a remote location, a conscious decision was made to employ current technology in the form of small pressure compensated hydraulics and electronic controls. The result of the design method is a modular assembly with each sub-component designed to be replaceable and adjustable independently from other machine components. The low profile remote roof bolting module is made up of six major sub-components. Each major component is painted a different color to help the operator make the visual connection from the drill module to the operator?s control/display monitor. The Stabilizer (White) secures the module between the mine top and floor. The Drillhead (Orange) produces the drilling rotation and torque but also must slide out of position to allow the machine to manipulate the drill consumables. The Drill Steel Carousel (Blue) holds and indexes the starter and finisher steel from position to position. The Manipulator Arm (Green) moves and guides the drill steels and drill consumables from position to position during installation. The Roof Reference Guide (Yellow) locates the mine roof and guides the drill steels and drill consumables into position. The Linear Bolt Carousel (Purple) holds the drill consumables. The low profile remote roof bolting module allows the operator to drill and install a roof bolt manually or automatically with the push of a button. This design substantially reduces the operator?s exposure in the mine environment, as well as inherent pinch points and rotary hazards. Positioning the operator safely in the confines of the air conditioned cab.

Jan 1, 2006

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 John Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has collected pillar and span data at 34 stone mines in 10 states to develop pillar and roof span design guidelines for underground stone mines. Pillar, floor and roof conditions each contribute significantly to the overall mine stability. The roof may be stable for a century, or it may appear to be stable but fail with little warning. Clearly, an assessment of the mine roof is critical to the safety of these operations. However, up until now, the assessment of the mine roof came primarily from exposures in roof falls or portal areas. This provides only limited information, and a more proactive approach is needed to assess changing mine conditions. A separate effort was undertaken to examine the roof structure with the use of a borescope. Numerous physical and geologic roof structures were observed with a stratascope/borehole video camera. The objective of this work was to develop a technique of assigning numerical values to features seen within the mine roof and apply those values to formulate a roof stability evaluation. Such features as changes in color, grain size, and general lithologies are readily observed. Minor structures in the rock, fossils and fractures are also readily identified. Each feature may have a variable and site-specific impact on roof quality. A number of mines were visited and a series of holes were borescoped in each mine. The features identified in the roof were classified and potential for roof damage related to each feature was assessed. A process is presented that relates the structures and their locations to provide a numerical value, or roof quality index that can be used to assess roof stability.

Jan 1, 2009

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 Alan J. Zeni

Drainage of water from abandoned underground mines has become important in recent years because of safety concerns when mining near abandoned works or reentering abandoned mines for further development. Seepage from inundated mines causes environmental damage and may threaten homes and communities. Where topography permits, horizontal drilling can provide a quick and cost effective method for solving trapped water problems. The mechanics of the horizontal drilling operation as applied to water drainage are discussed in this paper.

Jan 1, 1990

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 James Pile

Long, high capacity, highly tensioned, cable bolts (strand bolts), stiff enough to be installed with resin like a solid bolt, yet flexible enough to be used in any lengths, have been widely used in Australian coal mines since the early 1990?s. A brief history and update on recent developments of this technology is provided. The first major US trial of this Australian technology, has recently taken place at BHP Billiton?s San Juan longwall coal mine in New Mexico. Thirty-five foot long, high capacity, highly tensioned, post-grouted cable bolts are being used to provide active secondary support of intersections in areas where operator safety and life of mine considerations are paramount, and standing support cannot be installed for reasons of equipment access. The cable bolts are resin anchored into overlying sandstone channels, tensioned, and post-grouted. The installation procedure and support results for these highly tensioned cable bolts are given.

Jan 1, 2007

By Craig Compton

Falls of small pieces of rock from between roof bolts continue to cause fatalities and to injure hundreds of coal miners each year. Roof screen is the most effective way to prevent these incidents, but some mines are reluctant to use it because they believe installing screen can be awkward, expensive and time-consuming. The most common type of roof bolting machine used in mines today is the dual boom, outside-controlled roof bolter. Mines who install roof screen using these machines have developed some machine modifications and techniques to ease roof screen installation. This paper describes some successful techniques and machine modifications that are being used in productive mines to assist with roof screen installation. Since the material handling involved with roof screen installation can expose miners to an increased risk of musculoskeletal injuries, the National Institute for Occupational Safety and Health (NIOSH) ergonomists, designed machine modifications and implemented a series of tests to evaluate roof screen handling procedures for outside-controlled, dual boom roof bolting machines. Roof screen installation can also have a positive economic impact on a mine by reducing the cost of injuries, workers? compensation, required spot or re-bolting, and clean-up of long term travel and belt entries.

Jan 1, 2007

As the surface reserves are being depleted, more and more stone operators are seeking underground mining options. Stability of underground openings is a major concern for the safety and productivity. This paper relates to a field study conducted at a large underground limestone mine in Central Pennsylvania. Identification of hazards leading to ground failures and assessment of roof fall risks are important in mitigating injuries. Field observations at this mine were integrated with Roof Fall Risk Index (RFRI) mapping software recently developed by West Virginia University. The system is designed to combine field study data related to geologic factors, mining-induced damages, roof profile, and ground water influx to generate RFRI maps. The mine has adopted a proactive ground control program consisting of comprehensive examination of drilling and bolting logs; precision surveying of roof and rib; accurate mapping of cutters, falls, and geologic structures; and installation of extensometers and microseismic monitoring systems. Investment of time and resources has eliminated fatalities and lost-time injuries due to ground fall. This paper presents the geologic settings, mining conditions, ground control issues along with risk management and control techniques adopted at the study mine.

Jan 1, 2009

By Tara Bajpayee

Modern, highly productive underground mining operations need to assure the safety of their workforce by understanding where major strata fractures and roof failures could occur. Mines depend on a host of tools to assess ground instability. In-mine microseismic monitoring systems with sufficient number of sensors have also been used to provide this information. These in-mine systems must be robust in design and moved regularly to keep up with the mining advancement. System performance issues depend on maintaining ever increasing cable runs, preventing component damage from mining activities (scaling, blasting, etc.) and avoiding signal degradation due to interference from mining equipment (fans, trucks, electrical equipment, etc.). A potential solution is to use a surface based monitoring system having sensors placed in boreholes above and adjacent to the underground mine workings. These sensors must be capable of detecting and locating strata fracturing associated with rock failure events. A great advantage of a surface based monitoring system is its independence from the underground mine infrastructure. There is less concern with cable maintenance problems, signal degradation by interference from mining activity, or loss of power from the mine?s power grid. The disadvantages include the cost of drilling boreholes and challenges associated with placing sensors in the boreholes, maintaining radio communication and solar power, and public interference (vandalism, etc.). This paper describes a case study where a surface based microseismic system, using triaxial geophones in boreholes drilled from the surface, was deployed at a large limestone mine. The system was operational during a roof fall that occurred in October 2007. It detected the first rock fracture event 17 minutes before the rock fall event. The geophone array was sensitive enough to identify all large rock fracture, impact, and blast events as well as medium size rock fracture events occurring close to the geophone array.

Jan 1, 2008

By Roland C. Walker

Traditional headed rebar has existed in the mining industry as a ground support product for many years. The head of a rebar bolt is most commonly produced by hot upset die forging. Upset die forging produces a rugged, simple head that performs well. Certain elements of the head must comply with regulatory standards, such as ASTM F432-95, which offers a standard test procedure to ensure quality of the product. The head style produced by all major manufacturers has been optimized to reduce manufacturing costs, provide greater miner safety and provide an excellent performing product. Observations were made in some mines of heads failing under in-situ loading. These observations could not be replicated according to the ASTM testing method. This paper highlights the problem and how a solution was developed. The development includes pictures of in-situ failures from the underground operations, laboratory replication tests indicating how this loading occurs, and then the use of this calibrated test method to validate the new design created.

Jan 1, 2009

By Bill Stormes

This paper will discuss implementation of the Hilti OneStep rock anchor technology to improve face bolting operations during the May 2008 longwall move; increase rock anchor installation capacity; and improve safety by reducing employee exposure periods by utilizing the Hilti OneStep rock anchors.

Jan 1, 2008

By Srinivasulu Tadisetty

The productivity and safety of Indian longwall operations are insignificant because of complex geomechanical conditions. Therefore, the continuous monitoring and forecasting of abnormalities is imperative. Authors developed an innovative real time system and implemented in one of the Indian longwall mines. The real time system was successfully acquired data from underground chock shields and classified the behaviour in real time as per the standard norms. Furthermore, it forecasted the abnormal behaviour of chock shields promptly. The management was implemented appropriate precautionary measures based on real time forecast. Subsequently, the longwall operations were successfully completed with enhanced productivity and safety. This paper discuss the impact of the innovative system on productivity of experimental longwall mine.

Jan 1, 2006

By Francis Kendorski

The Skyline #3 Mine of Canyon Fuel Company in Utah was planning longwall mine development adjacent to and downdip of older mine workings known to be flooded. Barrier pillar geometries and widths were proposed by mining company personnel. Hydraulic conductivity testing suggested that the general characteristics of the local coal seam, dikes, and faults are only weakly conductive and that leakage through the barriers, if any, would be minor. The mechanical performance of the proposed barrier pillar design was evaluated according to (1) published empirical design methods and (2) numerical stress modeling. Comparison of the proposed design, with the range of barrier widths derived from the empirical methods, served as a first-order check on the adequacy of the proposed design. Numerical modeling was used to evaluate conditions site-specific to the Skyline #3 Mine and barrier geometry. Models were developed to quantify the relationship between barrier width and the abutment stresses onto the future workings. The models were also used to estimate the stress distribution within the barrier pillars. The hydraulic performance of the proposed barrier pillar design was evaluated according to (1) published empirical design methods for hydraulic impoundments, (2) empirical method estimates of seepage through coal barrier pillars, (3) numerical hydrogeologic flow modeling, and (4) numerical strain modeling. Using the techniques described above, and based on relevant industry experience, underground observations, knowledge of local geologic conditions, hydrogeologic measurements, and analytical results, the level of geomechanical risk associated with the barrier design as proposed was considered low and the level of hydraulic risk was considered moderate. The largest uncertainty that remained was the possibility that unfavorable geologic structures, such as faults and dikes, would act as conduits for leakage of impounded water into the new workings. Although the study indicated only minor steady-state leakage would occur through a small number of known structures, the presence and full leakage potential of threatening structures in the barrier could not be reliably known until the barrier was mined. One longwall panel has been completed adjacent to the barrier with no evidence of water flow even in fractured zones. Thus, as predicted by the modeling and analysis, it appears that abutment stresses imparted on the barrier during mining did not substantially alter the natural hydrogeologic characteristics, or leakage potential, of the barrier. The longwall coal mine water-barrier pillar design and performance are considered a success.

Jan 1, 2007

By Thomas Barczak

Roof support systems are designed for roof control to prevent unplanned falls. It sounds logical and has been the conventional thinking in support design since supports were first installed. In order to determine which support system should be used or which would be the most effective, the degree of control provided by the support system must be known. This question embodies the concept of a ground response curve, which is a measure of support control by assessment of the convergence in the mine entry as a function of the support capacity. In this National Institute for Occupational Safety and Health (NIOSH) study to optimize standing roof support design, ground response curves were developed for longwall tailgate conditions from numerical models of Pittsburgh Coal Seam geology. The models were calibrated against tailgate convergence measurements that were made in two Pittsburgh Coal Seam mines as the depth of cover varied during the panel extraction. Ground response curves were developed for four loading conditions: (1) development, (2) side abutment, (3) front abutment near the longwall face, and (4) full extraction inby the face. In general, the tailgate convergence and required support capacity increase through each of these loading stages. It was concluded that prior to failure of the rock mass, the support system regardless of its capacity has relatively little impact on tailgate convergence. Recognizing that the support cannot prevent much of the (pre-failure) convergence that is occurring is an important part of support design. Supports that exhibit high loading stiffness but cannot sustain that loading through a displacement compatible with the ground response curve will not provide adequate roof control because they can fail prematurely. The required capacity depends on the loading plus yielding characteristics of the support and the amount of rock failure that has occurred. The loading plus yielding character establishes where the support loading intersects the ground response curve. Design criterion for support capacity based on the identifying the onset of strain-soften rock response leading to ?damaged roof? is proposed as a foundation for assessing support design requirements. The capacity without accounting for the loading and yielding character of one support should not be used to assess the capacity requirement of an alternative support design, particularly when the loading and yielding characteristics are significantly different. A sensitivity study was made to evaluate the impact of mining height and overburden depth. As expected, the results show that convergence increases with increase in mining height and increase in overburden depth. Some specific support examples are also analyzed. Although this is a first step and the two-dimensional numerical models have limitations, these initial studies provide valuable insight into the control provided by the roof support system and will ultimately lead to optimizing support design based on specific mine site conditions.

Jan 1, 2008

By John Smyth

Mining through in panel entries and the full face recovery method has been successfully practiced at U.S. Stwl, Mine 50. Suppat of the cut-through entries and recovery room is critical. Traditionally, concrete crib or fly ash cement backfill has been utilized. However, these traditional methods are not feasible at Mine 50 due to the difficulties of the plow face mining through these types of support. Recent development of new cable support systems provides an option for cut-through entry support. Mine 50 and Jennmar Corporation personnel have worked together to design and apply the cable systems in the cut-through entries and full face recovery room to eliminate standing support. To date, a number of full face recovery rooms and cut-through inpanel entries have been successfully mined. The results indicate that the cable systems combined with high capacity shields, have provided sufficient capacity to support the roof. Utilizing the Jennmar Finite Element Analysis (JFEA) computer program (Stankus and Guo, 1996), an analysis of the stress and roof deflection changes as the longwall gradually cut through was conducted. From the finite element analysis data, a supplemental support plan was designed. The history and experiences at Mine 50 are presented. Based on the case experiences and modifications made to the finite element analysis, an improved design criteria for future use has been formulated.

Jan 1, 1998

By Joe F. T. Agapito

High stresses and adverse geology in deep coal mines in the state of Utah, USA, have caused numerous bumps. The larger bumps have been associated with seismic events with Richter magnitudes of 3.6 and greater, and in some cases have filled openings for lengths of 150 m. A better understanding of the mechanisms and stress levels involved in bumping is needed to help develop improved stress control design and bump mitigation methods. The geology of the area is notoriously bump-prone. The coal has poorly developed cleating and occurs in multiple seams that are often bounded by very strong roof and floor sandstone/siltstone beds. The overburden is formed by thick, competent strata with numerous sandstone channels This geology and deep cover are the major source of high stresses, causing bumps. This paper evaluates live common stress factors responsible for bump problems: depth, sandstone channels, arching, faults, and coal thickness. It uses ease study data from longwall panels with two-entry/yield pillar systems typical of deeper Utah mines. Results illustrate the importance of analyzing stress factor experience to allow a better understanding of the problem.

Jan 1, 2000

By Brian R. A. Wood

Any defect activity resulting from an applied stimulus on a structure will generate a stress wave which travels in the medium under the laws of acoustics, thus the name of acoustic emission (AE). AE techniques can therefore provide information on the deformation of both surface and below ground structures, Two operating coal mines were monitored from the surface, and both mining activity and post mining microseismic activity were detected. The nature of the detected AE signals demonstrated that there were a range of deformation mechanisms in operation and that some predictive capabilities were possible with the technique.

Jan 1, 1992

By Changsheng Song

With increasing depth of coal mining in China, the number of coal and gas outbursts has been increasing for years. By means of destressing the protective seam, the gas in the protected seam is drained to reduce gas concentration in the high gassy seams, thus ultimately attaining simultaneous extraction of coal and gas. Base on the simulated material scale modeling and numerical simulation, the protective seam mining method was practiced at No. 8 coal mine in Pingdingshan city. After extracting the No. 15 seam, the overlying strata movement and the deformation of No. 9-10 protected seam were studied. The experiments showed that it was feasible to destress the protected seams with large interburden thickness. As the protective seam was being mined, the protected seam experienced dynamic movement process and the change in vertical stress in the No. 9-10 seam assumed ?W? shape. When the face had advanced 656 ft (200m) from the setup room, the No. 9-10 seam was fully destressed, resulting in easy gas drainage in the destressed zone. Recommendations on mining sequence of multiple seams mining in the Pingdingshan coal areas were made.

Jan 1, 2008

By Sean Harvey

One of the manifestations of the dynamically induced failures in deep coal mines is the violent ejection of coal from the ribs. A possible protection against such failures is the use of a specifically designed dynamic support system with reinforcement units having the sufficient capacity to absorb the kinetic energy of the dynamic failure and a skin support that can adequately contain the disintegrated coal. With this method of mitigation in mind, a recently introduced yielding rock bolt, trade name Roofex, has been identified (Harvey and Ozbay, 2009) as a possible energy absorbing component of a dynamic support system. According to the manufacturer?s specification, the Roofex bolts can yield 300 mm (or more) displacement while holding 80 kN pull load. In order to assess the performance of this support unit in coal ribs, a series of in situ pull tests performed in two coal mines in Colorado. The results show that, under conventional pull test loading conditions, Roofex yielding bolts perform very close to their claimed specifications, thus showing a potential for their use as the main energy absorbing component of a dynamic support system. Further studies are required to confirm the in situ performance of these bolts under dynamic loading conditions and also to develop a containment component for a dynamic support system for the mitigation of dynamic coal rib failures.

Jan 1, 2009