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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 Jeffrey K. Whyatt

The Crandall Canyon accident investigation included an interesting and unexplained observation by rescuers that the ?barrier rib had shifted northward as a unit, as much as 10 feet.? This is not the first mention of such a movement, although such reports are rare. Historical accounts describe unusual movement and displacement of intact coal, cribs and timbers. Two movements of particular interest are the creation of gaps above coal pillars and falling of standing support elements without apparent damage. A dynamic boundary element program was used to explore movements induced by slip on geologic features removed from the affected panel. While the resulting models are much too simple to fully replicate these observations, they do show that the types of phenomena observed are possible. They can also provide insight into the types of motions that ground support elements are subjected to during large bump events. This was demonstrated for the case of a large bump in a Book Cliffs coal mine. Seismic information suggested the source mechanism to be normal slip on a fault. A model of that source showed initial dilation of the panel followed by dynamic compression and rebound ? consistent with underground observations. The initial dilation is important as it may allow slender standing support to shift or fall.

Jan 1, 2009

By Thomas M. Barczak

The resultant load vector is the representation of the forces applied to a longwall roof support element by caving strata into a single, quantifiable measure of support resistance. The relatively complex kinematics of the shield support prohibit a determination of support resistance simply by summation of leg forces. A method is being investigated by the Bureau of Nines to determine the resultant load vector by instrumenting supports with pressure transducers and strain gages to measure leg, canopy capsule, and lemnisoate link forces. This concept has been laboratory tested in the Bureau's Nine Roof Simulator. Functional relationships among variables have been assessed, and confidence intervals have been established for prediction of the resultant load vector parameters.

Jan 1, 1984

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 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 David Hill

The Australian underground coal mining industry is increasingly operating in an environment that challenges traditional concepts of coal pillar design, against a background of heightened expectations and demands from other stakeholders with regard to the protection of both natural surface features and burgeoning civil infrastructure. The result is greater possibility of conflict between mine operators, developers and regulatory bodies, with potential for unnecessary sterilisation of Australia?s coal resources and/or escalating surface development / infrastructure protection costs. On the positive side some significant advances have occurred in the general level of understanding of pillar behaviour and stability, both in Australia and overseas. This paper examines some current issues in coal pillar design, focussing particularly on the application and limitations of analytical models of pillar loading. Established methodologies are reviewed and alternative approaches explored, including examples of key geometrical, geological and probabilistic concepts. Practical experiences from the Southern Coalfield of New South Wales (NSW) are examined for the purpose of highlighting critical variables. Finally, some suggestions are made with regard to the direction and opportunities for future research, with particular regard to coal pillar loading, in the interests of all stakeholders.

Jan 1, 2008

By Scott A. Shapkoff

High strength cable strand supports are predominately used as a supplemental means of ground control in longwall and development operations in the U.S. The advantages of cable strand supports have been documented and include flexibility, strength, and ease of installation. Since the majority of cable supports installed in the U.S. today are partially grouted, and this same segment of supports has no inherent installed tension, the supports are considered currently arbitrarily strictly supplemental and therefore, cannot currently be deemed primary support (in most mining districts). This paper details and discusses two distinctly different configurations and the technical advantages of active partially grouted cable supports for certain primary support applications. The evolution of the designs for both systems is highlighted. Field data illustrates the amount of active load in relationship to installed torque and the affects that component variables have on this load measurement. Roof control patterns using active cable supports are examined as alternatives for combination patterns using primary and supplemental systems. The importance of maintaining consistent installation procedures plays an important role in the level of performance of the system. Economics comparing the installation of active cables with other primary/supplemental patterns is also included.

Jan 1, 2006

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 André C. Zingano

The coal pillars in room-and-pillar mining method are subjected to vertical stress due to overburden rock mass and horizontal stress relieve due to entries excavation around them. Some coal mining companies are applying rock bolts and steel straps to improve the pillar strength, consequently to reducing the pillar size and increasing coal recovery. This kind of approach deserves some discussion, whether it is pillar reinforcement or just rib support. The efficiency of pillar reinforcement (or rib support) is directly related to the initial load capacity of coal pillar, the type of excavation, and the width to high ratio of the pillar. The objective of this paper is to understand the mechanism of pillar reinforcement (or rib support) using rock bolts and steel straps on pillar ribs. Numerical simulation taking into account the relations listed above. The results showed that the increase of coal pillar strength is very short, and there is no influence on improvement of load capacity of pillar. However, the rib support avoids block falling, rib failure, and displacement, reducing the pillar failure propagation into the pillar center. Keywords: coal, pillar reinforcement, room and pillar.

Jan 1, 2006

By Mingju Liu

China has been suffering from coal and gas outbursts since 1950s. Nowadays, the situation has become worse than ever with the need to mine deeper coal deposits at higher production rates in a more complex underground mining environment. In the year 2006 alone, 37 outbursts occurred in underground coal mines with a death toll of 232. This paper describes a case study of an outburst event, which occurred in July 2006 during the shaft development of the Mengjin Coal Mine, Henan Province, China at a depth of 768m below surface. Detailed analysis of the possible causes that led to the occurrence of the outburst is reported. These include the study of complex geological features, gas storage conditions, possible improper technical management practice as well as incomplete prevention and control measures. Based on the case study and the observation of outburst occurrence in China in the year 2007, suggestions have been put forward to help prevent and control future coal and gas outbursts.

Jan 1, 2007

By Axel Preusse

Mining subsidence plays an increasing important role in the Chinese hard coal mining. Reasons for this are a substantial rise of production during the last decade, a dense settlement in many mining regions, the increasing emergence of private residential property as well as a grown environmental awareness. German companies and research institutions hold an over more than 150 years existing know-how in all subjects of mining subsidence engineering. Due to partially comparable preconditions regarding mining depth, mining methods and geological conditions, a close co-operation between German and Chinese mining subsidence engineering experts has been in existence for some years. This German-chinese co-operation is introduced in this paper.

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 John Popovich

The MICON RokLok binder system including component and consolidation processes and safety and its first aide are described. The implementation procedures and selection of injection parameters are discussed. Finally a case example of recovering a 30-ft. long roof fall in mainline haulage entry is used to illustrate the economic potential of preventive measures by RokLok injection.

Jan 1, 2001

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 Erik Westman

Coal bumps remain one of the industry?s most devastating but least understood phenomena. If changes to the internal conditions of the rock mass could be observed there is hope that bumps could be forecasted. Seismic tomography has been used over the past 25 years in an attempt to image these changing conditions in the vicinity of the longwall face. This paper describes the history and practical considerations of using seismic tomography in bump-prone coal mines. The implementation of seismic tomography at six different longwall mines will be described along with lessons learned. Early studies used mining independent sources of seismic energy, including explosives and sledgehammers. Subsequently more operator friendly sources, such as the mining equipment itself and mining-induced seismicity, were used. Seismic energy sensors have been placed in gateroads and on the surface above the mine. Each of these configurations has associated advantages and disadvantages. Although the results obtained have been encouraging, the application of seismic tomography to monitoring a bump-prone longwall face is still immature. The ultimate goal of clearly imaging changing stress conditions in the longwall panel in a manner that does not inhibit mining operation is still distant, yet drawing nearer. When this goal is achieved, the industry will have a capable tool for identifying locations where bumps are imminent.

Jan 1, 2008

By Jan A. Nemcik

Steel mesh has been used successfully for many years to control friable roof conditions and prevent loose roof and rib material from caving into the roadway. Despite its extensive use, the installation of steel mesh is difficult to automate and many other products have been evaluated as an alternative for skin control. Ideally, the properties of these new products should be similar or better than those of the steel mesh. In particular, development of a strong and resistant shell that minimizes movement along the fractured rock and coal surfaces between the bolt anchors is recommended. A strong surface adhesion and the strength of a reinforced polymer skin can provide the necessary toughening mechanism required to enhance roadway surface support by forming a reinforced polymer/rock surface layer. Most of the Thin Spray-on Liners (TSL) evaluated in the mines are weak with slow curing times, and the plastic mesh currently used to support the coal ribs is relatively weak. Therefore, neither material can seriously compete with steel mesh. Currently, a new polymer product that cures in seconds and forms an instantaneous strata binder that surpasses the properties of steel mesh is under development at the University of Wollongong.

Jan 1, 2009

By John H. Perry

Strata III is a full-spectrum roof control system which provides the mine operator the ability to upgrade his roof control in place. The procedure is easily adaptable with current roof bolting techniques. To use the system, the operator starts by installing a special hollow core mechanically anchored roof belt during the roof bolting cycle. This bolt can be upgraded to a fully grouted tensioned rebar bolt by injecting StrataGee G-300, a special polyurethane grout, around the bolt through an injection port in the roof plate. This same bolt can then be used to inject RokLok binders for roof consolidation by pumping the binders through the core of the bolt. This concept in roof control was developed to provide the mine operator a flexible way to maintain roof control in areas which deteriorate over time, or must be kept open for long periods of time. Possible locations of use would be: longwall set-up rooms, recovery rooms, headgates, intersections, and belt entries.

Jan 1, 1987

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 Murali M. Gadde

Welded wire mesh is the most commonly used skin control technique in the United States underground coal mines. Currently, selection of the mesh is based on trial and error rather than any rigorous design methodology. Several factors affect a wire mesh?s performance including the amount of rock material to be supported and how the load is distributed, wire gauge, aperture size, grade of steel, weld strength, bolting pattern and sheet size. Although the best way to choose the mesh is to conduct full-scale physical load tests, because of the limitations on the number of variables that can be considered in such tests and the requirement of an elaborate test set-up, it may not always be possible to use this option. An alternative would be to use numerical modeling to study the load-deformation behavior of a wire mesh. In addition to providing a very detailed stress, strain distribution in the entire mesh, the advantage of this approach is the ease with which parametric studies can be conducted. In this paper it is shown that the load-deformation curves generated by nonlinear numerical models match closely with laboratory derived ones. Then, based on parametric studies, design guidelines are developed to choose the mesh appropriate for different loading conditions. The modeling studies also bring forth the limitations of the current lab studies. For example, under some conditions it was noticed that the small sheet size considered in lab tests would produce higher mesh deformations than a full-size one used in actual practice.

Jan 1, 2006

By Stephen C. Tadolini

The International Ground Control Conference in Mining has always provided an open forum for the publication, presentation, discussion, and often heated debate on roof bolting systems mostly with attention to how, when, and why they work. During the last 24 Conferences at least 170 papers have been presented by International Experts on these ?simple? devices that range in length from 30-inches to 20 ft and from 5/8 to 2 inches in diameter. Roof bolts are primarily mechanically anchored, glued, cemented, or driven. Bolts are placed vertically, angled, or tied together with special fixtures (trusses and slings). They have been termed active, passive, stick-slip, and several other descriptive acronyms or mining slang expressions. This retrospective paper will present the changes in bolt types and usage, point out some of the biggest myths and hail the most significant advances (according to the authors? opinions of course).

Jan 1, 2006