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By Kazem Oraee, Mehran Gholincjad, Navid Hosseini

"Chain coal pillars are pans of the structure of longwall mining system that play a significant role in the stability of the entries. With mechanization and developments in the various aspects of the method, higher efficiency in optimization of the design of chain coal pillars seems appropriate. In this paper the three main methods of chain pillar design, namely the empirical analytical and numerical methods arc compared. Real data from the Tabaslran coal mines have been used in order to make the comparison process reliable. It is concluded that the most apparent advantage of the empirical method is the reliability of the results while the use of numerical methods enjoys the advantage of flexibility. On the other hand the analytical methods are complex unless simplifying assumptions arc made that can substantially decrease the accuracy of result which is thought to be the main advantage of the design method. A new method is therefore introduced here that combines all of the three presently used methods and by doing so the new method has the advantages of the three while minimizing complexities and inaccuracies associated with the use of the individual methods.INTRODUCTIONLongwall mining is a high production method that can use mechanization to its full extent. Its initial development can be traced to 17th century European collieries but it was not until the second half of the 20th century with the development of self advancing support systems that the method gained popularity in the USA (Oraec. 2001). During the last decade, the production of longwall faces has increased significantly (Peng. 2006). The ability for mechanization along with good safety condition is the main reasons for such development. In the longwall mining method a good design for panel and entries on both sides, are two essential parameters. In this method, the thickness of overburden (depth of mine) nom1ally ranges from 60 to 20 m (197 to 2.7 9 ft) (Hartman, 2000) and with increasing mine depth designing for stabilizing the entries on both sides of the panel become more difficult. Thus, for safety and stability the size of chain pillar must increase. This causes the overall coal recovery to decrease and increase mining costs per ton. As such wide pillars hinder the air flow and hence ventilation costs increases too. Therefore with accurate identification of loading conditions and stress analysis. pillar that are a minimum size and sufficient safety factor are carefully designed."

Jan 1, 2010

By Heather E. Lawson

Recent field data providing measured pillar loading from a deep western mine revealed that an increased load transfer distance existed compared to expected values (Larson et al., 2012). Additionally, recent work by Tulu and Heasley (2012) has shown that the abutment angle in deep longwall mines can be significantly reduced as the depth of mining increases. The load transfer distance and abutment angle are both used as parameters to estimate pillar loading in longwall design. To investigate the potential implications of these observations on pillar loading in deep longwall mines, a parametric study using ALPS loading estimation procedures was performed. A supplemental study was conducted using LaModel to further investigate pillar loading at the tailgate. The study found that the abutment angle is particularly important for panels with supercritical geometries. As panels reach a subcritical geometry with increasing depth, the primary influence of the abutment angle is reduced and matched by other factors, including load transfer distance for tailgate loading. An extended load transfer distance was shown to potentially reduce loading on pillars adjacent to the longwall as the loading is spread over a larger area. The results of these studies indicate a need for further field verification and research toward regional geologic influences on pillar loading and how these may impact ground stability.

Jan 1, 2013

By Timothy J. Matthews, Timothy J. Batchler, Ted M. Klemetti

"From 2011–2016, there were 1,511 documented injuries, including 7 fatalities, from ground falls in underground coal mines in the United States. The majority of these ground-fall injuries were not caused by a major roof collapse, but from falls of smaller rocks from the immediate roof. Roof screen can significantly reduce the number of these injuries and has been widely used in underground coal mines for surface control. Because of the potential of reducing ground-fall injuries, the National Institute for Occupational Safety and Health (NIOSH) is further evaluating the performance characteristics of welded wire screen as used in underground coal mines by conducting a laboratory testing program using the Mine Roof Simulator (MRS) in Pittsburgh, PA.The load-displacement characteristics of an 8-ft x 12-ft panel of 8-gauge welded screen were evaluated using a newly designed, large laboratory screen test frame with multiple load pull locations. This screen was tested in a configuration that simulates current installation practices in U.S. coal mines. In this study, the effects of varying the number and position of the load pull location on the screen performance were evaluated. Ultimately, the type of information obtained in this and similar studies can be used to aid in developing wire roof screen design criteria and to assist mine operators in the selection and use of roof screen in underground mines.INTRODUCTIONThe large majority of ground-fall injuries are caused by falls of smaller rocks from the immediate roof. Various controls are currently being used in mines to control this surface rock, including the use of wire roof screen. In mines where wire roof screen has been installed, injuries from rock falls have been reduced dramatically (Robertson and Hinshaw, 2001). Roof screen has the potential to prevent hundreds of injuries caused by the fall of small rocks between permanent roof supports (Compton et al., 2007). Because of this potential for reducing ground-fall injuries, the National Institute for Occupational Safety and Health (NIOSH) is evaluating the performance characteristics of welded wire screen as used in underground coal mines by conducting a laboratory testing program in the Mine Roof Simulator (MRS) in Pittsburgh, PA."

Jan 1, 2018

By James H. Fletcher

Much data and experience has been accumulated, especially in the last 5 or 6 years, to show that modern roof trusses, both of the Birmingham type and the bolt-and-channel type, perform well to support certain kinds of coal mine roof. For some time now, roof trusses of the Birmingham type have been effective in controlling thick shale roof, which tends to sag and fall over a period of about eight weeks. These falls leave dame-shaped cavities which are reminiscent of the pressure arch theory of roof failure. Lately there has also been significant interest in supporting some longwall panel entries with channel and-bolt systems. These systems utilize two vertical center bolts and two 45 degree end bolts through a heavy gage channel. Both types of truss systems have long bolts, which require 8 to 9 Foot holes on a 45 degree angle, and point anchor resin bolts, which utilize a 2 or 3 foot rebar grouted in a thin annulus of polyester resin. Consequently, many coal mining people have been impressed with the very good anchorage characteristics and tensionability features of these systems. However, the same people have been proportionately dismayed to find that very often the most difficult part of introducing a trussing program to a mine is deciding on the machine specifications which determine the truss installation rate. The purpose of this paper is to indicate aspects of truss bolter design and use which are not shared by roofbolter design and its vertical pattern of bolts. Of the two types of drill units conventionally accepted, arm feed units and mast units, only one 15 judged suitable. While the arm feed unit offers low collapsed height with a long drill feed length, a great attribute for low seam use, it has not lent itself to being rolled over to a 45 degree angle. One reason for this inability is that many items which are conventionally mounted on the arm feed unit cannot be allowed to rotate with the unit. These items include drilling controls, tool Cray, lights, canopy or safety post, panic bar, and stab jack(s). If these items were independent of the arm feed drilling unit, they would have to be out of the way of the boom arms, links, and feed cylinder as the unit moves. To ease this problem, the arm feed unit can be made very short and even mounted across the machine. However, the resulting short feed length is undesirable. Several short drill steel extensions are required to drill a relatively long hole, making this procedure time-consuming as well as increasing the likelihood of drilling a crooked hole. Another reason for abandonment of the arm feed unit is that a very stiff drill guide (or "centralizer") is required to start the hole. Using the mast tilt control, the roof bolter operator will force the bit against the roof and will rotate the bit without feeding to cut sideways into the roof to form a hole face. Omission of this step often results in the skidding of the bit toward the rib. To date, no such sturdy drill guide has been designed for an arm feed drill unit. The mast style unit has been the most successful design for angle drilling. It is easily rolled over to a 45 degree angle or to 90 degrees and, on single head models, often has been made to roll through 180 degrees. The mast style is compact, in plain view and is easily fitted with heavy duty manual or hydraulically operated drill guides. It can be mounted directly on a machine chassis, or onto a swinging boom, or onto a slider arrangement in a broad variety of configurations which will accommodate controls, tool trays, stab and roof support jacks, lights, canopies, and panic bars. The only significant difficulty with mast drilling units occurs in lower mining heights when desirable low mast height is accompanied by undesirable short feed length. While the lead extension in the drill string can be as large as the mining height will allow, a short feed length will permit only the use of shorter middle extensions and drivers, and perhaps will. require the use of two different length wrenches. The associated problem is the relatively short distance from the drillhead to the drillguide. This distance is further shortened when raising the drillhead to get the bit to the roof. One might keep longer lead extensions on the machine; drill a vertical pocket for the bit, starting the angle hole at this pocket; or modify the mast. If the mast is made so that the distance from the drillhead to the drillguide remains constant while the mast extends, after which the drillhead travels up the mast, the shorter mast can be made to start the hole more easily. However, short teed length will still result in longer drilling time. It is obvious from these points that a proper choice of mast is simply the highest one which will be moved easily in the given mining height. This, of course, goes against the grain of most experienced coal mining people who realize that a relatively small decrease in mining height can create a problem correctable only with the purchase and installation of shorter masts. If the mining

Jan 1, 1982

By Bruce Hebblewhite, Jim Galvin

"A coal burst occurred on 15 April, 2014 at the Austar Coal Mine, located west of Newcastle, NSW, Australia. The burst resulted in fatal injuries to two men working as part of the mining crew at the development face. At the time, a continuous miner was being used to mine a longwall development gate road through heavily structured coal, at a depth of approximately 550m. A number of pre-cursor bumps had occurred on previous shifts, emanating from the coal ribs of the roadway, in proximity to the coal face.This paper reviews the geological, geotechnical and mining conditions and circumstances leading up to the coal burst event; and presents and discusses the available evidence and possible interpretations relating to the geomechanical behaviour mechanisms that may have been critical factors in this incident. The paper also discusses some key technical and operational considerations of ground support systems and mining practices and strategies needed for operating in such conditions in the future.INTRODUCTIONAustar Coal Mine is an underground longwall coal mine located near Cessnock in the Hunter Valley of New South Wales (NSW), Australia and is the only underground mine still extracting the Greta Seam in this region. The mine was the first in Australia to adopt the Chinese-developed Longwall Top Coal Caving (LTCC) method for thick seam extraction. Typically seam thickness ranges from 4m to 7m and depth of mining from 480m to 560m, with future mining planned down to depths of up to 700m, making it one of the deepest operating coal mines in Australia.On 15 April 2014, a pressure burst occurred in the left hand rib at the active mining face of B Heading, 2 to 3 cut-through, Maingate A9 panel, during development of the gateroads for the ninth longwall top coal caving panel. Strata in the general vicinity was affected by disturbed geology and multiple geological structures. Figure 1 shows a section of the mine plan as at October 2014 (six months after the accident), indicating the current longwall extraction panel (AS) and the development panel A9 where the accident occurred. (Note: Neither development nor longwall face positions changed significantly between the time of the accident and the date of this plan). At the time of the accident the current longwall face was in excess of 1,000m away from the Maingate A9 development panel face position."

Jan 1, 2016

By Boqiang Wu, Ganggang Xu, Xiaodong Wang, Hai Wang, Shibin Zhu

"Using a paste-like slurry as a filling from the ground surface to the goaf, artificial pillars can be built in the cavity goaf to support the goaf roof to prevent roof collapse and ground subsidence. If the accumulation laws of paste-like slurry are known well, the filling technology can be optimized. The laws can establish the foundation for finding out the best methods for building artificial pillars and filling technology. In order to understand the accumulation laws of paste-like slurry in the cavity goaf, Aeolian sand with wide distribution in northwestern China was selected as aggregate, and Portland cement was used as cementitious material to prepare a paste-like slurry. A test model of the cavity goaf was built to simulate the filling process by pump from the ground surface to the goaf to study accumulation laws of the paste-like slurry. The results show that the whole process of expansion and accumulation of the paste-like slurry could be divided into stages: free expansion, fill tightening expansion, and stable expansion. The angle of repose of the accumulation body of the paste-like slurry is constant in the stage of stable expansion, and it is the final angle of repose of the accumulation body. The angle of repose of the accumulation body is highly relevant to the slump of the paste-like slurry, and it is unrelated to the distance between the floor and roof. Under certain conditions of the cavity goaf, reducing the slump of paste-like slurry is one of the effective ways to obtain cost-effective pillars.INTRODUCTIONA large number of cavity goafs of coal mines were left in northwestern China; they were formed in the mining process by room-pillar or strip mining. Safety coal pillars were used to support the roof to keep long-term stability in the goaf, so the ground subsidence was not obvious (Fu, Deng, and Zhang, 2011; Xie, 2014). With gradual weathering, destruction of coal pillars, or additional load from ground surface buildings and structures, there could be a risk of instantaneous collapse of the roof in the goaf. This would affect the safety of existing structures on the ground and adjacent mines (Qu et al., 2017; Li, Xiang, and Jia, 2011; Tan, Guo, and Zhao, 2016; Tan et al., 2017). For disaster prevention of the falling cavity goaf, the current common practice is full filling (Hu et al., 2008; Liu, Zhu, and Huang, 2015; Pan and Wu, 2010). However, for buildings and structures with lower deformation requirements, full filling is excessive. For example, it is acceptable for goaf treatment under general roads and industrial sites to control the maximum deformation and keep the deformation continuous (Liu, 2011; Liu, 2012)."

Jan 1, 2018

By Bolger Witthaus

"High performance production under challenging geological conditions is a product of several parameters. Main factors are the technical equipment and the knowledge in processing. The design and the support system of maingates must also be taken into consideration. The extraction of seams in German underground hard coal mines of RAG in a range of heights between approximately 1.1 and 4.0 m ( 4 and 13 ft) is done by plow and shearer technology. A complete system for quality management of production is required. The process examination of processes begins with procurement of the equipment. Education and training of the engineers and specialists, as well as for the miners are requirements for success as well.This report gives an overview of RAG measures for achieving high performance. Starting with examination of the processes accompanying procurement of equipment, the following important elements are described. Standards in education and training, technical design and elements of automated processing using stateof- the-art in control systems are critical to this system.The application of automation plays a decisive role in modem coal mines. The latest development in German coal mines is the fully-automated shearer system for longwalls. This helps to enhance safety, particularly in difficult ground conditions, by removing the shearer and shield operators from the immediate mining activity.INTRODUCTIONLongwall mining in carboniferous strata in Germany faces various challenges. Different longwall systems are used to mine up to 24 seams, each with a thickness between 1.1 m (3.6 ft) and 4.0 m (13 ft). High stresses due to a depth of up to 1,400 m (4,593 ft) requiring pillars between the panels, require special systems for roadway development, roadway support and reinforcemen.t as well as for longwall advance operations. Two main types of equipment for extraction are shearer and plow systems. While shearers are utilized for seam thicknesses above approximately 1.8 m (6 ft), plow systems can be used for smaller seam thicknesses between 1.8 m (6 ft) and approximately 1.0 m (3 ft)."

Jan 1, 2010

By W. C. Smith

Over the past several years, the Bureau of Mines has conducted studies on the performance of fully grouted resin bolts In underground mines. The effectiveness of resin-grouted bolts in maintaining a competent roof, even in weak rock, has been well documented in the literature. Recent Bureau studies have evaluated the affects of shortened grout columns on bolt behavior in a variety of rock types (1, 2, 3). The application of partial grout columns to primary roof support systems has been introduced as a viable alternative to fully grouted bolt systems in some mines. The point-anchored resin bolt and combination bolt represent such alternatives. While both systems usually employ some mechanical interlocking system, their primary difference lies in the length of resin column used. Point- anchor systems rely on an anchoring device that is a short length of resin column of 2 feet or leas. Combination bolts, on the other hand, often require 3 feat or more of resin column. Resin migration into voids and fractures intersecting the borebole wall is a major problem when installing fully grouted bolts in weak roof rock and can result in a reduced column length that provides reduced anchorage. The relationship between column length and anchorage integrity deserves closer study. The Denver Research Center of the Bureau of Mines has examined the load- carrying capabilities of resin-grouted bolts in weak strata. Pull tests were performed on about 900 resin-grouted, 4- foot-long bolts with column lengths ranging from 6 to 48 inches. The practical performances of 8 different column lengths were evaluated for 10 mines under in sit" conditions using data obtained from standard pull tests. In addition criteria are discussed, including engineering properties of the host rock for designing safer and more effective grouted bolt installations.

Jan 1, 1987

By Sheng Zhang

The compressive strength of coal in Yanlong mine area of China is less than 3 MPa. Basically, it is powdery. The roof and floor rocks are mudstone. Therefore this coal seam is a typical ?three soft? coal seam. Roof bolting cannot be used due to the very soft strata. When the conventional U-steel support was used, roadways deformation exceeded 30% of its original dimensions in some sections one half month after development. The required repair work was huge, seriously affecting the daily mine production. This paper describes how to support the soft coal roadways. The deformation characteristics of soft coal roadway were investigated. The conventional U-steel support is not subjected to uniform load, so the whole supporting structure must be strengthened. In addition, drilling holes in the coal seam can transfer high stress from near the ribs to the deeper parts of the surrounding strata, which is beneficial to the stability of the roadway. This new support method is to combine the strengthened U-steel support with pressure-relieving drill holes, providing an economic and efficient way to support the very soft coal roadways.

Jan 1, 2013

By D. M. Shu

A two-dimensional finite element modelling technique is applied to predict the subsidence movements on a sloping ground surface above a completely mined panel. The modelling is carried out for the surface both directly and using a combination of an equivalent horizontal surface and a rays projection method and the result compared. The rays projection method provides the subsidence components on a sloping surface from the corresponding ones on an assumed equivalent horizontal surface through the point of mean elevation of the relevant part of the sloping surface. This method is used next to compare the subsidences, horizontal displacements and horizontal strains from the mining of a given panel at the surface for different inclinations. The results show that the magnitudes of the subsidence components are greater on the dip side of the slope from the point of the mean elevation, than the corresponding ones on the rise side and the asymmetry increases with the inclination of the surface. The findings are also compared with those of other research workers.

Jan 1, 1992

By Ken Barish

Jane Mine of the Keystone Coal Mining Corporation was started in 1962 and is in the Lower Freeport seas located in Armstrong County, Pennsylvania, approximately 45 miles northeast of Pittsburgh. The mine is a slope/shaft complex with the cover ranging from 100 feet at the slope to 450 feet near the back end of the property. The seam height ranges from 48 to 72 inches. A variety of roof conditions have been en¬countered, and an equal variety of roof control systems have been tried with varying degrees of success. Initially, places could be mined as deep as 100 feet without any permanent roof support. After several problems and the advent of the Health and Safety Act of 1969, roof control became more and more prevalent and predominant at the Jane Mine. It started with 4 foot conventional oolts. As the mine progressed in the northeasterly direction, roof conditions changed from the extremely good roof that was encountered initially, to bad roof; and other types of roof support were required. Sixty pound rails and 6 inch 25 pound to the foot steel I-beams were used for supplemental support in long life headings such as belt and track. Resin bolting was done in many areas for long life support due to the deterioration of the shale roof over time. Several areas had to be abandoned because of uncontrollable top.

Jan 1, 1984

By Mark K. Larson

Panel-scale modeling of stress changes induced by longwall coal mining is a challenging problem, particularly in deep mines of the western United States. Two common approaches to this problem are the empirical and boundary element modeling methods. One boundary element method, LaModel, was recently calibrated to field measurements at a mine in Colorado (Larson and Whyatt, 2012). However, the calibrated overburden properties were extreme, resulting in very low estimates of gob loading. Next, the empirical method was calibrated to field measurements and applied to the problem, but this approach clearly extrapolated this method well beyond the characteristics of underlying cases. This paper examines a third option, the elastic continuum boundary element program Mulsim. The program was expanded to handle LaModel-size meshes in a version dubbed Mulsim/ Large. Properties for the elastic, homogeneous overburden model in Mulsim were estimated as a thickness-weighted average of typical published elastic properties for each major stratum. These properties, along with a commonly used pillar strength equation and coal properties, produced the measured load transfer distance and reasonable gob loading without further calibration. Given the number of input parameters required for all of these methods, it is not surprising that LaModel and the empirical approach could be adjusted further to incorporate similar gob loads. However, these extraordinary measures failed to bring gate road pillar loading in line with Mulsim results. Differences in pillar load estimates continued to be significant (as large as 65% in this study). These results demonstrate that, for this particular case, the choice of analysis method impacts results in ways that cannot be removed by calibration of input parameters. Finally, the natural fit between the Mulsim model and mine conditions suggests that approximating overburden at the particular site used for this research as an elastic continuum is superior to the alternatives examined.

Jan 1, 2013

By Pierre-Yves Declercq, Andre Vervoort

"After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—an upward movement. Although most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method, surface movement still occurs many decades after mining is terminated. After the closure and flooding of underground excavations and surrounding rock, this movement is reversed. This paper focuses on quantifying the upward movement in two neighboring coal mines (Winterslag and Zwartberg, Belgium). The study is based on data from a remote sensing technique: interferometry with synthetic aperture radar (INSAR). The results of the study show that the rate of upward movement in the decade after closure is about 10 mm/year on average. The upward movements are not linked directly to the past exploitation directly underneath a location. The amounts of subsidence at specific locations are linked mainly to their positions relative to an inverse trough shape situated over the entire mined-out areas and their immediate surroundings. Local features, such as geological faults, can have a secondary effect on the local variation of the uplift. The processes of subsidence and uplift are based on completely different mechanisms. Subsidence is initiated by a caving process, while the process of uplift is clearly linked to flooding. INTRODUCTION After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—upward movement in the area above the past exploitation and in the immediate surroundings. Of course, most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method (Peng, 1986). This downward movement continues to occur at a smaller rate many decades after mining has been terminated when the water in the underground excavations is pumped to the surface. This is the case as long as a mine is in operation. However, after the closure and flooding of the underground excavations and the surrounding rock, this movement is reversed, and the surface is uplifted. A total uplift of about half a meter has been recorded to date. This phenomenon has been described in several different coal basins in Europe, for example, in Belgium (Devleeschouwer et al., 2008), France (Samsonov, d’Oreye, and Smets, 2013), Germany (Baglikow, 2011), the Netherlands (Caro Cuenca, Hooper, and Hanssen, 2013), and Poland (Preusse, Kateloe, and Sroka, 2013). To date, research has focused mainly on understanding the phenomenon (e.g., Herrero et al., 2012) and identifying general trends, whereby the link with the rise in water levels is an important issue (Caro Cuenca, Hooper, and Hanssen, 2013). The most accepted explanation is linked to the swelling of clay minerals in the argillaceous rocks in the coal strata (Herrero et al., 2012). Without doubt, the full explanation is complex, and swelling is probably not the only cause of the residual movement. Most coal strata rock is weakened upon contact with water. The increase in water pressure also results in a decrease in the effective stress. Both aspects facilitate the fracturing of rock, which normally would result in further subsidence. However, decreases in the effective stresses also result in the relaxation or expansion of the rock, which induces an upward movement (Fjar et al., 2008). All of these aspects, including the long-term residual subsidence due to compaction under dry conditions, result in a net upward movement. The phenomenon discussed here is clearly different from the upsidence, sometimes observed on other continents (e.g., in Australia, Galvin, 2016). When upsidence occurs, the rock near the surface bends and buckles upwards due to the overstressing of the floors of the valleys."

Jan 1, 2017

By Ed Van Eeckhout

As part of a U.S. Bureau of Mines-sponsored project on the utilization of "stiff' backflll to minimize potential rock burst hazards, a finite element study was undertaken to predict stresses and displacements around a stope mined up from the 5400 level of the Sunshine mine, Kellogg, Idaho. This stope was to be backfilled using a high-modulus fill. In preparation for that configuration, certain instrumentation was installed near the 5400 level to monitor stress changes. Due to untenable economics, only 2 cuts of the stope were ever made. This paper presents the results of the instrument readings taken during the first cut and compares them with the numerical predictions, as well as presents the modeling results of closure and fill stresses in the stope if it had been mined.

Jan 1, 1984

By Stephen C. Tadolini, John Bolton, Bre-Anne Sainsbury, Tom Meikle

"The capacity of ground support components which have been affected by corrosion is reduced and may ultimately lead to dynamic failure of the component and the strata. In order to maintain an effective, long-term ground support system, significant campaigns of rehabilitation are often required in corrosion affected areas which also expose the workers to hazardous conditions. The most common corrosion protection for steel ground support utilises sacrificial systems such as galvanising. Galvanising has previously been proven to be susceptible to some corrosion processes. Stainless steel is the most effective in resistance to corrosion, but can be cost prohibitive, and its mechanical properties often make it unsuited to use in ground support components.Providing an outer protective plastic coating to bolts has proven to be an effective means of protecting the inner steel bar from corrosion. However, these support systems tend to be susceptible to coating damage, and require post cement grouting to provide full encapsulation. In comparison to a standard bolt/resin system, they can be slow to install and expensive. These systems have also been shown to reduce overall load transfer performance of the bolting system.In order provide a higher level of corrosion protection whilst maintaining current installation practices and bolting cycle times, Minova has developed the Enduro™ Steel Ground Support Range. The Enduro™ range consists of standard Minova steel ground support components which have been treated with a unique coating process. The Enduro™ coating has been tested in the harshest of conditions, in laboratory controlled conditions and in underground trials. It has been proven to effectively resist or completely eliminate the formation of corrosion, even in the most aggressive environments. This paper explains the process and provides the details of the laboratory and underground corrosion performance testing carried out on Enduro™ ground support products.INTRODUCTIONTraditionally, the most common form of corrosion protection in steel ground support consists of a sacrificial protective coating such as galvanising (Aziz, et al., 2013). Such coatings have proven to be ineffective in extreme high and low pH conditions with corrosion of the support system commonly encountered. There is also evidence that common forms of galvanising may actually increase the rate of corrosion in certain pH environments. Figure 1 shows typical corrosion and failure of a standard re-bar roof bolt."

Jan 1, 2016

By Jessica M. Wempen, William G. Pariseau, Michael K. McCarter

"Quantitative analysis of mine-wide subsidence at the kilometer scale and details of stress distribution about an advancing longwall face are estimated using an adaptation of the finite element method. The method is well suited to the tasks at hand. For greater realism, variability of strata properties is taken into account as are the effects of joints and cleats on elastic moduli and strengths. Evolution of pillar stress and entry closure remote from the face is readily quantified in a series of analyses that simulate face advance. Computed results compare favorably with the evolution of closure measurements about an instrumented pillar in a two-entry headgate. The appropriateness of the finite element method is confirmed. This method is based on first principles that avoid empirical schemes of uncertain applicability and numerical models “calibrated” by fitting computer output to mine measurements.INTRODUCTIONMine-induced seismicity is intimately related to strata mechanics associated with longwall coal mining and room and pillar mining, as well. Strata deformed beyond an elastic limit often tend to fail rapidly by brittle fracture and, thus, to emit acoustic signals in the form of elastic waves. Understanding the induced seismicity in coal mining offers the potential for early warning of hazardous conditions. Many years of experience with seismic phenomena associated with hardrock mining suggest considerable complexity should be expected. However, recent coal mine studies (Pariseau, 2014; Pariseau, 2015; Pariseau and McCarter, 2015; Pariseau and McCarter, 2016) have shown high correlations of seismicity with element failures in finite element simulations. The main lesson learned from these studies is one elaborated by Iannacchione et al (2005); deviation from normal activity is an early warning of ground control difficulty. For example, an abrupt increase in seismic event rate, or equivalently, an increase in element failure in finite element simulation of panel advance, is a cautionary indication for ground control. Present and past study results demonstrate the effectiveness of finite element simulations for mine design and early detection of potential ground control issues."

Jan 1, 2018

By Chase K. Barnard, Rahul Thareja, Sean Warren, Rajagopala Kallu

"Inflatable rock bolts are commonly utilized for ground support in Nevada underground mines, however, there is limited information regarding what factors influence the bond strength of these bolts. Bond strength is an important parameter in friction bolt support design, and this information is usually not available from manufacturers as a standard. Previous research has investigated the bond strength of friction bolts; however, these studies focused primarily on split set bolts and ground conditions more favorable compared to those commonly encountered in Nevada underground mines. Without site-specific bolt pull-out tests, ground control engineers are required to make assumptions regarding the in-situ bond strength of rock bolts. This is especially a concern for projects in the feasibility and initial development stages before site-specific experience is acquired.The purpose of this paper is to establish confidence in anticipated minimum bond strength for inflatable rock bolts by comparing the bond strength to variable geotechnical conditions using the Rock Mass Rating (RMR) system. To investigate a correlation between these parameters, the minimum bond strength of pull-out tested inflatable rock bolts was compared to the RMR of the rock in which these bolts were placed. Bond strength vs. RMR plots indicate that expected minimum bond strength is positively correlated with RMR, however the correlation is not strong. Cumulative percent graphs indicate that 97% of pull-out tests result in a minimum bond strength of 1 ton/ft and ½ ton/ft in RMR =45 and <45, respectively.Although lower bond strengths are more commonly encountered in low RMR ground, high bond strengths are possible as well, yielding higher variability in bond strengths in low RMR ground. Bond strength of friction bolts relies on contact between the rock bolt and drill hole. Experience in Nevada indicates that RMR is known to affect both the quality and consistency of drill holes which likely affects bond strength. Drilling and bolting in low RMR ground is more sensitive to drilling and bolting practices, and strategies for maximizing bond strength in these conditions are discussed."

Jan 1, 2015

By Thomas M. Barczak

A survey of the longwall industry was conducted to examine the performance of modern shield technology. The results of this survey indicate that state-of-the-art shields perform better and last longer than ever before, but premature failures do still occur. In addition, longwalI operators are now using shields longer than ever before and a greater number of fatigue related failures on aging shields are now occurring. A generic assessment of these shield failures is provided, describing the nature of the failures and how widespread they are throughout the industry. An engineering assessment of shield design and factors that cause shield failures is made using both strength of materials and fracture mechanics principles. From this analysis, design practices to improve structural margins of safety and extend the life of the shield are proposed. The existence of unexpected shield failures indicate that current shield performance testing is at times inadequate. Recommendations are made relative to improved shield performance testing protocols including the benefits of testing in an active load frame such as the National Institute for Occupational Safety and Health&apos;s (NIOSH) Mine Roof Simulator. Even the most rigorous shield testing procedures, which strive to simulate in-service loading conditions, fail to consider key environmental issues, such as corrosion which is often the cause of structural fatigue failures in modern shields. The survey also indicated that hydraulic failures occur much sooner in the life cycle of a shield than structural failures, and although they can significantly degrade support capability, often go undetected for long periods of time. Methods to detect the hydraulic failures are also provided. The paper concludes with key points to maximizing shield design and life expectancy and ideas for future generation shield designs.

Jan 1, 1999

By R. Karl Zipf

Stress analysis programs such as MULSIM/NL, LAMODEL, MinSim 2000, and EXAMINE TAB are used in the mining industry to analyze stresses and displacements in coal mines, platinum mines, gold reefs, and tabular-type deposits. These relatively simple numerical models can efficiently simulate yielding and failure of a rock mass near a mine opening and subsequent stress transfer. The main input parameters for these models are a family of nonlinear stress-strain curves for the in-seam material. For many applications of these models, such as in coal mining, extensive field measurements and observations exist from which to estimate these stress-strain curves and calibrate the numerical models; however, such measurements are lacking for other types of mines. A three-step method is presented to determine nonlinear stress¬strain curves for boundary-element (BE) programs used in many mining applications. The method requires a suite of laboratory¬scale strength tests at various confining pressures. Note that this analysis uses laboratory-measured strength and modulus properties of alluvium. The dependency of the mechanical properties of alluvium on scale has not been established, although more tests are being planned. However, it is believed that the relative comparisons performed are valuable regardless of scale effects. First, the FLAC2D computer program is used to model the laboratory tests and determine cohesion (c) and friction angle (0) for a Mohr-Coulomb material model. Second, another FLAC2D model uses the c and 0 values to calculate vertical stress and strain around a single opening in the rock mass. This model calculates the stress-strain path of points at various distances from the opening boundary. Finally, based on these stress-strain paths, the stress-strain curves needed for a BE analysis are derived. This three-step method was demonstrated in a large, flat-lying underground test facility in very weak rock. The BE analyses agreed well with observations of failure at the test facility and provided a basis for evaluating the behavior of alternative layouts.

Jan 1, 2003

By John Kucish

Roof bolting in the Pittsburgh Coalbed takes many forms today: coupled partially grouted bolts, fully grouted rebar, passive cable bolts, tensioned cable bolts, and torque tension rebar. The Federal No. 2 Mine of Peabody Energy has recently made a significant advance in longwall headgate primary roof support, with the change fiom an 8 ft long coupled partially grouted bolt to a 6 ft long torque tension system. The coupled partially grouted bolt consists of a 4 ft long 0.804 in diameter rolled deformation bar thread coupled to a smooth 718 in headed bar. The coupled partially grouted bolt is anchored into a 1 318 in borehole with 4 ft of high speed resin. The 718 in diameter smooth bar is tensioned by rotation into the treaded coupler, upon setting of the resin, compressing the immediate roof. The change was initially driven by occasional failure of the coupled partially grouted bolt at the threaded coupler, due to lateral movement caused by horizontal stress. A fully grouted % in diameter torque tension bolt, in a 1 in borehole, was proposed as a cost effective primary bolting for the high horizontal stress conditions. The upper 2 112 ft of the rebar is anchored in fast set, medium viscosity resin and the bottom 3 K ft is anchored in slow set, special low insertion force resin. The lower end of the rebar is threaded with a nut that permits the counter clockwise spinning of the bolt to mix the resin, at maximum rotational torque. Upon setting of the high speed top resin, the nut is spun clockwise to tighten the nut against a metal dome plate. The plate is forced against the roof to compress the immediate roof, A complete headgate entry has been successfully supported with the torque tension system. The mining of the adjacent longwall panel was completed in September 2004. The torque tension system provided a 25% savings over the previously employed coupled partially grouted bolt system.

Jan 1, 2005