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By Kevin W. Harris

Multiple-seam coal mining is a major issue in underground coal mining that adds additional complexity to the practice of ground control. As reserve exploitation is driven by economics, it is now difficult to mine coal without the existence of previous workings above and/or below the active seam; this is particularly true in the Appalachian coalfields. The frequency in which problematic multiple-seam interactions are now encountered has resulted in significant research regarding the underlying mechanisms of the problems and methods of mitigation. In the last few decades, a great deal has been learned about these interactions and the ability to minimize problems associated with these hazards has greatly advanced, resulting in better economics and improved safety. Minimizing the adverse effects of stress interactions often requires a thorough understanding of site-specific geotechnical parameters and previous mining geometry. This is especially evident in bump-prone ground, where pillar behavior is highly dependent on local conditions. This paper presents a case study in a deep cover, Central Appalachian coal mine where three bump events occurred. Initially, empirical design software was used in an effort to analyze the situation and prevent further instability. The empirical study proved ineffective. A detailed investigation of the bump events using the boundary element software LaModel 3.0 was then performed. This study resulted in a site-specific calibration standard that will be applied in numerical pillar design for the remaining reserve.

Jan 1, 2014

By Kudret Tutuk, Serkan Saydam, Sungsoon Mo

"This paper presents an overview of the floor heave management at the Glencore Bulga Underground Operations and investigates the contributing factors to the behaviour of the floor. The mine experienced a number of major floor heave events in gateroads on development. As the longwall face approached the roadways, the magnitude of floor heave frequently increased, while new floor heave also developed. Furthermore, severe floor heave events took place along the longwall face. The most observed failure mode was buckling. While regular floor measurements were conducted to better understand the nature of the phenomenon, and various measures were considered to control the deformation of floor, the mining height was increased for the predicted floor heave domains, which facilitated effective management of the floor issues. The experience in the mine indicates that mainly high horizontal stresses with greater depths of cover and certain types of floor lithology configuration are likely to contribute to the failures of floor strata.INTRODUCTIONThe Glencore Bulga Underground is located in the Southwest of Singleton in the Hunter Valley of New South Wales, Australia. Blakefield South, part of Bulga Underground, commenced longwall mining in 2010, extracting coal from the Blakefield seam, while the overlying longwall panels were previously mined from the Whybrow seam. The coal seam ranges from 4.5 m to 8.0 m thick with the mining heights varying from 2.9 m to 3.5 m. The longwall panels are 320 m and 400 m wide and up to 3.5 km long.The mine experienced a wide range of floor issues around longwall panels LW7 and LW8, both 400 m wide. For instance, the concrete floor in the travel road failed and lifted as the longwall retreated in close proximity as can be seen in Figure 1. Figure 2 shows the beam stage loader (BSL) buried into the lifted floor, and Figure 3 shows a timber prop distorted due to the movement of the floor. In addition, severe deformation of the floor along the longwall face impeded the longwall operation, as shown in Figure 4."

Jan 1, 2018

By Houquan Zhang, Yu Wu, Yanlong Chen, Zhenfu Luo, Aihong Lu

"A non-destructive detection scheme for bolt installation quality was designed by adopting the method of non-destructive detection on bolt installation quality. Based on the field measurements, a random non-destructive detection method for determining the characteristic length including anchoring length, free segment length and initial axial load of bolt was discussed, and the characteristics of increasing bolt axial load in the initial stable stage was obtained. The results show that the bolt axial load and deformation of surrounding rocks correlate each other. Compared with the traditional detection technique for bolt installation quality, the non-destructive detection method has obvious advantages such as random, accuracy, and economic. The method provides a reliable basis for bolt support optimization and stability control of surrounding rocks.INTRODUCTIONBolt support in roadway can make the surrounding rocks and bolt form a structure in the early stage of roadway development through the bolt pre-tension, which can maximize the self-bearing capacity of surrounding rocks. Thus it is a form of active support (Kang, 2007). In order to make the bolt play an active supporting role, it needs to ensure the installation quality of bolting. Thus the bolts can supply a high initial anchoring force and control the deformation of surrounding rocks (Shan, 2009; Wei, 2012). Moreover, bolt supporting is a concealed work. Once installation quality problems appear; it will be difficult for effective detection and screening (Xie, 2008).THE PROBLEMS OF BOLT INSTALLATION QUALITY IN COAL MINE AND ITS DETECTIONThe Problems of the In Situ Bolt InstallationDue to the problems in installation environment, machine, and technique, and workers proficiency (Li, 2003), there are many problems exist in bolt installation. If the borehole depth is too long, the anchoring agent will accumulate at the bottom of the bore, which reduces the anchoring length. Furthermore, if the borehole depth is short due to the poor geological condition, the exposed part of the bolt will be too Jong and the pallet cannot cling to the rock face, which prevent the pre-tension from being applied. The pre-tension is very important for bolt installation. If pre-tension is insufficient the rock bolt cannot control the deformation of surrounding rock efficiently and the bolt anchoring force will decrease considerably. The bolt length is another important parameter for anchoring quality, which will affect the control area of bolt supporting. In addition, the cleanness of drill hole has a great influence on the effective anchorage length of bolt. If the drill hole cleaning is not enough, it will directly affect the anchoring agent and the contact area of surrounding rocks, which will decrease the effective anchorage length and initial anchor-force."

Jan 1, 2016

By Stankusm John C., Xiaoting Li

"Quantitatively identifying the load on a steel king beam set is a complicated issue in designing the king beam structure at the slope bottom area. From an engineering perspective, understanding the nature and support mechanism of the king beam structure as a standing support is critical in identifying the calculation input parameters for the structure design software. Based on design practices in recent years and successful applications in several slope bottom intersections, the authors present a king beam design methodology, including qualitative analysis, quantitative calculation with a structural design software, and computer Finite Element Analysis (FEA) verification.Especially important is the concept of interaction and the confinement effect between the king beam structure and surrounding rocks, which is essential in designing a proper steel structure in terms of maintaining the roof stability, as well as saving the steel materials and costs of manufacture, labor, and delivery. This paper details the design methodology and applications in several underground slope bottom intersection designs, reaching the goals of improving roof stability and reducing costs.INTRODUCTION AND DESIGN METHODOLOGYCurrently, the load on a square or arch set is typically determined by either past empirical design experiences or known heights of roof falls in current or adjacent mines. These methods work but sometimes are too conservative from an engineering perspective. For example, when designing a king beam structure for an intersection, these methods may lead to a design with extremely large and heavy steel beams, which are not convenient for delivery and installation on-site, not to mention, costly. Therefore, accurately identifying the potential maximum load on the king beam structure is important for designing a safet, effective, and cost-efficient steel structure. Based on past several years’ practices and fundamentals of ground control and structural design engineering, the authors propose a design methodology including the following components:The nature and mechanism of effective roof support (qualitative analysis),Ground reaction curve,Interaction and confinement effect between bolts and steel standing support and surrounding rocks,Factors affecting the load conditions of the king beam set"

Jan 1, 2018

By Paul C. Hagan, Jianhang Chen, Serkan Saydam

"Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300 mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/ rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360 mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360 mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/ rock interface. Interestingly, increasing the borehole diameter from 42 mm to 52 mm in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.INTRODUCTIONFully grouted cable bolts have been used in underground mining industry for many decades. Initially, plain cable bolts manufactured by winding seven flexible steel wires together were commonly used (Thorne and Muller, 1964). Laboratory and field tests lead to a better understanding of the transfer process between grouted cable bolts and the surrounding rock mass, which is beneficial to improve the reinforcing performance of cable bolts in engineering practice (Fuller and Cox, 1975; Cox and Fuller, 1977; Schmuck, 1979; Stillborg, 1984). Nevertheless, as in many cases the initial cable was sourced from discarded steel with a smooth surface along the strand, plain cable bolts did not always meet performance expectations (Windsor, 1992). Over time, it was realised that cable surface geometry plays an important role in determining anchorage performance. Consequently, modified forms of cable bolts were developed. For instance, twin-strands which were made by using spacers to combine two plain cables together improved anchorage capacity two-fold (Matthews, Tillmann and Worotnicki, 1983). However, this required a large borehole to be drilled. Epoxycoated strands had remarkable performance in practice but were difficult to install without damaging the epoxy layer (Dorsten, Hunt and Kent, 1984). In the 1990s, there was a rapid development in the cable bolt design. A typical example was birdcaged cable bolts. They were initially suggested by Hutchins et al. (1990) via unwrapping traditional seven-wire strands and generating a number of nodes together with antinodes along the strand. But it was difficult to make birdcaged cable bolts that could be coiled for transporting."

Jan 1, 2015

By Denise Müller, Axel Preusse

"Due to multilateral underground use, mining subsidence is discussed more and more with regard to issues other than mining, such as post-mining topics, storage, or geothermal use. These discussions and the influence of the energy policy is why the situation in Germany is described as a transition from active to abandoned mining. Many PhD theses have been written within the last 20 years—every single one with a different topic but all with the same issue—the effects and challenges of mining subsidence.These challenges start with the research regarding geological and hydrogeological conditions, which is the basis for calculations like the angle of draw or predictions of subsidence due to water table drawdown. Furthermore, the research investigates issues like risk-management and financial planning. One of these issues is post-mining calculations; others include potential hazards of storage or the use of geothermal energy.This paper outlines an overview of German mining and the consequences due to subsidence. It deals with an overview of the circumstances related to hard coal mining, lignite mining, the use of geothermal energy, gas, and opportunities regarding underground storage. Basically, it describes the essential steps of processing mining subsidence, starting with monitoring to forecast and up to safety measures.With a glance back to the applied research of the prior two decades, we compare given forecasts with experiences already gained.Even though mining decreases, ground control does not.INTRODUCTIONAfter the Second World War, an economic recovery started in Germany, during which the mining industry provided an advantage. Areas like the Ruhr District or Saarland were characterized by hard coal mining. In addition, there was lignite mining in the Rhenish mining area and the middle German coal district. In the Lausitz area, there were gravel and sand companies for the construction industry and many other medium-sized mining companies. Especially in the Ruhr District and in Saarland, where large coal fields were built, mining played a major role. The number of mines increased, and the negative effects on the terrain got bigger. While minor open pits or quarries have influences on the landscape only, large open pits and underground mining lead to significant ground movements. This results in a larger importance of mining subsidence engineering. Calculations of subsidence and forecasts have been developed and refined within the last several years. Due to the fact that the mining areas were and still are densely populated, the forecasts are getting more and more precise."

Jan 1, 2018

By R. Quentin Eades, Kyle Perry

"Surface mines continue to implement highwalls for several reasons, such as increasing recovery, improving margins, and justifying higher stripping ratios. Highwall stability is a complex issue that is dependent upon a variety of mining and geologic factors, and a safe design is necessary for a successful surface operation. To improve highwall stability, it is important to understand the connection between local geology and blasting.Explosives are employed throughout the mining industry for primary rock breakage. There are a number of controlled blasting techniques that can be implemented to improve highwall stability. These include line drilling, smooth wall blasting, trim blasting, buffer blasting, air decking, and presplitting. Each of these techniques have associated advantages and disadvantages. Understanding local geology is necessary for selecting the appropriate controlled blasting technique. Furthermore, understanding the limitations and conditions for successful implementation of each technique is necessary. A discussion of the impact of geologic conditions on highwall stability is provided. Additionally, discussion is provided for the successful incorporation of the controlled blasting techniques listed above, and the associated mining and geologic factors that influence the selection and design of controlled blasting plans. Finally, a new methodology is proposed.INTRODUCTIONExplosives are used throughout the mining industry as the standard for primary rock breakage, a critical part of the mining cycle. In 2015, the U.S.. consumed 2.2 million tons of explosives (Apodaca, 2015). The coal mining industry accounted for the majority of explosive consumption, accounting for approximately 63% of total explosives. The quarrying and nonmetal mining industries accounted for 12% of explosive consumption, and the metal mining industry accounted for 9% (Apodaca, 2015). This constitutes 84% of the explosives used in the U.S. However, the energy released during the detonation process is often in excess of that required to adequately fragment the surrounding rock (Jhanwar and Jethwa, 2000). This excessive energy, along with over confinement and poor blast geometry, will cause damage to the undisturbed rock mass beyond the intended boundary of the blast. This event is known as overbreak or back-break (ISEE, 2011)."

Jan 1, 2018

By Hai Rong

EH-4 Electrical conductivity imaging system has been playing an important role in prospecting, looking for water resource, and determining coal mine gob boundary by virtue of its significant advantages such as collection of large volume of data, light weight, fast analysis, high-precision, and low cost. However, it has little application in determining the overburden failure zone and development of water flowing fractured zone and bed separation space in complex extra thick coal seam conditions. In order to confirm the overburden failure range and evolution of water flowing fractured zone and bed separations in complex extra thick coal seam conditions the panel 63005 of Laohutai coal mine in China was selected for study in this paper,. EH-4 magnetotelluric method was used to determine the overburden failure zones and bed separations in the overburden. Theoretical analysis and numerical simulation were employed to verify the EH4 survey results. The results show that the failure height in the overburden of the three coal splits was 120m to 170m before panel mining, and increased to 150m to 220m after panel mining. The overburden failure height was on average 7.4 times the equivalent mining height in complex extra thick coal seam condition of Laohutai coal mine when the sand backfilling and fully mechanized top coal caving methods were employed. After mining of panel 63005 started, bed separations were found on the hanging wall and footwall of the F18 fault where water bodies existed. The results obtained in this research not only provide a theoretical guidance for safe mining at Laohutai coal mine but also serve as a reference for predicting and preventing water inrush accidents for coal mines under similar condition in China.

Jan 1, 2014

By Keith D. Koper, Kris L. Pankow, Derrick J. A. Chambers, Michael K. McCarter

"In a mining environment, high quality seismic event catalogs are often useful in interpreting local stress states and identifying areas with elevated ground control risk. Installation, operation, and maintenance of local/in-mine seismic networks, however, may be prohibitively expensive, and mine operators may not anticipate the need for seismic data until unusual mining conditions arise. If regional, generally public, seismic instrumentation could be used to reproduce the results generated by local/in-mine instrumentation to an acceptable degree of fidelity, it would be a valuable and inexpensive tool for mine planners and ground control engineers.In this study we apply subspace detection—a non-traditional seismic event detection technique—to data from a sparse, regional network. The process is employed to identify mining induced seismicity (MIS) from an underground coal district. The results are compared to a seismic event catalog produced by a local seismic array operating within the permit boundaries. Additionally, we analyze the performance of two different stations of the same sparse, regional network in identifying blasts at a surface coal mine for which we have a detailed blasting log. In both cases data from stations between 10 and 60 km away from mining activity are used. We also explore the trade-offs between the number of detections and the number of false positives, which is influenced heavily by detection thresholds and the number of stations used in the process.For the surface mine we are able to identify more than 90% of the production blasts documented in the blasting log. We also find that the detected blasts from each of the four active pits produce waveforms similar to one another. The results are not dependent on source-receiver distance, and by requiring detections to occur on both stations the number of sure false positives is reduced to zero. In the underground mining district we identify between 30% and 75% of the events detected by the local array, depending on the acceptable number of false positives and the source-receiver distance. We also find many events that were missed by the local array, showing that regional subspace detection can usefully augment traditional local catalogs. Moreover, subspace detection can provide an estimate on event location and classification using data from as little as one seismic station."

Jan 1, 2015

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 Binchuan Zhang

Based on the theory of fracture mechanics and blasting, computer simulations were performed to predict the original rock mass structure and those rock mass structures damaged at different levels uing the properties of joints and fratures obtained in the field.

Jan 1, 2001

By M. V. Kurlenya

The experience of application of geomechanical investigations for substantiation of extraction of undermined ore deposit in the north of Siberia is discussed. The finite element method was used for forecasting the state of massif during the development of mining works. The results of calculations were compared with the experimental data of observations in situ for evaluation of authenticity of numerical modelling.

Jan 1, 1990

By Buddhima Indraratna

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks.

Jan 1, 1992

By Erik Westman

Tomographic imaging, commonly known as a CAT-scan or CT-scan, is used in the medical field to generate a picture of the interior of a body. Current research is applying this method to bodies of rock so that rock failure can be better understood and predicted. This paper discusses the basic principles of tomography and describes recent developments in the use of seismic tomography in longwall coal mines, including results from a bump-prone longwall coal mine and continuing research.

Jan 1, 2003

By Michael A. Berdine

In early 1997, Twentymile Coal Company took a dramatic departure from traditional longwall recovery methods. Previously, Twentymile's standard longwall meshing procedure included the use of chain link fencing material. Beginning with the 9-Right to 8-Right longwall recovery operation, Twentymile incorporated a polyester geogrid mesh system supplied by DYWIDAG-Systems International. This paper discusses the subsequent utilization of the polyester geogrid mesh system at Twentymile.

Jan 1, 2001

By Robert P. Kudlawiec

The use of Mobile Roof Supports (MRS) in pillar retreat mining has been a fairly recent occurrence in coal mining operations in the United States (1988). Determining the pillar block configuration, the crosscut and entry centers, the number of entries developed, the method of ventilation, and the type and mixture of equipment used arc all factors in establishing a successful pillar retreat operation. This paper will evaluate and report on the history of one coal company's relationship with MRS usage for pillar retreat mining and focus on the company's use of horizontal pillars (inverted-blocks) for the panel configuration.

Jan 1, 2000

By James Pile

Since C.C. White introduced coalmine roof trusses in 1967. many different systems have evolved. Beginning in the mid 1990's, several types of bar/cable truss systems have been introduced. A new and innovative bar/cable truss recently introduced offers several advantages. By eliminating many small parts, and simplifying the installation, this new truss increases operator safely. In order to fully benefit from the new truss system, a closer look at the individual components and their interaction was undertaken to understand critical implications on preload and overall support capabilities.

Jan 1, 2004

By Hamid Maleki

A new failure criteria is proposed for assessment of roof stability in underground sedimentary rocks. Unlike other stability assessment techniques, this method is simple and practical, having improved mine safety and productivity in several mines. It is shown that roof movements accelerate to a critical rate prior to actual roof collapse. Critical rates vary between 0.025 inches per day to 0.2 inches per minute depending on rock mass - pillar mechanical properties. Specific critical rates are determined for three rock mass - pillar categories which are common to mining in sedimentary rocks. Time-to-caving varies from one minute to ten days on rock structural controls and type of primary support utilized.

Jan 1, 1990

By Ross Seedsman

There are many models for how roof bolts behave, one of which is prevention of shear along bedding. The basis of this model is that a roof beam will not delaminate if bedding parallel shear can be prevented. A key point in such a model is that bedding discontinuities are present naturally in the roof and that mining induced stresses do not generate new surfaces. The presence of these discontinuities means that the rock mass must be modelled as a transversely isotropic material if continuum codes are used. A limit-equilibrium type design method for specifying roof bolts is being developed using voussoir beam and excess shear force concepts. The method incorporates much of what operators know intrinsically about roof reinforcement, and allows mining engineers to focus on the key uncertainty - the frequency and location of bedding surfaces in the immediate roof. Areas where more research is required are highlighted.

Jan 1, 2000

By Deren Zhu

Based on the results of field observation and physical model analyses, a computer simulation method, FEAEBP, has been developed for simulating the breakage of the main roof by considering it as a Kirchhoff plate on Winkler elastic foundation. By this method various parameters of rock property, different boundary conditions of the working face and the initiation, development and results of the breaking process of the main roof with different geometric dimensions can be effectively simulated. Since the main and immediate roofs, and the coal seams are treated as an integrated mechanical system, the interaction among them can be studied in detail.

Jan 1, 1988