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By Wen H. Su

Multiple seam mining and its associated problems are very serious in Southern West Virginia where poor planning or lack of knowledge in seam interaction often results in complete loss of coal properties. Several measures have been proposed to alleviate the interaction prob¬lems under various multiple seam conditions, but few of them are spec¬ific in terms of mining plan. Two parallel approaches are adopted for this paper. One is field investigation which consists mainly of mine site visits and gathering of data. The other is the two-dimensional finite element simulation of mine workings using the data gathered as input. Two typical cases which represent the past and present multi¬ple seam mining practice in West Virginia were investigated. Much attention is focused on the interaction problems experienced and the important parameters controlling interaction. Several key parameters controlling interaction in multiple seam mining were analyzed. Proper mining plans, by which the interaction problems can be minimized, are proposed for the individual cases after a better understanding of the interaction mechanism was realized through finite element analyses.

Jan 1, 1984

By J. Riefenberg

U.S. Bureau of Mines (USBM) researchers are developing a personal computer-based hazard mapping system for use in underground coal mines. Hazard mapping is rapidly gaining interest as delineating areas of possible instability is becoming more complex. The development of a hazard map is based on the consensus that combined effects of local and regional stresses, roof, floor, and rib quality, geology, faulting, presence of ground water, etc., all contribute to stability of the ground. In order to demonstrate the concept, a hazard map was developed that combined stresses anticipated to occur in a multiseam setting using numerical modeling, and the newly developed Coal Mine Roof Rating (CMRR) where roof quality ratings in untested areas were modeled using geostatistical methods. Ten borescope readings from an underground mine were used to compute CMRR's for the geostatistical analysis. CMRR's for the mine ranged from 57 to 77, and are considered in the moderate to strong range. A hazard map was developed by combining the stress effects of interseam mining, scaled to weigh equally with CMRR's, and CMRR geostatistical analysis results. This hazard map demonstrates a synergistic effect between the mining-induced stress fields and the roof quality ratings, indicating, as expected, a greater hazard index in areas of high stress and lesser roof quality.

Jan 1, 1994

By A. Wahab Khair

This paper presents an analysis of surface ground movements recorded from two longwall mines in Northern West Virginia which operate in the Pittsburgh Seam. The instrumentation areas consisted of four panels, two at each mine, covering various sections with unique topographic features. Instrumentation consisted of survey monuments on 25 to 120 ft. spacing. Vertical and horizontal ground movements were measured at specified intervals to determine subsidence rate, developing and final subsidence profiles, angle of draw, horizontal displacements and associated strain profiles. A relationship between panel depth and width and the subsidence factor was also developed.

Jan 1, 1988

By Bruce K. Hebblewhite

Tower Colliery is a longwall mine operated by BHP Coal Illawarra Collieries, Southwest of Sydney, Australia It mines the Bulli Seam at a depth of approximately 450m. The surface topography overlying the mine consists of several steep-sided river gorges, up to 68m in depth, which run at oblique angles across a sloping terrain Apart from some light rural development, the surface land is essentially natural bushland, but is traversed by a major freeway. This crosses one of the gorges on twin, six-span, box-girder bridges, with bridge piers up to 55m in height. Consequently, a major surface subsidence monitoring program has been in place for several years now, including intensive conventional, GPS and E DM surveying, plus real-time monitoring of critical components of the bridge structure. Although the bridge and freeway are outside the conventional angle of draw' subsidence influence criteria, and have seen only negligible vertical deformation as a result of mining, there has been widespread evidence of regional horizontal deformation of the land surface, large distances away from the mining area The effect at the bridge has been well within acceptable tolerances, primarily because of the regional nature of the movements, with very little differential movement observed at the bridge piers Gorge closure and evidence of large headlands of land moving `en masse' have been observed. Horizontal movements of hundreds of millimetres have been recorded in some locations where gorge closure has been monitored. Possible explanations of these movements include one or a combination of mechanisms such as pre-mining stress relaxation. valley notch effects, valley bulging, regional joint patterns, movement toward active goaf areas, vertical gorge shearing' and shear failure of horizontal bedding planes below the surface This paper presents a case study of the regional horizontal ?subsidence displacements? measured during mining. and a discussion of the possible explanations for these phenomena

Jan 1, 2000

By Z. T. Bieniawski

A major study was performed on determination of long term strength of coal strata directed to the design of coal pillars to ensure their stability beyond the life-time of a mine. Based on field investigations, numerical modeling and reliable data from 12 case histories, three aspects of time effect in the strength of coal strata were studied; (1) strength reduction based on the concept of volumetric strain estimate from in-situ tests, (2) strength reduction in terms of three failure criteria applicable to coal strata: original Hoek-Brown criterion, modified Hoek-Brown criterion, and Generalized criterion, and (3) long-term strength estimation as a function of coal strata quality RMR based on case studies. From these investigations, an integrated approach to long-term pillar stability is proposed, based on numerical modeling and empirical pillar strength formulation. The values of factors of safety for coal pillars are analyzed in terms of probability of failure and practical meaning for mine design purposes.

Jan 1, 1994

By John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By Gexin Sun

This paper presents the physical modelling results of subsurface fracture development associated with longwall mining operations and an application of image processing techniques to interpretion of the results. Physical modelling results have been obtained by employing a large sand and plaster model loaded purely by gravity as a means of fracturing the subsurface strata above the longwall excavation. The results have shown fracture development and crack propagation as a longwall face advances and demonstrated that the strength of stratified rocks, the presence and position of a weak band-, and the extraction thickness, have significant effects upon the overall fracture patterns above the excavation. Binary images of fracture patterns were scanned from the black and white photographs of the tested models using an image analyser coupled to a video camera. These images were manipulated by a custom written C program which allows the images to be processed in both vertical and horizontal direction. The images were filtered to quantitatively differentiate between vertical and horizontal fracture intensities. The vertical fracture intensity is particularly important as vertical fractures are considered to provide the main avenues for surface and subsurface water inflow into the excavation. The use of the image analysis techniques has shown a promising improvement in image enhancement and in model results interpretation in a quantitative manner. The numerical nature of the image analysis results will allow further statistical comparison with other modelling techniques to be undertaken and significantly increase the scope for use of the modelling technique results.

Jan 1, 1992

By D. Newman

An underground limestone quarry, developed in the 1940's, transitioned from a surface quarry to an underground room-and-pillar operation. The quarry expanded to a second underground level when the reserves on the first level reached the property boundaries. The historical basis for the sill pillar thickness between the first and second level, pillar dimensions, and mining height was lost during the course of several changes in ownership. When reserves on the second level reached a five-year life, and plans were initiated for a third level, quarry management embarked upon a thorough evaluation of i. current ground control practices for the support of the immediate roof, ii. roof support in the declines between the surface, Level 1, and Level II, iii. pillar centers and pillar safety factors, iv. pillar alignment between levels, v. mining height within a level, and vi. the locations of declines to access future reserves on level three. The evaluation began with detailed mapping of the location and orientation of calcite veins, joints, fractures, and ground conditions in the immediate roof of Level II. The orientation and location of calcite veins, joints, and fractures on Level II were compared with similar mapping on Level 1, completed in the 1970's to establish whether these features were continuous and would be expected to affect conditions on Level III. Rock strength, physical properties, and fracture spacing were obtained from two (2) diamond core holes drilled from Level I to Level II and two (2) holes drilled from Level II to define the vertical extent of future reserves on Level III. The effort to characterize geological conditions was expanded to a rock mechanics investigation of the: i. performance of the current roof support, ii. optimization of roof and rib scaling, iii. calculation of safety factors for 416 pillars on Level II, iv. evaluation of future pillar centers and mining height within a level based upon pillar strength and pillar geometry, v. development of beam safety factors for the immediate roof on Level II, vi. identification of the most suitable strata for the roof in the future level three, and vii. use of LAMODEL to develop a large numerical model of overburden, inter-level, and total stress on the pillars in levels one and Level II with a stress "footprint" on the proposed level three. The characterization of engineering parameters and geological conditions resulted in changes to the: ? active operations in Level II regarding scaling ? equipment and scaling practices, ? pillar centers, ? mining height, ? raising the immediate roof horizon to avoid ? laminated strata in the current roof, and ? a change in rock bolts from a friction based system ? to active tensioned rebar. The planning and design of level three is ongoing and relies heavily upon the geological and engineering investigations conducted during the past year and one-half.

Jan 1, 2001

By Tom Barczak

Early in my career, while giving a presentation on shield design for longwall mining at one of the International Conferences on Ground Control in Mining, the late Jim Scott told me that I was finally getting it, because ?I was starting to think like a rock!? Although I didn?t realize it at the time, those few words changed my career forever. At the start of my career, I did a lot of laboratory testing, taking advantage of the unique capabilities of the Mine Roof Simulator to study the performance and design characteristics of longwall shields. I had a pretty good handle on what made shields work and what made them fail, but I was still struggling with how and why they develop their in-service loads and what they were actually controlling as opposed to being passive bystanders in a world beyond their control. It became clear to me that the simplistic models of dead weight loading could not explain the behavior I saw and measured underground. The problem did not go away when I started studying standing support systems. Again, I learned much about the strength, stiffness, and stability of these supports and helped the industry develop several new support products that attempted to provide more efficient support designs. But here, too, the simplistic models of dead weight loading did not seem to fit for me. That?s when I began to incorporate the ground reaction concept as a way of integrating the support response and the ground behavior into a cohesive design methodology. The concept has its limitations, no doubt, but I believe some form of it will be the future of not only designing supports but also, in the broader sense, enhancing the engineering aspects of ground control. The paper presented at the International Conference on Ground Control last year by Esterhuizen (2010) on pillar behavior is evidence of this. Think like a rock. Jim Scott was right. It takes two to dance. You can?t dance alone and you will never understand ground control by thinking like a support! ?Dumb as a box of rocks?? I don?t think so. The answer lies in the rocks!

Jan 1, 2011

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 Guorui Feng, Min Zhang, Yi Luo, Jian Yang, Yujiang Zhang, Junwen Bai

"Safe mining of coal seam sandwiched between abandoned upper and lower room-and-pillared mines (ULPM) ·is increasingly becoming the focus of attention in recent years, which is closely related to the damage intensity of the upper and lower interburdens. The #7 seam with ULPM in Baijiazhuang coal mine was selected for this research. According to the mining and geological conditions, the vertical stress and plastic zone distributions were determined first by numerical simulation. Then, a theoretical model based on the elasti-plasticity theory was developed. The stress distribution and failure zone created in the interburden by mining the upper and lower seams were determined. Finally the feasibility of mining #7 seam between the abandoned upper and a lower seam was evaluated. The results show that: (1) the distributions of abutment pressure and plastic zone coincide, both of which can be used to evaluate the failure zone in ULPM. The failure zone penetrates the whole upper interburden, while only parts of the lower interburden fail. (2) the theoretical failure zone coincides with the maximum distribution zone of abutment pressure and plastic zone obtained by numerical simulation: the failure zone height in the lower interburden is less than its thickness, while that in the upper interburden is larger than its thickness. (3) the #7 seam is located in the failure zone created by the upper seam room-and-pillar mining, which leads to well-development fractures and blocky fragments in the roof, thereby it is unstabled. (4) when mining the #7 seam in the interburden, the key is to control the roof strata and proper support measures should be taken timely to ensure safe mining.INTRODUCTIONMany coal mines in China mine the thick, superior and easily mineable resources preferentially, while the thin, inferior and difficult-to-mine resources were abandoned (Chappells and Trentrnann, 2015; Zhenfu, 2012; Xu, et al., 2015; Guorui, 2009). As the results of deficient geological data and shortage of investment capital, the residual coal resources distributes in many mining areas in China.In recent years, many residual coal seams sandwiched between the abandoned upper and lower room-and-pillar mines (ULPM) were explored in Shanxi Province, especially in the mining area of Datang, Xishan and Luan (Figure.1). The reserves of residual coal seams in ULPM are considerable, increasingly becomes the focus of attention among scholars and engineers."

Jan 1, 2016

By Ihsan B. Tulu, Keith A. Heasley, Aanand Nandula

"The objective of this paper is to present the development of a software tool that will assist in an area calculation of the Analysis of Roof Bolting Systems (ARBS) support intensity while incorporating more detailed geology, stress, and intersection span input. In this area ARBS tool, a detailed CMRR distribution can be determined from the available geologic database at the mine. In addition, the overburden and multiple-seam stresses, as obtained from the boundary-element program LaModel, can be converted to a “pseudo-depth” which is then used as the depth input to the area ARBS calculation. Finally, the section-specific intersection spans can be determined from the mine design for input to the calculation. All this detailed area input information for ARBS is handled by the latest version of the Stability Mapping (StabMap) program, which now outputs an area ARBS suggested support density given the required input parameters. This final ARBS grid can be plotted, analyzed and overlaid on the mine map for optimum use in support design and for presentation to production personnel.INTRODUCTIONSince the inception of roof bolts in the 1940s, there has been a lack of a universally accepted roof bolting design method (Mark, 2002; Mark and Pappas, 2012). To address this situation, the National Institute of Occupational Safety and Hazard (NIOSH) developed roof support design guidelines, which were incorporated into a computer program called the Analysis of Roof Bolting Systems (ARBS). These guidelines were based on the effects of roof quality, stress, and mine geometry (Mark, Molinda and Dolinar, 2001). The roof rock quality in ARBS is represented by the Coal Mine Roof Rating (CMRR), the depth is used as a surrogate for the stress, and the intersection span serves to represent the mine geometry (Mark, Molinda and Dolinar, 2001). These three input parameters are used in the fundamental, empirically derived equation of ARBS to calculate a required support density factor, called ARBS, (Mark, Molinda and Dolinar, 2001):"

Jan 1, 2018

By Elizabeth Holley, Meriel Young, Gabriel Walton

"The coal mine roof rating (CMRR) was developed by Chris Mark and Greg Molinda to bridge the gap between geological variation in underground coal mines and engineering design. The CMRR accounts for the compressive strength of the immediate roof, the shear strength and intensity of any discontinuities present, and the moisture sensitivity of the immediate roof. The CMRR has been widely used and validated in Eastern US coal mines, but it has seen limited application in the Western US.This study focuses on roof behavior at a Western coal mine. Mine A shows significant lateral geological variation, along with localized faulting and a laterally extensive sandstone channel network. The CMRR is not used to predict roof instability at the mine.It is, therefore, hypothesized that there are other factors that are correlated with roof instability in underground coal mines that could potentially also be included in the CMRR.This hypothesis was tested by collecting 30 CMRR measurements at Mine A. At each measurement location, a binary record of the roof condition (stable or unstable) was made along with parameters, such as depth of cover, thought to also correlate with roof stability. A statistical analysis of the data was performed to determine the parameters in addition to those included in the CMRR, which can be correlated to roof stability.INTRODUCTION AND BACKGROUNDRoof fall is one of the greatest hazards facing underground coal miners (Barczak et al., 2000). In 2017, 91 lost-time injuries occurred due to roof fall in US underground coal mines. A further 48 roof falls occurred with no lost days (MSHA, 2018).The coal mine roof rating (CMRR) classification was developed by Molinda and Mark (1994) to quantify the geological description of mine roof into a single value that could indicate mine roof stability and be used in engineering design. It is widely used in the Eastern US as an input for the analysis of longwall pillar stability (ALPS) in underground coal mines. It provides an excellent start; however, it is arguably not fully comprehensive, nor is it widely used in the Western US."

Jan 1, 2018

By Soheil Bahrampour

Development of measurement while drilling (MWD) systems for ground control applications in mining applications has recently been an active research topic. The main goal is to utilize the drilling information for characterizing the ground condition. Such characterization is the key to the accurate and efficient mapping of hazards and proper planning of ground support. However, most of the existing drilling units and measurement while drilling systems are mainly developed for joints and voids detection, and much less research is done to estimate rock strength. This paper focuses on rock classification based on estimated strength from the drilling data. For this purpose, data from J.H. Fletcher roof bolters instrumented with the vibration and acoustic sensors as well as torque, thrust, penetration rate, RPM and position signals is used. All the available data was utilized for rock classification using pattern recognition algorithms. The algorithms were developed based on data from drilling, and into three different rock types. The classification is performed using a simple feature extraction algorithm with a well-known pattern recognition algorithm. The results demonstrate the suitability of the proposed algorithms in identifying the rock types based on their strength properties, which can be adopted in measurement while drilling systems.

Jan 1, 2014

By C. A. Vining, J. R. Nopola

"Evaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. Evaporites are subject to continual creep and no amount of ground support can successfully arrest this process. Common ground-support techniques that are useful for most rock types may reduce the effective life of bolts in evaporite mines and can create unseen hazards. A different approach to ground support must be applied to provide long-term support in areas where high creep rates are present. The goal of effective ground-support designs in these situations is not to contain the rock and prevent it from moving; rather, the ground-support design must be able to move along with the rock and contain any fully detached slabs until additional remediation measures can be undertaken. The most appropriate ground-support designs will consider both the expected movement of the rock and the duration that the area needs to remain accessible.INTRODUCTIONEvaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. The factors that must be considered in design stem from the deformation behavior of salt, which can fail in a brittle manner, akin to most rocks, but can also deform in a slow, ductile manner, resembling the stretching and elongation of steel. The deformation behavior of salt has been well established in technical circles where it has been studied for decades (Hardy and Langer, 1984). Much of the research in salt mechanics is related to repository science, specifically at the Waste Isolation Pilot Plant (WIPP) in southwest New Mexico. RESPEC has been continually involved in the WIPP from the initial design considerations (Gnirk, Grams, and Zeller, 1977) to the current day (Keffeler, 2016) and has been able to leverage cutting-edge research of salt mechanics into practical solutions for mining operators. While a detailed description of salt mechanics is not provided in this document, Fossum and Fredrich (2002) include a fine overview of salt mechanics. The following list paraphrases their chapter on basic facts about salt to provide some context for the reader unfamiliar with salt mechanics:"

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 Stephen C. Tadolini, Peter A. Gray, Robert Hawker, Andrew Sykes

"Self-drilling rock bolts (SDRBs) that use pumpable cement grout to anchor bolts have been used successfully in the tunnelling and civil engineering industries for many years. Pumpable cement grout offers a considerable advantage in that the SDRB can be fully encapsulated right down to the bearing plate, including filling cracks and cavities in the borehole. However, cement grout cannot provide the immediate rock support required for the coal industry. This paper describes a new SDRB system that uses pumpable resin to anchor the SDRB in a borehole, which not only provides all the advantages of a pumpable grout anchoring system, but also provides immediate roof and rib support as required in the coal industry. This paper further outlines the latest developments in SDRB technology including (1) extendable self-drilling rock bolts, which can almost double in length during the installation process and are particularly suited to low seam heights and rib support and (2) pre-tensioned SDRBs, where a pre-tensioned load can be applied during the installation process. These new SDRB systems offer both considerable advantages in rock support and improvements in mining productivity.INTRODUCTIONThe mining industry is continually striving to develop more efficient methods to support the roof and walls of roadways and tunnels, and rock bolting has proven to be one of the most significant improvements in ground support over the last 50 years. Rock bolting is now almost universally used in the underground mining industry worldwide, and there are many different rock bolts available.Rock bolts are traditionally made from steel or fiberglass and are anchored into a borehole with a friction-anchoring device, such as an expansion shell mechanism; cement grout; or chemical resin. Friction-anchored bolts are widely used in the Australian metal mining industry, predominantly as short-term support. They are used in conjunction with resin-anchored bolts where there is a requirement for longer life. Resin-anchored bolts are used in the coal mining industry.Friction-anchored bolts provide immediate rock support, but their load transfer capacity is poor compared to resin or cement anchored bolts. In addition, friction-anchored bolts are more susceptible to corrosion than resin or cement anchored bolts."

Jan 1, 2010

By Russell C. Frith

This paper examines the design of longwall shields, focusing on those aspects that are critical to either their successful application or contribution to failure during longwall extraction. In many instances, longwall shields are assessed and designed along lines similar to that of roadway ground support systems, namely according to typical or normal geotechnical conditions. However it will be contended that for longwall shields, this is inappropriate because unlike roadway ground support systems, longwall shields cannot be supplemented with additional or secondary support in localised areas of adverse geotechnical conditions. In other words, the longwall shield needs to be designed according to what are judged to be worst case or adverse geotechnical conditions?the outcome being that a well-designed shield will be significantly over-rated for the majority of its working life. Therefore, it will be argued that the additional capital cost of such shield design can be readily justified as a prudent riskbased outcome that is essential to minimising future business risks due to strata instability on the longwall face. Shield geometry will be discussed, including such relevant factors as leg angle, inclination of the top caving shield, canopy ratio, operating height range, and tip to face distance. These are all well-established longwall shield design considerations and, most importantly, areas whereby inadequate design can render a longwall shield as highly ineffective. The issue of tip to face distance will be considered in detail, in particular, the extent by which it is an important geotechnical design consideration, the conclusion being reached that, in fact, it is not. The critical importance of maximising set to yield ratio within practical operating limits will be justified, the wisdom of this having been subject to questioning in more recent technical literature. Overall, the aim is to provide the industry with a set of suggested guidelines for future use when designing longwall shields and, hopefully, to initiate discussion on a subject that, unlike roadway ground support design, is not well covered in its entirety in the published technical literature.

Jan 1, 2013

By Axel Preusse

This paper shows basic considerations about the prognosis and prediction of surface subsidence and other ground movements in the German hardcoal industry. The following case studies describe experiences collected in projects between university and industry.

Jan 1, 2000

By Anthony Lannacchione

There has been a persistent need to forecast roof falls so that miner's exposure to hazardous underground environments can be minimized. Several monitoring techniques have been developed and are used today with varying levels of acceptance in the mining industry. This paper examines the potential for monitoring microseismic emissions activity as a means of forecasting roof falls. The use of this activity to forecast roof falls has drawn only limited attention, resulting in a lack of published field performance data. This deficiency is being partially addressed by analyzing data obtained during longwall mining at Moonee Colliery in 1998. The Moonee data base contains a wealth of information concerning roof fall forecast parameters and the seismic alarm criteria used to develop these forecasts. Four seismic alarm criteria were developed and used by Moonee with varying degrees of success. Roof falls were forecasted 73% of the time with average forecast times of 54 minutes. Ninety percent of the predicted roof falls had a warning time, i.e., the time between warning and roof fall event, of greater than 1 minute. The fraction of forecasted roof falls decayed logarithmically as a function of warning time, until only 20% of events were predicted more than 100 minutes prior to roof falls. False alarms occurred in 50% of the warnings. Many of the false alarms were quickly followed by a cessation in mining which could have temporarily halted the on-going failure process. If mining had continued immediately after the false alarms, a roof fall may have occurred soon after. The microseismic activity collected .from Moonee Colliery demonstrates that techniques to forecast roof rock instabilities in underground mines are possible.

Jan 1, 2005