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By Anthony Spearing

There are approximately 68 million rockbolts installed annually in underground coal mines in the USA (Tadolini and Mazzoni, 2006). The most utilized is the passive fully grouted resin rebar bolt, which constitutes almost 90% of the primary support in US underground coal mines. Resin-grouted rockbolts are a very effective and safe support, but their performance still needs to be further optimized and improved. This paper investigates their performance and suggests operating conditions to optimize the performance of resin-grouted rockbolts. In addition, this paper discusses a small-scale test that can also be used to evaluate the performance of rockbolts that is quicker, easier, and less costly than short encapsulation tests.

Jan 1, 2014

By Josh Johnson, Stephen Tadolini, Mark Ferster, Curt Jacobs

"Stillwater Mining Company is the only U.S. producer of platinum group metals (PGMs) and the largest producer outside of South Africa and Russia. The Company controls a considerable portion of the J-M Reef and is currently developing the ""Blitz Project,"" which will provide a main haulage level, ore pass systems, backfill plants, and flow-through ventilation to increase production and ensure sustainable extraction of this critical strategic mineral. While driving two sub-parallel footwall laterals to about 8,125 m (26,000 ft) long, one of the development drifts experienced a massive fall in a geologically disturbed area created by a dike intrusion. Combinations of a void fill material (Tekseal), polyurethane injection, fore poling coupled with steel arches, and supplemental bolting allow safe and efficient advance through this geologically disturbed zone. This paper presents the details of this project.INTRODUCTIONStillwater Mining Company is the only U.S. producer of platinum group metals (PGMs) and the largest producer outside of South Africa and Russia. The Company controls a considerable portion of the J-M Reef and is currently developing the ""Blitz Project,"" which will provide a main haulage level, ore pass systems, backfill plants, and flow-through ventilation to increase production and ensure sustainable extraction of this critical strategic mineral.Tunnel Boring Machines (TBMs), shown in Figure 1, have been used extensively at both the Stillwater Mine and the East Boulder Mine to develop access to the J-M Reef. The J-M Reef is a PGMbearing reef deposit located in the Stillwater Complex near NyeMontana. The mining property and location are shown in Figure 2.The Blitz Project is named after the Blitz mining claims and is developing the Eastern extent of the J-M Reef Project., This consists of a 5.5 m (18 ft) diameter TBM drive on the SOE (1,562 m (5,000 ft) above sea level and east of the shaft collar) and a 4.7 x 4.7 m (15 x 15 ft) drill and blast heading on the 5,600E (1,750 m (5,600 ft) above sea level and east of the shaft collar). The Benbow Decline will be completed running perpendicular to the TBM by drilling and blasting methods. The TBM drift will be used for exploration drilling, ventilation, and will serve as the main haulage level. The 56E will be used for ventilation and exploration drilling. The Benbow decline will serve as the exhaust drift."

Jan 1, 2016

By Joe McQuerrey, William Kendall, Gene Wilson

"Roof bolting is the standard for ground control in underground coal mining. Rib and truss bolting is just as important when conditions are less than desirable. As a manufacturer of roof drills, J.H. Fletcher & Co. (""Fletcher"") promotes a walkthrough design machine to protect the machine operators from rib hazards.When installing rib and brow bolts in high mined heights, the operator's natural desire is to step from under the canopy onto the drill feed mechanism. This is done to allow a better reach to the drilled hole in the rib or brow. This, of course, is not an approved or acceptable practice.The effort to improve the operators' ability to reach the rib and brow has resulted in a new boom design. The design was first shown in September 2008 at the MINExpo in Las Vegas, NV. The design incorporates a step out area towards the rib side of the machine. The step provides improved access to the mine rib and brow.Having a solid concept does not guarantee an accepted design. Operator acceptance is the goal. When the machine was first tried in the underground environment, the machine operators did exactly as requested: run the machine as supplied and suggest improvements.In the fast pace job of roof bolting, any ergonomic improvements are welcome. These improvements usually increase the operator's productivity and safety. Based on operator feedback and final evaluation with Fletcher, machine revisions were generated. When the machine revisions were completed, Fletcher again asked the operators for comments, giving the boom design an evolving path into our industry.INTRODUCTIONIt's human nature to find the most comfortable position in which to do a task, especially for repetitive tasks. Designing a machine that will withstand the rigors of underground mining is a challenge in itself. With stringent safety requirements and limited real estate, there is a constant evolution in design to converge safety with productivity. With the ongoing development of the ""Rib Access Boom,"" one more step in the evolution process has been taken."

Jan 1, 2010

By Bob Coutts, Matt Martin, Dan Lynch, Dan Payne

"The Broadmeadow punch longwall coal mine in Central Queensland Australia has experienced significant highwall movement associated with the effect of longwall subsidence when the longwalls approach their final position close to the open-cut highwall. In response to this movement, Broadmeadow employed two types of broadscale highwall monitoring (radar and laser scanners) to provide full coverage measurement throughout three consecutive longwalls approaching the highwall. This was to attain a better understanding of the mechanism causing the movement and potentially enable prediction of instability. Results from the monitoring found the highwall is displaced to magnitudes unlike those typically measured in open-cut mining and in direct contrast with typical longwall subsidence behaviour.This paper discusses the ground movements measured, monitoring methods used, safety measures established, as well as theorising the failure mechanism. Recommendations are made for mine and pit designs for future punch longwall layouts. The paper shows how the movements measured are more aligned to some measurements made during stream valley closure studies previously presented at the ICGCM and challenge the mechanisms suggested by previous literature.BACKGROUNDThe mining of longwalls under or adjacent to large voids (e.g., stream valleys, escarpments, or cliffs) is commonly associated with heavily vegetated or steep surface areas. In areas of extreme topographic variance, access to traditional survey pegs and stations or even new radar or laser technologies is limited. In addition, seldom have the longwall layouts aligned themselves parallel or perpendicular to the surface feature, making interpretation of any available surface movement data more complicated.The punch longwall layout (Figure 1) is also quite uncommon (only undertaken at a small number of longwall mines in Australia) but creates the perfect configuration to enable a somewhat controlled study of the effect of longwall subsidence on a steeply dipping surface feature (an un-vegetated, evenly excavated open-cut highwall). The relatively recently developed radar and laser scanning technology has also enabled near continuous, real-time, sub-millimetre monitoring of a full 500m wide x 100m high highwall. Because the punch longwall enables access and a clear view, the technology could be easily deployed as compared to the highly vegetated and variable topography of stream valleys."

Jan 1, 2018

By Saeed Zhandi, Behdeen Oraee

"Rock bursts remain an important problem in longwall coal mining. These bursts are due to a sudden and severe failure of rocks from a high stress concentration in deep underground excavations that occur with the instantaneous release of strain energy stored in the rocks. They can potentially cause irrecoverable damage to equipment and personnel, thus accurate rock burst prediction and control is expected to be carried out by the mine design engineer. As a result, this can constitute major challenges for said engineer. In this paper, forensic engineering has been used to evaluate the possibility and extent of rock bursts in deep coal mining. For this purpose, established mining engineering principles, including factors influencing the severity of rock bursts, have been incorporated in the forensic engineering technique. The analyses took place in five steps:• Assessment of regional and local conditions prior to the event• Assessment of conditions after the event• Hypothesize plausible ways in which pre-event conditions can become post-event ones• Search for evidence that either denies or supports various hypotheses• Apply engineering knowledge to relate the various facts and evidence into a cohesive scenario of how the event may have occurred.The paper concludes by demonstrating a method for predicting rock bursts and preventing their reoccurrence. The methodology used in this paper, together with the results obtained, can serve as useful tools for the coal mine design engineer in the primary evaluation of rock burst potential in underground coal mines.INTRODUCTIONA rock burst is defined as the sudden and great failure of rocks due to a high stress concentration in deep underground mines. This phenomenon occurs with the instantaneous release of strain energy that is stored in the rocks, and can cause irrecoverable damages to both equipment and personnel (see Figure 1). Rock bursts pose a hazard owing to their lake of predictability; in fact, research into understanding rock burst mechanics and implementing techniques for their monitoring and prediction has been taking place for over 50 years."

Jan 1, 2015

By Christopher Mark

Coal bursts are typically associated with highly stressed coal. Most bursts occur during retreat mining (longwall mining or pillar recovery) in highly stressed locations like the tailgate corner of the longwall panel. Others are associated with multiple seam interactions. However, a small but significant percentage of coal bursts have occurred during development or in outby locations unaffected by active mining. Most development bursts have been relatively small, but some have been highly destructive. No theory of coal bursts can be complete if it does not account for this type of event. This paper focusses on the development mining coal burst experience in the US, putting it into the context of the entire US coal burst database. The first documented development coal burst occurred almost exactly 100 years ago during slope drivage at the Sunnyside Mine in Utah. Sunnyside subsequently had a long history of bursts, mainly during retreat mining but also during development. Several Colorado mines have also experienced multiple development bursts. Some, but by no means all, of the development bursts in these western US coalfields have been associated with faults. In the Central Appalachian coalfields, most development bursts have occurred in multiple seam situations. In some of these cases, however, there was no retreat mining in either seam. The paper closes with some lessons from this history, with implications for preventing such events in the future.

Jan 1, 2017

By Bruce K. Hebblewhite

BHP Billiton Illawarra Coal operates Dendrobium Mine in an area 10-20km west-northwest of Wollongong in the Southern Coalfield of New South Wales, Australia. The mine recently completed longwall mining in Area 3A adjacent to an overhanging rock feature known as Sandy Creek Waterfall, which had been identified as a significant, and potentially sensitive natural surface feature of the environment. This waterfall lies within the Sydney drinking water catchment area, in a surface terrain of variable topography where non-conventional subsidence behaviour such as valley closure, and far-field horizontal movements are known to occur, over and above conventional subsidence responses to mining. As a part of the conditions of approval to mine, Illawarra Coal undertook to protect the waterfall and the section of Sandy Creek immediately upstream of the waterfall from the effects of longwall mining using an innovative management process and an array of very high resolution monitoring systems. This paper outlines the type of non-conventional subsidence effects that can occur; the range of different monitoring regimes adopted in response to such effects in order to develop an understanding of the valley closure phenomenon in proximity to the waterfall and provide suitable early warning data. The paper also describes the management processes that were adopted, incorporating the use of these monitoring regimes in order to successfully protect the waterfall from the valley closure effects of mining four adjacent longwall panels in close proximity to the waterfall while continuing to maximise recovery of the coal resource in the area. The management structure adopted involved a technical committee, a steering committee, and an external independent reviewer. The technical committee was responsible for design of the monitoring systems, interpretation of the results, and making timely recommendations to the company steering committee who were then able to make informed decisions on when to cease mining each adjacent panel. The steering committee comprised Illawarra Coal management and technical personnel. Although the steering committee took advice from the technical committee, all decisions relating to mining were made by the steering committee. An external reviewer was engaged by the steering committee at the end of each longwall panel to review the results, interpretation and management decisions. This management structure was effective in successfully protecting the very sensitive structure of Sandy Creek Waterfall from potential impacts of nearby mining in the midst of active ongoing natural erosion processes.

Jan 1, 2014

By Keith A. Heasley, Christopher R. Newman

"Originally developed in 1993, LaModel has aided mining engineers and researchers alike in improving the design and safety of underground single- and multiple-seam mining operations (Heasley, 1998). Through the years (Heasley, 2011), the use and capabilities of the LaModel program have grown immensely while adapting to ground control concerns and problems within the mining industry. Presently, MSHA specifically mentions the use of LaModel for the analysis of, “complex non-typical roof control plans,” defined as either room-and-pillar retreat mining greater than 1000 feet, bump- or bounce-prone mines, or other criteria considered unusual by the District Manager (Stricklin, 2013).With an ever increasing number of users and an increase in available solution options and complex analyses, there is now a significant demand for better education and training in the practical application and detailed analysis using LaModel. This paper presents the development of an electronic user’s manual and comprehensive online training course created in an open online learning environment in order to better inform and train industry professionals as well as engineering students and new users. The development of both the user’s manual and online training course will ultimately increase the ground control design effectiveness of mining engineers, leading to safer and more productive mine designs.INTRODUCTIONOver the past 20 years, mining engineers and researchers have used the LaModel program for improving the design and safety of single- and multiple-seam mining operations. LaModel uses a laminated, displacement-discontinuity, boundary-element method which simplifies the overburden into a stack of frictionless homogeneous laminations and limits the detailed analysis to the seam itself. This approach is fundamentally simpler than either finite-element, finite-difference, or discrete-element approaches; and, therefore, the program provides a significant reduction in computational time and efficiently calculates the displacements, stresses, pillar safety factors, multiple-seam interactions, subsidence, etc. for thin-bedded deposits such as coal, salt, potash, limestone, etc. (Heasley and Salamon, 1996; Heasley and Agioutantis, 2001). The laminated overburden model in LaModel is different than previous homogeneous elastic formulations and more realistically represents the natural flexibilities of the stratified sedimentary overburden typically associated with coal seams (Heasley, 2008)."

Jan 1, 2015

Design and selection of support systems for longwall faces call for in-depth knowledge of strata mechanics and in particular of strata-support interactions. The paper presents the results of field investigations and analysis of a difficult-to-cave roof using powered supports under a massive dolerite sill in Central India using a state-of-the-art datalogger system. The mechanised longwall face was equipped with 4/680 tons chockshields operated on the IFS mode using a 300 K W DERD shearer. The datalogger system was capable of storing data simultaneously in 32 channels. The paper describes the monitoring system and the monitoring programme. The system provided useful information on support behaviour, build-up of leg pressures and face convergence during an intense dynamic periodic weighting phenomenon which destroyed the integrity of almost half of the face supports. At this point, the face had advanced by about 197 metres from the barrier. The paper outlines a post-mortem analysis of this dynamic event and provides a critical evaluation of the data collected for understanding strata support interaction and the behaviour of the massive sill in the overburden.

Jan 1, 1992

By Kazem Oraee-Mirzamani, Nikzad Oraee-Mirzamani, Arash Goodarzi

"Unexpected movement of ground can potentially endanger lives, damage equipment or destroy property. Occupational accidents frequently occur with fatal consequences in developing countries with significant economic dependence on industries such as mining. Therefore, there is increasing need for miners' protection against such hazards. Slope stability and roof support accidents are two of the major causes of fatalities at surface and underground mining operations. According to national codes, employers are obligated to protect workers against accidents; however, these rules fall short of the standards in developed countries. National safety regulations should clearly specify support systems. The International Labor Organization (ILO) prepared two codes of practice, aiming to guide those responsible for improving standards of safety and to provide guidelines for drafting of safety regulations for the coal mine industry and quarry open cast mines. The practical recommendations of these codes in the ground control area have been analysed and the advantages and disadvantages of ILO codes concerning ground control are summarized.INTRODUCTIONTwo million people die every year from work-related accidents and diseases. An estimated 160 million people suffer from work-related diseases, and there are an estimated 270 million fatal and non-fatal work-related accidents per year. In economic terms, the ILO has estimated that 4% of the world's annual GDP is lost as a consequence of occupational diseases and accidents (ILO, 2009).Mining has always been dangerous due to many potential risks such as those associated with using machinery in enclosed spaces, rock falls, blasting operations and explosion of flammable gases, which endanger miners' safety and health. However, it is suggested that implementing new laws along with strict adherence would reduce such risks in the workplace.The severity and frequency of accidents are trending downward in developed countries, where the focus is on large private companies who invest heavily in mechanization and developing economies of scale. But in many parts of the world, particularly in developing countries, minerals are extracted by people working with simple tools and equipment; they work on a smaller scale, are more flexible, and can exploit irregular ore bodies and steep, dipping seams."

Jan 1, 2010

By Khaled M. Mohamed, Michael M. Murphy, Ted M. Klemetti, Heather E. Lawson

"The design of rib support systems in U.S. coal mines is based primarily on local practices and experience. A better understanding of current rib support practices in U.S. coal mines is crucial for developing a sound engineering rib support design tool.The objective of this paper is to analyze the current practices of rib control in US coal mines. Twenty (20) underground coal mines were studied representing various coal basins, coal seams, geology, loading conditions, and rib control strategies.The key findings are: (1) any rib design guideline or tool should take into account external rib support as well as internal bolting; (2) rib bolts on their own cannot contain rib spall, especially in soft ribs subjected to significant load—external rib control devices such as mesh are required in such cases to contain rib sloughing; (3) the majority of the studied mines follow the overburden depth and entry height thresholds recommended by the Program Information Bulletin (PIB) 11-29; (4) potential rib instability occurred when certain geological features prevailed—these include draw slate and/or bone coal near the rib/roof line, claystone partings, and soft coal bench overlain by rock strata; (5) 47% of the studied rib spall was classified as blocky—this could indicate a high potential of rib hazards; and (6) rib injury rates of the studied mines for the last three years emphasize the need for more rib control management for mines operating at overburden depths between 500 ft and 1,000 ft.IntroductionEfforts to improve the stability of underground mine ribs have continued for decades. These efforts by mining professionals around the world persisted throughout the 1990s and 2000s as well, though much of this work focused on rib support methodology and selection of proper support for a specific location. Despite these efforts, it is unfortunate that there has been a continual occurrence of rib-related fatalities in underground coal mines. While overall improvements in rib safety have been achieved, the average fatality rate is still about 1.3 fatalities per year over the last 18 years (Jones, 2014).To manage and control the rib hazard, rib support is used in U.S. coal mines. The current rib support methods for U.S. mines fall into two main categories: rib control based on intrinsic support systems and those based on external support systems. Intrinsic or rib-bolt support requires a roof bolting machine that can install a bolt fixture into the rib. Furthermore, the appropriate level of support needed to control the ribs with an intrinsic-based support system as well as the design parameters for those support systems for various mine conditions have not been established in U.S. coal mines. Furthermore, surface control systems are often an integral part of these support systems and must be considered in the system design."

Jan 1, 2015

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 R. Kaund

"Quantifying the rockburst consequence is of critical importance to reduce the hazards with preventative measures in underground mines and deep tunnels. Contours of energy components within a pillar model are plotted at different rockmass damage stages, and plastic strain work and released energy are proposed as indicators of rockmass damage consequence. One pillar model under different loading stiffness is simulated to assess indicators of pillar burst and the resulting damages. The results show the rockmass damage under soft loading stiffness has larger magnitude of plastic strain work and released energy than that which is under stiff loading stiffness, indicating the rockburst consequence can be quantified with plastic strain work and released energy in numerical models. With the quantified rockburst consequence, preventative measures can be taken to avoid severe hazards to mine safety.INTRODUCTIONWith the increasing demand of mineral resources and depletion of near-surface ores, the depths of underground mines have made the mining activities one of mankind’s most dangerous types of work (Ortlepp, 2005). The Mine Safety and Health Administration (MSHA) defines rockburst as “a sudden and violent failure of overstressed rock resulting in the instantaneous release of large amounts of accumulated energy.”The types of rockburst can be classified into three types: (1) strain burst, (2) pillar burst, and (3) fault slip (Müller, 1991). Strain burst is the most common type of unstable rock failure in underground openings, where the intensity and scale are usually smaller than pillar burst and fault slip (White and Whyatt, 1999; White, Williams and Whyatt, 2002; Ortlepp, 2005). The cause of strain burst can range from shattering of rock under high stress concentration to buckling of discontinuities parallel to underground openings. The occurrence of pillar burst also depends on the stress and discontinuity conditions in the rockmass, but pillar burst usually involves the sudden loss of strength in the core or foundation of a pillar (Whyatt and Blake, 2002; Kias and Ozbay, 2013; Khademian et al., 2016). Fault slip or slip burst can be either slip along pre-existing discontinuities or shear failure in the rockmass, when the shear stress is larger than shear strength in the fault (Rice, 1983; White and Whyatt, 1999; Sainoki and Mitri, 2014). The consequence of fault slip can vary from very small rockmass damage to consequential rockmass damage, but large seismic magnitude does not always result in large rockmass damage (White and Whyatt, 1999; Whyatt and Blake, 2002)."

Jan 1, 2018

By Stephen P. Signer

A field test was conducted by the U.S. Bureau of Mines at the Cyprus Plateau Starpoint No. 2 Mine. near Price. UT. to study the support rock interaction mechanics. Twelve instrumented. fully grouted roof holts and eight sagmeters were installed in the roadway of a two-entry gate road to measure load changes and roof movements. Readings were taken during section development and as each longwall panel passed the test area. Loads that resulted from development of the en- tries were higher than would be expected if simple suspension of the roof rock was the sole cause of loading. When the first longwall face passed the test section massive beam behavior was exhibited over the entry and adjacent pillar. However, no significant localized bending was detected at any of the strain gauge locations. After the first panel passed the test section. all the roof bolts had at least one zone where the steel had yielded. Loading continued to increase. and when the face from the second panel was adjacent to the test section. 70 pct of the strain gauges at the 6-. 1%. and 30-in locations had passed the yield point of the steel roof bolt. However, even at these high load levels, the supporting pattern was sufficient to provide adequate reinforcement to the mine roof.

Jan 1, 1990

By Adam Mackenzie, Dan Payne, Yass-Marie Rutty

"Historically, the Crinum mine has experienced significant falls of ground when longwall production slowed to prepare for recovery in weak roof areas. These conditions continue through the recovery process and result in both safety concerns and delays. When mine plans and exploration revealed that most of the mine's future longwall recoveries were located in weak roof areas, a decision was made to try pre-driven recovery roads as a solution to the problem. After completing eight pre-driven recovery roads with varying degrees of success and numerous lessons, the Crinum North mine now utilizes a modified Pre-driven Recovery Roadway (PDRR) to improve the longwall take-off process in weak roof areas. During mine development, a standard roadway is driven where the final recovery location of each longwall is planned. After the installation of secondary support, the PDRR is backfilled with a cement-flyash mix to provide support to the roof, and confinement to the ribs and floor of the roadway. The method has been refined over the last four years to provide greater strata stability and improved operational and safety performance compared with conventional take-offs at Crinum, and has resulted in a site record of longwall relocation in 11.5 days (pull mesh to picks in coal). This paper describes the evolution of the PDRRs from Crinum East to Crinum North, including lessons from initial attempts and changes to the secondary support regime, the operational approach during the final stages of retreat, the backfill strategy and plans for the future.INTRODUCTIONThe Crinum mine is the underground component of BHP Billiton Mitsubishi Alliance's (BMA) Gregory Crinum mine, located northeast of Emerald, Queensland (Figure 1).The Crinum mine has a history of roof control problems coming into and during longwall recoveries. Weak roof, combined with the slow mining, which is a consequence of preparing for equipment recovery, pulling mesh (which also creates tip-to-face issues), and the lengthy bolt-up process, often results in loss of immediate roof and subsequent roof falls. In fact, approximately 50% of the first 13 longwall recoveries (which were at depths of 180 to 200m) experienced roof fall delays, the longest of which took four months to mine the last pillar and recover the longwall. This recurring problem prompted the mine to investigate options to reduce or eliminate the problem."

Jan 1, 2016

By Styles P.

To determine whether 130 felt earth tremors around Edwinstowe, Nottinghamshire U.K, which also experienced severe surface fissuring, were caused by coal extraction, a surface seismometer array was established around Thoresby Colliery. Over the next year, 785 microseismic events were detected. The spatio-temporal seismicity patterns are clearly associated with the commencement, continuing extraction and closure of faces. Of particular note are events which locate at the surface and appear to be related to the active fissuring. Events occur within days of commencement of production and cease when production finishes, with good correlation between face advance and hypocentral position. Naturally occurring microseismic events have also been detected up to 1 km ahead of active longwall faces in the Midlands using triaxial geophone packages grouted into the seam together with a surface seismometer in the top of the borehole. The quality of these data were very high and guided waves can clearly be seen with the dispersive characteristics associated with seam waves. In one experiment more than 2000 events were detected in only two days of monitoring even in a relatively noisy surface environment. This paper demonstrates how very accurate locations ( 5 10 metres) can be generated using three-component digital data from only one borehole. The surface seismometer and the borehole P-wave onset can give extra precision for the height of the event relative to the seam. The event distributions give a dynamic, three-dimensional image of the developing patterns of fracturing above, below and ahead of the longwall face with important implications for roof control, subsidence prediction and gas migration.

Jan 1, 1992

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'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 Jim Galvin

4 and 6 point timber chock constructions are used extensively on both a systematic and spot basis by Australian longwall operators to support tailgate roadways. In 1996, the School of Mining Engineering at UNSW published design and performance criteria for timber chocks constructed from Australian hardwoods. The research was funded by the Australian Coal Association Research Program (ACARP). In 1997, ACARP awarded funding for a stage 2 testing program. This testing focussed on examining the effect of using thinner timber elements in chock constructions to improve OH&S and comparing the full-scale performance of traditional 4 point chocks to Link-n-Lock constructions. It was established that chocks constructed from thinner elements have failure loads much less than thicker elements. However up to the failure loads of the thinner elements there was no significant difference in the response to convergence of chocks constructed from 25, 50 or 100mm thick elements. Chocks constructed from 150mm thick elements have a similar ultimate strength to the 100mm thick elements. However, they have a significantly lower resistance to load, thereby permitting more convergence to occur. There is more potential for chocks comprised of thin elements to be constructed 'out of plumb' and so suffer eccentric loading. The full scale testing program established that at any given level of convergence up to 10% strain, the timber in a Link-n-Lock chock generates twice the support resistance of the same quantity of the same timber in an equivalent size 4 pointer chock. On the basis of support resistance per cubic metre of timber, there is only a marginal difference in the performance of 1.0m Link-n-Lock chocks and 1.2m Link-n-Lock chocks constructed from select Blue Gum. The cost per tonne of support provided by a Link-n-Lock chock is effectively half of that provided by a 4 pointer chock of equivalent dimension and timber. Up to a strain level of 4%. Link-n-Lock chocks constructed from 1.2m long elements are slightly more cost effective than the same chocks constructed from 1.0m long elements. Thereafter, there is no significant difference in cost per tonne of support resistance. Beyond a strain level of about 4%, Link-n¬Lock chocks constructed from landscape grade Blue Gum are just as cost effective as Link-n-Lock chocks constructed from structural grade 4 Blue Gum.

Jan 1, 2000

By Radoslaw L. Michalowski

The operation of mobile mining equipment near the crest of stockpiles and wastedumps often results in localized slope failures. The Bureau of Mines has developed a computer code based on a rigorous limit plasticity approach which determines the admissible total weight for a concentrated loading near the crest of these otherwise stable structures. The program is based on a three- dimensional analysis and provides an upper bound solution to the load that can be applied to the top of the operating distances for mobile equipment from the slope edge. This presentation will discuss the application of limit plasticity to the development of the local three-dimensional collapse mechanism, and the use of the computer code based on this approach for mining applications.

Jan 1, 1990

By Peter Zhang, Dan Neilans, Scott Peterson, Scott Wade, Joe Pugh, Ryan Mcgrady

"A coal outburst is a severe safety hazard in room-and-pillar mining under deep cover. It is more likely to occur during pillar retreating. Multi-seam mining dramatically increases the risk of coal outburst within the influence zones created by remnant pillars and gob-solid boundaries. Though coal outburst is generally associated with heavy loading of coal pillars, its occurrence is difficult to predict. Risk management provides a proactive tool to minimize coal outburst in room-and-pillar mining under deep cover. Risk assessment is the first step in identifying and quantifying outburst risk factors. The primary risk factors for coal outburst are overburden depth, roof and floor strength, geological anomalies, mining type, multi-seam mining, and panel width. A risk assessment chart can be used to proactively screen out mining sections with high risk of coal outburst for further analysis. Gob-solid boundaries and remnant pillars are critical factors in evaluation of the coal outburst risk of multi-seam mining. Risk identification, risk assessment, geologic influence mapping, geotechnical evaluation, risk analysis, risk mitigation, and monitoring are essential elements of coal outburst risk management process. Training is an integral part of risk management for risk identification and communication between all the stakeholders including management, technical and safety personnel, and miners.INTRODUCTIONA coal outburst is a sudden violent burst of coal from a pillar with broken coal or blocks of coal forcibly ejected into open entries. A coal outburst is a severe safety hazard as the mining crew is highly exposed at the site when the event occurs. Though deep cover and strong roof and floor are underlying geologic conditions of a potential burst incident, its real occurrence is also the result of additional mining factors. In room-and-pillar mining, a coal outburst could occur during both development and pillar retreating, but the latter greatly increases the risk of outburst. The other risk factors of coal outburst also include mining layout, multi-seam mining, presence of adjacent gob, cutting sequence, and local abnormal geologic conditions. Over the years, the cases of coal outbursts have been studied by many researchers and mining practitioners(Peng, 2008; Newman, 2008; Iannacchione and Tadolini, 2008; Gauna and Phillipson, 2008; Hoelle, 2008; and Newman, 2002). It is commonly believed that the coal outburst is the result of a sudden release of elastic strain energy stored in coal pillars and is highly associated with cutting into heavily loaded coal pillars, but its occurrence is a rare event and is difficult to predict. A number of engineering controls have been recommended to mitigate outburst potential. For room-and-pillar mining, sufficient pillar sizes have been the primary control for coal outburst prevention. The pillar design tools such as ARMPS and AMSS developed by NIOSH have played an important role in the design of stable pillars to prevent pillar collapse and squeezing as well as coal outbursts. In fact, with the implementation of pillar design using proper stability factors, pillar collapse and squeezing have been almost eliminated, and the number of coal outbursts has been greatly reduced in the US over the past decade. However, after a few outbursts occurred during pillar retreating in the US over the past a few years (MSHA, 1996; MSHA, 2014; MSHA, 2013), it has been realized that sufficient pillar size is still not enough to prevent coal outbursts. The investigations of the incidents showed that other factors such as multi-seam mining, panel layout, cutting sequence, and local geologic factors also seemed critical in causing the events. Therefore, it has become imperative that additional proactive measures beyond proper pillar design be implemented to prevent coal outbursts."

Jan 1, 2015