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By Peng-Fei He

CT scanning tests were used to detect the pre-existing fracture systems of 36 cubical coal blocks. Systematic procedures were developed to construct the pre-existing fracture geometry of the cubical coal blocks from the CT images. Based on the constructed fracture systems, the fracture tensor components were calculated to capture the directional effects of the fracture system of the cubical coal blocks. The effect of the pre-existing fracture system and different combinations of confining stresses on the strength and deformability of coal masses was investigated at a preliminary level using some true triaxial test results and the computed fracture tensor components. The strength of the cubical coal blocks in the loading direction was found to be closely related to the summation of the fracture tensor components in the two perpendicular directions to the loading direction and the level of the confining stress system. The deformability of cubic coal blocks in the loading direction was found to be correlated to the fracture tensor component in the same direction.

Jan 1, 2014

By Morgan M. Sears, lhsan B. Tulu, Ted M. Klemetti

"Why do some room and pillar retreat panels encounter abnormal conditions? What factors deserve the most consideration during the planning and execution phases of mining and what can be done to mitigate those abnormal conditions when they are encountered? To help answer these questions, and to determine some of the relevant factors influencing the conditions of room and pillar (R&P) retreat mining entries, four consecutive R&P retreat panels were evaluated. This evaluation was intended to reinforce the influence of topographic changes, depth of cover, multiple-seam interactions, geological conditions, and mining geometry.This paper details observations made in four consecutive R&P retreat panels and the data collected from an instrumentation site during retreat mining. The primary focus was on the differences observed among the four panels and within the panels themselves. The instrumentation study was initially planned to evaluate the interactions between primary and secondary support, but produced rather interesting results relating to the loading encountered under the current mining conditions. In addition to the observation and instrumentation, numerical modeling was performed to evaluate the stress conditions. Both the LaModel 3.0 and Rocscience Phase2 programs were used to evaluate these four panels. The results of both models indicated a drastic reduction in the vertical stresses experienced in these panels due to the full extraction mining in overlying seams when compared to the full overburden load. Both models showed a higher level of stress associated with the outside entries of the panels. These results agree quite well with the observations and instrumentation studies performed at the mine.These efforts provided two overarching conclusions concerning R&P retreat mine planning and execution. The first was that there are four areas that should not be overlooked during R&P retreat mining: topographic relief, multiple-seam stress relief, stress concentrations near the gob edge, and geologic changes in the immediate roof. The second is that in order to successfully retreat an R&P panel, a three-phased approach to the design and analysis of the panel should be conducted: the planning phase, evaluation phase, and monitoring phase."

Jan 1, 2016

By Thierry Lavoie, Mark K. Larson

"Engineers at the National Institute for Occupational Safety and Health (NIOSH) and Itasca Consulting Group, Inc., conducted a study to determine the procedure for calibrating a caving model implemented in FLAC3D. The model is calibrated to measured surface subsidence. This sedimentary caving model was developed by Itasca for NIOSH for the purpose of more closely simulating mechanical processes during caving. When a model is considered calibrated to surface subsidence, the load between the extracted panel's roof and floor is a reasonable estimate of gob loading. Such information can also provide an estimate of the amount of overpanel weight that is transferred to the abutment. This load distribution is a parameter needed, for example, to estimate loading of pillars in longwall gate roads. For the current case, a calibrated model resulted in a calculated abutment angle of 30.6°considerably higher than the assumed value used in empirical methods, but reasonable considering the presence of massive stratigraphic members in the overburden. Because of additional information available after this study that the extracted seam height was actually greater in the first panel, a corrected range of abutment angle was estimated to be from 27.7° to 30.6°, still significantly higher than the empirical assumption. The expectation is that better estimates of abutment loading profiles will result in improved evaluations of mine layout design and, therefore, reduce the number of fatalities and injuries resulting from falls of ground.INTRODUCTIONThe safety of miners in underground coal mines depends on careful consideration of the mechanisms involved in failure and transfer of load. The successful design of pillars and panels in longwall and room-and-pillar coal mining requires thorough attention to any information and measurements available that help understand these mechanisms. Although empirical methods, such as Analysis of Pillar Stability (ALPS) Mark (1987); (Mark, 1992) or Analysis of Retreat Mining Pillar Stability (ARMPS) (Mark and Chase, 1997), can provide a sense of success or failure, complex geology and failure in the roof or floor material or complex material behavior of the coal can make evaluation by an appropriate numerical modeling tool necessary. The numerical tool must simulate important mechanisms and appropriate detail that underlie the mechanism. For example, Starfield and Cundall (1988) advised using ""the simplest model that will allow the important mechanisms to occur, and could serve as a laboratory for the experiments you have in mind."" To that end, data gathering about the geology, geometry, and material behavior is very valuable. Such data facilitate selection of an appropriate model and model inputs. Moreover, this data facilitate improved assessment of risk to miners and how to minimize that risk."

Jan 1, 2016

By Naj Aziz, Peter Craig

"Double shearing tests were carried out on the 28 mm (1 in) hollow strand Jennmar TG cable bolt to gain a better understanding of the shearing behavior of the cable. The first test was limited to a 50 mm (2 in) travel on the testing machine and produced a shear load of 900 kN (92 t) at the maximum 50 mm (2 in) displacement. The axial load generated on the cable bolt was 238 kN (24.3 t). In the second test, the machine travel was increased to 75 mm (3 in), and first strand failure of the cable, due to shear loading, occurred at 1,354 kN (138 t) vertical load at a vertical displacement of 59 mm (2.3 in). The cable axial load was in the order of 385 kN (39.3 t). Analysis of the failure mode and loads achieved showed that the cable strands bent and the concrete crushed along the shear plane; the shear loading across the concrete and grouted cable then reached the tensile strength of the steel wires.INTRODUCTIONCable bolts were introduced to the mining industry around 1970, initially to surface mining and underground metalliferous mining and then to coal mining in the early 1980s, mainly for roadway reinforcement as a secondary means of support. Cable bolts have since been used as both primary and secondary supports. As primary support, Fuller et al. (1994) described the application of cable bolts, known as FLEXIBOLT, for strata reinforcement in both Angus Place and Ellalong Collieries, in New South Wales (NSW), Australia. As secondary support, cable bolts have also been used as cable trusses, which act to support the immediate roof in a sling-like manner (Fabjanczyk and Tarrant, 1988; Fuller, et al., 1991; O'Grady and Fuller, 1992), and for reinforcement at higher stratification and beyond the rebar bolt length, mainly for anchoring lower strata layers immediately above the coal seam to the higher, competent bedding formation above. Initially cable bolt anchorage was by cementatious grouting and, since the 1990s, by chemical resin. The dominant type of cable bolts used for secondary support in Australian underground coal mines are 588 kN (60 t) capacity cables which are point anchored, pre-tensioned and post-grouted. Point anchored refers to a cable resin anchor. The cables are tensioned on the resin anchor; then the same cable is grouted by cementatious grout pumped to ·fin the annular gap around the cable from the collar of the hole to the resin anchor. Bulbs are located within the resin anchor section for mixing of the resin chemical components and efficient anchorage. The installation of these typically 8 m long cable bolts involves a lot of manual handling of the cables, lifting of heavy hydraulic tensioners, and exposure to resin grouts. Table 1 shows the specification of various cable bolts currently marketed and installed in Australian underground coal mines."

Jan 1, 2010

By Esterhuizen S. Gabriel, Su W. H. Daniel, Tulu B. Ihsan, Klemetti M. Ted, Mohammed M. Khaled, Gearhart F. Dave

"A numerical-model-based approach was recently developed for estimating the changes in both the horizontal and vertical loading conditions induced by an approaching longwall face. In this approach, a systematic procedure is used to estimate the model inputs. Shearing along the bedding planes is modeled with ubiquitous joint elements and interface elements. Coal is modeled with a newly developed coal mass model. The response of the gob is calibrated with back analysis of subsidence data and the results of previously published laboratory tests on rock fragments. The model results have been verified with the subsidence and stress data recently collected from a longwall mine in the eastern United States. INTRODUCTION In 2015, there were 40 longwall mines operating in the United States, each producing an average of 4.5 million tons of coal per year, and they supplied 60% of the U.S. underground coal production. This represents a substantial increase from 50% three years earlier (Sears, Tulu, and Esterhuizen, 2017). During this period, reportable roof fall rates in U.S. longwall mines also increased. Large roof falls that can block the gateroads are not only a ground-fall hazard, they can disrupt the ventilation system, block the escapeways, and increase the potential for elevated methane levels in the gob. To address these hazards, the National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Mining Research Division (PMRD) is conducting research to improve the design of ground control systems in longwall gateroads. Gateroad stability is primarily determined by the longwall pillar design. Generally, the required dimensions of the pillars around a longwall panel are determined first, which then dictates the location of the gate entries relative to the mined panel. The Analysis of Longwall Pillar Stability (ALPS) method is the most accepted gateroad pillar design procedure in the United States (Mark, 1992). The ALPS method accounts for local roof geology in the gateroad stability assessment by including the Coal Mine Roof Rating as an input parameter (Mark, 1992). The key assumption in the ALPS method is that unstable pillars result in unstable gate entries. However, experience provides examples of mines where pillar stability and gateroad stability are only loosely correlated (Su, Draskovich, and Thomas, 2003)."

Jan 1, 2017

By Tom Barczak, Julian Restrepo, Zach Agioutantis

"The Alpha Foundation for the Improvement of Mine Safety and Health, Inc. (Foundation) was established as part of a Non-Prosecution Agreement (Agreement) entered in December 2011 by the United States Attorney's Office for the Southern District of West Virginia, the United States Department of Justice and Alpha Natural Resources, Inc. (""Alpha"") and Alpha Appalachia Holdings, Inc. This Agreement was related to the explosion at Upper Big Branch Mine, an underground coal mine owned by Performance Coal Company, a former affiliate of Massey Energy Company, which Alpha acquired in June 2011. Pursuant to that agreement, Alpha agreed to establish a trust to fund projects designed to improve mine health and safety by providing $48,000,000 into the trust. The mission and vision of the Foundation are as follows:Mission: To improve mine health and safety through funding research and development projects by qualified academic institutions and other not-for-profit organizations.Vision: To enable miners in the future to be free of work-related injury or disease by the implementation of the results of the projects funded by the Foundation and undertaken by the best researchers from any discipline that can contribute.The focus of the research effort is not limited to coal mine explosion prevention, and in fact encompasses the entire spectrum of health and safety, addressing issues in both underground and surface mining operations in all mining sectors. Four major focus areas are identified (see Figure 1): 1) Health and Safety Interventions; 2) Mine Escape, Rescue, and Training; 3) Safety and Health Management and Training; and 4) Injury and Disease Exposure and Risk Factors.The Foundation is seeking to expand the knowledge base of mining safety and health information that can lead to resolving complex mining problems. To that end, the projects funded by this effort are designed to provide a spectrum of knowledge potentially encompassing the following:• Surveillance data - Collect and analyze surveillance data that better documents the prevalence and trends of occupational safety and health across the mining industry."

Jan 1, 2016

By Gabriel Esterhuizen

The roof rock in underground limestone mines in Northern Appalachia can be subject to high horizontal stresses in spite of the shallow depth of the workings. The high stresses can cause roof stability problems. The National Institute for Occupational Safety and Health staff have observed a distinctive asymmetrical mode of failure at the pillar-roof contact in two underground stone mines, which is different from the typical failure mode observed in response to excessive levels of horizontal stress. A dip of greater than 5° was identified as a possible cause of this mode of failure. Numerical model experiments showed that an increase in the dip of the workings can cause an increase in the stress at the up-dip comer of the roof beam. A case study is presented in which failure at the pillar-roof contact was observed where the dip of the workings was 7° in a high horizontal stress field. Numerical modeling showed that the relatively small dip of the workings could have induced stresses changes in the immediate roof that would explain the failures. The model results also showed that this mode of failure is less likely to occur if the limestone pillars contain weak bedding planes that provide stress relief. In addition, the results showed that for the conditions at the case study site, the stress in the roof beam was not sensitive to the thickness of the roof beam or to the excavation span. The high horizontal stresses at this site are an important contributing factor to the observed failures.

Jan 1, 2004

By Takashi Sasaoka, Hiroshi Takamoto, Akihiro Hamanaka, Pongpanya Phnthoudeth, Hideki Shimada, Jiro Oya

"Indonesia is the second largest coal exporter to Japan, accounting for about 30 Mt annually. They produced over 400 Mt in 2015, over 99% of which is from open-cut mines. However, the conditions of surface mining are becoming worse year by year: the stripping ratio is increasing, the economy is worsening, and the infrastructure for coal transportation from inland mining areas is not being maintained. To meet the demand for coal in Indonesia and the rest of the world, underground mines have to be developed. In consideration of growing need, our research program is conducting a study on the potential for an underground coal mine in Indonesia. This research is being done through the roadway heading research program, which is looking into developing an underground coal mine from an open pit highwall. The greatest challenge for underground coal mine development in Indonesia is the maintaining the safety of the mine roadway during heading because the roof rock is exceptionally weaker. There is a potential for heading below mobile, unconsolidated soft sediment, and instead mining a deep area. This increases the likelihood for problems in the stability of the support system due to increasing ground pressure. The ground behavior around the roadway was measured using telltales, extensometers, and convergence measurements. Moreover, rock property tests were also conducted on exploration drilling cores. Based on the geological dates and monitoring data, a series of numerical analyses were conducted in order to assess support needs in the main roadway under different depths. Results of numerical analyses indicate that the current arch support design is effective when the mining depth is lower than 100m. However, if the mining depth is larger than 100m, the current arch support design should be changed to include the application of higher yield strength material, shorter spacing, etc. Moreover, bolting may also be useful if ground pressure is high locally. Based on a series of measurement data of ground and support behavior, a control criterion for main roadways under weak geological conditions is proposed in this paper."

Jan 1, 2017

By Eric Zahl

Researchers at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, Spokane, WA, have collaborated with three Western longwall coal mines in an ongoing effort to develop technologies that will aid in providing safe and stable working areas. The goal of the research described here is to develop a stress monitoring system that will provide immediate information to mine managers for making daily safety decisions as areas of poor ground are mined through. Initial work has focused on answering preliminary questions regarding the reliability and use of stress change patterns. Research is concentrated on monitoring horizontal stress because horizontal stress is transmitted over long distances through stiff strata, thus allowing an extended length of entry to be monitored. This paper presents an explanation of the concept, key results from field tests at two mine sites, and a proposed process for imple¬menting a monitoring system. System design layouts, instrument use, data collection and interpretation methods, and processes to present findings to mine staff are described. Additional validation and correlation with actual failure mechanisms are required before this approach can be recommended at ongoing operations. How¬ever, initial results indicate that this approach shows promise in mines prone to bumps and roof falls associated with large stress changes. In particular, analyses of measured changes in horizontal stresses appear useful.

Jan 1, 2002

By Ihsan Berk Tulu, Keith A. Heasley, Chris Mark

"The Analysis of Retreat l\.fining Pillar Stability (ARJ\1PS) and the Lal\.fodel programs have been used successfully in the U.S. for designing safe pillar recovery operations for many years. However, the recent Crandall Canyon l\.fine collapse showed that further research is required to improve the pillar design for recovery under deep cover. To this end, the National Institute for Occupational Safety and Health (NIOSH) and the West Virginia University (WVU) l\.fining Engineering Department have been working together to improve both the ARJ\1PS and the Lal\.fodel programs.This paper compares and analyzes the overburden loads calculated by ARJ\1PS 2002, the new ARJ\1PS 2010, and the Lal\.fodel program with regard to the retreat pillar line, the gob, and the barrier pillars. The analysis shows a number of distinct differences between the ARJ\1PS and Lal\.fodel loading distributions. Ultimately, the programs, with their distinctive load distributions, are used to analyze a database of 52 deep cover pillar retreat case studies, and the ability of each program to discriminate between successful and unsuccessful retreat pillar plans is evaluated.INTRODUCTIONThe Analysis of Retreat l\.fining Pillar Stability (ARJ\.f PS) and the Lal\.fodel programs have been used successfully in the U.S. for designing safe pillar recovery operations for many years. However, the recent Crandall Canyon l\.fine collapse showed that further research is required to improve the pillar design for recovery under deep cover.Pillar recovery accounts for less than 10% of the coal produced from underground coal mines; however, in the period between 1989 and 1996, it accounted for more than 25% of all ground fatalities (l\.fark et al., 2003). In the past 15 years, the safety of retreat mining operations has greatly improved. l\.fark (2009) explains three steps promoted by l\1SHA and NIOSH for safer pillar recovery:1) Global stability through proper pillar design2) Local stability through proper roof support3) Worker safety through proper section managementThe ARJ\1PS and Lal\.fodel (Heasley, 1998) programs have been playing an important role in guiding mine operators in designing pillars to ensure the global stability of a retreat panel."

Jan 1, 2010

By Thomas L. Vandergrift

The type and severity of coal mine entry failures are affected by the strength and stiffness properties of the roof, floor, end coal. Knowledge of the mine-wide trends of these properties Is valuable in developing effective ground control plans. Within the context of a larger effort to Improve entry stability In high horizontal stress fields, the U.S. Department of the Interior, Bureau of Mines, has evaluated three portable, on-site test devices to determine their suitability for identifying mine-wide physical property trends. The devices evaluated were the borehole shear tester, the Schmidt harmer, and the point load tester. Despite the lack of standardized data Interpretation procedures, each device provided useful on-site physical property data. The Schmidt homer was preferred for trend identification because of Its speed and simplicity. Schmidt hammer data were collected over a large area of an underground coal mine to compare calculated roof strength trends with a known geologic feature.

Jan 1, 1990

By Lou Gaozhong, Guo Wenbing, Zhao Gaobo, Wang Shuren

"The height of a fractured zone due to high-intensity mining (HIM) plays a vital role in the safety analysis of coal mining under bodies of water. This paper described the characteristics of HIM. The processes of overburden failure transfer (OFT) are analyzed. The state of destruction transfers from a certain stratum to the upper stratum as face advance distance increases, and the processes of OFT are divided into two stages: transmission development stage and transmission termination stage. Through theoretical analysis, the limited suspension-distance and limited overhanging distance of each stratum are proposed to judge the damage of each layer. Based on this, the OFT criteria of fractured zone development are obtained. The mechanical model of strata suspended integrity and overhanging stability are established. Finally, a new method to predict the height of fractured zone due to HIM is put forward based on the processes of OFT, the limited suspension-distance, and the limited overhanging distance. Further, a HIM panel in the Datong coal mine is taken as an example. Theoretical methods are proposed, and engineering analogy is used to predict the height of the fractured zone, which is compared with in-situ measurements reported in the literature. The results show that the in-situ measurements for the height of the fractured zone are in close agreement with the theoretical calculation result. Therefore, the rationality and reliability of the theoretical method proposed to predict the height of fractured zone is verified.INTRODUCTIONThe height of the fractured zone (HFZ) due to the exploitation of coal is a key parameter for the safety analysis of coal mining under a body of water (Wenbing, Youfeng, and Hou, 2012). Accordingly, it is necessary to predict HFZ under the corresponding mining conditions before coal mining."

Jan 1, 2018

By Ed Van Eeckhout

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

Jan 1, 1984

By Takashi Sasaoka, Hiroshi Takamoto, Kikuo Matsui, Hideki Shimada

"Indonesia is the second largest coal exporter to Japan. Almost all the coal is produced from open cut mines. However, a great deal of coal is left in the highwalls of the mined-out pits. The use of highwall mining systems has increasingly come into play in the U.S. and Australia, when overburden depth exceeds economical recovery in an open cut mine. However, the stability of the highwalls is always a major safety concern in such mining. This paper describes the characteristics of the highwall mining system and discusses the appropriate highwall mining system for use in Indonesia.INTRODUCTIONIndonesia produces over 200 Mt of clean coal and is the second largest coal exporter to Japan, accounting for about 32 Mt of coal annually. Over 99% of the coal produced in Indonesia comes from surface mines. More open cut mines will be developed and more coal will be mined in order to fill the great demand both in Indonesia and the rest of the world.In open cut mines in Indonesia, there are many sites where mining operations have developed long highwalls that have been abandoned due to the current economics. Mining operations have been transferred to lower stripping ratio blocks of coal or overlying seams. In some cases, coal lies buried beneath spoil heaps or is covered with mud and water. It is estimated that there is a great deal of coal beneath abandoned and working highwalls. Considering these situations, it seems to be worth introducing highwall mining systems into Indonesian open cut mines.Final highwalls of open cut mines can form the starting point for other mining methods, such as highwall or underground mining. In its basic application, highwall mining is a technique utilized after an open cut portion of a reserve has been mined, sometimes prior to the introduction of underground mining (Seib, 1993). In this system, the coal seam is mined by remotely operated equipment, such as an auger machine or a modified continuous miner incorporated into a highwall mining system. The major problems with highwall mining systems are less coal recovery due to leaving coal as the pillars and the instability of openings and highwalls due to the pillar and roof failures."

Jan 1, 2010

By Wojciech Masny, Andrzej Walentek

"The most common extraction method in Polish coal mines utilize pillars between adjacent longwall panels. This is mainly due to the significant depth and difficulties in maintaining stability of the gateroads. This paper presents measurements of entry convergence and range of the fracture zone around a gateroad measured with a borehole camera. The entry where the measurements were taken was located in a parallel entry separated from the gateroad by a 5-m- (16-ft-) wide pillar. The gateroad was located in a hard coal seam 900 m (2,952 ft) deep with a dip of 5°. The extraction method was longwall mining with caving and the mining height was about 3 m (10 ft).Furthermore, the ""in situ"" measurements were compared to the convergence forecast for the gateroad and fracture zone by means of numerical modeling conducted using the Phase2 Version X.X finite element program from Rocscience. The determination of the fractured zone around the openings is of great importance to successful mining practice, especially in support planning and design for longwall gateroads. Moreover, proper determination of the fractured rock zone is one of the basic elements in the process of planning the roof bolting for mine workings.INTRODUCTIONIn the Polish. hard coal mining sector, retreat longwall extraction is most commonly used (Prusek and Masny, 2007). This method is connected with the necessity to carry out a number of development operations during the first stage, including among others, driving of gateroads and a setup room.Currently, most Polish mines eliminate gateroads behind the extraction front, using the ""U"" ventilation system (Szlitzak et al., 2008). In such cases, one of the roads for the next longwall panel is driven along the caving goafs, leaving a protective pillar of some chosen width (Figure 1). The question regarding the width of the coal pillar is especially important to the stability of the gateroads that define the panel for the subsequent longwall. When analyzing the previous experience relating to this type of pillar (Junker, 2006; Rajwa, 2008; Stranzet et al., 1973; Betriebsempfehlung, 1982), one can state that there exist two solutions for the determination of its width."

Jan 1, 2010

By G. R. Corbett

Between 1977 and 1984, No. 26 Colliery, at Glace Bay in the Sydney Coalfield, suffered from a series of 37 rock/gas outbursts. These events were characterized by the complete pulverization of a sandstone which was propelled into the mine roadway and an associated release of methane gas. The events began to appear at a depth of 700 m and increased in intensity and damage to the excavation as the mining depth increased. At the time of the closure of No. 26 Colliery (as a result of an unrelated fire), these outbursts were the subject of intensive investigation. Although No. 26 is now closed, there is concern that similar events could appear at Phalen Colliery where current development work is proceeding at a depth close to the 700 m threshold. Past investigations into rock/gas outbursts included studies of in situ stresses, the simulation of outburst events in the laboratory, investigations into the potential for destressing an& degassing of problem sandstone and the development of numerical models to assist in the design of openings in outburst prone ground. However, despite this work, methods of predicting outburst conditions are presently limited to examination of probe hole cores for core discing, and methods of working under such conditions are limited to boring and firing and using volley firing techniques to initiate any outbursts while workers are safely out of the heading. During this investigation of worldwide occurrences of this phenomena, literature obtained suggested that outbursts might generate microseismic precursors. An intrinsically safe (IS) microseismic monitoring system was designed and developed based on CANMET's rockburst monitoring system currently in use in hardrock mines, and installed in a development roadway at the deepest extent of the Phalen Colliery. The system will record microseismic activity associated with development work to try to identify characteristic precursors of outburst-prone conditions. This paper describes the steps taken to identify a microseismic system suitable for use at Phalen colliery, of which part of the requirement was that it be IS and approved for use in the Sydney Coalfield. The modifications to the selected system are described, along with its capabilities and the preliminary results of its performance over the first few months of use.

Jan 1, 1994

By Bruce Hebblewhite, Jim Galvin

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

Jan 1, 2016

By David F. Gearhart

A cooperative study was conducted between Signal Peak Energy and the National Institute for Occupational Safety and Health (NIOSH) to evaluate the behavior of a pre-driven recovery room under less than 200 ft of cover. The design of the support systems for a pre-driven recovery room must provide control of the roof and ribs during the changing stress environment experienced throughout the final advance of the longwall into the recovery room. It must also provide a stable area where mine personnel are not exposed to undue hazards during shield recovery operations. This recovery room was constructed in two phases; each phase mined a 21-ft-wide room that was heavily supported, and subsequently fully backfilled with 800-psi, cellular concrete. Instrumentation to monitor the recovery room performance included borehole pressure cells (BPCs) placed in the head gate and tailgate pillars, at four locations across the longwall panel, in the fill material, and in the first row of outby pillars near the center of the panel. Four roof-to-floor convergence sensors were installed in the back fill material and a vertical, multipoint borehole extensometer (MPBX) measured the displacement of the strata above the recovery room. Surface surveys were conducted to measure MPBX collar movement. The data show that the front abutment load was transferred onto the recovery room backfill and the out by pillars as the longwall panel was extracted. As the longwall approached the recovery area, the fender and shields yielded, but the backfill provided enough support to keep the room stable as the longwall advanced to its final position. The system worked as planned and the face was recovered in record time.

Jan 1, 2014

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 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