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By Stephen C. Tadolini

A joint research program was designed and conducted at Lodestar Energy?s Baker Mine to investigate the performance of a high-capacity tensioned cable supported gateroad. The secondary system consisted of a 4.5 m (14.5 ft) 7 strand cables that were 23 mm (.9 in) and 18 mm (.? in) in diameter with a designed capacity of 515 kN (58 tons) and 355 kN (40 tons), respectively. The cables ware installed with an effective resin column length of 1.25 m (4 ft) and then pre-tensioned using a hydraulic jacking system. Roof separations, caving characteristics, and support performance was monitored and evaluated during the critical extraction of the adjacent panel, complex geological conditions of the west Kentucky No. 13 seam were mapped and the support performance recorded. This paper will describe the theory of tensioned cable systems and present an assessment of the support performance during the longwall retreat mining process.

Jan 1, 2000

By Guangyi Sun

Wall rock classification method for coal mine support is the main factor affecting mine support. It is greatly affected by the combined effect of randomness, fuzziness and human being's incomplete knowledge. There are several coal rock classification methods. Using different rock classification methods the same engineering rock mass can be classified into different classification leading to different construction practices.

Jan 1, 2005

By Khaled Morsy

In the past two decades, high horizontal stress has been attributed to many ground control failures. Most believe that entries parallel to the high horizontal stress is most stable and least stable when perpendicular to it. Entry stability decreases when the angle between entry and horizontal stress increases. The concept of 2-D stress shadow has been proposed for selection of proper panel orientation, often with mixed results. In this paper 3-D finite element modeling was used to analyze the stress distribution at the headgate of a longwall panel subject to high horizontal stress at various angles. It was found that the directions of induced principal stresses at the longwall face and headgate are independent of the panel orientation. The induced principal stresses concentrate or relief at headgate based on the panel orientation with respect to the directions of the induced principal stresses. Accordingly, only 3-D modeling can fully account for the effects of high horizontal stress on panel orientation. The paper illustrates in details the changes in orientation and magnitude of all three principal stresses around the longwall face and T-junction area as panel orientation changes.

Jan 1, 2006

By Shuangsuo Yang

Based on field observations and physical modeling of simulated materials, the dynamic equilibrium principle on the whole interaction process between the surrounding rocks and support of large deformation roadways in coal mines has been developed. It explains the methods of forming the bolted strata and its changing load capacity with deformation. It also points out the factors and conditions for forming the bolted strata as follows: (1) many entries in coal mines are subject to large deformation. During their service life, it goes through the following process, elastic > plastic > fiacture > broken > loosening, etc. During this dynamic process, the mechanisms of supporting structure are changing and therefore the interaction between support and rocks experiences a process of dynamic equilibrium. Furthermore, the required supporting force for stabilizing the surrounding rocks shows overall decreasing trend; (2) the bolted strata is a special supporting structure consisting of bolt, rock skin protection devices and rock strata within the bolted horizon. As deformation increases, the supporting structure of the broken strata within the bolted horizon experiences a process of periodic change, from equilibrium > not equilibrium > equilibrium. Corresponding to this change, the supporting capacity of the bolted strata also fluctuates and gradually decreases; (3) under the bolted strata, the stability of a large deformation roadway is dynamic and step-wise; and (4) the formation of bolted strata, especially bolted broken strata depends on the existence of compressive force. Supporting the bolted strata is not constrained by the quality of the surrounding rocks. It has broad range of application.

Jan 1, 2005

By Adrian L. Moodie

Austar Coal Mine (formerly Southlands and Ellalong Collieries) has had a long association with difficult strata control conditions associated with mining depth and a highly jointed/cleated coal seam. The conditions include poor longwall face conditions, cyclic loading, heavy tailgate (TG) roadway conditions, and difficulties in maintaining stable roadways on development at 5.2 m (17 ft), as well as the geotechnical challenges of an 8.5-m (27.9 ft) -wide roadway required for installation faces. The introduction of Longwall Top Coal Caving (LTCC) to this environment has aided in the management of some of these issues, but LTCC has also given rise to other geotechnical considerations. Additional geotechnical issues associated with LTCC not only require management during operations, but also require consideration when evaluating new mining areas at Austar Coal Mine or potential LTCC extractable resources throughout Australia and the world. In September 2006, LTCC commenced at Austar Coal Mine in longwall panel A1. Since this point the LTCC face has been increased from 147 to 216 m (482 to 709 ft)and finally to 227 m (745 ft) in width, and it has also been rehanded and modified in the three fully extracted panels to date. The application of LTCC in panels A1, A2, A3, and now A4 has been very successful, both from a coal resource recovery point of view and in the management of the principal hazards of spontaneous combustion and strata control. This paper focuses on the geotechnical aspects of the application of LTCC at Austar Coal Mine and also reviews some advances in general strata control management at the mine.

Jan 1, 2011

By Michael Kelly

Inertial navigation technology has, for the first time, allowed the position of a longwall shearer to be mapped in three dimensions. Following the success of the technology in highwall mining and successful trials on a longwall face, the Australian Coal Association Research Program (ACARP) commissioned a three year "Landmark" project that aimed to advance longwall automation to the level of "on-face observation" by the end of 2004. This project has been completed with the following successful technical outcomes: Automatic face alignment was achieved, using an inertial navigation-based sensor on the shearer, to accurately measure face geometry and feedback signals used to move OEM (Original Equipment Manufacturer) roof supports. Operational on-line measurement of creep with creep information incorporated into face alignment corrections. Inertial Navigation System (INS)-based enhanced horizon control was bench-tested. Operational broadband communications system to shearer using wireless Ethernet on a commercial-product basis. Acquisition of located coal-face features for use in thermal infrared-based horizon control. Automated Longwall Information System (LIS) developed which integrates information from multiple systems and sensors and provides high quality visualisation and control interfaces. Automatic roadway and face convergence monitoring tools successfully tested. Condition monitoring tools developed and demonstrated for improved face reliability. This paper summarises the outcomes of the project and describes the work being undertaken for the 2 year Landmark Extension project approved in April 2005. Consistent with the conference theme, the paper focuses on the ground control aspects of the project.

Jan 1, 2005

By Shu Yan

The high resolution resistivity method (HRRM) is developed specifically for detecting underground cavities and tunnels. HRRM has a special feature in that it can recognize and cancel static shift, which makes it different &om the high density resistivity method (HDRM). In searching for various means for detecting underground cavities, we found that the detecting depth of the transient electromagnetic field method (EM) depends mainly on the measurement duration, not the loop area. In addition, TEM can also be used to locate the high-resistivity bodies. In order to use them for local working site conditions, we combine HRRM with TEM to become a combined electromagnetic method. Since the late 1980's this method has been used to locate abandoned mines and air passages and burning centers of mine fires with great success. This paper cites case studies for which this technique was successfully used.

Jan 1, 2005

By Jon Bowler

Daw Mill Colliery, UK Coal, near Coventry in England, mines the Warwickshire Thick Seam at 800 m/2500 ft depth using longwall retreat with gateroads supported by rockbolts. Over the last eight years the mine has worked longer and wider panels with extraction heights of over 5 m. Recently the mine has experienced high deformation conditions principally from the rib sides resulting in operational issues and production delays. Investigation of rib deformation was undertaken using new inspection and measurement techniques, including systematic laser surveying, and improved rib support consumables were developed and introduced. A new rib trussing system using textile webbing has proved very successful.. A rib support management system was also developed and introduced for face retreat. This allowed rib support to be more effectively managed both for production and safety. For the current developments, new 28-mm-diameter roof and rib bolt systems have been introduced and rib bolt length has been increased to 3 m, with the aim of achieving gateroad stability over the life of the panel. Significant reductions in roof and rib deformation have been confirmed by the latest monitoring results. These ground control innovations are allowing Daw Mill?s potential to be realised. Production from 302?s face in 2008 set new UK and European records. However the support enhancement process continues and further innovations are planned.

Jan 1, 2009

By Craig Compton

Falls of small pieces of rock from between roof bolts continue to cause fatalities and to injure hundreds of coal miners each year. Roof screen is the most effective way to prevent these incidents, but some mines are reluctant to use it because they believe installing screen can be awkward, expensive and time-consuming. The most common type of roof bolting machine used in mines today is the dual boom, outside-controlled roof bolter. Mines who install roof screen using these machines have developed some machine modifications and techniques to ease roof screen installation. This paper describes some successful techniques and machine modifications that are being used in productive mines to assist with roof screen installation. Since the material handling involved with roof screen installation can expose miners to an increased risk of musculoskeletal injuries, the National Institute for Occupational Safety and Health (NIOSH) ergonomists, designed machine modifications and implemented a series of tests to evaluate roof screen handling procedures for outside-controlled, dual boom roof bolting machines. Roof screen installation can also have a positive economic impact on a mine by reducing the cost of injuries, workers? compensation, required spot or re-bolting, and clean-up of long term travel and belt entries.

Jan 1, 2007

By Anil K. Ray

This paper discusses in situ studies conducted at an Illinois Basin room-and-pillar coal mine to evaluate the performances of three different types of bolts under similar geologic conditions. The bolt types considered were the fully grouted passive rebar (FGPR), the resin-assisted mechanical anchor (RMAB), and the fully grouted tension rebar (FGTR). During the evaluations, the three types of bolts were installed in a single entry covering a span of three contiguous pillars with almost identical geologic conditions. The performances of the three types of bolts were investigated by instrumentting the bolts prior to installation as primary supports on cycle. Additional data on roof behavior was collected with extensometers and shear meters placed near the instrumented bolts. Excluding some time lapse right after installation, the performance evaluation of the three bolt systems was based on the data collected for over a year. Based on this study, it was found that the axial loads measured in the different bolt types did not show any identifiable patterns under the prevailing geological conditions at the mine. Therefore, the assumption that active bolts must be superior to passive ones is not always valid. Even though the studied mine did not have any thinly laminated, weak rocks in the immediate roof, the passive bolts performed as well as the active systems.

Jan 1, 2012

By Dan Payne

The Crinum mine has recently completed workings in the southern area of the mine. Over the 10 years of longwall production 40 million tonnes have been extracted and the mine has experienced over 40 longwall face, 3 main gateend, and 2 tail gateend roof falls as well as 5 roadway roof falls away from the longwall. Weak roof (less than 10MPa (1450psi) in the bolted horizon) has been the principal roof control issue at the mine. However, weak, highly cleated coal, water inflow from overlying aquifers, some minor structure and a diatreme in the main headings have also contributed to the challenges at the mine. This paper describes the geotechnical experience over the 10 years, the mine?s approach to addressing the issues and the relative success of these approaches.

Jan 1, 2008

By D. Minney

The quality of resin used in South African collieries is currently controlled according to South African Bureau of Standards, "SABS-1534:2002". It has however been found that resin which passed the SABS tests failed in underground short encapsulation pull tests. An investigation was therefore conducted to determine the characteristics of resins that are currently being used in collieries. A series of in situ short encapsulated pull tests was conducted to determine the effect of expired resin on support performance. The results showed that the longer the time after the expiry date the poorer the resin performance. A series of resin-torque tests has been performed using Anglo Coal's testing facility. The following parameters were investigated on different resins: Spinning speed, Mixing time, and Setting time. An attempt was also made to develop a new quality control procedure for resin. This procedure enables mining personnel to make sure that the quality of resin can provide the required support performance. The effect of spinning speed on resin performance was also investigated. The results showed the resin gelling times of resin decreases as the spinning speed increases. It is found that over-spinning with slow speed roofbolters may result in decreased bond strengths. It was found that a 30-second resin (10-second spin, 30-second hold) should never be spun more than 20 seconds, even with very slow roofbolters (100 rpm).

Jan 1, 2005

By Lixun Kang

Bedded structure is the characteristics of coal measures strata. Bedding planes, coal streaks and soft rock bands existing in roof strata destroyed its continuity in the direction perpendicular to the coal seams. The statistics from a large number of long wall faces in which serious roof fall accidents occurred in Datong coal mining area suggest that beddings planes, coal streaks, soft rock bands, especially the first layer thickness of immediate roof have a great influence on its stability. As the first layer thickness increases, the probability of serious roof fall accidents decreases. Based on the investigations finite element method is used to analyze stress and displacement in the first layer of immediate roof. The results indicate that the thicker the first layer is, the less the roof convergence in the unsupported area is. Stress and strain features in stratified immediate roof are quite different f7om those in non- bedded immediate roof. In stratified immediate roof, every stratum has each own tensile zone in the upper portion and compressive zone in the lower portion, while tensile zone in the first layer is less, the tensile stress is the greatest.

Jan 1, 2005

By Yi Luo

The efforts to mitigate longwall subsidence influences on various residential and farming structures by many coal mining companies have been successful in the recent years (Luo, et al, 2003). The successes were largely dependent on the accurate subsidence predictions, correct assessments of potential influences, appropriate design and implementation of the mitigation measures. However, such efforts for protecting large industrial structures are still very rare. In this paper, the successful assessment and mitigation of the subsidence influences associated with a very intensive dynamic longwall subsidence process on a large workshop are presented.

Jan 1, 2005

By John Ellenberger

The nature of a stone deposit is rarely defined through exploration and engineering design, but more through the actual experience encountered as the mine develops; hence, issues with ground control are often at the forefront for a new mine. Ground control problems account for over 15 percent of lost-time injuries in underground stone mines. The objective of this work is to present actions taken during early development of a stone mine to mitigate the obvious effects of high horizontal stress on ground control. The mine was developed from a box cut with relatively little cover, and high horizontal stresses were not anticipated. Early recognition of possible high horizontal stress resulted in mapping of the mine using the NIOSH-developed Roof Fall Risk Index (RFRI) to provide a baseline to objectively analyze roof conditions. A program of borescoping the mine roof provided valuable roof beam thickness data and found that the roof beam was of minimal thickness, approximately 10 percent of drift width. As the mine developed, it became apparent that ground control problems related to high horizontal stress existed. A concentrated effort was made to improve ground conditions by implemeting a number of mitigation techniques. Actions taken by the mine included systematic roof support, reorientation of mining direction, a lower roof horizon, reduced span widths and reduced mining height in crosscuts to prevent running falls. These strategies resulted in signifcantly improved ground stability.

Jan 1, 2012

By Frank E. Chase

The Harris No. 1 Mine, located in Boone County, WV, has been longwalling the Eagle Coalbed for over 30 years. Harris has experienced numerous interactions associated with the extensive room-and-pillar and longwall mining operations which have been conducted in the overlying No. 2 Gas Coalbed. The problems have included roof falls, excessive rib sloughage, and gateroad and bleeder entry closure. A detailed evaluation of the multiple seam experiences at Harris No. 1 Mine was conducted as part of the National Institute for Occupational Safety and Health (NIOSH) nation-wide multiple seam mining case history data base. One observation from the Harris gateroad case histories was that smaller, critically loaded, upper seam pillars seemed to cause more severe ground conditions than did wider pillars. The LaModel program was used to investigate this supposition, and the results confirmed that "critical" sized pillars do transmit the highest amounts of stress to adjacent seams. In addition, the data suggest that the probability of a major multiple seam mining interaction increases when the depth of cover is 1,000 ft or greater and when the Eagle seam pillars have a Analysis of Longwall Pillar stability factor less than 1.50.

Jan 1, 2005

By Takashi Sasaoka

The PT. Adaro Coal Mine targeted in this research is located in South Kalimantan, Indonesia. The coal production of this mine is increasing year, with about 35 M tons of coal produced in 2006. This mine faces many ground control problems, especially instability of the footwall. Large-scale landslides occur often in the footwall; this may be because of weak rock and the effect of footwall rock weathering. Hence, prompt measures must be developed and followed in order to preserve the stability of the footwall and make this mine safe. Additionally, the final pit design is now being reconsidered because of the increasing demand for and high price of coal. With these issues in mind, this paper describes a solution for improving the stability of the footwall and discusses the final pit design based on the results of an on-site investigation and numerical analysis.

Jan 1, 2008

By Thomas M. Barczak

The era of the shield-supported longwall face began in the United States a little over 25 years ago. The most fundamental development of the shield in the past 25 years has been an increase in size and capacity and a progression toward two-legged designs in favor over four-legged shields. Is the ?bigger-the-better? design philosophy justified? The advantage of the two-legged design is largely attributed to its active horizontal force caused by the forward orientation of the leg cylinders. How do we know this? You might be surprised to learn that shield loading data may suggest otherwise. In addition, what about setting pressures? Should they be set as high as possible? Some people have advocated that setting pressures should equal the yield pressure. Does this make any sense? Is yielding a good or bad thing? These are some questions still worthy of investigation that are addressed in this paper. The first major change in gateroad support was the use of cable bolts to replace conventional wood cribbing particularly in Western mines. Standing support changed very little until the mid-1990s, when Strata Products1 introduced the Hercules and Propsetter supports in Eastern mines and Burrell Mining Products introduced the Can support in Western mines. Since then, there has been a dramatic increase in new roof support technologies for longwall gateroad support. Historically, standing support, including shield supports, have been designed based on a simplistic model requiring the support to have sufficient capacity to support the gravity loading of a perceived detached rock mass. This approach ignores the stiffness of the support and the resulting ground deformation that is known to be critical to roof stability. Recently, a new approach based on the ground reaction concept has been promoted by the National Institute for Occupational Safety and Health (NIOSH) to account for the interaction of the support with the rock mass behavior in achieving ground stability. In addition to gateroad support design, this concept has ramifications for shield capacities, setting pressures, and recovery room support design that challenge current practices in these areas. Although longwall mining has matured into the safest and most productive underground mining method used today, traditional concepts of roof support design and practice should still be evaluated and challenged. Ultimately, advancements in the science of ground control are made by recognizing deficiencies in current theories and thinking outside the box.

Jan 1, 2006

By Alan A. Campoli

The mining industry has long been searching for an effective structural application that prevents weathering and falls of loose rock, debris, and sediment. J-Crete is a fiber-reinforced, engineered, cementitious composite rock application that is designed, tested, and successfully utilized as such a material. Laboratory evaluation of J-Crete material properties has shown the final compressive strength to be in excess of 7,300 psi (50.34 MPa), with 24 hour strength in excess of 1,500 psi (10.34 MPa). The flexural tensile strength is in excess of 600 psi (4.1 MPa) and is provided through a unique combination of polyvinyl alcohol (PVA) fibers activated by a proprietary liquid additive that prevents them from clumping and enables even distribution throughout the grout. Tensile bond strength in excess of 350 psi (2.4 MPa) at 28 day cure tightens the strata and holds loose material in place. Practically impervious to gas and liquids, J-Crete protects strata against long-term weathering, does not require screening to be effective, and has proven to be a successful waterproofing material. J-Crete has demonstrated the potential to alleviate coal mine roof cutters in soft, wet clay formations. J-Crete reinforces coal mine ribs by bonding directly to the rib, conforms to the strata, strengthens the rib, and eliminates sloughing due to long-term weathering; stabilizes coal mine entries by bonding directly to the brow and roof, and creates a monolithic shell with double reinforced corners.

Jan 1, 2012

By Ted Klemetti

Roof deterioration in weak moisture-sensitive rock leads to roof falls in coal mines. An observation site was set up in intake air at an underground coal mine in central Ohio to evaluate the time-dependent deterioration of moisture-sensitive roof rock. By periodically collecting and weighing the rock falling from the roof, the National Institute for Occupational Safety and Health (NIOSH) was able to quantify the deterioration of the moisture-sensitive shale roof. The measurements showed that more roof rock is falling in the dry winter months. Movements in an intersection were monitored in order to determine if deformation correlated with seasonal changes in temperature and humidity. The rate of intersection roof-to-floor convergence was found to be cyclical and related to seasonal changes, with more convergence during the months of April through September. At the study site intersection, the roof beam did not show deflection indicating movement most likely due to mine floor deformation. The bolt load also increased in the warmer months before shedding load, possibly due to rock failure. Temperature and relative humidity sensors, placed in holes in the roof show that mine air does not mix with the air in the holes. Consistent saturation levels of relative humidity were measured in the holes and it appears that high levels of moisture may have migrated into the bolt holes from roof shales. A roof sealant applied to a portion of the test roof appears to be preserving the roof and reducing the amount of rock fall into the entry, while the screened area virtually eliminated rock falling from the roof.

Jan 1, 2009