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By Xinzhi Du

Coal strength is essential to analyze the stability of excavation in coal. It is well known that the strength of coal tested in the laboratory is size-dependent, and different size specimens will produce different strength. Many researchers have studied the uniaxial compressive strength of coal and developed a number of empirical formulae to describe the strength variation with specimen size. According to a comprehensive data base that includes most of the laboratory testing results in the last century in U.S. and South Africa, the size effect and shape effect were reviewed and analyzed. The results show that different groups of coal specimens have different size effect on the compressive strength. The critical size for the size effect from the data analysis is about 40 in. The shape (W/H) of the specimens has a large effect on the compressive strength. The pillar strength formulae obtained from the data regression have a larger strength than all other existing formulae. Most pillar strength formulae have their own place in coal pillar design field. In order to analyze the difference of those formulae, twelve design formulae have been selected to compare using the Pittsburgh coal seam. The intention is not to determine the preferred formulae, but only to compare them, and see how do the W/H ratio, width, length, angle, cross-section area and effective width affect different pillar strength formulae?

Jan 1, 2008

By John R. Koehler

Available barrier pillar design methods do not adequately account for overburden caving characteristics, the timing and magnitude of associated load transfers, the occurrence of unusual geologic features, or the effects of multiple-panel/multiple-seam mining. To address this problem, the Bureau of Mines, in cooperation with Cyprus Plateau Mining Corporation, conducted a barrier pillar study in a multiple-seam mine setting. Insufficient barrier width resulted in excessive load transfer to an adjacent/underlying section. Consequent entry and pillar deterioration resulted in the section being prematurely abandoned. This paper proposes an approach based on in situ measurements, which includes mine-specific loading conditions in the barrier design process.

Jan 1, 1989

By V. M. Shick

With gentle bedding within the zone of support pressure the delamination in roof rocks may occur only on those stratification planes where cohesiveness of rock is close to zero. Unlike this, at dip angles of more than 30-35° disintegration of rock occurs on the surfaces with cohesion up to 5 MPa. Therefore, the delamination in roof rocks at inclined beds is greater than at gently dipping ones. The original disintegration along the pressure fissures with the formation of block structure at inclined strata is not typical, and it may be observed only in roofs with included thick rocks; in this case, the orientation of fractures depends on the angle of seam inclination. Deformation of roof roocks above the mined-out space is determined by anisotropy of layers stiffness (hardness). With the increase of dip angle the own bearing capacity of layers will rise in the direction of strike. It was emphasized that the natural and technological factors are of great importance for rock pressure manifestation in longwalls.

Jan 1, 1996

By Stephen C. Tadolini

PC-Strand tendons, commonly referred to as cable bolts, have become a major rock reinforcement device for underground mining and civil applications. The most common configuration is a 1+6 strand, which designates a center king-wire, with six outer wires wound around it to form a 0.6-in (15-mm) diameter cable. The capacity of this tendon approaches 30 tons, but the performance is largely dependent on the anchorage medium and the ability of the system to transfer stresses back into the rock mass while reinforcing and suspending the immediate roof. Recent developments to improve the transfer mechanisms of the PCstrand via the anchorage medium by impregnating the individual wires with uniquely designed deformations have resulted in improved anchorage and subsequent cable bolt performance. This paper describes a series of laboratory tests used to develop the initial indentation patterns and numerical modeling simulations that were undertaken to refine and optimize the final designs. The new, indented PC-Strands, offered in both bright and properly galvanized, have been tested in the field and are being used effectively in two new cable bolt products. The results of the laboratory, computer modeling, field testing and in-situ installations provide a glance at the next generation of cable bolts. This suite of testing shows how they will improve the safety and serviceability of openings under adverse ground conditions.

Jan 1, 2012

By Kelvin K. Wu

A coal waste disposal dam failed on Buffalo Creek near Saunders, West Virginia, on February 26, 1972. This accident resulted in the loss of 125 lives, left over 4000 people homeless, and resulted in millions of dollars in damage to several downstream communities. As a result of this tragic event, major changes occurred in the mine waste disposal practices used by the U.S. coal industry. This paper summarizes changes in U.S. coal-waste disposal practices in the nearly 35 years since the Buffalo Creek dam failure and identifies safety issues that need to be addressed for continued improvement. While many organizations and individuals have played roles in the improvements that have occurred, this paper focuses on the role played by MSHA and its predecessor agencies.

Jan 1, 2006

By Gabriel S. Esterhuizen

A survey of pillar conditions was carried out at 21 operating limestone mines that use the room-and-pillar method. The surveyed pillars were all located in rock that was classified as ?Good? to ?Very Good? using the RMR rock mass classification system. The average pillar width was 14.5m and the width to height ratio varied between 0.53 and 3.52. Pillar conditions were rated using a visual rating system that accounts for the effect of geological structures as well as stress related fracturing. Nine cases of pillar instability were recorded. Pillar instabilities were typically limited to individual pillars or small groups of pillars within an otherwise stable layout. Local geological structures such as inclined discontinuities or weak bedding planes contributed to instability in seven of nine cases that were observed. Elevated pillar stresses contributed to instability in the two remaining cases. All the unstable pillars observed had width-to-height ratios of less than 1.5. It is concluded that pillar instability is most likely to be caused by unfavorable geological structures in pillars with width to height ratios of less than 1.5. Stress related instability such as rib spalling becomes more prevalent when the average pillar stress approaches approximately 20 MPa.

Jan 1, 2006

By Tristan H. Jones

Beginning in 1987, miniature dataloggers approved by the Mine Safety and Health Administration (MSHA) were publicly available for sale. The principal among these was the model CR21XQM, manufactured by Campbell Scientific, Inc. These dataloggers had been tested and were deemed permissible by MSHA and thus could be installed in the return air of an underground coal mine. In 1996, production of all permissible Campbell Scientific dataloggers ceased due to lack of sales. Data collection with these dataloggers has been a staple of ground control research in the United States for decades, and, without them, or a replacement, the capability to monitor ground behavior is severely impacted. Since that time, any party requiring a permissible datalogger has needed to borrow one that was purchased during the production years. In order to fill this technology gap and ensure its own data-gathering capabilities, the National Institute for Occupational Safety and Health (NIOSH) has developed and refined its own miniature data acquisition system (MIDAS). On February 16, 2010, MSHA deemed the MIDAS to be permissible and approved it for use in the return air of an underground coal mine. The MIDAS is an intrinsically safe, 16- or 24-bit datalogger, compatible with a wide range of resistive, single-ended, and differential-type instrumentation. It contains enough data storage memory to operate for over a year with typical data rates and is efficient enough to do this on a single C-sized alkaline battery for that entire period. It is menu-driven and requires no programming knowledge to operate. It supports wireless or hardwired communication for datalogger setup and data download. This paper describes the design, field and laboratory testing, and capabilities of the MIDAS datalogger. While not as fully -functioned as the CR21XQM, it is the only new permissible datalogger available, and there is intent to make it available to the mining industry for purchase.

Jan 1, 2012

By Rob Thomas

Adequate load transfer is considered essential for optimum ground support and is increasingly important in weak roof and/or high stress environments. If adequate load transfer is achieved not only can this have a positive impact on the resulting roof behavior, but it may also allow the use of lower and hence more economic densities of roof support. A wide variety of post-groutable cable bolts are currently in use in the Australian coal mining industry, each of varying capacity, design, and grouting methodology. Though all of the cables are supplied with the appropriate steel specification, strength, and modulus data, the Australian coal mining industry does not have access to a comprehensive and comparative database on the corresponding load transfer performance, which is a critical problem. On the basis of the above, this paper describes the methodology and results of a laboratory pull testing program that aims to assess the impact of cable bolt design on load transfer, and specifically, the impact on the capacity and stiffness of the anchorage. As part of the testing program, 19 cables were tested including (1) plain-strand cables installed in holes drilled with 28 to 55 mm diameter drill bits and (2) bulbed and nutcaged cables installed in holes drilled with 42 to 64 mm diameter drill bits. Other parameters assessed included the significance of smooth and profiled wire, wire packing, and expanded geometry bulbs. The main results of the testing indicate that in comparison to plain-strand cables, if the cable is bulbed or nutcaged, the capacity of the anchor can increase by up to 400%, and the peak stiffness by between one and two orders of magnitude. The results also indicate that increasing hole diameter may have a positive impact on load transfer in the case of bulbed or nutcaged cables and a negative impact on plain-strand cables. No conclusive results were obtained with regard to the impact of grout strength on load transfer.

Jan 1, 2012

By Peter Hatherly

As a discipline, geophysics covers an enormous range of scales and applications - from planetary geophysics through to the monitoring of acoustic emissions of rock samples. Although it is not a core discipline within the mining industry, geophysics has a useful role in providing information on the nature of the rock mass and the way it is responding to the mining process. For coal mining purposes, geophysical studies mainly seek to - delineate boundaries between the coal and the various layers within the surrounding strata - determine the physical properties of the rocks in-si tu, and - monitor the response of the rocks to the mining process Some of these applications may be associated primarily with mine planning, but once a mine is underway, most geophysical results can be used for the purposes of ground control. For example, insights into the nature of a geological fault mapped by a 3D seismic reflection survey will be relevant to determining support requirements while mining through it. In some instances there are parallels in the use of geophysics by the petroleum industry. Seismic reflection and wireline logging techniques have mainly been developed for petroleum exploration where their on-going development continues to provide spin-offs for the coal industry. The 3D seismic survey techniques that we now routinely employ were in their infancy 25 years ago and, almost certainly, the high quality borehole scanning tools that are now available would not have been developed without the need for petroleum well evaluation. Mining has also led to the development of specific techniques. An example is microseismic monitoring for the management of roof conditions during longwalling, and also the prediction of rock and coal bursts. The use and success of this technique have grown greatly over recent years. Another example lies with the use of geophysical wireline logs to predict rock strength and in rock mass classification. Coal seams also provide an unusual geophysical situation in that they form seismic and electromagnetic waveguides. The in-seam seismic and radio imaging methods have been developed to exploit these geological properties. In this paper I address the development and application of geophysical methods such as these with a particular emphasis on their role in ground control. We will see that there have been many developments over the past 25 years but we should also think about the future and how geophysics needs to be developed. From a miner?s perspective, one obvious area of need is in the detection of voids in the vicinity of current mine workings, be they old workings or karsts within nearby carbonate beds.

Jan 1, 2006

By Guoqi He

Common offset seismic data collected over two active longwall panels at different stages of mining reveal that significant arrival time delays are produced in reflections from different levels within the mine cover. Mining-induced arrival time delays are also observed beyond the edges of the mine over unmined areas. Reflection arrival time delays increase with reflector depth and are laterally variable. The arrival time delays are produced by reductions of seismic velocity within the overburden. Reflection arrival time delays from a major strong unit located neutrally within the cover are proportional to subsidence, however, delays in other events are not.

Jan 1, 1989

By X. S. Tan

This paper deals with the analysis of stress condition on the interface between two types of floor rock materials when the vertical stress is larger, or when the horizontal stress is larger in a rectangular opening. It indicates that the floor Failure and the failure stress of floor rocks are closely related. It also demonstrates that a larger horizontal stress is one of the most important factors in increasing the potential for floor rock failure and floor heave.

Jan 1, 1990

Recent improvements are described in several techniques for ground control in underground mining. Promising methods for analysis and mitigation of rockbursts are discussed. Methods of assessing damage to underground excavations from repetitive seismic loading are reviewed. Analysis of mining-induced surface subsidence is discussed.

Jan 1, 1989

By Haile Manteqi

In longwall coal mines, estimation of roof strata condition, sequence of loading, and supporting requirements are of a high degree of importance, and they are directly related to production and safety. Also, a prediction of the initial caving distance is rather complicated to calculate, because there are many parameters that effect caving process?roof and floor strata conditions, thickness of immediate roof, depth of cover, excavation geometry, and magnitude, and direction of principal stresses. This paper presents the results of 3D numerical modeling of the initial caving mechanism using the finite difference code FLAC3D at the E1 longwall panel of the Parvade 1 underground mine located in the Tabas area in the central part of Iran. The initial caving distance obtained was 10.4 m. Also, the results of numerical modeling have been compared to the analytical methods, such as Peng?s method. The results show good agreement with each other.

Jan 1, 2012

By Frank Chase

Longwall mining has gained the reputation as being the safest extraction method in underground coal mines. However, one of the most difficult tasks associated with longwall mining is moving the face once a panel is completed. Based on Mine Safety and Health Administration (MSHA) fatality reports since 1996, longwall face recovery operations have claimed the lives of 5 U.S. miners and have resulted in numerous injuries. Recovery operations can be hazardous because they involve moving large pieces of equipment in very confined spaces. They are also conducted in highly stressed ground conditions due to front abutment loads generated by panel extraction. Shield removal is the most hazardous operation during face recovery because miners are constantly exposed to the unpredictable gob edge. To protect the miners, one or more walking shields, cribbing and/or other supplemental roof and standing supports are typically employed as breaker line supports as each shield is removed. At the Harris No. 1 Mine in southern WV, mobile roof supports (MRS?s) have been used in lieu of traditional walking shields on 17 face moves since 1997. MRS?s are shield-like support units mounted on crawler tracks and are commonly used during room-and-pillar retreat mining operations. For longwall recovery, the two biggest advantages that MRS?s have over traditional walking shields are that they are remotely controlled and are highly maneuverable. MRS?s have contributed to safer shield recovery and shorter move times at the Harris No. 1 Mine. This paper will address both the safety and the operational issues associated with MRS usage during shield recovery. It will also discuss new developments, including the use of the inherently safer battery powered MRS?s, which have been recently certified by the Mine Safety and Health Administration.

Jan 1, 2007

By John C. Stankus

To improve mine roof bolt performance, Jennmar Corporation commenced the design and development of a new roof control sys- tem. This new system designated "Insta'l Compression", not only improves safety, but is more cost efficient due to reduced amounts of resin needed per bolt installation. In this paper the research and development of this new system will be outlined. Much of the information presented will be actual test data from various underground operations in Alabama, Kentucky, and Pennsylvania. The development of this system will also cover the design of a new expansion shell, resin compression ring, anti- friction washer, and upgrading of material specifications.

Jan 1, 1990

By Richard E. Gray

Subsidence due to coal mining is poorly understood by non-specialists. This has led to numerous misconceptions and myths based on limited observations and lack of knowledge. The three most common are: 1. Mine maps are inaccurate. 2. Deep mines are not a problem. 3. If no subsidence has occurred for many years after mining, there is no risk of future subsidence. Maps are important during mining and most are carefully prepared. Future use to evaluate conditions at mine level often includes drilling to confirm what the map shows. Usually, little or no effort is made to tie the surface survey of the property to the mine survey, to conduct a well designed drilling program to confirm the mine map, or to drill test borings vertically. When a mine entry is encountered rather than a coal pillar, or vice versa, and conditions at mine level appear different than anticipated, the first reaction is the mine map is inaccurate. The idea of a safe depth from subsidence is often based on the false premise that mining results in sufficient breakup of the overlying rock strata that bulking compensates for the coal extracted. The safe depth idea first appeared in the literature about 1880 and remained prevalent well into this century. Sadly, it is still encountered. The modern understanding of fragmentation of the immediate mine roof with the overlying beds sagging down on the broken roof rock was first described in 1900. With full extraction mining, either longwall or retreat room and pillar, surface subsidence occurs regardless of the depth of the mine. Subsidence over longwall mines at depths of 2000 feet can be 90 percent of the mined seam thickness. Numerous studies of undermined sites conclude that mining occurred many years ago and since no subsidence has occurred, there is no risk of future movement. This is true if sufficient coal pillars have been left to support the overlying strata. However, every year subsidence occurs over mines that have been closed for 100 years or more. In a study of subsidence incidents over the Pittsburgh Coal, the senior authors found that 50 percent of the incidents occurred above mines that had been closed for at least 50 years and 10 percent over mines closed for at least 80 years.

Jan 1, 1996

By David F. Gearhart

The Burrell Can1 is a thin, steel, tubular shell filled with aerated concrete that is used as a roof support in coal mines. The Can height is always shorter than the mining entry, so it is capped with wooden timbers and wedges when it is installed. This combination of the Can and the wooden header is a standing support system that is designed to yield in a controlled manner and exhibit a constant yield, or elastic-plastic behavior. The Burrell Can is capable of withstanding 20 in or more of vertical convergence and 15 in or more of horizontal displacement. Because of this exceptional ability to sustain load through extended convergence and displacement, it is one of the most robust standing supports in use today. The National Institute for Occupational Safety and Health (NIOSH) conducted testing on seventeen Burrell Cans in 1995, during development of the support where the ratio of the height to the diameter, or the aspect ratio, was between 2:1 and 4:1. Based on the results of that testing, it was suggested that the standard aspect ratio of the Can be less than 5:1. This suggestion is intended to preserve the overall stability of the Can, which will help to maintain the initial stiffness and prevent load shedding. Recently, the necessity of the aspect ratio has been questioned, and this issue will be addressed in this paper. Since the initial development and introduction, more than 130 tests of the Can have been conducted at the NIOSH Mine Roof Simulator (MRS), and 25 of those specimens had aspect ratios greater than 5:1. The 18-in-diameter by 10-ft-tall specimens had the largest aspect ratio, 6.67:1.However, the aspect ratio is not the only parameter that affects the performance characteristics of the Can. Multiple factors, such as the use of different wood species, the use of different header or footer configurations, and the use of pedestals, as well as biaxial loading conditions can also decrease the stiffness and capacity of the Can. Data from full-scale tests that include these other factors are analyzed to show that the initial stiffness is primarily affected by the roof and floor contact configurations. Furthermore, load shedding is more likely to occur or begin earlier when the aspect ratio is greater than 5:1 and one or more of the other factors are present.

Jan 1, 2012

By Meagan S. Boltz

From October 2000 to April 2001 a local array of fourteen seismograph stations was deployed at the Trail Mountain Mine (TMM), an underground coal mine in Emery County, Utah, to locate mining-induced seismicity (MIS). Original analysis of the data provided epicentral locations that coincided closely with the active longwall panel. The original analysis yielded poor quality hypocenters, however, preventing a useful relation from being made between mining activity and the event depths. For the original analysis, only stations at elevations above mine level were used for locating MIS. As a result, P- and S-arrival times from four of the stations in the local array were not used: an inmine station and three stations at elevations below mine level. This study compares mine workings and active mining to a revised TMM dataset that included P- and S-arrival times from the lower elevation stations that were not used in the previous study. In the revised locations, a modified master event methodology was used to calculate station delays. These station delays correct for errors in the shallow layers of the velocity model. The resulting hypocentral locations have improved depth resolution, determined by a lower root mean square (RMS) for the locations, compared to the original study. Most of the events occur in the roof of the mine and follow the longwall panel. When located using the velocity model determined in the original study, the elevations of the events increase as the longwall panel progresses. One possible explanation for the elevation increase is that it may be related to geology or the caving process. As presented here, a 10% decrease in the shallow velocity structure results in a more plausible location for the hypocenters. Consequently, it is hypothesized that progressive development of the TMM longwall panel(s) may have resulted in a progressive change in the velocity model.

Jan 1, 2012

By Shosei Serata

The Stress Control Method improves productivity and safety in underground coal mining. The method stabilizes the roofs and floors of mine openings in both shallow and deep coal beds, regardless of whether or not there are ground stability problems. This paper describes the means by which a tapered pillar experiment is used to collect in situ data, which is then used as input for a computer model. The resulting computer model is then used to optimize Stress Control design for the mine under study. The behavior of any underground coal mine is not elastic, as is generally conceived in the conventional view of rock mechanics, but rather is highly time-dependent and site-specific. Unfortunately, conventional rock mechanics methods based on the elastic principle are not able to capture the highly non-elastic nature of ground behavior which actually occurs. To capture actual behavior, a new method of study is required. In this regard, geomechanical study of an underground coal mine may be compared to studying the behavior of a living fish. Its true behavior zannot be understood by examining it "dead in the laboratory. It Is the "live" nature of the coal mine that is required for design optimization of the Stress Control Method in the given mine. The Stress Control Method is based on a time-dependent theory of the global behavior of the mine ground, which generally contradicts the conventional elastic theory of ground behavior. Therefore, site-specific adaptation of the Stress Control Method required a comprehensive understanding of the global behavior of the mine as a function of time and site-specific ground conditions.

Jan 1, 1986

By Gerald Stolarczyk

The Radio Imaging Method (RIM) is based upon electromagnetic (EM) wave propagation in the coal seam waveguide. The radio geophysics theory underlying RIM was developed by Hill of the national Institute for Science and Technology (Hill, 1984). Recent advances in radio geophysics instrumentation have increased operating range in the coal seam waveguide to almost 3,000 feet. This paper describes RIM longwall panel reconnaissance scans and tomography results.

Jan 1, 2006