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By Ismet Canbulat

"In order to ensure the stability of roadways, Anglo American Metallurgical Coal (AAMC) has developed a roof support design methodology that integrates analytical, numerical and empirical modelling. This methodology currently is based on a deterministic approach (a single factor of safety against failure is calculated). However, an improved methodology, based on stochastic modelling technique, has also been developed and currently being evaluated. The main advantage of this methodology is that the design is based on the probability distributions of input parameters; thus the outcome is based on a distribution of factors of safety rather than a single factor of safety. An evaluation of factors of safety may also be used to determine the likelihood of failure, which in turn may be utilized to quantitatively determine and evaluate the associated risks in decision making process. This methodology has been evaluated at Grasstree and Moranbah North Coal Mines in the designs of roof support in various critical areas and has been proven to be successful and a better way of determining the roof support requirements.This paper demonstrates the application of this methodology in a design of an install road roof support at Moranbah North Mine, where the 1,750 tonne capacity ''world's highest rated longwall"" has recently been installed.INTRODUCTIONAnglo American Metallurgical Coal Australia (AAMC) operates three longwall (LW) mines located in Central Queensland. There is an increasing emphasis on the reliability at these operations, as the longwalls are getting deeper and facing more geologically challenging conditions. In addition, the Anglo American Vision is to achieve ""Zero Harm"" through the effective management of safety at all businesses and operations. In order to accomplish this vision, AAMC has developed a proactive ground control management system for a safe and efficient production of underground reserves.Proactive ground control management involves understanding the impacts of the geotechnical environment on likely ground behavior, and it consists of approximately 15 major elements. One of the most essential fundamentals of proactive ground control management is to utilize a roof support design methodology that considers different failure mechanisms and also takes into account all important elements. The design methodology becomes even more important when the area in question is a critical area, which is defined as a high risk roadway where any failure may cause increased levels of safety and financial risks to underground workforce and operations."

Jan 1, 2010

By Morgan Sears

The LaModel program (Heasley, 1998) is a displacement discontinuity, boundary element model (BEM) for analyzing stresses and displacements in single and multiple-seam coal mines. After the Crandall Canyon mine collapse, the need for better design guidelines for LaModel was observed and a new calibration method for deep cover pillar retreat was developed (Heasley et al., 2010). However, the scope of this initial calibration was limited to single-seam mines deeper than 750 ft. This limited application of the initial calibration method lead directly to the research presented in this paper, which attempts to define appropriate LaModel calibration guidelines for pillar design where shallow cover and/ or multiple-seam mining scenarios are encountered. As part of this research, a database of shallow cover case histories has been developed, and this database is used to help validate the shallow cover calibration method. By adding the shallow cover database to the previous deep cover database, the LaModel calibration method has now been extended to the complete range of mining depths, as well as to multiple-seam cases. With this shallow cover calibration, the utility of the LaModel program continues to increase.

Jan 1, 2013

By David R. Hanson

Seismic tomographic imaging, based on the same principles as a medical CAT Scan (Computer-Aided-Tomography). has been used for many years in the oil industry for large-scale subsurface stratigraphic characterization. and has most recently been applied to various structure- and stress-related problems in the coal industry. New developments in signal processing have greatly enhanced the speed, resolution, and range of applications of topographic imaging in underground settings, and recent innovations in data acquisition hardware have further increased data reliability and repeatability. Whereas past structure mapping applications relied on Seismic velocity and/or attenuation tomopraphy within an enclosed seismic source gram receiver array, recent developments in "True Reflective Tomography- (TRT TM) have expanded the application of this technology considerably. For example, in-seam TRT TM seismic reflection tray now he used to image structure, and/or old workings from one general location in the mine face areas of mains and panel developments) well ahead of planned developments largely eliminating the need to probe-hole drill on regular intervals. This new reflective technology bas also been combined with traditional cross-hole seismic tomography-providing a comprehensive, powerful tool for velocity attenuation. and reflective imagine utilizing the same source/receiver array data sets (borehole-to-borehole. borehole-to-entry and entry-to-entry). These new developments in data acquisition and image processing have greatly expanded the variety of applications for tomographic site characterization allowing the mine operator considerable flexibility to cost-effectively characterize previously inaccessible areas of the property. This paper presents examples of 2-D and 3-D .seismic topographic imaging applied to (1) the location of old works in Appalachian room-and-pillar operations and (2) the identification of anomalous zones ahead of mining/tunneling developments these applications demonstrate the state-of-the-art in seismic ground imaging using velocity, attenuation. aril reflection techniques land combinations thereon, and further illustrate the flexibility of data acquisition from a variety of tinning arid tunneling settings. Recent examples are presented from projects in the United States and Europe.

Jan 1, 2000

By Christopher Mark

The Analysis of Retreat Mining Pillar Stability (ARMPS) program has been used to evaluate room and pillar retreat mining layouts since it was first released in the mid 1990s. After the Crandall Canyon mine disaster focused attention on the importance of pillar design, ARMPS became an essential component in most Roof Control Plans developed by mine operators. In 2010, NIOSH released ARMPS version 6, which features a ?pressure arch? loading model for deep cover room and pillar mining. NIOSH has also provided general guidance for using ARMPS, including design criteria based on statistical analyses of an extensive case history data base. Many real-world retreat mining scenarios entail mining configurations that ARMPS does not directly address. Some of these situations that the MSHA Roof Control Division (RCD) has encountered in the course of its reviews of more than 100 ?complex and non-typical? plans are ? Unmined pillars are left at the mouth of a retreat panel that function as a ?composite barrier pillar? ? The floor is extracted on retreat, increasing the mining height ? More than one row of bleeder pillars is left in an adjacent, previously mined panel ? A retreat panel is located above or below a gob area in a previously mined seam ? Bleeder pillars are partially extracted on retreat RCD has developed solutions that allow these situations to be fitted into the ARMPS framework, and these are presented in this paper. Also discussed are some of the rules of thumb that have been developed for various input parameters, such as the treatment of inseam rock and slab cuts into solid coal.

Jan 1, 2011

By Yuanping Cheng, Jingyu Jiang, Qiang Zhang

"With the increase of mining depths, deep-seated coal seams face threats from high ground stresses, high gas pressures, high gas content, and low seam permeability, which significantly increase the likelihood of coal and gas outbursts. Challenges continue to grow in coalbed methane (CBM) extraction fields, especially for short-distance gassy multiple-seams. Research has proved that the most effective way to increase coal seam permeability is to decrease the ground stress. Protective seam mining technology is a surrounding rock control technology, which can effectively reduce the ground stress and increase the permeability of the protected coal seam. In this paper, we try to use upper protective seam mining technology to simultaneously extract coal and gas. Findings are based on a FLAC3D simulation of stress variation during the mining of the No. 12 coal seam in the Daxing coal mine, Tiefa Coalfield, China. Results indicate that, after mining the upper protective seam, the pressure-relief effects of the rock mass generated three belts and five zones below the protective seam, providing the space and time for gas pre-extraction engineering. Within the protected area, the numerical simulation indicates that the stress of the No. 13/14 coal seam reduces 95.84% and 88.25% respectively, while the expansion deformation of the protective seams reach 6.87 ‰ and 6.70 ‰ respectively, which demonstrate that the protective mining technology can provide a significant pressure-relief effect to the outburst-prone coal seam. Combining stress relief in this manner with pressure-relief from engineered gas drainage (performed by crossing boreholes from the floor rock roadway) applied before the mining of the protective seam, it would not only be beneficial to CBM drainage but also can reduce the gas outburst risk of the protected seams.INTRODUCTION Internationally, since France used protective seam mining technology to control coal and gas outburst in 1939, it has been promoted and applied in many countries (?????, 1966). Since 1958, China has taken the mining test and engineering practice of protective seam mining technology in the Beipiao, Tianfu, Nantong, Zhongliangshan, Songzao, Huainan, Huaibei, and other coalfields (Yu, 1992). Long-term theoretical research and mining practice of outbursts and dangerous coal seams show that protective seam mining technology is the most effective, safest, and most economical outburst prevention measure (Cheng, Fu, and Yu, 2009; Hu et al., 2008; Cheng and Yu, 2007). Under the condition of high gas content coal seams, the safety and efficiency of simultaneous extraction of coal and gas can be realized through a reasonable mining sequence and an effective gas extraction method (Cheng and Yu, 2003). The in situ stress distribution of the protective seam (No. 1 coal seam) and protected seam (No. 2 coal seam) during protective seam mining in the Haishiwan coalfield formation (Figure 1) (Li et al., 2014). A comparison of the stress and displacement (five zones and three belts) of the rock mass below the protective seam, and in a multiple-seam context in the Dalong Mine, is shown in Figure 2 (Jiang et al., 2015). After mining protection coal in a close-distance seam group, the small spacing between the coal seams causes the protective coal and the protected coal between part of the rock to crack through each other until the protective layer of gob is through the area. This provides a desorption of gas to the protective layer of the mining space channel, making the coal seam permeability increase to several hundred times and improving the gas discharge capacity (Zhai, 2008)."

Jan 1, 2017

By Marvin B. Thompson

"The depositional model for major Illinois Basin coalbeds is well documented and defined on a regional scale. The model was a driving force during past exploration for lower sulfur and thicker coal deposits. The most attractive coal reserves were known to lie along the flanks of contemporaneous sandstone filled stream channels and were overlain by thick silty gray shale roof. At various distances away from the channels, the depositional model transitioned to black shale I limestone roof, sulfur content of the coal bed increased, and the coal bed often thinned. Understanding the character of roof rock types associated with this depositional model is essential to predicting ground control factors on both a regional scale, and on resolving roof control problems at the mine-site. Additionally, horizontal stress and overburden depth are significant factors that impact roof rock quality and ground control in Illinois Basin mines.Determining the character of several rock types, and defining the primary horizontal stress direction were crucial to a recent successful project at New Future Mine, located in southern Illinois. An inclined slope was constructed from the Illinois No. 5 Coalbed up to the Illinois No. 6 Coalbed through strata that ranged from extremely weak ""stackrock"" and a weak underclay unit, to a massive sandstone unit. Engineering geology, operations management, and Jennmar technical services personnel teamedup to select and install roof support for each strata transition. The project was completed with no injuries and no roof falls.INTRODUCTIONAbundant published information concerning coal deposition, geographic distribution, quantity, quality, and mining conditions exists for Illinois Basin coal reserves. The State of Illinois, through its State Geological Survey (ISGS), has provided decades of research, record keeping, and service to the coal industry. Other excellent sources of Illinois Basin coal information include the Indiana Geologiq1l Survey and the Kentucky Geological Survey. Past research by the U.S. Bureau of Mines (USBM) supplemented the work by the geological surveys, especially in the area of horizontal stress and mining hazards. The National Institute for Occupational Safety and Health (NIOSH) continues to produce publications focused on roof control issues common to many coal basins, including the Illinois Basin. Many of the illustrations in this paper originated in ISGS and USBM publications."

Jan 1, 2010

By Richard R. Wilding

The United States Coal Mining Indus- try had the opportunity to greatly improve their roof control techniques during the nineteen seventies by the use of fully grouted resin roof bolts. This new tool was first approved for underground coal mines by M.S.H.A. ten years ago and during the next decade has been perfected to meet many problems previously thought to be unsolveable. The initial approved cartridge was very similar to the ones used in Europe in the nineteen sixties and since then has undergone a great deal of modification for our mines. These refinements in cartridge construction have been obtained by the close cooperation of resin manufacturers with mining personnel in perfecting pro- ducts customized to particular needs. This type of continuing program naturally necessitates a responsibility upon the resin anchor manufacturer to maintain a competant technical department. These roof control technicians by constant exposure to many of the most demanding underground problems, have developed through the years into leaders in successful applications. Today, Celtite has over twenty technically trained underground technicians with an accumulation of over one hundred years experience in resin anchors. Our documented technical reports are in excess of five thousand, involving over six hundred mines representing every major coal mining area in most mineable seams. This department has been the implement used to work with our research and development people in perfecting a continual line of improved resin anchors. This program has been a significant factor in leading the resin anchor industry into a position where approximately one third of the 1980 under- ground coal production used the fully grouted system. However, as gratifying as this accomplishment is, our experience has lead us to believe that there is an additional demand for a roof control system capable of combining the mechanical bolts ability to apply tension with resins proven superiority in anchoring. A significant number mines visited during the last decade had problems that we believed could be better answered by applying tension rather than the fully grouted approach. In most in- stances, their current mechanical installations were doing an adequate job until anchor slippage occured. Fully grouted systems solved this bleed-off problem. However, an acceptable number of falls still occured. In these some- what unique areas which we believe may amount to as much as fifteen percent of the total bolt installations, the need for improved anchors was a necessity. In developing this program we listed the ultimate requirements necessary for both the bolt and the anchor. These are for the bolt: A. Have one end manufactured so it can be grouted to the mine roof with polyester resin, obtaining a variable length from 12" to 36". B. Have a mechanism which allows the operator to tension the roof from zero to 10,000 psi. This mechanism should indicate if the roof is in compression and the bolt in tension. C. Have a mechanism which allows the operator to thoroughly mix the resin. D. The head of the bolt should not extend below the roof line any more than a normal mechanical bolt. E. Not require any extra tools for installation. F. Be bendable for use in low seam coal. G. Be durable and inexpensive. The ultimate point anchor resin should have the following characteristics: A. Have a mix and gel cycle which would compete with a mechanical bolt for speed of installation. B. Have a pull out strength in excess of a normal 5/8" mechanical bolt.

Jan 1, 1981

By Chris Mark

Perhaps the most significant development in coal mine ground control during the last century was the introduction of roof bolting during the late 1940's and 1950's. From an engineering standpoint, roof bolts are inherently more effective than the wood timbers they replaced. Roof bolts promised to dramatically reduce the number of roof fall accidents, which then claimed hundreds of lives each year, and they were initially hailed as "one of the great social advances of our time." Roof bolting also emerged at a time of rapid technologic transformation of the coal industry, and greatly accelerated the transition to trackless, rubber-tired face haulage. The U.S. Bureau of Mines quickly became the new roof support's strongest advocate. Some state agencies and miners were skeptical at first, but nearly everyone was soon won over. Case histories were reported showing that roof falls could be largely eliminated while productivity increased dramatically. Little wonder that, in the words of one contemporary observer, "roof bolting has been adopted more rapidly than any other new technology in the history of coal mine mechanization." Yet by the end of the 1950's, it was clear that roof fall fatality incidence rates had actually increased. It would be another decade before the superior ground support provided by roof bolts would clearly save lives, The story of how roof bolting was implemented by the mining industry, but took so long to live up to its promise, is a fascinating example of the interaction between economics, technology, regulation, and science. It still has important lessons for today.

Jan 1, 2002

By Stephen C. Tadolini, Robert Hawker, Peter Mills, Colin Grubb, Tom Meikle

"In unfavorable strata conditions, it is often necessary to install additional long tendon support as part of a primary support cycle. Although these support systems provide the required additional stability to the roadway installation does have a significant effect on the rate of development. To overcome the slow cure time of Portland cement-type grouts, the use of two component resin systems, such as polyurethanes and urea silicates, have become favorable. However, concerns over low modulus and creep properties has led to the development of a two-component grouting medium that displays the properties of traditional cable grouting materials with the rapid reaction times of pumpable resin systems. This paper describes product testing, application, surface trials, underground trials, and full-scale applications of the Tekthix System in a dynamic underground environment. INTRODUCTION Single-component cement grouts have been used to anchor long tendon supports for many years. They range from high- to low-slump Portland cement and can be slow-setting to fast-setting grouts. However, current cement grouting systems, which includes the grout and equipment, have some disadvantages: 1. Low-slump grouts require pumps that can handle a thick, high-viscosity grout, so choice of pumps is limited. 2. Pumping distance is limited with current cable bolt grout pumps; operators often tend to add water to allow pumping long distance, which adversely impacts the quality and strength of the material. 3. Fast-setting grouts tend to cause issues with pump and line blockages due to the reduced working life of these single-component materials."

Jan 1, 2017

By Martin McKinnon, Guy Reed, Russell Frith

"It is arguable that the development of reinforcing roof bolting systems has largely stagnated in recent times, primarily due to the prevailing industry view that few, if any, further improvements can be made to what currently exists. However, this paper contends that reinforcing roof bolting systems can be further refined by considering both the specific manner by which horizontally bedded roof strata loses its natural self-supporting ability and the specific means by which reinforcing roof bolts act to promote or retain this natural self-supporting ability. The Australian coal industry insists on minimising bolt-hole diameter to maximise load transfer and on targeting full-encapsulation by any means necessary. This has led to a significant, albeit unintended, consequence in terms of overall roof bolting effectiveness, namely increased resin pressures during bolt installation and the associated potential for opening bedding planes that may have, otherwise, remained closed during the bolt installation process. Given that the natural self-supporting ability of roof strata is strongly linked to whether bedding planes are open or closed, logically, minimising resin pressures should be a significant benefit. This paper focuses primarily on three key issues that relate directly to the function of the roof bolting system itself: (i) the importance of proper resin mixing in the context of maximising load transfer strength and stiffness, (ii) the importance of minimising resin pressures developed during bolt installation, and (iii) the importance of maximising the effectiveness of the available bolt pre-tension. All mine operators should be invested in improving the individual effectiveness of each installed roof bolt, even by relatively small incremental amounts, so this is an important topic for discussion within the mining community."

Jan 1, 2017

By Yunqing Zhang

The 3-D numerical modeling using ABAQUS is performed to analyze the mechanisms of the tensioned bolts by taking into considerations excavation sequence, pre-tension, bedding planes and in-situ horizontal stresses. The effect of the pre-tension and the mechanisms of the tensioned bolts are analyzed based on the results from the modeling. Based on field observations, the failure modes for the tensioned bolted strata are classified by their causes and failure mechanisms. The support design consideration for each failure mode is given. Finally, a new design approach is proposed to do the tensioned bolting design using numerical method.

Jan 1, 2002

By Brian E. Perrine

Underground mining requires a variety of specialized equipment. With ever-changing mining conditions and methods, some mines have had to resort to purchasing existing industrial equipment and adapting it for underground use. Since this equipment is not made for underground mining or their specific task, the result is often lost time, money, and sometimes an unsafe working condition. One such application is the setting of a CAN. A CAN is a type of cylindrical secondary roof support made from a cement composite and encased in steel. The CAN has become popular because of its simple one-piece design that can handle heaving and sagging better than other available designs. ?Because of this exceptional ability to sustain load through extended convergence and displacement, the CAN is one of the most robust and stable standing supports in use today.?It also promotes better air flow due to the size and shape of the CAN. CAN size and weight have increased over time to handle more difficult mining conditions. Significant down time and inefficiencies with existing CAN setting machines at Twentymile Coal were directly due to the item in use not being designed to be task-specific. They were undersized with very limited movement. The miners had to take extra steps and precautions when setting CANs. Maintenance delays were significant. Twentymile Coal Company realized that they required a safer, more reliable and more efficient machine to handle and set CANs, and looked for a partner to develop a new design-specific machine. They worked jointly with J.H. Fletcher® engineering and sales to design and manufacture a machine specifically made for setting CANs. The objective was to produce a machine that would handle the conditions and methods that the mine required for setting CANs during longwall retreat. The resulting design had to be safe, stable, have a wide range of motion, be ergonomic, simple to use, and easy to maintain. The machine also had to improve efficiency. The resulting machine has been operating for approximately one year at Twentymile Coal and has been proven to improve safety, reliability, and efficiency. In conclusion, with the task-specific CAN Manipulator, the mine is able to better plan daily tasks with the knowledge that the machine will be available throughout a shift, and the employees will be safer because of the machine?s ergonomics, stability and safety. This has resulted in better productivity and a safer environment.

Jan 1, 2013

By Kewal K. Kohli, Stefanie Self

"Three cases of surface subsidence investigated over abandoned coal mines after flooding in one case and dcwatering in two cases are presented in this paper. Mining was conducted by the room and- pillar method at all the mines leaving stable pillars. This paper will discuss the background chain of events and the likely causes of the pillar failure resulting in surface subsidence.CASE NO.1BackgroundThe Pittsburgh scam was mined in 1960 by the room-and- pillar method, leaving pillars (Figure I) through Mine A. located in Monongalia County. West Virginia.The mining height in U1is area was 1.83 m (6 ft) with an average mining depth of 140 m (460 ft). The mining recovery was 62% and the safety factor of the pillars was calculated at 1.15 using the Bieniawski ( 1984) fom1ula. For stable pillar design. a 1.5 safety factor is recommended. A 1.15 safety factor is considered marginal. Pumping continued after the mine was shut down in 1960 to prevent water seeping into an adjoining mine owned by the same company. The pumping ceased on December 12. 2003, and the mine was allowed to flood. The water level rose about 17 m (55 ft) by August 3 1, 2004, based on observations in a nearby borehole. On September 3, 2004 surface subsidence occurred above this area causing damage 10 a residence (Figure 2).Summary of InvestigationThe floor of the Pittsburgh coal scam played a strong role in the pillar failure. In this area, the immediate floor of the Pittsburgh seam consists of about two feet of fireclay. Extensive damage to the surface house was an indication that a major collapse below the surface had occurred. The pillars supporting the overlying strata were evaluated to find the cause of their failure. The pillars in the Pittsburgh seam (sec Figure I) had a marginal safety factor of 1.15 before flooding and were stable for 26 years. Once the mine was flooded the water level in the Pittsburgh seam rose by 17 m (55 ft). Shortly thereafter the fireclay in the floor weakened, resulting in several pillars punching into the floor unevenly making the immediate roof unstable. The overburden strata supported by these pillars also subsided do"" n into the mine, causing damage to several house on the surface."

Jan 1, 2010

By James Pile

The San Juan Coal Mine, located near Farmington, New Mexico, supplies the San Juan Generating Station with more than 6 million tons of coal annually. To replace dwindling surface mine production, San Juan installed a longwall in 2002. The roof rock in much of the underground reserve consists of interbedded coals, carbonaceous shales, and mudstones. Roof support consists of fully-grouted roof bolts, both tensioned and non-tensioned, supplemented by modified cable trusses. Short-encapsulation pull tests conducted by San Juan's geotechnical engineering department showed that some of the immediate roof layers provided variable bolt anchorage. To ensure that the roof bolts were obtaining the necessary anchorage, additional tests were conducted in a variety of locations. The results have been used to determine the optimum support design for different areas of the mine. Another series of tests evaluated the performance of several bolt and hole diameter combinations. The experience at San Juan Mine shows that short-encapsulation pull tests can provide the information necessary to improve ground control in variable roof conditions. It also illustrates how a pro¬active geotechnical engineering program can be an essential component of a state of the art underground coal mine.

Jan 1, 2003

By Nicholas P. Kripakov

The unpredictable massive collapse of roof in openings developed under apparently safe and stable roof conditions in coal mines of the United States has been of much concern for many years. One type of massive caving phenomenon is often attributed to cutter roof, a failure that initiates at one and/or both upper corners of an entry and propagates nearly vertically resulting in overall failure with little or no warning. This paper reviews current methods being used to control cutter roof and suggests new alternatives. The impact of basic parameters that include mine geometry, overburden geology, rock properties, and in situ stress conditions is discussed in the context of their relationship to this problem. The effects of various stress control measures are investigated with model analysis to gain insight into means of reducing the high level of stress known to exist at the entry corners of a West Virginia coal mine prone to cutter roof. The results are presented in the paper and the feasibility of their practical implementation is discussed.

Jan 1, 1982

By Kot F. von Unrug

One of the major difficulties in the proper characterization of rocks surrounding excavations is the lack of data. The existing method for strength determination requires core drilling for sample collection, which, when performed from the surface is costly, and when performed underground, in addition to its cost, interferes with the mine operations. I-landling samples tested in the laboratory is troublesome and also influences the obtained results. Over several years. the author has been interested in developing a method of in situ strength determination that could rapidly generate strength data at low cost without a negative effect on mining activities. The recently developed borehole penetrometer is an improved version of the initial design ( 3 inches in diameter), which was used mainly for strength determination of coal pillars. The new penetrometer has been designed to work in 1.5 inch diameter holes. Such holes can be drilled at low cost with either hand held drilling machines or roof bolters. eliminating limitations of the previous system and opening the possibility of broad application. Strength testing of roof rock around longwall pillars is an example of an obvious benefit when designing secondary roof support. In the paper are given the principles of work of the apparatus and examples of the obtained strength data.

Jan 1, 1998

By Christopher Mark

Mine planning for a new reserve is based on information obtained from exploratory coreholes. A critical component of an exploration program is the geotechnical evaluation. Poor assumptions about roof conditions greatly add to the risks of mining. Rock mechanics testing is central to a geotechnical exploration program. Typically, 3 to 5 uniaxial compressive strength (UCS) tests are made to characterize a particular roof unit at a given corehole. The average (mean) of these tests is taken as "The UCS" for that location. Isopach contour maps are then used to show spatial trends in roof strength. Two issues are raised by this traditional approach. The first is due to the large variability in UCS values that is typical even within a single unit from a single hole. The average UCS might be higher at Corehole A than it is at Corehole B, but the difference may not be statistically significant. The second issue is whether widely spaced coreholes can identify valid spatial trends in rock strength. The answer depends upon whether rock strength changes over distances that are longer or shorter than the corehole spacing. This is a classical geostatistical problem. While geostatics have been used to investigate many coal quality parameters, they have seldom been used to evaluate rock strength. This paper describes an extensive investigation of these issues conducted by the National Institute for Occupational Safety and Health (NIOSH) in collaboration with Peabody Energy. The study employed the Peabody Rock Mechanics Data Base which contains more than 10,000 individual test results. Data from four important roof units were subjected to statistical analysis: ? Brereton Limestone above the Herrin 6 seam (Illinois) ? Turner Mine Shale above the No. 9 Seam (Kentucky) ? Sandstone above the Eagle Coal (West Virginia) ? Shale above the Eagle Coal (West Virginia) The study did not find significant spatial trends in rock strength in any of the cases. Perhaps there are none, or perhaps the exploration coreholes were just too far apart to see them. These results have valuable implications for the design of geotechnical exploration programs.

Jan 1, 2004

By Jay Hilary Kelley

This paper proposes a modification of longwall roof support to meet new difficult mining conditions that are anticipated in the future. A gob canopy is a movable appendage attached on the rear of a longwall shield to counterbalance the troublesome forward downward force couple (FDFC). The gob canopy can be swung in both vertical and horizontal directions, powered by hydraulic cylinders. Several functions and applications are de- scribed

Jan 1, 1997

By Brian Clifford

A wide range of different cablebolt systems are available and in use in hard rock and coal mines around the world. The birdcaged cablebolt was initially used in UK coal mines in conjunction with rockbolts in the late 1980's. More recently other cablebolt configurations have been used. Innovations in cablebolt design in the UK have been driven by a demand to find a system as effective as the birdcaged cable, but which can be installed more rapidly and offer immediate support. A laboratory short encapsulation pull test (LSEPT) has been developed for both rockbolt and tendon systems which measures bond strength and stiffness in rock and assesses the full range of potential failure modes. This test is now used along with conventional 'gun barrel' and shear tests to provide full assessment of bolt and tendon performance. This test may be incorporated into a new performance based British Standard for rockbolts and long tendon systems. This paper describes the performance of some of these systems as being applied to UK and South African coal mines.

Jan 1, 2001

By Dale A. Didcoct

Through the combined efforts of the C. S. Bureau of Mines, the coal industry, and Burrell Construction and Supply Company, New Kensington, PA, a steel fiber reinforced concrete crib block to improve coal mine safety in the area of roof control has been developed and is currently in use commercially. The development objective was to improve health and safety conditions with a stiffer, stronger, nonflammable roof support at a competitive cost. During the time this method of roof control has been in use in the mining industry, it has proven to be both functionally and economically effective.

Jan 1, 1982