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By Radoslaw L. Michalowski

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

Jan 1, 1990

By Xiaolin Wu

Most coal bump occurrences in the U.S. are directly related to mining operations under strong roofs. An understanding of the behavior of bridging or cantilevering strong roofs over working longwall faces is a key to a safe and productive longwall operation. In this paper, an attempt has been made to study the behavior of strong roof beds using the elastic beam theories. Four structural models are formulated to simulate the locations of strong roofs and their distinct weighting stages. For each model, analytical solutions for the deflection, bending moment, stress, and the strain energy stored in both the coal scam and the strong roof are obtained. Solutions for the critical spans, which are controlled by the caveability of the strong roof beds, are developed. The effects of rocks' differing elastic moduli, under tension and compression, on the actual flexural rigidity, and stress redistribution are analyzed.

Jan 1, 1993

By Douglas C. Peters

Part of the ground control research at the U.S. Bureau of Mines is involved with improving ground control data collection and analysis methods. A geographic information system (GIs) is being used within this research effort to standardize the data collected and provide a common graphical and tabular format and analysis system for researchers to access information on underground coal mines. In addition, GIS's can perform relational analyses on multiple map and tabular data sets which are not possible through map-creation or CAD packages or through data bases alone. In addition to standardizing ground control analyses, there is an increasing need for rapid analysis of ground control data in connection with increasing production rates at longwall mines. The examination of a large number of interrelated parameters and their impact on short-term mine safety and productivity under the time constraints imposed by rapid production requires computer-aided techniques. A GIs provides the means to quickly, quantitatively, and reproducibly evaluate mine-wide conditions. This overview of GI8 techniques for mine data integration and analysis, using a mine in Colorado as an example site, will cover the input of data into a GIs, including mine-wide monitoring system data and general data such as roof control plans and geology. The methods to geographically reference these data sets and retrieve and combine them also will be covered. Automation of such processes briefly will be reviewed to allow future users to automate their mine data analyses.

Jan 1, 1990

By Mao Bai

Mining in situations where there exists a potential hazard from inundation has always existed in coal mining, and often reserves were left unmined in favor of reserves with no such hazard. However, increased pressure on expanding the minable reserves and on producing low-sulfur coal, has made mining under water hazards necessary in many parts of the world. Recent joint efforts by the authors to develop case-history data bases of mining under water hazards has allowed the bringing together of Chinese and United States experience and guidelines for such mining. In China, mining under water hazards has been successfully accomplished for a number of years, allowing the development of a good data base from which to draw guidelines. In the United States, much more limited experience, and the longer delay in adopting longwall mining compared to the rest of the world, has forced reliance on a more limited data base and European experience. In the United States (and Canada), the data base is continuing to increase as more operations start-up. Developing a generally-applicable set of guidelines for international use, based upon extensive Chinese and other experience, appears feasible. The Chinese and United States experiences are detailed by case histories.

Jan 1, 1995

By David A. Newman

Strength and physical property testing of coal is conducted regularly by the Arch Mineral Corporation as part of reserve exploration. Pillar design and mine layout are the primary applications for the coal strength data. Distinct differences in pillar behavior were observed during longwall mining at a mine in Southeastern Kentucky. Horizontal and vertical variation in both triaxial and uniaxial compressive strength was observed within the four (4) benches of the Harlan seam. Similarly, geographical variations in coal strength were noted across the Harlan seam reserve. During the past three decades substantial research efforts have been focused upon coal pillar behavior and the refinement of pillar design methodologies. Ironically, little attention was directed to integrating the variability of coal strength into pillar design. The authors examine, through the presentation of case history data, i) typical industrial practice with respect to establishing a database of coal strength for use in pillar design and stability analysis, ii) approaches in which the observed variation in strength in the Harlan seam is integrated into pillar design and stability analyses, and to iii) propose standards for coal cube testing

Jan 1, 1993

By John L. Hill

Over 50 mines of the Appalachian and Illinois Basins are presently experiencing poor ground conditions believed to be caused by overlying stream valleys. The Bureau of Mines is conducting research into the causes of this problem with the aim of developing a predictive method for determining the safe limits of mining beneath stream valleys. In addition, the factors that control instability in these regions will be analyzed to provide design guidelines for adjusting support requirements. This paper presents the theorized causes of poor ground conditions beneath stream valleys and compares the theories with the results of a Bureau study of the Birchfield Mine No. 1 and a preliminary investigation of the Davidson Mine, both of southern West Virginia. The Birchfield Mine excavated four main entries beneath a 50-ft wide stream with only 50 ft of overburden. The valley flood plain is over 500 ft wide, flanked by valley wall slopes of 25 deg and relief of over 950 ft. In-mine monitoring of strata movement and detailed geologic mapping revealed that roof rock strength was greatest under high cover and weakest along outcrops and beneath the valley, with localized areas of decreases in rock mass quality. To improve the rock mass quality beneath the stream and reduce the threat of inundation, an extensive drilling program was undertaken to inject grout into the overburden from the surface. As a result, mining beneath the stream was practically routine from a ground control perspective, and water was not encountered until mining had advanced to a point beneath the floodplain that had not been grouted. The only ground control problems which were encountered were entirely a result of localized poor rock mass quality. In contrast to the Birchfield Mine, the Davidson Mine experienced failure of intact roof rock at a depth of 300 ft beneath a V-notch shaped valley with no change in rock mass quality. Finite element analysis was conducted to study the premining in situ stress at the two sites. The results are discussed within this paper and provide insight into the causes for the types of failure seen beneath valleys of varying shapes and depths.

Jan 1, 1988

By Vincent Scovazzo

Cost-effective underground mine configurations have been designed in highly fractured rock masses where the stone possesses a high material strength. A limestone mine in the eastern United States is used to demonstrate procedures for the geotechnical design of one such operation. The mine discussed is located in both limbs and the axis of a recumbent fold that fractured the rock mass. In a highly fractured rock mass, where the host rock has high strength, a design can be developed to alleviate intact rock failure. This is possible because in most cases as the stress levels needed for intact failures are approached, the stresses are quickly dissipated due to movement along discontinuities, which represent planes of weakness. As a result most underground failures in highly fractured rock are due to sliding or toppling. Even when intact rock fails in a highly fractured rock mass, it is the movement along an existing joint or other weakness plane that often initiates or causes this failure. It is therefore important to obtain the characteristics of weakness planes that are the discontinuities within the mass. The required information on weakness planes, geology, and strength of materials is discussed and recommendations for obtaining this information are presented. A discussion is provided on failure modes and the use of developed information for their evaluation. The impact on geotechnical design is demonstrated through pillar sizing procedures; however, this approach can also be applied to mine roof and pillar foundations.

Jan 1, 2002

By Guorui Feng, Min Zhang, Yi Luo, Jian Yang, Yujiang Zhang, Junwen Bai

"Safe mining of coal seam sandwiched between abandoned upper and lower room-and-pillared mines (ULPM) ·is increasingly becoming the focus of attention in recent years, which is closely related to the damage intensity of the upper and lower interburdens. The #7 seam with ULPM in Baijiazhuang coal mine was selected for this research. According to the mining and geological conditions, the vertical stress and plastic zone distributions were determined first by numerical simulation. Then, a theoretical model based on the elasti-plasticity theory was developed. The stress distribution and failure zone created in the interburden by mining the upper and lower seams were determined. Finally the feasibility of mining #7 seam between the abandoned upper and a lower seam was evaluated. The results show that: (1) the distributions of abutment pressure and plastic zone coincide, both of which can be used to evaluate the failure zone in ULPM. The failure zone penetrates the whole upper interburden, while only parts of the lower interburden fail. (2) the theoretical failure zone coincides with the maximum distribution zone of abutment pressure and plastic zone obtained by numerical simulation: the failure zone height in the lower interburden is less than its thickness, while that in the upper interburden is larger than its thickness. (3) the #7 seam is located in the failure zone created by the upper seam room-and-pillar mining, which leads to well-development fractures and blocky fragments in the roof, thereby it is unstabled. (4) when mining the #7 seam in the interburden, the key is to control the roof strata and proper support measures should be taken timely to ensure safe mining.INTRODUCTIONMany coal mines in China mine the thick, superior and easily mineable resources preferentially, while the thin, inferior and difficult-to-mine resources were abandoned (Chappells and Trentrnann, 2015; Zhenfu, 2012; Xu, et al., 2015; Guorui, 2009). As the results of deficient geological data and shortage of investment capital, the residual coal resources distributes in many mining areas in China.In recent years, many residual coal seams sandwiched between the abandoned upper and lower room-and-pillar mines (ULPM) were explored in Shanxi Province, especially in the mining area of Datang, Xishan and Luan (Figure.1). The reserves of residual coal seams in ULPM are considerable, increasingly becomes the focus of attention among scholars and engineers."

Jan 1, 2016

By Ernest A. Curth

An estimated 49 billion tons or 29 percent, of the coal reserve base to a depth of 1,000 feet in the eastern United States fall in the 28- to 42-inch range. Often left out as a consequence of selective mining, it is a paramarginal source that will become increasingly important. At present, all active thin seam longwalls are located in the Eastern Coal Province, and a number of operations have been suspended due to the erosion of the market for metallurgical coal. Extensive premining studies, appropriate mine planning, reliable equipment that is designed with ergonomics in mind, adequate roof support, intensive training, and strong face discipline are the ingredients that provided a measure of consistency in production for the last 10 to 16 years, particularly in the Pennsylvania bituminous coalfield where single drum shearers dominate. Plows, which were found to be very productive in suitable strata in Europe, are used in the Pocahontas field. Low seam four- and six-legged shields have been introduced recently. The future holds automation at progressive levels. The first step is batch control with the benefits of quickly applied roof support and reduction of workmen exposure to environmental hazards.

Jan 1, 1981

By Duk-Won Park

A vast majority of the operating longwall sections use shield-type face supports to provide ground control in the United States. As a co-operative research program between the University of Alabama (UA) and Jim Walter Resources, Inc.(JWR), a joint research program is underway to study interaction of shields with surrounding rock masses. A series of analyses on the response of shields are performed using monitored data at a longwall face in a deep coal mine. In this paper, various effects of parameters, such as leg pressure, setting load, developing load, maximum loading rate, pressure in capsule, and convergence between canopy and base, are discussed and recommended as stability indices. A microcomputer program is being developed for instantaneous interpretation of the shield monitored data, and for giving warnings of potential problems. The preliminary results define the effects of the setting load, developing load, and rate of developing load or leg closure rate. The objective of the study is to understand the shield characteristics and their interaction with surrounding strata in real time, then develop a reliable program for maintaining longwall stability.

Jan 1, 1992

By Mario G. Karfakis

Mining related subsidence is a major concern over abandoned shallow coal mines. Many of the subsidence prone areas are presently used or will be used in the future for residential housing development. Concern over mine subsidence is inhibiting the development of surface land uses on previously mined area. This concern prompted public officials to realize the need to develop guidelines which can be used to assess and reduce the risk of mine subsidence damage to building and other improvements. Subsidence and the associated ground movements must be defined before one can relate them to building damage. Surface movements resulting from underground coal mining occurs in two phases termed as: active subsidence and residual subsidence. Active subsidence refers to all surface movements occur in^ simultaneously with the mining operations, while residual subsidence is-that part of the surface deformation which occurs following the cessation of mining. Residual subsidence is the major concern over abandoned mine lands. The duration of residual subsidence is of particular importance from the standpoint of damage at the surface to structures built after the mine has been abandoned. Time spans during which surface subsidence may occur, vary markedly with the mining method being used. In total extraction methods, such as longwalls, subsidence is induced rapidly, beginning almost immediately after mining. Subsidence concurrent with coal extraction is desirable for a safe longwall mining operation. Usually subsidence is complete within a relatively short time. The remaining settlement occurs as residual subsidence due to the gradual compaction of the subsided ground. The relative proportion of the residual subsidence movements is dependent upon the presence of remnant pillars and roadside packs. rate of face advance and depth of workings. In partial extraction methods, such as with room-and-pillar systems. major occurrences of surface subsidence may be delayed for decades until the support pillars and/or the roof have substantially deteriorated and collapsed. The actual time involved depends on a number of factors such as the strength of coal, roof, and floor, extent of fracturing. presence of water, depth of workings, pillar size and percentage extraction. Consequently residual subsidence is more likely to occur over abandoned room-and-pillar mines. The following sections define subsidence over abandoned mines, review the proposed mechanisms, identify the controlling factors and examine the prediction methods.

Jan 1, 1987

By J. Riefenberg

U.S. Bureau of Mines (USBM) researchers are developing a personal computer-based hazard mapping system for use in underground coal mines. Hazard mapping is rapidly gaining interest as delineating areas of possible instability is becoming more complex. The development of a hazard map is based on the consensus that combined effects of local and regional stresses, roof, floor, and rib quality, geology, faulting, presence of ground water, etc., all contribute to stability of the ground. In order to demonstrate the concept, a hazard map was developed that combined stresses anticipated to occur in a multiseam setting using numerical modeling, and the newly developed Coal Mine Roof Rating (CMRR) where roof quality ratings in untested areas were modeled using geostatistical methods. Ten borescope readings from an underground mine were used to compute CMRR's for the geostatistical analysis. CMRR's for the mine ranged from 57 to 77, and are considered in the moderate to strong range. A hazard map was developed by combining the stress effects of interseam mining, scaled to weigh equally with CMRR's, and CMRR geostatistical analysis results. This hazard map demonstrates a synergistic effect between the mining-induced stress fields and the roof quality ratings, indicating, as expected, a greater hazard index in areas of high stress and lesser roof quality.

Jan 1, 1994

By Clifford J. Roblee

In this paper, various seismic indicators are evaluated for ability to discern fractures induced into a rock mass by blasting. This is accomplished by presenting results from in situ crosshole seismic studies performed prior to and after a sequence of controlled blasts within limestone and dolomite rock near Austin, TX. Indicators investigated are compression (P-) and shear (S-) wave travel times, first peak and maximum amplitudes, first arrival rise rate, and the linear spectrum of the received signal. Compression wave travel time yielded the most consistent results, while the various amplitude indicators yielded the poorest results for the equipment configuration utilized. In addition, this study documents magnitudes of change in seismic measurements which occurred following blasting. Both P- and S-wave travel times exhibited a wide range of variation due to blasting. Near blast centers below the surficial fractured rock, velocity decreases of about 10% were noted, significantly less than the 50% decrease observed for previously fractured near-surface rock through which the explosive energy was vented. Signal amplitude measurements also varied in magnitude but were less consistent.

Jan 1, 1989

By Ding Xu Chu

There are many methods developed for measuring the crustal stress in China. Among them only two methods have been adopted in practice, i.e., overcoring method (for which the sensors include: Piezo-magnetic stress gauge, steel-ring strain cell, 3-dimensional hollow-inclusion stress ,gauge, and Leeman's-like stress gauge) and hydraulic fracturing method. For the former method, one of those sensors is installed in a borehole, 36 mm in diameter, then prestressed, and finally overcored with a drill bit of 130 mm in diameter. After that the deformation of the borehole wall is measured, from which the in-situ stresses are determined. In order to avoid the errors resulting from the difference in elastic modulus for various kinds of rocks as well as from the installment of the cells, we have developed a technique to calibrate the sensor on-spot using a confining pressure calibrator and produced a very good result. For the later method, the measurements are achieved by isolating a certain section of a borehole and then injecting water to pressurize it till a tensile break is induced. It is identical to the typical technology used internationally. For many years, through field observations and a large number of laboratory experiments, we believe that the most efficient way of stress measurement for engineering purposes is the overcoring method using Piezo-magnetic stress gauge. It is characterized by stable and reliable measurement values, high accuracy and high adaptability. Taking the measurements in the intact rock mass as an example, the measured stress magnitudes are within accuracies of 8% and the errors in orientation are about 5 degrees. It can even be used perfectly in deep water. Up to now, we have performed in-situ stress measurements in more than 70 sites and obtain more than one thousand sets of data. The deepest depth used in the overcoring method is 80 m in vertical borehole and 780 m in tunnels.

Jan 1, 1987

By Dennis R. Dolinar

A room and pillar coal operation in central Pennsylvania was experiencing roof cutters and long running roof falls caused by high horizontal stresses. The roof conditions created hazards for the miners, and caused several production panels to he abandoned prematurely. The mine requested assistance from the National Institute for Occupational Safety and Health (NIOSH) in applying the "advance and relieve" mining to reduce the affects of the horizontal stress during panel development. The "advance and relieve" plan that was developed called for removing a pillar on one side of the panel as it was being advanced, thus creating a cave. The caving then relieved a portion of the horizontal stress across the panel. Because the horizontal stress direction is key to the success of this method, stress mapping as well as mining experience was used to establish the direction of the horizontal stress. Subsequent field measurements provided an estimate of the stress magnitude. Three panels were mined using this technique, and good roof conditions were achieved in all these panels. Instrumentation was used to monitor the stress changes in the roof created by the cave. Stress relief of over 1.000 psi was measured 120 ft from the cave and to depths of 20 ft into the roof. The lateral extent of the stress relief zone across the panels appears to be about 400 to 500 ft. This case history has provided much insight into the practical application of the "advance and relieve" method discussed in this paper.

Jan 1, 2000

By Brian E. Travis

Lightweight polymer grids have recently been introduced to the mining industry. They are currently being used for roof and rib control, longwall shield recovery, safety guarding or barricade, highwall screening, internal reinforcement of gunnite/shotcrete applications, sub-base reinforcement, construction of steepened slopes, retaining walls, mine reclamation (i.e. erosion control, turf reinforcement), and in numerous mining waste related projects. These polymer mining grids utilize a strong, lightweight polymer, usually special grades of polypropylene or polyethylene. High tensile strengths are achieved by molecular orientation of these polymers in the manufacturing process. The first step in the manufacturing process is to extrude the polymers into thick continuous sheets. These sheets are then punched, and drawn into open grid structures. Aperture size and configuration are controlled by the geometry of the punched hole and the degree to which the grids are stretched. The drawing process is the key to the molecular orientation required for the tensile strengths. These long chain polymer molecules in their natural state have a random orientation. When stretched, the molecules are pulled into a parallel alignment, thus creating a lightweight product with tensile strength comparable to steel. The finished polymer mining grid product is a viable alternative to conventional supplemental roof control meshes such as chain link fence and welded wire. Polymer mining grids are a fraction of the weight, have comparable load support qualities, and will not rust or corrode in even acidic or alkaline conditions. Specific flame retardant additives can also be incorporated to make the polymer grid self extinguishing for use in underground "gassy" environments.

Jan 1, 1992

By Yong-Ming Jiang

In cooperation with Cyprus Twentymile Coal Co., researchers from the National Institute for Occupational Safety and Health (NIOSI-I), Spokane Research Laboratory, conducted a study at the Foidel Creek Mine, an underground coal mine near Oak Creek, CO, to evaluate the stability of underground working conditions as a longwall advanced through a series of backfilled cross-panel entries. Instruments installed in the cross-panel entry pillars, backfill, longwall panel, and adjacent headgate entry were continuously monitored by a computerized data acquisition system as the longwall approached and advanced through the backfilled section of the panel. Data collected from these instruments were analyzed to determine the magnitude and direction of the secondary principal stress changes ahead of the longwall face, deformation and closure of the headgate entry, and stability of the cross-panel entry pillars and backfill. A three-dimensional, boundary element model was calibrated to accurately predict mining-induced stress changes and the distance they occurred ahead of the longwall face. As the longwall advanced through the hackfillcd entries, most of the mining-induced load was supported by the cross-panel entry pillars. The backlill provided stability for the root and Moor of the in-panel entries and also confined the entry pillars significantly improving their load-carrying capacity. Comprehensive information derived from backfill material property tests, instrument data, underground observations, and numeric modeling results document the safety and stability of the 8-Right minethrough and should provide beneficial case study information fur other longwall backfilling applications in the future.

Jan 1, 1998

The properties of coal or rock containing gas are very important both for studying coal and methane outburst mechanism and for controlling underground rock in mining. The theological fracture properties of coal containing gas were studied to obtain its rheological fracture principle both in microscopic scale and in macroscopic scale. The application of this principle is successful in explaining coal and methane outburst mechanism and for putting forward a rheological hypothesis of coal and methane outburst.

Jan 1, 1992

By Lewis A. Martin

Instrumented cable bolts developed at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health were used in conjunction with existing ground control systems to monitor rock mass loads at Tg Soda Ash's trona mine, Granger, WY. Axial and shear loads were determined to levels of strain gauge accuracy of ±5 N or ±5 microstrain. These gauges were embedded in a remanufactured king wire that replaced the conventional king wire. Cable bolt performance, quality of grout, and installation techniques were also assessed. By using instrumented cables, a mine operator can determine axial load along the cable at predefined gauge locations. These data provide necessary input for an operator to design a safer working environment for miners.

Jan 1, 2001

By Eric R. Bauer

Skin failures of roof and rib in underground coal mines continue to be a significant safety hazard for mine workers. Skin failures do not usually involve failure of the support systems but result from rock or coal spalling from between the support elements. For instance in 1997, over 800 miners were injured by roof and rib falls, of which 98 pet were the result of skin failures. Also, nearly 80 pct of the roof and rib failure injuries occurred at or near the working faces in development sections. The face area is a zone where the potential for skin failure accidents and injuries and roof and rib failures, in general, is high because of mining activity, ground readjustment due to changing stress conditions, and the higher exposure of mine workers. In addition, failures occur where the roof and rib are unsupported. This paper features a review of the roof and rib accident statistics resulting from skin failure, highlighting the incidences by average days lost, in-mine location, state, MSHA District, and worker activity at the time of injury. A detailed analysis of recent fatalities caused by skin failure is included. Also discussed are the causes of roof and rib skin failures, current methods and support materials for skin surface control, as well as a historical literature review of skin failures and control methods.

Jan 1, 1999