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By M. O. Serbousek

In 1975 the Bureau of Mines Spokane Research Center began a series of laboratory and field tests to investigate the feasibility of developing a substitute for the organic chemicals used in resin roof-bolting systems. An inorganic system, comprised of calcinated gypsum, an accelerator, and microcapsules of water was developed and patented by the Bureau in 1977. The system has several advantages including economics, non- toxicity, nonallergenic, inflammability, and is not based on petroleum products. Two roof-bolting systems utilizing inorganic cements are under development at the Spokane Research Center--a cartridge system, using the microcapsules of water and accelerator, and a slurry system, using a mix-inject method which combines dry powder cement with water for simultaneous injection into a predrilled roof-bolt hole. Several underground field tests have been conducted in eastern coal mines. Test results indicate that inorganic cartridge systems are adequately supporting mine roof in several test sections. Economic potential for the bolt systems appears favorable.

Jan 1, 1981

By David P. Conover

The effects of stress distribution on pillar stability were evaluated through rock mechanics studies conducted in three underground room-and-pillar coal mines. Results of the three field instrumentation programs are presented to illustrate the use of borehole pressure cells to determine vertical stress distribution in coal pillars at various stages of mine development. Vertical stresses and stress changes are determined using methods developed at the Bureau of Mines. Measured stress distributions are compared against theoretical distributions and possible explanations for observed behavior are discussed. Results indicate that stresses and stress changes calculated from pressure cell data are inconsistent with theoretical values; however, the data are useful for qualitative evaluation of trends over time. Calculated values are found to be sensitive to coal properties and initial setting pressures of the cells.

Jan 1, 1989

By Jinsheng Chen

The major functions of longwall headgate entries are to provide access ways to the outby submains or mains for coal transportation and to the longwall face for fresh air and for transporting men and material during longwall operations. Any roof control problems such as cutter roofs or roof falls in the outby headgate entries could result in significant production delays to the longwall face and may be a threat to miners' safety Under a thinly laminated immediate roof and a high horizontal stress field, even a longwall headgate entry that has an angle of less than 45 degree to the maximum horizontal stress may experience roof deterioration in the outby headgate if the longwall panel mining sequence and retreat direction are not properly considered. This paper attempts to analyze the causes of the roof control problems experienced in the longwall gate entries at our Cumberland and Emerald mines in Pennsylvania and to determine the optimal longwall panel mining sequence and retreat direction for the future mining area at Cumberland

Jan 1, 1998

By Uros Bajzelj

When driving mine roadways in the vicinity of tectonic disturbances and fault zones, sudden outbursts of gas, along with ejections of fine coal, occur. Substantial dynamic changes occur around the roadways in the coal and around the support. The problem has several aspects. Studies have been carried out on the technical analyses of the phenomena which occurred in the Velenje Lignite Mine from 1957 to 1988, The second part of the investigation involves systematic laboratory studies of strength and deformability characteristics of lignite samples and "in-situ" measurements of stress changes during the driving of some roadways. Analyses of events (using the model of limit convergence and the finite element method) showed the possibility of gas outbursts, taking into account the deformability conditions when crossing over from compact lignite to the disturbed region. The rate of advance of roadways and the method of support strongly influence the occurrence of the phenomenon. The preventative measures used in driving roadways under such conditions include trial drilling and the use of blasting instead of machine cutting.

Jan 1, 1992

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By Richard E. Thill

Two Cross-borehole systems were developed by the Bureau of Mines, one operating at a central frequency of 20 kHz for close-in spacing between boreholes (2-20m) and another operating at a dominant frequency between 1-2 kHz for more distant range (40-300+m) between boreholes. The systems are specially designed for the expected ranges and high resolution required in mining applications. Potential applications of the crosshole systems are presented for a variety of geophysical measurements related to underground mining problems.

Jan 1, 1989

By Klaus-Dieter Beck

RAG American Coal Holding, Inc. affiliates operate two longwall mines in the Pittsburg seam in Pennsylvania, the Cumberland mine and the Emerald mine. A unique situation in longwall operation occurred at these mines as the mining activities reached the property boundary between the two mines. The retreat direction in the longwall panel in Cumberland mine is West-to-East while in Emerald mine, it is East-to-West. This unique condition required an adequate design for barrier pillar between the two mines to minimizing the mutual effects of longwall retreat when both longwall faces met and an adequate roof control plan for gateroads. Field studies that involved mainly roof-to-floor convergence monitoring were conducted to assess the effectiveness of the employed barrier pillar in isolating the mutual effect of longwall retreat. Three-dimensional finite element numerical modeling was used to evaluate the gateroad roof and pillar stability. The modeling was conducted such that it duplicated the actual mining process as realistically as possible. The modeling results predicted that the employed barrier pillar and the roof control plan would ensure stable gateroad systems in both mines during and after the two longwall faces met. The prediction was confirmed because mining operations in the two longwall panels were completed without any noticeable ground control problems.

Jan 1, 2004

By Y. P. Chugh

About 60-70% of the underground mined coal in Illinois is extracted using a room-and-pillar mining method that permits extraction of about 50% of the coal. The remaining coal is left behind in the form of support pillars to control surface and subsurface movements. Illinois coal seams are typically associated with water sensitive weak floor strata whose thickness varies 0.31 - 1.3 m. The current mining geometry mostly uses pillars of constant size within a mining area. This traditional practice suffers from the disadvantage that pillars within the center region are subject to much higher loads than the pillars located on the outer edges of the panel. This results in a higher incidence of rib spalling, and roof falls, creating a potentially hazardous work environment. In addition, this concentration of loading at the center of the panel provides initiation points for floor failure to occur. The primary goal of this study was to reduce ground control problems and production costs in Illinois mines by considering alternate mining geometries. The development and subsequent field demonstration of an alternate geometry is presented in this paper.

Jan 1, 2004

By Bob Thomas

The roof drill operator productivity can be enhanced by the introduction of control technology The operators control station can be improved by considering human factors such as handle forces and control location. The smaller electrical controls can be located much easier than the traditional hands on valves. The operators control can he improved by using processors with programmable subroutines that assist the operator in the decision making process. For example to increase feed rate - to slow rotation - to avoid clogged steels - etc. The overall affect is to improve drilling and bolting consistency. The control system uses a dedicated processor packaged as an intrinsically safe control station. The roof drill operator is assisted in the drilling; arid bolting by a closed loop feedback system. The processor monitors the drilling parameters such as feed force, feed rate, rotation torque, RPM, drill position, and vacuum condition Based on the drilling parameters the processor executes pre-determined condition statements intended to enhance drilling The drill operator has an active display to alert him to the real time machine status. For example if the feed were to slow or delay due to a high vacuum level the display would show "high vacuum". The dedicated processor is available on the arm feed style machines, The prototype machines used programmable logic controllers to develop the feedback relations. The ability to record drilling parameters was extremely helpful in this development project. The extended use of the recording feature will aid operators in mapping the mine roofs Interpreting the recorded drilling parameters can reveal a void in the top or hard competent drilling The prototype machines also showed the importance of vacuum monitoring This feature is extremely valuable in softer materials, The benefits of the control system range from efficient use of the consumable bolting materials to uniform operator performance.

Jan 1, 1999

By Anthony Iannacchione

As the amount of new fractured surfaces or "damaged rock layers" within roof rock increases, the stability of the rock mass decreases. While direct measurements of this phenomenon are not easily made, there is good circumstantial evidence to support this hypothesis. For example, it is common to observe increased cracks or fractures in the immediate mine roof rock before a roof fall. Likewise, roof drill holes placed in areas that later fail often reveal increased numbers and/or separations of fractures within the rock column through time. And finally, the frequency of microseismic activity, representative of rock fracturing, increases before a roof fall. For this study, more than 700 microseismic emissions were collected from two underground limestone mine roof fall areas in southwestern Pennsylvania. Microseismic events were located and magnitudes determined using the moment magnitude technique. Moment magnitude is based on the event seismic moment, which is a measure of the seismic deformation. The amount of new fracture surface length was calculated based on the stored strain energy within the rock prior to fracture. In the two case studies presented, a significant amount of microseismic activity was observed as much as two days before the first signs of failure in the roof fall areas. Additionally, results from this analysis reveal much about the behavior of strata prone to failure and allows for the construction of hazard maps based on microseismic emissions. The potential use of this technique as a means of anticipating roof falls is analyzed and discussed.

Jan 1, 2004

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

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 M. Karmis

mining factors and representative ground movement parameters. Based on this analysis, the previously developed prediction methods were modified and tested for their applicability in the Appalachian coalfield. Finally, in order to facilitate the implementation of the prediction methods, as well as the data processing of the monitoring program, a number of computer software were developed and will be discussed in the paper. Surface deformations above underground mines and their prediction and control have been the topics of an extensive research effort by Virginia Polytechnic Institute and State University over the past few years. During the initial stages of this research, a number of case studies were collected from the literature, the mining industry and various government and state agencies and, based on the analysis of this information, appropriate prediction methods were proposed (Goodman, 1980; Webb, 1982; Hasenfus, 1984; Karmis et al., 1984). Although the prediction methods were based on sound concepts, it became evident that their accuracy and implementation were a reflection of the size and quality of the collected case studies. In this respect, the data bank was rather insufficient and posed certain questions regarding the development of subsidence information due to inappropriate length and spacing of monuments used in subsidence surveys, density and accuracy of such surveys, lack of monitoring horizontal movements, etc. In an effort to alleviate these problems, a comprehensive monitoring program was initiated in the coal mines of southwest Virginia, which included: - Two longwall and five room and pillar mines. - In total, seven longwall and sixteen room and pillar panels. - Monitoring lines in excess of 35,000 feet. - Vertical as well as horizontal measurements. A high accuracy digital computer tacheometer was used in all surveys and the type and spacing of all monuments were in accordance with well accepted guidelines. The data from this monitoring program completed the initially collected data bank and, thus, enabled a much needed statistical analysis between

Jan 1, 1987

During the last decade mechanical rock excavation has become increasingly widespread in the mining industry and, more in general, in excavation engineering. This technology, especially the tools and their operating mode, has developed according to empirical procedures and for this reason their validity should be ascertained by systematically re-examining the rock breakage processes. This topic has been approached in the framework of a joint research project involving the Universities of Cagliari (Italy) and Wollongong (Australia). The investigation has been carried out experimentally as well as with the methods of numerical modelling which is based on fracture mechanics principles. The first results of this integrated research led to the proposal of an interpretation of rock breakage by means of drag tools and to the identification of a basic correlation between the rake angle of the tool and the grain size of the rock fragments produced and, therefore, the energy efficiency of the operation.

Jan 1, 1990

By Amirhossein Molavi Tabrizi

In this paper we introduce a realistic model for mine openings that explicitly considers the effects of local irregular topography. Also, we consider the effect of entry orientation and rock anisotropy on the induced stress fields around mine openings under the given in-situ stress field. A triangular mesh formulation is derived for representing complicated irregular surface topography and underground openings. Each element contains three internal collocation nodes. Parts of the surface which are far from the mine openings are discretized by infinite elements. The rectangular infinite elements with three internal nodes are introduced specifically for this problem so they are compatible with the triangular elements. With the corresponding boundary element method (BEM) formulation, we then investigate the combined effect of the topography and entry orientation on the stress fields induced by openings in anisotropic rocks under in-situ stresses. A single rectangular opening in the anisotropic ground with topography and in-situ stresses is simulated using data from the Carroll Hollow mine in Ohio. The results of these simulations show that topography, rock anisotropy, orientation of the regional stress field and mine opening geometry all have appreciable effects on the stress fields around underground openings, yet these factors can all be accounted for in an efficient code with no need to discretize the model domain. Future applications using this new BEM formulation will provide useful guidance on selecting the optimal opening orientation in the mine in order to minimize undesirable stress concentrations around underground mine workings.

Jan 1, 2014

By William Kendall

"A tremendous amount of research, analysis, and engineering time is used to design a proper roof support system including the type, length, and location of the roof bolts. Unfortunately, since the roof bolts are being installed under unsupported roof, the exact location for the bolt holes cannot easily be determined and then marked onto the roof. The actual location where the roof bolts are installed is typically left to visual estimates by the operator. This frequently results in either (1) the operator overestimating the space between roof bolts and installing more bolts than required, thereby increasing cost and reducing efficiency; or (2) the operator underestimating the space between roof bolts and installing fewer bolts than required, thereby reducing safety, leading to potential roof failure or possible violations.A patented laser light grid system has been developed by J .H. Fletcher and Monitech, a South African Company, to aid in proper machine positioning in the heading and proper bolt spacing. The system can also show when an entry is over width, requiring additional roof support. The system is relatively simple and inexpensive. The concept is applicable across the full range of roof bolters from low to high seams and from single boom to multiple boom bolters. The system can be configured to meet most bolting patterns.The system consists of several very small 1 milliwatt, class 2, line type lasers that project a line onto the roof. The actual laser is 27mm in diameter x 47mm long (1"" x 1.8""). In South Africa it is approved as intrinsically safe and is mounted inside a 50mm diameter x 64mm long (2""x 2.5"") protective housing. In the USA, we are seeking an X/P approval and the laser will be mounted in a headlight enclosure 110mm diameter x 125mm long (4.3"" x 5""). Fifteen of the systems have been installed in South African coal mines with good results.The system has aided the mines in meeting the bolt spacing tolerance used in the Republic of South Africa, which is typically 15% of the nominal bolt spacing or 7 in for 4-ft bolt spacing. Fletcher has submitted the system to MSHA for approval in the US."

Jan 1, 2016

By Alan Campoli

The Eclipse System was developed by Minova USA Inc. formerly Fosroc Inc. to improve the performance of 5/8-inch deformed rebar fully grouted into a one inch borehole. An 1/8-inch offset bolt head combined with specially formulated Eclipse Lokset resin cartridge results in reduced gloving and increased mixing efficiency. The benefits of the Eclipse System have been demonstrated, under the supervision of MSHA, in Kentucky, West Virginia and Illinois coal mines in partnership with Excel Mining Systems, a leading roof bolt manufacturer. Over 100,000 units have been installed to date. Eclipse system bolts exposed during overcast construction at the Panther Mine of Cumberland Resources displayed an extremely well mixed, uniform resin anchorage over the entire bolt length.

Jan 1, 2003

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 Michael A. Berdine

In early 1997, Twentymile Coal Company took a dramatic departure from traditional longwall recovery methods. Previously, Twentymile's standard longwall meshing procedure included the use of chain link fencing material. Beginning with the 9-Right to 8-Right longwall recovery operation, Twentymile incorporated a polyester geogrid mesh system supplied by DYWIDAG-Systems International. This paper discusses the subsequent utilization of the polyester geogrid mesh system at Twentymile.

Jan 1, 2001

By Kikro Matsui

Longwall mining 1s more productive in comparison to other methods but requires particular conditions for its effective use This paper relates to longwall roof instability at Ikeshima Colliery In order to make clear the cause of the longwall roof instability, scientific investigation in terms of geomechanical factors and longwall support system was planned and carried out

Jan 1, 1997