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By Greg Hendon

Longwall developments in the UK have historically consisted of single entry gate roads. Adjacent developments were separated from existing panels by large barrier pillars (designed of sufficient width to get away from the longwall abutments of the previous panel) or by small barriers driven in the shadow, or de-stressed zone, of the previous panel. Some 2nd panel tailgates were also driven skin to skin leaving no coal barrier between the newly driven entry and the heavily supported existing gateroad. With the developement and wide acceptance of fully bolted entries and the pressure to reduce production costs, alternatives to single entry drivage, particularly yield pillar developments, were examined. Through the Rock Mechanics Branch of British Coal, a cooperative study was begun with Jim Walter Resources, Inc., USA, (JWR) to look at the yield pillar alternative in detail. This study was to determine the feasibility of utilizing yield pillars in the UK and to determine, through monitoring, the possibility of reducing stable pillar widths at JWR. The study has included extensive monitoring of the yield-stable-yield pillar system at JWR No. 7 Mine and an underground trial of a two entry yield pillar test area at Welbeck Colliery in the UK. This paper describes results from the JWR study and the subsequent results of the first advancing yield pillar development in the UK at Welbeck Colliery.

Jan 1, 1995

By Chao-jiong Hou

The necessity of using tensible rockbolt in the roadway with soft rock, high in-situ stresses, mining influence and other complex conditions was discussed, the structures of three kinds of tonsibte rockbolt and design principle were introduced in this paper. The tests of analogue material simulation under plane strain condition and underground industrial tests indicated that the effect of controlling deformation surrounding roadway by use of tensible rockbolt was good.

Jan 1, 1989

By Duk-Won Park

The excavation of a longwall panel causes a continuous stress redistribution in the surrounding strata for every face advancement. The understanding of the behavior of rock to high level of stresses is instrumental to the structural design of underground mines. In order to achieve a realistic analysis of the behavior of strata, the three-dimensional finite element method is frequently used. Also the model developed for the analysis has to be designed large enough to represent the influenced strata in the study area and a sufficient number of elements has to be used to provide detailed information on the stresses and displacements. Due to the size of the model and its numerical solution the CRAY X-MP/24 supercomputer has been used for this study. The mine site selected for the analysis is a coal mine located in the Black Warrior Basin in Alabama. The mine has two longwall sections at a depth of 2,000 feet with various dimensions of chain pillars, including yield pillars. Various research programs at this mine have been carried out by Park et. al. ( 1984 ). Therefore data were already available on the geology, and physical properties of rock and coal. In addition a rock testing program has been carried out to obtain rock mass classification parameters. The progressive failure technique has been utilized to simulate stress redistributions. The failure criterion after Hoek and Brown ( 1980 ) which is based on rock mass behavior was used for the iterative technique. Simulated stress redistributions are therefore very realistic. In order to automate the process involved in the application of progressive failure technique, a computer program has been developed. The combination of a large scale three-dimensional finite element model and the automation of progressive failure simulation provided a tool that can generate a large amount of information related to the behavior of rock and coal as a response to a longwall mining operation.

Jan 1, 1989

By Wolfgang Schott

The propagation velocity of in-seam seismic (ISS ) waves depends on frequency which is also known as dispersion. This dispersion is not only determined by the thickness of the coal seam and its dirt hand content (i.e. thickness and position), but also by the rock type and its condition (soft or compact) above and below the coal. A coal panel completely surrounded by roadways was investigated using the in-seam seismic transmission survey technique. Along the roadways the immediate roof was either sandstone and shale- The seam thickness varied between 1.20 m (sandstone in the roof) and 1.40 m (shale in the roof) and a fault system (normal faults) crossed the panel with a total throw of about the seam thickness. Below this coal seam another coal scam with a thickness of about 80 cm exists. Its interval shown by exploratory drilling changes from about 60 cm to more than 6 m (scam splitting). Tomographic inversion was applied to the dispersion of the measured ISS waves and velocity distributions were calculated at frequencies found to he most sensitive to changes in the seam thickness and the distance to the coal seam below. By correlating the velocity distributions with known values along the roadways approximate distributions of the coal seam thickness and the seam splitting within the panel were produced. The mining of the coal panel had already been finished and comparison was made between the survey results and the actual situation as far as it was documented. Prediction by the seismic survey was partly in good agreement with the actual held conditions or showed at least the same trends.

Jan 1, 2003

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 Axel Preusse

This paper shows basic considerations about the prognosis and prediction of surface subsidence and other ground movements in the German hardcoal industry. The following case studies describe experiences collected in projects between university and industry.

Jan 1, 2000

By Dennis Dolinar

In general, the direction of the maximum horizontal stress in the eastern United States is fairly well defined. However, the variation of the magnitudes of the horizontal stresses is not very well understood. Because the horizontal stresses cause severe ground control problems in underground coal and limestone mines throughout the eastern United States a more complete understanding of how the magnitudes vary would be useful for developing mine design strategies to combat horizontal stress related ground control problems. Therefore, in this National Institute for Occupational Safety and Health (NIOSH) study, the variation of the magnitude of the horizon stresses in sedimentary deposits in the eastern and Midwestern United States are examined with respect to two factors, the elastic modulus of the rock and the site depth. Stress measurements from thirty-seven sites are used in the evaluation. Examining the applied excess strains indicates that the eastern United States can be, separated into high and low strain zones. For most of the eastern United States the maximum applied excess or tectonic strain ranges from only 300 to 550 micro strains. However, there is one area, a portion of the Beckley seam in the central Appalachian region where the strains are significantly higher than the other regions. In this higher strain zone, the maximum applied tectonic strains range from 700 to 1,000 micro strains. Regression models for each zone based on the elastic modulus can explain between 83 to 85% of the variation of the maximum horizontal stress. Because one region, the northern Appalachian district, has strains that are about 20% higher than the other regions in the low strain zone, multiple strain models based on geographic regions were developed for the low strain zone that can explain 87 to 91% of the maximum horizontal stress variation with the elastic modulus. Depth was found not to be a significant causal factor in any increase in the horizontal stress even though the site depths ranged from 275 to 2,500 ft. Beyond a theoretical increase, based on Poisson's effect and gravity, no other increase in the horizontal stress with depth can be justified with this data. The most significant factor controlling the variation of the maximum horizontal stress is the elastic modulus of the rock, not the overburden depth.

Jan 1, 2003

By C. J. H. Brest van Kempen

The techniques developed should provide a useful tool, not only during the initial formulation of a suitable bolting plan for a new section, but also for periodic check¬ing on the utilization factor of hardware installed in the roof. Indications are that with present practice, utilization seldom exceeds 507.. Retro-fittable bolter equipment now available (FCBS?) allows doubling of the utilization factor in many cases. We are also introducing a computer pro¬gram based on the material outlined in this paper. This program can save a great deal of time in examining the real effect of implementing alternate bolting plans or using different installation techniques. It allows fine-tuning of bolt lengths, spacing and installation tension to mini¬mize roof support cost, while maximizing safe life of the mine opening. We have developed a procedure which allows easy, site-specific quantification of the need for reinforcement of a mine roof. The concept is based on the forma¬tion of a direct link between theory and empirical data to custom-fit each mine section. In its simplest form, the proce¬dure requires only recording of roof bolt torques in a sample working place (16 to 25 bolts) several times during the first week or so after initial bolting. The way in which the torque pattern changes during this time can reveal whether or not the bolts carry enough tension to maintain stability in that specific roof. From the same data, the nature of the anchorage horizon is derived. Specifically, shale or limestone can be distinguished from sandstone. If the procedure is repeated a few times (within the same mine section) with different bolt installation tensions, we can numerically characterize the roof in terms of one single value: an "effective angle of internal friction". This number can then serve to calculate trade-offs in bolting plan design, such as bolt length, bolt spacing and optimum bolt tension. Additional refinement is possible if a sagmeter is installed in the sample work¬ing place being monitored and if bolt anchorage capacity is determined. The lat¬ter can be done automatically on each bolt as it is being installed, using a system we developed and patented. Alternately, a number of pull tests could be performed. Using the sagmeter, bolt torque and anchor¬age data, it is possible to project the safe life of the supported opening. The details of the procedures described apply specifically to tensioned roof bolting, but a brief discussion paints the way for a similar analysis of full-column resin bolting also.

Jan 1, 1986

By Stephen C. Tadolini

The risk of injury caused by minor roof falls continues to increase in underground mining. Most ground control injuries result from small rock falls that occur in a supported area, but do not involve the failure of the support system. In the roof, these are the "skin failures" that occur between previously installed primary or secondary bolting systems. To minimize skin failures and the associated accidents, several devices were designed and tested in the laboratory to maximize stiffness, minimize material thickness and weight, and ease material storage and handling on roof bolting machines. The goal of this project was to design and evaluate devices that can be installed with traditional primary or secondary roof supports, and that arc easy to handle/install under temporary or previously supported roof.

Jan 1, 2001

By Hani Mitri

A new monitoring device called SAFEBOLT (Stress And Force Evaluator in Bolts) has been recently developed. The device SAFEBOLT, essentially replaces the load cell, it permits the monitoring of bolt axial load from the time of its installation underground. The SAFEBOLT system consists of a portable readout unit, connecting cable and the SAFEBOLT device, which is installed in the bolt head. SAFEBOLT can be installed on any industry standard rockbolt. No special installation method is required; the conventional method of bolt installation by the stoper or roofbolter remains unchanged. The device is completely encapsulated in the bolt head, which makes it resistant to corrosion, rugged, and free of any electrical wires. Pilot tests at three underground Quebec mines were carried out and helped improve the initial design of the device connector. The paper reports on two of such pilot tests.

Jan 1, 2001

By J. M. Girard

Ground control problems at surface mining operations can occur for a variety of reasons. Stress, gravity loading, rock strength, geology, pore pressure, weather effects, underground workings, and many other factors contribute to slope instabilities that range from small rock falls to massive slides of material. While some of these failures can he predicted or controlled by preventive measures, each year many completely unexpected failures occur. Current methods for monitoring generally involve measuring displacements at a few, selected points in and around the suspected area of instability. While most of the displacements along these points will be in a downslope direction, freeze-thaw cycles of water-filled joints, horizontal stresses or pressure, buoyancy in saturated soils, human measurement errors, or other situations can produce deformation in almost any direction, even without any instability in the slope. Determination of which, if any of the observed movements represents a potential hazard is essential.

Jan 1, 1998

By H. N. Maleki

This paper presents a field evaluation of six tailgate secondary support systems, with the objective of optimizing support efficiency for maintaining open gate roads. Support systems consisted of a combination of three wooden crib configurations, with either tension rebars or truss bolts. The effectiveness of the support systems was evaluated through measurements of ground movements, support load, and observations along 2000 linear ft of a tailgate. It was concluded that floor heave, crib width to height ratio, cribbing pattern, and support spacing have significant impact on tailgate stability and access after the passage of the first longwall face. Guidelines were suggested for selection of secondary support for other applications.

Jan 1, 1986

By Ross Seedsman

There are many models for how roof bolts behave, one of which is prevention of shear along bedding. The basis of this model is that a roof beam will not delaminate if bedding parallel shear can be prevented. A key point in such a model is that bedding discontinuities are present naturally in the roof and that mining induced stresses do not generate new surfaces. The presence of these discontinuities means that the rock mass must be modelled as a transversely isotropic material if continuum codes are used. A limit-equilibrium type design method for specifying roof bolts is being developed using voussoir beam and excess shear force concepts. The method incorporates much of what operators know intrinsically about roof reinforcement, and allows mining engineers to focus on the key uncertainty - the frequency and location of bedding surfaces in the immediate roof. Areas where more research is required are highlighted.

Jan 1, 2000

By Kanaan Hanna

The integrity factor approach was developed by the Bureau of nines to assess the stability of mine pillars and has been applied primarily to longwall chain pillars. Recently this approach was applied to a room-and-pillar mine in the Illinois Coal Basin. A special configuration of hydraulic borehole pressure cells was installed in a pillar to determine the in situ strength-and- stress relationship of the coal pillar and to verify the applicability of the integrity factor approach. Results indicate that the integrity factor approach is applicable to the assessment of pillar st6b:lity and nay bs a realistic and effective approach for room-and-pillar mine design.

Jan 1, 1988

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

By Andrew P. Schissler

Cyprus Shoshone is an underground coal mine in southern Wyoming owned by Cyprus Coal Company, a subsidiary of Cyprus Minerals Company. To date, seven longwall panels have been extracted from the pitching seam under increasing depth of cover from 150 ft to 1200 ft. The purpose of this paper is to describe the geology, rock mechanics needs identification, design, testing and successful application of the current headgate/tailgate cribbing system. The immediate roof of the Shoshone is unique in that the first 50 ft of lithology is interbedded mudstones. The mudstone is slickensided, highly friable and has characteristics of unconsolidated fill relative to self-spanning and beaming abilities. Finite element modelling analysis of the overall geomechanical environment showed the mudstone to yield under applied longwall abutment stress. Roof bolt system design methodology also pinpointed the relative weakness of the mudstone roof. In-mine entry vertical convergence and weight response from the mudstone identified the need to design a crib which had superior weight capacity over a wide range of convergence. Design of a structure ensued to achieve the superior strength properties of wood parallel to the grain and the displacement properties of wood perpendicular to the grain. Testing of the prototype structure along with other alternative wooden cribs was done at the U.S. Bureau of Reclamation facility in the Denver Federal Center, Colorado. The test results verified the design. Crib placement pattern was chosen to maximize support with a confined core crib under the high convergence area. The design has been successfully installed in over 5,000 ft of headgate entry in the 11 left panel.

Jan 1, 1993

The purpose and current practice of cable trusses in underground coal mines is briefly outlined and details of various cable truss designs are presented. The main part of the paper describes the in-situ performance of cable trusses installed as secondary supports in gateroads and recovery roads of longwall coalfaces. The performance analysis is based on the results of in-situ monitoring carried out at a number of sites at two underground coal mines within Australia. Truss performance has been assessed as load developed in relation to roof convergence, face position and time. Additionally, the influence of strata conditions, installation pattern/density, roadway layout and geometry have been taken into account. An analytical model has been developed to predict truss loads. The principal variables are listed and the basis of the model is described. Based upon behaviour observed in the field the model has been progressively upgraded and validated against measured performance. Guidelines on truss performance and factors which influence it are presented and the implications of these to the use of cable trusses is discussed. Possible methods for improving truss performance in certain strata conditions are outlined and some comments are made on the value of cable trussing compared to cable bolting in these situations,

Jan 1, 1992

By Eric Zahl

Researchers at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, Spokane, WA, have collaborated with three Western longwall coal mines in an ongoing effort to develop technologies that will aid in providing safe and stable working areas. The goal of the research described here is to develop a stress monitoring system that will provide immediate information to mine managers for making daily safety decisions as areas of poor ground are mined through. Initial work has focused on answering preliminary questions regarding the reliability and use of stress change patterns. Research is concentrated on monitoring horizontal stress because horizontal stress is transmitted over long distances through stiff strata, thus allowing an extended length of entry to be monitored. This paper presents an explanation of the concept, key results from field tests at two mine sites, and a proposed process for imple¬menting a monitoring system. System design layouts, instrument use, data collection and interpretation methods, and processes to present findings to mine staff are described. Additional validation and correlation with actual failure mechanisms are required before this approach can be recommended at ongoing operations. How¬ever, initial results indicate that this approach shows promise in mines prone to bumps and roof falls associated with large stress changes. In particular, analyses of measured changes in horizontal stresses appear useful.

Jan 1, 2002

By John C. Stankus

ldentifcation of potential roof problems in coal mines has long been a complex issue due to the wide variety of mining and geological conditions. It is well known that mine roof falls are related to many different factors such as roof strata properties, sandstone channels, regional and localized horizontal stress, vertical stress, and tectonic stress. Many times, a combination of these factors causes roof problems. Through years of ground control experiences, Keystone Mining Services LLC, (KMS) has developed a Roof Instability Rating (TUR) system to predict potential roof problems. The first RIR analysis was presented at the 19th Ground Control Conference (Watts, et al., 2000). The RIR system includes: quantifying each individual factor, weighting each factor, combining individual factors, finalizing the RIR, and setting the criteria for the areas where special support is needed. In this paper, the application of the Roof Instability Rating (RIR) system at Enlow Fork mine will be presented.

Jan 1, 2001

By Bret E. Leisemann

The geomechanical environment in which highwall mining must operate represents a hybrid between surface and underground operations. Mine design must take into consideration the complexities associated with unsupported spans, pillars with extreme aspect ratios and the instability related to the interaction between coal extraction and the highwall stability. The rock mass adjacent to the highwall, and for a significant distance in, is affected by lateral stress relief and blast damage. This paper describes a rigorous site investigation program aimed at providing some of the input parameters for the design of highwall mining layouts at the Moura Mine and focuses upon defining the nature and extent of the relaxed zone behind the highwall. The combination of borehole imaging and C factor analysis allows delineation of blast damage and relaxation of the rock mass immediately adjacent to the highwall excavation, as well as the more subtle relaxation effects deeper into the rock mass. This has led to the formulation of a proposed geomechanical model for the relaxed zone for direct input into highwall instability analysis and mine design.

Jan 1, 1993