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By Thomas P. Mucho

To document the performance of mine roof and roof support systems the U.S. Bureau of Mines (IJSBM) has been installing instrumentation at selected sites in U.S. coal mines. Much of this support and roof instrumentation has been installed in high stress conditions to maximize differences in performance. Summarized in this paper are the results of four such site investigations. The roof geology at the four sites is quite different and is quantified using the USBM's Coal Mine &f Rating (CMRR). The studies included detailed measurements of roof movement using multi-point extensometers, as well as measurements and monitoring of bolt loading. The investigations include developmental loading and abutment loading from longwall and room-and-pillar mining operations. Some of the issues examined are the effect of the horizontal stress field, the effect of installed tension (torque-tension versus resin bolts), the effect of reduced annulus bolt holes, and the differences between similar bolts supplied by different manufacturers.

Jan 1, 1995

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 Winston Gale

This paper is presents a summary of recent investigations into fracture and caving about longwall panels. The results of these investigations indicate that rock failure initiates well ahead of the longwall face. Rock fracture typically forms in response failure through the material and bedding planes. Tensile fractures also form in massive units. These fracture patterns typically create a fracture network which determines the caving characteristics encountered at the faceline. The action of longwall face supports under such conditions is to maintain confinement to the fractured ground and develop a consistent caving line, The confinement developed above the canopy under these conditions can be variable on a shear by shear basis and the operational face support procedures play an important role in stability about the face area.

Jan 1, 2004

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 Olayemi Akinkugbe

As coal production in the United States continues to increase, the availability of unexploited or virgin fields continue to diminish. As a result, mine operators are forced to mine in unfavorable or multiple seam conditions. The safe, productive exploitation of coal in multiple-scam conditions requires specific design technology. At present, there are two general design approaches that the mining engineer can use to analyze the multiple-seam problem in question, either simple empirical relationships or complex numerical models. Often, the empirical approaches are too general for the specific problem and the numerical models require too much time and expertise for the practicing engineer. This paper introduces a new computer program, Lam2D, designed specifically to enhance multiple seam mining operation by quickly and easily calculating, stress, displacement, subsidence, horizontal strain and safety factors associated with multiple seam mining situations. The program implements a simplified, two-dimensional boundary-element method in order to model the complex multiple-seam stress and displacement interactions. Most importantly, the program is greatly simplified by incorporating automatic overburden, interburden, coal and gob property generation thereby reducing typical modeling and solution time to a few minutes.

Jan 1, 2004

By Karl Brandt

Longwall panel 580 has recently been retreated in the Zollverein 1/2 seam at a depth of approximately 1000m at Auguste Victoria mine with rectangular rockbolted roadways used for both the maingate and tailgate. Although Auguste Victoria mine has been increasingly using rockbolted support since the mid 1990's. this has previously been restricted to tailgates for single use. This panel represents the first use of a rectangular rockbolted roadway as a maingate at Auguste Victoria mine. As such, it represents a significant change in support strategy. As part of the strategy numerical modelling was used to examine expected behaviour of rockbolted roadways at the site, stress changes during face retreat, the feasibility of adopting rockbolted support for the maingate and the reinforcement pattern required. Monitoring instrumentation was installed in both gates to confirm that the support systems were performing satisfactorily as the gates were driven and as the face was retreated. Although the panel is isolated from others in the Zollverein 1/2 seam, markedly different roadway conditions have been experienced in the two gate roads. Favourable conditions were obtained in the maingate, but particularly difficult conditions were encountered in the tailgate. Previous work for the mine had examined interaction with old workings in other seams and shown a vertical stress of 28MPa for the maingate and up to 45MPa for the tailgate. The values from the earlier work were used in the modelling work described, which showed contrasting results given the different stresses. The difference between the gates can be attributed to the interaction. The paper describes the numerical modelling work, the reinforcement systems adopted, the monitoring results, the interaction effects and the comparison between expectations from the numerical modelling results and the conditions actually experienced.

Jan 1, 2002

By David Bigby

A remote reading dual height telltale system has been developed to provide mine management with early warning of impending roof failure. This system is a logical development from the visual dual height and triple height telltales which are increasingly being used worldwide for routine monitoring of mine roof. The paper describes the principles and history of telltales from their origins in France, their refinement and routine use in the UK and their subsequent spread around the world. They are now being used in various forms in USA, Canada, Germany, Spitzbergen (Norway), Poland, Ukraine, Russia, Australia, South Africa, Mexico, India, Egypt and China. The remote reading telltale system has coal mine intrinsic safety approval in Europe and the United States (MSHA) and the first commercial system was installed in a German colliery at the beginning of the year. Up to 100 dual height telltales can be connected, in a simple "daisy chain" configuration, to each of 4 underground interrogation units. These are connected through a twisted pair link to a surface computer which displays the readings, updates the database, generates warnings and provides the user interface. The paper describes the features and principles of operation of the system which differs considerably from conventional mine-wide monitoring systems. A European Community funded research programme is currently underway with the aim of developing improved alarm generating algorithms including evaluation of interfacing a neural network to the system. The paper concludes by exploring other potential applications of the system for ground control monitoring currently in development, including extensometry, stope closure monitoring and support load measurement.

Jan 1, 2001

By John Cassie

The condition of roadways in the Deep Main seam at Asfordby mine, operated by RJB Mining (UK) Ltd, is strongly dependant on drivage direction relative to the maximum horizontal stress. The mine is laid out with gate roads driven approximately parallel to the maximum horizontal stress giving favourable drivage conditions. This has led to particulary difficult conditions for face installation headings which have to be driven to an increased width and at a high angle to the horizontal stress. A solution to the formation of face headings is described making use of stress relief with the face heading being driven in the zone of reduced stress surrounding a previous heading. In implementing this solution use was made of results from computer modelling and detailed monitoring of roadway behaviour. High reinforcement densities including cable bolts were used in supporting the headings. The experience provides an example of a traditional technique being applied more scientifically. The results obtained demonstrate the effect of stress relief and reinforcement methods in altering drivage conditions and suggest possible alternative strategies for the formation of future face headings in these conditions.

Jan 1, 1997

By E. H. R. Schumann

The local stratigraphy and composition of the coal measures, including competent dolerite sill and massive sandstone lavers is shown to have a restricting influence on the development and magnitude of surface subsidence above total extraction panels in South Africa. The critical panel width necessary to induce nonviolent failure of a dolerite sill is quantified. Small subsidence due to the deflection of a 70 metre thick dolerite above subcritical panels is shown by example. Maximum surface subsidence tilt an; ground strain is compared to the NCB model. An exponential subsidence profile curve, SPC, describes a trough half-profile. A parabolic relationship between horizontal displacement and tilt, which has been discovered to exist can be substituted in the first derivative of the SPC in order to generate transverse displacement and strain profiles.

Jan 1, 1988

By Robert C. Garson

The Mine Roof Simulator at the DOE'S Mining Equipment Test Facility is described. Its use in the research and development on the behavior and response of longwall roof supports to simulated mine environment is investigated. Advanced technology analysis methods such as finite element modeling, strain gage instrumentation, and photoelastic techniques are investigated for application in roof support design two case studies are presented of test data and results.

Jan 1, 1982

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

By Patrick S. Artrip

A deep mining operation in the Lower Kelly (Imboden) seam in Wise County, Virginia experienced severe ground control conditions in its attempt to develop reserves situated under increasingly higher cover conditions. Ultimately one of only three mines capable of providing access to a large reserve area had to be abandoned. A thorough geotechnical exploration program was carried out to allow characterization of the critical roof and floor rock strata in accordance with the modified geomechanics classification system. The principal roof and floor rock stratigraphic sequence types were defined and mapped throughout the reserve area along with other critical geological factors. A ground control study based on the geotechnical findings provided a quantitative analysis of the causes of previous entry instability. Further analysis of pillar and entry stability identified configurations which would remain stable for the variety of strata types and range of overburden depths within the remainder of the reserve. Mine designs based on the geotechnical evaluation have, to-date, provided the stability required for long-term future development - of the reserve area. However, some areas of difficult ground were encountered which could not be anticipated with the drill data available at the time of the original study.

Jan 1, 1993

By N. P. Kripakov

The U.S. Bureau of Mines (USBM) is conducting research to develop a practical computer-based tool that will allow coal mine planners to anticipate rock mass behavior surrounding mine entries prior to actual mining. More specifically, this tool estimates to what degree and extent the immediate roof and floor strata and the coal pillar ribs surrounding an opening will weaken or fail for a given set of mining parameters. The technical approach couples together two numerical techniques: the boundary-element method and the finite-element method to allow for an approximate three-dimensional simulation of actual mining situations. Outputs from plan-view, linear-elastic, boundary-element models executed at different mining stages are used as input to a detailed, section-view, finite-element model to estimate the degree and extent of failure expected to occur around the periphery of an entry system. Using a pseudo-elastic approach. elastic rock mass properties are continually updated as different zones are predicted to fail. The procedure is relatively easy to use and is being set up to work interactively with the user who will not be expected to be a computer expert. Inputs include easily understood parameters obtainable from the field and rock mechanics laboratories. Examples of computer-generated output from generic sample models representing typical mining situations are presented and results discussed.

Jan 1, 1994

By K. Y. Haramy

Deep coal mines with strong roof and floor strata frequently encounter face and rib bursts. The burst problem becomes more severe with increased depth. While the exact causes of bursts are often difficult to determine, localized high stress zones are common to their occurrence. This Bureau of Mines report describes a study at a mine using longwall systems that experienced both face and floor bursts. The study correlates shield loading, entry closure, and forward abutment pressure increases to observed burst occurrences. The results indicate that strong roof strata that overhang behind the longwall supports create additional abutment stress. When caving lags behind the face and the abutment zone does not advance ahead of the face during mining, stresses on the face may increase to a critical level, resulting in a burst. The critical level occurs when the stress in the abutment zone exceeds the ability of the coal and/or adjacent strata to store strain energy. Caving characteristics may be improved by inducing fractures in the roof or by providing sufficient support resistance to shear the roof. Data analysis of a major burst indicated a dramatic increase in floor heave, with a maximum occurring 80 to 100 ft inby the face, and a lag in the cave line behind the face supports. Analysis of the effects of caving characteristics and floor heave on burst occurrences is presented.

Jan 1, 1987

By Liang Yinhuai

The hydraulic-stowing mining method is most effective for mining thick coal seams. It is superior to the sliced cover-caving and artificial mining method and the fully mechanized sublevel-caving mining method for the following reasons - a much higher recovery rate is achieved, there is much less surface subsidence and roof pressure, there is no hidden danger of gob fire, and much less methane emission and coal dust problems, etc.. However, its superiority is overshadowed by its backward stowing technique which makes mechanisation of mining and stowing difficult, causes bleed water interference to production and to the environment and is uneconomic because of the high cost of stowing material. These disadvantages limited its application to coal seams that are difficult and not suitable to be mined by other mining methods. The remedy is in reforming it thoroughly to permit mining of otherwise unminable seams and this paper presents the reformation.

Jan 1, 1992

By D. W. H. Su

A two-year study was conducted by CONSOL in a northern West Virginia coal mine to evaluate pillar design and support requirements to improve roof control in right-hand longwall headgates subject to high in situ horizontal stresses. Horizontal stress measurements conducted within undisturbed areas of the subject mine revealed a pre-existing regional horizontal stress field with a significant difference in magnitude between the east-west maximum and the north-south minimum stresses. Three¬dimensional stress cells installed to monitor horizontal stress concentration in the headgate roof during mining revealed that a narrow (60-foot center) headgate pillar reduces the east-west horizontal stress concentration in the roof ahead of the face such that the headgate is more stable and requires less support. This 60 foot x 140-foot pillar design, which has been employed in subsequent panels, still provides adequate stress attenuation to protect future tailgates under a maximum cover depth of 1000 feet. Roof geology derived from underground roof reconnaissance and underground bolt load and roof displacement measurements confirmed the effectiveness of 12-foot long cable bolts for controlling the headgate roof subject to high horizontal stresses. With the new headgate pillar design and the supplemental cable bolts, headgate roof conditions in the subsequent panels improved significantly, longwall production delay due to adverse headgate roof condition was totally eliminated, and record coal production was achieved by the subject mine in the subsequent years.

Jan 1, 2003

By Daniel W. H. Su

This paper presents the results of an extensive geomechanical testing and monitoring program conducted at a longwall panel. The field program included the monitoring of roof caving sequence and mechanism, surface subsidence, change of pillar stress, roof displacement and entry convergence, and strength characterization of the overburden rocks. The response of eight headgate pillars and the adjacent entries was monitored as the longwall face approached and passed the instrumented locations. Overburden response to longwall mining was monitored using time domain reflectometry (TDR) technology; and, the height of the highly fractured zone above the gob was determined by postmining drilling. Good correlation was found between the height of the highly-fractured zone and the TDR, subsidence, and pillar stress data. The results provide a detailed picture of the response of overburden strata and chain pillars to longwall mining. A preliminary model representing the overburden strata behavior at the test site in response to the longwall mining is proposed.

Jan 1, 1987

By D. W. H. Su

This paper describes geomechanical criteria employed by Consol Energy for selecting mine-specific longwall face supports in the past decade. The criteria include immediate and main roof rock characteristics, floor rock characteristics, critical tip-to-face distance and shield operating range. Detailed roof rock properties from high-density core holes as well as in-mine scope holes are used to evaluate the set load density and shield stiffness required to control the immediate roof and the yield load density required to absorb the load density developed from main roof weighting. Detailed floor rock properties from core holes and in-mine floor bearing capacity tests are used to specify the maximum peak toe pressure as calculated by the Jackson method. The critical tip-to-face distance is evaluated based on the immediate roof rock strength, layering and thickness. The shield operating range is evaluated based on main bench thickness, the presence of thick draw slate, the potential of minor roof falls at the face and transportation restriction. Three case examples representing three different geologic conditions are presented to illustrate the successful application of these geomechanical criteria for selecting longwall face supports within Consol Energy in the past decade.

Jan 1, 2004

By Chris Haycocks

The relevant literature concerning ground control, mine planning and reserve conservation in multi- seam mining is reviewed. Factors affecting interaction, interaction mechanisms, and design guidelines are presented in detail. The impact of multi-seam mining on future mining, measures for ameliorating negative interaction effects, and future research needs are also discussed.

Jan 1, 1990

By P. K. Kaiser

The purpose of deformation monitoring for the evaluation of the stability of underground openings is reviewed briefly and some results from laboratory tests of small tunnels are presented. The ground behaviour is described by comparison with the response of structural systems and illustrated by the data from the testing of small tunnels. It is concluded that the main objective of a deformation monitoring system for stability evaluation is to determine the actual process of deformation and instability in the area surrounding the opening. Only when this process has been established is it possible to use deformation measurements to design support systems or to improve construction or mining procedures. Present monitoring practice is not sufficiently orientated toward the aim of process description.

Jan 1, 1981