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By Eric G. Zahl

Researchers at the Spokane Research Laboratory of the National institute for Occupational Safety and health are evaluating stress and displacement measurements in a longwall coal mine. The objective is to determine the best indicators of coal bump hazards during operations so that mine foremen and technical stall can make more informed intervention decisions to protect miners from coal bumps. Instruments were clustered in two panels, a yield pillar, and the immediate roof and floor of a two-entry longwall coal mine. Data were gathered continuously from a variety of instruments as mining proceeded toward and past the instrument cluster. Researchers and the mine's technical staff evaluated these data with respect to coal bumps and bump mechanisms. Selected graphs were then presented to mine foremen on a periodic basis to test their usefulness in identifying hazards. Of particular interest was the use of horizontal stress data from the immediate roof. To measure changes in horizontal stress, a new technique was devel¬oped for installing a grouted biaxial stressmeter in the roof. An evaluation of the development of this stress monitoring system in producing data beneficial to mine foremen is presented.

Jan 1, 2000

By Daniel W. H. Su

Surface subsidence induced by multiple-panel coal extraction was calculated with finite element stress analysis. The use of nonlinear material behavior and GAP elements, which provide a realistic representation of shearing along existing planes of weakness, allows the finite element program to accurately reproduce observed subsidence profiles. A reduction factor of 1/6 is applied to the measured rock strength and modulus for use in the finite element model. The models satisfactorily predict a significant change in maximum subsidence and subsidence profile associated with an increase from subcritical to near critical panel width. Modeled shearing along planes of weakness is in good agreement with available time domain reflectometry field observations and is demon¬strated to have a very profound effect on the maximum subsidence and the shape of the subsidence profile. When both material nonlinearity and planes of weakness are incorporated, the finite element model appears to possess great potential for accurately predicting surface subsidence and subsurface strata deformation in environments where little subsidence information is available.

Jan 1, 1990

By M. K. Larson

The U.S. Bureau of Mines and Cyprus Shoshone Coal Corp. conducted a study of deformation mechanisms around a longwall gate-road system in an underground coal mine. Of particular interest was roof deformation over the headgate entry. The strata above and below the coal seam are very weak, carbonaceous mudstones that have planes of weakness oriented along the bedding. Visual observations of previous gate roads over several years suggested that deformation was time dependent and resulted in creep along joints or crack growth. Instruments were installed at a test site to measure deformation around the entry, strew changes in the pillar, and loads transferred to support systems. Multipoint extensometers, instrumented bolts, instrumented trusses, biaxial stressmeters, borehole pressure cells, and single-point extensometers were installed. Monitoring continued until the longwall face passed the test site by 38.7 m (127 ft). Results show that fracture zones arched over the entry from both ribs, most bolts and trusses were loaded past their yield point, and the largest change in secondary principal stress was almost horizontal in the pillar. Displacement measurements in the roof showed classic primary creep.

Jan 1, 1995

By Leslie Carver

The effects of longwall mine subsidence on water resources were studied at a West Virginia coal mine (mine Z) by Carver (1994). Data were obtained for 137 domestic supply wells and springs and 12 baseflow streams. Subsidence caused short-term dewatering (of a few days to weeks) of wells located on lower hillslopes and valleys, and long-term dewatering of hillslope and hilltop springs (with no recovery yet). New or increased spring discharge was observed at lower elevations. Unsubsided water supplies experienced no dewatering beyond a nearby panel's 27°-38° angle of dewatering influence zone. Subsidence from longwall mining beneath streams commonly reduced discharge when panel ages were less than two to three years. However, "normal" discharge was typically observed over panels greater than three years in age; such streams exhibited lower baseflow recession constants, moderated winter baseflow, and increased summer baseflow compared to unsubsided streams.

Jan 1, 1994

By John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By Frank E. Chase

A massive pillar collapse occurs when undersized pillars fail and rapidly shed their load to adjacent pillars which in turn fail. This chain reaction-like failure may involve hundreds, even thousands, of pillars and the consequences have been catastrophic. One effect of a massive pillar collapse can be a powerful, destructive, and a potentially hazardous airblast. On eleven recent occasions, massive pillar collapses have occurred in six southern West Virginia coal mines. Two other instances of massive pillar collapses in U.S. mines have been documented in the literature. Research was conducted at four mines where massive pillar collapses occurred. Geotechnical evaluations of roof rock, coalbed, and floor conditions were made. Evidence indicates that in each case a massive and competent roof rock unit was able to bridge a relatively wide span, creating a pressure arch. Eventually, the pressure arch apparently broke down, and the structural characteristics of the pillar system were such that sudden, massive pillar failures occurred. Data collected at the failure sites also indicates that all the massive collapses occurred where the pillars width-to-height ratio was 3.0 or less. Numerical modeling, performed with a modified version of the MULSIM/NL computer program, supports the conclusions that the extent of the mined-out area, the bridging capability of the main roof, and the width-to-height ratio of the pillars are probably all significant factors in the occurrence of massive pillar failures.

Jan 1, 1994

By Thomas M. Barczak

The engineering of a support system is driven largely by economics. The goal is to provide effective support at minimal cost. For this reason, wood has been extensively used as a support material for centuries. The cost of timber for mining applications has risen by more than 50% in the past 5 years and is likely to continue to increase as the supply diminishes and further environmental restrictions are placed on timber harvesting. The increased cost and lack of high strength wood in the Western United States has provided incentives to improve the utilization of wood and to develop alternative support materials. This U.S. Bureau of Mines' (USBM) report discusses the basic engineering requirements for the application of various materials for supplemental roof support construction and provides an overview of support systems that use materials other than wood. A comparison of support performance and cost for various material constructions is made to assist the mine operator in support selection. The scope of materials discussed in the report include: (1) conventional mine timber loaded perpendicular to the grain, (2) parallel to the grain timber loading, (3) steel fiber reinforced and air entrained concrete, (4) steel, (5) granular fill materials, and (6) combinations of these as composite support structures. The range of support systems analyzed include wood and concrete cribs, concrete columns and props, steel beams, friction props, and several novel support concepts.

Jan 1, 1994

By Axel Preusse

To detect improper deformations of a roof bolted roadway in order to reduce inner mine damages in the most early stage and to initiate suitable countermeasures if necessary, several different measurement systems were used for monitoring a roadway's stability on the hardcoal mine "Auguste Victoria" (DSK AG). In this paper the different convergence measurement procedures are described and their respective results are introduced. Subsequently the results of the single measurement methods and procedures are correlated with each other.

Jan 1, 2000

By Erik C. Westman

The objective of this Bureau of Mines study is to determine the structural integrity and stress state of coal structure using seismic techniques. Bureau personnel conducted seismic transmission and refraction surveys on yield pillars and seismic tomography on rigid pillars. The pillars studied were in the headgate at increasing distances from the working face of a longwall coal mine, effectively evaluating pillars at lowering stress states and increasing levels of structural integrity. By examining characteristics of the seismic wave, including velocity and frequency, the survey results are used to infer yield zone thickness, core mss state, and stress distribution. An accurate definition of these parameters can not only provide mine engineers a useful tool for evaluating mine design, but also give valuable insight into the inherent safety of the mine structure.

Jan 1, 1993

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 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 Stephen C. Tadolini

The U.S. Bureau of Mines (USBM) is conducting research to provide alternatives for traditional secondary support methods. These cost-saving methods are proving to afford safer installation, improve entry stability, eradicate material-handling injuries associated with crib supports, and enhance ventilation. In cooperation with Mountain Coal Company, the USBM replaced timber cribbing with high- strength cable supports to provide stability in a gate road that was used for two longwall panels. The support design consisted of 2.1-m (7-ft) full-column resin-grouted bolts and 4.8-m- (16-ft-) long high-strength resin-grouted cable supports. Three support concepts were evaluated: passive, stiff, and tensioned systems. Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during panel extraction. The stress and loading histories for both panels and the abutment and yield pillars were monitored to assist in evaluating the stress transfer and pillar performance in conjunction with the roof behavior. The test results indicated that resin- grouted cable support systems successfully maintained the immediate and main roofs during the extraction of two longwall panels. This paper describes the theory, application, and advantages of resin-grouted cable supports and presents the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1994

By John W. Fredland

Conducting mining operations under a body of Water can create hazardous conditions for miners. The potential for a sudden inrush must be considered and abnormal ground control conditions can be encountered. This paper synopsizes the factors which mine operators should take into account when evaluating whether an inrush may occur. It addresses the applicable Federal regulations and discusses some items which coal operators should consider in applying for a permit, Through collecting and analyzing the pertinent information, the likelihood of encountering problems can be judged. Through proper mine planning, the potential for creating hazardous conditions can be mitigated.

Jan 1, 1982

By J. K. Whyatt

A comprehensive survey of mine seismicity and rock bursting during development of two sublevels at the Lucky Friday Mine, Mullan, ID, USA, was conducted to better define rock failure mechanisms and sources of ground control hazards. Survey data included rock burst damage reports, seismic event locations and magnitudes and, for the most energetic events, first-motion information. Several subsets of this database, including large seismic events, rock bursts, and microseismic activity near the face, were analyzed. Elements of the large-event and rock burst data sets were nearly independent. That is, there was no relationship between the risk posed by a seismic event and its energy, although such a relationship is well established for the mine as a whole. All data sets showed that certain geologic features appear to control the spatial distribution of events and the spatial distribution of rock burst risk. The data also suggest that. within the scope of this study, the greatest risk of injury occurs when a pocket of heightened rock burst risk is first encountered and that this risk is controlled when miners adapt their practices to these conditions. Recognition of the role of particular geologic features in the spatial distribution of rock burst hazards provides an opportunity for anticipating, rather than only reacting to, a changing level of rock burst hazard. However, much work will be required to make good on this promise.

Jan 1, 1998

By Andre Vervoort

The roof in coal room and pillar panels is normally stratified and can be thought of as beams or plates supported by the pillar sides in the roadways and by pillar corners in the intersections Underground measurements showed the effect of the face advance on the additional total movement of the roof skin and on the relative movement between various layers in the roof. The latter consists of the opening of bedding planes and of the lateral sliding of layers over one another. Both phenomena increase locally the axial load along full column anchors. Four models have been used to come to a better understanding of the roof behavior: a beam either clamped above the pillar side or laying on the pillar sides and a three-dimensional boundary clement model with and without bedding planes. The shape ref the curvature of the roof deflection is host approached by a clamped beam: the other models overestimate the movement at the sides. The largest roof deflection for a given Young's modulus is estimated by the model incorporating bedding planes. This model estimates that the bed separation occurs close to the face (within the first meter) and within 0.2 m of the sidewalk. In some simulations, openings are even recorded just ahead of the face. Further underground measurements should focus on these findings, as the effectiveness of the support is directly related to the amount of bed separation and sliding, which have occurred prior to the support installation and which still cart occur abler installation.

Jan 1, 2000

By Mictck Rataj

Methods of rock reinforcement in Australian coal mines are being advanced progressively through the better understanding of rock failure mechanisms around roadways and the-development of improved bolting hardware and installation techniques. It has been recognised that to improve the performance of rock reinforcement systems, the stiffness of the system as a whole has to be improved. New style bolts and accessories together with high pre-tensioned roof and cable bolts are being developed as a result. A number of the new techniques used to stabilise the roof, especially in weak roof/high horizontal stress areas have been site proven and are now currently used on a routine basis as a means of improving development rates and gate road stability during longwall retreat.

Jan 1, 1997

By P. E. Christensen

Due to ever-increasing stripping ratios associated with the No. 8 Seam at the San Juan Mine surface operation, BHP Minerals decided to implement longwall mining for the extraction of the substantial future reserve base. In preparation for longwall developments, BHP opted to first develop a small-scale pilot operation within the No.8 Seam on an adjacent lease to address geotechnical/geological concerns related to ground control, subsidence, caving, panel and entry design, and operational issues related to entry development and proper face equipment selection. Room-and-pillar mining methods are currently being utilized to develop a full extraction panel for a wide range of geological, geotechnical, and geomechanical evaluations aimed at characterizing and assessing the mineability of the No.8 Seam. Variable roof and floor conditions, coupled with a multi-bench coal seam, require that a number of mining horizons, opening geometries, and roof and rib support fixtures be evaluated relative to expediting gate and panel developments. Issues related to development of timely caves in areas subject to overlying sand channel deposits will also be addressed through underground in-seam stress and deformation measurements, and interburden and surface subsidence monitoring. This information will ultimately be used to evaluate face support and gate and panel pillar/entry development requirements. The current depositional model for the property, developed from an aggressive drilling, highwall mapping, and lineament evaluation program, will also be refined during the Pilot Mine study through extensive in-trine mapping, additional roof and floor drilling and physical property testing, and determination of the in situ principal horizontal stress field.

Jan 1, 1998

By John C. Stankus

In this paper, an innovative concept in longwall tailgate support is discussed. This new concept utilizes a new high capacity roof truss system in lieu of conventional wooden crib block. The first field test was conducted at Bailey Mine and was very successful. Similar tests have been installed in other mines and seams. An extensive instrumentation plan was designed and implemented in the truss test area. The test data has shown that the dynamic roof truss system effectively controls the various abutment pressures caused by longwall mining thus maintaining a stable tailgate entry.

Jan 1, 1994

By Noor Mohammad

Comprehensive numerical modelling of the subsidence associated with longwall mining in UK Coal Measures strata has been conducted and validated against UK data. The Rock Mass Classification System (RMR) was used as the main basis to derive representative in-situ rock mass properties for stiffness and strength parameters for the numerical modelling input. A Finite Difference Method, Fast Lagrangian Analysis of Continua (FLAC) has been used to design a suitable non¬linear model and simulate the behaviour of the caved zone with a unique modification of the post failure properties within the zone of influence. The established model was validated for different configurations, varying depth, panel width and extraction thickness, in a sensible and realistic range. The Subsidence Engineers Handbook (SEH) and a stress abutment model were used as a validation tool in this respect. The approach developed will be modified for application to a number of different International Coalfields with different geological environments.

Jan 1, 1997

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