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By Chris Adams

The application of pre-tensioned tendon roof support assisted Harworth Colliery in their revival, from impending closure to lowest cost deep-mined producer in the UK, over just six months last year. The paper describes this success story, how it came about, the reasons "why", and the method "how"

Jan 1, 2003

The U.S. Bureau of Mines (USBM) has developed the coal Mine Roof Rating (CMRR) to aid in ground control. This paper describes the application of the CMRR at four coal mines operated by a major U.S. coal company. The mines are widely separated geographically, and cover a wide variety of geologic conditions and mining methods. The study found that the CMRR was a useful predictor of roof performance. In mines where the CMRR varied, and other parameters held constant, roof falls were associated with low CMRR values. Important similarities in roof performance were found in mines with similar CMRR values, despite radically different geologic environments. The analyses also indicated that statistics can be used to evaluate ground control performance. For these mines, a "roof fall probability" can be estimated from the CMRR and the intersection span. A spreadsheet program has been developed to facilitate treatment of CMRR data, and it is briefly describe

Jan 1, 1994

By Ken Mills

This paper describes the first successful use of hydraulic fracturing to induce caving events "on demand" in Australia. Moonee Colliery operate a longwall immediately below a thick conglomerate strata. This strata temporarily bridges across the extracted longwall panel to create a large area of standing goal' When this standing goat' eventually collapses, the windblast generated presents a significant hazard to men working on and around the longwall face. Hydraulic Fracturing has been successfully introduced to take control of the timing of these caving events so as to eliminate the risk of windblast injury, The longwall face area is completely evacuated during the treatment. Water is pumped into an injection point located in the conglomerate strata above the standing goat. A horizontal fracture is generated and grows outward from the in point, separating the conglomerate strata below the fracture horizon_ At some point the strata can no longer span and a goat fall is initiated. After a treatment, mining can be recommenced with the windblast hazard eliminated.

Jan 1, 2000

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 J. D. Cole

In order to reduce the number of longwall face moves and to increase the rate of gateroad construction, super longwall panels, 1000 feet wide and 12000 to 14000 feet long, are planned for Wolf Creek Collieries No. 4 mine. The development of the longwall panels at the mine has evolved from 5-entry, to 4-entry, to 3-entry systems. A three-entry system including pillar size and entry supporting method for the super longwall panels was designed for long-term stability. The design procedures included field observation and studies, laboratory properties tests, in-situ stress measurement, and computer analysis. Three pillar design methods consisting of PILLAR, ALPS, and the Wilson Method, and one entry supporting method, ROOFBOLT, were adopted in this study. The procedures, methods, and results are discussed in detail.

Jan 1, 1990

By Wm. Mark Hart

Under a specific geological condition, roof supporting method, and pillar-entry system, an entry convergence concept may be the most effective means that can be used to effectively indicate the longwall entry stability (i.e. pillar bumps, roof falls, or floor beaver This concept may also be used to verify the effectiveness of the current pillar-entry system and roof supporting method. Furthermore, convergence measured under different mining phases may lead to a better understanding of the longwall mining-induced abutment loads and may be used to estimate stress change in pillars and thus pillar stability. In this paper, a remote entry activity monitoring system was designed; a comprehensive entry convergence monitoring strategy was proposed; thee abutment loading phases in longwall mining were defined; the relationship between roof deflection and stress change in a pillar was derived, and finally the extent and magnitude of the mining-induced abutment loads as well as their impact on entry convergence were discussed using the field data obtained so far.

Jan 1, 1995

By Gexin Sun

This paper overviews the recent research progress in predicting cutting performance using fracture mechanics principles. It is emphasised that rock fragmentation due to cutting is mainly a process of chip formation resulting from crack initiation and propagation or crack interaction for multi-tool cutting. This process can be simulated by using the well developed fracture mechanics criteria coupled to numerical methods. Fracture toughness is shown to be an important and useful parameter in quantifying cutting performance and in designing cutting tools. The paper concludes with discussions on the dynamic effects involved in rock cutting processes which should be taken into account for optimum cutting efficiency and tool design.

Jan 1, 1992

By Thomas R. Scotese

A rock mechanics instrumentation and monitoring program was implemented during pillar extraction at Gulf Mineral Resources' Mt. Taylor Mine, the deepest uranium mine in the U.S. Three types of monitoring were employed: (1) drift convergence around stopes (using a portable tube extensometer), (2) stress changes in pillars (using vibrating wire stressmeters in horizontal boreholes), and (3) load changes in haulage ways under stopes (using vibrating wire load cells in jack stands). Results include convergence-time graphs, convergence contour maps, stress-time graphs, a stress increase contour map, load-time graphs, and abutment load limit maps. Major factors influencing results include (1) size of pillars around the extraction area, and (2) presence and orientation of a major fault adjacent to one extraction area. These factors, combined with pillar extraction, produced evidence of load transfer over 120 m from a stope measuring only 55 m x 45 m. Vertical stress increases of 7 to 17 MPa (1000 to 2500 psi) within 60 m of the stope were obtained using the stress- meter manufacturer's method. The fault apparently prevented load transfer in the down-dip direction but increased load transfer in the up-dip direction. Convergence rates of up to 1.3 cm/day (0.5 in/day) were measured at two stations. Instruments were generally reliable despite adverse underground conditions. The instrumentation and monitoring program at Mt. Taylor provided a warning system against ground control problems and a characterization of ground behavior for future development and extraction of pillars. Recommendations are made for future mining and instrumentation practices.

Jan 1, 1984

By Mikko Ahola

This paper describes recent innovative analyses conducted by the Bureau of Mines to demonstrate the application of the boundary-element method in evaluating the effectiveness of caving chambers or sacrificial entries to relieve the stress conditions initiating cutter roof failure in underground coal mines. The analysis assumes that cutter roof failure results when the shear strength of the roof rock at an entry corner is exceeded by the applied shear stress. The applied shear stress is due to large differences between the horizontal and vertical in situ stresses known to exist in many coal mining districts experiencing cutter roof failure. The results of boundary-element analysis suggest that driving an arched lead entry in advance of adjacent rectangular entries and allowing it to cave significantly reduces the difference between the principal stresses at the corner of the rectangular entry, thereby lowering the maximum shear stress. Slippage of the immediate roof along horizontal bedding planes into the caved entry is shown to further reduce the stress conditions in adjacent entries. Practical applications of this stress control technique to longwall and room-and-pillar mining are discussed.

Jan 1, 1987

By Gregory M. Molinda

An investigation was conducted to determine the nature and frequency of coal mine roof failure beneath valleys. A mechanism for this failure, and suggestions for controlling this problem are presented. Hazardous roof conditions identified in a number of mines were positively correlated with mining activities beneath stream valleys. Mine maps with overlays of unstable roof and locations of stream valleys show that 52% of the instances of all unstable roof in the surveyed mines occurred directly beneath the bottom-most part of the valley. The survey also showed that broad, flat-bottomed valleys were more likely to be sites of hazardous roof than narrow-bottomed valleys. Evidence of valley stress relief was found beneath a number of valleys in the form of bedding-plane faults and low-angle thrust faults. This type of failure, previously believed to be only a shallow phenomenon, was found at mining depths as great as 300 ft. In situ horizontal stress measurements in a mine beneath a valley and the adjacent hillsides confirmed valley stress re1ief. Underground mapping showed that roof falls in excess of 100 ft in length and up to 25 ft high closely aligned with the valley axis. Numerical analysis of 13 valleys overlying one mine property showed the effect of the valley excavation on horizontal and vertical stress. Thickness of cover, valley shape, and the orientation of the valley relative to the maximum regional horizontal stress all influence the "valley effect" on roof stability.

Jan 1, 1990

By Yong-Ming Jiang

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

Jan 1, 1998

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

By G. K. Rogers

Portals, which are the surface entrances to underground excavations, are frequently overlooked and often difficult areas In terms of ground control. Failures commonly occur In the typically high angle approach cuts and Initial subsurface portion of the portal. In order to permit rigorous design of all types of portal structures, failures are Identified and subsequently classified. Based on the failure classification and suggested analysis methodology, optimum ground and support loading can be determined.

Jan 1, 1988

By Ian W. Farmer

A description of the factors causing outbursts and rockbursts in coal mines is given. The mechanics of both outbursts and rockbursts can be described in terms of conversion of stored strain energy in the coal and surrounding rocks into kinetic energy through failure. The major difference between the two phenomena is that gases adsorbed onto the coal structure and desorbed during deformation contribute to the energy available during outbursts and may also affect the loading and unloading characteristics associated with failure. The amount of energy involved and the type of phenomenon are related principally to gas content depth and rock behavior.

Jan 1, 1988

By Brian Clifford

Thoresby Colliery is in the process of recovering an old roadway that was flooded following the closure of an adjacent mine. The aim is to re-use the roadway as a gate road for a longwall retreat panel and it is therefore important to establish the condition of the existing rockbolt support during the recovery operation. Two NDT methods, ultrasonics and RMT's newly developed RF (Radio Frequency) system were used in combination to interrogate the rockbolts to determine their condition. This paper describes the condition of the rockbolt supports, the NDT methods used and the problems that had to be overcome to successfully apply the two methods.

Jan 1, 2002

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 Gregory M. Molinda

The U.S. Bureau of Mines has developed a rock mass classification system for coal mine roof control. The Coal Mine Roof Rating System (CMRR) evaluates the structural competence of mine roof using simple field tests and observations obtained from underground exposures in roof falls or overcasts. The basis of the WRR is a weighing system which converts descriptive geologic roof rock data to a numerical value, facilitating its use in engineering design. A full suite of data collection and hand calculation sheets accompanies the CHRR A data base is currently being collected from all major coal basins in the United States to validate the CMW. To date, CHRR values are available from 96 roof exposures in 59 coal mines representing these basins. The system can be used to help solve many practical ground control problems, including longwall gate road design, roof support selection, and decisions regarding extended cut length.

Jan 1, 1993

By Thomas L. Vandergrift

With the inevitable expansion of homes, businesses, and infrastructure in coal mining regions, the potential for future subsidence above abandoned mines is of increasing concern. Of particular concern are areas underlain by room-and-pillar coal mines. Unlike areas above longwall mines, where total extraction causes subsidence in a predictable and timely manner, subsidence above room-and-pillar mines is difficult to predict and may be active for many decades. As part of a geotechnical assessment of a water pipeline replacement project, NSA Engineering recently performed an evaluation of tong-terns subsidence potential along the proposed alignment due to time¬dependent failure of underlying room-and-pillar workings. The alignment of the replacement pipeline closely follows an existing concrete pipeline, which lies 150 11 to 300 fl above workings that both pre- and post-date the pipeline. Annual surveys along the existing alignment indicate that some subsidence has occurred, primarily from partial pillaring performed in the early 1980's, but that this subsidence is not active. Using the boundary-clement code LAMODEL as the primary analysis tool, NSA followed a three-step approach to estimate future subsidence impacts on the replacement pipeline. First, historical information for the mine was compiled, allowing for calibration of models to actual field conditions. Then, numerical models of workings directly under and adjacent to the proposed alignment were nun, with model input parameters varied until resulting loading patterns matched historical data, and subsidence in the model approximated that measured along survey lines. Finally, time-dependent deterioration of coal pillars was simulated by decreasing the coal strength in the calibrated model until a realistic "worst-case" condition (stability of barriers, but Yielding of production pillars and pillar remnants) was attained. Subsidence from the previous calibrated model was then subtracted from that of the worst case model to estimate future subsidence and related ground movement parameters that slay occur along the proposed alignment. The replacement pipeline design, incorporating couplings capable of withstanding the ground movements predicted, is now being finalized.

Jan 1, 2000

By Hamid Maleki

Mining in the Book Cliffs Coal Field of Utah has historically been associated with seismicity because of high-stress environments and the presence of stiff, competent rocks. Recognizing the geotechnical risks in this coal field, mine engineers have dedicated considerable resources to the study of mine seismicity. In spite of these efforts, there are no engineering procedures for assessing mine seismicity for a new property and in developing mine designs. This paper presents the results of ongoing geotechnical monitoring and stress analyses at a western U.S. coal mine. The intent is to demonstrate the application of geotechnical methodologies in evaluating mine seismicity and stress-control technologies including evolutions in yield pillar designs for variable topographic conditions of the Book Cliffs Coal Field. We show that narrow yield pillars are quite effective in limiting strain energy accumulation along the gateroads and thus the potential for pillar seismicity. At the same time, the pillars provide sufficient resistance to control roof-floor convergence at the headgate, gradually unloading to residual strength inby the face position. Lagging, nonuniform caving of overburden rocks results in periodic stress concentrations at the tailgate comer. The sudden release of strain energy triggers sudden failure of critically loaded coal in this area of the high stress gradient. The three-entry gate pillar configuration promotes formation of long cantilevers and load transfer to the tailgate, contributing to formation of cutters and coal bumps. The source of seismicity is investigated while simulating slip along three sets of joints in two stiff stratigraphic units during multipanel extraction.

Jan 1, 2003

By Tetsuro Esaki

This paper describes the present condition of mining-induced damages in Japan and the characteristics of them. The change of mining operation and environmental condition has caused various significant impacts on the ground environment: subsidence, cave-in, springing out of ground water etc. The characteristics and the influences of these phenomena are discussed by investigation of the actual conditions and by fundamental studies. Some counter measures are also introduced.

Jan 1, 1989