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Coal Abstracts
Analytical Investigation of Shaft Damages at West Elk Mine
(click to view entire
paper in PDF format) Several shear failures were observed in Shaft #1 at the Mountain Coal Company, LLC, West Elk Mine, after mining longwall Panel 23, 1,100 ft to the east of Shaft #1. It was speculated that this shear damage could be related to differential ground movement caused by in situ stress relief from the “stress shadow” of the caved zone above longwall Panel 23. A numerical study was conducted to assess the possibility of the shaft shear damage being caused by in situ stress relief and the potential for additional damage to Shaft #1 and two other nearby shafts, due to mining nearby longwall Panel 24. Three-dimensional (3D) models were built in FLAC3D to simulate past and future mining near shafts, the estimated local anisotropic and directional horizontal stresses, and the overlying variable surface topography. The numerical analyses indicated that stress relief due to mining Panel 23 caused the shear damage to Shaft #1and that additional damage to Shaft #1 and the other two shafts, would likely result from mining longwall Panel 24. Additional shear damage was documented in Shaft #1 when longwall Panel 24 was mined, confirming the results of the numerical analyses. |
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The Application of Microcomputers
in Geotechnical Analysis of Coal Mine Excavations and Layouts Today it is practical to perform geotechnical analyses of mining excavations using microcomputers. This paper describes three computer programs that may be used for that purpose and their practical application in assessing pillar and entry stability in a coal mine. Emphasis is placed on the computer resources required.
A Critical Evaluation of Coal Mine Ground
Control Expert Systems Expert systems have found applications in a number of engineering problems within the last decade. Continuous ground control monitoring systems are currently being installed in coal mines for research purposes. This paper presents a critical evaluation of the existing expert systems and proposes a methodology for development of a practical expert system for detection of roof control problems and for allocation of secondary support. Existing mine-wide ground control monitoring systems are reviewed and shown to have potential for improving coal mine stability. Effective monitoring and data evaluation techniques need to be developed to permit cost-effective use of monitoring systems for stability evaluation. |
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Dealing with Rock Bursts at the Deer Creek Mine
(click to view entire
paper in PDF format) Experience at Energy West Mines in the Wasatch Plateau Coal Field, Utah, has shown that longwall face bursts occur mostly near the tailgate when extracting the third adjacent longwall in a panel section where depths exceed 610 m (2000 ft). It is believed that this is due to a combination of cover depth and side abutment stresses created by the widening of a pressure arch formed by interlocking and bridging of large blocks of very competent strata. The 60-m-thick (200-ft-thick) Castlegate Sandstone probably plays a key role in pressure arching. This paper presents a back-analysis of the location and magnitude of the stresses associated with face bursts that occurred when mining the 7th Right Longwall at the Deer Creek Mine. Improvement of these conditions may be achieved by widening the longwalls to attain critical subsidence width sooner while mining the second longwall, and to reduce stresses at the face. Wider longwall faces, however, increase the chance of sandstone channel interception often associated with bursts. Satisfactory mitigation of face bursts beyond cover depths of 610 to 670 m (2000 to 2200 ft) may be difficult without using destressing techniques. The hydraulic fracturing/liquid infusion technique seems to offer the best possibilities for application in longwall mining.
Design and Performance of a Longwall Coal Mine Water-Barrier Pillar
(click to view entire
paper in PDF format) The Skyline #3 Mine of Canyon Fuel Company in Utah was planning longwall mine development adjacent to and downdip of older mine workings known to be flooded. Barrier pillar geometries and widths were proposed by mining company personnel. Hydraulic conductivity testing suggested that the general characteristics of the local coal seam, dikes, and faults are only weakly conductive and that leakage through the barriers, if any, would be minor. The mechanical performance of the proposed barrier pillar design was evaluated according to (1) published empirical design methods and (2) numerical stress modeling. Comparison of the proposed design, with the range of barrier widths derived from the empirical methods, served as a first-order check on the adequacy of the proposed design. Numerical modeling was used to evaluate conditions site-specific to the Skyline #3 Mine and barrier geometry. Models were developed to quantify the relationship between barrier width and the abutment stresses onto the future workings. The models were also used to estimate the stress distribution within the barrier pillars. The hydraulic performance of the proposed barrier pillar design was evaluated according to (1) published empirical design methods for hydraulic impoundments, (2) empirical method estimates of seepage through coal barrier pillars, (3) numerical hydrogeologic flow modeling, and (4) numerical strain modeling. Using the techniques described above, and based on relevant industry experience, underground observations, knowledge of local geologic conditions, hydrogeologic measurements, and analytical results, the level of geomechanical risk associated with the barrier design as proposed was considered low and the level of hydraulic risk was considered moderate. The largest uncertainty that remained was the possibility that unfavorable geologic structures, such as faults and dikes, would act as conduits for leakage of impounded water into the new workings. Although the study indicated only minor steady-state leakage would occur through a small number of known structures, the presence and full leakage potential of threatening structures in the barrier could not be reliably known until the barrier was mined. One longwall panel has been completed adjacent to the barrier with no evidence of water flow even in fractured zones. Thus, as predicted by the modeling and analysis, it appears that abutment stresses imparted on the barrier during mining did not substantially alter the natural hydrogeologic characteristics, or leakage potential, of the barrier. The longwall coal mine water-barrier pillar design and performance are considered a success.
Economic Benefits Gained by Rock Mechanics: Three Case Studies
(click to view entire paper in PDF Significant economic benefits can result when rock mechanics is applied within a practical framework and integrated within the other engineering functions of the mine organization. This requires management support and understanding of rock mechanics as a practical tool to help attain high standards of safety, productivity and resource recovery. For such an approach, a long-term geotechnical program is needed in most operations to build an adequate data base and to ensure that design and ground control issues are handled in a cost-effective manner. This paper presents a summary of three case studies where significant economic benefits were realized with the help of rock mechanics. Although cost savings were not computed, the economic benefits are obvious and clearly impacted the mine operations favorably.
Effect of Full-Extraction Underground Mining on Ground and Surface Waters A
25-Year Retrospective
(click to view entire paper in PDF Over the past 25 years, the present author has written with several co-authors a series of papers beginning with the landmark paper in the 1st International Conference on Ground Control in Mining in 1981, in turn, based upon U. S. Bureau of Mines (USBM) funded contract research managed by the author with a final report issued in 1979, all concerning this paper’s title subject, Effect of Full-Extraction Underground Mining on Ground and Surface Waters. Initially the work was a summary of British, Russian, and Hungarian experience tailored to United States strata conditions, but has evolved into a consistent and well-documented model of the behavior of strata influenced by full-extraction underground mining such as longwall coal mining. The several strata zones recognized in these works, Surface Fracture Zone, Constrained or Aquiclude Zone, Dilated Zone, Fractured Zone, and Caved Zone, have been observed by several workers in the field. The concepts presented initially 25 years ago have been adopted, rightly or wrongly, by state regulators, ground and surface water researchers, and mining practitioners. The development and utility of these concepts and recent findings will be summarized.
Evaluation of Roof Bolt Tension
Measuring Techniques Performance characteristics of an instrumented bolt utilizing the vibrating wire gages were evaluated through 140 field and laboratory tests. Evaluations consisted of instrumented bolt stability, repeatability and accuracy as affected by field installation factors, i.e., tension, torsion and bending moments. Bolt bending moments are present in field installations due to non-squareness of the mine roof and the bolt hole. The testing revealed several problems with the instrumented bolt, requiring redesign of the instrumented bolt assembly. Improvements in the instrumented bolt design included using a stronger bolt unit coupled to a standard 5/8-in. (15.8-mm) shank and anchor and modifying the instrumented bolt installation procedure to eliminate the bending effects. The elimination of bolt bending was achieved by the design and field application of an 8.5-in. (216-mm) hole facing tool. The modified instrumented bolt assembly and bolt installation procedure allowed bolt tension measurement under field conditions up to 12,500 lbs (55.6 N) with good long-term stability and no significant impact on the bolting operation. Also, as part of this study, we assured ourselves that the installed instrumented bolt was representative of other un-instrumented bolts. The elimination of bending in the installation of conventional bolts would also enhance their operating range. Application of the facing tool to all mechanical bolt installations is recommended.
Five Stress Factors Conducive to Bursts in
Utah, USA, Coal Mines High stresses and adverse geology in deep coal mines in the state of Utah, USA, have caused numerous bursts. The larger bursts have been associated with seismic events with Richter magnitudes of 3.6, and in some cases have filled openings for lengths of 150 m. A better understanding of the mechanisms and stress levels involved in bursting is needed to help develop improved stress control design and burst mitigation methods. The geology of the area is notoriously burst-prone. The coal has poorly developed cleating and occurs in multiple seams that are often bounded by very strong roof and floor sandstone/siltstone beds. The overburden is formed by thick, competent strata with numerous sandstone channels. This geology and deep cover are the major source of high stresses, causing bursts. This paper evaluates five common stress factors responsible for burst problems: depth, channels, arching, faults, and coal thickness. It uses case study data from longwall panels with two-entry/yield pillar systems typical of deeper Utah mines. Results illustrate the importance of analyzing stress factor experience to allow a better understanding of the problem.
Gate Road
Layout Design for Two-Seam Longwall Mining Results of instrumentation, test mining, and computer-aided stability analyses were combined to design the most stable gate road layout for two-seam longwall mining at the Plateau Mining Company. Five layouts were evaluated; these layouts used a combination of yield and/or large pillars with either three-entry or two-entry development systems. A layout which used a yield pillar within a two-entry development system resulted in the most stable gate roads for two-seam mining. Another layout using a yield and large pillar was shown to be most stable for single-seam mining of the top seam, but it was unsuitable for two-seam mining because of seam interaction problems. The size of the yield pillar was determined by satisfying several design requirements, as well as by limited test mining; test results showed good, medium-term stability for the yield pillars, with a possible need for rib control. It was shown that the yield pillars might be effective in controlling the floor heave, and could minimize interaction problems in two-seam mining.
Geomechanical
Evaluation of a Coal Mine Arched Entry In an effort to minimize cutter roof failure in longwall development entries of a West Virginia coal mine, a three-entry system was developed with the center entry arched into the strata above the coal seam. The purpose of the arched entry was to modify the horizontal stress envelope associated with the three-entry system and improve roof conditions in the adjacent outside entries. Contrary to the predictions of elastic analyses, the presence of the arched entry improved roof conditions by eliminating cutter roof failures in the outside entries. In situ stress determinations and numerical modeling techniques were employed to quantify the impact of the arch and investigate the mechanism of stress relief associated with the arched entry. Stress determinations and underground observations indicated that the roofs of the outside entries were within the zone of stress relief generated by the arch, and that failure or movement along bedding plane discontinuities played a major role in the formation of the stress relieved zone. The numerical model incorporated horizontal joints to simulate bedding plane discontinuities. The properties of the joints were adjusted until roof stresses predicted by the model were in agreement with measured stress profiles. This process resulted in a calibrated analytical tool that could be used to evaluate alternate mining geometries.
Geotechnical
Mine Design of the Foidel Creek Mine The results of five years of geotechnical investigations are presented to develop productive and stable longwall layouts for the Foidel Creek Mine. The program was initiated during the pre-mining stage, and has continuously provided the data required for mine design. From several stress determinations at the mine, the relationship between geologic structure and the stress field/depth was established. The results were used for orienting the entries for improved stability. Cave conditions and load transfer to panel boundaries were evaluated from closely monitored full extraction mining and computer analysis. Back-analyses of ground movements, as compared with the actual measurements, indicated that cave conditions were favorable in spite of the presence of competent, thick-bedded sandstones in the mine roof. Gate pillar design using a yield-rigid concept was developed through computer analyses and underground instrumentation, resulting in 35- and 75-ft-wide pillars. The influence of a slickensided bedding plane at the roof contact on pillar yield zones, core confinement, and pillar behavior was evaluated.
Ground Stability and Support in Block Caving Operations at Molycorp's Questa
Mine Evaluation of ground stability and support requirements during block caving operations at Molycorp's Questa Mine was conducted with the help of a simple rock mechanics program consisting of underground mapping and classification of ground conditions, and stability and support monitoring by extensometer measurements. This paper describes how rock quality evaluations based on the Q system, convergence measurements, and computer-aided analysis have proven useful in helping establish the support for a wide variety of rock conditions. The type of supports used consisted basically of three systems, which were generally installed according to the location with respect to the cave, as follows: (1) cast concrete and steel beneath the cave; (2) shotcrete and fully grouted bolts in the peripheral areas; and (3) shotcrete and split sets outside the cave abutment areas. Exceptions to this general rule occur in some locations. Support performance assessment was based on Q evaluations and convergence measurements, as well as roof fall experience. Results indicated that areas of very poor to poor ground need to be supported with steel and concrete or shotcrete and 6-m-long, untensioned, fully grouted bolts, depending upon the location in relation to the cave. Shotcrete and split sets performed well in areas of fair to good quality rock outside the cave abutments. Roof falls were structurally controlled, wedge-type failures, usually associated with water. No falls have occurred where the shotcrete is 50 mm or more in thickness.
Highwall Mining in a Multiple-Seam, Western United States Setting--Design
and Performance
(click to view entire paper in PDF With advances in system design driving higher productivity, safety, and coal recovery, highwall mining is becoming an attractive option for extending reserve life at surface mines. Typically, highwall mining is performed by contract, with the system owner charging the mining company on a per ton basis, plus a mobilization fee. For this arrangement to benefit both parties, geotechnical planning is required to minimize risk to highwall mining personnel and equipment, while maximizing coal recovery. This paper discusses how highwall mining has been successfully implemented at a large surface coal mine in Colorado, including design procedures, operational factors, and productivity. The operation includes an area where four seams are mined in succession from the lowermost upward, requiring careful attention to seam interaction issues.
Horizontal Stresses as Indicators of Roof
Stability (click to view entire
paper in PDF format) High horizontal stresses were recognized to impact roof stability more than 60 years ago. Since then, numerous measurements associated high horizontal stresses with difficult ground conditions. This paper presents case histories illustrating the practical usage of roof stress determinations for helping assess stability, not only in the case of high horizontal stresses but also of low stresses. Examples are given of high stresses associated with faults, mine design changes, quantification of stress shadow effect, and anistrophy. The paper concludes with a comparative evaluation on the effects of various stress fields on ground support requirements.
Implication of Highly Anisotropic Horizontal Stresses on Entry Stability at the
West Elk Mine, Somerset, Colorado Overcore measurements at the North Fork Valley (NFV) coal mines in western Colorado have shown that horizontal stresses are highly anisotropic. Measurements have been made in four mines at various depths. In many measurements, the maximum horizontal stress is three to four times higher than the minor horizontal stress. At the West Elk Mine, maximum and minimum horizontal stresses of 24 MPa and 6 MPa, respectively, have been measured at depths of 640 meters (m). Under highly anisotropic stress conditions, ground control problems associated with both high and low horizontal stresses can develop. While high horizontal stresses can produce cutter failures and floor heave, low horizontal stresses can allow block fallouts. This paper summarizes the horizontal stress measurements in NFV mines and, in particular, in the West Elk Mine where their role in roof fall and floor heave failures is discussed. This experience has led to an improved use of ground support and safer mining operations at depths of 640 to 700 m.
Impact of Mechanical Bolt Installation Parameters on Roof Stability To evaluate the influence of uniformly tensioned roof bolts on roof stability, mechanically anchored roof bolts were installed using the conventional bolter and a specially developed thrust and torque controlled retrofitable box mounted on a bolting machine. Thirteen thousand bolts were installed over 19 crosscuts in a 4-heading development in an operating coal mine. Geological, bolt and mine parameters were recorded at bolt installation, and bolt tension histories, roof sag and other changes evident in the rooms, crosscuts and intersections were recorded over a 2-month period. In all, 24 variables and 11 responses were quantified at 67 bolt clusters. This data was analyzed statistically to determine the significant variables influencing roof stability. As the mine development was safe and stable during the monitoring period, roof sag after two months was used as an index to quantify "stability." The variables of significance were shown to be coefficient of variation of bolt tension, bolt anchorage capacity, roof lithology, roof shape, standard deviation of bolt tension, and dip of roof lithologies. It was concluded that of the variables with which the mine operator has control, bolt tension uniformity was the most significant parameter at this mine. Other variables, such as intersection type and time between mining and bolting, were not significant, whereas, some other potentially significant variables such as horizontal stress or roof joint frequency did not vary sufficiently for their influence on stability to be evaluated.
In-situ Pillar Strength Determination for Two-entry Longwall Gates
Extensive measurements and underground observations in three Western U.S. coal mines are integrated in this paper to determine in-situ pillar load-deformation characteristics for narrow (30-ft-wide, 80-ft-long) pillars in two-entry gate road systems. In spite of similarities in the regional geology, coal pillar laboratory mechanical properties and gate pillar geometries, the pillar peak strength, post-failure behavior, and failure mechanism were shown to be significantly different. Pillar peak strength was shown to be dependent on depth at one site, approaching burst-prone stress levels of 4000 psi. At another site, the pillar peak strength was lower because of lower confinement; this was related to the higher frequency of cleats and the lower frictional properties of the roof/floor and coal contact. Two failure mechanisms were identified — one in the pillar, and the other in the mine floor. The roof stability was good at all three sites because of the thick-bedded nature of the roof strata and limited total gate span in a two-entry system. Existing pillar design techniques were shown to be inadequate for design, requiring adjustments for depth of cover, cleat frequency, and roof/floor frictional properties.
Interpanel Barriers for Deep Western U.S. Longwall Mining
(click to view entire paper in PDF Western U.S. longwall operators face increasing challenges with optimizing ground control and productivity as mines reach greater depths and coal bursting hazards increase. Some western U.S. mines, many known to be bump-prone, achieved a successful balance between ground control and productivity by transitioning to side-by-side longwall panel mining combined with a yield pillar gateroad system. With this design, development footages could be minimized and pillar bumping averted by controlled yielding at moderate depths, generally in the range of 450 to 600 m. Over the past four decades, the yield pillar system has won wide acceptance among western mines facing pillar bursting hazards, particularly those in the Wasatch Plateau and Book Cliff coal fields of central Utah. However, recent attempts among the deepest Utah operators to mine side-by-side panels with yield pillars at depths in excess of 600 m have been met with mixed success and, in some circumstances, with serious difficulty. Challenges include violent face bursting and excessive tailgate convergence outby the face, which can be crippling to ventilation. The use of interpanel barriers, i.e. barriers left between longwall panels, offers one possible solution to mining under deep cover with bump-prone geology. Interpanel barriers have already been adopted by Utah's deepest longwall mine, and others are considering their use. The geomechanical implications of mining with and without interpanel barriers, and the competing tradeoffs between ground control and ventilation are discussed.
Long Load
Transfer Distances at the Deer Creek Mine Energy West Mining Company operates the Deer Creek coal mine located in the Wasatch Plateau about 40 km (25 miles) southwest of Price, Utah, USA. Longwall panels at depths in excess of 610 m (2000 ft) have been successfully retreated. Visual observations and stress measurements indicate that load transfer distances greater than 229 m (750 ft) have been experienced in the west side panels off the 3rd North mains. During retreat of longwall panels, ground conditions deteriorated significantly in the development section for the next longwall in the sequence located approximately 229 m (750 ft) away. Deteriorated ground conditions included a pillar burst, floor heave, rib spalling, and roof falls. Stress measurements suggest 13% excess load near this gateroad over the anticipated vertical stress. The long load transfer distances observed are believed to be primarily due to the massive and strong Castlegate Sandstone in the overburden strata.
Longwall Mining Through a Graben with Anomalous Stresses at the Deer Creek
Mine Energy West Mining operates the Deer Creek coal mine in the Wasatch Plateau about 40 km (25 miles) southwest of Price, Utah, United States of America (USA). The mine extracts coal from the Blind Canyon Seam located within the Mesa Verde Group strata. A graben (Left Fork Graben) approximately 305.0 m (1000 ft) in width was encountered during development mining in the Rilda Canyon portion of the mine. Ground problems during gateroad development, including pillar-rib yielding and small bounces, occurred on the outside margins of the graben, but no abnormal mining difficulties were encountered inside the graben. Stress measurements were made using overcoring techniques along the 5th West gateroad at a depth of 579.5 m (1900 ft) to help evaluate stresses across the graben. While stress concentrations of nearly twice the magnitude of the expected in situ vertical stress were measured on the outside flanks of the graben, the measured vertical stresses within the graben were 17% less than anticipated. Stress analyses using the displacement discontinuity method were conducted for adjacent longwall panels to approximate the stress redistribution due to retreat of the longwall face through the boundary fault and into the graben. Results were calculated at multiple face positions and were compared to conditions experienced at other locations within the mine where longwall mining and stress analyses had been completed for an assessment of future minability and bounce potential.
Mine Layout Design for
Coal Bump Control A comprehensive study consisting of stress determinations, core logging, laboratory testing, and numerical analyses was conducted to investigate the cause and potential alternatives to the coal bump conditions that developed in the Second North Panel of the Little Dove Mine near Huntington, Utah. Roof rock characteristics were such that a cave did not develop above this 420-ft-wide panel. After 700 ft of pillar retreat, severe coal bump conditions developed during pillar mining. The coal bump conditions resulted from excessive pillar stress which was caused by load transfer from the extracted, noncaving, portion of the panel. Site-specific data and numerical modeling techniques were used to back-analyze the bump prone condition. This back-analysis lead to the identification of critical stress, closure, and energy release levels. These critical levels were then used to evaluate alternative pillar mining sequences in the hope that the potential for coal bumps could be reduced. It was found that while alternative sequences offered some potential improvement over the pillar mining sequence used in the mine, alternative pillar mining sequences would not significantly reduce the potential for bumps. For the roof characteristics at this mine, coal bumps are best avoided by developing narrow panels or significantly wider panels, which incorporate properly sized barrier pillars. Narrow panels, on the order of 300 ft, would not cave, but panel pillar stresses and the potential for bumps would be moderated by the reduced panel width. Wider panels, of approximately 650 ft, would allow a cave to develop, also reducing panel pillar stresses and coal bump potential.
A Novel
Ground Control Program at Plateau Mining Company A novel ground control program has been developed at Plateau Mining Company through five years of geotechnical investigations. The study was initiated to determine the causes for roof stability problems and to develop alternative support and monitoring systems for increased safety and productivity. From systematic roof fall observations, extensive geologic mapping, roof drilling, and monitoring ground movements along gate entries of four longwall panels, a sandstone channel system was detected and shown to impact roof stability. In particular, underlying mudstones up to 7-ft-thick experienced instability due to differential compaction and load transfer from the gob. Roof bolter operators were trained to report roof lithology for mapping channel positions into the roof and for selective use of longer bolts. This boosted operator morale, and improved ground conditions. Monitoring rates of roof movement is used in these areas for detection of instability and for application of secondary support. This has significantly increased safety and productivity.
Prefailure Pillar Yielding (click to view
Mining Engineering paper in PDF format) Yield pillars have been used for many years to help reduce stresses near mine openings and improve roof and floor stability. A yield pillar is often defined as a pillar that fails but retains residual strength. Stress transfer occurs through the roof and floor after the peak strength of the pillar is reached. High stresses are transferred from around the openings onto abutments that can be barrier pillars or unmined ground. This mechanism, often referred to as pressure arching, is possible as long as the width of yield pillars is less than the critical width above which stresses cannot be carried by the overburden. Significant stress transfer also can occur due to small amounts of pillar and/or floor yielding before the peak strength is reached. This is accomplished in a quasi-elastic manner with little or no visible roof and pillar fracturing or floor heave. Long-term stability may be achieved when stresses and mechanical properties are favorable to pre-failure yielding. This paper gives practical examples where improvements in stability and resource recovery were achieved with this mechanism. Yielding was assessed by comparing measured and calculated vertical pillar stresses. Results indicated that calculated stresses in the pillars were 25 to 40 percent higher than the measured stresses, demonstrating significant arching load transfer to the aubtments. Pre-failure yielding is probably often present but unintentional in both development and production areas. Better recognition and use of this mechanism should lead to improved designs as mines become deeper.
Pressure Measurements in the Gob
Gob pressure measurements were made in a Western U.S. coal mine as part of a long-term program to evaluate cave progress and to determine the influence of geological discontinuities on caving conditions, load transfer, and resulting instabilities. Gloetzl cells were selected for the measurements due to their simple, robust construction and a history of being able to monitor pressures in a broken medium. A methodology was developed for the successful installation and protection of the cells and hydraulic lines in the gob as the face retreated. The measurements indicated a cave progress controlled by the frequency of major faults. The pressure-mining progress profile was compared to those observed in other parts of the world. It was concluded that the significant differences in profiles was caused by the thick-bedded strata, the existence of high lateral stresses, and the spacing between faults. Recommendations are given for future applications of Gloetzl cells for gob pressure measurements.
Problems in Void Detection in Coal Mine Water Hazards (click to view
Ground Control paper in PDF
format) One of the most dangerous events in underground coal mining is unexpectedly encountering water inrushes from undetected abandoned mines in the same seam. The surest and most confident method is probe drilling either from the mine or from the surface. However, drilling is expensive, and may even miss the suspected mine voids entirely by drilling through pillars. Many operators rely upon one or more of several remote sensing techniques for detecting mine voids. However, mine "voids" often are not air- or water-filled open cavities, but are collapsed, rubble-filled chimneyed columns in the strata. Geophysical techniques such as seismic reflection and refraction, electrical resistivity, magnetics, ground penetrating radar, and others, often assume a continuous or fractured rock mass that has varying properties which provide the signatures that allow discrimination of one strata from another, or of strata from voids. However, a rubble-filled cavity has rock block-to-rock block contact throughout its volume, and can still respond as a continuous rock mass with the rock blocks allowing signal transmission or mass detection, rather than a void space. Hydraulically, a rubble-filled cavity is essentially as transmissive to water as an open mine void. Thus, the problem of detecting a mine void with confidence is extremely compounded.
Rock Reinforcement Longevity
Rock reinforcement has been accepted in mining and construction since the 1950s. The longevity of rock reinforcement is often questioned in engineering design because it's historical performance is measured in just decades, relies on natural systems to function, and is not easily inspected. The elements of a typical rock reinforcement fixture include an anchoring device, a bar or tube, a plate, a head bolt, and possibly grout.
19th Conference on Ground Control in Mining, Morgantown, West Virginia,
August 8–10, 2000 Rock reinforcement has been in widespread use and generally has been accepted in underground mining and tunneling since the 1950s. The first rock reinforcement technologies employed were mechanical anchors such as split wedges or expansion shells with ⅝-inch-diameter steel bolts. Failures occurred in weak strata which provided poor anchorage or in ground with corrosive waters. Friction rock fixtures consisting of relatively thin-walled tubes have been in use for about 15 to 20 years. While generally performing adequately, longevity problems have developed from corrosion in water bearing ground. Longevity of rock reinforcement is much enhanced with grouted bolts. Portland-cement-grouted rock reinforcement has been in use since the mid-1950s, primarily in tunneling and other civil engineering underground construction. Tests of decades-old installations have revealed few problems except in ground with aggressive waters. Polyester-resin-grouted rock reinforcement was introduced in the United States to mining in the late 1960s and to tunneling in the early 1970s. Experience from 30 years of resin-grouted bolt installations and field tests have identified longevity problems associated with degradation of steel reinforcing bars, but in generally unusual situations. Improvements continue in resin chemistries, packaging, corrosion protection, grout quantities, and mixing and distribution in the drilled hole to achieve long-term performance. Specific case histories cited with resin- or Portland-cement-grouted rock reinforcement longevity or performance problems, upon close examination, reveal that the causes of the problems were quality control procedures being inadequate and not in accordance with good practice. Manufacturers' recommendations and engineering specifications, if followed in the field, and with competent inspection and supervision, would have prevented most, if not all, reported longevity or performance difficulties.
Stress-Induced Stability Problems—A Coal Mine Case Study This paper presents the results of stress measurements, underground observations, and stability monitoring in a southern Colorado mine. The objective of the program was to assess the causes for panel stability problems and to develop a mine layout design to improve ground conditions. This geotechnical program was implemented in cooperation with the U.S. Bureau of Mines (USBM). It consisted of laboratory mechanical property testing, overcoring stress determination at two sites, underground observations/convergence monitoring at problem areas, and numerical modeling for back analyses of stability problems and for development of improved mine design. Stability problems were shown to have been caused by (1) variability in rock mass strength due to the depositional environment of the seam and development of finely laminated rock strata; (2) in situ anisotropic horizontal stresses exceeding bedding plane strength of coal measure rocks, contributing to time-dependent delamination failure; and (3) time-dependent failure in the mine floor, reducing pillar effectiveness and resulting in a chain-type reaction. An improved mine layout design was developed consisting of (1) panel orientation, (2) pillar design, and (3) support design.
A Study of Ground Control Problems in Coal Mines with High Horizontal
Stresses The concept of vertical stress as being equal to the weight of the overburden is intuitive and relatively accurate in most cases. In contrast, the magnitude of the horizontal in situ stress field generally cannot be predicted without measurements. Brown and Hoek (1978) found that the ratio of the average of the two horizontal stresses to the vertical stress varies widely, from 0.5 to greater than 3.5 near the surface and from 0.3 to 1.0 at depths below 10,000 feet. Many underground mines experience ground control problems that are due in part to high in situ horizontal stresses. One group of mines with such problems operates in the Beckley coalbed. The Beckley coalbed underlies an area of approximately 600 square miles in portions of Fayette, Raleigh, and Wyoming Counties in south central West Virginia. More than 2 billion tons of mineable metallurgical-grade, low-volatile bituminous coal, ranging from 3 to 10 feet in thickness, make the Beckley coalbed an economically significant unit. However, ground control problems in the Beckley coalbed have been encountered throughout most of the underground mines in the area, resulting in a significant potential loss of exploitable resources. In recent work by the United States Bureau of Mines, it was determined that observed ground control problems were due in part to high horizontal stresses (Aggson 1978b). This conclusion was reached after conducting strain relief overcoring in the Beckley #1 coal mine of Ranger Fuels, Inc. As a result of these findings, the Bureau initiated a program to determine the horizontal stresses throughout the Beckley coalbed and to evaluate the effectiveness of changes in mining geometry in alleviating the stress-induced roof control problems. This study, conducted under Bureau of Mines contract J0285020, developed in two stages: (1) the determination of horizontal stresses in the roof by overcoring to verify the regional existence of high stresses, and (2) numerical analyses based on these stress determinations to investigate alternate mine geometries that might reduce ground control problems.
A Study of
Periodic Weighting of Longwall Supports Periodic weighting of longwall supports occurs during retreat of the face under certain geologic conditions. Strong strata in the immediate and main roof tend to cantilever over the gob, weighting the supports periodically. The distance between weighting peaks, as well as the intensity of periodic weighting, is determined by the strength and thickness of roof members, their location relative to the seam, the frequency of jointing, and characteristics of the gob. In this paper the effects of varying several of these variables are investigated using block modeling. The models are calibrated against a published case history of periodic weighting in the Pittsburgh Seam. It is concluded that block modeling is a useful tool for correlating measured support pressure histories with expected modes of strata behavior in the roof, with potential for predicting shield behavior in new geologic environments.
A Study of Potential Fault
Reactivation and Water Intrusion in a Longwall Coal Mine in Appalachia
(click to view entire
paper in PDF format) A longwall coal mine in Appalachia about 1,500 ft deep encountered a fault while developing a new longwall panel. The fault extended from mining depth to the surface near a secondary road and drainage. The fault was located inside the anticipated angle of draw within the mined panel and gob. The fault extended vertically out and up, away from the panel, caved zone, and gob at nearly the angle of draw; the fault very nearly following the angle of draw. It was initially thought that “fault reactivation” could possibly occur. Fault reactivation is the phenomenon of having mining subsidence localized along a fault leading to a “reactivation” of the fault and shearing and displacement along the fault. Such fault reactivation would disrupt and deform the fault plane beyond the normal angle of influence of the subsidence trough, and may provide a conduit for any ground and surface waters to reach the mine. We contacted all operators of longwall mines in Appalachia to determine if any Appalachian longwall mines had ever experienced fault reactivation, and learned that none had experienced the phenomenon. After studies of possible water intrusion quantities and rates based upon in-fault pump tests, which indicated that water intrusion rates should be manageable, and the prior experience that faults in this particular area were usually barriers to water flow, mining proceeded with caution and monitoring. Mining was successful with no noticeable increase in water inflow rates, and no measurable off-setting of the fault exposure on the surface. It can be concluded that the fault did not reactivate due to its relationship to the mining sequence.
Subsidence
Behavior at the SUFCO Coal Mine, Utah Canyon Fuel Company operates the SUFCO coal mine located in the southern Wasatch Plateau Coal Field approximately 48 km (30 miles) east of Salina, Utah, USA. The mine produces coal from the Upper Hiawatha Seam of the Mesa Verde Group (Cretaceous) strata. Annual subsidence monitoring, mostly by aerial photogrammetric methods, has been conducted for the last 22 years. In 1998, detailed survey measurements of vertical and horizontal displacements over a longwall panel were conducted in order to better understand the response of the 61-m-thick (200-ft-thick) Castlegate Sandstone and to develop a failure criteria for naturally occurring rock deformations. Measurements indicate magnitudes and directions of transient and permanent deformation over the panels and gateroads as a result of longwall panel retreat. Vertical subsidence magnitude ranges up to approximately 1.62 m (5.3 ft). Magnitudes of horizontal movement measured over the panel have been up to 0.19 m (0.62 ft).
Tailgate
Support Evaluation at Plateau Mining Company 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.
Tension-Torque Relationship for Mechanical Anchored Roof Bolts Tension-torque relationships were investigated in a coal mine for both standard 5/8-in. (16-mm) bolts and a vibrating wire instrumented bolt, using conventional and controlled installation techniques. The instrumented bolts differed from the standard bolts by their head size and hole preparation procedures. The controlled installation technique developed by the U.S. Bureau of Mines (USBM) involved using hardened washers and lubrication in the bolt assembly, and maintaining a minimum thrust level during bolt installations. The object of this study was to determine the tension to torque ratio (lb/ft-lbs) for standard bolts and instrumented bolts for both conventional and controlled installation techniques, and to modify the bolt assembly so that the ratios would be equal for the instrumented and standard bolts for both installation techniques. It was shown that the instrumented bolts installed in faced holes had a similar tension-torque ratio to that of standard 5/8-in. (16-mm) bolts installed in unfaced holes for each installation technique, and thus they were representative of other instrumented bolts. This was achieved by evaluating the influence of several factors on the relationship, including facing the hole surface, introducing washers and lubrication in the bolt assembly, and applying different levels of thrust at installation due to bolter operator practice. The tension to torque ratio for conventional bolts was only 21 (lb/ft-lb) for the coal mine, resulting in an installation tension of 4000 lbs (17.8 kn) with the typical installation torque of 150 to 200 ft-lb. This tension level was approximately 50 percent of the required bolt tension, and thus, it was concluded that torque standards are not sufficient to assure adequate bolt tension. The low tension levels were due to the particular bolt hardware in use, operator practice, and bolt bending.
Two-Entry Longwall Gate Road Experience in a Burst-Prone Mine Severe pillar bursting at the Utah Power & Light Company's Deer Creek Mine, Utah, has caused serious safety and ground control problems. The bursts occurred in pillars 40- to 80-ft wide in three- and multi-entry systems as a result of high abutment stresses from longwall mining. Some of the major bursts have been 300 to 400 ft long, and have completely filled the entries. High pillar stresses have also caused a massive roof fall in a three-entry development 60 ft below in a different seam. To reduce the burst-prone stress levels, a 30-ft-wide yield pillar was introduced in a two-entry gate road system in 1979. During the last eight years, no bursting or any other significant ground control problem has been encountered in more than 60,000 ft of two-entry development. Back analysis of two major burst events indicates that burst-prone stress levels can be developed easily in pillars 40-ft wide or wider, and that 30-ft-wide pillars yield just before the burst-prone stress levels are reached. There is a lack of understanding about pillar loading and yielding, and as a consequence, there is no rational criterion for yield pillar design. Yield pillar systems should be used within a framework of comparative experience after careful geotechnical evaluation and under controlled test mining conditions. The Deer Creek experience is a positive case study of a two-entry, yield pillar system where significant safety and productivity benefits were obtained.
Use of Block Models for Longwall Shield Capacity Determinations Simplified design methods based on statics have traditionally been used for determining the capacity of longwall shield supports. However, these methods are mechanically oversimplified in the statically indeterminate loading environment at the longwall face. Computer block models, which have found many applications in rock mechanics in recent years, are ideally suited to analyzing the mechanics of shield loading, but have not yet been widely applied to this problem. In this paper the topic of using block models to determine longwall shield capacity is addressed. Agapito Associates, Inc. (AAI) has performed these analyses for major coal and trona projects in the past several years. Block analysis has proven to be a useful supplement to traditional methods in determining support capacities, especially under special loading conditions such as superincumbent loading by massive sandstones that are nearly impossible to analyze traditionally. Applications of the block model and examples of its uses from project case histories are selected to illustrate the mechanics of shield loading. Although assumptions are required in all types of modeling, this method provides an accurate, mechanically correct framework for analysis. This enables a clearer definition of the sensitivity of the problem to the assumed parameters that can be achieved using simplified analysis. Specific data needed to calibrate block models and improve the reliability of support prediction are outlined. |
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