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By Mao Bai

Mining in situations where there exists a potential hazard from inundation has always existed in coal mining, and often reserves were left unmined in favor of reserves with no such hazard. However, increased pressure on expanding the minable reserves and on producing low-sulfur coal, has made mining under water hazards necessary in many parts of the world. Recent joint efforts by the authors to develop case-history data bases of mining under water hazards has allowed the bringing together of Chinese and United States experience and guidelines for such mining. In China, mining under water hazards has been successfully accomplished for a number of years, allowing the development of a good data base from which to draw guidelines. In the United States, much more limited experience, and the longer delay in adopting longwall mining compared to the rest of the world, has forced reliance on a more limited data base and European experience. In the United States (and Canada), the data base is continuing to increase as more operations start-up. Developing a generally-applicable set of guidelines for international use, based upon extensive Chinese and other experience, appears feasible. The Chinese and United States experiences are detailed by case histories.

Jan 1, 1995

By Dennis Dolinar

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

Jan 1, 2003

By Bruce W. King, David A. Newman

"Underground mines have always been an integral part of the limestone industry. However, beginning in the 1990s, the number of underground limestone mines has increased for many reasons including• Stripping ratios at surface quarries have become uneconomic.• Existing surface quarries have reached the mineral boundaries, and the remaining stone reserves are underground.• Surface access to the entirety of the limestone reserve is limited by development or infrastructure.• The once-rural setting of the surface quarry is now surrounded by subdivisions and development. An underground mine is less intrusive and takes the mining operation out of the public eye.• For larger markets, an underground mine enables year-round production without the sequence of overburden stripping in the winter to expose mineable stone for summer production. This is common at many surface quarries.Multiple-level operations are a natural extension of the decision to mine underground. Most limestone reserves, for example, the Tyrone-Oregon-Camp Nelson, Hermitage-CartersLebanon- Ridley, and Salem Limestones, are sufficiently thick to accommodate multiple levels. Multiple-level mines should be planned by considering each level separately and then integrating the individual levels into a larger mine plan. Failure to plan all the levels of a multiple-level mine in concert can result in ground control (back, pillar, floor, and inter-level sill pillar) problems attributable to stress transfer and interaction between the levels or the necessity to leave a thicker sill pillar between adjacent levels.Multiple-level mine planning begins with geology. Once the upper level back horizon is identified, the geology, rock strength, and physical properties in each level are used to• identify the optimal back horizon• quantify whether back support is required and, if so, the type, spacing, and length of the back support• determine the pillar centers• determine the total mining height, including floor shots"

Jan 1, 2016

By Guorui Feng, Min Zhang, Yi Luo, Jian Yang, Yujiang Zhang, Junwen Bai

"Safe mining of coal seam sandwiched between abandoned upper and lower room-and-pillared mines (ULPM) ·is increasingly becoming the focus of attention in recent years, which is closely related to the damage intensity of the upper and lower interburdens. The #7 seam with ULPM in Baijiazhuang coal mine was selected for this research. According to the mining and geological conditions, the vertical stress and plastic zone distributions were determined first by numerical simulation. Then, a theoretical model based on the elasti-plasticity theory was developed. The stress distribution and failure zone created in the interburden by mining the upper and lower seams were determined. Finally the feasibility of mining #7 seam between the abandoned upper and a lower seam was evaluated. The results show that: (1) the distributions of abutment pressure and plastic zone coincide, both of which can be used to evaluate the failure zone in ULPM. The failure zone penetrates the whole upper interburden, while only parts of the lower interburden fail. (2) the theoretical failure zone coincides with the maximum distribution zone of abutment pressure and plastic zone obtained by numerical simulation: the failure zone height in the lower interburden is less than its thickness, while that in the upper interburden is larger than its thickness. (3) the #7 seam is located in the failure zone created by the upper seam room-and-pillar mining, which leads to well-development fractures and blocky fragments in the roof, thereby it is unstabled. (4) when mining the #7 seam in the interburden, the key is to control the roof strata and proper support measures should be taken timely to ensure safe mining.INTRODUCTIONMany coal mines in China mine the thick, superior and easily mineable resources preferentially, while the thin, inferior and difficult-to-mine resources were abandoned (Chappells and Trentrnann, 2015; Zhenfu, 2012; Xu, et al., 2015; Guorui, 2009). As the results of deficient geological data and shortage of investment capital, the residual coal resources distributes in many mining areas in China.In recent years, many residual coal seams sandwiched between the abandoned upper and lower room-and-pillar mines (ULPM) were explored in Shanxi Province, especially in the mining area of Datang, Xishan and Luan (Figure.1). The reserves of residual coal seams in ULPM are considerable, increasingly becomes the focus of attention among scholars and engineers."

Jan 1, 2016

By William Gray

It has been ten years since the Mine Safety and Health Administration (MSHA) promulgated a safety standard requiring the use of Automated Temporary Roof Support (ATRS) systems in underground coal mines. The use of ATRS systems with roof bolting machines and continuous miners equipped with integral bolters has all but eliminated fatal roof fall accidents associated with manually setting temporary roof support. The last ten years have seen an evolution of the ATRS systems themselves and of MSHA's policy regarding ATRS systems. These changes are a response to operational and safety issues regarding ATRS systems often brought about by the ever-changing coal mining environment. As a result, MSHA's Technical Support personnel receive numerous inquiries from all segments of the coal mining industry on various ATRS topics. These requests for technical assistance range from an interpretation of a facet of the ATRS standard to an in-depth evaluation of a prototype ATRS system. This paper will address the aforementioned evolution of ATRS systems and will also summarize the factors MSHA Technical Support uses to evaluate these systems in the field. In addition, some of the more common misconceptions regarding the design, use, and regulatory requirements regarding ATRS systems will be covered.

Jan 1, 1998

By Hideki Shimada

In underground coal mines, grouting method is employed in order to reinforce the floor and to control the movement of groundwater into the roadway. However, the seepage and the reinforcement behavior of the grouting material into the floor are not clearly understood. Therefore, grouting is today normally based on empirical relationships and on the experience of the grouting personnel. To assist grouting projects, it must be acknowledged that many of the flow and the hardening processes occurring during grouting so far defy mathematically analysis. From this point of view. this paper discusses the behavior of grouting material, when it is injected into the roadway floor, by means of 3-D Eulerian-Lagragian seepage analysis. Moreover, the effectiveness of reinforcement grouting on the mechanical properties of slaking-prone mine tunnel floor are studied by means of several laboratory tests. Keywords: Mine tunnel floor, Grout, Seepage, Reinforcement

Jan 1, 2002

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 Revendra Awasthi

The application of a horizontal inclinometer and a tiltmeter to measure tilt, curvature, and deformations of shallow footings during mine subsidence is described. The measurements were taken during mining of a longwall panel in Southern Illinois and were supplemented with precision level survey data. Monitoring was conducted under a cooperative agreement between the Institute for Geotechnology at The University of Tennessee, Knoxville, the U.S. Bureau of Mines, the National Science Foundation, the Illinois Mine Subsidence Insurance Fund, and Old Ben Coal Company. Six different footings were constructed and instrumented at two sites; one above the panel centerline, and the second inside the edge of the panel in the anticipated zone of maximum tension. A brief description of the inclinometer casing and tiltplate installation is described, and the results obtained from the two instruments on a plain concrete footing compared. Observations of footing deformations and curvatures calculated from the inclinometer and tiltmeter data are also presented, and the results compared with observed structural damage.

Jan 1, 1990

By Christopher Mark

The demographics and safety record of the pillar retreat segment of the U.S. coal industry was analyzed using statistics collected by the Mine Safety and Health Administration. Pillar recovery is practiced primarily by mines in Appalachia and the Midwest. Using 1993 data, the accident rates and productivity of a large sample of pillar re- mines were found to be similar to other room-and-pillar mines in the same geographic areas. Pillar recovery apparently accounts for about 10% of all U.S. underground production, but has been associated with about 25% of the mf and rib fatalities during 1989-96. However, of the 28 fatalities that were analyzed, only 4 occurred for which no citations were issued for violations of mining law. Nearly one- half of the fatal incidents occurred during the mining of the last lift or pushout. All four no-citation incidents occurred during the removal of the last lift during a "Christmas tree" extraction sequence.

Jan 1, 1997

By Winton J. Gale

The design of the reinforcement systems required to stabilise mine roadways has developed through the application of field measurement techniques. These techniques have allowed the level of security of a given opening to be determined from an assessment of the loading within the reinforcement, the extent of roof deformation and experience. The design systems developed are now used extensively in a wide variety of geological environment. Recent interpretative techniques have been developed to improve the level of confidence when designing roadway reinforcement. The techniques are particularly applicable for roadways subject to either weak materials or extreme stress levels. A second area of application is where roadway advance rates and hence the timing of reinforcement placement are critical. The newly developed techniques include both analytical and computational models. The validated computational techniques allow a more rational approach to investigation of the effect of varying reinforcement type, timing of placement and density on the level of stability achieved.

Jan 1, 1992

By M. Karmis

For several centuries surface subsidence has been recognized as an inevitable consequence of most underground mining. In fact, British court records of disputes and litigations related to property damage due to mining subsidence can be traced to the early fifteenth century (Shadbolt, 1978). The first systematic investigations of the mechanisms involved in mining subsidence can be credited to Belgian Mining Engineers. The re- search effort was initiated after the widespread movements and the resulting structural damages suffered by the City of Liege in the 1920's. The publication of Gonot's 1871 treatise "Loie de la NomZ" (normal theory) was one of the most not- able achievements of the Belgian school of that era (Gonot, 1871). During the past 100 years the phenomenon of mining subsidence has been the subject of intensive research, particularly in Europe where long- wall is the most common method of underground coal mining. Many theories have been advanced to explain the mechanism of ground movement as it develops from the excavation, through the superincumbent strata, to the surface. Such theories include the original concepts of beams, arches, domes and the contemporary mathematical modeling principles of strata movement assuming two extreme concepts-- a continuous elastic medium and a stochastic medium for the superincumbent strata. An excellent review of the development of these theories ?-s given by Shadbolt (Shadbolt, 1978). In conjunction with the theories of mass strata movement in the vicinity of mine workings, the subject of surface subsidence predictions has also been the subject of extensive research. The development of reliable and rational techniques of subsidence prediction is imperative, if this phenomenon is to be recognized and controlled within acceptable environmental levels. Empirical as well as mathematical methods have been proposed to fulfill this objective and a comprehensive analysis of these methods has been presented in the literature (Bramer, 1973; Shad- bolt, 1978; Virginia Polytechnic Institute and State University, 1980). As a result of this intensive research effort, it is now accepted in many )coal fields-- mainly in Europe-- that the state of art in subsidence prediction and control has approached a level where mining induced surface displacements can be predicted to a suggested accuracy of less than + 20 percent. Furthermore, such movements can be translated into anticipated structural failures using established damage criteria, thus enabling appropriate precautionary measures to be implemented. In this country, subsidence is rapidly gaining emphasis as an important environmental problem of underground coal mining. Damage resulting from this phenomenon ranges from land settlement to severe structural damage and has been witnessed in rural (Illinois, Colorado, West Virginia, Virginia, Pennsylvania) as well as in urban areas (Pennsylvania, West Virginia, Illinois, Wyoming). Yet, the art of subsidence prediction and hence control in this country, for both longwall and room-and-pillar coal mining, is far from approaching the maturity reached by the Europeans. In this paper, field measurments and computer modeling techniques have been used to develop some basic relationships of longwall subsidence and its related parameters. Because the objective of this study was to develop regional subsidence trends, all processed information pertains to the Appalachian coal region.

Jan 1, 1981

By Klaus-Dieter Beck

RAG American Coal Holding, Inc. affiliates operate two longwall mines in the Pittsburg seam in Pennsylvania, the Cumberland mine and the Emerald mine. A unique situation in longwall operation occurred at these mines as the mining activities reached the property boundary between the two mines. The retreat direction in the longwall panel in Cumberland mine is West-to-East while in Emerald mine, it is East-to-West. This unique condition required an adequate design for barrier pillar between the two mines to minimizing the mutual effects of longwall retreat when both longwall faces met and an adequate roof control plan for gateroads. Field studies that involved mainly roof-to-floor convergence monitoring were conducted to assess the effectiveness of the employed barrier pillar in isolating the mutual effect of longwall retreat. Three-dimensional finite element numerical modeling was used to evaluate the gateroad roof and pillar stability. The modeling was conducted such that it duplicated the actual mining process as realistically as possible. The modeling results predicted that the employed barrier pillar and the roof control plan would ensure stable gateroad systems in both mines during and after the two longwall faces met. The prediction was confirmed because mining operations in the two longwall panels were completed without any noticeable ground control problems.

Jan 1, 2004

By Ross W. Seedsman

The in situ behaviour of claystone floors associated with the Wallarah, Great Northern and Fassifem Seams has been studied using a comprehensive suite of stress and displacement monitors. The instrumentation was supplemented by in situ and laboratory testing of the claystone materials. The instrumentation demonstrated that the presence of claystone floors is associated with floor heave, extra rib spall, and settlement of the pillar into the floor. Horizontal floor extensometers showed that the depth of extrusion of clay was limited to 1-2 in. A pore pressure transducer installed in a claystone floor demonstrated the possible existence of short-term undrained behaviour of the claystone. Long term swelling of destressed claystone was also documented. Settlement of pillars into claystone floors can be analysed using simple elastic theory. The assumption of plane strain is valid for most claystone thickness/pillar width aspect ratios. Floor heave can be analysed using bearing capacity concepts. However, it is considered that heave is the result of local bearing capacity failure in the rib spall zone. The advantage of this model is that the bearing capacity equation can be applied without resort to unrealistically high factors of safety.

Jan 1, 1992

By Behdeen Oraee-Mirzamani

Since the advent of New Austrian Tunneling Method (NATM), shotcrete as a primary means of support in tunnels has been widely applied. It?s most important features are durability, speed of application and cost effectiveness. This paper introduces some tables that provide guidelines for the thickness of shotcrete required in some common situations of mine roadways. In order to devise such tables, two different arch sections, together with three different overburden types, were considered. Geotechnical parameters such as apparent cohesion and angle of internal friction of surrounding rocks were chosen, based on the five-category classification of Bieniawski. Two K0 factors (the ratio of horizontal stress to vertical stress) and an average rock density were utilized. Using numerical methods, 60 models were then devised in this way. By applying interaction diagrams of axial force and the bending moment for different thicknesses of shotcrete, appropriate shotcrete thickness for these models were calculated. The results of this research, as well as the methodology applied, can be used in mining roadway support design and all types of civil engineering tunnels.

Jan 1, 2011

By John L. Edwards

The underground coal mines in the Lake Macquarie area of the Newcastle Coalfield, New South Wales, Australia are constrained by surface subsidence restrictions as low as 150 mm. The mining environment is shallow, generally between 100 m and 200 m, often multi-seam and commonly overlain by residential development or tidal waters. Most mines use partial or panel and pillar extraction techniques to meet the subsidence restrictions and allow maximum resource recovery. This paper gives an overview of the results of a recent two year government funded research project aimed at developing a mechanistic, geomechanically based mine design criteria for subsidence control. Three extraction layouts were monitored in detail to assess the relationship between mining geometry, pillar behaviour and surface subsidence. Pillars at each site were instrumented with stress cells, convergence monitors and extensometers. Field data was used to initially calibrate, and subsequently, verify stress and displacement predictions made by the three-dimensional displacement discontinuity code, SUBSOL. Back-analysis of the modelling results indicate SUBSOL's capability as a tool to satisfactorily predict surface subsidence magnitudes and pillar loads for alternative pillar and panel layouts.

Jan 1, 1993

By H. Gene Wilson

Mobile Roof Supports (MRS) look similar to a longwall shield on crawler tracks. The MRS units offer a safe, productive method of extracting the 40% to 60% product left normally unrecovered in room and pillar mining. The units feature radio control, self-contained power units, crawler track drive, and a 600 ton yield. Cribbing and setting posts in retreat mining has been a necessary burden to production. The MRS reduces this burden. Long blocks of coal can be developed to further reduce move and tram time, thus increasing production. The MRS units have successful safety records. The workers are removed from the immediate fall area, and we feel back injuries have been reduced by the workers not setting posts. While MRS units have been damaged in falls, the other capital equipment has been unharmed. The MRS product line has undergone normal product improvement and evolution. Product evolution is seen in our development of machines for particular mine requirements. Through the continuing development of the MRS, Fletcher presents the coal industry with a safe, productive method of retreat mining with potential to develop new mining methods.

Jan 1, 1990

By Vasundhara

Underground coal mining in soft floor environments has often led to strata control problems ranging from small scale issues such as floor heave which disrupts traffic and ventilation, rib spalling and induced fractures in pillars, through to large scale instability and long term subsidence. Past investigations of the problems encountered in Newcastle Coalfields of New South Wales, Australia, have revealed that the soft floor unit comprises of claystone which is very sensitive to moisture. It is prone to swelling, reduction in stiffness and loss of strength in the presence of moisture. Long term stability of mine panels in such an environment is governed by the extent of deterioration of the floor strata. The paper reports the field investigations conducted at Cooranbong Colliery to assess the extent of softened floor zone in the roadway and under a pillar.

Jan 1, 2001

By Robert C. Garson

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

Jan 1, 1982

By Keith A. Heasley

Pillar recovery in deep coal mines with competent roof and floor can concentrate stresses and generate hazardous bumps. Actual calculation of the geologic strain energy released in association with these coal bumps has been rare, although energy release values have been utilized successfully for indicating burst potential in numerous hard- rock mines. This paper advances the current knowledge of energy release calculations as applied to coal bumps through an extensive analysis of an actual bump occurrence. The U.S.B.M, displacement- discontinuity code, MULSIM/NL, is used to produce a numerical model which generates stress, displacements and energy values associated with a pillar retreat section. This model is calibrated with actual field data, and then dissipated energy values from the model are examined as to their suit- ability for indicating bump potential. Ultimately, it is determined that prudent application of dissipated energy calculations can be used as a tool to investigate the coal bump potential of a mining plan or cut sequence.

Jan 1, 1990

By Keith D. Koper, Kris L. Pankow, Derrick J. A. Chambers, Michael K. McCarter

"In a mining environment, high quality seismic event catalogs are often useful in interpreting local stress states and identifying areas with elevated ground control risk. Installation, operation, and maintenance of local/in-mine seismic networks, however, may be prohibitively expensive, and mine operators may not anticipate the need for seismic data until unusual mining conditions arise. If regional, generally public, seismic instrumentation could be used to reproduce the results generated by local/in-mine instrumentation to an acceptable degree of fidelity, it would be a valuable and inexpensive tool for mine planners and ground control engineers.In this study we apply subspace detection—a non-traditional seismic event detection technique—to data from a sparse, regional network. The process is employed to identify mining induced seismicity (MIS) from an underground coal district. The results are compared to a seismic event catalog produced by a local seismic array operating within the permit boundaries. Additionally, we analyze the performance of two different stations of the same sparse, regional network in identifying blasts at a surface coal mine for which we have a detailed blasting log. In both cases data from stations between 10 and 60 km away from mining activity are used. We also explore the trade-offs between the number of detections and the number of false positives, which is influenced heavily by detection thresholds and the number of stations used in the process.For the surface mine we are able to identify more than 90% of the production blasts documented in the blasting log. We also find that the detected blasts from each of the four active pits produce waveforms similar to one another. The results are not dependent on source-receiver distance, and by requiring detections to occur on both stations the number of sure false positives is reduced to zero. In the underground mining district we identify between 30% and 75% of the events detected by the local array, depending on the acceptable number of false positives and the source-receiver distance. We also find many events that were missed by the local array, showing that regional subspace detection can usefully augment traditional local catalogs. Moreover, subspace detection can provide an estimate on event location and classification using data from as little as one seismic station."

Jan 1, 2015