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By Kevin Andrews, Scott Nelson, Terry Hamrick

"In the process of Geologic Hazard Mapping (GHM) for underground coal mining, geologists evaluate the effects of various geologic and geotechnical conditions to assist with optimization of mine productivity and with selection of more favorable areas for future development. The methodology involves correlation of multiple geologic factors (such as overburden depth, proximity to sandstone channels, and areas with weak immediate roof strata) with conditions encountered during mining (roof falls, floor heave, pillar spalling, etc.). Such studies yield maps that identify areas where mining is likely to be difficult and allow mining engineers to foresee favorable and unfavorable mining areas when planning new or expanding mines. To expand the benefits of GHM, work has been done to correlate geologic factors with room-and-pillar mining advance rates. Average mining advance rates are mapped and then correlated to different areas, as characterized by GHM. This process exposes the geological characteristics that have a material impact on mining rates of advance, improving the rates of advance into future mining areas with similar geological conditions. The final result of the work allows mine planners to not only design the mine qualitatively accounting for areas of unfavorable conditions, but also allows the mine planner to more accurately apply rates of advance for mine timing and production sequencing. The use of GHM to predict mining advance rates in unmined areas enhances the benefits of traditional hazard mapping and provides quantitative data that can be incorporated into mine sequencing models.INTRODUCTIONThe role of a geologist in the coal mining industry has evolved significantly over the last 30 to 40 years. Geologic Hazard Mapping (GHM) mapping for coal mines began in the late 1970s and early 1980s (Chase, Newman, and Rusnak, 2006). Since its beginnings, the GHM process has evolved significantly and continues to take on new levels of sophistication. Numerous case studies of GHM application to predict future mining conditions exist in the literature. For example, Artrip, Nelson, and Newman (1993), documents the geotechnical characterization of roof and floor for mining in the Imboden Seam in Virginia where previous mining efforts had been abandoned due to adverse mining conditions. In addition, Keim and Miller (1999) summarize evaluation of geological influences on a longwall mine in West Virginia. In both examples, significant geotechnical and geological information was available, including the opportunity to compare predicted mining conditions against actual conditions as mining progressed."

Jan 1, 2018

By Andy L. Shaffer

A novel liquid settlement system (LSS) was installed in advance of mining at a western United States (U.S.) longwall mine to measure total roof sag from the time of cutting to days after mining during gate road development. The system was used to compare differences in roof sag between conventional place-change mining using a Joy continuous miner and a Fletcher roof bolter and mining using a miner-bolter. The LSS was installed with an intrinsically safe, miniature data acquisition system (MIDAS) to record roof response at four locations in the same gate road. The system utilized pressure transducers to measure roof sag by the principle of falling head (fluid pressure) in fluid-filletubes installed in the immediate roof. Details of the LSS design, installation, data collection, post-processing, and future applications are discussed.

Jan 1, 2014

By Yang Li

For the study of overburden movement in longwall mining of shallow coal seams, most scholars merely consider the thickness of bedrock and top soils. In this paper, the overburden movement in longwall mining of shallow coal seams with thin top soils and key strata group was studied. By using numerical simulation, the development of fractures in the overburden was analyzed for different ratios of panel width to depth (W/D) and depth to mining height. The results show that the overburden movement changed with the variation of mining height, panel width and depth under the specific geological conditions. It also explain how different overburden disturbance structures, such as ?three zones? (caved, fractured, and continuous deformation zones), ?two zones?(caved and fractured zones) or ?steps-like collapse? are formed under different mining conditions. The results broaden the vision for research on overburden movement in longwall mining of shallow coal seams.

Jan 1, 2014

By Michael Gauna

In 2010, the number of fatal rib falls in U.S. underground coal mines exceeded fatal roof falls for the first time ever. While recent trends have seen roof fall fatalities reduced by about 62% since the 1990s (from an average of 9.6 per year in the 1990s to 3.6 per year during the last five years), the incidence of rib fall fatalities has remained approximately constant (about 1.5 per year). In addition, injury incidents associated with pillar rib failure have been steady at approximately 100 per year. The persistence of rib fall injuries, underlined by the recent increase in rib fall fatalities, has led to this investigation of the coal rib failure parameters and methods available to mitigate rib failure in U.S. underground coal mines A detailed study of the 21 fatal rib fall incidents that have occurred since 1995 (excluding coal burst incidents) indicates that the two most important risk factors are the mining height and the depth of cover. The depth of cover was 700 ft (213 m) or greater in 81% of the fatalities, and the mining height was at or exceeded 7 ft (2 m) in every instance but one. Other risk factors included multiple seam interactions, rock partings in the seam, and retreat mining. The accident history reveals that joints or slickenside features within the rib increase the hazard. Approximately threequarters of the rib fatality victims were roof bolting machine operators and continuous mining machine operators. This paper includes a brief overview of techniques for rib control; including bolts, standing support, bent steel straps, and rib strapping. The available mining equipment features for installing rib support and protecting operators from rib falls are also discussed. A proactive approach to rib control using available technologies will lower the risk to miners and is an endeavor that must be pursued to reduce rib fall accidents.

Jan 1, 2011

By Gexin Sun

This paper presents the physical modelling results of subsurface fracture development associated with longwall mining operations and an application of image processing techniques to interpretion of the results. Physical modelling results have been obtained by employing a large sand and plaster model loaded purely by gravity as a means of fracturing the subsurface strata above the longwall excavation. The results have shown fracture development and crack propagation as a longwall face advances and demonstrated that the strength of stratified rocks, the presence and position of a weak band-, and the extraction thickness, have significant effects upon the overall fracture patterns above the excavation. Binary images of fracture patterns were scanned from the black and white photographs of the tested models using an image analyser coupled to a video camera. These images were manipulated by a custom written C program which allows the images to be processed in both vertical and horizontal direction. The images were filtered to quantitatively differentiate between vertical and horizontal fracture intensities. The vertical fracture intensity is particularly important as vertical fractures are considered to provide the main avenues for surface and subsurface water inflow into the excavation. The use of the image analysis techniques has shown a promising improvement in image enhancement and in model results interpretation in a quantitative manner. The numerical nature of the image analysis results will allow further statistical comparison with other modelling techniques to be undertaken and significantly increase the scope for use of the modelling technique results.

Jan 1, 1992

By Simon H. Prassetyo

Violent failures of coal pillars, known in practice as coal mine bumps, have long been a subject of investigation. Many field investigations have considered geological conditions that create high stress in the pillar to be the main causative factor leading to bumps. In recent years, constraint at interfaces has been observed to be associated with the occurrence of coal bursts. This paper shows how interface friction and width-to-height (W/H) ratio affect the degree of violence of coal specimen failure. A series of UCS tests on 178 coal samples from bump-prone (Utah, called UT coal) and non-bump-prone (West Virginia, called WV coal) mines were conducted. Three violent failure parameters? peak sound pressure level, core zone failure, and ultimate stress? were used to assess the violence of failure. The degree of violence were investigated at varying interface frictions (high: µ = 0.40, medium: µ = 0.22, and low: µ = 0.13) and W/H ratios (W/H = 2, 3, 4, 5, 6, 7, 8, 10, and 12). A simulation series of modeled coal specimen under uniaxial loading was also conducted using the finite element method to observe the equivalent plastic strain distribution experienced by the modeled coal specimens. It was found that the violence of coal specimen failure depends on W/H ratio and interface friction. Interface friction has more significant influence on the violence of failure at high W/H ratios than it does at low W/H ratios.

Jan 1, 2011

By Kenny D. Basnett

An analysis of the Roof Falls Data Base (RFDB) for underground coal mines in Illinois for the period 2004?2008 indicated that about 80% of the roof falls occur at intersections and are associated with geologic anomalies or a rapidly changing geologic environment. Most falls are associated with anomalies such as clay dikes, differential compaction faults, shear zones, joints and fractures, and geologic structure. Therefore, a better analysis of the geology with emphasis on mapping anomalies such as joints and fractures and their application is necessary to improve ground conditions and safety. Such studies within Illinois Coal Basin are limited. This paper presents a mapping study of the joint orientations with documentation of dip of the several lithological units that lie directly above the Herrin No. 6 Coal seam around southern Illinois. These units include the Energy Shale, Anna Shale, and Brereton Limestone. The mapping was done in three mines and the data were plotted on a stereographic projection to determine the average characteristics of joint planes. The study mines included two deep underground mines and one shallow highwall mine. This study has helped identify favorable mining and support orientations in areas of highly variable geology. These are currently under review by the cooperating coal company for experimental roof control studies.

Jan 1, 2011

By Liang Yinhuai

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

Jan 1, 1992

By K. Y. Haramy

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

Jan 1, 1987

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By J. R. M. Hill

The U.S. Bureau of Mines is involved in research to improve geological sensing (or geo-sensing) techniques. This paper presents results from field trials of two research projects concerned with new technologies for geosensing. The goal of the first project was to develop a "smart" roof bolter drill so that it could probe into roof rock to evaluate rock conditions. The goal of the second project was to modify an existing mine-wide monitoring system in a continuous mining operation so that the system could interface with geotechnical instruments to provide near real-time geosensing. Integration of this mine-wide monitoring system with the smart roof drill to collect, reduce, and analyze geotechnical data and the potential applications of the resulting technology in mining, are described.

Jan 1, 1992

By Ernest A. Curth

An estimated 49 billion tons or 29 percent, of the coal reserve base to a depth of 1,000 feet in the eastern United States fall in the 28- to 42-inch range. Often left out as a consequence of selective mining, it is a paramarginal source that will become increasingly important. At present, all active thin seam longwalls are located in the Eastern Coal Province, and a number of operations have been suspended due to the erosion of the market for metallurgical coal. Extensive premining studies, appropriate mine planning, reliable equipment that is designed with ergonomics in mind, adequate roof support, intensive training, and strong face discipline are the ingredients that provided a measure of consistency in production for the last 10 to 16 years, particularly in the Pennsylvania bituminous coalfield where single drum shearers dominate. Plows, which were found to be very productive in suitable strata in Europe, are used in the Pocahontas field. Low seam four- and six-legged shields have been introduced recently. The future holds automation at progressive levels. The first step is batch control with the benefits of quickly applied roof support and reduction of workmen exposure to environmental hazards.

Jan 1, 1981

By S. Jayanthu

This paper presents analysis of strata behaviour with special reference to convergence and stress variation during an experimental trial of extraction of pillars with cable bolts as major support system in 6.5 - 8.0 m thick coal seam. Critical span for roof falls was estimated through empirical models and also evaluated by numerical models. In situ strata behaviour studies during 1992-96 for about 70 local/major falls revealed inadequacy of the available literature for interpretation and prediction of the roof falls. Convergence acceleration and variation of induced stress based on continuous monitoring data showed distinct anomalies and potential for better understanding of strata mechanics during pillar extraction.

Jan 1, 1998

By Yunqing Zhang

Roof failure occurs much more frequently at intersections than entries and crosscuts. Using a commercial finite element package, four-way and three-way intersections reinforced with the tensioned bolts are modeled. The 3-D model considers bolt pre-tension, bolt installation procedure, bedding planes and in-situ horizontal stresses. The modeling results from both four-way and three-way intersections are analyzed with regard to the intersection stability. The mechanisms of tensioned bolting at the intersections are investigated and some tensioned bolting guidelines for intersections are given. Based on this study, a better way to enhance intersection stability is to use longer bolts than entries or crosscuts and increase the bolt density near the intersection corners.

Jan 1, 2003

By D. F. Howarth

Two innovative ground reinforcement products have been developed for use in the mining industry. One of these products is a high strength polyethylene (HOPE) bar for use as a rib bolt in underground coal mining and as a corrosion proof tendon in open pit mines. The other product is a cable bolt (Ultrastrand) which demonstrates enhanced load transfer and is available in continuous lengths. Ultrastrand's exceptional in¬situ performance and versatility makes it ideal for mechanised installation in both underground and surface mining operations. Details of these products are presented and results from labortaory and field trials are discussed.

Jan 1, 1992

By Yi Luo

The responses of a section of railroad to ground subsidence process was monitored as it was undermined by a longwall panel. The subsidence data collected and the observations made through this monitoring program could help us understand betteer the ground subsidence process and its impact on linear structures such as railroads. The data analysis techniques used and the assessment methods presented could be useful to researchers practitioners in the field of mining subsidence.

Jan 1, 1994

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 Chris Mark

Fully-grouted roof bolts comprise more than 80% of the primary roof supports used in U.S. coal mines. However, nearly 1,500 MSHA reportable, non-injury roof falls occur each year, and most of these are attributable to failure of the roof bolt system. Anchorage failure is one failure mechanism for fully-grouted bolts. As roof deformation works its way upward, the bolts can become heavily loaded near their upper ends, If the applied load exceeds the anchorage, the bolts will simply pull out. Research dating back 30 yrs indicates that this type of anchorage failure is most likely when the roof rock is weak, just where roof support is most critical. In soft shale or coal roof, the small amount of data available in the literature indicates that 20-30 in of resin anchorage may be required to achieve the full capacity of the bolt. In other words, the "full resistance zone" of a 60 in bolt may actually be just 30-40 in. Despite its potential importance, there is no widely accepted anchorage test for fully grouted bolts. Standard pull tests have sometimes been employed, but they provide no information on the anchorage near the top of the bolt. An alternative, first described more than 25 yrs ago in the U.S., is the short-encapsulation pull test (SEPT). With this test, the bolt is installed with only a short (1 ft or less) tube of resin. In recent years variations of this test have become international standards. This paper describes recent studies using short encapsulation pull tests in the U.S. Tests were conducted in the National Institute for Occupational Safety and Health (NIOSH) Safety Research Coal Mine at Bruceton and at underground mines in Pennsylvania and West Virginia. The study found that the SEPT can be used to make a simple evaluation of resin bolt anchorage. Suggested procedures for conducting SEPT are included. The study also confirmed that poor anchorage can be an issue, particularly where the roof rock is very weak. Some simple techniques for improving anchorage, and thereby the effectiveness of fully grouted bolts, are discussed.

Jan 1, 2002

By Yunxin Qiu

A new technology has been introduced - the use of the High-Water Solidifying Material for providing pumped roadside supports to maintain a roadway for re-use in longwall retreating. As a gob edge support, its initial use at Hebi No. I Colliery was successful, providing acceptable performances in support and control of convergences and lateral closures of the gob-edge roadways. This acceptability applied to both first use and re-use when the roadway became a gateroad for the adjoining longwall, remaining stable throughout the associated loadings and convergences. The technology promises economic and social benefits.

Jan 1, 1992

By Kazem Oraee-Mirzamani

Blasting operations generate seismic effects in underground mines. These effects apply additional dynamic loads on the support system, which should bear both static and dynamic loads. Static loads are caused by the weight of the superincumbent strata, while dynamic loads occur as a result of blasting in the mining area. Identification of the origin and determination of the support system behavior in natural frequencies is crucial in assessing the stability of underground mines. This is because resonance occurs when a support is vibrated with its natural frequencies, which can cause a vibration with the maximum amplitude and subsequently cause extreme deformations. The mechanism of support system deformation during dynamic load displacement has been studied and numerical simulation for the impact of the dynamic loads on stability of supports is carried out using finite element method. The paper introduces a simple technique for improving stability and safety of mining operations. Results obtained and the methodology adopted in this research can help mining design engineers make decisions on adequate support for active mining operations.

Jan 1, 2011