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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 Anthony Iannacchionc, Jessica Benner, Harold Miller, Megan Witkowski

"Longwall bituminous coal mining has been practiced in Pennsylvania since the 1970""s. Hundreds of longwall panels have been mined during the last 40 years but only 24 have actually undermined Pennsylvania's Interstate Highway System. The impact to interstate highways are summarized in this paper and the factors that influence the magnitude of these impacts are examined.INTRODUCTIONOver the last 30 years, interstate highways in Pennsylvania have been undermined by portions of 21 bituminous coal mine longwall panels. From 2003 to 2008, three of the panel were extracted by the Emerald Mine and six by the Cumberland Mine both in Greene County and both owned by Alpha Resources. Previous assessments of longwall mining under Pennsylvania’s interstates have identified impacts to the highways but no danger to public safety. In general the magnitude and extent of impacts during the recent undermining of Interstate 79 or 179 appears to be similar as those identified in previous studies. The nine longwall panels are among some of the widest in the U.S. They are supercritical in character with overburdens ranging from greater than 152 m (500 ft) to less than 305 m (1,000 ft).The Pennsylvania Department of Environmental Protection (PA DEP) requested that the University of Pittsburgh summarize observations collected by their staff during the recent undermining of 179. Observations and photographs of impacts to the highway were made by PA DEP staff and entered into reports prepared by California District Mining Office (COMO) staff and statements written into the Bituminous Underground Mining Information System (BUMIS) database. In addition to the observations by the PA DEP, the Pennsylvania Department of Transportation (PennDOT) conducted a series of detailed land surveys along portions of the highway undermined from 2003 to 2008. To characterize the subsidence basin fanned by longwall mining. PennDOT information has been analyzed by another research group within the university (Guticrre7, et al., 20 I 0)."

Jan 1, 2010

By B. N. Whittaker

The paper briefly reviews the type of support and stability problems encountered in mining tunnels in U.K. Carboniferous rock conditions, particularly the Coal Measures where almost all such tunnels are located. Stability prediction of mining tunnels is discussed and a computer-based model which deals with stratified rock conditions is described. Various schemes for monitoring the stability of mining tunnels are examined and the application of the results discussed. Comparisons between predicted and measured Closure values are presented for a major Coal mining tunnelling project at Selby in the North Yorkshire Coalfield. Conclusions are drawn concerning the application of the present prediction technique together with the importance of underground monitoring. Examples of different types of mine tunnel stability problems encountered in the U.K. Coal Measures are discussed and comments made on the secondary effects of tunnel peripheral rock weathering and corrosion of the associated support systems.

Jan 1, 1987

By Thomas Barczak

A new generation of hydraulic mine support prestressing devices has been developed. These thin-walled steel shells are machine-welded and can be inflated with water or any liquid to provide prestressing for a wide variety of roof support products ranging from Can supports to props and cable bolts. With an expansion capability of several inches, depending on the geometry and size of the unit, they can also establish roof contact eliminating the need for wooden wedges or crib blocks that are commonly employed with several standing roof support products. Capable of withstanding pressures from 1,000 to 6,000 psi, these cells are able to transfer roof loading into the support structure without rupturing. A recent development has been the addition of an inexpensive yield valve that provides control of the maximum pressure and load development on the support. This paper examines the performance capabilities of these inflatable prestressing units and the impact they have on the performance of various support systems, including an evaluation of the overall stiffness of the support system and the load control during yielding of the prestressing unit. Recommendations are made regarding the design requirements of the prestressing unit to optimize the support performance. A summary of mine applications using this technology is provided with general comments regarding their impact on ground control.

Jan 1, 2004

By Z. T. Bieniawski

A major study was performed on determination of long term strength of coal strata directed to the design of coal pillars to ensure their stability beyond the life-time of a mine. Based on field investigations, numerical modeling and reliable data from 12 case histories, three aspects of time effect in the strength of coal strata were studied; (1) strength reduction based on the concept of volumetric strain estimate from in-situ tests, (2) strength reduction in terms of three failure criteria applicable to coal strata: original Hoek-Brown criterion, modified Hoek-Brown criterion, and Generalized criterion, and (3) long-term strength estimation as a function of coal strata quality RMR based on case studies. From these investigations, an integrated approach to long-term pillar stability is proposed, based on numerical modeling and empirical pillar strength formulation. The values of factors of safety for coal pillars are analyzed in terms of probability of failure and practical meaning for mine design purposes.

Jan 1, 1994

By Yi Luo

Regulating the face advance rate in longwall mining operation can be an effective means to reduce the disturbance potential to surface structures associated with a longwall subsidence process. However, there are two seemingly different views about the relation between the face advance rate and the disturbance potential associated with dynamic subsidence process. The US mining industry prefers a faster but a constant face advance rate while the German counter- part is forced to select a slower rate due to its mining conditions, especially while undermining sensitive objects. In order to gain a better understanding about these two different approaches, the experiences in longwall mining subsidence in both Germany and US are presented in this paper with the fundamental differences in geological conditions and mining practices.

Jan 1, 2001

Tailgate pillar and tailgate-face corner humps have been a major threat to longwall coal mines where hump-prone conditions exist. The yield pillar design concept, in which the longwall chain pillars are designed in such way that they can yield in a nonviolent or controlled manner during longwall retreat, has been used in many longwall mines to prevent tailgate pillar humps. However, this practice often shifts the side abutment load onto the tailgate-face corner especially when a massive and strong sandstone member is present within the caving zone or in the vicinity of the coal seam. Therefore, the abutment pillar design concept has also been used in some hump-prone coal mines to minimize the stress concentration on the tailgate panel rib for the control of tailgate-race corner humps. The disadvantage of using the abutment pillar design concept is that it is more costly and puts additional pressure on mine operations for gate entry development and significantly reduces the recovery of coal reserve. In addition to pillar size, panel layouts. such as the panel width and panel orientation also play a critical role in pillar or face humps and should he taken into account. Based Upon the Cyprus mine design and operational experiences at Willow Creek and case studies at other bump-prone longwall nines in the U.S., the authors attempt to identify the critical factors causing coal mine bumps and to introduce some guidelines and criteria for the design of both yield and abutment pillar systems and panel layouts.

Jan 1, 1999

By Alan Peacock

Shield supports have been in use in the USA underground mines since 1975, and their development in the intervening time has been progressive and inovative. Specialized techniques are used to optimize their performance in a particular application and to design the component parts.

Jan 1, 1981

By Gustavo Herrera Rico

As part of a general effort to increase safety, efficiency, and profitability of its mining operations, MICARE personnel with the support of NorWest Mine Services, Inc. determined that the key to improving development mining was the implementation of roof bolting as the means of primary roof support for both main entry and longwall gate road development. Until mid-1994, MICARE had relied exclusively upon primary roof support systems composed of heavy steel beams and wooden posts. Support systems were designed, implemented, and evaluated that were successful in controlling the mine roof under extreme geological conditions. This paper describes the geological challenges, support selection criteria, and presents the instrumentation and 'mine measurements made to assess the bolting performance during the development and retreat mining process of longwall mining.

Jan 1, 1997

By E. Bauer

During 1987 and 1988, the Eighty Four Complex Mine of BethEnergy Mines, Inc. used the open entry longwall recovery method to recover one partial and three complete longwall faces. Various cement- concrete supports were utilized to support the open recovery entry until the approaching longwall face cut into this entry. This recovery method and the supports used were studied by the mine personnel and the Bureau of Mines. This paper discusses the findings, results and conclusions of that study.

Jan 1, 1988

By Patrick S. Artrip

A deep mining operation in the Lower Kelly (Imboden) seam in Wise County, Virginia experienced severe ground control conditions in its attempt to develop reserves situated under increasingly higher cover conditions. Ultimately one of only three mines capable of providing access to a large reserve area had to be abandoned. A thorough geotechnical exploration program was carried out to allow characterization of the critical roof and floor rock strata in accordance with the modified geomechanics classification system. The principal roof and floor rock stratigraphic sequence types were defined and mapped throughout the reserve area along with other critical geological factors. A ground control study based on the geotechnical findings provided a quantitative analysis of the causes of previous entry instability. Further analysis of pillar and entry stability identified configurations which would remain stable for the variety of strata types and range of overburden depths within the remainder of the reserve. Mine designs based on the geotechnical evaluation have, to-date, provided the stability required for long-term future development - of the reserve area. However, some areas of difficult ground were encountered which could not be anticipated with the drill data available at the time of the original study.

Jan 1, 1993

By Thomas Barczak

Pumpable roof supports provide an alternative longwall tailgate roof support and have grown in usage during the past few years. Heintzmann Corporation has been installing pumpable roof supports at the RAG Resources Emerald Mine in western PA since 1998, where they have provided effective roof control in their longwall tailgates. Despite the success of these supports in this application, questions remain regarding critical design issues for optimizing the use of this support technology. The support loading profile is characterized by a high initial stiffness with peak loading occurring at less than one inch of convergence, followed by significant load shedding with a post failure capacity comparable to that of wood cribbing. Therefore, the key to optimizing the support utilization is to provide sufficient load density to prevent convergence from occurring beyond the peak loading capacity. This requires an understanding of how the supports interact with the ground conditions, hence measurement of the ground reaction curve. In order to obtain this information, pumpable roof supports were instrumented to measure support loading. Roof deformation and roof-to-floor convergence measurements in the vicinity of the instrumented supports were also made. The experimental parameters for the installation were the support spacing and water¬to-solids ratio of the grout, which controls the grout strength and ultimately the maximum capacity of the support. The study clearly shows that at this mine a 24-inch diameter support is fully capable of providing adequate ground control under depths of cover of 750 ft as only 50 pct of the available support capacity was utilized outby the longwall face. It was also shown that the 2.00 to 1 water¬solids ratio, despite providing a slightly softer support response, is sufficient for maintaining the same degree of roof control provided by the traditional 1.75 to 1 grout mix. The 10-ft spacing of the supports did not cause any ground control problems outby the face. However, inby the face the performance of the support is degraded once the peak capacity of the supports is exceeded. It appears that the large load shedding behavior, which is characteristic of this support following peak loading, allows the immediate roof to separate. When this happens, the support is unable to regain control of the roof in time to prevent failure of the immediate roof beam.

Jan 1, 2003

By A. Wahab Khair

This paper presents an analysis of surface ground movements recorded from two longwall mines in Northern West Virginia which operate in the Pittsburgh Seam. The instrumentation areas consisted of four panels, two at each mine, covering various sections with unique topographic features. Instrumentation consisted of survey monuments on 25 to 120 ft. spacing. Vertical and horizontal ground movements were measured at specified intervals to determine subsidence rate, developing and final subsidence profiles, angle of draw, horizontal displacements and associated strain profiles. A relationship between panel depth and width and the subsidence factor was also developed.

Jan 1, 1988

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 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 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 David Bigby

The paper describes research undertaken on an ECSC/K HSE project which investigated several potential physical principles which might be harnessed to provide a non-intrusive means of determining the extent of failure in the near roof of mine roadways and tested the most promising in the laboratory and in the field. It was concluded that the principles which warranted further development were thermal response and laser vibration measurement. In the latter case there is a need to develop an efficient means of imparting energy to the seam to induce sufficient vibration. A new, SIMRAC funded Project will progress work in the field of thermal response.

Jan 1, 2004

By P. K. Kaiser

The purpose of deformation monitoring for the evaluation of the stability of underground openings is reviewed briefly and some results from laboratory tests of small tunnels are presented. The ground behaviour is described by comparison with the response of structural systems and illustrated by the data from the testing of small tunnels. It is concluded that the main objective of a deformation monitoring system for stability evaluation is to determine the actual process of deformation and instability in the area surrounding the opening. Only when this process has been established is it possible to use deformation measurements to design support systems or to improve construction or mining procedures. Present monitoring practice is not sufficiently orientated toward the aim of process description.

Jan 1, 1981

By Daniel W. H. Su

This paper presents the results of an extensive geomechanical testing and monitoring program conducted at a longwall panel. The field program included the monitoring of roof caving sequence and mechanism, surface subsidence, change of pillar stress, roof displacement and entry convergence, and strength characterization of the overburden rocks. The response of eight headgate pillars and the adjacent entries was monitored as the longwall face approached and passed the instrumented locations. Overburden response to longwall mining was monitored using time domain reflectometry (TDR) technology; and, the height of the highly fractured zone above the gob was determined by postmining drilling. Good correlation was found between the height of the highly-fractured zone and the TDR, subsidence, and pillar stress data. The results provide a detailed picture of the response of overburden strata and chain pillars to longwall mining. A preliminary model representing the overburden strata behavior at the test site in response to the longwall mining is proposed.

Jan 1, 1987

By Bruce Hebblewhite

An ACARP-funded industry research project in Australia recently investigated the problem of premature rock bolt failures in underground coal mines. The project carried out limited sampling from 9 different collieries in NSW and Queensland, followed by a more comprehensive, controlled sampling from one colliery. Over 50 broken bolts were collected and subjected to metallurgical analysis at UNSW to determine both the steel composition and to analyse the fracture surfaces. Prior to this study, there had been anecdotal evidence of similar bolt failures from a number of sites within the industry. The sampling and analysis conducted by this project - whilst being below a statistically representative sample of the entire industry bolt population - highlighted the fact that bolt failures were continuing to occur at a number of sites, in both older and recently installed bolts. The failures were almost entirely attributable to stress corrosion cracking (SCC). It appears that a combination of factors - including steel composition, bolt manufacturing process, loading environment, geological and hydrological environment, bacterial corrosion - can all contribute to this problem. There is no simple solution, however, as with similar research findings from the UK, it appears that steels with higher values of fracture toughness, or Charpy Index, are less susceptible to SCC. Testing to date has been limited to bolt failures below the resin encapsulation horizon. A real concern is the potential extent of the problem at higher horizons, within the encapsulation. These may require some form of non-destructive testing (NDT) for detection. Initial trials in Australia of the ultrasonic device developed by DMT in Germany have proved encouraging. Further testing using this device is anticipated in future research. This paper describes the outcomes of the project, completed in 2002.

Jan 1, 2003