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By H. Yavuz

A Numerical modelling study using the two dimensional finite difference code "FLAC" was performed for investigating the ability of numerical modelling to predict the performance of yielding pillars. The model was verified by comparing the stress and deformation data obtained from a yield pillar trial application at Welbeck Colliery, a UK coal mine. Two entries with-a 10 m intervening pillar were modelled. Both for development and for side abutment loading stages, the vertical stress measurement data over the pillar and side abutment and deformation measurement data in the roof and floor of the trial entry were compared with the results obtained from the model. Two difficulties were encountered, unavailability of data for caved rock and for the yielding pillar material. These were overcome by back analysis to estimate the unknown field properties of the pillar material and caved rock from known properties of these materials. Results from this model are given and the predictive capability of a finite difference model for two entry yield pillar applications is discussed.

Jan 1, 2001

By Anthony T. Iannacchione

Longwall mining produces surface subsidence basins that affect streams by water diminution and contamination. These effects can ultimately impair the biological character of the plants and animal communities living in the streams. The combination of flow, water quality, and ecological effects threaten the environmental sustainability of longwall mining and, ultimately its social acceptability. Pennsylvania has developed new standards, released in 2005 and implemented in 2007, to help protect the commonwealth?s valuable streams from the detrimental effects of subsidence. These standards are aimed at eliminating environmental problems and will therefore help to make underground bituminous coal mining more sustainable. The engineering solutions necessary to comply with these standards are fundamentally affected by the geologic nature of coal-bearing strata, the existing in-situ stress conditions, and the ability of selected species to survive and adapt to changes in their environment caused by longwall coal mining subsidence. Needless to say, geologists, biologists, and engineers are all playing a pivotal role in developing prevention controls and recovery measures necessary to protect streams and sustain underground coal mining. The challenges facing scientists and engineers in sustaining the development of underground coal mining in Pennsylvania are examined through case studies. These challenges all arise from the subsidence effects of longwall mining. The key effects of subsidence, discussed through case studies, are: a) tension cracks, b) springs in headwater stream areas, c) stream gradients and subsidence basins, and d) stream compression ruptures and horizontal stress. All of these potential threats demonstrate the need for high-level scientific knowledge and innovative engineering solutions. One could argue that Pennsylvania?s stringent stream protection standards put the coal mining industry at a competitive disadvantage compared to other coal producing states. It is equally likely that compliance with these standards help to mitigate the effect of subsidence, thereby helping the coal industry achieve a sustainable future.

Jan 1, 2011

By Eric G. Zahl

Researchers at the Spokane Research Laboratory of the National institute for Occupational Safety and health are evaluating stress and displacement measurements in a longwall coal mine. The objective is to determine the best indicators of coal bump hazards during operations so that mine foremen and technical stall can make more informed intervention decisions to protect miners from coal bumps. Instruments were clustered in two panels, a yield pillar, and the immediate roof and floor of a two-entry longwall coal mine. Data were gathered continuously from a variety of instruments as mining proceeded toward and past the instrument cluster. Researchers and the mine's technical staff evaluated these data with respect to coal bumps and bump mechanisms. Selected graphs were then presented to mine foremen on a periodic basis to test their usefulness in identifying hazards. Of particular interest was the use of horizontal stress data from the immediate roof. To measure changes in horizontal stress, a new technique was devel¬oped for installing a grouted biaxial stressmeter in the roof. An evaluation of the development of this stress monitoring system in producing data beneficial to mine foremen is presented.

Jan 1, 2000

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 Chris Mark

Pillar recovery continues to be a significant ground control hazard. During the past decade, 10 miners were killed during pillar extraction operations in southern West Virginia. Studies conducted during the past decade have identified a number of "risk factors" that can be used to evaluate pillar extraction plans: Cut sequence Final stump Timber or Mobile Roof Supports Roof bolting Intersection span Depth of cover Roof quality Age of workings For each of these factors, rock mechanics science suggests which alternative would be expected to be more risky. For example, numerical models were used to evaluate different cut sequences, and indicated that less roof convergence occurred with the outside lift method than with the Christmas tree in the particular environment simulated. For many of the risk factors, accident statistics confirm the science. One finding was that currently almost 70% of the retreat coal in southern WV is being mined with MRS. In contrast, timber supports were used in 70% of the past decade's pillaring fatalities. This paper discusses each of the risk factors in turn, presents the relevant accident statistics, and shows how the risk factors can be combined to estimate the overall hazard. It also addresses the use of pillar design to minimize the risk of global stability hazards including squeezes. massive collapses, and bumps.

Jan 1, 2002

By John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By Marc T. Filigenzi

Two of the major ground control safety issues confronting underground mine operations today are shaft pillar stability and the failure of rock around active mine openings. Failure of a mine shaft can lead to the entrapment of workers. Failure of rock around active underground mine openings can lead to roof falls, which in turn can result in worker injuries and fatalities. Finite-element analysis has proven to he a reliable method for predicting stresses and displacements around underground mine openings. However, this is a complex and time consuming technique and is not used as often as it could be in mine planning. The purpose of this paper is to demonstrate one technique developed at the Spokane Research Center that allows the user to create a finite-element model of a two-dimensional section of an underground mine in a relatively straightforward manner. This model is then used to calculate stresses, displacements, and safety factors around mine openings. With this information, mine planners can evaluate the stability of mine openings as well as the stability of pillars and mine shafts. This analysis will help develop mining sequences and layouts that minimize stresses in that section of the mine. This, in turn, should minimize the occurrence of shaft failure, roof falls, and other hazards associated with underground mining.

Jan 1, 1997

By Li-min Liu

The Semi-Analytical method can be used to solve a variety of problems in geomechanics and civil engineering. When it comes to layered rock strata and rock masses, it is superior to all other analytical methods. This paper presents the application of a 3-D semi- analytical method to 3-D surface subsidence and stress analysis caused by mining. In this semi-analytical method the layered elements and triangular elements ate used alternatively. Their displacement pattern, strain mahix, elastic matrix, stiffiness matrix, load matrix and stress matrix are presented. A numerical simulation program based on the semi-analytical method bas been developed using Fortran 90. This program is suitable for subsidence prediction and stress analysis of mom and pillar mining, strip longwall and mechanized longwall mining methods. It is applicable to all geological conditions and mining methods. Comparing with the finite method, this method reduces significantly the number of simultaneous equations to be solved and the amount of data preparation. It not only reduces the computer memory, but also runs faster and cost less. Key Words Semi-analytical method, surface subsidence, stress analysis, layered element, triangular prism element

Jan 1, 2002

During the last decade mechanical rock excavation has become increasingly widespread in the mining industry and, more in general, in excavation engineering. This technology, especially the tools and their operating mode, has developed according to empirical procedures and for this reason their validity should be ascertained by systematically re-examining the rock breakage processes. This topic has been approached in the framework of a joint research project involving the Universities of Cagliari (Italy) and Wollongong (Australia). The investigation has been carried out experimentally as well as with the methods of numerical modelling which is based on fracture mechanics principles. The first results of this integrated research led to the proposal of an interpretation of rock breakage by means of drag tools and to the identification of a basic correlation between the rake angle of the tool and the grain size of the rock fragments produced and, therefore, the energy efficiency of the operation.

Jan 1, 1990

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

By S. S. Peng

The hydraulic powered supports have been employed in U. S. longwall mining for the past two decades. Yet the sciences of roof control at the powered supported faces remain largely unknown. In order to improve longwall roof control and increase production, a series of questions in longwall roof control should be studied and answered: what is the basic rules of roof behavior in longwall coal faces equipped with the hydraulic powered supports? What is the interaction between the roof, supports, and the floor What is the contact condition of the canopy with the roof, and consequently the load distribution on the canopy? Des the periodic roof weighting really exist in the powered supported face and if so, how does it affect the face area? This paper presents the preliminary analysis of underground observations and instrumentations in a longwall panel with 4-leg shield supports designed to address the questions mentioned above.

Jan 1, 1982

By H. Maleki

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 measurement 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.

Jan 1, 1984

By Xiaolin Wu

Most coal bump occurrences in the U.S. are directly related to mining operations under strong roofs. An understanding of the behavior of bridging or cantilevering strong roofs over working longwall faces is a key to a safe and productive longwall operation. In this paper, an attempt has been made to study the behavior of strong roof beds using the elastic beam theories. Four structural models are formulated to simulate the locations of strong roofs and their distinct weighting stages. For each model, analytical solutions for the deflection, bending moment, stress, and the strain energy stored in both the coal scam and the strong roof are obtained. Solutions for the critical spans, which are controlled by the caveability of the strong roof beds, are developed. The effects of rocks' differing elastic moduli, under tension and compression, on the actual flexural rigidity, and stress redistribution are analyzed.

Jan 1, 1993

By Michael J. Friedel

The fundamental problems associated with ground control reflect the inability of mining personnel to "see" the development of anomalous conditions. Toward this end, the U. S. Bureau of Mines is investigating the application of seismic refraction tomography for imaging the process of pillar yielding, at the Foidel Creek coal mine, near Steamboat Springs, CO. The inherent three-dimensional problem of imaging coal pillar stress is reduced to a single plane by considering only the stress transmitted from the pillar to the adjacent floor rock. This approach exploits the fact that refracted seismic energy is essentially confined to the adjacent higher velocity sandstone strata. The tomographic analysis using ray-tracing based on wavefront migration resulted in a robust P-wave tomogram from which regions of high velocity indicate zones of high stress concentration, whereas low velocity zones indicate regions of stress relief. The interpreted region of high stress concentration occurs at the pillar end nearest to the working face of the adjacent longwall panel. This finding is corroborated by vertical pressure cell measurements recorded in the pillar. The stress diminishes toward the pillar perimeter with local minima correlating to the locations of observed pillar spalling. Time-sequence monitoring, as the longwall face advances, is recommended for evaluating changing stress conditions.

Jan 1, 1993

By Tim Burgess

Patton Mining, LLC and J.H. Fletcher Company recently collaborated and mechanized the sometimes hazardous, and always labor intensive, process of installing standing steel supports during the construction of a slope entry of a new mine. The new innovation is a beam setting machine design that can lift and maneuver heavy steel beams into place over and around the belt and vent tubing. In addition, the machine is designed with an integral double extending lifting work platform to allow workers a stable work area from which to install metal sheeting between the beams. The previous method to install the heavy steel supports and the steel sheeting included multiple workers lifting heavy loads frequently on stepladders. This paper will inform the reader of how Patton?s major goal to develop the safest, most continuous slope development process possible was achieved and the beam setting machine that evolved from the project.

Jan 1, 2011

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999

By David B. Plumeau

The Cayuga Mine, a deep rock salt mining, operation, changed from a standard room and pillar mine design to a yielding pillar/pressure arch design five years ago. In general the roof conditions improved. However, entries adjacent to the abutment pillars continued to fail. It was noticed that where abutment pillars were accidentally "notched" (by driving cross- cuts one cut into the pillars), the abutment entries had better roof conditions than where there were no "notches". Experiments with intentionally notched abutment pillars showed that notching did improve abutment entry conditions. During the past four years notch configuration has been varied to suit local ground conditions. Recently, "pre-cutting" of notches has proven itself as a means of protecting abutment entries under severe ground conditions. As an added bonus, the notches improve productivity by providing more places to work and reduce costs because it is not necessary to bolt the notches.

Jan 1, 1981

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 Eddie Martin

Jim Walter Resources, Mining Division, employs the retreat longwall mining system as the primary method of coal production. The strata control aspects of developing and maintaining roadways for retreat longwall mining have continued to improve and advance at Jim Walter Resources. New and innovative concepts in pillar design, roof and rib support, as well as longwall face support, have been combined to provide better strata control. Control of immediate roof breakup in the headgate and tailgate entries has been a problem at Jim Walter Resources' mines. The application of roof trusses as supplemental roof support failed to provide the desired strata stability. The immediate roof deterioration has been controlled through the installation of increased density of roof bolts in the affected roadways. Control of the rib sides has been a documented problem at Jim Walter Resources' No. 4 Mine since its start-up. The installation of 3/4 inch diameter roof bolts at 2 ft. spacing failed to control rib rolls. The development and application of a yieldable rib support system provided the desired results. Rib rolls have been virtually eliminated where the yieldable system of rib support has been installed. An evaluation of longwall panel development using the total yield pillar mining plan has been completed. Results of the strata stability and other operational data have been compiled. Coal production and gate entry stability were not adversely affected using the total yield pillar mining plan for longwall panel development. Strata Control on the longwall face is provided by the face support system. Excessive convergence and roof breakup along the longwall face have been experienced at several of the Jim Walter mines. In an effort to eliminate this problem, an evaluation of higher setting pressures for longwall face supports is being conducted. Results have been inconclusive to date. Each of the above strata control projects will be presented as case studies. Since Jim Walter Resources business is coal production, emphasis will be directed toward the application rather than the theoretical aspect of the projects.

Jan 1, 1988

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