Search Documents

By Dave Forrester

Mining subsidence over shallow abandoned coal mine workings is unfortunately not an uncommon experience in the coalfields of North America and there is a significant knowledge and literature base addressing the phenomenon and related issues. From time to time, however, a case history arises which is outside the more routine occurrence. This paper addresses one such case and presents an overview of a subsidence Event above abandoned coal workings, which badly damaged a two-story High School building. The subsidence at the Cape Breton & Victoria Regional School Board (CBVRSB)'s MacDonald High School in Dominion, N.S., Canada on October 30, 2002 led to a detailed investigation. It was concluded that the ground movement and associated building damage had been caused by mining subsidence in underlying old underground coal mine workings of the 1890s, only 150 ft deep at the School. The Event was not isolated and a similar but smaller event happened nearby a few months later. The paper describes the events and summarizes the subsequent investigations and findings. The 17-year old High School building was eventually demolished.

Jan 1, 2004

By Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

By Bruce W. Jack

An extensive underground roof-monitoring program was conducted in order to determine the roof strata behavior under various conditions. In total 29 sites at 5 different collieries were monitored using the sonic probe extensometer. Results showed that, in drill and blast sections, there are often pre-existing openings in the roof prior to the installation of the support. An estimated 42 per cent of the total roof displacement monitored in drill and blast sections took place prior to the installation of support. A comparison between roadways and intersections indicated that, for a 40 per cent increase in the span, taken across the diagonal of art intersection, relative to the roadway width, the magnitude of the displacement in the roof increased by a factor of 3. Total displacements measured were relatively small, only 2 reaching double figures of 12 mm. The remainder were all less than 7 mm. The results also showed no evidence of a substantial increase in the height of the bed separated, potentially unstable roof strata, as is the case in the high horizontal stress-driven beam-buckling mechanism experienced in for instance some Australian coal nines. Therefore, it was concluded that, in South African collieries, the effects of horizontal stresses is generally relatively small compared to overseas collieries.

Jan 1, 2000

By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

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 W. A. Cincilla

The use of resin-grouted rock anchors continues to gain popularity in underground wall mines in the United States. It is estimated that during 1985, more than 30% and as many as 35% of all roof bolts used employed sane from of resin anchor. The so-called 'combination' bolting systems, which generally utilize a resin pint-anchor, have also found increased application. This increased application is particularly true in poor ground, where more 'traditional' support measures have performed poorly. Virtually all resin bolts in use today in U.S. mines employ cartridge-type delivery systems, which in general, have produced good results under a variety of conditions. However, a host of inconsistent and sometimes uncontrollable variables may significantly alter the optimum bolt performance. These variables have been identified and are well documented in the literature (Snyder, et al., 1979; Kwitoweki and Wade, 1980; Serbcusek and Signer, 1985). The beau of Mines, Denver Research Center (DRC) conducted a major research project aimed at investigating the effects of selected aspects of installation procedures on resin-bolt performance. One objective of the project was to establish anchorage criteria for full column resin bolts installed in actual mining conditions. This piper describes the results of a particular study which focused on quantifying, through field tests, possible reductions in bolt anchorage due to installation of less-than optimum resin columns. These columns might result from losses of resin to surrounding strata, or non-compliance with recommended installation procedures. Over 1000 resin bolts having grout columns ranging from 12 inches up to 48 inches (full column) were installed and pull-tested at 13 underground test sites located in a total of 11 mines (Fig. 1). Four basic roof-rock types were included: shale, top coal, sandstone, and limestone. The majority of the bolts employed 3/4-in-diameter, 48-in-long, type 40 rebar having a minimum '~ated' yield point of 17,600 lb (8.8 tons). Actual yield strength may vary by manufacturer.

Jan 1, 1986

By N. P. Kripakov

The U.S. Bureau of Mines (USBM) is conducting research to develop a practical computer-based tool that will allow coal mine planners to anticipate rock mass behavior surrounding mine entries prior to actual mining. More specifically, this tool estimates to what degree and extent the immediate roof and floor strata and the coal pillar ribs surrounding an opening will weaken or fail for a given set of mining parameters. The technical approach couples together two numerical techniques: the boundary-element method and the finite-element method to allow for an approximate three-dimensional simulation of actual mining situations. Outputs from plan-view, linear-elastic, boundary-element models executed at different mining stages are used as input to a detailed, section-view, finite-element model to estimate the degree and extent of failure expected to occur around the periphery of an entry system. Using a pseudo-elastic approach. elastic rock mass properties are continually updated as different zones are predicted to fail. The procedure is relatively easy to use and is being set up to work interactively with the user who will not be expected to be a computer expert. Inputs include easily understood parameters obtainable from the field and rock mechanics laboratories. Examples of computer-generated output from generic sample models representing typical mining situations are presented and results discussed.

Jan 1, 1994

By Dennis R. Dolinar

A room and pillar coal operation in central Pennsylvania was experiencing roof cutters and long running roof falls caused by high horizontal stresses. The roof conditions created hazards for the miners, and caused several production panels to he abandoned prematurely. The mine requested assistance from the National Institute for Occupational Safety and Health (NIOSH) in applying the "advance and relieve" mining to reduce the affects of the horizontal stress during panel development. The "advance and relieve" plan that was developed called for removing a pillar on one side of the panel as it was being advanced, thus creating a cave. The caving then relieved a portion of the horizontal stress across the panel. Because the horizontal stress direction is key to the success of this method, stress mapping as well as mining experience was used to establish the direction of the horizontal stress. Subsequent field measurements provided an estimate of the stress magnitude. Three panels were mined using this technique, and good roof conditions were achieved in all these panels. Instrumentation was used to monitor the stress changes in the roof created by the cave. Stress relief of over 1.000 psi was measured 120 ft from the cave and to depths of 20 ft into the roof. The lateral extent of the stress relief zone across the panels appears to be about 400 to 500 ft. This case history has provided much insight into the practical application of the "advance and relieve" method discussed in this paper.

Jan 1, 2000

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 Fred Leighton

Outbursts, or the sudden and sometimes catastrophic expulsion of coal and gas into the working area of a coal mine, are a little understood and growing problem. The AE/MS (Acoustic Emission/Microseismic), or rock talk, activity prior to and associated with several outbursts has been recorded using ultrasonic and micro- seismic equipment and has been analyzed by the Bureau of Mines. Analysis of these data indicates that outbursts are comprised of several failures in rapid succession and that anomalous AE/MS data suitable for providing failure warning are generated 15 to 45 minutes prior to the failure. This paper discusses the techniques for recording and analyzing these data and presents sane case histories.

Jan 1, 1982

By Hani Mitri

A new monitoring device called SAFEBOLT (Stress And Force Evaluator in Bolts) has been recently developed. The device SAFEBOLT, essentially replaces the load cell, it permits the monitoring of bolt axial load from the time of its installation underground. The SAFEBOLT system consists of a portable readout unit, connecting cable and the SAFEBOLT device, which is installed in the bolt head. SAFEBOLT can be installed on any industry standard rockbolt. No special installation method is required; the conventional method of bolt installation by the stoper or roofbolter remains unchanged. The device is completely encapsulated in the bolt head, which makes it resistant to corrosion, rugged, and free of any electrical wires. Pilot tests at three underground Quebec mines were carried out and helped improve the initial design of the device connector. The paper reports on two of such pilot tests.

Jan 1, 2001

By A. Bortolussi

The paper discusses the state of the progress reached in the development of the diamond wire tool for cutting hard rocks. To the advances of this relatively new technology a substantial contribution has been provided by the research being carried out at the University of Cagliari on samples of granite rocks from different Sardinian stone quarries. On the basis of field experiments and laboratory test data, the mechanism of diamond wire cutting has been assessed. In particular the influence of the rock features and operational variables an tool performance (cutting rate and bead wear) has been elucidated.

Jan 1, 1992

By Thomas M. Barczak

A theoretical evaluation of the impact of horizontal load on shield support capacity is made from a static rigid body analysis of the shield structure. Parameters affecting horizontal support resistance are investigated for a typical too-leg shield design. Horizontal forces acting on a roof support element are found to be both roof-generated and shield-generated. The impact of shield-generated horizontal forces on roof strata is also discussed from a theoretical viewpoint in relation to desired ground control. A technique is described whereby horizontal loading of shield supports can be measured underground. Horizontal load measurements from two different longwall faces were found to be significant, ranging from 20-35 pct of measured vertical load.

Jan 1, 1984

By Donald P. Richards

West Elk Coal Company operates its Mt. Gunnison No. I mine on the western slope of the Rocky Mountains in Western Colorado. The mine is located along the north fork of the Gunnisson River near Somerset, Colorado as shown in Figure 1. The mine is designed for a sustained production of 1.4 million tons per year out of the "F" coal seam in the Mesa Verde formation. The coal seam is nominally 7 feet in thickness and dips at about 3 ½ degrees to the northeast. Mining of the seam is up dip with the entry portals on the north facing slope of Mount Gunnison. The mine is designed as a five entry system with four entries for ventilation and access and one entry for a coal-handling conveyor. The mine entry layout is shown in Figure 2. Initial mine planning began in 1979, with "permitting" complete and construction starting in the fall of 1981. A "turnkey" contract for design and construction of all five parallel mine entry tunnels was awarded by West Elk Coal Company to a Joint Venture of Jenny Engineering Corporation (design and contract management responsibility) and Affholder, Inc. (construction responsibility). The first entry access into the coal seam was completed in the early spring of 1982. The mine has been operating at full service (all five entries) for nearly 3 years.

Jan 1, 1984

By Thomas M. Barczak

Wood cribs are used extensively to stabilize mine openings by providing resistance to deflections of the immediate roof and floor and by supporting the weight of unstable rock masses. While the unit costs of these supports are relatively low, their extensive use results in significant costs to coal mining. The Bureau of Mines has developed a model that computes the load capacity of wood cribs as a function of the displacement of the crib structure induced by mine roof and floor convergence. This permits comparison of the loading characteristics of various crib constructions and enables systems to be designed with consideration of the load conditions imposed by the mine environment. The design method matches the stiffness, strength, and stability of the crib structure with expected rock mass behavior to determine a crib design and employment spacing that will provide the lowest cost support.

Jan 1, 1993

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By Y. Luo

An extensive subsidence research program conducted by the authors has greatly improved the accuracy and the capabilities of subsidence prediction under complicated mining and surface conditions. The techniques developed for assessing and mitigating subsidence influences on various surface structures have been applied in numerous cases with a very good re- cord of success.

Jan 1, 1997

By John L. Hill

Cutter roof failure is a specific type of ground control problem which frequently results in massive roof failure. It is a cannon occurrence in coal mines of the Northern Appalachian Coal Sasin, causing delays in production and posing a safety hazard to mine personnel. The Bureau of Mines is conducting research on the causes of cutter roof failure togain a basis from which to prevent its occurrence and to support such roof when failure does occur. Research conducted in a coal mine of central Pennsylvania has revealed a correlation between the occurrence of clastic dikes and fonation of cutter roof failure. Inmine mapping of ground conditions showed an increase in roof failure in areas of high frequencies of clastic dikes. Rock pressure monitoring around clastic dires registered the greatest amount of roof loading near the inter- section of dikes with the rib. Load cells measurim horizontal pressure changes in the roof indicated that the greitest pressure changes were occurring perpendicular to entry headings when clastic dikes were present. Analysts of rock pressure nwnitiring shows that the roof behaved as two cantilever beams when severed by a clastic dike. Additional roof supports such as trusses and cribbing were found to effectfvelv suooort the roof in areas of clastic dikes and prevent cutter roof iailure. However, there lnethads were only successful when employed shortly after mining.

Jan 1, 1984

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 John Feddock

This study deals with underground mines and ground control and the prediction of seats strength characteristics based upon ultrasonic logging. Exploration drill holes, both rotary and core holes are used for proving coal reserves and determining mining parameters. Drill hole cores can be subjected to geologic study and geotechnical evaluation. In addition, the use of geophysical logging techniques is well established in determining certain strata characteristics and ground parameters. Since the characterization of the rock mass in mines remains insufficient in most cases, the idea of possibly characterizing the strength parameters without expensive core retrieval and laboratory testing is very attractive. The elastic properties of the rock can be deduced from measurement of the sonic velocity if the density is known. Through the use of in-hole geophysical density logging combined with an ultrasonic logging tool, the estimation of strata properties is possible. An investigation was conducted at two Indiana underground coal mines using geologic logging, geophysical logging, ultrasonic logging and laboratory strength testing to establish the strength properties of the roof rock. A specialized ultrasonic logging tool with one transmitter and two receivers enables nearly continuous readings along the length of the hole. The results of correlating portions of the data are presented. It is anticipated that the ultrasonic logging will be a supplemental tool for extending laterally the strata characteristics obtained from the existing program to surrounding areas. This may be obtained from the relatively inexpensive logging data obtained by ultrasonic logging.

Jan 1, 2003