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By T. L. Dobecki

As a means of demonstrating the effectiveness of reflection seismology in determining continuity of coal seams, three U. S. field projects are reviewed. The three projects involve coals of varied thickness (2-14m) and age (Pennsylvanian to Eocene) from three coal areas of the U. S. (Pennsylvania, Wyoming, and Washington). Each project also employed its own particular seismic technique, recording system, and seismic energy source although all are considered state-of-the-art, high resolution, digital seismic surveys. Project 1 (thin, Pennsylvania coal) sought detection of sand channels using dynamite and standard in-line (2-D) seismic technique. Project 2 (thick, Wyoming underground coal gasification) involved a gas-explosion (Dinoseis) source with areal (3-D) acquisition methods. Project 3 (thick, Washington underground coal gasification) employed a shotgun-type source and standard in- line methods. Data processing was handled by different contractors for each project. Each project was successful in accomplishing its own particular objective; however, data quality and interpretation seem to be more a function of thickness of the target seam, complexity of the overburden, and processing contractor than a seismic source, acquisition scheme (2-D versus 3-D), or recording instrumentation.

Jan 1, 1981

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 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 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 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 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 Erik C. Westman

The objective of this Bureau of Mines study is to determine the structural integrity and stress state of coal structure using seismic techniques. Bureau personnel conducted seismic transmission and refraction surveys on yield pillars and seismic tomography on rigid pillars. The pillars studied were in the headgate at increasing distances from the working face of a longwall coal mine, effectively evaluating pillars at lowering stress states and increasing levels of structural integrity. By examining characteristics of the seismic wave, including velocity and frequency, the survey results are used to infer yield zone thickness, core mss state, and stress distribution. An accurate definition of these parameters can not only provide mine engineers a useful tool for evaluating mine design, but also give valuable insight into the inherent safety of the mine structure.

Jan 1, 1993

By Jay Keng

During the construction of shaft on the fractured and weak rock strata, the stability of construction plays an important role. This study includes earth stress estimation, support method, and stability analysis. Stresses on the site of three shafts include structural residual stresses, earth pressures, and hydrostatic pressures were determined. The basic method of estimating the support strength is dependent upon the' size of the plastic ring after rock excavation. During the construction of the shaft, geotechnical measurements, physical properties of the rock, and the change after excavation have to be evaluated again. Geology and construction methods need further consideration.

Jan 1, 1988

By Leslie Carver

The effects of longwall mine subsidence on water resources were studied at a West Virginia coal mine (mine Z) by Carver (1994). Data were obtained for 137 domestic supply wells and springs and 12 baseflow streams. Subsidence caused short-term dewatering (of a few days to weeks) of wells located on lower hillslopes and valleys, and long-term dewatering of hillslope and hilltop springs (with no recovery yet). New or increased spring discharge was observed at lower elevations. Unsubsided water supplies experienced no dewatering beyond a nearby panel's 27°-38° angle of dewatering influence zone. Subsidence from longwall mining beneath streams commonly reduced discharge when panel ages were less than two to three years. However, "normal" discharge was typically observed over panels greater than three years in age; such streams exhibited lower baseflow recession constants, moderated winter baseflow, and increased summer baseflow compared to unsubsided streams.

Jan 1, 1994

By Greg Hendon

Longwall developments in the UK have historically consisted of single entry gate roads. Adjacent developments were separated from existing panels by large barrier pillars (designed of sufficient width to get away from the longwall abutments of the previous panel) or by small barriers driven in the shadow, or de-stressed zone, of the previous panel. Some 2nd panel tailgates were also driven skin to skin leaving no coal barrier between the newly driven entry and the heavily supported existing gateroad. With the developement and wide acceptance of fully bolted entries and the pressure to reduce production costs, alternatives to single entry drivage, particularly yield pillar developments, were examined. Through the Rock Mechanics Branch of British Coal, a cooperative study was begun with Jim Walter Resources, Inc., USA, (JWR) to look at the yield pillar alternative in detail. This study was to determine the feasibility of utilizing yield pillars in the UK and to determine, through monitoring, the possibility of reducing stable pillar widths at JWR. The study has included extensive monitoring of the yield-stable-yield pillar system at JWR No. 7 Mine and an underground trial of a two entry yield pillar test area at Welbeck Colliery in the UK. This paper describes results from the JWR study and the subsequent results of the first advancing yield pillar development in the UK at Welbeck Colliery.

Jan 1, 1995

By Thomas M. Baraak

This paper evaluates factors which influence longwall support and strata interaction. The longwall system is composed of an immediate and main roof structure and three supporting foundations: 1) longwall panel, 2) powered roof supports, and 3) gob waste. The main roof forms a structure that is generally supported by all three foundations, while the immediate roof acts as a beam that cantilevers from the coal face to the powered support. In most cases, shield loading involves a complex interaction of both main roof and immediate roof behavior and is a combination of loads produced from subsidence of the main roof and displacements of the immediate roof caused by deformations of the cantilevered roof beam. Since the shield stiffness remains constant for all leg pressures and main roof convergence is irresistible in terms of shield capacity, the shield must be able to control the behavior of the immediate roof or floor structure for shield loading to be sensitive to setting pressures. If the goal is to minimize total shield loading, any active setting force must be offset by reduced passive shield loading to justify the active setting loads. Field data suggest that the typical reductions in passive loading do not justify the required increases in setting pressure in some applications.

Jan 1, 1990

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 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 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 Mario G. Karfakis

Mining related subsidence is a major concern over abandoned shallow coal mines. Many of the subsidence prone areas are presently used or will be used in the future for residential housing development. Concern over mine subsidence is inhibiting the development of surface land uses on previously mined area. This concern prompted public officials to realize the need to develop guidelines which can be used to assess and reduce the risk of mine subsidence damage to building and other improvements. Subsidence and the associated ground movements must be defined before one can relate them to building damage. Surface movements resulting from underground coal mining occurs in two phases termed as: active subsidence and residual subsidence. Active subsidence refers to all surface movements occur in^ simultaneously with the mining operations, while residual subsidence is-that part of the surface deformation which occurs following the cessation of mining. Residual subsidence is the major concern over abandoned mine lands. The duration of residual subsidence is of particular importance from the standpoint of damage at the surface to structures built after the mine has been abandoned. Time spans during which surface subsidence may occur, vary markedly with the mining method being used. In total extraction methods, such as longwalls, subsidence is induced rapidly, beginning almost immediately after mining. Subsidence concurrent with coal extraction is desirable for a safe longwall mining operation. Usually subsidence is complete within a relatively short time. The remaining settlement occurs as residual subsidence due to the gradual compaction of the subsided ground. The relative proportion of the residual subsidence movements is dependent upon the presence of remnant pillars and roadside packs. rate of face advance and depth of workings. In partial extraction methods, such as with room-and-pillar systems. major occurrences of surface subsidence may be delayed for decades until the support pillars and/or the roof have substantially deteriorated and collapsed. The actual time involved depends on a number of factors such as the strength of coal, roof, and floor, extent of fracturing. presence of water, depth of workings, pillar size and percentage extraction. Consequently residual subsidence is more likely to occur over abandoned room-and-pillar mines. The following sections define subsidence over abandoned mines, review the proposed mechanisms, identify the controlling factors and examine the prediction methods.

Jan 1, 1987

By Z. C. Song

[THIS PAPER WAS TRANSLATED FROM CHINESE TO ENGLISH BY S. S. PENG, WEST VIRGINIA UNIVERSITY] Immediately after coal extraction, the pressure that causes the movement of the surrounding strata is called mine pressure. Through the strata movement, certain features such as supports taking load arise. This is called "manifestation of mine pressure". The existence of mine pressure is ab¬solute whereas its manifestation is relative and conditional. The visible movement of the surrounding strata occurs only when the applied pressure exceeds the plastic breaking strength. Me support begins to take load only when it offers resistance to the strata movement. The key to investigating mine pressure and its control hinges on understanding the rules and conditions for the manifestation of mine pressure. Since the face moves continuously, the mine pressure and its manifestations are also constantly changing. 'Therefore, it should not only emphasize to obtain its instantaneous magnitude, but more importantly the rules of development changes and their relations to the movement of overlying strata. Once this is resolved, one can estimate the movement of the overlying strata through mani¬festation of mine pressure and predict the time and magnitude of incoming roof weighting. Consequently the location and timing of rib excavation can be correctly selected and thus solve several major problems in the design of mining operation.

Jan 1, 1982

By Thomas Barczak

Hydraulic cylinders are used in several roof support systems, such as longwall shields and mobile roof supports, to provide critical ground control Many state-of-the-art hydraulic support system operate at pressures as high as 7,000 psi to provide the necessary support capacities Pressure in the upper stages of multi-stage cylinders can be intensified by a factor of 2 or 3 providing pressures of 20,000 psi or greater. A proper understanding of the operation of these high pressure systems is necessary to safely maintain and repair these support systems. Likewise, the performmce characteristics need to be fully understood to evaluate the impact of design parameters on the capability of the support to maintain effective ground control This paper examines the basic operating principles of state-of-the- art hydraulic cylinders and discusses relative issues pertaining to. (1) setting loads, (2) support stiffness, (3) yielding behavior, (4) errors in assessing support loading, and (5) hydraulic failure mechanisms and how to detect them

Jan 1, 1998

By Anthony Iannacchione

As the amount of new fractured surfaces or "damaged rock layers" within roof rock increases, the stability of the rock mass decreases. While direct measurements of this phenomenon are not easily made, there is good circumstantial evidence to support this hypothesis. For example, it is common to observe increased cracks or fractures in the immediate mine roof rock before a roof fall. Likewise, roof drill holes placed in areas that later fail often reveal increased numbers and/or separations of fractures within the rock column through time. And finally, the frequency of microseismic activity, representative of rock fracturing, increases before a roof fall. For this study, more than 700 microseismic emissions were collected from two underground limestone mine roof fall areas in southwestern Pennsylvania. Microseismic events were located and magnitudes determined using the moment magnitude technique. Moment magnitude is based on the event seismic moment, which is a measure of the seismic deformation. The amount of new fracture surface length was calculated based on the stored strain energy within the rock prior to fracture. In the two case studies presented, a significant amount of microseismic activity was observed as much as two days before the first signs of failure in the roof fall areas. Additionally, results from this analysis reveal much about the behavior of strata prone to failure and allows for the construction of hazard maps based on microseismic emissions. The potential use of this technique as a means of anticipating roof falls is analyzed and discussed.

Jan 1, 2004

By S. M. Hsiung

In late September 1984, a longwall panel in West Virginia lost 60 powered supports when the face had advanced for 250 ft. from the setup entry. An in-mine investigation showed that it was a first weighting failure resulting from inadequate Support capacity. This paper presents the results of finite element modeling, which have led to a better understanding of the first weighting failure mechanism. The dynamic impact of the first weighting failure on the powered supports is analyzed. The optimum capacity for the powered supports in order to control the roof at face area under the geological condition is discussed.

Jan 1, 1984

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