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By S. M. Hsiung

A model (support selection model) for the selection of suitable types of powered supports under various geological conditions is proposed. This model is developed by evaluating the support, performance under different roof conditions at longwall faces in terms of five controlling parameters: (1) roof Falls, (2) front abutment pressure, (3) Face convergences, (4) gob materials entering into working area, and (5) roof caving line. This model suggests that more than one type of powered support. may be used under the same roof condition and the best selection should he based on the magnitude of suitability index of each support- type. The development and application of suitability index arcs discussed. Fourteen panels in seven coal seams are discussed in the paper to illustrate the applicability and effectiveness of this model. A roof classification system at longwall faces developed specifically for the construction of the selection model is also discussed. A simple example fur determining roof classification index is given.

Jan 1, 1988

By Thomas L. Vandergrift

With the inevitable expansion of homes, businesses, and infrastructure in coal mining regions, the potential for future subsidence above abandoned mines is of increasing concern. Of particular concern are areas underlain by room-and-pillar coal mines. Unlike areas above longwall mines, where total extraction causes subsidence in a predictable and timely manner, subsidence above room-and-pillar mines is difficult to predict and may be active for many decades. As part of a geotechnical assessment of a water pipeline replacement project, NSA Engineering recently performed an evaluation of tong-terns subsidence potential along the proposed alignment due to time¬dependent failure of underlying room-and-pillar workings. The alignment of the replacement pipeline closely follows an existing concrete pipeline, which lies 150 11 to 300 fl above workings that both pre- and post-date the pipeline. Annual surveys along the existing alignment indicate that some subsidence has occurred, primarily from partial pillaring performed in the early 1980's, but that this subsidence is not active. Using the boundary-clement code LAMODEL as the primary analysis tool, NSA followed a three-step approach to estimate future subsidence impacts on the replacement pipeline. First, historical information for the mine was compiled, allowing for calibration of models to actual field conditions. Then, numerical models of workings directly under and adjacent to the proposed alignment were nun, with model input parameters varied until resulting loading patterns matched historical data, and subsidence in the model approximated that measured along survey lines. Finally, time-dependent deterioration of coal pillars was simulated by decreasing the coal strength in the calibrated model until a realistic "worst-case" condition (stability of barriers, but Yielding of production pillars and pillar remnants) was attained. Subsidence from the previous calibrated model was then subtracted from that of the worst case model to estimate future subsidence and related ground movement parameters that slay occur along the proposed alignment. The replacement pipeline design, incorporating couplings capable of withstanding the ground movements predicted, is now being finalized.

Jan 1, 2000

By Kot F. v. Unrug

The depletion of thicker coal reserves has resulted in the thinner seams being economically attractive. Surface protection against subsidence limits the extraction ratio to 50% for the room and pillar method as a "no subsidence condition". That rule was developed for pillars with smaller width/height ratio because seams mined at the time were commonly 6 feet thick. Over time pillar ribs deteriorate around the pillar perimeter and thus decrease the area of the pillar. Using 50% extraction as the no subsidence condition in the squat pillar category (below 42 in with w/h > 7) causes unnecessary losses of reserves because squat pillars are stronger. In this paper a method is presented for design of long-term pillars. Based on the analysis of pillar deterioration and field observations, an increase of the "no subsidence" extraction rate to 60% for squat pillars is proposed.

Jan 1, 2001

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 E. M. Williams

The U.S. Bureau of Mines is investigating the use of in-seam seismic methods to monitor stress-induced changes in coal near the working face of longwall mines. Seismic surveys performed through a longwall panel and pillars near the face can provide valuable information on geologic anomalies, high stress regions, tailgate support performance and other conditions that could endanger miners or interrupt production. This paper describes the details of the equipment and data analysis techniques used, and the significant results obtained and their implications for ground control. Using seismic tomography, it is possible to conduct measurements on the pillar and reconstruct an image representing the internal velocity distribution. Successive measurements over several days identify changes in velocity attributable to changes in stress. Several field measurements were conducted at a western U.S. longwall mine to observe stress induced changes within the panel and pillars during mining. A permissible data acquisition system was designed to meet mine safety requirements for operation in gassy air. Certification from the Mine Safety and Health Administration (MSHA) was obtained, and this approved system was used to monitor stress changes in several pillars as the longwall face passed during first panel mining. Tomographic surveys were completed across the longwall panel to map the apparent abutment stresses ahead of mining. Velocity distributions show an increase in vertical stress across the panel from the headgate to the tailgate. Additionally. a yield zone along the face near the tailgate and the forward abutment approximately 100 feet outby the face in the tailgate side of the panel were detected. These results agree well with data obtained from other instrumentation emplaced in the panel near the tailgate. Using velocities for both compressional and shear waves, the Poisson's ratio was calculated for the panel.

Jan 1, 1994

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 John Cassie

The condition of roadways in the Deep Main seam at Asfordby mine, operated by RJB Mining (UK) Ltd, is strongly dependant on drivage direction relative to the maximum horizontal stress. The mine is laid out with gate roads driven approximately parallel to the maximum horizontal stress giving favourable drivage conditions. This has led to particulary difficult conditions for face installation headings which have to be driven to an increased width and at a high angle to the horizontal stress. A solution to the formation of face headings is described making use of stress relief with the face heading being driven in the zone of reduced stress surrounding a previous heading. In implementing this solution use was made of results from computer modelling and detailed monitoring of roadway behaviour. High reinforcement densities including cable bolts were used in supporting the headings. The experience provides an example of a traditional technique being applied more scientifically. The results obtained demonstrate the effect of stress relief and reinforcement methods in altering drivage conditions and suggest possible alternative strategies for the formation of future face headings in these conditions.

Jan 1, 1997

By Paul Cartwright

A rockbolted roadway risk assessment procedure has been developed specifically for the Parkgate seam at Thoresby Colliery by Rock Mechanics Technology and colliery engineers. The purpose of this assessment is to identify any areas of significant risk in relation to roof instability due to potential failure or overloading of the rockbolt support system. This allows appropriate additional support to be installed in sections of roadway considered to be at increased risk. The assessment of risk in rockbolted coal mine roadways, in terms of fall of material, has to be based on an understanding of the rock mass behaviour at that site. The development of an appropriate risk assessment system is therefore site specific. A consistent approach to developing such a system can, however, be applied by adopting the same general logic to each individual site. The use of risk assessment to categorise areas with increased risk of roof instability has been undertaken for four longwall panels at Thoresby. The procedures used for these assessments are discussed and the operational benefits examined. The approach successfully used at Thoresby has been developed into a routine tool to assist mine personnel in assessing areas of potential roof instability in rockbolted roadways and as a means of communicating the position of these areas to both supervisors and workmen. The system provides a rational and objective interpretation of the current underground conditions based upon a logical, consistent and structured approach.

Jan 1, 1999

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 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 Christopher Mark

The past decade has provided a wealth of practical experience with longwall pillar design. Design formulas developed specifically for longwalls have proven their value in numerous applications. Analytic methods, including numerical models, have a1 so made important contributions. These advances have been based on basic research into abutment loads, coal pillar mechanics, and the interaction between pillar performance and entry stability. The goal of this paper is to review the state of the art in longwall pillar design from the standpoint of the practitioner. Actual case histories are presented to illustrate the current capabilities and limitations of pillar design methods. The paper also discusses the advances that can be expected in the future.

Jan 1, 1990

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 John C. Stankus

ldentifcation of potential roof problems in coal mines has long been a complex issue due to the wide variety of mining and geological conditions. It is well known that mine roof falls are related to many different factors such as roof strata properties, sandstone channels, regional and localized horizontal stress, vertical stress, and tectonic stress. Many times, a combination of these factors causes roof problems. Through years of ground control experiences, Keystone Mining Services LLC, (KMS) has developed a Roof Instability Rating (TUR) system to predict potential roof problems. The first RIR analysis was presented at the 19th Ground Control Conference (Watts, et al., 2000). The RIR system includes: quantifying each individual factor, weighting each factor, combining individual factors, finalizing the RIR, and setting the criteria for the areas where special support is needed. In this paper, the application of the Roof Instability Rating (RIR) system at Enlow Fork mine will be presented.

Jan 1, 2001

By Reha Ozel

The stability in longwall faces depends on the interaction beman the roof strata, face supports and floor strata. The load distribution on the other hand, depends upon relative stiffness of the three supporting elements namely, the coal face, support and gob. Therefore, in defining support requirements, it is particularly important to analyze the strata loading, the face convergence and the controlling mechanism provided by supports. The controlling mechanism however, depends on the type and characteristics of supports used in long- wall faces. The design guidelines presented in this paper have bean developed based on a number of strata control oriented projects carried out in Turkey, load and convergence measurements carried out in 10 longwall faces, in Zonguldak Coal Region, and procedures given in literature. Evaluation of load and convergence measurements, and analyses of face supports were carried out by means of a computer program, called L0NGWAI.L-MEW, developed by the authors. The output of the design analyses include the following: (1) estimation of support capacity for roof supports namely; individual hydraulic props, chock-type power supports and various types of shield supports, and (2) analysis of loading conditions and response of shield support components in terms of stability and structural integrity.

Jan 1, 1990

By Lorraine Kent

Comparison of in-situ stress overcore measurement results with those determined from laboratory evaluation of Acoustic Emission (AE) testing on oriented sub-core samples has been carried out for a variety of mining environments. These include UK and German Coal mines at moderate depth, shallow-level South African Coal mines and deep-level South African gold mines. Results show that the `Kaiser Effect' or 'Stress Memory-Damage is readily determinable throughout the brittle deformation process, although particular care is required in interpretation of results during the 'bedding-in' phase of testing and the region associated with unstable crack growth prior to failure. The results suggest that AE provides a useful complimentary method for comparison with existing methods of stress determination. The success of the method can, however, be dependent on both rock type and the likely magnitude of in-situ stress in comparison to the strength of the rock. Keywords: in-situ stress, acoustic emission, Kaiser Effect

Jan 1, 2002

By Eric R. Bauer

Skin failures of roof and rib in underground coal mines continue to be a significant safety hazard for mine workers. Skin failures do not usually involve failure of the support systems but result from rock or coal spalling from between the support elements. For instance in 1997, over 800 miners were injured by roof and rib falls, of which 98 pet were the result of skin failures. Also, nearly 80 pct of the roof and rib failure injuries occurred at or near the working faces in development sections. The face area is a zone where the potential for skin failure accidents and injuries and roof and rib failures, in general, is high because of mining activity, ground readjustment due to changing stress conditions, and the higher exposure of mine workers. In addition, failures occur where the roof and rib are unsupported. This paper features a review of the roof and rib accident statistics resulting from skin failure, highlighting the incidences by average days lost, in-mine location, state, MSHA District, and worker activity at the time of injury. A detailed analysis of recent fatalities caused by skin failure is included. Also discussed are the causes of roof and rib skin failures, current methods and support materials for skin surface control, as well as a historical literature review of skin failures and control methods.

Jan 1, 1999

By Rao Pothini

Measurement of pressure distribution under the base plate of the Hemscheidt 2-leg 800-ton shield was performed underground for two shifts using 12 pressure cells installed in two rows, one each under each split base plate. Pressure distribution was uneven depending on contact condition. In general the highest base pressure occurred at 1/3 of the base plate contact length from the rear edge. The resultant was near vertical, and the pressure under the tail side split base plate was always higher than the head side. For comparison, pressure distribution on the canopy and under the base plate was also measured under controlled conditions at the USBM's Mine Simulator Testing Facilities in Pittsburgh, PA. A total of 45 tests were conducted under several simulated contact conditions. The results showed that the actual contact area is highly dependent on the compliance of the immediate roof and floor due to the sloped canopy tip and a base lifting device under the base tip. The loading distribution tends to change in magnitude and profile as the leg pressure increases. The base lifting device exaggerates the high toe loading on the base plate characteristics of this type of shield support. This paper describes in detail the testing methods, data analysis results obtained, and recommended changes.

Jan 1, 1992

By Liang Yin-Huai

In Fuxin Coal Mines, there are more than 20 million tons of coal buried under seasonal streams. The coal seams are mostly shallow. We have mined more then 50 mining blocks distributed in 5 coal mines. Their natural conditions are different, and the consequences are also more or less different. However, no stream-water-inrush accidents under regular mining conditions are the same. There were 3 exceptions. They were caused by unexpected roof caving which made valleys or alluvia subside as funnel-like cavities and let in inrush of stream flow. The key to our success in mining shallow under-stream coal seams, according to our analysis, is that valleys or alluvia subside gently and basically evenly with contractible tension fissures and that fissures both in valleys or alluvia and in rock seams are blocked up or sealed by cohesive deposits either from alluvia or rock. This paper treats: (1) some typical illustrations of mining shallow coal seams of different occurrences under streams of different sizes, flowing capacities and valley structures, (2) observations of the height of caved and fissured zones and subsidence of valleys or alluvia, (3) observations of underground water flow before, during, and after mining and the fluctuation with respect to stream flow, (4) observations and analyses of blocking up of fissures caused by mining, and (5) feasibility of mining shallow under-river coal seams.

Jan 1, 1990

By B. G. D. Smart

The development of a softwood alternative to the hardwood chockpiece used to build the waste-edge breakers commonly incorporated in gateroad packs is described. The results of laboratory tests on softwood prototypes and two underground trials are presented. The results of the last trial suggest that a viable alternative chockpiece has been designed, offering advantages over availability of wood, weight and given reduced wood content, price.

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