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By Patrick S. Artrip

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

Jan 1, 1993

By 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 Robert C. Garson

The Mine Roof Simulator at the DOE'S Mining Equipment Test Facility is described. Its use in the research and development on the behavior and response of longwall roof supports to simulated mine environment is investigated. Advanced technology analysis methods such as finite element modeling, strain gage instrumentation, and photoelastic techniques are investigated for application in roof support design two case studies are presented of test data and results.

Jan 1, 1982

By Takashi Sasaoka

EGAT Mae Moh Lignite Mine in Thailand produces about 16 million tons of lignite annually from open-cut mining. The coal is used to generate 2,400 MW of electricity. In the near future, however, the coal must be extracted from underground at 400? 500 m (1,312?1,640 ft) deep from the surface. The problem is the thickness of the coal seams. There are two coal seams that are about 25 m (82 ft) thick with 20?25 m (66?82 ft) of interburden between them. Up to now, many mining systems for thick coal seams have been used and improved in the world. However, where the thickness of coal seams is beyond the limitation of the current mining technology and system, an applicable mining system has to be introduced, considering influencing factors such as caving and subsidence. In order to investigate thick seam mining systems and propose a new mining system in ultra-thick coal seams in Thailand, a joint research group was established by Japanese and Thai researchers and mining engineers in early2009. This paper describes underground mining systems for thick coal seams and proposes an innovative underground mining system for thick coal seam, ?eco-friendly underground mining system?, considering geological, geotechnical, and environmental conditions in this mine. This paper discusses its applicability to this mine by means of numerical analysis from the geotechnical point of view.

Jan 1, 2011

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 E. Bauer

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

Jan 1, 1988

By Z. T. Bieniawski

This paper presents the results of a survey of room and pillar dimensions and design practice in U.S. coal mines aimed at improving the design procedures in room and pillar mining. A review is given of the current pillar strength formulas and suggestions are made for better utilization of research results in engineering practice.

Jan 1, 1981

By Zhaohui Wang, Jiachen Wang

"Effective surrounding rock control is a prerequisite for realizing safe mining in underground longwall faces. In the past three decades, longwall top-coal caving mining (LTCC) and single pass large height longwall mining (SPLL) found expanded usage in extracting thick coal seams in China. The two mining methods lead to large void space left behind the working face, which increase the difficulty of ground control. Longwall face failure is a common problem in underground mining, which is induced due to low strength and high fracture density of the coal seam. Moreover, the stiffness of main components included in the surrounding rock system also greatly influences longwall face instability. Correspondingly, the surrounding rock system is developed for LTCC and SPLL faces in this paper. Mechanical conditions for simultaneous balance of roof structure and longwall face are put forward by taking the stiffness of coal seam, roof strata, and hydraulic support into account. The safety factor of the longwall face is defined as the ratio between its ultimate bearing capacity and the load imposed on it. The influence of coal strength, stiffness of the coal seam, roof strata, and hydraulic support, as well as the movement of roof structure, are analyzed. Finally, the key elements dominating longwall face stability are identified for improving surrounding rock control effectiveness in LTCC and SPLL faces.INTRODUCTIONThe concentration of coal mining in China is gradually shifting to the west region, and there are a large number of thick coal seams in the newly formed coal mining area. Therefore, the surrounding rock control problems related to mining of thick coal seams will become the focus of research in the field of mining engineering in the future. There are two kinds of mining methods for thick coal seams in China, including single pass large height longwall mining (SPLL) and longwall top-coal caving mining (LTCC). Both methods can accomplish the goal of excavating the full thickness of thick coal seams at one time. SPLL mining benefits from the improvement of the development level of associated equipment. The appearance of large hydraulic supports and high power shearers ensure that the height protected by the hydraulic supports and the height of the sheared coal are the same as the coal thickness. LTCC mining benefits from mining technology advantages. The thick coal seam is divided into two sub-layers, and the working face is normally installed in the bottom layer, while the top coal is caved under the compressive forces due to mining. Therefore, there is a scraper conveyor at the front of the hydraulic support for transporting the sheared coal and another one at the tail for transporting caved top coal. Both SPLL and LTCC methods realize one-off recovery of thick coal seams."

Jan 1, 2018

By Gustavo Herrera Rico

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

Jan 1, 1997

By Vladimir Adamek

Two existing predictive methods were chosen for evaluation; an influence function (Gals Theory) and a profile function (hyperbolic). These were applied to several field measured subsidence pro¬files over United States coal mines. The effect of homogeneous versus non-homogeneous overburden on surface subsidence is demonstrated and the utilization of Bals Theory as a subsidence predictive method for both types of overburden is examined. The major factor affecting subsidence profile characteristics is the migration of the inflection point toward the centerline of the longwall panel. A computer program was written to aid in the analysis.

Jan 1, 1981

By M. Y. Fisekci

This paper concentrates on subsidence measurements, applied over the thick and steep seam mining in the Rocky Mountains Region of Western Canada. The studies to date indicate that two new subsidence monitoring techniques appear to be the most suitable methods for these conditions. The computerized telemetry and aerial photogrammetry systems are being tested for the reliability of the systems during the winter months within the net- work of laser surveying over the workings of the new hydraulic mine.

Jan 1, 1981

By P. K. Kaiser

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

Jan 1, 1981

By Clifford J. Roblee

In this paper, various seismic indicators are evaluated for ability to discern fractures induced into a rock mass by blasting. This is accomplished by presenting results from in situ crosshole seismic studies performed prior to and after a sequence of controlled blasts within limestone and dolomite rock near Austin, TX. Indicators investigated are compression (P-) and shear (S-) wave travel times, first peak and maximum amplitudes, first arrival rise rate, and the linear spectrum of the received signal. Compression wave travel time yielded the most consistent results, while the various amplitude indicators yielded the poorest results for the equipment configuration utilized. In addition, this study documents magnitudes of change in seismic measurements which occurred following blasting. Both P- and S-wave travel times exhibited a wide range of variation due to blasting. Near blast centers below the surficial fractured rock, velocity decreases of about 10% were noted, significantly less than the 50% decrease observed for previously fractured near-surface rock through which the explosive energy was vented. Signal amplitude measurements also varied in magnitude but were less consistent.

Jan 1, 1989

By Stephen C. Tadolini

The U.S. Bureau of Mines (USBM) is conducting research to provide alternatives for traditional secondary support methods. These cost-saving methods are proving to afford safer installation, improve entry stability, eradicate material-handling injuries associated with crib supports, and enhance ventilation. In cooperation with Mountain Coal Company, the USBM replaced timber cribbing with high- strength cable supports to provide stability in a gate road that was used for two longwall panels. The support design consisted of 2.1-m (7-ft) full-column resin-grouted bolts and 4.8-m- (16-ft-) long high-strength resin-grouted cable supports. Three support concepts were evaluated: passive, stiff, and tensioned systems. Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during panel extraction. The stress and loading histories for both panels and the abutment and yield pillars were monitored to assist in evaluating the stress transfer and pillar performance in conjunction with the roof behavior. The test results indicated that resin- grouted cable support systems successfully maintained the immediate and main roofs during the extraction of two longwall panels. This paper describes the theory, application, and advantages of resin-grouted cable supports and presents the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1994

By Thomas Barczak

Longwall shields provide essential ground control in Iongwall mining, yet a high percentage of shields are operating at less than peak capacity and many at well below the rated support capacity due to defective hydraulic cylinders or malfunctions in other hydraulic components. Leg pressure data are currently collected on state-of¬the-art longwall shields, but typically are not analyzed to evaluate shield performance. The National Institute for Occupational Safety and Health (NIOSH) Shield Hydraulics Inspection and Evaluation of Leg Data (SHIELD) program is a Visual Basic software system that is designed to analyze leg pressure data and identify shields that are not performing to rated specifications. The program analysis is configured to detect the following conditions: (I ) loss of leg pressure, (2) imbalance in leg pressure, (3) low set pressure, (4) low yield pressure, and (5) full extension of the bottom stage. Other performance assessment measures include the percentage of the support capacity utilized (ratio of peak load to yield load), the percent of time that a shield operates at yield load, the ratio of the set load to the yield load, and the amount of support capacity that is lost due to leaking cylinders. Historical record keeping will allow the user to select a particular shield and review the performance record as developed by the program for that shield. In addition to these performance assessment measures, the software will include an animated description of shield hydraulic systems that will describe the operation and significance of each hydraulic component and the impact of component failures on the shield's capability to provide the required roof support. A general overview of the SHIELD program is provided in the paper together with an example analysis of a 2.5-year-old Australian longwall face.

Jan 1, 2002

By R. Karl Zipf

Highwall mining is an important coal mining method. Upwards of 60 highwall miners are presently in operation, and they may account for approximately 4% of total U.S. coal production. A review of the Mine Safety and Health Administration (MSHA) accident data over the 20 year period from 1983 to 2002 identified 9 fatalities attributable to auger and highwall mining of which inadequate ground control accounted for 1/3. In the past 5 years, 1 fatality occurred in highwall mining. Estimates of the manpower requirements in highwall mining suggest that its fatality rate is essentially the same as for surface coal mining, thus highwall mining appears to be a very safe modern mining method. Highwall stability is the major ground control related safety concern, and operators are required to develop and follow an appropriate highwall mining ground control plan. The plans usually specify the following geotechnical parameters: hole width, maximum hole depth, maximum overburden depth, seam thickness, web pillar width, barrier pillar width and number of holes between barriers. Calculated web pillar stability factor exceeded 1.3 for most designs evaluated. This study examined records from 5,289 highwail miner holes with a total completed footage of about 2,560,000 feet to understand the reasons for early pull out. Average lost footage is typically about 20% of planned footage. Only 35% of the holes reached planned depth, and 20% were short due to rockfalls. Water and adverse geology accounted for 15% of the losses. Mechanical/electrical problems, guidance, and slope stability problems accounted for the remaining 30%. Web pillar stability factor for these holes also exceeded 1.3 in 95% of cases. Trapped highwall miners present a safety hazard to workers during their recovery. Best practices to avoid trapped highwall miners include: avoid mining in stream valleys, avoid mining near outside corners, careful alignment of each hole and the use of an onboard guidance system, Several issues in highwall mining ground control require further investigation including highwall mining through old auger workings, highwall mining near old underground mines, multiple-seam and multiple lift highwall mining and finally the size and frequency of barrier pillars.

Jan 1, 2004

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 P. Tsang

In order to deal with ground control problems such as roof falls, floor heaves and pillar failures in underground coal mines, a new pillar design method based on the "yield pillar" concept is proposed. Using the finite element model simulation, the functions and mechanisms of the "yield pillar" are studied. The important variables that affect the stability of longwall entry-pillar system are identified. To simplify the geological and mining conditions, a new pillar design method classifies the in-situ roof and floor conditions into four categories: 1) strong roof and strong floor, 2) strong roof and weak floor, 3) weak roof and strong floor, and 4) weak roof and weak floor conditions. The design formulae are derived based on the finite element model simulation, stability study and nonlinear regression analysis. The finite element models used consider the plastic failure properties and time-dependent behaviors of rock. To better organize the finite element model simulation, the orthogonal experiment design is used to arrange the finite element models and proved to be very effective. An application example is used to illustrate the basic design procedures involved in the new pillar design method.

Jan 1, 1993

By Gregory J. Chekan

Developing a coal seam that has been influenced by previous mining in seams either above or below can result in severe ground control problems. In many instances, interactions between operations are inevitable. but improvements in ground stability can usually be achieved by-predicting potential problem areas in advance and adjusting the mining plans accordingly. This report investigates four case studies involving interactions between operations that used both room-and-pillar and longwall mining methods. In each case, geologic and mine design parameters are addressed, and those parameters found critical to the interaction are further defined in relation to observed ground problems.

Jan 1, 1990

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