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By Richard N. Campbell

The world?s ever-expanding demand for energy and coking coals has resulted in the underground mining industry looking at taking on increasingly marginal and geotechnically challenging deposits. The drivers for this change include a reduced availability of open cut reserves and a lack of remaining ?good coal leases?. The lack of plum deposits is forcing some companies wanting to enter the coal supply market to purchase and operate in higher risk, lower margin areas. As geotechnical professionals, we are expected to develop strategies to facilitate safe and productive mining in ground that once would have been avoided. To be successful in these reserves, we are required to use every tool available, from exploration data to longwall automation, and invent or reinvent some new tools along the way. This paper presents geotechnical case studies of challenges in longwall mining and some of the methods developed and used to allow longwall mining to succeed where others said we would fail.

Jan 1, 2014

By Robert Kimutis, Ke Yang, Yi Luo, Jianwei Cheng

"Surface subsidence processes, especially those associated with a longwall mining operations, could cause operational and/or safety problems to railroads. For railroads with normal amounts of traffic, both the limited time window between traffic and the limited space available make the efforts to return the railroad back to its original state nearly impossible. In order to ensure the safe operation of a subsiding railway, the partial lifting method has been developed and applied. This paper describes the principle and application of the method. A case of applying this method to mitigate subsidence effects on a long section of railway has been demonstrated.INTRODUCTIONMaintaining an operational railway as it is experiencing a longwall subsidence process is not an easy task. The subsidence process may induce sufficient strain and curvature to cause problems to the railroad structures. Localized subsidence-induced slope often is significantly larger than the permissible ruling gradient of 0.7% for a locomotive to haul a loaded train reliably. In order to ensure safe operation, it would be ideal to instantly lift the railroad track back to its original level as it is subsiding. However, it is often impractical to do so due to the limited time windows available between railway traffic and the available space on the railroad base.A partial lifting method has been developed to keep the railroad operational under such limitations. The subsiding railroad track is raised by only a determined, partial amount of the occurred subsidence at a given time. By making such a determined adjustment, the railroad track can be maintained on a smooth and operational profile, and the high strain and curvature on the railroad structures can be reduced as well. To define a smooth railroad track profile in a subsided section the following two conditions should be met: (1) the maximum railroad gradient should be less than the permissible ruling gradient, and (2) the recommended amount of adjustment should be able to be made by the repair crew within the available time windows between scheduled railway traffic.The subsidence effects on a long section of railroad over two longwall panels have been successfully mitigated using the partial lifting method. The railroad experienced a maximum subsidence 89 of about 5 .0 ft (1.5 m) with the average subsidence along the entire length being about 3.45 ft (1.05 m). However, the average adjustment due to lifting that was made along the entire length of the subsidence-influenced railway was only 0.5 ft (152 mm)."

Jan 1, 2010

By C. A. Vining, J. R. Nopola

"Evaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. Evaporites are subject to continual creep and no amount of ground support can successfully arrest this process. Common ground-support techniques that are useful for most rock types may reduce the effective life of bolts in evaporite mines and can create unseen hazards. A different approach to ground support must be applied to provide long-term support in areas where high creep rates are present. The goal of effective ground-support designs in these situations is not to contain the rock and prevent it from moving; rather, the ground-support design must be able to move along with the rock and contain any fully detached slabs until additional remediation measures can be undertaken. The most appropriate ground-support designs will consider both the expected movement of the rock and the duration that the area needs to remain accessible.INTRODUCTIONEvaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. The factors that must be considered in design stem from the deformation behavior of salt, which can fail in a brittle manner, akin to most rocks, but can also deform in a slow, ductile manner, resembling the stretching and elongation of steel. The deformation behavior of salt has been well established in technical circles where it has been studied for decades (Hardy and Langer, 1984). Much of the research in salt mechanics is related to repository science, specifically at the Waste Isolation Pilot Plant (WIPP) in southwest New Mexico. RESPEC has been continually involved in the WIPP from the initial design considerations (Gnirk, Grams, and Zeller, 1977) to the current day (Keffeler, 2016) and has been able to leverage cutting-edge research of salt mechanics into practical solutions for mining operators. While a detailed description of salt mechanics is not provided in this document, Fossum and Fredrich (2002) include a fine overview of salt mechanics. The following list paraphrases their chapter on basic facts about salt to provide some context for the reader unfamiliar with salt mechanics:"

Jan 1, 2018

By André Vervoort

To learn more about the fundamental roof behaviour in South African coal mines, accurate underground measurements of roof deflection were conducted in bord and pillar panels during development and pillar extraction. To conduct these measurements a survey levelling technique was adapted. The overall system reading accuracy was established as being within 0,4 mm on a 15 m sighting distance. The underground sites were modelled and, for a roadway development, similar symmetrical curvatures were calculated by a linear elastic model, as were measured underground. In a pillar extraction panel, a significant amount of bed separation was measured and the roof underwent an a-symmetric movement due to the close presence of the mined out area at one side of the roadway.

Jan 1, 1992

By Kot Unrug

The selection of tensioned versus non-tensioned roof bolts, for primary coal mine roof support, has been debated for well over 25 years. The wide spread use of fully grouted rebar marked the beginning of the discussion of the relative importance of bolt tension and the resultant immediate roof compression. The focus is on roof conditions encountered in coal mines however the discussion may be extended to hard rock mines as well. The theoretical bases for both systems are presented, with the analysis focused on the bolt's ability to actually fulfill the theoretical assumptions. Of specific concern are assumptions about bolt tension upon installation and the likelihood of maintaining useful bolt tension over time. The usefulness of tension roof bolting is limited by the mechanical interaction of roof bolt assembly and the rock, which may be valid at the time of installation only. Ambient mine atmosphere and fluctuation in moisture content changes the physical condition of rock, which may have a significant impact on the effectiveness and longevity of bolt tension. Changes in the material properties of the surface of the immediate roof can invalidate the assumptions, which were made in the design stage of roof support system. This means that the roof support in the long term excavations may be inadequate requiring costly rehabilitation. The presented arguments are illustrated with the examples of some relevant experimental data. Tension primary roof bolting is recommended where the immediate roof is: ? Stiff with a high modulus of elasticity ? Surface layers are continuous and fairly thin ? Not easily weathered

Jan 1, 2004

By M. Karmis

mining factors and representative ground movement parameters. Based on this analysis, the previously developed prediction methods were modified and tested for their applicability in the Appalachian coalfield. Finally, in order to facilitate the implementation of the prediction methods, as well as the data processing of the monitoring program, a number of computer software were developed and will be discussed in the paper. Surface deformations above underground mines and their prediction and control have been the topics of an extensive research effort by Virginia Polytechnic Institute and State University over the past few years. During the initial stages of this research, a number of case studies were collected from the literature, the mining industry and various government and state agencies and, based on the analysis of this information, appropriate prediction methods were proposed (Goodman, 1980; Webb, 1982; Hasenfus, 1984; Karmis et al., 1984). Although the prediction methods were based on sound concepts, it became evident that their accuracy and implementation were a reflection of the size and quality of the collected case studies. In this respect, the data bank was rather insufficient and posed certain questions regarding the development of subsidence information due to inappropriate length and spacing of monuments used in subsidence surveys, density and accuracy of such surveys, lack of monitoring horizontal movements, etc. In an effort to alleviate these problems, a comprehensive monitoring program was initiated in the coal mines of southwest Virginia, which included: - Two longwall and five room and pillar mines. - In total, seven longwall and sixteen room and pillar panels. - Monitoring lines in excess of 35,000 feet. - Vertical as well as horizontal measurements. A high accuracy digital computer tacheometer was used in all surveys and the type and spacing of all monuments were in accordance with well accepted guidelines. The data from this monitoring program completed the initially collected data bank and, thus, enabled a much needed statistical analysis between

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 Michael Kelly

Current research in longwall mining in CSIRO Exploration & Mining is focused within five major projects and several minor projects. The major projects are: 1 3D Aspects of Longwall Geomechanics. This was reported to the conference last year and a final report will be available by mid-2001. 2 Mine Gas Control. This just completed project centred on Dartbrook Colliery which has a longwall gas make of more than 8 cubic metres per second. 3 Rapid Roadway Development This includes the construction of an autonomous conveying and bolting module (ACBM) that sits between the miner and haulage system. 4 Top Coal Caving In conjunction with the YanKuang group in China, CSIRO and the University of NSW are conducting a prefeasibility study of the Top Coal Caving method in Australian conditions. 5 Longwall Automation. In partnership with the Cooperative Research Centre for Mining Technology and Equipment (CMTE), the CSIRO has conducted a scoping study on this issue and will commence an initial three-year demonstration project in July this year. This paper describes these protects in more detail with particular focus on the last three in the above list.

Jan 1, 2001

By Erdal Unal

In gate roads where supports are subjected to dynamic loading four times due to retreating longwall panels, it is a real challenge to replace the rigid arches with flexible reinforcement systems and yet to provide safety and stability of these openings within better economical limits. The results presented in this paper are based on a three years of research project aimed at developing safe and economical support systems for gate roads in Çayìrhan coal mine. Çayìrhan -Mine Project includes laboratory and field studies, design analysis, pilot scale design applications and performance monitoring. Based on outputs of the design approach developed, four types of bolts, namely, Split Set, Yielding Super Swellex, Sis-Resin, and Fosroc-Resin bolts were installed in a gate road together with widely spaced yielding steel-arches. In order to compare the performance of these flexible support systems and to provide feed back for the design approach, a monitoring program was initiated.

Jan 1, 1993

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 Sheng Zhang

The compressive strength of coal in Yanlong mine area of China is less than 3 MPa. Basically, it is powdery. The roof and floor rocks are mudstone. Therefore this coal seam is a typical ?three soft? coal seam. Roof bolting cannot be used due to the very soft strata. When the conventional U-steel support was used, roadways deformation exceeded 30% of its original dimensions in some sections one half month after development. The required repair work was huge, seriously affecting the daily mine production. This paper describes how to support the soft coal roadways. The deformation characteristics of soft coal roadway were investigated. The conventional U-steel support is not subjected to uniform load, so the whole supporting structure must be strengthened. In addition, drilling holes in the coal seam can transfer high stress from near the ribs to the deeper parts of the surrounding strata, which is beneficial to the stability of the roadway. This new support method is to combine the strengthened U-steel support with pressure-relieving drill holes, providing an economic and efficient way to support the very soft coal roadways.

Jan 1, 2013

By Takashi Sasaoka, Pinyo Meechumna, Hiroshi Takamoto, Pipat Laowattanabandit, Akihiro Hamanaka, Kikuo Matsui, Hideki Shimada

"The EGAT Mae Moh coal mine is an open-cut coal mine in Thailand that produces about 16 million tons of lignite annually to generate 2,400 mw of electricity. The surface mine pit covers is 4 by 7 km (2 by 4 mi) with mining depths ranging up to 260 m (853 ft). A large pit slope is formed with the progression of the mining operation. However, massive slides in the pit slope often occur due to weak planes, such as faults and bedding, and the weak mechanical properties of the rocks. Hence, a 200-300 m (656-984 ft) boundaries of rock block including coal seams is left in front of faults in order to prevent slides and maintain the stability of the pit slope. As a result, there are significant coal reserves beneath the abandoned area along the pit slope.The focus of this paper is on the applicability and design of the highwall mining for recovering much of the coal left along the pit slope.INTRODUCTIONThe EGAT Mae Moh Mine is located about 630 km (186 mi) north of Bangkok, Thailand (see Figure 1). It is the largest coal mine in Thailand and produces about 16 million tons of lignite annually from open-cut mining (70 % of the total coal produced in Thailand). The coal is used to generate 2,400 mw of electricity at the Mae Moh power plant, which is situated near the mining area. The mine pit covers an area of 4 by 7 km (2 by 4 mi) at varied depths up to 260 m (583 ft) as shown in Figure 2. As the mining operation progresses, the pit becomes to be deeper and deeper. In the year 2035, the bottom of the pit is expected to reach depths of 500-600 m and the open-cut mining operation will be finished. The possibility of an underground mining operation is being investigated.A large pit slope is formed with the progression of open-cut mining. However, massive slides often occur in the pit slope along weak planes including faults and bedding planes due to the weak nature of the rock. As a result, 200 to 300 m (656 to 984 ft) boundaries of rock including coal seams are left in place in front of faults to prevent slides and to maintain the stability of pit slope. Moreover, the dip of the slope is less than that of the planned design. As a result, there significant coal reserves abandoned along the pit slope."

Jan 1, 2010

By David Bartsch

The Ohio Valley Coal Company (Ohio Valley) has operated the Powhatan No. 6 Mine in Belmont County, Ohio since 1972. Ohio University owns a 455-acre research farm over a portion of the Ohio Valley reserve containing 51 acres of old-growth forest known as Dysart Woods. In 1988, when Ohio Valley applied for a new mining area for its first longwall located several miles away from Dysart Woods, Ohio University claimed that mining at that distance would lower the regional water table and kill the old trees. In response, Ohio Valley began to study potential impacts from longwall mining on hydrology, soil moisture, and tree growth. Early studies examined the impacts of longwall mining on hydrology, soil moisture, and tree growth directly over mined areas compared to woodlots that were not undermined. A subsequent study examined tree-ring growth for a five-year period before and after longwall mining for several woodlots, some directly over panels and others at distances of up to 2000 ft away. No significant differences in growth rate associated with longwall mining were observed. New research was begun that compared longwall-mining impacts on groundwater hydrology, available soil moisture, and tree growth in two stands of mature trees located near Dysart Woods in order to confirm the earlier results. Using a test technique known as "Before-and-After-Controlled-Impact-Pairs," data was collected from one woodlot (test) that was undermined and from another (control) that was not undermined. Data collection began several years prior to longwall mining and has continued for several years after mining. The data for the test site shows that although the shallow and deep groundwater levels were affected by longwall mining, the soil moisture, vigor rating, or the growth rate of trees did not decline significantly compared to the control site where there were hydrologic impacts. The study shows that there are no known significant impacts to trees from longwall mining. The results of these studies were used by Ohio Valley to obtain State approval to longwall to within 300 ft of Dysart Woods

Jan 1, 2005

By Xu Linsheng

Datong coal Mine suffers from delays and sudden caving of its massive competent conglomerate roof. Detailed geological studies of the nature of pebbles and matrix of the conglomerate and its sparse bedding, joint and fracture structure, gave rise to investigation of water injection to destroy its strength and to control caving. Comparative investigations, including porisity, permeability, compressive strengths and water injection of conglomerate and sandstone led to empirical expressions relating various properties and to guidance for effective injection and caving practice. This new safe roof control practice should be even more improved with further research, including modelling.

Jan 1, 1992

By Eric G. Zahl

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

Jan 1, 2000

By S. Paul Singh

The implications of blast damage are harmful in both economic and physical sense. To understand the mysterious nature of blast damage assessment and prediction, field work was done by driving an experimental tunnel in the bench of a quartzite quarry. On the basis of the blast vibration monitoring records, a model was developed to predict the PPV at different distances from a blasthole. Critical particle velocity for damage and the output of the prediction model were used to delineate the extent of damage for different explosives. The predictory model was tested by designing a special blast and the burdens for the back holes have been recommended. The relationship between the critical particle velocity for "Fall Off' and the critical particle velocity for damage has been established. The existing criteria for blast damage have also been renewed in this paper.

Jan 1, 1993

By David C. Johnson

Coal mining in Illinois has resulted in many structures being exposed to subsidence damage. Damage mitigation techniques (DMT) applied in the foundation design/construction have proven successful in minimizing damage in several test cases. To determine the practicality of these techniques, the Costs of their application are investigated. The AMT costs are compared to foundation and total house costs for various classes and sizes of houses. An economical and effective subsidence resistant foundation construction is proposed.

Jan 1, 1993

By K. F. Unrug

A new testing technique for the determination of the rock strength from portable rock tester is proposed. It is a quick and economical method that can determine the rock strength with reasonable accuracy in different conditions. This method is based on the application of portable rock tester working at the principle of forcing steel ball into the rock surface. The correlation between laboratory measured compressive and/or tensile strength and rack tester reading has been determined in terms of the three-controlling parameters: (1). diameter of indentation &'the testing sample, (2). diameter of indentation-on the reference bar, (3). Brine11 hardness number of reference bar.

Jan 1, 1990

By Khaled Morsy

In this paper a new method to evaluate the stability of yield pillars was introduced. The evaluation of yield pillar stability was conducted in three steps: (1) Estimation of pillar loading - numerical modeling is a good tool to estimate the pillar loading for most of the geological, geometrical and mining conditions. In this study. finite element technique was used to estimate the state of stress, strain, elastic strain energy and plastic energy, etc. at every point within the pillar. (2) Defining the yield pillar loading zones¬. The proposed method considered the non-uniform stress distribution of yield pillars by dividing the area of the yield pillar into three loading zones, namely core, transition and rib zones. (3) Estimation of stability indicators - an appropriate stability measure was proposed for each loading zone. such as; Core Stability Factor. CSF, Pillar Bump Index, PBI and Rib Instability Index. RIF. A case study of successful and unsuccessful yield pillar application was used to assess the applicability of the proposed method.

Jan 1, 2003

By Nikolaos Polysos

To drive and utilise gate roads economically requires accurate planning and risk assessment considering the variable geomechanical requirements. The geomechanical part of roadway planning is secured by geotechnical investigations of drill cores and geophysical borehole logging. The rock specific parameters determined in this way allow a statement about the quality (defomtation and strength characteristics) of the rock that has to be cut across. They set up the basis for a standardised rock assessment. Combined with the prevailing rock mechanical situation, technical boundary conditions for advance and support techniques can be identified already in the planning phase. Detailed geotechnical investigations prior to the planning and driving of roadways can lead to a technically and economically optimised roof support system. Geotechnical core logging and a standardised assessment scheme based on the data acquired will be introduced. Geophysical borehole logging methods provide additional physical data. Different methods and their validity, which may lead to a further improvement of the planning reliability, will also be presented. The geomechanical information gained and their relevance fir the planning will he illustrated. Keywords: rock classification, geotechnical rock parameters. geophysical well logs. support planning

Jan 1, 2002