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By Gregory M. Molinda

Difficult mining conditions in deep western U.S. Iongwalls have necessitated the use of novel tailgate support systems. Extreme ground movement in these yielding gate systems has caused operators to utilize secondary support systems which match yield and strength characteristics with expected ground reaction. Other benefits include ventilation and safety gains, as well as lower costs. Recently, the use of novel support has been tried at longwall mines in other U.S. coal basins. One of the main considerations is the capability of the support to maintain an adequate airway for gob ventilation in gassy eastern coal mines. In one large Pennsylvania mine working the Pittsburgh seam, a number of support configurations were monitored over a 2,300 ft tailgate test area. The test included 4-pt wood cribs, Link-N-Lock cribs, Propsetter yieldable posts, cable trusses, and vertical cable bolts. Ground reaction, and thus support performance, was documented by roof sag and convergence instruments, ventilation measurements, and photographs. Various combinations of standing support and cable truss and vertical cable bolts provided adequate support for ventilation and safe escape. A cribless configuration consisting of only cable trusses and vertical cable bolts was inadequate to provide inby gob ventilation to the next open crosscut. Instrument ground movement data and visual documentation are presented.

Jan 1, 1997

By Peter Zhang

"Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground-fall fatalities in the underground coal mines of the United States. The risk of ground falls during pillar recovery increases in multiple-seam mining conditions. The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave. When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction. This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 70 ft of interburden under about 1,000 ft of depth of cover.The study finds that, for interburden thickness of 70 ft, the multiple seam mining influence zone in the lower seam is directly under the barrier pillar within about 100 ft from the gob edge of the upper seam. The peak stress in the interburden transfers down at an angle of approximately 20° away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat. The study also suggests that, for full pillar recovery in close distance multiple seam scenarios, it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars. If the entries and / or crosscuts in the lower seam are developed outside the gob line of the upper seam, additional roof and rib support needs to be considered to account for the elevated stress in the multiple seam influence zone. INTRODUCTION Room-and-pillar mining accounted for about 40% of underground coal production in the United States in 2016. Pillar recovery, practiced in about one-third of the room-and-pillar mines, represents about 10% of the coal mined underground, yet it has historically been associated with more than 25% of all ground fall fatalities (Mark, Chase, and Pappas, 2003). In some U.S. coal fields, particularly central Appalachia, many coal mines are operating under geological conditions with multiple coal seams. The risk of ground falls during pillar recovery increases under multiple-seam mining conditions. The hazards of pillar recovery associated with multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave (Mark and Tuchman, 2007; NIOSH, 2010b). Pillar retreating creates abutment pressure, not only in the currently mined seam, but also in the overlying or underlying seams. Multiple-seam interactions become more pronounced as overburden depth increases and interburden thickness decreases. To safely recover the pillars in multiple seams, it is critical to properly plan the mining sequence and panel layout to minimize potential multiple-seam interaction."

Jan 1, 2017

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 Winton Gale

"The aim of this paper is to summarise the results and conclusions of Australian Coal Association Research Project (ACARP) Report Cl3013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks. The study concluded that flow into mines is typically via an interconnected network of pre-existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia.INTRODUCTIONWater inflow into coal mines has been a design issue for many years. Guidelines as to the potential for water inflow have been developed in many countries based on local experience and the form of mining being undertaken. In most instances, the guidelines relate to inflows which would endanger underground personnel and operations.In more recent times, water inflow criteria for mines has been widened to include lesser inflows which may not impact on mine safety or operations, but have the potential to reduce water flow within streams and surface aquifers. For the purpose of this paper the larger inflows which are readily measured in a mine or may impact on mining safety are defined as mine inflow and the lesser inflow relating to aquifer water loss as environmental inflow.The water losses referred to are those which enter the mine from the surface or from near surface aquifers.The empirical relationships developed as mine design guidelines or regulations typically relate to high flow (mine inflow) situations and in this paper, these have been reviewed as to their application to environmental inflow. In many cases environmental inflows were unlikely to have been noted or recorded in many mining operations in the past."

Jan 1, 2010

By Dennis Dolinar

"Tailgate entries in U.S. longwalls can be subjected to high loads and deformations. Therefore, tailgate entries are generally supported with varying types and amounts of standing support. However, even with standing support, roof falls and blockages can occur in the tailgate when the support system is not properly designed. Safe and efficient operation of a longwall face requires the selection of the appropriate type and amount of standing support. Because of the importance of maintaining a safe and functional tailgate entry, the National Institute for Occupational Safety and Health (NIOSH) is currently developing a standing support design methodology based on the ground reaction curve. Using the ground reaction curve for design requires that the ground and support interaction be quantified. This can be accomplished by the installation and analysis of data from instrumented field sites. Although necessary, the field results cannot develop the complete ground reaction curve. Numerical models, calibrated to the field results, are used to establish the complete ground reaction curve. In the present study, data was obtained and evaluated to establish points for calibration of the ground reaction curve for two western longwall operations.One test site, Mine A, was located at a depth of 1,550 ft. At this site, the floor consisted of siltstone, while the roof was an interbedded siltstone and sandstone. Because of the depth, a double row of22-in diameter CANs1 were installed to support the tailgate. Both the first and second longwall panels have mined past the instrumented site.The second site in Mine B is at a depth of only 760 ft. The floor at this mine consists of a weak shale/mudstone and the roof consists of thick, top coal overlain by weak shales. At this site, a double row of pumpable cementitious cribs was installed. The second test site has only been subjected to side abutment loading from first panel longwall mining.The measurements indicate that the support at both mines was heavily loaded when subjected to the side abutment loading from first panel mining. At the deeper site, the support began to yield when the face was more than 300 ft outby during second panel mining. In comparison to results from previous test sites in the Pittsburgh Seam and Illinois Basin, the amount of support loading and roof-to-floor convergence was an order of magnitude higher at the western sites. This has resulted in a significant difference in the position and shape of the ground reaction curves."

Jan 1, 2010

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 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 T. P. Mucho

Mine roof failure due to excessive horizontal stress has been recognized as a major cause of hazardous roof conditions in some mines. Stress measurements gathered by the U.S. Bureau of Mines (USBM) at 24 different eastern mines show horizontal stress to be typically 2 to 3 times as great as vertical stress. The focus of current Bureau work is to develop detailed design parameters to assess and control the effects of horizontal stress. To facilitate the recognition of horizontal stress effects and to easily determine principal stress direction without resorting to cumbersome, expensive, and time consuming field measurements, the Bureau has developed a stress mapping methodology. Stress recognition and direction determination are required to successfully employ a number of control strategies such as mine layout, mining sequences, and support optimization to control the effects of horizontal stress, thereby increasing the safety of U.S. coal mines. This approach has been used in other major coal producing countries, most notably Australia and the UK, and has greatly enhanced the safety of their mines (1). This paper discusses horizontal stress, directional based control techniques, and provides guidelines for the use of the stress mapping technique. A case study of a mine where the technique has been used is also presented.

Jan 1, 1994

Virtually all mineable Appalachian coal seams exist in a multi-seam environment. This makes it inevitable that most seams will eventually experience interaction induced ground control and mining problems. Indeed, it has been estimated that every working face will experience interaction effects from previous or current workings at least once during its lifetime (1), and that these problems are considerably exaggerated when seams are in close proximity -- a distance of less than 110 feet. The continued increase in multi-seam problems has resulted in considerable research activities over many years (2,3,4,1). Theoretical analysis, numerical and physical model studies, statistical and empirical analysis of numerous interaction related field data have been applied to multi-seam mining situations and have identified many useful trends and design criteria (4,5,6,7). To fully utilize research results and transfer multi-seam mining technology to field engineers, a design model -- USEAM was constructed (8). This model, designed for close seam under-mining situations, was computerized to facilitate utilization. In the current research, a new computer program "MSEAM" was developed to produce a model for both under- and overmining close seam situations. It is hoped that both of these software packages will aid field engineers in using important interactive design criteria to gain a better understanding of interaction mechanisms. Through advance planning with these programs, detrimental interaction may be avoided and minimized, and occasionally a design can be selected which will allow interaction effects to be utilized to the benefit of mining operations.

Jan 1, 1987

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 Yoginder Paul Chugh, Jason E. Tinsley

"Over the last five years, the authors have compiled a database of Short Encapsulation Pull Test (SEPT) evaluations from Interior Basin (Basin) coal mines. Typically, SEPT data are analyzed to obtain peak load carrying capacity per unit length of resin encapsulation designated as the ""Grip Factor,"" or GF. In a previous paper, ""An Analysis of Short Encapsulation Bolt Pull Test (SEPT) Data From Interior Basin Coal Mines,"" authors analyzed tangent value of the load-displacement curve as Anchorage Stiffness at 50% (AS-50) of the peak load, or GF value, to assess reinforcement potential of an installed bolt system. This paper attempts to correlate the secant AS VALUES (of SEPT on 6 ft (1.8 m) long installed bolts at a lower load of about four tons from several different mines in the Basin with their MSHA-approved primary support requirements. Since the current regulatory environment requires plan changes dynamically to ensure that roof falls are minimized, the authors hypothesize that the practiced roof control plan must meet the minimum requirement of adequately supporting immediate roof strata. The analyses are based on 181 SEPT studies performed by Minova professionals. Analyses indicate a macrolevel correlation for both roof support density and roof support cost that may have the potential to be used by mine operators to compare their performance with other mines in the area. Limitations of the analyses include ignoring mining depth, mining height and required secondary supports as part of the requirements for a roof control plan. The authors recommend additional analyses to develop this concept further to develop guidelines that can be confidently used by coal companies.INTRODUCTIONPartially- and fully-grouted bolts are extensively used in coal mines for roof support as part of MSHA-approved roof control plans. Since grouted bolts have a larger area of contact with the rock, they can develop much greater anchorage capacity than conventional expansion shell (point-anchored) bolts. Load transfer characteristics and interaction mechanics between steel rebar, grout and rock mass around the bolt hole for grouted bolts are generally assessed in the laboratory or field though short-encapsulation pull tests (SEPT), which have been extensively researched by Aziz and Jalalifar (2005), Aziz (2004a), Serbousek and Signer (1987), Signer (1990), and Mark et al. (2002), among others. In a SEPT, about 12 in. (30 cm) of bolt length is typically encapsulated in resin and a pull test is performed to collect load-displacement data prior to failure and, in some cases, post-failure. A typical load-displacement data set for a bolt is shown in Figure 1. SEPT characteristics are designated by peak load carrying capacity, or grip factor, which is the peak load carrying capacity per unit length of resin encapsulation. SEPT is routinely used to assess anchorage performance characteristics of different lithologies overlying a coal seam to identify type of bolt, bolt diameter, length and spacing across and along an excavation. Anchorage stiffness, or AS, at any load may also be calculated as the tangent or secant value of the slope of the load displacement curve at a particular load level. Anchorage Stiffness characteristics of bolts from SEPT data have typically not been considered in any depth until recently (Chugh et al., 2014; Chugh et al., 2015)."

Jan 1, 2016

By Christopher Mark

Despite more than half a century of experience with roof bolting, no design method has received wide acceptance. To begin to improve this situation, National Institute for Occupational Safety and Health (NIOSH) conducted a statistical study of roof bolt performance at a number of mines throughout the U.S. Case histories were collected from 37 mines with a variety of roof bolt types and patterns in a wide range of geologic environments. Performance was measured in terms of the number of roof falls that occurred per 10,000 ft of drivage. The study found that roof falls are rare when the roof is strong and the stress is low, even with light roof bolting patterns. The focus of this paper is on the more difficult conditions, where the roof is weaker and/or the stress is higher. Analysis of the results led to guidelines that can be used to make preliminary estimates of the required bolt length, capacity, and pattern. The guidelines are based on the depth of cover (which correlates with stress) and the roof quality (measured by the Coal Mine Roof Rating (CMRR), and the intersection span. Another contribution is a formula for estimating the horizontal stress level in the eastern U.S. coalfields as a function of the depth of cover. The design guidelines are currently being implemented into a computer program called Analysis of Roof Bolt Systems (ARBS).

Jan 1, 2001

By Christopher Mark

Coal bursts are typically associated with highly stressed coal. Most bursts occur during retreat mining (longwall mining or pillar recovery) in highly stressed locations like the tailgate corner of the longwall panel. Others are associated with multiple seam interactions. However, a small but significant percentage of coal bursts have occurred during development or in outby locations unaffected by active mining. Most development bursts have been relatively small, but some have been highly destructive. No theory of coal bursts can be complete if it does not account for this type of event. This paper focusses on the development mining coal burst experience in the US, putting it into the context of the entire US coal burst database. The first documented development coal burst occurred almost exactly 100 years ago during slope drivage at the Sunnyside Mine in Utah. Sunnyside subsequently had a long history of bursts, mainly during retreat mining but also during development. Several Colorado mines have also experienced multiple development bursts. Some, but by no means all, of the development bursts in these western US coalfields have been associated with faults. In the Central Appalachian coalfields, most development bursts have occurred in multiple seam situations. In some of these cases, however, there was no retreat mining in either seam. The paper closes with some lessons from this history, with implications for preventing such events in the future.

Jan 1, 2017

By Al Campoli

The majority of the 100 million roof bolts installed each year in the United States are 5/8 inch deformed rebar fully grouted into a one inch borehole. Commonly referred to as B-Series or No. 5 bar, this system is proven effective and cost efficient in coal mines with average to above average roof conditions. One of the critical variables affecting resin bolt performance is the size of the resin annulus, calculated as the difference between the hole and bolt diameters. The optimum annulus has been proven to be '/4 inch, which provides for effective mixing of the two polyester resin components and incorporation of the plastic film packaging within the cured resin. Glove fingering occurs when the plastic film is forced against the borehole wall potentially interfering with the mechanical interlock of the resin and the rock mass. Glove fingering is not a problem with the optimum '/n inch annulus but has been know to occur with the B-Series 3/8 inch annulus. Laboratory testing correlated the degree of Glove fingering with resin formulation and the bolt head to shaft axes alignment. Glove fingering occurred over 45 % of the grouted length of standard B¬Series bolts. The newly formulated Eclipse B-Series resin when combined with a 1/8 inch offset between the bolt head and shaft axes reduced the glove fingering 70%. The Eclipse system allows the 3/8 inch B-Series annulus to achieve the positive mixing characteristics associated with the / inch optimal annulus systems.

Jan 1, 2002

By D. G. Lynch, M. R. Martin, R. J. Coutts

"Broadmeadow Mine is an underground longwall mine located in Central Queensland, Australia. The mine introduced a top coal caving (TCC) longwall face in 2013 but since then has experienced severe convergence events at the start of each panel after 60–70m of retreat, resulting in equipment damage and the longwall almost becoming iron bound. A significant geotechnical monitoring effort during LW10 and LW11 to characterise the caving mechanics and operational changes during LW11 were successful in mitigating the risk posed by these initial convergence events. This paper describes the geotechnical monitoring programme undertaken during LW11, the operational methods used to mitigate these initial convergence events, and the contribution to these events of the TCC technique.A thick sandstone unit close to the extraction roof was likely the cause of the convergence events at the start of each longwall panel. However, stress change monitoring cells, surface extensometers, an inclinometer, and the blockage of holes designed for pre-conditioning using hydraulic fracturing indicated that the convergence events were not caused by the sudden collapse of sandstone bridging across the goaf. Rather, they were caused by the propping action of strong units within the overburden strata. Equipment design modifications were made to allow the longwall to remain operational during the convergence events and an operational strategy was implemented to mitigate convergence in LW11. The initial convergence event was successfully negotiated, and Broadmeadow went on to be the highest producing longwall for CY16 in Australia.INTRODUCTIONBroadmeadow Mine is an underground longwall mine located in Central Queensland, Australia, owned and operated by BHP Mitsubishi Alliance (BMA). The mine is a punch longwall mine using open cut pit access provided by Goonyella/Riverside Mine. Broadmeadow commenced longwall operations in 2005 using a conventional high reach longwall, but later changed to the top coal caving (TCC) method of extraction. Broadmeadow extracts the easterly dipping 5.5–7.5m-thick Goonyella Middle Seam in ranging in depth from 90m to 330m. The coal is washed and blended at the adjacent Goonyella/Riverside open cut mine to produce a premium coking coal product for export."

Jan 1, 2018

By Gift Makusha

The Witbank Coalfield in South Africa contains up to five economically viable coal seams. These seams have been mined continuously for over 100 years and the reserve in now becoming depleted. The normal mining method has been bord and pillar with examples of stooping. The situation now occurs where the formed pillars, left to maintain stability, have a higher quality than the coal that is left in the unmined reserves. Where the reserves are less than 70m deep and continuous over several kilometers, opencast mining is being successfully applied. After several years of investigation, Anglo Coal has identified blocks of coal that warrant investigation with regard to pillar extraction by underground methods The proposed method is pillar extraction using Mobile Roof Supports (MRS) It is proposed to mine three panels at a depth of 120m, by keeping the panels narrow and sub-critical. The sub-critical layout does not allow caving to surface and more stress is thrown onto the barrier pillar rather than the face pillars. The sub-critical span is anticipated to be 100m. The paper describes the full geotechnical investigation, including numerical modelling, instrumentation and drawing on the local South African experience of shortwall mining. The project will be unique in that pillars were not designed for pillar extraction and have low factors of safety (l.4 - 1.8) and width to height ratios of less than 4. The mining height will be approximately 4-4 5m, and this represents "new" operating heights for Mining Roof Supports. The required equipment was ordered in late 2002 and pillar extraction is anticipated to begin in June 2003.

Jan 1, 2003

By Joseph Hirschi, Naga S. Y. Tirumalaraju, Aditya Mishra, Anthony (Sam) J. S. Spearing

"Primary roof support in underground coal mines in the USA consists mostly of fully grouted passive rebar or grouted (partially or fully) tensioned rebar. Research conducted at two room-and pillar coal mines in the Illinois Basin and one longwall mine in Colorado monitored the performance of these passive and active rock bolts under similar geologic and mining conditions and found no significant initial loads in either passive or active rock bolts (Spearing et al., 2011). Active rock bolts showed significant loss of initial pre-load, although the exact reasons for this were not fully investigated. Subsequent laboratory testing has also shown that in weak rock, resin creep occurs, caused by slippage of the mechanical key between resin column and rock interface. These creep tests were studied in the laboratory in an effort to quantifythis phenomenon.INTRODUCTIONResin-grouted rock bolts are the first and main line of defense to counter roof and rib falls in underground coal mines of the United States. Rock bolts are categorized as active or passive. Active bolts are tensioned during installation whilst no (or limited) tension is applied to passive bolts, which need subsequent strata movement to generate reactive loads. During installation of passive bolts, some reactive pre-load may be generated by a significant up-thrust from the rock bolter boom.The selection of active or passive bolts is critical and is mostly dependent on the conditions and properties of the immediate roof (Unrug, 2009). This decision is frequently made based on personal experience, bias, and most recently cost. Active bolts are generally used in cases of highly laminated roof rock. When active bolts are tensioned, they develop compressive forces between these layers forming a beam, which helps to stabilize weak individual layers thereby increasing frictional forces between layers and decreasing effects of horizontal stresses. The fully grouted tensioned rebar system is high in stiffness as compared to mechanical bolts and provides higher surface area of contact along the length of the bolt due to the grout, which permits it to develop a strong anchorage mechanism."

Jan 1, 2015

By P. E. Christensen

Due to ever-increasing stripping ratios associated with the No. 8 Seam at the San Juan Mine surface operation, BHP Minerals decided to implement longwall mining for the extraction of the substantial future reserve base. In preparation for longwall developments, BHP opted to first develop a small-scale pilot operation within the No.8 Seam on an adjacent lease to address geotechnical/geological concerns related to ground control, subsidence, caving, panel and entry design, and operational issues related to entry development and proper face equipment selection. Room-and-pillar mining methods are currently being utilized to develop a full extraction panel for a wide range of geological, geotechnical, and geomechanical evaluations aimed at characterizing and assessing the mineability of the No.8 Seam. Variable roof and floor conditions, coupled with a multi-bench coal seam, require that a number of mining horizons, opening geometries, and roof and rib support fixtures be evaluated relative to expediting gate and panel developments. Issues related to development of timely caves in areas subject to overlying sand channel deposits will also be addressed through underground in-seam stress and deformation measurements, and interburden and surface subsidence monitoring. This information will ultimately be used to evaluate face support and gate and panel pillar/entry development requirements. The current depositional model for the property, developed from an aggressive drilling, highwall mapping, and lineament evaluation program, will also be refined during the Pilot Mine study through extensive in-trine mapping, additional roof and floor drilling and physical property testing, and determination of the in situ principal horizontal stress field.

Jan 1, 1998

By Binchuan Zhang

Based on the theory of fracture mechanics and blasting, computer simulations were performed to predict the original rock mass structure and those rock mass structures damaged at different levels uing the properties of joints and fratures obtained in the field.

Jan 1, 2001

By David Bigby, Tim Ross, David Conover

"Extensive arrays of telltale roof monitoring instruments were recently installed in a Mexican copper mine and a large underground mined storage facility in the eastern U.S. The data were used to evaluate roof stability during development and retreat mining and after installation of supplemental supports. Both manual-reading and automated systems were employed and consisted of up to 64 instruments. This paper will discuss the practical issues involved with installation and monitoring, including reliability and maintenance. Examples will be given for telltale response in relation to known events affecting roof stability, including nearby pillar extraction roof caving, installation of supplemental cable bolts, and separation of the immediate roof layer. The strategy for processing the large quantity of data, presenting the data for review, monitoring the system remotely, and identifying and reporting critical events will be described.INTRODUCTIONTelltales have been used for many years with many varied applications. They are relatively inexpensive, easy to install and interpret, robust, and adaptable. The term ""telltale"" generally denotes a strata extensometer that incorporates a visual indication (and warning) of strata movement. The most common dual-height telltale (Figure 1) was developed by British Coal in the early 1990s as roof bolting was replacing steel arch support (Bigby, 2001). Dual-height telltales provide immediately visible measurements, distinguish between movements above and below the bolted height, and are an established means for providing preemptive warnings of roof falls.Many improvements and modifications on the basic design have been developed to adapt to different mining conditions. For example, triple-height telltales are commonly used where roof bolts of different lengths are installed. The standard manuallyread telltales are limited by the difficulty of obtaining frequent, consistent, and accurate readings, especially in high openings. The instruments may also be located in unstable or inaccessible areas. Consequently, an intrinsically safe electronic telltale system was developed that combines high accuracy and both local and remote reading options. The system supports a network of up to 100 dualheight telltales on each of four separate branches. The instruments are connected with a two-conductor cable and can be read both underground, using a portable readout, and on a surface computer (Figure 2). The automated system still provides the visual warning indication underground."

Jan 1, 2010