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By Thierry Lavoie, Mark K. Larson

"Engineers at the National Institute for Occupational Safety and Health (NIOSH) and Itasca Consulting Group, Inc., conducted a study to determine the procedure for calibrating a caving model implemented in FLAC3D. The model is calibrated to measured surface subsidence. This sedimentary caving model was developed by Itasca for NIOSH for the purpose of more closely simulating mechanical processes during caving. When a model is considered calibrated to surface subsidence, the load between the extracted panel's roof and floor is a reasonable estimate of gob loading. Such information can also provide an estimate of the amount of overpanel weight that is transferred to the abutment. This load distribution is a parameter needed, for example, to estimate loading of pillars in longwall gate roads. For the current case, a calibrated model resulted in a calculated abutment angle of 30.6°considerably higher than the assumed value used in empirical methods, but reasonable considering the presence of massive stratigraphic members in the overburden. Because of additional information available after this study that the extracted seam height was actually greater in the first panel, a corrected range of abutment angle was estimated to be from 27.7° to 30.6°, still significantly higher than the empirical assumption. The expectation is that better estimates of abutment loading profiles will result in improved evaluations of mine layout design and, therefore, reduce the number of fatalities and injuries resulting from falls of ground.INTRODUCTIONThe safety of miners in underground coal mines depends on careful consideration of the mechanisms involved in failure and transfer of load. The successful design of pillars and panels in longwall and room-and-pillar coal mining requires thorough attention to any information and measurements available that help understand these mechanisms. Although empirical methods, such as Analysis of Pillar Stability (ALPS) Mark (1987); (Mark, 1992) or Analysis of Retreat Mining Pillar Stability (ARMPS) (Mark and Chase, 1997), can provide a sense of success or failure, complex geology and failure in the roof or floor material or complex material behavior of the coal can make evaluation by an appropriate numerical modeling tool necessary. The numerical tool must simulate important mechanisms and appropriate detail that underlie the mechanism. For example, Starfield and Cundall (1988) advised using ""the simplest model that will allow the important mechanisms to occur, and could serve as a laboratory for the experiments you have in mind."" To that end, data gathering about the geology, geometry, and material behavior is very valuable. Such data facilitate selection of an appropriate model and model inputs. Moreover, this data facilitate improved assessment of risk to miners and how to minimize that risk."

Jan 1, 2016

By Chris Kelly

The Monroe County Quarry is an open pit granite quarry located near the town of Bolingbroke. Monroe County. Georgia. The quarry has been in continuous operation for approximately 10 years. The quarry covers approximately 27 acres and has an annual production rate of 1.0 million tons. On the morning of April 26. 2000 a sudden rapid highwall failure occurred at the west end of the north wall of the quarry pit. The failure occurred approximately one hour after a three inch-wide crack was observed in an access road located at the crest of the highwall. No indications of failure had been observed prior to the development of the crack. The active mining area had been at the base of the failed area. The pit was successfully evacuated prior to the failure and there were no losses of either personnel or equipment. The amount of failed material was estimated to be in excess of two million tons. In order to ensure a safe working environment at the quarry a slope monitoring program was established in the area of the failure. The program consisted of using an EDM survey instrument to periodically measure the location of survey prisms installed at carious points on the north wall. The locations of the prisms were recorded over several months and changes in location were analyzed in order to identify and characterize the movement of the failed mass. The results of the analysis provided a basis for recommendations regarding access to various areas of the quarry. This paper discusses the factors contributing to the failure, the method of post-failure slope monitoring, anti the interpretation of the monitoring data.

Jan 1, 2002

By Karsten Zimmermann, Peter Cain

"To receive regulatory approval for a new coal mining operation in North America, extensive investigations of feasibility and potential environmental impacts, including ground deformations, are required. Figure 1 shows pictures from mine sites and an example mine map with the planned panel layout for one seam of the multi-seam longwall operation considered.The cumulative mined coal thickness of 22 meters in some areas of the operation requires a comprehensive forecast of expected ground movements and subsidence impacts (e.g., subsidence, slope, displacement, curvature and strain). In the frame of the impact study, the prediction of these impact factors was conducted by DMT using the computer program SPS 4D, based on the Ruhrkohle modeling method by Ehrhardt and Sauer (1961).This paper briefly presents the modeling method and explains the benefits and possibilities for its international application. Information gathered from literature studies on subsidence in North America were incorporated into the initial model parameterization. The article shows the ways in which the observed influences of mountainous topography on surface displacements and deformations can be considered by an extension of the subsidence prediction model and integration of DEM information.SUBSIDENCE MODELFor the calculation of mining-induced subsidence and deformation, DMT has developed subsidence prediction software SPS 4D, which is built according to the stochastic prediction model by (Ehrhardt and Sauer, 1961). The model is based on the subsidence theory by (Knothe, 1953) with a specific parametrization of the stochastic influence function and includes enhancements for dynamic subsidence calculations after (Sroka, 1978). In Europe, mainly in Germany and Poland, the model is approved by the authorities as a standard model for subsidence prediction in mining areas. However, extensive research within R&D activities has shown the possibility to predict mining-induced subsidence considering the specific mining conditions in North America (Zimmermann and Fritschen, 2007)."

Jan 1, 2016

By Phil R. Ames, Anil K. Ray, Murali M. Gadde, John A. Rusnak

"Many factors can be responsible for coal mine roof falls including, mining geometry, stress conditions and local geology. In the Illinois Basin coal mines, roof instability is commonly attributed to the low strength of the immediate roof, which is also moisture sensitive. Recently, in order to understand the mechanisms of roof instability, numerical modeling has often been the preferred ground control analysis tool. Since roof materials can exhibit significant spatial variability, such modeling results may not represent every section of the mine. At the same time, it is not feasible to carry out numerical modeling for the full range of geological and mining conditions possible at a mine. Operationally, it is more convenient and highly practical to have a simple map that depicts the expected roof conditions, determined through either complex numerical models or simple empirical analysis throughout the reserve area. In this paper, a case history is presented that shows how a combination of simple empirical analyses of geologic data and past ground control experiences at a mine can be synthesized into a roof stability map, which can then be used to forecast future ground conditions. The usefulness of such a roof stability map will be examined in terms of the predicted and actual ground conditions encountered over a period of more than a year after the map was created.INTRODUCTIONA roof fall, which can result in fatalities and lost work time, is considered to be one of the most severe ground control hazards in underground coal mines. In general, there are many factors responsible for roof falls, including mining geometry, stress conditions and local geology. In the Illinois Basin, roof falls are commonly attributed to the weak nature of immediate roof rocks, and some superficial falls may be initiated due to moisture sensitivity. Some unstable areas also occur when the immediate roof is highly laminated.Numerical modeling is a useful tool used to understand the mechanism of a roof fall by incorporating site-specific parameters. In the past, numerical modeling has been used to explain the mechanism of roof falls as well as simulate· roof falls and cutter behavior, but such work had been strictly site-specific (Gadde and Peng, 2005; Ray, 2009). Because of the restrictions imposed by the input parameters, numerical modeling results cannot represent the entire mine. In the real world, mining conditions vary from place to place, and even in a single panel, modeling results valid at one site may not apply at another nearby site. At the same time, due to the lack of input data pertaining to rock characteristics and in-situ stress variations, it is not practical to carry out numerical modeling for each local area of the entire mine site. In addition to these difficulties, mine operators prefer simple tools to understand the ground conditions at their mine sites. One simple and commonsense based tool is the roof stability map. A stability map not only synthesizes the geologic and stress analysis data empirically, but it can also be used to predict future ground control issues at that mine. Because of this practical appeal, stability maps are more likely to gain mine operators' acceptance and be used in their planning operations. In this paper, a case study is presented on the development of a 'Roof Stability Map' that was principally used for roof fall prediction for a mine in the Illinois Basin."

Jan 1, 2010

By Joel Bradley, Kot Unrug, Kyle Perry, Mark Klimek

"A West Kentucky mine operation in No. 11 seam, encountered floor heave, due to the localized increase of the thickness in the fireclay mine floor. Floor heave has overridden seals installed in two mined out panels. The third seal’s location was planned for isolating that area from the Mains. A plan of support has been developed to prevent repetition of the floor heave and related problems outby the seals. The applied ground control measures were successful. An attempt of a 3D numerical modeling was made, that it would match the observed behavior of the mine floor and could be used as a design tool in similar conditions. In the paper described are sequence of events, a mitigation ground control system applied, and the first stage of numerical modeling.IntroductionWest Kentucky coal mine operation in the No. 11 seam has approached an area where the fireclay floor thickness increases to about 5 ft. Delayed floor heave has been observed. In the first sealed area (Panel KK), floor heave has overridden the seals and has begun to progress outby. This could not be allowed at the new seal location PP described in this paper because such override would affect the mains. Previously numbers of standing support systems were used, but their effectiveness was not satisfactory as is illustrated in Figure 1, Figure 2, and Figure 3. Therefore, with sealing of the next area approaching, mine management was looking for a better solution that would eliminate the problem. In that stage, our team looked closely at the geology and strength of roof and floor, as well as the characteristics of ground failure around the seal at the first site. The technical literature has been searched as well Haramy et al. 1986 Hsiung and Peng 1987, Peng 2008, Santos & Bieniawski 1987. This led to the identification of factors responsible for the ground failure. It has been determined that increased fireclay thickness from typically less than 2 ft to about 5 ft, when compared to the previously mined parts of reserve, and the strong limestone strata just above the immediate roof composed of 2 ft of black shale, created conditions of failure. It appeared that, when pillars were punching into the mine floor, the strong roof did not break but was bending, exerting high pressure on outby pillars, causing rib failures and closure. This is a similar mechanism to a strong roof affecting longwall shields. The remedial action planned was additional support."

Jan 1, 2015

By Tony Iannacchione

A close look at the Loyalhanna Limestone of southwestern Pennsylvania reveals a complex structural environment. Most exposures of the Loyalhanna occur along Chestnut Ridge and Laurel Hill within the Allegheny Mountain Section of the Appalachian Plateaus Province. Because the Loyalhanna is rated as a super pavement aggregate by the Pennsylvania Department of Transportation and because of its proximity to the population centers of southwestern Pennsylvania, the Loyalhanna has been extensively mined along these prominent anticlinal structures. Geologic and engineering analyses were performed using gas well and core logs, outcrop examinations, underground observations, and mine maps. Strata exposures from both outcrops and quarries were used to construct geologic maps, stereonet, and rose diagram projections so that the Loyalhanna's structural environment could be understood better. Many of these structural conditions cause ground control problems at local quarries. These problems ranged from small-to-moderate sized rock falls associated with roof jointing to pillar failures associated with dipping discontinuities. Stresses ranged from tensional on structural domes where weathering dominates, to high levels of compression within structural saddles. A greater understanding of these characteristics is a prerequisite to develop engineering controls, such as improved mine layouts. pillar sizes, etc., that lessen miner exposure to these hazards.

Jan 1, 2002

By David F. Gearhart, Mark Van Dyke, Heather Dougherty, Ted Klemetti, Gabriel S. Esterhuizen

"Coal mine longwall gateroads are subject to changing loading conditions induced by the advancing longwall face. The ground response and support requirements are closely related to the magnitude and orientation of the stress changes, as well as the local geology. This paper presents the monitoring results of gateroad response and support performance at two longwall mines at a 600-ft and 2,000-ft depth of cover. At the first mine, a three-entry gateroad layout was used. The second mine used a four-entry, yield-abutment-yield gateroad pillar system. Local ground deformation and support response were monitored at both sites. The monitoring period started during the development stage and continued during first panel retreat and up to second panel retreat. The two data sets were used to compare the response of the entries in two very different geotechnical settings and different gateroad layouts. The monitoring results were used to validate numerical models that simulate the loading conditions and entry response for these widely differing conditions. The validated models were used to compare the load path and ground response at the two mines. This paper demonstrates the potential for numerical models to assist mine engineers in optimizing longwall layouts and gateroad support systems.INTRODUCTIONThe need for roof stability in coal mine longwall gateroads places significant demands on the gateroad ground support system. The variable loading conditions associated with the approaching longwall face can cause excessive ground deformations that need to be accommodated by the support system. In coal mines in the United States several different types of supports are used to maintain the stability of gateroads. The supports may include grouted roof bolts, cable bolts, and roof trusses as intrinsic support. Standing supports include timber cribs, engineered timber props, and various types of cement-based cribs. The intensity of support is governed by the expected geologic conditions and the expected stress changes that will occur during longwall mining. At present, the design of ground support for gateroads is largely based on local experience. In-mine trials are usually conducted when new support technologies become available or ground conditions dictate a change in the support system. The research reported in this paper has the objective to assist mining engineers in evaluating alternative gateroad supports and to design suitable support systems for the local ground conditions."

Jan 1, 2018

By Jun Lu

This paper presents the evaluation, enhancement and comparison of two hydraulic fracturing techniques which were employed to mitigate the impact of a massive sandstone channel on two successive longwall faces. The hydraulic fracturing technique and Longwall Visual Analysis (LVA) software were initially employed to mitigate the impact of thick, massive sandstoneroof during mining of longwall Panel 21. Improved longwall face conditions and productivity were observed as Panel 21 mined through the sandstone channel. However, moderate sandstone fractures and falls at the face were still present in areas where hydraulic fracture influence was not present. Such adverse conditions caused two to three weeks of production delays due to large face cavities, jammed face conveyor and subsequent face gluing. Based on the mining experience in Panel 21, and site-specific roof geology in Panel 22, a modified and more effective hydraulic fracturing program was employed. Fewer well-placed holes were drilled, and larger volume of water was pumped into each hole in order to propagate the individual horizontal pancake fractures over a larger area, thereby enhancing caving and relieving face pressure caused by overhang of the massive sandstone. Longwall Visual Analysis (LVA) software was also employed at Panel 22 to predict the cavity risk index and guide the longwall face crew in double-pulling shields behind the shearer drum. This helped to reduce the tip-to-face distance at the critical spots along the longwall face, which helped mitigate the formation of the roof cavities and failure of massive sandstone onto the face conveyor. Underground observations, face advance rate and face rock delay analysis confirmed that longwall face conditions and advance rate in Panel 22were significantly improved compared to those of Panel 21. Consequently, safety and productivity were improved.

Jan 1, 2014

By Gennaro G. Marino

As part of the development of a shopping center in West Virginia, a significant bench had to be cut into a mountainside that was over 700 ft. high. As a result, a steeply stepped, over 400-ft.- high wall was created. The high wall exposed old abandoned room-and-pillar coal workings at the toe of the cut. Because of exposure to the elements, the mine openings began to severely deteriorate with significant potential of undermining of the rock cut face and causing a large rock fall(s). Exacerbating this wall problem was a set of fairly planar sub-parallel and sub-vertical joints creating, in essence, thin rock slabs. Also present were stress relief fractures that were observed in the exposed pillars. Given the proximity of the project building to this high rock face, mitigation of the rock fall potential was deemed necessary. In order to prevent continued damage and deterioration of these exposed mine entries, the rubblized openings were filled with bulkhead grout and pressure injected with grout. The total amount of grout installed in these openings was about 3,945 CY. Also, a rock drape was attached to the entire face to mitigate small rock falls from natural causes. There also existed another level of mining below the ground surface and under an existing building at a depth of about 160 ft. Under a portion of the building, the pillars in these old works were second mined, and the building seemed to have been already affected by limited subsidence above this area. Also, calculated pillar safety factors in this high extraction section of the mine were too low. Consequently, these old works were stabilized by saturation grouting. An indication of overall roof distress was the significant fracturing encountered in the bedrock overburden during the drilling of the injection holes and the volume of grout placed in these holes. The total grout installed in the high extraction section of the lower mine was about 6,660 CY. This paper describes the investigation and the grouting techniques employed at the rock cut face and under the existing building to mitigate continued damage to avoid future subsidence problems at the face and the building premises.

Jan 1, 2013

By Jimenez Leon, Christopher Newman, Boede Gabriel, Zacharias Agioutantis

"Ground movements due to longwall mining operations have the potential to damage the hydrological balance within as well as outside the mine permit area in the form of increased surface ponding and changes to hydrogeological properties. Recently, the Office of Surface Mining, Reclamation and Enforcement has completed a public comment period on a newly proposed rule for the protection of streams and groundwater from adverse impacts of surface and underground mining operations (80 FR 44435). With increased community and regulatory focus on mining operations and their potential to adversely affect streams and groundwater, there is now a greater need for better prediction of the possible effects mining has on both surface and sub surface bodies of water.As mining induced stress and strain within the overburden is correlated to changes in the hydrogeological properties of rock and soil, this paper investigates the evaluation of the hydrogeological system within the vicinity of an underground mining operation based on strain VALUES (calculated through a surface deformation prediction model. Through accurate modeling of the pre- and post-mining hydrogeological system, industry personnel can better depict mining induced effects on ground water flows through the overburden material aiding in the optimization underground extraction sequences while maintaining the integrity of surface and subsurface bodies of water.INTRODUCTIONThe utilization of high-recovery underground mining methods, such as longwall or high-extraction room-and-pillar operations, have the potential to cause adverse impacts to both surface and subsurface bodies of water as strata movement and deformations propagate from the mined seam through the overburden to the surface (Peng, 2008).Previous research has indicated that mining induced strains are the most damaging to surface streams (Singh, 1992) as well as greatly affecting the integrity of subsurface bodies of water and groundwater flow conditions (Booth, 1986). On the surface, adverse effects to the stream can occur due to the development of either tensile or compressive strain in the stream bed. The development of tensile cracks along the bedrock allows for a potential loss of stream flow through developed fissures. In fact, water flow in the Cataract River of Australia ceased in 1994 as a result of mining-induced strains from longwall operations in the Bulli seam 430 meters (1320 feet) below the river gorge (McNally and Evans, 2007). On the other hand, the development of compressive strains within the rock layers can cause rupturing or buckling of the stream bed, blocking stream flow and/or diverting flow into the fractures at the base (Iannacchione et al, 2010). While these localized fractures can contribute to the loss of stream flows, given time, damaged streams have the ability to self-heal through the regeneration of near-surface aquifers as well as the sealing of mining-induced fractures with rock debris, gravel, sand, clay or other soil particles carried from upstream sources and deposited in the river bed (Waddington and Kay, 2002)."

Jan 1, 2016

By Jesse Puller, Ken W. Mills, Owen Salisbury

"An underground coal mine located in New South Wales has a target coal seam located 160-180 m deep directly below a 16-20 m thick conglomerate unit that has been associated with significant periodic weighting events on the longwall face. As part of the investigations to better understand the causes of periodic weighting at the mine, inclinometers capable of measuring horizontal shear movements through the full section of the overburden strata were installed ahead of mining at two locations approximately 1 km apart above the centre of two longwall panels. These inclinometers were monitored as the longwall approached each site. This paper presents the details of the installation, the results of the inclinometer monitoring at both sites, and the insights that these measurements provide for overburden behaviour about longwall panels.Horizontal shear movements were observed to develop on shear horizons that correlate closely across the two sites suggesting a mechanism that is consistent across a large area of the mine. Shear movements were observed to develop on a single horizon near the top of the conglomerate strata that was mobilised almost immediately after initial formation of the longwall goaf at a distance of 425 m ahead of the longwall face. The direction of horizontal movement was consistent with the relief of the major principal horizontal stress. As the longwall face approached each inclinometer, other horizons within the upper part of the overburden strata begin to shear with the upper strata moving toward the approaching goaf more than the lower strata. Within the last 30 m of longwall approach, tilting of the strata associated with the increase in vertical subsidence toward the extracted goaf caused a change in the style of deformation with tilting and reverse shear offsets.INTRODUCTIONLongwall mining is recognised from subsidence monitoring, surface extensometer measurements, and various other observations to cause significant disturbance to the overburden strata directly above the extracted goaf of each longwall panel. Disturbance to the overburden strata beyond the limits of each longwall panel is not so well understood. Subsidence monitoring indicates that horizontal movements beyond the edges of the extracted longwall panel are generally greater in magnitude than vertical subsidence and horizontal movements may extend up to several kilometres from the edge of the panel in some circumstances (Reid 1998, Reid 2001, Hebblewhite et al 2000, Mills 2001, Mills 2014). The mechanics of how these movements are accommodated within the overburden strata is only poorly understood and there are few direct measurements."

Jan 1, 2015

By Zach G. Agioutantis

The Illinois Basin has an extensive history with the use of mechanized longwall mining dating back to the early 1970?s. In recent times, technological advances have resulted in increased panel widths from 800 feet to over 1400 feet and daily panel advance rates have also increased significant l. The prediction and control of subsidence and surface deformation due to underground mining are important considerations in the permitting, planning, and monitoring of these coal mining operations. The prediction of mining-induced ground movements using the influence function method is a mature technology, well utilized in the Illinois basin, and widely used by researchers and planning engineers around the world. This paper utilizes subsidence data, recently measured in longwall operations in Illinois, to develop an updated ground movement prediction capability implemented by the Surface Deformation Prediction System (SDPS) software package.

Jan 1, 2014

By Tristan H. Jones

Rib-related accidents in underground coal mines continue to cause injuries and take lives at a rate of 1.3 fatalities per year for the last 17 years. Prevention of these accidents cannot simply be a matter of installing more rib support, though the eventual final solution will undoubtedly include that component. Ultimately, rib safety will be brought about through an understanding of the causes and mechanisms that cause rib fatalities, and by fine-tuning mining methods and procedures that are currently in place. This paper investigates the factors likely to be most influential in the occurrence of rib fatalities. A careful analysis has been conducted of the rib fatality reports documented by the Mine Safety and Health Administration between 1996 and 2013. The primary outcome is the determination of the relative impact of each type of rib fatality factor and a better understanding of the impact each factor has. Among the rib fatality factors considered, the influence of rib profileis considered for the first time. Mining height, rib stratigraphy, mining cycle, and the standing position of a miner next to a pillar are also considered. Questions are also raised about the current interpretation of rib fatalities due to mining depth, with the indication being that mines operating at the greatest depths may actually have the most respectable rib fatality record among all mining depths. Additionally, a new method of considering rib exposure is developed. Based on the results, recommendations are made for future work. By gaining a better understanding of how particular factors contribute to rib fatalities as opposed to rib failure or stability, it is hoped that future efforts can be more precisely directed towards the desired outcome of fatality reduction.

Jan 1, 2014

By Mark K. Larson, Douglas R. Tesarik, Heather E. Lawson

"Load transfer distance (LTD) is simple in concept and usually evident in underground coal mines. Many people in the mining industry recognize LTD as the distance from the panel to where the mine ribs and pillars show evidence of renewed spalling, floor heave, or other effects of increased weight due to longwall mining or pillaring. Barrier pillars, in particular, are designed to shield mains and other relatively permanent facilities from this loading. LTD is also important in determining the extent and degree to which mining increases the load on pillars and ribs in the gate roads. Generally, a long LTD reduces loads in gate roads, but may require larger barrier pillar widths to protect mains or adjacent panels from excessive loads. Therefore, a long LTD may make it easy or difficult to meet various objectives of mine layout design . LTD reflects overburden geology in two ways: First, overburden that is weak and readily caves will limit both the amount and distance of load transfer. Second, stronger, stiffer, sandier, and more massive overburden increases both the amount and distance of load transfer. As such, LTD is central to coal mine layout design and calibration of numerical models that seek to anticipate these transfers of load during mining. However, the term does not have a consistent definition in the literature, and various instruments and observational techniques with varying sensitivities and thresholds have been used to indicate the onset of mining-induced stress increase and, subsequently, to determine an LTD. Consistency is important for meaningful comparison of ground response between mining sites and for accurate calibration of models.This paper adds to the body of observed and reported LTDs by presenting cases with relatively sandy and massive overburden at two western coal mines. These two cases include observation and measurement of LTD with multiple methods and provide insight into how distances reported relative to different criteria and instrumentation could be adjusted for comparison against a common base. This base is the criteria used by Peng and Chiang (1984) to determine their empirical equation. In one of these two cases, LTD was recently measured at a western U.S. coal mine (Mine A) to be approximately four times that calculated using the Peng and Chiang empirical equation. LTD measurements were examined using various instruments at Mine A and at adjacent Mine B. The measurements at Mine B showed that LTD was significantly greater than that calculated by the Peng and Chiang equation. Difficulties with BPC installation and fittings at Mine B permitted only ranges of LTD to be determined from some BPCs. Even so, the LTDs and ranges of LTDs determined were consistent with the LTDs determined at Mine A."

Jan 1, 2015

By L. A. Sandbak, Yan Xing

"The purpose of this study is to evaluate the rock mass stability around the tunnels by three-dimensional (3-D) numerical modeling. Complex geological and tunnel features have been included in the model using the software package 3DEC™. Based on the available information on stratigraphy, geological structures, and mechanical properties of rock masses and discontinuities from the underground mine, 3-D stress analyses were performed on various cases with different K0 (lateral stress ratio) values, constitutive models, as well as support systems. Relations between these factors and the distributions of stress, displacement, and failure zones around the tunnels were obtained and discussed. Finally, comparisons between the field deformation measurements and numerical simulations are used as validations for the simulations, helping to determine the appropriate K0 value and material constitutive models for the underground mine.INTRODUCTIONThe case study site is an underground mine in the USA with a monthly ore production between 20,000 and 30,000 tons. The regional geology of the mine is structurally and stratigraphically complex. Rocks in the area include carbonaceous mudstone and limestone, tuffaceous mudstone and limestone, polylithic megaclastic debris flows, fine-grained debris flows and basalts, all part of the Cambrian-Ordovician Comus formation and related dikes. Both the mineralization and faults have general orientations striking N-S to NW-SE with high dip angles. The ore body in the mine has an incline of about 25---45 degrees located in relatively low quality rock; hence, cut-and-fill mining has been used.This underground mine has two vertical shafts used for ventilation, ore hoisting, and for personnel and material transportation. Development drifts were designed and driven to extract and transport the ore, as shown in Figure I. Bolts, mesh, and shotcrete have been applied in this mine for ground control. In order to reach various locations, the tunnel system is designed with different heading directions and inclines, which will be described in detail later. The area selected for modeling is a cubic region including all the tunnels inside the black square (Figure 1)."

Jan 1, 2016

By Khaled M. Mohamed, Michael M. Murphy, Ted M. Klemetti, Heather E. Lawson

"The design of rib support systems in U.S. coal mines is based primarily on local practices and experience. A better understanding of current rib support practices in U.S. coal mines is crucial for developing a sound engineering rib support design tool.The objective of this paper is to analyze the current practices of rib control in US coal mines. Twenty (20) underground coal mines were studied representing various coal basins, coal seams, geology, loading conditions, and rib control strategies.The key findings are: (1) any rib design guideline or tool should take into account external rib support as well as internal bolting; (2) rib bolts on their own cannot contain rib spall, especially in soft ribs subjected to significant load—external rib control devices such as mesh are required in such cases to contain rib sloughing; (3) the majority of the studied mines follow the overburden depth and entry height thresholds recommended by the Program Information Bulletin (PIB) 11-29; (4) potential rib instability occurred when certain geological features prevailed—these include draw slate and/or bone coal near the rib/roof line, claystone partings, and soft coal bench overlain by rock strata; (5) 47% of the studied rib spall was classified as blocky—this could indicate a high potential of rib hazards; and (6) rib injury rates of the studied mines for the last three years emphasize the need for more rib control management for mines operating at overburden depths between 500 ft and 1,000 ft.IntroductionEfforts to improve the stability of underground mine ribs have continued for decades. These efforts by mining professionals around the world persisted throughout the 1990s and 2000s as well, though much of this work focused on rib support methodology and selection of proper support for a specific location. Despite these efforts, it is unfortunate that there has been a continual occurrence of rib-related fatalities in underground coal mines. While overall improvements in rib safety have been achieved, the average fatality rate is still about 1.3 fatalities per year over the last 18 years (Jones, 2014).To manage and control the rib hazard, rib support is used in U.S. coal mines. The current rib support methods for U.S. mines fall into two main categories: rib control based on intrinsic support systems and those based on external support systems. Intrinsic or rib-bolt support requires a roof bolting machine that can install a bolt fixture into the rib. Furthermore, the appropriate level of support needed to control the ribs with an intrinsic-based support system as well as the design parameters for those support systems for various mine conditions have not been established in U.S. coal mines. Furthermore, surface control systems are often an integral part of these support systems and must be considered in the system design."

Jan 1, 2015

By Renn Oler

Corrosion can often cause significant strength reduction and failure of steel rockbolts, thus demanding continuous rehabilitation and posing challenges to underground mines in maintaining long term ground support. The mining industry has traditionally applied galvanized and epoxy coatings for protection of ground support and reinforcement in corrosive environments due to cost considerations. However, under a harsh and corrosive groundwater condition with extreme pH levels, failure of the existing coated rockbolts often results in failure and falls of the mine roof, especially in long-term mine drifts or entries. To prevent the roof falls induced by corrosive bolt failure, DSI developed a new polymer coating, called ?Obduro AP coating,? to combat harsh and corrosive ground conditions. The new polymer coating was tested in extreme conditions to identify weaknesses and was found to be capable of handling highly corrosive mine water solutions. This paper highlights the challenges faced by underground mines with corrosive bolt failure and shows that the DSI Omega BoltTM can withstand the most corrosive ground conditions when it is coated in the new, economic Obduro AP coating.

Jan 1, 2014

By Takashi Sasaoka

A great deal of coal will be left under the final end-walls in Chinese surface coal mines because of lower slope angle in shoveltruck mining systems and larger mining depths. In traditional surface mining systems, the coal under the end-walls will be buried by the inner dumping site, and these coal resources are generally abandoned. Considering these situations, the application of end-wall mining systems was considered in order to recover the residual coal around the end-wall. This mining system can improve economic benefits by exploiting haulage and ventilation roadways from end-walls and utilizing the existing transportation systems. In order to develop the appropriate pillar design methodology for the end-wall mining system, several empirical formulas and methods were discussed based on the formulas developed so far and the mechanics properties of coal and rocks in the Chinese coal mine. From the results of a series of theoretical and numerical analyses, it can be concluded that the coal pillar width calculated by the Mark- Bieniawski formula may serve as an important reference for pillar design in end-wall mining systems, and the end-wall mining system can increase the coal recovery from residual coal resources around highwall and reduce the effect of underground mining operation on slope stability

Jan 1, 2013

By Peter Zhang, Daniel Su, Essie Esterhuizen, Mark Van Dyke, Berk Tulu, Dave Gearhart

"Roof falls in longwall headgate can occur when weak roof and high horizontal stress are present. To prevent roof falls in the headgate under high horizontal stress, it is important to understand the ground response to high horizontal stress in the longwall headgate and the requirements for supplemental roof support. In this study, a longwall headgate under high horizontal stress was instrumented to monitor stress change in the pillars, deformations in the roof, and load in the cable bolts. The conditions in the headgate were monitored for about six months as the longwall face passed by the instrumented site. The roof behavior in the headgate near the face was carefully observed during longwall retreat. Numerical modeling was performed to correlate the modeling results with underground observation and instrumentation data and to quantify the effect of high horizontal stress on roof stability in the longwall headgate. This paper discusses roof support requirements in the longwall headgate under high horizontal stress in regard to the pattern of supplemental cable bolts and the critical locations where additional supplemental support is necessary.INTRODUCTIONLongwall mining is the primary underground coal mining method in the United States and currently accounts for more than 60% of the underground coal production. The longwall headgate, as a passageway for the longwall crew, intake air, material supplies, and coal belt transportation, is critical for both safety and continuous production of the longwall panel. A roof fall in the longwall headgate would not only result in substantial interruption of production but could potentially cause injuries or fatalities. Rehabilitation of failed roof in the headgate would also expose miners to the risk of injuries. Roof falls in the headgate, though infrequent, mostly occur in the belt entry near the face. Considerable research has been conducted to determine the effects of various factors on headgate roof stability, as well as effective measures to support the roof in longwall mines in the Pittsburgh seam (Mark, Mucho, and Dolinar, 1998; Chen et al., 1998; Su and Hasenfus, 1995; Su, Thomas, and Hasenfus, 1999; Su, Morris, and McCaffrey, 2002; Su, Draskovich, and Thomas, 2003; Hasenfus and Su, 2006; Van Dyke et al., 2015). Previous studies have demonstrated that high horizontal stress and transitional roof geology are primary factors that cause headgate roof failure. These studies also showed that panel orientation, retreat direction, pillar sizes, and roof support played an important role in the stability of a headgate.In underground coal mines located in the eastern United States, the magnitude of the maximum horizontal stress is typically three times greater than the vertical stress, and about 40% greater than the minimum horizontal stress. Mark, Mucho, and Dolinar (1998) studied seven cases of headgate failures caused by high horizontal stress in different coal seams in the United States and stated that roof stability is affected, to a large extent, by rock type, entry orientation, and longwall orientation. The effects of horizontal stress can be summarized in these statements: (1) A laminated roof is very vulnerable to high horizontal stress. (2) Entries that are aligned with the maximum horizontal stress will suffer less damage on development than those perpendicular to it. (3) Horizontal stress concentration and relief depends on panel orientation, the direction of retreat, and the sequence of longwall panel extraction."

Jan 1, 2018

By Elia Elias, Alan Crosky, Paul Hagan, Damon Vandermaat, Serkan Saydam, Peter H. Craig

"The University of New South Wales (UNSW Australia) had been involved in the study of premature failure of rock bolts in Australian coal mines from the initial identification of the problem in 1999 (Crosky et al 2002). Rock bolt steel changes over the last decade appear to have not reduced the incidence of failures. A broadened UNSW research project funded by the Australian Research Council (ARC) and Industry has targeted finding the environmental causes through extensive field and laboratory experiments. This paper describes the field studies conducted in underground coal mines, in particular attempts to measure the contribution to corrosion from groundwater, mineralogy and microbial activity. Various underground survey techniques were used to determine the extent of broken bolts, with the presence of both stress corrosion cracking (SCC) and localized deep pitting making no single technique suitable on their own. Groundwater found dripping from bolts across various coalfields in Australia were found to be not aggressive and known groundwater corrosivity classification systems did not correlate to where broken bolts were found. In-hole coupon bolts placed in roof strata containing claystone bands confirmed the clay as being a major contributor to corrosion. Microbes capable of contributing to steel corrosion were found to be present in groundwater, and culturing of the microbes taken from in-situ coupon bolts proved that the bacteria was present on the bolt surface. An ‘in-hole bolt corrosion coupon’ development by the project may have multiple benefits of 1) helping quantify newly developed corrosivity classification systems, 2) providing an in-situ ground support corrosion monitoring tool, and 3) for testing possible corrosion protection solutions.introductionThe problem of premature rock bolt failure in Australian coal mines was published in 2002 and 2004 by UNSW Australia from Australian Coal Association Research Program (ACARP) funded projects. The majority of the broken bolts examined had steel Charpy impact toughness values of 4 – 7 Joules, and the failure mechanism was most often stress corrosion cracking (SCC). Steel fracture mechanics predicts that an increase in impact toughness will increase the length of the crack before sudden brittle failure. In the final report of 2004, anecdotal evidence from one coal mine indicated that the problem may be eliminated in some environments by a change to steel grades with higher Charpy impact values of ~16. (Crosky et al, 2002 and 2004)"

Jan 1, 2015