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By Yi Luo

The severity of disturbances caused by longwall mining subsidence to various residential structures can be re¬duced. The success of such mitigation efforts depends on accurate subsidence prediction, correct assessments of the subsidence influences, and careful design and implementa¬tion of the proper mitigation measures. This paper presents the systematic approach involved in the subsidence mitiga¬tion efforts with a case demonstration.

Jan 1, 2003

By Yi Luo

Subsurface strata movements and deformations associated with underground mining activities could cause problems to subsurface structures and water bodies. By incorporating the methods for surface subsidence prediction with the subsidence parameters derived from collected subsurface subsidence data, methods have been proposed for the prediction of final and dynamic subsurface movements and deformations induced by underground longwall mining. Two new' types of strata deformations in the field of subsidence research, vertical and total strains that could he useful in assessing subsidence influences to the subsurface structures and water bodies, have peen introduced.

Jan 1, 2000

By D. W. H. Su

Following a roof fall in the B12 headgate of a CONSOL Pennsylvania Coal Company (CCPC) underground coal mine in February 1998, CONSOL R&D and Exploration initiated an underground roof geology reconnaissance program along the B13 and B14 developments to define potential roof instability areas and to recommend appropriate supplemental headgate support. This reconnaissance program revealed the presence of a thick, massive Pittsburgh sandstone channel overlying the B13 and B14 panels. Surface and underground core holes confirmed the close proximity and the very massive, nonlaminated nature of the sandstone. To mitigate anticipated adverse conditions at the B 13 and B 14 longwall faces due to delayed caving of the massive sandstone behind the shields, a task force consisting of Operations, Exploration, and R&D personnel was formed in May 1998. Several operational adjustments were made by mine management, including repairing damaged lemniscate links, maintaining immediate forward shield support, and implementing measures recommended by the task force intended to boost the shield set load density at the face. After reviewing available geotcchnical information, the task force also recommended fracing of the massive sandstone over the B13 and B14 panels to enhance caving characteristics and further reduce the bending-induced stresses in the sandstone near the face. Horizontal fracs were created at a horizon about 25 feet above the Pittsburgh seam main bench over the two panels in June and November 1998, respectively. To evaluate the effect of sandstone fracing and operational adjustments on the bending-induced stresses in the sandstone, three finite element models were constructed and analyzed. Results from the finite element analyses indicate that one horizontal frac, 25 feet above the Pittsburgh seam, would reduce the bending-induced stresses in the sandstone by approximately 50 percent, maintaining them at a level below the ultimate strength of the sandstone. Operational adjustments to increase the support set load density and support stiffness could reduce the bending-induced stresses in the sandstone by a maximum of an additional 10 percent. Shield pressure monitoring at the B14 midface confirmed the presence of a lower roof pressure zone inby and outby the 1314 frac holes. This lower face pressure significantly contributed to improving the B13 and B14 longwall productivity and safety. The total production downtime due to bad top at the B 12 face within the interval of channel influence was 30,600 minutes compared to about 3,500 minutes each for the B13 and B14 faces. The average longwall advance per machine shift for the B13 and B14 faces within the channel was about 70 percent higher than that for the B 12 face.

Jan 1, 2001

By A. Chrzanowski

The integrated approach to monitoring surveys is based on a simultaneous utilization of geodetic, photogrammetric and geotechnical measurements of displacements, tilts, strains and any other geometrical parameters of deformation. New techniques for the integrated surveys are being developed at the University of New Brunswick in cooperation with the Canada Centre for Mineral and Energy Technology. A telemetry system for continuous monitoring of geotechnical measurements under difficult climatic and topographic conditions has been constructed and successfully tested in a coal mining area in western Canada. A generalized method for analysing the integrated deformation measurements has been developed and applied in the test area to yield three dimensional empirical modelling of surface movements from geodetic, photogrammetric and tilt surveys. The most appropriate model can be chosen from a statistical assessment of several modelling possibilities. The empirical model of the deformation can be employed in a "calibration" of the prediction (deterministic) model of the ground movements. A computer programme for two- and three-dimensional finite element modelling of elasto-plastic deformations has been developed at UNB as a part of the integrated approach. The :EM has been tested in predicting deformation in the test area in difficult geological conditions which were associated with an extraction of a steeply inclined coal seam. The results gave good agreement with the actual displacements obtained from geodetic measurements and confirmed the existence of a suspected fault.

Jan 1, 1984

By Joao Felipe Costa, Jair C. Koppe, Andre Zingano, Cleber Gomes

"Run-of-mine Brazilian coal has high ash content and low calorific power. Therefore, the coal must be washed to achieve the power plant specifications for burning coal. A large amount of waste results from the coal preparation, and it must be disposed of in waste piles or waste dams, depending on the material size. These waste disposals are the cause of serious environmental problems, not only in the short and medium terms, but mainly in the long term, due to the risk of provoking acid drainage or spontaneous combustion. Waste disposal requirements also demand large areas. Other problems include the pillar and roof stability in abandoned and mined out areas, building of edifices on the old and active mines, and mining under urban areas. To solve these problems simultaneously, backfilling the entries in mined out areas began with two objectives: to understand the mechanical behavior of the filling material and to model the backfilling of room-and-pillar coal mine entries through numerical simulation. Laboratory tests were performed for different filling material (with and without cement) to determine its mechanical properties and failure criteria. Based on this study, it was possible to conclude that the backfill method is a good strategy to improve the ground stability in room-and-pillar coal mines for the long term, especially for weak coal seams, and to reduce the quantity of material on waste piles on the surface.INTRODUCTIONRun-of-mine (ROM) Brazilian coal has high ash content and low calorific power. Therefore, the coal must be washed to achieve the power plant specifications for burning coal. Two types of waste are associated with power plants that bum coal for energy generation: 1) the waste from the coal preparation plant and 2) the ash from the power plant after coal burning.The Brazilian coal ash content in the ROM coal is between 45 and 55%. The clean coal is 20 to 30% ash for 4,500 kcal/kg (18,000 BTU). The coal preparation plant recovery is only about 35%. Santa Catarina State (SC) produces about 7 million tons/year of ROM coal, which results in a total of 4.5 million tons of waste. This is a large amount of waste that must be disposed of in waste piles or waste dams, depending on the material size. The waste and ash content is represented by silicate materials, such as clay and sand, and by sulfide materials, like pyrite and organic sulfur (between 1and5%)."

Jan 1, 2010

By Ken Mills

Dartbrook Mine (Australia) works the Wynn seam, which has a moderate propensity for spontaneous combustion. Prior to any mining a commitment was made to place a segregation barrier pillar between longwall blocks 4 and 5. This was to segregate goaves and improve management controls in the event of a spontaneous combustion event within previous longwall goaves. An alternative to the segregation barrier known as a Consolidated Barrier was developed; firstly as a concept, then computer modelled and finally field tested. It consists of a wider than normal chain pillar with concrete Mega-Seals placed in cut-throughs. Permeability and stress measurements were conducted in the surrounding coal of the seal. Remote monitoring took place as the longwall face approached and then passed by the seal. Results indicate that as the longwall retreats past the seal, horizontal stress increases. The additional stress leads to an increase in confinement and a subsequent reduction in permeability.

Jan 1, 1999

By Hanjie Chen

A mine panel collapse may occur when pillar sizes are too small or the surrounding rock strata (roof or floor) yields. After a small pillar fails, its loading is rapidly transferred to adjacent pillars that in turn fail. If the surrounding rock strata yield, the loading on pillars will redistribute. which may result in some pillars under high stress. This paper analyzes the causes of two cases of mine panels failures in two room and pillar mines. One case of massive pillar failure occurred in a phosphate mine and the other, mine panels failure, is in a coal mine. In the phosphate mine, both the roof and floor strata were a thick competent shale stratum. The finite element analysis shows that the major reason for the massive pillar failure is that the pillar sizes were too small in a panel. In addition, the high cliff topography has some influence on the failure. In the coal mine, the failed area was about 1,000 ft long by 760 ft wide (covering more than 3 panels) involving 390 pillars. The harrier pillars between the panels were punched on retreat into small chain pillars and the extraction ratio in this failed area was more than 80%. The roof remained intact, and pillars were crushed and punched into the floor. Based on the stress analysis, it is found that multiple panel collapse occurred because the barrier pillars were punched into small pillars and the loading on panel edges increased due to floor failure.

Jan 1, 1999

By David Gearhart, Peter Zhang, Gabriel Esterhuizen

"Researchers from the National Institute for Safety and Health (NIOSH) installed instruments in the track entry and in the pillars of a northern West Virginia longwall coal mine between two planned panels for the purpose of monitoring the behavior of the ground. The instruments included four cable bolt load cells to measure the change in gateroad loading, six borehole pressure cells to measure the loading in the pillars, three six-channel roof extensometers to measure roof sag, four convergence measurements on the cribs to measure the crib loading, and a multipoint borehole extensometer to measure the pillar expansion. The instrumentation was all connected through two multiplexers to a single Campbell Scientific permissible datalogger located about 350 feet outby the instruments. The goal of the project was to record all of the measurements as both longwall faces passed the site of the instrumentation and to use the data to calibrate numerical models.This report describes the recording of all of the measurements taken as the first panel approached and then passed the instrumentation site. This study provided valuable data related to the ground response and support performance that will be used to improve safety in longwall gateroads.INTRODUCTIONA longwall mine in northern West Virginia was selected as a second study site for an ongoing research project into gateroad layout and support design. The area was affected by a transition from limestone to sandstone in the roof. The entries were 16 feet wide, and the height was approximately 80 inches. The layout for the longwall panels was 1,170 feet wide by 4,000 feet long. The panel configuration was right-handed, and a three-entry gateroad was used between panels. The pillars next to the headgate were 137.5 feet by 115 feet center-to-center, and the pillars next to the tailgate were 275 feet by 100 feet center-to-center, with the crosscuts at 60 degrees. The width of the gateroad was a total of 215 feet."

Jan 1, 2018

By G. Skybey

The Skybolt is a new bolt concept, point anchored with resin in large diameter holes of specific sizes 38 mm and 45 mm. The new design achieves the optimum annular spacing required for mixing the resin anchor components which is essential for optimum resin bonding of the bolt to the surrounding rock. This paper explains the load transfer mechanisms between bolt/resin/rock, unique to this bolt concept. It compares the pull-out test results to those of paddle and expansion shell type bolts, and highlights the versatility of Skybolt over existing, currently used, rock support systems and the benefits mine operators can achieve when using the Skybolt.

Jan 1, 1992

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 Linda K. Killoran

The U.S. Bureau of Mines has developed a prototype cross well acoustic logging system and complementary tomographic image reconstruction software for the detection and delineation of abandoned underground mines and associated subsidence failure in relatively shallow environments. The logging system uses a piezoelectric, cylindrical bender as an acoustic source. A triaxial accelerometer assembly, designed to be rigidly locked in the borehole, is the receiver portion of the logging system. Both the source and receiver probes operate at high frequencies, thus providing short wavelengths capable of resolving small features in most rock types. A modified van with a dual-drum wireline winch, 4-channel data acquisition system, power amplifiers, and waveform generator supports field operation of the logging system. A field study was conducted at a test site in San Antonio, Texas, to demonstrate the cross well acoustic logging system. The underlying rock at the test site is similar to stratified sediments in coal measures. First¬arrival travel times were interpreted by tomographic reconstruction of the velocity field in the two-dimensional cross section between the source and receiver boreholes. The resulting image correlates well with geological borehole logging data for the test site.

Jan 1, 1992

By Joachim Leonhardt

Hundreds ofbolt-indicators were already used successfully in the German deep hard coal-mines to increase the safety and economy. The bolt-indicator offers the simple possibility to show roof-bolt (anchor-) loads. to indicate weak points as well as to optimize the anchor density and the dimension: The bolt-indicator is mushroom shaped and consists of coated metal. It is inserted between the anchor plate and the flanged nut. The loads appearing at the anchor are transferred to the bolt- indicator and can be recognized visually at the cylindrical part by the amount of the surface coating. Examples are used to clarify the application aims, operation and usage of the bolt-indicator and its yielding advantages in practice.

Jan 1, 2001

By Frank Schmidt

Due to changing geology, rib bolting is an ever-changing problem in ground control. With the economic status of the coal market, the need for safely increasing productivity in rib bolting is at an all-time high. The current solutions used in the market often require a long installation time (e.g. if collapsed holes have to be redrilled) or result in poor quality installations that can lead to insufficient ground control. A new bolting product based on the simple screw principle has been developed to bolt ribs faster and with a more repeatable, higher level of installation quality. The paper covers the product invention and development, the fastening principle and its advantages over current systems and first experiences in the market. The first underground trials have shown that the screw anchors can be installed safely and efficiently, often faster than traditional methods. The screw anchor does not suffer from conventional bolting issues such as collapsed holes due to bad ribs. The screw anchors are designed to obtain steel failure and load testing underground shows consistent fastening in all types of conditions.

Jan 1, 2013

By Andrew Hyett

Over the last 30 years, a transition away from end-anchored bolts to fully grouted rock bolt systems has occurred in many underground mines. It seems geomechanically intuitive that the success of such bolts, which interfere with rock movements around underground excavations through a variety of complex interactions, is precisely why they remain challenging to instrument and monitor. Whereas measuring the load profile for an end-anchored mechanical bolt is trivial, for a fully grouted system, the load may vary dramatically due to discrete rock movement at cracks and joints. Despite the challenges, measuring the strain (i.e., load) along fully grouted rock bolts remains very appealing, and a lot of valuable research has been conducted using foil resistance strain gauges, particularly in US coal mines (Signer and Jones, 1990). Despite good intentions, more widespread adoption of this instrumentation remains unlikely for the following reasons: (i) The technology is expensive, partly because Intrinsically Safe (IS) requirements limit the number of suppliers and solutions. (ii) The technology is perceived as challenging to successfully implement except by those who are skilled in the art. (iii) The reliability of the technology is questionable in harsh environments. (iv) The technology relies on discrete pairs of strain gauges that may not capture the complete strain distribution along the bolt. No existing alternative technology can offer a solution to all of these limitations. However, in this paper, we introduce results from a new Distributed Optical Sensing technology that promises significant potential. Since the technology is capable of distributed sensing, our results suggest that this new technology can, for the first time, reveal the detailed strain profile along fully grouted rock bolts.

Jan 1, 2013

By David A. Newman

In the 1950s, an underground limestone mine in the Camp Nelson Formation was developed from an outcrop exposure. The Camp Nelson Limestone met the requirements for DOT stone and aggregate. The mine ownership changed several times, which resulted in distinct mine designs in Level 1. Based upon the production requirements and an attempt to maximize limestone recovery from a single level, a variety of pillar dimensions and three mining heights (28 feet, 68 feet and 83 feet) were developed. This was sufficient prior to reaching the mineable extent of Level 1. Level 2 introduced previously unknown ground control challenges. These included: ??Pillar spalling and a significant back fall on the Level 1 - Level 2 decline; ??Laminated immediate back and the need to bolt the back on Level 2; ??Delamination of the back adjacent to the Level 1 - Level 2 decline resulting in bolt shearing, and floor heave in isolated areas of Level 2; ??Multiple level interaction, the need to design pillars based upon numerical modeling as columnization between Levels is infeasible: and ??Water inflow as the mine approached river. To meet the challenges, a testing program to determine rock strength and physical properties was implemented along with numerical (LAMODEL and FEM) modeling, and borescope inspections of the immediate back. The efforts to implement rock mechanics were re-focused as Level 2 approached its mineable limit and the initial attempt to drive a decline to Level 3 was unsuccessful. A second decline site was selected based on inter burden stability, a consistent shale parting for the Level 3 back horizon, results from numerical models of the immediate back and multiple level interactions, and the projected low water inflow. The mine is operating on Levels 2 and 3 with long-term planning and ground monitoring for Level 3.

Jan 1, 2014

By David H. Y. Tang

The reinforcement mechanism of suspension effect is analyzed based on beam-column theory. The equations of the maximum bending stress, deflection and transferred bolt load for the bolted roof strata are derived. In the analysis, the bolt load is assumed to be point load and the horizontal stress is uniformly applied to each stratum. The reinforcement mechanism of friction effect is also analyzed. The major function of roof bolting in this case is to create the friction¬al resistance by tensioning the roof bolts, thereby the individual thin strata are "welded" into one single thick stratum. Based on the results, equal spacing of roof bolts is used in the suspension effect while for friction effect, the bolts should be spaced based on equal shear force concept. The design nomographs for the proper bolting pattern and bolt ten¬sion of mechanical roof bolting systems are presented for practical applications.

Jan 1, 1984

By Peter Zhang

Room and pillar mining under shallow depth is usually conducted with a high extraction ratio, which leaves small pillars with a low safety factor in some old workings in abandoned mines. Pillar sloughage, squeezing or even collapse may have occurred in those old workings with small pillars, either at the time of mining or later. The conditions of the pillars in abandoned mines can affect mining above them. Subsidence fractures and abutment pressure are indications of multi-seam interaction caused by squeezing or collapse of small pillars. To mine over areas with potentially unstable small pillars, the panel layout in the upper seam should be such that multi-seam interaction can be minimized and proper roof and rib support should be installed in the influence zones. This case study presents an example of multi-seam mining above small pillars under a shallow depth of cover. Though the interburden is relatively thick and competent, interaction is still present over the influence zones. Pillar stability and multi-seam interaction analysis, numerical modeling and in-situ monitoring are presented to demonstrate how the upper seam can be mined safely over the small pillars.

Jan 1, 2014

By Wei Yin, Yu Wu, Xiexing Miao

"For engineering problems of “three under” coal (coal trapped under buildings, waters-bodies and railways) exploitation, environment protection and gangue separation from raw coal underground exist in Tangshan Coal Colliery. By analyzing concrete technical problems, a technical approach of closed cycle “coal mining-gangue washing-backfilling-coal mining”, i.e. integration of coal mining, gangue separation and solid backfilling, is put forward with key technologies including gangue transportation with great depth, gangue separation from raw coal underground and integration of coal mining and backfilling. Based on the specific geological conditions of Tangshan Coal Colliery, 591m vertical feeding transportation system with great depth is designed; meanwhile, for efficient gangue transportation, ancillary wear-resisting pipe size and buffer equipment are optimized. Underground gangue separation system is created to be joined with the existing production system and backfilling system; besides, as per separation characteristics of coal and gangue, buddle jig is adopted for gangue roughing of raw coal underground. By cooperation of backfilling hydraulic support and other key equipment, the designed stope backfilling system completely realizes integration of coal mining and backfilling. Engineering practice shows that, through the research and implementation of the above key technologies, integration of coal exploitation, gangue separation and solid backfilling is successfully implemented in Tangshan Coal Colliery; moreover, it completely solves the three major engineering problems and realizes waste disposal, underground washing and separation, and ground surface subsidence control.INTRODUCTIONSo far, “three under” coal (coal trapped under buildings, watersbodies and railways) and key state-owned coal resources trapped under villages in China have reached 13.7 billion t and 5.22 billion t, respectively. With the development of coal enterprises, China currently has more than 1,600 gangue dumps (Miao et al., 2010a) with accumulated stack amount of 4.5 billion t, occupying a land area of 150,000 km2; moreover, surface subsidence caused by mining activities has damaged the environments of industry, agriculture, transportation and living, while new environmental pollution sources formed by coal related industries also result in damage to ecological environment (Huang et al., 2011a). On the one hand, due to large scale coal mining, some coal collieries suffering from resource exhaustion are faced with survival problems; on the other hand, coal mining has caused and been faced with increasingly prominent environmental problems, such as “three under” coal exploitation, waste disposal and ground surface subsidence control (Miao et al., 2010b; Zhang et al., 2009)."

Jan 1, 2015

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 Steve P. Signer

Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health installed 39 instrumented, fully grouted bolts at six test sites in a trona mine retreat panel to study mine roof stability for the improvement of workplace safety. Variables at each test site included bolt spacing, bolt length, roof span, and location in the panel layout. At most test sites, two rows of instrumented bolts were installed, one in or near the intersection created during development and the other in or near the intersection created during second-pass mining. The most significant factor affecting bolt load was roof span. The highest loads were on the bolts installed in the intersections of the 6 m-wide entry. Mining-induced stress resulting from panel layout was the next most significant factor affecting bolt loading. Minor varia¬tions in bolt loading could be attributed to changes in bolt spacing and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressional loading during gas bleed-off.

Jan 1, 2001