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By Guoqi He

Common offset seismic data collected over two active longwall panels at different stages of mining reveal that significant arrival time delays are produced in reflections from different levels within the mine cover. Mining-induced arrival time delays are also observed beyond the edges of the mine over unmined areas. Reflection arrival time delays increase with reflector depth and are laterally variable. The arrival time delays are produced by reductions of seismic velocity within the overburden. Reflection arrival time delays from a major strong unit located neutrally within the cover are proportional to subsidence, however, delays in other events are not.

Jan 1, 1989

By Victor V. Nazimko

Long term stability of the pillars that have been remained after R&P extraction causes postponed subsidence. To prevent unforeseen subsidence and to diminish postponed subsidence 25-50% new method for its control has been developed by the author. It supposes to use drilling and blasting technology and other original approaches. New method has been fulfilled through the holes being drilled from the surface or from the active underground openings. Explosives had been charged into the holes and were blasted. This had increased bulking factor dramatically and prevented natural caving of the strata. Physical scale modeling corroborated efficiency of the new approach. The bulking factor has grown from 0.0623 to 0.1185 and surface subsidence has been diminished from 1.1 m to 0.2 m or 5.5 times.

Jan 1, 1996

By Andreas Hucke

For the roadway design in the German hard coal mining industry, an advanced combined planning system has been applied during the past 20 years. The German mining industry is working with single entry roadways system. After first longwall passage the roadways will be stabilised with side wall building material packages. Later this roadway will be used again for the second retreating longwall. The intention of the implementation of several different design methods for the planning of the roadways is to enhance the reliability of the results. On one hand the planning system uses numerical simulation and physical modelling in addition to analytical and empirical methods. On the other hand in-situ monitoring and analysis during the roadway development will be used to verify and calibrate the numerical and physical models. Especially before the implementation of new support elements in underground, this system is suitable and reasonable because the new support elements will be tested in the physical and numerical simulation prior to cost intensive underground tests. The interaction between the numerical and physical modelling in comparison with underground monitoring and analysis during development and the later use of roadways will be shown with some examples. The first example shows a rock bolted rectangular roadway which will be mined at both sides of the roadway. Different support and roadway building material package systems are investigated under various stress conditions. The results are utilised to optimise the roadway support and the roadway side package systems. The influence of different points in time of the installation of several support elements in a combined arch and bolted roadway is shown as another example. Two different types of support systems are compared within the physical models. These models are used as a base for ongoing numerical analysis and simulation to find out the ideal point of time to assemble the different support elements. The last example handles the influence of slickensides in the roof of arch shaped roadway on their stability. Different slickenside positions and densities are investigated and classified.

Jan 1, 2006

By V. M. Shick

With gentle bedding within the zone of support pressure the delamination in roof rocks may occur only on those stratification planes where cohesiveness of rock is close to zero. Unlike this, at dip angles of more than 30-35° disintegration of rock occurs on the surfaces with cohesion up to 5 MPa. Therefore, the delamination in roof rocks at inclined beds is greater than at gently dipping ones. The original disintegration along the pressure fissures with the formation of block structure at inclined strata is not typical, and it may be observed only in roofs with included thick rocks; in this case, the orientation of fractures depends on the angle of seam inclination. Deformation of roof roocks above the mined-out space is determined by anisotropy of layers stiffness (hardness). With the increase of dip angle the own bearing capacity of layers will rise in the direction of strike. It was emphasized that the natural and technological factors are of great importance for rock pressure manifestation in longwalls.

Jan 1, 1996

By A. M. Richardson

The significance of a massive sandstone unit in the immediate roof to longwall support weighting is well known. If the unit lies in the main roof above the zone of caving, its influence becomes more difficult to calculate using traditional methods. Much of the reserves at Cyprus Plateau's new, multi-seam Willow Creek Mine are located in the A Seam. The massive Kenilworth Sandstone is typically situated 21.3 m (70 ft) above the A Seam in the main roof and is on average 14.6-m (48-ft) thick, ranging to 29.0-m (95-ft) thick. This paper describes the application of distinct-element modeling to determination of the appropriate yield capacity for supports at Willow Creek. The modeling was determined to be useful for understanding support loading mechanisms and as an aid to engineering judgment in selecting supports.

Jan 1, 1996

By Kelvin K. Wu

A coal waste disposal dam failed on Buffalo Creek near Saunders, West Virginia, on February 26, 1972. This accident resulted in the loss of 125 lives, left over 4000 people homeless, and resulted in millions of dollars in damage to several downstream communities. As a result of this tragic event, major changes occurred in the mine waste disposal practices used by the U.S. coal industry. This paper summarizes changes in U.S. coal-waste disposal practices in the nearly 35 years since the Buffalo Creek dam failure and identifies safety issues that need to be addressed for continued improvement. While many organizations and individuals have played roles in the improvements that have occurred, this paper focuses on the role played by MSHA and its predecessor agencies.

Jan 1, 2006

By Lawrence M. Gochioco

The interpretation of coal seismic data requires a certain precision and accuracy to make the results more reliable. The signature of the seismic wavelet provides a host of valuable geologic information. Subtle features and anomalies, not easily observed in conventional black and white seismic sections, are enhanced through color attribute displays in terms of frequency, amplitude and phase. Coal seam anomalies like sandstone washouts produce a distinct pattern in frequency and amplitude displays due to the change in the net acoustic impedance contrast. Faults produce frequency and amplitude variations due to interference within the Fresnel zone that straddles the fault. The magnitude of displacement is estimated by studying the effects of faults on reflection amplitudes and frequency from computer-generated models.

Jan 1, 1989

By Michael Amick

Over the last few years as the coal industry has seen an increase in the price of coal, a renewed interest in the additional mining of pre-law highwall sites has occurred. Much premium coal and property remains behind these pre-law highwalls but access to mining is limited because prior mining used auger machines to remove coal from under the highwall. Older augers seldom reached more than 200 feet and often no more than 100 feet but this zone of auger holes destabilizes the highwall to where additional mining is often unsafe. If the reserve is large enough, the highwall can be stabilized with common flyash and cement grouts to form a face-up for an underground mine but this expensive method is usually not practical for continuous use with a highwall mining machine. It is estimated there are hundreds of miles of highwall riddled with auger holes where the coal reserve available is ill-suited for deep mining for a variety of reasons. However, these same properties are ideal for additional mining with state-of-the-art highwall mining machines that have a reach up to 1200 feet, except to the relatively thin auger mined de-stabilized zone. The renewed interest in highwall mining these properties has driven research into finding an economical way to stabilize the highwall so the highwall mining machine can recover additional coal. This research discovered and fine-tuned a process that is economical and is successfully supporting the highwall at a highwall mining operation. This paper discusses the research involved in determining the strengths need, the methodology used to place any grouts, the effect on mining plans and the increased recoveries. ARMPS-HWM is used extensively in this research. The failure of the highwall, when not grouting, and the success, when grouting using the new materials and equipment utilized, confirmed the ARMPS-HWM method.

Jan 1, 2007

By Jeff Whyatt

Deformation of underground salt, trona and potash mines is generally time dependent, providing for gradual adjustment of strata to mining induced stresses. Time dependence can allow for higher extraction ratios provided eventual failure can be tolerated. However, this eventual failure can be violent if creep deformation can shift stress and potential energy to strong, brittle geologic units. The mine failure case studies reviewed here illustrate this process. Yield pillars and defects in bridging strata figure prominently in these cases. Yield pillars provide local and temporary support to the roof, temporarily delaying the cave; and allowing extraction ratios and overburden spans to increase beyond the long term capacity of overlying strata. Defects (faults, voids, thinning) of strong overburden strata reduce the critical span, sometimes to less than panel width. Analyses of many of these cases have focused on a cascading pillar failure mechanism, but recent work and this review point to failure of strong overburden strata as the essential element. The suddenness of failure and attendant seismic events pose hazards to miners and, in some cases, to those on the surface. Characterizing these failures is a first step towards recognizing and managing the risk of catastrophic collapse in underground mines.

Jan 1, 2008

By Xinzhi Du

Coal strength is essential to analyze the stability of excavation in coal. It is well known that the strength of coal tested in the laboratory is size-dependent, and different size specimens will produce different strength. Many researchers have studied the uniaxial compressive strength of coal and developed a number of empirical formulae to describe the strength variation with specimen size. According to a comprehensive data base that includes most of the laboratory testing results in the last century in U.S. and South Africa, the size effect and shape effect were reviewed and analyzed. The results show that different groups of coal specimens have different size effect on the compressive strength. The critical size for the size effect from the data analysis is about 40 in. The shape (W/H) of the specimens has a large effect on the compressive strength. The pillar strength formulae obtained from the data regression have a larger strength than all other existing formulae. Most pillar strength formulae have their own place in coal pillar design field. In order to analyze the difference of those formulae, twelve design formulae have been selected to compare using the Pittsburgh coal seam. The intention is not to determine the preferred formulae, but only to compare them, and see how do the W/H ratio, width, length, angle, cross-section area and effective width affect different pillar strength formulae?

Jan 1, 2008

By Doug Tesarik

Researchers from the National Institute for Occupational Safety and Health1 (NIOSH), together with employees from The Doe Run Company, are monitoring and evaluating the stability of a pillar retreat section at the Fletcher Mine, a lead/zinc, room-and-pillar mine near Bunker, MO. Cemented rockfill is being used to provide roof support as high-grade, remnant ore pillars are mined. Earth pressure cells and deformation meters placed in backfill panel pillars monitor stress change and vertical deformation, respectively, as nearby ore-bearing pillars are extracted. Similarly, biaxial stressmeters and borehole extensometers are used to monitor mining-induced stress change and deformation, respectively, in long-term support pillars. Data from these instruments indicate that when ore-bearing pillars were mined, stress was redistributed from support pillars to a backfill panel pillar. The data also indicate that the backfill and support pillars have remained stable for two years after pillar extraction was completed in the area. This research demonstrates the importance of using reliable instruments to document the geomechanical behavior of pillars and backfill. Gaining a better understanding of roof support and stress redistribution mechanisms is a fundamental part of NIOSH's goal to reduce and/or predict rockfalls, which are still among the leading causes of injuries in metal/nonmetal underground mines.

Jan 1, 2006

By Dakota D. Faulkner

Cable bolts have been very effective in mine ground control. Non-tensioned and tensioned cable bolts are now used for both primary and supplemental bolting (Suder, 2001; Mirabile, 2010). In the United States, cable bolts are typically installed in either 1 or 1-3/8 in. (25 or 35 mm) diameter drill holes and, if point anchored only, are utilized with 4 - 5 equivalent feet (1.2 - 1.5 m) of resin. Cable bolts can be tensioned by various methods such as using an expansion shell mounted on the cable or post-tensioned with a hydraulic cable tensioning unit. In recent years there has been a desire to completely encapsulate the cable bolt for corrosion protection. Fully encapsulating a cable bolt with traditional resins can be very difficult. Keystone Mining Services, LLC (KMS) and Jennmar Corporation have developed a new cable bolt that can be fully encapsulated during installation or post-installation with polyurethane (PUR). The Fully-Grouted Cable Bolt (FGCB)* is equipped with a special head that allows the injection of PUR up through the cable bolt head and cable strands, completely encapsulating the cable and filling the drill hole. Although the FGCB was initially developed to fully encapsulate the cable for corrosion protection, it was also found that by modifying the cable bolt head and injection pressure, PUR could be injected into cracks and fissures of the strata to stabilize the roof. This paper details the development of this system and three underground applications under different ground conditions.

Jan 1, 2012

By Richard D. Begley

This paper summarizes the development of a mechanistic model to predict longwall mining induced surface subsidence. The model was originally developed from data acquired from a recent field study conducted in Northern West Virginia. Additional data has been collected from published sources and used to refine the model. The original model utilizes a negative exponential equation to predict the subsidence profile after the maximum possible subsidence is predicted based on the following input parameters: mine geometry, angle of draw and expansion factors for the deformation zones that have been identified above a longwall excavation. The model refinement involved the development of relationships between physical properties of the overburden strata and angle of draw in addition to the expansion or bulking factors. Therefore the refined model now requires only mine geometry and a typical geologic column from the site to predict the subsidence profile. This paper provides the methodology that enables the model to determine the angle of draw and expansion factors breed on physical properties of the site geology i. e. percent of strong rock and Rock Quality Designation of the overburden strata. In addition :comparisons of predicted results with measured data for over H) different sites with various geological and geometrical parameters located throughout the United States will be presented.

Jan 1, 1989

By Holger Witthaus

Various seam conditions had been examined during longwall production at RAG German hard coal mines. Based on the field data a prediction method was developed considering separation planes as influencing factors for roof falls at the face. The characteristics, orientation, and density of separation are important factors on the occurrence of roof falls. In addition to geological influences, the performance parameters and geometry of support must be considered and optimized for the prediction and prevention of roof falls in a longwall face. Based on detailed macro- and microscopic investigations of e coal seams considering various ranks of carbonization, the origin and characteristics of separation planes are determined and a strata-specific influence on roof fall is interpreted. It was assessed that the coal face fracturing and displacement caused by the stress depend on the floor and roof strata conditions due to activation of the existing separation planes. Field studies are shown giving an idea of influences by advance rates of the longwall face, support delay and hydraulic pressure to the roof control in longwall faces in depth of more than 3000 ft. The analysis includes the application of specific software developed for longwall roof fall prediction as a result of a European research project. The software allows a laminar graphic display of roof fall probability, which is used for recognition of risk areas and risk management.

Jan 1, 2009

By David Newman

In Eastern Kentucky, the Darby seam has had an extensive history of coal bumps and pillar bursts. The combination of high overburden, strong rock and coal, subjacent and superjacent mines, and retreat mining has produced incidences of violent pillar failure. The influence of these factors on pillar stability and the potential to bump is illustrated in two case histories. In the first case history, a bump occurred unexpectedly during the retreat mining of a five heading continuous haulage panel. Alternative cut sequences and pillar dimensions were analyzed to identify a geometry that would reduce the potential for future coal bumps or bounces during retreat mining. The viability of the alternatives was constrained by the reach of the continuous haulage system, the overburden depth, and the presence of thick sandstones in the main roof. Each alternative was numerically modeled cut-by-cut to examine the mechanism of stress transfer, as a pillar row is retreat mined. This approach highlighted which cuts were being made in highly stressed portions of a pillar and therefore more prone to coal outbursts. Validation of the numerical modeling was accomplished by comparing the cuts where bumps and outbursts had occurred underground to the highly stressed cuts shown within the models. Recommendations of the barrier pillar width between adjacent panels, pillar dimensions, and changes in the retreat mining sequence were proposed based upon the overburden thickness, geology, and the result of the numerical modeling. The second case history documents a nine-entry panel 3,600-feet long that was retreat mined a distance of 2,064-feet without a significant roof fall or roof-floor convergence. Mine management became concerned about the potential for a coal bump or a large roof collapse within the gob and the resulting air blast. The stress on the pillar line and within the remnant pillars in the gob was quantified through numerical modeling. Changes in mining practices on the current panel were implemented to avoid a bump in the active panel. A barrier was established between the current and the future panels to protect the future panel against abutment pressures from the void area. A change in pillar centers was proposed in the adjacent panel to absorb the side abutment pressure from the gob of the active panel.

Jan 1, 2008

By Jiulong Cheng

Underground coal mining causes overburden deformation and failure, one major form of which is bed separation in the overburden. Surface subsidence can be controlled by injecting grout into the bed separations. For a long time, the grouting results of bed separation can not be validated. This is the problem in the application of this technique. This paper analyzes the conditions leading to bed separation, develops mathematical models for seismic wave fields generated before and after bed separation, and bed separations grouted with different materials, and computes the seismic wave field characteristics using the finite difference method. The results show that the existence of bed separation causes changes in the characteristics of seismic wave field, and that different grouting materials generate different seismic wave field characteristics. Intact strata, bed separation, and bed separation filled with air or water all display distinctive seismic wave field in amplitude and frequency spectra. After mining, the original location that provides strong seismic reflection wave no long exists. It is replaced by a weak and syncline-shaped reflection wave. The area, where the seam wave disappears, is larger than that of the mined area. The location, where a bed separation occurs, generates a set of reflection waves stronger than the normal one. The energy of the reflection waves varies with the materials filling the separations. Bed separations filled with air generate stronger reflection waves than those filled with water. The major frequencies for those filled with air are lower than those filled with water. Therefore, it is possible to determine the property of filling materials by analyzing and comparing the amplitude and frequency spectra of the reflection waves. Case studies of application of seismic method for exploring bed separations and grouting were analyzed. The results show that it is feasible to use seismic technique for exploring bed separations and grouting.

Jan 1, 2006

Continuing with the effort of providing new and more effective exploration methods to the mining industry, where direct methods (drilling, cross-cutting, etc.) are not applicable, either for logistical or environmental reasons or because they are costly and sometimes ineffective, we present here a new seismic technique based on full wave recording of 3-D channel waves generated within the coal seam across a longwall. In the past two years we reported the application on In-Seam High Resolution Seismic Reflection (ISHRSR) and In-Seam Seismic Tomography (ISST) to map non-coal inclusions that disrupt the coal seam continuity. In the present paper we present a 3-D In-Seam Seismic Tomography (3-DTSST) method using 3-D channel waves recorded by three component high resolution geophones. The method allows to distinguish longitudinal and transversal components of the channel waves, similar to the spheroidal and toroidal modes for the free oscillations of the earth. The recording of these waves allows a more complete analysis of the free modal vibrations of channel waves, permitting a more complete inversion of the structure of the coal seam through which they travel. The full wave analysis provides additional insights of the detailed coal structure for larger, distances than for the ones previously reported using the one dimensional method. The refinement of the inversion software is also a contribution towards the better definition of the coal seam geometry and structures disturbing them such as sandstone channels, sandstone rolls, partings, mine voids, faults, fractures, etc. The method is illustrated with three case studies where subsequent mining revealed the virtues and limitations. The first example deals with the problem of mapping mine works in one side of a longwall panel which geometry is well known, to verify its applicability to map old mine works. The second example deals with a sandstone body that partially scoured the coal seam withing a longwall panel. The third shows an application of the method to map roof conditions in a longwall panel where this problem critically affected production scheduling and costs

Jan 1, 1996

By Khaled Morsy

Cementitious seals are made of foamed cements with strength ranging from 100 to 600 psi. These cementitious seals are usually used at locations where the seal will be subjected to some convergence, assuming that this material can absorb entry closure without compromising the structural integrity of the seal. Prior to 2006, Lake Lynn Experimental Mine (LLEM) conducted full-scale tests on various cementitious seals designed to meet or exceed the 20-psi horizontal static load requirements of the old 30 CFR1 75.335. Both hydrostatic and dynamic loading tests were conducted. The structural response (pressure and displacement readings) obtained from these tests were recorded. The stability of the cementitious seal is based on balancing the load applied to the face of the seal from the explosion against the shearing resistance at the seal/rock interface. Therefore, the properties of the seal/rock interface, such as stiffness, cohesion, friction, etc. are critical to determine the stability of a cementitious seal. These properties are usually assumed by the researchers in their models. This paper emphasizes the importance of having the correct properties of seal/rock interface in the numerical modeling. FLAC 3D models were developed to simulate Celuseal2 cementitious seals subjected to both hydrostatic and dynamic loadings. Using the hydrostatic and dynamic loading test data conducted at LLEM, the static and dynamic properties of the seal/rock interface were back calculated. The FLAC 3D models showed good agreement with the test results.

Jan 1, 2009

By Dennis R. Dolinar

Controlling coal mine roofs subjected to high-horizontal stress conditions has always been difficult and uncertain. Traditional supports such as wooden cribs and posts, concrete donut cribs, and standing supports collapse and fail when the roof and floor move horizontally as mining progresses. The former U.S. Bureau of Mines (USBM) (currently the U.S. Department of Energy (DOE)), in cooperation with Western Fuels-Utah, Incorporated, conducted research to provide an alternative for traditional secondary support systems in a 3-entry gate road system subjected to high horizontal movements. The support system used in several other coal mine operations, consisted of internal high-strength galvanized resin-grouted cable supports. The system virtually eliminates the necessity for external crib, timber, or concrete supports. The support system consisted of 2.4 m (8 ft) full-column resin grouted bolts and 4.8 m (16 ft) long cable supports installed in conjunction with wire mesh and "Monster-Mats." Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during first and second panel extractions. Additionally, cable trusses were installed on the longwall headgate to protect the coal conveyance system from roof and pillar falls created by the formation of cutters and gutters. The test results indicated that the designed support system successfully maintained the roof during the extraction of two longwall panels and dramatically reduced the cost of secondary support. This paper describes the theory of high-horizontal roof movements, the advantages of vertical cable supports and cable trusses, and presents the roof and cable measurements made to assess the support performance during longwall retreat mining.

Jan 1, 1996

By Brad R. Blair

This paper will review impacts on longwall design and ground stability assessment due to advances in mine automation and sensing technology. Several new approaches to the prediction and monitoring of longwall panel and gateroad stability have recently become mature, and are ready for application in problematic ground control situations. These methods are revealing significant detail regarding deformation and failure mechanics of critical underground structures. Examples of how these technologies have worked successfully will be given, and applications for the use of these tools for cost-effective engineering design and monitoring will be suggested.

Jan 1, 1996