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By Peter Altounyan

The rate of decay of the energy from a hammer blow to a coal mine roof is related to the size and nature of the contacts between the rock being struck and its surroundings The use of this principle as a basis for testing roof condition in South African coal mines has been investigated under a SIMRAC funded project, with the objective of reducing the number of roof falls. ( I ) A prototype instrument having a simple numeric display related to the energy decay rate (the Acoustic Energy Meter) was constructed and used to carry out surveys at mines covering the range of coal mine roof geology and condition. The equipment was simple to use and provided excellent discrimination between visually intact roof and less stable conditions, once the normal/abnormal threshold had been established for a given site. Areas of potential roof failure were identified close to roof slips and igneous dykes, and confirmed in areas of visible deterioration. Directional stress effects on roof condition were also identified at some sites, with consistent differences in readings obtained depending on the direction of drivage. Repeat surveys also indicated areas where roof conditions were deteriorating with time. Based on the success of the prototype, a hand held version of the instrument. has been developed to a specification from Anglo Coal. This incorporates a simple 'traffic light' green / amber/ red display, making the device suitable for routine use by mine operatives This version is now undergoing trials at an Anglo Coal mine. It is concluded that the Acoustic Energy Meter shows great potential as a test instrument providing output which relates to the stability of the immediate roof.

Jan 1, 2000

By Michael Gauna, Christopher Mark

"For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines. Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by a roof fall than other coal miners. Since 2007, however, there has been just one fatal roof fall on a pillar line. This paper describes the process that resulted in this historic achievement. It covers both the key research findings and the ways in which those insights, beginning in the early 2000s, were implemented in mining practice.One key finding was that safe pillar recovery requires both global and local stability. Global stability is addressed primarily through proper pillar design, and became a major focus after the 2007 Crandall Canyon mine disaster. But the most significant improvements resulted from detailed studies that showed that local stability, defined as roof control in the immediate work area, could be achieved with three interventions:• Leaving an engineered final stump, rather than extracting the entire pillar;• Enhancing roof bolt support, particularly in intersections, and• Increasing the use of Mobile Roof Supports (MRS)A final component was an emphasis on better management of pillar recovery operations. This included a focus on worker positioning, as well as on the pillar and lift sequences, MRS operations, and hazard identification. As retreat mines have incorporated these elements into their Roof Control Plans, it has become clear that pillar recovery is not ""inherently unsafe.""The paper concludes with a discussion of the challenges that remain, including the problems of rib falls and coal bursts.INTRODUCTIONPillar recovery has always been an integral part of underground coal mining in the US. When room-and-pillar methods are employed, large blocks of coal in the form of pillars are initially left in place to support the weight of the overburden. Unless these pillars are subsequently recovered, the coal they contain will never be mined."

Jan 1, 2016

By Sam Spearing

Free-standing supports are commonly used in coal mines in the USA, particularly when longwalling. A recent trend has been the development of prop supports. Prop supports have many advantages over bulky, crib-type secondary standing supports, particularly in regard to material handling and installation. The challenge in prop design has been to develop load capacities that can compete with crib type systems and to develop reliable (stable) yielding capability. This paper will focus on the design aspects of steel prop supports, specifically, and evaluate the various concepts that have been developed to provide easy height adjustment and sustainable stable yielding under load. The design and resulting improvement provided by recent support developments called the Quik Stik and Ball Buster are discussed in detail. Performance areas that still need improvement in the design and usage of free standing supports in general are identified and possible solutions outlined.

Jan 1, 2008

By Karsten Zimmermann

The paper presents a study on the influence of face retreat rate in longwall coal mining on surface subsidence. It shows possibilities to influence dynamic ground movement by using a specific and controlled winning regime. For this, a dynamic stochastic prediction model used by DMT in Germany will be proved and adjusted on the specific character of geology in the Northern Appalachian coal field. Occurring ground movements such as inclination, curvature and deformation can be analyzed under static and dynamic points of view. It is theoretically shown that the face retreat rate has only a small influence on the static deformation values, but directly affects rate and acceleration of subsidence and deformation processes. The study shows that because of low mining depth and high mined thickness, static deformation values have the largest impact to surface objects. This implies that high face retreat rates don?t affect the impact on surface objects.

Jan 1, 2007

By Stephen C. Tadolini

The International Ground Control Conference in Mining has always provided an open forum for the publication, presentation, discussion, and often heated debate on roof bolting systems mostly with attention to how, when, and why they work. During the last 24 Conferences at least 170 papers have been presented by International Experts on these ?simple? devices that range in length from 30-inches to 20 ft and from 5/8 to 2 inches in diameter. Roof bolts are primarily mechanically anchored, glued, cemented, or driven. Bolts are placed vertically, angled, or tied together with special fixtures (trusses and slings). They have been termed active, passive, stick-slip, and several other descriptive acronyms or mining slang expressions. This retrospective paper will present the changes in bolt types and usage, point out some of the biggest myths and hail the most significant advances (according to the authors? opinions of course).

Jan 1, 2006

By Nikolay Boykov

Subsurface imaging technologies are part of the electromagnetic (EM) geophysical methods. EM technologies such as radio imaging method (RIM), ground penetrating radar (GPR), and drill string radar (DSR) have great application potentials in the mining industry. However, the success for applying the EM methods to subsurface observations is largely dependant on the correct evaluation of the electrical properties (i.e., resistivity and dielectric constant) of the rocks in the domain of interest. The depth of penetration of the EM signal in coal, as well as the electrical contrast between coal and subsurface boundary rocks or anomalies, are critical parameters for any successful radar system. Based on the measured electrical properties of the coal seam, the proper operating frequency of the radar system must be chosen in order to successfully detect features such as the coal-roof rock boundary, dykes, and voids ahead of mining. The electrical properties of the rocks in a coal seam are frequency dependent. Even though there has been a great advancement in the hardware and data processing software in the past twenty years, there have not been many examinations of the high-frequency properties of coal measure rocks. In this paper, the laboratory setups, the testing procedure, and case studies will be presented for a wide spectrum of frequencies for coal measure rock samples collected from various US coal fields. Tests have been conducted to study the different conditions, including moisture content, that affect the EM properties.

Jan 1, 2006

By Murali M. Gadde

Perimeter mining is a special form of very productive partial extraction method practiced at some mines in the United States. In the Illinois Basin mines where all major coal seams have weak underclay in the immediate floor, experience shows that any long-term ground control issues will arise due to floor failures rather than instability of the coal pillar itself. While there is a significant amount of work done on floor stability associated with regular pillars, no work has been done so far on the stability of perimeter pillars. This paper reports the results of a comprehensive study that was conducted for the first time on floor stability during perimeter mining in the Illinois Basin mines. A special plate template, which mimicked a common perimeter pillar configuration was created to conduct in-situ bearing capacity tests at five mines in three states of the Basin. Additionally, because of the complex shape of pillars created in perimeter mining that defies any closed-form solution, floor bearing capacity was studied using three-dimensional nonlinear numerical modeling. Based on these in-situ and numerical studies, some guidelines are provided for perimeter pillar stability analysis.

Jan 1, 2009

By Erik Westman

Coal bumps remain one of the industry?s most devastating but least understood phenomena. If changes to the internal conditions of the rock mass could be observed there is hope that bumps could be forecasted. Seismic tomography has been used over the past 25 years in an attempt to image these changing conditions in the vicinity of the longwall face. This paper describes the history and practical considerations of using seismic tomography in bump-prone coal mines. The implementation of seismic tomography at six different longwall mines will be described along with lessons learned. Early studies used mining independent sources of seismic energy, including explosives and sledgehammers. Subsequently more operator friendly sources, such as the mining equipment itself and mining-induced seismicity, were used. Seismic energy sensors have been placed in gateroads and on the surface above the mine. Each of these configurations has associated advantages and disadvantages. Although the results obtained have been encouraging, the application of seismic tomography to monitoring a bump-prone longwall face is still immature. The ultimate goal of clearly imaging changing stress conditions in the longwall panel in a manner that does not inhibit mining operation is still distant, yet drawing nearer. When this goal is achieved, the industry will have a capable tool for identifying locations where bumps are imminent.

Jan 1, 2008

By Rob Thomas

Main gate roof control during longwall retreat is subject to a significant increase in horizontal stress and risk. The implementation and management of an appropriate roof support design is as a result, an imperative requirement for any longwall mine and must in addition to the likely consequences of a roof fall, also consider the variables of the geotechnical environment, associated support costs and practicalities. This paper will therefore summarise a number of recent advances in main gate roof control in the Australian coal industry. Points considered include; (i) the ?Longwall Acceleration? concept, (ii) macro-scale horizontal stress notching, (iii) micro-scale horizontal stress notching, (iv) bedded versus laminated roof types and (v) the type of cable reinforcement used.

Jan 1, 2006

By Ebrahim Ghasemi

Problem statement: One of the most challenging safety problems in room-and-pillar coalmines is controlling roof falls during retreat mining. Roof falls not only cause both fatal and non-fatal injuries to miners, stoppages in mining operations, and equipment breakdowns, but also results in the inefficient recovery of coal reserves. Approach: Sufficient awareness of effective parameters related to roof falls is an important factor in roof control. In this study, the major contributing parameters on roof falls during retreat mining are introduced based upon experience and judgment acquired from extensive retreat mining case histories and a review of relevant literature. Key findings: The main roof fall parameters in retreat mining can be divided into three categories: geological, design, and operational. Key conclusions: The design and operational parameters can be controlled by the engineer and/or operator. By the appropriate selection of these parameters in the design and implementation stages of retreat mining, the roof fall risk can be decreased to an acceptable extent.

Jan 1, 2012

By Rob Thomas

Pre-driven recovery roads are used to speed up the longwall recovery process by reducing bolt-up requirements and by improving ground conditions on the face during the removal of the powered supports. To lower the geotechnical risks associated with the use of pre-driven recovery roads, and so improve its overall acceptance in the industry as a viable alternative to more conventional, potentially difficult and costly longwall recoveries, this paper outlines (i) a geomechanical model for the use and design of pre-driven recovery roads, (ii) an updated version of the worldwide database on pre-driven recovery roads, (iii) a summary of the various methods of holing a longwall into a pre-driven recovery road and (iv) a series of relevant ground support and strata management guidelines.

Jan 1, 2008

By Nic Slatalla

Resin anchored rock bolts are an effective tool to control the deformation of an underground opening because they combine high support capacity with fast installation. However, there are still numerous shortcomings concerning the understanding and interaction of the system bolt-resin-rock. Field and laboratory test series have been conducted for evaluating the load capacity and bond strength of resin grouted rock bolts in different host rocks under axial load. This paper describes that the behavior and shear strength of the system bolt-resin-rock depends on the rock type and discusses a simple correlation between peak shear strength and shear stiffness in the ratio 1:1000 which may moreover be helpful for estimating rock bolt capacity after installation.

Jan 1, 2006

By Brijes Mishra

This paper presents a correlation of acoustic emission (A.E.) with physical and mechanical properties of different types of rock and coal specimens which were tested under aegis of a consultancy project from a Columbian Mining Company. In this study, three types of rocks which are sandstone (Arneisca), siltstone (Arciolita) and claystone (Limolita) and two different types of coal were tested. The tests were carried out under constant loading rates according to ASTM Standards for determining Young?s Modulus and Poisson?s Ratio. In both cases a MTS electro-hydraulic loading system was used. Mode of failure in each material was characterized by the acoustic emission and the results were substantiated by the deformation characteristics of the materials during failure.

Jan 1, 2006

By Scott A. Shapkoff

High strength cable strand supports are predominately used as a supplemental means of ground control in longwall and development operations in the U.S. The advantages of cable strand supports have been documented and include flexibility, strength, and ease of installation. Since the majority of cable supports installed in the U.S. today are partially grouted, and this same segment of supports has no inherent installed tension, the supports are considered currently arbitrarily strictly supplemental and therefore, cannot currently be deemed primary support (in most mining districts). This paper details and discusses two distinctly different configurations and the technical advantages of active partially grouted cable supports for certain primary support applications. The evolution of the designs for both systems is highlighted. Field data illustrates the amount of active load in relationship to installed torque and the affects that component variables have on this load measurement. Roof control patterns using active cable supports are examined as alternatives for combination patterns using primary and supplemental systems. The importance of maintaining consistent installation procedures plays an important role in the level of performance of the system. Economics comparing the installation of active cables with other primary/supplemental patterns is also included.

Jan 1, 2006

By Hideki Shimando

Periodic weighting and windblast which are caused by the dynamic effects of overburden in a longwall panel significantly affect longwall mining This study describes past experience with wind blast at Miike Colliery, proposes a new loosening system using hydraulic fracturing and moistening, and also discusses strength deterioration in hard-to-collapse roof rocks due to water in order to solve the problems caused by the effects of dynamic roof rocks behavior in a longwall face

Jan 1, 1998

By Hakan Gurgenli

Many coal seams have weak shale immediate roofs that cause ground control problems. Therefore, it is important to know the properties of these shales so that preventive measures can be developed in advance. However, it is sometimes hard to understand this rock type due to its unpredictable behavior. A good example of this situation is found in two shale coal mine roofs located in the Illinois coal basin (Herrin No. 6 seam). Although, these mines are located very close to each other, one of them has many roof falls, whereas the other has no measurable ground control problems. The aim of this paper is to explain why and how one shale roof behaves worse than the other. Laboratory tests, including point load strength index, slake durability index, moisture activity index, weatherability index, water content determination, swelling strain and x-ray diffraction tests were performed to analyze the shale properties. According to these analyses, some useful correlations were found between the engineering properties, moisture-sensitivity indices, and weathering properties. Finally, mine and support design considerations based on the test results were discussed and recommendations to prevent ground control problems were given.

Jan 1, 2006

By D. N. Bigby

This paper concentrates on ground control innovation in Europe since the first Morgantown conference, mainly from a UK perspective. In particular, it describes the rock engineering and ground control tools developed to address the mine support and design problems associated with coal mining at great depth in areas influenced by previous workings. The differences in approach across the Western European industries are also examined. Perhaps the most significant change in the period has been in the science of coal mine rock mechanics itself, with support design now based firmly on measurement, analysis and understanding. The ground control innovations described here have played their part in increasing the efficiency with which coal is mined and in increasing safety to the extent that fall of ground accidents are now rare.

Jan 1, 2006

By John E. Feddock

Retreat mining plays an important role in the coal production within Kentucky and the Appalachian Coal Basin. In Kentucky, the majority of retreat mining is performed in the Eastern coal field. However, since September 2004, Kentucky has suffered four mining fatalities during retreat mining operations, and the coal industry has focused attention on the safety concerns associated with retreat mining practices. The Kentucky Department of Natural Resources (KYDNR) engaged Marshall Miller & Associates, Inc. (MM&A) to conduct a study to ensure that retreat mining is utilizing the best technology currently available and that the current available employee training programs afford maximum safety to miners. MM&A studied 34 coal mines that were conducting active retreat mining operations and represent about one third of all the mines with approved retreat mining plans in Kentucky. This paper summarizes the major research work conducted (Feddock and Ma, 2006) and recommends the practical methods for improving safety during retreat mining operations.

Jan 1, 2006

By Murali M. Gadde

Welded wire mesh is the most commonly used skin control technique in the United States underground coal mines. Currently, selection of the mesh is based on trial and error rather than any rigorous design methodology. Several factors affect a wire mesh?s performance including the amount of rock material to be supported and how the load is distributed, wire gauge, aperture size, grade of steel, weld strength, bolting pattern and sheet size. Although the best way to choose the mesh is to conduct full-scale physical load tests, because of the limitations on the number of variables that can be considered in such tests and the requirement of an elaborate test set-up, it may not always be possible to use this option. An alternative would be to use numerical modeling to study the load-deformation behavior of a wire mesh. In addition to providing a very detailed stress, strain distribution in the entire mesh, the advantage of this approach is the ease with which parametric studies can be conducted. In this paper it is shown that the load-deformation curves generated by nonlinear numerical models match closely with laboratory derived ones. Then, based on parametric studies, design guidelines are developed to choose the mesh appropriate for different loading conditions. The modeling studies also bring forth the limitations of the current lab studies. For example, under some conditions it was noticed that the small sheet size considered in lab tests would produce higher mesh deformations than a full-size one used in actual practice.

Jan 1, 2006

By Wenbing Guo

Strip pillar mining method is one of the most important techniques for mining subsidence control in China. Mining depth is one of the most important factors that affect subsidence in strip pillar mining method. Six numerical simulation models were set up in this paper to simulate surface subsidence of strip pillar mining at different mining depths. The mining depth in these models ranged from 300 to 800 m. The simulated results were compared and analyzed. The results showed that the subsidence factor of strip pillar mining is related to mining depth. While other things being equal, the subsidence factor of strip pillar mining increases logarithmically with increase in mining depth. Their quantitative relationship was established based on the simulated results.

Jan 1, 2007