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By S. Paul Singh

Rock burst is one of the most serious mining hazards, adversely affecting the safety, economy and productivity of mines. It is a sudden manifestation of the release of strain energy stored in the rock mass. At present, there is no universally accepted method for predicting the timing of the rock burst. However, if the seismically hazardous regions in mines can be identified before they are exposed to mining then precautionary measures can be planned before the work begins. A study was conducted in Sudbury Nickel Belt and an approach has been proposed to identify rock burst prone workings on the basis of (a) Rock properties and bursting indices (b) In-situ approaches (c) Geological features (d) Mining conditions (e) Observations during mining operations (f) Comparison and analogy with previous case histories of bursting. This approach will facilitate the judicious application of the burst control techniques to seismically hazardous areas.

Jan 1, 2008

By Yun Zhang, Haijun Jiang, Shenggen Cao, Chen Wang

The distribution of front abutment pressure is closely related to the force, deformation and energy distribution of hard roof before periodic weighting. So it is necessary to carry out research on the relationship between them. According to front abutment pressure distribution feature, using the location of peak front abutment pressure as the dividing point, coal seam is divided into two parts along the direction of mining: the yield zone that is the area between the faceline and the point where the maximum front abutment pressure occurs, and the elastic zone that is the part before the point of peak front abutment pressure. The proposed mechanical model of unit width hard roof at the panel center periodic weighting consists of five parts including the yield zone. All parameters of the deflection equations for each of the five parts that satisfy the continuity conditions and natural boundary conditions are obtained by using the Matlab software. The continuous curves of front abutment pressure, deflection, bending moment and bending strain energy density distribution of hard roof are obtained by iterative approximation method. The following conclusions can be drawn through analyzing those curves: (1) The position of peak front abutment pressure is located at the transition of coal seam foundation from yield zone to elastic zone (elastoyield critical point), the distance of it to the faceline is equal to the width of yield zone. The peak front abutment pressure increases with yield zone with. (2) The peak bending moment in front of the faceline, and the distance of it to the faceline increase with the yield zone width. The deflection of hard roof at the face and midspan also increase with yield zone width. (3) As the yield zone width increases, the accumulated elastic energy of hard roof in front of the faceline increases, and the core zone of energy moves forward away from the face. (4) When the yield zone width is zero, the whole coal foundation is regarded as elastic foundation, and the peak front abutment pressure occurs at the faceline.

Jan 1, 2015

By Jun Han

Coal has been mined in the Kailuan mining area for more than 130 years. The remaining reserves consist of, in large part, coals that would have been considered economically unminable or technically not feasible by the previous generations. Some of the current operations have to resort to overmining because the lower seams had been extracted in the past. This paper deals with the feasibility and proper layout of mining roadways of overmining carried out in the No. 5 seam of Qianjiaying coal mine of Kailuan mining area using numerical and physical simulation. FLAC2D was used to investigate the characteristics of stress and displacement in the overburden after extraction of the lower seams. It also analyzed the proper layouts of gateroads in the upper seams, i.e. locating them in the stress-relief zone of the gob so that their stability can be easily maintained. In addition, a two-dimensional physical model was constructed for investigation of damages to the upper seams due to mining of the lower seams. Finally, based on the results of investigations, proper gateroads layout was designed and successful extraction was implemented for panel 1357 in the No.5 seam.

Jan 1, 2009

By Khaled Morsy

The tragic incidents at Sago and Darby coal mines in 2006 pointed out the need for stronger seals to resist the blast effects of violent gob explosions. Mine Safety and Health Administration (MSHA) has published the final rule on Sealing of Abandoned Areas in April 2008. According to the final rule, the mine seals for unmonitored gob areas should withstand an instantaneous pressure of 120 psi. Reinforced concrete (RC) seal is one of the feasible options that can be used to meet the final rule. Concrete is a heterogeneous, cohesive-frictional material and exhibits complex non-linear inelastic behavior under multi-axial state of stress. The increased use of concrete as primary structural material requires the development of sophisticated material models to have an accurate prediction of the material response to a variety of loading situations. The constitutive modeling of concrete has advanced considerably in the past three decades. Choosing a reasonable concrete material model is important in order to obtain an accurate mine seal design. This paper attempts to address the effect of different concrete material models on the response of RC seal subjected to dynamic loads from methane explosion. Three concrete models, from ABAQUS library, were selected to address the effect of non-linear behavior of concrete and damages due to tensile and compressive stresses. In addition, a typical and verified concrete material model was used for conducting 3D finite element models simulating the RC seal by ABAQUS, EXPLICIT ver. 6.6.

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 Nic Slatalla

Acoustic emission monitoring techniques are a powerful tool for analysing deformation behaviour and fracture characteristics of rock samples under uniaxial loading. Uniaxial compression tests were carried out on three different types of sandstone for a better understanding of acoustic emission response with respect to rock deformation and damage develop-ment. All sandstone samples revealed congruent characteristic acous-tic emission patterns which correlated directly with distinct stress-strain stages. However, the acoustic response and development within the individual stages proved to be entirely different due to variations in their microstructure.

Jan 1, 2008

By David Hill

The Australian underground coal mining industry is increasingly operating in an environment that challenges traditional concepts of coal pillar design, against a background of heightened expectations and demands from other stakeholders with regard to the protection of both natural surface features and burgeoning civil infrastructure. The result is greater possibility of conflict between mine operators, developers and regulatory bodies, with potential for unnecessary sterilisation of Australia?s coal resources and/or escalating surface development / infrastructure protection costs. On the positive side some significant advances have occurred in the general level of understanding of pillar behaviour and stability, both in Australia and overseas. This paper examines some current issues in coal pillar design, focussing particularly on the application and limitations of analytical models of pillar loading. Established methodologies are reviewed and alternative approaches explored, including examples of key geometrical, geological and probabilistic concepts. Practical experiences from the Southern Coalfield of New South Wales (NSW) are examined for the purpose of highlighting critical variables. Finally, some suggestions are made with regard to the direction and opportunities for future research, with particular regard to coal pillar loading, in the interests of all stakeholders.

Jan 1, 2008

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 A. G. Pasamehmetoglu

This paper describes the conver¬gence and load measurements carried out in inclined seams at five longwall faces, two of which were supported by hydraulic props and the other three by conventional timber supports in Karadon-Gelik mine. The minimum support capacities determined in this study are compared with the results obtained from two empirical approaches suggested by Wilson (1975) and Jacobi (1981). The characteristics of hydraulic props to be used and support patterns in the longwalls are found, conclu¬sions are drawn and suggestions are made. Design nomograms are prepared for field engineers, in order to be able to approach the problem more confidently and realistically.

Jan 1, 1992

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 Anthony Iannacchione

A Major Hazard Risk Assessment (MHRA) was developed in Australia after a series of mine disasters in the 1990?s. A MHRA is used to help prevent major hazards, i.e. fire, explosion, wind-blast, outbursts, spontaneous combustion, roof instability and chemical and hazardous substances, from injuring miners. A MHRA is a structured process that identifies the characteristics of major hazards, assesses and ranks the risk they present, and evaluates engineering and administrative controls to mitigate them. These controls typically consists of a broad spectrum of prevention, monitoring, first response, and emergency response techniques and helps to move an operation from a reactive to a proactive approach towards safety. This paper documents a MHRA performed at an underground mine where strata instabilities and fire hazards may threaten the condition of its escapeways. The objective of this MHRA is to 1) identify what hazards could affect the egress through the mine?s escapeways, 2) determine what unwanted events pose the greatest threat for the mine, and 3) recommend a plan to prevent or recover from the potential disruption of egress through the escapeway. The plan provides information on the key existing controls that should be monitored and audited, and makes recommendation of new potential controls to further reduce related risks. By documenting the use of MHRA to this specific ground control issue, this paper provides a framework for others to judge the merits of this approach and to help design and perform these activities.

Jan 1, 2007

By Arun Kumar Singh

Wide variation in the nature of the coal seams and the surrounding rock mass of the Indian coalfields and very little information about the associated stress fields were observed to be the major hurdles in the straight forward application of a continuous miner (CM) based mechanised depillaring approach. Due to various techno-economic reasons, a considerably large number of coal seams have been developed and are standing on pillars in India. The development of pillars is done as per Indian coal mines regulation, which results in pillars of relatively large size. In fact, the large size of the pillars provides stiff natural support to control caving of the massive overlying strata. However, the size of these pillars is not conducive for the single pass pillar stripping technique and, therefore, there is a need for pillar splitting before pillar extraction/retreating. Splitting of the pillar may reduce the competency/stiffness of the pillar, which creates a chance of overriding during final extraction below a massive roof stratum. This paper first discusses the existing coal mining scenario in India followed by a brief presentation of geo-technical details of pillar extraction at the three sites where a CM has been introduced for depillaring/pillar retreating in the country to date. Based on past rock mechanics experiences with conventional depillaring faces, stability issues of the pillars, fenders, and stumps in and around the depillaring face are discussed. This may prove to be an important factor in the success of future CM faces. It is recognized that the application of the modern strata movement monitoring techniques may improve operation safety by providing valuable qualitative and quantitative information that can be used to improve the design of modern concepts like roof bolt based breaker line support during pillaring.

Jan 1, 2009

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 Collin Stewart

Bowie Resources, LLC (Bowie) has been using the Stability Mapping System (SMS) developed by Dr. Keith Heasley, West Virginia University, since 2005. Seven three-entry longwall gateroads, four longwall panels and a five-entry set of mains have been mined in two different seams in areas originally analyzed with the SMS. This paper compares predictions of mining conditions based on the SMS with actual mining conditions. Specific comparisons include longwall and continuous miner (CM) productivity, longwall tailgate response and support and CM roof bolting requirements. The paper also discusses current thoughts at Bowie on the use of different weighting factors for CM development, longwall face conditions and longwall tailgate support requirements.

Jan 1, 2007

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 Dennis Dolinar

In this National Institute for Occupational Safety and Health (NIOSH) investigation, numerical modeling is used to evaluate the degree the pillar strength is increased with a rectangular pillar over a square pillar. Three-dimensional models are developed with pillar sizes and geometries typical of underground limestone mines in the United States. Physical properties for the models are based on geotechnical surveys of nearly 30 underground limestone mines. To examine the effects of pillar length on pillar strength with the modeling, a parametric study was conducted where the pillar width was held constant while the length and height were varied resulting in width-to-height ratios ranging from 0.5 to 1.5 and width to length ratios ranging from 1 to 6. There are two aspects to pillar strength and failure. At relatively low stresses, pillars can fail as a result of geologic structure. At greater depths and sufficient extraction ratios, the pillars can also fail as a result of the vertical stresses exceeding the pillar strength. The effects of increasing pillar length regarding both strength aspects are presented in this study. Equations were developed for the increase in pillar strength with both height and length based on the model results. The results from these equations were compared to those from previously developed empirical equations for rectangular pillars. For the more slender limestone pillars with a width-to-height ratio below one, there is little increase in pillar strength for length-to-width ratios above 2 and almost no increase with length for a width-to-height ratio of 0.5. Only with the more squat pillars is there a significant improvement in strength with length. The gain in pillar strength with length for the pillars is explained by the change in the ratio of the area of the pillar affected by the ends to the area of the pillar. For the squattest pillar, the change in strength is also affected by an increase in pillar confinement along the central length of the pillar as the pillar transitions from a square to a long rectangular pillar. With geologic structure, added pillar length can significantly improve the pillar strength, depending on the strike of that structure with respect to the long pillar axis.

Jan 1, 2007

By Guo Junfeng

The strength of coal mass saturated with gas is an essential parameter in studying the mechanism of gas and coal outburst, and its prediction method. Up to now, there has not been a method for determining the strength of coal mass saturated with gas. This paper describes a new method whereby the strength of in-situ coal mass can be determined and stored continuously and automatically. The simulation results obtained in the laboratory indicate that the new method allows a fast, reliable, inexpensive and convenient determination of the strength of coal mass.

Jan 1, 1992

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 Thomas A. Bailey

Since the advent of longwall mining, various methods and materials have been used during relocation to control the intrusion of the gob in the shield/panline area. (figure 1) Controlling this gob is critical to ensuring a safe, speedy, and productive longwall move. An uncontrolled gob can cause a number of problems, from increased worker exposure to entrapment of equipment to lost production. With an eye toward worker safety and increased productivity, several different methods have been developed to minimize gob intrusion. This presentation will take a look at the history and development of longwall ?screens? and ?mesh?, from chain-link fencing to high-modulus polyester geogrids. The benefits and advantages of these developments will be defined. Since 1994, longwall coal mines have used polymer geogrids during longwall recovery to control intrusion of gob material into the shield/pan line area. As described by Brandon and Collins (1996) in their paper presented at the 15th Ground Control Conference in Golden, CO., mines had used various forms of support, such as welded wire mesh, chain-link fencing, wooden cribbing, props, and steel cable to accomplish this. These methods worked, but were labor-intensive, slow, and provided varying degrees of gob stoppage. Although a few mines still use chain-ink fencing to control the gob, longwall coal mines have generally settled on polyester geogrids to control gob and roof and these mines have been instrumental in providing input in order to develop and improve these geogrids products. he trend during the past twelve years has been to incorporate stronger grids and to eliminate auxiliary reinforcements, such as steel cables and nylon or polyester webbing. A job that once required heavy steel cables, props, and fencing is now accomplished with strong, lightweight, high-modulus polyester grids. When we discuss high-modulus grids, we refer to the ratio between load and strain, as illustrated in figure 2. These grids are woven or knitted from yarns that have been pre-stressed. When these grids come under load, they pick up the load more quickly. This allows less deformation in, in this case, the mine roof. The same principle applies when these grids are used to control the ribs. With less elongation, the grids will hold rib material tighter, thereby keeping travel ways more open.

Jan 1, 2006

By Vincent Scovazzo

This paper compares the two most common pillar design equations used worldwide by John T. Boyd Company; the Mark-Bieniawski Equation, an empirical approach, and the Wilson Equation, an analytical approach. The equations are compared in two ways; by their stress profiles and by the Mark-Bieniawski Equation stability factor. Contrasting the two equations based on the visual comparison of their stress profiles is straight forward but evaluating their resulting pillar designs is problematic. Complications arise due to the differences between the use and development of stability factors for empirical equations and safety factors for analytical equations. To work around these differences, comparisons are made between the two equations by first calculating pillar sizes using the Wilson Equation and then determining the Mark-Bieniawski Equation stability factor for these pillars. Pillars designed using the Wilson Equation were based on site-specific data from 23 mines located in the Americas and Australia. Comparing sizes of pillars designed by the Wilson Equation to the Mark-Bieniawski Equation; the Wilson Equation pillar sizes were smaller in 77% of the cases, similar in 3%, and larger in 20% of the cases.

Jan 1, 2008