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By Wenbing Guo

The stability of strip pillar is the key to successful strip pillar mining. In analyzing the stability of the strip pillar, the width of yield zone must be determined. The determination of the width of yield zone has been an important subject for researchers. In this paper, the numerical method was used to study the relationship between the width of yield zone in strip pillar and coal recovery ratio. The results showed that as the recovery ratio increases, the width of yield zone increases exponentially. Accordingly, the formula for determining the width of yield zone was updated and the effect of changing coal recovery ratio on strip pillar deformation is presented.

Jan 1, 2006

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 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

By Keith Heasley

In many practical situation where the LaModel program in used to analyze the mechanical response and failure of the mining environment, there are very few actual field measurement from the site available for deriving the appropriate input parameters. For coping with this typical lack of field data, this paper presents some recommended calibration techniques derived from on an in-depth understanding of the laminated model formulation and from many years of experience applying LaModel to practical situations. For each of the critical input parameters (rock mass stiffness, gob stiffness and coal strength) in LaModel, a specific calibration technique is recommended and presented along with the justification for the technique and some appropriate derivations/equations for assistance. It is hoped that this frank discussion of recommended calibration techniques can help novice LaModel user develop better models and can initiate a discussion among rock mechanics experts on how to improve upon the presented techniques.

Jan 1, 2008

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 Khaled Morsy

The studied mine extracts the Sewickley seam using room-and-pillar method. The overburden at study site ranges from 400 to 800 ft. Eight years earlier, the Pittsburgh seam was mined 80 ft underneath the Sewickley seam. Parts of the mains at the study site experienced problems of rib spalling associated with floor heave. Numerical modeling was used to explain the mechanism of ground control problems encountered at the mains of the upper seam. The modeling results show that the ground control problems encountered were caused by multiple seam interactions with the previously mined longwall panel of the lower seam. Based on the modeling results, a criterion to predict the probability of multiple seam interactions was proposed.

Jan 1, 2006

By Rob Thomas

Strata management is a critical and now necessary requirement in the New South Wales underground coal mining industry. As a formal and structured process, strata management allows ground support and mining methods to be tailored to the anticipated and encountered geotechnical environment in a controlled manner. In its application, strata management therefore enables operators to identify and control geotechnical hazards and the risks they pose to the mining operation, including risks to personnel safety and mine production. The paper considers recent developments in strata management in the New South Wales underground coal mining industry. Points covered include the statutory requirements stipulated in the Coal Mine Health and Safety Regulations, Geotechnical Hazard Plans, tiered support plans, the Authority to Mine concept, Trigger Action Response Plans and monitoring. Several successful and unsuccessful case studies are included which make specific reference to roadway development and longwall retreat and in doing so, the unique demands imposed by these two different aspects of the mining operation on strata management.

Jan 1, 2007

By Marc Arl

Failures of the face and roof above longwall shields have caused days and sometimes weeks of lost production. A common recovery method for these failures has been to pump light weight cementitious grout into the roof and face cavity to stabilize the area This approach while effective is very time consuming and costly. Mines in several coal producing nations have over the past few years been applying phenolic foam grouts in these situations in lieu of cementitious materials. Phenolic foam is a two-component injection product intended for cavity filling, air and gas sealing and the consolidation of highly fractured strata. The properties of phenolic foam; rapid application, quick setting, high expansion rate, high compressive strength and excellent anti-static properties, produce a fast, safe and cost effective cavity filing solution. The product is also classified as fire resistant and so is also suitable for sealing off underground heatings and for aiding fire fighting operations in deep mines. This paper will discuss the application and use of phenolic foam in various underground coal mines in the world over the past few years.

Jan 1, 2005

By H. Nan

Using the case of Qianqiu Mine, Yima Coal Mining Administration, theoretical analysis, laboratory physical simulation, and underground observation were performed to study the major parameters for fully mechanized sublevel caving mining method, including drawing spacing, drawing sequence, drawing numbers, and number of shields performing simultaneous drawing. The effects of those parameters on coal recovery and amount of waste rocks were also investigated. The optimal coal drawing technology for this case was obtained.

Jan 1, 2006

By Frank E. Chase

During the past quarter century, coal mine geologists have evolved from providing a qualitative description of drill core and coal reserves into quantitative geotechnical specialists who bridge the gap between geology and engineering. In the late 1970?s and early 1980?s, hazard prediction mapping was in its infancy. At best, hazard maps identified rudimentary structures and conditions such as stream valley locations, sandstone/shale transition zones, and lineaments. A ?technical disconnect? had existed between geologists and mining engineers as each profession operated largely independent of each other. Engineers did not readily integrate geology and geologic conditions, with the exception of coal thickness, into mine planning. Geologists were relied upon for reserve estimates and did not participate in mine planning or design. The employment and market slump of the 1980?s and 1990?s combined with mining more challenging reserves mandated that this gap be bridged and forced geologists to learn to speak more ?numerically? to engineers so they could be understood. The purpose of this paper is to describe the procedures and thought processes used by coal mine geologists when determining various mineability and reserve extraction feasibility issues. The paper will also address underground geologic hazard mapping and prediction techniques. To achieve these objectives, the opinions and experiences of leading coal mine geologists, geotechnical and strata control specialists across the United States were solicited.

Jan 1, 2006

By Carl Tom Blevins

Resin was first introduced to U. S. mines in 1971 and first produced in 1973. The original polyester roof bolt resin was very expensive and limited to a single cartridge size and a single slow gel time. Early resin applications were limited to very difficult ground conditions. Today approximately 80% of the over 100 million roof bolts installed in U.S. mines, tunnels, and construction projects employ polyester roof bolt resin. The industry has evolved from hand packing samples for trial testing to producing enough resin, if converted to a .9 inch (22.9mm) diameter cartridge, to encircle the world at the equator approximately three (3) times each year. Manufacturing and design improvements allow for a more consistent product with a wide range of applications at a price that is approximately 30% below our 1974 sales price. An inflation adjusted dollar buys six (6) times more resin in 2006 than in 1974. Modern gel times range from a few seconds to over 15 minutes. Viscosity is matched to bolt type and drilling machine characteristics. Multiple cartridge diameters and a wide range of cartridge lengths are offered by two operating companies. Resin manufacturers have also gone through a period of restructuring and change to meet the challenges of the mining industry. DuPont Fasloc has become Fasloc, Inc., a stand-alone company. Celtite became Fosroc then Minova, as it is known today. During this transformation, several manufacturers have either discontinued production or were absorbed. The industry led the development of the chemical grouting materials specifications contained in ASTM F432-95, Standard Specifications for Roof and Rock Bolts and Accessories. A history of the industry will be presented within a discussion of the evolution of the strength index, speed index, cartridge volume, class designation, and product marking requirements of ASTM F432-95.

Jan 1, 2006

By J. B. Lizak

This paper is not available for inclusion in the Proceedings at the time of printing, but it will be included as an insert.

Jan 1, 1984

By Jun Lu, Mark Van Dyke, Daniel W. H. Su, Greg Hasenfus

"This paper presents the geologic and ground control challenges that were encountered by CONSOL Energy’s mining operations in southwestern Pennsylvania. A sandstone-to-limestone geology transition was encountered by Mine A, where the primary 8-foot bolts were anchored in claystone. Upon experiencing roof sags and a roof fall in the return entry, 6-foot high-tension, fully-grouted primary bolts were employed along with 16-foot center cable bolts at every other strap to improve beam building and to ensure proper anchorage into the competent limestone. No longwall delay was reported within this geologic transition zone, although there was some difficulty in maintaining an open #2 entry airway behind the longwall face.The geologic encounter in Mine B was more complex, where fluvial deposits of massive sandstone channels, shale channels and pyritic-rich, green claystones were encountered on both sides of the mains, and which impacted both development and longwall ground control. In particular, poor transitional roof areas near the edges of the sandstone and shale channels often posed significant ground control challenges. In addition, hydraulic fracturing was sometimes utilized to enhance caving of massive sandstone behind the shields to relieve pressure at the face. Longwall face conditions and advance rates were documented to illustrate the effects of sandstone channels, shale channels, and green claystones on safety and productivity and to demonstrate the effectiveness of the hydraulic fracturing.Shale channels, slickensided zones, laminated roof zones, soft floor, and a regional syncline were encountered in Mine C, the combination of which posed unique ground control challenges during both development and longwall mining. The presence of soft floor, coupled with the presence of thick floor coal and deep cover, resulted in headgate convergence during retreat of the first right-hand longwall panel. Two-piece 8-foot bolts were sometimes observed to fail at the coupler within the laminated roof zone, and 6-foot, 1-piece, fully-grouted point-anchor bolts were employed as the primary bolts thereafter."

Jan 1, 2015

By Gennaro Marino

Documented case histories are studied where data were found on the initial collapse of the overburden above the longwall panels. Under this scenario, the most controlled conditions exist for assessing the span capacity of the overburden (i.e., before failure the roof spans to fin coal abutments on two to all four sides). In an attempt to correlate theory to field observations, data are used on the overburden lithology, and the depth, width, and face advance of the panel, as well as, the subsidence conditions noted on the surface. The subsidence measured on the surface is used to determine when complete failure in the over- burden rock has occurred.

Jan 1, 1987

By Jin He

Based on the simple-beam theory, a stress analysis model was established for undermining in multiple seam strip-pillar mining when the strip pillars were not columnized; the effects of various mining factors (e.g., mining depth, inter-burden thickness, recovery ratio, etc.) on inter-burden stability were analyzed; the changes in inter-burden stress distribution were investigated; and the important role that the inter-burden spacing and the mining width of the lower seams on inter-burden stability was studied. Finally, the actual practice used in Feng Feng Mine area was used as an example to demonstrate the rational mining width for a fixed mining depth.

Jan 1, 2006

By Daniel W. H. Su

Current longwall pillar design relies heavily upon past experience and many assumptions about pillar and entry response to longwall abutment pressures. Caving characteristics of the immediate and main roof strata are known to play an important role in the overall pressure redistribution, which determines the maximum abutment pressure, abutment pressure at the T-junction, and the width of influence zone. This paper presents the results of an extensive geomechanical study in a longwall panel at a West Virginia coal mine. The response of eight headgate pillars and the adjacent entries was monitored as the longwall face approached and passed by the instrumented locations. Overburden response to longwall mining was also monitored, and height of the highly fractured zone above the gob was determined by postmining drilling. Good agreement was observed between the measured stress changes and those computed using the known height of the highly fractured zone. The results provide a detailed picture of the response of overburden strata and chain pillars to longwall mining.

Jan 1, 1987

By David Dye

Good morning, everyone, and thanks Syd for that fine introduction. I am honored to be here to help you celebrate 25 years of advancement and success in the important field of ground control. I would also like to recognize Dr. Peng for his significant contributions in longwall mining and ground control. His contributions have had a major impact on improving safety and reducing fatalities in America?s mines. Thank you, Dr. Peng, for the important work you have carried on throughout the years. Many miners are alive today because of your dedicated work to protect them.

Jan 1, 2007

By Yongjie Yang

Experimental study on the strength and deformation characteristics of coal under cyclic loading provides guidance for designing coal pillar which is subjected to repeated mining influence during mining. This paper studies the strength, deformation and fatigue process of No.3 coal from Xinhe Colliery under cyclic loading by the MTS815.03 servo-controlled rock mechanics test system. The results indicate that coal is more susceptible to fatigue damage than other hard rocks. Under uniaxial cyclic loading, the fatigue failure ?threshold value? of coal is smaller than 78% of its uniaxial strength, and there is also some fatigue damage when cyclic loading and unloading experiment is carried out below the fatigue failure ?threshold value?. Axial deformation can be divided into three stages: Initial deformation, steady-state deformation and accelerating deformation. Transverse deformation can be divided into two stages: Stable deformation and accelerating deformation. When the transverse deformation begins to increase rapidly and the deformation recovers only slightly upon unloading, it?s a precursor to failure of coal. Under cyclic loading, fatigue failure develops through a process of compaction, strain hardening, and strain softening.

Jan 1, 2006

By John A. Rusnak

"This paper discusses the variation of coal strength based on the megascopic coal lithotypes for Central Appalachian high-volatile A coal beds. The composition of coal beds is complex and variable, depending on the depositional environment associated with original plant material, inflow of clastic material, and water level fluctuations in the original swamp/mire environment. Thus, different types of coal and sediment are formed within each coal bed. These differences in coal composition were first classified by Marie Stopes, a paleobotanist, in 1919. The classic coal megascopic lithotypes are vitrain, clarain, and durain, which correspond to bright, bright banded, and dull coal, respectively. Stopes also identified that the lithotype fusain, which is mineral charcoal, has no inherent strength and can act as a plane of discontinuity. In this study, two other coal-bed lithologies are also considered—bone and carbonaceous shale. This makes for a comprehensive suite of coal-bed stratigraphic components apart from rock partings. A total of 1,000 uniaxial compressive strength (UCS) and 440 indirect tensile strength (ITS) tests were conducted on cores from southern West Virginia that were specifically logged and described using the megascopic coal lithotype nomenclature. Statistical analysis is presented showing the results for each lithotype group with mechanical properties correlated to the lithotype. Previous studies have analyzed the mechanical breakage properties associated with coal lithotypes, which correlate well with these results. However, coal rank does appear to be a contributing factor, and this study is confined to only high-volatile A bituminous rank. Application of these results would be well suited to a rock mass classification system to address coal pillar rib stability. This would allow for rib support design and implementation to consider relative coal strength by visual observation of the seam stratigraphy through ongoing observations within a mine. Changes in seam composition can be readily identified, allowing the adjustment in rib support design. Another application would be for the determination of numerical modeling inputs based on detailed seam stratigraphy. However, these results, even with the standard strength reduction criteria, may not be applicable to currently accepted empirical coal pillar design procedures."

Jan 1, 2017