Search Documents

By Kevin J. Ma, John Stankus

"In order to access remote reserve areas, some U.S. coal mines have to maintain aged underground entries for a great distance. However, high humidity, warm temperature, and time dependent deterioration can cause progressive roof deterioration and unexpected roof falls can, pose a great challenge to ground control engineers. With an active belt structure in place and limited space, re-bolting becomes very costly, less effective, and, sometimes, impractical and unfeasible. To gain long-term entry stability and serviceability, operators typically rehabilitate the aged belt entries by installing standing steel set supports. In the last several years, Keystone Mining Services, LLC, (KMS) has assisted many coal mines with their belt entry rehabilitation projects, evaluated the ground condition of various aged belt entries, and designed different standing steel set support systems. This paper presents a case study of a large-scale roof fall that occurred at an aged belt entry in a mine located in an eastern coalfield, analyzes root causes of excessive deformation of square sets that were installed in an adjacent entry, evaluates the adequacy of an existing rehabilitation square set, and develops remedial recommendations for future rehabilitation practice. Based on the case study, the paper outlines design guidelines for rehabilitation steel sets that include field evaluation, engineering considerations, design assumptions, steel structural analysis, and field installation quality control. INTRODUCTION With gradual depletion of shallow coal reserves, some underground coal mines have to maintain long-term entries (belt, track, return airway, etc.) for a great distance in order to access remote coal reserve areas. However, due to high humidity, warm temperature, and aging in the belt entry, roof strata weathering and pillar degradation become more and more severe. Progressive roof deterioration poses a great challenge to ground control engineers. Pillar rib sloughs at certain locations, entry width increases in some areas, immediate roof sags, immediate roof rock falls out between bolts, existing bolts lose functionality due to corrosion, cutters develop deeper in corners, and strata separation develops deeper up into the main roof. As a result, unexpected roof falls occur in a random manner resulting in loss of production, damage of belt structure, and even injuries to underground personnel."

Jan 1, 2017

By K. Biswas

So far, all pillar design methods or approaches consider the coal seam as a uniform structure From some mine visits in northern Appalachian Region. it is found that some coal seams have binder or parting material within the coal scan with varying thicknesses and numbers- And it has also been found that these kinds of hinder materials tire prone to weathering when exposed to ambient mine moisture In this paper an attempt has been made to study the weathering susceptibility of parting materials and coal when exposed to moisture and a two dimensional finite element analysis has been carried out to study the effect of the presence of partings on the structural integrity of the coal pillar in terms of short-term and long-term stability.

Jan 1, 1995

By Wen H. Su

Multiple seam mining and its associated problems are very serious in Southern West Virginia where poor planning or lack of knowledge in seam interaction often results in complete loss of coal properties. Several measures have been proposed to alleviate the interaction prob¬lems under various multiple seam conditions, but few of them are spec¬ific in terms of mining plan. Two parallel approaches are adopted for this paper. One is field investigation which consists mainly of mine site visits and gathering of data. The other is the two-dimensional finite element simulation of mine workings using the data gathered as input. Two typical cases which represent the past and present multi¬ple seam mining practice in West Virginia were investigated. Much attention is focused on the interaction problems experienced and the important parameters controlling interaction. Several key parameters controlling interaction in multiple seam mining were analyzed. Proper mining plans, by which the interaction problems can be minimized, are proposed for the individual cases after a better understanding of the interaction mechanism was realized through finite element analyses.

Jan 1, 1984

By J. Nielen van der Merwe

During the process of pillar extraction at the Welgedacht Colliery, an unexpected pillar collapse resulted in the entrapment of equipment. The area was visited soon after the event to evaluate the stability of the area for the purposes of extracting the equipment. It was found that the area had stabilised sufficiently to allow work without undue risk to the safety of workers. The equipment was reclaimed successfully. A subsequent investigation into the reasons for the collapse was conducted. It entailed detailed underground and surface subsidence observations and numerical modeling. It was found that none of the perceived negative factors that were present at the time could result in pillar loads that were sufficient to cause the collapse. It was concluded that the local pillar strengths were less than the nationally used value and that they were possibly further reduced by aging.

Jan 1, 1999

By Anthony Iannacchionc, Jessica Benner, Harold Miller, Megan Witkowski

"Longwall bituminous coal mining has been practiced in Pennsylvania since the 1970""s. Hundreds of longwall panels have been mined during the last 40 years but only 24 have actually undermined Pennsylvania's Interstate Highway System. The impact to interstate highways are summarized in this paper and the factors that influence the magnitude of these impacts are examined.INTRODUCTIONOver the last 30 years, interstate highways in Pennsylvania have been undermined by portions of 21 bituminous coal mine longwall panels. From 2003 to 2008, three of the panel were extracted by the Emerald Mine and six by the Cumberland Mine both in Greene County and both owned by Alpha Resources. Previous assessments of longwall mining under Pennsylvania’s interstates have identified impacts to the highways but no danger to public safety. In general the magnitude and extent of impacts during the recent undermining of Interstate 79 or 179 appears to be similar as those identified in previous studies. The nine longwall panels are among some of the widest in the U.S. They are supercritical in character with overburdens ranging from greater than 152 m (500 ft) to less than 305 m (1,000 ft).The Pennsylvania Department of Environmental Protection (PA DEP) requested that the University of Pittsburgh summarize observations collected by their staff during the recent undermining of 179. Observations and photographs of impacts to the highway were made by PA DEP staff and entered into reports prepared by California District Mining Office (COMO) staff and statements written into the Bituminous Underground Mining Information System (BUMIS) database. In addition to the observations by the PA DEP, the Pennsylvania Department of Transportation (PennDOT) conducted a series of detailed land surveys along portions of the highway undermined from 2003 to 2008. To characterize the subsidence basin fanned by longwall mining. PennDOT information has been analyzed by another research group within the university (Guticrre7, et al., 20 I 0)."

Jan 1, 2010

By Tony Iannacchione

A close look at the Loyalhanna Limestone of southwestern Pennsylvania reveals a complex structural environment. Most exposures of the Loyalhanna occur along Chestnut Ridge and Laurel Hill within the Allegheny Mountain Section of the Appalachian Plateaus Province. Because the Loyalhanna is rated as a super pavement aggregate by the Pennsylvania Department of Transportation and because of its proximity to the population centers of southwestern Pennsylvania, the Loyalhanna has been extensively mined along these prominent anticlinal structures. Geologic and engineering analyses were performed using gas well and core logs, outcrop examinations, underground observations, and mine maps. Strata exposures from both outcrops and quarries were used to construct geologic maps, stereonet, and rose diagram projections so that the Loyalhanna's structural environment could be understood better. Many of these structural conditions cause ground control problems at local quarries. These problems ranged from small-to-moderate sized rock falls associated with roof jointing to pillar failures associated with dipping discontinuities. Stresses ranged from tensional on structural domes where weathering dominates, to high levels of compression within structural saddles. A greater understanding of these characteristics is a prerequisite to develop engineering controls, such as improved mine layouts. pillar sizes, etc., that lessen miner exposure to these hazards.

Jan 1, 2002

By Pieter-Jan Kleiterp

Caterpillar has been involved in highwall mining since the purchase of Bucyrus in 2011. The Cat® HW300 is a continuation of the Superior Highwall Miner that was first sold in the USA in 1994. Currently, there are more than 80 machines supplied worldwide, and Caterpillar envisions increasing the population steadily in the coming years. The Cat highwall miner product group, based in Beckley, WV, has been involved in the sales of all miners to date. With the dealer model that Caterpillar is using, more people, dealers in particular, need to be trained on the application of highwall mining. A site investigation is a highwall specialist?s work, and Caterpillar assists dealers and customers in determining the technical feasibility of highwall mining on mine sites. With the ever-increasing amount of requests, it proved to be important that people without highwall mining expertise, yet who are familiar with mining, could perform a very quick analysis of a potential highwall mining site to determine at an early stage if it is worthwhile to invest the time and money of all parties?customer, dealer, and Caterpillar?in further follow-up. Therefore, Caterpillar would like to introduce the Cat HW300 QuickScan tool.

Jan 1, 2014

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 Morgan M. Sears, Mark Van Dyke, Gamal Rashed, John Rusnak, Khaled Mohamed

"In 2016, room-and-pillar mining provided nearly 40% of underground coal production in the United States. Over the past decade, rib falls have resulted in 12 fatalities, representing 28% of the ground-fall fatalities in U.S. underground coal mines. Nine of these 12 fatalities (75%) have occurred in room-and-pillar mines. The objective of this research is to study the geomechanics of bench room-and-pillar mining and the associated response of high pillar ribs at overburden depths greater than 1,000 ft. This paper provides a definition of the bench technique, the pillar response due to loading, observational data for a case history, a calibrated numerical model of the observed rib response, and application of this calibrated model to a second site. INTRODUCTION Bench mining is a an underground mining technique typically applied to room-and-pillar mines where full seam extraction on development presents an unacceptably high risk of injury from high pillar ribs or where the mining equipment is not designed to extract the full seam thickness. To mitigate the increased risk associated with high ribs and slender pillars, a more modest thickness is extracted from the top of the seam on development. This is followed by grading the floor and recovering the bottom of the seam with or without extraction of the pillars during retreat mining. A mine located in Eastern Kentucky extracts two seams at depths ranging from outcrop to 2,000 ft. In some areas of the mine, Seam A and Seam B are close enough together to be mined simultaneously. Seam A (top) is mined on development 7 to 9 ft. high for the entire length of the panel. On retreat, as each pillar is extracted, the continuous miner is ramped down into the floor (Seam B), two pairs of Mobile Roof Support (MRS) units are set, and the pillars are extracted at the full mining height of 14+ ft. During the pillar recovery process, the floor is extracted in each entry, from a ramp initiated outby the retreat line, sequentially just prior to extracting the lifts from the pillars (see Figure 1)."

Jan 1, 2017

By Ismet Canbulat, John Naylor, Jason Emery, Peter Craig, Andy Sykes

"Longwall mining is by far the most common method of underground coal extraction in Australia. The industry trends and expectations are placing increasing emphasis on the reliability and productivity of these operations. The longwalls are becoming wider and longer while retreat rates are continuously increasing due to significant improvements achieved in longwall equipment reliability and automation. This increased longwall productivity is placing significant emphasis on the reliability of longwall panel development.Although there have been significant improvements in the reliability of continuous miners and hydraulic drill rigs, the traditional resin encapsulated bolt installation is still the principal method used in all major coal-producing countries around the world. Anglo American realised an opportunity existed to introduce an alternative roof bolt installation technique called “spin to stall” in Australia. Spin to stall was first introduced in Anglo American Coal in South Africa where it has been successfully used for over a decade, though implementation of South African spin to stall resin in Australia has proven to be near impossible due to a significant difference in geotechnical conditions, mining method, and, subsequently, roof bolting equipment. Therefore, a new spin to stall development project was initiated between Anglo American Coal and Jennmar Australia (SPIN2STALL®). This paper summarises the journey of this project, the results, and the successful implementation of spin to stall achieved at Grasstree Mine.INTRODUCTIONThe Anglo American Coal business unit operates Grasstree and Moranbah North underground longwall mines in the Bowen Basin of Central Queensland, with a third longwall mine, Grosvenor, currently being developed, and two more (Aquila and Moranbah South) in the project pipeline. The vision for Anglo American is to achieve “Zero Harm” through the effective management of safety at all operations, while for the coal business unit, at the same time to achieve industry-leading operational performance. Currently, both Grasstree and Moranbah North longwalls are operating at industry best practice rates. This is placing increased pressure on the development process to allow such high productivity levels to be sustainable."

Jan 1, 2015

By Sandin Phillipson

During the course of ground control evaluations conducted by the Mine Safety and Health Administration's Roof Control Division, a number of surface and underground coal mines were visited that exhibited instances of ground failure associated with joints. The evaluations were conducted over a broad geographic area, including the Appalachian Basin from Pennsylvania to Tennessee, and they documented that joints occur in a wide variety of mining conditions and exhibit a wide variety of characteristics. Despite minor local variability in orientations, the joints fit into a cohesive pattern when evaluated on a regional scale in context with the system of thrust faults that define the Alleghany Front. Joints documented for this study, together with joints in sandstone and coal and the trends of drag folds compiled from other studies, were plotted on regional maps using GIS software. The trends of joints, including those that could be described colloquially as "hill seams," were not only parallel to regional fault systems and local trends of drag folds, but were also parallel to the regional patterns of previously mapped joints in sandstone and coal. These observations indicate that the features referred to informally as "hill seams" are in fact joints formed in response to tectonic processes associated with regional deformation of the Cumberland-Allegheny Plateau. The 286-266 million-year-old age of regional deformation significantly pre-dates the time when the Cumberland-Allegheny Plateau was subjected to deeply incised stream valley erosion, which began 25 million years ago. This challenges an existing interpretation that "hill seams" represent local ·stress relief joints that formed when stream valley erosion removed confinement and allowed tensile failure of the rock. Although relaxation of confining stress can reasonably be expected to occur near outcrop, observations indicate that this results in the dilation of existing joints. This paper presents instances of ground failure associated with joints, discusses methods of mitigation, and recommends that the term "hill seam" be abandoned in technical communication because it simply refers to a kind of joint that may exhibit certain weathering and aperture characteristics.

Jan 1, 2010

By Eugene D. Krupa

Mine 33 of Beth Energy has serious and complex roof cutter problems causing delay of the advance rate of both the entry development and longwall face retreat. The cost of maintaining these entries is very high due to the requirement of wry heavy artificial supports. It was suspected that this problem was caused by multiple factors including high in-situ horizontal stresses and weak roof in the problem areas. In order to alleviate the problem, proper design of panel entry and roof reinforcement were considered. This paper presents the results of extensive measurements in experimental panel entries of Mine 33 in order to assess the performance of the designed structure and roof support system during the entry development and longwall retreat. An experimental panel entry has been designed and implemented. The designed parameters were varied seeking for the most ideal parameters which provide stable roadways. Changing these parameters accompanied instrumentation of roof, floor, and pillars. Typical instrumentation, including stress meters, convergence measurement stations, and borehole borescope were utilued. Gnensive monitoring of the behavior of the designed structure and support system led to the development of a complete stress-deformation history of the roadway and an ideal roof reinforcement, support system, which reduced the cost of entry development by less than a third of the previous method.

Jan 1, 1990

By Zach Agioutantis, Christopher Mark

"Thirty years ago, the Analysis of Longwall Pillar Stability (ALPS) inaugurated a new era in coal pillar design. ALPS was the first empirical pillar design technique to consider the abutment loads that arise from full extraction, and the first to be calibrated using an extensive database of mining case histories. ALPS was followed by the Analysis of Retreat Mining Stability (ARMPS) and the Analysis of Multiple Seam Stability (AMSS). These methods incorporated other innovations, including the Coal Mine Roof Rating (CMRR), the Mark-Bieniawski pillar strength formula, and the pressure arch loading model. They also built upon ever larger case history databases and employed more sophisticated statistical methods.Today, these empirical methods are used in nearly every underground coal mine in the US. However, the piecemeal manner in which these methods have evolved resulted in some weaknesses. For example, in certain situations, it may not be obvious which program is the best to use. Other times the results from the different programs are not entirely consistent with each other. The programs have also not been updated for several years, and some changes were necessary to keep pace with new developments in mining practice.The Analysis of Coal Pillar Stability (ACPS) now integrates all three of the older software packages into a single pillar design framework. ACPS also incorporates the latest research findings in the field of pillar design, including an expanded multiple seam case history data base and a new method to evaluate room and pillar panels containing multiple rows of pillars left in place during pillar recovery. ACPS also includes updated guidance and warnings for users and features upgraded help files and graphics.INTRODUCTIONPillar design is one of the fundamental elements of mining engineering. Pillars are necessary to control the great weight of the overburden during all phases of underground mining. Without stable pillars, ground control is impossible."

Jan 1, 2018

By Tim Miller, Michael M. Murphy, John L. Ellenberger, Gabriel S. Esterhuizen

"A limestone mine in Ohio has had instability problems that have led to massive roof falls extending to the surface. This study focuses on the role that weak, moisture-sensitive floor has in the instability issues. Previous NIOSH research related to this subject did not include analysis for weak floor or weak bands and recommended that when such issues arise they should be investigated further using a more advanced analysis. Therefore, to further investigate the observed instability occurring on a large scale at the Ohio mine, FLAC3D numerical models were employed to demonstrate the effect that a weak floor has on roof and pillar stability. This case study will provide important information to limestone mine operators regarding the impact of weak floor causing the potential for roof collapse, pillar failure, and subsequent subsidence of the ground surface.IntroductionMassive roof falls extending to the surface at a limestone mine in Ohio have been studied collaboratively by the National Institute for Occupational Safety and Health (NIOSH) and the mine owners. This paper provides a summary of the sequence of events leading to observed instability and presents a numerical modeling analysis that investigates the role a weak floor can have on roof and pillar stability.This study builds upon previous NIOSH research in underground limestone mines undertaken by way of a survey that analyzed performance of pillars in 34 different stone mines in the Eastern and Midwestern United States between 2005 and 2009 (Esterhuizen et al., 2011). This survey led to the development of underground stone mine pillar design guidelines based upon a “stability factor” for the pillar system. The factor is compared to operational experience to determine ranges of values that are typical of stable pillar systems. A software package titled Stone Mine Pillar Design (S-Pillar) was developed for easy application of these guidelines (Esterhuizen and Murphy, 2011).Weak floor in the present case study departs significantly from cases included in the previous NIOSH research, in that none of the 34 study mines had weak floor issues. Moreover, this case study is the first in which a weak floor has been monitored routinely as roof falls are occurring. A detailed report on the observations and measurements taken at the mine has been published (Murphy et al., 2015). The previous observations led to the conclusion that although there were weak bands in the pillar, the weak floor was the significant defect that led to the instability issues. The pillars were initially mined in 2004, approximately 10 years before the instability issues began to occur. The objective of this study is to use numerical models to analyze a variety of floor strengths and the impact of moisture content to find the critical point at which the floor becomes unstable, leading to long-term instability issues."

Jan 1, 2015

By Sandin E. Phillipson

"On April 5, 2010 a massive dust-fueled explosion at a longwall mine in southern West Virginia claimed the lives of 29 miners. The mine had experienced large gas inrushes from the floor on two known previous occasions, and natural gas was found emanating from floor fractures behind the longwall shields after the explosion, indicating a likely fuel source for the initial ignition. In succeeding months, another longwall mine operating in the same seam 15 miles away encountered smaller inflows of natural gas that halted production from two different longwall faces. Production was halted at a third longwall mine, 30 miles south of the first, in May 2011, when explosive levels of methane were detected inby the longwall face on the headgate. The same mine was evacuated on March 20, 2014 when a series of floor gas feeders were encountered on the longwall face. A literature review indicated that there has been little documentation or description of these events, and that their size and frequency are unknown. The most significant floor gas inrushes in the United States are believed to have been associated with longwall mining in the Pocahontas No. 3 Seam of western Virginia, in which an estimated 5.8 x 107 ft3 of natural gas was expended from a floor feeder on the longwall face over a two-week period (Aul, pers. comm.).A variety of mechanisms and circumstances are associated with methane emanations from underground mines. Hyman (1987) defined an outburst as a violent, simultaneous release of gas and comminuted rock material into a working face or the interior of a borehole, and indicated that certain soft or fractured coals have a propensity for outburst based on gas desorption rates. In this mechanism, reservoirs of autogenic gas hosted in pockets of soft, crushed coal surrounded by harder coal are released suddenly as confining stress is removed by mining. Hyman (1987) also suggested that highly stressed, relatively gassy coal that is penetrated by mining-induced fractures could be outburst-prone. Clayton et al. (1993) and Ulery (2008) recognized the importance of faults as potential conduits for gas from adjacent strata into coal mines, and suggested that biogenic gas trapped in adjacent strata could migrate into mined coal seams due to a pressure differential. Ulery (2008) distinguished between an outburst and a blower, the latter defined as an event expending a large amount of gas over an extended time period, most often emanating from underlying strata, but without expulsion of coal or rock. Methane feeders are further defined as a subset of blowers that continually expend gas over a long period of time but at a lower rate. Many of the mechanisms discussed by Ulery (2008) relate to geologic features that can impound gas within the seam, releasing a voluminous amount when mining breaches the elastic dike, fault gouge, or other geologic structure that had restricted and compartmentalized coalbed methane migration. Thus, the events discussed in this paper can be described as blowers and feeders. However, detailed geological observations of the three longwall mines discussed in this paper appear to indicate a mechanism that is different from those described by Hyman (1987) or Ulery (2008), in that they did not involve coalbed-sourced gas, and the reservoirs were not hosted by coal seams."

Jan 1, 2016

By Lightfoot. Nicholas, Na Liu

"Mining in multi-seam workings has been undertaken for many years in the VK Remnant pillars above and/or below current working areas have an important influence on retreat gateroad conditions at five of the remaining six deep coal mines. These interaction effects need lo be predicted ahead of mining so that longwall panels can be positioned to minimize roadway damage and to identify areas where additional support should be installed early into competent strata when and where it is most effective.Currently, three-dimensional elastic stress analysis using MAP3D is undertaken to predict local vertical and horizontal stress magnitudes during gateroad development and in front of the face on retreat. The predicted stress fields are often used as input to FLAC for roadway modelling.Using Maltby Colliery in South Yorkshire as a case study, this paper illustrates the MAP3D approach, together with its current limitations and uncertainties. Possible future improvements in the analysis method are proposed and the authors would welcome comments on these, or suggestions for alternative approaches.MODELLING OF MULTl-SEAM LAYOUT & MULTI-SEAM INTERACTIONThe remaining large coal mines in the UK arc operating at depths of 800-1, 150 m (2,625-3,773 ft). Production panels are now mined on retreat after the accompanying single entry access roadways have been completed. These gateroads are typically supported with a system of steel rock bolts and cable in the roof and steel bolts and glass re in forced polymer (G RP) dowels in the sidewalls.In addition to new panels being mined in the vicinity of old goafed districts, a significant amount of extraction has usually already taken place in coal seams above and/or below the current working areas. This means that there is also interaction between the current workings and the remnants of the old workings in other seams. As a consequence of the mining methods employed, old workings can have a complex geometry compri ing collapsed and partially collapsed excavations intersper ed with small and large pillars of unmined coal. The unmined coal pillar act as stress concentrators that carry the overburden load while the mined out excavations create stress shadows of varying degrees depending on how much collap e and reconsolidat ion has occurred. This is illustrated by a simple elastic model as shown in Figure I."

Jan 1, 2010

By Khaled M. Mohamed, Michael M. Murphy, Ted M. Klemetti, Heather E. Lawson

"The design of rib support systems in U.S. coal mines is based primarily on local practices and experience. A better understanding of current rib support practices in U.S. coal mines is crucial for developing a sound engineering rib support design tool.The objective of this paper is to analyze the current practices of rib control in US coal mines. Twenty (20) underground coal mines were studied representing various coal basins, coal seams, geology, loading conditions, and rib control strategies.The key findings are: (1) any rib design guideline or tool should take into account external rib support as well as internal bolting; (2) rib bolts on their own cannot contain rib spall, especially in soft ribs subjected to significant load—external rib control devices such as mesh are required in such cases to contain rib sloughing; (3) the majority of the studied mines follow the overburden depth and entry height thresholds recommended by the Program Information Bulletin (PIB) 11-29; (4) potential rib instability occurred when certain geological features prevailed—these include draw slate and/or bone coal near the rib/roof line, claystone partings, and soft coal bench overlain by rock strata; (5) 47% of the studied rib spall was classified as blocky—this could indicate a high potential of rib hazards; and (6) rib injury rates of the studied mines for the last three years emphasize the need for more rib control management for mines operating at overburden depths between 500 ft and 1,000 ft.IntroductionEfforts to improve the stability of underground mine ribs have continued for decades. These efforts by mining professionals around the world persisted throughout the 1990s and 2000s as well, though much of this work focused on rib support methodology and selection of proper support for a specific location. Despite these efforts, it is unfortunate that there has been a continual occurrence of rib-related fatalities in underground coal mines. While overall improvements in rib safety have been achieved, the average fatality rate is still about 1.3 fatalities per year over the last 18 years (Jones, 2014).To manage and control the rib hazard, rib support is used in U.S. coal mines. The current rib support methods for U.S. mines fall into two main categories: rib control based on intrinsic support systems and those based on external support systems. Intrinsic or rib-bolt support requires a roof bolting machine that can install a bolt fixture into the rib. Furthermore, the appropriate level of support needed to control the ribs with an intrinsic-based support system as well as the design parameters for those support systems for various mine conditions have not been established in U.S. coal mines. Furthermore, surface control systems are often an integral part of these support systems and must be considered in the system design."

Jan 1, 2015

By William J. Gray

The Roof Control Division of MSHA's Pittsburgh Safety and Health Technology Center has been conducting a program to evaluate methods employed by the U.S. coal mining industry during recovery of longwall faces. The objective of this effort is to provide information on safe and efficient roof control methods currently being used to recover longwall faces. In addition to a literature review and extensive contacts with MSHA and industry personnel, 14 mines were visited to observe firsthand various aspects of longwall recovery. It is hoped that the dissemination of this information will aid in the prevention of accidents and a reduction in costly downtime during recovery.

Jan 1, 1993

By Kirk W. McCabe

RokLok binder is a two-component polyurethane system consisting of a polymeric isocyanate (Component A) and a polyol resin (Component B). The two chemicals are mixed and injected into the mine rock under pressure. The initial low viscosity enables the RokLok material to permeate many of the smallest fractures. About 3-5 minutes after injection, the reacting material begins to expand in the fractures; the reaction is completed after 1-2 hours, and the material sets to a mechanically strong binder which adheres tightly to the rock. The consolidated rack provides resistance to any further f movement w thin the strata. The RokLok binder injection system is fast becoming an important engineering tool for coping with adverse underground geological conditions. Two case histories discussed in this paper, a longwall mining problem and an entry-roof problem, demonstrate the effectiveness of using the RokLok binder system to help solve complex ground-control problems in a cost-effective manner.

Jan 1, 1981

By Christopher Mark

After the United States and Australia, South Africa has the largest. modern underground coal mining industry in the world. Historically, South Africa has been the cradle of many innovations in ground control, particularly in the area of pillar design. Today, South African mines are grappling with many of the same issues faced by their U.S. counterparts, including safety during pillar retreat operations, selecting the proper roof support, and maintaining long-term pillar stability. The author recently visited seven underground mines in the Mpumalanga, Free State, and KwaZulu-Natal provinces. Mining methods included longwall, room-and-pillar with continuous miners, and drill-and-blast. The mines represented a cross-section of geology and ground support practices. At each mine evaluations were made of the Coal Mine Roof Rating (CMRR), roof support practices, pillar design methods, and general ground control experience. The coal seams observed were generally thick (2-5 m), the depths of cover moderate (less than 200 m), and the roof rocks relatively strong (CMRR greater than 45). Unfortunately, the South African roof fall fatality rate is approximately three times greater than in U.S., perhaps in part because roof bolts seem to be more widely spaced in most South African mines. Roof bolting equipment was generally outdated, and the quality of bolt installation was a major concern. Pillar design, on the other hand, is quite advanced, and pillar failures are rare. Several novel partial pillaring techniques are employed, including "checkerboard" mining in which every other pillar is removed, 3-cut systems, and high-extraction mining where engineered final stumps are left by design. The paper also offers observations on ground control management techniques (including recently introduced Codes of Practice), the role of the mine inspectorate, and the status of the research community.

Jan 1, 1999