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By Guang Ping Cui

In this paper the imminent relationship between rock burst and its phenomenon with normal rock pressure and its appearance, as well as the factors of one changing into another have been analyzed. Based on this, the necessary and sufficient appearing factors of rock bursts have been put forward, and the term "burst pressure" has been defined. According to the statistics data of tremendous rock bursts which appear in collieries in Hunan Province, the futures of high frequency, great strength and shallow depth of the beginning burst have been found. There ere some special structures in the coal seams, which are closely related with the burst phenomenon. A hypothesis of "star structure" has been suggested for the appearing mechanism of bursts. Inside and around this star structure the distribution rule of burst pressure and rock strength has been researched depending upon the theory of the limited stress field. A new description of the appearing causes of bursts has been raised.

Jan 1, 1984

By James Pile

Since C.C. White introduced coalmine roof trusses in 1967. many different systems have evolved. Beginning in the mid 1990's, several types of bar/cable truss systems have been introduced. A new and innovative bar/cable truss recently introduced offers several advantages. By eliminating many small parts, and simplifying the installation, this new truss increases operator safely. In order to fully benefit from the new truss system, a closer look at the individual components and their interaction was undertaken to understand critical implications on preload and overall support capabilities.

Jan 1, 2004

By A. H. PH. D Wilson

Compared to the rocks encountered in, say, metalliferous mining or tunnel drivage in igneous and metamorphic rocks, the strata associated with the coal measures are relatively soft. If excavations are made at depths greater than one or two hundred metres, zones of fractured or yielded rock develop around the openings. The stress which can be taken by yielded rock at the boundary of an opening is low, but because of the friction present between the fragments or particles, the sustainable stress rises rapidly with distance from the opening until sufficient confinement has been developed to prevent failure of the rock in the first place. Thereafter the laws of elasticity will apply and increasing distance will reduce the stress field until conditions consistent with the cover load are reached. For rocks which maintain their elastic condition right to the edge of an opening, finite element analysis and similar techniques have proved successful in determining the distribution of stress around the opening. This approach, however, is not successful when a yield zone is introduced between the elastic zone and the opening. Nevertheless, some estimate is required if help is to be given in the siting of roadways and in the determination of adequate pillar sizes. This matter is of particular importance in the planning of new mines especially if retreat faces are involved, as extensive roadway drivage must be planned and executed before experience of the effects of face extraction is gained. In 1977, the author published [I] a method based on 'stress balance'. The total aggregate downward load over a large area will remain that of the cover load even after part of the coal seam has been removed. Any rise in stress in ribsides and pillars must be offset by an equivalent stress reduction across roadways and other areas of extraction, and vice versa. Knowledge of one can lead to an estimate of the other, provided the general form of the stress distribution can be postulated. It is the object of this paper to revue the method in more detail and to give further examples of its successful application.

Jan 1, 1981

By Ross W. Seedsman

The logical framework recognises that a coal mine roof can be intrinsically stable or may collapse in one of four ways. Suspension of the immediate roof is one of fundamental ground control strategies available to a coal mine engineer and is recommended for three of the four collapse modes. Correctly implemented suspension offers the greatest improvement in roof stability and greatest reduction in longwall risk. For the control of maingates ahead of retreating longwall faces the ideal suspension support (if required) consists of angled, partially debonded, medium-length tendons installed as far behind the development face as practical. For situations where the roof may be subjected to tensile horizontal stress, immediate support by equally spaced short vertical tendons is required. The step away from fully-grouted tendons improves their survivability during the onset of compressive failure in the roof, minimises the risk of isolated loading, and allows a more robust TARP for roof movement. All suspension systems require a strap, sling or truss between the suspension elements.

Jan 1, 2014

By Markus Papst, Denise Millier, Daniel Beckers, Axel Preusse

"At an open-pit lignite extraction mine in Germany, groundwater has to be pumped over a large area, which leads to subsidence at the surface. As long as the geology of the area of influence is homogeneous, the subsidence is uniform and has marginal influence on buildings.Since the geology of many regions is characterized by faults, there are different rates of subsidence along those faults, which can lead to considerable consequences. Those different rates of subsidence on both sides of a geological fault are associated with the structure of the geology and soil mechanical properties. Horizons with various thicknesses and types of soil are affected differently by the groundwater pumping because of the offset of the faults, and thus react dissimilarly to dewatering.The smaller the grain of the geological horizon, the stronger the chance of shrinking, which leads to a stronger effect at the surface; therefore, dewatering impacts on clayey soil are different to those on sand and gravel. Regions consisting of meadows with turf inclusions are endangered as well; compared to the surrounding geology, those areas react in an entirely different way to water removal, which leads to immense differences in subsidence.The differences in area and respective subsidence impacts, as well as if it is possible to localize those areas, is an issue which needs to be analyzed. Furthermore, detailed mapping can help in the development of future forecasts about those subsidence differences and can also help with monitoring geological faults. In the context of subsurface spatial planning, this mapping has to be kept under review medium- to long-term.INTRODUCTIONThe utilization of underground mining is of increasing significance and of high interest, both for the common good and for future generations. For decades, humanity has been extracting resources from the subsurface, and salt, coal, lignite and water - just to mention a few resources - have been extracted from underground (Sailer, 2013)."

Jan 1, 2016

By Sean C. Farrell

In underground mining and tunneling, machine design is predominantly dictated by mine conditions and individual customer desires. In partnership with Australian companies WDS Limited and Cram Fluid Power, the engineering department of J.H. Fletcher & Company was tasked to design and manufacture roof bolting modules to be mounted off both sides of a roadheader, providing in-cycle bolting. The objective was to produce a boom and bolt module that would allow an operator to bolt from a remote location, eliminating the need for miner to work in unsupported ground. The bolt module needed to operate in front of the cutting boom, but retract to a parked position out of the way during the cutting cycle. Due to Australian electrical approval requirements, it was desirable to design the module with minimal use of electrical controls. The need for multiple, complex, drill steel and bolt handling steps led to the need for a custom hydraulics package to automate several of the movements. The result of the design is dual drill and bolt modules mounted to a multistage boom with over 24 feet (7.31 m) of extension. Each unit rotates and tilts to meet the bolt pattern requirements in conjunction with the stow and load requirements. The bolt module builds on previous designs incorporating a modular assembly with each sub-component designed to be replaceable and adjustable independently from other machine components. Each major component is painted a different color to help the operator make the connection visually from the module to the operator?s control valves. Hydraulic package valves were designed to automatically control several of the sub-components during the drill cycle, reducing the number of steps a drill operator would, otherwise, have to do manually. This design removes the operator from working in unsupported roof and away from other inherent hazards, putting them at a safe distance on the operator?s platform. The cutting and bolting equipment has been combined into one machine, which is advantageous in single entry drifts because there is no need to place change equipment. The hydraulic package valves eliminate the need for any electronics, thus avoiding any electrical regulations with respect to the controls. The addition of these modules to the roadheading machine increases miner safety and streamlines production while meeting the ground control support requirements.

Jan 1, 2014

By Changshou Sun

The geotechnical properties of mining zones at Leeville Underground mine (or simply called Leeville mine through this paper) are different from one zone to another. The typical ground condition is poor to very poor. Therefore, an appropriate ground control practice is the key for the successful and smooth ore production at Leeville mine. Based on the available geotechnical data, the empirical and numerical methods are used to design the right ground support systems. The main ground control practice at Leeville mine includes geotechnical data collection, evaluation of the stiffness of the support system, selection of rock bolt types, design bolt length and bolting patterns, development of the ground support standards, and performance of quality controls on bolt installation to ensure proper rock anchorage, and strength of shotcrete and backfill. Through all these activities, an appropriate ground support system has been established and applied at Leeville mine for the past four years. So far, a good result related to successful ground control and smooth ore production has been achieved, although the system is still being improved. The authors of this paper will summarize the ground control practice conducted in the last four years and will discuss the procedures and standards of establishing the appropriate ground support systems for Leeville mine.

Jan 1, 2013

By Rudrajit Mitra, Tien Dung Le, Chengguo Zhang, Bruce Hebblewhite, Joung Oh

"Geomechanical and geotechnical characteristics of coal seam are important parameters that directly affect the cavability of top coal in the longwall top-coal caving (LTCC) method. The impact of these parameters such as coal strength, coal deformation modulus, coal discontinuities and top-coal thickness on cavability has not been fully understood in previous numerical analyses. The limitation and selection of the numerical modelling codes were two of the main reasons. In this paper, detailed caving simulations are performed using a 2D distinct element method (DEM) code, UDEC, to study the effect of various critical parameters on top-coal cavability. The numerical analysis evidently shows that the coal deformation modulus, coal discontinuities, and top-coal thickness have considerable influences on cavability. Coal strength is also a factor but to a lesser extent. The results presented in this paper contribute to the understanding of top-coal cavability in different mining environments and facilitate the development of an advanced cavability evaluation criterion. INTRODUCTION The longwall top-coal caving (LTCC) method was originally developed in the 1950s in the former Soviet Union and France (Tien, 1998). The method was nearly abandoned in Europe after the mid-1980s; however, it has been significantly improved and widely applied in China since the late 1980s. A conceptual model of the LTCC method is shown in Figure 1 in which a thick coal seam is divided into two sections including the lower or cutting section, and the upper or top coal section. The lower section of coal seam is mined by conventional longwall method. Meanwhile, the upper section is recovered by caving through the face support. The LTCC method is considered to be the most viable option for mining general thick seams. Compared to the multi-slice longwall (MSL) method, LTCC has the major advantages (Zhongming, 2001; cited by Vakili, 2009):"

Jan 1, 2017

By Joseph A. Cybulski

In response to the mining of reserves with increasingly adverse round conditions and the roof support recommendations of current rock mechanics theories, roof support systems have continued to improve. A few of these newer bolting systems. while giving improved performance, require a new method to check bolt installation. Previous1 , a torque wrench had been the sole means to check the installed torque of a roof bolt. However, due to the characteristics of soma of the newer bolting systems which can prevent a reliable manual torque check and a recent change in the federal roof control regulations, an alternative method to a manual torque check is gaining acceptance. This alternative method improves monitoring the hydraulic pressure of the roof bolting machine torque circuit and provides a safe, reliable and quick wav to check bolt installation in those instance when an accurate manual torque check cannot be made.

Jan 1, 1990

By Jessica M. Wempen

Differential Interferometric Synthetic Aperture Radar (DInSAR), a satellite-based remote sensing technique, has been demonstrated as a potentially practical method for measuring mining induced surface subsidence. Unlike traditional methods of subsidence monitoring, DInSAR has the capacity to generate subsidence data on a regional mining scale with a relatively high data density. Using DInSAR to evaluate subsidence over large regions with relatively long time scales has the potential to help quantify the impact of subsidence in the mining region, and provide a means for validating whole-mine geotechnical models. In this study, nine pairs of interferometric data from the Japanese Advance Land Observing Satellite (ALOS) were processed using DInSAR to generate displacement maps for a group of trona mines in southwest Wyoming. The data cover a time span from December 2007 to March 2011. The maximum subsidence for one of the mines totaled 1.3 meters for this period. Cumulative DInSAR displacement maps show the development of subsidence over time. In this mining region, surface subsidence is generated primarily by extraction of trona using retreating longwalls. Additionally, trona is extracted by solution mining, which also generates measurable surface subsidence.

Jan 1, 2014

By Joel Bradley, Kot Unrug, Kyle Perry, Mark Klimek

"A West Kentucky mine operation in No. 11 seam, encountered floor heave, due to the localized increase of the thickness in the fireclay mine floor. Floor heave has overridden seals installed in two mined out panels. The third seal’s location was planned for isolating that area from the Mains. A plan of support has been developed to prevent repetition of the floor heave and related problems outby the seals. The applied ground control measures were successful. An attempt of a 3D numerical modeling was made, that it would match the observed behavior of the mine floor and could be used as a design tool in similar conditions. In the paper described are sequence of events, a mitigation ground control system applied, and the first stage of numerical modeling.IntroductionWest Kentucky coal mine operation in the No. 11 seam has approached an area where the fireclay floor thickness increases to about 5 ft. Delayed floor heave has been observed. In the first sealed area (Panel KK), floor heave has overridden the seals and has begun to progress outby. This could not be allowed at the new seal location PP described in this paper because such override would affect the mains. Previously numbers of standing support systems were used, but their effectiveness was not satisfactory as is illustrated in Figure 1, Figure 2, and Figure 3. Therefore, with sealing of the next area approaching, mine management was looking for a better solution that would eliminate the problem. In that stage, our team looked closely at the geology and strength of roof and floor, as well as the characteristics of ground failure around the seal at the first site. The technical literature has been searched as well Haramy et al. 1986 Hsiung and Peng 1987, Peng 2008, Santos & Bieniawski 1987. This led to the identification of factors responsible for the ground failure. It has been determined that increased fireclay thickness from typically less than 2 ft to about 5 ft, when compared to the previously mined parts of reserve, and the strong limestone strata just above the immediate roof composed of 2 ft of black shale, created conditions of failure. It appeared that, when pillars were punching into the mine floor, the strong roof did not break but was bending, exerting high pressure on outby pillars, causing rib failures and closure. This is a similar mechanism to a strong roof affecting longwall shields. The remedial action planned was additional support."

Jan 1, 2015

By Hideki Shimada

A low cost methodology that allows the determination of in- situ stress using oriented core specimens has been investigated. The technique can be used to determine the stresses during the early stages of a project, even in undeveloped areas of a mine. The research aim was to compare the experimental results estimated by the AE and DRA methods to those estimated by conventional ANZI (Australia and New Zealand Inflatable) cell measurements. The studies have been focused on fill stress tensor determination from a single oriented cored rock. In all cases the rock core specimens recollected the in-situ stress reasonably well compared with the standard overcoring method.

Jan 1, 2001

By Joe E. Bryan, John P. McDonnell

"Underground mine rib stability is an ongoing safety issue in all mining applications. Rib-related accidents present at least as many problems as roof-related ground control. Underground coal mine rib safety is continually emphasized as a primary safety concern at every mining operation. The application of rib support materials is increasing as more difficult mining conditions are experienced due to deeper cover, more variable depositional environments, thicker seam heights, and multi-seam mining effects. As the coal seams become thicker, the need for rib control increases. A variety of rib control methodologies has been documented and research continues on mine design methods and support techniques to reduce or eliminate rib-related accidents.This paper presents an alternative rib bolting technology using proven conventional rib support systems that allows mine personnel to more efficiently install rib support during development operations. This alternative rib bolting method allows for additional rib support with a single support installation.INTRODUCTIONRib control continues to garner the watchful eye of both concerned operators and government officials due to many factors: inherent safety within entries and crosscuts, actual rib stability history, safety improvement, cost control, improved mining methods, and maintaining production goals. As demonstrated through many years of actual practice, desired outcomes can be achieved by scrutiny, innovation, and combining improved practice methodologies.Rib surface control continues to be a necessary part of the production process. However, production and safety issues persist at the point of installation using current control apparatuses and methods to install rib controls. Among others, current issues of rib control are quantity of bolts required, overall time required for installation, ease of apparatus installation, flexibility of the system for appendage installation and operator exposure. A need exists to decrease the time miners spend installing rib control apparatuses, while increasing the safety of such installation processes. At present, installing rib control devices (primary rib bolts, plates, mesh, and, in most cases, secondary rib bolts and plates) is performed in a very congested area and is time-consuming process at the primary coal-winning area: the face. The MatLok system addresses these problems."

Jan 1, 2015

By Gamal Rashed

This research is divided into two parts: the first part of this research focuses on understanding coal mine bump mechanisms, and the second part focuses on mitigating the violent failure associated with a bump. Many researchers believe that the coal bumps happen because of sudden loss of constraint between pillar/ roof and pillar/floor or the hammering effect due to the breaking of thick, massive strata above the coal seam, creating a sudden impact load on the pillars and causing them to bump. However, very little work has been done to explore and understand the consequences of these proposed mechanisms for coal bumps. In the first part of this research, computer simulation of coal specimen behavior have been used to explore two issues: 1) How would an instantaneous loss of constraint or a small sudden impact load cause violent failure? 2) How does the strength of the coal specimen control coal mine bumps? Eliminating or reducing the violent failure is the main objective of the second part of this research. The coal specimen needs to be softened either in the rib zone or the core zone and then its mode of failure studied to see whether it is violent or not. Instantaneous loss of constraint and the hammering effect are associated with sudden release of elastic energy, increase in the kinetic energy, and very rapid transient vertical stresses of the coal specimen. These are three of the major proposed mechanisms for bumps. The potential for violent failure increases with increasing the unconfined compressive strength, UCS, of the coal specimen. Failure of the rib zone is not the main factor for the violent failure of a coal specimen. The energy stored in the core zone is the key factor for the violent failure.

Jan 1, 2013

By Peter Zhang

"Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground-fall fatalities in the underground coal mines of the United States. The risk of ground falls during pillar recovery increases in multiple-seam mining conditions. The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave. When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction. This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 70 ft of interburden under about 1,000 ft of depth of cover.The study finds that, for interburden thickness of 70 ft, the multiple seam mining influence zone in the lower seam is directly under the barrier pillar within about 100 ft from the gob edge of the upper seam. The peak stress in the interburden transfers down at an angle of approximately 20° away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat. The study also suggests that, for full pillar recovery in close distance multiple seam scenarios, it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars. If the entries and / or crosscuts in the lower seam are developed outside the gob line of the upper seam, additional roof and rib support needs to be considered to account for the elevated stress in the multiple seam influence zone. INTRODUCTION Room-and-pillar mining accounted for about 40% of underground coal production in the United States in 2016. Pillar recovery, practiced in about one-third of the room-and-pillar mines, represents about 10% of the coal mined underground, yet it has historically been associated with more than 25% of all ground fall fatalities (Mark, Chase, and Pappas, 2003). In some U.S. coal fields, particularly central Appalachia, many coal mines are operating under geological conditions with multiple coal seams. The risk of ground falls during pillar recovery increases under multiple-seam mining conditions. The hazards of pillar recovery associated with multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave (Mark and Tuchman, 2007; NIOSH, 2010b). Pillar retreating creates abutment pressure, not only in the currently mined seam, but also in the overlying or underlying seams. Multiple-seam interactions become more pronounced as overburden depth increases and interburden thickness decreases. To safely recover the pillars in multiple seams, it is critical to properly plan the mining sequence and panel layout to minimize potential multiple-seam interaction."

Jan 1, 2017

By Stephen R. Iverson, Jeffrey C. Johnson, Donovan J. Benton, Michael J. Raffaldi, Lewis A. Martin

"The NIOSH ground control safety research program at Spokane, Washington, is exploring applications of photogrammetry to rock mass and support monitoring. This paper describes two ways photogrammetric techniques are being used. First, photogrammetric data of laboratory testing is being used to correlate energy input and support deformation. This information can be used to infer remaining support toughness after ground deformation events. This technique is also demonstrated in a field application. Second, field photogrammetric data is compared to crackmeter data from a deep underground mine. Accuracies were found to average 8 mm, but have produced results within 0.2 mm of true displacement, as measured by crackmeters. Application of these techniques consists of monitoring overall fault activity by monitoring multiple points around the crackmeter. A case study is provided in which a crackmeter is clearly shown to have provided insufficient information regarding overall fault ground deformation. Photogrammetry is proving to be a useful ground monitoring tool due to its unobtrusiveness and ease of use.INTRODUCTIONNIOSH Mine Safety ResearchPhotogrammetry systems have been implemented by the National Institute for Occupational Safety and Health (NIOSH) as part of its ground control research to improve mine safety. Conventional monitoring of ground movement has typically focused on movement of a few discrete points that are marked or anchored to instruments. Loss of a designated point or anchor, or the ability to identify a stable point in an unstable area, have often frustrated monitoring efforts. NIOSH researchers are using photogrammetry to conduct full-field measurements of rock surfaces underground. Periodic measurements of the entire surface of a ramp allow patterns of displacement to be observed. Full-field measurements provide much more insight into ground behavior than point measurements.BackgroundPrevious work by Benton, et al. (2014) discussed laboratory calibration and verification testing of two photogrammetry systems being used by NIOSH researchers. Both systems utilize stereoscopic image pairing techniques for photogrammetric reconstruction. This paper discusses how a laboratory system was used to conduct volumetric analyses of testing of shotcrete panels with varying types of reinforcement. The laboratory system was found to be accurate to within 2 mm. Additionally, a field photogrammetry system was found to produce linear measurements within 1 mm of known lengths in laboratory conditions. This system is currently being used at a deep underground mine to observe ground support conditions and deformations that have occurred. This paper discusses comparison of preliminary volumetric measurements of rib deformation to laboratory tests, as well as comparison of the field system against a crackmeter installed across a fault surface where it intersects the ramp."

Jan 1, 2015

By C. Tom Blevins

The purpose of this paper is to discuss production and roof control problems associated with directional lateral stresses in an in situ stress field and the approach that Inland Steel Coal Company made in trying to cope with them. Before proceeding, a description of the mine location, product, geology, restraints, and mine history is in order. The mine is located in Hamilton County in Southeastern Illinois. It produces a high volatile metalurgical coal and is slated to produce 2.1 MTY. Product ion is in the Illinois No. 5 seam at a depth of 930 feet, approximately 490 feet below sea level. The scam height averages from 5.5 to 6.0 feet throughout the reserves with height increasing in the eastern reserve area to approximately feet and a 5 Foot average in the West. The scam tends to dip to the North, North East, but is locally very undulating. The Galatia channel washout outlines the eastern boundary of the reserves and there is a split coal area toward the central part of the property than runs south-southeast. The top above the #5 seam is laminated medium gray shale called Dykersburg. Individual shale bedding planes vary locally From ½ to 1" in thickness to a more normal thickness of 1 to 1 ½ with some areas being interlayed by thin carboncous material. The top normally must be considered as materially competent. Even though the mine is still relatively small in area with only 1.3 million tons of total production to (late, we have encountered dense zones of slicker sides and premining, stress cracks (joints). There are also areas of siltstone, that vary from a few inches to several feet thick, and areas of rider coal, clay instrusions, and rolls. We have crossed lineament concentrations that show strong evidence of corelation to localise top problems. These are being monitored closely to see their predictability on our present and future mining conditions. The face cleat runs N 55° E and butt cleat runs S. 25° E. On a regional basis, there are several anticlinal belts that trend in a northerly direction. The one major restraint is that Inland does not own surface subsidence rights. Therefore, a partial extraction mining plan is being followed. The Holland-Gaddy formula for pillar strength design is used to calculate theortical pillar sides. When production began in March of 1979, it was noted that ground control problems occurred as we drove north entries. East-west cross cuts tended to have better working conditions. This phenomia occurred from a couple hundred feet, but seemed to dispate as we got away from the men and material shafts. Once the production shaft was connected and enough entries extended for installation of belts, ground control problems were again experienced in north-south headings. It was also observed that when mining north-south headings with east-west crosscuts, if the crosscut was driven through ahead and supported, the intersection was stable upon entry connection. This held true even where fairly extreme ground control, problems were encountered. The reverse held true. when driving east-west headings with north-south crosscuts. At this time, we started searching in earnest for a working theory to explain this roof phenomia. It also became the first step of what eventually was a four step sequence to derive a workable solu¬tion to the problem. The combination of the many geological changes and the fact that north-south condition was at times intermitent, helped to disguise the stress field. The mine also being in a fairly virgin area with no mines in approximately 25 milt, radius, meant we didn't have a reliable case history to examine. Various theories were researched before it was concluded that our primary ground control problem was a directional lateral stress Field, with the major stress axis in an east-west direction. The second stop of the procedure was then to test. mining methods and determine if these stresses could be modified to a

Jan 1, 1982

By Jinhua Wang

Based on engineering examples and theoretical analysis, the mechanism of chemical consolidation for fractured roadway surrounding rock was discussed. The chemical consolidating materials and three consolidating methods, that is, capsule, forced infusion and spraying, were introduced. Finally the application effects of the chemical consolidating technique to reinforce the fractured surrounding rock of roadways were analysed.

Jan 1, 1996

By Christopher Mark

The past decade has provided a wealth of practical experience with longwall pillar design. Design formulas developed specifically for longwalls have proven their value in numerous applications. Analytic methods, including numerical models, have a1 so made important contributions. These advances have been based on basic research into abutment loads, coal pillar mechanics, and the interaction between pillar performance and entry stability. The goal of this paper is to review the state of the art in longwall pillar design from the standpoint of the practitioner. Actual case histories are presented to illustrate the current capabilities and limitations of pillar design methods. The paper also discusses the advances that can be expected in the future.

Jan 1, 1990

By Nielen van der Merwe

The data base of failed pillar cases as used by Salamon and Munro (1967) in their original analysis to determine the strength of coal pillars empirically, has been updated for this study. It is shown that the Vaal Basin and Klip River coal fields exhibit similar behaviour, i.e. failure at high safety factors within a short period of time after their creation. Those collapse cases have been omitted from the new data base. New failures that occurred after 1967 have been added, except for those that occurred in the Vaal Basin and Klip River coal fields. (1967) a technique to minimise the area of overlap between the populations of failed and intact pillar cases, the new data base was re-analysed in conjunction with the original Salamon and Munro (1967) data base of intact pillar cases. It was found that a linear formula reduced the overlap area by 22%. The new formula predicts higher strength for pillars with a width-to-height ratio greater than approximately 2S and a lower strength for smaller pillars. For most current South African coal mining conditions, using the new formula for pillar strength will result in leaving smaller pillars without sacrificing stability. The reason for this is that the Salamon and Munro (1967) formula underestimated the strength of larger pillars and overestimated the strength of smaller pillars. The potential benefit for the South African coal mining industry amounts to saving reserves equal to the total production of two large mines per year.

Jan 1, 2002