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By Floyd D. Varley

Outbursts, the sudden and violent out- pouring of material into mine openings, is a ground control problem increasingly encountered in the United States as deeper seams are developed. Outbursts, caused by several mechanisms, are the result of the loading of ground support systems be- yond their designed capacity to failure, manifested in a violent release of stored energy. There are several types of outbursts typical to deep underground coal mines. - Outbursts of the coal face due to the presence of pressure abutments in close proximity to the face. These outbursts are usually accompanied by liberation of large amounts of methane. - Instantaneous convergence of the ribs and floor due to overloading and failure of ribs which lengthens the beam of floor strata sometimes resulting in a chain reaction failure of the mine floor. - violent failure of the confined core of a pillar due to rapid loading beyond the yield point of the pillar.

Jan 1, 1986

By Tim Ross

Several shear failures were observed in Shaft #1 at the Mountain Coal Company, LLC, West Elk Mine, after mining longwall Panel 23, 1,100 ft to the east of Shaft #1. It was speculated that this shear damage could be related to differential ground movement caused by in situ stress relief from the ?stress shadow? of the caved zone above longwall Panel 23. A numerical study was conducted to assess the possibility of the shaft shear damage being caused by in situ stress relief and the potential for additional damage to Shaft #1 and two other nearby shafts, due to mining nearby longwall Panel 24. Three-dimensional (3D) models were built in FLAC3D to simulate past and future mining near shafts, the estimated local anisotropic and directional horizontal stresses, and the overlying variable surface topography. The numerical analyses indicated that stress relief due to mining Panel 23 caused the shear damage to Shaft #1and that additional damage to Shaft #1 and the other two shafts, would likely result from mining longwall Panel 24. Additional shear damage was documented in Shaft #1 when longwall Panel 24 was mined, confirming the results of the numerical analyses.

Jan 1, 2006

By Sandin E. Phillipson

The Mine Safety and Health Administration?s Roof Control Division responded to a massive ground failure at an underground marble mine in northern Georgia. The ground failure occurred in a benched area that had been abandoned in the early 1990s. It involved the failure of an estimated 19 pillars, in conjunction with a massive roof fall of an undetermined extent that was at least 60 ft high. Pillars in the benched area had been mined nominally 40 x 40 feet to a height of 50 feet ft, with 40-ft mining widths, although it seems likely from measurements and accounts from mining personnel that pillars were even smaller than intended. Average overburden depth was 500 ft, and the rock mass was characterized by three sets of intersecting geological discontinuities that not only defined very blocky conditions that were conducive to pillar spalling and volume loss, but they were also adversely oriented in such a way as to cut diagonally through pillars at a high angle. The recently released S-Pillar software from NIOSH was used to determine pillar safety factors in the failure area and indicated that pillars were undersized for the conditions. S-Pillar was used to assess pillar stability in active areas to evaluate the likelihood of experiencing similar collapses in the future. S-Pillar correctly identified the pillars in the bench area as having been at risk for instability, in light of their ultimate failure. A two-dimensional finite element model was used to evaluate the possible stages of failure and to assess the likelihood of continued failure in the benched area that might affect more distal mine openings. Although stone mines are generally characterized as having more forgiving conditions than those found in softer sedimentary rocks, the failure shares some characteristics with domino-style massive pillar collapses experienced in coal and trona mines. First, the minimum dimension of the benched area approached 350 ft; second, the pillars defined by benching had small width-to-height ratios (0.8), and; third, the pillar failure area was slightly greater than 4 acres. The massive pillar collapse and associated roof fall illustrate the importance of incorporating pillars that have been properly sized for depth and for the final bench height. Furthermore, the designed dimensions must be adhered to during the actual mining process. Finally, the importance of understanding the geological conditions and their effect on the rock mass is critical.

Jan 1, 2012

By Steve Tadolini

Underground roof falls can have devastating effects on the safety of operations personnel, and equipment, and seriously impact production. Immediate steps are usually taken to danger off and secure the area while clean-up efforts are designed and implemented. A critical step, often overlooked due to production demand, is the determination of - why the roof fall occurred so that additional occurrences can be avoided in future developments or what measures can be taken to identify areas that have similar ?symptoms?? Forensic engineering includes the investigation of materials, products, structures or components that fail or do not operate/function as intended. The authors define forensics in ground control engineering as the study of evidence discovered while examining underground roof falls by applying expert observational techniques and established mining engineering principles to determine the probable cause, hypothesize solutions, and design engineering safeguards to prevent reoccurrences. Simply stated, the primary steps in the forensics of underground roof fall techniques are: ? Assess regional and local conditions prior to the event. ? Assess post-event conditions. ? Hypothesize plausible ways in which the pre-event conditions ? became the post-event conditions.Search for evidence that either denies or supports the various ? hypotheses.Apply engineering skill to relate the various facts and evidence into a cohesive scenario of how the event may have occurred. This paper details the proposed technique by utilizing a roof fall case study that required the examination of five primary factors: mine design parameters, excavation geology, method of excavation, method of support, and mining cycle times.

Jan 1, 2009

By Stephen C. Tadolini

The U.S. Bureau of Mines, in cooperation with the Cyprus- Plateau Mining Company, has conducted research to provide an alternative to traditional secondary support systems in a two-entry, yield pillar gate road. As recent as two years ago, it was impossible to imagine a gate road supported solely with internal high-strength resin-grouted cable supports that would virtually eliminate the necessity for crib or concrete external supports. The support system evaluated consisted of 1.6-m (5-ft) full-column resin-grouted bolts and 4.8-m (164) long cable supports installed in conjunction with wire mesh and "monster-mats." Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during first and second panel extractions. The stress and loading histories for the panels and yield pillar were monitored to evaluate the stress transfer and pillar performance in conjunction with the roof and floor behavior. The test results indicated that the designed support system successfully maintained the roof during the extraction of two longwall panels and dramatically reduced the cost of secondary support. This paper will describe the theory of a cribless support system, the advantages of cable supports, and present the pillar, roof, and floor measurements made to assess the support performance during longwall retreat mining.

Jan 1, 1995

By Andre Vervoort

Falls of ground, such as falls of roof and sidewall collapses, are a major cause of fatalities and injuries in South African collieries. Based on information obtained from the accident reports of the various Chief Inspectors of nines, a statistical analysis was conducted covering the period 1970 to 1988. This study shows that the volume of rock in a fatal fall of ground is often quite small. In intersections, fatal falls are on average larger than in roadways and at the face. It was also found that in 36 % of all fatal falls of ground no temporary or permanent support was installed at the site of the accident.

Jan 1, 1990

By Keith A. Heasley

In the past, numerous displacement-discontinuity (DD) models of stratified rock masses have been proposed using a homogeneous isotropic elastic overburden. In this paper, it is postulated that a laminated overburden model might be more accurate for describing the displacements and stress in these stratified deposits. In order to investigate the utility of using a laminated overburden in a DD model, the fundamental mathematical basis of the laminated model is presented and graphically compared with the fundamental behavior of homogeneous isotropic elastic overburden and with field data. Specifically, the stresses and displacements surrounding an idealized longwall panel as determined from the laminated overburden model are presented and compared with results from the homogeneous isotropic overburden and with measured abutment stress data. Also, the remote displacements and surface subsidence as calculated by the laminated overburden model are compared with homogeneous isotropic calculations and with measured subsidence data. The results of this investigation show that the laminated overburden is more supple, apt to propagate displacements and stress further, and better able to fit observed data than the classic homogeneous isotropic overburden. Ultimately, it is suggested that the laminated model has the potential to increase the accuracy of displacement and stress calculations for a variety of mining situations.

Jan 1, 1996

By Gilles Rousset

This paper presents laboratory experiments conducted on hollow cylinders samples. The advantage of such a geometry is to approach in situ conditions around cylindrical openings (lined galleries, boreholes, ...). Many authors have been interested in this kind of problem, especially for borehole stability applications (for references see Guenot 1987). However, as long as we know, time-dependent behavior has not been well studied. For this purpose, we have set up two different types of tests on a deep clay : - Quick unloading test, to assess the influence of unloading rate on mechanical response and the failure modes. - Creep and relaxation test, to compute the time dependent overall deformation of the hole wall as well as to plot long term convergence-confinement pressure curve. The experimental results are then compared with those given by a theoretical model developed in our laboratory, well fitted with the tested material.

Jan 1, 1989

By P. Gaskell

The paper examines the development of rock movement between a longwall extraction horizon and the surface for a fully caved mining situation. Physical modelling has been used to examine the mechanics of ground movement and the propagation of fractures around the longwall working. The paper discusses dimensional analysis, model construction and measurement technique and data processing of the results into effective graphical forms for further study.

Jan 1, 1988

By Abouzar Vakili

Longwall Top Coal Caving (LTCC), as the most attractive method for thick coal seams, has been suffering from insufficient geomechanical understanding for the past few years. Cavability without doubt is the most critical geomechanical issue associated with the top coal caving method. A discontinuum method has been used for comprehensive modeling of LTCC operation by using the Universal Distinct Element Code (UDEC). In addition Particle Flow Code (PFC) and Phase2 were used as support for the caving mechanics analysis. Although the previous researches of gravity flow in caving operations neglected the effect of continuous span expansion, it has been observed in this study that face advancement has a great impact on roof symmetrical behavior. The amount of symmetry fluctuates between different face distances. Horizontal displacement seems to be the best indication for roof symmetrical evaluations. Two critical caving conditions have been modeled and parameters such as horizontal displacement, caving angle and recovery rate have been compared thoroughly. Results show that unsymmetrical behavior on the roof is closely related to face stability.

Jan 1, 2007

By Andrew P. Schissler

The Citronen Zinc-Lead Project is one of the world?s largest undeveloped zinc-lead resources, containing more than 13 billion pounds of zinc and lead metal. The project is located in northern Greenland, a self-governed region of the Kingdom of Denmark (Figure 1). Citronen is the northern-most mine in the world, operating at a latitude of 83o 3? N. In excess of 67,000 meters of diamond drilling have been completed to delineate the resource. As of January 2012, the measured, indicated, and inferred reserves for the Citronen project totaled 70.8 million tonnes, consisting of 5.7% zinc + lead (3.5% zinc cut-off) (Ironbark, 2012). For the underground mine plan, room-and-pillar and long holestoping mining methods were selected. The beneficial application of structural fill was considered for increased recovery, efficient tailings disposal, and ground control. Three types of fill were compared - paste fill, cemented rock fill, and iced tailings. An environmental and economic feasibility study concluded that iced tailings was the preferred alternative provided it met the ground control requirements. The purpose of this paper is to explain the investigatory steps that led to the possibility for the use of iced tailings as structural fi

Jan 1, 2012

By Ian Misich

A research programme has been undertaken to evaluate and predict the effect of total extraction of coal by underground methods on the superimposed strata, aquifer systems and ground surface in the Collie Basin. Western Australia. The research work has integrated comprehensive field monitoring of surface and subsurface subsidence with empirical, analytical and physical modelling in an effort to develop an effective predictive tool which can be used in all mining scenarios. Early results indicate that although each method has certain limitations, this approach has enabled the researchers to develop a very good understanding of the subsidence mechanisms from the mine panel through to the surface in the weak, water saturated sediments in the Collie Basin. Given that both subsurface and surface subsidence characteristics have tied in well, greater confidence can be held for predictions made by representative models.

Jan 1, 1993

By Richard D. Begley

This paper summarizes the development of a mechanistic model to predict longwall mining induced surface subsidence. The model was originally developed from data acquired from a recent field study conducted in Northern West Virginia. Additional data has been collected from published sources and used to refine the model. The original model utilizes a negative exponential equation to predict the subsidence profile after the maximum possible subsidence is predicted based on the following input parameters: mine geometry, angle of draw and expansion factors for the deformation zones that have been identified above a longwall excavation. The model refinement involved the development of relationships between physical properties of the overburden strata and angle of draw in addition to the expansion or bulking factors. Therefore the refined model now requires only mine geometry and a typical geologic column from the site to predict the subsidence profile. This paper provides the methodology that enables the model to determine the angle of draw and expansion factors breed on physical properties of the site geology i. e. percent of strong rock and Rock Quality Designation of the overburden strata. In addition :comparisons of predicted results with measured data for over H) different sites with various geological and geometrical parameters located throughout the United States will be presented.

Jan 1, 1989

By Victor V. Nazimko

Multiple mining generates gob-interaction displacements. They redistribute stress m strata and obey laws of irreversibility thermodynamics. An irreversible state causes inhomogeneity of the stress distribution on the mass. A destressed zone develops along a working panel, and worked out panels are deprived of the destressing. The inhomogeneity is smoothed out during elapsed time.

Jan 1, 1996

By T. P. Mucho

Over the past several years, two types of roof control problems have been en- countered at Beth Energy's Eighty Four Complex mine in Southwestern Pennsylvania. These problems have been (1) poor roof conditions encountered while advancing submain entries and longwall panel entries and (2) deterioration and failure of long- wall gateroads, principally the tailgate entry. This paper discusses the efforts by mine personnel to analyze the origins of the various attempts to-deal with the problems, and the success (or failure) of these efforts. Highlighted are the use of various common roof Control materials, variations in mining techniques, cementatious pillars, and changes in pillar design using yield and abutment pillars to address these problems.

Jan 1, 1986

In this paper a numerical formulation is presented for determination of the axial load along a cable bolt for a prescribed distribution of rock mass displacement. Results using the program CABLE indicate that during excavation, the load distribution that develops along an untensioned fully grouted cable bolt depends on three main factors: (i) the properties of the cable itself, (ii) the shear force that develops due to bond at the cable-grout interface (i.e. bond stiffness), and (iii) the distribution of rock mass displacement along the cable bolt length. In general, the effect of low modulus rock and mining induced stress decreases in reducing bond strength as determined from short embedment length tests, is reflected in the development of axial loads significantly less than the ultimate tensile capacity even for long cable bolts. However, the load distribution is also dependent on the deformation distribution in the reinforced rock mass. Higher cable bolt loads will be developed for a rock mass that behaves as a discontinuum with deformation concentrated on a few fractures, than for one which behaves as a continuum, either due to a total lack of fractures or a very high fracture density. This result suggests that the stiffness of a fully grouted cable bolt is not simply a characteristic of the bolt and grout used, but also of the deformation behavior of the ground. In other words, the same combination of bolt and grout will be stiffer if the rock behaves as a discontinuum than if it behaves as a continuum. This paper also explains the laboratory test programme used to determine the constitutive behavior of the Garford bulb and Nutcase cables bolts. Details of the test setup as well as the obtained results are summarized and discussed.

Jan 1, 1996

By Erik Westman

Few studies have been conducted regarding the location, movement, and relative magnitude of the rear abutment of a longwall coal mine. The rear abutment, or the abutment pressure arch in the gob area, is controlled primarily by the quality of pack of the gob; the more compacted the gob is, the larger the stress would be theoretically. While there is no definitive location for the rear abutment, early studies showed that it could be located as far back in the gob as 300 m (1,000 ft). Like the location of the rear abutment, there has been little research conducted and very few answers as to the exact magnitude of the rear abutment load. Due to the fact that the rear abutment has been studied considerably less than the front or side abutment, the need for further studies is evident. From a ground control standpoint, the location and magnitude of a rear abutment could reveal information on the caving process associated with longwall mining, such as when the initial cave could be expected to occur in future panels. Besides the possible importance of the rear abutment from a ground control perspective, evidence related to the longwall rear abutment could be vital from a ventilation standpoint. The peak rear abutment location could serve as an area for harmful gases, such as methane, to accumulate because forcing air through repacked rock is more difficult than through loose rock. The objective of this study is to determine whether passive seismic tomography could image the location, movement, and relative magnitude of the rear abutment as the longwall face retreated, in addition to the forward abutment and gob. The results of a passive seismic imaging study at a western US longwall show evidence for the location, movement, and relative magnitude of a rear abutment. A clear zone of highlystressed strata is shown where the forward abutment is expected and a low-velocity, damaged zone is shown behind the face where the gob is located. These two zones retreat with the face over a three-week period. In addition, the images show a return to stress levels behind the gob that are slightly higher than overburden stress levels, which is presumably the rear abutment. This occurs at a distance behind the face that is approximately the same as the face width for this mine. We can conclude that passive seismic imaging can be used to identify the location, movement, and relative magnitude of the rear abutment of a longwall mine. This conclusion is dependent on the use of appropriate data collection and processing methods. The information provided can benefit the both ground control and ventilation engineers as they strive to improve the safety and efficiency of longwall mines. The knowledge associated with the rear abutment of a longwall could not only help increase production through proper initial planning and reduce down time, but it could also make positive strides in improving the overall safety surrounding a longwall mining operation.

Jan 1, 2012

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

By Vincent A. Scovazzo

Many accepted and useful methods have been developed and used for pillar design. Each method has its own particular strengths and weaknesses. Three methods addressed in this paper are; empirical, analytical, and numerical. Each of these methods have their place in pillar design. Analytical methods result in close approximation of the ideal pillar (most economical, stable, or yielding) and are suitable for use in final mine design and in analyzing existing problems. BOYD uses the analytical method to develop site specific mine pillars design equations. An analytical method, one of many developed by BOYD, is presented here and is based on the procedure used by Wilson (1) and others in development of their empirical equations.

Jan 1, 1995

By Morgan M. Sears

In recent years, one large mine collapse and numerous smaller bump events have resulted in a renewed interest in coal bump research. With numerical modeling, the Energy Release Rate (ERR) calculation quantifies the ?release? of the gravitational potential energy of the rock mass into the environment as mining progresses. This release of energy can occur passively in the form of heat and sound or dynamically in the form of pillar bumps or rock bursts. From the initial application of a calculated energy release rate in the deep hard-rock mines of South Africa, the ERR was found to have a significant correlation with the risk or potential of damaging coal bumps or rock bursts. In this research, an ERR calculation is incorporated into the modern LaModel program to facilitate the analysis for potential coal bumps. Initially the ERR calculations in LaModel are verified using a case study of cut sequences originally modeled using the MULSIM/NL program. Then, the ERR calculations are applied to a bump-prone mine in Southern Appalachia were a number of different pillar recovery cut sequences were used in order to minimize the risk of bumps.

Jan 1, 2009