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By Jesse L. Craft

Field investigation of surface subsidence events associated with both active and abandoned underground coal mines in the United States has established criteria that enable the prediction and classification of mine-related subsidence. The key was establishing the role of geology in all types of subsidence and the recognition of beam subsidence in specific geologic settings. The classification is based on overburden thickness, variations in stratigraphy, topography and mine plan. Types of subsidence are Pit, Room, Sag, and Beam. Pit subsidence is associated with three geologic settings: shale overburden, seasonally wet unconsolidated overburden, and perched water table in unconsolidated overburden. Room subsidence occurs where the roof across the width of the room fails to the surface and a shallow depression develops. Sag subsidence develops where the pillars are failing in conjunction with roof failure. Overburden thickness is generally greater than 150 feet. Beam subsidence can occur in all terrains. It is associated with retreat mining and room and pillar mining where pillar crushing or pillar compression has taken place and barriers are left inplace. They occur as beam arch or one-sided beam subsidence. Beam arch failure occurs where mining has taken place on both sides of a coal barrier. One-sided beam failure is associated with retreat mining moving from a barrier towards the entry in hilly topography. This type of subsidence occurs where a hard, thick, rock unit is acting as a bridge supporting the overburden and outcrops on the hill side.

Jan 1, 1992

By John G. Oldsen

Passive cable bolting and active cable trusses have been used in roof support for more than two years Excellent roof control has been experienced by many coal operations. This paper covers the new and innovative developments in cable support systems and their application in mines. Multiple cable bolts, tensioned cable bolts, coal rib wrapping, and other cable applications will be covered.

Jan 1, 1997

By John M. Karhnak

The Bureau of Mines has a long history of research in Ground Control. For many years, this work was done at Bureau facilities by Bureau researchers. As the tiles changed, however, the area of research interest broadened, and additional methods of implementation, such as con- tracts, were added. 'Ibis paper outlines the organization, goals, and methods of implementation of the Bureau Ground Control Program, along with procedures for project selection. This program organization is followed by several specific examples of Bureau research that will be soon ready for use in the mines.

Jan 1, 1981

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 Nicholas Chlumecky

One of the most dangerous jobs in mining is that of resupporting the roof after a fall has occurred. The resulting cavity may be more than 30 feet high, with relatively unstable sides and roof. It is often impossible to proceed with rehabilitation without exposing men and equipment to unsupported rock. Temporary supports provide margin- ally effective protection because of the lack of available footing. Furthermore, the usual supports in rehabilitated fall locations are not permanent, require frequent maintenance, and fail to eliminate some remaining serious safety hazards. This is typically the case for high cavities where loose rock drops from considerable heights from between roof bolts. Supports made of cribbing, beams and additional cribbing to the high points of the fall in many cases do not provide good protection due to the deterioration and shrinkage of timber. In mines, high cribbing frequently loses contact with the roof and thus provides no protection against rocks peeling off the sides. This is true in spite of the fact that at time of installation the cribbing was wedged tightly to the roof. Therefore, when considering the difficulties which occur with current practices, the objective is to find alternate methods of resupporting the high roof after a fall.

Jan 1, 1981

Results of direct shear tests on rock discontinuities and triaxial compression tests on intact rocks are presented and - discussed. The main parameters considered area stress functions, angle of internal friction and cohesion variations. Primary objective of this work is to determine relationships that adequately describe the rock behaviour for design of mining excavations, while still maintaining a certain degree of simplicity.

Jan 1, 1987

By Jinsheng Chen

Longwall mining-induced abutment loads on the surrounding coal pillars can be grouped into two categories in terms of the relative position of the coal pillars and the longwall face. They are the front and side abutment loads. Even though a lot of research and efforts have been made to study the nature and behavior of these longwall mining-induced abutment loads, their influence zones and magnitude are still not well defined and fully understood due to the complexity of geological conditions and the longwall muting-induced overburden strata movement. The uncertainty about the mining-induced abutment loads often forces consultants and mining engineers to employ a conservative approach in designing coal pillars and entry roof control plans. The problems with this approach are that: (a) it increases CM development¬longwall retreat footage ratio, (b) it decreases coal recovery, and (c) it increases !Ill' operating costs. However, a more liberal approach may create safety issues and result in loss of production. Therefore, the importance of accurately defining the influence zones and magnitude of the front and side abutment loads induced by longwall mining can never be over emphasized. The authors in this paper attempt to define the influence zones and discuss the magnitude of the longwall mining-induced abutment loads by analyzing the field data measured at RAG American Coal Company's longwall mines. In addition, the relationship between entry stability and the balance among roof/floor conditions, pillar design, and roof control plan is also briefly discussed.

Jan 1, 2002

By S. S. Peng

In recent years many roof falls have been conveniently attributed to the adverse existence of a high horizontal stress. The normal practice of not conducting a follow-up study in a roof fall investigation usually leads to a wrong conclusion. This paper describes two cases of massive oriented roof falls in two separate room and pillar mines. The investigation in each case involved the analysis of past history and current conditions from which remedial measures were recommended and implemented. A follow-up period of up to three years was performed to evaluate the effectiveness of the recommended remedial measures. In both cases, high horizontal stress was proven positively not the cause, contrary to the original conviction of mine management Rather, a weak immediate roof was the basic cause Under a weak immediate roof, a proper roof bolting plan and adequate entry width and intersection span must be strictly observed.

Jan 1, 1999

By Erdal Unal

In underground mining today, safety and economical aspects demand a better understanding of the rock-mass conditions, particularly for design of underground mine openings excavated in weak and stratified rock mass. The rock mass classification systems, in essence, are empirical approaches utilized during preliminary design stage. These systems have been developed for specific purposes and rock mass types, therefore, direct utilization of the classification systems in their original form, for characterization of complex rock-mass conditions is not always possible. This is probably one of the main reasons why designers continue to originate new systems, or modify and extend the ones already existing. RMR and Q systems, for example, although widely used in mining and tunnelling, can not fully describe the specifications of the weak, stratified and flay bearing rock mass. Consequently, the engineering applications that would be carried out based on original RMR and Q ratings could be inadequate for making design decisions even during the preliminary design stage.

Jan 1, 1990

By K. Haramy

Coal mine bursts or bumps involve the violent, rapid failure of coal and rock in or around a mine excavation. Failure is normally associated with high stress and brittle or brittle-elastic materials; in coal mining, bursting may also be associated with desorbing gas, and this type of failure is termed an outburst. This paper discusses factors affecting coal mine bursts and several methods to predict bursts, including microgravity, microseismic, rheological, rebound, and drilling-yield methods. Methods to avoid burst conditions using volley firing, hydraulic fracturing, and auger drilling are also discussed. A case study is presented in which the redistribution of stresses and displacements was found to be associated with a burst at a deep longwall coal mine. The research showed that stresses are redistributed following the burst.

Jan 1, 1984

By Thomas M. Barczak

Roof support systems are necessary to provide stable mine openings and much research has been conducted to design a variety of roof support systems that will function in various manners to ensure that stable ground conditions are achieved. Despite these advancements in technology, mistakes continue to be made in the evaluation and/or installation that significantly degrade the support capability or lead to erroneous determinations of support expectations. The purpose of this paper is to discuss misconceptions about how roof supports perform and factors that impact their performance. The goal is to present practical information that will assist mine operators and engineers in selecting, installing, and evaluating roof support systems properly, and help them to avoid mistakes that can lead to erroneous expectations and potentially catastrophic results that may lead to roof falls. The paper is limited to a discussion of secondary roof support systems and powered roof supports such as longwall shields.

Jan 1, 2001

By Jessica M. Wempen, William G. Pariseau, Michael K. McCarter

"Quantitative analysis of mine-wide subsidence at the kilometer scale and details of stress distribution about an advancing longwall face are estimated using an adaptation of the finite element method. The method is well suited to the tasks at hand. For greater realism, variability of strata properties is taken into account as are the effects of joints and cleats on elastic moduli and strengths. Evolution of pillar stress and entry closure remote from the face is readily quantified in a series of analyses that simulate face advance. Computed results compare favorably with the evolution of closure measurements about an instrumented pillar in a two-entry headgate. The appropriateness of the finite element method is confirmed. This method is based on first principles that avoid empirical schemes of uncertain applicability and numerical models “calibrated” by fitting computer output to mine measurements.INTRODUCTIONMine-induced seismicity is intimately related to strata mechanics associated with longwall coal mining and room and pillar mining, as well. Strata deformed beyond an elastic limit often tend to fail rapidly by brittle fracture and, thus, to emit acoustic signals in the form of elastic waves. Understanding the induced seismicity in coal mining offers the potential for early warning of hazardous conditions. Many years of experience with seismic phenomena associated with hardrock mining suggest considerable complexity should be expected. However, recent coal mine studies (Pariseau, 2014; Pariseau, 2015; Pariseau and McCarter, 2015; Pariseau and McCarter, 2016) have shown high correlations of seismicity with element failures in finite element simulations. The main lesson learned from these studies is one elaborated by Iannacchione et al (2005); deviation from normal activity is an early warning of ground control difficulty. For example, an abrupt increase in seismic event rate, or equivalently, an increase in element failure in finite element simulation of panel advance, is a cautionary indication for ground control. Present and past study results demonstrate the effectiveness of finite element simulations for mine design and early detection of potential ground control issues."

Jan 1, 2018

By Timothy Ross

The Pittsburg & Midway Coal Mining Co.'s Kemmerer Mine is one of the deepest surface coal operations in the world, with the highwall extending to approximately 1,000 ft above the pit floor. To increase recovery, auger mining is being evaluated beneath the highwalls of several seams representing the current economic limit of pit development. The primary augered seam is over 50 ft thick, can approach 100 ft thick, and dips into the highwall at an average of 17 degrees. The seam thickness and dip present many geotechnical design and operational challenges for multiple Sup to five) lift augering. This paper presents the design criteria and methodology applied to determine appropriate web and septum thicknesses for the various cover depths anticipated, as well as operational measures that can be taken to increase overall coal recovery. The primary tool used in the design was the finite difference code FLAG. Models incorporating typical strata sequences associated with the coal seam were run at several different cover depths corresponding to the maximum anticipated auger penetration. Web and septum thicknesses were varied until the minimum thickness satisfying a given safety factor criterion was determined. Using this procedure, design curves were developed for web and septum thickness versus cover depth. Operationally, several recommendations were developed to ensure that overall coal recovery is maximized. By angering across the scam dip, auger penetration per hole can he increased dramatically, and by carefully planning maximum planned auger penetration versus hole spacing, increased coal recovery can be safely attained.

Jan 1, 1999

By Alan Bugden

Goedehoop Colliery produces 8 million tonnes of coal a year, principally from room and pillar mining, and is situated in the Witbank Coalfield in the Republic of South Africa. The mine has a long and successful record of producing export quality coal from the 2,4 and 5 seams at depths ranging from 20 to 100 metres using modem mining equipment. Following a number of fatalities caused by large and unexplained roof falls in roadways and intersections of supported ground, a review of the roof control system at Goedehoop was carried out. This led in turn to a programme of underground measurements and surveys. This included detailed investigation of the roof falls, horizontal stress mapping, stress measurement and short encapsulation pull testing on roofbolts. These investigations led to a number of important conclusions in relation to the state of stress in the ground, the roof failure mechanisms, standard of rootbolt installation and performance of the roofbolt system. As a result of the work Goedehoop Colliery has made a number of substantive changes to the roof control management system at the mine. These include: 1. A recognition of the importance of horizontal stress as the main factor responsible for roof falls. 2 The forward planning of roof support based on known features which cause the stresses to be elevated. 3. The introduction of a more effective roofbolt system, which can be more easily installed to a high standard. 4. A response system based on risk assessment and monitoring roof movement on a routine basis. The paper describes the knowledge acquired, the conclusions drawn, the changes made and progress to date.

Jan 1, 2001

By Yi Luo

The severity of disturbances caused by longwall mining subsidence to various residential structures can be re¬duced. The success of such mitigation efforts depends on accurate subsidence prediction, correct assessments of the subsidence influences, and careful design and implementa¬tion of the proper mitigation measures. This paper presents the systematic approach involved in the subsidence mitiga¬tion efforts with a case demonstration.

Jan 1, 2003

By Michael Kelly

Longwall mining is the principal method of underground coal mining in Australia, with 70 million tonnes being produced by longwall mines in 1999. However, the mechanisms that control longwall geomechanics are still not clearly understood resulting in stoppages costing several hundred million dollars per annum The CSIRO has been investigating longwall geomechanics with consultant groups and minesites through several ACARP supported projects since 1994. The research has aimed to improve longwall support design and control methods by defining strata failure mechanisms and their occurrence about underground extraction panels, It also aimed to develop better predictive tools bur a broad range of underground environments. As a result of site studies at more than 8 locations some of the aspects that have been touted to control longwall geomechanics, especially from a 3D aspect are highlighted. These aspects include stress controls, geometry, faulting, depth. longwall width and changes in lithology.

Jan 1, 2000

By Thomas M. Barczak

The decade of the 90's brought an unprecedented increase in the development of innovative technologies to provide more effective and easier to install roof support in underground mines. To facilitate the application of these technologies to improve mine safety. National Institute for Occupational Safety and Health (NIOSH) developed the Support Technology Optimization Program (STOP). STOP is a windows based software program that provides mine operators with a simple and practical tool to make engineering decisions regarding the selection and placement strategy of these various standing roof support technologies. This program includes a complete data base of the support characteristics and loading profiles obtained through safety performance testing of these supports at the NIOSH Safety Structures Testing Laboratory. Support design criteria in the form of the required support load density at a specified convergence can he established from four options: ( 1 ) a data base of measured ground reaction data from various mines or ground behavior information inputted by the user. (2) determination of loading requirements based on a detached roof block or rock failure height. or (3) criteria based on the current roof support system, and (4) arbitrary criteria set the by user. Using these design criteria, the program will determine the installation requirements for a particular support technology that will provide the necessary support load density and convergence control. Optimization routines are also available to determine the most efficient support design for a user specified support installation- In addition to these performance measures. STOP can be used to compare material handling requirements and installation costs. Comparisons among the various support technologies are easily made including graphical analysis of relevant support parameters. This paper describes the STOP and its application to optimize standing secondary roof support systems.

Jan 1, 2000

By Satyendra K. Singh

If the extraction is beneath massive roof strata, the roof tends to 'bridge' across large spans without failing, the caving behaviour is complex and irregular. Understandably, these give rise to many geotechnical problems with safety implications and involving sometime huge direct and indirect costs. Only two important problems, 'feather-edging' in pillar mining and `severe periodic weighting' in longwalling, are discussed in this paper in the background of geomechanics investigations in Australia at Blue Mountain Colliery and in South Bulga Colliery respectively. `Feather edging'- a major source of injury and fatal accidents in Australia and South Africa in such situations, refers to a phenomenon where the roof, at the goat edge, falls as a thin wafer of rock often suddenly, over-riding conventional timber breakerlines. The results of the first comprehensive, monitored trial in an Australian pillar extraction coal mine, suggests how the roof falls into feather-edging caving (generally) because of multiple shear-planes (interpreted) and how it may be alleviated. Because of complex fracturing behaviour of massive roof strata, periodic weighting at a longwall face is certain to manifest itself as 'yielding supports', heavy conditions', rapid convergence and in the worst case, as a cause of face stoppage. After briefly discussing the programme of instrumentation (in a chain-pillar and over the would-be goal) and results, this paper attempts to improve mechanistic understanding of the related geotechnical issues associated with the weighting behaviour and aims at establishing a 'threshold' value of weighting severity index (suggesting intensity of periodic weighting) with an objective to optimise operational controls before mining through potential (future) weighting areas.

Jan 1, 2000

Numerical simulation for microseismicity due to mining at one of the collieries in Australia was carried out and the results were compared to the observed microscismicity. 3-D Stress analysis showed stress concentration on the edges of the mined areas and on the coal pillars- The stress concentration ahead of the coal face was biased to the tailgate. This is similar to the observed distribution of microseismic events. Stress reduction above and below the mined area was seen. Tensile stress was observed mainly above the mined area. It was realized that compressive failure due to nearly vertical maximum principal stress and nearly horizontal minimum principal stress in retreat direction (NS) might occur ahead of the face. Tensile failure due to nearly vertical minimum principal stress above the mined area was expected. Compressive failure due to nearly horizontal maximum principal stress in EW direction and nearly vertical minimum principal stress above and below the mined area were also expected.

Jan 1, 2001

By R. Fraher

Hazard analysis utilizes interaction mechanisms in combination with geologic, structural and mining conditions to delineate areas of potential damage in seams subject to interaction. Using characterizations of conditions peculiar to multi-seam mining, such as innerburden thickness, geology and structures, initial interaction contour maps can be constructed. Geologic hazard analysis is then carried out on roof, pillar and floor conditions in the affected seam using lower seam rock properties and layout geometries in the overlying seam. Either safety factors or damage ratings can then be developed representing degree of hazard in the lower seam. The final hazard map shows both the aerial extent and magnitude of potential damage from interaction.

Jan 1, 1992