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By Karl Brandt

Longwall panel 580 has recently been retreated in the Zollverein 1/2 seam at a depth of approximately 1000m at Auguste Victoria mine with rectangular rockbolted roadways used for both the maingate and tailgate. Although Auguste Victoria mine has been increasingly using rockbolted support since the mid 1990's. this has previously been restricted to tailgates for single use. This panel represents the first use of a rectangular rockbolted roadway as a maingate at Auguste Victoria mine. As such, it represents a significant change in support strategy. As part of the strategy numerical modelling was used to examine expected behaviour of rockbolted roadways at the site, stress changes during face retreat, the feasibility of adopting rockbolted support for the maingate and the reinforcement pattern required. Monitoring instrumentation was installed in both gates to confirm that the support systems were performing satisfactorily as the gates were driven and as the face was retreated. Although the panel is isolated from others in the Zollverein 1/2 seam, markedly different roadway conditions have been experienced in the two gate roads. Favourable conditions were obtained in the maingate, but particularly difficult conditions were encountered in the tailgate. Previous work for the mine had examined interaction with old workings in other seams and shown a vertical stress of 28MPa for the maingate and up to 45MPa for the tailgate. The values from the earlier work were used in the modelling work described, which showed contrasting results given the different stresses. The difference between the gates can be attributed to the interaction. The paper describes the numerical modelling work, the reinforcement systems adopted, the monitoring results, the interaction effects and the comparison between expectations from the numerical modelling results and the conditions actually experienced.

Jan 1, 2002

By Mario G. Karfakis

Mining related subsidence is a major concern over abandoned shallow coal mines. Many of the subsidence prone areas are presently used or will be used in the future for residential housing development. Concern over mine subsidence is inhibiting the development of surface land uses on previously mined area. This concern prompted public officials to realize the need to develop guidelines which can be used to assess and reduce the risk of mine subsidence damage to building and other improvements. Subsidence and the associated ground movements must be defined before one can relate them to building damage. Surface movements resulting from underground coal mining occurs in two phases termed as: active subsidence and residual subsidence. Active subsidence refers to all surface movements occur in^ simultaneously with the mining operations, while residual subsidence is-that part of the surface deformation which occurs following the cessation of mining. Residual subsidence is the major concern over abandoned mine lands. The duration of residual subsidence is of particular importance from the standpoint of damage at the surface to structures built after the mine has been abandoned. Time spans during which surface subsidence may occur, vary markedly with the mining method being used. In total extraction methods, such as longwalls, subsidence is induced rapidly, beginning almost immediately after mining. Subsidence concurrent with coal extraction is desirable for a safe longwall mining operation. Usually subsidence is complete within a relatively short time. The remaining settlement occurs as residual subsidence due to the gradual compaction of the subsided ground. The relative proportion of the residual subsidence movements is dependent upon the presence of remnant pillars and roadside packs. rate of face advance and depth of workings. In partial extraction methods, such as with room-and-pillar systems. major occurrences of surface subsidence may be delayed for decades until the support pillars and/or the roof have substantially deteriorated and collapsed. The actual time involved depends on a number of factors such as the strength of coal, roof, and floor, extent of fracturing. presence of water, depth of workings, pillar size and percentage extraction. Consequently residual subsidence is more likely to occur over abandoned room-and-pillar mines. The following sections define subsidence over abandoned mines, review the proposed mechanisms, identify the controlling factors and examine the prediction methods.

Jan 1, 1987

By Xiaolin Wu

Most coal bump occurrences in the U.S. are directly related to mining operations under strong roofs. An understanding of the behavior of bridging or cantilevering strong roofs over working longwall faces is a key to a safe and productive longwall operation. In this paper, an attempt has been made to study the behavior of strong roof beds using the elastic beam theories. Four structural models are formulated to simulate the locations of strong roofs and their distinct weighting stages. For each model, analytical solutions for the deflection, bending moment, stress, and the strain energy stored in both the coal scam and the strong roof are obtained. Solutions for the critical spans, which are controlled by the caveability of the strong roof beds, are developed. The effects of rocks' differing elastic moduli, under tension and compression, on the actual flexural rigidity, and stress redistribution are analyzed.

Jan 1, 1993

By Douglas C. Peters

Part of the ground control research at the U.S. Bureau of Mines is involved with improving ground control data collection and analysis methods. A geographic information system (GIs) is being used within this research effort to standardize the data collected and provide a common graphical and tabular format and analysis system for researchers to access information on underground coal mines. In addition, GIS's can perform relational analyses on multiple map and tabular data sets which are not possible through map-creation or CAD packages or through data bases alone. In addition to standardizing ground control analyses, there is an increasing need for rapid analysis of ground control data in connection with increasing production rates at longwall mines. The examination of a large number of interrelated parameters and their impact on short-term mine safety and productivity under the time constraints imposed by rapid production requires computer-aided techniques. A GIs provides the means to quickly, quantitatively, and reproducibly evaluate mine-wide conditions. This overview of GI8 techniques for mine data integration and analysis, using a mine in Colorado as an example site, will cover the input of data into a GIs, including mine-wide monitoring system data and general data such as roof control plans and geology. The methods to geographically reference these data sets and retrieve and combine them also will be covered. Automation of such processes briefly will be reviewed to allow future users to automate their mine data analyses.

Jan 1, 1990

The formation of tension cracks in slopes is well known. Analytical studies require the determination of a critical slip surface for any slope and, in doing so, it is necessary to consider an appropriate tension crack. A general yet simple approach is proposed for identifying the critical slope failure surfaces associated with critical tension cracks. The minimum factor of safety consequently reflects both safety against shear sliding and tensile failure. In general, the critical slope failure surface for a slope determined in this way differs from that which is obtained without optimizing the position and depth of the tension crack. In particular situations, the difference is significant.

Jan 1, 1989

By Paul Cartwright

A rockbolted roadway risk assessment procedure has been developed specifically for the Parkgate seam at Thoresby Colliery by Rock Mechanics Technology and colliery engineers. The purpose of this assessment is to identify any areas of significant risk in relation to roof instability due to potential failure or overloading of the rockbolt support system. This allows appropriate additional support to be installed in sections of roadway considered to be at increased risk. The assessment of risk in rockbolted coal mine roadways, in terms of fall of material, has to be based on an understanding of the rock mass behaviour at that site. The development of an appropriate risk assessment system is therefore site specific. A consistent approach to developing such a system can, however, be applied by adopting the same general logic to each individual site. The use of risk assessment to categorise areas with increased risk of roof instability has been undertaken for four longwall panels at Thoresby. The procedures used for these assessments are discussed and the operational benefits examined. The approach successfully used at Thoresby has been developed into a routine tool to assist mine personnel in assessing areas of potential roof instability in rockbolted roadways and as a means of communicating the position of these areas to both supervisors and workmen. The system provides a rational and objective interpretation of the current underground conditions based upon a logical, consistent and structured approach.

Jan 1, 1999

By Jingyi Cheng

Many longwall panels in China are laid out along the dip direction such that the direction of longwall retreat mining is along the strike direction of coal seams. Downward mining longwall panels in steeply inclined seams refers to those that are laid out in a non-uniformly developed fold structure where the seam is not only steeply dipping, but also highly undulated along the strike direction, causing longwall mining extremely difficult because it has to deal with large dip angles that vary both in the mining and faceline directions. Obviously, the characteristics of mine pressure and overlying strata movement in downward mining longwall panels in steeply dipping seams have unique patterns that are very much different from those cases of mining the flat or gently inclined coal seams. The effects of those unique patterns presented a big challenge to production and safety. Almost all previous and current researches on ground control have been concentrated on issues related to the flat and gently inclined coal seams. Conversely, research on steeply inclined coal seams, especially those with large angle variation both along the seam dip and strike directions is almost none. Therefore, it is very important to initiate a study to better understand the characteristics of mine pressure and overburden strata movement for this special type of geological condition. The objective of this paper is to monitor and analyze the characteristics of mine pressure and overlying strata movement in downward mining longwall panels in steeply inclined seams, the angle of which varies greatly along both the seam dip and strike directions. The measured data has conclusively demonstrated that the characteristics of mine pressure and overlying strata movement in downward mining longwall panels in steeply inclined seams are different from those for the flat and gently inclined coal seams. The large downward angles of coal seam undulation along the strike (mining) direction can obviously affect the roof movement and reduce strata pressure. Study in this research showed that appropriately increasing the rate of mining (or face advance) is positive to roof control.

Jan 1, 2013

By David Newman

Coal refuse impoundments and underground mines are frequently sited in close vertical and horizontal proximity in the valleys of the Southern Appalachian coalfield. The steep V-shaped valleys provide the volume necessary for slurry storage against a coarse refuse embankment that is constructed across the valley mouth. The presence of coal outcrops on the valley walls provides economic access to mineable coal seams that may be located under several thousand feet of overburden beneath the adjacent ridges. In some instances where the underground mine precedes the slurry impoundment, the type of mining (partial or complete extraction) and pillar centers were selected without consideration for the potential future use of the valley as an impounding structure. The high standard established for the design and stability analysis of the coarse refuse embankments is clearly illustrated by the absence of an embankment failure in the thirty-one years since Buffalo Creek. However, the recent Martin County slurry spill focused attention on a thorough geotechnical analysis to prevent a connection between the flowable slurry and the mine void. The overburden, roof, pillar, floor, and outcrop barrier stability are examined to quantify the break through potential. The strength and physical properties of the consolidated fine refuse are also important since this material is often the most well-defined and quantifiable barrier separating the slurry and mine void. Because each site is unique, the appropriate analysis ranges from the calculation of roof, pillar, and floor safety factors to detailed numerical modeling of the mine workings and the determination of ground deformation and strains resulting from mine subsidence. The method of refuse disposal may be altered to slurry cells where the accuracy of the mine maps, extent of mining, type of mining (development or retreat) cannot be reliably verified or the break through analysis does not produce an acceptable level of risk. Case histories from the Southern Appalachian coalfield are used to illustrate the application of methodologies to quantify the break through potential of coal refuse impoundments.

Jan 1, 2003

By Mao Bai

Mining in situations where there exists a potential hazard from inundation has always existed in coal mining, and often reserves were left unmined in favor of reserves with no such hazard. However, increased pressure on expanding the minable reserves and on producing low-sulfur coal, has made mining under water hazards necessary in many parts of the world. Recent joint efforts by the authors to develop case-history data bases of mining under water hazards has allowed the bringing together of Chinese and United States experience and guidelines for such mining. In China, mining under water hazards has been successfully accomplished for a number of years, allowing the development of a good data base from which to draw guidelines. In the United States, much more limited experience, and the longer delay in adopting longwall mining compared to the rest of the world, has forced reliance on a more limited data base and European experience. In the United States (and Canada), the data base is continuing to increase as more operations start-up. Developing a generally-applicable set of guidelines for international use, based upon extensive Chinese and other experience, appears feasible. The Chinese and United States experiences are detailed by case histories.

Jan 1, 1995

By Bruce W. King, David A. Newman

"Underground mines have always been an integral part of the limestone industry. However, beginning in the 1990s, the number of underground limestone mines has increased for many reasons including• Stripping ratios at surface quarries have become uneconomic.• Existing surface quarries have reached the mineral boundaries, and the remaining stone reserves are underground.• Surface access to the entirety of the limestone reserve is limited by development or infrastructure.• The once-rural setting of the surface quarry is now surrounded by subdivisions and development. An underground mine is less intrusive and takes the mining operation out of the public eye.• For larger markets, an underground mine enables year-round production without the sequence of overburden stripping in the winter to expose mineable stone for summer production. This is common at many surface quarries.Multiple-level operations are a natural extension of the decision to mine underground. Most limestone reserves, for example, the Tyrone-Oregon-Camp Nelson, Hermitage-CartersLebanon- Ridley, and Salem Limestones, are sufficiently thick to accommodate multiple levels. Multiple-level mines should be planned by considering each level separately and then integrating the individual levels into a larger mine plan. Failure to plan all the levels of a multiple-level mine in concert can result in ground control (back, pillar, floor, and inter-level sill pillar) problems attributable to stress transfer and interaction between the levels or the necessity to leave a thicker sill pillar between adjacent levels.Multiple-level mine planning begins with geology. Once the upper level back horizon is identified, the geology, rock strength, and physical properties in each level are used to• identify the optimal back horizon• quantify whether back support is required and, if so, the type, spacing, and length of the back support• determine the pillar centers• determine the total mining height, including floor shots"

Jan 1, 2016

By D. F. Howarth

Two innovative ground reinforcement products have been developed for use in the mining industry. One of these products is a high strength polyethylene (HOPE) bar for use as a rib bolt in underground coal mining and as a corrosion proof tendon in open pit mines. The other product is a cable bolt (Ultrastrand) which demonstrates enhanced load transfer and is available in continuous lengths. Ultrastrand's exceptional in¬situ performance and versatility makes it ideal for mechanised installation in both underground and surface mining operations. Details of these products are presented and results from labortaory and field trials are discussed.

Jan 1, 1992

By Salah Badr

Longwall mine layouts with their entries, chain pillars, face support systems, advancing mining faces and compacting gobs represent a geomechanically complex mining operation. Geomechanical aspects are further complicated if mining occurs at depths exceeding 350 m, as stresses induced adjacent to the excavations exceed the unconfined strength of the coal and result in fracturing of the sidewalls of entries and chain pillars during development. Traditional methods of geomechanical analysis, whether empirical or numerical, are unable to represent such behavior satisfactorily. Achieving some meaningful results requires explicit representation of the three-dimensional boundary conditions and implementation of appropriate constitutive models for the rock mass. This paper describes a numerical modeling methodology for deep longwall mines operation using the FLAC3D finite difference de, which incorporates a strain softening constitutive law for modeling failure of coal. An important aspect of the modeling method includes accounting for stress relief provided to the pillars and entries due to the increasing load applied to the compacting gob.

Jan 1, 2003

By David J. Reddish

Longwall systems are inflexible in layout, intolerant of disruption and represent a substantial investment. It is therefore essential drat all understanding of the likely strata behaviour and support requirements is obtained prior to panel development. For many tears rock mechanics engineers have applied numerical modelling based kill finite element techniques to the design of the support and layout of underground excavations. Whilst such design methodology has worked well in civil engineering and hard rock situations, it has been less reliable when applied to excavation design in the soft rocks of the coal measures. This lack of reliability has been identified largely as being the result of the utilisation of unsatisfactory input data, This paper describes the results of an ongoing research programme at the University of Nottingham in which a novel rock mass rating system known as the UK Coal Mine Classification system was developed to enable the prediction of the strata properties that are required as input parameters for use in numerical modelling of the deformation of soft rocks around excavations such as roadways and longwall faces in coal mines. The paper describes the development of the system. the way in which classifications are applied to derive input data for numerical models and gives two case studies ill which deformations and stress re-distributions around coal mine excavations have been simulated by numerical modelling using the FLAC finite difference code.

Jan 1, 2000

By Vincent Scovazzo

Cost-effective underground mine configurations have been designed in highly fractured rock masses where the stone possesses a high material strength. A limestone mine in the eastern United States is used to demonstrate procedures for the geotechnical design of one such operation. The mine discussed is located in both limbs and the axis of a recumbent fold that fractured the rock mass. In a highly fractured rock mass, where the host rock has high strength, a design can be developed to alleviate intact rock failure. This is possible because in most cases as the stress levels needed for intact failures are approached, the stresses are quickly dissipated due to movement along discontinuities, which represent planes of weakness. As a result most underground failures in highly fractured rock are due to sliding or toppling. Even when intact rock fails in a highly fractured rock mass, it is the movement along an existing joint or other weakness plane that often initiates or causes this failure. It is therefore important to obtain the characteristics of weakness planes that are the discontinuities within the mass. The required information on weakness planes, geology, and strength of materials is discussed and recommendations for obtaining this information are presented. A discussion is provided on failure modes and the use of developed information for their evaluation. The impact on geotechnical design is demonstrated through pillar sizing procedures; however, this approach can also be applied to mine roof and pillar foundations.

Jan 1, 2002

By H. N. Maleki

This paper presents a field evaluation of six tailgate secondary support systems, with the objective of optimizing support efficiency for maintaining open gate roads. Support systems consisted of a combination of three wooden crib configurations, with either tension rebars or truss bolts. The effectiveness of the support systems was evaluated through measurements of ground movements, support load, and observations along 2000 linear ft of a tailgate. It was concluded that floor heave, crib width to height ratio, cribbing pattern, and support spacing have significant impact on tailgate stability and access after the passage of the first longwall face. Guidelines were suggested for selection of secondary support for other applications.

Jan 1, 1986

By Brian E. Travis

Lightweight polymer grids have recently been introduced to the mining industry. They are currently being used for roof and rib control, longwall shield recovery, safety guarding or barricade, highwall screening, internal reinforcement of gunnite/shotcrete applications, sub-base reinforcement, construction of steepened slopes, retaining walls, mine reclamation (i.e. erosion control, turf reinforcement), and in numerous mining waste related projects. These polymer mining grids utilize a strong, lightweight polymer, usually special grades of polypropylene or polyethylene. High tensile strengths are achieved by molecular orientation of these polymers in the manufacturing process. The first step in the manufacturing process is to extrude the polymers into thick continuous sheets. These sheets are then punched, and drawn into open grid structures. Aperture size and configuration are controlled by the geometry of the punched hole and the degree to which the grids are stretched. The drawing process is the key to the molecular orientation required for the tensile strengths. These long chain polymer molecules in their natural state have a random orientation. When stretched, the molecules are pulled into a parallel alignment, thus creating a lightweight product with tensile strength comparable to steel. The finished polymer mining grid product is a viable alternative to conventional supplemental roof control meshes such as chain link fence and welded wire. Polymer mining grids are a fraction of the weight, have comparable load support qualities, and will not rust or corrode in even acidic or alkaline conditions. Specific flame retardant additives can also be incorporated to make the polymer grid self extinguishing for use in underground "gassy" environments.

Jan 1, 1992

By M. Karmis

For several centuries surface subsidence has been recognized as an inevitable consequence of most underground mining. In fact, British court records of disputes and litigations related to property damage due to mining subsidence can be traced to the early fifteenth century (Shadbolt, 1978). The first systematic investigations of the mechanisms involved in mining subsidence can be credited to Belgian Mining Engineers. The re- search effort was initiated after the widespread movements and the resulting structural damages suffered by the City of Liege in the 1920's. The publication of Gonot's 1871 treatise "Loie de la NomZ" (normal theory) was one of the most not- able achievements of the Belgian school of that era (Gonot, 1871). During the past 100 years the phenomenon of mining subsidence has been the subject of intensive research, particularly in Europe where long- wall is the most common method of underground coal mining. Many theories have been advanced to explain the mechanism of ground movement as it develops from the excavation, through the superincumbent strata, to the surface. Such theories include the original concepts of beams, arches, domes and the contemporary mathematical modeling principles of strata movement assuming two extreme concepts-- a continuous elastic medium and a stochastic medium for the superincumbent strata. An excellent review of the development of these theories ?-s given by Shadbolt (Shadbolt, 1978). In conjunction with the theories of mass strata movement in the vicinity of mine workings, the subject of surface subsidence predictions has also been the subject of extensive research. The development of reliable and rational techniques of subsidence prediction is imperative, if this phenomenon is to be recognized and controlled within acceptable environmental levels. Empirical as well as mathematical methods have been proposed to fulfill this objective and a comprehensive analysis of these methods has been presented in the literature (Bramer, 1973; Shad- bolt, 1978; Virginia Polytechnic Institute and State University, 1980). As a result of this intensive research effort, it is now accepted in many )coal fields-- mainly in Europe-- that the state of art in subsidence prediction and control has approached a level where mining induced surface displacements can be predicted to a suggested accuracy of less than + 20 percent. Furthermore, such movements can be translated into anticipated structural failures using established damage criteria, thus enabling appropriate precautionary measures to be implemented. In this country, subsidence is rapidly gaining emphasis as an important environmental problem of underground coal mining. Damage resulting from this phenomenon ranges from land settlement to severe structural damage and has been witnessed in rural (Illinois, Colorado, West Virginia, Virginia, Pennsylvania) as well as in urban areas (Pennsylvania, West Virginia, Illinois, Wyoming). Yet, the art of subsidence prediction and hence control in this country, for both longwall and room-and-pillar coal mining, is far from approaching the maturity reached by the Europeans. In this paper, field measurments and computer modeling techniques have been used to develop some basic relationships of longwall subsidence and its related parameters. Because the objective of this study was to develop regional subsidence trends, all processed information pertains to the Appalachian coal region.

Jan 1, 1981

By Yong-Ming Jiang

Longwall mining systems are extremely complex and expensive and require great caution for reliable and efficient operation. In many cases, ground control problems are the primary concern in longwall mining. For example, unexpected threats such as roof falls or shield damage can cause sudden interruption of coal production and safety problems, which can originate from many parameters such as shield capacity, setting load, shield type, conditions of immediate and main roofs, pillar stability, geological anomalies, etc. However, it is difficult to correlate ground-control problems at a longwall face with the related mining and geological factors. In this paper, a method of correlation analysis, called tree classification, which permits the use of a large and variable number of features, is introduced The method provides an effective approach to produce an accurate classifier or uncover a knowledge structure, which will be used to predict future consequences depending on past and current data sets. The application of the tree classification method in longwall stability analysis is discussed, and the major procedures of constructing a knowledge structure are included as an example utilizing monitored leg pressure data in a longwall face. Also, the importance and ranking of the variables related lo longwall shield stability is discussed This is a major step towards developing an artificial intelligence system for real-time interpretation of shield monitoring data and hazard prevention.

Jan 1, 1993

By Jennifer Riefenberg

Underground coal mines often experience ground control problems related to weak floor. Developing a methodology for rating floor quality can aid in understanding and delineating ground hazards. U.S. Bureau of Mines researchers are currently exploring methods for generating ground control hazard potential maps. One component of hazard potential in floor-heave-prone mines is delineating floor quality and its correlation with failure. Although field exposures are limited, significant information on the roof and floor quality can be gained through analysis of existing drill core data. Drill core data and roof and floor exposures were analyzed for quality. Laboratory tests were run on 50 core samples to evaluate, compressive and tensile strengths of various roof and floor lithologies. Extensive geologic and lithologic examinations of the cores and samples were also completed. Results of this work involve the formation of a preliminary quality rating for coal mine floor using a modified Coal Mime Roof Rating (CMRR).

Jan 1, 1995

By Kot Unrug

The selection of tensioned versus non-tensioned roof bolts, for primary coal mine roof support, has been debated for well over 25 years. The wide spread use of fully grouted rebar marked the beginning of the discussion of the relative importance of bolt tension and the resultant immediate roof compression. The focus is on roof conditions encountered in coal mines however the discussion may be extended to hard rock mines as well. The theoretical bases for both systems are presented, with the analysis focused on the bolt's ability to actually fulfill the theoretical assumptions. Of specific concern are assumptions about bolt tension upon installation and the likelihood of maintaining useful bolt tension over time. The usefulness of tension roof bolting is limited by the mechanical interaction of roof bolt assembly and the rock, which may be valid at the time of installation only. Ambient mine atmosphere and fluctuation in moisture content changes the physical condition of rock, which may have a significant impact on the effectiveness and longevity of bolt tension. Changes in the material properties of the surface of the immediate roof can invalidate the assumptions, which were made in the design stage of roof support system. This means that the roof support in the long term excavations may be inadequate requiring costly rehabilitation. The presented arguments are illustrated with the examples of some relevant experimental data. Tension primary roof bolting is recommended where the immediate roof is: ? Stiff with a high modulus of elasticity ? Surface layers are continuous and fairly thin ? Not easily weathered

Jan 1, 2004