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By Radoslaw L. Michalowski

The operation of mobile mining equipment near the crest of stockpiles and wastedumps often results in localized slope failures. The Bureau of Mines has developed a computer code based on a rigorous limit plasticity approach which determines the admissible total weight for a concentrated loading near the crest of these otherwise stable structures. The program is based on a three- dimensional analysis and provides an upper bound solution to the load that can be applied to the top of the operating distances for mobile equipment from the slope edge. This presentation will discuss the application of limit plasticity to the development of the local three-dimensional collapse mechanism, and the use of the computer code based on this approach for mining applications.

Jan 1, 1990

By D. N. Cleaver

Like other influence function methods, the SWIFT subsidence prediction program, developed within the Mineral Resources Engineering Department at the University of Nottingham, requires calibration to regional data in order to produce accurate predictions of ground movements. Previously, this software had been solely calibrated to give results consistent with the Subsidence Engineer's Handbook (NCB, 1975). This approach was satisfactory for the majority of cases based in the United Kingdom, upon which the calibration was based. However, in certain circumstances within the UK and, almost always, in overseas case studies, the predictions did not correspond to observed patterns of ground movement. Therefore, in order that SWIFT, and other subsidence prediction packages, can be considered more universal, an improved and adaptable method of regional calibration must be incorporated. This paper describes the analysis of a large database of case histories from the UK industry and international publications. Observed maximum subsidence, mining geometry and Geological Index for several hundred cases have been statistically analysed in terms of developing prediction models. The models developed can more accurately predict maximum subsidence than previously used systems but also, are capable of indicating the likely range of prediction error to a certain degree of probability. Finally, the paper illustrates how this statistical approach can be incorporated as a calibration system for the influence function program, SWIFT.

Jan 1, 1995

By Leigh J. Wardle

Significant coal reserves in New South Wales lie under areas with steep topography such as valley slopes and cliffs. Subsidence predictions are difficult to make for these locations as existing empirical databases cannot cover all permutations of topography and mining geometry. Two dimensional numerical model alternatives are of limited applicability. Consequently a novel three-dimensional numerical model is developed including the detailed panel layout, realistic rock mass properties including the full stratigraphy, goat behaviour and actual surface topography. Model results show the same trends as subsidence measurements from Baal Bone Colliery in the Western Coalfield of New South Wales

Jan 1, 1992

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 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 Thomas Barczak

Pumpable roof supports provide an alternative longwall tailgate roof support and have grown in usage during the past few years. Heintzmann Corporation has been installing pumpable roof supports at the RAG Resources Emerald Mine in western PA since 1998, where they have provided effective roof control in their longwall tailgates. Despite the success of these supports in this application, questions remain regarding critical design issues for optimizing the use of this support technology. The support loading profile is characterized by a high initial stiffness with peak loading occurring at less than one inch of convergence, followed by significant load shedding with a post failure capacity comparable to that of wood cribbing. Therefore, the key to optimizing the support utilization is to provide sufficient load density to prevent convergence from occurring beyond the peak loading capacity. This requires an understanding of how the supports interact with the ground conditions, hence measurement of the ground reaction curve. In order to obtain this information, pumpable roof supports were instrumented to measure support loading. Roof deformation and roof-to-floor convergence measurements in the vicinity of the instrumented supports were also made. The experimental parameters for the installation were the support spacing and water¬to-solids ratio of the grout, which controls the grout strength and ultimately the maximum capacity of the support. The study clearly shows that at this mine a 24-inch diameter support is fully capable of providing adequate ground control under depths of cover of 750 ft as only 50 pct of the available support capacity was utilized outby the longwall face. It was also shown that the 2.00 to 1 water¬solids ratio, despite providing a slightly softer support response, is sufficient for maintaining the same degree of roof control provided by the traditional 1.75 to 1 grout mix. The 10-ft spacing of the supports did not cause any ground control problems outby the face. However, inby the face the performance of the support is degraded once the peak capacity of the supports is exceeded. It appears that the large load shedding behavior, which is characteristic of this support following peak loading, allows the immediate roof to separate. When this happens, the support is unable to regain control of the roof in time to prevent failure of the immediate roof beam.

Jan 1, 2003

By S. Y. Liu

Based on the underground measurements, physical model test of simulated materials in the laboratory and finite element analysis, this paper demonstrates the pattern and process of roof fall over the tip-to-face area in the fully mechanized coalfaces. The main factors that affect the stability of the immediate roof are thoroughly studied including the fracture position and tilt angle of the main roof, the tensile strength (Rt) and cohesive force (c) of the immediate roof, and type and resistance of the powered support. Based on these factors, this paper proposes the basic process of immediate roof breakage a. follows: As the longwall farm advances, the main roof breaks and then rotates. A tensile fracture zone will be formed in the immediate roof under the fracture line of the main roof. A caving zone (plastic zone) will be formed in unsupported area between coalface and canopy tip. If these two zones overlap the immediate roof will be completely unstable. Under this condition, powered support will have difficulties lo control the rod. Therefore it is necessary to study the process of immediate roof breakage and look for effective methods for controlling the roof.

Jan 1, 1990

By John Feddock

This study deals with underground mines and ground control and the prediction of seats strength characteristics based upon ultrasonic logging. Exploration drill holes, both rotary and core holes are used for proving coal reserves and determining mining parameters. Drill hole cores can be subjected to geologic study and geotechnical evaluation. In addition, the use of geophysical logging techniques is well established in determining certain strata characteristics and ground parameters. Since the characterization of the rock mass in mines remains insufficient in most cases, the idea of possibly characterizing the strength parameters without expensive core retrieval and laboratory testing is very attractive. The elastic properties of the rock can be deduced from measurement of the sonic velocity if the density is known. Through the use of in-hole geophysical density logging combined with an ultrasonic logging tool, the estimation of strata properties is possible. An investigation was conducted at two Indiana underground coal mines using geologic logging, geophysical logging, ultrasonic logging and laboratory strength testing to establish the strength properties of the roof rock. A specialized ultrasonic logging tool with one transmitter and two receivers enables nearly continuous readings along the length of the hole. The results of correlating portions of the data are presented. It is anticipated that the ultrasonic logging will be a supplemental tool for extending laterally the strata characteristics obtained from the existing program to surrounding areas. This may be obtained from the relatively inexpensive logging data obtained by ultrasonic logging.

Jan 1, 2003

By George Mishra

In mid-1979, J&L Steel Corporation experienced rapid failure of pillars after only two weeks' operation with a combination of full retreat and partial mining in the 154 Road Section of its Nemacolin Mine, Greene County. The rate of advance of the "squeeze line" was very rapid for the first 500 feet, but it slowed down considerably as it approached some large coal blocks and gas well barrier pillars. After moving all section equipment out tc a safe distance, all accessible headings and crosscuts were filled with cribs and tri-set posts to prevent further spread of the squeeze. At one time, there was great apprehension by mine management that without implementation of some immediate corrective measures the safety of #3 airshaft and the only haulage and access road to the 8 Road mining area would be in jeopardy. In September, 1979, the U.S. Bureau of Mines established convergence monitoring stations outby the squeeze area. Readings were taken at weekly intervals until January, 1980, and monthly thereafter. Mathematical calculations, plot- ting of contour maps, and graphing of total convergence at different stations were used to follow and analyze the progress of the squeeze and provide guidelines for mining plans in the adjoining areas.

Jan 1, 1981

By Robert A. Stansbury

The No. 14 Mine of United States Steel Corp- oration is located at Munson, West Virginia, in &Dowell County. The mine was originally assigned 7,530 acres of Pocahontas 3, 4 and 5 Seams. Production began in 1948 with a listed reserve of 64,500,000 in place tons. It was originally intended to superimpose the 4 Seam workings with the 3 Seam and mine the two simultaneously to minimize the effects of one seam on the other. Roof control problems and economic conditions led to the discontinuance of 4 Seam workings in 1958 and again in 1980, after reopening in 1972. With room and pillar mining continuing in the 3 Seam, the majority of the 4 Seam reserves have been undermined. The mine began operation using track mounted conventional equipment. Off-track conventional equipment was next used, and in 1957 continuous miners were introduced. Since this tie, continuous miners on room and pillar work have been exclusively used with the exception of a brief shortwall trial in the 4 Seam in 1975. By mid 1982, 3 Seam mining will have been completed, leaving an estimated 42 million in place tons of 4 and 5 Seam coal as the focus of mining efforts. Plans for the subsequent reopening of the 4 and 5 Seam workings have been scheduled for June of 1981. With the resumption of mining, management is faced with solving the roof support problems caused by both the inherent physical conditions and undermining.

Jan 1, 1981

By Bruce K. Hebblewhite

Underground pillar extraction continues to be practiced in Australia in various forms, in spite of the dominance of the longwall method, which accounts for over 90% of underground production. Australia has been a leader in various forms of pillar extraction, particularly since mechanisation was introduced to the industry during the middle of the 20th century. Historical developments have included a range of thick seam mining pillar extraction methods; the widely adopted Wongawilli method; early adoption of the use of mechanized breaker line supports; and a number of more recent partial extraction systems including what is known as rib stripping. Current pillar extraction practice varies from total extraction panels to various forms of partial extraction, driven by either surface subsidence constraints or by underground conditions. This paper reviews a number of different pillar extraction methods and mining layouts, and also highlighting a number of fundamental geotechnical principles which have been found to be critical for safe pillar extraction ? often through lessons learned the hard way, as a result of back analysis of different accident scenarios. These principles include the all-important understanding of the location of the goaf (gob) edge at any point in the operation, and the management of that goaf edge; the impact of geological structures on local and regional stability; the critical importance of extraction lifting sequences; the role of stooks (remnant pillars); and a number of other factors. The paper will also briefly reference the educational and training practices adopted in Australia aimed at the continuous improvement of pillar extraction safety.

Jan 1, 2011

By Edward S. Custer

Recent investigations of mine roof falls have demonstrated that roof failures are generally associated with geologic conditions. The roof falls can be predicted by using the techniques of depositional modeling for the coal bed and roof rocks, and by lineament analysis. The ability to predict potential roof failure areas will allow the engineer and mine planner to develop a mine plan to avoid the roof failure areas and thus reduce overall mining costs. The ability to avoid roof failure areas also will permit maximum recovery of the coal resources.

Jan 1, 1981

By André Zingano

This paper analyses possible causes for floor heaving in room-and-pillar coal mining. The coal mines are in the southern part of Brazil, where the Barro Branco Coal Seam varies in depth from 20m to 400m, and a very thick volcanic formation overlies part of the sedimentary basin. The erosion of part of the volcanic formation causes this large range of overburden thickness, which formed large V-type valleys. The mine in study is located near the slope of one of these valleys. The mine inclined shaft is at the lower overburden thickness area of the valley and the mining is developing toward the thicker overburden area. The floor heave occurred in the crosscuts, which are parallel to the slope valley direction. Numerical modeling was applied to study the stress distribution around the openings; and fieldwork on the floor heaved entries was carried out. The geometry of mining (pillar and openings width) also was analyzed. The results showed that the very weak rock and the high horizontal stress were the causes of floor heave. As the mining gets near the valley slope, the stress distribution around the crosscuts changes considerably. In addition, the stress distribution changes as the mining goes far from the crosscut. Keywords: floor failure, room and pillar, finite element model

Jan 1, 2002

By Al Campoli

One of the critical variables affecting resin bolt performance is the size of the resin annulus, defined as the difference between the hole and bolt radius. The vast majority of the millions of resin roof bolts installed each year in the United States employ an annulus between 0.125 and 0.188 inch. As the annulus size decreases the resin flow around the bolt becomes more difficult, increasing the resin pressure induced by bolt insertion. As the resin pressure increases the loss of resin into bedding planes and vertical fractures in the rock increases. As the annulus size increases the mixing of the resin paste and mastic becomes less efficient. Thus, the lower and upper limits of the optimum annulus size is determined by insertion pressure and mixing considerations, respectively. Previous U.S. Bureau of Mines researchers have concluded that a 0 125 inch annulus is optimal. However, others have recently reported that a 0.095 inch annulus with special rolled deformation (SRO) bolts provide superior anchorage to standard rebar with a very large 0.250 inch annulus, attributing SRD resin loss in weak ground to oversized holes. The effect of annulus size on unit length pull strength and resin loss to fractured rock was investigated. Short encapsulation pull tests with resin annuluses of 0.125 and 0.095 inch resisted an average of 2.25 tons per grouted inch, with no significant variation. However, annulus size has shown a dramatic effect on the magnitude of resin lost to simulated rock fractures. No. 5 standard . No. 6 standard, and No 7 J-Bar rebar were inserted into two different viscosity resin cartridges designed for 4 foot boll encapsulation into a one inch inside diameter metal tube, To simulate a rock fracture, a 0.250 inch diameter orifice was located near the capped end of the tube Resin loses increased as annulus size decreased, with a minimum loss of 35% and a maximum loss of 60% associated with 0.125 and 0.095 inch annuluses, respectively.

Jan 1, 1999

By Paul L. Tyrna

The Fola Coal Co., LLC No 2 surface mine, located in Nicholas and Clay Counties, WV, exploits up to nine coal seams by contour, highwall, and mountain top removal mining methods. After encountering faults, areas of highly fractured coal, and subsidence failures. the mine requested a lineament analysis from the Mine Safety & Health Administration's (Pittsburgh Safety and Health Technology Center) Roof Control Division. The 31 km2 study area was viewed on a LANDSAT-5 band 4 (near infrared, 0.76-0.90 µm wavelength) satellite image with ERDAS Imagine 8.4 software. The software allows image examination under artificial sun azimuths and elevations, resulting in variable tonal characteristics. The study area image was viewed in 45° azimuthal increments from 000° to 315° at artificial sun angles of 10°, 30°, 50°, and 70°, yielding 32 separate images. The interpretation process developed by MSHA Roof Control personnel identified 16 northeast-striking and one northwest-striking lineament that intersected mine property. Faults and areas of roof instability corresponded well with lineaments identified in the study. Successful lineament identification prompted mine management to incorporate lineament analysis as a mine-planning tool for adjacent property, thereby identifying in advance areas that could adversely affect safety and production.

Jan 1, 2001

By Peter Cain

In 1993 an opportunity presented to monitor the effects of undermining an abandoned roadway by a longwall panel in the Sydney. N.S. coal field. Interaction subsidence data was collected by survey leveling to benchmarks in the undermined gateroad as well as by a standpipe system equipped with 15 vibrating wire piezometers measuring changes in elevation in the gateroad relative to an outby header tank. The piezometers were monitored at 30 minute intervals using a datalogger. while the leveling surveys were conducted weekly over the life of the project. The monitoring program resulted in the collection of a very large both of data. relating vertical subsidence to the relative position of the underlying face. The most basic analyses derived a travelling subsidence curve from the leveling data only. which hinted at a relationship between rate of extraction in the panel below and the amount of strain developed in the undermined strata. The complexity of the relationships between the amount of subsidence obtained, the location of' the individual monitoring stations, the rate of face advance below and the potential for "periodic" subsidence due to geological conditions has made analysis difficult. and the amount of data has compounded this problem. Recently. an analytical approach using a commercial neural network analysis package has been applied. which appears to have resulted in the ability to draw sonic meaningful conclusions from the data set. In this paper a short history regarding the collection of the data set is followed by a brief' description of neural networks. This is followed by a description of the several analyses performed. some of the results obtained, and conclusions drawn.

Jan 1, 1998

By Garth Kuhnhein

The Black River and Maysville underground limestone mines operated by Carmeuse Lime and Stone, Inc. in northeastem Kentucky are two of the largest underground limestone operations in the United States. The combined output of these two options has consistently averaged over 5.5- million tons per year for the last decade. The mines operated beneath 650-ft and 1,200-ft of overburden and are affected by high horizontal stress. In 1982 changes to the mine design were made at the Maysville mine to improve ground conditions. A series of design changes were initiated in 2002 at Black River to improve ground conditions. This paper reviews the results of the changes made at Maysville, recent horizontal stress measurements conducted at the Black River mine, and the recent modifications to the Black River mine design. Roof fall statistics at both operations are used to compare the distinctly different mine layouts used at the two mines.

Jan 1, 2004

By Bruce Hebblewhite

An ACARP-funded industry research project in Australia recently investigated the problem of premature rock bolt failures in underground coal mines. The project carried out limited sampling from 9 different collieries in NSW and Queensland, followed by a more comprehensive, controlled sampling from one colliery. Over 50 broken bolts were collected and subjected to metallurgical analysis at UNSW to determine both the steel composition and to analyse the fracture surfaces. Prior to this study, there had been anecdotal evidence of similar bolt failures from a number of sites within the industry. The sampling and analysis conducted by this project - whilst being below a statistically representative sample of the entire industry bolt population - highlighted the fact that bolt failures were continuing to occur at a number of sites, in both older and recently installed bolts. The failures were almost entirely attributable to stress corrosion cracking (SCC). It appears that a combination of factors - including steel composition, bolt manufacturing process, loading environment, geological and hydrological environment, bacterial corrosion - can all contribute to this problem. There is no simple solution, however, as with similar research findings from the UK, it appears that steels with higher values of fracture toughness, or Charpy Index, are less susceptible to SCC. Testing to date has been limited to bolt failures below the resin encapsulation horizon. A real concern is the potential extent of the problem at higher horizons, within the encapsulation. These may require some form of non-destructive testing (NDT) for detection. Initial trials in Australia of the ultrasonic device developed by DMT in Germany have proved encouraging. Further testing using this device is anticipated in future research. This paper describes the outcomes of the project, completed in 2002.

Jan 1, 2003

By T. L. Dobecki

As a means of demonstrating the effectiveness of reflection seismology in determining continuity of coal seams, three U. S. field projects are reviewed. The three projects involve coals of varied thickness (2-14m) and age (Pennsylvanian to Eocene) from three coal areas of the U. S. (Pennsylvania, Wyoming, and Washington). Each project also employed its own particular seismic technique, recording system, and seismic energy source although all are considered state-of-the-art, high resolution, digital seismic surveys. Project 1 (thin, Pennsylvania coal) sought detection of sand channels using dynamite and standard in-line (2-D) seismic technique. Project 2 (thick, Wyoming underground coal gasification) involved a gas-explosion (Dinoseis) source with areal (3-D) acquisition methods. Project 3 (thick, Washington underground coal gasification) employed a shotgun-type source and standard in- line methods. Data processing was handled by different contractors for each project. Each project was successful in accomplishing its own particular objective; however, data quality and interpretation seem to be more a function of thickness of the target seam, complexity of the overburden, and processing contractor than a seismic source, acquisition scheme (2-D versus 3-D), or recording instrumentation.

Jan 1, 1981

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