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By Steve P. Signer

Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health installed 39 instrumented, fully grouted bolts at six test sites in a trona mine retreat panel to study mine roof stability for the improvement of workplace safety. Variables at each test site included bolt spacing, bolt length, roof span, and location in the panel layout. At most test sites, two rows of instrumented bolts were installed, one in or near the intersection created during development and the other in or near the intersection created during second-pass mining. The most significant factor affecting bolt load was roof span. The highest loads were on the bolts installed in the intersections of the 6 m-wide entry. Mining-induced stress resulting from panel layout was the next most significant factor affecting bolt loading. Minor varia¬tions in bolt loading could be attributed to changes in bolt spacing and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressional loading during gas bleed-off.

Jan 1, 2001

By Reha Ozel

The stability in longwall faces depends on the interaction beman the roof strata, face supports and floor strata. The load distribution on the other hand, depends upon relative stiffness of the three supporting elements namely, the coal face, support and gob. Therefore, in defining support requirements, it is particularly important to analyze the strata loading, the face convergence and the controlling mechanism provided by supports. The controlling mechanism however, depends on the type and characteristics of supports used in long- wall faces. The design guidelines presented in this paper have bean developed based on a number of strata control oriented projects carried out in Turkey, load and convergence measurements carried out in 10 longwall faces, in Zonguldak Coal Region, and procedures given in literature. Evaluation of load and convergence measurements, and analyses of face supports were carried out by means of a computer program, called L0NGWAI.L-MEW, developed by the authors. The output of the design analyses include the following: (1) estimation of support capacity for roof supports namely; individual hydraulic props, chock-type power supports and various types of shield supports, and (2) analysis of loading conditions and response of shield support components in terms of stability and structural integrity.

Jan 1, 1990

The applicability of four empirical strength criteria for rock masses, namely, Hoek and Brown, Bieniawski - Yudhbir et al., Rarnamurthy - Arora and Johnston - Sheorey et al. has been tested against the triaxial test data for jointed plaster of Paris of Arora. The following modified Bieniawski criterion shows best agreement with the triaxial test data for jointed plaster of Paris:- where st = axial compressive stress at failure, MPa; s3 = confining pressure, MPa; scm = uniaxial compressive strength of jointed plaster of Paris, MPa; and B = - 3.988 + 9.674 (scm )-0? 185 The modified Bieniawski strength criterion can be used to estimate the strength of a coal pillar, based on a knowledge of stress distribution in it.

Jan 1, 1992

By Y. P. Chugh

About 60-70% of the underground mined coal in Illinois is extracted using a room-and-pillar mining method that permits extraction of about 50% of the coal. The remaining coal is left behind in the form of support pillars to control surface and subsurface movements. Illinois coal seams are typically associated with water sensitive weak floor strata whose thickness varies 0.31 - 1.3 m. The current mining geometry mostly uses pillars of constant size within a mining area. This traditional practice suffers from the disadvantage that pillars within the center region are subject to much higher loads than the pillars located on the outer edges of the panel. This results in a higher incidence of rib spalling, and roof falls, creating a potentially hazardous work environment. In addition, this concentration of loading at the center of the panel provides initiation points for floor failure to occur. The primary goal of this study was to reduce ground control problems and production costs in Illinois mines by considering alternate mining geometries. The development and subsequent field demonstration of an alternate geometry is presented in this paper.

Jan 1, 2004

By George J. Karabin

Since the inception of a qualitative pressure arch theory by Haak4 in 1911, much work has been done in an effort to apply this concept to underground mining. It has been used in both longwall and room and pillar mining to determine roadway and pillar stability (with varying degrees of success). This paper presents a summary of a case study where the pressure arch concept was extended in a totally qualitative manner to define the source of roof control problems in the mains of a Pennsylvania mine. It was concluded that, due to the method of mining employed, a pressure arch formed over a portion of the entries, and the high abutment stresses accompanying it resulted in extreme roof deterioration. The following discussion will include a general presentation of the arching mechanism, the physical response of the roof strata under mining-induced abutment stresses (both from the case study and in general), a computer simulation of ground stability under both "normal" and "abutment" stress conditions, and finally, potential means of alleviating such conditions.

Jan 1, 1982

By David Newman

The Southern Appalachian coalfield has a long history of coal bumps that are attributable to a unique combination of topography, geology, and multiple seam mining. The high pillar stresses that generate a coal bump result from: i. mountainous topography where the overburden commonly exceeds 1,500 feet beneath ridges, ii. multiple seam mining, iii. thin interburden between active and abandoned mines, iv. thick beds of competent sandstone and sandy shale strata throughout the stratigraphic section, v. strong coal seams, and vi. irregular pillar and mining geometry in older room-and¬pillar mines. Coal has been intensively mined for more than 100 years in Eastern Kentucky, Southwestern Virginia, and Southern West Virginia. As a result, it is commonplace to have abandoned mines above and/or below active operations. Therefore, those conditions that generate coal bumps will be more prevalent in the future, as underground mining continues in Southern Appalachia. The focus of this paper concerns two coal bumps that occurred in the Kellioka seam where both undermining and overmining had occurred. The purpose of the investigation is to determine the magnitude and distribution of vertical stress that initiated the bumps, to identify bump-prone areas in the unmined reserve, and to recommend pillar centers and mining layouts to avoid future bumps. The state of stress in three vertically adjacent seams was examined using the LAMODEL boundary element program. Large, detailed numerical models were constructed of both bump areas and of the remaining unmined reserve block. The results of the numerical analyses were compared with those obtained using analytical equations.

Jan 1, 2002

By John Latilla

Following its introduction on underground collieries, the mandatory Code of Practice to Combat Rockfall Accidents (CoP) was applied to all opencast operations of lngwe Collieries (the South African division of BHP Billiton Energy Coal). The CoP includes, but is not limited to, requirements to assess hazards, manage them and monitor compliance. Information on methods to measure highwall conditions and compliance with control measures on opencast strip mines, as opposed to open pit mines, was found to be anything but freely available. However, on the Ingwe underground mines, a system of rating physical conditions in each section and how well production personnel were managing the situation, has been used with success for some time. It was decided that a similar approach would be taken, and, as with the underground rating system, the resultant rating forms should not be aimed at rock engineering specialists only but should be able to be used by production personnel as well. This paper discusses the highwall risk rating method as evolved over a couple of years and highlights that it is a flexible tool that can easily be changed to suit an individual mine's requirements. It also shows how operational aspects are being measured to determine how well the identified hazards are being managed. Allied to the method is a relatively easy, and certainly "low capex," mapping technique which allows between 500 and 1000m of highwall to be covered in a day. Features generally recorded include lithology, bench height, work still to be completed, coal losses or "toes", presence of water, dolerite intrusions, existing and potential failures, prominent joints, declared special and cautionary areas, cracks on upper benches and condition of spoil piles. Highwall mapping and photography as well as basic geological data collection, enable highwall risk and operational ratings to be determined. The same rating sheets may also be completed by production personnel, but without detailed mapping.

Jan 1, 2002

By Buddhima Indraratna

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks.

Jan 1, 1992

By Deren Zhu

Based on the results of field observation and physical model analyses, a computer simulation method, FEAEBP, has been developed for simulating the breakage of the main roof by considering it as a Kirchhoff plate on Winkler elastic foundation. By this method various parameters of rock property, different boundary conditions of the working face and the initiation, development and results of the breaking process of the main roof with different geometric dimensions can be effectively simulated. Since the main and immediate roofs, and the coal seams are treated as an integrated mechanical system, the interaction among them can be studied in detail.

Jan 1, 1988

By M. Karmis

mining factors and representative ground movement parameters. Based on this analysis, the previously developed prediction methods were modified and tested for their applicability in the Appalachian coalfield. Finally, in order to facilitate the implementation of the prediction methods, as well as the data processing of the monitoring program, a number of computer software were developed and will be discussed in the paper. Surface deformations above underground mines and their prediction and control have been the topics of an extensive research effort by Virginia Polytechnic Institute and State University over the past few years. During the initial stages of this research, a number of case studies were collected from the literature, the mining industry and various government and state agencies and, based on the analysis of this information, appropriate prediction methods were proposed (Goodman, 1980; Webb, 1982; Hasenfus, 1984; Karmis et al., 1984). Although the prediction methods were based on sound concepts, it became evident that their accuracy and implementation were a reflection of the size and quality of the collected case studies. In this respect, the data bank was rather insufficient and posed certain questions regarding the development of subsidence information due to inappropriate length and spacing of monuments used in subsidence surveys, density and accuracy of such surveys, lack of monitoring horizontal movements, etc. In an effort to alleviate these problems, a comprehensive monitoring program was initiated in the coal mines of southwest Virginia, which included: - Two longwall and five room and pillar mines. - In total, seven longwall and sixteen room and pillar panels. - Monitoring lines in excess of 35,000 feet. - Vertical as well as horizontal measurements. A high accuracy digital computer tacheometer was used in all surveys and the type and spacing of all monuments were in accordance with well accepted guidelines. The data from this monitoring program completed the initially collected data bank and, thus, enabled a much needed statistical analysis between

Jan 1, 1987

By Michael Gauna

Ground fall injuries and fatalities in United States underground coal mines have shown a diminishing trend. Advances in bolting and standing support systems have led to improved safety in underground coal mines. However, the Crandall Canyon Mine disaster in 2007, in which 9 individuals were fatally injured as a result of a widespread collapse, emphasized that proper mine design is a critical issue that must be addressed. The Mine Safety and Health Administration (MSHA) has instituted technical review procedures structured to facilitate the implementation of safe mine designs in underground coal mines. The technical review process and the typical information required are outlined in this paper. The range of mining situations that lead to a technical review, common design methods submitted for review, and their relevant guidelines and limitations are also discussed. Additionally, the relationship between design criteria and mine site historical experience is discussed.

Jan 1, 2011

By Jinsheng Chen

For underground openings, many factors can cause roof failure and thus roof falls. The most common factors related to Mother Nature are weak roof lithology, geological structures, and an abnormal in-situ stress field. Theoretically, any underground opening can be successfully supported under any geological and mining conditions if the opening width is limited and the roof supporting system is well designed. However, roof control cost must be taken into account. In practice, to minimize the roof control cost, different roof control plans have to be considered to take care of the uncertainty or variation in geological and mining conditions. For most underground mines, it is unsafe or economically unrealistic to use one fixed roof control plan for the entire mine. Therefore, local, abnormal geological and mining conditions must be identified and a site- or area-specific roof control plan must be employed. This study tries to identify the major factors that have significant impact on entry roof stability by analyzing the historical roof fall data at Rofomex mine. Finally, the appropriate roof control plans or roof bolting systems for different geological and mining conditions are recommended.

Jan 1, 2011

By Erdal Unal

In gate roads where supports are subjected to dynamic loading four times due to retreating longwall panels, it is a real challenge to replace the rigid arches with flexible reinforcement systems and yet to provide safety and stability of these openings within better economical limits. The results presented in this paper are based on a three years of research project aimed at developing safe and economical support systems for gate roads in Çayìrhan coal mine. Çayìrhan -Mine Project includes laboratory and field studies, design analysis, pilot scale design applications and performance monitoring. Based on outputs of the design approach developed, four types of bolts, namely, Split Set, Yielding Super Swellex, Sis-Resin, and Fosroc-Resin bolts were installed in a gate road together with widely spaced yielding steel-arches. In order to compare the performance of these flexible support systems and to provide feed back for the design approach, a monitoring program was initiated.

Jan 1, 1993

By David A. Newman

Strength and physical property testing of coal is conducted regularly by the Arch Mineral Corporation as part of reserve exploration. Pillar design and mine layout are the primary applications for the coal strength data. Distinct differences in pillar behavior were observed during longwall mining at a mine in Southeastern Kentucky. Horizontal and vertical variation in both triaxial and uniaxial compressive strength was observed within the four (4) benches of the Harlan seam. Similarly, geographical variations in coal strength were noted across the Harlan seam reserve. During the past three decades substantial research efforts have been focused upon coal pillar behavior and the refinement of pillar design methodologies. Ironically, little attention was directed to integrating the variability of coal strength into pillar design. The authors examine, through the presentation of case history data, i) typical industrial practice with respect to establishing a database of coal strength for use in pillar design and stability analysis, ii) approaches in which the observed variation in strength in the Harlan seam is integrated into pillar design and stability analyses, and to iii) propose standards for coal cube testing

Jan 1, 1993

By Wm. Mark Hart

Under a specific geological condition, roof supporting method, and pillar-entry system, an entry convergence concept may be the most effective means that can be used to effectively indicate the longwall entry stability (i.e. pillar bumps, roof falls, or floor beaver This concept may also be used to verify the effectiveness of the current pillar-entry system and roof supporting method. Furthermore, convergence measured under different mining phases may lead to a better understanding of the longwall mining-induced abutment loads and may be used to estimate stress change in pillars and thus pillar stability. In this paper, a remote entry activity monitoring system was designed; a comprehensive entry convergence monitoring strategy was proposed; thee abutment loading phases in longwall mining were defined; the relationship between roof deflection and stress change in a pillar was derived, and finally the extent and magnitude of the mining-induced abutment loads as well as their impact on entry convergence were discussed using the field data obtained so far.

Jan 1, 1995

By T. N. Singh

With a view to producing increased target output every year, Indian coal mining industry is fast moving towards mechanisation but the present approach of mechanisation suffers from lack of standardisation, mainly due to nonavailability of the cutting data for various machine/rock combinations. The coal seams of the country are relatively hard in nature and the coaling machines are reported to be affecting production and productivity of the coal mine and the very trend of mechanisation. This paper presents the result of extensive study performed to visualise the cutting characteristics of coal seams of compressive strength ranging from 20 MPa to 65 MPa. In a specially designed drag bit type coal plough rig, continuous recording of cutting forces was done with the help of a microprocessor based data logger and the specific energy of cuttability was evaluated at different depth of cut and speed of cutting. The geomechanical properties such as compressive strength, orientation and frequency of cleats etc. and different indices of coal seams were measured in the laboratory and field and relationship was developed for easy determination of specific energy.

Jan 1, 1992

By Kenny D. Basnett

An analysis of the Roof Falls Data Base (RFDB) for underground coal mines in Illinois for the period 2004?2008 indicated that about 80% of the roof falls occur at intersections and are associated with geologic anomalies or a rapidly changing geologic environment. Most falls are associated with anomalies such as clay dikes, differential compaction faults, shear zones, joints and fractures, and geologic structure. Therefore, a better analysis of the geology with emphasis on mapping anomalies such as joints and fractures and their application is necessary to improve ground conditions and safety. Such studies within Illinois Coal Basin are limited. This paper presents a mapping study of the joint orientations with documentation of dip of the several lithological units that lie directly above the Herrin No. 6 Coal seam around southern Illinois. These units include the Energy Shale, Anna Shale, and Brereton Limestone. The mapping was done in three mines and the data were plotted on a stereographic projection to determine the average characteristics of joint planes. The study mines included two deep underground mines and one shallow highwall mine. This study has helped identify favorable mining and support orientations in areas of highly variable geology. These are currently under review by the cooperating coal company for experimental roof control studies.

Jan 1, 2011

By Anthony Iannacchione

As the amount of new fractured surfaces or "damaged rock layers" within roof rock increases, the stability of the rock mass decreases. While direct measurements of this phenomenon are not easily made, there is good circumstantial evidence to support this hypothesis. For example, it is common to observe increased cracks or fractures in the immediate mine roof rock before a roof fall. Likewise, roof drill holes placed in areas that later fail often reveal increased numbers and/or separations of fractures within the rock column through time. And finally, the frequency of microseismic activity, representative of rock fracturing, increases before a roof fall. For this study, more than 700 microseismic emissions were collected from two underground limestone mine roof fall areas in southwestern Pennsylvania. Microseismic events were located and magnitudes determined using the moment magnitude technique. Moment magnitude is based on the event seismic moment, which is a measure of the seismic deformation. The amount of new fracture surface length was calculated based on the stored strain energy within the rock prior to fracture. In the two case studies presented, a significant amount of microseismic activity was observed as much as two days before the first signs of failure in the roof fall areas. Additionally, results from this analysis reveal much about the behavior of strata prone to failure and allows for the construction of hazard maps based on microseismic emissions. The potential use of this technique as a means of anticipating roof falls is analyzed and discussed.

Jan 1, 2004

By Larry Stolarczyk

The mining industry would benefit greatly by imaging geologic conditions well in advance of mining. In layered deposits such as coal, trona, quartz, and potash, natural waveguides form and enable electromagnetic seam waves to travel great distances. In the early 1980s, the Radio Imaging Method (RIM) was developed to capitalize on electromagnetic seam wave propagation in the assessments of seam conditions in longwall panels. The initial applications of RIM tomography and comparisons to in-mine mapping of geologic conditions proved that faults, paleochannels, and rapidly thinning coal could be detected and imaged with seam waves. This paper describes the application of electromagnetic seam waves in mapping the margins of a paleochannel crossing a longwall panel and the interesting possibility of adopting the newly developed Full Wave Inversion Code (FWIC) )Newman 1995) to significantly improve imaging resolution. When tomography images are combined with ground control science, a significant reduction in cost, roof fall potential, and waste rock in underground mining may be realized.

Jan 1, 1999

By Tim Ross

Slurry injection into mined-out panels is an attractive alternative for disposal of coal preparation plant fines. however, in areas where surface subsidence cannot be tolerated, either by law or necessity to protect surface structures or aquifers, special consideration trust he given to the effect of slurry injection on panel stability. Slurry injection can lead to panel failure and unplanned subsidence by degrading roof, pillar, and floor strength. Water degradation is of particular concern where clay is present in the floor. Recently. unplanned subsidence related to slurry injection occurred at an Indiana room-and-pillar coal mine subject to weak floor conditions. This paper discusses the efforts of the mine operator to determine the causes of the panel failure and to design panels and injection procedures to allow for future economic slurry injection without subsidence. The numerical and empirical/analytical techniques applied give the operator a rational approach to pillar panel design for future injection panels under various cover depths and weak floor conditions.

Jan 1, 1998