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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 Michael Alber

Coal resource extraction reaches higher depth of overburden and rockbursts as well as associated seismic events appear to become an additional problem to he addressed by ground control engineers. For example, German coal mines approach a depth of overburden of some 1400 m (4200 ft). Certain strata at those depths and associated high stresses may react upon resource extraction and subsequent deformation by violent release of strain energy, termed rodcbursts. Rock mechanics tests on numerous rock types have been conducted and three types of brittle rock behavior were classified. This paper describes different types of rock behavior in uniaxial compression and discusses a possible classification of rock with respect to their likelihood of violent response upon loading.

Jan 1, 2004

By Kenneth L. Johnson

The effects of underground longwall coal mining on flow in perennial streams within the Pittsburgh seam basin were evaluated. Four streams above four different longwall mines were studied. Overburden depths at the study sites ranged from 500 to 600 feet. At all four locations, premining streamfow measurements were made to establish baseline flow conditions. Measurements were then repeated at each location after the longwall face passed beneath the stream. At one location, in which the longwall panel axis was roughly parallel to the stream, the streamflow was monitored as the longwall face advanced. Measurements at this location showed that temporary flow reduction occurred as the face advanced underneath; however, after the tensile portion of the traveling subsidence profile passed, this effect was lessened. At two of the sites in which the stream was roughly perpendicular to the panel axis, postmining streamfow increased across the panel and decreased over the development headings, such that the net lateral change in streamflow across the fully subsided panel and associated development headings was small compared to the measured flow.

Jan 1, 1997

By Dale S. Preece

In 1975 Congress passed the Energy Conservation Act to establish a U. S. Strategic Petroleum Reserve (SPR) with a capacity of 750 million barrels of crude oil. The most economic storage medium was determined to be salt caverns leached in salt domes in Louisiana and Texas. Salt caverns existed at several sites when the reserve was created. These were obtained by the U. S. Department of Energy (DOE) and used to initiate SPR oil storage. In order to meet the storage capacity approved by Congress, new caverns also had to be leached. To support the resulting design effort, finite element computer program have been used to determine the creep closure and structural stability of salt caverns. Using site specific material properties including creep models, elastic moduli and fracture data, the finite element analyses have replaced earlier empirical approaches to cavern design. This report presents results of such finite element analyses to determine the best cavern roof shape and the minium pillar to dimeter ratio. , P/D. These numerical predictions indicate that the current cavern design is safe.

Jan 1, 1984

By Mingju Liu

In this research, we measured fractal characteristics of over ten typical outburst areas on different scales and related them to their outburst proneness, in attempts to provide new methods for outburst prediction. It was shown that both faults and folds follow the power law on coal basin scale, mine scale or mining district scale, whichever it is. Fractal dimensions are appropriate indices to describe geological structures quantitatively. Preliminary relationships have been established between outburst proneness and fractal dimension. Comparisons have been made among fractal characteristics and degrees of outburst proneness on all three scales. The results indicate that multi-scale assessment of outburst proneness can be made based on fractal measurements independently or supplemented by other assessment methods.

Jan 1, 2000

By Neil Styler

This paper presents an analysis of measurements of inter-burden de- formations above six longwall faces. An attempt is made to demonstrate some correlation between the movements at the various sites, and to examine their importance with respect to predicting caving height, disruption of overlying seams, and disruption of aquifers. This analysis demonstrates some significant differences between predicted surface subsidence, and inter-burden deformation. In addition it is shown that the caving height above a longwall face is equal to 8 to 12 times the extraction height, with a zone of fractured rock ex- tending to approximately 50 times the extraction height above the seam.

Jan 1, 1984

By Holger Witthaus

The conditions of great mining depth and multi-seam¬longwalling require a special method of rockbolting, It can be described as a control loop which is based on the elements of planning, drivage and measurement. Different planning elements such as the results of the observation of physical models, numerical programmes for calculation of stress-distribution and support success, lab and underground support testing and the knowledge of strata behaviour underground including geotechnical classification are used to get best results. The increasing geotechnical information collected during drivage of roadways are used to estimate and minimise the risks of support-overload. The repeated plan/acutal comparison during the use of roadway and continuously monitoring of support and roadways provided the opportunity of optimising support design during the process of drivage and if necessary in front and after of the longwall face.

Jan 1, 2000

By Tim Ross

J. M. Huber Corporation's Quincey Mine. an underground limestone mine located near Quincy. Illinois, has operated successfully and safely for many years utilizing roof bolts on a very limited basis. Over the years, the mine had experienced several rool falls that did not result in injury to personnel. Nearly all of the roof falls were limited to the first 2 feet of the massive limestone roof. Investigation of the falls indicated the major contributing factor to these roof falls was a randomly occuring thin. horizontal clay layer in the immediate roof. Over time, separation would occur at the clay layer. causing the root to sag. and eventually causing the root to fail. J. M. Huber Corporation contracted with NSA Engineering. Inc. to assist in a study to determine the effectiveness of Ground Penetrating Radar (GPR ) for locating these potential root fall areas so that mitigating action could he taken prior to roof failure. Multiple tests were conducted in areas of known roof conditions. GPR accurately predicted the existence or absence of root separation up to 1.5 meters into the immediate roof for 100 % of the known lest site conditions. As a consequence of this successful studs, J. M. Huber Corporation has used GPR to evaluate roof conditions at both its Quince and Marble Hill Mines.

Jan 1, 2002

By James M. Kramer

This paper introduces an analytical approach for the determination of the vertical stress in the chain pillar system between two adjacent longwall panels. The development of this approach is based on the principles of fracture mechanics and superposition. Some empirical formulae have also been incorpomted into the development of the mathematical model. The mine entries and longwall gobs are treated as Mode I cracks in an infinite plate while the resistance of gob material and the characteristics of the pillar yield zone are considered as the distributed forces on the surface of the cracks. An user- friendly computer program has been developed to facilitate the calculation involved in this task. This approach is as accurate as the numerical analysis methods but requires much less expertise and computer hardware and software to apply

Jan 1, 1998

By Vasundhara

A chance detection of a seismic event of unexplained origin in the mining district of Appin in New South Wales. Australia, led to a detailed investigation of recording and analyzing seismic events occurring in the area. Multiple events were recorded each day on the seismometer stations. The larger events were strong enough to be recorded on a number of stations spread out in the district. Background investigation of seismicity stimulated due to engineering activities revealed that similar cases of mining triggered seismicity have been reported previously from many mining districts around the world. If the tectonic stresses are close to failure in a region then a small perturbation of the stress state may lead to instability in the region which may manifest itself as seismicity. The parameters that control the occurrence of such siesmicity include tectonic stress magnitudes and orientations. geological weaknesses or structural features and the amount of stress contribution due to mining. This paper reports on the seismic investigative study conducted in the Appin area and considers its association with the geological layout of the region and the mining activity.

Jan 1, 2001

By Khaled Morsy

Most of the available numerical codes simulating the reinforcing effect of fully grouted bolt are accomplished by a series of bar elements which are connected along their length to the host rock by shear springs and sliders. The spring-slider system represents the shear bonding effect between the bolt and the host rock. This paper describes a three dimensional finite element modeling technique for the 3D simulation of fully grouted bolt reinforcement. The proposed technique introduced a physical simulation for the fully grouted bolts. The bolt simulation is composed of a series of beam elements (representing the steel rebar) embedded in brick elements (representing the grout). Because of the complexity of the grout/rock interface, ABAQUS explicit code was used. A modification of the ABAQUS default friction model was conducted to consider the cohesion of the grout/rock interface. The interaction between the grout and the host rock is simulated by the modified friction model. To verify the proposed technique, a simulation of the pull out test we conducted. The results from the simulation were compared with former studies

Jan 1, 2004

By Collin L. Stewart

A summary of the results of a limited subsidence monitoring program at Cyprus Amax Coal Company's Shoshone Mine No. 1 in the Hanna Basin of south central Wyoming is presented. The monitoring program was designed to meet regulatory requirements for confirmation of subsidence predictions made in the mining permit. The longwall panels were 380 ft to 600-ft wide. Mining heights varied from 11 ft to 13.5 ft. Gate road development included three-entry and two-entry head and tail gates. Overburden depth varied from 150 to 945 ft. The coal seam and overburden dip at approximately 11 degrees. The overburden is very low to low strength mudstone and carbonaceous shale with low to moderate strength, silty sandstone interbeds. Panel W/h ratios varied from subcritical to supercritical. Smax was 0.9 of the mining height. The subsidence ratio was about 0.6 for subcritical width. The draw angle on the up-dip side averaged 20 deg. The draw angle on the down-dip side averaged 12 deg.

Jan 1, 1995

By H. N. Maleki

Tension-torque relationships were investigated *n a coal mine for bath standard 5/8 in. (16 mm) bolts and a vibrating wire instrumented bolt, using conventional and controlled installation techniques. The instrumented bolts differed from the standard bolts by their head size and hole preparation procedures. The controlled installation technique developed by the U. S. Bureau of Nines (USBN) involved using hardened washers and lubrication in the bolt assembly, and maintaining a minimum thrust level during bolt installations. The object of this study was to determine the tension to torque ratio (lb/ft-lbs; for standard bolts and instrumented bolts for both conventional and controlled installation techniques, and to modify the bolt assembly so that the ratios would be equal for the instrumented and standard bolts for both installation techniques. It was shown that the instrumented bolts installed in faced holes had a similar tension torque ratio td that of standard 5/8 in. (16 mm) bolts installed in unfaced holes for each installation technique, and thus they were representative of other uninstrumentea bolts. This was achieved by evaluating the influence of several factors oh the relationship, including: facing the hale surface, introducing washers and lubrication in the bolt assembly, and applying different levels of thrust at installation due to bolter operator practice. The tension to torque ratio fur conventional bolts was only 21 (lb/ft-lb) for the coal mine, resulting in an installation tension of 4000 lbs (17.8 kg) with the typical installation torque of 150 to 400 ft-lb. This tension level was approximately 50 percent of the required bolt tension, and thus, it was concluded that torque standards are not sufficient to assure adequate bolt tension. .he low tension levels were due to the particular wait hardware in use, operator practice, and bolt fencing.

Jan 1, 1984

By Tom Denton

The bolt design criteria, since it was first published by Dr. John Stankus (Stankus, 1997), has been recognized as an important tool in ground control design. This concept is based on the Optimum Beam Building Effect (OBE), which is defined as the roof beam that has no separation above or within the bolted range and having the shortest bolt possible. This paper will present an application of this concept at the American Coal Company. Galatia mine. This mine previously used a 6' and 7' length bolt. However, since seam thickness has gotten smaller in the Southern area of the nine, a two-piece bolt is required. A two-piece bolt increases ground control costs and delays production. Therefore, a one-piece, shorter bolt, if feasible, can reduce costs and expedite development. In this analysis, Computer modeling is first conducted to determine bolt length and installed load. Second, a special bolt is developed, that can achieve the proper installed load as determined by the computer modeling. Third, underground bolt pull and load cell tests were conducted to evaluate bolt performance. Finally, an underground "bench" test was conducted. The test results have shown that entry convergence in the short bolt (5) test area is less than the 6' bolt area. Overall roof condition in the test area was as good as the 6' bolt area. Based on this analysis, the mine has converted to a shorter bolt in the low seam area and it is working well.

Jan 1, 2000

By Hamid Maleki

A novel ground control program has been developed at Plateau Mining Company through five years of geotechnical investigations. The study was initiated to determine the causes for roof stability problems and to develop alternative support and monitoring systems for increased safety and productivity. From systematic roof fall observations, extensive geologic mapping, roof drilling and monitoring ground movements along gate entries of four longwall panels, a sand- stone channel system was detected and shown to impact roof stability. In particular, underlying mudstones up to seven feet thick experienced instability due to differential compaction and load transfer from the gob. Roof bolter operators were trained to report roof lithology for mapping channel positions into the roof and for selective use of longer bolts. This boosted operator morale, and improved ground conditions. Monitoring rates of roof movement is used in these areas for detection of instability and for application of secondary support: This has significantly increased safety and productivity.

Jan 1, 1987

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 Anthony T. Iannachione

The location and time of 2,007 microseismic emissions from a limestone mine in southwestern Pennsylvania were compared with the development of mine faces and the characteristics of the mine layout. Based on analyses of these results, the occurrence of roof failure zones appears to be associated with certain characteristics of the mine plan. It was determined that significant relationships exist between the intensity of the microseismic activity and the scale of the roof failures. Microseismic activity associated with these roof falls occurs in distinct episodes, with the final failure event occurring during approximately a 12-hour period. Each roof fall episode appears to be composed of dozens of distinct roof beam failures. As each beam fails in shear and tension, tens to hundreds of audible noises representing rock fracturing or bedding plane separation can occur. While every roof fall can be viewed as unique, certain mechanistic similarities can be realized through careful observation and monitoring of these complex systems. Understanding these similarities in characteristics allows mine personnel to design the most effective and efficient control technique.

Jan 1, 2001

By Erdal Akyol

The rockbolting method of support is employed widely in several countries in different of mines. Hard rock mines use this method extensively. Soft rock mines, such as coal mines, have employed standing supports (steel sets) as the main method, although some countries such as the U.S.A. and Australia have preferred rockbolting. There is increasing interest in extending the rockbolting method in coal mines. The application of rockbolting techniques in an industry which traditionally has not applied such techniques, has required the development of a systematic design approach. This design approach uses extensive monitoring, to asses the viability of the support system in differing conditions. Currently at a number of new and existing mine developments in the U.K., rockbolting is being successfully applied. Although, inevitably some localised geotechnical problems have been encountered and have needed to be taken into account. The effect of faulting on roadway conditions is discussed in this paper and a design approach has been applied using a numerical analysis method, namely the Finite Element Method. The results presented in the paper serve as a useful guide to roadway support designers, and in particular the role played by different bolts in a range of conditions.

Jan 1, 1992

By K. Itakura

Information about geostructure for effective rock bolting must be obtained before setting the bolt into rock in a coal mine roadway. Especially, rock type variation and distribution of discontinuities such as layer boundaries, and separation of strata and cracks all represent indispensable knowledge. To achieve in-situ evaluation of roof rock, we developed an MWD (measurement while drilling) system using mechanical data from a drilling machine for rockbolt. System hardware detects torque, thrust, revolution, and stroke of the machine, and software analyzes the mechanical data log and displays locations of discontinuities using Neural Network techniques. Also, this system is able to estimate roof rock 3-D geostructure for varied rock types and discontinuity distribution in cases of a drill hole array arrangement in roof rock. We conducted a feasibility study for it using hydraulic drilling at roadways of the Taiheiyo Coal Mine, Japan. Simultaneously, some core drillings were also performed to confirm system analysis data. For core samples, distribution of cracks was observed and the borehole wall was investigated by OFS (optical fiber scope). Must discontinuity locations estimated from roof logging were conformed with data obtained from core inspection. However, the core sample contained a number of cracks which were not found in the mechanical data log. This indicates that additional cracks were generated during and/or after core drilling by the influence of rotational force of the machine and water supply. Furthermore, in comparison with the OFS image, resolution of the discontinuity's log estimated by this system was clarified; that is, it is difficult to detect small size cracks (hairline cracks) from a mechanical data log and the core inspection by OFS. This paper outlines the new hardware system for hydraulic drilling machine and compares the discontinuity map obtained from the software system with drilling core inspection.

Jan 1, 2001

By J. K. Owens

As part of a larger research effort focused on ground control, the U.S. Bureau of Mines is currently evaluating the effectiveness of using stereological analysis for characterizing mine roof strata. Input for a stereological analysis of a rock mass is best obtained by scanning (imaging) the walls of clean drill holes using a video-imaging borescope or other video device. Two classes of features are being investigated: (1) Lithologic types that comprise the overall rock mass and (2) structural discontinuities (e.g., joints, foliation, bedding planes) within a given lithologic unit. In the former, the volume fraction of a given Lithologic unit in relation to the entire rock mass can be estimated. In the latter, the mean intercept length between adjacent structures and surface area density of the geologic structures can be estimated. Examples of sampling procedures and subsequent computations are presented for several situations using information collected from drill holes in roofs of underground mines. One such example is based on the trace of a cycloid curve used as a stereological sampling path on the cylindrical wall of a drill hole. Stereological measures, such as intercept length, volume fraction, and surface area density, are explained in the context of rock mass characterization.

Jan 1, 1994