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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 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 Lu Jiam Zhang

Coal and gas outburst is one of the serious natural disasters in the coal mines of China. Since 1950, more than 10,000 coal or gas outbursts have occurred in more than 200 underground coal mines of different types. The outburst prone coal mines are widely spread out. It occurs in different types of coal -ma and is very serious in some areas. Most outbursts occurred at the time of roadway driving in coal seam, serious outbursts occurred mainly when entering the coal seam during the cross-cut driving; of the outbursts, coal and gas outbursts We UP major proportion, while pressing out and pouring out ah account for a large proportion; outburst. occur under different operating conditions, but mostly occur during shotfiring. This paper introduces the prevention and control measures as practiced in China and illustrates the three types of outburst cla8sification. Both technology and management are equally important in outburst prevention and control. Measurement of the volume of drilling cuttings and the initial gas outflow rate fm- boreholes have proven to be effective. A special team and organization must be set up to deal with the problems.

Jan 1, 1987

By Nikolaos Polysos

For the planning and driving of gateroads supported by roof bolts a practical concept has been developed in order to determine the geological and rock mechanical parameters derived from exploration drillholes. Based on this, an assessment scheme for the design of roof bolting has been established. The new methods of investigation comprise of geotechnical logging of drill cores for the determination of structural-geological rock characteristics and complementary geophysical well-logging which enables the evaluation of petro-physical and rock mechanical properties. The tools and their measurements will be briefly presented. The relationship between the geophysical log readings and the geotechnical parameters will be illustrated. The aim of the data acquisition is to improve roof bolting design which is based on the assessment scheme. This will be adjusted to the prevailing rock mechanical conditions and stands up to the required stability of the gate roads. In order to ensure consistent data acquisition, a method of core logging has been standardized for the respective demands. To achieve this it was necessary to introduce specific parameters and new methods. For the recording of linear and planar structural elements at the drill core, a special system for the documentation of the angles in relation to each other has been set up. The rock mechanical parameters and their acquisition will be explained. In order to carry out an objective analysis as fast as possible, software has been developed for the acquisition, management and presentation of the data. These programs and their functions will also be presented. The software is characterized by being user-friendly and the flexibility of its interfaces, Different options for the combined presentation of input and evaluated data give a more objective basis for the planning of roof bolt design. Examples will illustrate the operational implementation of the results during the planning and driving phases.

Jan 1, 2001

By Stanley J. Michalek

Good planning is critical to operating a safe and efficient mining operation. As the shallow coal scams become depleted, attention turns toward deeper mineable seams or remnant coal reserves. It is becoming more common to place a mine in a scam that is below or adjacent to existing mine workings. Significant research has been conducted regarding the ground control issues of multiple scam mining. However, safety aspects of the operation may not be fully considered. Safety issues include working below or adjacent to a flooded mine and controlling gas Communication between the active mine and the nearby mine. This paper will discuss the safety issues a mine operator should consider when working under or adjacent to mines that may be flooded with water.

Jan 1, 2000

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By Dennis Dolinar

"Tailgate entries in U.S. longwalls can be subjected to high loads and deformations. Therefore, tailgate entries are generally supported with varying types and amounts of standing support. However, even with standing support, roof falls and blockages can occur in the tailgate when the support system is not properly designed. Safe and efficient operation of a longwall face requires the selection of the appropriate type and amount of standing support. Because of the importance of maintaining a safe and functional tailgate entry, the National Institute for Occupational Safety and Health (NIOSH) is currently developing a standing support design methodology based on the ground reaction curve. Using the ground reaction curve for design requires that the ground and support interaction be quantified. This can be accomplished by the installation and analysis of data from instrumented field sites. Although necessary, the field results cannot develop the complete ground reaction curve. Numerical models, calibrated to the field results, are used to establish the complete ground reaction curve. In the present study, data was obtained and evaluated to establish points for calibration of the ground reaction curve for two western longwall operations.One test site, Mine A, was located at a depth of 1,550 ft. At this site, the floor consisted of siltstone, while the roof was an interbedded siltstone and sandstone. Because of the depth, a double row of22-in diameter CANs1 were installed to support the tailgate. Both the first and second longwall panels have mined past the instrumented site.The second site in Mine B is at a depth of only 760 ft. The floor at this mine consists of a weak shale/mudstone and the roof consists of thick, top coal overlain by weak shales. At this site, a double row of pumpable cementitious cribs was installed. The second test site has only been subjected to side abutment loading from first panel longwall mining.The measurements indicate that the support at both mines was heavily loaded when subjected to the side abutment loading from first panel mining. At the deeper site, the support began to yield when the face was more than 300 ft outby during second panel mining. In comparison to results from previous test sites in the Pittsburgh Seam and Illinois Basin, the amount of support loading and roof-to-floor convergence was an order of magnitude higher at the western sites. This has resulted in a significant difference in the position and shape of the ground reaction curves."

Jan 1, 2010

By E. Bane Kroeger

Rib failure is a hazard that has resulted in many injuries and fatalities in underground coal mines, especially in soft coal seams. Conventional rib control methods including rib boarding, shotcreting. steel meshing and geogridding, are fairly expensive and time consuming to install. Some methods, such as rib boarding, have met with marginal success at controlling rib failure. Under this research, the Rib Strap System (RSS) introduced by Dr. Chugh in 1999, was modified and new components were added. Both bench scale and full size prototypes were tested in the labs at SIUC to determine the best materials for fabrication and easier methods for assembly and installation. This laboratory research has developed new methods of securing the strap ends to smaller rib hoards, a new device to post-tension two straps together, and a technique for replacing straps without having to drill new rib bolts or for retrofitting straps to rib hoards that are already in place. The method used to secure the strap end to rib boards developed during bench-scale testing was dubbed the wrapped staple technique. It is very simple, requiring only 10-15 staples, with the end of a strap secured to a rib board in about 45 seconds. Several different prototype designs for a new device to join the strap ends together and tension the strap were fabricated and tested in the labs at SIUC. The device, dubbed the rib strap buckle, was very easy to install and held the straps firmly. One specific buckle design worked extremely well during testing and exceeded all expectations. The laboratory research also developed a method of replacing a damaged rib strap that can also be used to add rib straps to existing rib boards. The technique uses nails, spacer blocks, and band clamps to secure the new board and strap. This technique was developed to allow one person to replace a strap using only a hammer and screwdriver, negating the need to drill and install a new rib bolt using mine equipment such as a roof bolter or scoop. In addition to the technical feasibility, the cost of materials for the rib strap system was compared to the rib boarding method currently being used at a mine in the Illinois Basin. The mine is using 20 rib boards on each pillar at cost of about $223.60. The costs for the rib strap system depend on the materials chosen for fabrication but for the base case, were about $97 per pillar.

Jan 1, 2004

By Gregory J. Chekan

Developing a coal seam that has been influenced by previous mining in seams either above or below can result in severe ground control problems. In many instances, interactions between operations are inevitable. but improvements in ground stability can usually be achieved by-predicting potential problem areas in advance and adjusting the mining plans accordingly. This report investigates four case studies involving interactions between operations that used both room-and-pillar and longwall mining methods. In each case, geologic and mine design parameters are addressed, and those parameters found critical to the interaction are further defined in relation to observed ground problems.

Jan 1, 1990

By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

Jan 1, 2002

By P. E. Christensen

Due to ever-increasing stripping ratios associated with the No. 8 Seam at the San Juan Mine surface operation, BHP Minerals decided to implement longwall mining for the extraction of the substantial future reserve base. In preparation for longwall developments, BHP opted to first develop a small-scale pilot operation within the No.8 Seam on an adjacent lease to address geotechnical/geological concerns related to ground control, subsidence, caving, panel and entry design, and operational issues related to entry development and proper face equipment selection. Room-and-pillar mining methods are currently being utilized to develop a full extraction panel for a wide range of geological, geotechnical, and geomechanical evaluations aimed at characterizing and assessing the mineability of the No.8 Seam. Variable roof and floor conditions, coupled with a multi-bench coal seam, require that a number of mining horizons, opening geometries, and roof and rib support fixtures be evaluated relative to expediting gate and panel developments. Issues related to development of timely caves in areas subject to overlying sand channel deposits will also be addressed through underground in-seam stress and deformation measurements, and interburden and surface subsidence monitoring. This information will ultimately be used to evaluate face support and gate and panel pillar/entry development requirements. The current depositional model for the property, developed from an aggressive drilling, highwall mapping, and lineament evaluation program, will also be refined during the Pilot Mine study through extensive in-trine mapping, additional roof and floor drilling and physical property testing, and determination of the in situ principal horizontal stress field.

Jan 1, 1998

By Edward A. Barrett

A significant number of injuries resulting from fa118 of roof and rib may be avoided by modifying the reactive behavior of miners as they encounter potentially hazardous ground conditions. In 1984-85, the Bureau investigated barriers which prevent miners from recognizing and correcting groundfall hazards. One basic problem from which these barriers originate is the inability of miners to recognize dangerous roof and rib conditions. This study is a report of experimental research on the use of stereo scopic (3-D slides for presenting visual cues associated with hazardous roof and rib conditions. Findings reveal that, in comparison to conventional slides, 3-D provides for significantly greater hazard recognition among both experienced and inexperienced miners. Details such as the presence of t cracks and gaps in the surrounding rock are more accurately and realistically perceived. Acceptance by the mine training community has been affirmative and is additionally encouraged by the fact that 3-D slides stimulate trainees' overall interest.

Jan 1, 1986

By Dennis R. Dolinar

A room and pillar coal operation in central Pennsylvania was experiencing roof cutters and long running roof falls caused by high horizontal stresses. The roof conditions created hazards for the miners, and caused several production panels to he abandoned prematurely. The mine requested assistance from the National Institute for Occupational Safety and Health (NIOSH) in applying the "advance and relieve" mining to reduce the affects of the horizontal stress during panel development. The "advance and relieve" plan that was developed called for removing a pillar on one side of the panel as it was being advanced, thus creating a cave. The caving then relieved a portion of the horizontal stress across the panel. Because the horizontal stress direction is key to the success of this method, stress mapping as well as mining experience was used to establish the direction of the horizontal stress. Subsequent field measurements provided an estimate of the stress magnitude. Three panels were mined using this technique, and good roof conditions were achieved in all these panels. Instrumentation was used to monitor the stress changes in the roof created by the cave. Stress relief of over 1.000 psi was measured 120 ft from the cave and to depths of 20 ft into the roof. The lateral extent of the stress relief zone across the panels appears to be about 400 to 500 ft. This case history has provided much insight into the practical application of the "advance and relieve" method discussed in this paper.

Jan 1, 2000

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 Hamid Maleki

This study presents a historic overview of the role of mobile roof support (MRS) technologies in improving stability and worker safety and presents the results of recent field evaluations of the MRS load rate monitoring device and other remote deformation-monitoring techniques. Field studies were implemented at two sites in cooperation among researchers from the National Institute for Occupational Safety and Health (NIOSH), Maleki Technologies, Inc., and J. H. Fletcher & Co. The objective of the field programs were to (l) study the interaction between MRS's and coal mine strata and (2) develop and test suitable monitoring systems for assessing roof and pillar stability. An MRS consists of a roof canopy, four hydraulic cylinders, a caving shield canopy, and associated electromechanical systems mounted on crawler tracks. The machines are controlled by radio from a remote location and operate on self-contained power units. Typically, MRS's have capacities of 5,340 and 7,120 kN (600 and 800 tons). In comparison to posts, an MRS is capable of maintaining the yield load after significant amounts of roof-floor deformation. Because the mining cycle is accelerated, MRS's help reduce the potential for time-dependent roof falls. MRS performance has been monitored in the laboratory under controlled static loading conditions and in the field under deep, two-scam mining conditions. Laboratory studies have quantified support capacity and system stiffness as a function of machine height. Field investigations have focused on determination of optimum operating conditions and development of warning systems that indicate excessive load on the machine and/or impending roof-pillar stability problems. Analyses of field data show that roof instabilities are influenced by (1) pillar failure, (2) pillar yielding, (3) mine seismicity, (4) geologic structures, and (5) panel layout designs and mining practice. Pillar yielding and failure (unloading) and seismicity can be conveniently monitored by the load rate monitoring device, but for consistent detection of roof falls, additional deformation measurements directly within the cuts ate needed.

Jan 1, 2001

By K. Y. Haramy

Strong roof helps minimize roof fall problems in coal mine entries. However, the Inability of strong roof to cave readily may contribute to major ground control problems In Iongwall and retreat mining operations. The concern expressed by mine operators prompted this Bureau of Mines study to analyze the effects of strong roof members on ground stability around Iongwall openings. The problem was approached from three directions: development of a theory to analyze the strain energy conditions In the coal and surrounding strata, modeling the effects of different thicknesses and strengths of Coal and roof strata, and field Instrumentation of powered supports In two Iongwall mines. Results from the theoretical and modeling analysis confirm the accepted belief that a strong roof does, In fact, play a major role in high stress concentration problems In and around the Iongwall panel. Analysis of the effect of different roof overhang dimensions and properties on relative strain energy buildup In the coal and roof Is presented. Shield pressure data Indicate possible methods for detecting noncaving roof behind the shields. Strategies to monitor roof overhang and control dangerous conditions caused by the noncaving roof are also presented.

Jan 1, 1988

By J. S. Oram

Maltby Colliery, operated by RJB Mining (UK) Ltd is successfully mining the Parkgate seam in Yorkshire at a depth of 1000m using longwall retreat. Extraction in the Parkgate seam is influenced by interaction from previous workings in the Swallow Wood Seam 160m above. The paper describes in-situ stress measurements and simple gate road convergence measurements carried out to assess the extent of the interactive influence. The stress measurements, obtained using the stress relief overcore technique, were carried out from a maingate development roadway in an area where the stress redistribution under an overlying pillar and adjacent panel goat could be measured. The measured vertical stress distribution confirmed a significant interactive influence with high imposed vertical stresses under the overlying panel edges. Simple roadway convergence monitoring stations established in the gateroads of a retreating panel and in the development gateroads of the adjacent panel, confirmed higher rates of roadway deformation relating to overlying panel edges and pillars, particularly in the areas where a vertical stress window developed during panel retreat. In addition to highlighting a close relationship between closure rate and the measured vertical stresses, the convergence monitoring proved valuable in establishing characteristic deformation behaviour under specific interactive influences and also in defining the optimum location, extent and time of installation u£ additional roadway support and reinforcement. The information from the measurement programme has enabled the Mine Management to gain a better understanding of the interactive influences on roadway behaviour and was instrumental in designing the layout of panels in an area of the North East district of the mine where previous experience had indicated the combined influence of high interactive stresses and weak roof lithology resulted in difficult strata conditions both at development and on panel retreat. The planned location of panels and gateroads took into account both mining operational considerations and the need to avoid potentially high risk interaction zones as highlighted from analysis of the monitoring results. To obtain additional data on parameters influencing roadway deformation, extended roadway convergence monitoring has been established.

Jan 1, 1997

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 C. J. H. Brest van Kempen

The techniques developed should provide a useful tool, not only during the initial formulation of a suitable bolting plan for a new section, but also for periodic check¬ing on the utilization factor of hardware installed in the roof. Indications are that with present practice, utilization seldom exceeds 507.. Retro-fittable bolter equipment now available (FCBS?) allows doubling of the utilization factor in many cases. We are also introducing a computer pro¬gram based on the material outlined in this paper. This program can save a great deal of time in examining the real effect of implementing alternate bolting plans or using different installation techniques. It allows fine-tuning of bolt lengths, spacing and installation tension to mini¬mize roof support cost, while maximizing safe life of the mine opening. We have developed a procedure which allows easy, site-specific quantification of the need for reinforcement of a mine roof. The concept is based on the forma¬tion of a direct link between theory and empirical data to custom-fit each mine section. In its simplest form, the proce¬dure requires only recording of roof bolt torques in a sample working place (16 to 25 bolts) several times during the first week or so after initial bolting. The way in which the torque pattern changes during this time can reveal whether or not the bolts carry enough tension to maintain stability in that specific roof. From the same data, the nature of the anchorage horizon is derived. Specifically, shale or limestone can be distinguished from sandstone. If the procedure is repeated a few times (within the same mine section) with different bolt installation tensions, we can numerically characterize the roof in terms of one single value: an "effective angle of internal friction". This number can then serve to calculate trade-offs in bolting plan design, such as bolt length, bolt spacing and optimum bolt tension. Additional refinement is possible if a sagmeter is installed in the sample work¬ing place being monitored and if bolt anchorage capacity is determined. The lat¬ter can be done automatically on each bolt as it is being installed, using a system we developed and patented. Alternately, a number of pull tests could be performed. Using the sagmeter, bolt torque and anchor¬age data, it is possible to project the safe life of the supported opening. The details of the procedures described apply specifically to tensioned roof bolting, but a brief discussion paints the way for a similar analysis of full-column resin bolting also.

Jan 1, 1986

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