Search Documents

By Matthew J. DeMarco

Research personnel at the U.S. Bureau of Mines have conducted longwall gate stability studies at four western U.S. mines attempting to employ yield pillars for improved ground conditions. Located in the Book Cliffs and Wasatch Plateau coalfields of north-central Utah, these operations are generally characterized by multiple-seam mining, cover depths averaging 450 - 600 m (1,500 - 2,000 fl), occasionally extending beyond 760 m (2,500 ft), abrupt cover depth variation due to the overlying canyon/mesa topography, and massive, rigid sandstone roof units. Such conditions encourage severe gate and panel coal bumping, as well as various types of roof instabilities and occasional floor-heave problems. To alleviate these adverse conditions, several mines have employed yield pillars to-reduce hazard-inducing excessive gateroad stress concentrations. This paper describes the evolution of gateroad designs at four of these operations, the specific mining conditions involved, and the general stability performance attained by using yielding gate systems. Mine experiences are "compared" in terms of basic setting conditions, and the general concepts of yielding system applicability and success potential are summarized.

Jan 1, 1993

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 Changshou Sun

Excessive deformation occurred in the underground openings of Jinchuan Nickel Mine, the largest nonferrous underground mine in China. Developed in weak rock formations at great depth, the underground openings are under great in-situ stresses with the horizontal stress being twice as high as the vertical one. Based on in-mine observations and in-situ testing results at the mine, time-dependent mite element simulations with coupled creep and plasticity constitutive models were conducted in this study to analyze the stability of underground openings In the study, the Burgers creep model and the Drucker-Prager plastic model were coupled to simulate underground drifts and cut-and-fill stopes. Two-dimensional finite element models were developed with ABAQUS - a comprehensive finite element simulation package The study investigated the behavior of underground drifts at great depth with high horizontal stress and the behavior of cut-and-fill stopes with various stope heights and fill conditions. The numerical simulations revealed important characteristics of opening deformations as a function of time after excavation and location relative to the wall of opening The results had good agreement with field monitoring data, They provided useful information to aid mine engineers in the design of mine openings, the selection of opening supports and the determination of support installation procedures.

Jan 1, 1999

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

By Keith Heasley

An eastern longwall mine was required to mine through a set of three entries located midway down the length of a panel. Of course, the mine desired to design a successful support system for the mine-through area at the least cost. To further complicate the design process, the mine-through entries were located directly under the end of a previously mined, overlying longwall panel and the entries were close enough to be subject to stress interactions from the overlying seam. In order to accurately evaluate the multiple-seam stresses and modify the mine-through support system appropriately, the boundary element program LaModel was used to model the stress and displacements in the two mines. Using the results of the model's stress analysis, the support density was systematically varied throughout the mine- through entries for optimum sup@ efficiency and economics. During the actual mining of the cross panel entries, the longwall experienced few, if any, additional delays, and overall, the support and mine through of the cross-panel en- tries at Harris #1 was very successful, saving the mine confineable time and money.

Jan 1, 2003

By William Gray

It has been ten years since the Mine Safety and Health Administration (MSHA) promulgated a safety standard requiring the use of Automated Temporary Roof Support (ATRS) systems in underground coal mines. The use of ATRS systems with roof bolting machines and continuous miners equipped with integral bolters has all but eliminated fatal roof fall accidents associated with manually setting temporary roof support. The last ten years have seen an evolution of the ATRS systems themselves and of MSHA's policy regarding ATRS systems. These changes are a response to operational and safety issues regarding ATRS systems often brought about by the ever-changing coal mining environment. As a result, MSHA's Technical Support personnel receive numerous inquiries from all segments of the coal mining industry on various ATRS topics. These requests for technical assistance range from an interpretation of a facet of the ATRS standard to an in-depth evaluation of a prototype ATRS system. This paper will address the aforementioned evolution of ATRS systems and will also summarize the factors MSHA Technical Support uses to evaluate these systems in the field. In addition, some of the more common misconceptions regarding the design, use, and regulatory requirements regarding ATRS systems will be covered.

Jan 1, 1998

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999

By S. I. Al-Ameen

Monolithic packing systems have gained in popularity as a gate side packing method, However, there has been no detailed investigation to assess their underground strength behaviour. This work includes the laboratory and in situ investigation for strength behaviour determined on various monolithic packing materials namely Tekpak-XX, Hydropak-20, Hydropak-30, Astrapak-3R, Flashpak, Thyssen pack and Aquablends. A simple technique for assessing the pack strength was developed at the laboratory by using a penetrometer, of which penetration resistance can be directly correlated with the uniaxial strength of the pack. The in situ strength investigations were made on core samples obtained from three pipes inserted inside the pack, while the penetration test were made on the pack surface. The in situ results indicated that the packing material develops an inconsistent strength on various part of the pack which was mainly related to an inadequate mixing of grouts inside the pack.

Jan 1, 1992

By Noor Mohammad

Comprehensive numerical modelling of the subsidence associated with longwall mining in UK Coal Measures strata has been conducted and validated against UK data. The Rock Mass Classification System (RMR) was used as the main basis to derive representative in-situ rock mass properties for stiffness and strength parameters for the numerical modelling input. A Finite Difference Method, Fast Lagrangian Analysis of Continua (FLAC) has been used to design a suitable non¬linear model and simulate the behaviour of the caved zone with a unique modification of the post failure properties within the zone of influence. The established model was validated for different configurations, varying depth, panel width and extraction thickness, in a sensible and realistic range. The Subsidence Engineers Handbook (SEH) and a stress abutment model were used as a validation tool in this respect. The approach developed will be modified for application to a number of different International Coalfields with different geological environments.

Jan 1, 1997

By John W. Fredland

Conducting mining operations under a body of Water can create hazardous conditions for miners. The potential for a sudden inrush must be considered and abnormal ground control conditions can be encountered. This paper synopsizes the factors which mine operators should take into account when evaluating whether an inrush may occur. It addresses the applicable Federal regulations and discusses some items which coal operators should consider in applying for a permit, Through collecting and analyzing the pertinent information, the likelihood of encountering problems can be judged. Through proper mine planning, the potential for creating hazardous conditions can be mitigated.

Jan 1, 1982

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 Edward McHugh

While less than 1% of reported accidents are associated with slope stability problems. slope failures were responsible for about 15% of all U.S. surface mine fatalities between 1995 and 2001. Small rockfalls, which may involve hand-sized rocks weighing only a few kilograms, can cause fatal injuries to workers away from the protection of large machinery. Massive highwall failures containing a million cubic meters of material or more can be fatal even for heavy equipment operators. As part of an ongoing study at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, several remote-sensing technol¬ogies are being evaluated as tools to monitor slopes for hazards and to assess slope stability. Field tests of a hyperspectral imager were conducted to assess its value for improving geologic maps of poten¬tially unstable alteration zones on mine slopes. An interferometric radar device capable of detecting very small displacements on slopes awaits final assembly and field testing. Computerized moni¬toring methods using images from digital and video cameras are being assessed for application to mine slope surveillance. These and other techniques will eventually provide new tools to augment current methods for monitoring ground control hazards in mines.

Jan 1, 2002

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

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 Y. P. Chugh

The paper discusses laboratory and field studies to analyze time-dependent deformation behavior of immediate weak floor strata in an Illinois coal mine. In the laboratory, core samples of the immediate floor strata were subjected to incremental creep tests under unconfined compressive stress. Both axial and lateral creep deformations were recorded. Time-dependent behavior was reasonably characterized by a Burger model with one Kelvin-Voigt unit. The elastic and viscous parameter8 of the model were found to be stress-dependent, but in the range of 40% to 80% of the failure stress, the effect of stress was relatively small. Therefore, a linear visco-elastic model was developed to represent the laboratory time-dependent behavior of immediate floor strata. Such a model may be acceptable for the design of partial extraction mining systems having extraction ratios of 40% to 60%. Field observations of roof-floor convergence (floor heave) at the mine also indicated a time-dependent behavior similar to that observed in the laboratory. Field time-dependent behavior was also characterized in terms of a Burger model with one Kelvin-Voigt unit to obtain scaling factors between the field and laboratory parameters. This approach should help in predicting likely floor heave in the mine based on laboratory creep tests. It was concluded that additional studies in the field are required to arrive at more reliable correlations between the two sets of parameters.

Jan 1, 1987

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 Yi Luo

The potential influences of mining subsidence on structural stability, integrity and functionality of guyed tower structures have not been systematically studied before. This paper presents a ease study where a guyed steel tower of a telecommunication company was mined under by a longwall operation. Prior to mining, every aspect of the subsidence influences to this structure was studied. The pre-mining assessment indicated that the structure could experience vibration problem in the second half of the dynamic subsidence process. Bawd on the study, a simple mitigation measure was recommended and implemented and the structure was successfully protected.

Jan 1, 2003

By H. Maleki

Gob pressure measurements were made in a Western U. S. coal mine as part of a long-term program to evaluate cave progress and to determine the influence of geological discontinuities on caving conditions, load transfer, and resulting instabilities. Gloetzl cells were selected for the measurements due to their simple, robust construction and a history of being able to monitor pressures in a broken medium. A methodology was developed for the successful installation and protection of the cells and hydraulic lines in the gob as the face retreated. The measurement indicated a cave progress controlled by the frequency of, major faults. The pressure-mining progress profile was compared to those observed in other parts of the world. It was concluded that the significant differences in profiles was caused by the thick-bedded strata, the existence of high lateral stresses and the spacing between faults. Recommendations are given for future applications of Gloetzl cells for gob pressure measurements.

Jan 1, 1984

By Richard E. Thill

Two Cross-borehole systems were developed by the Bureau of Mines, one operating at a central frequency of 20 kHz for close-in spacing between boreholes (2-20m) and another operating at a dominant frequency between 1-2 kHz for more distant range (40-300+m) between boreholes. The systems are specially designed for the expected ranges and high resolution required in mining applications. Potential applications of the crosshole systems are presented for a variety of geophysical measurements related to underground mining problems.

Jan 1, 1989

By Nikolaos Polysos

Besides considering the given geomechanical conditions, the geological/geotechnical logging of drill cores and heading faces during the driving of parallel entries and the resulting rock classification form the basis of the assessment of the proneness to caving of the roof strata in the actual mining phase. Both types of separation planes, parallel to the stratification as well as intersecting the bedding, will be defined and assessed within the immediate roof strata depending on the lithological structure. These data may hold the potential to plan the winning and design its operational organization in a way that - to a major extent - future damage of the workings by increased caving or cavities can be predicted in advance. Following the general description of the separation plane structure, the combined influence of the petrographic and structural geological roof properties on the operating cycle of the planned working face will be presented and illustrated.

Jan 1, 2003