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By Duk-Won Park

A vast majority of the operating longwall sections use shield-type face supports to provide ground control in the United States. As a co-operative research program between the University of Alabama (UA) and Jim Walter Resources, Inc.(JWR), a joint research program is underway to study interaction of shields with surrounding rock masses. A series of analyses on the response of shields are performed using monitored data at a longwall face in a deep coal mine. In this paper, various effects of parameters, such as leg pressure, setting load, developing load, maximum loading rate, pressure in capsule, and convergence between canopy and base, are discussed and recommended as stability indices. A microcomputer program is being developed for instantaneous interpretation of the shield monitored data, and for giving warnings of potential problems. The preliminary results define the effects of the setting load, developing load, and rate of developing load or leg closure rate. The objective of the study is to understand the shield characteristics and their interaction with surrounding strata in real time, then develop a reliable program for maintaining longwall stability.

Jan 1, 1992

By Douglas Morrison

Hardrock mines in Ontario generally operate at between 3,000 and 7,500 ft below surface and generally experience significant rockburst activity below 5,000 ft. Since the introduction of bulk mining techniques around 1980, the safety record of Canadian hardrock mines has improved dramatically. However, the process of improvement was not a smooth one and, in the early years, the number of accidents and fatalities actually increased. One incident in 1984 that resulted in 4 fatalities in Ontario initiated a complete re-examination of mining practice and by the mid 1980s many changes had been introduced, driven by new non-prescriptive mining legislation. These changes affected not only the technical discipline of mining engineering but also the management of operational procedures and, together, they resulted in the significant improvements that have continued to this day. The analysis of the rock-fall related fatalities showed 5 major causes (loose, scaling, installing support, collapse and rockburst) and by the mid 1990s fatalities in all of these categories, except rockbursting, had been eliminated and even then, all these rockburst fatalities occurred in one mine which was closed shortly afterward. This paper discusses both the technical and social reasons for the initial failure and the ultimate success in improving the safety of these mines, with implications for mines in other major mining jurisdictions.

Jan 1, 2003

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 Leo Gilbride

Western U.S. longwall operators face increasing challenges with optimizing ground control and productivity as mines reach greater depths and coal bursting hazards increase. Some western U.S. mines, many known to be bump-prone, achieved a successful balance between ground control and productivity by transitioning to side-by-side longwall panel mining combined with a yield pillar gateroad system. With this design, development footages could be minimized and pillar bumping averted by controlled yielding at moderate depths, generally in the range of 450 to 600 m. Over the past four decades, the yield pillar system has won wide acceptance among western mines facing pillar bursting hazards, particularly those in the Wasatch Plateau and Book Cliff coal fields of central Utah. However, recent attempts among the deepest Utah operators to mine side¬by-side panels with yield pillars at depths in excess of 600 m have been met with mixed success and, in some circumstances, with serious difficulty. Challenges include violent face bursting and excessive tailgate convergence outby the face, which can be crippling to ventilation. The use of interpanel barriers, i.e., barriers left between longwall panels, offers one possible solution to mining under deep cover with bump-prone geology. Interpanel barriers have already been adopted by Utah's deepest longwall mine, and others are considering their use. The geomechanical implications of mining with and without interpanel barriers, and the competing tradeoffs between ground control and ventilation are discussed.

Jan 1, 2004

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 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 Axel Preusse

A high advance rate of the longwall face increases inevitably the mining dynamics within the rock and on the surface. Measures to limit the mining dynamics are the adjustment of the face advance rate to stretch the influences of mining over time and continuous winning to maintain a uniform influence of mining. This paper deals with the dynamics of mining subsidence within the range of a transmission overhead line for transportation of gasoline, diesel, kerosene, heating oil, mineral oil and other hydrocarbons. In a certain segment of the 24"-pipeline it was possible to predict, due to technical, operational and laying conditions, that undermining the pipeline would cause local stress maxima within the pipeline string. To limit these expected local stress maxima, the face advance rate of panel 497 was adjusted to the sensitivity of the pipeline towards the influences of mining. Comparison of the predicted mining dynamics, using the theory of stochastic media, with the measured ground movements led to the conclusion that the effects of dynamical influences which occurred within the range of the transmission overhead line and acted on it were low. This confirmed that the decision to adjust the face advance rate to the sensitivity of the transmission overhead line was correct (Kateloe et a)., 2002). A later relaxation cut in range of the pipeline siphon scarcely produced relaxation movements of the pipeline. The appointed sensitivity data of the undermined pipeline and the according maximum admissible face advance rate were thus correctly measured.

Jan 1, 2003

By P. M. Lin

The problem of abandoned mine land (AML) subsidence is getting more severe because abandoned mine lands which used to be located in the remote areas have been gradually developed into suburbs or even urban areas. Abandoned mine subsidence is also becoming more common than before because the time-dependent effect of exposed underground materials. These materials will deteriorate gradually and cause subsidence as time passes. Subsurface conditions of abandoned mines are seldom known to the public. Mine maps are not always available. If the mine mans are available, their accuracy are very - often questionable. In addition, the most common damages to surface structure and ground surface due to mine subsidence can also be created by other non-mining related geological hazards. These factors make the study of abandoned mine subsidence significantly complicated, tedious, and inconclusive. For instance, the cause, time, and amount of surface subsidence can be predicted with relative accuracy during active mining. However, it will be a considerable challenge to do so on abandoned mine subsidence. If abandoned mine subsidence can be predicted in advance, then the damages can be held to a minimum. However, this prediction technique is not yet well developed. Therefore, remedial measures require more attention. In order to have effective remedial measures, the real cause of the damage should be determined first. This paper describes the approach which was employed for the determination of abandoned mine subsidence. A site investigation was made first. According to the information obtained from the site investigation, a hypothesis was set up. Then the instrumentation was used to prove the hypothesis. By going through the procedures for several cases, a guideline which can be used by the inspectors to inspect abandoned mine subsidence will be established.

Jan 1, 1987

By W. G. Pariseau

A new approach to the direct inclusion of the effects of joints and geologic structure in rock mechanics analyses is applied to an elastic analysis of stability of a Lucky Friday undercut and 4111 longwall stope (WL) using the finite element method (FW). The approach is base+ on the concept of equivalent properties of a heterogeneous test volume, but does not depend on the assumption that a test volume is a representative volume element. Comparison with conventional FW results show greater variability in fields of stress, strain, displacement, safety factor and energy densities. Stability requires a knowledge of rock mass strengths as well as elastic moduli.

Jan 1, 1989

By Kevin Beauchamp

In many cases traditional support designs cover the many variations in ground conditions and bolt performance by a conservative factor of safety. The result is that most bolts are ineffectual most of the time and yet the response to occasional roof collapses is an even more conservative design. The probabilistic analysis presented in this paper takes several important parameters of a support design into account such as span, bolt strength, bolt length, water pressure, seismic accelerations, joint orientation, bed thickness and joint strength and allows the effect of variations in these parameters to be evaluated simultaneously. The analysis demonstrates the application of the probabilistic approach to both blocky, wedge-dominated conditions and bedded deposits dominated by laminations. All mine excavations have a designed stand-up time. By taking into account of the possible variations in the design parameters explicitly, it is possible to produce a support system designed to ensure stability over a given period of time. Rather than use a factor of safety criterion, the approach uses a percentage reliability or probability of failure. By understanding the probability of failure of the support system, it is possible to improve the performance of ground support without any increase in cost or conversely to decrease the cost while maintaining the performance of ground support. This approach reduces reliance on empirical experience and leads to a better understanding of the key factors that influence stability and can provide a much more accurate and optimized assessment of support design and performance. The result is a more cost-effective support system and we hope that a broader application of this approach to ground support in underground mines will lead to a greater acceptance in the industry of probabilistic analyses and of terms such as Probability of Failure and Reliability of Support.

Jan 1, 2003

By Thomas L. Hutchinson

The mechanical design of a roof support is basically a matter of a statics and dynamics problem, assuming of course, that the imposed loads and mining conditions are known. Here in the Appalachian coal fields the general conditions are known well enough to allow the majority of the proposed new faces of longwall to be furnished with standard commercially available supports equipped with suitable variants and options. While not trying to negate the primary importance of thoroughly investigating a potential face and the selection of the correct equipment for that face, it still becomes clear that an equally important aspect of the design of an adequate ground control program has in the past too of ten been overlooked, having been overshadowed by the ?endless? selection process of the roof supports proper. The equally important aspect which this paper discusses is the overall design of a mining system, which design incorporates not only the size and capacity of the equipment but the following points as well, namely: Equipment adaptability to mining scheme, Equipment va. human constraints, Operation at the designed levels, and Coordination of operation, maintenance and support design. Unless the coordination of these factors is considered in the design and selection of the equipment, a lopsided and inefficient operation is possible. How can we afford to put an 8-10 million dollar investment up to chance as to whether it will yield optimum results or not?

Jan 1, 1981

By Linsheng Xu

When coal seam is under the integral thick sandstone (or conglomerate), the structure of thick sandstone is integral and its strength is rather high. After coal is extracted, the roof is hanging and caving is difficult. When the method of full caving is adopted in the longwall working face to control this kind of hard and difficult caving roof, the effect of roof-controlling affects directly the production and safety. In China Datong Coal Mines the extracting coal deposit is Datong Jurassic period. Most of the roof is sandstone and conglomerate. It is hard and difficult to cave. Having used special hydraulic power supports and other special ways to deal with the roof, the hard and difficult caving roof has been controlled.

Jan 1, 1986

By Al Campoli

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

Jan 1, 1999

By 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

By Bob Thomas

The roof drill operator productivity can be enhanced by the introduction of control technology The operators control station can be improved by considering human factors such as handle forces and control location. The smaller electrical controls can be located much easier than the traditional hands on valves. The operators control can he improved by using processors with programmable subroutines that assist the operator in the decision making process. For example to increase feed rate - to slow rotation - to avoid clogged steels - etc. The overall affect is to improve drilling and bolting consistency. The control system uses a dedicated processor packaged as an intrinsically safe control station. The roof drill operator is assisted in the drilling; arid bolting by a closed loop feedback system. The processor monitors the drilling parameters such as feed force, feed rate, rotation torque, RPM, drill position, and vacuum condition Based on the drilling parameters the processor executes pre-determined condition statements intended to enhance drilling The drill operator has an active display to alert him to the real time machine status. For example if the feed were to slow or delay due to a high vacuum level the display would show "high vacuum". The dedicated processor is available on the arm feed style machines, The prototype machines used programmable logic controllers to develop the feedback relations. The ability to record drilling parameters was extremely helpful in this development project. The extended use of the recording feature will aid operators in mapping the mine roofs Interpreting the recorded drilling parameters can reveal a void in the top or hard competent drilling The prototype machines also showed the importance of vacuum monitoring This feature is extremely valuable in softer materials, The benefits of the control system range from efficient use of the consumable bolting materials to uniform operator performance.

Jan 1, 1999

By Maleki. Hamid

The U.S. Bureau of Mines implemented an inte¬grated static and geophysical monitoring program in a southern Wyoming mine to study changes in seismic wave propagation properties in roof strata. The purpose was to develop techniques to assess roof stability problems. The work consisted of record¬ing deformation and stress histories and calcu¬lating seismic velocity profiles. In addition. geology and roof conditions were mapped; the in¬struments were initially installed at the develop¬ment face; monitoring continued until the mecha¬nized retreat face retreated within 60 m of the instruments. Mining geometry consisted of a three-¬entry gate system using 5.2-m-wide pillars at depths of 426 m. Results were analyzed using stan¬dard history plots, velocity tomographs, and numer¬ical modeling techniques. Changes in velocity. frequency, amplitude, dynamic elastic modulus, and dynamic Poisson's ratio were related to roof defor¬mation, fracturing, and support requirements. A reduction in seismic velocity of 17 pct occurred after 60 mm of roof deformation following installa¬tion of supplementary support. Static measurements revealed significant dilation in the mine floor, spreading to the pillar and the mine roof. Failure was initiated in the floor as a result of uniform pillar penetration.

Jan 1, 1993

By Lance R. Barron

The U.S. Bureau of Mines, in cooperation with a central Utah coal .mine operator, began a study in July 1988 into longwall gateroad designs applicable to deep, bump- prone mine conditions. Prior to the study, four adjacent longwall panels, incorporating a variety of panel entry configurations using "yielding' chain pi liars, experienced severe bumps and coal outbursts at the face and in the tailgate pillars during panel extraction, resulting in several lost-time accidents and a fatality. To quantify those factors primari1y responsible for bump occurrences during panel retreat, a field investigation was initiated to confirm the anticipated performance of a two-entry gateroad system employing a large, nonyielding chain pillar design. Hydraulic borehole pressure cells and roof-to-floor closure [convergence) measurement stations were installed to monitor gateroad abutment loading and entry closure during panel extraction. Study results indicate that very high confinement of the coal seam by strong roof and floor members existing across the entire property, in conjunction with overburden depths in excess of 1.600 feet, provides for bump-scale energy storage in not only gate pillars, but along the entire periphery of the active mining area. Insufficient gate pillar designs were also identified as possibly enhancing the occurrence of bumps in the tailgate and adjacent face areas.

Jan 1, 1990

By Yong-Ming Jiang

Longwall mining systems are extremely complex and expensive and require great caution for reliable and efficient operation. In many cases, ground control problems are the primary concern in longwall mining. For example, unexpected threats such as roof falls or shield damage can cause sudden interruption of coal production and safety problems, which can originate from many parameters such as shield capacity, setting load, shield type, conditions of immediate and main roofs, pillar stability, geological anomalies, etc. However, it is difficult to correlate ground-control problems at a longwall face with the related mining and geological factors. In this paper, a method of correlation analysis, called tree classification, which permits the use of a large and variable number of features, is introduced The method provides an effective approach to produce an accurate classifier or uncover a knowledge structure, which will be used to predict future consequences depending on past and current data sets. The application of the tree classification method in longwall stability analysis is discussed, and the major procedures of constructing a knowledge structure are included as an example utilizing monitored leg pressure data in a longwall face. Also, the importance and ranking of the variables related lo longwall shield stability is discussed This is a major step towards developing an artificial intelligence system for real-time interpretation of shield monitoring data and hazard prevention.

Jan 1, 1993

By Steve P. Signer

A study of rock support interaction mechanics was conducted by the U.S. Bureau of Mines at the Jim Walter Resources No. 7 Mine, Brookwood, AL. Bolt loads and entry deformation were monitored at four sites in a tailgate roadway. Two sites were areas supported by fully grouted resin bolts, and two sites were supported by tensioned bolts that used resin-assisted mechanical anchors. The yield capacity of the tensioned bolts was 37 pct higher than that of the grouted bolts. To monitor loads, instruments were mounted on 16 bolts, which were installed during entry development in the standard bolt pattern. Additional instrumented bolts were installed as supplemental support just prior to the headgate pass of the first longwall. Instruments to measure roof movements included three-point roof extensometers and convergence stations. Readings were taken during section development and as each longwall panel passed the test area. The fully grouted bolts showed localized loads that were higher than loads on the tensioned bolts; however, overall loading magnitudes were similar. Many sections of the fully grouted bolts passed the yield point of the bolt steel (98 kN or 22,000 lb). However, the maximum strain measured at any loca¬tion was 50 pct of the ultimate strain, and there was adequate bolt length to transfer the high loads into the rock. Significant bending was detected at many points along the length of the bolts and seemed to be the most significant mechanism in support-rock interaction. Loads on the tensioned bolts did not exceed the yield point of 160 kN (35,000 lb). Entry deforma¬tion, supplemental bolt loading, and general entry degradation appeared to be slightly less at the tensioned bolt sites. Local geology and in situ horizontal stress were also observed to affect con¬ditions at the test sites.

Jan 1, 1993

By C. P. Mangelsdorf

Between June of 1975 and November of 1978 Stateham and Radcliffe of the Denver Research Center of the Bureau of Mines in cooperation with officials of the Bear Coal Company of Somerset. Colorado Conducted extensive studies of the roof conditions in the Bear Mine. The purpose of the investigation was to determine the relationship if any between the length of time the roof was left unbolted and the subsequent stability of the roof. Using the comprehensive information available from Stateham and Radcliffe (1), (2), (3), (4) the author has proposed a design for a roof truss installation which might have been used in lieu of some of the bolting in the areas where bad roof was encountered. Historically roof truss installations like bolting patterns have been applied rather than designed. The purpose of the present paper is to demonstrate how with the information available, one might proceed through a semi-rational design. In spite of the very extensive documentation by Stateham and Radcliffe, it was still necessary to make some working assumptions. It is, therefore, not intended that the process here illustrated be used without modification in any other mine nor is there any criticism implied or otherwise, of the way in which Bear Coal Company conducted its operations. The author has never visited the mine in question. This paper is meant to be an encouragement award a more rational use of roof trusses in situations where it is economically and structurally reasonable to do so.

Jan 1, 1984