Search Documents

By E. M. Williams

The U.S. Bureau of Mines is investigating the use of in-seam seismic methods to monitor stress-induced changes in coal near the working face of longwall mines. Seismic surveys performed through a longwall panel and pillars near the face can provide valuable information on geologic anomalies, high stress regions, tailgate support performance and other conditions that could endanger miners or interrupt production. This paper describes the details of the equipment and data analysis techniques used, and the significant results obtained and their implications for ground control. Using seismic tomography, it is possible to conduct measurements on the pillar and reconstruct an image representing the internal velocity distribution. Successive measurements over several days identify changes in velocity attributable to changes in stress. Several field measurements were conducted at a western U.S. longwall mine to observe stress induced changes within the panel and pillars during mining. A permissible data acquisition system was designed to meet mine safety requirements for operation in gassy air. Certification from the Mine Safety and Health Administration (MSHA) was obtained, and this approved system was used to monitor stress changes in several pillars as the longwall face passed during first panel mining. Tomographic surveys were completed across the longwall panel to map the apparent abutment stresses ahead of mining. Velocity distributions show an increase in vertical stress across the panel from the headgate to the tailgate. Additionally. a yield zone along the face near the tailgate and the forward abutment approximately 100 feet outby the face in the tailgate side of the panel were detected. These results agree well with data obtained from other instrumentation emplaced in the panel near the tailgate. Using velocities for both compressional and shear waves, the Poisson's ratio was calculated for the panel.

Jan 1, 1994

By Anthony Spearing

There are approximately 68 million rockbolts installed annually in underground coal mines in the USA (Tadolini and Mazzoni, 2006). The most utilized is the passive fully grouted resin rebar bolt, which constitutes almost 90% of the primary support in US underground coal mines. Resin-grouted rockbolts are a very effective and safe support, but their performance still needs to be further optimized and improved. This paper investigates their performance and suggests operating conditions to optimize the performance of resin-grouted rockbolts. In addition, this paper discusses a small-scale test that can also be used to evaluate the performance of rockbolts that is quicker, easier, and less costly than short encapsulation tests.

Jan 1, 2014

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 Hamid Maleki

Mining in the Book Cliffs Coal Field of Utah has historically been associated with seismicity because of high-stress environments and the presence of stiff, competent rocks. Recognizing the geotechnical risks in this coal field, mine engineers have dedicated considerable resources to the study of mine seismicity. In spite of these efforts, there are no engineering procedures for assessing mine seismicity for a new property and in developing mine designs. This paper presents the results of ongoing geotechnical monitoring and stress analyses at a western U.S. coal mine. The intent is to demonstrate the application of geotechnical methodologies in evaluating mine seismicity and stress-control technologies including evolutions in yield pillar designs for variable topographic conditions of the Book Cliffs Coal Field. We show that narrow yield pillars are quite effective in limiting strain energy accumulation along the gateroads and thus the potential for pillar seismicity. At the same time, the pillars provide sufficient resistance to control roof-floor convergence at the headgate, gradually unloading to residual strength inby the face position. Lagging, nonuniform caving of overburden rocks results in periodic stress concentrations at the tailgate comer. The sudden release of strain energy triggers sudden failure of critically loaded coal in this area of the high stress gradient. The three-entry gate pillar configuration promotes formation of long cantilevers and load transfer to the tailgate, contributing to formation of cutters and coal bumps. The source of seismicity is investigated while simulating slip along three sets of joints in two stiff stratigraphic units during multipanel extraction.

Jan 1, 2003

By J. K. Whyatt

A comprehensive survey of mine seismicity and rock bursting during development of two sublevels at the Lucky Friday Mine, Mullan, ID, USA, was conducted to better define rock failure mechanisms and sources of ground control hazards. Survey data included rock burst damage reports, seismic event locations and magnitudes and, for the most energetic events, first-motion information. Several subsets of this database, including large seismic events, rock bursts, and microseismic activity near the face, were analyzed. Elements of the large-event and rock burst data sets were nearly independent. That is, there was no relationship between the risk posed by a seismic event and its energy, although such a relationship is well established for the mine as a whole. All data sets showed that certain geologic features appear to control the spatial distribution of events and the spatial distribution of rock burst risk. The data also suggest that. within the scope of this study, the greatest risk of injury occurs when a pocket of heightened rock burst risk is first encountered and that this risk is controlled when miners adapt their practices to these conditions. Recognition of the role of particular geologic features in the spatial distribution of rock burst hazards provides an opportunity for anticipating, rather than only reacting to, a changing level of rock burst hazard. However, much work will be required to make good on this promise.

Jan 1, 1998

Since the initial development and trials of FLEXIBOLT flexible roof bolts, further comparative testing and product development has been completed as part of the changeover to this roof support system at two Australian longwall operations, Ellalong and Angus Place Collieries. Details of the practical implementation of FLEXIBOLTS at both collieries are outlined together with their impact on roadway development rate productivity. At Ellalong where fully encapsulated 2.1 m long bolts are used, the reduction in bolt density is discussed with respect to both support system design and work force acceptance. The impact of shear deformation on bolt loading is described. Bolts longer than the working height have been required at Angus Place to achieve a suitable bolt anchorage horizon. Details of the long hole drilling and the benefits of a two stage grouting procedure are outlined. Improvements in roof conditions which have resulted from the inclusion of FLEXIBOLTS into the overall roof support strategy at Angus Place are described. Their importance to the viability of the Angus Place operation is also highlighted.

Jan 1, 1994

By Craig Collins

The objective was to develop a Real Time Drilling Display System for Rotary Roof Bolting using Analyzing Software to display mapped holes as they are being drilled in real time. The Analyzing Software was developed to analyze and interpret drill sensor data produced during a rotary drilled roof bolt hole by using algorithms and drill sensor data trends to find and locate roof separations or voids. After this software was validated through lab and field testing, its application needed to be tested in a production environment. However, the current method used to record the drill sensor data proved to be cumbersome for the mine to use on a continuous production basis. With different more practical mediums being explored and considered, a Drilling Display System was developed that will both map holes in real time and record the data for later review. With the Analyzing Software's current level of accuracy and with the convenience offered by the real time Drilling Display, the Drill Display System was implemented along with a test mine's current roof control plan and tested as a roof separation "Early Warning System". After installing the Drilling Display System onto a machine, input and suggestions from the mine environment were considered and design changes were made. The real world application provided a design direction that made the information produced by the Analyzing Software both convenient and significant for the test mine. Also, using the Drilling Display System increased the number of drilled holes that were both mapped and recorded. The results from studying these recorded data files lead to refinements in the current algorithms used by the Analyzing Software which increased the overall accuracy and capability of the system A fairly accurate representation of void or separation locations in the mine roof can be determined fromsensor data recorded during production bolting cycle. The immediate feedback on the local roof condition that the Drilling Display System provides, amounts to a quicker and safer alternative to the scratch hole and the video bore-scope examinations that arc performed after the bolting is completed. This project has shown that making the alternative method of examining the roof condition practical for the mine to use is just as important as the accuracy of its results. Based on the results of this field test, this or other methods of reviewing the analyzed drill hole data will be explored.

Jan 1, 2004

By James J. Scott

Over the past two years research, development ad field tests have been conducted on a new product called the "Scott Cable Sling". This work resulted in the awarding of U.S. Patent No. 4265571 on May 5, 1981, to the inventor. Work on this' project was done by the staff of Scott Mine Technical Services, Inc., and is in keeping with its philosophy of advancing mine grad control technology. Recent acceptance by industry has resulted in volume purchases of the product and indications are that it will gain wide acceptance by the mining industry. The system features flexibility in its application and is useful in both roof and rib control. This paper describes the product, discusses its range of applications and cites specific field examples of its use.

Jan 1, 1981

By Nikolaos Polysos

To drive and utilise gate roads economically requires accurate planning and risk assessment considering the variable geomechanical requirements. The geomechanical part of roadway planning is secured by geotechnical investigations of drill cores and geophysical borehole logging. The rock specific parameters determined in this way allow a statement about the quality (defomtation and strength characteristics) of the rock that has to be cut across. They set up the basis for a standardised rock assessment. Combined with the prevailing rock mechanical situation, technical boundary conditions for advance and support techniques can be identified already in the planning phase. Detailed geotechnical investigations prior to the planning and driving of roadways can lead to a technically and economically optimised roof support system. Geotechnical core logging and a standardised assessment scheme based on the data acquired will be introduced. Geophysical borehole logging methods provide additional physical data. Different methods and their validity, which may lead to a further improvement of the planning reliability, will also be presented. The geomechanical information gained and their relevance fir the planning will he illustrated. Keywords: rock classification, geotechnical rock parameters. geophysical well logs. support planning

Jan 1, 2002

By Chris Adams

The application of pre-tensioned tendon roof support assisted Harworth Colliery in their revival, from impending closure to lowest cost deep-mined producer in the UK, over just six months last year. The paper describes this success story, how it came about, the reasons "why", and the method "how"

Jan 1, 2003

By Kanaan Hanna

The integrity factor approach was developed by the Bureau of nines to assess the stability of mine pillars and has been applied primarily to longwall chain pillars. Recently this approach was applied to a room-and-pillar mine in the Illinois Coal Basin. A special configuration of hydraulic borehole pressure cells was installed in a pillar to determine the in situ strength-and- stress relationship of the coal pillar and to verify the applicability of the integrity factor approach. Results indicate that the integrity factor approach is applicable to the assessment of pillar st6b:lity and nay bs a realistic and effective approach for room-and-pillar mine design.

Jan 1, 1988

By Mictck Rataj

Methods of rock reinforcement in Australian coal mines are being advanced progressively through the better understanding of rock failure mechanisms around roadways and the-development of improved bolting hardware and installation techniques. It has been recognised that to improve the performance of rock reinforcement systems, the stiffness of the system as a whole has to be improved. New style bolts and accessories together with high pre-tensioned roof and cable bolts are being developed as a result. A number of the new techniques used to stabilise the roof, especially in weak roof/high horizontal stress areas have been site proven and are now currently used on a routine basis as a means of improving development rates and gate road stability during longwall retreat.

Jan 1, 1997

By Jay Keng

During the construction of shaft on the fractured and weak rock strata, the stability of construction plays an important role. This study includes earth stress estimation, support method, and stability analysis. Stresses on the site of three shafts include structural residual stresses, earth pressures, and hydrostatic pressures were determined. The basic method of estimating the support strength is dependent upon the' size of the plastic ring after rock excavation. During the construction of the shaft, geotechnical measurements, physical properties of the rock, and the change after excavation have to be evaluated again. Geology and construction methods need further consideration.

Jan 1, 1988

By Kot F. v. Unrug

The depletion of thicker coal reserves has resulted in the thinner seams being economically attractive. Surface protection against subsidence limits the extraction ratio to 50% for the room and pillar method as a "no subsidence condition". That rule was developed for pillars with smaller width/height ratio because seams mined at the time were commonly 6 feet thick. Over time pillar ribs deteriorate around the pillar perimeter and thus decrease the area of the pillar. Using 50% extraction as the no subsidence condition in the squat pillar category (below 42 in with w/h > 7) causes unnecessary losses of reserves because squat pillars are stronger. In this paper a method is presented for design of long-term pillars. Based on the analysis of pillar deterioration and field observations, an increase of the "no subsidence" extraction rate to 60% for squat pillars is proposed.

Jan 1, 2001

By Stephen C. Tadolini

A new innovation in coal mine roof support has been developed by the Bureau of Mines at the Denver Research Center. The technique, called "Thrust Bolting,"1 converts a traditional passive roof support system, a full or partial column rebar bolt, into an active support system by following a specific installation process. The thrust bolting system provides two advantages from a ground control standpoint. First, because the system is active, no roof movement is necessary to put the bolt into tension. This eliminates the possibility of strata separations in sedimentary roots, minimizing progressive-type roof failures. Secondly, the tension placed on the system is independent of mechanical devices or torques. This eliminates the highly variable frictional losses that occur between the bearing plate and the bolt head and in the threaded portions of traditional active support systems. These two improvements create a safer working environment for mining personnel. An additional advantage of this improved support system is the reduced cost of installation and maintenance. The initial support cost is reduced by a minimum of 30 percent per bolt when compared to similar conventional support systems. The labor intensive and potentially hazardous torque¬tension test, required for most tensionable active support systems, is not required for thrust bolts. The potential for improved support performance and cost reduction of thrust bolting has generated a considerable amount of interest from both industry and MSHA. This paper will describe, in detail, the theory and application of thrust bolting and present the results of two underground coal mine case studies.

Jan 1, 1990

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 John L. Edwards

The underground coal mines in the Lake Macquarie area of the Newcastle Coalfield, New South Wales, Australia are constrained by surface subsidence restrictions as low as 150 mm. The mining environment is shallow, generally between 100 m and 200 m, often multi-seam and commonly overlain by residential development or tidal waters. Most mines use partial or panel and pillar extraction techniques to meet the subsidence restrictions and allow maximum resource recovery. This paper gives an overview of the results of a recent two year government funded research project aimed at developing a mechanistic, geomechanically based mine design criteria for subsidence control. Three extraction layouts were monitored in detail to assess the relationship between mining geometry, pillar behaviour and surface subsidence. Pillars at each site were instrumented with stress cells, convergence monitors and extensometers. Field data was used to initially calibrate, and subsequently, verify stress and displacement predictions made by the three-dimensional displacement discontinuity code, SUBSOL. Back-analysis of the modelling results indicate SUBSOL's capability as a tool to satisfactorily predict surface subsidence magnitudes and pillar loads for alternative pillar and panel layouts.

Jan 1, 1993

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

By D. J. Reddish

This paper illustrates how numerical modelling has been utilised to predict water and gas pathways around active longwall panels using two case studies. This information has proved invaluable m the design of methane drainage schemes and for risk assessments undertaken at the mining planning stage. The modelling approach described has now been applied successfully to several situations in UK coalmines in order to optimise the position, length and orientation of methane drainage boreholes. The first case study describes the numerical modelling of the stress and shear plane development mund a longwall panel. The simulation was used to predict potential gas pathways through regions of low stress confinrment which are conducive to fluid flow. The conclusions from the modelling were utilised to allow an increase in the volumes of gas drainage and allow the efficient mining of the panel. A detailed discussion is provided on the model construction, determination of input paramters and optimisation of the borehole positions Details are provided on how the models allow a prediction of the fracture patterns, stress distributions and zones of reduced permeability around an active longwall panel. The second case study describes the numerical modelling of a fault adjacent to a face line drivage of a panel that had just started to be retreated. An inrush of water had occurred and a concern was whether further mining would lead to more extensive inrushes or whether the water inrush was associated with more local, limited capacity water bearing features. The role of a fault that was positioned close to the locality of the mine workings was examined. Fracture patterns stresses, strata permeability and water flows were predicted using mechanical and hydro-mechanical coupled models.

Jan 1, 2003

By Dennis Dolinar

In general, the direction of the maximum horizontal stress in the eastern United States is fairly well defined. However, the variation of the magnitudes of the horizontal stresses is not very well understood. Because the horizontal stresses cause severe ground control problems in underground coal and limestone mines throughout the eastern United States a more complete understanding of how the magnitudes vary would be useful for developing mine design strategies to combat horizontal stress related ground control problems. Therefore, in this National Institute for Occupational Safety and Health (NIOSH) study, the variation of the magnitude of the horizon stresses in sedimentary deposits in the eastern and Midwestern United States are examined with respect to two factors, the elastic modulus of the rock and the site depth. Stress measurements from thirty-seven sites are used in the evaluation. Examining the applied excess strains indicates that the eastern United States can be, separated into high and low strain zones. For most of the eastern United States the maximum applied excess or tectonic strain ranges from only 300 to 550 micro strains. However, there is one area, a portion of the Beckley seam in the central Appalachian region where the strains are significantly higher than the other regions. In this higher strain zone, the maximum applied tectonic strains range from 700 to 1,000 micro strains. Regression models for each zone based on the elastic modulus can explain between 83 to 85% of the variation of the maximum horizontal stress. Because one region, the northern Appalachian district, has strains that are about 20% higher than the other regions in the low strain zone, multiple strain models based on geographic regions were developed for the low strain zone that can explain 87 to 91% of the maximum horizontal stress variation with the elastic modulus. Depth was found not to be a significant causal factor in any increase in the horizontal stress even though the site depths ranged from 275 to 2,500 ft. Beyond a theoretical increase, based on Poisson's effect and gravity, no other increase in the horizontal stress with depth can be justified with this data. The most significant factor controlling the variation of the maximum horizontal stress is the elastic modulus of the rock, not the overburden depth.

Jan 1, 2003