Search Documents

By Greg Smith

This paper documents the use of bench testing equipment developed to enhance the understanding of, and improve upon, common roof bolt support systems. Most roof bolt systems consist of components that are covered within the scope of ASTM (American Society for Testing and Materials) standards. However, ASTM does not specify testing methods or performance standards for complete roof bolt ?systems?. It only designates testing methods and minimum performance standards for roof bolt ?components?. Improved understanding and continuous improvement of these systems is essential given the enormous impact that they have on the safety and profitability of underground mining operations. Bench testing equipment has been developed that effectively measures and records multiple points of deformation in a rock bolt support system while load is being applied. With this testing capability, an improved understanding of component and system stress distribution can be obtained and devices and methods for visual load indication can be developed. This paper is formatted as case studies where testing was performed to demonstrate load versus deformation profiles and to develop system improvement products.

Jan 1, 2012

By J. Brasfield

Jim Walter Resources, along with the Rock Mechanics Branch of British Coal and Strata Control Technology Pty. Ltd., is presently studying the yield-stable-yield pillar design in use at the Blue Creek No. 7 Mine located in Brookwood, Alabama. This paper describes the steps taken to determine stresses on the strata surrounding the pre-driven development entries. It also describes the installation and monitoring of stress cells and extensometers used to quantify the stress build-up and strata movement associated with the headgate pass of a longwall panel. The findings of this study will be used to design and predict the performance of a 2 entry yield pillar system in the Parkgate Seam at Welbeck Colliery in the United Kingdom. Stress measurements and strata movement data from No. 7 Mine and Welbeck will be used to develop a computer model that can predict the performance of such a pillar system in British mines. Jim Walter Resources will use this data to study the performance of the present yield-stable-yield pillar system in order to determine if conditions will allow the stable pillar width to be decreased while maintaining current standards of safety and production.

Jan 1, 1994

By Mike Watts

Since the first longwall panel development started in the new mining, (Southern) area, a number of roof falls have occurred in L1 and L2 gate-entries. The longwall was delayed for a total of two months due to roof falls in the gate-entries. To prevent roof falls in future gate-entries, an extensive ground control analysis has been conducted which incorporated regional horizontal stress and panel orientation, tectonic stress, roof strata, surface stream valleys, etc. Results of the analysis indicate that a combination of these factors caused the roof falls. Therefore, a Roof Instability Rating (RIR) system was constructed to predict the roof condition and identify areas where supplemental support should be implemented. The L3 gate-entries, supported by the prediction using the RIR system, have not experienced any roof problems.

Jan 1, 2000

Historically, the selection of mining sequence for multiple seam conditions has been based on many factors other than ground control considerations and minimization of interaction effects. Today, numerous operations are finding themselves mining above existing or abandoned operations, some of which exhibit large scale caving, pressure arches and subsidence troughs. Development of design criteria for the upper seam that would alleviate damage due to lower seam interaction effects can contribute substantially to the safety and productivity of many mining operations. The primary objective of this re search has been to identify methods of improving ground control conditions in the upper seam when mining through either passive or active subsidence waves. These methods concentrate on improved ground control practices for the upper seam, rather than reducing the amount of subsidence in the upper seams by improved practices in the lower seam. Design factors such as pillar size, pillar and opening orientation relative to the subsidence trough, offsetting intersections, and different support methods have been studied for the upper seam in a variety of typical geologic environments. Analysis has demonstrated criteria for ameliorating negative interaction effects and facillitating design in the superincumbent seam.

Jan 1, 1986

By W. C. Smith

Over the past several years, the Bureau of Mines has conducted studies on the performance of fully grouted resin bolts In underground mines. The effectiveness of resin-grouted bolts in maintaining a competent roof, even in weak rock, has been well documented in the literature. Recent Bureau studies have evaluated the affects of shortened grout columns on bolt behavior in a variety of rock types (1, 2, 3). The application of partial grout columns to primary roof support systems has been introduced as a viable alternative to fully grouted bolt systems in some mines. The point-anchored resin bolt and combination bolt represent such alternatives. While both systems usually employ some mechanical interlocking system, their primary difference lies in the length of resin column used. Point- anchor systems rely on an anchoring device that is a short length of resin column of 2 feet or leas. Combination bolts, on the other hand, often require 3 feat or more of resin column. Resin migration into voids and fractures intersecting the borebole wall is a major problem when installing fully grouted bolts in weak roof rock and can result in a reduced column length that provides reduced anchorage. The relationship between column length and anchorage integrity deserves closer study. The Denver Research Center of the Bureau of Mines has examined the load- carrying capabilities of resin-grouted bolts in weak strata. Pull tests were performed on about 900 resin-grouted, 4- foot-long bolts with column lengths ranging from 6 to 48 inches. The practical performances of 8 different column lengths were evaluated for 10 mines under in sit" conditions using data obtained from standard pull tests. In addition criteria are discussed, including engineering properties of the host rock for designing safer and more effective grouted bolt installations.

Jan 1, 1987

By Thomas L. Vandergrift

With the inevitable expansion of homes, businesses, and infrastructure in coal mining regions, the potential for future subsidence above abandoned mines is of increasing concern. Of particular concern are areas underlain by room-and-pillar coal mines. Unlike areas above longwall mines, where total extraction causes subsidence in a predictable and timely manner, subsidence above room-and-pillar mines is difficult to predict and may be active for many decades. As part of a geotechnical assessment of a water pipeline replacement project, NSA Engineering recently performed an evaluation of tong-terns subsidence potential along the proposed alignment due to time¬dependent failure of underlying room-and-pillar workings. The alignment of the replacement pipeline closely follows an existing concrete pipeline, which lies 150 11 to 300 fl above workings that both pre- and post-date the pipeline. Annual surveys along the existing alignment indicate that some subsidence has occurred, primarily from partial pillaring performed in the early 1980's, but that this subsidence is not active. Using the boundary-clement code LAMODEL as the primary analysis tool, NSA followed a three-step approach to estimate future subsidence impacts on the replacement pipeline. First, historical information for the mine was compiled, allowing for calibration of models to actual field conditions. Then, numerical models of workings directly under and adjacent to the proposed alignment were nun, with model input parameters varied until resulting loading patterns matched historical data, and subsidence in the model approximated that measured along survey lines. Finally, time-dependent deterioration of coal pillars was simulated by decreasing the coal strength in the calibrated model until a realistic "worst-case" condition (stability of barriers, but Yielding of production pillars and pillar remnants) was attained. Subsidence from the previous calibrated model was then subtracted from that of the worst case model to estimate future subsidence and related ground movement parameters that slay occur along the proposed alignment. The replacement pipeline design, incorporating couplings capable of withstanding the ground movements predicted, is now being finalized.

Jan 1, 2000

By Leigh J. Wardle

This paper describes a numerical method for calculating the three¬-dimensional stresses and displacements induced by tabular excavations in a rock mass consisting of parallel layers of distinct material properties. The method is based on the superposition of numerical solutions for elementary displacement discontinuities in a parallel layered rock mass. The method can be used to analyse complicated excavation layouts such as those encountered in room-and-pillar or longwall mining. The method can automatically take into account the effect of the earth's surface. The method offers an economic and efficient solution to stress analysis problems which cannot be adequately modelled using two¬-dimensional methods and for which the expense of three-dimensional finite element or general boundary element analysis cannot be justified.

Jan 1, 1984

By David C. Oyler

The Bureau of Mines conducted research at the 9-Right Section of the Olga Mine to determine the cause of bumps and aid in the development of appropriate control techniques. The purpose of this paper is to identify the factors which affect the occurrence of bumps at the Olga Mine. These factors have been broken down into two categories, in situ and mining related. The in situ factors include those conditions which exist prior to mining, such as variations in lithology and overburden, presence of jointing, the composition and the physical properties of the rock, and the original stress field existing in the coalbed and adjacent strata. The mining factors include the mining sequence, changes in stress in the coalbed and adjacent strata during mining, rates of convergence, and the pattern of failure of the coal and adjacent strata. The factors which appeared to have the greatest effect on bumping were the overburden, the lithology of the roof and floor rock and the physical properties of the rock. The roof rock is generally a massive, stiff sandstone which does not break until coal pillar extraction is complete. The immediate floor is a strong shale which generally grades into siltstone and then into sandstone. The overburden at 9-Right ranged from 990 to 1,600 ft. Secondary factors appeared to be the in situ (pre-mining) stress and the mining sequence. When barrier pillars were split., pressures measured in hydraulic cells (Borehole platen flatjacks) located in those barriers showed large pressure increases, and the incidence of bumps increased, especially when the barriers were near old gob areas. The pillar extraction system used at Olga, where three rows of pillars are mined concurrently, appears to reduce pillar loads at the face areas, thereby reducing the exposure of men and equipment to the risks of bumps during mining operations. The horizontal components of in situ stresses at Olga were found to be nearly the same as those at 5 other mines in southern WV, where humps were not common, so it appears that horizontal in situ stress alone is not a major influence on bumping at the Olga Mine.

Jan 1, 1987

By Ebrahim Ghasemi

Roof fall risk is a common problem in coal mines, and it is generally unpredictable due to variability in geological and mining parameters. In this study, a new fuzzy logic model was developed to predict roof fall rate in coal mines. Parameters such as coal mine roof rating (CMRR), primary roof support (PRSUP), intersection span, and depth of cover were considered as the model inputs. The model was developed based on experts? knowledge and also a database including about 109 datasets of roof performance from U.S. coal mines. Approximately 20% of these datasets were used to assess the performance of the fuzzy model. In each case, comparison between results from model and real roof fall rate showed that this model can predict roof fall rate to an acceptable extent.

Jan 1, 2011

By G. C. Marino

The performance of the V-Day Mine near Danville, Illinois has been evaluated from data extending over about a 20 year period. The area included in this study involves over 16 acres. Essentially this entire area has progressively subsided. Extensive data has been collected on the geologic and mining conditions, the subsidence movements, as well as on the response of the surface structures to the subsidence. In this paper a detailed summary of the site conditions is given, and the mechanics of progressive failure of the studied room and pillar mine is postulated. These progressive long-term failures of the V-Day are then compared to other similar mine failures in Illinois. Some overall, but tentative, long-term strength evaluations are presented as well.

Jan 1, 1990

By Barry Voight

The law [ ] where [ ] is a measurable quantity such as strain and A and cu are empirical constants, describes the behavior of materials in terminal stages of failure under conditions of approximately constant stress and temperature. Applicable to intact rock and rock masses, the relation may be extended to conditions of variable stress and cyclic stress, and may be used to predict time to failure. Applications in rock mechanics include slope and excavation stability and various topics in ground control.

Jan 1, 1989

By Daniel W. Su

This paper presents the implementation and evaluation of the hydraulic fracturing technique and Longwall Visual Analysis (LVA) software to mitigate the impact of a 1,000-ft (305-meter) widemassive sandstone channel on a 1,500-ft-wide (457-m-wide) longwall face. Based on a underground roof geology reconnaissance program, four frac holes were drilled and fraced along the center axis of the sandstone channel. To further provide detailed monitoring of the longwall face, the Longwall Visual Analysis (LVA) software was installed to track the face pressure and cavity formation index. In mid-December 2011, the longwall face approached and entered into the western edge of the sandstone channel. The longwall face mined through the center axis of the sandstone channel in mid-January 2012 and advanced outby the eastern edge of the sandstone channel in early February 2012. Results from daily underground observations and production delay analysis confirmed the effectiveness of the hydraulic fracturing program and the validity of the LVA Software.

Jan 1, 2012

In recent years, improvements made to the longwall system have accelerated at a faster pace than those made to the gate entry development system. As a consequence, unless planned years ahead of time, extending the panel length is nearly impossible without some innovative approach. At Cyprus Amax Coal's Emerald mine the G panel length was extended by 3,500 ft- by developing three entries across it to facilitate development of its headgate and tailgate entries simultaneously when the adjoining F panel was in operation. After evaluating alternative supporting methods and associated risks, determination was made to backfill these entries and crosscuts using a low strength flyash cement mixture prior to mining thru. Instrumentation was also used to monitor the stress change or front abutment stress in both the coal and the backfilled material ahead of the face. Longwall mining thru these three entries took about four eight-hour shifts and proved to be very successful. The designed flyash cement mixture provided adequate roof stability and caused no difficulty in the shearer's cutting while eliminating an in-panel longwall face move and minimizing the risk of roof or floor failure in the in-panel openings, or in the worst scenario loss of the face during the longwall cutting thru. This approach provided the intended result of extending G panel and recovered about one million tons of coal. This paper summarizes the engineering design of the cutting-thru project and flyash cement mixture, front abutment stress changes within the longwall coal block, backfilled material, and coal pillars ahead of the longwall face, and our underground observations

Jan 1, 1997

By Lawrence R. Artler

The North American Coal Corporation, its subsidiaries and other coal companies, have been mining the Pittsburgh #8 seam in Eastern Ohio for several decades. Underground mining by the "room and pillar" method, and more recently by longwall methods, have been practiced. This presentation deals with ground control problems and subsequent solutions to those problems for both mining methods. But first, let me briefly discuss some pertinent points about the geologic conditions of the coal seam near the southwest edge of the Pittsburgh Coal Basin. The Eastern Ohio region of the Pittsburgh Coal Basin is situated along the western edge of the unglaciated Allegheny Plateau. The local terrain is quite steep, with topographic relief of 430-500 feet being common. The coal thickness in the region ranges from 36"to 120", with the average being 60" in the areas being currently mined. Regional dip of the seam is toward the southeast in an amount of 10 to 40 feet per mile. A cross-section of the mineable seam generally consists of the main bench (60"), a rider coal (6"-12"), and a claystone parting (6"-12") for a total excavated height of 6-7 feet. Greater excavated height occurs where the fireclay floor is so soft and friable that movement of mechanical equipment breaks up the first 4-8 inches. Generally, the main bench and rider coal thin and become more variable toward the southwestern portion of the mining area. The rider coal also grades laterally into a coaly carbonaceous shale. In those areas where the roof coal can no longer be counted on to help support the immediate top, some of the main seam (6") is left to protect the overlying 8'-12' of mudstone/claystone strata. These strata are virtually non-bedded, highly slickensided units and are "hung in suspension" by roof bolts from a more competent limestone stratum above. These mudstone/claystone units are very susceptible to moisture and thus to the weathering cycles. The rider coal or 6" of the main bench is left in place to "seal" the mudstone/claystone units in Mains and Submains development. For many years, it was thought that the strength of the limestone member was detrimental in attempts to fully extract the seam by conventional means; however, longwall methods that have been more recently employed have broken the "myth" that the Redstone Limestone prevents full extraction of pillars.

Jan 1, 1984

By Andre Zingano

The coal seam strength and the failure criteria that represent the behavior of the coal seam are very important for mine section and pillar design. Additionally, mine section and pillar design take into account roof and floor layers, which form the roof-pillar-floor system. Therefore, underground ground control design begins, most importantly, with rock mass characterization. Another issue is the distribution or behavior of the coal seam strength for the whole mine area. Sometimes, the coal seam strength is assumed to be the same for an entire area, but this is not true because the coal seam is heterogenic and anisotropic, and also the coal strength is affected by faults and spacing between fractures. The objective of this paper is to demonstrate the use of the methodology for geomechanics characterization to determine the strength for the Bonito coal seam, and discuss the coal seam strength distribution around the entire mine area. The intact coal strength was determined by laboratory tests that were completed during the last 17 years, including triaxial and uniaxial strength tests. It was noted that there is a strong correlation between all laboratory strength tests with 90% confidence interval. The explanation for this coal seam strength variation is the presence of the discontinuities and their variation in quantity and quality at each location in the mine area, because the coal seam rock classification and its behavior will be affected by the quantity and quality of fractures.

Jan 1, 2014

By W. A. Cincilla

The use of resin-grouted rock anchors continues to gain popularity in underground wall mines in the United States. It is estimated that during 1985, more than 30% and as many as 35% of all roof bolts used employed sane from of resin anchor. The so-called 'combination' bolting systems, which generally utilize a resin pint-anchor, have also found increased application. This increased application is particularly true in poor ground, where more 'traditional' support measures have performed poorly. Virtually all resin bolts in use today in U.S. mines employ cartridge-type delivery systems, which in general, have produced good results under a variety of conditions. However, a host of inconsistent and sometimes uncontrollable variables may significantly alter the optimum bolt performance. These variables have been identified and are well documented in the literature (Snyder, et al., 1979; Kwitoweki and Wade, 1980; Serbcusek and Signer, 1985). The beau of Mines, Denver Research Center (DRC) conducted a major research project aimed at investigating the effects of selected aspects of installation procedures on resin-bolt performance. One objective of the project was to establish anchorage criteria for full column resin bolts installed in actual mining conditions. This piper describes the results of a particular study which focused on quantifying, through field tests, possible reductions in bolt anchorage due to installation of less-than optimum resin columns. These columns might result from losses of resin to surrounding strata, or non-compliance with recommended installation procedures. Over 1000 resin bolts having grout columns ranging from 12 inches up to 48 inches (full column) were installed and pull-tested at 13 underground test sites located in a total of 11 mines (Fig. 1). Four basic roof-rock types were included: shale, top coal, sandstone, and limestone. The majority of the bolts employed 3/4-in-diameter, 48-in-long, type 40 rebar having a minimum '~ated' yield point of 17,600 lb (8.8 tons). Actual yield strength may vary by manufacturer.

Jan 1, 1986

Recent improvements are described in several techniques for ground control in underground mining. Promising methods for analysis and mitigation of rockbursts are discussed. Methods of assessing damage to underground excavations from repetitive seismic loading are reviewed. Analysis of mining-induced surface subsidence is discussed.

Jan 1, 1989

By Andre Zingano

An underground mine using room and pillar and retreat mining methods was developed during the 1950?s. In 2009 an open cast mine was opened in the same reserve with the objective of mining five coal seams including the one previously mined. Recently, a series of slope failures were detected at the open cast mine. The surface mine map was overlain on the underground mine map, and it was observed that the slope failures were located above a retreat mined area where the overburden had caved and the ground subsided. The deformation and displacement attributable to subsidence caused a loss of strength in the overburden strata. The objective of this paper is to study the causes of slope sliding (or failure) due to the ground subsidence of rock layers above oldmined out areas. A map was created to overlay the two mines -- the abandoned underground mine and the active surface mining. Two- and three-dimension numerical modeling was conducted to simulate the slope behavior and to verify whether the mine subsidence was the cause of slope instability. This study indicated that the subsided rock layers were the cause of slope instability and sliding.

Jan 1, 2013

By John P. Dunford

U.S. Bureau of Mines researchers are investigating the ability of mine-wide monitoring systems already in place at many coal operations to provide a direct link between geotechnical instruments and the mine-wide monitoring console on the surface. This paper describes a number of field tests using instruments such as-string potentiometers to measure roof-to-floor closure and bed separation and pressure transducers to monitor pillar loading. The advantages of using existing systems include on-site data collection and analysis, real-time understanding of geological conditions, and long-term data recovery from areas that have become inaccessible to mine personnel. However, because data are generated constantly, large files develop very quickly. In many cases, the data are redundant, such as during times of reduced mining activity or in areas not yet in the vicinity of active mining. To reduce the volume of data, a software program was written that eliminates unnecessary readings and generates a more concise file that can be output to a mine-wide monitoring system, personal computer, or as hard copy.

Jan 1, 1993

By George J. Karabin

Since the inception of a qualitative pressure arch theory by Haak4 in 1911, much work has been done in an effort to apply this concept to underground mining. It has been used in both longwall and room and pillar mining to determine roadway and pillar stability (with varying degrees of success). This paper presents a summary of a case study where the pressure arch concept was extended in a totally qualitative manner to define the source of roof control problems in the mains of a Pennsylvania mine. It was concluded that, due to the method of mining employed, a pressure arch formed over a portion of the entries, and the high abutment stresses accompanying it resulted in extreme roof deterioration. The following discussion will include a general presentation of the arching mechanism, the physical response of the roof strata under mining-induced abutment stresses (both from the case study and in general), a computer simulation of ground stability under both "normal" and "abutment" stress conditions, and finally, potential means of alleviating such conditions.

Jan 1, 1982