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Virtually all mineable Appalachian coal seams exist in a multi-seam environment. This makes it inevitable that most seams will eventually experience interaction induced ground control and mining problems. Indeed, it has been estimated that every working face will experience interaction effects from previous or current workings at least once during its lifetime (1), and that these problems are considerably exaggerated when seams are in close proximity -- a distance of less than 110 feet. The continued increase in multi-seam problems has resulted in considerable research activities over many years (2,3,4,1). Theoretical analysis, numerical and physical model studies, statistical and empirical analysis of numerous interaction related field data have been applied to multi-seam mining situations and have identified many useful trends and design criteria (4,5,6,7). To fully utilize research results and transfer multi-seam mining technology to field engineers, a design model -- USEAM was constructed (8). This model, designed for close seam under-mining situations, was computerized to facilitate utilization. In the current research, a new computer program "MSEAM" was developed to produce a model for both under- and overmining close seam situations. It is hoped that both of these software packages will aid field engineers in using important interactive design criteria to gain a better understanding of interaction mechanisms. Through advance planning with these programs, detrimental interaction may be avoided and minimized, and occasionally a design can be selected which will allow interaction effects to be utilized to the benefit of mining operations.

Jan 1, 1987

By Keith Heasley

In this paper, a state-of-the-art, three-dimensional, full waveform, microseismic system was used to analyze the rock failure around a deep (> 750 in (2500 ft) of cover) bump-prone longwall panel. The microseismic system consisted of both underground and surface geophones coupled through radio telemetry and a fiber-optic network to produce pseudo real-time detection and location of seismic events surrounding the coal seam. This was the first microseismic installation of this scope at a U.S. coal mine, and the system was intended to help determine the exact strata mechanics associated with the redistribution of stress and the associated gob formation at one of the U.S.'s deepest longwall mines. Overall, 5,000 calibrated seismic events were recorded during the mining of one panel, including a Richter magnitude 4.2 event which occurred inside of the array. Analysts of these events provides a number of notable insights into the rock mass behavior. For instance, the longwall panel was widened during the retreat process, and the mining-induced seismicity shows a distinctly different behavior between the start of the panel, the mining of the narrow part of the panel and the mining of the wider part of the panel. Also, in itself, the acquisition of the ML 4.2 event was a major milestone in geomechanical and bump research. This is the first time that such an event has been recorded with this detail and accuracy at a bump-prone coal mine in the U.S. `The analysis of this single event has set distinct new limits on the relationship between mining seismicity and coal bumps.

Jan 1, 2001

By Yanfa Gao

Abstract In order to control and reduce ground subsidence due to coal mining, and to protect building, railway and other surface construction, a new subsidence control method is proposed. Ground subsidence is reduced by grouting bed separations in overlying strata with fine-coal ash. Engineering application tests have been performed for 5 years in Tangshan Coal Mine. Kailuan Group Corporation. China. The technology has been applied to 4 fully mechanized sublevel caving mining faces. Its effect in reducing subsidence has been significant. The technology including grouting engineering design method and ground subsidence prediction theory is introduced in detail. Keywords: overlying strata separation, multi-layer grouting, subsidence control

Jan 1, 2002

By Vince Caggiano

The newly developed Hilti OneStep roof bolt combines the standard 5-step anchoring process (i.e. drilling, transfer of drill steel, insertion of resin, insertion of rod and tightening) into one easy operation. It also replaces 4 items (drill head and steel, adhesive cartridge and anchor) for simplified material management. This paper describes its history of development, and bolt, bolt drilling and bolt resin specifications, experimental test results in Germany where it achieved a reduction of 36-42% in roof bolt cycle time and 25-30% improvement in gateroad development rate. In the U.S. coal mines, a time study showed that OneStep can save approximately 50 seconds in installation time per bolt.

Jan 1, 2005

By Rusty Mchenry, Craig Dickerson, Robin L. Oldham

"It has been proven that longwall faces can be moved safely and efficiently. However, abutment pressures and poor ground control conditions can halt operations and be hazardous to coal miners. Recently at a mine in Southwestern Pennsylvania, roof material collapsed above shields that created two large voids and caused major challenges for shield recovery. A unique, engineering solution was developed that utilized a modified concrete material to fill the voids, creating stability in the affected area. The many phases of this project included the construction phase, void pumping, cutting out, and bolting of the concrete material. This project eliminated the hazards associated with bolting the recovery face and removing shields in adverse conditions, making it possible for the mine operator to safely complete the longwall move.INTRODUCTIONFrequently in the mining process, especially during the recovery phase, voids above the longwall supports have occurred. These voids are simply roof cavities caused by falling rock strata above the longwall face. Most often, this condition slows down or halts mining operations and creates hazardous work conditions for coal miners. Numerous methods of supporting void areas on longwall faces have been attempted and utilized throughout the mining industry since the advent of the longwall mining system. These methods have utilized many different materials and processes and have included the installation of steel or wooden beams, crib blocks, and various types of roof bolts. However, these methods can be dangerous due to exposure of personnel to the mine voids, falling roof material, and the necessity to handle large amounts of heavy support materials.This paper describes a unique case study in Southwestern Pennsylvania where longwall recovery methods, through two void fills, were successfully completed, eliminating exposure of coal miners to the rock voids above. At the start of this endeavor, normal recovery operations were halted, and hazardous conditions became a concern when the voids were encountered. However, in the interest of safety, the decision was made to try a unique method to completely fill the void from the bottom of the coal seam to the top of the void with a cementitious material and then mine through it. This is in contrast to traditional methods where beams and support materials are used to stabilize the area above and in front of the mine shields, which can expose miners to the void above."

Jan 1, 2015

By Eddie Martin

Jim Walter Resources, Mining Division, employs the retreat longwall mining system as the primary method of coal production. The strata control aspects of developing and maintaining roadways for retreat longwall mining have continued to improve and advance at Jim Walter Resources. New and innovative concepts in pillar design, roof and rib support, as well as longwall face support, have been combined to provide better strata control. Control of immediate roof breakup in the headgate and tailgate entries has been a problem at Jim Walter Resources' mines. The application of roof trusses as supplemental roof support failed to provide the desired strata stability. The immediate roof deterioration has been controlled through the installation of increased density of roof bolts in the affected roadways. Control of the rib sides has been a documented problem at Jim Walter Resources' No. 4 Mine since its start-up. The installation of 3/4 inch diameter roof bolts at 2 ft. spacing failed to control rib rolls. The development and application of a yieldable rib support system provided the desired results. Rib rolls have been virtually eliminated where the yieldable system of rib support has been installed. An evaluation of longwall panel development using the total yield pillar mining plan has been completed. Results of the strata stability and other operational data have been compiled. Coal production and gate entry stability were not adversely affected using the total yield pillar mining plan for longwall panel development. Strata Control on the longwall face is provided by the face support system. Excessive convergence and roof breakup along the longwall face have been experienced at several of the Jim Walter mines. In an effort to eliminate this problem, an evaluation of higher setting pressures for longwall face supports is being conducted. Results have been inconclusive to date. Each of the above strata control projects will be presented as case studies. Since Jim Walter Resources business is coal production, emphasis will be directed toward the application rather than the theoretical aspect of the projects.

Jan 1, 1988

By Kudret Tutuk, Serkan Saydam, Sungsoon Mo

"This paper presents an overview of the floor heave management at the Glencore Bulga Underground Operations and investigates the contributing factors to the behaviour of the floor. The mine experienced a number of major floor heave events in gateroads on development. As the longwall face approached the roadways, the magnitude of floor heave frequently increased, while new floor heave also developed. Furthermore, severe floor heave events took place along the longwall face. The most observed failure mode was buckling. While regular floor measurements were conducted to better understand the nature of the phenomenon, and various measures were considered to control the deformation of floor, the mining height was increased for the predicted floor heave domains, which facilitated effective management of the floor issues. The experience in the mine indicates that mainly high horizontal stresses with greater depths of cover and certain types of floor lithology configuration are likely to contribute to the failures of floor strata.INTRODUCTIONThe Glencore Bulga Underground is located in the Southwest of Singleton in the Hunter Valley of New South Wales, Australia. Blakefield South, part of Bulga Underground, commenced longwall mining in 2010, extracting coal from the Blakefield seam, while the overlying longwall panels were previously mined from the Whybrow seam. The coal seam ranges from 4.5 m to 8.0 m thick with the mining heights varying from 2.9 m to 3.5 m. The longwall panels are 320 m and 400 m wide and up to 3.5 km long.The mine experienced a wide range of floor issues around longwall panels LW7 and LW8, both 400 m wide. For instance, the concrete floor in the travel road failed and lifted as the longwall retreated in close proximity as can be seen in Figure 1. Figure 2 shows the beam stage loader (BSL) buried into the lifted floor, and Figure 3 shows a timber prop distorted due to the movement of the floor. In addition, severe deformation of the floor along the longwall face impeded the longwall operation, as shown in Figure 4."

Jan 1, 2018

By John Kucish

Roof bolting in the Pittsburgh Coalbed takes many forms today: coupled partially grouted bolts, fully grouted rebar, passive cable bolts, tensioned cable bolts, and torque tension rebar. The Federal No. 2 Mine of Peabody Energy has recently made a significant advance in longwall headgate primary roof support, with the change fiom an 8 ft long coupled partially grouted bolt to a 6 ft long torque tension system. The coupled partially grouted bolt consists of a 4 ft long 0.804 in diameter rolled deformation bar thread coupled to a smooth 718 in headed bar. The coupled partially grouted bolt is anchored into a 1 318 in borehole with 4 ft of high speed resin. The 718 in diameter smooth bar is tensioned by rotation into the treaded coupler, upon setting of the resin, compressing the immediate roof. The change was initially driven by occasional failure of the coupled partially grouted bolt at the threaded coupler, due to lateral movement caused by horizontal stress. A fully grouted % in diameter torque tension bolt, in a 1 in borehole, was proposed as a cost effective primary bolting for the high horizontal stress conditions. The upper 2 112 ft of the rebar is anchored in fast set, medium viscosity resin and the bottom 3 K ft is anchored in slow set, special low insertion force resin. The lower end of the rebar is threaded with a nut that permits the counter clockwise spinning of the bolt to mix the resin, at maximum rotational torque. Upon setting of the high speed top resin, the nut is spun clockwise to tighten the nut against a metal dome plate. The plate is forced against the roof to compress the immediate roof, A complete headgate entry has been successfully supported with the torque tension system. The mining of the adjacent longwall panel was completed in September 2004. The torque tension system provided a 25% savings over the previously employed coupled partially grouted bolt system.

Jan 1, 2005

By Weidong Pan, Yinchao Yang, Baodong Zuo

Fully mechanized mining technology, with a large cutting height in thick coal seam, has been developed to be large-scale, mechanized, and intelligent. Furthermore, it is also superior to other mining methods because it operates more efficiently and has higher recovery rates of coal resources. Dongpang coal mine (located in north China) is one of the largest coal mining productions in Hebei province. It is significant because it is the first set of coal mines in China to adopt fully mechanized mining technology that has large cutting height potential. When looking at the coal seams of deep mining, specifically in the mine No. 2612 where the working face is 4.5–6.5 m cutting height, the coal walls have low stability due to the way the coal seam fluctuates in a large working face. This paper adopts mechanical analysis, numerical simulation, and field observation research methods to control the rib fall of coal walls in a coal mine. By adopting research methods that analyze the mechanical characteristics of face roof, we are able to conclude the main roof structure of the masonry seam stability criterion in overhead mining with large cutting height. This is done through employing the numerical simulation method, which helped to determine the main factors that influence the rib fall of a coal wall and weighting factor. Through field observations, we discovered the technology method to relieve, control, and mitigate rib falls of coal seam in a coal mine. We also discovered methods that would ensure the smooth mining of the working face of the mine by strengthening the bolting and grouting reinforcement methods used to improve the working face of stope surrounding rock conditions. This paper discusses research results that enrich the stope wall rock control method by expressing how fully mechanized mining technology, with a large cutting height, is meaningful to solve the complex conditions of rock fall caused by thick coal seam.

Jan 1, 2017

By Peter Zhang

"Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground-fall fatalities in the underground coal mines of the United States. The risk of ground falls during pillar recovery increases in multiple-seam mining conditions. The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave. When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction. This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 70 ft of interburden under about 1,000 ft of depth of cover.The study finds that, for interburden thickness of 70 ft, the multiple seam mining influence zone in the lower seam is directly under the barrier pillar within about 100 ft from the gob edge of the upper seam. The peak stress in the interburden transfers down at an angle of approximately 20° away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat. The study also suggests that, for full pillar recovery in close distance multiple seam scenarios, it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars. If the entries and / or crosscuts in the lower seam are developed outside the gob line of the upper seam, additional roof and rib support needs to be considered to account for the elevated stress in the multiple seam influence zone. INTRODUCTION Room-and-pillar mining accounted for about 40% of underground coal production in the United States in 2016. Pillar recovery, practiced in about one-third of the room-and-pillar mines, represents about 10% of the coal mined underground, yet it has historically been associated with more than 25% of all ground fall fatalities (Mark, Chase, and Pappas, 2003). In some U.S. coal fields, particularly central Appalachia, many coal mines are operating under geological conditions with multiple coal seams. The risk of ground falls during pillar recovery increases under multiple-seam mining conditions. The hazards of pillar recovery associated with multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave (Mark and Tuchman, 2007; NIOSH, 2010b). Pillar retreating creates abutment pressure, not only in the currently mined seam, but also in the overlying or underlying seams. Multiple-seam interactions become more pronounced as overburden depth increases and interburden thickness decreases. To safely recover the pillars in multiple seams, it is critical to properly plan the mining sequence and panel layout to minimize potential multiple-seam interaction."

Jan 1, 2017

By E. Bane Kroeger

For many years, researchers around the world have been investigating coal pillar stability. Many have focused on trying to optimize the size of the pillars by examining stable and failed pillars in underground coal mines. For mines that are already in operation or companies that are opening a new mine adjacent to an existing mine where the pillar dimensions and stability can easily be determined, this is the preferred technique. However, when a new mine is planned for an area where there has been relatively little mining, then samples of the coal need to be drilled and tested for strength. Because of the fractured nature of coal, this lab testing is often problematic and may often yield poor results because of the limited size and number of samples that can be obtained from the core. To remedy this problem, research was conducted in the labs at SIUC to determine if improvements could be made when optimizing the size of pillars for underground coal mining in Illinois. Through a greater understanding of the relationship between dimensions and strengths of coal samples tested in the lab, it was hoped the in situ strength of coal pillars could be determined with greater accuracy. This could allow the extraction ratio to be increased and more of the coal resources in Illinois recovered without impacting the safety of the mine workers. Many different agencies around the world have published recommended minimum numbers of samples for strength determination based on the size of the coal samples. For two inch cubes, uniaxial testing of at least seven samples is recommended and this minimum number decreases to four when testing four inch cubes. However, the laboratory testing of coal from two seams thus far has suggested this number should be almost doubled to accurately determine the coal strength. Testing also showed there is a pronounced decrease in the variation in the strength of the samples tested as the dimensions of the sample were increased. From the testing results thus far, it is recommended that a sample size versus strength curve be constructed for every coal seam for Illinois. Creating such a curve will better determine both the minimum number of samples and minimum sample dimensions for that seam or region of a particular scam. A second set of tests were conducted in the laboratory to determine the increase in strength with reduction in height for samples with the same width and length. In Illinois, typical pillar dimensions are 40 to 80 feet (12.2 to 24.4 m) wide and long, with coal thickness varying from about 4 to 9 feet (1.22 to 2.74 m). This provides width/height ratios ranging from 4.5 to 17.5. Coal samples having width/height ratios ranging from 0.5 to 13 have been tested thus far. As with most pillar design formulae, the strength of the Murphysboro coal versus width/height ratio was linear and closely matched shape factors published by other researchers. The laboratory testing thus far has identified three factors that affect the strength of laboratory samples, the number of discontinuities, the angles of critical discontinuities, and the shape of the samples. The factor that probably had the most pronounced effect on the strength of the coal samples was the angle of critical discontinuities. This testing has confirmed that there is still a gap that needs to be bridged between the strength obtained from laboratory testing and the in situ coal strength predicted.

Jan 1, 2004

By William Kendall

"A tremendous amount of research, analysis, and engineering time is used to design a proper roof support system including the type, length, and location of the roof bolts. Unfortunately, since the roof bolts are being installed under unsupported roof, the exact location for the bolt holes cannot easily be determined and then marked onto the roof. The actual location where the roof bolts are installed is typically left to visual estimates by the operator. This frequently results in either (1) the operator overestimating the space between roof bolts and installing more bolts than required, thereby increasing cost and reducing efficiency; or (2) the operator underestimating the space between roof bolts and installing fewer bolts than required, thereby reducing safety, leading to potential roof failure or possible violations.A patented laser light grid system has been developed by J .H. Fletcher and Monitech, a South African Company, to aid in proper machine positioning in the heading and proper bolt spacing. The system can also show when an entry is over width, requiring additional roof support. The system is relatively simple and inexpensive. The concept is applicable across the full range of roof bolters from low to high seams and from single boom to multiple boom bolters. The system can be configured to meet most bolting patterns.The system consists of several very small 1 milliwatt, class 2, line type lasers that project a line onto the roof. The actual laser is 27mm in diameter x 47mm long (1"" x 1.8""). In South Africa it is approved as intrinsically safe and is mounted inside a 50mm diameter x 64mm long (2""x 2.5"") protective housing. In the USA, we are seeking an X/P approval and the laser will be mounted in a headlight enclosure 110mm diameter x 125mm long (4.3"" x 5""). Fifteen of the systems have been installed in South African coal mines with good results.The system has aided the mines in meeting the bolt spacing tolerance used in the Republic of South Africa, which is typically 15% of the nominal bolt spacing or 7 in for 4-ft bolt spacing. Fletcher has submitted the system to MSHA for approval in the US."

Jan 1, 2016

By Michael J. Peacock

The Powhatan No. 4 Mine is located in southeastern Ohio along the Ohio River in Monroe County. The mine produces approximately 3.2 million tons of steam coal annually from the Pittsburgh No. 8 seam. The Pittsburgh No. 8 seam in this area dips to the southeast in varying amounts from 10-feet to 40-feet per mile. The seam is made up of a main bench coal (4' to 5f' ) containing several thin and variable partings and a rider ("roof") coal (0" to 18") which is separated from the main bench by a parting (0" to 18") that is reasonably consistent in occurrence and position. Strata overlying the seam varies from a bedded to nonbedded calcareous mudstone-claystone (6' to 12') in the north and east regions of the mine to a sandstone (Pittsburgh Sandstone C10' to 40'1 in the south and west regions. The Redstone Limestone (10' to 18' ) overlying the mudstone-claystone in the north and east thins and rises toward the area of sandstone deposition in the south and west. From its opening in April of 1971 until late 1981, primary roof support in the long-1 ived entries at Quarto Mining Company's Powhatan No. 4 Mine had been achieved with 8-foot and 10-foot. 518-inch mechanical roof bolts using a Single expansion shell. These bolts were installed so that the expansion shell achieved a competent anchorage in the limestone. However, as the mine developed westward, the limestone began to trend upward becoming inaccessible as an anchorage medium at the 8-foot and 10-foot horizons. Anchorage achieved in the underlying mudstone-claystone strata was failure-prone due to weathering (after exposure to air and moisture) and inconsistencies in the anchorage horizon, leading to an increase in the incidence of reportable roof falls at the mine. Responding to the threat to safety and productivity posed by the increase in roof falls. Mine Management through its Safety and Engineering Departments, sought solutions to the problem. In seeking a solution to the problem, the safest and en engineering groups examined the reportable roof falls to determine the cause of those failures. Their examination pinpointed two items which were found to be a common link in the majority of those falls; namely, anchor integrity and geologic trends. Analysis of the roof falls indicated that due to effects of air and moisture on the anchoraae zones in the mudstones and clay- itones, the integrity of the anchorage over time could not be maintained. Geologic trends noted included the upward trend of the -main limestone as mining progressed westward, inconsistencies in the amount of roof coal and draw slate in the immediate mine roof and increasing evidence of heavily slickensided immediate mine roof. In the absence of the protective barrier provided by the roof coal, the immediate strata and particularly the draw slate, is susceptible to deterioration due to the weathering effects of air and moisture and thus is prone to "eat out" around the roof bolts and cause roof falls. The presence of heavily slickensided roof creates a need for increased contact surface such as that provided by plank and header blocks to aid in holding up the roof and tends to make the behavior of such roof very unpredictable. Having determined the causes of the roof falls. Mine Management began to identify the kind of roof support which would best solve these problems, while at the same time providing the mine with a safe, cost effective and efficient roof support system. This paper will provide an analysis of how this decision was reached, the background that was researched to aid in forming the decision, the considerations that played key roles in the formulation of the plan, the mechanics of following through and implementing the new roof support system, and an assessment of the resultant gains in safety, productivity and improved costs derived from the implementation of the new roof support system.

Jan 1, 1986

By Charles D. Albright

FMC built its first angle bolter in 1969. This was a single mast unit which could drill holes for conventional bolts or angle holes for installing roof trusses. From this drill we learned very quickie that the mast design was an important factor to the success or failure of any angle bolter. The first mast installed on this machine (EXHIBIT 1) was fixed in length with endless chain and hydraulic motor feed. It took less than a shift underground to determine that the fixed length was not going to work. The undulating top created conditions where the mast was too long to be positioned at a 450 angle. Several mast designs were tried and discarded before we felt we had the answer to our problems. (EXHIBIT 2} We developed a mast that would extend or retract without moving the position of the drill unit. This allowed the operator to "wedge" the mast between the floor and roof before starting his drill feed. This arrangement worked exceptionally well, but unfortunately the demand, as perceived by our Sales Department, for angle bolters did not materialize. This unit became an "orphan" and was used by many coal mines until it died a natural death. FMC did not decide to re-enter this market again until 1979. At this time many coal producers were contacted to determine what features should he incorporated into a dual mast angle bolter. From this information we were able to arrive at three main areas of concern that we should consider when designing a new machine. 1. Eliminate overturning moment on drill unit. 2. Provide some sort of steel support near roof line to aid in starting a 15° hole. 3. Dual mast units need a method of raising center of mast rotation to facilitate various mining heights and still maintain a 45° angle and proper distance from rib. I would now like to discuss each of these individually. EXHIBIT 3 indicates the overturning moment resulting from the feed cylinder thrust and the reaction of the drill hit against the roof. To eliminate this moment a mast was designed using two feed cylinders mounted in the plane of the drill center line, thus removing all overturning moments on the drill unit carriage. This mast has been used extensively in vertical drilling with no downtime attributed to wear on mast guides and drill unit supports. The cylinders are designed to provide uniform thrust and feed speed throughout the total stroke. For manufacturing and economic reasons, the final mast design on the first drill was not used for this new drill. A drill steel support mounted on the mast, and positioned by a hydraulic cylinder provides the needed guidance near the roof for starting a hole at a 45° angle. (EXHIBIT 4) This support does not surround the drill steel but simple cradles the drill steel for "collaring" the hole. Once the hole is started, the support can he lowered. The support plate is hinged to allow the drill mast to pass. The advantage of having vertical motion of the center of rotation on a dual mast drill is shown in EXHIBIT 5. Following the criteria recommended by Birmingham Bolt Company, the truss bolts should he at a 45° angle and one fifth of the width of the entry from the rib. Therefore, in an entry 20 ft. wide, the hole should be drilled 4 ft. from the rib and at the 45° angle previously mentioned. With the limitations inherent to dual boom drills, that is, the masts cannot pass each other nor can one mast pass the center line of the machine even if the other is positioned out of the way, it is impossible to accommodate much variation in seam height and maintain the recommended criteria. As shown in EXHIBIT 5, a center of rotation 17 inches above the floor will start a hole 4 ft. and 45°angle at a height of 67 inches. Any height above 67 inches

Jan 1, 1982

By Anil Mahyera

Under contract to the Bureau of Mines, a roofbolting machine has been developed which will: control torque on the bolthead to within 4 percent of a set point, control thrust to a low level (150 lbf), and implement a tightening method to minimize variability due to bolt tension decay of a group of bolts caused by differing anchor creep rates. This bolter was used to determine bolthead and thread friction factors, and to determine the effect of adding a hardened washer to the bolt assembly on the uniformity of bolt tension in the laboratory and field. Recommendations for torquing speed, bolt lubrication, use of hardened washers, and thrust level on the bolthead are made. The relationship of this system to Improved roof control is discussed, along with additional work that remains to be performed. Patent Notice Patents are pending which protect the hardware and methods of controlling tension decay as discussed in this paper.

Jan 1, 1981

By Roy H. Grau

The trend in underground room-and-pillar cod mining is to employ remote-control continuous mining machines to take extended cuts of 40 A or more. Roof fall safety hazards to the mine worker associated with 4ed cutting are not completely de6ned but are known to include removal of the miner operator from the protective canopy, non-compliance with approved cutting depth and CM operator positioning. This report summarizes ground control research performed by the National Institute for Occupational Safety and Health (?-JIOSH) researchers at the Pittsburgh Research Laboratory concerning extended cuts. A combination of statistical analysis and underground investigations have been conducted. Statistical analysis of extended cut use, along with roof fall accidents and fatalities data delineate the extent of the problem in U.S. coal mines. Additionally, an underground study is described where the long-term roof stability of extended cut areas are compared to standard cuts.

Jan 1, 1997

By Robert M. Cox

Abrupt tailgate roadway ground failures, described as floor bumps, sometimes occur in the tailgate entry outby the face during the high-speed extraction of coal from mechanized longwall panels. These floor bumps typically damage the tailgate entry support structures for a distance of from 12.2 to 24.4 m (40 to 80 ft) outby the face. The relative closure of the tailgate entry may be a key indicator of potential tailgate ground control problems. Regression analyses of tailgate entry convergence data conducted by the U.S. Bureau of Mines (USBM) indicate that entry closure may be characterized as an exponential function of the distance outby the longwall face. Assuming that the entry closure is caused by the abutment loads created by the excavation of the longwall panel, the resulting convergence equation can be integrated to locate the centroid of abutment loading. This information can be used to document zones of potential ground failure ahead of the operating face.

Jan 1, 1994

By Jian Yong Chen

The results of a laboratory investigation of pullout tests conducted on grouted, plain cable bolts are reported. The main purpose of the study is to determine experimentally the shear bond stiffiess of cement grouted and resin grouted cable bolt systems, which is an important input parameter for numerical modeling of cable bolt support. Three nominal embedment lengths of 152mm (6 inch), 203mm (8 inch) and 254mm (10 inch) were used and two types of grout namely cement and Anchortite resin were used. Two water- to-cement ratios of 0.3 and 0.4 were employed for the cement grout. The concepts of effective shear stress and effective shear stiffiess are explained and adopted to calculate the shear bond strength and the elastic shear bond stiffiess per unit length. A comparison of results with previous research is presented to reveal the effect of water-to-cement ratio and embedment length on the shear bond characteristics of grouted cable bolts.

Jan 1, 2005

By David A. Newman

Strength and physical property testing of coal is conducted regularly by the Arch Mineral Corporation as part of reserve exploration. Pillar design and mine layout are the primary applications for the coal strength data. Distinct differences in pillar behavior were observed during longwall mining at a mine in Southeastern Kentucky. Horizontal and vertical variation in both triaxial and uniaxial compressive strength was observed within the four (4) benches of the Harlan seam. Similarly, geographical variations in coal strength were noted across the Harlan seam reserve. During the past three decades substantial research efforts have been focused upon coal pillar behavior and the refinement of pillar design methodologies. Ironically, little attention was directed to integrating the variability of coal strength into pillar design. The authors examine, through the presentation of case history data, i) typical industrial practice with respect to establishing a database of coal strength for use in pillar design and stability analysis, ii) approaches in which the observed variation in strength in the Harlan seam is integrated into pillar design and stability analyses, and to iii) propose standards for coal cube testing

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