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By David R. Hanson

Seismic tomographic imaging, based on the same principles as a medical CAT Scan (Computer-Aided-Tomography). has been used for many years in the oil industry for large-scale subsurface stratigraphic characterization. and has most recently been applied to various structure- and stress-related problems in the coal industry. New developments in signal processing have greatly enhanced the speed, resolution, and range of applications of topographic imaging in underground settings, and recent innovations in data acquisition hardware have further increased data reliability and repeatability. Whereas past structure mapping applications relied on Seismic velocity and/or attenuation tomopraphy within an enclosed seismic source gram receiver array, recent developments in "True Reflective Tomography- (TRT TM) have expanded the application of this technology considerably. For example, in-seam TRT TM seismic reflection tray now he used to image structure, and/or old workings from one general location in the mine face areas of mains and panel developments) well ahead of planned developments largely eliminating the need to probe-hole drill on regular intervals. This new reflective technology bas also been combined with traditional cross-hole seismic tomography-providing a comprehensive, powerful tool for velocity attenuation. and reflective imagine utilizing the same source/receiver array data sets (borehole-to-borehole. borehole-to-entry and entry-to-entry). These new developments in data acquisition and image processing have greatly expanded the variety of applications for tomographic site characterization allowing the mine operator considerable flexibility to cost-effectively characterize previously inaccessible areas of the property. This paper presents examples of 2-D and 3-D .seismic topographic imaging applied to (1) the location of old works in Appalachian room-and-pillar operations and (2) the identification of anomalous zones ahead of mining/tunneling developments these applications demonstrate the state-of-the-art in seismic ground imaging using velocity, attenuation. aril reflection techniques land combinations thereon, and further illustrate the flexibility of data acquisition from a variety of tinning arid tunneling settings. Recent examples are presented from projects in the United States and Europe.

Jan 1, 2000

By Jiachen Wang

China is the largest coal producing country in the world. Coal is widely distributed in China and thus the mining conditions are complex and diverse. This paper summarizes the important achievements in the last 20 years in various mining methods including the extra thick seam with large mining height top coal caving, the thick seam with large mining height and top coal caving mining, the steep seam with top coal caving and fully-mechanized mining, the thin seam automated mining, and the backfill mining. At the same time, the rapid advance in the development of mining machinery and manufacturing has also contributed to the advancement of coal mining technology and the great improvement of coal mine safety. The future direction of China?s coal mining is to further improve mine safety as to improve the coal recovery rate, promote the pillar less mining technology, develop high-efficiency backfilling technology, develop steep seam high-efficiency mining technology for seams less than 10m thick, and high recovery mining techniques for extra thick seams, improve the reliability of mining equipment, and strengthen the protection and restoration of surface environments and water resources.

Jan 1, 2014

By Brent A. Slaker, Khaled M. Mohamed

"Understanding coal mine rib behavior is important for inferring pillar loading conditions as well as ensuring the safety of miners who are regularly exposed to ribs. Due to the variability in the geometry of underground openings and ground behavior, point measurements often fail to capture the true movement of mine workings. Photogrammetry is a potentially fast, cheap, and precise supplemental measurement tool in comparison to extensometers, tape measures, or laser range meters, but its application in underground coal has been limited. The practical use of photogrammetry was tested at the Safety Research Coal Mine, National Institute for Occupational Safety and Health (NIOSH). A commercially available, Digital Single-Lens Reflex (DSLR) camera was used to perform the photogrammetric surveys for the experiment. Several experiments were performed using different lighting conditions, distances to subject, camera settings, and photograph overlaps, with results summarized as follows: The lighting method was found to be insignificant if the scene was appropriately illuminated. It was found that the distance to the subject has a minimal impact on result accuracy, and that camera settings have a significant impact on the photogrammetric quality of images. An increasing photograph resolution was preferable when measuring plane orientations; otherwise a high point cloud density would likely be excessive. Focal Ratio (F-stop) changes affect the depth of field and image quality in situations where multiple angles are necessary to survey cleat orientations. Photograph overlap is very important to proper three-dimensional reconstruction, and at least 60% overlap between photograph pairs is ideal to avoid unnecessary post-processing. The suggestions and guidelines proposed are designed to increase the quality of photogrammetry inputs and outputs as well as minimize processing time, and serve as a starting point for an underground coal photogrammetry study.INTRODUCTIONThe stability of underground mine ribs has been a safety concern for underground coal mines for decades. Unfortunately, the average rib fatality rate has been about 1.3 per year between 1996 and 2013 (Jones et al., 2014). In an effort to better understand coal rib behavior, photogrammetry is being investigated in this study as a measurement tool for surveying ribs in an underground coal mine environment."

Jan 1, 2016

By Marvin B. Thompson

"The depositional model for major Illinois Basin coalbeds is well documented and defined on a regional scale. The model was a driving force during past exploration for lower sulfur and thicker coal deposits. The most attractive coal reserves were known to lie along the flanks of contemporaneous sandstone filled stream channels and were overlain by thick silty gray shale roof. At various distances away from the channels, the depositional model transitioned to black shale I limestone roof, sulfur content of the coal bed increased, and the coal bed often thinned. Understanding the character of roof rock types associated with this depositional model is essential to predicting ground control factors on both a regional scale, and on resolving roof control problems at the mine-site. Additionally, horizontal stress and overburden depth are significant factors that impact roof rock quality and ground control in Illinois Basin mines.Determining the character of several rock types, and defining the primary horizontal stress direction were crucial to a recent successful project at New Future Mine, located in southern Illinois. An inclined slope was constructed from the Illinois No. 5 Coalbed up to the Illinois No. 6 Coalbed through strata that ranged from extremely weak ""stackrock"" and a weak underclay unit, to a massive sandstone unit. Engineering geology, operations management, and Jennmar technical services personnel teamedup to select and install roof support for each strata transition. The project was completed with no injuries and no roof falls.INTRODUCTIONAbundant published information concerning coal deposition, geographic distribution, quantity, quality, and mining conditions exists for Illinois Basin coal reserves. The State of Illinois, through its State Geological Survey (ISGS), has provided decades of research, record keeping, and service to the coal industry. Other excellent sources of Illinois Basin coal information include the Indiana Geologiq1l Survey and the Kentucky Geological Survey. Past research by the U.S. Bureau of Mines (USBM) supplemented the work by the geological surveys, especially in the area of horizontal stress and mining hazards. The National Institute for Occupational Safety and Health (NIOSH) continues to produce publications focused on roof control issues common to many coal basins, including the Illinois Basin. Many of the illustrations in this paper originated in ISGS and USBM publications."

Jan 1, 2010

By Uwe Wyink, Guenther Thiele, Isabel Gollnick, Klaus Thyrock

"The application of a thin spray on liner was implemented in longwall T-junction sections in 2009 for the first time. The development of the material and the application is based on a technical cooperation between RAG Deutsche Steinkohle AG and BASF Corporation. The report gives an overview of the first trials. The underground test occurred in a seam with a height of 1.5 m (5 ft) in a depth of 1,100 m (3,609 ft). The roadway is dual-use entry and the roof strata is composed of shale and sandy clay. In some sections geological faults impact the surrounding strata. The poor conditions induce separating and loosening of roof rock. Several remedial actions have to be implemented for decreasing deformations at the face entrance.The application of thin spray on liner (TSL) provides simplification and cost reduction of supporting work at longwall T-junction. The installation of TSL to avoid roof and coal fall operations for lagging are omitted.INTRODUCTIONFor safe operation of face retreat in dual-use roadways, the processes of securing and stabilizing the face end area are very important in order to minimize deformation and stress. Avoiding roof and coal fall are the main concerns for safe operation. For that reason different methods of reinforcement are tested in underground operation. In this paper the application of TSL is presented.The specifications of TSL are described, followed by the case study results. The underground study is located in one mine of RAG Deutsche Steinkohle. The extraction of the hard coal in this mine is characterized by great depth and high stress. There are poor geological and geotechnical conditions. The roadway used for the underground application was headed with the support system combination type A, which will be described later in the paper. After the first face retreat, deformation of steel arch support and bagging destroyed the roadway, so for the second face retreat remedial actions had to be undertaken. For this a new system of reinforcement, simplifying the supporting work at the longwall T-junction was tested. The reinforcement consisted of different parts that are described in the following report. Furthermore two different versions were applied. One method is the operation with TSL and the other one is the use of concrete for sealing. Both methods are compared in the text."

Jan 1, 2010

By Jesse Puller, Ken W. Mills, Owen Salisbury

"An underground coal mine located in New South Wales has a target coal seam located 160-180 m deep directly below a 16-20 m thick conglomerate unit that has been associated with significant periodic weighting events on the longwall face. As part of the investigations to better understand the causes of periodic weighting at the mine, inclinometers capable of measuring horizontal shear movements through the full section of the overburden strata were installed ahead of mining at two locations approximately 1 km apart above the centre of two longwall panels. These inclinometers were monitored as the longwall approached each site. This paper presents the details of the installation, the results of the inclinometer monitoring at both sites, and the insights that these measurements provide for overburden behaviour about longwall panels.Horizontal shear movements were observed to develop on shear horizons that correlate closely across the two sites suggesting a mechanism that is consistent across a large area of the mine. Shear movements were observed to develop on a single horizon near the top of the conglomerate strata that was mobilised almost immediately after initial formation of the longwall goaf at a distance of 425 m ahead of the longwall face. The direction of horizontal movement was consistent with the relief of the major principal horizontal stress. As the longwall face approached each inclinometer, other horizons within the upper part of the overburden strata begin to shear with the upper strata moving toward the approaching goaf more than the lower strata. Within the last 30 m of longwall approach, tilting of the strata associated with the increase in vertical subsidence toward the extracted goaf caused a change in the style of deformation with tilting and reverse shear offsets.INTRODUCTIONLongwall mining is recognised from subsidence monitoring, surface extensometer measurements, and various other observations to cause significant disturbance to the overburden strata directly above the extracted goaf of each longwall panel. Disturbance to the overburden strata beyond the limits of each longwall panel is not so well understood. Subsidence monitoring indicates that horizontal movements beyond the edges of the extracted longwall panel are generally greater in magnitude than vertical subsidence and horizontal movements may extend up to several kilometres from the edge of the panel in some circumstances (Reid 1998, Reid 2001, Hebblewhite et al 2000, Mills 2001, Mills 2014). The mechanics of how these movements are accommodated within the overburden strata is only poorly understood and there are few direct measurements."

Jan 1, 2015

By Jonathan P. Furniss

The paper describes a range of laboratory tests undertaken on full column resin anchored glass fibre rockbolt systems, to assess all aspects of their performance. Laboratory instrumentation has been developed for the accurate study of strain transferral from the Glass Fibre Reinforced Plastic (GRF') bolt, through the surrounding resin annulus, and into the rock. Experience in laboratory testing to the new British Standard on glass fibre rockbolting is discussed. A critical comparison is made of the influencing parameters affecting . underground rockbolted roadway performance to the parameters assessed by BS 7861 : Part 1 The work has been co-funded by MA1 Systems UK Ltd. who supply GRP rockbolt components to the British coal mining industry, mainly for the reinforcement of coal ribsides. In rockbolting, glass fibre has a number of useful * qualities which are discussed in the paper. These properties, make the product more suitable than steel in certain applications, both in mines, and other engineering structures such as tunnels, soil stabilisation, and coastal defences.

Jan 1, 1997

By Jimenez Leon, Christopher Newman, Boede Gabriel, Zacharias Agioutantis

"Ground movements due to longwall mining operations have the potential to damage the hydrological balance within as well as outside the mine permit area in the form of increased surface ponding and changes to hydrogeological properties. Recently, the Office of Surface Mining, Reclamation and Enforcement has completed a public comment period on a newly proposed rule for the protection of streams and groundwater from adverse impacts of surface and underground mining operations (80 FR 44435). With increased community and regulatory focus on mining operations and their potential to adversely affect streams and groundwater, there is now a greater need for better prediction of the possible effects mining has on both surface and sub surface bodies of water.As mining induced stress and strain within the overburden is correlated to changes in the hydrogeological properties of rock and soil, this paper investigates the evaluation of the hydrogeological system within the vicinity of an underground mining operation based on strain VALUES (calculated through a surface deformation prediction model. Through accurate modeling of the pre- and post-mining hydrogeological system, industry personnel can better depict mining induced effects on ground water flows through the overburden material aiding in the optimization underground extraction sequences while maintaining the integrity of surface and subsurface bodies of water.INTRODUCTIONThe utilization of high-recovery underground mining methods, such as longwall or high-extraction room-and-pillar operations, have the potential to cause adverse impacts to both surface and subsurface bodies of water as strata movement and deformations propagate from the mined seam through the overburden to the surface (Peng, 2008).Previous research has indicated that mining induced strains are the most damaging to surface streams (Singh, 1992) as well as greatly affecting the integrity of subsurface bodies of water and groundwater flow conditions (Booth, 1986). On the surface, adverse effects to the stream can occur due to the development of either tensile or compressive strain in the stream bed. The development of tensile cracks along the bedrock allows for a potential loss of stream flow through developed fissures. In fact, water flow in the Cataract River of Australia ceased in 1994 as a result of mining-induced strains from longwall operations in the Bulli seam 430 meters (1320 feet) below the river gorge (McNally and Evans, 2007). On the other hand, the development of compressive strains within the rock layers can cause rupturing or buckling of the stream bed, blocking stream flow and/or diverting flow into the fractures at the base (Iannacchione et al, 2010). While these localized fractures can contribute to the loss of stream flows, given time, damaged streams have the ability to self-heal through the regeneration of near-surface aquifers as well as the sealing of mining-induced fractures with rock debris, gravel, sand, clay or other soil particles carried from upstream sources and deposited in the river bed (Waddington and Kay, 2002)."

Jan 1, 2016

By John R. Poland, Benjamin Mirabile

"There are significant ground support hazards to both personnel and equipment when longwall equipment must mine through cross-panel entries. Backfilling support strategies are often cost prohibitive. Additionally, it is often very difficult to install backfill material in full contact with the mine roof in backfilled entries. This paper will demonstrate the use of fiber-reinforced concrete crib material as a method for standing support in longwall cut-through entries. Despite conceptions of poor performance in potentially high convergence environments, fiber-reinforced concrete crib material can be used in longwall support environments subject to high abutment loads and convergence. Central to the proper design of these supports is the use of yield material between crib layers and at the mine roof/crib interface. The placement location of cribs within the cut-through entry has a significant effect on the main roof behavior and associated ground conditions in the longwall abutment zones. It is particularly important to engineer the proper load displacement characteristics of the installed crib to support expected ground conditions. The cost of fiber-reinforced crib supports is less than 50% of backfilling methods.INTRODUCTIONThe design and execution of a longwall cut-through is perhaps one of the most difficult and risky endeavors in the field of ground control. The magnitude of abutment loads and potential for ground failure present a significant safety hazard to underground personnel. In addition, there is significant financial risk, with replacement costs of face equipment exceeding $200 million. Faced with these consequences, it is both advisable and common for an engineer to design ground support with a significant margin of safety. This paper outlines a robust support system that possesses a significant margin of safety along with significantly lower costs than most other support systems employed in similar conditions.SITE PARAMETERSThe mine site described in this study operates in the Herrin No. 6 seam in southern Illinois. The seam height is typically 76- 84 inches. Mining height in gateroads and mains is typically over 96 in, and typical mining height on the longwall face is 82 in. The overburden depth is 950-970 feet in the area of the longwall cutthrough."

Jan 1, 2016

By Chris Mark

Perhaps the most significant development in coal mine ground control during the last century was the introduction of roof bolting during the late 1940's and 1950's. From an engineering standpoint, roof bolts are inherently more effective than the wood timbers they replaced. Roof bolts promised to dramatically reduce the number of roof fall accidents, which then claimed hundreds of lives each year, and they were initially hailed as "one of the great social advances of our time." Roof bolting also emerged at a time of rapid technologic transformation of the coal industry, and greatly accelerated the transition to trackless, rubber-tired face haulage. The U.S. Bureau of Mines quickly became the new roof support's strongest advocate. Some state agencies and miners were skeptical at first, but nearly everyone was soon won over. Case histories were reported showing that roof falls could be largely eliminated while productivity increased dramatically. Little wonder that, in the words of one contemporary observer, "roof bolting has been adopted more rapidly than any other new technology in the history of coal mine mechanization." Yet by the end of the 1950's, it was clear that roof fall fatality incidence rates had actually increased. It would be another decade before the superior ground support provided by roof bolts would clearly save lives, The story of how roof bolting was implemented by the mining industry, but took so long to live up to its promise, is a fascinating example of the interaction between economics, technology, regulation, and science. It still has important lessons for today.

Jan 1, 2002

By Yunqing Zhang

The 3-D numerical modeling using ABAQUS is performed to analyze the mechanisms of the tensioned bolts by taking into considerations excavation sequence, pre-tension, bedding planes and in-situ horizontal stresses. The effect of the pre-tension and the mechanisms of the tensioned bolts are analyzed based on the results from the modeling. Based on field observations, the failure modes for the tensioned bolted strata are classified by their causes and failure mechanisms. The support design consideration for each failure mode is given. Finally, a new design approach is proposed to do the tensioned bolting design using numerical method.

Jan 1, 2002

By Christopher Mark

Mine planning for a new reserve is based on information obtained from exploratory coreholes. A critical component of an exploration program is the geotechnical evaluation. Poor assumptions about roof conditions greatly add to the risks of mining. Rock mechanics testing is central to a geotechnical exploration program. Typically, 3 to 5 uniaxial compressive strength (UCS) tests are made to characterize a particular roof unit at a given corehole. The average (mean) of these tests is taken as "The UCS" for that location. Isopach contour maps are then used to show spatial trends in roof strength. Two issues are raised by this traditional approach. The first is due to the large variability in UCS values that is typical even within a single unit from a single hole. The average UCS might be higher at Corehole A than it is at Corehole B, but the difference may not be statistically significant. The second issue is whether widely spaced coreholes can identify valid spatial trends in rock strength. The answer depends upon whether rock strength changes over distances that are longer or shorter than the corehole spacing. This is a classical geostatistical problem. While geostatics have been used to investigate many coal quality parameters, they have seldom been used to evaluate rock strength. This paper describes an extensive investigation of these issues conducted by the National Institute for Occupational Safety and Health (NIOSH) in collaboration with Peabody Energy. The study employed the Peabody Rock Mechanics Data Base which contains more than 10,000 individual test results. Data from four important roof units were subjected to statistical analysis: ? Brereton Limestone above the Herrin 6 seam (Illinois) ? Turner Mine Shale above the No. 9 Seam (Kentucky) ? Sandstone above the Eagle Coal (West Virginia) ? Shale above the Eagle Coal (West Virginia) The study did not find significant spatial trends in rock strength in any of the cases. Perhaps there are none, or perhaps the exploration coreholes were just too far apart to see them. These results have valuable implications for the design of geotechnical exploration programs.

Jan 1, 2004

By James Pile

The San Juan Coal Mine, located near Farmington, New Mexico, supplies the San Juan Generating Station with more than 6 million tons of coal annually. To replace dwindling surface mine production, San Juan installed a longwall in 2002. The roof rock in much of the underground reserve consists of interbedded coals, carbonaceous shales, and mudstones. Roof support consists of fully-grouted roof bolts, both tensioned and non-tensioned, supplemented by modified cable trusses. Short-encapsulation pull tests conducted by San Juan's geotechnical engineering department showed that some of the immediate roof layers provided variable bolt anchorage. To ensure that the roof bolts were obtaining the necessary anchorage, additional tests were conducted in a variety of locations. The results have been used to determine the optimum support design for different areas of the mine. Another series of tests evaluated the performance of several bolt and hole diameter combinations. The experience at San Juan Mine shows that short-encapsulation pull tests can provide the information necessary to improve ground control in variable roof conditions. It also illustrates how a pro¬active geotechnical engineering program can be an essential component of a state of the art underground coal mine.

Jan 1, 2003

By Kot F. von Unrug

One of the major difficulties in the proper characterization of rocks surrounding excavations is the lack of data. The existing method for strength determination requires core drilling for sample collection, which, when performed from the surface is costly, and when performed underground, in addition to its cost, interferes with the mine operations. I-landling samples tested in the laboratory is troublesome and also influences the obtained results. Over several years. the author has been interested in developing a method of in situ strength determination that could rapidly generate strength data at low cost without a negative effect on mining activities. The recently developed borehole penetrometer is an improved version of the initial design ( 3 inches in diameter), which was used mainly for strength determination of coal pillars. The new penetrometer has been designed to work in 1.5 inch diameter holes. Such holes can be drilled at low cost with either hand held drilling machines or roof bolters. eliminating limitations of the previous system and opening the possibility of broad application. Strength testing of roof rock around longwall pillars is an example of an obvious benefit when designing secondary roof support. In the paper are given the principles of work of the apparatus and examples of the obtained strength data.

Jan 1, 1998

By Nicholas P. Kripakov

The unpredictable massive collapse of roof in openings developed under apparently safe and stable roof conditions in coal mines of the United States has been of much concern for many years. One type of massive caving phenomenon is often attributed to cutter roof, a failure that initiates at one and/or both upper corners of an entry and propagates nearly vertically resulting in overall failure with little or no warning. This paper reviews current methods being used to control cutter roof and suggests new alternatives. The impact of basic parameters that include mine geometry, overburden geology, rock properties, and in situ stress conditions is discussed in the context of their relationship to this problem. The effects of various stress control measures are investigated with model analysis to gain insight into means of reducing the high level of stress known to exist at the entry corners of a West Virginia coal mine prone to cutter roof. The results are presented in the paper and the feasibility of their practical implementation is discussed.

Jan 1, 1982

By Jay Hilary Kelley

This paper proposes a modification of longwall roof support to meet new difficult mining conditions that are anticipated in the future. A gob canopy is a movable appendage attached on the rear of a longwall shield to counterbalance the troublesome forward downward force couple (FDFC). The gob canopy can be swung in both vertical and horizontal directions, powered by hydraulic cylinders. Several functions and applications are de- scribed

Jan 1, 1997

By D. E. Winston

Roof falls are the major cause of fatalities in the, coal mining industry and the prevention of roof failure is a major concern of mine management. Many ground failures observed underground are associated with geological discontinuities such as fractures, faults, clay veins and sandstone channels inherent in the earth's crust (Rinkenberger, 1977). To properly design a mine requires an advance knowledge of such geologic discontinuities and this is not always available from exploration boreholes. The use of remote sensing for mine planning is a relatively new development, and is at present being used primarily by the Roof Control Support Branches of the Mane Safety and Health Administration (MSHA) in Denver and Pittsburgh. However, technical advances, coupled with public access to previously unavailable photographic products, make remote sensing a useful aid in the difficult process of mine planning (Jansky and McCabe, 1979). Remote sensing techniques are being used to predict the location of geological features in advance of mining which influence roof conditions. The features appear on photographic images as linears, which are plotted on a mine map and serve as a planning tool for mine management. Also, during development of entries toward a linear, mining personnel will be on alert for signs of unstable roof conditions which might normally go undetected. Preventive action can then be taken before a fall occurs. A confusing aspect in remote sensing, analyses is terminology such as linear, lineament, fracture trace and joint. In this report the term "linear" refers to a line-like appearance on an aerial photograph or satellite imagery which signifies a naturally occurring geologic feature no the earth's surface. The objective of this paper is to describe the use of remote sensing techniques as an aid in predicting geological discontinuities which might produce hazardous roof conditions to a mine operating in the Pocahontas No. 4 Seam in southern West Virginia. Mine projections and roof control plans will he developed based on this and other available information such as data from cored drill holes.

Jan 1, 1982

By Brian Clifford

A wide range of different cablebolt systems are available and in use in hard rock and coal mines around the world. The birdcaged cablebolt was initially used in UK coal mines in conjunction with rockbolts in the late 1980's. More recently other cablebolt configurations have been used. Innovations in cablebolt design in the UK have been driven by a demand to find a system as effective as the birdcaged cable, but which can be installed more rapidly and offer immediate support. A laboratory short encapsulation pull test (LSEPT) has been developed for both rockbolt and tendon systems which measures bond strength and stiffness in rock and assesses the full range of potential failure modes. This test is now used along with conventional 'gun barrel' and shear tests to provide full assessment of bolt and tendon performance. This test may be incorporated into a new performance based British Standard for rockbolts and long tendon systems. This paper describes the performance of some of these systems as being applied to UK and South African coal mines.

Jan 1, 2001

By Dale A. Didcoct

Through the combined efforts of the C. S. Bureau of Mines, the coal industry, and Burrell Construction and Supply Company, New Kensington, PA, a steel fiber reinforced concrete crib block to improve coal mine safety in the area of roof control has been developed and is currently in use commercially. The development objective was to improve health and safety conditions with a stiffer, stronger, nonflammable roof support at a competitive cost. During the time this method of roof control has been in use in the mining industry, it has proven to be both functionally and economically effective.

Jan 1, 1982

By Stephen C. Tadolini, John P. McDonnell

"High-strength cable bolts have been used for ground support in underground coal mines for many years. Cable bolts are routinely used as secondary ground support in numerous longwall gateroad support applications, areas that possibly experience the most severe conditions in current underground coal mining. Additionally, when mining ground conditions deteriorate, engineers oftentimes recommend cable supports as a principal solution because of the established credibility for successful ground control. Nevertheless, there is still a reluctance to utilize non-tensioned cable supports as an effective primary roof support alternative.This paper provides historical and recently documented examples of non-tensioned cable supports utilized as an effective primary ground support method in a western U.S. underground coal mine, and it will continue the technical discussion between the differences of various degrees of tension in cable supports with respect to ground behavior and reactions.INTRODUCTIONResin-grouted cable supports are widely used to support difficult ground conditions throughout the world. Through the use of innovative applications and configurations they have proven to be a major paradigm shift in roof support and ground control technology in the U.S. (Tadolini and Koch, 1993; McDonnell et al., 1995; Tadolini and Gallagher, 1994; Tadolini and Trackemas, 1994)Today, the basic cable bolt support consists of a high-strength steel cable installed in bore holes that range in diameter from 25 to 50 mm (1 to 2 in) depending on the strand size and drilling equipment. Cables are offered in both bright and galvanized, for corrosion protection, and range in capacity from about 30 to 60 tons. An example of a resin-grouted cable bolt and the key components is shown in Figure 1. The steel cables are flexible and the support length is not limited by the height of the opening. Lengths of up to 10 m (32 ft) have been supported with high pattern densities when standing supports were not operationally feasible. The most common lengths are 3 to 5 m (10 to 16 ft). An important physical characteristic is that PC-strand has an ASTM minimum elongation requirement of 4%. For comparison purposes, the ASTM 432-08 Standard Specifications for Roof and Rock Bolts and Accessories specifies elongation requirements for traditional roof bolts are a minimum of 12% for grade 40, 9% for grade 60, and 8% for grade 75 materials, respectively. Traditional roof bolts can stretch and elongate twice the length that cable bolts can, which makes design considerations challenging."

Jan 1, 2010