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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 Antoni Kidybinski

Coal mining both with longwall and room-and-pillar method often encounters severe operational difficulties due to had roof conditions. Rooffalls at entry cross-cuts or along the rib line in longwalls as welt as dynamic and excessive loads on supports due to overhanging of strong roof layers in a goab - are well known phenomena to the mine operators. Also, a number of practical examples may be quoted of dramatic improvement of working conditions after changing of coal winning technology or supports type. It is therefore a question to be answered how far we are able to recognize roof strata properties before mining starts and adjust both coal extraction technology and supports of entries to rock stability foreseen. Furthermore, engineering principles and criteria for decision making should be established. In recent years several in situ rock strength and discontinuities detection testing techniques have been developed both indirect (geophysical) and direct thus making possible closer roof-strata examination. In this paper critical roof span criterion is discussed as a basis for selection of proper mining system in terms of desirable stability of mine opening .

Jan 1, 1982

By Thomas M. Barczak

A survey of the longwall industry was conducted to examine the performance of modern shield technology. The results of this survey indicate that state-of-the-art shields perform better and last longer than ever before, but premature failures do still occur. In addition, longwalI operators are now using shields longer than ever before and a greater number of fatigue related failures on aging shields are now occurring. A generic assessment of these shield failures is provided, describing the nature of the failures and how widespread they are throughout the industry. An engineering assessment of shield design and factors that cause shield failures is made using both strength of materials and fracture mechanics principles. From this analysis, design practices to improve structural margins of safety and extend the life of the shield are proposed. The existence of unexpected shield failures indicate that current shield performance testing is at times inadequate. Recommendations are made relative to improved shield performance testing protocols including the benefits of testing in an active load frame such as the National Institute for Occupational Safety and Health's (NIOSH) Mine Roof Simulator. Even the most rigorous shield testing procedures, which strive to simulate in-service loading conditions, fail to consider key environmental issues, such as corrosion which is often the cause of structural fatigue failures in modern shields. The survey also indicated that hydraulic failures occur much sooner in the life cycle of a shield than structural failures, and although they can significantly degrade support capability, often go undetected for long periods of time. Methods to detect the hydraulic failures are also provided. The paper concludes with key points to maximizing shield design and life expectancy and ideas for future generation shield designs.

Jan 1, 1999

By John G. Oldsen

Passive cable bolting and active cable trusses have been used in roof support for more than two years Excellent roof control has been experienced by many coal operations. This paper covers the new and innovative developments in cable support systems and their application in mines. Multiple cable bolts, tensioned cable bolts, coal rib wrapping, and other cable applications will be covered.

Jan 1, 1997

By K. Haramy

Coal mine bursts or bumps involve the violent, rapid failure of coal and rock in or around a mine excavation. Failure is normally associated with high stress and brittle or brittle-elastic materials; in coal mining, bursting may also be associated with desorbing gas, and this type of failure is termed an outburst. This paper discusses factors affecting coal mine bursts and several methods to predict bursts, including microgravity, microseismic, rheological, rebound, and drilling-yield methods. Methods to avoid burst conditions using volley firing, hydraulic fracturing, and auger drilling are also discussed. A case study is presented in which the redistribution of stresses and displacements was found to be associated with a burst at a deep longwall coal mine. The research showed that stresses are redistributed following the burst.

Jan 1, 1984

By Ross Seedsman

Discussion on the design of roof support in tailgates has often been conducted without a clear statement of the stress and failure conditions acting. There is general agreement that in the tailgate the vertical stresses above the chain pillar increase. Within the stress 'bulb' associated with this vertical stress increase there will be an increase in horizontal stress. This does not mean that the horizontal stresses acting across the roof line increase. In retreat longwalling, the proximity of the tailgate to the adjacent goof results in a substantial reduction in the horizontal stress field acting in the immediate roof. Furthermore, at the face/tailgate corner the onset of significant compression of the chain pillar. if it is designed to yield, will result in a further reduction in the horizontal stresses in the immediate roof. It is suggested that this latter mechanism can cause the horizontal stresses in the immediate roof to go to zero. Whatever, the horizontal stress acting across the roof in the face/tailgate corner will be significantly lower than in the face/maingate corner. The implication of the model is that support design in tailgates should be based on the assumption of zero horizontal (in fact tensile) stresses. How the roof behaves in a tensile stress environment is a function of joint spacing and orientation compared to excavation width and direction. It is speculated that the empirical relationship between support density and roof rating results from the relationship between joint spacing and bedding spacing. The onset of stress reductions has major implications to the design of cable and bolt anchorages in the tailgate environment.

Jan 1, 2001

By Thomas M. Barczak

Roof support systems are necessary to provide stable mine openings and much research has been conducted to design a variety of roof support systems that will function in various manners to ensure that stable ground conditions are achieved. Despite these advancements in technology, mistakes continue to be made in the evaluation and/or installation that significantly degrade the support capability or lead to erroneous determinations of support expectations. The purpose of this paper is to discuss misconceptions about how roof supports perform and factors that impact their performance. The goal is to present practical information that will assist mine operators and engineers in selecting, installing, and evaluating roof support systems properly, and help them to avoid mistakes that can lead to erroneous expectations and potentially catastrophic results that may lead to roof falls. The paper is limited to a discussion of secondary roof support systems and powered roof supports such as longwall shields.

Jan 1, 2001

By I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

By Gang Ren, Behrooz Ghabraie

"Observational data suggest that the ground surface subsidence profile from multiple-seam longwall mining is rather different from that of single seam. Despite the increase of coal production from multiple-seam mining, available predictive methods are mostly unable to predict a reliable subsidence profile for multiple-seam extractions. In this paper, a case study of multiple-seam subsidence is modelled using physical modelling techniques. The subsidence mechanism as revealed in the physical model is explained and the differences between multiple and single-seam subsidence are highlighted. On the same case study, a commonly used influence function method (IFM) and Generalised Influence Function Method (GIFM) are utilised to predict the subsidence profile. The results are compared with predictions achieved by a newly developed predictive method based on the understanding of mechanism of the multiple-seam subsidence. Results show a significantly improved correlation between the newly developed method in comparison with the conventional IFM and GIFM. INTRODUCTIONField observations and subsidence monitoring data from multiple-seam coal mining cases show a different subsidence profile as compared with single-seam ones (Sheorey et al., 2000; Li, et al, 2007; Luo and Qiu, 2012; Unlu et al, 2013). The multiple-seam subsidence curve is generally deeper and steeper than of single-seam cases, and varies from one mining configuration to another. Various researchers had made efforts to understand the mechanism behind this phenomenon and the interaction between the two mining horizons (Li et al., 2010, Sui et al., 2014; Zhang et al., 2014). However, the mechanism of the multiple-seam subsidence is yet to be comprehensively understood (Suchowerska, 2014; Ghabraie et al., 2015b).In addition, differences between the single and multiple-seam subsidence profiles make it difficult to adapt single-seam subsidence prediction tools for predicting the subsidence over multiple-seam cases, unless significant modifications are applied This requires understanding the mechanism of the multiple-seam subsidence as the first step to achieve reliable predictive results.The mechanism of the subsidence due to a multiple-seam case study can be investigated via physical modelling. The physical model allows observing the substrata movement as well as the interaction of the two mining activities. An understanding of the mechanism of the multiple-seam subsidence can be developed based on these observations, which would provide basis for developing a subsidence prediction tool."

Jan 1, 2016

By William Bookshar

The development of increasingly more sophisticated and powerful retreat longwall equipment has led to ever wider and longer panels. To maximize gateroad development efficiency, provide for effective face recovery, andlor reduce ventilation pressures, entries are cut across longwall panels in advance of longwall retreat mining. These entries must be supported to prevent falls of immediate roof in advance of the face. Recently, cut through entries and predriven recovery room entries have been supported with various combinations of roof bolts and passive support. The cost and relative success of these support systems has varied considerably. Eighty-Four Mining Company, Fosroc, and Sub-Technical have developed and implemented an efficient, yielding, multiple component, cement crib to support longwall panel cut through entries and pre-driven recovery rooms. The cutable crib consists of an inexpensive flyash cement pillar poured in a bag, within a temporary metal form. The form is moved to a subsequent cutable crib location after the initial hardening of the flyash pillar. The remaining entry height between the mine roof and the top of the flyash cement pillar is filled with a specially designed bag filled with Fosroc's Tekset cement formulation. The diameter of the pillar and thickness of the Tekset bag are designed to accommodate the anticipated front abutment loading and entry convergence.

Jan 1, 1998

By Zach Agioutantis, Christopher Mark

"Thirty years ago, the Analysis of Longwall Pillar Stability (ALPS) inaugurated a new era in coal pillar design. ALPS was the first empirical pillar design technique to consider the abutment loads that arise from full extraction, and the first to be calibrated using an extensive database of mining case histories. ALPS was followed by the Analysis of Retreat Mining Stability (ARMPS) and the Analysis of Multiple Seam Stability (AMSS). These methods incorporated other innovations, including the Coal Mine Roof Rating (CMRR), the Mark-Bieniawski pillar strength formula, and the pressure arch loading model. They also built upon ever larger case history databases and employed more sophisticated statistical methods.Today, these empirical methods are used in nearly every underground coal mine in the US. However, the piecemeal manner in which these methods have evolved resulted in some weaknesses. For example, in certain situations, it may not be obvious which program is the best to use. Other times the results from the different programs are not entirely consistent with each other. The programs have also not been updated for several years, and some changes were necessary to keep pace with new developments in mining practice.The Analysis of Coal Pillar Stability (ACPS) now integrates all three of the older software packages into a single pillar design framework. ACPS also incorporates the latest research findings in the field of pillar design, including an expanded multiple seam case history data base and a new method to evaluate room and pillar panels containing multiple rows of pillars left in place during pillar recovery. ACPS also includes updated guidance and warnings for users and features upgraded help files and graphics.INTRODUCTIONPillar design is one of the fundamental elements of mining engineering. Pillars are necessary to control the great weight of the overburden during all phases of underground mining. Without stable pillars, ground control is impossible."

Jan 1, 2018

By Dion Pastars

Kestrel Coal has undertaken a surface-to-seam consolidation program of faulted ground prior to longwall retreat in the 304 panel. Micro fine cement is used to improve the rock mass quality and therefore reduce the risk to personnel and equipment whilst increasing previously assumed sterilised reserves. The program is based on information attained from previously successful grouting campaigns in the Queensland coal area and follows the decision making process to ensure there is sufficient spatial permeation of cement across the faulted area. This data is used to assess the residual risk to the business in terms of health, safety and production

Jan 1, 2007

By Yi Luo

Accurate mechanical and geological information of the roof strata is vital for roof bolting design in underground mines. In order to obtain such information in a timely manner, a research has been under way to develop a technology for mapping the roof geology using the drilling parameters acquired during the roof bolting operations. In this paper, a systematic approach has been proposed for determining the rock strengths using the acquired real-time drilling parameters. A computer simulation program based on the mathematic model has been developed to verify the approach and to study its sensitivity. The method has been used to interpret the acquired drilling data.

Jan 1, 2002

By Rocky Wu

Rock bursts are one of the greatest challenges to ground control in the mining industry. With increasing mining depth and mining scale, there are more and more industry requirements on yielding rock support. Since 2008, Jennmar has been conducting large scale research and development works to develop a technically reliable and cost-effective yielding rock bolt. This paper introduces a new yielding rock support: the Yield-Lok bolt. The bolt is characterized by the designed yielding ability to produce 150?250 mm (6?10 in) of deflection at 8?10 metric tons (MT) (7.9?9.8 t) loads for every 16.4kJ (12,096 ft lb) energy input. The bolt?s performance characteristics are consistent through multiple and varying amplitude of impacts. In this paper, the design criteria of the bolt and the principle of performance are described; the dynamic testing results are discussed; and the features and application of the bolt are presented.

Jan 1, 2011

By Francis S. Kendorski

Rock reinforcement has been in widespread use and generally has been accepted in underground mining and tunneling since the 1950s. The first rock reinforcement technologies employed were mechanical anchors such as split wedges or expansion shells with 5/8-inch-diameter steel bolts. Failures occurred in weak strata which provided poor anchorage or in ground with corrosive waters. Friction rock fixtures consisting of relatively thin-walled tubes have been in use for about 15 to 20 years. While generally performing adequately, longevity problems have developed from corrosion in water bearing ground Longevity of rock reinforcement is much enhanced with grouted bolts. Portland-cement-grouted rock reinforcement has been in use since the mid-1950s, primarily in funneling and other civil engineering underground construction. Tests of decades-old installations have revealed few problems except in ground with aggressive waters. Polyester-resin-grouted rock reinforcement was introduced in the United states to musing in the late 1960s and to tunneling in the early 1970s. Experience from 30 years of resin-grouted bolt installations and field tests have identified longevity problems associated with degradation of steel reinforcing bars, but in generally unusual situations. Improvements continue in resin chemistries, packaging, corrosion protection, grout quantities, and mixing and distribution in the drilled hole to achieve long-term performance. Specific case histories cited with resin- or Portland-cement-grouted rock reinforcement longevity or performance problems, upon close examination, reveal that the causes of the problems were qualify control procedures being inadequate and not in accordance with good practice. Manufacturers' recommendations and engineering specifications, if followed in the field, and with competent inspection and supervision, would have prevented must, if not all, reported longevity or performance difficulties.

Jan 1, 2000

By Hamid Maleki

The U.S. Bureau of Mines implemented a monitoring program in a western U.S. coal mine for the evaluation of both conventional and alternative support systems. The conventional support consisted of 2.5-m-long vertical bolts installed in an 5.5-m-wide entry. The alternative system consisted of 2.5- and 3-m vertical and inclined bolts installed at a higher density during widening of a cut from 3.6 to 5.5 m. Roof deformation, roof stress, rib deformation, and bolt loads were monitored during the 2-month test period. In addition, roof failure patterns and lithology were characterized within the six-entry main test section. Results were analyzed using roof deformation and failure profiles, roof stress profiles. bolt loading and bending moment diagrams, and numerical models. The mechanics of roof deformation, failure, and support reaction in three zones in the mine roof were characterized. Significant improvements were shown in roof stability in the section using the alternative system. Results are encouraging and indicate the effectiveness of alternative support systems and excavation techniques for limiting inelastic deformation of roof in highly stressed underground mines.

Jan 1, 1994

By Gerald I. Finfinger

Identifying the properties of overlying rocks in underground mining operations is important to ensure the appropriate roof support design is used to maintain stability of the mine entries. Recently J. H. Fletcher & Co. developed a monitoring and control system for roof bolters for the underground mining industry. The system records the drilling parameters used during roof bolt drilling and the information can provide insight into the physical properties of the roof strata. The parameters include thrust, rotational speed, torque and velocity and the measurements are collected every 0.1 second during the operation. The drilling parameters were analyzed to determine the application of identifying the strength of rocks being drilled from the measurements. The data was converted into the specific energy of drilling which is a measure of the amount of energy required for removing a given unit of rock during a drilling operation. The laboratory studies completed to date indicate a fairly high correlation between the specific energy of drilling and the unconfined compressive strength of the rocks that were drilled. Additionally, the drilling parameters were shown to be effective for identifying the presence of fractures or bed separations between rock ]avers. The thrust, torque and specific energy of drilling were all good indicators for identifying the fractures or separations. Regardless of the drilling parameters used during the drilling experiments, the location of the fractures were identified, In order to determine the application of the drilling parameters for identifying rout rock properties, two series of experiments were conducted. The first series of experiments used three "manufactured" roof layers that had various rock samples embedded in concrete blocks. The rock samples included three types of sandstone, marble. and argillite. Another concrete block was poured with foam inserts to simulate large bedding separations (2 to 8-in). Two other manufactured blocks were constructed using high-strength concrete with cardboard layers embedded to simulate smaller fractures or bedding separations. The size of the cardboard layers varied from 1/8- to 1-in thick. One of the blocks had the cardboard layers embedded at an inclined angle to determine the effect of the orientation or, the drilling parameters. A series of experiments was conducted with the rotational speed and the penetration rate held constant and the thrust and torque allowed to vary. The information collected from the experiments is used to determine the application of the drilling parameter measurements for identifying rock properties, fractures and bedding separations.

Jan 1, 2000

By John Shepherd

Pillar extraction carried out using various methods (split and lift, Wongawilli and old Ben) involves the formation of narrow fenders (commonly 7 m of coal) formed by driving "splits" for lifting off with a continuous miner. The strata above a split and fender adjacent to the goaf consists of a cantilever beam (usually >3 m thick) that extends from the solid coal and bridges across the fender. Lifting off the fender from the split in safe conditions depends upon the bridging capability of the cantilever and the fender strength. The nature and competence of the immediate and near roof, the support capability of the coal fender and the presence of faults and or joints can adversely affect the bridging behaviour of the cantilever. Insufficient knowledge of these parameters can lead to extraction under dangerous conditions. Remnants of fenders ("stooks") also support the cantilever and their dimensions are critical to obtain regular caving and goaf falls. Stook area, goaf roof span and roof stand-up time after lifting can be related and only a relatively small increase in Btook area of 10-15 m2 can lead to a significantly increased stand-up time, especially under a massive roof. Such conditions strongly influence the redistribution of abutment stresses and can create increased outbye pillar loading. Based on detailed underground geotechnical investigations, it is concluded that strata caveability, roof cantilever behaviour, fender H:W ratio and remnant stook size are all critical parameters for successful and safe pillar extraction.

Jan 1, 1992

By Douglas Scott

Highly stressed rock in stopes continues to be a primary safety risk for miners in underground mines because it can result in failures of ground that lead to both injuries and death. Spokane Research Laboratory personnel investigated electromagnetic (EM) emissions in a deep underground mine in an effort to determine if these emissions could be used as indicators of impending catastrophic ground failure. Results suggest that (1) there is no increase in the number of EM emissions prior to recorded seismic activity, (2) some EM signals are generated during blasting, (3) interference from mine electrical sources mask seismic-generated EM signals, (4) EM emissions do not give enough warning (compared to seismic monitoring) to permit miners to leave a stope, (5) the distance an EM signal can travel in the rock is between 17.7 and 39.6 m (58 and 130 ft), and (6) current data acquisition systems do not differentiate between EM signals generated from seismic activity and random mine electrical noise. These results preclude monitoring EM emissions as precursors of impending catastrophic ground failure.

Jan 1, 2004

By Robert Trueman

This paper details the results of an assessment aimed at understanding the shield loading mechanisms associated with strata-related issues on a longwall face. Shield load cycle analysis theories developed by the authors were used to quantify the interaction between the shields and strata. Shield pressure data was back-analyzed using a version of the Longwall Visual Analysis (LVA) software that has been extended to enable efficient load cycle analysis to be undertaken. The software outputs enabled the shield strata interactions to be characterized. The results of the analyses indicated that the major cause of the roof falls was high-level periodic weighting, resulting in periodic shield overload. Exploration borehole data was analysed to identify and characterize the overburden unit that was the likely cause of the shield overloading.

Jan 1, 2011