A semi-quantitative coal burst risk classification system

Safety is the highest priority in the mining industry as underground mining in particular poses high safety risks to its workers. In underground coal mines, coal bursts are one of the most catastrophic hazards, which involves sudden and violent dynamic coal mass failure with rapid ejection of the broken material into the mine workings. Despite decades of research, the contributing mechanisms of coal bursts are still not completely understood. Hence, it remains challenging to forecast coal bursts and quantify their likelihood of occurrence. However, a range of geological and geotechnical factors are associated with coal bursts and can increase the coal burst proneness. This paper introduces a semi-quantitative coal burst risk classification system, namely, BurstRisk. Based on back-analysis of case histories from Australia, China and the United States, BurstRisk classifies the coal burst risk into three categories:low, medium and high risk. In addition, it allows mining engineers to modify the weighting of the selected factors based on specific conditions. The risk classification charts introduced are for both longwall retreat and development sections of long-wall mining operations. This paper also provides a set of risk management strategies and control measures for effective coal burst mitigation.     

A review of the geomechanics aspects of a double fatality coal burst at Austar Colliery in NSW,Australia in April 2014

A coal burst occurred on 15 April, 2014 at the Austar Coal Mine, located west of Newcastle, NSW,Australia. The burst resulted in fatal injuries to two men working as part of the mining crew at the development face. At the time, a continuous miner was being used to mine a longwall development gate road through heavily structured coal, at a depth of approximately 550 m. A number of pre-cursor bumps had occurred on previous shifts, emanating from the coal ribs of the roadway, in proximity to the coal face.This paper reviews the geological, geotechnical and mining conditions and circumstances leading up to the coal burst event; and presents and discusses the available evidence and possible interpretations relating to the geomechanical behaviour mechanisms that may have been critical factors in this incident. The paper also discusses some key technical and operational considerations of ground support systems and mining practices and strategies needed for operating in such conditions in the future.     

Assessment of energy release mechanisms contributing to coal burst

Coal burst is a dynamic release of energy within the rock(or coal) mass leading to high velocity expulsion of the broken/failed material into mine openings. This phenomenon has been recognised as one of the most catastrophic failures associated with the coal mining industry, which can often lead to injuries and fatalities of miners as well as significant production losses. This paper aims to examine the mechanisms contributing to coal burst occurrence, with an emphasis on the energy release concept. In this study, a numerical modelling study has been conducted to evaluate the roles and contributions of difference energy components. The energy analysis presented in this paper can help to improve the understanding of energy release mechanisms especially under Australian conditions.     

A new criterion of coal burst proneness based on the residual elastic energy index

To evaluate the coal burst proneness more precisely, a new energy criterion namely the residual elastic energy index was proposed. This study begins by performing the single-cyclic loading-unloading uniaxial compression tests with five pre-peak unloading stress levels to explore the energy storage characteristics of coal. Five types of coals from different mines were tested, and the instantaneous destruction process of the coal specimens under compression loading was recorded using a high speed camera. The results showed a linear relationship between the elastic strain energy density and input energy density, which confirms the linear energy storage law of coal. Based on this linear energy storage law, the peak elastic strain energy density of each coal specimen was obtained precisely. Subsequently, a new energy criterion of coal burst proneness was established, which was called the residual elastic energy index(defined as the difference between the peak elastic strain energy density and post peak failure energy density).Considering the destruction process and actual failure characteristics of coal specimens, the accuracy of evaluating coal burst proneness based on the residual elastic energy index was examined. The results indicated that the residual elastic energy index enables reliable and precise evaluations of the coal burst proneness.     

Spatial context in the calculation of gas emissions for underground coal mines

The prediction of gas emissions arising from underground coal mining has been the subject of extensive research for several decades, however calculation techniques remain empirically based and are hence limited to the origin of calculation in both application and resolution. Quantification and management of risk associated with sudden gas release during mining(outbursts) and accumulation of noxious or combustible gases within the mining environment is reliant on such predictions, and unexplained variation correctly requires conservative management practices in response to risk. Over 2500 gas core samples from two southern Sydney basin mines producing metallurgical coal from the Bulli seam have been analysed in various geospatial context including relationships to hydrological features and geological structures. The results suggest variability and limitations associated with the present traditional approaches to gas emission prediction and design of gas management practices may be addressed using predictions derived from improved spatial datasets, and analysis techniques incorporating fundamental physical and energy related principles.     

Application of coal mine roof rating in Chinese coal mines

The coal mine roof rating(CMRR) is a measure of roof quality or structure competency for bedded roof types typically of underground coal mines. The CMRR has been used widely in the US, South Africa,Canada and Australia. In order to investigate the application of the CMRR system in Chinese coal mines,two coal mines in China located in Panjiang Coal Field in Guizhou Province were investigated. Field data were collected which is required to calculate the CMRR value based on underground exposure. The CMRR values of 11 locations in two coal mines were calculated. The investigations demonstrated that the chance of mine roof failure is very low if the CMRR value is more than 50, given adequate support is installed in mine. It was found that the CMRR guideline are useful to preliminarily investigate stability in Panjiang Coal Field mines.     

Cooling pathways for deep Australian longwall coal mines of the future

Cooling of coal mines in the Bowen Basin, characterized by steep geothermal gradient, is presently achieved mostly through rental surface bulk air cooling in summer months. This paper argues that future mines will be required to focus their cooling resources more intensively to manage a challenging thermal environment where virgin coal temperatures over 50 °C at a depth of 500 m are expected. Currently, mine cooling systems are employed to maintain the wet bulb temperatures(WBT) to below 27 °C at which point the risks of heat stroke or other heat related issues are manageable. The capacities of these systems are in the range of 6–10 MW refrigeration power. The relationship between high working temperature environment and injury frequency rates is well established. Therefore, provision of appropriate cooling strategies and understanding their optimum performance and suitability are important to Australian coal mines of the future. This paper evaluates the underground temperature profiles of deep, gassy coal mines with propensity for spontaneous combustion and proposes the long term cooling pathways to effectively manage the thermal hazards. Thermodynamic modeling is performed on a longwall face and includes air leakage effects from goaf streams at various locations along the longwall face. The strategy summarizes the application of underground bulk air cooling, chilled water sprays on the shearer and the resulting temperature profiles. Considering the new mining projects planned for the Bowen Basin region, a review of implementable cooling strategies such as mid-gate mobile bulk air coolers(BACs), spot coolers, underground bulk air cooling and the use of chilled water to enhance the positional efficiency of cooling plants,are discussed in this paper. Finally, the comparison of ‘rental' versus ‘ownership' of cooling plants is analysed as part of long-term cooling strategies.     

Occurrence,predication,and control of coal burst events in the U.S.

Coal burst represented a major hazard for some U.S. mining operations. This paper provides an historical review of the coal burst hazards,identifies the fundamental geological factors associated with these events,and discusses mechanisms that can be used to avoid their occurrences. Coal burst are not common in most underground mines. Their occurrence almost always has such dramatic consequences to a mining operation that changes in practice are required. Fundamental factors influencing coal burst events include strong strata,abnormal strata caving,elevated stresses,critical size pillars and the lack of sufficiently sized barrier pillars during extraction. These factors interact to produce excessive stress,seismic shock and loss of confinement mechanisms. Over the 90 years of dealing with these hazards,many novel prevention controls have been developed including novel mine designs and extraction sequences,most of which are site specific in their application. Without an accurate assessment of the fundamental factors that influence coal burst and knowledge of their mechanisms of occurrence,control techniques may be misapplied and risk inadequately mitigated.     

Quantitative hazard assessment for Zonguldak Coal Basin underground mines

Underground coal mining is one of the most dangerous occupations throughout the world. The reasons behind an underground occupational accident are too complex to analyze mainly due to many uncertainties which may arise from geological, operational conditions of the mine or individual characteristics of employees. This study proposes implementing a quantitative methodology for the analysis and assessment of hazards associated with occupational accidents. The application of the proposed approach is performed on the mines of Turkish Hard Coal Enterprises(TTK). The accidents in TTK between the years 2000 and 2014 are firstly statistically analyzed with respect to the number, type and location of accidents, age,experience, education level and main duty of the casualties and also injuries resulting from such accidents. The hazards are classified as individual, operational and locational hazards and quantified using contingency tables, conditional and total probability theorems. Lower and upper boundaries of hazards are determined and event trees for each hazard class are prepared. Total hazard evaluation results show that Armutcuk, Karadon and Uzulmez mines have relatively high hazard levels while Amasra and Kozlu mines have relatively lower hazard values.     

An updated empirical model for ground control in U.S. multiseam coal mines

Multiple seam interactions are a major source of ground instability in several U.S. coalfields. Empirical methods are well suited for this problem, because while the mechanics multiple seam interactions are very complex and poorly understood, many mining case histories are available for analysis. This study makes use of an updated database that includes 356 multiseam case histories, including 67 unsuccessful designs. The paper describes in detail the process used to design the study, collect the data, conduct the statistical analysis, and develop the quantitative model. The model can be used for mine planning in multiple seam situations, and has been made available as a module within the Analysis of Coal Pillar Stability(ACPS) computer program.     

A dynamic ejection coal burst model for coalmine roadway collapse

In this study, we established a dynamic ejection coal burst model for a coalmine roadway subject to stress, and held that the stress concentration zone at the roadway side is the direct energy source of this ejection. The formation and development of such burst undergoes three stages:(1) instability and propagation of the cracks in the stress concentration zone,(2) emerging of a layered energy storage structure in the zone, and(3) ejection of coal mass or coal burst due to instability. Moreover, we figured out the initial strength of periodic cracks is parallel to the maximal dominant stress direction in the stress concentration zone and derived from the damage strain energy within the finite area of the zone based on the Griffith energy theory. In addition, we analyzed the formation process of the layered energy storage structure in the zone, simplified it as a simply supported restraint sheet, and calculated the minimum critical load and the internally accumulated elastic energy at the instable state. Furthermore, we established a criterion for occurrence of the coal burst based on the variational principle, and analyzed the coal mass ejection due to instability and coal burst induced by different intensity disturbances. At last, with the stratum conditions of Junde Coalmine as the model prototype, we numerically simulated the load displacement distribution of the stress concentration zone ahead of the working face disturbed by the main roof-fracture-induced dynamic load during the mining process as well as their varying characteristics,and qualitatively verified the above model.     

Protecting miners from coal bursts during development above historic mine workings in Harlan County,KY

In order to reach a large, untapped reserve of high-quality coal, D8 Cloverlick Mine proposed to mine a corridor nearly 600 m deep beneath the Benham Spur of Black Mountain, Kentucky's highest peak. D8 Cloverlick Mine was extracting the Owl seam, but the corridor's route lay approximately 20 m above century-old mine workings in the C–(Darby) seam. Adding to the concern, three serious coal bursts had recently occurred in nearby Owl seam workings. Maps of the old workings seemed to indicate that the underlying C–seam had been fully extracted. However, two of the coal bursts had occurred above areas where the C–Seam was also shown as mined out. Mine Safety and Health Administration(MSHA) Technical Support therefore investigated the records of past mining to better understand the old mine maps. Underground conditions observed in current Owl seam workings were also compared with the maps of the old C–seam workings. The study concluded that the presence of hazardous underlying remnants could not be ruled out. To mitigate the burst risk, D8 Cloverlick Mine adopted a strategy of stress probe drilling. A self-propelled coal drill was used to auger 11.5-m-long, small diameter holes in advance of mining. As each hole was drilled, the cuttings were measured to detect the presence of highly stressed coal. Ultimately the crossing was successfully completed without incident.     

A field study of a roof bolter canopy air curtain (2nd generation) for respirable coal mine dust control

A 2nd generation roof bolter canopy air curtain(CAC) design was tested by National Institute for Occupational Safety and Health(NIOSH) at a Midwestern underground coal mine. During the study,the roof bolter never operated downwind of the continuous miner. Using a combination of personal Data Rams(p DR) and gravimetric samplers, the dust control efficiency of the roof bolter CAC was ascertained. Performance evaluation was determined using three methods:(1) comparing roof bolter operator concentrations underneath the CAC to roof bolter concentrations outside the CAC,(2) comparing roof bolter operator concentrations underneath the CAC to the concentrations at the rear of the bolter, and finally,(3) using the gravimetric data directly underneath the CAC to correct roof bolter operator concentrations underneath the CAC and comparing them to the concentrations at the rear of the bolter. Method 1 dust control efficiencies ranged from -53.9% to 60.4%. Method 2 efficiencies ranged from -150.5% to 52.2%,and Method 3 efficiencies ranged from 40.7% to 91%. Reasons for negative and low dust control efficiencies are provided in this paper and include: incorrect sampling locations, large distance between CAC and operator, and contamination of intake air from line curtain. Low dust concentrations encountered during the testing made it difficult to discern whether differences in concentrations were due to the CAC or due to variances inherent in experimental dust measurement. However, the analyses, especially the Method 3 analysis, show that the CAC can be an effective dust control device.     

Pillar design and coal burst experience in Utah Book Cliffs longwall operations

Longwall mining has existed in Utah for more than half a century. Much of this mining occurred at depths of cover that significantly exceed those encountered by most other US longwall operations. Deep cover causes high ground stress, which can combine with geology to create a coal burst hazard. Nearly every longwall mine operating within the Utah's Book Cliffs coalfield has been affected by coal bursts. Pillar design has been a key component in the burst control strategies employed by mines in the Book Cliffs.Historically, most longwall mines employed double-use two-entry yield pillar gates. Double-use signifies that the gate system serves first as the headgate, and then later serves as the tailgate for the adjacent panel. After the 1996 burst fatality at the Aberdeen Mine, the inter-panel barrier design was introduced.In this layout, a wide barrier pillar protects each longwall panel from the previously mined panel, and each gate system is used just once. This paper documents the deep cover longwall mining conducted with each type of pillar design, together with the associated coal burst experience. Each of the six longwall mining complexes in the Book Cliffs having a coal burst history is described on a panel-by-panel basis.The analysis shows that where the mining depth exceeded 450 m, each design has been employed for about 38000 total m of longwall panel extraction. The double-use yield pillar design has been used primarily at depths less than 600 m, however, while the inter-panel barrier design has been used mainly at depths exceeding 600 m. Despite its greater depth of use, the inter-panel barrier gate design has been associated with about one-third as much face region burst activity as the double-use yield pillar design.     

A practical application of photogrammetry to performing rib characterization measurements in an underground coal mine using a DSLR camera

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.     

Fundamentals of modern ground control management in Australian underground coal mines

Underground coal mining is inherently hazardous, with uncontrolled ground failure regarded as one of only several critical risks for multiple fatality events. Development, implementation and management of overarching systems and procedures for maintaining strata control is an important step to mitigating the impact of ground failure hazards at a mine site operational level. This paper summarised the typical pro-active ground control management system(PGCMS) implemented in various Australian underground coal mines. Australia produces approximately 100 million tonnes a year of metallurgical and thermal coal from approximately 30 of the world's safest longwall mines operating in New South Wales and Queensland. The increased longwall productivity required to achieve both high levels of safety and profitability, places significant emphasis on the reliability of pro-active ground control management for longwall mining operations. Increased depths, adverse geological conditions, elevated variable stress regimes and weaker ground conditions, coupled with an industry wide need for increased development rates continue to make ground control management challenging. Ground control management is not only about ground support and pillar design though but also a structured process that requires a coordinated effort from all levels of the workforce to both minimise the occurrence of adverse geotechnical events and mitigate the potential risks when they do occur. The PGCMS presented in this paper is proven to provide both a safer and more productive mine environment through minimisation of unplanned delays. The critical elements of the method are presented in detail and demonstrate the utility and value of a ground control management system that has potential for implementation in underground coal mining globally.     

Proposing a novel comprehensive evaluation model for the coal burst liability in underground coal mines considering uncertainty factors

Coal burst is a severe hazard that can result in fatalities and damage of facilities in underground coal mines. To address this issue, a robust unascertained combination model is proposed to study the coal burst hazard based on an updated database. Four assessment indexes are used in the model, which are the dynamic failure duration(DT), elastic energy index(WET), impact energy index(KE) and uniaxial compressive strength(RC). Four membership functions, including linear(L), parabolic(P), S and Weibull(W)functions, are proposed to measure the uncertainty level of individual index. The corresponding weights are determined through information entropy(EN), analysis hierarchy process(AHP) and synthetic weights(CW). Simultaneously, the classification criteria, including unascertained cluster(UC) and credible identification principle(CIP), are analyzed. The combination algorithm, consisting of P function,CW and CIP(P-CW-CIP), is selected as the optimal classification model in function of theory analysis and to train the samples. Ultimately, the established ensemble model is further validated through test samples with 100% accuracy. The results reveal that the hybrid model has a great potential in the coal burst hazard evaluation in underground coal mines.     

Upward surface movement above deep coal mines after closure and flooding of underground workings

After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—an upward movement.Although most surface movement(i.e., subsidence) occurs in the months and years after mining by the longwall method, surface movement still occurs many decades after mining is terminated.After the closure and flooding of underground excavations and surrounding rock, this movement was reversed.This paper focuses on quantifying the upward movement in two neighboring coal mines(Winterslag and Zwartberg, Belgium).The study is based on data from a remote sensing technique: interferometry with synthetic aperture radar(INSAR).The results of the study show that the rate of upward movement in the decade after closure is about 10 mm/year on average.The upward movements are not linked directly to the past exploitation directly underneath a location.The amounts of subsidence at specific locations are linked mainly to their positions relative to an inverse trough shape situated over the entire mined-out areas and their immediate surroundings.Local features, such as geological faults, can have a secondary effect on the local variation of the uplift.The processes of subsidence and uplift are based on completely different mechanisms.Subsidence is initiated by a caving process, while the process of uplift is clearly linked to flooding.     

Dynamic failure in coal seams: Implications of coal composition for bump susceptibility

As a contributing factor in the dynamic failure (bumping) of coal pillars, a bump-prone coal seam has been described as one that is “uncleated or poorly cleated, strong…that sustains high stresses.” Despite extensive research regarding engineering controls to help reduce the risk for coal bumps, there is a paucity of research related to the properties of coal itself and how those properties might contribute to the mechanics of failures. Geographic distribution of reportable dynamic failure events reveals a highly localized clustering of incidents despite widespread mining activities. This suggests that unique, contributing geologic characteristics exist within these regions that are less prevalent elsewhere. To investigate a new approach for identifying coal characteristics that might lead to bumping, a principal component analysis (PCA) was performed on 306 coal records from the Pennsylvania State Coal Sample database to determine which characteristics were most closely linked with a positive history of reportable bumping. Selected material properties from the data records for coal samples were chosen as variables for the PCA and included petrographic, elemental, and molecular properties. Results of the PCA suggest a clear correlation between low organic sulfur content and the occurrence of dynamic failure, and a secondary correlation between volatile matter and dynamic failure phenomena. The ratio of volatile matter to sulfur in the samples shows strong correlation with bump-prone regions, with a minimum threshold value of approximately 20, while correlations determined for other petrographic and elemental variables were more ambiguous. Results suggest that the composition of the coal itself is directly linked to how likely a coal is to have experienced a reportable dynamic failure event. These compositional controls are distinct from other previously established engineering and geologic criteria and represent a missing piece to the bump prediction puzzle.     

Constructing an assessment index system for strategic risk management in coal science and technology enterprises

Coal occupies a dominant position in China’s national economy and is an essential energy source for future industrial development.To inform the efficient and sustainable development of the coal industry,this paper analyzes the sources of strategic risk for coal science and technology enterprises using 3 firstclass indicators,10 second-level indicators,and 37 observation points established through the existing research literature and experience.Moreover,in accordance to the obtained initial index data,the selected indicators have been tested and screened using reliability and membership degree analyses to remove redundant variables,avoid juxtaposition of risk factors at different levels,and reduce the influence of some tiny risk factors for enterprise strategic risk.Then,factor analysis of external environment factor sub-scale was carried out.Factors are extracted according to a standard characteristic value greater than 1.Variables with high coefficients are classified into one factor category;and finally,3 first-class indicators,8 second-level indicators,and 37 observation points are reconstructed.