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.     

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.     

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.     

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.     

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.     

Effect of faulting on coal burst——A numerical modelling study

Coal burst occurrence on roadways has always been a major concern in deep underground coal mines,especially under complex geological conditions. To evaluate the effect of faulting on coal burst, the stress concentration in the vicinity a reverse fault was analysed considering the geological history of the fault formation where high horizontal stresses led to the initiation and propagation of the reverse fault.Various in situ stresses and mechanical parameters of the fault, including the ratio of horizontal stress to vertical stress were used to analyse the state of fault. Numerical modelling was conducted using two and three dimensional distinct element models(UDEC and 3 DEC) based on a geotechnical conditions of an Australian underground coal mine. The formation process of reverse fault was simulated to evaluate the stress characteristics in the coal seam and the immediate roof and floor near the fault. The results show that, both the horizontal and vertical stress in footwall were higher than those in hanging wall after the formation of the reverse fault. The stress condition near fault was complicated due to complex geology in the coal measures, and the vertical stress peaked in the footwall at a distance of about 160 m from the fault. When a roadway was excavated, stress concentration occurred at both the roadway face and ribs, which reached as high as 38 MPa in the ribs at a depth of 500 m. This will significantly elevate the risk of dynamic instability of the roadway such as coal burst. The stress concentration zone in the footwall can be considered as a hazardous zone near the reverse fault. This study provides a general reference for analysis of roadway stability affected by faults.     

Comprehensive early warning of rock burst utilizing microseismic multi-parameter indices

Rock bursts have become one of the most severe risks in underground coal mining and its early warning is an important component in the safety management. Microseismic(MS) monitoring is considered potentially as a powerful tool for the early warning of rock burst. In this study, an MS multi-parameter index system was established and the critical values of each index were estimated based on the normalized multi-information warning model of coal-rock dynamic failure. This index system includes bursting strain energy(BSE) index, time-space-magnitude independent information(TSMII) indices and timespace-magnitude compound information(TSMCI) indices. On the basis of this multi-parameter index system, a comprehensive analysis was conducted via introducing the R-value scoring method to calculate the weights of each index. To calibrate the multi-parameter index system and the associated comprehensive analysis, the weights of each index were first confirmed using historical MS data occurred in LW402102 of Hujiahe Coal Mine(China) over a period of four months. This calibrated comprehensive analysis of MS multi-parameter index system was then applied to pre-warn the occurrence of a subsequent rock burst incident in LW 402103. The results demonstrate that this multi-parameter index system combined with the comprehensive analysis are capable of quantitatively pre-warning rock burst risk.     

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.     

Review of preventive and constructive measures for coal mine explosions: An Indian perspectiveOA

Firedamp and coal dust explosion constitute a lion’s share in mine accidents in a global mining scenario.This paper reports a list of mine explosion disasters since last two decades, a critical review of the different prevention and constructive measures, and its recent development to avoid firedamp and coal dust explosion. Preventive legislation in core coal-producing countries, viz. China, USA, Australia, South Africa, and India related to firedamp and coal dust explosion are critically analys...     

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.     

Effects of coal damage on permeability and gas drainage performance

Coal permeability is a measure of the ability for fluids to flow through coal structures. It is one of the most important parameters affecting the gas drainage performance in underground coal mines. Despite the extensive research conducted on coal permeability, few studies have considered the effect of coal damage on permeability. This has resulted in unreliable permeability evaluation and prediction. The aim of this study is to investigate the effect of coal damage on permeability and gas drainage performance. The Cui-Bustin permeability model was improved by taking into account the impact of coal damage on permeability. The key damage coefficient of the improved permeability model is determined based on the published permeability data. A finite-element numerical simulation was then developed based on the improved permeability model to investigate the damage areas and the permeability distribution around roadway. Results showed that the tensile failure occurs mainly on the upper and lower sides of the roadway while the shear failure symmetrically occurs on the left and right sides. With the increase in the friction angle value, the damage area becomes small. A good agreement was obtained between the results of the improved permeability model(c = 3) and the published permeability data. This indicated a more accurate permeability prediction by the improved permeability model. It is expected that the findings of this study could provide guidance for in-seam gas drainage borehole design and sealing, in order to enhance the gas drainage performance and reduce gas emissions into underground roadways.     

New risk assessment methodology for coal mine excavated slopes

This paper presents a new risk assessment methodology for coal mine excavated slopes. This new empirical-statistical slope stability assessment methodology(SSAM) is intended for use by geotechnical engineers at both the design review and operational stages of a mine's life to categorise the risk of an excavated coal mine slope. A likelihood of failure is determined using a new slope stability classification system for excavated coal mine slopes developed using a database of 119 intact and failed case studies sourced from open cut coal mines in Australia. Consequence of failure is based on slope height and stand-off distance at the toe of the excavated slope. Results are presented in a new risk matrix, with slope risk being divided into low, medium and high categories. The SSAM is put forward as a new risk assessment methodology to assess the potential for, and consequence of, excavated coal mine slope failure.Unlike existing classification systems, assumptions about the likely failure mode or mechanism are not required. Instead, the SSAM applies an approach which compares the conditions present within the excavated slope face, with the known past performance of slopes with similar geotechnical and geometrical conditions, to estimate the slope's propensity for failure. The SSAM is novel in that it considers the depositional history of strata in an excavated slope and how this sequence affects slope stability. It is further novel in that it does not require explicit measurements of intact rock, rock mass and/or defect strength to rapidly calculate a slope's likelihood of failure and overall risk. Ratings can be determined entirely from visual observations of the excavated slope face. The new SSAM is designed to be used in conjunction with existing slope stability assessment tools.     

Prediction of rock-burst-threatened areas in an island coal face and its prevention:A case study

The island coal face arises in coal mines with the purpose of preventing gas explosion or maintaining the balance between mining and tunneling. However, its particular stress conditions in the surrounding rock may increase the difficulty of stress control in the coal face and in its mining roadways, especially when the coal seam, the roof, and the floor have rock-burst propensities. The high energy accumulated in the island coal face and in its roof and floor will intensify rock-burst propensity or even induce rock burst,which further result in great casualties and financial losses. Taking island coal face 2321 in Jinqiao coal mine as a case, we propose a method for the prediction of rock-burst-threatened areas in an island coal face with weak rock-burst propensity. Based on the analysis of the movement of the overlying roof and characteristics of stress distribution, this method combined numerical simulation with drilling bits to ensure the prediction accuracy. The effects of coal pillars with different widths on the mitigation of stress concentration in the coal face and on the prevention of rock burst are analyzed together with the mechanism behind. Finally, corresponding measures against the rock burst in the island coal face are proposed.     

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.     

A review of energy associated with coal bursts

A coal burst is defined as a rapid expulsion of coal(and potentially gas) from the boundary of the roadway. Rock and coal fractures together with micro seismic vibration is a common occurrence during mining, however, it is very uncommon for coal and rock to be propelled into the roadway. Irrespective, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy which can contribute to the propulsion of coal from the face or ribs are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area. The aim of this paper is to briefly review the bursts which may be related to strain energy, seismic energy and gas energy.     

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.     

基于GIS的东滩煤矿顶板突水预测预报

以东滩煤矿 3煤开采中顶板突水预测预报的研究工作为例 ,阐述了从建立突水的概念模型到运用GIS的手法进行建模并得出最终结果的全过程     

煤矿冲击矿压的分级预测研究

针对煤矿冲击矿压灾害,研究了冲击危险性的监测原理和冲击矿压危险的分级预测准则.通过连续监测预警技术和系统集成,应用综合指数法、微震法、电磁辐射法和钻屑法,形成冲击矿压的时空分级预测技术体系,即在时间上对冲击危险进行早期综合分析预测与即时预测相结合,在空间上进行区域预测与局部监测、点预测相结合,对冲击矿压的危险性根据危险指数的大小,按无、弱、中等和强冲击危险4级进行预测.根据预测的冲击矿压危险等级,采取加强监测、解危、甚至撤人等防治对策.工程实践表明,采用该技术,可大大提高冲击危险预测的准确性,取得良好的应用效果.     

Mechanics of rib deformation—Observations and monitoring in Australian coal mines

The risk of fatalities from rib failure is still prevalent in the coal mining industry. This risk has prompted further research to be conducted on rib deformation in order to understand the mechanisms of rib failure, with the long-term objective being to improve rib support design. This paper presents the results of ACARP research project C25057, which investigated the mechanics and drivers of rib failure. The results of rib deformation monitoring at three different mines in Australia provide rib deformation characteristics for overburden depths ranging from 160 to 530 m. Monitoring includes deformation during development drivage conditions and during the longwall retreat abutment stress environment. The rib deformation monitoring covered three different seams: the Goonyella Middle Seam, Ulan Seam, and Bulli Seam in the Bowen Basin, Western Coalfield, and Southern Coalfield, respectively. The observed mechanisms driving the rib deformation ranged from bedding shear failure along weak claystone bands to vertical shear fractures to kinematic failures driven by shear failure dilation. The variation in mechanisms of rib failure, together with the seemingly consistent method of rib support design, prompts the question: what exactly is the role of rib support?     

Effect of discontinuities characteristics on coal mine stability and sustainability: A rock fall prediction approach

Rock fall related accidents continue to occur in coal mines, although artificial support mechanisms have been used extensively. Roof stability is primarily determined in many underground mines by a limited number of methods that often resort to subjective criteria. It is argued in this paper that stability conditions of mine roof strata, as a key factor in sustainability in coal mines, must be determined by a survey which proactively investigates fundamental aspects of said mine. Failure of rock around the opening happens as a result of both high rock stress conditions and the presence of structural discontinuities. The properties of such discontinuities affect the engineering behavior of rock masses causing wedges or blocks to fall from the roof or sliding out of the walls. A practical rule-based approach to assess the risk of a roof fall is proposed in the paper. The method is based on the analysis of structural data and the geometry and stability of wedges in underground coal mines. In this regard, an accident causing a huge collapse in a coal mine leading to 4 fatalities is illustrated by way of a case study. Horizontal and vertical profiles are prepared by geophysical methods to define the falling zone and its boundaries. The collapse is then modeled by the use of sophisticated computer programs in order to identify the causes of the accident.