Loading characteristics of mechanical rib bolts determined through testing and numerical modeling

Underground coal mines use mechanical bolts in addition to other types of bolts to control the rib deformation and to stabilize the yielded coal ribs. Limited research has been conducted to understand the performance of the mechanical bolts in coal ribs. Researchers from the National Institute for Occupational Safety and Health(NIOSH) conducted this work to understand the loading characteristics of mechanical bolts(stiffness and capacity) installed in coal ribs at five underground coal mines. Standard pull-out tests were performed in this study to define the loading characteristics of mechanical rib bolts. Different installation torques were applied to the tested bolts based on the strength of the coal seam. A typical tri-linear load-deformation response for mechanical bolts was obtained from these tests. It was found that the anchorage capacity depended mainly on the coal strength. Guidelines for modeling mechanical bolts have been developed using the tri-linear load-deformation response. The outcome of this research provides essential data for rib support design.     

A coal rib monitoring study in a room-and-pillar retreat mine

The National Institute for Occupational Safety and Health(NIOSH) conducted a comprehensive monitoring program in a room-and-pillar mine located in Southern Virginia. The deformation and the stress change in an instrumented pillar were monitored during the progress of pillar retreat mining at two sites of different geological conditions and depths of cover. The main objectives of the monitoring program were to better understand the stress transfer and load shedding on coal pillars and to quantify the rib deformation due to pillar retreat mining; and to examine the effect of rib geology and overburden depth on coal rib performance. The instrumentation at both sites included pull-out tests to measure the anchorage capacity of rib bolts, load cells mounted on rib bolts to monitor the induced loads in the bolts, borehole pressure cells(BPCs) installed at various depths in the study pillar to measure the change in vertical pressure within the pillar, and roof and rib extensometers installed to quantify the vertical displacement of the roof and the horizontal displacement of the rib that would occur during the retreat mining process.The outcome from the monitoring program provides insight into coal pillar rib support optimization at various depths and geological conditions. Also, this study contributes to the NIOSH rib support database in U.S coal mines and provides essential data for rib support design.     

Mechanics of rib spalling of high coal walls under fully-mechanized mining

In order tO solve the problem of rib spalling of high coal walls in fully-mechanized (HCWFM) mines, we used the principle of damage mechanics to analyze coal wall rib spalling. The results show that coal wall rib spalling is, to a certain degree, a macro-performance of the development of micro-cracks. We built a mechanical model to simulate the damage to the front of coal walls, carried out theoretical calculations of the damage parameters, analyzed the effect of mining height, original cracks, seam strength, horizontal stress, vertical displacement of the coal walls and other parameters on coal wall rib spalling,which conform well with the results of our field measurements and numerical simulation. The key to control coal wall rib spalling is to control the development of cracks in coal walls. Accelerating the speed of advancing the working face, improving the setting load of support and the horizontal force of the guard board, strengthening coal walls and other technical measures can effectively reduce the degree of damage to the coal walls and control coal wall rib spalling at HCWFMVl faces.     

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?     

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.     

Coal rib response during bench mining: A case study

In 2016, room-and-pillar mining provided nearly 40% of underground coal production in the United States.Over the past decade, rib falls have resulted in 12 fatalities, representing 28% of the ground fall fatalities in U.S.underground coal mines.Nine of these 12 fatalities(75%) have occurred in room-andpillar mines.The objective of this research is to study the geomechanics of bench room-and-pillar mining and the associated response of high pillar ribs at overburden depths greater than 300 m.This paper provides a definition of the bench technique, the pillar response due to loading, observational data for a case history, a calibrated numerical model of the observed rib response, and application of this calibrated model to a second site.     

Effect of bolt rib spacing on load transfer mechanism

Rock bolting has firmly used as the coal mine roof reinforcement in underground coal mine. The bolting effect of fully grouted rebar bolt is closely related to the bolt surface profile. This paper provides an experimental study to confirm that bolt rib internal length has great influence on bolting effect. Pull-out tests were conducted using rebar bolt with different rib spacings of 12, 24, 36 and 48 mm representatively from steel tube and from concrete. Results show that peak load increases 25.3% for bolt with large rib spacing. For pull out using concrete block, the increment of peak load between large and small rib spacing is not significant, but the bolt with large rib spacing has great absorption of deformational energy than small rib spacing bolt. This study provides experimental evidence towards optimum design of rock bolt for understand coal mining industry.     

Developing coal burst propensity index method for Australian coal mines

Coal burst is the violent failure of overstressed coal, and it is often accompanied by sound, coal ejection and seismic events. It is subsequently recognized as a serious safety risk of Australia after double fatalities coal burst happened at Austar Coal Mine. Considering the increasing trend of coal burst severity and frequency with mining depth, it is an urgent task to develop the coal burst risk assessment methods for Australia underground coal mines. Coal burst propensity index method is a widely used method of burst risk evaluation of coal as it is summed up from the coal burst research and practice of many countries.This paper presents the experimental and theoretical research of coal burst propensity index method for coal burst risk assessment in Australia. The definition of four indexes including elastic strain energy index(W_(ET)), bursting energy index(K_E), dynamic failure time(DT) and uniaxial compression strength(RC)is introduced in the first part. Then, the standard laboratory test process and test parameter of coal burst propensity index is presented. DT test is conducted with 0.3 mm/min displacement control loading rate while other test is with 0.5 mm/min. Besides, modified data processing and risk classification method of test are proposed. Differentiate analysis of stress-strain curve is adopted in the data processing of DT and KEindex. A four level risk classification form of burst risk is recommended for Australian underground coal mines. Finally, two likely improvement methods of W_(ET) test, including volumetric strain indicator method and theoretical calculation method, are discussed.     

Rib stability:A way forward for safe coal extraction in India

Ensuring rib stability during pillar extraction is of prime importance in bord and pillar(BP) method of underground coal mining with caving. Rib stability has been assessed here by way of assessing factor of safety(FOS), a ratio of the strength of rib to stress on it. Earlier formulations for rib stability when applied to case studies gave very low FOS value suggesting significant ground control problems, which were contrary to the field observations. This has necessitated the need to revisit the concept of rib stability. The stress coming on the rib is estimated with the use of numerical modeling technique using the FLAC~(3D) software. The methodology of assessing rib-stability with the help of suggested rib-strength formulation has been validated at eight Indian coal mines. The outcome of this study finds relevance and importance in ensuring underground coal liquidation with improved safety and conservation.     

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.     

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.     

New advances in coal structure model

Although human beings have come to understand and utilize coal for a very long history, no theoretical breakthrough in the study of coal structure has been made, which still needs continuous efforts of coal chemical workers. Based on the viewpoint of ‘vague/clear', the species classification and accurate analysis on coal were conducted by using the natural clustering all-component separation method. A more systematic and detailed coal embedded structure model theory which is suitable for coal of all ranks was developed from the previous one and a more complete theoretical system about the component and structure of coal was constructed. The whole establishment process of the theory was summarized and some of the main support data and analysis test results, including TEM, AFM, FTIR, GC/MS, MALDI/TOF/MS, DART/MSD, fractal analysis and so on were provided. The coal embedded structure theory fully considers both the identity and the particularity of all-rank coal, reflects the coal component and structure in the full range of coal rank, solves the systematic cognitive problem of coal component and structure on macro and micro level, and provides a valuable and meaningful theoretical approach for the coal processing and conversion technology.     

A mathematical model of gas flow during coal outburst initiation

A proposed concept of outburst initiation examines the release of a large amount of gas from coal seams resulted from disintegrating thermodynamically unstable coal organic matter(COM). A coal microstructure is assumed to getting unstable due to shear component appearance triggered by mining operations and tectonic activities considered as the primary factor while COM disintegration under the impact of weak electric fields can be defined as a secondary one. The energy of elastic deformations stored in the coal microstructure activates chemical reactions to tilt the energy balance in a ‘‘coal–gas" system.Based on this concept a mathematical model of a gas flow in the coal where porosity and permeability are changed due to chemical reactions has been developed. Using this model we calculated gas pressure changes in the pores initiated by gas release near the working face till satisfying force and energy criteria of outburst. The simulation results demonstrated forming overpressure zone in the area of intensive gas release with enhanced porosity and permeability. The calculated outburst parameters are well combined with those evaluated by field measurements.     

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.     

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.     

Utilising the scientific method to demonstrate that slender beam/column behaviour is the dominant behavioural mechanism leading to roof/rib failure

As per most other earth science engineering problems, the underground coal geotechnical environment and the way in which roof and rib support interacts with the rock mass are complex issues. It is therefore generally recognised that without prudent simplification, the complexity of the problem will overwhelm all current geotechnical methods of modelling, not least for the reason that a rock mass can never be characterised to a level that allows a ‘‘non-simplified" analysis. The fact that numerical models, which are commonly purported to be a ‘‘simulation" tool and the so-called epitome of advanced geotechnical engineering, always need to be ‘‘calibrated" to a known reality is taken to be conclusive proof of this statement. While the problem should not be oversimplified(i.e. the dominant failure mechanisms or critical data input parameters should not be ignored), without question judicious simplification is at the heart of all engineering design, to the point that it has a well-established name –‘‘reductionism". The hypothesis addressed in this paper, is that horizontal and vertical stress-driven slender beam and column behaviour(which includes unstable Euler Buckling) are respectively the dominant(but not only) roadway roof and ribline behavioural mechanism that(if not controlled) can lead to excessive deformation,failure and eventual collapse. As a part of the Scientific Method, a hypothesis can only be tested via real-world observations, measurements and analyses in establishing it is a credible Theory. Utilising the Scientific Method, this paper demonstrates that slender beam/column behaviour is the dominant instability mechanism within a coal mine roof/rib subject to elevated horizontal/vertical stress conditions and therefore, must be representatively accounted for in any credible empirical, analytical, or numerical approach to coal mine roof/rib stability assessment and associated ground support design.     

Using LaModel to analyze coal bumps

Coal bumps have long been a safety hazard in coal mines, and even after decades of research, the exact mechanics that cause coal bumps are still not well understood. Therefore, coal bumps are still difficult to predict and control. The LaModel program has a long history of being used to effectively analyze displacements and stresses in coal mines, and with the recent addition of energy release and local mine stiffness calculations, the LaModel program now has greatly increased capabilities for evaluating coal bump potential. This paper presents three recent case histories where coal stress, pillar safety factor, energy release rate and local mine stiffness calculations in LaModel were used to evaluate the pillar plan and cut sequencing that were associated with a number of bumps. The first case history is a longwall mine where a simple stress analysis was used to help determine the limiting depth for safely mining in bump-prone ground. The second case history is a room-and-pillar retreat mine where the LaModel analysis is used to help optimize the pillar extraction sequencing in order to minimize the frequent pillar line bumps. The third case history is the Crandall Canyon mine where an initial bump and then a massive pillar collapse/bump which killed 6 miners is extensively back-analyzed. In these case histories, the calculation tools in LaModel are ultimately shown to be very effective for analyzing various aspects of the bump problem, and in the conclusions, a number of critical insights into the practical calculation of mine failure and stability developed as a result of this research are presented.     

应力参数对圆柱壳声辐射影响的初探

对加筋圆柱壳由于肋骨数目改变所引起的结构应力参数变化对辐射噪声的影响进行了探索．运用Ansys软件建立不同结构参数的加筋圆柱壳有限元模型，计算了模型在激振力作用下的耦合振动响应，提取结构表面的位移，在Sysnoise软件中计算各种声学物理量．得到了各工况下模型在水下受激振动时的结构表面速度、水中辐射声压和辐射声功率等分布规律．找出了肋骨数目改变引起的结构应力参数与加筋圆柱壳振动和结构声辐射之间的关系，对舱段结构振动与噪声的控制有重要的参考意义．     

肋片强化矩形截面螺旋通道换热特性研究

应用CFD软件研究肋片对矩形截面螺旋通道湍流换热的强化作用,基于正交螺旋坐标系分析通道内流场和温度场分布.结果表明:对于单一矩形截面螺旋通道,换热壁面中心线附近受二次流影响较弱,换热效果较差.在此处安装扰流肋片后,矩形截面中心处产生了附加的二次流.研究范围内,加装肋片后的对流换热系数α是未加装肋片的1.03~1.2倍,流动阻力系数f是未加装肋片的1.003~1.033倍;强化传热因子j在0.911~1.067之间.低雷诺数下的低高度肋片综合强化换热效果较好.     

Fast recognition using convolutional neural network for the coal particle density range based on images captured under multiple light sources

A method based on multiple images captured under different light sources at different incident angles was developed to recognize the coal density range in this study. The innovation is that two new images were constructed based on images captured under four single light sources. Reconstruction image 1 was constructed by fusing greyscale versions of the original images into one image, and Reconstruction image 2 was constructed based on the differences between the images captured under the different light sources. Subsequently, the four original images and two reconstructed images were input into the convolutional neural network AlexNet to recognize the density range in three cases: −1.5 (clean coal) and + 1.5 g/cm³ (non-clean coal); −1.8 (non-gangue) and + 1.8 g/cm³ (gangue); −1.5 (clean coal), 1.5–1.8 (middlings), and + 1.8 g/cm³ (gangue). The results show the following: (1) The reconstructed images, especially Reconstruction image 2, can effectively improve the recognition accuracy for the coal density range compared with images captured under single light source. (2) The recognition accuracies for gangue and non-gangue, clean coal and non-clean coal, and clean coal, middlings, and gangue reached 88.44%, 86.72% and 77.08%, respectively. (3) The recognition accuracy increases as the density moves further away from the boundary density.