Bumps and other types of dynamic failure have been a persistent, worldwide problem in the underground coal mining industry, spanning decades.For example, in just five states in the U.S.from 1983 to 2014,there were 388...Bumps and other types of dynamic failure have been a persistent, worldwide problem in the underground coal mining industry, spanning decades.For example, in just five states in the U.S.from 1983 to 2014,there were 388 reportable bumps.Despite significant advances in mine design tools and mining practices,these events continue to occur.Many conditions have been associated with bump potential, such as the presence of stiff units in the local geology.The effect of a stiff sandstone unit on the potential for coal bumps depends on the location of the stiff unit in the stratigraphic column, the relative stiffness and strength of other structural members, and stress concentrations caused by mining.This study describes the results of a robust design to consider the impact of different lithologic risk factors impacting dynamic failure risk.Because the inherent variability of stratigraphic characteristics in sedimentary formations,such as thickness, engineering material properties, and location, is significant and the number of influential parameters in determining a parametric study is large, it is impractical to consider every simulation case by varying each parameter individually.Therefore, to save time and honor the statistical distributions of the parameters, it is necessary to develop a robust design to collect sufficient sample data and develop a statistical analysis method to draw accurate conclusions from the collected data.In this study,orthogonal arrays, which were developed using the robust design, are used to define the combination of the(a) thickness of a stiff sandstone inserted on the top and bottom of a coal seam in a massive shale mine roof and floor,(b) location of the stiff sandstone inserted on the top and bottom of the coal seam,and(c) material properties of the stiff sandstone and contacts as interfaces using the 3-dimensional numerical model, FLAC3D.After completion of the numerical experiments, statistical and multivariate analysis are performed using the calculated results from the orthogonal arrays to analyze the effect of these variables.As a consequence, the impact of each of the parameters on the potential for bumps is quantitatively classified in terms of a normalized intensity of plastic dissipated energy.By multiple regression, the intensity of plastic dissipated energy and migration of the risk from the roof to the floor via the pillars is predicted based on the value of the variables.The results demonstrate and suggest a possible capability to predict the bump potential in a given rock mass adjacent to the underground excavations and pillars.Assessing the risk of bumps is important to preventing fatalities and injuries resulting from bumps.展开更多
While faults are commonly simulated as a single planar or non-planar interface for a safety or stability analysis in underground mining excavation, the real 3D structure of a fault is often very complex, with differen...While faults are commonly simulated as a single planar or non-planar interface for a safety or stability analysis in underground mining excavation, the real 3D structure of a fault is often very complex, with different branches that reactivate at different times. Furthermore, these branches are zones of nonzero thickness where material continuously undergoes damage even during interseismic periods. In this study, the initiation and the initial evolution of a strike-slip fault was modeled using the FLAC3D software program. The initial and boundary conditions are simplified, and mimic the Riedel shear experiment and the constitutive model in the literature. The FLAC3D model successfully replicates and creates the 3D fault zone as a strike-slip type structure in the entire thickness of the model. The strike-slip fault structure and normal displacement result in the formation of valleys in the model. Three panels of a longwall excavation are virtually placed and excavated beneath a main valley. The characteristics of stored and dissipated energy associated with the panel excavations are examined and observed at different stages of shear strain in the fault to evaluate bump potential. Depending on the shear strain in the fault, the energy characteristics adjacent to the longwall panels present different degrees of bump potential, which is not possible to capture by conventional fault simulation using an interface.展开更多
This paper was developed as part of an effort by the National Institute for Occupational Safety and Health(NIOSH)to identify risk factors associated with bumps in the prevention of fatalities and accidents in highly s...This paper was developed as part of an effort by the National Institute for Occupational Safety and Health(NIOSH)to identify risk factors associated with bumps in the prevention of fatalities and accidents in highly stressed,bump-prone ground conditions.Changes of failure mechanism with increasing confinement,from extensional-to shear-dominated failure,are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses.In the previous analysis conducted in 2018,both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of some specimens of a Utah coal,including the spalling limits,the ratio of apparent unconfined compressive strength(AUCS)to unconfined compressive strength(UCS),the damage characteristics,and the postyield dilatancy.These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction.However,the transition from extensional to shear failure at the given confinements was not clearly identified.In this study,a total of 20 specimens were additionally prepared from the same coal sample used in the previous study and then tested under both unconfined and triaxial compressive conditions.The different confining stresses are used as analogs for different width-to-height(W/H)ratios of pillar strength.Although the W/H ratios of the specimens were not directly considered during testing,the equivalent W/H ratios of a pillar as a function of the confining stresses were estimated using an existing empirical solution.According to this relationship,theW/H atwhich in-situ pillar behavior would be expected to transition from brittle to ductile is identified.展开更多
This contribution describes development and application of a user-friendly finite element program,UT3PC, to address three important problems in underground coal mine design:(1) safety of main entries,(2) barrier pilla...This contribution describes development and application of a user-friendly finite element program,UT3PC, to address three important problems in underground coal mine design:(1) safety of main entries,(2) barrier pillar size needed for entry protection, and(3) safety of bleeder entries during the advance of an adjacent longwall panel.While the finite element method is by far the most popular engineering design tool of the digital age, widespread use by the mining community has been impeded by the relatively high cost of and the need for lengthy specialized training in numerical methods.Implementation of UT3PC overcomes these impediments in three easy steps.First, a material properties file is prepared for the considered site.Next, mesh generation is automatic through an interactive process.A third and last step is simply execution of the program.Examples using data from several western coal mines illustrate the ease of using the application for analysis of main entries, barrier pillars, and bleeder entry safety.展开更多
文摘Bumps and other types of dynamic failure have been a persistent, worldwide problem in the underground coal mining industry, spanning decades.For example, in just five states in the U.S.from 1983 to 2014,there were 388 reportable bumps.Despite significant advances in mine design tools and mining practices,these events continue to occur.Many conditions have been associated with bump potential, such as the presence of stiff units in the local geology.The effect of a stiff sandstone unit on the potential for coal bumps depends on the location of the stiff unit in the stratigraphic column, the relative stiffness and strength of other structural members, and stress concentrations caused by mining.This study describes the results of a robust design to consider the impact of different lithologic risk factors impacting dynamic failure risk.Because the inherent variability of stratigraphic characteristics in sedimentary formations,such as thickness, engineering material properties, and location, is significant and the number of influential parameters in determining a parametric study is large, it is impractical to consider every simulation case by varying each parameter individually.Therefore, to save time and honor the statistical distributions of the parameters, it is necessary to develop a robust design to collect sufficient sample data and develop a statistical analysis method to draw accurate conclusions from the collected data.In this study,orthogonal arrays, which were developed using the robust design, are used to define the combination of the(a) thickness of a stiff sandstone inserted on the top and bottom of a coal seam in a massive shale mine roof and floor,(b) location of the stiff sandstone inserted on the top and bottom of the coal seam,and(c) material properties of the stiff sandstone and contacts as interfaces using the 3-dimensional numerical model, FLAC3D.After completion of the numerical experiments, statistical and multivariate analysis are performed using the calculated results from the orthogonal arrays to analyze the effect of these variables.As a consequence, the impact of each of the parameters on the potential for bumps is quantitatively classified in terms of a normalized intensity of plastic dissipated energy.By multiple regression, the intensity of plastic dissipated energy and migration of the risk from the roof to the floor via the pillars is predicted based on the value of the variables.The results demonstrate and suggest a possible capability to predict the bump potential in a given rock mass adjacent to the underground excavations and pillars.Assessing the risk of bumps is important to preventing fatalities and injuries resulting from bumps.
文摘While faults are commonly simulated as a single planar or non-planar interface for a safety or stability analysis in underground mining excavation, the real 3D structure of a fault is often very complex, with different branches that reactivate at different times. Furthermore, these branches are zones of nonzero thickness where material continuously undergoes damage even during interseismic periods. In this study, the initiation and the initial evolution of a strike-slip fault was modeled using the FLAC3D software program. The initial and boundary conditions are simplified, and mimic the Riedel shear experiment and the constitutive model in the literature. The FLAC3D model successfully replicates and creates the 3D fault zone as a strike-slip type structure in the entire thickness of the model. The strike-slip fault structure and normal displacement result in the formation of valleys in the model. Three panels of a longwall excavation are virtually placed and excavated beneath a main valley. The characteristics of stored and dissipated energy associated with the panel excavations are examined and observed at different stages of shear strain in the fault to evaluate bump potential. Depending on the shear strain in the fault, the energy characteristics adjacent to the longwall panels present different degrees of bump potential, which is not possible to capture by conventional fault simulation using an interface.
基金The authors would like to thanks to Steve Berry at Montana Tech for his working on the lab testing.
文摘This paper was developed as part of an effort by the National Institute for Occupational Safety and Health(NIOSH)to identify risk factors associated with bumps in the prevention of fatalities and accidents in highly stressed,bump-prone ground conditions.Changes of failure mechanism with increasing confinement,from extensional-to shear-dominated failure,are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses.In the previous analysis conducted in 2018,both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of some specimens of a Utah coal,including the spalling limits,the ratio of apparent unconfined compressive strength(AUCS)to unconfined compressive strength(UCS),the damage characteristics,and the postyield dilatancy.These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction.However,the transition from extensional to shear failure at the given confinements was not clearly identified.In this study,a total of 20 specimens were additionally prepared from the same coal sample used in the previous study and then tested under both unconfined and triaxial compressive conditions.The different confining stresses are used as analogs for different width-to-height(W/H)ratios of pillar strength.Although the W/H ratios of the specimens were not directly considered during testing,the equivalent W/H ratios of a pillar as a function of the confining stresses were estimated using an existing empirical solution.According to this relationship,theW/H atwhich in-situ pillar behavior would be expected to transition from brittle to ductile is identified.
文摘This contribution describes development and application of a user-friendly finite element program,UT3PC, to address three important problems in underground coal mine design:(1) safety of main entries,(2) barrier pillar size needed for entry protection, and(3) safety of bleeder entries during the advance of an adjacent longwall panel.While the finite element method is by far the most popular engineering design tool of the digital age, widespread use by the mining community has been impeded by the relatively high cost of and the need for lengthy specialized training in numerical methods.Implementation of UT3PC overcomes these impediments in three easy steps.First, a material properties file is prepared for the considered site.Next, mesh generation is automatic through an interactive process.A third and last step is simply execution of the program.Examples using data from several western coal mines illustrate the ease of using the application for analysis of main entries, barrier pillars, and bleeder entry safety.
