Development of a fault-rupture environment in 3D: A numerical tool for examining the mechanical impact of a fault on underground excavations

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.

Influence of longwall mining on the stability of gas wells in chain pillars

Longwall mining has a significant influence on gas wells located within longwall chain pillars.Subsurface subsidence and abutment pressure induced by longwall mining can cause excessive stresses and deformations in gas well casings.If the gas well casings are compromised or ruptured,natural gas could migrate into the mine workings,potentially causing a fire or explosion.By the current safety regulations,the gas wells in the chain pillars have to be either plugged or protected by adequate coal pillars.The current regulations for gas well pillar design are based on the 1957 Pennsylvania gas well pillar study.The study provided guidelines for gas well pillars by considering their support area and overburden depth as well as the location of the gas wells within the pillars.As the guidelines were developed for room-andpillar mining under shallow cover,they are no longer applicable to modern longwall coal mining,particularly,under deep cover.Gas well casing of failures have occurred even though the chain pillars for the gas wells met the requirements by the 1957 study.This study,conducted by the National Institute for Occupational Safety and Health(NIOSH),presents seven cases of conventional gas wells penetrating through longwall chain pillars in the Pittsburgh Coal Seam.The study results indicate that overburden depth and pillar size are not the only determining factors for gas well stability.The other important factors include subsurface ground movement,overburden geology,weak floor,as well as the type of the construction of gas wells.Numerical modeling was used to model abutment pressure,subsurface deformations,and the response of gas well casings.The study demonstrated that numerical models are able to predict with reasonable accuracy the subsurface deformations in the overburden above,within,and below the chain pillars,and the potential location and modes of gas well failures,thereby providing a more quantifiable approach to assess the stability of the gas wells in longwall chain pillars.

The design of a laboratory apparatus to simulate the dust generated by longwall shield advances

A laboratory apparatus (shield dust simulator) was designed and constructed to simulate the dust generated during the advance of longwall hydraulic roof supports,or shields.The objective of the study was to develop a tool that could be used to test the hypothesis that foam applied to a mine roof prior to a shield advance could be used to reduce the respirable dust generated during shield advances.This paper will outline the design parameters for the development of thesystem,as well as describe baseline testing of coal and limestone dust.Results show that the average instantaneous respirable dust concentrated during simulated shield advance.Confidence intervals were calculated from the instantaneous respirable dust data to determine the repeatability of the data produced by the device.

Laboratory investigation of the anisotropic confinement-dependent brittle-ductile transition of a Utah coal

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.

Investigation of the anisotropic confinement-dependent brittleness of a Utah coal

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 an analysis published in 2018,both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of 84 specimens of a Utah coal,including the spalling limits,the ratio of apparent unconfined compressive strength to unconfined compressive strength(UCS),the damage characteristics,and the post-yield dilatancy.These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction,defined as the included angle.A total of four different included angles were used in the work performed in 2018.The authors found that the degree of anisotropic strength differed according to the included angle.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.Because the authors already knew the most contrasting cases of the included angles from the previous work using the four included angles,they chose only two of the included angles(0°and 30°)for this study.For the triaxial compressive tests,a greater confining stress than the mean UCS was applied to the specimens in an attempt to identify the brittle-ductile transition of the coal.The new results have been compiled with the previous results in order to re-evaluate the confinement-dependency of the coal behavior.Additionally,the different confining stresses are used as analogs for different width-to-height(W/H)conditions 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,the W/H at which in situ pillar behavior would be expected to transition from brittle to ductile is identified.

A test method for evaluating the thermal environment of underground coal mine refuge alternatives

Since 2009,the Mine Safety and Health Administration(MSHA)has required mines to install refuge alternatives(RAs)in underground coal mines.One of the biggest concerns with occupied RAs is the possible severity of the resulting thermal environment.In 30 CFR 7.504,the maximum allowable apparent temperature(AT)for an occupied RA is specified as 35℃(95°F).Manufacturers must conduct heat/humidity tests to demonstrate that their RAs meet the 35℃(95°F)AT limit.For these tests,heat input devices are used to input the metabolic heat of actual miners.A wide variety of test methods,sensors,and heat input devices could be used when conducting such tests.Since 2012,the National Institute for Occupational Safety and Health(NIOSH)has conducted over thirty 96-hour heat/humidity tests on four different RAs.This paper discusses the test equipment and procedures used during these investigations.This information is useful for RA manufacturers conducting RA heat/humidity tests,for other researchers investigating RA heat/humidity buildup,and for those who need to assess the thermal environment of any confined space where people may be trapped or are seeking refuge.

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(pDR)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.

The current perspective of the PA 1957 gas well pillar study and its implications for longwall gas well pillars

Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells.The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span.At the time,coal was mined using the room-and-pillar mining method with full or partial pillar recovery,and square or rectangle pillars surrounding the gas wells were left to protect the wells.The study provided guidelines for pillar sizes under different overburden depths up to 213 m(700 ft).The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s.The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover.However,under deep cover,severe deformations in gas wells have occurred in longwall chain pillars.Presently,with a better understanding of coal pillar mechanics,new insight into subsidence movements induced by retreat mining,and advances in numerical modeling,it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars.In this paper,the data from the 1957 study is analyzed from a new perspective by considering various factors,including overburden depth,failure location,failure time,pillar safety factor(SF),and floor pressure.The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery.A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining.Through analyzing the data from the 1957 study,the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for roomand-pillar mining and their applicability to longwall mining.Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings.Results of this study indicate that the guidelines in the 1957 study may be appropriate for pillars protecting conventional gas wells in both room-and-pillar mining and longwall mining under overburden depths up to 213m(700 ft),but may not be sufficient for protective pillars under deep cover.The current evaluation of the 1957 study provides not only insights about potential gas well failures caused by retreat mining but also implications for what critical considerations should be taken into account to protect gas wells in longwall mining.

Guest editorial–special issue on ground control in mining in 2020

Ground control is the science of studying and controlling the behavior of rock strata in response to mining operations.Ground-control-related research has seen significant advancements over the last 40 years,and these accomplishments are well documented in the proceedings of the annual International Conference on Ground Control in Mining(ICGCM)[1].The ICGCM is a forum to promote closer communication among researchers,consultants,regulators,manufacturers,and mine operators to expedite solutions to ground control problems in mining[2–7].Fundamental research and advancements in ground control science define the central core of the conference mission.Providing information to mine operators is a priority,as the conference goal is to offer solutions-oriented information.In addition,the conference has included innovative technologies and ideas in miningrelated fields such as exploration,geology,and surface and underground mining in all commodities.Many new ground control technologies and design standards adopted by the mining industry were first discussed at ICGCM.This conference is recognized as the leading international forum for introducing new ground-control-related research and products.

Review of current coal rib control practices

The instability of coal ribs in underground mines continues to result in the injuries and fatalities of mine workers.The proper esti-mation and evaluation of primary and secondary support for coal ribs is still a challenging problem in the field of ground control science and requires further research and study.Although mining operations have various support design criteria and support methodologies for strata control,most rib support designs are still based on experience and local practices.This review study is intended to summarize the currently applied practices for rib support and control in various countries and mining conditions.Firstly,critical parameters that con-trol the amount and type of required rib support are considered and evaluated.The study revealed that among these parameters that control the stability of coal ribs,mining depth,rib height,cleat orientation/condition,and coal strength are the most significant param-eters.Secondly,current rib support application methods were also summarized.Similar to rock mass classification systems,some studies proposed a rib control rating system for practical estimation of the current rib condition and to estimate primary support requirements.These studies are classified and summarized into two groups(categorical and empirical)based on the required inputs and methodologies.Empirically based coal rib rating systems were closely examined,and the usefulness and intuitive aspects of each rating system were com-pared.This comprehensive literature review demonstrates that the Australian rating system,Analysis and Design of Rib Support(ADRS),and the new U.S.rating system,Coal Pillar Rib Rating(CPRR),are highly applicable for their regions.