Investigation of factors influencing roof stability at a Western U.S.longwall coal mine

The coal mine roof rating(CMRR) was developed to bridge the gap between geological variation in underground coal mines and engineering design. The CMRR accounts for the compressive strength of the immediate roof, the shear strength and intensity of any discontinuities present, and the moisture sensitivity of the immediate roof. The CMRR has been widely used and validated in Eastern US coal mines, but it has seen limited application in the Western US. This study focuses on roof behavior at a Western coal mine(Mine A). Mine A shows significant lateral geological variation, along with localized faulting and a laterally extensive sandstone channel network. The CMRR is not used to predict roof instability at the mine. It is, therefore, hypothesized that there are other factors that are correlated with roof instability in underground coal mines that could potentially also be considered in conjunction with the CMRR.This hypothesis was tested by collecting 30 CMRR measurements at Mine A. At each measurement location, a binary record of the roof condition(stable or unstable) was made, and other parameters such as depth of cover, presence of faulting, and sandstone channels were also recorded. ANOVA tests showed that the CMRR values and the roof conditions were not strongly correlated, indicating that the CMRR input criteria are not fully predictive of roof stability at this mine. The CMRR values showed statistically significant correlations(p less than 0.05) with faulting as well as with location at an intersection. For areas that had previously experienced roof fall but were currently stable, faulting was correlated with roof condition(p less than 0.05) only when the condition was classified as unstable.     

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.     

Understanding roof deformation mechanics and parametric sensitivities of coal mine entries using the discrete element method

Although conventional coal mine designs are conservative regarding pillar strength, local failures such as roof-falls and pillar bursts still affect mine safety and operations. Previous studies have identified that discontinuous, layered roof materials have some self-supporting capacity. This research is a preliminary step towards understanding these mechanics in coal-measure rocks. Although others have considered broad conceptual models and simplified analogs for mine roof behavior, this study presents a unique numerical model that more completely represents in-situ roof conditions. The discrete element method(DEM) is utilized to conduct a parametric analysis considering a range of in-situ stress ratios, material properties, and joint networks to determine the parameters controlling the stability of single-entries modeled in two-dimensions. Model results are compared to empirical observations of roof-support effectiveness(ARBS) in the context of the coal mine roof rating(CMRR) system. Results such as immediate roof displacement, overall stability, and statistical relationships between model parameters and outcomes are presented herein. Potential practical applications of this line of research include:(1) roof-support optimization for a range of coal-measure rocks,(2) establishment of a relationship between roof stability and pillar stress, and(3) determination of which parameters are most critical to roof stability and therefore require concentrated evaluation.     

Coal mine roof rating(CMRR), rock mass rating(RMR) and strata control:Carborough Downs Mine,Bowen Basin,Australia

The rock mass rating(RMR) has been used across the geotechnical industry for half a century. In contrast,the coal mine roof rating(CMRR) was specifically introduced to underground coal mines two decades ago to link geological characterization with geotechnical risk mitigation. The premise of CMRR is that strength properties of mine roof rock are influenced by defects typical of coal measures stratigraphy.The CMRR has been used in longwall pillar design, roof support methods, and evaluation of extended cuts,but is rarely evaluated. Here, the RMR and CMRR are applied to a longwall coal mine. Roof rock mass classifications were undertaken at 67 locations across the mine. Both classifications showed marked spatial variability in terms of roof conditions. Normal and reverse faulting occur across the mine, and while no clear relationships exist between rock mass character and faulting, a central graben zone showed heterogeneous rock mass properties, and divergence between CMRR and RMR. Overall, the CMRR data fell within the broad envelope of results reported for extended cuts at Australian and U.S. coal mines. The corollary is that the CMRR is useful, and should not be used in isolation, but rather as a component of a strata control programme.     

Application of the coal mine floor rating(CMFR) to assess the floor stability in a Central Appalachian Coal Mine

Estimating the overall floor stability in a coal mine using deterministic methods which require complex engineering properties of floor strata is desirable, but generally it is impractical due to the difficulty of gathering essential input data. However, applying a quantitative methodology to describe floor quality with a single number provides a practical estimate for preliminary assessment of floor stability. The coal mine floor rating(CMFR) system, developed by the University of New South Wales(UNSW), is a rockmass classification system that provides an indicator for the competence of floor strata. The most significant components of the CMFR are uniaxial compressive strength and discontinuity intensity of floor strata. In addition to the competence of the floor, depth of cover and stress notch angle are input parameters used to assess the preliminary floor stability. In this study, CMFR methodology was applied to a Central Appalachian Coal Mine that intermittently experienced floor heave. Exploratory drill core data,overburden maps, and mine plans were utilized for the study. Additionally, qualitative data(failure/non-failure) on floor conditions of the mine entries near the core holes was collected and analyzed so that the floor quality and its relation to entry stability could be estimated by statistical methods. It was found that the current CMFR classification system is not directly applicable in assessing the floor stability of the Central Appalachian Coal Mine. In order to extend the applicability of the CMFR classification system, the methodology was modified. A calculation procedure of one of the CMFR classification system's components, the horizontal stress rating(HSR), was changed and new parameters were added to the HSR.     

Preliminary rib support requirements for solid coal ribs using a coal pillar rib rating(CPRR)

Researchers from the National Institute for Occupational Safety and Health(NIOSH) are developing a coal pillar rib rating(CPRR) technique to measure the integrity of coal ribs. The CPRR characterizes the rib composition and evaluates its impact on the inherent stability of the coal ribs. The CPRR utilizes four parameters: rib homogeneity, bedding condition, face cleat orientation with respect to entry direction,and rib height. All these parameters are measurable in the field. A rib data collecting procedure and a simple sheet to calculate the CPRR were developed. The developed CPRR can be used as a rib quality mapping tool in underground coal mines and to determine the potential of local rib instabilities and support requirements associated with overburden depth. CPRR calculations were conducted for 22 surveyed solid coal ribs, mainly composed of coal units. Based on this study, the rib performance was classified into four categories. A preliminary minimum primary rib support density(PRSD) line was obtained from these surveyed cases. Two sample cases are presented that illustrate the data collection form and CPRR calculations.     

深井巷道顶板锚固体破坏特征及稳定性分析

结合深井回采巷道围岩锚固体变形破坏特征,分析了深井回采巷道顶板锚固体失稳机理,建立了巷道顶板不同破坏形态的稳定性模型,得出了深井回采巷道锚杆、锚索支护条件下顶板锚固体稳定性力学方程,并结合古汉山矿深井回采巷道锚杆、锚索耦合支护方案开展验证性研究,对现场试验巷道顶板锚固体的稳定性进行了力学求解及数据分析,提出的深井回采巷道锚杆、锚索耦合支护方案支护参数合理,顶板离层量小于25mm、顶底板及两帮位移收敛量小于180mm,均保持在可控范围内,锚杆、锚索耦合支护有利于巷道围岩的稳定.     

Dust removal efficiency of high pressure atomization in underground coal mine

To master theoretical calculation for dust removal efficiency of high pressure atomization in an underground coal mine, the corresponding atomization characteristics and dust removal efficiency were both comprehensively studied in theory by virtue of related theories of hydromechanics and aerosol.According to actual measurements of flow coefficients and atomization angles of X-type swirl nozzle,computational formula was derived for atomized particle sizes of such a nozzle in conjunction with relevant empirical equation. Moreover, a mathematical model for applying high pressure atomization to dust removal in underground coal mine was also established to deduce theoretical computation formula of fractional efficiency. Then, Matlab was adopted to portray the relation curve between fractional efficiency and influence factors. In addition, a theoretical formula was also set up for removal efficiency of respirable dust and total coal dust based on dust size and frequency distribution equations. In the end,impacts of dust characteristic parameters on various dust removal efficiencies were analyzed.     

特殊岩层顶板煤巷锚杆支护应用研究

针对九龙矿152下07工作面回风巷顶板岩芯结构特殊性,确定了锚杆支护参数并进行试验研究。试验巷道经历了回采动压的影响而获得成功,取得了良好的技术与经济效益。     

口孜东矿11-2煤岩巷顶板离层稳定性分析

淮南矿区口孜东矿11－2煤埋置深,顶板地质条件复杂,根据已有研究结果将多点位移计实测的顶板总离层分离为层间离层和塑性变形。比较分离后层间离层、塑性变形值与其临界值,得出了该煤层岩巷顶板失稳特征,并分析了顶板岩层组成变化对其产生的影响,可为顶板合理支护提供依据。     

渗流作用下露天煤矿边坡稳定性数值模拟研究

对露天煤矿均质边坡的渗流场及其对边坡稳定性的影响规律进行研究,根据温度场和渗流场的可比拟性,利用有限元分析软件ANSYS提供的岩土非线性弹塑性Druker-Prager模型及其前后处理程序,对边坡在渗流影响下的内部应力、位移和稳定性进行分析,采用强度折减有限元方法计算边坡的安全系数并得到边坡可能产生破坏的滑移面,这有助于滑坡治理方案的制定。     

Analysis of diffusion behavior of harmful emissions from trackless rubber-wheel diesel vehicles in underground coal minesOA

To define the diffusion behavior of harmful exhaust substances from diesel vehicles and support safety risk assessments of underground coal mines, we performed a multi-species coupling calculation of the emission and diffusion of harmful substances from a trackless rubber-wheel diesel vehicle. A computational fluid dynamics(CFD) model of the diffusion of harmful emissions was hence established and verified. From the perspective of risk analysis, the diffusion behavior and distribution of hazardo...     

Numerical simulation of roof cavings in several Kuzbass mines using finite-difference continuum damage mechanics approach

An essential stage of mine design is an estimation of the steps of the first and periodic roof caving in longwall mines. Generally, this is carried out using the field experience and can be much enhanced by numerical simulation. In this work, the finite-difference method was applied coupled with the continuum damage mechanics(CDM) approach to simulate the stress-strain evolution of the rock mass with the underground opening during coal extraction. The steps and stages of roof caving were estimated relying on the numerical simulation data, and they were compared with the field data from several operating mines in the south of the Kuznetsk Basin, Russia. The dependence of the first roof caving step in simulation linearly correlates with field data. The results correspond to the actual roofs of longwall panels of the flat-dipping coal seams and the average rate of face advancement is approximately 5 m/day.     

Computer simulation of evacuation in underground coal mines

In order to study the evacuation behavior of miners during accidents,we analyzed the rational layout of a safe chamber,and improved evacuation efficiency.A visual model of evacuation was developed and used to simulate the evacuation process of mines,given their special underground environments.In our simulation,the working faces of a coal mine and nearby tunnels were divided into 0.5 m×0.5 m grids to reflect the location of occupants and obstacles.Based on a"flow potential of the occupant",we determined the direction of movement and routes of occupants.In the model,evacuation speed changed as a function of crowd density,with an average speed of 1.30 m/s.The model also takes into account detection time of the disaster and the personnel response time.Evacuation time,exit flow rate and evacuation path were obtained by simulation.The results indicate that using simulation technology can present a more visual evacuation process and show the behavior of occupants.Our establishment of a mine safety evacuation system has merit as reference value.     

Investigation of stability of a tunnel in a deep coal mine in China

Stability level of tunnels that exist in an underground mine has a great influence on the safety, production and economic performance of mines. Ensuring of stability for soft-rock tunnels is an important task for deep coal mines located in high in situ stress conditions. Using the available information on stratigraphy, geological structures, in situ stress measurements and geo-mechanical properties of intact rock and discontinuity interfaces, a three-dimensional numerical model was built by using 3DEC software to simulate the stress conditions around a tunnel located under high in situ stress conditions in a coal rock mass in China. Analyses were conducted for several tunnel shapes and rock support patterns. Results obtained for the distribution of failure zones, and stress and displacement fields around the tunnel were compared to select the best tunnel shape and support pattern to achieve the optimum stability conditions.     

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     

A case study of multi-seam coal mine entry stability analysis with strength reduction method

In this paper,the advantage of using numerical models with the strength reduction method(SRM) to evaluate entry stability in complex multiple-seam conditions is demonstrated.A coal mine under variable topography from the Central Appalachian region is used as a case study.At this mine,unexpected roof conditions were encountered during development below previously mined panels.Stress mapping and observation of ground conditions were used to quantify the success of entry support systems in three room-and-pillar panels.Numerical model analyses were initially conducted to estimate the stresses induced by the multiple-seam mining at the locations of the affected entries.The SRM was used to quantify the stability factor of the supported roof of the entries at selected locations.The SRM-calculated stability factors were compared with observations made during the site visits,and the results demonstrate that the SRM adequately identifies the unexpected roof conditions in this complex case.It is concluded that the SRM can be used to effectively evaluate the likely success of roof supports and the stability condition of entries in coal mines.     

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.     

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.     

Time-depth conversion of transient electromagnetic method used in coal mines

Accuracy of time-depth conversion in data processing of transient electromagnetic prospecting always affects the accurate positioning of water bodies in coal mines. In order to improve the accuracy of time-depth conversion, we established a mathematical model of time-depth conversion for a transient electromagnetic method based on the theory of "double smoke ring effect"of full space transient electromagnetic field transmission. Using a 3-layer as well as a 4-layer geo-electric model for roadway floors,we performed the time-depth conversion of theoretical curves of apparent resistance varying over time. In these curves, the depth corresponding to extreme value points is nearly the same as the depth of a geo-electric model. The position of water body determined by our time-depth conversion method agrees well with the result of borehole drilling, indicating that the established time-depth conversion model can clearly improve the accuracy of spatial positioning of water bodies in coal mines.