Design concerns of room and pillar retreat panels

Why do some room and pillar retreat panels encounter abnormal conditions? What factors deserve the most consideration during the planning and execution phases of mining and what can be done to mitigate those abnormal conditions when they are encountered? To help answer these questions, and to determine some of the relevant factors influencing the conditions of room and pillar (R &; P) retreat mining entries, four consecutive R &; P retreat panels were evaluated. This evaluation was intended to reinforce the influence of topographic changes, depth of cover, multiple-seam interactions, geological conditions, and mining geometry. This paper details observations were made in four consecutive R &; P retreat panels and the data were collected from an instrumentation site during retreat mining. The primary focus was on the differences observed among the four panels and within the panels themselves. The instrumentation study was initially planned to evaluate the interactions between primary and secondary support, but produced rather interesting results relating to the loading encountered under the current mining conditions. In addition to the observation and instrumentation, numerical modeling was performed to evaluate the stress conditions. Both the LaModel 3.0 and Rocscience Phase 2 programs were used to evaluate these four panels. The results of both models indicated a drastic reduction in the vertical stresses experienced in these panels due to the full extraction mining in overlying seams when compared to the full overburden load. Both models showed a higher level of stress associated with the outside entries of the panels. These results agree quite well with the observations and instrumentation studies performed at the mine. These efforts provided two overarching conclusions concerning R &; P retreat mine planning and execution. The first was that there are four areas that should not be overlooked during R &; P retreat mining: topographic relief, multiple-seam stress relief, stress concentrations near the gob edge, and geologic changes in the immediate roof. The second is that in order to successfully retreat an R &; P panel, a three-phased approach to the design and analysis of the panel should be conducted: the planning phase, evaluation phase, and monitoring phase.     

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.     

Preventing roof fall fatalities during pillar recovery: A ground control success story

For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines.Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by a roof fall than other coal miners. Since 2007, however, there has been just one fatal roof fall on a pillar line. This paper describes the process that resulted in this historic achievement. It covers both the key research findings and the ways in which those insights, beginning in the early 2000 s, were implemented in mining practice. One key finding was that safe pillar recovery requires both global and local stability.Global stability is addressed primarily through proper pillar design, and became a major focus after the2007 Crandall Canyon mine disaster. But the most significant improvements resulted from detailed studies that showed that local stability, defined as roof control in the immediate work area, could be achieved with three interventions:(1) leaving an engineered final stump, rather than extracting the entire pillar,(2) enhancing roof bolt support, particularly in intersections, and(3) increasing the use of mobile roof supports(MRS). A final component was an emphasis on better management of pillar recovery operations.This included a focus on worker positioning, as well as on the pillar and lift sequences, MRS operations,and hazard identification. As retreat mines have incorporated these elements into their roof control plans,it has become clear that pillar recovery is not ‘‘inherently unsafe." The paper concludes with a discussion of the challenges that remain, including the problems of rib falls and coal bursts.     

Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

A gas–solid coupling model involving coal seam deformation,gas diffusion and seepage,gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining.The research results indicate:(1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase,thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient,which will cause a decrease in gas pressure.(2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam,which can effectively reduce the coal and gas outburst risk.The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.     

Geotechnical considerations for concurrent pillar recovery in close-distance multiple seams

Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground fall fatalities in underground coal mines in the United States.The risk of ground falls during pillar recovery increases in multiple-seam mining conditions.The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave.When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction.This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 21 m of interburden under about 305 m of depth of cover.The study finds that, for interburden thickness of 21 m, the multiple-seam mining influence zone in the lower seam is directly under the barrier pillar within about 30 m from the gob edge of the upper seam.The peak stress in the interburden transfers down at an angle of approximately 20°away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat.The study also suggests that, for full pillar recovery in close-distance multiple-seam scenarios,it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars.If the entries and/or crosscuts in the lower seam are developed outside the gob line of the upper seam,additional roof and rib support needs to be considered to account for the elevated stress in the multiple-seam influence zone.     

Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining

To investigate the abnormal ground pressures and roof control problem in fully mechanized repeated mining of residual coal after room and pillar mining,the roof fracture structural model and mechanical model were developed using numerical simulation and theoretical analysis.The roof fracture characteristics of a repeated mining face were revealed and the ground pressure law and roof supporting conditions of the repeated mining face were obtained.The results indicate that when the repeated mining face passes the residual pillars,the sudden instability causes fracturing in the main roof above the old goaf and forms an extra-large rock block above the mining face.A relatively stable ‘‘Voussoir beam" structure is formed after the advance fracturing of the main roof.When the repeated mining face passes the old goaf,as the large rock block revolves and touches gangue,the rock block will break secondarily under overburden rock loads.An example calculation was performed involving an integrated mine in Shanxi province,results showed that minimum working resistance values of support determined to be reasonable were respectively 11,412 kN and 10,743 kN when repeated mining face passed through residual pillar and goaf.On-site ground pressure monitoring results indicated that the mechanical model and support resistance calculation were reasonable.     

Strata behavior investigation for high-intensity mining in the water-rich coal seam

This paper describes a specific case of mining in a water-rich coal seam in western China. Water inrushes, roof caving and other disasters induced by intensive mining operation could pose great threats to the safety of coal mines. The strata behavior during the high-intensity extraction in the water-rich coal seam is analyzed by employing the numerical simulation method and in situ monitoring. The results show that about 10 m ahead of the workface, the front abutment pressure peaks is at 34.13 MPa, while the peak of the side abutment pressure is located about 8 m away from the gateway with the value of 12.41 MPa; the height of the fracture zone, the first weighting step and the cycle weighting step are calculated to be 45, 50 and 20.8 m, respectively; pressure distribution in the workface is characterized by that the vertical pressure in the center occurs earlier and is stronger than those on both ends. Then, the results above are verified by in situ measurement, which may provide a basis for safe mining under similar conditions.     

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.     

Numerical investigation into the effect of backfilling on coal pillar strength in highwall mining

This paper attempts to quantify the effect of backfilling on pillar strength in highwall mining using numerical modelling. Calibration against the new empirical strength formula for highwall mining was conducted to obtain the material parameters used in the numerical modelling. With the obtained coal strength parameters, three sets of backfill properties were investigated. The results reveal that the behavior of pillars varies with the type and amount of backfill as well as the pillar width to mining height ratio(w/h). In case of cohesive backfill, generally 75% backfill shows a significant increase in peak strength, and the increase in peak strength is more pronounced for the pillars having lower w/h ratios. In case of noncohesive backfill, the changes in both the peak and residual strengths with up to 92% backfill are negligible while the residual strength constantly increases after reaching the peak strength only when 100%backfill is placed. Based on the modelling results, different backfilling strategies should be considered on a case by case basis depending on the type of backfill available and desired pillar dimension.     

倾斜煤层区段煤柱变形破坏规律

煤层倾角是影响区段煤柱稳定性的关键因素之一.利用理论分析、相似模拟、数值模拟等方法研究了倾斜煤层开挖后倾向覆岩结构演化特征、煤柱变形及失稳破坏形式.研究结果表明,0~45°范围内随着煤层倾角增大,区段煤柱发生剪切失稳破坏的可能性增大;煤柱两侧覆岩结构呈现不对称分布,煤柱上侧砌体梁结构形成层位较低,煤柱下侧形成冒空区,砌体梁结构形成层位高于上侧;与水平煤层煤柱破坏以挤压变形为主不同,倾斜煤柱以沿着弱面剪切滑移破坏为主;不同倾角煤层煤柱围岩变形量呈不对称分布,煤柱下侧围岩变形量大于上侧,煤层倾角越大煤柱围岩变形量不对称分布趋势越明显.     

Investigations on longwall mining subsidence impacts on Pennsylvania highway alignments

Over 600 longwall panels have been mined in Pennsylvania in the last 50 years. Of those 600 panels, 25 panels undermined interstates highways. Despite this quantity of panels, much is still unknown regarded the detailed effects of undermining highways. The Gateway Mine, the Emerald Mine, and the Cumberland Mine undermined I-79 with 17 panels in 1982–1989 and in 2003–2008, respectively; Mine 84 undermined I-70 with 4 panels in 1987–2000. Through the examination of the panels that undermined I-70 and I-79, it is possible to determine which factors have most impacted the highway alignments. In some locations, the highway intersects with panels at angles ranging from 45° to 80°; and at the others, it runs between two panels, which simulates the effect of gateroads on the subsidence. The panel width to overburden ratio varies between 0.64 and 1.7, meaning that the interstates were influenced by both subcritical and supercritical subsidence basins. The face advance rates and overburden depths also vary between the panels. Unfortunately, specific information detailing highway impacts associated with unique characteristics of the subsidence basins are limited. In this paper, using the profile function model and influence function model within the surface deformation prediction system(SDPS), the effects of overburden,panel size, and orientation of the road on the highway can be indirectly assessed.     

A study of mining fatalities and coal price variation

It has long been postulated that a relationship exists between commodity price cycles and fatalities in the mining industry. Previous studies have found only weak correlations in this area. This study analyses the fatalities recorded in coal mines over the period 1985–2016 in the State of Queensland as a function of thermal coal price variation. The study finds that the relationship between fatalities and coal prices is not linear. One to two fatalities occur in most years independent of the thermal coal price. When the price of coal falls below AUD 55/tonne(non-inflation adjusted), the likelihood of an incident involving multiple fatalities increases. The probability can be estimated at 2 in 18 events(equivalent to 11%). This paper postulates that in difficult economic times, mining companies react by downsizing direct employees. If not carefully managed, this can result in loss of knowledge around safety systems, and reduced effectiveness of safety supervision. Because of labour cost advantages, some jobs previously undertaken by direct employees will be replaced by contractors. Increased contractor numbers contribute to increased risk of fatalities occurring, as contractors are over-represented in accident categories involving vehicle accidents, tire handling and crushing incidents. Mine inspectorates, mining, and mining contractor companies need to be especially vigilant to enforce health and safety management systems during periods of low coal prices.     

煤与瓦斯突出矿井煤层群联合开采动态接替研究

针对高瓦斯突出矿井煤层群联合开采的特点,提出了动态编制矿井采掘接替计划的方法.通过对影响矿井采掘工程接替的安全因素和经济因素的分析,建立了影响采掘工程接替的因素指标体系,利用层次分析方法得出了因素的指标权重,采用模糊综合优选法确定备选工作面中的最优接替面,依次类推,直到安排所有的备选工作面.根据采煤工作面生产计划,综合考虑矿井瓦斯治理技术,采用时间反推方法,编制采煤工作面相关的掘进计划.应用研究结果表明,采掘动态接替法可以适时调整采掘接替计划,所得采掘工程接替方案科学合理、切实可行.     

Surface movement above old coal longwalls after mine closure

Although most subsidence occurs in the months and years after mining by the longwall method, surface movement is still occurring many decades after the mining. The aim of the study is to quantify the long term behavior. Satellite data(radar-interferometry) were analyzed to study an area of about 2 km~2 during the 18 years following the closure of the underground infrastructure and the flooding of the underground workings and rock mass. It was observed that, on average, a residual downward movement took place till 7–12 years after the closure, followed by a clear uplift. However, the first signs of an uplift occurred in certain sub-areas 3–4 years after the closure. Zones within the area studied were identified with either larger or smaller movements. However, the spatial variation of the surface subsidence or uplift could not be directly explained by the characteristics of mining.     

Upward surface movement above deep coal mines after closure and flooding of underground workings

After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—an upward movement.Although most surface movement(i.e., subsidence) occurs in the months and years after mining by the longwall method, surface movement still occurs many decades after mining is terminated.After the closure and flooding of underground excavations and surrounding rock, this movement was reversed.This paper focuses on quantifying the upward movement in two neighboring coal mines(Winterslag and Zwartberg, Belgium).The study is based on data from a remote sensing technique: interferometry with synthetic aperture radar(INSAR).The results of the study show that the rate of upward movement in the decade after closure is about 10 mm/year on average.The upward movements are not linked directly to the past exploitation directly underneath a location.The amounts of subsidence at specific locations are linked mainly to their positions relative to an inverse trough shape situated over the entire mined-out areas and their immediate surroundings.Local features, such as geological faults, can have a secondary effect on the local variation of the uplift.The processes of subsidence and uplift are based on completely different mechanisms.Subsidence is initiated by a caving process, while the process of uplift is clearly linked to flooding.     

Top coal caving mining technique in thick coal seam beneath the earth dam

It is important to study the mining technology under structures for raising the coal resources recovery ratio. Based on the geological and mining conditions, the top coal caving harmonic mining technique in thick coal seam beneath the earth dam was put forward and studied. The 5 factors such as the panel mining direction, panel size, panel location, panel mining sequence and panel advance velocity were taken into account in this technique. The dam movement and deformation were predicted after the thick coal seam mining and the effects of mining on the dam were studied. By setting up the surveying stations on the dam, the movement and deformation of the dam were observed during mining. By taking some protective measures on the dam, the top coal caving mining technique in thick coal seam beneath the earth dam was carried out successfully. The study demonstrates that harmonic mining in thick coal seam is feasible under the dam. The safety of the earth dam after mining was ensured and the coal resources recovery ratio was improved.     

采动区建筑物下保护煤柱开采优化设计研究

采用岩层移动角进行留设保护煤柱的传统方法,增大了保护煤柱呆滞量,造成了煤炭资源的巨大浪费.为了探讨综放开采条件下保护煤柱留设的最优尺寸,基于潞安矿区王庄煤矿的地质采矿条件,提出了根据建筑物采动损害允许的临界变形值进行优化设计保护煤柱的新思路.按照概率积分法,预测计算开采工作面位于不同停采线位置时保护煤柱留设的合理尺寸,分析了不同开采方案下建筑物所受采动的影响面积、地表下沉程度及倾斜变形、拉伸变形等地表移动变形对建筑物采动损害的影响程度.通过11个方案的分析比较,结合经济与社会效益,给出了建筑物下保护煤柱留设的最优方案,为综放开采条件下建筑物压煤开采提供了科学依据.     

复合水体下急倾斜煤层开采的水害防治研究

淮南孔集煤矿山西组A1、A3煤层总厚6m,为第四系含水砂层下底板岩溶水上复合含水体威胁的急倾斜煤层。对于砂层水害防治,采用合理留设防水煤柱,沿用“小阶段、长走向、间歇开采”的方式方法;对于底板岩溶水患防治,采用“疏水降压、限压开采”的防治水方法;对于局部抽冒及流水钻孔涌砂造成地表抽冒漏斗及岩溶塌陷漏斗,采取了“按尺核产、严禁超限出煤”、“小流量、长历时、控砂疏水”的有力措施。实现了安全开采,取得了良好的效果。     

“三软”极不稳定煤层综采技术研究

偃龙矿区开采的山西组二．煤层属于“三软”极不稳定煤层,其突出特征为煤层松软、厚度和倾角变化大,顶板及底板松软、破碎,赋存条件复杂,开采极其困难．嵩山煤矿以多项支护技术专利为支撑,采用底板岩巷的“一巷三用”,合理选择高强度轻型放顶煤支掩式液压支架,以及综采防片帮和防冒顶等技术,实施综采机械化开采,实现了“三软”煤层的安全高效开采,取得了显著效果．     

The influence of mining factors on seismic activity during longwall mining of a coal seam

In this article an attempt to determine the influence of mining factors on the seismic activity during the longwall mining of the upper layer of coal seam no. 405/2 in one of the Polish hard coal mines in the Upper Silesian Coal Basin was conducted. Two longwall panels were mined in analogous geological conditions and based on the same mining system and technology. However, there was significant difference with regards to the mining factors, which was reflected in the observed seismic activity. Some tools used in mining seismology were applied to illustrate the aforementioned influence of mining factors, e.g. the frequency-energy distribution, the frequency-magnitude distribution, the 2 D distribution of released seismic energy, the relationship between released seismic energy and the volume of mined coal, the Benioff strain release, and the Gutenberg-Richter(GR) b coefficient distribution(b is the proportion between high and low energy tremors). Concerning the Benioff strain release, a new solution, based on the slope of a fitted line in a moving time window, is proposed.