Area wide pillar failures, or “squeezes,” were once fairly common in US underground coal mines. While rare today, they do occur. This paper describes nine case histories that were investigated by MSHA Technical Support. They occurred across the range of US coalfields, in both longwall and room-and-pillar mines, and involved development mining, pillar recovery, longwall panel extraction, and multiple seam interactions. The analysis showed that just three of the nine squeezes were “false negatives” that occurred where the pillar stability factor exceeded the accepted design criteria. The observed mechanism of failure in these cases was sliding along bedding planes, highlighting the importance of their frictional strength in developing pillar confinement. The six other squeezes resulted from a variety of causes, including unanticipated multiple seam interactions and as-built pillar dimensions that differed from the design values.

Six decades ago, the most dangerous job in the USA was mining coal underground. Roof falls were responsible for half of the deaths, killing about 100 miners every year. Fast forward to 2016 and zero roof fall fatalities. Just three miners were killed by roof falls during the following 6 years. How did the mining community achieve this historic goal? This paper starts by analyzing the roof fall fatalities in 1968, categorizing them by their fundamental cause. Then, it shows how each type of roof fall was reduced over time, using snapshots of the fatalities occurring in subsequent decades. Along the way, it evaluates the influence of the regulatory environment, changing mining methods, and better ground control technology. The study found that in 1968 more than half of roof fall fatalities at large mines were attributable to an inadequate safety culture. The immediate effect of the 1969 Coal Mine Health and Safety Act was to reduce the riskiest activities, like needlessly going under unsupported roof. Other hazards, like large roof falls, required technological developments before they were brought under control. Roof Control Plans, which the US Bureau of Mines had been advocating since the 1920s, played a significant role throughout the process.

From 2015 to 2021, five massive pillar collapses occurred at four underground stone mines in the eastern USA. These events resulted in powerful airblasts that damaged mine infrastructure and mobile equipment, seriously injured miners, and disrupted underground travelways. Each of these pillar collapses propagated through the overlying strata, causing a subsidence sinkhole on the surface. Pillar collapses are particularly hazardous for miners because they can occur with little warning and can affect miners that are far from the pillar collapse area. The following case studies show that certain factors may increase the likelihood of a pillar collapse. For instance, each of the 5 events involved at least 12 benched pillars with width-to-height ratios ( w/h ) of 0.8 or less. This study elaborates on these factors and proposes a framework to assist mine operators where they identify similar hazards.

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.

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.

Longwall mining has existed in Utah for more than half a century. Much of this mining occurred at depths of cover that significantly exceed those encountered by most other US longwall operations. Deep cover causes high ground stress, which can combine with geology to create a coal burst hazard. Nearly every longwall mine operating within the Utah’s Book Cliffs coalfield has been affected by coal bursts. Pillar design has been a key component in the burst control strategies employed by mines in the Book Cliffs. Historically, most longwall mines employed double-use two-entry yield pillar gates. Double-use signifies that the gate system serves first as the headgate, and then later serves as the tailgate for the adjacent panel. After the 1996 burst fatality at the Aberdeen Mine, the inter-panel barrier design was introduced. In this layout, a wide barrier pillar protects each longwall panel from the previously mined panel, and each gate system is used just once. This paper documents the deep cover longwall mining conducted with each type of pillar design, together with the associated coal burst experience. Each of the six longwall mining complexes in the Book Cliffs having a coal burst history is described on a panel-by-panel basis. The analysis shows that where the mining depth exceeded 450 m, each design has been employed for about 38000 total m of longwall panel extraction. The double-use yield pillar design has been used primarily at depths less than 600 m, however, while the inter-panel barrier design has been used mainly at depths exceeding 600 m. Despite its greater depth of use, the inter-panel barrier gate design has been associated with about one-third as much face region burst activity as the double-use yield pillar design.

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 room-and-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 213 m (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.

During the past 15 years, roof fall rates have fallen dramatically in US coal mines, particularly in regions where the roof is weakest. The remarkable reduction in the number of roof falls has been accomplished with more effective roof support systems. The purpose of this paper is to present a design methodology that builds on and quantifies the basic roof support concepts that have been successful in the USA. The methodology starts by defining three modes of roof support, based on the roof strength relative to the stress level: (1) suspension, where roof bolts mainly provide skin control for strong roof; (2) beam building, where moderate strength roof can be supported by roof bolts alone; and (3) supplemental support for weak roof. Next, a large database of roof fall histories at a number of mines is used to define the approximate boundaries of these three regimes based on the coal mine roof rating and the depth of cover. Finally, guidelines are presented for site-specific design of support systems within each regime. The new computer package, analysis of mine roof support (AMRS), implements the design methodology.

The proliferation of unconventional gas well development in the Northern Appalachian coalfields has raised a number of mine safety concerns. Unconventional wells, which extract gas from deep shale formations, are characterized by gas volumes and pressures that are significantly higher than those observed at many conventional wells. The gas is composed largely of methane as well as other hydrocarbons. Hundreds of planned and actively producing wells penetrate protective coal pillars or barriers within active mine boundaries, including chain pillars located between longwall panels. Gas released from a well damaged by mining-induced ground movements could pose a risk to miners by flowing into the mine atmosphere. The mining-induced ground movements that may cause well damage include conventional subsidence, non-conventional subsidence (e.g. bedding plane slip), pillar failure, and floor instability. This paper describes the known risk factors for each of the four failure mechanisms. It includes a framework that can guide the risk assessment process when mining takes place near gas or oil wells.

Thirty years ago, the analysis of longwall pillar stability (ALPS) inaugurated a new era in coal pillar design. ALPS was the first empirical pillar design technique to consider the abutment loads that arise from full extraction, and the first to be calibrated using an extensive database of longwall mining case histories. ALPS was followed by the analysis of retreat mining stability (ARMPS) and the analysis of multiple seam stability (AMSS). These methods incorporated other innovations, including the coal mine roof rating (CMRR), the Mark-Bieniawski pillar strength formula, and the pressure arch loading model. They also built upon ever larger case history databases and employed more sophisticated statistical methods. Today, these empirical methods are used in nearly every underground coal mine in the US. However, the piecemeal manner in which these methods have evolved resulted in some weaknesses. For example, in certain situations, it may not be obvious which program is the best to use. Other times the results from the different programs are not entirely consistent with each other. The programs have also not been updated for several years, and some changes were necessary to keep pace with new developments in mining practice. The analysis of coal pillar stability (ACPS) now integrates all three of the older software packages into a single pillar design framework. ACPS also incorporates the latest research findings in the field of pillar design, including an expanded multiple seam case history data base and a new method to evaluate room and pillar panels containing multiple rows of pillars left in place during pillar recovery. ACPS also includes updated guidance and warnings for users and features upgraded help files and graphics.

Coal bursts are typically associated with highly stressed coal. Most bursts occur during retreat mining (longwall mining or pillar recovery) in highly stressed locations like the tailgate corner of the longwall panel. Others are associated with multiple seam interactions. However, a small but significant percentage of coal bursts have occurred during development or in outby locations unaffected by active mining. Most development bursts have been relatively small, but some have been highly destructive. No theory of coal bursts can be complete if it does not account for this type of event. This paper focusses on the development mining coal burst experience in the US, putting it into the context of the entire US coal burst database. The first documented development coal burst occurred almost exactly 100 years ago during slope drivage at the Sunnyside Mine in Utah. Sunnyside subsequently had a long history of bursts, mainly during retreat mining but also during development. Several Colorado mines have also experienced multiple development bursts. Many, but by no means all, of the development bursts in these western US coalfields have been associated with known faults. In the Central Appalachian coalfields, most development bursts have occurred in multiple seam situations. In some of these cases, however, there was no retreat mining in either seam. The paper closes with some lessons from this history, with implications for preventing such events in the future.

Underground coal mining in the U.S. is conducted in numerous regions where previous workings exist above and/or below an actively mined seam. Miners know that overlying or underlying fully extracted coal areas, also known as gob regions, can result in abutment stresses that affect the active mining. If there was no full extraction, and the past mining consists entirely of intact pillars, the stresses on the active seam are usually minimal. However, experience has shown that in some situations there has been sufficient yielding in overlying or underlying pillar systems to cause stress transfer to the adjoining larger pillars or barriers, which in turn, transfer significant stresses onto the workings of the active seam. In other words, the overlying or underlying pillar system behaves as a “pseudo gob.” The presence of a pseudo gob is often unexpected, and the consequences can be severe. This paper presents several case histories, summarized briefly below, that illustrate pseudo gob phenomenon: (1) pillar rib degradation at a West Virginia mine at 335m depth that contributed to a rib roll fatality, (2) pillar rib deterioration at a Western Kentucky mine at 175m depth that required pillar size adjustment and installation of supplemental bolting, (3) roof deterioration at an eastern Kentucky mine at 400m depth that stopped mine advance and required redirecting the section development, (4) coal burst on development at an eastern Kentucky mine at 520m depth that had no nearby pillar recovery, and (5) coal burst on development at a West Virginia mine at the relatively shallow depth of 335m that also had no nearby pillar recovery. The paper provides guidance so that when an operation encounters a potential pseudo gob stress interaction the hazard can be mitigated based on an understanding of the mechanism encountered.

Pillar recovery is an integral part of underground coal mining in many areas where longwalls are not employed. Historically considered an "inherently unsafe" mining method, the safety record of pillar recovery has recently improved significantly in the US and Australia. Better safety can be attributed to new ground control theory and practice that emphasize both global and local stability. Global stability is addressed primarily through engineered pillar design, and became a major focus after the 2007 Crandall Canyon mine disaster. Local stability, defined as ground control in the immediate work area, has improved through the application of three concepts: Leaving an engineered final stump, rather than extracting the entire pillar; Enhancing roof bolt support, particularly in intersections; and Increasing the use of Mobile Roof Supports (MRS) A final component is better management of pillar recovery operations. This includes a focus on worker positioning, as well as on standardized pillar and lift sequences, MRS operations, and pre-mining hazard identification. Safe pillar recovery also requires control of other core hazards including rib falls, coal bursts, and violent caving events that can trigger airblasts or explosions.

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 2000s, 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 the 2007 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.

Coal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are a particular hazard because they typically occur without warning. During the past 2 years three US coal miners were killed in two coal bursts, following a 6-year period during which there were zero burst fatalities. This paper puts the US experience in the context of worldwide research into coal bursts. It focuses on two major longwall mining coalfields which have struggled with bursts for decades. The Utah experience displays many of the “classic” burst characteristics, including deep cover, strong roof and floor rock, and a direct association between bursts and mining activity. In Colorado, the longwalls of the North Fork Valley (NFV) also work at great depth, but their roof and floor strengths are moderate, and most bursts have occurred during entry development or in headgates, bleeders, or other outby locations. The NFV bursts also are more likely to be associated with geologic structures and large magnitude seismic events. The paper provides a detailed case history to illustrate the experience in each of these coalfields. The paper closes with a brief discussion of how US longwalls have managed the burst risk.

Many problems in rock engineering are limited by our imperfect knowledge of the material properties and failure mechanics of rock masses. Mining problems are somewhat unique, however, in that plenty of real world experience is generally available and can be turned into valuable experimental data. Every pillar that is developed, or stope that is mined, represents a full-scale test of a rock mechanics design. By harvesting these data, and then using the appropriate statistical techniques to interpret them, mining engineers have developed powerful design techniques that are widely used around the world. Successful empirical methods are readily accepted because they are simple, transparent, practical, and firmly tethered to reality. The author has been intimately associated with empirical design for his entire career, but his previous publications have described the application of individual techniques to specific problems. The focus of this paper is the process used to develop a successful empirical method. A six-stage process is described: identification of the problem, and of the end users of the final product; development of a conceptual rock mechanics model, and identification of the key parameters in that model; identification of measures for each of the key parameters, and the development of new measures (such as rating scales) where necessary; data sources and data collection; statistical analysis; and packaging of the final product. Each of these stages has its own potential rewards and pitfalls, which will be illustrated by incidents from the author’s own experience. The ultimate goal of this paper is to provide a new and deeper appreciation for empirical techniques, as well as some guidelines and opportunities for future developers.

Coal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. Despite decades of research, the sources and mechanics of these events are not well understood, and therefore they are difficult to predict and control. Experience has shown, however, that certain geologic and mining factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present, and then identifying appropriate control measures to mitigate the hazard. This paper summarizes the U.S. and international experience with coal bursts, and describes the known risk factors in detail. It includes a framework that can be used to guide the risk assessment process.

BACKGROUND:Coal plays a crucial role in the U.S. economy yet underground coal mining continues to be one of the most dangerous occupations in the country. In addition, there are large variations in both profitability and the incidence of occupational injuries across mines. OBJECTIVE:The objective of this study was to examine the association between profitability and the incidence rate of occupational injuries in U.S. underground coal mines between 1992 and 2008. DATA AND METHOD:We used mine-specific data on annual hours worked, geographic location, and the number of occupational injuries suffered annually from the employment and accident/injury databases of the Mine Safety and Health Administration, and mine-specific data on annual revenue from coal sales, mine age, workforce union status, and mining method from the U.S. Energy Information Administration. A total of 5669 mine-year observations (number of mines×number of years) were included in our analysis. We used a negative binomial random effects model that was appropriate for analyzing panel (combined time-series and cross-sectional) injury data that were non-negative and discrete. The dependent variable, occupational injury, was measured in three different and non-mutually exclusive ways: all reported fatal and nonfatal injuries, reported nonfatal injuries with lost workdays, and the 'most serious' (i.e. sum of fatal and serious nonfatal) injuries reported. The total number of hours worked in each mine and year examined was used as an exposure variable. Profitability, the main explanatory variable, was approximated by revenue per hour worked. Our model included mine age, workforce union status, mining method, and geographic location as additional control variables. RESULTS:After controlling for other variables, a 10% increase in real total revenue per hour worked was associated with 0.9%, 1.1%, and 1.6% decrease, respectively, in the incidence rates of all reported injuries, reported injuries with lost workdays, and the most serious injuries reported. CONCLUSION:We found an inverse relationship between profitability and each of the three indicators of occupational injuries we used. These results might be partially due to factors that affect both profitability and safety, such as management or engineering practices, and partially due to lower investments in safety by less profitable mines, which could imply that some financially stressed mines might be so focused on survival that they forgo investing in safety.