• 1.9k Downloads • Abstract 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 Official BurstCoin Cryptocurrency Hub. When us exchanges ask for id. Hi everyone can someone help me i need 1 burst to start mining all is completed. This activity is called “mining” and because of that the entire. Allowing only big mining farms to thrive. Burst’s Proof of. Tweets by Burstcoin. This activity is called “mining” and because of that the entire. Allowing only big mining farms to thrive. Burst’s Proof of. Tweets by Burstcoin. Mining crews should be trained to observe coal burst warning signs, particularly the occurrence of small bursts, which are often the best indication that an area is becoming more burst prone. A record-keeping system should be maintained, and management processes developed to ensure that warning signs receive appropriate responses.

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. As long ago as the early 1930s, coal bursts were classified into two types according to their apparent cause (Rice ). “Pressure bursts” were thought to originate in the seam itself and were associated with high stress and direct mining activity.

Typical pressure bursts occur while a continuous miner is extracting a pillar or a longwall shearer is cutting the tailgate corner of the face. “Shock bursts”, on the other hand, were thought to be caused by “the breaking of a thick, massive, rigid strata at a considerable distance above the coal bed, causing a great, hammerlike blow to be given to the immediate roof of the mine opening, (transmitting) a shock wave to the coal pillar or pillars” (Rice ). These events could occur on off shifts, outby the face, in bleeder pillars, or other unexpected times and locations. Today, we understand that the sudden, dynamic failure in the overlying (or underlying) strata releases elastic energy in the form of seismic waves. The failures include sudden downward movements of the rock above the worked-out areas, shear slip motion on faults or fractures in the overburden, or some combination of these two mechanisms (Pankow et al.

Shear slip motion can occur on newly-created fractures, or on reactivated pre-existing faults (Swanson et al.; Alber et al. The seismic energy released by such events can cause damage both underground and on the surface. The long history of coal bursts in the US has been well-documented. Iannacchione and Zelanko ( ) analyzed a database of 172 bursts that had occurred between 1936 and 1993. These bursts had resulted in a total of 87 fatalities and 163 injuries.

Reflecting the mining technology in use during this period, 61% of the events occurred during pillar recovery operations, and 25% during longwall mining. The remaining 14% occurred during development. Three main coalfields have accounted for the vast majority of bursts.

Iannacchione and Zelanko reported that 65% of the bursts in their database occurred in the Central Appalachian coalfields of West Virginia, Virginia, and Kentucky. Most of the rest occurred in Colorado (17%) or Utah (15%).

Bursts occurred at more than 50 different mines during this period. Iannacchione and Zelanko found that nearly all bursts occurred at depths greater than 300 m, and most were greater than 400 m. A database of coal bursts that occurred during the period 1994–2013 was recently developed. It includes a total of 140 events that were reported to MSHA. An additional 13 events involved roof “thumps”, but since they did not result in violent ejection of coal or rock they were not included in the total. Four of the 140 events in the new database resulted in a total of five fatalities, two on longwalls, and three during two pillar recovery events.

Note that the Crandall Canyon mine disaster which claimed six lives is not included in the database. That event was unique and can be best described as a catastrophic, mine-wide, pillar system failure. The area that collapsed at Crandall Canyon was enormous, encompassing 20 rows of pillars over approximately one square kilometer. It has more in common with the massive pillar collapses described by Mark et al. ( ) and Zipf ( ) than with a typical coal burst.

Figure shows that since the 1980’s there has been a generally declining trend in the number of reported bursts. Some of this improvement can be attributed to changes in mining methods and geologic environments. Longwall mining has largely replaced pillar recovery in the western US, and much less coal is being mined from the burst prone Pocahontas coalfields of Virginia and West Virginia. On the other hand, longwalls work deeper seams today than they did 30 years ago, and multiple seam interactions are much more prevalent. Therefore it seems evident that much of the improvement must be due to the better mining practices, some of which will be discussed further on. Figure shows that during the past 20 years Utah mines have accounted for the largest number of bursts, followed by Colorado. Virginia and West Virginia only recorded one burst each, though together those two states had accounted for 54% of the bursts during the earlier period analyzed by Iannacchione and Zelanko ( ).

Unfortunately, in 2014 a coal burst killed two West Virginia miners. Most coal mining in Central Appalachia is conducted by room-and-pillar methods, though the experience of one burst prone longwall has been described in detail by Hoelle ( ). Figure shows that 41% of the bursts during the past 20 years have occurred on the longwall face. Another 20% affected the tailgate entry at the corner of the longwall face, and 12% occurred during retreat mining. All of these locations are subject to very high stresses, and they are directly affected by mining activity, and so might be considered likely locations for bursts.

On the other hand, 14% of the bursts occurred during entry development, and another 13% affected pillars in the headgate, bleeder, or other outby locations. Fig. 5 Coal bursts reported to MSHA, by location in the mine, 1994–2013 There are also some significant regional trends. In Utah, 76% of the total 68 events occurred on the longwall face, and another 10% occurred either in the longwall tailgate or during pillar recovery.

Similarly, in Central Appalachia, 81% of the 21 reported bursts occurred on the longwall face, tailgate, or pillar line. In Colorado, on the other hand, nearly half of the bursts occurred during entry development or in the headgate, bleeder, or other outby location. And although 40 of the 46 Colorado events took place in longwall mines, only two occurred on a longwall face. 2.1 Utah experience. How To Make A BURST Miner.

Fig. 6 Mountainous terrain in the Utah coalfields. Note the thick, cliff-forming sandstone units The first coal bursts were recorded in Utah almost 100 years ago. They were typically associated with pillar recovery under deep cover.

However, Peperakis ( ) noted that severe bumps at the Sunnyside Mine in Utah had occurred in virgin development a long distance away from active workings, and were attributed to geologic faults. Bursts were one reason why Sunnyside was one of the early longwall pioneers in the US. Through a lengthy process during which many gateroad configurations were trialed, Sunnyside engineers developed a two-entry, yield pillar system that virtually eliminated pillar bursts. The yield pillars were typically 9 m wide in 2.5–3 m thick seams (DeMarco et al. Mining engineers also learned to avoid “critical” pillars which are too large to yield non-violently yet too small to support large abutment loads. The width-to-height ratios of such burst-prone, critical pillars normally exceeded 4 or 5 (DeMarco et al.

Longwall face bursts continued to be a problem however, typically once the cover depth exceeded 450 m. Seismicity induced by mining operations in Utah has also been extensively studied. The University of Utah operates a regional seismic system which recorded 148 mining induced events with Local Magnitude M L >2.5, including 18 with M L >3.0, between 1978 and 2000 (Arabasz and Pechmann ). Of the larger events, three were judged to have shear–slip mechanisms, while 13 had possible collapse-type mechanisms. Few of these large events coincided with longwall bursts underground. The largest mining induced event ever recorded in Utah was the M = 4.2 shear-slip event that was located 150 m above face of the second panel the Willow Creek mine.

This event was large enough to cause rock falls that closed a railroad and major highway, but it resulted in only minor damage in the vicinity of the longwall (Ellenberger and Heasley ). Case history Mine A was located in the Book Cliffs region of central Utah (MSHA ). The massive Kenilworth sandstone formation lies 6–12 m above the seam, and the strata between it and the seam includes other strong siltstones and sandstones with typical strengths of about 100 MPa. Another massive sandstone, the Aberdeen, lies directly beneath the seam.

The seam dip was 6–12 degrees, so each successive panel was about 40 m deeper than the previous one. Some mining had been conducted in a coal seam lying approximately 85 m above Mine A, but there were no noticeable stress transfers. Longwall mining began in 1995. Panels were developed 225 m wide in the 3 m thick seam. A three-entry yield pillar system, with entries on 15 by 36 m centers, separated the first longwall panel from the second. The cover above the tailgate of the second panel was about 480 m. As the second panel retreated, “bounces” consisting of sudden forceful vibrations became increasingly common on the tailgate end of the panel.

Five of these events resulted in broken shearer torque shafts. A major coal burst occurred when the panel had been retreated 225 m. The shearer had just begun the double-cut at the tailgate that began the return pass towards the headgate. Approximately 30 m of the face blew out, propelling coal across the conveyor and into the shields, and causing fatal injuries to the shearer operator. Floor, roof, and rib damage from the burst was also visible for 45 m along the tailgate entry.

This event registered M = 2.2 on the regional seismic network (UUUS ). In the wake of this incident, the remainder of the second panel was abandoned. A new tailgate was driven for the third panel, leaving the remainder of the second panel as an interpanel barrier pillar protecting the third panel tailgate from abutment loads arising from the first panel. Subsequent panels were also developed with the interpanel barrier design, leaving solid pillars up to 180 m wide between adjoining panels.

Two independent sets of two-entry yield pillar gates were driven for each new panel. Almost 10 years after the first fatal burst, the seventh longwall panel was being retreated at Mine A under almost 840 m of cover (MSHA ). Bounces were common along the longwall face, ranging from thumps in the roof or floor to coal being blown from the face. These events occurred all along the longwall face, but were most common near the headgate and tailgate entries. “Bounce procedures” were in place to protect the workers by limiting access to the face when the shearer was near the gate entries and specifying that shearer operators should position themselves behind the 8 m long deflector shields mounted above the shearer.

The deflector shields were hinged off the shearer frame to be lowered or raised to accommodate mining clearance. In addition, expanded metal guards were attached to the armored face conveyor periodically along the walkway, and sheets of conveyor belting were suspended from the bottom of shield canopies along the walkway. Despite these precautions, a fatal bounce killed a shearer operator located approximately 15 m from the headgate corner as the shearer was double-cutting towards the headgate (Fig. The burst extended approximately 15 m along the face, with the largest cavity about 1 m deep directly in front of the victim.

This event measured M = 1.2 on the seismic network. Mine A closed not long afterward, because the burst hazard could not be effectively managed at the even greater depths above the remaining reserves. Fig. 7 Sketch of the second burst accident scene at Utah Mine A (MSHA ) The experience at Mine A demonstrated both the advantages and limitations of the two Utah longwall pillar design techniques. While the yield pillar system used on the first two panels typically performs well at depths up 600 m or so, it concentrates the load on the tailgate corner of the longwall face, and this can make it unworkable at greater depths. After the interpanel barrier method was introduced at Mine A, it was adopted at several other Utah longwalls (Gilbride and Hardy ). In some cases, rather than leave a full barrier, these mines elected to make mid-panel moves around the area of deepest cover, thus providing a local interpanel barrier for the next panel (Maleki ). The interpanel barrier effectively protects the tailgate corner from the influence of previous panels, but at greater depths the single-panel stresses on the longwall face reach the same levels as were present with abutment loads and yield pillars.

This limitation led one major Utah operator to announce in 2008 that it would write off reserves at depths exceeding 900 m as unmineable (Foy ). 2.2 Colorado experience While coal bursts have occurred in several different coalfields in central and western Colorado, in recent years the problems have focused on operations in the North Fork Valley (NFV) of the Gunnison River. The NFV is an area of extremely mountainous topography where drift mines can encounter depths of cover that exceed 600 m.

Past mining also gives rise to multiple seam interactions in some areas. The geology of the NFV differs from Utah in that the immediate roof of the most common mining horizons is of weak to moderate strength. Usually composed of interbedded siltstone, fossiliferous shale, and thin layers of sandstone, Coal Mine Roof Rating (CMRR) values typically range between 40 and 60, with typical UCS values of 50–80 MPa (Maleki et al.; Stewart et al. The immediate floor usually contains a considerable thickness of coal.

Massive sandstone units, with strengths exceeding 100 MPa, are typically found about 15 m beneath the mineable seams (Maleki et al. Faulting and joint zones are present throughout the coalfield, and active tectonism continues to occur in the region today (Swanson et al. Case history Mine B began longwall mining in 2002 (Mark et al. In 2004, a series of three bursts occurred in the tailgate of the active longwall face as it was passing over underlying works at greater than 450 m of overburden. A fourth event occurred the following year in the adjacent panel tailgate in the absence of underlying workings. A fifth burst occurred 3 years later beneath some of the deepest overburden encountered up to that time (540 m). No multiple seam interaction was present in this instance.

The pillars in this district were developed on 33 by 60 m centers, and maintained Analysis of Longwall Pillar Stability (ALPS) stability factor (SF) of less than 0.6. Late in 2009, the mine began longwalling in a new district, with depths of cover that consistently exceeded 600 m. The pillars were significantly larger than any that had been used at the mine in the past, with three entries driven on 57 m centers, and they maintained an ALPS SF of 1.12 (bleeder loading) even at a depth of 680 m.

The first panel (panel D-1 in Fig. ) was extracted without serious incident, but, midway through the D-2 panel in 2011, a powerful burst that registered M = 3.1 caused extensive pillar failure and floor heave over a 3 ha area of tailgate pillars. Ventilation was also severely affected, and the panel was abandoned. Fig. 8 Map of Colorado Case History Mine B, showing the locations of the bursts discussed in the texts ( red stars) The burst in the D-1 tailgate was centered at least 150 m outby the tailgate corner, so it is unlikely that front abutment stresses were a significant contributing factor. The burst did occur within the linear projection of a densely jointed and slickensided joint zone that had been identified in the mains more than 1000 m away several years before.

It was one of a series of joint zones that were arrayed with roughly constant spacing across the mine reserve, exhibiting an approximate N 70°E trend. Another of these other structural features was associated with a burst that resulted in an injury during the development of the headgate for the D-3 panel. This feature was cause for concern because it crossed the tailgate of the D-3 panel at about mid-panel. When another large tailgate burst occurred during the mining of the D-3 panel, however, it was well in by this zone. While not as destructive as the burst that occurred on the D-2 panel, it destroyed 1.6 ha of tailgate pillars and again forced the abandonment of the panel. This event was recorded as M = 3.2.

A new gateroad was developed in order to leave an additional 75 m interpanel barrier to isolate the D-4 panel from the D-3 panel. Mining had advanced approximately 400 m in the D-4 panel, and the face was beneath almost 800 m of cover, when the largest burst yet, with M = 3.4, destroyed 4 ha of headgate pillars in by the face. Ventilation was again severely affected. Several weeks later, before the face could be recovered, a heating event developed in the gob. Ultimately the face was abandoned and the mine was sealed. Managing the risk of coal bursts begins with an evaluation of the factors that increase the likelihood of bursts. These include the depth of cover, the presence of past mining above or below, the roof and floor geology, and the presence of faults and other geologic factors.

A past history of bursts is one of the most powerful indicators of burst risk during any type of mining. Major bursts have often been preceded by smaller ones. Often these “precursors” have occurred at the same stage in the mining process as the subsequent large event (for example, in the same location on the longwall face). Also, once a mine has experienced bursts, later situations with similar geology and mining methods should also be considered high risk. Once zones at elevated risk of bursts are identified, the next step is to determine appropriate control techniques to employ within each one. According to risk management principles, the most effective way to reduce a risk is to eliminate it entirely (Iannacchione et al.

In the context of burst control, this would be achieved by not mining at all in the areas of greatest risk. Where the risk is not great enough to indicate complete avoidance, mining may be limited to development only. For example, in a mountainous area, the main entries might be developed beneath the ridgeline where the cover is deepest. Physical barriers can be used to protect miners from the full force of a burst event.

They can be helpful against small bursts, but are likely incapable of absorbing the energy from the largest events. Examples of physical barriers that have been used on longwalls include conveyor belting secured between the shields and the face conveyor, and metal plate burst protectors installed on shearing machines. Miners can also be provided with personal protective equipment (PPE), such as helmets, face shields, or body armor, though such devices can only protect miners from small events. The value of administrative controls, physical barriers, and PPE is also compromised if they are not correctly and consistently employed. Therefore, such techniques require worker training and constant management attention. Destressing techniques, including drilling, water infusion, hydrofracturing, and blasting, have occasionally been used to reduce the burst risk (Varley and Whyatt; Maleki et al.

While some of these techniques are used routinely in German mines (Baltz and Hucke ), their performance in the US has been mixed. The difficulties of identifying optimum distressing times and ability to assess the effectiveness of each destressing attempt, the limited time available for face destressing (to avoid production delays) and adverse drilling conditions reduced the overall success of the efforts. Hydrofracturing is perhaps the most promising technique for modern high-production longwalls (Hoelle ). Underground observations and monitoring are critical components of a burst risk management program. Mining crews should be trained to observe coal burst warning signs, particularly the occurrence of small bursts, which are often the best indication that an area is becoming more burst prone. A record-keeping system should be maintained, and management processes developed to ensure that warning signs receive appropriate responses. 4 Conclusions.

In the US, coal mines must report to MSHA any “coal or rock outburst that causes withdrawal of miners or which disrupts regular mining activity for more than 1 h” (Code of Federal Regulations, Title 30, Part 50.2). However, there is no special data field which identifies an accident report as a burst. The burst database employed in the present study was constructed by searching MSHA accident report narratives for terms such as “burst”, “bump” and “bounce”. Only those incidents where the narrative clearly indicated that a burst had occurred were retained in the database.