Christopher Mark,Michael Gauna

Mine Safety and Health Administration Technical Support,Pittsburgh,PA 15236,USA

Keywords:Longwall Ground control Coal burst Pillar design Yield pillar

ABSTRACT 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.

1.Introduction

Underground coal mining has been conducted in central Utah since the late 1800s.Since the early 1960s,longwall mining has been employed at depths of cover that significantly exceed those encountered by most other U.S.longwall operations.Deep cover causes high ground stress,which can combine with geology to create a coal burst hazard.The burst hazard has been most severe in mines operating within the Book Cliffs coalfield in Carbon County,Utah.Historical burst prone longwall mines in this coalfield include:Castle Gate No.3,Willow Creek,Aberdeen,Dugout Canyon,West Ridge,Sunnyside No.1,and Sunnyside No.3.

The Book Cliffs extend in a north to southeast arc around Price,Utah (Fig.1).The Book Cliffs coal seams are within the Blackhawk formation of the Mesaverde group.The Blackhawk formation is overlain by the Castle Gate sandstone and underlain by the Mancos shale.Sandstone dominates the stratigraphy associated with the coal seams of the Blackhawk formation (Fig.2),forming the burst prone geology.

Historically,pillar design has been a key component in the burst control strategies employed by mines in the Book Cliffs.After the earliest longwall experience at the Sunnyside mines in the 1960s,most longwall operations 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.The narrow single pillar between the two entries has minimal core strength where pillars yield rather than accept load when subjected to the longwall abutment loading.Consequently,it is designated as a yield pillar and has historically had an approximate rib-to-rib width of 7-9 m.The yielding nature reduces the potential for pillar bursts alongside miner’s work areas during longwall retreat.

After the 1996 burst fatality at the Aberdeen Mine,a second design evolved where each longwall panel has protection from the previously mined panel by a wide barrier pillar.Two-entry yield pillar systems flank both sides of the barrier pillar.This concept is termed the inter-panel barrier design[3,4].The objective of the design is to isolate the active longwall panel from the abutment loading from the previously extracted longwall panel.Protection from the side abutment loading reduces the potential for coal bursts in the miners’ work areas,particularly near the tailgate corner.

Fig.1.Location of the mines in the Book Cliffs and Wasatch Plateau regions of Utah [1].

Fig.2.Geologic cross-section of the mineable coal seams of the Book Cliffs coalfield,showing the approximate locations of the longwall mining operations [2].

This publication compares the burst history of the double-use,two-entry yield pillar gates to the inter-panel barrier design.It evaluated the amount of deep cover longwall mining that was conducted with the two types of longwall gate design,together with the associated coal burst experience.Three primary sources were used:published literature;Mine Safety and Health Administration(MSHA) technical support field investigations;and a Coal Burst Database created using the MSHA part 50 accident/injury records going back to 1983.

2.Sunnyside mines

The Sunnyside property conducted underground mining east and north of Sunnyside,Utah(Fig.1),beginning in 1896.A century ago,the mine was already very familiar with bursts during pillar recovery [5].Continuing burst problems led Sunnyside in 1961 to become one of the first US mines to install a longwall.Sunnyside also pioneered the two-entry yield pillar design which became the Utah standard [6].

The Sunnyside seam consisted of the upper and lower Sunnyside splits,which when combined could create a single seam that was up to 7 m thick.In other places on the property,as much as 22 m of siltstone separated the seam splits.Sometimes mining was only in the upper Sunnyside seam,sometimes in just the lower Sunnyside seam,and other times in the combined seam,and still other times in both seams in a variety of multiple seam configurations.Single seam mining heights were generally in the 1.5-2.5 m range [2,6].

The immediate roof above the Sunnyside coal seam was also highly variable and could change substantially over a few hundred lateral m(Fig.3).It consisted variously of a hard,dark brown mudstone,lenses of sandy siltstone,interbedded siltstones and sandstones,and/or irregular lenses of fine-grained sandstones.Historically the roof could be so weak that roof falls were a serious concern.Faults were also associated with both roof instability and coal bursts[7].The thick Castlegate sandstone was present approximately 45 m above the coal.The main floor was the 30-45 m thick Sunnyside sandstone.The immediate floor was commonly a hard mudstone.

Koehler documented the entire longwall mining history at Sunnyside,based on interviews with 30 long-time mining veterans.His report focused on the performance of the yield pillar designs employed at the Sunnyside No.1 and No.3 Mines,and it provides considerable information on the burst history as well.The information is particularly valuable because much of the history that occurred prior to 1983 would have been lost otherwise.About 35 longwall panels were extracted at the Sunnyside mines before they were closed in 1994 [6].

Fig.3.Generalized stratigraphy for the Sunnyside Mine area [6].

Koehler found that bursts were rare during longwall mining when the depths were less than 450 m,unless a multiple seam interaction was involved.About half of the Sunnyside complex longwall panels were,at least partially,mined at depths that exceeded 450 m,and essentially all of them employed doubleuse two-entry gates [6].A summary of the longwall experience with the double-use gates at deep cover in the No.1 Mine follows below:

(1) The 13th left through 18th left panels(Koehler Area 6)were mined primarily with two-entry gates with 7.5 m-wide yield pillars.Mining involved approximately 2800 m of panel mined at 450-600 m of cover,with 480 m mined at more than 600 m of cover.No significant coal bursts were encountered [6].

(2) The 19th left panel (Koehler Area 6),immediately adjacent to 18th left described above,was extracted with a 30 m barrier pillar flanked by two-entry yield pillar systems as the tailgate pillar system.Conditions were considered ‘‘excellent”but the depth of cover never exceeded 480 m [6].

(3) The 10th left panel(Koehler Area 7)employed a 45 m barrier flanked by 7.5 m yield pillars (Fig.4).Koehler reported that‘‘severe bumps along the face and in the tailgate along the barrier pillar began soon after longwall mining was initiated.Several of the bumps were apparently recorded as earthquake-scale events at the University of Utah seismograph center in Salt Lake City.Many lost-time accidents from bumps or bump-related roof falls were incurred”.Following a devastating burst in the tailgate near the face corner,the panel was abandoned.The depth of cover was fairly uniform along the tail side of the panel ranging from 570 to 600 m,and a fault system was noted approximately 600 m outby the final longwall face location [6].

(4) The 2nd and 3rd panels off 15th right (Koehler Area 8)involved about 1000 m of double use two-entry gates at depths greater than 450 m,with about 25%of that at depths greater than 600 m.No problems with bursts were reported[6].

(5) The last longwall panels mined at Sunnyside were the 19th through 22nd left panels(Koehler Area 11)in the No.1 Mine.A total of about 1800 m was mined at greater than 450 m of depth along the double-use gates at the time of the Koehler report,with about 10% of that distance at depths greater than 600 m.Koehler reported that mining ‘‘proceeded relatively smoothly in regard to bump activity.Bumps have reportedly been limited to the longwall face area within approximately 30 m of the tailgate corner,which is typical of the quietest panels at Sunnyside”.Nevertheless,between 1986 and 1992 a total of 11 bursts were reported to MSHA,almost evenly divided between events that caused tailgate blockages and events that resulted in lost-time injuries [6].

In the No.3 Mine deep cover experience with double-use gates was limited to five panels.The 17th and 18th right panels(Koehler Area 10) encountered significant burst activity (Fig.5).Approximately 1200 m of these panels had more than 450 m of cover,and roughly half of that length was with more than 600 m of cover.Koehler reported that ‘‘rather than tailgate bumps,17th right was plagued with face bumps,and one miner reported that he felt the worst bumps in the area were encountered during the mining of this panel.As mining progressed to the 18th right panel,losttime injuries from the bump activity were common,and many of those interviewed concurred that this was one of the toughest single-seam blocks to be extracted in the No.3 Mine”.These panels were situated to the northeast of the ‘‘Water Canyon slopes”that are also shown on Fig.5.‘‘Hard bumping”caused the abandonment of the development of these slopes at a depth of about 670 m.The bursts encountered when these slopes were being driven were reportedly so severe that ‘‘heavy machinery was often displaced or overturned”[6].

About 1000 m of the 15th and 17th right extension panels(Koehler Area 9) had more than 450 m of cover with few reported problems (Fig.5) [6].Between these two panels,the 16th right extension panel was mined,separated from the other two panels by a tapered barrier pillar flanked by two-entry pillar systems on each side.The barrier pillar increased in width from roughly 30 to about 45 m as the cover deepened to about 670 m in the retreat direction towards the main.Also,the tail-side of the 16th right extension two-entry pillar increased in width from roughly 9-22 m.With the retreat towards the mains,reportedly ‘‘coal bump frequency and severity increased with increasing pillar size”.

In summary,a total of about 2000 m of longwall panel involved extraction adjacent to double-use gates under cover that exceeded 600 m at the Sunnyside operations.In at least three panels,severe bursts occurred.On the other hand,approximately 8000 m of longwall panel extraction had depths between 450 and 600 m with few significant burst problems.

None of the Sunnyside panels employed inter-panel barrier pillars,though some of the strongest events occurred when a 45 m wide barrier pillar separated the active panel from an adjacent longwall district.The 45 m wide barrier was much less than the barrier widths used in the inter-panel barrier designs discussed in later sections of this report,but it was similar to the pillar sizes employed in the abutment-yield pillar designs used in the eastern US [8,9].

3.West Ridge Mine

Fig.4.Sunnyside Mine No.1,10th left longwall panel (Area 7) [6].

Fig.5.The 15th through 18th right panels at the Sunnyside No.3 (Area 9 and Area 10) and Area 9 in the upper right as the extension panels [6].

The West Ridge Mine operated in the lower Sunnyside seam in the Book Cliffs coalfield approximately five miles northwest of Sunnyside,UT.Longwall retreat mining began in May 2001 in Panel #1 and continued northeast,to Panel #7 on the southeast side of the northeast trending North Main.Panel extraction relocated to the northwest of the North Main,towards the portal,with Panel#8 in October 2006.Subsequent panels progressed down dip to the northeast.The topography also increases to the northeast allowing the depth of cover to significantly increase (Fig.6).The minimum depth with Panels #1 and #8,towards the portal,was roughly 300 m.The panels situated farthest to the northeast are Panel#23 and Panel#21.After Panel#17,the longwall panel numbers do not represent the extraction sequence.Panel #23 had depths of cover ranging from 950 to 1020 m.Panel#21 had depths of cover ranging from 800 to 930 m.The deep cover longwall mining temporarily ceased with the completion of Panel#18 in February 2013.

Then,panel extraction continued with nine longwall panels completed near the outcrop,southwest of the original longwall Panels #1 and #8 at relatively shallow depths.Near the end of the sequence of these shallow panels,the last deep cover Panel#19 completed extraction in mid-2015.Afterwards,mining relocated to the last shallow depth panel,Panel#34,where mining terminated in November 2015 (Fig.6).

Gate entry design has been two-entry yield pillar using 15 m center width and 36 m center length gate pillars.Development mining was roughly 2.3-2.6 m height and 5.3-6 m width.Panel center-to-center widths have ranged from 200 to 270 m with all mining in a single seam environment.Longwall mining with two-entry,double-use gates having side-by-side longwall panels existed from May 2001 to March 2009.As a consequence of coal bursts in the tailgate area of the longwall face,mining in Panel 13 terminated and West Ridge converted to an inter-panel barrier design [10].

Panels #1 to#13 represented the double-use gate system.Portions of these panels had no longwall retreat because:the early termination of Panel #4 and the barrier to Panel # 5 resulted from a heating event and a need to seal and re-establish the longwall district;the mid-panel barrier in Panels #5 was to move around a fault which had too much displacement to longwall through;and the Panel #11 significant deviation in start position reportedly resulted from the need to avoid a seam split to the northwest.

Fig.6.West Ridge Mine longwall mining,with depth of cover and burst locations shown with red labeling.

Fig.6 shows the burst events obtained from the MSHA technical support archives.Other events may exist that are not in those records.Fig.6 suggests that the tailgate corner burst events have a tendency to occur with only one previously extracted longwall panel.While tailgate corner events occurred with depths of cover as shallow as 400 m,analysis shows that they become much more likely under deep cover.Of the 14 significant coal bursts recorded at West Ridge,eight (57%) occurred at depths greater than 600 m.However,that deeper mining only represents 26%of the two-entry double-use design.

In contrast,the inter-panel barrier design was used for 11300 m of total panel extraction,with no reported burst events.Depths of cover exceeding 600 m accounted for 96%of that mining,with 13%at depths greater than 900 m.

4.Dugout Canyon Mine

Dugout Canyon is an underground mining complex,northeast of Price Utah (Fig.1) that began production in 1998,initially as a room-and-pillar operation and then converted to a longwall operation in 2001.In 2012,Dugout Canyon completed mining longwall panels and now operates as a room-and-pillar operation with the option to resume longwall mining in the future in an adjacent reserve block (Wolverine 2018).

Longwall mining at the Dugout Canyon initially utilized two and three-entry gate systems with 8.7 m wide yield pillars and 220 m wide panels.The initial four panels extracted the Rock Canyon Seam prior to January 2004,under gradually increasing depths of cover (Fig.7).During the mining of the Rock Canyon 3rd West panel,two reported bursts damaged the shearer,and then a third caused a serious injury.The panel,which was then under 480-540 m depth of cover,stopped early and relocated to the next panel.On the next panel,Rock Canyon 4th West-A,two bursts that damaged the shearer resulted in a mid-panel longwall move at 430 m depth of cover.When mining was resumed on the other side of the in-panel barrier,one observer noted that ‘‘Conditions improved significantly,providing the first evidence of barrier pillar effectiveness in this seam”[4].The mining in the Rock Canyon Seam after the move was continued with the inter-panel barrier design for 870 m of the total 1180 m in length.The recorded burst events occurred in the Rock Canyon Seam at depths less than 540 m.

Fig.7.Dugout Canyon longwall panels in the Rock Canyon Seam.

In 2004 longwall operations relocated into the Gilson Seam(Fig.8).At shallower depths,two-entry double-use gates with 8.7 m×30 m yield pillars were employed.Inter-panel barriers were employed to control seismicity,considered indicative of excessive abutment loading,at depths exceeding 450 m.The panel barrier pillars measured 150 m (GIL-1 to GIL-2,maximum depth 600 m),115 m (GIL-5 to GIL-6,maximum depth 600 m),130 m(GIL-6 to GIL-7,maximum depth 720 m) and 180 m (GIL-7 to GIL-8,maximum depth 780 m).No bursts damaged the longwall mining equipment during the mining of the first seven Gilson Seam panels,though a small burst in September 2007 at roughly 660 m depth caused an injury to a shearer operator.

The GIL-8 longwall panel started with an 840 m mining depth in April 2009 and in early May 2009,had progressed under an overlying sandstone channel system.A reportable face burst occurred near the headgate at a mining depth of 2400 m in late June that damaged the shearing machine.A second serious longwall face burst occurred in early August as the shearing machine approached the tailgate entry beneath approximately 600 m of cover.The shearer trapping shoe was broken and the machine was forced off the face conveyor into the face conveyor hosecable tray.Both events were associated with an overlying sandstone channel system.

In July of 2008 the then owner of the Dugout Canyon Mine,Arch Coal,announced that due to the burst risk no mining would be conducted at depths in excess of 3000 m [11].As a consequence,the GIL-8 panel was the last deep panel with retreat ending in January 2010.

In total,the inter-panel barrier design was used for 43,350 m of panel extraction where depth of the cover predominately exceeded 450 m.For 30%of that mining the depth of cover exceeded 600 m.The three face burst events occurred at depths of cover ranging from 600 to 2400 m.

After the GIL-8 panel,longwall mining relocated nearer to the outcrop where the GIL-10 through GIL-12 panels were mined with inter-panel barriers.The depth of cover was mostly less than 450 m,with a maximum of approximately 510 m.Inter-panel barriers of 95 and 105 m were used.The last longwall panel at Dugout Canyon,GIL-4A,was completed in December 2012.No burst activity was recorded for mining after the GIL-8 panel.Since 2012 only room-and-pillar mining has been conducted,all in reserves at lower depths of cover.

5.Aberdeen Mine

Aberdeen Mine initiated the inter-panel barrier design concept.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°,so each successive panel was about 40 m deeper than the previous one.Some mining existed in a coal seam lying approximately 75 m above the Aberdeen Mine,but there were no noticeable stress transfers.

Longwall mining began in 1995.Longwall retreat involved panels 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 ranged from 420 to 480 m (Fig.9).

As the second panel retreated,bursts became increasingly common on the tailgate end of the panel.Five of these events resulted in broken shearer torque shafts,though none were initially reported to MSHA.Then a major coal burst occurred when the panel had retreated about 240 m.The shearer had just begun the double-cut at the tailgate that initiates 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 [12].A month later a second powerful burst caused major damage to the shearer.

Fig.8.Dugout Canyon longwall panels in the Gilson Seam.

Fig.9.Aberdeen Mine longwall mining with depth of cover and the sites of the two fatal coal bursts.

These incidents led to the abandonment of the remainder of the second panel in January 1997.Fig.9 shows that the inby portion of the third panel was also abandoned,so that the unmined remainder of the second panel could serve as an inter-panel barrier pillar protecting the third panel tailgate from abutment loads arising from the first panel.This was the inception of the inter-panel barrier design.Subsequent panels were also developed with the design.Panels to the north (down dip) of the third panel had 140-180 m barriers between adjoining panels.Each panel was developed with two independent sets of two-entry yield pillar gates driven on either side of the panel coal block.The mining of the third through eighth longwall panels had no reported longwall face bursts.

Almost 10 years after the first fatal burst,the ninth longwall panel was being retreated with depth of cover ranging from 780 to 840 m[13].Bursts were common along the longwall face,ranging from thumps in the roof or floor to coal 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 on top of the shearer frame.Deflector shields are plates hinged off the top of the shearer frame,lowered or raised to accommodate mining clearance.In addition,the armored face conveyor had expanded metal guards periodically attached to the conveyor walkway.The conveyor walkway also had sheets of conveyor belting suspended from the bottom of shield canopies with the belt bottom attached to the armored face conveyor framework.These procedures and fixtures have since been commonly used throughout Utah for deep cover longwall mining.

Despite these precautions,a fatal burst in January 2006 killed a shearer operator located approximately 15 m from the headgate corner as the shearer was making the double-cut procedure towards the headgate (Fig.9).The burst extended approximately 15 m along the face,with the largest cavity about 1 m deep directly in front of the victim.The depth of cover at the accident site was approximately 825 m [13].

During 2 years following the second fatal burst,12 additional bursts were reported to MSHA in the vicinity of the longwall.Four of these events damaged the shearer or injured the shearer operator while cutting near the gate ends.Four non-injury events affected the stageloader area.Of the other events that occurred away from the face (in the headgate belt line,headgate travel way,and in the tailgate entry),two resulted in injuries.With all its remaining reserves at even greater depths,the mine ceased operation in March 2008 with Panel 10 only partially mined.

In summary,seven burst events were encountered in only 360 m of a three entry,yield pillar,double-use gate design.The inter-panel barrier design was then used for 11200 m of total panel extraction with depths of cover greater than 600 m representing of 79% of that mining.The inter-panel barrier was quite successful until depths of cover exceeded 830 m.

6.Willow Creek Mine

The Willow Creek Mine opened in 1996 with longwall mining beginning in July 1998.Willow Creek mined the Castle Gate ‘‘D”seam,when combined with the Kenilworth seam in the longwall area had a total thickness averaging 9 m.Mining heights ranged from 2.2 to 3.6 m,with the remainder of the coal left in the floor.The depth of cover for the longwall panels ranged from 600 to 840 m [14,15].

The geology immediately above and below the seam consisted of thinner(3 m)layers of siltstones,mudstones,shales,sandstones,and coal.Above the coal bearing portion of the Blackhawk Formation,approximately 150-165 m of braided stream deposits with numerous lenticular channel sandstones occurred.These lenticular deposits make up the immediate and main roof over the mine[14].

Willow Creek while in operation mined three longwall panels(Fig.10).The D-1 panel had retreated approximately 600 m when an explosion and fire occurred in late November 1998.All miners evacuated safely,and the longwall subsequently relocated to the D-2 panel [16].Since the D-1 panel gate could not be re-used as the D-2 panel tailgate,a new tailgate was developed leaving an 80 m wide barrier pillar.In late November 1999,longwall mining resumed on the D-2 panel.Once past the D-1 extraction,the D-2 face extended to its full 250 m width,and the remainder of the panel was successfully retreated until its planned stop at a seam split.

Longwall mining in the D-3 panel resumed in July 2000 [16].The double-use gate,D-3 panel tailgate side had a single row of yield pillars developed 7.9 m wide.The projected D-3 longwall panel length was approximately 1400 m,but after mining had progressed just 75 m a series of three explosions resulted in two fatalities and the mine was abandoned.

The Willow Creek Mine reported six coal bursts to MSHA during development and longwall mining.The record shows two events from the longwall face,one event along the longwall belt entry,and three events during section development.

The D-1 panel explosion and fire converted the D-1 and D-2 panels into an inter-panel barrier design that we included with the inter-panel barrier design statistics.These two panels extracted a total of 1800 m with depths in excess of 180 m representing 89%of that mining.

Fig.11.Longwall panels at the Castle Gate No.3 Mine [2].

7.Castle Gate No.3 Mine

The Castle Gate No.3 Mine is the westernmost of the mines located in the Book Cliffs,and it suffered from perhaps the most severe burst issues.Mining in the Sub-3 Seam began in 1896.As mining progressed under deeper cover,coal bursts occurred with increasing frequency.By the 1970s even development mining was considered hazardous.The first longwall was introduced in 1975 with a three-entry yield pillar design,but it was almost immediately subjected to violent coal bursts,and the fourth panel was abandoned (Fig.11).After leaving a panel barrier,a second attempt at longwall mining was made during the late 1980s with the fifth and sixth panels.Despite de-stressing efforts with water infusion,the mine was finally idled in 1989 [2].

The immediate roof above the Sub-3 seam at the Castle Gate Mine was highly variable.The most common rock was thinly laminated siltstone.Locally sandstone channels,9-60 m thick,were located directly on the coal.The main roof was 12-30 m above the seam,and consisted of the extremely strong and massive Aberdeen Sandstone,24-45 m thick.The 48 m thick Spring Canyon sandstone formed the immediate floor.

A variety of pillar designs were employed in the tailgates at the Castle Gate No.3 Mine,including two-and three-entry yield pillars,three-entry ‘‘large”pillars,and the inter-panel barrier (Fig.11).Only two longwall panels were successfully completed,the 1st Panel,and the 5th Panel which was adjacent to the 135 m wide inter-panel barrier.In every other longwall panel the tailgate was located adjacent to an extracted panel,and bursts were common in the tailgate pillars and on the longwall face near the tailgate corner.These conditions persisted despite the fact that the depth of cover at the Castle Gate No.3 Mine never exceeded 480 m above any of the double-use gates.

MSHA accident reports are only available from 1983 onwards,and therefore only reflect the last two panels at Castle Gate No.3.No bursts were reported on the second-to-last panel,the 5th Panel,which was adjacent to the inter-panel barrier pillar.According to Barron et al.,the generally favorable ground conditions encountered in this panel are probably attributable to the massive 7th east panel barrier pillar protecting the tailgate pillars and working face from excessive loading [2].The depth of cover above the panel barrier was also only 240-330 m,however.In contrast,a total of 10 coal bursts were reported to MSHA during the mining of the final 6th panel.Seven events occurred on the longwall face near the tailgate,five of which resulted in injuries and two in significant equipment damage.The other three resulted in tailgate entryblockages.The two-entry tailgate pillars in this last panel were mined on 26 m by 36 m center distances,at 360-480 m of cover.

Table 1.ouble-use longwall gates in Book Cliffs,depths exceeding 450 m.

Table 2.nter-panel barrier longwall panels in Book Cliffs,depths exceeding 450 m.

The mining where double-use gates existed accounts for 2200 m of panel extraction,with 80% of the mining at depths between 300 and 450 m,and 6% at depths over 450 m.The 5th Panel with an inter-panel barrier design had 1200 m of extraction,with 58%of the mining at depths of 300-450 m and 7%of the mining at depths over 450 m.

8.Summary

Table 1 summarizes the Book Cliffs mines’ experience with the double-use gate design,and Table 2 summarizes the experience with the inter-panel barrier design.The tables show that at depths exceeding 450 m,each design was employed for about 37500 m of longwall panel extraction.However,the depths of cover for the double-use gate design were significantly lower than those for the inter-panel barrier design.Only 16%of double-use gate mining had depths over 600 m,with only 5%at over 750 m.In contrast,the 65% of the mining with the inter-panel barrier design had depths over 600 m,with 35% at depths over 750 m.

Despite its shallower depths of use,the double-use design has been associated with at least three times as much face region burst activity as the inter-panel barrier design.The evidence strongly suggests that the inter-panel barrier design has been far more successful than the double-use yield pillar design for controlling the burst risk at depths exceeding 600 m.