新庄矿0.6Mt每年新井设计

目录一般部分1矿井概况与地质特征 页英文原文《IntroductionofLongwallMining》ExecutiveSummaryLongwallminingisoneoftheprincipalundergroundminingmethodsintheUnitedStates.Itsimportanceasacoalproductiontechniquehasgrownsteadilysincetheintroductionofmodernlongwalltechnologyintothiscountryinthe1950'sand1960's.Inthepastdecadelongwallproductionandproductivitygrewrapidly,asaresultofsignificantimprovementsinlongwallequipmentandoperatingpractices.By1993,longwallminingaccountedfor40percentoftheNation'sundergroundcoalproduction—upfrom27percentin1983.Laborproductivityatlongwallminesmorethandoubledbetween1983and1993.Productivityisnowhigherforlongwallminingthanforotherundergroundproductionmethods,andproductivityisexpectedtokeepgrowingasnewtechnologicaladvancesareintroduced.Longwallminingisoneoftwobasicmethodsofundergroundcoalmining.Theothermethodisroom-and-pillarmining,historicallythetraditionalmethodusedintheUnitedStates.Inroom-and-pillarmining,“rooms”areexcavated,andpillarsofcoalareleftinplacebetweentheroomstosupportthemineroof.Incontrast,longwallmininginvolvestheessentiallycompleteextractionofthecoalcontainedinalargerectangularblockor“panel”ofcoal,andtheroofinthemined-outareaisallowedtocollapse.Thesequenceofoperationsinlongwallminingisbasi-callysimple.Therectangularlongwallpanel,averagingnearly800feetwide,7,000feetlong,and7feethigh,is“blockedout”byexcavatingpassagewaysarounditsperimeter.Room-and-pillarminingisusedtoblockoutthepanel.Excavationofthecoalinthepanelisanalmostcontinuousoperation.Workingunderthesteelcanopiesofhydraulic,movableroofsupports,acoalcuttingmachinerunsbackandforthalongthe800-footface,takingacutranginganywherefromafewinchesto3-1/2feetdeepduringeachpass.Thecutcoalspillsintoanarmoredchainconveyorrunningalongtheentirelengthoftheface.Thisfaceconveyordumpsthecoalontobeltconveyorsfortransportoutofthemine.Asthecuttingmachinepasseseachroofsupport,thesupportismovedclosertothenewlycutfacetopropuptheexposedroof.Theroofisallowedtocollapsebehindthesupportsastheyareadvancedtowardstheface.Miningcontinuesinthismanneruntiltheentirepanelofcoalisremoved.Becauselongwallminingisessentiallyacontinuous,highlymechanizedoperation,longwallproductivityispotentiallyhigherthanroom-and-pillarproductivity.Longwallminingalsooffersimprovedsafetythroughbetterroofcontrol,morepredictablesurfacesubsidence,andbetteropportunityforfullautomation.Ontheotherhand,capitalcostsforlongwallequipmentaremuchhigherthanforroom-and-pillarequipment,productivityduringdevelopment(“blockingout”)ofthelongwallpanelsistypicallylow,andlargeamountsofdustandmethanearegeneratedduringtheminingprocess.IntroductionLongwallminingisoneoftheprincipalundergroundminingmethodsintheUnitedStates.In1993,longwallminesaccountedfor40percentoftheNation'sunder-groundcoaloutput—comparedwith27percentin1983.Althoughbasiclongwallminingtechniqueswerede-velopedinEnglandinthe17thcentury,therewaslittleinterestinlongwallminingintheUnitedStatesuntilthe1950's,whennewGermantechnologywasintroduced.AsthetechnologywasdevelopedfurtherintheUnitedStates,longwallproductiongrewsteadily.By1993,85longwallunitswereoperatingin12States.Laborproductivityatlongwallminesmorethandoubledbetween1983and1993.Productivityisnowhigherforlongwallminingthanforotherundergroundproductionmethods,andproductivityisexpectedtokeepgrowingasnewtechnologicalimprovementsareintroduced.Thepurposesofthisreportaretodescribethelongwallminingprocess,analyzethemostimportantchangesinlongwallminingoverthepastdecade,anddiscussfactorsthatwillshapethefutureoflongwallmining.TheHistoryofLongwallMiningLongwallminingisnotanewapproachtocoalmining.Infact,thebasicprinciplesoflongwallmininghavebeentracedbacktothelatterpartofthe17thcenturytoShropshireandothercountiesinEngland,whereitwasdescribedasa“totallydifferentmethodofmining”calledthe“Shropshiremethod.”Manymodificationsintheoriginalmethodshaveoccurred,butalllongwallmininghasinvolvedextractingcoalfromalongwallorface.Theareafromwhichthecoalwasextracted,the“gob”(fromaCelticwordforcaveorhollow),waspartlyorwhollyfilledwithstoneandrefuse,uponwhichtheoverlyingstratasettled.Untiltheearly1900's,coalmininginEnglandwasmostlybythe“bordandpillar”method(equivalentto“room-and-pillar”).The“bords,”orpassages,wereareas12to20feetwidefromwhichthecoalwasextracted;thepillarsweremadeofcoal,some50feetwideandasmanyas100feetlong,thatwasleftunminedtosupporttheoverlyingstrata.EffortstoextractsomeorallofthecoalleftinthepillarsatalaterstageofminingeitherwerenotattemptedorwerenotalwayssuccessfulAsthedemandforcoalincreased,bordandpillarminingsoonwasregardedaswasteful,andtheadvantagesofthelongwalltechniquewerenoted:“Itenablesacollierytobeopenedwithlesscapitalexpenditures...andtobecomeremunerativeinthesmallestpossibletime...Theyieldperacreisgreater...Ventilationiseasier,theworkmenareconcentrated,andtheexpenseofsupervisionisreduced...inseamsgivingofflargequantitiesofexplosivegas...shotfiringcanalmostentirelybedispensedwith[becausethe]weightonthefaceisinitselfsufficienttobringdownthecoal”Theoveralllayoutofearlylongwallmineswasgenerallycircular,withminingradiatingoutfromacentralshaf.Themainroadwaysrandiagonallyfromtheshaftpillarlikethespokesofawheel,whiletheinterveningareasweresubdividedintosmallerandsmallersectorsbysubsidiaryroadways.Theroadwaywerekeptopenby“packwalls”ofwasterockconstructedoneithersideofthem.Theroofintheworkingarea,orface,wassupportedbyalineoftimbers,whichweremovedforwardasminingadvanced,andby“packs”or“cribs”ofwasterock,whiletheroofintheminedoutareawasallowedtocollapse.LongwallminingwaspracticedonaverysmallscaleintheUnitedStatesinthelate1800'sandearly1900's.Thepioneeringlongwallattemptsweregenerallyinthincoalbedsthatcouldnotbeminedeffectivelybyroom-and-pillartechniques,andthatrequiredaminimumofpackwallconstructionandbackfillingforroofsupport.Wheresuccessful,thoseearlylongwalloperationsresultedincompleteremovalofthecoalatminimalexpense,withlesstimbering,morecontrolledsubsidence,andbetterventilationintheworkingareathanroom-and-pillarmethods.Untilundercuttingmachinesbecameavailableintheearly1900's,longwallminersundercutthecoalbyhandwithpicks.Theearlyworkingfacesgenerallywereintheformofanarcabout40feetacross,butasminesbecamedeeperandmechanized,straightfaceswerefoundtobemoreefficient.Theundercutcoalbedwastemporarilysupportedbyshortwoodenpropsor“sprags”setevery4to6feet.Whenthepropswereknockedout,theundercutcoalfellbecauseofitsownweightandroofpressure,butifnecessary,itwasknockeddown;explosiveswereseldomused.Thebrokencoalwasthenloadedbyhandintocars(tubs)fortransportoutofthemine.By1924,productivitywasimprovedwhenconveyorswereinstalledalongthelongwallfaceinsomemines.Overall,theexperienceofearlylongwallminingintheUnitedStatesshowedthatitwasnotcompetitivewiththeroom-and-pillarmethod.Althoughsteeljacksreplacedwoodenpropsforroofcontrolaround1912,andmineswerebecomingmechanized,thelargenumberofworkersrequiredtomovethejacksandtoconstructothertypesofroofsupportsmadelongwallminingalabor-intensiveeffort.Withlongwallproductivityaveragingonlyabout3tonsorlessperworkerpershift,U.S.undergroundminingtechnologyfocusedonimprovingroom-and-pillarmining,abettermethodforextractingcoalfromrelativelythickbeds.Incontrast,longwallminingremainedpredominantinEurope,whereconditionsweremoresuitableforthetechniquebecausethecoalbedsweredeeperandoverlainbythinlylayeredstratathatcavedmoreeasilythanthoseinthetypicalU.S.coalmine.1950-1960AfterWorldWarII,U.S.interestinlongwallminingwasrenewedbythepossibilitiesofusingtheGermandevelopedplow(orplaner)and“panzer,”orarmoredfaceconveyor.Theplowispulledacrossthecoalfacewhileridingonabasethatslidesundertheconveyor.Itshavesoff2to4inchesofcoalthatspillsontotheconveyor.In1952,EasternGasandFuelAssociates,withsupportfromtheU.S.BureauofMines,testedlongwallminingwithaplowandfaceconveyorattheStatesburymine,nearBeckley,WestVirginia,tolearnifthistypeofequipmentcouldbeusedtoextractsomeU.S.coal.RoofsupportwasprovidedbymechanicalpropswithI-beamcapsandwoodcribs.Thetestwassuccessfulandtheequipmentwasusedinthreeotherlongwalloperationsbetween1952and1958.Duringthe1950-1960period,therewereanaverageofsixlongwalloperationsperyear,mostlyinWestVirginiaandPennsylvania,butalsoinArkansas.Theplowwasheprincipalcoal-cuttingmachine,replacingtheearlylabor-intensivemechanicalundercuttingmethod.However,aboutthree-fourthsofthelongwalloperationswerenotsuccessfulbecausethecoalbedswerenotfriableenoughforextractionwithaplow,orbecauseroofcontrolpresentedproblems.Althoughthehydraulicpropsintroducedinthelate1950'swereanimprovementovertheearliermechanicalfrictionprops,alargeamountofmanuallaborwasstillneededtorecoverandresettheprops.Moreover,asthefaceadvanced,woodcribshadtobeconstructedforadditionalroofsupport,requiringadditionalmanuallabor.Asaconsequence,by1960longwallminingwasgenerallyconsideredalastresort,usedonlyforextractingthinbedsofpremiumcoalwhenroom-and-pillarmethodsfailed.OtherfactorsalsoconstrainedtheuseoflongwallminingintheUnitedStates.TheseincludedthelackoffamiliaritywiththemethodonthepartoftheU.S.coalindustry,andthehighcapitalinvestmentrequiredfortheequipment.Furthermore,bythistime,continuousminingmachineswereimprovingtheefficiencyofroom-and-pillarmining.1960-1970InterestinlongwallminingintheUnitedStatesrevivedinthe1960's,andthenumberofinstallationsrosetoabout20before1970,duemainlytotheintroductionofself-advancinghydraulicroofsupports.Thesepoweredsupportsreplacedjacksandwoodcribs,eliminatingtheneedforsubstantiallabor.Theyalsohadtheadvantageofbeingabletopushtheconveyorforwardautomaticallyasthefaceadvanced.Self-advancinghydraulicroofsupportswerefirstused,togetherwithaplow,in1960toexcavatea52-inchcoalbedinEasternAssociates'KeystoneminenearWelch,WestVirginia.Thefirstself-advancingroofsupportswereframes.Aframeconsistedoftwosinglehydraulicjacksconnectedtoabeam,andtwoframeswerelinkedtogethertooperateasapair.Theyadvancedintwosteps.Whileoneframeremainedsetbetweentheroofandfloor,theotherwasloweredandthenpushedforwardbyaram;theprocedurewasreversedtomovetheotherframe.Frameswithatwo-legdesigncouldsupportasmuchas88tonsbeforeyielding;thosewithfour-legdesignswereabouttwiceasstrong.Framessuccessfullysupportedtheroofwhentheoverlyingstratacavedeasily,buttheywereofteninadequateifthestrata“hungup.”AnumberoflongwallinstallationsintheIllinoisBasinwerediscontinuedbecauseframescouldnotcontrolthemineroof.Inthemid1960's,betterdesigned,high-capacity,self-advancingroofsupports,capableofholdingabout700tons,becameavailableintheformofthechock.Describedasamobilecrib,thechockconsistsoftwoframesupportstiedtogetherwitharigidcanopyandsemi-rigidbase.Morestablethanframesupports,thechockisalsosaferbecauseithasacanopythatprovidesprotectionagainstmaterialfallingfromthemineroof.Thechockcanalsobeadvancedasasingleunitbyahydraulicramattachedtothefaceconveyor.Althoughthechockrepresentsagreatimprovementinroofcontroltechnology,itcanbecomeunstablewhentheroofcavesinlargepiecesandcreatesrotationalorhorizontalstresses.Theinstabilitycanoccurbecausethechock'scanopyisconnectedtoitsbaseonlybythehydrauliclegcylinders.SeverallongwalloperationsinsouthernIllinoiswereabandonedbecausechocksfailedastheresultofseriousroofcontrolproblems.The1960'salsosawtheintroductionoftheshearingmachineintheUnitedStates,firstatKaiserSteelCorporation'sSunnysidemineinUtahin1961,andlaterinminesintheEast.Theshearingmachineisanelectricallypoweredrotatingdrumthatnotonlyexcavateshardercoal,butalsocutsawiderstrip(24to28inches)fromthecoalbedthantheplow.However,theearlyshearerswerenotfreeofproblems.Ashearer'sperformancecouldbereducedifthesupportswerenotadvanceduniformly,resultinginpooralignmentoftheshearerwiththecoalface.Furthermore,theshearer'sheavierweightrequiredtheuseofstrongerarmoredfaceconveyorstosupportit.Shearersalsoproducedfinersizedcoalthanplows,andthistendedtojamfaceconveyors,reducingproductiveminingtime.Healthproblemsbecameaconcernbecausethesheareralsogeneratedmorerespirabledust.Nevertheless,by1966,afterimprovementsweremade,shearersproduced42percentofthecoalatU.S.longwalloperations.By1970,shearersoutnumberedplows,andthefirstdoubledrumrangingshearerwasinserviceinthenorthernAppalachians.1970to1980Inthisperiod,thelastmajorimpedimenttotheacceptanceoflongwallminingintheUnitedStateswasovercomethroughtheintroductionofshieldsupports,amajorstepintheevolutionofroofcontrol.AlthoughnewtotheU.S.coalminingscene,shieldshadbeenusedsuccessfullysincethe1960'sintheSovietUnionandtherEasternEuropeancountries.Safetyandproductivityfactorsfavoredtheshieldoverthechock.Theaveragesupportcapabilityofashieldandchockareaboutthesame,buttheshieldismorestable.Theshieldprovidesadditionalroofsupportbecauseitscanopyandbaseareconnectedbystructuralmembersotherthanthehydrauliclegcylinders.Asaresult,thelegcylindersoftheshield,unlikethoseofthechock,arenotsubjectedtodamagingbendingmovements.ThefirstshieldsintheUnitedStateswereinstalledin1975intheShoemakermineofConsolidationCoalCompany,nearWestVirginia.Shortlyafterwards,shieldswereappliedtootherU.S.longwalloperations,provingsuccessfulinareaswhereotherroofsupportsfailed.Thebasicshielddesignwasimproved,andbythelate1970s,shieldsweretheleadingroofsupportsinlongwallinstallations.Advancesmadetothedouble-drumandrangingarmshearersdevelopedinthe1960'smadethemmoreadaptable.Theircuttingheightcouldbequicklyadjustedwhencoalbedthicknesschangedorwhenitwasnecessarytoleavealayerofcoalatthetopofthebedtostrengthenthemineroof.Improvementswerealsomadeinthemethodofhaulingthesheareracrossthecoalface.Theearlyshearerswerepulledbychainsstretchedalongthelengthoftheface.Ifthechainbroke,itcouldcauseseriousinjuries.Bytheearly1970's,shearersmovedbysafer“chainless”methodsusingself-containedtractionunits.Althoughdevelopmentconcentratedontheshearer,theplowwasalsoimproved.Aplowdesignedtobestrongerandmoreruggedwasplacedinservicein1974atClinchfieldCoalCompany'sNo.2mine,nearDante,Virginia.Itoperatedsuccessfullyinathincoalbedthathadbeentoohardforearlierplows.1980-1994Since1980,anaverageofmorethan100longwallinstallationshavebeeninoperationannuallyintheUnitedStates.Inrecentyears,however,thenumberhasdeclinedslightly,reflectingpartlyeconomicandmarketconditionsforcoalandpartlytheabilityofthecurrentlongwalloperationstomeetdemandwithouttheneedforadditionalinstallations.ShieldshavebecomethepredominanttypeofroofsupportsinU.S.longwallmines,andshearerstheprincipalcuttingmachines.Thereliabilityofarmoredfaceconveyors,likethatofroofsupports,hasbeenimprovedtotheextentthattheyarenolongerresponsibleformajorinterruptionsinlongwallmining.Thelistofwide-rangingadvancesinlongwalltechnologyincludesshearersthataredesignedtominerelativelythincoalbeds(lessthan42inches).Betterdustcontrolhasbeenachievedwithwaterspraysandimproveddesignofcuttingdrumsandcuttingbits.Powersupplyproblemsforlargemultipleshearermotorsandlongerfaceconveyorshavebeenovercome.Becauselongwallminingisarepetitiveprocess,ithasthepotentialtobeautomated.Amongthehealthandsafetybenefitsfromanautomatedlongwallinstallationaretheremovalofpersonnelfromhazardssuchasdustexposure,rooffalls,andnoise.Theeconomicbenefitsincludeimprovedcoalquality,higherproductivity,reducedmaintenancecosts(forexample,reducedwearontheshearer'scuttingbits),increasedspeedofoperation,andbetteruseofpersonnel.Automationisbeingincorporatedinallphasesoflongwallmining.Push-buttoncontroltobeginasequenceofpredeterminedpatternsisnowbecomingthenorm.Shieldadvancecanbeautomaticallycontrolledbyasignalfromtheshearer.Sensorsandcontrolsystemshavebeendevelopedtodetectthecoal-rockinterfaceandprovideautomaticverticalrangingoftheshearerdrums.Anexampleofthematurestatethatlongwallmininghasreachedisthe15-milliondollarsysteminstalledbytheCONSOLCoalGroupin1994atitsRobinsonRunmine,nearShinnston,WestVirginia.Reportedlytheworld'smostadvancedlongwallsystem,itintegratessophisticatedcomputertechnology,instrumentation,androboticcontrolstoautomatemostoftheroutinetasksoflongwallmining,usinga42-inchcoalshearerand172hydraulicroofsupportshields.LongwallMiningComparedwithOtherUndergroundCoalMiningTechniquesLongwallminingisoneoftwobasicmethodsofminingcoalunderground.Theotherisroom-and-pillarmining,historicallythestandardmethodintheUnitedStates.Bothofthesemethodsarewellsuitedtoextractingtherelativelyflatcoalbeds(orcoalseams)typicaloftheUnitedStates.Althoughwidelyusedinothercountries,longwallmininghasonlyrecentlybecomeimportantintheUnitedStates,itsshareoftotalundergroundcoalproductionhavinggrownfromlessthan5percentbefore1980to40percentin1993.1Currently,85longwallsoperateintheUnitedStates,mostofthemintheAppalachianregion.Thebasicprincipleoflongwallminingissimple.Acoalbedisselectedandblockedoutintoapanelaverag-ingnearly800feetinwidth,7,000feetinlength,and7feetinheight,byexcavatingpassagewaysarounditsperimeter.Apanelofthissizecontainsmorethan1millionshorttonsofcoal,mostofwhichisrecovered.Intheextractionprocess,numerouspillarsofcoalareleftuntouchedincertainpartsofthemineinordertosupporttheoverlyingstrata.Themined-outareaisallowedtocollapse,generallycausingsomesurfacesubsidence.Extractionbylongwallminingisanalmostcontinuousoperationinvolvingtheuseofself-advancinghydraulicroofsupports,asophisticatedcoal-shearingmachine,andanarmoredconveyorparallelingthecoalface.Workingunderthemovableroofsupports,theshearingmachineridesontheconveyorasitcutsandspillscoalontotheconveyorfortransportoutofthemine.Whentheshearerhastraversedthefulllengthofthecoalface,itreversesdirection(withoutturning)andtravelsbackalongthefacetakingthenextcut.Astheshearerpasseseachroofsupport,thesupportismovedclosertothenewlycutface.Thesteelcanopiesoftheroofsupportsprotecttheworkersandequipmentlocatedalongtheface,whiletheroofisallowedtocollapsebehindthesupportsastheyareadvanced.Extractioncontinuesinthismanneruntiltheentirepanelofcoalisremoved.Bycontrast,thetypicalundergroundU.S.coalmineislaidoutinacheckerboardofroomsandpillars,andtheminingoperationinvolvescyclical,step-by-stepminingsequences.Theroomsaretheemptyareasfromwhichcoalhasbeenmined,andthepillarsareblocksofcoal(generally40to80feetonaside)lefttosupportthemineroof.Room-and-pillarmininggenerallyislimitedtodepthsofabout1,000feetbecauseatgreaterdepthslargerpillarsareneeded,resultinginsmallercoalrecovery.The“continuous”versionofroom-and-pillarminingisthemostcommon,representing56percentofallundergroundproductionin1993.Inthismethod,acon-tinuousminingmachineexcavatesthecoalandloadsitontoaconveyororshuttlecarinasinglestep.Despitetheterm“continuous,”themachineoperatesonlypartoftheworkingtime,becauseafterminingadvancesabout20feet,themachineiswithdrawnfromthefacesothatroofboltscanbeinstalledtobondthestrataandpreventcaving.In“conventional”room-and-pillarmining(whichrep-resents12percentofundergroundproduction),productionoccursinfivesteps:mechanicallyundercut-tingthecoalbed,drillingholesintothebedforexplosives,blastingthecoal,loadingthebrokencoalintoshuttlecarsfordeliverytoaconveyor,andthenboltingthemineroofintheexcavatedarea.Toprovideasteadyflowofcoalinaroom-and-pillarmine,severalstagesofminingoccursimultaneouslyindifferentrooms.Afinalphaseofminingtermed“retreatmining”maybeperformedtorecoveradditionalcoalbyextractingpillarsandallowingtherooftofall.However,thisisacomplexprocedurethatrequiresadditionalplanning.AdvantagesofLongwallMiningLongwallminingisaveryefficientcoal-producingtechnique.Longwallproductivityispotentiallyhigherthanthatofroom-and-pillarmining,becauselongwallminingisbasicallyacontinuousoperationrequiringfewerworkersandallowingahighrateofproductiontobesustained.Theamountofcoalrecoveredisalsohigh,currentlyreaching57percentasanationwideaverage,butachievinghigherpercentagesatsomemines.Room-and-pillarrecoveryratesareslightlylower.However,longwallcoalrecoverymaynotbesignificantlydifferentfromroom-and-pillarminesprac-ticing“retreatmining.”Thelongwallsystemalsooffersanumberofotheradvantagesoverroom-and-pillarmining.Itconcentratesminersandequipmentinfewerworkingsections,makingthemineeasiertomanage.Safetyimprovesthroughbetterroofcontrolandareductionintheuseofmovingequipment.Thismethodeliminatesroofboltingattheworkingfacetosupportthemineroof,anditminimizestheneedfordustingminepassageswithinertmaterialtopreventcoaldustexplosions.Itinvolvesnoblasting,withitsconsequentdangers.Italsorecoversmorecoalfromdeepercoalbedsthandoesroom-and-pillarmining.Thecoalhaulagesystemissimpler,ventilationisbettercontrolled,andsubsidenceofthesurfaceismorepredictable.Overall,aswell,longwallminingoffersthebestopportunityforautomation.DisadvantagesofLongwallMiningForemostamonglongwallmining'sdrawbacksarecapitalcostsforequipmentandinstallationthataresubstantiallyhigherthanthoseforroom-and-pillarmining.Inadditiontolongwallequipment,continuousminingmachinesandotherequipmentusedinroom-and-pillarminingarerequiredforthedevelopmentworkneededtoblockoutapanelofcoalforlongwallmining.Becausealargeinitialcapitaloutlayisrequiredwithnoimmediatereturnfromcoalproduction(apartfromthecoalproducedduringdevelopmentwork),economicsgenerallyrestrictslongwallminingtolargecoalcompanies.Moreover,smallcoalcompaniesinexperiencedinlongwallminingmaynotbeabletoprovidetimeforthespecializedtrainingneededforthisminingmethod.Longwallminingisamethodinwhichallpartsmustoperateasanintegratedsystem.Afailureofonepartcandisrupttheentireoperation,andtheimpactonmeetingcontractsforcoalsalescanbesubstantial.Longwallminingalsorequiresawell-maintainedven-tilationsystembecauseofthelargeamountsofdustandmethaneproduced.Dustlevelsoftenexceedthemaximumallowablelimitdespiteimprovementsindust-controltechnology.WhenthisisnotedduringaFederalmineinspection,atemporaryvarianceisgrantedsothatthedustlevelscanbeloweredbymodifyingthecoal-cuttingsequenceand/orbyincreasingtheairflowacrosstheface.ChangesinLongwallMiningOverthePastDecadeAsof1993,atotalof85longwallunitsoperatedin73U.S.coalmines.Mostofthesemines(53)werelocatedinAppalachia.WestVirginiawastheleadinglongwallState,with21mines.In1993,therewere13longwallminesintheWestand7operationsintheIllinoisBasin.Relativetothetotalundergroundminepopulationofroughly1,200mines,thelongwallminepopulationisquitesmall.However,becauselongwallminesarealmostinvariablylargeoperationswithhighannualproductionrates,theirshareoftotalundergroundproductionisdispropor-tionatetotheirsmallnumbers.In1993,40percentofthetotalU.S.undergroundcoaloutputwasproducedatlongwallmines.Thiswasconsiderablyhigherthanthe27-percentproductionsharecontributedbylongwallminesin1983.Longwallminesnowaccountfor80percentofundergroundproductionintheWest,37percentinAppalachia,and27percentintheIllinoisBasin.Theriseinlongwallproductionwaslargelyduetoadramaticincreaseinlongwalllaborproductivity.Between1983and1993,theaverageproductivityatU.S.longwallminesincreased108percent,from1.59tonsto3.30tonsperworker-hour.Althoughtheproductivityofroom-and-pillaroperationsalsoincreasedrapidlyduringthisperiodofdecliningcoalpricesandhighlycompetitivemarkets,operatorsofroom-and-pillarmineswerenotabletokeeppacewithlongwalloperators.Asaresult,averagelongwalllaborproductivity,whichwas2percentlowerthantheavera