平煤二矿2.4Mt-a新井设计-大断面硬岩巷道快速掘进技术

页英文原文StressanalysisoflongwalltopcoalcavingHabibAlehossein，BrettA.PoulsenCSIROExploration&Mining,Brisbane,AustraliaUniversityofQueensland,Brisbane,AustraliaABSTRACTLongwalltopcoalcaving(LTCC)isarelativelynewmethodofminingthickcoalseamsthatiscurrentlyachievinghighproductivityandefficiencyinapplication,particularlyinChina.Thetechniqueissimilartotraditionallongwallmininginthatacuttingheadslicescoalfromthelowersectionofthecoalseamontoaconveyorbeltinstalledinfrontofthehydraulicsupportnearthecuttingface.InmodernLTCCanadditionalrearconveyorbeltislocatedbehindthesupport,towhichtheflowofthecavedcoalfromtheupperpartoftheseamcanbecontrolledbyamoveableflipperattachedtothecanopyofthesupport.Theminingmethodreliesonthefracturingofthetopcoalbythefrontabutmentpressuretoachievesatisfactorycavingintotherearconveyor.Thispaperdevelopsayieldandcaveabilitycriterionbasedoninsituconditionsinthetopcoalinadvanceoftheminingface(yield)andbehindthesupports(caveability).Yieldingandcavingeffectsarecombinedintoonesinglenumbercalledcavingnumber(CN),whichisthemultiplicationresultofcavingfactor(CF)andyieldfactor(YF).Analyticalderivationsarebasedoninsitustressconditions,Mohr–Coulomband/orHoek–Brownrockfailurecriteriaandanon-associatedelastoplasticstrainsofteningmaterialbehaviour.Theyieldandcaveabilitycriteriaareinagreementwithresultsfrombothnumericalstudiesandminedata.ThecavingnumberisnormalisedtominingconditionsofareferenceChinesemine(LMXmine)andisusedtoassessLTCCperformanceatfourteenotherChineseworkinglongwallsthathavehadvaryingsuccesswiththeLTCCtechnology.ThecavingnumberisfoundtobeingoodagreementwithobservationsfromworkingLTCCmines.Asapredictivemodel,resultsofthisanalytical/numericalstudyareusefultoassessthepotentialsuccessofcavinginnewLTCCoperationsandindifferentminingconditions.INTRODUCTIONTopcoalcavingisaneconomicalundergroundminingmethod,whichhasrecentlybeenintroduced,modifiedandpracticedintheundergroundcoalminesofChina.ThemethodanditsconsecutivemodificationscanbelinkedtotheoriginalmethodofsoutirageminingdevelopedinFranceduringthe1960s[1].Longwalltopcoalcaving(LTCC)iscosteffectivebecauseonlythelowerpartofacoalseamiscutbyamechanicalcutterandtheupperpartisallowedtocaveundergravity,providedthegroundconditionsareappropriate[2–14].Chineseexperiencehashighlightedtheimportanceofthestressabutmentpeakinfracturingthecoalinadvanceofthefacetoallowthecoaltocaveontotherearconveyor.Fig.1illustratestheconceptofatypicalabutmentstresschangeasaresultofminingbythemodernLTCC[5].Asshowninthefigure,amajordifferencebetweenthemodernLTCCandtraditionallongwallminingmethodsistheexistenceofanadditionalrearconveyorbeltbehindthesupport,onwhichthecavedcoalfromtheupperpartoftheseamisdrawnbyamoveableflipperattachedtothecanopyattherearofthesupport.Theminingmethodreliesonthefracturingofthetopcoalbythefrontabutmentpressuretoachievesatisfactorycavingintotherearconveyor.Becauseofthehighefficiencyandoutput,thistechniqueis,atpresent,popularforthickcoalseams,particularlyinChina.ThereareseveralfavourablecharacteristicsobservedinLTCC.Supportloadnormallyincreasewithcoalseamheight,however,ariseinthecuttingheightofthecoalseammayreducetheLTCCsupportload.Themovementoftheoverburdenrockstratumoverthecoalfaceshowsperiodicityduetocantileverbeameffects,althoughthisperiodiceffectisnotverystrong.However,distributionofthenormalandshearloadingofthesupportstructuremaynotbeeitheruniformorthesameasconsideredinthedesign,whichcancausenon-uniformreactionforcesandmomentsinthesupportsystem.Inotherwords,anexcessforceormomentinanelementofthesupportstructurecancausepermanentdamageresultinginexcessiveoperationalcosts.Inpractice,thereisarelationbetweenthesupportloadandthesupportstructuralbehaviour.Inparticular,thereisamechanicalrelationbetweentherecordedhydraulicallypoweredforceofthesupportsystemandthenormalandfrictionforcesgeneratedinthepropsinrelationtothefracturing,yieldingandcavingprocessesofthetopcoal.ChineseexperienceindicatesthatapplicationofmodernLTCCinthickcoalseamsislimitedbytheoverburdenrock,coalstrengthandthicknessanddipangleofthecoalseam.Forexample,ifcoalistoostrong,orthefrontabutmentstresspeakistoolow,thenthecoalaheadofthefacemaynotbesufficientlyfracturedtocaveintotherearconveyor.Asaremedy,althoughincreasingtheproductioncost,artificialtechniquessuchasdrillandblastmaybeusedtoweakenthecoalaboveandaheadofthesupport.Therefore,thepatternsofthemovement,deformationandfailureofthetopcoalandthesurroundingrockandtheforcesexperiencedbytheelementsofthesupportstructurearerecordedandstudiedtosolvetheproblemoftheperformanceoftheLTCCmethod.Thesupport–coal–rockinteractionproblemistoodifficulttobesolvedanalyticallyatpresent.However,numericalmodelscanbeusedtoapproximateandsimulatetheactualproblemtoprovideusefulinformationtounderstandsomeofthecomplexmechanismsinvolvedinLTCC[5–14].Forinstance,Xieetal.[9]andZhang[4]usedFLAC3D,whichisbasedoncontinuummechanicsandfinitedifference,tosimulatedeformationsandfailuremechanismscausedbytheLTCCminingmethod.Theyalsodetermineddistributionoftheabutmentpressureanddiscussedresultsofthevariationsindeformationandfailureofthetopcoalbeforeandaftertheweakeningprocess.Whilecalculatingindetaildeformation,stressesandfailurezonesinthecoalseamandthesurroundingrockmass,thesenumericalsimulationsgenerallyconfirmandrecommendanadditionalartificialweakeningpracticeofthetopcoalinordertoimprovetherateofcoalrecovery[4–14].ModernLTCChasbeenverypopularinChinawhereitisappliedincoalseamsfrom5to12mthickness[12–14].Chineseresearchinstituteshavedevelopednumerousempiricalmethodsforassessingthecavabilitycharacteristicsoftheirminesmainlybasedon:(i)coalstrength,(ii)coverdepthand(iii)caveability,althoughthefullnumberofparametersconsideredintheircalculationofcaveabilityindexincludecoalstrength(mainlyUCS),verticalstress(gravity),topcoalthickness,inter-burdenrockmasslayerthickness,coverdepthortotaloverburdendepth,anddegreeoffracturing.Chineseexperiencesuggeststhatthecoalintactstrength(UCS)shouldbeinarangeof10–25MPaor,whenthestrengthis425MPa,welldevelopedjointsandcleatsarerequired.Therequiredcoverdepthdependsonthespecificcoalseamchar-acteristicsandstrengthinvolved.Itisacriticalparameterandshouldbe4150m.Theyfoundthatcoverdepths<150mwouldbetoodifficultforthetopcoaltobepre-fracturedbytheabutmentstress.Inthenumericalinvestigationsofthispaper,thedatafromDMNmineareusedasthebasisofthenumericalmodel.DMNmineincludesa7mthickcoalseamwithtypicalinsitustressconditionsofChina.KeycaveabilityindicatorsmeasuredinDMmineareasfollows.Cavingofthetopcoalfirstoccurredafterapproximately20mofminingandthecavingofthefirstroofbeamafterapproximately40–50mandperiodicweightingthereafteratapproximately15–20mretreat.Distancetothefrontabutmentstresspeak(Fig.1)wasapproximately7–8m.Thecoalseamextractionwasabout90%[9].Oncethemechanismandgeotechnicalconditionsofcavinghavebeenassessedtheappropriateequipmentisselected.Table1showsacavingindexgeneratedfromthecavingparametersoftherock[12].TheChinesecavingindex(CCI)relatestoseamrecoveryandagenericassessmentofthecavingconditions.ItshouldbenoticedthatAustralianminingconditionsvarymarkedlyfromtheChinesecoalfields,ChineseLTCCminesarenormallydeeper,ataround600–700mdepth,withmoderatelyhardcoals,whereasAustralianminesarerelativelyshallow,at250–350mindepth,withrelativelysoftercoals.EvenifthetechniqueisassumedtobegenerallyapplicabletoAustralianmines,animportantquestionwillbewhetherapreconditionsuchasdrillandblastisnecessary,whichaffectsmineproductioncost.Duetothelackofpreviousexperienceinthistypeofmining,the‘degreeoffracturing’parameterisdifficulttomeasureorcalculateforother(e.g.Australian)mines.Therefore,toolsmustbedevelopedtoenableacomparisonofother(e.g.Australian)miningparametersagainstcurrentChineseLTCCmeasurementindexestoobtainanindicationastowhetherLTCCisapplicabletoprospectiveother(e.g.Australian)sites.ThisgeneralisationissueofthespecificallymeasuredCCIhasbeenaddressedinthispaperbydevelopingasite-independent,genericmodeloftheLTCC,todemonstrateaveryimportantfinding,thatthereisindeedastrongcorrelationbetweenthecavingindex(CCI)andtheplasticstraindevelopedatthetopcoalface,whichextendsitsapplicationtocomparableundergroundAustralian(oranyother)coalmines.Inthismodel,anassessmentofdeformationsandstressesbehind,aboveandaheadofthecoalfaceismadeandanalyticalformulationsaredevelopedforyieldingandcavingrequirementsoftheLTCCmethodinasinglecavingnumber(CN),whicharediscussedinSections2–4.ThisisfollowedbynumericalapplicationscoveredinSection5andcomparisonsofthesetheoreticalresultswithrealdata,takenfromfourteenminesinChina,arediscussedinSection6.GeometricalproblemPostminingstressesaroundexcavationsdependonexcavationshapeandsizeandpre-mininginsitustresses.InAustralia,thecoefficientofinsituhorizontalstresstoverticalstress(K)isgenerally41(oftenaround2).Fig.2demonstratestheeffectofpre-mininginsituverticalandhorizontalstressesonthe2Delasticpost-mininghoopboundarystress(syy)forbothcircularandellipticalcavitiesatthetwopointsofA(onthexaxis)andB(ontheyaxis).Inthisfigure,lrepresentstheratiooftheminorandmajoraxes,whichisunityforacircle.Itisdemonstratedinthisfigurethatnotonlyhightensileandcompressivestressconcentrationscanoccuraroundexcavationboundaries,butalsoboundarystressescanbeordersofmagnitudelargerthantheirpre-mininginsitustressescausingfracture,damageandfailureoftherockmassbothlocally(nearboundary)andglobally(farfromboundary).Thelargerthesemininginducedstresses,themorevolumeoftherockmasswillbeaffected.ThemajorquestioninthecurrentunderstandingofLTCCishowthetopcoalaheadofsupportfailsandwhatmechanismdrivesitlatertocavebehindthesupport.Itisalsounknownwhetherthefronttopcoalyieldsfirstfromthesmallercleats,bandsandjointsorbyalargerblockystructureduetothechangeofstresses.Isthefronttopcoalinapassiveoractivestressmode,andifactive,inwhatdirections(horizontalorvertical)arethemovements?Therefore,thereareseveralhypotheses,whichcanbemadeforthefailurebehaviourofthefronttopcoal.SomeofthesepossiblemechanismsareschematicallyillustratedinFig.3.Finiteelement(FE)modellinghasbeenusedtoaddressanswerstosuchhypothesesassumingarangeofstressratio,K=(horizontalstress)/(verticalstress).Onlyresultsfortheunitinsitustressratio,K=1,areshowninFigs.4–6.Twodimensional,eight-noded,isoparametricMohr–Coulomb,elastoplastic(withoutsoftening)finiteelementshavebeenusedintheseanalyses.Strainsofteningmaterialsarenormallymesh-dependentandrequirespecialtreatments[15].Theseelasticandelastoplasticfiniteelementanalysesindicatethatalltheassumptionsandparticularlyeitheroftheverticallyorthattheymaybeassumedtoremainasprincipalstressesintheseformulations.Extensionofthetheorytoincludeprincipalstressrotationispossibleanddoesnotrequiremucheffort.YieldfactorandinelasticdeformationArockbrittlenesscoefficientk,whichisanalogousandidenticalinvaluetothemorefamiliarsoilmechanicstermsofactive(Ka)andpassive(Kp)earthpressurecoefficients(i.e.materialstresslimitratios),nowcanrepresenteitherofthefactorsKaorKp,dependingonthedominantcoalfailuremechanisminEq.(1).scistheuniaxialcompressivestrength(UCS),stisthetensilestrength(e.g.Braziliantensilestrength;BTS),s1ands3arethemajorandminorprincipalstresses,whicharelimitedbymaterialstrength.Theactiveandpassivecoeffi-cientsofrockfailurearerelatedtoeachotherviatherelation,whereeachrepresentsthelinearMCenvelopeslopeofoneprincipalstressagainsttheother.Duetothematerialstrengthlimitations,themagnitudeofanynegativestresscannotbegreaterthanthevalueoftherockuniaxialtensilestrength(st)andthemagnitudeofanypositivestress,atanynegativeconfinementstress,cannotbegreaterthanthevalueoftherockuniaxialcompressivestrength(sc),i.e.:σ3min=-σr当σ1Intermsofthenon-linearHoekandBrown(HB)criterion,theffunctioninEq.(1a)becomesfThepowerfactornisnormallyo1.Forexample,n=0.5formosthardrocks,andn=0.65forafewAustraliancoalseams.Noticethatcriterion(3)transformstothesamelinearMCcriterioninEq.(1a)whenn=1,s=1andm=1.Theparametersdependsontherockfracturingcondition,withamaximumvalueofunityfortheintactrockandtheparametermidentifiestherockstrengthforaparticularrocktype.Itissimilartothepassivestrengthcoefficient(Kp),whichactivatesrockfrictionalbehaviouroncecohesivebondshavebeenbroken.Ingeneralthefollowingrelationsholdtrueforthetwomethods:m=sσThereisnocomparableUCSstrengthreductionfactorsintheMCcriteriontobeapplicabletofracturedrockmassesaswell.Becausetherockstrengthparameter(sc)hastwodifferentinterpretationsinthetwocriteria,itisbettertodistinguishthetwostrengthtermsbythesubscriptsMCandHB,particularly,whentheyaremeanttobedifferent,asshowninEq.(4b).ThefrictionangleintheHBcriterionisnotconstantanddependsonthemagnitudesoftheprincipalstresses.InsitustressandsolutionsExcludinganypossiblesurchargeloadings,theundergroundverticalstressbeforeanyminingisnormallyassumedtobeduetothegravityforce.Insitupre-mininghorizontalstressisusuallyrelatedtoverticalstressbyalinehavingaslopeKandaconstantq:σσwheretheindex0emphasisestheinsitupre-miningcondition,zisthedepthbelowgroundsurfaceandgisrockunitweight.AsshowninFig.1,cavingattherearofthesupportoccursinanLTCCmethodifandonlyiftherockaheadofthefacehasalreadycracked,failedoryielded.Whenthishappens,thestressesintherocksatisfytheyieldcriterion(1)or(3)causinginelasticdeformationsgovernedbythestressstrainrelations(11).Inotherwords,theinitialstateofstress,sh0andsv0,havebeendisturbedtonewstressesshandsvasaresultoftheinducedexcavationstressesDshandDsv.Therefore,wecanwrite:σhσIntermsofprincipalstresses,thesecanbewrittenasσσUsingEqs.(15)–(18)inEqs.(1)and(3),weobtainthefollowingseparatesystemofequationsintermsoftheoriginalinsitustresses(13)and(14):YF=YF=Forexample,atopcoalwillyieldaheadoftheface(Fig.1),ifthefollowingpropertiesandstressconditionsareassumedforthecoalseam,whichispossibleinpractice:N=0.65,σc=20MPa，s=0.25，σt=1MPa，σv0fguNumericalanalysisMixeddistinct–discreteelementmodellingallowsforrocktobemodelledafterithasbrokenasthemodelledrockmassismadeupofsmallerindividualelementsbondedtogether.Fig.13showsaUDECmodelillustratingtheformationoffracturesaheadoftheminingfaceandcoalbreakageoverthetopofthesupportsfollowedbycavingattherear.ThemodelinFig.14showsadifferentdiscreteelementcodeusingsphericalelementsandisabletoaccuratelysimulatetheflowofthefracturedcoalinthegoafduetoLTCC.Themodelautomaticallyextractsthecoalthatfallswithin2moftherearofthesupporttosimulatetherearfaceconveyor.(TheLTCCsupportisnotshowninthismodel).Depthofmining,horizontaltoverticalstressratioandUCSstrengthofthetopcoalwerevariedfromthebasevalueswiththeresultspresentedinFigs.15–17.ThepatternsoftheresultingfunctionsareingoodagreementwiththeanalyticalresultspresentedinFigs.7–9.ThenumericalresultsofFigs.15–17generallyconfirmthechangepatternsoftheanalyticalpredictionsoftheyieldfunctionpresentedinFigs.7–9.Insummarythenumericalmodellingresultsindicatethatyieldincreaseswithincreasingdepth(Fig.15),decreaseswithincreasingcoalstrength(Fig.16)andisnon-monotonicwithvariationinK(Fig.17),aspickedupbytheanalyticalmodelofFig.9.TheresultsforKeffectsinFig.17,however,needsmorecarefulconsiderations,asitalsorepresentnon-uniformandcomplexresponsesfromacollectionofelementsformingthetopcoalblockinthenumericalmodel,inwhicheachelementcanbeunderanyactive,passive,compression,tensile,shearfailuremode.Inotherwords,theyieldfunctionhereisastrongfunctionofnotonlythestressratioK,butalsotheelementsizeandthetypeofthedominantfailuremechanismexplainedbyEqs.(1).ApplicationtorealcoalminesTwofactorsthatassessthecavingperformanceofLTCC,thecavingfactor(CF)andtheyieldfactor(YF)arepresentedinanalyticalandnumericalstudies.ToconsidertheusefulnessofthesefactorstheyarecomparedtotheChineseexperiencewithLTCC.ThesuccessassessmentofLTCCinChinaisdocumentedforfourteenmines/longwallsandcanbeclassifiedfrom1(excellent),2(good),3(medium)and4(poor).InTable3,thesefactors,inrelativeterms,havebeencalculatedfortheseminestobecomparedwiththeirfinalcavingperformancedatapresentedinthelastcolumn.Notice,coalandrockgeologicalstructurehavenotbeentakenintoaccountexceptforcoalUCSanddepthofcover,allotherpropertiesrequiredforCFandYFcalculationsarekeptconstantandaresimilartotheparametersusedinexampleprobleminSection3.CalculatedCFandYFforeachminearenormalisedwithrespecttothosefortheLMXmine,ashighlightedinthetable.TheCF’sarecalculatedbyEq.(35)andYF’sbyEq.(19).Thecavingnumber(CN)isfinallycalculatedbymultiplyingthesetwofactors,asshowninEq.(36).Whencomparedwiththecavingperformance,itisapparentthatbothcavingfactorandyieldfactorneedtobetakenintoconsideration.ResultspresentedinTable3showthemoredominanteffectofthecoalstrengthforcavingcomparedtothedepthofcover.AsdiscussedinSection4,thereareseveralotherfactors,likethestressratio(K),cohesion,frictionanddilationangleofthecoal,andmoreimportantlythesupport-coalinteractionsandtherockgeologicalstructureareimportantfactorsthatneedtobeconsideredforsuccessfulpredictions.ConclusionByconsideringtheinsitugeological,geometricalandgeo-technicalconditionsinadvanceofaLTCCfaceayieldfunctionisdevelopedtoaidintheassessmentofLTCCinanewminingoperation.TheyieldfunctionisdevelopedforbothMohrCoulombandHoekBrownfailurecriteriaandconsidersdepthofmining,horizontaltoverticalstressratioandtheUCSofthecoalseam.Numericalmodellingconfirmsthetrendspredictedbytheyieldfunctionandgivestheoptionofconsideringothergeotechnicalvariables.Acavingfunctionisdevelopedbasedonconsiderationsattherearofthechock.BoththeyieldfunctionandcavingfunctionhavebeenappliedtofourteenChineseLTCCminesthathavehadvaryingsuccesswithtopcoalcaving.NotallrequiredparametersareavailableforalltheChinesemines,however,theYFandCFtogetherappeartogivepredictionsinagreementwithChineseexperience.REFERENCES[1]UDECusersmanual.Minneapolis:ItascaConsultingGroup，2000[2]SenGC.CoalmininginFrance.CollieryGuardian，1961[3]PengSS、ChiangHS.Longwallmining.NewYork:Wiley，1983[4]WoldMB，PalaJ.Aspectsofsupportandstrataperformanceonlongwallno.1atEllalongColliery.CSIROdivisionofgeomechanicsreportno.62，1986[5]ZhangD.Groundpressurecontroloffacewithfully-mechanizedsub-levelcavingmining.Privatecommunication，2003[6]CaiY，HebblewhiteB，etal.ApplicationoflongwalltopcoalcavingtoAustralianoperations.CSIRO–ACARPreportC11040，2003

中文译文长壁放顶煤采煤法应力分析HabibAlehossein,BrettA.Poulsen联邦科学与工业研究组织，布里斯班，澳大利亚昆士兰大学，布里斯班，澳大利亚摘要:长壁放顶煤采煤法是开采厚煤层的相对较新的方式，目前应用该方式已取得了较高的生产率和效率，尤其是在中国。该技术类似于传统的长壁采煤法，工作面采煤机从煤层底部将煤炭割下落在液压支架前面的刮板输送机上。现代长壁放顶煤采煤法在支架后方还设有一架输送机，用于运输经支架顶部的放煤口控制冒落的顶煤。顶煤的超前支撑压力特征决定了该采煤方法是否能获得令人满意的冒放性。本文基于采煤工作面前方和支架后方的顶煤的现场条件建立了屈服和冒放性的标准。屈服性和冒放性的影响可构成一个由冒放因素（CF）和屈服因素（YF）综合作用结果的冒放因数（CN），分析推导基于原岩应力条件下，莫尔-库仑和/或虎克-布朗岩石破裂准则和非连续弹塑性应变软化材料的行为。屈服和冒放性标准与数值模拟和现场数据的结果均相符。正常的冒放因数根据中国煤矿提出，并用于评估中国其他14个应用长壁放顶煤技术取得不同成果的工作面。作为一个预测模型,数值分析研究结果评估在不同的综放开采条件下的冒放性是有效地。1引言如前所述，在中国煤矿改进和实践的综放采煤法是一种经济的地下采矿方法。该方法以及其持续的改进可以追溯到20世纪60年代在法国出现的soutirage采矿法。由于采煤机只采煤层底部，顶煤在重力作用下垮落，所以综放采煤法成本是高效的。中国的经验强调了为了使顶煤垮落到输送机上，工作面前方的应力峰值的重要性。图1说明了现代综放开采引起的应力的典型变化。如图所示，现代和传统长壁综放开采的方法一个主要的区别就是存在支架后方一个额外的传送带，用于运输从支架顶部的放煤口溜下的顶煤。该方法依赖于顶煤的特征，输送机前方的超前支承压力影响冒放性。由于高效率和高产量，目前该技术流行于厚煤层开采中，尤其是在中国。在综放开采中显示了一些有力的特征。支承载荷随着煤层厚度增加，然而随着截割煤层厚度的增加可能降低综放支架载荷。工作面上方的覆岩地层的运动由于悬臂梁的影响显示了周期性，尽管这种周期性不是很强烈。然而，支撑结构上的应力分布可能既不均衡也不是设计过程中考虑过的，它可能导致支撑系统中的非均匀反应力和时刻。换句话说，支撑结构元素过度的应力或时刻会引起永久的损害从而导致过多的损失。实践中，支撑载荷和支撑结构行为之间存在着联系。液压驱动的支撑系统和支柱产生的摩擦力存在着机械联系，并关系到顶煤的破裂，屈服和冒落过程。中国的经验表明现代综放采煤法在厚煤层中的应用是有限的，因为覆岩，煤层强度，厚度和下沉角度。例如，如果煤的强度太大，或超前支承压力峰值过低，那么工作面前面的煤炭不足以垮落到输送机上。作为补救，尽管增加了生产成本，诸如钻眼爆破将用于弱化支架上方和前方的煤炭。图一因此，这种运动模式，顶煤及围岩的变形和失效以及支持结构元素的应力经记录和研究来解决综放开采的难题。目前支架—煤层—围岩交互问题还难以分析的解决。但是，数值模型可用来近似和模拟实际的问题，为理解综放开采的复杂力学问题提供有效的信息。例如谢和张曾用基于连续介质力学和有限差分法的FLAC3D模拟综放开采引起的变形和失效力学。他们还确定了支撑压力的分布并讨论了在弱化前后的顶煤变形失效的结果。在计算煤层和围岩的详细变形，力学和失效区域时，数值模拟通常需要确定一额外人为削弱顶煤的工作以提高煤炭的回收率。现代综放开采在中国已十分流行，应用于5-12m厚的煤层开采。中国研究机构提出了大量经验方法来评估矿山的冒放特征，主要基于：（1）煤层强度，（2）保护层厚度，（3）冒放性，但在他们评估冒放指数是考虑的完整参数包括煤层强度，垂直应力，顶煤厚度，覆岩厚度，保护层厚度及破裂程度。中国的经验表明，完好煤岩体的压力应在10-25MPa之间，当压力大于25MPa时，就需要良好发育的解理。所需保护层的厚度取决于特定煤层的特征及压力。临界值需大于150m。他们发现保护层的厚度小于150m时，顶煤将难以在支承压力作用下预裂。在本文的数值研究，来自于DMN煤矿的数据作为数值模型的基础。DMN矿煤层厚度为7m，且具有中国比较典型的原岩应力情况。衡量DM矿冒落性的关键指标如下：开采大约20m后，顶煤首次垮落，直接顶的垮落在开采大约40-50m后发生，之后每隔15-20m将会周期性的垮落。超前支承压力的峰值（图1）的距离大约在7-8m之间。煤层采出率约为90%。一旦确定了综放开采的原理和岩土工程条件，相应的设备就能选出来。表1显示了由岩石垮落参数产生的冒落指数。中国的冒放指数与新生裂隙相关，并作为冒放情况的一般评估。应该注意到，澳大利亚的采矿条件与中国煤炭存在明显差异，中国的综放开采通常更深一些，大约在600-700m深处的中硬煤层。相比之下，澳大利亚采矿则较浅，在250-350m深处，煤层也较软。即使该技术被认为适用于澳大利亚煤矿，一个重要的问题就是诸如钻眼爆破的先决条件是否需要，从而影响采矿生产成本。由于缺乏这种采矿方法的经验，破裂程度参数难以评估其他（例如澳大利亚）煤矿情况。因此，必须开发出与当前中国综放开采评估指数不同的开采参数相对照的工具，以此确定综放开采是否适合其他地区。表一中国冒放性评估示例冒放等级12345开采条件优良中差极差冒放指数>0.90.8-0.90.7-0.80.6-0.7<0.6回收率(%)>8065-8050-6530-50<30本文通过开发独立通用的综放开采模型，解决了特定的中国冒放指数的归纳问题，证明了一个非常重要的发现：冒放指数和顶煤内的塑性应力存在极强的关联，使其可类比的应用到澳大利亚地下煤矿中。在本模型中，提出了对工作面前中后变形和应力的评估方式以及综放开采引起的屈服和冒落的分析公式，将在第2-4部分讨论。数值应用在第五部分涉及，并在第六部分讨论理论结果和从中国十四个煤矿得到的数据的对比。几何问题开挖岩体周围的采动应力依赖于开挖体的形状和大小及原岩应力。在澳大利亚，原岩水平应力和垂直应力的比值一般大于1（通常在2左右）。表2演示了圆形和椭圆形上的A点（位于x轴）和B两点（位于y轴）的二维弹性区的开采前垂直应力和水平应力和开采后的边界应力。在图中，λ代表圆形短轴和长轴的比率。图中了证明不仅高拉伸和压缩应力出现在开挖边界周围，同时边界应力的数量级也比开挖前原岩应力大，能引起周围（边界附近）和远处（距边界远处）的岩石破断，损坏和失效。采动应力越大，受影响的岩体的体积也就越大。图二弹性模型图三塑性模型当前地下综放开采的主要问题是支架前方的顶煤是如何失效的以及什么机理驱使其落后于支架后方的煤层垮落。顶煤是否会由于小的裂隙，解理或是由于压力变化大的块体结构。顶煤是处于被动还是主动的压力形式，如果是主动地，向哪个方向移动呢？因此，有几个假设可用于解释顶煤的失效行为。其中一些可能的机制在表3中示意出来。有限元(FE）模型已经被用来解决假设一系列应力比的假说，K=（水平应力）/（垂直应力）。只有原岩应力状态下的比率，K=1，如表4-6所示。这些分析运用了两个维度，八个节点，莫尔-库仑准则，弹塑性（没有软化）有限介质。应力软化材料通常啮合依赖和需要特殊对待。这些弹性和弹塑性有限元分析表明，所有的假设和特别要么垂直的，或者认为继续担任主应力。理论的延伸包括主应力循环是可能的，并且不需要太多的努力。屈服因素和非弹性变形当最大主应力（例如垂直应力）等于或大于最小主应力失效函数和岩石强度特性（k，m，s，n）时，支