洞室开挖稳定分析如何建设隧洞毕业论文外文文献翻译.docx
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洞室开挖稳定分析如何建设隧洞毕业论文外文文献翻译.docx
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洞室开挖稳定分析如何建设隧洞毕业论文外文文献翻译
Tunnel
StabilityAnalysisofTunnelExcavation
Aspillwaytunnelforanembankmentdamistobeconstructedinapoorqualitysandstone.Theexcavateddiameterofthetunnelisabout13mandthecoverovertheroofis8m.Thetunnelistohavea1.3mthickun-reinforcedconcreteliningand,afterplacementofthislifting,a28tohighportionoftherockfilldamwillheovertheconstructedtunnel.
Thequestionstobeaddressedare:
(1)Whatsupportisrequiredinordertoexcavatethetunnelsafelyundertheveryshallowcover?
(2)Istheproposedtopheadingandbenchexcavationsequence,usingdrillandblastmethods,appropriateforthistunnel?
(3)Howwilltheconcreteliningrespondtotheloadingimposedbytheplacementof28mofrockfilloverthetunnel?
Inordertoanswerthesequestionsaseriesoftwo-dimensionalfiniteelementanalyseswerecarriedusingtheprogramPHASE'`.Thefirstoftheseanalysesexaminedthestabilityandsupportrequirementsforthetopheadingexcavation.Thefinalanalysisincludedtheentireexcavationandsupportsequenceandtheplacementoftherockfilloverthetunnel.
Therockmassisapoorqualitysandstonethat,beingclosetosurface,isheavilyjointed.ThemechanicalpropertiesassumedforthisrockmassareacohesivestrengthC=0.04Mpa,africtionangleof40andamodulusofdeformationE=1334MPa.Noinsitustressmeasurementsareavailablebut,becauseofthelocationofthetunnelinthevalleyside,ithasbeenassumedthatthehorizontalstressnormaltothetunnelaxishasbeenreducedbystressrelief.Themodelisloadedbygravityandaratioofhorizontaltoverticalstressor0.5isassumed.
Asimplifiedversionofthemodelwasusedtoanalysethestabilityandsupportrequirementsforthetopheading.Thismodeldidexcludetheconcreteliningandthebenchexcavations.
Thefirstmodelwasusedtoexaminetheconditionsforafull-faceexcavationofthetopheadingwithoutanysupport.Thisisalwaysausefulstartingpointinanytunnelsupportdesignstudysinceitgivesthedesigneraclearpictureofthemagnitudeoftheproblemsthathavetobedealtwith.
Themodelwasloadedintwostages.Thefirststageinvolvedthemodelwithoutanyexcavationsandthiswascreatedbyassigningthematerialwithintheexcavationboundarythepropertiesofthesurroundingrockmass.Thisfirststageiscarriedoutinordertoallowthemodeltoconsolidateundergravitationalloading.Itisrequiredinordertocreateareferenceagainstwhichsubsequentdisplacementsinthemodelcanbemeasured.
TheresultsoftheanalysieareillustratedinFigure18.1,thatshowntheextentofyieldintherockmasssurroundingthetopheading,andFigure18.2thatshowstheinduceddisplacementsaroundthetunnel.
Thelargeamountofyieldintherockmassoverlyingthetopheadingsuggeststhatthisexcavationwillbeunstablewithoutsupport.ThisviewissupportedbythedisplacementsshowninFigureI8.2.
Thereadermaybesurprisedthatthedisplacementintheroofofthetunnelisonly26mmwhentheextentoftheyieldzonesuggestscompletecollapseoftheroof.IthastoberememberedthatPHASEisasmallstrainfiniteelementmodelandthatitcannotaccommodatetheverylargestrainsassociatedwiththecompletecollapseofatunnel.InexaminingFigure18.2itismoreimportanttolookattheshapeoftheoveralldisplacementprofilethanthemagnitudeofthedisplacements.Arockmasswillnottoleratethedifferentialdisplacementsillustratedandprogressiveravellingleadingtoultimatecollapsewouldalmostcertainlyresultfromexcavationofanunsupportedtopheading.
Ageneralruleofthumbusedbyexperiencedtunnellersisthatanundergroundexcavationwillnotbeself-supportingunlessthecoveroverthetunnelexceeds1.5timesthespanoftheopening.Thisisatypicalsituationthatoccurswhenexcavatingtunnelportalsarethereareseveraloptionsavailablefordealingwiththeproblem.Oneoftheseoptionsistouseashotcreteliningtostabilizetherockmassabovethetunnel.Afiniteelementanalysisofthisoptionshowsthata50mmthicklayeroffullyhardenedshotcrete(uniaxialcompressivestrengthof30MPa)issufficienttostabilizethetunnel.Theproblemishowtogetofshotcreteintoanadvancingtunnelheading.Asecondproblemiswhethertheworkerswouldhavesufficientconfidenceinsuchasolutiontoworkinthetunnel.
Oneprojectonwhichthissolutionwasusedwastheconstructionofan8mspandiversiontunnelforadam.Therockmasswasaveryweaklycementedlimestonethatcouldbeexcavatedbyhandbutwhichhadsufficientstrengththatitwasmarginallyself-supporting.TheScandinaviancontractorontheprojecthadusedshotcreteformanyyearsandtheveryexperiencedtunnellershadcompleteconfidenceinworkingunderacoverofshotcrcte.Thetunnelwasnotonthecriticalpathoftheprojectandsoconstructioncouldproceedatasufficientlyslowpacetoallowtheshotcretetosetbeforethenextadvance.Alayerofun-reinforcedshotcretewasthesolesupportusedinthistunnel,withoccasionalsteelsetsembeddedintheshotcretewheregroundconditionswereparticularlydifficult.
Inthecaseofthetopheadinginsandstoneunderconsiderationhere,theshotcretesolutionwasrejectedbecause,inspiteofthefiniteelementanalysis,thedesignersdidnothavesufficientconfidenceintheabilityoftheshotcretelayertosupportthelargespanofblockysandstone.Inaddition,thecontractoronthisdamprojectdidnothaveagreatdealofexperienceinusingshotcreteintunnelsanditwasunlikelythattheworkerswouldhavebeenpreparedtooperateunderacoverofshotcreteonly.
Anotheralternativethatiscommonlyusedinexcavatingtunnelportalsistousesteelsetstostabilisetheinitialportionofthetunnelunderlowcover.Thissolutionworkswellinthecaseofsmalltunnelsbut,inthiscase,a13mspantunnelwouldrequireveryheavysets.Anadditionaldisadvantageinthiscaseisthattheinstallationofsetswouldpermittoomuchdeformationintherackmass.Thisisbecausethesteelsetsareapassivesupportsystemandtheyonlycarryaloadwhentherockmasshasdeformedontothesets.Sincethistunnelisindeformationofadam,excessivedeformationisclearlynotacceptablebecauseoftheadditionalleakagepathswhichwouldbecreatedthroughtherockmass.
Thesolutionfinallyadoptedwas"borrowed"fromtheminingindustrywhereuntensionedfullygrouteddowelsarefrequentlyusedtopre-supporttherockmassaboveundergroundexcavations.Inthiscase,apattern3mx3mpatternof15mlong60toncapacitycableswereinstalledfromthegroundsurfacebeforeexcavationofthetopheadingwascommenced.Whenthesecableswereexposedintheexcavation,faceplateswereattachedandtheexcesscablelengthwascutoff.Inaddition,a2mx2mpatternof6mlongmechanicallyanchoredrockboltswereinstalledradiallyfromtheroofofthetopheading.
TheresultsofananalysisofthissupportsystemareillustratedinFigure18.3andFigure18.4whichshowtheextentoftheyieldzoneandthedeformationsintherockmassabovethetopheading.
ComparingFigure18.1andFigure18.3showsthattheextentoftheyieldzoneisonlyreducedbyasmallamountbytheenstallationofthesupportsystem.Thisisnotsurprisingsincesomedeformationoftherockmassisrequiredinordertomobilizethesupportingloadsintheuntensionedcables.Thisdeformationoccursasaresultoffailureoftherockmass.
Figure18.4showsthatthedisplacementsintheroofofthetopheadinghavebeenreducedsubstantiallyasaresultoftheplacementofthesupport.However,asmallproblemremainsandthatistheexcessivedisplacementoftherockbetweentherockboltfaceplateswhicharespacedona2mx2mgrid.Unlessthisdisplacementiscontrolleditcanleadtoprogressiveravellingoftherockmass.
Onlyasmallsurfacepressureisrequiredtocontrolthisravellingandthiscouldbeachievedbymeansofalayerofmeshorshotcreteofbytheinstallationoflightsteelsets.Inthiscasethelattersolutionwasadoptedbecauseofthesenseofsecuritywhichthesegavefortheworkersinthetunnel.
洞室开挖稳定分析
某土石坝工程在质量差的砂岩区开挖溢洪隧洞。
其开挖直径13m,顶部理深8rn。
隧洞有1.3m厚的普通混凝土衬砌。
衬砌浇筑后在隧洞上修建28m高的土石坝出水口。
面临的问题是:
(1)在浅部开挖时,为保证溢洪隧洞的安全所需的支护形式?
(2)在隧洞开挖时比较适用的是钻爆法施工,开挖过程中如何确定顶部上导洞和分步开挖的顺序?
(3)混凝土衬砌与隧洞上28m高的堆石荷载之间的作用是怎样的?
为解决上述问题,采用PHASE2程序进行了一系列的平面有限元分析。
首先对上导洞开挖的稳定性及所需要的支护形式进行分析,然后对全断面开挖、支护顺序和堆石对隧洞衬砌的影响进行分析
岩体为质量差的砂岩,近地表浅部节理发育。
其力学特性建议为:
黏聚力c=0.04Mpa.内摩擦角为40度,变形模量E=1334MPa。
没有现场地应力量测资料,由于隧洞位于谷坡,可以假定垂直隧洞轴线的水平应力由于应力释放而减小,并且认为水平与垂直应力之比为0.5,计算模型考虑自重荷载.
一简化的计算模型可用于洞室顶拱稳定性分析和支护要求。
该模型不包括混凝土衬砌和台阶开挖。
初始模型用来检查在没有任何支护情况下的上导洞全断面开挖的情况。
该模型常用于隧洞支护设计研究开始阶段,因为模型可以为设计者提供较清楚的将要处理问题的难度。
模型的加载分两个阶段。
首先是没有开挖情况下的模型,此模型是在开挖边界内用给定岩石材料特性建立起来。
第一阶段实施是为了在重力荷载条件下对模型加固相对于模型其后的位移可以量测到,则创建一个参考系是必要的。
分析结果如图18.1所示,图中显示了上导洞围岩的屈服范围;而图18.2则显示了旋隧洞次生位移的情况。
上导洞洞顶岩体的大范围屈服说明在没有支护条件下的开挖是不稳定的。
图18.的位移也说明了这一点。
大家感到奇怪的是当屈服带的范围被暗示洞顶会全部坍塌时,隧洞顶部仅有26mm的位移。
但应记住PHASE“是一个关于小应变模型的有限元程序,并不适用于隧洞整体塌落的应变工况。
从图18.2可看出,考虑整体位移形状远比位移量重要,而且岩体将不能承受图所示的差异性位移,上导洞不做支护开挖,由于渐进式剥落.最终导致洞石完全塌落是肯定。
经验丰富的隧洞建设者采用的经验法则是隧洞上部覆盖层厚度大于1.5倍洞室跨度,地下开挖才可达到自承支护。
对所涉及的问题来说,当开挖洞日有几种选取方案时,上面情况才会发生。
这些方案之一是采用喷混凝土衬砌来维持隧洞顶部岩体的稳定。
这一方案有限元分析表明,50mm厚的充分硬化喷混凝土层(单轴压缩强度30MPa)对于隧洞的稳定已经足够。
问题是如何把充分硬化的喷射混凝土注人到前方的洞室端部。
其次是施工者是否有信心来解决隧洞的这一问题。
某工程在为大坝建一条8m跨度的导流洞时采用了喷混凝土衬砌。
围岩为弱胶结的灰岩。
可用手工掘进,但有足够的强度在一定程度上达到自承作用。
斯堪的纳维亚(Scandinavian)承包商在工程上采用喷混凝土技术已多年,非常有经验的隧道建设者完全有信心在喷射过混凝土的隧道中施工。
该隧洞并不是工程的关键隧洞,建设者有充分的时间进行喷混凝土施工。
在隧洞中喷混凝土技术是常用的支护措施,对隧洞地质条件特别差的地段,采用预埋在喷混凝土层中的钢拱架支护。
在砂岩中进行上导行洞开挖,未采用喷混凝土方案,尽管有限元分析结果表明是可以的,但设计者没有充分的信心认为喷混凝土层可以对块状砂岩大跨度洞室进行支护。
除此以外,大坝项目的承包商没有隧洞喷混凝土施工的丰富经验,施工人员也不准备在仅有喷混凝土支护条件下施工。
在开挖洞口时另一个通常采用的方法是在洞顶覆盖层较薄的地方用钢
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