The-Face-Stability-of-Slurry-shield-driven-Tunnels

TUNNELSANDDEEPSPACE0886-7798(94)E0006-5TheFaceStabilityofSlurry-shield-drivenTunnelsG.AnagnostouandK.KovdriAbstract--Duringtheexcavationofatunnelthroughsoftwater-bearingground,atemporarysupportisoftenrequiredtomaintainthestabilityoftheworkingface.Inaslurryshield,thissupportisprovidedbyapressurizedmixtureofbentoniteandwater.Slurry-shieldtunnellinghasbeenappliedsuccessfullyworldwideinrecentyears.Underextremelyunfavorablegeologicalconditions,however,faceinstabilitiesmayoccur.Thispaperaimsatabetterunderstandingofthemechanicsoffacefailurewhenusingabentoniteslurrysupport.Thecomplexinterrelationsbetweenthevariousparameters(shearstrengthandgroundpermeability,suspensionparameters,slurrypressure,geometricdataofthetunnel,safetyfactor)arestudied.Attentionispaidtothetime-dependenteffectsassociatedwiththegradualinfiltrationofslurryintothegroundaheadofthetunnel.Relatedtopics,suchasthestand-uptime,soilpropertiesandtheeffectofadvancerate,arediscussedquantitatively.R$sumg,--Lecreusementd'untunneldartsunterrainmeublendcessitesouventunsout}nementtemporaireafindestabiliserlefrontdetaille.Danelecasd'unbouclierg~bone,cettepressiondestabilisationestr~alisdeparunmdlangedebentoniteetd'eausouspression.Durantcesdernikresanndes,grdveg~unedvolutiontechnologiqueimportante,l'utilisationdeboucliereg~bouspourlar~alisationd'ouvragessouterrainsaconnuunsaccgsmondial.Pourtant,faceddesconditionsg~ologiquesextr~mementcl~favorables,desprobl~mesmajeursdestabilitddufrontdetaillepeuventseprdsentsr.L'articleprdsentapourobjectifd'am~liorerlacompr~heneiondelamdcaniquedelarupturedufrontdetaillepourlecasd'unfrontstabilisdpardelabous~basedebentonite.On~tudielesrelationscomplexesentrelesdiffdrentsparam$trss(r~sistanceaucisaillementetperm~abilit~dusol,pressiondeboue,donndesg~omdtriquesdutunnel,coefficientdesdcurit~)entenantcomptedeseffetstransitoiresdtls~l'infiltrationprogressivedelabouedartslesolenavantdel'avancement.Enoutre,d'autressujetscommeletempspendantlequellefrontdemillerestestableoul'influencedelavitessed'avancementsontdiscut~squantitativement.1.IntroductionDuringtheexcavationofatunnelthroughsoftwater-bearingground,theexcava-tionfacebecomesinstablewhenthesoillackssufficientcohesion.Atempo-rarysupportisthereforerequiredtomaintaingroundstabilityinthework-ingarea.Inshieldtunnelling,collapseofthewallsandroofispreventedbytheshieldandthesubsequentsegmentlining.Apartfromratherimpracticabletypesofsupportsuchasbreastingplates,stabilityoftheworkingfacecanbeachievedbyusingcompressedair.However,theuseofcompressedairintroducestheriskofablowout,i.e.,asuddenreductionofsupportpressureonaccountofrapidlossofair.ThedifferenceinthehydrostaticheadatthecrownandthefloorofthetunnelPresentaddress:Dr.G.AnagaostouandProf.If.Kovdri,InstituteforGeotechnics,SwissFederalInstituteofTechnologyZurich,ETH-H6nggerbergPF-133,CH-8093Zurich,Switzerland.resultsinexcessairpressureandmaycauseanescapeofairtothesurface--eitherbyleakagethroughsoilporesorbyaheavingofthegroundmassabovetheshield.Thisphenomenonispar-tieularlylikelytooccurwhenthetun-nelisshallowanditsdiameterlarge.Theproblemsassociatedwiththeuseofcompressedairhaveledtotheincreasinguseofslurry-shieldtunnel-lingoverthelasttwentyyears(see,e.g.,BeckerandSawinski1982).Inaslurryshield,thetemporarysupportofthefaceisprovidedbyapressurizedmixtureofbentoniteorclayandwater.Becauseofthehighviscosityoftheslurry,theriskofanuncontrolledes-capeoffluidbyleakageisgenerallyreduced.Asslurryisonlyslightlyheavierthanwater,theexcessfluidpressureinthecrownissmall.Theriskofanupheavalofthegroundthere-foreiseliminatedaswell.Inthelastdecade,slurry-shieldtun-nellinghasbeenappliedsuccessfullyworldwideonseveralprojects.Thelargestexistingtunnel(inGrauholz,Switzerland),excavatedbytheslurryshieldmethod,hasadiameterof11.65m(seeFig.1).However,underex-tremelyunfavourabiegeologicalcon-ditions,faceinstabilitiesmayoccurlocallywhenthistechniqueisused(BellingandEisenbach1989;Baben-dererde1991;Balmer1992).Figure2showsschematicallytwotypicalpatternsoffailure.Inthefirstcase,majorsoilmovementsarere-Figure1.Thecutting-headofthehydroshieldintheGrauholzTunnelhadadiameterof11.65m(fromKovdrietal.1993).TunnellingandUndergroundSpaceTechnology,Vol.9,No.2,pp.165-174.1994Copyright(~)1994ElsevierScienceLtdPrintedinGreatBritain.Allrightsreserved0886-7798/94$6.00+.O0~Pergamon165NotationThefollowingsymbolsareusedinthispaper:c=cohesion;D=diameteroftunnel;dlo=characteristicgrainsize;e=penetrationdistance;e=~=maximumpenetrationdis-tance;F=safetyfactor;f=criticalpressuregradient;f=pressuregradient;f=stagnationpressuregra-dient;H=overburden;Hw=elevationofwatertable;k=soilpermeabilityw.r.t.water;n=soilporosity;p=pressure;Pb=pressureinbentonite;p,=pressureingroundwater;q=macroscopicfiltrationve-locity;r=ratioofvolumetosurfaceofprismaticblock;S=supportforce;So=supportforceofmem-brane-model;t=stand-uptime;v=excavationadvancerate;v=criticalexcavationadvancerate;=submergedunitweight;7b=unitweightofslurryincl.aggregatedsolidmatter;7d=dryunitweight;7s=unitweightofslurrywith-outaggregates;7.=unitweightof