CHAPTER 12 OF ROCK ENGINEERING

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1、12Tunnelsinweakrock12.1IntroductionTunnellinginweakrockpresentssomespecialchallengestothegeotechnicalengineersincemisjudgementsinthedesignofsupportsystemscanleadtoverycostlyfailures.Inordertounderstandtheissuesinvolvedintheprocessofdesigningsupportforthistypeoftunnelitisnecessarytoexaminesome

2、verybasicconceptsofhowarockmasssurroundingatunneldeformsandhowthesupportsystemsactstocontrolthisdeformation.Oncethesebasicconceptshavebeenexplored,examplesofpracticalsupportdesignsfordifferentconditionswillbeconsidered.12.2DeformationaroundanadvancingtunnelFigure12.1showstheresultsofathree-di

3、mensionalfiniteelementanalysisofthedeformationoftherockmasssurroundingacirculartunneladvancingthroughaweakrockmasssubjectedtoequalstressesinalldirections.Theplotshowsdisplacementvectorsintherockmassaswellastheshapeofthedeformedtunnelprofile.Figure12.2givesagraphicalsummaryorthemostimportantfe

4、aturesofthisanalysis.Deformationoftherockmassstartsaboutonehalfatunneldiameteraheadoftheadvancingfaceandreachesitsmaximumvalueaboutoneandonehalfdiametersbehindtheface.Atthefacepositionaboutonethirdofthetotalradialclosureofthetunnelhasalreadyoccurredandthetunnelfacedeformsinwardsasillustratedi

5、nFigures12.1and12.2.Whetherornotthesedeformationsinducestabilityproblemsinthetunneldependsupontheratioofrockmassstrengthtotheinsitustresslevel,aswillbedemonstratedinthefollowingpages.Notethatitisassumedthatdeformationprocessdescribedoccursimmediatelyuponexcavationoftheface.Thisisareasonableap

6、proximationformosttunnelsinrock.Theeffectsoftimedependentdeformationsupontheperformanceofthetunnelandthedesignofthesupportsystemwillbenotbediscussedinthischapter.12.3TunneldeformationanalysisInordertoexploretheconceptsofrocksupportinteractioninaformwhichcanreadilybeunderstood,averysimpleanaly

7、ticalmodelwillbeutilised.Thismodelinvolvesacirculartunnelsubjectedtoahydrostaticstressfieldinwhichthehorizontalandverticalstressesareequal.Tunneldeformationanalysis205DeformedprofileFigure1:Verticalsectionthroughathree-dimensionalfiniteelemen